Photochemical and electrochemical assessment of UIO-66-NH₂/g-C₃N₄ thin-film heterostructures as potential candidates for hydrogen evolution: an experimental study augmented by DFT insights

The global shift towards carbon-neutral energy systems has catalyzed an intensified focus on sustainable hydrogen production, with photo and electrochemical water splitting emerging as a particularly promising pathway. This study elucidates the design, simulation, and synthesis of advanced photo and electrocatalytic materials tailored for the hydrogen evolution reaction (HER), concentrating on heterostructures formed by zirconium-based metal-organic frameworks (MOFs)-specifically, UiO-66 and its amine-functionalized derivative, UiO-66-NH₂-in conjunction with graphitic carbon nitride (g-C₃N₄). Employing density functional theory (DFT) simulations, we pre-screened the electronic properties of the MOFs, revealing that amine functionalization significantly narrows the bandgap and optimizes band alignment, thereby enhancing photocatalytic activity. Guided by DFT-derived analyses of electronic structure and density of states, UiO-66-NH₂ was selected for experimental synthesis. Thin-film catalysts comprising UiO-66-NH₂ and g-C₃N₄ in varying weight ratios (60:40, 70:30, and 50:50) were deposited onto fluorine-doped tin oxide (FTO) substrates and subsequently evaluated in a standard three-electrode photochemical setup using a 0.5 M Na₂SO₃ electrolyte, followed by testing in an electrochemical configuration with 1 M KOH. Comprehensive material characterization techniques-including X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM/TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS)-coupled with rigorous electrochemical assessments (linear sweep voltammetry (LSV), cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS)), demonstrated that the 70:30 UiO-66-NH₂/g-C₃N₄ composite exhibited superior HER performance. This composite achieved the highest stable photocurrent response, a low overpotential of 135 mV, a favorable Tafel slope of 98 mV/dec, and the smallest semicircle diameter, indicating the lowest charge transfer resistance and enhanced electron transport efficiency. These findings confirm the synergistic enhancement realized through the hybridization of MOFs and g-C₃N₄, providing critical insights into the role of interfacial interactions in augmenting HER activity. The integration of theoretical and experimental methodologies in this research paves the way for the rational design of high-efficiency MOF-based photocatalysts, thereby advancing the development of green hydrogen technologies.