Prof A. Robert Hillman
Professor of Physical Chemistry
B.Sc., (London) D.Phil. (Oxford).
Tel: 0116 2522144
Member of the International Editorial Board of:
Front page article January 14th, 2009
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Innumerable natural processes – from biology to corrosion - take place at interfaces. Interfacial properties are also widely exploited in man-made devices as diverse as sensors, optical displays and lightweight batteries. The ability to understand the relationships between interfacial composition, structure and properties is central to our appreciation of these processes and their exploitation. Since interfacial properties are determined by structure and composition over a distance of a few nanometres or less, interfacial science is necessarily nanoscience.
Present research involves thin films of polymers, biological molecules, metals and inorganic complexes on electrode surfaces. Through the use of electrochemistry as a control vehicle and a range of spectroscopic and other in situ physical probes, the generic goal is the control and determination of interfacial architecture at the nanoscale. By combining the results of diverse techniques, one can correlate interfacial composition, structure and physical / chemical properties in a manner that will permit rational design of interfaces for specific purposes.
Electrochemistry allows precise control over film deposition and subsequent manipulation of film charge state. Acoustic wave methods allow one to monitor the extent and nature of film deposition and subsequent redox switching of the resulting electroactive films. When a piezoelectric resonator launches an acoustic wave into the film, changes in the frequency and amplitude of the resonance are highly sensitive to the mass and viscoelastic properties of the film. For mechanically rigid films, the resonator functions as a microbalance: the frequency shift provides a highly sensitive (nanogram) and rapid (millisecond) measure of the film mass. Thus, when the film is oxidised or reduced, the dynamics of insertion or ejection of ions and solvent can be monitored; this is relevant to lithium insertion in battery materials, capture of an analyte species or release of a drug. For “soft matter” films, the resonator response provides viscoelastic properties and thereby an entry into polymer dynamics, which commonly control the response time in electronic, optical and sensor devices. Viscoelasticity can be correlated with film solvation and microstructure, as illustrated for the crown-ether functionalised polymer poly[Ni(3-Mesalophen-b15-c5)]: the thin film (left) is smooth, compact and stiff, while the thicker film (right) is rough, solvated and soft.
X-ray and neutron methods are used to probe structural details of electroactive thin films. X-rays are more sensitive to the heavier elements (notably metals) and neutrons to the lighter elements (notably hydrogen). Thus, in a metallopolymer film, EXAFS measurements provide information on the local structure around the metal atom and neutron methods provide information on the polymer matrix and the permeating solvent. In each case, this can be explored as a function of electrochemically controlled charge state. Neutron reflectivity (NR), with the benefit of isotopic sensitivity, can provide the spatial distributions of electroactive matrix and solvent with charge state. Recent work has dramatically improved the time resolution of NR measurements, from hours to a few seconds.
The figure right shows the evolution of solvent volume fraction in a polyvinylferrocene film, measured as a function of distance outwards from the underlying electrode surface, during the oxidation (upper panel) and reduction (lower panel) of the ferrocene redox sites.
Since interfaces are where transfer of material occurs when two objects come into contact, their study is of direct relevance to forensic science. We have recently been exploring the development of latent fingerprints on metal surfaces. Contact between a finger and a metal surface brings together metal, water, salt and oxygen – all the constituents of a corrosion reaction. For appropriate metals, this can lead to a localised corrosion reaction that indelibly etches the fingerprint into the metal surface. We have been studying this process for the case of brass, the alloy used for bullet casings.
The AFM image shows a high magnification view of a single ridge on a fingerprint deposit. With appropriate development conditions, this generates a trench. In collaboration with Dr. John Bond of Northamptonshire Police, we are studying the formation of the final etched fingerprint using a combination of spectroscopic and microscopy methods.
Dynamics of polymer films
- “Timescale- and temperature-dependent properties of viscoelastic PEDOT films”, A.R. Hillman, I. Efimov and K.S. Ryder, J. Am. Chem. Soc., 127 (2005) 16611-16620.
- “Solvent effects on the electrochemical p-doping of PEDOT”, A.R. Hillman, S.J. Daisley and Stanley Bruckenstein, PCCP, 9 (2007) 2379-2388.
- “Kinetics and mechanism of the electrochemical p-doping of PEDOT”, A.R. Hillman, S.J. Daisley and S. Bruckenstein, Electrochem. Commun., 9 (2007) 1316-1322.
- “Viscoelastic characterization of benzo-crown ether functionalised electroactive films” , J.A. Tedim, A.C. Freire and A.R. Hillman, Phys. Chem. Chem. Phys., 11 (2009) 268-277.
- “Quartz crystal microbalance determination of trace metal ions in solution”, A.M. Etorki, A.R. Hillman, K.S. Ryder and A.Glidle, J. Electroanal. Chem., 599 (2007) 275-287.
Structural studies of electroactive films
- “Unusual coordination environment for barium cations in ion recognition conducting poly[Ni(salen)(receptor)] films”, J. Tedim, R. Bessada, S. Patrício, A. L. Magalhães, C. Freire, S. J. Gurman and A. R. Hillman, Langmuir, 24 (2008) 8998-9005.
- “Modulation of electroactive polymer film dynamics by metal ion complexation and redox switching”, J.A. Tedim, A.C. Freire and A.R. Hillman, Soft Matter, 5 (2009) 2603–2613.
- "Use of neutron reflectivity to measure the dynamics of solvation and structural changes in polyvinylferrocene films during electrochemically controlled redox cycling", A. Glidle, A. R. Hillman, K.S. Ryder, E.L. Smith, J.M. Cooper, N. Gadegaard, J.R.P. Webster, R.M. Dalgliesh, R. Cubitt, Langmuir, 25 (2009) 4093-4103.
- “High resolution imaging of latent fingerprints on brass surfaces”, A.J. Goddard, A.R. Hillman and J.W. Bond, J. For. Sci., 55 (2010) 58-65.