Elsevier

Applied Catalysis B: Environmental

Volumes 140–141, August–September 2013, Pages 283-288
Applied Catalysis B: Environmental

Improved visible-light-driven photocatalytic activity of rutile/titania-nanotube composites prepared by microwave-assisted hydrothermal process

https://doi.org/10.1016/j.apcatb.2013.04.001Get rights and content

Highlights

  • The composites including rutile nanoparticles and titania-nanotubes were grown by a facile and green approach based on microwave irradiation.

  • The nanocomposites enhance the light-harvesting efficiency in photocatalytic reactions.

  • By H2-thermal treatment, the optical absorption of the nanocomposite extended to the visible light region.

  • The nanocomposite catalysts exhibit superior photocatalytic activity, nearly 100% of NO conversion with 81.7% of N2 selectivity, have been achieved under natural solar-light irradiation.

Abstract

This study demonstrates a facile approach based on microwave irradiation for the preparation of rutile/titania-nanotube composites that exhibit highly efficiency in visible light induced photocatalysis. The obtained nanocomposites were characterized using XRD, Raman, FESEM, TEM, N2 physisorption isotherms at 77 K and UV–vis DRS techniques. The results show that the nanocomposites exhibit multilayer-wall morphologies with open-ended cylindrical structures. The presence of the rutile phase in the titania nanotubes enhanced the light-harvesting efficiency in photocatalytic reactions. By H2-thermal treatment, the optical absorption of the nanocomposite extends to the visible light region up to 600 nm. It is believed that thermal-treatment gives rise to create active surface oxygen vacancies, which are responsible for visible light absorption and the promotion of electrons from the localized states to the conduction band. The catalytic results revealed that the nanocomposites exhibited higher photocatalytic activities toward the decomposition of nitric oxide and the degradation of methylene blue compared with commercial P25 TiO2.

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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