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When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g. AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination http://eprints.soton.ac.uk UNIVERSITY OF SOUTHAMPTON FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES SCHOOL OF CHEMISTRY Nanostructured Palladium Hydride Microelectrodes: from the Potentiometric Mode in SECM to the Measure of Local pH during Carbonation Mara Serrapede Thesis for the degree of Doctor of Philosophy March 2014 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES SCHOOL OF CHEMISTRY Doctor of Philosophy NANOSTRUCTURED PALLADIUM HYDRIDE ELECTRODES: FROM THE POTENTIOMETRIC MODE IN SECM TO THE MEASURE OF LOCAL PH DURING CARBONATION. By Mara Serrapede The detection of local variations of the proton activity is of interest in many fields such as corrosion, sedimentology, biology and electrochemistry. Using nanostructured palladium microelectrodes Imokawa et al. fabricated for the first time a reliable and miniaturized sensor with high accuracy and reproducibility of the potentiometric-pH response. In absence of oxygen, the nanostructured palladium hydride tips are sensitive only to the activity of the protons close to their local environment and they have an almost Nernstian theoretical response with a slope of 58.7 mV/pH (25C) from pH 2 to 14. In the bulk, the lifetime of the palladium hydride sensor is 60 times longer when the solution is saturated with argon than with oxygen. Besides, the open circuit potential (OCP) recorded during the discharge of the hydride is more positive in an oxygenated solution. To unravel the influence of oxygen on the potentiometric response of these tips, we carried out a series of potentiometric and amperometric scanning electrochemical microscopy (SECM) experiments over a range of tip-substrate distances against an inert substrate. Potentiometric SECM experiments in aerated solutions demonstrate that the duration of the hydrogen discharge and tip potential depend on the tip-substrate distance: the closer the tip is to an inert substrate, the longer the lifetime of the sensor is, and the more cathodic the open circuit potentials are. Linear sweep voltammetry (LSV) near the OCP values reveals that the polarization resistance decreases when the tip approaches the substrate. These trends are confirmed by Tafel plots recorded over a range of tip-substrate distances. Potentiometric and amperometric measurements are found to be in good agreement. These results can be analysed in terms of a mixed potential theory as used in corrosion. They reveal that in the potentiometric mode, despite being held at zero current, the tips promote the reduction of oxygen which in turns leads to the rapid discharge of hydrogen from the palladium hydride. The closer the tip is to the substrate, the smaller is the flux of oxygen, the longer is the duration of the discharge and the more negative is the OCP. This dissertation will therefore show that even in a potentiometric SECM experiment where the tip is supposed to be a passive probe, hindered diffusion can affect the tip potential and produce a dependence on the tip-substrate distance. In aerated solutions, a simple correction can be made to bulk experiments. In this study the exceptional potentiometric properties of pH microprobes made with nanostructured palladium hydride microelectrodes are reported to demonstrate their application by monitoring pH variations resulting from a reaction confined in a porous medium. Their properties were validated by detecting pH transients during the carbonation of Ca(OH)2 within a fibrous mesh. Experimental pHs recorded in situ were in excellent agreement with theoretical calculations for the CO2 partial pressures considered. Results also showed that the electrodes were sufficiently sensitive to differentiate between the formation of vaterite and calcite, two polymorphs of CaCO3. These nanostructured microelectrodes are uniquely suited to the determination of pH in highly alkaline solutions, particularly those arising from interfacial reactions at solid and porous surfaces. I List of contents List of symbols ................................................................................................................. XV Abbreviations ................................................................................................................ XVII 1. Introduction .................................................................................................................. 1 1.1. The carbonation reaction ....................................................................................... 2 1.1.1. The theory about the carbonation .................................................................. 2 1.1.2. The measurement of the carbonation ............................................................. 4 1.2. The basic theory on potentiometric probes ......................................................... 10 1.3. pH measurements with potentiometric sensors ................................................... 12 1.3.1. Conventional ionophore based pH electrodes .............................................. 13 1.3.2. Ionophore based pH electrodes with solid contact ...................................... 16 1.3.3. Ion-selective field effect transistors (ISFET) pH sensors ............................ 17 1.3.4. Metal and metal oxides pH electrodes ......................................................... 18 1.4. The palladium-hydrogen system ......................................................................... 24 1.4.1. The solid-gas system .................................................................................... 24 1.4.2. The phases of the palladium hydride electrode............................................ 28 1.5. Scanning Electrochemical Microscopy ............................................................... 37 1.5.1. Historical Background ................................................................................. 37 1.5.2. Principles of SECM ..................................................................................... 38 1.5.3. Potentiometric probes in SECM .................................................................. 41 1.5.4. Applications of potentiometric pH detection in SECM ............................... 42 1.6. Structure of the thesis .......................................................................................... 44 2. Experimental .............................................................................................................. 45 2.1. Reagents .............................................................................................................. 45 2.2. Preparation of the solutions for pH measurements ............................................. 46 2.2.1. Phosphate buffers ......................................................................................... 46 2.2.2. Neutralization of 1 M NaOH with H2SO4.................................................... 46 II 2.2.3. Titration in a solution containing 1 M NaOH and 50 mM Na3PO4 ............. 46 2.3. Scanning electron microscope ............................................................................. 46 2.4. Electrochemical instrumentation ......................................................................... 47 2.5. Electrochemical cells........................................................................................... 47 2.6. Electrodes ............................................................................................................ 52 2.6.1. Reference electrodes .................................................................................... 52 2.6.2. Counter electrodes ....................................................................................... 53 2.6.3. Working electrodes ...................................................................................... 53 2.7. Deposition and characterization of the palladium film ....................................... 56 2.7.1. Preparation of the plating mixture ............................................................... 56 2.7.2. Electrochemical deposition of H1-e Pd film on platinum microdiscs .......... 58 3. PdH microelectrodes as pH probes: the alkaline region and the role of oxygen. ...... 64 3.1. Introduction to the PdH microelectrodes ............................................................ 64 3.2. How to obtain a PdH-pH probe? ......................................................................... 65 3.2.1. Potentiostatic loading ................................................................................... 66 3.2.2. Galvanostatic loading................................................................................... 67 3.3. How does the PdH-pH probe work? ..................................................................