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Title:Investigation of the Surface-Electrolyte Interface of Semiconductors and Metals
Author(s):Butcher, Dennis
Director of Research:Gewirth, Andrew A.
Doctoral Committee Chair(s):Gewirth, Andrew A.
Doctoral Committee Member(s):Suslick, Kenneth S.; Girolami, Gregory S.; Murphy, Catherine J.
Department / Program:Chemistry
Discipline:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):Shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS)
Surface
Adsorption
Photocatalysis
Electroreduction
Nitrate
Semiconductors
Copper
Gold
Abstract:This thesis focuses upon investigating the interface between semiconductors or metals with electrolyte. Understanding the interfacial dynamics are important to improving process performance, whether investigating photocatalytic systems or the nature of adsorption and reaction dynamics on metal surfaces. The first project presents the photoelectrochemical characteristics of TlVO4, a previously uncharacterized candidate for photocatalytic water splitting. Additionally, a composite of InVO4 and TlVO4 was synthesized by a facile solution method using orthorhombic InVO4 as a seed for growth of crystallographically similar orthorhombic TlVO4. Photoelectrochemical measurements indicate an increase in photocurrent and more negative flatband potentials for TlVO4 and the InVO4:TlVO4 composite relative to InVO4. Diffuse reflectance UV-visible measurements were used to determine bandgaps of 3.50 eV, 2.94 eV, and 2.98 eV for InVO4, TlVO4, and InVO4:TlVO4, respectively. DFT calculations were performed to elucidate the band structures and correlate well with experimental data. The results indicate higher photoelectrochemical activity for TlVO4 and the InVO4:TlVO4 composite relative to InVO4. The second project investigates the potential dependent assembly of 2,2’-bipyridine molecules on both Au(100) and Au(111) surfaces using a newly developed SHINERS technique. We present potential dependent SHINERS spectra of 2,2’-bipyridine adsorbed on both surfaces collected under anodic as well as cathodic polarization. A series of processes were characterized by the analysis of the data set with Perturbation Correlation Moving Window Two Dimensional Spectroscopy (PCMW2D) and Two Dimensional Correlation Spectroscopy (2DCOS). Exquisite spectral detail was achieved and allowed for the characterization of complicated ring breathing mode and C-C inter-ring stretching modes that are diagnostic of molecular orientation on the surfaces. Detection of several occluded vibration peaks was also made possible with SHINERS. Analysis reveals that in very negative potentials, 2,2’-bipyridine adsorbs in a disordered, mixed state with both π-flat cis and several different vertically N-bound cis orientations, in contrast to previously published reports. Our findings provide insight into 2,2’-bipyridine adsorption on Au single crystals and also powerfully combine SHINERS with two dimensional correlation analysis to yield a more detailed view of spectral transitions. The third project details the origin of differential nitrate reduction activity between the (100), (111), and (110) faces of Cu using vibrational spectroscopy and calculations. Shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) reveals a suite of intermediates from the nitrate reduction process on Cu(100), Cu(111), and Cu(110) including NO2- and HNO. All three faces show similar intermediates, suggesting the same mechanism is operative on all of them. Critical to the reduction pathway on the bare Cu surfaces is the autocatalytic reduction of nitrate to nitrite concomitant with partial oxidation of the Cu surface. This priming action facilitates nitrate reduction and reduces overpotentials, particularly on the Cu(111) and Cu(110) faces, which are more susceptible to oxidation. Calculations show that the trend in activity between the different single crystal faces of Cu follows the strength of the adsorption of intermediates on the corresponding oxides; this trends was not found in calculations on the bare metal surfaces. Decoration of the surfaces with Cl- suppresses nitrate autocatalysis, resulting in higher overpotentials and lower current density. NH3 is observed by SHINERS as a direct nitrate reduction product in the presence of Cl-, rather than NOx species observed on the bare Cu surfaces, indicating a reaction pathway unique from the bare, undecorated surface. The fourth project examines the coadsorption of water and potassium on a Au(100) surface using variable temperature Scanning Tunneling Microscopy (STM). The two-layer system initially formed on the reconstructed Au(100) through addition of K is converted into a labyrinthine row structure upon the introduction of water. This structure features KOH molecules likely covered with a water adlayer. Density Functional Theory (DFT) calculations provide further insight into the observed labyrinthine striped structures formed by adsorbed KOH. Images obtained following limited introduction of water feature larger unreacted areas and a more disordered row structure.
Issue Date:2013-02-03
URI:http://hdl.handle.net/2142/42434
Rights Information:Copyright 2012 Dennis Butcher
Date Available in IDEALS:2013-02-03
2015-02-03
Date Deposited:2012-12


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