Paper SE-TuA9
Nb-doped TiO₂ Thin Films for Solar Energy Conversion

Tuesday, October 21, 2008, 4:20 pm, Room 204

A deeper understanding of synthesis, structure and function is needed to improve the design of materials tailored to solar energy conversion and storage. This study highlights the efficacy of reactive sputtering as a means to fabricate cation-doped TiO₂ films having controlled properties tailored to the generation of energy rich fuels such as CH₄ or CH₃OH by photoreduction of CO₂. It is our hypothesis that cation substitution in the TiO₂ lattice is an effective way to shift the photoresponse of the material further into the visible light region without deleteriously modifying its photochemical properties. Unbalanced reactive dc magnetron sputtering (UBMS) with partial pressure control of oxygen was utilized to synthesize a series of pure and mixed phase TiO₂ films. Films were doped with Nb to evaluate the effect of cation doping on optical, chemical and physical properties. Nb doping was achieved by altering a pure Ti target in a pieced manner by adding slugs of dopant material at regular intervals. The films were interrogated structurally and functionally using SEM, EDS, XPS, XRD and UV-vis spectroscopy. The ability of these materials to selectively and efficiently reduce CO₂ to energy rich fuels was evaluated in a gas phase reactor coupled with a GC/FID. While earlier work established the relationships between sputtering process parameters and the film structure, including phase identity and distribution in pure TiO₂¹, this work is focused on how the addition of Nb cations in the range of 0-20%Nb change the film growth and phase formation relative to the pure material. The work also tries to characterize the cation valence and location in the TiO₂ lattice, but this is still under investigation. The parametric response of film structure still suggests that in the mixed phase system, greater energy input favors the formation of rutile and lower energy favors anatase, but the Nb additions shift the regions of phase stability compared to the pure TiO₂ case. The shift in optical absorption to the visible wavelength range as a function of Nb concentration and anatase-rutile phase distribution is also presented.

¹ L. Chen, et al., Fabricating Highly Active Mixed Phase TiO₂ Photocatalysts by Reactive DC Magnetron Sputter Deposition. Thin Solid Films, 2006. 515(3): p. 1176-1181.