Study on Optimum Preparation Conditions of ZnIn₂S₄ to Effectively Reduce Cr(VI) under Visible Light Radiation

Abstract

Previous studies have displayed various conclusions about the effect of preparation factors on the photoreduction property of ZnIn₂S₄. Therefore, it is not easy to figure out the optimal preparation conditions of ZnIn₂S₄ for Cr(VI) photoreduction. To ensure Cr(VI) reduction efficiency, various ZnIn₂S₄ photocatalysts were prepared in different solvents (i.e., water and ethylene glycol) and temperatures (i.e., 120 °C, 150 °C and 180°C). Different characterization methods were used to explain the difference in optical performance and photocatalytic property among the obtained samples. The results show that all the samples exhibit a similar band gap. The reaction solvent and temperature have a great influence on the surface morphology and optical property, leading to the different photocatalytic properties. ZnIn₂S₄ synthesized at 120 °C in the solvothermal condition shows the optimal efficiency on Cr(VI) photoreduction due to the effective utilization of photo-induced carriers. The reasonable analysis and effective conclusion presented may provide the optimal synthesis method of ZnIn₂S₄ to effectively remove Cr(VI) from water environment.

1. Introduction

With the rapid development of society, compounds containing hexavalent chromium (Cr(VI)) have been used in industries, including electroplating, mining, pigment manufacturing, and smelting [1,2,3]. Cr(VI) can enter water environment through sewage effluent, rainfall, and runoff and possesses the characteristic of easy migration in environment and high solubility in water [4]. Cr(VI) can be untaken by animals, plants, and even humans via the food chain, causing potential and persistent threat for humans and ecosystem [5]. Therefore, various strategies have been developed to remove Cr(VI) from water environments, such as physical adsorption, chemical precipitation, chemical reduction, and membrane filtration [5]. However, the traditional approaches have the disadvantages of high cost and power consumption, complicated operation, and membrane pollution [6]. It is urgent to find sustainable, renewable, and effective ways to remove Cr(VI). In the situation of environmental pollution and energy crisis, photocatalytic reduction is an extremely efficient method to reduce Cr(VI) by photogenerated electrons due to high efficiency, low cost, and environmental friendliness [2].

In recent years, layered ZnIn₂S₄, especially hexagonal ZnIn₂S₄, has exhibited excellent photocatalytic activity in hydrogen generation, Cr(VI) reduction and carbon dioxide reduction under visible light radiation [7,8]. ZnIn₂S₄ nanopowder synthesized via hydrothermal method as a visible light catalyst was applied to reduce Cr(VI) in aqueous solution for the first time [9]. After that, various methods for the reduction of aqueous Cr(VI) were developed. Among the variety of methods for synthesizing micro/nano-structure ZnIn₂S₄, both hydrothermal and solvothermal approaches are prevalently used in reported studies. Gou et al. [10] adjusted reaction solvent (i.e., pyridine, alcohol, acetonitrile) and temperatures (120–180 °C) to control the shape of ZnIn₂S₄. However, the photocatalytic activity of these ZnIn₂S₄ photocatalysts has not been investigated, which is significant on its application. Tian et al. [11] used the hydrothermal method to synthesize ZnIn₂S₄ by changing reaction temperature from 120 °C to 200 °C for hydrogen generation, and ZnIn₂S₄ synthesized at 160 °C showed the best activity. Based on the above researches, it can be obtained that solvent and temperature have a great influence on the shape and photoreduction property of ZnIn₂S₄. In the previous research, the influence of preparation conditions on the morphology and property of ZnIn₂S₄ has been studied. However, the suitable conditions for synthesizing bulk ZnIn₂S₄ with efficient reduction activities have not systematically been concluded and elaborated. Therefore, it is difficult to figure out the optimum synthesis conditions of ZnIn₂S₄ for Cr(VI) reduction from water environment. However, this is convenient and essential to evaluate material properties.

In the work, various ZnIn₂S₄ photocatalysts were prepared in different synthesis solvent and temperatures, in order to investigate the optimum condition for the application of Cr(VI) reduction. The photocatalytic properties of different ZnIn₂S₄ photocatalysts were evaluated by Cr(VI) reduction under visible light radiation and the difference in reduction efficiency has been analyzed and discussed.

2. Results and Discussion

2.1. Character of Different ZnIn₂S₄ Photocatalysts

XRD was performed to study the phase structures of the ZnIn₂S₄ photocatalysts. Figure 1 displays that all the samples show the same characteristic peaks at 2θ of 22.2°, 27.7°, 32.5°, 41.3°, 47.3°, 54.5, 57.1°, and 77.5°, which correspond to (006), (102), (104), (108), (110), (116), (022), and (123) crystal faces of ZnIn₂S₄, respectively (JCPDS, No. 65-2023) [12]. Besides, no other impurity peaks can be found in the XRD patterns. This result indicates that ZnIn₂S₄ can be successfully prepared via both the hydrothermal process after reacting at 120 °C for 2 h (DI-120) and solvothermal process after reacting at 120 °C (EG-120), 150 °C (EG-150) and 180 °C (EG-180) for 2 h. From the diffraction peak intensity, it should be noted that diffraction peaks (i.e., the crystal faces of (006), (104), (116), (022)) of DI-120 are narrower and sharper in comparison with EG-120, EG-150, and EG-180. This means that the hydrothermal system is beneficial to the crystallization of ZnIn₂S₄ as compared with the solvothermal system.

The morphologies of different ZnIn₂S₄ photocatalysts were characterized by SEM (Figure 2a–d). It can be seen that all the samples display spherical structures consisting of numerous sheets due to the Oswald ripening [8]. DI-120 displays marigold-like spherical structure, which is similar with that prepared at 180 °C by the hydrothermal method [13]. However, EG-120, EG-150 and EG-180 show rose-like spherical structure. The petal gap of EG-120 further increases when compared with DI-120, which can provide more active sites to enhance photocatalytic activity. However, the previous study reported that the reaction temperature should be increased to 200 °C in the hydrothermal process to achieve wider petal gap [11]. The thickness of petal in synthesized ZnIn₂S₄ is increased when raising temperature from 120 °C to 180 °C, which is similar with the result of TEM (Figure S1a,b). This means that the higher temperature may lead to the increased thickness in the ZnIn₂S₄ synthesis process. Besides, the lattice fringes and good crystallinity can be clearly observed in the corresponding TEM image of EG-120 (Figure S2). The lattice spacing of 0.333 nm and 0.410 nm corresponds to the (102) and (006) planes of the tetragonal ZnIn₂S₄, respectively [14].

2.2. Comparison on Photocatalytic Activities of Different ZnIn₂S₄ Photocatalysts

To evaluate the photocatalytic performance of different ZnIn₂S₄ photocatalysts, the removal efficiencies of Cr(VI) under visible light irradiation were investigated. First, adsorption removal in the dark by different ZnIn₂S₄ photocatalysts was studied. Figure S3 shows that only less than 5% of Cr(VI) can be removed in the adsorption processes. When exposed to visible light for 30 min, the removal efficiencies of Cr(VI) by DI-120, EG-120, EG-150, and EG-180 are 53.3%, 94.3%, 57.1%, and 78.4% (Figure 3a). The result indicates that Cr(VI) is mainly removed by the photoreduction action of different ZnIn₂S₄ due to the highest valence state of chromium and the limited adsorption effect of ZnIn₂S₄. Photocatalytic processes by DI-120, EG-120, EG-150, and EG-180 all obey the first order dynamic models and the kinetic constant in EG-120 process is 4, 3.43, and 1.85 times than that in DI-120, EG-150, and EG-180 processes, respectively (Figure 3b). The result shows that ZnIn₂S₄ prepared via solvothermal process in 120 °C exhibits the excellent photoreduction activity, which is inconsistent with the previous study [15]. This may be due to the different raw materials, resulting in different effects of solvents on the photoreduction performance of ZnIn₂S₄.

Besides, different quenching agents were added in the photoreduction process to investigate the mechanism of Cr(VI) removal by EG-120. Tertiary butanol (t-BuOH, 1 mM), benzoquinone (BQ, 1 mM), ethylenediamine tetraacetate (EDTA-2Na, 1 mM) and AgNO₃ (1 mM) were employed as scavengers for OH·, ·O₂⁻, holes, and photo-induced electrons, respectively [5,16]. Figure S4 shows that photo-induced electrons play a major role during the removal process of Cr(VI) by EG-120. ·O₂⁻ shows a minor effect on Cr(VI) removal and the other two active substances almost never work. The quenching experimental result indicates that Cr(VI) is mainly removed by the reducing action of photo-induced electrons in EG-120 photocatalytic system.

2.3. Comparison on Optical Property of Different ZnIn₂S₄ Photocatalysts

When ZnIn₂S₄ and Cr(VI) solution are simultaneously irradiated by visible light, Cr(VI) is reduced by photo-generated electrons due to the separation of photo-generated electrons and holes. The generation, separation, and combination of electron-hole pairs in ZnIn₂S₄ influence the Cr(VI) reduction efficiency. Therefore, the optical properties of different ZnIn₂S₄ were investigated to expound the utilization of electron-hole pairs and confirm the difference in Cr(VI) reduction.

First, UV–vis DRS was used to investigate the visible light absorption of ZnIn₂S₄. Commonly, the wide visible light region is beneficial to the increase of photocatalytic activity. As shown in Figure 4a, all the ZnIn₂S₄ photocatalysts exhibit strong absorption intensity in the UV and partially visible region, and the absorption edges with a steep shape locate at 500~554 nm. The corresponding band gaps (Figure 4b) of ZnIn₂S₄ photocatalysts were estimated via the Kubelka–Munk method [17]. As shown, the Eg values are calculated as 2.74 eV for EG-180, 2.71 eV for EG-150, 2.58 eV for EG-120, and 2.47 eV for DI-120. Relatively, DI-120 is superior to absorb visible light but demonstrates little difference with EG-120.

Besides, electrochemical impedance spectroscopy (EIS) and photoluminescence (PL) spectra were conducted to figure out the combination of electron-hole pairs. Normally, the small radius in an EIS Nyquist indicates the inhibited recombination of photoinduced carriers, suggesting the effective separation of electron-hole pairs [18,19]. Figure 5a presents EIS of different photocatalysts irradiated with visible light. As shown, the radius of EG-120 is significantly smaller than the other three ZnIn₂S₄ photocatalysts, suggesting that EG-120 can exhibit the excellent electroconductivity. EG-120 is expected to possess the effective transfer and separation efficiency of photoinduced electron-hole pairs when irradiated with visible light, which is beneficial for the enhancement of photocatalytic activity. Meanwhile, the PL spectra of different ZnIn₂S₄ photocatalysts at an excitation wavelength of 315 nm have been measured and compared under visible light irradiation. As shown in Figure 5b, the emission intensity of DI-120 is similar with that of EG-180 and exhibits high PL intensity, indicating the fast recombination rate of photogenerated pairs. EG-150 and EG-120 also show the similar PL intensity. Compared with DI-120 and EG-180, a decreased intensity can be obviously obtained, suggesting the more effective separation of photoinduced carriers. Furthermore, transient photocurrent responses were performed to investigate the transportation behaviors of electron-hole pairs. As shown in Figure 5c, all the samples exhibit a quick response to the light on, but the generated photocurrent responses are different. In comparison with that in the dark, the photocurrent density of EG-120, DI-120, EG-150, and EG-180 can be increased by 0.17, 0.10, 0.08, and 0.05 μA·cm⁻² at 0 V vs. RHE when irradiated with visible light, respectively. The response value of EG-120 is 1.7, 2.1, and 3.4 times higher than that of DI-120, EG-150, and EG-180 at the same experimental condition. In addition, the photocurrent intensity does not change significantly after four light on-off cycles, which indicates the excellent stability of the four ZnIn₂S₄ photocatalysts [20]. However, among the methods for improving ZnIn₂S₄ activity, Au doping and Sn²⁺/⁴⁺ co-doping ZnIn₂S₄ shows the decreased photocurrent intensity with increasing cycle number [21,22]. The above synthesis regarding optical property shows that the photogenerated electrons and holes of EG-120 are separated effectively and the carrier life is extended, which is be beneficial to utilize photo-induced electrons for Cr(VI) reduction. This is consistent with the results of photoreduction activity of ZnIn₂S₄ (Figure 3).

3. Materials and Methods

3.1. Reagents

Thioacetamide, indium chloride, and zinc chloride were purchased from Aladdin Chemical Reagent. Ethylene glycol, benzoquinone, ethylenediamine tetraacetate, silver nitrate, and tertiary butanol were purchased from Sinopharm Chemical Reagent. Potassium bichromate was purchased from Tianjin Fengchuan Chemical Reagent Technology Co. Ltd. (Tianjin, China). All the reagents were in analytically grade and used without any further processing.

3.2. Preparation of Different ZnIn₂S₄

Briefly, 0.442 g of indium chloride and 0.1363 of zinc chloride were mixed in 60 mL ethylene glycol (EG) and treated by ultrasonic wave for 30 min. Then 0.3 g of thioacetamide was added into the above solution. After stirring for 1 h, the obtained solution was transferred to a 100 mL Teflon-lined autoclave and heated at 120 °C for 2 h. While cooling down to room temperature, ZnIn₂S₄ was obtained by centrifugation, washed with deionized water/ethanol, dried, and marked as EG-120. When ZnIn₂S₄ was prepared in ethylene glycol at 150 °C and 180 °C, the as-prepared ZnIn₂S₄ was marked as EG-150 and EG-180, respectively. While ZnIn₂S₄ was prepared in deionized water instead of ethylene glycol at 120 °C, the obtained ZnIn₂S₄ was marked as DI-120.

3.3. Characterization Analysis of ZnIn₂S₄

Crystal phases and structures of the samples were investigated by X-ray diffraction analysis (XRD, Bruker D8 ADVANCE, Bruker AXS GmbH., Karlsruhe, Germany) with Cu-Kα radiation, λ = 0.15406 nm. Morphology was analyzed by transmission electron microscopy (TEM, JEM 2100, JEOL, Tokyo, Japan) and scanning electron microscopy (SEM, JSM-7001F, JEOL, Tokyo, Japan). Light adsorption was characterized by UV–vis diffuse reflection spectra (DRS, U-3900H, Hitachi, Tokyo, Japan). The transient photocurrent responses and electrochemical impedance spectroscopy (EIS) were performed by using an electrochemical workstation (CHI660E, Shanghai CH Instrument Company, Shanghai, China) with three electrodes. In addition, a 300 W xenon lamp (PLS-SXE300, Beijing, China) equipped with a 420 nm cut-off filter was adopted as the optical source. The photoluminescence (PL) spectra were conducted by a fluorescence spectrophotometer (FLS980, Bain Square, UK) at an excitation wavelength of 315 nm.

3.4. Photoreduction Cr(VI) by ZnIn₂S₄ under Visible Light Irradiation

The photoreduction experiments were performed in a 150 mL beaker at room temperature. Hence, 10 mg of ZnIn₂S₄ was dispersed in 100 mL of 10 mg L⁻¹ Cr(VI) solution by ultrasonic wave for 5 min and stirred in the dark for 30 min to achieve adsorption equilibrium between Cr(VI) and the photocatalyst. Afterwards, the obtained suspension was exposed to visible light irradiation and the illumination radiation intensity was approximately 100 mW cm⁻². After irradiation for some time, 2 mL of reaction solution was taken out, filtered, and analyzed by ultraviolet spectrophotometry.

3.5. Determination of Cr(VI) Concentration

Briefly, 1 mL of sample solution, 0.5 mL of H₂SO₄, 0.5 mL H₃PO₄, 1 mL 1,5-diphenylcarbazide/acetone/water ternary solution, and 47 mL deionized water were uniformly mixed. The mixed solution stood for 10 min and measured at 540 nm by ultraviolet spectrophotometry and the measured absorbance at 540 nm was denoted as A_i. For comparison, 0.5 mL of H₂SO₄, 0.5 mL H₃PO₄, 1 mL 1,5-diphenylcarbazide/acetone/water ternary solution, and 47 mL deionized water were uniformly mixed as well to eliminate the interference of deionized water. After standing for 10 min, the mixed solution was measured at 540 nm and the absorbance was denoted as B₀. The removal efficiency of Cr(VI) was calculated as Equation (1).

$$\mathsf{\eta }{\mathrm{C}\mathrm{r}}_{(\mathrm{VI})} (\%)=\frac{{(\mathrm{A}}_0-{ \mathrm{B}}_0{) - (\mathrm{A}}_{\mathrm{i} }-{ \mathrm{B}}_0)}{{\mathrm{A}}_0-{ \mathrm{B}}_0} \times \%$$

(1)

where A₀ represents the absorbance of initial concentration, B₀ represents the absorbance of deionized water, and A_i represents the absorbance of reaction solution after being irradiated for a certain time.

4. Conclusions

To identify the optimum preparation conditions, a series of ZnIn₂S₄ photocatalysts were synthesized at 120 °C in water and at different temperatures (120 °C, 150 °C and 180 °C) in ethylene glycol. All the synthesized ZnIn₂S₄ photocatalysts display spherical structures. Among them, DI-120 shows a marigold-like spherical structure. EG-120, EG-150, and EG-180 exhibit a rose-like spherical structure. Therein, EG-120 shows the widest petal gap, providing more active sites to improve photocatalytic activity. UV-vis DRS indicates there is no obvious difference among the absorption edges of all the samples, which are located at 500~554 nm. EIS and PL spectra show that EG-120 exhibits the effective transfer and low combination efficiency of photo-induced electron pairs. Besides, transient photocurrent response also reveals that EG-120 shows the effective separation of photo-induced carriers, the photocurrent value of which is 1.7, 2.1, and 3.4 times higher than that of DI-120, EG-150, and EG-180, respectively. As expected, EG-120 exhibits the optimum reduction efficiency, due to the surface morphology and the excellent optical property.