Novel Ag-Bridged Z-Scheme CdS/Ag/Bi2WO6 Heterojunction: Excellent Photocatalytic Performance and Insight into the Underlying Mechanism

The construction of semiconductor heterojunction photocatalysts that improve the separation and transfer of photoinduced charge carriers is an effective and widely employed strategy to boost photocatalytic performance. Herein, we have successfully constructed a CdS/Ag/Bi2WO6 Z-scheme heterojunction with an Ag-bridge as an effective charge transfer channel by a facile process. The heterostructure consists of both CdS and Ag nanoparticles anchored on the surface of Bi2WO6 nanosheets. The photocatalytic efficiency of the CdS/Ag/Bi2WO6 system was studied by the decontamination of tetracycline (TC) and Rhodamine B (RhB) under visible light irradiation (λ ≥ 420). The results exhibited that CdS/Ag/Bi2WO6 shows markedly higher photocatalytic performance than that of CdS, Bi2WO6, Ag/Bi2WO6, and CdS/Bi2WO6. The trapping experiment results verified that the •O2− and h+ radicals are the key active species. The results of photoluminescence spectral analysis and photocurrent responses indicated that the CdS/Ag/Bi2WO6 heterojunctions exhibit exceptional efficiency in separating and transferring photoinduced electron−hole pairs. Based on a series of characterization results, the boosted photocatalytic activity of the CdS/Ag/Bi2WO6 system is mostly due to the successful formation of the Ag-bridged Z-scheme heterojunction; these can not only inhibit the recombination rate of photoinduced charge carriers but also possess a splendid redox capacity. The work provides a way for designing a Z-scheme photocatalytic system based on Ag-bridged for boosting photocatalytic performance.


Introduction
The rapid advancement of human society has led to an increasing focus on the issues of energy scarcity and environmental pollution.As a promising and eco-friendly technology that uses solar energy to address environmental problems and energy scarcity, semiconductor-based photocatalysis has recently received significant attention [1][2][3][4].However, the traditional photocatalysts are only photoexcited in the ultraviolet, which only accounts for 4% of the total solar spectrum, thus considerably restraining their practical application [5,6].Therefore, it is imperative to develop new visible-light-driven (VLD) photocatalysts [7].
Bismuth tungstate (Bi 2 WO 6 ), a representative aurivillius oxide, possesses a unique layered structure composed of alternating [WO 4 ] 2− octahedral layers and [Bi 2 O 2 ] 2+ layers; this was advantageous for the transmission of photogenerated carriers.Bi 2 WO 6 has a suitable band gap of approximately 2.7 eV, which has been regarded as a promising VLD photocatalyst [8,9].In addition, Bi 2 WO 6 possesses numerous advantages, such as chemical stability, nontoxicity, and corrosion resistance.However, the UV-to-visible photo-absorption region of Bi 2 WO 6 is shorter than approximately 450 nm.And swift recombination of photoinduced charge carriers extremely restricts its energy conversion efficiency [10,11].To surmount these problems and improve the photocatalytic properties of pristine Bi 2 WO 6 , various techniques have been developed, such as morphological control [12,13], noble metal element doping [14,15], non-noble metal element doping [16,17], building heterojunction nanocomposite [18,19], and so on.Among these, the construction of heterostructures is a promising method, particularly the construction of Z-scheme photocatalytic systems [20,21].The Z-scheme photocatalytic system can not only improve the transfer efficiency of photoexcited electrons and holes but also ensure a powerful redox capacity [22].
Cadmium sulfide (CdS) is a narrow-bandgap semiconductor with a band gap of about 2.4 eV, which has been attracting much attention for environmental contaminant purification and hydrogen generation [36][37][38].As a consequence, CdS is usually coupled with various photocatalysts to enhance visible light absorption performance and the separation ability of photoinduced charge carriers [39][40][41].Besides, CdS is a very suitable semiconductor for assembling heterojunctions based on Bi 2 WO 6 , because the CB and VB of CdS are matched well with Bi 2 WO 6 [42][43][44].Zhang et al. [45] prepared Z-scheme CdS/Bi 2 WO 6 heterojunction via a simple hydrothermal method, and they found that 15% CdS/Bi 2 WO 6 photocatalysts could remove 60.82% of Cr(VI) and photodegrade almost all Rhodamine B within 1 h.
In this study, we design and synthesize ternary CdS/Ag/Bi 2 WO 6 Z-scheme heterojunction by a facile process.The photocatalytic experiments exhibited that this ternary CdS/Ag/Bi 2 WO 6 Z-scheme displayed excellent photocatalytic performances toward photodegrading Rhodamine B (RhB) and tetracycline (TC) under visible-light irradiation.The Ag nanoparticles can act as a charge transfer bridge between Bi 2 WO 6 and CdS; this could boost the transfer rate of photoinduced electrons and holes in this Z-scheme system.Moreover, a plausible mechanism was investigated and proposed for explaining the excellent photocatalytic performance of CdS/Ag/Bi 2 WO 6 heterojunctions.

Preparation of Bi 2 WO 6
Bi 2 WO 6 photocatalysts were synthesized via a simple hydrothermal method.Dissolve 2 mmol Bi(NO 3 ) 3 •5H 2 O, 1 mmol Na 2 WO 4 •2H 2 O, and 0.05 g of cetyltrimethylammonium bromide (CTAB) in diluted nitric acid and vigorously stir for 30 min to acquire a uniform suspension.The pH of the aforementioned suspension was adjusted to approximately 7 by NaOH solutions.After stirring for 30 min, the resultant solution was poured into a 100 mL Teflon-lined stainless autoclave and heated at 180 • C for 24 h.The precipitate was subsequently filtered and washed with distilled water and ethanol several times, then dried at 80 • C for 8 h.

Preparation of Ag/Bi 2 WO 6
The Ag/Bi 2 WO 6 photocatalyst was prepared using a photo-reduction method.To be specific, a certain amount of AgNO 3 (0.05 mmol) was added to 50 mL of distilled water and then stirred until AgNO 3 was completely dissolved in the dark.Afterward, the assynthetized Bi 2 WO 6 (1 mmol) was added to the AgNO 3 solution and irradiated by 500 W Xe light (1 h) with vigorous stirring.Then the Ag/Bi 2 WO 6 was collected and dried in a vacuum oven at 60 • C for 8 h.

Preparation of CdS/Ag/Bi 2 WO 6
The CdS/Ag/Bi 2 WO 6 was prepared using a precipitation method.A certain amount of Cd(NO 3 ) 2 •4H 2 O and 0.2 g of Ag/Bi 2 WO 6 were dispersed in 30 mL of distilled water with ultrasonic vibration for 30 min.Subsequently, 20 mL of Na 2 S solution was dropwise added to the above solution and stirred for 4 h.The precipitate was filtered and washed with distilled water and ethanol several times, and finally dried at 80 • C overnight.The mass ratio of CdS:Ag/Bi 2 WO 6 was controlled to be 0.04.Similarly, CdS/Bi 2 WO 6 were prepared under the same conditions.

Characterization of Photocatalysts
The phase structures of the prepared samples were measured by X-ray diffraction (XRD) (D/MAX-RB; Rigaku, Tokyo, Japan).The diffraction patterns were examined in the 2θ range from 20 • to 80 • with a Cu Kα source (λ = 0.15405) running at 40 kV and 30 mA.The morphology of the samples was examined by scanning electron microscopy (SEM; S-4800; Hitachi, Hitachi-shi, Japan).Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were analyzed with a transmission electron microscope (FEI Talos F200X G2, Thermo Scientific, Waltham, MA, USA) at an accelerating voltage of 200 kV.The UV-vis diffuse reflectance spectra (DRS) were collected via a UV-vis spectrophotometer (T9s; Persee, Beijing, China) equipped with an integrating sphere.Barium sulfate (BaSO 4 ) was used as the reference.Photoluminescence (PL) spectra data of the samples were recorded by a fluorescence spectrophotometer (F-4500; Hitachi, Japan).

Photocatalytic Experiments
The photocatalytic performances of the photocatalysts were assessed via the photodegradation of Rhodamine B (RhB) and tetracycline (TC) under visible light irradiation.A 400 W Xe lamp with a cut-off filter λ ≥ 420 nm served as a light source.In each photocatalytic degradation test, 40 mg of the as-obtained photocatalyst was dispersed in 40 mL of 10 mg/L RhB solution (or 20 mg/L TC).Prior to light irradiation, the prepared suspensions were stirred in the dark for 1 h to reach adsorption-desorption equilibrium.The RhB and TC concentrations were recorded by a UV-vis spectrophotometer at 553 and 357 nm, respectively.The degradation performance was evaluated using the ratios (C/C 0 ) of the RhB and TC concentrations (C 0 was the initial concentration, and C was the concentration at a given time).

Photoelectrochemical Measurements
Photoelectrochemical measurements were performed on an electrochemical workstation (5060F; RST, Zhengzhou, China) in a conventional three-electrode system with a 0.5 M Na 2 SO 4 aqueous solution.The samples, a saturated calomel electrode, and a Pt wire were used as the working, reference electrode, and counter electrodes, respectively.The light source was provided by a 100 W incandescent lamp with a 420 nm cut-off filter.The working electrode was manufactured as follows: 5 mg of photocatalyst was dispersed homogeneously in a certain amount of Nafion solution and ethanol (v/v = 30:1).Finally, the as-prepared samples were loaded onto the bottom middle of ITO glass with a diameter of 6 mm.Then the photocurrents of the samples with the light on and off were measured at 0.8 V.
light source was provided by a 100 W incandescent lamp with a 420 nm cut-off filter.working electrode was manufactured as follows: 5 mg of photocatalyst was dispersed mogeneously in a certain amount of Nafion solution and ethanol (v/v = 30:1).Finally, as-prepared samples were loaded onto the bottom middle of ITO glass with a diamete 6 mm.Then the photocurrents of the samples with the light on and off were measure 0.8 V.

Crystal Structure Analysis
The crystal structures of Bi2WO6, Ag/Bi2WO6, CdS/Bi2WO6, CdS/Ag/Bi2WO6, and C were investigated using XRD.From Figure 1A, all diffraction peaks were completely responding to the structure of orthorhombic Bi2WO6 (JCPDS Card No. 39-0256).The ch acteristic peaks at 2θ = 28.3°,32.8°, 47.1°, 55.8°, and 58.5° were attributed to the (1 3 1 0 0), (2 0 2), (3 3 1), and (2 6 2) crystal planes, respectively.Comparing the curves of Bi2W and Ag/Bi2WO6, it can be observed that they have similar patterns.This finding was c sistent with the previous results [34].This may be because of the low loading amoun Ag nanoparticles in the heterojunction.Furthermore, no characteristic diffraction pe for CdS were observed in CdS/Bi2WO6 and CdS/Ag/Bi2WO6, which could be caused by high dispersion, small particles, and small amount of CdS dopant.Similar results w found in CdS/Bi2WO6 [42] and CdS/BiOCl [46]. Figure 1B displays the XRD pattern of p CdS, which can be assigned to the cubic phase of CdS (JCPDS Card No. 89-0440) [47].diffraction peaks at 26.4°, 43.9°, and 51.9° were well-matched with the crystal planes o 1 1), (2 2 0), and (3 1 1) of CdS, respectively.The existence of Ag nanoparticles and/or C in CdS/Ag/Bi2WO6 composites was further identified by TEM analysis.

Morphology Characterization
The morphologies of Bi2WO6, Ag/Bi2WO6, CdS/Bi2WO6, and CdS/Ag/Bi2WO6 com sites were studied by scanning electron microscope (SEM; S-4800; Hitachi, Hitachi-shi pan).As could be observed from Figure 2A, the Bi2WO6 showed an aggregated nanosh like microstructure.It was worth noting that Ag nanoparticles on the surface of Bi2W were found, indicating that Ag nanoparticles were successfully deposited on the Bi2W surface (Figure 2B).The SEM images of CdS/Bi2WO6 and CdS/Ag/Bi2WO6 composites ( ure 2C,D) were found to be similar to pure Bi2WO6.This similarity could be attribute the use of the same original Bi2WO6 material, as well as high dispersion and the sm particle size of CdS in the composites.

Morphology Characterization
The morphologies of Bi 2 WO 6 , Ag/Bi 2 WO 6 , CdS/Bi 2 WO 6 , and CdS/Ag/Bi 2 WO 6 composites were studied by scanning electron microscope (SEM; S-4800; Hitachi, Hitachishi, Japan).As could be observed from Figure 2A, the Bi 2 WO 6 showed an aggregated nanosheet-like microstructure.It was worth noting that Ag nanoparticles on the surface of Bi 2 WO 6 were found, indicating that Ag nanoparticles were successfully deposited on the Bi 2 WO 6 surface (Figure 2B).The SEM images of CdS/Bi 2 WO 6 and CdS/Ag/Bi 2 WO 6 composites (Figure 2C,D) were found to be similar to pure Bi 2 WO 6 .This similarity could be attributed to the use of the same original Bi 2 WO 6 material, as well as high dispersion and the small particle size of CdS in the composites.
The morphologies of the CdS/Ag/Bi 2 WO 6 were further observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM).From Figure 3A,B, the CdS/Ag/Bi 2 WO 6 sample showed an irregular and nanosheetlike microstructure.The HRTEM image (Figure 3C) displays that some nanoparticles have grown on the Bi 2 WO 6 nanosheet.It is worth noting that the edges of Bi 2 WO 6 nanosheets appear to have two different types of nanoparticles, which may be Ag and CdS nanoparticles.The HRTEM image (Figure 3D) shows the crystal plane spacing of 0.23, 0.34, and 0.27 nm; these correspond to the (1 1 1) plane of Ag nanoparticles, the (1 1 1) plane of CdS, and the (2 0 0) plane of Bi 2 WO 6 , respectively.Furthermore, Ag nanoparticles loaded on Bi 2 WO 6 nanosheets are in close contact with CdS.It shows that CdS and Ag nanoparticles were successfully supported on the Bi 2 WO 6 nanosheet, which could be beneficial for charge separation within the Z-scheme CdS/Ag/Bi 2 WO 6 heterojunctions in comparison to pure Bi 2 WO 6 .Moreover, the elemental mapping method was employed to investigate the composition distribution in CdS/Ag/Bi 2 WO 6 samples.As depicted in Figure 3E1-E6, W, O, Bi, Ag, Cd, and S elements are evenly distributed throughout the CdS/Ag/Bi 2 WO 6 sample.And the element mapping distribution of Ag, Cd, and S proved evidence that CdS, Ag, and Bi 2 WO 6 are closely combined.Combined with the TEM results, it can be indicated that CdS and Ag nanoparticles were evenly loaded on the surface of Bi 2 WO 6 nanosheets.The morphologies of the CdS/Ag/Bi2WO6 were further observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM).From Figure 3A,B, the CdS/Ag/Bi2WO6 sample showed an irregular and nanosheet-like microstructure.The HRTEM image (Figure 3C) displays that some nanoparticles have grown on the Bi2WO6 nanosheet.It is worth noting that the edges of Bi2WO6 nanosheets appear to have two different types of nanoparticles, which may be Ag and CdS nanoparticles.The HRTEM image (Figure 3D) shows the crystal plane spacing of 0.23, 0.34, and 0.27 nm; these correspond to the (1 1 1) plane of Ag nanoparticles, the (1 1 1) plane of CdS, and the (2 0 0) plane of Bi2WO6, respectively.Furthermore, Ag nanoparticles loaded on Bi2WO6 nanosheets are in close contact with CdS.It shows that CdS and Ag nanoparticles were successfully supported on the Bi2WO6 nanosheet, which could be beneficial for charge separation within the Z-scheme CdS/Ag/Bi2WO6 heterojunctions in comparison to pure Bi2WO6.Moreover, the elemental mapping method was employed to investigate the composition distribution in CdS/Ag/Bi2WO6 samples.As depicted in Figure 3E1-E6, W, O, Bi, Ag, Cd, and S elements are evenly distributed throughout the CdS/Ag/Bi2WO6 sample.And the element mapping distribution of Ag, Cd, and S proved evidence that CdS, Ag, and Bi2WO6 are closely combined.Combined with the TEM results, it can be indicated that CdS and Ag nanoparticles were evenly loaded on the surface of Bi2WO6 nanosheets.

Optical Properties
The light absorption properties of the obtained samples were characterized by UV-vis DRS, as illustrated in Figure 4.The absorption edges were observed at approximately 450 nm and 650 nm for Bi 2 WO 6 and CdS, respectively.The optical absorption edge of the CdS/Bi 2 WO 6 composite was distinctly red-shifted compared with Bi 2 WO 6 , which could be assigned to the forming heterojunction between CdS and Bi 2 WO 6 .Meanwhile, after Ag nanoparticles growth on the surface of Bi 2 WO 6 nanosheets, Ag/Bi 2 WO 6 had a wide absorption in the visible light region; this may be attributed to the surface plasmon resonance effect of spatially confined electrons in Ag nanoparticles [48].Compared with all other samples, the obtained CdS/Ag/Bi 2 WO 6 ternary system exhibited enhanced visible light absorption, which may be on account of the synergetic effect of CdS and Ag.These results reveal that the as-prepared CdS/Ag/Bi 2 WO 6 heterojunction photocatalyst had an excellent visible-light absorption range and thus produced more photoinduced electron−hole pairs, as demonstrated subsequently.The Kubelka-Munk formula: ahv = A(hv − Eg) n/2 was used to estimate the band gap values of CdS and Bi 2 WO 6 [49].For CdS and Bi 2 WO 6 , the values of n are 1 and 4, respectively [50,51].From the inset of Figure 4, the band gap values of CdS and Bi 2 WO 6 are approximately 2.17 eV and 2.81 eV, respectively.

Optical Properties
The light absorption properties of the obtained samples were characterized by UVvis DRS, as illustrated in Figure 4.The absorption edges were observed at approximately 450 nm and 650 nm for Bi2WO6 and CdS, respectively.The optical absorption edge of the CdS/Bi2WO6 composite was distinctly red-shifted compared with Bi2WO6, which could be assigned to the forming heterojunction between CdS and Bi2WO6.Meanwhile, after Ag nanoparticles growth on the surface of Bi2WO6 nanosheets, Ag/Bi2WO6 had a wide absorption in the visible light region; this may be attributed to the surface plasmon resonance effect of spatially confined electrons in Ag nanoparticles [48].Compared with all other samples, the obtained CdS/Ag/Bi2WO6 ternary system exhibited enhanced visible light absorption, which may be on account of the synergetic effect of CdS and Ag.These results reveal that the as-prepared CdS/Ag/Bi2WO6 heterojunction photocatalyst had an excellent visible-light absorption range and thus produced more photoinduced electron−hole pairs, as demonstrated subsequently.The Kubelka-Munk formula: аhv = A(hv − Eg) n/2 was used to estimate the band gap values of CdS and Bi2WO6 [49].For CdS and Bi2WO6, the values of n are 1 and 4, respectively [50,51].From the inset of Figure 4, the band gap values of CdS and Bi2WO6 are approximately 2.17 eV and 2.81 eV, respectively.

Photocatalytic Performances of the Samples
The photocatalytic performance of pure CdS, Bi2WO6, Ag/Bi2WO6, CdS/Bi2WO6, and CdS/Ag/Bi2WO6 was assessed via monitoring the photodegradation of RhB under visible light illumination (λ ≥ 420 nm).All catalysts were dispersed in RhB solution and then

Photocatalytic Performances of the Samples
The photocatalytic performance of pure CdS, Bi 2 WO 6 , Ag/Bi 2 WO 6 , CdS/Bi 2 WO 6 , and CdS/Ag/Bi 2 WO 6 was assessed via monitoring the photodegradation of RhB under visible light illumination (λ ≥ 420 nm).All catalysts were dispersed in RhB solution and then magnetically stirred in a dark environment for 60 min to attain an adsorption-desorption equilibrium.The adsorption capacity of all photocatalysts for RhB is shown in Figure 5A.The results indicated that the pure Bi 2 WO 6 photocatalyst exhibits the highest adsorption capacity among all the photocatalysts, while the pure CdS photocatalyst has the lowest adsorption capacity.Additionally, Ag/Bi 2 WO 6 , CdS/Bi 2 WO 6 , and CdS/Ag/Bi 2 WO 6 samples demonstrate similar capacities for RhB adsorption.From Figure 5B, it was found that the RhB degradation efficiencies over the Bi 2 WO 6 , CdS, Ag/Bi 2 WO 6 , CdS/Bi 2 WO 6 , and CdS/Ag/Bi 2 WO 6 under visible light illumination for 40 min are about 45.8%, 53.2%, 82.2%, 90.5%, and 98.6%, respectively.Obviously, of all the as-prepared photocatalysts, Zscheme CdS/Ag/Bi 2 WO 6 heterojunction exhibited the greatest photocatalytic degradation effect, which was 2.15 and 1.85 times higher than the pure Bi 2 WO 6 and CdS, respectively.The superior photocatalytic activity of CdS/Ag/Bi 2 WO 6 could be because this Z-scheme ternary system has a stronger absorption capacity for visible light and excellent separation and transmission of photoinduced carriers compared to other photocatalysts.In addition to RhB, other colorless pollutants, such as Tetracycline (TC), were also selected to estimate the photocatalytic efficiency of Bi2WO6, CdS, Ag/Bi2WO6, CdS/Bi2WO6, and CdS/Ag/Bi2WO6 under visible-light illumination (λ ≥ 420 nm).Before irradiation, the adsorption capacities of Bi2WO6, CdS, Ag/Bi2WO6, CdS/Bi2WO6, and CdS/Ag/Bi2WO6 were also subjected to 60 min in a dark environment.As depicted in Figure 5C, the adsorption In addition to RhB, other colorless pollutants, such as Tetracycline (TC), were also selected to estimate the photocatalytic efficiency of Bi 2 WO 6 , CdS, Ag/Bi 2 WO 6 , CdS/Bi 2 WO 6 , and CdS/Ag/Bi 2 WO 6 under visible-light illumination (λ ≥ 420 nm).Before irradiation, the adsorption capacities of Bi 2 WO 6 , CdS, Ag/Bi 2 WO 6 , CdS/Bi 2 WO 6 , and CdS/Ag/Bi 2 WO 6 were also subjected to 60 min in a dark environment.As depicted in Figure 5C, the adsorption ratios of Bi 2 WO 6 , CdS, Ag/Bi 2 WO 6 , CdS/Bi 2 WO 6 , and CdS/Ag/Bi 2 WO 6 are approximately 0.318, 0.330, 0.410, 0.430, and 0.422, respectively.Unlike the adsorption of RhB, Ag/Bi 2 WO 6 , CdS/Bi 2 WO 6 , and CdS/Ag/Bi 2 WO 6 have a higher adsorption capacity for TC.From Figure 5D, after the adsorption-desorption equilibrium, it can be discovered that the Z-scheme CdS/Ag/Bi 2 WO 6 heterojunction presented the optimal photocatalytic activity, with a TC photodegradation efficiency of about 78% after 45 min of visible light illumination.
From the perspective of future practical applications, the repeatability and stability of the catalysts play a crucial role.Therefore, to evaluate the repeatability and stability of the CdS/Ag/Bi 2 WO 6 , recycling experiments were conducted using the heterojunction photocatalyst for RhB photodegradation (Figure 5G).The recovery process of the photocatalyst is as follows: after each degradation cycle, collect the photocatalyst powder, wash it three times with distilled water and ethanol, dry it, and proceed with subsequent photocatalysis.In the cyclic experiment, the error bars were derived from three batches of CdS/Ag/Bi 2 WO 6 samples, representing the standard deviation.It can be observed that CdS/Ag/Bi 2 WO 6 maintains high degradation efficiency after four cycles.As a comparison, in the cyclic experiment, no regeneration treatment was applied to directly use the collected photocatalyst powder.The results are shown in Figure 5H.After four cyclic experiments, there is a slight decrease in photocatalytic degradation efficiency compared to previous regeneration procedures.This may be attributed to pollutants adsorption on its surface after the experiment, which reduces active site availability and performance deterioration.This finding indicated remarkable stability of the Z-scheme CdS/Ag/Bi 2 WO 6 heterojunction.

Photocatalytic Mechanism of CdS/Ag/Bi 2 WO 6 Heterojunction Photocatalyst
Trapping experiments were performed to identify the key reactive radicals and further understand the probable catalytic mechanism.In the CdS/Ag/Bi 2 WO 6 photocatalytic reaction system, isopropanol (IPA, 10 mM), sodium oxalate (Na 2 C 2 O 4 , 10 mM), and benzoquinone (BQ, 1 mM) were used as scavengers of hydroxyl radicals ( • OH), holes (h + ), and superoxide ions ( • O 2 − ), respectively.As displayed in Figure 6, the degradation efficiency of CdS/Ag/Bi 2 WO 6 was slightly inhibited when IPA was added, implying that • OH may not play a main role in the photodegradation of RhB.After adding BQ or Na 2 C 2 O 4 , the degradation performance of the CdS/Ag/Bi 2 WO 6 photocatalyst was dramatically inhibited, which confirmed that • O 2 − and h + were the key active species in the CdS/Ag/Bi 2 WO 6 photocatalytic reaction.
superoxide ions ( O2 ), respectively.As displayed in Figure 6, the degradation efficiency of CdS/Ag/Bi2WO6 was slightly inhibited when IPA was added, implying that • OH may not play a main role in the photodegradation of RhB.After adding BQ or Na2C2O4, the degradation performance of the CdS/Ag/Bi2WO6 photocatalyst was dramatically inhibited, which confirmed that • O2 − and h + were the key active species in the CdS/Ag/Bi2WO6 photocatalytic reaction.The recombination efficiency of photoinduced electron-hole pairs was analyzed using photoluminescence (PL) spectroscopy.The weaker PL intensity typically indicates a lower possibility of photoinduced electron-hole recombination [52].From Figure 7, the PL spectra of pure Bi2WO6, Ag/Bi2WO6, CdS/Bi2WO6, and CdS/Ag/Bi2WO6 are in the range of 350-700 nm, and all intense emission peaks are at about 468 nm.It is observed that the order of the emission intensity of the catalysts is: Bi2WO6 > Ag/Bi2WO6 > CdS/Bi2WO6 > The recombination efficiency of photoinduced electron-hole pairs was analyzed using photoluminescence (PL) spectroscopy.The weaker PL intensity typically indicates a lower possibility of photoinduced electron-hole recombination [52].From Figure 7, the PL spectra of pure Bi 2 WO 6 , Ag/Bi 2 WO 6 , CdS/Bi 2 WO 6 , and CdS/Ag/Bi 2 WO 6 are in the range of 350-700 nm, and all intense emission peaks are at about 468 nm.It is observed that the order of the emission intensity of the catalysts is: Bi 2 WO 6 > Ag/Bi 2 WO 6 > CdS/Bi 2 WO 6 > CdS/Ag/Bi 2 WO 6 .The results confirmed that the CdS/Ag/Bi 2 WO 6 heterojunctions possess the strongest separation efficiency of photoinduced charge carriers, suggesting they have superior photocatalytic performance.To further recognize the transmission and separation of photogenerated charge in CdS/Ag/Bi2WO6, the photocurrent response measurement is also employed under visible light irradiation.The higher photocurrent intensity means that the transmission efficiency of photogenerated carriers is higher, which results in outstanding photocatalytic activity [53].Figure 8 shows the regular photocurrent responses of pure Bi2WO6, Ag/Bi2WO6, CdS/Bi2WO6, and CdS/Ag/Bi2WO6 in the dark and light.The photocurrent density increases in the order Bi2WO6, Ag/Bi2WO6, CdS/Bi2WO6, and CdS/Ag/Bi2WO6, which corresponds with their PL and photocatalytic properties.Therefore, given the above results, the as-prepared CdS/Ag/Bi2WO6 ternary heterojunction system can considerably enhance photogenerated charge transfer and separation efficiency, thereby improving photocata- To further recognize the transmission and separation of photogenerated charge in CdS/Ag/Bi 2 WO 6 , the photocurrent response measurement is also employed under visible light irradiation.The higher photocurrent intensity means that the transmission efficiency of photogenerated carriers is higher, which results in outstanding photocatalytic activity [53].Figure 8 shows the regular photocurrent responses of pure Bi 2 WO 6 , Ag/Bi 2 WO 6 , CdS/Bi 2 WO 6 , and CdS/Ag/Bi 2 WO 6 in the dark and light.The photocurrent density increases in the order Bi 2 WO 6 , Ag/Bi 2 WO 6 , CdS/Bi 2 WO 6 , and CdS/Ag/Bi 2 WO 6 , which corresponds with their PL and photocatalytic properties.Therefore, given the above results, the as-prepared CdS/Ag/Bi 2 WO 6 ternary heterojunction system can considerably enhance photogenerated charge transfer and separation efficiency, thereby improving photocatalytic performance.To further recognize the transmission and separation of photogenerated charge in CdS/Ag/Bi2WO6, the photocurrent response measurement is also employed under visible light irradiation.The higher photocurrent intensity means that the transmission efficiency of photogenerated carriers is higher, which results in outstanding photocatalytic activity [53].Figure 8 shows the regular photocurrent responses of pure Bi2WO6, Ag/Bi2WO6, CdS/Bi2WO6, and CdS/Ag/Bi2WO6 in the dark and light.The photocurrent density increases in the order Bi2WO6, Ag/Bi2WO6, CdS/Bi2WO6, and CdS/Ag/Bi2WO6, which corresponds with their PL and photocatalytic properties.Therefore, given the above results, the as-prepared CdS/Ag/Bi2WO6 ternary heterojunction system can considerably enhance photogenerated charge transfer and separation efficiency, thereby improving photocatalytic performance.To systematically explore the photocatalytic mechanism, it is necessary to calculate the position of the conduction band (CB) and valence band (VB) using the following equation: χ value is the Mulliken electronegativity of Bi 2 WO 6 (6.39 eV [54]) and CdS (5.18 eV [55]).− was the key active species in the CdS/Ag/Bi 2 WO 6 photocatalytic reaction system.This means that the CdS/Ag/Bi 2 WO 6 system has a different photoinduced electron and hole transport itinerary from the Bi 2 WO 6 /CdS system, which may be derived from the formation of the Z-scheme system with Ag-bridge as an efficient charge transfer medium.
Based on the analysis of the above result, a plausible mechanism for illustrating the transport path of photogenerated electron−hole pairs over the CdS/Ag/Bi 2 WO 6 heterojunctions was proposed, as presented schematically in Figure 9. Upon exposure of the CdS/Ag/Bi 2 WO 6 photocatalyst to visible light, both Bi 2 WO 6 and CdS can be excited and then generate photogenerated electrons and holes.Because the CB position of Bi 2 WO 6 is more negative than the Fermi level of silver nanoparticles, electrons on its CB will be injected into the silver nanoparticles through the Schottky barrier.Meanwhile, the holes on the VB of CdS will transfer to silver nanoparticles.Therefore, electrons generated from the CB of Bi 2 WO 6 and holes generated from the VB of CdS can directly annihilate through the Ag nanoparticle bridge.The strong reductive electrons on the CB of CdS can react with dissolved oxygen molecules to form the active species • O 2 − , which can oxidize organic contaminants into decomposed products.And the holes on the VB of Bi 2 WO 6 oxidize organic contaminants directly.
transport path of photogenerated electron−hole pairs over the CdS/Ag/Bi2WO6 heterojunctions was proposed, as presented schematically in Figure 9. Upon exposure of the CdS/Ag/Bi2WO6 photocatalyst to visible light, both Bi2WO6 and CdS can be excited and then generate photogenerated electrons and holes.Because the CB position of Bi2WO6 is more negative than the Fermi level of silver nanoparticles, electrons on its CB will be injected into the silver nanoparticles through the Schottky barrier.Meanwhile, the holes on the VB of CdS will transfer to silver nanoparticles.Therefore, electrons generated from the CB of Bi2WO6 and holes generated from the VB of CdS can directly annihilate through the Ag nanoparticle bridge.The strong reductive electrons on the CB of CdS can react with dissolved oxygen molecules to form the active species • O2 − , which can oxidize organic contaminants into decomposed products.And the holes on the VB of Bi2WO6 oxidize organic contaminants directly.The Z-scheme with Ag-bridge in the CdS/Ag/Bi2WO6 ternary system not only promotes the spatial isolation of the photoinduced electron−hole pairs but also can maintain The Z-scheme with Ag-bridge in the CdS/Ag/Bi 2 WO 6 ternary system not only promotes the spatial isolation of the photoinduced electron−hole pairs but also can maintain powerful redox capability, thus significantly boosting the quantum yield and photocatalytic activity.

Conclusions
In summary, the CdS/Ag/Bi 2 WO 6 Z-scheme heterojunction photocatalysts were successfully synthesized by hydrothermal, photoreduction, and precipitation methods.Compared with single-and two-component systems such as CdS, Bi 2 WO 6 , Ag/Bi 2 WO 6 , and CdS/Bi 2 WO 6 samples, the CdS/Ag/Bi 2 WO 6 Z-scheme heterojunction exhibited remarkably boosted photocatalytic performance for the degradation of RhB and TC under visible light irradiation (λ ≥ 420 nm).The plausible photocatalytic mechanism was raised to explain the superior photocatalytic performance based on DRS and PL analysis, photocurrent responses, band edge positions, and the active species trapping experiment.In the CdS/Ag/Bi 2 WO 6 Z-scheme heterojunctions system, the introduced Ag nanoparticles can be used as a bridge for the transportation of photogenerated charge carriers between CdS and Bi 2 WO 6 , thus accelerating photogenerated charge carrier separation and enhancing redox capacity.This work provided an effective method for the design and construction of extremely efficient photocatalysts based on semiconductor/noble-metal/semiconductor Z-scheme heterojunction composites.

Data Availability Statement:
The data presented in the study are available from the corresponding author.
14, x FOR PEER REVIEW 8 of 14

Figure 5 .
Figure 5. Adsorption ratio of RhB (A) and TC (C) in the dark; photodegradation of RhB (B) and TC (D) with different photocatalysts under visible light irradiation (λ ≥ 420 nm); the pseudo-first-order reaction kinetics of the RhB (E) and TC (F) degradation over different photocatalysts; cyclic photodegradation of RhB by CdS/Ag/Bi2WO6 photocatalyst with (G) and without (H) the regeneration procedure.

Figure 5 .
Figure 5. Adsorption ratio of RhB (A) and TC (C) in the dark; photodegradation of RhB (B) and TC (D) with different photocatalysts under visible light irradiation (λ ≥ 420 nm); the pseudofirst-order reaction kinetics of the RhB (E) and TC (F) degradation over different photocatalysts; cyclic photodegradation of RhB by CdS/Ag/Bi 2 WO 6 photocatalyst with (G) and without (H) the regeneration procedure.

Figure 6 .
Figure 6.The species trapping experiments of CdS/Ag/Bi2WO6 under visible light irradiation.

Figure 6 .
Figure 6.The species trapping experiments of CdS/Ag/Bi 2 WO 6 under visible light irradiation.

Nanomaterials 2024 ,
14, x FOR PEER REVIEW 10 of 14CdS/Ag/Bi2WO6.The results confirmed that the CdS/Ag/Bi2WO6 heterojunctions possess the strongest separation efficiency of photoinduced charge carriers, suggesting they have superior photocatalytic performance.

Figure 9 .
Figure 9. Schematic diagram of the proposed photogenerated charge separation and transmission over the CdS/Ag/Bi2WO6 heterojunctions under visible light irradiation.

Figure 9 .
Figure 9. Schematic diagram of the proposed photogenerated charge separation and transmission over the CdS/Ag/Bi 2 WO 6 heterojunctions under visible light irradiation.

Author Contributions:
Conceptualization, F.W., L.J. and G.Z.; methodology, Z.Y. and F.W.; validation, Q.H. and J.L.; formal analysis, P.L. and G.Z.; investigation, Y.C. and L.J.; data curation, X.Z. and R.S.; writing-original draft preparation, F.W.; writing-review and editing, F.W. All authors have read and agreed to the published version of the manuscript.Funding: This research was funded by the Natural Science Foundation of Shandong Province (ZR2022QB136), Shandong Key Research and Development Program (2022SFGC0302, 2023RKY06020), Central Guiding Local Science and Technology Development Special Project (YDZX2022152, YDZX2023013).
[45]band gap values of Bi 2 WO 6 and CdS were 2.81 and 2.17 eV, respectively.The calculated E VB values of Bi 2 WO 6 and CdS were 3.29 and 1.76 eV, respectively, and their corresponding E CB values were 0.48 and −0.41 eV.With the determination of the CB and VB values of Bi 2 WO 6 and CdS, the transport itinerary of photoinduced electron−hole pairs gradually becomes clear.According to the charge carrier transfer mode of a typical type-II heterostructure, the photogenerated electrons on the CB of CdS will migrate to the CB of Bi 2 WO 6 .Because the conduction band of potential Bi 2 WO 6 is more positive than the E(O 2 / • O 2 − ) (−0.33 eV)[45], electrons on its CB cannot reduce O 2 into • O 2 − .However, from the results of trapping experiments, • O 2