Paper EN+BI+SS+SE-TuM5
Reducing Carbon Dioxide to Methane under Visible Light Illumination by Non-stoichiometric Mixed Phase Titania Thin Films

Tuesday, October 21, 2008, 9:20 am, Room 203

Non-stoichiometric mixed phased titania composites were deposited by reactive DC magnetron sputtering. Previously we^1-3 demonstrated that there are solid-solid interfaces with highly reactive interfacial sites created within mixed phase titania thin films, and we observed by EPR measurement that the bulk composition of the films was not fully stoichiometric. The objective of this study is to explore the role of non-stoichiometry in mixed phase titania in terms of photoresponse and photocatalytic performance in reducing CO₂ to methane. The control of oxygen partial pressure during film deposition yielded different levels of non-stoichiometry in films deposited mostly in the transition mode. Trace amounts of nitrogen were introduced during the sputtering process to stabilize the reactive sputtering process at the turning point of the transition mode and metallic mode and without incorporation in the films. The photocatalytic results showed that there was an optimal non-stoichiometry of titania films in terms of methane yield from CO₂ reduction. Under UV illumination, the best CO₂ conversion percentage was around 22%. In addition, both from reaction tests under visible light and the optical measurements, we determined that non-stoichiometric mixed phase titania films showed a strong light absorption shift into the visible range compared to commercial standard Degussa P25, which has a similar phase composition. SEM and TEM results showed film morphology with a high density of solid-solid interfaces developed in the films. Both EELS and XPS results identified the Ti³⁺ species in addition to Ti⁴⁺. Most of the Ti³⁺ species were located at the interfaces of titania columns, where they might serve as the reactive interfacial sites for visible light harvesting or electron trapping.

¹L. Chen, et al., Photoreduction of CO₂ by TiO₂ Nanocomposties Synthesized through Reactive DC Magnetron Sputter Deposition. Thin Solid Films, 2008. in review.
²L. 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.
³Hurum, D.C., et al., Probing reaction mechanisms in mixed phase TiO₂ by EPR. Journal of Electron Spectroscopy and Related Phenomena, 2006. 150: p. 155-163.