Abstract

The ZnWO4/Ag3PO4 nanocomposites synthesized by simple precipitation processes were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and UV-vis diffuse reflectance spectra. The results indicated that the ZnWO4 nanorods dispersed well on the surface of Ag3PO4 particles and ball-and-rod structure p-n heterojunctions were successfully fabricated. In subsequent degradation experiments of methyl orange (MO), ZnWO4/Ag3PO4 composites showed the highest photocatalytic activity compared to pure Ag3PO4 and ZnWO4, due to the presence of ZnWO4/Ag3PO4 heterojunctions, which could separate and transfer the electron–hole pairs generated by visible light and enhance the photocatalytic performance of the catalysts. The band gap structure and degradation mechanism of the enhanced photocatalytic materials are also discussed in this article. In conclusion, the ZnWO4/Ag3PO4 composite is a promising and excellent photocatalyst for the degradation of dye wastewater under visible light irradiation.

INTRODUCTION

Pollutants in wastewater are dangerous to human health and survival. With worsening environment pollution, degrading organic pollutants in wastewater is an important societal challenge. Semiconductor photocatalysis is an advanced oxidation process that can use clean solar energy to degrade target pollutants efficiently without harming the environment, playing a critical role in the degradation of refractory chemical wastewater (Fujishima 1972; Fujishima et al. 2000). Silver phosphate (Ag3PO4), a new and efficient photocatalytic material, is of great interest due to its strong ability to oxidate light and degrade organic pollutants (Yi et al. 2010; Wang et al. 2012). Ag3PO4, with a narrow band gap (2.36 eV), can achieve quantum efficiencies of up to 90% under visible light, observably higher than other photocatalysts (Bi et al. 2011). However, limitations of the Ag3PO4 material itself, such as terrible photocorrosion during photodegradation and the production of weakly active Ag on the surface of the catalyst, can reduce absorption of light energy (Wang et al. 2012). The photocatalytic degradation efficiency decreases gradually in practical application. Therefore it is still a challenging task to use Ag3PO4 as a recyclable and highly efficient photocatalyst.

Ag3PO4 photocatalysts can achieve better stability and activity after being modified by metal particles, such as Ag0 species, which most likely suppress electron–hole recombination and promote solar energy efficiency (Linic et al. 2011; Bi et al. 2012; Wei et al. 2012). Coupling Ag3PO4 with carbon materials such as carbon nanotubes (CNTs) (Liu et al. 2014), graphene oxide (GO) (Liang et al. 2012) and carbon quantum dots (CQDs) (Zhang et al. 2012), which possess excellent chemical and physical properties, have also been reported in recent years. In addition, combining two types of semiconductors, Ag3PO4 and another ideal semiconductor, to form a heterojunction such as AgX/Ag3PO4 (Bi et al. 2012), Ag3PO4/CeO2 (Yang et al. 2014), Ag3PO4/Bi2MoO6 (Xu & Zhang 2013), Ag3PO4/TiO2 (Teng et al. 2013; Tang et al. 2014), Ag3PO4/BiVO4 (Li et al. 2013a, 2013b) etc., is also a promising strategy. The advantages of coupled semiconductors include extending the light responsive range and boosting the transferring abilities of the photo-generated charge carrier. ZnWO4, a kind of zinc tungstate of prototype wolframite structure with high chemical stability, low photocorrosion, and inexpensive commercial availability (Fu et al. 2006; Siriwong 2011), has been found to be an excellent catalytic material. However, the performance of ZnWO4 in applications for photocatalytic degradation is only passable, as it is subject to its large band gap energy. To improve the photocatalytic activity of ZnWO4, many strategies, such as shape control (Lin et al. 2007), doping (Huang & Zhu 2007), noble metal deposition (Yu & Jimmy 2009) and compounding semiconductors (Li et al. 2013a, 2013b), have been reported in the last few years.

Based on this background, we proposed coupling Ag3PO4 with ZnWO4, to achieve complementary advantages. ZnWO4 nanorods were chosen because they had higher photocatalytic activity than ZnWO4 nanoparticles, which was attributed to the anisotropic structure of the nanorod (Lin et al. 2007). In this paper, we successfully coupled ZnWO4 nanorods with Ag3PO4 nanoparticles to synthesize a novel ZnWO4/Ag3PO4 nanocomposite and assessed its photocatalyst activity with a variety of detection techniques. The experimental results indicated that the novel composite photocatalyst material showed a favorable cycle performance and excellent photocatalytic performance.

MATERIAL

Synthesis of photocatalyst materials

Rod-like ZnWO4 was synthesized with a temperate hydrothermal process (Yu & Jimmy 2009). A total of 30 mL of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) aqueous solution (0.2 M) was slowly added to 30 mL sodium tungstate dihydrate (Na2WO4·2H2O) aqueous solution (0.2 M) on a magnetic stirrer to form a milky white mixture at room temperature. Next, the solution pH was brought to 9 by adding 0.5 M NaOH solution. Afterwards, the solution was poured into a 100 mL Teflon-lined stainless steel autoclave heated at 180 °C for 24 hours. Finally, the white precipitate was collected and rinsed several times with ethanol and deionized water. Then the precipitate was dried in a vacuum oven at 80 °C all night to obtain purified ZnWO4 powder.

The ZnWO4/Ag3PO4 composite was prepared via a simple precipitation process. In summary (sample ZA-2), 0.01 g ZnWO4 was dispersed in 40 mL of distilled water and sonicated for 10 min to get a dispersed solution. Then, 20 mL of 0.05 M silver nitrate (AgNO3) solution was added to the ZnWO4 dispersed suspension and stirred magnetically for 10 min. After this, 10 mL of 0.1 M disodium phosphate dodecahydrate (Na2HPO4·12H2O) was added dropwise, and stirred magnetically for 30 min. Finally, the obtained precipitate was centrifuged at the rate of 4,000 rpm with ethanol and deionized water several times to remove the remaining material. The precipitate was then dried in a vacuum oven at 80 °C for 12 hours. In the experiment, four samples with initial ZnWO4 contents of 0.03, 0.02, 0.005, and 0.0 (denoted as ZA-4, ZA-3, ZA-1, and Ag3PO4) were also prepared.

Characterization of materials

The morphology of ZnWO4, Ag3PO4, and ZnWO4/Ag3PO4 were studied using a Hitachi S4800 scanning electron microscope (SEM). An X-ray diffraction (XRD) study was carried out using Cu Kα radiation (λ = 0.15418 nm) at a scanning rate of 0.02° per min to elucidate structural information. Diffused reflectance UV-vis spectra of the samples were obtained from a UV-vis spectrophotometer (UV2550, Shimadzu, Japan). MO concentrations were determined using the TU-1800 UV–vis spectrophotometer (Shanghai AoXi Technology Instrument CO., Ltd) by recording the variations of the maximum absorption band at λ = 434 nm.

Photocatalytic activity test

The potential application of the ZnWO4/Ag3PO4 nanocomposite was evaluated by degradation of organic dye methyl orange (MO) with a 300 W Xe lamp with a cut-off filter of 420 nm. Briefly, ZnWO4/Ag3PO4 powder (40 mg) was placed in a quartz tube with circulating water jacket containing 50 mL of the MO aqueous solution (10 mg/L). The quartz tube with the catalyst and MO mixed liquor were first stirred magnetically for 30 min in a dark room to reach desorption–adsorption equilibrium. Afterwards, the degradation reaction occurred under irradiation from a Xe lamp, and a circulating cooling-water system was switched on to maintain a constant temperature. The suspension was collected at regular intervals and centrifuged for 10 min to obtain a supernatant without remnant particles. The MO concentration of the supernatant was measured with a UV–visible spectrophotometer. In addition, the stability and reusability of the ZnWO4/Ag3PO4 nanocomposite was tested with recycling experiments by following the previous procedure. After each exposure the nanocomposite was centrifuged and washed for the next experiment with deionized water three times. The degradation rate of MO was estimated with the following formula: D = (1– Ct/C0) × 100%, where C0 was the initial concentration of MO and Ct was the final concentration of MO after exposure.

RESULTS AND DISCUSSION

Characterization of as-synthesized materials

X-ray diffraction (XRD) analysis

Figure 1 illustrates the XRD patterns of the ZnWO4 sample, Ag3PO4 sample, and ZnWO4/Ag3PO4 nanocomposites with different ZnWO4 contents denoted as ZA-1, ZA-2, ZA-3, and ZA-4, respectively. The diffraction peaks shown in Figure 1(f) were well-indexed to monoclinic ZnWO4 (JCPDS card No. 73-0554), while those of Figure 1(a) correspond to the cubic phase of Ag3PO4 (JCPDS card No. 06-0505). The sharp diffraction peaks and less impure peaks of both ZnWO4 and Ag3PO4 XRD patterns demonstrated their good crystallinity and high purity. All curves of the XRD patterns in Figure 1(b)1(e) clearly display the characteristic peaks of cubic phase Ag3PO4. In addition, the new addition peaks at 23.77°, 24.51°, 30.55°, and 53.71° are also found in the curves (b)–(e), which were assigned to (011), (110), (111), and (202) crystal planes of ZnWO4 (JCPDS card no.73-0554), respectively. Furthermore, it is notable that the diffraction peaks of ZnWO4 gradually weakened with reducing the ZnWO4 content, while the peak intensities of Ag3PO4 strengthened. This indicates that the ZnWO4/Ag3PO4 composite was successfully prepared.

Figure 1

XRD patterns of (a) Ag3PO4; ZnWO4/Ag3PO4 nanocomposites with different ZnWO4 contents: (b) ZA-1, (c) ZA-2, (d) ZA-3, (e) ZA-4; and (f) ZnWO4.

Figure 1

XRD patterns of (a) Ag3PO4; ZnWO4/Ag3PO4 nanocomposites with different ZnWO4 contents: (b) ZA-1, (c) ZA-2, (d) ZA-3, (e) ZA-4; and (f) ZnWO4.

SEM analysis

The typical SEM photographs of prepared ZnWO4, Ag3PO4 and ZnWO4/Ag3PO4 samples are presented in Figure 2. The pure ZnWO4 sample with a uniform and rod-like morphology is displayed in Figure 2(a). The pure Ag3PO4 particles (Figure 2(b)) were irregular globular particles. Figure 2(c) and 2(d) indicate ZnWO4 nanorods were well dispersed on the surface of the Ag3PO4 particles, which could be further proved by the subsequent high-resolution transmission electron microscopy (HRTEM) analysis. In addition, with ZnWO4 decreasing from 0.03 g to 0.005 g, the number of ZnWO4 nanorods on the surface of Ag3PO4 particles were also reduced gradually, corresponding to the observed XRD patterns. Figure 2(f) shows the energy-dispersive X-ray spectroscopy (EDS) analysis of ZnWO4/Ag3PO4 samples. Zn, W, Ag and P elements were clearly observed on the nanocomposite surface, and also provided evidence of the successful preparation of composite photocatalytic materials.

Figure 2

SEM images of (a) ZnWO4, (b) Ag3PO4; (c) and (d) ZnWO4/Ag3PO4 nanocomposite (ZA-2); (e) ZA-2 after photoreaction; and (f) EDS spectrum of ZA-2.

Figure 2

SEM images of (a) ZnWO4, (b) Ag3PO4; (c) and (d) ZnWO4/Ag3PO4 nanocomposite (ZA-2); (e) ZA-2 after photoreaction; and (f) EDS spectrum of ZA-2.

Transmission electron microscopy analysis

Figure 3 shows transmission electron microscopy (TEM) and HRTEM photographs of the ZnWO4/Ag3PO4 heterostructure ZA-2. As shown in Figure 3(a) and 3(c), the ZnWO4 nanorods were well dispersed on the surface of the Ag3PO4 nanoparticles. Furthermore, Figure 3(b) and 3(d) clearly show that the lattice spacing at the interface region of 0.247 and 0.373 nm was consistent with the (021) and (011) planes of ZnWO4, respectively, and those of 0.150 nm and 0.300 nm were consistent with the (400) and (200) planes of Ag3PO4, respectively. These results showed that close heterojunction between Ag3PO4 and ZnWO4 could be efficiently generated, enhancing the efficiency of charge separation.

Figure 3

(a) and (c) TEM images and (b) and (d) HRTEM images of the ZnWO4/Ag3PO4 (ZA-2) nanocomposite.

Figure 3

(a) and (c) TEM images and (b) and (d) HRTEM images of the ZnWO4/Ag3PO4 (ZA-2) nanocomposite.

UV–vis analysis

As shown in Figure 4, the UV–vis diffuse reflectance spectra of ZnWO4, Ag3PO4 and ZnWO4/Ag3PO4 (ZA-2) nanocomposites were described clearly from 200 nm to 800 nm. The absorption edge of pure Ag3PO4 particles was located at 500 nm, while ZnWO4 absorbed light less than 400 nm due to its wide band gap. In the case of the ZnWO4/ Ag3PO4 composite photocatalyst, we identified not only the feature absorption band edge (approximately 500 nm) of Ag3PO4 in the visible light region, but also the characteristic absorption peaks (230 and 285 nm) in the UV light range of pure ZnWO4 from its UV-vis diffuse reflectance spectra. It indicated that Ag3PO4 and ZnWO4 were successfully composited together. Our calculations showed that the as-prepared ZnWO4 had a narrower band gap of 3.14 eV compared with that of previous reports, due to its special rod-like structure (Lin et al. 2007). The band gap of Ag3PO4 was calculated to be 2.31 eV, and this agreed with previous reports (Dinh et al. 2011).

Figure 4

UV-vis diffuse reflectance spectra of ZnWO4, Ag3PO4 and ZnWO4/Ag3PO4 (ZA-2).

Figure 4

UV-vis diffuse reflectance spectra of ZnWO4, Ag3PO4 and ZnWO4/Ag3PO4 (ZA-2).

Photocatalytic degradation of methyl orange (MO)

To measure the photocatalytic activity of the as-prepared ZnWO4/Ag3PO4 nanocomposites, we tested its ability to decompose MO dye in simulated visible light with a 300 W Xe lamp. Figure 5(a) illustrates the degradation curves of MO by ZnWO4/Ag3PO4 nanocomposites with different ZnWO4 amounts, the performances of pure ZnWO4 and Ag3PO4 were also measured for comparison. As shown in Figure 5(a), pure Ag3PO4 degraded 54% of MO dye after 35 min of visible light irradiation. At the same time pure ZnWO4 exhibited invisible degradation efficiency. Meanwhile, the photodegradation efficiencies of ZA-4, ZA-3, ZA-2, and ZA-1 for MO are 72%, 79%, 95%, and 93%, respectively after 35 min of irradiation with visible light. Therefore, it was clearly shown that all ZnWO4/Ag3PO4 composite photocatalysts have higher degradation rates than that of only ZnWO4 or Ag3PO4 degradation of the organic dye MO. Based on these points, we concluded that the ZnWO4/Ag3PO4 heterojunction could improve photodegradation efficiency. It is true, however, that an increase in the percentage of ZnWO4 lowers photocatalytic activity, and the nanorods took the place of catalytic active sites of Ag3PO4, which was responsible for decreasing decomposition of MO. As a result, ZA-2 nanocomposite with 0.01 g ZnWO4 content performed higher photocatalytic activity in degrading dye.

Figure 5

(a) Photocatalytic activities of ZnWO4/Ag3PO4 nanocomposites, pure Ag3PO4 and pure ZnWO4 for degradation of MO dye under visible light irradiation; (b) kinetic fit for the degradation of MO under visible light irradiation.

Figure 5

(a) Photocatalytic activities of ZnWO4/Ag3PO4 nanocomposites, pure Ag3PO4 and pure ZnWO4 for degradation of MO dye under visible light irradiation; (b) kinetic fit for the degradation of MO under visible light irradiation.

Figure 5(b) shows the kinetic fit for degradation of the MO solution of only Ag3PO4, ZnWO4 and ZnWO4/Ag3PO4 photocatalysts under visible light irradiation. From the curve graph, the degradation kinetics curve of the natural logarithm of normalized MO concentration versus irradiation time was almost a straight line. As such, the kinetic curve of decomposition of MO could be well represented with a first order kinetic model:  
formula
where C0 is the initial concentration of MO, Ct is the instantaneous concentration of MO at t time, and k is the apparent rate constant (min−1). Figure 5(b) shows that k was 0.0234, 0.0020, 0.0454, 0.0372, 0.0443, 0.0898 and 0.0826 min−1 for single Ag3PO4, ZnWO4 and ZA-4, ZA-3, ZA-2, and ZA-1 photocatalysts, respectively. ZA-2 had the highest k and displayed the highest photocatalytic activity, which was consistent with previously described analysis.

Five recycling experiments were performed under identical conditions to confirm the cycle performance of the ZnWO4/Ag3PO4 nanocomposite for practical application. Figure 6 shows that the degradation efficiency for MO dye still reached 81% and did not significantly decrease after five cycling experiments. As shown in Figure 2(f), the SEM of the morphology of the ZnWO4/Ag3PO4 nanocomposite was still intact after reaction. The results indicated that ZnWO4/Ag3PO4 nanocomposite was a type of potential and stable photocatalytic materials in repeated photocatalytic degradation experiments.

Figure 6

The cycling efficiency for MO degradation of ZA-2 sample under visible light irradiation.

Figure 6

The cycling efficiency for MO degradation of ZA-2 sample under visible light irradiation.

In the case of the ZnWO4/Ag3PO4 heterojunction photocatalyst, the photocatalytic activity relied on the process of excitation, transfer and separation of the photogenerated electron pairs, which were determined by the band positions. The valence band (VB) and conduction band (CB) edge positions of ZnWO4 and Ag3PO4 were calculated using the following equations (Butler & Ginley 1978).  
formula
(1)
 
formula
(2)
where χ was the absolute electronegativity of the semiconductor (χ was 6.31 eV and 5.96 eV for ZnWO4 and Ag3PO4, respectively), Ee was the energy of free electrons on the hydrogen scale (ca. 4.5 eV) and Eg was the band gap energy. Therefore, the EVB of ZnWO4 and Ag3PO4 were calculated to be 3.38 and 2.62 eV (vs normal hydrogen electrode; NHE), respectively, by Equation (1). According to Equation (2), the ECB of ZnWO4 and Ag3PO4 could be separately calculated to be 0.24 and 0.31 eV (vs NHE). The energy band structure illustrations of the ZnWO4/Ag3PO4 heterojunction is shown in Figure 7(a) based on theoretical analysis and the previously described study.
Figure 7

Schematic diagrams of the energy band positions of p-type Ag3PO4 and n-type ZnWO4 before contact (a) and after contact (b). EF is the Fermi level of the semiconductor.

Figure 7

Schematic diagrams of the energy band positions of p-type Ag3PO4 and n-type ZnWO4 before contact (a) and after contact (b). EF is the Fermi level of the semiconductor.

Schematic diagrams for energy band positions of Ag3PO4 and ZnWO4 are displayed in Figure 7. The as-prepared Ag3PO4 was a p-type semiconductor with the Fermi level close to the VB (Reunchan & Umezawa 2013), while ZnWO4 was a typical n-type semiconductor whose Fermi level was close to the CB. Before contact, the band positions of the pure ZnWO4 and Ag3PO4 displayed the nested band structure by computation. After coupling the Ag3PO4 and ZnWO4 semiconductors, a novel p–n heterojunction structure was formed, and an internal electric field was established that was directed from ZnWO4 to Ag3PO4 and improved the charge transfer efficiency simultaneously. As illustrated in Figure 7(b), an equilibrium state of Fermi levels of Ag3PO4 and ZnWO4 was achieved with an upward shift of the energy band positions of Ag3PO4 and a downward shift of ZnWO4 during the combination process. As a result, the VB and CB of semiconductor Ag3PO4 were higher than that of semiconductor ZnWO4, and this type of heterojunction led to efficient charge carrier separation. The photo-generated electrons could disengage freely from the CB of the Ag3PO4 semiconductor toward the CB of the ZnWO4 semiconductor. Meanwhile, the photo-generated holes could conveniently transfer from the VB of ZnWO4 to the VB of Ag3PO4 under visible light irradiation.

As shown in Figure 8, photo-generated holes (h+) produced in the VB top of Ag3PO4 oxidized the OH ionized from H2O to produce active hydroxyl radicals (OH·). Meanwhile, a superoxide radical ion (·O2) was formed by the reaction of photo-generated holes (e) in a conduction band with O2. Therefore, active groups of h+, OH· and ·O2 were closely associated with the degradation of organic dye MO. The process is described as follows:  
formula
(3)
 
formula
(4)
 
formula
(5)
 
formula
(6)
 
formula
(7)
 
formula
(8)
 
formula
(9)
Figure 8

Photocatalytic mechanism of the novel ZnWO4/Ag3PO4 nanocomposite under visible light irradiation.

Figure 8

Photocatalytic mechanism of the novel ZnWO4/Ag3PO4 nanocomposite under visible light irradiation.

CONCLUSIONS

In conclusion, this new ZnWO4/Ag3PO4 composite photocatalyst was fabricated by an in situ precipitation method under mild conditions. We found that this as-prepared ZnWO4/Ag3PO4 nanocomposite exhibited high photocatalytic activities and good photostability for degradation of MO dye, compared with pure Ag3PO4 particles under visible light irradiation. Furthermore, according to the calculation and study based on the theory of band gaps, the p–n heterojunction constituted with semiconductor interfaces of ZnWO4 and Ag3PO4 improved the photoelectric efficiency of the original catalyst. This paper presents a new approach to composite photocatalyst synthesis, which could provide promising catalytic materials with excellent catalytic performance and good applicability.

ACKNOWLEDGEMENTS

This work was supported by Fundamental Research Funds for the Central Universities (No. 310829161016 and 310829171004) and PhD Start-up Fund of Xi'an University of Science and Technology (No. 6310117055).

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