Critical assessment of the exsolution process in Cu-doped SrTiO3 by a combined spectroscopic approach

The surface transformation and defect evolution of Cu-doped SrTiO3 upon copper exsolution have been studied by exploiting a multi-technique approach which integrates, for the first time, common methods describing exsolution like XAS, XPS and STEM with unconventional strategies, namely electron paramagnetic resonance (EPR) and UV-Vis diffuse reflectance (UV-DRS). XAS and EPR indicated that copper is present in the matrix in a disordered coordination environment as amorphous Cu2O and CuO located at the surface and as substitutional Cu2+ lattice species with a distorted octahedral structure. Interestingly, EPR unveiled that, during exsolution, Cu2+ surface sites with disordered coordination primarily migrate undergoing selective reduction, while a delay is observed for the lattice defects. UV-DRS resulted in a valid alternative to HRTEM to determine the size of exsolved nanoparticles by tracking the plasmon resonance effect. Moreover, when XANES showed the complete regain of the pristine state of Cu after reoxidation, both UV-DRS and EPR highlighted that the original features are not entirely restored. These outcomes suggest that the chemical environment of exsolvable species is much more heterogeneous and the exsolution process much less straightforward than expected. Thus, alternative and original characterization techniques should be exploited to provide a solid methodological benchmark for an effective evaluation of this phenomenon.