Cation-Exchange-Derived Wurtzite-Type Cobalt Sulfide Nanocrystal Improves Both Onset Potential and Photocurrent of Hematite Photoanode for Efficient Solar Water Splitting.

Hematite (α-Fe2O3) photoanode material holds significant promise for practical applications. However, its performance is severely limited by low photocurrent density (Jph) and high onset potential (Von). Although high-valence metal doping can boost Jph, it usually shifts Von positively. Herein, we report an unconventional strategy that utilizes wurtzite-type cobalt sulfide (w-CoS) nanocrystals, synthesized via cation exchange, to modify Nb-doped hematite (Nb:Fe2O3) photoanodes. This approach simultaneously achieves two beneficial effects: enhancing Jph and reducing Von. More importantly, w-CoS nanocrystals form a type-II heterojunction with Nb:Fe2O3, which facilitates the spatial separation and interfacial transfer of photogenerated carriers, with electrons preferentially directed toward Nb:Fe2O3 and holes toward the w-CoS-modified surface for water oxidation. As a result, the optimized Nb:Fe2O3/w-CoS photoanode delivers a Jph of 3.46 mA cm-2 at 1.23 VRHE, along with a cathodic Von shift of ∼140 mV. This surface engineering strategy demonstrates broad universality across various hematite-based photoanodes, offering a distinct pathway for the design of efficient photoelectrodes.