Enhanced visible-light absorption of mesoporous TiO 2 by co-doping with transition-metal/nitrogen ions
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Enhanced visible-light absorption of mesoporous TiO2 by co-doping with transitionmetal/nitrogen ions J. E. Mathis,1,2 Z. Bi,2 C. A. Bridges,2 M. K. Kidder,2 and M. P. Paranthaman2 1 Physical Sciences Dept., Embry-Riddle Aeronautical University, Daytona Beach, FL 32114 2 Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 ABSTRACT Titanium (IV) oxide, TiO2, has been the object of intense scrutiny for energy applications. TiO2 is inexpensive, non-toxic, and has excellent corrosion resistance when exposed to electrolytes. A major drawback preventing the widespread use TiO2 for photolysis is its relatively large band gap of ~3eV. Only light with wavelengths shorter than 400 nm, which is in the ultraviolet portion of the spectrum, has sufficient energy to be absorbed. Less than 14 percent of the solar irradiation reaching the earth’s surface has energy exceeding this band gap. Adding dopants such as transition metals has long been used to reduce the gap and increase photocatalytic activity by accessing the visible part of the solar spectrum. The degree to which the band gap is reduced using transition metals depends in part on the overlap of the d-orbitals of the transition metals with the oxygen p-orbitals. Therefore, doping with anions such as nitrogen to modify the cation-anion orbital overlap is another approach to reduce the gap. Recent studies suggest that using a combination of transition metals and nitrogen as dopants is more effective at introducing intermediate states within the band gap, effectively narrowing it. Here we report the synthesis of mesoporous TiO2 spheres, co-doped with transition metals and nitrogen that exhibit a nearly flat absorbance response across the visible spectrum extending into the near infrared. INTRODUCTION Modification of the composition and morphology of titanium (IV) oxide, TiO2, continues to be an active area of research for applications in energy production and storage. In the area of energy production, TiO2 can be used in photovoltaic or photocatalysis applications, particularly in splitting water into hydrogen and oxygen. TiO2 is being investigated for use in energy storage as an electrode in lithium-ion batteries, to replace the graphite anodes presently used. A major obstacle in using TiO2 for photolysis is its relatively large band gap of ~3eV. To excite an electron from the valence band to the conduction band requires the energy of the light striking TiO2 to exceed this value. Only light with wavelengths shorter than 400 nm, which is in the ultraviolet portion of the spectrum, fulfills this requirement. Other intrinsic properties that hinder TiO2’s employment as an anode for electrochemical cells include poor ionic conductivity and high electrical resistance. Adding cationic dopants such as transition metals to TiO2 has long been used to reduce this gap and increase photocatalytic activity [1-6]. In particular, Li, et al., found that, chromium-, nitrogen-codoped TiO2 (denoted as (Cr, N)TiO2 ) exhibited promising photocatalytic properties[4]. An
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