V.I. Birss

In situ mass and ellipsometric study of hydrous oxide film growth on Pt in alkaline solutions

Abstract The growth and reduction of compact (α-) and hydrous (β-) oxide films on polycrystalline Pt electrodes in aqueous 0.1 M NaOH solutions have been investigated using cyclic voltammetry, as well as in situ ellipsometry and the quartz crystal microbalance (QCMB) techniques. All α-oxide films, formed in base with time at constant potentials up to 1.9 V, or by multi-cycling of the potential, are non-hydrated in nature, even when covered by a thick β-oxide film. Two different forms of α-oxide film are generated, suggested to be PtO at lower potentials (≤1.6 V) and PtO 2 at higher potentials (>1.6 V), based on their different growth rates and optical properties. All β-oxide films are different from the α-oxide, being hydrated to varying extents. Based on the measured refractive index of 2.5, and their mass, thin β-oxide films are suggested to be PtO 2 .H 2 O, when formed using standard growth conditions (2 V/s between 0.5 and 2.8 V). As more β-oxide film is formed, the outer regions become more hydrated ( n as low as 1.8), and the suggested film composition is PtO 2 .1.2H 2 O. Pt β-oxide films are even more hydrated when formed using more negative E − and E + limits during growth (while maintaining the other limit constant at either 0.5 or 2.82 V), resulting in film masses and a refractive index consistent with PtO 2 .3.5H 2 O.

Hydrous oxide film growth on Pt. I. Type I β-oxide formation in 0.1 M H2SO4

Abstract The objective of this work has been to establish the compositional properties of hydrous oxide films formed at polycrystalline Pt electrodes in 0.1xa0mol/l sulfuric acid by multicycling to moderately positive potentials, i.e., less than 1.8xa0V vs. RHE. Under these conditions, both a compact ( α ) and an overlaying hydrous ( β ) oxide film (“Type I”) form. Both QCMB and ellipsometric methods have been used, along with electrochemical techniques, to characterize the α -oxide and the Type I β -oxide film. It has been shown that the α -oxide films are non-hydrated under all conditions, being either PtO or PtO 2 . Very thin β -oxide films formed in the first ca. 30xa0s of growth are suggested to be Pt(OH) 4 . As the β -oxide film thickens with time of growth, it becomes increasingly hydrated, particularly towards its outer surface, with a mass consistent with Pt(OH) 4 ·H 2 O and later with Pt(OH) 4 ·2H 2 O. The increasing water content of the film with thickness can also be viewed as reflecting an increasing extent of film porosity in the outer regions of the film.

Comparison of Ir oxide film redox kinetics in sulfuric and p-toluene sulfonic acid solutions

Ir oxide films were grown and studied electrochemically in 0.4 M H2SO4 and in 0.3 M para-toluenesulfonic acid (TsOH). The equilibrium CV characteristics for films formed in these two solutions were very similar, even though the kinetics of the Ir(+III)/Ir(+IV) charge transfer reaction were ca. 10 times more rapid for films grown and studied in H2SO4 versus TsOH. From the ac impedance response of these films, the same equivalent circuit was found to describe them both. In both solutions, the Ir(+III)/Ir(+IV) reaction rate was found to be inversely proportional to the square of the film charge density (film thickness) and to increase exponentially with increasing potential. Kinetic differences of up to 10 times were again found for films studied at the same dc potential and of similar film charge density, but grown and studied in the two different solutions. Analysis of the impedance data could not distinguish between an electron hopping versus a coupled electron-counter ion transport model. Differences in the nanostructure of the Ir oxide films grown and studied in H2SO4 and TsOH are believed to be at least partly responsible for the observed kinetic differences. Field Emission Scanning Electron Microscopy (FESEM) studies revealed a highly ordered pore structure, with pores of ca. 20–30 nm in diameter, for Ir oxide films grown in H2SO4, while the pore size of films formed in TsOH must be much smaller than this.

Complications associated with kinetic studies of hydrous Ir oxide films

The kinetics of the oxidation and reduction of electrochemically formed Ir oxide films have been examined as a function of the potential limits used, oxide film thickness, solution pH and the degree of reversible aging (diminished kinetics) of the oxide. It is shown first that the oxidation kinetics of the principal Ir(III) to Ir(IV) redox reaction are diminished in acidic solutions if the lower potential limit is set so as to include the kinetically slow process which occurs in the anodic cyclic voltammetric pre-peak, A0. Consistent with this, in alkaline solutions, little evidence for the prepeak is found, reversible aging does not occur and the kinetics do not depend significantly on the lower potential limit. Other results show that, if kinetic measurements are made employing a lower limit more positive than the A0 pre-peak and an upper potential limit below that required to generate Ir(V) and/or Ir(VI) states, the kinetics of the Ir(III)/Ir(IV) process are independent of film thickness in acidic solutions. If anodic steps are made to 1.25 V or more in acidic solutions, the Ir(V)/(VI) states generated can mediate and, hence, accelerate, the oxidation of Ir(III) to Ir(IV), as seen by the unusual shapes of the j/t transients.

Towards a reliable and high sensitivity O2-independent glucose sensor based on Ir oxidenanoparticles

The primary goal of this work is the development of a rapidly responding, sensitive, and biocompatible Ir oxide (IrOx)-based glucose sensor that regenerates solely via IrOx-mediation in both O₂-free and aerobic environments. An important discovery is that, for films composed of IrOx nanoparticles, Nafion® and glucose oxidase (GOx), a Michaelis-Menten constant (K(m)) of 20-30 mM is obtained in the case of dual-regeneration (O₂ and IrOx), while K(m) values are much smaller (3-5 mM) when re-oxidation of GOx occurs only through IrOx-mediation. These smaller K(m) values indicate that the regeneration of GOx via direct electron transfer to the IrOx nanoparticles is more rapid than to O₂. Small K(m) values, which are obtained more commonly when Nafion® is not present in the films, are also important for the accurate measurement of low glucose concentrations under hypoglycemic conditions. In this work, the sensing film was also optimized for miniaturization. Depending on the IrOx and GOx surface loadings and the use of sonication before film deposition, the i(max) values ranged from 5 to 225 μA cm⁻², showing very good sensitivity down to 0.4 mM glucose.

Electrochemical characteristics of dip-coated poly (2-cinnamoylethyl methacrylate) (PCEMA) films on Au surfaces

The electrochemical behavior of poly (2-cinnamoylethyl methacrylate) (PCEMA) coatings, dip-coated on Au electrodes from PCEMA/THF solutions, was investigated primarily in pH 7 buffer solution. The thickness and mass of the PCEMA coatings on Au sputter-coated quartz crystals, established using ellipsometry and mass measurement (QCMB) techniques, were found to initially increase rapidly and then linearly with increasing PCEMA concentration of the deposition solution, while the coating density was found to be higher at the PCEMA/Au interface. The PCEMA coverage of Au can be established from the magnitude of the compact Au oxide film (α-Au oxide) oxidation/reduction charges in pH 7 buffer solution and was found to increase with the concentration of PCEMA in the THF solution. The presence of the PCEMA film on Au was found to promote the growth of a hydrous β-Au oxide film, normally formed at bare Au only when much higher potentials are applied. The stability of the PCEMA coating in basic and acidic solutions, and the resistance of the PCEMA coating to degradation by extreme anodic and cathodic polarization, are also discussed.

Properties of thin polystyrene-poly(2-cinnamoylethyl methacrylate) (PS-PCEMA) copolymeric coatings on gold electrodes

Abstract The characteristics of dip-coated polystyrene–poly(2-cinnamoylethyl methacrylate) (PSn–PCEMAm) copolymeric coatings on Au wire and sputtered Au (on quartz crystals) electrodes have been investigated using TEM, cyclic voltammetry, mass measurements and ellipsometry. TEM studies of PS800–PCEMA600 films, formed from an 86% cyclopentane (CP)/14% THF solvent, without subsequent rinsing, are 70–85 nm thick and are in the form of micelles, ca. 40 nm in diameter. The coverage of Au by the PSn–PCEMAm micellar coating, established using cyclic voltammetry in pH 7 solutions, increases with deposition time and with the concentration of PSn–PCEMAm in the deposition solution. Also, the coverage is higher on smooth sputtered Au versus Au wires and for copolymers containing the greatest number of styrene units, independent of the ratio of the number of PS to PCEMA units. Rinsing the PSn–PCEMAm coating with the block selective solvent after coating deposition rapidly decreases the coating density, thickness and surface coverage. An interesting feature of these coatings is that the copolymer (likely the PCEMA block) promotes the formation of a hydrous Au β-oxide film at anomolously low potentials.