|M.Sc Student||Tsyganok Anton|
|Subject||Effect of Fe(1-x)Ni(x)OOH Overlayer on Photo-Electrochemical|
Performance of Hematie (Alpha-Fe2O3) Photoandes
|Department||Department of Materials Science and Engineering||Supervisor||Professor Avner Rothschild|
|Full Thesis text - in Hebrew|
Solar water splitting holds a great promise for direct conversion of solar energy into storable hydrogen fuel. One of the proposed solutions is the usage of photoelectrochemical cells that utilize photoelectrochemical reactions to split water to hydrogen and oxygen. One of the most promising candidates for producing an effective photoanode is a - Fe2O3, also known as hematite. Hematite was chosen due to its stability, abundance, and a bandgap suitable for effective sunlight absorption in the visible range. However, a - Fe2O3 also has some major drawbacks including: low mobility and lifetime of charge carriers, low rate of holes reaction with the solution at the photoanode/electrolyte interface, and high external voltage for solar driven water splitting.
To improve hematite performance different approaches were investigated, one of which is the deposition of various overlayers. One of the most promising materials for use as an overlayer is earth abundant Fe1-xNixOOH. Fe1-xNixOOH is considered a favorable material because of its cheap cost, high catalytic activity, and stability under water oxidation reaction conditions in alkaline solutions.
Despite numerous studies of this overlayer, the underlying mechanisms for improvement in photoelectrochemical performance remain unclear, 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 thin (~1.5nm) Fe1-xNixOOH overlayers on hematite photoanodes as a function of the hematite photoanode properties.
First, the role of the Fe1-xNixOOH overlayer film was examined as a function of the doping of the hematite layer. Ti, Sn, and Zn doped hematite films together with undoped hematite photoanodes were examined. Photoelectrochemical results obtained using intensity modulated photocurrent spectroscopy (IMPS) revealed that the overlayer can play different roles depending on the doping type. One of the possible mechanisms that can explain this result is surface passivation that depends on the dopant properties. This hypothesis was further supported by measuring photoelectrochemical performance of hematite photoanodes with different Ti concentrations.
Next, the effect of the overlayer was examined for heteroepitaxial hematite photoanodes with different crystallographic orientations. The obtained results showed that the overlayer performance depends on the hematite orientation. In case of c-plane hematite photoanode, which possesses the highest surface states density, the surface recombination rate was reduced. For the three other photoanodes with a and m-plane orientations the hole current was found to increase by the overlayer, in addition to the reduction in surface recombination rate.
The obtained results suggest that in case of partial surface passivation the surface recombination is reduced, but the Fermi level remains pinned at surface states. Only when the surface states are effectively suppressed by the overlayer the Fermi level gets unpinned, leading to higher band bending and thus improved charge separation within the photoanode.
Finally, the ability to combine several overlayers of different co-catalyst materials on top of hematite photoanodes was examined. RuO2 and IrO2 layers were deposited on top of the Fe1-xNixOOH overlayer. The results showed that additional enhancement in the photo-electrochemical performance can be achieved.