Nucleation current transients

By electrodeposition of CuInSe2 thin films on glassy carbon disk substrates in acidic (pH 2) baths of cupric ions and sodium citrate, under potentiostatic conditions [176], it was established that the formation of tetragonal chalcopyrite CIS is entirely prevalent in the deposition potential interval -0.7 to -0.9 V vs. SCE. Through analysis of potentiostatic current transients, it was concluded that electrocrystallization of the compound proceeds according to a 3D progressive nucleation-growth model with diffusion control. 

Johans et al. derived a model for diffusion-controlled electrodeposition at liquid-liquid interface taking into account the development of diffusion fields in both phases [91]. The current transients exhibited rising portions followed by planar diffusion-controlled decay. These features are very similar to those commonly observed in three-dimensional nucleation of metals onto solid electrodes [173-175]. The authors reduced aqueous ammonium tetrachloropalladate by butylferrocene in DCE. The experimental transients were in good agreement with the theoretical ones. The nucleation rate was considered to depend exponentially on the applied potential and a one-electron step was found to be rate determining. The results were taken to confirm the absence of preferential nucleation sites at the liquid-liquid interface. Other nucleation work at the liquid-liquid interface has described the formation of two-dimensional metallic films with rather interesting fractal shapes [176]. 

Structural changes on surfaces can often be treated as first-order phase transitions rather than as adsorption process. Nucleation and growth of the new phase are reflected in current transients as well as dynamic STM studies. Nucleation-and-growth leads to so-called rising transients whereas mere adsorption usually results in a monotonously falling transient. In Fig. 10 are shown the current responses to potential steps across all four current peaks in the cyclic voltammogram of Fig. 8a [44], With the exception of peak A, all structural transitions yield rising current transients sug-... 

The models incorporate two microscopic parameters, the site density and the critical nucleus size. A fit of experimental current transients to the models allows conclusions, for example, concerning the effect of additives on nucleation rate. Fabricus et al. found by analysis of current transients that thiourea increases the nucleation density of copper deposited on glassy carbon at low concentration, but decreases it at higher concentration [112], Schmidt et al. found that Gold nucleation on pyrolytic graphite is limited by the availability of nucleation sites [113], Nucleation density and rate were found to depend on applied potential as was the critical nucleus size. Depending on concentration, critical nuclei as small as one atom have been estimated from current transient measurements. Michailova et al. found a critical nucleus of 11 atoms for copper nucleation on platinum [114], These numbers are typical, and they are comparable to the thermodynamic critical radii [86],... 

Figure 10.6 Normalized current transients for instantaneous and progressive nucleation.

The two current transients are shown in Fig. 10.6. The curve for progressive nucleation rises faster at the beginning because not only the perimeter of the clusters increases but also their number it drops off faster after the maximum. Such dimensionless plots are particularly useful as a diagnostic criterion to determine the growth mechanism. Real current transients may fit neither of these curves for a number of reasons, for example, if the growth starts from steps rather than from circular clusters. 

Figure 10.9 Current transient at r = —4 mV after application of a nucleation pulse of r) — —17 mV for 120 ps. Data taken from Ref. 6.

Diagnostic Relationships Between Current, Maximum Current, and Time. Scharifker and Hills (26) developed a theory that deals with the potentiostatic current transients for 3D nucleation with diffusion-controlled growth. According to this theory, the theoretical diagnostic relationship in a nondimensional form is given by... 

Equations (7.16) and (7.17) are used in an analysis of experimental data. Eor example, R5mders and AUdre (32) used these equations to analyze copper electrodeposition on platinum. They concluded that at the intermediate overpotentials (120 and 170 mV), the dimensionless current transients are consistent with the theoretical predictions for progressive nucleation, Eq. (7.17). At overpotentials higher than 220mV, nucleation shifted to the instantaneous nucleation theory. 

Current transient studies have also demonstrated instantaneous nucleation and two-dimensional growth mechanism of f-Pb02 [123] and a higher rate of reduction of small PbSOd crystals, compared to greater ones [124]. 

Fig. 29. Electrodeposition of Ag from 0.017 M AgCN + 0.92 M KCN + 0.11 M K2CO3 solution dimensionless analysis of experimental potentiostatic current transients (/, and tm are the current and time corresponding to the maximum on the current transient curve, respectively). Upper curve calculated for the instantaneous nucleation mechanism lower curve, for the progressive nucleation mechanism. Different symbols/experimental points relating to different potentials [136], Reproduced by permission of The Electrochemical Society, Inc.

Monatomic steps at real surfaces do significantly influence not only the nucleation act, but also the spreading and overlapping of 2D islands, which determine the shape of potentiostatic current transients. Neglecting the adsorptive contribution in eq. (3.64), i.e., ep Qcd, and assuming a barrierless nucleation and growth of a condensed 2D Mcads phase only at steps with a regular pattern characterized by a... 

First, Harrison et al. [3.36] studied Me UPD in the systems Ag(lll)/Pb, Cf, Ag(polycrystalline)/Tl, Cf, and Ag (polycrystalline)/Pb, acetate by cyclic voltammetry and potentiostatic pulse measurements. The authors claimed that a non-monotonous current transient represents a necessary criterion for 2D nucleation and growth involved in the 2D Meads overlayer formation. However, the experimental results presented did not give evidence for a first order phase transition. 

Bewick and Thomas [3.110-3.114, 3.270] measured electrochemically and by optical means different Me UPD systems Ag(A 0/Pb, H, ClOd", acetate and citrate, CnQikt)/ h H C104, acetate, and AgQikt)m SOd with Qikt) = (111), (100), and (110). Potentiostatic pulse measurements showed non-monotonous current transients for Ag(lll) substrates which are attributed to a first order phase transition. As an example, a current transient in the system Pig hkt)/Vf, H, SOd is shown in Fig. 3.46. In the case of Ag(lOO) and Ag(llO) substrates, higher order phase transitions were supposed. Clear evidence of a participation of 2D nucleation and growth steps in the 2D Meads phase formation process was found in the system Cu(lll)/Pb H", ClOd", acetate [3.270]. Non-monotonous current transients and a discontinuity in the q(lsE,fi) isotherm were observed (Fig. 3.13). 

Lorenz et al. [3.92-3.94, 3.98, 3.101] demonstrated that potentiostatically measured non-monotonous current transients represent a necessary, but not a sufficient criterion for 2D nucleation and growth. For example, such transients observed in the systems Ag(lll)/Pb, H, CIO4 and Ag(lll)/TT, If, S04 were quantitatively explained by Mefj g charge transfer and bulk diffusion as rate determining steps as illustrated in Fig. 3.48 [3.92, 3.93]. 

However, the given interpretation of current transients is based on a kinetic model in which both Meg iv adsorption and 2D nucleation and growth processes are assumed to operate independently of each other in parallel at different substrate... 

The kinetics of the initial stage of 3D Me-S bulk alloy formation process can be affected by nucleation and growth phenomena. A typical example is the formation of the fi phase of 3D Li-Al bulk alloy in the systems Al(polycrystalline)/molten Li, Cf and A1 (polycrystalline)/LP, 0104, propylene carbonate [3.345, 3.346]. In both systems, non-monotonous current transients were observed in the initial stage of alloy formation as shown in Fig. 3.66 [3.345]. 

In the case of progressive nucleation- and diffusion-controlled growth, the initial part of the current transient follows a dependence and the nucleation rate can be... 

Analysis of the form of the current transients at growth after nucleation pulse excitation nuc-... 

Figure 5.22 Current transient following a nucleation pulse on a quasi-perfect Ag (100) face in the standard system Ag (100)/AgNO3 [5.381. grovrth = 4 mV j nuc = - 14 mV pulse duration fnuc = 012 ms time scale 10 ms div current scale 0.4 pA div electrode surface area A = 3.14 x 10"4.

Metal deposition on the silicon surface may follow an instantaneous or a progressive nucleation process followed by a diffusion-limited growth of the nuclei. The growth of nuclei can be either kinetically limited, diffusion limited, or under a mixed control. The current transients measured by Oskam el at various potentials of... 

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