Selective formation of monodisperse CdSe nanoparticles on functionalized self-assembled monolayers using chemical bath deposition

Using CdSe chemical bath deposition (CBD) we demonstrate the selective growth and deposition of monodisperse nanoparticles on functionalized self-assembled monolayers (SAMs) using time-of-flight secondary ion mass spectrometry and scanning electron microscopy. We show that the deposition mechanism involves both ion-by-ion growth and cluster-by-cluster deposition. On -COOH terminated SAMs strongly adherent CdSe nanoparticles form via a mixed ion-by-ion and cluster-by-cluster mechanism. Initially, Cd²⁺ ions form complexes with the terminal carboxylate groups. The Cd²⁺-carboxylate complexes then act as the nucleation sites for the ion-by-ion growth of CdSe. After a sufficient concentration of Se²⁻ has formed in solution via the hydrolysis of selenosulfate ions, the deposition mechanism switches to cluster-by-cluster deposition. On -OH and -CH₃ terminated SAMs monodisperse CdSe nanoparticles are deposited via cluster-by-cluster deposition and they do not bind strongly to the surface. Finally, under the appropriate experimental conditions we demonstrate the selective deposition of CdSe nanoparticles on patterned -CH₃/-COOH SAMs.     

Catalytic and inhibitory effects of dissolved Cd2+ on C3H8 electro-oxidation

Studies have been made of the effect of Cd²⁺ on the oxidation rate of C₃H₈ in 80% H₃PO₄ at 130 C on platinized Pt electrodes.Dissolved Cd²⁺ discharges, probably as Cd⁰, and forms a stable sub-monolayer on a Pt electrode at potentials as high as 0·5 V versus RHE. Although this Cd⁰ layer is apparently substantially desorbed in favor of adsorbed hydrocarbon, C₃H₈ adsorption is inhibited in its presence. C₃H₈ oxidation is also inhibited. However, the C₃H₈ oxidation rate is observed to increase under some circumstances, and this appears to be because Cd isabsorbed into the Pt lattice. Co-plating of Cd and Pt leads to an electrode which, once the excess Cd has been removed, appears to be stable, and which catalyses C₃H₈ oxidation by about a factor of three at 0·25 V. Above 0·3 V, this absorbed Cd somewhat inhibits C₃H₈ oxidation.     

Enhanced photoelectrocatalytic activity of FTO/WO3/BiVO4 electrode modified with gold nanoparticles for water oxidation under visible light irradiation

Gold nanoparticles were successfully deposited on FTO/WO₃/BiVO₄ electrode surface by means of electrolysis of AuCl₄⁻ ions. The composite films were characterized by SEM, XPS and XRD techniques. An increase in photocurrent and a negative shift of onset potential for water oxidation were observed upon modification of the electrode surface with the Au particles. The electrochemical impedance spectroscopy was used to confirm the acceleration of charge transfer process by Au deposition at the electrode surface. The photocurrent action spectrum did not correlate with the plasmonic absorbance of Au nanoparticles at 560 nm, suggesting that the Au nanoparticles increased charge separation without undergoing a plasmon resonance effect under visible light irradiation.     

Underpotential deposition of cadmium adatoms on Te and CdTe

Underpotential deposition (UPD) of Cd adatoms onto the surface of Te and CdTe films in Cd²⁺-containing solutions has been studied using electrochemical methods and AFM. The electrochemical deposition of Cd adatoms on Te and CdTe begins at potentials 400 mV more positive than the reversible potential of Cd²⁺/Cd couple and proceeds irreversibly. A strong chemical interaction of Cd adatoms with the surface Te atoms is the driving force of the UPD process. The deposition of Cd adatoms on the tellurium surface is accompanied by their stepwise interaction with tellurium to give CdTe nanophase.     

Electrodissolution of aluminium thin film microband electrodes

This paper discusses the electrodissolution of aluminium thin films as microband electrodes (length = 5 × 10^–3 m) in terms of mass transfer determined by voltammetry and a.c.-impedance techniques as a function of bandwidth (20 to 2000 nm) in 0.1m NaOH solution. The anodic polarization curves of the aluminium microband electrodes show that current density is enhanced with decreasing bandwidth. The ac impedance response suggests that a steady-state diffusion layer appears the more markedly, the smaller the bandwidth. The anodic polarization curves are analysed on the basis of the combined Butler-Volmer high field approximation and the semi-cylindrical diffusion field approximation. As a result of the analysis, the electrodissolution proceeds by a mixed kinetic-mass transfer controlled reaction. The analysis also makes it possible to distinguish the semi-cylindrical diffusive mass transfer contribution to the electrodissolution from the kinetic contribution, i.e., mass transfer index linearly diminishes with decreasing bandwidth. The increased current density is attributable to the decreased mass transfer contribution, i.e., the more predominant semi-cylindrical diffusive mass transfer as compared to laminar diffusive mass transfer.Nomenclature k _a anodic kinetic constant - k _c cathodic kinetic constant - F Faraday constant - _s kinetic transfer coefficient for anodic reaction - _c kinetic transfer coefficient for cathodic reaction - c ^s surface concentration - V anodic polarization - D diffusion coefficient - d diffusion layer thickness - z number of electrons transferred - l length of microband electrode - w bandwidth of microband electrode - r radius of cylinder     

Electrodics of methanol oxidation on Pt-f-multiwalled carbon nanotube composite, prepared by γ-radiolysis

We report the electrodics of methanol oxidation on Pt-multiwalled carbon nanotube composites (Pt-f-MWCNTs), prepared by γ-radiolysis of K₂PtCl₆ in the presence of HOOC-functionalized multiwalled carbon nanotubes. The electrocatalytic activity for the methanol oxidation was studied using cyclic and linear sweep voltammetric techniques on the stationary indium tin oxide and rotating gold disc electrodes, respectively. Higher values of oxidative (anodic) current were obtained using Pt-f-MWCNTs compared to the polycrystalline Pt electrode. This phenomenon is attributed to the synergistic effect of oxy groups on MWCNTs, which alleviate CO poisoning. The electrodics of the reaction at various temperatures was studied using linear sweep voltammetry (LSV) on a rotating disc gold electrode, modified with the composite. From the Koutecky-Levich plots, the standard rate constant (k⁰) was determined to be 7.9 ± 1.9 × 10⁻⁸ cm s⁻¹.     

Anodic film on lead-arsenic alloy in sulphuric acid medium

The phase composition, anodic behaviour and semiconducting properties of the anodic film on Pb-6 at % As alloy have been investigated. The linear relationship between i and t ^–1/2 shows that the anodic film growth is controlled by a diffusion mechanism. The experimental results show that the anodic film contains t-PbO, PbO · PbSO₄, 3PbO · PbSO₄ and some arsenic compounds. The dielectric constant of the film is 2.8 × 10² obtained from the capacitance-time relationship measured at 2500 Hz. The Mott-Schottky plot shows that the film is an n-type semiconductor. The flat-band potential is –0.56 V vs Hg/Hg₂SO₄, and the donor density is 7.2 × 10¹⁵ cm^–3 for the film obtained at 0.9 V vs Hg/Hg₂SO₄ for 2 h on a Pb-6 at % As electrode.     

Influence of surface inhomogeneities of boron doped CVD-diamond electrodes on reversible charge transfer reactions

The electrochemical behaviour of reversible charge transfer reactions on boron doped diamond (BDD) was studied by cyclic voltammetry and electrochemical impedance spectroscopy using rotating disc electrodes under defined convection. Diamond films of 5 m thickness with doping levels of 200, 3000 and 6000 ppm were prepared by hot filament chemical vapour deposition on niobium substrate. The electrochemical measurements were carried out on BDD electrodes in deaerated 0.5 M Na₂SO₄ + 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] solution at rotation frequencies 0 < f _rot < 4000 rpm. Comparative measurements were carried out on an active Pt electrode. The BDD electrodes exhibit distinct irreversibilities indicated by a larger peak potential difference in the cyclic voltammograms, lower diffusion limiting current densities and an additional impedance element at high frequencies. Mechanical polishing with carbon fleece and SiC paper strongly affects the irreversible behaviour of the BDD electrodes. The experimental results are explained by a partial blocking of the diamond surface with reversible charge transfer at active sites. The impedance spectra are analysed quantitatively using a transport impedance model for reversible reactions on partially blocked electrode surfaces.     

Electrochemical impedance study of the reaction mechanism for electro-oxidation of isosorbide

Electrochemical impedance measurements in the anodic range were made in 0.1 and 1 M solutions of sodium bromide using a rotating platinum disc electrode. It has been shown that bromide is oxidized in a rapid transfer reaction accompanied by diffusion of a bromine intermediate species, which was thought to be bromine itself. At 1 M, a further reaction was observed involving formation of the tribromide ion Br ₃ ^– in the vicinity of the electrode. In a second set of experiments, impedance was measured in the presence of isosorbide. The overall shape of the diagrams was unchanged, indicating that oxidation of isosorbide takes place essentially in the bulk of the solution and not on the electrode.This paper is dedicated to the memory of Michel Duprat.     

Tunable electrochemical properties of liquid phase deposited TiO2 films

Titanium dioxide (TiO₂) films on glassy carbon (GC) electrode surface were prepared by the liquid phase deposition (LPD) process for different deposition times. The morphological structure, interfacial property and electrocatalytic activity of as-prepared LPD TiO₂ films on GC surface were studied by field-emission scanning electron microscopy (FE-SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The FE-SEM observation showed that the deposition time controlled the morphology of film on GC surface. With increasing deposition time, TiO₂ formed nanoparticles at the initial 5-h stage and compact thick films after 20 h. Due to the semiconducting properties of TiO₂, the LPD films inhibited the electron transfer process of [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ on GC by increasing the redox reaction peak potential separation of CV curve and electron transfer resistance of EIS. The inhibition was increased with TiO₂ film thickness. Nevertheless, the onset reduction potential of maleic acid decreased with increasing LPD TiO₂ film thickness while the cathodic and anodic currents increased, demonstrating the useful electrocatalytic activity of LPD TiO₂ films.     

Studies of stoichiometry of electrochemically grown CdSe deposits

The proper deposition bath composition for electrochemical synthesis of the CdSe deposit in the hexagonal structure of the right elemental stoichiometry, and photoreacting as an n-type semiconductor which can be used as a stable photoanode is investigated. The deposits were prepared by a cyclic potentiodynamic technique and the concentration of Cd²⁺ and SeO₃²⁻ in the deposition baths varied from 10⁻⁴ M to 0.1 M, and from 10⁻⁵ M to 10⁻³ M, respectively. The electrochemical, the X-ray diffraction (EDS and XRD), and the photoactivity studies of a number of deposits have shown that application of the solution composition following Cd:Se = 5:1 results in deposition of the stoichiometric CdSe. The detected ratio of reagents is explained on the base of reaction mechanism and necessary excess of cadmium ions preventing CdSe deposit dissolution. The procedure of CdSe electrosynthesis was developed to yield of a direct semiconductor in the hexagonal structure. The necessity for cadmium cations excess is explained on the basis of the mixed electrochemical/chemical deposition mechanism.     

Electrochemical detection of Cd ions by a self-assembled monolayer of 1,9-nonanedithiol on gold

The application of 1,9-nonanedithiol (NDT) self-assembled monolayer (SAM) on gold for the electrochemical determination of Cd²⁺ was studied. Interestingly, we found that a NDT SAM strongly affects the stripping wave of Cd, resulting in a sharp peak that was used for electroanalytical determination of Cd²⁺ in aqueous solutions. The different parameters, such as potential and time of deposition of Cd, were examined. Furthermore, polarization-modulated infrared reflection absorption spectroscopy (PM IRRAS) and X-ray photoelectron spectroscopy (XPS) were used for exploring the interaction between the deposited Cd and the thiol groups on Au. FTIR measurements clearly indicate that NDT is assembled in a disordered liquid type monolayer interacting with the Au electrode via both thiol moieties. XPS reveals that Cd is stripped at two different potentials and that the signal of sulfur is almost unchanged by deposition and desorption of Cd. All these finding allude to the interesting conclusion that Cd is deposited on Au lifting to some extent the thiol groups.     

In Situ Soft X‐ray Microscopy Study of Fe Interconnect Corrosion in Ionic Liquid‐Based Nano‐PEMFC Half‐Cells

This in situ soft X‐ray scanning microscopy electrochemical study of model proton exchange cathodic and anodic nano‐fuel cells is exploring the evolving structure and chemical composition of key cell components represented by Au and Fe electrodes in contact with Nafion‐ionic liquid composite electrolyte containing Pt black catalyst particles. Morphological and chemical changes of the electrodes as well as the chemical state and fate of the Fe species released into the electrolyte are monitored in short circuit and with applied cathodic or anodic polarization. The in situ X‐ray absorption images of the cathodic cell fed with 2.5 × 10^–5 mbar O₂ have revealed corrosion‐induced morphology changes in the Fe electrode, being more pronounced in the vicinity of Pt‐black particles, and deposition of the Fe species released into the electrolyte, onto the intact Au counter electrode upon cathodic polarization. The Fe electrodes of the anodic cell containing NaBH₄ in the electrolyte appear relatively more corrosion resistant. The Fe L₃ absorption spectra taken in different locations within the Fe electrode have shown lateral variations in the relative ratio between Fe²⁺ and Fe^3&4+ oxidation states, whereas the Fe species released into the RTIL electrolyte are only in the high Fe^3&4+ oxidation states.     

Electrooxidation of methanol on NiMn alloy modified graphite electrode

Nickel and nickel-manganese alloy modified graphite electrodes (G/Ni and G/NiMn) prepared by galvanostatic deposition were examined for their redox process and electrocatalytic activities towards the oxidation of methanol in alkaline solutions. The methods of cyclic voltammetery (CV), chronoamperometry (CA) and impedance spectroscopy (EIS) were employed. In CV studies, in the presence of methanol NiMn alloy modified electrode shows a significantly higher response for methanol oxidation. The peak current of the oxidation of nickel hydroxide increase is followed by a decrease in the corresponding cathodic current in presence of methanol. The anodic peak currents show linear dependency upon the square root of scan rate. This behavior is the characteristic of a diffusion controlled process. Under the CA regime the reaction followed a Cottrellian behavior and the diffusion coefficient of methanol was found to be 4 × 10⁻⁶ cm² s⁻¹. A mechanism based on the electro-chemical generation of Ni³⁺ active sites and their subsequent consumptions by methanol have been discussed and the corresponding rate law under the control of charge transfer has been developed and kinetic parameters have been derived. The charge transfer resistance accessible both theoretically and through the EIS have been used as criteria for derivation of the rate constant.     

Electrochemical behaviour of tin in bicarbonate solution at pH 8

The anodic oxidation of tin in 0.1 to 1M bicarbonate solutions at pH 8 has been studied. The process may be divided into three potential regions: (1) a short active dissolution (Tafel) region; (ii) a dissolution-precipitation region; and (iii) a large region of electrode passivity. The rate-determining step of the reaction in the active-dissolution region is attributed to the diffusion of an ionic species into the solution, the diffusing species being generated at the metal surface. In the region of the first oxidation peak, the reaction rate is controlled by diffusion of CO ₃ ^2– species in solution. When the potential becomes more positive than –0,1 V_sce, a highly passivating (most likely SnO₂) film is formed on the electrode surface.     

Underpotential alloy formation in the system Ag(hkl)/Cd2+

The underpotential deposition (UPD) of Cd on “real” and “quasi-ideal” silver single crystal surfaces of (111) and (100) crystallographic orientation as well as on polycrystalline silver substrate has been investigated in 0.5 M Na₂SO₄ supporting electrolyte solutions. At high underpotentials 100 < ΔE < 350 mV, the UPD is characterized by a quasi-reversible adsorption/desorption of Cd whereas at low underpotentials. ΔE < 50 mV, an increase of the anodic stripping charge with the polarization time is found due to the formation of an AgCd alloy at the substrate surface. The time dependence of this process can be described by a parabolic rate law, the rate constant of which is a function of ΔE and temperature T. Relatively low activation energies of about 70 kJmol^?1 were determined from measurements at 293 ? T ? 338 K. The results are discussed in terms of a semi-infinite-linear diffusion model. The alloy formation process is assumed to be initiated by a place exchange between Ag substrate atoms and vacancy sites within a mobile Cd adsorbate layer thus forming the initial stage of a highly distorted AgCd alloy. The further growth will take place by the movement of Ag atoms through the vacancy-rich surface alloy and the simultaneous deposition of Cd at the interface AgCd/Cd²⁺.     

XPS investigation of surface oxidation layers on a platinum electrode in alkaline solution

A thick oxidation layer on a platinum electrode has been grown in 1 N NaOH at 3 V vs Ag/AgCl reference electrode. After transferring the Pt electrode into an ultrahigh vacuum chamber the surface layer was analysed by X-ray photoelectron spectroscopy. Pt4f_5/2 amd O1s electron binding energies of 74.3, 77.6 and 530.9 eV respectively, as well as the broad peak shape of the O1s signal and the oxygen-to-platinum intensity ratio of 3.08 point towards a platinum—oxyhydroxide PtO(OH)₂. This formula is in good agreement with cyclic voltammetry curves, measured for the same electrode, that revealed two cathodic reduction peaks for oxygen surface coverages equivalent to more than two hydrogen monolayers. These two peaks were assigned to PtOH and PtO. XPS analysis at elevated temperatures showed that the thick (5 nm) oxidation layer decomposes at 400 K to a mixture of several oxides and hydroxides of Pt⁴⁺ and Pt²⁺ and Pt metal with a ratio of O-to-Pt of 1. This mixture further gradually decomposes to only a monolayer of oxygen at 770–870 K. Sodium cations were found to be present in trace amounts in the adlayer and to strongly shift the O1s binding energy to lower values.     

Electrocatalytic oxidation of some amino acids on a nickel-curcumin complex modified glassy carbon electrode

This study investigated the electrocatalytic oxidation of alanine, l-arginine, l-phenylalanine, l-lysine and glycine on poly-Ni(II)-curcumin film (curcumin: 1,7-bis [4-hydroxy-3-methoxy phenyl]-1,6-heptadiene-3,5-dione) electrodeposited on a glassy carbon electrode in alkaline solution. The process of oxidation and its kinetics were established by using cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy techniques. Voltammetric studies indicated that in the presence of amino acids the anodic peak current of low valence nickel species increased, followed by a decrease in the corresponding cathodic current. This indicates that amino acids were oxidized on the redox mediator which was immobilized on the electrode surface via an electrocatalytic mechanism. Using Laviron's equation, the values of α and k_s for the immobilized redox species were determined as 0.43 ± 0.03 and 2.47 ± 0.02 × 10⁶ s⁻¹, respectively. The rate constant, the electron transfer coefficient and the diffusion coefficients involved in the electrocatalytic oxidation of amino acids were determined.     

Anodic depassivation of potassium electrode in propylene carbonate medium

Depassivation of potassium electrode in a highly dried propylene carbonate-KPF₆ electrolyte (< 0.01 ppm H₂O) was studied by using galvanostatic E-t and polarization i-E curves at various scan rates. The results suggest that the passivating layer is strongly absorbed on the electrode surface even during anodic dissolution of potassium, which proceeds only on a certain depassivated fraction of the surface. A mechanism of the anodic depassivation is proposed.