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 CH 3 -terminated p-GaP(111)A photoelectrodes toward reductive degradation under illumination. The steady-state photoelectrochemical results showed modification of GaP(111)A with −CH 3 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 CH 3 -terminated p-GaP(111)A. Further, sensitized photocurrents of p-GaP photoelectrodes with Brilliant Green were systematically higher after modification with −CH 3 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.

The sensitized hole injection to the p-type gallium phosphide (p-GaP) electrode was evaluated for several triarylmethane (6O+), acridine (4O+, 2O+, Me2N-Acr+, T-Acr+), and flavin dyes (Et-Fl+). Thermodynamics for sensitized charge injection were evaluated using steady-state ultraviolet–visible absorption and cyclic voltammetry experiments. All dyes (except 4O+) had strong absorption at wavelengths above 550 nm (absorption cutoff for GaP), and their highest occupied molecular orbital energies were below the valence band of GaP (+1.20 V vs normal hydrogen electrode), indicating that the electron transfer from p-GaP electrode to the excited dye molecules was thermodynamically favorable. Photoelectrochemical measurement conducted on p-GaP electrodes immersed in aqueous electrolytes and dye showed sensitization for only two dyes (2O+ and Et-Fl+), and the sensitization efficiencies were found to depend on the chemical nature of differently prepared p-GaP electrodes. Femtosecond pump–probe measurements revealed that the “inefficient” dyes had short-lived excited states (few picoseconds), preventing the successful charge transfer into p-GaP surface. Collectively, this work provides insight on time scales of hole-injection rates during dye-sensitization processes.

We have examined the catalytic activity of an iron(III) complex bearing the 14,28-[1,3-diiminoisoindolinato]phthalocyaninato (diiPc) ligand in oxidation reactions with three substrates (cyclohexane, cyclooctane, and indan). This modified metallophthalocyaninato complex serves as an efficient and selective catalyst for the oxidation of cyclohexane and cyclooctane, and to a far lesser extent indan. In the oxidations of cyclohexane and cyclooctane, in which hydrogen peroxide is employed as the oxidant under inert atmosphere, we have observed turnover numbers of 100.9 and 122.2 for cyclohexanol and cyclooctanol, respectively. The catalyst shows strong selectivity for alcohol (vs. ketone) formation, with alcohol to ketone (A/K) ratios of 6.7 and 21.0 for the cyclohexane and cyclooctane oxidations, respectively. Overall yields (alcohol + ketone) were 73% for cyclohexane and 92% for cyclooctane, based upon the total hydrogen peroxide added. In the catalytic oxidation of indan under similar conditions, the TON for 1-indanol was 10.1, with a yield of 12% based upon hydrogen peroxide. No 1-indanone was observed in the product mixture.

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.

Chemically modified Si(111) surfaces have been prepared through a series of wet chemical surface treatments that simultaneously show resistance towards surface oxidation, selective reactivity towards chemical reagents, and areal defect densities comparable to unannealed thermal oxides. Specifically, grazing angle attenuated total reflectance infrared and X-ray photoelectron (XP) spectroscopy were used to characterize allyl-, 3,4-methylenedioxybenzene-, or 4-[bis(trimethylsilyl)amino]phenyl-terminated surfaces and the subsequently hydroxylated surfaces. Hydroxylated surfaces were confirmed through reaction with 4-(trifluoromethyl)benzyl bromide and quantified by XP spectroscopy. Contact angle measurements indicated all surfaces remained hydrophilic, even after secondary backfilling with CH 3-groups. Surface recombination velocity measurements by way of microwave photoconductivity transients showed the relative defectcharacter of as-prepared and aged surfaces. The relative merits for each investigated surface type are discussed.