Z-scheme CdS–Au–BiVO₄ with enhanced photocatalytic activity for organic contaminant decomposition

Abstract

A Z-scheme heterogeneous photocatalyst CdS–Au–BiVO₄ was synthesized for the first time by photo-reduction and deposition–precipitation methods. The microstructures and optical properties of the as-prepared samples were investigated by means of scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectroscopy (DRS). Due to the oriented accumulation of electrons on the {010} facets of BiVO₄ crystals, Au nanoparticles were successfully anchored on the {010} facets of BiVO₄ crystals via the photo-reduction process. CdS was further selectively deposited on the surface of Au nanoparticles, benefitting from the strong S–Au interaction. Photocatalytic degradations of tetracycline and Rhodamine B indicated that CdS–Au–BiVO₄ exhibits much higher photocatalytic activity than BiVO₄, Au–BiVO₄, and CdS–BiVO₄. Radical trapping experiments confirmed that in the case of CdS–Au–BiVO₄, the main reactive species responsible for organic contaminant degradation are h⁺, ˙OH, and ˙O₂⁻, while only h⁺ can be produced in the case of CdS–BiVO₄. Based on the photoelectrochemical analysis and radical trapping experiments, it can be deduced that the Z-scheme structure of CdS–Au–BiVO₄ not only decreases the recombination rate of photo-generated carriers but also makes the holes and electrons keep a higher redox ability.

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1. Introduction

With the rapid development of industry, a large amount of industrial effluents containing various organic pollutants such as antibiotics, drugs, dyes, and agrochemicals have been released into water sources,^1–5 which has become a severe threat to human survival and development. For instance, the accumulation of antibiotics in the environment can disrupt ecosystem equilibrium and induce bacterial resistance against drugs, threatening human health subsequently.⁶ Poisonous dyes in natural water can cause the variation or even extinction of animals and plants.^7–9 Therefore, it has become an imperative task to remove these pollutants from industrial effluences. In this context, traditional, biological and physical adsorption methods cannot effectively eliminate these organic pollutants, due to their low efficiency and secondary pollution.^10–12

Alternatively, semiconductor photocatalysis has received much attention due to its high efficiency in degrading and mineralizing organic pollutants in environmental water systems.^13–19 Amongst various semiconductor materials, bismuth vanadate (BiVO₄) has been considered as an ideal candidate, owing to its narrow band gap for visible light absorption, low cost, and good stability.^20–22 However, its low charge transportation efficiency leads to a very high recombination rate of electron–hole pairs, severely restricting its practical applications.^23–25 In recent years, many efforts have been devoted to improving the separation rate of the photo-generated carriers of BiVO₄.^26–28 Li et al. reported that depositing noble metals and transition metal oxides on the different facets of BiVO₄ single crystals can effectively reduce the recombination rate of photo-generated carriers.^23–27 Besides, constructing a p–n heterojunction is another effective approach to separate the photo-generated carriers.^29–32 Song et al. found that the BiOCl/BiVO₄ p–n heterojunction showed significantly enhanced photocatalytic activity for methyl orange degradation.³¹ Wang et al. also reported that the p–n heterojunction Cu₂O/BiVO₄ nanocrystals can effectively reduce the recombination rate of charge carriers.³² Although both loading a co-catalyst and establishing a p–n heterojunction can improve the separation efficiency of photo-generated electrons and holes, they will lower down the redox performance of photo-generated carriers at the same time.³³ Hence, it is desirable to fabricate a structure for BiVO₄, which can enhance the separation rate of photo-generated electrons and holes under the premise of keeping their redox ability.

In this work, a Z-scheme heterogeneous photocatalyst CdS–Au–BiVO₄ was synthesized for the first time by anchoring Au nanoparticles (NPs) on the {010} facets of BiVO₄ crystals and further depositing CdS on Au NPs. Meanwhile, a composite semiconductor CdS–BiVO₄ was also prepared as a contrast photocatalyst. The morphology, crystalline structure, and chemical composition of the as-prepared samples were characterized by means of scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The light absorption properties of the samples were analyzed by UV-vis diffuse reflectance spectroscopy (DRS). The photoelectric properties of the samples and the separation rate of charge carriers were investigated using transient photocurrents, electrochemical impedance, photoluminescence (PL) spectra, and time-resolved fluorescence decay spectra. Moreover, the photocatalytic activities of the different samples were evaluated by the photocatalytic degradations of tetracycline (TC) and Rhodamine B (RhB). On the basis of the above results and radical trapping experiments, the possible photocatalytic mechanisms of Z-scheme CdS–Au–BiVO₄ and heterojunction CdS–BiVO₄ were proposed.

2. Experimental section

2.1. Synthesis of the different photocatalysts

2.1.1. Au–BiVO₄. BiVO₄ single crystals were beforehand synthesized according to the procedures of our previous work.²⁰ Here, the selective photo-reduction method was employed to deposit Au NPs on the surface of BiVO₄ {010} facets. The detailed procedure is as follows: 25 mg of BiVO₄ single crystals was added into a quartz tube containing 10 mL of H₂C₂O₄ aqueous solution (0.2 g L⁻¹) at room temperature. Under constant stirring, 250 μL of HAuCl₄ aqueous solution (10 g L⁻¹) was injected into the above suspension and irradiated by a 300 W Xenon arc lamp with a 420 nm filter for 3 h. Gradually, the colour of the mixture turned from the original bright yellow to brown, indicating the generation of metallic Au on the surface of BiVO₄. The resulting product was collected by centrifugation and dried at 70 °C for 8 h to obtain Au–BiVO₄.

2.1.2. CdS–Au–BiVO₄. CdS–Au–BiVO₄ was synthesized by a deposition–precipitation method, similar to previous studies.^34,35 In a typical procedure, a 25 mg as-prepared Au–BiVO₄ sample was dispersed in a 10 mL aqueous solution containing 0.01 mmol CH₃CSNH₂ and stirred for 30 min in the dark to achieve the selective adsorption of CH₃CSNH₂ on the surface of Au NPs. Then, a 1 mL aqueous solution containing 0.01 mmol CdCl₂ (0.2 M) was added to the above suspension. The reaction mixture was heated to 60 °C and stirred at this temperature for 30 min. After being cooled to room temperature, the product was separated by centrifugation, washed with deionized water, and dried at 70 °C for 8 h.

2.1.3. CdS–BiVO₄ and CdS. With the same procedure for synthesizing CdS–Au–BiVO₄, CdS–BiVO₄ was prepared using BiVO₄ instead of Au–BiVO₄. CdS was also synthesized with the same procedure in the absence of BiVO₄ and Au–BiVO₄.

2.2. Characterization

The morphologies of the prepared samples were observed using a TESCAN VEGA 3 SBH scanning electron microscope (SEM), a FEI NOVA Nano SEM450 field emission scanning electron microscope (FE-SEM), and a JEOL JEM2100 high resolution transmission electron microscope (HR-TEM). X-ray diffraction measurements were carried out on a Rigaku D/max 2550 VB/PC X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm). The XPS spectra were recorded on a Thermo Fisher ESCALAB 250Xi X-ray photoelectron spectroscope with Al Kα radiation, operated at 250 W. The UV-vis diffuse reflectance spectra (DRS) were measured with a SHIMADZU UV-2450 spectrometer equipped with an integrating sphere assembly, using BaSO₄ as the reference material. The PL spectra were measured with a SHIMADZU RF5301PC by using the 380 nm line of a xenon lamp as the excitation source at room temperature. The time-resolved fluorescence decay spectra were recorded on a FLS 980 spectrometer (EDINBURGH INSTRUMENTS) using a 360 nm LED laser as the light source and the detection wavelength is 540 nm.

2.3. Transient photocurrent and electrochemical impedance measurements

The transient photocurrent and electrochemical impedance measurements were conducted on a Zahner electrochemical work station with a standard three-electrode system consisting of a working electrode, a Pt wire as the counter electrode, and a saturated calomel electrode as the reference electrode. The working electrode was prepared via the following steps: firstly, the FTO glass was washed with acetone to remove the residual oil on its surface. Then, the surface of the FTO glass was covered with masking tape to leave an interspace of 1 cm × 1.5 cm. Subsequently, a 0.2 mg sample dispersed in 20 μL pure water was dropped in the interspace. Finally, the FTO glass containing the sample was dried in a drying oven and the masking tape was removed to obtain the working electrode. The transient photocurrents of the different samples were measured in 0.5 M Na₂SO₄ aqueous solution using a 300 W Xe lamp with a 420 nm filter as the light source. The working current was set at 15 A. The electrochemical impedance spectra were measured in a mixed aqueous solution containing 25 mM K₃[Fe(CN)₆], 25 mM K₄[Fe(CN)₆] and 0.1 M KCl. The potential and amplitude were fixed at 0.4 V (vs. NHE) and 10 mV, respectively. The frequency range was from 100 KHz to 10 mHz.

2.4. Photocatalytic activity measurements

The photocatalytic activities of the samples were evaluated by the degradations of tetracycline (TC) and Rhodamine B (RhB) under visible light irradiation. Briefly, 0.025 g and 0.1 g photocatalysts were suspended in the aqueous solutions of TC (50 mL, 10 mg L⁻¹) and RhB (50 mL, 10 mg L⁻¹), respectively. Prior to light irradiation, the suspension was stirred for 30 min in the dark to obtain the adsorption–desorption equilibrium of the organic contaminant and dissolved oxygen on the surface of the photocatalyst. Then, the suspension was irradiated by a 300 W Xe lamp with a 420 nm filter under magnetic stirring. The working current was set at 15 A. At a given time interval, the suspension was withdrawn from the reaction vessel and the photocatalyst was removed by centrifugation. Finally, the concentration of TC or RhB in the solution was monitored using a UV-vis spectrophotometer. For the recycling experiments, the used photocatalyst was washed with deionized water several times and retested in a fresh contaminant solution under the same experimental conditions as mentioned above.

3. Results and discussion

3.1. Synthetic route and morphology

The synthetic route of the Z-scheme photocatalyst CdS–Au–BiVO₄ is illustrated in Fig. 1A. Firstly, BiVO₄ single crystals with a decagonal shape were prepared according to our previous work.²⁰ Secondly, Au NPs were deposited on the {010} facets of BiVO₄ crystals by a selective photo-reduction process using HAuCl₄ as the Au source and H₂C₂O₄ as the hole scavenger (step 1). The reaction equations for depositing Au NPs are shown in Fig. 1B. After absorbing light, BiVO₄ crystals produce photo-generated electrons and holes, which transfer to the {010} and {110} facets, respectively, due to their different energy levels in the conduction bands and valence bands.^23–28 Subsequently, the Au³⁺ ions are reduced to metallic Au⁰ by the electrons on the BiVO₄ {010} facets. Meanwhile, the holes on the {110} facets are consumed by H₂C₂O₄. In step 2, CH₃CSNH₂ are preferentially adsorbed on the surface of Au NPs, benefitting from the strong interaction between the sulfur-containing compound and Au surface.^36,37 With the increase of temperature, CH₃CSNH₂ adsorbed on Au NPs are gradually hydrolyzed, as illustrated in Fig. 1C. Meanwhile, S²⁻ ions are released and react with Cd²⁺ to form CdS on the surface of Au NPs (step 3).

Fig. 1 (A) Schematic diagram illustrating the synthetic route of the Z-scheme photocatalyst CdS–Au–BiVO₄. (B) Reaction equations for depositing Au NPs by selective photo-reduction. (C) Reaction equations for forming CdS NPs.

The morphologies of the as-prepared samples were observed by SEM (Fig. 2A–C and A′–C′), FE-SEM (D and E), and HR-TEM (Fig. 2F–H). As shown in Fig. 2A and A′, BiVO₄ crystals exhibit a decagonal shape with an average side length of 8 μm and their surface seems to be clean and smooth. After depositing Au by photo-reduction, the {010} facets of BiVO₄ crystals contain many Au NPs with size of ∼200 nm, while the {110} facets still remain bald (Fig. 2B and B′), which is because the photo-generated electrons selectively accumulate on the {010} facets of BiVO₄ crystals. This is also evidence confirming the transfer pathway of photo-generated electrons. As shown in Fig. 2C and C′, the surface morphology of CdS–Au–BiVO₄ has no evident change compared with that of Au–BiVO₄. To further explore the state of CdS, the FE-SEM and HR-TEM images of Au–BiVO₄ and CdS–Au–BiVO₄ were further observed. As shown in Fig. 2D, the surface of Au particles on BiVO₄ {010} facets looks bald before depositing CdS. In contrast, many hillocks appear on the surface of Au particles after depositing CdS (Fig. 2E). From the side view of the HR-TEM image (Fig. 2F and G), it can be clearly seen that CdS NPs (the grey ones) have been successfully deposited on the surface of Au NPs (the black ones). The lattice fringe pattern of CdS NPs was measured to be 0.34 nm (Fig. 2H), consistent with the interplanar spacing of the CdS (111) plane.³⁴ Unfortunately, we failed to observe the lattice fringes of Au NPs and BiVO₄ crystals because they are too thick to be transmitted by an electron beam.

Fig. 2 SEM images of the as-prepared BiVO₄ (A and A′), Au–BiVO₄ (B and B′), and CdS–Au–BiVO₄ (C and C′). FE-SEM images of the as-prepared Au–BiVO₄ (D) and CdS–Au–BiVO₄ (E). HR-TEM images of CdS–Au–BiVO₄ (F–H). The insert of Fig. 2H is the corresponding lattice fringes of CdS.

3.2. Crystalline structure and surface composition

The X-ray diffraction patterns (XRD) of the different samples were analyzed to determine their crystallographic structures. As shown in Fig. 3A, all the samples BiVO₄, Au–BiVO₄, CdS–BiVO₄, and CdS–Au–BiVO₄ show the characteristic peaks of monoclinic scheelite phase BiVO₄ (JCPDS file: 14-0688). Both Au–BiVO₄ and CdS–Au–BiVO₄ present two peaks at 38.2° and 44.3° (Fig. 3C and D), ascribed to the Au (111) and (200) diffraction peaks, respectively (JCPDS file: 80-0019). This result indicates that Au NPs have been successfully anchored on the BiVO₄ surface. After CdS was deposited on the surface of Au NPs, CdS–Au–BiVO₄ shows the (002) diffraction peak of cubic phase CdS (Fig. 3B) (JCPDS file: 41-1049). Similarly, the contrast sample CdS–BiVO₄ also exhibits the (002) diffraction peak of CdS (Fig. 3B).

Fig. 3 (A) X-ray diffraction patterns of the as-prepared CdS (a), BiVO₄ (b), CdS–BiVO₄ (c), Au–BiVO₄ (d), and CdS–Au–BiVO₄ (e). (B–D) Y-axis enlarged X-ray diffraction patterns in the range of 25–28° (B), 37.5–39° (C), and 44–45° (D).

To identify the elemental composition and chemical state of the as-prepared CdS–Au–BiVO₄, X-ray photoelectron spectroscopy (XPS) analysis was performed. As shown in Fig. 4A, the XPS survey spectrum confirms the presence of Bi, O, V, Au, Cd, S, and C elements, in which the C comes from adventitious carbon. In Fig. 4B, the two peaks with binding energies of 159.1 eV and 164.4 eV can be attributed to Bi 4f_7/2 and Bi 4f_5/2, respectively. As displayed in Fig. 4C, the binding energies of V 2p_3/2 and V 2p_1/2 are located at 516.6 eV and 524.3 eV, respectively. In Fig. 4D, the peak at 529.8 eV can be ascribed to the lattice oxygen while the peak at 532.3 eV can be assigned to the adsorbed H₂O and OH on the surface of CdS–Au–BiVO₄.^38,39 The binding energies of Bi, V, and O are consistent with those of monoclinic scheelite BiVO₄.⁴⁰Fig. 4E shows the XPS spectrum of Au 4f. The two peaks at 84.6 eV and 88.2 eV correspond to Au 4f_7/2 and Au 4f_5/2, respectively, of Au⁰, which confirms the existence of Au NPs in the composite.³⁴ In Fig. 4F, the S 2s peak at 225.7 eV should be ascribed to the S²⁻ in CdS.⁴¹ However, the S 2p peak cannot be found because it is covered by the strong Bi 4f peak. In Fig. 4G, the two peaks at 405.1 eV and 411.8 eV are attributed to Cd 3d_5/2 and 3d_3/2, respectively, which confirms the formation of CdS.³⁵

Fig. 4 XPS spectra of the as-prepared CdS–Au–BiVO₄: (A) survey spectrum; (B) Bi 4f; (C) V 2p; (D) O 1s; (E) Au 4f; (F) S 2s; (G) Cd 3d.

3.3. Absorption and PL spectra

The light absorption properties of the samples BiVO₄, CdS, Au–BiVO₄, CdS–BiVO₄ and CdS–Au–BiVO₄ were analysed via UV-vis diffuse reflectance spectroscopy (DRS), as shown in Fig. 5A. Compared to BiVO₄, both Au–BiVO₄ and CdS–Au–BiVO₄ exhibit a broad absorption peak around 620 nm, attributed to the surface plasmon resonance (SPR) absorption of Au NPs loaded on the {010} facets of BiVO₄ crystals. Although the absorption edge of CdS exhibits an evident red shift compared with that of BiVO₄, the absorption edge of CdS–BiVO₄ is very near to that of BiVO₄, which is due to the low CdS content in CdS–BiVO₄. Compared to Au–BiVO₄, the SPR absorption peak of CdS–Au–BiVO₄ shows a red shift of 30 nm, which is due to the strong electromagnetic coupling between Au and CdS.⁴² Moreover, the SPR absorption intensity of CdS–Au–BiVO₄ is slightly lower than that of Au–BiVO₄, because the formed CdS NPs can hinder the SPR effect of Au NPs.⁴³ The above results further confirm the formation of CdS NPs on the surface of Au NPs.

Fig. 5 (A) UV-vis diffuse reflectance spectra of BiVO₄, CdS, Au–BiVO₄, CdS–BiVO₄, and CdS–Au–BiVO₄. (B) PL spectra of BiVO₄, Au–BiVO₄, CdS–BiVO₄, and CdS–Au–BiVO₄.

Photoluminescence emission spectroscopy (PL) is considered to be a very useful technique to investigate the efficiency of charge carrier trapping, migration, and transfer of the photo-generated charge carriers in semiconductor particles.^44–46Fig. 5B shows the PL spectra of BiVO₄, Au–BiVO₄, CdS–BiVO₄, and CdS–Au–BiVO₄. All samples CdS–BiVO₄, Au–BiVO₄, and CdS–Au–BiVO₄ exhibit a weaker PL intensity compared with BiVO₄, implying that the recombination rates of the photo-generated charge carriers in these samples have been restrained. The PL intensity of Au–BiVO₄ is much lower than that of CdS–BiVO₄, which indicates that depositing Au is a more effective strategy to reduce the recombination rate of charge carriers compared with coupling CdS. CdS–Au–BiVO₄ displays the lowest PL intensity, implying that the photo-generated electron–hole pairs possess the highest transfer efficiency among the samples. For semiconductor materials, the lifetime of photo-generated charge carriers can be estimated by their fluorescence decay time. Table 1 shows the fluorescence decay times τ₁ and τ₂ of the different samples determined by their time-resolved fluorescence decay spectra. According to previous reports,^47–49τ₁ corresponds to the direct recombination of electrons and holes in the valence band, while τ₂ is assigned to the charge carrier recombination during their transfer process. As shown in Table 1, the value of τ₂ gradually increases in the order of BiVO₄ < CdS–BiVO₄ < Au–BiVO₄ < CdS–Au–BiVO₄, demonstrating that CdS–Au–BiVO₄ with the Z-scheme structure possesses the highest ability for separating the photo-generated electrons and holes.^50,51

Table 1 Fluorescence decay times of BiVO₄, CdS–BiVO₄, Au–BiVO₄ and CdS–Au–BiVO₄

	τ 1 (ns)	A 1 (%)	τ 2 (ns)	A 2 (%)	χ ^2
BiVO4	0.146	97.0	2.617	3.0	1.124
CdS–BiVO4	0.136	97.2	2.901	3.8	1.124
Au–BiVO4	0.148	96.8	3.162	3.2	1.182
CdS–Au–BiVO4	0.132	97.7	3.456	2.3	1.273

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3.4. Photoelectrochemical properties

The transient photocurrents and electrochemical impedance spectra (EIS) were obtained to further investigate the transfer and separation efficiency of photo-generated carriers in the different samples. As shown in Fig. 6A, the photocurrents of the different samples follow the order of CdS–Au–BiVO₄ > Au–BiVO₄ > CdS–BiVO₄ > BiVO₄. Although all the samples CdS–BiVO₄, Au–BiVO₄, and CdS–BiVO₄ show improved photocurrents compared to BiVO₄, their mechanisms for enhancing photocurrents are different. For CdS–BiVO₄, the separation efficiency of photo-generated electrons and holes is improved by forming a heterojunction structure between CdS and BiVO₄, i.e., the photo-generated electrons transfer to the conduction band of BiVO₄ while the holes transfer to the valence of CdS. With regard to Au–BiVO₄, the Au NPs deposited on the BiVO₄ {010} facets can form a Schottky barrier on the Au–BiVO₄ interface, which can enhance the separation efficiency of charge carriers by trapping photo-generated electrons.^52,53 In the case of CdS–Au–BiVO₄, CdS and BiVO₄ can form the Z-scheme with the participation of Au NPs, which can effectively separate the photo-generated electron–hole pairs.^33–35Fig. 6B displays the Nyquist plots of the electrochemical impedance spectra of the different samples, which can be used to explore the dynamics of the mobile and bound charges in the interfacial or bulk region of the semiconductors.^54,55 As shown in Fig. 6B, the semicircle diameters of CdS–Au–BiVO₄, Au–BiVO₄, and CdS–BiVO₄ are smaller than that of BiVO₄, revealing that these composite structures are more favorable to the carrier transfer than the single BiVO₄.

Fig. 6 (A) Transient photocurrents and (B) Nyquist plots of electrochemical impedance spectra of the as-prepared samples: (a) BiVO₄, (b) CdS–BiVO₄, (c) Au–BiVO₄, and (d) CdS–Au–BiVO₄. A 300 W Xe lamp with a 420 nm filter was used as the light source for the photocurrent tests.

3.5. Photocatalytic activity and mechanism

The photocatalytic activities of BiVO₄, CdS–BiVO₄, Au–BiVO₄, and CdS–Au–BiVO₄ were evaluated in terms of the degradations of TC and RhB under visible light irradiation. As shown in Fig. 7A and B, both Au–BiVO₄ and CdS–Au–BiVO₄ show evidently enhanced photocatalytic activity compared to BiVO₄ for the degradations of TC and RhB. For instance, after visible light irradiation for 90 min, only 47% TC was degraded over BiVO₄ while the degradation rates of TC over Au–BiVO₄ and CdS–Au–BiVO₄ reached 76% and 91%, respectively. For Au–BiVO₄, the Au NPs can effectively improve the separation rate of photo-generated electrons and holes by forming a Schottky barrier,^56,57 enhancing the photocatalytic activity of BiVO₄. In the case of CdS–Au–BiVO₄, the Z-scheme structure not only decreases the recombination rate of the photo-generated carriers but also makes the holes and electrons keep higher oxidation and reduction abilities, respectively (the reason will be discussed later). Therefore, CdS–Au–BiVO₄ shows much higher photocatalytic activity than Au–BiVO₄. Strangely, the heterojunction photocatalyst CdS–BiVO₄ exhibits a lower photocatalytic activity than the single BiVO₄, which is inconsistent with the results of photocurrent tests, implying that the photocatalytic activity is not totally determined by the separation rate of photo-generated carriers (the details will be discussed later). As photostability is very important to a photocatalyst for its practical application, here cyclic photocatalytic degradation experiments were carried out to investigate the photostability of CdS–Au–BiVO₄. As displayed in Fig. 7C and D, the degradation rates of TC and RhB show only a little decrease after four cycles. Excluding the loss of photocatalyst in the cycling tests, CdS–Au–BiVO₄ can be considered to be a stable photocatalyst.

Fig. 7 Photocatalytic degradations of TC (A) and RhB (B) over BiVO₄, CdS–BiVO₄, Au–BiVO₄, and CdS–Au–BiVO₄. Cyclic photocatalytic degradations of TC (C) and RhB (D) over CdS–Au–BiVO₄. All photocatalytic experiments were carried out using a 300 W Xe lamp with a 420 nm filter as the light source.

It is generally considered that organic pollutants can be degraded by photocatalytic oxidation processes, in which a series of reactive species, such as holes (h⁺), hydroxyl radicals (˙OH), and superoxide radicals (˙O₂⁻), are involved in the degradation reactions of organic pollutants. To investigate the main reactive species responsible for the degradation of organic pollutants over heterojunction CdS–BiVO₄ and Z-scheme CdS–Au–BiVO₄, a series of contrast experiments in the presence of different quenchers were carried out. Here, tert-butyl alcohol (TBA) was used to quench ˙OH, EDTA-2Na for h⁺, and p-benzoquinone (BQ) for ˙O₂⁻. As shown in Fig. 8A, after addition of EDTA-2Na or BQ, the photocatalytic activity of CdS–Au–BiVO₄ decreases dramatically, indicating that all h⁺ and ˙O₂⁻ are the key reactive species responsible for the degradation of organic pollutants. The degradation rate of RhB shows a certain decrease in the presence of TBA, implying that ˙OH also takes part in the photocatalytic degradation of organic pollutants. In the case of CdS–BiVO₄, the degradation rate of RhB exhibits an evident decrease after the addition of EDTA-2Na, while it almost has no change in the presence of MT and BQ (Fig. 8B), revealing that h⁺ plays a vital role in organic degradation.

Fig. 8 (A and B) Photocatalytic degradation curves of RhB over (A) CdS–Au–BiVO₄ and (B) CdS–BiVO₄ in the presence of TBA (5 mM), EDTA-2Na (2 mM), and BQ (1 mM). (C) Proposed photocatalytic mechanisms of Z-scheme CdS–Au–BiVO₄ and heterojunction CdS–BiVO₄.

On the basis of the above experimental results, the degradation mechanisms of organic pollutants over CdS–BiVO₄ and CdS–Au–BiVO₄ were proposed and illustrated in Fig. 8C. For the Z-scheme photocatalyst CdS–Au–BiVO₄, when it is irradiated with visible light, the electrons will be excited to the conduction bands of BiVO₄ and CdS while the holes will be left on their valence bands. Due to the excellent conductivity of the Au joint, the electrons on the BiVO₄ conduction band (with weaker reduction ability) will combine with the holes on the CdS valence band (with weaker oxidation ability), while the holes on the BiVO₄ valence band (with stronger oxidation ability) and the electrons on the CdS conduction band (with stronger reduction ability) will be preserved. Therefore, the electrons on the CdS conduction band easily combine with absorbed O₂ to produce ˙O₂⁻, while the holes on the BiVO₄ valence band easily oxidize hydroxyl to form ˙OH. The formation of ˙O₂⁻ and ˙OH has been confirmed by the reactive species trapping experiments. Since the Z-scheme structure not only decreases the recombination rate of electrons and holes but also favors the formation of reactive species ˙OH and ˙O₂⁻, CdS–Au–BiVO₄ shows excellent photocatalytic activity for organic pollutant degradation. In the case of heterojunction CdS–BiVO₄, the photo-generated electrons and holes will transfer to the conduction band of BiVO₄ and the valence band of CdS, respectively. Due to the weak reduction ability of electrons as well as the poor oxidation ability of holes, almost no ˙OH and ˙O₂⁻ can be produced. Moreover, due to the weak oxidation ability of h⁺ on the CdS valence, CdS–BiVO₄ exhibits lower photocatalytic activity.

4. Conclusions

In this work, a Z-scheme heterogeneous photocatalyst CdS–Au–BiVO₄ was synthesized for the first time by photo-reduction and deposition–precipitation methods. By photo-reduction, Au NPs with an average size of 200 nm were successfully anchored on the {010} facets of BiVO₄ crystals, in which the selective loading of Au NPs results from the oriented accumulation of electrons on the {010} facets. By the deposition–precipitation process, CdS was further selectively deposited on the surface of Au NPs via the strong S–Au interaction between the S precursor and Au NPs. CdS–Au–BiVO₄ exhibited much higher photocatalytic activity for the degradation of TC and RhB when compared with BiVO₄, Au–BiVO₄, and CdS–BiVO₄, which is because the Z-scheme structure of CdS–Au–BiVO₄ not only decreases the recombination rate of photo-generated carriers but also makes the holes and electrons keep a higher redox ability. Radical trapping experiments proved that in the case of CdS–Au–BiVO₄, the main reactive species responsible for organic contaminant degradation are h⁺, ˙OH, and ˙O₂⁻, while only h⁺ can be produced in the case of CdS–BiVO₄. As a highly efficient photocatalyst, the Z-scheme heterogeneous photocatalyst CdS–Au–BiVO₄ can be potentially applied in wastewater treatment and other photocatalytic fields.

Acknowledgements

This work has been supported by the National Natural Science Foundation of China (21277046, 21573069), the Shanghai Committee of Science and Technology (13NM1401000), and the National Basic Research Program of China (973 Program, 2013CB632403). We also thank the TEM Lab of Research Center of Analysis and Test of East China University of Science and Technology for kindly providing the TEM analysis.

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Footnote

† Shenyuan Bao and Qiangfang Wu contributed equally to this work.

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