Charge transport in monolayer protected Au and Ag clusters in electrolyte environment and transport properties of a redox-active ionic liquid
Creators: name: Bodappa, Nataraju
Publisher: name: Universität Bern
Year: 2014
Subjects: 540 Chemistry

description: This dissertation compiles interdisciplinary topics broadly in Raman spectroscopy, nanoparticle synthesis, nanoparticle electrochemistry and ionic liquids. Chapter one describes the overview on the two topics Monolayer protected gold and silver cluster synthesis and investigation of their electrochemical and electrical studies and, secondly, physicochemical and charge transport studies of ionic liquids (ILs) ranging from conventional (redox-inactive) to redoxactive ILs. At the end of the chapter the scope of this thesis is described in a problem solving based approach. Chapter two describes the principles and techniques which rationalize in the designed problems and the experiments. In chapter three, the general procedures used in preparing the experiments and the sample preparation for the voltammetric, electrochemical Raman, and Scanning Tunneling Microscopy/ Spectroscopy (STM/STS) experiments are discussed. In chapter four, results are presented in the form of the three publications and one chapter of first results. In the first publication, we have studied Au(55 nm)@SiO2 nanoparticles (NPs) on two low-index phases of gold and platinum single crystal electrodes in ClO4- and SO42- ion-containing electrolytes by both electrochemical methods and in-situ shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS). The blocking of the electrode with surfactants originating from the synthesis of as-prepared SHINERS NPs is shown and an efficient procedure to overcome this problem is introduced, which provides a fundamental platform for the application of SHINERS in surface electrochemistry and beyond. The method is based on a hydrogen evolution treatment of the SHINERS-NP-modified single-crystal surfaces. The reliability of our preparation strategy is demonstrated in electrochemical SHINERS experiments on the potential-controlled adsorption and phase formation of pyridine on Au(hkl) and Pt(hkl). High-quality Raman spectra on these well-defined and structurally carefully characterized single-crystal surfaces were obtained. The analysis of the characteristic A1 vibrational modes revealed perfect agreement with the interpretation of single-crystal voltammetric and chronoamperometric experiments. The study demonstrates that the SHINERS protocol developed in this work qualifies this Raman method as a pioneering approach with unique opportunities for in situ structure and reactivity studies at well-defined electrochemical solid/liquid interfaces. In the second publication, a proof-of-concept study is presented by electrochemically controlled scanning probe experiments performed on tailor-made Au particles of narrow dispersity. In particular the charge transport characteristics through chemically synthesized hexane-1-thiol and 4-pyridylbenzene-1-thiol mixed monolayer protected Au144 clusters (MPCs) by differential pulse voltammetry (DPV) and electrochemical scanning tunneling spectroscopy (EC-STS) are reported. The pyridyl groups exposed by the Au-MPCs enable their immobilization on Pt(111) substrates. By varying the humidity during their deposition, samples coated by stacks of compact monolayers of Au-MPCs or decorated with individual, laterally separated Au-MPCs are obtained. DPV experiments with stacked monolayers of Au144-MPCs and EC-STS experiments with laterally separated individual Au144-MPCs are performed both in aqueous and ionic liquid electrolytes. Lower capacitance values were observed for individual clusters compared to ensemble clusters. This trend remains the same irrespective of the composition of the electrolyte surrounding to the Au144-MPC. The resolution of the energy level spacing of single clusters however, is strongly affected by the proximity of neighboring particles. In the third publication, a redox-active ionic liquid (IL), 1-butyl-1’-heptyl-4,4’-bipyridinium bis(trifluoromethanesulfonyl) imide has been synthesized and its transport processes were investigated. The conductivity and viscosity of the IL, as well as the diffusion coefficients of its components were studied over a 50 °C wide temperature range: for the diffusivity studies, both the pulsed-gradient spin-echo (PGSE)–NMR technique and voltammetric measurements have been applied. The measured data are presented in the paper and are compared to each other. It was found that the diffusion coefficients determined by means of NMR and chronoamperometry measurements are, within the range of experimental error, equal — and they are (in accordance with other ionic liquid studies) higher than what the conductivity or viscosity measurements indicate. The results are interpreted in the light of the existing theories. The measured diffusion coefficients and bulk conductivities can be well interrelated based on the “ionicity” concept (that is, by treating the ionic liquid as a weak electrolyte). In agreement with the empirical Walden rule, a direct comparison between the measured conductivities and viscosities is also possible, for which a hole conduction model is utilized. Based on the fact that both the electrochemical and the NMR measurements yield practically the same diffusion coefficients in the system, there is no evidence that interpretations based in other redox-active IL systems on “homogeneous electron transfer” apply to the system studied here. In chapter 4.4, first results are presented on stabilization of monolayer protected silver clusters (“Ag9MSA7”, MSA= Mercpatosuccinic acid) and their electrochemical and electrospray ionisation mass spectrometry (ESI-MS) investigations. Here, it has been shown that an enhanced stability of “Ag9MSA7” MPCs can be achieved by submitting them to phase transfer into an organic solvent in the presence of sterically bulky counterions. The resulting phase transferred clusters, “Ag9MSA7(TOA)x” show a very high stability in ambient conditions that has enabled for the first time to perform voltammetric investigations. Size exclusion chromatography reveals also a minor yield of clusters with a smaller size during the synthesis of “Ag9MSA7” clusters. The electrochemical studies of the “Ag9MSA7(TOA+)x” (1.63 nm) clusters show an electrochemical gap of 1.19 V with a HOMO-LUMO gap of 0.87 eV. However, as an important result, the combination of the MS investigations with the electrochemically determined HOMO-LUMO gap strongly suggest that the real cluster sizes are larger than the empirically used formula “Ag9MSA7(TOA+)x”. In chapter five, the findings are summarized and an outlook is given of the investigated problems in the view of broader range.