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Paper published in J. Phys. Chem. C

High Water-Splitting Efficiency Through Intentional In and Sn Co-Doping in Hematite Photoanodes

Ali Kaouk, Tero-Petri Ruoko, Myeongwhun Pyeon, Yakup Gönüllü, Kimmo Kaunisto, Helge J. Lemmetyinen, and Sanjay Mathur

Abstract

The effects of intermittent thin ITO layers on the water splitting efficiency of α-Fe2O3 films grown by PECVD on FTO substrates are reported. The α-Fe2O3 was co-doped with indium and tin by temperature-driven ionic transport and diffusion from the ultrathin ITO layer sputtered between the α-Fe2O3 layer and FTO substrate. The α-Fe2O3/ITO/FTO photoanodes showed a remarkable interdependence between the thickness of the ITO layer and PEC efficiency. Hematite photoanodes with a 32 nm thick ITO under-layer showed the highest photocurrent density of 2.5 mA.cm-2, corresponding to an approximate 3-fold enhancement over pristine α-Fe2O3 at 1.23 V vs RHE, whereas the thinner (8 nm) ITO under-layer yielded the lowest onset potential at 0.6 V vs RHE. Although the electrode with a thicker 72 nm ITO under-layer showed a higher onset potential of 0.9 V vs RHE, it still showed an enhancement in the photocurrent density at higher bias voltages. α-Fe2O3 was also deposited on metallic titanium substrates with intermittent sputtered tin and ITO layers. The co-doping with indium and tin from ITO was observed to yield greatly enhanced performance when compared with both α-Fe2O3 alone and tin doped α-Fe2O3. Transient absorption decays in the sub-nanosecond timescale were not affected by the doping, indicating that the doping had little effect on the primary charge carrier generation and recombination. On the other hand, fewer trapped electrons on the microsecond to millisecond timescale and a greatly increased amount of long-lived surface photoholes were observed for the ITO doped samples. The transient absorption results imply that the large increases in photoelectrochemical efficiency were obtained due to higher electron mobility, which reduces recombination and leads to more efficient electron extraction from the electrodes.

J. Phys. Chem. C, Just Accepted Manuscript.

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