Construction of one dimensional ZnWO₄@SnWO₄ core-shell heterostructure for boosted photocatalytic performance

1. Introduction

Currently, the abundant discharge of textile industrial effluent is becoming a huge challenge to public health and environment with the development of the economy and industry [1,2]. Hence, an efficient pollution-free processing technology to remove the toxic contaminants from water waste is highly imperative and desired [[3], [4], [5]]. Semiconductor-based photocatalysis is regarded as one of the most promising technologies [[6], [7], [8]]. Since 2006, Zhu et al. [9] first found that ZnWO₄ owned the high photocatalytic activity for the degradation of RhB, ZnWO₄-based photocatalysts began to be used for the degradation of organic pollutants. Due to its unique physicochemical properties, ZnWO₄ can be used as scintillating materials, optical fibers, gas sensors, photocatalysts, etc. [10]. However, ZnWO₄ is a wide band-gap semiconductor and has lower charge transfer efficiency, which severely limits its practical applications [11]. Hence, many strategies such as doping, semiconductor compositing, and sensitization have been carried out to overcome the existing problems. Recently, Chen et al. [12] reported that ZnWO_4-x sample with oxygen vacancies can degrade 91% and 78% tetracycline under the irradiation of UV and ultraviolet-visible-near infrared (UV-Vis-NIR) light, respectively. Ran et al. [13] found that Si-doped ZnWO₄@ZnO nanocapsules with open-shaped structures displayed the high effective photocatalytic activity for degradation of RhB under visible light irradiation. In addition, GO/ZnWO₄ [14], ZnWO₄/ZnFe₂O₄ [10] and ZnWO₄/Cu₂O [15] composites also showed the enhanced photocatalytic activity. Therefore, it can be concluded safely that the semiconductor heterostructure by coupling a suitable semiconductor is a good way to improve photocatalytic performance

Stannous tungstate (SnWO₄) is a promising candidate as a visible-light-sensitive photocatalyst for its desired band-gap and unique electronic catalytic property [16,17]. It is well known that SnWO₄ has two crystalline phases (including α-phase and β-phase), and both of them can be appropriate for the photocatalysis [16,18]. Compared with the other photosensitizers, the SnWO₄ material displays several distinct advantages: (1) SnWO₄ material is a narrow band gap semiconductor and has appropriate band-gap position; (2) It can be easily prepared by in-situ synthesis method; (3) SnWO₄ and ZnWO₄ materials are tungstate with the same ABO₄ structure. Thus, it is very intriguing to explore the performance of photocatalyst that the SnWO₄ material is introduced as photosensitizer on surface of ZnWO₄ nanocrystal to fabricate the heterostructure.

In this work, the ZnWO₄@SnWO₄ heterostructure composite with a core-shell structure was successfully fabricated by a simple two-step method. The as-prepared ZnWO₄@SnWO₄ material exhibits an excellent photocatalytic activity for RhB degradation under visible light irradiation. Furthermore, the detailed structure and properties of photocatalysts were also analyzed by a series of characterizations. Thereinto, the uniform core-shell structure and a clear heterogenous interface between ZnWO₄ and SnWO₄ could be observed by SEM and TEM measurements. It was found that the incorporation of SnWO₄ shell layer could not only enhance photo-absorption efficiency of ZnWO₄ nanorods, but also drastically improve its photocatalytic performance under visible light irradiation. Meanwhile, PEC and ESR measurements were also used for the photocatalytic reaction mechanism, suggesting that the enhanced photocatalytic activity was attributed to the more efficient separation and transfer of photo-generated charge carriers, and they could produce more hydroxyl radical (•OH) as the main active species in photocatalytic reaction process.

2. Experimental

2.1. Catalyst preparation

ZnWO₄ nanorods were prepared by a simple hydrothermal synthesis [19]. In a typical process, 3 mmol Zn(NO₃)₂•6H₂O and 3 mmol Na₂WO₄•2H₂O were dissolved together in 28 mL deionized water under 30 min vigorous stirring. Then, the mixed solution was transferred into a stainless-steel autoclave with a Teflon cup of 50 mL inner volume. Subsequently, the pH value of mixed solution was adjusted to 7 using 0.5 mol/L NaOH, and the mixture was stirred for 1 h at room temperature. After being sealed, the autoclaves were heated in a convection oven at 180 °C for 48 h. Finally, the white precipitate was obtained and washed with distilled water and ethanol by centrifugation, and then dried in an oven at 60 °C for 12 h.

ZnWO₄@SnWO₄ nanorod composite was prepared by an in-situ synthetic method. 2 mmol ZnWO₄ nanorods and 80 mL of distilled water were added into a 150 ml three-necked flask, and the dispersion was refluxed at 120 °C. Then, the pH of mixed solution was adjusted to 5 using 0.1 mol/L HCl. Subsequently, 0.2 mmol SnCl₂•2H₂O and 1 mL of 0.2 mol/L Na₂WO₄ solution were added into the mixed solution each time at an interval of 3 h for several times. After being added, the mixed solution was continuously refluxed at 120 °C for 3 h. The obtained yellow sample was washed with distilled water and ethanol by centrifugation, and then dried in an oven at 60 °C for 12 h. SnWO₄ sample using the above synthetic method was prepared, except that ZnWO₄ nanorods didn’t be added. The sample denoted as ZnWO₄/SnWO₄ was prepared by the mechanical mixing of SnWO₄ with ZnWO₄, and the mass ration of ZnWO₄ and SnWO₄ is equal for ZnWO₄/SnWO₄ and ZnWO₄@SnWO₄ samples.

2.2. Characterization of samples

X-ray diffraction (XRD) plots were measured on a Bruker D8 Advance X-ray diffractometer with CuKα radiation (1.5406 Å). The Hitachi New Generation SU8010 field emission scanning electron microscope (SEM) was employed to obtain the SEM images. High resolution transmission electron microscopy (HRTEM) images were obtained by a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) were carried out on a VGESCALAB 250 XPS system with a monochromatized AlKα X-ray sources (15 kV 200 W 500 μm pass energy = 20 eV). All binding energies were referenced to the C 1s peak at 284.6 eV of surface adventitious carbon. UV-vis DRS spectra were taken on a Varian Cary 500 Scan UV-vis-NIR spectrometer with BaSO₄ as the reference. Electron spin resonance (ESR) spectra were measured using a Bruker model A300 spectrometer with a 500 W Xe-arc lamp equipped with an UV-cutoff (≥ 420 nm) as visible light source. Typically, the water or methanol solution of DMPO was added to 5 mg samples, and the mixture was sampled using a capillary tube and placed in an ESR tube for the experiment.

2.3. Photoelectrochemical measurement

Photoelectrochemical tests were performed using the electrochemical workstation (CHI 660E). Photocurrent was measured using the conventional three-electrode electrochemical cell with a working electrode, a platinum foil counter electrode and an Ag/AgCl electrode as reference electrode. The working electrode was fabricated by a special way to deposit a certain amount of catalyst onto the fluorine-doped tin oxide (FTO) glass. The working electrode was fabricated on the FTO glass: 10 mg of sample was mixed with 0.5 ml ethanol with continuous stirring for 3 h to get slurry. Then, 20 μL of slurry was spreading onto FTO glass, which side part was previously protected by Scotch tape, and the electrode was dried at room temperature for 24 h. In this experiment, the three electrodes were immersed in a mixed solution with a 0.2 mol/L Na₂SO₄ aqueous solution, and the working electrode was irradiated by visible light (≥ 420 nm).

2.4. Photocatalytic activity measurement

The photocatalytic performance of the as-prepared samples was evaluated by the degradation of RhB in aqueous solution. A 300 W Xe lamp with a 420 nm cutoff filter was used as the light source to ensure the irradiation with visible light only. Typically, 80 mg of photocatalyst was added into 80 mL of 10 mg/L RhB aqueous solution in a container. Prior to irradiation, the suspensions were magnetically stirred in dark for 60 min to ensure the establishment of an adsorption/desorption equilibrium between the photocatalyst and RhB. At given irradiation time intervals, 4 mL of suspension was collected and centrifuged to remove the photocatalyst particles, then the residual RhB solution was analyzed by monitoring variations at the wavelength of maximal absorption (554 nm) in the UV-vis spectra.

3. Results and discussion

3.1. Characterization of photocatalysts

Fig. 1 shows SEM images of ZnWO₄ and ZnWO₄@SnWO₄ nanorods. For the nude ZnWO₄ sample, it can be observed that the ZnWO₄ nanorods were in the range of 500-1000 nm length with a smooth surface, as shown in Fig. 1(a) and (b). Intriguingly, an uneven SnWO₄ shell layer is completely covered on the smooth surface of ZnWO₄ nanorods, as displayed in Fig. 1(c). The SnWO₄ nanoparticles are uniformly dispersed on the surface of ZnWO₄, indicating that the heterostructure between ZnWO₄ and SnWO₄ can be fabricated. In addition, Fig. 1(d) shows that the great majority of ZnWO₄ nanorods can be uniformly coated with SnWO₄ nanoparticles by the in-situ synthetic method.

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Fig. 1.   SEM images of (a, b) ZnWO₄ nanorods and (c, d) ZnWO₄@SnWO₄ heterostructure.

The ZnWO₄ and ZnWO₄@SnWO₄ samples were further examined by transmission electron microscopy (TEM). Fig. 2(a) shows the smooth and well-proportioned surface for the pure ZnWO₄ nanorods, which agrees well with the SEM results. Meanwhile, the core-shell structure of ZnWO₄@SnWO₄ sample is clearly display in Fig. 2(b) and (c). It is can be found that the SnWO₄ nanoparticles are fully coated on the entire surface of ZnWO₄ nanorods, which presents the ideal shell layer. Moreover, the core-shell structure was further investigated by HRTEM, as shown in Fig. 2(d). The distinct interface between ZnWO₄ and SnWO₄ can be clearly observed, suggesting that the heterostructure was successfully fabricated. Additionally, the interplanar spacing of 0.373 nm can be attributed to the (011) crystal plane of ZnWO₄, which corresponds to the previous report [20]. However, the lattice fringe of SnWO₄ shell layer can’t be observed in HRTEM image, implying that the SnWO₄ shell layer is amorphous.

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Fig. 2.   TEM images of (a) ZnWO₄ nanorods and (b, c) ZnWO₄@SnWO₄ heterostructure, (d) HRTEM image of ZnWO₄@SnWO₄ heterostructure and (e) energy dispersive spectroscopy mapping images of ZnWO₄@SnWO₄ heterostructure.

Fig. 3 shows the XRD pattern of all as-prepared samples. Apparently, SnWO₄ sample displays an amorphous phase with a weak and broad characteristic peak, which agrees with the result of HRTEM measurement. Meanwhile, the characteristic diffraction peaks of ZnWO₄ sample can be well assigned to pure monoclinic ZnWO₄ (JCPDS card no. 73-0554) [21]. Interestingly, the intensity of diffraction peaks of ZnWO₄ sample becomes weaker after the SnWO₄ shell layer coated, suggesting that SnWO₄ nanoparticles are deposited on the surface of ZnWO₄ nanorods. This is in line with the results of SEM and TEM measurements. Ultraviolet-visible diffuse reflectance absorption spectra (DRS) were employed to investigate the optical absorption properties of SnWO₄, ZnWO₄ and ZnWO₄@SnWO₄ samples, as shown in Fig. 4. It can be seen that the SnWO₄ sample displays remarkable absorption capability in the visible light region, and the absorption band edge of ZnWO₄ sample is observed in the UV region [22]. As expected, the absorption band edge of ZnWO₄@SnWO₄ sample appears the red-shift to visible light region, which can be due to the introduction of SnWO₄ shell layer.

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Fig. 3.   XRD patterns of ZnWO₄, SnWO₄ and ZnWO₄@SnWO₄ samples.

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Fig. 4.   UV-vis DRS spectra of ZnWO₄, SnWO₄ and ZnWO₄@SnWO₄ samples.

XPS was employed to investigate the chemical and binding state of SnWO₄ nanoparticles on the surface of ZnWO₄ nanorods, as shown in Fig. 5. Fig. 5(a)-(d) displays the high resolution XPS spectra for O 2p, W 4f, Zn 2p and Sn 3d, respectively. Fig. 5(a) shows the binding energy of O 1s at 530.8 eV and 533.0 eV. The lower binding energy peak at 530 eV can be attributed to the metal bound oxide component (O^2-) of [WO₄]^2-, and the peak at 533.0 eV is assigned to the surface adsorbed or structural water molecules [[23], [24], [25]]. The XPS spectrum of W 4f shows two characteristic peaks at around 35.2 eV for W 4f_7/2 and 37.3 eV for W4f_5/2, respectively, suggesting that W exists as W⁶⁺ in ZnWO₄@SnWO₄ nanorods [24,26]. Meanwhile, the binding energy of Zn 2p also can be observed at 1021.6 eV and 1044.7 eV, which is attributed to Zn²⁺ in ZnWO₄ nanorods [23]. In addition, the XPS spectrum of Sn 3d can be clearly observed, which further demonstrates the existence of amorphous SnWO₄ nanoparticles. However, the Sn 3d XPS spectrum of ZnWO₄@SnWO₄ nanorods displays two core level peaks at ca. 486.8 eV and 488.9 eV, which is assigned to Sn²⁺ in SnWO₄ and Sn⁴⁺ species, respectively [24,26]. It suggests that a small parts of Sn²⁺ ion is oxidized into Sn⁴⁺ species in the prepared process. Combining with the result of XRD pattern, the SnO, SnO₂ or Sn₃O₄ samples did not be detected in the SnWO₄ sample or ZnWO₄@SnWO₄ nanorods. Those results demonstrate that the Sn⁴⁺ species may be Sn⁴⁺ ion, which is adsorbed on the surface of ZnWO₄@SnWO₄ nanorods, so it can be detected by XPS measurement.

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Fig. 5.   High resolution XPS spectra of (a) O 2p, (b) W 4f, (c) Zn 2p and (d) Sn 3d for ZnWO₄@SnWO₄ heterostructure.

3.2. Photocatalytic performance of all samples

Photocatalytic activity of the Blank, ZnWO₄, ZnWO₄/SnWO₄ and ZnWO₄@SnWO₄ samples was evaluated for RhB degradation under visible light irradiation (λ ≥ 420 nm), as shown in Fig. 6. The blank experiment for photolysis of RhB was carried out. Apparently, the decomposition of RhB is inappreciable in the absence of photocatalyst, indicating that the RhB is stable under the experiment condition. Similarly, ZnWO₄ sample also displays the weak photocatalytic activity after 120 min of irradiation, because it is a wide-band-gap semiconductor. Interestingly, when ZnWO₄ and SnWO₄ semiconductors were coupled together, it presented an excellent photocatalytic activity for the degradation of RhB under visible light irradiation. It can be seen that the removal rate of RhB reached to 100% within 2 h. It suggests that the SnWO₄ shell layer acting as a visible light photocatalyst plays an important role in the photocatalytic reaction process. In order to testify the advantage of the ZnWO₄@SnWO₄ core-shell structure, the ZnWO₄/SnWO₄ sample used as the reference samples was prepared and evaluated for RhB degradation. Obviously, the ZnWO₄/SnWO₄ sample with the mechanical mixing displayed a lower photocatalytic performance, suggesting that the enhanced performance is dependant on their type of unique and excellent structure.

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Fig. 6.   Photocatalytic activity of RhB degradation for Blank, ZnWO₄, ZnWO₄/SnWO₄ and ZnWO₄@SnWO₄ samples under visible light irradiation.

3.3. Mechanism of photocatalytic performance enhancement

In order to explore the mechanism for enhanced photocatalytic performance of ZnWO₄@SnWO₄ sample, the photoelectrochemical measurement was carried out, as shown in Fig. S1 in supporting information. Generally speaking, the photo-generated current intensity can indirectly reflect separation efficient of photo-induced electrons and holes [27]. Evidently, the photocurrent density of ZnWO₄@SnWO₄ sample is much higher than that of ZnWO₄/SnWO₄ and ZnWO₄ sample. The increased photocurrent response suggests that the separation and migration efficiency of photogenerated charge carries are greatly improved, which is beneficial to the enhancement of photocatalytic activity.

The specific active species of ZnWO₄@SnWO₄ sample was further explored by electron spin resonance (ESR) measurement in photocatalytic reaction process, as shown in Fig. 7. Obviously, the four characteristic peaks of DMPO-•OH adducts can be clearly observed, indicating that the OH^- can be oxidized into •OH by the photogenerated holes in the valence band of ZnWO₄@SnWO₄ sample. On comparison, there is no signal of DMPO-^•OH adducts under dark test condition. It indicates that the hydroxy radical (^•OH) are indeed produced in the reaction system of ZnWO₄@SnWO₄ photocatalyst in aqueous solution under visible light irradiation. Simultaneously, in order to verify whether the superoxide radical (^•O₂^-) can be generated in the photocatalytic process, the ESR measurement of ZnWO₄@SnWO₄ photocatalyst can be carried out in DMPO methanol solution under visible light irradiation, as displayed in Fig. S2. However, there is no signal of DMPO-^•OH adducts in DMPO methanol dispersion. In conclusion, it demonstrates that the ^•OH is main active species in the photocatalytic process.

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Fig. 7.   ESR signal of DMPO-^•OH spin adducts for ZnWO₄@SnWO₄ heterostructure under visible light irradiation.

Based on the above results, the reaction mechanism for enhanced photocatalytic activity can be proposed in Scheme 1. The SnWO₄ sample is stimulated to produce the photo-induced electrons and holes under visible light irradiation. Because the flat band potential of SnWO₄ sample is higher than that of ZnWO₄ sample (Fig. S3), the photogenerated electrons on conduction band (CB) of SnWO₄ can be quickly transferred to conduction band (CB) of ZnWO₄. And the holes on valence band (VB) of SnWO₄ can react with OH^- and produce more hydroxyl radical (^•OH) as main active species. Finally, the organic pollutant was completely degraded by the hydroxyl radical (^•OH).

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Scheme 1..   Proposed mechanism of ZnWO₄@SnWO₄ heterostructure for enhanced photocatalytic activity.

4. Conclusion

In this work, we designed and fabricated the one-dimensional ZnWO₄@SnWO₄ heterostructure composite with a core-shell structure, which displayed an excellent visible-light photocatalytic activity for RhB degradation. The introduction of SnWO₄ shell layer can not only enhance the absorption and utilization of visible light for ZnWO₄ nanorods, but also promote charge transfer and separation efficiency so that to improve its photocatalytic activity. The enhanced performance can be attributed to the matched level structure and the intimately contacted interface. In addition, the hydroxyl radical (^•OH) can be regarded as the main active species in the photocatalytic reaction process. This work exhibits a novel ZnWO₄@SnWO₄ core-shell heterostructure for efficient visible light photocatalysis, so we can believe that it can be further applied to the designing of other semiconductor photocatalysts for improving their photocatalytic performance.