Synthesis, crystallinity control, and photocatalysis of nanostructured titanium dioxide shells
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In this article, we review our recent research efforts on the synthesis, crystallinity control, and photocatalysis of nanostructured titanium dioxide (TiO2) shells. First, we introduce several synthetic methods for preparing TiO2 shell structures using either template-free or template-assisted approaches. Several methods to change the structures from amorphous to crystalline and subsequently ways to enhance the crystallinity are then discussed, including those involving the “silica-protected calcination” and “partial etching and recalcination” strategies. We also discuss the photocatalytic applications of the TiO2 nanoshells and the methods for improving their catalytic activities. Finally, we conclude with a summary and our perspective on the further development of the nanostructured TiO2 shells. It is believed that more rational design and modification strategies such as well-controlled nonmetal doping, plasmonic metal decoration and the hybridization with other semiconducting materials will significantly enhance the photocatalytic efficiency of TiO2-based catalyst materials.
I. INTRODUCTION
Titanium dioxide (Titania, TiO2) has been considered to be one of the most important semiconductor materials due to its wide practical uses in several applications including energy storage, solar cells, catalysis and photocatalysis. It has a wide-band-gap energy (3.0–3.2 eV) that allows absorption of photon energy to excite electron- (e ) hole (h1) pairs that can induce redox reactions.1 Since Honda and Fujishima first discovered its ability to decompose water and produce hydrogen under ultraviolet (UV) irradiation,2 TiO2-based materials have been intensively studied and used as photocatalysts.1,3–5 Over the past decades, tremendous efforts have been devoted to improving the physical, chemical and catalytic properties of TiO2 for enhanced photocatalytic efficiency.6–10 With continued development in nanomaterials synthesis, a variety of tailored TiO2 nanostructures have been suggested and investigated for practical application in photocatalysis.11 13 Among various TiO2 nanostructures, there has been increasing interest in colloidal TiO2 nanostructured shells due to their advantageous characteristics including facile functionalization, relatively large surface area-to-volume ratio, and improved accessibility for reactant molecules.14 19 Various forms of hollow TiO2 shells and metal (or metal oxide)-TiO2 core–shell nanostructures with spherical or nonspherical morphologies a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.280 362
J. Mater. Res., Vol. 28, No. 3, Feb 14, 2013
http://journals.cambridge.org
Downloaded: 12 Jul 2014
have been studied from both the synthetic and application viewpoints.6,19 In this article, we intend to summarize our recent research activities on the synthesis and photocatalytic applications of TiO2 nanostructured shells. We organize this article as follows: we first introduce synthetic strategies for preparing TiO2 shell structures using both template-free
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