Melting point of dried gold nanoparticles prepared with ultrasonic spray pyrolysis and lyophilisation

Abstract

A coupled process of ultrasonic spray pyrolysis and lyophilisation was used for the synthesis of dried gold nanoparticles. Two methods were applied for determining their melting temperature: uniaxial microcompression and differential scanning calorimetry (DSC) analysis. Uniaxial microcompression resulted in sintering of the dried gold nanoparticles at room temperature with an activation energy of 26–32.5 J/g, which made it impossible to evaluate their melting point. Using DSC, the melting point of the dried gold nanoparticles was measured to be around 1064.3°C, which is close to pure gold. The reason for the absence of a melting point depression in dried gold nanoparticles was their exothermic sintering between 712 and 908.1°C.

Optical Characteristics of Directly Deposited Gold Nanoparticle Films

“…The chosen parameters (gold concentration in the precursor solution, heating temperatures, gas flows) for the given USP system provided adequate control over the AuNP deposition on the glass substrate. It was determined from previous experimental studies [30] that a gold concentration in the precursor solution of 0.5 g/L Au produces relatively small AuNPs, which is beneficial for an evaluation of their LSPR occurrence, as this optical property is more evident in small nanoparticles. Using higher gold concentrations in the precursor solution, or higher gas and material flows, produces larger AuNPs, or a greater number of nanoparticles, which would cover the glass substrate faster, losing some control over the deposition rates in the conducted experiments.…”

“…The precursor used was Hydrogen tetrachloroaurate (III) trihydrate (HAuCl 4 •3H 2 O, molecular weight: 393.83 g/mol, Glentham Life Sciences, UK), dissolved in deionised water at a concentration of 0.5 g/L Au (1.0 g/L HAuCl 4 •3H 2 O; Au with a molar weight of 196.967 g/mol represents 50% of the weight of Hydrogen tetrachloroaurate (III) trihydrate). The USP process was carried out using a proprietary device from Zlatarna Celje d.o.o., Slovenia [30], consisting of a custom built tube furnace, an ultrasonic generator, a precursor supply system, and a gas washing system for nanoparticle collection. The ultrasonic generator is constructed with a 1.6 MHz piezoceramic membrane for generating the ultrasound (Liquifog II, Johnson Matthey Piezo Products GmbH, Germany), which is submerged in a chamber for the aerosolization of the precursor solution.…”

“…Steric stabilisers are added to nanoparticles ′ suspensions to stabilise the solution and prevent agglomeration and aggregation when the repulsive forces are not sufficient [5,26,27]. Stabilisers can also prevent agglomeration and aggregation of nanoparticles, while removing the dispersion media slowly [28][29][30]. Without the presence of stabilisers, the nanoparticles agglomerate and aggregate during the removal of the dispersion media, which changes the colour of the gold nanoparticles directly.…”

“…A similar phenomenon is expected when AuNPs are layered on each other with direct deposition within the reaction chamber of an ultrasound spray pyrolysis (USP) device. No sintering is expected to occur, since the temperatures inside the USP device in the deposition zone are not near the sintering temperature of the AuNPs [30]. No direct deposition of AuNPs from an USP process has been observed in the available literature, as this process is used mainly for the production of discrete nanoparticles.…”

Optical Characteristics of Directly Deposited Gold Nanoparticle Films

“…Steric stabilisers are added to nanoparticles` suspensions to stabilise the solution and prevent agglomeration and aggregation when repulsive forces are not sufficient [5,19,20]. Stabilisers can also prevent agglomeration and aggregation of nanoparticles, while removing the dispersion media slowly [21][22][23]. Without the presence of stabilisers the nanoparticles agglomerate and aggregate during the removal of the dispersion media, which changes the colour of the gold nanoparticles directly.…”