Multiscale charge-carrier dynamics governed by surface energetics in facet-engineered NaNbO₃/FeVO₄ photoanodes

Efficient photoelectrochemical water splitting requires the simultaneous extension of visible-light absorption and effective suppression of photogenerated charge-carrier recombination. While heterostructure formation and morphology control are widely employed to improve PEC performance, the role of crystallographic facet-interface coupling in regulating charge-carrier dynamics across ultrafast and electrochemical time scales remains insufficiently understood. In this work, a facet-dependent heterojunction engineering strategy was implemented through the construction of cube and truncated-cube NaNbO₃/FeVO₄n-n heterostructures. Ultrafast transient absorption spectroscopy revealed a pronounced facet-dependent reorganization of excited-state relaxation pathways, with the truncated-cube heterostructure exhibiting enhanced populations of long-lived charge carriers. Time-resolved electrochemical analysis based on the distribution of relaxation times further identified accelerated interfacial charge-transfer processes accompanied by a markedly reduced charge-transfer resistance. Consistently, Mott-Schottky analysis indicated an increased donor density and stronger band bending at the facet-engineered interface. First-principles surface energy calculations reveal that facet engineering induces a strong energetic anisotropy, providing a driving force for directional charge separation. The synergistic effects of optimized charge separation, prolonged carrier lifetimes, and improved interfacial charge transport resulted in a significantly enhanced PEC response, delivering a photocurrent density of 0.547 mA cm^-2 at 1.7 V vs. RHE, along with improved photostability and photoresponsivity for the NaNbO₃-truncated cube/FeVO₄ photoanode. Overall, this study establishes facet-guided heterojunction engineering as an effective approach to directly correlate nanoscale charge-carrier dynamics with enhanced charge separation and improved charge-transfer kinetics in photoelectrochemical water splitting systems.