Showing papers by "Nathan S. Lewis published in 1995"

A chemically diverse conducting polymer-based "electronic nose".

Abstract: We describe a method for generating a variety of chemically diverse broadly responsive low-power vapor sensors. The chemical polymerization of pyrrole in the presence of plasticizers has yielded conducting organic polymer films whose resistivities are sensitive to the identity and concentration of various vapors in air. An array of such sensing elements produced a chemically reversible diagnostic pattern of electrical resistance changes upon exposure to different odorants. Principal component analysis has demonstrated that such sensors can identify and quantify different airborne organic solvents and can yield information on the components of gas mixtures.

Routes to Conjugated Polymers with Ferrocenes in Their Backbones: Synthesis and Characterization of Poly(ferrocenylenedivinylene) and Poly(ferrocenylenebutenylene)

Abstract: Ring opening metathesis polymerization (ROMP) has been used to prepare conjugated polymers that contain ferrocene moieties as part of their backbones. The conjugated polymer poly-(ferrocenylenedivinylene) (PFDV) and the analogous unconjugated polymer poly(ferrocenylenebutenylene) (PFB) with chain lengths of greater than 10 were found to be insoluble in typical organic solvents. The conductivities of oxidatively doped films of PFDV and PFB were found to be 10 -4 and 10 -5 Ω -l cm -l , respectively. Oxidative doping of the monomer 1,4-(1,1'-ferrocenediyl)-1-butene was found to yield conductivities on the order of 10 -5 Ω -l cm -l , supporting interchain hopping as the dominant mechanism for charge carrier movement through these films. The ROMP of monomers octamethyl-1,4-(1,1'-ferrocenediyl)-1,3-butadiene and 1,4-(1,1'-ferrocenediyl)-1-methoxy-1,3-butadiene was also studied. Although soluble polymer was successfully generated from the latter monomer, no conditions were found under which the octamethyl compound could be polymerized.

An Analytical Description of the Consequences of Abandoning the Principles of Detailed Balance and Microscopic Reversibility in Semiconductor Photoelectrochemistry

Abstract: Key differences between the conventional and «irreversible» models of semiconductor photoelectrochemistry are identified and discussed within the framework of experimental observations. Conceptual differences between these two models appear to lie in the treatment of interfacial charge-transfer processes for photogenerated charge carriers. The conventional model utilizes detailed balance principles for obtaining rate constant relationships for all interfacial charge-transfer events at semiconductor/liquid contacts and uses the principle of microscopic reversibility to evaluate these rate constants for situations away from equilibrium. In contrast, the irreversible model postulates that local statistical detailed balance does not apply to charge-transfer events in photoelectrolysis, and that such charge-transfer events are highly irreversible, like photoemission into a vacuum. It is shown analytically that the two models predict differences in the behavior of the available free energy produced by a photoelectrochemical cell at a fixed incident light intensity. The conceptual implications of these differences are evaluated analytically and are also compared to experimental results for semiconductor/liquid junctions

Limits on the corrosion rate of Si surfaces in contact with CH3OH-ferrocene+/0 and CH3OH-1,1′-dimethylferrocene+/0 solutions

Abstract: Although Si/CH^3OH contacts have been extensively investigated and reported to provide highly efficient photoelectrochemical energy conversion devices, a recent study using the scanning electrochemical microscope (SECM) has claimed that, in CH_3OH solutions, Si surfaces in contact with 4.57 mM ferrocenium (Fc^+) were etched in the dark at a mass-transport-limited rate. The reported etching rate constant of > 0.37 cm s^-1) at 4.57 mM ferrocenium corresponds to an equivalent corrosion current density of > 240 mA cm^(-2) and to a Si etch rate of > 75 nm s^(-1). The presence of such severe corrosion was inferred from an unexpectedly large feedback current in an SECM experiment. The present work describes a search for corrosion of Si in contact with CH_3OH-ferrocene^+/0) and CH_3OH-dimethylferrocene(Me2Fc)^(+/0) solutions through the use of very sensitive electrochemical, chemical, and physical methods. For CH_3OH - 1.0 M LiClO_4 - 100 mM Me_2Fc- 80 mM Me_2Fc^+ solutions, an upper limit on the etch rate of 6.6 x10^(-6) nm s^-1) has been established through direct experimental measurements; thus, a 400 pm thick Si photoelectrode in contact with the CH_3OH-Me_2-Fc^(+/0) electrolyte would require over 1500 years to corrode completely at room temperature. An alternative explanation for the SECM data, based on the documented existence of an inversion layer at the Si/liquid contact, is presented and shown to be consistent with the available data.

Stability of n-Si/CH3OH Contacts as a Function of the Reorganization Energy of the Electron Donor

Abstract: Predictions of the Marcus/Gerischer theory for photoelectrode stability have been investigated experimentally for n-Si/CH_3OH photoelectrochemical cells. Specifically, a semiconductor electrode is predicted to be more stable if the reorganization energy of the stabilizing agent is decreased (in the normal region of the Marcus behavior), thereby increasing the rate of minority carrier capture by the stabilizer. This prediction was quantified experimentally by monitoring the branching ratio between two competing reactions at a semiconductor/liquid interface: hole transfer from a Si photoanode to the electron donor in solution vs passivation of the Si photoanode through hole transfer to water. Deliberate addition of water to n-Si/CH_3OH contacts provided a constant, known passivation pathway that competed with charge transfer to the stabilizing agent. Dimethylferrocene (Me_2Fc), ruthenium(II) pentaamrnine 4,4'-bipyridine (Ru(NH_3)_5(4,4'-bpy)^(2+)), and cobalt(II) tris( 2,2'-bipyridine) (Co(2,2'-bpy)_3^(2+)) provided three outer sphere electron donors with very similar standard electrochemical potentials but varying solvent reorganization energies. At constant electron donor concentration, constant driving force for reaction, constant photocurrent density, and constant water concentration in CH_3-OH, the stability of n-Si photoelectrodes decreased in the order Me_2Fc^(+/0) > Ru(NH_3)_5(4,4'-bpy)^(3+/2+) > Co(2,2'-bpy)_3^(3+/2+). This observation can be consistently explained through the theoretically predicted influence of the minority carrier acceptor reorganization energy on the interfacial charge transfer rate constant.

Efficient Reductive or Oxidative Current Flow across 100 μm Using an Si Photoelectrode under High Level Injection

Abstract: The photoelectrochemistry of Si/CH 3 OH contacts has been investigated under high level injection conditions. Exposure of Si samples having low doping levels (10 13 cm -3 ) and long minority carrier lifetimes (>1 ms) to modest steady-state illumination levels has produced high level injection conditions and minimal electric fields at the solid-liquid contact. Under these conditions, the solid is analogous to a large molecule, with both electrons and holes available for interfacial charge transfer (and recombination) at the solid/liquid contact. In principle, photocurrents of either sign are possible in such samples. Vectorial charge separation with a high quantum yield has been effected by selective removal of one carrier type at the back of the sample, in a fashion analogous to either oxidative or reductive quenching of a molecular excited state. Through specifying the back contact connections, the sign of the photocurrent at the solid/liquid interface, 100 μm from the initial quenching site, has been manipulated. Charge separation with high quantum yields and with 10% photoelectrode energy conversion efficiencies under solar simulation has been achieved using this approach