Efficient Absorption of Sunlight Using Resonant Nanoparticles for Solar Heat Applications

Summary Black color is a typical choice to absorb sunlight. To realize black color, paints or coatings of organic or inorganic materials have been used. In contrast, here we show with analytical calculations as well as experiments that resonant nanoparticles can act as efficient sunlight absorbers even though their bulk materials are not black. The important role of nanoparticles is to excite either plasmon or Mie resonances, which are excited at metallic and dielectric materials, respectively. Among the materials we have considered, highly conductive titanium nitride nanoparticles exhibit strong and broad absorption which covers the majority of the solar spectrum. For Mie resonances, germanium nanoparticles exhibit strong but rather narrow absorption peaks. However, if these nanoparticles have size variations, they can still absorb broad spectrum as a whole. In experiment, we examine the photothermal effect of these nanoparticles dispersed in water and demonstrate that they are efficient in heating and vaporizing water under the illumination of sunlight. The nanoparticle concentration can be as small as 0.1 vol% to improve the water heating and vaporization by a factor of few. These resonant nanoparticles can be applied in solar water heating systems as well as in solar water distillation systems for the efficient usage of solar heat for a sustainable society.