Spontaneous formation of gold nanostructures in aqueous microdroplets

Abstract

The synthesis of gold nanostructures has received widespread attention owing to many important applications. We report the accelerated synthesis of gold nanoparticles (AuNPs), as well as the reducing-agent-free and template-free synthesis of gold nanoparticles and nanowires in aerosol microdroplets. At first, the AuNP synthesis are carried out by fusing two aqueous microdroplet streams containing chloroauric acid and sodium borohydride. The AuNPs (~7 nm in diameter) are produced within 60 µs at the rate of 0.24 nm µs^-1. Compared to bulk solution, microdroplets enhance the size and the growth rate of AuNPs by factors of about 2.1 and 1.2 × 10⁵, respectively. Later, we find that gold nanoparticles and nanowires (~7 nm wide and >2000 nm long) are also formed in microdroplets in the absence of any added reducing agent, template, or externally applied charge. Thus, water microdroplets not only accelerate the synthesis of AuNPs by orders of magnitude, but they also cause spontaneous formation of gold nanostructures.

Fig. 1

Kinetically controlled AuNP growth using fused microdroplets. a Schematic of the experiment setup. Microdroplets containing 100 µM HAuCl₄ solution were fused with the ones containing 400 µM NaBH₄ solution. The crystallization process was kinetically controlled on the microsecond timescale by adjusting the traveling distance of the fused microdroplets. b TEM images of gold nanoparticles at different time points. Scale bar is 20 nm. c Time-course change in the diameter of the gold nanoparticles. d TEM image of a nanoparticle showing multiple crystalline structures. e Portion of nanoparticles with single or multiple crystalline structures synthesized in microdroplets. Error bars represent one standard deviation of three replicates

Fig. 2

Reducing-agent-free synthesis of AuNPs in the microdroplets with no externally applied charges. a schematic of the experimental setup for reducing-agent-free synthesis of AuNP in the microdroplets. Scale bar is 1 µm. b, c TEM images of the synthesized AuNPs and AuNP aggregates. d SEM image of the AuNP aggregates. e TEM image showing an intermediate aggregated status of AuNP aggregates. Scale bars for b–e are 50 nm

Fig. 3

Reducing-agent-free and template-free synthesis of Au nanowires (AuNWs) in the aqueous microdroplets. a AuNWs with several micrometer length. Scale bar is 1 µm. b Formation of AuNW network. Scale bar is 100 nm. c Junction between two AuNWs. Scale bar is 20 nm. d A TEM image showing the lattice spacing of ~2.3 Å, corresponding to the Au(111) structure, confirming that the nanowire is made of pure Au. e, f AuNWs accompanying with AuNP and AuNP aggregates. Scale bars for e and f are 300 nm and 100 nm. g, h Two different types of AuNWs formed in the microdroplets with a smooth surface (g) and beaded surface (h). Scale bars for g and h are 20 nm

Fig. 4

TEM images at different time points of AuNPs and AuNWs grown in the aqueous microdroplets. Time-course changes in the individual AuNPs (a, e, i, m), AuNP aggregates (b, f, j, n), and AuNWs (c, d, g, h, k, l, o, p). Scale bars for a, b, e, f, i, j, m, n are 40 nm. Scale bars for c, g, k, o are 80 nm. Scale bars for d, h, l, p are 80 nm

Fig. 5

Kinetics of AuNPs and AuNWs growth in the reducing-agent- and template-free microdroplets. a The diameter of AuNP aggregates as a function of time in the microdroplets. b The change in the diameters of individual AuNP and AuNP aggregates over time. c The change in the thickness of AuNW over time. d The bimodal distribution of the thickness of AuNWs formed in the microdroplets. Error bar represents standard deviation of the three replicates

Fig. 6

Possible mechanism for nanowire growth in the microdroplets. a AuNW formation by the growth of an initial seed at the surface of the microdroplet. b AuNW grown by the self-assembly of AuNPs