Photoelectrochemical Processes on Fe2O3 Based Model Anodes

Doctoral thesis, 2014

One possibility to partially satisfy the ever-increasing energy needs of modern society, without compromising the Earth’s environment through e.g. global warming, consists in harvesting solar energy and storing it as hydrogen produced from water splitting. This can potentially be achieved on a large scale using photoelectrochemical (PEC) cells. Hematite (Fe2O3) is a promising anode material for the oxidation half reaction (which leads to oxygen evolution), despite the low values of energy conversion efficiency reported so far. In order to exploit the full potential of hematite, the processes of light absorption, charge separation/transport, and oxygen evolution at the hematite/electrolyte interface need to be understood in greater detail. In this work, we employed model anodes based on hematite thin films towards the following, interconnected goals: (i) to deepen our knowledge of the aforementioned PEC processes, and (ii) to test promising strategies to enhance the oxygen evolution on hematite. In the first part, the influence of functionalization with Ti/Au nanoparticles on the photoelectrochemistry of hematite was explored. When certain conditions are met, these nanoparticles support localized surface plasmon resonances (LSPRs), i.e., collective, non-propagating oscillations of electrons. At resonance, the electric field distribution around the nanoparticles was strongly altered causing an enhanced charge generation in hematite, which led to a considerable enhancement of photocurrent and oxygen evolution on the functionalized anodes. In the second part, the oxygen evolution was investigated on bare hematite anodes, and on the role played by surface states in this re- action. Moreover, it was found that the energy required to initiate the oxygen evolution can be decreased upon appropriate thermal treatment during the fabrication process. Finally, electrochemical oxygen evolution from water was compared to oxidation of other molecules of interest in the field by a combination of first-principles calculations and electrochemical measurements. The results presented here are expected to be helpful in future design of hematite based PEC cells, where the strategies to increase the energy conversion efficiency discussed in this work are further combined with other approaches like material doping and nanostructuring, as well as addition of co-catalysts.