Directed wet-chemical synthesis of metallic nanoparticles of different sizes and shapes: control mechanisms

Directed wet-chemical synthesis of metallic nanoparticles of different sizes and shapes: control mechanisms Síntesis química de nanopartículas metálicas de diferentes tamaños y formas: mecanismos de control Yasser Attia Attia February 2012 Thesis doctor University of Santiago de Compostela Faculty of Chemistry Department of physical chemistry Directed wet-chemical synthesis of metallic nanoparticles of different sizes and shapes: control mechanisms Síntesis química de nanopartículas metálicas de diferentes tamaños y formas: mecanismos de control A thesis submitted to the University of Santiago de Compostela In partial fulfillment of the requirements for the degree of doctor in chemistry in the department of physical chemistry, Faculty of Chemistry February 2012 By Yasser Attia Attia Santiago de Compostela University Faculty of Chemistry Approval Sheet Title of thesis: Directed wet-chemical synthesis of metallic nanoparticles of different sizes and shapes: control mechanisms Name of student: Yasser Attia Attia Awad Khalifa Approved by: Prof. Dr. M. Arturo López-Quintela Dpt. Physical Chemistry, Fac. Chemistry University of Santiago de Compostela Date Approved: 12 /2011 Santiago de Compostela, 2011 MANUEL ARTURO LÓPEZ QUINTELA, CATEDRÁTICO DE QUÍMICA DEL DEPARTAMENTO DE QUÍMICA FÍSICA DE LA UNIVERSIDAD DE SANTIAGO DE COMPOSTELA CERTIFICA: La presente tesis titulada¨ Síntesis química de nanopartículas metálicas de diferentes tamaños y formas: mecanismos de control¨ que presenta Yasser Attia Attia para optar al grado de doctor en Química, ha sido realizada bajo mi dirección, en el Departamento de Química Física de la facultad de Químicas de la Universidad de Santiago de Compostela, y hallándose concluida, autorizo su presentación para que sea evaluada por el tribunal correspondiente: Y para que así conste, firmo en la presente en Santiago de Compostela, Diciembre, 2011. Dedication This work is dedicated to my parents, my wife, my kids and my country Egypt. ix 5-4: Effect of Laser irradiation on the gold nanorods. 245 I) Laser in cw (continuous wave) mode. 247 II) Pulsed Laser mode. 247 5-4-1: Effect of Nd: YAG laser and Nd:YVO 4 (continues wave mode(CW)). 249 5-4-2: Nanosecond pulsed-mode laser. 251 IEffect of Pulsed Nd: YVO 4 laser, working in Q-switch regime. 251 IIEffect of Pulsed Nd:YAG laser, working in Q-switch regime. 253 5-5: Conclusion. 263 5-6: References. 264 x List of Figures: Figures Pages 1-1 A picture showing the nanoscale in context. The length scale at the top ranges from 1 m to 10 -10 m. The section from 10-7 m (100 nm) to 10 -9 m (1nm) is expanded on the length scale below. The typical length scale of interest for nanoscience is from ≈ 100 nm down to the atomic scale. 7 1-2 A sketch diagram of the gold nanoparticle growth. 10 1-3 A diagram of the experimental set up for laser ablation to form gold nanoparticles. 12 1-4 Water-in-oil and oil-in-water microemulsions. 13 1-5 (A) TEM image of gold nanotriangles synthesized by the reduction of aqueous HAuCl 4 solution with lemon grass extract. B) UV-Vis -nIR spectra of gold nanoparticles synthesized by adding different amounts of lemongrass leaf extract to a HAuCl 4 solution. (C) TEM images of gold nanoplates synthesized by the reduction of aqueous AuCl 4 - by sea weed extract. D) Singlecrystalline Ag nanoplates synthesized in aqueous medium at room temperature using an extract of the unicellular green alga Chlorella vulgaris. Inset shows the SEM image of a single Ag nanoplate. 14 1-6 Silver nanoparticles and silver nanotriangles after illumination with a fluorescent lamp. 15 1-7 A diagram of the set-up for preparation of gold nanorods via the electrochemical method. 16 1-8 (A and B) FESEM images of an alumina membrane. (C) Schematic representation of the successive stages during formation of GNRs via the template method. (D) TEM micrographs of GNRs obtained by the template method. 18 1-9 (a) Image of photochemically prepared gold nanorods solution using different concentration of AgNO3 , and (b) corresponding UV-Vis spectrum. 20 1-10 A sketch explaining surface plasmon absorption. The electric field E of the incident light induces polarization of the free surface electrons with respect to the heavier ionic core. 25 1-11 Size dependence of the plasmon absorption of spherical gold nanoparticles of different sizes. 28 1-12 Absorption spectra of spherical and rod shape nanoparticles. 32 1-13 Absorption spectra showing the dependence of SP L of the gold nanorods on the aspect ratio. 33 1-14 A representation diagram illustrating the optical response of rodlike nanoparticles to an electric field E. Two oscillating modes can be possible: (a) the transverse oscillation along the B or C axis and (b) the longitudinal oscillation along the A axis. 33 1-15 (A) Scheme of a single GNR. (B) HRTEM showing the different crystal planes and lattice structure. (C) Scheme showing the different ways of arranging the NRs. 43 2-1 Absorption spectra showing the Nanogap Ag-clusters with an 52 xi AFM picture of the clusters deposited on mica (mean square roughness ~ 150 pm) and the profiles throughout the red and green lines depicted on the AFM picture. 2-2 A diagram showing the components of a typical UV-Vis spectrometer. 54 2-3 A diagram showing the components of a typical TEM. 55 2-4 A diagram showing the components of a typical AFM. 56 2-5 A diagram showing the pulsed Nd:YAG laser setup. 58 2-6 Relation between the applied intensity (“corriente”) and the output power. 59 2-7 A diagram showing the pulsed Nd: YVO4 laser setup. 60 2-8 A picture showing the exposure of the gold nanoparticles in a quartz cuvette to laser radiation. 60 3-1 TEM images of gold nanorods, prepared by the Murphy's method (a) a.r.= 4.6; (b) a.r.=13; (c) a.r.=18. The scale bar (100 nm) applies to all three images. 74 3-2 A cartoon illustrating the “zipping” mechanism: the formation of a bilayer of Cn TAB (squiggles) on the nanorod (black rectangle) surface may assist the growth of the nanorod as more gold ions (black dots) are introduced. 76 3-3 Sketch showing the mechanism for gold nanorod formation. The transport of the gold ions bound to the CTAB micelles to the growing seed particles is controlled by the double layer interaction. 78 3-4 A cartoon illustrating of the mechanism of nanorod growth from CTAB protected gold seed particles in the presence of Ag + . 85 3-5 Absorption spectrum of gold seeds after 10 min, 3h, 7h, 20h and 40h of preparation. Please note that the Au Plasmon band is absent even at 3h from preparation. 87 3-6 AFM images of the Au seeds after 10 min of preparation. Average size: 0.6±0.2nm. 87 3-7 TEM images of gold nanoparticles prepared by the citrate method (A), for gold nanoparticles produced without AgNO3 (B) and using gold seeds after 24h (C), (D) UVVis absorption spectra of gold nanoparticles prepared by the citrate method (red line) and by the CTAB metho d without addition of AgNO3 (black line). (E) UV - Vis absorption spectra of gold nanoparticles prepared without addition of AgNO 3 at different addition times of the seed solution. 88 3-8 Absorption spectra showing the effect of Ag ions on the growth of gold nanoparticles, (A) from 40 to 100μM AgNO 3 and (B) from 120 to 320μM AgNO3. 90 3-9 TEM images of gold nanorods produced with (A) 40μM, (B) 80 PM and (C) 100PM of AgNO3 , respectively. Gold nanoprisms produced for 120μM AgNO3 (D); 160μM AgNO3 (E); 200μM AgNO 3 (F). Aggregated particles produced for 240μM AgNO3 (G) and 320μM AgNO3 (H). 91 3-10 Absorption spectra for large volume synthesis (5-500 ml) using 92 xii 80μM AgNO3 and 0.1 M CTAB. 3-11 Absorption spectra for large volume synthesis (20 ml) with different concentrations of AgNO3 (40, 80, 160, and 200 μM). 92 3-12 Absorption spectra showing the growth of the formed particles after addition of AA and the seeds to the growth solution without AgNO3. 93 3-13 Absorption spectra showing the growth of the formed particles after addition of AA and the seeds to the growth solution with 40μM of AgNO 3. 93 3-14 Absorption spectra showing the growth of the formed particles after addition of AA and the seeds to the growth solution with 80μM of AgNO 3. 94 3-15 Absorption spectra showing the growth of the formed particles after addition of AA and the seeds to the growth solution with 16 0μM of AgNO3. 94 3 -16 Absorption spectra showing the effect of the seed concentration using 40μM of AgNO3. 95 3 -17 Absorption spectra showing the effect of the seed concentration using 60μM of AgNO3. 96 3 -18 Absorption spectra showing the effect of the seed concentration using 80μM of AgNO3. 96 3 -19 Absorption spectra showing the effect of the seed concentration using 100μM of AgNO3. 97 3 -20 (A) TEM image of the NPs obtained using 0.113 μM of seeds and 40 μM AgNO 3 (a.r.= 3.4) and (B) TEM image for a similar synthesis using 80μM AgNO3 (a.r.= 4.2). 97 3 -21 Absorption spectra showing the effect of the aging time of the seeds on the growth of NPs without AgNO3. 99 3-22 Absorption spectra showing the effect of the aging time of the seeds on the growth of NPs using 40μM AgNO3. 99 3-23 Absorption spectra showing the effect of the aging time of the seeds on the growth of NPs using 80μM AgNO3. 100 3-24 TEM images showing the produced nanoparticles with 40μM AgNO3 (A) and 80μM AgNO3 (B) using 24h aged seed solution. 100 3-25 Absorption spectra showing the effect of the aging time of the seeds on the growth of NPs using 160μM AgNO3. 101 3-26 Absorption spectra showing the reduction of Ag+ ions at neutral pH by ascorbic acid. Please, note the absence of the Ag plasmon band even at 60 minutes. 102 3-27 AFM picture of Ag-clusters (prepared in situ after 10 min. from the addition of ascorbic acid to the growth solution in the absence of Au salt) deposited on mica with the profiles throughout the red and green lines and 3D AFM picture showing that the size of Ag-clusters is below 1 nm. 103 3-28 AFM picture of Ag-clusters deposited on mica with the profiles throughout the red and green lines ( A) and, HRTEM for picture of Ag nanoparticles of approx.4nm with a pyramidal shape (B). 104 3-29 Absorption spectra showing the reduction by ascorbic acid (AA) 105 xiii of Au3+ to Au+ in the growth solution (without AgNO 3 ) at neutral pH. 3-30 Absorption spectra showing the reduction of the growth solution by AA at neutral pH. 105 3-31 Absorption spectra showing the absorption of CTAB, HAuCl4, and growth solution. 106 3-32 A picture of the change in the color of the samples as the [AgNO3] increases from 40μM to 240μM. 106 3-33 Absorption spectra showing the influence of the concentration of Ag NO3 on the formation of gold nanorods showing a faster growth with increasing concentra tion (only displayed the final spectra at the end of the reaction). 107 3-34 Catalytic effect of Ag-ions based on the increase of the transverse Plasmon band at different concentrations of AgNO3. (A) Absorbance at O = 538nm vs time. (B) Fitting to a pseudofirst order reaction: log (A -A0) vs time. A0 : initial absorbance. (C) Linear dependence of the inverse of the relaxation time ( W -1 /s -1) with the Ag+ concentration. Slope = kcat =(2.6±0.6)x10 2 M -1 s -1 108 3-35 Catalytic effect of Ag-ions based on the increase of the longitudinal Plasmon band at different concentrations of AgNO 3. (A) Absorbance at O = 724nm vs time. (B) Fitting to a pseudo -first order reaction: log (A-A0) vs time. A0 : initial absorbance. (C) Linear dependence of the inverse of the relaxation time ( W-1 /s-1) with the Ag+ concentration. Slope = kcat =18±2 M -1 s -1 110 3-36 Absorption spectra of gold nanoparticles with different shapes at different concentration of silver ions : spheres (Rcluster≈0), rods (Rclusters≈1) and prisms (Rclusters≈3). 113 3-37 (A) Absorption spectra showing the effect of Ag-clusters at different concentrations (6.6x10 -11 – 9.5x10-7 M). (B) Picture show ing the time for the appearance of the color in the samples, which is faster as the [Ag-clusters] increases. (CN) TEM images of gold nanorods and nanoprisms obtained with different concentrations of Ag-clusters. 115 3-38 Absorption spectra showing the effect of NaCl in the absence and the presence of Ag-clusters. 117 3-39 TEM images showing the samples without NaCl and Agclusters (A); with 10-6 M NaCl without Ag-clusters (B); and with 10 -6 M NaCl in the presence of 6.5x10 -10 M Ag-clusters (C). 117 3-40 Absorption spectra showing the effect of NaCl with different concentrations (0 – 8.5x10-3 M) (A), and TEM images of the corresponding products (B-K). 119 3-41 Absorption spectra showing the effect of the concentration of CTAB (from Aldrich) on the growth of gold nanorods. 122 3-42 Absorption spectra showing the effect of the concentration of AgNO 3 on the growth of gold nanorods using CTAB from Fluka. 122 3-43 Absorption spectra showing the formation of gold nanorods at 123 xiv the same experimental conditions using CTAB from Aldrich and Fluka. 3-44 Absorption spectra showing the effect of AA concentration with 0.048 μM seed solution. 126 3-45 Absorption spectra showing the effect of [AA] with 0.097 μM seed solution. 127 3-46 TEM images showing the effect of AA concentration (0.4 – 1.6 mM ). Gold nanorods with different aspect ratios (2.7 to 5.8) are observed. 127 3-47 TEM images showing the effect of AA concentration; (A) 1.1m M ascorbic acid with 0.048μM seed concentration; (B) 1.1mM ascorbic acid with 0.097μM seed concentration. 128 3-48 Absorption spectra showing the effect of citrate-seed concentrations aged 10 min.(6 to 1000 μl) using 80μM AgNO3. 129 3-49 Absorption spectra showing the effect of citrate-seed concentration aged 3h (6 to 1000 μl) using 80μM AgNO3. 130 3-50 Absorption spectra showing the effect of AgNO 3 concentration from 0 to 320 μM using a fixed amount of 100μl of citrateseeds (aged 10min.). 131 3-51 Absorption spectra showing the effect of AgNO 3 concentration from 0 to 320 μM using a fixed amount of 100μl of citrateseeds (aged 3h.). 131 3-52 Absorption spectra showing the effect of [Ag-clusters] using 100μl of citrate-seeds (aging 10min.). 132 3-53 Absorption spectra showing the effect of [Ag-clusters] using 100μl of citrate-seeds (aging 3h.). 133 3-54 TEM images showing the effect of [Ag clusters] (0.175 μM); (A) citrate -seeds aged during 10 min and (B) citrate-seed s aged during 3h. 133 4-1 Mass spectra of the solutions aged at different times (30, 60, 120, 240, and 300 Secs) respectively. 142 4-2 Absorption spectra showing the growing of gold clusters and seeds up to 43 hours. 146 4-3 Absorbance intensity of gold clusters at λ= 520 nm from 30 to 10000 s. 147 4-4 First fitting of the absorbance of gold clusters and seeds at λ= 520 nm with time, from 30 to 10000 s. 149 4-5 Three exponential fitting of the absorbance of gold clusters and seeds at λ= 520 nm with time, from 30 to 10000 s. 149 4-6 Relations of concentration and molar fractions of Au 2 , Au 6 and Au55 with time (A and B, respectively). 152 4-7 AFM images of the Au seeds after 10 min of preparation. Average size: 1.8±0.2nm. 155 4-8 Absorption spectra showing the formation of high aspect ratio gold nanorods by the addition of gold clusters with different concentrations, i.e., aged at different times without addition of seeds; A) 10 s; B) 30 s; C) 1min; D) 5 min. Please note that without Auclusters, only aggregated particles are formed. 157 4-9 Absorption spectra showing the formation of high aspect ratio gold nanorods by the addition of 5 ml of gold clusters with different 158 xv concentrations (different aging times) without addition of seeds. For comparison purposes the spectra of the same reaction without addition of gold clusters are included. 4-10 Absorption spectra showing the formation of high aspect ratio gold nanorods by the addition of 5 ml of gold clusters with different concentrations (different aging times) with addition of seeds. For comparison purposes the spectra of the same reaction without addition of gold clusters are included. 160 4-11 Absorption spectra showing the formation of high aspect ratio gold nanorods by the addition of 5 ml of gold clusters with different concentrations, (different times) in the presence of seeds. 161 4-12 (A&B) TEM images showing the particles prepared using Au clusters aged during 10sec (without & with seeds, respectively); (C&D) with Au clusters aged during 30sec (without & with seeds, respectively); (E&F) with Au clusters aged during 60sec (without & with seeds, respectively); (G&H) with Au clusters aged during 5min (without & with seeds, respectively), and (I&J) with Au clusters aged during 5min (with double concentration of seeds). 162 4-13 Absorption spectra showing the influence of CTAB concentration (105 to 0.1 M) in gold clusters (1min) on the growth of gold nanoparticles in the absence of seed. 166 4-14 Absorption spectra showing the influence of CTAB concentrations (10 -5 to 0.1M) using gold clusters aged during 1 min on the growth of gold nanoparticles in the presence of seeds. 167 4-15 TEM images, showing the formation of gold nanorods with [CTAB] = 10 -4 (A) and 8 x 10-4 M (B) using the gold clusters aged during 1 min without adding seeds, and formation of gold nanorods with [CTAB] = 10 -4 (C) and 8 x 10-4 M (D) using gold clusters aged during 1 min with the addition of adding seeds. 167 4-16 Absorption spectra showing the effect of diluted concentration of seeds aged during 1h on the growth of gold nanoparticles using gold clusters aged during 1min and 5min. 168 4-17 Absorption spectra showing the effect of diluted concentration of seeds aged during 3h on the growth of gold nanoparticles using gold clusters aged during 1min and 5min. 169 4-18 Absorption spectra showing the effect of diluted concentration of seeds aged during 5h on the growth of gold nanoparticles using gold clusters aged during 1min and 5min. 169 4-19 TEM-images of gold NRs prepared with seeds (aged during 1h) in a dilution 1:1000 using gold clusters aged during 1min (A) and 5min (B). TEM images of gold NRs prepared with seeds (aged during 3h) in a dilution 1:1000 using gold clusters aged during 1min (C) and 5min (D). TEM images of gold NRs prepared with seeds (aged during 5h) in a dilution 1:1000 using gold clusters aged during 1min (E) and 5min (F). 170 4-20 Absorption spectra showing the effect of different concentrations of AgNO3 on the growth of gold nanoparticles using Ni-seeds. 172 4-21 Absorption spectra showing the effect of different concentrations of AgNO3on the growth of gold nanoparticles using Co-seeds. 172 4-22 Absorption spectra showing the effect of different concentrations of Ni-seeds on the growth of gold nanoparticles using 80μM AgNO3. 173 4-23 Absorption spectra showing the effect of different concentrations of Co-seeds on the growth of gold nanoparticles using 80μM AgNO3. 173 xvi 4-24 TEM images showing the effect of Ni-seeds on the growth of gold nanoparticles (A -C for 40, 100 and 200μM of AgNO3, respectively); and (D -F) effect of Coseeds on the growth of gold nanoparticles (40, 100 and 200μM of AgNO3, respectively). 174 5-1 Absorption spectra of gold nanorods (40, 80 and 120μM of AgNO3) with different aspect ratios and gold nanoprisms (160 and 200 μM of AgNO 3 ), using CTAB as a capping agent in the seed solution. Gold nanorods prepared with 80μM of AgNO 3 using citrate as capping agent in the seed solution are also shown by comparison. 183 5-2 A and B: TEM images of gold nanorods prepared using 40 and 80μM of AgNO 3 respectively. C: gold nanorods prepared with 80μM AgNO3 using trisodium citrate as capping agent in the seed solution. D: gold nanoprisms prepared with 200 μM AgNO3. 184 5-3 Change in the absorption spectra of gold rods of aspect ratio 2.27 (A) and 3.1 (B) at room temperature (27 0 C) from 30 min to 90 days. 185 5-4 Absorption spectra showing the effect of thermal heating on gold nanorods of aspect ratio 2.2, 3.4, 4.5 (A, B, and C), and 3.4 (D) prepared with citrate - seeds at different temperatures. TEM images showing gold nanorods prepared using CTAB - seeds with a.r. =3.4 aged at 160 0 C (E) and gold nanorods prepared by citrate-seeds with a.r. =3.4 aged at 160 0 C (F). 188 5-5 Absorption spectra showing the effect of temperature on the gold nanorod´s absorption of an aqueous solution sample with a few drops of glycerol: (a) 25 0C, (b) 110 0C, (c) 120 0C, (d) 140 0 C, (e) 150 0 C, (f) 155 0C, and (g) 160 0 C. The disappearance of the transverse surface plasmon band ( λmax = 520 nm) at 160 0 C suggests that even the spherical nanoparticles are not present, which indicates that the micelles have been decomposed at this temperature 189 5-6 (A) Absorption spectra of a gold nanorod´s solution as a function of time after being placed in the thermostat at 100 0 C; and (B) Arrhenius plot for the determination of the activation energy for the thermal reshaping of gold nanorods (21.0 ±1.0 Kcal mol -1 ). 190 5-7 Absorption spectra showing the effect of thermal heating on the gold nanorods of a.r . =2.2 at temperatures 40, 60, 80, and 1000C with time (A - D, respectively). The activation energy for the decomposition of gold nanorods (a.r. = 2.2) is showing in (E). 193 5-8 Absorption spectra showing the effect of thermal heating on the decomposition of gold nanorods of a.r . =3.4 at temperatures 40, 60, 80, and 100 0C (AD, respectively). The activation energy for the decomposition of gold nanorods (a.r . = 3.4) is showing in (E). 195 5-9 TEM images showing the decomposing of gold nanorods (a.r.= 2.2) at different temperatures 27, 120, 160 and 180 0 C. 196 5-10 (A) TGA data showing GNRs, CTAB, and GNRs after washing 2 times at 6000 rpm for 30 min with distilled water and dried in desiccators under vacuum (green line) from 45 0C to 6000 C at 10 0 C/min. (B) is the zooming of figure A from 50 0 C to 350 0 C. 197 5-11 DSC data showing GNRs and CTAB dried in desiccators under vacuum. 197 xvii 5-12 Absorption spectra showing the effect of thermal heating on gold snapped prisms and prisms from 40 0C to 1800 C (A and B, respectively). 199 5-13 Absorption spectra showing the effect of the thermal heating on gold nanoprisms at temperatures 40 (A), 60 (B), 80 (C), and 100 0 C (D) with time. The activation energy for the decomposition of gold nanoprisms is shown in (E). 201 5-14 TEM images showing the decomposing of gold nanoprisms at different temperatures: 27, 120, 160 and 180 0 C. 202 5-15 Absorption spectra showing the effect of thermal heating on gold nanorods with 30% PVP (A) and with 15% PVP (B) from 40 0 C to 220 0 C. 204 5-16 Absorption spectra showing the effect of thermal heating on gold nanorods containing 30% PVP with time, at temperatures 40 -1000C (A-D, respectively). The activation energy for the decomposition of gold nanorods containing 30% PVP is shown in (E). 206 5-17 Normalized absorption spectra showing the effect of thermal heating on rods with different aspect ratios; rods containing 30%PVP; and nanoprisms. 207 5-18 Absorption spectra showing the effect of addition of 30 % (A) and 15% (B) PVP on the thermal stability of the gold nanoprisms. TEM images (C and D) showing gold nanoprisms with 15% and 30 % PVP, respectively. 209 5-19 Absorption spectra showing that UV-irradiation of gold nanospheres prepared in the absence of AgNO 3 during 25h has almost no noticeable effect. 210 5-20 Absorption spectra showing the effect of UV-light on the gold nanoprisms at different times, from 0 to 24h. 211 5-21 Absorption spectra showing the effect of UV-light on gold nanoprisms at different times: 1h, 3h, 1 day, 2 days, and 3 days. 211 5-22 Absorption spectra showing the changes of gold nanorods of (a.r= 2.5 and 3.7) after irradiation by UV - light (A & B, respectively); (C) normalized absorption spectra of gold nanoparticles with different shapes at different irradiation times and (D) absorption spectra showing that exposure to UV light for ~ 25 h leads to almost the complete decomposition of the rods. 214 5-23 Absorption spectra showing the changes of gold nanorods (a.r . ≈ 3.9 at pH = 3.9) after irradiation under UVlight, at T= 28 0 C, showing again the complete decomposition of the rods. 215 5-24 Absorption spectra showing the changes of gold nanorods of (a.r. ≈ 3.9, at pH 3.9) after irradiation under UV - light for ~ 30 h leading to the complete decomposition of the rods and the formation of Au(III) - CTAB complex, as it can be seen by the appearance of the band at 398 nm. 216 5-25 TEM images showing the effect of the UV-irradiation on gold nanorodsat different times of irradiation: (A) 0, (B) 8h, (C) 12h, (D) 18h, (E) 22h and (F) 28h. 217 xviii 5-26 Absorption spectra showing the similarity between the absorption spectra of 0.5mM Au(III) -CTAB complex (growth solution of NRs without addition of AA) and GNRs after 30h of UV-irradiation. 217 5-27 Absorption spectra showing the changes of gold nanorods with addition of a small amount of ethanol and irradiating the sample with UV-light during ~ 30 h. 220 5-28 Absorption spectra showing the changes of gold nanorods preheated to 130 0C for 10min and irradiated with UVlight during ~ 30 h. 222 5-29 Picture showing the difference between the initial GNRs sample and irradiated gold nanorods during 30 hours (1) with pale yellow color characteristic of the formation of Au (III) - CTAB complexes, (2) for gold nanorods pre -heated to 1300 C and then irradiated during 30 hours with a pink color characteristic of the formation of gold nanospheres and (3) for gold nanorods with ethanol showing almost no change in the color of the initial GNRs solution. 222 5-30 Absorption spectra showing the small changes of gold nanorods pre -heated to 1300C for 2h and then irradiated with UVlight during ~ 40 h. 223 5-31 Absorption spectra showing the changes of gold nanorods preheated to 130 0 C for 2h in the presence of external synthesized Ag clusters and then irradiated with UV-light during 30h. 223 5-32 Absorption spectra showing the changes of gold nanorods (a.r . ≈ 3.7, at pH 12.6) after irradiation under UVlight during ~ 30 h. 224 5-33 Normalized absorption intensities of the SP L and SP T bands of gold nanorods at both pHs (3.9 and 12.6) under UV light irradiation during 30 h. 225 5-34 Absorption spectra showing the changes of gold nanorods after being washed 2 times and irradiated with UV - light during ~ 30 h. 226 5-35 Absorption spectra showing the changes of gold nanorods after being washed 2 times and preheated to 130 0C. The sample was then irradiated with UV-light during ~ 30 h. 226 5-36 Absorption spectra showing the changes of gold nanorods after being washed 2 times. A small amount of ethanol was added to the sample and then irradiated with UV-light during ~ 30 h. 227 5-37 Normalized absorption intensities of the SP L bands of gold nanorods, washed NRs, washed NRs and preheated to 130 0 C, and washed NRs with a small amount of ethanol, under UV light irradiation during ~30 h. 227 5-38 Absorption spectra showing the dissolution of gold nanorods (pH=3.9) with stirring, irradiated with UV - light during ~ 12 h; and the reformed NRs after addition of AA & seeds solution. The including picture shows the difference in color between before UV irradiation, after irradiation and after addition AA & seeds solution. 229 xxv Summary The successful applications of nanoparticles require the ability to tune their properties by controlling size and shape at the nanoscale. Therefore, it is very important to prepare nanoparticles of well-defined sizes and shapes in order to control their physicochemical characteristics. In this work we described an improved seed mediated synthesis to prepare gold nanoparticles of different shapes (Spheres, Rods, Prisms,…..). New absorption features different from that of the classical surface plasmon absorption bands of rods and spheres are found to be associated with the appearance of nanoprisms, and snapped prism shaped gold nanoparticles. The effect of [Ag-ions], [seed], [ascorbic acid], [Cl-] and the aging of the seed solution were studied in order to get a deeper understanding of the growth mechanism. Our results indicate that the ratio of [Ag+] to [seed] is one of the key parameters for controlling the shape of the particles, e.g. rods, prisms or snapped prisms. We proposed a mechanism for the formation of the different gold shapes that takes in account the reduction of Ag ions by ascorbic acid forming intermediate small Ag clusters, which can then act as catalysts for the formation of gold nanoparticles. The mechanism has been proved using externally prepared Ag clusters instead of adding Ag ions. The results also indicate that, to get such different shapes, the used CTAB concentration should be above the CMC and is independent of the CTAB supplier. It seems therefore that, the catalytic activity of clusters needs the presence of micelles, whose micellar surface can be viewed as a kind of catalyst nanosupport. These results open a new way of thinking in the interpretation of the mechanisms involved in the anisotropic growth of nanoparticles. In order to confirm that mechanism, we developed a simple method to synthesize gold nanorods with high aspect ratios using gold clusters (Au2- xxvi Au6) stabilized by CTAB. These clusters were added to gold ions in the growth solution without adding any surfactant or polymer. The mechanism proposed for the formation of gold nanorods with high aspect ratios in the absence / presence of the seeds is based on the deposition of these catalytic clusters on active sites of the gold surface. The deposition and the corresponding reduction reaction mainly occur at the end (tips) of the nanorods. The concentration of these clusters was calculated from the UVvis absorption of the solutions at different reaction times. The effects of the dilution of CTAB in the clusters solution and the dilution of the seed solution on the growth of gold nanorods were also studied. The results showed that gold nanorods can be formed without addition of the seeds indicating that the seed particles do not directly affect the growth of gold nanorods. The effect of the gold nanoparticle shape on the stability (thermal heating and photo stability by UV-light and laser) was also studied. It was found that the mechanism of the particle dissociation in the case of nanoprisms is different from than that of nanorods under thermal heating. Great enhancement of the thermal stability has been achieved by adding specific amounts of polyvinyl pyrrolidone (PVP) to the gold nanoparticles of different shapes capped with cetyltrimethylammonium bromide (CTAB). It is worth to mention that gold nanorods stabilized by PVP are totally stable up to 220 0C. The effect of irradiation with UV-light on gold nanoparticles also showed different mechanisms depending on the shape of the NPs. Under long irradiation times (up to approx. 30 h), gold nanorods totally decomposed and another band started to appear at 398nm , corresponding to the formation of the Au(III)-CTAB complex, indicating that the gold(0) NRs are dissolved and transformed to gold-ions. The photocorrosion of the Au nanoparticles was explained assuming that semiconducting Ag-clusters are attached to the tips of the NRs as we assumed in the growth xxvii mechanism. This photocatalytic activity of Ag clusters was confirmed adding ethanol to the irradiated rods, which acts as hole scavenger and avoids in this way the dissolution of the rods. The inhibition of the Au photocorrosion can also be done by pre-heating the irradiated rods to 1300C which corresponds to the fusion temperature. Further experiments were used to analyze the Au ions produced during the photocorrosion and to confirm the high stability of the photocatalytic Ag clusters, which can be reused to direct again the formation of Au NRs after its photodissolution. Finally, the effect of different types of nanosecond lasers (CW and pulsed) on different aspect ratio gold nanorods was studied and the corresponding mechanisms (fragmentation and melting) were proposed to explain the different observed nanosecond pulsed laser effects. xxviii Resumen En la actualidad, la nanotecnología ocupa un lugar preferente tanto en lo que se refiere a los importantes retos científicos que tiene planteados, como a las enormes potencialidades de aplicación que ofrece esta disciplina científica. Dentro de esta nueva gran área de investigación, la preparación de los nanomateriales (nanoparticulas, NPs) es uno de sus puntos clave, requiriéndose un perfecto control de su tamaño y forma a escala nanométrica para que puedan ser aplicadas con éxito. Por ello, es de gran importancia desarrollar métodos adecuados que permitan obtener nanopartículas de tamaño y forma controladas. Aunque se han desarrollado un gran número de métodos para la fabricación de nanoestructuras anisotrópicas, el proceso de crecimiento anisotrópico utilizando semillas es uno de los más ampliamente utilizados debido a la posibilidad que ofrece de producir nanoestructuras muy diferentes, tales como nanobarras, nanoalambres, nanofibras, nanotriángulos, nanoprismas, nanoestrellas, nanoflores, etc., mediante el control de un número determinado de sus variables experimentales. Este popular procedimiento de siembra ha venido utilizándose -con sus diversas variacionespara la producción de diferentes tipos de nanomateriales, entre los que destacan las partículas metálicas. Aunque durante los últimos años se han hecho importantes avances en esta técnica, respecto al aumento del rendimiento de la reacción, sin embargo, la preparación de distribuciones muy monodispersas de tamaño y forma es todavía un reto científico, sin cuya resolución resultan inviables la mayoría de sus potenciales aplicaciones. El método de siembra implica un proceso de, al menos, dos pasos. El primer paso es la síntesis de nanopartículas que han de servir como semillas para el posterior crecimiento anisotrópico de las mismas. En el xxix caso de los metales este proceso consiste en una simple reducción de la correspondiente sal del metal mediante reductores adecuados en presencia de agentes estabilizantes o complejantes. El segundo paso consiste en el crecimiento de las semillas para la obtención de las nanopartículas con las formas y tamaños deseados. La disolución de crecimiento contiene generalmente un agente tensioactivo (para orientar el crecimiento), y/o un agente que limita el crecimiento (como por ej. un polímero), así como un reductor suave que ayuda a controlar cinéticamente la formación de estructuras poco favorables desde el punto de vista termodinámico. En este proceso, las sales de los metales se reducen preferentemente en la superficie de las nanopartículas utilizadas como semillas, en un proceso favorecido por la menor energía implicada en la nucleación heterogénea. Agentes externos, tales como varios tipos de moléculas o iones, e incluso el propio disolvente o la temperatura de reacción, pueden también alterar la dirección del crecimiento de las nanopartículas ayudando o, más bien, complicando en muchos casos, el control del crecimiento anisotrópico de las mismas. Conocer, pues, con detalle, los complejos mecanismos implicados en el crecimiento de las nanopartículas es una condición sine qua non para poder obtener procedimientos con altos rendimientos de nanopartículas monodispersas en cuanto a su tamaño y forma, que puedan ser fácilmente escalables con vistas a su aplicación industrial. En el presente trabajo se ha realizado una investigación sistemática del método de siembra para la síntesis de nanopartículas anisotrópicas (preferentemente en forma de nanobarras) lo que ha permitido descubrir aspectos muy importantes del complejo mecanismo implicado no reconocidos hasta la fecha. Ello ha permitido no sólo la preparación de diferentes tipos de nanopartículas con variada morfología (nanoesferas, nanobarras, nanoprismas, ..) con mayores rendimientos y relaciones de xxx aspecto (relaciones entre los ejes menores/mayores de las NPs) mucho mayores que las conocidas hasta la fecha, sino también simplificar enormemente los protocolos de síntesis lo que permite un más fácil escalado de los mismos. Como comentamos, a pesar de la gran cantidad de bibliografía existente acerca de la síntesis química de estructuras anisótropas, los mecanismos que determinan el tipo de estructura cristalina final del nanomaterial y la morfología no son bien conocidos, e incluso cambios sutiles en las condiciones experimentales, como por ejemplo, la empresa proveedora de algunos productos químicos utilizados en la síntesis (disolvente, surfactantes, etc.) parece tener una gran influencia en la forma, tamaño y rendimiento final de las nanopartículas. De forma concreta, en el método de siembra para la preparación de nanopartículas anisotrópicas de oro, mediante semillas obtenidas con AgNO3, no se conoce actualmente todavía la naturaleza de las especies de plata formadas durante la síntesis, a pesar de ser un tema crítico en dicho método y haberse debatido ampliamente por muchos grupos de investigación pioneros en el desarrollo de estos métodos. Varios grupos han propuesto diferentes razones del por qué las especies de plata pueden ayudar al crecimiento anisotrópico de nanobarras de oro, siendo una de las posibilidades más aceptadas que los complejos de bromuro de plata juegan un papel fundamental en este mecanismo. A pesar de carecerse de una comprensión completa de este método de síntesis, se aceptan actualmente dos mecanismos principales como los más relevantes. El primer mecanismo considera que la estructura rígida de los monómeros del surfactante CTAB (bromuro de cetiltrimetilamonio) adsorbidos sobre algunas de las caras del nanocristal de oro que está creciendo, ayuda no sólo a mantener un crecimiento unidimensional, sino que también sirve para controlar la velocidad de la reducción de oro, de una manera similar al xxxi mecanismo que se propone para la formación de nanovarillas muy largas en ausencia de iones de plata. Se supone en este mecanismo, tal como se comentó, que los iones de plata no se reducen por el ácido ascórbico presente en el medio de reacción, sino que se forma bromuro de plata durante la síntesis. Murphy y colaboradores propusieron que la adsorción de bromuro de plata sobre algunas de las caras de los nanocristales de oro, frenan la reducción del mismo sobre esas caras contribuyendo de esa forma al crecimiento cristalino en forma de nanobarras. La presencia de especies de bromuro de plata se basa en los datos de XPS que sugieren la presencia de Ag en forma de Ag(I). A favor de esta hipótesis se han utilizado resultados de NMR 1H realizados con nanobarras de oro obtenidas por este procedimiento observándose que sus espectros eran idénticos a los de mezclas de AgBr-CTAB. Sin embargo, la presencia de bromuro de plata libre (ya que las condiciones de la reacción se producen por encima del producto de solubilidad del AgBr) pone en duda estos resultados. Otros autores encontraron pruebas, por espectrometría de masas, de la existencia de AgBr2 -, así como AuBr2, en la superficie de nanobarras de oro obtenidas por síntesis fotoquímica. Hafner y colaboradores obtuvieron por espectroscopía Raman pruebas de la formación del enlace Au-Br a 180 cm-1, que desaparece cuando se añaden tioles para reemplazar los bromuros adsorbidos en la superficie de las nanobarras de oro. Otros grupos, sin embargo, proponen un segundo modelo en el que el bromuro de plata no juega ningún papel en el crecimiento anisotrópico argumentando en su lugar que la deposición a sub-potenciales (UPD) de una sola capa o una sub-monocapa de plata elemental en la superficie de las nanobarras de oro es de gran importancia. Así, Guyot-Sionnest y colaboradores propusieron que la deposición de la plata a subpotenciales tiene lugar preferentemente sobre las caras de oro {110} frente a las {111} y {100}, en las que la deposición es menor. En este modelo, una monocapa de plata xxxii protege fuertemente la cara {110} y, aunque la plata se acaba oxidando y se sustituye por el oro, esta oxidación-sustitución es más lenta que la propia reacción de crecimiento de las NPs, por lo que dicha reacción procede preferentemente sobre las otras caras del nanocristal no recubiertas por la plata. Mediante espectroscopia de emisión atómica se ha observado que hasta cuatro monocapas de plata están presentes en las barras, aunque estos datos no son concluyentes ya que esta técnica no hace diferencias entre Ag (0) y Ag (I). Finalmente conviene indicar que estudios de espectroscopía de absorción de Rayos X (EXAFS) han permitido deducir que la plata se encuentra en forma de Ag (0) en nanobarras de plata obtenidas mediante métodos fotoquímicos, por lo que se ha concluido que la presencia de plata elemental es de vital importancia no sólo para la técnica estándar de siembra, sino también para otros tipos de métodos de síntesis disponibles para producir nanobarras de oro. Para tratar de dilucidar los complejos mecanismos implicados en la formación de estas nanopartículas de Au, se procedió al estudio del efecto de la concentración de Ag+, semillas, ácido ascórbico, Cl-, así como el tiempo de envejecimiento de la disolución de semillas. Los resultados indicaron que la relación de la concentración de Ag+ frente a la de las semillas utilizadas es uno de los parámetros clave para el control de la forma de las NPs (nanobarras, nanoprismas, nanoprismas truncados, etc). A partir de los resultados obtenidos se propuso un mecanismo que permite explicar la formación de las nanopartículas con diferentes estructuras basado en la formación de pequeños clústeres de Ag que actúan como catalizadores en la síntesis de dichas partículas. Este mecanismo, propuesto al inicio de la tesis, se pudo comprobar que explicaba adecuadamente muchos otros resultados obtenidos a lo largo de la misma, por lo que xxxviii clústeres en el crecimiento anisotrópico de las nanopartículas. Las nuevas características en las bandas de absorción que aparecen en estas partículas, se asignan a la absorción de resonancia cuadripolar a lo largo del eje perpendicular corto de los prismas y los prismas truncados. En el capítulo 4, se presentan los experimentos que se llevaron a cabo para explicar y probar el papel de los clústeres en el mecanismo de crecimiento de las nanobarras de oro. Estos experimentos se basaron en el uso de clústeres de oro (Au2-Au6), estabilizados por bromuro de cetiltrimetilamonio, los cuales se añadían a los iones de oro en la disolución de crecimiento en ausencia de agentes tensioactivos o polímeros. El mecanismo propuesto para la formación de las nanobarras de oro con altas relaciones de aspecto en la ausencia / presencia de la disolución de las semillas se basa en el depósito de los clústeres catalíticos en los lugares activos de la superficie de oro. El capítulo 5 se dedicó al estudio de la estabilidad de nanopartículas de Au frente al calentamiento térmico, radiación UV y tratamiento con láser. Este nuevo método propone estabilizar las nanopartículas de oro hasta 220 0C en presencia de PVP como agente protector. El resultado más importante obtenido en esta parte es la de descubrir que nanobarras de oro mostraron un efecto de fotocorrosión con tiempo largo de irradiación con luz UV. Se realizaron varios experimentos para estudiar este efecto, lo que muestra claramente la importancia de la presencia de los clústeres en los fenómenos observados. Por último, se ha estudiado el efecto de la radiación con láser de las diferentes dispersiones de nanobarras variando la longitud de onda y la energía. Mecanismos de fragmentación y la fusión de las nanopartículas de oro se han propuesto para explicar la remodelación de la nanopartículas sometidas a esta irradiación láser. xxxix ﺺﺨﻠﻣ ﺔﺤﺟﺎﻨﻟﺍ ﺕﺎﻘﻴﺒﻄﺘﻟﺍ ﻰﻠﻋ ﺓﺮﻄﻴﺴﻟﺍ ﺚﻴﺣ ﻦﻣ ﺎﻬﺼﺋﺎﺼﺧ ﻂﺒﺿ ﻰﻠﻋ ﺓﺭﺪﻘﻟﺍ ﺐﻠﻄﺘﺗ ﺔﻳﻮﻧﺎﻨﻟﺍ ﺕﺎﻤﻴﺴﺠﻠﻟ ﻮﻧﺎﻨﻟﺍ ﺱﺎﻴﻘﻣ ﻰﻓ ﻞﻜﺸﻟﺍﻭ ﻢﺠﺤﻟﺍ .ﺢﺒﺻﺃ ﺗ ﻥﺃ ﻢﻬﻤﻟﺍ ﻦﻣﺮﻴﻀﺤ ﺟﺕﺎﻤﻴﺴ ﻞﻜﺸﻟﺍ ﻦﻣ ﺍﺪﻴﺟ ﺓﺩﺪﺤﻣ ﺔﻳﺮﺼﺒﻟﺍ ﺺﺋﺎﺼﺨﻟﺍ ﻒﺻﻮﻟ ﺢﻴﺤﺻ ﻞﻜﺸﺑ ﻢﺠﺤﻟﺍﻭ. ﺑ ﺎﻨﻤﻗ ﺪﻗﻭﻒﺻﻮ ﺔﻘﻳﺮﻁ ﺔﻁﺎﺳﻮﺑ ﺕﺎﺒﻴﺒﺣ ﺔﻳﻮﻧﺎﻨﻟﺍ ﺐﻫﺬﻟﺍ ﺔﻨﺴﺤﻤﻟﺍ ﺟ ﺮﻴﻀﺤﺘﻟﻤﻴﺴ ﺕﺎ ﺔﻔﻠﺘﺨﻤﻟﺍ ﻝﺎﻜﺷﻷﺍ ﻦﻣ ﺐﻫﺬﻟﺍ)ﺔﻳﻭﺮﻜﻟﺍ ،ﺔﻳﻮﺼﻌﻟﺍ ، ﺔﻳﺭﻮﺸﻨﻤﻟﺍ ، .(.....ﺪﺟﻭ ﺪﻘﻟ ﻙﺎﻨﻫ ﻥﺃﺃ ﻰﺘﻟﺍ ﻦﻋ ﻒﻠﺘﺨﺗ ﺕﺎﻤﻴﺴﺠﻟﺍ ﻩﺬﻬﻟ ﺓﺪﻳﺪﺟ ﺹﺎﺼﺘﻣﺃ ﻑﺎﻴﻁ ﻞﻜﺸﻟﺍﻭ ﻯﻮﺼﻌﻟﺍ ﻞﻜﺸﻟﺍ ﺕﺍﺫ ﺐﻫﺬﻟﺍ ﺕﺎﻤﻴﺴﺟ ﻦﻣ ﻞﻜﻟ ﻰﺋﻮﻀﻟﺍ ﺹﺎﺼﺘﻣﻸﻟ ﺔﺒﺴﻨﻟﺎﺑ ﺖﻈﺣﻮﻟ ﻯﻭﺮﻜﻟﺍ . ﻞﻜﺷ ﺕﺍﺫ ﺔﻳﺭﻮﻠﺑ ﺎﻬﻟﺎﻜﺷﺃ ﻥﺃ ﻰﻠﻋ ﺎﻬﺤﻴﺿﻮﺗ ﻢﺗ ﺪﻘﻟﻭ ﻡﺍﺪﺨﺘﺳﺎﺑ ﺎﻬﺘﺳﺍﺭﺪﺑ ﻚﻟﺫﻭ ﻯﺭﻮﺸﻨﻣ ﺬﻓﺎﻨﻟﺍ ﻰﻧﻭﺮﺘﻜﻟﻷﺍ ﺏﻮﻜﺳﻭﺮﻜﻴﻤﻟﺍ. ﻞﻜﺸﻟﺍ ﺕﺍﺫ ﺐﻫﺬﻟﺍ ﺕﺎﻤﻴﺴﺟ ﻦﻣ ﺓﺮﻴﺒﻛ ﺕﺎﻴﻤﻛ ﻰﻠﻋ ﻝﻮﺼﺤﻟﺍ ﻢﺗ ﺎﻬﺑ ﻡﺪﺨﺘﺴﻤﻟﺍ ﺔﻘﻳﺮﻄﻟﺍ ﻰﻓ ﻞﻳﺪﻌﺘﻟﺍ ﺍﺬﻬﻟ ﺔﺠﻴﺘﻨﻟ ﻯﻮﺼﻌﻟﺍ ﺮﻐﺼﻟﺍ ﻪﻴﻫﺎﻨﺘﻤﻟﺍ ﺕﺎﻤﻴﺴﺠﻟﺍ.ﺔﺳﺍﺭﺩ ﻢﺗ ﺮﻴﺛﺄﺗﺕﺍﺰﻴﻛﺮﺗ ]ﺔﻀﻔﻟﺍ ﺕﺎﻧﻮﻳﺍ [ ،]ﺔﻳﻮﻧﺎﻨﻟﺍ ﺐﻫﺬﻟﺍ ﺕﺎﺒﻴﺒﺣ [ ،]ﻴﻓﺃ ﻦﻴﻣﺎﺘ [ ،] ﺔﻳﻮﻧﺎﻨﻟﺍ ﺐﻫﺬﻟﺍ ﺕﺎﺒﻴﺒﺣ ﻡﺍﺪﺨﺘﺳﺎﺑ ﺓﺮﻀﺤﻤﻟﺍ ﻟﺍﺕﺍﺮﺘﻴﺴ [ ،]ﺭﻮﻠﻜﻟﺍ [ ﺔﺳﺍﺭﺩ ﻭ ،ﺮﻐﺼﻟﺍ ﺔﻴﻫﺎﻨﻤﻟﺍ ﺕﺎﻤﻴﺴﺠﻟﺍ ﺮﻤﻋ ﺓﺩﺎﻳﺯ ﻦﻣ ﻮﻤﻨﻟﺍ ﺔﻴﻟﺁ ﻲﻓ ﺮﺜﻛﺃ ﻢﻬﻓ ﻞﺟﺃ . ﺔﺒﺴﻧ ﻥﺍ ﻰﻟﺍ ﺮﻴﺸﺗ ﺎﻨﺠﺋﺎﺘﻧ]ﺔﻀﻔﻟﺍ ﺕﺎﻧﻮﻳﺍ[ ﻭ ]ﺔﻳﻮﻧﺎﻨﻟﺍ ﺐﻫﺬﻟﺍ ﺕﺎﺒﻴﺒﺣ [ ﻰﻫ ﻠﻟ ﺔﻴﺳﺎﺳﻷﺍ ﺮﻴﻳﺎﻌﻤﻟﺍ ﻦﻣ ﺓﺪﺣﺍﻭﻢﻜﺤﺘ ﻰﻓ ﻞﻜﺷﺔﻳﻮﻧﺎﻨﻟﺍ ﺕﺎﻤﻴﺴﺠﻟﺍ ﻩﺬﻫ ﻝﺎﺜﻤﻟﺍ ﻞﻴﺒﺳ ﻰﻠﻋ ، ﻞﻜﺸﻟﺍ ﻯﻮﺼﻌﻟﺍ ،ﻯﺭﻮﺸﻨﻤﻟﺍ ﻭﺃﻞﻤﺘﻜﻣ ﺮﻴﻐﻟﺍ ﻯﺭﻮﺸﻨﻤﻟﺍ.ﺪﻗﻭ ﺔﻴﻟﺁ ﺎﻨﺣﺮﺘﻗﺍﺍﻮﻤﻨﻟ ﻝﺎﻜﺷﻷﻟﺍﻦﻣ ﺔﻔﻠﺘﺨﻤ ﺕﺎﻤﻴﺴﺟ ﺐﻫﺬﻟﺍ ﺗ ﻰﺘﻟﺍﻭ ﺭﺎﺒﺘﻋﻻﺍ ﻲﻓ ﺬﺧﺄﺃﻝﺍﺰﺘﺧ ﺔﻄﺳﺍﻮﺑ ﺔﻀﻔﻟﺍ ﺕﺎﻧﻮﻳﺃ ﺃ ﻦﻴﻣﺎﺘﻴﻓ ) ﺾﻤﺣ ﻚﺑﺭﻮﺳﻻﺍ( ﻟﺘﻦﻳﻮﻜ ﺔﻄﻴﺳﻭ ﺕﺎﻋﻮﻤﺠﻣﻟﺍ ﺕﺎﻤﻴﺴﺟ ﻦﻣﺮﻐﺼﻟﺍ ﻰﻓ ﺍﺪﺟ ﺔﻴﻫﺎﻨﺘﻤﻟﺍ ﻪﻀﻔ ﻦﻜﻤﻳ ﻲﺘﻟﺍﻭ ، ﻛ ﻞﻤﻌﺗ ﻥﺃﻤﺟ ﻦﻳﻮﻜﺘﻟ ﺰﻔﺤﺕﺎﻤﻴﺴ ﺐﻫﺬﻟﺍ ﺔﻳﻮﻧﺎﻨﻟﺍ .ﺕﺎﻋﻮﻤﺠﻣ ﻡﺍﺪﺨﺘﺳﺎﺑ ﺔﻴﻟﻵﺍ ﺖﺘﺒﺛﺃ ﺪﻗﻭ ﺕﺎﻤﻴﺴﺟ ﻦﻣ ﺓﺮﻀﺤﻤﻟﺍﺮﻐﺼﻟﺍ ﺔﻴﻫﺎﻨﺘﻤﻟﺍ ﺔﻀﻔﻟﺍ ﺔﻀﻔﻟﺍ ﺕﺎﻧﻮﻳﺃ ﺔﻓﺎﺿﺇ ﻦﻣ ﻻﺪﺑ ﺎﻴﺟﺭﺎﺧ . ، ﺎﻀﻳﺃ ﺞﺋﺎﺘﻨﻟﺍ ﺮﻴﺸﺗﻭ ﻴﻛﺮﺗﻭ ،ﻞﻴﺒﻘﻟﺍ ﺍﺬﻫ ﻦﻣ ﺔﻔﻠﺘﺨﻣ ﻝﺎﻜﺷﺃ ﻰﻠﻋ ﻝﻮﺼﺤﻠﻟ ﺰﺔﻔﻠﻐﻤﻟﺍ ﺓﺩﺎﻤﻟﺍ ﺔﻳﻮﻀﻌﻟﺍ ﺕﺎﻳﻭﺮﻐﻟﺎﺑ ﻯﺍﺮﺗ ﻞﻴﺘﻴﺳ ﺪﻴﻣﻭﺮﺑ ﻡﻮﻴﻧﻮﻣﺃ ﻞﻴﺜﻴﻣ ﻕﻮﻓ ﻥﻮﻜﺗ ﻥﺃ ﻲﻐﺒﻨﻳ ﺔﻣﺪﺨﺘﺴﻤﻟﺍ ﻦﻋ ﺮﻈﻨﻟﺍ ﻑﺮﺼﺑ ﻪﺑ ﺡﻮﻤﺴﻤﻟﺍ ﺪﺤﻟﺍ ﻩﺭﺪﺼﻣ . ﻥﻮﻜﺗ ﻥﺃ ﻞﺟﺃ ﻦﻣ ، ﻪﻧﺃ ﻭﺪﺒﻳ ﻚﻟﺬﻟﻭ ﺮﻐﺼﻟﺍ ﺔﻴﻫﺎﻨﺘﻤﻟﺍ ﺔﻀﻔﻟﺍ ﺕﺎﻤﻴﺴﺟ ﻙﻮﻠﺴﻟﺍ ﻲﻓ ﺔﻟﺎﻌﻓ ﺮﻘﺘﺴﺗ ﻥﺃ ﻰﻟﺇ ﺝﺎﺘﺤﺗ ﺎﻬﻧﺈﻓ ، ﺰﻔﺤﻤﻟﺍ ﻩﺩﺎﻣ ﺩﻮﺟﻮﺑﺔﻔﻠﻐﻣ ﻦﻣ ﻉﻮﻧ ﺎﻬﻧﺍ ﻰﻠﻋ ﺎﻬﻴﻟﺍ ﺮﻈﻨﻟﺍ ﻦﻜﻤﻳ ﻲﺘﻟﺍﻭ ، ﻯﻮﻧﺎﻨﻟﺍ ﺪﻧﺎﺴﻤﻟﺍ ﻟﺍﺯﺎﻔﺤ . ﻮﻤﻧ ﻲﻓ ﻙﺭﺎﺸﺗ ﻲﺘﻟﺍ ﺕﺎﻴﻟﻵﺍ ﺮﻴﺴﻔﺗ ﻲﻓ ﺮﻴﻜﻔﺘﻠﻟ ﺍﺪﻳﺪﺟ ﺎﻘﻳﺮﻁ ﺢﺘﻔﺗ ﺞﺋﺎﺘﻨﻟﺍ ﻩﺬﻫ ﺔﻳﻮﻧﺎﻨﻟﺍ ﻦﻣ ﻦﻳﺎﺒﺘﻣ. ﺪﻗﻭ ﻫﻭ ، ﺔﻴﻟﻵﺍ ﻩﺬﻫ ﺪﻴﻛﺄﺗ ﻞﺟﺃ ﻦﻣ ﺎﻧﺭﻮﻁﻡﺍﺪﺨﺘﺳﺃ ﻮ ﺘﻟ ﺔﻄﻴﺴﺑ ﺔﻘﻳﺮﻁﺕﺎﻤﻴﺴﺟ ﺮﻴﻀﺤ ﺐﻫﺬﻟﺍ ﺔﻳﻮﺼﻌﻟﺍ ﻧ ﻊﻣﺝﺎﺘ ﻟﺎﻋﻰ ﻡﺍﺪﺨﺘﺳﺎﺑﺔﻳﻮﻧﺎﻧ ﺕﺎﺒﻴﺒﺣ ﺐﻫﺬﻟﺍ ﻦﻣﻟﺍﻤﺓﺮﻀﺤ ﻭ ﺔﻔﻠﻐﻤﻟﺍ ﺓﺩﺎﻤﻟﺎﺑ ﺔﻴﻤﺤﻤﻟﺍ xl ﺔﻳﻮﻀﻌﻟﺍ ﺕﺎﻳﻭﺮﻐﻟﺎﺑ ﺪﻴﻣﻭﺮﺑ ﻡﻮﻴﻧﻮﻣﺃ ﻞﻴﺜﻴﻣ ﻯﺍﺮﺗ ﻞﻴﺘﻴﺳ ﻭﻭﻥﺄﺑ ﺎﻬﻔﺻ ﻦﻴﺑ ﺡﺍﻭﺮﺘﻳ ﺎﻬﺑ ﺕﺍﺭﺬﻟﺍ ﺩﺪﻋ ﺕﺍﺭﺫ ﺔﺘﺳ ﻰﻟﺍ ﻦﻴﻨﺛﺍ ﻟﺍ ﻮﻤﻨﻟﺍ ﻝﻮﻠﺤﻣ ﻲﻓ ﺐﻫﺬﻟﺍ ﺕﺎﻧﻮﻳﺃ ﻰﻟﺇ ﺖﻔﻴﺿﺃﻭﻯﺬ ﻻﻳ ﻱﺃ ﻰﻠﻋ ﻱﻮﺘﺤ ﺓﺩﺎﻣ ﺔﻔﻠﻐﻣ ﺔﻳﻮﻀﻌﻟﺍ ﺕﺎﻳﻭﺮﻐﻟﺎﺑ ﺪﻴﻣﻭﺮﺑ ﻡﻮﻴﻧﻮﻣﺃ ﻞﻴﺜﻴﻣ ﻯﺍﺮﺗ ﻞﻴﺘﻴﺳ . ﺣﺮﺘﻗﺍﻭﺎﻨ ﻞﻴﻜﺸﺘﻟ ﺓﺪﻳﺪﺟ ﺔﻴﻟﺁ ﺕﺎﻤﻴﺴﺟ ﺐﻫﺬﻟﺍ ﺔﻳﻮﺼﻌﻟﺍ ﻊﻣﻊﻔﺗﺮﻣ ﺞﺗﺎﻧ ﺏﺎﻴﻏ ﻲﻓ /ﻭﻭ ﺩﻮﺟﻚﻟﺫﻭ ﺐﻫﺬﻟﺍ ﺕﺎﺒﻴﺒﺣ ﻰﻠﻋ ﺍﺩﺎﻤﺘﻋﺍ ﺯﺍﺰﺘﻣﺃ ﺯﺎﻔﺤﻟﺍ ﺕﺎﻋﻮﻤﺠﻤﻟﺍ ﻩﺬﻫﺓ ﺐﻫﺬﻟﺍ ﺢﻄﺳ ﻦﻣ ﺔﻄﺸﻨﻟﺍ ﻊﻗﺍﻮﻤﻟﺍ ﻰﻠﻋ. ﻻﺍﺯﺍﺰﺘﻣ ﻭﻝﺍﺰﺘﺧﻻﺍ ﻳ ﺙﺪﺤ ﻴﺋﺭ ﻞﻜﺸﺑﻳﺎﻬﻨﻟﺍ ﻲﻓ ﻲﺴﺕﺎ ﻯﻮﺼﻌﻟﺍ ﻞﻜﺸﻟﺍ ﻦﻣ. ﺒﻴﻛﺮﺗﻭ ﻞﺘﻜﻟﺍ ﻩﺬﻫ ﺰﻴﻛﺮﺗ ﺏﺎﺴﺣ ﻢﺗﺎﻬ ﺔﻄﺳﺍﻮﺑ ﺓﺎﻛﺎﺤﻣ ﻮﻟﺭﺎﻛ ﻲﺘﻧﻮﻣ. ﻦﻣ ﻒﻴﻔﺨﺘﻟﺍ ﻰﻠﻋ ﺔﺒﺗﺮﺘﻤﻟﺍ ﺭﺎﺛﻵﺍ ﺔﺳﺍﺭﺩ ﺖﻤﺗﻭ ﻪﻔﻠﻐﻤﻟﺍ ﺓﺩﺎﻤﻟﺍ ﺕﺎﻳﻭﺮﻐﻟﺎﺑ ﺔﻳﻮﻀﻌﻟﺍ ﺪﻴﻣﻭﺮﺑ ﻡﻮﻴﻧﻮﻣﺃ ﻞﻴﺜﻴﻣ ﻯﺍﺮﺗ ﻞﻴﺘﻴﺳ ﻲﻓ ﺔﻴﻫﺎﻨﺘﻤﻟﺍ ﺕﺎﻤﻴﺴﺠﻟﺍ ﻦﻣ ﺕﺎﻤﺠﺘﻟﺍ ﻩﺬﻫ ﺮﻴﻀﺤﺗ ﺐﻫﺬﻟﺍ ﻦﻣ ﺮﻐﺼﻟﺍ ﻦﻣ ﻒﻴﻔﺨﺘﻟﺍﻭﺐﻫﺬﻠﻟ ﺔﻳﻮﻧﺎﻨﻟﺍ ﺕﺎﺒﻴﺒﺤﻟﺍ ﻮﻤﻧ ﻰﻠﻋ ﺔﻳﻮﺼﻌﻟﺍ ﺐﻫﺬﻟﺍ ﺕﺎﻤﻴﺴﺟ. ﻥﺃ ﺎﻀﻳﺃ ﺞﺋﺎﺘﻨﻟﺍ ﺕﺮﻬﻅﺃﻭ ﺔﻳﻮﺼﻌﻟﺍ ﺐﻫﺬﻟﺍ ﺕﺎﻤﻴﺴﺟ ﺔﻓﺎﺿﺇ ﻥﻭﺩ ﻦﻣ ﺎﻬﻠﻴﻜﺸﺗ ﻦﻜﻤﻳ ﺐﻫﺬﻟﺍ ﺕﺎﺒﻴﺒﺣ ﺔﻳﻮﻧﺎﻨﻟﺍ ﺎﻀﻳﺃ ﻭﺔﻳﻮﻧﺎﻨﻟﺍ ﺕﺎﺒﻴﺒﺤﻟﺍ ﻩﺬﻫ ﺩﻮﺟﻭ ﻮﻤﻧ ﻰﻠﻋ ﺓﺮﺷﺎﺒﻣ ﺮﺛﺆﺗ ﻻ ﺪﻗ ﺕﺎﻤﻴﺴﺟ ﺐﻫﺬﻟﺍ ﺔﻳﻮﻧﺎﻨﻟﺍ ﺔﻳﻮﺼﻌﻟﺍ. ﺍﺭﺩ ﻢﺗ ﺔﺳ ﺮﻴﺛﺄﺗﺕﺎﻤﻴﺴﺠﻟ ﻞﻜﺸﻟﺍ ﻰﻠﻋ ﺐﻫﺬﻟﺍﺕﺎﺒﺜﻟﺍ ﺹﺍﻮﺧ )ﻟﺍﺕﺎﺒﺜ ﻭ ﻱﺭﺍﺮﺤﻟﺍﻰﺋﻮﻀﻟﺍ ﺕﺎﺒﺜﻟﺍ ﻟ ﺔﻌﺷﻸﻟﺍﺭﺰﻴﻠﻟﺍﻭ ﺔﻴﺠﺴﻔﻨﺒﻟﺍ ﻕﻮﻔ(. ﻚﻜﻔﺗ ﺔﻴﻟﺁﺕﺎﻤﻴﺴﺠﻟﺍ ﻩﺬﻫ ﺔﻟﺎﺣ ﻲﻓ ﻯﺭﻮﺸﻨﻤﻟﺍ ﻞﻜﺸﻟﺍ ﻦﻋ ﻒﻠﺘﺨﻳ ﻦﻣ ﻚﻟﺫ ﻯﻮﺼﻌﻟﺍ ﻞﻜﺸﻟﺍ ﺖﺤﺗﺕﺎﺒﺜﻟﺍ ﺮﻴﺛﺄﺗ ﻱﺭﺍﺮﺤﻟﺍ ﺔﺟﺭﺩ ﻰﻓ ﺮﻴﺒﻛ ﻦﺴﺤﺗ ﻙﺎﻨﻫ ﻥﺃ ﺪﺟﻭ ﺪﻗﻭ ﺭﺍﺮﺤﻟﺍ ﺕﺎﺒﺜﻟﺍ ﻚﻟﺫﻭ ﻥﻭﺪﻴﻠﻳﺮﻴﺑ ﻞﻴﻨﻴﻓ ﻰﻟﻮﺒﻟﺍ ﻞﺜﻣ ﺕﺍﺮﻤﻴﻟﻮﺒﻟﺍ ﻦﻣ ﺕﺎﻔﻠﻐﻣ ﺩﻮﺟﻭ ﻰﻓ ﻝﺎﻜﺷﻷﺍ ﻩﺬﻬﻟ ﻯ ﺪﻴﻣﻭﺮﺑ ﻡﻮﻴﻧﻮﻣﺃ ﻞﻴﺜﻴﻣ ﻯﺍﺮﺗ ﻞﻴﺘﻴﺳ ﻞﺜﻣ ﺔﻳﻮﻀﻌﻟﺍ ﺕﺎﻳﻭﺮﻐﻟﺎﺑ ﺔﻔﻠﻐﻤﻟﺍ ﻚﻠﺘﻟ ﺔﺒﺴﻨﻟﺎﺑ ﺮﻛﺬﻟﺎﺑ ﺮﻳﺪﺠﻟﺍ ﻦﻣﻭ ﻰﺘﺣ ﻞﺼﺗ ﻥﻭﺪﻴﻠﻳﺮﻴﺑ ﻞﻴﻨﻴﻓ ﻰﻟﻮﺒﻟﺎﺑ ﺔﻔﻠﻐﻤﻟﺍ ﺔﻳﻮﺼﻌﻟﺍ ﺐﻫﺬﻟﺍ ﺕﺎﻤﻴﺴﺠﻟ ﻯﺭﺍﺮﺣ ﺕﺎﺒﺛ ﺔﺟﺭﺩ ﻙﺎﻨﻫ ﻥﺃ 220ﻡ◦ . ﻩﺬﻫﻭ ، ﺎﻳﺮﺼﺑﻟﺍﺠﺔﻳﻮﻧﺎﻨﻟﺍ ﺕﺎﻤﻴﺴ ﻟ ﻲﺘﻟﺍ ﺐﻫﺬﻠﺖﺘﺒﺛ ﻅﻮﺤﻠﻣ ﻞﻜﺸﺑ ﻦﻣ ﺕﺎﻔﻠﻐﻣ ﺩﻮﺟﻭ ﻰﻓ ﻥﻭﺪﻴﻠﻳﺮﻴﺑ ﻞﻴﻨﻴﻓ ﻰﻟﻮﺒﻟﺍ ﻞﺜﻣ ﺕﺍﺮﻤﻴﻟﻮﺒﻟﺍ ﺕﺎﺒﺜﻟﺍ ﺔﺟﺭﺩ ﻰﻓ ﻭ ﻱﺭﺍﺮﺤﻟﺍﺒﺜﻟﺍ ﺖﺤﺗ ﺕﺎ ﻕﻮﻓ ﺔﻌﺷﻷﺍ ﺔﻴﺠﺴﻔﻨﺒﻟﺍ. ﺟ ﻰﻠﻋ ﺔﻴﺠﺴﻔﻨﺒﻟﺍ ﻕﻮﻓ ﺔﻌﺷﻷﺍ ﺮﻴﺛﺄﺗﺕﺎﻤﻴﺴ ﺮﺧﺁ ﻰﻟﺇ ﻞﻜﺷ ﻦﻣ ﺔﻔﻠﺘﺨﻣ ﺔﻴﻟﺁ ﺕﺮﻬﻅﺃ ﺐﻫﺬﻟﺍ. ﺭﺎﻁﺇ ﻲﻓﻞﻳﻮﻁ ﺖﻗﻮﻟ ﻉﺎﻌﺷﻼﻟ ﺾﻳﺮﻌﺘﻟﺍ ﻳ ﻰﻟﺇ ﻞﺼ30 ، ﺔﻋﺎﺳﻠﻟ ﺔﻳﻮﺼﻌﻟﺍ ﺕﺎﻤﻴﺴﺠﻟﺎﻓﺐﻫﺬ ﻚﻜﻔﺘﺗ ﻭ ﺎﻣﺎﻤﺗ ﻰﺟﻮﻤﻟﺍ ﻝﻮﻄﻟﺍ ﺪﻨﻋ ﻰﺋﻮﺿ ﺹﺎﺼﺘﻣﺍﺭﻮﻬﻅ ﺃﺪﺒﻳ398 ﻭ ﺲﻔﻧ ﺎﺒﻳﺮﻘﺗ ﻮﻫﺍ ﻰﺟﻮﻤﻟﺍ ﻝﻮﻄﻟ ﻟﺍ ﺹﺎﺼﺘﻣﻻﺪﻴﻣﻭﺮﺑ ﻡﻮﻴﻧﻮﻣﺍ ﻞﻴﺜﻴﻣ ﻞﻴﺘﻴﺳ ﺔﻳﻮﻀﻌﻟﺍ ﺕﺎﻳﻭﺮﻐﻟﺍ ﻊﻣ ﺐﻫﺬﻟﺍ ﺕﺎﻧﻮﻳﺍ ﺐﻛﺮﻤﻟ ءﻮﻀ ﻮﻫﻭ ﺐﻫﺬﻟﺍ ﻥﺃ ﻲﻨﻌﻳ ﺎﻣ)0 (ﺗﻟﻮﺤﺖ ﻳ ﺍﺬﻫﻭ ﺐﻫﺬﻟﺍ ﺕﺎﻧﻮﻳﺃ ﻰﻟﺇ ﻩﺬﻬﻟ ﻰﺋﻮﺿ ﻞﻛﺄﺗ ﺙﻭﺪﺣ ﻰﻠﻋ ﻝﺪ ﻮﻤﻨﻟﺍ ﺔﻴﻟﺍ ﻰﻓ ﺎﻫﺭﻭﺩﻭ ﺎﻫﺩﻮﺟﻭ ﺎﻨﺿﺮﺘﻓﺍ ﺎﻤﻛ ﺔﻀﻔﻟﺍ ﺕﺎﻌﻤﺠﺗ ﺪﺟﺍﻮﺗ ﺔﺠﻴﺘﻧ ﺕﺎﻤﻴﺴﺠﻟﺍ . ﺪﻗﻭﺗ ﺍﺬﻫ ﺪﻛﺄ ﻝﻮﻠﺤﻣ ﺔﻓﺎﺿﺄﺑ ﻞﻛﺄﺘﻟﺍ ﻰﻟﺇ ﻝﻮﻧﺎﺜﻳﻻﺍ ﺔﻳﻮﺼﻌﻟﺍ ﺕﺎﻤﻴﺴﺠﻟﺍ ﻭ ﺪﻌﺑ ﺕﺎﻤﻴﺴﺠﻟﺍ ﻩﺬﻫ ﻉﺎﻌﺷﺍ ﻖﻳﺮﻁ ﻦﻋ ﺓﺭﺍﺮﺣ ﺔﺟﺭﺪﻟ ﺎﻬﻨﻴﺨﺴﺗ130 ﻡº ﺕﺎﺒﻴﺒﺤﻟﺍ ﻩﺬﻫ ﻥﺎﺑﻭﺫ ﻪﺟﺭﺩ ﺲﻔﻧ ﻰﻫﻭ ﺔﺟﺭﺩ ﺲﻔﻧ ﺭﻮﻬﻅ ﻡﺪﻋﻭ xli ﻝﺪﻳ ﺍﺬﻫﻭ ﻖﺑﺎﺴﻟﺍ ﻰﺟﻮﻤﻟﺍ ﻝﻮﻄﻟﺍ ﺪﻨﻋ ءﻮﻀﻟﺍ ﺹﺎﺼﺘﻣﺍﻰﻠﻋ ﺃ ﺔﻴﻟﺍ ﻰﻓ ﺕﺎﻌﻤﺠﺘﻟﺍ ﻩﺬﻫ ﺩﻮﺟﻭ ﺔﻴﻤﻫ ﻮﻤﻨﻟﺍ. ﺳﺭﺩ ﺪﻗﻭﺎﻨ ﺮﻴﺛﺄﺗﺏﺭﺎﺠﺗ ﻡﺍﺪﺨﺘﺳﺍ ﻝﻼﺧ ﻦﻣ ﺕﺎﻤﻴﺴﺠﻟﺍ ﻩﺬﻬﻟ ﻰﺋﻮﻀﻟﺍ ﻞﻛﺄﺘﻟﺍ ﺔﻴﺋﺎﻴﻤﻴﻛﻭﺮﻬﻜﻟﺍ ﺪﻛﺆﺗ ﻲﺘﻟﺍ ﺙﻭﺪﺣ ﺓﺪﺴﻛﻷﺍﻟﺠﻟﺍ ﻩﺬﻬ ﻰﻟﺍ ﺎﻬﻟﻮﺤﺗﻭ ﺕﺎﻤﻴﺴ ﻲﺘﻟﺍ ﻭ ، ﺔﻴﻧﺎﺛ ﺕﺎﻧﻮﻳﻷﺍ ﻩﺬﻫ ﺩﻮﺟﻭ ﺎﻬﺘﺒﺒﺳ ﺕﺎﻌﻤﺠﺘﻟﺍ. ﻛﻻﺍ ﺙﻭﺪﺣ ﻰﻫﻭ ﺔﺳﺍﺭﺪﻟﺍ ﻩﺬﻫ ﻝﻼﺧ ﺞﺋﺎﺘﻧ ﻦﻣ ﻪﻟ ﺎﻨﻠﺻﻮﺗ ﺎﻣ ﻢﻫﺃ ﻦﻣ ﺔﺠﻴﺘﻨﻟﺍ ﻩﺬﻫ ﺪﻌﺗﻭ ﺓﺪﺴ ﺩﻮﺟﻮﺑ ﻰﺋﻮﻀﻟﺍ ﻞﻛﺎﺘﻟﺍ ﻖﻳﺮﻁ ﻦﻋ ﻯﻮﺼﻌﻟﺍ ﻞﻜﺸﻟﺍ ﺕﺍﺫ ﺮﻐﺼﻟﺍ ﺔﻴﻫﺎﻨﺘﻤﻟﺍ ﺐﻫﺬﻟﺍ ﺕﺎﺌﻳﺰﺠﻟ ﺕﺎﻋﻮﻤﺠﻣ)ﺯﺮﺘﺴﻠﻛ( ﺔﻀﻔﻟﺍ. ﺪﻨﻜﻴﺳﻮﻧﺎﻨﻟﺍ ﺭﺰﻴﻠﻟﺍ ﻦﻣ ﺔﻔﻠﺘﺨﻣ ﻉﺍﻮﻧﺃ ﺮﻴﺛﺄﺗ ﺔﺳﺍﺭﺩ ﻢﺗ ، ﺍﺮﻴﺧﺃ )ﺮﻤﺘﺴﻤﻟﺍ ﻭﺾﻣﺍﻮﻟﺍ ( ﻰﻠﻋ ﺕﺎﻤﻴﺴﺟ ﺔﻳﻮﺼﻌﻟﺍ ﺐﻫﺬﻟﺍ ﻦﻴﻴﻔﻠﺘﺨﻣ ﺽﺮﻋﻭ ﻝﻮﻁ ﺐﺴﻧ ﺕﺍﺫ ﺔﻔﻠﺘﺨﻣ ﺕﺎﻴﻟﺁ ﺡﺍﺮﺘﻗﺍ ﻢﺗ ﻭ)ﻥﺎﺑﻭﺬﻟﺍﻭ ﺖﺘﻔﺘﻟﺍ ( ﺪﻨﻜﻴﺳﻮﻧﺎﻨﻟﺍ ﺭﺰﻴﻠﻟﺍ ﺮﻴﺛﺄﺘﻟﺔﻔﻠﺘﺨﻣ ﺔﻴﺟﻮﻣ ﻝﺍﻮﻁﺎﺑﻭ ﺾﻣﺍﻮﻟﺍ. xlii Chapter I Introduction - 1 - Chapter one Introduction Chapter I Introduction - 2 - Chapter I Introduction - 3 - An Overview of the Dissertation Work In this dissertation work, we have carefully re-examined the process of synthesis of gold nanoparticles of different shapes (e.g. spheres, rods, etc) by the seed's mediated method(64,142), to have a better understanding and insight into the mechanisms. We modified this method in order to produce gold nanoparticles with selected shapes in high yields. We determined the very important role of subnanometric clusters (Ag and Au) on the anisotropic growth of Au nanoparticles. The optical and thermal properties, as well as the photostability of these particles of different shapes and compositions were studied in detail showing that clusters again play an important role in the "photocorrosion" of gold observed for the first time in this work. It was found that this unexpected behavior of gold is related with the photocatalytic activity of the subnanometric clusters, which remain mainly attached to some particular planes of the gold nanoparticles after their synthesis. The thesis is divided in the following chapters. After the first introduction chapter, the experimental methods regarding the sample preparation, characterization, and optical measurements are discussed in details in Chapter 2. Chapter 3 describes the modified seed-mediated synthesis of gold nanorods and nanoprisms developed in the thesis using a rational choice of the experimental conditions and additives to select the shape (and consequently the optical properties) of the NPs. A novel mechanism of the formation of Chapter I Introduction - 4 - these particles is proposed showing the important role of the presence of Ag-clusters in the anisotropic growth of nanoparticles. New absorption features appear for these particles, which are assigned to the quadruple resonance absorption along the short axis perpendicular of the prisms and snapped nanoparticles. In chapter 4, other experiments were carried out to explain and confirm the role of clusters in the growth mechanism of gold nanorods. Chapter 5 was devoted to the study of the stability of Au NPs against thermal heating, UV-irradiation and Laser treatment. A new method is proposed to stabilize gold nanoparticles up to 220 0C using PVP solution as protecting agent. The most important result obtained in this part is the discovering that gold nanorods showed a photocorrosion effect under long irradiation time of UV-light. Several experiments were performed to study this effect, which clearly shows the importance of the clusters in the observed phenomena. Finally, the effect of nanosecond laser at different wavelengths and power on the prepared gold nanorods with different aspect ratios was studied. Fragmentation and melting mechanisms for the gold nanoparticles were proposed for explaining the reshaping of the NPs with the laser irradiation. . Chapter I Introduction - 5 - Chapter I 1-1: General Introduction: Metallic nanoparticle´s (especially gold and silver nanoparticles) research is currently an area of great intense scientific research, because of their unusual chemical and physical properties that make them suitable for many potential and promising applications such as catalysis, electronics, optics, imaging and biotechnology. Nanoparticles contain a small number of atoms or molecules that they differ from the properties inherent to their bulk counterparts. However, they contain sufficiently large number of atoms or molecules so that their properties differ from isolated groups of atoms or molecules (clusters). Nanoparticles can exhibit physicochemical properties that are different from both the bulk and the constituent atoms or molecules. For example, the striking colors of metallic nanoparticle solutions (such as gold and silver) are due to the red shift of the plasmon band to visible frequencies, unlike that for bulk metals where the plasmon absorption is in the UV region (a plasmon is a quantum of collective oscillation of free electrons in the metals). Although nanoparticles are generally considered an invention of modern science, they actually have a very long history. Egyptians, Greeks and Romans used many colored pigments for the decoration of their buildings, ceramics and glass-ware. The use of gold and silver particles in glassblowing can be seen in the famous Lycurgus Cup exposed in the British Museum(1). Gold Juice, Kim Yeh, alkimiya, alchemy, and nanogold are names called for colloidal gold, which was known since ancient times. The synthesis of colloidal gold was originally used as a method of staining glass. Modern scientific studies of colloidal gold did not begin until Michael Faraday's work in the middle of the XIX century(2). It is known that colloidal gold was used in roman times to color glass with intense shades of yellow, red, or mauve, depending on Chapter I Introduction - 12 - Electrolysis(39, 40) has also been used successfully to prepare noble and transition metallic nanoparticles of different shapes. Metal nanoparticles can be also prepared by reducing metal salts in reversed micelles (water-inoil microemulsions). Strong reducing agents such as NaBH4, N2H4, and sometimes hydrogen gas were used for such purposes. Pt, Rh, Pd, Ir, Cu(41), Ag(16, 42-44), Au, Co(45), Ni, FeNi(46), Cu3Au(47), CoNi(48), etc., have been synthesized using this method in which the size of the droplet containing the reactants is controlled by the ratio of the surfactant to the water concentration. Oil-in-water microemulsions have been also used for the synthesis of nanomaterials. For example, CdS and Cu nanoparticles were successfully produced using these types of microemulsions(49, 50). Figure 1-3: A diagram of the experimental set up for laser ablation to form gold nanoparticles. Chapter I Introduction - 13 - Figure 1-4: Water-in-oil and oil-in-water microemulsions. In biological systems, shape controlled synthesis of nanomaterials has been achieved either by growth in constrained environments such as membrane vesicles or through functional molecules such as polypeptides that bind specifically to different crystallographic planes of inorganic surfaces. For example, triangular gold nanoprisms (Figure 1-5A) can be synthesized biologically(51) in high yield at room-temperature by the reduction of aqueous chloroaurate ions (AuCl4 -) by the extract of the plant lemongrass (Cymbopogon flexuosus). During the reaction, a visible color change occurred from pale yellow to a ruby red, indicating the formation of gold nanoparticles. The reducing sugars (aldoses) present inside the lemongrass extract were found to be responsible for the reduction of Au3+. By simple variation in the concentration of the lemongrass extract in the reaction medium, it is possible to vary the size of the nanoprisms(52), thereby the longitudinal SPR band in the nIR region can be easily tuned (Figure 1-5B). It was reported that tamarind leaf extract can also be used as the reducing agent for making gold nanotriangles(53). Chapter I Introduction - 14 - Light has been found to play an important role in shaping the nanoparticles(40). For example, Ag triangles have been prepared photochemically by irradiation of very small Ag clusters at different wavelengths. Lasers have been also used to melt and reshape the nanocrystals formed in solution(54-56). Spherical silver nanoparticles were converted into silver nanoprisms by exposure to UV and visible light(55,56). It is possible to control the photochemical growth of metal nanoparticles by choosing the color of the light used to drive the reaction. The size and shape of the resulting nanoparticles can be controlled by selectively exciting the plasmon resonance of a given class of particles(56). Figure 1-5: (A) TEM image of gold nanotriangles synthesized by the reduction of aqueous HAuCl 4 solution with lemon grass extract. B) UV-Vis-n IR spectra of gold nanoparticles synthesized by adding different amount s of lemongrass leaf extract to a HAuCl 4 solution. (C) TEM images of gold nanoplates synthesized by the reduction of aqueous AuCl 4by seaweed extract. D) Singlecrystalline Ag nanoplates synthesized in aqueous medium at room temperature using an extract of the unicellular green alga Chlorella vulgaris. Inset shows the SEM image of a single Ag nanoplate. Chapter I Introduction - 15 - 1-3: General methods to the synthesis gold nanorods. One of the current major challenges in material´s research is to develop experimental recipes for the systematic control of the size and shape of metallic nanoparticles (gold nanoparticles). The size and shape of nanorods (NRs) are determined by the various experimental parameters that affect the growth mechanisms. However, the specific mechanisms governing the morphology and geometry control of the particle growth are not yet explicitly understood. Here we will summarize the four major methods for the synthesis of gold nanorods. 1-3-1: Electrochemical method. Gold nanorods were first prepared by the electrochemical method by Wang and co-workers(57). This method provides a synthetic route for preparing high yields of gold nanorods. The synthesis is conducted within a simple two-electrode-type electrochemical cell, as shown in the figure (1-7). A gold metal plate is used as an anode while the cathode is a platinum plate with similar dimensions. Both electrodes are immersed in an electrolytic solution containing a cationic surfactant above its critical micelle concentration (CMC), hexadecyltrimethylammonium bromide (CTAB), Figure 1-6: Silver nanoparticles and silver nanotriangles after illumination with a fluorescent lamp. Chapter I Introduction - 16 - and a small amount of a much more hydrophobic cationic surfactant, tetradodecylammonium bromide (TCAB), which acts as a rod-inducing cosurfactant. The CTAB serves not only as the supporting electrolyte but also as the stabilizer for the nanoparticles, to prevent their aggregation. This method is simple, and NRs can be prepared in a short time. The electrolytic cell containing the mixed solution is then placed inside an ultrasonic bath at 360C. Before the electrolysis, appropriate amounts of acetone and cyclohexane are added into the electrolytic solution. Acetone is used for loosening the micellar framework facilitating the incorporation of the cylindrical-shape-inducing co-surfactant into the CTAB micelles, and cyclohexane is necessary for enhancing the formation of elongated rod-like CTAB micelles. Controlled-current electrolysis is used throughout the process with a typical current of 3mA and a typical electrolysis time of 30min. During the synthesis, the bulk gold metal anode is initially Power S upply Electrochemical cell Pt Cathode Au Anode Teflon spacer Ultrasonic cleaner Oxidation Reduction Surfactant solutions ions Ad-atoms Figure 1 -7: A diagram of the setup for preparation of gold nanorods via the electrochemical method. Chapter I Introduction - 17 - consumed forming AuBr4 - . These anions are complexed to the cationic surfactants and migrate to the cathode where reduction occurs. It is unclear at present whether nucleation occurs on the cathode surface or within the micelles. Sonication is needed to shear the resultant rods as they form away from the surface or possibly to separate the rod from the cathode surface. Another important factor controlling the aspect ratio of the Au nanorods is the presence of a silver plate inside the electrolytic solution, which is gradually immersed behind the Pt electrode. The redox reaction between gold ions generated from the anode and silver metal leads to the formation of silver ions. Wang and co-workers found that the concentration of silver ions and their release rate determined the length of the nanorods. The complete mechanism, as well as the role of silver ions, is still unknown. 1-3-2: Template method. Gold nanorods were first synthesized by Martin and co-workers(58-60) using the template method. This method is based on the electrochemical deposition of Au within the pores of nanoporous polycarbonate or alumina template membranes. These authors showed that the Au/alumina composites can be optically transparent in the visible and also that, by changing the aspect ratio of the prepared nanocylinders, the color of composite membrane can be varied. Initially, the template method was employed to prepare microscopic electrodes by depositing Au on a polycarbonate membrane using electrochemical plating methods. Subsequently, the method has been applied not only to the synthesis of nanocomposites but also to the redispersion of the template-synthesized gold nanorods into water. Alternatively, the rods could be dispersed into organic solvents through the dissolution of the appropriate membrane followed by polymer stabilization. Schematically, the method can be explained as follows: Chapter I Introduction - 18 - Initially a small amount of Ag or Cu is sputtered onto the alumina template membrane to provide a conductive film for electrode position. This is then used as a substrate onto which the Au nanoparticles can be electrochemically grown (stage I). Subsequently, Au is electrodeposited within the nanopores of alumina (stage II). The next stage involves the selective dissolution of both, the alumina membrane and the copper or silver film, in the presence of a polymeric stabilizer such as polyvinyl pyrrolidone (PVP) (stages III and IV). In the last stage, the rods are dispersed either in water or in organic solvents by means of sonication or agitation. The diameter of the gold nanoparticles synthesized coincides with the pore diameter of alumina membrane. This means that Au nanorods (GNR) with different diameters can be prepared by controlling the pore diameter of the template. The length of the nanorods can be controlled through the amount of gold deposited within the pores of the membrane. Figure 1-8: (A and B) FESEM images of an alumina membrane. (C) Schematic representation of the successive stages during formation of GNRs via the template method. (D) TEM micrographs of GNRs obtained by the template method. Chapter I Introduction - 19 - Similar techniques have been successfully applied to the synthesis of gold nanotubes, and nanostructured composites, including tubular composites, which comprise coaxial nanotubes made of different materials. The growth mechanism is straightforward and the size and shape of the NRs are predetermined by the size and shape of the template. This method has advantages for obtaining monodisperse particles. But, the fundamental limitation of the template method is the yield. Since only monolayers of rods are prepared, even milligram amounts of rods are hard to prepare. 1-3-3: Photochemical method. In the first observation of rod-like gold particles, UV-irradiation was used as a reducting agent and alkyltrimethylammonium chloride as a stabilizer. Synthesis conditions such as, the type and concentration of surfactant and the duration of UV-irradiation were modified to prepare NRs in great yield. Kim et al.(61) also used UV-irradiation to produce NRs. They changed the type of stabilizer to hexadecyltrimethylammonium bromide (CTAB) and tetradodecylammonium bromide. They were able to prepare NRs as the major component with the aspect ratio up to 5. The aspect ratio of the rods was controlled by the amount of silver ions (in the form of AgNO3) added to the system. This process itself is highly promising for producing uniform nanorods and, more importantly, it will be useful in resolving the growth mechanism of anisotropic metal nanoparticles due to its simplicity and the relatively slow growth rate of the nanorods. In this method, a growth solution containing AgNO3 and a small amount of cyclohexane (added to loosen the micellar structure) is putted in a quartz cell under UV-light for certain time. The irradiation time is the key factor to control the aspect ratio of the nanorods because with long times of irradiation, short gold nanorods are formed, which is possibly due to the transformation of the rods into the thermodynamically more stable spherical form. Chapter I Introduction - 20 - Ahmed and Narain(62) were using a photoinitiator I-2959 as a source of ketyl radicals in addition to the growth solution and irradiated the solution with UV-light. Figure 1-9: (a) Image of photochemically prepared gold nanorods solution using different concentrations of AgNO3, and (b) corresponding UV-Vis spectrum. Scheme 1-1: Scheme showing the preparation of gold nanorods using UV-light irradiation. Chapter I Introduction - 21 - Scheme 1-2: Scheme showing the photochemical method for the generation of gold nanorods using photoinitiator. By this UV-irradiation method, gold NRs having aspect ratio less than 5 were obtained. However, this method has a limitation to produce large quantities of NRs since the reactor size is limited. 1-3-4: Seed-growth method. Even though a large number of methods have been used for making anisotropic nanostructures, the seed-mediated growth process is a widely used method that can yield various nanostructures such as rods, wires, triangles, stars, flowers, and so on. The seeding-growth procedure is the most popular technique that has been used for a number of years. This method is a modified form of Zsigmondy’s ¨nuclear¨ method, which involved a two-step process for making nanoparticles(63). In 2001, Jana et al. produced NRs in good yield by the seed-mediated method(64). This method involves two steps. The first step is the synthesis of ¨seed nanoparticles¨ by a simple reduction process in which the metal salt is reduced by reducing agents in the presence of stabilizers. Sodium borohydride is the commonly used Chapter I Introduction - 28 - techniques have been developed that give high yields of nonspherical particles, especially rods(94) and triangles(56). The shapes and sizes of these particles are better characterized than in the past using electron and scanning probe microscopies allowing in some cases, the determination of the optical properties of individual nanoparticles(95). Mie’s theory and experimental spectra agree well in the size regime >20 nm until the normal incidence absorption no longer shows a plasmon resonance for bulk metals. The spectrum is composed of the sum of sizedependent absorption and scattering modes. Higher order modes become more dominant with increasing the particle size, causing the plasmon absorption band to red shift and resulting in increased bandwidth, because for larger particles, the light cannot polarize homogeneously the nanoparticles and retardation effects lead to the excitation of higher order modes(83). The optical absorption spectra depend directly on the size of the nanoparticles, which is called the extrinsic size effect(83). Figure (1-11) shows the absorption spectra of Au spherical nanoparticles with different sizes. As the size increases, the plasmon band shifts to the red. Figure 1-11: Size dependence of the plasmon absorption of spherical gold nanoparticles of different sizes. Chapter I Introduction - 29 - When the size of nanoparticles is much smaller than the wavelength (<20 nm) of the interacting light, only the dipole oscillation contributes significantly to the extinction cross section. In this case the Mie’s theory can be approximately expressed by the following equation (6): where, J = extinction coefficient, V =particle volume, N = number of particles, O = wavelength of the light, Hm = medium dielectric constant, ε1 and ε2 represent the real and imaginary parts of the material dielectric function, respectively (ε (ω) = ε1 (ω) + i ε2 (ω), where ω is the angular frequency of the light). Even in this most primitive model, it is perfectly clear that the localized surface plasmon resonance spectrum of an isolated metallic nanosphere embedded in an external dielectric medium will depend on the nanoparticle material (ε2 and ε1), and the nanoenvironment's dielectric constant (εm).The resonance condition for the plasmon absorption is roughly fulfilled when ε1(ω) = -2εm if ε2 is small or weakly(83) dependent on ω. The plasmon bandwidth mainly depends on ε2(ω). Within this dipole approximation (eq 6), the surface plasmon resonance is independent of the particle size. This is contrary to the experimental results for metallic nanoparticles much smaller than 10 nm, where the plasmon band shows size dependence for small particles and even disappears completely for nanoparticles of ≤ 2 nm (clusters). Thus, the assumption of a free electron gas is no longer valid in the size range below 2 nm. It is in fact well established that the bandwidth is inversely proportional to the radius r of the particle for sizes smaller than about 20 nm(83, 96-98).Since Mie’s theory has found wide applicability and has generally been successful in explaining optical absorption spectra of metallic nanoparticles 22 2/3 21 2 ]2[ 3 2 HHH H O HS J     m m VN (6) Chapter I Introduction - 30 - (99, 83-85), a size dependence for the quasi-static regime is introduced in eq 6 by assuming a size dependent material dielectric function ε (ω, R)(100). The dielectric function can be written as a combination of an interband term εIB(ω), accounting for the response of the d electrons, and a Drude term εD(ω) considering only the free conduction electrons (ε(ω) = εIB(ω) + εD(ω))(88,101). The Drude term is given, within the free electron model, by the following expression(88). εD (ω) = 1 – (ωp 2 / ω2 + i κ ω) (7) where ωp 2 = [ne2/(ε0meff)] is the bulk plasmon frequency expressed in terms of the free electron density n, the electron charge e, the vacuum permittivity ε0, the electron effective mass meff, and κ, which is a phenomenological damping constant equals to the plasmon bandwidth. The constant κ is related to the lifetimes of all the electron scattering processes in occurring in the bulk material that are mainly due to electron-electron, electron-phonon, and electron-defect scattering. For a small particle, electron-surface scattering also becomes important, since the mean free path of the conduction electrons, typically in the range of tens of nanometers in noble metals, is limited because of the particle’s boundaries. κ therefore becomes a function of the particle radius r (96,97), κ (r) = κ0 + A υf/r (8) where κ0 is the bulk damping constant, υf is the velocity of the electrons at the Fermi energy, and A is a theory-dependent parameter that includes details of the scattering process (e.g., isotropic or diffuse scattering)(83, 96-98). Kreibig was the first in introducing this classical picture of the limitation of the electron mean free path and found good agreement with experimental results(96). An early quantum mechanical model developed by Kawabata and Kubo(102) also predicts a 1/r dependence of the plasmon bandwidth. However, their model does not treat the surface as a scatterer for the Chapter I Introduction - 31 - electrons; rather the surface determines the energy of the system. Another more recent quantum mechanical model considers the adsorbed molecules on the nanoparticle surface(103). It is suggested that the surface plasmon energy is transferred into excitation modes of the surface metal-adsorbate complex (chemical interface damping). Many more theories exist(83, 98,104107), but all of them find a 1/r dependence, reflecting the importance of the ratio between the surface area and the volume. However, the slope parameter A (on the order of 1) depends on the theory(98). Since the size dependence of the plasmon absorption in the quasi-static regime is introduced assuming a size dependent material dielectric function ε(ω, R) , the related changes in the optical absorption spectra are referred to as intrinsic size effects(83). For larger nanoparticles (>25 nm for gold particles), the extinction coefficient explicitly depends on the nanoparticle size as higher order terms contribute, which are functions of r (84, 85, 108). For these large particles the plasmon bandwidth increases with increasing size as the wavelength λ of the interacting light becomes comparable to the dimension of the nanoparticle. This leads to an inhomogeneous polarization of the nanoparticle by the electromagnetic field. The broadening of the plasmon band is then usually ascribed to retardation effects(83). The increased line width is also caused by the excitation of different multipole modes, which peak at different energies. This behavior is referred to as an extrinsic size effect because the size dependence enters through the full expression of Mie’s theory(83). Furthermore, when the nanoparticles are not spherical, as is always the case in real samples, the extinction spectrum will depend on the nanoparticle’s in-plane diameter, out-of-plane height, and shape. The absorption spectrum of gold nanoparticles not only depends on the size and the dielectric constant of the surrounding medium but also the shape of the particles has a Chapter I Introduction - 32 - very pronounced effect(9). The rod shape dependence of the surface plasmon absorption has been studied by Gans(9). It has been shown that the longitudinal plasmon band of the rod shaped particle is very sensitive to the aspect ratio of the rods as shown by the following equation (9)(84): where Pj are the depolarization factors for the three axes A, B, C of the rod with A > B = C. These factors are defined as: e = (1- (B/A)2)1/2 = (1-1/R2)1/2 It is then predicted that increasing the aspect ratio, the band is shifted to lower energies. The ratio A / B is known as the aspect ratio R (a.r). 400 500 600 700 800 900 Longitudinal SP Transverse SP Absorbance/a.u Wavelength/nm 0.0 0.5 1.0 1.5 2.0 ¦       j m j j jm P P PVN 22 22/3 21 2 ) 1 ( )/1( 3 2 HHH H O HS J 2 1 ;1 1 1 ln 2 11 2 2A CBA P PP e e e e e P » ¼ º « ¬ ª ¸ ¹ · ¨ © §       Figure 1-12: Absorption spectra of spherical and rod shape nanoparticles. (9) Chapter I Introduction - 33 - Figure 1-14: A representation diagram illustrating the optical response of rod-like nanoparticles to an electric field E. Two oscillating modes can be possible: (a) the transverse oscillation along the B or C axis and (b) the longitudinal oscillation along the A axis. Figure 1-13: Absorption spectra showing the dependence of SP L of the gold nanorods on the aspect ratio. 400 500 600 700 800 900 1000 iv iii ii i Absorbance /a.u Wavelength/nm 0.0 0.4 0.8 1.2 1.6 Chapter I Introduction - 34 - As it was previously mentioned, the plasma resonance for nanorods splits into two bands. In the case of nanoprisms(55,56), the plasmon resonance splits into three bands, a transverse mode, a longitudinal mode and a band between them. The third band and the longitudinal band depend on the length of the three axes of the prism. As the size (length and thickness) of these particles increases, these bands shift to lower energy. The higher energy bands are assigned to oscillation along the C3 axis perpendicular to the triangular faces and the broad low energy band is assigned to the inplane oscillation on the triangular faces. 1-5: Applications of Gold Nanoparticles (nanorods). The longitudinal surface plasmon wavelengths (LSPWs) of gold nanorods are tunable from the visible to infrared regions. Their absorption cross sections are at least five orders larger than those of conventional dyes, and the light scattering by Au nanorods is several orders larger than the light emission from strongly fluorescent dyes(109-111).The tunability of the LSPW, together with the strongly enhanced scattering and absorption at the LSPW, makes GNRs useful for the formation of many functional composite materials, for example, with hydrogels(112,113), polymers(114,115), silica(116), and bacteria(117). The axial (transversal) surface Plasmon resonance (SSPR), though one-third that of the LSPR, is still many orders of magnitude greater than quantum dots and nanoshells, which can also be used for the described purposes. GNRs also offer advantages of good biocompatibility, facile preparation, and conjugation with a variety of biomolecular ligands, antibodies, and other targeting moieties(118). They have therefore found wide applications in biochemical sensing(119), biological imaging, medical diagnostics, and therapeutics(120-124). Further, GNRs have found application in materials and optics, including polarizers, filters, and to improve the storage density in compact disks. Chapter I Introduction - 35 - The effectiveness of GNRs as scattering-based biomedical imaging contrast agents and as photothermal therapeutic agents is strongly dependent on their scattering and absorption cross sections. In general, high scattering cross sections are favorable for cellular and biological imaging based on dark field microscopy, while large absorption cross sections with small scattering losses allow for photothermal therapy with a minimal laser dosage. In addition, the LSPWs of GNRs are strongly desired because of their tunability in the spectral range of 650–900 nm. Light irradiation in this region can penetrate deeper in tissues and cause less photodamage than UV–visible irradiation(125). Therefore, the ability to tailor both scattering and absorption of GNRs with different LSPWs is of ultimate importance for practical in vivo biomedical imaging and therapeutic applications(126-128). Recent results show that laser writing can also be readily applicable to PVA films containing gold nanorods of various aspect ratios, and thus expanding the possibilities for practical applications of such films(129). Further applications related to the nonlinear optical response of the nanorods can also be envisaged, but still need to be developed. However, the main use of GNRs is in electronics, catalysis and biomedical applications, which will be discussed in more in details below. 1-5-1: Electronics. Gold is the material of choice in many electronic applications, especially in telecommunications, information technology and other high performance and safety critical applications. In the early 2000s it was estimated that around 200 tons of gold found its way into electronics and electrical components. Where the voltages are small, the circuitry complex or the required reliability is high; gold is usually the preferred choice. Gold-plated connectors are an integral part of plugs and sockets for cable terminations, integrated circuit sockets and printed circuit boards. In general, the more sophisticated the equipment and the greater the need for reliability, the Chapter I Introduction - 36 - greater the requirement to exploit the advantages of gold as the material of choice. This means that in telecommunications, computers, automotive electronics and defense systems where safety is critical, gold is indispensable(130). Gold’s other main electronic use is in fine wires to connect parts of semiconductors such as transistors and integrated circuits to ensure reliable connections between components. This bonding wire is specially refined to high purity (99.99 % gold) and would typically be thinner than a human hair (10 - 200 microns). Gold’s use in electronics is based upon its high electrical conductivity along with excellent tarnish resistance. Its main applications are as conductive pastes, bonding wires, connectors and low duty contacts. Where high quality, durable, safety and reliability critical applications operating in arduous environments are concerned, gold is the optimal material. Despite the substantial growth in electronics, growth in gold demand has not kept pace due to miniaturization of devices, coupled with the use of cheaper, competing materials in the low-end, high-volume consumable electronics. The use of palladium as a cheaper alternative to gold in connectors is a typical example of this trend. The growth in automotive electronics continues; here, the electronics operate in arduous under-bonnet conditions, necessitating the use of gold. With environmental restrictions on traditional lead-tin coatings increasing, the use of electro less nickel/immersion gold protective coatings for printed circuit boards is reported to be growing(131). 1-5-2: Catalysis. Catalysts drive many reactions, with the ability to lower the activation energy of the reaction, and thus increase the rate of reaction and the yield of the desired products. The use of nanoparticles as catalysts has increased exponentially as nanoparticle properties and reactions are better understood. The possibility of using less material and having different Chapter I Introduction - 37 - properties for different shapes of nanoparticles is very attractive. Nanoparticle catalysis has been investigated for both homogeneous and heterogeneous systems. In homogeneous catalysis, Narayanan and El-Sayed(132) have shown that shapes with more corners and edge atoms have a higher reactivity than similar nanoparticles with fewer corner and edge atoms. Thus shape and crystal structure differences can lead to different catalytic rates. Research continues to observe the connection between structure and function for nanoscale catalysts. Small clusters are also found to be very catalytically active, even for materials that display very limited reactivity on the bulk scale(133). Bulk gold is considered a noble metal, and is very unreactive in the bulk state. However, small clusters (nanoparticles smaller than approx. 2 nm) of gold are found to be catalytically active. The explanations to this include the different electronic and chemical properties of clusters than bulk material and nanoparticles. The surface support of the catalysis is also suggested to have an important influence on the catalytic activity. The crystal structure of gold has also been proposed to be important in the catalytic properties. This demonstrates the appearance of new properties for nanoparticles, which are unexpected based on bulk (and even on nanoparticle) behavior, since bulk gold has no catalytic activity, and clusters are efficient catalysts, generating further interest in nanomaterials as new functionalities are present on the nanoscale(134). Catalysts are important to the chemical industry, with over 90 % of all the chemicals manufactured using catalysts. Among other chemicals, gold has been traditionally viewed as the exception to the other precious metals in not being very catalytically active. All that changed in the mid-to-late 80’s when Hutchings and Yamada et al. demonstrated that, when prepared properly, gold can be a very active catalyst(134). The key to this is preparation of gold as nanosized particles of around 5 nm in diameter on an Chapter I Introduction - 44 - conditions like concentration, pH, ionic strength, and so on. Self assembly can be modified by controlling the interactions between the constituents. This is usually done by regulating the environmental conditions such as pH and ionic strength and is often referred to as ¨programmed self-assembly¨. Chemical modifications or functionalization are usually done in order to control the assembly. Surface-functionalized GNRs, with specific molecular groups, can lead to precisely controlled self-assembled structures. It is generally observed that programmed self-assembly with functionalized NRs can be an efficient mean of nanofabrication owing to its simplicity, versatility, and low cost. The only drawback of this method is that normally provides ordered structures in small areas, and do not allow the control over large distances. 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The experimental methods used for the characterization of these particles will be also discussed in this chapter together with the explanation of the different methods used to study the thermal and photostability of these particles. We will start firstly writing the materials used in the preparation methods. 2-1: Materials. The materials used were: 1Surfactants: x Cetyltrimethylammonium bromide (CTAB, Fluka, 99%) x Cetyltrimethylammonium bromide (CTAB, Aldrich, ≥ 98%) 2Gold (III) chloride hydrate, (Aldrich, 99.999% metal basis). 3Reducing agents: x Sodium borohydride ((NaBH4), Riedel-de Haën, 95%) x L-ascorbic acid (Sigma-Aldrich, 99%) 4Trisodium citrate (Sigma-Aldrich, 99%) 5Silver nitrate (AgNO3, Aldrich, A.C.S. reagent, 99%) 6Polyvinyl pyrrolidone (PVP-K30, with an average molecular weight = 30000 to 40000, Fluka) 7Silver atomic quantum clusters dispersed in water at a concentration of 1 mg/l (Ag-clusters, NGAP AQC Ag-1102-W, Nanogap, Spain). Chapter II Experimental chapter - 52 - The details of the characterizations and the properties of these silver clusters, provided by the supplier, are : Figure 2-1: Absorption spectra showing the Nanogap Ag-clusters with an AFM picture of the clusters deposited on mica (mean square roughness ~ 150 pm) and the profiles throughout the red and green lines depicted on the AFM picture. Chapter II Experimental chapter - 53 - Distilled water (18 MΩ) was used in all the experiments. All glassware was firstly washed with aqua regia (3:1 ratio by volume of HCl and HNO3), and then several times with distilled water before using. 2-2: Instrumentation. 2-2-1: Nanoparticle characterization and optical absorption. One of the significant challenges in the gold nanoparticle synthesis is the right characterization in order to establish the average size and shape, ligand shell composition and impurity profile of the synthesized material. Developing an accurate picture of the formed gold nanoparticles allows for a greater understanding of the optical and electronic properties and assists in the development of structure-function relationships. To determine the size and shape of the produced gold nanoparticles, transmission electron microscopy (TEM) and UV-Visible spectroscopy (UV-Vis) were used. Atomic Force Microscopy (AFM) was also used to determine the size of the clusters and seeds of gold and silver particles. Thermal gravimetric analysis (TGA) and Differential scanning calorimetry (DSC) were used to determine changes in the heat flow and weight of Au NRs capped with CTAB. Using this string of characterization techniques, it is possible to develop a fairly accurate picture of the gold nanoparticle samples, though the development of new characterization methods is an active area of research. A) Absorption spectra were recorded with a Perkin-Elmer Lambda 40 spectrometer, Thermo Evolution300 UV-Visible spectrophotometer and Hewlett-Pack 8452A Diode - Array spectrophotometer to analyze the optical properties of gold nanoparticles, primarily through the plasmon resonances. For spherical particles the plasmon band is located at ~520 nm; however, the location is shifted due to changes in the dielectric constant of the solvent, ligand shell, core Chapter II Experimental chapter - 60 - 2-2-2-3: Nd: YAG laser (continuum SLI-10). The laser (10 Hz) generated from the second harmonic generation of the Nd-YAG laser (continuum SLI-10) at λ = 1064/2 = 532 nm were focused onto a quartz cell containing the sample solution of gold nanoparticles. The energy of the Nd-YAG laser was 100mJ measured with a power energy meter (molectron-EPM 2000:Tar5). The photostability of the gold nanoparticles was followed by measuring the changes in absorption spectra as a function of time. Figure 2-8: A picture showing the exposure of the gold nanoparticles in a quartz cuvette to laser radiation. Figure 2-7: A diagram showing the pulsed Nd: YVO 4 laser setup. Chapter II Experimental chapter - 61 - 2-3: Preparation methods. 2-3-1: Preparation of spherical gold nanoparticles using the citrate method. Spherical gold nanoparticles in aqueous solution were prepared according to the method described by Turkevich. Simply, the method is a chemical reduction of gold ions by sodium citrate in aqueous solution. Sodium citrate serves also as a capping material to prevent aggregation and further growth of the particles. 5 ml of 1 % sodium citrate solution were added to 40 ml of a boiling solution of chlorauric acid (HAuCl4) containing 5 mg of gold ions. The solution was boiled for 30 minutes and was then left to cool down to room temperature. The produced gold particles have an average diameter of 15 nm as determined by TEM analysis. The standard deviation of the average particle diameter was found to be about 10 % or less. 2-3-2: Preparation of different shaped gold nanoparticles. Different shaped gold nanoparticles were prepared by a modified version of the seed-mediated method. In order to study the effect of Ag-ions and seed concentration, the seed and the growth solutions were made as described below: IInfluence of AgNO3 concentration. Seed Solution: A CTAB solution (2.5 ml, 0.20 M) was mixed with 2.5 ml of 5x10-4 M HAuCl4. To the stirred solution, 0.3 ml of ice-cold 0.01 M NaBH4 was added, which resulted in the formation of a brownish yellow solution. Vigorous stirring of the seeds was continued for 2 min. Then, after the solution was stirred, it was kept at 25 0C. Chapter II Experimental chapter - 62 - Growth Solution: A CTAB (2.5 ml, 0.20 M) was added to specific amounts (0 - 400μl) of 4x10-3 M AgNO3 solution at 250C. To this solution, 2.5 ml of 10-3M HAuCl4 was added and, after gentle mixing of the solution, 35 μl of 0.0788 M ascorbic acid (freshly prepared) was added. The color of the growth solution changed from dark yellow to colorless. The final step was the addition of 6 μl of the seeds to the growth solution at 27-30 0C. The color of the solution gradually changed within 10-20 minutes. For lower concentration of Ag-ions, the change in color took place more slowly. The temperature of the growth medium was kept constant at 27-30 0C in all the experiments. IIInfluence of the seed solution concentration on the growth process. Seed Solution: A CTAB solution (2.5 ml, 0.20 M) was mixed with 2.5 ml of 5x10-4 M HAuCl4. To the stirred solution, 0.3 ml of ice-cold 0.01 M NaBH4 was added, which resulted in the formation of a brownish yellow solution. Vigorous stirring of the seeds was continued for 2 min. After the solution was stirred, it was kept at 25 0C. Growth Solution: A CTAB (2.5 ml, 0.20 M) was added to 100μl of 4x10-3 M AgNO3 solution at 250C. To this solution, 2.5 ml of 10-3M HAuCl4 was added and, after gentle mixing of the solution, 35 μl of 0.0788 M ascorbic acid (freshly prepared) was added. The color of the growth solution changed from dark yellow to colorless. The final step was the addition of (2 μl - 14 μl) of the seeds to the growth solution at 27-30 0C. The color of the solution gradually changed within Chapter II Experimental chapter - 63 - 10-20 minutes. The temperature of the growth medium was kept constant at 27-30 0C in all the experiments. 2-3-3: Influence of aging of the seeds. In the method 2-3-2, the final step for the addition of the seeds was to use this solution aged at different times. 2-3-4: Influence of CTAB suppliers. To study the effect of CTAB supplier, two sources of CTAB (one from Fluka and other from Aldrich) were used in the 2-3-2 method. 2-3-5: Influence of Ascorbic acid on the growth of gold nanoparticles. The effect of ascorbic acid concentration was studied using the same method of preparation (2-3-2): Seed Solution: A CTAB solution (2.5 ml, 0.20 M) was mixed with 2.5 ml of 5x10-4 M HAuCl4. To the stirred solution, 0.3 ml of ice-cold 0.01 M NaBH4 was added, which resulted in the formation of a brownish yellow solution. Vigorous stirring of the seeds was continued for 2 min. After the solution was stirred, it was kept at 25 0C. Growth Solution: A CTAB (2.5 ml, 0.20 M) was added to 100μl of 4x10-3 M AgNO3 solution at 250C. To this solution, 2.5 ml of 10-3M HAuCl4 was added and, after gentle mixing of the solution, different amount (25, 35, 50, 70, and 100 μl) of 0.0788 M ascorbic acid (freshly prepared) was added. The growth solution color changed from dark yellow to colorless. The final step was the addition of different amounts (2, 6, and 12 μl) of the seeds to the growth solution at 27-30 0C. The color of the solution gradually changed within 10-20 minutes. Chapter II Experimental chapter - 64 - 2-3-6: Effect of the nature of the capping agent in the seed solution using trisodium citrate. IChange of the concentration of the seeds. Seed Solution: A trisodium citrate solution (10 ml, 2.5x10-4 M) was mixed with 10 ml of 2.5x10-4 M HAuCl4. To the stirred solution, 0.6 ml of ice-cold 0.1 M NaBH4 was added, which resulted in the formation of a pink solution. This solution was aged and used after 10 minutes and 3 hours of addition of NaBH4. It was kept at 27 0C. Growth Solution: A CTAB (2.5 ml, 0.20 M) was added to 100 μl of 4x10-3 M AgNO3 solution at 250C. To this solution, 2.5 ml of 10-3M HAuCl4 was added and, after gentle mixing of the solution, 35 μl of 0.0788 M ascorbic acid (freshly prepared) was added. The growth solution color changed from dark yellow to colorless. The final step was the addition of different amounts (6, 12, 25, 50, 100, 250, 500, 1000 12 μl) of the seeds to the growth solution at 27-30 0C. The color of the solution gradually changed within 5-25 minutes. IIChange of the silver ions concentration. Seed Solution: A trisodium citrate solution (10 ml, 2.5x10-4 M) was mixed with 10 ml of 2.5x10-4 M HAuCl4. To the stirred solution, 0.6 ml of ice-cold 0.1 M NaBH4 was added, which resulted in the formation of a pink solution. This solution was aged and used after 10 minutes and 3 hours of addition of NaBH4. It was kept at 27 0C. Growth Solution: A CTAB (2.5 ml, 0.20 M) was added to different solutions (0 - 400μl) of 4x10-3 M AgNO3 at 250C. To this solution, 2.5 ml of 10-3M HAuCl4 was Chapter II Experimental chapter - 65 - added and, after gentle mixing of the solution, 35 μl of 0.0788 M ascorbic acid (freshly prepared) was added. The growth solution color changed from dark yellow to colorless. The final step was the addition of 50μl of the seeds to the growth solution at 27-30 0C. The color of the solution gradually changed within 10-20 minutes. 2-3-7: Preparation of large volume of gold nanoparticles. The method to prepare gold nanorods in large volumes (up to 500ml) was as follows: Seed Solution: A CTAB solution (2.5 ml, 0.20 M) was mixed with 2.5 ml of 5x10-4 M HAuCl4. To the stirred solution, 0.3 ml of ice-cold 0.01 M NaBH4 was added, which resulted in the formation of a brownish yellow solution. Vigorous stirring of the seeds was continued for 2 min. After the solution was stirred, it was kept at 25 0C. Growth Solution: (2.5, 5, 10, 25, 50 and 250 ml) of 0.20 M CTAB were added to (0.1, 0.2, 0.4, 1, 2, 10 ml) of 4x10-3 M AgNO3 solution at 250C. To this solution, (2.5, 5, 10, 25, 50 and 250 ml) of 10-3M HAuCl4 were added and, after gentle mixing of the solution, (35, 70, 140, 350, 700 and 3500μl) of 0.0788 M ascorbic acid (freshly prepared) were added. The color of the growth solution changed from dark yellow to colorless. The final step was the addition (6, 12, 24, 60, 120 and 600μl) of the seeds to the growth solution at 27-30 0C. The color of the solution gradually changed within 10-50 minutes. The temperature of the growth medium was kept constant at 27-30 0C in all the experiments. Chapter II Experimental chapter - 66 - 2-3-8: Preparation of different shaped gold nanoparticles using silver clusters. The effect of commercial Ag-clusters on the growth process of gold nanoparticles was carried out as described below: Seed Solution: A CTAB solution (2.5 ml, 0.20 M) was mixed with 2.5 ml of 5x10-4 M HAuCl4. To the stirred solution, 0.3 ml of ice-cold 0.01 M NaBH4 was added, which resulted in the formation of a brownish yellow solution. Vigorous stirring of the seeds was continued for 2 minutes. After the solution was stirred, it was kept at 25 0C. Growth Solution: A CTAB (2.5 ml, 0.20 M) was added to varied amounts of 1 mg/l of Agclusters (Nanogap, Spain) to have a final concentration ranging from 6.65 x 10-11 to 9.5x10-7 M at 250C. To this solution, 2.5 ml of 10-3M HAuCl4 was added and, after gentle mixing of the solution, 35 μl of 0.0788 M ascorbic acid (freshly prepared) was added. Ascorbic acid changed the growth solution color from dark yellow to colorless. Finally, 6 μl of the seeds added and the color of the solution gradually changed within 2-10 minutes. 2-3-9: Effect of [Cl-] on the growth of gold nanoparticles. The effect of NaCl on the growth process was studied in the absence and the presence of Ag-clusters (Nanogap, Spain) as follows: Seed Solution: A CTAB solution (2.5 ml, 0.20 M) was mixed with 2.5 ml of 5x10-4 M HAuCl4. To the stirred solution, 0.3 ml of ice-cold 0.01 M NaBH4 was added, which resulted in the formation of a brownish yellow solution. Vigorous stirring of the seeds was continued for 2 minutes. After the solution was stirred, it was kept at 25 0C. Chapter II Experimental chapter - 67 - Growth Solution: A CTAB (2.5 ml, 0.20 M) was added to varied amounts of 0.0973 M of NaCl to get a final concentration [Cl-] ranging from 10-7 to 3.14x10-3 M at 250C. To this solution, 2.5 ml of 10-3M HAuCl4 was added and, after gentle mixing of the solution, 35 μl of 0.0788 M ascorbic acid (freshly prepared) was added. Ascorbic acid changed the growth solution color from dark yellow to colorless. Finally, 6 μl of the seeds added and the color of the solution gradually changed within 30-120 minutes. 2-3-10: Preparation of different shaped gold nanoparticles using gold clusters. For the preparation of gold nanorods of different aspect ratios, seeds, clusters and growth solutions were made as described below: Cluster solution: 5 ml of 0.2 M CTAB solution was mixed with 5 ml of 5x10-4 M HAuCl4•3H2O. To the stirred solution, 60 μl of ice-cold freshly prepared 0.1M NaBH4 was added, which results in the formation of a solution with brownish yellow color. Vigorous stirring of the clusters was continued and used after 10 sec., 30sec., 1min. and 5min. of preparation. Seed solution: 2.5 ml of 0.2 M CTAB solution was mixed with 2.5 ml of 5x10-4 M HAuCl4•3H2O. To the stirred solution, 30 μl of ice-cold freshly prepared 0.1M NaBH4 was added, which results in the formation of a solution with brownish yellow color. Vigorous stirring of the seed solution was continued for 2 minutes. The seeds were used after 10 minutes of preparation. Chapter II Experimental chapter - 68 - Growth solution: 5 ml of the gold clusters solution was added to 2.5 ml of deionized water at 25 0C. To this solution, 2.5 ml of 10-3M HAuCl4•3H2O was added and, after gentle mixing of the solution, 35 μl of 0.0788 M ascorbic acid was added. The color of growth solution changed from pale yellow to colorless within few seconds. After that, the color of the solution changed gradually within 20-30 minutes. For comparison experiments, 6 μl of the seeds were added at the same time of the addition of ascorbic acid to the growth solution. The color of the solution gradually changed within 10-20 minutes. Chapter III Chapter III Results & discussion - 76 - create an asymmetric electric field initially, possibly in the form of a twinning plane or stacking fault(2,18).The influence of CnTAB analogues, in which the length of the hydrocarbon tails was varied, keeping the headgroup and the counter ion constants, was also studied(19). It was found that the length of the surfactant tail is critical for controlling not only the length of the nanorods but also the yield. Shorter chain lengths produce shorter nanorods, whereas longer chain lengths lead to longer nanorods in higher yields. Considering the preferential adsorption of C16TAB to the different crystal faces forming a bilayer(15,16,18,19), a "zipping" mechanism was proposed taking into account the van der Waals interaction between surfactant tails within the surfactant bilayer, that may promote the formation of longer nanorods from more stable bilayers(18). The group of Liz-Marzán also investigated the factors affecting the nucleation and growth of gold nanorods under similar conditions(2). They showed that when temperature and CTAB concentration are reduced, it is also possible to synthesize short gold nanorods of aspect ratios ranging up to six with a yield up to ~ 50%. The control of the aspect ratio, as well as the monodispersity and the yield were demonstrated to be influenced by a number of factors, such as the stability of the seed, temperature and the Figure 3-2: A cartoon illustrating the “zipping” mechanism: the formation of a bilayer of C n TAB (squiggles) on the nanorod (black rectangle) surface may assist the growth of the nanorod as more gold ions (black dots) are introduced (16) . Chapter III Results & discussion - 77 - nature and concentration of surfactant. It is observed that the yield of nanorods prepared from CTAB capped seeds is much higher than that from naked (or citrate stabilized) seeds. This indicates that the more colloidally stable the gold seed nanoparticles are, the higher the yield of rods. As expected, the lower the amount of seeds added the higher the aspect ratio of the nanorods formed. In addition, the authors proposed another mechanism for the nanorod growth, which is based on a series of observations related to the binding of the gold salt ions to the cationic micelles, which are summarized below: i. The yield of rods improves with increasing the colloidal stability of the seeds; hence dimmers or coalescent seeds are not good precursors to the rod formation. ii. Bromide ions are much better than chloride ions as rod-inducing agents in the presence of CnTA+. As the length of the surfactant tail, Cn, increases the yield and the aspect ratio of the nanorods increase (18). The addition of NaCl, NaNO3, or NaBr reduces the aspect ratio of the nanorods, being this effect similar for each ion. An increase in the ionic strength produces a decrease in the yield of rods. iii. Under optimal conditions, the aspect ratio can be controlled through the ratio of seeds to the HAuCl4 concentration. An increase in the amount of seeds gives rise to a decrease in the aspect ratio of the rods because of the nuclei increase. iv. Both AuCl4 - and AuCl2 - are quantitatively adsorbed to CTAB(20). The optimal CTAB: HAuCl4 concentration ratio is in a narrow window since precipitation of CTAB-HAuCl4 occurs at < 10:1 ratio. This can be avoided by increasing the temperature. However, the yield of rods decreases gradually at higher temperatures. v. The presence of CTAB not only directs Au ions to the tips, but also drastically retards the rate of metallic gold formation compared with Chapter III Results & discussion - 78 - the absence of CTAB(18). The higher the curvature of the gold surface, the faster the rate of growth. The proposed mechanism can be summarized in the following steps: AuCl4 - + 2e- ↔ AuCl2 - + Cl- (1) 3AuCl2 - ↔ AuCl4 - + 2Au0 + 2Cl- ; (2) AuCl2 - + e- ↔ 2Au0 + 2Cl- (3) AuCl2 - −CTABmic + Aum ↔ Aum+1 –CTABmic + CTABmic + 2Cl- (4) In the presence of CTAB, ascorbic acid reduces Au(III) to Au(I) via reaction (1). However, no colloidal gold is formed, i.e., disproportionation of AuCl2 - −CTAB does not occur (reaction 2). In fact, it is possible to prepare solutions of AuCl2 - - CTAB by adding HAuCl4 to colloidal gold in the presence of CTAB. Consequently, the reduction of Au(I) can proceed through electron transfer at the surface of the electron–rich gold seeds (reaction 3). In the presence of CTAB, the reduction can be described by reaction (4). During the typical microelectrode-type deposition(21), electrons are transferred to the gold particle while adsorbed AuCl2 - ions may pick up electrons at any favorable adsorption site. Usually spherical growth is obtained under these circumstances(21,22), but in the case of CTAB containing solutions, the gold seeds are encapsulated in CTAB, and the gold ions are likewise bound to CTAB (21). It was postulated that the rate of nanorod formation is determined by the frequency of collisions of AuCl2 - laden cationic micelles with the cationic gold seed particles. The following sketch might explain this mechanism: Figure 3-3: Sketch showing the mechanism for gold nanorod formation. The transport of the gold ions bound to the CTAB micelles to the growing seed particles is controlled by the double layer interaction (21) . Chapter III Results & discussion - 79 - This interaction will be controlled by the electrical double layer interaction between micelles and gold nanorods. The rate of the reaction is then controlled by the collision of the micelles. A faster rate of collision of the micelles at the tips than at the sides will induce rod formation which was confirmed through calculations of the potential distribution and potential gradient around an ellipsoid(2). These calculations showed that the potential decays more rapidly near the tip which means that it will be easier for a micelle to approach to a given distance at the tip rather than from the sides of the rods. The initial change in morphology could only be explained assuming that a stacking fault or twinning plane in the seeds should be present to create an initial electric field asymmetry. Zubarev found that Au(III)–CTAB complexes are capable of dissolving gold nanostructures in a shape-dependent manner; this process can be used to purify gold high aspect ratio nanorods and greatly improve their monodispersity(23). There were many groups working on different ways in the chemically synthesis of gold nanorods, or nanowires; for example, Giersig and LizMarzán reported that HAuCl4 can be reduced with oleylamine, followed with thermal and solvent processing, to yield 1.6 nm diameter singlecrystalline gold nanowires that can have lengths up to 4 μm(24). In this case the control of aspect ratio was limited, and the growth mechanism was, like for CTAB, attributed to preferential adsorption of oleylamine to different crystal facets of the growing nanorod/nanowire(24). Murray et al. used a gold(I) precursor, AuCl, with oleylamine and carbon monoxide as a reducing agent in a related manner to produce gold nanowires that are 2.5 nm in diameter and up to several microns long, albeit with limited control over aspect ratio(25). Chapter III Results & discussion - 80 - 3-1-ii: Synthesis with AgNO3. The presence of silver nitrate allows a better control of the shape of gold nanorods synthesized via the electrochemical method(26). In the seedmediated growth method, Murphy and co-workers modified their initial procedure for long nanorods by adding a little amount of AgNO3 (14) in order to increase the yield of rod-shaped nanoparticles (up to 50%) and to control the aspect ratio of shorter nanorods and spheroids(27). Under identical experimental conditions, a small amount of silver nitrate is added prior to the growth step. The aspect ratio of the spheroids and nanorods can be controlled by varying the ratio of seed to metal salt. They found that the presence of the seed particles is crucial in the growth process and that is an increase in the aspect ratio when the concentration of seed particles is decreased. The mechanism by which Ag+ ions modify the metal nanoparticle shape is not really understood. It has been hypothesized that Ag+ adsorbs at the particle surface in the form of AgBr (Brcoming from CTAB) and restricts the growth of the AgBr passivated crystal facets(27). The possibility that the silver ions themselves are reduced under these experimental conditions (pH ~ 2.8) was neglected since the reducing power of ascorbic is too positive at low pHs(28). Nikoobakht and El-Sayed modified the seed-mediated growth method to overcome its drawbacks and limitations, such as the formation of noncylindrical nanorods, unshaped particles and contamination by spherical particles. Their modification essentially consisted in the use of CTAB-capped seeds rather than the citrate-capped ones(9). This resulted in the formation of 99% gold nanorods with aspect ratios, which could be turned from 1.5 to 5. The replacement of sodium citrate with CTAB in the seed´s formation step produces small spherical particles (<4nm)(9). Differences in yield and shape of nanorods could be due to the slightly Chapter III Results & discussion - 81 - different size of the CTAB and sodium citrate capped seeds, but it may also be attributed to different crystal structure in the two kinds of seed particles. By simply adjusting the amount of silver ions in the growth solution, a fine-tuning of the aspect ratio of the nanorods can be achieved so that an increase in silver concentration (keeping the amount of seed solution constant) leads to a red-shift in the longitudinal plasmon band indicating an increase of the aspect ratio. There is a critical silver ion concentration above which the aspect ratio of the nanorods decreases again(9). The aspect ratio can be controlled by adjusting the amount of the seeds added to the growth solution in the presence of constant Ag+ concentration. Contrary to expectations, an increase in the amount of seeds produces a red-shifted in the longitudinal plasmon band position pointing towards an increase in aspect ratio. With the increase of the concentration of seeds the length of the nanorods decreases slightly while the width decreases(29,30). A similar trend is observed when the gold ion concentration is increased(30). Nanorods with higher aspect ratios could be obtained for gold ion concentrations up to [Au3+] ~ 6 x 10-4 M. The concentration of ascorbic acid has also been shown to influence the morphology with decreasing length and rod yield when the ascorbic acid concentration is increased maintaining other parameters constant. By adjusting the silver ion or gold seed concentrations in this single-component surfactant (CTAB) solution, the longitudinal plasmon band can be tuned up to 825nm corresponding to an aspect ratio of ~ 4.5. Nikoobakht and El-Sayed proposed the use of a binary surfactant mixture containing CTAB and benzyldimethylammoniumchloride (BDAC) to grow gold nanorods with larger aspect ratios (>5)(9). Two different approaches have been proposed for explaining this fact: firstly, by changing the BDAC: CTAB ratio from 16 to 2, the aspect ratio of the nanorods can be tuned from 5 up to 8. The process in this binary mixture is complex. There Chapter III Results & discussion - 82 - is an initial fast growth stage that occurs over the first hour. This is followed by a much slower growth step that takes place over one week. The approach presents two main disadvantages related to the ageing of the solutions, namely the low reproducibility and the formation of a large amount of spherical particles as the surfactant ratio is increased. A second approach, which employs a low BDAC: CTAB surfactant ratio, has been used to ensure that few nanospheres are formed. After the first stage of the growth is completed (fast growth step), different amounts of the growth solution are added gradually. These gradual additions result in a red-shift of the longitudinal plasmon band position, which means that the length of the nanorods is increased after each addition. The mechanism in this case must be related to the presence of silver nitrate, since this is essential for the preparation of nanorods in high yields. Two mechanisms have been proposed to account for the gold nanorod formation although the role of silver ions is not clearly understood at this moment. In the first one, the surfactant forms a soft template with a certain size that depends on surfactant concentration and ionic strength of the solution(9). The growth solution contains a mixture of gold and silver ions and when ascorbic acid is added only gold ions are reduced since it is assumed that silver ions can only be reduced at basic pH values(31). Nikoobakht and El-Sayed proposed that silver ions located between the headgroups of the capping surfactant (CTAB) can be considered as Ag-Br pairs, decreasing the charge density on the bromide ions and, therefore, the repulsion between neighboring headgroups on the gold surface giving rise to NRs with lower aspect ratios(9). This possibility is supported by the stronger affinity of CTAB monomers for the side facets compared to the end facets(32). High-resolution TEM images show that nanorods have four facets(33). In the case of the BDAC: CTAB surfactant mixtures, the template is considered to be more flexible than the single component surfactant. Due to the larger affinity of Chapter III Results & discussion - 83 - CTAB monomers for the side facets, it has been assumed that there is a higher probability of having BDAC monomers bound to the end facets of the nanorods promoting a faster growth in the longitudinal direction due to the weaker bonding in Ag-Cl pairs relative to that in Ag-Br pairs(9). The second mechanism considers a rigid structure of CTAB monomers, which helps maintain an one-dimensional growth but also serves to control the rate of gold reduction(9), in a similar way to the mechanism previously proposed for the formation of long rods in the absence of silver ions(32). Again, it is assumed in this mechanism that silver ions are not being reduced by ascorbic acid, but will form silver bromide during the synthesis. Murphy and co-workers proposed the adsorption of silver bromide to the facets of the gold nanocrystals, which slow down the gold reduction and induces the single crystalline growth of the nanorods. In spite of the large amount of literature about the chemical synthesis of anisotropic structures the mechanisms that determine the crystal habit and morphology(34) are not well-understood and even subtle changes in the experimental conditions, like surfactant supplier(35), seems to have a large influence in the final nanoparticle shape. As we discussed in the introduction part, two main mechanisms are accepted at this moment as the more relevant ones, although a complete understanding is still lacking (36). The critical nature of the silver species formed during the synthesis has been even more heavily discussed, and several groups have proposed different reasons of why silver species can assist in the anisotropic growth of gold nanorods. One possibility is that silver bromide complexes play a critical role in this mechanism(37). In the Murphy´s group mechanism, the deposition of AgBr on a specific facet of gold nanorods during seedmediated growth stabilizes the rods and directs their growth by allowing a more rapid incorporation of gold onto the less hindered facets. The presence of silver bromide species is further evidenced by XPS data Chapter III Results & discussion - 84 - suggesting the presence of Ag(I) . Also it was observed that 1H NMR spectra of CTAB capped gold nanorods was identical to those of pure AgBr–CTAB(37). However, the presence of free silver bromide (as the reaction occurs above the Ksp of AgBr) could lead to spurious NMR spectra interpretation. Other authors found mass spectrometric evidence that AgBr2 −, as well as AuBr2 −, exist at the gold surface for well-purified nanorods that result from the photochemical synthesis method (38,39). Hafner reported Raman evidence of the Au–Br bond formation at 180 cm−1, which disappears if thiols replace the adsorbed bromine on the gold nanorod surface(40). Other groups, however, propose that silver bromide complexes may not explain the anisotropic growth, and instead argue that underpotential deposition (UPD) of a monolayer or a submonolayer of elemental silver on the gold nanorod surface is of significance(41). GuyotSionnest and colleagues proposed that, in the seed-mediated synthesis, underpotential deposition of silver preferentially occurs on the {110} gold facets compared to the {111} and {100} facets(41). In this model, a silver monolayer strongly protects the {110} facet, and, although the silver may be oxidized and replaced by gold, other facets grow fastest due to the fact that they are less covered with silver. Differing degrees of silver passivation on the {110} facets should lead to varying ratios of growth on this facet and the nanorod's end facets, and this is consistent with what is observed for the standard silver-assisted growth technique (figure 3-4). Inductively coupled atomic emission spectroscopy has been used to quantify the amount of silver in the rods after synthesis, and it was estimated that four monolayers of silver are present in the rods, though the technique does not differentiate between Ag (0) and Ag (I)(42). Extended Xray absorption fine structure (EXAFS) studies(43) have suggested that Ag(0) is the final form of silver for rods grown via a photochemical method, consistent with the possibility that elemental silver is of crucial importance Chapter III Results & discussion - 85 - not just for the standard technique, but for the variety of synthetic methods available to produce gold nanorods. To clarify the mechanisms involved in the growth of gold NRs, and to achieve a better understanding of the role of AgNO3 we undertook a systematically study of the effect of the Ag+ ions and the seed concentration on the final shape of gold particles. This would allow to prepare gold nanostructures in high yields and to get a more precise control on the final particle shape. Because the experimental conditions used in many of the above mentioned methods are particularly suitable to produce metal clusters (kinetic control(44)), we investigate in this work the possibility that metal clusters may play a role in the anisotropic growth of nanoparticles. For this purpose, we firstly studied the influence of some factors, which mainly affect the nanoparticle directed growth, such as Ag+, seed and Figure 3-4: A cartoon illustrating of the mechanism of nanorod growth from CTAB protected gold seed particles in the presence of Ag +. Chapter III Results & discussion - 92 - We also prepared large volumes (20 ml) of different shaped gold nanoparticles by changing the concentration of AgNO3 with constant concentration of CTAB and the other reagents, as it is shown in figure (3-11). We found similar results than those obtained with 5ml (figure 3-8): prisms are produced when the Ag-ion concentration is increased. Figure 3-10: Absorption spectra for large volume synthesis (5-500 ml) using 80μM AgNO3 and 0.1 M CTAB. 200 300 400 500 600 700 800 900 1000 1100 0 1 2 3 Absorbance/a.u Wavelength/nm V5ml V10ml V20ml V50ml V100ml V500ml Figure 3-11: Absorption spectra for large volume synthesis (20 ml) with different concentrations of AgNO3 (40, 80, 160, and 200 μM). 400 500 600 700 800 900 1000 1100 Absorbance/a.u Wavelength/nm At 40μM Ag + At 80μM Ag + At 160μM Ag + At 200μM Ag + 0.0 0.5 1.0 1.5 2.0 2.5 Chapter III Results & discussion - 93 - The growth of the gold nanoparticles after addition of AA and seeds to the growth solution containing different concentrations of AgNO3, was followed spectrophotometrically, and the results are shown in the figures (3-12 to 3-15). 300 400 500 600 700 800 900 Absorbance/a.u Wavelength/nm Growth solution AA at 30s Seed 30s seed 1min seed 2min) seed 5min seed 10min seed 15min seed 20min 0.0 0.5 1.0 1.5 2.0 2.5 3.0 300 400 500 600 700 800 900 1000 Absorbance/a.u Wavelength/nm Growth solution AA 30s Seed 1min Seed 3min seed 5min Seed 10min seed 15min Seed 20min Seed 25min Seed 30min 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Figure 3-13: Absorption spectra showing the growth of the formed particles after addition of AA and the seeds to the growth solution with 40μM of AgNO3. Figure 3-12: Absorption spectra showing the growth of the formed particles after addition of AA and the seeds to the growth solution without AgNO3. Chapter III Results & discussion - 94 - It can be seen that the longitudinal band characterizing the rod´s shape appears faster with increasing AgNO3 concentrations. 300 400 500 600 700 800 900 1000 1100 Absorbance/a.u Wavelength/nm Growth solution AA 30s Seed 1min Seed 3min Seed 5min Seed 10min Seed 15min Seed 18min Seed 20min Seed 25min 0.0 0.5 1.0 1.5 2.0 2.5 3.0 300 400 500 600 700 800 900 1000 1100 Absorbance/a.u Wavelength/nm Growth solution AA 30s Seed 1min Seed 2min Sed 3min Seed 5min Seed 10min Seed 12min Seed 15min Seed 20min 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Figure 3-14: Absorption spectra showing the growth of the formed particles after addition of AA and the seeds to the growth solution with 80μM of AgNO3. Figure 3-15: Absorption spectra showing the growth of the formed particles after addition of AA and the seeds to the growth solution with 160μM of AgNO3. Chapter III Results & discussion - 95 - 3 -3: Effect of seeds concentration. Figures (3-16 to 3-20) show the effect of the seed concentration on the nanoparticle shape. To emphasis the effect of the amount of silver ions concentration on the growth rate of the different facets of gold particles, and because we are mainly interesting in the gold nanorods formation, we used a fixed concentration of Ag + ions (40, 60, 80, and 100 μM) and change the amount of the seed gold particles, which act as nuclei for the growing particles. We can see that, in the absence of Au seeds, the color remains transparent indicating that Au nanorods are not formed; while using the smallest amount of seeds nanoprisms are formed, as can be deduced from the presence of the 3rd band. Increasing the amount of seeds leads to the formation of gold rods. It is also to be noticed that, for high concentration of silver ions (100μM), gold nanorods are formed for all the concentrations of the seeds used. 400 500 600 700 800 900 1000 0 1 2 3 4 Absorbance/a.u Wavelength/nm Seed2 Seed4 Seed6 Seed8 Seed10 Seed12 Seed14 Figure 3-16: Absorption spectra showing the effect of the seed concentration using 40μM of AgNO 3. Chapter III Results & discussion - 96 - 400 500 600 700 800 900 0 1 2 3 4 Absorbance/a.u Wavelength/nm Seed2 Seed4 Seed6 Seed8 Seed10 Seed12 Seed14 400 500 600 700 800 900 Absorbance/a.u Wavelength/nm Seed2 Seed4 Seed6 Seed8 Seed10 Seed12 Seed14 0.0 0.5 1.0 1.5 2.0 Figure 3-18: Absorption spectra showing the effect of the seed concentration using 80μM of AgNO 3. Figure 3-17: Absorption spectra showing the effect of the seed concentration using 60μM of AgNO 3. Chapter III Results & discussion - 97 - 400 500 600 700 800 900 1000 0 1 2 3 4 Absorbance/a.u Wavelength/nm Seed2 Seed4 Seed6 Seed8 Seed10 Seed12 Seed14 A B Figure 3-20: (A) TEM image of the NPs obtained using 0.113 μM of seeds and 40 μM AgNO 3 (a.r.= 3.4) and (B) TEM image for a similar synthes is using 80μM AgNO3 (a.r .= 4.2). Figure 3-19: Absorption spectra showing the effect of the seed concentration using 100μM of AgNO3. Chapter III Results & discussion - 98 - The effect of aging the seeds at different times (10 min., 4h, 7h, and 24 h.) were also studied with different concentration of AgNO3. The following results were obtained: In the absence of AgNO3, spheres were produced for all the aging times (figure 3-21). Scheme 3-3: Scheme showing a summary of the results obtained for the synthesis of the formation of gold nanoparticles with different shapes using different experimental conditions. Chapter III Results & discussion - 99 - 300 400 500 600 700 800 900 Absorbance/a.u Wavelength/nm zeroat10min At4h At7h At24h 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Figure 3-21: Absorption spectra showing the effect of the aging time of the seeds on the growth of NPs without AgNO 3 . Figure 3-22: Absorption spectra showing the effect of the aging time of the seeds on the growth of NPs using 40μM AgNO3. 300 400 500 600 700 800 900 1000 0 1 2 3 4 5 Wavelength/nm Absorbance/a.u At10min At4h At7h At24h Chapter III Results & discussion - 100 - The addition of the seeds aged for 10 min., 4h and 7h to the growth solution in the presence of AgNO3 (40μM and 80 μM) yields to the formation of gold nanorods, while some snapped prisms and prisms mixed with spheres are formed using seeds aged for 24 hours. Figure 3-24: TEM images showing the produced nanoparticles with 40μM AgNO3 (A) and 80μM AgNO3 (B) using 24h aged seed solution. Figure 3-23: Absorption spectra showing the effect of the aging time of the seeds on the growth of NPs using 80μM AgNO3. 300 400 500 600 700 800 900 1000 1100 0 1 2 3 4 5 Wavelength/nm Absorbance/a.u At10min At4h At7h At24h A B Chapter III Results & discussion - 101 - Nanoprisms and snapped prisms mixed with gold nanorods are formed, when a high concentration of AgNO3 (160μM) was added in the growth solution using seeds at different aging times (figure 3-25). One can then conclude that, at low aging times the seeds (from 10 min to 3h in which no Plasmon band appears in the UV-Vis absorption spectrum) well-shaped Au nanorods are produced with high yields (׽90%) using low concentrations of AgNO3, while largely snapped prisms are formed with 24h aging seeds. Therefore, the aged Au seeds at 24h are not suitable for the synthesis of well-defined Au nanorods. Figure 3-25: Absorption spectra showing the effect of the aging time of the seeds on the growth of NPs using 160μM AgNO3. 300 400 500 600 700 800 900 1000 1100 0 1 2 3 4 Wavelength/nm Absobance/a.u At10min At4h At7h At24h Chapter V Results & Discussion - 204 - Figure 5-15: Absorption spectra showing the effect of thermal heating on gold nanorods with 30% PVP (A) and with 15% PVP (B) from 400C to 2200C. 400 500 600 700 800 900 2.0 1.6 1.2 0.8 0.4 0.0 B 160 0 C 130 0 C 100 0 C 80 0 C 60 0 C 40 0 C Rods with 15% PVP Absorbance/a.u Wavelength/nm 400 500 600 700 800 900 2.0 1.6 1.2 0.8 0.4 0.0 A PVP0 At0min At10min At20min At40min At60min At80min At100min At120min At150min Absorbance/a.u Wavelength/nm Chapter V Results & Discussion - 205 - 400 500 600 700 800 900 2.4 2.0 1.6 1.2 0.8 0.4 0.0 B Absorbance/a.u Wavelength/nm Rods At0 At10 At20 At40 At60 At80 At100 At120 At150 400 500 600 700 800 900 2.4 2.0 1.6 1.2 0.8 0.4 0.0 C Absorbance/a.u Wavelength/nm Rods At0 At10 At20 At40 At60 At80 At100 At120 At150 At175 Chapter V Results & Discussion - 206 - Figure 5-16: Absorption spectra showing the effect of thermal heating on gold nanorods containing 30% PVP with time, at temperatures 401000C (A -D, respectively). The activation energy for the decomposition of gold nanorods containing 30% PVP is shown in (E). 400 500 600 700 800 900 2.4 2.0 1.6 1.2 0.8 0.4 0.0 D Absorbance/a.u Wavelength/nm Rods At0 At5 At10 At15 At20 At25 At30 At40 At50 At60 At90 At120 0.25 0.20 0.15 0.10 0.05 0.00 3.2 3.1 3.02.9 2.8 2.7 2.6 E ln k (a.u) 1/T X10-3 (K-1) NRs+PVP a.r.= 3.4 Eact= 8.3±0.1 KJ/mol Chapter V Results & Discussion - 207 - 5-2-3-2: Gold nanoprims. In the case of nanoprisms, adding 30% of PVP, the particles are completely stable up to 100 0C. Then, a slight decrease in the absorbance is observed with almost no shift in the absorption band up to 140 0C. At 160 0C a large decrease of the intensity is observed, which corresponds to the aggregation of nanoparticles. A picture of the aggregated nanoparticles at 1800C is shown in figure (5-18A). Using lower concentrations of PVP (15%), the thermal heating leads first to the dissolution of the tips of these prisms yielding snapped prisms. Then, particles decompose leading to a decrease in the absorbance intensity and a shift to higher wavelengths, but the particles remain stable after such reorganization until 180 0Csee figure (5-18B). Figure 5-17: Normalized absorption spectra showing the effect of thermal heating on rods with different aspect ratios ; rods containing 30%PVP; and nanoprisms. 0 102030405060 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 A t /A 0 Time/min. a.r2.2 ar3.4 PVP Prisms Chapter V Results & Discussion - 208 - 400 500 600 700 800 900 2.0 1.6 1.2 0.8 0.4 0.0 A 180 0 C 40 0 C prisms with 30% PVP Absorbance/a.u Wavelength/nm 400 500 600 700 800 900 1.6 1.2 0.8 0.4 0.0 B 180 0 C 160 0 C 140 0 C 120 0 C 100 0 C 80 0 C 60 0 C 40 0 C prisms with 15% PVP Absorbance/a.u Wavelength/nm Chapter V Results & Discussion - 209 - Part II: Photostability of gold nanoparticles. 5-3: Effect of UV irradiation. In this section, we report the effect of UV-light and Laser on the shape of gold particles of different shapes. 5-3-1: Effect of UV-light irradiation on gold nanospheres. As it can be seen in Figure (5-19), irradiation of spherical nanoparticles prepared in the absence of AgNO3 shows only a very slight decrease without any remarkable shifts in the corresponding SP band. This implies that spherical particles are very stable against UV-light irradiation. C D Figure 5-18: Absorption spectra showing the effect of addition of 30 % (A) and 15% (B) PVP on the thermal stability of the gold nanoprisms. TEM images (C and D) showing gold nanoprisms with 15% and 30 % PVP, respectively. Chapter V Results & Discussion - 210 - 5-3-2: Effect of UV-light irradiation on gold nanoprisms. As it can be seen in Figure (5-20), nanoprisms are also very stable towards UV light. After exposure for long times (about 24 hrs) only small changes - probably associated with the morphology change from prisms to snapped prisms are observed. The three absorption bands, characteristic of nanoprisms, are blue shifted. However, after 2 days of irradiation (see figure 5-21) almost all prisms are converted into rods, as it can be observed by the disappearance of the intermediate plasmon band. Figure 5-19: Absorption spectra showing that UV-irradiation of gold nanospheres prepared in the absence of AgNO3 during 25h has almost no noticeable effect. 300 400 500 600 700 800 900 1.2 0.9 0.6 0.3 0.0 25h 0 Absorbance/a.u Wavelength/nm At0min At30min At60min At90min At2h At3h At4h At5h At7h At9h At12h At15h At17h At17n At18h At19h At20h At22h At24h At25h Chapter V Results & Discussion - 211 - Figure 5-20: Absorption spectra showing the effect of UV-light on the gold nanoprisms at different times, from 0 to 24h. Figure 5-21: Absorption spectra showing the effect of UV-light on gold nanoprisms at different times: 1h, 3h, 1 day, 2 days, and 3 days. 400 500 600 700 800 900 2.0 1.6 1.2 0.8 0.4 0.0 24 hrs. 3 hrs. 1 hrs. 30 min. Absorbance/a.u Wavelength/nm 400 500 600 700 800 900 2.0 1.6 1.2 0.8 0.4 0.0 3day 2day 1day Absorbance/a.u Wavelength/nm After30min After1hUV After3hUV After1dayUV After2days After3days Chapter V Results & Discussion - 212 - 5-3-3: Effect of UV-light irradiation on gold nanorods. The effect of the irradiation on gold nanorods encapsulated in micelles, with average aspect ratios of a.r. =2.5 and 3.7, (with absorption maxima at 701and 762 nm, respectively), was investigated. Figures (5-22A and, 5-22B) show the absorption spectra of gold nanorods of a.r. = 2.5 before and after irradiation by UV-light. The optical density at both, longitudinal and transverse plasmon bands, decreases with increasing the irradiation time and the absorption maxima of the SPL are gradually shifted to shorter wavelengths (λmax. = 672nm versus 700nm). The observed decrease in the absorption spectra-and the shift in SPLare mainly caused by the decrease in the length of the nanorods. Figure (5-22C) illustrates that the normalized absorption spectra changes almost linearly with time. This change may be due to the regular reshaping of the nanorods inside the capping micelles. Increasing the irradiation time (for the two types of gold nanorods), produces an increase of the decomposition of rods, decreasing their lengths. But, totally unexpected, it is also observed that exposure to UV-irradiation for longer times (24h), leads to almost the complete decomposition of the rods (see figure 5-22D). Chapter V Results & Discussion - 213 - 400 500 600 700 800 900 1.5 1.2 0.9 0.6 0.3 0.0 A 240 mins 180 mins 60 mins. 120 mins. 0 min. Absorbance/a.u Wavelength/nm 400 500 600 700 800 900 1.0 0.8 0.6 0.4 0.2 0.0 B 120 mins. 100 mins. 80 mins. 60 mins. 40 mins. 20 mins. 0 mins. Absorbance/a.u Wavelength/nm Chapter V Results & Discussion - 220 - In order to check that this explanation of cluster-mediated metal photocorrosion can takes place, we carried out several experiments to confirm such hypothesis. a) Influence of ethanol. It is well-known that ethanol is very easy to oxidize and, therefore, is normally used as a hole scavenger. If the proposed mechanism is correct dissolution of Au NRs should be avoided (or largely inhibited) by introduction of ethanol into the system. Therefore, we carried out an irradiation experiment with a mixture NRs to ethanol ratio = 2:1 by volume. Figure (5-27) shows that, in fact, the dissolution of Au NRs is greatly inhibited, and only some dissolution of the Au NR tips occurred giving rise to a decrease of the a.r. of the Au NRs, as it can be deduced by the red shift of the SPL band. Figure 5-27: Absorption spectra showing the changes of gold nanorods with addition of a small amount of ethanol and irradiating the sample with UV-light during ~ 30 h. 300 400 500 600 700 800 900 1.8 1.5 1.2 0.9 0.6 0.3 0.0 Absorbance/a.u Wavelength/nm CTeh0 at12h at15h at18h at20h at22h at25h at26h at28h at29h at30h Chapter V Results & Discussion - 221 - b) Influence of NR´s preheating. It is known that small metal clusters fuse at low temperatures. In particular, Ag clusters employed for the synthesis of Au NRs (see chapter III) fuse at around 1200C-1500C, when they are unprotected in solution (unpublished results). Therefore, a second experiment for the proof of the mechanism here proposed for the dissolution of Au NRs was to heat the solution of Au NRs to 130ºC before irradiation with UV-light. It can be assumed that at such temperatures the Ag clusters fuse and are incorporated into the NR's lattice. Therefore, in such a case the mechanism of clustermediated UV-dissolution of Au NRs would be avoided. Figure (5-28) shows that when NRs were preheated at 1300C for short time (10min), although the dissolution was not completely inhibited, it was slowed down. In fact, the preheated NRs solutions after of 30h irradiation are only reshaped to spheres, in contrast to the untreated NRs, which were totally dissolved. The non-complete inhibition of the NR solution implies that the short preheating time was not enough to fuse all the clusters (maybe because clusters are more stable when they are supported over a substrate and protected with a ligand like CTAB). However, when NRs were preheated for long time (≈ 2h) all the Ag clusters fuse and the gold nanorods are very stable even under long time of UV irradiation (≈40h), which confirms the role of the clusters in the dissolution mechanisms (see figure 5-30). To further confirm this hypothesis a new experiment was carried out adding a small amount (3nM) of Ag-clusters (from Nanogap - see Chapter III-) to a preheated sample of NRs sample and irradiated this sample with UV-light. It can be seen in figure (5-31) that the dissolution of NRs occurs after a short time (4h only) compare with the results shown in figure (5-24). These results further confirm the hypothesis putted forward in Chapter III for the growth mechanism assuming that clusters would be preferentially located at the tips of the rods. Figure (5-29) shows a Chapter V Results & Discussion - 222 - comparison of colours of the final samples after 30h irradiation showing the influences of preheating the samples and adding ethanol to the solutions. 300 400 500 600 700 800 900 Wavelength/nm Absorbance/a.u at26h rods reshaping to spheres Rods At 130C at 18h at 20h at 22h at23h at 24h at 25h at 26h at 27h at 28h at 29h at 30h 0.0 0.5 1.0 1.5 2.0 2.5 Figure 5-28: Absorption spectra showing the changes of gold nanorods pre-heated to 130 0 C for 10min and irradiated with UV-light during ~ 30 h. 30h UV-light Figure 5-29: Picture showing the difference between the initial GNRs sample and irradiated gold nanorods during 30 hours (1) with pale yellow color characteristic of the formation of Au (III) -CTAB complexes, (2) for gold nanorods preheated to 1300C and then irradiated during 30 hours with a pink color characteristic of the formation of gold nanospheres and (3) for gold nanorods with ethanol showing almost no change in the color of the initial GNRs solution. Chapter V Results & Discussion - 223 - 300 400 500 600 700 800 900 1000 1100 Absorbance/a.u Wavelength/nm NRs after 130 0 c at 2h at 4h at 8h at 10h at 12h at 15h at 20h at 22h at 25h at 28h at 30h at 32h at 35h at 40h 0.0 0.5 1.0 1.5 2.0 2.5 400 600 800 1000 Absorbance/a.u Wavelength/nm with clusters at 30min at 45min at 1h at 2h at 3h at 4h at 5h at 6h at 8h at 10h at 12h at 15h at 18h at 20h at 22h at 25h at 30h 0.0 0.5 1.0 1.5 2.0 Figure 5-30: Absorption spectra showing the small changes of gold nanorods preheated to 130 0 C for 2h and then irradiated with UV-light during ~ 40 h. Figure 5-31: Absorption spectra showing the changes of gold nanorods pre-heated to 130 0C for 2h in the presence of external synthesized Ag clusters and then irradiated with UV-light during 30h. Chapter V Results & Discussion - 224 - c) Influence of pH. The previous experiments were conducted at acid pHs (3.9). At basic pHs, the oxidation of water to give oxygen is favoured (H0 = 0.68V) in comparison with the oxidation of Au (H0=0.85V, for the bromide salt), so that at these pH values the oxidation of Au NRs should be largely suppressed. Figure (5-32) shows that even after 30h irradiation NRs do not decompose and only their length is decreased. Figure (5-33) shows how the normalized absorption intensities of the SPL and SPT bands change with the irradiation time at acid and basic pHs. It can be clearly seen that at basic pHs the dissolution is inhibited and that the diameter of the rods does not change with the irradiation time, as it can be deduced from the constant value of the SPT band. Figure 5-32: Absorption spectra showing the changes of gold nanorods (a.r ≈ 3.7, at pH 12.6 ) after irradiation under UV-light during ~ 30 h. 300 400 500 600 700 800 900 1000 1100 pH = 12.6 Absorbance/a.u Wavelength/nm Rods At 4h At 6h At 8h At10h At12h At15h At18h At20h At22h At25h At30h 0.0 0.3 0.6 0.9 1.2 1.5 1.8 Chapter V Results & Discussion - 225 - d) Influence of NR’s washing. We carried out similar irradiation experiments with NR’s samples after centrifuging 2 times at 6000rpm for 20min and washing the samples with distilled water. Also two similar experiments were carried out washing the samples and then either pre-heated to1300C or in the presence of a small amount of ethanol. By this procedure (in the first experiment) some CTAB -and also the clusters linked to this ligandwould be removed, which would also inhibit in some extension the dissolution of Au NRs. In the second and third experiments more clusters would be removed/fused, which would further prevent the dissolution effect. As it can be observed in Figures (5-34, 35, and 36) after 30h, only a decrease of the length is observed, which indicates that an appreciable amount of the photocatalytic clusters, responsible for the dissolution of NRs, were effectively removed by washing. Figure 5-33: Normalized absorption intensities of the SP L and SP T bands of gold nanorods at both p Hs (3.9 and 12.6) under UV light irradiation during 30 h. 0 5 10 15 20 25 30 A t /A 0 Time/h Transverse band at pH= 3.9 Longitudinal band at pH= 3.9 Transverse band at pH= 12.6 Longitudinal band at pH= 12.6 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Chapter V Results & Discussion - 226 - 200 300 400 500 600 700 800 900 1000 1100 30hours 0 mins. Absorbance/a.u Wavelength/nm 0.0 0.2 0.4 0.6 1.0 0.8 1.2 Figure 5-34: Absorption spectra showing the changes of gold nanorods after being washed 2 times and irradiated with UV-light during ~ 30 h. 200 300 400 500 600 700 800 900 1000 1100 30h 0 mins. Absorbance/a.u Wavelength/nm 0.0 0.4 0.8 1.2 1.6 Figure 5-35: Absorption spectra showing the changes of gold nanorods after being wash ed 2 times and preheated to 1300C. The sample was then irradiated with UVlight during ~ 30 h. Chapter V Results & Discussion - 227 - 200 300 400 500 600 700 800 900 1000 1100 30 h 0 mins Absorbance/a.u Wavelength/nm 0.0 0.2 0.4 0.6 1.2 0.8 1.0 Figure 5-36: Absorption spectra showing the changes of gold nanorods after being washed 2 times. A small amount of ethanol was added to the sample and then i rradiated with UV -light during ~ 30 h. 0 5 10 15 20 25 30 At/A0 Time/h GNRs Washing GNRs Washing GNRs preheated to 1300C Washing GNRs+Ethanol 0.0 0.2 0.4 0.6 0.8 1.0 Figure 5-37: Normalized absorption intensities of the SP L bands of gold nanorods, washed NRs, washed NRs and preheated to 130 0C, and washed NRs with a small amount of ethanol, under UV light irradiation during ~ 30 h. Chapter V Results & Discussion - 228 - e) Influence of stirring. The dissolution of NRs (at pH=3.9) increases when the irradiation of NRs is made with stirring the sample. The band of the gold (III)-CTAB complex appears at shorter times (≈12h) compared with the unstirred samples (see Figure 5-38). It was noticed that the final pH of the solution after complete dissolution of the NRs is more acid than at the begining: pH = 2.7. In order to show that the Ag clusters remain after the dissolution of the NRs a new experiment was performed using the final solution after irradiation for the formation of Au NRs adding AA and seeds solution. As it can be seen in Figure (5-38) NRs are formed again, confirming that Ag clusters play an important role on both, the formation and dissolution of gold nanorods. UV-light AA + Seeds Au-NRs Au(III)-ions Chapter V Results & Discussion - 229 - f) Influence of PVP. Adding 15 % by weight of PVP to the nanorods solution leads to an increase of the photostability of the gold nanorods (see Figure 5-39). Upon addition of PVP, a slight shift to the red (~4nm) is observed due to the change of the dielectric constant of the surrounding medium and the absorbance slightly decreases due to the decrease of the rods concentration by adding PVP. Afterwards, there is not any absorption shift upon exposure the NRs to UV light during 4 hrs. NRs are not decomposed even after 72h of irradiation (compare this with the complete dissolution of NRs without PVP after 24h). The protection of PVP can be due to the oxidation of PVP by the generated holes avoiding in this way the oxidation of the Au Figure 5-38: Absorption spectra showing the dissolution of gold nanorods (pH=3.9) with stirring, irradiated with UV-light during ~ 12 h; and the reformed NRs after addition of AA & seeds solution. The including picture shows the difference in color between before UV irradiation, after irradiation and after addition AA & seeds solution. 300 400 500 600 700 800 900 1000 1100 Absorbance/a.u Wavelength/nm At0h At12h Reformed 0.0 0.5 1.0 1.5 2.0 AA+Seeds Chapter V Results & Discussion - 236 - 5-3-4: Electrochemical analysis of the Au ions produced by the photocorrosion of Au NRs. Because in the spectrophotometric experiments described above to follow the dissolution of the Au NRs, the bands which show the presence of Au ions in the solution were observed only at the end of the experiments, we decided to carry out an electrochemical analysis of the Au ions produced during the photocorrosion experiment. For this purpose we designed a cell, which can be illuminated with UV light. The design of the cell is schematically shown in Figure (5-47). The cell is made up of Teflon and consisted of one quartz window on the top (W). One platinum wire is used as the working electrode or the source (S); one 3.5 cm ring of stainless steel wire centering on the source is used as counter electrode (C), while a gold wire is used as quasi-reference electrode. The cell is approximately 6 cm in diameter and 3 cm dip. There is a small hillock at the center to control the volume of the cell. When the lid is clasped the volume of the cell is roughly 60 ml. The working electrode sits on the top of the hillock. There is one inlet on the lid to introduce the electrolyte into the cell and another passage on the side wall to take out the electrolyte. One hole of 3 cm diameter on the lid, which is covered by a 2 mm thick quartz plate, serves as window to radiate light inside the cell. Chapter V Results & Discussion - 237 - A commercially available UV lamp with a power of 4 mwatt/cm2 (λmax = 254nm) was kept at a distance of almost 10 cm away from the window of the cell. All this set up was covered by a box which was completely wrapped with aluminum foil. All the measurements were made with Metrohm Autolab potentiostat using GPES software. The Au quasi-reference electrode was chosen (99.95% pure, Good fellow Cambridge Limited, England) to minimize the effect of ion exchange of the gold ions formed upon irradiation by UV light with the supporting electrolyte. But, even with this disposition we saw that the open circuit voltage was not constant. A gradually increase in negative potentials was observed whenever the experiment was started. This may be attributed to formation of a double layer on the electrodes. With time the layer thickness increases hindering the ions in solution to exchange electrons with electrode. Platinum (99.95% pure, Good fellow Cambridge Limited, England) was chosen as working electrode due its purity and inertness. Figure 5-47: Design of the electrochemical cell used for the analysis of the Au ions released during the photocorrosion experiments. Chapter V Results & Discussion - 238 - With this set up we carried out a standard salt (Fe2+/Fe3+) characterization. For this we used a 2 mM aqueous solution of potassium ferricyanide (K3Fe(CN)6,3H20) which has a standard reduction potential (Fe3+→Fe2+) at 0.77 V(RHE). In our non-standard set up the hump of the reduction potential is located at +0.27 V, which shows that the non-standard reference potential of this cell is +0.5 V. From the datasheet supplied by the manufacturer the intensity of the radiation at a distance of 10 cm is approximately ~ 40 μwatt/cm2. Taken the area of the cell, it can be assumed that a power of 40x7=280 μwatt of light was illuminating the solution. This means E=hc/λ =8x10-19J/photon, and 2.8x10-4W=2.8x10-4J/s ؞ 2.8x10-4/8x10-19 =3.5x1014 photons/s, which means, 3.5x1014/6x1023 = 5.8x10-10 mole of photons/s ≈ 0.6 nanomoles of photons/s. Firstly, we performed the experiment with a similar sample than the used one in figure (5-24), corresponding to Au NRs with a.r ≈ 3.9. After the UV -2.0x10-6 -1.0x10-6 1.0x10-6 Current/A Voltage/V 0.0 0.2 0.4 0.6 0.8 2.0x10-6 0.0 Figure 5-48: Cyclic voltammogram of 2 mM K3Fe(CN)6 taken at a scan rate of 0.05 V/s. Chapter V Results & Discussion - 239 - lamp was turned on the potential was recorded at a constant current of 1μA for 30 hours. The results are shown in the next figure. We can observe from the figure that the voltage firstly decreased very slightly from 0 to 4 hours and then increased approaching a limit at around 30 hours. The difference in the potential is approximately 0.1V.The first part of the curve is an artifact due to the precision of the electrochemistry measurements, usually in the range of nanomolar. This means that the time needed for the production of such an amount of Au ions needed to be detected by this technique is circa 4 hours. We also see that there is an increase of the potential during the illumination, which agrees with the idea that there is an oxidation process in the system, confirming the oxidation of the gold nanorods. From these data one can make some calculations. As it was proposed in the scheme (5-1), when the clusters attached to the tips of the NRs are excited oxidizing holes that generated in the clusters Figure 5-49: Potential recorded for gold nanorods under UV irradiation during 30hours at a constant current of 1 μA. Chapter V Results & Discussion - 240 - and transferred to the Au rods, eventually oxidizing Au atoms, which dissolved and go to the solution, giving rise to the corrosion of the rods. This process can be summarized as follows: Ag(clusters) → Ag*(c) → h+ + eAun(r) + h+ → Aun-1(r) + Au3+ In general: Au0 + 3h+→Au3+ The electrochemical potential at time t and time zero are related by the expressions; Ɛt=Ɛ0 + (0.059/n). log ([Au3+]t / [Au3+]0), where n = 3. Then ΔƐ = Ɛt-Ɛ0 =0.02 x log ([Au3+]t / [Au3+]0). Taking into account that ΔƐt→∞ = 0.1V(from fig.5-49) =0.02 x log ([Au3+] t→∞ / ([Au3+] t→0 ), then log ([Au3+] t→∞ / [Au3+]0) = 5 . Then, log[Au3+] t→∞ = 5+log[Au3+] t→0 From the spectrophotometric experiments (see figure 5-26) we know that the final amount of Au(III) at t→∞ is, [Au3+] t→∞ = 3x10-4M . Then, we can deduce that [Au3+] t→0 ≈ 3x10-9M, which agrees with the previous comment that this concentration is in the limit to be detected by this electrochemistry technique, and explains the first part of the curve voltage-time represented in figure (5-50). In order to make a rough estimation of the photon conversion efficiency, we can compare the amount of photons used in the experiments (0.6 nanomols/s) with the amount of ions generated, which are 3x10-4M/(30x3600) ≈ 5nM/s, which means, in the total volume of the 60ml cell, 5x(60/1000) = 0.3 nanomols of Au(III)/s. Because the production of one Au-III ion needs 3 photons, the photons needed for the production of the Au III ions produced should be: 0.9 nanomols/s, which is approx. the same as the photons calculated from the illumination source. Therefore, one can say that the photon conversion in electron and holes is a very efficient process. Chapter V Results & Discussion - 241 - The second experiment that we carried out was to measure the voltage at a constant current of 1μA turning the light ON/OFF after 4 hours of illumination. The results are shown in figure (5-50). It is clear from the figure that the voltage increases when the UV lamp is turned on and only a very little decrease is observed when the light is turned off. This confirms that light causes the production of ions, and that the photocorrosion is inhibited when the light is off. After these experiments we tried to see if we could perform some experiments during the first minutes of irradiation. For this purpose we recorded the voltage values as a function of ON/OFF light cycles, as we did before (figure 5-50), but at lower constant currents a) 0 A and b) 10 nA. Figure 5-50: Potential recorded for gold nanorods under UV irradiation after 4hours at a constant current of 1 μA and turning on the light for 20 min and off for 10 min during 2 h. 0 1000 2000 3000 4000 5000 6000 7000 0.98 0.97 0.96 0.95 Voltage (V)/RHE Time/s current 1 P A time counted after 4 hrs 0.94 Chapter V Results & Discussion - 242 - The results shown in figure (5-51) indicate that, contrary to what happens at longer times, now the voltage after illuminating goes down to the value before illuminating. This indicates that the concentration of ions generated during the illumination is very low (near the limit of the detection of the electrochemical set-up) and that they are not well homogeneously distributed inside the solution, because not all the solution is homogeneously irradiated. Therefore, during this initial time (less than approx. ~ 4 h) the change in the voltage when the light is ON is due to the local increase (near the electrodes) of the concentration of Au ions. The voltage will turn to approx. the original value when the light in OFF 0 300 600 900 1200 1500 1800 -0.410 -0.405 -0.400 -0.395 Voltage (V) Time (s) UV on UV off (a) -0.390 0 500 1000 1500 2000 -0.4164 -0.4166 -0.4168 Voltage (V) Time (s) current 10 nA as prepared sample -0.4170 0 300 600 900 1200 1500 1800 -0.421 -0.420 -0.419 -0.418 -0.417 Voltage (V) Time (s) (b) -0.416 1300 1400 1500 Voltage (V) Time (s) 45 s 20 s 0 500 1000 1500 2000 -0.3930 -0.3928 -0.3926 -0.3924 -0.3922 Voltage (V) Time (s) current 0A as prepared sample -0.3920 c d Figure 5-51: Constant current characteristics of as prepared Au nanorods solution, (a) at 0 A and (b) at 10 nA. The interval in time scale is 5 sec. Up and down arrows show UV on and off respectively. (c and d) after background corrections of (a and b). Chapter V Results & Discussion - 243 - because of the diffusion of ions from the proximities of the electrodes to the bulk decreases again the concentration of ions to the sensitivity detection limit (see scheme 5-2). To confirm the effects of the different species present in the solution we repeated the experiments, but for pure nanorods after centrifuged at 6000 rpm for 30 min and then washed and redispersed in water to separate nanorods from the other components. This washing procedure was repeated three times. Figures (5-52a) and b)) show the results for constant current at 0 A and 10 nA, respectively. As expected, clusters were washed away and there is almost no change in the recorded potential. The slope is almost the same, but the starting point of the potential is little less/more than previous one. This is because we took out CTAB from the electrolyte solution which can provide some boost in ion concentrations and also the concentration of nanorods was changed during the process by centrifugation and redispersion. The small hump like ripples seen in the characteristics regularly with turning on or off the UV light may be due to presence of very small quantity of clusters which were attached to the surface of the rods and could not been removed during the washing process. B) 4h A) 1h Scheme 5-2: Difference between the electrochemical measurements carried out before 1h of irradiation with UV light (A) and after 4h of irradiation (B). Chapter V Results & Discussion - 244 - In order to show that the presence of clusters is the key parameter for these experiments, we added Ag clusters from Nanogap (o.5 ml of 0.01mg/l) to the washed NRs solution, and repeated the previous experiment. The results can be seen in Figure (5-53). Both figures (5-52 and 5-53) clearly show that there is a significant increase in the observed signals. While in the absence of clusters there is almost no response to UV light by increasing the concentration of clusters the change between ON and OFF values of the potential are clearly seen. To increase the conductivity of the solution we added a little amount of Figure 5-53: Constant current characteristics, (a) at 0 A and (b) 10nA for a mixture of 58 ml of washed nanorods and 0.5 ml cluster solution from Nanogap. 0 100 200 300 400 500 600 -0.56 -0.52 -0.48 Voltage (V) Time (s) (a) -0.44 0 400 800 1200 -0.576 -0.572 -0.568 Voltage (V) Time (s) (b) -0.564 Figure 5-52: Constant current characteristics, (a) at 0 A and (b) 10nA for a washed nanorods sample. 0 100 200 300 400 500 600 -0.4190 -0.4185 -0.4180 -0.4175 -0.4170 Voltage (V) Time (s) current 0A nanorods + 0,5 ml clusters (a) -0.4165 0 200 400 600 800 1000 1200 -0.4140 -0.4135 -0.4130 -0.4125 Voltage (V) Time (s) current 10nA nanorods + 0,5 ml clusters (b) -0.4120 Chapter V Results & Discussion - 245 - CTAB (1 ml of 1 mM). While CTAB itself does not have any response to UV light (Figure 5-54b) it stabilizes the current at ON state. The instability at ON state potential is clearly visible. 5-4: Effect of Laser irradiation on gold nanorods. Many scientific groups(15-18) studied the interaction between metal nanoparticles and laser light. Koda et al.(19,20) reported the irradiation effect of nanosecond laser pulses (532 nm) on gold nanospheres, which leads to fragmentation of the nanodots. The slow heat transfer of the deposited laser energy into the surrounding solvent was used to explain the results. This slow heat transfer then leads to the melting and vaporization of the nanoparticles as estimated from the deposited laser energy. More information about the structural dynamics of metal nanoparticles after laser excitation can be obtained when nanoparticles having different shapes than spheres are used. Link et al.(17) studied the effect of high power (femtosecond and nanosecond) lasers on the structure of gold nanorods as a comparison study. They found that nanorods are easily converted to nanospheres by laser irradiation. For femtosecond pulses, typically pulse energies on the order of 10 mJ are sufficient to completely convert the rods into spheres. However, the exact value of the nanosecond pulse energy 0 50 100 150 200 250 300 -0.31 -0.30 -0.29 -0.28 Voltage (V) Time (s) (b) -0.27 0 200 400 600 800 -0.365 -0.360 Voltage (V) Time (s) (a) -0.355 Figure 5-54: (a) Potential vs time curve for 10 nA constant current of a solution of washed nanorod s + cluster + CTAB. (b) Potential vs time curve for 10 nA of only a 0.1M CTAB solution. Up and down arrows indicate on and off of UV light. Chapter V Results & Discussion - 252 - some effect on the SPT fundamental (transverse) band of Au nanorods, within the visible region of the spectrum, is observed. This is in contrast to the null influence that CW irradiation has over this visible transverse mode resonance band. Figure 5-59: Absorption spectra showing the effect of pulsed Nd: YVO 4 laser on gold nanorods after changing the intensity from 25 to 32 A (from ≈ 4 to 10.5 W) , ʋ = 100 mm/s, f = 50 KHz, the superposition 90 %, and orientation with 00. Figure 5-60: Absorption spectra showing the effect of pulsed Nd: YVO 4 laser on gold nanorods after changing the intensity from 20 to 32 A (from ≈ 1.5 to 10.5 W), ʋ = 50 mm/s, f = 5 KHz, the superposition 20 %, and orientation with 0 0 . 400 500 600 700 800 900 1.6 1.2 0.8 0.4 0.0 Wavelength/nm Absorbance/a.u Rods At I = 20A At I = 22A At I = 25A At I = 28A At I = 30A At I = 32A 400 500 600 700 800 900 1.5 1.2 0.9 0.6 0.3 Wavelength/nm Absorbance/a.u Rods At I= 25A At I= 28A At I= 30A At I= 32A 0.0 Chapter V Results & Discussion - 253 - IIEffect of pulsed Nd:YAG laser, working in Q-switch regime. Experiments carried out with a pulsed Nd:YAG nanosecond regime emission laser yielded significantly different results for Au nanorods of different aspect ratios (1.8, 2.5, and 4.2), as follows from the discussion below. 1Fundamental line at 1064 nm / 9 ns / 12.5 W / Irr ≈ 3.54 x 107 Wcm-2/pulse. 400 500 600 700 800 900 1.5 1.2 0.9 0.6 0.3 Rod 2 Absorbance/a.u Wavelength/nm a0sec a7sec a22sec a57sec a117sec a177sec a267sec a402sec a522sec a642sec a762sec 400 500 600 700 800 900 1.6 1.2 0.8 0.4 0.0 Rod1 Absorbance/a.u Wavelength/nm a0sec a7sec a22sec a57sec a117sec a177sec a267sec a402sec a522sec a642sec a762sec Chapter V Results & Discussion - 254 - Figure 5-61: Absorption spectra showing the effect of pulsed Nd: YAG laser (at 1064 nm / 9 ns / 12.5 W / Irr ≈ 3.54 x 10 7 Wcm-2/pulse ) on gold nanorods with different aspect ratios (Rod1= 1.8, Rod2= 2.5 and Rod3= 4.2) from 0 to 760sec. with illustration of the possible fragmentations which can occur under illumination. 400 500 600 700 800 900 1.6 1.2 0.8 0.4 0.0 Rod 3 Absorbance/a.u Wavelength/nm a0sec a7sec a22sec a57sec a117sec a177sec a267sec a402sec a522sec a642sec a762sec Chapter V Results & Discussion - 255 - This pulsed laser Nd: YAG (1064 nm / 9 ns / 12.5 W / Irr ≈ 3.54 x 107 Wcm-2/pulse) is used in a direct output configuration (without movement of the laser beam), so that the Au NR suspension is irradiated under cylinder type geometry with a 1 cm diameter (see illustration in figure 5-63). Fragmentation is observed under these irradiation conditions (figures 5-61 & 5-62), as reported by Link et al.(17), where laser pulses are claimed to cause excitation of the hot lattice (stage1) in the experiments with nanosecond pulses. One thus expects that absorption of more photons by the hot lattice occurring in the nanosecond experiment is what leads to an increase in the lattice internal energy and fragmentation of the gold NRs (stage 2). The longitudinal absorption band of GNR changes its maximum to shorter wavelength but retains its width, suggesting that the laser exposure changes the distribution into smaller rods. [(Au)NP]vib [(Au)NP]vib** (1) [(Au)NP]vib** [(Au)NP]SNS (2) Figure 5-62: Normalized absorption spectra of rods with different aspect ratios (Rod1= 1.8, Rod2 = 2.5 and Rod3 = 4.2) for the transverse band (A ) and longitudinal (B) under the effect of pulsed Nd: YAG laser ( 1064 nm / 9 ns / 12.5 W / Irr ≈ 3.54 x 10 7 Wcm-2/pulse). mhʋ(ns) fragmentation 0 100 200 300 400 500 600 700 800 1.04 1.00 0.96 0.92 0.88 0.84 0.80 Long. Band Rod1 Rod2 Rod3 A t /A 0 Time/s B 0 100 200 300 400 500 600 700 800 1.16 1.12 1.08 1.04 1.00 0.96 Transverse band Rod1 Rod2 Rod3 A t / A 0 Time/s A Chapter V Results & Discussion - 256 - 2SH laser with 532 nm / 7 ns / 2.9 W / Irr ≈ 1.65 x 107 Wcm-2/pulse. Figure 5-63: Illustration diagram showing the difference between CW and pulsed nanosecond laser effect . As one can see the pulsed laser is more concentrated to the NR s than the CW laser. 400 500 600 700 800 900 2.0 1.6 1.2 0.8 0.4 0.0 Rod1 Wavelength/nm Absorbance/a.u a0Sec a7Sec a17Sec a32Sec a67Sec a127Sec a187Sec a277Sec a412Sec a532Sec Chapter V Results & Discussion - 257 - 400 500 600 700 800 900 1.2 0.8 0.4 0.0 Rod2 Wavelength/nm Absorbance/a.u a0sec a7sec a17sec a32sec a67sec a127sec a187sec a277sec a412sec a532sec a652sec 400 500 600 700 800 900 1.6 1.2 0.8 0.4 0.0 Wavelength/nm Absorbnace/a.u Rod3 a0sec a7sec a17sec a32sec a67sec a127sec a187sec a277sec a412sec a532sec a652sec a772sec Chapter V Results & Discussion - 258 - A drastically different behavior is observed under green (532nm) laser irradiation, as compared to similar conditions in the nIR (1064 nm). Although the pulse width was slightly shorter (7 vs. 9 ns) it should not cause a significant effect on the irradiation results in terms of nanorods modification. The irradiance values are of the same magnitude, thus other type of mechanism (melting) must be responsible for such a change in the laser-nanorods interaction. In fact, the changes observed are very similar to those reported for irradiation with fs lasers (Link et al.(17)). In addition, this laser output wavelength coincides with the transverse resonance plasmon band of Au, so that it is not surprising to observe considerable effects on the corresponding Vis band in the UV-Vis spectrum recorded just upon 532 nm irradiation. Figure 5-64: Absorption spectra showing the effect of pulsed Nd: YAG laser (SH with 532 nm / 7 ns / 2.9 W / Irr ≈ 1.65 x 10 7 Wcm-2/pulse ) on gold nanorods with different aspect ratios and illustration showing the possible melting mechanism. Chapter V Results & Discussion - 259 - 0 100 200 300 400 500 600 700 800 0.2 0.4 0.6 0.8 1.0 Long. band Rods3 Rods2 Rods1 A t /A 0 Time/s B Figure 5-65: Normalized absorption spectra of rods with different aspect ratios (Rod1= 1.8, Rod2= 2.5 and Rod3 = 4.2) for the transverse band (A ) and longitudinal (B) under the effect of pulsed Nd: YAG laser ( SH with 532 nm / 7 ns / 2.9 W / Irr ≈ 1.65 x 10 7 Wcm -2 /pulse). Figure 5-66: TEM images showing the effect of pulsed Nd: YAG laser (SH with 532 nm / 7 ns / 2.9 W / Irr ≈ 1.65 x 10 7 Wcm-2/pulse) on gold nanorods and transformation to nanospheres through the melting mechanism (scale bar 200nm). 0 100 200 300 400 500 600 700 800 1.6 1.4 1.2 1.0 Transverse band Rods 3 Rods 2 Rods 1 A t /A 0 Time/s A Chapter V Results & Discussion - 260 - 33rd Harmonic with 355 nm / 6 ns / 0.55 W / Irr ≈ 3.67 x 106 Wcm-2/pulse. 400 500 600 700 800 900 1.6 1.2 0.8 0.4 0.0 Rods 1 Absorbance/a.u Wavelength/nm a0sec a7sec a22sec a57sec a117sec a177sec a267sec a402sec a522sec a642sec a762sec 400 500 600 700 800 900 1.6 1.2 0.8 0.4 0.0 Rods 2 Wavelength/nm Absorbance/a.u a0sec a7sec a22sec a57sec a117sec a177sec a267sec a402sec a522sec a642sec a762sec Chapter V Results & Discussion - 261 - 0 100 200 300 400 500 600 700 800 1.2 1.0 0.8 0.6 0.4 0.2 0.0 long. band Rod1 Rod2 Rod3 At/A0 Time/s B 400 500 600 700 800 900 1.6 1.2 0.8 0.4 0.0 Rods 3 Wavelength/nm Absorbance/a.u a0sec a7sec a22sec a57sec a117sec a177sec a267sec a402sec a522sec a642sec a762sec Figure 5-67: Absorption spectra showing the effect of pulsed Nd: YAG laser (3 rd harmonic with 355 nm / 6 ns / 0.55 W / Irr ≈ 3.67 x 106 Wcm-2/pulse) on gold nanorods with different aspect ratios (Rod1= 1.8, Rod2= 2.5 and Rod3= 4.2) . 0 100 200 300 400 500 600 700 800 1.30 1.25 1.20 1.15 1.10 1.05 1.00 0.95 Transverse band Rod1 Rod2 Rod3 A t /A 0 Time/s A Figure 5-68: Normalized absorption spectra of rods with different aspect ratios (Rod1= 1. 8, Rod2= 2.5 and Rod3= 4.2) for the transverse band (A ) and longitudinal (B) under the effect of pulsed Nd: YAG laser (3rd harmonic with 355 nm / 6 ns / 0.55 W / Irr ≈ 3.67 x 10 6 Wcm -2 /pulse).