Localized Surface Plasmon Enhanced Passive Radiative Cooling Effects

Radiative cooling,a passive cooling technique,has shown great potential in reducing refrigeration energy consumption in recent years.Significant efforts have been devoted to realizing daytime radiative cooling and the reported cooling power kept increasing over years.However,the theoretical cooling limit of such a technology is still unclear.Therefore,we proposed a theoretical limit imposing an upper bound for the attainable cooling power,and enhanced the emissivity of polymer-based materials using the localized surface plasmon resonance（LSPR）of metal oxides to obtain radiation cooling power close to this theoretical limit.Tin-doped indium oxide（ITO）nanoparticles were synthesized by the one-step liquid phase method,and the LSPR peak’s position of the nanocrystals was controlled by reducing the doping concentration of Sn.Finally,the LSPR was found to be red-shifted to the atmospheric window（8-13μm and 16-28μm）in the vacancy intrinsic doped indium oxide（In₂O₃）.In addition to indium oxide,we have also managed to realize LSPR in the atmospheric window using aluminum-doped zinc oxide（AZO）and gallium-doped zinc oxide（GZO）.Indium oxide nanoparticles with desired infrared absorption spectrum were obtained and mixed into poly（methyl methacrylate）（PMMA）to form thin films.By extracting and fitting the measured spectral data of indium oxide and PMMA,the thickness and concentration of indium oxide needed to maximize the emissivity of the composite films were determined.The final composite film contained 4.5%volume fraction of In₂O₃nanoparticles with a thickness of28.9μm,which successfully improved the emissivity of the film in both primary and secondary atmospheric windows.The limiting cooling power of the blackbody transmitter in the whole spectrum（2-28μm）was calculated by using the atmospheric spectrum provided by ATRAN modeling software,and the empirical relationship between the calculated limiting power and the ambient temperature was fitted by the least square method.Finally,the cooling power of the thin film was measured by the thermal compensation method.The results were very close to the limit with the closest value only about 0.4 W/m² below the limit.