Porous versus compact hematite nanorod photoanode for high performance photoelectrochemical water oxidation

Hematite (alpha-Fe2O3) is an attractive photoanode material for photoelectrochemical (PEC) water oxidation due to its high chemical stability, earth abundance, and suitable bandgap. However, the practical application of hematite in PEC water oxidation is severely limited by its short diffusion length of holes and high charge recombination rate. This work describes the synthesis of porous hematite nanorod (Fe2O3-PN) photoanodes via a facile surfactant assisted hydrothermal method. With dicyandiamide-formaldehyde (DF) resin as a surfactant, uniform hematite nanoarrays with a porous nanostructure are successfully grown on the surface of FTO (F-doped tin oxide) glass and exhibit enhanced charge separation for improved PEC water oxidation with comparison to that of compact hematite nanorods (Fe2O3-CNs). Photo-electrochemical impedance spectroscopy (PEIS) and PEC analysis reveal that the porous nanostructure is crucial to promote the photogenerated charge separation in the bulk and also accelerate the charge separation on the surface by providing enlarged electrochemical surface area. A boosted photocurrent density of 1.06 mA/cm(2) for Fe2O3-PN photoanodes is delivered at 1.23 V vs RHE under AM 1.SG illumination in 0.1 M KOH solution, which is 2-fold of that of Fe2O3-CN photoanodes. Furthermore, the PEC water oxidation kinetics of Fe2O3-PN photoanodes is further enhanced by incorporation of a cobalt phosphate (CoPi) cocatalyst, attaining a photocurrent density of 1.6 mA/cm(2) at 1.23 V vs RHE. This study provides an effective pathway for rationally synthesizing a highly active hematite photoanode for efficient PEC water oxidation.