Preparation of Low-Density Macrocellular Tin Dioxide Foam with Variable Window Size

Low-density porous tin dioxide (∼0.5 g/cm 3 ) was prepared from a SnCl 4 /ethanol/H 2 O mixture and polystyrene (PS) template. Such a low density made the as-synthesized tin dioxide act as a promising target material for laser-induced extreme ultraviolet (EUV) emission. The resulting structure of the as-synthesized tin dioxide (SnO 2 ) was a cellular foam, which was composed of large cells (ca. 10 3 nm, large macropores) interconnected by windows (ca. 10 2 nm, small macropores). Scanning electron microscope images showed that SnO 2 particles of ∼10 nm constituted the cell wall. The space among particles formed mesopores, which were about 7 nm estimated by nitrogen adsorption−desorption isotherm measurements . Therefore, the as-prepared SnO 2 showed a hierarchical porous system from macropores to mesopores. The window size on a submicrometer scale was tunable in the range of ∼480−200 nm by changing the molar ratio of ethanol to SnCl 4 in tin source solution from 2:1 to 10:1. By consideration that the windows originated from the tight contact area among the PS spheres, some parameters affecting the contacting area among the PS spheres were investigated in order to analyze the effect of ethanol content on the widow size. The wettability of the tin source solution on the PS particle film and the viscosity of the tin source solution were found to be the main reasons responsible for the variable window size. Another observed phenomenon was the change in the SnO 2 filling area from the complete filling of voids to a mere coating of the colloidal spheres as the ethanol content was increased, that is, a template transition from a volume template to a surface template was realized by changing the ethanol content. The mechanism was discussed in detail.