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.