Because of their intrinsic polarization and related properties, ferroelectrics have been considered as good and promising catalysts to address current energy and environmental issues. As photocatalysts or piezocatalysts, they can use the free and inexhaustible sunlight source or waste mechanical energies, respectively, as stimulus to produce at their surface reactive species, which in turn decompose organic molecules into harmless byproducts in wastewater. Here, we investigated the photocatalytic, piezocatalytic and combined piezo-photo-catalytic activity of BiFeO3-based nanoparticles. We show these nanoparticles are extremely efficient and very versatile photo- and/or piezo-catalysts making them very competitive when compared to other catalysts reported in the literature. We discuss the reasons of such performances considering the key parameters at play in the mechanism.

There is an increasing awareness of the necessity to protect the environment and the need to find innovative solutions to replace or remove different industrial pollutants present in wastewater, complementing the existing physical, chemical and biological techniques used for wastewater treatment. Recently, piezoelectric nanomaterials have been used as catalysts for water purification, a processdubbed piezocatalysis. Piezocatalytic reactions are generally excited by ultrasonication and occur simultaneously with other catalytic reactions caused by different mechanisms such as sonocatalysis or tribocatalysis. A cruial undertaking is to discriminate these effects against each other and quantify each contribution. In order to study the contribution of these different effects, we followed two pathways: We compared the catalytic performance of piezoelectric BaTiO3 and non-piezoelectric anatase TiO2 nanoparticles for different concentrations and temperatures to degrade the model pollutants methyl orange rhodamine B. Alternately, we also studied the temperature-dependent catalytic properties of piezoelectric BaTiO3 nanoparticles of different sizes to compare their catalytic properties below and close to the phase transition temperature, at which the crystalline structure changes from the tetragonal ferroelectric and piezoelectric phase to the cubic paraelectric and non-piezoelectric phase. Transmission electron microscopy, X-ray diffraction and Raman spectroscopy provided insights into the nano- and microstructural properties, while temperature-dependent Raman allowed the ferroelectric phase transition temperature to be estimated. The catalytic activities of the BaTiO3 and TiO2 nanoparticles were determined by analyzing the dependence on the ultrasonication time of the optical absorption of the solution containing the model pollutants and the dispersed catalytic particles. A clear relationship has been found between the catalytic reaction rates and the tetragonal distortion of the BaTiO3 crystalline structure and the related piezoelectric properties of the nanoparticles. It has been established that at temperature well below the tetragonal to cubic phase transition temperature, while 10% of the catalytic reaction of BaTiO3 is related to either sonocatalysis or tribocatalysis, 90% of the overall catalytic activity could be ascribed to the piezocatalytic contribution. The piezoelectric properties of the nanoparticles used as catalysts therefore significantly improve their catalytic performance, highlighting the potential of piezoelectric and ferroelectric nanomaterial for environmental mitigation and remediation applications.

Solar to electric energy conversion technology is making such a huge progress that it rises a concern about how to store or reconvert the energy generated during the sunny periods. One possibility is to develop materials, in which the solar energy would be used to produce hydrogen from water. It has been shown that photoexcited silicon could be used to run the hydrogen evolution reaction in water, provided that the surface is capped by a protective layer, additionally allowing the photo-excited electron carriers to travel to the device surface. Various such protective layer materials have been proposed in the past, including perovskite SrTiO3. Here we address a theoretical possibility that the protective layer would be made by an isotructural perovskite form of SrGeO3. In particular, we contribute by exploring anticipated structural and electronic properties of the SrGeO3/Si(001) interface. We acknowledge funding by the Czech Science Foundation (project no. 21-20110K) and by the Volkswagen Foundation (project “dandelion”).