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Mild solution-based method for synthesizing wurtzite CuInS2 nanoplates at low temperature
Materials Letters 123 (2014) 169–171
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Mild solution-based method for synthesizing wurtzite CuInS2 nanoplates at low temperature Jianyong Guo a,b, Gang Chang a,n, Wei Zhang a, Xiong Liu a, Taosheng Zhou a, Yunbin He a,n a Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, China b School of Electronic and Electrical Engineering, Wuhan Textile University, Wuhan 430074, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 December 2013 Accepted 27 February 2014 Available online 11 March 2014
Wurtzite copper indium disulfide (CuInS2) nanoplates were synthesized via a low-cost, facile solutionbased method. The reaction was carried out at 140 1C for an hour under nitrogen atmosphere by using the solvent of monoethanolamine (MEA). The experimental results revealed that the products had good crystallinity, monomorphology and stoichiometric composition. The solvent material, reaction time and temperature played important roles in the formation of the wurtzite CuInS2 nanoplates. The possible growth mechanism was also proposed and discussed. And the wide stoichiometry of the wurtzite-CuInS2 implies that this work may offer a new strategic approach for the synthesis of I–III–VI2 ternary semiconductor nanocrystals as new solar energy materials. & 2014 Elsevier B.V. All rights reserved.
Keywords: CuInS2 Solar energy materials Nanocrystalline materials Hot-injection Wurtzite
1. Introduction Among the I–III–VI2 ternary semiconductor nanocrystals, CuInS2 has been extensively investigated in recent years. With a band gap of 1.53 eV and a high absorption coefficient of 105 cm 1, CuInS2 shows great potential applications as absorbers for low-cost and high-efficiency solar cells [1]. Due to its low toxic composition, CuInS2 is much “greener” than the other photovoltaic materials such as CdSe, CdTe, CuInSe2, etc. It is well known that bulk CuInS2 has three crystal structures [2,3]: chalcopyrite, zincblende and wurtzite. For bulk CuInS2 materials, both zincblende and wurtzite structures are metastable at room temperature. However, CuInS2 nanocrystals can be stable in all three phases at room temperature [4]. Due to the disordered occupancy of Cu þ and In3 þ in the lattice sites, the wurtzite CuInS2 allows large flexibility in stoichiometry. This implies that the wurtzite CuInS2 materials could have tunable band gap in even wider range, which is much beneficial to the fabrication of optoelectronic devices. Recently, there has been considerable interest devoted to realizing the potential applications in thin film solar cells for wurtzite CuInS2 materials. Several groups had synthesized wurtzite CuInS2 nanocrystals based on solution phase methods, such as precursors thermolysis [5], solvothermal [6,7], hot-injection [8,9], etc. However, most of these synthesis processes
n
Corresponding authors. Tel./fax: þ86 27 8866 1803. E-mail addresses: [email protected] (G. Chang), [email protected] (Y. He). http://dx.doi.org/10.1016/j.matlet.2014.02.096 0167-577X/& 2014 Elsevier B.V. All rights reserved.
were under harsh conditions including high reaction temperatures (180–300 1C) and expensive solvents (usually the oil system, e.g., oleylamine or octadecylene) [5,10,11]. Although new solvents such as polyalcohol system were attempted to replace the expensive oil system [12,13], it is still a challenge to synthesize pure and well-controlled wurtzite CuInS2 nanocrystals by milder approaches. In this work, we developed a low-cost and facile method to synthesize wurtzite CuInS2 materials. By using the monoethanolamine as solvent, wurtzite CuInS2 nanoplates were obtained at 140 1C in 1 h. The method also has the potential for the scalability of manufacturing CuInS2 nanoparticles.
2. Experimental methods In a typical synthesis process, 0.6 mmol copper sulfate pentahydrate (CuSO4 5H2O, Z99.0%) and 0.6 mmol indium chloride tetrahydrate (InCl3 4H2O, Z 99.999%) were mixed in 30 ml monoethanolamine (MEA, C2H7NO, Z99.0%). A transparent blue solution was formed by stirring for 15 min at 80 1C. After being degassed by nitrogen for 15 min, the mixture was heated up to 140 1C and the color of solution changed from dark blue to colorless. Then the thiourea (TA, N2H4CS, Z99.0%) solution (1.8 mmol TA dissolved in 5 ml MEA) was injected into the former mixture under flowing nitrogen. During the injection process, the solution turned to brown and soon changed to black within one minute. The temperature was kept at 140 1C during the whole process and the reaction solution was cooled down to room
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J. Guo et al. / Materials Letters 123 (2014) 169–171
temperature afterwards. The products were isolated by precipitation with ethanol, and then centrifuged with ethanol and acetone for three to five times. The final black products were dried at 45 1C for 4 h under vacuum. X-ray diffraction (XRD, Bruker, D8 Advance) was used to analyze the phase structure of the products by using Cu Kα radiation (λ ¼1.5418 Å). The morphology of the nanoplates was observed by scanning electron microscopy (SEM, ZEISS) and transmission electron microscopy (TEM, FEI TECNAI20, USA). To prepare the samples for SEM observation, the suspension of CuInS2 nanocrystals was dispersed on the surface of silicon slide and dried in ambient conditions. Energy dispersive X-ray analysis (EDX, EDAX Inc., Genesis XM2) was applied to determine the chemical compositions of the nanoplates.
A typical SEM image of the products is displayed in Fig. 2(a), which indicates that the nanocrystals have the morphology of hexagonal plate. The average diameter of the plates is about 200 nm, with a thickness of 30 nm (as illustrated in Fig. 2(b)). To investigate the growth mechanism, the synthesis of wurtzite CuInS2 nanoplates was repeated as control experiment with the same conditions except for different reaction time of 5, 15, 30, and 60 min. Fig. 3 shows XRD patterns of the four samples produced with different reaction time. The characteristic peaks of wurtzite CuInS2 can be found even at 5 min, which means wurtzite CuInS2 has been synthesized. Meanwhile, two peaks of Cu2S show up in the pattern for this 5-min reaction time sample, as indicated by the asterisk and triangle. With increase of reaction time, the peaks
3. Results and discussion Fig. 1(a) shows XRD pattern of the CuInS2 products that were synthesized at 140 1C for 1 h. A simulated XRD pattern of wurtzite CuInS2 (calculated by using CrystalMaker 1.2, with the lattice parameters a ¼b¼3.897 Å , c ¼6.441 Å and space group of P63mc) is also presented in the same figure. Both peak positions and intensity distribution of the experimental XRD result match well to the simulated data, indicating that the as-prepared nanoplates have a wurtzite structure. The size of the nanoplates is about 160 nm, calculated by Scherrer formula, which is in line with the result estimated from the SEM image (see Fig. 2(a)). The elemental composition of the nanoplates, obtained by the EDX technique, is shown in Fig. 1(b). It indicates that the product is composed of Cu, In and S with molar ratio of 1:1:2.1, which is very close to the stoichiometric composition of CuInS2.
Fig. 3. XRD patterns of the samples produced with different reaction time (CIS2 represents CuInS2): (a) 5 min, (b) 15 min, (c) 30 min, and (d) 1 h.
Fig.1. (a) XRD pattern of the CuInS2 nanoplates synthesized at 140 1C, 1 h (black curve) and the simulated result (discrete vertical lines); and (b) EDX analysis of CuInS2 nanoplates synthesized at 140 1C, 1 h. The inset shows the results of quantitative elemental analysis.
Fig. 2. (a) SEM image of CuInS2 nanoplates synthesized at 140 1C, 1 h; and (b) TEM image of a hexagonal plate and a corresponding selected area electron diffraction (SAED) pattern (inset at the right bottom corner).
J. Guo et al. / Materials Letters 123 (2014) 169–171
of Cu2S become weaker and finally disappear, whereas those of the wurtzite CuInS2 gradually enhance and eventually evolve into a single phase when the reaction time was 60 min. The evolution of the Cu2S and CuInS2 peaks suggests Cu2S as an intermediate product toward the formation of wurtzite CuInS2. The wurtzite structure of Cu2S can play an important role as soft model for the growth of wurtzite CuInS2 [5,11]. As displayed in Fig. 3, the intensities of (102) and (103) peaks of Cu2S are comparable to those of CuInS2 for the reaction time of 5 min. With further reaction of longer time, the peaks of Cu2S become weaker while those of CuInS2 grow higher. Finally, the Cu2S peaks vanish in the spectra. The interplay between the Cu2S and CuInS2 peaks indicates the growth process, i.e., the evolution from wurtzite Cu2S to CuInS2. MEA played an important role during the synthesis of the wurtzite CuInS2. First of all, MEA acted as the reductant, which reduced Cu2 þ to Cu þ . This may be proved by the color change of the solution: the Cu2 þ showed blue while Cu þ was colorless. Secondly, MEA acted as a chelating agent. Before the TA was injected, MEA can chelate with Cu þ and In3 þ to form the metal precursors. Since the reaction activity of Cu þ was higher than that of the In3 þ [9], the chelating may inhibit the reaction rate of Cu þ , which can be beneficial to the disordered occupancy of Cu þ and In3 þ in the lattice sites equally. This was important for the synthesis of wurtzite CuInS2 nanocrystals. 4. Conclusions In this work, the synthesis of wurtzite CuInS2 nanoplates has been performed via a low-cost, facile solution-based method. Compared with the other previously reported solution-based processes, the MEA solvent employed here is more economical. And more importantly the reaction process takes place under relatively mild conditions (lower reaction temperature and shorter reaction time). XRD results demonstrate a wurtzite structure of the products, while both SEM and TEM images show correspondingly a typical hexagonal morphology of the nanocrystals. EDX reveals that the wurtzite nanoplates have a composition well close to CuInS2. While MEA plays key roles during the reaction, the reaction time also has important influence on the formation of CuInS2 nanoplates.
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Acknowledgments This investigation was supported by the NSFC (Nos. 50972041, 51102085, and 61274010), Program for New Century Excellent Talents in University (NCET-09-0135), Research Fund for the Doctoral Program of Higher Education of China (20124208110005), Scientific Research Foundation for Returned Scholars, Ministry of Education of China, NSF of Hubei Province (Nos. 2011CDB057, 2011CDA81), Science Foundation from Hubei Provincial Department of Education (No. Q20111002), and Wuhan Municipal Academic Leaders Program (200951830550).
References [1] He YB, Polity A, Alves HR, Oesterreicher I, Kriegseis W, Pfisterer D, et al. Structural and optical characterization of RF reactively sputtered CuInS2 thin films. Thin Solid Films 2002;403–404:62–5. [2] Binsma JJM, Giling LJ, Bloem J. Phase relations in the system Cu2S–In2S3. J Cryst Growth 1980;50:429–36. [3] Abrahams SC, Berbstein JL. Piezoeletric nonlinear optic CuGaS2 and CuInS2 II IV V crystal structure: sublattice distortion in AIBIIICVI 2 and A B C2 type chalcopyrites. J Chem Phys 1973;59:5415–22. [4] Nose K, Soma Y, Omata T, Matsuo S, Otsuka Yao. Chem Mater 2009;21: 2607–13. [5] Batabyal SK, Tian L, Venkatram N, Ji W, Vittal JJ. Phase-selective synthesis of CuInS2 nanocrystals. J Phys Chem C 2009;113:15037–42. [6] Huang WC, Tseng CH, Chang SH, Tuan HY, Chiang CC, Lyu LM, et al. Solvothermal synthesis of Zincblende and Wurtzite CuInS2 nanocrystals and their photovoltaic application. Langmuir 2012;28:8496–501. [7] Guo J, Zhou WH, Li M, Hou ZL, Jiao J, Zhou ZJ, Wu SX. Synthesis of bullet-like wurtzite CuInS2 nanocrystals under atmospheric conditions. J Cryst Growth 2012;359:72–6. [8] Pan DC, An LJ, Sun ZM, Hou W, Yang Y, Yang ZZ, et al. Synthesis of Cu–In–S ternary nanocrystals with tunable structure and composition. J Am Chem Soc 2008;130:5620–1. [9] Xie RG, Rutherford M, Peng XG. Formation of high-quality I–III–VI semiconductor nanocrystals by tuning relative reactivity of cationic precursors. J Am Chem Soc 2009;131:5691–7. [10] Koo B, Patel RN, Korgel BA. Wurtzite–chalcopyrite polytypism in CuInS2 nanodisks. Chem Mater 2009;21:1962–6. [11] Connor Stephen T, Hsu CM, Weil BD, Aloni S, Cui Y. Phase transformation of biphasic Cu2S–CuInS2 to monophasic CuInS2 nanorods. J Am Chem Soc 2009;131:4962–6. [12] Sheng X, Wang L, Luo YP, Yang DR. Synthesis of hexagonal structured wurtzite and chalcopyrite CuInS2 via a simple solution route. Nanoscale Res Lett 2011;6:562. [13] Qi YX, Liu QC, Tang KB, Liang ZH, Ren ZB, Liu XM. Synthesis and characterization of nanostructured wurtzite CuInS2: a new cation disordered polymorph of CuInS2. J Phys Chem C 2009;113:3939–44.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Mild solution-based method for synthesizing wurtzite CuInS2 nanoplates at low temperature Jianyong Guo a,b, Gang Chang a,n, Wei Zhang a, Xiong Liu a, Taosheng Zhou a, Yunbin He a,n a Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, China b School of Electronic and Electrical Engineering, Wuhan Textile University, Wuhan 430074, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 December 2013 Accepted 27 February 2014 Available online 11 March 2014
Wurtzite copper indium disulfide (CuInS2) nanoplates were synthesized via a low-cost, facile solutionbased method. The reaction was carried out at 140 1C for an hour under nitrogen atmosphere by using the solvent of monoethanolamine (MEA). The experimental results revealed that the products had good crystallinity, monomorphology and stoichiometric composition. The solvent material, reaction time and temperature played important roles in the formation of the wurtzite CuInS2 nanoplates. The possible growth mechanism was also proposed and discussed. And the wide stoichiometry of the wurtzite-CuInS2 implies that this work may offer a new strategic approach for the synthesis of I–III–VI2 ternary semiconductor nanocrystals as new solar energy materials. & 2014 Elsevier B.V. All rights reserved.
Keywords: CuInS2 Solar energy materials Nanocrystalline materials Hot-injection Wurtzite
1. Introduction Among the I–III–VI2 ternary semiconductor nanocrystals, CuInS2 has been extensively investigated in recent years. With a band gap of 1.53 eV and a high absorption coefficient of 105 cm 1, CuInS2 shows great potential applications as absorbers for low-cost and high-efficiency solar cells [1]. Due to its low toxic composition, CuInS2 is much “greener” than the other photovoltaic materials such as CdSe, CdTe, CuInSe2, etc. It is well known that bulk CuInS2 has three crystal structures [2,3]: chalcopyrite, zincblende and wurtzite. For bulk CuInS2 materials, both zincblende and wurtzite structures are metastable at room temperature. However, CuInS2 nanocrystals can be stable in all three phases at room temperature [4]. Due to the disordered occupancy of Cu þ and In3 þ in the lattice sites, the wurtzite CuInS2 allows large flexibility in stoichiometry. This implies that the wurtzite CuInS2 materials could have tunable band gap in even wider range, which is much beneficial to the fabrication of optoelectronic devices. Recently, there has been considerable interest devoted to realizing the potential applications in thin film solar cells for wurtzite CuInS2 materials. Several groups had synthesized wurtzite CuInS2 nanocrystals based on solution phase methods, such as precursors thermolysis [5], solvothermal [6,7], hot-injection [8,9], etc. However, most of these synthesis processes
n
Corresponding authors. Tel./fax: þ86 27 8866 1803. E-mail addresses: [email protected] (G. Chang), [email protected] (Y. He). http://dx.doi.org/10.1016/j.matlet.2014.02.096 0167-577X/& 2014 Elsevier B.V. All rights reserved.
were under harsh conditions including high reaction temperatures (180–300 1C) and expensive solvents (usually the oil system, e.g., oleylamine or octadecylene) [5,10,11]. Although new solvents such as polyalcohol system were attempted to replace the expensive oil system [12,13], it is still a challenge to synthesize pure and well-controlled wurtzite CuInS2 nanocrystals by milder approaches. In this work, we developed a low-cost and facile method to synthesize wurtzite CuInS2 materials. By using the monoethanolamine as solvent, wurtzite CuInS2 nanoplates were obtained at 140 1C in 1 h. The method also has the potential for the scalability of manufacturing CuInS2 nanoparticles.
2. Experimental methods In a typical synthesis process, 0.6 mmol copper sulfate pentahydrate (CuSO4 5H2O, Z99.0%) and 0.6 mmol indium chloride tetrahydrate (InCl3 4H2O, Z 99.999%) were mixed in 30 ml monoethanolamine (MEA, C2H7NO, Z99.0%). A transparent blue solution was formed by stirring for 15 min at 80 1C. After being degassed by nitrogen for 15 min, the mixture was heated up to 140 1C and the color of solution changed from dark blue to colorless. Then the thiourea (TA, N2H4CS, Z99.0%) solution (1.8 mmol TA dissolved in 5 ml MEA) was injected into the former mixture under flowing nitrogen. During the injection process, the solution turned to brown and soon changed to black within one minute. The temperature was kept at 140 1C during the whole process and the reaction solution was cooled down to room
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temperature afterwards. The products were isolated by precipitation with ethanol, and then centrifuged with ethanol and acetone for three to five times. The final black products were dried at 45 1C for 4 h under vacuum. X-ray diffraction (XRD, Bruker, D8 Advance) was used to analyze the phase structure of the products by using Cu Kα radiation (λ ¼1.5418 Å). The morphology of the nanoplates was observed by scanning electron microscopy (SEM, ZEISS) and transmission electron microscopy (TEM, FEI TECNAI20, USA). To prepare the samples for SEM observation, the suspension of CuInS2 nanocrystals was dispersed on the surface of silicon slide and dried in ambient conditions. Energy dispersive X-ray analysis (EDX, EDAX Inc., Genesis XM2) was applied to determine the chemical compositions of the nanoplates.
A typical SEM image of the products is displayed in Fig. 2(a), which indicates that the nanocrystals have the morphology of hexagonal plate. The average diameter of the plates is about 200 nm, with a thickness of 30 nm (as illustrated in Fig. 2(b)). To investigate the growth mechanism, the synthesis of wurtzite CuInS2 nanoplates was repeated as control experiment with the same conditions except for different reaction time of 5, 15, 30, and 60 min. Fig. 3 shows XRD patterns of the four samples produced with different reaction time. The characteristic peaks of wurtzite CuInS2 can be found even at 5 min, which means wurtzite CuInS2 has been synthesized. Meanwhile, two peaks of Cu2S show up in the pattern for this 5-min reaction time sample, as indicated by the asterisk and triangle. With increase of reaction time, the peaks
3. Results and discussion Fig. 1(a) shows XRD pattern of the CuInS2 products that were synthesized at 140 1C for 1 h. A simulated XRD pattern of wurtzite CuInS2 (calculated by using CrystalMaker 1.2, with the lattice parameters a ¼b¼3.897 Å , c ¼6.441 Å and space group of P63mc) is also presented in the same figure. Both peak positions and intensity distribution of the experimental XRD result match well to the simulated data, indicating that the as-prepared nanoplates have a wurtzite structure. The size of the nanoplates is about 160 nm, calculated by Scherrer formula, which is in line with the result estimated from the SEM image (see Fig. 2(a)). The elemental composition of the nanoplates, obtained by the EDX technique, is shown in Fig. 1(b). It indicates that the product is composed of Cu, In and S with molar ratio of 1:1:2.1, which is very close to the stoichiometric composition of CuInS2.
Fig. 3. XRD patterns of the samples produced with different reaction time (CIS2 represents CuInS2): (a) 5 min, (b) 15 min, (c) 30 min, and (d) 1 h.
Fig.1. (a) XRD pattern of the CuInS2 nanoplates synthesized at 140 1C, 1 h (black curve) and the simulated result (discrete vertical lines); and (b) EDX analysis of CuInS2 nanoplates synthesized at 140 1C, 1 h. The inset shows the results of quantitative elemental analysis.
Fig. 2. (a) SEM image of CuInS2 nanoplates synthesized at 140 1C, 1 h; and (b) TEM image of a hexagonal plate and a corresponding selected area electron diffraction (SAED) pattern (inset at the right bottom corner).
J. Guo et al. / Materials Letters 123 (2014) 169–171
of Cu2S become weaker and finally disappear, whereas those of the wurtzite CuInS2 gradually enhance and eventually evolve into a single phase when the reaction time was 60 min. The evolution of the Cu2S and CuInS2 peaks suggests Cu2S as an intermediate product toward the formation of wurtzite CuInS2. The wurtzite structure of Cu2S can play an important role as soft model for the growth of wurtzite CuInS2 [5,11]. As displayed in Fig. 3, the intensities of (102) and (103) peaks of Cu2S are comparable to those of CuInS2 for the reaction time of 5 min. With further reaction of longer time, the peaks of Cu2S become weaker while those of CuInS2 grow higher. Finally, the Cu2S peaks vanish in the spectra. The interplay between the Cu2S and CuInS2 peaks indicates the growth process, i.e., the evolution from wurtzite Cu2S to CuInS2. MEA played an important role during the synthesis of the wurtzite CuInS2. First of all, MEA acted as the reductant, which reduced Cu2 þ to Cu þ . This may be proved by the color change of the solution: the Cu2 þ showed blue while Cu þ was colorless. Secondly, MEA acted as a chelating agent. Before the TA was injected, MEA can chelate with Cu þ and In3 þ to form the metal precursors. Since the reaction activity of Cu þ was higher than that of the In3 þ [9], the chelating may inhibit the reaction rate of Cu þ , which can be beneficial to the disordered occupancy of Cu þ and In3 þ in the lattice sites equally. This was important for the synthesis of wurtzite CuInS2 nanocrystals. 4. Conclusions In this work, the synthesis of wurtzite CuInS2 nanoplates has been performed via a low-cost, facile solution-based method. Compared with the other previously reported solution-based processes, the MEA solvent employed here is more economical. And more importantly the reaction process takes place under relatively mild conditions (lower reaction temperature and shorter reaction time). XRD results demonstrate a wurtzite structure of the products, while both SEM and TEM images show correspondingly a typical hexagonal morphology of the nanocrystals. EDX reveals that the wurtzite nanoplates have a composition well close to CuInS2. While MEA plays key roles during the reaction, the reaction time also has important influence on the formation of CuInS2 nanoplates.
171
Acknowledgments This investigation was supported by the NSFC (Nos. 50972041, 51102085, and 61274010), Program for New Century Excellent Talents in University (NCET-09-0135), Research Fund for the Doctoral Program of Higher Education of China (20124208110005), Scientific Research Foundation for Returned Scholars, Ministry of Education of China, NSF of Hubei Province (Nos. 2011CDB057, 2011CDA81), Science Foundation from Hubei Provincial Department of Education (No. Q20111002), and Wuhan Municipal Academic Leaders Program (200951830550).
References [1] He YB, Polity A, Alves HR, Oesterreicher I, Kriegseis W, Pfisterer D, et al. Structural and optical characterization of RF reactively sputtered CuInS2 thin films. Thin Solid Films 2002;403–404:62–5. [2] Binsma JJM, Giling LJ, Bloem J. Phase relations in the system Cu2S–In2S3. J Cryst Growth 1980;50:429–36. [3] Abrahams SC, Berbstein JL. Piezoeletric nonlinear optic CuGaS2 and CuInS2 II IV V crystal structure: sublattice distortion in AIBIIICVI 2 and A B C2 type chalcopyrites. J Chem Phys 1973;59:5415–22. [4] Nose K, Soma Y, Omata T, Matsuo S, Otsuka Yao. Chem Mater 2009;21: 2607–13. [5] Batabyal SK, Tian L, Venkatram N, Ji W, Vittal JJ. Phase-selective synthesis of CuInS2 nanocrystals. J Phys Chem C 2009;113:15037–42. [6] Huang WC, Tseng CH, Chang SH, Tuan HY, Chiang CC, Lyu LM, et al. Solvothermal synthesis of Zincblende and Wurtzite CuInS2 nanocrystals and their photovoltaic application. Langmuir 2012;28:8496–501. [7] Guo J, Zhou WH, Li M, Hou ZL, Jiao J, Zhou ZJ, Wu SX. Synthesis of bullet-like wurtzite CuInS2 nanocrystals under atmospheric conditions. J Cryst Growth 2012;359:72–6. [8] Pan DC, An LJ, Sun ZM, Hou W, Yang Y, Yang ZZ, et al. Synthesis of Cu–In–S ternary nanocrystals with tunable structure and composition. J Am Chem Soc 2008;130:5620–1. [9] Xie RG, Rutherford M, Peng XG. Formation of high-quality I–III–VI semiconductor nanocrystals by tuning relative reactivity of cationic precursors. J Am Chem Soc 2009;131:5691–7. [10] Koo B, Patel RN, Korgel BA. Wurtzite–chalcopyrite polytypism in CuInS2 nanodisks. Chem Mater 2009;21:1962–6. [11] Connor Stephen T, Hsu CM, Weil BD, Aloni S, Cui Y. Phase transformation of biphasic Cu2S–CuInS2 to monophasic CuInS2 nanorods. J Am Chem Soc 2009;131:4962–6. [12] Sheng X, Wang L, Luo YP, Yang DR. Synthesis of hexagonal structured wurtzite and chalcopyrite CuInS2 via a simple solution route. Nanoscale Res Lett 2011;6:562. [13] Qi YX, Liu QC, Tang KB, Liang ZH, Ren ZB, Liu XM. Synthesis and characterization of nanostructured wurtzite CuInS2: a new cation disordered polymorph of CuInS2. J Phys Chem C 2009;113:3939–44.