Surface-Enhanced Spectroelectrochemistry using Synchrotron Infrared Radiation

Electrochemical reactions are inherently heterogeneous, occurring at the interface between a solid electrode and an electrolyte solution. Therefore, detailed mechanistic understanding requires the electrode/solution interface (ESI) to be interrogated. Doing so with spectroelectrochemical techniques generally encounters several analytical challenges. Sampling the ESI requires a surface-sensitive spectroscopy capable of addressing a buried interface, placing strong limitations on photon energy and spectroelectrochemical cell design. Furthermore, dynamic measurements are fundamentally limited by the finite rise time of the electrode. For many important processes with characteristic timescales in the milli- to microsecond regime, achieving a suitably low rise time requires the use of an electrode with critical dimensions in the hundreds of micrometers, i.e. a microelectrode. In this thesis, I develop the spectroscopic platform necessary to perform surface-sensitive, time-resolved infrared measurements in the milli- to microsecond regime. I will make the case that an infrared spectroelectrochemical technique, namely attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), is applicable because it is intrinsically surface-sensitive, yields detailed information on molecular structure, and is compatible with a range of electrocatalytic metals. I will show that the small size of the microelectrode requires an unconventional infrared source, namely highly focused synchrotron radiation. This thesis will present the characterization of a new internal reflection element which is fully compatible with ATR-SEIRAS and easily amenable to microfabrication. A custom horizontal microscope endstation will be developed at the mid-IR beamline at the Canadian Light Source. Its general utility beyond the primary goal of this thesis will be demonstrated with imaging experiments of a simple interfacial reaction in a microfluidic device. Finally, a 500 micrometer wide linear microelectrode compatible with ATR-SEIRAS will be fabricated and preliminary kinetic measurements of a model electrochemical process, namely the potential-induced desorption of 4-methoxypyridine, will be discussed

Electrochemical ATR-SEIRAS Using Low-Cost, Micromachined Si Wafers

Thin, micromachined Si wafers, designed as internal reflection elements (IREs) for attenuated total reflectance infrared spectroscopy, are adapted to serve as substrates for electrochemical ATR surface enhanced infrared absorption spectroscopy (ATR-SEIRAS). The 500 μm thick wafer IREs with groove angles of 35° are significantly more transparent at long mid-IR wavelengths as compared to conventional large Si hemisphere IREs. The appeal of greater transparency is mitigated by smaller optical throughput at larger grazing angles and steeper angles of incidence at the reflecting plane that reduce the enhancement factor. Through use of the potential dependent adsorption of 4-methoxypyridine (MOP) as a test system, the microgroove IRE is shown to provide relatively strong electrochemical ATR-SEIRAS responses when the angle of incident radiation is between 50 and 55°, corresponding to refracted angles through the crystal of ∼40°. The higher than expected enhancement is attributed to attenuation of the reflection loss of p-polarized light and multiple reflections within the wafer-based IRE. The micromachined IREs are shown to outperform a 25 mm radius hemisphere in terms of S/N at wavenumbers less than ca. 1400 cm^–1 despite the weaker signal enhancement derived from the steeper angle incident on the IRE/sample interface. The high optical transparency of the new IREs allows the spectral observation of displaced water libration bands at ca. 730 cm^–1 upon solvent replacement by adsorbed MOP. The results are highly encouraging for the further development of low-cost, Si wafer-based IREs for electrochemical ATR-SEIRAS applications

Surface Enhanced Infrared Spectroelectrochemistry using a Microband Electrode

The successful use of a microband electrode printed on a silicon internal reflection element to perform time resolved infrared spectroscopy is described. Decreasing the critical dimension of the microband electrode to several hundred micrometers provides a sub-microsecond time constant in a Kretschmann configured spectroelectrochemical cell. The high brilliance of synchrotron sourced infrared radiation has been combined with a specially designed horizontal attenuated total reflectance (ATR) microscope to focus the infrared beam on the microband electrode. The first use of a sub-microsecond time constant working electrode for ATR surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) is reported. Measurements show that the advantage afforded by the high brilliance of the synchrotron source is at least partially offset by increased noise from the experimental floor. The test system was the potential induced desorption of an adsorbed monolayer of 4-methoxypyridine as measured using step-scan interferometry. Based on diffusion considerations alone, the expected time scale of the process was less than 10 microseconds but was experimentally measured to be three orders of magnitude slower. A defect-mediated dissolution of the condensed film is speculated to be the underlying cause of the unexpected slow kinetics.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Femtomole Infrared Spectroscopy at the Electrified Metal–Solution Interface

Characterization of surface adsorbed species using infrared (IR) spectroscopy provides valuable information concerning interfacial chemical and physical processes. However, <i>in situ</i> infrared studies of surface areas approaching the IR diffraction limit, such as micrometer scale electrodes, require a hitherto unrealized means to obtain high signal-to-noise (S/N) spectra from femtomole quantities of adsorbed molecules. A major methodological breakthrough is described that couples the high brilliance of synchrotron-sourced infrared microscopy with attenuated total reflection surface enhanced infrared spectroscopy (ATR-SEIRAS). The method is shown to allow the spectral measurement of a monolayer of 4-methoxypyridine (MOP) adsorbed on a surface enhancing gold film electrode under fully operational electrochemistry conditions. A factor of 15 noise improvement is achieved with small apertures using synchrotron IR relative to a thermal IR source. The very low noise levels allow the measurement of high quality IR spectra of 2.5 fmol of molecules confined to a 125 μm² beam spot

Spatial Mapping of Methanol Oxidation Activity on a Monolithic Variable-Composition PtNi Alloy Using Synchrotron Infrared Microspectroscopy

The use of synchrotron-sourced infrared radiation to map the electrochemical activity of a binary metal (Pt and Ni) alloy is demonstrated. The alloy is created in such a way that its metal concentration varies along one of its dimensions thus creating a continuum of electrocatalyst compositions on a single electrode. Localized methanol oxidation activity is determined spectroscopically by measuring the rate of CO₂ production at variable positions along the alloy concentration gradient using an infrared microscope. Numerical simulations of the kinetically controlled reaction demonstrate that qualitative assessment of relative reaction rates is possible as long as the reaction is followed on time scales smaller than those that lead to diffusional broadening. Characterization of the alloy before and after electrochemical experiments reveals significant levels of base metal leaching. Highly dealloyed regions of the sample show the highest rates of methanol activity and have a final Ni atomic composition of approximately 5%. Surface roughening from the dealloying process is shown to be at least partially responsible for enhanced activity

Service employee burnout and engagement: the moderating role of power distance orientation

Studies show that service employees are among the most disengaged in the workforce. To better understand service employees’ job engagement, this study broadens the scope of the job demands-resources (JD-R) model to include power distance orientation (PDO). The inclusion of PDO enriches the JD-R model by providing a key piece of information that has been missing in prior JD-R models: employees’ perceptions of the source of job demands (i.e., supervisors) or employees’ views of power and hierarchy within the organization. Study 1 uses a survey-based field study to show that employees with a high (compared to low) PDO feel more burnout due to supervisors when they are closely monitored by their supervisors. Study 1 further supports the finding that employees with high (compared to low) PDO feel less disengagement despite burnout due to supervisors. Study 2, using a lab experiment, and Study 3, relying on a survey-based field study, unveil why these effects were observed. Stress and job satisfaction emerge as mediators that explain the findings from Study 1. Implications of the role of PDO are discussed to improve the current understanding of how job engagement can improve customer service performance