Tailoring Porous Templated Inorganic Oxide Films

Abstract

Mesoporous materials have been widely applied in the eld of separation and catalysts. Surfactant templating is one of the widely used methods for the preparation of mesoporous materials. Many methods based on surfactant templating, such as spin coating, dip coating and co-precipitation, have been developed to prepare mesoporous materials.

Free-standing lm synthesis method using the surfactant templating method, which has been extensively studied by our group, enables the investigation of the change in the surface structure while the lm is growing using surface sensitive techniques. Previously used silica precursors (tetramethyl orthosilicate and tetraethyl orthosilicate), produce organic species that disrupt the organisation of the micelles. Sodium silicate solution, used in this work produces water instead so is a potential precursor that could give better control of the pore size and the strength of the lm. The growth of the lm was followed using cutting-edge techniques: in situ X-Ray Reflectivity (XRR) and in situ Grazing Incidence Small Angle X-Ray Scattering (GISAXS), which suggest a possible growth mechanism.

Functionalisation of mesoporous materials is required to improve their catalytic properties. Novel surfactants (POM-2Cn, n is the carbon number of the hydrocarbon chain length, n= 12, 14, 16 and 18) were designed and were prepared through grafting double hydrocarbon chains onto a Dawson structured polyoxometalates ([P2W17O61]10 ), to use these species as both templates and a source of polyoxometalate to functionalize the mesoporous materials prepared. The self-assembly of the prepared surfactants and their mixture with commercial nonionic surfactants (C12EO6 and C12EO8) in water have been studied using Small Angle Neutron Scattering (SANS) and Small Angle X-Ray Scattering(SAXS).

The pure POM-2Cn molecules form less elongated micelles in water with increasing hydrophobic tail length, suggested by the SANS analysis. The micelles formed by POM-2Cn were used to template titanium dioxide. The prepared materials, denoted as POM-TiO2, have been proven to bear POM units well-dispersed in the porous TiO2 materials using Energy Dispersive X-Ray Analysis (EDX). The POM-TiO2 materials showed enhanced photodegradation properties for rhodamine B compared to the unfunctionalised TiO2 materials. This is attributed to the charge transfer from TiO2 conduction band to POM that prevent the recombination of the photo-excited holes and electrons during the photodegradation process.

The insertion of the nonionic surfactant into the POM-2Cn system induces a micellar growth. The comparison of SANS studies of the two binary surfactant systems (POM-2Cn/C12EO6 and POM-Cn/C12EO8) have shown that the micellar growth caused by nonionic surfactant with a smaller polyethylene oxide group is more pronounced than that caused by that with a larger hydrophilic headgroup. The POM-2Cn/C12EO8 mixtures form Liquid Crystal Phases (LCP) when the total concentration is increased to 133 mM. The type of LCP formed depends on the mixing mole ratio of the POM-2Cn and C12EO8. The POM-2Cn rich mixtures form a cubic phase, while the C12EO8 rich mixtures form a lamellar phase. The lamellar phase formed by the mixture was then used to template long-range ordered silica. Porous silica was obtained after the organic part of the template was removed, with a relatively small surface area (ca. 313 m2/g) and small pore size (1 nm- 4 nm).

This work includes several individual systematic investigations, the understanding of the growth of the silica lm using sodium silicate solution, the basic understanding of surfactant behaviour of the POM-2Cn surfactants and their mixtures with C12EOm (m = 6 and 8). Their applications in fabricating hybrid porous materials with enhanced properties were also studied. It is hoped that these results could give a good understanding of the surfactant behaviour of both traditional and novel surfactants and would contribute to the future study of the functionalisation of inorganic materials using a surfactant templating method.

Date of Award	15 Jan 2020
Original language	English
Awarding Institution	University of Bath
Supervisor	Karen Edler (Supervisor) & Asel Sartbaeva (Supervisor)