ZnWO₄/Ag₃PO₄ composites with an enhanced photocatalytic activity and stability under visible light

Abstract

A simple and renewable approach to synthesize ZnWO₄/Ag₃PO₄ composites has been explored for the enhancement of the photocatalytic activity and stability of Ag₃PO₄. The XRD, SEM and TEM spectra show that the ZnWO₄ particles are deposited on the Ag₃PO₄ substrate. The photocatalytic experiments demonstrate that the 4% ZnWO₄/Ag₃PO₄ composite exhibits a high photocatalytic activity, which is higher than that of pure Ag₃PO₄. The photoluminescence measurements suggest a more efficient photoinduced charge separation and transfer in the ZnWO₄/Ag₃PO₄ composite. The photocatalytic mechanism of the ZnWO₄/Ag₃PO₄ composite is discussed according to its band structure. The constructed ZnWO₄/Ag₃PO₄ composite exhibited a promising and desirable photocatalytic activity, as well as a good stability under visible light, and thus may find potential applications in environmental decontamination and conversion of solar energy.

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1. Introduction

The degradation of organic contaminants by semiconductor photocatalysts can potentially be an advantageous technology for environmental decontamination.^1–10 Amongst all known photocatalysts, TiO₂ is the most promising and economical material due to its photocatalytic activity, stability and innocuousness.^11–13 Nevertheless, TiO₂ is active only under UV light, which represents just a small portion of solar radiation (ca. 4%), whereas visible light occupies a much larger percentage (ca. 46%).¹² Therefore, for the conversion of solar energy, it is significant for researchers in material science to explore photocatalysts that possess a good activity under visible light.

Recently, a tremendous breakthrough was achieved by Yi et al., who investigated a novel utilization of the Ag₃PO₄ semiconductor,^14–16 which exhibited a high-efficient usage (up to 90%) of the visible light for O₂ resolved from water and an effective photodegradation of organic materials. Nevertheless, Ag₃PO₄ can be slightly dissolved in water solution, which enormously diminishes its structural stability. Unfortunately, the low structural stability of pure Ag₃PO₄ intensely constrains its environmental applications in the practice. Therefore, a great amount of effort and attempts have been emphasized on morphology transformations,^17–19 electronic-structure computation,²⁰ as well as composite construction.^21,22 Lately, some photocatalytic composites based on Ag₃PO₄, e.g., AgX/Ag₃PO₄ (X = Cl, Br, I),²³ Ag₃PO₄/TiO₂,^24–27 Fe₃O₄/Ag₃PO₄,²⁸ Ag₃PO₄/SnO₂,²⁹ and graphene oxide/Ag₃PO₄,³⁰ have successfully been synthesized and have exhibited an improved photocatalytic activity. More recently, an Ag₃PO₄/graphene composite,^31–37 Ag₃PO₄/g-C₃N₄,^38,39 and other complex Ag₃PO₄ photocatalysts^40,41 have been extensively studied and reported. Nevertheless, the efficiency and effectiveness of Ag₃PO₄ for degrading organic pollutants still needs to be investigated.

Some investigations reported that under UV irradiation, ZnWO₄ exhibits a high photocatalytic activity for the mineralization of organic contaminants.^42–45 Its advantages in physical as well as chemical properties, such as chemical inertness, photostability and other environmentally friendly characteristics contribute to our conviction that it can be a suitable photocatalytic material. Consequently, the enhancement of the photocatalytic activity of ZnWO₄ for practical utilizations is of remarkable significance. In this study, a new photocatalytic composite based on Ag₃PO₄ and ZnWO₄ is designed to achieve efficient and effective degradation of rhodamine B (RhB) under simulated solar radiation. The results reveal that under visible light the new photocatalytic composite exhibits a much improved activity with respect to pure Ag₃PO₄. In addition, the prepared ZnWO₄/Ag₃PO₄ composite shows good stability. Correspondingly, a rational model was established to demonstrate the core roles of ZnWO₄ during the photocatalytic progression. It is notable that this is the first investigation to report semiconductor ZnWO₄/Ag₃PO₄ composites for the photodecomposition of RhB under visible light. This study also provides a possible approach for developing novel visible light-driven photocatalysts with good activity and stability, satisfying the demands from future environmental and energy technologies.

2. Experimental

2.1. Photocatalyst synthesis

The synthesis of ZnWO₄ was completed via a hydrothermal process.⁴⁶ During the preparation of the ZnWO₄/Ag₃PO₄ composite, a quantity of as-prepared ZnWO₄ powder was dispersed in 20 mL water, ions were removed, and the solution was sonicated for 20 min. Immediately after that, 0.4 g of AgNO₃ was added to the ZnWO₄ dispersion. Right after 15 min of stirring, 20 mL of deionized water containing 0.284 g of dissolved Na₂HPO₄ was added dropwise to the abovementioned dispersion and the mixture was stirred for 5 h. The attained ZnWO₄/Ag₃PO₄ composites (with molar ratios of 2%, 4%, 6% and 8%) were flushed several times with ethanol and deionized water, and then dried at 80 °C for the following photocatalytic reaction and characterization.

2.2. Characterization

The structure of the composites was identified by X-ray diffraction (XRD, Bruker D8 advance; Cu Kα radiation). Their morphology was characterized by scanning electron microscopy (SEM, JEOL JSM-7001F) and transmission electron microscopy (TEM) using a FEI Model Tecnai G220 instrument. UV-vis diffuse reflectance spectra (DRS) were collected on a Hitachi UV-3100. The UV-vis spectrophotometer was equipped with an integrating sphere and BaSO₄ was used as the reference. A thin film of the ZnWO₄/Ag₃PO₄ composites deposited on indium-tin oxide (ITO) was used as the working electrode for investigation. The photoelectrochemical response was recorded with a CHI 660 electrochemical system.

2.3. Photocatalytic activity test

The activity of the photocatalyst was assessed by degrading RhB in water solution under visible-light using a 500 W Xe lamp equipped with a cutoff filter (λ > 420 nm). In a representative reaction process, 40 mL RhB (10 mg L⁻¹) solution was added to a flask containing 20 mg of photocatalyst. The reaction was maintained at room temperature with cooling water to avoid any thermal influence. Over time, a small amount of the abovementioned solution was extracted, and RhB concentration was determined by measuring the absorbance at 553 nm by UV-vis spectrophotometry. Every time before the final examination, the extracted solution was centrifuged at 8000 rpm min⁻¹ for 10 min to separate the photocatalytic powder. Finally, a standard curve was obtained to show the linear relationship between RhB concentration and its absorbance at 553 nm.

3. Results and discussion

3.1. Phase structures and compositions

Fig. 1 displays the XRD forms of the Ag₃PO₄, ZnWO₄ and ZnWO₄/Ag₃PO₄ composites. The XRD form of the as-prepared Ag₃PO₄ fitted well in the normal card (JCPDS no. 06-0505), indicating that cubic Ag₃PO₄ had been synthesized. The diffraction peaks regarding the as-prepared ZnWO₄ could be attributed to the monoclinic crystal (JCPDS no. 15-0774), demonstrating that ZnWO₄ had been prepared; the diffraction peaks were broad due to its low crystallinity. The mean crystallite dimension, computed from the broadening of the (111) X-ray diffraction peak according to the Scherrer's equation, was approximately 30 nm. For the composite samples, apart from the diffraction peaks for Ag₃PO₄, a small diffraction peak corresponding to the {111} plane of ZnWO₄ could be observed, confirming the formation of the ZnWO₄/Ag₃PO₄ composites.

Fig. 1 XRD patterns. Ag₃PO₄, 2% ZnWO₄/Ag₃PO₄, 4% ZnWO₄/Ag₃PO₄, 6% ZnWO₄/Ag₃PO₄, 8% ZnWO₄/Ag₃PO₄ and ZnWO₄.

The morphology of the obtained ZnWO₄/Ag₃PO₄ photocatalysts was characterized by SEM. Fig. 2(a–g) shows the SEM images of pure Ag₃PO₄, ZnWO₄/Ag₃PO₄ composites, and pure ZnWO₄. An association of cubic particles are found in pure ZnWO₄, and the thickness of the cubes is around 2 μm. In the ZnWO₄/Ag₃PO₄ composites, the ZnWO₄ particles were deposited on the substrate of the Ag₃PO₄ cubic particles, showing the combination of ZnWO₄ and Ag₃PO₄. Moreover, the particle size of the Ag₃PO₄ becomes smaller with increasing the proportion of ZnWO₄ in the catalyst. On the other hand, the mean size of the ZnWO₄ particles is around 60 nm. The corresponding EDS results are shown in Fig. 2(h). The elemental mapping analysis of the ZnWO₄/Ag₃PO₄ composites was necessary. The analysis showed that 4% ZnWO₄/Ag₃PO₄ was composed of Ag, P, Zn, W and O, which further proves that ZnWO₄ existed in all the ZnWO₄/Ag₃PO₄ composites. In addition, no other impurity was observed in the system.

Fig. 2 SEM images. (a) Pure Ag₃PO₄, (b) 2% ZnWO₄/Ag₃PO₄, (c) 4% ZnWO₄/Ag₃PO₄, (d) 6% ZnWO₄/Ag₃PO₄, (e) 8% ZnWO₄/Ag₃PO₄, (f) ZnWO₄, (g) single 4% ZnWO₄/Ag₃PO₄ composite particle (h) EDS spectrum of the 4% ZnWO₄/Ag₃PO₄ composite.

A representative TEM image of the ZnWO₄/Ag₃PO₄ composites is shown in Fig. 3. Fig. 3(a) shows the transmission electron microscopy (TEM) image of the obtained ZnWO₄/Ag₃PO₄ photocatalysts. The evident difference between the ZnWO₄ particles and the Ag₃PO₄ substrate (the thickness is about 20 nm) confirms the formation of ZnWO₄/Ag₃PO₄ composites. To further confirm the formation of the ZnWO₄/Ag₃PO₄ composite structure, a high-resolution transmission electron microscopy (HRTEM) image was investigated and is shown in Fig. 3(b). The interplanar gaps of 0.421 and 0.296 nm were distinctly observed, which are attributed to the (110) and (111) crystallographic planes of ZnWO₄ and Ag₃PO₄, respectively, and fitted well in the card JCPDS no. 06-0505 and 15-0774.

Fig. 3 TEM images. (a) TEM and (b) HRTEM images of the ZnWO₄/Ag₃PO₄ composite photocatalyst.

3.2. Photocatalytic activity

The degradation of RhB under UV light is illustrated in Fig. 4. All solutions were stirred in darkness for 2 h to achieve an adsorption–desorption balance between catalyst and RhB. The blank experiment (without catalyst) indicated a negligible degradation of RhB. The results demonstrated that the ZnWO₄/Ag₃PO₄ photocatalytic composite possessed better activity than pure Ag₃PO₄. As illustrated in Fig. 4(a), the 4% ZnWO₄/Ag₃PO₄ composite showed more photocatalytic activity than the other samples, and RhB was completely degraded within 8 min under visible light. Upon increasing the amount of ZnWO₄, as in 6% ZnWO₄/Ag₃PO₄, the photocatalytic activity diminished, although it was still higher than the photocatalytic activity of pure Ag₃PO₄. The temporal progression of the spectral alterations that occur during the photodegradation of RhB over the 4% ZnWO₄/Ag₃PO₄ composite photocatalyst is displayed in Fig. 4(b). The absorption peaks corresponding to RhB absorption progressively decrease in intensity as the reaction progresses in time, demonstrating high photocatalytic activity of the 4% ZnWO₄/Ag₃PO₄ composite photocatalyst. For a photocatalyst to be useful, it has to be robust, i.e., it has to maintain its activity after repeated usage. As shown in Fig. 4(c), the photocatalytic activity for the photodegradation of RhB did not suffer any considerable loss after recycling the catalyst four times. Fig. 4(d) shows that the crystalline structure of the photocatalyst did not change after the photocatalytic process, which indicates that the 4% ZnWO₄/Ag₃PO₄ composite photocatalyst is highly stable and does not suffer photocorrosion during the photocatalytic process.

Fig. 4 (a) Photocatalytic degradation efficiencies of RhB using ZnWO₄/Ag₃PO₄, (b) change over time of the UV-vis spectra of the RhB aqueous solution during degradation with 4% ZnWO₄/Ag₃PO₄, (c) cycling degradation curve for 4% ZnWO₄/Ag₃PO₄, (d) XRD patterns of 4% ZnWO₄/Ag₃PO₄ before and after the degradation experiment.

Fig. 5 shows the transformed absorption spectrum of the ZnWO₄/Ag₃PO₄ composite photocatalyst obtained from DRS based on the Kubelka–Munk (K–M) principle.⁴⁷ The light absorbance of the ZnWO₄/Ag₃PO₄ composite photocatalyst lies in the area within ca. 480 nm. Thus, the absorption spectrum indicates that these composites are able to absorb visible light, which indicates their potential usage as visible light-driven photocatalyst. The relevant energy band gap for these samples can be assessed from the (αhν)²–(hν) graph by extrapolating the linear portion of the (αhν)² curve to zero, as shown in the graph in Fig. 5. After computation, the band gap value was found to be 2.41 eV and 2.38 eV for pure Ag₃PO₄ and 4% ZnWO₄/Ag₃PO₄. These findings also confirm the construction of a connected structure between ZnWO₄ and Ag₃PO₄ in the photocatalytic composite. The energy band gaps for the ZnWO₄/Ag₃PO₄ photocatalytic composites range from 2.41 eV to 2.38 eV, depending on the ZnWO₄ content, and hence it appears reasonable to assume that the as-synthesized photocatalytic composites could be excited in visible light and lead to an improved photocatalytic activity.

Fig. 5 UV-vis diffuse reflectance spectrum of the ZnWO₄/Ag₃PO₄ composite photocatalyst. The inset shows the calculation of the band gap.

For understanding the abovementioned phenomena, the photoluminescence (PL) spectrum was recorded. The enhanced charge carrier segregation as well as the extended lifetime of photoinduced electron–hole pairs can be observed in the PL spectrum. As shown in Fig. 6, under an excitation wavelength of 355 nm, for Ag₃PO₄, a broad band at 430–530 nm is detected, whereas for the 4% ZnWO₄/Ag₃PO₄ composite, the distinctly decreased emission intensity indicates an effective and efficient interfacial charge-transfer process, implying that an additional channel for the vanishing of charge carriers dominates due to the interaction between the activated ZnWO₄ and Ag₃PO₄. This result indicated that the composite possesses a lower recombination rate for electrons and holes under visible light irradiation.

Fig. 6 Photoluminescence (PL) spectra of Ag₃PO₄ and the 4% ZnWO₄/Ag₃PO₄ composite with an excitation wavelength of 355 nm.

The separation efficiency of electrons and holes plays a vital role in the photocatalytic reaction. In the photocurrent experiment, for the test conditions, a 500 W xenon lamp equipped with a 420 nm cut-off filter and an 0.1 M Na₂SO₄ electrolyte are used. The current density–time curves (Fig. 7) indicate that the 4% ZnWO₄/Ag₃PO₄ composite produced a higher photocurrent than Ag₃PO₄ under the same conditions, indicating a lower recombination rate and a more efficient separation of the photoinduced electron–hole pairs generated at the interface between ZnWO₄ and Ag₃PO₄ in the ZnWO₄/Ag₃PO₄ composite.

Fig. 7 Photocurrent density of Ag₃PO₄ and 4% ZnWO₄/Ag₃PO₄ under visible light irradiation (electrode area: 1 cm²).

Since substantial success was achieved in computing band position as well as photoelectric thresholds for plenty of materials from the electronegativity of the component atoms,^48,49 we computed the conduction band (CB) as well as VB potentials of ZnWO₄ and Ag₃PO₄ at zero charge using the empirical equation:

E_VB = X − E^e + 0.5E_g

where E_VB is the VB edge potential, X is the electronegativity for the semiconductor, which is the geometric average of the electronegativity for the component atoms and the mathematical average for the atomic electron affinity as well as the first ionized energy. E^e is the free electron energy (−4.5 eV), and E_g is the band gap energy for the semiconductor. On the other hand, E_CB can be calculated according to the formula E_CB = E_VB − E_g. In order to obtain the band gap energy value for ZnWO₄ and Ag₃PO₄, the UV/Vis absorption spectra of ZnWO₄ and Ag₃PO₄ particles were recorded, as shown in Fig. 5. Based on the above investigation, the energy band graph for ZnWO₄/Ag₃PO₄ is shown in Fig. 8. The band structure could also explain the improved stability of the ZnWO₄/Ag₃PO₄ heterostructures, compared to the pure Ag₃PO₄ particles. The predicted valence band (VB) edges for ZnWO₄ and Ag₃PO₄ according to the above equation were 3.0 eV and 2.43 eV. Ag₃PO₄ can be excited by visible light, and thus the photocatalytic process is activated by the absorption of visible-light photons, which leads to the generation of photoinduced holes in the VB as well as electrons in the CB. On the basis of the band border position, the activated holes in the VB of Ag₃PO₄ shift to that of ZnWO₄ in the potential for the difference in valence band energy. Thus, the recombination for electron–hole pairs can be diminished, and the photocatalytic process can be improved. In the meanwhile, it is assumed that as regards the amount of ZnWO₄ in the composite, there is an optimum ZnWO₄ versus Ag₃PO₄ ratio that affords the highest photocatalytic activity.

Fig. 8 Schematic of the band diagrams of the ZnWO₄/Ag₃PO₄ composite.

4. Conclusions

In summary, we have used a simple method for synthesizing ZnWO₄/Ag₃PO₄ composites so as to enhance the photocatalytic activity of Ag₃PO₄. Significantly, compared with pure Ag₃PO₄, these photocatalytic composites exhibit an improved photocatalytic activity and an excellent structural stability when degrading RhB under visible light. More interestingly, the optimized molar ratio of ZnWO₄ with respect to Ag₃PO₄ is 4% for the positive synergistic influence between the two constituents. After 5 rounds of repetition tests, the efficiency and effectiveness for RhB degradation were retained. Hence, the high photocatalytic performance of ZnWO₄/Ag₃PO₄ may offer a new approach for the design of novel catalysts for new energy resources and green chemistry, to address environmental issues.

Acknowledgements

This study was supported by the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY15E020002, LY15E080002, LQ14E020004, and LY14B070011), the National Natural Science Foundation of China (Grant No. 51502188, 51572183 and 21502007), and the Science and technology personnel training project personnel from Xinjiang Uygur Autonomous Region of China (Grant No. qn2015bs014).

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