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

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. Electronic supplementary material The online version of this article (doi:10.1186/s11671-015-0996-y) contains supplementary material, which is available to authorized users.


Background
Ternary I-III-VI 2 groups of compounds are important players in solar energy-harvesting materials [1][2][3]. Among them, CuInS 2 is a direct gap semiconductor with a bulk band gap of approximately 1.5 eV and a high extinction coefficient of around 10 5 cm −1 [4,5]. It is noteworthy that bulk CuInS 2 at room temperature has the chalcopyrite structure, whereas CuInS 2 nanocrystals can be additionally synthesized in zincblende and wurtzite structure [6]. Since Pan et al. [7] reported the colloidal synthesis of CuInS 2 nanocrystals with wurtzite structure via hot injection, numerous research works on the field of metastable wurtzite CuInS 2 nanocrystals have been reported [4], including the synthesis, phase transformation, and photovoltaic application. Kolny-Olesiak et al. [8] demonstrated the phase transforming from Cu 2 S to wurtzite CuInS 2 nanocrystals.
The wurtzite CuInS 2 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 CuInS 2 nanocrystals for the following device fabrication [9]. While most reports describe the preparation of CuInS 2 nanocrystals, limited work is available for onedimensional CuInS 2 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][12][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][15][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 growth in which the catalysts remain solid [20]. In these researches, metallic bismuth and indium nanocrystals usually acted as the catalysts [21]. Recently, sulfide Ag 2 Se and Cu 2 S 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 Ag 2 Se nanocrystals could be used as catalysts for the growth of semiconductor heterostructures, such as dimeric Ag 2 Se-CdSe and trimeric Ag 2 Se-CdSe-ZnSe. Further, Tang et al. [24] successfully fabricated Cu 2 S-In 2 S 3 heterostructures by djurleite Cu 1.94 S-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, Ag 2 S-CdS, Cu 2 S-ZnS, and Ag 2 Se-ZnSe heterostructures were prepared [26,27]. In the growth process of one-dimensional nanomaterials, Ag 2 S and Cu 2 S 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 CuInS 2 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 Cu 1.94 S 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 Cu 1.94 S nanocrystals played the catalytic roles for the CuInS 2 nanoleaf growth. The structure and composition of CuInS 2 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 CuInS 2 nanoleaf film were also evaluated.

Synthesis of In(dedc) 3 Precursors
In a typical synthesis of In(dedc) 3 , 3 mmol Na(dedc) and 1 mmol In(NO 3 ) 3 were dissolved in 50 mL ionized water, respectively. Then, In(NO 3 ) 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 CuInS 2 Nanoleaves
In a typical synthesis of CuInS 2 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.

Characterization
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 CuInS 2 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 darkfield (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-filmsupported 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 CuInS 2 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
Typical STEM micrographs of the resultant CuInS 2 nanomaterials were shown in Fig. 1. The representative micrographs of the as-synthesized samples indicate that the CuInS 2 nanomaterials have a leaf shape, about 500 nm in length and 100 nm in width. The nanoparticle is observed at the tip of almost every nanoleaf (red circles in Fig. 1b), which is the typical morphology of catalyst-assisted growth. The crystal structure of CuInS 2 nanoleaves is determined as hexagonal wurtzite phase, originated from wurtzite ZnS by randomly replacing zinc ions with copper or indium ions [7,8]. The diffraction pattern of wurtzite CuInS 2 is simulated (a = 3.9080 Å, c = 6.4250 Å) and listed as reference line in Fig. 1c, for the standard card has not been established. The latticeresolved high-resolution TEM micrograph (Fig. 1d) shows the single crystal nature of the individual nanoleaves with lattice spacing of 3.38 Å. Accordingly, the (100) plane of the wurtzite CuInS 2 is resolved. The regularity of lattice spacing in the nanoleaves is also seen from the line profile as inset in Fig. 1d, enclosed from the yellow rectangle area. The distance of 6.76 nm can be represented as 20 × 3.38 Å, which is the lattice spacing of (100) plane in CuInS 2 . Figure 2 displays the STEM micrograph and EDS spectrum of CuInS 2 nanoleaves. The composition of the nanoleaves was mainly identified as Cu, In, and S elements. Chemical analyses with nanoscale spatial resolution are performed to clarify the sample compositions and elemental distributions in individual. The 2Dprojected elemental maps for three atoms, shown in Fig. 2d-f and Additional file 1: Fig. S1, demonstrate the reasonable separated distribution of three atoms among the nanoleaves. Indicated by yellow circles in Fig. 2b, d-f and Additional file 1: Fig. S1, enhanced copper content in head part of nanoleaves is thought to be attributed to copper sulfide catalyst and will be discussed later. The composition of the head part in the nanoleaf was identified as Cu and S elements with ratio of 61/39. Meanwhile, element ratio 26/29/45 of Cu/In/S was identified in the body part of nanoleaf. Also, line scan across the single CuInS 2 nanoleaf testified the difference between head and body parts (Additional file 1: Fig. S2 and S3). Copper content has considerable high ratio in the head part than body part, while indium content could be negligible in the head part compared with that in the body part.
A typical HRTEM micrograph of the tip in CuInS 2 nanoleaf is shown in Fig. 3a. The interface between head Fig. 1 a, b STEM micrographs of CuInS 2 nanoleaves. The red circles indicate the typical end terminal of the nanoleaves. c XRD pattern of as-synthesized CuInS 2 nanoleaves. The reference line below is simulated according to wurtzite CuInS 2 lattice constants (a = 3.9080 Å, c = 6.4250 Å). d HRTEM micrograph of CuInS 2 nanoleaves. Inset represents line profile integrated along the region enclosed in the yellow rectangle area and body parts in the nanoleaf, indicated by the parallel dotted lines, is clearly revealed. The two FFT patterns corresponding to head and body parts are noticeably dissimilar and are indexed to monoclinic Cu 1.94 S and wurtzite CuInS 2 , respectively. Lattice spacing with 3.40 Å in the head and 3.38 Å in the body is corresponded to the (042) plane of Cu 1.94 S crystal and (100) plane of CuInS 2 crystal, respectively. An interplanar distance analysis based on the HRTEM and FFT pattern shown in Fig. 3 suggests that the (210) plane of CuInS 2 nanoleaf epitaxially attaches to the (046) plane of the Cu 1.94 S head, which demonstrates the grain boundary is composed of (210) planes of CuInS 2 and (046) planes of Cu 1.94 S. Based on above analysis, an atomic packing model that depicts the epitaxial attachment of the CuInS 2 body to Cu 1.94 S head is proposed. The corresponding interface orientation relationships between the CuInS 2 and Cu 1 To explore the growth of CuInS 2 nanoleaves, the phase and morphological evolution of samples at different reaction stages was investigated by XRD and TEM (Figs. 4 and 5). Metal copper and indium hydroxide were the main phase of the solid product within 15 s after the reaction system immersed into the oil bath at 180°C, and the latter was originated from the reaction between oleylamine and indium precursors. The signal of djurleite Cu 1.94 S emerged as the reaction lasted for 60 s. The coexistence of wurtzite CuInS 2 , indium hydroxide, and djurleite Cu 1.94 S was observed in the product after 120 s. It is supposed that djurleite Cu 1.94 S catalyzed the growth for the nanoleaves. By contrast, copper phase kept unchanged at the early stage under nitrogen atmosphere (the absence of oxygen) in the controlled experiment. The transformation from copper nanoparticle to copper oxide in oxygen atmosphere underwent at elevating temperature process, and then to Cu 1.94 S rapidly after the reaction with dodecanethiol. It is thought that sulfidation has higher reaction rate than oxidation, though the latter possesses the priority, which also explains the fail trial on the confirmation of copper oxide by XRD analysis. In vapor system, the role of oxygen pressure during the transformation from metal copper to copper sulfide has been clarified [29,30]. Monoclinic Cu 2 S nanowire arrays were grown on Cu foil substrates with the aid of H 2 S gas. In consideration that catalytic roles played by Cu 1.94 S were validated only if wurtzite CuInS 2 and djurleite Cu 1.94 S coexist, the sample at 2 min was chose as the target. As shown in Fig. 5a, the samples at the early stage appear as acorns. Lattice-resolved HRTEM micrographs (Fig. 5b) clearly reveal the interface between the head and body parts of samples, indicated by the yellow dotted lines. With the growth proceeding (Fig. 5c-e), the body parts of CuInS 2 samples grew with the gradual increase of length, whereas the Cu 1.94 S head parts and the interfaces kept.
The Cu 1.94 S head part in CuInS 2 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 Ag 2 S, Ag 2 Se, and Cu 2 S, are found to be efficient catalysts in the growth of nanowires and heteronanostructures for their intrinsic nature [22,[25][26][27]32]. Also, new mechanism has been proposed as solutionsolid-solid mode [25]. 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 Cu 1.94 S nanocrystal can  catalyze the growth of Cu 2 S-In 2 S 3 , Cu 1.94 S-CdS, and Cu 2 S-PbS heterostructures for the intrinsic cationic deficiencies [23,24,33,34]. In the present work, the catalyst Cu 1.94 S 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 Cu 1.94 S head and wurtzite CuInS 2 body is formed.
Comparatively, size enlargement of Cu 1.94 S, from 10 nm in width at 2 min (Fig. 5a, b) to 100 nm at 60 min (Fig. 3a), provided further evidence for catalystassisted growth of CuInS 2 nanoleaves. If seed-mediated growth model is employed in the present system, the size of Cu 1.94 S should be stable as the targeted material  only grows epitaxially on the specific facet of Cu 1.94 S 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 CuInS 2 nanoleaves by seed-mediated growth makes no difference to the seed, either in composition or in size.
To further evaluate the optoelectronic properties of the CuInS 2 nanoleaves, thin films by drop-casting nanoleaves solutions on inter-digitated electrode (IDE) silicon substrate test chips were fabricated [35]. After that, annealing process was conducted at 400°C to remove the attached ligands, OA, and DT, facilitating the carrier transportation among nanoleaves. Shown in Fig. 6, the I-V curve demonstrates the photoresponsive property of thin films composed of CuInS 2 nanoleaves. The drop-casting CuInS 2 film exhibits observable photoresponsive property, 11-fold increase from 7.2 × 10 −8 A at 10 V in the dark to 8.1 × 10 −7 A under illumination. The enhancement of photoresponsive current of CuInS 2 films in this work is deduced to be beneficial from fast carriers transport for their nature of single crystalline. Tang and Sargent [35] reported that colloidal CuInSe 2 , CuGaSe 2 , and Cu(InGa)Se 2 nanoparticle-based thick films showed photoresponse, from 2-10-fold current increase compared dark with lamp. And also, ZnO nanocrystal-based field-effect transistors exhibited much improved semiconducting properties with spin-coated ZnO thin-films by tuning the shape from zero-dimensional nanocrystals to one-dimensional nanorods [36]. In contrast to nanocrystals, nanoleaves with one-dimensional morphology do not have grain boundary along their length [12,36,37], which might have faster carrier transport than percolation through a random polycrystalline network in nanocrystals, and thus enhancing photoresponse.

Conclusions
We demonstrated the catalyst-assisted growth of wurtzite CuInS 2 nanoleaves in solution by using commercial copper nanoparticles as staring materials. The transformation from copper nanoparticle to copper oxide and then copper sulfide Cu 1.94 S underwent quickly in the presence of oxygen atmosphere at the elevated temperature. Then, Cu 1.94 S nanocrystals played the catalytic roles for the growth of wurtzite CuInS 2 nanoleaves. The 2D-projected elemental maps for three elements demonstrated the evenly distribution of those elements among CuInS 2 nanoleaves. Photoresponses of CuInS 2 nanoleaves were evaluated by I-V measurements, 11-fold increase compared with that in the dark. The enhancement of photoresponsive current of CuInS 2 film is believed to be attributed to one-dimensional single crystalline nature of CuInS 2 nanoleaves.

Additional file
Additional file 1: Figure S1. HAADF-STEM images of CuInS 2 nanoleaf. (a) the composite STEM-EDS micrograph, (b) EDS spectra collected from head and body parts of nanoleaf. Mo and Si element peaks attributed to molybdenum grid and EDS detector. (c-e) STEM-EDS elemental maps of Cu, In, and S, respectively. The yellow cycles indicates the head part of nanoleaf. Figure S2. HAADF-STEM image of CuInS 2 nanoleaf. (a) SE image, (b) EDS line scan profile. The analysis was made from the head part toward the body part of nanoleaf, as indicated by hollow blue arrow. Figure S3. HAADF-STEM image of CuInS 2 nanoleaf. (a) SE image, (b) EDS line scan profile. The analysis was made from the head part toward the body part of nanoleaf, as indicated by hollow blue arrow.