Transformation of Li4P4O12 rings into LiPO3 chains by CoO or CuO doping: Crystallization-induced reduction of photoluminescent Cu+ to plasmonic Cu° glass-ceramics

Abstract

Lithium-metaphosphate is an imperative matrix for the evolution of Li4P4O12 rings and LiPO3 chains, as well as worthy host matrix for holding photoluminescent Cu+, plasmonic Cu° nanoparticles and Co2+ color center. This has been verified by studying the mutual influence between the crystallization process and transition metal doping in 50Li2O·50P2O5, 1CoO·49Li2O·50P2O5 and 1CuO·49Li2O·50P2O5 glasses. Samples were subjected to heat treatment, and the crystallization and molecular arrangements were examined by XRD, TEM, HRTEM, SAED and FTIR. In 50Li2O·50P2O5 glass, Li4P4O12 crystallizes with increasing the time of heat-treatment, whereas the concentrations of Q1 and Q2 increase. These rings are transformed into LiPO3 in 1CoO·49Li2O·50P2O5 with upturn and downturn points in concentrations of Q2 and Q1 at 3 and 4 h, respectively. In 1CuO·49Li2O·50P2O5 glass, LiPO3 is precipitated after 2 h, and the concentration of Q2 increases whereas that of Q1 exhibits a quite small decrease with the time of heat-treatment. Furthermore, 1CuO·49Li2O·50P2O5 exhibits a higher crystallization rate than 1CoO·49Li2O·50P2O5, which is attributed to a higher rate of cluster formation, as revealed by SEM and TEM micrographs. This enhancement is attributed to the coordination of Co ions by 4 and/or 6 oxygen atoms, while Cu ions have a redox state of 0 or +1, as deduced from UV–Vis spectroscopy. Also, the existed intrinsic defects in 50Li2O·50P2O5 are reduced by the addition of CoO. In contrast, photoluminescent Cu+ ions contribute to such defects, and are reduced into Cu° during the crystallization process.