M.Sc. Candidate Anton Tsyganok
Department of Materials Science and Engineering, Technion – Israel Institute of Technology, Haifa, Israel email@example.com
Hematite (α-Fe2O3) is an attractive material for solar water splitting based on its favorable properties as a photoanode material in photoelectrochemical (PEC) cells. However, the performance of state-of-the-art hematite photoanodes is still far short of the maximum theoretical efficiency, both in terms of photocurrent and photovoltage. One route for improving photoanode performance is through use of various co-catalysts that lead to a cathodic shift in the applied bias. One of the most promising materials for use as a co-catalyst is earth abundant Fe1-xNixOOH. It has recently been shown that using a photoelectrochemical deposition method, very thin and transparent Fe1-xNixOOH overlayers can be deposited to avoid optical (absorption) losses in the photoanode. Generally, the changes in performance have been attributed to a reduction in the recombination either as a result of surface passivation, hole storage in the overlayer, or p-n junction formation. For ultrathin photoelectrodeposited Fe1-xNixOOH overlayers, improved catalysis has been suggested as the reason for improvement. Despite significant study of this material, the underlying mechanisms for improvement in photoelectrochemical performance are not clear, and comparisons between different Fe1-xNixOOH coated hematite photoanodes prove difficult due to the different techniques used to fabricate both the co-catalyst and hematite layers.
This work examines the effect of Fe1-xNixOOH co-catalyst overlayer on thin film hematite photoanodes. Transparent Fe1-xNixOOH overlayers (~2 nm thick) were deposited photoelectrochemically on (001) oriented heteroepitaxial Sn- and Zn-doped hematite (α-Fe2O3) thin film photoanodes. Intensity modulated photocurrent spectroscopy (IMPS) was applied to study the changes in the hole current and recombination current induced by the Fe1-xNixOOH overlayers. For the Sn-doped hematite photoanode, the improvement in performance after deposition of the Fe1-xNixOOH overlayer was entirely due to reduction in the recombination current. For the Zn-doped hematite photoanode, in addition to a reduction in recombination current, an increase in the hole current to the surface was also observed after the Fe1-xNixOOH overlayer deposition, leading to a cathodic shift in the onset potential as well as an enhancement in the plateau photocurrent. These results demonstrate that Fe1-xNixOOH co-catalysts can play different roles depending on the underlying hematite photoanode.