Electrocrystallization of Au nanoparticles on glassy carbon from HClO4 solution containing [AuCl4]

The mechanism and kinetics of electrocrystallization of Au nanoparticles on glassy carbon (GC) were investigated in the system GC/1 mM KAuCl₄ + 0.1 M HClO₄. Experimental results show that the gold electrodeposition follows the so-called Volmer-Weber growth mechanism involving formation and growth of 3D Au nanoparticles on an unmodified GC substrate. The analysis of current transients shows that at relatively positive electrode potentials (E ≥ 0.84 V) the deposition kinetics corresponds to the theoretical model for progressive nucleation and diffusion-controlled 3D growth of Au nanoparticles. The potential dependence of the nucleation rate extracted from the current transients is in agreement with the atomistic theory of nucleation. At sufficiently negative electrode potentials (E ≤ 0.64 V) the nucleation frequency becomes very high and the nucleation occurs instantaneously. Based on this behaviour is applied a potentiostatic double-pulse routine, which allows controlled electrodeposition of Au nanoparticles with a relatively narrow size distribution.     

Electrodeposition of Te onto monocrystalline n- and p-Si(1 0 0) wafers

Electrochemical deposition of Te onto n- and p-Si(1 0 0) wafers from 0.1 M HNO₃ + 1 mM TeO₂ solution was studied using cyclic voltammetry (CV), chronoamperometry, ex situ SEM and XRD. Electrodeposition of Te onto n-Si takes place both in the dark and under illumination. Electrodeposition of Te onto p-Si proceeds only under illumination, when the photoelectrons are generated in silicon substrate and reduce Te(IV) species in solution. Electrochemical reduction of Te(IV) on n- and p-Si occurs with large cathodic overvoltage (0.22-0.62 V). Nucleation of Te on n- and p-Si proceeds via 3D island growth, it is characterised correspondingly by progressive and instantaneous nucleation mechanisms followed by diffusion limited growth. Cathodic deposition of Te onto n- and p-Si is irreversible. Anodic stripping of Te electrodeposited onto p-Si occurs both in the dark and under illumination and anodic stripping of Te from n-Si proceeds only under illumination.     

Stability of iodine on ruthenium during copper electrodeposition and its effects on the nucleation behavior of electrodeposited copper

Cyclic voltammetry, current-time-transient measurements, and X-ray photoelectron spectroscopy (XPS) have been used to study the nucleation behavior of electrochemically deposited Cu films on Ru substrates as a function of Ru pre-treatment. Pre-treatment consisted of cathodic polarization in either 1 M H₂SO₄ or in 1 M H₂SO₄ + 1 mM KI, followed by sample emersion and placement in a 1 M H₂SO₄ + 50 mM CuSO₄ plating bath. XPS measurements confirmed the presence of adsorbed I on the Ru surface following pre-treatment in the KI/H₂SO₄ solution. Cyclic voltammogram (CV) data for electrodes either as-received or pre-reduced in H₂SO₄ and then immersed in the plating solution exhibited a broad peak in the overpotential region consistent with oxide reduction followed by Cu deposition. No underpotential deposition (UPD) feature was observed for these electrodes. In contrast, the sample pre-reduced in I-containing electrolyte exhibited a narrow Cu deposition peak in the overpotential region and a UPD Cu feature centered at 80 mV vs. Ag/AgCl. Current-time-transient (CTT) measurements of Cu deposition on as-received electrodes or electrodes pre-reduced in I-free solution exhibited potential-independent kinetics that are not well described by either progressive or instantaneous nucleation models and which at long times indicate a combination of diffusion and kinetic control. In contrast, CTT measurements of deposition kinetics for samples reduced in I-containing electrolyte exhibited complex, potential-dependent behavior and that at long times indicates diffusion control. XPS results also indicated that the iodine adlayer on Ru reduced in I-containing electrolyte is stable upon polarization to at least −200 mV vs. Ag/AgCl. These data indicate that a protective I adlayer may be deposited on an air-exposed Ru electrode as the oxide surface is electrochemically reduced, and that this layer will inhibit reformation of an oxide during the Cu electroplating process. Therefore, electrochemical pre-treatment in I-containing electrolyte may be of practical utility under industrial conditions for Cu electroplating.     

Electrodeposition of PbSe onto n-Si(1 0 0) wafers

PbSe was electrodeposited onto monocrystalline n-Si(1 0 0) wafers from 50 mM Pb(NO₃)₂ + 2 mM SeO₂ + 0.1 M HNO₃ solution. The mechanism of PbSe electrocrystallization on n-Si was studied. At initial stage, 3D Pb and 3D Se nuclei are simultaneously codeposited onto Si at potentials more negative than Si flat band potential and chemically interact resulting in PbSe formation. When n-Si/PbSe heterostructure is formed, the overvoltage of bulk lead deposition increases, as a result of redistribution of electrode potential. Further growth of PbSe is realized due to underpotential deposition (UPD) of Pb and overpotential deposition (OPD) of Se onto formed PbSe nuclei. With Pb UPD shift increase, amorphous Se inclusion is registrated in the deposit. When 2D Pb nucleation mechanism is changed to 3D mode, metal Pb cubic phase is codeposited with PbSe. Electrodeposition of PbSe onto n-Si is irreversible. PbSe anodic stripping does not take place in the dark due to the barrier on solid interface. Oxidation of PbSe on n-Si is observed only under illumination, when photoholes are generated in silicon substrate.     

Different steps in the electrosynthesis of poly(3,4-ethylenedioxythiophene) on platinum

The initial stages of poly(3,4-ethylenedioxythiophene) (PEDOTh) film growth on platinum electrodes from TBAClO₄/acetonitrile solution are investigated by means of current-time transient measurements and tapping mode atomic force microscopy. It is shown that, for growth potentials in the range 1.17-1.29 V vs. SCE, the deposition process involves the formation of oligomers in the solution, progressive nucleation of centres of the new phase and three-dimensional growth of a first compact layer followed by a non-uniform distribution of granular-like clusters, whose number and size increase with the synthesis potential and charge. The obtained results reveal that PEDOTh films prepared at distinct potentials but with the same growth charge (Q_g) display similar electroactivities. They also depict that the electrochemical behaviour of the films is a function of the charge used for the synthesis, namely the reduction of tick PEDOTh layers (Q_g > 20 mC cm⁻²) includes more that one step, as a consequence of the formation of a two-layered polymer film.     

The kinetics of copper island growth on ruthenium oxide in perchlorate solution

The nucleation potential for the deposition of copper on ruthenium oxide is very small and hence island growth can occur in the kinetic regime. Previously, we have reported on island growth in sulfate solution where anion adsorption results in the formation of hexagonal, disk-shaped islands. Here we report on island growth in perchlorate solution, in the absence of specific anion adsorption. All islands have a 〈1 1 1〉 surface orientation dictated by the amorphous substrate. The islands are hemispherical in shape and island growth follows power law kinetics in both lateral and vertical directions with exponents 0.50 and 0.26, respectively. From analysis of the time-dependent extended area using Avrami analysis, analysis of the near-neighbor distribution, and the correlation between the island size and the spatial distribution, we conclude that island growth is controlled by surface diffusion-mediated lateral growth.     

Pulsed electrodeposition of bismuth telluride films: Influence of pulse parameters over nucleation and morphology

Pulsed electrodeposition methods were applied to the preparation of bismuth telluride films. Over the potential ranges from −170 mV to −600 mV, the formation of Bi₂Te₃ nuclei proceeded through a three-dimensional instantaneous nucleation mode. The nuclei densities for several values of potential were ranged between ∼10⁶ nuclei cm⁻² and ∼10⁸ nuclei cm⁻². For a pulsed galvanostatic electroplating, the best covering percentage and a stoichiometry close to the desired Bi₂Te₃ were obtained with the parameters t_on, t_off and J_c, respectively, equal to 10 ms, 1000 ms and −100 mA cm⁻².     

Electrochemical properties of modified copper-thallium hexacyanoferrate electrode in the presence of different univalent cations

The preparation of copper(II) hexacyanoferrate (CuHCF) films on the surface of gold electrodes as well as their characterization in solutions of various alkali metal and NH₄⁺ cations and in the presence of thallium(I) are described. The electrochemical quartz crystal microbalance and cyclic voltammetric techniques were used. In 0.50 M lithium nitrate, even at submillimolar concentration of Tl(I), the formal potential of CuHCF was shifted to more positive values. At higher Tl(I) concentrations, the formal potential of the CuHCF redox reaction changed linearly with the logarithm of Tl(I) concentration (in the 0.50 M solution of lithium or another alkali metal nitrate). From such dependencies, selectivity coefficients K_Tl/M were calculated, and they show that the CuHCF film on the gold electrode interacts preferentially with Tl(I). High affinity of Tl(I) to copper hexacyanoferrate, that was observed in the presence of alkali metal cations, was explained by relatively strong donor-acceptor interactions of Tl(I) ions with nitrogen in CN groups of the CuHCF film.It was also shown for simple M₄[Fe(CN)₆] metal ferrocyanate salts (where M = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and Tl⁺) that there is a preferential interaction of Tl⁺ with CN group consistent with formation of a Tl-NC-Fe bridge.     

The nucleation, crystallization and dispersion behavior of PET-monodisperse SiO2 composites

To more accurately investigate the nucleation, crystallization and dispersion behaviors of silica particles in polymers, the composites of PET with monodisperse SiO₂-PS core-shell structured particles were prepared with SiO₂ size from 380 nm to 35 nm.For these SNPET samples, DSC results showed that the nucleation rate of silica particles increased as their size decreased, in which 35 nm SiO₂ particles produced the most obvious nucleation effect. At 2.0 wt.% load of 35 nm silica, Avrami equation proved that the isothermal crystallization rate G of SNPET was ca. 30% higher than that of pure PET and the crystallization activation energy for SNPET was −218.7 kJ mol⁻¹ lower than −196.1 kJ mol⁻¹ for PET. While, the non-isothermal crystallization ΔE for SNPET was −199.8 kJ mol⁻¹ lower than −185.5 for PET.On non-isothermal crystallization, Jeziorny equation presented the primary and secondary crystallization stages in PET and SNPET, in which nano SiO₂ accelerated the crystallization rate. Their Ozawa number m was from 2.1 to 2.7, which was smaller than that of Avrami number n.The nucleation and dispersion behaviors of SiO₂ particles were directly observed. POM results demonstrated that SNPET samples crystallized more quickly from melt and their crystallization rate increased as silica load increases but accelerated at 2-3 wt.%. The spherulites grew well in PET but their size was smaller in SNPET due to the silica barrier on their growth. SEM and TEM observed the homogeneous silica dispersion morphology and the vivid ordered patterns formed in SNPET. The monodisperse particles are highly expected to give more accurate and valuable references than multi-scale ones in obtaining novel advanced PET composites.     

In situ analysis of oxygen partial pressure at the cathode catalyst layer/membrane interface during PEFC operation

To understand the concentration overpotential in the polymer electrolyte fuel cell (PEFC), we have performed an in situ analysis of the oxygen partial pressure (p[O₂]_CL/PEM) at the interface between the cathode catalyst layer (CL) and the polymer electrolyte membrane (PEM). Diffusion-limited oxygen reduction current was measured, with Pt probes inserted into the PEM, during cell operation by supplying H₂ to the anode and O₂ + N₂ to the cathode at 80 °C. It was found that the p[O₂]_CL/PEM decreased by ca. 20% when the current density was stepped from 0 to 2.0 A cm⁻² at p[O₂]_gas = 54 kPa and 100% RH at the cathode inlet, irrespective of the oxygen utilization UO₂ (from 10% to 50%). Such a change in p[O₂]_CL/PEM might result in a concentration overpotential of ca. 10 mV, based on the Tafel slope of 120 mV decade⁻¹ in the high current density region. It was also found that ohmic losses in the ionomer phase of the CL increased with decreasing humidity, from 100% to 80% RH, and became a dominant factor in the increased total overpotential, while the corresponding concentration overpotential was unchanged. The present results provide new insight into the transport of oxygen and water at the CL/PEM interface, especially at the high current densities required for the electric vehicle application.     

Pd deposition onto Au(111) from nitrate solution

The initial stages of palladium deposition onto Au(111) from 0.1 M HNO₃ + 0.2 mM Pd(NO₃)₂ have been studied by cyclic voltammetry and in situ scanning tunnelling microscopy. It is demonstrated that nucleation starts exclusively at surface defects such as monoatomic high steps, which is at variance with recently published work. From this and our previous work it thus appears that surface defects are the preferred nucleation sites indeed for nitrate, sulphate and chloride containing solutions.     

Electrochemistry of terbium in the eutectic LiCl-KCl

The electrochemical behaviour of terbium at a tungsten electrode, in the eutectic LiCl-KCl molten was investigated in the temperature range 673-823 K, by means of cyclic voltammetry, chronopotentiometry and chronoamperometry. It was found that during cathodic polarization, deposition of metallic Tb from the chloride mixture onto the tungsten surface proceeds in a single step, and electrocrystallization plays an important role in the whole process. Experimental current-time transients followed the theoretical models based on instantaneous nucleation with three-dimensional growth of the nuclei whatever the applied overpotential. From chronopotentiometric measurements, the diffusion coefficient of the Tb(III) ions was determined by applying the Sand equation and modifying the immersion dept of the working electrode in stages. The validity of the Arrhenius law was also verified and the activation energy for diffusion was found to be 33.4 ± 1.5 kJ mol⁻¹.The standard apparent potential value of the Tb(III)/Tb(0) system has been determined by potentiometry at several temperatures, and the activity coefficient of Tb(III) in the molten chloride media based on a pure supercool reference state has also been calculated.     

Electroformed iron and FeCo alloy

Iron and FeCo alloys were electroformed from additive-free acidic chloride baths. Film stress and magnetic properties were strongly influenced by deposition current density and operating temperature. In general, low film stress and low coercivity (H_C) was achieved with low current density and high operating temperature baths. SEM micrographs indicated that these conditions promote large grain growth. Coercivity of electroformed iron films linearly increased with increasing film stress, indicating that magnetoelastic energy is a dominant anisotropy. The addition of 0.25 M CaCl₂ improves current efficiency while maintaining low film stress. The lowest iron film stress of 5 MPa was achieved from 1.5 M FeCl₂ in the absence of CaCl₂ at 20 mA cm⁻² with a current efficiency of 91%. A “normal” codeposition of FeCo was observed in acidic chloride baths, where the deposition rate of Co²⁺ was faster than Fe²⁺. Film compositions also played an important role in magnetic properties of FeCo films in addition to film stress. Magnetic saturation (M_S) of FeCo films increased linearly with an increase in deposited Fe content. High magnetic saturation with low-stress (M_S of 2.3 T and σ=70 MPa) were achieved from 71Fe29Co films.     

Copper electrodeposition on pyrolytic graphite electrodes: Effect of the copper salt on the electrodeposition process

The electrodeposition of copper on pyrolytic graphite from CuSO₄ or Cu(NO₃)₂ in a 1.8 M H₂SO₄ aqueous solution was investigated. The Cu deposits were formed potentiostatically and characterized by electrochemical methods, scanning electron microscopy, energy dispersive X-ray and X-ray photoelectron spectroscopy. It was found that the deposition of copper in the presence of CuSO₄ induced the codeposition of sulfate anions. In addition, electrochemical quartz crystal microbalance revealed that the increase of the Cu mass was higher than expected from Faraday's law with the CuSO₄/H₂SO₄ solution. These results confirmed the specific adsorption of anions during the Cu deposition. On the other hand, the use of Cu(NO₃)₂ resulted in a non-contaminated surface with different surface morphologies. The Cu nuclei size, the population density and the surface coverage were monitored as a function of the deposition potential. From the analysis of the chronoamperometric curves, the nucleation kinetics was studied by using various theoretical models. Independently of the Cu source, the nucleation mechanism follows a three-dimensional (3D) process. Copper nucleates according to an instantaneous mode when the deposition potential is more negative than −300 mV versus Ag/AgCl, while the nucleation was interpreted in terms of a progressive mode at −150 mV. The nuclei population densities were also determined by using two common fitting models for 3D nucleation and growth (Scharifker-Mostany and Mirkin-Nilov-Heerman-Tarallo). Their values are reported here as a function of the deposition potential.     

Overpotential deposition of copper on an iodine-modified Au(1 1 1) electrode

Cyclic voltammetry, chronoamperometry and in situ electrochemical scanning tunneling microscopy were used to study the kinetics of nucleation and crystal growth during the initial stages of copper overpotential deposition (OPD) on a previously iodine-modified Au(1 1 1) electrode, from an aqueous solution 10⁻³ M CuSO₄ in 0.05 M H₂SO₄. The starting potential during step experiments was chosen in the region where the gold electrode was completely free of the copper deposit. The recorded current transients for copper deposition onto the iodine-modified Au(1 1 1) electrode surface appear to be very complex, with the unusual presence of two or more current maxima. A new method was used for quantitative evaluation of current transients that involves the transition UPD-OPD, developed by our group [M. Palomar-Pardavé, I. González, N. Batina, J. Phys. Chem. B 104 (2000) 3545], was used for the quantitative interpretation. Our results show that, within a single current transient, copper adsorption and two types of nucleation process: two-dimensional (2D) and three-dimensional (3D) limited by lattice incorporation of copper adatoms and diffusion of Cu(II) ion, respectively, take place simultaneously. STM images revealed the enhanced growth of 3D copper on edge of I-Au(1 1 1) during the early stages of deposition. Moreover, our results strongly suggest that the iodine adlayer is constantly present, even after the striping Cu that was overpotential deposited.     

Electrochemical micromachining using a solid electrochemical reaction at the metal/β″-Al2O3 microcontact

A simple solid state technique for electrochemical micromachining of metal substrates using a metal ion conductor (Na-β″-Al₂O₃) was proposed. The fundamental solid electrochemical cell consists of a (anode) metal substrate (M = Ag, Cu, Zn, and Pb)/pyramidal Na-β″-Al₂O₃/Ag (cathode) system, where the contact diameter between M/Na-β″-Al₂O₃ was extremely small, on the order of a few micrometer. Under an applied electric field, the metal substrate was electrochemically oxidized to metal ions (Mⁿ⁺) at the M/Na-β″-Al₂O₃ microcontact. These Mⁿ⁺ ions migrated into the Na-β″-Al₂O₃. As a result of continuous electrolysis, the metal substrate was locally consumed at the microcontact, and thus solid state electrochemical micromachining was accomplished. As expected, the machining size or depth depended on the electrolysis conditions (current, operating time) and the apex configuration of pyramidal Na-β″-Al₂O₃. Moreover, the scanning of the Na-β″-Al₂O₃ pyramid during electrolysis produced a fine patterned metal substrate. In the present paper, solid state electrochemical micromachining was performed for several metal substrates, and its advantages and disadvantages vis-a-vis the conventional electrochemical micromachining method are discussed in detail.     

Electrochemical investigation of electrodeposited Fe-Pd alloy thin films

In the present study, the electrodeposition of Fe, Pd and Fe-Pd alloys, in alkaline solutions, has been investigated. Using ammonium hydroxide and trisodium citrate as the complexing agents, it has been shown that the co-deposition of Fe and Pd is achieved due to diminishing the difference between the reduction potentials of these two metals. Cyclic voltammetry results clearly show that the electrodeposition processes are diffusion-controlled and the diffusion coefficients of Fe²⁺ and Pd²⁺ are 1.11 × 10⁻⁶ and 2.19 × 10⁻⁵ cm² s⁻¹, respectively. The step potential experiments reveal that nucleation mechanism is instantaneous with a typical three-dimensional (3D) growth. At low overpotentials, addition of Pd²⁺ to Fe²⁺ solution leads to a dramatic reduction in the number of nucleation sites, due to this fact that at such overpotentials, the electrodeposition behavior of Pd²⁺ governs on the overall process. The analysis of chemical composition of the electrodeposited films and the number of nucleation sites indicate that at higher overpotential, Fe²⁺ is deposited preferentially, thus the electrodeposition of iron-palladium alloys was classified as an anomalous co-deposition.     

Silver-zinc electrodeposition from a thiourea solution with added EDTA or HEDTA

This paper shows the study of silver-zinc electrodeposition from a thiourea solution with added (ethylenedinitrilo)tetraacetic acid (EDTA), disodium salt and N-(2-hydroxyethyl)ethylenediaminetriacetic acid (HEDTA), trisodium salt. Voltammetric results indicated that silver-zinc alloy can be obtained applying overpotential higher than 0.495 V, in Tu solution containing 1.0 × 10⁻¹ mol L⁻¹ Zn(NO₃)₂ + 2.5 × 10⁻² mol L⁻¹ AgNO₃. This was due to silver(I) ion complexation with thiourea, which shifted the silver deposition potential to more negative value and due to silver-zinc alloy deposition, which occurred at potentials more positive than the potential to zinc deposition alone. EDTA or HEDTA did not significantly affect the silver and zinc deposition potentials, but decreased the current density for silver-zinc deposition. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses of the silver-zinc deposits showed that the morphology and composition changed as a function of the conditions of deposition, viz, deposition potential (E_d), deposition charge density (q_d) and solution composition (silver, EDTA and HEDTA concentrations). EDS analysis of the deposits showed sulphur (S) incorporated into the silver-zinc deposit, while SEM images showed that this sulphur content seemed to improve the silver-zinc morphology, as did the presence of EDTA and HEDTA in the solution, which enhanced the sulphur incorporation into the silver-zinc deposit. X-ray diffraction (XRD) analysis of the silver-zinc deposit showed that it was amorphous, irrespective of its composition and morphology.     

Effects of PVA content on the synthesis of LaFeO3 via sol-gel route

LaFeO₃ were synthesized via a sol-gel route based on polyvinyl alcohol (PVA). Differential scanning calorimetry (DSC), Thermogravimetric (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Raman spectroscopy and field emission scanning electron microscopy (FESEM) techniques were used to characterize precursors and derived oxide powders. The effect of the ratios of positively charged valences to hydroxyl groups of PVA (Mⁿ⁺/-OH) on the formation of LaFeO₃ was investigated. XRD analysis showed that single-phase and well-crystallized LaFeO₃ was obtained from the Mⁿ⁺/-OH = 4:1 molar ratio precursor at 700 °C. For the precursor with Mⁿ⁺/-OH = 2:1, nanocrystalline LaFeO₃ with average particle size of ∼50 nm was formed directly in the charring procedure. With increase of PVA content to Mⁿ⁺/-OH = 1:1, phase pure LaFeO₃ was obtained at 500 °C.