Controllable zeolite films on electrodes – comparing dc voltageelectrophoretic deposition and a novel pulsed voltage method

Bin Yu, Soo Beng Khoo *

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore

Received 5 July 2002; received in revised form 14 August 2002; accepted 14 August 2002

Abstract

We report here a novel application of pulsed voltage electrophoretic deposition (EPD) of zeolite 13X particles on glassy carbon

surfaces. Our initial studies employing dc voltage EPD indicated that while adjustments of solution parameters (pH, supporting

electrolyte concentration) and other experimental parameters (dc voltage, deposition time) allowed control of the amount of zeolite

coated (from sub-monolayer to multilayer), difficulties of surface deactivation, controllability, uniformity, and reproducibility oc-

curred under conditions suitable for the various films. These difficulties can be alleviated by utilizing a pulsed voltage program for

EPD, an approach which has not been previously used for the fabrication of zeolite modified electrodes. � 2002 Elsevier Science

B.V. All rights reserved.

Keywords: Glassy carbon; Pulsed voltage electrophoretic deposition; Zeolite 13X; Modified electrode

1. Introduction

The controlled assembly of colloidal particles hasreceived much attention in recent years because of thepotential applications of nano- and micro-structuredmaterials in many fields [1]. The constructions of two-dimensional colloidal arrays on surfaces have beendeveloped by different techniques, including microli-thography, self-assembly, and electrophoretic deposition(EPD) [2,3]. Such controlled deposition of colloidalparticles on surfaces can be used to create a variety ofsensors, etc. [4].

In this work, we focused on the EPD of zeolite par-ticles on glassy carbon electrodes (GCEs). EPD is apractically simple, yet effective method for coating ofcharged particles on surfaces, which has gained accep-tance for coatings in various industrial applications,including the automative, appliance, and industrial or-ganic coating industries. Its advantages include uniformdeposition, control of deposit thickness, low levels ofcontamination, and continuous processing [5]. In spite

of these advantages and the extensive previous usage ofEPD, it was only recently that EPD has been employedfor the fabrication of clay [6] and zeolite [7] coatedelectrodes. Zeolites have also been coated on varioussubstrates as thin films but not for the purposeful fab-rication of zeolite modified electrodes [8,9]. Zeolites arecrystalline aluminosilicates organized into regular threedimensional networks with intracrystalline void spacesconsisting of channels and cages which may be inter-connected [10]. Such pores and channels allow the in-gress and egress of molecular and ionic speciescontrolled by factors such as size, charge, and shape.Thus, zeolites are charged particles possessing interest-ing properties including sieving, analyte preconcentra-tion, ion-exchange with applications which mesh withthe ability to form organized zeolite assemblies on sur-faces, e.g., sensors. In recent years, there has been muchinterest on the applications of zeolite modified elec-trodes and, while various methods of coating of zeolitefilms on electrodes have been used [11], the search fornew approaches/directions are ongoing. Since zeolitesare charged particles, EPD should be a suitable methodfor their deposition on electrode surfaces. Our literaturesearch indicated that, to-date, there have only been fourreports [6,7,12,13] on the deposition of zeolite films on

Electrochemistry Communications 4 (2002) 737–742

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*Corresponding author. Tel.: +65-68742919; fax: +65-67791691.

E-mail address: chmksb@nus.edu.sg (S.B. Khoo).

1388-2481/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S1388-2481 (02 )00444-7

electrodes by EPD. Of these, three [6,7,12] employed dcvoltage EPD while Ke et al. [13] mentioned briefly thatvoltage step application gave more uniform films. Also,in a different context, Zhao et al. [14,15] have studied thesquare wave EPD of gold nanoparticles. So far, there isvery little systematic study on the controllability andreproducibility of zeolite coatings deposited on electrodesurfaces by EPD.

Therefore, the objectives of this work are to system-atically investigate the factors affecting EPD on GCEand the controllability, uniformity, and reproducibilityof zeolite films on electrodes fabricated by EPD. The dcvoltage EPD process was compared to a novel pulsedvoltage EPD method developed here for zeolite coatingon GCE.

2. Experimental

2.1. Reagents

All chemicals were of analytical reagent grade unlessotherwise specified. Water was obtained from a Milli-pore Alpha-Q water purification system (18:2 MX;Millipore, USA). Zeolite 13X was purchased fromAldrich Chemical (Milwaukee, WI, USA). The manu-facturer�s specifications gave the nominal particle size as2 lm but our observations showed that there was a widevariation of sizes with a significant fraction differingsubstantially from 2 lm. By trial and error, we devel-oped a simple sedimentation and decantation procedureto obtain a narrower particle size distribution in theregion of 2 lm. It should be noted that a zeolite 13Xsuspension in water increased the pH of the aqueousphase to between 10 and 11, depending on the amountof suspended zeolite. This is due to hydrolysis of alu-minum present in the zeolite network [16]. This hydro-lysis effect led to some difficulties in measurements andadjustments of solution pH and longer times were usu-ally needed to obtain stable values.

2.2. Apparatus

Containers (glassware, polyethylene bottles, etc.)were soaked overnight in 10% HNO3 prior to use.Electrochemical experiments were performed with anAutolab PGSTAT 30 (Eco Chemie, Netherlands) con-trolled by a personal computer. For EPD of zeoliteparticles, a single compartment cell with a flat, taperingbottom (�200 ml capacity) and a plastic top with holesfor electrodes and nitrogen purging was used. The dis-tance between counter and working electrode was about2 cm. Glassy carbon disks (3 mm diameter) were usedas working electrodes, while counter electrodes wereplatinum disks (3 mm diameter). The reference elec-trode was Ag/AgCl (saturated KCl). Scanning electron

micrographs of the GC electrode surfaces were obtainedusing a JEOL Model JSM-5200 (JEOL, Japan) scanningelectron microscope (SEM). pH measurements weremade with a Hanna Model HI 9318 meter (Hanna In-struments, Woonsocket, RI, USA) while conductivitymeasurements were carried out with a Model GM-115conductivity meter (Kyoto Electronics, Japan).

2.3. Procedure

Before each experiment, the glassy carbon electrode(GCE) was polished with alumina ð0:3 lmÞ-water slurryon polishing cloth. The electrode surface was then rinsedcopiously with Millipore water and wiped with soft tis-sue wetted with water. This was followed by 5 min so-nication in a Millipore water bath (extra care was takento avoid leaving any alumina particles on the electrodesurface).

For EPD of zeolite 13X, the prepared GCE wasplaced in the single compartment cell with 50 ml of asolution of KNO3 (deaerated for 15 min with purifiednitrogen) containing 3:00 g l�1 of suspended zeolite13X. EPD was performed potentiostatically at dc po-tential (lower and upper potential limits were )5.00 and+5.00 V, respectively) for a fixed time period (typically30 min) while the suspension was magnetically stirred.The dc potential was varied, as well as the time durationof EPD. Other parameters studied were concentrationand type of supporting electrolytes.

3. Results and discussion

EPD can be viewed as a two-step process, where inthe first step the suspended charged particles move to-wards the electrode under the attractive influence of theapplied electric field (electrophoresis). In the secondstep, the particles reaching the electrode surface aredeposited in a controlled manner according to theamount desired.

There are many factors, which may be interrelated,which influence the EPD of colloidal particles on sur-faces [17]. It is known that in electrolyte solutions con-taining suspended charge particles, where the ionicconcentrations are relatively high, the major chargecarriers are the ions because of their higher nobilitiescompared to the charged, colloidal particles. Therefore,high ionic concentrations may not be beneficial forEPD. Additionally, a high ionic strength (concentration)has been demonstrated to be detrimental to colloidalstability. Further, ionic concentration may affect thezeta potential of the colloidal particles and thereforetheir electrophoretic mobility [18].

Considerations of the above interrelated factors arecrucial to effect EPD under dc potentiostatic conditions.It can be observed in Table 1 that the presence of KNO3

738 B. Yu, S.B. Khoo / Electrochemistry Communications 4 (2002) 737–742

concentrations of 1:00� 10�4, 1:00� 10�3, and 0.10 Mresulted in the absence of zeolite deposition between�5 V. However, at 1:00� 10�5 M KNO3, depositionwas successful at +1.80 V and above but not below thatpotential. Thus, the presence of 1:00� 10�5 M KNO3

was judged to be favorable and chosen for furtherstudies. Figs. 1(a)–(d), depict the SEM images of de-posited zeolites ranging from sparse to multilayer fordifferent dc voltages, all at 30 min EPD. While controlof the amount of deposits was possible as shown in Fig.1, reproducibility and uniformity are more difficult fromour experience. Additionally, deposition was achieved atthe cost of electrode surface changes – cyclic voltam-mograms (CVs) of 1.00 mM FeðCNÞ3�6 in 1.0 M KClbefore and after EPD with removal of zeolite particlesby rinsing with a forceful jet of Millipore water, revealed

that there were considerable surface changes as indi-cated by significant changes in the CVs as shown inFigs. 2(a) and (b). These changes were attributed toexcessive surface oxidation. The control, uniformity,reproducibility, and surface changes of the dc EPDmethod could be improved as will be discussed laterwhen using the pulsed voltage method.

All the above discussions were based on KNO3 assupporting electrolyte. For comparison, we also studiedMgðNO3Þ2 and AlðNO3Þ3 as supporting electrolytes.Two concentrations were studied here, these being1:00� 10�5 and 1:00� 10�3 M for both Mg2þ and Al3þ

(Table 2). Considering the conductivities (Table 2), at1:00� 10�5 M, these were not too much different fromthose for KNO3. With regard to EPD (30 min),the presence of 1:00� 10�3M Mg2þ and Al3þ gave no

Fig. 1. SEM images of dc EPD fabricated in 3:00 g l�1 suspension of zeolite 13X, 10�5 M KNO3 for 30 min: (a) E ¼ 1:9 V; (b) E ¼ 2:0 V; (c) E ¼2:2 V; (d) E ¼ 2:5 V.

Table 1

Solution/suspension parameters and their effects on dc EPD

Zeolite 13X

(g l�1)

Concentration of

KNO3 (10�5 M)

pH Conductivity

(mS/cm)

EPD potential

(V)

EPD average

current ðlAÞRemark

3 10,000 7.80 7.25 )2.0 )210 No EPD (not even �5 V)

3 100 10.15 0.098 )2.0 200 No EPD (not even �5 V)

3 10 10.32 0.051 4.0 52 No EPD (not even �5 V)

3 1 10.31 0.0362 2.2 25 EPDa (higher than 1.8 V)

0 1 5.60 0.0019 – – –

0 1 (Adjust pH of

KNO3 by KOH)

10.45 0.115 – – –

aClose to monolayer.

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zeolite deposits in the range �5 V. At 1:00� 10�5 M, inthe case of Mg2þ, no deposit was observed for potentialsless than 2.20 V, whereas for 2.20, 3.00, and 4.00 V,sparse, submonolyer, and multilayer zeolite depositswere found, respectively. For 1:00� 10�5M Al3þ, at3.00 V and below, no EPD was found but a sparselypopulated layer of zeolite particles was seen at 4.00 V.From these results, it can be concluded that EPD ofzeolite was more difficult and required higher positivepotentials for Mg2þ and Al3þ than for KNO3, the dif-ficulty increasing from Mg2þ to Al3þ (i.e., with in-creasing charge on the cations). The reason for this issimply because the adsorption of cations in the doublelayer region of the negatively charged zeolite particlesled to a decrease in magnitude of the negative zeta po-tential. This lowering is more substantial going from Kþ

to Mg2þ to Al3þ, due to the increased shielding of thehigher charge cations. For Al3þ and Mg2þ at the highpH of 10.30, there could be a problem of the precipi-tations of AlðOHÞ3 ðKsp ¼ 2� 10�32Þ and MgðOHÞ2ðKsp ¼ 1� 10�11Þ but we think this was alleviated by theadsorption of Al3þ and Mg2þ on the zeolite surfaces.Therefore, for better control and improved deposition,

Kþ is still the most favorable of the three cations inaqueous suspensions but in all cases, surface changesoccurred under the present conditions.

While our objectives of controlling zeolite coatings onGCEs can be attained with dc EPD, our subsequentexperiments indicated that surface deactivation, con-trollability, uniformity, and reproducibility could besignificantly improved by using a novel pulsed voltagemethod. Fig. 3 shows the rectangular pulse train em-ployed as waveform for pulsed EPD. At the beginningof EPD, )2.00 V was applied for 30 s for the purpose ofelectrode cleaning and conditioning. After this initialinterval was over, subsequent voltage step sequenceswere at E1 for t1 s followed by E2 for t2 s and thisE1 � E2 sequence was repeated n times (steps) as desired.

The present discussion is for 3:00 g l�1 suspension ofzeolite 13X in 1:00� 10�5 M KNO3. The main functionof E2 was to enable deposition while E1 was utilized forelectrode surface cleaning/reactivation. The results fromvarious E2 are consistent with those for dc EPD. No de-position was observed for E2 < 2:00 V while above 2.50V, surface deactivation occurred. In the case of E1, neg-ative potentials exhibited no zeolite deposition, likely dueto destabilization from the negative zeta potential of theparticles. Typically, for E1 < 0 V, no zeolite deposition

Table 2

Effect of different supporting electrolytes on dc EPD

Supporting electrolyte

solution (all at 1� 10�5 M)apH Conductivity

(mS/cm)

EPD potential

(V)

EPD average

current ðlAÞRemark

KNO3 10.31 0.0362 2.20 25 EPDb

MgðNO3Þ2 10.31 0.032 2.20 14.3 Sparse EPD

MgðNO3Þ2 10.31 0.032 3.00 48 Sparse EPD

MgðNO3Þ2 10.31 0.032 4.00 70 Multilayer EPD

AlðNO3Þ3 9.90 0.0418 3.00 38 No EPD (not even �5 V)

AlðNO3Þ3 9.90 0.0418 4.00 44 Sparse EPD

aAll containing 3:00 g l�1 suspension of zeolite 13X.bClose to monolayer.

Fig. 3. Pulse waveform.

Fig. 2. CVs ð50 mV s�1Þ in 1.00 mM FeðCNÞ3�6 , 1.00 M KCl: (a) bare

GCE, no EPD; (b) EPD in 3:00 g l�1 suspension of zeolite 13X,

10�5 M KNO3 at 2.2 V for 30 min; (c) EPD in 3:00 g l�1 suspension of

zeolite 13X, 10�5 M KNO3 by pulsed voltage (E1 ¼ 0 V, t1 ¼ 50 s,

E2 ¼ 2 V, t2 ¼ 200 s, 10 steps). For (b) and (c), the CVs were obtained

after removing the deposited zeolite particles by rinsing strongly and

copiously with Millipore water. While (b) and (c) are for close to

monolayer depositions, similar results were obtained for all levels of

depositions studied.

740 B. Yu, S.B. Khoo / Electrochemistry Communications 4 (2002) 737–742

occurred while above 0.50 V, electrode surface deactiva-tion is likely to have happened. These effects are depen-dent on the times t2 and t1 as well. Generally, the longerthe t2, the larger the amount of zeolite deposit but withmore likelihood of electrode surface oxidation whilelower t2 may result in the absence of zeolite deposit (typ-ically the t2 used in this study ranged from140 to 300 s).Asfor t1, a larger value resulted in the absence of depositwhile a smaller value gave larger amounts of depositedzeolites but with more likelihood of surface oxidation(t1 ranged typically from 20 to 50 s for the present inves-tigation). While the roles of E2 and t2 are fairly straight-forward, those of E1 and t1 are more complex. E1

apparently served two purposes, firstly to reactivate theelectrode surface after oxidation through mitigation ofthe effect of the high voltage of E2. For example, surfaceoxide formation could be removed at the lower potentialsof E1. Secondly, with respect to deposition, E1 is slightlydestabilizing so that during t1 duration, loosely heldparticles may be lost. Additionally, lateral movement ofthe deposited particles may occur [3], resulting in moreuniform deposits. Therefore, too long a t1 will lead to lesszeolite deposited while too short a t1 means more depositbut if excessively short, then reactivation would not besignificant, resulting in the occurrence of surface oxida-tion. Fig. 4(a), (b) and (c) show the SEM images for

submonolayer, close to monolayer and multilayer de-posits of zeolites. Fig. 4(b) and (d) show similar, close tomonolayer deposits which could be achieved by tuning t1and t2. The CV (see Fig. 2(c)) of the GCE in ferricyanide,after removing the zeolite coating, was essentially thesame as a freshly cleaned GCEwithout undergoing EPD.For submonolayer and multilayer deposits, the same re-sults were obtained. Further, our results indicate thatuniformity (observing SEM images at different positions)and reproducibility were improved. Another advantageof the pulsed EPD method over dc EPD is the controlla-bility – the possibility of controlling the amount of depositby tuning the parametersE2, t2,E1, and t1 provides amoreversatile and powerful method compared with dc EPDwhere varying only the voltage and time of EPD gavecoarser control. Another avenue for control of amount ofdeposit in pulsed EPD is the number of steps. Increased inthe numbers of step (n) resulted in increased zeolite de-posits but with more likelihood of surface deactivation.For this work, an n value of 10 was typically used.

All the above discussions were for the presence of1:00� 10�5 M KNO3 as supporting electrolyte. We alsoperformed the same experiments with just the zeolitesuspensions in the absence of any supporting electrolyte.The results indicated that controllable deposits werepossible for dc EPD but in all cases, surface damage

Fig. 4. SEM images of pulsed EPD fabricated in 3:00 g l�1 suspension of zeolite 13X, 10�5 M KNO3 for 30 min: (a) E1 ¼ 0 V, t1 ¼ 50 s, E2 ¼ 2 V,

t2 ¼ 190 s; (b) E1 ¼ 0 V, t1 ¼ 50 s, E2 ¼ 2 V, t2 ¼ 200 s; (c) E1 ¼ 0 V, t1 ¼ 20 s, E2 ¼ 2 V, t2 ¼ 200 s; (d) E1 ¼ 0 V, t1 ¼ 30 s, E2 ¼ 2 V, t2 ¼ 190 s;

In all cases, the number of steps was 10.

B. Yu, S.B. Khoo / Electrochemistry Communications 4 (2002) 737–742 741

occurred. However, similar results were observed forpulsed EPD as with the presence of supporting electro-lyte. Due to a difference in zeta potential in the absenceof supporting electrolyte, voltages used for both dc andpulsed EPD were somewhat different from those for thepresence of KNO3.

4. Conclusion

We have shown here the applicability of a novel ap-proach to controllable zeolite deposition on electrodesurface by pulsed voltage EPD. While dc voltage EPDalso affords control of zeolite deposition, it is less tun-able and convenient as different solutions have to beused and, more importantly, at the expense of electrodesurface integrity. In contrast, pulsed voltage EPD issimple, yet more powerful and convenient to give con-trol of the amount of zeolite deposited (from sub-monolayer to multilayer) by tuning the pulse widths,heights and number of pulses.

Acknowledgements

This work was supported by a grant from the Na-tional University of Singapore.

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