Improved water oxidation performance of ultra-thin planar hematite photoanode: Synergistic effect of In/Sn doping and an overlayer of metal oxyhydroxides

摘要

Hematite is a promising photoanode candidate with many favorable material properties, such as stability and suitable band-gap. However, there are some severe challenges, including high losses due to charge recombination and slow oxidation kinetics, which can be addressed by doping and addition of co-catalysts. Here, the effects of temperature driven diffusion of substrate impurities (doping) and subsequent surface modification by metal oxy-hydroxides (co-catalysts) have been studied for enhanced water-oxidation performance in photoelectrochemical (PEC) measurements. Diffusion of indium and tin from the indium-doped tin oxide (ITO) substrate into planar films of alpha-Fe2O3 photoanodes results in a photocurrent density (J(ph)) of 0.09 mA/cm(2), corresponding to an approximate 9-fold enhancement over the control pristine alpha-Fe2O3 (0.01 mA/cm(2)) at 1.23 V-RHE. A thin amorphous FeOOH coating over the In/Sn co-doped alpha-Fe2O3 photoanode improves the water oxidation performance further, with a 211 % enhancement in J(ph) at 1.23 VRHE and a 0.21 V cathodic shift in onset potential. Thin layers of NiOOH and FeNiOOH co-catalysts exhibit 100 and 155 % enhancement in J(ph), respectively. Characterization and electrochemical measurements reveal that the enhanced performance is a result of reduced bulk recombination by temperature driven In/Sn substrate impurity doping and improved surface oxidation kinetics by the metal oxy-hydroxide overlayer. Especially deposition of FeOOH onto In/Sn co-doped alpha-Fe2O3 significantly reduces resistance at the semiconductor/electrolyte interface, leading to the shift in onset potential. Further, the results indicate that all the samples exhibit a quantitative correlation between the cathodic shift in photocurrent onset potential (V-onset) and flat band potential (V-fb).