Hydrogen fuels are one of many approaches to combat the climate crisis that is caused by the emission of CO2 into the Earth's atmosphere. In order for hydrogen to be a low-emission fuel, it has to be produced in low-emission processes. One currently highly researched low emission process is photocatalytic water splitting. In photocatalysis, light induced charge carrier separation in semiconductors is utilised to promote redox reactions. For many of those reactions, including water splitting, additional co-catalysts are necessary to avoid charge recombination by extracting charge carriers and to act as catalytic sites. In this work, the method of redox exsolution is employed to form nanoparticle co-catalyst on a semiconductor material. For redox exsolution, the host material is doped with the components of the desired nanoparticles, which are then formed through reduction at high temperatures. Due to their socketed geometry, exsolved nanoparticles are generally more resistant to agglomeration and catalyst poisoning than deposited ones. Additionally, the metal can be re-absorbed into the host lattice under oxidising conditions and exsolved again through reduction, enabeling regeneration of the catalytic system. These properties make exsolved nanoparticles promising candidates for water splitting co-catalysts. Exsolution is benchmarked against photodeposition using the photocatalyst/co-catalyst couple SrTiO3/Cu and the hydrogen reduction half of the water splitting reaction. We show that Cu co-catalyst nanoparticles can be grown on SrTiO3 through exsolution, as the catalytic activity of Cu-doped SrTiO3 is significantly improved after reduction of the material. During the catalytic reaction, the activity of the nonexsolved Cu-doped SrTiO3 increases, which could be due to in-situ reduction of the Cu-dopant. When comparing the activity development of exsolved and photodeposited Cu co-catalysts, we see that the exsolved system is more stable over time. In terms of initial hydrogen evolution rate, the photodepositedsystem is about 3 times as effective as the exsolved system, and even though the activity drops overtime, the photodeposited system outperforms the exsolved system overall. This can potentially be traced back to charge trapping effects caused by the Cu-doping. Post-catalytic regeneration was only tested with one set of conditions, in which case the activity of the catalyst decreased after the regeneration process. In summary, exsolved Cu co-catalysts on SrTiO3 outperform photodeposited Cu co-catalysts in terms of stability, but do not reach the same overall catalytic activity, which might be due to charge trapping effects in the bulk caused by the Cu-doping.
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