CRYSTAL GROWTH & DESIGN
Copper-Indium Sulfide Hollow Nanospheres Synthesized by a Facile Solution-Chemical Method
2008 VOL. 8, NO. 7 2402–2405
Aiyu Zhang, Qian Ma, Mengkai Lu,* Guangwei Yu, Yuanyuan Zhou, and Zifeng Qiu State Key Laboratory of Crystal Materials, Shan Dong UniVersity, Jinan 250100, P. R. China ReceiVed December 20, 2007; ReVised Manuscript ReceiVed March 1, 2008
ABSTRACT: Chalcopyrite CuInS2 is an important photovoltaic material. CuInS2 hollow nanospheres with diameters of 80-100 nm have been synthesized by a surfactant-assisted solution-chemical route. Structural characterization indicated that shells of the hollow spheres are composed of CuInS2 nanoparticles of about 10 nm in size. A vesicle-template mechanism was proposed to explain the formation process of the hollow structure, during which amorphous hollow structures are first formed on the surfactant template and then crystallize in the refluxing process. Introduction Ternary I-II-VI2 Chalcopyrite semiconductors Cu(In/Ga)(S/ Se)2 (CIS) are well-known as photovoltaic materials,1 for their excellent properties such as high absorption coefficients, structural defect tolerance and good energy matching between their band gaps (ranging from 1.5 eV to 1.1 eV) and the solar spectrum. Efficiency of 18.8% has been recorded for solar cells based on CIS materials.2 Copper indium disulfide (CuInS2) with a band gap of 1.5 eV has the potential to attain high conversion efficiencies (CE)3 and is low cost compared to other CIS materials. Several ways have been proposed to improve CE of the solar cells, since it is one of the most important parameters to optimize for implementing photovoltaic cells on a truly large scale.4 In these routes, nanotechnology is considered as an effective one.5 In contrast to the large number of research papers on quantum dot and nanostructure synthesis of other materials, relatively few reports have been published about the synthesis of nanosized CIS structures. CuInS2 nanoparticles have been obtained by a molecular single-source precursor method,6 which has a drawback in that it involves complicated organic reactions and toxic organic reagents. Hydrothermal, solvothermal, one-pot synthesis and copper indium sulfide methods have also been applied to synthesize different nanostructures such as nanorods, nanotubes, nanoacorns, foam-like and flower vase-like nanostructures.7,8 To the best of our knowledge, however, no report is related to the synthesis of CuInS2 hollow nanospheres, which has more potential to improve the performance of the CIS apparatus. Hollow spheres have a low density and high surface area, and spherically monodisperse morphology is an important factor for the low-light scattering at the surface. Nanomaterials with hollow spherical structure are quite important in the wide range of applications include catalysis, drug delivery, chemical storage, optoelectronics, photonic crystals, and microcavity resonance,9 owing to their unique structural, optical, and surface properties. Compared with solid spheres, the design and fabrication of hollow spheres is more difficult. In this study, we aimed to synthesize nanosized CuInS2 hollow spheres. Considering the complication of the organic reaction, common inorganic metal salts were used as raw materials instead of the hazardous and expensive organic reagents. Surfactant cetyltrimethylammonium bromide (CTAB) was applied to control the microstructure of the product. The * Corresponding author. E-mail:
[email protected].
formation mechanism of these spherical structures was studied systematically. Experimental Section Chemical. Copper acetate (Cu(CH3COO)2 · H2O), (indium nitrate In(NO3)3 · 4.5H2O), thioacetamide (TAA), and CTAB were used as starting materials. All the reagents were of analytical grade without further purification. Cu(CH3COO)2 · H2O, In(NO3)3 · 4.5H2O, and TAA were dissolved in ethylene glycol (EG) to make stock solutions with concentrations of 0.1 M, 0.02 M, and 0.1 M, respectively. Synthesis. In a typical synthesis procedure, CTAB (1.822 g, 5mmol) was dissolved in EG (40 mL) by stirring and heating. TAA/EG (5 mL) was added into this CTAB/EG solution under stirring. The mixture was preheated to 80 °C and then In(NO3)3/EG (1 mL) was added. The formed solution was kept at 80 °C for about 15 min, before Cu(CH3COO)2/EG (5 mL) was quickly injected. The final concentration of CTAB was about 0.1 M. The as-obtained solution was drastically stirred and transferred into a three-necked flask equipped with a condenser. An aliquot of the solution was removed at this time and marked as S1. The residual mixture was then heated and refluxed. Another aliquot (S2) was removed as soon as the solution began to boil. The reaction was terminated in 1.5 h by rapidly cooling the mixture to room temperature in a water bath. The final product together with S1 and S2 was centrifuged to separate precipitates, which were washed thoroughly with acetone and ethanol in that order, dried in atmosphere, and then collected for characterization. Parallel experiments were also carried out with different surfactants and copper source. Characterization. X-ray powder diffraction (XRD) measurements were carried out on a Germany Bruker Axs D8-Avance X-ray diffractometer with graphite monochromatized Cu KR irradiation (λ ) 1.5418 Å). The morphologies and microstructures of the samples were studied by scanning electron microscopy (SEM) (Hitachi, S-4800), transmission electron microscopy (TEM) (JEM-100CX) and highresolution transmission electron microscopy (HRTEM) (JEOL, JEM 2010). Stoichiometry data were obtained from energy dispersive spectrometry (EDS) (Horiba EMAX Energy, EX-350).
Results and Discussion The morphology and microstructure of CuInS2 hollow nanospheres are demonstrated by TEM and SEM images. As shown in Figure 1a, the strong contrast between the bright edge and the dark center is evidence for the hollow nature. The diameter of the hollow spheres is in the range of 80-100 nm. Some of the hollow spheres are connected and show a peanut-like structure. In addition, a few fragments can be found during the TEM observation, indicating that the spheres are not very compact and some of them may be destroyed by intensive postsonication. From the SEM images shown in Figure 2,
10.1021/cg701257x CCC: $40.75 2008 American Chemical Society Published on Web 05/28/2008
Copper-Indium Sulfide Hollow Nanospheres
Crystal Growth & Design, Vol. 8, No. 7, 2008 2403
Figure 1. TEM images of (a) the samples synthesized with CTAB concentration (CCTAB) of 0.1 M and the refluxing time (Tr) of 1.5 h, (b) S1, (c) S2, and (d) sample synthesized also with CCTAB ) 0.1 M, Tr ) 1.5 h, but using Cu(NO3)3 instead of Cu(CH3COO)2 as the Cu source.
Figure 3. (a-c) HRTEM images, (d) XRD, and (e) SAED patterns of CuInS2 hollow spheres (CCTAB ) 0.1 M, Tr ) 1.5 h). Arrow 1 denotes the single spheres, and arrow 2 indicates the connected spheres showing a peanut-like structure. (The scale is the same for panels (a-c), and the bar is shown in (b) indicating 100 nm.)
Figure 2. SEM micrographs of CuInS2 spheres with different magnification. (The synthesis condition: CCTAB ) 0.1 M, Tr ) 1.5 h.)
spherical nanostructures with a rough exterior have been observed. The diameter of the spheres is about 80-100 nm, in accordance with the TEM results. A broken sphere is marked by a white arrow in Figure 2a-c, further demonstrating the hollow nature of the products. HRTEM provides further insight into the microstructure of the CuInS2 nanospheres. As shown in Figure 3a-c, the walls of the CuInS2 nanospheres are really constructed by nanocrystals of about 10 nm in size. The peanutlike structures can also be observed clearly from the HRTEM images. The phase composition of the hollow spheres was characterized by XRD and selected area electron diffraction (SAED). The XRD pattern in Figure 3d confirms the formation of the CuInS2 phase. All the diffraction peaks can be indexed to the tetragonal CuInS2. The SAED pattern in Figure 3e shows concentric diffracted rings indicating the polycrystalline nature of the product. The rings can also be indexed to (112), (204), (220), (312), and (116) reflections of CuInS2. Furthermore, energy dispersive spectroscopy (EDS) analysis was utilized to determine the chemical composition of the nanospheres. EDS
Figure 4. EDS spectrum of CuInS2 hollow spheres (CCTAB ) 0.1 M, Tr ) 1.5 h). Table 1. Element composition of CuInS2 Hollow Spheresa elements
S
Cu
In
weight percentage (%) atom percentage (%)
28.34 51.94
27.56 25.49
44.09 22.57
a
CCTAB ) 0.1 M, Tr ) 1.5 h.
in Figure 4 reveals that the sample is exactly composed of copper, indium, and sulfur elements. No element of organic matter such as C, H, O, or Br was detected in our test. The peak around 1.5 KeV is attributed to the substrate element Al. The calculated element composition is shown in Table 1. The average atomic ratio of Cu/In/S is about 1:0.89:2.04, and thus the product is Cu-rich. The excess Cu might exist in the form of CuS, Cu2S, or Cu. In the XRD analysis, however, no other phase has been observed except CuInS2. It may be inferred that
2404 Crystal Growth & Design, Vol. 8, No. 7, 2008
Zhang et al.
Scheme 1. I-V Indicate the Formation Process of the Hollow Structurea
a
A, B denote two different vesicle templates.
the final product is the mixture of CuInS2 nanocrystals and a small quantity of Cu-rich amorphous phase. It is known that TAA can combine with free H+ to release H2S in water, and then H2S decomposes to give S2-, which can react with metal cation to form metal sulfide.10 However, it is difficult to obtain pure CuInS2 phase from aqueous solution through the common chemical routes, because of the solubility product difference between In2S3 and CuS (or Cu2S). Cu2+ and S2- will form black precipitate immediately when the Cu2+, In3+, and S2- are mixed in one aqueous solution, without the formation of CuInS2. Furthermore, the reaction rate is too high in the aqueous solution to produce well-crystallized particles with uniform size. So we chose EG as the reaction solvent in the present work. The reaction rate is much lower in EG than in aqueous solution, because H2S forms much more slowly in EG than in water due to the smaller quantity of H free-radical in glycol. Polyol is also a reducing agent, and the polyol method has been well-used to prepare different elemental metals and alloys.11 In the present work, EG acted both as solution and reductant, and it reduced Cu(II) to Cu(I) in the reaction. The formation routes of CuInS2 can be summarized as follows:
H2O f H· + OH·
(1)
H2O- trace water is contained in EG and from the crystal water in the metal salts.
C2H6O2 f H· + C2H5O2 ·
(2)
2H· + RS f H2S + R · (RS ) CH3CSNH2)
(3)
H2S f H+ + S2-
(4)
Cu2+ f Cu+
(5)
2-
S
+
+ Cu + In
3+
f CuInS2
(6)
In the above reaction process, the existence of trace water is required, because the generation of H2S is mainly dependent on the water. Hardly any H2S was detected when anhydrous TAA/EG solution was refluxed. A probative experiment is described in the Supporting Information. To better understand the formation mechanism of CuInS2 hollow spheres, a series of parallel experiments were carried out. Keeping other experimental conditions constant, when no surfactant was used, or when sodium dodecylbezenesulfonate
(DBS) was used to replace CTAB, only irregular CuInS2 nanoparticles were obtained. This indicates that the use of surfactant CTAB is the prerequisite for the formation of hollow nanospheres. The concentration of CTAB (CCTAB) in the system also plays an important role. Distributed nanocrystals were the main product at low CCTAB (
