Hydrothermal Synthesis and Photocatalytic Property of β-Ga2O3 Nanorods
© Reddy et al. 2015
Received: 9 June 2015
Accepted: 6 September 2015
Published: 16 September 2015
Gallium oxide (Ga2O3) nanorods were facilely prepared by a simple hydrothermal synthesis, and their morphology and photocatalytic property were studied. The gallium oxide hydroxide (GaOOH) nanorods were formed in aqueous growth solution containing gallium nitrate and ammonium hydroxide at 95 °C of growth temperature. Through the calcination treatment at 500 and 1000 °C for 3 h, the GaOOH nanorods were converted into single crystalline α-Ga2O3 and β-Ga2O3 phases. From X-ray diffraction analysis, it could be confirmed that a high crystalline quality of β-Ga2O3 nanorods was achieved by calcinating at 1000 °C. The thermal behavior of the Ga2O3 nanorods was also investigated by differential thermal analysis, and their vibrational bands were identified by Fourier transform infrared spectroscopy. In order to examine the photocatalytic activity of samples, the photodegradation of Rhodamine B solution was observed under UV light irradiation. As a result, the α-Ga2O3 and β-Ga2O3 nanorods exhibited high photodegeneration efficiencies of 62 and 79 %, respectively, for 180 min of UV irradiation time.
In recent years, various fabrication methods of photocatalytic products have been developed for the photodegradation of organic and inorganic pollutants in the environment [1, 2]. Particularly, inorganic semiconductor nanomaterials have been considered to be promising for photocatalyst applications because they provide good physical and chemical properties with large surface area, and a variety of morphologies, such as nanorods, cubes, spheres, and flowers, could be achieved by the chemical synthesis [3–8]. In addition to the morphology and surface area of particles, the other factors which can influence the catalytic activity are pore volume, pore size, crystallinity, defect sites, exposed facets, etc. The electron transport mechanism and the exposed facets are related to the morphology of particles. In the one-dimensional (1D) morphology, the generation of electron charge carriers is higher along the elongated nanostructures and gives rise to fast transport of charge carriers, due to the hampering of recombination of charge carriers. Hence, 1D nanostructures are gaining more importance for their use in different applications as seen by latest reports [9, 10]. For example, Liu et al. studied the morphology-dependent photocatalytic properties of bare zinc oxide nanocrystals , which indicated that the rod-shaped ZnO nanostructures have higher photocatalytic activity than the multi-layer disks or truncated hexagonal cones. Similarly, Han et al. also studied the morphology-related properties of nano/microstructured ZnO crystallites . Gallium oxide (Ga2O3) nanostructures have been recognized as an important material for several applications including catalysts, gas sensors, solar cells, and photodetectors due to their wide bandgap energy (E g = 4.2 to 4.7 eV) and good luminescence properties [13–16]. Typically, the Ga2O3 nanostructures could be obtained by calcination of gallium oxide hydroxide (GaOOH) which has been synthesized via various fabrication routes including thermal evaporation, hydrothermal, sol-gel, and microwave-assisted methods [17–19].
In order to obtain such GaOOH nanostructures by using a facile hydrothermal method, gallium nitrate was chemically reacted with various alkalis such as NaOH, NH4OH, KOH, and Na2CO3 [20, 21]. Then, the phase of the GaOOH nanostructures was changed into the rhombohedral crystal structure (α-Ga2O3) and monoclinic crystal structure (β-Ga2O3) after calcination [22, 23]. The crystal structures of Ga2O3 nanostructures strongly affect the chemical property and photocatalytic activity, but they still have not been completely studied. In this work, we prepared and characterized two kinds of Ga2O3 nanorods (α- and β-crystal structures) via a facile hydrothermal synthesis and proper calcination. By changing the temperature of the growth solution, the morphological properties of GaOOH nanostructures were investigated. Previously, there are so many reports on the preparation of Ga2O3 nanorods by hydrothermal and solvothermal methods which include complex instruments, making it an expensive approach. Girija et al. synthesized Ga2O3 nanostructures by the reflux method , Li et al. and Wang et al. used hydrothermal synthesis (autoclave) [25, 26], and Zhang et al. used the solvothermal method (autoclave) to obtain the Ga2O3 nanostructures . However, in this work, we chose a simple and cost-effective process in which a beaker on a hotplate is used to produce crystalline Ga2O3 nanostructures. To characterize the structural property and functional group of the prepared samples, field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FT-IR) analyses were employed. Also, the photocatalytic feasibility of Ga2O3 nanorods was evaluated by measuring the UV-vis absorption spectrum with a Rhodamine B (RhB) solution for environmental applications.
The GaOOH nanorods were synthesized by the hydrothermal synthesis method using analytical pure grade chemicals. To make the growth solution, 0.1 M of gallium(III) nitrate hydrate (Ga(NO3)3·nH2O) was dissolved in 100 ml of de-ionized (DI) water. Then, the aqueous solution was heated on a hot plate at different temperatures from room temperature to 95 °C. While the temperature of the growth solution was maintained, ammonium hydroxide (NH4OH) was slowly added into the solution until a pH of 9 is reached. The final solution was then heated for 5 h to get a white precipitate of GaOOH nanorods. After the solution was naturally cooled down to room temperature, the precipitate was filtered and washed with DI water. Then, the sample was dried in an oven at 70 °C for 5 h under ambient atmosphere. Further, the as-prepared GaOOH nanorods were calcinated at different temperatures of 500, 800, and 1000 °C for 3 h to obtain the α- and β-Ga2O3 powders.
For the morphological and structural analysis, FE-SEM (LEO SUPRA 55, Carl Zeiss), TEM (JEM-2100F, JEOL), and XRD (M18XHF-SRA, Mac Science) measurements were utilized. The thermal behavior of the GaOOH nanorods was investigated by thermogravimetric analysis-differential scanning calorimetry (TGA-DSC: SDT Q600 V8.3 Build 101). The FT-IR spectrum was scanned and analyzed in the wavenumber range of 4000–400 cm−1 by using a FT-IR measurement system (Spectrum 100, PerkinElmer). The photocatalytic properties of α- and β-Ga2O3 nanorods for the degradation of Rhodamine B (RhB) aqueous solution were characterized by measuring the absorbance of the irradiated solution. For UV irradiation, the 100-W UV lamp (SXT-20-M, UVitec) was used as a light source. To prepare the suspension of the photocatalyst, 50 mg of Ga2O3 nanorod powders was mixed with 50 ml of RhB aqueous solution (2 × 10−4 M), which was continuously stirred at room temperature in the dark for 30 min. Then, the suspension was irradiated at different illumination times from 30 to 180 min. After that, the aliquot was separated from Ga2O3 nanorod powders by using a filter paper and then the absorbance of RhB solution was obtained using a UV-vis spectrophotometer (CARY 300 Bio, Varian).
Results and Discussion
The GaOOH nanorods with a length of ~1 μm and a width of ~300–400 nm were successfully prepared by a simple hydrothermal synthesis at 95 °C of growth temperature. Then, the as-prepared GaOOH nanorods were calcined at different temperatures of 500–1000 °C for converting into single crystalline α-Ga2O3 and β-Ga2O3 nanorods, and their crystal structures were confirmed by the XRD analysis. Also, the dehydration processes were studied by thermal analysis with consideration of the phase changes of the as-prepared GaOOH precursors. At 1000 °C of calcination temperature, the β-Ga2O3 nanorods with good crystallinity and porous surface were formed by the removal of water molecules during the dehydration. Additionally, these β-Ga2O3 nanorods provided a relatively stable and high photocatalytic activity, compared with the α-Ga2O3 nanorods. Under UV irradiation for 180 min, the β-Ga2O3 nanorods exhibited a relatively high photodegradation efficiency of 79 % compared to the α-Ga2O3 nanorods (62 %). This fabrication process and analysis can be useful to produce a good inorganic semiconductor nanomaterial-based photocatalyst.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015-023255).
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