Showing papers by "Nathan S. Lewis published in 1997"

Source of Image Contrast in STM Images of Functionalized Alkanes on Graphite: A Systematic Functional Group Approach

Abstract: A series of functionalized alkanes and/or alkyl alcohols have been prepared and imaged by scanning tunneling microscopy (STM) methods on graphite surfaces. The stability of these ordered overlayers has facilitated reproducible collection of STM images at room temperature with submolecular resolution, in most cases allowing identification of individual hydrogen atoms in the alkane chains, but in all cases allowing identification of molecular length features and other aspects of the image that can be unequivocally related to the presence of functional groups in the various molecules of concern. Functional groups imaged in this study include halides (X = F, Cl, Br, I), amines, alcohols, nitriles, alkenes, alkynes, ethers, thioethers, and disulfides. Except for −Cl and −OH, all of the other functional groups could be distinguished from each other and from −Cl or −OH through an analysis of their STM metrics and image contrast behavior. The dominance of molecular topography in producing the STM images of alkanes and alkanols was established experimentally and also was consistent with quantum chemistry calculations. Unlike the contrast of the methylene regions of the alkyl chains, the STM contrast produced by the various functional groups was not dominated by topographic effects, indicating that variations in local electronic coupling were important in producing the observed STM images of these regions of the molecules. For molecules in which electronic effects overwhelmed topographic effects in determining the image contrast, a simple model is presented to explain the variation in the electronic coupling component that produces the contrast between the various functional groups observed in the STM images. Additionally, the bias dependence of these STM images has been investigated and the contrast vs bias behavior is related to factors involving electron transfer and hole transfer that have been identified as potentially being important in dominating the electronic coupling in molecular electron transfer processes.

Sensors for detecting analytes in fluids

Abstract: Chemical sensors for detecting analytes in fluids comprise first and second conductive elements (e.g., electrical leads) electrically coupled to and separated by a chemically sensitive resistor which provides an electrical path between the conductive elements. The resistor comprises a plurality of alternating nonconductive regions (comprising a nonconductive organic polymer) and conductive regions (comprising a conductive material) transverse to the electrical path. The resistor provides a difference in resistance between the conductive elements when contacted with a fluid comprising a chemical analyte at a first concentration, than when contacted with a fluid comprising the chemical analyte at a second different concentration. Arrays of such sensors are constructed with at least two sensors having different chemically sensitive resistors providing dissimilar such differences in resistance. Variability in chemical sensitivity from sensor to sensor is provided by qualitatively or quantitatively varying the composition of the conductive and/or nonconductive regions. An electronic nose for detecting an analyte in a fluid may be constructed by using such arrays in conjunction with an electrical measuring device electrically connected to the conductive elements of each sensor.

Electrochemical production of hydroxyl radical at polycrystalline Nb-doped TiO2 electrodes and estimation of the partitioning between hydroxyl radical and direct hole oxidation pathways

Abstract: The use of TiO_2 as a photocatalyst for the destruction of organic chemical pollutants in aqueous systems has been extensively studied. One obstacle to the effective utilization of these systems is the relatively inefficient use of the solar spectrum by the photocatalyst. In addition, light delivery to the photocatalyst can be impeded by UV-absorbing components in mixed effluent streams. We present a novel use of TiO_2 as a catalyst for the oxidative degradation of organic compounds in water that uses a potential source instead of light to generate reactive oxidants. Application of an anodic bias of >+2 V vs NHE to titanium electrodes coated with niobium-doped, polycrystalline TiO_2 particles electrochemically generates hydroxyl radicals at the TiO_2 surface. This process has been demonstrated to efficiently degrade a variety of environmentally important pollutants. In addition, these electrodes offer a unique opportunity to probe mechanistic questions in TiO_2 catalysis. By comparing substrate degradation rates with increases in current density upon substrate addition, the extent of degradation due to direct oxidation and •OH oxidation can be quantified. The branching ratio for these two pathways depends on the nature of the organic substrate. Formate is shown to degrade primarily via a hydroxyl radical mechanism at these electrodes, whereas the current increase data for compounds such as 4-chlorocatechol indicate that a higher percentage of their degradation may occur through direct oxidation. In addition, the direct oxidation pathway is shown to be more important for 4-chlorocatechol, a strongly adsorbing substrate, than for 4-chlorophenol, which does not adsorb strongly to TiO_2.

Array-Based Vapor Sensing Using Chemically Sensitive, Carbon Black−Polymer Resistors

Abstract: We describe herein the construction of a simple, low-power, broadly responsive vapor sensor. Carbon black−organic polymer composites have been shown to swell reversibly upon exposure to vapors. Thin films of carbon black−organic polymer composites were deposited across two metallic leads, and swelling-induced resistance changes of the films signaled the presence of vapors. To identify and classify vapors, arrays of such vapor-sensing elements were constructed, with each element containing the same carbon black conducting phase but a different organic polymer as the insulating phase. The differing gas−solid partition coefficients for the various polymers of the sensor array produced a pattern of resistance changes that can be used to classify vapors and vapor mixtures. This type of sensor array resolved common organic solvents, including molecules of different classes (such as aromatics from alcohols) as well as those within a particular class (such as benzene from toluene and methanol from ethanol). The r...

Photoelectrochemical Degradation of 4-Chlorocatechol at TiO2 Electrodes: Comparison between Sorption and Photoreactivity

Abstract: The TiO_2-catalyzed photodegradation of a strongly adsorbing substrate, 4-chlorocatechol, has been investigated as a function of solution concentration and pH at illuminated polycrystalline TiO_2 electrodes operated at a constant current density. The results are compared to the previously determined sorption behavior of 4-chlorocatechol. The initial rates of 4-chlorocatechol photodegradation measured at pH 3, 5, and 8 and solution concentrations of 20, 50, and 200 μM show a linear correlation with the concentration of the sorbed substrate.

Fermi Golden Rule Approach to Evaluating Outer-Sphere Electron-Transfer Rate Constants at Semiconductor/Liquid Interfaces

Abstract: Fermi's golden rule is used to formulate rate expressions for charge transfer of delocalized carriers in a nondegenerately doped semiconducting electrode to localized, outer-sphere redox acceptors in an electrolyte phase. If the charge-transfer rate constant is known experimentally, these rate expressions allow computation of the value of the electronic coupling matrix element between the semiconducting electrode and the redox species. This treatment also facilitates comparison between charge-transfer kinetic data at metallic and semiconducting electrodes in terms of parameters such as the electronic coupling to the electrode, the attenuation of coupling with distance into the electrolyte, and the reorganization energy of the charge-transfer event. Within this framework, rate constant values expected at representative semiconducting electrodes have been evaluated from experimental data for charge transfer from Au electrodes to ferrocene-terminated thiols, to Ru(NH3)53+/2+-terminated thiols, and through bl...

Free-Energy Dependence of Electron-Transfer Rate Constants at Si/Liquid Interfaces

Abstract: Differential capacitance vs potential and current density vs potential measurements have been used to characterize the interfacial energetics and kinetics, respectively, of n-type Si electrodes in contact with a series of one-electron, outer-sphere redox couples. The differential capacitance data yielded values for the electron concentration at the surface of the semiconductor as well as values for the driving force of the interfacial electron-transfer event at Si/CH3OH−viologen2+/+ junctions. The differential capacitance vs potential measurements were essentially independent of the ac frequency imposed on the interface, with linear Bode plots (log|impedance| vs log frequency, at a fixed potential) between ≈103 and ≈105 Hz, with slopes typically between −0.99 and −1.00. The slopes of C-2−E (Mott−Schottky) plots were in excellent agreement with theory, and little frequency dispersion was observed in the x-intercepts of such plots. The conduction band edge of the n-type Si anodes was invariant to within ±40...

Theoretical description of the stm images of alkanes and substituted alkanes adsorbed on graphite

Abstract: A theoretical model based on perturbation theory has been developed to predict the scanning tunneling microscopy (STM) images of molecules adsorbed on graphite. The model is applicable to a variety of different molecules with reasonable computational effort and provides images that are in qualitative agreement with experimental results. The model predicts that topographic effects will dominate the STM images of alkanes on graphite surfaces. The computations correlate well with the STM data of functionalized alkanes and allow assessment of the structure and orientation of most of the functionalized alkanes that have been studied experimentally. In addition, the computations suggest that the highly diffuse virtual orbitals of the adsorbed molecules, despite being much farther in energy from the Fermi level of the graphite than the occupied orbitals, may play an important role in determining the STM image contrast of such systems. I. Introduction The previous article 1 reports high-resolution scanning tunneling microscopy (STM) images of organic molecules adsorbed in ordered monolayers onto highly ordered pyrolytic graphite (HOPG) surfaces. The images reported therein reveal features as small as single atoms and/or functional groups in the adsorbed molecules and provide atomic level information on the electronic coupling profiles of such overlayers. The STM images observed in this work and in studies of related systems 2-5 are, however, dependent on subtle structural changes in the adsorbed molecules and appear to be the result of both electronic and geometric effects. Thus, to fully exploit this imaging technique, it is important to understand how different aspects of the adsorbate and the experimental conditions blend together to create the observed images. In order to study and explain such phenomena for a large number of molecules, this article describes a theoretical model that has been developed to simulate the STM process. The method is accurate enough to be useful and reliable, it is simple enough to provide a facile method for interpretation of the available STM results, and it is cheap enough to be applicable to the systems of interestssmall functionalized alkanes on

Array-based vapor sensing using chemically sensitive, polymer composite resistors

Abstract: We describe herein the construction of simple, low-power, broadly responsive vapor sensors. Insulating polymer-conductor composites have been shown to swell reversibly upon exposure to vapors. Thin films of polymer composites have been deposited across two metallic leads, with swelling-induced resistance changes of the films signaling the presence of vapors. To identify and classify vapors, arrays of such vapor-sensing elements have been constructed, with each element containing either carbon black or poly(pyrrole) as the conducting phase mixed with one of several different organic polymers as the insulating phase. A convenient chemical polymerization of poly(pyrrole) which allows a high degree of processibility is also described. The differing gas-solid partition coefficients for the various polymers of the sensor array produce a pattern of resistance changes that can be used to classify vapors and vapor mixtures. This type of sensor array has been shown to resolve common organic solvents, including molecules of different classes (such as aromatics from alcohols) as well as those within a particular class (such as benzene from toluene and methanol from ethanol). The response of an individual composite to varying concentrations of solvent is shown to be consistent with the predictions of percolation theory. Accordingly, significant increases in the signals of array elements have been observed for carbon black-polymer composites that were operated near their percolation thresholds.

Measurement of interfacial charge-transfer rate constants at n-type InP/CH3OH junctions

Abstract: Steady-state current density vs potential methods have been used to measure interfacial electron-transfer rate constants at n-type indium phosphide/liquid junctions. n-InP/CH3OH-1,1‘-dimethylferrocene+/0, n-InP/CH3OH-ferrocene+/0, n-InP/CH3OH-tetrahydrofuran-decamethylferrocene+/0, and n-InP/CH3OH-1,1‘-diphenyl-4,4‘-dipyridinium2+/+• contacts displayed bimolecular kinetic behavior in which the observed current density was first order in the concentration of electrons at the semiconductor surface and in the concentration of acceptors in the solution. Differential capacitance potential measurements were used to determine the energetics for the charge-transfer process as well as to determine the concentration of electrons at the semiconductor surface as a function of applied potential. These measurements indicated that the voltage dropped across the semiconductor space charge region varied linearly with changes in the Nernst potential of the solution, as expected for an ideally behaving semiconductor/liquid ...

Behavior of Si photoelectrodes under high level injection conditions. 1. Steady-state current-voltage properties and quasi-Fermi level positions under illumination

Abstract: The behavior of the quasi-Fermi levels of electrons and holes at various semiconductor/liquid interfaces has been probed through the use of thin, high purity, low dopant density single crystal Si photoelectrodes. Since standard Air Mass 1.5 illumination is sufficient to produce high level injection conditions in such samples, minimal electric fields can be present near the solid/liquid interface. Under these conditions, efficient charge separation relies on establishment of kinetic asymmetries at the back contacts while effectively sustaining photogenerated carrier concentration gradients in the photoelectrode. These conditions were achieved for Si/CH3OH interfaces in contact with the 1,1‘-dimethylferrocene+/0, cobaltocene+/0, methyl viologen2+/+, and decamethylferrocene+/0 redox couples. For redox couples having energies near the top of the Si valence band, such as 1,1‘-dimethylferrocene+/0, the sample acted like an n-type photoelectrode, yielding large photovoltages for collection of electrons at the ba...

Behavior of Si Photoelectrodes under High Level Injection Conditions. 2. Experimental Measurements and Digital Simulations of the Behavior of Quasi-Fermi Levels under Illumination and Applied Bias

Abstract: Use of thin, nearly intrinsically doped Si electrodes having implanted, interdigitated n+ and p+ back contact points has allowed electrical control over the potential of either electrons or holes in the solid. During potential control at the n+ point contacts, the open-circuit potential of holes could be monitored, while during potential control of the p+ point contacts, the open-circuit potential of electrons was measured. In combination with current density−voltage measurements of either electrons or holes passing through the back contact points, these data allowed a comparison of the behavior of a given carrier type when generated by an applied bias (i.e., as majority carriers) relative to their behavior when generated with band gap illumination of the solid (as minority carriers). Data have been collected for Si/CH3OH junctions having 1,1‘-dimethylferrocene+/0, decamethylferrocene+/0, methyl viologen2+/+, and cobaltocene+/0 as redox couples. These data have been used to validate certain key prediction...

Behavior of Si Photoelectrodes under High Level Injection Conditions. 3. Transient and Steady-State Measurements of the Quasi-Fermi Levels at Si/CH3OH Contacts

Abstract: Real-time measurements of the photovoltage rise and decay at the back of lightly doped, thin, long lifetime Si photoelectrodes were recorded subsequent to a variety of spatial and temporal carrier generation impulses. The functional form of the rising portion of the photovoltage signal is sensitive to charge transport processes, and this signal was used to validate experimentally the hypothesis that charge transport in these samples under high level injection is primarily driven by diffusion, as opposed to drift. The decay of the photovoltage signal back to its equilibrium value yielded information concerning the surface recombination velocity, Sf, of the various Si/CH3OH redox couple contacts. These data validated the relatively high surface quality of the Si/liquid interface in contact with a variety of redox species. Furthermore, the low surface recombination velocities are in agreement with prior theoretical and experimental estimates of interfacial charge-transfer rate constants for semiconductors in...

Electron-Transfer Reactions at Silicon/Liquid Interfaces

Abstract: This article describes theoretical models for, and experimental data on, the rates of interfacial electron-transfer processes at semiconductor/liquid contacts. These systems are of practical interest because such electron transfers are a critical factor in understanding the behavior of photoelectrochemical cells as energy conversion devices. The general principles of these processes, a discussion of past and present experimental data, and a comparison between theoretical expectation and experimental observations on a variety of semiconductor electrode systems, are the main focus of this article.

Sondes de detection d'analytes dans des fluides

Abstract: La presente invention concerne des sondes chimiques permettant de detecter des analytes dans des fluides, lesquelles sondes comprennent un premier et un second element electroconducteur (par exemple, des conducteurs electriques) electriquement couples et separes par une resistance chimiosensible qui cree un passage electrique entre les elements electroconducteurs. La resistance comporte, dans le travers du passage electrique, une alternance de regions non conductrices (faites d'un polymere organique non conducteur) et de regions conductrices (faites d'un materiau conducteur). Cette resistance cree une difference des resistances, entre d'une part les elements conducteurs en contact avec un fluide contenant l'analyte chimique sous une premiere concentration, et d'autre part les elements conducteurs en contact avec un fluide contenant l'analyte chimique sous une seconde concentration differente. Les matrices de telles sondes possedent par construction au moins deux sondes a resistances chimiosensibles differentes creant des differences non homogenes de resistance. La variation de chimiosensibilite entre sondes s'obtient par modification quantitative ou qualitative de la composition des regions conductrices et non conductrices. Il est ainsi possible de construire un renifleur electronique capable de detecter l'analyte dans un fluide en utilisant de telles matrices en relation avec un appareil electrique de mesure electriquement connecte aux elements conducteurs de chaque sonde.