As the need for clean, cheap, and reliable sources of energy intensifies, the photoreduction of CO2 to value-added fuels presents an attractive solution. Carbon dioxide, a greenhouse gas and combustion byproduct, is converted into energy-rich molecules, such as methane and methanol, in the presence of water and a light-activated photocatalyst, providing a new source of fuels and consuming an undesirable waste product. One potential photocatalyst is the semiconductor titanium dioxide (TiO2), a low-cost, nontoxic, and stable material.
When TiO2 is activated by UV radiation, an electron is promoted from the valence band to the conduction band, forming a hole at the valence band. The electron can migrate to the surface of the catalyst to react with adsorbed CO2 while the hole can split water at the valence band to form H+ and O2. Successive reactions of H+ and CO2 can form a variety of reduction products such as methane, methanol, carbon monoxide, and higher-order hydrocarbons.
Recombination of the photogenerated charge carriers is the greatest factor limiting the efficiency of this reaction. The separation and trapping of charge carriers can be enhanced by the addition of oxygen vacancies into the TiO2 matrix, which has been shown experimentally to improve the conversion of CO2. To develop an understanding of the mechanism of charge formation and migration within the TiO2 crystal structure, electron paramagnetic resonance (EPR) spectroscopy, a sophisticated tool for the study of charge transfer, has been employed.
Methods
A commercial mixed-phase TiO2 photocatalyst was heat treated under a helium atmosphere at 300°C for two hours to promote the formation of oxygen vacancies. Treated and untreated samples were characterized using X-band continuous wave EPR spectroscopy both in the dark and under UV-visible irradiation.
Results
It was found that the heat treated samples displayed a ten-fold increase in the concentration of trapped charge carriers on activation compared to untreated samples. Additionally, heat treatment promoted the trapping of electrons at interfacial defect sites within the mixed-phase TiO2 structure.
Conclusions
A simple heat treatment process forms oxygen vacancy defect sites within TiO2, drastically increasing the trapping of photogenerated charges and greatly improving the activity of these photocatalysts for the conversion of CO2 to fuels. This process enhances the potential of this reaction to eliminate an undesirable waste product while simultaneously providing a sustainable avenue for hydrocarbon fuel synthesis.