Synthesis of the ZnGa 2 S 4 Nanocrystals and Their Visible-Light Photocatalytic Degradation Property

Nanostructured ZnGa 2 S 4 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 ZnGa 2 S 4 was evaluated by the degradation of methylene blue (MB) aqueous solution. The high photoactivities of ZnGa 2 S 4 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.


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][8][9][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 Zn 2 GeO 4 , ZnGa 2 O 4 , and Sr 2 Sb 2 O 7 , 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][17][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 ZnGa 2 S 4 photocatalysts with nanostructure were prepared via a simple method 2 Journal of Nanomaterials 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 ZnGa 2 S 4 photocatalyst exhibits good photocatalytic activity and stability for methylene blue degradation.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.

Experimental Section
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 ZnGa 2 S 4 powders were cooled naturally down to the room temperature and collected for test.

Photocatalytic Activity Experiment. Methylene blue (AR,
Beijing Chemical Reagent Factory) was selected as pollutant to investigate the photocatalytic activity of the ZnGa 2 S 4 .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 ZnGa 2 S 4 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 ZnGa 2 S 4 photocatalyst was calculated by the following formula: where  0 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 ZnGa 2 S 4 photocatalyst reach adsorption-desorption equilibrium.[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].and ZnGa 2 S 4 photocatalyst.For ZnGa 2 O 4 precursors, the main peak is located between 200 and 300 nm, corresponding to a wide band gap with approximately 4.7 eV, showing that ZnGa 2 O 4 precursors have photocatalytic activity only in the ultraviolet region [16].However, after thiourea reduction reaction, the peak position of ZnGa 2 S 4 photocatalyst extends to the visible area, and its band gap becomes narrow, which is beneficial to the absorption of visible light.

Visible-Light Degradation.
To investigate the photocatalytic performance of nanostructured ZnGa 2 S 4 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 ZnGa 2 S 4 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 ZnGa 2 S 4 nanocrystalline visible photocatalyst exhibits good photocatalytic properties.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 ZnGa   All active groups, such as • OH, • HO 2 , and valence band holes, to a certain extent, can oxidize organic pollutants [31].

Degradation
The MB photocatalytic degradation mechanism is shown in Figure 5(b).

Conclusion
The nanostructured ZnGa 2 S 4 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 ZnGa 2 S 4 on organic contamination was investigated.The ZnGa 2 S 4 photocatalyst with high catalytic activity and good stability can be applied to purification of water contamination.

2 O 4
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 ZnGa 2 O 4 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 ZnGa 2 S 4 ; the calculation parameters are used the same as the ZnGa 2 O 4 .The study finds that ZnGa 2 S 4 also shows similar properties to indirect semiconductor ZnGa 2 O 4 .However, ZnGa 2 S 4 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 ZnGa 2 S 4 band gap becomes narrow after thiourea reduction, it can absorb visible light.Furthermore, under visible-light

Figure 5 :
Figure 5: (a) Unit cell of the ZnGa 2 S 4 (ZnGa 2 O 4 ) structure.The blue and green balls represent the Zn and Ga atoms, respectively, while the red ball corresponds to S or O atom.(b) The schematic diagram of the photocatalytic degradation of MB mechanism using ZnGa 2 S 4 photocatalysts.Band structures of (c) ZnGa 2 O 4 and (d) ZnGa 2 S 4 calculated by the HSE06 functional.The Fermi level is set at zero.
Photocatalyst Preparation.0.1 g ZnGa 2 O 4 2.4.Characterization.The X-ray powder diffraction (XRD) patterns were carried out on a Rigaku Company D/Max 2200 PC using Cu K 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 The electronic structures of ZnGa 2 O 4 and ZnGa 2 S 4 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) ). Nanostructured ZnGa 2 S 4 photocatalyst is obtained at 800 ∘ C without adding catalyst.Instead of using traditional high toxicity catalyst H 2 S and explosive gas H 2 , we use the thiourea as sulfur source, and ZnGa 2 O 4 serves as precursor.Figures1(a) and 1(b) show the morphology of nanostructured ZnGa 2 S 4 photocatalyst.The synthesized size of ZnGa 2 S 4 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 ZnGa 2 S 4 .The chemical elements of ZnGa 2 O 4 precursors and ZnGa 2 S 4 photocatalyst were analyzed by EDX.As shown in Figures1(c) and 1(d), there are three kinds of elements Zn, Ga, and O in ZnGa 2 O 4 precursors, whereas ZnGa 2 S 4 contains only Zn, Ga, and S.3.2.The Powder X-Ray Diffraction (XRD).The XRD patterns of ZnGa 2 O 4 precursors and ZnGa 2 S 4 photocatalyst are shown in Figure2, in which all the diffraction peaks 30.3 ∘ , 35.7 ∘ , 43.4 ∘ , 53.8 ∘ , 57.4 ∘ , 63.0 ∘ , and 74.6 ∘ , corresponding to two materials are pure.After thiourea reaction, ZnGa 2 O 4 precursors have been completely transformed into ZnGa 2 S 4 nanocrystals.3.3.UV-Visible Diffuse Reflectance (UV-Vis).Figure 3 displays UV-Vis diffuse reflectance spectra of ZnGa 2 O 4 precursors [25,26]sm.The high catalytic performance of ZnGa 2 S 4 photocatalyst could be explained by band structure calculations.At atmospheric pressure, ZnGa 2 S 4 crystal is cubic spinel structure, which belongs to the space group 3 number 227 (Figure5(a)).Zn cations form a tetrahedron, and the Ga cations locate in GaS 6 (GaO 6 ) octahedron.For comparison, we discuss the ZnGa 2 O 4 properties first.Optimized lattice constant of the ZnGa 2 O 4 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 ZnGa 2 O 4 has an indirect band gap of 2.33 eV.Usually, the LDA and GGA underestimate the irradiation, the electrons in the valence band of ZnGa 2 S 4 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 ZnGa 2 S 4 and react with oxygen molecules, forming in • O 2 − .The remaining holes in the valence band are easier to react with adsorbed water molecules and produce • OH active radicals.Produced • O 2 − can continue to combine with H + and form • HO 2 ; they can capture the electrons and produce H 2 O 2 .The • OH radicals also can be produced when H 2 O 2 molecules trap electrons.