One-pot solvothermal synthesis of wurtzite Cu2ZnSnS4 nanocrystals
Graphical abstract
Introduction
The stable physical and chemical properties of semiconductor powder to photocatalytic degradation of pollutants have attracted considerable attention [1]. In recent decades, the n-type semiconductor photocatalysts have been researched in-depth, such as TiO2 [2], ZnO [3], and CdS [4]. However, these n-type photocatalysts have wide band gaps and can be used in UV illumination. As for p-type semiconductor photocatalysts, there are very few related researches.
CZTS is a p-type semiconductor, which is a kind of very promising material owing to its low toxicity, high abundance and low cost [5]. The kesterite and wurtzite CZTS nanocrystals have been synthesized based on the hot-injection method using oleylamine and oleic acid as capping agents [6], [7]. Peng et al. first prepared W-CZTS nanocrystals using 1-dodecanethiol as the sulfur source by hot-injection [8]. Singh et al. [5] and Regulacio et al. [9] demonstrated the importance of sulfur source in obtaining W-CZTS nanocrystals. However, the above-mentioned method often uses oleylamine as the solvent, and the reaction mixture has to be degassed under vacuum followed by purging for several times. Ethylenediamine-assisted hydrothermal method was carried out to synthesize orthorhombic CZTS at a high pressure [10]. Moreover, enthylenediamine has been usually used as a solvent during the synthesis of wurtzite chalcogenide nanoparticles. It is thought that ethylenediamine plays an important role in the chalcogenide phase transition processes between the tetragonal and hexagonal structures [11], [12]. Recently, W-CZTS has been used as a photocatalyst to fabricate CZTS–metal (Au or Pt) structure [6], [13], [14]. It is demonstrated that the structure is capable of enhancing photocatalytic activity towards the degradation of pollutions, but the noble metal would increase the cost.
Herein, we report a one-pot approach to synthesize wurtzite CZTS nanocrystals using thiourea as the sulfur source and ethylenediamine as the solvent. In this method, thiourea could be used as not only a ligand for the in situ metal complexes formation but also essential sulfur source through the thermos-decomposition process under specific temperatures. In addition, the p-type semiconductor W-CZTS could be used as a photocatalyst directly without the assistance of a noble metal.
Section snippets
Experimental section
In a typical synthesis of W-CZTS NPs, Cu(Ac)2 (0.6 mol), Zn(Ac)2 (0.4 mmol), SnCl2 (0.3 mmol) and thiourea (4 mmol) were dissolved in 50 mL of PEG-400 and 10 ml of ethylenediamine (en) by stirring for 4 h in a flask at room temperature. Then the mixed solution was continually stirred at 80 °C for 30 min. After that, the flask was heated to 185 °C and lasted for 1 h, then cooled down naturally. The as-synthesized NCs were washed with deionized water and ethanol for several times and then dried in vacuum
Results and discussion
The XRD pattern (Fig. 1a) clearly reveals that all of the diffraction peaks match well the W-CZTS [7], [8]. The synthesis of the metastable W-CZTS could be ascribed to the change in the chemical growth environment and usage of solvent, which result in a temporary phase reversion [11], [12]. Similar crystal phase reversion phenomenon was also observed in the case of hexagonal wurtzite ZnS and ε-Co colloids [15], [16]. It is suggested that the key factor for deciding the formation of wurtzite
Conclusion
In summary, wurtzite Cu2ZnSnS4 nanocrystals have been synthesized through a one-pot solvothermal route using thiourea as the sulfur source and ethylenediamine as the solvent. The ethylenediamine content and the reaction time are both important to synthesize high-quality wurtzite CZTS nanocrystals. The as-synthesized CZTS exhibits the significant photocatalytic performance under visible-light irradiation. The photocatalytic activities' comparative experiment results revealed that the CZTS
Acknowledgment
The authors acknowledge the financial support from the National Natural Science Foundation of China (Project no. 51162005), the Program for New Century Excellent Talents in University (No. NCET-12-0655) and Guangxi Natural Science Foundation (2012GXNSFFA060007).
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