Synergistic Enhancement of Hydrogen Production Through Photo-Piezocatalytic BaTiO₃/ZnIn₂S₄ Heterostructures

Solar-to-Fuel technologies are crucial for the energy transition, enhancing green energy availability and mitigating climate change by offsetting emissions. Hydrogen energy, notable for its zero emissions, relies on solar irradiation to drive essential photocatalytic reactions for fuel production. Improving charge transfer and electron-hole pair separation is key to boosting catalytic efficiency. Additionally, integrating the piezoelectric effect, triggered by mechanical strain, enhances charge carrier separation, thus enhancing the efficiency of Solar-to-Fuel systems. In this study, we synthesized visible-driven ZnIn₂S₄ nanosheets as photocatalysts and integrated them with nanosized BaTiO₃ piezocatalysts for hydrogen production via water splitting and biomass reforming. To develop a highly active primary photocatalyst, we optimized the synthesis conditions of ZnIn₂S₄ to control its preferred crystal phase, phase ratio, and planar spacing effectively. The optimal conditions, identified at a stabilizing agent concentration of 75% and a temperature of 120°C, allowed the photocatalyst to exhibit superior performance.We then engineered a heterostructure to create BaTiO₃/ZnIn2S₄ photo-piezocatalysts, observing a well-dispersed distribution of nanocubes onto nanosheets. Significant modifications in the morphological and crystal structures, as well as optical properties, were observed as the heterostructure was developed. In photo-piezocatalysis, the optimal BaTiO₃/ZnIn₂S₄ achieved a hydrogen production rate of 154.04 μmol●g^-1●h^-1, which is 3.55 times higher than that of pristine BaTiO₃ or ZnIn₂S₄ alone. This increase is attributed to the synergistic effects of photocatalysis and piezocatalysis. To elucidate the synergistic pathways of energy-induced charge carriers, we employed in-situ X-ray photoelectron spectroscopy (XPS) under Xenon illumination. Notable upshifts in the binding energies of elemental orbitals in BaTiO₃ indicated a decrease in electron density upon irradiation, suggesting that photogenerated holes from ZnIn₂S₄ migrate to the interface and attract localized electrons within BaTiO₃, thereby enhancing its self-polarization effect. This mechanism highlights the dynamic interaction at the heterojunction, crucial for enhancing photocatalytic activity. Further investigating the dynamics of charge carriers, we used a photo-assisted Kelvin probe analyzer to measure the contact potential difference (CPD) under UV illumination and in the dark. We observed a gradual decrease in the CPD shift of BaTiO₃/ZnIn₂S₄ compared to pristine ZnIn₂S₄, which we attribute to the coverage of polarized BaTiO₃ nanocubes on the composite surface, affecting the electrostatic environment. Additionally, ultrasonic treatment combined with illumination effectively inhibited the recombination of photogenerated electron-hole pairs in ZnIn₂S₄. Simultaneously, a high concentration of anions attracted onto the BaTiO₃ enhanced the reduction activities due to the screening charge effect of piezocatalysis. These observations confirm the charge carrier interaction and underscore the synergistic effect within this heterostructure. Consequently, the BaTiO₃/ZnIn₂S₄ heterostructure demonstrates efficient hydrogen production through photo-piezocatalysis, applicable not only in water splitting but also in the reforming of biomass into hydrogen. This photo-piezocatalyst holds promising potential for future applications in green energy production, offering a sustainable and versatile solution for renewable energy technologies.