Rational design of BiVO4/CoFe-LDH@Ni2(NO3)2(OH)2 ternary photoanodes: synergistic built-in electric field and oxygen vacancies for high-efficiency solar water oxidation

The rational design of efficient photoanodes is essential for advancing solar-driven water oxidation. This study addresses the intrinsic challenges of bismuth vanadate (BiVO 4 ), such as sluggish charge kinetics and severe surface recombination, through the construction of a ternary composite photoanode via synergistic interfacial modification and cocatalyst integration. A hierarchical architecture was fabricated through sequential low-temperature chemical bath deposition and electrodeposition, integrating a CoFe-layered double hydroxide (CoFe-LDH) interlayer and nickel-hydroxynitrate (Ni 2 (NO 3 ) 2 (OH) 2 ) onto BiVO 4 substrates. The optimized BiVO 4 /CoFe-LDH@Ni 2 (NO 3 ) 2 (OH) 2 photoanode achieves a photocurrent density of 5.57 mA/cm 2 at 1.23 V vs. reversible hydrogen electrode (V RHE ) under Air Mass 1.5 Global (AM 1.5G) illumination, accompanied by a 251 mV cathodic shift in onset potential. Comprehensive characterizations-including X-ray photoelectron spectroscopy (XPS), open-circuit photovoltage (OCP) decay, time-resolved photoluminescence (TRPL) decay, electron paramagnetic resonance (EPR) and Kelvin probe force microscopy (KPFM)-reveal that the oxygen vacancy (O v )-rich CoFe-LDH@Ni 2 (NO 3 ) 2 (OH) 2 layer facilitates efficient charge transfer within BiVO 4 and generates a pronounced built-in electric field, thereby significantly enhancing interfacial charge separation. Concurrently, density functional theory (DFT) calculations demonstrate that the interfacial effect of CoFe-LDH@Ni 2 (NO 3 ) 2 (OH) 2 substantially lowers the thermodynamic barrier for the oxygen evolution reaction (OER). This study offers fundamental insights for fabricating high-efficiency BiVO 4 -based photoanodes.