Synthesis and layer-by-layer self-assembly of titania nanosheets controllably doped with binary transition metal ions
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Here, we describe the synthesis of novel titania nanosheets controllably doped with binary transition metal ions and their layer-by-layer self-assembly. The tailored Mn and Fe doping in exfoliated Ti0.6Mnx/2Fe(0.8-x)/2O2 (x 5 0.0–0.4) nanosheets is achieved by systematically changing the molar ratio of Mn/Fe in K0.8Ti1.2MnxFe0.8-xO4 using a codoping strategy. The protonated layered crystals exhibit a delaminated behavior in the tetrabutylammonium hydroxide solution and are exfoliated into colloidal single sheets, which are characterized by a large lateral size and a thickness in molecular dimension. The resulted nanosheets are able to be layer-by-layer deposited with oppositely charged polymers into a composite organic/inorganic system.
I. INTRODUCTION
Titanium dioxide (TiO2), a wide band gap semiconductor, is a highly attractive material for a wide variety of industrial applications, because of its cheapness, easy availability, environmental friendliness, and high resistance to corrosion and photocorrosion in aqueous solution.1 It has been reported in processes for photocatalytic degradation of various organic pollutants2–4 and as a part of gas sensors, solar cells,5 diluted magnetic semiconductors (DMSs),6,7 and numerous optical devices.8 However, the large band gap (3.2 eV) of TiO2 lying in the ultraviolet (UV) region of solar radiation and the high recombination rate of the photoexcited electron-hole pairs hamper its utilization in photocatalysis.9,10 Doping with foreign ions is one of the most efficient strategies for sensitizing TiO2 to visible-light region and also for forming charge traps to keep electron-hole pairs separate.11 Compared with the larger O2 anions, the smaller Ti4+ cations are much more easily substituted by other transition metal ions with similar ionic radius. Choi et al.12 reported a systematic investigation of the photocatalytic activity of TiO2 doped with different transition metal ions. Among them, Fe and Mn ions doped into TiO2 were demonstrated to enhance the photocatalytic activities because they can not only extend the region of light response but also trap both photoexcited electrons and holes. TiO2 is a nonmagnetic semiconductor. However, after being doped with some transition metal ions, it was discovered to exhibit ferromagnetism at and above room temperature, which makes them potential DMSs applied a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.101 J. Mater. Res., Vol. 26, No. 10, May 28, 2011
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in spintronic devices.6,7 With the initial report of room temperature ferromagnetism in codoped TiO2, the TiO2based DMSs have been extensively studied, such as FeTiO2,13 Mn-TiO2,14 Cr-TiO2,15 and so on. Recently, considerable research interests were focused on low-dimensional nanoscalar materials because of their distinctive physical and chemical properties.16 Metal oxide nanosheets, the new classes of nanostructured semiconductors, have been synthesized by the delamination of starti
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