Mechanistic Study of Sn Electrodeposition on TiO2 Nanotube Layers

Nov 9, 2009 - Quımica Inorgánica, UniVersidad de Córdoba, Edificio Marie Curie, Campus de Rabanales, 14071 Córdoba,. Spain. ReceiVed: March 2, 200...
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Mechanistic Study of Sn Electrodeposition on TiO2 Nanotube Layers: Thermodynamics, Kinetics, Nucleation, and Growth Modes Ilie Hanzu,† Thierry Djenizian,*,† Gregorio F. Ortiz,†,‡ and Philippe Knauth† UniVersity of Aix-Marseille I, II, III-CNRS Laboratoire Chimie ProVence (UMR 6264), Electrochemistry of Materials Research Group, Centre Saint-Je´roˆme, F-13397 Marseille Cedex 20, France, and Laboratorio de Quı´mica Inorga´nica, UniVersidad de Co´rdoba, Edificio Marie Curie, Campus de Rabanales, 14071 Co´rdoba, Spain ReceiVed: March 2, 2009; ReVised Manuscript ReceiVed: September 16, 2009

The electrodeposition of Sn nanowires from citrate buffered baths on titania nanotubes is investigated. Electrochemical studies are combined with scanning electron microscopy (SEM) observations to get an insight into nucleation and growth modes. The electrochemical potential of Sn citrate complexes, exchange current density ((4 ( 1) 10-7 A/cm2), and the diffusion coefficient of Sn citrate complex in solution ((8 ( 1) 10-6 cm2/s) are determined. The high transfer coefficient (R ) 0.8 ( 0.1) of the Sn reduction indicates a highly asymmetrical reaction, which is most likely coupled with a physical adsorption-desorption process on the surface of the growing Sn crystallite. Nucleation and growth appears to be instantaneous according to the model developed by Scharifker and Hills. nanorods. Apart from that, the removal of the template may raise limitations, and the dissolving reactive used may alter the desired nanowires in the case of non noble metals. The template membrane might be fragile and difficult to handle, posing problems when scaling up the procedure.

1. Introduction Tin nanowires are particularly interesting through the prism of current technologies. They can be used as connections in microelectronics, catalysts, and, in their oxidized form, as field emission tips in electron guns.1,2 Tin oxides have already been used to design new-type chemical sensor arrays3,4 successfully employed for the detection of many gases, including nitric oxide,5,6 ethanol,7,8 and acetone9 vapor, CO,6 H2,8,10 and isobutane.11 SnO2-based compounds are nowadays used on a large scale in the technology of liquid crystal displays (LCD). Transparent electrodes and current collectors made of ITO (indium-doped tin oxide) or FTO (fluorine-doped tin oxide)12 are practically present in every small portable electronic device featuring a display. Nanostructured SnO2 was also recently used to design dye-sensitized solar cells.13 The nanowire morphology proves to be particularly beneficial for solar cells when coupled with another semiconductor oxide (TiO2).14 Tin dioxide has also been recently used for the design of new nanoscaled electronic devices such as nano-FETs (field effect transistors) that are fully transparent15 and present a tubular or wire morphology,16,17 LEDs (light emitting diodes),18 and many others. Sn and its oxides are also known to be high capacity negative electrode materials for Li-ion batteries.19,20 Titania- and Sn-based nanomaterials21,22 seem particularly interesting for future applications in the field of microbatteries.23 Electrodeposition is an efficient and reliable technique for preparing metal nanocrystallites,24 nanowires, and nanofibers by employing hard templates such as mesoporous silicas,25 alumina templates,26 polycarbonate membranes,27,28 and carbon nanotubes.29,30 Although the use of templates may have the advantage of a predefined 1D morphology and controllable wire diameters, there are also a number of intrinsic shortcomings and limitations. The dimensions are dictated by the template pore size, and it is very difficult to prepare single-crystal

The formation of tin nanowires by electrodeposition on selforganized titania nanotubes has been previously reported,32,33 and it is based on a two-step electrochemical process. The first step is the anodic growth of self-organized TiO2 nanotubes from sputtered Ti thin film,34 while the second step is the galvanostatic electrodeposition of vertical Sn nanocrystallites on the nanotubular TiO2 layer. The electrode used for electrodeposition has a capital influence on the morphology of the Sn deposit. The nanowire morphology is only obtained when the deposition is performed on a nanotube substrate. On a compact titania layer, only large crystallites without any particular orientation were observed, showing that nanotube thin films can be used as seed layer for guiding the growth of Sn nanowires.32,33 This situation is clearly illustrated by the SEM photographs shown in Figure 1. The nanotube-induced wire morphology has been also reported in the case of the electrochemical growth of ZnTe.35

* Corresponding author. Tel.: +33(0)491637075. Fax: +33(0)491637111. E-mail: [email protected]. † University of Aix-Marseille I,II,III-CNRS. ‡ Universidad de Co´rdoba.

The aim of this work is to study the nucleation and growth mechanism of tin on nanotubular titania and the influence of electrochemical parameters on the morphology of the tin deposits.

Investigating template-free approaches while controlling the size and the shape of the nanostructures is still challenging. Among all recent techniques that have been explored for the fabrication of such vertical nano-objects, the electrochemical deposition process is strongly emerging because of significant technical and economical advantages such as simplicity, lowcost, and even more importantly the ability to be performed at low temperature and in different media. Thus, the electrodeposition process is a solution of choice for the formation of vertical structures with tailored dimensions as reported for the synthesis of ZnO nanowire arrays.31

10.1021/jp906070v CCC: $40.75  2009 American Chemical Society Published on Web 11/09/2009

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Figure 1. SEM cross-sectional views of Sn crystallites electrochemically grown onto (a) thin compact TiO2 film and (b) TiO2 nanotube layer. The morphology and the density of Sn crystallites are strongly modified by the presence of the nanotubes leading to the formation of nanowires.

2. Experimental Section Mirror polished Si wafers (SSP/E-Prime grade quality, SiTech Inc.) doped with boron (p-type dopant, F ) 1-10 Ω cm) cut along the (100) crystallographic plane were cleaned with acetone, isopropanol, and methanol, in this order, for 5 min in an ultrasonic bath. The native oxide layer was then removed by dipping the Si wafers in HF 1 wt % aqueous solution for 30 s followed by rinsing with water and quick drying in a stream of Ar. The cleaned and etched Si substrates were immediately fit into the PVD (physical vapor deposition) chamber, which was subsequently pumped down to around 10-6 mbar. A Ti thin layer was DC sputtered on the as-prepared Si substrate using a laboratory-made PVD device. A Ti 99.9% sputtering target was used, and an ultrapure Ar atmosphere was maintained inside the deposition chamber at a pressure of 8 × 10-4 mbar during deposition. Using a deposition current of 150 mA, a 2 µm thick Ti deposit was obtained after 2 h of sputtering. The Ti-sputtered Si wafers were cleaved in smaller rectangular pieces using a diamond tip. Prior to any electrochemical treatment, the small pieces were embedded at the level of the liquid/air interface in a double component commercial epoxy-resin (Araldite, Sigma-Aldrich). This resin protects against intense electrochemical corrosion that would take place during anodization at the level of the three-phase contact if this area was left unprotected. To enhance the electric contact between the as-prepared electrode and the sample holder, the contact zone was smeared with In-Ga eutectic alloy (Aldrich, mp 15.7 °C). The Ti thin film was anodized in potentiostatic regime by applying a voltage of 20 V for 25 min across the two electrode cell. The working electrode (Ti thin film) was placed at 3 cm from the counter electrode (a large Pt grid). The anodization bath contained H3PO4 1M, NaOH 1 M, and HF ∼0.4 wt %. This procedure leads to the formation of a self-organized array of amorphous TiO2 nanotubes (80 nm wide, 600 nm long, wall thickness 20 nm).34 All electrochemical deposition experiments were carried out in a three-electrode cell having the anodized sample as the working electrode and a large platinum grid as the counter electrode. A commercial saturated calomel electrode (SCE) was used for all deposition experiments as reference electrode. The schematic illustration of the electrochemical cell is shown in Figure 2. SnCl2 (Sigma-Aldrich, 98% reagent grade) was used to prepare an 18 mM solution that was buffered with sodium citrate (Na3C6H5O7 · 2H2O, 98% reagent grade, Sigma-Aldrich). The concentration of sodium citrate in the electrodeposition bath was 50 mM, and the resulting pH was 6. The citrate is used to stabilize Sn2+ species in solution and avoid the precipitation of tin hydroxide. For some experiments, this bath was further

Figure 2. Schematic representation of the electrochemical cell used for the electrodeposition of Sn nanowires.

diluted after preparation. The volume of the deposition bath was kept at 200 mL.32 These baths were carefully deaerated by argon bubbling. To optimize the formation of tin nanowires, the deposition bath was moderately stirred (500 rpm) during galvanostatic experiments. The potentiodynamic experiments were made without stirring according to the corresponding measuring protocol (Cottrell, Scharifker, and Hills) that requires a nondisturbed diffusion layer. The electrochemical measurements were carried out using a Princeton EG&G Parstat 2273 potentiostat. The scan rate for potentiodynamic measurements was 1 mV/s; this is a compromise value, which is sufficiently low to be near steady state without increasing noise levels. The deposits were investigated using a Philips ESEM 130 scanning electron microscope (SEM). 3. Results and Discussion 3.1. Electrochemical Thermodynamics: Complexation of Electroactive Species. Although the electrolyte bath is highly diluted, nucleation and growth of tin occur on titania nanotube layers. The concentration of the electrodeposition bath has a tremendous influence on the morphology of the deposit and the Sn wire dimensions. The initial electrodeposition bath had a concentration of 18 mM SnCl2 and 50 mM sodium citrate. To evaluate the influence of the concentration, the bath was further diluted by adding distilled water. Hence, the ratio between the Sn2+ ions and the sodium citrate ions was kept constant all the time. As it can be seen in Figure 3, the deposits shift from a rod and wire morphology (Figure 3a,b,e) to a rather spherical

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Figure 3. The influence of SnCl2 and sodium citrate concentration on the morphology of the Sn deposit: (a,b) 18 mM SnCl2 in 50 mM sodium citrate; (c,d) 4.5 mM SnCl2 in 12.5 mM sodium citrate; (e) 9 mM SnCl2 in 25 mM sodium citrate. At low concentrations, the nanowire morphology is replaced by a spherical morphology. The maximum aspect ratio of the nanowires is obtained at intermediate concentrations.

morphology (Figure 3c,d). When the electrodeposition bath is diluted to 4.5 mM SnCl2 and 12.5 mM sodium citrate, the Sn deposit seems to have no preferential direction of growth, and round Sn aggregates are formed. Although their shape may vary significantly, the deposits are generally characterized by the absence of well faceted crystallites. At certain intermediary concentrations, the electrodeposited Sn nanowires have the highest aspect ratios as it can be seen in Figure 3e and also relatively little dispersion of the wire width. The rest of the deposit, which does not adopt wire shape, consists of well faceted Sn crystallites of relatively low height. To investigate the mechanism responsible for the formation of Sn nanowires, cyclic voltammetry experiment was performed at a scan rate of 1 mV s-1 (Figure 4) and without stirring. After the initial increase of the cathodic current that is attributed to the nucleation and growth of tin, a plateau is observed at around -0.8 mA cm-2 for cathodic potential lower than -0.6 V vs NHE. This reduction plateau corresponds to the limiting current, suggesting that the electrodeposition process is under mass control. The standard electrode potential for the Sn2+/Sn redox couple is -0.137 V vs NHE, assuming the Sn2+ ions are hydrated ions. According to literature data,36,37 dissolution of tin chloride in citrate solutions leads to different soluble Sn(II)-citrate chelate species, whereas only a very small fraction of Sn ions is in the form [Sn(H2O)6]2+. Citric acid has three carboxylic functions and one isolated hydroxyl function. The proton on the hydroxyl

Figure 4. Typical cyclic voltammogram obtained on a nanotubular titania electrode. The deposition bath contained 18 mM SnCl2 in 50 mM sodium citrate and was purged with Ar before data were acquired. Scan rate ) 1 mV/s.

group has the lowest tendency to dissociate. Thus, the citrate ligands will further on be represented as HL3-, where L4represents the tetravalent citrate ligands. Han et al. have investigated Sn(II)-citrate complexes in solution37 and concluded that at room temperature the configuration and electric charge of Sn(II)-citrate complexes depend mainly on the pH of the solution with Sn(II)-citrate chelate

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species bearing negative, neutral, or positive electric charge. The measured pH of all of the Sn-citrate electrodeposition baths was around 6 ( 0.25. At pH ) 6 in the Sn2+-citrate electrodeposition bath, the dominant chelate species is negatively charged.37 The equilibrium that leads to this species

Sn2+ + HL3- f SnHL-

(1)

has a controversial equilibrium constant. At a temperature of 298 K, Survila et al.38 have found a value of log K ) 19.5, while a more recently determined value39 is log K ) 10.3. Nevertheless, the equilibrium constants are reasonable high in both cases and allow us to consider that nearly all Sn2+ ions are forming complexes with citrate ions. Thus, the concentration of the SnHL- species is equal to the initial concentration of Sn2+ species before adding the sodium citrate. The electrochemical reaction corresponding to the electrodeposition of Sn at the cathode should then be written:

SnHL-+ 2e- f Sn + HL3-

RT [SnHL-] ln 2F [HL3-]

(3)

The standard electrode potential for reaction 2 was determined by Han et al.37 as E° ) -0.41 V vs NHE. A typical reduction potential of Sn(II)-citrate complexes was determined by slow polarization measurements: -0.496 V vs NHE (Figure 4). In this experiment, the concentration of SnHL- is 18 mM, while the concentration of HL3- is 32 mM, representing the citrate ligands in excess; a Nernst potential value E ) -0.417 V vs NHE can be calculated, which is close to the experimental reduction potential. The overpotential is thus quite small (see below). The standard Gibbs free energy corresponding to the formation of Sn-citrate bonds can be determined using a simple thermodynamic approach. The standard Gibbs free energy corresponding to the electrochemical reaction 2 is: 0 0 0 ∆G0 ) -zFE0 ) ∆fGHL 3- + ∆fGSn - ∆fGSnHL- ) 79.1 kJ/mol (4)

By definition, the standard Gibbs free energy of formation of crystalline elementary tin is zero. The standard Gibbs free energy of the electrochemical reaction can thus be also expressed as: 0 0 ∆G0 ) -∆fGSn 2+ - ∆G[Sn-HL]-

temperature

equilibrium constant

[SnHL-] mM/L

[Sn2+] (aquated) mM/L

[HL3-] mM/L

25 °C 38 °C 56 °C 70 °C

2.0 × 1012 6.3 × 1011 1.4 × 1011 5.0 × 1010

18 18 18 18

2.81 × 10-10 8.93 ×10-10 4.02 × 10-9 1.13 × 10-8

32 32 32 32

formed, in other words, when complexation of Sn2+ ions by trivalent citrate ions takes place. The Gibbs standard free energy corresponding to the Sn-citrate complex formation reaction (1) can also be expressed as 0 0 0 ∆G0 ) -RT ln K ) ∆fGSnHL - - ∆fGHL3- - ∆fGSn2+ ) -70.2 kJ/mol (6)

(2)

As Sn complex ions are negatively charged, it is expected that the tin reduction reaction exhibits a large activation energy. The theoretical equilibrium potential can be calculated according to Nernst law:

E ) E0 +

TABLE 1: Calculated Values of the Equilibrium Constant Corresponding to the Sn-Citrate Complex Formation (Eq 1) at Experimental Temperatures and the Corresponding Compositions at Equilibrium of an Electrodeposition Bath Containing 18 mM SnCl2 and 50 mM Sodium Citrate

(5)

0 2+ Here, ∆fGSn represents the standard Gibbs free energy of 0 - represents formation of hydrated Sn2+ ions, while ∆G[Sn-HL] the standard Gibbs free energy corresponding to bond formation between tin divalent ions and citrate trivalent ions. A value 0 2+ ) -8.9 kJ/mol was found in the literature.40 Conse∆fGSn 0 - ) -70.2 kJ/mol is calculated, quently, a value of ∆G[Sn-HL] which is released when bonds between tin and citrate ions are

where K represents the equilibrium constant of reaction 1. The values calculated for each temperature are shown in Table 1. Although the equilibrium constant decreases by 2 orders of magnitude between room temperature and 70 °C, the value is still reasonably high, and it can be concluded that all Sn ions are complexed by citrate ions even at 70 °C and that the Sn decomplexation process is thermodynamically hindered at all working temperatures. The K value is in reasonable agreement with the literature value reported by Tselesh, whereas the older data of Survila et al. appear too high. 3.2. Electrochemical Kinetics: Charge Transfer. The kinetics of the Sn electrodeposition process can be described according to the Butler-Volmer equation:

{ [

j ) j0 exp (1 - R)

]}

nF nF η - exp -R η RT RT

]

[

(7)

where j represents the current density, η is the overpotential, j0 is the exchange current density, R is the cathodic transfer coefficient, n is the number of exchanged electrons, T is the temperature, and R is the gas constant. For large cathodic overpotentials (|η| > 0.1 V), the anodic branch is very small and can be neglected. The resulting Tafel law corresponding to electrochemical reaction 2 can then be written:

ln |j| ) ln j0 - R

nF η RT

(8)

A typical Tafel plot corresponding to Sn electrodeposition onto a titania nanotube layer is shown in Figure 5. The linear part of the graph is rather small as the mass transport phenomenon quickly becomes the rate-limiting step. The exchange current density has a value j0 ) (4 ( 1) × 10-7 A/cm2, while the transfer coefficient is R ) (0.8 ( 0.1). This rather low value of the exchange current density is expected, considering that the reaction requires two electrons to be transferred and the tin-citrate complex is negatively charged. The value of the transfer coefficient is unusually high, pointing toward a highly asymmetrical electrochemical reaction. A simplified graphical representation of the Gibbs free energy versus the reaction coordinate assuming a single step charge

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Figure 5. Tafel plot corresponding to the electrochemical reduction reaction of Sn from a bath containing 18 mM SnCl2 in 50 mM sodium citrate. The scan rate was 1 mV/s. The overpotential is expressed with respect to the calculated equilibrium potential (-0.417 V vs NHE).

Figure 7. Typical current transient obtained for the electrodeposition of tin from a bath containing 18 mM SnCl2 and 50 mM sodium citrate.

is the minimum distance allowed at equilibrium between complex ions and the surface of the electrode. After the charge transfer step, the reduced species exist as mobile adatoms on the surface of the electrode, and they can spend some time in this state until they settle on energetically favorable positions, such as surface defects (steps, kinks, etc.), and lose their entire solvation sphere. The transfer coefficient might also be enhanced by the relatively high cleavage energy of the Sn-citrate bonds (70.2 kJ/mol according to the previous section). 3.3. Electrochemical Kinetics: Diffusion of Electroactive Species in Solution. When a constant reducing potential is applied to an electrode, the relationship between current density j and time t is known as the Cottrell equation. For a reduction reaction and before the occurrence of the limiting current, it can be written: Figure 6. Simplified linear representation of the standard free energy of reaction as a function of reaction coordinate (in arbitrary units) corresponding to a transfer coefficient of 0.8. The charge transfer of two electrons involved was considered as a single step.

transfer and a transfer coefficient R ) 0.8 is shown in Figure 6. The value of R was demonstrated to depend on the relative slopes of the free energy-reaction coordinate curves according to the relation:41

R)

tan γ tan γ + tan θ

(9)

For a cathodic reaction, tan γ represents the slope of the free energy curve corresponding to the activation of reactants, while tan θ represents the slope of free energy curve corresponding to the decaying of the activated complex to the reaction products. It is obvious from the schematic representation in Figure 6 that the slope corresponding to the reactants (the aqueous tin citrate chelate species) is significantly higher than the slope corresponding to the products (Sn atoms on the electrode surface and trivalent citrate ions in solution). In other words, the transition between the tin-citrate chelate and the activated complex is spatially more confined than the decaying of the transition state toward the reaction products (Figure 6). A good assumption would be to consider that the charge transfer step occurs at the level of the outer Helmholtz plane (OHP), which

-1/2 -1/2 |j| ) nFD1/2 t o C*π o

(10)

j is the cathodic current density, n represents the number of electrons exchanged, F is the Faraday constant, Do is the diffusion coefficient, and C*o is the concentration of the [SnHL-] species to be reduced. By plotting |j| ) f(t-1/2), a linear graph is obtained until the growing diffusion layer has reached its maximum width. The increase of cathodic current is related to the decrease of average diffusion distance, due to the growth of a large number of Sn crystallites. The diffusion coefficient can then be determined by analysis of the potentiostatic current transient displayed in Figure 7. Right at the beginning of the potentiostatic experiment, a non-Faradic current can be noticed due to the charging of the double layer. Therefore, the linear domain to be analyzed is after this non-Faradic part. The linear part of the experimental Cottrell plot can be seen in Figure 8 as well as the corresponding data fit. A diffusion coefficient of D ) (8 ( 1) × 10-6 cm2/s was found for SnHLspecies. This value is of the same order of magnitude as that of common divalent cations, for instance,40 D ) 7.9 × 10-6 cm2/s for hydrated Ca2+ ions and D ) 9.4 × 10-6 cm2/s for Pb2+ ions. As Sn2+ ions form negatively charged Sn-citrate complexes, it is expected that diffusion-limited mass transport will be slower than in the case of hydrated Sn2+ ions due to the increase of ionic radius. In a classical approach, the drag force that acts

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Figure 8. Cottrell plot used for the determination of the diffusion coefficient of SnHL- species. A value of D ) 8 × 10-6 cm2/s was found.

Figure 9. Citrate concentration dependence of Sn electrodeposition. The measurements are done at 25 °C with a scan rate of 1 mV/s.

upon an ionic species moving through a viscous medium is directly proportional to the species radius and inversely proportional to its mobility. The relation that links the diffusion coefficient to the apparent ion radius is known as the Stokes-Einstein equation:

D)

kBT 6πr0η

(11)

D represents the diffusion coefficient, kB is the Boltzmann constant, T is the absolute temperature, η is the dynamic viscosity of the fluid medium, and r0 is the radius of the ionic species. The value of the dynamic viscosity of water at 25 °C was found in the literature:40 η ≈ 900 µPa s. The apparent radius r0 of the hydrated SnHL- ions was calculated assuming that the complex has a spherical geometry: r0 ≈ 3 Å. This value is in good accordance with the semiempirical simulation performed with MOPAC (molecular orbital package);42,43 assuming that the Sn2+ ion is coordinated by three negatively charged carboxylic groups of the citrate ion, a value of r0 ≈ 3.8 Å was calculated. This corresponds to a diffusion coefficient of (7 ( 1) × 10-6 cm2/s, which is reasonably close to the experimental value of the SnHL- diffusion coefficient. The citrate concentration dependence shows that the limiting current increases with increasing citrate concentration (Figure 9) and underlines the importance of the presence of citrate. 3.4. Nucleation and Growth of Tin. Mechanisms of nucleation of tin crystallites and nanowires can be investigated using electrochemical methods. Scharifker and Hills44 have developed in the 1980s theoretical models of nucleation and have established relations that can be used to distinguish clearly between nucleation and growth modes. Their approach is based on the analysis of current transients when a constant potential is applied to the electrode. Such an experimental current transient is shown in Figure 7. The formalism of Scharifker and Hills is based on the fact that the mass transport process toward forming and growing nuclei is achieved by spherical rather than by unidimensional (linear) diffusion. This case is commonly known as 3D nucleation with diffusion-controlled growth. The cathodic current increases rapidly as the nuclei surface increases or new nuclei are formed. This is seen on the current transient curve (Figure 7) before the time tmax. This current increase can be

Figure 10. Comparison between the theoretical model of 3D nucleation with diffusion control growth and the experiment (continuous line). The observed behavior fits with the model of instantaneous nucleation.

also due to the nuclei edge effect responsible for the decreasing of the diffusion zone observed during the nucleation step. This typical behavior, which is observed in the case of microelectrodes, is consistent with the formation of nanowires behaving as microelectrodes. During this stage of growth, the nuclei develop diffusion zones around themselves. As the diffusion zones overlap, the diffusion regime will change from a spherical to a rather linear diffusion. This corresponds to the decrease in the cathodic current down to the limiting current that corresponds to linear diffusion. This theory provides diagnose relationships that allow distinguishing between nucleation modes. It is possible to plot experimental curves in a nondimensional manner by representing (i/imax)2 ) f(t/tmax) and to compare them with theoretical curves. imax and tmax are the experimental values of the maximum current and the associated time of occurrence. According to Scharifker and Hills, a perfect instantaneous nucleation process should follow the law

( ) i imax

2

)

{

[

( )]}

t 1.9542 1 - exp -1.2564 t/tmax tmax

2

(12)

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Figure 11. Influence of the bath temperature upon the morphology of Sn deposits: (a) 26 °C; (b) 38 °C; (c) 56 °C; (d) 70 °C. The crystallite size increases with temperature.

Figure 12. Potential transients corresponding to galvanostatic Sn electrodeposition from tin-citrate baths at different temperatures. The current density was set at -2.5 mA/cm2.

while a truly progressive nucleation process should be described by the relation

( ) i

imax

2

)

{

[

( ) ]}

t 1.2254 1 - exp -2.3367 t/tmax tmax

2

2

(13)

According to this theory, all experimental curves lay between these two extreme cases. The experimental curve plotted in Figure 10 in nondimensional form is obtained from the current transient shown in Figure 8. As compared to the theoretical curves, the experimental data fit with eq 12. This result clearly shows that the nucleation of Sn on the nanotubular titania substrate is instantaneous. This

Figure 13. Potentiodynamic experiments at different temperatures. The scan rate was set at 1 mV/s.

is not in contradiction with our previous work; it has been found that apparently the number of crystallites increases with time, which is in accordance with a progressive nucleation mechanism.32 In this case, the SEM observations provide information after the emergence of Sn crystallites from the nanotube layers. This is in agreement with the long deposition time (5 s) measured before apparition of the first crystallites corresponding in fact to the crystallite growth along the nanotubes. The initial anomaly occurring immediately after the application of the reducing potential is due to the charging of the double layer. Because of the limited power capability of the potentiostat, the charging of the double layer takes 0.3 s in the present experiments. Nucleation starts only after this time has passed (0.3 s), as charging of the double layer is a nonfaradic process. Thus, the whole graph is actually shifted to the right with this time interval.

Sn Electrodeposition on TiO2 Nanotube Layers Given that mass transport is thermally activated, the limiting current increases with temperature. Figure 11 shows different morphologies obtained when the electrodeposition bath temperature was varied between 25 and 70 °C. The crystallite size increaseswithtemperature,because,accordingtotheButler-Volmer eq 6, the electrodeposition current is smaller at higher temperature for identical overpotential. Qualitatively, if the deposition current is kept constant, the overpotential should increase with temperature, a situation that is clearly illustrated in Figure 12. In potentiodynamic experiments at these temperatures (Figure 13), Sn deposition starts at very similar potentials, which is expected given the weak influence of temperature on deposition potential, but the deposition current increases significantly with temperature in parallel with increase of the crystallite size. Another important fact that can be noticed is that the deposit seems to have the best homogeneity if deposition is performed at 38 °C (Figure 11b), and the distribution of the crystallite size is the narrowest. The width of the wires is relatively large, around 0.5 µm. Naturally, the adatom diffusion length increases with increasing temperature, and there is a higher probability that all available low energy sites are occupied leading to a more uniform crystallite growth. Although for most metals the diffusion coefficients of adatoms do not vary significantly within the small investigated temperature range, this is not the case of tin, because the melting point of β-tin (232 °C) is sufficiently low with respect to room temperature to allow relatively high surface diffusion rates of tin atoms even at room temperature. It has been already proven that Sn atom diffusion via defect sites governs the mass transport in the case of tin whisker formation.45 4. Conclusions The electrodeposition of Sn from citrate buffered solutions presents some particularities. The electroactive species, Sn-citrate complex, is negatively charged. The measured electrodeposition potential is not very different from the theoretical equilibrium potential when the Sn-citrate complex formation is taken into account. The determined high transfer coefficient of tin reduction indicates a highly asymmetrical reaction. The diffusion length of Sn adatoms is relatively long until they occupy their final position and increases with temperature. Furthermore, the highly asymmetrical electrochemical reaction is an indication of a slow citrate desorption step from the tin electrodeposit, because bond cleavage between tin and citrate ions requires rather large energy. The nucleation of Sn on titania nanotubes is instantaneous according to the theory of Scharifker and Hills, consistent with the model of 3D nucleation with diffusion-controlled growth. Acknowledgment. The financial support by ANR Programme Blanc (LIBAN project) and European Research Institute ALISTORE is gratefully acknowledged. We thank A. Tonetto and R. Notonier (Centre Commun de Microscopie Electronique, Universite´ de Provence, Centre St. Charles) for helpful assistance in SEM measurements. References and Notes (1) Bhise, A. B.; Late, D. J.; Ramgir, N. S.; More, M. A.; Mulla, I. S.; Pillai, V. K.; Joag, D. S. Thin Solid Films 2008, 516, 6388. (2) Bhise, A. B.; Late, D. J.; Walke, P. S.; More, M. A.; Pillai, V. K.; Mulla, I. S.; Joag, D. S. J. Cryst. Growth 2007, 307, 87. (3) Harrison, P. G.; Willett, M. J. Nature 1988, 332, 337. (4) Lee, D. S.; Shim, C. H.; Lim, J. W.; Huh, J. S.; Lee, D. D.; Kim, Y. T. Sens. Actuators, B 2002, 83, 250.

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