Facile synthesis of highly luminescent lithium silicate nanocrystals with varying crystal structures and morphology
Lithium silicates have received noteworthy interest as a class of materials with significant potential in lithium ion batteries, ionic conductors, optical waveguides and sensors, and efficient sorbents for CO2 capture. Herein we report the optical properties, electronic structures, and surface characteristics of two distinct lithium silicate crystal phases using first-principles hybrid density functional theory (DFT) calculations and in-depth experimental characterization studies. Orthorhombic Li2SiO3 (space group Cmc21) and Li2Si2O5 (space group Ccc2) nanoparticles (NPs) passivated with alkylamine and alkane surface functionalities were produced by reacting SiI4 with n-butyllithium in the presence of 1,2-hexadecanediol. The as-synthesized nanostructures exhibit poor crystallinity, which upon annealing at 600 °C adopt phase-pure orthorhombic crystal structures with spherical (Li2SiO3), polyhedral or rod-shaped (Li2Si2O5) morphologies. The surface analysis of Li2SiO3 and Li2Si2O5 NPs reveals distinct chemical states for Li+, Sin+, and On−, consistent with their stoichiometry and higher binding energies for constituent elements in Li2Si2O5 NPs. Hybrid functional calculations predict indirect and direct energy gaps of 7.79 and 7.80 eV for Li2SiO3 and Li2Si2O5, respectively, signifying the insulating nature of extended solids. Nonetheless, the as-synthesized Li2SiO3 and Li2Si2O5 NPs exhibit high intensity visible photoluminescence (quantum yields = 10–30%) with nanosecond timescale decays at 15 K and 295 K, which we attribute to radiative recombination from surface/interface traps. The facile colloidal synthesis provides control over the crystal structure and composition of nanostructured lithium silicates, which will potentially widen their applications as visible to IR transparent optical materials, chromophores, waveguides, sensors, and high surface area CO2 sorbents.