Christopher E. D. Chidsey

Free energy and temperature dependence of electron transfer at the metal-electrolyte interface.

The rate constant of the electron-transfer reaction between a gold electrode and an electroactive ferrocene group has been measured at a structurally well-defined metal-electrolyte interface at temperatures from 1� to 47�C and reaction free energies from -1.0 to +0.8 electron volts (eV). The ferrocene group was positioned a fixed distance from the gold surface by the self-assembly of a mixed thiol monolayer of (η5C5H5)Fe(η5C5H4)CO2(CH2)16SH and CH3(CH2)15SH. Rate constants from 1 per second (s–1) to 2 x 104 s–1 in 1 molar HClO4 are reasonably fit with a reorganization energy of 0.85 eV and a prefactor for electron tunneling of 7 x 104 s–1 eV–1. Such self-assembled monolayers can be used to systematically probe the dependence of electron-transfer rates on distance, medium, and spacer structure, and to provide an empirical basis for the construction of interfacial devices such as sensors and transducers that utilize macroscopically directional electron-transfer reactions.

Atomic layer-deposited tunnel oxide stabilizes silicon photoanodes for water oxidation

A leading approach for large-scale electrochemical energy production with minimal global-warming gas emission is to use a renewable source of electricity, such as solar energy, to oxidize water, providing the abundant source of electrons needed in fuel synthesis. We report corrosion-resistant, nanocomposite anodes for the oxidation of water required to produce renewable fuels. Silicon, an earth-abundant element and an efficient photovoltaic material, is protected by atomic layer deposition (ALD) of a highly uniform, 2 nm thick layer of titanium dioxide (TiO(2)) and then coated with an optically transmitting layer of a known catalyst (3 nm iridium). Photoelectrochemical water oxidation was observed to occur below the reversible potential whereas dark electrochemical water oxidation was found to have low-to-moderate overpotentials at all pH values, resulting in an inferred photovoltage of ~550 mV. Water oxidation is sustained at these anodes for many hours in harsh pH and oxidative environments whereas comparable silicon anodes without the TiO(2) coating quickly fail. The desirable electrochemical efficiency and corrosion resistance of these anodes is made possible by the low electron-tunnelling resistance (<0.006 Ω cm(2) for p(+)-Si) and uniform thickness of atomic-layer deposited TiO(2).

Electroactive polymers and macromolecular electronics

Electrodes can be coated with electrochemically reactive polymers in several microstructural formats called sandwich, array, bilayer, micro-, and ion-gate electrodes. These microstructures can be used to study the transport of electrons and ions through the polymers as a function of the polymer oxidation state, which is essential for understanding the conductivity properties of these new chemical materials. The microstructures also exhibit potentially useful electrical and optical responses, including current rectification, charge storage and amplification, electron-hole pair separation, and gates for ion flow.

A Cytochrome c Oxidase Model Catalyzes Oxygen to Water Reduction Under Rate-Limiting Electron Flux

We studied the selectivity of a functional model of cytochrome c oxidases active site that mimics the coordination environment and relative locations of Fea3, CuB, and Tyr244. To control electron flux, we covalently attached this model and analogs lacking copper and phenol onto self-assembled monolayer–coated gold electrodes. When the electron transfer rate was made rate limiting, both copper and phenol were required to enhance selective reduction of oxygen to water. This finding supports the hypothesis that, during steady-state turnover, the primary role of these redox centers is to rapidly provide all the electrons needed to reduce oxygen by four electrons, thus preventing the release of toxic partially reduced oxygen species.

Molecular order at the surface of an organic monolayer studied by low energy helium diffraction

We demonstrate that the surface structure of organic monolayers can be determined by low energy helium diffraction at low surface temperatures. This uniquely surface‐sensitive and nondestructive technique shows that the CH3‐terminated surface of a monolayer of docosane thiol (CH3(CH2)21SH) on Au(111) is composed of small, ordered domains (lattice constant 5.01±0.02 A), a large fraction of which share a common orientation. The helium diffraction intensities decrease monotonically with increasing temperature and vanish around 100 K, due to thermal motion of the CH3 groups. Surface order is observed for chains as short as ten carbons (CH3(CH2)9SH) but a shorter chain, (CH3(CH2)5SH), gave no diffraction.

Superlattice structure at the surface of a monolayer of octadecanethiol self‐assembled on Au(111)

We report direct evidence of a unit mesh containing more than one hydrocarbon chain at the surface of a self‐assembled monolayer of long‐chain n‐alkanethiols. Our helium diffraction measurements for a monolayer of n‐octadecanethiol on Au(111) are consistent with a rectangular primitive unit mesh of dimensions 8.68×10.02 A containing four crystallographically distinct hydrocarbon chains. This packing arrangement can also be described as a c(4×2) superlattice with respect to the fundamental simple hexagonal [(√3×√3)R30°] array of lattice parameter 5.01 A previously observed for monolayers of other n‐alkanethiols on gold. No temperature‐dependent phase behavior is observed in the temperature range where surface diffraction is measurable (30–100 K) and cycling up to temperatures as high as 50 °C caused no observable change in the diffraction. It is proposed that this larger unit mesh is the result of a patterned arrangement of rotations of the hydrocarbon chains about their molecular axes. This patterned arrangement must be different than the herringbone structure expected by simple analogy to bulk n‐alkanes.

Polar orientation of dyes in robust multilayers by zirconium phosphate-phosphonate interlayers.

Polar orientation of molecules in solids leads to materials with potentially useful properties such as nonlinear optical and electrooptical activity, electrochromism, and pyroelectricity. A simple self-assembly procedure for preparing such materials is introduced that yields multiple polar dye monolayers on solid surfaces joined by zirconium phosphate-phosphonate interlayers. Second harmonic generation (SHG) shows that the multilayers have polar order that does not decrease with increasing numbers (up to a large number) of monolayers in the film. The inorganic interlayers, as determined by SHG, impart excellent orientational stability to the dye molecules, with the onset of orientational randomization above 150�C.

Monolayer vibrational spectroscopy by infrared-visible sum generation at metal and semiconductor surfaces

Abstract IR-visible sum generation spectroscopy, an interface-selective probe of molecular vibrations, is used to obtain vibrational spectra of molecular monolayers on metal and semiconductor surfaces. The spectra obey electric dipole selection rules: vibrational modes must be both Raman and infrared active to show sum frequency resonances. The orientation of molecules at the interface can be determined by interference between the resonant molecular signal and a substrate background signal. Sum generation is also observed at a buried interface in the absence of a dielectric discontinuity, suggesting uses at buried molecular structures such as polymer-polymer interfaces.

STM study of the surface morphology of gold on mica

Abstract The scanning tunneling microscope (STM) promises to be a useful tool for the study of thin film deposition processes. We have imaged the growth surface of vapor-deposited gold on mica with an STM. Monatomic steps, dislocations, grain boundaries and grain topographies have been mapped for various gold thicknesses and deposition temperatures. Complementary X-ray diffraction and transmission electron microscopy experiments have been used to determine the orientation, size, shape and defectiveness of the crystallites. Near room temperature, the nuclei do not fuse together into larger single crystallites as the film coalesces, and the mounded topography of the individual crystallites evolves only slightly as the film thickens. The mounds are about 500 A across and 75 A high. At higher temperatures (150°C to 300°C), the nuclei apparently fuse as the film coalesces, giving larger crystallites containing holes. The dislocations observed by STM are apparently created as these holes fill in, due to frequent misregistry of the gold lattice around the holes. In the latter stages of deposition, there is a temperature-dependent flattening of the irregular topography caused by the island growth mode, which we attribute to surface diffusion. Consequently, starting the deposition at lower temperatures and increasing the temperature during the deposition gives very flat surfaces. In contrast, depositing gold under the same conditions on a thin underlayer of silver results in large islands separated by grooves, presumably due to extensive fusion of the silver nuclei into larger crystallites before a complete film is formed.

Etch-pit initiation by dissolved oxygen on terraces of H-Si(111)

Dissolved oxygen in 40% aqueous ammonium fluoride solution initiates the formation of etch pits in the terraces of the otherwise ideal H-Si(111) surface. The etch pits are observed byex situ scanning tunneling microscopy in an argon atmosphere following emersion from the aqueous fluoride solution. Removal of O2 from the fluoride solution by sparging with argon substantially reduces the initiation of etch pits. We propose the following mechanism of etch-pit initiation. Oxygen molecules are reduced to superoxide anion radicals at the negative open-circuit potential of the silicon surface. A small fraction (less than 0.4%) of these superoxide anions abstract hydrogen atoms from the H-Si(111) terraces to form silicon radicals (dangling bonds), which are then susceptible to etching in neutral to basic aqueous solutions. Hydrogen atom abstraction by aqueous superoxide anion radical also explains the known enhancement by water of oxide growth on hydrogen-terminated silicon surfaces.