The oscillatory adsorption of self‐assembled organosilane films on aluminium oxide surfaces

Abstract

The time‐dependent oscillatory growth mechanism of organosilane film self‐assembly on aluminium oxide has been investigated using X‐ray photoelectron spectroscopy. While this unusual oscillatory process has been reported for the trifunctional silane, propyltrimethoxysilane, we report here, for the first time, that this oscillatory behaviour is also present during the self‐assembly of the difunctional silane propylmethyldimethoxysilane. The presence of multiple oscillations in this growth mechanism is also first reported for propyltrimethoxysilane and propylmethyldimethoxysilane as a function of exposure time. Multiple oscillations indicate that the 3‐component model that is used to describe and fit a single coverage oscillation must be reconsidered and contain additional components to account for the multiple oscillations seen experimentally. The absence of such oscillatory behaviour in the growth of the monofunctional organosilane propyldimethylmethoxysilane, which in fact follows a Langmuir‐type growth mechanism, indicates that this measurable oscillatory behaviour is because of the ability of multifunctional silanes to oligomerise both on the substrate and in solution.

The Role of Physisorption and Chemisorption in the Oscillatory Adsorption of Organosilanes on Aluminium Oxide

“…If the original θ 1 and final θ 2 were not a result of a conversion of the same molecule from hydrogen bound to covalently bound species and rather the result of a displacement of θ 1 by another PDMMS molecule creating a new θ 1 which was then effectively converted to θ 2 , the same logic could be applied. As the Langmuir type adsorption profile of PDMMS has been reported previously [8], it was directly compared to the PDMMS coverage for rinsed films and is shown in Figure 3.…”

“…The conversion of θ 1 to θ 2 as described in the kinetic model is supported by the thermodynamically favourable mechanism of covalent adsorbate-substrate bond formation, which occurs via an intermediate hydrogen bond. The potential energy diagram originally described by Prutton [23] and adapted for silane condensation by Sims et al [8] describes chemisorption on a planar substrate via the conversion of hydrogen bound to covalently bound adsorption when an equilibrium is forced to completion. While the formation of the covalently bound θ 2 is thermodynamically favourable, it can only occur once the adsorbate is in the hydrogen bound potential well as θ 1 , when it can then cross the diffusion energy barrier to become θ 2 .…”

“…Previous studies have shown this method to result in a reproducible oxide layer [16]. 1% v / v silane solutions were prepared in a 1:1 H 2 O:EtOH solvent altered to pH 3 with glacial acetic acid and stirred until the silane solution was considered hydrolysed through observing complete dissolution of silane in the aqueous solvent [8]. Silane films were prepared by submerging aluminium substrates in the silane-solvent solution for the prescribed time.…”

“…While the mechanism of alkoxysilane self-assembly in solution is well known [5], the mechanism of alkoxysilane substrate condensation is not as simple. The presence of an oscillatory behaviour has been reported for silane coatings with more than one hydrolysable group, such as Propyltrimethoxysilane (PTMS) [6,7,8] and Propylmethyldimethoxysilane (PMDMS) [8], as a function of both film growth time and silane concentration in solution. In contrast, restricting the number of hydrolysable groups to one in the case of Propyldimethylmethoxysilane (PDMMS) results in a Langmuir-type adsorption profile on the native oxide of polycrystalline aluminium [8].…”

“…Studies that over extended silane-substrate exposure times revealed the presence of multiple oscillations in surface coverage, which required the addition of components to the model shown above in order to describe the behavior [8]. While the kinetics of a multilayer system with multiple oscillations in substrate coverage becomes increasingly complex and a 2-component model can no longer be used to represent the kinetics of self-assembly, the thermodynamics requiring the conversion of hydrogen bound to covalently bound species remain the same.…”