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

Abstract

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.

1. 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 (Bi₂WO₆), a representative aurivillius oxide, possesses a unique layered structure composed of alternating [WO₄]²⁻ octahedral layers and [Bi₂O₂]²⁺ layers; this was advantageous for the transmission of photogenerated carriers. Bi₂WO₆ has a suitable band gap of approximately 2.7 eV, which has been regarded as a promising VLD photocatalyst [8,9]. In addition, Bi₂WO₆ possesses numerous advantages, such as chemical stability, nontoxicity, and corrosion resistance. However, the UV-to-visible photo-absorption region of Bi₂WO₆ 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 Bi₂WO₆, 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].

Among the connection modes of Z-scheme photocatalytic systems, ternary semiconductor/conductor/semiconductor Z-scheme heterojunctions in which two different semiconductors have a matching band structure can prominently enhance the photocatalytic performance [23,24]. Recently, noble metals (such as Ag, Pt, and Au) have been used as charge-carrying mediators, which can quickly transfer interfacial charge between two semiconductors. Besides, noble metal nanoparticles have a surface plasmon resonance during photocatalytic reactions. In recent years, various semiconductor/noble-metal/semiconductor Z-scheme heterojunctions, such as CdS/Ag/g-C₃N₄ [25], g-C₃N₄@Ag/BiVO₄ [26], Co₃O₄/Ag/Bi₂WO₆ [27], CdS/Au/BiVO₄ [28], BaTiO₃/Au/g-C₃N₄ [29], g-C₃N₄/Ag/MoS₂ [30], g-C₃N₄/Pt/Bi₂WO₆ [31], BiVO₄/Au/CdS [32], MoS₂/Au/g-C₃N₄ [33], etc., have been successfully synthesized. Xiao et al. [34] constructed C₃N₄@Ag-Bi₂WO₆ by a facile process, and the ternary system showed a boosted photocatalytic capacity for degrading RhB and producing H₂ than that of single- and two-component systems; this was mainly due to the Z-scheme delivery mechanism. Gao et al. [35] successfully prepared BiVO₄/Ag/CdS Z-scheme heterojunction, which attained improved ability in synergistic adsorption and photocatalytic degradation of fluoroquinolones. Hence, constructing a ternary semiconductor/noble metal/Bi₂WO₆ Z-scheme heterojunction could be a very promising strategy to obtain excellent photocatalytic activity.

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 Bi₂WO₆, because the CB and VB of CdS are matched well with Bi₂WO₆ [42,43,44]. Zhang et al. [45] prepared Z-scheme CdS/Bi₂WO₆ heterojunction via a simple hydrothermal method, and they found that 15% CdS/Bi₂WO₆ 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/Bi₂WO₆ Z-scheme heterojunction by a facile process. The photocatalytic experiments exhibited that this ternary CdS/Ag/Bi₂WO₆ 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 Bi₂WO₆ 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/Bi₂WO₆ heterojunctions.

2. Materials and Methods

2.1. Preparation of Bi₂WO₆

Bi₂WO₆ photocatalysts were synthesized via a simple hydrothermal method. Dissolve 2 mmol Bi(NO₃)₃·5H₂O, 1 mmol Na₂WO₄·2H₂O, 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.

2.2. Preparation of Ag/Bi₂WO₆

The Ag/Bi₂WO₆ photocatalyst was prepared using a photo-reduction method. To be specific, a certain amount of AgNO₃ (0.05 mmol) was added to 50 mL of distilled water and then stirred until AgNO₃ was completely dissolved in the dark. Afterward, the as-synthetized Bi₂WO₆ (1 mmol) was added to the AgNO₃ solution and irradiated by 500 W Xe light (1 h) with vigorous stirring. Then the Ag/Bi₂WO₆ was collected and dried in a vacuum oven at 60 °C for 8 h.

2.3. Preparation of CdS/Ag/Bi₂WO₆

The CdS/Ag/Bi₂WO₆ was prepared using a precipitation method. A certain amount of Cd(NO₃)₂·4H₂O and 0.2 g of Ag/Bi₂WO₆ were dispersed in 30 mL of distilled water with ultrasonic vibration for 30 min. Subsequently, 20 mL of Na₂S 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/Bi₂WO₆ was controlled to be 0.04. Similarly, CdS/Bi₂WO₆ were prepared under the same conditions.

2.4. 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 (BaSO₄) was used as the reference. Photoluminescence (PL) spectra data of the samples were recorded by a fluorescence spectrophotometer (F-4500; Hitachi, Japan).

2.5. 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/C₀) of the RhB and TC concentrations (C₀ was the initial concentration, and C was the concentration at a given time).

2.6. 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 Na₂SO₄ 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.

3. Results and Discussion

3.1. Crystal Structure Analysis

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

3.2. Morphology Characterization

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

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.

3.3. 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 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: ahv = 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.

3.4. 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 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 Bi₂WO₆ photocatalyst exhibits the highest adsorption capacity among all the photocatalysts, while the pure CdS photocatalyst has the lowest adsorption capacity. Additionally, Ag/Bi₂WO₆, CdS/Bi₂WO₆, and CdS/Ag/Bi₂WO₆ samples demonstrate similar capacities for RhB adsorption. From Figure 5B, it was found that the RhB degradation efficiencies over the Bi₂WO₆, CdS, Ag/Bi₂WO₆, CdS/Bi₂WO₆, and CdS/Ag/Bi₂WO₆ 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, Z-scheme CdS/Ag/Bi₂WO₆ heterojunction exhibited the greatest photocatalytic degradation effect, which was 2.15 and 1.85 times higher than the pure Bi₂WO₆ and CdS, respectively. The superior photocatalytic activity of CdS/Ag/Bi₂WO₆ 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 ratios of Bi₂WO₆, CdS, Ag/Bi₂WO₆, CdS/Bi₂WO₆, and CdS/Ag/Bi₂WO₆ are approximately 0.318, 0.330, 0.410, 0.430, and 0.422, respectively. Unlike the adsorption of RhB, Ag/Bi₂WO₆, CdS/Bi₂WO₆, and CdS/Ag/Bi₂WO₆ 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/Bi₂WO₆ heterojunction presented the optimal photocatalytic activity, with a TC photodegradation efficiency of about 78% after 45 min of visible light illumination.

The kinetic behavior of the Bi₂WO₆, CdS, Ag/Bi₂WO₆, CdS/Bi₂WO₆, and CdS/Ag/Bi₂WO₆ photocatalysts in the RhB and TC degradation processes is displayed in Figure 5E and Figure 5F, respectively. Moreover, the pseudo-first-order equation was employed to fit the kinetic process. The Ln(C/C₀) ∼ reaction time (t) curves exhibited linear variations, indicating that the photocatalytic degradation data of RhB and TC followed the first-level reaction kinetics law. The rate constants (RhB) of Bi₂WO₆, CdS, Ag/Bi₂WO₆, CdS/Bi₂WO₆, and CdS/Ag/Bi₂WO₆ were calculated as 0.0171, 0.0261, 0.0571, 0.0634, and 0.1075 min⁻¹, respectively. Similarly, the rate constants (TC) of the Bi₂WO₆, CdS, Ag/Bi₂WO₆, CdS/Bi₂WO₆, and CdS/Ag/Bi₂WO₆ were determined as 0.0161, 0.0286, 0.0278, 0.0261, and 0.0319 min⁻¹, respectively. It was evident that, compared with single- and two-component photocatalysts, the reaction rate of CdS/Ag/Bi₂WO₆ heterojunction has been significantly improved.

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/Bi₂WO₆, 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/Bi₂WO₆ samples, representing the standard deviation. It can be observed that CdS/Ag/Bi₂WO₆ 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/Bi₂WO₆ heterojunction.

3.5. Photocatalytic Mechanism of CdS/Ag/Bi₂WO₆ Heterojunction Photocatalyst

Trapping experiments were performed to identify the key reactive radicals and further understand the probable catalytic mechanism. In the CdS/Ag/Bi₂WO₆ photocatalytic reaction system, isopropanol (IPA, 10 mM), sodium oxalate (Na₂C₂O₄, 10 mM), and benzoquinone (BQ, 1 mM) were used as scavengers of hydroxyl radicals (^•OH), holes (h⁺), and 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 > CdS/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.

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:

$${ \mathrm{E}}_{\mathrm{VB}}=\mathrm{\chi }-{\mathrm{E}}_{\mathrm{e}}+0.5{\mathrm{E}}_{\mathrm{g}}$$

(1)

$${ \mathrm{E}}_{\mathrm{CB}}={\mathrm{E}}_{\mathrm{VB}}-{\mathrm{E}}_{\mathrm{g}}$$

(2)

χ value is the Mulliken electronegativity of Bi₂WO₆ (6.39 eV [54]) and CdS (5.18 eV [55]). The band gap values of Bi₂WO₆ and CdS were 2.81 and 2.17 eV, respectively. The calculated E_VB values of Bi₂WO₆ 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 Bi₂WO₆ 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 Bi₂WO₆. Because the conduction band of potential Bi₂WO₆ is more positive than the E(O₂/^•O₂⁻) (−0.33 eV) [45], electrons on its CB cannot reduce O₂ into ^•O₂⁻. However, from the results of trapping experiments, ^•O₂⁻ was the key active species in the CdS/Ag/Bi₂WO₆ photocatalytic reaction system. This means that the CdS/Ag/Bi₂WO₆ system has a different photoinduced electron and hole transport itinerary from the Bi₂WO₆/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/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 powerful redox capability, thus significantly boosting the quantum yield and photocatalytic activity.

4. Conclusions

In summary, the CdS/Ag/Bi₂WO₆ Z-scheme heterojunction photocatalysts were successfully synthesized by hydrothermal, photoreduction, and precipitation methods. Compared with single- and two-component systems such as CdS, Bi₂WO₆, Ag/Bi₂WO₆, and CdS/Bi₂WO₆ samples, the CdS/Ag/Bi₂WO₆ 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/Bi₂WO₆ Z-scheme heterojunctions system, the introduced Ag nanoparticles can be used as a bridge for the transportation of photogenerated charge carriers between CdS and Bi₂WO₆, 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.