Novel In₂S₃/ZnWO₄ heterojunction photocatalysts: facile synthesis and high-efficiency visible-light-driven photocatalytic activity

Abstract

Novel heterojunction photocatalysts In₂S₃/ZnWO₄ were prepared by a hydrothermal and surface-functionalized method, and the optimized In₂S₃/ZnWO₄ ratio was tuned to explore their visible-light photocatalytic activity for rhodamine B (RhB) degradation. The heterojunction structure formed by the In₂S₃ nanoparticles grew on the primary ZnWO₄ nanorods. Remarkably, In₂S₃/ZnWO₄ composites exhibited much higher photocatalytic activity than that of the individual In₂S₃ and ZnWO₄. The enhanced activity could be attributed to the strong visible-light absorption and the effective separation and transportance of the photogenerated charges. Moreover, the main active species for the degradation of RhB were also investigated. Then, a possible reaction mechanism for the excellent photocatalytic activity of the In₂S₃/ZnWO₄ composites was proposed.

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Introduction

Photocatalytic degradation of organic contamination by semiconductor photocatalysts has the potential to be a beneficial and green technology for solving global environmental purification and energy problems.^1–3 Recently, metal tungstate photocatalysts^4–6 have attracted considerable interest, and have been considered as promising candidates for their potential application in the use of solar energy to eliminate contaminant in water. As an important type of tungstate, zinc tungstate (ZnWO₄) with a monoclinic wolframite structure has been extensively investigated due to its unique physical and chemical properties, molecular and electronic versatility, high chemical stability, relatively high catalytic activity and inexpensive commercial availability.^4,7–9 In some case, ZnWO₄ photocatalyst exhibited even better activity than that of TiO₂ (P-25) for the photodegradation of formaldehyde under UV light irradiation.⁷ However, high recombination ratios of photogenerated electron–hole pairs and poor response to visible-light (the band gap is 3.75 eV)¹⁰ have restrained the practical application of ZnWO₄ in pollutants degradation.

Over the past several years, many efforts have been made to develop effective strategies to improve the photocatalytic activity of ZnWO₄, such as tuning morphology,¹¹ controlling crystallinity,⁶ ion doping,^12,13 coupling with noble metals¹⁴ and other semiconductors.¹⁵ Among these, the construction of a semiconductor heterojunction attracted a lot of attention due to its significantly benefits to enhance the photocatalytic activities.^16–19 Compared to single photocatalysts, several advantages of the heterostructures can be obtained: an extended light responsive range and a rapid charge transfer to the catalyst, as well as the enhanced photogenerated charge carrier separation and a longer lifetime of the charge carriers. However, the matching of interface lattice and energy levels of two components is the key factors to obtain the desirable heterojunction photocatalysts. Accordingly, coupling ZnWO₄ with narrow band gap semiconductors with matching band-potential to design and fabricate heterojunctions is an active strategy of improving the photocatalytic activities under visible-light irradiation. ZnWO₄/BiOI heterostructures²⁰ have been investigated and showed high photocatalytic activities in degradation of MO and photocurrent response under visible light irradiation. The findings indicated that the high photocatalytic activities owing to the lattice and energy levels between the ZnWO₄ and BiOI phases matching well with each other. In addition, ZnWO₄-based heterojunction photocatalysts by coupling with other semiconductors were also successfully synthesized, such as g-C₃N₄/ZnWO₄,²¹ BiOBr/ZnWO₄ ²² and graphene–ZnWO₄.²³ These composites was demonstrated to be effective in improving the photocatalytic activity of pure ZnWO₄.

However, the visible light responding of the ZnWO₄-based heterojunction photocatalysts is still limited, and fabricated ZnWO₄-based composites do not yet meet the requirements for practical application. Thus, the exploration of the new ZnWO₄-related heterojunction photocatalysts with high-efficiency visible light photocatalytic activities is highly desired. Furthermore, the detailed mechanism of the photogenerated charge carrier transfer-separation and the nature of the visible-light-driven heterojunction photocatalysts in improving the photocatalytic activity of ZnWO₄ have not yet been entirely understood. Currently, indium sulfide (In₂S₃), which is an extensively researched narrow band gap (2.0–2.3 eV) semiconductor²⁴ due to its high photosensitivity, photoconductivity, low toxicity and high stability. Particularly, comparing with CdS, In₂S₃ is more friendly to environment because In₂S₃ does not contain heavy toxic Cd metal. For instance, In₂S₃ was served as a buffer layer to replace the environmentally unfriendly CdS buffer layer in Cu(In,Ga)Se₂(CIGS) thin film solar cells.²⁵ In₂S₃ was also used as a visible-light sensitizer of ZnO²⁶ and TiO₂ ²⁷ to obtain a high performance in photodecomposition. These In₂S₃-coupled photocatalysts were confirmed to be outstanding visible-light-driven photocatalysts. Whereas, the conduction band (CB) of the In₂S₃ is higher than that of ZnWO₄, which allows that photogenerated electrons transfer effectively from In₂S₃ into ZnWO₄ to separate the photogenerated electron–hole pairs. Therefore, it may be an excellent candidate for In₂S₃ to couple with ZnWO₄ to form In₂S₃/ZnWO₄ heterostructure with high visible-light photocatalytic activities. To the best of our knowledge, however, there is no report regarding In₂S₃/ZnWO₄ composites as a high-efficiency visible-light-driven photocatalyst.

In this work, novel In₂S₃/ZnWO₄ heterojunction photocatalysts were synthesized by a surface functionalization method.^26,28 Their photocatalytic activities were evaluated by studying the degradation of RhB under visible light irradiation. Furthermore, the possible mechanism for the enhanced photocatalytic activity was also proposed based on trapping experiments and calculated band-edge potential positions.

Experimental

Synthesis of ZnWO₄

All the chemicals were of analytical grade and used without further purification. ZnWO₄ was prepared by a hydrothermal method. In a typical synthesis, sodium tungstate (Na₂WO₄·2H₂O, 5 mmol) and zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 5 mmol) were dissolved in Milli-Q water in separate beakers. The sodium tungstate solution was then added to the zinc nitrate solution dropwise, accompanied by vigorous stirring at room temperature. After stirring for 30 minutes, the white slurry was transferred to 100 mL Teflon-lined autoclaves and heated at 180 °C for 24 h. After autoclaves were cooled to room temperature naturally, the samples were collected by filtration and washed several times with Milli-Q water and alcohol followed by dried at 80 °C for several hours.

Synthesis of In₂S₃/ZnWO₄ composites

Surface functionalized synthesis route was used to prepare In₂S₃ coupled ZnWO₄ nanorods. Firstly, 0.47 g of ZnWO₄ nanorods was dispersed in 50 mL of Milli-Q water containing citric acid (CA) 0.08 g under constant magnetic stirring. The solution was heated at 60 °C for 2 h and cooled to room temperature gradually. Subsequently, 25 mL aqueous solutions of containing different amounts of InCl₃ were slowly added to the previous solution dropwise respectively under constant stirring for 2 h. Finally, 25 mL Na₂S aqueous solutions (the mole ratio of InCl₃/Na₂S is 2 : 3) were added dropwise into the above systems respectively. The mixture was stirred for another 1 h at room temperature. Then the products were separated by centrifugation and washed with water and alcohol. The obtained samples were dried at 80 °C for several hours. The samples with different In/Zn mole ratio of 0.05, 0.4, 0.6, 0.8 and 1.0 were denoted as 0.05-In₂S₃/ZnWO₄, 0.4-In₂S₃/ZnWO₄, 0.6-In₂S₃/ZnWO₄, 0.8-In₂S₃/ZnWO₄ and 1.0-In₂S₃/ZnWO₄, respectively. For comparison, the pure In₂S₃ sample were also fabricated under the same conditions.

Characterization of photocatalysts

The crystalline phases of pure ZnWO₄ and In₂S₃/ZnWO₄ composites were determined using X-ray diffraction (XRD) (D/MAX-RB, Rigaku, Japan). The diffraction patterns were recorded in the 2θ = 20–80° range with a Cu Kα source (λ = 0.15405) running at 40 kV and 30 mA. The specific surface area of pure ZnWO₄ and In₂S₃/ZnWO₄ composites were determined by Brunauer–Emmett–Teller (BET) method (NOVA4200e, Quantachrome, USA). The samples were outgassed at 300 °C under vacuum for 4 h prior to measurement. Their N₂ adsorption and desorption isotherms at 77 K under different partial pressures were then measured. The morphology of the samples was studied using an emission scanning microscopy (SEM, S-4800, Hitachi, Japan). The transmission electron microscopy (TEM) and the high-resolution TEM (HRTEM) images were performed with a transmission electron microscope (F-20, FEI, USA) at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an X-ray photoelectron spectrometer (AXIS ULTRA^DLD, Kratos, Japan) using the Mg Kα radiation. The UV-vis diffuse reflectance (DRS) spectra of the samples were recorded at room temperature using a UV-vis spectrophotometer (U-3900H, Hitachi, Japan) equipped with an integrating sphere. BaSO₄ was used as the reference. The photoluminescence (PL) spectra were recorded using a fluorescence spectrophotometer (F-4500, Hitachi, Japan) with a Xe lamp as the excitation light source.

Photocatalytic experiments

The photocatalytic activities of samples were evaluated by the degradation of rhodamine B (RhB) under visible-light irradiation at ambient temperature using a 400 W Xe lamp with a 420 nm cut off filter as the light source. In each experiment, powder photocatalyst (40 mg) was dispersed in 40 mL of RhB aqueous solution (10 mg L⁻¹). Before illumination, the suspensions were stirred for 2 h to ensure the establishment of an adsorption–desorption equilibrium between the photocatalysts and RhB. During the photoreactions, 5 mL suspensions from each sample were taken at a period of interval, followed by centrifugation to remove the photocatalyst. Finally, the centrifuged solution were recorded using a UV-vis spectrophotometer (U-3900H, Hitachi, Japan) at its maximum absorption wavelength of 553 nm.

Photoelectrochemical measurements

The measurements of photocurrents were carried out with an electrochemical workstation (5060F, RST, China) in a standard three-electrode system with the sample, Ag/AgCl electrode (saturated KCl), and a Pt wire used as the working electrode, the reference electrode, and the counter electrode, respectively. And 0.5 M Na₂SO₄ aqueous solution was introduced as the electrolyte. A 100 W incandescent lamp with a 420 nm cut off filter was used as the light source. For the preparation of the working electrodes, 5 mg samples were mixed with a certain amount of ethanol and Nafion on solution homogeneously. The as-prepared samples were spread on the bottom middle of ITO glass with a size of 6 mm × 6 mm. The photocurrents of the photocatalysts with light on and off were measured at 0.8 V.

Results and discussion

XRD analysis

The crystallographic structure of as-prepared ZnWO₄ and In₂S₃/ZnWO₄ composites were investigated by XRD analysis. Fig. 1(A) shows the XRD patterns of ZnWO₄ and a series of In₂S₃/ZnWO₄ composites. All of the diffraction peaks in Fig. 1(A) could be well-indexed to the monoclinic ZnWO₄ (JCPDS Card no. 73-0554), and the distinctive peaks at 2 theta of 23.8°, 24.5°, 30.5°, 36.3°, 41.2°, 53.7° and 64.9° were well-matched with the lattice planes of (0 1 1), (1 1 0), (−1 1 1), (0 0 2), (−1 2 1), (2 0 2) and (−1 3 2), respectively. As shown in Fig. 1(A), there was a new addition weak peak at 28.6°, which was assigned to (1 0 1) lattice planes of In₂S₃ (JCPDS Card no. 33-0623). The appearance of this peak indicated the formation of In₂S₃ crystals. However, it was difficult to identify other peaks of In₂S₃ (Fig. 1(A)) probably because its high dispersity.²² Moreover, the intensity of the diffraction peaks of In₂S₃ gradually increased with the increasing In₂S₃ contents, whereas the intensity of all peaks of ZnWO₄ decreased simultaneously. Fig. 1(B) shows the peak at about 2 theta of 30.5°, which corresponded to the (−1 1 1) planes. Different levels of decrease in peak intensity was observed depending on the coupling of the In₂S₃ component on the ZnWO₄ surface, which suggested that lattice planes of ZnWO₄ (0 1 1) were covered by the In₂S₃ composition on the In₂S₃/ZnWO₄ composites surface.

Fig. 1 (A) XRD patterns and (B) XRD peaks in (−1 1 1) lattice plane of ZnWO₄ and In₂S₃/ZnWO₄ composites.

Table 1 presents the results of the surface area and lattice parameters of the ZnWO₄ and In₂S₃/ZnWO₄ composite photocatalysts. Little change of lattice parameters could be observed as shown in Table 1, reasonably implying that the compressing of In₂S₃ on the surface of the ZnWO₄ to form a heterojunction. Furthermore, all of the In₂S₃/ZnWO₄ composites photocatalysts kept the same monoclinic structure as for pure ZnWO₄.

Table 1 Surface area and lattice parameter of pure ZnWO₄ and In₂S₃/ZnWO₄ composites

Photocatalysts	Surface area (m^2 g^−1)	Lattice parameters
		a (Å)	b (Å)	c (Å)	α (°)	β (°)	γ (°)
ZnWO4	33.30	4.682	5.736	4.947	90	90.587	90
0.05-In2S3/ZnWO4	32.99	4.688	5.731	4.951	90	90.581	90
0.40-In2S3/ZnWO4	61.92	4.690	5.748	4.912	90	90.657	90
0.60-In2S3/ZnWO4	62.16	4.692	5.738	4.930	90	90.374	90
0.80-In2S3/ZnWO4	40.66	4.686	5.727	4.937	90	90.625	90
1.0-In2S3/ZnWO4	27.64	4.696	5.719	4.927	90	90.514	90

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Morphology characterization

The morphology of the pure ZnWO₄ and composites was observed by SEM. Fig. 2 shows the SEM images of the pure ZnWO₄ and In₂S₃/ZnWO₄ composite photocatalysts. The morphology of the pure ZnWO₄ (Fig. 2(A)) presented rod-like nanoparticles, which have an average length of about 60 nm and an average diameter of about 30 nm. The composite photocatalysts in Fig. 2(B)–(F) mainly kept the same rod-like morphology with pure ZnWO₄. However, some irregularly shaped nanoparticles could be found after In₂S₃ was introduced. Compared with pure ZnWO₄, the size of In₂S₃/ZnWO₄ composites increased and the surface of the ZnWO₄ nanorods was not smooth, especially when the mole ratio of In/Zn was up to 0.6 (Fig. 2(E)). However, the overload of In₂S₃ was easy to aggregate and cannot be effectively coupled with ZnWO₄, which was unfavorable to the formation of heterojunctions. Meanwhile, the surface area in Table 1 also confirmed that when the mole ratio of In/Zn was 0.6 and 1.0, the surface area of In₂S₃/ZnWO₄ composites decreased to 62.16 and 27.64 m² g⁻¹ respectively, corresponding to the size of nanoparticles.

Fig. 2 SEM images of the as-prepared samples: (A) pure ZnWO₄; (B) 0.05-In₂S₃/ZnWO₄; (C) 0.4-In₂S₃/ZnWO₄; (D) 0.6-In₂S₃/ZnWO₄; (E) 0.8-In₂S₃/ZnWO₄ and (F) 1.0-In₂S₃/ZnWO₄.

In order to obtain the detailed information of the heterojunction nanostructure of the composite photocatalysts, TEM and HRTEM analysis was carried out, which is an efficient and widely used characterization in terms of heterojunction.^20,21,29 Fig. 3 shows the TEM images and HRTEM images of 0.6-In₂S₃/ZnWO₄ composite photocatalysts. The TEM images show that the surface of the rod-like ZnWO₄ nanoparticles was covered by In₂S₃ (Fig. 3(A) and (B)), which was corresponded with the surface area that the coupled In₂S₃ made the surface area of ZnWO₄ increase from about 33.30 m² g⁻¹ up to 62.16 m² g⁻¹ (Table 1). Fig. 3(C) and (D) illustrate the interface between ZnWO₄ and In₂S₃, where two crystal structures with different interplanar spacing were clearly presented together in the HRTEM images. It is clear from Fig. 3(C) that the interplanar spacing of the distinct lattice fringes is about 0.37 nm and 0.31 nm, which corresponded to the lattice planes of ZnWO₄ (0 1 1) and In₂S₃ (1 0 1), respectively. As can be seen from Fig. 3(D), In₂S₃, that possessed distinctly different lattice fringes with that of ZnWO₄, located at the boundary of ZnWO₄ nanorods. This result demonstrates the formation of heterojunction nanostructure composed by ZnWO₄ and In₂S₃.

Fig. 3 (A and B) TEM images, and (C and D) HRTEM images of the 0.6-In₂S₃/ZnWO₄ composite.

Moreover, the BET surface area of the samples were determined by the nitrogen sorption tests (Table 1), and the corresponding values were calculated to be 33.30, 32.99, 61.92, 62.16, 40.66 and 27.64 m² g⁻¹ for ZnWO₄, 0.05-In₂S₃/ZnWO₄, 0.4-In₂S₃/ZnWO₄, 0.6-In₂S₃/ZnWO₄, 0.8-In₂S₃/ZnWO₄ and 1.0-In₂S₃/ZnWO₄, respectively. Generally, a larger BET surface area means the presence of more active center sites and beneficial to photocatalytic activities.³⁰ The 0.4-In₂S₃/ZnWO₄, 0.6-In₂S₃/ZnWO₄ and 0.8-In₂S₃/ZnWO₄ composite possess larger specific surface areas than that of pure ZnWO₄. This finding could be explained as that the BET of the In₂S₃/ZnWO₄ composites were increased by In₂S₃-coupled and a suitable content of In₂S₃ could be well dispersed on the surface of ZnWO₄ nanorods. When the mole ratio of In/Zn increased to 1.0, the specific surface area was decreased from 62.16 to 27.64 m² g⁻¹, which was mainly due to the overloading of In₂S₃ leads to the agglomeration of composites on the surface of ZnWO₄ (Fig. 2(E) and (F)).

In₂S₃/ZnWO₄ composites were formed by the well known surface functionalization reactions of carboxylic acid. Firstly, the citric acid molecules were adsorbed on the surfaces of ZnWO₄ nanorods by the interaction of carboxyl groups. Then the negative charges on the functionalized surfaces of the ZnWO₄ nanorods attracted the positive charged In³⁺ ions supplied by the aqueous solution of InCl₃. As a result, In(OH)₃ formed on the surface of ZnWO₄ nanorods. Subsequently, In₂S₃ deposited on the surface of ZnWO₄ nanorods by substitution of the OH⁻ ions by S²⁻ ions after injection of the sulfur source.

Chemical state analysis

The XPS analysis provided further information of the chemical state and surface chemical composition of the In₂S₃/ZnWO₄ composite photocatalysts. The survey scan spectra displays Zn 2p, O 1s, W 4f, In 3d and S 2p signals (Fig. 4(A)), which were consistent with the chemical composition of ZnWO₄, In₂S₃ and 0.6-In₂S₃/ZnWO₄. The carbon peaks come from the adventitious hydrocarbon from the XPS instrument.³¹ Before the analysis, all binding energies of the other elements in the XPS spectra were calibrated by using the standard of C 1s at 284.8 eV. Fig. 4(B)–(F) shows the high-resolution XPS spectra of Zn 2p, O 1s, and W 4f for pure ZnWO₄ and In₂S₃/ZnWO₄ composites, and the S 2p and In 3d spectra for pure In₂S₃ and In₂S₃/ZnWO₄ composites. The binding energy of Zn 2p_1/2 and Zn 2p_3/2 were observed at 1044.6 and 1021.4 eV (Fig. 4(B)), respectively, which can be ascribed to Zn²⁺ ions of the ZnWO₄.^32,33 However, The binding energy of Zn 2p_1/2 and Zn 2p_3/2 for In₂S₃/ZnWO₄ composites were 1044.9 and 1021.7 eV, respectively, both of which had a shift of 0.3 eV to high energy region compared with those of the pure ZnWO₄. Fig. 4(C) shows that the peak for O 1s at 530.2 eV, which corresponded to the O²⁻ anion. Compared with ZnWO₄, the signal of O 1s in composites had a 0.3 eV shift to higher binding energy. The high resolution XPS spectra of W 4f for pure ZnWO₄ and In₂S₃/ZnWO₄ composites displayed in Fig. 4(D). The peaks corresponding to W 4f (W 4f_7/2 and W 4f_5/2) in the spectra of In₂S₃/ZnWO₄ composites lightly shifted toward higher binding energies as about 0.2 eV in comparison to pure ZnWO₄.³⁴ For In₂S₃, two strong peaks at 444.7 and 452.2 eV shown in Fig. 4(E) were attributed to In 3d_5/2 and In 3d_3/2, respectively.^27,35 After the sub-peak processing, the peaks at 161.3 and 162.4 eV (Fig. 4(F)) were attributed to the binding energy of S 2p_3/2 and S 2p transition.³⁵ It is clear that a slight shifts to higher binding energy of In 2p signal could be observed, whereas the peaks of S 2p almost maintained the same binding energy in In₂S₃/ZnWO₄ composites. The XPS spectra analysis indicated the strong interaction between In₂S₃ and ZnWO₄, confirming the existence of chemical bonds between In₂S₃ and ZnWO₄ in the composites.²⁰

Fig. 4 (A) overall XPS spectra of ZnWO₄, In₂S₃ and 0.6-In₂S₃/ZnWO₄ composite; and high-resolution XPS spectra of (B) Zn 2p; (C) O 1s; (D) W 4f; (E) In 3d and (F) S 2p.

Optical properties

The optical properties of the ZnWO₄, In₂S₃ and In₂S₃/ZnWO₄ composites were investigated using UV-vis diffuse reflectance spectroscopy. As illustrated in Fig. 5(A), the pure ZnWO₄ only showed an absorbance in the UV-light region, with an absorption edge of about 350 nm. Compared with ZnWO₄ nanorods, the visible-light absorption ability of the In₂S₃/ZnWO₄ composites was obviously enhanced, which indicated that In₂S₃ was a good visible light sensitizer to ZnWO₄. The efficient visible light absorption abilities ensured that In₂S₃/ZnWO₄ composites generated sufficient electron–hole pairs under visible light irradiation. Furthermore, with increasing of In₂S₃ content, the absorbance intensity of the In₂S₃/ZnWO₄ composites increased in the visible region. The band gap energy of the In₂S₃ and ZnWO₄ can be estimated using the following equation:

Ahv = c(hv − E_g)^n/2 (1)

where A, h, v, E_g and c are absorption coefficient, Planck's constant, the light frequency, absorption band gap energy and proportionality constant, respectively. The parameter n is a pure number represented 1 and 4 for the direct and indirect band gap semiconductors. According to the ref. 7 and 24, the n values of ZnWO₄ and In₂S₃ were both 1. Therefore, the band gap energy of ZnWO₄ and In₂S₃ could be calculated from a plot of (Ahv)² versus photon energy hv and was found to be about 3.27 and 2.35 eV, respectively (Fig. 5(B)). As shown in Table 2, E_gs of In₂S₃/ZnWO₄ heterostructures were estimated to be 2.72, 2.33, 2.26, 2.31 and 2.28 eV for 0.05-In₂S₃/ZnWO₄, 0.4-In₂S₃/ZnWO₄, 0.6-In₂S₃/ZnWO₄, 0.8-In₂S₃/ZnWO₄ and 1.0-In₂S₃/ZnWO₄, respectively. The results indicated that the In₂S₃/ZnWO₄ composites can be excited by visible light.

Fig. 5 (A) DRS spectra of the ZnWO₄, In₂S₃ and In₂S₃/ZnWO₄ series of composite photocatalysts; (B) band gap energy of In₂S₃, ZnWO₄ and In₂S₃/ZnWO₄ composite photocatalysts.

Table 2 Band gaps of pure ZnWO₄ and In₂S₃/ZnWO₄ composites

Photocatalysts	ZnWO4	0.05-In2S3/ZnWO4	0.4-In2S3/ZnWO4	0.6-In2S3/ZnWO4	0.8-In2S3/ZnWO4	1.0-In2S3/ZnWO4	In2S3
Eg/(eV)	3.27	2.72	2.33	2.26	2.31	2.28	2.35

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Enhancement of photocatalytic activity and photocurrent

The photoactivity of pure In₂S₃, ZnWO₄ and In₂S₃/ZnWO₄ composites was evaluated by examining the photodegradation of RhB under the visible light irradiation (λ ≧ 420 nm) (Fig. 6(A)). Pure ZnWO₄ presented almost no visible light photocatalytic activity, and pure In₂S₃ exhibited a weak photodegradation rate of 9% after 110 min of visible light irradiation, whereas all of the In₂S₃/ZnWO₄ composites clearly showed a better photocatalytic performance. Pure ZnWO₄ had negligible photocatalytic activity under visible-light irradiation due to its no absorbance in the visible-light region, whereas the introduction of In₂S₃ into ZnWO₄ enhanced the photocatalytic activity of the In₂S₃/ZnWO₄ composites. This observation implied that the heterojunction formed between ZnWO₄ and In₂S₃ played an important role in improving the photocatalytic activity. Fig. 6(A) also demonstrated that the degradation rate of RhB using the In₂S₃/ZnWO₄ composites initially increased along with the increase of the In₂S₃ content in the order of 0.05-In₂S₃/ZnWO₄ (30%), 0.4-In₂S₃/ZnWO₄ (81%) and 0.6-In₂S₃/ZnWO₄ (96%) after 110 min of visible-light irradiation. Whereas the degradation ratio of RhB decreased to 74% and 63% when the mole ratio of In/Zn in In₂S₃/ZnWO₄ composites were 0.80 and 1.0. The kinetics of degradation of RhB under visible-light irradiation were also investigated. A pseudo-first-order kinetic model was used to fit the degradation data using ln(C₀/C) = kt + a, where k is the apparent reaction rate constant, C₀ is the concentration of RhB after the adsorption–desorption equilibrium before irradiation, and C is the concentration of RhB at different illumination interval. Fig. 6(B) revealed that the plot of ln(C₀/C) and reaction time (t) is approximate linear, indicating that the photocatalytic degradation process of RhB followed the first-order kinetics model. The kinetic parameters for each photocatalyst were calculated and listed in the inset of Fig. 6(B). The order of the rate constants was consistent with the conclusions of photocatalytic degradation curves presented in Fig. 6(A). The figure also revealed that 0.6-In₂S₃/ZnWO₄ had the best photodegradation performance. Moreover, further increasing the In₂S₃ content in the In₂S₃/ZnWO₄ composites will lead to decrease of the photocatalytic performance under visible-light. The reason for the result in this finding can be explained by the following aspects. Firstly, excess In₂S₃ could lead to the decrease of the effective heterojunction interface on the surface of ZnWO₄. Secondly, excess In₂S₃ might act as recombination centers of the photogenerated electron-holes. Thirdly, the BET surface area of the In₂S₃/ZnWO₄ composites decreased when the mole ratio of In/Zn was over 0.6 (Table 1), which lead to the reduction in active center sites of photocatalysts.

Fig. 6 (A) Photocatalytic activities of ZnWO₄, In₂S₃ and In₂S₃/ZnWO₄ composites for the degradation of RhB under the visible light irradiation (λ ≧ 420 nm); (B) kinetics of the RhB decomposition over ZnWO₄, In₂S₃ and In₂S₃/ZnWO₄ composites; (C) photocurrent responses of pure ZnWO₄ and 0.6-In₂S₃/ZnWO₄ composite under the visible light irradiation; (D) cycling tests of photocatalytic activity of In₂S₃/ZnWO₄ composites for RhB degradation.

The photocatalytic activity of the previously literature reported of various ZnWO₄-based heterojunction systems are summarized in Table 3. From Table 3, it is obviously that the In₂S₃/ZnWO₄ heterojunctions in this work showed enhanced visible-light activity compared to the previous reports.

Table 3 Comparison of the ZnWO₄-based heterojunction photocatalysts

Photocatalysts	Preparation	Morphology	Photocatalytic applications	Ref.
BiOI/ZnWO4	Hydrothermal and chemical bath method	ZnWO4 nanorods are spread on the surface of the BiOI plates	86% MO degraded within 4 h irradiation with visible light	20
C3N4/ZnWO4	Chemisorption	ZnWO4 rod-like nanostructures deposited on the surface of g-C3N4 sheets	86% MB degraded within 150 min irradiation with visible light	21
Bi2WO4/ZnWO4	Hydrothermal process	Bi2WO6 nanoparticles grow on the ZnWO4 nanorods	About 90% RhB degraded within 80 min irradiation with UV light	15
WO3/ZnWO4	Microwave-solvothermal method	Irregular nanoplates and nanoparticles and flower-like structure	55.54% MB degraded within 480 min irradiation with UV light	36
Ag/AgBr/ZnWO4	Deposition–precipitation method	Quasi-spherical nanoparticles (AgBr or Ag or their mixture) adhered to the surface of the ZnWO4 nanorods	About 91% AR18 degraded within 60 min irradiation with visible light	37
Ag/AgCl/ZnWO4	Precipitation–photoreduction method	Ag/AgCl nanoparticles deposited on ZnWO4 nanorods	Completely MO and MB degraded within 25 min irradiation with visible light	43

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To better understand the effect of the heterojunction on the high photocatalytic activity of the In₂S₃/ZnWO₄ composites, we investigated its photocurrent responses in an electrolyte under visible light, which may indirectly correlate with the generation and transfer of the photoinduced charge carriers in the photocatalytic process.^20,38 Fig. 6(C) shows the photocurrent responses of pure-ZnWO₄ and 0.6-In₂S₃/ZnWO₄ under visible-light irradiation. Compared with pure-ZnWO₄, 0.6-In₂S₃/ZnWO₄ composite exhibited an obviously enhanced photocurrent response. The result demonstrated that a more efficient separation of photogenerated electron–hole pairs and fast transfer of photoinduced charge carriers occurred in the 0.6-In₂S₃/ZnWO₄ composite, which could be attributed to close interfacial connections and the synergetic effect existing between ZnWO₄ and In₂S₃. Meanwhile, the photocurrent changes of the pure ZnWO₄ and In₂S₃/ZnWO₄ composite are well corresponded to their photocatalytic performance.

In addition, stability is very important for photocatalysts in practical applications. To test the stability of the In₂S₃/ZnWO₄ composites, cycling experiments in the photodegradation of RhB under visible light irradiation were evaluated. Fig. 6(D) shows that only a little loss of the photocatalytic efficiency appeared after four successive cycles. This finding indicated that In₂S₃/ZnWO₄ composites holds excellent stability during the degradation.

PL analysis

The migration and recombination processes of electron–hole pairs of the In₂S₃/ZnWO₄ composites was also confirmed by PL emission spectra. Generally, the lower PL intensity often implies the lower recombination of free electron and hole carriers. The pure ZnWO₄ and In₂S₃/ZnWO₄ composites were excitated at 300 nm wavelength. Fig. 7 presents the PL spectra. The peak with the largest intensity attributed to the pure ZnWO₄ appeared at around 470 nm, which originated from the wolframite structure of WO₆⁶⁻.³⁹ The intensity of the PL peaks decreased after In₂S₃-coupled ZnWO₄ to form heterojunction nanostructure in the In₂S₃/ZnWO₄ series of composites (Fig. 7), which implied that the recombination of the photogenerated charge carriers was greatly suppressed. With the mole ratio of In/Zn in In₂S₃/ZnWO₄ composites increasing from 0.05 to 0.60, the PL peak intensity declined and the lowest peak intensity was observed for the 0.6-In₂S₃/ZnWO₄. However, the peak intensity raised again when the In₂S₃ content further increased. This intensity has a good correspondence with the photocatalytic performance and photocurrent response of the samples (Fig. 6(A) and (C)). Moreover, the results further confirmed that the effective charge separation could lead to the high photocatalytic activity of In₂S₃/ZnWO₄ composites.

Fig. 7 PL spectra of the pure ZnWO₄ and In₂S₃/ZnWO₄ series of composites.

Photocatalytic mechanism

To understand the degradation mechanism, it is necessary to determine the main active species^40–42 in the process of photoreaction for the degradation of RhB. For detecting the active species generated in the photocatalytic reaction, 0.2 mM benzoquinone (BQ, a quencher of ˙O₂⁻),^40–43 10 mM sodium oxalate (Na₂C₂O₄, a quencher of h⁺),^43,44 10 mM isopropanol (IPA, a quencher of ˙OH radical)^43,45,46 and 10 mM sodium bicarbonate (NaHCO₃, a scavenger for the adsorbed ˙OH radical and h⁺)⁴⁷ were adopted. Fig. 8 shows the variation of RhB degradation with different scavenger added. The degradation efficiency of RhB changed slightly with the addition of IPA, indicating that ˙OH radicals were not the dominant active species in the photocatalytic reaction system of In₂S₃/ZnWO₄. After addition of Na₂C₂O₄ or NaHCO₃, the degradation efficiency of RhB was remarkably depressed, which suggested that h⁺ was an important active species in the RhB photocatalytic degradation. In addition, when BQ was added into the reaction system, the degradation efficiency of RhB was almost completely depressed, whose apparent reaction rate constant k was decelerated obviously from 2.98 × 10⁻² to 1.84 × 10⁻³ min⁻¹. Considering the above results, h⁺ and ˙O₂⁻ were main active species in the degradation process, and ˙O₂⁻ was acted as the dominant active species responsible for RhB degradation under visible-light irradiation.

Fig. 8 Effects of different scavengers on the degradation of RhB in the presence of 0.6-In₂S₃/ZnWO₄ composite under visible light irradiation (λ ≧ 420 nm).

As we know, ZnWO₄ has no visible-light absorption due to its wide band gap. Thus, the visible light absorption was contributed by the In₂S₃ component in In₂S₃/ZnWO₄ heterojunction composites. Band-edge potential positions are crucial to determine the flowchart of photoinduced charge carriers in a heterojunction composite, which is important to approach the photocatalytic mechanism. A possible mechanism for the RhB photodegradation of the enhanced activity of the In₂S₃/ZnWO₄ composites under visible-light irradiation could be proposed. The conduction band (CB) and valence band (VB) potentials of a semiconductor can be calculated by the following equation:⁴⁸

E_VB = χ − E_e + 0.5E_g (2)

E_CB = E_VB − E_g (3)

Where χ is the absolute electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, E_e is the energy of free electrons in the hydrogen scale (about 4.5 eV), E_g is the band gap energy of the semiconductor. The χ values for ZnWO₄ and In₂S₃ are 6.230 ²⁰ and 4.704 eV,^27,49 respectively, and the band gap energies of ZnWO₄ and In₂S₃ are 3.27 and 2.35 eV, respectively. The calculation results show that the E_VB values of ZnWO₄ and In₂S₃ were 3.36 and 1.38 eV, and then their homologous E_CB values were 0.09 and −0.97 eV, respectively. Thus, the conduction band (CB) and the valence band (VB) of In₂S₃ are higher than that of ZnWO₄, and an efficient heterostructure for the separation and transportation of photogenerated charge carriers could be formed when these two semiconductor coupled together.

On the basis of the above analysis results, the possible photocatalytic mechanism was supposed as illustrated in Fig. 9. The In₂S₃ in the heterojunction photocatalysts acted as a sensitizer to absorb photons as well as excites electron–hole pairs, when the In₂S₃/ZnWO₄ composites were under visible-light irradiation. Since the conduction band (CB) edge potential values of In₂S₃ (−0.97 eV) is more negative than that of ZnWO₄ (0.09 eV), photoinduced electrons on the In₂S₃ surface can easily transfer to ZnWO₄ conduction band via heterojunction interfaces,⁴¹ leaving the holes on the In₂S₃ valence band. The dissolved O₂ molecules adsorbed on the surface of the photocatalyst could be reduced to ˙O₂⁻ by electrons derived from the more negative E (e⁻) of ZnWO₄ than E_(O2/˙O2⁻) (0.13 eV vs. NHE),⁵⁰ which was assigned to the degradation of RhB.⁵¹ At the same time, the photogenerated holes left on the valence band of In₂S₃ could also oxidize organic pollutant directly⁵² (Fig. 9). Therefore, it was leading to efficient separation of electron–hole pairs and inhibiting recombination of charge carriers, which was confirmed by photocurrent responses measurement and PL spectra analysis. As a summary, In₂S₃/ZnWO₄ composites possessed strong visible-light absorbing capacity, as well as the ability of efficiently separating and carrying the photogenerated charges, which resulted in the high-efficiency visible-light-driven photocatalytic activity.

Fig. 9 Schematic diagram of the transfer and separation of photogenerated charges in the In₂S₃/ZnWO₄ composites under visible light irradiation (λ ≧ 420 nm).

Conclusion

In summary, novel In₂S₃/ZnWO₄ heterojunction photocatalysts were synthesized using hydrothermal and surface-functionalized method. The results of XRD, morphological studies and XPS analysis revealed that In₂S₃ nanoparticles successfully grew on the monoclinic ZnWO₄ nanorods. Compared with pure ZnWO₄, the heterojunction composites possess larger specific surface area and stronger visible-light absorption. All of the In₂S₃/ZnWO₄ composites exhibited much higher photocatalytic activity than that of pure ZnWO₄ and In₂S₃ under visible-light irradiation, and the optimal photocatalytic activity was obtained from 0.6-In₂S₃/ZnWO₄ composite. The results of photocurrent and PL revealed that the formation of the heterojunction between In₂S₃ and ZnWO₄ induced the excellent separation and transportation of the photogenerated charges. Moreover, the possible mechanism of the enhanced photocatalytic activity was discussed.

Acknowledgements

We gratefully acknowledge the financial support provided by the Project of the National Natural Science Foundation of China (Grant No. 21271022).

References

1. M. W. Kanan and D. G. Nocera, Science, 2008, 321, 1072–1075.
2. Z. G. Zou, J. H. Ye, K. Sayama and H. Arakawa, Nature, 2001, 414, 625–627.
3. C. L. Yu, G. Li, S. Kumar and K. Yang, Adv. Mater., 2014, 26, 892–898.
4. X. Zhao and Y. F. Zhu, Environ. Sci. Technol., 2006, 40, 3367–3372.
5. D. Li, R. Shi, C. S. Pan, Y. F. Zhu and H. J. Zhao, CrystEngComm, 2011, 13, 4695–4700.
6. R. Shi, Y. J. Wang, D. Li, J. Xu and Y. F. Zhu, Appl. Catal., B, 2010, 100, 173–178.
7. H. B. Fu, J. Lin, L. W. Zhang and Y. F. Zhu, Appl. Catal., A, 2006, 306, 58–67.
8. P. Li, X. Zhao, Y. L. Li, H. G. Sun, L. M. Sun, X. F. Cheng, X. P. Hao and W. L. Fan, CrystEngComm, 2012, 14, 920–928.
9. J. Lin, J. Lin and Y. F. Zhu, Inorg. Chem., 2007, 46, 8372–8378.
10. M. Bonanni, L. Spanhel, M. Lerch, E. Füglein and G. Müller, Chem. Mater., 1998, 10, 304–310.
11. C. L. Zhang, H. L. Zhang, K. Y. Zhang, X. Y. Li, Q. Leng and C. G. Hu, ACS Appl. Mater. Interfaces, 2014, 6, 14423–14432.
12. G. L. Huang, R. Shi and Y. F. Zhu, J. Mol. Catal. A: Chem., 2011, 348, 100–105.
13. T. T. Dong, Z. H. Li, Z. X. Ding, L. Wu, X. X. Wang and X. Z. Fu, Mater. Res. Bull., 2008, 43, 1694–1701.
14. C. L. Yu and J. C. Yu, Mater. Sci. Eng., B, 2009, 164, 16–22.
15. D. Q. He, L. L. Wang, D. D. Xu, J. L. Zhai, D. J. Wang and T. F. Xie, ACS Appl. Mater. Interfaces, 2011, 3, 3167–3171.
16. H. L. Wang, L. S. Zhang, Z. G. Chen, J. Q. Hu, S. J. Li, Z. H. Wang, J. S. Liu and X. C. Wang, Chem. Soc. Rev., 2014, 43, 5234–5244.
17. J. L. Shi, Chem. Rev., 2012, 113, 2139–2181.
18. H. W. Huang, S. B. Wang, N. Tian and Y. H. Zhang, RSC Adv., 2014, 4, 5561–5567.
19. X. N. Li, R. K. Huang, Y. H. Hu, Y. J. Chen, W. J. Liu, R. S. Yuan and Z. H. Li, Inorg. Chem., 2012, 51, 6245–6250.
20. P. Li, X. Zhao, C. J. Jia, H. G. Sun, L. M. Sun, C. F. Cheng, L. Liu and W. L. Fan, J. Mater. Chem. A, 2013, 1, 3421–3429.
21. L. M. Sun, X. Zhao, C. J. Jia, Y. X. Zhou, X. F. Cheng, P. Li, L. Liu and W. L. Fan, J. Mater. Chem., 2012, 22, 23428–23438.
22. X. C. Song, W. T. Li, W. Z. Huang, H. Zhou, Y. F. Zheng and H. Y. Yin, Mater. Chem. Phys., 2015, 160, 251–256.
23. L. Rao, J. L. Xu, Y. H. Ao and P. F. Wang, Mater. Res. Bull., 2014, 57, 41–46.
24. Y. H. He, D. Z. Li, G. C. Xiao, W. Chen, Y. B. Chen, M. Sun, H. J. Huang and X. Z. Fu, J. Phys. Chem. C, 2009, 113, 5254–5262.
25. R. Verma, D. Datta, A. Chirila, D. Güttler, J. Perrenoud, F. Pianezzi, U. Müller, S. Kumar and A. N. Tiwari, J. Appl. Phys., 2010, 108, 074904.
26. S. Khanchandani, S. Kundu, A. Patra and A. K. Ganguli, J. Phys. Chem. C, 2013, 117, 5558–5567.
27. B. Chai, T. Y. Peng, P. Zeng and J. Mao, J. Mater. Chem., 2011, 21, 14587–14593.
28. K. Das and S. K. De, J. Phys. Chem. C, 2009, 113, 3494–3501.
29. S. N. Gu, W. J. Li, F. Z. Wang, S. Y. Wang, H. L. Zhou and H. D. Li, Appl. Catal., B, 2015, 170, 186–194.
30. J. Y. Xiong, G. Cheng, G. F. Li, F. Qin and R. Chen, RSC Adv., 2011, 1, 1542–1553.
31. H. Q. Wang, Z. B. Wu and Y. Liu, J. Phys. Chem. C, 2009, 113, 13317–13324.
32. V. V. Atuchin, E. N. Galashov, O. Y. Khyzhun, A. S. Kozhukhov, L. D. Pokrovsky and V. N. Shlegel, Cryst. Growth Des., 2011, 11, 2479–2484.
33. J. H. Bi, L. Wu, Z. H. Li, Z. G. Ding, X. X. Wang and X. Z. Fu, J. Alloys Compd., 2009, 480, 684–688.
34. X. Zhao, W. Q. Yao, Y. Wu, S. C. Zhang, H. P. Yang and Y. F. Zhu, J. Solid State Chem., 2006, 179, 2562–2570.
35. S. Rengaraj, S. Venkataraj, C. Y. H. Kim, E. Repo and M. Sillanp, Langmuir, 2011, 27, 5534–5541.
36. Y. Keereeta, S. Thongtem and T. Thongtem, Powder Technol., 2015, 284, 85–94.
37. K. B. Li, J. Xue, Y. H. Zhang, H. Wei, Y. L. Liu and C. X. Dong, Appl. Surf. Sci., 2014, 320, 1–9.
38. Y. Zhang, J. N. Lu, M. R. Hoffmann, Q. Wang, Y. Q. Cong, Q. Wang and H. Jin, RSC Adv., 2015, 5, 48983–48991.
39. Y. Koltypin, S. I. Nikitenko and A. Gedanken, J. Mater. Chem., 2002, 12, 1107–1110.
40. Y. L. Tian, B. B. Chang, Z. C. Yang, B. C. Zhou, F. G. Xi and X. P. Dong, RSC Adv., 2014, 4, 4187–4193.
41. H. L. Lin, H. F. Ye, S. F. Chen and Y. Chen, RSC Adv., 2014, 4, 10968–10974.
42. J. Cao, B. D. Luo, H. L. Lin, B. Y. Xu and S. F. Chen, J. Hazard. Mater., 2012, 217, 107–115.
43. J. Ke, C. G. Niu, J. Zhang and G. M. Zeng, J. Mol. Catal. A: Chem., 2014, 395, 276–282.
44. C. H. Zhang, L. H. Ai, L. L. Li and J. Jiang, J. Alloys Compd., 2014, 582, 576–582.
45. L. S. Zhang, K. H. Wong, H. Y. Yip, C. Hu, J. C. Yu, C. Y. Chan and P. K. Wong, Environ. Sci. Technol., 2010, 44, 1392–1398.
46. P. Ju, P. Wang, B. Li, H. Fan, S. Y. Ai, D. Zhang and Y. Wang, Chem. Eng. J., 2014, 236, 430–437.
47. C. H. An, J. Z. Wang, C. Qin, W. Jiang, S. T. Wang, Y. Li and Q. H. Zhang, J. Mater. Chem., 2012, 22, 13153–13158.
48. M. A. Butler and D. S. Ginley, J. Electrochem. Soc., 1978, 125, 228–232.
49. Z. M. Mei, S. Ouyang, D. M. Tang, T. Kako, D. Golberg and J. H. Ye, Dalton Trans., 2013, 42, 2687–2690.
50. S. Kumar, T. Surendar, A. Baruah and V. Shanker, J. Mater. Chem. A, 2013, 1, 5333–5340.
51. J. Zhou, G. H. Tian, Y. J. Chen, Y. H. Shi, C. G. Tian, K. Pan and H. G. Fu, Sci. Rep., 2014, 4, 4027.
52. M. Q. Yang, B. Weng and Y. J. Xu, Langmuir, 2013, 29, 10549–10558.

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