Richard L. McCreery

Advanced Carbon Electrode Materials for Molecular Electrochemistry

3.6.1. Polishing and Cleaning 2663 3.6.2. Vacuum and Heat Treatments 2664 3.6.3. Carbon Electrode Activation 2665 3.7. Summary and Generalizations 2666 4. Advanced Carbon Electrode Materials 2666 4.1. Microfabricated Carbon Thin Films 2666 4.2. Boron-Doped Diamond for Electrochemistry 2668 4.3. Fibers and Nanotubes 2669 4.4. Carbon Composite Electrodes 2674 5. Carbon Surface Modification 2675 5.1. Diazonium Ion Reduction 2675 5.2. Thermal and Photochemical Modifications 2679 5.3. Amine and Carboxylate Oxidation 2680 5.4. Modification by “Click” Chemistry 2681 6. Synopsis and Outlook 2681 7. Acknowledgments 2682 8. References 2682

Progress with Molecular Electronic Junctions: Meeting Experimental Challenges in Design and Fabrication

Molecular electronics seeks to incorporate molecular components as functional elements in electronic devices. There are numerous strategies reported to date for the fabrication, design, and characterization of such devices, but a broadly accepted example showing structure-dependent conductance behavior has not yet emerged. This progress report focuses on experimental methods for making both single-molecule and ensemble molecular junctions, and highlights key results from these efforts. Based on some general objectives of the field, particular experiments are presented to show progress in several important areas, and also to define those areas that still need attention. Some of the variable behavior of ostensibly similar junctions reported in the literature is attributable to differences in the way the junctions are fabricated. These differences are due, in part, to the multitude of methods for supporting the molecular layer on the substrate, including methods that utilize physical adsorption and covalent bonds, and to the numerous strategies for making top contacts. After discussing recent experimental progress in molecular electronics, an assessment of the current state of the field is presented, along with a proposed road map that can be used to assess progress in the future.

Photoresist‐Derived Carbon for Microelectromechanical Systems and Electrochemical Applications

Photopatterned resists pyrolyzed at different temperatures and different ambient atmospheres can be used as a carbonaceous material for microelectromechanical systems. Carbon films were prepared by pyrolysis of photoresists at temperatures ranging from 600 to 1100°C. The carbon films were characterized by several analytical techniques, viz., profilometry, thermogravimetric analysis. four-point probe measurements, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. In addition, cyclic voltammetry was performed on the carbon film electrodes, and the carbon films were compared to glassy carbon (GC for their electrochemical behavior. Electron-transfer rate constants for the benchmark Fe(CN) 3-/4- 6 and Ru(NH 3 ) 3+/2+ 6 redox systems increased with increasing heat-treatment temperature and approached those observed on GC following-treatment at 1100°C. The pyrolyzed films have low capacitance and background current, approximately one-fourth of that observed on GC. The oxygenicarbon atomic ratio determined from XPS was low (∼1% for I 100°C pretreatment), and increased more slowly upon exposure to air than that for GC treated under identical conditions. Pyrolysis of photoresist films permits photolithographic fabrication of carbon electrode devices, and also appears to yield a carbon film with a smooth surface and unusual surface chemistry.

Corrosion Protection of Untreated AA‐2024‐T3 in Chloride Solution by a Chromate Conversion Coating Monitored with Raman Spectroscopy

This work was supported by the Air Force Office of Scientific Research, contract no. F49620-96-1-0479.

Covalent Bonding of Organic Molecules to Cu and Al Alloy 2024 T3 Surfaces via Diazonium Ion Reduction

Cu surfaces and polished aluminum alloy 2024 T3 substrates were derivatized at open-circuit potential with aryl diazonium salts in both aprotic and aqueous media. Raman spectroscopy confirmed the presence of a derivatized film on the substrates before and after exposure to boiling water and sonication in acetone. Two different Cu substrate surfaces were prepared and used for X-ray photoelectron spectroscopy (XPS) analysis of the derivatization results. One surface was native oxide Cu, predominantly in the form of Cu 2 O, and one surface was predominantly Cu 0 . Results of the XPS analysis indicate the presence of both a Cu-O-C linkage and a Cu-C covalent bond between the aryl ring and the Cu substrate, and a high coverage of the organic layer. XPS results also indicate the formation multilayers on both types of Cu surfaces with different percentages of azo coupling within the multilayers on the two surfaces. Applications of a covalently bonded organic film on copper and alloy surfaces include adhesion promotion, corrosion protection, and possibly inhibition of oxygen reduction.

Effects of Chromate and Chromate Conversion Coatings on Corrosion of Aluminum Alloy 2024-T3

Various effects of chromate conversion coatings (CCCs) and chromate in solution on the corrosion of AA2024-T3 and pure Al are studied in this work. Raman spectroscopy was used to investigate the nature of chromate in CCCs through a comparison with the spectra of known standards and artificial Cr(III)/Cr(VI) mixed oxides. Chromate was shown to be released from CCCs and to migrate to and protect a nearby, uncoated area in the artificial scratch cell. However, experiments investigating the effect of chromate in solution on anodic dissolution kinetics under potentiostatic control indicated that large chromate concentrations were needed to have an effect.

Elucidation of the Mechanism of Dioxygen Reduction on Metal‐Free Carbon Electrodes

On a glassy carbon (GC) surface covered with a covalently bonded methylphenyl monolayer, O 2 reduction to superoxide was observed and shown to be chemically reversible above pH 10. The voltammetry is completely explained by electron tunneling through the organic monolayer, then degradation of O .- 2 in aqueous solution by known homogeneous mechanisms. As the pH is decreased below about 10, O .- 2 in solution decays to O 2 and H 2 O 2 by routes previously deduced from pulse radiolysis experiments. In contrast, a GC surface cleaned with isopropanol and activated carbon is very active toward adsorption, and a two electron reduction to peroxide is observed. An adsorbed intermediate is proposed to be a surface hydroperoxide analogous to stable organic peroxides of the general formula ROOH. On clean, unmodified GC, the pH dependence of the O 2 reduction mechanism is consistent with control of the reduction process by adsorbed O .- 2 or . O 2 H. In the absence of adsorption sites on the carbon surface, degradation of electrogenerated superoxide occurs entirely in solution.

Storage and Release of Soluble Hexavalent Chromium from Chromate Conversion Coatings Equilibrium Aspects of Cr VI Concentration

This work was supported by the Air Force Office of Scientific Research, contract F49620-96-1-0479.

Chemistry of a Chromate Conversion Coating on Aluminum Alloy AA2024‐T3 Probed by Vibrational Spectroscopy

Infrared and Raman spectroscopy were used to characterize a chromate conversion coating (CCC) on 2024-T3 aluminum aircraft alloy with a long range objective of determining the anticorrosion mechanism of the CCC. Spectra were compared to those from synthetic mixed oxides of aluminum, Cr(III), and Cr(VI) made by treating pure reagents with NaOH. The Fourier transform infrared (FTIR) and Raman spectra of the CCC showed similar behavior to the chromium III/VI mixed oxide for both the initial materials and after heat-treatment. Analysis of the CCC and chromium mixed oxide by UV-vis spectroscopy indicated that both have a 3:1 ratio of Cr(III) to Cr(VI). When the chromium mixed oxide was immersed in pH 4 HNO 3 , the ratio of released H + to released Cr(VI) ranged from 0.98 to 1.07. In addition, a compound with a Raman spectrum very similar to that of a CCC was formed by a reaction of Cr(III) hydroxide with Cr 2 O 7 -2 , CrO 4 -2 , or the Alodine chromating bath. The results indicate a strong structural similarity between the Cr-mixed oxide and a major component of the CCC. A likely structure for this common material involves covalent bonding between polymeric Cr(III) hydroxide and Cr 2 O 7 -2 or CrO 4 2 , This mixed oxide structure may hydrolyze to release H + and soluble chromate.

Isotope and surface preparation effects on alkaline dioxygen reduction at carbon electrodes

The reduction of dioxygen in base was examined on several carbon electrode surfaces, particularly polished and modified glassy carbon (GC). Electrochemical pretreatment, fracturing, and vacuum heat treatment shifted the O2HO−2 reduction peak positive, while adsorption of several covalent and physisorbed organic compounds shifted it negative. A reverse wave for O−2 oxidation was observed in tetraethylammonium hydroxide electrolyte, and on GC surfaces preadsorbed with Co(II) phthalocyanine. An HD isotope effect was observed when H2O + KOH and D2O + KOD electrolytes were compared, with the largest effect observed on surfaces exhibiting the most positive reduction peak potential. The results indicate involvement of proton transfer in the rate limiting step of reduction, and a strong dependence of the O2O−2 electron transfer rate on the carbon surface condition. The results support a mechanism involving adsorption of O−2 and associated enhancement of proton transfer from water to O−2. Activation of the dioxygen reduction by surface pretreatment is attributed to increasing the concentration of adsorbed O−2.