A facile approach for preparing densely-packed individual p-NiO/n-FeO heterojunction nanowires for photoelectrochemical water splitting.

Innovative design of electrode materials is crucial for efficient conversion of solar energy into chemical fuel through photoelectrochemical (PEC) water splitting. Herein, we report the development of a p-n heterojunction nanowire (NW) based photoanode made of low cost earth-abundant materials. Densely-packed and freestanding individual p-NiO/n-Fe2O3 heterojunction NWs are fabricated through consecutive electrodeposition of Fe and Ni NWs inside the pores of the anodic alumina template followed by controlled oxidation. Heterojunction formation in individual NWs is confirmed through energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM), along with elemental mapping on individual NWs through electron energy loss spectroscopy (EELS). An inverted 'V' shape nature of the Mott-Schottky curve suggests p-n diode like characteristics of the heterojunction NWs. These p-n heterojunction NWs demonstrate a significantly enhanced photocurrent density (∼24 times at a potential of 1.23 V vs. RHE) and a cathodic shift (∼0.4 V) of the photocurrent onset potential compared to the pristine Fe2O3 NW electrode, which can be attributed to the synergistic combination of n-Fe2O3 with the co-catalyst p-NiO facilitating the generation and transfer of photogenerated holes into the electrolyte for water oxidation. This study validates the feasibility of developing Fe2O3 based heterojunction photoelectrodes for efficient PEC water splitting.