Noseung Myung

A combined voltammetry and electrochemical quartz crystal microgravimetry study of the reduction of aqueous Se(IV) at gold

Abstract The reduction of Se(IV) in 0.5 M Na 2 SO 4 is complex and does not proceed via the Se(IV) → Se(0) → Se(II-) scheme considered by most previous authors. A combined use of voltammetry and electrochemical quartz crystal microgravimetry techniques shows that the direct 6e − Se(IV) → Se(II-) reduction pathway competes with the initial four-electron process. Coupling with a subsequent (fast) chemical reaction between Se(II-) and Se(IV) results in the further deposition of Se(0) at the electrode surface. The electrochemical behavior at more negative potentials reflects a complex interplay of the four-electron and six-electron reduction processes and the chemical reaction along with the effect of Se(II-) ions stripped from the initial selenium layer. Thus the delicate balance between these is influenced by two variables, namely the potential and the Se(II-) concentration in the electrolyte. Further data in support of our mechanistic scheme are furnished by measurements with a Au disk electrode oriented “face-down” in the cell as well as by hydrodynamic voltammetry experiments.

Electrochemical deposition and stripping of copper, nickel and copper nickel alloy thin films at a polycrystalline gold surface: a combined voltammetry-coulometry-electrochemical quartz crystal microgravimetry study

Abstract Cyclic voltammetry, anodic stripping voltammetry (ASV) and coulometry were used in conjunction with electrochemical quartz crystal microgravimetry (EQCM) and ex situ X-ray photoelectron spectroscopy (XPS) for a study of the electrodeposition and stripping of Cu, Ni and CuNi alloy thin films at a polycrystalline Au surface. A deaerated Watts bath was employed for the electrodeposition of Ni and CuNi thin films, the bath containing Cu 2+ ions (Ni:Cu = 200:1 mole ratio) in the latter case. These films showed the most satisfactory stripping in either 0.1 M HCl or 0.1 M H 2 SO 4 . Coulometry was used in conjunction with EQCM data for mapping the (potentiostatic) plating efficiency as a function of deposition potential in the Cu and Ni systems. For the CuNi alloy, similar data were acquired by additionally utilizing XPS compositional information and assuming current additivity in the (non-interactive) co deposition of Cu and Ni. Finally, the feasibility of employing the combined ASV-EQCM technique for analysis of CuNi/Ni bilayers is presented.

Electrosynthesis of cadmium selenide films on a selenium-modified gold surface

A two-step method for the growth of CdSe films is described that is based on the initial chemical modification of a polycrystalline gold surface with a selenium overlayer. In the second step, this overlayer is cathodically stripped as Se2− in a Se(IV)-free electrolyte medium (0.1 M Na2SO4) that is dosed with the requisite amount of Cd2+ ions. Unlike the classical cathodic route, this new approach does not suffer from problems with excess Se admixed with CdSe. The two-step approach is validated using a combination of voltammetry, microgravimetry, and photoelectrochemical experiments.

A High Energy and Power Novel Aluminum/Nickel Battery

Recent advancements in aluminum materials and aluminum electrochemistry open a current density compatibility domain in which aluminum anodes and nickel oxide cathodes can simultaneously discharge at high faradaic efficiency. The resultant overlapping comparability region permits demonstration of an aluminum/nickel battery capable of unusually high specific power density and high specific energy density: Al+3NiOOH+NaOH+3H{sub 2}O {yields} NaAl(OH){sub 4}+3Ni(OH){sub 2}. The operational cell maintains a steady-state discharge voltage in excess of 2.3 V. Aluminum/nickel battery discharges are presented which yield a power density of 915 W/kg, and a specific energy density of up to 140 Wh/kg.

Electrochemical Preparation of Cadmium Selenide Nanoparticles by the Use of Molecular Templates

We report the electrochemical preparation of cadmium selenide nanoparticles at gold electrodes modified with molecular templates. The molecular templates were obtained by properly arranging thiolated β-cyclodextrin self-assembled monolayers (SAMs) on gold electrodes. Selenium was first deposited on a SAM-modified gold electrode at an appropriate potential, followed by reduction to HSe in a solution containing Cd 2+ , leading to the deposition of CdSe. The Cd codeposited at the time of CdSe deposition was removed by anodic dissolution at a potential where it was oxidized. Scanning electron microscopic pictures showed that Se 0 and cadmium selenide particles of smaller than 60 and 150 nm diam, respectively, were deposited on the SAM-modified gold electrodes. Selenium particles of as small as 1-2 nm diam and ∼0.2 nm height were observed by in situ electrochemical scanning tunneling microscopy experiments during the early stages of electrochemical deposition.

In situ compositional analysis of electrosynthesized cadmium telluride thin films by electrochemical quartz crystal microgravimetry

Anodic stripping voltammetry was used in conjunction with electrochemical quartz crystal microgravimetry to develop a new electroanalytical protocol for the in situ compositional analysis of semiconductor thin films. Electrosynthesized cadmium telluride at a gold support electrode was used as the model system. The thin film composition could be graded from pure tellurium → CdTe + Te + Cd → pure cadmium by varying the deposition potential from − 0.30 V to − 0.80 V (vs. Ag/AgCl reference). Correspondingly, the mass loss associated with the stepwise anodic stripping of the thin film constituents was used for their selective quantitation. Supporting voltammetry and coulometry data are presented along with an investigation of the effect, on the thin film composition, of the Cd2+ concentration in the electrosynthesis medium.

Compositional analysis of electrodeposited bismuth telluride thermoelectric thin films using combined electrochemical quartz crystal microgravimetry--stripping voltammetry.

Bismuth telluride (Bi 2Te 3 ) is a benchmark material for thermoelectric power generation and cooling applications. Electrodeposition is a versatile technique for preparing thin films of this material; however, it affords films of variable composition depending on the preparation history. A simple and rapid assay of electrodeposited films, therefore, has both fundamental and practical importance. In this study, a new protocol for the electroanalysis of Bi 2Te 3 thin films is presented by combining the two powerful and complementary techniques of electrochemical quartz crystal microgravimetry (EQCM) and stripping voltammetry. First, any free (and excess) tellurium in the electrodeposited film was reduced to soluble Te ( 2- ) species by scanning to negative potentials in a 0.1 M Na 2SO 4 electrolyte, and the accompanying frequency increase (mass loss) was used to determine the content of free tellurium. The film was again subjected to cathodic stripping in the same medium (to generate Bi (0) and soluble Te (2-) from the Bi 2 Te 3 film component of interest), and the EQCM frequency change was used to determine the content of chemically bound Te in the Bi 2Te 3 thin film and thereby the compound stoichiometry. Finally, the EQCM frequency change during Bi oxidation to Bi (3+) and the difference between total Bi and Bi in Bi 2Te 3 resulted in the assay of free (excess) Bi in the electrodeposited film. Problems associated with the chemical/electrochemical stability of the free Bi species were circumvented by a flow electroanalysis approach. Data are also presented on the sensitivity of electrodeposited Bi 2Te 3 film composition to the electrodeposition potential. This newly developed method can be used for the compositional analysis of other thermoelectric thin-film material candidates in general.

CHAPTER 11:Electro- and Photocatalytic Reduction of CO2: The Homogeneous and Heterogeneous Worlds Collide?

This chapter focuses on the use of molecular catalysts and/or electrode materials (electrocatalytic, semiconductor) to sustain the reduction of CO2. It includes a comparison of molecular catalysts for both electrochemical and photochemical systems as well as a review of the progress made in our own laboratories on semiconductor photocatalysts. Only a few molecular catalysts are capable of deeper reduction than the two-electron reduced products of CO2 (such as CO and formic acid) and the generation of value-added reduction products such as methanol and methane are needed. The challenge to overcome is the overpotential for these electrochemical reactions and short-lived one-electron reduced species for the photochemical systems. Incorporation of a chromophore with the real catalyst in either intermolecular or intramolecular photochemical systems has demonstrated the feasibility of CO2 photoreduction. However, photoinduced electron transfer from the chromophore to the catalyst or from the semiconductor to the solution still account for much of the inefficiency in these systems. Semiconductor-based photocatalyst systems (nanoparticles and electrodes) have shown formation of two-electron reduced products as well as deeper reduction pathways although with limited efficiency. It is our hope that this chapter will contribute to further progress and stimulate future generations of scientists to develop new electro-/photocatalyst design paradigms.

The deposition of Group 6A-derived inorganic semiconductor films as studied by quartz crystal microgravimetry

Abstract This paper focuses on the use of QCM for the study of semiconductor film deposition processes. Specifically the electrosynthesis of metal chalcogenides (In 2 S 3 , CdS, and CdTe) is considered. A brief background is first given for electrodeposition as a process candidate for semiconductor film preparation. Previous studies are reviewed on the use of QCM (and specifically EQCM) in this area. New combined voltammetry-EQCM data are presented for the oxidative deposition of sulfur on polycrystalline Au surfaces from alkaline sulfide baths. The anodic growth of CdS films and the cathodic electrosynthesis of In 2 S 3 are studied by the combined approach. Finally, data are presented on the cathodic electrosynthesis of CdTe films using EQCM in a rotating disc electrode (RDE) configuration. From an instrumental perspective, the presented data illustrate the virtues of combining the QCM technique with cyclic voltammetry, coulometry, and hydrodynamic (RDE) voltammetry for studies of semiconductor film deposition.

Potential Enhancement of Polyiodide Redox Couples via Solution Modification

The stability and high reversibility of the polyiodide equilibrium results in a positive redox couple useful in fuel, battery, and photoelectrochemical solar cells I{sup {minus}} + I{sub 2} {r_reversible} I{sub 3}{sup {minus}}; I{sub 3}{sup {minus}} + 2e{sup {minus}} {yields} 3I{sup {minus}}; E(0.01 m I{sub 2}, 1 m NaI) = 298 mV vs. Ag/AgCl. A further positive shift of polyiodide redox couples which can make polyiodide more attractive as a storage cathode is accomplished by a two-step solution modification process. Solution-phase iodide is bound by complexation with cadmium salts. Then the complexation product, unstable at neutral pH, is stabilized by acidification. The redox potential is increased while maintaining facile charge transfer and electrolyte stability. For example, redox potentials of 425, 430, or 440 mV are sustained in solutions of 0.01 m I{sub 2}, 1 m NaI, and 1 m Cd(NO{sub 3}){sub 2} containing either 1 m H{sub 2}SO{sub 4}, 1 m HNO{sub 3}, or 1 m HClO{sub 4}, respectively.