Reduction Mechanisms of Cu2+-Doped Na2O–Al2O3–SiO2 Glasses during Heating in H2 Gas

Controlling valence state of metal ions that are doped in materials has been widely applied for turning optical properties. Even though hydrogen has been proven effective to reduce metal ions because of its strong reducing capability, few comprehensive studies focus on practical applications because of the low diffusion rate of hydrogen in solids and the limited reaction near sample surfaces. Here, we investigated the reactions of hydrogen with Cu2+-doped Na2O-Al2O3-SiO2 glass and found that a completely different reduction from results reported so far occurs, which is dominated by the Al/Na concentration ratio. For Al/Na < 1, Cu2+ ions were reduced via hydrogen to metallic Cu, distributing in glass body. For Al/Na > 1, on the other hand, the reduction of Cu2+ ions occurred simultaneously with the formation of OH bonds, whereas the reduced Cu metal moved outward and formed a metallic film on glass surface. The NMR and Fourier transform infrared results indicated that the Cu2+ ions were surrounded by Al3+ ions that formed AlO4, distorted AlO4, and AlO5 units. The diffused H2 gas reacted with the Al-O-···Cu+ units, forming Al-OH and metallic Cu, the latter of which moved freely toward glass surface and in return enhanced H2 diffusion.