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International Journal of Photoenergy
Volume 2016 (2016), Article ID 4054351, 11 pages
Research Article

Ag/AgCl Loaded Bi2WO6 Composite: A Plasmonic Z-Scheme Visible Light-Responsive Photocatalyst

Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur Private, Ottawa, ON, Canada K1N 6N5

Received 16 November 2015; Accepted 24 February 2016

Academic Editor: Ying Dai

Copyright © 2016 Xiangchao Meng and Zisheng Zhang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Hierarchical flower-like Bi2WO6 was successfully synthesized by facile hydrothermal method at low pH. And Ag/AgCl was loaded by photoreduction on its surface. As-prepared photocatalysts were characterized by various techniques. Bi2WO6 was successfully synthesized at a size of 2-3 μm. Depositing Ag/AgCl did not destroy the crystal structure, and both Ag+ and metallic Ag0 were found. The band gap of the composite was 2.57 eV, which indicates that visible light could be the activating irradiation. In the photocatalytic activity test, the composite with 10 wt% Ag/AgCl boasted the highest removal efficiency (almost 100%) in 45 min. The significant enhancement can be attributed to the surface plasmon resonance (SPR) effect and the establishment of heterostructures between Ag/AgCl and Bi2WO6. A possible mechanism of photocatalytic oxidation in the presence of Ag/AgCl-Bi2WO6 was proposed. This work sheds light on the potential applications of plasmonic metals in photocatalysis to enhance their activities.

1. Introduction

Surface plasmon resonance (SPR) has been widely applied in the conversion of solar to chemical energy. It may be described as the resonant photon-induced collective oscillation of valence electrons when the frequency of photons matches the natural frequency of surface electrons oscillating against the restoring force of positive nuclei [1]. The wavelength of a resonant photon depends on the type of metal. For silver nanoparticles, surface electrons may be activated by ultraviolet (UV) and visible (Vis) photons.

Ag/AgCl was first reported by Wang et al. to exhibit an excellent photocatalytic activity due to the SPR effect [2]. This study shed light on the potential implementation of Ag/AgCl for the photocatalytic oxidation of organic pollutants. To date, researchers have designed composites by depositing Ag/AgCl on photocatalysts, such as Ag/AgCl-TiO2 [3, 4] and Ag/AgCl-BiOX (X=Cl, Br) [5], aiming to enhance their performances. Meanwhile, numerous novel photocatalytic materials have also been reported. Among these materials, Bi2WO6, with a band gap of 2.70 eV, has been found to exhibit a high visible-light-driven photocatalytic activity with regard to the degradation of organic contaminants [6]. This is due to the valence band of bismuth-based photocatalysts consisting of not only O 2p orbits but also Bi 6s orbits. It has been confirmed that moderately well-dispersed orbits are able to accelerate the mobilities of photogenerated carriers and narrower band gap [7]. However, one of the main problems surrounding Bi2WO6 implemented in the degradation of organic pollutants in wastewater is the high recombination rate of photogenerated carriers. One of the most effective approaches to deal with this issue is to establish heterostructures such as BiOBr-Bi2WO6 [8, 9], WO3/Bi2WO6 [10, 11], and Bi2O3/Bi2WO6 [10]. At present, there is still no published work establishing the Ag/AgCl-Bi2WO6 heterostructure applied in the degradation of organic pollutants.

Based on the above analysis, depositing Ag/AgCl on Bi2WO6 should enhance its photocatalytic activity due to the SPR effect and the heterostructure. The depositing method (the photoreduction method) can be implemented as reported [12]. In this work, a facile method of preparing an Ag/AgCl-Bi2WO6 composite was developed. As-synthesized photocatalysts were investigated in terms of their crystal structures, morphologies, optical properties, and visible-light-induced photocatalytic activities with regard to the degradation of the organic contaminant Rhodamine B (RhB). A possible mechanism of the photocatalytic oxidation process in the presence of the Ag/AgCl-Bi2WO6 composite was also explored and proposed. The enhanced photocatalytic activity may be regarded as solid evidence of the potential application of Ag/AgCl in photocatalytic oxidation processes.

2. Experimental

2.1. Preparation of Bi2WO6 and Ag/AgCl-Bi2WO6 Composites

All of the reagents were purchased from the Sigma-Aldrich Company, were of analytical purities, and were used as received. Bi2WO6 was synthesized by a hydrothermal method. In a typical process, 0.9702 g Bi(NO3)3·5H2O were dissolved in 20 mL acetic acid, termed solution A, which was magnetically stirred for 10 min. Next, 0.3298 g Na2WO4·2H2O were dissolved in 40 mL distilled deionized water (DDW), termed solution B. Solution B was added dropwise into solution A and stirred magnetically for 30 min. The suspension was then transferred to a 100 mL Teflon-lined stainless steel autoclave (approximately 60% of its maximum volume) and heated at 180°C for 20 h. After the autoclave was allowed to cool to room temperature, the precipitate was separated by centrifugation and washed once with ethanol and then twice with DDW. The precipitate was then dried at 60°C for 12 h.

The Ag/AgCl-Bi2WO6 composite was prepared by the photoreduction method. Typically, 0.32 g Bi2WO6 were dispersed in 40 mL DDW and sonicated for 30 min. Next, 0.24 g NaCl (excess amount) was added to the suspension, which was termed solution C. Meanwhile, 0.0421 g AgNO3 were added to 20 mL DDW, termed solution D. Solution D was added dropwise to solution C with vigorous stirring for 30 min. Products were separated by centrifugation and dispersed in 40 mL DDW and then illuminated under a 300 W halogen tungsten projector lamp (Ushio) for 30 min. Final products were separated via centrifugation, washed twice with DDW, and dried at 60°C for 12 h. The sample was then collected and labelled as Ag/AgCl (10 wt%)-Bi2WO6. Similarly, Ag/AgCl (1 wt%)-Bi2WO6, Ag/AgCl (2 wt%)-Bi2WO6, Ag/AgCl (4 wt%)-Bi2WO6, Ag/AgCl (8 wt%)-Bi2WO6, and Ag/AgCl (20 wt%)-Bi2WO6 were also prepared.

2.2. Characterization

X-ray diffraction (XRD) analysis was performed using a Rigaku Ultima IV Diffractometer with CuKα radiation ( nm) at 40 kV and 44 mA. Surface elements and the chemical states of samples were tested using a XSAM-800 X-ray Photoelectron Spectroscope (XPS). A field-emission scanning electron microscope (FE-SEM, JEOL JSM-7500F) using energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM, JEM-2100F) were applied to explore the morphologies of samples. Acceleration voltages of the SEM and TEM were 3.00 kV and 200 kV, respectively. The Thermo Evolution 300 spectrophotometer was used to evaluate the ultraviolet-visible (UV-Vis) diffuse reflectance spectra (DRS) of the photocatalysts.

2.3. Photocatalytic Activity Test

The target organic pollutant was Rhodamine B (RhB). The characteristic peak in UV-Vis spectroscopy of RhB is found at a wavelength of 554 nm, which was used to calculate its concentration based on the Beer-Lambert law. A 500 mL beaker with a cooling jacket maintaining the system at 20°C served as the photocatalytic reactor. The visible-light source was a 300 W halogen tungsten projector lamp (Ushio) with a cut-off (Kenko Zeta, transmittance > 90%) to filter out irradiation with a wavelength below 400 nm. The distance between the irradiation source and the top of the beaker was 10 cm, and the irradiation intensity was measured by a quantum meter (Biospherical QSL-2100, 400 nm < < 700 nm) to be 1.1 × 10−2 Einsteins m−2 s−1. In each batch, 100 mL fresh solution with a concentration of RhB at 10 mg/L (10 ppm) was added to the reactor and mixed with as-prepared photocatalysts in the absence of light for 30 min to ensure that adsorption-desorption equilibrium was achieved. The quantity of photocatalyst added was 0.5 g/L. After mixing, the light was turned on to begin the photocatalytic process. An aliquot of 1 mL suspension was taken every 3 min for 45 min and tested using a Genysys 10-UV spectrophotometer (Geneq Inc.). To explore the roles of radical species, EDTA-2Na and 2-Butanol with concentrations of 0.01 mol/L were added to the reactor.

To study the degradation of phenol, 100 mL phenol solution with a concentration of 10 mg/L (ppm) was mixed with an Ag/AgCl-Bi2WO6 composite. Samples taken during the photocatalytic process were tested using UV-Vis spectroscopy with the peak absorption at a wavelength of 270 nm. Other procedures used the same process as that of the degradation of RhB.

To recycle the photocatalysts, photocatalysts were separated by centrifugation between experiments and then used in the following experiment without a wash. The final used products were separated via centrifugation and dried at 60°C for 12 h. Products were collected and tested using XRD to measure their crystal stabilities.

3. Results and Discussions

3.1. XRD and XPS

The X-ray diffraction (XRD) patterns of as-prepared Ag/AgCl-Bi2WO6 composites and pure Bi2WO6 and Ag/AgCl are shown in Figure 1. The diffraction peaks of pure Bi2WO6 agreed well with the orthorhombic structure (space group: Pbca (61), PDF card number 00-039-0256) [13]. Specifically, peaks at = 28.3°, 32.7°, 47.1°, 55.8°, and 58.54° were assigned to the (131), (060)/(200)/(002), (260)/(202), (191)/(331)/(133), and (262) planes of Bi2WO6, respectively. Samples with deposit amounts of Ag/AgCl larger than and including 4 wt% showed characteristic peaks of Ag and AgCl. As for the small amount of Ag0, the peak was weak. In Figure 2, the XRD pattern of Ag/AgCl (10 wt%)-Bi2WO6 was enlarged and each species in the composite was identified. Excluding peaks from Bi2WO6, peaks at = 27.8° (Figure 2(a)), 32.2°, 46.2°, 54.8°, and 57.5° (Figure 2(c)) were found, indicating the existence of AgCl (cubic structure, space group Fm-3m (225), PDF card number 00-006-0480). Meanwhile, the peak at = 38.2° (Figure 2(b)) was attributed to the (111) crystal plane of Ag (PDF card number 01-087-0719). The lattice parameters of Bi2WO6 are  Å,  Å, and  Å. In regard to these parameters shown in Table 1, negligible differences between them were observed. This indicates that loading of Ag/AgCl may not destroy the main structure of Bi2WO6 and merely laid upon its surface as opposed to covalently anchoring to its lattice [14].

Table 1: Lattice parameters of Bi2WO6 and Ag/AgCl-Bi2WO6 composites.
Figure 1: XRD patterns of as-prepared samples: pure Bi2WO6, Ag/AgCl, and Ag/AgCl-Bi2WO6 composites.
Figure 2: XRD pattern of Ag/AgCl (10 wt%)-Bi2WO6.

The elemental compositions and chemical states of Ag/AgCl-Bi2WO6 composites were determined by X-ray photoelectron spectroscopy. In Figure 3(a), survey spectra and high-resolution scans of typical orbits in Ag/AgCl (10 wt%)-Bi2WO6 are illustrated. Specifically in the survey spectra, all elements of the composite (Bi, W, O, Ag, and Cl) were demonstrated. The spectra were calibrated using C 1s, the binding energy of which was 284.6 eV. As for high-resolution XPS spectra in Figure 3(b), two strong peaks centered at 164.50 eV and 159.19 eV indicate that the valence of Bi in the composite was +2 [15]. Furthermore, binding energies of 37.57 eV and 35.41 eV for W 4f5/2 and W 4f7/2, respectively, were found in Figure 3(c), which were confirmed W in the composite with valence of +6 [15, 16]. The largest peak in Figure 3(d) could be separated into 3 smaller peaks which are corresponding well with O in the composite with forms of Bi2WO6 at 530.28 eV, –OH at 531.28 eV, and H2O at 531.96 eV. The existence of a negligible fraction of hydroxyl anion and H2O in the composite may be attributed to the reaction of atmospheric water with the oxide surface and the chemisorbed water vapor on top of the oxide surface [17]. Silver peaks at 373.39 eV and 367.30 eV in Figure 3(e) were ascribed to binding energies of Ag 3d3/2 and Ag 3d5/2, respectively, in the presence of Ag+ in AgCl [18]. Smaller peaks located at 374.39 eV and 368.32 eV for binding energies of Ag 3d3/2 and Ag 3d5/2 were assigned to that of metallic Ag0 [18]. This may suggest that metallic Ag0 can be found in the composite.

Figure 3: XPS spectra of Ag/AgCl (10 wt%)-Bi2WO6 composite: (a) survey spectra, and high-resolution orbits scan of (b) Bi 4f, (c) W 4f, (d) O 1s, (e) Ag 3d, and (f) Cl 2p.
3.2. SEM, EDS, and TEM

Morphologies of as-prepared samples were measured by SEM and TEM, the results of which are shown in Figure 4. Bi2WO6 synthesized by the hydrothermal method described in this work exhibited a nanoplate-built hierarchical flower-like structure (Figure 4(a)). Similar structures were also reported in the literatures [19, 20]. A potential mechanism for the formation of this superstructure is shown in Scheme 1. The scheme shows that an irregular structure is formed through self-aggregation at the first step. As for the low pH (<1), and ions are generated in solution and may be nucleated onto protuberances on the surface of solid spheres. This dissolution-recrystallization process is called Ostwald ripening. The plate-like structure is owed to the intrinsic anisotropic growth habits of Bi2WO6 [21]. According to the Gibbs-Thomson law, larger particles grow at the cost of small particles as for the energy difference in solubility. Flower-like superstructures eventually self-organize after sufficient duration of hydrothermal treatment. The size of each hierarchical microsphere was around 2-3 μm. Ag/AgCl growing on the surface of Bi2WO6 is shown in Figure 4(b). Smaller plate-like AgCl could be observed as randomly loaded on its surface and metallic silver (Ag0) was not observed. However, Ag0 was shown to be present in the composite based on TEM imaging results (Figure 4(c)). In the inset of Figure 4(c), nanoparticles with a size of no more than 5 nm could possibly be identified as metallic silver (Ag0). Furthermore, EDS was performed on the composite and is illustrated in Figure 5. Characteristic peaks of all elements in the composite were found, which further confirms the successful synthesis of Ag/AgCl-Bi2WO6 in this work.

Scheme 1: Potential mechanism of forming the hierarchical flower-like superstructure of Bi2WO6.
Figure 4: SEM images of pure Bi2WO6 (a) and Ag/AgCl (10 wt%)-Bi2WO6 composite (b), and TEM images of Ag/AgCl (10 wt%)-Bi2WO6 composite (c).
Figure 5: EDS of Ag/AgCl (10 wt%)-Bi2WO6 composite.
3.3. UV-Vis Diffused Reflectance Spectra (DRS)

The optical absorbent properties of the Ag/AgCl-Bi2WO6 composite were measured using UV-Vis diffused reflectance spectroscopy. Spectra of pure Bi2WO6, Ag/AgCl, and Ag/AgCl (10 wt%)-Bi2WO6 are illustrated in Figure 6(a). After loading Ag/AgCl onto Bi2WO6, the absorbent ability of the photocatalyst in the visible-light range was enhanced. This is mainly due to the plasmon resonance of photoreduced silver (Ag0) from AgCl [2], which could also be evidence for the existence of Ag0 in the composite. To further explore the band gap () of samples, the classical Tauc equation (1) was employed as follows:where , , , and are the absorption coefficient, the Planck constant, the irradiation frequency, and the constant for semiconductors (usually equal to 1), respectively. The constant depends on the type of band gap, where is for direct transition and is for indirect transition. The band gap of the as-prepared Bi2WO6 in this work is characteristic of a direct transition (Figure 7), so is equal to 4. By plotting versus as illustrated in Figure 6(b), the intercept at the horizontal axis is the band gap value. This indicates that Ag/AgCl in composites narrowed the band gap from 2.70 eV to 2.57 eV compared to pure Bi2WO6, which could widen the edge wavelength of incidence from 459.3 nm to 482.5 nm. This indicates that Ag/AgCl-Bi2WO6 composites could be effectively activated under visible-light irradiation.

Figure 6: (a) UV-Vis diffused reflectance spectra and (b) curves of as-prepared pure Bi2WO6, Ag/AgCl, and Ag/AgCl (10 wt%)-Bi2WO6.
Figure 7: Band gap of Bi2WO6 (simulated by Quantum Espresso [22]).
3.4. Photocatalytic Activity Test under Visible Light

Photocatalytic activities of as-prepared samples were tested by degrading RhB (10 mg/L). All of the screening test results are plotted in Figure 8. The removal efficiencies for pure Bi2WO6 and Ag/AgCl were 38.8% and 65.3% in 45 min, respectively. By increasing the loading amount of Ag/AgCl in the composites from 1 wt% to 10 wt%, removal efficiencies of the composites were increased up to 100% after 45 min under visible light. This enhancement effect may be attributed to the establishment of a heterojunction between Ag/AgCl and Bi2WO6 [3, 14, 23]. However, further increasing the amount of Ag/AgCl in the composites deteriorates the photocatalytic degradation performances. This may be interpreted as larger nanoparticles which are loaded onto Bi2WO6 possibly weakening the anchoring forces between them and destroying the heterostructure.

Figure 8: Photocatalytic degradation of RhB (10 mg/L) under visible light with as-prepared pure Bi2WO6, Ag/AgCl, and Ag/AgCl-Bi2WO6 composites.

Langmuir-Hinshelwood kinetic analysis was applied to RhB degradation in the presence of as-prepared photocatalysts. Trials performed with diluted concentrations of RhB ( mol/L) showed that the Langmuir-Hinshelwood model could be simplified to a pseudo-first-order reaction model [24], expressed as follows:where , , and are the time, the initial concentration, and concentration at each specific reaction time of RhB, is the reaction rate constant, is the adsorption coefficient of the pollutant on the photocatalyst, and is pseudo-first-order reaction kinetics constant used as the parameter to evaluate the performances of as-prepared photocatalysts in degrading pollutants. A plot of as a function is found in Figure 9, where the slope of the line of best fit is the pseudo-first-order kinetics constant, the results of which are summarized in Table 2. The excellent linearity of each line of best fit indicates that the photocatalytic degradation of RhB under visible-light irradiation in the presence of Ag/AgCl-Bi2WO6 follows first-order kinetics. The process carried out with pure Bi2WO6 as the photocatalyst exhibited the lowest reaction constant, while loading of Ag/AgCl onto Bi2WO6 increased the constant up to 5.4 times compared to that of pure Bi2WO6 at 10 wt% Ag/AgCl. These results confirm that the presence of Ag/AgCl in the composites significantly increases photocatalytic activity with regard to the degradation of organic pollutants.

Table 2: Pseudo-first-order reaction rate constants for various processes.
Figure 9: Photocatalytic kinetics of each process.

The stepwise mechanism for the degradation of RhB was observed by taking UV-Vis spectra, which are illustrated in Figure 10. In the inset table, the positions of characteristic peaks for each intermediate species are summarized [25]. It can be concluded that two processes of RhB degradation exist, namely, deethylation and destruction of the aromatic ring. In the deethylation process, RhB is decomposed into N,N,N′-Triethyl-Rhodamine (TER), N,N′-Diethyl-Rhodamine (DER), N-Ethyl-Rhodamine (MER), and Rhodamine, the UV-Vis characteristic peaks of which are located at wavelengths of 539 nm, 522 nm, 510 nm, and 498 nm, respectively. In the first 33 min, peak positions blue-shifted from 554 nm to 498 nm, indicating that RhB degraded into Rhodamine. After that, peak positions stabilized at 498 nm and gradually reduced in intensity. This may be attributed to the degradation of Rhodamine into smaller species such as H2O and CO2.

Figure 10: UV-Vis spectra of solutions taken during the photocatalytic process in the presence of Ag/AgCl (10 wt%)-Bi2WO6 (inset table shows characteristic wavelength for each intermediate species in water [25]).

The photocatalytic activity with respect to the degradation of the colourless organic compound, phenol, was investigated and is illustrated in Figure 11. These results aimed to confirm the increased photocatalytic activity in the presence of the Ag/AgCl-Bi2WO6 composite under visible light attributed to the surface plasmon resonance effect instead of photosensitization of the dye (RhB in this work) [26]. The composite performed well in the degradation of phenol (approximately 30% of phenol was converted in 45 min), which was a better performance than that exhibited by pure Bi2WO6 (only about 5% of phenol was converted in 45 min). This confirmed that the enhancement effect of loading Ag/AgCl onto Bi2WO6 exists under visible-light irradiation.

Figure 11: Degradation of phenol (10 mg/L) by using Ag/AgCl-Bi2WO6 composite.
3.5. Roles of Radical Species

Holes (h+) and free radicals, especially , are regarded as the main oxidative species in photocatalytic degradation of organic pollutants [27]. In this study, EDTA-2Na and 2-Butanol were chosen as the hole and hydroxyl radical scavengers, respectively [28]. When holes were quenched, only 15% of RhB was removed (Figure 12). Comparatively, there was little influence when 2-Butanol was added to quench hydroxyl radicals. This suggests that the hydroxyl radical is not the main oxidative species in the degradation of RhB and that holes govern the photocatalytic process. Similar results were also reported in the literature [9, 29]. This phenomenon may be explained knowing that the standard redox potential of Bi2O4/BiO+ (+1.59 eV) is much more negative than that of /–OH (+2.30 eV) [9, 29]. This suggests that “nonselective” could not be produced in this photocatalytic oxidation process.

Figure 12: Photocatalytic activity in presence of Ag/AgCl/-Bi2WO6 and quenchers.
3.6. Reusability

Reusability is a significant parameter used to evaluate photocatalysts implemented in practical applications. As-prepared composites were investigated by taking 4 runs where RhB was degraded. Removal efficiencies after 45 min were plotted in Figure 13. After 4 runs, the removal efficiency decreased slightly from 97.8% to 91.5%, but this relatively high removal efficiency still implies the high stability of the Ag/AgCl-Bi2WO6 composite. Furthermore, used photocatalysts were observed by XRD, the patterns of which are shown in Figure 14. Lattice parameters of Bi2WO6 after four runs were  Å,  Å, and  Å. All the characteristic peaks of Bi2WO6 were found with high intensities, indicating that the main crystal structure of Bi2WO6 was stable and was not destroyed during the recycle. However, the characteristic peaks of AgCl were observed with reduced intensities. On the other hand, intensities of other peaks, especially the peak at = 38.2° (Figure 14(b)), for metallic Ag0, became stronger. This may be attributed to the reduction of AgCl to metallic Ag0 in the photocatalytic process under visible-light irradiation, as photogenerated electrons are produced and easily react with Ag+. This may be an explanation for the decreased removal efficiency of RhB. Another possible explanation is that the intermediate species of RhB were adsorbed on the composite and refractory to be degraded.

Figure 13: Removal efficiencies for 4 runs of 45 min.
Figure 14: XRD patterns of Ag/AgCl (10 wt%)-Bi2WO6 before and after 4 runs.
3.7. Proposed Mechanism of Photocatalytic Degradation

The band structure of the Ag/AgCl-Bi2WO6 composite is illustrated in Figure 15. The proposed mechanism of using the composite to degrade RhB could be summarized as follows: Bi2WO6 with a band gap of 2.70 eV is activated by visible-light irradiation, generating electrons and holes; Ag0 nanoparticles induced by surface plasmon resonance under visible light could produce electrons and leave holes on the valence band [1], and electrons could rapidly transfer to Bi2WO6 or AgCl; RhB could be activated by photons (photosensitization) and generate electrons which could transfer to the conduction band of either Bi2WO6 or AgCl; and Cl could be oxidized into a radical which is supposed to oxidize organic pollutants. Holes on the valence band of Bi2WO6 would transfer to that of AgCl, and the separation efficiency of photogenerated electrons and holes would improve [30]. Meanwhile, electrons induced by the surface plasmon resonance and photosensitization effects would be beneficial to produce more oxidative species. The integrated effect results in an enhanced photocatalytic activity to degrade organic pollutants.

Figure 15: Schematic of possible mechanism of degrading RhB in the presence of Ag/AgCl-Bi2WO6.

4. Conclusion

An Ag/AgCl-Bi2WO6 composite was successfully synthesized using a hydrothermal and photoreduced method. Its morphology was hierarchical and flower-like with a size of 2-3 μm. As-prepared photocatalysts were performed with increased photocatalytic activities and high stabilities when degrading RhB under visible-light irradiation. The 10 wt% Ag/AgCl sample exhibited the best performance. A mechanism of photocatalytic oxidation was proposed, and it was deduced that the enhancement effect is mainly due to the establishment of a heterostructure, as well as the surface plasmon resonance and photosensitization effects. This work is evidence for the potential application of plasmonic metals in photocatalytic oxidation processes with enhancement effects.

Competing Interests

The authors declare that they have no competing interests.


Xiangchao Meng was financially supported by the China Scholarship Council (CSC). This work was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC).


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