Raspberry-Like Microspheres of Core–Shell Cr₂O₃@TiO₂ Nanoparticles for CO₂ Photoreduction

To promote the interaction of p–n semiconductors, raspberry-like microspheres of core–shell Cr₂O₃@TiO₂ nanoparticles have been fabricated through a five-step process. Raman spectroscopy of products calcined at various temperatures reveal that the titania shell causes crystal distortion of the Cr₂O₃ core, without changing the microstructures of the fabricated core–shell microspheres. In situ and time-resolved synchrotron-based powder XRD reveals the formation of monoclinic TiO₂ in the fourth step, but these monoclinic TiO₂ nanocrystals undergo a phase transition when the applied calcination temperature is above 550 °C. As a result, TiO₂(B), a magnéli phase of Ti₄O₇ and Cr₂Ti₆O₁₅ compounds, resulting from inner doping between Cr₂O₃ and TiO₂, is formed. The close interaction of Cr₂O₃ and TiO₂ forms a p–n junction that decreases the recombination of photogenerated electron–hole pairs, leading to enhanced production of CH₄ by photocatalytic reduction of CO₂.