The Role of the Interactions at the Tungsten Disulphide Surface in the Stability and Enhanced Thermal Properties of Nanofluids with Application in Solar Thermal Energy.

Transition metal dichalcogenides (TMCs) exhibit unique properties that make them of interest for catalysis, sensing or energy storage applications. However, few studies have been performed into nanofluids based on TMCs for heat transfer applications. In this study, nanofluids based on 2D-WS2 are prepared by liquid phase exfoliation to analyze their potential usage in concentrating solar power plants. Periodic-Density Functional Theory (DFT) calculations were performed to rationalize the success of the exfoliation process. The hydrogen bond interaction between the hydroxyl group from PEG, which acts as a surfactant, and the S atoms of the WS2 surface stabilizes the nanosheets in the fluid. Electron localization function (ELF) analysis is indicative of the stability of the S-H interaction from WS2 with the molecules of surfactant due to the tendency to interact through weak intermolecular forces of van der Waals solids. Moreover, improvements in thermal properties were also found. Isobaric specific heat increased by up to 10% and thermal conductivity improved by up to 37.3%. The high stability of the nanofluids and the thermal improvements were associated with the high surface area of WS2 nanosheets. These results suggest that these nanofluids could be a promising heat transfer fluid in concentrating solar power plants.

A Solvothermal Synthesis of TiO2 Nanoparticles in a Non-Polar Medium to Prepare Highly Stable Nanofluids with Improved Thermal Properties.

Nanofluids are systems with several interesting heat transfer applications, but it can be a challenge to obtain highly stable suspensions. One way to overcome this challenge is to create the appropriate conditions to disperse the nanomaterial in the fluid. However, when the heat transfer fluid used is a non-polar organic oil, there are complications due to the low polarity of this solvent. Therefore, this study introduces a method to synthesize TiO2 nanoparticles inside a non-polar fluid typically used in heat transfer applications. Nanoparticles produced were characterized for their structural and chemical properties using techniques such as X-ray Diffraction (XRD), Raman spectroscopy, Transmission Electron Microscopy (TEM), Fourier Transform Infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The nanofluid showed a high stability, which was analyzed by means of UV-vis spectroscopy and by measuring its particle size and ζ potential. So, this nanofluid will have many possible applications. In this work, the use as heat transfer fluid was tested. In this sense, nanofluid also presented enhanced isobaric specific heat and thermal conductivity values with regard to the base fluid, which led to the heat transfer coefficient increasing by 14.4%. Thus, the nanofluid prepared could be a promising alternative to typical HTFs thanks to its improved thermal properties and high stability resulting from the synthesis procedure.

Revealing at the molecular level the role of the surfactant in the enhancement of the thermal properties of the gold nanofluid system used for concentrating solar power.

A molecular dynamics study based on gold nanofluids performed with and without the presence of tetraoctylammonium halide as a surfactant in a base fluid is presented. The base fluid consisting of a mixture of biphenyl and diphenyl oxide is used in concentrating solar power (CSP) plants. The radial distribution functions (RDFs) and spatial distribution functions (SDFs) were analysed with the temperature. Theoretical results indicate that the surfactant acts as a kind of net around the nanoparticle that plays an active role in enhancing the thermal properties of the gold nanofluid system. A greater lability of the base fluid-surfactant interactions than the base fluid-gold nanoparticle interactions is observed. At lower temperatures, there is an inner layer around the gold nanoparticle with two surfactant molecules close to the metal. At a higher temperature a ratio of gold nanoparticles : diphenyl oxide molecules of 1 : 4 is maintained in the inner layer for the systems with and without the presence of a surfactant. At the highest temperatures, the presence of the surfactant in a second shell impedes the approximation of the fifth diphenyl oxide molecule. Thus, the surfactant affects the macroscopic properties of the gold nanofluid system at the molecular level.

MoS2/Cu/TiO2 nanoparticles: synthesis, characterization and effect on photocatalytic decomposition of methylene blue in water under visible light.

Photodegradation processes are of great interest in a range of applications, one of which is the photodecomposition of pollutants. For this reason, analysing nanoparticles that improve the efficiency of these processes under solar radiation are very necessary. Thus, in this study, TiO2 was doped with Mo and Cu using low-temperature hydrolysis as the method of synthesis. Pure TiO2 and x%MoS2/Cu/TiO2 nanoparticles were prepared, where x is the theoretical quantity of MoS2 added (0.0%, 1.0%, 5.5%, 10.0%), setting the nominal quantity of Cu at 0.5 wt.%. The samples obtained were characterized by X-ray diffraction, Raman spectroscopy, X-ray electron spectroscopy and UV-Vis spectroscopy in diffuse reflectance mode. The results suggest that the TiO2 structure was doped with the Mo6+ and Cu2+ ions in the position of the Ti4+. The x%MoS2/Cu/TiO2 samples presented lower band gap energy values and greater optical absorption in the visible region than the pure TiO2 sample. Lastly, the photocatalytic activity of the samples was assessed by means of the photodegradation of methylene blue under visible light. The results show that when the quantity of Mo in the co-doped samples increased (x%MoS2/Cu/TiO2) there were significant increases of up to 93% in the photocatalytic activity.