Cu1.94S-Assisted Growth of Wurtzite CuInS2 Nanoleaves by In Situ Copper Sulfidation

Cai et al. Nanoscale Research Letters (2015) 10:294 DOI 10.1186/s11671-015-0996-y NANO EXPRESS Open Access Cu1.94S-Assisted Growth of Wurtzite CuInS2 Nanoleaves by In Situ Copper Sulfidation Chunqi Cai, Lanlan Zhai, Chao Zou*, Zhensong Li, Lijie Zhang, Yun Yang and Shaoming Huang* Abstract Wurtzite CuInS2 nanoleaves were synthesized by Cu1.94S-assisted growth. By observing the evolution of structures and phases during the growth process, Cu1.94S nanocrystals were found to be formed after uninterrupted oxidation and sulfidation of copper nanoparticles at the early stage, serving as catalysts to introduce the Cu and In species into CuInS2 nanoleaves growth for inherent property of fast ionic conductor. The obtained CuInS2 nanoleaves were characterized by scanning transmission electron microscopy, transmission electron microscopy, fast Fourier transform, X-ray diffraction, and energy dispersive X-ray spectroscopy mapping. The enhancement of photoresponsive current of CuInS2 nanoleaf film, evaluated by I-V curves of nanoleaf film, is believed to be attributed to the fast carrier transport benefit from the nature of single crystalline of CuInS2 nanoleaves. Keywords: CuInS2; Wurtzite; Catalyst; Nanoleaves Background Ternary I-III-VI2 groups of compounds are important players in solar energy-harvesting materials [1–3]. Among them, CuInS2 is a direct gap semiconductor with a bulk band gap of approximately 1.5 eV and a high extinction coefficient of around 105 cm−1 [4, 5]. It is noteworthy that bulk CuInS2 at room temperature has the chalcopyrite structure, whereas CuInS2 nanocrystals can be additionally synthesized in zincblende and wurtzite structure [6]. Since Pan et al. [7] reported the colloidal synthesis of CuInS2 nanocrystals with wurtzite structure via hot injection, numerous research works on the field of metastable wurtzite CuInS2 nanocrystals have been reported [4], including the synthesis, phase transformation, and photovoltaic application. Kolny-Olesiak et al. [8] demonstrated the phase transforming from Cu2S to wurtzite CuInS2 nanocrystals. The wurtzite CuInS2 is constructed as randomly distributed copper and indium over the cation sites of the wurtzite ZnS lattice [6]. The cation disorder allows flexibility of the stoichiometry and a tunable Fermi energy over a wide range, which feature particularly in wurtzite CuInS2 nanocrystals for the following device fabrication * Correspondence: ; Zhejiang Key Laboratory of Carbon Materials, College of Chemistry and Material Engineering, Wenzhou University, Wenzhou 325027, People’s Republic of China [9]. While most reports describe the preparation of CuInS2 nanocrystals, limited work is available for onedimensional CuInS2 nanomaterials [4, 10]. Semiconductor nanomaterials in one-dimensional morphology provide ideal models to study the relationship between electrical transport, optical, and other properties with dimensionality and size confinement [11–13]. Specifically, one-dimensional nanomaterials could offer continuous charge carrier transport pathways and efficiently promote charge separation, which makes them highly attractive for photocatalytic and photovoltaic applications [14–16]. Thus, one-dimensional nanomaterials comprise an important class of nanomaterials used in electronic and photoelectronic devices, including field-effect transistors, energy harvesting, and sensors [12, 17]. To synthesize one-dimensional nanomaterials in solution, several mechanisms have been developed [11], including catalyst-assisted growth, template-directed growth, and oriented attachment growth. Among them, catalyst-assisted growth [18] exhibited wonderful features to acquire one-dimensional nanomaterials with high crystallinity, tolerating big lattice mismatch between catalysts and targeted nanomaterials. During the growth process, catalyst either formed a liquid eutectic in solution-liquid-solid growth [19], which induces nanowire formation after supersaturation, or enables solid-phase diffusion in supercritical-fluid-liquid-solid © 2015 Cai et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Cai et al. Nanoscale Research Letters (2015) 10:294 growth in which the catalysts remain solid [20]. In these researches, metallic bismuth and indium nanocrystals usually acted as the catalysts [21]. Recently, sulfide Ag2Se and Cu2S nanocrystals have also been found to be the effective catalysts in the synthesis of onedimensional nanomaterials for the intrinsic nature of fast ionic conductor [22, 23]. Wang et al. [22] reported Ag2Se nanocrystals could be used as catalysts for the growth of semiconductor heterostructures, such as dimeric Ag2Se-CdSe and trimeric Ag2Se-CdSe-ZnSe. Further, Tang et al. [24] successfully fabricated Cu2S-In2S3 heterostructures by djurleite Cu1.94S-assisted growth model, in which the catalyst underwent transformations in crystal structure and composition. Accordingly, Wang et al. [25] proposed the novel solution-solid-solid mechanism for nanowire growth catalyzed by superionic (fast ionic) conductor nanocrystals. By using solution-solidsolid growth, Ag2S-CdS, Cu2S-ZnS, and Ag2Se-ZnSe heterostructures were prepared [26, 27]. In the growth process of one-dimensional nanomaterials, Ag2S and Cu2S nanocrystals were usually decomposed from single-source molecular precursors and used as catalysts. Then, the target species dissolved into the catalysts and dissolved out after supersaturation. The complicated process in these cases makes one aware that further investigation is needed, for the solubility and fluidity of intermediate species in the catalysts and the supersaturation and condensation of target substances are unique [27, 28]. Thus, there is much room in the exploration of catalysts for the growth of the desired nanomaterials. Here, we report the catalyst-assisted growth of wurtzite CuInS2 nanoleaves in solution by using commercial copper nanoparticles as staring materials. The transformation from copper nanoparticle to copper oxide in oxygen atmosphere underwent quickly at elevated temperature, and then to copper sulfide Cu1.94S with the presence of dodecanethiol. Detailed investigation on the growth by monitoring the structures and morphologies of the nanoleaves during the process implied that the formed Cu1.94S nanocrystals played the catalytic roles for the CuInS2 nanoleaf growth. The structure and composition of CuInS2 nanoleaves were also investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDS). Furthermore, the photoresponsive characteristics of the CuInS2 nanoleaf film were also evaluated. Methods Materials All chemicals were used as received without further purification. Sodium diethyldithiocarbamate t (...truncated)