High Temperature Stability and Optical Measurements of Plasmonic Nanoparticles for Energy Conversion

Use of nanoparticles to convert photons into thermal energy necessitates the heating of the suspending fluid embedding the particles. High temperature media can influence the energy and bandwidth of the nanoparticle plasmon resonance thus altering the absorption capabilities of the nanoparticles as they absorb more photons. Additionally, heated nanoparticles can reshape due to unstable surface energies or they can fall out of suspension due to surfactant boiling or instability at high temperature. Here, we directly measured the optical properties of Au nanorods and indium tin oxide (ITO) nanocrystals at a range of temperatures up to 300 degrees Celsius in the heat transfer fluids Duratherm S and Dynalene MT. Measurements were done using a combination of UV-Vis and Fourier transform infrared spectroscopy (FTIR) to obtain optical absorbance from 300 to 4000 nm. Changes to optical properties are reported from ambient to heated conditions. Effects of thermal cycling were also analysed by taking nanoparticles to 300 degrees Celsius and back to ambient multiple times and comparing the optical properties for lifetime testing. Applications for direct heating of nanoparticles include solar conversion, catalysis, steam generation, photothermal storage, and biological systems including photothermal imaging and therapy.