Preparation and photocatalytic properties of Cu2ZnSnS4 for H2 production

Cu2ZnSnS4 (CZTS) thin film photocatalytic water splitting for hydrogen production under visible light irradiation has been reported together with CZTS nanoparticles prepared by ultrasonic spray pyrolysis and hydrothermal method, respectively. CZTS thin film provided higher H2 production rate (68.68 μmol · g−1 · h−1), which was 32 times higher than that of CZTS nanoparticles (2.08 μmol · g−1 · h−1) without loading any noble metals. What’s more, photocatalytic activity of CZTS thin film remained 94% after 48 h which confirmed the good stability and reusability of CZTS thin film. CZTS thin film is a potential and durable candidate for photocatalysis.


Introduction
H 2 is an ideal fuel with high combustion calorific value and pollution-free combustion product. Photocatalytic water splitting by solar energy, as an ideal way of hydrogen production, has attracted extensive attention of researchers. How to obtain hydrogen efficiently, cheaply and safely is research hotspot. H 2 production efficiency of most photocatalysts such as TiO 2 [1,2], ZnO [3,4], ZnS [5][6][7] and g-C 3 N 4 [8] are limited due to wider band gap which results in larger portion of solar spectrum transmission.
As a kind of narrow band-gap semiconductor, Cu 2 ZnSnS 4 (CZTS) owns high light absorption coefficient (∼10 4 cm −1 ) and direct band gap (∼1.5 eV). Based on this, Cu 2 ZnSnS 4 is considered as candidates for solar cell light absorption materials, and has obtained high photoelectric conversion efficiency (12.6%) [9] at laboratory scale. The high photoelectric conversion efficiency demonstrates that the number of photoelectron-hole pairs of CZTS based material is considerable in visible light range. Therefore, researchers began to explore the application of CZTS in the photocatalytic hydrogen generation field, since 2010 by Yokoyama [10].
Much work so far has focused on CZTS nanoparticles with various shapes and sizes for photocatalytic water splitting, and improving H 2 evolution activity decorating noble metals nanoparticles on surface [11,12], composting other semiconductors [13,14]. There are relatively few studies devoted to application of CSTS thin film for photocatalytic water splitting directly rather than photoelectrochemical water splitting. Herein, in this work, CZTS thin film photocatalytic water splitting for hydrogen production under visible light irradiation was reported together with CZTS nanoparticles prepared by ultrasonic spray pyrolysis and hydrothermal method, respectively. In addition, long-term photostability of Cu 2 ZnSnS 4 thin film was also studied in this work. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. yellow. CH 4 N 2 S was excess in the solution because the sulfur was volatile. The distance between the heating plate and the nozzle, the carrier gas flow rate and the velocity of the nozzle, intensity of oscillation of the PZT, the scan length and the times of scanning motion were all kept constant. In addition, the substrate temperature was fixed at 400°C. After deposition, the sample cooled naturally to room temperature in air.

Synthesis of Cu 2 ZnSnS 4 nanoparticles
Cu 2 ZnSnS 4 photocatalyst was prepared via a hydrothermal process. First, 4 mmol CuCl 2 ·2H 2 O, 2 mmol ZnCl 2 and 2 mmol SnCl 2 ·2H 2 O were added to 70 mL deionized water under magnetic stirring, and ultrasonic treatment 10 min to make it dissolved fully. Next, 8 mmol Na 2 S and 4 mmol EDTA-2Na were scattered into the above mentioned mixture successively under magnetic stirring. And then, the blend was transferred to a 100 mL Teflon-lined autoclave and reacted at 180°C for 8 h. The autoclave cooled to room temperature naturally. After that, the precipitates were centrifuged, washed with deionized water and absolute ethanol until the filtrate was neutral, and dried at 80°C for 4 h. Finally, Cu 2 ZnSnS 4 powders were prepared successfully.

Characterization
The crystal structure was analyzed by x-ray diffraction (XRD, D/MAX2500PC Rigaku, Cu Kα). The surface morphology was investigated by scanning electron microscopy (SEM, Hitachi S-4800) and high-resolution transmission electron microscope (HRTEM, Joel 2100F). Raman spectrum was detected on a LabRAM Aramis (France) and excited with the wavelength of 552 nm. The atomic valence states were analyzed by x-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250 XI).

Photocatalytic reactions
In photocatalytic H 2 evolution experiment, 50 mg of Cu 2 ZnSnS 4 nanoparticles and 10 mg of Cu 2 ZnSnS 4 thin film were dispersed and placed on a holder in 100 mL deionized water containing Na 2 S (0.1 mol L −1 ) and Na 2 SO 3 (0.1 mol L −1 ), respectively. The visible light irradiation (λ>400 nm) was supplied by 100 W xenon lamp (50 mW cm −2 ) with UV-cut filter. The amounts of H 2 production were determined every 2 h by online gas chromatography (GC 2002, Shanghai Kechuang Chromatograph instruments Co., Ltd) equipped with a thermal conductivity detector (TCD), using argon was used as the carrier gas.

Results and discussion
The diffraction patterns of nanoparticles and thin film prepared by hydrothermal method and ultrasonic spray pyrolysis have three peaks at 28.5°, 47.3°and 56.2°, corresponding to (112), (220) and (312) of tetragonal Cu 2 ZnSnS 4 (ICDD No. 26-575) [15], respectively. The broad peaks especially for Cu 2 ZnSnS 4 prepared by hydrothermal method indicate that crystallite size is small. The average crystallite sizes were calculated by Scherrer equation and were found to be about ∼3 nm, ∼9 nm for nanoparticles and thin film. The intensity of nanoparticles is ostensibly higher inferring that nanoparticles maybe have better crystal quality, but the difference of peaks intensity between nanoparticles and thin film is mainly due to the difference mass during measurement. The thickness of thin film is only 700 nm. Furthermore, no other second phases were detected by XRD as shown in figure 1.
Because the possible existence of ZnS and Cu 2 SnS 3 phases with similar XRD peaks to that of the CZTS phase, Raman scattering was performed to confirm the CZTS phase. Raman spectrum for thin film was depicted in figure 2. Characteristic peak at 331 cm −1 belonged to CZTS [16,17]. Furthermore, no other second phases were detected by Raman spectrum. By combining XRD and Raman analysis, pure CZTS phase was prepared successfully by ultrasonic spray pyrolysis. Figure 3 shows SEM micrographs of CZTS thin film and nanoparticles, respectively. Both of them were agglomerate to reduce surface energy. CZTS thin film was consist of irregular shape particles roughly in a range of tens to hundreds nanometers. Meanwhile, the thickness of the film is about 700 nm which can be seen from inset of figure 3(a). CZTS nanoparticles consist of spherical roughly 20 nanometers in diameter similar to [13]. This is consistent with the observation from XRD which has broad diffraction peaks. Obviously, the particle sizes of them were larger than the average crystallite size calculated by Scherrer formula. This result suggested that these particles may the aggregations of fine CZTS nanocrystals.
In order to further investigate the morphologie, high-resolution transmission electron microscopy (HRTEM) was used to characterize CZTS nanoparticles. TEM images in figure 4 show that the subglobular nanoparticles are dense with size about a few nanometers ∼20 nm. HRTEM image in figure 4(b) shows the characteristic spacing of 0.31 nm for the (112) [16] lattice plane of tetragonal Cu 2 ZnSnS 4 . This result is consistent with the observation from XRD ( figure 1).
The surface compositions and chemical states of Cu 2 ZnSnS 4 were studied by XPS spectra, and the results are shown in figure 5. Cu, Zn, Sn, S, C and O were clearly displayed in the survey spectra as shown in figure 5(a), and identical result was reported by Ha [11]. Two signals were displayed in figure 5(b) at 951.9 eV and 932.0 eV,      [11,17,18].
Combined with XRD and SEM results, it can be concluded that the CZTS nanoparticles prepared by hydrothermal method have smaller size. So when Cu 2 ZnSnS 4 nanoparticles dispersed to form aqueous suspension, the contact area with water was larger than that of the film. Usually, smaller size could be beneficial to light absorption and reaction sites number. Nevertheless, contrary to expectation, CZTS thin film provided higher H 2 production rate (68.68 μmol·g −1 ·h −1 ), which was 32 times higher than that of CZTS nanoparticles (2.08 μmol·g −1 ·h −1 ). H 2 production rate of CZTS nanoparticles was lower because it had larger grain boundary ratio as a result of smaller crystallite size. Recombination is particularly problem for polycrystalline materials with small grain sizes, owing to the exceptionally large density of dangling bonds at the grain boundaries [19] which act as carrier recombination centers according to literature [20]. Therefore, crystallite size is not as small as possible; especially considering carrier diffusion length was 350 nm of CZTS [21]. It should not increase specific surface area unilaterally, but balance between it and carrier recombination probability [22]. The research of Chang [22] indicated H 2 evolution rate increased with the increase of crystalline size in a certain range. H 2 production rate of CZTS nanoparticles is significantly different by different researcher. Jiang [13] considered CZTS without activity for photocatalytic H 2 evolution, because in his work no H 2 was measured when CZTS irradiation for 6 h. Ha [11] reported H 2 evolution rate of CZTS nanoplate and nanorods were about 16 μmol g −1 ·h −1 and 43 μmol g −1 ·h −1 , respectively. Yu [12] reported H 2 evolution rate of CZTS nanocrystals was 0.13 mmol g −1 ·h −1 . For comparison purpose, photocatalytic water splitting for hydrogen production rates of different researcher over different kinds of CZTS form are list in table 1. So far to our best knowledge, almost all CZTS film are used as photocathode for photochemical water splitting for hydrogen production [23], rather than photocatalytic water splitting.
To examine the photostability of Cu 2 ZnSnS 4 thin film, three cycles of the photocatalytic experiment of hydrogen production had been conducted, and one cycle lasted 16 h. After each cycle, Cu 2 ZnSnS 4 thin film was  washed and re-placed in a new deionized water solution containing fresh scavengers. The photocatalyst remained with 94% activity after 48 h of reaction as shown in figure 6, which signified efficient reusability. CZTS nanoparticles also exhibited analogous long term activity for photocatalytic H 2 evolution, when loaded with expensive Pt or MoS 2 as a co-catalyst [12,14]. Undoubtedly, photocatalyst in the form of thin film is very easy to recycle.

Conclusions
In summary, CZTS thin film has been studied photocatalytic water splitting for hydrogen production under visible light irradiation for the first time together with CZTS nanoparticles prepared by ultrasonic spray pyrolysis and hydrothermal method. H 2 production rate is higher for CZTS film (68.68 μmol·g −1 ·h −1 ) than for CZTS nanoparticles (2.08 μmol·g −1 ·h −1 ) without loading any noble metals. More importantly, photocatalytic activity of CZTS thin film remains 94% after 48 h. CZTS thin film shows good candidate for further applications in photocatalytic fields.