A Direct Z-Scheme for the Photocatalytic Hydrogen Production from a Water Ethanol Mixture on CoTiO₃/TiO₂ Heterostructures

Photocatalytic H₂ evolution from ethanol dehydrogenation is a convenient strategy to store solar energy in a highly valuable fuel with potential zero net CO₂ balance. Herein, we report on the synthesis of CoTiO₃/TiO₂ composite catalysts with controlled amounts of highly distributed CoTiO₃ nanodomains for photocatalytic ethanol dehydrogenation. We demonstrate these materials to provide outstanding hydrogen evolution rates under UV and visible illumination. The origin of this enhanced activity is extensively analyzed. In contrast to previous assumptions, UV-vis absorption spectra and ultraviolet photoelectron spectroscopy (UPS) prove CoTiO₃/TiO₂ heterostructures to have a type II band alignment, with the conduction band minimum of CoTiO₃ below the H₂/H⁺ energy level. Additional steady-state photoluminescence (PL) spectra, time-resolved PL spectra (TRPLS), and electrochemical characterization prove such heterostructures to result in enlarged lifetimes of the photogenerated charge carriers. These experimental evidence point toward a direct Z-scheme as the mechanism enabling the high photocatalytic activity of CoTiO₃/TiO₂ composites toward ethanol dehydrogenation. In addition, we probe small changes of temperature to strongly modify the photocatalytic activity of the materials tested, which could be used to further promote performance in a solar thermophotocatalytic reactor.