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People in the Department |
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Professor Stephen FletcherHead of Section and Professor of Physical Chemistry
Electrons and MatterHello. My name is Stephen Fletcher and I am a researcher and teacher here in the Department of Chemistry at Loughborough. My personal goal is to understand how electrons interact with matter. This field of activity is called electrochemistry. On the following pages you will find information about my research that is specific to electrochemistry. There is a selection of my recent publications, plus some brief commentaries about various research highlights. My hope is that you will find these pages useful and informative. I begin with a description of my current research interests. There is also a list of my lecture courses, and a list of selected publications from the past decade. A full list of my most recent publications may be found under the heading “hot off the press”, and also includes the ability to download them as pdf files. Finally, just for fun, I have also compiled my all-time top-ten favourite theory papers, and my all-time top-ten favourite experimental papers. Enjoy browsing! Stephen Fletcher
Current Research InterestsUnderstanding how electrons interact with living matter is one of the great scientific challenges of the 21st century. With this in mind, I have recently embarked on a reformulation of the dielectric and quantum theories of electron transfer. This has led to some exciting new results concerning the activation of electron transfer processes, and I am currently applying these results to the photosynthetic reaction centre, with the hope of gaining new insights into that extraordinary biological machinery. Equivalent circuit modelling also continues to excite my interest. In essence, this mathematical technique replaces coupled differential equations with electrical circuit elements, which are easy to visualize and which greatly simplify the modelling of complex systems (provided they are linear). Fortunately, the laws of newtonian mechanics are linear, the laws of quantum mechanics are linear, and the laws of electric circuit theory are linear, so in practice the method has very wide application. In recent times I have successfully applied the method to the design of three-terminal electrochemical cells, to the study of fluctuations in electron transfer theory, and to the characterisation of tunnelling pathways in photosystem II. It is wonderfully adaptable, and a rich source of new ideas. How do crystals form and grow? That question has tantalized the human mind for millennia, and, despite all our efforts, we’re still not sure! I therefore continue to grapple with the development of nucleation-and-growth theory, and have recently completed a thorough study of the thermodynamics of solid-solid interfaces, including a new expression for the temperature and pressure dependence of the surface energy. This result has implications in fields as diverse as the smelting of iron ore, the physics of planetary interiors, and the pressure dependence of electrochemical reactions. In my laboratory, the prevailing ethos is to apply singular and novel technologies to real-world, energy-related problems. In conjunction with my colleagues and students, I have developed microelectrode arrays containing up to 5000 electrodes in parallel (a world record I think), and these are being applied to problems in analytical chemistry, biochemistry, and fundamental research. We have also developed screen-printed porous electrodes (which, unlike bulk porous electrodes, can be addressed by cyclic voltammetry), and we have also built a differential quartz crystal microbalance that generates massograms in parallel with voltammograms. Armed with these technologies, we have recently completed studies on lithium/thionyl chloride batteries, supercapacitors, and redox flow cells. Materials of special interest have included TCNQ, Prussian Blue, carbon nanofibers, ferritin, sodium polysulfide, viologens, quinones and lithium. Lecture Courses
Selected PublicationsRandom assemblies of microelectrodes (RAM™ electrodes) for electrochemical studies. Stephen Fletcher & Michael David Horne, Electrochemistry Communications, 1, 502-512 (1999). The relationship between the electrochemistry and the crystallography of microcrystals. The case of TCNQ (7,7,8,8-tetracyanoquinodimethane). Alan M. Bond, Stephen Fletcher and Peter G. Symons, Analyst, 123, 1891-1904 (1998). Directed assembly of multilayers —the case of Prussian Blue. Roy C. Millward, Claire E. Madden, Ian Sutherland, Roger J. Mortimer, Stephen Fletcher and Frank Marken, Chem. Commun., 19, 1994-1995 (2001). Voltammetry at carbon nanofiber electrodes. Frank Marken, Mark L. Gerrard, Ian M. Mellor, Roger J. Mortimer, Claire E. Madden, Stephen Fletcher, Katherine Holt, John S. Foord, Ralf H. Dahm, Frank Page, Electrochemistry Communications, 3, 177-180 (2001). The use of massograms and voltammograms for distinguishing five basic combinations of charge transfer and mass transfer at electrode surfaces. Graeme A. Snook, Alan M. Bond, and Stephen Fletcher, Journal of Electroanalytical Chemistry, 526, 1-9 (2002). The direct electrochemistry of ferritin compared with the direct electrochemistry of nanoparticulate hydrous ferric oxide. Frank Marken, Dimple Patel, Claire E. Madden, Roy C. Millward and Stephen Fletcher, New J. Chem., 26, 259-263 (2002). The deconvolution of nucleation and growth rates from electrochemical current–time transients. Linear-Scan Anodic Stripping Voltammetry with Thin-Film Electrodes: Theory of the Stripping Stage and Experimental Tests. Jörg Schiewe, Keith B. Oldham, Jan C. Myland, Alan M. Bond, Victoria A. Vicente-Beckett, and Stephen Fletcher, Anal. Chem., 69, 2673-2681 (1997). Extracting nucleation rates from current-time transients. Comments on three papers by Abyaneh and Fleischmann published in this issue. Stephen Fletcher, Journal of Electroanalytical Chemistry, 530, 105-107 (2002) A voltammetric study of direct electron transfer to cytochrome c using a very large assembly of carbon microelectrodes. Michael Kudera, Yasue Nakagawa, Stephen Fletcher and H. Allen O. Hill, Lab on a Chip, 1, 127-131 (2001). The two-terminal equivalent network of a three-terminal electrochemical cell. Stephen Fletcher,
Electrochemistry Communications, 3, 692-696 (2001). Nucleation-growth kinetics of the oxidation of silver nanocrystals to silver halide crystals. Ulrich Hasse1, Stephen Fletcher and Fritz Scholz, Journal of Solid State Electrochemistry, 10, 833-840 (2006). The thermodynamics of solid-solid interfaces in systems of fixed mass. Stephen Fletcher, Australian Journal of Chemistry, 58, 302–305 (2005). The catalysis of solid state intercalation processes by organic solvents. Graeme A. Snook, Alan M. Bond and Stephen Fletcher, Journal of Electroanalytical Chemistry, 554, 157-165 (2003). [Special issue in memory of Professor M.J. Weaver.] HOT OFF THE PRESS!These papers are available for download from our institutional repository. FLETCHER, S., 2007. A non-Marcus model for electrostatic fluctuations in long range electron transfer. Journal of Solid State Electrochemistry, 11(7), pp 965-969 (2007). FLETCHER, S., 2007. The new theory of electron transfer. Thermodynamic potential profiles in the inverted and superverted regions. Journal of Solid State Electrochemistry, 12(6), pp 765-770 (2008). FLETCHER, S., 2008. The new theory of electron transfer. Application to the photosynthetic reaction centre. Journal of Solid State Electrochemistry, 12(11), 1511-1520 (2008). FLETCHER, S., 2008. Tafel Slopes from First Principles. Journal of Solid State Electrochemistry, Recent Book ChapterNot a chapter per se, but actually a series of contributions to the brand new Electrochemical Dictionary (edited by Allen Bard, György Inzelt, and Fritz Scholz, and published by Springer). If the spirit moves you, check my entries out. They can be found at http://dspace.lboro.ac.uk/dspace/handle/2134/3108. Better yet, buy the book! It's terrific. My Favourite Theory PapersHere are my favourite theory papers. Six of them involve nucleation, two involve equivalent circuits, one involves conducting polymers, and one involves electron transfer theory. Although the list largely reflects my own personal interests, it also reflects (to a small extent) the history of electrochemistry in the late twentieth century. From 1945-1965, mainstream electrochemistry was dominated by the study of soluble reactants, while phase transformations were generally viewed as a quirky sub-field. After 1965, however, there was a correction, and the study of phase transformations moved towards centre-stage. Today, some decent models of electrochemical phase transformations have finally been sketched out, but much of the published literature remains deeply flawed, and there are many gaps in our knowledge. For example, the voltammetric theory of solid state phase transformations lags far behind the corresponding theory of soluble species. Over the years I have mainly focussed on the early stages of first order phase transformations, which are dominated by nucleation. The coupling of nucleation processes with subsequent phase growth has also proved to be fertile ground. Regarding equivalent circuit analysis, the real action is not in the actual components, as most people think, but in the way the components are connected. Thus, by varying the connections, it is possible to explore large numbers of models in a very short space of time. The technique has some similarities with the Laplace transform method. Rather than solve a complex problem by lengthy analysis, one simply takes Laplace transforms, solves the problem quickly in Laplace space, and then reverts to the original problem armed with the answer. It is exactly the same with equivalent circuits. One maps a problem into equivalent circuit components, quickly tests all the possible connections, and then finally revert backs to the original problem armed with the right answer, without having to crunch masses of equations. Its neat. The synthesis of electroactive polyaniline films by Diaz and Logan in 1980 initiated an explosion of interest in conducting polymers. Before that, electrochemists had only two types of charge carrier to play around with, electrons and holes. Suddenly, with the advent of air-stable conducting polymers, polarons became readily available, and a whole new field of activity was born. One of my contributions to the ensuing frenzy was to explain why the capacitance of conducting polymers increased with film thickness, when the classical formula for the capacitance of a thin film suggested just the opposite. It was quite a puzzle at the time, but, like all good puzzles, the solution was “obvious” once you’d got the answer. Another important contribution was to show that conducting polymers in electrolyte solutions behaved as porous electrodes, an idea which has survived the test of time. Finally, I come to electron transfer theory. This is a difficult area to get into, because the interaction between quantum mechanics and dielectric theory is both obscure and highly nuanced. Or, as Lars Onsager once remarked, “There are more pitfalls in the theory of dielectrics than there are zeroes in the Riemann zeta function”. However, in the last couple of years, I’ve finally started to get to grips with this slippery subject, and research is now going well. As I mentioned at the top of this page, understanding how electrons interact with living matter is going to be one of the great scientific challenges of the 21st century, and if we are going to make any progress at all we will need a sustained effort to merge quantum mechanics with bio-electrochemistry. My personal efforts are currently directed at photosynthesis, although the ultimate target must be the human brain. That is the most fascinating electrochemical device of them all. Which is my favourite theoretical paper? Probably, “The two-terminal equivalent network of a three-terminal electrochemical cell.” I like this paper so much because it fulfils my psychological expectation that a good theory should be a “surprise machine”. In this case the main result is still a shock even when you know the answer. (The three terminal system exhibits inductive behaviour, even though there is no inductor in the system!) And all the results have great practical utility, because they tell you how to obtain data from electrochemical cells while minimizing the effects of artefacts. So here’s my final list. See what you think. 1. S. Fletcher, D. Gates, C.S. Halliday, T. Lwin, G. Nelson and M. Westcott, Response of Some Nucleation-Growth Processes to Triangular Scans of Potential. Journal of Electroanalytical Chemistry, 159, 267 (1983). 2. S. Fletcher, A New Formula for the Electrical Current-Time Behaviour of Two-Dimensional Nucleation/Growth/Collision Processes. Journal of Electroanalytical Chemistry, 195, 417 (1985). 3. S. Fletcher, Electrochemical Deposition of Hemispherical Nuclei Under Diffusion Control: Some Theoretical Considerations. Journal of the Chemical Society, Faraday Transactions I, 79, 467 (1983). 4. R.L. Deutscher and S. Fletcher, Nucleation on Active Sites: 5. The Theory of Nucleation Rate Dispersion. Journal of Electroanalytical Chemistry, 277, 1 (1990). 5. S. Fletcher, Contribution to the Theory of Conducting Polymer Electrodes in Electrolyte Solutions. J. Chem. Soc. Faraday Trans (I). 89, 311-320 (1993). 6. S. Fletcher, Tables of Degenerate Electrical Networks for use in the Equivalent Circuit Analysis of Electrochemical Systems. J. Electrochem. Soc. 141, 1823-26 (1994). 7. A.M. Bond, S. Fletcher, F. Marken, S.J. Shaw and P.G. Symons, Electrochemical and x-ray diffraction study of the redox cycling of nanocrystals of 7,7,8,8-tetracyanoquinodimethane. Observation of a solid–solid phase transformation controlled by nucleation and growth, J. Chem. Soc., Faraday Trans. (I), 92, 3925-3933 (1996). 8. R.L.Deutscher and S. Fletcher, The Deconvolution of Nucleation and Growth Rates from Electrochemical Current-Time Transients. J. Chem. Soc Faraday Trans. (I), 94, 3527-3536 (1998). 9. S. Fletcher, The two-terminal equivalent network of a three-terminal electrochemical cell, Electrochem. Commun., 3, 692-696 (2001) 10. S. Fletcher, A non-Marcus model for electrostatic fluctuations in long range electron transfer. Journal of Solid State Electrochemistry, 11, 965-969 (2007). My Favourite Experimental PapersHere are my favourite experimental papers. Looking at them in retrospect, I find it interesting to note that they were all driven by industrial problems. This reveals the value of maintaining strong industrial links even if you are a theorist. The first paper, showing the occurrence of nucleation and growth in the deposition of viologens, was important at the time because it explained the slow response of this class of electrochromic devices. There was also a hidden sub-text to this paper — not only had nucleation been missed by previous workers, but also it had probably been missed in many other systems too. This idea was resisted by many senior electrochemists at the time (who regarded electrochemical theory as essentially complete) but eventually they were forced to change their minds. The arrival of high power mosfets in the early nineteen eighties made fast switching of high currents possible. Together with my colleagues John Hamilton and Rex Deutscher we developed a new device called a “resistometer”, which allowed the resistance of electrodes to be measured while cyclic voltammetry was actually being carried out. The basic trick was to use such brief current pulses that electrodes didn’t have time to respond faradaically. They did, however, have time to respond ohmically, which allowed their resistance to be measured about thirty times per second. The discovery of nucleation rate dispersion pleased me as much as any discovery in my life. Having the idea was one thing, but proving it was another. This was an epic struggle, which not only required the development of functioning microelectrode arrays, but also required experimental skills of a very high order. Fortunately, in the person of Rex Deutscher, I found a first-class colleague to help me, and between us we managed to accomplish our goal. Basically, we found that electrode surfaces present a patchwork of different surfaces to solutions, on which the nucleation rate differs by many millions of times, depending upon location. The reason for this extraordinary behaviour is that the nucleation rate is phenomenally sensitive to small (and unavoidable) variations in surface energy. Another project which also involved Rex, was the development of pH-sensitive surfaces on microelectrode arrays. This was brought to a high state of development by immobilizing quinones on the exposed edge planes of carbon fibre electrodes, an idea which seems to get re-discovered about every five years or so. The miniaturisation of electrodes and cells was a major goal for my research group throughout the nineteen eighties, and this inevitably led us to confront issues of stray capacitance and mains interference (noise). Basically, as electrodes get smaller, the faradaic currents get smaller, and so the pick-up of mains interference becomes more and more problematic. In the limit, experimental data are buried in noise. I finally developed two strategies for minimizing this problem in 1990/91. The first was to design a digital filter that actually discriminated against mains interference, and the second was to design a new kind of reference capillary to replace the traditional Luggin capillary, which is horribly sensitive to mains pick-up. In combination, these two methods greatly enhanced the signal-to-noise ratio of microelectrode measurements. For me, the main highlight of the decade 1990-2000 was the development of random assemblies of microelectrodes. Mike Horne and I successfully developed that technology despite a chorus of voices telling us that “microelectrode arrays don’t work”. Previously, several famous research groups had tried to make working arrays and failed. The problem was that everyone was placing the electrodes too close together, and so our 1999 paper went to great lengths to spell out the importance of proper inter-electrode spacing. If you got that right, they worked like a dream! Another application of microelectrode assemblies, which somehow got lost along the way, was that they could be used as viscometers, since, by the Stokes-Einstein relation, the diffusional steady state current was inversely proportional to the viscosity of the solution. Vicky Vicente-Beckett and I explored that idea in some detail, but to our dismay our paper got rejected by the Electrochemical Society, for reasons we couldn’t fathom. So all we have to show from that beautiful idea is a little abstract. The Becher process involves the aerial oxidation of reduced ilmenite to synthetic rutile, which is a crucial feedstock for the Australian titanium industry. My earlier experience with quinones as pH sensitive probes led me to the idea that anthraquinones might be adapted for use as redox catalysts for accelerating the aeration step of the Becher process, and so it proved. For many years, screen-printed electrodes were restricted to non-porous carbons and low currents. In collaboration with Nick van Dijk, we have recently developed thin-film, porous carbon electrodes that have high reproducibility (± 10%). These novel electrode architectures have immediately found a range of uses, including supercapacitor development, biosensors, ac impedance measurements, and, for the first time, cyclic voltammetry on porous electrodes. These electrodes have a very bright future. This completes my list of top-ten experimental papers. I’ve only got one more thing to add — and that is to tell the moral of the tale. Basically, if you want to do first class laboratory work, make sure you have first class colleagues! 1. Nucleation and Charge-Transfer Kinetics at the Viologen/SnO2 Interface in Electrochromic Device Applications. S. Fletcher, L. Duff and R.G. Barradas, Journal of Electroanalytical Chemistry, 100, 759 (1979). 2. Invention of Cyclic Resistometry. R.L. Deutscher, S. Fletcher and J.A. Hamilton, Electrochimica Acta, 31, 585 (1986). 3. Nucleation on Active Sites: 4. Invention of an Electronic Method of Counting the Number of Crystals as a Function of Time; and the Discovery of Nucleation Rate Dispersion. R.L. Deutscher and S. Fletcher, Journal of Electroanalytical Chemistry, 239, 17 (1988). 4. R.L. Deutscher, and S. Fletcher, Undercutting of Painted Metals: development of a pH-Sensitive Surface on a Microelectrode Array, CSIRO-MPC/M-216, Dec. 1990, 13 pp. (restricted report). 5. Magic Sampling — A Digital Sampling Strategy that Discriminates against Mains Interference (Noise). R.L. Deutscher and S. Fletcher, Electrochimica Acta, 35, 1797 (1990). 6. A Reference Half-Cell Capillary that Improves the High Frequency Performance of the Potentiostat/Whole-Cell Combination. S. Fletcher and M. D. Horne. J. Electroanal. Chem., 297, 297 (1991). 7. A New Kind of Viscometer Based on the Electrochemical Measurement of Diffusion-limited Currents at Microelectrodes and Microelectrode Arrays. S. Fletcher, R.L. Deutscher, and V.A. Vicente-Beckett, in “Extended Abstracts”, 184th Meeting of the Electrochemical Society, New Orleans, 10-15 Oct. 1993 (Electrochemical Society, Pennington, N.J. 1993) abstr. vol. 93-2, pp 923-924. 8. Random Assemblies of Microdisks (RAM electrodes) for Electrochemical Studies. S. Fletcher and M.D. Horne. Electrochemistry Communications, 1, 502-512 (1999). 9. W.J. Bruckard, C. Calle, S. Fletcher, M.D. Horne, G.J. Sparrow, A.J. Urban, The application of anthraquinone redox catalysts for accelerating the aeration step in the Becher process. Hydrometallurgy, 73, 111-121 (2004). 10. S. Fletcher and N.J van Dijk, A Screen-Printing Method for the Testing of Activated Carbons, Extended Abstract, Electrochem (2003), Southampton UK.
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