Fabrication and electrochemical behavior of a sol±gel-derivedcarbon ceramic composite electrode entrapping

2:18-molybdodiphosphate

Peng Wang, Xiangping Wang, Yi Yuan, Guoyi Zhu *

Laboratory of Electroanalytical Chemistry, National Analytical Research Center of Electrochemistry and Spectroscopy, Changchun

Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People's Republic of China

Received 2 February 2000; received in revised form 19 May 2000

Abstract

A new type of inorganic±organic hybrid material incorporating carbon powder and a-type 2:18-molybdodiphos-

phate (P2Mo18) in a methyltrimethoxysilane (MTMOS) based gel has been produced by a sol±gel process and used to

fabricate a chemically modi®ed electrode. The P2Mo18-doped carbon ceramic composite electrode was characterized

using SEM and cyclic voltammetry. Square-wave voltammetry with an excellent sensitivity was exploited to conve-

niently investigate the dependence of current and half-wave potential (E1=2) on pH. The chemically modi®ed electrode

has some advantages over the modi®ed ®lm electrodes constructed by the conventional methods, such as long-term

stability, reproducibility, and especially repeatability of surface-renewal by simple polishing in the event of surface

fouling or dopant leaching. In addition, the modi®ed electrode shows a good catalytic activity for the electrochemical

reduction of bromate in an acidic aqueous solution. Ó 2000 Elsevier Science B.V. All rights reserved.

1. Introduction

The sol±gel process provides a versatile meansfor the production of inorganic and inorganic±organic hybrid materials via the hydrolysis andcondensation of suitable metal alkoxides [1,2].These materials possess notable advantages overother inorganic±organic materials for the encap-sulation of various dopants and the developmentof practical sensors and catalytic supports [3±5].The ®eld of silica-modi®ed electrodes is narrow intime, beginning slowly in 1989 and then increasing

exponentially in the last years. Recent advances inthe various ®elds and applications of sol±gel elec-trochemistry were described in four excellent re-view articles [6±9]. Carbon ceramic compositeelectrodes (CCEs) are a new class of carbon ma-terial electrodes ®rst introduced by LevÕs group[10]. An important feature of the CCEs is theability to produce surface renewable, bulk modi-®ed electrodes for electroanalytical and electro-catalytic applications. The preparation protocol ofCCEs allows the incorporation of other com-pounds during the preparation stage itself andhence they can be modi®ed throughtout the sam-ple volume. An additional advantage is that theelectrochemically active, wetted surface of theelectrode in aqueous solutions can be manipulated

Journal of Non-Crystalline Solids 277 (2000) 22±29

www.elsevier.com/locate/jnoncrysol

* Corresponding author. Tel.: +86-431 568 2801; fax: +86-431

568 5653.

E-mail address: zhuguoyi@ns.ciac.jl.cn (G. Zhu).

0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 2 8 7 - 8

by the judicious choice of monomers to preparethe silicate matrix [11±14].

Polyoxometalates (POMs) are a large and rap-idly growing class of compounds that have at-tracted much attention in catalysis, medicine,bioanalysis and material science owing to theirchemical, structural and electronic versatility [15].One of the most important properties of POMs istheir ability to accept various numbers of electronsgiving rise to mixed-valency species. Sadakane andSteckhan [16] have presented detailed accounts ofelectrochemical properties and electrocatalyticapplications of POMs and various procedures forthe attachment of POMs onto electrode surfaces.Because of some inherent disadvantages of theconventional methods [16,17], it seems imperativeto explore and develop a simple and reliablemethod to immobilize and stabilize POMs, withtheir structures and properties retained to a largeextent.

The recent explosive growth of sol±gel scienceand technology o�ers several examples of utiliza-tion of POMs as additives in inorganic or organicmatrices. Through a sol±gel approach, Judeinsteinand Schmidt [18] trapped POM anions such asPW12O3ÿ

40 , SiW12O4ÿ40 , or W10O3ÿ

32 into gel matricesto construct conducting materials with electro-chromic and photochromic properties. Asukaet al. [19,20] patented siloxane±POM acid (Keggintype) networks as coatings with antire¯ective andantistatic properties, and the coatings exhibitedgood adhesion, transparency, abrasion resistance,hardness, and low resistivity. Sol±gel glass thin®lms containing PMo12O3ÿ

40 were also obtained onthe surface of a glassy carbon electrode, and themodi®ed ®lm electrode was used as an ampero-metric sensor for iodate [17]. Recently, Tess andCox [21] have reported electrocatalytic oxidationof methionine and cystine on a CCE modi®ed withbis(acetato)dirhodium-11-phosphotungstate, how-ever, redox behavior of the section of 11-phos-photungstate was not studied and the catalytic sitewas rhodium(II) rather than phosphotungstate.

In the present work, we incorporated carbonpowder and a-type 2:18-molybdodiphosphate(P2Mo18) in a methyltrimethoxysilane (MTMOS)based glassy gel to fabricate a new type of inor-ganic±organic hybrid material and employed it to

fabricate a P2Mo18-doped CCE. To our knowl-edge, the electrode is the ®rst POM-modi®ed CCEwhich combines renewable ability and bulk mod-i®cation of CCEs with the excellent electrochemi-cal behavior of POMs. In addition, the modi®edelectrode showed a good catalytic activity of elec-trochemical reduction of bromate in an acidicsolution.

2. Experimental

2.1. Reagents

Methyltrimethoxysilane (MTMOS, 97%) was aproduct of ACROS Chemical and used withoutfurther puri®cation. High purity carbon powderwas purchased from Shanghai Carbon Plant. PureK6P2Mo18O62�14H2O was received as a gift fromProfessor Enbo Wang (Northeast Normal Uni-versity). Other chemicals were all analytical grade.Solutions with di�erent pH (0.25±3.42) were pre-pared from the aqueous solution of 0.1 mol lÿ1

Na2SO4 by adjustment with concentrated H2SO4.Ultrapure water was obtained from a MilliporeMilli-Q water puri®cation system was usedthroughout the experiments. All experimental so-lutions were thoroughly deoxygenated by bubblingpure argon through them for at least 15 min.

2.2. Apparatus and physical measurements

A scanning electron microscope was employedfor the observation of morphology. The backs ofunmodi®ed and modi®ed CCEs cut to be thelength of 0.5 cm were connected to the sampleplatform with electrically conductive paint. Sincecarbon powder and P2Mo18 are both conductive,sputtering of the sample with gold was not neces-sary for the SEM study.

An CHI 660 Electrochemical Workstationconnected to a Digital-586 personal computer wasused for control of the electrochemical measure-ments and for data collection. A conventionalthree-electrode system was used. The workingelectrode was a glassy carbon electrode (d� 3mm), an unmodi®ed CCE, or a modi®ed CCE.A Ag/AgCl (saturated KCl) electrode was used as

P. Wang et al. / Journal of Non-Crystalline Solids 277 (2000) 22±29 23

a reference electrode and a Pt gauze as a counterelectrode. The glassy carbon was polished with 0.5,0.3, 0.1, and 0.05 lm a-Al2O3 paste, respectively,washed with distilled water then ultrasonicated indeionized water and acetone successively, and ®-nally dried at room temperature. All potentialswere measured and reported versus the Ag/AgClelectrode. A pH meter was used for pH measure-ments. All the experiments were conducted atroom temperature (�20°C).

2.3. Fabrication of unmodi®ed CCEs and P2Mo18-doped CCEs

Unmodi®ed CCEs were prepared according tothe procedure described by Lev and co-workers[22±25]. The fabrication of P2Mo18-doped CCEs isdescribed as follows. The solution of 0.75 mlmethanol containing 3.75 mg P2Mo18, 0.25 mlMTMOS, and 0.025 ml hydrochloric acid (11 mollÿ1) was ultrasonically mixed for 2 min, then 1.875g carbon powder was added and shaken on avortex agitator for an additional 3 min. The mix-ture was added to glass tubes with 3 mm innerdiameter and 8 cm length, and the length ofcomposite material in the tubes was controlled tobe about 0.8 cm. In addition, a little extra mixturewas retained on the top of the electrodes, and themixture in the tubes was pressed lightly on asmooth plastic paper with a copper stick throughthe back. After drying at 30°C for 24 h, the elec-trodes were polished with a 3# emery paper toremove extra composite material and wiped gentlywith a weighing paper. Electric contact was madeby silver paint through the back of the electrode.

3. Results

3.1. Fabrication and SEM of the unmodi®ed andP2Mo18-doped CCEs

In the process of fabrication of P2Mo18-dopedCCEs, the composite material became fragile andthus it was di�cult to obtain smooth electrodesurfaces if the gelation temperature was higherthan 60°C.

Surfaces of modi®ed and unmodi®ed CCEs areboth mirror-like and homogeneous. A drop ofwater deposited at the surfaces does not spreadindicating the apparent hydrophobic nature of thesurfaces. Fig. 1 shows SEM micrographs of thesurfaces of typical unmodi®ed CCEs and P2Mo18-doped CCEs. The uniform surface microstructureof the modi®ed electrode is seen from Fig. 1(b),and the almost same structure (Fig. 1(a)) was ob-tained for an unmodi®ed CCE polished in thesame manner as that for the P2Mo18-doped CCE.

3.2. Voltammetric behavior of the P2Mo18-dopedCCE

Fig. 2 shows comparative cyclic voltammo-grams for P2Mo18 at glassy carbon electrode andCCE, and P2Mo18-doped CCE in a 0.5 mol lÿ1

H2SO4 aqueous solution. Four successive waveswere observed in the potential range from 0.8 to)0.2 V. The shape of the cyclic voltammogram for

Fig. 1. SEM micrographs of the surfaces of typical unmodi®ed

(a) and P2Mo18-doped (b) CCEs.

24 P. Wang et al. / Journal of Non-Crystalline Solids 277 (2000) 22±29

P2Mo18-doped CCE is almost the same as those ofP2Mo18 with a glassy carbon electrode and CCE.

Fig. 3 presents cyclic voltammograms obtainedat di�erent scan rates for a P2Mo18-doped CCEimmersed in a 0.5 mol lÿ1 H2SO4 aqueous solutionand the inset is the corresponding dependence ofcathodic current of the ®rst reduction wave on

scan rate. At a scan rate up to 100 mV sÿ1, all thehalf-wave potentials (E1=2) remain unchanged, andpeak potential separation (DEp) does not increasewith increasing scan rate. In the scan rate rangesfrom 150 to 500 mV sÿ1, E1=2 is still the same asthat obtained at low scan rates, but DEp graduallyincreases along with the increase of scan rate.Moreover, the cathodic peak current is almost thesame as the corresponding anodic peak current.The inset of Fig. 3, presents a good linearity ofcathodic current of the ®rst reduction wave versusscan rate from 10 to 100 mV sÿ1, however, theslope becomes smaller in the scan rate range from150 to 500 mV sÿ1 and a good linearity is re-mained. The slopes of the linear regression linesdrawn through the data at slow and fast scan ratesare 0.094 and 0.054, respectively.

3.3. pH-dependent electrochemical behavior of theP2Mo18-doped CCE

Fig. 4 shows Osteryoung square-wave voltam-mograms for the P2Mo18-doped CCE in acidicaqueous solutions with di�erent pH. It can beclearly seen that along with increasing pH, thethree peak potentials all shift gradually to thenegative potential direction and the peak currentsalso decrease. A plot of E1=2 of the three successive

Fig. 3. Cyclic voltammograms of a P2Mo18-doped CCE in the

aqueous solution (0.5 mol lÿ1 H2SO4 + 0.1 mol lÿ1 Na2SO4) at

di�erent scan rates (from inter to outer: 10, 20, 40, 60, 80, 100,

150, 200, 250, 300, 350, 400, 450, and 500 mV sÿ1, respectively).

The inset is the plot of cathodic current of the peak I versus

scan rate.

Fig. 4. Osteryoung square-wave voltammograms of the

P2Mo18-doped CCE in aqueous solutions (H2SO4 + 0.1 mol lÿ1

Na2SO4) with di�erent pH: 0.25 (a), 0.54 (b), 1.08 (c), 1.60 (d),

2.02 (e), 2.49 (f), 2.97 (g) and 3.42 (h), respectively. Scan rate is

100 mV sÿ1.

Fig. 2. Comparative cyclic voltammograms of P2Mo18 at glassy

carbon electrode (a) and CCE (b), and doped in CCE (c) in the

aqueous solution (0.5 mol lÿ1 H2SO4 + 0.1 mol lÿ1 Na2SO4).

Scan rate is 200 mV sÿ1.

P. Wang et al. / Journal of Non-Crystalline Solids 277 (2000) 22±29 25

redox processes versus pH for the P2Mo18-dopedCCE is shown in Fig. 5. Good linearity in the pHrange from 0.25 to 3.42 was obtained.

3.4. Electrocatalysis of bromate by the P2Mo18-doped CCE

Fig. 6 presents cyclic voltammograms for theelectrocatalytic reduction of bromate at a P2Mo18-doped CCE and bromate at an unmodi®ed CCE.

No obvious voltammetric response is observed inthe potential range from 0.8 to 0.0 V for bromateat an unmodi®ed CCE. It can be clearly seen thatthe gradual increase of bromate concentrationcauses a dramatic change in the cyclic voltammo-grams with an increase in cathodic currents and aconcomitant decrease in anodic current. The cat-alytic waves appear on the second (peak II) andthird (peak III) reductions of P2Mo18, but the ®rstredox wave was almost una�ected by bromate.The catalytic current of peak III is proportional tobromate concentration in the range of 1±20 mmollÿ1 as shown in Fig. 7.

3.5. Renewal repeatability and long-term stability

First of all, the stability of the modi®ed CCEwas examined by measuring the decrease in vol-tammetric currents during potential cycling. Forexample, the electrode was subjected to 5000 po-tential cycles in the potential range from 0.8 to 0.0V in the aqueous solution (0.5 mol lÿ1 H2SO4 + 0.1mol lÿ1 Na2SO4), a decrease in the cathodic cur-rent of less than 0.5% was observed. When theP2Mo18-doped CCE was stored in dry atmosphereat 4°C for 4 months, it was also proven to be verystable. In the potential range from +0.8 to 0.0 V, inthe aqueous solution (0.5 mol lÿ1 H2SO4 + 0.1 mollÿ1 Na2SO4) and at the scan rate of 40 mV sÿ1, thecurrent response remained almost unchanged.Little leakage was found even when the electrode

Fig. 5. Plots of E1=2 of the ®rst (a), the second (b) and the third

(c) redox processes versus di�erent pH.

Fig. 6. Cyclic voltammograms of a P2Mo18-doped CCE in 0.5

mol lÿ1 H2SO4 + 0.1 mol lÿ1 Na2SO4 aqueous solutions con-

taining 0 (b), 1 (c), 2 (d), 5 (e), 10 (f), 15 (g) and 20 (h) mmol lÿ1

NaBrO3, respectively, and an unmodi®ed CCE (a) in the same

solution as (e). Scan rate is 40 mV sÿ1.

Fig. 7. Plot of catalytic current of peak III versus bromate

concentration.

26 P. Wang et al. / Journal of Non-Crystalline Solids 277 (2000) 22±29

was immersed in the aqueous solution (0.5 mol lÿ1

H2SO4 + 0.1 mol lÿ1 Na2SO4) for 20 days. TheP2Mo18-doped CCE had the almost same voltam-metric behavior in di�erent solutions with thesame composition. Ten successive polishings re-sulted in a relative standard deviation (RSD) of5.4% for a P2Mo18-doped CCE.

4. Discussion

4.1. Voltammetric behavior of the P2Mo18-dopedCCE

It is known that P2Mo18 undergoes three suc-cessive reversible two-electron and a four-electronreduction in acidic solutions [16]. So the electro-chemical experiments were all performed in acidicaqueous solutions with pH varied from 0.25 to3.42 (Na2SO4±H2SO4). As shown in Fig. 2, redoxpeaks I±I0, I±II0, I±III0, and I±IV0 corresponds toreduction and oxidation through two-, four-, six-,and ten-electron processes, respectively. Thesimilar electrochemical behaviors of P2Mo18 at aglassy carbon electrode and CCE, and doped inCCE suggest that the entrapment of P2Mo18 inCCE introduce no di�erence in the electronicstructures of the oxidized or reduced forms of theredox couple. The modi®ed CCE can provide asuitable environment for P2Mo18 to performelectron and proton transfer. The electrochemicalreaction of P2Mo18-doped CCEs can be shown asfollows:

P2Mo18O6ÿ62 � 2eÿ � 2H�H2P2Mo18O6ÿ

62 ;

H2P2Mo18O6ÿ62 � 2eÿ � 2H�H4P2Mo18O6ÿ

62 ;

H4P2Mo18O6ÿ62 � 2eÿ � 2H�H6P2Mo18O6ÿ

62 ;

H6P2Mo18O6ÿ62 � 4eÿ � 4H�H10P2Mo18O6ÿ

62 :

Many literature reports [10±14,21±25] revealedthat the stable potential range for CCEs is from1.1 to )0.2 V. Thus we selected the potential rangefrom 0.8 to 0.0 V for the study of cyclic voltam-metric behavior of P2Mo18-doped CCE at di�erentscan rates. At slow scan rate, a good linearity inthe plot of peak current versus scan rate up to 100mV sÿ1 reveals that the electrochemical behavior

of P2Mo18 doped in CCE shows prominently afast, di�usionless electron transfer process. How-ever, DEp is less than 35 mV instead of the valuezero expected for a reversible surface redox process[26], which might arise due to non-ideal behavior.The CCEs can be treated as ¯at geometry asshown earlier [27]. Thus, the electroactive coverage(Cc) of the gel-immobilized P2Mo18 is estimated tobe 3:6� 10ÿ10 mol cmÿ2 for n� 2 by the followingequation [26]:

Ip � n2F 2mACc=4RT ;

where v is the scan rate, A is the electrode surfacearea, which in our case is close to the electrodegeometric area, and the other symbols have theirusual meaning. However, the peak current de-pendence on scan rate shows two di�erent slopes,the higher scan rates leading to small slopes asshown in the inset of Fig. 3. Similar results werealso reported by Wang et al. [28] for inorganic±organic hybrid ®lms prepared from ferrocenemodi®ed silanes, and by Sampath and Lev [29]for electrochemical oxidation of NADH on un-modi®ed CCEs. The reason for this rather com-plex process described here is unknown atpresent.

4.2. pH-dependent electrochemical behavior of theP2Mo18-doped CCE

In general, the reduction of POMs is accom-panied by protonation, therefore, the electro-chemical behavior of POMs relies on the pH of thesolution to a large extent [16]. In order to inves-tigate the pH-dependent electrochemical behaviorof the P2Mo18-doped CCE, square-wave voltam-metry with an excellent sensitivity is adopted toaccurately measure the half-wave potentials. Re-dox of P2Mo18 immobilized in the CCE matrix isaccompanied by the process of uptake of protonsfrom solution to the wetted electroactive section ofthe electrode to maintain charge neutrality. Alongwith increasing pH, the concentration gradient ofprotons becomes smaller, which is described byFickÕs ®rst Law [30]. The slower penetration ofprotons to the wetted section of the P2Mo18-dopedCCE may be the reason for the current decrease.

P. Wang et al. / Journal of Non-Crystalline Solids 277 (2000) 22±29 27

The more negative reduction potentials along withthe decrease of proton concentration can be elu-cidated by the Nernst equation [31]. Slopes (Fig. 5)of the three redox couples in this pH range are)74.3 (a), )68.6 (b), and )72.8 (c) mV pHÿ1, re-spectively, which are close to the theoretical value)60 mV pHÿ1 for the 2eÿ/2H� redox process. Theresults suggest the charge compensation by othercations is unnecessary for P2M18-doped CCEs. Asa matter of fact, analogous behavior in PMo12-modi®ed thin ®lm electrode has been describedand discussed previously [17].

4.3. Electrocatalysis of bromate by the P2Mo18-doped CCE

Considerable interest has been given to the de-velopment of materials capable of catalyzing theoxidation or reduction of solution species[16,17,21,25]. With this in mind, we have examinedthe catalytic reduction of bromate at the P2Mo18-doped CCE to evaluate the feasibility of usingthese hybrid materials in electrocatalysis. Keitaet al. [32] have fabricated P2Mo18-doped poly(4-vinylpyridine) or polyaniline composite electrodesto electrocatalytically reduced nitrite. In experi-ments, we ®nd that the P2Mo18-doped CCE has acatalytic e�ect on the reduction of bromate shownin Fig. 6. The catalytic behavior is similar to thatof the electrocatalytic reduction of oxygen bySiW11 [33], but it is di�erent from the electrocat-alytic reduction of bromate and chlorate by PMo12

[34]. The bromate is reduced by the four-electron-and six-electron-reduced species in the presence ofprotons to yield bromide, and the two-electron-reduced species is regenerated. It can also be seenfrom Fig. 6 that the six-electron-reduced specieshas a larger catalytic activity toward bromate thanthe four-electron-reduced species. So the catalyticactivity of P2Mo18 toward bromate increases withthe extent to which P2Mo18 is reduced. The cata-lytic behavior of a P2Mo18-doped CCE towardsbromate can be explained by the following mech-anism:

3H4P2Mo18O6ÿ62 �BrOÿ3 ! 3H2P2Mo18O6ÿ

62 �3H2O�Brÿ;

3H6P2Mo18O6ÿ62 �2BrOÿ3 ! 3H2P2Mo18O6ÿ

62 �6H2O�2Brÿ:

4.4. Renewal repeatability and long-term stability

Compared with POM-modi®ed ®lm electrodesfabricated by the conventional methods, theP2Mo18-doped CCE based on the sol±gel tech-nique has certain advantages. One of the mainattractions of the use of P2Mo18-doped CCEs forthe electroreduction of bromate is that the surfaceof the electrodes can be renewed after every use.The hydrophobic methyltrimethoxysilane mono-mer results in a controlled wetting of the com-posite electrode in aqueous solutions. Hence, abulk modi®ed electrode can be polished usingemery paper and a fresh surface exposed in theevent of surface fouling and dopant leaching. Thisis especially useful for the electrocatalytic studysince the catalytic activity is known to decreasewhen the electrode is fouled.

For small reagents such as electron-transfermediators, dye molecules, complexing agents,physically doping in the silicate matrix may resultin signi®cant leaching. Thus the design and syn-thesis of the organically modi®ed silicates has beenproposed to construct leak-free chemical sensors[9]. We think that the high stability of the P2Mo18-doped CCEs is probably related to the stability ofthe silicate matrix, the limited wetted sectioncontrolled by methyl group, and the possiblebonding interactions between the doped P2Mo18

and silanol groups [35].

5. Conclusions

The hydrophobic sol±gel-derived CCEs modi-®ed with POMs, which are exempli®ed here by theP2Mo18-doped CCE, promise to compete well withtraditional POM-modi®ed ®lm electrodes. TheP2Mo18-doped CCE can catalyze the electrore-duction of bromate, and exhibits a distinct ad-vantage of polishing in the event of surfacefouling, as well as simple preparation, goodchemical and mechanical stability and good re-producibility. One of the basic ideas behind thepresent work was to transfer to CCEs the homo-geneous catalytic activities of other unsubstitutesas well as substituted Keggin-type and Dawson-type POMs. These goals were achieved, and the

28 P. Wang et al. / Journal of Non-Crystalline Solids 277 (2000) 22±29

results will be published in our future papers. Inaddition, the fundamental research is an importantand useful step to demonstrate their viability of theelectrocatalytic system in large scale electrolysis.

Acknowledgements

Financial support of this work by the Ministryof Science and Technology of China is gratefullyacknowledged. We also thank Professor EnboWang for the supply of P2Mo18.

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