Exploring Photo-Piezocatalysis in Heterostructured Barium Titanate and Titanium Dioxide: Insights from Photo-Triggered Contact Potential Differences

Pollutants perpetually generated by various industries intensify the burden on the environment and progressively damage our living spaces. The combination of photocatalysis and piezocatalysis presents an intriguing technique for achieving pollutant degradation and potential in versatility and energy efficiency. However, the significance influence of catalysis reaction is sufficient electron-hole pairs to generate the reactive radicals. The piezocatalyts, triggered by mechanical strain and stress possesses a spontaneous polarization field of ferroelectric bodies on charge transport, and attract external charge on the interface. The effective modulation of the catalyst dielectric properties and the internal static electric field, improve the piezoelectric-induced catalytic performace. In this study, we synthesized a nanosized photo-piezocatalyst by incorporating titanium dioxide (TiO₂) with barium titanate (BaTiO₃) using an in situ hydrothermal method. Utilizing the spontaneous polarization of BaTiO₃ under strain to form a built-in electric field and high voltage coefficient, this promotes the separation of electron-hole pairs and enhances photocatalytic activity. As the reaction temperature increased, BaTiO₃ gradually grew onto the TiO₂ nanoparticle, promoting a phase transition in the crystal structure. At a critical temperature of 120°C, both the primary crystalline structures of BaTiO₃ and TiO₂ coexisted, revealing a heterostructure evident in microstructure observation. When compared to pristine TiO₂ and BaTiO₃, the BaTiO₃-TiO₂ photo-piezocatalyst demonstrated superior degradation activity towards methyl orange dye under both illumination and ultrasonic vibration. This resulted in a reaction rate constant of 0.01326 min^-1. It also proves that the incorporation of photocataylsyt and peizocatalyst is beneficial for the overall activities. To further explore strain-induced polarization in the photo-piezocatalyst, we employed Raman spectroscopy and noted a significant change in the typical phonon mode (E(LO + TO) + B1(309.0 cm-1)) for BaTiO₃ following compression. This suggests that lattice compression in BaTiO3 reduces the vibration of oxygen atoms and alters the dipole properties in the BaTiO₃-TiO₂ photo-piezocatalyst. To understand the enhancement mechanism of photo-assisted piezocatalysis, we used an innovative photo-induced in-situ Kelvin probe analyzer to measure the contact potential difference (CPD) of the material. The generation of electron-hole pairs through photo-assisted excitation of photosensitive materials causes a shift in the Fermi energy band, yielding a greater surface potential difference, indicating increased photosensitive properties.Tests on the optimal BaTiO₃-TiO₂ photo-piezocatalyst revealed that the change in CPD of BaTiO₃-TiO₂ exhibits a delayed behavior under continuous UV LED illumination in both short-term and long-term illumination switch tests. We infer that the hysteresis of CPD could be caused by photo-induced charge driven by TiO₂, which reduces the polarization of BaTiO₃. This phenomenon likely contributes to interfacial charge migration under photo and ultrasonic vibration, possibly extending the charge life and maintaining a sufficient surface charge concentration for radical generation. Consequently, the overall piezoelectric and photocatalytic characteristics are significantly improved. The photo-peizocatalyst is expected to be applied in cyclic wastewater treatment in the future, using the surface local stress generated by water flow to achieve the goal of degrading pollutants.