Highly Efficient Photoelectrochemical Water Splitting: Surface Modification of Cobalt‐Phosphate‐Loaded Co3O4/Fe2O3 p–n Heterojunction Nanorod Arrays

Abstract Hematite (α‐Fe 2 O 3 ) as a photoanode material for photoelectrochemical (PEC) water splitting suffers from the two problems of poor charge separation and slow water oxidation kinetics. The construction of p–n junction nanostructures by coupling of highly stable Co 3 O 4 in aqueous alkaline environment to Fe 2 O 3 nanorod arrays with delicate energy band positions may be a challenging strategy for efficient PEC water oxidation. It is demonstrated that the designed p‐Co 3 O 4 /n‐Fe 2 O 3 junction exhibits superior photocurrent density, fast water oxidation kinetics, and remarkable charge injection and bulk separation efficiency (η inj and η sep ), attributing to the high catalytic behavior of Co 3 O 4 for the oxygen evolution reaction as well as the induced interfacial electric field that facilitates separation and transportation of charge carriers. In addition, a cocatalyst of cobalt phosphate (Co‐Pi) is introduced, which brings the PEC performance to a high level. The resultant Co‐Pi/Co 3 O 4 /Ti:Fe 2 O 3 photoanode shows a photocurrent density of 2.7 mA cm −2 at 1.23 V RHE (V vs reversible hydrogen electrode), 125% higher than that of the Ti:Fe 2 O 3 photoanode. The optimized η inj and η sep of 91.6 and 23.0% at 1.23 V RHE are achieved on Co‐Pi/Co 3 O 4 /Ti:Fe 2 O 3 , respectively, corresponding to the 70 and 43% improvements compared with those of Ti:Fe 2 O 3 . Furthermore, Co‐Pi/Co 3 O 4 /Ti:Fe 2 O 3 shows a low onset potential of 0.64 V RHE and long‐time PEC stability.