CuInS2 Flower Vaselike Nanostructure Arrays on a Cu Tape Substrate

For the first time, well-crystalline single-phase CuInS2 flower vaselike nanostructure arrays have been successfully prepared on Cu-tape using CISCuT ...
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CuInS2 Flower Vaselike Nanostructure Arrays on a Cu Tape Substrate by the Copper Indium Sulfide on Cu-Tape (CISCuT) Method: Growth and Characterization Kajari Das, Anuja Datta, and Subhadra Chaudhuri*

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 8 1547-1552

Department of Materials Science and DST unit of Nanoscience, Indian Association for the CultiVation of Science, Kolkata- 700 032, India ReceiVed December 6, 2006; ReVised Manuscript ReceiVed May 18, 2007

ABSTRACT: CuInS2 flower vaselike nanostructure arrays were successfully prepared for the first time on Cu-tape substrates by a facile copper indium sulfide on Cu-tape (CISCuT) method without using any template or catalyst. The sulfurization time and the thickness of the In layer deposited on the Cu-tape were the influential factors for obtaining single phase, well-crystalline, nanostructured CuInS2. Formation of the CuInS2 flower vaselike nanostructures was initiated by a self-catalyzed vapor-liquid-solid (VLS) technique, whereas the vapor-solid (VS) process played a crucial role in defining the shapes of the nanoflower vases. Optical reflectance and Raman spectroscopy were adopted to characterize the CuInS2 flower vaselike nanostructures. Introduction Nanocrystalline materials have attracted a great deal of attention from researchers in various fields for their fundamental size-dependent properties and many important technological applications.1-7 Specifically, research on heterostructured nanocrystals and microcrystals has attracted much attention during the past few years because of their fundamental importance in understanding the dependence of properties on size and dimensionality as well as their novel properties and potential applications in fabricating nanoscale electronic, optoelectronic, and sensing devices.8-10 CuInS2 is an important I-III-IV2 ternary semiconductor material used as an absorber layer for highefficiency and radiation-hard solar cell applications11 due to its direct band gap of 1.5 eV and a large absorption coefficient (R ∼ 105 cm-1). In addition, CuInS2 is a candidate for the cathode material of photochemical devices owing to its high performance and high output stability.12 Various fabrication methods proposed for the preparation of CuInS2 include spray pyrolysis,13,14 rf sputtering,15,16 sulfurization,17 electrodeposition,18 chemical process,19,20 and sequential deposition of Cu2S and In2S3.21 Using nanostructure semiconductors instead of their bulk counterpart as an absorber layer in the solar cells, high efficiency has already been achieved.22 The preparation of nanocrystals with noble and novel morphologies provides an opportunity to explore the dependence of material properties on crystal structure, size, and shape. Synthesis of CuInS2 nanostructures, such as nanorods, nanotubes, nanoacorns, nanobottles, and larva shapes nanostructures have been reported using different synthetic routes, including hydrothermal and solvothermal routes and a one-pot synthesis method.23-25 As a continuation of the development of the designed synthesis of uniform-sized and -shaped nanocrystals, the synthesis of CuInS2 heterostructured nanocrystals with flower vase shapes on Cu-tape is a critical step toward achieving the goal of nanoscale device fabrication; this is the basis of the present work. In the paper, we synthesized CuInS2 (CIS) flower vaselike nanostructure arrays on Cu-tape substrates by a copper indium sulfide on Cu-tape (CISCuT) method. The CISCuT method, first * Corresponding author. Telephone: +9133 24734971, Ext: 377. Fax: +9133 24732805. Corresponding author: e-mail: [email protected].

Figure 1. XRD spectra of Cu-tape substrate and Cu-In alloy and XRD spectra of CuInS2 samples CIS-1, CIS-2, CIS-3, and CIS-4 prepared with different thicknesses of In layer on Cu-tape.

reported in 1998,26 is a technique in which an In layer is deposited on Cu-tape and sequential sulfurization is carried out to form a CIS absorber layer. Here Cu-tape serves both as a back electrical contact and as a Cu source for the CuInS2 layer formation. This is a very cost-effective technique for large-scale industrial production of the thin film solar cell. In our work, the In layer was deposited on the Cu-tape by a simple electrodeposition technique, which was annealed to prepare a Cu-In alloy, and then sequential sulfurization was carried out in a H2S + N2 gas atmosphere. The thickness of the In layer on the Cu-tape and the sulfurization time were crucial factors to form the single-phase CuInS2 flower vaselike nanostructure arrays. Previous researchers prepared phase pure CuInS2 films by this CISCuT method using highly toxic KCN for the surface etching of the film to remove the Cu2-XS phases on the top of the CuInS2 layer.26,27 In our work, by optimizing the basic experimental parameters, single-phase tetragonal CuInS2 nanoflower vase arrays were prepared on Cu-tape substrate without any harmful surface etching of the films. To the best of our knowledge, this is the first time single-

10.1021/cg0608918 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/11/2007

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Table 1. Effects of the Experimental Parameters on the Phases, Morphologies and Sizes of the CuInS2 Micro- and Nanostructures

sample

electrodeposition time of In layer on Cu-tape (min)

sulfurization time (min)

morphology and average body diameters of the nanostructures

CIS-1 CIS-2 CIS-3 CIS-4

20 40 60 90

60 60 60 60

Cu2-XS, CuInS2 Cu2-XS, CuInS2 Cu2-XS, CuInS2 CuInS2

CIS-5

90

30

CIS-6 CIS-7

90 90

90 120

CumInn, Cu2-XS, CuInS2, InS, In6S7 Cu2-XS, CuInS2 Cu2-XS, CuInS2

phase identified

phase CuInS2 nanoflower vase arrays have been prepared through a very simple and convenient pathway. A possible growth mechanism has also been proposed. Experimental Section The electrodeposition process was chosen to deposit In layers on the cleaned Cu-tapes. The process was carried out in an electrodeposition system containing an aqueous solution of 0.9 mM InCl3 under a potentiostatic condition -0.4 V against the reference electrode. To obtain the In layers of different thicknesses on Cu-tape substrates, electrodeposition of the In layer was carried out for different durations from 20-90 min. All the deposited In films on Cu-tape substrates were heated at 130 °C for 4 h in vacuum to form the desired Cu-In alloys.28 Preparation of Cu-In alloy was an important step toward the growth of the nanoflower vases. The resulting Cu-In alloy was then transferred to a closed chamber in a 60/40 v/v H2S and N2 atmosphere and heated at 450 °C for 60 min. The four CuInS2 films obtained by varying the thickness of the In layer on Cu-tape substrates were named as CIS-1, CIS-2, CIS-3, and CIS-4 as shown in Table 1. A higher thickness of the In layer on the Cu-tape substrate was not chosen for sulfurization, as the surface of the film after annealing became fragile. Likewise, the sulfurization time of the prepared Cu-In alloys was also varied from 30 to 120 min for the 90 min indium deposited films. The samples obtained thereof are named as CIS-5, CIS-6, and CIS-7, respectively (see Table 1). The crystalline phases of the products were determined by X-ray powder diffraction by using a Seifert 3000P diffractometer with Cu KR radiation (λ) 1.54 Å). The morphologies of the samples were studied by a scanning electron microscope (SEM; Hitachi S-2300) and field emission scanning electron microscope (FESEM, JEOL, JSM- 6700F). The purity and composition of the prepared samples were confirmed by an energy dispersive X-ray analyzer

Figure 2. XRD of the samples CIS-5, CIS-6, and CIS-7 sulfurized for 30, 90, and 120 min, respectively.

650 nm (incomplete nanoflower vases) 580 nm (incomplete nanoflower vases) 270 nm (nanoflower vases with mixed head morphologies) 100 nm (complete growth of nanoflower vases with three distinct parts) hexagonal and tetragonal nanocrystals microcolumns microcrystals

attached to the FESEM. Microstructural properties were obtained using transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM; JEOL 2010). For the TEM observations, the films were scratched out of the substrate and then dispersed in 2-propanol and ultrasonicated for 15 min. A few drops of this ultrasonicated solution were taken on a carboncoated copper grid for TEM study. Optical reflectance of the samples was recorded by a UV-vis-NIR spectrophotometer (Hitachi, U-3410). The Raman spectra were recorded at room temperature using a T64000 Jobin-Yvon triple spectrometer, equipped with a liquid nitrogen-cooled charge coupled device (CCD) and a microscope. The 532.1 nm line of a crystal laser was used for the excitation.

Results and Discussion For the formation of the CuInS2 nanostructures on Cu-tape substrate from the In-coated Cu-tape, the following processes were adopted step by step: (i) heating of the In-coated Cutapes at 130 °C to form Cu-In alloy and (ii) sulfurization of

Figure 3. (a) Optical reflectance spectra of CuInS2 samples CIS-3 and CIS-4; (b) the differential reflectance spectra of the samples CIS-3 and CIS-4.

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Figure 4. (a-d) SEM and FESEM micrographs for CuInS2 samples CIS-1, CIS-2, CIS-3, and CIS-4, respectively; (e) high-resolution FESEM image of the sample CIS-4; inset of (e) is the image of a single nanoflower vase; (f) FESEM micrograph for sample CIS-5 annealed for 30 min.

Cu-In alloys at 450 °C in H2S + N2 gas mixture where the following reactions took place.

CumInn + H2S + N2 f Cu2-XS + InS + In6S7 + CuInS2 + H2 + N2 (1) Cu2-XS + In + H2S + N2 f CuInS2 + H2 + N2

(2)

InS + Cu + H2S+ N2 f CuInS2 + H2 + N2

(3)

In6S7 + Cu + In + H2S + N2 f CuInS2 + H2 + N2

(4)

The presence of inert N2 gas in the system was favorable to control the reaction of H2S with the Cu-In alloy at a slow and steady rate. Controlling the phase purity and at the same time preparing the arrayed flower vases structures were the two main objectives in the present work. The optimum condition for the preparation of the phase pure CuInS2 product was first determined from the XRD measurements. Figure 1c-f shows the X-ray diffraction spectra of the CIS samples CIS-1 to CIS-4. Figure 1a,b shows the XRD spectra of the Cu-tape substrate and Cu-In alloy (CumInn), respectively. A layer sequence Cu-CuInS2Cu2-XS is found for all the samples except CIS-4, where Cu2-XS indicates the different phases of copper sulfide. For the flower vaselike nanostructure arrays in CIS-4, all the peaks corresponding to the reflections from (112), (004), (204), and (312) planes can be indexed to tetragonal CuInS2, which are consistent

with the standard reported values (JCPDS file No. 27-0159). A few reflections of Cu were also detected originating from the Cu-tape substrate. No other secondary phases of the Cu-Insulfide system or Cu2-XS were observed, indicating that the flower vaselike nanostructures in CIS-4 are phase pure. Lattice constants a ) 5.53 Å and c ) 11.09 Å, calculated from the XRD peaks for CIS-4 nanoflower vases, matched well with the reported values of a ) 5.517 Å and c ) 11.06 Å (JCPDS file No. 27-0159) for pure CuInS2. The high crystallinity of CuInS2 flower vase structures is evident from the intense and sharp peaks. Figure 2 shows the XRD spectra of the samples CIS-5 to CIS-7 prepared at different sulfurization times. It was observed that when the sulfurization time was 30 min (CIS-5), impure phases of Cu, Cu2-XS, CumInn, InS, and In6S7 occur with CuInS2. No phases of indium sulfide are observed for the films annealed at 90 and 120 min (CIS-6, CIS-7). It can therefore be assumed that the sulfurization time of 60 min and the thickness of the In layer on Cu-tape corresponding to the electrodeposition time of 90 min, as in CIS-4, are optimum conditions for the formation of the single-phase, well-crystalline CuInS2 nanovases. In the next part of this paper, our main focus will be on the evolution of the structure, growth, and optical properties of the pure CuInS2 vaselike nanostructures. The stoichiometry and purity of the composition of the CuInS2 nanovases were determined by energy-dispersive X-ray analysis (EDAX). The EDAX spectrum of the pure phase sample CIS-4 is shown in Figure 5d, which indicates an atomic ratio of Cu/ In/S ) 1:0.91:2. A little excess of Cu is probably due to the

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Figure 5. (a) TEM Low-resolution image of incomplete growth of nanovase structures of the sample CIS-3; (b) TEM low-resolution image of complete nanoflower vase of the sample CIS-4; inset of (b) is the SEM image of a single nanoflower vase; (c) high-resolution lattice images; inset of (c) is the SAED pattern of CuInS2 sample CIS-4; and (d) EDAX analysis of CuInS2 sample CIS-4.

contribution from the Cu-tape substrate. This was also confirmed by the XRD analysis mentioned above. From optical reflectance measurements, too, the formation of pure phase CuInS2 nanovases was confirmed in CIS-4. Figure 3a shows the reflectance (%R) vs wavelength (λ) plot of the samples CIS-3 and CIS-4. Two reflectance bands are present in the reflectance trace of the sample CIS-3, whereas only a single sharp drop can be detected from the reflectance trace of CIS-4. The band gaps of the materials were determined from the differential reflectance spectra,29,30 which are shown in Figure 3b. The differential spectrum of the sample CIS-3 revealed the two band gaps at 1.23 and 1.48 eV, which are close to the bulk reported values of Cu2-XS and copper-rich CuInS2, respectively.30,31 The band gap calculated for the CIS-4 sample is 1.53 eV, which confirms that the nanoflower vases in CIS-4 are composed of single-phase CuInS2.32 The growth process of the flower vase nanostructures was monitored by SEM, FESEM, and TEM. From the SEM studies, the unsulfurized Cu-In alloy showed no development of flower vaselike structures, which clearly indicated that the vaselike growth initiated from the Cu-In alloy only during sulfurization (see Supporting Information). Figure 4a-d shows the SEM and FESEM images of the samples CIS-1 to CIS-4. Figure 4a,b shows incomplete growth of vaselike morphology in CIS-1 and CIS-2. The SEM image of mixed phase product in CIS-3 is shown in Figure 4c. The presence of two distinctly different shapes of heads, hexagonal and tetragonal, may indicate the formation of the two phases: hexagonal Cu2-XS and tetragonal CuInS2. In the figure, arrows I and II indicate the hexagonal and tetragonal heads of the flower vaselike nanostructures, respectively. However, the complete development of nanovase structures was not found in CIS-3. Proper nanovase structures with three parts, (i) head, (ii) neck, and (iii) body, were obtained only in CIS-4. Figure 4d shows the low-resolution FESEM image of beautifully grown nanovase bunches in CIS-4, which

Figure 6. Raman spectra of CuInS2 nanoflower vase sample CIS-4.

gives a fair idea of the high yield of the vases on the substrate surface. Figure 4e shows the high-resolution FESEM image of the same sample. It can be well observed that the flower vaselike nanostructures in CIS-4 have only tetragonal heads of average dimension 95 × 75 nm, indicating the formation of the single phase tetragonal CuInS2. The inset of the figure shows an enlarged picture of a single nanovase. From the FESEM images, the average diameters of the neck and body of the nanovases were measured to be about 82 and 110 nm, respectively. The wide body part was found to be slightly tapered at the bottom, giving a real vaselike appearance. An interesting observation is that the single-phase CuInS2 nanovases self-assemble to form flowers, which are shown in Figure 4d,e. The evolution of the vase morphology was again very evident from Figure 4f, which shows the partial growth of the vaselike morphologies in CIS5. Only tetragonal and hexagonal heads of the vase nanostructures were found to form here, which reveals that the head parts are first to form in these nanovase structures. Figure 5a shows the TEM image of the incomplete growth of the nanovase

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Figure 7. Schematic model showing different stages for the growth of CuInS2 flower vaselike nanostructures on the Cu-tape.

structure in CIS-3, where only the head and neck parts are observed, and the development of the body part is incomplete. The TEM image of the pure phase flower vase in CIS-4 is shown in Figure 5b. The inset of the figure also shows the enlarged SEM image of a single nanovase. Dimension of the tetragonal head and the diameters of the neck and body of the flower vaselike nanostructure measured from TEM image are observed to be well matched with those calculated from the SEM and FESEM images. A HRTEM image of a nanovase is shown in Figure 5c. The lattice fringe spacing is calculated to be 0.32 nm, which corresponds to the (112) lattice planes of the tetragonal CuInS2. Large-scale continuity of the lattice fringes and the spotted SAED pattern (inset of Figure 5c) also indicate the well-crystalline nature of the nanostructures. Raman study is an effective tool to determine the crystallinity, crystal orientation, and lattice defect in the sample. Raman active modes of CuInS2 flower vases in CIS-4 have been shown in Figure 6. It may be observed that the five peaks at 240, 266, 290, 320, and 340 cm-1 are assigned to the E, B2, A1, E, and B2 modes of the chalcopyrite phase of CuInS2.33,34 The Raman peak at 290 cm-1 was fitted to the Gaussian curve, and the fullwidth at half-maximum (fwhm) was estimated to be 8.6 cm-1. Defect-free CuInS2 single crystals and thin films are reported to have fwhm of 3.5 and 5 cm-1, respectively.35 Hence, it may be commented here that almost defect-free CuInS2 flower vase nanostructure arrays were prepared using our process. The growth process toward the formation of defect-free CuInS2 flower vaselike nanostructure arrays on the Cu-tape can be explained from the microstructural evidence. Figure 7 is the simplified schematic model showing the different stages for the growth of CuInS2 flower vaselike nanostructures. When the CuIn alloy on Cu-tape was exposed to the H2S + N2 atmosphere at high temperature, hexagonal copper sulfide, orthorhombic InS, monoclinic In6S7, and tetragonal CuInS2 were formed according to eq 1. At high temperature, In metal formed liquid nanodroplets (melting point of In ∼ 157 °C). These liquid In metal droplets reacted with copper sulfide in stoichiometric cation-anion ratio and formed tetragonal CuInS2 such that the proper local charge is balanced and the structural symmetry is maintained (eq 2). At the same time, the liquid In reacted with H2S, and indium sulfide vapor was formed at the high temperature. Then, In metal droplets absorbed the incoming source of molecular vapor of indium sulfide, and simultaneous diffusion of Cu from Cu-tape facilitated the formation of CuInS2 further (eqs 3 and 4). Finally, upon supersaturation, solid CuInS2

nanostructures started to appear with the tetragonal heads at their tips. With further intake of the molecules, low-energy surfaces started to form. Thus, the key characteristic of the selfcatalyzed VLS36,37 growth process, i.e., the existence of the liquid In nanodroplets, triggers the growth of the vaselike morphology. This is also the reason behind the appearance of only heads when the sulfurization time was low as 30 min in CIS-5. However, the formation of the wide body parts in the CuInS2 flower vase nanostructures suggested that the conventional VLS process alone could not give rise to the vaselike morphology. A secondary growth process must be operative. We propose that the diameters of the nanostructures increased by the influence of the VS38-40 method. At high indium sulfide vapor concentration, the indium sulfide vapor was not only absorbed by the liquid In droplets but also deposited below, i.e., on the Cu-tape that was still in a high energy state and the width of the nanostructures increased at the body part. The In supply from the Cu-In alloy during sulfurization decreased with increasing time, whereas the continuous supply of Cu from the Cu-tape still proceeded by the solid-state diffusion. The high growth rate at the first stage slowed down as the indium sulfide vapor concentration decreased gradually. The diameters of the nanostructures gradually decreased again at the bottom of the body part, and the real flower vaselike nanostructures were formed. In the process, the formation of CuInS2 flower vases were initiated by the self-catalyzed vapor-liquid-solid (VLS) method, whereas the vapor-solid (VS) process played a crucial role in defining the shapes of the nanovases.41 Conclusion In conclusion, for the first time well-crystalline single-phase CuInS2 flower vaselike nanostructure arrays have been successfully prepared on Cu-tape using the CISCuT method. The condition for obtaining pure phase nanovases was skillfully optimized. Generally, the typical layer sequence from the bottom to the top of the films is Cu-CuInS2-Cu2-XS. The formation of all these phases depends critically on the thickness of the In layer on the Cu-tape substrate and the annealing/sulfurization conditions. By adjusting these parameters, single-phase tetragonal CuInS2 nanoflower vase arrays were formed on Cu-tape substrate without any harmful surface etching of the films. Because of the phase purity of the nanostructure arrays, it will certainly help in building nanostructure-based photovoltaic and photochemical devices. This direct growth of flower vaselike nanostructure arrays from Cu-tape substrate by the non-etched

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CISCuT method is very simple, cost-effective, and facile and also may be used for large-scale industrial production of the thin film solar cell. Acknowledgment. The authors thank Ministry of Non Conventional Energy Sources (MNES), Department of Science and Technology (DST) and Council of Scientific and Industrial Research (CSIR), Government of India, for financial assistance during the tenure of the work. The authors also thank Mr. K. K. Das of IACS for recording the SEM micrographs. Supporting Information Available: SEM micrographs of the CuIn alloy and CIS-6 and CIS-7 samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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