Low-temperature synthesis of visible-light active fluorine/sulfur co-doped mesoporous TiO2 microspheres.

TiO2 nanomaterials with controlled morphologies have attracted extensive interest because of potential applications in solar energy conversion, environmental cleanup and hydrogen generation. Nanoscale titanium dioxides have been produced as nanotubes, nanosheets, nanowires and mesoporous microspheres by a variety of approaches. Among the various nanomaterials, mesoporous microspheres have drawn considerable attention due to the intrinsic shape-dependent properties, including high surface area, tuneable pore structure, large pore volume and good surface permeability, in addition to unique optical, electronic, magnetic and catalytic properties. Mesoscale TiO2 spheres can be used in catalysis, separation, ionic intercalation, photonic devices and size-selective reactions. Mesoporous TiO2 microand nanostructures are also considered promising candidates for harvesting optical absorption with enhanced energy-conversion efficiency. There is, however, a drawback that limits more extensive applications of mesoscale TiO2 microspheres; this photo-catalyst exhibits activity predominantly under ultraviolet (i.e. , l<388 nm) irradiation, leading to only a narrow utilisation of the visible-light spectrum that accounts for 43% of the total incoming solar energy. A lot of effort has therefore been devoted to modify the band gap of TiO2 by doping to extend the optical absorption properties into the visible spectrum. For example, it was reported that mono-doping of TiO2 with N, [7] F and certain transition metals expands the photo-response of TiO2 into the visible region. However, studies also show that mono-doping alone does not guarantee improved photo-catalytic activity because the partially occupied impurity bands can act as recombination centres and therefore reduce the photo-generated current. More recently, band-gap engineering of TiO2 nanospheres by codoping with two non-metals has received more attention due to the fact that the defect bands are passivated and hence not effective as carrier recombination centres. However, no related mesoporous TiO2 microspheres codoped with non-metals have been reported to date. The ability to control the morphology and chemical composition of the mesoporous microspheres would enable a great light-harvesting capacity and high activity for the synthesised TiO2 photo-catalyst. Herein, we propose a simple and effective method for the fabrication of anatase TiO2 microspheres at a low temperature. The preparation conditions are much milder than those of conventional methods. Meanwhile, a co-doping approach redshifts the threshold of the TiO2 adsorption into the visible-light region and improves the photo-catalytic efficiency for degradation of organic dyes. The nitrogen adsorption–desorption isotherm and Barrett–Joiner–Halenda (BJH) pore size distribution curves of H-400 and H-600 (calcinated at 400 and 600 8C, respectively) are shown in Figure 1. The adsorption isotherms of both H400 and H-600 show typical IV isotherms, indicating that the obtained TiO2 microspheres are characteristic for mesoporous material and the adsorption hysteresis loop of the isotherm is close to the type H4 according to the IUPAC classification; this suggests the presence of slit-like pores in the samples. The pore size distribution curves (inset in Figure 1), calculated by the BJH method based on the desorption branch, show that the TiO2 microspheres have a very clear mesoporous structure. As the calcination temperature increases from 400 to 600 8C, the average pore size of [a] Dr. G. Yang, Prof. Z. Yan State Key Laboratory of Heavy Oil Processing China University of Petroleum Qingdao, 266555 (P.R. China) Fax: (+86)532-8698-1295 E-mail : [email protected] [b] Dr. G. Yang, Dr. T. Xiao, Dr. G. Li Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford, OX1 3QR (UK) Fax: (+44)1865-272690 E-mail : [email protected] [c] Dr. J. Sloan Department of Physics, University of Warwick Coventry CV4 7AL (UK) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201001676.