Solar-Thermal Conversion Characteristics of Surfactant-Free Gold Nanoparticles Produced By Plasma-Based Synthesis Method

Abstract

Gold nanoparticles (Au NPs), with their unique plasmonic absorption in the visible spectrum, are promising candidates for the use as nanofluids for enhancing the solar-thermal conversion (STC) process in direct-absorption solar collectors (DASCs) [1-4]. Their absorption performance mainly depends on their size and morphology [5] controlled by the synthesis technique used [4], such as the widely-used wet chemistry-based methods. These chemical methods rely on the use of surfactants for controlling the nanoparticles size and shape, as well as achieving stability within the hosting fluid. However, utilizing chemical additives results in undesired by-products, which require washing, purification and recycling processes and also they eliminate the possibility of further functionalization of the produced Au NPs [6, 7]. Besides, surfactants increase the thermal resistance between the nanoparticle and the hosting fluid [8] and could also decompose and degrade over time, at operating temperatures as low as 70 °C [9].In the present work, the optical properties and STC characteristics of three types of Au NPs produced by different synthesis methods were experimentally investigated. The first type was produced using a plasma-induced non-equilibrium electrochemistry system, to form surfactant-free nanoparticles (sf-Au NPs). The second type was synthesized using Turkevich method to form PEGylated nanoparticles (p-Au NPs), while the third type is citrate-capped nanoparticles (c-Au NPs), supplied by Sigma. After washing and drying the Au NPs powder of each type, they were separately re-dispersed in ethylene glycol (EG) as a base fluid to give Au NPs concentration of 39.4 mg/L forming nanofluids of sf-Au NPs/EG, p-Au NPs/EG and c-Au NPs/EG.XPS results of the untreated and treated gold precursors, used for the synthesis process, showed that no gold ions were found in the treated sample, indicating the full reduction of the gold precursors. In addition, TEM images showed that all samples have the same average nanoparticle size of ~32 nm. However, the size and morphology of sf-Au NPs showed a broad range of sizes and shapes, while the other two samples have nanoparticles of fine spherical shapes and narrow size distribution. Furthermore, the optical properties results for a wavelength range of 280-900 nm, using UV-VIS spectroscopy, showed sf-Au NPs/EG nanofluid exhibited a broad plasmonic absorption peak over the visible spectrum, while the other two nanofluids have similar narrow peaks. Moreover, measurements from Zetasizer Nano ZS system showed that sf-Au NPs have a zeta potential of -25.8 mv, while the p-Au NPs and c-Au NPs have slightly higher values of -35.6 mV and -38.6 mV, respectively, which could be considered as good stabilization.A lab-scale DASC was built for investigating the STC performance of each nanofluid, under a uniform radiation intensity of ∼1000 W/m2 for 25 min, during which the nanofluids temperature change was recorded. Results revealed that sf-Au NPs/EG nanofluid achieved the highest solar-thermal conversion efficiency and specific absorption rate, compared to the other two surfactant-based Au NPs/EG nanofluids. The STC efficiency was enhanced to above 80%, compared to the efficiency of the base fluid, while 50-60% enhancement was reached by the surfactant-based nanofluids, highlighting the advantages of using plasm-liquid treatment method for the synthesis of sf-Au NPs for the use as nanofluids for solar energy harvesting in DASCs. 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