Monodisperse colloidal gallium nanoparticles: synthesis, low temperature crystallization, surface plasmon resonance and Li-ion storage

Abstract

We report a facile colloidal synthesis of gallium (Ga) nanoparticles with the mean size tunable in the range of 12-46 nm and with excellent size distribution as small as 7-8%. When stored under ambient conditions, Ga nanoparticles remain stable for months due to the formation of native and passivating Ga-oxide layer (2-3 nm). The mechanism of Ga nanoparticles formation is elucidated using nuclear magnetic resonance spectroscopy and with molecular dynamics simulations. Size-dependent crystallization and melting of Ga nanoparticles in the temperature range of 98-298 K are studied with X-ray powder diffraction, specific heat measurements, transmission electron microscopy, and X-ray absorption spectroscopy. The results point to delta (δ)-Ga polymorph as a single low-temperature phase, while phase transition is characterized by the large hysteresis and by the large undercooling of crystallization and melting points down to 140-145 and 240-250 K, respectively. We have observed size-tunable plasmon resonance in the ultraviolet and visible spectral regions. We also report stable operation of Ga nanoparticles as anode material for Li-ion batteries with storage capacities of 600 mAh g(-1), 50% higher than those achieved for bulk Ga under identical testing conditions.

Figure 1

An outline of the synthesis of Ga NPs via thermal decomposition of Ga-alkylamides. Dioctylamine acts as a surfactant and also engages into partial or complete transamination of Ga₂(NMe₂)₆, thus controlling the reaction kinetics. (A,B) Low- and high-resolution TEM images of 24.0 nm Ga NPs with narrow size distribution of 7.4%.

Figure 2

(A,B) Electron diffraction patterns of ∼32 nm Ga NPs at room temperature and at 103 K. (C) Radial integrals of (A) and (B) indicating the appearance of diffraction peaks upon cooling. (D) High-resolution TEM image of a single Ga NP crystalline at 103 K, coated with a native oxide layer. (E) HAADF-STEM, (F) EDX pseudocolor map of Ga NPs, revealing an O-rich thin shell around a Ga core (Ga, green; O, purple).

Figure 3

Wide-angle powder X-ray diffraction pattern (Cu Kα1 irradiation) for 24 nm Ga NPs at 113 K presented together with theoretical pattern for δ-Ga and a fit obtained by Rietveld refinement. Inset illustrates a crystal structure of δ-Ga polymorph.

Figure 4

Temperature-dependent XRD (Mo Kα) patterns for Ga NPs. (A) Rough scan over the temperature range of 103–293 K for 46 nm Ga NPs. (B,C) More detailed study near the temperatures of phase transitions for 24 and 46 nm Ga NPs.

Figure 5

Temperature-dependent specific heat measurements for 24 nm Ga NPs.

Figure 6

Ga–Ga distances for two shells, extracted from the best fits of EXAFS spectra at different temperatures. CN is coordination number.

Figure 7

UV–vis absorption spectra of Ga NPs dispersed in hexane. Inset shows SPR values of all measured samples.

Figure 8

Electrochemical performance of ∼20 nm Ga NPs and of bulk Ga as anode materials for Li-ion batteries. Two-electrode half-cells with metallic Li as counter electrode were assembled. (A) Galvanostatice charge/discharge curves; (B) cycling stability tests; (C) rate-capability tests (0.5–20C rates, where 1C is a current density of 769 mA g^–1 based on the theoretical capacity of pure Ga). All batteries were cycled in the voltage window of 0.02–1.5 V.