Structural evolution of reduced GeOx nanoparticles.

GeOx nanoparticles (NPs) are of growing interest in lithium storage and optoelectronics. GeOx NPs prepared by chemical reduction, exposed to air or retained under N2, then annealed under H2 at various temperatures are studied herein using soft X-ray spectroscopy. We find that fresh and air-exposed GeOx NPs evolve rather differently under annealing. The fresh GeOx NPs start as a very amorphous heterogeneous mixture of GeOx and Ge, and during annealing both the valence band and conduction band edges evolve. In contrast, the air-exposed GeOx NPs initially contain quartz-phase GeO2, and during annealing only the conduction band edge evolves due to increased oxygen vacancies forming unoccupied defect states (the valence band does not change until annealing at high temperture, at which point almost all of the GeO2 is removed). These findings suggest a preparation and annealing strategy that could be used to tailor GeOx NPs for their intended use in lithium storage or optoelectronic applications.