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J. Phys. Chem. B 2005, 109, 20676-20679
ARTICLES InOOH Hollow Spheres Synthesized by a Simple Hydrothermal Reaction Hongliang Zhu,*,†,‡ Kuihong Yao,† Hui Zhang,‡ and Deren Yang‡ Center of Materials Engineering, Zhejiang Sci-Tech UniVersity, Xiasha UniVersity Town, Hangzhou 310018, People’s Republic of China, and State Key Lab of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China ReceiVed: March 8, 2005; In Final Form: September 11, 2005
Novel micrometer-sized indium oxyhydroxide (InOOH) hollow spheres were successfully synthesized via a citric acid (CA) assisted hydrothermal process. The morphology, crystal structure, and optical properties of the product were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, and UV-vis diffuse reflectance spectroscopy (DRS). The optical band gap, Eg, was estimated to be 3.5 eV from the DRS spectrum, which is almost equal to that of indium oxide. Furthermore, on the basis of a series of SEM observations, phenomenological elucidation of a mechanism for the growth of the InOOH hollow spheres has been presented; key factors for the formation of the structures have been proposed.
1. Introduction In recent years there has been great interest in the synthesis of nanostructures with specific morphologies and functionality.1,2 Due to their unique structural, optical, and surface properties, inorganic hollow spheres with nanometer to micrometer sizes have various potential applications in drug delivery, chemical storage, photonic crystals, catalysis, optoelectronics, and microcavity resonance.3-8 A variety of methods have been employed to fabricate hollow spheres of inorganic materials, including liquid droplets,9 latex templates,10 polymer templates,11 polymer beads,12 emulsion and microemulsion droplets,13,14 and inorganic nanoparticles.15 However, these methods require template materials to build sphere architectures, and the templates needed to be removed later. Under certain conditions nanoparticles can undergo self-assembly and form three-dimensional structures. The optical and electronic properties of these structures are dependent on both the initial nanoparticles and the manner in which they organized. More recently, many self-assembly processes have been used to assemble inorganic nanoparticles into hollow spheres.16-19 Herein, we report that well-crystallized micrometer-sized InOOH hollow spheres can be synthesized by a simple citric acid (CA) assisted hydrothermal process, and CA plays an important role in the formation of hollow spheres. The importance of indium, indium oxide, and indium hydroxide is related to the semiconducting and optical properties of these materials.20-22 Indium oxyhydroxide (InOOH) is the hydrate of indium sesquioxide (In2O3). R. Roy and M. W. Shafer23 reported that In(OH)3 decomposed at 245 °C to directly yield InOOH and that InOOH decomposed into In2O3 at 435 °C. Recently, nanostructures of In2O3, an important n-type transparent semiconductor with a direct band gap of ∼3.6 eV, have been prepared by annealing its InOOH counterparts.24,25 * To whom correspondence should be addressed. Phone/fax: +86 571 86843266. E-mail:
[email protected]. † Zhejiang Sci-Tech University. ‡ Zhejiang University.
More importantly, the Eg of the InOOH hollow spheres in our study was estimated to be 3.5 eV, which is typical for wide band-gap semiconductors. Therefore, the novel InOOH hollow spheres reported here have potential applications in nanoscience and nanotechnology. Due to simplicity, high efficiency, low cost, and suitability for environmentally friendly large-scale production, the CAassisted hydrothermal synthesis of the hollow spheres reported here has proved to be a promising synthesis route for InOOH hollow spheres. Furthermore, a phenomenological growth mechanism for the hollow spheres has been presented, and key factors have been proposed. 2. Experimental Section All reagents were of analytical grade without further purification. A 1.7g amount of InCl3‚3H2O (0.0064 mol) and 0.5 g of citric acid (CA, C6O7H8‚H2O) were added to 160 mL of deionized water with stirring. Then 0.256 g of NaOH (0.0064 mol) was added to the aforementioned solution, meaning that the mole ratio of InCl3:CA:NaOH was 3:1:3. No precipitate was formed, and the final solution was clear. The pH of the solution (pH 2.4) was measured using a Thermo Orion pH meter. After 2 min of stirring the solution was put into a Teflon-lined stainless steel autoclave of 200 mL capacity and sealed. The autoclave was maintained at 220 °C for 24 h and then cooled to room temperature. The mixture turned white due to formation of the InOOH precipitate. The product was centrifuged, filtered out, and rinsed with alcohol and deionized water several times and then dried at 60 °C for 1 h in air. The X-ray diffraction (XRD) pattern was obtained on a Thermo ARL XTRA X-ray diffractometer with Cu KR radiation (λ ) 1.54178 Å). Transmission electron microscopy (TEM) observation was performed with a JEM 200 CX transmission electron microscope operated at 160 kV. A JEOL JSM-5610LV scanning electron microscope (SEM) and an Oxford Instrument’s INCA energy-dispersive spectrometer (EDS) were
10.1021/jp0511911 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/18/2005
InOOH Hollow Spheres
J. Phys. Chem. B, Vol. 109, No. 44, 2005 20677
Figure 1. XRD pattern of the InOOH hollow spheres.
employed to observe the morphology and analyze chemical composition. To characterize the chemical composition and the optical properties, Fourier transform infrared spectroscopy (FTIR) and UV-vis diffuse reflectance spectroscopy (DRS) were performed on two Perkin Elmer machines, namely, a Spectrum One FT-IR spectroscopy machine and a Lambda 900 UV/Vis spectroscopy machine. 3. Results and Discussion The XRD pattern of the product is shown in Figure 1. All the peaks can be indexed to orthorhombic InOOH with lattice constants a ) 5.26 Å, b ) 4.56 Å, and c ) 3.27 Å, which is in good agreement with values from the standard card (JCPDS No. 71-2277). The strong and sharp diffraction peaks indicate that the product is well crystallized. Average crystalline size has been estimated by Scherrer’s formula
D ) K λ/(β cos θ)
(1)
where K ) 0.9 is the shape factor, λ is the X-ray wavelength of Cu KR radiation (0.154 nm), θ is the Bragg angle, and β is the experimental full-width half maxium (fwhm) of the respective diffraction peak (in units of radians). The grain size of the hollow spheres was estimated to be around 20 nm. SEM and TEM were used to characterize the morphology and crystal structure of the product. Typical SEM images are shown in Figure 2, which reveals that the product is composed of InOOH hollow spheres with an average diameter of 5 µm. Cavities are occasionally found in the spheres as shown in Figure 2a. The hollow sphere structure can be vividly observed by the magnified SEM image of a broken hollow sphere inserted in Figure 2a. More interestingly, as shown in Figure 2b, the hollow spheres are assembled from thousands of nanoparticles with a diameter of decades of nanometers, and the crystalline size observed from the SEM image is consistent with that estimated by Scherrer’s formula. A typical bright-field TEM image and selected area electron diffraction (SAED) pattern on a hollow sphere are shown in Figure 3. Figure 3 clearly confirms that the diameter of the InOOH hollow spheres is around 5 µm, consistent with the value shown in the SEM images. Additionally, white contrast was found at the center of the InOOH spheres in the TEM image. Consequently, it can be concluded that the InOOH spheres are hollow inside. The SAED pattern shows that the InOOH nanoparticles assembling in the halosphere walls have hexagonal symmetry. The first and second rings of the strong spots in the SAED pattern correspond to the (011) and (301) reflections of the orthorhombic InOOH structure, respectively. Due to the hollow sphere structure it is possible that the reactants and complexing agent (citric acid) might remain inside. Thus, the chemical composition was analyzed by EDS and FTIR, and the spectra are shown in Figures 4 and 5, respectively. The strong peaks from In and O are found in the EDS spectrum.
Figure 2. SEM images of the InOOH hollow spheres: (a) several hollow spheres and (b) magnified image.
Figure 3. TEM image of the InOOH hollow spheres with its SAED pattern.
The Au peaks came from the thin gold layer for conductive coating. The In/O atomic ratio was ∼1:2, calculated using the Oxford Instruments INCA software package (INCA Energy 2000), consistent with the theoretical In/O atomic ratio of InOOH. The FT-IR spectrum of Figure 5 exhibits strong broad absorption bands at 3435 cm-1 and a narrow band at 1635 cm-1, confirming the presence of crystallization water molecules.26,27 The water molecules might reside inside the spheres. Normally, the VCH bands are located at 3000-2700 cm-1. There are no bands between 3000 and 2500 cm-1, indicating that no citric acid residues reside inside the hollow spheres. It is important to determine the optical properties of the InOOH hollow spheres, which are assembled of the nano-
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Figure 4. EDS spectrum of the InOOH hollow spheres.
Figure 5. FT-IR spectrum of the InOOH hollow spheres.
Figure 6. UV-diffuse reflectance spectrum (DRS) of the InOOH hollow spheres.
particles. To measure the band gap DRS characterization was employed. The DRS spectrum was obtained on a Perkin Elmer Lambda 900 UV/Vis spectroscopy machine at room temperature, and a typical spectrum is shown in Figure 6. On the basis of the DRS absorbance, estimation of the optical band gap, Eg, is 3.5 eV. Importantly, the Eg of 3.5 eV, which is typical for wide band-gap semiconductors, is almost equal to that of In2O328 and is much smaller than that of In(OH)3 (5.15 eV).29,30 Citric acid (CA) is a versatile, widely used, cheap, and safe complexing agent.31-34 In this work the hydrothermal temperature, CA, and pH play critical roles in hydrothermal formation of the InOOH hollow spheres. To verify this point, the final solution mentioned in the Experimental Section was hydrothermally processed at 200, 220, 250, and 270 °C. The InOOH hollow spheres were only obtained at hydrothermal temperatures above 220 °C, while In(OH)3 nanocrystals formed at 200 °C. In addition, the hydrothermal temperature made no difference in the shape of the hollow spheres. We speculate that formation of the hollow spheres is related to the transformation of In(OH)3 into InOOH. When sodium citrate instead of CA was used as complexing agent in the same conditions, only micrometer-sized In(OH)3 spheres were obtained (shown in Figure 7a). Only In(OH)3 nanorods were formed by a non-CA-assisted hydrothermal process in the same conditions, as we previously reported.35 Interestingly, the products change with the concentration of CA
Figure 7. SEM images of the products prepared with different complexing agent: (a) sodium citrate instead of CA as complexing agent, (b) low concentration of CA (mol ratio of CA:InCl3 was 1:6), and (c) high concentration of CA (mol ratio of CA:InCl3 was 1:1).
(shown in Figure 7b and c). Figure 7b reveals that the product prepared with a low concentration of CA has two structures, hollow spheres and cube-shaped crystals. Formation of the cubeshaped crystals was due to shortage of CA to react with In3+ to yield indium citrate complexes. While there was excessive CA in solution, as shown in Figure 7c, the hollow spheres are irregular. Therefore, CA is extremely critical for formation of hollow spheres. Likewise, the products change with the concentration of NaOH. On increasing the NaOH/InCl3 mole ratio of the solution to 3:1 (pH 3.6), In(OH)3 submicrometer spheres instead of InOOH hollow spheres were obtained. On increasing the pH to 4.8, In(OH)3 nanocrystals were obtained. When no
InOOH Hollow Spheres
J. Phys. Chem. B, Vol. 109, No. 44, 2005 20679 4. Conclusions Novel InOOH hollow spheres with a diameter of several micrometers have been successfully prepared by a simple citric acid assisted hydrothermal process. The morphology, crystal structure, and optical properties were characterized by XRD, TEM, SEM, FT-IR, and UV-vis. The optical band gap, Eg, was estimated from the DRS spectrum to be 3.5 eV, which is almost equal to that of indium oxide. The hollow spheres were assembled of thousands of InOOH nanoparticles with a diameter of several decade nanometers. A phenomenological mechanism for the growth of InOOH hollow spheres has been presented. Hydrothermal temperature, citric acid, and pH are three key factors for formation of the structures. Acknowledgment. We appreciate the Natural Science Foundation of China (No. 50442025). We also thank Lina Wang for SEM and UV-vis measurements. References and Notes
Figure 8. Evolution of the InOOH hollow spheres: (a) hydrothermally processed for 2, (b) 3, and (c) 4 h.
NaOH was added to the solution, no solid product was obtained. Therefore, the pH of the solution is another critical factor. Evolution of the InOOH hollow spheres was also investigated in our study, and Figure 8 shows the formation process of the structure. Figure 8a clearly reveals the formation of irregular colloid rings. Importantly, the diameter of the rings is 1-5 µm, a bit smaller than that of the InOOH spheres. Figure 8b shows that the colloid rings become much more regular. Subsequently, on increasing the hydrothermal treatment time to 4 h, the rings transformed into hollow spheres as shown in Figure 8c. However, the hollow sphere structure of Figure 8c is still not “grown up”. On the basis of the above discussion it can be concluded that formation of the InOOH hollow spheres was due to the aggradation of colloids along the radial direction and the circumferential direction of the rings. It is well documented that metallic ion can react with citric acid and OH- to yield citrate complexes.34,36,37 In this study the mole ratio of InCl3:CA:NaOH was 3:1:3 and the final solution was clear. Therefore, the indium citrate complexes were formed in solution. During the hydrothermal process hydrolysis of the indium citrate complexes occurred, and the InOOH colloids were generated. Then the colloids were assembled into the rings, and the hollow spheres were formed.
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