A. J. Arvia

Anodisation of copper in thiourea- and formamidine disulphide-containing acid solution.: Part I. Identification of products and reaction pathway

Abstract The anodic behaviour of copper in aqueous 0.5 M sulphuric acid containing different amounts of dissolved thiourea or formamidine disulphide was investigated at 298 K, combining data from electrochemical polarisation, chemical analysis, UV–vis spectroscopy, XPS and EDAX analysis, and structural information on copper–thiourea complexes. The main reactions depend on the applied potential and initial thiourea concentration. In the potential range −0.30≤E≤0.075 V (versus SCE), the electro-oxidation of thiourea to formamidine disulphide, the formation of Cu(I)–thiourea soluble complexes, and Cu(I)–thiourea complex polymer-like films, are the most relevant processes. The formation of this film depends on certain critical thiourea/copper ion molar concentration ratios at the reaction interface. At low positive potentials, the former reaction is under intermediate kinetic control, with the diffusion of thiourea from the solution playing a key role. For E≥0.075 V, soluble Cu(II) ions in the solution are formed and the anodic film is gradually changed to another one consisting of copper sulphide and residual copper. The new film assists the localised electrodissolution of copper. A complex reaction pathway for copper anodisation in these media for the low and high potential range is advanced.

The dynamic behavior of butanethiol and dodecanethiol adsorbates on Au(111) terraces

The dynamics of butanethiol and dodecanethiol monolayers adsorbed on Au(111) studied by ex situ and in situ sequential scanning tunneling microscopy (STM) shows, at room temperature and in the range of seconds, √3×√3 R30°⇔c(4×2) transitions. High-resolution STM imaging also shows that these transitions can be explained by a displacement of adsorbed molecules from hollow to bridge sites and vice versa. Transitions from the p(n×1) superlattice to the √3×√3 R30° lattice were also imaged in real time. These processes appear to be coupled with fluctuations of the hole size of the Au(111) terrace.

Dynamic characteristics of adsorbed monolayers of 1-dodecanethiol on gold (111) terraces from in-situ scanning tunneling microscopy imaging

Abstract The dynamics of 1-dodecanethiol self-assembled monolayers (SAMs) on Au(111) produced by contacting pure thiol was followed by in situ STM at 298 K. Initially, these SAMs constituted a heterogeneous surface consisting of disordered and ordered adsorbate domains at terraces, step edges and pits, forming a p(6×1) superlattice which later changed to the ( 3 × 3 )R30° structure. Subsequently, the c(4×2) superlattice was also observed. Surface dynamics involves the coalescence and Ostwald ripening phenomena at pits occurring simultaneously with adsorbate ordering at different surface domains. The difference in surface mobility between adsorbate-free and 1-dodecanethiol-covered Au(111) can be explained taking into account both gold lattice relaxation due to adsorption and alkyl chain interaction at SAM. Data analysis in terms of clustering theory allowed us to conclude that a single mechanism is likely involved in the overall surface phenomena in which the mass transport is either a nonsteady state surface diffusion or an interface transfer along step edges as rate-determining step.

X-ray diffraction study of copper(I) thiourea complexes formed in sulfate-containing acid ­solutions

The formation of three different copper(I) thiourea complexes in sulfate-containing acid solutions was observed. The ratio between Cu(I) and thiourea (tu) in these complexes depends on the amount of thiourea and copper sulfate in the solution. The crystal and molecular structure of a new complex, [Cu(2)(tu)(6)](SO(4)).H(2)O, was determined, and the formation and structures of [Cu(2)(tu)(5)](SO(4)).3H(2)O and [Cu(4)(tu)(7)](SO(4))(2).H(2)O were confirmed. The compound [Cu(2)(tu)(6)](SO(4)).H(2)O crystallizes in the P1; space group, with a = 11.079 (2), b = 11.262 (1), c = 12.195 (2) A, alpha = 64.84 (1), beta = 76.12 (1), gamma = 66.06 (1) degrees, and Z = 2. The Cu-thiourea complex is arranged as a Cu(I) tetranuclear ion, [Cu(4)(tu)(12)](4+), sited on a crystallographic inversion center. All copper ions are in a tetrahedral coordination with thiourea ligands and located at alternate sites on an eight-membered, crown-like ring.

Anodisation of copper in thiourea-containing acid solution: Part II. In situ transversal imaging observations. Kinetics of anodic film growth

Abstract The formation of anodic films during the anodisation of copper, at different applied potentials E, in aqueous 0.5 M sulphuric acid containing different amounts of dissolved thiourea was investigated following the corroding electrode profile by on line in situ imaging. For E 0.07 V, the main reactions are the electro-decomposition of formamidine disulphide and Cu(I)–thiourea complexes yielding a copper sulphide-containing film (film II) and the electrodissolution of copper as aqueous Cu(II) ions through film II. The relative contribution of these processes depends on thiourea concentration in the solution, the applied electric potential and anodisation time. The growth kinetics of films I and II were determined from the evolution of the average film height 〈h〉 obtained from in situ imaging. The kinetics of film I fit a parabolic rate law, whereas those of film II approach a linear 〈h〉 versus anodisation time relationship. The rupture of film II assists the localised corrosion of copper. Likely physical mechanisms for the formation of these anodic films are discussed.

Roughening kinetics of chemical vapor deposited copper films on Si(100)

The roughening kinetics of copper films synthesized by low pressure chemical vapor deposition (LPCVD) on Si(100) substrates was investigated by scanning tunneling microscopy (STM). By applying the dynamic scaling theory to the STM images, a steady growth roughness exponent α=0.81±0.05 and a dynamic growth roughness exponent β=0.62±0.09 were determined. The value of α is consistent with growth model predictions incorporating surface diffusion. The value of β, while higher than expected from these models, can be related to LPCVD processing conditions favoring growth instabilities.

Two thiourea-containing gold(I) complexes.

The crystal structures of two salts of bis(thiourea)gold(I) complexes, namely bis(thiourea-kappaS)gold(I) chloride, [Au(CH(4)N(2)S)(2)]Cl, (I), and bis[bis(thiourea-kappaS)gold(I)] sulfate, [Au(CH(4)N(2)S)(2)](2)SO(4), (II), have been determined. The chloride salt, (I), is isomorphous with the corresponding bromide salt, although there are differences in the bonding. The Au(I) ion is located on an inversion centre and coordinated by two symmetry-related thiourea ligands through the lone pairs on their S atoms [Au-S 2.278 (2) A and Au-S-C 105.3 (2) degrees ]. The sulfate salt, (II), crystallizes with four independent [Au(CH(4)N(2)S)(2)]+ cations per asymmetric unit, all with nearly linear S-Au-S bonding. The cations in (II) have similar conformations to that found for (I). The Au-S distances range from 2.276 (3) to 2.287 (3) A and the Au-S-C angles from 173.5 (1) to 177.7 (1) degrees. These data are relevant in interpreting different electrochemical processes where gold-thiourea species are formed.

Dynamic scaling analysis of scanning force microscopy images of electrochemically formed polyaniline films in the oxidized form scale-dependent roughening kinetics

The morphology of the oxidized form of electrodeposited polyaniline films (PANI) grown on platinum under potentiodynamic cycling has been studied by scanning force microscopy (SFM). For average film height 〈h〉 1000 nm, a highly branched deposit was produced. The dynamic scaling analysis of the granular-deposit SFM image scans revealed the characteristic features of a self-affine fractal surface with two sets of roughness exponents α and β, depending whether the length of the SFM scan, Ls, involved in the analysis was smaller or larger than the average grain size ds of the polymer film. For Ls < ds, α(I)= 0.90 and β(I)= 0.4, whereas for ds⩽Ls⩽ 1000 nm, α(II)= 0.4 and β(II)= 0.6 were obtained. Both high values of β are consistent with the contribution to the growth process of a directional electric field, which at advanced stages of growth leads to the development of the highly branched deposit. A correlation between the analysis of PANI morphology and potentiostatic PANI electroformation current transient data is also presented.

Voltammetric anodic stripping of silver from platinum electrodes consisting of an array of nm-sized voids

Abstract Silver voltammetric anodic stripping from stabilized rough platinum electrodes in 1 M H 2 SO 4 + c o Ag 2 SO 4 (5×10 −4 M≤ c o ≤10 −3 M) and 1 M H 2 SO 4 aqueous solutions, at 298 K, was investigated. The topography of these electrodes was determined by scanning tunneling microscopy (STM). The roughness factor, R , was varied from 10 to 71. The analysis of STM images showed that the rough structure consists of an array of nm-sized protrusions and voids as a columnar structured surface. For each value of R , the average size and volume of voids were determined. The voltammetric features of silver anodic stripping depend on R and the electrodeposited silver charge, Q Ag . The value of Q Ag (vs), the saturation value of Q Ag for voids, was estimated. The anodic stripping of underpotential deposited silver occurs in the potential range where oxygen electroadsorption on platinum takes place. By using data derived from STM imaging and equations that have been derived for the voltammetric anodic stripping reactions, in which the reactant is confined to nm-sized voids, the voltammetric behaviour of the system can be reproduced reasonably.

COMPARATIVE STUDY OF THIOL FILMS ON C(0001) AND Au(111) SURFACES BY SCANNING PROBE MICROSCOPY

The structures resulting from 1-dodecanethiol, 1-butanethiol and 1,9-nonanedithiol films produced on highly oriented pyrolytic graphite (HOPG) and gold(111) have been comparatively studied by scanning probe microscopies. Molecular resolution images resulting from atomic force microscopy (AFM) and scanning tunneling microscopy (STM) of different thiol films show the formation of arrays of molecules parallel to the HOPG surface. The electrochemical response of the ferro-ferricyanide reaction was used to test the characteristics of electron transfer processes in thiol-covered HOPG as compared to the bare substrate. The decrease in the heterogeneous rate constant for the test reaction appears to be directly related to the degree of film thickness uniformity. For comparison, films with the same kind of thiols were produced on Au(111). Although the electrochemical characteristics of these films appear to be the same irrespective of the substrate nature, the structure of the films on Au(111) is different from that produced on HOPG.