Abstract
Nanostructured ZnGa2S4 with indirect band gap was prepared by a simple thiourea reduction method. As-prepared samples were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX), and UV-Vis diffuse reflectance spectra (UV-Vis). The visible-light photocatalytic activity of the ZnGa2S4 was evaluated by the degradation of methylene blue (MB) aqueous solution. The high photoactivities of ZnGa2S4 can be attributed to the small band gap and large surface area, which is beneficial for degradation of pollutants and enhancement of transfer of photogenerated carriers.
1. Introduction
One of the major sources of water contamination is the dyeing and printing dye wastewater from various textile industries [1, 2]. The dye wastewater contains high levels of organic pollutants, color rendering and alkaline [3, 4]. In the past few decades, photocatalytic degradation is considered to be a feasible method to remove dyes from wastewater. Compared with the traditional processing methods, it can effectively and thoroughly destroy much structural stability of nonbiodegradable pollutants [5, 6]. The semiconductor oxide usually can be used as photocatalyst, which is due to its special electronic structure, that is, a filled valence band and an empty conduction band [7–10]. However, these semiconductor oxide catalysts are usually synthesized at high temperature, and they only have ultraviolet photocatalytic activity. For the solar spectrum, only a small portion of solar energy (about 4%) lies in the UV region, and visible solar spectrum represents a greater percentage (about 46%) [11]. Recently, in the development of new catalysts in the field of visible light with rapid progress, these photocatalysts have proper band gap which can be used to drive a photocatalytic reaction and thus are suitable for sewage treatment and manufacturing of hydrogen energy [12, 13]. A large amount of nanostructured materials can be used for the photocatalytic degradation, due to their inherent properties, such as increasing photon absorption, improving charge transfer rate, high carrier separation, and surface reactivity, arising from their large specific surface area [14, 15].
Recently, the semiconductor oxides of p-block, such as Zn2GeO4, ZnGa2O4, and Sr2Sb2O7, are considered to be new semiconductor photocatalysts which can catalyze the oxidation of organic pollutants [16, 17]. In addition to having higher pollutant degradation ability, these catalysts are also extremely stable and they are not inactive in the long process of photocatalytic reaction [16]. And these p-block semiconductor metal oxides have high catalytic performance, due to their special electronic structure [16–18]. However, the research of p-block metal oxide semiconductor photocatalyst is very limited, and most of the photocatalysts are only responsive to ultraviolet light. Therefore, to develop smaller size, large specific surface area, and accessible visible catalyst is a very effective method to promoting the photocatalytic degradation of organic pollutants.
In this paper, we reported that ZnGa2S4 photocatalysts with nanostructure were prepared via a simple method of thiourea reactions and investigated the catalyst for visible-light photocatalytic degradation of organic pollutants. Thiourea reduction method is suitable for synthesizing a large amount of materials, and the synthetic materials are uniform nanocrystalline particles with high surface area. The prepared ZnGa2S4 photocatalyst exhibits good photocatalytic activity and stability for methylene blue degradation.
2. Experimental Section
2.1. ZnGa2O4 Precursors Preparation
The ZnGa2O4 precursor was synthesized by coprecipitation method. A typical experimental procedure was as follows: 0.001 mol Zn(NO3)2·6H2O (AR, Beijing Chemical Reagent Factory) and 0.002 mol Ga(NO3)3·xH2O (AR, Sigma Company) were mixed in 10 mL distilled water. Under vigorous stirring, the pH value of mixed solution was adjusted to 9.0 with ammonia solution. The obtained suspension was thoroughly stirred for 24 h at a temperature of 80°C. The precipitation was obtained, then washed with distilled water and anhydrous ethanol (AR, Beijing Chemical Reagent Factory), and dried in air. The obtained products were annealed at 600°C for 5 h in air.
2.2. ZnGa2S4 Photocatalyst Preparation
0.1 g ZnGa2O4 precursors were put in the quartz boat, and the quartz boat was put in the middle of quartz tube. Another quartz boat containing 1 g thiourea powders (AR, Beijing Chemical Reagent Factory) was put in the upper part of the airflow in the quartz boat, and it was made sure that the temperature at this place was stabilized at 250–350°C where thiourea could be decomposed at this temperature. The tube was evacuated by vacuum pump then filled with high-purity nitrogen in order to ensure the quartz tube without oxygen in the reaction. Heating tube furnace at the rate of 25°C/min, until the center position was about 700–800°C and kept at this temperature for 30 minutes. After the reaction, the ZnGa2S4 powders were cooled naturally down to the room temperature and collected for test.
2.3. Photocatalytic Activity Experiment
Methylene blue (AR, Beijing Chemical Reagent Factory) was selected as pollutant to investigate the photocatalytic activity of the ZnGa2S4. 300 W high pressure mercury lamp was selected as a light source, and all experiments were carried out in air. In a typical procedure, 0.1 g/L MB and a certain quantity of ZnGa2S4 powder were stirred in 100 mL solution under visible light. Under certain time interval, 5 mL solution was taken out and centrifuged and then analyzed by UV-Vis spectrophotometer. MB decoloration efficiency by ZnGa2S4 photocatalyst was calculated by the following formula:where and represented the concentration of MB (g/L) at the initial time and the time in solution, respectively. In order to avoid the influence of MB absorption on catalytic reaction, the photocatalyst suspension was placed in the darkness for 30 minutes before using, to make the MB in ZnGa2S4 photocatalyst reach adsorption-desorption equilibrium.
2.4. Characterization
The X-ray powder diffraction (XRD) patterns were carried out on a Rigaku Company D/Max 2200 PC using Cu Kα radiation at 50 kV and 50 mA. Data were recorded at a scanning rate of 2°/min ranging from 10° to 90°. UV-visible diffuse reflectance (UV-Vis) was measured by Beckman-DU-8B-type UV-Vis-NIR spectrophotometer; the scanning range was 200–800 nm. The morphology of products was observed by H-9000 field emission transmission electron microscope at accelerating voltage of 100–300 kV (Hitachi). Energy-dispersive X-ray spectroscopy (EDX) was performed by the instrument attached on transmission electron microscope.
2.5. Computational Method
The electronic structures of ZnGa2O4 and ZnGa2S4 were calculated by density functional theory (DFT). Exchange-correlation energy was calculated by generalized gradient approximation (GGA) which was functionalized by Perdew Burke-Ernzerhof (PBE) [19]. In the calculation, the cut-off energy we used was 500 eV; Monkhorst-Pack grid-5 × 5 × 1 was used for Brillouin zone integrations [20]. Geometric structure was fully relaxed until the Hellmann-Feynman force of each atom was less than 0.02 eV/Å. We have implemented these algorithms within a powerful package called VASP [21, 22].
3. Results and Discussion
3.1. Transmission Electron Microscopy (TEM)
Nanostructured ZnGa2S4 photocatalyst is obtained at 800°C without adding catalyst. Instead of using traditional high toxicity catalyst H2S and explosive gas H2, we use the thiourea as sulfur source, and ZnGa2O4 serves as precursor. Figures 1(a) and 1(b) show the morphology of nanostructured ZnGa2S4 photocatalyst. The synthesized size of ZnGa2S4 nanocrystals is very small and uniform and their diameters are about 3–5 nm. Layer space is approximately 3.03 nm, corresponding to the (112) plane of ZnGa2S4. The chemical elements of ZnGa2O4 precursors and ZnGa2S4 photocatalyst were analyzed by EDX. As shown in Figures 1(c) and 1(d), there are three kinds of elements Zn, Ga, and O in ZnGa2O4 precursors, whereas ZnGa2S4 contains only Zn, Ga, and S.
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3.2. The Powder X-Ray Diffraction (XRD)
The XRD patterns of ZnGa2O4 precursors and ZnGa2S4 photocatalyst are shown in Figure 2, in which all the diffraction peaks 30.3°, 35.7°, 43.4°, 53.8°, 57.4°, 63.0°, and 74.6°, corresponding to ZnGa2O4 precursors (220), (311), (400), (422), (511), and (533) planes, occurred (JCPDS number 38-1240). For Figure 2(b), all the peaks 18.7°, 29.3°, 39.0°, 49.1°, 58.0°, and 79.5°, corresponding to the ZnGa2S4 photocatalyst (001), (112), (121), (024), (132), and (136) planes (JCPDS number 40-1462), can be seen. As we can see, there is no other phase peak existing in the XRD patterns of two materials, indicating that the two materials are pure. After thiourea reaction, ZnGa2O4 precursors have been completely transformed into ZnGa2S4 nanocrystals.
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3.3. UV-Visible Diffuse Reflectance (UV-Vis)
Figure 3 displays UV-Vis diffuse reflectance spectra of ZnGa2O4 precursors and ZnGa2S4 photocatalyst. For ZnGa2O4 precursors, the main peak is located between 200 and 300 nm, corresponding to a wide band gap with approximately 4.7 eV, showing that ZnGa2O4 precursors have photocatalytic activity only in the ultraviolet region [16]. However, after thiourea reduction reaction, the peak position of ZnGa2S4 photocatalyst extends to the visible area, and its band gap becomes narrow, which is beneficial to the absorption of visible light.
3.4. Visible-Light Degradation
To investigate the photocatalytic performance of nanostructured ZnGa2S4 in visible light, MB was selected as an organic contaminant. Many studies have reported that the quantity of catalyst can significantly influence the degradation efficiency of photocatalyst [1, 23, 24]. Therefore, a series of comparative tests were carried out to explore the influence of catalyst quantity on MB decolorizing efficiency, by changing the ZnGa2S4 quantity from 0.2 to 0.6 g/L. As shown in Figure 4, with increasing the catalyst quantity from 0.2 to 0.6 g/L, decolorization efficiency is increased from 61.3% to 99.2% after 60-minute irradiation under visible light. The catalytic results show that the ZnGa2S4 nanocrystalline visible photocatalyst exhibits good photocatalytic properties.
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3.5. Degradation Mechanism
The high catalytic performance of ZnGa2S4 photocatalyst could be explained by band structure calculations. At atmospheric pressure, ZnGa2S4 crystal is cubic spinel structure, which belongs to the space group number 227 (Figure 5(a)). Zn cations form a tetrahedron, and the Ga cations locate in GaS6 (GaO6) octahedron. For comparison, we discuss the ZnGa2O4 properties first. Optimized lattice constant of the ZnGa2O4 is 8.46 Å, which is consistent with other reports (8.44 and 8.41 Å) [25, 26]. Electronic structure calculations are calculated with GGA-PBE; it suggests that ZnGa2O4 has an indirect band gap of 2.33 eV. Usually, the LDA and GGA underestimate the semiconductor band gap, so we adopt HSE06 hybrid functional calculations, which have been proved more accurately to predict the experimental results of energy gap [27]. The ZnGa2O4 band structure calculated by the HSE06 is shown in Figure 5(c). Compared with GGA-PBE calculations, we obtain the large band gap of 4.04 eV; the result is similar to the experimental band gap value (4.4~5.0 eV) [28]. Previously, Sampath et al. reported that calculated ZnGa2O4 direct band gap was 2.79 eV by the TB-LMTO; Lopez et al. found that the calculated indirect band gap was 2.78 eV by the PAW-LDA, and Karazhanovw and Ravindran’s calculated indirect band gap was 3.09 eV by the LDA+U [29, 30]. Based on the GW and modified MBJ approximation, Dixit et al.’s obtained indirect band gaps were 4.57 and 4.71 eV, respectively [31]. It is found that our calculated results are consistent with the previously reported ones. Then, we also calculate the property of ZnGa2S4; the calculation parameters are used the same as the ZnGa2O4. The study finds that ZnGa2S4 also shows similar properties to indirect semiconductor ZnGa2O4. However, ZnGa2S4 has a smaller band gap, which is calculated by GGA-PBE about 1.41 eV. Using HSE06, the calculated band gap value correction is 2.27 eV (Figure 5(d)). Because the ZnGa2S4 band gap becomes narrow after thiourea reduction, it can absorb visible light. Furthermore, under visible-light irradiation, the electrons in the valence band of ZnGa2S4 can be excited to the conduction band, thus leaving a large number of holes in the valence band. Some photoinduced electrons transfer to the surface of the ZnGa2S4 and react with oxygen molecules, forming in . The remaining holes in the valence band are easier to react with adsorbed water molecules and produce active radicals. Produced can continue to combine with H+ and form ; they can capture the electrons and produce H2O2. The radicals also can be produced when H2O2 molecules trap electrons. All active groups, such as , , and valence band holes, to a certain extent, can oxidize organic pollutants [31]. The MB photocatalytic degradation mechanism is shown in Figure 5(b).
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4. Conclusion
The nanostructured ZnGa2S4 with indirect band gap semiconductor was successfully synthesized via a simple thiourea reduction reaction. The obtained samples had small particle size and larger specific surface area. In the photocatalytic degradation of MB, the material exhibited a significant stability and activity. Meanwhile, the photocatalytic degradation mechanism of ZnGa2S4 on organic contamination was investigated. The ZnGa2S4 photocatalyst with high catalytic activity and good stability can be applied to purification of water contamination.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This paper is supported by the Science and Technology Research Project Foundation of Jilin Province Education Department ([2013] 249), Natural Science Foundation of Changchun Normal University ([2010] 006), and National Natural Science Foundation of China (21041007).