Selective doping of titanium into double layered hematite nanorod arrays for improved photoelectrochemical water splitting

Abstract

Element doping is effective to improve the photoelectrochemical (PEC) performance of photoelectrodes, as it can increase the carrier density and then enhance electrical conductivity for efficient charge transfer. In this study, titanium (Ti) was selectively doped into the bottom and/or top layer of the double layered hematite (α-Fe2O3/α-Fe2O3) nanorod arrays grown on conductive transparent substrate (F:SnO2, FTO) via a two-step hydrothermal method to optimize the electron donor distribution and improve the charge separation efficiency for a remarkable enhancement in PEC water splitting. It was demonstrated that, by selectively doping Ti into the bottom layer, the obtained FTO/α-Fe2O3:Ti/α-Fe2O3 nanorod photoanode showed the highest PEC performance for water splitting, with photocurrent density reaching 1.69 mA/cm2 at 1.9 V vs. RHE under AM 1.5G illumination, which was 4.3 times that of undoped α-Fe2O3/α-Fe2O3 nanorod film (0.39 mA/cm2) and even much higher than the top layer and double layer doped α-Fe2O3 nanorod films (FTO/α-Fe2O3/α-Fe2O3:Ti and FTO/α-Fe2O3:Ti/α-Fe2O3:Ti). By introducing the Ti electron donor dopants into α-Fe2O3, the electron density will be increased in the α-Fe2O3:Ti layer, raising the Fermi level. For the FTO/α-Fe2O3:Ti/α-Fe2O3 nanorod film, the band realignment will create a terraced band structure and then build an internal electric field at the interface of the bottom and top layers. As a result, the photoexcited electrons and holes will transfer to the FTO substrate and the photoanode surface, respectively, as driven by the internal electric field. This study demonstrated an alternative approach to the design of novel photoanodes with improved PEC performances, by engineering the electron density distribution and the band structure for efficient charge carrier separation.