Tin electrodeposition from choline chloride based solvent: Influence of the hydrogen bond donors

Journal of Electroanalytical Chemistry 703 (2013) 80–87

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Tin electrodeposition from choline chloride based solvent: Influence of the hydrogen bond donors Sónia Salomé, Nuno M. Pereira 1, Elisabete S. Ferreira, Carlos M. Pereira 1, A.F. Silva ⇑,1 CIQ-L4, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal

a r t i c l e

i n f o

Article history: Received 21 December 2012 Received in revised form 9 April 2013 Accepted 7 May 2013 Available online 28 May 2013 Keywords: Electrodeposition Deep Eutectic Solvents Tin Nucleation mechanism

a b s t r a c t In this work we present a fundamental study of the electrodeposition of tin from Deep Eutectic Solvents (DES) formed by a mixture of choline chloride and different hydrogen bond donors (HBD). Results shows that choline chloride based solvents can be successfully used for the electrodeposition of tin. Furthermore we demonstrate that the choice of hydrogen bond donor does not affect, significantly, the chemistry of tin in solution and we characterize the first stages of tin deposits at glassy-carbon (GC) electrode. The electrochemical characterization of tin deposits is carried out using cyclic voltammetry and chronoamperometry. The comparison of the theoretically and experimentally obtained current transients via dimensionless plots based on Bewick–Fleischman–Thirsk (BFT) theory, Scharifker and Hills (SH) and Scharifker and Mostany (SM) models and a non-linear fitting method showed that tin nucleation on GC surface occurs though a 3D instantaneous process with growth controlled by diffusion. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Electrodeposition of metals is essential for a variety of industries and among them tin has been generally applied as a coating because it is a non-toxic, ductile and corrosion resistant metal [1]. Generally this metal is electrodeposited through an aqueous based solutions, generally acid or basic [2] and cyanide containing baths [3]. Due to the toxicity of aqueous baths alternative baths has been investigated using different additives [4–6]. A different approach to replace aqueous baths consists in the use of ionic liquids/Deep Eutectic Solvents [7] which present a wider potential window [8] reducing the effects of hydrogen evolution and allowing the electrodeposition of some elements such as the light and refractory metals, elemental and compound semiconductors [9]. Room-temperature ionic liquids, RTILs, can also be considered a greener alternative to the aqueous based electrodeposition baths [10,11]. RTILs have many favorable properties, such as negligible vapor pressure at elevated temperature, good thermal stability, however their industrial application is not widespread due to their moisture sensitivity nature. Not even the synthesis of water stable ILs seems to be the solution because the metal halides present low solubility in those liquids [12]. An alternative class of ionic liquids, called Deep Eutectic Solvent, based on combinations of choline chloride with hydrogen bond donors, such ethylene glycol, was introduced by Abbott ⇑ Corresponding author. Tel.: +351 220402613; fax: +351 220402649. 1

E-mail address: [email protected] (A.F. Silva). ISE member.

1572-6657/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2013.05.007

et al. [13]. These eutectic mixtures have properties similar to RTILs but they are cheaper, moisture stable and exhibit high metal solubility [2]. Choline chloride based DES have been successfully applied for electrodeposition of different metals [14,15] and/or alloys [13,16] on different substrates and in electropolishing [17,18]. It is well established that the properties of the coatings are strongly affected by the morphological and structural characteristics of the deposits that depend on the operating conditions, substrate and the nature or composition of the electrodeposition bath [6,19–21]. In particular the process of nucleation and growth of metal deposits are closely related with the active sites existing on the substrate [22]. The cations and anions constituting the liquid have a strong effect on the physical and chemical properties of ILs and therefore could affect the electrodeposition process [10]. In fact Endres and co-workers [23–25] demonstrated that by changing the cation of the ionic liquid the deposit obtained had different characteristics. Furthermore Abbott and co-workers studied the electrodeposition of zinc–tin alloy [13] and zinc [26] from urea and ethylene glycol/choline chloride based DES and they showed that the electrochemical response and the deposit morphology depend on the DES used. Recently Abbott et al. [27] reported an initial study about the nucleation mechanism only for the deposition of zinc using reline and ethaline. Aiming to improve the knowledge on tin electrodeposition from Deep Eutectic Solvents we present a thorough study on the nucleation mechanism and the kinetic parameters for tin deposition. Furthermore the effect of DES composition will be presented and discussed. The initial stages of tin deposition, monitored by

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chronoamperometric technique, were analysed using theoretical models and correlated with the deposit morphology characterized by SEM. Moreover this work reports for the first time the use of propylene glycol/choline chloride based DES for tin deposition.

2. Experimental Choline chloride (CH3)3N(Cl)CH2CH2OH(ChCl) (Aldrich, 99%) and urea (Aldrich, >99%) were dried in the oven during 16 h before use and ethylene glycol (EG) (Aldrich, 99+%), propylene glycol (Aldrich, 99.9%) and tin chloride (Aldrich, 97%) were used as received. The eutectic mixtures were formed by stirring the two components together directly in the electrochemical cell, at 75 °C, in the proportions indicated in Table 1, until a homogeneous, colourless liquid was formed. Before the experiments the solutions were de-aerated with nitrogen and the cell was kept under a nitrogen atmosphere during electrochemical measurements. A three-electrode system consisting of a GC electrode (Methrom, diameter 2 mm), a titanium counter electrode and a saturated ChCl calomel reference electrode (CE(ChCl)) were used. The working electrodes were polished to a mirror finish before each experiment using 1 lm diamond suspension (Buehler) followed by 0.5 lm alumina powder (Buehler). The electrodes were sonicated for about 5–10 min, rinsed with water (purified through a Milli-Q Millipore system, to a specific resistance of 18 M Ohm cm) and dried with a nitrogen flow prior to all measurements. For the GC surface this method of surface preparation provides both electrochemically reproducible surface properties and roughness that are suitable for the nucleation studies [28]. After determining the potential window of the DES, the appropriate amount of tin chloride was dissolved in the cell under agitation and nitrogen atmosphere. The electrochemical measurements were performed in a threeelectrode cell using a computer-controlled AUTOLAB PSTAT 10 potentiostat/galvanostat from Eco Chemie, controlled with GPES 4.8 software. The experiments were performed at 75 °C. Voltammetric experiments were carried out at 20 mV s1, starting at 0 V and scanning towards negative potentials. Only one cycle was performed in each voltammetric experiment. Surface analysis was carried out using scanning electron microscopy FEI Quanta 400 FEG/ EDAX Genesis X4 M at CEMUP. The viscosity of the DES was measured using a viscometer Brookfield model DV-I+ at 75 °C. 3. Results 3.1. Voltammetric experiments Fig. 1 shows the voltammetric profile of Sn(II) on GC electrode for the three DES used. When Sn (II) is added to reline (urea: choline chloride), propeline (propylene glycol (PG): choline chloride) and ethaline (ethylene glycol (EG): choline chloride) a well-defined peak is observed in the cathodic scan that can be attributed to the reduction of tin since it is absent in the tin free DES. The peak potential for tin

Table 1 Composition and commercial designation of the Deep Eutectic Solvents (DES) used and viscosity at 75 °C. DES

Reagents

Ratio

Viscosity g (cP)

Ethaline Propeline

Choline chloride:ethylene glycol Choline chloride:propylene glycol Choline chloride:urea

1 ChCl:2 EG 1 ChCl:2 PG

16 20

1 ChCl:2 U

167

Reline

Fig. 1. Voltammograms (scan rate 20 mV s1) in reline (red dot line), propeline (green dash line) or ethaline (black solid line) containing 5  102 mol dm3 SnCl2 for GC electrode at 75 °C. (For interpretation of the references to the colour in this figure legend the reader is referred to the web version of this article.)

reduction is similar both in ethaline and propeline (0.88 V and 0.83 V respectively). In contrast the reduction of tin from reline is negatively shifted towards more cathodic potential but the onset of the reduction occurs at 0.78 V which is close to the onset potential in ethaline and propeline (0.76 V). In reline the reduction current reaches a maximum at 1.01 V. Grinbank et al. [29] reported that increasing the viscosity the convection decreased and the concentration gradients were more pronounced, while electric resistance and voltage increased. Therefore the differences observed in reline may be ascribed to the higher viscosity of this liquid. Abbott et al. [13] studied the electrolytic deposition of Zn, Sn and Zn/Sn alloys from a solution of the metal salts separately in urea and ethylene glycol based ionic liquids. They showed that the potential window of the reline, ethaline and propeline is relatively small on platinum electrode (1.20 V to +1.25 V) and considerably larger (2 V to +1.2 V) on a glassy carbon electrode. In principle it is not possible to attribute this difference to the existence of different tin speciation on each of the DES studied since Abbott et al. [13] reported data obtained from fast atom bombardment (FAB) mass spectra indicating that [SnCl3] is the only tin specie present in ethaline and reline solutions. The oxidative scan shows one peak whose peak potential is almost invariant for the three HBDs used and the highest current density is obtained in ethaline and follows the order jox.(ethaline) > jox.(propeline) > jox.(reline). The cyclic voltammograms shows a crossover between the cathodic and the anodic scan which is a characteristic of metal electrodeposition through a nucleation and growth process [30,31]. 3.2. Chronoamperometry Carbon is an ideal substrate for nucleation studies by minimizing the hydrogen evolution interference, as the overvoltage for hydrogen evolution on a carbon substrate is higher than that for metal electrodes [32] because the amorphous, mechanically polished glassy carbon surface has randomly distributed active sites [33]. Fig. 2 shows a set of potentiostatic current transients obtained during the electrochemical deposition of tin onto glassy carbon from three choline chloride based ionic liquids. The current density of the chronoamperograms presented in Fig. 2 strongly decreases, goes through a minimum and then

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Fig. 2. Chronoamperograms of Sn deposition on GC electrode in (A) reline, (B) propeline and (C) ethaline containing 1102 mol dm3 SnCl2, at 75 °C and at different potentials. The initial potential prior to each potential step was 0.50 V. The inserts shows the representations of j vs. t1/2 and linear fitting at 1.25 V.

increases until a current maximum, jm, is reached at time, tm, from which it decreases. As the potential becomes more negative the value of jm increases and tm decreases. This is the typical behavior of metal deposition involving a nucleation and growth process [30,34], furthermore the transients converge to the limiting current corresponding to linear diffusion of the electroactive ions to a planar electrode [35]. Replacing the HBD did not cause significant changes in the j–t profile, however the value of tm is higher in reline and follow the order tm(reline) > tm(propeline) > tm(ethaline) (the tm values for the different potentials are summarized in Table 2), which is the same order of the liquid viscosity. Considering these results it is possible to assume that the HBD influences the nucleation and growth process. As seen from the voltammetric data the HBD did not change significantly the chemistry of tin in solution therefore the differences in the tm values could be caused by the adsorption of the HBD on the GC surface [20,34,36] or on the nuclei formed. The transients merged to a common current at longer time therefore the data could be fitted to the Cottrell equation excluding Table 2 tm Values obtained chronoamperometry. Liquids

Reline Propeline Ethaline

using the different

DES

for the potentials

used

E/V vs. CE(ChCl) 1.150 tm (s)

1.175

1.200

1.225

1.250

0.014 0.0064 0.0026

0.011 0.0048 0.0022

0.0081 0.0042 0.0020

0.0063 0.0038 0.0015

0.0050 0.0033 –

in

the possibility of a kinetic limitation for the nucleation and growth of the nuclei of tin in these electrochemical systems. The diffusion coefficients of tin species in reline, propeline and ethaline could be obtained by the representation of j vs. t1/2 in the range where the typical behaviour of diffusion controlled reduction is found as illustrated by the insets in Fig. 2A–C. The resulting values of the diffusion coefficients (D) of Sn(II) in reline, propeline and ethaline were 4.1  107 cm2 s1, 1.2  106 cm2 s1 and 1.4  106 cm2 s1, respectively, at 75 °C. The value of D increases by the order DSn(II),ethaline > DSn(II),propeline > DSn(II),reline which is in agreement with the inverse order of liquid’s viscosity reported on Table 1 and the Walden’s rule(D1g1  D2g2) is only verified for ethaline and propeline and not for reline which confirms previous reports found in the literature [13]. These diffusion coefficients values for Sn (II) species in ethaline and propeline are in the same order of magnitude as those reported for Sn (II) species in aqueous solution [37,38] but larger than those reported for Sn (II) species in different RTILs [10,39]. On the other hand the values of the diffusion coefficients for the Sn (II) species in reline are smaller than those reported using aqueous solution [37,38] but in the same order of magnitude as those reported for Sn (II) species in different RTILs [10,39]. To identify the nucleation mechanism the experimental transients were compared with those obtained using theoretical models. Innumerous models were developed to predict the nucleation mechanism [35,40–46]. The Scharifker and Hills (SH) model [35] is largely used to identify the nucleation mechanism of metal deposition from water based baths [20,47,48] and from DES [12,27]. Metal deposition on foreign substrate usually occurs via

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three-dimensional (3D) nucleation process [30] and the SH model considers two limiting cases, one called instantaneous nucleation (IN) and the other called progressive nucleation (PN). In the first case all the nuclei are formed immediately after the potential step, in the second case the nuclei formation is time dependent, i.e. the number of nuclei increases during the deposition process. The theoretical transients for IN or PN nucleation were plotted using the following equations:

 2  1   2 i t t IN-3D ¼ 1:9542 1  exp 1:2564 im tm tm "  2  1 (  2 #)2 i t t ¼ 1:2254 1  exp 2:3367 PN-3D im tm tm

deposition on a platinum electrode in BMPTFSI follows a progressive nucleation model. An intermediate case between instantaneous and progressive nucleation was obtained by Leong et al. [10] using a GC electrode in EMI-DCA on which the experimental data lies between the two theoretical curves. Dimensionless models use only one point (tm, jm) to estimate the electrocrystallization parameters which could restrain the accuracy of the fitting and therefore this may be the cause of the poor fitting in the rising part of the chronoamperogram [48,52,53]. The model proposed by Scharifker and Hills (SH model) [35] consider two limiting cases for the nucleation and growth process (instantaneous or progressive) since in the present study the nucleation and growth follows an instantaneous process the change of the current density with time can be described by following equation:

ð1Þ ð2Þ

The comparison of the experimental plots with the theoretical curves is illustrated in Fig. 3A–C. For the three DES the rising part of the chronoamperogram did not fit the theoretical curve but for t/ tm > 0.9 the experimental data follows the instantaneous (IN-3D) behaviour. Tin electrodeposition was previously studied in different ionic liquids [10,39,49–51] and from an analysis of the published data it is not possible to identify a common mechanism for the nucleation and growth of tin electrodeposits on ionic media. For example Xu et al. [49] using AlCl3–MeEtimCl and Huang et al. [50] using ZnCl2–EMINCl found out that the best fitting for the experimental data obtained using a GC electrode was the instantaneous model. In contrast Tachikawa et al. [39] suggest that the tin

1.00

jSH ¼ zFD1=2 cðpt Þ1=2 ½1  exp ðNpkDtÞ

ð3Þ

1=2

where k ¼ ð8pM=qÞ , zF the molar charge transferred during electrodeposition (C mol1), D the diffusion coefficient (cm2 s1), c the concentration of the metal ions in solution, M the atomic weight (g mol1), q density of the deposit (g cm3) and N is number of nucleus formed on the surface of the electrode (cm2). Later Scharifker and Mostany (SM model) [52] proposed a model on which the need for two limiting separated cases were eliminated. The analytical expression for the current transient obtained by the SM model is given by following equation:

A

B

1.00

0.95

0.95

(j/jm)2

(j/jm)2

0.90 0.85

0.90

0.85 0.80 0.80

0.75 0.70

0.75 0.6

0.8

1.0

1.2

1.4

1.6

0.6

0.8

1.0

1.2

1.4

1.6

t/tm

t/t m

1.0

C

(j/jm)2

0.8

0.6

0.4

0.2

0.0 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

t/t m Fig. 3. Dimensionless plots of (j/jm)2 vs. t/tm using the data of Fig. 2: (A) reline, (B) propeline and (C) ethaline (step potential: 1.20 V vs. CE(ChCl)). Instantaneous (solid line), progressive (dash line) and experimental data (dot line).

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Fig. 4. Experimental current density transient (j vs. t) (s) recorded at 1.20 V, in (A) reline, (B) propeline and (C) ethaline, and a corresponding theoretical curve (blue solid line). The individual contributions to the overall current have also been plotted that are due to an adsorption process (jad, red dot line) and due to the SM model (jSM, green dash line). The inserts shows a zoom in of the overlapping area. (For interpretation of the references to the colour in this figure legend the reader is referred to the web version of this article.)

    ð1  expðAtÞÞ jSM ¼ zFD1=2 cðptÞ1=2 1  exp N 0 pkD t  A

ð4Þ

where N0 is density of active sites (cm2) and A is the nucleation rate (s1). Later Heerman and Tarallo [41] and Mirkin and Nilov [46] (MNHT model) proposed an improvement to the SM model based on the argument that the thickness of the diffusion layer should be a function of time and nucleation rate (A) in contrast to the SM model that considers the thickness of the diffusion layer is only

a function of time. However both SM and MNHT models use the same nucleation law N(t) = N0[1 – exp (At)]. Several authors [52,54] reported that there are no significant differences between the values of the free parameters obtained using both SM and MNHT models. For that reason and being the MNHT model more complicated to solve from the mathematical and computational perspective than the SM model [52] we decided to use the SM model to fit the experimental transients and compare them with the values obtained from the SH model.

Table 3 Potential dependence of the free parameters for the tin deposition according to the SM and SH models. DES

Parameters

SM model

SH model

E/V vs. CE(ChCl) 1.150

1.175

1.200

1.225

1.250

1.150

1.175

1.200

1.225

1.250

Reline

D (107 cm2 s1) N0 (109 cm2) A (s1)

2.58 1.56 1192

2.67 2.05 1535

2.66 2.94 1373

2.65 3.90 1681

2.64 5.36 1706

2.48 1.66 –

2.61 1.96 –

2.65 2.46 –

2.66 3.18 –

2.66 4.18 –

Propeline

D (107 cm2 s1) N0 (109 cm2) A (s1)

8.65 1.09 3007

8.71 1.62 2047

8.75 1.74 3630

8.81 1.94 4076

8.89 2.16 4090

8.63 0.98 –

8.74 1.28 –

8.80 1.49 –

8.86 1.67 –

8.94 1.85 –

Ethaline

D (107 cm2 s1) N0 (109 cm2) A (s1)

11.12 2.23 5628

11.23 2.67 4960

11.27 3.08 6322

– – –

– – –

11.18 1.95 –

11.28 2.28 –

11.32 2.67 –

– – –

– – –

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22.6 22.4

A

B

22.2

21.8

Ln (A)

Ln (N0)

22.0

21.6 21.4 21.2 21.0 20.8 20.6 -1.26

3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0 2.9 2.8 2.7 2.6 2.5 -1.26

-1.24

-1.22

-1.20

-1.18

-1.16

-1.14

-1.24

E/V vs CE (ChCl)

-1.22

-1.20

-1.18

-1.16

-1.14

E/V vs CE (ChCl)

Fig. 5. (A) Plot of ln (N0) vs. E for tin electrodeposition on reline (N), propeline (d) and ethaline (j) and (B) plot of ln (A) vs. E for tin electrodeposition on reline (N), propeline (d) and ethaline (j).

The high falling cathodic current was neglected in order to avoid the time region within which the electric double layer may be not fully charged therefore we only interpreted the rising part of the current transient as done by Milchev et al. [55] on their fittings. The transients depicted in Fig. 4 displayed a shape which could not be described only by the SM model (Fig. 4 dash line), unless it is taken into consideration the contribution of the Sn adsorption (jad),

which may be expressed in terms of a Langmuir-type adsorption– desorption equilibrium [56,57]:

jad ¼ kads exp ðkdes tÞ

ð5Þ

where kads = kdesQads. The overall current density comprises an adsorption current (jad) plus the current obtained by using the SH model (jSH) or the

Fig. 6. SEM images of tin deposits obtained potentiostatically on GC electrode at E = 1.25 V during 60 s in 5102 mol dm3 of SnCl2 in: (A) reline; (B) propeline; and (C) ethaline.

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SM model (jSM) and thereby the transients obtained can apparently be explained by the nucleation models used. Table 3 summarizes the values of the free parameters (A, N0 and D) that best fitted the experimental transients when the electrode overpotential is changed using both SH and SM models. As mentioned before the tm for ethaline is lower than the tm obtained for the other DES. Furthermore tm is also influenced by the deposition potential. As the potential becomes more negative there is a reduction on tm therefore, in the case of ethaline and for potentials <1.20 V the tm is almost overlapping with the double layer charging current. Because of that the fittings of the experimental transients obtained in ethaline were only done for potentials less negative than 1.20 V. The quality of the fitting can be evaluated comparing the diffusing coefficient values extracted from the fitting model and those obtained from Cottrell equation. The average values for the diffusion coefficients of Sn (II) species in the different liquids obtained are 2.64  107 cm2 s1, 8.76  107 cm2 s1 and 1.12  106 cm2 s1 for reline, propeline and ethaline, respectively which are in accordance with those obtained using Cottrell equation. The value of N0, for reline, propeline and ethaline, increases as the metal deposition becomes more cathodic and the representation of ln (N0) vs. E (Fig. 5A) gives a straight line which shows that the number of active sites increases exponentially as the potential becomes more negative. This behavior is typical for an instantaneous nucleation as reported by Khelladi et al. [58]. The nucleation rate constant also increases exponentially as the potential becomes more negative (Fig. 5B) and the higher values for the nucleation rate were obtained for ethaline and diminish as the viscosity increases. Albeit there is a significant variation of A this does not cause a substantial change of the fitting curve. Similar observation was reported by Brylev et al. for Rh deposition [52]. The fittings were also performed using the SH model (fittings not shown) and the values obtained for D and N0 (see Table 3) are similar to those obtained using the SM model, therefore both models could be used to fit the experimental transients but only the SM model allows to extract the nucleation rate.

4. Conclusion Tin deposition onto GC electrode from non-aqueous solutions based on choline chloride and three different hydrogen bond donors, namely ethylene glycol, propylene glycol and urea, containing SnCl2, were studied by cyclic voltammetry and chronoamperometry. Cyclic voltammograms show well-defined voltammetric peaks than can be related to the deposition of Sn (II) onto GC electrodes. This shows that ionic liquids based on eutectic mixtures of choline chloride and hydrogen bond donors can be used as greener solvents for electrochemical deposition of tin. Chronoamperometric analysis indicate that tin nucleation on GC surface occurs through a 3D instantaneous process with growth controlled by diffusion. The time for overlapping of diffusion layers increases in the order ethaline < propeline < reline, for less negative potentials, which is the opposite of the tin diffusion coefficient. The number of active sites increases when the potential becomes more cathodic however it is not possible to find any correlation with the physical properties of the DES under study. The values of the nucleation rate are strongly dependent on the nature of the DES used and it seems to be linked to the increase of the DES viscosity. SEM images show an increase in the deposit material in the order ethaline < propeline < reline which seems to contradict the results obtained by chronoamperometric results. However these results can be rationalized if we consider that the number of nuclei (active sites) initially formed are controlled by adsorption of the HBD and the stronger adsorption is the order ethaline > propeline > reline. The reason for this behaviour may be linked with the different adsorption of OH groups of the ethylene glycol and propylene glycol with GC surface. A more extended study is required in order to understand the processes governing the growth of the tin deposit in different Deep Eutetic Solvents and elucidate what are the phenomena controlling the nucleation rate constant and how to improve the modeling of such electrochemical processes.

3.3. SEM analysis

Acknowledgments

The morphological study of the tin deposits obtained on vitreous carbon for reline, propeline and ethaline was performed by SEM and is displayed in Fig. 6A–C. Fig. 6A shows that the deposit obtained from reline covers almost the entire surface with not well defined crystals but uniformly distributed in the surface and on top of which few crystallites of defined parallelepiped shape are visible. In the deposit obtained from propeline, Fig. 6B, the number of crystallites decreases when compared with the deposit obtained from reline. Also the parallelepiped crystallites are no longer visible, in the area observed, being the deposit formed by not well defined crystallites that shows coalescence. Using ethaline to deposit tin, Fig. 6C, results in a deposit also formed by undefined crystallites with a few of the parallelepiped crystallites but the number of particles constituting the deposit is lower when compared with propeline and even lower when compared with reline. Furthermore SEM images of the deposits obtained using ethaline and propeline show the presence of other small particles which could indicate a certain degree of progressive nucleation. The SEM images are not in complete agreement with the values on N0 obtained by the nonlinear fit because the higher values for N0 were obtained for ethaline and we obtained higher surface coverage with reline. This could be caused by the adsorption of the organic component of the liquid that may reduce the number of nuclei or reduce their adhesion to the surface.

The authors want to thanks the FP7 project IONMET for the financial support of this work and Dr. Vera Pinto for the viscosity measurements at Centro Tecnológico do Calçado de Portugal. Elisabete S. Ferreira also wants to thanks FCT for the Ph.D grant SFRH/BD/31494/2006. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

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