Enhanced photocatalytic activity of Au/TiO2 nanoparticles against ciprofloxacin

catalysts Article Enhanced Photocatalytic Activity of Au/TiO2 Nanoparticles against Ciprofloxacin Pedro Martins 1,2,* , Sandro Kappert 3, Hoai Nga Le 3,4, Victor Sebastian 5,6 , Klaus Kühn 3, Madalena Alves 1, Luciana Pereira 1, Gianaurelio Cuniberti 3,7,8, Manuel Melle-Franco 9and Senentxu Lanceros-Méndez 1,10,11,* 1Department of Physics/Centre of Biological Engineering, University of Minho, 4710-057 Braga, Portugal; [email protected] (M.A.); [email protected] (L.P.) 2IB-S—Institute for Research and Innovation on Bio-Sustainability, University of Minho, 4710-057 Braga, Portugal 3Institute for Materials Science and Max Bergmann Center of Biomaterials, Technische Universität Dresden Dresden, 01062 Dresden, Germany; sandr[email protected] (S.K.); [email protected]esden.de (H.N.L.); [email protected] (K.K.); [email protected] (G.C.) 4Department of Chemical Engineering, Hanoi University of Science and Technology, Hanoi 10000, Vietnam 5Department of Chemical Engineering, Aragon Institute of Nanoscience (INA), University of Zaragoza, Campus Río Ebro-Edificio I+D, C/Poeta Mariano Esquillor S/N, 50018 Zaragoza, Spain; [email protected] 6Networking Research Centre on Bioengineering, Biomaterials and Nanomedicine, Centro de Investigacion Biomédica en Red—Bioengenharía, Biomateriales e Nanomedicina, 28029 Madrid, Spain 7Dresden Center for Computational Materials Science, Technische Universität Dresden Dresden, 01062 Dresden, Germany 8Center for Advancing Electronics Dresden, Technische Universität Dresden Dresden, 01062 Dresden, Germany 9 Centro de Investigaç ã o em Materiais Cer â micos e Comp ó sitos, Aveiro Institute of Materials, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal; manuelmelle.resear[email protected] 10 BCMaterials, Basque Center for Materials, Applications, and Nanostructures, Universidad del País Basco—Euskal Herriko Unibertsitatea, Science Park, 48940 Leioa, Spain 11 IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain *Correspondence: [email protected] (P.M.); [email protected] (S.L.-M.) Received: 14 January 2020; Accepted: 11 February 2020; Published: 15 February 2020   Abstract: In the last decades, photocatalysis has arisen as a solution to degrade emerging pollutants such as antibiotics. However, the reduced photoactivation of TiO 2 under visible radiation constitutes a major drawback because 95% of sunlight radiation is not being used in this process. Thus, it is critical to modify TiO 2 nanoparticles to improve the ability to absorb visible radiation from sunlight. This work reports on the synthesis of TiO 2 nanoparticles decorated with gold (Au) nanoparticles by deposition-precipitation method for enhanced photocatalytic activity. The produced nanocomposites absorb 40% to 55% more radiation in the visible range than pristine TiO 2 , the best results being obtained for the synthesis performed at 25 ◦ C and with Au loading of 0.05 to 0.1 wt. %. Experimental tests yielded a higher photocatalytic degradation of 91% and 49% of ciprofloxacin (5 mg/L) under UV and visible radiation, correspondingly. Computational modeling supports the experimental results, showing the ability of Au to bind TiO 2 anatase surfaces, the relevant role of Au transferring electrons, and the high affinity of ciprofloxacin to both Au and TiO 2 surfaces. Hence, the present work represents a reliable approach to produce efficient photocatalytic materials and an overall contribution in the development of high-performance Au/TiO 2 photocatalytic nanostructures through the optimization of the synthesis parameters, photocatalytic conditions, and computational modeling. Keywords: Au-TiO2; antibiotics; emergent contaminants; nanocatalyst; photocatalysis; GFN-xTB Catalysts 2020,10, 234; doi:10.3390/catal10020234 www.mdpi.com/journal/catalysts Catalysts 2020,10, 234 2 of 20 1. Introduction The resilience of specific emerging pollutants such as pharmaceuticals to the traditional wastewater treatments makes them spread in variable concentrations in surface and groundwater [ 1 ]. Dissemination of antibiotics in nature is one of the most significant environmental concerns as they affect biological metabolism and induce the presence of bacterial resistance among drinking water sources [ 2 ]. Photocatalysis has received considerable attention from the scientific community as a possible solution to degrade these compounds [3,4]. Typically, the photocatalytic process takes place when a catalyst is UV irradiated and electron-hole pairs are created that will react with H 2 O, OH − , and O 2 to generate oxidizing species such as the hydroxyl radical (OH • ), superoxide radical anions (O 2•− ), and hydrogen peroxide (H 2 O 2 ). These species will initiate a series of reactions that will degrade pollutants into harmless compounds (e.g., CO2and H2O). Photocatalysis presents several advantages when compared with other methods, such as the low cost, and the eco-friendly and straightforward processing conditions [ 5 , 6 ]. Many photocatalysts have been reported in the last decades [ 7 , 8 ]. Among them, titanium dioxide (TiO 2 ) is the most studied and applied in photocatalysis, mainly because of its remarkable optical and oxidizing properties, superhydrophilicity, chemical stability, and durability [ 9 , 10 ]. Despite the compelling advantages of TiO 2 , there are also some drawbacks. One of the main hurdles is the low spectral activation of TiO 2 , caused by its wide bandgap (3.0–3.2 eV) excitation that only occurs under radiation in the UV or near the UV region (410–387 nm) [11]. For this reason, solar radiation cannot be efficiently used because only less than 5% of this radiation corresponds to UV [ 3 ]. Additionally, the process becomes less cost effective as the UV lamps are required to provide the radiation. Another limitation is the electron-hole pair recombination that decreases the photocatalytic efficiency [12,13]. The research developed in the last decades has been mainly devoted to surpassing those limitations by producing new and more efficient photocatalytic materials. Strategies for metallic and nonmetallic doping, co-doping [ 14 , 15 ], dye sensitization, semiconductor combination, co-catalyst loading, and nanocomposite materials [ 16 , 17 ] have been used and tested. These approaches allow us to reduce the electron-hole recombination rate and enhance the absorption of visible radiation of TiO 2 by introducing intermediate energy levels inside the bandgap [ 18 ]. In this scope, several works have reported the functionalization of TiO 2 nanoparticles surfaces with metals such as Au [ 19 ], Cu [ 20 ], Co [ 21 ], and Ag [ 22 ]. When irradiated, noble metals nanoparticles at the TiO 2 surface can receive electrons and prevent the recombination of the photo-generated electron-hole pairs [23,24]. Metals such as Au and Ag can increase visible light absorption due to the surface plasmon resonance effect [ 25 , 26 ]. Gold (Au) nanoparticles have attracted considerable attention, mainly because they possess exceptional stability, nontoxicity, and biocompatibility [ 3 ]. Their properties are highly dependent on the size and shape of the nanoparticles, allowing a broad range of applications [ 27 , 28 ]. For instance, the literature shows that gold nanoparticles in the range of 5 to 10 nm present an enhanced catalytic activity [ 29 , 30 ]. In this sense, some works focused on the photocatalytic activity of Au/TiO 2 nanocomposite have been published, including interesting review articles [3,29,31]. Different physical-chemical techniques have been exploited to produce Au/TiO 2 nanocomposites with enhanced catalytic properties. For instance, chemical vapor deposition [ 32 ], sol-gel [ 33 ], spray pyrolysis [ 34 ], electrophoretic approach [ 35 ], deposition-precipitation (DP) [ 36 ], deposition-precipitation using urea [ 37 ], impregnation [ 38 ], hybridization [ 39 ], and surface functionalization [ 40 ], among others [ 41 , 42 ]. However, many of these techniques are time-consuming, and few of them have focused on the optimization of the nanocomposite and the computational modeling of its nanostructure. Thus, this work focused on the optimization of a DP, converting the Au/TiO 2 nanocomposite production into a cost-effective and straightforward technique, with enhanced photocatalytic activity, under UV and visible radiation. The method optimization aims for cost reduction, using the lowest Au loading that endows visible spectra photocatalytic activity to the nanocomposite. The computational studies Catalysts 2020,10, 234 3 of 20 provide further information about the electronic mechanism behind the enhanced photocatalytic activity of the Au/TiO2nanocomposite, as well as the interaction with the target compound. The target compound is the fluoroquinolone ciprofloxacin (CIP) (chemical formula in Supplementary Material, Figure S1), belonging to a class of synthetic broad-spectrum antibiotics [ 43 ], which is mostly used in medicine (e.g., tuberculosis, pneumonia, or digestive disorders). It is also one of the most prescribed fluoroquinolones in the world and studies has shown its presence in potable water and wastewater, as well as in sewage sludge at variable concentrations from milligrams to nanograms per liter [2,44]. In this work, photocatalytic efficiency during the degradation of CIP under UV and visible illumination was assessed. To the best of our knowledge, this is the first work that combines an optimization process of Au/TiO 2 nanocomposite with photocatalytic experiments for CIP degradation and computational modeling that addresses the interaction between Au and TiO 2 nanoparticles, as well as the interaction of CIP with the produced nanocomposites. 2. Results and Discussion 2.1. Nanocomposite Characterization The Au/TiO 2 nanocomposites were produced by nanoprecipitation method, and the temperature (25, 60, and 80 ◦ C) and the Au loading (ranging from 0.025 to 0.5 wt. %) were changed to understand how these parameters affect the morphology of the nanocomposites and relate it to the photocatalytic efficiency. In this sense, scanning transmission electron microscopy-high-angle annular dark-field imaging (STEM-HAADF) analysis was performed, and the micrographs of the different nanocomposites are displayed in Figure 1. Catalysts 2020, 10, x FOR PEER REVIEW 3 of 20 mechanism behind the enhanced photocatalytic activity of the Au/TiO2 nanocomposite, as well as the interaction with the target compound. The target compound is the fluoroquinolone ciprofloxacin (CIP) (chemical formula in Supplementary Material, Figure S1), belonging to a class of synthetic broad-spectrum antibiotics [43], which is mostly used in medicine (e.g., tuberculosis, pneumonia, or digestive disorders). It is also one of the most prescribed fluoroquinolones in the world and studies has shown its presence in potable water and wastewater, as well as in sewage sludge at variable concentrations from milligrams to nanograms per liter [2,44]. In this work, photocatalytic efficiency during the degradation of CIP under UV and visible illumination was assessed. To the best of our knowledge, this is the first work that combines an optimization process of Au/TiO2 nanocomposite with photocatalytic experiments for CIP degradation and computational modeling that addresses the interaction between Au and TiO2 nanoparticles, as well as the interaction of CIP with the produced nanocomposites. 2. Results and Discussion 2.1. Nanocomposite Characterization The Au/TiO2 nanocomposites were produced by nanoprecipitation method, and the temperature (25, 60, and 80 °C) and the Au loading (ranging from 0.025 to 0.5 wt. %) were changed to understand how these parameters affect the morphology of the nanocomposites and relate it to the photocatalytic efficiency. In this sense, scanning transmission electron microscopy-high-angle annular dark-field imaging (STEM-HAADF) analysis was performed, and the micrographs of the different nanocomposites are displayed in Figure 1. Figure 1. Scanning transmission electron microscopy-high-angle annular dark-field imaging micrographs of Au/TiO2 nanocomposites synthesized with different Au loadings at 60 °C (a–c), and Au/TiO2 nanocomposites obtained at different temperatures with an Au loading of 0.05 wt. % (d–f). The Au loading study (Figure 1a–c) was assessed producing different nanocomposites using the same experimental conditions (temperature = 60 °C) and changing the loading of gold exclusively, from 0.025 to 0.5 wt. %. The STEM-HAADF micrographs show that for the sample with 0.025 wt. % of Au (Figure 1a), the presence of Au nanoparticles over the surface of the TiO2 nanoparticles was almost inexistent (Figure 1a). With the increase of Au loading to 0.05 wt. % (Figure 1b), it was possible to observe a homogeneous distribution of predominantly small Au nanoparticles (bright contrast Figure 1. Scanning transmission electron microscopy-high-angle annular dark-field imaging micrographs of Au/TiO 2 nanocomposites synthesized with different Au loadings at 60 ◦ C ( a – c ), and Au/TiO 2 nanocomposites obtained at different temperatures with an Au loading of 0.05 wt. % ( d – f ). The Au loading study (Figure 1a–c) was assessed producing different nanocomposites using the same experimental conditions (temperature =60 ◦ C) and changing the loading of gold exclusively, from 0.025 to 0.5 wt. %. The STEM-HAADF micrographs show that for the sample with 0.025 wt. % of Au (Figure 1a), the presence of Au nanoparticles over the surface of the TiO 2 nanoparticles was almost inexistent (Figure 1a). With the increase of Au loading to 0.05 wt. % (Figure 1b), it was possible to observe a homogeneous distribution of predominantly small Au nanoparticles (bright contrast Catalysts 2020,10, 234 4 of 20 nanoparticles below 5 nm in diameter) over the TiO 2 nanoparticles. Similar results were obtained for 0.1 wt. % (data not shown). For the concentrations of 0.25 and 0.5 wt. % (Figure 1a–c), agglomerates of Au over theTiO 2 nanoparticles (brightest areas of the micrograph) were identified as well as large Au nanoparticles. Analogously, the effect of temperature on the synthesis product was also performed maintaining all the synthesis parameters (Au loading =0.05 wt. % yielded a homogeneous distribution and size of Au nanoparticles) and changing the temperature of the different samples. STEM-HAADF images (Figure 1d–f) indicate that although the used Au loading was the same in the three temperatures tested when the nanocomposite was synthesized at 80 ◦ C, larger Au nanoparticles appeared more frequently on the nanocomposite (Figure 1f). Conversely, at lower temperatures (25 and 60 ◦ C), the Au nanoparticles size was smaller (Figure 1d,e). The study of the effect of Au loading and temperature in the nanocomposites morphology indicates that the samples produced at 60 ◦ C and with an Au loading of 0.05 wt. % possessed the more homogeneous distribution and size of Au nanoparticles. In this way, a more detailed STEM-HAADF analysis (Figure 2) was performed on this sample. Figure 2a,b reveal a homogeneous dispersion of Au nanoparticles (white arrows) over TiO 2 nanoparticles’ surface. The representation of the sphere-like shape of Au nanoparticles in Figure 2c, where an high-resolution scanning transmission electron microscopy – high-angle annular dark field shows that single-crystal nanoparticles with high crystallinity were produced by the proposed method. Size distribution, ranging from 1 to 7 nm, and the average size of 3.2 ± 1.13 nm (Figure 2d), were quantified using Image J software applied to 400 nanoparticles. The size distribution of Au nanoparticles for synthesis at 25 ◦ C and 80 ◦ C is provided in Supplementary Material (Figure S2). All the images show Au nanoparticles with similar sizes, which is in good agreement with the size distribution histogram that presents a sharp size distribution. Catalysts 2020, 10, x FOR PEER REVIEW 4 of 20 nanoparticles below 5 nm in diameter) over the TiO2 nanoparticles. Similar results were obtained for 0.1 wt. % (data not shown). For the concentrations of 0.25 and 0.5 wt. % (Figure 1a–c), agglomerates of Au over theTiO2 nanoparticles (brightest areas of the micrograph) were identified as well as large Au nanoparticles. Analogously, the effect of temperature on the synthesis product was also performed maintaining all the synthesis parameters (Au loading = 0.05 wt. % yielded a homogeneous distribution and size of Au nanoparticles) and changing the temperature of the different samples. STEM-HAADF images (Figure 1d–f) indicate that although the used Au loading was the same in the three temperatures tested when the nanocomposite was synthesized at 80 °C, larger Au nanoparticles appeared more frequently on the nanocomposite (Figure 1f). Conversely, at lower temperatures (25 and 60 °C), the Au nanoparticles size was smaller (Figures 1d and e). The study of the effect of Au loading and temperature in the nanocomposites morphology indicates that the samples produced at 60 °C and with an Au loading of 0.05 wt. % possessed the more homogeneous distribution and size of Au nanoparticles. In this way, a more detailed STEMHAADF analysis (Figure 2) was performed on this sample. Figure 2a,b reveal a homogeneous dispersion of Au nanoparticles (white arrows) over TiO2 nanoparticles’ surface. The representation of the sphere-like shape of Au nanoparticles in Figure 2c, where an high-resolution scanning transmission electron microscopy – high-angle annular dark field shows that single-crystal nanoparticles with high crystallinity were produced by the proposed method. Size distribution, ranging from 1 to 7 nm, and the average size of 3.2 ± 1.13 nm (Figure 2d), were quantified using Image J software applied to 400 nanoparticles. The size distribution of Au nanoparticles for synthesis at 25 °C and 80 °C is provided in Supplementary Material (Figure S2). All the images show Au nanoparticles with similar sizes, which is in good agreement with the size distribution histogram that presents a sharp size distribution. Figure 2. STEM-HAADF micrographs of Au/TiO2 nanocomposites (produced at 60 °C and Au loading of 0.05 wt. %) at different scales (a) 50 and (b) 200 nm; detail of Au nanoparticle over TiO2 Figure 2. STEM-HAADF micrographs of Au/TiO 2 nanocomposites (produced at 60 ◦ C and Au loading of 0.05 wt. %) at different scales ( a ) 50 and ( b ) 200 nm; detail of Au nanoparticle over TiO 2 nanoparticles’ surface and single Au nanoparticle amplification (inset) ( c ); size distribution of 400 Au nanoparticles with the respective average size (d). Catalysts 2020,10, 234 5 of 20 The STEM-HAADFenergy-dispersive X-ray spectroscopy (EDX) measurements allowed us to identify the elements present in the Au/TiO 2 sample in two different points, 1 and 2 (signaled in Figure 3a). STEM-HAADF-EDX spectra in Figure 3b in point 1 indicate the presence of Au and Cu (copper), which can be respectively addressed to Au nanoparticles and copper grid. In point 2, the signatures of Ti (titanium) and O (oxygen) were identified, corresponding to TiO 2 nanoparticles. Thus, EDX measurements confirmed the presence of all the elements of the Au/TiO2nanocomposite. Catalysts 2020, 10, x FOR PEER REVIEW 5 of 20 nanoparticles’ surface and single Au nanoparticle amplification (inset) (c); size distribution of 400 Au nanoparticles with the respective average size (d). The STEM-HAADFenergy-dispersive X-ray spectroscopy (EDX) measurements allowed us to identify the elements present in the Au/TiO2 sample in two different points, 1 and 2 (signaled in Figure 3a). STEM-HAADF-EDX spectra in Figure 3b in point 1 indicate the presence of Au and Cu (copper), which can be respectively addressed to Au nanoparticles and copper grid. In point 2, the signatures of Ti (titanium) and O (oxygen) were identified, corresponding to TiO2 nanoparticles. Thus, EDX measurements confirmed the presence of all the elements of the Au/TiO2 nanocomposite. Figure 3. The STEM-HAADFenergy-dispersive X-ray spectroscopy (EDX) image of Au/TiO2 nanocomposites with the identification of the measured points: Au (1) and TiO2 (2) (a), EDX spectra with elemental identification (Au, Ti, O, and C) for points 1 and 2 (b). The Au/TiO2 nanocomposite synthesized at 60 °C and with an Au loading of 0.05 wt. % was used. X-ray diffraction was performed to assess the crystal structure of the pure TiO2 nanoparticles and Au/TiO2 nanocomposite, Figure 4a. Both samples show the typical reflexes from anatase (25.3°, 37.8°, and 48.0°) and rutile (27.49°). There was no significant difference between the intensities or positions of the reflexes from both samples. Moreover, no reflexes of Au were detected, which can be explained by the low amount of Au present in the nanocomposite (below detection limit). Figure 4b shows the study of hydrodynamic size for TiO2 and Au/TiO2 nanocomposites obtained by dynamic light scattering (DLS). The results indicated nanoparticles diameters of 1023 nm and 342 nm, for the pristine TiO2 and the Au/TiO2 nanocomposites, respectively. The results suggest that the presence of Au nanoparticles over TiO2 nanoparticles surface may prevent the formation of nanoparticles’ aggregates. On the other hand, the size distribution was broader for the nanocomposites regarding the pristine TiO2. Previous work equally showed that the presence of erbium (Er) on TiO2 nanoparticles contributed to reducing the hydrodynamic size when compared with bare TiO2 [15]. Figure 3. The STEM-HAADFenergy-dispersive X-ray spectroscopy (EDX) image of Au/TiO 2 nanocomposites with the identification of the measured points: Au (1) and TiO 2 (2) ( a ), EDX spectra with elemental identification (Au, Ti, O, and C) for points 1 and 2 ( b ). The Au/TiO 2 nanocomposite synthesized at 60 ◦C and with an Au loading of 0.05 wt. % was used. X-ray diffraction was performed to assess the crystal structure of the pure TiO 2 nanoparticles and Au/TiO 2 nanocomposite, Figure 4a. Both samples show the typical reflexes from anatase (25.3 ◦ , 37.8 ◦ , and 48.0 ◦ ) and rutile (27.49 ◦ ). There was no significant difference between the intensities or positions of the reflexes from both samples. Moreover, no reflexes of Au were detected, which can be explained by the low amount of Au present in the nanocomposite (below detection limit). Figure 4b shows the study of hydrodynamic size for TiO 2 and Au/TiO 2 nanocomposites obtained by dynamic light scattering (DLS). The results indicated nanoparticles diameters of 1023 nm and 342 nm, for the pristine TiO 2 and the Au/TiO 2 nanocomposites, respectively. The results suggest that the presence of Au nanoparticles over TiO 2 nanoparticles surface may prevent the formation of nanoparticles’ aggregates. On the other hand, the size distribution was broader for the nanocomposites regarding the pristine TiO 2 . Previous work equally showed that the presence of erbium (Er) on TiO 2 nanoparticles contributed to reducing the hydrodynamic size when compared with bare TiO2[15]. The zeta potential was studied at different pH values (3, 5, 7, 9, and 11) for TiO 2 and Au/TiO 2 samples and the results are displayed in Figure 4c. The pristine and the Au/TiO 2 presented very similar profiles, with higher zeta potential values ≈ |20|mV for pH below 3 and 9. These data were in good agreement with the literature [ 45 ], with positive zeta potential values for acidic conditions and negative values for basic pH. The more significant difference between the two samples occurred at pH =7, with the nanocomposite presenting higher zeta potential values than the pure TiO 2 . Higher zeta potential values mean that nanoparticles possess higher periphery surface charge, which promotes nanoparticles’ repulsions, avoiding aggregates’ formation and enhanced stability [ 46 ]. In this context, and relating it with DLS-obtained results, the smaller hydrodynamic size was probably obtained for the Au/TiO 2 because repulsions endowed by Au on TiO 2 nanoparticles surface prevented the formation of the aggregates. Catalysts 2020,10, 234 6 of 20 Catalysts 2020, 10, x FOR PEER REVIEW 6 of 20 Figure 4. X-ray diffraction reflexes of pristine TiO2 and Au/TiO2 nanocomposite and identification of the representative peaks for anatase (A) and rutile (R) phases (a); dynamic light scattering, intensity size distribution of the pristine TiO2 and the Au/TiO2 nanocomposite and respective Z-average hydrodynamic size (b); zeta potential measurements, performed at different pHs (3, 5, 7, 9, and 11) for pristine TiO2 nanoparticles and Au/TiO2 nanocomposite (c); UV–vis reflectance spectra of pristine TiO2 and Au/TiO2 and (inset) the estimation of the bandgap for both samples at (F(R))1/2 = 0 (d). The Au/TiO2 nanocomposite synthesized at 60 °C and with an Au loading of 0.05 wt. % was used. The zeta potential was studied at different pH values (3, 5, 7, 9, and 11) for TiO2 and Au/TiO2 samples and the results are displayed in Figure 4c. The pristine and the Au/TiO2 presented very similar profiles, with higher zeta potential values ≈ |20| mV for pH below 3 and 9. These data were in good agreement with the literature [45], with positive zeta potential values for acidic conditions and negative values for basic pH. The more significant difference between the two samples occurred at pH = 7, with the nanocomposite presenting higher zeta potential values than the pure TiO2. Higher zeta potential values mean that nanoparticles possess higher periphery surface charge, which promotes nanoparticles’ repulsions, avoiding aggregates’ formation and enhanced stability [46]. In this context, and relating it with DLS-obtained results, the smaller hydrodynamic size was probably obtained for the Au/TiO2 because repulsions endowed by Au on TiO2 nanoparticles surface prevented the formation of the aggregates. To understand the differences in the photocatalytic performance of TiO2 and Au/TiO2 nanocomposite, the optical properties of these materials were studied by UV-visible diffuse reflectance spectra (DRS), depicted in Figure 4d. In the visible range (400–800 nm), the pure TiO2 nanoparticles reflect the radiation almost entirely (≈ 95%). However, the nanocomposite displays reflectance below 64% for the same range. Additionally, a minimum reflectance (≈ 44%) was obtained at 545 nm, indicating a maximum of absorbance band that can be associated with the surface plasmon of Au nanoparticles, typically in the wavelength range between 520 and 560 nm [47,48]. These results show that the nanocomposite presented a broad absorbance spectrum when compared to the pristine Figure 4. X-ray diffraction reflexes of pristine TiO 2 and Au/TiO 2 nanocomposite and identification of the representative peaks for anatase (A) and rutile (R) phases ( a ); dynamic light scattering, intensity size distribution of the pristine TiO 2 and the Au/TiO 2 nanocomposite and respective Z-average hydrodynamic size ( b ); zeta potential measurements, performed at different pHs (3, 5, 7, 9, and 11) for pristine TiO 2 nanoparticles and Au/TiO 2 nanocomposite ( c ); UV–vis reflectance spectra of pristine TiO 2 and Au/TiO 2 and (inset) the estimation of the bandgap for both samples at (F(R))1/2=0 ( d ). The Au/TiO2nanocomposite synthesized at 60 ◦C and with an Au loading of 0.05 wt. % was used. To understand the differences in the photocatalytic performance of TiO 2 and Au/TiO 2 nanocomposite, the optical properties of these materials were studied by UV-visible diffuse reflectance spectra (DRS), depicted in Figure 4d. In the visible range (400–800 nm), the pure TiO 2 nanoparticles reflect the radiation almost entirely ( ≈ 95%). However, the nanocomposite displays reflectance below 64% for the same range. Additionally, a minimum reflectance ( ≈ 44%) was obtained at 545 nm, indicating a maximum of absorbance band that can be associated with the surface plasmon of Au nanoparticles, typically in the wavelength range between 520 and 560 nm [ 47 , 48 ]. These results show that the nanocomposite presented a broad absorbance spectrum when compared to the pristine TiO 2 nanoparticles, which is also consistent with the purple/pink color exhibited by the produced nanocomposite. In the ultraviolet range (200–400 nm), both samples showed similar behavior. From DRS spectra it was possible to estimate the band gap, shown in the inset graph of Figure 4d, for pure TiO 2 and Au/TiO 2 nanocomposite was converting the reflectance to Kubelka–Munk units through Equation (1) and Equation (2). The obtained values show that the nanocomposites possessed a lower bandgap (2.84 eV) than the pristine TiO 2 nanoparticles (2.96 eV). The decrease of the bandgap in Au/TiO 2 was related to the shift absorption to longer wavelengths. Similar results have been reported in the literature [49,50]. Catalysts 2020,10, 234 7 of 20 2.2. Nanocomposites’ Optimization and Photocatalytic Experiments The photocatalytic activity of all the produced Au/TiO 2 nanocomposites was assessed by monitoring the degradation of CIP under artificial UV and visible irradiations. Process conditions were varied depending on the studying purposes. Nanocomposite Optimization As gold is a noble metal, cost-effectiveness should be considered, and the amount of gold used in the nanocomposite is one of the most paramount parameters. In this study, Au loading was varied by using different concentrations of the gold precursor. The tested Au loadings were 0.025, 0.05, 0.1, 0.25, and 0.5 wt. %. These nanocomposites were employed for the photocatalytic degradation of CIP under both UV and simulated visible radiation. Figure 5a shows the data of photocatalytic experiments under UV light. Accordingly, all produced samples and the pristine TiO 2 used as a control showed photocatalytic activity, proven by the decrease of CIP concentration along with the irradiation time. As confirmed by the diffuse reflectance spectroscopy (Section 2.1), the bandgap of the nanocomposites was 2.84 eV, corresponding to the wavelength of 437 nm. Here, the used UV lamp had the mode wavelength of 365 nm, which was shorter than the bandgap. It means that the photon energy was adequate to excite the photocatalytic materials, and photocatalytic reaction occurred in all experiments. Pristine TiO 2 was compared with the synthesized photocatalysts. After 30 min, 77% of CIP was degraded in the presence of pristine TiO 2 , whereas higher degradation of 80–90% was achieved in the same time of irradiation, using the synthesized photocatalysts, 0.05 wt. %. This efficiency can be assigned to the presence of gold particles on the surface of the photocatalysts, confirmed by the TEM and EDX characterization (Section 2.1). The further quantitative inspection was obtained using the Langmuir–Hinshelwood kinetics (Equation (3)), and data are shown in Table 1. The apparent reaction rate constant k of the experiment with the bare TiO 2 was found to be 0.047 min −1 , while the decoration with gold particles improved the photocatalytic activity by 2–3 times. As predicted, in the presence of gold, the excited electrons may be conducted to the gold particles, and the electron-hole recombination may be reduced, which prolongs the lifetime of generated holes [51,52]. Consequently, the photocatalytic activity of the composites increased. Additionally, the increase of the used chloroauric acid concentration might induce a more significant number of gold particles distributed on the TiO 2 surface. In other words, the number of electron absorption centers was increased, which explains the increase of k from 0.078 to 0.131 min −1 when increasing the Au loading from 0.025 to 0.5 wt. %. However, the further increase in the Au loading caused a decrease in k. These results can be addressed to the loss of photocatalytic active sites on the surface of TiO 2 nanoparticles. Based on the TEM images shown in Figure 1, when the Au loading was very high both the amount and the size of Au nanoparticles over the surface of TiO 2 nanoparticles were larger, which contributed to a reduction of the adsorption and probably to mitigate the radiation absorbance by the catalytic nanoparticles. Together, these limitations contributed to reducing the photocatalytic efficiency of the nanocomposite towards the samples with lower amounts of Au and demonstrated the relevance of optimizing the Au loading. Catalysts 2020,10, 234 8 of 20 Catalysts 2020, 10, x FOR PEER REVIEW 8 of 20 Figure 5. Photocatalytic degradation of ciprofloxacin (5 mg/L) with bare TiO2 and Au/TiO2 nanocomposite with different Au concentrations under 30 and 180 minutes of UV (a) and visible (b) radiation. The degradation with bare TiO2 and Au/TiO2 nanocomposites synthesized at different temperatures and Au loading of 0.05 wt. % under 30 and 180 minutes of UV (c) and simulated visible light radiation (d), respectively. Consequently, the photocatalytic activity of the composites increased. Additionally, the increase of the used chloroauric acid concentration might induce a more significant number of gold particles distributed on the TiO2 surface. In other words, the number of electron absorption centers was increased, which explains the increase of k from 0.078 to 0.131 min-1 when increasing the Au loading from 0.025 to 0.5 wt. %. However, the further increase in the Au loading caused a decrease in k. These results can be addressed to the loss of photocatalytic active sites on the surface of TiO2 nanoparticles. Based on the TEM images shown in Figure 1, when the Au loading was very high both the amount and the size of Au nanoparticles over the surface of TiO2 nanoparticles were larger, which contributed to a reduction of the adsorption and probably to mitigate the radiation absorbance by the catalytic nanoparticles. Together, these limitations contributed to reducing the photocatalytic efficiency of the nanocomposite towards the samples with lower amounts of Au and demonstrated the relevance of optimizing the Au loading. The photocatalytic assays performed under visible illumination are shown in Figure 6b. Regarding these assays, it is essential first to mention the controls (Supplementary Material, Figures S3 and S4), which have shown that the CIP solution was stable under simulated visible radiation, demonstrating its photostability. Moreover, another control was performed by adding the Au/TiO2 nanocomposites to CIP solution in the dark for 180 minutes. In this case, approximately 11% of CIP was removed from the solution by adsorption to the Au/TiO2 nanocomposites. With the information from controls, it is possible to understand the photocatalytic efficiency of the tested materials better. Similarly, to the UV light experiments, the degradation rates of all produced nanocomposites were faster than that with the bare TiO2. TiO2 could remove ≈ 33% of CIP after 180 min of simulated visible irradiation. This CIP removal may be assigned to adsorption, confirmed by controls performed in the dark (as above mentioned). Additionally, the sun simulator device had a small percentage (≈ 3%) of UV radiation (to mimic sunlight radiation). This radiation can induce a low photocatalytic activity on bare TiO2, which, together with the adsorption of CIP, is responsible for its removal from the solution. Figure 5. Photocatalytic degradation of ciprofloxacin (5 mg/L) with bare TiO 2 and Au/TiO 2 nanocomposite with different Au concentrations under 30 and 180 minutes of UV ( a ) and visible ( b ) radiation. The degradation with bare TiO 2 and Au/TiO 2 nanocomposites synthesized at different temperatures and Au loading of 0.05 wt. % under 30 and 180 minutes of UV ( c ) and simulated visible light radiation (d), respectively. Table 1. Apparent reaction rates (k) for photocatalytic degradation of ciprofloxacin (CIP) (5 mg/L) with bare TiO 2 and Au/TiO 2 nanocomposite with different Au loadings, over 30 and 180 minutes of UV and simulated visible radiation, respectively. Au loading (wt. %) UV Simulated Visible k(min−1)k(h−1) 0 0.047 0.073 0.025 0.078 0.211 0.05 0.099 0.242 0.1 0.131 0.211 0.25 0.089 0.195 0.5 0.076 0.202 The photocatalytic assays performed under visible illumination are shown in Figure 6b. Regarding these assays, it is essential first to mention the controls (Supplementary Material, Figures S3 and S4), which have shown that the CIP solution was stable under simulated visible radiation, demonstrating its photostability. Moreover, another control was performed by adding the Au/TiO 2 nanocomposites to CIP solution in the dark for 180 minutes. In this case, approximately 11% of CIP was removed from the solution by adsorption to the Au/TiO2nanocomposites. Catalysts 2020,10, 234 9 of 20 Catalysts 2020, 10, x FOR PEER REVIEW 10 of 20 Both under UV and visible radiation, another control was performed (Figures 5c,d) by testing single Au nanoparticles at the very same amount of Au (corresponding to 0.05 wt. % obtained at 60 °C) and TiO2 nanoparticles on CIP degradation. The results confirmed that the photocatalytic efficiency obtained by the nanocomposites should be assigned to the interface between Au and the TiO2 surface. 2.3. Photocatalytic Degradation The rate of photocatalytic degradation depends on the availability of the catalyst surface for the photo-generation of electron-hole pairs that produce hydroxyl radicals. Thus, in these experiments, the amount of catalyst was kept constant, and the number of hydroxyl radicals generated remained the same, while CIP concentration increased. The influence of CIP initial concentration of 5, 10, and 25 mg/L was studied under visible irradiation. It was observed that the CIP concentration impacted by the degradation rate and efficiency (Figure 6). With the lowest CIP concentration, 40% of CIP degradation was obtained after 30 min. With the increase of concentrations by 2 and 5 times, the efficiencies achieved were 22% and 8%, respectively. In these tests, while using the photocatalyst concentration of 0.3 g/L, the adsorption of the CIP on the Au/TiO2 nanoparticles surface might be halted due to surface saturation. Additionally, the presence of organic compounds such as CIP can generate an increased number of intermediates and products, which will compete with CIP for adsorption on the photocatalyst surface [57]. This competition caused a lower reaction rate for high CIP concentration. The following assays, focused on the photocatalytic activity of the produced nanocomposites, were performed using the lowest CIP concentration, 5 mg/L. Figure 6. Degradation efficiency (%) (a) and ln (C/C0) vs. time (b) for different initial ciprofloxacin concentrations (5, 10, and 25 mg/L), using Au/TiO2 nanocomposites produced at 60 °C and with an Au loading of 0.05 wt. %, under 3 hours of simulated visible radiation. Photocatalytic degradation of ciprofloxacin (5 mg/L) in 45 mL of aqueous solution with different Au/TiO2 concentrations (0.1, 0.3, 1.0, and 1.3 g/L). The Au/TiO2 nanocomposite synthesized at 60 °C and with an Au loading of 0.05 wt. % was used. The tests were performed over 30 minutes under UV irradiation (c). Table 2. Apparent reaction rates (k) for photocatalytic degradation of CIP (5 mg/L) with bare TiO2 and Au/TiO 2 nanocomposite synthesized at different temperatures, over 30 and 180 minutes of UV and simulated visible radiation, respectively. The Au loading of 0.05 wt. % was used for the tested materials. Temperature (°C) UV Simulated visible k (min-1) Degradation (%) k (h-1) Degradation (%) TiO2 0.66 80 0.073 20 25 0.131 88 0.221 48 60 0.117 91 0.226 49 80 0.047 80 0.176 41 Figure 6. Degradation efficiency (%) ( a ) and ln (C/C0) vs. time ( b ) for different initial ciprofloxacin concentrations (5, 10, and 25 mg/L), using Au/TiO 2 nanocomposites produced at 60 ◦ C and with an Au loading of 0.05 wt. %, under 3 hours of simulated visible radiation. Photocatalytic degradation of ciprofloxacin (5 mg/L) in 45 mL of aqueous solution with different Au/TiO 2 concentrations (0.1, 0.3, 1.0, and 1.3 g/L). The Au/TiO 2 nanocomposite synthesized at 60 ◦ C and with an Au loading of 0.05 wt. % was used. The tests were performed over 30 minutes under UV irradiation (c). With the information from controls, it is possible to understand the photocatalytic efficiency of the tested materials better. Similarly, to the UV light experiments, the degradation rates of all produced nanocomposites were faster than that with the bare TiO 2 . TiO 2 could remove ≈ 33% of CIP after 180 min of simulated visible irradiation. This CIP removal may be assigned to adsorption, confirmed by controls performed in the dark (as above mentioned). Additionally, the sun simulator device had a small percentage ( ≈ 3%) of UV radiation (to mimic sunlight radiation). This radiation can induce a low photocatalytic activity on bare TiO 2 , which, together with the adsorption of CIP, is responsible for its removal from the solution. More importantly, the decoration of gold particles on the TiO 2 surface resulted in the faster degradation rate of CIP under visible radiation. The bandgap of the composites was lowered, from 2.96 eV to 2.84 eV (Section 2.1). Similar results were obtained for methylene blue degradation using Au/TiO 2 nanoparticles. The authors obtained higher degradation efficiencies and ability to use visible radiation [ 37 ]. Thus, the materials could absorb the longer wavelength in the visible range (up to 437 nm). The reaction rates’ constant increased from 0.073 h −1 , without Au, to 0.195 − 0.224 h −1 , with different Au loadings (Table 1). The obtained results, for UV and visible radiation, confirmed that the photocatalytic efficiency of the TiO 2 nanoparticles was enhanced with the Au loading, until a specific plateau. When the Au loading was higher than 0.1 and 0.05 wt. %, respectively, for UV and visible radiation, the gold nanoparticles can block the surface-active sites of TiO 2 nanoparticles [ 53 , 54 ]. Furthermore, an excessive amount of Au nanoparticles can play as recombination centers for photo-induced electrons and holes. Both situations can contribute to a significant reduction of pollutant adsorption and, consequently, the photocatalytic efficiency [ 55 ]. The remaining assays of this study will be performed with an Au loading of 0.05 wt. %. Another critical parameter that is worth to stress and study is temperature, which can affect the surface charge phenomenon and the dispersity of the TiO 2 particles in the solution during the synthesis. It can also influence the nucleation and the gold particles’ crystal growth on the TiO 2 nanoparticle surface. In this study, the synthesis was operated at 25, 60, and 80 ◦ C, and the photocatalytic degradation of CIP, with the nanocomposites produced at different temperatures, was performed under UV and visible radiation (Figure 5c,d and Table 2). Regardless of the synthesis temperature, the photocatalytic activity of the nanocomposites (Au loading =0.05 wt. %) was equal or higher than that of the bare TiO 2 . Here, the synthesis at the room and medium temperatures (25 and 60 ◦ C) yielded the more efficient photocatalytic materials, for UV and visible radiation, towards higher temperature synthesis (80 ◦C). Catalysts 2020,10, 234 16 of 20 CIP was assessed using the same amounts of Au and TiO 2 nanoparticles, not as a nanocomposite, but separately added to the solution. The photocatalytic efficiencies were tested by degrading CIP in aqueous solution under UV and visible radiation and monitoring the maximum absorption peak (277 nm) using a Shimadzu UV-2501PC UV/Vis spectrophotometer. The degradation fit the Langmuir-Hinshelwood model, expressed by Equation (3): C/C0=exp−kt (3) where C 0 and Crepresent the concentration of the pollutant at time 0 min and at time t, respectively, and kis the first-order rate constant of the reaction. 5. Conclusions An Au/TiO 2 nanocomposite was produced, characterized, and applied in the photocatalytic degradation of ciprofloxacin (CIP). The characterization results changing the synthesis conditions (temperature and Au loading) indicated that the synthesis performed at 60 ◦ C with the Au loading of 0.05 wt. % yielded the most homogeneous distribution of Au nanoparticles ( ≈ 3 nm) over TiO 2 nanoparticles surface, after TEM inspection. Additionally, these samples absorbed more radiation in the visible range ( ≈ 66% at 545 nm) and presented a lower bandgap (2.84 eV vs. 2.96 eV from bare TiO 2 ). The photocatalytic results confirmed that all the manufactured nanocomposites possessed higher photocatalytic efficiency in the UV and simulated visible radiation towards the pristine TiO 2 . It was also possible to understand the impact of the synthesis parameters envisaging the optimal photocatalytic efficiency conditions. In this way, with the Au/TiO 2 nanocomposite, it was possible to enhance the photocatalytic degradation efficiency in 13% and 145% under UV and simulated visible light radiation, respectively. The gold nanoislands played a paramount role transferring electrons from Au to the anatase from TiO 2 nanoparticles. Additionally, Au endowed the nanocomposite with the ability to absorb the visible radiation. Computational modeling supported the experimental data, showing the ability of Au to bind TiO 2 anatase surfaces as well as the relevant role of Au transferring electrons. The fundamental importance of the interface between TiO 2 and Au nanoparticles regarding the enhanced photocatalytic activity was also rationalized. Moreover, models indicated a high affinity of CIP to both Au and TiO 2 surfaces, which favors the adsorption process and consequently may also be cause for enhanced photocatalytic efficiency in the presence of Au nanoparticles. According to the results obtained through systematic experimental data and modeling results, the simple method herein presented constitutes a reliable approach to produce efficient photocatalytic materials. Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/10/2/234/s1: Figure S1: Size distribution of Au nanoparticles for synthesization at 25 and 80 ◦ C; Figure S2: Photostability of CIP solution under UV; Figure S3: Photostability of CIP solution under visible radiation; Figure S4: Synthesis reproducibility on CIP degradation. Author Contributions: Conceptualization, P.M. and S.L.-M.; data curation, P.M., S.K., and H.N.L.; formal analysis, H.N.L., L.P., M.M.-F., and S.L.-M.; investigation, P.M., S.K., H.N.L., and M.M.-F.; methodology, P.M., S.K., and V.S.; project administration, S.L.-M.; resources, V.S., M.A., G.C., and S.L.-M.; software, M.M.-F.; supervision, K.K., M.A., G.C., and S.L.-M.; validation, V.S.; visualization, P.M. and M.M.-F.; writing—original draft, P.M. and H.N.L.; writing—review & editing, P.M., M.A., and S.L.-M. All authors have read and agreed to the published version of the manuscript. Funding: The authors acknowledge funding from the Basque Government Industry Department under the ELKARTEK Program and the Spanish Ministry of Economy and Competitiveness (MINECO) through the project MAT2016-76039-C4-3-R (AEI/FEDER, UE) (including the FEDER financial support). This work was also supported by the Graduate Academy of the Technische Universität Dresden. Centro de Investigacion Biom é dica en Red – Bioengenhar í a, Biomateriales e Nanomedicina (CIBER-BBN) is an initiative funded by the 6th National R&D&i Plan 2008–2011, Iniciativa Ingenio 2010, Consolider Program, and CIBER Actions and financed by the Instituto de Salud Carlos III (Spain) with assistance from the European Regional Development Fund. S. Kappert and H.N. Le acknowledge fruitful discussions with Nadia Licciardello. Catalysts 2020,10, 234 17 of 20 Acknowledgments: This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the strategic projects UID/FIS/04650/2013 by Fundo Europeu de Desenvolvimento Regional (FEDER) funds through the COMPETE 2020—Programa Operacional Competitividade e Internacionalizaç ã o (POCI) with the reference project POCI-01-0145-FEDER-006941, project PTDC/CTM-ENE/5387/2014, as well as UID/BIO/04469 unit through COMPETE 2020 (POCI-01-0145-FEDER-006684) and BioTecNorte operation (NORTE-01-0145-FEDER-000004) funded by the European Regional Development Fund under the scope of Norte2020—Programa Operacional Regional do Norte. P.M. Martins thanks the FCT for the grant SFRH/BD/98616/2013 and Luciana Pereira for the grant SFRH/BPD/110235/2015. M. 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