Effect of the Cross-Section Morphology in the Antimicrobial Properties of α -Ag 2 WO 4 Rods: An Experimental and Theoretical Study

: In this work, α -Ag 2 WO 4 particles with different cross-sections were obtained using the co-precipitation method at different synthesis temperatures. The samples were characterized by X-ray diffraction (XRD), ﬁeld-scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The antimicrobial activity was analyzed using the Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) methods against the Escherichia coli and Salmonella spp. gram-negative bacteria. The antimicrobial tests against Escherichia coli and Salmonella spp. indicated that concentrations of 2.5–5 mg/mL and 5 mg/mL completely inhibit its growth, respectively. The antimicrobial activity was analyzed employing band-edge positions for ROS generations and the superﬁcial distribution of Ag + species that contribute to antimicrobial activity. Quantum-chemical calculations were used at the DFT level to investigate the surface-dependent reactivity of α -Ag 2 WO 4 , and we demonstrated how the antimicrobial properties could be tailored by the geometry and electronic structure of the exposed surfaces, providing guidelines for the morphology design.


Introduction
Diseases caused by microbes are a global problem, and it is even more severe in underdeveloped countries due to lack of hygiene or even proper treatment [1,2].In addition, treating microbes with conventional remedies results in making them more resistant over time, and it is increasingly necessary to use stronger medications for the treatment to promote the expected result [3,4].Metallic oxide particles are a well-recognized strategy because they are toxic to microbes, but present low toxicity to humans [5][6][7].The use of semiconductor oxides enables the manipulation of their properties by increasing the separation process of electron/hole charge pairs (e − /h + ), which enhances the formation of reactive oxygen species (ROS), such as hydroxyl radical ( • OH) and superoxide radical ( • O 2 − ), that degrade the constituent proteins and membranes of the microorganisms [8].
In this work, nanoparticles of α-Ag 2 WO 4 with rod morphology were obtained by a co-precipitation method.The cross-section of the rods was changed from hexagonal to quadratic through an increase in the synthesis temperature from 30 to 70 • C. The crosssection effect in the antimicrobial properties was analyzed against Escherichia coli and Salmonella spp.In addition, DFT calculations, on realistic surface models, were carried out to investigate the geometry, electronic structure, and properties of α-Ag 2 WO 4 .Based on these results, we hope to understand how the different surfaces change their energies throughout the synthesis process and propose a mechanism by which the experimental and theoretical morphologies of the α-Ag 2 WO 4 perform the antimicrobial activity more efficiently.We believe that these novel results are of significant relevance since they may inspire the efficient synthesis of this material and provide critical information to expand our fundamental understanding of this property in this compound.
Firstly, 4 mmol of AgNO 3 and 10 mmol of PVP were dumped in 40 mL of deionized water and in a glass beaker and continuously stirred.Then, 2 mmol of Na 2 WO 4 .2H 2 O and 10 mmol of PVP were dissolved in 40 mL of deionized water in another glass beaker.After 10 min, the solution containing Ag + cations was immersed in the W 6+ solution and was maintained under vigorous stirring for 30 min at 30 • C (AW30 sample).The same procedure was repeated, increasing the precipitation temperature to 70 • C (AW70 sample).After 30 min, the supernatant was separated by centrifugation, washed with deionized water, and dried at 100 • C for 24 h.

Characterization
The crystalline phase of the powder was analyzed by X-ray diffraction using CuKα radiation (1.5418 Å), scanning from 10 to 80 • , using a speed of 1 • /min and a step of 0.02 • in a Shimadzu diffractometer (XRD-6000).Rietveld refinement was performed in the General Structure Analysis System (GSAS) program (Developed by the Advanced Photon Source, Argonne National Laboratory, Developed by the Advanced Photon Source, Argonne National Laboratory, Lemont, IL, USA) with a EXPGUI graphical interface version 1166 [28] using background, scale factor, microstructure, crystal, texture, and strain parameters for refinement.The α-Ag 2 WO 4 rods morphology was observed by a field emission scanning electron microscope (FE-SEM, Carl-Zeiss-microscope, ZEISS Microscopy, Oberkochen, Germany).X-ray photoelectron spectroscopy (XPS) was performed in a ES-CALAB 250Xi (Thermo Fisher Scientific, Waltham, MA, USA) device operating with AlKα (hν = 1486.68eV) radiation at 225 W and 15 kV.The XPS spectra were collected at 200 eV and 20 eV for survey spectra and individual elements, respectively.

Antimicrobial Tests
The α-Ag 2 WO 4 powders' antimicrobial capacity was analyzed using the Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) methods.
Minimal inhibitory concentration (MIC) of the α-Ag 2 WO 4 powders was determined using the broth microdilution method [29] with modifications using 96-well microtiter plates.The pre-inoculum culture consisted of a bacterial colony cultivated in 5 mL Muller-Hinton medium (MH) at 37 • C for 16-18 h under 180 rpm agitation.After this period, 5 µL of pre-inoculum was transferred to 5 mL of MH and incubated at 37 • C with agitation until optical density reached 0.1 at 600 nm measured by UV-VIS (Quant, Biotek, American Laboratory Trading, San Diego, CA, USA).The α-Ag 2 WO 4 samples were firstly dissolved in sterile distilled water and added to the MH medium for a final dose of 5 µg/mL.The samples were serially diluted, and 50 µL was added to each well.The bacterial inoculums were adjusted to 10 6 CFU/mL concentration and inoculated with samples and MH at a final volume of 100 µL/well.The plates were incubated at 37 • C for 16-20 h with no agitation.MIC was considered as the lowest concentration which inhibited visual growth.Next, an aliquot of each growth well content was inoculated in an MH agar plate and incubated at 37 • C for 24 h for the Minimum Bactericidal Concentration (MBC) test.MBC was considered as the lowest concentration at which no bacterial colonies were observed.The bacteria used were gram-negative Escherichia coli and Salmonella spp.Negative and sterility controls were included, and all experiments were performed in triplicate.

Computational Details
Computational methods and theoretical procedures were employed to study the bulk and surfaces related to α-Ag 2 WO 4 .Density Functional Theory (DFT) calculations were carried out using the periodic ab initio CRYSTAL17 [30] package within B3LYP [31,32] hybrid functional.This computational technique has been successfully applied to study the electronic and structural properties of various Ag-based materials [33][34][35][36][37].The Ag and W atoms in all calculations were described using effective core pseudopotential HAYWSC-311d31G and HAYWSC-11d31G, respectively, while O atoms were described using atomcentered all-electron Gaussian basis 8-411G [38][39][40].Low-index (100), (001), (001) and (101) slab models were employed in order to evaluate the physical and chemical properties associated with α-Ag 2 WO 4 surfaces.Herein, the previous optimized surface models [11,41] obtained combining semi-local PBE exchange-correlation functional and plane-wave basis set were considered as first guess for electronic structure analysis with atom-centered basis set and a hybrid functional approach.The electronic structure analysis was carried out combining band-edge positions, electrostatic potential isosurfaces, and surface-dependent chemical environment from undercoordinated cations.

Results and Discussion
The diffractograms for the Ag 2 WO 4 powders obtained at 30 and 70 • C are shown in Figure 1a,b, respectively.According to diffractograms, all peaks corresponding to the alpha phase of the silver tungstate, which have an orthorhombic system and space group P2n2 (no.34), are characterized by the ICSD 243987 card.The absence of secondary peaks indicates that the co-precipitation method at different temperatures is efficient for the α-Ag 2 WO 4 phase obtention.The ICSD 243987 card was used for Rietveld refinement in the GSAS software and Table 1 shows the obtained data.According to data obtained by the refinement, the increase in the precipitation temperature provided a little increase in the crystallite size, followed by a reduction in the microstrain.The growth of crystallites occurred due to the increase in the lattice parameters of the material, as shown in Table 1.These changes occurred because of the greater energy during the synthesis procedure, enabling the greater lattice accommodation and energy reduction associated with crystallites.Table S1 shows the fractional coordinates of the atoms, indicating that the increase in temperature favors the displacement of the atoms, facilitating their accommodation in the crystalline lattice.The low values of the refinement coefficients, in parallel with the good fit between the theoretical and practical diffractograms, indicate the reliability of the data obtained by the refinement.
Figure 1c,d show the micrographs for the AW30 and AW70 samples, respectively.The growth of particles in well-defined morphologies can be explained by the energy associated with the surface planes.According to the highlighted images, it is clear that the temperature increase changed the cross-section of the rods, changing them from hexagonal to square form.Roca et al. [42] showed the possible morphologies of α-Ag 2 WO 4 through DFT calculations using the Wulff construction obtained by a microwave-assisted hydrothermal method and according to the surface energy of the crystallographic planes.According to this study, the markings 1, 2, 3, and 4 refer to the (010), (001), (101), and (100) planes.Therefore, we suggest that the rods obtained in this work at 30 • C preferentially grow in the [010] direction, while the increase in precipitation temperature to 70 • C favors growth in the [100] direction.The increase in precipitation temperature provides greater energy to the system, allowing the growth of particles in a morphology which has less energy associated with its crystallographic planes.As seen through the diffractograms, α-Ag 2 WO 4 has an orthorhombic crystalline structure, justifying the growth of the particles in the [100] direction when increasing the precipitation temperature.The chemical environment for (100), (010), (001), and (101) surfaces were analyzed based on the Wulff construction [42], as presented in the Figure 2. In this case, the (100) surface exposed a 5-fold Ag and W center, while (010) exposed a 4-fold Ag-center.On the other hand, the (001) and (101) surfaces exhibited Ag 5c and Ag 4c centers summed to W 5c (101) and W 4c (001).Therefore, the (100) and (010) surfaces showed a more regular environment in comparison to the (001) and (101) surfaces that presented a higher undercoordinated degree.temperature increase changed the cross-section of the rods, changing them from hexago-nal to square form.Roca et al. [42] showed the possible morphologies of α-Ag2WO4 through DFT calculations using the Wulff construction obtained by a microwave-assisted hydrothermal method and according to the surface energy of the crystallographic planes.
According to this study, the markings 1, 2, 3, and 4 refer to the (010), ( 001), (101), and (100) planes.Therefore, we suggest that the rods obtained in this work at 30 °C preferentially grow in the [010] direction, while the increase in precipitation temperature to 70 °C favors growth in the [100] direction.The increase in precipitation temperature provides greater energy to the system, allowing the growth of particles in a morphology which has less energy associated with its crystallographic planes.As seen through the diffractograms, α-Ag2WO4 has an orthorhombic crystalline structure, justifying the growth of the particles in the [100] direction when increasing the precipitation temperature.The chemical environment for (100), ( 010), (001), and (101) surfaces were analyzed based on the Wulff construction [42], as presented in the Figure 2. In this case, the (100) surface exposed a 5-fold Ag and W center, while (010) exposed a 4-fold Ag-center.On the other hand, the (001) and (101) surfaces exhibited Ag5c and Ag4c centers summed to W5c (101) and W4c (001).Therefore, the (100) and (010) surfaces showed a more regular environment in comparison to the (001) and (101) surfaces that presented a higher undercoordinated degree.Figure 3 shows the XPS spectra for the AW70 sample.According to Figure 3b, Ag 3d had two peaks at 367.2 and 373.2 eV, which could be assigned to Ag 3d5/2 and Ag 3d3/2, respectively.These peaks were ascribed to Ag + and the absence of minor peaks after deconvolution of these peaks indicated the non-formation of metallic silver (Ag 0 ) [43].The high-resolution spectra for W 4f are shown in Figure 3c.The peaks at 34.1 and 36.3 eV correspond to W 4f7/2 and W 4f5/2, respectively, relative to W 6+ [44].Figure 3d shows the deconvolution of the O 1s peak in two, 529.5 and 530.7 eV, which can be attributed to lattice oxygen and the oxygen of water molecules adsorbed on the surface, respectively [45]. Figure 3 shows the XPS spectra for the AW70 sample.According to Figure 3b, Ag 3d had two peaks at 367.2 and 373.2 eV, which could be assigned to Ag 3d 5/2 and Ag 3d 3/2 , respectively.These peaks were ascribed to Ag + and the absence of minor peaks after deconvolution of these peaks indicated the non-formation of metallic silver (Ag 0 ) [43].The high-resolution spectra for W 4f are shown in Figure 3c.The peaks at 34.1 and 36.3 eV correspond to W 4f 7/2 and W 4f 5/2 , respectively, relative to W 6+ [44].Figure 3d shows the deconvolution of the O 1s peak in two, 529.5 and 530.7 eV, which can be attributed to lattice oxygen and the oxygen of water molecules adsorbed on the surface, respectively [45].
The antimicrobial capacity of the α-Ag 2 WO 4 powders was estimated by MIC and MBC methods.Table 2 shows the methodology of presentation of the inhibitory results, and Tables 3 and 4 show the results against Escherichia coli and Salmonella spp.according to the plates shown in Figures S1 and S2 (Supplementary Materials).The negative control refers to a well without α-Ag 2 WO 4 samples, while the sterility controls refer to a well without bacteria or α-Ag 2 WO 4 .According to Tables 3 and 4, the growth in all wells in columns 8 and 9, and the non-growth in the wells in columns 10 and 11, indicate the correct sterilization during the preparation process, indicating the reliability of the results.The results against E. coli indicate that the 5 and 2.5 mg/mL concentrations of AW30 and AW70 samples inhibited the growth of the bacteria, while lower concentrations did not hinder them.On the other hand, only 5 mg/mL concentration against Salmonella spp.resulted in the inhibition of bacterial growth for the AW30 and AW70 samples.The antimicrobial capacity of the α-Ag2WO4 powders was estimated by MIC and MBC methods.Table 2 shows the methodology of presentation of the inhibitory results, and Tables 3 and 4 show the results against Escherichia coli and Salmonella spp.according to the plates shown in Figures S1 and S2 (Supplementary Materials).The negative control refers to a well without α-Ag2WO4 samples, while the sterility controls refer to a well without bacteria or α-Ag2WO4.According to Tables 3 and 4, the growth in all wells in columns 8 and 9, and the non-growth in the wells in columns 10 and 11, indicate the correct sterilization during the preparation process, indicating the reliability of the results.The results against E. coli indicate that the 5 and 2.5 mg/mL concentrations of AW30 and AW70 samples inhibited the growth of the bacteria, while lower concentrations did not hinder them.On the other hand, only 5 mg/mL concentration against Salmonella spp.resulted in the inhibition of bacterial growth for the AW30 and AW70 samples.

Table 2. Presentation of antimicrobial results
, where C1 to C7 refer to concentrations of 5, 2.5, 1.25, 0.625, 0.3125, 0.156, and 0.078 mg/mL, respectively.NC and SC are negative and sterility controls, respectively.From now on, our major interest is devoted to explaining the antimicrobial capacity of the α-Ag 2 WO 4 powders based on electronic structure analysis.It is well known that Ag-based semiconductors can exhibit superior antimicrobial activity due to their capability of generating ROS from O 2 adsorption, activation, and evolution along with the exposed surfaces [11,13,46,47].In addition, other authors argue that the presence of exposed Ag + ions are toxic and are able to kill the bacteria through the denaturation or oxidation mechanism [48,49].In both cases, morphological modulations seem to be the best alternative to tailor the biological activity and provide superior behaviors.
In this context, Ag-and Cu-based nanoparticles have recently had their interest renewed, as experimental results indicate the antiviral potential of semiconductor materials against SARs-CoV-2 virus, also known as COVID-19 [8,[50][51][52].In particular, ROS generation summed to the metal-rich surface exposure induces the superior biological activity of such materials, contributing to designing innovative materials and reducing the drastic effects of COVID-19 and similar pathogens.Moreover, theoretical results have also confirmed the role of surface exposure to provide superior ROS generation, indicating that the rational design of solid-state materials plays a key role in present and future medical applications [53][54][55][56].
Aiming to complement such discussion, the vacuum band-edge positions were computed using the expressions: in which E CB and E VB correspond to the VB and CB potential, E e is the energy of free electrons vs. hydrogen electrode potential (NHE = 4.5 eV) [57], χ is the Mulliken electronegativity calculated as 5.989 eV for α-Ag 2 WO 4 , and E gap is the band-gap value estimated here by DFT calculations.In this context, it is important to mention that the calculated band-gap values showed good agreement with the experimental data for AW30 and AW70 (Figure S3-Supplementary Materials), where the role of exposed surfaces explain the reduced band-gap values in comparison with the bulk.Figure 4  reactions, respectively, at different pH levels, such as 0 and 7.
According to Figure 4, the position of VB shows that the bulk and surface models of α-Ag 2 WO 4 can be used to generate • OH radicals from H 2 O oxidation at pH = 0, since the VBM are more positive than the redox potential +1.23 V for H 2 O/ • OH versus NHE.Only the (101) cannot be used to generate • OH radical at pH = 7, since the VBM is more negative that the redox potential of +2.72 V (NHE).With regard to the reduction reaction involving adsorbed oxygen species O 2 / • O 2 − , only the CB positions for the bulk and (010) surfaces are properly able to active the adsorbed species and generate • O 2 − , since the CB values are higher than the redox potential of −0.33 V (NHE).ure S3-Supplementary Materials), where the role of exposed surfaces explain the reduced band-gap values in comparison with the bulk.Figure 4 exhibits the band edge potential for bulk and surfaces of α-Ag2WO4 with respect to NHE.In order to verify if the bulk and different surfaces of α-Ag2WO4 are capable of generating ROS species, i.e., hydroxyl ( • OH) and superoxide radicals ( • O2 − ), from adsorbed H2O and O2 molecules, it is possible to compare the VB/CB position with the potential of H2O/ • OH and O2/ • O2 − reactions, respectively, at different pH levels, such as 0 and 7.According to Figure 4, the position of VB shows that the bulk and surface models of α-Ag2WO4 can be used to generate • OH radicals from H2O oxidation at pH = 0, since the VBM are more positive than the redox potential +1.23 V for H2O/ • OH versus NHE.Only the (101) cannot be used to generate • OH radical at pH = 7, since the VBM is more negative that the redox potential of +2.72 V (NHE).With regard to the reduction reaction involving adsorbed oxygen species O2/ • O2 − , only the CB positions for the bulk and (010) surfaces are properly able to active the adsorbed species and generate • O2 − , since the CB values are higher than the redox potential of −0.33 V (NHE).
Based on these results, we can argue that the bulk and (010) surfaces band-edge potential of α-Ag2WO4 are the most favorable in provoking the formation of ROS considering both oxidative and reductive reactions, being that the VBM and CBM potential, associated with the superficial chemical environment represented in Figure 2, contributes as holetrapping centers which activate adsorbed H2O and O2 species to generate • OH and • O2 − , respectively.Moreover, we can argue that all investigated surfaces of α-Ag2WO4 Based on these results, we can argue that the bulk and (010) surfaces band-edge potential of α-Ag 2 WO 4 are the most favorable in provoking the formation of ROS considering both oxidative and reductive reactions, being that the VBM and CBM potential, associated with the superficial chemical environment represented in Figure 2, contributes as holetrapping centers which activate adsorbed H 2 O and O 2 species to generate • OH and • O 2 − , respectively.Moreover, we can argue that all investigated surfaces of α-Ag 2 WO 4 contribute to generate at least one type of ROS ( • OH) that help us to explain the biological activity reported in this work and by previous experimental results [12,13,24,26,47].
Very recently, our research group proposed a new concept to describe the atomic coordination environment of surface atoms.We found a relationship between the material properties (biological and catalytic behavior) and the exposed surface at the morphology, as well as finding that the ROS generation in the α-Ag 2 WO 4 , β-Ag 2 MoO 4 , and Ag 2 CrO 4 [9,21,58] was sustained.On the other hand, the reactivity and antimicrobial behavior of shape-oriented Ag 2 O nanoparticles were attributed to the presence of (100) surfaces with increased distribution of Ag + species [48,49,59].Moreover, the electrostatic potential isosurface contributes to depicting the charge density distribution to describe the reactivity of exposed surfaces in the morphology of different materials [60,61].Therefore, the calculated charge density distributions for α-Ag 2 WO 4 surfaces are shown in Figure 5.
An analysis of the results point out that the (001) and (101) surfaces exhibit a large positive charge concentration, while (100) and (010) show the presence of negative and positive charge centers along the exposed surfaces.In addition, we can interpret that the most positive surfaces induce reduced formation energy for Ag vacancies, contributing to Ag + release.Therefore, the (001) and (101) can attach to the bacteria membrane and release Ag + cations that contribute to oxidative stress and cell death.
Moreover, the electrostatic potential surface and band-edge positions can be combined to understand the activation process of adsorbed H 2 O and O 2 in ROS generation.The more positive surfaces exhibit a metal-rich environment which can contribute to localizing the excited hole after the exciton dissociation.On the other hand, increasing the oxygen contribution, the semiconductor surfaces can trap both excited electrons and holes at different crystalline sites.In fact, the (101) and (001) surfaces exhibit a characteristic bandedge for VB which contributes to generate • OH radicals from oxidation mediated by the excited holes trapped on the undercoordinated silver and tungsten clusters.Nonetheless, the (010) surface exhibits a chemical environment with Ag-O-W bond paths where the excited electrons and holes can be dissociated and located at different sites, contributing to both the oxidation and reduction reactions responsible for ROS generation.
contribute to generate at least one type of ROS ( • OH) that help us to explain the biological activity reported in this work and by previous experimental results [12,13,24,26,47].
Very recently, our research group proposed a new concept to describe the atomic coordination environment of surface atoms.We found a relationship between the material properties (biological and catalytic behavior) and the exposed surface at the morphology, as well as finding that the ROS generation in the α-Ag2WO4, β-Ag2MoO4, and Ag2CrO4 [9,21,58] was sustained.On the other hand, the reactivity and antimicrobial behavior of shape-oriented Ag2O nanoparticles were attributed to the presence of (100) surfaces with increased distribution of Ag + species [48,49,59].Moreover, the electrostatic potential isosurface contributes to depicting the charge density distribution to describe the reactivity of exposed surfaces in the morphology of different materials [60,61].Therefore, the calculated charge density distributions for α-Ag2WO4 surfaces are shown in Figure 5.An analysis of the results point out that the (001) and (101) surfaces exhibit a large positive charge concentration, while (100) and (010) show the presence of negative and positive charge centers along the exposed surfaces.In addition, we can interpret that the most positive surfaces induce reduced formation energy for Ag vacancies, contributing to Ag + release.Therefore, the (001) and (101) can attach to the bacteria membrane and release Ag + cations that contribute to oxidative stress and cell death.
Moreover, the electrostatic potential surface and band-edge positions can be combined to understand the activation process of adsorbed H2O and O2 in ROS generation.The more positive surfaces exhibit a metal-rich environment which can contribute to localizing the excited hole after the exciton dissociation.On the other hand, increasing the oxygen contribution, the semiconductor surfaces can trap both excited electrons and holes at different crystalline sites.In fact, the (101) and (001) surfaces exhibit a characteristic band-edge for VB which contributes to generate • OH radicals from oxidation mediated by the excited holes trapped on the undercoordinated silver and tungsten clusters.Nonetheless, the (010) surface exhibits a chemical environment with Ag-O-W bond paths where Thus, the electronic structure analysis, combined with the α-Ag 2 WO 4 surface environment, helps us to rationalize the role of the cross-section morphology in the antimicrobial properties.In addition, the obtained results can also contribute to designing new solid-state materials based on Ag-exposed surfaces for both antimicrobial and antiviral applications.

Conclusions
This work provides a valuable strategy for understanding how ROS are generated in an α-Ag 2 WO 4 semiconductor with photoenhanced catalytic and antimicrobial activity.The temperature variation in the co-precipitation synthesis was efficient in controlling the cross-section of the α-Ag 2 WO 4 rods.The diffractograms also showed that there was no formation of secondary phases, confirming the efficiency of this method in obtaining α-Ag 2 WO 4 particles.The cross-section variation in the α-Ag 2 WO 4 sticks did not show significant differences in the antimicrobial tests.Escherichia coli bacteria were completely inhibited at concentrations of 2.5-5 mg/mL for both samples, while only a concentration of 5 mg/mL completely inhibited Salmonella spp.growth.These low α-Ag 2 WO 4 concentrations indicate that both cross-sections are efficient for treating environments that contain such bacteria.The theoretical results based on DFT calculations confirmed that the bulk and (010) and surfaces are responsible for the biological activity due to the band-edge position in ROS generation and cation distribution along the exposed surfaces, which contributes to enhancing the reactivity and biological response.

Figure 4 .
Figure 4. Band-edge values for bulk and surfaces models of α-Ag2WO4 with respect to hydrogen electrode potential (NHE).

Figure 4 .
Figure 4. Band-edge values for bulk and surfaces models of α-Ag 2 WO 4 with respect to hydrogen electrode potential (NHE).

Table 1 .
Refinement data obtained by the GSAS software (EXPGUI graphical interface version 1166).

Table 1 .
Refinement data obtained by the GSAS software (EXPGUI graphical interface version 1166).
exhibits the band edge potential for bulk and surfaces of α-Ag 2 WO 4 with respect to NHE.In order to verify if the bulk and different surfaces of α-Ag 2 WO 4 are capable of generating ROS species, i.e., hydroxyl ( • OH) and superoxide radicals ( • O 2 − ), from adsorbed H 2 O and O 2 molecules, it is possible to compare the VB/CB position with the potential of H 2 O/ • OH and O 2 / • O 2