Mn-doped TiO 2 nanopowders with remarkable visible light photocatalytic activity Q.R. Deng a , X.H. Xia a,b , M.L. Guo b , Y. Gao b , G. Shao a,c, ⁎ a Institute for Materials Research and Innovation, University of Bolton, Bolton BL3 5AB, UK b Faculty of Physics and Electronic Technology, Hubei University, Wuhan 430062, China c School of Materials Science and Engineering, Zhengzhou University, Science Road 100, Zhengzhou 450001, China abstract article info Article history: Received 13 January 2011 Accepted 1 April 2011 Available online 8 April 2011 Keywords: TiO 2 Sol–gel Mn doping Red shift Photocatalysis TiO 2 nanocrystalline powders with various Mn-doping levels were synthesized by the sol–gel process using tetrabutyl titanate and manganese nitrate as precursors. The crystal structure, morphology, doping concentration, optical absorption property, and elemental state of the obtained samples were analyzed. TEM results showed that the synthesized TiO 2 powders were anatase nanoparticles about 7 nm in size. EDX and XPS analyses proved the incorporation of Mn ions into the TiO 2 lattice. A remarkable red shift of the absorption edge was achievable by increased Mn content, leading to gigantically narrowed energy gap to permit absorption well into the infrared spectral region. The dramatic optical absorbance of the doped TiO 2 nanopowders in the visible spectral region led to strong photocatalytic activity under visible light illumination, which was observed by measuring the degradation of methylene blue. In contrast, little degradation was observed for the pure TiO 2 powder. The optimum Mn/Ti ratio was observed to be 0.2 at.% for photocatalytic applications. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Semiconductor photocatalysts have been extensively exploited in recent years owing to their promise in addressing the pressing task to curb the current rapid deterioration of the living environment. The primal process for photocatalysis decomposition of organic and inorganic compounds is through the generation of electron-hole pairs. When a semiconductor photocatalyst is illuminated with light of energy higher than its band gap, electrons are excited from the valence band to the conduction band, leaving holes in the valance band. These photo- excited electrons and holes could help to deoxidize or oxidize adsorbates on the surface of the catalyst. Among various photocatalysts, titania (TiO 2 ), particularly the anatase phase which is reported as the most sensitive phase of TiO 2 has gained most attention owing to its outstanding photocatalytic properties [1–3], together with the benefit in terms of its low-cost, high stability, and low biological toxicity. However, the photocatalyic functionality for pure titania can be excited only by ultraviolet (UV) light that only accounts for less than 5% of solar irradiance because of the wide band gaps (3.2 eV for anatase and 3.0 eV for rutile). This wide gap nature of titania phases badly limits their practical applications in most circumstances, as the UV light can be readily absorbed even by pure water vapor and clear glass. In order to extend the photocatalytic functionality into the visible light range, extensive efforts have been made to narrow the band gaps of TiO 2 phases by doping the compounds with metal [4–6] or nonmetal atoms [7,8]. In ideally doped TiO 2 materials, the dopants could induce shallow donor or acceptor states for effective ionization under photon illumination, leading to prolonged carrier diffusion length before they are recombined, and thus offers good photocatalytic activity [9]. On the other hand, photocatalytic activity of some of the doped catalysts did not improve despite observable red shift in optical absorption edges due to doping, because the doping induced defect states acted as carrier recombination centers when carriers migrated from the inside of the photocatalyst to the surface [10]. For these reasons, it is rather difficult yet urgent to identify appropriate doping elements for TiO 2 . Metal dopants such as Fe [11], Cr [12], Co [13], Mn [14,15],V [16], and Ni [17], have been investigated. Theoretical modeling showed that among the 3d metals, Mn has the greatest potential in permitting significant optical absorption in the visible or even the infrared solar light, through the combined effects of narrowed band gap and the introduction of intermediate bands (IBs) within the forbidden gap [18–20]. Unlike most of the 3d dopants which tend to induce defect states in the forbidden gap of TiO 2 , the IBs owing to Mn doping are of significant curvature and hence adequate carrier mobility. This makes them effective stepping stones for relaying low energy photons from the valence band into the conduction band, thus extending the optical absorption power of TiO 2 from the limited ultraviolet spectral region well into the major visible and even infrared region [18–20]. This work has been inspired by the theoretical work of refs [18] and [19], in order to develop a sol–gel technique to synthesize Mn-doped TiO 2 for enhanced photocatalytic activity. The sol–gel method has been chosen, since it offers considerable advantages in terms of low cost owing to the simple process, excellent compositional and stoichiometry control, and large specific surface area associated with the resultant nanopowders. Materials Letters 65 (2011) 2051–2054 ⁎ Corresponding author at: Institute for Materials Research and Innovation, University of Bolton, Bolton BL3 5AB, UK. Tel.: +44 1204 903592. E-mail address: G.Shao@bolton.ac.uk (G. Shao). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.04.010 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet