Improved visible-light-driven photocatalytic activity of rutile/titania-nanotube composites prepared by microwave-assisted hydrothermal process
Graphical abstract
Introduction
One-dimensional (1-D) nanotubular materials, such as carbon nanotubes, have attracted worldwide attention in both fundamental and applied sciences, particularly for application in electronic, mechanic, and optoelectronic devices [1]. Tubular structures not only provide large internal and external surfaces for reactions but also facilitate electron transfer, which results in improved device performance. Among the 1-D nanotubular architectures, titania nanotubes (Tnt) have attracted increasing interest due to their tubular shape, unique size and excellent physicochemical durability; these properties have led to broad potential applications in catalysis, photocatalysis, gas storage, photoelectric water splitting and dye-sensitized solar cells [2], [3], [4], [5], [6]. Thus far, the main approaches used to synthesize TiO2-based nanotubes have included template-assisted, alkaline hydrothermal and anodic oxidation methods [7], [8], [9]. Among these methodologies, the hydrothermal soft-chemical synthesis involving the treatment of TiO2 nanoparticles with NaOH followed by subsequent acid washing is a relatively effective route to cost-effectively manufacture nanotubes [8].
Solar light is an abundant natural energy source that can be conveniently used to excite semiconducting materials. Because of the bandgap limitation, TiO2 is used as a good photocatalyst under UV irradiation that can utilize only approximately 5% of the incoming solar energy incident to the Earth's surface. The smaller bandgap of rutile titania can absorb more visible light than anatase titania; however, the photocatalytic activity of rutile titania is limited because of its low surface area and because of the rapid electron–hole recombination that occurs in this material [10]. Titania nanotubes can provide a significantly larger surface area; however, their large intrinsic bandgap energy (3.3–3.8 eV) [11] due to the quantum size effect in their isolated layered structure limits their ability to adsorb solar light. The rutile/Tnt composites might be interesting for practical applications in environmental and renewable energy. Pure rutile nanotubes have been prepared via the sol–gel template technique using a sacrificial carbon nanotube template [12]. However, these studies were mostly carried out under UV irradiation, and the more significant break through toward visible light response by the surface fluorination has not been achieved. Here, we demonstrate a facile approach based on microwave radiation for the preparation of rutile/titania-nanotube composites. In contrast to conventional thermal treatments, heat treatments using microwaves are reportedly an economic, rapid, and homogeneous heating method for green processes [13], [14]. By hydrogen-thermal treatment, the obtained nanocomposites catalyst exhibited higher photocatalytic activities toward the decomposition of nitric oxide and the degradation of methylene blue compared with commercial P25 TiO2.
Section snippets
Catalyst preparation
Herein, titania nanotubes with rutile-phase crystalline structures were prepared using microwave technology. Briefly, 0.50 g of rutile-TiO2 (Aldrich, SBET ∼ 2 cm2/g) was dispersed in 25.0 mL of 10 M NaOH solution under vigorous stirring. The mixture was transferred to a Teflon-lined digestion autoclave and hydrothermally treated at 200 °C for 45 min under microwave (START D, Milestone) irradiation. The solid was separated by filtration after the hydrothermal treatment. The precipitate was washed with
Characterization of prepared nanotubes
Fig. 1a and b show the morphology of the rutile/Tnt before and after hydrothermal treatment, respectively, as characterized using field-emission scanning electron microscopy (FESEM). The raw material of rutile TiO2 powder exhibited particles sizes ranged from 0.5 to 1 μm. After hydrothermal treatment, the morphology of this material became tube-like (Fig. 1b). The length of randomly tangled nanotubes was up to several hundred nanometers. The nanotubular structure of the product was further
Conclusion
This study demonstrates a green technology for the preparation of nanocomposites of rutile titania-nanotubes via microwave hydrothermal treatment. The rutile/Tnt composites exhibited high photocatalytic activity because of the larger number surface active sites on the nanotubes in conjunction with the highly crystalline rutile-TiO2 phase. The presence of the rutile phase in the titania nanotubes enhanced the light-harvesting efficiency and the new absorption band at 400–600 nm induced the
Acknowledgements
We acknowledge the financial supports from Academia Sinica and National Science Council of Taiwan, ROC.
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