Steve J. Roser

Growth and characterization of mesoporous silica films

The synthesis of surfactant-templated silicate materials has developed rapidly over the past decade. The uniform controlled pore sizes created in the amorphous silicate framework by this method show promise as catalyst supports, sensors, filtration membranes and in a variety of optoelectronic applications. Formation of these materials in a thin-film or membrane geometry is therefore an active area of research. This review covers the methods currently used to produce thin-film silicate?surfactant composites, with an emphasis on the mechanism of mesostructure formation and the types of composite structure formed in each case. Solvent evaporation methods such as dip coating, spin coating and casting are treated first, followed by methods involving the spontaneous growth of the surfactant?silicate composite as either a self-supporting film or on a substrate such as mica, graphite or silicon.

Structural studies on surfactant-templated silica films grown at the air/water interface

In situ characterisation of a growing surfactant-templated silica film has been carried out by X-ray reflectivity, diffuse X-ray scattering from the surface, Brewster angle microscopy and surface pressure measurements. The results indicate an unexpected film growth mechanism where layered structures form in solution and diffuse to the interface after some critical induction period.

Formation of CTAB-templated mesophase silicate films from acidic solutions

We have examined the effect of the different components in the spontaneous formation of surfactant-templated mesostructured silicate films grown at the air/solution interface. The rate of film formation shows differing dependencies on the concentrations of the silica precursor, tetramethoxysilane, the surfactant template, cetyltrimethylammonium bromide (CTAB), water and the solution pH. Further, we relate the nature of the film formation to the properties of the dried film using small angle scattering methods. From this data a general formation mechanism for the interaction between silica and surfactant micelles in this system is proposed. The polymerising silica can be considered as a cationic polyelectrolyte which interacts with the positively charged CTAB micelles through the bromine anion of the surfactant and via adsorption of the micelles as the polymer becomes more hydrophobic. This polyelectrolyte–surfactant system undergoes a liquid–liquid phase separation (coacervation) at a critical point dependant on the charge and molecular weight of the polymer and the charge on the surfactant micelle.

NEUTRON REFLECTIVITY DETERMINATION OF BURIED ELECTROACTIVE INTERFACE STRUCTURE : PBT/PPY AND PBT/PXV BILAYERS

Structure−property correlations of two electroactive polymer bilayers fabricated by different methods are investigated using neutron reflectivity to probe the buried polymer/polymer interfaces. Both bilayers utilize electropolymerized poly(2,2‘-bithiophene) as an inner, mediating layer, while the outer layer consists of either electropolymerized polypyrrole or electroprecipitated polyxylylviologen, respectively. These devices have very different current−potential behaviors, which neutron reflectivity shows is a consequence of the fabrication method and the resulting polymer/polymer interfacial structure. The polybithiophene/polypyrrole film is found to be of composite structure while polybithiophene/polyxylylviologen forms a film comprised of segregated polymer layers.

Multiple thin film formation from dilute mixtures of polyethyleneimine (PEI) and cetyltrimethylammonium bromide (CTAB)

Dilute mixtures of the water soluble polymer polyethyleneimine (PEI) and the cationic surfactant cetyltrimethylammonium bromide (CTAB) form mesostructured thin films at the air/solution interface. In this paper we show that these films form spontaneously, reaching an equilibrium composition. When the film is removed, a new solid film rapidly reforms, with a similar morphology when inspected by Brewster angle microscopy (BAM). Successive removal of films lead to a series of structurally similar films, until the amount of surfactant in solution approaches the lower limit of film forming concentration. The results obtained using surfactant-selective potentiometry suggest that the incipient polymer:surfactant aggregates are in a position to migrate to the surface rapidly after film removal, prior to mesostructure formation. X-ray reflectivity indicates that films formed at different PEI:CTAB compositions retain mesophase regular structures with the same d-spacing feature, equal to 52.2A. Grazing incidence diffraction measurements indicate that films are composed of small crystallites arranged in a random powder fashion, developing a rough surface morphology evidenced by BAM. The central finding is that PEI:CTAB films form when the amount of CTAB in solution is higher than a critical film formation concentration, very close to the critical aggregation concentration (CAC), allowing the formation of several equilibrated films from the same growing solution.

In situ Brewster angle microscopy and surface pressure studies on the interfacial growth of mesostructured silica thin films

Growth of a mesostructured silica thin film at the air/water interface was observed in situ using Brewster angle microscopy and surface pressure measurements allowing real time observation of nucleation of the film and its rapid growth to full surface coverage at the end of the induction period.

The effect of hydrogen on the morphology of n-type silicon electrodes under electrochemical conditions

We study the electrochemical roughening of a silicon electrode surface during the hydrogen evolution reaction in a fluoride electrolyte using neutron reflection. We demonstrate that as the roughening process modifies the morphology of the silicon surface we can follow the changes by observing the changes in the shape of the total reflection feature. We assume that the change in the morphology of the surface is due to the diffusion of hydrogen in the silicon electrode. This assumption allow us to model the changes in the reflected intensity at two different angles and find the diffusion exponent for the diffusion of hydrogen in the silicon lattice.

A tale of two mechanisms: comparing mesostructure formation in cationic and non-ionic surfactant-templated silicas

Abstract The formation of surfactant templated silica films at the air-solution interface has been investigated in situ using a number of time-resolved surface-sensitive and bulk sensitive techniques. Results from templating solutions prepared using a non-ionic surfactant (C16EO8) are compared to those obtained when a cationic surfactant (CTAB) is used. In both cases specific solution phase interactions drive mesostructure organisation, in the precipitates formed in the subphase and in the thin films at the solution surface, however the templating interaction between silica and surfactant appears to occur differently in each case.

Formation of mesophase surfactant-templated silica thin films from acidic solutions

Abstract The formation of mesophase silica-surfactant thin films at the air/solution interface has been studied in situ using off-specular X-ray reflectivity, Brewster angle microscopy and small angle scattering. Results for cetyltrimethylammonium bromide-templated films suggest that the formation mechanism is strongly dependent on the silica:surfactant ratio, and this is confirmed by studies on the subphase solutions using time-resolved small angle X-ray and neutron scattering. Results of similar investigations for films templated with Pluronic® P123 triblock copolymer surfactant also show a strong dependence of mesostructure development on silica:surfactant ratio. A general formation mechanism for mesophase growth and thin film development is proposed.