Photothermal Capture and Shell-Controlled Release Kinetics in Au@TiO₂ Core–Shell Nanorod / Phase-Change Composites

In nanorod-modified phase-change media, optical energy capture and controlled heat release can be built into a single structure. We report laboratory measurements on composites in which plasmonic gold nanorod cores absorb near-infrared radiation, conformal TiO₂ shells of 2–12 nm thickness modify heat flow between rod and matrix, and an n-eicosane matrix (melting point ≈ 37 °C) stores and releases latent heat. Aspect-ratio control tunes the longitudinal plasmon resonance linearly from 590 to 1065 nm (108 nm per unit aspect ratio). At 808 nm and 1.5 vol % loading, the measured capture efficiency is 78 ± 3 %, and the steady-state temperature rise is linear in irradiance at 31.0 ± 0.8 K per W cm⁻². The shell lowers the onset melting temperature by up to 3.6 K (12 nm shell). It also changes the release kinetics: the isothermal Avrami exponent falls from 2.6 (neat) to 1.35 (12 nm shell), and the time to 90 % release at 3 K undercooling increases 3.8-fold between 2 and 12 nm shells. Isoconversional analysis of DSC melting peaks gives apparent activation energies of 142 kJ mol⁻¹ (neat) and 186 kJ mol⁻¹ (12 nm shell). We treat these as empirical kinetic signatures, not as energy barriers, because melting is not normally a thermally activated process. Over 800 laboratory charge–release cycles, capture efficiency decreased by 1.2 % and the release time increased by 6 %. All values are laboratory measurements. The laboratory test programme began on 4 April 2025 at laboratories in Tokyo and Incheon; first results were obtained on 13 February 2026.