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Covalent Functionalization of Carbon Surfaces: Diaryliodonium versus 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.