Vibrational studies of metal cation reduction at oxide glass surfaces

Reduction of metal oxide metaphosphate glasses (M₂O · P₂O₅, or MO · P₂O₅, where M is a metal cation) is observed above ca. 100°C when the glass is treated with one several gaseous reducing agents such as H₂, atomic hydrogen, Hg or K. Such amorphous systems are characterized by rapid cation diffusion, with cation reduction occurring principally at the glass surface, which leads to metal cluster formation and eventual continuous metal film growth. Structural changes induced in the glass by cation migration and subsequent reduction are studied by transmission infrared spectroscopy on thin-film samples during the reaction. Scanning electron microscopy and Raman spectroscopy are also employed to study and characterize the glass surfaces. The effect of water vapor on increasing the reaction rate for the mercuric metaphosphate/hydrogen reaction is demonstrated by continuous measurements of vibrational intensity changes. Surface-adsorbed water is seen to reduce the reaction induction period with little effect on the subsequent reaction rate. These results suggest that the rate-determining step of metal ion reduction at glass surface is a diffusion-limited process.