High Water-Splitting Efficiency through Intentional In and Sn Codoping in Hematite Photoanodes

The effects of intermittent thin ITO layers on the water-splitting efficiency of alpha-Fe2O3 films grown by PECVD on FTO substrates are reported. The alpha-Fe2O3 was codoped with indium and tin by temperature-driven ionic transport and diffusion from the ultrathin ITO layer sputtered between the alpha-Fe2O3 layer and FTO substrate. The alpha-Fe2O3/ITO/FTO photoanodes showed a remarkable interdependence between the thickness of the ITO layer and PEC efficiency. Hematite photoanodes with a 32 nrn thick ITO underlayer showed the highest photocurrent density of 2.5 mA cm(-2), corresponding to an approximate 3-fold enhancement over pristine alpha-Fe2O3 at 1.23 V vs RHE, whereas the thinner (8 nm) ITO underlayer yielded the lowest onset potential at 0.6 V vs RHE. Although the electrode with a thicker 72 nm ITO underlayer showed a higher onset potential of 0.9 V vs RHE, it still showed an enhancement in the photocurrent density at higher bias voltages. alpha-Fe2O3 was also deposited on metallic titanium substrates with intermittent sputtered tin and ITO layers. The codoping with indium and tin from ITO was observed to yield greatly enhanced performance when compared with both alpha-Fe2O3 alone and tin-doped alpha-Fe2O3. Transient absorption decays in the sub-nanosecond time scale 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 time scale 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.