Abstract
Chemical transformations at metal oxide interfaces that are triggered by mechanical energy set the basis for applications in the fields of tribo- and mechanochemistry, ceramic and composite processing, and piezoelectric devices. We investigated the early stages of tribochemically initiated radical chemistry of structurally well-defined TiO(2) and BaTiO(3) nanoparticles in argon or in oxygen atmosphere. Electron paramagnetic resonance spectroscopy enabled the determination of the chemical nature and concentration of paramagnetic surface species which form upon uniaxial powder compaction at room temperature. Trapped hole centers (O(-)) as well as trapped or scavenged electrons (Ti(3+) or O(2)(-), respectively) were analyzed as products of mechanical surface activation. For ferroelectric BaTiO(3) nanoparticles, we found that the spontaneous polarization effects of the oxide lattice increase the yield of paramagnetic surface species by a factor >20 as compared to paraelectric TiO(2) nanoparticles. Comparison with UV excitation experiments, where the energy required to drive the corresponding charge separation phenomena is hν ≥ 3.2 eV, indicates that the paramagnetic species that originate from uniaxial powder compaction in the dark result from mechanically induced surface redox processes that are supported by local flexoelectric potential differences.