Microwave fabrication of Cu2ZnSnS4 nanoparticle and its visible light photocatalytic properties

Cu2ZnSnS4 nanoparticle with an average diameter of approximately 31 nm has been successfully synthesized by a time effective microwave fabrication method. The crystal structure, surface morphology, and microstructure of the Cu2ZnSnS4 nanoparticle were characterized. Moreover, the visible light photocatalytic ability of the Cu2ZnSnS4 nanoparticle toward degradation of methylene blue (MB) was also studied. About 30% of MB was degraded after 240 min irradiation when employing Cu2ZnSnS4 nanoparticle as a photocatalyst. However, almost all MB was decomposed after 90 min irradiation when introducing a small amount of H2O2 as a co-photocatalyst. The enhancement of the photocatalytic performance was attributed to the synergetic effect between the Cu2ZnSnS4 nanoparticle and H2O2. The detailed photocatalytic degradation mechanism of MB by the Cu2ZnSnS4 was further proposed.


Background
Organic dyes widely used in textile and plastic industries are one of the chief sources of contaminants in wastewater. They have induced serious environmental problems due to their potential toxicity to living organisms. Degradation and total removal of such contaminants are keys to ensuring a protected environment. A photocatalytic technique is considered to be a promising method for treating organic dyes in wastewater [1]. However, an obvious challenge for degradation of organic dyes is that most photocatalysts, such as TiO 2 or BiVO 4 [2,3], are only effective in the UV range. To broaden their light absorption range, various methods including dye sensitizing [4], metal doping [5] non-metal doping [6,7], and noble metal decorating [8] have been developed. However, stable and efficient dyes are rare and expensive. Moreover, dopant impurity atoms in photocatalysts often serve as recombination centers for photogenerated holes and electrons [9]. To avoid these problems, great efforts have also been put into the development of alternative undoped photocatalysts which work under visible light irradiation. Until now, many materials with attractive visible light photocatalytic performance, such as Bi 2 TiO 4 F 2 [10], Bi 2 O 3 [11], AgNbO 3 [12], and graphene oxide enwrapped Ag/AgX (X = Br, Cl) nanocomposite [13] have been investigated. However, the supply of rare elements of Ag, Bi, and Nb is a critical issue for widespread use. Thus, it is crucial to investigate alternative cost-effective visible-light-driven photocatalysts. Cu 2 ZnSnS 4 is a direct bandgap p-type semiconductor with a high optical absorption coefficient of about 10 5 cm −1 [14,15]. Its elements are environmentally friendly and abundant in the earth's crust. As its bandgap is around 1.5 eV, it can absorb most of the visible light. It has been reported that Cu 2 ZnSnS 4 possesses high photocorrosion resistance in air and aqueous solution [16]. Both of these superior properties of Cu 2 ZnSnS 4 enrich its potential use in solar-energy-related applications.
In this work, we have fabricated the Cu 2 ZnSnS 4 nanoparticle by a facile microwave fabrication method. The advantages of this method are the following: it is economical of time and cost effective. The crystal structure and surface morphology of the prepared Cu 2 ZnSnS 4 nanoparticle were characterized. Moreover, the photocatalytic performance of the Cu 2 ZnSnS 4 nanoparticle toward the degradation of methylene blue (MB) under visible light irradiation was also investigated. The Cu 2 ZnSnS 4 nanoparticle showed noteworthy visible light photocatalytic ability.

Methods
The Cu 2 ZnSnS 4 nanoparticle was synthesized by a facile microwave fabrication method. Cu(CH 3 COO) 2 · H 2 O, Zn (CH 3 COO) 2 , Sn(CH 3 COO) 2 , and thiocarbamide with a molar ratio of 2:1:1:4 were employed as source materials. All the reagents were analytically pure and bought from Sinopharm Chemical Reagent Co., Ltd, Shanghai, China. Typically, 1.123 g of the source materials was dissolved in 20 mL ethylene glycol solution as precursor. Then the precursor was stirred gently and heated in a microwave reactor (MCR-3, Gongyi City Yuhua Instrument Co., Ltd, Gongyi City, China) at 180°C for 10 min. After the vacuum filtration and drying process, the Cu 2 ZnSnS 4 nanoparticle sample was obtained.
The photocatalytic properties of the prepared Cu 2 Zn SnS 4 nanoparticle were investigated by employing MB as a model dye. The Cu 2 ZnSnS 4 nanoparticle (20 mg) was dispersed in 100 mL of MB aqueous solution (10 mg/L). Prior to irradiation, the MB solution over the catalyst was gently stirred in the dark for 30 min to reach equilibrium adsorption state. Then the solution was illuminated with a 100-W xenon light source (Shanghai Yaming Lighting Co., Ltd., Shanghai, China). The concentration change of MB was monitored by measuring UV-vis absorption of the extracted MB solution at regular intervals. The characteristic peak absorbance of MB at 665 nm was used to determine its concentration. In addition, the photocatalytic properties of the Cu 2 ZnSnS 4 nanoparticle with the assistance of 0.1 mL H 2 O 2 (30% aqueous solution) were further investigated in the same measurement process. For comparison, a control experiment without adding Cu 2 ZnSnS 4 and H 2 O 2 was also carried out.

Results and discussion
The crystal structure of the prepared Cu 2 ZnSnS 4 nanoparticle is shown in Figure 1. The observed diffraction peaks at 2θ = 28.48°, 32.77°, 47.38°, and 56.26°correspond to the Cu 2 ZnSnS 4 crystal planes (112), (200), (220), and (312), which match well with the standard XRD data file of Cu 2 ZnSnS 4 (JCPDS No. . No other crystalline by-products were observed in the pattern, suggesting that the as-prepared sample was pure Cu 2 ZnSnS 4 . Additionally, the strong relative intensity of the (112) and (220) lines indicates the Cu 2 ZnSnS 4 nanoparticle is preferentially oriented in the (200) and (110) directions during the growing process.
In addition, it has been reported that the spectra of Cu 2 ZnSnS 4 and β-ZnS are very similar in the XRD analysis results [17]. Raman spectroscopy analysis is a feasible method to distinguish Cu 2 ZnSnS 4 from β-ZnS [18]. Therefore, we employed Raman spectroscopy to further confirm the structure of the prepared Cu 2 ZnSnS 4 nanoparticle. A Raman spectrum of the prepared Cu 2 ZnSnS 4 nanoparticle over the wave number range of 200 to 450 cm −1 is shown in Figure 2. There is an intensive peak located at approximately 331 cm −1 , which suggests the existence of Cu 2 ZnSnS 4 [17,19]. The characteristic peaks of β-ZnS located at 348 and 356 cm −1 are not observed in the spectrum [20], indicating the absence of β-ZnS.
Surface morphology and microstructure of inorganic semiconductor materials are of vital importance to their optoelectronic properties. Accordingly, the surface morphology of the Cu 2 ZnSnS 4 nanoparticle was studied by SEM. Figure 3a demonstrates a representative surface morphology of the Cu 2 ZnSnS 4 nanoparticle. It can be seen that the Cu 2 ZnSnS 4 nanoparticle possesses similar sizes in diameter and packs uniformly. The average diameter of the Cu 2 ZnSnS 4 nanoparticle is approximately 31 nm calculated from randomly selected 100 nanoparticles. High-resolution transmission electron microscopy (HRTEM) was further employed to investigate the microstructure of the Cu 2 ZnSnS 4 nanoparticle. Figure 3b shows a typical HRTEM image of the Cu 2 ZnSnS 4 nanoparticle. The interplanar spacing of 2.7 Å corresponds to the (200) plane of kesterite Cu 2 ZnSnS 4 . The selected area electron diffraction (SAED) pattern shown in Figure 3c suggests the polycrystalline nature of the nanoparticle.
To evaluate the photocatalytic performance, we analyzed the decomposition of the (MB) dye in aqueous solution over the Cu 2 ZnSnS 4 nanoparticle under visible light irradiation. Figure 4a presents the time-dependent absorption spectra of MB degradation over the Cu 2 ZnSnS 4 nanoparticle upon visible light irradiation. The five curves in the pattern are the UV-vis spectra of MB solutions extracted at 0 min, 30 min, 60 min, 150 min, and 240 min. It can be observed that the absorbance peak at 665 nm, which is the characteristic absorption peak of MB, reduced slowly with increasing irradiation time. After 240 min, only about 30% of MB was degraded. In addition, it was difficult to further degrade MB by increasing the irradiation time. However, with the addition of 0.1 mL H 2 O 2 (30% aqueous solution), as shown in Figure 4c, almost all MB was degraded after 90 min irradiation. In general, peroxymonosulfate, peroxydisulfate, and H 2 O 2 are often employed to assist in evaluating the photocatalytic properties of semiconductor materials. Both peroxymonosulfate and peroxydisulfate can be driven by visible light for photochemical oxidation [21], while H 2 O 2 can hardly be activated [22]. Additionally, as a typical organic pollutant, MB is stable under visible light irradiation if no photocatalysts are involved. Therefore, the degradation of MB molecules was attributed to the synergetic effect of Cu 2 ZnSnS 4 nanoparticle and H 2 O 2 . The H 2 O 2 enhanced the photocatalytic ability through an  Figure 4b,d illustrates the curves of C/C 0 , where C 0 is the initial concentration of MB and C is the concentration of MB at time t. Insets in Figure 4b,d present the curve of the corresponding ln(C 0 /C) versus irradiation time. No linear relationship between irradiation time and ln(C 0 /C) can be observed when employing the Cu 2 ZnSnS 4 as a photocatalyst solely. However, with the assistance of H 2 O 2 , a linear relationship between irradiation time and ln (C 0 /C) is well established, suggesting that the photodegradation of MB over the Cu 2 ZnSnS 4 nanoparticle and H 2 O 2 proceeded through the pseudo-first-order kinetic reaction [23]. The first-order reaction rate constant k 1 was 0.04 min −1 , which is comparable to that of TiO 2 -C hybrid aerogel photocatalysts (0.01~0.06 min −1 ) toward the degradation of MB driven by UV light irradiation [24].
According to the experiment results and previous reports, the photocatalytic degradation mechanism by the Cu 2 ZnSnS 4 nanoparticle under visible light irradiation was illustrated in Figure 5 and proposed as follows: When the photogenerated carriers emigrate to the surface of the Cu 2 ZnSnS 4 nanoparticle, the generated Cu 2 ZnSnS 4 h þ vb À Á provides holes, decomposing the absorbed contaminant.