Effect of synthesis pH on the physicochemical properties of a synthesized Bi2WO6 and the type of substrate chosen, in assessing its photo-catalytic activities

ORIGINAL ARTICLE Effect of synthesis pH on the physicochemical properties of a synthesized Bi 2 WO 6 and the type of substrate chosen, in assessing its photo-catalytic activities C. Jaramillo-Pa ´ez, J.A. Navı´o * , M.C. Hidalgo Instituto de Ciencia de Materiales de Sevilla, Centro Mixto Universidad de Sevilla-CSIC, Ame ´rico Vespucio 49, 41092 Sevilla, Spain Received 15 March 2017; accepted 20 May 2017 Available online 26 May 2017 KEYWORDS Bismuth tungstate; Photocatalysis; Phenol; Dyes; Rhodamine B; TiO 2 (P25) Abstract Crystalline orthorhombic Bi 2 WO 6 powders were synthesized by a hydrothermal method from aqueous solutions of Bi(NO 3 ) 3 5H 2 O and Na 2 WO 4 2H 2 O over a range of three selected pH values (2.0, 5.0 and 7.0), using NaOH as precipitating agent. The as-prepared catalysts were characterized by XRD, BET, FE-SEM, TEM, XPS and UV-vis spectroscopy. The effect of pH-synthesis on crystallinity, morphologies, surface area and optical absorption properties, were investigated. Although the pH has a marked influence on morphology, the nature of the precipitating agent (NaOH or TEA) also influences the morphology and surface structure composition, as it is observed in the present work. Three different probe molecules were used to evaluate the photocatalytic properties under two illumination conditions (UV and Visible): Methyl Orange and Rhodamine B were chosen as dye substrates and Phenol as a transparent substrate. The photo-catalytic activities are strongly dependent not only on the pH used in the synthesis but also on the nature of the chosen substrate in assessing the photo-catalytic activities. Results were compared with those obtained when using TiO 2 (P25, Evonik) in the same experimental conditions. The photocatalytic activity of one of the synthesised samples has been evaluated by exposing a mixture of Rhodamine B and Phenol in water, to different illumination conditions. Our results provide new evidences about the issue of whether dyes are suitable substrates to assess the activity of a photo-catalyst. Ó2017 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article underthe CCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction It is extensively recognized that TiO 2 is one of the most significance inorganic photo-catalytic materials (Nakata and Fujishima, 2012). However, the large band gap of TiO 2 (3.2 eV for Anatase and Brookite, 3.0 eV for Rutile) requires an excitation wavelength that falls in the UV region. Nonetheless, TiO 2 has proven to be useful material for *Corresponding author. E-mail address: [email protected] (J.A. Navı´o). Peer review under responsibility of King Saud University. Production and hosting by Elsevier Arabian Journal of Chemistry (2020) 13, 431–443 King Saud University Arabian Journal of Chemistry www.ksu.edu.sa www.sciencedirect.com http://dx.doi.org/10.1016/j.arabjc.2017.05.014 1878-5352 Ó2017 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). environmental and disinfection applications (Abdel-Maksoud et al., 2016; Fagan et al., 2016; Malato et al., 2016). Therefore, it is desirable and urgent to develop highly efficient visible-light driven photocatalysts for pollutant degradation. In principle, two approaches can be exploited to develop the visible-light-irradiation photo-catalysts: modification of TiO 2 and development of a novel material. The former has been largely investigated by doping different ions (Di Paola et al., 2012; Fagan et al., 2016; Mohamed et al., 2012; Shaham-Waldmann and Paz, 2016) to extend absorption wavelength range to visible region. Though a significant number of new photo-catalysts adequately perform under visible light, they typically display poor performance with respect to TiO 2 commercial references (e.g. Evonik P25) under sunlight. This has generated some controversy (ShahamWaldmann and Paz, 2016) as to whether it is or not convenient, to move away from the use of TiO 2 and seek new alternatives in the field of synthesizing new photo-catalysts, operating in the visible region. It is widely accepted that the use of visible light photons and the adequate overall handing of UV-vis radiation constitute the key point for a good photo-catalytic performance under sunlight illumination (Shaham-Waldmann and Paz, 2016). However, an increase in visible absorption, in principle, does not guarantee visible-light induced activity, since photo-catalytic reactions proceed by photo-generated charge carriers and poor photo-catalytic activity may be observed whether these charge carriers are highly recombined. Among others, bismuthbased materials and particularly Bi 2 WO 6 seem to be suitable alternatives to widespread used TiO 2 (Colon et al., 2010; Yu et al., 2005). Bismuth Tungstate (Bi 2 WO 6 ) is a typical n-type direct band gap semiconductor with a band gap of 2.8 eV and has prospective applications for the degradation of organic pollutants under visible light illumination due to its low valence band and high chemical stability (Hu et al., 2013). Considerable effort has been taken to synthesize these hierarchical photo-catalysts with small size so as to increase the separation rate of photo-induced charge carriers and consequently achieve high activities (Shang et al., 2008). At the same times, it is widely accepted that the photo-catalytic performance of a catalyst is very closely related to its surface structure and crystallinity, including its morphology, crystallite sizes, specific surface area, crystal planes, defects, structure and composition of the surface and the synthesis pH values. All these factors, determine the result of the photo-catalytic activity that exhibits the photo-catalyst. Obviously, all these factors depend not only on the synthesis method, but also on the synthesis variables and preand post-treatment to the synthesis procedure. Thus, the photo-catalytic activity of Bi 2 WO 6 prepared by solid state reaction in the leading works was relatively weak because of a small surface area of 0.64 m 2 g 1 . Afterward, considerable progress has been achieved in the preparation of Bi 2 WO 6 nanoparticles (Shang et al., 2008; Xu et al., 2009; Zhang and Zhu, 2005) and hierarchical superstructures with much increased surface areas and thus enhanced photo-catalytic behaviors. Notably, Bi 2 WO 6 has been prepared by a wide variety of synthesis methods, some of them are reported in a recent work (Wu et al., 2016). However, in most of these studies, Rhodamine B (RhB) and other dyes are used in the evaluation of photo-catalytic activity of the synthesized Bi 2 WO 6 samples. RhB and other dyes are commonly used as model molecules, in part because their concentration can be easily monitored by UV-vis spectrometry. However, as pointed out by Yan et al. (2006) since dyes absorb light, especially in the visible range, the influence of this photo-absorption by dyes should be excluded for evaluation of the real photo-catalytic activity of the photo-catalysts. Also, as a remark, one should be careful when determining the dye concentration (for instance Methyl Orange, MO) during a photo-catalytic reaction because the pH may not be constant (acidic properties of the photodegradation intermediates). Hence, the absorption spectra of a dye as MO may change (MO is a pH indicator!) which would cause error in the determination of the MO concentration. In this study, Bi 2 WO 6 was synthesized by following the same hydrothermal procedure which was described in our previous work (Murcia-Lo ´pez et al., 2013b) but varying the pH values of synthesis. Thus, samples that, once subjected to different calcination treatments, should have different physicochemical properties and therefore should also exhibit different photo-catalytic activities. Although there is some work on the effect of pH-synthesis on the morphology, structure or photo-catalytic activity of Bi 2 WO 6 (Chunmei et al., 2009; Shang et al., 2008), however the effect of the pH-synthesis on the surface characteristics of Bi 2 WO 6 has not been investigated. Regardless of this, the photo-catalytic properties of this material, obtained at different pH values, have been evaluated using RhB or other dyes, thus that the results of the photo-catalytic activities could be exalted. The objective of this work was to synthesize Bi 2 WO 6 , through a hydrothermal route, to three pH values selected using NaOH (instead of TEA) as precipitating agent and to deepen not only on the structural and morphological characteristics of the obtained materials but also on the effects that the pH of synthesis exerts on the surface characteristics thereof. At the same time, the aim was to use three different pollutants substrates, as probe molecules. For the photo-catalytic evaluation of the synthesized materials, we used two dyes: Rhodamine B (RhB) and Methyl Orange (MO) and a transparent substrate, Phenol (Ph). The obtained results provide important information about the choice of substrate for evaluation of photo-catalytic activity of the same material, in this case Bi 2 WO 6 , but having different physico-chemical properties. Results were compared with those obtained when using TiO 2 (P25, Evonik) in the same experimental conditions. 2. Experimental details 2.1. Preparation of Bi 2 WO 6 Bi 2 WO 6 was prepared by following a hydrothermal procedure described elsewhere (Murcia-Lo ´pez et al., 2013b). In brief, about 0.01 mol of Bi(NO 3 ) 3 5H 2 O was dissolved in 10 mL of glacial acetic acid, and the stoichiometric amount of the tungsten precursor (0.005 mol of Na 2 WO 4 2H 2 O) was dissolved in 90 mL of distilled water. These two solutions were mixed forming a white suspension (pH 2), which was kept under stirring for 1 h. In another sequence preparations, the pH was adjusted to values of ca. 5 and 7 respectively by adding, drop wise to the suspension, a 2 M aqueous solution of NaOH (Panreac, >98.0%). The obtained white suspensions, for each of the indicated pH values, were transferred independently into Teflon recipient inside a stainless steel autoclave. The hydrothermal treatment was done at 140 °C for 20 h, and then the precipitate was centrifuged, washed several times with portions of bidistilled water and dried overnight at 120 °C. Finally the samples were submitted to a calcination treatment at 300 °C for 4 h. The synthesized Bi 2 WO 6 samples will hereafter be named as BW-x, where x is the pH of preparation. Commercial TiO 2 (P25, Evonik) with anatase/rutile structure, was used as a reference photo-catalyst. 2.2. Characterization of the photo-catalysts X-ray diffraction (XRD) patterns were obtained on a Siemens D-501 diffractometer with Ni filter and graphite monochromator using Cu Karadiation k= 0.1541 nm. Crystallite sizes were calculated from the line broadening of the main X-ray diffraction peak (131) of the Bi 2 WO 6 russellite synthesized (2b= 28.3°), with space group Pbca, using the Scherrer equation. Peaks were fitted using a Voigt function. X-ray photoelectron spectroscopy (XPS) studies were carried out on a Leybold-Heraeus LHS-10 spectrometer, working 432 C. Jaramillo-Pa ´ez et al. with constant pass energy of 50 eV. The spectrometer main chamber, working at a pressure <2 10 9 Torr, is equipped with an EA-200 MCD hemispherical electron analyzer with a dual X-ray source working with Al Ka(hm= 1486.6 eV) at 120 W and 30 mA. Bi 4f 7/2 signal (159.6 eV) was used as internal energy reference in all the experiments. Samples were outgassed in the pre-chamber of the instrument at 150 °Cuptoa pressure <2 10 8 Torr to remove chemisorbed water. All photoelectron spectra were analyzed using Casa-XPS software. Light absorption properties of the samples were studied by UV–Vis spectroscopy. The Diffuse Reflectance UV–Vis Spectra (UV–Vis DRS) were recorded on a Varian spectrometer model Cary 100 equipped with an integrating sphere and using BaSO 4 as reference. Band gaps values were calculated from the corresponding Kubelka–Munk functions, F(R1), which are proportional to the absorption of radiation, by plotting (F (R1)hm) 1/2 against hm. The morphology for all the samples was analyzed by Field Scanning electron microscopy (FE-SEM) using a Hitachi S 4800 microscope. The samples were dispersed in ethanol using an ultrasonicator and dropped on a copper grid. Transmission electron microscopy (TEM) was performed in a Philips CM 200 microscope. The samples for the microscopic analyzes were dispersed in ethanol using an ultrasonicator and dropped on a carbon grid. The porosity of the samples was characterized by measuring the N 2 adsorption isotherms at 196 °C in a volumetric analyzer (ASAP 2020, Micromeritics). Before the experiments, the samples were outgassed under vacuum (ca. 10 3 Torr) at 120 °C for 30 min in He flow. The isotherms were used to calculate the specific surface area, S BET and total pore volume, V total . 2.3. Photo-degradation tests The photo-catalytic activity of the catalysts prepared was tested in the photo-assisted degradation of two chosen dyes: Methyl Orange and Rhodamine B and of a transparent substrate, the phenol. Methyl Orange, Phenol and Rhodamine B [Reagent Plus >99%] were supplied by Sigma-Aldrich. From this section we will use the abbreviations of the reagents used that will hereafter be named along the text as MO (Methyl Orange), as RhB (Rhodamine B) and as Ph (Phenol) occasionally. Photo-catalytic tests were carried out using a discontinuous batch system, this includes a 250 mL Pyrex reactor enveloped by an aluminum foil, filled with an aqueous suspension (100 mL) containing the single substrates (concentrations: 20 ppm of MO, 50 ppm of Phenol or 10 ppm of RhB) and the photo-catalyst (1 g/L). Two different illumination conditions were tested, using an Osram Ultra-Vitalux lamp (300 W) with a sun-like spectrum and a main line in the UVA range at 365 nm. The visible photo-catalytic experiments were performed using a polyester UV filter sheet (Edmund Optics) showing 99.9% of absorbance below 400 nm whereas under UV-conditions systems were illuminated through a UV-transparent PlexiglasÒtop window (threshold absorption at 250 nm). The intensity of the incident UVA light on the solution was measured with a PMA 2200 UVA photometer (Solar Light Co.) being ca 90 W m 2 (UVA PMA2110 sensor; spectral response 320–400 nm). On the other hand, the intensity of light in the visible range measured in this case was 110 W m 2 (Photopic PMA21300 sensor; spectral response 400–700 nm). In order to favor the adsorption–desorption equilibrium, prior to illumination the suspension was magnetically stirred for 20 min in the dark. Magnetic stirring and a constant oxygen flow of 20 L/h, as an oxidant, were used to produce a homogeneous suspension of the photo-catalyst in the solution. A tank bubbler was used as a source of natural oxygen. All photo-catalytic tests started at pH ca. 5.5 and the total reaction time was 120 min. During the Methyl Orange and Rhodamine B photoreactions, samples were collected at different times and in order to evaluate the dye discoloration rate, the concentrations of MO and RhB during the photo-degradation reactions were analyzed by UV–Visible spectroscopy, considering the main peak of each dye in the visible range, located at 465 nm (for MO) and 554 nm (for RhB). For this analysis a UV–Vis spectrometry with a Cary 100 (Varian) spectrometer was used. Phenol concentrations were followed using HPLC (Agilent Technologies, 1200 Series) equipped with UV-vis detector using an Eclipse XDB-C18 column (5 lm, 4.6 mm 150 mm; Agilent) at 40 °C. Samples of about 2 mL were removed periodically during the experiments and filtered (Millipore Millex 25 0.45 mm membrane filter) previous to HPLC measurements. Mobile phase was water/methanol (65:35) at a flow rate of 0.8 mL/min. On the other hand, following the recommendations of Ohtani (2014), we have proven that the initial concentrations of the substrates do not suffer variations, either under direct photolysis (negligible) or in the single presence of the catalyst, except in the latter case, in which the variations of concentration are due to an adsorption process in the dark until equilibrium is reached before illumination. Reproducibility of the measurements was ensured by double testing of selected samples. Total organic carbon was followed by means of a TOC analyzer Shimadzu 500. Mineralization degrees (%) were evaluated by the TOC values upon 2 h of illumination, for all the photo-assisted processes studied, using the formula [1 (final TOC/initial TOC)] 100. 3. Results and discussion 3.1. Characterization Fig. 1 shows the XRD patterns of the photo-catalysts; the diffraction patterns of Bi 2 WO 6 were well indexed to the russellite synthetic phase (JCPDS 39-0256, orthorhombic phase), constituted of a layered structure containing WO 6 octahedra and (Bi 2 O 2 ) 2+ layers and H 2 O molecules (Amano et al., 2009; Babu et al., 2014a, 2014b). No diffraction peaks were observed for the starting precipitate precursors obtained by simple inorganic precipitate reaction before hydrothermal treatment. Therefore, it can be concluded that the hydrothermal treatment is essential for the formation of crystalline Bi 2 - WO 6 . Likewise, no other possible impurities, such as WO 3 or Bi 2 O 3 , were detected, indicating that the as-prepared Bi 2 WO 6 Effect of synthesis pH on the physicochemical properties of a synthesized Bi 2 WO 6 433 was pure. From Fig. 1 it can be clearly seen that increasing the pH of synthesis, the width of the diffraction peaks becomes gradually narrower, and thus indicating a better crystallinity. In fact, with the improvement of crystallization, the crystallite sizes grow larger as has been estimated according to the Scherrer equation, and the surface areas of the samples decrease (Table 1). Based on the standard data, the intensity of the (131) peak is approximately five times that of the (200) or (020). However, the intensity ratio of the (131) peak to the (200) or (020) peak in the as-prepared Bi 2 WO 6 samples was less than 2.5. This fact reveals that the synthesized BW-x samples exhibited anisotropic grow in the (200) or (020) direction. The XPS technique was used to identify the surface chemical composition and the oxidation state of the synthesized Bi 2 - WO 6 samples. Table 2 shows the results obtained by XPS of the elemental atomic quantification of the synthesized samples and Fig. 2 shows the surface features of the synthesized BW-x series. In none of the samples, the presence of sodium on the surface was detected. The two peaks into de Bi 4f region (Fig. 2A–C) at 164.8 eV and 159.6 eV can be attributed to Bi 4f 5/2 and Bi 4f 7/2 respectively (Jaramillo-Pa ´ez et al., 2017) and are characteristic of Bi 3+ . The binding energies of 37.8 eV and 35.6 eV with a spin–orbital separation of 2.2 eV, as shown in Figs. 2 D, 2 E and 2 F, could be assigned to the +6 oxidation state of tungsten for the W 4f 5/2 and W 7/2 respectively (Wang et al., 2013). The O 1s region (Fig. 3A–C) can be fitted using two peaks, which are consistent to different chemical environments of oxygen element in BiAO and WAO bonds, the relative areas of which vary as the pH of synthesis changes. The XPS peaks in the O 1s at 529.4 eV and 530.2 eV are attributable to BiAO and WAO bonds in the BW-x series respectively (Jaramillo-Pa ´ez et al., 2017; Wang et al., 2013). Table 2 also shows the percentages of the relative areas of the two oxygen peaks of the O 1s region of the XPS spectrum due to contribution BiAO and WAO bonds in for each sample. Taking into account the relative contributions of the peaks assigned to OAW (530.2 eV) and OABi (529.4 eV) links, we have made estimations of the O/W and O/Bi ratios on the surfaces of each sample, establishing species cationic and anionic that are compatible with the oxidation states Bi (3+) and W (6 +) in the stoichiometric Bi 2 WO 6 at the surface. According to these estimates, the surface of each of the samples can be understood as formed by anionic and cationic species as follows: BW-2 ([Bi 2 O 2 ] 2+ and [WO 4 ] 2 ; BW-5 ([Bi 2 O 2 ] 2+ and [WO 4 ] 2 ; BW-7 ([WO 2 ] 2+ and [Bi 2 O 4 ] 2 ). Thus, the surface structure of the samples prepared at acidic pH values is consistent with a layer structure formed by octahedral species WO 6 and [Bi 2 O 2 ] 2+ layers (Babu et al., 2014a; Ohtani, 2014) while the surface of the BW-7 samples should be understood in terms of [WO 2 ] 2+ and [Bi 2 O 4 ] 2 species. This result implies that at pH = 7, the Bi 3+ and W 6+ species have been rearranged on the surface of the BW-7 sample, differently than the BW-2 samples and BW-5, without any changes in the structure. In fact, the existence of WO 2 2+ species is well established; thus, for example, WO 2 Cl 2 is a layered material, consisting of distorted octahedral W centers (Abrahams et al., 1993; Bortoluzzi et al., 2016). The effect of the pH during the synthetic procedure on the morphology of the as-obtained Bi 2 WO 6 samples was investigated by FE-SEM. Fig. 4 shows selected SEM micrographs of the synthesized samples. Clearly, the morphologies and dimensions of the samples are strongly depending on the pH value. BW-2 showed 3D flower-like spherical superstructures (Fig. 4 A) with a diameter ranging between 5 and 8 lm constructed by sheets aligned perpendicularly to the spherical surface (Fig. 4B). Fig. 4 A further reveals that the flower-like superstructures were occasionally assembled by twodimensional micrometer sheets (marked in Fig. 4A). However, for the Bi 2 WO 6 samples synthesized at higher pH values (pH ca. 5.0 or 7.0) a sheet-like morphology was observed being heterogeneous in sizes, although the sample prepared at pH = 7.0 displays bigger sheet-sizes than the sample prepared at pH = 5.0, by comparing Fig. 4D and E. The thickness of the thin sheet-shapes was estimated at 0.04 lm(Fig. 4F). The detailed morphology of the BW-x samples was further investigated with TEM. Fig. 5 shows the TEM photographs of BW-x samples. Fig. 5A shows that the structure of the sheets is grouped together following a circular tangential direction, which is in accordance with the SEM photographs (Fig. 4A). A large proportion of sheets can be seen in lateral position Figure 1 X-ray diffraction patterns of the prepared photocatalysts. Table 1 Some of the physicochemical properties of the synthesized BW-x samples. Sample Crystallite size (nm) Band gap (eV) S BET (m 2 g 1 ) Pore volume (cm 3 /g nm) BW-2 11.7 2.78 24.0 0.0032 BW-5 28.1 2.98 19.2 0.0027 BW-7 64.5 2.94 8.0 0.0018 TiO 2 (P25) – 3.20 43.8 0.0081 434 C. Jaramillo-Pa ´ez et al. Table 2 Proposed of surface species based on XPS results for BW-x samples. Bi (4f) (atomic %) W (4f) (atomic %) O (1 s) (atomic %) Contribution area (%) Atomic ratio BW-2 (26.95) (9.87) (63.17) OAW (60.53) O/W = 38.24/9.87 = 3.87 (4:1) [WO 4 ] 2 OABi (39.47) O/Bi = 24.74/25.67 = 0.92 (1:1) [Bi 2 O 2 ] 2+ BW-5 (25.62) (9.04) (65.34) OAW (51.83) O/W = 33.86/9.04 = 3.60 (4:1) [WO 4 ] 2 OABi (48.17) O/Bi = 31.47/25.62 = 1.22 (1:1) [Bi 2 O 2 ] 2+ BW-7 (26.09) (9.57) (64.33) OAW (24.21) O/W = 15.57/9.57 = 1.62 (2:1) [WO 2 ] 2+ OABi (75.79) O/Bi = 44.80/23.97 = 1.87 (2:1) [Bi 2 O 4 ] 2 Figure 2 XPS results, in the Bi 4f and W 4f region of the synthesized BW-x samples. Effect of synthesis pH on the physicochemical properties of a synthesized Bi 2 WO 6 435 (indicated with arrows). However, it can be seen that BW-5 and BW-7 appear the square morphology with the size 50– 200 nm. The corners and the surfaces of the BW-5 and BW-7 nanosheets are smooth. In a previous work (Colon et al., 2010) we report the synthesis of Bi 2 WO 6 following the same procedure but adjusting the pH by adding TEA (triethylamine). In that work, we also report that the morphology of the Bi 2 WO 6 catalyst appears highly influenced by the precipitation pH. Thus, at low pH the particles show sheet-like morphology while by increasing the precipitation pH with TEA a regular 3D hierarchical superstructure is formed through an Ostwald ripening process (Amano et al., 2008; Zhang et al., 2007). However, in the present work, at low pH the particles show a tendency to flowerlike superstructure, while by increasing the precipitation pH with NaOH the particles show a sheet-like morphology, at least until pH = 7.0. This result suggests that, regardless of the effect of pH, other factors such as the presence of either hydroxyls or surfactants in the synthesis medium can affect the morphology of the samples. In any case, it is evident that, from the results of characterization, the pH of synthesis of Bi 2 - WO 6 sample has an effect on the physico-chemical properties of the obtained samples (Table 1), which can influence on the photo-catalytic activity. On one hand, it can be seen (Table 1), from crystallite sizes, that the samples prepared at low pH are less crystalline than those obtained at a higher value, and this effect influences the values of specific surfaces areas, being higher for BW-2 relative to BW-7. The study and control of morphology has been extensively studied in Bi 2 WO 6 photo-catalysts. The nanosheets growth mechanism of this compound in neutral pH conditions has been proposed by Zhang and Zhu (2005). Thus, a crystalline structure is formed from Bi 2 WO 6 small amorphous particles, thanks to the typical hydrothermal process ripening in which small crystal nuclei are initially formed in the supersaturated middle. Because of the difference in solubility between small and large particles (according to the Gibbs-Thomson law), growth is favored at the first cost of the dissolution of the latter this tends to be anisotropic growth and parallel to WO 6 octahedra layer and has been explained as a consequence of the intrinsic structural characteristics of tungstates associated with the chemical potential of some faces on underlying distorted octahedral chains [WO 4 ] 2 (Zhang and Zhu, 2005). However, using Pluronic P123 surfactant, it is possible to suppress the anisotropic growth and formation of nanosheets of Bi 2 WO 6 (Zhang et al., 2011). Possibly, in our synthesis conditions, the NaOH (instead of TEA) is facilitating the formation of sheet-like morphology. The influence of pH on the synthesis of such structure materials has been explained through two effects: first, a relationship between the concentrations of ions in the system during the aging process, influencing the rate of one species over the other and, secondly, the possible formation of oxyanions [WO 4 ] 2 or polyanions of other species due to the presence of hydroxyl OH  ions. In any case, it is clear that there is a deep dependence of morphology with the method and conditions of preparation, even in the case of procedures performed by hydrothermal synthesis. The optical properties of the samples were also explored by UV–Vis diffuse reflectance spectroscopy (Fig. 6). Commercial TiO 2 (P25, Evonik) is also shown as reference. As can be seen, the synthesized Bi 2 WO 6 samples as well as the commercial TiO 2 (P25, Evonik), presents the photo-absorption property from the UV light region to visible light shorter than 450– 475 nm corresponding to the band gap values reported in Table 1. The band edge of Bi 2 WO 6 shift with different pH. As shown, the sample BW-2 has the lowest value of band gap, when compared with the values obtained for the other samples (Table 1). In fact, this sample presents a very pale yellow-green color, as opposed to the other samples which exhibit a whitish color. Self-assembled layered structure can cause multiple scattering of the light, which would increase optical path of light propagation through the layered structure. Multiple scattering of UV-vis light increases their light Figure 3 XPS results, in the O 1s region of the synthesized BW-x samples. 436 C. Jaramillo-Pa ´ez et al. absorption and light utilization rate, resulting in a decrease in band gap. Accordingly it is found that the difference in morphology between the BW-2 sample and the other ones, BW-5 and BW-7, can do affect the optical absorption and band gap energy of the samples, as can be observed (Fig. 6). Fig. 7 shows the evolution of pore diameter distributions for samples prepared in this study; TiO 2 (P25, Evonik) is also shown as reference. As it can be observed, BW-x samples present a sharp pore family at about 2–3 nm (microporous) whose volume slightly decreases as the pH increases. These pores at about 2–3 nm are probably slits produced by sheets stacking. These results suggest that the pH of synthesis influences the texture of the samples as expected. On the other hand, the pore volume is bigger in TiO 2 (P25, Evonik) than in BW-x samples. Moreover, BW-2 sample exhibited additional pore family at higher diameters, between 5 and 20 nm (mesoporous) that is more or less absent in samples BW-5 and BW-7, although BW-5 sample shows a broad distribution of pore size between 20 and 100 nm similar to that presented by TiO 2 (P25, Evonik) in the same region. Mesoporous in BW-2 is likely caused by the flower structure according to Fig. 4A and B. Although the pH of synthesis and the nature of the precipitating agent (NaOH or TEA) seem to have marked influence on the physicochemical properties of Bi 2 WO 6 , the nature of the substrate chosen to evaluate the photo-catalytic activity is another factor to consider when establishing the photocatalytic properties of a catalyst. In our studies, we have chosen three substrates of different nature, which they have been widely used in scientific literature due to its toxic nature. 3.2. Photo-catalytic experiments The photo-catalytic activity of the as-obtained samples was evaluated by the degradation of Rhodamine B (RhB), Methyl Figure 4 FE-SEM images of the as-prepared BW-x samples. (A) and (B) BW-2; (C) and (D) BW-5; (E) and (F) BW-7. Effect of synthesis pH on the physicochemical properties of a synthesized Bi 2 WO 6 437 Orange (MO) and Phenol (Ph) aqueous solutions under the experimental conditions indicated previously. Fig. 8 shows the conversion plots for photo-chemical discoloration of MO (Fig. 8A) and photo-chemical disappearance of Phenol (Fig. 8B) and RhB (Fig. 8C) under UV and Visible illumination using the synthesized Bi 2 WO 6 samples. It is clear that the percentages of degradation depend not only on the nature of the substrate studied, but also on the pH of the catalyst synthesis and obviously on the illumination conditions. Thus, while the synthesized catalysts show low values of conversion for MO substrate, they however showed high conversion values when the RhB is used, being the values for Phenol conversion, moderate. The results also indicate that the synthesized catalysts showed better results under lighting conditions in the UV, than under visible illumination. Fig. 9 shows the results of the conversion percentages of the three selected substrates, using the commercial TiO 2 (P25, Evonik), Figure 5 TEM images of the as-prepared BW-x samples. (A) BW-2, (B) BW-5 and (C) BW-7. Figure 6 UV-vis diffuses reflectance spectra of the as-prepared samples and TiO 2 (P25). A magnification in the region of 400– 500 nm is shown (inset). Figure 7 Pore diameter distributions of the studied materials. 438 C. Jaramillo-Pa ´ez et al. under two illumination conditions. As can be seen (Fig. 9A), TiO 2 (P25, Evonik) showed high values of conversion for three substrates, especially for RhB, under illumination in the UV. However, under Visible illumination (Fig. 9B), only conversions for RhB was obtained but not for Phenol or for MO. This last fact is not surprising, since according to our previous results (Abrahams et al., 1993; Colon et al., 2010), the TiO 2 (home-prepared) presented photo-catalytic activity in the visible, when Rhodamine B is used, due to added photo-sensitizing effect. In order to compare the photo-catalytic activity of the synthesized samples and the commercial titania, in Fig. 10A the values of the initial reaction rates of degradation, under UVillumination, of each of the substrates with the indicated catalysts and the values of the reaction rates per surface area unit (Fig. 10B) are shown. Similarly, Fig. 11 shows the values of reaction rates under lighting conditions in the visible. The reaction rates were calculated from the slopes of the conversion plots at the first 15 min of reaction, and assuming zero-order kinetics at this stage of the reaction. The differences between the BW-x series could be attributed to structural (crystallinity), morphology and surface areas features. Crystallinity, morphology and the surface area of the photo-catalyst are two important factors influencing the photo-catalytic activity. The higher the crystalline quality, the smaller the amount of defects. The defects operate as trapping and recombination centers between photo-generated Figure 8 Conversion plots for photoassisted transformations of Methyl Orange (A), Phenol (B) and Rhodamine B (C) using the indicated BW-x catalyst under UV and visible illumination. Figure 9 Conversion plots for photochemical discoloration of Methyl Orange (MO), Phenol (Ph) and Rhodamine B (RhB) disappearance, under (A) UV or (B) sun-like illumination: with only the presence of commercial TiO 2 (P25) photo-catalyst. Effect of synthesis pH on the physicochemical properties of a synthesized Bi 2 WO 6 439