Side-by-Side In(OH)3 and In2O3 Nanotubes: Synthesis and Optical Properties
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A simple and mild wet-chemical approach was developed for the synthesis of one-dimensional (1D) In(OH)3 nanostructures. By calcining the 1D In(OH)3 nanocrystals in air at 250 °C, 1D In2O3 nanocrystals with the same morphology were obtained. TEM results show that both 1D In(OH)3 and 1D In2O3 are composed of uniform nanotube bundles. SAED and XRD patterns indicate that 1D In(OH)3 and 1D In2O3 nanostructures are single crystalline and possess the same bcc crystalline structure as the bulk In(OH)3 and In2O3, respectively. TGA/DTA analyses of the precursor In(OH)3 and the final product In2O3 confirm the existence of CTAB molecules, and its content is about 6%. The optical absorption band edge of 1D In2O3 exhibits an evident blueshift with respect to that of the commercial In2O3 powders, which is caused by the increasing energy gap resulted from decreasing the grain size. A relatively strong and broad purple-blue emission band centered at 440 nm was observed in the room temperature PL spectrum of 1D In2O3 nanotube bundles, which was mainly attributed to the existence of the oxygen vacancies.
KeywordsSide-by-side In(OH)3 In2O3 Nanotubes Wet-chemical approach
One-dimensional nanostructures (1D: nanostructures with nanometer-sized diameters but much longer lengths), such as nanorods/nanowires, nanotubes, and nanobelts have been extensively prepared and investigated, owing to their unusual chemical and physical properties that differ from those of the bulk materials and potential utilization in nanoelectronic and optoelectronic devices [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. In recent years, much attention has been paid to the fabrication and self-assembly of 1D wide-bandgap semiconducting oxides because of their interesting optical and electronic feature [10, 11, 12, 13], including fabrication of semiconductor materials with hollow and core–shell structures [14, 15, 16]. Especially, indium hydroxide In(OH)3 (with a wide-bandgap about 5.15 eV)  and indium oxide In2O3 (with a direct bandgap around 3.6 eV and an indirect bandgap around 2.5 eV) [18, 19, 20, 21, 22, 23, 24], as two important wide-bandgap semiconductor, are of great importance for fundamental research and many device applications such as solar cell , organic light-emitting diodes [26, 27, 28], architectural glasses , gas sensors , and flat-panel display [25, 30]. Accordingly, various type of synthetical strategies have been established for synthesizing one-dimensional In(OH)3 and In2O3 nanostructures, such as sonohydrolysis, chemical vapor deposition, hydrothermal method, and these methods often require special equipment and rigorous experimental condition [17, 31, 32, 33, 34]. Moreover, among the reported literatures, the research on the solution phase fabrication of 1D In(OH)3 and 1D In2O3 nanotubes is rather limited. Therefore, the solution phase synthesis of 1D In(OH)3 and 1D In2O3 nanotubes remains a challenging task.
In this study, we report a facile wet-chemical synthesis of well-defined 1D In(OH)3 nanotubes from the hydrolysis of the In(NO3)3·4.5H2O precursor in the presence of surfactant cetyltrimethylammonium bromide (CTAB). Meanwhile, we found that the as-synthesized 1D precursor In(OH)3 nanostructures easily turned into In2O3 nanostructures with the same shape. The optical properties of 1D In2O3 nanotube bundles were also studied.
Preparation of the Precursor In(OH)3 Nanostructures
First, 0.38 g (0.001 mol) of In(NO3)3·4.5H2O was dissolved in 80 mL of distilled water, and 0.36 g (0.001 mol) cetyltrimethylammonium bromide (CTAB) was dissolved in 20 mL of ethanol (95%). Secondly, two solutions were introduced into a round-bottom three-neck flask equipped with the condensator and magnetic stirring. Then, the mixture was heated to reflux. Gradually, the solution turned from colorless and transparent to white turbid. After the reaction was lasted for 12 h, the white turbid solution was centrifugated at 10,000 rpm and washed with anhydrous ethanol several times. Finally, the white powders obtained were dried in an air atmosphere at 60 °C for 12 h.
Transformation of In(OH)3 Nanostructures into In2O3 Nanostructures
The appropriate amount of the precursor In(OH)3 nanostructures were coated on a clean glass flake as thin as possible. This glass flake was transferred into an oven (air atmosphere), in which temperature was kept at 250 °C for 8 h. Under the current condition, the dehydration of the precursor In(OH)3 nanostructures was complete and the light yellow In2O3 nanostructures were obtained.
The X-ray powder diffraction pattern was recorded with an X-ray diffractometer (Philips) using Cu Kα (40 kV × 40 mA) radiation (λ = 0.154056 nm). Low-magnification and high-magnification TEM images were taken on JEM-100CXII (using an accelerating voltage of 100 Kv) and JEM-2010 (using an accelerating voltage of 200 Kv) transmission electron microscope, respectively. SAED images were carried out on JEM-100CXII. A UV–visible spectrophotometer, HEλIOSa was used to carry out the optical measurement of the sample dispersed in CHCl3. The room temperature PL spectrum was performed on a SPEX F212 fluorescence spectrophotometer with a Xe lamp upon excitation at 300 nm.
Results and Discussion
In conclusion, a simple and mild wet-chemical route we proposed is propitious to the preparation of 1D In(OH)3 nanotube bundles. Furthermore, the morphology of the nanotube bundles was also inherited in the transformation from In(OH)3 to In2O3 successfully. Their composition and single-crystalline structures were confirmed using XRD, TEM, and SAED. The optical determinations imply that the UV–Visible and PL behaviors of In2O3 nanotube bundles were different from those of the bulk. This means that the as-prepared In2O3 nanotube bundles may perform better in optoelectronic devices and nanoscale gas sensors. In addition, on the basis of the present work, we conclude that through the appropriate modification of the synthetic condition, In(OH)3 nanobelts are possibly obtained and further transform into In2O3 nanobelts. This work is currently in progress.
This work is supported by the National Natural Science Foundation of China (Grant No. 50701016).
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