Reduced GeO(2) Nanoparticles: Electronic Structure of a Nominal GeO(x) Complex and Its Stability under H(2) 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.