Transition metal oxides for photoelectrochemical water splitting
Transient absorption spectroscopy (TAS), Surface enhanced Raman spectroscopy, X-Ray photoelectron spectroscopy (XPS).
Hydrogen production via electrolysis of water has been known for more than a century, however the required catalysts needed to achieve practical efficiencies in the process are too expensive for mass production of hydrogen using renewable energy sources.
From the thermodynamics point of view the oxidation of water is an uphill process,
2H2O → O2 + 4H+ + 4e- E°= 1.23
This reaction can potentially be driven by solar energy using suitable photoactive semiconductor materials in a photoelectrochemical (PEC) cell, without the need for additional energy input.
Nevertheless the sluggish kinetics for the oxygen evolution reaction (OER), where 4 e- are involved, is partly responsible for the large overpotentials needed in a real PEC cell.
Even though the photoelectrocatalytic production of hydrogen at the surface of TiO2 was demonstrated 40 years ago , a single semiconductor material suitable for overall water splitting that is efficient, robust and inexpensive has not yet been identified.
In the context of "artificial photosynthesis", several strategies inspired in nature's photosynthetic systems have been actively pursued in recent years, for both water splitting (generation) and CO2 reduction (production of hydrocarbon fuels like CH3OH and CH4).
Recently, the discovery of a very active electrocatalyst for OER made of abundant Cobalt and phosphate , which can be coupled with photoactive semiconductors, demonstrated the feasibility of "artificial leaves" and opened the door for the search of novel, efficient, stable and low-cost (photo)catalysts.
In this framework, a fundamental understanding of structure-activity relationships in semiconductor-based photoelectrodes is crucial for a knowledge-based development of new artificial photosynthesis devices. In particular, the dynamics of photogenerated charges at the bulk and interfaces of absorber/catalyst and catalyst/electrolyte are key in rationalising the limiting factors responsible for efficiency losses and the high overvoltages required to drive the OER.
 K. Honda and A. Fujishima, Nature 238, 37 - 38 (1972)
 M. Kanan and D. Nocera, Science 321, 1072-1075 (2008)