Electrochimica Acta 158 (2015) 121–128

Kinetics Study of Silver Electrocrystallization on (3-mercaptopropyl)trimethoxysilane-grafted Indium Tin Oxide Plastic Substrate

Nga Yu Hau 1, Ya-Huei Chang 1, Shien-Ping Feng *Department of Mechanical Engineering, The University of Hong Kong, Pokfulam, Hong Kong

A R T I C L E I N F O

Article history:Received 7 November 2014Received in revised form 3 January 2015Accepted 23 January 2015Available online 24 January 2015

Keywords:electrocrystallizationAC voltammetryITO-PENchronoamperometryShariker-Hills

A B S T R A C T

3-mercaptopropyl-trimethoxysilane (MPS) self-assembled monolayer (SAM) has been demonstrated aseffective promoters to enable direct electroplated metallization on indium tin oxide (ITO) plasticsubstrate. In this paper, the detail kinetics in Ag electrocrystallization on MPS-grafted ITO-PEN isreported. Contact angle measurement provides evidence of bridging-link effect between the sulfur headgroups of MPS and the Ag+ ions in the electrolyte. Electrochemical techniques including cyclicvoltammetry and Tafel plot were used to investigate the redox kinetics. Quantitative evaluation wasconducted by alternating current voltammetry to determine the rate constant of electron transfer. Thechronoamperograms and their fitting results suggest a combined model with two-dimensional/three-dimensional nucleation transition and Shariker-Hills model for electroplated Ag on blank ITO-PEN andMPS-grafted ITO-PEN respectively.

ã 2015 Elsevier Ltd. All rights reserved.

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1. Introduction

Electroplating is a very commonly used metallization processfor electronic components, printed circuit boards, and semicon-ductor industries because of its inherently low cost, lowtemperature, and high throughput. As known, the electrocrystal-lization kinetics can be divided in few steps [1]. It starts with iondiffusion from bulk electrolyte onto the electrode surface,followed by electron transfer between electrode and ad-ions.After partially or completely loss of solvation sheath, ad-atomsare formed and gradually clustered to form critical nuclei. Finalstep is the lattice incorporation and crystallographic growth.Fundamental aspects of electrocrystallization are subject to thenucleation mechanism, which governs morphological, structuraland adhesive properties for metal electrodeposition. Therefore, adetailed understanding of the nucleation mechanism, such asinstantaneous/progressive mode, two-dimensional/three-dimen-sional (2D/3D) growth, and its rate-determining step, is ofsignificant importance.

* Corresponding author at: Department of Mechanical Engineering TheUniversity of Hong Kong, 7-24, Haking Wong Building, Pokfulam Road, Hong Kong.Tel.: +852 2859 2639; fax: +852 2858 5415.

E-mail address: hpfeng@hku.hk (S.-P. Feng).1 These authors carry equal contribution.

http://dx.doi.org/10.1016/j.electacta.2015.01.1170013-4686/ã 2015 Elsevier Ltd. All rights reserved.

Indium tin oxide (ITO) plastic substrate, such as indium tinoxide polyethylene naphthalate (ITO-PEN), is indispensable whenthe device goes flexible and rollable. Reliable metallization on ITOplastic substrate is one of the important processes. The currentsputtering method of fabricating metal on ITO-PEN requires costlyvacuum equipment and has a low production rate. People haveworked on electrodeposition because it is a cost-effective methodand has been used as metallization process for electroniccomponents, printed circuit boards, and semiconductor industries[2–4]. As known, electrocyrstallization is usually dominated by theinitial nucleation process. However, in the case of electroplating onITO-PEN, the low surface energy and poor wettability do notprovide opportunity to strongly interlock with electroplatedmetals because a sparse distribution of nucleation sites leads topoor adhesion and irregular grain growth. In general, a conformaland conductive seed layer coated on ITO-PEN by sputtering orchemical vapor deposition (CVD) is needed before electroplating toachieve a good contact performance [5]. We recently presented anew technique to electroplate an adhesive and uniform metalliclayer on ITO-PEN using 3-mercaptopropyl-trimethoxysilane (MPS)self-assembled monolayers (SAMs) as a promotion layer [6,7]. Theuse of an MPS thin layer, which consists of methoxy group adhereto the substrate surface and its sulfur functional group can form abridging link to metal ions in the electrolyte, provides a strongcovalent binding and creates more nucleation sites when electro-plating. The deposition of MPS thin layer followed by the

122 N.Y. Hau et al. / Electrochimica Acta 158 (2015) 121–128

electroplating metallization can be achieved by low temperatureand solution-based processing, which is significantly more cost-effective than the previously reported methods. This findingpresents a great potential for the preparation of cost-effective andreliable metallization on ITO-coated plastic substrates, which canfacilitate the development of flexible, rollable, and lightweightdevices in the future.

As this electroplating technology is still in its infancy, this papertherefore aims to stress a quantitative study to advance ourfundamental understanding for the kinetics of electron transferacross the electroactive MPS thin layer grafted on ITO-PEN. Staticwater contact angles were used to compare the surface wettabilityon blank, MPS-grafted and octadecyl-trimethoxysilane (ODS)grafted ITO-PEN samples. Electrochemical techniques includingcyclic voltammetry (CV) and Tafel plot were conducted tounderstand the redox kinetics of Ag electrodeposition on MPS-grafted layer. Alternating current voltammetry (ACV) [8] was usedto determine the rate of electron transfer for redox species in a MPSlayer. Chronoamperometry (CA) was measured and fitted tocharacterize the electrochemical nucleation/growth models ofelectroplating Ag on blank and MPS-grafted ITO-PEN.

2. Experimental

2.1. Pretreatment of ITO-PEN

ITO-PENs (Peccell Co., Japan) with a sheet resistance of 15 V/&and optical transmission of 75% were cleaned in acetonefor 30 minutes with sonication. For the MPS-grafted samples,ITO-PENs (2 � 1.5 cm) were immersed into a solution of 1% MPS inethanol for 50 minutes, and were then rinsed by ethanol and driedby air gun. ODS-grafted samples were made by immersingITO-PENs into a solution of 1% of ODS in toluene for 50 minutes,

Fig. 1. Contact angle of DI water and Ag electrolyte on (A)

and were then rinsed by toluene and dried. Chemical-resist sticker(Max Bepop, CM-200E) was used as a mask with an interior 1 cm2

hollow circle to predetermine the electroplated area on ITO-PEN.Static contact angle was measured by contact angle goniometer(Sindatek model 100SB).

2.2. Electrochemical measurements

Electrochemical measurements were performed by a CHI 660 Epotentiostat in standard three-electrode system (Pt mesh ascounter electrode, Ag/AgCl electrode as reference electrode) atroom temperature. A Ag electrolyte (Metaler MetSil 500CNF R-T-U;contents: 0.28 M of Ag+ ions, cyanide free) was used for CV and CA.CV was measured on blank and MPS-grafted ITO-PEN from -1 to 1 Vat a scanning rate of 20 mV/s. CAs were recorded by plating currentdensity (mA/cm2) versus time (sec) at constant voltage of -0.7 V,-0.75 V, -0.8 V, -0.85 V, and -0.9 V. OriginPro was used for theexperimental data fitting. A 50 times diluted Ag electrolyte(0.0056 M) was used in ACV measurement to avoid a large amountof Ag coated on substrate to block the active sites. ACV wasmeasured by a DC potential scan from -0.3 to -0.9 V with asuperimposed sinusoidally oscillating AC wave of 25 mV in astepped mode every 10 mV. A series of AC frequencies from 0.1 to1000 Hz were collected to obtain the ratio of peak current (ip) tobackground current (ib). The surface morphologies of as-plated Agon blank and MPS-grafted substrates were investigated by field-emission scanning scanning electron microscope (SEM, S-4800,Hitachi).

3. Results and Discussion

Fig. 1A shows that the static contact angles of DI water is 63.2�

on blank ITO-PEN and the contact angle is reduced to 61.9� while

blank, (B) MPS-grafted and (C) ODS-grafted ITO-PENs.

Fig. 2. SEM images of Ag deposition at early stage (deposition time: 0.7 s) on (A) blank and (B) MPS-grafted ITO-PENs, and completed stage (deposition time: 40 s) on (C) blankand (D) MPS-grafted ITO-PENs under -0.8 V applying potential with respect to Ag/AgCl electrode at room temperature.

N.Y. Hau et al. / Electrochimica Acta 158 (2015) 121–128 123

using Ag electrolyte as a liquid probe. This slight difference in staticcontact angle can be explained by the higher polarity of Agelectrolyte than DI water. Fig. 1B shows the contact angles onMPS-grafted ITO-PEN are 53.2� and 46.2� for DI water and Agelectrolyte respectively. Taking Fig. 1A as a reference, the greaterdifference of contact angles on MPS-grafted should not be onlycontributed by the increase in polarity [9,10]. According to theYoung’s equation [11], a smaller contact angle corresponds to ahigher surface energy. Our previous study [2,3] provides theevidence that the decrease in contact angle after MPS-treatmentcan be explained by the bridging link effect as the ligand formedbetween Ag+ ions and sulfur functional groups of MPS layer. Asshown in Fig. 1C, a counter example of contact angle measurementon octadecyl-trimethoxysilane (ODS)-grafted ITO-PEN was used toprove the significance of using an appropriate SAM with correct

-1.0 -0.5 0. 0 0.5 1.0-10

-5

0

5

10

Cur

rent

Den

sity

(mA

/cm

2 )

Potential vs Ag/AgCl (V)

blank IT O MPS -grafted ITO

(A)

Fig. 3. (A) Cyclic voltammogram for electroplating Ag on blank and MPS

head group. As seen, contact angles of DI water and Ag electrolyteon ODS-grafted ITO-PEN are 90.8� and 75.3� respectively, which aremuch larger than that on blank and MPS-grafted samples. Thisresult shows that ODS treatment makes hydrophilic ITO-PENbecome hydrophobic because the —CH3 head group owns a relativelow surface energy [9,10].

At the early stage of electrocrystallization, nucleation competeswith the grain growth, which affects the uniformity andgranularity of metal deposit. In general, a high nucleation rateresults in fine grains [12]. The electroplating nucleation processcan be distinguished to instantaneous or progressive mode. Theinstantaneous mode means that a fixed number of nuclei areformed right after applying an electrical potential. The progressivemode represents that the number of nuclei is time-dependent.Hence, uniform nuclei size can be observed in instantaneous

-1.0 -0 .8 -0.6 -0.4-10

-5

0

Cur

rent

Den

sity

(mA

/cm

2 )

Potential vs Ag/AgCl (V )

blank IT O MPS-graft ed IT O

(B)

-grafted ITO-PEN. (B) Cathodic waves of the cyclic voltammogram.

0 10 20 30 40 50 60

0.0

0.1

Cur

rent

Den

sity

(mAc

m-2

)

Times (s)

blank ITOMPS-graft ed ITO

(A) (B)

-10 -8 -6 -4 -2 00.0

-0.2

-0.4

-0.6

-0.8

-1.0

slope: -0.298

E v

s A

g/A

gCl (

V)

log (i mAcm-2)

blank IT OMPS-grafted IT O

slope : -0 .318

Fig. 4. (A) Tafel plots with cathodic slopes between �0.5 V to �0.2 V, and (B) the current responses upon the exposure of blank and MPS-grafted ITO-PENs in Ag electrolytewithout the application of electrical potential.

124 N.Y. Hau et al. / Electrochimica Acta 158 (2015) 121–128

nucleation while in progressive nucleation, nuclei are found insizes with a wide range [13,14]. Fig. 2 shows the SEM images ofelectroplated Ag on blank and MPS-pretreated samples at initial(0.7 seconds) and completed (40 seconds) stages under a constantvoltage of �0.8 V. As seen in Fig. 2A and B, a higher density ofnucleation sites with smaller radius were found on MPS-graftedITO-PEN while less dense nucleation sites with larger radiusappeared on blank ITO-PEN. It indicates that the nucleation

Fig. 5. (A) Randles equivalent circuit for the impedance of an electrode coated with a rfrequency of 0.1 Hz. ACV data plot of ip/ib vs. frequency and their fitting curves for (C)

reaction occurred much faster on MPS-grafted surface than onblank surface. As seen, in the beginning of electroplating, the nucleiwithin each sample have similar sizes, indicating that both casesbelong to the instantaneous nucleation mode. Considering at thecolour depth, the nuclei on MPS-grafted ITO-PEN are thehemispherical dome-shape, and the nuclei on blank substrateare close to the disc-like. Fig. 2C shows a poor surface coveragewith uneven agglomerated Ag clusters on blank ITO-PEN after

edox-active thin layer. (B) AC voltammogram of blank and MPS-grafted ITO-PEN atblank and (D) MPS-grafted ITO-PENs.

Table 1Kinetics Parameters for Tafel plot and AC voltammetry.

Cathodic Tafel slope ac Cr RctCad kETV/dec s�1

Blank �0.318 0.172 7.29 0.32 1.55MPS-grafted �0.298 0.184 9.32 0.127 3.93

N.Y. Hau et al. / Electrochimica Acta 158 (2015) 121–128 125

electroplating because of the irregular grain growth on a smallnumber of scattered Ag nuclei. In contrast, Fig. 2D shows a uniformAg film with fine grains was electrodeposited on MPS-graftedsample, suggesting that the Ag+ ions were grown evenly on a largenumber of Ag nuclei.

Fig. 3shows CVs for electroplating Ag on ITO-PEN with andwithout MPS pretreatment. The cathodic/anodic crossover iscorrelated to the formation energy of a new phase [14]. Thus, amore positive potential shift of the crossover point suggeststhat the formation energy of electroplated Ag on MPS-graftedITO-PEN is reduced [15]. A more positive onset voltage in ananodic wave for MPS-grafted surface means that a moreoxidized energy is needed to strip off the Ag deposit fromthe substrate, indicating the deposited Ag atoms bond morestrongly on ITO-PEN after MPS pretreatment. The cathodicwaves in Fig. 3B show the same cathodic onset potentials forboth cases, but an obvious transition hump ranging from�0.6 to �0.8 V can be seen in MPS-grafted sample. Thistransition hump indicates a Ag seed layer was deposited prior

Fig. 6. Chronoamperoograms of Ag electrodeposition on (A) blank and (B) MPS-graftedelectrodeposition on (C) blank and (D) MPS-grafted ITO-PENs at different operating vo

to bulk electroplating because the MPS bridging link to Ag+ ionsreduces the formation energy [7,16].

Fig. 4A is Tafel plot. Our case is an irreversible reaction so that acathodic charge transfer coefficient can be obtained by thefollowing equation in the condition of high overpotential [17]:

logð�icÞ ¼ logio � acnFh2:303RT

(1)

where ic and io are the cathodic and exchange current densities (A/m2) respectively, ac is cathodic charge transfer coefficient, h is theoverpotential (V), n is the number of electrons involved, R is gasconstant (J K�1 mol�1), T is temperature (K), F is Faraday constant(Cmol�1). The cathodic Tafel slopes of blank and MPS-graftedsamples are �0.318 V/dec and �0.298 V/dec respectively, corre-sponding to the cathodic charge transfer coefficients (ac) of0.172 and 0.184 respectively. A larger cathodic charge transfercoefficient represents a more favorable cathodic reaction [18],which supports the reduced formation energy of Ag electrodepo-sition on MPS-grafted ITO-PEN. Fig. 4B are the current responsesupon the exposure of blank and MPS-grafted ITO-PEN in Agelectrolyte. Without the application of electrical potential, nocurrent response was observed in blank sample while a smallpositive current response was collected in MPS-grafted sample.Recent evidence suggests that this pseudocapacitive current isinduced by an interfacial ion-dipole interaction when the aqueouspositive ions absorb on the specific functionalities of electrodes[19,20], which is consistent with the above-mentioned bridging-link effect between MPS and Ag+ ions.

ITO-PEN at different operating voltages. Plots of current density versus t1/2 of Agltages.

126 N.Y. Hau et al. / Electrochimica Acta 158 (2015) 121–128

To further investigate the ITO/MPS/liquid interface, ACV waschosen as the electrochemical method to evaluate the kinetics ofelectron transfer in a redox-active MPS layer because it isstraightforward to obtain a rate constant (kET) for the redoxreaction by fitting experimental data using Randles equivalentcircuit as illustrated in Fig. 5A [8,21]. CDL is double-layercapacitance, RCT is charge transfer resistance, CAD is adsorptionpseudocapacitance, RSOL is the resistance of electrolyte [22]. Thepeak current (ip) and the background current (ib) can bedetermined for each frequency, as shown in an example inFig. 5B. The ratio of ip/ib decreases with the increase of frequencyfor both blank and MPS-grafted samples, as shown in Fig. 5C, D. Asseen, there are two limiting regions, one is at low frequency whereip/ib approaches a constant value and the other at high frequencywhere ip/ib approaches one. Between these two limits, there is atransition region where ip/ib is strongly correlated with frequency.The fitting procedure for the plot of ip/ib vs. frequency involvesadjustment of only the electron-transfer rate constant kET andcapacitance ratio Cr, which can be calculated from the followingequations. A good fitting result can be achieved by treating thesetwo parameters independently because kET primarily affects thebreakpoint and Cr primarily affects the limiting value at lowfrequency.

ipib

� �2

¼ ðv=2KETÞ2 þ Cr

ðv=2KETÞ2 þ 1

!2

� ðv=2KETð1 � CrÞÞ2 (2)

Fig. 7. The fitted curve of chronoamperogram of Ag electrodeposition on (A) blank

instantaneous mode. The fitted curve of the chronoampermetry of Ag electrodepositiodiagram of Shariker-Hills model.

Cr ¼ CAD þ CDL

CDL(3)

kET ¼ 12RctCad

(4)

The fitting results are summarized in Table 1. The fitted kETproves that the electron-transfer rate on MPS-grafted surface isenhanced by 2.5 times as compared to that on blank surface. Asnoted, the value of kET is in the same scale as the previouslyreported kET for other electrochemical redox reactions [8,23].

Subsequently, CA was performed to collect the current responseversus time for electroplating Ag on blank and MPS-grafted ITO-PEN at different operating voltages. As seen in Fig. 6A and B, sharppeaks were found in the current response of MPS-grafted ITO-PENwhile no noticeable peak was shown in the case of blank ITO-PEN.In general, the sharp peak in CA is a symbol of rapid electroplatingunder diffusion-controlled condition, which can be checked byCottrell equation [24],

i ¼ nFD1=2Co

p1=2t1=2(5)

where i is the current density (mA/cm2), n is the number ofelectrons involved, D is the diffusion coefficient (cm2/s), Co is thebulk concentration of the reaction species (mol/cm3), t is the

ITO-PEN under �0.7 V and its (B) corresponding schematic diagram of 2D + 3Dn on (C) MPS-grafted ITO-PEN under �0.7 V and its (D) corresponding schematic

Table 3The best-fit parameters of the current transient fitting for silver deposition on MPS-grafted ITO-PEN.

N3 D im tmcm�2 cm2s�1 mAcm�2 s

4.39 � 105 6.22 � 10�7 3.66 5.46

N.Y. Hau et al. / Electrochimica Acta 158 (2015) 121–128 127

reaction time (second). Fig. 6C and D are the plots of currentdensity versus t1/2 for Ag electrodeposition [25] on blank andMPS-grafted ITO-PEN respectively. As expected, MPS-graftedITO-PEN has a better linearity of Cottrell equation than blankITO-PEN. To advance our fundamental understanding of theelectrochemical nucleation and crystal growth, we presenttheoretical expressions capable of describing CAs arising fromAg deposition on blank and MPS-grafted ITO-PEN. As mentionedabove, the nucleation of Ag electrodeposition on blank ITO-PENfollows the instantaneous mode. The initial disc-like nuclei are 2Dstructure and then grow into 3D islands (Fig. 2A and C). The relativepoor linearity of Cottrell equation suggests a surface-controlledrather than diffusion-controlled mode in the beginning ofelectroplating on blank substrate. We therefore proposed thatAg electrodeposition on blank ITO-PEN is 2D instantaneous modefollowed by 3D instantaneous mode with a rate determining stepof lattice incorporation by surface ad-atoms [26–28]. According toStranski-Kratanow study [29], the current response of lattice-incorporation 2D and 3D instantaneous mode can be representedas below:

I2Dinstantaneous ¼2pnFMhN1k1

2tr

exp �pM2N1k1

2

r2 t2 !

(6)

I3Dicintantaneous ¼ zFk02 1 � exp �pM2N2k2

2

r2 t2 !" #

(7)

where M is the molecular mass of the deposited species (g/mol�1),h is the layer height (cm), r is the density of the deposited species(g/cm3), N1 & N2 are the nucleation densities (cm�2), k1 is the rateconstant of disc-like nuclei growth (s�1), k2&k02 are the rateconstant of horizontal and vertical growth of 3D nuclei (s�1)respectively. A good fitting result can be seen in Fig. 7A for Agelectrodeposition on blank ITO-PEN under the operating voltage of�0.7 V. The blue and green curves reveal the correspondingcontribution by lattice-incorporation 2D and 3D instantaneousmode respectively. We make the assumptions in the modelling asItotal ¼ aI2Dinstantaneous þ bI3Dicintantaeous. The best-fit ratio of a/b isapproximately one and the resulted parameters are summarized inTable 2. Fig. 7B is the schematic diagram showing that the Ag+ ionswere firstly deposited on blank ITO-PEN via 2D instantaneousmode to form the disc-like nuclei and then ad-atoms werepreferentially reduced on the formed nucleation sites following 3Dinstantaneous lattice-corporation controlled mode.

In the case of Ag electrodeposition on MPS-grafted ITO-PEN, itfits well to Shariker-Hills model shown in Fig. 7C. The Shariker-Hills model [30] is based on 3D instantaneous diffusion-controllednucleation by taking the random overlapping of hemisphericalnuclei into account. The equation is as follows:

I3Ddcinstantaneous ¼zFD1=2C0

p1=2 t1=2½1 � expð�pk3DN3tÞ� (8)

where k = ((8pcM)/r)1/2,N3 & k3 are the nucleation density (cm�2)and the rate constant for 3D instantaneous diffusion-controllednucleation (s�1) respectively. Fig. 7D is the correspondingschematic diagram of Shariker-Hills model. The best-fit param-eters are listed in Table 3. As noted, the calculated nucleation

Table 2The best-fit parameters of the current transient fitting for Ag deposition on blankITO-PEN.

h N1 N2 k1 k2 k02cm cm�2 cm�2 s�1 s�1 s�1

5.25 �10�7 1.27 � 104 1.82 � 104 1.27 � 10�4 2.74 �10�5 3.95 �10�8

density of Ag deposition on MPS-grafted ITO-PEN is 30 times morethan that on blank ITO-PEN, which is in a good agreement with theobservation on SEM images at nucleation stage.

4. Conclusions

In this paper, a quantitative kinetic study in Ag electro-crystallization on blank and MPS-grafted ITO-PENs was conducted.The SEM observation shows that Ag electrodeposition on MPS-grafted ITO-PEN has a significantly high nucleation density andfine grains as compared to that on blank ITO-PEN. The comparisonof the contact angles measured using DI water and Ag electrolyteprovides evidence of the bridging-link effect between the sulfurhead groups of MPS and the Ag+ ions in the electrolyte. Theincreased cathodic charge transfer coefficient obtained by Tafelplot demonstrates that MPS is an effective promoter to enableelectroplated Ag on ITO-PEN. The ion-dipole current responseupon the exposure of MPS-grafted ITO-PEN in the electrolyteproves the Ag+ ions adsorption on MPS layer, which supportsthe bridging-link model. ACV measurement is used to analyse theelectron-transfer rate at ITO/MPS/electrolyte interface, theobtained kET shows that the electron-transfer rate on MPS-graftedsurface is enhanced by 2.5 times as compared to that on blanksurface. The fitting results of CAs proposed that the Agelectrodeposition on MPS-grafted ITO-PEN is 3D instantaneousmode under diffusion-controlled (Shariker-Hills model) while thaton blank ITO-PEN is 2D instantaneous mode followed by 3Dinstantaneous mode under surface lattice-corporation controlled.

Acknowledgments

This work was supported by the General Research Fund fromResearch Grants Council of Hong Kong Special AdministrativeRegion, China, under Award Number HKU 719512E.

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