Effects of halogen substitutions on the formation of copper colloids by hydrogen reduction in sodium borate glasses

The substitution of small amounts of halogens (< 1 mol%) for oxygen in sodium borate glasses with 10 to 35 mol% soda that contain 0.1 or 0.2 wt% CuO dramatically influences the ligand environment around copper in these glasses and the formation of copper colloids during hydrogen treatment near Tg. The suppression of copper colloid formation in glasses that contain halogen ions has not previously been reported.; The interaction of halogen ions and copper depends on the identity of the halogen ion, the ratio of the molar concentrations halogen to copper ions and the soda content of the glass. The heavier halogen ions, Cl, Br and I, preferentially associate with copper ions in the melt. Four bromine ions associate with each cupric ion, reducing it to the cuprous state. The reduction power of the ions increases in the order Cl- < Br- < I-. Cl, Br and I associate with cupric ions in combination with oxygen ions in the glass. These mixed anion complexes, which probably contain multiple halogen ions, give rise to intense charge transfer absorption bands in the UV-Vis spectra which cause a color change in the glass. The concentration of the mixed anion complexes is very small and depends on the ratio of halogen to copper.; The reduction of cupric ions has been separated from the growth of copper colloids during hydrogen reduction through use of the ESR. The reduction of cupric ions by hydrogen is described by the tarnishing model. The presence of Cl, Br and I completely prevents the formation of copper colloids during hydrogen reduction near Tg. CuBr nanoparticles form in glasses with ratios of Br:Cu of 1:1 or 2:1. Support for the view that cuprous ions are the diffusing species in these glasses following hydrogen reduction is presented, with a requirement that hydrogen be present at the growing nuclei to further reduce the ion to the atomic state. Bonding between mobile cuprous ions and halogen ions is suggested to slow or prevent the formation of copper colloids and the growth of CuBr nanocrystals by limiting copper diffusion to growing nuclei.