In Situ and Real Time Characterization of Spontaneous Grafting of Aryldiazonium Salts at Carbon Surfaces

Aryldiazonium cations are widely used to covalently functionalize carbon substrates that display a wide range of composition, from 100% sp 2 such as graphite or graphene to 100% sp 3 such as diamond and nanodiamond. In this work we investigated the effect that changes in carbon composition have on aryldiazonium adsorption rates and surface reaction mechanism. Quartz crystal microbalance (QCM) was used to investigate the rates of adsorption in situ and in real time at two amorphous carbon substrates, one with high sp 2 content (a-C) and one with high sp 3 content (a-C:H). A reversible Langmuir adsorption model was found to satisfactorily describe adsorption at a-C:H, yielding an adsorption rate coefficient k a = 3.1 M –1 s –1 and a free energy of adsorption Δ G a = −20.1 kJ mol –1 . This model, on the other hand, could not be applied for the interpretation of adsorption curves at a-C. Using electrochemical methods and X-ray photoelectron spectroscopy (XPS), we found that adlayers formed at a-C:H and a-C surfaces differ considerably in composition; in particular, a-C surfaces were found to display higher rates of dediazoniation with respect to a-C:H surfaces. Our findings are interpreted and discussed in the context of current proposed mechanisms for aryldiazonium reactions at surfaces that consist of an adsorption/desorption step followed by a chemisorption via dediazoniation step. Our observations are consistent with proposed mechanisms and strongly suggest that differences in carbon composition result in differences in the relative magnitude of adsorption and chemisorptions rate coefficients.