Infrared analysis of catalytic CO₂ reduction in hydrogenated germanium

The oxidation and carbisation kinetics of porous amorphous and nano-crystalline hydrogenated germanium (a-Ge:H and nc-Ge:H) films exposed to ambient air and deionized water have been studied using vibration modes observed by infrared spectroscopy. Based on infrared analysis, a two-step process of first oxidation by water and secondly carbisation by carbon dioxide (CO₂) is proposed that partly mimics the (photo-)catalytic processes in artificial (photo)synthesis. It is shown that water acts like the precursor for oxidation of porous a-Ge:H and nc-Ge:H in the first step. The incorporation of oxygen in a-Ge:H and nc-Ge:H alloys occurs preferentially at Ge-dangling bonds and not at the Ge-Ge back bonds like in hydrogenated silicon alloys (next of kin IV-valence element). The formation of germanium oxide (GeO) tissue at void surfaces locally creates Ge alloys with significantly lower energy levels for the valence band that can align with the half reaction for water reduction. The heterogeneous nature of a-Ge:H and nc-Ge:H oxidation will result in local catalytic generation of electrons and protons. It is proposed that these charge carriers and ions act as precursors for the second-step reaction based on carbisation that includes both the adsorption of CO₂ and formation of CO and formaldehyde.