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Materials Chemistry and Physics 117 (2009) 365–372

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

lectrochemical behavior of two and one electron redox systems adsorbed on toicro- and mesoporous silicate materials: Influence of the channels and the

ationic environment of the host materials

. Senthil Kumar, P. Natarajan ∗

ational Centre for Ultrafast Processes, University of Madras, Taramani Campus, Chennai – 600113, India

r t i c l e i n f o

rticle history:eceived 11 November 2008eceived in revised form 6 May 2009ccepted 11 June 2009

eywords:henosafranineodified electrodeCM-41

eolite-YSM-5

a b s t r a c t

Electrochemical behavior of two electron redox system, phenosafranine (PS+) adsorbed on to micro- andmesoporous materials is investigated by cyclic voltammetry and differential pulse voltammetry usingmodified micro- and mesoporous host electrodes. Two redox peaks were observed when phenosafranineis adsorbed on the surface of microporous materials zeolite-Y and ZSM-5. However, only a single redoxpeak was observed in the modified electrode with phenosafranine encapsulated into the mesoporousmaterial MCM-41 and when adsorbed on the external surface of silica. The observed redox peaks for themodified electrodes with zeolite-Y and ZSM-5 host are suggested to be primarily due to consecutive twoelectron processes. The peak separation �E and peak potential of phenosafranine adsorbed on zeolite-Y and ZSM-5 were found to be influenced by the pH of the electrolyte solution. The variation of thepeak current in the cyclic voltammogram and differential pulse voltammetry with scan rate shows that

electrodic processes are controlled by the nature of the surface of the host material. The heterogeneouselectron transfer rate constants for phenosafranine adsorbed on to micro- and mesoporous materialswere calculated using the Laviron model. Higher rate constant observed for the dye encapsulated into theMCM-41 indicates that the one-dimensional channel of the mesoporous material provides a more facilemicro-environment for phenosafranine for the electron transfer reaction as compared to the microporoussilicate materials. The stability of the modified electrode surface was investigated by multisweep cyclic voltammetry.

. Introduction

Electrodes modified with electrochemically active dyes, metalons and metal complexes adsorbed into the porous silicate mate-ials such as zeolites and MCM-41 have potential application inlectrochemical sensors and electrocatalysis [1,2]. Over the pastecades, the electrochemistry of transition metal ions [3,4], dyes5] and metal complexes [6,7] incorporated into the pores of micro-nd mesoporous silicates have been studied. Silicate materialsrovide more rigid and highly structured environment. In theserdered environments, the characteristics of the guest moleculesre modified as a result of the confinement in a restricted space.he active sites of the zeolites driven by weak and specific elec-

rostatic interaction enable specified catalytic reactions to occurith the encapsulated guest molecules [8]. Notably, Walcarius [9]

nd Zanjanch [10] have recently reported the electrochemical char-cteristics of ferrocene, thionine and metal complexes entrapped

∗ Corresponding author. Tel.: +91 44 24540962/5006; fax: +91 44 24546709.E-mail address: pnatrajan@hotmail.com (P. Natarajan).

254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2009.06.008

© 2009 Elsevier B.V. All rights reserved.

into the pores of micro- and mesoporous silicate materials. Molec-ular transformations of organic molecules have been extensivelystudied using zeolites as catalysts. Recently a preliminary reportof the electrochemical properties of some dyes present in mod-ified electrodes with silicates as hosts has been reported [11,12].The voltammetric investigation of immobilized guest molecules inzeolite hosts is useful to understand the electrochemical reactionmechanisms. Several extensive mechanistic details on the electrontransfer in microheterogeneous silicate electrodes and modifiedporous electrodes have shown different pathways for the substratesadsorbed on the external surface and at the interior channels andvoids of silicates [8,3].

At present the dye modified electrodes attract much attentionfor the potential applications in electrochromic devices [13,14],photogalvanic cells and sensors [15–18]. Phenosafranine, PS+, isknown to be an efficient excited state redox system and the chemi-

cal stability, redox properties and excited state reactivity of the dyehave been reported earlier [19]. The dye has also been employedas a probe in miceller system; the property of the dye covalentlybound to the macromolecules has also been investigated [19]. Thephotochemical and electrochemical properties of phenosafranine

366 K. Senthil Kumar, P. Natarajan / Materials Chemistry and Physics 117 (2009) 365–372

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ncorporated into nafion@ film and macromolecular bound systemsave been extensively studied [19–21].

In the present report, the electrochemical behavior ofhenosafranine adsorbed on to modified electrodes using micro-nd mesoporous materials is outlined. In addition to this the elec-rochemical behavior of cresyl violet, CV+, a one electron reductionye incorporated in the silicate hosts has been studied for com-arison. The influence of pH of the electrolyte solution and scanate on the behavior of modified electrode is investigated. Theeterogeneous electron transfer rate constant for the micro- andesoporous materials are calculated and a mechanism is proposed

or the two electron redox processes of the dye.

. Experimental methods

Phenosafranine (PS+) and cresyl violet (CV+) were obtained from Aldrich andsed after recrystallising in ethanol. Zeolite-Y and ZSM-5 were purchased from Sudhemie India. The graphite used as plates was obtained from Graphite India sup-lied as a gift from CECRI, Karaikudi. Cetyltrimethyl ammoniumbromide (CTAB) andetramethylorthosilicate (TEOS) were obtained from Lancaster. All other chemicalsere purchased from Qualigens and Merck fine chemicals.

1.

MCM-41 was synthesized and characterized by the procedure already reportedin the literature [22]. 2.4 g of CTAB was dissolved in 120 g of deionised water andstirred until the solution was homogeneous and clear. Ammonium hydroxide (8 ml)was added to this mixture and stirred for 5 min after which tetraethylorthosili-cate (10 ml) was added to give a molar composition of the gel (0.5 M TEOS:0.82 MNH4OH:0.075 M CTAB:53 M H2O). The reaction mixture was stirred overnight, afterwhich the solution was filtered and washed consecutively with deionised waterand ethanol; calcination was performed at 823 K for 5 h. The obtained material wascharacterized by SAXRD and surface area analysis and found to be showing identicalcharacteristics as reported earlier [22].

Phenosafranine and cresyl violet dyes adsorbed on to zeolites were prepared bystirring 30 ml of (1 × 10−5 M) aqueous solution of the dye with 1 g of zeolite for about4 h. The resulting coloured solid was filtered and washed several times with excessamount of water until the filtrate shows no absorbance at 520 nm for phenosafra-nine and at 580 nm for cresyl violet. The samples were then dried in air at roomtemperature under vacuum. Phenosafranine encapsulated MCM-41 was preparedby the addition of 30 ml of (1 × 10−5 M) aqueous solution of the dye to MCM-41 (1 g)and stirred at room temperature for about 4 h. The resulting coloured solid was fil-

tered and washed several times with excess amount of water until the filtrate showsno absorption for phenosafranine after which the materials were stored in air tightcontainer.

Modified electrodes were prepared by droplet evaporation method. 3 mg of dye-loaded zeolite samples was dispersed in 1 ml of methanol to obtain a suspension.12 �l of suspension was added on freshly polished 6 mm diameter graphite elec-

K. Senthil Kumar, P. Natarajan / Materials Che

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ig. 1. Diffuse reflectance spectra of PS+ in (a) zeolite-Y, (b) ZSM-5, (c) MCM-41 andd) silica.

rode surface. 6 �l of binder (35% of polystyrene in THF) was added for providingechanical stability of the electrode; the electrode prepared was allowed to air dry.

he solutions were prepared using doubly distilled water.Cyclic voltammograms and pulse voltammograms were carried out in a CH-

20B instrument using conventional three-electrode electrochemical cell with thelatinum foil as the auxiliary electrode, saturated calomel electrode as the refer-nce electrode and modified graphite electrode as the working electrode with 0.1 MCl as the electrolyte; all experiments were performed at ambient temperature25 ± 0.1 ◦C) in argon atmosphere.

. Results and discussion

Zeolite-Y and ZSM-5 are microporous silicates with Si/Al ratiof 2.6 and 53.9, respectively. The other silicates used, MCM-41, is aesoporous material with channel diameter of 4.2 nm while silica

s a nonporous host. The molecular dimensions of phenosafranine:1.54 Å × 9.56 Å and cresyl violet: 11.54 Å × 7.5 Å (obtained fromrguslab) shown in Scheme 1 are larger than the pore openingsf both the zeolite-Y (7.4 Å) and ZSM-5 (5.4 Å) and hence the dye

olecules are adsorbed exclusively on the external surface of the

eolite and not entrapped inside the cavities and channels. In thease of MCM-41 the pore size (4.2 nm) is larger than the molecularize of PS+ and in this case the dye is encapsulated into the channelsf the host material. The dye adsorbed at the external surface in this

ig. 2. Cyclic voltammogram of PS+ in phthalate buffer solution (pH = 3.5); scan rate of 50pH = 3.5).

mistry and Physics 117 (2009) 365–372 367

case is removed by repeated washing with the solvent water. TheUV–vis absorption spectra of phenosafranine in aqueous solutionand adsorbed on to micro- and mesoporous materials are shownin Fig. 1. Phenosafranine dye shows a broad peak when adsorbedon to zeolite-Y and ZSM-5 surface with maximum at 520 nm as inthe case of aqueous solution; phenosafranine also shows absorp-tion peak with maximum at 520 nm when the dye is encapsulatedinto MCM-41 and also when adsorbed on to silica.

3.1. Electrochemical studies at modified silicate electrodes

The modified electrode with the host silicate containing thedye shows in the SEM image that the host particles are dis-tributed uniformly (Supporting material-1). Cyclic voltammogramof phenosfranine (1 × 10−5 M) obtained in the buffer solution(pH = 3.5) using graphite electrode at the scan rate of 50 mV s−1,is shown in Fig. 2. The redox peak observed at −265 mV versusSCE and the peak-to-peak separation �Ep of 34 ± 5 mV correspondsto a two electron concerted redox process of phenosafranine asreported earlier [20]. In the case of the modified electrodes withthe dye adsorbed on to the silicates: zeolite-Y (PS+/ZYME) and ZSM-5 (PS+/ZSME), the cyclic voltammograms obtained are shown inFig. 3i. In the case of PS+/ZYME electrode two redox potentials areobserved at −340 and −481 mV with the corresponding �Ep val-ues of 48 and 61 mV, respectively. Similarly the modified electrodePS+/ZSME also shows two redox couples with peak potentials of−221 and −330 mV with �Ep values 35 and 40 mV (Table 1). In allthese cases, the ohmic drop is observed to be negligible in 0.1 MNaCl as electrolyte. It has also been discussed earlier [8] that theporous silicates used are inherent charge carriers and ionic speciesand the ohmic drop is reduced as a result. A few experiments car-ried out using glassy carbon electrode showed results similar tothe graphite plates used in the present investigation as shown inTable 1.

In the case of modified electrode PS+/MCM-41 and the modifiedelectrode with the dye adsorbed on silica the cyclic voltammogramshows only one redox peak Fig. 3ii. The one-dimensional silicateMCM-41 host has channels with 4.2 nm diameter and hence the dye

is adsorbed inside the channel. Earlier report on the electrochemi-cal behavior for the phenosafranine covalently bound to a polymerand made into a film electrode showed that electrodes coated withpolymer films show two peaks in the cyclic voltammogram, whichwas attributed to the two consecutive one electron redox processes

mV s−1; inset: differential pulse voltammogram of PS+ in phthalate buffer solution

368 K. Senthil Kumar, P. Natarajan / Materials Chemistry and Physics 117 (2009) 365–372

F a) PS+

i ) PS+/M

[t

twopcaw

TEr

S

E

�

T

ig. 3. Cyclic voltammogram of (i) (a) PS+/ZYME, (b) PS+/ZSME and (c) ZY/ME, (ii) (nset: differential pulse voltammogram of (i) (a) PS+/ZYME and (b) PS+/ZSME, (ii) (a

19]. The pulse volammograms shown in Fig. 3 as insets also indicatehe potentials similar to those seen in the cyclic voltammogram.

In general, for many two electron processes occurring consecu-ively in organic [23] or inorganic [24] systems only one redox peakas observed in the cyclic voltammograms. However depending

n the characteristics of the depolarizer, complexing agents, one-eak or two-peak response has also been obtained. In the presentase the observed redox peaks for the silicate modified electrodere presumably due to following reasons (i) the dye is presentith different structural orientations at the silicate surface or (ii)

able 1lectrochemical data of PS+ in phthalate buffer solution, pH = 3.5; scanate = 50 mV s−1. Reference electrode: SCE.

ample PS+/NaY PS+/ZSM-5 PS+/MCM-41 PS+/silica

pc ± 5 mV−340, −328a −221 −328 −330−481, −436a −330

−303b −230b

−340b −337b−427b −344b

Ep, mV48, 51b 35

34 3061, 55a 40

he values are from an average of three experiments.a Glassy carbon electrode.b Differential pulse voltammogram.

/MCME, (b) PS+/silicaME in phthalate buffer solution (pH 3.5); scan rate 50 mV s−1;CME and (b) PS+/silicaME in phthalate buffer solution (pH 3.5).

two electron processes, showing only one redox peak in the cyclicvoltammogram in aqueous solutions, are separated into two sepa-rate one electron processes due to the change in the kinetics of theelectrodic process when the substrate is adsorbed onto a surface.In order to further elucidate the nature of the electron transfer pro-cess at the electrode, the cyclic voltammogram of another dye, a oneelectron redox system, cresyl violet, CV+, adsorbed on to zeolite-Yand ZSM-5 was investigated. The size of the CV+ molecule is largerthan the pore size of the zeolites as shown in Scheme 1 and there-fore adsorbed on zeolite surface and cannot be entrapped into thecages of the zeolites. The cyclic voltammograms of the modifiedelectrodes, CV+/ZYME and CV+/ZSME show only a single redox peak

in the modified electrodes as shown in Fig. 4; the differential pulsevoltammograms also show the corresponding peak potential. Theobserved redox peak potentials for the dye in homogenous solu-tion and in the modified electrodes, CV+/ZYME and CV+/ZSEM aregiven in Table 2. In the case of crystal violet adsorbed on to the sili-

Table 2Electrochemical data of CV+ in phthalate buffer solution, pH = 3.5; scanrate = 50 mV s−1.

Sample CV+ CV+/ZSM-5 CV+/NaY

Epc ± 5 mV −243 −251 −270�Ep, mV 67 57 58

K. Senthil Kumar, P. Natarajan / Materials Chemistry and Physics 117 (2009) 365–372 369

Fig. 4. Cyclic voltammogram of (a) CV+, (b) CV+/ZSME and (c) CV+/ZYME in phthalatebuffer solution pH 3.5; scan rate 50 mV s−1.

cfnsr[et

seepei

Fs

Scheme 2.

ate surface, the observed shift in E1/2 values towards negative sideor CV+/ZYME and CV+/ZSME as compared to the CV+ in homoge-ous solution is attributed to the dye strongly adsorbed on theilicate surface. Similar electrochemical behavior of one electronedox system cytochrome c on zeolite surface is reported earlier1]. These experimental results clearly show that in the case of onelectron redox system, cresyl violet, the peak separation is not dueo different orientation of the dye on the silicate surface.

The electrochemical behavior of organic molecules and tran-ition metal chelates adsorbed on the external surface andncapsulated in the porous silicates zeolite-Y have been discussed

xtensively [8,15]. The electrodic behavior of PS+ adsorbed in micro-orous materials given in Scheme 2, indicates that after the firstlectron and proton transfer to form the intermediate (PSH+) whichs stabilized by the zeolite surface, the second electron transfer pro-

ig. 5. Cyclic voltammogram of PS+ at different pH (a) 1.5, (b) 2.5, (c) 3.5, (d) 4.5 and (e)olution and (ii) variation of current ratio (Ipa/Ipc) with pH.

Fig. 6. Heterogeneous rate constant determined at graphite electrode dependencewith pH (comparison of k versus pH plot for PS+).

cess takes place. The observed behavior is in contrast to the redoxproperty of the dye in homogeneous solution and in nafion@ filmmodified electrode as well where only one redox peak is observed[20]. However in homogenous solution the intermediate (PSH+) isnot electrochemically observable and only a net two electron trans-fer process is observed. Earlier reports of two electron processesfor phenosafranine at the polymer modified electrodes have indi-cated that, at the polymer film which was hydrophilic in nature thecovalently bound dye showed two distinct redox peaks in the cyclicvoltammogram [19]. In addition to variations in the zeolite sur-faces, other parameters such as the Si/Al ratio, surface area, particlesize and the dimensions of the channels are known to be differentfor ZSM-5 and zeolite-Y. Both zeolite-Y and ZSM-5 are hydrophilicmaterials and depending up on the framework cations and Si/Alratio, the charge is known to be different for the atoms presentin the lattice. The polarity of the external surface and the surfaceof the channels and voids depend up on Si/Al ratio which influ-ences the catalytic activity and conductivity of zeolites [25,26].Zeolite-Y is a three-dimensional network and have Si/Al ratio of2.6 which adsorbs the cationic dye molecule exchanging with the

cations on its surface. The ZSM-5 is a two-dimensional networksilicate zeolite with Si/Al ratio in this case of 53.9 leading to anincrease in the surface acidity of the materials. It could be seenin Fig. 3i, that in the case of zeolite-Y and ZSM-5 modified elec-

5.5; scan rate 50 mV s−1 (i) plot of cathodic peak potential versus pH of electrolyte

370 K. Senthil Kumar, P. Natarajan / Materials Chemistry and Physics 117 (2009) 365–372

ntials

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3

covi

Fp

Fig. 7. Dependence of cathodic peak pote

rodes, the cathodic peak current C1 is lower than C2 which isttributed to the first electron and proton transfer process. Cyclicoltammograms of phenosafranine encapsulated into the meso-orous material MCM-41 and adsorbed on silica surface shown inig. 3ii, indicate only a single redox peak at −328 and −330 mV,espectively. The dye is adsorbed inside the channels of MCM-41nd in the case of silica the dye is adsorbed on the external surface.n both cases the redox peak potentials are shifted towards negativeide as compared to those in homogenous solution and peak-to-eak separation �Ep values are 34 and 30 mV, respectively. TheCM-41 and silica which do not have any Al3+ ions in the framework

re known to be more hydrophobic and shows the electrochemicalehavior of phenosafranine similar to that observed in homoge-ous solution. Two electron redox systems, thionine and methylenelue encapsulated in side the channels of zeolite-Y show only oneedox peak as reported earlier [12]. The channel network present ineolite-Y and ZSM-5 play a key role to separate the redox processesf the adsorbed dye at the external surface.

.2. Influence of pH

In order to understand the nature of the electron transfer pro-esses of PS+ at PS+/ZYME and PS+/ZSME electrodes the influencef pH on the redox process was investigated. The dye, PS+ with pKa

alues of 0.5 and 8.5, is essentially present as PS+ in the pH rangenvestigated. The cyclic voltammograms of PS+ obtained at different

ig. 8. Cyclic voltammogram of PS+/ZYME modified electrode at different scan rate, insetlot (log � versus Ep); scan rate range: 0.1–6 V.

with pH (i) PS+/ZYME and (ii) PS+/ZSME.

pH at the scan rate of 50 mV s−1 in homogeneous solution are shownin Fig. 5. The formal potential of PS+ was evaluated as the averagevalue of anodic and cathodic potential. Increasing the pH from 1.5to 5.5 shows a negative shift at the rate of −65 mV/pH as shownin Fig. 5i. The observed rate is very close to the theoretical value of−60 mV/pH which is expected for the electrode reaction involving atwo electron and two proton processes. The shift in redox potentialwith pH for PS+ corresponds to the following reaction

PS+ + 2e− + 2H+ → PSH2+

The observed increase in the anodic and cathodic peak cur-rent ratio (Ia/Ic) with pH as shown in Fig. 5ii indicates that inthe acidic solution, the reduction of PS+ is thermodynamicallymore favorable. The apparent heterogeneous electron transfer rateconstant for the phenosafranine at different pH was determinedfor understanding the nature of electron transfer mechanism ofphenosafranine at different pH and the results are shown in Fig. 6.Applying the methodology described by Nicholson [27], at acidic pHthe PS+ follows the ECCE (e−/H+/H+/e−) mechanism, whereas abovepH = 3.5 the mechanism changes to ECEC (e−/H+/e−/H+) and higher

electron transfer rate constants were observed above pH = 3.5. Inthe present investigation, the change in the slope is observed atpH = 2.5. Similar, observation has been reported for the electrontransfer processes for the nitrosoaromatic derivatives is electrolytesolutions at varying pH [28].

(i) plot of cathodic and anodic peak current versus scan rate and inset (ii) Laviron

K. Senthil Kumar, P. Natarajan / Materials Chemistry and Physics 117 (2009) 365–372 371

Table 3Heterogeneous electron transfer rate constant of PS+ at modified electrode surface.

Sample PS+ PS+/NaY PS+/ZSM-5 PS+/MCM-41

R23.87 19.54

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3p

(titcwectgpt

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ate constant (s−1) 10.27a 92.257.39 34.43

a Homogeneous solution, pH = 3.5. (The ˛ value used in the calculation of rateonstant is ˛ = 0.5).

In the case of PS+/ZYME and PS+/ZSME electrodes only a singleedox peak was observed below pH = 2 in contrast to the two dis-inct redox peaks observed at higher pH. The variation of cathodiceak potential with pH of electrolyte solution is shown in Fig. 7or the modified silicate electrodes PS+/ZYME and PS+/ZSME. Whenhe pH of the solution is increased, the cathodic part of the waveroadens and eventually a second cathodic wave appears with twoistinct cathodic and anodic peaks. The observed single redox peakt acidic pH is due to the increasing proton concentration on zeoliteurface which presumably suppresses the interaction between dyeolecule and zeolite surface so that the adsorbed PS+ behaves as

n homogenous solution with the dye loosely bound to the surfacef the silicate hosts.

.3. Influence of scan rate and the kinetics of the electroderocesses

Cyclic voltammograms of PS+/ZYME in phthalate buffer solutionpH = 3.5) at different scan rates are shown in Fig. 8. Separa-ion between anodic and cathodic peak potential increased withncrease in the scan rate. The peak current varies linearly withhe scan rate which corresponds to an adsorption controlled pro-esses, as discussed by Laviron [29]. Such a linear relationshipas observed in the case of PS+/ZSME and PS+/MCME modified

lectrodes whereas in homogeneous solution of the dye the peakurrent increases linearly with square root of scan rate suggestinghat the reaction is a diffusion controlled process. The hetero-eneous electron transfer rate constant for the electron transferrocesses for phenosafranine bound to the silicate modified elec-rodes were calculated using Laviron equation

og ks = ˛ log(1 − ˛) + (1 − ˛) log ˛

− log(

RT

nF�

)− ˛(1 − ˛)nF �Ep

2.3RT(1)

here ks and ˛ are heterogeneous electron transfer rate constantnd diffusion coefficient, respectively. The diffusion coefficient (˛)s determined by estimating the variation of the peak potentialEp = Epa − Epc with scan rate (�). A plot of Ep versus log � yields twotraight lines with slopes equal to 2.3RT/˛nF and 2.3RT/(1 − ˛)nF forhe cathodic and anodic peaks, respectively. The plots are shown innset of Fig. 8, using such a plot and Eq. (1), the value of ks was cal-ulated. The results given in Table 3 show that higher rate constantsre observed in the case of PS+/MCM-41 which reveal that MCM-41rovide more facile micro-environment for PS+ to undergoes theedox reaction at electrode surface; in this environment the mobil-ty of cation in the wider pores of MCM-41 is enhanced as comparedo the dye adsorbed at the external surface of zeolite-Y and ZSM-. In the case of the PS+/ZYME electrode a higher rate constant forhe first redox couple is observed as compared to the second elec-ron transfer process for PS+/ZSME. This behavior is attributed tohe variation of acidity of the zeolite surface.

.4. Stability of electrodes

Stability of the adsorbed dye on to the surface of the porousaterials is investigated recording multisweep cyclic voltam-ograms extending to several continuous sweeps. The cyclic

Fig. 9. Multisweep cyclic voltammogram of PS+/ZYME recorded in phthalate buffersolution at scan rate 50 mV/s (a) immediate measurement and (b) after 20 min ofequilibration.

voltammograms of PS+ adsorbed onto microporous zeolite-Y andZSM-5 and mesoporous MCM-41 show that even after severalcycles, the dye is not leached into the electrolyte solution to anyappreciable extent (<5%). The cathodic and anodic currents arefound to be restored to the original level after repeated cycles asshown in Fig. 9. The dye with the positive charge is exchanged withthe sodium ions present in the silicates and the adsorbed dye is sol-vated and present in the double layer even after the electrochemicalprocesses at the electrode are repeated for many cycles. As indicatedby the �E values, the adsorption is driven by electrostatic inter-action of the dye with the host with the solvent present aroundthe dye in contact with host surface. In certain instances where theguest molecules are neutral the adsorption is so strong and the elec-trochemical processes at the electrode indicate that the differencebetween the cathodic processes are negligible, i.e., �E = 0, as in thecase of azo compounds, phenazine and benzocinnoline studied byLaviron et al. [29].

4. Conclusion

Phenosafranine adsorbed onto the external surface of the micro-porous materials and encapsulated into the mesoporous materialsundergo electron transfer reactions at electrodes. The cyclic voltam-mograms of PS+/ZYME and PS+/ZSME exhibit independent tworedox couples corresponding to consecutive one electron processes.The peak separation and peak potential of phenosafranine adsorbedonto zeolite-Y and ZSM-5 are influenced by the pH of the electrolytesolution. The hydrogen ions exchanged in the silicate lattice play acrucial role for the electrochemical behavior of phenosafranine at

+

low pH. The electrodic reaction of PS is interpreted to be due to theinfluence of the nanopores of the materials and to a larger extent onthe orientation of the substrate at the adsorbed surface. The meso-porous materials provide more facile micro-environment for PS+ toundergoes the specific redox reaction. The stability of the bound

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72 K. Senthil Kumar, P. Natarajan / Materia

ye is also investigated in the materials of the modified electrodehich the dye is adsorbed both at the external surface and at the

nterior of the channels.

cknowledgements

The authors acknowledge the financial support received fromhe Department of Science and Technology, Government of Indiahrough the Raja Ramanna Fellowship to PN. The centre is sup-orted by DST-IHRPA programme.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.matchemphys.2009.06.008.

eferences

[1] Z. Dai, S. Liu, H. Ju, Electrochim. Acta 49 (2004) 2139–2144.

[2] B.J. Privett, J.H. Shin, M.H. Schoenfisch, Anal. Chem. 80 (2008) 4499–4517.[3] A. Domenech, J. Phys. Chem. 108 (1995) 20471–20478.[4] C.A. Bessel, D.R. Rolison, J. Am. Chem. Soc. 119 (1997) 12673–12674.[5] M.A. Zanjanch, Sh. Sohrabnezhad, M. Arvand, M.F. Mousavi, Russ. J. Electrochem.

43 (2007) 758–763.[6] C. Liu, X. Te, W. Wu, Catal. Lett. 36 (1996) 263–266.

[[[[

[

mistry and Physics 117 (2009) 365–372

[7] S. Kim, W. Zhang, J. Pinnavaia, J. Catal. Lett. 43 (1997) 149–153.[8] D.R. Rolison, C.A. Bessel, Acc. Chem. Res. 33 (2000) 737–744.[9] C. Delacote, J. Philippe Bouillon, A. Walcarious, Electrochem. Acta 51 (2006)

6373–6383.[10] M.A. Zanjanch, S. Sohrabnezhad, M. Arvand, M.F. Mousavi, Russ. J. Electrochem.

43 (2007) 758–763.[11] S. Easwaramoorthi, P. Natarajan, J. Porous Mater. 15 (2008) 343–349.12] S. Easwaramoorthi, P. Natarajan, Mater. Chem. Phys. 107 (2008) 101–109.

[13] S. Kuwabata, K. Mitsui, H. Yoneyama, J. Electroanal. Chem. 281 (1990) 97–107.

[14] R.O. Lenza, S. Junato, J.H. Zagal, J. Electroanal. Chem. 389 (1995) 197–200.[15] A. Domenech, H. Garcia, M.T. Domenech-Carbo, M.S. Galletero, Anal. Chem. 74

(2002) 562–569.[16] S. Eswaramoorthy, P. Natarajan, Micropor. Mesopor. Mater. 86 (2005) 185–190.[17] A. Domenech, M.T. Domenech-Carbo, H. Garcia, M.S. Galletero, Chem. Commun.

(1999) 2173–2174.[18] R.W. Murray, Acc. Chem. Res. 13 (1980) 135–141.[19] P. Natarajan, J. Macromol. Sci. Chem. A 25 (1988) 1285–1306.20] K.R. Gopidas, P.V. Kamat, J. Phys. Chem. 94 (1990) 4723–4727.21] S. Abraham John, R. Ramaraj, J. Electroanal. Chem. 424 (1997) 49–59.22] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Lenowicz, et al., J. Am. Chem. Soc. 114 (1992)

10834–10843.23] D.E. Richardson, H. Taube, Coord. Chem. Rev. 60 (1984) 107–129.24] D.T. Pierce, W.E. Geiger, J. Am. Chem. Soc. 111 (1989) 7636–7638.

25] A. Corma, J. Catal. 216 (2003) 298–312.26] M. Alvaro, J.F. Cabeza, D. Fabuel, H. Garcia, Chem. Mater. 18 (2006) 26–33.27] R.S. Nicholson, Anal. Chem. 37 (1965) 1351–1355.28] S. Bollo, S. Finger, J.C. Sturm, J.A. Squella, Electrochim. Acta 52 (2007)

4892–4898.29] E. Laviron, A. Vallat, R. Maunier-prest, J. Electroanal. Chem. 379 (1994) 427–435.