Selective synthesis of cubic and hexagonal phase of CuInS₂ nanocrystals by microwave irradiation

Abstract

Metastable cubic zincblende and hexagonal wurtzite CuInS₂ nanocrystals were successfully synthesized by a facile microwave radiation method. The morphology, structure and phase composition of the as-prepared products were examined, and the results demonstrated that high-purity and uniform CuInS₂ nanocrystals were obtained. Further investigation revealed a structural evolution process from cubic zincblende to hexagonal wurtzite with increasing volume ratio of ethylenediamine and ethanol from 1 : 30 to 1 : 1 at 160 °C. In addition, the optical absorption properties of samples were also studied. It was found that the as-synthesized cubic and hexagonal CuInS₂ nanocrystals had band gaps of 1.503 and 1.470 eV, respectively. Thanks to the superior optical absorption properties for visible light, CuInS₂ nanocrystals may have potential applications in various nanostructured optoelectronic devices. The possible formation mechanism of the product, namely “phase transformation”, was systematically studied.

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1. Introduction

Nanocrystals (NCs) of multinary chalcogenides have attracted considerable attention owing to their potential use in photovoltaic solar cells, light-emitting diodes, bio-tagging, photocatalysts, and optical devices.^1–5 In contrast to the bulk materials, nanocrystals have unique optical and electronic properties.^6–8 As one of the most attractive nanocrystals, copper indium disulfide (CuInS₂ or CIS) is an important ternary I–III–VI₂ semiconductor material with a direct band gap of 1.45–1.5 eV. It is considered as a potential alternative for solar cells.⁹ Previous experiments have revealed that CuInS₂ has good stability against radiation and high optical absorption coefficients (∼10⁵ cm⁻¹).¹⁰ Furthermore, it does not contain any toxic elements (Cd, Pb, Hg, Se and As), rendering it a promising environmentally-friendly solar cell material.

Research has shown that there are three crystal structures for CuInS₂: chalcopyrite, zincblende and wurtzite.¹¹ The chalcopyrite tetragonal structure as the low-temperature phase is common and stable, which has already been widely applied in CIS solar cells. On the contrary, the cubic zincblende and hexagonal wurtzite structures as the high-temperature phases are metastable at room temperature because of the random distribution of Cu(I) and In(III) in the cation sublattices. Since the cubic zincblende CuInS₂ (c-CIS) and hexagonal wurtzite CuInS₂ (h-CIS) can result in stable nonstoichiometric Cu–In–S nanocrystals due to the flexibility of the chemical components, they may be helpful in equipment manufacturing. Up to now, studies have been focused on tetragonal chalcopyrite CuInS₂ NCs, because c-CIS and h-CIS NCs are thermodynamically metastable at ambient conditions.

Recently, much effort has been devoted to the synthesis of c-CIS and h-CIS NCs^12–16 after their successful synthesis via a hot-injection method using mixed generic precursors by Lu et al.¹⁷ Many current methods require complex procedures, high temperature, long operation time and inert gas protection. The reagents used are often highly toxic. In an attempt to improve the synthesis of c-CIS and h-CIS NCs, we have developed a facile, green and economical method using microwave-assisted solvothermal synthesis, which effectively reduces side reactions, shortens reaction time, increases yields and improves reproducibility.¹⁸ The structural evolution from cubic zincblende to hexagonal wurtzite has been achieved for the first time by simply altering the volume ratios of a mixed solvent system of ethylenediamine and ethanol.

2. Experimental section

2.1. Materials

CuCl₂·2H₂O (99%, Tianjin Guangfu Co., China), InCl₃·4H₂O (99%, Sinopharm Chemical Reagent Co., China), thiourea (CH₄N₂S, AR, Beijing Fine Chemical Company, China), ethanol (AR, Tianjin Tiantai Co., China) and anhydrous ethylenediamine (AR, Shantou Xilong Co., China) were used as starting materials. All chemical reagents in the experiments were of analytical grade and used as received without any further purification.

2.2. Synthesis of CIS NCs

In a typical synthetic procedure for CIS NCs with the cubic zincblende structure, 1 mmol CuCl₂·2H₂O, 1 mmol InCl₃·4H₂O and 2 mmol thiourea were dissolved in 30 mL ethanol to form a solution. Subsequently, 1 mL ethylenediamine was introduced into the solution. After 30 min of magnetic stirring, 20 mL of the mixture was transferred into a microwave vial. The vial was sealed, put into a single-mode microwave synthesizer and irradiated at 160 °C for 20 min under magnetic stirring. Afterwards, the solution was cooled down to room temperature naturally. The reaction products were washed with distilled water and absolute ethanol, and then collected by centrifugation at 8000 rpm for 5 min. Finally, the purified nanocrystals were dried under vacuum. The CIS NCs with the hexagonal wurtzite structure were synthesized by the reaction of 1 mmol CuCl₂·2H₂O, 1 mmol InCl₃·4H₂O, 2 mmol thiourea, 15 mL ethanol and 15 mL ethylenediamine under the same conditions mentioned above.

2.3. Characterization methods

The powder X-ray diffraction (PXRD) data of the samples were collected on a Rigaku Model D/Max 2550 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) at 50 kV and 200 mA at room temperature. The general morphology and chemical composition of the materials were studied using a JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM) equipped with an energy dispersive X-ray (EDX) spectrometer. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL JEM-3010. UV-vis absorption spectra were obtained by a Lambda 800 UV-vis spectrophotometer. All measurements are done at room temperature and under ambient conditions. The microwave synthesis experiments were performed using a single-mode cavity microwave synthesizer at 2450 MHz (Biotage AB, Uppsala, Sweden).

3. Results and discussion

The XRD patterns of as-obtained CIS NCs, as shown in Fig. 1, do not match any pattern in the standard Joint Committee on Power Diffraction Standards (JCPDS) card database. To clarify the crystal structures of as-obtained CIS NCs, we simulated diffraction patterns of zincblende and wurtzite CuInS₂ using the lattice parameters reported in the previous study.¹⁷ In Fig. 1a, the XRD pattern of CIS NCs obtained at 160 °C with a solvent volume ratio (ethylenediamine : ethanol) of 1 : 30 shows an excellent agreement with that of the simulated cubic zincblende structure. The peaks around 2θ = 27.90°, 32.36°, 46.42°, 55.02°, 57.62°, 67.74°, 74.80° and 77.40° can be assigned to the (111), (200), (220), (311), (222), (400), (331) and (420) planes, respectively. The strong and sharp diffraction peaks indicate that the as-obtained products are well crystallized. No diffraction peaks from impurities, e.g. CuS, Cu₂S or In₂S₃, have been detected, suggesting the high purity of the as-obtained products.

Fig. 1 Experimental and simulated XRD patterns of CuInS₂ synthesized with (a) zincblende and (b) wurtzite structures. Insets show corresponding crystal structures.

In Fig. 1b, the XRD pattern of CIS NCs obtained at 160 °C with a volume ratio of 1 : 1 agrees well with that of the simulated hexagonal wurtzite structure. The peaks around 2θ = 26.14°, 27.54°, 29.62°, 38.42°, 46.22°, 50.14°, 54.74°, 56.00° and 70.96° correspond to the (100), (002), (101), (102), (110), (103), (112), (201) and (203) planes, respectively. There are also no characteristic peaks for the impurities.

In order to further understand formation mechanism of phase transformation, we have added the structural evolution process at the volume ratio of ethylenediamine and ethanol of 1 : 20, 1 : 10 and 1 : 5, respectively (Fig. S1†). We found that the sample which obtained at the volume ratio (ethylenediamine : ethanol) of 1 : 20 and 1 : 30 have same crystal structure, the cubic zincblende structure. However, the XRD pattern of sample which obtained at the volume ratio (ethylenediamine : ethanol) of 1 : 10 revealed the presence of both wurtzite and zincblende phases. When the volume ratio (ethylenediamine : ethanol) decreased to 1 : 5, only wurtzite phase was formed, but the product had lower crystallinity than the sample produced from the volume ratio of 1 : 1. Hence it may be concluded that the formation of these two phases can be controlled by changing the volume ratio of ethylenediamine and ethanol while keeping the other experimental conditions the same.

Fig. 2 shows the EDX spectrum of the as-obtained CIS NCs. The Cu–In–S compositional ratios (%) are 23.62 : 26.56 : 49.82 and 25.73 : 27.39 : 46.88 for the zincblende and wurtzite CIS, respectively. These values are close to the 1 : 1 : 2 ratio in the CuInS₂ formula.

Fig. 2 Typical EDX pattern of the CuInS₂ prouducts: (a) zincblende and (b) wurtzite structures.

The morphology and sizes of the as-synthesized CIS NCs were examined by SEM, TEM and HRTEM. In the SEM image of c-CIS NCs shown in Fig. 3a number of larger irregular-shaped agglomerates with diameters between 200 and 500 nm can be seen. The TEM image (Fig. 3b) reveals that the agglomerates are composed of some smaller nanoparticles. The nanoparticles have severe aggregation, which should be attributed to the small size and huge surface energy of the particles.¹⁹ The detailed microstructure of c-CIS NCs was further investigated by HRTEM, as shown in Fig. 3c. An average d spacing of about 0.310 nm can be observed from the lattice resolved TEM image. This d-spacing is consistent with the spacing between (111) planes of cubic zincblende CuInS₂.

Fig. 3 SEM (a), TEM (b), and HRTEM (c) images of the zincblende CuInS₂ nanocrystals; SEM (d), TEM (e), and HRTEM (f) images of the wurtzite CuInS₂ nanocrystals.

The typical SEM image of h-CIS NCs, as shown in Fig. 3d, shows that the products are hexagonal nanosheets. The diameter of the hexagonal nanosheets is measured to be approximately 200 nm from the TEM image shown in Fig. 3e. The HRTEM image (Fig. 3f) exhibits clear lattice fringes with a spacing of 0.338 nm, which is consistent with the lattice parameters of (100) plane of the hexagonal-phase CuInS₂.

Fig. 4 shows the UV-vis absorption spectra of CIS NCs with the zincblende (Fig. 4a) and wurtzite (Fig. 4b) structures. It is clear that the CuInS₂ NCs have superior absorption in the region of 400–850 nm, indicating their efficient photoabsorption ability. The optical band gap of the as-synthesized CIS NCs can be estimated in the UV-vis spectra by plotting (αhν)² versus hν, using the relation

αhν = A₀[hν − E_g]ⁿ (1)

where α represents the absorption coefficient, h is the Planck's constant and ν is the frequency, E_g is the band gap energy, A₀ is a constant which is related to the effective masses associated with the bands. The value of n is equal to 1/2 for a direct band gap material.^20,21 From linear extrapolation of the plots, direct band gap E_g values are determined to be 1.503 (the inset of Fig. 4a) and 1.470 eV (the inset of Fig. 4b) for the CIS NCs with zincblende and wurtzite structures, respectively. These values are near the optimum for photovoltaic solar conversion in a single band gap device.

Fig. 4 UV-vis absorption spectra of the CuInS₂ nanocrystals: (a) zincblende and (b) wurtzite structures. Insets show extrapolation of the spectra for the determination of the band gap.

The vast differences of c-CIS and h-CIS are often attributed to the different coordination modes of metal ions in their structures. In the cubic phase, Cu atoms are tetrahedrally coordinated and In atoms are only octahedrally coordinated by sulfur atoms. However, in the hexagonal phase, all Cu and half of the In atoms are tetrahedrally coordinated by sulfur atoms, while the other half of the In atoms are octahedrally coordinated.²² According to the solution coordination model (SCM) proposed in the literature, the coordination of the ions formed in solution can serve as a template to maintain the same coordination in the solid.²³ Accordingly, the phase of the solid product is determined by the coordination status of the ions in the solution. Typically, Cu⁺ primarily exhibits tetrahedral four-fold coordination mode and can be complexed by thiourea to form [Cu(Tu)₄]⁺ in solution and form Cu–S₄ under solvothermal conditions. In³⁺ usually adopts octahedrally six-fold coordination mode. However, it can adopt a variety of coordination modes depending on the pH of the solution. Ethylenediamine is an alkaline solvent, which is pivotal in this work. At the solvent volume ratio (ethylenediamine : ethanol) of 1 : 30, In³⁺ adopts six-fold coordination [In(Tu)₆]³⁺ to form In–S₆. As a result, Cu–S₄ and In–S₆ can combine to form zincblende c-CIS phase (Fig. 5). Under the condition of equal volume of ethylenediamine and ethanol, however, both [In(Tu)₆]³⁺ and four-fold coordination [In(Tu)₄]³⁺ can coexist with the increase of pH. The Cu–S₄, In–S₆ and In–S₄ can combine to generate hermodynamically stable wurtzite h-CIS.

Fig. 5 Schematic illustration showing the formation of the cubic and hexagonal CuInS₂ using the solution coordination model. The charges of the complexes are omitted in the figure. Tu is the abbreviation of thiourea.

4. Conclusion

In summary, cubic and hexagonal CuInS₂ nanocrystals have been successfully synthesized using microwave-assisted solvothermal method. Specifically, the effects of the volume ratios of mixed solvent ethylenediamine and ethanol on CuInS₂ structure were systematically studied in an attempt to elucidate the phase formation mechanism. In the synthesis, ethylenediamine plays a very important role in controlling the phase. Both phases exhibit strong absorption in a wide wavelength range from visible to UV light. The band gaps of the zincblende and wurtzite CIS nanocrystals are determined to be 1.503 and 1.470 eV, respectively. Potential application of both products may extend in the fields of solar cell and linear or nonlinear optical devices.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 21271082).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45486j

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