Surface modification of metal oxide materials with aryldiazonium ions.

Abstract

This thesis describes the development of methods to modify metal oxide surfaces with aryldiazonium ions and the characterisation of the effect of modification on the properties of these materials. SU-8 2005 photoresist surfaces were used as a model system for modifying poorly conducting metal oxides. SU-8 surfaces were modified with nitrophenyl (NP) and methoxyphenyl (MP) groups at pH 7 and pH 10 in aqueous conditions. The thickness of the resulting films was measured by atomic force microscopy (AFM). The grafting reaction at pH 10 was found to have minimal dependence on the aryl substituent whereas at pH 7, MP-films required over 30 min to reach uniform coverage. It is proposed that the formation of the diazohydroxide intermediate at pH 7 is more favoured for the aryldiazonium ions with nitro substitution than methoxy substitution. In non-aqueous conditions, no grafting was observed by AFM after 1 h reaction time in the absence of reducing agent. In the presence of ferrocene as a reducing agent, scattered aggregates were found across the surface, suggesting the grafting reaction was rapid and uncontrolled. These conditions were then adapted to modify MnO₂ nanoparticles and electrodeposited films and the effect on their electrochemical behaviour as pseudocapacitance materials was investigated. X-ray photoelectron spectroscopy (XPS) showed that attachment of aryl layers to the MnO₂ nanoparticles occurred via covalent bonding though surface O species. The capacitance of MnO₂ films was found by cyclic voltammetry (CV) to generally increase after modification at 50 mV s⁻¹ scan rate but decrease at 500 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) measurements indicated that the aryl layer introduced a diffusive resistance in the electrode, limiting charge storage at high scan rates. The origin of the increase in capacitance at low scan rates could not be identified, however, it is proposed that modification of the surface may introduce gap states that provide additional charge storage. MBE-grown and bulk single crystal ZnO surfaces were modified at pH 8 in aqueous conditions with aryldiazonium ions. MBE-grown surfaces were modified electrochemically and non-electrochemically with a range of aryldiazonium ions, with electrochemical modification giving increased grafting density. These electrochemical modification conditions were then extended to single crystal O-polar, Zn-polar and m-plane ZnO surfaces, however, no signs of aryl layer attachment were observed. XPS analysis of modified MBE-grown and single crystal surfaces showed increased P-content after electrochemical cycling in the presence of aryldiazonium ions, most likely due to adsorption of phosphates from the modification solution. At single crystal surfaces this was found to vary with the extent of O-termination of the crystal phase. Changes in valence band edge position were observed by XPS for some modified single crystal surfaces, however, this did not directly correlate to P-content at the surface, suggesting further modifications are occurring at the surface.