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Template-free one-step fabrication of porous CuInS2 hollow microspheres
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Microporous and Mesoporous Materials 114 (2008) 395–400 www.elsevier.com/locate/micromeso
Template-free one-step fabrication of porous CuInS2 hollow microspheres Yunxia Qi, Kaibin Tang *, Suyuan Zeng, Weiwei Zhou Nanomaterial and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, PR China Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR China Received 6 September 2007; received in revised form 13 January 2008; accepted 16 January 2008 Available online 1 February 2008
Abstract Hollow CuInS2 microspheres have been successfully prepared via a simple solution-based route without employing templates and surfactants. Transmission electron microscope (TEM) images clearly show the hollow nature of the as-obtained products. Field emission scanning electron microscopy (FESEM) images reveal that the shells of as-prepared CuInS2 microspheres were constructed by nanoparticles or nanoflakes. The specific surface area and pore-size distribution of the obtained product as determined by gas-sorption measurements show that the CuInS2 hollow microspheres exhibit high Brunauer–Emmett–Teller (BET) surface area and porosity properties. TEM observations of intermediate products at different reaction stages indicate that these hollow CuInS2 microspheres are formed mainly via Ostwald ripening. Ó 2008 Elsevier Inc. All rights reserved. Keywords: CuInS2; Hollow microspheres; Aggregation; Ostwald ripening; Solvothermal
1. Introduction Materials with nanometer-to-micrometer hollow sphere structures have been received intense interest due to their potential applications in various areas. For example, in biotechnology, the hollow spheres can be used as controlled drug (molecule) release system [1,2], protection of biologically active agents [3]; in engineering, such structured materials can be utilized as low density materials; and in chemistry, hollow spheres can be employed as high surface-materials for catalysis and structure materials [4,5]. Driven by the technological importance of such structured materials, many efforts have been devoted to the synthesis of such hollow spheres. The most popular approach to obtain hollow nano(micro)spheres is template-directed synthetic methodology, which usually use removable template such as polystyrene beads, silica sol–gel [6–8], microemul*
Corresponding author. E-mail address: [email protected] (K. Tang).
1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.01.027
sion droplets [9], and so on. Other synthetic methods making use of some physical phenomena such as Ostwald ripening [10–12], the Kirkendall effect [13–16] and the oriented attachment process [17], have also been developed for the generation of hollow inorganic structures. I–III–VI2 chalcopyrite structure compounds have gained much attention because of their tunable electronic and optical properties [18–23]. As a typical ternary chalcopyrite material, CuInS2 has been considered as one of the most popular and promising candidates as absorber materials for photovoltaic applications because of its high absorption coefficient and environmental consideration [22]. Various CuInS2 structures, such as nanoparticles [24,25], nanorods [26,27], nanotubes [28], foam-like nanocrystallites [29], and microspheres [30–32] have been successfully fabricated by various methods. Nevertheless, it is still a grand challenge to prepare CuInS2 materials with controlled size and shape. Herein we report a template and surfactant free synthetic method to synthesize CuInS2 hollow structures by
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2. Experimental section The synthesis of CuInS2 hollow spheres proceeded through a simple solvothermal route. InCl3 4H2O, CuSO4 5H2O, thiourea, N,N-dimethylformamide (DMF) were all of analytical grade and purchased from Shanghai Chemical Reagent Ltd without further purification. In a typical synthesis procedure, 0.5 mmol InCl3 4H2O (0.147 g), 0.5 mmol CuSO4 5H2O (0.125 g) and a slight excess of thiourea (0.10 g) were dissolved in 40 mL of N,N-dimethylformamide (DMF) to form a solution under vigorous stirring. The resulting solution was then transferred into a 50 mL teflonlined stainless steel autoclave and maintained at 160 °C for 25 h. The obtained black solid products were collected by centrifugation, which were then washed several times with deionized water and absolute ethanol and dried in a vacuum at 70 °C for 4 h. The phase purity of the as-prepared products was determined by X-ray diffraction (XRD) using a Philips X’Pert PRO SUPER X-ray diffractometer with Cu Ka radiation ˚ ). Transmission electron microscope (k = 1.54178 A (TEM) images were taken with a Hitachi H-800 transmission electron microscope at an acceleration voltage of 200 kV. High-resolution transmission electron microscope (HRTEM) images were performed on a JEOL-2010 transmission electron microscope. The scanning electron microscopy (SEM) images were taken using a Sirion 200 field emission scanning electron microscope (FESEM, 20 kV). Nitrogen adsorption analysis was carried out with a Micromeritics ASAP2020 system equipment (BET and BJH models, respectively, for specific surface area and porosity evaluation). The ultraviolet and visible spectra were recorded on a JGNA Specord 200 PC UV–visible spectrophotometer.
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the reaction of CuSO4 5H2O, InCl3 4H2O, thiourea and N, N-dimethylformamide (DMF) at a suitable concentration and temperature, in which DMF performed both as a solvent and a reducing agent. To the best of our knowledge, this is the first report on the synthesis of CuInS2 hollow spheres. The mechanism of growth hollow sphere morphologies was investigated on the ground of a series of control experiments on various reaction times.
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2θ (degree) Fig. 1. XRD pattern of the hollow CuInS2 microspheres synthesized at 160 °C for 25 h.
3. Results and discussion The XRD pattern of the as-prepared samples at 160 °C for 25 h is shown in Fig. 1. All the diffraction peaks can be indexed as tetragonal phase CuInS2 (JCPDS 75–0106) with ˚ and c = 10.99 A ˚, the calculated cell parameters a = 5.52 A which are in accordance with the reported values. No characteristic peaks were detected as the other impurities such as CuS, Cu2S or In2S3. The morphology of the products obtained under the condition at 160 °C for 25 h was studied by field emission scanning electron microscopy (FESEM). Fig. 2a shows a panoramic FESEM image of a typical sample composed
Fig. 2. Images of the hollow CuInS2 microspheres synthesized at 160 °C for 25 h: (a) Low magnification FESEM image of overall products; (b) FESEM image of the hollow nanoparticle-microspheres, and an image of the cavity of a single microsphere (inset); (c, d) FESEM images of the hollow nanoflake-microspheres, and an image of the cavity of a single microsphere (inset); (e, f) TEM images of the hollow nanoparticlemicrospheres and their SAED patterns (inset); (g) HRTEM image of the nanoparticle-microspheres; (h, i) TEM images of the nanoflake-microspheres and their SAED patterns (inset); (j) HRTEM images of the hollow nanoflake-microspheres.
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ent shapes and sizes. Fig. 5a shows that one kind of them are nanoparticle-spheres with diameters of 600–700 nm. In addition, spheres with pale edges and dark centers (Fig. 5a, inset) suggest the existence of size/density variations intrinsic inside the starting nanoparticle-spheres. The formation reason for these size/density variations will be discussed later. Fig. 5e shows another nanoflake-spheres with diameters of 2–3 lm. The XRD pattern of the sample collected after three hours (Fig. 4a) presented very weak peaks, characteristic of a poorly crystallized compound. However, further analysis from the XRD patterns (Fig. 4a) shown that the sample contains CuInS2 (JCPDS 75–0106) along with CuS(JCPDS 75-2235), which is in accordance with SAED results. The diffraction rings insetted in Fig. 5a can be indexed to (1 1 2), (2 2 0), and (3 1 2) of CuInS2, whereas the diffraction rings insetted in Fig. 5e can be indexed to (1 0 2), (1 0 3), and (1 1 0) of CuS. Summarizing, the samples collected at 3 h are comprised of CuInS2 phase nanoparticle-spheres and CuS phase nanoflakespheres. As the reaction time extended, the CuInS2 nanoparticle-spheres were found to be divided into discrete
Intensity (a.u)
of microspheres. Furthermore, by detailed observation (Fig. 2b–d), these microspheres could be distinguished into two different categories. A sort of these spheres are composed of nanoparticles, as shown in Fig. 2b. The diameters of such nanoparticle-spheres range from about 500 nm to 1 lm. Moreover, as indicated by the inset in Fig. 2b, many broken spheres and their apparent cavities can be observed (Fig. 2a), indicating that these spheres are of hollow structures. As shown in Fig. 2c and d, the other kinds of spheres are comprised of nanoflakes standing perpendicularly to the surfaces. The diameters of such nanoflake-spheres range from about 1 lm to 3 lm. Many of these spheres have also open pores (Fig. 2d, inset), indicating that these spheres are of hollow structures. In addition, the number of broken spheres in nanoparticle-spheres is more than that in nanoflake-spheres, which can be attributed to the shells of nanoparticle-spheres are thinner and then more breakable by post-sonication for FESEM observation than those of nanoflake-spheres. Further observation was provided by TEM images shown in Fig. 2e and h. Calculated from these TEM images, the shell thickness of nanoparticle-spheres (Fig. 2e) ranges from about 100 nm to 150 nm, and the shell thickness of nanoflake-spheres (Fig. 2h) is about 300 nm to 600 nm. In addition, TEM observations indicate the hollow nature of both nanoparticle-spheres and nanoflake-spheres with a high yield of above 90%. Meanwhile, the insets in Fig. 2f and i show the corresponding SAED patterns, indexed to the (1 1 2), (2 2 0), (3 1 2) planes of CuInS2, with a high polycrystalline structure. HRTEM images taken from edges of the hollow spheres are shown in Fig. S1 and Fig. S2, respectively, which clearly indicate that they are composed of many small nanocrystals with different orientations. Furthermore, the clear contrast difference as indicated by arrows in Fig. S1 and Fig. S2 shows many pores with size less than 5 nm presented. The lattice fringes are clearly visible with a spacing of 0.32 nm (Fig. 5g and j), which is corresponding to the (1 1 2) planes of tetragonal phase CuInS2. Fig. 3a shows the nitrogen adsorption–desorption isotherms. The isotherms are of type IV, which confirms the characteristics of mesoporous materials. The Brunauer– Emmett–Teller (BET) of the sample is about 25 m2 g 1. Barrett–Joyner–Halenda (BJH) model analysis of these as-prepared hollow spheres is shown in inset of Fig. 3, which gives one narrow peak centered at 3.5 nm in the pore size distribution and another broad peak in the region of 10–50 nm with an obvious maximum at 23 nm. The first pore diameter is in good agreement with the value of the nanopores investigated by HRTEM. And the larger pores result from the aggregation of nanoparticles and nanoflakes of these hollow spheres, which indicate that they are not tightly adhered to each other. In order to understand the formation process of the hollow spheres, samples collected at different reaction times were characterized by TEM, SAED, and XRD. As shown in Fig. 5a and e, at the early stage (3 h), the obtained samples are composed of two types of solid spheres with differ-
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2θ (degree) Fig. 4. XRD patterns of samples synthesized at 160 °C for different reaction times: (a) 3 h; (b) 4 h; (c) 7.5 h; (d) 15 h. Symbol arrowheads indicate the diffraction peaks from CuS phase.
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Fig. 5. TEM images of samples synthesized at 160 °C for different reaction times: (a, e) 3 h; (b, f) 4 h; (c, g) 7.5 h; (d, h) 15 h. The insets show SAED patterns of the as-obtained samples. The inset at the bottom right corner of Fig. 5a shows an image of a single nanoparticle-microsphere.
regions (Fig. 5b). As the reaction proceeded, the cores shrank gradually (Fig. 5c and d), and eventually no cores remained (Fig. 2e). The corresponding SAED patterns are shown in the inset of Fig. 5b–d respectively. All of the SAED patterns can be indexed to (1 1 2), (2 2 0), and (3 1 2) of CuInS2. The diffraction rings in inset of Fig. 5b– d become progressively sharper with compared to Fig. 5a with aging time, suggesting a recrystallization process took place simultaneously. At the same time, the CuS nanoflakespheres were also followed by a solid core evacuation. As evidenced in Fig. 5f–h, the void space in these microspheres is getting bigger and bigger with aging time. The corresponding SAED patterns are shown in inset of Fig. 5f–h, respectively. However, the diffraction rings in Fig. 5 f and g show some characteristic of CuInS2 diffractions superimposed on CuS phase. When the reaction time was prolonged to 15 h, the SAED (Fig. 5h, inset) rings show that CuS phase is almost completely disappeared, and the SAED rings can be indexed to (1 1 2), (2 2 0), and (3 1 2) of CuInS2. Thus, it is obvious that the CuS nanoflake-microspheres did not only undergo a hollowing process but also go through a further phase transformation, which was also detected by XRD patterns (Fig. 4b–d). Moreover, with the reaction time increasing, peak intensities of CuInS2 in Fig. 4b–d increase accordingly. DMF has been proved to be a versatile solvent and reducing agent for controlling the crystallization of nanoparticles. The rapid diffusion of ions in DMF lead to the acceleration of the nucleation and the subsequent deposition of aggregations comprised of small nanoparticles [12,33,34]. In the present case, it can be proposed that DMF played a critical role in controlling the composition and microstructure of CuInS2, and DMF worked as both reductant [12] and solvent. Zeng and co-workers [10–12] proposed the Ostwald ripening process model for the for-
mation of hollow nanospheres. The Ostwald ripening involves ‘‘the growth of larger crystals from those of smaller size which have a higher solubility than the larger ones” [35,36]. Based on the above results, a mechanism of Ostwald ripening could be proposed for the formation of CuInS2 hollow spheres. However, in our synthetic system, the formation process of hollow nanoparticle-microspheres and hollow nanoflake-microspheres are somewhat different. Hence, two concrete ripening processes were produced, which were schematically outlined respectively in Fig. 6. The schematic diagram of possible ripening process for the nanoparticle-microspheres is shown in Fig. 6a. At the early reaction stage, some Cu2+ ions are reduced to Cu+ by DMF, and then Cu+ reacts with In3+ and S2 to form CuInS2 nanoparticles. For the minimization of the total energy of the system, these small nanoparticles aggregated together to form nanoparticle-spheres (step 1). Moreover, it is obvious to observe the size/ density variation inside these nanoparticle-spheres, which can be attributed to a
Fig. 6. Schematic illustration of the formation process of the two types of CuInS2 hollow microspheres: (a) nanoparticle-microspheres; (b) nanoflake-microspheres.
Y. Qi et al. / Microporous and Mesoporous Materials 114 (2008) 395–400
kinetically controlled process [37]. In the initial stage, a higher supersaturation solution provides a sufficient driving force to give a fast growth of a lot of poorly crystalline nanoparticles which comprised a primary core. When the supersaturation of mother solution is lower than a critical value, a comparatively lower driving force will give birth to many better crystalline nanoparticles on the external surface of the primary core. The poorly crystalline nanoparticles underneath the better crystalline shell will dissolve because of its higher solubility, and then second nucleation on the external surface (step 2). Thus, during this dissolverecrystallize process, the shell continued to grow at the expense of the core. At the end, no core existed and a hollow microphere was formed (step 3). The schematic diagram of the possible ripening process for nanoflake-microspheres is shown in Fig. 6b. In the primal solution, there are still many unreduced Cu2+ ions, which react with S2 to form CuS. Similarly, for the minimization of the total energy of the system, these CuS small nanoparticles aggregate together to form nanoflake-microspheres (step 1). These nanoflake-microspheres will act as both copper source and template for the subsequent formation of CuInS2 hollow microspheres. Compared with the anterior nanoparticle-microspheres, they will suffer a more complicated ripening process. Such a process includes two key points: (a) CuS nanoparticles on the surface of the primary spheres are reduced to Cu2S, which then in-situ reacted with In3+ and S2 in the solution to form CuInS2 nanoparticles coated on the surface of the primary CuS spheres (step 2); (b) cavum is formed in the interior spheres by dissolution of the CuS nanoparticles. Dissolved CuS is also reduced to Cu2S, then reacts with In3+ and S2 to form CuInS2, finally second crystallizes on the external surface (step 3). It should be mentioned that the reaction temperature has a significant effect on the yield of the products. A high yield of products could be produced over a temperature range from 160 to 170 °C. CuS always coexisted when
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the temperature was lower than 160 °C. This suggests that CuS could not be reduced drastically at a lower temperature. However, Cu always coexisted when the temperature was higher than 170 °C, indicating that CuS could be overly reduced at a higher temperature. The optical absorption properties of the porous CuInS2 hollow spheres were investigated by the UV–vis spectra (Fig. 7). From the full spectra, it is clear that the spectra show a broad and strong absorption peak at 900 nm. The band-gap energies can be estimated by a related curve of a2 versus photo energy plotted in figure based on the equation (aht)n = B(ht Eg), where a is the absorption coefficient, ht is the photo energy, B is a constant relative to the material, Eg is band gap, and n is either 1/2 for an indirect transition or 2 for a direct transition. From the intersection of the extrapolated linear portion, the Eg values of the porous CuInS2 can be determined as 1.50 eV, slightly lower than CuInS2 single crystals Eg = 1.53 eV [38], owing to largest nanoparticles and nanoflakes on the surface. 4. Conclusion In conclusion, ternary chalcogenide CuInS2 hollow microspheres have been successfully synthesized though a simple solvothermal procedure. A mechanism of Ostwald ripening and two concrete ripening processes were proposed for the formation of CuInS2 hollow spheres. Taking into account the unique properties of hollow spheres, such as low density, high specific surface area, and good permeation, the CuInS2 hollow microspheres developed in the present work have great potential applications. Given the versatility of this approach, we hope to extend it to the growth of other ternary chalcopyrite material. Further research will be performed on the well control of the homogeneous nucleation and heterogenous nucleation of the reaction system to tune the shape and size of building blocks of hollow CuInS2 microstructures. Acknowledgments
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Financial support by the National Natural Science Foundation of China (No. 20671086, 20621061), the Program for New Century Excellent Talents in University (NCET), and the 973 Projects of China is gratefully acknowledged.
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Appendix A. Supplementary data
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Wavelength (nm) Fig. 7. UV/Vis absorbance spectra of the porous CuInS2 hollow microspheres prepared at 160 °C for 25 h. The inset shows the plot of (aht)2 ht curve. Symbol arrowheads indicate the peaks from apparatus.
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Microporous and Mesoporous Materials 114 (2008) 395–400 www.elsevier.com/locate/micromeso
Template-free one-step fabrication of porous CuInS2 hollow microspheres Yunxia Qi, Kaibin Tang *, Suyuan Zeng, Weiwei Zhou Nanomaterial and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, PR China Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR China Received 6 September 2007; received in revised form 13 January 2008; accepted 16 January 2008 Available online 1 February 2008
Abstract Hollow CuInS2 microspheres have been successfully prepared via a simple solution-based route without employing templates and surfactants. Transmission electron microscope (TEM) images clearly show the hollow nature of the as-obtained products. Field emission scanning electron microscopy (FESEM) images reveal that the shells of as-prepared CuInS2 microspheres were constructed by nanoparticles or nanoflakes. The specific surface area and pore-size distribution of the obtained product as determined by gas-sorption measurements show that the CuInS2 hollow microspheres exhibit high Brunauer–Emmett–Teller (BET) surface area and porosity properties. TEM observations of intermediate products at different reaction stages indicate that these hollow CuInS2 microspheres are formed mainly via Ostwald ripening. Ó 2008 Elsevier Inc. All rights reserved. Keywords: CuInS2; Hollow microspheres; Aggregation; Ostwald ripening; Solvothermal
1. Introduction Materials with nanometer-to-micrometer hollow sphere structures have been received intense interest due to their potential applications in various areas. For example, in biotechnology, the hollow spheres can be used as controlled drug (molecule) release system [1,2], protection of biologically active agents [3]; in engineering, such structured materials can be utilized as low density materials; and in chemistry, hollow spheres can be employed as high surface-materials for catalysis and structure materials [4,5]. Driven by the technological importance of such structured materials, many efforts have been devoted to the synthesis of such hollow spheres. The most popular approach to obtain hollow nano(micro)spheres is template-directed synthetic methodology, which usually use removable template such as polystyrene beads, silica sol–gel [6–8], microemul*
Corresponding author. E-mail address: [email protected] (K. Tang).
1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.01.027
sion droplets [9], and so on. Other synthetic methods making use of some physical phenomena such as Ostwald ripening [10–12], the Kirkendall effect [13–16] and the oriented attachment process [17], have also been developed for the generation of hollow inorganic structures. I–III–VI2 chalcopyrite structure compounds have gained much attention because of their tunable electronic and optical properties [18–23]. As a typical ternary chalcopyrite material, CuInS2 has been considered as one of the most popular and promising candidates as absorber materials for photovoltaic applications because of its high absorption coefficient and environmental consideration [22]. Various CuInS2 structures, such as nanoparticles [24,25], nanorods [26,27], nanotubes [28], foam-like nanocrystallites [29], and microspheres [30–32] have been successfully fabricated by various methods. Nevertheless, it is still a grand challenge to prepare CuInS2 materials with controlled size and shape. Herein we report a template and surfactant free synthetic method to synthesize CuInS2 hollow structures by
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400 (008)
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2. Experimental section The synthesis of CuInS2 hollow spheres proceeded through a simple solvothermal route. InCl3 4H2O, CuSO4 5H2O, thiourea, N,N-dimethylformamide (DMF) were all of analytical grade and purchased from Shanghai Chemical Reagent Ltd without further purification. In a typical synthesis procedure, 0.5 mmol InCl3 4H2O (0.147 g), 0.5 mmol CuSO4 5H2O (0.125 g) and a slight excess of thiourea (0.10 g) were dissolved in 40 mL of N,N-dimethylformamide (DMF) to form a solution under vigorous stirring. The resulting solution was then transferred into a 50 mL teflonlined stainless steel autoclave and maintained at 160 °C for 25 h. The obtained black solid products were collected by centrifugation, which were then washed several times with deionized water and absolute ethanol and dried in a vacuum at 70 °C for 4 h. The phase purity of the as-prepared products was determined by X-ray diffraction (XRD) using a Philips X’Pert PRO SUPER X-ray diffractometer with Cu Ka radiation ˚ ). Transmission electron microscope (k = 1.54178 A (TEM) images were taken with a Hitachi H-800 transmission electron microscope at an acceleration voltage of 200 kV. High-resolution transmission electron microscope (HRTEM) images were performed on a JEOL-2010 transmission electron microscope. The scanning electron microscopy (SEM) images were taken using a Sirion 200 field emission scanning electron microscope (FESEM, 20 kV). Nitrogen adsorption analysis was carried out with a Micromeritics ASAP2020 system equipment (BET and BJH models, respectively, for specific surface area and porosity evaluation). The ultraviolet and visible spectra were recorded on a JGNA Specord 200 PC UV–visible spectrophotometer.
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the reaction of CuSO4 5H2O, InCl3 4H2O, thiourea and N, N-dimethylformamide (DMF) at a suitable concentration and temperature, in which DMF performed both as a solvent and a reducing agent. To the best of our knowledge, this is the first report on the synthesis of CuInS2 hollow spheres. The mechanism of growth hollow sphere morphologies was investigated on the ground of a series of control experiments on various reaction times.
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Y. Qi et al. / Microporous and Mesoporous Materials 114 (2008) 395–400
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2θ (degree) Fig. 1. XRD pattern of the hollow CuInS2 microspheres synthesized at 160 °C for 25 h.
3. Results and discussion The XRD pattern of the as-prepared samples at 160 °C for 25 h is shown in Fig. 1. All the diffraction peaks can be indexed as tetragonal phase CuInS2 (JCPDS 75–0106) with ˚ and c = 10.99 A ˚, the calculated cell parameters a = 5.52 A which are in accordance with the reported values. No characteristic peaks were detected as the other impurities such as CuS, Cu2S or In2S3. The morphology of the products obtained under the condition at 160 °C for 25 h was studied by field emission scanning electron microscopy (FESEM). Fig. 2a shows a panoramic FESEM image of a typical sample composed
Fig. 2. Images of the hollow CuInS2 microspheres synthesized at 160 °C for 25 h: (a) Low magnification FESEM image of overall products; (b) FESEM image of the hollow nanoparticle-microspheres, and an image of the cavity of a single microsphere (inset); (c, d) FESEM images of the hollow nanoflake-microspheres, and an image of the cavity of a single microsphere (inset); (e, f) TEM images of the hollow nanoparticlemicrospheres and their SAED patterns (inset); (g) HRTEM image of the nanoparticle-microspheres; (h, i) TEM images of the nanoflake-microspheres and their SAED patterns (inset); (j) HRTEM images of the hollow nanoflake-microspheres.
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Pore Volume
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Relative Pressure (P/Po) Fig. 3. Nitrogen adsorption–desorption isotherms of the CuInS2 hollow spheres prepared at 160 °C for 25 h. The inset shows its BJH pore size.
400 (008)
d
224
200
312 (116)
220 (204)
112
ent shapes and sizes. Fig. 5a shows that one kind of them are nanoparticle-spheres with diameters of 600–700 nm. In addition, spheres with pale edges and dark centers (Fig. 5a, inset) suggest the existence of size/density variations intrinsic inside the starting nanoparticle-spheres. The formation reason for these size/density variations will be discussed later. Fig. 5e shows another nanoflake-spheres with diameters of 2–3 lm. The XRD pattern of the sample collected after three hours (Fig. 4a) presented very weak peaks, characteristic of a poorly crystallized compound. However, further analysis from the XRD patterns (Fig. 4a) shown that the sample contains CuInS2 (JCPDS 75–0106) along with CuS(JCPDS 75-2235), which is in accordance with SAED results. The diffraction rings insetted in Fig. 5a can be indexed to (1 1 2), (2 2 0), and (3 1 2) of CuInS2, whereas the diffraction rings insetted in Fig. 5e can be indexed to (1 0 2), (1 0 3), and (1 1 0) of CuS. Summarizing, the samples collected at 3 h are comprised of CuInS2 phase nanoparticle-spheres and CuS phase nanoflakespheres. As the reaction time extended, the CuInS2 nanoparticle-spheres were found to be divided into discrete
Intensity (a.u)
of microspheres. Furthermore, by detailed observation (Fig. 2b–d), these microspheres could be distinguished into two different categories. A sort of these spheres are composed of nanoparticles, as shown in Fig. 2b. The diameters of such nanoparticle-spheres range from about 500 nm to 1 lm. Moreover, as indicated by the inset in Fig. 2b, many broken spheres and their apparent cavities can be observed (Fig. 2a), indicating that these spheres are of hollow structures. As shown in Fig. 2c and d, the other kinds of spheres are comprised of nanoflakes standing perpendicularly to the surfaces. The diameters of such nanoflake-spheres range from about 1 lm to 3 lm. Many of these spheres have also open pores (Fig. 2d, inset), indicating that these spheres are of hollow structures. In addition, the number of broken spheres in nanoparticle-spheres is more than that in nanoflake-spheres, which can be attributed to the shells of nanoparticle-spheres are thinner and then more breakable by post-sonication for FESEM observation than those of nanoflake-spheres. Further observation was provided by TEM images shown in Fig. 2e and h. Calculated from these TEM images, the shell thickness of nanoparticle-spheres (Fig. 2e) ranges from about 100 nm to 150 nm, and the shell thickness of nanoflake-spheres (Fig. 2h) is about 300 nm to 600 nm. In addition, TEM observations indicate the hollow nature of both nanoparticle-spheres and nanoflake-spheres with a high yield of above 90%. Meanwhile, the insets in Fig. 2f and i show the corresponding SAED patterns, indexed to the (1 1 2), (2 2 0), (3 1 2) planes of CuInS2, with a high polycrystalline structure. HRTEM images taken from edges of the hollow spheres are shown in Fig. S1 and Fig. S2, respectively, which clearly indicate that they are composed of many small nanocrystals with different orientations. Furthermore, the clear contrast difference as indicated by arrows in Fig. S1 and Fig. S2 shows many pores with size less than 5 nm presented. The lattice fringes are clearly visible with a spacing of 0.32 nm (Fig. 5g and j), which is corresponding to the (1 1 2) planes of tetragonal phase CuInS2. Fig. 3a shows the nitrogen adsorption–desorption isotherms. The isotherms are of type IV, which confirms the characteristics of mesoporous materials. The Brunauer– Emmett–Teller (BET) of the sample is about 25 m2 g 1. Barrett–Joyner–Halenda (BJH) model analysis of these as-prepared hollow spheres is shown in inset of Fig. 3, which gives one narrow peak centered at 3.5 nm in the pore size distribution and another broad peak in the region of 10–50 nm with an obvious maximum at 23 nm. The first pore diameter is in good agreement with the value of the nanopores investigated by HRTEM. And the larger pores result from the aggregation of nanoparticles and nanoflakes of these hollow spheres, which indicate that they are not tightly adhered to each other. In order to understand the formation process of the hollow spheres, samples collected at different reaction times were characterized by TEM, SAED, and XRD. As shown in Fig. 5a and e, at the early stage (3 h), the obtained samples are composed of two types of solid spheres with differ-
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c
b
a 10
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2θ (degree) Fig. 4. XRD patterns of samples synthesized at 160 °C for different reaction times: (a) 3 h; (b) 4 h; (c) 7.5 h; (d) 15 h. Symbol arrowheads indicate the diffraction peaks from CuS phase.
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Y. Qi et al. / Microporous and Mesoporous Materials 114 (2008) 395–400
Fig. 5. TEM images of samples synthesized at 160 °C for different reaction times: (a, e) 3 h; (b, f) 4 h; (c, g) 7.5 h; (d, h) 15 h. The insets show SAED patterns of the as-obtained samples. The inset at the bottom right corner of Fig. 5a shows an image of a single nanoparticle-microsphere.
regions (Fig. 5b). As the reaction proceeded, the cores shrank gradually (Fig. 5c and d), and eventually no cores remained (Fig. 2e). The corresponding SAED patterns are shown in the inset of Fig. 5b–d respectively. All of the SAED patterns can be indexed to (1 1 2), (2 2 0), and (3 1 2) of CuInS2. The diffraction rings in inset of Fig. 5b– d become progressively sharper with compared to Fig. 5a with aging time, suggesting a recrystallization process took place simultaneously. At the same time, the CuS nanoflakespheres were also followed by a solid core evacuation. As evidenced in Fig. 5f–h, the void space in these microspheres is getting bigger and bigger with aging time. The corresponding SAED patterns are shown in inset of Fig. 5f–h, respectively. However, the diffraction rings in Fig. 5 f and g show some characteristic of CuInS2 diffractions superimposed on CuS phase. When the reaction time was prolonged to 15 h, the SAED (Fig. 5h, inset) rings show that CuS phase is almost completely disappeared, and the SAED rings can be indexed to (1 1 2), (2 2 0), and (3 1 2) of CuInS2. Thus, it is obvious that the CuS nanoflake-microspheres did not only undergo a hollowing process but also go through a further phase transformation, which was also detected by XRD patterns (Fig. 4b–d). Moreover, with the reaction time increasing, peak intensities of CuInS2 in Fig. 4b–d increase accordingly. DMF has been proved to be a versatile solvent and reducing agent for controlling the crystallization of nanoparticles. The rapid diffusion of ions in DMF lead to the acceleration of the nucleation and the subsequent deposition of aggregations comprised of small nanoparticles [12,33,34]. In the present case, it can be proposed that DMF played a critical role in controlling the composition and microstructure of CuInS2, and DMF worked as both reductant [12] and solvent. Zeng and co-workers [10–12] proposed the Ostwald ripening process model for the for-
mation of hollow nanospheres. The Ostwald ripening involves ‘‘the growth of larger crystals from those of smaller size which have a higher solubility than the larger ones” [35,36]. Based on the above results, a mechanism of Ostwald ripening could be proposed for the formation of CuInS2 hollow spheres. However, in our synthetic system, the formation process of hollow nanoparticle-microspheres and hollow nanoflake-microspheres are somewhat different. Hence, two concrete ripening processes were produced, which were schematically outlined respectively in Fig. 6. The schematic diagram of possible ripening process for the nanoparticle-microspheres is shown in Fig. 6a. At the early reaction stage, some Cu2+ ions are reduced to Cu+ by DMF, and then Cu+ reacts with In3+ and S2 to form CuInS2 nanoparticles. For the minimization of the total energy of the system, these small nanoparticles aggregated together to form nanoparticle-spheres (step 1). Moreover, it is obvious to observe the size/ density variation inside these nanoparticle-spheres, which can be attributed to a
Fig. 6. Schematic illustration of the formation process of the two types of CuInS2 hollow microspheres: (a) nanoparticle-microspheres; (b) nanoflake-microspheres.
Y. Qi et al. / Microporous and Mesoporous Materials 114 (2008) 395–400
kinetically controlled process [37]. In the initial stage, a higher supersaturation solution provides a sufficient driving force to give a fast growth of a lot of poorly crystalline nanoparticles which comprised a primary core. When the supersaturation of mother solution is lower than a critical value, a comparatively lower driving force will give birth to many better crystalline nanoparticles on the external surface of the primary core. The poorly crystalline nanoparticles underneath the better crystalline shell will dissolve because of its higher solubility, and then second nucleation on the external surface (step 2). Thus, during this dissolverecrystallize process, the shell continued to grow at the expense of the core. At the end, no core existed and a hollow microphere was formed (step 3). The schematic diagram of the possible ripening process for nanoflake-microspheres is shown in Fig. 6b. In the primal solution, there are still many unreduced Cu2+ ions, which react with S2 to form CuS. Similarly, for the minimization of the total energy of the system, these CuS small nanoparticles aggregate together to form nanoflake-microspheres (step 1). These nanoflake-microspheres will act as both copper source and template for the subsequent formation of CuInS2 hollow microspheres. Compared with the anterior nanoparticle-microspheres, they will suffer a more complicated ripening process. Such a process includes two key points: (a) CuS nanoparticles on the surface of the primary spheres are reduced to Cu2S, which then in-situ reacted with In3+ and S2 in the solution to form CuInS2 nanoparticles coated on the surface of the primary CuS spheres (step 2); (b) cavum is formed in the interior spheres by dissolution of the CuS nanoparticles. Dissolved CuS is also reduced to Cu2S, then reacts with In3+ and S2 to form CuInS2, finally second crystallizes on the external surface (step 3). It should be mentioned that the reaction temperature has a significant effect on the yield of the products. A high yield of products could be produced over a temperature range from 160 to 170 °C. CuS always coexisted when
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the temperature was lower than 160 °C. This suggests that CuS could not be reduced drastically at a lower temperature. However, Cu always coexisted when the temperature was higher than 170 °C, indicating that CuS could be overly reduced at a higher temperature. The optical absorption properties of the porous CuInS2 hollow spheres were investigated by the UV–vis spectra (Fig. 7). From the full spectra, it is clear that the spectra show a broad and strong absorption peak at 900 nm. The band-gap energies can be estimated by a related curve of a2 versus photo energy plotted in figure based on the equation (aht)n = B(ht Eg), where a is the absorption coefficient, ht is the photo energy, B is a constant relative to the material, Eg is band gap, and n is either 1/2 for an indirect transition or 2 for a direct transition. From the intersection of the extrapolated linear portion, the Eg values of the porous CuInS2 can be determined as 1.50 eV, slightly lower than CuInS2 single crystals Eg = 1.53 eV [38], owing to largest nanoparticles and nanoflakes on the surface. 4. Conclusion In conclusion, ternary chalcogenide CuInS2 hollow microspheres have been successfully synthesized though a simple solvothermal procedure. A mechanism of Ostwald ripening and two concrete ripening processes were proposed for the formation of CuInS2 hollow spheres. Taking into account the unique properties of hollow spheres, such as low density, high specific surface area, and good permeation, the CuInS2 hollow microspheres developed in the present work have great potential applications. Given the versatility of this approach, we hope to extend it to the growth of other ternary chalcopyrite material. Further research will be performed on the well control of the homogeneous nucleation and heterogenous nucleation of the reaction system to tune the shape and size of building blocks of hollow CuInS2 microstructures. Acknowledgments
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Financial support by the National Natural Science Foundation of China (No. 20671086, 20621061), the Program for New Century Excellent Talents in University (NCET), and the 973 Projects of China is gratefully acknowledged.
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Appendix A. Supplementary data
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Absorbance
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Wavelength (nm) Fig. 7. UV/Vis absorbance spectra of the porous CuInS2 hollow microspheres prepared at 160 °C for 25 h. The inset shows the plot of (aht)2 ht curve. Symbol arrowheads indicate the peaks from apparatus.
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