Macroporous honeycomb films of surfactant-encapsulated polyoxometalates at air/water interface and their electrochemical properties

A series of surfactant-encapsulated polyoxometalates which have different compositions, shapes, and sizes, are able to self-assemble to the highly ordered honeycomb-structured macroporous films at the air/water interface without any extra moist airflow across the solution surface. The honeycomb film pores in the average diameter of 2-3 μm are obtained, which are independent of the polyoxometalates. It is speculated that the cooled micrometer water droplets act as the necessary templates for the formation of macropores, and the stability of these water droplets is crucial during the self-assembly. With increasing the concentration of surfactants, various morphologies from lowly ordered honeycomb films to highly ordered honeycomb films and then to disordered fragments can be modulated. The interfacial tension between chloroform solution and water droplets induces the changes of films. High-resolution TEM observations indicate a close-packed lamellar structure in the ordered honeycomb film walls. The self-assembly successfully performs the transfer of functional polyoxometalates from bulk solutions to interfacial films. Consequently, the produced honeycomb films present electronic activities, such as ferromagnetism and electrochemical properties. These detailed researches will enrich the studies based on materials obtained by encapsulations in cationic surfactants to construct newly nanostructures of polyoxometalates at interfaces, and promote the potential applications of the honeycomb films of surfactant-encapsulated polyoxometalates in advanced materials.