Covalent Functionalization of Carbon Surfaces: Diaryliodoniumversus Aryldiazonium Chemistry

Covalently modified carbon surfaces remain central to a number of applications. Surface modification is often achieved using aryldiazonium chemistry, which has become the gold standard for the functionalization of carbon-based surfaces. However, the higher reactivity and thus the lower stability of diazonium precursors at carbon interfaces have prompted a search for alternative chemistries. Diaryliodonium salts have emerged as stable alternatives to the conventional aryldiazonium precursors. In this contribution, we provide a detailed comparison between the two types of chemistries by carrying out electrochemical covalent grafting of graphite substrates using nitrophenyl groups generated from the corresponding aryldiazonium and diaryliodonium salts under identical experimental conditions. The electrochemical process was studied using cyclic voltammetry, whereas the covalently grafted substrates were characterized using a battery of surface analytical techniques spanning multiple length scales. Analysis of the modified substrates using Raman spectroscopy revealed that the efficiency of covalent grafting is higher for iodonium chemistry than that for aryldiazonium chemistry. This observation was further corroborated by detailed morphological characterization of the covalent films using atomic force microscopy and molecular-resolution scanning tunneling microscopy, which revealed higher surface coverage for iodonium modified substrates. The chemical composition of the films was probed using X-ray photoelectron spectroscopy. The detailed and systematic comparison presented here clearly demonstrates that diaryliodonium chemistry presents a robust and reliable alternative to the widely used aryldiazonium chemistry.