Photothermal Conversion Performance of Surfactant-Free Gold Nanoparticles Synthesised By Plasma-Induced Non-Equilibrium Electrochemistry and Compared with Surfactant-Based Samples
Authors: Moghaieb Hussein Sayed , McGlynn Ruairi James , Mariotti Davide 
Published: May 2020

Most of the wet-chemistry synthesis routes in particular rely on the use of reducing, capping and stabilising agents to control the produced nanomaterials morphology and to provide steric stabilisation for their colloids. However, they introduce several undesired consequences, such as undergoing a multi-step process with a high level of control and time consumption for purifying and recycling their by-products, as incomplete purification may eliminate further functionalization. In addition, they contribute to the morphology and optical and thermal properties of the synthesised nanomaterials resulting in inaccurate analyses such as overestimation. Furthermore, the possibility of their decomposition and degradation at operating temperatures as low as 70 °C weakens the dependence on them in practical applications. Alternatively, plasma-induced non-equilibrium electrochemistry (PiNE) with its recent improvements in nanomaterial synthesis and surface functionalization offers a great opportunity to prepare nanomaterials in a single-step, safe, cost-effective and versatile method within a relatively short processing time. In this method, no reagents are needed as the plasma’s electrons initiate the reduction process and also provide electrostatic stabilisation for the nanomaterials formed. Therefore, in this work, we synthesised surfactant-free gold nanoparticles (sf-Au NPs) and re-dispersed them in ethylene glycol (EG) at a concentration of 39.4 mg/L in order to evaluate their optical properties and photothermal conversion (PTC) performance for direct absorption solar collectors (DASCs). The synthesis process was performed by generating an atmospheric-pressure microplasma on the surface of an aqueous solution of gold (III) chloride trihydrate (HAuCl4.3H2O) dissolved in distilled water at a concentration of 0.2 mM. The operating conditions used are a 25-sccm helium gas flow rate and a constant direct current of 5 mA applied to a carbon rod immersed in the solution. At a sample volume of 10 mL, a treatment time of only 15 min was sufficient to completely reduce the gold precursor into Au NPs, which was confirmed by fitting X-ray photoelectron spectroscopy results. For further evaluation, we compared the absorption and scattering characteristics of our sf-Au NPs with two additional surfactant-based Au NPs samples at same average particle size; a PEG-coated Au NPs sample synthesised by Turkevich-Frens method in a two-step process and a commercial citrate-capped sample. The morphology analysis, using transmission electron microscopy images, showed a wide range of sizes and several shapes of sf-Au NPs, whilst monodispersed and fine spherical shapes for both surfactant-based samples. Thus, after re-dispersion in EG at the same concentration, sf-Au NPs/EG nanofluid sample exhibited a broad plasmonic absorption peak over the visible spectrum, while p-Au NPs and c-Au NPs in EG had similar narrow peaks. As a result, the analysis of theoretical and experimental PTC results, under a solar irradiation of 1000 W/m2, revealed that sf-Au NPs/EG nanofluid achieved the highest PTC efficiency and specific absorption rate. The PTC process was enhanced by more than 80%, compared to EG as a base fluid, and 50-60 % was achieved by the surfactant-based samples. These results confirm the benefits of using PiNE for the synthesis of surfactant-free nanomaterials, particularly for direct volumetric absorption in solar thermal applications.