Nanostructured metal oxide semiconductors for photoelectrocatalytic conversion of solar energy

The direct conversion of solar energy by means of photoelectrocatalytic processes, such as hydrogen generation via photoelectrochemical water splitting or photo-oxidation of biomasses into higher value chemical products, represents an important pillar of the clean energy transition. Metal oxide semiconductors are the materials of choice for the fabrication of efficient photoanodes for sun-driven oxidation in a photoelectrochemical cell [1]. The pathway that leads from the photoexcitation of electron-hole pairs to the production of the energy carrier is particularly complex and requires the optimization of several materials-related factors, particularly broadband light harvesting, charge transport toward the semiconductor-electrolyte interface, and charge transfer to reactive species, generally assisted by catalysts deposited at the photoelectrode surface. Therefore, the metal oxide semiconductors must be at the same time nanoporous to increase active area, heavily doped to enhance charge transport, and coupled to heterogenous nano-catalysts at the surface to facilitate charge transfer. In this presentation, we will report on recent progresses achieved within a running H2020 project [2] in relation to two classes of materials: WO
3
/BiVO
4
heterojunctions for solar hydrogen generation [3] and Ti-doped hematite for photo-oxidation of biomasses into valuable chemicals [4, 5]. The combination of two semiconductors with different energy gaps (WO
3
and BiVO
4
) allows for a broader harvesting of the solar spectrum while maintaining a good capacity to separate and collect photogenerated charges, which significantly improves the solar-to-hydrogen efficiency. Doping hematite with Ti greatly suppresses recombination, while a careful choice of the coupled catalyst steers the selectivity toward the desired photo-oxidation product. The charge carrier dynamics and the role of the nano-catalysts in these materials was studied by modulation techniques (intensity-modulated photocurrent and electrochemical impedance spectroscopies), pump-probe transient absorption spectroscopy, and operando x-ray absorption spectroscopy.