Associate Professor Kaido Tammeveski, University of Tartu
Associate Professor Lasse Murtomäki, Aalto University, Finland
The main purpose of the PhD thesis was to modify the surface of different electrode materials including glassy carbon (GC), gold, highly oriented pyrolytic graphite (HOPG) and a novel carbon material, graphene, grown by chemical vapour deposition (CVD) method via electrochemical reduction of aryldiazonium salts to further investigate the morphological and electrochemical properties of these aryl-modified electrodes. For the characterisation, various surface analytical (X-ray photoelectron spectroscopy, atomic force microscopy, electrochemical quartz crystal microbalance, high-resolution scanning electron microscopy, Raman spectroscopy, ellipsometry) and electrochemical methods (linear sweep voltammetry, cyclic voltammetry, electrochemical impedance spectroscopy, the rotating disk electrode method) were employed. The electrochemical behaviour of different redox probes including ABTS, Fe(CN)63-/4- and in addition, the oxygen reduction reaction in alkaline medium was studied. In the first part of the work, the electrochemical results revealed that the response of ABTS was independent of pH on bare and 4-nitrophenyl-modified GC electrodes with higher surface coverage, whereas some differences were observed for other, 4-carboxyphenyl and reduced nitrophenyl films, modified GC electrodes. In the next part, a comparative study between GC and Au surface electrografted with three different azobenzene diazonium salts indicated the multilayer formation on both substrates and from all these aryl films studied, the azobenzene-modified GC and Au electrodes showed the best blocking action towards the ferricyanide redox probe and oxygen reduction. In general, the electrochemical behaviour between aryl-modified GC and Au substrates was different indicating more loosely packed aryl films on Au than on GC surface. In addition, it was found that the degradation of aryl layers on GC and Au electrodes by OH● radicals generated by UV photolysis and hydrogen peroxide was faster from Au than from GC surface. The final part of the work revealed that the electrochemical behaviour between HOPG and multilayer graphene grown by CVD was rather similar. After the redox grafting of thick 9,10-anthraquinone (AQ) layers, the AQ layer thickness on different carbon substrates varied between 7 and 60 nm. In addition, the GC electrodes modified with thick AQ layers had excellent blocking properties towards the Fe(CN)63-/4- redox probe compared with AQ-grafted HOPG and graphene-based electrodes. In addition, the AQ-film increased the electrocatalytic activity of HOPG and graphene for oxygen reduction. In conclusion, the morphological and electrochemical properties of these aryl films depended greatly on the aryldiazonium salt, modification procedure and the underlying substrate used.