title: Evolution of microstructure in mixed niobia-hybrid silica thin films from sol-gel precursors.
abstract: The evolution of structure in sol-gel derived mixed bridged silsesquioxane-niobium alkoxide sols and drying thin films was monitored in situ by small-angle X-ray scattering. Since sol-gel condensation of metal alkoxides proceeds much faster than that of silicon alkoxides, the incorporation of d-block metal dopants into silica typically leads to formation of densely packed nano-sized metal oxide clusters that we refer as metal oxide building blocks in a silica-based matrix. SAXS was used to study the process of niobia building block formation while drying the sol as a thin film at 40-80°C. The SAXS curves of mixed niobia-hybrid silica sols were dominated by the electron density contrast between sol particles and surrounding solvent. As the solvent evaporated and the sol particles approached each other, a correlation peak emerged. Since TEM microscopy revealed the absence of mesopores, the correlation peak was caused by a heterogeneous system of electron-rich regions and electron poor regions. The regions were assigned to small clusters that are rich in niobium and which are dispersed in a matrix that mainly consisted of hybrid silica. The correlation peak was associated with the typical distances between the electron dense clusters and corresponded with distances in real space of 1-3 nm. A relationship between the prehydrolysis time of the silica precursor and the size of the niobia building blocks was observed. When 1,2-bis(triethoxysilyl)ethane was first hydrolyzed for 30 min before adding niobium penta-ethoxide, the niobia building blocks reached a radius of 0.4 nm. Simultaneous hydrolysis of the two precursors resulted in somewhat larger average building block radii of 0.5-0.6 nm. This study shows that acid-catalyzed sol-gel polymerization of mixed hybrid silica niobium alkoxides can be rationalized and optimized by monitoring the structural evolution using time-resolved SAXS.
authors.affiliation: MESA(+) Institute for Nanotechnology, University of Twente, Enschede, The Netherlands.
authors.forename: Rogier
authors.initials: R
authors.lastname: Besselink
authors.forename: Tomasz M
authors.initials: TM
authors.lastname: Stawski
authors.forename: Hessel L
authors.initials: HL
authors.lastname: Castricum
authors.forename: Johan E
authors.initials: JE
authors.lastname: ten Elshof
mesh_terms: D008567:Membranes, Artificial; D009556:Niobium / Q000737:chemistry*; D012822:Silicon Dioxide / Q000737:chemistry*; D012997:Solvents; D013499:Surface Properties
chemical_list: D008567:Membranes, Artificial; D012997:Solvents; D009556:Niobium; D012822:Silicon Dioxide