Thermal And Kinetic Study of Electrochemical Deposition of Lead On FTO Substrate

We report the electrochemical deposition of lead (Pb) onto fluorine-doped tin oxide (FTO) electrodes in a sodium nitrate bath (0.4M NaNO 3 ) at constant potential conditions. The kinetics electrodeposition processes have been in situ monitored for advanced nucleation stages by chronoamperometry for various temperature at fixed concentration of Pb 2+ , that is 0.1M. The microstructure and morphological characteristics of the deposit layers were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-Ray EDX techniques. The results show that the current density as well as the deposits density strongly depend on the temperature. The correlation between the experimental results and the theoretical process of the lead deposits was discussed and verified.


I. Introduction
Lead (Pb), as an electrodeposit, is a low cost material and extremely attractive due to its distinguishable chemical and electronic properties mainly the high conductivity and the considerable overpotential for the evolution of the oxygen reaction in comparison to other metal's deposits [1,2]. The Pb also shows a long lifetime and a good resistance to corrosion factors. These properties provide ample opportunities for the application of lead deposits of different sizes in sensing platforms and in microelectronics application [3]. More specifically, the electronic properties of isolated Pb particles change considerably when their sizes are evolving from the nanometer to the micrometer scale or contrary. For these reasons, there is a large demand to develop the growth techniques used to produce Pb clusters with controlled size and morphology and to understand the related growth kinematics. In this regard, electrochemical deposition technique (ECDT) is an attractive approach that has been used to produce lead in nano as well as in micro clusters. The advantages of the ECDT is a simple and non-expensive technique which can be implemented under ambient conditions and easily scalable to industrial level [4]. It presents a high deposition rate and allows a precious control of the growing nanostructure properties (dimension and morphology) by adjusting the electrical parameters [5]. Nevertheless, a good knowledge of the growth parameters involved during the electrodeposition process is required to achieve a precise control of the Pb cluster's synthesis. In this regard, sodium nitrate solution (NaNO3) have been the preferred systems to electrodeposit lead nanoclusters onto graphite substrate [6,7], indium-tin oxide [8], platinum [9], silver [10], and lead foil electrodes [11][12][13].
We consider that a good understanding of the kinetics of lead electrodeposition will provide a good control of size and morphology of Pb nanoparticles. Therefore, in this work, we investigate the advanced nucleation phases by ECDT, of lead onto FTO electrode in a NaNO3 solution using cyclic voltammetry (CV) and chronoamperometry measurements. The electrodeposits were characterized by X-Ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-Ray (EDX). The correlation between the experimental results and the theoretical process of the lead deposits was discussed as well.

II.1 Electrodeposition process
The electroplating of metals on a conductive substrate consists in reducing the metal ions of various possible electrolytes. The lowering of ions generally present as cation (positive charge) in the solution, to produce a neutral metal atom on the cathode as a deposit in the following way.
The charge is provided by an external current source and a layer of atoms is deposited at a

II.1.2 Kinetic analysis
The chronoamperograms recorded for electrolytes containing 0.1M lead nitrate in 0.4M sodium nitrate at the potential -0.8 V at different temperatures are shown in Figure. 3. These curves present an in-situ evolution of the current transient. They clearly show that the nucleation rate changes versus temperature. The chronoamperograms present three visibly different parts describing the whole process. Each current density reaches its threshold noted as jm (maximum density of current) at time tm. Then, they decrease slightly, practically they remain constant. The first portion is characterized by a relatively stable current intensity noted as silent part. The duration of this stability is strongly affected by the temperature. Indeed, it decreases as the temperature increases. The second portion corresponds to the nucleation process. It starts when the current intensity begins to have a significant increase. It is clearly shown that, the current density versus time slope increases when the temperature increases. The third portion is the part in which the density of current is practically stable. We may say that during this plateau like shape, the Pb deposits across the FTO surface reaches its equilibrium state. As reported by González-García and his coworkers [15][16][17] , the electrochemical-deposition is a very complex process and is strongly influenced by the experimental conditions. Hence, to understand this mechanism properly, these experimental results are computed and discussed.

III. Theory of nucleation and crystal growth
The elaboration of the crystalline materials involves phenomena of nucleation and growth.
Electro-crystallization is the study of these two phenomena under the influence of an electric field.
According to Amblard [18], these two phenomena are in competition with each other. They influence on kinetics, structure, and properties of the deposits. For example, the faster the nucleation speed, the finer the grains that form the deposit.
Theoretical electro-crystallization models have been proposed to demonstrate the nucleation and growth mode during the electrodeposition process. The nucleation process is generally described by two types: instantaneous and progressive process. The growth process is typically divided into three categories [19]:  Two-dimensional (2D) growth mode or Frank-Van der Merve mechanism which is generally found in the case where the metal and the substrate are of the same chemical nature.  3D growth mode or Volmer-Weber mechanism, it can be exploited to produce nanostructures.  Stranski-Krastanov mechanism which begin by 2D growth mode followed by 3D growth.
According to the ref [20]:  In 2D growth process the current density can be expressed by Eq.1 and Eq.2 for instantaneous and progressive nucleation, respectively.
where k2D is the lateral growth rate constants (mol cm -2 s -l ), h the layer height (cm), N0 the total number of active centers (cm -2 ), A2D the nucleation rate (nuclei cm -2 s -1 ), M the atomic weight (gmol -1 ) and ρ the density (g cm -3 ) of the deposit.
 In 3D growth process, the current density can be expressed by Eq.3 and Eq.4 for instantaneous and progressive nucleation, respectively.
where k and k' are, respectively, the lateral and vertical growth rate constants and A3D the nucleation rate.
 In Stranski-Krastanov mechanism, the current density can be expressed by a combination of Eq.1 or Eq.2 and Eq.3 or Eq.4.
Furthermore, in Harrison and Thirsk [21] studies, the current density is divided in two parts: the first one is due to 3D crystal growth which can be given from Eq.3 or Eq.4. The second part is the current caused by an outward growth on a substrate base plane at a free surface uncovered by growing nuclei, if. Their equations in the case of an instantaneous or progressive nucleation are given by Eq.5. or Eq.6, respectively.
where k0 is the growth rate constant on the base plane of the substrate.

III.1 Approximations of current-time transients: nucleation and crystal growth models
As shown in Figure 3, the current-time transients are different in shapes at different temperatures. This indicates that the temperature strongly affects the nucleation and growth mechanism. The behavior of current-time characteristics for three temperature, 5°C, 20°C and 35°C is analyze theoretically. Experimental results have been simulated and tested by 2D and 3D models from Eq. 1 to Eq. 6.
In figure 3a, the current fit is described by combination of three parts, Eq.7: Where: The best fit confirms that the nucleation and crystal growth at 5°C starts by a progressive nucleation and 2D crystal growth. Then, at an induction time 0 , a second process progressive nucleation and 3D crystal growth as describe by Stranski-Krastanov mechanism.
The chronoamperometric curve of the electrodeposition of lead, obtained at 20°C, (Figure 3b) shows that two quasi-plateaus appear at shorter and longer times which follows the model of 3D growth [22]. To obtain further information regarding this process, different equations (from Eq.1 to Eq.6) where tested. The best fit of the experimental results is obtained by Eq.8 which is a combination of Eq.5, Eq.3 and Eq.4. This global process begins by an instantaneous nucleation and 3D growth followed by a progressive nucleation and 3D growth after the induction time tind.
Where: is the induction time, and ′ are respectively the lateral and vertical growth rate constants for secondary growth process.
At 35°C, the current-time transient curve is characterized by higher slope (Figure 3c). This mechanism can be described by progressive nucleation and 3D growth or Volmer-Weber model [19]. The modelling resumed on Eq.9, which is the combination of Eq.6 and Eq.4 leads to the best fit. Figure 4 shows the values of log(k0), log(k ' ) and A3D as functions of temperature which are extracted from the best fits. For the rate constant, k0, the dependence was linear and increasing by varying the temperature from 5°C to 35°C (black curve). Concerning the vertical growth rate constant, k ' , it is found to be nonlinear for the first 3D growth process (red curve). Indeed, k' increases as temperature increases from 5°C to 20°C and then becomes, practically, stable from 20°C to 35°C. We also investigated the nucleation rate, A3D assuming that the growth rates in two different directions are same. It decreases as temperature increases, as shown in blue curve.  F is missed maybe due to its low doped percentage. Si is present with low percentage because is totally covered by FTO substrate. The presence of Pb is the result of the electrodeposition process.

V. X-Ray diffraction Analysis
The powder X-ray diffraction (XRD) of the obtained electrodeposits samples was performed on a PanAlytical MPDPRO diffractometer (Bragg-Brentano geometry) equipped with a linear X'celerator detector using copper anticathode (λKα1/α2= 1.540560/1.544330Å). X-ray diffraction Since the Pb (II) cations were electrochemically reduced at the potential of -0.8 V, the obtained deposit should be lead (lead oxide is excluded). Deep X-ray diffraction study is in progress to determine the structure of this new phase.

VI. Conclusion
Pb (II) cations were electrochemically reduced at the potential of -0.8 V on FTO substrate at different temperature. The kinetic parameters of electrodeposition processes were determined from the analysis of chronoamperometry data. The good agreement of the values of diffusion coefficients determined applying Cottrell equation and non-linear fitting method was observed.
The density number of active sites and the nucleation rate constant have been discussed. A correlation between the morphology of the deposit particles and the chronoamperometry curves exist. After comparison with was reported in the literature, X-ray diffraction and energy dispersive X-ray analysis suggest the formation of new lead phase.