Successful Synthesis of Gold Nanoparticles through Ultrasonic Spray Pyrolysis from a Gold(III) Nitrate Precursor and Their Interaction with a High Electron Beam

Abstract

Herein, we report for the first time the successful preparation of a gold(III) nitrate [Au(NO₃)₃] water-based precursor for use in a bottom-up ultrasonic spray pyrolysis (USP) process. Due to its limited solubility in water, the precursor was prepared under reflux conditions with nitric acid (HNO₃) as the solvent and ammonium hydroxide (NH₄OH) as a neutralizer. This precursor enabled the USP synthesis of gold nanoparticles (AuNPs) and the in situ formation of low concentrations of NO₂^- and NO₃^- ions, which were caught directly in deionized water in a collection system. These ions were proven to act as stabilizers for the AuNPs. Investigations showed that the AuNPs were monodispersed and spherically shaped with a size distribution over three groups: the first contained 5.3 % AuNPs with diameters (2 r) <15 nm, the second contained 82.5 % AuNPs with 2 r between 15 and 200 nm, and the third contained 12.2 % AuNPs with 2 r>200 nm. UV/Vis spectroscopy revealed the maximum absorbance band of the AuNPs at λ=528 nm. Additionally, scanning transmission electron microscopy (STEM) observations of the smallest AuNPs (2 r<5 nm) revealed atomically resolved coalescence phenomena induced by interaction with the electron beam. Four stages of the particle-growth process were distinguished: 1) movement and rotation of the AuNPs; 2) necking mechanism; 3) orientated attachment at matching facets; 4) reshaping of the AuNPs by surface diffusion. This provided important insight into the formation/synthesis process of the AuNPs.

Keywords: characterization; gold; interactions; nanoparticles; ultrasonic spray pyrolysis.

Figure 1

Schematic representation of the synthesis of AuNPs from the Au(NO₃)₃ precursor. a) initial material and b) solubility testing of the Au(NO₃)₃ precursor: HNO₃ was added to the initial Au(NO₃)₃ powder. The mixture was heated at reflux at T≤100 °C and a yellow‐colored solution was obtained with pH 0–1. NH₄OH was added to neutralize the refluxing solution to pH 6–7. A solution of the Au(NO₃)₃ precursor was achieved with a gold concentration of 2.5 g L⁻¹. c) Experiments with USP for AuNPs synthesis: an ultrasonic generator with a frequency of 1.6 MHz created aerosol droplets from the precursor, and these droplets were carried away in quartz transport tubes with the help of N₂ as the carrier gas (1 L min⁻¹) to the evaporation zone (maintained at 150 °C) and reaction zone (maintained at 600 °C). H₂ gas (1 L min⁻¹) entered directly the reaction zone and was used as a reducing agent. The AuNPs were collected in deionized water in bottles (seen as a pink‐colored solution). A digital camera located at the end of the reaction zone was used to measure the area covered by aerosol droplets in the evaporation zone. d) Structural representation: it represents schematically the initial ionic state, as well as their changes from the precursor solution to AuNPs at different stages: precursor preparation, aerosol droplet generation, evaporation of water and shrinkage of aerosol droplet, thermal decomposition, hydrogen reduction, and collection of AuNPs in deionized water with NO₃ ⁻ and NO₂ ⁻ ions.

Figure 2

a) DLS size distribution of the AuNPs into three groups: group 1: 5.3 % AuNPs in the size range of 0 to 15 nm, group 2: 82.5 % AuNPs in the size range of 15 to 200 nm, and group 3: 12.2 % AuNPs in the size range of above 200 nm. TEM characterization of AuNPs belonging to group 2: b) TEM bright field, c) STEM annular dark field (ADF) images of the synthesized AuNPs from Au(NO₃)₃, d) EDX spectroscopy Au elemental mapping of a group of AuNPs. e) TEM image of a single AuNP and its electron diffraction pattern displaying several spots matching the lattice distances of Au and indicating a polycrystalline nature. f) STEM ADF image of a single AuNP. g) EDX spectroscopy Au elemental mapping of the single AuNP showing a hollow center.

Figure 3

AuNPs (belonging to group 1) coalescence and reshape as a result of exposure to the electron beam, as observed by STEM. a, b) STEM ADF image of a small‐sized AuNP and after beam exposure. c) AuNPs with crystal lattice planes indicated. d–i) STEM ADF images displaying the coalescence and reshaping of the AuNPs. It shows the sequence of the recorded images, each roughly after another demonstrating two particles that initiated necking but, however, did not merge into one. j–o) Atomic models of the coalescence of the AuNPs.