Mark J. Henderson

Ion and solvent transfer discrimination at a nickel hydroxide film exposed to LiOH by combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) techniques

Abstract A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) instrument was used to monitor the mobile species transfers associated with the redox processes of thin ( Γ ≅100–150 nmol cm −2 ) α- and β-nickel hydroxide films exposed to aqueous LiOH solution. A comparison of the measured PBD signal with the predicted PBD profiles, calculated by temporal convolution analysis of the current and mass responses, enabled the contributions to redox switching of anion (OH − ) and solvent (H 2 O) transfers to be discriminated quantitatively. The responses from the combined instrument are reconciled in terms of H + deintercalation/intercalation within the nickel hydroxide structure as OH − ions enter/exit the film. Hydroxide ion movement is associated with a counterflux of water. Thin nickel hydroxide films show a gradual α→β phase transformation with continuous voltammetric cycling, especially when the films are exposed to high concentrations of electrolyte. α-Films are characterised by OH − transfers that dominate the H + and H 2 O movements; β-films are characterised by an increased participation of water and protons to the exchange dynamics.

A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) study of a poly(o-toluidine) modified electrode in perchloric acid solution

A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) instrument was used to monitor the effects of film history and redox state of an unhydrated poly(o-toluidine) modified electrode immersed in aqueous perchloric acid (pH=0) solution. The degree of film hydration affected the relative contributions of protons and anions exchanged during the first redox process and the potential of the first anodic voltammetric peak. A distinct proton contribution to the ion exchange during the first redox process was detected during the second and subsequent cycles. Electroneutrality during the second charge transfer step was maintained predominantly by proton expulsion. The determination of the diffusion coefficient of the exchanged species in the first redox process by the time dependence of laser beam deflection delay and by temporal convolution of the current signal are compared. The concomitant changes in the gravimetric and optical signals during film hydration demonstrate the participation of both proton and solvent during redox cycling of poly(o-toluidine).

Chronoamperometric resolution of ion and solvent transfers at a poly(o-toluidine) modified electrode by combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD)

Combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) measurements on short time scales following the application of a potential step are presented. The procedure for evaluating the exchange of mobile species (ions and solvent) is illustrated by application of the combined technique to a poly(o-toluidine) (POT) film redox switching in aqueous HClO4 solutions. A comparison of the predicted PBD profiles calculated from the current and mass responses to that of the measured PBD signal, by temporal convolution analysis, enabled the contributions of anion (ClO4−) and solvent (H2O) transfer at the first redox step to be discriminated quantitatively. The results are consistent with a mechanism where anion exchange involves a counter flux of solvent, and suggest that a film volume constraint is applicable on the time scale of the experiment. During oxidation and reduction, the EQCM indicates a non-equilibrium movement of anions, confirmed by convolution analysis of the associated current and deflection signals.

Combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD): validation of the technique by a study of silver ion mass transport

Abstract A quantitative combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) analysis of the Ag+/Ago deposition/dissolution redox process in aqueous perchloric acid solution was achieved by comparing the cyclic -voltammetric, -gravimetric and -beam deflection responses. The recorded current and mass variation signals were superimposed upon the beam deflection profile by using the mathematical tool of quantitative convolution. The good correlation between the coulometric, gravimetric and optical signals, displayed in either differential or integral form, highlights the complementary nature of the responses from the combined instrument and demonstrates how current, mass and beam deflection represent a single electrochemical process.

Temporal Resolution of Ion and Solvent Transfers at Nickel Hydroxide Films Exposed to LiOH

Abstract A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) instrument was used to monitor mobile species transfers on short time scales following the application of a potential step to potentiostatically deposited α-nickel hydroxide films exposed to aqueous LiOH solution. Upon film oxidation, hydroxide ions enter the film and protons are deintercalated. However, the protons are not detected directly since, upon ejection from the film, they immediately combine with hydroxide ions from the solution to yield water. The relatively rigid film lattice (which allows gravimetric interpretation of the EQCM response) imposes volume constraints. As ions are driven into/out of the film in order to satisfy electroneutrality, this volume constraint causes solvent to be driven out of/into the film by internal pressure. Consequently, solvent transfer is forced into the same time scale as ion transfer and is no longer simply diffusionally controlled. Combining the EQCM/PBD-derived fluxes of individual mobile species with their molar volumes, one can estimate the implied changes in film volume driven by redox switching. These volume changes are relatively small and correspond to a monotonic expansion on film oxidation and a transient expansion then eventual contraction upon film re-reduction. These observations are in the opposite sense to those predicted on the basis of unit cell volumes for crystalline α-Ni(OH)2 and γ-NiOOH, suggesting that the material studied is poorly represented by these idealised structures.

Lead Underpotential Deposition on Polycrystalline Gold Electrode in Perchloric Acid Solution: A Combined Electrochemical Quartz Crystal Microbalance and Probe Beam Deflection Study

A quantitative combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) analysis of Pb underpotential deposition (Pb upd ) in aqueous 0.1 mol dm -3 perchloric acid solution at polycrystalline gold was achieved by comparing the cyclic-voltammetric, -gravimetric, and -beam deflection responses The refractive index responses (PBD) were commensurate with the large variation in index of refraction of Pb(ClO 4 ) 2 with change in concentration (30.5 cm 3 mol -1 ), demonstrating that the technique is remarkably sensitive to monitor the formation of submonolayers of Pb upd . In this work the current and mass variation signals were projected upon the beam deflection profile by using the mathematical tool of quantitative temporal convolution. We show that the overall process of Pb upd occurs in three distinct stages. The first stage consists of water ejection from gold. In the second stage, metal underpotential deposition is accompanied by OH ad displacement followed, in the last step, by water formation. The coupled diffusion coefficient of Pb(ClO 4 ) 2 transferring in 0.1 mol dm -3 HClO 4 solution was evaluated to be 6.25 × 10 -6 cm 2 s -1 .

Multi-approach Electron Paramagnetic Resonance Investigations of UV-Photoinduced Ti3+ in Titanium Oxide-Based Gels

EPR investigations of the photoreduction of Ti(4+) into Ti(3+) under UV irradiation were carried out on three titanium-based materials for which the initial concentration of Ti(4+) was defined in the ternary phase diagram (TiOCl(2), H(2)O, DMF). The kinetics of this photoreduction was monitored at 200 K and related to the initial concentration of Ti(4+) in the solution. This study was complemented by a multi-approach EPR method (pulsed electron paramagnetic resonance (EPR), pulsed electron nuclear double resonance, and hyperfine sublevel correlation spectroscopy (HYSCORE)) with the aim of probing the proton environment of the Ti(3+) ions. Indeed, many species such as H(2)O, OH(-), HCOO(-) are located in the immediate vicinity of Ti(3+). Although we found that a distribution of g tensors was involved, for simplicity, two types of g tensor were used to describe the main features of the EPR signal related to the paramagnetic ions. Additionally, we have evidenced that two kinds of protons are identified next to Ti(3+) species, with specific distances determined from the hyperfine coupling parameters obtained by the HYSCORE method.

The Growth of Self-Assembled Titania-Based Films at the Air–Water Interface

The surfactant-assisted assembly of new titania-based films at the air–water interface has been monitored by time-resolved energy-dispersive reflectometry using X-rays and neutrons. Two films are described. The first, prepared from an aqueous alkaline solution containing Ti(OBun)4 and the cationic surfactant tetradecyltrimethylammonium chloride (C14TAC), shows short-range ordering. In moderately alkaline solution (pH ~ 10), triethanolamine (TEA) was required to complex the titanium alkoxide precursor to prevent immediate heterogeneous precipitation of titania. Reflectometry shows that the assembly is partly structured and comprises only a few layers. The film forms within 30 min of mixing the reagents. No further significant structural changes were detected until, after several hours, severe compression buckling prevented further reflectivity measurement. The second film, obtained from aqueous acidic solution containing Ti(OBun)4 and sodium dodecyl sulfate (SDS), contains material that is highly ordered. Strongly acidic conditions (pH < 1) enabled immediate heterogeneous precipitation of titania to be avoided prior to reaction with the dilute aqueous SDS solution. The film prepared by this route displayed a Bragg peak corresponding to a d-spacing of 35 A. The formation of these films is reconciled in terms of the attraction between the surfactant headgroup and titania above and below the pH of the formal isoelectric point of titania.

Grating induced micelle alignment of mesostructured silica films

A two-dimensional hexagonal film was produced by the evaporation-induced self-assembly route using the triblock copolymer (EO20PO70EO20) as the structure directing agent. The film was deposited between the walls of a grating of 450nm period and 55nm average line depths by dip coating the substrate in the direction perpendicular to the lines of the grating. In the case where film thicknesses were less than the height of grating walls, the orientation of the micelles was directed by the walls of the grating. Observation of the mesostructured film using atomic force microscopy after chemical etching revealed the internal structure.

Study of Titanium Oxide Sol-Gel Condensation Using Small Angle X-ray Scattering

Transparent gels prepared from an acid solution of TiOCl(2) in N,N-dimethylformamide (DMF) and water have been studied by small-angle X-ray scattering (SAXS). The sol-gel transformation of the titanium inorganic polymer was studied as a function of chemical composition of the sol and of the annealing time. Quantitative information was obtained by modeling the SAXS data with the Burford and Beaucage models. From the fits to the data, the radius of gyration of the primary particles, the so-called building blocks, the size xi of the homogeneous objects forming a fractal network in the gel, and the fractal dimension of the gel were obtained. We found fractal dimensions varying between D(f) = 1.75 and 2.2 and a radius of gyration of the building blocks equal to R(g) = 0.46 nm, which remained almost constant for all studied samples. The analysis of the homogeneous domain size xi as a function of the annealing time shows the existence of an incubation time preceding the rapid growth of the aggregates at high titanium concentration.