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Synthesis and reaction pathway investigation of chalcopyrite CuInSe2 nanoparticles for one-pot method
Superlattices and Microstructures 62 (2013) 156–165
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Synthesis and reaction pathway investigation of chalcopyrite CuInSe2 nanoparticles for one-pot method Zhaoxia Han a,b, Dawei Zhang a,c,⇑, Daohua Zhang c, Ruijin Hong a, Qinmiao Chen a, Chunxian Tao a, Yuanshen Huang a, Zhengji Ni a, Songlin Zhuang a a Engineering Research Center of Optical Instrument and System, Ministry of Education and Shanghai Key Lab of Modern Optical System, University of Shanghai for Science and Technology, No. 516 JunGong Road, Shanghai 200093, China b Physics and Engineering School, Henan University of Science and Technology, Luoyang 471003, China c School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
a r t i c l e
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Article history: Received 28 May 2013 Accepted 23 July 2013 Available online 6 August 2013 Keywords: CuInSe2 Nanoparticle Reaction pathway Chalcopyrite One-pot method
a b s t r a c t Highly dispersed and near stoichiometric chalcopyrite CuInSe2 nanoparticles were successfully synthesized via a facile and rapid one-pot method. For understanding the reaction pathway, the solid intermediates obtained at different stages of CuInSe2 nanoparticles synthesis process were investigated in detail by powder X-ray diffraction (XRD), X-ray photoelectron spectrometer (XPS), energy dispersive spectroscopy (EDS) and Raman spectroscopy. The XRD patterns showed that the phase formation sequence was CuSe ? CuInSe2. The XPS results indicated that the valences of Cu and Se in CuSe were +1 and 1, respectively. The chemical composition of the solid intermediates revealed the presence of the solidstate In–Se secondary phases in the synthesis process. However, no XRD signals or Raman signals of the solid-state In–Se secondary phase were observed. Based on the experimental results, the possible reaction pathway was proposed. Ó 2013 Elsevier Ltd. All rights reserved.
⇑ Corresponding author at: Engineering Research Center of Optical Instrument and System, Ministry of Education and Shanghai Key Lab of Modern Optical System, University of Shanghai for Science and Technology, No. 516 JunGong Road, Shanghai 200093, China. Tel./fax: +86 21 55276078. E-mail address: [email protected] (D. Zhang). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.07.023
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1. Introduction The I–III–VI chalcopyrite CuInSe2 (CISe) and its related alloys Cu(InxGa1 x)(S, Se)2 (CIGS) materials continue to receive attention for applications in thin film solar cell because of their high absorption coefficient (105 cm 1), good photostability and demonstrated high solar cell conversion efficiency of 20.3% [1–3]. However, the absorber layer of high efficiency solar cell is usually prepared by vacuum processes, which have many shortcomings such as high production cost, complex process and difficulty in scaling up production [4,5]. In recent years, nanoparticle-based printing technology has been developed and gradually attracted the attention of researchers because of its important advantages, e.g., low processing equipment cost, well-controlled stoichiometry, high material utilization efficiency (close to 100%) and compatibility with roll-to-roll production process. For the nanoparticle-based printing technology, one of the key steps is the synthesis of ordered chalcopyrite and near stoichiometric CISe nanoparticles to have a good photovoltaic conversion. Generally, large particles and agglomerates will lead to high porosity, which hinder the formation of dense CISe layer and degrade the photovoltaic properties of the solar cell. Therefore, the as-synthesized CISe nanoparticles should also have good dispersibility to obtain a good photovoltaic conversion. Several techniques have been reported for the synthesis of CISe and related ternary and quaternary nanoparticles, such as solvothermal [6], thermal decomposition [7], hot injection [8,9], ball milling [10], and polyol reflux method [11]. Among the several methods, the rapid hot injection method is the most successful and widely used technology in the nanoparticles synthesis. The nanoparticles synthesized by hot injection method are small, uniform and dispersed. However, it is difficult to mass production because the injection rate of monomers and mass transfer are limited in large quantities [12]. To overcome these difficulties, a one-pot strategy, in which the reagents and solvent are mixed at room temperature and heated up to a certain temperature to initiate the crystallization reaction, has been used to synthesize highly dispersed nanoparticles. The one-pot solution process has been used for the synthesis of many binary sulfides such as CdSe and CdS [13]. However, this mild and facile method has rarely been applied in the synthesis of ternary or quaternary chalcogenide nanoparticles. The growth process of CISe nanoparticles for one-pot method is a complex process, which involves a number of chemical and physical events. Moreover, phase transformation accompanies the crystal growth. An in-depth understanding of the reaction pathway can improve nanocrystal growth and discover new synthesis routes. The reaction pathways of CISe nanoparticles have been reported in the literatures for solvothermal method [6] and hot injection method [9], but there is little analytical data of solid intermediates to support the proposed reaction pathway. To the best of our knowledge, the reaction pathway of chalcopyrite CISe nanoparticles synthesized by one-pot method has not yet been reported. Here we present the XRD, XPS, Raman spectroscopy and EDS results of solid intermediates to reveal the possible reaction pathway. In this paper, highly dispersed and near stoichiometric chalcopyrite CISe nanoparticles were successfully synthesized via a facile and rapid one-pot method. Moreover, the solid intermediates obtained at different stages of CISe nanoparticles synthesis process by one-pot method were investigated in detail using XRD, XPS, Raman spectroscopy and EDS. Based on the experimental results, the possible reaction pathway was proposed.
2. Experimental 2.1. Materials Copper (II) chloride dihydrate (CuCl22H2O, AR), indium (III) chloride tetrahydrate (InCl34H2O, 99.99%), selenium powder (Se, 99.95%), hexane (AR), ethanol (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Oleylamine (OLA, 80–90%) was the product of Aladdin. All reagents were used as received without further purification.
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2.2. Synthesis of CISe nanoparticles In a typical experimental procedure, 1 mmol copper (II) chloride dihydrate, 1 mmol indium (III) chloride tetrahydrate and 10 ml OLA were added to a 50 ml three-neck ground bottom flask and magnetic stirred at 60 °C for 3 h. Then, the solution was cooled down to room temperature and the selenium suspension in OLA (2 mmol selenium powders dissolved in 10 ml OLA) was quickly added into the three-neck flask and mixed. The mixture solution was heated to 120 °C and purged with N2 gas for three times. The temperature of the mixture solution was slowly raised to 265 °C and kept for 1 h to let the particles grow. After the reaction finished, the mixture solution was cooled down to 60 °C. 10 ml non-polar hexane and 10 ml polar ethanol were successively added into the flask to disperse and flocculate the nanoparticles. The obtained mixture was centrifuged at 8000 rpm for 10 min without sizeselective precipitation and then washed with ethanol for several times to remove the excess organic solvent and by-products. Finally, the dark precipitate was collected and dried at 60 °C for several hours for the characterizations. For the solid intermediates experiments, the reactions were stopped at different stages of reaction process and quickly cooled down to room temperature. The obtained mixture was treated via the same method as above. 2.3. Materials’ characterization The crystalline phases were investigated by XRD (D8 ADVANCE, Bruker) using Cu ka radiation. The morphology and detailed microstructure were observed by TEM (Tecnai F30, FEI). The element composition was analyzed by EDS (Genesis Apollo X, AMETEK). The optical absorption spectrum was measured with an UV–Vis–IR double beam spectrophotometer (Lambda 1050, Perkin–Elmer). The valences of the product were examined by XPS (K-Alpha, Thermo Scientific). The Raman spectra were measured using the dispersive Raman spectroscopy (Raman Station 400/400F, Perkin–Elmer), the excitation source was the 785 nm line of semiconductor laser. All the measurements were carried out at room temperature. 3. Results and discussion 3.1. Characterization of the as-synthesized CISe nanoparticles The XRD pattern of the CISe nanoparticles synthesized at 265 °C for 1 h by one-pot method is shown in Fig. 1. The diffraction peaks are narrow and strong, indicating good crystallinity. The major diffraction peaks, which match very well with the reference JCPD data (PDF card # 40-1487) for chal-
Fig. 1. XRD pattern of the CISe nanoparticles synthesized at 265 °C for 1 h by one-pot method.
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copyrite CISe, are indexed to the (1 1 2), (2 0 4)/(2 2 0), (1 1 6)/(3 1 2), (0 0 8)/(4 0 0), (3 1 6)/(3 3 2), (2 2 8)/ (4 2 4) and (3 3 6)/(5 1 2), respectively. In addition to these major diffraction peaks, the minor peaks such as (1 0 1), (1 0 3), (2 1 1), (1 0 5) and (3 0 1) are also observed in the XRD pattern, which distinguish the chalcopyrite phase from the sphalerite phase [8]. The presence of these peaks indicated that the as-synthesized CISe nanoparticles had perfect chalcopyrite structure. It was found that adding the Se suspension at low temperature (from room temperature to 130 °C) and controlling the temperature rise slowly could easily form the ordered chalcopyrite phase. However, disordered sphalerite phase was formed either adding the Se suspension at higher temperature (over 200 °C) or adding the Se solution. The reasons may be that adding the Se suspension at low temperature can prevent large supersaturation (Se powder is difficult to dissolve in OLA at low temperature) and allow the atoms to form the thermodynamically favored chalcopyrite phase. However, either adding the Se suspension at higher temperature or adding the Se solution can lead to large supersaturation and tend to form sphalerite phase caused by disordering of the cation lattice. The average grain size of the as-synthesized CISe nanoparticles estimated from full width at half maximum (FWHM) of the (1 1 2) peak by the Scherrer formula was about 66 nm. Fig. 2 shows the large area TEM image, high resolution transmission electron microscopy (HRTEM) image and selected area electron diffraction (SAED) pattern of the as-synthesized CISe nanoparticles.
Fig. 2. (a) Large area TEM image, (b) HRTEM image, (c) SAED pattern of the as-synthesized CISe nanoparticles, and (d) TEM image of large-sized CISe nanocrystal, the insets show the corresponding HRTEM image and SAED pattern.
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According to Fig. 2a, the as-synthesized CISe nanoparticles are highly dispersed with the particle size in the range of 50–300 nm. The nanoparticles are comprised of two morphology types: large-sized nanocrystallites and nanoparticles comprised of several small-sized nanocrystallites. Therefore, the average particle size is larger than that of estimated from XRD data. The polydispersity is consistent with the nature of the reaction. Since all reagents were added at low temperature, followed by a slowly increase to the final growth temperature, the nucleation and growth of the nanoparticles were not expected to be uniform in both size and shape. The HRTEM image shown in Fig. 2b clearly indicates a lattice fringes with a spacing of 0.1746 nm, which was consistent with the planar spacing of tetragonal chalcopyrite CISe (3 1 2). There are twins in the as-synthesized CISe nanocrystals, the twin defects may stem from stacking faults [14]. Fig. 2c shows the SAED pattern. The maxima of the SAED radial intensity profile match very well with the XRD measurements and the reference data for tetragonal CISe, corresponding to the (1 1 2), (2 2 0) and (3 1 2) reflection direction. The presence of ring diffraction pattern reveals the polycrystalline nature of the as-synthesized CISe nanoparticles. Fig. 2d shows the TEM image of large-sized CISe nanocrystal. The left inset shows the corresponding HRTEM image, the fringes (spacing 0.2813 nm) correspond to the plane of tetragonal chalcopyrite CISe (2 0 0). The right inset shows the corresponding SAED pattern, confirming good crystallinity of CISe nanocrystals. The crystal face is along the (1 0 0) direction. The size of the as-synthesized CISe nanoparticles in this experiment was larger than that of reported by Guo using hot-injection method [8]. This is mainly because that adding the reagents at lower room temperature and controlling the temperature rise slowly prevent large supersaturation. Moreover, the electron affinity of selenium is lower, which results in a lower driving force for the reduction of selenium to the reactive selenide. Therefore, the monomer (metal ions, selenide anions) concentration is lower than that of hot-injection in the initial reaction solution. According to the kinetic size control model reported by Yin and Livisatos [15], the critical size of nanoparticles (at which the nanoparticles neither grow nor shrink, over which the nanoparticles grow and below which the nanoparticles redissolve) is higher when the monomer concentration is lower, the diameter of a smaller number of particles is above the critical size, which grow until all the monomers are consumed. Thus, the average particle size is larger in this experiment. Table 1 Chemical composition of the as-synthesized CISe nanoparticles. Element
Wt%
At%
Cu In Se
19.5 32.4 48.1
25.6 23.6 50.8
Fig. 3. Vis–IR absorption spectrum of the as-synthesized CISe nanoparticles.
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The chemical composition of the as-synthesized CISe nanoparticles was analyzed by EDS. The EDS data in Table 1 shows that the atom percentages in the sample were 25.6% (Cu), 23.6% (In) and 50.8% (Se), which were close to the stoichiometry. Fig. 3 shows the Vis–IR absorption spectrum of the as-synthesized CISe nanoparticles. Using a simple band gap value (Eg) measurement method of Eg = h C/k (h is Planck’s constant, C is the speed of light and k is the absorption cutoff wavelength that can be obtained from the absorption spectrum) [16], the calculated band gap value of the sample is 0.96 eV, which is agreement with the reported value of 0.88–1.02 eV [17]. The sample exhibits strong absorption from the entire visible light region to the near-infrared region. Therefore, the as-synthesized CISe nanoparticles are good light absorption layer material for thin film solar cell. Based on the above analysis, it is found that highly dispersed and near stoichiometric chalcopyrite CISe nanoparticles can be successfully synthesized by a facile and rapid one-pot method.
3.2. Reaction pathway of chalcopyrite CISe nanoparticles To understand the reaction pathway of chalcopyrite CISe nanoparticles synthesized by one-pot method, the solid intermediates obtained at different stages of synthesis process were investigated in detail by XRD, XPS, Raman spectroscopy and EDS. Fig. 4 shows the XRD patterns of the solid intermediates obtained at different stages of CISe nanoparticles synthesis process. According to Fig. 4, only the diffraction peaks of CuSe are observed when the mixture solution was heated to 130 °C and kept for 10 min. When the mixture solution was heated to 180 °C, the diffraction peaks of CuSe and CISe coexist. When the mixture temperature was raised to 246 °C, all diffraction peaks are attributed to CISe. With the further increase of the reaction temperature, the intensities of the diffraction peaks increase. The XRD patterns showed that the phase formation sequence was CuSe ? CISe, which was different from that of other synthesis methods such as solvothermal. The diffraction peaks of solid-state In–Se secondary phases were not observed as expected. Some similar phenomena have been reported [18,19], however, no in-depth explanation and discussion were found in those papers. To investigate the valences of Cu and Se in CuSe, the product obtained at 130 °C for 10 min was examined by XPS. Fig. 5 shows the XPS survey spectrum, Cu 2p, Se 3d, and In 3d core-level spectrum, respectively. The XPS survey spectrum shown in Fig. 5a identifies the presence of Cu and Se, and In is also present. The Cu 2p core-level spectrum is illustrated in Fig. 5b. Two intense peaks centered at 932 eV and 951.7 eV, corresponding to Cu 2p3/2 and Cu 2p1/2, are observed. The Cu 2p3/2 satellite peak characterizing Cu2+, which was usually centered at 942 eV [20], is not observed in the present Cu 2p core-level spectrum. Therefore, the chemical valence of copper in CuSe was +1 (Cu+), indicating that the Cu2+ ions of the starting material were reduced by OLA. For Se (-II) core-level spectrum, the
Fig. 4. XRD patterns of the solid intermediates obtained at different stages of CISe nanoparticles synthesis process.
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Fig. 5. (a) XPS survey spectrum, (b) Cu 2p, (c) Se 3d, and (d) In 3d core-level spectrum of the product obtained at 130 °C for 10 min.
branching ratio 3d5/2:3d3/2 equaled to 1.5, the distance between the 3d5/2 and 3d3/2 was about 0.85 eV [21]. However, the Se 3d5/2 and Se 3d3/2 double peak have not been observed in our Se 3d core-level spectrum (shown in Fig. 5c). The Se 3d core-level spectrum is similar to previous literature report for Cu+Se [22]. Thus, the chemical valence of selenium in the product obtained at 130 °C for 10 min should be 1 (Se1 ), which may be due to the weak reduction ability of OLA for Se powders at a lower reaction temperature. Fig. 5d shows the In 3d core-level spectrum. The peaks centered at binding energy of 444.6 eV and 452 eV coincide well with In 3d5/2 and In 3d3/2. The In 3d core-level spectrum is very similar to those reported for CISe [23]. Therefore, the chemical valence of indium in the product obtained at 130 °C for 10 min was +3 (In3+). Based on the XPS analysis, it can be concluded that the valences of Cu and Se in CuSe were +1 and 1, respectively. In addition to CuSe, one or more In-containing species were present in the sample. However, the nature of the In-containing species was unknown. The In-containing species must be amorphous, because no XRD signals from product were observed. Raman scattering for amorphous In1 xSex alloys [24] has been reported. In order to identify the phase of amorphous In–Se compound, the solid intermediates obtained at different stages of CISe nanoparticles synthesis process were analyzed by Raman spectroscopy. Fig. 6 shows the Raman spectra of the solid intermediates. According to Fig. 6, a major intense peak at 236 cm 1 and a broad peak at 260 cm 1 are observed when the mixture solution was heated to 130 °C and kept for 10 min. The two Raman peaks were also attributed to CuSe [25]. When the mixture solution was heated to 180 °C, two broad peaks at 236 cm 1 and 173 cm 1 are observed, which were assigned to CuSe and CISe, respectively. When the mixture temperature was raised to 246 °C, only the Raman peak of CISe is observed. With the further increase of the reaction temperature, the intensity of the CISe Raman
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Fig. 6. Raman spectra of the solid intermediates obtained at different stages of CISe nanoparticles synthesis process.
peak increases. The Raman spectra indicated that the formation pathway of CISe nanoparticles synthesized by one-pot method was CuSe ? CISe, which was consistent with the XRD results. No Raman signals from solid-state In–Se secondary phases were detected. The chemical composition of the solid intermediates obtained at different stages of CISe nanoparticles synthesis process were also analyzed by EDS, as shown in Table 2. The EDS results reveal the presence of solid-state In–Se secondary phases in early solid intermediates, which was consistent with the XPS results. However, no XRD signals or Raman signals of solid-state In–Se secondary phases were observed. This may be because that the solid-state In–Se secondary phases were composed of amorphous clusters, which lacked molecular characteristics and resulted in the absence of XRD signals and Raman signals. Based on the XRD, XPS, Raman and EDS results, the crystalline phase and possible material compositions of the solid intermediates obtained at different stages of CISe nanoparticles synthesis process were given in Table 3. Since the reducing ability of OLA highly depended on the reaction temperature, the formation process of CISe nanoparticles for one-pot method was a complex process, which involved a number of chemical and physical events (e.g., formation of nuclei, formation of amorphous clusters with a magic number of atoms, reactions between the solid intermediates, crystallization and growth of nanoparticles.). Based on the experimental results, the possible formation process was as follows. OLA acted as both high boiling point solvent and reducing agent during the reaction. Firstly, Cu2+ ions were reduced to Cu+ ions, Se powders dissolved partially and were reduced to Se ions by OLA at a lower reaction temperature. The Se species (Se0, Se , Se2 and Se2n ) depend on the reaction temperature and the reduction ability of the solvent. Meanwhile, OLA also behaved as a complex ligand and formed stable complexes with CuCl2, InCl3 and Se powders. Only OLA–Cu complex was decomposed and released monomers containing Cu+ ions at lower reaction temperature for a shorter period because the reactivity of OLA–Cu complex was much higher than that of OLA–In complex [26]. Then, the monomers containing Cu+ ions reacted with Se ions to form CuSe nanocrystallites. With the reaction temperature increase, the OLA–In complex was decomposed and released monomers containing In3+ ions. The
Table 2 Chemical composition of the solid intermediates obtained at different stages of CISe nanoparticles synthesis process. Reaction temperature (°C)
Reaction time (min)
Mole ratio (Cu:In:Se)
130 180 246 265
10 0 0 10
26.9:11.3:61.8 21.6:18.1:60.3 15.4:21.7:62.9 24.7:23.6:51.7
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Table 3 Crystalline phase and possible material compositions of the solid intermediates obtained at different stages of CISe nanoparticles synthesis process. Reaction temperature (°C)
Reaction time (min)
Crystalline phase
Possible material compositions
130 180 246 265
10 0 0 10
CuSe CuSe and CISe CISe CISe
CuSe and InSe3 CuSe, Se-rich CISe and InSex Se-rich CISe CISe
Fig. 7. Diagram of the reaction pathway of CISe nanoparticles synthesized by one-pot method.
monomers containing In3+ ions reacted with Se ions to form amorphous InSe3 clusters. With the further increase of reaction temperature, Se ions were further reduced to Se2n ions by OLA, amorphous InSe3 clusters were converted into InSex (1.5 < x < 3) clusters. As the reaction proceeded, the CuSe nanocrystallites reacted with InSex clusters to form Se-rich CISe phase. With the reaction temperature increase and the reaction proceeding, the Se2n ions were reduced to Se2 ions, the composition of the Se-rich CISe nanoparticles became close to stoichiometry by interdiffusion of Cu, In and Se. The diagram of the reaction pathway of CISe nanoparticles synthesized by one-pot method was shown in Fig. 7. The reaction pathway was different from that reported in the literatures [6,9], which may be because that the solvent or the order of precursor addition were different.
4. Conclusion Highly dispersed and near stoichiometric chalcopyrite CISe nanoparticles were successfully synthesized via a facile and rapid one-pot method. The phase formation sequence of CISe nanoparticles synthesized by one-pot method was CuSe ? CISe, which was different from that of other synthesis methods such as solvothermal and hot injection. As the reaction proceeded, the valence of Se in solid intermediates was from 1 to 2 because the reducing ability of OLA highly depended on the reaction temperature. The solid-state In–Se secondary phases existed in early solid intermediates. However, no XRD signals and Raman signals of solid-state In–Se secondary phases were observed as expected. Study of the nature of In–Se secondary phases is underway. Based on the experimental results, the possible reaction pathway of chalcopyrite CISe nanoparticles synthesized by one-pot method was proposed.
Acknowledgments This work was partially supported by National Natural Science Foundation of China (61176085, 11105149), National Science Instrument Important Project (2011YQ15004003, and 2011YQ14014704), Dawn Program of Shanghai Education Commission (11SG44), Programs
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Synthesis and reaction pathway investigation of chalcopyrite CuInSe2 nanoparticles for one-pot method Zhaoxia Han a,b, Dawei Zhang a,c,⇑, Daohua Zhang c, Ruijin Hong a, Qinmiao Chen a, Chunxian Tao a, Yuanshen Huang a, Zhengji Ni a, Songlin Zhuang a a Engineering Research Center of Optical Instrument and System, Ministry of Education and Shanghai Key Lab of Modern Optical System, University of Shanghai for Science and Technology, No. 516 JunGong Road, Shanghai 200093, China b Physics and Engineering School, Henan University of Science and Technology, Luoyang 471003, China c School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
a r t i c l e
i n f o
Article history: Received 28 May 2013 Accepted 23 July 2013 Available online 6 August 2013 Keywords: CuInSe2 Nanoparticle Reaction pathway Chalcopyrite One-pot method
a b s t r a c t Highly dispersed and near stoichiometric chalcopyrite CuInSe2 nanoparticles were successfully synthesized via a facile and rapid one-pot method. For understanding the reaction pathway, the solid intermediates obtained at different stages of CuInSe2 nanoparticles synthesis process were investigated in detail by powder X-ray diffraction (XRD), X-ray photoelectron spectrometer (XPS), energy dispersive spectroscopy (EDS) and Raman spectroscopy. The XRD patterns showed that the phase formation sequence was CuSe ? CuInSe2. The XPS results indicated that the valences of Cu and Se in CuSe were +1 and 1, respectively. The chemical composition of the solid intermediates revealed the presence of the solidstate In–Se secondary phases in the synthesis process. However, no XRD signals or Raman signals of the solid-state In–Se secondary phase were observed. Based on the experimental results, the possible reaction pathway was proposed. Ó 2013 Elsevier Ltd. All rights reserved.
⇑ Corresponding author at: Engineering Research Center of Optical Instrument and System, Ministry of Education and Shanghai Key Lab of Modern Optical System, University of Shanghai for Science and Technology, No. 516 JunGong Road, Shanghai 200093, China. Tel./fax: +86 21 55276078. E-mail address: [email protected] (D. Zhang). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.07.023
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1. Introduction The I–III–VI chalcopyrite CuInSe2 (CISe) and its related alloys Cu(InxGa1 x)(S, Se)2 (CIGS) materials continue to receive attention for applications in thin film solar cell because of their high absorption coefficient (105 cm 1), good photostability and demonstrated high solar cell conversion efficiency of 20.3% [1–3]. However, the absorber layer of high efficiency solar cell is usually prepared by vacuum processes, which have many shortcomings such as high production cost, complex process and difficulty in scaling up production [4,5]. In recent years, nanoparticle-based printing technology has been developed and gradually attracted the attention of researchers because of its important advantages, e.g., low processing equipment cost, well-controlled stoichiometry, high material utilization efficiency (close to 100%) and compatibility with roll-to-roll production process. For the nanoparticle-based printing technology, one of the key steps is the synthesis of ordered chalcopyrite and near stoichiometric CISe nanoparticles to have a good photovoltaic conversion. Generally, large particles and agglomerates will lead to high porosity, which hinder the formation of dense CISe layer and degrade the photovoltaic properties of the solar cell. Therefore, the as-synthesized CISe nanoparticles should also have good dispersibility to obtain a good photovoltaic conversion. Several techniques have been reported for the synthesis of CISe and related ternary and quaternary nanoparticles, such as solvothermal [6], thermal decomposition [7], hot injection [8,9], ball milling [10], and polyol reflux method [11]. Among the several methods, the rapid hot injection method is the most successful and widely used technology in the nanoparticles synthesis. The nanoparticles synthesized by hot injection method are small, uniform and dispersed. However, it is difficult to mass production because the injection rate of monomers and mass transfer are limited in large quantities [12]. To overcome these difficulties, a one-pot strategy, in which the reagents and solvent are mixed at room temperature and heated up to a certain temperature to initiate the crystallization reaction, has been used to synthesize highly dispersed nanoparticles. The one-pot solution process has been used for the synthesis of many binary sulfides such as CdSe and CdS [13]. However, this mild and facile method has rarely been applied in the synthesis of ternary or quaternary chalcogenide nanoparticles. The growth process of CISe nanoparticles for one-pot method is a complex process, which involves a number of chemical and physical events. Moreover, phase transformation accompanies the crystal growth. An in-depth understanding of the reaction pathway can improve nanocrystal growth and discover new synthesis routes. The reaction pathways of CISe nanoparticles have been reported in the literatures for solvothermal method [6] and hot injection method [9], but there is little analytical data of solid intermediates to support the proposed reaction pathway. To the best of our knowledge, the reaction pathway of chalcopyrite CISe nanoparticles synthesized by one-pot method has not yet been reported. Here we present the XRD, XPS, Raman spectroscopy and EDS results of solid intermediates to reveal the possible reaction pathway. In this paper, highly dispersed and near stoichiometric chalcopyrite CISe nanoparticles were successfully synthesized via a facile and rapid one-pot method. Moreover, the solid intermediates obtained at different stages of CISe nanoparticles synthesis process by one-pot method were investigated in detail using XRD, XPS, Raman spectroscopy and EDS. Based on the experimental results, the possible reaction pathway was proposed.
2. Experimental 2.1. Materials Copper (II) chloride dihydrate (CuCl22H2O, AR), indium (III) chloride tetrahydrate (InCl34H2O, 99.99%), selenium powder (Se, 99.95%), hexane (AR), ethanol (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Oleylamine (OLA, 80–90%) was the product of Aladdin. All reagents were used as received without further purification.
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2.2. Synthesis of CISe nanoparticles In a typical experimental procedure, 1 mmol copper (II) chloride dihydrate, 1 mmol indium (III) chloride tetrahydrate and 10 ml OLA were added to a 50 ml three-neck ground bottom flask and magnetic stirred at 60 °C for 3 h. Then, the solution was cooled down to room temperature and the selenium suspension in OLA (2 mmol selenium powders dissolved in 10 ml OLA) was quickly added into the three-neck flask and mixed. The mixture solution was heated to 120 °C and purged with N2 gas for three times. The temperature of the mixture solution was slowly raised to 265 °C and kept for 1 h to let the particles grow. After the reaction finished, the mixture solution was cooled down to 60 °C. 10 ml non-polar hexane and 10 ml polar ethanol were successively added into the flask to disperse and flocculate the nanoparticles. The obtained mixture was centrifuged at 8000 rpm for 10 min without sizeselective precipitation and then washed with ethanol for several times to remove the excess organic solvent and by-products. Finally, the dark precipitate was collected and dried at 60 °C for several hours for the characterizations. For the solid intermediates experiments, the reactions were stopped at different stages of reaction process and quickly cooled down to room temperature. The obtained mixture was treated via the same method as above. 2.3. Materials’ characterization The crystalline phases were investigated by XRD (D8 ADVANCE, Bruker) using Cu ka radiation. The morphology and detailed microstructure were observed by TEM (Tecnai F30, FEI). The element composition was analyzed by EDS (Genesis Apollo X, AMETEK). The optical absorption spectrum was measured with an UV–Vis–IR double beam spectrophotometer (Lambda 1050, Perkin–Elmer). The valences of the product were examined by XPS (K-Alpha, Thermo Scientific). The Raman spectra were measured using the dispersive Raman spectroscopy (Raman Station 400/400F, Perkin–Elmer), the excitation source was the 785 nm line of semiconductor laser. All the measurements were carried out at room temperature. 3. Results and discussion 3.1. Characterization of the as-synthesized CISe nanoparticles The XRD pattern of the CISe nanoparticles synthesized at 265 °C for 1 h by one-pot method is shown in Fig. 1. The diffraction peaks are narrow and strong, indicating good crystallinity. The major diffraction peaks, which match very well with the reference JCPD data (PDF card # 40-1487) for chal-
Fig. 1. XRD pattern of the CISe nanoparticles synthesized at 265 °C for 1 h by one-pot method.
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copyrite CISe, are indexed to the (1 1 2), (2 0 4)/(2 2 0), (1 1 6)/(3 1 2), (0 0 8)/(4 0 0), (3 1 6)/(3 3 2), (2 2 8)/ (4 2 4) and (3 3 6)/(5 1 2), respectively. In addition to these major diffraction peaks, the minor peaks such as (1 0 1), (1 0 3), (2 1 1), (1 0 5) and (3 0 1) are also observed in the XRD pattern, which distinguish the chalcopyrite phase from the sphalerite phase [8]. The presence of these peaks indicated that the as-synthesized CISe nanoparticles had perfect chalcopyrite structure. It was found that adding the Se suspension at low temperature (from room temperature to 130 °C) and controlling the temperature rise slowly could easily form the ordered chalcopyrite phase. However, disordered sphalerite phase was formed either adding the Se suspension at higher temperature (over 200 °C) or adding the Se solution. The reasons may be that adding the Se suspension at low temperature can prevent large supersaturation (Se powder is difficult to dissolve in OLA at low temperature) and allow the atoms to form the thermodynamically favored chalcopyrite phase. However, either adding the Se suspension at higher temperature or adding the Se solution can lead to large supersaturation and tend to form sphalerite phase caused by disordering of the cation lattice. The average grain size of the as-synthesized CISe nanoparticles estimated from full width at half maximum (FWHM) of the (1 1 2) peak by the Scherrer formula was about 66 nm. Fig. 2 shows the large area TEM image, high resolution transmission electron microscopy (HRTEM) image and selected area electron diffraction (SAED) pattern of the as-synthesized CISe nanoparticles.
Fig. 2. (a) Large area TEM image, (b) HRTEM image, (c) SAED pattern of the as-synthesized CISe nanoparticles, and (d) TEM image of large-sized CISe nanocrystal, the insets show the corresponding HRTEM image and SAED pattern.
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According to Fig. 2a, the as-synthesized CISe nanoparticles are highly dispersed with the particle size in the range of 50–300 nm. The nanoparticles are comprised of two morphology types: large-sized nanocrystallites and nanoparticles comprised of several small-sized nanocrystallites. Therefore, the average particle size is larger than that of estimated from XRD data. The polydispersity is consistent with the nature of the reaction. Since all reagents were added at low temperature, followed by a slowly increase to the final growth temperature, the nucleation and growth of the nanoparticles were not expected to be uniform in both size and shape. The HRTEM image shown in Fig. 2b clearly indicates a lattice fringes with a spacing of 0.1746 nm, which was consistent with the planar spacing of tetragonal chalcopyrite CISe (3 1 2). There are twins in the as-synthesized CISe nanocrystals, the twin defects may stem from stacking faults [14]. Fig. 2c shows the SAED pattern. The maxima of the SAED radial intensity profile match very well with the XRD measurements and the reference data for tetragonal CISe, corresponding to the (1 1 2), (2 2 0) and (3 1 2) reflection direction. The presence of ring diffraction pattern reveals the polycrystalline nature of the as-synthesized CISe nanoparticles. Fig. 2d shows the TEM image of large-sized CISe nanocrystal. The left inset shows the corresponding HRTEM image, the fringes (spacing 0.2813 nm) correspond to the plane of tetragonal chalcopyrite CISe (2 0 0). The right inset shows the corresponding SAED pattern, confirming good crystallinity of CISe nanocrystals. The crystal face is along the (1 0 0) direction. The size of the as-synthesized CISe nanoparticles in this experiment was larger than that of reported by Guo using hot-injection method [8]. This is mainly because that adding the reagents at lower room temperature and controlling the temperature rise slowly prevent large supersaturation. Moreover, the electron affinity of selenium is lower, which results in a lower driving force for the reduction of selenium to the reactive selenide. Therefore, the monomer (metal ions, selenide anions) concentration is lower than that of hot-injection in the initial reaction solution. According to the kinetic size control model reported by Yin and Livisatos [15], the critical size of nanoparticles (at which the nanoparticles neither grow nor shrink, over which the nanoparticles grow and below which the nanoparticles redissolve) is higher when the monomer concentration is lower, the diameter of a smaller number of particles is above the critical size, which grow until all the monomers are consumed. Thus, the average particle size is larger in this experiment. Table 1 Chemical composition of the as-synthesized CISe nanoparticles. Element
Wt%
At%
Cu In Se
19.5 32.4 48.1
25.6 23.6 50.8
Fig. 3. Vis–IR absorption spectrum of the as-synthesized CISe nanoparticles.
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The chemical composition of the as-synthesized CISe nanoparticles was analyzed by EDS. The EDS data in Table 1 shows that the atom percentages in the sample were 25.6% (Cu), 23.6% (In) and 50.8% (Se), which were close to the stoichiometry. Fig. 3 shows the Vis–IR absorption spectrum of the as-synthesized CISe nanoparticles. Using a simple band gap value (Eg) measurement method of Eg = h C/k (h is Planck’s constant, C is the speed of light and k is the absorption cutoff wavelength that can be obtained from the absorption spectrum) [16], the calculated band gap value of the sample is 0.96 eV, which is agreement with the reported value of 0.88–1.02 eV [17]. The sample exhibits strong absorption from the entire visible light region to the near-infrared region. Therefore, the as-synthesized CISe nanoparticles are good light absorption layer material for thin film solar cell. Based on the above analysis, it is found that highly dispersed and near stoichiometric chalcopyrite CISe nanoparticles can be successfully synthesized by a facile and rapid one-pot method.
3.2. Reaction pathway of chalcopyrite CISe nanoparticles To understand the reaction pathway of chalcopyrite CISe nanoparticles synthesized by one-pot method, the solid intermediates obtained at different stages of synthesis process were investigated in detail by XRD, XPS, Raman spectroscopy and EDS. Fig. 4 shows the XRD patterns of the solid intermediates obtained at different stages of CISe nanoparticles synthesis process. According to Fig. 4, only the diffraction peaks of CuSe are observed when the mixture solution was heated to 130 °C and kept for 10 min. When the mixture solution was heated to 180 °C, the diffraction peaks of CuSe and CISe coexist. When the mixture temperature was raised to 246 °C, all diffraction peaks are attributed to CISe. With the further increase of the reaction temperature, the intensities of the diffraction peaks increase. The XRD patterns showed that the phase formation sequence was CuSe ? CISe, which was different from that of other synthesis methods such as solvothermal. The diffraction peaks of solid-state In–Se secondary phases were not observed as expected. Some similar phenomena have been reported [18,19], however, no in-depth explanation and discussion were found in those papers. To investigate the valences of Cu and Se in CuSe, the product obtained at 130 °C for 10 min was examined by XPS. Fig. 5 shows the XPS survey spectrum, Cu 2p, Se 3d, and In 3d core-level spectrum, respectively. The XPS survey spectrum shown in Fig. 5a identifies the presence of Cu and Se, and In is also present. The Cu 2p core-level spectrum is illustrated in Fig. 5b. Two intense peaks centered at 932 eV and 951.7 eV, corresponding to Cu 2p3/2 and Cu 2p1/2, are observed. The Cu 2p3/2 satellite peak characterizing Cu2+, which was usually centered at 942 eV [20], is not observed in the present Cu 2p core-level spectrum. Therefore, the chemical valence of copper in CuSe was +1 (Cu+), indicating that the Cu2+ ions of the starting material were reduced by OLA. For Se (-II) core-level spectrum, the
Fig. 4. XRD patterns of the solid intermediates obtained at different stages of CISe nanoparticles synthesis process.
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Fig. 5. (a) XPS survey spectrum, (b) Cu 2p, (c) Se 3d, and (d) In 3d core-level spectrum of the product obtained at 130 °C for 10 min.
branching ratio 3d5/2:3d3/2 equaled to 1.5, the distance between the 3d5/2 and 3d3/2 was about 0.85 eV [21]. However, the Se 3d5/2 and Se 3d3/2 double peak have not been observed in our Se 3d core-level spectrum (shown in Fig. 5c). The Se 3d core-level spectrum is similar to previous literature report for Cu+Se [22]. Thus, the chemical valence of selenium in the product obtained at 130 °C for 10 min should be 1 (Se1 ), which may be due to the weak reduction ability of OLA for Se powders at a lower reaction temperature. Fig. 5d shows the In 3d core-level spectrum. The peaks centered at binding energy of 444.6 eV and 452 eV coincide well with In 3d5/2 and In 3d3/2. The In 3d core-level spectrum is very similar to those reported for CISe [23]. Therefore, the chemical valence of indium in the product obtained at 130 °C for 10 min was +3 (In3+). Based on the XPS analysis, it can be concluded that the valences of Cu and Se in CuSe were +1 and 1, respectively. In addition to CuSe, one or more In-containing species were present in the sample. However, the nature of the In-containing species was unknown. The In-containing species must be amorphous, because no XRD signals from product were observed. Raman scattering for amorphous In1 xSex alloys [24] has been reported. In order to identify the phase of amorphous In–Se compound, the solid intermediates obtained at different stages of CISe nanoparticles synthesis process were analyzed by Raman spectroscopy. Fig. 6 shows the Raman spectra of the solid intermediates. According to Fig. 6, a major intense peak at 236 cm 1 and a broad peak at 260 cm 1 are observed when the mixture solution was heated to 130 °C and kept for 10 min. The two Raman peaks were also attributed to CuSe [25]. When the mixture solution was heated to 180 °C, two broad peaks at 236 cm 1 and 173 cm 1 are observed, which were assigned to CuSe and CISe, respectively. When the mixture temperature was raised to 246 °C, only the Raman peak of CISe is observed. With the further increase of the reaction temperature, the intensity of the CISe Raman
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Fig. 6. Raman spectra of the solid intermediates obtained at different stages of CISe nanoparticles synthesis process.
peak increases. The Raman spectra indicated that the formation pathway of CISe nanoparticles synthesized by one-pot method was CuSe ? CISe, which was consistent with the XRD results. No Raman signals from solid-state In–Se secondary phases were detected. The chemical composition of the solid intermediates obtained at different stages of CISe nanoparticles synthesis process were also analyzed by EDS, as shown in Table 2. The EDS results reveal the presence of solid-state In–Se secondary phases in early solid intermediates, which was consistent with the XPS results. However, no XRD signals or Raman signals of solid-state In–Se secondary phases were observed. This may be because that the solid-state In–Se secondary phases were composed of amorphous clusters, which lacked molecular characteristics and resulted in the absence of XRD signals and Raman signals. Based on the XRD, XPS, Raman and EDS results, the crystalline phase and possible material compositions of the solid intermediates obtained at different stages of CISe nanoparticles synthesis process were given in Table 3. Since the reducing ability of OLA highly depended on the reaction temperature, the formation process of CISe nanoparticles for one-pot method was a complex process, which involved a number of chemical and physical events (e.g., formation of nuclei, formation of amorphous clusters with a magic number of atoms, reactions between the solid intermediates, crystallization and growth of nanoparticles.). Based on the experimental results, the possible formation process was as follows. OLA acted as both high boiling point solvent and reducing agent during the reaction. Firstly, Cu2+ ions were reduced to Cu+ ions, Se powders dissolved partially and were reduced to Se ions by OLA at a lower reaction temperature. The Se species (Se0, Se , Se2 and Se2n ) depend on the reaction temperature and the reduction ability of the solvent. Meanwhile, OLA also behaved as a complex ligand and formed stable complexes with CuCl2, InCl3 and Se powders. Only OLA–Cu complex was decomposed and released monomers containing Cu+ ions at lower reaction temperature for a shorter period because the reactivity of OLA–Cu complex was much higher than that of OLA–In complex [26]. Then, the monomers containing Cu+ ions reacted with Se ions to form CuSe nanocrystallites. With the reaction temperature increase, the OLA–In complex was decomposed and released monomers containing In3+ ions. The
Table 2 Chemical composition of the solid intermediates obtained at different stages of CISe nanoparticles synthesis process. Reaction temperature (°C)
Reaction time (min)
Mole ratio (Cu:In:Se)
130 180 246 265
10 0 0 10
26.9:11.3:61.8 21.6:18.1:60.3 15.4:21.7:62.9 24.7:23.6:51.7
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Table 3 Crystalline phase and possible material compositions of the solid intermediates obtained at different stages of CISe nanoparticles synthesis process. Reaction temperature (°C)
Reaction time (min)
Crystalline phase
Possible material compositions
130 180 246 265
10 0 0 10
CuSe CuSe and CISe CISe CISe
CuSe and InSe3 CuSe, Se-rich CISe and InSex Se-rich CISe CISe
Fig. 7. Diagram of the reaction pathway of CISe nanoparticles synthesized by one-pot method.
monomers containing In3+ ions reacted with Se ions to form amorphous InSe3 clusters. With the further increase of reaction temperature, Se ions were further reduced to Se2n ions by OLA, amorphous InSe3 clusters were converted into InSex (1.5 < x < 3) clusters. As the reaction proceeded, the CuSe nanocrystallites reacted with InSex clusters to form Se-rich CISe phase. With the reaction temperature increase and the reaction proceeding, the Se2n ions were reduced to Se2 ions, the composition of the Se-rich CISe nanoparticles became close to stoichiometry by interdiffusion of Cu, In and Se. The diagram of the reaction pathway of CISe nanoparticles synthesized by one-pot method was shown in Fig. 7. The reaction pathway was different from that reported in the literatures [6,9], which may be because that the solvent or the order of precursor addition were different.
4. Conclusion Highly dispersed and near stoichiometric chalcopyrite CISe nanoparticles were successfully synthesized via a facile and rapid one-pot method. The phase formation sequence of CISe nanoparticles synthesized by one-pot method was CuSe ? CISe, which was different from that of other synthesis methods such as solvothermal and hot injection. As the reaction proceeded, the valence of Se in solid intermediates was from 1 to 2 because the reducing ability of OLA highly depended on the reaction temperature. The solid-state In–Se secondary phases existed in early solid intermediates. However, no XRD signals and Raman signals of solid-state In–Se secondary phases were observed as expected. Study of the nature of In–Se secondary phases is underway. Based on the experimental results, the possible reaction pathway of chalcopyrite CISe nanoparticles synthesized by one-pot method was proposed.
Acknowledgments This work was partially supported by National Natural Science Foundation of China (61176085, 11105149), National Science Instrument Important Project (2011YQ15004003, and 2011YQ14014704), Dawn Program of Shanghai Education Commission (11SG44), Programs
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