John B. Henry

The formation and characterisation of redox active and luminescent materials from the electrooxidation of indolizine

Indolizine has been synthesised on the small scale with enhanced yield using a novel Flash Vacuum Pyrolysis method. Electrooxidation of indolizine results in the formation of a redox-active film on the electrode surface. Excellent agreement is found between calculated and experimental indolizine oxidation potentials; a combination of fluorescence and electrochemical studies are consistent with the computational prediction that electroxidation results in the formation of three specific and redox active indolizine dimers. The insoluble redox active film is considered to be polymeric and to arise from the further oxidation of these dimers at the electrode. This combination of methods can be used for the characterisation of products formed from the electrooxidation of novel luminescent heteroaromatics synthesised on a small scale and particularly those of interest as redox active species for electrochemical processes and devices.

Calculation of the Redox Properties of Aromatics and Prediction of Their Coupling Mechanism and Oligomer Redox Properties

The B3LYP density functional theory method has been used to determine theoretical values for the peak oxidation potentials for a range of redox-active aromatics in acetonitrile at room temperature. Excellent agreement to within 37 mV is found between these values and those observed experimentally. The calculated electron spin density distributions of indole monomer and indole oligomer radical cations also enable a plausible mechanism to be advanced by which the experimentally observed asymmetric trimer product is formed. Theoretical values for the peak oxidation potential of the indole trimer also show excellent agreement with those observed previously in electrochemical studies, again consistent with this asymmetric trimer product. Together, with the previously demonstrated ability of this approach to predict the coupling mechanisms and redox properties of the oligomers formed from indolocarbazole, these calculations provide a method for the in silico screening of molecular properties to inform molecular materials design and electrosynthesis.

Electrochemical Characterisation of Copper Thin‐Film Formation on Polycrystalline Platinum

Electrochemically formed thin films are vital for a broad range of applications in virtually every field of modern science and technology. Understanding the film formation process could provide a means to aid the characterisation and control of film properties. Herein, we present a fundamental approach that combines two well-established analytical techniques (namely, electrochemical impedance spectroscopy and electrogravimetry) with a theoretical approach to provide physico-chemical information on the electrode/electrolyte interface during film formation. This approach allows the monitoring of local and overall surface kinetic parameters with time to enable an evaluation of the different modes of film formation. This monitoring is independent of surface area and surface concentrations of electroactive species and so may allow current computational methods to calculate these parameters and provide a deeper physical understanding of the electrodeposition of new bulk phases. The ability of this method to characterise 3D phase growth in situ in more detail than that obtained by conventional approaches is demonstrated through the study of a model system, namely, Cu bulk-phase deposition on a Pt electrode covered with a Cu atomic layer (Cu(ad)/Pt).

Fluorescence enhancement by symmetry breaking in a twisted triphenylene derivative.

1,4,5,8,9,12-hexamethyltriphenylene (HMTP) shows a high photoluminescence quantum yield (PLQY) of 31% in the solid state, making it of interest for luminescence applications. The detailed photophysical properties of HMTP have been investigated by using time-resolved and steady-state luminescence, PLQY, and molar absorption coefficient measurements. An enhancement of the transition dipole moment for fluorescence and absorption was demonstrated compared to the case of unsubstituted triphenylene, which resulted in a 20-fold increase in the radiative decay rate. This is attributed to a breaking of triphenylene symmetry as a result of the necessarily twisted structure induced by steric crowding. In addition, it was shown that HMTP shows similar photoluminescence energies in solution, powder, and film, indicating a reduced propensity for intermolecular π-stacking compared to the case of triphenylene, as a result of this twisted structure. This work also develops a method for calculating the photoluminescence quantum yield of powders by using a calibrated photodiode in combination with an uncalibrated CCD spectrometer.

In depth analysis of complex interfacial processes: in situ electrochemical characterization of deposition of atomic layers of Cu, Pb and Te on Pd electrodes

A combination of cyclic voltammetry, electrogravimetry, and electrochemical impedance spectroscopy has been used to characterize, in situ, the underpotential deposition (UPD) of atomic layers of Cu, Pb and Te on Pd electrode surfaces. This approach provides co-adsorption and competitive adsorption of anions to be measured and quantified during the UPD processes, highlighting the complex competitive processes that can e.g. hinder the design of new catalysts. The formed Cu, Pb and Te atomic layers on the Pd electrode showed no evidence of anion co-adsorption or surface alloying effects, which indicates that these systems, when formed in a perchlorate medium, could act as building blocks for catalysts. The mode of deposition was found to vary greatly for each overlayer. Cu was found to form a compact monolayer on the Pd surface, while Te formed a bilayer structure on the Pd surface, of which ∼1/4 of a monolayer was found to be irreversibly adsorbed. The formation of Pb overlayers was complicated by background UPD of hydrogen and its absorption to the underlying Pd substrate. While perchloric acid media are suitable for the formation of the overlayer, catalytic application of the formed Pb-layers would require a higher pH to negate such processes.

Multiparametric characterization of nonelectroactive self-assembled monolayers during their formation.

The formation of nonelectroactive self-assembled monolayers (SAMs) at the electrode/electrolyte interface was characterized with simultaneous impedance, gravimetric, and direct current measurements. In the presence of specifically adsorbing inorganic ions, this provides key information about the formation of SAMs. Gravimetric measurements allow an estimation of the adsorbate surface coverage; and completion of the assembly process can then be monitored in real-time. Electrochemical impedance spectroscopy measurements play a multifunctional role: they enable elucidation of the physical models of the interface, provide the information about the effective capacitance of SAMs thus probing the dielectric properties of the adsorbed layers, and evaluate the ability of charged electrolyte components to approach the electrode surface through the SAM (using adsorbing/desorbing SO4(2-) as an electroactive probe). The latter is important to assess the extent of defects in the formed organic layers. Finally, monitoring the direct current during SAM formation together with the collected gravimetric data can give additional important information about the process. A series of n-mercaptoalcohols with different hydrocarbon chain length adsorbing at Au electrodes was used as the model object to evaluate the proposed approach.

Electrochemical formation and surface characterisation of Cu2−xTe thin films with adjustable content of Cu

Electrochemically driven “intercalation” of Cu into Te was used to prepare Cu2−xTe (0.2 < x ≤ 2) thin films and accurately control the composition of the resulting samples. A thorough theoretical analysis of the system using density functional theory (DFT) calculations showed that in the absence of external electric fields the driving forces for Cu atoms to move into the subsurface layers of the Te electrodes depend on the surface coverage of copper atoms. The Cu atoms tend to preferentially occupy the subsurface layers in the telluride films. The effective electric charge on Cu atoms inside the Te-electrodes is positive. These effective charge differences with respect to pure Cu and pure Te are only 0.2 e−. Scanning Kelvin probe (SKP), atomic force microscopy (AFM) and electrochemical techniques were used to characterise the surface status of the obtained samples. Both, DFT-calculated work function differences and the SKP-measured contact potential differences (CPD) change non-linearly with the variation of the film composition. Interfacial (solid/liquid) properties evaluated using electrochemical impedance spectroscopy depend on the nominal composition of the samples and display an abrupt change that correlates with a large change in the work function and CPD. While the proposed electrochemical synthetic route can efficiently and accurately control the composition of the Cu2−xTe thin films, SKP-measurements performed under close to ambient conditions in combination with DFT calculations can provide a promising tool to link fundamental surface properties and parameters which define the interface between solids and liquids.

Characterisation of non-uniform functional surfaces: towards linking basic surface properties with electrocatalytic activity

Functional materials, particularly heterogeneous catalysts, are often non-uniform at a microscopic level making their detailed characterisation extremely complex. This complexity inhibits the design and implementation of novel functional materials as such characterisation is a key to understanding interfaces for heterogeneous catalysis. We demonstrate that a combination of Scanning Kelvin Probe (SKP) and Scanning Electrochemical Microscopy (SECM) experiments made over the same sample surface using an integrated SKP–SECM system provides a powerful and robust tool to link basic surface properties with the observed electrocatalytic activity. As the SKP-response can be accurately assessed using modern quantum chemical approaches to benchmark analytical signals for different surface structures with varying compositions, application of an integrated SKP–SECM system can offer valuable insight into the origin of the observed electrocatalytic activity. As model objects, we used Pt(111)-like thin films modified with sub-monolayer and monolayer amounts of Cu atoms located at the electrode surface and in the sub-surface region. The exact position of the Cu atoms relative to the topmost Pt layer greatly affects basic surface properties and governs the electrocatalytic activity of the surface towards various reactions, i.e. the oxygen reduction reaction. SKP–SECM appeared to be a very sensitive tool to monitor those changes as a function of the spatial coordinates.