Photoreduction of CO₂ by TiO₂ nanocomposites synthesized through reactive direct current magnetron sputter deposition

The photoreduction of CO₂ into methane provides a carbon-neutral energy alternative to fossil fuels, but its feasibility requires improvements in the photo-efficiency of materials tailored to this reaction. We hypothesize that mixed phase TiO₂ nano-materials with high interfacial densities are extremely active photocatalysts well suited to solar fuel production by reducing CO₂ to methane and shifting to visible light response. Mixed phase TiO₂ films were synthesized by direct current (DC) magnetron sputtering and characterized by X-ray diffraction (XRD), atomic force microscope (AFM), scanning electron microscope (SEM) and transmission electron microscope (TEM). Bundles of anatase-rutile nano-columns having high densities of two kinds of interfaces (those among the bundles and those between the columns) are fabricated. Films sputtered at a low deposition angle showed the highest methane yield, compared to TiO₂ fabricated under other sputtering conditions and commercial standard Degussa P25 under UV irradiation. The yield of methane could be significantly increased (~ 12% CO₂ conversion) by increasing the CO₂ to water ratio and temperature (< 100 °C) as a combined effect. These films also displayed a light response strongly shifted into the visible range. This is explained by the creation of non-stoichiometric titania films having unique features that we can potentially tailor to the solar energy applications.