Nikola Batina

Detailed characterization of potentiostatic current transients with 2D-2D and 2D-3D nucleation transitions

Abstract Cyclic voltammetry, chronoamperometry and atomic force microscopy were used to study the kinetics of nucleation and crystal growth in the initial stages of silver overpotential deposition onto a vitreous carbon electrode from an aqueous solution of 10 −3 M Ag(I) in 1.6M NH 4 OH and 1M KNO 3 (pH 11). The silver deposition process was found to be complex and occurred via two different pathways as a function of the applied deposition potential. The identification and characterization of each pathway was based on a quantitative analysis of the current transient curves, which show that within a single potentiostatic current transient, initial progressive nucleation and two-dimensional (2D) growth was followed either by other 2D nucleation and growth processes (lower overpotentials) or by three-dimensional (3D) growth controlled by incorporation of adatoms into the lattice (higher overpotentials). In order to properly identify and interpret each step during the deposition process, the characteristic features of the current transient curve were treated separately using existing mathematical formalisms. After proper identification of all processes within the recorded transients, a general equation for the whole process including contributions from adequate 2D-2D or 2D-3D nucleation transitions and double-layer charging effects was derived. A comparison of theoretically calculated and experimentally obtained current transients via a non-linear fitting method showed that the different consecutive nucleation and growth processes actually overlap. AFM analysis, which was used to characterize the surface morphology of the silver deposit, confirmed the electrochemical findings. AFM images revealed a different morphology of the silver deposit in relation to the existence of 2D or 3D silver deposits, depending on the applied electrode potential. The enhanced growth of 3D silver clusters around surface imperfections on the vitreous carbon electrode surface observed during the early stages of deposition provides an explanation for the origin of the overlap of the nucleation processes.

Determination of elemental sulphur, sulphide and their mixtures in electrolyte solutions by a.c. voltammetry

The electrochemical behaviour of elemental sulphur, sulphide and their mixtures on a mercury electrode in 0.5 mol dm−3 NaCl-NaOH supporting electrolyte at pH 10 with and without the addition of 10% (v/v) methanol and 1 × 10−5 mol dm−3 toluene was studied. Investigations were carried out over a wide concentration range (from 1 × 10−8 to 1 × 10−3 mol dm−3) of sulphur species in solution by a.c. voltammetry. Differences in the electrochemical behaviour during the formation of HgS at the electrode surface were observed between the two investigated species under different experimental conditions such as accumulation at different potential and voltammetric scan directions. Based on these differences, a method for the determination of sulphide, elemental sulphur and their mixtures in the aqueous solution is proposed. The solubility of elemental sulphur in aqueous medium at room temperature was determined as 1.7 × 10−6 mol dm−3. The usefulness of the proposed method for the direct determination of sulphide and elemental sulphur in natural waters is discussed.

Interfacial structure of iodine electrodeposited on Au(111): studies by LEED and in situ STM

Abstract Iodine adlayers formed on Au(111) in aqueous KI solution have been studied by ex situ Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and in situ scanning tunneling microscopy (STM). The LEED patterns are described as c ( p × 3 R − 30 ° ) where p decreases continuously with increasing emersion potential. By in situ STM, a series of rotated hexagonal structures was observed at positive potentials in addition to the c ( p × 3 R − 30 ° ) structures. This rotated hexagonal adlattice was also compressed with increasing electrode potential. These results in general confirm the phenomenon of electrocompression of the iodine adlayer on Au(111) observed by in situ surface X-ray scattering.

Formation Mechanisms and Characterization of Black and White Cobalt Electrodeposition onto Stainless Steel

Cobalt electrodeposition onto a stainless steel substrate from 1.17 M Co(II) aqueous solution containing 0.98 M H 2SO4, 0.56 M KCl, and 0.2 M H3BO3 was evaluated in the absence (i) and presence (ii) of 0.1 M KNO3. Cobalt electrodeposited from the electrolytic bath (i) was white-gray colored, whereas deposition from bath ( ii) formed a black-colored surface. SEM-WDX, AFM, and XRD analysis of the steel surfaces covered with these two deposits revealed distinct characteristics for black and white cobalt fil ms. Although both deposits were composed of metallic cobalt, the white cobalt deposit was a smooth, 2D film while the black deposit consisted of many dispersed, nano-sized clusters of 150 to 250 nm in diameter. Analysis of potentiostatic current transients ( I-t curves) indicated that formation of white cobalt was carried out by multiple 3D nucleation limited by lattice incorporation of cobalt a datoms to the growth centers. Formation of black cobalt was shown to involve the simultaneous processes of 3D nucleus formation and growth, limited by mass transfer, and the reduction of nitrates in the medium onto the surfaces of these nuclei. It is shown that , beside this cobalt-nitrate interaction, NO 3 ions in solution can block active sites for cobalt reduction and the effect of this phenomenon

Identification of different silver nucleation processes on vitreous carbon surfaces from an ammonia electrolytic bath

We performed an electrochemical study of silver electrodeposition from an electrolytic bath containing 1 M NH4OH and 1 M KNO3 (pH=11), over a Ag(I) concentration range of 10−4 to 0.3 M, and identified silver nucleation processes on the vitreous carbon surface. Depending upon the silver concentration in the deposition bath, growth occurred either two-dimensionally (2D), controlled by adatom incorporation, or three-dimensionally (3D), controlled by diffusion or lattice incorporation. Qualitative and quantitative characterization of the observed nucleation processes were based on the results of cyclic voltammetry and chronoamperometry analysis. For quantitative characterization, different theoretical models related to electrocrystallization processes were used. Atomic force microscopy (AFM) was employed to probe the surface morphology of the silver deposit. AFM images revealed that silver deposits formed from ammonia baths with different concentrations of Ag(I) ion possess different morphological characteristics. Indeed, many of the surface characteristics clearly corroborated the mechanism of silver deposition proposed by electrochemical analysis.

Effect of the Electrosynthesis Method on the Surface Morphology of the Polypyrrole Film An Atomic Force Microscopy Study

The surface morphology of polypyrrole (ppy) films prepared by potentiostatic and voltammetric methods at the platinum electrode was studied by ex situ atomic force microscopy (AFM). The study was mainly focused on the early stages of polymer film growth up to a film thickness of 30 to 50 nm. AFM analysis revealed that polymer film morphology depends on the electrosynthesis method used. Electrosynthesized films produced by the voltammetric method consisted of nano-size ppy nodules, which during the polymer oxidation process become associated forming larger surface aggregates. However, in thicker films and those prepared using a large anion, this association was not observed. The ppy films formed by the potential step method (potentiostatic) allowed a better control of the early stages of the electropolymerization process. Thus, the AFM study showed that the first ppy nodules grow three-dimensionally during the application of the potential pulse steps, and simultaneously their number increase on the electrode surface according to the progressive nucleation and growth model. Negligible association of ppy nodules was noticed for the film prepared by the potentiostatic method, which suggests that this synthesis method is a better alternative for controlling ppy film growth.

Cobalt oxide films grown by a dipping sol-gel process

Cobalt oxide thin films were prepared by the dipping sol-gel process, using two different inorganic precursors: cobalt chloride and cobalt nitrate salts. Also, samples with a different number of dipping-annealing cycles (3, 5, and 7) where prepared. Composition, structure, surface morphology and optical properties of such films have been characterised by means of X-ray diffraction, differential thermogravimetric analysis (DTA), transmittance spectra and atomic force microscopy (AFM). The results show that starting from distinct precursors leads to different properties: film water contents, surface roughness, crystallite size, total film transmittance, absorption coefficient and refractive index. Absorption coefficients higher than 104 cm−1 where found for all the samples. Refractive indices vary from n ~ 1.9–2.8 in the near infrared region. Our study shows that using a relatively simple preparation method like the sol-gel process, cobalt oxide films with specific properties, can be made.

Micro/nanomachining of Polymer Surface for Promoting Osteoblast Cell Adhesion

Interactions between cells and biomaterials are affected by surface properties. Therefore, various approaches have been introduced for surface modifications. Here a technique based on ion beam lithography to improve osteoblast cell adhesion on polymeric materials is reported. We have demonstrated that exposing the polymer to P+ or Ar+ ions through masks can generate micro/nano-scale patterns. Our results illustrate that after exposure to an ion beam, the amount of osteoblast cells attached to the polymer was enhanced as a consequence of the roughened surface as well as due to the implanted ions. This indicates that masked ion beam lithography (MIBL) can not only generate nanostructures on the surface of a biocompatible polymer, but can also selectively modify the surface chemistry by implanting with specific ions. These factors can contribute to an osteogenic environment.

Oriented adsorption at well-defined electrode surfaces studied by Auger, leed, and eels spectroscopy

Abstract Quantitation by use of Auger spectroscopy and cyclic voltammetry of molecular layers adsorbed at Pt(111) and Pt(100) surfaces from aqueous electrolytes is examined in this work for the following compounds: hydroquinone (HQ); phenol (PL); catechol (CT); 3,4-dihydroxyphenylacetic acid (DOPAC); L-3,4-dihydroxyphenylalanine (DOPA); l -tyrosine (TYR); l -phenylalanine (PHE); nicotinic acid (NA); 2,5-dihydroxy-4-methyl-benzyl mercaptan (DMBM); thiophenol (TP); benzylmercaptan (BM); 3-thiophene carboxylic acid; and 2,5,2′,5′-tetrahydroxybiphenyl (THBP). Two independent methods of measurement of packing density based upon Auger spectroscopy, and one based upon cyclic voltammetry are employed and the results compared. Voltammetric oxidation/reduction of adsorbed layers formed from these compounds at Pt surfaces in aqueous electrolyte is found to be essentially the same whether carried out before or after lengthy evacuation. Therefore, the results of surface spectroscopy in UHV are directly applicable to the liquid—solid chemistry and electrochemistry of these adsorbed compounds. Packing densities measured by means of two Auger spectroscopic methods were in good agreement with each other and with the voltammetric measurements.

The adsorption of sodium dodecyl sulphate at the mercury electrode and its effect on the electrochemical processes of cadmium(II) in sodium chloride solution

Abstract The adsorption of sodium dodecyl sulphate (SDS) at the mercury electrode in 0.55 mol dm −3 sodium chloride has been studied on the basis of capacity measurements using ac polarography. Investigations were carried out in a wide concentration range of SDS (3.8 × 10 −7 −1.9 x 10 −3 mol dm −3 ) at the dropping mercury electrode (DME) and at the hanging mercury drop electrode (HMDE) with a prior accumulation of surfactant at the potential E = −0.6 V vs. SCE. The effect of SDS upon the electrode reaction of cadmium was studied by differential pulse polarography. The rate constant of the electrode reaction of cadmium, k s , was estimated from the shape and the height of differential pulse polarograms using theoretical curves obtained by digital simulation. It was found that association processes both in the bulk solution as well as at the electrode surface play an important role in the adsorption and inhibition processes of SDS. An inhibition effect for cadmium is considerably decreased at the HMDE and at bulk concentrations near to the CMC value.