Surface & Coatings Technology 252 (2014) 93–101

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Adsorption of gelatin during electrodeposition of copper and tin–copperalloys from acid sulfate electrolyte

Charline Meudre a, Laurence Ricq a,⁎, Jean-Yves Hihn a, Virginie Moutarlier a,Alexandra Monnin b, Olivier Heintz c

a Institut UTINAM, Université de Bourgogne-Franche-Comté, UMR 6213, 16 Route de Gray, 25030 Besançon Cedex, Franceb Institut FEMTO-ST, Université de Franche-Comté, UMR 6174, 32 Avenue de l'Observatoire, 25044 Besancon Cedex, Francec Laboratoire Interdisciplinaire Carnot de Bourgogne, Université de Bourgogne-Franche-Comté, UMR 6303, 9, avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France

⁎ Corresponding author. Tel.:+33 3 81 66 20 30; fax: +E-mail address: laurence.ricq@univ-fcomte.fr (L. Ricq)

http://dx.doi.org/10.1016/j.surfcoat.2014.04.0500257-8972/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 January 2014Accepted in revised form 19 April 2014Available online 4 May 2014

Keywords:Cu–Sn alloyElectrodepositionAdsorptionGelatinElectrochemical analysis

An acid Cu–Sn deposition bath was developed, and copper and copper–tin coatings were electrodeposited onpolycrystalline platinum. The effect of gelatin on copper and copper–tin electrodeposition from acid sulfatesolutions has been investigated by a variety of electrochemicalmethods (voltammetric studies and electrochemicalquartz crystal microbalance) as well as by morphologic technique (scanning electron microscopy). The electro-chemical results have shown that the overpotential is requiredwhen gelatin is added, indicating the presence of in-teraction between the additive and the coating. From the results of X-ray photoelectron spectroscopy, PM-IRRASand cyclic voltammetry, gelatin was found to react with metal ions and platinum substrate. Adsorption of gelatinon the coating and platinum substrate is shown. Gelatin adsorption seemed to inhibit the initial nucleation of thecopper and copper–tin electrodeposition, allowing homogeneous and smaller crystallites. Addition of gelatin wasfound to have an effect on reducing copper–tin alloys. The presence of gelatin impacts the crystal sizeand morphology of Cu–Sn deposits.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

To date, nickel electrodeposition plays a key role in a variety ofdecorative, engineering and electroforming applications due to itsworldwide dissemination, its costs that are maintained at an acceptablelevel, and their excellent functional properties (corrosion resistance[1,2]). Environmental constraints lead to increasingly severe regula-tions [3]. Recent updates concern nickel salts which have generatedenvironmental concerns and health hazards and have been classifiedas CRM since 2010 [3]. Thus, several alternatives have to be investigated,and the main ones grouped into three categories: copper-based alloys,gold-based alloys, and palladium-based alloys [4]. Copper-based alloysare proposed for their functional properties, and used in technical coat-ings for their corrosion resistance similar to nickel. Copper and its alloyscan be electrodeposited from various electrolytes including aqueouscitrate [5], sulfate [6–8], pyrophosphate [9], oxalate [10] etc. The sulfateelectrolyte is often considered for Ni-free plating electrolyte, due to itslow cost and low toxicity [6–8,11,12]. Previousworks have then been un-dertaken recently on the influence of various additives in copper sulfateelectrolytes [13,14].

Commonly, organic additiveswere added to copper–tin plating solu-tions in small quantities to stabilize the bath and obtain the required

33 3 81 66 20 33..

physical properties and quality of coatings. Matt to bright surfaceappearance is obtained with various brightening agents [13] as well asa leveling effect. Oxidation inhibitors are added to maintain stability oftin-based electrolyte at a low pH and to reduce the formation rate ofstannic ion by complexes such as hydroquinone [6,15,16]. Other addi-tives can be identified from the literature such as gluconic acid [8,17–20], sorbitol [21], laprol 2402C [6,7,22], PEG [23] or thiourea [13,24–26]. Recently, considerable attention has been paid to substancesthat can be used in industry with a low toxicity [16,27–31]: gelatinfalls into this category [32]. The first reported use of gelatin or animalglue in industrial electrodeposition appears with Betts at Trail, BC, in1903 [33]. Since then, gelatin and glues have become commonplace inmetal electrodeposition. Gelatin iswidely used as an additive in electro-deposition of metals such as copper [20,34–36], lead, bismuth [37,38],tin [39] and zinc [40,41] from aqueous solutions.

Many papers are available concerning Cu–Sn coating depositionprocesses [6,16,22,27,37] and the utilization of methanesulfonic acid(MSA) [15,16,27–31,42]. However, works concerning the action ofgelatin on electrodeposition of Cu–Sn alloys are less common. Forexample, despite its widespread use in the plating industry, the exactrole of gelatin in the copper-based alloy deposition process has notyet been fully explained. In the present study, deposition of Cu andCu–Sn on a platinum substrate was studied, and the role of gelatin atthe interface was investigated. The end goal is to identify optimumoperating conditions based on gelatin action. Therefore, mechanisms

94 C. Meudre et al. / Surface & Coatings Technology 252 (2014) 93–101

for gelatin–surface interactions during copper and copper–tin electro-deposition were studied by electrochemical techniques of cyclicvoltammetry and chronoamperometry in static conditions. The presentwork also describes the analyses of the electrochemical quartz micro-balance (EQCM) technique [43,44], supplying sensitive data on masschanges associated with the deposition process and the different inter-actions with additives. The morphology and structure of copper andcopper–tin deposits, obtained by chronoamperometry, are investigatedby scanning electronmicroscopy (SEM), polarizationmodulation infraredreflection absorption spectroscopy (PM-IRRAS), and X-ray photon elec-tron spectroscopy (XPS).

2. Experimental

2.1. Electroplating bath constituents

The composition of the electroplating bath used to deposit Cu andCu–Sn alloys is described in Table 1. It mainly consists of metal saltbased on the ratio Cu:Sn, with Na2SO4 as the background electrolyte.Gelatin (Fisher Scientific UK, MW= 2.1 ∙ 102–1.1 ∙ 105) is used as an or-ganic additive at weight concentrations of 1 g/L, 5 g/L and 10 g/L. In acidicsolution, gelatin is positively charged (i.e. p 8) with a density of 0.68 atroom temperature, where this denaturation is usually negligible. Asmall pH range has been tested according to the Pourbaix diagram(from 1 to 3), but precipitation occurs very early on in our conditions(even for pH N 1.1). All solutions are prepared from high quality MilliQwater (18 mΩ.cm), and are agitated until all themetal salt was dissolved.

2.2. Electrochemical studies

A conventional rotating disk electrode (RDE) is used in cyclicvoltammetric measurements and chronopotentiometry. These experi-ments are performed under static conditions. Experiments are carriedout in a glass cell with three electrodes. The working electrode is a plati-numdiskwith an area S=0.33 cm2. Before eachmeasurement, thework-ing electrode (substrate) is etched with 10% sulfuric acid for 30 s andrinsed carefully with distilled water. Electrode potentials are measuredand reported respectively to the saturated mercurous sulfate electrode(MSE, 650 mV vs. standard hydrogen electrode SHE), which is used as areference for all potentials in this paper. A platinum disk is used as acounter electrode. Electrochemical studies are conducted with aPGZ-301 radiometer potentiostat controlled by a computer, and resultsare processed by VoltaMaster 4 software. Voltamograms are carried outat a sweep rate |dE/dt| = 10 mV/s.

2.3. Preparation of gelatin adsorption film on polycrystalline platinum

Themethod used in this paper is the one used byMagali Quinet et al.[14], which showed the adsorption of thiourea on the platinum sub-strate. The substrate is a polycrystalline platinum disk (S = 0.33 cm2),similar to the one used for electrochemical studies. For the modification,

Table 1Electrolyte composition and operating condition for Cu, and Cu–Sn co-deposition experi-ments.

Chemicals Concentrations Function

Sulfuric acid (H2SO4) 0.6 mol/L Supporting electrolyteSodium sulfate (Na2SO4) 0.5 mol/L Background electrolyteCopper(II) sulfate (CuSO4·5H2O) 0.04 mol/L Source of Cu2+ ionsTin (II) sulfate (SnSO4) 0.04 mol/L Source of Sn2+ ionsGelatin (when added) 0.1 to 5 g/L AdditivepH 1.15 ± 0.03Conductivity 127 ± 5 mS cm−1

Temperature 25 °C

the layer was formed by immersing the platinum substrate in the gelatinsolution for different periods of time (from 5 min to 20 h). Immediatelyaftermodification, theplatinumsubstratewas removed from the solutionand rinsed with high quality Millipore Milli-Q water dried in nitrogenflow.

2.4. Characterization of gelatin adsorption on a polycrystalline platinumsubstrate

Characterization of the gelatinfilmon platinum substrate is obtainedby severalmethods: polarizationmodulation infrared reflection absorp-tion spectroscopy (PM-IRRAS), X-ray photon electron spectroscopy(XPS), and contact angle measurements.

2.4.1. PM-IRRAS analysisInfrared reflection absorption spectroscopywith photoeleasticmod-

ulation (PM-IRRAS) is a useful technique to study thin films on metallicsubstrate. In PM-IRRAS experiment, twomodulations are applied simul-taneously: the Fourier modulation produced by FT-IR interferometerwith a grazing angle and the polarization modulation by photoelasticmodulator.

The PM-IRRAS spectra are recorded using a Vertex 70 spectrometerwith polarization set (PMA 50, Bruker). For photoelastic modulation,the incident IR-beam is modulated at 50 kHz at 80° relative to the sub-strate surface normal. PM-IRRASmeasurements are carried out in air. Aliquid nitrogen cooledMCT detector is used in all experiments. Samplesare composed of gelatin films on platinum surface. All samples arescanned for 250 scans, with a 4 cm−1 resolution.

2.4.2. X-ray photon electron spectroscopy (XPS)X-ray photon electron spectroscopy (XPS) is used to investigate the

gelatin adsorption process on the polycrystalline platinum surface.Depth profile XPS measurements are taken using Versaprobe 5000(PHI) with the aluminum Al Kα1 monochromatic radiation source. Inall cases, spectra are referenced to hydrocarbon contamination C1speak at a binding energy (BE) of 284.5 eV. High-resolution scans ofthe C1s and O1s regions are carried out on all samples.

2.4.3. Contact angle measurementsHydrophobicity of a solid material is usually expressed in terms of a

contact angle (θ) or a surface tension (γ). Although they are closelyrelated, they need to be distinguished. Surface tension describes the in-terface forces between two phases, while contact angle describes theedge of the two-phase boundary where it ends at a third phase. Hencetwo phases must be specified to describe surface tension, while threeare needed to describe contact angle. Our aim is to study the latter,which is given by the Young equation in thermodynamic equilibriumEq. (1):

cos θ ¼ γSV−γSL

γLVð1Þ

where γSV and γLV are the surface tensions of the solid and the liquidwith the vapor of the liquid, and γSL is the tension of the solid–liquidinterface. The most widely used method (sessile drop) is a direct mea-surement of θ on a liquid drop deposited on a surface, where the angleis determined by constructing a tangent to the profile at the point ofcontact of the drop with the solid surface. This can be carried out on aprojected image or a photograph of the drop, or directly by using a tele-scope fitted with a goniometer eyepiece.

2.5. Quartz crystal microbalance

This is a reliablemethod for studying interfacial processes that occuron surfaces and thin films. Electrochemical quartz microbalance exper-iments are conducted with a Q-sense E1 and the module QEM 401

Fig. 1. Cyclic voltammograms for copper deposition in the absence (—) and presence of1 g/L of gelatin (—). Inset: cathodic voltammograms of copper deposition in the absence(—) and presence of 1 g/L of gelatin (—). Experimental conditions: scan rate, 10 mV/s, stat-ic conditions.

95C. Meudre et al. / Surface & Coatings Technology 252 (2014) 93–101

equipment connected potentiostat. The quartz crystal has a geometricarea of 1.3267 cm2, and is operated at the fundamental frequency of5 MHz (referring to air). The QCM system characterizes the interfacein terms of structure and mass during and after interaction. All experi-ments are carried out at room temperature. The quartz crystals aredipped in dilute sulfuric acid, rinsed with Millipore Milli-Q water, anddried in a nitrogen stream prior to the experiments. Platinum is usedas the counter electrode, and a saturated silver electrode as the refer-ence electrode. The results are referred against the saturatedmercuroussulfate electrode.

According to Sauerbrey, the following ratio of mass modification,Δm, over resonant frequency, Δf, is expressed as [45,46]:

ΔmΔ f

¼−A μq ρq

� �1=22 f 20� � ð2Þ

where f0 is the resonant frequency, A is the area of platinum disk coatedon the crystal (1.3267 cm2) and μq and ρq are the shear modulus (2.947× 1011 g cm−1 s−2) and density (2.648 g.cm−3) of the quartz crystal, re-spectively. In liquid applications of the QCM technique, resonant fre-quency depends on the viscosity and density of the liquid in contactwith the quartz crystal according to the following equation [45,46]:

Δ f ¼ − f320

η1ρ1

πμqρq

!1=2

ð3Þ

where η1 and ρ1 are the viscosity and density, respectively, of the liquidin contact with the quartz crystal.

2.6. Morphology of deposits surface

Scanning electron microscopy (SEM) images are obtained fromcopper coatings, and copper–tin alloy deposited on a polycrystallineplatinum disk substrate. Copper coatings and copper–tin coatingsare deposited by polarization at a potential of −950 mV vs. MSEand −1200 mV vs. MSE respectively, over a period of 30 min forSEM analyses in a three-electrode cell. Cathodic potentials are chosenaccording to polarization curves. SEM experiments are conductedwith an Environmental FEI Quanta 450 W microscope model at theFEMTO institute.

3. Results and discussion

3.1. Deposition of copper

3.1.1. Electrochemical studies

3.1.1.1. Open circuit potential. Potentials in a solution of cupric ions and ina solution of cupric ions containing gelatin at pH 1 are measured fora period of 30 min. The open circuit potential of free additive solu-tion is stabilized at 100 ± 30 mV vs. MSE after 15 min. However,when solution contains gelatin, open circuit potential is lower andis equal to 60 ± 30 mV vs. MSE. The presence of gelatin in thesolution and/or on the platinum surface impacts the substrate/electrolyte interface.

3.1.1.2. Cyclic voltammetry. Cyclic voltammetry is performed in thepotential range from 0.2 to −1.1 V vs. MSE onto Pt from solutionswith andwithout gelatin (1 g/L). Typical cyclic voltammograms obtainedfor copper are presented in Fig. 1.

In the additive free solution (—) at pH 1, cathodic current densitystarts to increase at a potential of −0.48 V and reaches a maximumof −1.3 mA/cm2 at about −0.71 V. From −0.48 V up to −0.64 Vvs. MSE the cathodic process is under activation control, whereasfrom −0.64 V to −0.71 V vs. MSE, it follows a mixed kinetics. At this

voltage, cathodic current forms the peak Ic, which corresponds to copperreduction according to Reaction (1).

Cu2þ þ 2e‐→Cu Reaction ð1Þ

After −0.71 V, current density decreases to −0.3 mA/cm2 with aminor plateau and becomes mass-transport controlled. The plateaumay be associated with a depletion of copper species at the interfaceof the working electrode and indicates that there is a nucleation andgrowth mechanism controlled by diffusion [39,47]. Further increasesin current density, at a potential more negative than −0.85 V vs. MSE,are attributed to hydrogen evolution as a secondary Reaction (2):

2Hþ þ 2e‐→H2 Reaction ð2Þ

Upon sweep reversal, cathodic current density gradually decreasesuntil it crosses zero and turns into anodic current. Anodic current thenincreases to form a peak Ia at E =−0.31 V vs. MSE, which correspondsto Reaction (1) in the reverse direction. After peak Ia, current drops tozero, indicating the completion of oxidative dissolution ofmetallic copperat the electrode surface.

The equilibrium Nernst potential of the Cu/Cu(II) redox couple is —0.353 V vs. MSE at 298 K, which is different to the onset of copper depo-sition. This is due to the nucleation overpotential on platinum.

In the case of solution with gelatin as an additive (—), the cyclicvoltammogram presents the following characteristics: a negligiblecathodic current is observed from Eocp to — 0.65 V vs. MSE, which sug-gests no reaction in this potential range. Continuing with the potentialscan in the negative direction, one cathodic peak I′c is observed. Gelatinaddition to the electrolyte decreases the current density of the peak I′cto −0.9 mA/cm2 at E = −0.82 V vs. MSE. Reduction of copper movestoward a more negative potential than the reduction without gelatin.When gelatin is added, the hydrogen reaction occurs at a more negativepotential (E=−0.93 V/MSE). This increase in hydrogen overpotential isdue to the inhibitory effect of gelatin on hydrogen evolution. These obser-vations indicate that the presence of gelatin creates an overpotential,which is required to initiate nucleation followed by subsequent growthof the copper deposit.

When the scan direction is reversed in the switching potential, twoanodic peaks, I′a and II′a, appear at potentials of −0.3 V and −0.25 Vvs. MSE, respectively, for j = 2.75 mA/cm2. The first peak is attributedto Reaction (1) in the reverse direction. The other peak could be dueto oxidation of complexes between gelatin and copper, copper species(free complex ions and adsorbed state) or gelatin layer desorption [37].

96 C. Meudre et al. / Surface & Coatings Technology 252 (2014) 93–101

For copper electrolyte without gelatin, the cathodic limiting currentdensity reached for E = −0.705 V vs. MSE is j = −1.3 mA/cm2,whereas the current density of copper reduction peak decreases(j =−0.9 mA/cm2) when gelatin is added, indicating an inhibitor effectof gelatin on the reduction of cupric ions.

These observations lead us to suggest two assumptions: a possiblecomplexion effect of the Cu2+ by gelatin [48,49], leading to a delay inelectrodeposition kinetics and a possible adsorption of additive on theplatinum substrate blocking active sites, inhibiting the crystal growthprocess of copper.

On the basis of these results, we propose in this paper to understandhow gelatin acts on the electrodeposition of copper and copper–tincoatings.

3.1.2. Study of interactions with additive

3.1.2.1. Effect of gelatin concentration. In the presence of gelatin, bothcupric ions and gelatin additive are concerned by electrodeposition.Gelatin is a protein obtained by partial hydrolysis of collagen [32]. Con-sequently, themain components of gelatin reflect the amino acids of theparent collagen. The most common amino acids are glycine (Gl),proline, hydroxyproline, alanine and glutamic acid. As reported in theliterature [48,49], copper (II) is known to form a complex with glycine:CuGl+ (pH 1.5–3), CuGl2 (pH 4–7.5) and CuGl3− (pH ≥ 8.5). In thiswork, only CuGl+ can be found in the electrolyte because pH is equalto 1. The study of the effect of gelatin concentration in this paper willallow the presence of complexes with copper and gelatin.

Polarization curves (Fig. 2) are used to define the characteristics ofcopper electrodeposition and to identify the presence of Cu(II)-gelatincomplexes. In this linear voltammograms (Fig. 4), one cathodic peak isobserved for the additive free solution, at E =−0.62 V vs. MSE. Concen-tration of gelatin affects the position of the cathodic peak mentioned,which is negatively shifted with the increase in gelatin concentration(E = −0.82 V vs. MSE). However, in the presence of 5 g/L gelatin (dotline), the value of the current density peak is lower (j =−0.7 mA/cm2)than for the additive free solution (j =− 0.5 mA/cm2).

Without other cathodic peaks or plateaux, it can be concluded thatcomplexes between cupric ions and gelatin are not significant at pH 1.Several authors have shown that formation of complexes with glycineand cupric ions exists in alkaline solutions but not in acid electrolytes.Kublanovsky and Litovchenko [49] have shown that only, CuGl+ isformed at pH 1.5–3 in the case of excess ligand, while Saban et al. [50]have shown that gelatin undergoes decomposition in a sulfuric acidCu electrolyte. It is possible that in the case of gelatin depletion, the

Fig. 2. Effect of the concentration of gelatin on the cathodic polarization curves. (—) Cu2+

solution, (—) Cu2++ 1 g/L of gelatin, (∙∙∙) Cu2++ 5 g/L of gelatin. Scanning rate: 10 mV/s.

complexing bonds between gelatin and metal ions no longer exist.Considering the pH conditions present in this study, the complexingphenomenon is not predominant. This paper therefore deals with theinvestigation into adsorption of gelatin.

Fig. 2 shows that peak current density decreases when gelatin con-centration increases. This observation can suggest that an adsorptionphenomenon takes place [37]. Surface coverage θ of the electrodesurface can be determined according to the Eq. (2) [51,52]:

θ ¼ 1− iaddi

ð2Þ

where i is the current density in the absence of gelatin and iadd is thecurrent density in the presence of a certain concentration of gelatin.Coverage of the cathode surface increases from 0.1 to 0.4 when the con-centration of gelatin increases from 0.5 g/L to 5 g/L. Gelatin moleculescan be adsorbed on the platinum substrate either by forming a surfacefilm that acts as a physical barrier, or by interacting with the surfaceand participating to the electrochemical reactions [53]. Proteins, asgelatin, adsorbed on solid surfaces, form various specified configurationsthrough a variety of interactions, hydrophobic attractive (Van DerWaalsinteractions and hydrogen bonds) and electrostatic interactions, inparticular [54,55]. The blocking effect reduces the number of active sitesand induces a decrease in Cu reduction causing a drop in current density.

3.1.2.2. EQCM (electrochemical quartz crystal microbalance). To under-stand these modifications, the thickness of the deposit recorded at thesame time as the polarization curves is illustrated in Fig. 3.

Similar behaviors are observed for the additive-free solution and inthe presence of gelatin, but the potential levels are different. Curvescan be divided into two regions. Without gelatin, in the first region(0.1 to −0.350 V vs. MSE), there is no modification and no reactionoccurs. In the second region from −0.35 V to −0.25 V vs. MSE, thick-ness increaseswhich corresponds to the cathodic peak related to copperdeposition. Then, thickness increases yet again up to −0.8 V vs. MSE.From this potential, the reaction corresponds to hydrogen evolutionand the deposit continues to grow. When gelatin is added, the cathodiccurrent density peak is at a potential −0.75 V vs. MSE, where copperdeposition begins. However, thickness is smaller for the solution con-taining gelatin (0.72 μm) than for the solution without additives(1.35 μm). Even if there is no complex, gelatin has an influence on thethickness of copper deposition. The presence of gelatin in the electrolyteallows smaller growth of the copper coating, an effect that may lead tochanges in nucleation of copper.

Fig. 3. Linear potential scan obtained on platinum substrate immersed in copper electrolytewithout additive (—), and containing 1 g/L of gelatin (—), and thickness obtained by masschange recorded simultaneously.

Table 2Data of techniques showing the phenomenon of adsorption [45,56–59].

Techniques Principles References

Quartz crystal microbalance (QCM) Change in oscillating frequency of piezoelectric devices upon mass loading (information about the adsorption anddesorption processes)

[45]

Ellipsometry Change in the state of polarized light upon reflection (optical method to obtain the amount of protein adsorbed) [56]Fluorescence spectroscopy Change in fluorescence spectrum of protein on adsorption [58]Fourier transform infrared spectroscopy (FTIR) Change in infrared absorption spectrum of protein on adsorption (information on the structure of proteins adsorbed) [57]Atomic force microscopy (AFM) Atomic interaction between surface and scanning probe (information on the structural change during adsorption) [59]

97C. Meudre et al. / Surface & Coatings Technology 252 (2014) 93–101

3.1.3. Characterization of gelatin adsorption film on the working electrodeProtein adsorption on metal surface can be observed by several

methods listed in Table 2, such as optical techniques (ellipsometry,total internal reflection fluorescence (TIRF)) [56–58], Fourier transforminfrared reflection (FTIR) [57] or Fluorescence [58] which providesinformation on the structure or conformation of adsorbed proteins(summarized in Table 2). Among these methods, the electrochemicaltechnique, XPS, PM-IRRAS and contact angle measurements are com-plementary for observation of surface modifications and adsorptionphenomenon.

3.1.3.1. Open circuit potential. Thefirstmethod consists in characterizationof gelatin layers on polycrystalline platinum. Open circuit potentials aremeasured for 30 min to observe evolution of potentials on the platinumsurface modified at various times for the non-modified surface (Fig. 4).

The open circuit potential of the non-modified platinum surface isstabilized at 100 ± 10 mV vs. MSE after 15 min in support electrolytecontaining cupric ions. However, when the surface is modified withgelatin, the open circuit potential is lower. Surface modification is in-stantaneous and changes depending on the immersion time.

3.1.3.2. Electrodeposition of copper onmodified surface. To understand therole of gelatin in the copper deposition process, electrochemical deposi-tion of Cu on gelatin layers has been studied [45,54,55]. The presentpaper shows that preliminary adsorption of gelatin before the platingstepmodifies the platinum surface and thus influences electrodeposition,even if there are no organic additives in the electrolyte. Two parametersare studied: the effect of immersion time of the substrate in a solutionof gelatin, and the effect of gelatin concentration.

Fig. 5 shows the polarization curves obtained in a solution containingcupric ions without gelatin on modified and non-modified platinumsubstrates. When the platinum surface is modified when immersed for

Fig. 4. Open circuit potential of non-modified platinum surface and modified platinumsurface by immersion in 10 g/L gelatin solution for various times of immersion: 5 min,15 min and 20 h.

20 h in the solution containing 1 g/L of gelatin, we observe that thecathodic potential is moved toward a more negative potential. Thisoverpotential is similar to the one reported in Fig. 1, corresponding tocyclic voltammetry of solutions with and without gelatin on non-modified platinum electrode. We can thus associate these observationswith the phenomenon of adsorption of gelatin on platinum surface.

Then, when immersion time is set at 20 h, Fig. 6 shows the influenceof gelatin concentration on the modified surface. Depending on gelatinconcentration, the reduction peak potential of the cupric ions shiftsslightly towardmore negative values. For non-modified surfaces, the re-duction peak potential is approximately – 600 mV vs. MSE. For surfacesmodified after 20 h in the solution containing 0.5 g/L of gelatin, the re-duction peak potential is approximately — 700 mV vs. MSE. With10 g/L of gelatin, the reduction peak potential is −820 mV vs. MSE.This means that the potential is shifted to more negative potentialswith a higher gelatin concentration, and implies that there is an increasein the inhibitory effect of gelatin when the active sites are blocked. Thisis typically characteristic of adsorption on the platinum surface [37–40].If we increase gelatin concentration so as to reduce the repulsion inter-actions, the adsorbed amount is probably increased.

Interactions between gelatin and the platinum substrate are affectedby the variations in immersion time. Thus, gelatin tends to adsorb on allplatinum surfaces, due to a gain in conformational entropy resultingfrom adsorption.

3.1.3.3. PM-IRRAS. PM-IRRAS is used in this study to investigate gelatinadsorption on platinum surface. The spectral region of interest includesamide bands, which are characteristic of all proteins and of gelatin inparticular. In Fig. 7, we can observe the spectra of the non-modifiedplatinum surface and the platinum surface modified by gelatin. In thisstudy, our analysis is focused on Amide I (ca. 1600–1700 cm−1) andAmide II bands (ca. 1480–1580 cm−1). The carbonyl CO double bond-stretching mode, with contributions from in-phase bending of the

Fig. 5. Linear potential scan obtained on non-modified platinum substrate (—) andmodifiedplatinum substrate with 1 g/L of gelatin (—) after 20 h; in solution containing 0.04 M ofCuSO4, 0.5 M of Na2SO4, and 0.6 M of H2SO4, at pH 1.

Fig. 6. Linear potential scan obtained on non-modified platinum substrate (—) andmodifiedplatinum substrate after 20 h with gelatin 0.5 g/L (—) 10 g/L (∙∙∙); in solution containing0.04 M of CuSO4, 0.5 M of Na2SO4, and 0.6 M of H2SO4, at pH 1.

Fig. 8. XPS spectra of a platinum substrate non-modified and modified for 20 h withgelatin.

98 C. Meudre et al. / Surface & Coatings Technology 252 (2014) 93–101

N\H bond and stretching of the C\N bond, occurs in the frequencyrange 1660–1620 cm−1 region, which is often referred to as theAmide I band. The frequency range 1660–1650 cm−1 was known as theα-helical structure, and the 1640–1620 cm−1 frequency range as theβ-sheet. The frequency range of 1550–1520 cm−1 is due to Amide IIwith a α-helical structure between 1550 and 1540 cm−1, andβ-sheets at 1525–1520 cm−1. Amide II vibration is caused by defor-mation of the N\H bonds. Several authors [57] have attributed1500–1200 cm−1 to CH2 deformation. It is a known fact that this regioncontains vibrations corresponding to groups present in fatty acids,proteins, polysaccharides, and phosphate derivatives. Consequently,this technique reveals the presence of Amide I, Amide II and Amide IIIbands on the platinum surface modified by gelatin.

3.1.3.4. XPS analysis. XPS spectra were obtained for the non-modifiedsurface and for the surface modified after immersion for 20 h in the so-lution containing gelatin. The general spectrum is shown in Fig. 8 todemonstrate the various elements contained in gelatin. The intensityof the O1s signal in modified platinum is much stronger than that inthe non-modified surface. N1s and S2p signals are quite clear, whilealmost no nitrogen and sulfur are detected in non-modified platinum.After modification by gelatin, oxygen and carbon contents increase,

Fig. 7. PM-IRRAS spectra of a platinum substrate non-modified and recorded after 20 h ofimmersion of the substrate in a solution containing 10 g/L of gelatin.

and nitrogen and sulfur are present in the substrate surface. The XPSspectrum of the non-modified surface indicates the platinum signal bythe presence of Pt4f and Pt4d, while the other spectra do not show plat-inum signals, characteristic of the presence of a layer on the modifiedsubstrate. This result is confirmed by the atomic ratio of platinum:24% is detected when the platinum surface is not modified, while 0.4%is the ratio of the surface modified by gelatin.

The platinum substrate is washed with distilled water, and driedbefore and after modification by gelatin for XPS testing. First, the non-modified surface shows this contamination on oxygen and carbon inthis surface. After modification, the O1s spectrum and data (Table 3)shows a broad feature, which can be fitted by two peaks of different in-tensities at BE 531.5 ± 0.2 eV and 532.9 ± 0.2 eV, respectively. This is acharacteristic for a carboxylic acid group with two non-equal types ofoxygen atoms, one at low BE (531.5 eV) assigned to C_O (\COOH)and one at high BE (532.9 eV) assigned to\OH (\COOH) [63,67]. Thisspectrum demonstrates the existence of\OH groups on a modifiedplatinum surface, which is present in the gelatin molecule. The groupsmay provide more active sites in the surface, and thus modify the elec-trochemical reaction. For the blank platinum, the C1s spectrum showsthat decomposition takes place in three peaks of different intensities,so that when the substrate decomposition is modified, four peaksoccur. The corresponding C1s spectrum, for modified platinum, revealsseveral carbon functional groups [63,67]. Decomposition of this spectrashows that the main peak located at 284.5 ± 0.2 eV corresponds to theBE of the aromatic/aliphatic carbon atoms, i.e. the carbon atomsbondingwith carbon and hydrogen atoms (C\C, C\H). The component appearsat 286.1± 0.2 eV for C1s, which can be ascribed to oxygen atoms boundwith carbon (C\O bonds) or bonding with nitrogen (C\N), character-istic of gelatin with peptide bonds. The feature at 287.7 ± 0.2 eV can beassociated with the carbon atom of the carboxylic group. The peak at288.9 ± 0.2 eV is attributed to the carboxylic group (O\C_O bonds).The presence of this group and of C\N bonds contributes to identifyinggelatin on the substrate surface. The Pt4f data (Table 3) shows that thesurface core level shifts are also affected by the zwitterionic species[60–67]. Adsorption of gelatin first leads to a decrease in surface signal,and the binding energies are shifted toward lower bonding energy [63].Presence of different atoms, identified by XPS, on the platinum substrateconfirms the phenomenon of gelatin adsorption on polycrystallineplatinum as expected according to PM-IRRAS measurements.

3.1.3.5. Contact angle measurements. Contact angle measurements ofplatinum substrates with and without pre-adsorbed layers of gelatin(from 1 g/L to 10 g/L) are shown in Table 4. The adsorbed layer of gelatinhas considerable influence on contact angle, which expresses alterationin surfacewettability. Gelatin adsorption (20h of time immersion) affectsmeasured contact angle: when there is no gelatin, contact angle is equal

Table 3XPS data of platinum substrate non-modified and modified for 20 h with gelatin after the C1s, the O1s, and for the Pt4f [60–67].

Relative area (%)

XPS peaks C1s O1s Pt4fBinding energy (eV) 284.4 286.1 288 288.9 531.4 532.9 70.8 74Bonds C\C C\O C_O COO− C\OH C_O

C\H C\NRefs [60–67] [63,67] [63,67] [63,67] [60–67] [60–67]Platinum surface 73 16 11 – 53 47 53 47Platinum surface 54 26 16 4 77 23 54 46Modified by gelatin 54 26 16 4 77 23 54 46

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to 73°, and this value decreases as gelatin concentration increases, i.e. asgelatin content is adsorbed on the platinum surface.

To confirm the adsorption effect, time immersion is modified and alsohas an effect: if time immersion is reduced to 1 min, angle contact isalready modified in the presence of gelatin (Table 4). The phenomenonis instantaneous, thus confirming the results obtained for open circuitpotential depending on immersion time (Fig. 4). The adsorption gelatinproduces a hydrophilic platinum surface, and modifications in surfacewettability prove that gelatin molecules are oriented toward the airinterface.

The gelatin molecule structure consists of a series of different aminoacids linked by peptide bonds, with the presence of electro-rich donoratoms such as nitrogen. This implies that several different amide nitrogenatoms and hydrogen bonds should be available for adsorption onplatinum surface by electrostatic interactions [54]. Unlike small mole-cules, gelatin has a complex composition with several amine groups,this structure being the cause of its adsorption mode. On the basis ofthese results, we propose that adsorbed gelatin forms a film on theworking electrode, considering the increase in overpotential and thepresence of nitrogen, carbon and oxygen at the electrode surface. Thismay indicate that the adsorption effect decreases surface activity ofthe electrode, which in turn should inhibit the crystal growth process.As a result, the interface is apt to be thermodynamically stabilized byadsorbing gelatin.

3.2. Co-deposition of copper and tin

To understand the role of gelatin as an organic additive in the copper–tin deposition process, electrochemical deposition of Cu and Sn has beenstudied, respectively. The tin reduction mechanism is not easy to explainbased on instability of tin in acidic electrolyte (pH = 1). According tomany authors, the presence of organic additives, such as gelatin, is neces-sary to improve the feasibility, quality and characteristics of deposits. Theprevious results reveal gelatin adsorption on platinum substrate by elec-trostatic and hydrophobic interactions and show that this phenomenonhas an effect on copper deposition. This is thefirst paper to show that pre-liminary adsorption of gelatin influences copper-tin electrodeposition.This part will demonstrate that gelatin can play an important part inthe copper–tin electrodeposition process.

Fig. 9 shows the cathodic polarizations for a solution containing bothcopper and tin ions, both additive free (solid line) andwith gelatin as anadditive (dash line) under the same conditions as before. It can be seenthat metal ion reduction begins at a potential of approximately −0.4 V

Table 4Contact angle measurements of non-modified surface and surface modified by gelatin atseveral concentrations (1 g/L to 10 g/L) and at different immersion times (1 min and20 h).

Time immersion 1 min 20 hGelatin concentration Contact angle θ (±3°) Contact angle θ (±3°)None (without gelatin) 73° 73°l g/L 52° 55°5 g/L 42° 46°10 g/L 28° 27°

vs. MSE without gelatin. The peak starts to increase and reaches a maxi-mum of current density (j = −1 m A/cm2). Low current density at thispotential is similar to that found in copper deposition (Fig. 1). At this po-tential, the deposit layer is expected to be copper. An increase in cathodiccurrent is observed at potentials more negative than −0.9 V vs. MSE,most likely at the cathodic potential of tin. The rapid increase in currentdensity after the plateau is due to hydrogen evolution (E ≥ −1.2 V vs.MSE). The decrease in current densities after each reduction peak is dueto the depletion of electroactive species to the platinum surface as condi-tions are static. It can be concluded that codeposition of the two metalsoccurs at potentials below−0.4 V vs.MSE. The similarity in electrochem-ical behavior of the individual metals with that of co-deposition indicatesan independent alloy plating system.

In the case of gelatin (—), a significant difference is noted with fourcathodic peaks appearing at Ep1 = −0.73 V vs. MSE, Ep2 = −0.81 Vvs. MSE, Ep3 =−0.97 V vs. MSE, and Ep4 =−1.14 V vs. MSE. Further-more, in the presence of gelatin, current densities of peaks are only slight-ly lower than for the solution without additive. As reported in theliterature [49,50], complexes betweenmetal ions and gelatin in acid elec-trolyte are not leading. This may indicate that inter-metallic compoundscould be formed in tin–copper alloys [16], or the presence ofunderpotential (UPD) and overpotential (OPD) deposition [8,16,27].Moreover, potential reduction of copper and tin is shifted towardnegativevalues when gelatin is added. These observations indicate that gelatinforms a layer adsorbed in the platinum substrate, in accordance withXPS results (Fig. 8).

The overall deposition reaction of the elements takes place in amorenegative potential range: at present, hydrogen evolution initiates atabout E = −1.35 V vs. MSE, which is more electronegative than theCu–Sn bathwithout gelatin (E=−1.2 V vs.MSE). These results demon-strate that gelatin may have formed an adsorbed layer on the platinumsurface that is expected to block the active sites and/or decrease the rateof ions transferred across the electric double layer [37–40]. Gelatin,consisting of different amino acids, is detected by XPS analysis (Fig. 7),showing the presence of various atoms on the platinum surface after

Fig. 9. Cathodic voltammograms of copper–tin depositionwith composition of the solution:H2SO4 0.6M, Na2SO4 0.5M, CuSO4 0.04M, SnSO4 0.04Mwithout gelatin (—) andwith 1 g/Lgelatin (—).

100 C. Meudre et al. / Surface & Coatings Technology 252 (2014) 93–101

modification. These results reveal that gelatin adsorptionmay be due tonitrogen or sulfur atom bonding with the metallic surface [45,54].Adsorption of gelatin could affect transport of ionic species such as cupricand tin ions to the cathode platinum surface.

Without additives, the reduction potential gap between twometallicelements is 280 mV (Fig. 9). It is difficult for alloy co-deposition to beinitiated if the reduction potential difference is not within 200 mV[55]. When gelatin is added to the plating bath, there is a substantialreduction in the potential gap (Fig. 9), and the potential difference be-tween cathodic peaks (when current density reaches its maximum) ofcopper and tin is decreased to 120 mV, corresponding to a potentialgap of 160 mV. These investigations show that gelatin–surface interac-tions help co-deposition of Cu–Sn by reducing the deposition potentialgap between the two metallic elements (copper and tin) [55]. Theadsorption phenomenon of this additive can improve and/or modifythe properties of copper–tin coatings. Gelatin may adopt a new confor-mation during the adsorption process by an entropy gain, which canpromote deposition of the copper–tin alloy.

3.3. Morphological studies

Morphological examinations of copper and copper–tin deposits onplatinum were analyzed by the SEM to confirm the electrochemicalresults, and obtain structural information on the influence of gelatinon the nucleation process. Fig. 10(a)–(b) shows the SEM micrographsof deposits obtained in Cu plating solutions without and with 1 g/L of

Fig. 10. SEM micrographs of copper deposit obtained for 30 min at−950 mV/MSE, froman electrolyte containing (a) no additive, (b) 1 g/L of gelatin.

gelatin, while Fig. 11(a)–(b) shows copper–tin coatings without andwith 1 g/L of gelatin. The copper and copper–tin deposits are obtainedby chronoamperometry after 30 min at a potential of E = −950 mVvs. MSE (Fig. 10(a)–(b)) and at a potential of E = −1200 mV vs. MSE(Fig. 11(a)–(b)) respectively. Without gelatin, the SEM micrograph(Fig. 10(a)) shows a growth in the form of aggregates (a few μm to50 μm) within elongated shapes. Grains retain their overall elongatedshape but tend to occur in clusters at certain preferred sites, indicatingprogressive nucleation. In the presence of gelatin (Fig. 10(b)), the coppercoating has a different shape and is formed to crystallites of similar size(5–10 μm), and homogeneous over the entire surface. These grainstended to be interconnected, and became densely packed, which indi-cated instantaneous nucleation.

A similar phenomenon is observed with copper–tin deposits(Fig. 11(a)–(b)). Without additives in the electrolyte, dendritic growthabove previous deposits occurs at certain preferred sites. The structureof this deposit is formed in elongated shapes likened to needles,which is characteristic of tin whiskers [68–70]. With additives, thestructure of the coating is different, and similar to the copper depositwith gelatin. The Cu–Sn deposit is homogeneous over the platinumsurface, and the grain structure ismodified. The formation of crystallitesis observed and regular. These grains have a smaller size (a few μm to10–15 μm), and the growth of this deposit is modified, indicating thatnucleation is instantaneous.

Gelatin was used as an organic additive to achieve a smooth coating[39,40] and a good platinum substrate coverage. It is also clear that the

Fig. 11. SEM micrographs of copper–tin deposits obtained for 30 min at−1200 mV/MSEfrom an electrolyte containing (a) no additive, (b) 1 g/L of gelatin.

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grain sizes of the copper and copper–tin coatings were smaller thanthose obtained without additives. Gelatin as an additive modifies thenucleation mode, thus identifying instantaneous nucleation andenabling smaller grains. Therefore, gelatin has a grain refining effect aswell as leveling properties on electrodeposits of copper and copper–tinalloys.

4. Conclusion

In this paper, copper and copper–tin electrodeposition processes onplatinum substrate from acid sulfate electrolyte are investigated in theabsence and in the presence of gelatin as an additive, which is a low-cost and easily supplied product. The influence of gelatin on copperand copper–tin deposition is analyzed by electrochemical studies, com-bined with other surface characterization analyses.

Electrochemical studies have shown the existence of surface modifi-cation by the presence of cathodic overpotential,which is very significant.The presence of a reduction peak, irrespective of gelatin content, is char-acteristic of a system not limited by the decomplexation step. Therefore,overpotential can only be attributed to the presence of an adsorptionlayer, which is consistent with all surface analysis results (PM-IRRAS,XPS, contact angles). These investigations have allowed us to reveal thepresence of adsorbed gelatin on platinum surface.

Adsorption behavior often results in a combination of transport,adsorption and electrostatic processes. An analysis with XPS andPM-IRRAS shows that gelatin has a strong affinity to platinum surface,including significant conformational reorientations. Surface-gelatincontact induces an entropy gain, and gelatin tends to adopt a conforma-tional re-organization on active growth sites at the cathode, whichcauses cathodic overpotential detected by electrochemical analysis.

In the second part of this paper, we have shown the influence ofgelatin on copper–tin deposition. Polarization curves have shownthat adsorption of gelatin at the double layer may affect transport ofionic species as cupric or tin ions to the cathode surface of platinum,and play a role in the electrodeposition mechanism. Adsorption ofgelatinmodifies the copper–tin deposition process by enhancing nucle-ation and growth of coatings. Gelatin is electrophoretically transportedto the cathode during electrodeposition, and reduces the reductionpotential gap between tin and copper. Co-deposition of Cu and Sn is fa-cilitated by the closer deposition potentials of Cu and Sn in the presence ofgelatin on bare Pt with an improvedmorphology, in particular exhibitingfar less denditric growth. Finally, commercial gelatin can be used as an ad-ditive at an optimum concentration of 1 g/L in acid copper-tin electrolytein a potential range between −1.25 V/MSE and−1.45 V/MSE to obtainCu–Sn alloys.

Acknowledgments

The authors would like to thank the Franche-Comté region forsupporting a Ph.D scholarship. We thank the FEMTO-ST Institute,Besançon, France, for their access to clean room equipment on theMIMENTO technological platform for SEM images, and the ICB Laboratory,Dijon, France for their access to equipment for XPS analyses.

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