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Solvothermal synthesis and optical characterization of chalcopyrite CuInSe2 microspheres
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Materials Chemistry and Physics 106 (2007) 296–300
Solvothermal synthesis and optical characterization of chalcopyrite CuInSe2 microspheres Li Zhang, Jing Liang, Shengjie Peng, Yunhui Shi, Jun Chen ∗ Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, PR China Received 2 October 2006; received in revised form 28 May 2007; accepted 2 June 2007
Abstract CuInSe2 microspheres were synthesized by a solvothermal method with a mixed solvent of ethylenediamine and ethanol (1:1, v/v). Besides, other morphological CuInSe2 such as platelets and rods were obtained by selecting different solvothermal conditions. The crystalline structure and morphology of the as-obtained products were characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effects of the experimental parameters including solvents, reaction time, and reactant concentration have also been studied. The near IR absorption spectra showed that CuInSe2 microstructures had strong absorption and their energy band gaps were tunable in the range of 1.03–1.13 eV, which was somewhat relevant to the size and morphology. © 2007 Elsevier B.V. All rights reserved. Keywords: CuInSe2 ; Chemical synthesis; Crystal growth; Optical property; Solar cells
1. Introduction In recent years, the synthesis of nano- and microstructured semiconductors has been of interest to materials scientists due to their outstanding physical or chemical properties [1]. Semiconductor materials of ternary chalcogenide compounds ABmCn (A = Cu, Ag, Zn, etc.; B = Al, Ga, In; C = S, Se, Te) have been extensively studied because of their tunable electronic and optical characteristics [2–9]. The chalcogenide compound CuInSe2 is a candidate as a promising material for solar cell applications due to its high absorption coefficient, suitable band gap, good radiation stability, and easy conversion of n/p carrier type [10]. Many techniques have been established to prepare CuInSe2 , including evaporation [11], spraying [12,13], pyrolysis of molecular single-source precursors [14] and electrodeposition [15]. Castro and Banger synthesized CuInSe2 nanocrystals by thermal deposition of molecular single-source precursors in a noncoordinating solvent [14]. Guillenmoles et al. [15] reported that CuInSe2 thin film can be prepared by electrodeposition method. Malik et al. [16] conducted monodispersed CuInSe2 nanopartilces by reacting tri-n-octylphosphine
∗
Corresponding author. Tel.: +86 22 23506808; fax: +86 22 23506808. E-mail address: [email protected] (J. Chen).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.06.005
selenide (TOPSe), CuCl and InCl3 in tri-n-octylphosphine oxide (TOPO). Peng and co-workers obtained CuInSe2 nanowires by Au-catalyzed vapo-liquid-solid (VLS) growth at 700 ◦ C [17]. In general, these techniques usually require either a high processing temperatures or special devices, and some of them even use toxic agents such as H2 Se or organometallic compounds. Recently, solution-based methods have been used to fabricate CuInSe2 at much lower temperature [18,19]. The solvothermal route has emerged as a powerful tool for controlled synthesis of nano/microstructures. For example, Qian and co-workers reported the solvothermal synthesis of CuInSe2 nanowhiskers and nanorods by using structure-directing organic amine solvents [20,21]. Since the shape, size, and dimensionality of semiconductors are important parameters to affect their properties, developing a facile method to prepare ternary chalcogenide materials with various morphologies is vital for further applications. We have been interested in the use of a solvothermal route to produce various kinds of nano/micromaterials [22]. In a previous work, we have described shape-controlled synthesis of ternary chalcogenide ZnIn2 S4 and CuInS2 nano/microstructures in the presence of surfactants or thioglycolic acid TGA [23,24]. Herein, we report on the solvothermal synthesis of CuInSe2 microspheres in an ethylenediamine–ethanol binary solution without surfactants or templates. In addition, the optical properties
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Table 1 The effects of experimental conditions on the morphology, particle sizes, and phase(s) of CuInSe2 Sample
[Cu2+ ]a (mM)
Solventb
t (h)
Products
Morphology
1 2 3 4 5 6 7
10 10 10 10 10 10 50
En–ethanol(1:1, v/v) Ethanol Benzene Ethylene glycol En–ethanol (1:1, v/v) En–ethanol (1:1, v/v) En–ethanol (1:1, v/v)
12 12 12 12 2 48 12
CuInSe2 CuInSe2 + In2 Se3 c CuInSe2 + In2 Se3 c CuSe + Se CuInSe2 CuInSe2 CuInSe2
Microsphere Sparse and thick plates Flakes and fine granules Flakes with microrods Dispersive plates Interconnected sheets Conjoint short rods
a b c
The concentrations of the starting agents, Cu:In:Se = 1:1:2. En refers to ethylenediamine. Minor by-products.
of the as-synthesized CuInSe2 microsphere were studied. The present solvothermal method is mild and effective for large-scale production of CuInSe2 microstructures. 2. Experimental All chemicals were of analytical grade and used without further purification. In a typical synthesis, stoichiometric amounts (0.1 mmol) of CuCl2 , InCl3 ·4H2 O, and selenium powder were mixed into a Teflon-lined stainless steel autoclave of 25 mL capacity, which was filled with solvent up to 40% of the total volume. The autoclave was sealed, maintained at 180 ◦ C for 12 h, and then cooled to room temperature naturally. The resulting precipitates were centrifuged and washed several times with distilled water and absolute ethanol to remove any dissoluble by-products. After being vacuum-dried at 60 ◦ C for 6 h, the final dark product was collected. Different solvent systems including ethanol, benzene, ethylene glycol, and the mixed solvent of ethylenediamine and ethanol (1:1, v/v) were investigated, respectively. The experimental results under different reaction conditions are listed in Table 1. The obtained samples were characterized by X-ray powder diffraction (XRD), operating on a Rigaku RINT-2000 X-ray diffractometer with graphitemonochromated Cu K␣ radiation. Scanning electron microscopy (SEM) images were taken using a Philips XL-30 and a JEOL JSM-6700F microscope. The near IR absorption spectra of the products were recorded at normal incidence at room temperature by using a computer-aided double-beam spectrophotometer (JASCO model V-570 UV–vis-NIR).
3. Results and discussion Fig. 1 shows the XRD patterns of four representative samples (samples 1–4 in Table 1) synthesized by altering various
Fig. 1. XRD patterns of the four products by using various solvents: (a) ethylenediamine–ethanol, (b) ethanol, (c) benzene, and (d) ethylene glycol.
solvents including ethylenediamine–ethanol, ethanol, benzene, and ethylene glycol. The diffraction peaks of sample 1 (Fig. 1a) can be indexed to pure phase of CuInSe2 with chalcopyrite structure (JCPDS No. 89-5647). This pattern is in good agreement with the reported features of stoichiometric CuInSe2 with tetragonal phase [13]. No peaks of other impurities were detected, indicating the high purity of sample 1. When ethanol, ethylene glycol, or benzene was used as the reaction medium, the as-synthesized sample contained impurities such as In2 Se3 , CuSe, or selenium (Fig. 1b–d). This result indicates that the ethylenediamine–ethanol mixed solvent plays the dominant role in the structural control of CuInSe2 . Fig. 2 shows the SEM images of CuInSe2 (sample 1) obtained from ethylenediamine–ethanol media. Fig. 2a indicates that the obtained product is mainly composed of monodispersive microspheres with diameters of about 4–5 m. As seen in Fig. 2b, the SEM image of sample 1 at higher magnification reveals that the microspheres are essentially composed of randomly arranged flexible nanosheets, which possess an average thickness of approximately 100 nm. To further understand the formation mechanism of CuInSe2 microspheres in the mixed solvent, we have investigated the samples obtained at different reaction times using the SEM and XRD techniques. Fig. 3 shows the XRD patterns of the products obtained after the reaction for 2 and 48 h. The diffraction peaks of sample 5 (Fig. 3a) are greatly broadened as compared to those of sample 1 (Fig. 1a), indicating the poor crystallinity of CuInSe2 at the initial reaction stage. When the reaction was elongated to 48 h, the obtained sample (sample 6) was well crystallized and the XRD pattern (Fig. 3b) coincided well with the chalcopyrite CuInSe2 . Fig. 4 shows the SEM images of the samples 5 and 6. As shown in Fig. 4a, sample 5 mainly consists of dispersive plates coexisting with some smaller granules, resemble the surface morphology of CuInSe2 grown on glass substrates by MOCVD method [13]. After the reaction proceeded for 12 h, the plates and granules gradually assembled into monodispersive microspheres. While prolonging the holding time up to 48 h, the nanosheets of the microspheres reassembled into stacked platelike aggregates (Fig. 4b). According to the above results, it is deduced that the evolution of CuInSe2 microspheres follow three distinct steps: (1) small granules were formed at the beginning of the reaction; (2) primary granules grew up to uniform nanosheets
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Fig. 2. SEM images of CuInSe2 microspheres prepared from ethylenediamine–ethanol mixed solvent (sample 1) at low (a) and higher (b) magnification.
Fig. 3. XRD patterns of samples obtained at different growth stages: (a) 2 h (sample 5) and (b) 48 h (sample 6).
by Ostwald ripening; (3) nascent nanosheets self-assembled into microspheres with prolonging reaction time. The effect of precursor concentration in ethylenediamine–ethanol mixed solvent was investigated by fixing the reaction time for 12 h. The concentration of the starting reagent (sample 7) was five times higher than that of sample 1 in Table 1. As shown in Fig. 5, a typical SEM image of sample 7 illustrated that some aggregating short rods were obtained, instead of spheres or sheet clusters. Since ions or ion groups of the reactants easily reach supersaturate state under high precursor concentration, more nucleus were produced, and the nucleation and growth of CuInSe2 crystal occurred fast, leading to a quick precipitation of conjoint nanorods from the solvothermal solution.
We also investigated the effect of the volume ratio of ethylenediamine to ethanol. The result showed that microspheres could be obtained only with the volume ratio of 1:1. Otherwise, irregular aggregates were formed. When ethylenediamine is completely replaced by ethanol, some by-products coexisted with CuInSe2 (as shown in Fig. 1). According to the present results and that reported previously [17,21], the following equations (where “en” represents bidentate ethylenediamine) can be concluded: 2InCl3 + 3Se2− → In2 Se3 + 6Cl− In2 Se3 + Se2− → 2(InSe2 )−
Fig. 4. SEM images of samples obtained at different growth stages: (a) 2 h (sample 5) and (b) 48 h (sample 6).
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Fig. 5. SEM images of sample 7 at low (a) and higher (b) magnification.
Cu+ + 2en → [Cu(en)2 ]+ (InSe2 )− + [Cu(en)2 ]+ → CuInSe2 + 2en Therefore, only solvents with suitable reducing ability or coordination ability can activate selenium powder and reduce Cu2+ ions. This means that when the reaction time and reactant concentration were kept constant, the products in ethylenediamine–ethanol would grow higher crystallinity than those in benzene or ethylene glycol. It is noted that ethylenediamine–ethanol mixed solvent showed the optimal reaction medium to build up chalcopyrite CuInSe2 microspheres. In the mixed reaction medium, the growth of CuInSe2 microspheres is most likely to be controlled by the well-known solution–liquid–solid (SLS) mechanism [25,26]. As mentioned by Li et al. [21], the intermediate product, In2 Se3 , has low solubility in ethylenediamnie. When an appropriate amount of weakly polar ethyl alcohol was mixed with ethylenediamine, the polarity of the mixed solvent decreased to the similar value of In2 Se3 . This probably resulted in the increasing of the solubility of In2 Se3 in the mixed solvent. As a result, the site of nucleation became bigger and the fast growth of the individual suppressed the anisotropic growth of the particles, leading to the formation of CuInSe2 microspheres with sheet-like crystalline [27]. Furthermore, the toxicity of the reaction medium was lowered in this mixed reaction medium by substituting a part of toxic ethylenidiamine to harmless ethanol. Fig. 6 shows the absorption spectra of the as-prepared CuInSe2 samples (samples 1, 6, and 7). It can be seen that all the spectra show characteristic and well-defined excitation absorption of CuInSe2 , which is similar to previously reported results [17]. The band gap of CuInSe2 is determined from the fundamental absorption edge of the spectra [28], which are shown in the inset of Fig. 6. The straight line fit indicates that the optical transition is direct. The band gaps of the as-synthesized CuInSe2 samples are in the range of 1.03–1.13 eV. It is established that the optical absorption edge and the optical band gap are related to the shape and the size of semiconductors [29]. In our experiment, the band gaps shrink in the order of sample 7, 1, 6 with increasing particle size. Similar size- and shape-dependent optical properties have also been found in our previous studied ZnIn2 S4 system [23]. Sample 6 has the narrowest band gap (1.03 eV) because of its largest building blocks. Sample 1 with micrometer-scale
Fig. 6. Absorption spectra of CuInSe2 samples synthesized in the ethylenediamine–ethanol system: (a) sample 1, (b) sample 7, and (c) sample 6.
structure has the band gap of 1.05 eV similar to that of bulk materials (1.04 eV) [30]. In contrast, sample 7 of aggregating short rods possesses the widest band gap at 1.13 eV. Further analysis is still needed to understand the correlation between the optical properties and the size/shape of CuInSe2 . 4. Conclusions In conclusion, we described a solvothermal method for controlling the morphologies and microstructures of chalcopyrite CuInSe2 microspheres by using binary solution ethylenediamine–ethanol as solvent. Compared with ethanol, benzene, and ethylene glycol as single solvent, ethylenediamine–ethanol mixed system provides a mild and homogeneous condition for a high yield of CuInSe2 microspheres. The evolution of phase and morphology of the products are tuned by adjusting the solvent composition, duration and reactant concentration. The absorption spectra show that the band gaps of the obtained CuInSe2 microstructures are tailored in the range of 1.03–1.13 eV, which is of interest for their photovoltaic applications. Acknowledgements This work was supported by 973 Program (2005CB623607) and Tianjin City Project (05YFJMJC00300).
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Materials Chemistry and Physics 106 (2007) 296–300
Solvothermal synthesis and optical characterization of chalcopyrite CuInSe2 microspheres Li Zhang, Jing Liang, Shengjie Peng, Yunhui Shi, Jun Chen ∗ Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, PR China Received 2 October 2006; received in revised form 28 May 2007; accepted 2 June 2007
Abstract CuInSe2 microspheres were synthesized by a solvothermal method with a mixed solvent of ethylenediamine and ethanol (1:1, v/v). Besides, other morphological CuInSe2 such as platelets and rods were obtained by selecting different solvothermal conditions. The crystalline structure and morphology of the as-obtained products were characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effects of the experimental parameters including solvents, reaction time, and reactant concentration have also been studied. The near IR absorption spectra showed that CuInSe2 microstructures had strong absorption and their energy band gaps were tunable in the range of 1.03–1.13 eV, which was somewhat relevant to the size and morphology. © 2007 Elsevier B.V. All rights reserved. Keywords: CuInSe2 ; Chemical synthesis; Crystal growth; Optical property; Solar cells
1. Introduction In recent years, the synthesis of nano- and microstructured semiconductors has been of interest to materials scientists due to their outstanding physical or chemical properties [1]. Semiconductor materials of ternary chalcogenide compounds ABmCn (A = Cu, Ag, Zn, etc.; B = Al, Ga, In; C = S, Se, Te) have been extensively studied because of their tunable electronic and optical characteristics [2–9]. The chalcogenide compound CuInSe2 is a candidate as a promising material for solar cell applications due to its high absorption coefficient, suitable band gap, good radiation stability, and easy conversion of n/p carrier type [10]. Many techniques have been established to prepare CuInSe2 , including evaporation [11], spraying [12,13], pyrolysis of molecular single-source precursors [14] and electrodeposition [15]. Castro and Banger synthesized CuInSe2 nanocrystals by thermal deposition of molecular single-source precursors in a noncoordinating solvent [14]. Guillenmoles et al. [15] reported that CuInSe2 thin film can be prepared by electrodeposition method. Malik et al. [16] conducted monodispersed CuInSe2 nanopartilces by reacting tri-n-octylphosphine
∗
Corresponding author. Tel.: +86 22 23506808; fax: +86 22 23506808. E-mail address: [email protected] (J. Chen).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.06.005
selenide (TOPSe), CuCl and InCl3 in tri-n-octylphosphine oxide (TOPO). Peng and co-workers obtained CuInSe2 nanowires by Au-catalyzed vapo-liquid-solid (VLS) growth at 700 ◦ C [17]. In general, these techniques usually require either a high processing temperatures or special devices, and some of them even use toxic agents such as H2 Se or organometallic compounds. Recently, solution-based methods have been used to fabricate CuInSe2 at much lower temperature [18,19]. The solvothermal route has emerged as a powerful tool for controlled synthesis of nano/microstructures. For example, Qian and co-workers reported the solvothermal synthesis of CuInSe2 nanowhiskers and nanorods by using structure-directing organic amine solvents [20,21]. Since the shape, size, and dimensionality of semiconductors are important parameters to affect their properties, developing a facile method to prepare ternary chalcogenide materials with various morphologies is vital for further applications. We have been interested in the use of a solvothermal route to produce various kinds of nano/micromaterials [22]. In a previous work, we have described shape-controlled synthesis of ternary chalcogenide ZnIn2 S4 and CuInS2 nano/microstructures in the presence of surfactants or thioglycolic acid TGA [23,24]. Herein, we report on the solvothermal synthesis of CuInSe2 microspheres in an ethylenediamine–ethanol binary solution without surfactants or templates. In addition, the optical properties
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Table 1 The effects of experimental conditions on the morphology, particle sizes, and phase(s) of CuInSe2 Sample
[Cu2+ ]a (mM)
Solventb
t (h)
Products
Morphology
1 2 3 4 5 6 7
10 10 10 10 10 10 50
En–ethanol(1:1, v/v) Ethanol Benzene Ethylene glycol En–ethanol (1:1, v/v) En–ethanol (1:1, v/v) En–ethanol (1:1, v/v)
12 12 12 12 2 48 12
CuInSe2 CuInSe2 + In2 Se3 c CuInSe2 + In2 Se3 c CuSe + Se CuInSe2 CuInSe2 CuInSe2
Microsphere Sparse and thick plates Flakes and fine granules Flakes with microrods Dispersive plates Interconnected sheets Conjoint short rods
a b c
The concentrations of the starting agents, Cu:In:Se = 1:1:2. En refers to ethylenediamine. Minor by-products.
of the as-synthesized CuInSe2 microsphere were studied. The present solvothermal method is mild and effective for large-scale production of CuInSe2 microstructures. 2. Experimental All chemicals were of analytical grade and used without further purification. In a typical synthesis, stoichiometric amounts (0.1 mmol) of CuCl2 , InCl3 ·4H2 O, and selenium powder were mixed into a Teflon-lined stainless steel autoclave of 25 mL capacity, which was filled with solvent up to 40% of the total volume. The autoclave was sealed, maintained at 180 ◦ C for 12 h, and then cooled to room temperature naturally. The resulting precipitates were centrifuged and washed several times with distilled water and absolute ethanol to remove any dissoluble by-products. After being vacuum-dried at 60 ◦ C for 6 h, the final dark product was collected. Different solvent systems including ethanol, benzene, ethylene glycol, and the mixed solvent of ethylenediamine and ethanol (1:1, v/v) were investigated, respectively. The experimental results under different reaction conditions are listed in Table 1. The obtained samples were characterized by X-ray powder diffraction (XRD), operating on a Rigaku RINT-2000 X-ray diffractometer with graphitemonochromated Cu K␣ radiation. Scanning electron microscopy (SEM) images were taken using a Philips XL-30 and a JEOL JSM-6700F microscope. The near IR absorption spectra of the products were recorded at normal incidence at room temperature by using a computer-aided double-beam spectrophotometer (JASCO model V-570 UV–vis-NIR).
3. Results and discussion Fig. 1 shows the XRD patterns of four representative samples (samples 1–4 in Table 1) synthesized by altering various
Fig. 1. XRD patterns of the four products by using various solvents: (a) ethylenediamine–ethanol, (b) ethanol, (c) benzene, and (d) ethylene glycol.
solvents including ethylenediamine–ethanol, ethanol, benzene, and ethylene glycol. The diffraction peaks of sample 1 (Fig. 1a) can be indexed to pure phase of CuInSe2 with chalcopyrite structure (JCPDS No. 89-5647). This pattern is in good agreement with the reported features of stoichiometric CuInSe2 with tetragonal phase [13]. No peaks of other impurities were detected, indicating the high purity of sample 1. When ethanol, ethylene glycol, or benzene was used as the reaction medium, the as-synthesized sample contained impurities such as In2 Se3 , CuSe, or selenium (Fig. 1b–d). This result indicates that the ethylenediamine–ethanol mixed solvent plays the dominant role in the structural control of CuInSe2 . Fig. 2 shows the SEM images of CuInSe2 (sample 1) obtained from ethylenediamine–ethanol media. Fig. 2a indicates that the obtained product is mainly composed of monodispersive microspheres with diameters of about 4–5 m. As seen in Fig. 2b, the SEM image of sample 1 at higher magnification reveals that the microspheres are essentially composed of randomly arranged flexible nanosheets, which possess an average thickness of approximately 100 nm. To further understand the formation mechanism of CuInSe2 microspheres in the mixed solvent, we have investigated the samples obtained at different reaction times using the SEM and XRD techniques. Fig. 3 shows the XRD patterns of the products obtained after the reaction for 2 and 48 h. The diffraction peaks of sample 5 (Fig. 3a) are greatly broadened as compared to those of sample 1 (Fig. 1a), indicating the poor crystallinity of CuInSe2 at the initial reaction stage. When the reaction was elongated to 48 h, the obtained sample (sample 6) was well crystallized and the XRD pattern (Fig. 3b) coincided well with the chalcopyrite CuInSe2 . Fig. 4 shows the SEM images of the samples 5 and 6. As shown in Fig. 4a, sample 5 mainly consists of dispersive plates coexisting with some smaller granules, resemble the surface morphology of CuInSe2 grown on glass substrates by MOCVD method [13]. After the reaction proceeded for 12 h, the plates and granules gradually assembled into monodispersive microspheres. While prolonging the holding time up to 48 h, the nanosheets of the microspheres reassembled into stacked platelike aggregates (Fig. 4b). According to the above results, it is deduced that the evolution of CuInSe2 microspheres follow three distinct steps: (1) small granules were formed at the beginning of the reaction; (2) primary granules grew up to uniform nanosheets
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Fig. 2. SEM images of CuInSe2 microspheres prepared from ethylenediamine–ethanol mixed solvent (sample 1) at low (a) and higher (b) magnification.
Fig. 3. XRD patterns of samples obtained at different growth stages: (a) 2 h (sample 5) and (b) 48 h (sample 6).
by Ostwald ripening; (3) nascent nanosheets self-assembled into microspheres with prolonging reaction time. The effect of precursor concentration in ethylenediamine–ethanol mixed solvent was investigated by fixing the reaction time for 12 h. The concentration of the starting reagent (sample 7) was five times higher than that of sample 1 in Table 1. As shown in Fig. 5, a typical SEM image of sample 7 illustrated that some aggregating short rods were obtained, instead of spheres or sheet clusters. Since ions or ion groups of the reactants easily reach supersaturate state under high precursor concentration, more nucleus were produced, and the nucleation and growth of CuInSe2 crystal occurred fast, leading to a quick precipitation of conjoint nanorods from the solvothermal solution.
We also investigated the effect of the volume ratio of ethylenediamine to ethanol. The result showed that microspheres could be obtained only with the volume ratio of 1:1. Otherwise, irregular aggregates were formed. When ethylenediamine is completely replaced by ethanol, some by-products coexisted with CuInSe2 (as shown in Fig. 1). According to the present results and that reported previously [17,21], the following equations (where “en” represents bidentate ethylenediamine) can be concluded: 2InCl3 + 3Se2− → In2 Se3 + 6Cl− In2 Se3 + Se2− → 2(InSe2 )−
Fig. 4. SEM images of samples obtained at different growth stages: (a) 2 h (sample 5) and (b) 48 h (sample 6).
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Fig. 5. SEM images of sample 7 at low (a) and higher (b) magnification.
Cu+ + 2en → [Cu(en)2 ]+ (InSe2 )− + [Cu(en)2 ]+ → CuInSe2 + 2en Therefore, only solvents with suitable reducing ability or coordination ability can activate selenium powder and reduce Cu2+ ions. This means that when the reaction time and reactant concentration were kept constant, the products in ethylenediamine–ethanol would grow higher crystallinity than those in benzene or ethylene glycol. It is noted that ethylenediamine–ethanol mixed solvent showed the optimal reaction medium to build up chalcopyrite CuInSe2 microspheres. In the mixed reaction medium, the growth of CuInSe2 microspheres is most likely to be controlled by the well-known solution–liquid–solid (SLS) mechanism [25,26]. As mentioned by Li et al. [21], the intermediate product, In2 Se3 , has low solubility in ethylenediamnie. When an appropriate amount of weakly polar ethyl alcohol was mixed with ethylenediamine, the polarity of the mixed solvent decreased to the similar value of In2 Se3 . This probably resulted in the increasing of the solubility of In2 Se3 in the mixed solvent. As a result, the site of nucleation became bigger and the fast growth of the individual suppressed the anisotropic growth of the particles, leading to the formation of CuInSe2 microspheres with sheet-like crystalline [27]. Furthermore, the toxicity of the reaction medium was lowered in this mixed reaction medium by substituting a part of toxic ethylenidiamine to harmless ethanol. Fig. 6 shows the absorption spectra of the as-prepared CuInSe2 samples (samples 1, 6, and 7). It can be seen that all the spectra show characteristic and well-defined excitation absorption of CuInSe2 , which is similar to previously reported results [17]. The band gap of CuInSe2 is determined from the fundamental absorption edge of the spectra [28], which are shown in the inset of Fig. 6. The straight line fit indicates that the optical transition is direct. The band gaps of the as-synthesized CuInSe2 samples are in the range of 1.03–1.13 eV. It is established that the optical absorption edge and the optical band gap are related to the shape and the size of semiconductors [29]. In our experiment, the band gaps shrink in the order of sample 7, 1, 6 with increasing particle size. Similar size- and shape-dependent optical properties have also been found in our previous studied ZnIn2 S4 system [23]. Sample 6 has the narrowest band gap (1.03 eV) because of its largest building blocks. Sample 1 with micrometer-scale
Fig. 6. Absorption spectra of CuInSe2 samples synthesized in the ethylenediamine–ethanol system: (a) sample 1, (b) sample 7, and (c) sample 6.
structure has the band gap of 1.05 eV similar to that of bulk materials (1.04 eV) [30]. In contrast, sample 7 of aggregating short rods possesses the widest band gap at 1.13 eV. Further analysis is still needed to understand the correlation between the optical properties and the size/shape of CuInSe2 . 4. Conclusions In conclusion, we described a solvothermal method for controlling the morphologies and microstructures of chalcopyrite CuInSe2 microspheres by using binary solution ethylenediamine–ethanol as solvent. Compared with ethanol, benzene, and ethylene glycol as single solvent, ethylenediamine–ethanol mixed system provides a mild and homogeneous condition for a high yield of CuInSe2 microspheres. The evolution of phase and morphology of the products are tuned by adjusting the solvent composition, duration and reactant concentration. The absorption spectra show that the band gaps of the obtained CuInSe2 microstructures are tailored in the range of 1.03–1.13 eV, which is of interest for their photovoltaic applications. Acknowledgements This work was supported by 973 Program (2005CB623607) and Tianjin City Project (05YFJMJC00300).
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