Designing a Novel Photothermal Material of Hierarchical Microstructured Copper Phosphate for Solar Evaporation Enhancement

Hierarchical microstructured copper phosphate (HCuPO), which could accelerate water evaporation was well designed based on d–d transition of 3d electrons in Cu²⁺ and fabricated via a solvothermal method. A very strong vis–NIR absorption with the maximum at 808 nm was observed for the HCuPO. Upon irradiation of 808 nm NIR laser light, the HCuPO generated heat with a light-to-heat converting efficiency of 41.8%. The reason for this high efficiency was investigated and assigned to a high probability of nonradiative relaxation, which released the energy in form of heat, happened to the excited 3d electrons of Cu²⁺. The proposed photothermal mechanism was quite different from the surface–plasmon mechanism of other Cu-based photothermal materials. By adding HCuPO into polydimethylsiloxane (PDMS), HCuPO–PDMS composite sheets were fabricated. Due to the intrinsic hydrophobicity of PDMS matrix, the sheets were floatable on water surface and the heat generated by HCuPO was confined within water–air interface region. A much sharper temperature gradient and more rapid increase of surface temperature were observed compared with the HCuPO–water dispersion in which the HCuPO particles were dispersed in water. Porous HCuPO–PDMS sheets were fabricated in order to further accelerate water evaporation. Under 808 nm laser irradiation with power density of 1000–2000 W·m^–2, water evaporation rate of salt water (3.5 wt %) was measured to be 1.13–1.85 kg·m^–2·h^–1 for porous floating HCuPO–PDMS, which was 2.2–3.6 times of that measured for ordinary salt water without HCuPO. By using a solar simulator as a light source, a very high solar thermal conversion efficiency of 63.6% was obtained with a power density of 1000 W·m^–2, indicating that solar evaporation of salt water could be greatly enhanced by the well-designed HCuPO.