Cu1.94S-Assisted Growth of Wurtzite CuInS2 Nanoleaves by In Situ Copper Sulfidation
© Cai et al. 2015
Received: 20 April 2015
Accepted: 30 June 2015
Published: 15 July 2015
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.
KeywordsCuInS2 Wurtzite Catalyst Nanoleaves
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 . Since Pan et al.  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 , including the synthesis, phase transformation, and photovoltaic application. Kolny-Olesiak et al.  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 . 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 . While most reports describe the preparation of CuInS2 nanocrystals, limited work is available for one-dimensional 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 , including catalyst-assisted growth, template-directed growth, and oriented attachment growth. Among them, catalyst-assisted growth  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 , which induces nanowire formation after supersaturation, or enables solid-phase diffusion in supercritical-fluid-liquid-solid growth in which the catalysts remain solid . In these researches, metallic bismuth and indium nanocrystals usually acted as the catalysts . Recently, sulfide Ag2Se and Cu2S nanocrystals have also been found to be the effective catalysts in the synthesis of one-dimensional nanomaterials for the intrinsic nature of fast ionic conductor [22, 23]. Wang et al.  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.  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.  proposed the novel solution-solid-solid mechanism for nanowire growth catalyzed by superionic (fast ionic) conductor nanocrystals. By using solution-solid-solid 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.
All chemicals were used as received without further purification. Sodium diethyldithiocarbamate trihydrate (Na(dedc), 99 %), chloroform (99.9 %), and n-hexane (95 %) were obtained from J&K, indium nitrate (In(NO3)3, 99.9 %) from ABCR, oleylamine (OA, C18 content 80–90 %) from Acros, and copper nanoparticle (99.9 %) and 1-dodecanethiol (DT, 98 %) from Alfa.
Synthesis of In(dedc)3 Precursors
In a typical synthesis of In(dedc)3, 3 mmol Na(dedc) and 1 mmol In(NO3)3 were dissolved in 50 mL ionized water, respectively. Then, In(NO3)3 aqueous solution was mixed with Na(dedc) solution by drop adding, washed three times at least with ionized water and ethanol followed by drying. As-synthesized precursors were stored in desiccator at room temperature.
Synthesis of CuInS2 Nanoleaves
In a typical synthesis of CuInS2 nanoleaves, 0.1 mmol (6.4 mg) copper nanoparticles and 0.05 mmol (28.0 mg) In(dedc)3 were dispersed in 6.0 mmol (2.0 mL) OA and 16.5 mmol (4.0 mL) DT-loaded flask under magnetic stirring. Then, the flask was vacuumed and filled with oxygen. The procedures were repeated three times and the oxygen flow was maintained during the following reaction. The flask containing the mixture was immersed in an oil bath at 180 °C. The heated solution in the flask showed the color evolution within 1 min, from transparent yellow to light brownish red, implying the formation and decomposition of the Cu-DT complex. After keeping the mixture at the temperature for 60 min, the resulting solution was cooled to room temperature and the samples were washed with n-hexane followed by further centrifugation. Aliquots were taken out during the synthesis for monitoring the size and shape evolution of nanoleaves.
The obtained crystalline phases were identified using powder XRD (Bruker, D8 advance, Cu Ka radiation using a curved graphite receiving monochromate), with a step of 0.02° at a speed of 4°/min. The simulated XRD patterns of CuInS2 were obtained by using CrystalMaker 2.5.5 programs. Morphology analyses were undertaken using scanning transmission electron microscopy (STEM, FEI Nova NanoSEM 200). TEM, high-angle annular dark-field (HAADF), STEM, and EDS were performed on JEOL 2100F microscope. The samples for TEM, HAADF-STEM, and STEM-EDS were collected by placing a drop of dilute solution of sample in hexane onto carbon-film-supported nickel grids. Composition analysis was performed by EDS (oxford INCA). The two parallel gold electrodes on silicon substrate with quartz layer were used to evaluate electrical property of CuInS2 nanoleaves. The interval and length of the two gold electrodes is 5 and 100 μm. Sample was made by drop-casting nanoleaves in chloroform onto the substrate. Annealing process was conducted at 400 °C to remove the attached ligands. The current-voltage characteristics were recorded using a Keithley 4200 Source Meter in the dark and under illumination. The scan voltage was tuned from −10 to 10 V.
Results and Discussion
The Cu1.94S head part in CuInS2 nanomaterial is deduced to be catalyst, which is the typical role in the synthesis of one-dimensional nanomaterial by the mechanisms of solution-liquid-solid and vapor-liquid-solid growth [19, 31]. Recently, superionic conductor nanocrystals, such as Ag2S, Ag2Se, and Cu2S, are found to be efficient catalysts in the growth of nanowires and heteronanostructures for their intrinsic nature [22, 25–27, 32]. Also, new mechanism has been proposed as solution-solid-solid mode . The superionic conductor catalysts have enough cation vacancies in their lattice with high cation mobility in the rigid anionic sublattice. It has been demonstrated that djurleite Cu1.94S nanocrystal can catalyze the growth of Cu2S-In2S3, Cu1.94S-CdS, and Cu2S-PbS heterostructures for the intrinsic cationic deficiencies [23, 24, 33, 34]. In the present work, the catalyst Cu1.94S nanocrystal introduces Cu(I) and In(III) species into the vacant sites of the crystal lattice, then condenses and crystallizes successively after saturation from the favorable facet of the catalyst to minimize the interfacial energy. As calculated from the proposed atomic packing model, lattice mismatches are as small as 0.59 %. We can deduce that grain boundary with low interfacial energy between djurleite Cu1.94S head and wurtzite CuInS2 body is formed.
Comparatively, size enlargement of Cu1.94S, from 10 nm in width at 2 min (Fig. 5a, b) to 100 nm at 60 min (Fig. 3a), provided further evidence for catalyst-assisted growth of CuInS2 nanoleaves. If seed-mediated growth model is employed in the present system, the size of Cu1.94S should be stable as the targeted material only grows epitaxially on the specific facet of Cu1.94S seed. From this point, the growth of the targeted materials just involves the first atomic layer epitaxial growth on seed, then transforms into conventional crystal growth. Thus, the subsequent growth of CuInS2 nanoleaves by seed-mediated growth makes no difference to the seed, either in composition or in size.
We demonstrated 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 and then copper sulfide Cu1.94S underwent quickly in the presence of oxygen atmosphere at the elevated temperature. Then, Cu1.94S nanocrystals played the catalytic roles for the growth of wurtzite CuInS2 nanoleaves. The 2D-projected elemental maps for three elements demonstrated the evenly distribution of those elements among CuInS2 nanoleaves. Photoresponses of CuInS2 nanoleaves were evaluated by I-V measurements, 11-fold increase compared with that in the dark. The enhancement of photoresponsive current of CuInS2 film is believed to be attributed to one-dimensional single crystalline nature of CuInS2 nanoleaves.
This work was supported by the funds from the NSFC (51102186, 51302194, 21101120, 61471270, 51025207), the NSFZJ (LQ12E02006), and the Research Fund of College Student Innovation of Zhejiang Province (2014R424027).
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