PhD Students, University of Cape Town
To further understand the metal support interaction of Pt with carbon/graphite.
This work is a fundamental study to better understand how platinum "sticks" to carbon. First of all, a good catalyst comprises of an active metal on which a reaction takes place (this active metal reduces the transition energy of the reaction, i.e. creates an alternative pathway for a reaction to proceed). In order to increase the surface area of the active metal, small (nanoparticles) are made and this are then supported on an inert material (like alumina, carbon, titanic, silica, ceria, etc.). However, because we are working at such a small scale (1000 times smaller than the width of a human hair), there are physical forces occurring between the active metal and the support. My project looks at understanding what those forces are, how does the platinum actually "stick" to the carbon material, does the carbon influence the physical properties of the platinum, etc.
Further to this, we modify the carbon by removing some of the graphite sites (graphite is like a metallic form of carbon) and replacing them with oxygen groups (either alcohols and ethers or acids and ethers). How do these modifications change the local electronics and physical properties of the carbon/graphite and does this extend to the platinum metal?
Overall, with the work we can better understand how to engineer catalysts for future energy applications. We can better understand the properties and together with operating conditions of these reactions we can develop better, cheaper and more efficient energy conversion devices (fuel cells, batteries, redox flow cells, etc.).
Abstract: The electrochemical quartz-crystal nanobalance (EQCN) measures in situ mass changes associated with interfacial electrode processes. Real electrodes are not atomically flat, thus their surface roughness affects the conversion of frequency variations (Δf) to mass changes (Δm) associated with electrochemical processes. Here, we analyze Δm associated with the electrochemical H adsorption/desorption and surface oxide formation/reduction on Pt electrodes of gradually increasing surface roughness using the EQCN and cyclic-voltammetry in an aqueous H2SO4 solution. These two interfacial processes are ideal to probe changes in the electrochemically active surface area. The surface roughness of Pt-coated resonators is fine-tuned through Pt electrodeposition and examined using atomic force microscopy. The results acquired using Pt electrodes of increasing roughness factor (1.61 ≤ R ≤ 13.0) reveal a linear relationship between Δm and R. Extrapolation of this relationship to R = 1.00 leads to the determination of Δm associated with H adsorption/desorption and oxide formation/reduction on an atomically flat polycrystalline Pt electrode. The values of Δm associated with these processes are analyzed in terms of the number of H, O, water, and ionic species interacting with each Pt atom of the electrode surface. We find that the charge densities associated with these electrochemical processes and mass variations do not scale up by the same factor. This leads to a modified version of the Sauerbrey equation for Pt electrodes, which takes into account the intrinsic surface roughness.
Pub.: 06 Jul '17, Pinned: 31 Jul '17
Abstract: A unique molecular junction design is described, consisting of a molecular mono- or multilayer oriented between a conducting carbon substrate and a metallic top contact. The sp2 hybridized graphitic carbon substrate (pyrolyzed photoresist film, PPF) is flat on the scale of the molecular dimensions, and the molecular layer is bonded to the substrate via diazonium ion reduction to yield a strong, conjugated C-C bond. Molecular junctions were completed by electron-beam deposition of copper, titanium oxide, or aluminium oxide followed by a final conducting layer of gold. Vibrational spectroscopy and XPS of completed junctions showed minimal damage to the molecular layer by metal deposition, although some electron transfer to the molecular layer resulted in partial reduction in some cases. Device yield was high (>80%), and the standard deviations of junction electronic properties such as low voltage resistance were typically in the range of 10-20%. The resistance of PPF/molecule/Cu/Au junctions exhibited a strong dependence on the structure and thickness of the molecular layer, ranging from 0.13 ohms cm2 for a nitrobiphenyl monolayer, to 4.46 ohms cm2 for a biphenyl monolayer, and 160 ohms cm2 for a 4.3 nm thick nitrobiphenyl multilayer. Junctions containing titanium or aluminium oxide had dramatically lower conductance than their PPF/molecule/Cu counterparts, with aluminium oxide junctions exhibiting essentially insulating behavior. However, in situ Raman spectroscopy of PPF/nitroazobenzene/AlO(x)/Au junctions with partially transparent metal contacts revealed that redox reactions occurred under bias, with nitroazobenzene (NAB) reduction occurring when the PPF was biased negative relative to the Au. Similar redox reactions were observed in PPF/NAB/TiO(x)/Au molecular junctions, but they were accompanied by major effects on electronic behavior, such as rectification and persistent conductance switching. Such switching was evident following polarization of PPF/molecule/TiO2/Au junctions by positive or negative potential pulses, and the resulting conductance changes persisted for several minutes at room temperature. The "memory" effect implied by these observations is attributed to a combination of the molecular layer and the TiO2 properties, namely metastable "trapping" of electrons in the TiO2 when the Au is negatively biased.
Pub.: 02 Jun '06, Pinned: 31 Jul '17
Abstract: Electrochemical deposition of aromatic organic molecules by reduction of diazonium reagents enables formation of molecular layers with sufficient integrity for use in molecular electronic junctions of interest to microelectronics. Characterization of organic films with thicknesses in the 1-10 nm range is difficult with Raman spectroscopy, since most molecular structures of electronic interest have Raman cross sections which are too small to observe as either thin films on solid electrodes or within intact molecular junctions. Layer formation on a 10-nm thick Ag island film on a flat carbon surface (eC/Ag) permitted acquisition of structural information using surface enhanced Raman spectroscopy (SERS), in many cases for molecules with weak Raman scattering. Raman spectra obtained on eC/Ag surfaces were indistinguishable from those on carbon without Ag present, and the spectra of oligomeric molecular layers were completely consistent with those of the monomers. Layer growth was predominantly linear for cases where such growth was sterically allowed, and linear growth correlated strongly with the linewidth and splitting of the C=C phenyl ring stretch. Molecular bilayers made by successive reduction of different diazonium reagents were also observable, and will be valuable for applications of 1-20 nm organic films in molecular electronics.
Pub.: 23 May '17, Pinned: 31 Jul '17
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