Bicrystalline TiO2 with controllable anatase–brookite phase content for enhanced CO2 photoreduction to fuels

Abstract

Among the three naturally existing phases of TiO₂, brookite is the least studied as a photocatalyst. In this study, single-phase anatase and brookite, and mixed-phase anatase–brookite TiO₂ nanomaterials were synthesized through a hydrothermal method. The anatase–brookite phase content was controlled by adjusting the concentration of urea in the precursor solution. XRD, Raman spectroscopy, and high-resolution TEM were used to confirm the crystal structures. SEM and TEM analyses demonstrated that anatase TiO₂ were nearly spherical nanoparticles while brookite TiO₂ were rod-shaped nanoparticles. UV-vis diffuse reflectance spectroscopy showed a blue shift in absorption spectra with increasing brookite content. The photocatalytic activities of the prepared bicrystalline TiO₂ were evaluated for CO₂ photoreduction in the presence of water vapor for production of solar fuels (CO and CH₄). The activities were compared with those of pure anatase, pure brookite, and a commercial anatase–rutile TiO₂ (P25). The results showed that bicrystalline anatase–brookite was generally more active than single-phase anatase, brookite, and P25. The bicrystalline mixture with a composition of 75% anatase and 25% brookite showed the highest photocatalytic activity, likely due to the enhanced interfacial charge transfer between anatase and brookite nanocrystals. In situ DRIFTS analysis showed that CO₂⁻ and HCO₃⁻ species were active reaction intermediates for CO₂ photoreduction while the accumulation of non-reactive CO₃²⁻ species on the TiO₂ surface may be detrimental.