Raman studies for alpha-Fe2O3 based thin-films photoanodes for water oxidation
Wal, L.I. van der
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α-Fe2O3, better known as hematite, has gained interest to be used as semiconductor for the anode in photoelectrochemical cells, since it has a suitable valence band position to catalyze the oxygen evolution reaction (OER) of water splitting. Other favorable properties of hematite are its abundance, making it a low cost material, its stability in water and its low toxicity. The material has, however, also two major drawbacks, which are its short photo-generated charge carrier lifetime, resulting in fast recombination of electron-hole pairs and its slow charge carrier mobility, leading to problems in the kinetics for the transfer of holes across the hematite-water interface. In this research, the first drawback was overcome by the addition of Ti dopant-species. Three different doping percentages (1 mole%, 5 mole%, 10 mole%) were used during the research, and PEC measurements clearly showed that upon introduction of Ti dopant species, a higher photocurrent was obtained and that the performance towards the OER was increased. Within the range of samples tested, the 10% Ti doped samples were the most efficient towards the OER. The second drawback was overcome by adding a co-catalyst, NiFeOx, on top of the hematite material to enhance the kinetics of hole transfer across the interface of the electrode with water, resulting in higher photocurrent for hematite samples on which a co-catalyst was deposited compared to bare hematite samples. There is, however, no consensus yet on what the active phase is of NiFeOx, and how it improves the performance of hematite towards the OER. This research tried to answer these questions by the means of in-situ Raman spectroscopy. The research showed that Raman spectroscopy measurements were possible for both materials, but when in-situ conditions were simulated by adding a thin layer of water on top of the electrodes, Raman signals could not be collected for NiFeOx. Therefore, surface-enhanced Raman spectroscopy measurements were performed in order to overcome the troubles upon simulating in-situ conditions. Since the surface-enhancement effect could not be properly obtained, in-situ Raman spectroscopy measurements, in which an alkaline electrolyte should be used and a potential should be applied, were not yet performed during this research. Investigating the possibilities to enhance the NiFeOx Raman signal, however, might lead to opportunities for in-situ surface-enhanced Raman spectroscopy measurements for the NiFeOx – α-Fe2O3 samples. Then, NiFeOx – α-Fe2O3 samples could be properly studied with Raman spectroscopy under in-situ conditions, possibly giving an answer on what the active phase in NiFeOx is.