BaTiO<sub>3</sub> Nanoparticle Interfaces in Contact:\nFerroelectricity Drives Tribochemically Induced Oxygen Radical Formation

Chemical transformations\nat metal oxide interfaces that\nare triggered\nby mechanical energy set the basis for applications in the fields\nof tribo- and mechanochemistry, ceramic and composite processing,\nand piezoelectric devices. We investigated the early stages of tribochemically\ninitiated radical chemistry of structurally well-defined TiO<sub>2</sub> and BaTiO<sub>3</sub> nanoparticles in argon or in oxygen atmosphere.\nElectron paramagnetic resonance spectroscopy enabled the determination\nof the chemical nature and concentration of paramagnetic surface species\nwhich form upon uniaxial powder compaction at room temperature. Trapped\nhole centers (O<sup>–</sup>) as well as trapped or scavenged\nelectrons (Ti<sup>3+</sup> or O<sub>2</sub><sup>–</sup>, respectively)\nwere analyzed as products of mechanical surface activation. For ferroelectric\nBaTiO<sub>3</sub> nanoparticles, we found that the spontaneous polarization\neffects of the oxide lattice increase the yield of paramagnetic surface\nspecies by a factor >20 as compared to paraelectric TiO<sub>2</sub> nanoparticles. Comparison with UV excitation experiments, where\nthe energy required to drive the corresponding charge separation phenomena\nis <i>h</i>ν ≥ 3.2 eV, indicates that the paramagnetic\nspecies that originate from uniaxial powder compaction in the dark\nresult from mechanically induced surface redox processes that are\nsupported by local flexoelectric potential differences.