Effect of solids concentration on pore structure of ZnO-foams prepared by particle-stabilized foaming route

An initially hydrophilic zinc oxide (ZnO) surface with a contact angle (θ) of ~32° has been changed to hydrophobic (θ~90°) through preferential adsorption of anionic sodium dodecul sulfate molecules on the particle surface by electrostatic interactions. Macroporous ZnO foams are prepared by anchoring the hydrophobic ZnO nanoparticles at the air/water interface to stabilize the air-in-water bubbles produced by mechanical frothing, followed then by sintering at elevated temperatures. Solids concentration (ϕ) of the ZnO suspensions is found critically important to pore structure of the sintered ZnO foams; in which, a broad pore-size distribution (~20–160 μm) results and the pore volume decreases pronouncedly from 2.41 mL g−1 to 0.85 mL g−1 when ϕ was increased from 3 vol% to 6 vol%. The pore-volume difference stems primarily from the degree of coverage of the nanoparticles on the bubble surface. This is particularly pronounced for the suspension with ϕ of 3 vol%; to which, interconnected spherical pores with a highly porous wall structure are formed because of the insufficient coverage. The process is indeed facile and is readily applicable to “one-pot” preparation of composite foams, such as ZnO foams decorated with discrete distribution of silver particles.