Interfacial Regulation by a NiO _x Overlayer Enables Enhanced Near-Infrared Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting provides a promising pathway for sustainable hydrogen production. However, the inefficient use of near-infrared (NIR) light, which accounts for nearly half of the solar spectrum, remains a major limitation to overall energy conversion efficiency. Here, we present a NIR-responsive dye-sensitized photoanode enabled by a boron-dipyrromethene-carbazole-based organic sensitizer (1), combined with a thin NiO _x overlayer that acts as both an oxygen evolution reaction (OER) cocatalyst and an interface regulator. The NiO _x layer, deposited via low-temperature pulsed laser deposition, exhibits low crystallinity and mixed Ni²⁺/Ni³⁺ valence states, as confirmed by transmission electron microscopy and X-ray photoelectron spectroscopy. Adding the NiO _x overlayer results in a 1.9-fold increase in photocurrent density and significantly better photostability. Electrochemical testing indicates that NiO _x alters interfacial charge-transfer kinetics and the local electrochemical environment, thereby improving carrier utilization and reaction efficiency. The multilayer photoanode retains a measurable photocurrent response at 850 nm, demonstrating its capability to extend PEC activity into the NIR region. This work elucidates the synergistic functions of NiO _x in interfacial charge regulation and catalytic kinetics, offering a viable strategy for NIR-driven solar fuel conversion. Additionally, it establishes the longest-wavelength photoelectrocatalytic performance reported to date for non-noble-metal dye-sensitized systems.