Hierarchical porous/hollow tin oxide nanostructures mediated by polypeptide: Surface modification, characterization, formation mechanism and gas-sensing properties

Successful synthesis of silica shells templated by polypeptide spheres stabilized by citric acid inspired us to explore the possibility of realizing aqueous self-assembly of metal oxides using the same template. As a specific case in the present study, we have begun with tin (IV) oxide. Initially, we failed to control the assembly process due to strong hydrolysis of Sn ⁴⁺ ions and agglomeration of tin hydrates. After modification by a proper amount of citric acid which was trapped onto the surface of the tin hydrates through the chelating bond between RCOO^- groups and Sn ⁴⁺ ions, finely dispersed and stabilized tin oxide precursors have been obtained through electrostatic repulsion or/and steric hindrance. As a result, aqueous self-assembly of surface functional SnO₂ building blocks mediated by the pre-formed polypeptide templates has been successfully achieved through interfacial columbic forces (COO^-/NH ₃⁺). Samples fabricated from systematic processing control were characterized by thermogravimetric (TG) analysis, field-emission scanning electron microscopy (SEM), x-ray powder diffraction, high-resolution transmission electron microscopy (HRTEM), infrared (IR) absorption and N ₂ gas sorption experiments. Unique coral-and sea worm-like multilevel porous/hollow frameworks consisting of SnO₂ nanocrystallites (4-5nm) were synthesized. The samples also demonstrated improved H₂ gas sensing property due to the large specific surface area (180-210m ²g^-1) as well as easy gas diffusion.