Reactivity of Functionalised Surfaces with Atmospheric Radicals

Self-assembled monolayers (SAMs) are widely used in various technologies and as model systems for studying interfacial reactions. However, their stability can be compromised by autoxidation, where molecular oxygen and organic molecules undergo a free-radical chain reaction, leading to monolayer degradation. This project aims to understand the surface oxidation mechanisms following the radical initiation of organic chains. Upon radical initiation, the carbon-centred radical formed reacts with oxygen to generate a peroxyl radical. The propagation of these radical reactions via the peroxyl radical was investigated in monolayer systems with varying functionalities, including alkyl, branched alkyl, ether, and perfluorinated chains. The results indicate that H-abstraction of ether-containing chains by a peroxyl radical occurs more efficiently than alkyl chains, likely due to their lower C-H bond dissociation energies. The lack of H-abstraction by peroxyl radicals in alkyl chains suggests that peroxyl radicals undergo an alternative reaction pathway. A statistical model showed that the probability of two peroxyl radicals being close enough to react to form stable products is approximately 0.1%, due to the limited mobility of molecules in monolayer systems. Experiments with added NO, which readily reacts with peroxyl radicals to form more reactive alkoxyl radicals, indicated that NO contributes to reaction spreading but is not the dominant fate for peroxyl radicals. This suggests an unknown alternative reaction pathway for peroxyl radicals in the monolayer system, potentially involving reaction with the surface or atmospheric contaminants. This project provides insights into autoxidation mechanisms in monolayer systems, which can inform the stability of SAM-based technologies, such as chemical sensing, and enhance our understanding of oxidation processes on surfaces. Moreover, this project is part of a research programme funded by the European Commission, which aims to develop sensors based on Si junctionless nanowire transistor (Si JNT) devices for the real-time detection of short-lived atmospheric radicals (OH and NOx). The interaction of OH radicals with functionalised planar silica substrates as a model system for Si JNT devices has shown that ether- or allylic-containing chains degrade faster than alkyl or perfluorinated chains. Initial atmospheric chamber tests demonstrated that functionalised sensor surfaces can detect OH radicals through a change in current.