Abstract
In this study, we found that it could be an universal case that an elemental carbon overlayer would be induced when hematite electrodes are prepared using electrodeposition with additional organic acid additives in the electrolyte. Their photocurrent densities are strongly dependent on the electrolyte pH where they were prepared as the thickness of elemental carbon ovrelayers is regulated by the amount of adsorbed organic acids on electroreduced iron oxyhydroxides during electrodeposition. An optimal carbon overlayer approximately 2~5 nm would lead the PEC performances up to around 2 mA/cm2 (0.4 V vs SCE), which is almost 10 times higher than those without carbon overlayers. While carbon overlayers would effectively passivate surface states, their degradation in the end of 90 minutes of illumination would lead a capacitor feature occurring at hematite/carbon overlayer interface and therefore compromises the PEC performance. Based on our results, characteristic of an effective carbon overlayer might include: (i) passivating surface states to reduce onset photocurrent potential, (ii) doping additional charges to enhance photocurrent plateau, and (iii) enhanced stability to sustain a long period of sunlight illumination.