Enhanced Photoelectrochemical Water Oxidation Performance in Bilayer TiO2/α‐Fe2O3 Nanorod Arrays Photoanode with Cu : NiOx as Hole Transport Layer and Co−Pi as Cocatalyst

Abstract Efficient charge transfer and excellent surface water oxidation kinetics are key factors in determining the photoelectrochemical (PEC) water splitting performance in photoelectrodes. Herein, a bilayer TiO 2 /α‐Fe 2 O 3 nanorod (NR) arrays photoanode was prepared with deposited Cu‐doped NiO x (Cu : NiO x ) hole transport layer (HTL) and Co−Pi oxygen evolution reaction (OER) cocatalyst for PEC water oxidation. The hierarchical TiO 2 /α‐Fe 2 O 3 composite obtained by a secondary hydrothermal process exhibited an inapparent bilayer structure by embedding the underlayer TiO 2 NR arrays at the bottom part of the post‐grown α‐Fe 2 O 3 NR arrays. The underlayer TiO 2 NRs acted as an effective shuttling pathway for transferring photoelectrons generated in the upper hematite light absorber layer. A p‐type inter‐Cu : NiO x HTL was introduced to form a build‐in p–n electric field between Cu : NiO x and α‐Fe 2 O 3 NRs, which improved the hole extraction from α‐Fe 2 O 3 to Co−Pi OER catalyst. As expected, the as‐engineered TiO 2 /α‐Fe 2 O 3 /Cu : NiO x /Co−Pi photoanode displayed an excellent photocurrent density of 2.43 mA cm −2 at 1.23 V versus the reversible hydrogen electrode (V RHE ), up to 4.05 and 2.23 times greater than those of the bare α‐Fe 2 O 3 (0.60 mA cm −2 ) and TiO 2 /α‐Fe 2 O 3 , respectively. The results demonstrate that the bottom‐up engineering of electron‐hole transport channels and cocatalyst modification is an attractive maneuver to enhance the PEC water oxidation activity in hematite and other photoanodes.