Hydrogen-Accelerated Phase Transition and Diffusion in TiO2 Thin Films

TiO2 thin films have received attention as intermetallic diffusion barriers in hydrogen-permeable membranes used for the extraction of purified H-2 from chemical reactors. We report the fabrication of thin (similar to 100 nm) TiO2 films on Si(111) substrates by pulsed laser deposition. Anatase and rutile phases are found to coexist in the as-deposited TiO2 films by X-ray diffraction. Upon H-2 annealing at 773-973 K, the phase transformation from anatase to rutile occurs and completes at a strongly (100-380 K) reduced temperature as compared to bulk TiO2. Atomic force microscopy shows that the surface roughness of the thin films increases only slightly (from 3 to 6 nm) in this H-accelerated phase transition. The hydrogen depth distributions in the TiO2 thin films are revealed with resonant H-1(N-15,alpha gamma)C-12 nuclear reaction analysis and evaluated in terms of a one-dimensional diffusion model, which allows the determination of the temperature-dependent diffusion coefficient of hydrogen as D(T) = (5.27 +/- 0.89) x 10(-8) exp(-0.86 +/- 0.02 eV/k(B)T) m(2) s(-1). This H diffusivity is slightly smaller than that in the single-crystal rutile phase, and has a relatively larger activation energy, which is compatible with H trapping at the residual defects in the polycrystalline TiO2 films.