Secondary Functionalization of Allyl-Terminated GaP(111)A
Surfaces via Heck Cross-Coupling Metathesis, Hydrosilylation, and
Electrophilic Addition of Bromine
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Abstract
The functionalization
of single crystalline gallium phosphide (GaP)
(111)A surfaces with allyl groups has been performed using a sequential
chlorine-activation/Grignard reaction process. Increased hydrophobicity
following reaction of a GaP(111)A surface with C<sub>3</sub>H<sub>5</sub>MgCl was observed through water contact angle measurements.
Infrared spectra of GaP(111)A samples after reaction with C<sub>3</sub>H<sub>5</sub>MgCl showed the asymmetric CC and CC–H
modes diagnostic of surface-attached allyl groups. The stability of
allyl-terminated GaP(111)A surfaces under ambient and aqueous conditions
was investigated. XP spectra of allyl-terminated GaP(111)A highlighted
a significant resistance against interfacial oxidation both in air
and in water relative to the native interface. Electrochemical impedance
spectroscopy indicated a change in the flat-band potential of allyl-terminated
GaP(111)A electrodes immersed in water relative to native GaP(111)A
surfaces. Further, the flat-band potentials for allyl-terminated electrodes
were insensitive to changes in solution pH. The utility of surface-bound
allyl groups for covalent secondary functionalization of GaP(111)A
interfaces was assessed through three separate reactions: Heck cross-coupling
metathesis, hydrosilylation, and electrophilic addition of bromine
reactions. Addition of aryl groups across the olefins on allyl-terminated
GaP(111)A via Heck cross coupling was performed and confirmed through
high-resolution F 1s and C 1s XP spectra and IR spectra. Control experiments
with GaP(111)A surfaces functionalized with short alkanes indicated
no evidence for metathesis. Hydrosilylation reactions were separately
performed. Si 2s XP spectra, in conjunction with infrared spectra,
similarly showed secondary evidence of surface functionalization for
allyl-terminated GaP(111)A but not for CH<sub>3</sub>-terminated GaP(111)A
surfaces. Similar analyses showed electrophilic addition of Br<sub>2</sub> across the terminal olefin on an allyl-terminated GaP(111)A
surface after exposure to dilute Br<sub>2</sub> solutions in CH<sub>2</sub>Cl<sub>2</sub>. The work presented herein establishes a set
of secondary reaction strategies utilizing allyl-terminated surfaces
to modify chemically protected GaP surfaces