Enhanced Photocatalytic Activities of Ag₂WO₄ Modified Ag₆Si₂O₇ through a Comprehensive p-n Heterojunction S-Scheme Process

Abstract

The S-scheme photocatalyst system has become increasingly popular in recent years for its ability to efficiently degrade various pollutants, including organic dyes, pesticides, and other harmful substances. This system uses two semiconductor photocatalysts with different bandgap energies, working together in a redox reaction to produce a highly reactive species capable of pollutant breakdown. Here, an S-scheme Ag₂WO₄/Ag₆Si₂O₇ p-n heterojunction nanocomposite was successfully developed by a coprecipitation method. By decomposing Rhodamine B (RhB) under visible-light irradiation, the photocatalytic activities of Ag₆Si₂O₇/Ag₂WO₄ showed enhanced photocatalytic degradation performance of organic dyes, especially at a 4% molar ratio of the Ag₂WO₄-modified Ag₆Si₂O₇ sample, whose degradation rate was 23.7 and 4.65 times those of Ag₂WO₄ and Ag₆Si₂O₇, respectively. The physical and chemical properties of the samples were determined by identifying the physical structure, chemical element composition, and optical responsiveness. The optimum composite amongst the prepared materials was AgSW-4, achieving the maximum RhB degradation efficiency of 97.5%, which was higher by 60% and 20% than its counterparts Ag₆Si₂O₇ and Ag₂WO₄, respectively. These results showed that in the nanocomposite structure, Ag₆Si₂O₇ was a p-type semiconductor and Ag₂WO₄ was an n-type semiconductor. Based on the analysis data, a comprehensive p-n heterojunction S-scheme process was proposed to demonstrate the enhanced photocatalytic performance of the Ag₆Si₂O₇/Ag₂WO₄ nanocomposite.

1. Introduction

Environmental disaster and water pollution can be caused by various factors, including but not limited to fossil fuel burning and industrial wastewater. The staggering problems faced by the earth in the coming century include the depletion of fossil fuel energy and a shortage of clean water. Many efforts are in progress to address environmental disasters and water pollution, as well as to find alternative solutions to the root causes of these problems [1]. To prevent water pollution, water treatment technologies have become a persistent interest of the scientific and industrial community because without water, life is not possible on earth. Shifting to renewable and clean energy sources such as solar, wind, and hydro power is an important step in reducing our reliance on fossil fuels and addressing the issue of climate change. These technologies allow us to generate energy in a way that is more sustainable and has lower environmental impact. However, one of the challenges with renewable energy sources is that they are often intermittent; that is, they generate energy only when the sun is shining, the wind is blowing, or water is flowing. Thus, energy storage technologies are needed to ensure that renewable energy can be used when needed even when not being generated at that moment. Efficient and long-lasting batteries are some of the most promising energy storage technologies currently being studied. Advances in battery technology have enabled the storage of larger amounts of energy in smaller and more portable devices and at lower costs [2]. Advanced photooxidation processes (AOPs) are a group of water treatment technologies that involve the use of highly reactive hydroxyl radicals to degrade and remove pollutants from water. A common method used in AOPs is semiconductor photocatalysis, which can enable light-energy absorption and create electron–hole pairs that can generate hydroxyl radicals through reactions with water and oxygen. These hydroxyl radicals are powerful oxidising agents that can break down organic contaminants and convert them into harmless by-products, such as carbon dioxide and water [3,4,5]. Visible-light-driven semiconductor photocatalysts harvest freely available sunlight and convert the absorbed photons into electron–hole pairs, which induce reduction and oxidation reactions to treat the pollutant species [6]. This technology is cost effective, efficient, and convenient to operate compared with its counterparts. Traditional semiconductors such as ZnO and TiO₂ possess a wide bandgap and absorb only ultraviolet (UV) radiation, accounting for around 4% of the solar spectrum [7]. Given that visible light makes up a massive fraction of the solar spectrum (approximately 43%), creating semiconductor photocatalysts that respond to visible light is a major environmental priority [8]. Recently, various modification methods have been developed to expand the light absorption range and improve the photocatalytic performance of photocatalysts, including ion doping, noble metal recombination, and heterostructure construction. For example, Niu et al. [9] doped TiO₂ nanotubes with the transition metal ion Nb. The defect states present in TiO₂ lead to the formation of local intermediate bandgap states (MS) in the bandgap, thereby adjusting the band structure of TiO₂ and extending the light absorption to the visible region. Lu et al. [10] used a one-step hydrothermal method to synthesise CdS@ZnO nanocomposites with a wide visible-light absorption range, greatly improving the visible-light utilisation.

Silver-based semiconductor materials have received significant attention for photocatalysis applications owing to their excellent bandgap and structural stability [11,12,13]. For instance, AgX-based (Cl, Br, and I) [14,15,16,17,18,19,20] semiconductors exhibit notable visible-sunlight harvesting and could be used to remove organic contaminants from water systems. Silver tungstate (Ag₂WO₄) has also been extensively explored as a semiconductor photocatalyst for the decontamination of organic pollutants [21,22]. However, pure Ag₂WO₄ always shows a low adsorption and high recombination rate, inhibiting its ability to function as a photocatalyst. Ag₆Si₂O₇, which has a suitable band structure and bandgap, is reportedly a possible visible-light-responsive photocatalyst [23]. A distinctive internal electric field exists in the Ag₆Si₂O₇ crystal structure, and the photogenerated holes and electrons could be quickly unpaired [13,24], which is conducive to enhancing photocatalytic performance. Forming heterojunction structures could further enhance the charge separation and carrier life span and achieve better photocatalysis performance than single-component semiconductors [25,26].

Research on the degradation of Rhodamine B (RhB) contaminants by using a combination of Ag₆Si₂O₇ and Ag₂WO₄ photocatalysts is limited. However, the use of various photocatalysts for RhB degradation in wastewater has been extensively explored, and investigations on the development of new and more effective photocatalysts are ongoing. Some recent studies have examined the use of bimetallic or hybrid photocatalysts for RhB degradation, such as Ag/AgBr/TiO₂, BiVO₄/TiO₂, and Fe₂O₃/TiO₂. In the future, researchers may also explore the use of a combination of Ag₆Si₂O₇ and Ag₂WO₄ for RhB degradation. Notably, the effectiveness of a photocatalyst for the degradation of a particular contaminant can depend on many factors, including the properties of the contaminant, the properties of the photocatalyst, and the conditions under which the photocatalysis is performed. Therefore, researchers must continue exploring different photocatalytic systems and optimising the conditions for their use to develop more efficient and effective treatment methods for wastewater containing RhB contaminants. In the current work, we synthesised a Ag₂WO₄-modified Ag₆Si₂O₇ nanocomposite structure by a coprecipitation method. Under visible-light irradiation, the Ag₆Si₂O₇/Ag₂WO₄ nanocomposite samples showed enhanced photocatalytic activity compared with pure Ag₂WO₄ or Ag₆Si₂O₇ samples, and the 4% (molar ratio) Ag₂WO₄-modified Ag₆Si₂O₇ (AgSW-4) had the optimum performance. After carefully analysing the Ag₆Si₂O₇/Ag₂WO₄ nanocomposite’s structure, morphology, and optical characteristics, a potential charge-transfer mechanism, i.e., a p-n heterojunction S-scheme process, was put forward to explain how the Ag₆Si₂O₇/Ag₂WO₄ nanocomposite heterojunction assisted in the breakdown of organic contaminants.

2. Experimental Section

2.1. Materials

Silver nitrate (AgNO₃), sodium tungstate dehydrate (Na₂WO₄·2H₂O), sodium silicate (Na₂SiO₃·9H₂O), RhB, tertiary butanol, benzoquinone, ethylene diamine tetra acetic acid disodium salt, and ethanol were acquired from Tian in Fuyu Fine Chemical Co., Ltd., Tianjin, China. Without any other addition, all chemical reagents were used in their original form.

2.2. Synthesis of Ag₆Si₂O₇

Ion exchange synthesis was used to synthesise Ag₆Si₂O₇. Initially, 2.038 g of AgNO₃ was dissolved in DI water at room temperature to prepare a 0.2 mol/L solution placed in darkness. Then, a 0.2 mol/L solution of Na₂SiO₃·9H₂O with a typical mass of 1.136 g was prepared under continuous stirring. Afterwards, both solutions were mixed slowly under stirring and the mixture was further stirred for 40 min whilst shielding from light. The mixture was finally centrifuged at 7000 r/min and then washed with water. After ethanol washing and drying at 60 °C for 20 h in an oven, the obtained Ag₆Si₂O₇ powder was ground and collected.

2.3. Synthesis of Ag₆Si₂O₇/Ag₂WO₄ Composites

We weighed a certain amount of AgNO₃, Na₂WO₄, and Na₂SiO₃ solids, prepared them into a 0.1 mmol/L solution, and placed the solution in darkness. Magnetic stirring was performed for 15 min to ensure its complete dissolution, followed by slowly adding a certain amount of Na₂WO₄ solution to AgNO₃ solution dropwise in darkness. White flocculent precipitate can be observed in the solution at this time. Stirring was continued for 30 min to ensure that the Na₂WO₄ had completely reacted. After slowly adding a specific quantity of Na₂SiO₃ solution to the suspension solution, the dosage ratio was determined according to the proportion of silver ions from Ag₂WO₄ in the Ag₆Si₂O₇/Ag₂WO₄ composite sample, i.e., 2%, 3%, 4%, 5%, and 6% (denoted as AgSW-2, AgSW-3, AgSW-4, AgSW-5, and AgSW-6, respectively). Brown precipitate can be observed at this time. Continuous stirring was performed for 45 min to ensure that the solute reacted completely. Finally, the obtained mixed liquid was centrifuged at a speed of 7000 r/min, the supernatant was discarded, three water washes and one ethanol wash were performed, and the obtained Ag₆Si₂O₇/Ag₂WO₄ heterojunction powder was ground and collected after drying at 60 °C for 20 h in an oven.

2.4. Characterisation

The morphology and microstructure of the prepared materials were characterised by scanning electron microscopy (SEM; Hitachi, SU8000, Tokyo, Japan) and transmission electron microscopy (TEM; FEI Talos F200X, Hillsboro, OR, USA). An XPERT-PRO diffractometer (Bruker D8 Advance, Bremen, Germany) was used for X-ray diffraction (XRD) analysis and Cu K-alpha radiation (1.5406) to reveal the structural properties for 2θ values from 10° to 90°. To compute the Brunauer–Emmett–Teller (BET) specific surface areas of the materials, N₂ adsorption–desorption isotherms were used with an ASAP2460 (Micromeritics instrument Ltd., Norcross, GA, USA) aperture analyser. An electron spectrometer (model number 250Xi, Thermo Fisher, Waltham, MA, USA) was used to perform XPS measurements to ascertain the elemental composition of the surface. Using UV–visible (UV–vis) diffusion reflectance spectroscopy (DRS), PerkinElmer Lambda 35 UV/Vis (Waltham, MA, USA) spectrophotometer data of freshly produced powders were collected to analyse optical bandgaps. The investigation of electron spin resonance (ESR) was completed using a Bruker A300-10/12 equipment.

2.5. Photocatalytic Degradation Experiment

Photocatalytic activities were evaluated by observing the change in RhB solution concentration under visible light. A schematic of the photocatalytic experimental equipment is shown in Figure S1. Typically, 100 mL of RhB solution (20 mg/L) had 10 mg of catalyst added to it. To balance the adsorption–desorption, the solution was agitated for 30 min whilst placing it in complete darkness. As the light source with a 400 nm cutoff filter, a 300 W Xenon lamp was operated. To extract the photocatalyst, 6 mL of the solution was collected taken from each photocatalysis experiment at different irradiation durations (10, 20, 30, 60, and 90 min) and centrifuged. Finally, the concentration of residual organic dyes was determined by UV–vis absorption spectroscopy.

3. Results and Discussion

3.1. XRD Analysis

XRD analysis was used to examine the crystal phase and structure of the synthesised materials. The XRD patterns of Ag₂WO₄, Ag₆Si₂O₇, and Ag₂WO₄-modified Ag₆Si₂O₇ composite photocatalysts (AgSW-2, AgSW-3, AgSW-4, AgSW-5, and AgSW-6) are shown in Figure 1. Data revealed that Ag₂WO₄ exhibited six characteristic peaks at 2θ values of 27.3°, 30.1°, 32.3°, and 44.7°, respectively, which agreed with the crystal planes in the literature of hexagonal β-Ag₂WO₄ (JCPDS # 33-1195) [27,28,29]. Meanwhile, Ag₆Si₂O₇ exhibited a broad peak at around 33° owing to the broadening of small particles and low crystallinity, corresponding with the XRD pattern of (JCPDS#47-0704) in the literature [24]. After composite Ag₆Si₂O₇ with Ag₂WO₄, no obvious change was found in the XPS curves, which may be due to the very low ratio of Ag₂WO₄ to Ag₆Si₂O₇.

3.2. Morphology and Composition Analyses

Scanning electron microscopy (SEM) techniques were used to reveal the surface morphology of the as-synthesised materials. The resulting patterns of the Ag₂WO₄-modified Ag₆Si₂O₇ with different mole ratios and pure Ag₂WO₄ and Ag₆Si₂O₇ are shown in Figure 2a–g. Figure 2f clearly shows that the Ag₂WO₄ possessed a rod-like morphology, indicating an inhomogeneous size having an average diameter of about 168.5 nm. Figure 2g is the SEM image of Ag₆Si₂O₇, inferring a smooth spherical morphology with an average particle size of 75.6 nm. After compositing Ag₂WO₄ with Ag₆Si₂O₇ at different ratios, the average sizes of the particles were smaller; for example, the 4% samples had the smallest particle sizes at 39.1 ± 6.5 nm.

The microstructure of the 4% samples was further determined by high-resolution TEM (HRTEM). As shown in Figure 3a, small nanoparticles aggregated together to form larger irregular particles in Figure 2c. In Figure 3b, when enlarging some zones in high resolution, some small particles showed a lattice fringe spacing of 0.201 nm, corresponding with the peak of 2θ at 44.7° (JCPDS # 33-1195). This finding indicated that these particles were Ag₂WO₄ nanocrystals. Meanwhile, some small particles showed a lattice fringe spacing of 0.270 nm, which matched the 2θ peak at 33.0° (JCPDS # 47-0704) well, meaning that these particles were Ag₆Si₂O₇ nanocrystals. To further illustrate the composite structure of the Ag₂WO₄-modified Ag₆Si₂O₇, the element mapping of the composition elements were determined. As shown in Figure 3c, all tested elements, Ag, Si, W, and O, were detectable and uniformly distributed, suggesting that both the Ag₂WO₄ and Ag₆Si₂O₇ structures existed in the particles, and the Ag₂WO₄-modified Ag₆Si₂O₇ composite was developed. Meanwhile, the signal intensity of W was much weaker than that of the other elements, suggesting a much lower concentration of Ag₂WO₄ in the composite.

3.3. BET Analysis

To gain further insights into the as-prepared composite, BET was performed to reveal the specific surface area that anchored the chemical reaction ability of the materials. The enhanced surface area could provide more reaction sites on the particle surface, thereby improving the photocatalytic activity. Nitrogen adsorption and desorption coupled with BET was performed, and the results demonstrated specific surface areas of 0.9141 and 12.9647 m²/g for Ag₂WO₄ and Ag₆Si₂O₇, respectively.

Table 1. BET-specific surface area of the photocatalysts.

Serial Number	Samples	BET Surface Area (m2/g)
1	AgSW-2	14.0788
2	AgSW-3	13.5775
3	AgSW-4	15.0416
4	AgSW-5	12.4154
5	AgSW-6	14.3216
6	Ag2WO4	0.9141
7	Ag6Si2O7	12.9647

shows that for the samples of Ag₂WO₄-modified Ag₆Si₂O₇ composite, the specific surface area data of the composite materials were larger than the Ag₆Si₂O₇ sample (except for AgSW-5, whose specific surface area was smaller than that of Ag₆Si₂O₇). AgSW-4 had the largest specific surface area of 15.0416 m²/g.

3.4. XPS Analysis

XPS involves a photoelectric effect-based technique to characterise the surface chemistry and density of electronic states residing on the photocatalyst surface. AgSW-4 was chosen as the representative composite to study the composition information. Figure 4A–D show the XPS spectra of AgSW-4, Ag 3d, Si 2p, and W 4f, respectively. We can infer the coexistence of Ag, Si, O, and W from Figure 4A. Figure 4B shows the Ag spectra of AgSW-4, and two characteristic peaks located at binding energies of 374.2 and 368 eV were found, corresponding with 3d_3/2 and 3d_5/2 of Ag⁺, respectively [30]. Similarly, Figure 4C depicts the existence of 2p orbitals of Si⁴⁺ associated with a binding energy of 101.9 eV, and Figure 4D exhibits two peaks at 34.8 and 36. 8 eV owing to an electronic states density of W⁶⁺ in 4f_5/2 and 4f_7/2, respectively [31,32]. Thus, the XPS spectra also confirmed the existence of Ag₂WO₄ and Ag₆Si₂O₇ and the successful formation of the Ag₂WO₄-modified Ag₆Si₂O₇ composite structure.

3.5. UV–vis Optical Properties and Valence Band (VB)/Conduction Band (CB) Potential Positions

The optical properties of the synthesised materials were assessed through UV–vis DRS, and the absorbance spectra are shown in Figure 5a. Pure Ag₂WO₄ absorbed photons with wavelengths lower than 397 nm, whereas pure Ag₆Si₂O₇ absorbed photons with wavelengths lower than 536 nm. All composite materials also exhibited similar curves in the visible region to Ag₆Si₂O₇, with a little lower optical absorbance intensity, especially within the range of 380–510 nm. The DRS data were closely related to the bandgap of the materials. The optical bandgap was calculated utilising the wood Tauc equation for direct band evaluation as the studied materials were regarded as direct bandgap materials [33].

$${\left(\mathsf{\alpha }\mathrm{h}\mathsf{\nu }\right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\mathrm{n}$}\right.}=\mathrm{A}(\mathrm{h}\mathsf{\nu }-{\mathrm{E}}_{\mathrm{g}})$$

(1)

where α, h, ν, A, hν, and E_g are the absorption coefficient, Planck constant, photon frequency, constant of proportionality, photon energy, and energy bandgap of the material, respectively. The proportionality constant had no effect on optical absorbance, and n = 2 for the indirect bandgap and 1/2 for the direct bandgap. Ag₆Si₂O₇ and Ag₂WO₄ are reportedly direct transition semiconductors, and their n value is ½ [34,35]. Bandgaps can be calculated by plotting (αhν)² along the y-axis and $h\nu ={\mathrm{E}}_g$ along the y-axis, as shown in Figure 5b. By setting (αhν)² to zero, the x-intercept provided the bandgaps of 3.12, 2.31, 2.38, 2.42, 2.61, 2.56, and 2.46 eV for Ag₂WO₄, Ag₆Si₂O₇, AgSW-2, AgSW-3, AgSW-4, AgSW-5, and AgSW-6, respectively (

Table 2. The bandgap of the samples.

Serial Number	Bandgap	Samples
1	Ag2WO4	3.12 eV
2	Ag6Si2O7	2.31 eV
3	AgAW-2	2.38 eV
4	AgSW-3	2.42 eV
5	AgSW-4	2.61 eV
6	AgSW-5	2.56 eV
7	AgSW-6	2.46 eV

).

3.6. RhB Dye Degradation Analysis

After revealing the CB alignment of precursors in the Ag₂WO₄-modified Ag₆Si₂O₇ composite, the suitable light absorption, and excellent specific area, the photocatalytic performance was evaluated through RhB dye photodegradation under visible-light irradiation. RhB dye exhibited a characteristic peak at 554 nm in the UV–vis spectrum in the absence of a photocatalyst [36], and the very moment the photocatalyst was introduced, the peak intensity declined. After certain time intervals, the UV–vis spectrum of the solution was obtained to reveal the degradation. As shown in Figure S2, an obvious absorption peak at 554 nm gradually weakened with prolonged illumination time and almost disappeared at 90 min, indicating that the skeleton structure of RhB was destroyed during photocatalysis and the content of RhB in the solution was continuously reduced. Figure 5c clearly shows that the Ag₆Si₂O₇/Ag₂WO₄ composite photocatalysts exhibited much better degradation ability than the pure Ag₂WO₄ or Ag₆Si₂O₇ samples. Notably, the AgSW-4 photocatalyst had the optimum photodegradation performance amongst all catalysts, with a degradation rate that reached 97.5% within 90 min. Moreover, the experiment was performed three times to ensure reproducibility, and the degradation efficiency was 97.5%, 94.9%, and 95.4%, respectively (mean = 95.9%; SD = 0.0138), indicating that the Ag₆Si₂O₇/Ag₂WO₄ composite was reproducible (Figure S3).

Equation (2) was introduced to understand the reaction kinetics, where C₀ C, t, and k are the initial concentration of RhB, concentration after time t, and reaction rate, respectively.

$$\mathrm{l}\mathrm{n}({\mathrm{C}}_0/\mathrm{C})=\mathrm{k}\mathrm{t}$$

(2)

As shown in Figure 6d, the degradation rate value of this reaction was k = 0.03603 min^–1 for the AgSW-4 photocatalyst, which was 23.7 times that of Ag₂WO₄ and 4.65 times that of Ag₆Si₂O₇. The prepared Ag₂WO₄/Ag₆Si₂O₇ Z-scheme heterostructure also exhibited better photocatalytic activity for RhB degradation compared with most Ag₆Si₂O₇-based and Ag₂WO₄-based photocatalysts reported in recent years (Table S1).

Meanwhile, the cycle photocatalytic degradation performance of Ag₆Si₂O₇/Ag₂WO₄ was studied. As shown in Figure S4, no significant decrease in the photodegradation efficiency of RhB with AgSW-4 was observed through six cycles. To further demonstrate the stability and reusability of Ag₆Si₂O₇/Ag₂WO₄, we characterised the composition and structure of the used sample AgSW-4. The XRD spectra in Figure S5 showed no significant changes. As shown in Figure S6, the XPS spectrum of each element in AgSW-4 showed slight changes after use, but the valence states of each element were still the same as before the reaction. These results suggested that the structure of AgSW-4 was very stable during photocatalysis. Thus, AgSW-4 was selected to systematically analyse and reveal the probable mechanism of photocatalysis.

3.7. Electrochemical and Band Position Analysis

Photo-electrochemical properties could indicate the separation and transfer efficiency of the photogenerated electron–hole pairs in the semiconductor structures. The transient photocurrent analysis results of Ag₆Si₂O₇, Ag₂WO₄, and AgSW-4 are shown in Figure S7. The photocurrent signal curves of the samples indicated that the AgSW-4 photocatalyst had much higher photocurrent intensity than Ag₆Si₂O₇ and Ag₂WO₄, implying that the electron transfer and separation efficiency of photocharges were efficiently enhanced. This finding ultimately suggested that AgSW-4 had a higher capacity in the separation of photocharges, which was beneficial for the photocatalysis performance in accordance with the photodegradation results. EIS was performed to monitor the ability of photocatalysts to convert incident photons into charge carriers and transfer rate. The composite with the smaller arc radius in the EIS plots and smaller EIS reflection had lower movement resistance and thus are considered beneficial for redox processes [37]. Figure 6a clearly shows that AgSW-4 possessed much lower impedance than the pure Ag₂WO₄ or Ag₆Si₂O₇ samples, with an improved ability to convert incident photons into electron–hole pairs. Moreover, the lower impedance indicated that AgSW-4 had a higher charge-transfer efficiency than its individual parts.

The band positions such as the VB and CB values of the photocatalysts were investigated to infer the photocatalysis mechanism of the composite material. The flat-band potential and naturally prepared semiconductors were investigated by Mott–Schottky (MS) plots. A negative MS slope represents an n-type semiconductor, whereas a positive slope represents a p-type one. Figure 6b,c show that Ag₆Si₂O₇ is a p-type and Ag₂WO₄ is an n-type semiconductor [38]. By extrapolating the x-intercept of the linear region in the MS curve, the flat-band potential (E_f) of Ag₂WO₄ was calculated at 0.99 eV vs. Ag/AgCl, and was calculated to be 1.19 eV vs. NHE. For Ag₆Si₂O₇, it was 0.97 eV vs. Ag/AgCl and 1.17 eV vs. NHE, respectively. Typically, the CB edge of the n-type semiconductors was 0.1 eV more negative than the E_f edge, whereas the VB edge of the p-type semiconductors was 0.1 eV more positive than the E_f edge [39]. On this basis, the CB potential (E_CB) of Ag₂WO₄ was approximately 1.09 eV vs. NHE, and the VB potential (E_VB) of Ag₂WO₄ was 1.27 eV vs. NHE.

Consequently, the following equation was used to calculate the VB or CB edge of Ag₂WO₄ or Ag₆Si₂O₇:

$${\mathrm{E}}_{\mathrm{V}\mathrm{B}}-{\mathrm{E}}_{\mathrm{C}\mathrm{B}}={\mathrm{E}}_{\mathrm{g}}$$

(3)

From the UV–vis DRS analysis, the ${\mathrm{E}}_{\mathrm{g}}$ values of Ag₂WO₄ and Ag₆Si₂O₇ were 3.12 and 2.31 eV, respectively, so the E_VB of Ag₂WO₄ was calculated to be 4.21 and the E_CB of Ag₆Si₂O₇ was −1.04 eV.

3.8. Reactive Species Study

To gain insights into how dye degradation occurred during photocatalysis, ESR was performed to infer about the reactive species participating in the degradation and their corresponding roles. Once the photocatalyst was exposed to light, electron–hole pairs were generated and further produced superoxide (·O₂⁻) or hydroxyl radicals (·OH) that interacted with dyes to degrade them [40,41,42]. Apart from these reactive species, holes were also considered to be the crucial candidates reacting directly with dyes to destroy and degrade them. Figure 6d shows the ESR graph demonstrating the production of (·O₂⁻) radicals as a consequence of light irradiation. No peaks were observed when the photocatalyst was in darkness, and six peaks emerged after light irradiation, indicating the existence of reactive (·O₂⁻) radicals in the presence of light. The intensity of peaks was further increased with prolonged irradiation exposure time, thereby confirming that they were the active species during the photocatalysis of dyes. Figure 6e depicts the subsequent hydroxyl radical (·OH) scenario when the photocatalyst was in darkness and light. They were also produced as a consequence of photogenerated carries. The increased peak intensity with increased irradiation time confirmed that they were also photo-induced reactive species. Finally, Figure 6f depicts the photogeneration of holes (h⁺). ESR results suggested that during the photocatalytic degradation of dyes, the ·O₂⁻, ·OH, and holes all participated and played important roles.

3.9. Photocatalysis Mechanism

During advanced photocatalysis, which has been demonstrated to be more efficient for the degradation of organic contaminants, further studying the proposed process is necessary to better understand it. Generally, organic pollutants first adsorb onto the surface of photocatalytic material, and then they are broken down into nontoxic end-products by secondary reactive substances produced by photocatalysts during reaction.

Scheme 1 shows that based on the previous data and analysis, before composition, the separated Ag₂WO₄ or Ag₆Si₂O₇ nanocrystals absorbed the photons with suitable energy and generated photoelectrons and holes. However, the photocatalysis efficiency was low.

When in contact with each other, the p-type Ag₆Si₂O₇ and the n-type Ag₂WO₄ formed a p-n heterojunction. Along with this formation, an electric field was established between the interface of the two different nanostructures. With the effectiveness of the electric field at the interface, the E_f band positions of the two nanostructures Ag₆Si₂O₇ and Ag₂WO₄ were unified at the same level. After exposure to visible light, the photogenerated electrons were transported to the CB levels of Ag₂WO₄ and Ag₆Si₂O₇ and followed the traditional process of the heterojunction structure. The photogenerated electrons at a higher CB level of Ag₆Si₂O₇ transferred to the lower CB level of Ag₂WO₄, whereas the photogenerated holes transferred from Ag₂WO₄ to Ag₆Si₂O₇. However, data analysis revealed that the CB level of Ag₂WO₄ nanostructures (1.09 eV vs. NHE) was less negative than that of O₂/·O₂⁻ (−0.33 eV vs. NHE) and the VB level of Ag₆Si₂O₇ (1.27 eV vs. NHE) was less negative than that of ·OH (+2.4 eV vs. NHE) [43]. Given that ·O₂⁻ and ·OH were detected, other more rational mechanisms should be proposed.

When exposed to visible light, the photogenerated electrons’ Ag₂WO₄ nanostructures clearly aggregated in the CB level of Ag₂WO₄, whereas the holes of the Ag₆Si₂O₇ nanostructures aggregated in the VB level of Ag₆Si₂O₇, thereby enhancing the electric field at the p-n heterojunction.

The CB level of the Ag₂WO₄ nanostructures (1.09 eV vs. NHE) and the VB level of Ag₆Si₂O₇ (1.27 eV vs. NHE) were very close. With the S-scheme photocatalysis mechanism [44,45,46,47,48], the photogenerated electrons in the CB level of the Ag₂WO₄ nanostructures and the holes in the VB level of Ag₆Si₂O₇ very easily crossed the bandgap with the help of the electric field at the p-n heterojunction, and the electrons in the CB level of Ag₆Si₂O₇ (−1.04 eV vs. NHE) and the holes in the VB level of Ag₂WO₄ (4.21 eV vs. NHE) were preserved and stable. They reacted with O₂ and H₂O to produce ·O₂⁻ and ·OH, consistent with the ESR results. The photogenerated h⁺, ·O₂⁻, and ·OH further reacted with the adsorbed organic dyes and degraded the organic dyes to small molecules with much lower toxicity to the environment. This finding confirmed that the p-n heterojunction S-scheme photocatalytic degradation was valid and reliable.

4. Conclusions

A coprecipitation method was used to prepare Ag₂WO₄-modified Ag₆Si₂O₇ nanocomposites. XRD, XPS, and TEM elemental mapping tests confirmed the composition of these two nanocrystals. Photodegradation performance tests suggested that the mole ratio of 4% of the Ag₂WO₄-modified Ag₆Si₂O₇ (AgSW-4) photocatalyst had the optimum photocatalysis efficiency. Physical and chemical analyses confirmed that the AgSW-4 photocatalyst had appropriate light absorption, a large specific surface area, high efficiency to generate photocarriers, and enhanced the stability of the p-n heterojunction, which were the main factors for improving the photocatalysis performance. With the ESR data, all active species h⁺, ·O₂⁻, and ·OH in the photodegradation were detected. Based on these data, an S-scheme photocatalysis degradation process was proposed.