Multivariate Synthesis of Tin Phosphide Nanoparticles: Temperature, Time, and Ligand Control of Size, Shape, and Crystal Structure

Tin phosphides make up a class of materials that have received a noteworthy amount of interest in photocatalysis, charge storage, and thermoelectric devices. Dual stable oxidation states of tin (Sn 2+ and Sn 4+ ) allow tin phosphides to exhibit different stoichiometries and crystal phases. However, the synthesis of such nanostructures with control over morphology and crystal structure has proven to be a challenging task. Herein, we report the first colloidal synthesis of size-, shape-, and phase-controlled, narrowly disperse rhombohedral Sn 4 P 3, hexagonal SnP, and trigonal Sn 3 P 4 nanoparticles (NPs) displaying tunable morphologies and size-dependent physical properties. The control over NP morphology and crystal phase was achieved by tuning the nucleation/growth temperature, Sn/P molar ratio, and incorporation of additional coordinating solvents (alkylphosphines). The absorption spectra of Sn 3 P 4 NPs (3.0 ± 0.4 to 8.6 ± 1.8 nm) exhibit size-dependent blue shifts in energy gaps (1.38–0.88 eV) compared to the theoretical value of bulk Sn 3 P 4 (0.83 eV), consistent with quantum confinement effects. The trigonal Sn 3 P 4 NPs adopt rhombohedral Sn 4 P 3 and hexagonal SnP crystal structures at 180 and 250 °C, respectively. Structural and surface analysis indicates consistent bond energies for phosphorus across different crystal phases, whereas the rhombohedral Sn 4 P 3 NPs demonstrate Sn oxidation states distinctive from those of the hexagonal and trigonal phases because of the complex chemical structure. All phases exhibit N(1s) and ν (N–H) energies suggestive of alkylamine surface functionalization and are devoid of tetragonal Sn impurities.