Redox Properties of Mixed Methyl/Vinylferrocenyl Monolayers on Si(111) Surfaces

We report the redox properties of Si(111) surfaces functionalized with a mixed monolayer of vinylferrocenyl and methyl moieties that have been characterized using spectroscopic, electrical, and electrochemical techniques. The silicon was functionalized using reaction conditions analogous to those of hydrosilylation, but instead of a H-terminated Si surface, a chlorine-terminated Si precursor surface was used to produce the linked vinyl-modified functional group. The functionalized surfaces were characterized by time-resolved photoconductivity decay, X-ray photoelectron spectroscopy, electrochemical measurements, and photoelectrochemical measurements. The functionalized Si surface was well passivated, exhibited high surface coverage and few remaining reactive Si atop sites, had a very low surface recombination velocity, and displayed little initial surface oxidation. The surface was stable toward atmospheric and electrochemical oxidation. The surface coverage of vinylferrocene (or fluorostyrene) was controllably varied from 0 up to 30% of a monolayer. Interfacial charge transfer to the attached ferrocene group was relatively rapid, and a photovoltage of 0.4 V was generated upon illumination of functionalized n-type silicon surfaces in CH_(3)CN

Assembly, characterization, and electrochemical properties of immobilized metal bipyridyl complexes on silicon(111) surface

Silicon(111) surfaces have been functionalized with mixed monolayers consisting of submonolayer coverages of immobilized 4-vinyl-2,2′-bipyridyl (1, vbpy) moieties, with the remaining atop sites of the silicon surface passivated by methyl groups. As the immobilized bipyridyl ligands bind transition metal ions, metal complexes can be assembled on the silicon surface. X-ray photoelectron spectroscopy (XPS) demonstrates that bipyridyl complexes of [Cp*Rh], [Cp*Ir], and [Ru(acac)2] were formed on the surface (Cp* is pentamethylcyclopentadienyl, acac is acetylacetonate). For the surface prepared with Ir, X-ray absorption spectroscopy at the Ir LIII edge showed an edge energy as well as post-edge features that were essentially identical with those observed on a powder sample of [Cp*Ir(bpy)Cl]Cl (bpy is 2,2′-bipyridyl). Charge-carrier lifetime measurements confirmed that the silicon surfaces retain their highly favorable photoelectronic properties upon assembly of the metal complexes. Electrochemical data for surfaces prepared on highly doped, n-type Si(111) electrodes showed that the assembled molecular complexes were redox active. However the stability of the molecular complexes on the surfaces was limited to several cycles of voltammetry

Functionalization of Si(111) Surfaces and the Formation of Mixed Monolayers for the Covalent Attachment of Molecular Catalysts in Photoelectrochemical Devices

The functionalization of silicon surfaces with molecular catalysts for proton reduction is an important part of the development of a solar-powered, water-splitting device for solar fuel formation. The covalent attachment of these catalysts to silicon without damaging the underlying electronic properties of silicon that make it a good photocathode has proven difficult. We report the formation of mixed monolayer-functionalized surfaces that incor- porate both methyl and vinylferrocenyl or vinylbipyridyl (vbpy) moieties. The silicon was functionalized using reaction conditions analogous to those of hydrosilylation, but instead of a H-terminated Si surface, a chlorine-terminated Si precursor surface was used to produce the linked vinyl-modified functional group. The functionalized surfaces were characterized by time-resolved photoconductivity decay, X-ray photoelectron spectroscopy (XPS), electro- chemical, and photoelectrochemical measurements. The functionalized Si surfaces were well passivated, exhibited high surface coverage and few remaining reactive Si atop sites, had a very low surface recombination velocity, and displayed little initial surface oxidation. The surfaces were stable toward atmospheric and electrochemical oxidation. The surface coverage of ferrocene or bipyridine was controllably varied from 0 up to 30% of a monolayer without loss of the underlying electronic properties of the silicon. Interfacial charge transfer to the attached ferrocene group was relatively rapid, and a photovoltage of 0.4 V was generated upon illumination of functionalized n-type silicon surfaces in CH3CN. The immobilized bipyridine ligands bound transition metal ions, and thus enabled the assembly of metal complexes on the silicon surface. XPS studies demonstrated that [Cp∗Rh(vbpy)Cl]Cl, [Cp∗Ir(vbpy)Cl]Cl, and Ru(acac)2vbpy were assembled on the surface. For the surface prepared with iridium, x-ray absorption spectroscopy at the Ir LIII edge showed an edge energy and post-edge features virtually identical to a powder sample of [Cp∗Ir(bipy)Cl]Cl (bipy is 2,2 ́-bipyridyl). Electrochemical studies on these surfaces confirmed that the assembled complexes were electrochemically active

Molecular proton-reduction catalysis at silicon semiconductor surfaces

Driving catalysis at semiconductor:liq. junctions with light enables the efficient conversion and storage of solar energy in chem. fuels. Hydrogen evolution from water is one possible path to fuel, as there are known heterogeneous and homogeneous catalysts for proton redn. Homogeneous catalysts show the greatest promise for clarifying both (1) key mechanistic aspects of the hydrogen-evolution reaction at the surface and (2) energetic considerations of the semiconductor band structure. Along this line, we are studying a family of rhodium complexes as model catalysts for hydrogen evolution at semiconductor:liq. junctions. Both diffusional and silicon surface-attached systems have been developed, and will be compared. This approach has yielded new information concerning the ambiguities of homogeneous vs. Heterogeneous catalysis, as well as the possible catalytic mechanism(s) of the surface-attached species

The Exemplar Programme for integrated out-of-hours care: evaluation report

Report of an independent evaluation, by the University of Southampton for the Department of Health, of an exemplar programme for single call access to out-of-hours care through NHS Direct. This evaluation was commissioned to examine several aspects of the Programme including the experiences of patients, the performance and costs of the new services, and their impact on other immediate care providers

Effect of introduction of integrated out of hours care in England: observational study

Objectives: To quantify service integration achieved in the national exemplar programme for single call access to out of hours care through NHS Direct, and its effect on the wider health system. Design: Observational before and after study of demand, activity, and trends in the use of other health services. Participants: 34 general practice cooperatives with NHS Direct partners (exemplars): four were case exemplars; 10 control cooperatives. Setting: England. Main outcome measures: Extent of integration; changes in demand, activity, and trends in emergency ambulance transports; attendances at emergency departments, minor injuries units, and NHS walk-in centres; and emergency admissions to hospital in the first year. Results: Of 31 distinct exemplars, 21 (68%) integrated all out of hours call management. Nine (29%) achieved single call access for all patients. In the only case exemplar where direct comparison was possible, a higher proportion of telephone calls were handled by cooperative nurses before integration than by NHS Direct afterwards (2622/6687 (39%) v 2092/7086 (30%): P < 0.0001). Other case exemplars did not achieve 30%. A small but significant downturn in overall demand for care seen in two case exemplars was also seen in the control cooperatives. The number of emergency ambulance transports increased in three of the four case exemplars after integration, reaching statistical significance in two (5%, -0.02% to 10%, P = 0.06; 6%, 1% to 12%, P = 0.02; 7%, 3% to 12%, P = 0.001). This was always accompanied by a significant reduction in the number of calls to the integrated service. Conclusion: Most exemplars achieved integration of call management but not single call access for patients. Most patients made at least two telephone calls to contact NHS Direct, and then waited for a nurse to call back. Evidence for transfer of demand from case exemplars to 999 ambulance services may be amenable to change, but NHS Direct may not have sufficient capacity to support national implementation of the programme

Reversible Alkaline Hydrogen Evolution and Oxidation Reactions Using Ni–Mo Catalysts Supported on Carbon

Unitized regenerative fuel cells based on hydroxide exchange membranes are attractive for long duration energy storage. This mode of operation depends on the ability to catalyze hydrogen evolution and oxidation reversibly, and ideally using nonprecious catalyst materials. Here we report the synthesis of Ni–Mo catalyst composites supported on oxidized Vulcan carbon (Ni–Mo/oC) and demonstrate their performance for reversible hydrogen evolution and oxidation. For the hydrogen evolution reaction, we observed mass-specific activities exceeding 80 mA/mg at 100 mV overpotential, and additional measurements using hydroxide exchange membrane electrode assemblies yielded full cell voltages that were only ~100 mV larger for Ni–Mo/oC cathodes compared to Pt–Ru/C at current densities exceeding 1 A/cm2. For hydrogen oxidation, Ni–Mo/oC films required <50 mV overpotential to achieve half the maximum anodic current density, but activity was limited by internal mass transfer and oxidative instability. Nonetheless, estimates of the mass-specific exchange current for Ni–Mo/oC from micropolarization measurements showed its hydrogen evolution/oxidation activity is within 1 order of magnitude of commercial Pt/C. Density functional theory calculations helped shed light on the high activity of Ni–Mo composites, where the addition of Mo leads to surface sites with weaker H-binding energies than pure Ni. These calculations further suggest that increasing the Mo content in the subsurface of the catalyst would result in still higher activity, but oxidative instability remains a significant impediment to high performance for hydrogen oxidation

New Architectures for Designed Catalysts: Selective Oxidation using AgAu Nanoparticles on Colloid-Templated Silica

A highly modular synthesis of designed catalysts with controlled bimetallic nanoparticle size and composition and a well-defined structural hierarchy is demonstrated. Exemplary catalysts—bimetallic dilute Ag-in-Au nanoparticles partially embedded in a porous SiO2 matrix (SiO2-AgxAuy)—were synthesized by the decoration of polymeric colloids with the bimetallic nanoparticles followed by assembly into a colloidal crystal backfilled with the matrix precursor and subsequent removal of the polymeric template. We show that these new catalysts architectures are significantly better than nanoporous dilute AgAu alloy catalysts (nanoporous Ag0.03Au0.97) while retaining a clear predictive relationship between their surface reactivity with that of single crystal Au surfaces. This paves the way for broadening the range of new catalyst architectures required for translating the designed principles developed under controlled conditions to designed catalysts under operating conditions for highly selective coupling of alcohols to form esters. Excellent catalytic performance of the porous SiO2-AgxAuy structure for selective oxidation of both methanol and ethanol to produce esters with high conversion efficiency, selectivity, and stability was demonstrated, illustrating the ability to translate design principles developed for support-free materials to the colloid-templated structures. The synthetic methodology reported is customizable for the design of a wide range of robust catalytic systems inspired by design principles derived from model studies. Fine control over the composition, morphology, size, distribution and availability of the supported nanoparticles was demonstrated.Chemistry and Chemical Biolog