CdS-sensitized single-crystalline TiO 2 nanorods and polycrystalline nanotubes for solar hydrogen generation
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We report results of comparative study of photocatalytic properties of polycrystalline TiO2 nanotubes and single-crystalline nanorods. It is demonstrated that single-crystalline nanorods show superior photocatalytic properties compared to polycrystalline nanotubes due to low recombination of photoexcited carriers. Grain boundaries in polycrystalline nanotubes act as a barrier of the effective carrier pathway. Visible light activity of the TiO2 nanostructures is enhanced by the sensitization of CdS nanoparticles on TiO2. Subsequent heat treatment of the CdS/TiO2 heterostructures led to the dramatically enhanced photoresponse under both white and visible light irradiation, which was attributed to the improved crystallinity of CdS nanoparticles and TiO2 nanostructures.
I. INTRODUCTION
Photoelectrochemical (PEC) solar energy conversion on semiconductor surface has been extensively studied since the first report of Fujisima in 1972 using titanium dioxide (TiO2) photoanode.1 Titanium dioxide (TiO2) is well known as a candidate for water photocatalyst as it is abundant, stable in aqueous solution under irradiation, and has strong photocatalytic activity. However, due to its large band gap (;3.2 eV), it is only active in the ultraviolet (UV) region, which contributes less than 5% of the total energy of the solar spectrum. One of the prerequisites of enhancing the solar energy conversion efficiency of titania is to enhance the visible light activity of TiO2, which composes a greater portion of the solar spectrum (45%).2,3 Another requirement of an effective photomaterial is to obtain good electron–hole separation characteristics, which can be improved by reducing recombination centers and increasing charge transfer. TiO2 has a short hole diffusion length (;10 nm for Rutile).4 Therefore, it is advantageous to reduce the TiO2 layer thickness to decrease the diffusion pathway of photoholes to the electrode/ electrolyte interface. Thus, nanostructured TiO2 has better photocatalytic properties, compared to the bulk. Moreover, in a PEC cell, electrons generated in TiO2 photoanodes have to travel within the TiO2 to the F-doped SnO (FTO) contact and then transfer to the cathode. Therefore, one-dimensional single-crystalline structures on conducting substrate are considered as an optimum morphology to enable electrons to travel to the FTO a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.373 418
J. Mater. Res., Vol. 28, No. 3, Feb 14, 2013
http://journals.cambridge.org
Downloaded: 28 Jun 2014
contact and holes to diffuse to the electrode/electrolyte interface in the easiest manner without any recombination lost at a grain boundary.5 On the other hand, visible light activity of the TiO2 electrodes has been achieved by several strategies such as doping with impurities and sensitizing with low band gap semiconductors having proper conduction band levels.6–9 Recently, low band gap semiconductors such as CdS, CdSe, and CdTe have been successfully studied as a sensitizer for TiO2 nanostruc
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