In situ PM‐IRRAS studies of organothiols and organosilane monolayers–ZnO interfaces at high water activities

Abstract

In situ photoelastic-modulated Fourier transform infrared reflection absorption spectroscopy has been applied for the investigation of interfacial stability of organothiol and organosilane monolayer films on nanocrystalline zinc oxide thin films. It has been shown that for octadecyltriethoxysilane films, exposure to high water activities results in physisorption of water in the crosslinked film. This high water activity at the interface leads to a reversible wet de-adhesion of the interfacial silanol groups from the ZnO surface. However, the organothiol seems to form a denser monolayer and a stable by S-Zn bond that is resistant to the competition with adsorbed water. The reversible attachment for cross-linked organosilanol films has been demonstrated for the first time by means of an in situ spectroscopic method on model ZnO surfaces. Copyright © 2016 John Wiley & Sons, Ltd.Keywords: interfacial stability; photoelastic-modulated Fourier transform infrared reflection absorption spectroscopy (PM-IRRAS); organofunctional monolayer films; zinc oxide; organosilane Thin organic films play an important role in microsensors, electronics and optical devices and offer wide possibilities in tuning surface and interface properties for the development of protective coatings and biocompatible materials. [1][2][3][4][5][6][7] Organosilanols are a class of functional molecules that are able to react with organic and inorganic surfaces through surface hydroxyl groups to form an interface dominated by van der Waals and hydrogen bonding interactions that could be converted to covalent bonds via hydrolysis reactions, usually achieved by means of a thermal treatment. [1,8,9] Even though the exact mechanism of the film formation is still not fully understood, the consensus in the literature points to a stepwise evolution of the cross-linked siloxane network via the hydrolysis of halogen or alkoxy groups to silanols in contact with water, which can then further react to form the Si-O-Si and Si-O metal bonds. [10,11] While the Si-O-Si bond formation can be easily proven, the covalent bond formation to metal and oxide-covered metal substrates especially in the presence of water is questionable.In a recent study, Naik and coworkers have investigated the effect of the degree of polymerization on the film formation of alkylsilane monolayers on silica surfaces and have concluded that the inherent stress caused by the mismatch of alkyl-chain van der Waals radius and Si-O-Si bond distance results in the adsorption of structures resembling snow moguls or closely packed umbrellas.[11] Organothiols and organophosphonic acids, which can form stable interfaces on zinc oxide surfaces, have been investigated as adhesion promoters on various metal-metal oxide surfaces, as insulating layers on circuit materials, as well as possible linkers for sensor and biosensor applications. [4,5,7,12,13] In most of the aforementioned application fields, the stability of the thin functional films is strongly dependent on the strength of the interfacial interactions...

New details of assembling bioactive films from dispersions of amphiphilic molecules on titania surfaces

“… 73 The release of alkylorganosilanes was reported by Torun and co-workers, who revealed that alkylorganosilanes anchor by few points on ZnO surface leading to a small number of covalent bonds. 53 Thereby, well ordered DMOAP monolayers might not be suitable for antibacterial coatings since the quaternary ammonium groups are not easily accessed by the bacteria membrane.…”

“…40,51,52 Starting from a long-chain alkyltrialkoxysilane, such as octadecyltriethoxysilane (ODS) on zinc oxide surfaces, the formation of Si-O-Si was reported to dominate the formation of Si-O-Zn bonds, leading to a crosslinked siloxane network that adheres on the substrate surface via hydrogen bonding or with few anchoring points along the surface. 53 Interestingly, acidic properties of both zinc oxide and titania are related to Lewis acid surface sites rather than to Bronsted sites. 52,54 The complexation of the acidic substrate surface sites with oxoanions or esters may be driven by Lewis basic electron pair donor properties of the respective adsorbent providing the oxo-bridge.…”

Molecular Characterization of Multiple Bonding Interactions at the Steel Oxide–Aminopropyl triethoxysilane Interface by ToF-SIMS

“…Although the bonding mechanism illustrated in Figure has been often cited in the literature, reactions (c,d) in the mechanism, which involve the actual interactions between the silanes and the substrate, are still under debate. On the one hand, it is questioned whether SiOM + fragments obtained from ToF-SIMS measurements can be considered as sufficient evidence for covalent bonding. , On the other hand, direct evidence for hydrogen bond interactions at the metal oxide–silane interface has never been provided. In this work, ToF-SIMS will be applied to study the bonding mechanism of aminopropyl triethoxysilane (APS) on steel.…”

“…On the one hand, it is questioned whether SiOM + fragments obtained from ToF-SIMS measurements can be considered as sufficient evidence for covalent bonding. 3,10 On the other hand, direct evidence for hydrogen bond interactions at the metal oxide− silane interface has never been provided. In this work, ToF-SIMS will be applied to study the bonding mechanism of aminopropyl triethoxysilane (APS) on steel.…”

A Complementary Experimental and Theoretical Approach for Probing the Surface Functionalization of ZnO with Molecular Catalyst Linkers

“…Reports in this direction are usually restricted to the adsorption of long chain molecules (12-18 carbon atoms) on monocrystalline ZnO surfaces. [47,48] It is also important to note that macroscopic assemblies of ZnO are generally polycrystalline, an aspect that has been largely overlooked in previous studies. In a seminal work, Perkins showed that the thermal stability of n-hexanethiol SAMs on polycrystalline ZnO surfaces was lower than that of their phosphonic acid counterparts.…”