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.