Facile synthesis of highly luminescent lithium silicate nanocrystals with varying crystal structures and morphology

Abstract

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.