On the influence of the nucleation overpotential on island growth in electrodeposition

Electrochemical deposition of a metal onto a foreign substrate usually occurs by an island growth mechanism. A key feature of island growth for a material M on a foreign substrate S is that the onset potential for deposition is shifted negative from the equilibrium potential for the metal ion couple. The nucleation overpotential, defined as η_n(M⁺/S) = |U_n(M⁺/S) − U_eq(M⁺/M)|, influences key aspects of deposition of a metal on a foreign substrate. Here we discuss how the nucleation overpotential influences the kinetics of island growth, the implications of the nucleation overpotential on island shape and orientation, and the consequences of the coupling between the island density (applied potential) and the island size at coalescence (grain size). We then discuss the kinetics of island growth in terms of the contributions to vertical and lateral growth. Finally, we present examples of experimental methods to manipulate the nucleation overpotential and overcome some of the limitations imposed by the nucleation overpotential.     

Electrochemical deposition on surface nanometric defects: Thermodynamics and grand canonical Monte Carlo simulations

A thermodynamic analysis is performed on electrochemical metal deposition in the cavity of a foreign substrate. In particular, the deposition of Cu and Ag in nanometer-sized holes on Au(1 1 1) is studied by means of off-lattice atomistic Grand Canonical Monte Carlo simulations, using embedded atom method potentials. The present simulation conditions emulate experiments of electrochemical metal deposition in nanocavities, as performed in the literature. Depending on the system, remarkable differences are found in the way in which the defects are decorated, as well as in their energetics. When the interaction of the adsorbate atoms with the substrate is less favorable than the bulk interaction of the adsorbate, clusters are found that grow stepwise over the level of the surface. In the opposite case, the filling of the cavity occurs stepwise, without the occurrence of cluster growth above the surface level. The results of the simulations present a good qualitative agreement with experimental results from the literature.     

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.     

Aptamer-based electrochemical detection of protein using enzymatic silver deposition

In this work an electrochemical protein detection based on enzymatic silver deposition has been proposed and applied to the detection of thrombin. The target protein, thrombin, was first captured by thrombin binding thiolated aptamer self-assembled monolayers (SAMs) on the gold electrode surface, and then sandwiched with another biotinylated thrombin binding aptamer for the association of alkaline phosphatase (Av-ALP). The attached Av-ALP enzymatically converts the nonelectroactive substrate p-aminophenyl phosphate (p-APP) to p-aminophenol (p-AP) which can reduce silver ions in solution leading to deposition of the metal onto the electrode surface. Finally, linear sweep voltammetry (LSV) is used to detect the amount of deposited silver. The peak current during the anodic scan was found to reflect the amount of the target protein captured into the sandwich configuration. The proposed approach has been successfully implemented for the detection of thrombin in the range of 0.1 nM to 1 μM. Therefore, the current work has demonstrated enzymatic silver deposition for detection at aptamer-modified electrode.     

Tuning the architectures of lead deposits on metal substrates by electrodeposition

Different morphologies of lead (Pb) deposited on different metal substrates have been prepared via electrochemical deposition in aqueous solution. The morphologies of as-deposited lead were determined by scanning electron microscope (SEM). It is found that the various morphologies of the products are dependent on the electrodeposition conditions, including the deposition current densities, concentration of additives, substrates and deposition time. X-ray diffraction (XRD) and transmission electron microscope (TEM) results reveal that all these lead deposits with different morphologies can be assigned to the space group Fm-3m (2 2 5).     

Single molecule force spectroscopy of smart poly(ferrocenylsilane) macromolecules: Towards highly controlled redox-driven single chain motors

We investigated various stimuli-responsive poly(ferrocenylsilane) (PFS) polymers as model systems for the realization of molecular motors powered by a redox process. Covalently end-grafted PFS on gold, as well as PFS homopolymers and block copolymers in ultrathin films, were studied by AFM-based single molecule force spectroscopy (SMFS). Surface confined PFS macromolecules were chemically oxidized by addition of tetracyanoethylene or were completely and reversibly oxidized (and reduced) in situ by applying an electrochemical potential. Chemical oxidation was successful only for the block copolymers. The entropic elasticity of neutral PFS chains (Kuhn length I_K∼0.40 nm) was found to be larger than that of oxidized PFS chains (I_K∼0.65 nm) in the lower force regime. The elasticities could be reversibly controlled in situ by adjusting the applied potential in electrochemical SMFS experiments. For a single PFS macromolecule (DP=80) operating cycle, a work output and an efficiency of 3.4×10⁻¹⁹ J and ∼5%, respectively, were estimated based on the single chain experimental data.     

Anodic stripping voltammetric measurement of trace heavy metals at antimony film carbon paste electrode

The antimony film carbon paste electrode (SbF-CPE) was prepared in situ on the carbon paste substrate electrode as a “mercury-free” electrochemical sensor. Its aptitude for measuring some selected trace heavy metals has been demonstrated in combination with square-wave anodic stripping voltammetry in non-deaerated model solutions of 0.01 M hydrochloric acid with pH 2. Some important operational parameters, such as deposition potential, deposition time, and concentration of antimony ions were optimized, and the electroanalytical performance of the SbF-CPE was critically compared with both bismuth film carbon paste electrode (BiF-CPE) and mercury film carbon paste electrode (MF-CPE) using Cd(II) and Pb(II) as test metal ions. In comparison with BiF-CPE and MF-CPE, the SbF-CPE exhibited superior electroanalytical performance in more acidic medium (pH 2) associated with favorably low hydrogen evolution, improved stripping response for Cd(II), and moreover, stripping signals corresponding to Cd(II) and Pb(II) at the SbF-CPE were slightly narrower than those observed at bismuth and mercury counterparts. In addition, the comparison with antimony film electrode prepared at the glassy carbon substrate electrode displayed higher stripping current response recorded at the SbF-CPE. The newly developed sensor revealed highly linear behavior in the examined concentration range from 5 to 50 μg L⁻¹, with limits of detection (3σ) of 0.8 μg L⁻¹ for Cd(II), and 0.2 μg L⁻¹ for Pb(II) in connection with 120 s deposition step, offering good reproducibility of ±3.8% for Cd(II), and ±1.2% for Pb(II) (30 μg L⁻¹, n = 10). Preliminary experiments disclosed that SbF-CPE and MF-CPE exhibit comparable performance for measuring trace concentration levels of Zn(II) in acidic medium with pH 2, whereas its detection with BiF-CPE was practically impossible. Finally, the practical applicability of SbF-CPE was demonstrated via measuring Cd(II) and Pb(II) in a real water sample.     

Characterization of electrode/electrolyte interface using in situ X-ray reflectometry and LiNi0.8Co0.2O2 epitaxial film electrode synthesized by pulsed laser deposition method

An in situ experimental technique was developed for detecting structure changes at the electrode/electrolyte interface of lithium cell using synchrotron X-ray reflectometry and two-dimensional model electrodes with a restricted lattice plane. The electrode was constructed with an epitaxial film of LiNi_0.8Co_0.2O₂ synthesized by the pulsed laser deposition method. The orientation of the epitaxial film depends on the substrate plane; the 2D layer of LiNi_0.8Co_0.2O₂ is parallel to the SrTiO₃ (1 1 1) substrate ((0 0 3)_{LiCo0.2Ni0.8O2}//(1 1 1)SrTiO₃), while the 2D layer is perpendicular to the SrTiO₃ (1 1 0) substrate ((1 1 0)_{LiCo0.2Ni0.8O2}//(1 1 0)SrTiO₃). These films provided an ideal reaction field suitable for detecting structure changes at the electrode/electrolyte interface during the electrochemical reaction. The X-ray reflectometry indicated a formation of a thin-film layer at the LiNi_0.8Co_0.2O₂ (1 1 0)/electrolyte interface during the first charge-discharge cycle, while the LiNi_0.8Co_0.2O₂ (0 0 3) surface showed an increase in the surface roughness without forming the surface thin-film layer. The reaction mechanism at the electrode/electrolyte interface is discussed based on our new experimental technique for lithium batteries.     

Electrochemical synthesis and characterization of mixed molybdenum-rhenium oxides

The electrodeposition of Mo_xRe_1−xO_y films (0.6 ≤ x ≤ 1) on indium-tin oxide (ITO) coated glass substrates from acidic peroxo-polymolybdo-perrhenate solutions is described. Trends in film growth were established as a function of potential from +0.4 V to −0.7 V vs Ag/AgCl by analyzing the composition and stoichiometry of the deposit using inductively coupled plasma mass spectrometry (ICPMS) and X-ray photoelectron spectroscopy (XPS). These experiments show that the concentration of rhenium increases linearly with the deposition potential and that the deposits are mixed-valent containing up to five different metal oxidation states (i.e., Mo^IV, Mo^V, Mo^VI, Re⁰, Re^IV). Electroanalytical techniques were used to explore the deposition mechanism, including chronocoulometry, cyclic voltammetry, spectroelectrochemistry, and electrochemical quartz crystal nanogravimetry (EQCN). At potentials positive to −0.26 V, perrhenate (Re^VIIO₄⁻) behaves as a redox mediator to accelerate the deposition of a mixed-valent molybdenum oxide, but at more negative potentials mixed molybdenum-rhenium oxides are produced.     

Carbon monoxide oxidation on Au(1 1 1) surface decorated by spontaneously deposited Pt

Platinum is deposited spontaneously on Au(1 1 1) surface from 1 mM H₂PtCl₆ + 1 M HClO₄ solution using multiple deposition procedure. X-ray photoelectron spectroscopy (XPS) analysis has shown that after immersion into the Pt containing solution and rinsing with water, Pt(OH)₂ resides on the Au(1 1 1) substrate. Consecutive depositions as well as in situ scanning tunneling microscopy (STM) and electrochemical measurements are performed on previously electrochemically reduced Pt/Au(1 1 1) surfaces. Only homogeneous distribution of thus deposited Pt islands is observed by in situ STM. With subsequent depositions, the width of deposited Pt islands increases, but stays lower than 10 nm, while a significant increase of Pt islands height is observed, leading to moderate increase of the coverage. Cyclic voltammetry (CV) profiles of obtained Pt/Au(1 1 1) surfaces, and CO stripping curves are recorded in 0.5 M H₂SO₄ solution. CO oxidation takes place only at higher potentials shifting negatively with increasing coverage. This is discussed with respect to Pt islands width and height distributions and to the influence of the Au(1 1 1) substrate surface.     

Molybdenum alloy electrodeposits for magnetic actuation

This paper deals with the preparation and characterisation of alloys containing molybdenum applicable in MEMS. An electrodeposition process for achieving a homogeneous, low-stressed, soft-magnetic Co-Ni-Mo alloy is described. The electrochemical study allows setting bath composition and deposition conditions useful to perform the deposition process. Deposits with low nickel and molybdenum percentages (11-15 wt%) are virtually useful for magnetic actuation applications. Saccharine is an effective antistress agent and it also decreases grain size and surface roughness of the Co-Ni-Mo layers. The mechanical and magnetic properties of the ternary alloy have been compared with those shown by a binary Co-Mo alloy containing similar molybdenum percentages previously electrodeposited in our laboratory. Thin Co-Ni-Mo films exhibited higher microhardness values than Co-Mo films and better magnetic properties for magnetic actuation (H_c = 50 Oe, M_r = 90 emu g⁻¹ and μ_r = 670). Furthermore, film appearance (such as brightness) and corrosion resistance improved due to nickel presence. Electrodeposition has been tested on silicon/seed-layer substrates and the process selectivity has been investigated on photolithographed silicon. Both alloys are compatible with fabrication techniques involved in MEMS technology. No damage was observed when silicon was removed over alloy films to get stand-alone layers. The response of a microsized valve (silicon + seed-layer + electrodeposited Co-Mo or Co-Ni-Mo film) under an external magnetic field has been explored with good results.     

Correlation of morphology and barrier properties of thin microwave plasma polymer films on metal substrate

The barrier properties of thin model organosilicon plasma polymers layers on iron are characterised by means of electrochemical impedance spectroscopy (EIS). Tailored thin plasma polymers of controlled morphology and chemical composition were deposited from a microwave discharge. By the analysis of the obtained impedance diagrams, the evolution of the water uptake ?, coating resistance and polymer capacitance with immersion time were monitored and the diffusion coefficients of the water through the films were calculated. The impedance data correlated well with the chemical structure and morphology of the plasma polymer films with a thickness of less than 100 nm. The composition of the films were determined by means of infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The morphology of the plasma polymer surface and the interface between the plasma polymer and the metal were characterised using atomic force microscopy (AFM). It could be shown that, at higher pressure, the film roughness increases which is probably due to the adsorption of plasma polymer nanoparticles formed in the plasma bulk and the faster film growth. This leads to voids with a size of a few tens of nanometers at the polymer/metal interface. The film roughness increases from the interface to the outer surface of the film. By lowering the pressure and thereby slowing the deposition rate, the plasma polymers perfectly imitate the substrate topography and lead to an excellent blocking of the metal surface. Moreover, the ratio of siloxane bonds to methyl-silyl groups increases which implies that the crosslink density is higher at lower deposition rate. The EIS data consistently showed higher coating resistance as well as lower interfacial capacitance values and a better stability over time for the film deposited at slower pressure. The diffusion coefficient of water in thin and ultra-thin plasma polymer films could be quantified for the smooth films. The measurements show that the quantitative evaluation of the electrochemical impedance data requires a detailed understanding of the film morphology and chemical composition. In addition, the measured diffusion coefficient of about 1.5×10⁻¹⁴ cm² s⁻¹ shows that plasma polymers can act as corrosion resistant barrier layers at polymer/metal interfaces.     

Seed-mediated electrochemical growth of gold nanostructures on indium tin oxide thin films

Two-dimensional gold nanostructures (Au NSs) were fabricated on amine-terminated indium tin oxide (ITO) thin films using constant potential electrolysis. By controlling the deposition time and by choosing the appropriate ITO surface, Au NSs with different shapes were generated. When Au NSs were formed directly on aminosilane-modified ITO, the surface roughness of the interface was largely enhanced. Modification of such Au NSs with n-tetradecanethiol resulted in a highly hydrophobic interface with a water contact angle of 144°. Aminosilane-modified ITO films further modified with colloidal Au seeds before electrochemical Au NSs formation demonstrated interesting optical properties. Depending on the deposition time, surface colors ranging from pale pink to beatgold-like were observed. The optical properties and the chemical stability of the interfaces were characterized using UV-vis absorption spectroscopy. Well-defined localized surface plasmon resonance signals were recorded on Au-seeded interfaces with λ_max = 675 ± 2 nm (deposition time 180 s). The prepared interfaces exhibited long-term stability in various solvents and responded linearly to changes in the corresponding refractive indices.     

Novel mixed method for the electrochemical deposition of thick layers of Bi2+xTe3−x with controlled stoichiometry

A new method for the electrochemical deposition of Bi_2+xTe_3−x is presented, which combines voltage-controlled deposition pulses with current-controlled resting pulses. This method is based on results of a comprehensive electrochemical investigation including cyclic voltammetry, chronoamperometry and chronopotentiometry, which has been performed on the system Bi and Te on Pt in 2 M HNO₃. The influence of electrolyte composition, deposition potential and deposition pulse duration on morphology and stoichiometry of the deposited material as well as the variation of the composition over the thickness of the layer has been investigated by means of SEM and EDX. The crystal structure was examined with XRD. Layers deposited with the new method show a constant and reproducible stoichiometry over their entire thickness. Layers of up to 800 μm thickness deposited with deposition rates of up to 50 μm/h have been achieved. The composition and hence the thermoelectric behavior may be adjusted via electrolyte composition or the deposition potential. Fabrication of n-type and, for the first time, p-type Bi_2+xTe_3−x is demonstrated and verified by measurements of the Seebeck coefficients. The suitability of the proposed method for low-cost fabrication of micro-thermoelectric devices is shown. The advantages of this method may also apply for electrochemical deposition of other binary or ternary compounds.     

Cathodic electrodeposition of MnOx films for electrochemical supercapacitors

Cathodic electrosynthesis has been utilized for the fabrication of MnO_x films. The use of polyethylenimine (PEI) as an additive enabled the formation of adherent films, which exhibited enhanced resistance to cracking during drying. The polymer content in the deposits can be varied by the variation of the polymer concentration in the solutions. The mechanism of PEI deposition was proposed which is based on the use of PEI-Mn²⁺ complexes. The deposition yield has been studied at different deposition durations. X-ray diffraction analysis showed the crystallization of Mn₃O₄ phase at 300 °C and Mn₂O₃ at 500 °C. The electrochemical performance of the MnO_x films sintered at different temperatures was studied by cyclic voltammetry (CV), chronopotentiometry and impedance spectroscopy in Na₂SO₄ solutions. The films showed excellent pseudocapacitive behavior. The specific capacitance (SC) of 425 F/g in a potential window of 0-0.9 V was obtained from the CV data at a scan rate of 10 mV/s. The SC calculated from the chronopotentiometry data is about 445 F/g. The SC decreased by ∼20% after 1000 cycles. SEM investigations revealed changes in the film morphology during cycling. Obtained results indicate that the proposed method can be used for the fabrication of electrodes for electrochemical supercapacitors.     

Electrochemical growth and dissolution of Ni on bimetallic Pd/Au(1 1 1) substrates

In this work we study the electrochemical growth and dissolution of a Ni on Pd-Au(1 1 1) bimetallic surfaces using in situ scanning tunnelling microscopy. We also compare Ni deposition on monometallic electrodes, i.e. Au(1 1 1) and Pd(1ML)/Au(1 1 1), using electrochemical characterizations. Results evidence that the first Ni monolayer grows preferentially on Au(1 1 1) in a wide potential range, and that a full Ni monolayer covering the entire Pd-Au surface can be selectively dissolved from Pd islands. No such selectivity is observed upon growth of subsequent Ni atomic planes. We demonstrate that the Ni-substrate interactions play a key role in the above mentioned selectivity. The binding energy of Ni to Pd is found to be 80 meV smaller than of Ni to Au. The sign and the amplitude of this difference are discussed in light of the d band filling of the Pd-Au(1 1 1) bimetallic surface and the presence of adsorbed H on Pd before deposition.     

Electrochemistry of Sn(II)/Sn in a hydrophobic room-temperature ionic liquid

The electrochemical reaction of Sn(II)/Sn was investigated in a room-temperature ionic liquid, 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI) at 25 °C. The anodic dissolution of Sn metal proceeded by a two-electron transfer reaction with a current efficiency of nearly 100%. Electrodeposition of Sn on a Cu substrate is possible in BMPTFSI containing Sn(II). The formal potential of the Sn(II)/Sn is −0.57 V vs. Ag/Ag(I). The diffusion coefficient of Sn(II) was estimated to be ∼1 × 10⁻⁷ cm² s⁻¹ from chronoamperometric and chronopotentiometric techniques. The initial stage of nucleation of Sn on a polycrystalline Pt substrate was found to be classified into a three-dimensional progressive nucleation under diffusion control by chronoamperometry, suggesting the rate of nucleation is faster than that of crystal growth.     

Strategy for the syntheses of isolated fine silver nanoparticles and polypyrrole/silver nanocomposites on gold substrates

In this work, isolated fine silver nanoparticles and polypyrrole/silver nanocomposites with diameters of about 10 nm on gold substrates were first prepared by electrochemical methods. First, an Ag substrate was cycled in a deoxygenated aqueous solution containing 0.1 M HCl from −0.30 to +0.30 V versus Ag/AgCl at 5 mV/s with 30 scans. Subsequently the Ag working electrode was immediately replaced by an Au electrode and a cathodic overpotential of 0.2 V was applied under controlled sonication to synthesize Ag nanoparticles on the Au electrode. Then pyrrole monomers were encouragingly found to be polymerized on the deposited Ag nanoparticles. This polymerization is distinguishable from the known chemical or electrochemical one, due to the electrochemical activity of unreduced species of Ag_n⁺ clusters inside the nanoparticles. Also, this polymerization may be ascribed to the oxidizing agent of AuCl₄⁻, which is present on the Au electrode.     

A comparative electrochemical study of iron deposition onto n- and p-type porous silicon prepared from lightly doped substrates

Electrodeposition of iron from acidic sulfate solutions onto porous silicon (PS) prepared from n- and p-type (1 0 0) substrates is studied by electrochemical measurements. Results from current-potential curves show that deposition of iron on p-type PS can only be achieved under illumination and cathodic polarization, whereas the deposition is found to proceed on n-type even in the dark. The measurements of the cathodic current efficiency indicate that the fraction of current used for iron deposition decreases with the applied potential due to hydrogen evolution reaction which is a competing reaction to metal deposition. Scanning electron microscopy shows that very fine iron crystallites with an average size of 40-70 nm are formed under double potential step conditions. The energy band diagrams of silicon-solution interfaces determined by electrochemical impedance measurements reveal that the iron deposition mechanism on both substrates is electron transfer from the conduction band.     

Electrophoretic deposition of nanocomposites formed from polythiophene and metal oxides

An electrophoretic deposition (EPD) procedure was adopted for the cathodic preparation of thin films of conducting polymer/metal oxide nanocomposites with a core-shell structure. The deposition process was investigated at different potentials and in various solvents. The mechanism and kinetics of the electrophoretic deposition were studied via quartz crystal microbalance (QCM) and zeta-potential measurements.The properties of the composite layers were studied by electrochemical methods (cyclic voltammetry, impedance spectroscopy) and photocurrent measurements. The reversible redox potential of polythiophene films was about 0.75 V_SCE. The p-type semiconducting behaviour of the reduced polythiophene was studied by photocurrent measurements. In the case of using TiO₂ (n-type semiconductor) as a core material, an n/p heterojunction was observed. In the photocurrent spectra the maximum of the cathodic peak of polythiophene was found around λ = 500 nm (2.5 eV), depending on the applied potential. It is in agreement with the results of UV-vis optical spectra of deposited layers and of pressed pellets. The flatband potential of polythiophene in the heterojunction with TiO₂, obtained from photocurrent measurements, was 0.53 V_SCE.