Reduced GeO2 Nanoparticles: Electronic Structure of a Nominal GeOx Complex and Its Stability under H2 Annealing

A nominal GeO x (x ≤ 2) compound contains mixtures of Ge, Ge suboxides and GeO 2 , but the detailed composition and crystallinity could vary from material to material. In this study, we synthesize GeO x nanoparticles by chemical reduction of GeO 2 and comparatively investigate the freshly prepared sample and the sample exposed to ambient conditions. Although both compounds are nominally GeO x , they exhibit different X-ray diffraction patterns. X-ray absorption fine structure (XAFS) is utilized to analyse the detailed structure of GeO x . We find that the two initial GeO x compounds have entirely different compositions: the fresh GeO x contains large amorphous Ge clusters connected by GeO x , while after air exposure; the Ge clusters are replaced by a GeO 2 -GeO x composite. In addition, the two GeO x products undergo different structural rearrangement under H 2 annealing, producing different intermediate phases before ultimately turning into metallic Ge. In the fresh GeO x , the amorphous Ge remains stable, with the GeO x being gradually reduced to Ge, leading to a final structure of crystalline Ge grains connected by GeO x . The air-exposed GeO x on the other hand, undergoes a GeO 2 →GeO x →Ge transition, in which H 2 induces the creation of oxygen vacancies at intermediate stage. A complete removal of oxides occurs at high temperature.