AVS 56th International Symposium & Exhibition
    Advanced Surface Engineering Monday Sessions
       Session SE1-MoM

Paper SE1-MoM3
The Effect of Nb Additions on the Synthesis, Photo-Response, and Photo-Activity of TiO2 Thin Films

Monday, November 9, 2009, 9:00 am, Room B1

Session: Photocatalytic Coatings
Presenter: P.A. DeSario, Northwestern University
Authors: P.A. DeSario, Northwestern University
M.E. Graham, Northwestern University
K.A. Gray, Northwestern University
Correspondent: Click to Email
A deeper understanding of the relationships between synthesis, structure and function is needed to improve the design of materials tailored to solar energy conversion and storage. The objective of this research is to synthesize TiO2 composites with properties tailored to the harvesting and conversion of solar energy. Unbalanced reactive dc magnetron sputtering with partial pressure control of oxygen was utilized to synthesize a series of pure and mixed phase TiO2 films. Films were doped with Nb (Ti1-xNbxO2) in the range of 0<x<0.45 to evaluate the effect of cation doping on optical, chemical and physical properties. The films were interrogated structurally and functionally using SEM, EDX, XPS, AFM, UV-Vis absorption spectroscopy, and XRD. Selected films were evaluated with respect to gas phase CO2 reduction and gas phase oxidation of acetaldehyde and the action spectra were determined.

Improvements in catalytic efficiency can be made by shifting the photo-response of tiania-based materials to utilize a larger portion of the solar spectrum. Many attempts have been made to red-shift the photoresponse of TiO2 through cation doping or anion doping. It is not yet proven, however, if the shift in photoresponse correlates to enhanced photoactivity under visible light or simply the creation of color centers. Preliminary findings show that Nb doping enhances oxidative reactivity under visible light. Reductive chemistry does not seem to be enhanced, however, which suggests how Nb substitution is altering band gap structure.

While earlier work established the relationships between sputtering process parameters and the film structure, including phase identity and distribution in pure TiO2, this work is focused on how the addition of Nb cations 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. 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 Nb additions shift the regions of phase stability compared to pure TiO2 and favor the growth of rutile. The shift in optical absorption to the visible wavelength range as a function of Nb concentration and anatase-rutile phase distribution is also presented.

Our work on pure TiO2 indicates that the films with the highest photoreactivity were nanocrystalline, mixed phase structures (~70% anatase) with substoichiometric oxygen concentration. Results are presented comparing the photoreactivity of the Nb doped films with the pure TiO2 mixed phase films of similar structure and phase proportions.