Secondary Functionalization of Allyl-Terminated GaP(111)A Surfaces via Heck Cross-Coupling Metathesis, Hydrosilylation, and Electrophilic Addition of Bromine

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 C₃H₅MgCl was observed through water contact angle measurements. Infrared spectra of GaP(111)­A samples after reaction with C₃H₅MgCl showed the asymmetric CC and CC–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 CH₃-terminated GaP(111)­A surfaces. Similar analyses showed electrophilic addition of Br₂ across the terminal olefin on an allyl-terminated GaP(111)­A surface after exposure to dilute Br₂ solutions in CH₂Cl₂. The work presented herein establishes a set of secondary reaction strategies utilizing allyl-terminated surfaces to modify chemically protected GaP surfaces

Photoelectrochemical Properties of CH₃‑Terminated p‑Type GaP(111)A

The photoelectrochemical properties of p-type gallium phosphide (GaP) (111)­A electrodes before and after a two-step chlorination/Grignard reaction sequence have been assessed. Electrochemical impedance spectroscopy indicated both a change in the flat-band potential in water and decreased sensitivity of the band edge energetics toward pH for GaP(111)­A surfaces following modification. Separate stability tests were performed to gauge the susceptibilities of unmodified and CH₃-terminated p-GaP(111)­A photoelectrodes toward reductive degradation under illumination. The steady-state photoelectrochemical results showed modification of GaP(111)­A with −CH₃ groups significantly enhanced p-GaP stability. Separately, sub-bandgap photocurrent measurements were collected to assess relative changes in surface states acting as recombination centers. In the absence of any dye, the sub-bandgap photocurrent from trapping/detrapping of charge carriers in surface states was higher for unmodified p-GaP photoelectrodes than for CH₃-terminated p-GaP­(111)­A. Further, sensitized photocurrents of p-GaP photoelectrodes with Brilliant Green were systematically higher after modification with −CH₃ groups, indicating a deactivation of a surface recombination pathway after surface modification. Collectively, this work illustrates a rational chemical strategy to modify and augment the pertinent interfacial properties of p-GaP photocathodes in an aqueous photoelectrochemical system