Morphology-controlled synthesis of Cu2O encapsulated phase change materials: Photothermal conversion and storage performance in visible light regime

Microencapsulated phase change materials (MPCMs) are usually limited in photothermal conversion due to their poor visible light absorbability and low thermal conductivity. Owing to a direct band gap of 2.0-2.2 eV, the semiconductor cuprous oxide (Cu2O) has attracted intense interest in solar energy harvest. Shape-dependent optical properties of Cu2O semiconductors are mainly focused on crystals enclosed by three low-index facets ({1 0 0}, {1 1 0} and {1 1 1}). Here, we successfully design and fabricate the Cu2O encapsulated MPCMs from cube, truncated cube, 26-hedron, and truncated octahedron to octahedron under precise control of NaOH. A possible growth mechanism to explore the correlation between selective adsorption of OH- on Cu2O facets and MPCMs shape evolution is suggested based on density functional theory calculations. The thermal analysis shows that the octahedral MPCMs enclosed by {1 1 1} facets possess latent heat of 148.9 J/g and photothermal conversion efficiency of 82.65 % under irradiation of visible light. Differential scanning calorimeter (DSC) profiles of the MPCMs maintain good coincidence with only a slight fluctuation of phase transition temperatures and the associated enthalpies during the 200-cyclic scans, demonstrating excellent phase change reversibility and thermal durability. Our studies unambiguously provide a strategy for tailoring the optical properties of MPCMs to greatly harvest solar energy for green building materials, anti-ice coating etc. in the future.