Photocatalytic functionalization of carbon surfaces

The core of this research is the oxidation of graphitic carbon materials by a completely new approach. Oxidation-resistant carbon materials like graphite are oxidized by using photocatalysis. Present-day carbon oxidation methods make use of very high temperatures, polluting chemicals and aggressive oxidants, which impact the environment badly. The photocatalytic oxidation method could potentially provide an alternative to the commonly used wet chemical oxidation methods for graphite oxidation. The Hummers method and its abundant modifications for example involve strong mineral acids, highly oxidative potassium permanganate (risk of explosions) and hydrogen peroxide. Carbon oxidation would strongly benefit from a green alternative to this non-sustainable oxidation method that generates much liquid waste. The photocatalytic oxidation of graphite is simply based on using UV light, a photocatalyst material and some assisting oxidative gases. In contrast to other graphite oxidation processes, this eco-friendly method can functionalize graphitic carbon surfaces at relatively low temperature with spatial oxidation control and good reaction reproducibility. Remote graphite photo-oxidation is also a promising research area which follows from the successful carbon black oxidation by photocatalytically generated mobile oxidative species.

In view of optimizing the photoreaction, the photo-oxidized carbon materials are characterized by multiple advanced techniques. Structural analyses are carried out by Raman spectroscopy, revealing a molecular fingerprint of the carbon microstructure. After the photoreaction, oxidation-related carbon bonds formed at the surface are detected by recording detailed Raman spectra. The effect of the photocatalyst activity on the carbon surface topography and morphology is elucidated by two microscopic tools, namely atomic force microscopy (AFM) and scanning electron microscopy (SEM). AFM shows topographical differences between non-oxidized and oxidized carbon surfaces at the nanoscale. SEM analysis gives high-resolution images of photo-oxidized carbon materials, leading to the discovery of new oxidation-induced structural features like surface blisters at the edges and basal planes of graphite. X-ray photoelectron spectroscopy (XPS) is furthermore a useful tool to reveal the chemical composition of the top carbon layers of the graphite particles following photo-oxidation treatment.