Single-Crystalline Indium Hydroxide and Indium Oxide Microcubes

of inorganic particles with a uniform size and shape have been of great interest in ... sulfate (SDS), were analytical grade from the Beijing Chem...
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J. Phys. Chem. C 2008, 112, 18426–18430

Single-Crystalline Indium Hydroxide and Indium Oxide Microcubes: Synthesis and Characterization Xiaohe Liu,* Libin Zhou, Ran Yi, Ning Zhang, Rongrong Shi, Guanhua Gao, and Guanzhou Qiu* Department of Inorganic Materials, Central South UniVersity, Changsha, Hunan 410083, People’s Republic of China ReceiVed: April 1, 2008; ReVised Manuscript ReceiVed: April 25, 2008

Uniform single-crystalline indium hydroxide microcubes can be successfully synthesized in large quantities via a convenient hydrothermal route using hydrated indium nitrate and sodium borohydride as reagents under mild conditions. The morphology and size of indium hydroxide microcubes can be controlled by varying the synthetic parameters such as hydrothermal time, reaction temperature, and surfactant. Single-crystalline indium oxide microcubes also can be successfully prepared by a thermal decomposition method using indium hydroxide microcubes as the precursor. The phase structures, morphologies, and optical properties of the final products were investigated in detail by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, differential scanning calorimetric analysis, thermogravimetric analysis, and photoluminescence spectroscopy. These uniform single-crystalline microcubes may be useful in microelectronic and optoelectronic devices. Introduction During the past few decades, inorganic particles with a uniform size and shape have drawn considerable attention due to their unique size- and shape-dependent physicochemical properties.1–6 The design and controlled synthesis of inorganic particles with a uniform size and shape have been of great interest in recent years based on their importance in basic scientific research and potential technology applications in electronic and optoelectronic devices.7–9 Until now, many kinds of inorganic particles with a novel shape and controlled size have been successfully prepared. Recently, a particularly significant breakthrough in the synthesis of cubic particles was made by Sun and Xia via reducing silver nitrate with ethylene glycol in the presence of poly(vinyl pyrrolidone).10 Various kinds of cubic particles such as metals,11 alloys,12 oxides,13 chalcogenide,14 borides,15 and fluorides16 also have been reported via varied synthetic reactions at room or slightly elevated temperatures. However, the development of synthetic methods for the fabrication of cubic particles with a uniform size remains a highly sophisticated challenge to chemists and material scientists. Cubic particles expose a specific surface, which provides an ideal model for the study of surface related properties. In particular, connecting these cubic particles into microscale devices may provide future projects. Indium compounds such as indium hydroxide and indium oxide were investigated intensively for many years because of their novel optical and electrical properties and potential applications in various fields. Indium hydroxide is an important semiconductor with a wide band gap (Eg ) 5.15 eV) and has promising applications as a photocatalyst.17 Indium oxide, an important n-type transparent semiconductor with a wide band gap of 3.55-3.75 eV, has been used widely as ultrasensitive sensors (e.g., in detecting NO2, NH3, and DNA, etc.),18–20 transparent conductors,21 solar cells,22 and solid-state optoelec* To whom correspondence should be addressed. E-mail: (X.L.) liuxh@ mail.csu.edu.cn and (G.Q.) [email protected]; tel.: +86-731-8830543; fax: +86-731-8879815.

tronic devices.23 In recent years, various methods such as chemical vapor deposition, hot-injection techniques, organic solution synthetic routes, hydrothermal methods, and solvothermal methods have been exploited to synthesize indium hydroxide and indium oxide with varied shapes (e.g., nanotubes,24 nanowires,25 nanorods,26 nanobelts,27 and hollow spheres,28 etc.). In particular, cubic nanoparticles of indium hydroxide and indium oxide also were successfully synthesized.29 However, the production of indium hydroxide and indium oxide microcubes has not been realized up to now. Herein, we demonstrated that uniform single-crystalline indium hydroxide microcubes could be successfully synthesized in large quantities via a convenient, inexpensive, and reproducible hydrothermal synthetic route under mild conditions. The influences of hydrothermal time, reaction temperature, and surfactant on the shape and size of microcubes were carefully investigated. Furthermore, single-crystalline indium oxide microcubes also could be successfully fabricated via calcining the corresponding indium hydroxide counterparts. Single-crystalline indium hydroxide and oxide microcubes may exhibit some novel properties, which can lead to new and important applications in microelectronic and optoelectronic devices. Experimental Procedures All chemicals in this work, such as hydrated indium nitrate (In(NO3)3 · 9H2O), sodium borohydride (NaBH4), poly(vinyl pyrrolidone) (PVP), and sodium dodecyl sulfate (SDS), were analytical grade from the Beijing Chemical Factory. They were used without further purification. Deionized water was used throughout. Synthesis. In a typical procedure, 0.1910 g of In(NO3)3 · 9H2O was first put into a Teflon-lined autoclave of 50 mL capacity and dissolved in 20 mL of deionized water at room temperature. Then, the solution of 0.0128 g of NaBH4 dissolved in 10 mL of deionized water was gradually added into the autoclave under vigorous stirring. The autoclave was filled with deionized water up to 75% of the total volume, sealed, and maintained at 100-160 °C for a period of 2-12 h without shaking or stirring.

10.1021/jp802778p CCC: $40.75  2008 American Chemical Society Published on Web 11/04/2008

Indium Hydroxide and Indium Oxide Microcubes

Figure 1. XRD pattern of as-prepared In(OH)3 microcubes synthesized in the absence of surfactants at 140 °C for 12 h.

The autoclave was allowed to cool to room temperature naturally after heat treatment. The resulting white solid products were filtered, washed with distilled water and absolute ethanol, and finally dried in vacuum at 70 °C for 6 h. Indium oxide microcubes could be obtained via a thermal decomposition method using indium hydroxide microcubes as the precursor in air at 400 or 600 °C for 2 h. Characterization. The obtained samples were characterized on a D/max2550 VB+ powder X-ray diffraction (XRD) instrument with Cu KR radiation (λ ) 1.54178 Å). The operation voltage and current were kept at 40 kV and 40 mA, respectively. The size and morphology of the as-synthesized products were determined at 20 kV by a XL30 S-FEG scanning electron microscopy (SEM) instrument at 160 kV by a JEM200CX transmission electron microscopy (TEM) instrument and a JEOL JEM-2010F high-resolution transmission electron microscopy (HRTEM) instrument. Selected area electron diffraction (SAED) was further performed to identify the crystallinity. Photoluminescence (PL) experiments were conducted on a Hitachi F-4500 fluorescence spectrophotometer with a Xe lamp as the excitation light source at room temperature. Results and Discussion Powder X-ray diffraction (XRD) was used to characterize the composition and phase structure of the as-synthesized products. Figure 1 shows a typical XRD pattern of the as-synthesized In(OH)3 microcubes prepared with the absence of surfactants at 140 °C for 12 h. All of the reflections of the XRD pattern could be indexed to body-centered cubic In(OH)3 with a lattice constant a ) 7.97 Å, which are consistent with reported data (JCPDS Card No. 76-1464). No peaks of impurities were observed, indicating that the final product is a pure phase compound. The narrow and strong peaks showed good crystallinity of In(OH)3 microcubes. The size and morphology of the as-synthesized products were characterized by using SEM and TEM. Figure 2A shows the low magnification SEM image of the as-synthesized In(OH)3 microcubes, in which microcubes with good uniformity were clearly observed, indicating that a large amount of In(OH)3 microcubes was successfully obtained by using the simple approach. Figure 2B is a higher magnification SEM image obtained from a selected area of Figure 2A. The edge length of

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Figure 2. (A) Low- and (B) high-magnification SEM images of asprepared In(OH)3 microcubes synthesized at 140 °C for 12 h. (C) TEM and (D) HRTEM images of In(OH)3 microcubes obtained using the same sample batch; inset shows the SAED pattern of the corresponding In(OH)3 microcube.

the In(OH)3 microcubes ranges from 500 nm to 2 µm. The surfaces of In(OH)3 microcubes are smooth, and the angle between the adjacent edges is relatively close to 90°, indicating that their cubic shape is quite regular. Figure 2C shows a typical TEM image of the In(OH)3 microcubes with length of ∼500 nm. The left part of Figure 2C shows two single microcubes that are close to each other. The inset of Figure 2C is the SAED pattern taken on an individual In(OH)3 microcube, revealing the single-crystal nature of the sample, which can be indexed to the pure cubic phase of In(OH)3 microcubes. HRTEM images provide further insight into the structure of the In(OH)3 microcubes. A typical HRTEM image recorded near the edge of the In(OH)3 microcube is given in Figure 2D. The clear lattice fringe further confirms that the In(OH)3 microcubes are structurally uniform and of satisfactory crystallinity. The interlayer spacing was calculated to be ∼3.96 Å, which corresponds to an interlayer distance of a (200) crystal plane in body-centered cubic In(OH)3. Further studies indicate that the morphology and size of the final products are strongly associated with reaction conditions such as hydrothermal temperature, reaction time, and surfactant. Figure 3 shows the influence of reaction time on the morphology and size of the final products. When the reaction time was reduced to 2 h, the product contained both floccule structures with irregular shapes and cubic particles with edge lengths of 0.1-1 µm (Figure 3A). Prolonging the reaction time to 4 h, the product is mainly composed of microcubes with edge lengths from 200 nm to 1.2 µm besides a few floccule structures, as shown in Figure 3B. Figure 3C shows a SEM image of the as-prepared In(OH)3 microcube synthesized at 140 °C for 6 h. The size of the final product was increased to ∼1 µm, and the morphology improved. However, further elongating the reaction time to 8 h mostly formed uniform In(OH)3 microcubes with a mean edge of 1.2 µm (iFigure 3D). The corners and edges of In(OH)3 microcubes can be clearly observed. This result suggests that body-centered cubic In(OH)3 microcubes gradually formed with an increasing reaction time. Hydrothermal temperature influenced the morphology of the final products with similar results. Figure 4A indicates the SEM image of In(OH)3 synthesized at 100 °C for 12 h. It can be

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Figure 5. DSC and TG curves of as-prepared In(OH)3 microcubes. Figure 3. SEM images of In(OH)3 microcubes synthesized at 140 °C for different reaction times: (A) 2 h, (B) 4 h, (C) 6 h, and (D) 8 h.

Figure 4. SEM images of In(OH)3 microcubes synthesized in the absence of surfactants under different reaction temperatures for 12 h: (A) 100 °C and (B) 160 °C and those synthesized using different surfactants at 140 °C for 12 h: (C) SDS and (D) PVP.

clearly seen that the products are comprised of a few microcubes and a large amount of floccules, indicating that the reaction conditions are not strong enough to form uniform In(OH)3 microcubes at lower temperatures. If the hydrothermal temperature is elevated to 160 °C, as shown in Figure 4B, the morphology of the final product mostly formed microcubes, and the length of the microcubes increased to 3 µm. The surfactant is another factor that influences the morphology of the final products effectively. Figure 4C indicates a typical SEM image of as-prepared In(OH)3 synthesized with the surfactant SDS process at 140 °C for 12 h, which reveals that the product is mainly composed of floccules and nanocubes with sizes ranging from 50 to 250 nm. Figure 4D shows the representative SEM image of products obtained in the presence of PVP and indicates that a large number of In(OH)3 submicrometer cubes with a mean edge of 500 nm were obtained under the current conditions. The sizes of the as-prepared products are smaller as compared to the microcubes synthesized in the absence of surfactant. It is believed that the change of the product

Figure 6. Comparison of XRD patterns of In2O3 obtained by calcination of as-prepared In(OH)3 microcubes in air at 400 °C (A) and 600 °C (B) for 2 h, respectively.

morphology and size may be caused by the adsorption of surfactants on the crystallographic planes of In(OH)3. The thermal behavior of In(OH)3 oxidized to In2O3 was investigated with TG and DSC measurements in the temperature range of 25-900 °C. Figure 5 shows the TG and DSC curves of In(OH)3. It can be seen from the TG curve that the dominant mass loss in the range of 150-400 °C is ∼16%, which can be attributed to conversion from In(OH)3 to In2O3, in good agreement with a theoretical weight loss (16.3%). The gradual mass loss from room temperature to ∼150 °C may be due to evaporation of the adsorbed water species on In(OH)3 surfaces. The DSC curve indicates that the endothermic peak at 278 °C corresponds to the dominant mass loss, which is evidenced by the TG curve. The possibility of using In(OH)3 microcubes as a precursor for the synthesis of In2O3 microcubes by the thermal decomposition method was investigated on the basis of TG and DSC measurements. We demonstrate here that In2O3 microcubes could be selectively obtained by calcination of as-prepared In(OH)3 microcubes in air at 400 and 600 °C for 2 h, respectively. Figure 6 shows XRD patterns of In2O3 obtained by calcination of as-prepared In(OH)3 microcubes in air at 400 °C (A) and 600 °C (B) for 2 h, respectively. Both pattern A

Indium Hydroxide and Indium Oxide Microcubes

Figure 7. SEM (A) and TEM (B) images of In2O3 microcubes obtained by calcination of as-prepared In(OH)3 microcubes in air at 400 °C for 2 h. HRTEM (C) image of individual In2O3 microcubes corresponding to panel B. Inset of panel C is a FFT image originating from the lattice of In2O3 microcubes. (D) SEM image of In2O3 microcubes obtained by calcination of as-prepared In(OH)3 microcubes in air at 600 °C for 2 h.

and pattern B exhibit multiple intense peaks that can be wellindexed to crystalline In2O3 with a lattice constant a ) 10.11 Å, consistent with reported data (JCPDS Card No. 71-2194). Careful comparison reveals that the diffraction peaks of pattern B are stronger and sharper than those of pattern A, indicating that the crystallinity of products calcined at 600 °C is improved over that calcined at 400 °C. Figure 7A shows the SEM image of In2O3 microcubes obtained by calcination of as-prepared In(OH)3 in air at 400 °C for 2 h. These microcubes have a relatively regular shape with a mean edge length of ∼1 µm. Closer observation reveals that there are cracks on the surface of the microcubes, which may be attributed to the removal of -OH during the calcination process. The result indicates that In2O3 microcubes successfully were obtained and that the morphologies were well-inherited from In(OH)3 microcubes except for the cracks on the surface and micropores on the inside. TEM images of as-prepared In2O3 microcubes prepared at 400 °C are displayed in Figure 7B. The crystalline structure of as-prepared In2O3 microcubes was further explored by HRTEM. Figure 7C shows a partly magnified image obtained from a selected area of Figure 7B. The clear lattice fringe indicates that the In2O3 microcube is structurally uniform and single-crystalline. The interplanar spacing of In2O3 microcubes was measured to be 2.92 Å, which is consistent with the ideal values of (222) planes of cubic In2O3. The inset of Figure 7C is the fast Fourier transform (FFT) image of HRTEM, which shows that the microcubes are of single-crystalline structure with a preferential [111] growth direction. Figure 7D shows the SEM image of In2O3 microcubes obtained by calcination of the as-prepared In(OH)3 microcubes in air at 600 °C for 2 h. It is obvious that most of the microcubes have been destroyed, but many broken microcubes still existed when the calcination temperature was increased to 600 °C. Room-temperature PL spectra of as-prepared In2O3 microcubes obtained at different calcination temperatures are shown in Figure 8. It is well-known that bulk In2O3 cannot emit light at room temperature.30 However, the relatively strong PL emission spectra from both samples can be clearly observed

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Figure 8. PL spectra of In2O3 microcubes obtained at different calcination temperatures under excited wavelengths of 290 nm at room temperature: (A) 400 °C and (B) 600 °C.

under the excited wavelength of 290 nm at room temperature, in which four distinct emission peaks are located at 452, 470, 484, and 494 nm, respectively. Although the PL emission mechanism of In2O3 is still ambiguous, it is believed that the different emission peaks are related to the different energy levels caused by oxygen vacancies created from the thermal oxidation process and result from the recombination of a photogenerated hole with an electron occupying an oxygen vacancy in In2O3 microcubes.c,31 In addition, In2O3 microcubes with a high surface-to-volume ratio may be beneficial to the formation of quantities of oxygen vacancies. A similar mechanism was proposed for other oxides that do not give out light in a bulk state such as La2O332 and ZnO.33,34As compared to In2O3 microcubes obtained at 600 °C, the intensity of all emission peaks gradually strengthened for In2O3 microcubes obtained at 400 °C, while the peak positions in the emission spectra are identical to each other. We considered that the intensity of emission peaks of In2O3 microcubes obtained at 400 °C increased as a result of more oxygen vacancies and defects. With an increasing calcination temperature, the crystallinity of the samples was improved, which caused a rapid reduction of oxygen vacancies and defects. Furthermore, the intensity of emission peaks of In2O3 microcubes also are directly related to their size and shape. Conclusion We successfully synthesized uniform single-crystalline indium hydroxide microcubes in large quantities via a convenient hydrothermal synthetic method using hydrated indium nitrate and sodium borohydride as reagents under mild conditions. The study of the influences of experimental parameters shows that the morphology and size of the final products can be controlled by adjusting hydrothermal temperature, reaction time, and surfactant. Single-crystalline indium oxide microcubes also were obtained by a thermal decomposition method using indium hydroxide microcubes as the precursor. The structure, shape, size, and optical properties of the final products were investigated. Because of their optical and electrical properties, indium oxide microcubes may have potential applications in microelectronic and optoelectronic devices. This strategy may provide more opportunities for the preparation of other metal hydroxide

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