Electrochemically controlled deposition of nanomaterials

Abstract

The modification of ammonium-based surfactants used for preparing mesoporous silica films by the electrochemically assisted surfactant assembly (EASA) were investigated by varying either the surfactant chain length (as described in Chapter 3) or by increasing the size of the surfactant head group (as described in Chapter 4). The silica films are formed by using an applied potential to organise the cationic surfactants and the resultant current drives silica condensation in the sol electrolyte. They contain a hexagonal array of pores in vertical orientation, i.e. perpendicular to the substrate surface.
Grazing incidence small-angle X-ray scattering revealed that extending the alkyl chain length from C14 to C24 resulted in the pore diameter measured by ellipsometric porosimetry shifting from 2.82 nm to 4.40 nm. Increasing the surfactant chain length to from C22 to C24 saw a collapse in hexagonal ordering, however, vertical pore orientation remained. Following surfactant removal, this work showed that mass transport was faster in the films obtained using larger surfactants.
Mesoporous silicas were prepared from sols containing surfactants with increasing head sizes, specifically using [C18H37NMe3-xEtx]Br, [C20H41NMe3-xEtx]Br and [C22H45NMe3-xEtx]Br surfactants. Grazing incidence small-angle X-ray scattering data showed a significant drop in peak intensity, indicating the effect of the larger surfactants on the film’s porosity. Electron microscopy revealed uniform and crack free films along with a gradual reduction in hexagonal pore structure as one, two or three methyl substituents were replaced with less hydrophilic ethyl groups in the head group of the surfactant. The CVs showed no faradaic current for films templated by [C18H37NMe3-1Et2]Br and [C18H37NMe3-0Et3]Br. The charge transfer resistance was influenced by the ordering of pores as the film formed using [C18H37NMe3-2Et1]Br showed an increase in charge transfer resistance (830.2 Ω) relative to those formed using the [C18H37NMe3-3Et0]Br surfactant (416.6 Ω). Ellipsometric porosimetry showed that the pore diameter of the silica films increased with increasing head group sizes.
The effect of a surfactant on electrodeposition of nickel was also examined. Electroplating from NiCl2.6H20 in an acidified aqueous electrolyte containing [C16H33NMe3-3Et0]Br and boric acid is described in Chapter 5. EDX and SEM analysis indicated the presence of elemental nickel and existence of pores within the CTAB-templated films. The electrochemical surface area was larger for the CTAB-templated nickel (6.0 × 10-4 cm2) than that of non-templated nickel (3.9 × 10-4 cm2). The specific capacitance determined from CV studies of CTAB-templated nickel (1565 F g-1) was also higher than non-templated nickel (822 F g-1) at 5 mV s-1.

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Identifiers

ORCID for Andrew Hector: orcid.org/0000-0002-9964-2163

ORCID for Gill Reid: orcid.org/0000-0001-5349-3468

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