Composites Communications

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Composites Communications

journal homepage:www.elsevier.com/locate/coco

Short Communication

Early stages of copper microparticles electrodeposition on polypyrrole ﬁ lm

Charif Dehchar

^a,b,⁎

, Imene Chikouche

^c

, Ali Sahari

^c

, Ahmed Zouaoui

^c

aResearch Center in Industrial Technologies CRTI, P.O. Box 64, Cheraga 16014, Algiers, Algeria

bLaboratory of Environmental Engineering, Badji Mokhtar University, Annaba, Algeria

cLaboratoire Croissance et Caractérisation de Nouveaux Semi-conducteurs, Université Ferhat Abbas, Sétif-1, Algeria

A R T I C L E I N F O

Keywords:

Composites Copper Electrodeposition Polymer Thinﬁlm

A B S T R A C T

In this work, we studied the electrodeposition of copper (Cu) microparticles on polypyrrole (PPy)films using cyclic voltammetry and chronoamperometry techniques. The initial stages of Cu deposition were investigated by performing current transients. Models based on Scharifker and Hills calculations were established to determine the nucleation and growth type. The results suggest that the deposition of Cu proceeds via an instantaneous nucleation followed by three-dimensional (3D) diffusion-limited growth. The values of the number density of active sitesN_∞and diffusion coefficientDwere also determined.

1. Introduction

Composites of conductive polymers with metal particles embedded inside the polymer matrix have aroused great interest during the last decades for making advanced materials that require specific physical and chemical properties[1]. Interest in these materials is linked to their versatile promising applications in various fields including catalysis, sensors, capacitors and fuel cells[2–4]. In recent years, electrodeposition has been extensively used for the development of such materials since of its several advantages such as simplicity, high deposition rate, low synthesis temperature and cost-efficiency in comparison with other methods[5]. Furthermore, this method offers opportunities to study the evolution of morphology and microstructure of grains in different growth processes by adjusting the deposition parameters such as pH, electrolyte composition, overpotential, etc. Recently, it has been found that properties of the electrodeposits are very sensitive to their struc- tural and crystallographic properties[6,7]. It is then well reported that a more detailed understanding of the nucleation-growth kinetics at early stages of deposition may be of primary interest in establishing a correlation between deposition, structure and properties exhibited by the material[8].

This paper reports studies intended to examine the mechanism and kinetics of copper particles at early stages of electrodeposition onto polypyrrole ﬁlm electrogenerated on silicon substrate. Potentiostatic current transient measurements are used. The obtained data were analyzed by applying theoretical model of Scharifker and Hills. The copper nucleation and growth process was conﬁrmed by means of morphological characterization using scanning electron microscopy.

2. Experimental

PPy ﬁlms were synthesized on n-doped silicon (Si) substrate (1×1 cm²) acting as working electrode. Before deposition, the Si sub- strates were treated in 5% HF solution for 3 min and then cleaned in acetone. This treatment aims to improve the adhesion of PPy deposit on the Si substrate by increasing the surface roughness of the latter. The synthesis solution was composed of 5 mM pyrrole and 0.1 M tetra- butylammonium perchlorate (TBAP) dissolved in acetonitrile.

Polymerization was accomplished under galvanostatic control using an EC-Lab SP 300 potentiostat-galvanostat and three-electrode cell with a platinum wire as counter electrode and a saturated calomel electrode as reference electrode. After synthesis, the samples were accurately wa- shed with distilled water to remove possible excess of monomer. The nucleation and growth processes involved in the initial times of copper electrodeposition on PPyﬁlm were investigated by chronoamperometry in an aqueous solution of 0.1 M copper chloride (CuCl2) as the source of metal ions and 0.1 M KCl as the supporting electrolyte. Morphological characterization was performed using a JEOL JCM-5000 scanning electron microscope.

3. Results and discussion 3.1. Deposition of PPyﬁlm

Fig. 1a shows the potential transient for the polymerization of pyrrole on Si electrode under galvanostatic control at current density of 0.3 mA/cm². The potential evolution can be divided into three domains:

https://doi.org/10.1016/j.coco.2018.12.001

Received 31 August 2018; Received in revised form 6 December 2018; Accepted 10 December 2018

⁎Corresponding author at: Research Center in Industrial Technologies CRTI, P.O. Box 64, Cheraga 16014, Algiers, Algeria.

E-mail address:c.dehchar@crti.dz(C. Dehchar).

Composites Communications 11 (2019) 52–55

Available online 11 December 2018

T

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first, the potential increases very quickly and reaches a maximum value. This is related to the oxidation of the monomer pyrrole on the electrode surface and the formation of thefirst nucleus of PPy. Then, the potential decreases slowly with time and finally it remains fairly constant, indicating the continuous growth of PPy.Fig. 1b shows a SEM image taken after deposition of PPy film. As can be seen, the film morphology is homogeneous and characterized by a granular microstructure with the presence of some grains in toroidal shape. The same microstructure was reported by other authors[9].

3.2. Growth of copper microparticles on PPyﬁlm

Fig. 2a shows a cyclic voltammogram run between 1.0 and−.8 V at the scan rate of 150 mV/s for copper electrodeposition onto the PPy/Si electrode. As can be seen, in the negative scan, a sharp cathodic peak due to the reduction of Cu²⁺ions to metallic Cu is observed, followed by a current decrease characteristic of the diﬀusion controlled Cu electrodeposition process. The crossover between cathodic current branches observed during the reverse scan at−1.0 V is characteristic of the nucleation and growth phenomena of Cu onto PPy/Si electrode.

There is also an anodic current shoulder at 0.6 V probably due to the oxidation of Cu previously deposited during the direct scan. In order to verify the type of control limiting the Cu electrodeposition process, a plot of the variation of Cu reduction peak current (ip) as a function of the square root of scan rate (v^1/2) is presented inFig. 2b. A linear relationship was obtained, and then the nucleation process can be deﬁned as diﬀusion controlled process.

Formation of new phases usually occurs through nucleation and growth mechanisms, and chronoamperometry measurements can pro- vide helpful information about the kinetics of deposition process.

Fig. 3a shows a series of current-time transients recorded at diﬀerent applied potentials during the initial stages of Cu electrodeposition on PPy/Si electrode. The potential was stepped from the rest potential,

where the Cu is not deposited on the electrode surface, to a working potential range in the Cu deposition region to initiate the nucleation of Cu on the electrode surface. The curvesfit closely to the typical shape of a metal deposition process involving nucleation followed by diffusion- controlled three-dimensional (3D) growth. At the initial stages, a sharp decrease in the current due to double layer charging is observed, then the current increased with time and reached a maximum value,imax, at the timetmax, followed by an exponential decay due to diffusion lim- itation following the Cottrell equation[10]. The rising portion of the current transients corresponds to the density before the growth of the first monolayer of deposit and thus can be used to determine the kinetics of nuclei growth.

Scharifker and Hills have developed theoretical models to describe the nucleation process during the initial few seconds using chronoamperometric curves, and proposed the following current-time re- lationships:

=

i zFD^{3/2 1/2}C Nkt^1/2 (1)

for instantaneous nucleation in which the nuclei are formed in- stantaneously on many active sites; and:

= ∞ ′

i zFD^{3/2 1/2}C AN k t^1/2 (2)

for progressive nucleation in which the nuclei are continuously formed during the deposit growth.

In Eqs.(1) and (2),^k=

(

⁸^πCM^ρ

)

^1/2^and^k^{′ =}⁴³

(

⁸^πCM^ρ

)

^1/2^,ⁱis the current density,zFis the molar charge transferred during electrodeposition,Dis the diﬀusion coeﬃcient,Cis the bulk concentration of the electroactive species,Ais the steady-state nucleation rate per site,Nis the total number of nuclei,N∞is the number density of active sites,M andρare the molecular weight and density of the deposit, respectively andtis the time.

Analysis of the early stages of deposition is possible by representing, for the initial transient portion,ivst^1/2, for instantaneous (Eq.(1)), and 0

0 0 0 0 0 0

E (V)

0 5

0.3 0.4 0.5 0.6 0.7 0.8 0.9

0 100

t (s)

150 200

(

250

(a)

Fig. 1.(a) Potential transient for the galvanostatic deposition of PPy on Si. (b) SEM micrograph of the obtained PPyﬁlm.

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

-6 -4 -2 0 2

(a)

i (mA/cm)

E (V)

4 5 6 7 8 9 10

2.0 2.5 3.0 3.5 4.0 4.5

(b)

-i(mA/cm)

v (mV/s) y = 0.5258 + 0.37694 * x R = 0.9925

Fig. 2.(a) Cyclic voltammogram recorded at PPy/Si electrode in 0.1 M CuCl2+0.1 M KCl. (b) Plot of reduction peak current vs square root of scan rate.

C. Dehchar et al. Composites Communications 11 (2019) 52–55

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ivst^3/2for progressive nucleation (Eq.2). For the experimental data shown inFig. 3a, plots ofivst^1/2show a linear relationship (Fig. 3b).

On the other hand, plots ofivst^3/2(the insert) show poor linearity, indicating that the nucleation of Cu on PPy ﬁlm follows an instantaneous nucleation mode.

The maximum values of the current transients can be used to determine the mechanism for Cu nucleation.Fig. 3c shows a comparison between the experimental data fromFig. 3a plotted in normalized co- ordinates, (i/imax)²vs (t/tmax), and theoretical plots derived from the model proposed by Scharifker and Hills for the instantaneous and progressive 3D nucleation given by Eqs.(3)and(4), respectively:

⎜ ⎟ ⎜ ⎟

⎛

⎝

⎞

⎠

= ⎛

⎝

⎞

⎠

⎡

⎣⎢ − ⎛

⎝

− ⎞

⎠

⎤

⎦⎥ i

i

t t

t 1.9542 1 exp 1.2564t

max 2

max

max 2

(3)

⎜ ⎟

⎛

⎝

⎞

⎠

= ⎛

⎝

⎞

⎠

⎡

⎣⎢ − ⎛

⎝

− ⎞

⎠

⎤

⎦⎥ i

i

t t

t 1.2254 1 exp 2.3367t

max max 2

max 2

2 2

(4) The obtained curves indicate good agreement of experimental data with the instantaneous limiting case, and therefore we can consider that the Cu nucleation follows an instantaneous mechanism in which the

nuclei occupy all of the available nucleation sites on the substrate si- multaneously during the initial few seconds of deposition process. This behavior is in good agreement with morphological observation. The SEM micrograph inFig. 3d shows clearly the presence of a large number of Cu grains of very similar sizes, which appear homogeneously dis- persed over the entire surface of the PPyﬁlm. In this model, the number of active sites (N∞) can be determined using the following equation (Eq.

5):

∞= −

N 0.065(8πCM ρ/ ) ^1/2(zFC i/_{max max}t )² (5) Values ofimax, tmax andN_∞ at diﬀerent potentials are shown in Table 1. These values show that by increasing the potential, the value of imaxincreases and shifts towards shorter times. TheN∞values depend on the overpotential, and increase as the applied potential is made more negative. This can be understood by the activation of more nucleation sites with higher overpotential, which is consistent with instantaneous nucleation model.

To further verify the nucleation type of Cu deposition, the diﬀusion coeﬃcient was determined from Eq.(6)characteristic of instantaneous nucleation model:

= i t D zFC

0.1629( )

max2 max

2 (6)

Values ofDat different potentials are shown inTable 1. The average diffusion coefficient was 6.04 × 10⁻⁶ cm²/s. This value compare fa- vourably with those values reported in the literature[11].

Fig. 3.(a) Potentiostatic transients for Cu deposition onto PPy/Si electrode from a solution of 0.1 M CuCl₂+0.1 M KCl. (b) Plots ofivst^1/2constructed from the rising portion of the transients. (Insert is the plots ofivst^3/2). (c) Non-dimensional plots of the experimental transients compared with the theoretical curves for instantaneous and progressive limiting cases. (d) SEM micrograph of the Cu-PPyﬁlm.

Table 1

Electrochemical parameters obtained from the current transients inFig. 3a.

E (V) −imax(mA/cm²) tmax(s) 10⁻⁴N_∞(cm⁻²) 10⁶D(cm²/s)

−1.1 5.98 12.9 3.04 7.60

−1.2 6.24 9.96 4.69 6.39

−1.4 7.26 4.76 15.19 4.13

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4. Conclusion

In this work, the electrodeposition of copper particles onto polypyrrole ﬁlm has been investigated from chloride solution.

Electrochemical nucleation and growth of copper at initial stages of deposition was studied using chronoamperometric measurements. The Scharifker and Hills model was used to analyze current transients.

Results suggest that copper electrodeposition is achieved through a 3D instantaneous nucleation process with controlled growth. The diﬀusion coeﬃcientDwas, in average, 6.04 × 10⁻⁶cm²/s.

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