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Journal of Electroanalytical Chemistry 495 (2000) 51–56

Sol-gel-derived a2-K7P2W17VO62/graphite/organoceramic compositeas the electrode material for a renewable amperometric hydrogen

peroxide sensor

Peng Wang a, Xiangping Wang a, Lihua Bi b, Guoyi Zhu a,*a Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,

Changchun 130022, People’s Republic of Chinab Department of Chemistry, Northeast Normal Uni6ersity, Changchun 130024, People’s Republic of China

Received 11 April 2000; received in revised form 28 August 2000; accepted 5 September 2000

Abstract

The conductive a2-K7P2W17VO62/graphite/organoceramic composite was prepared by dispersing a2-K7P2W17VO62 and graphitepowder in a propyltrimethoxysilane-based sol-gel solution; it was used as the electrode material for an amperometric hydrogenperoxide sensor. The modified electrode had a homogeneous mirror-like surface and showed well defined cyclic voltammograms.Square-wave voltammetry was employed to study the pH-dependent electrochemical behavior of a2-K7P2W17VO62 doped in thegraphite organoceramic matrix, and the experiment showed that both protons and sodium cations participated in the redoxprocess. A hydrodynamic voltammetric experiment was performed to characterize the electrode as an amperometric sensor for thedetermination of hydrogen peroxide. The sensor can be renewed easily in a repeatable manner by a mechanical polishing step andhas a long operational lifetime. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Sol-gel; Composite; Polyoxometalate; Amperometric sensor; Hydrogen peroxide

1. Introduction

Polyoxometalates (POMs), metal–oxygen clusters,have attracted much attention in catalysis, medicine,and materials science in the last decade [1]. POMs canaccept and release a certain number of electrons with-out decomposition, thus serving as multi-electron relays[2]. Recently, Sadakane and Steckhan [3] have pre-sented detailed accounts of electrochemical propertiesand electrocatalytic applications of POMs. Attachmentof these species to electrode surfaces can be achieved byelectrodeposition, adsorption, entrapping in organicand inorganic polymer matrices, self-assembly, layer-by-layer deposition, and the LB method [3,4]. To ourknowledge, almost all these POMs were used directly inaqueous solutions or immobilized on electrode surfacesas catalysts [3,4]. A serious drawback in the application

of these modified film electrodes is that electrode sur-faces cannot be renewed in the case of leakage, contam-ination and passivation.

Sol-gel technology provides an attractive route forthe preparation of three-dimensional inorganic net-works [5]. The basic sol-gel process involves the sequen-tial hydrolysis and polycondensation of silicon alkoxideat low temperature [6,7]. The applications of sol-gelchemistry to produce sensing material are attractingconsiderable interest [8–12]. This is due to a number ofadvantages, including low-temperature encapsulation ofactive species, tunability of physical characteristics, op-tical transparency, mechanical rigidity, and low chemi-cal reactivity. Most of the activity in sol-gel basedsensors has been directed toward optical devices. How-ever, the versatility of sol-gel processing is also provinguseful in the realm of electrochemical sensors [13–16].In particular, renewable [17–23] and disposable [24,25]amperometric sensors have been developed recently bydispersing various catalysts in sol-gel carbon com-posites or inks (prepared by adding graphite powder tothe sol-gel solution).

* Corresponding author. Tel.: +86-431-5682801; fax: +86-431-5685653.

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

0022-0728/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S 0022 -0728 (00 )00371 -5

P. Wang et al. / Journal of Electroanalytical Chemistry 495 (2000) 51–5652

The recent explosive growth of sol-gel science andtechnology offers several examples of the utilization ofPOMs as additives in silica matrices. In 1989, Tatsumis-ago and Minami [26] first applied the sol-gel method toprepare proton-conducting amorphous silica gel filmscontaining H3PMo12O40. Through a sol-gel approach,Judeinstein and Schmidt [27] trapped POM anions suchas PW12O40

3−, SiW12O404−, or W10O32

3− in gel matrices toconstruct conducting materials with electrochromic andphotochromic properties. After reports [28,29] on thesolid-state voltammetric characterization of PW12O40

3−

and SiW12O404− doped in tetraethoxylsilane-derived gels,

Song et al. [30,31] applied the sol-gel technique tofabricate POM-modified thin film electrodes. Our re-cent interest was focused on the fabrication of graphiteorganoceramic electrodes bulk modified with POMs[32,33]. In this study, we prepared a new kind ofa2-K7P2W17VO62/graphite/organoceramic composite bythe sol-gel process and employed it to fabricate athree-dimensional POM-modified electrode. In thepresent configuration, the graphite powder contributesto the conductivity by a percolation mechanism, a2-K7P2W17VO62 provides the electrocatalysis of hydrogenperoxide reduction, the crosslinked silicate backboneprovides rigidity in the matrix, and the propyl groupsimpart hydrophobicity and limit the wetting section ofthe modified electrodes when immersed in the elec-trolyte. The amperometric sensor can easily be renewedin a repeatable manner using a quick and simple polish-ing step, thus exposing a new electroactive layer. Inaddition, hydrodynamic voltammetric experiments wereperformed to characterize the electrode as an ampero-metric sensor for the determination of hydrogenperoxide.

2. Experimental

2.1. Materials and solutions

Propyltrimethoxysilane (]97%) was purchased fromFluka and used without purification. High puritygraphite powder (average particle 1–2 mm) was ob-tained from Aldrich. 600-grit emery paper was suppliedby Shanghai Sand Wheel Plant. a2-K7P2W17VO62·26H2O was prepared and characterized by the literaturemethods [34]. Other chemicals were of analytical gradeand were used as received.

Ultrapure water obtained from a Millipore Milli-Qwater purification system was used throughout the ex-periments. Solutions with different pH (0.00–2.49) wereprepared by mixing 0.1 M Na2SO4 aqueous solutionwith 0.1 M Na2SO4+0.5 M H2SO4 aqueous solution.Solutions were deaerated by argon bubbling priorto the experiments and the electrochemical cell waskept under an argon atmosphere throughout theexperiments.

2.2. Apparatus

An EG&G PARC model 273 galvanostat/poten-tiostat with PARC M270 electrochemical software wasused for voltammetric and amperometric studies. A 50ml, three-electrode compartment electrochemical cellequipped with a Pt flag counter electrode and anAg � AgCl � KClsat reference electrode was used for allelectrochemical experiments. The working electrodewas a glassy carbon electrode (self-made, 3 mm indimameter) or a modified electrode. Prior to the exper-iment, the glassy carbon electrode was immersed in 0.1M nitric acid for 5 min and polished sequentially with1, 0.3 and 0.05 mm alumina and cleaned ultrasonicallyfor 1 min at the end. All potentials are reported versusthe reference electrode. A Cole-Parmer 3™ pH meterwas used for pH measurements. All the experimentswere conducted at 1590.2°C.

2.3. Fabrication of the a2-K7P2W17VO62-modifiedelectrodes

The solution of 0.75 ml methanol containing 9 mga2-K7P2W17VO62, 0.35 ml propyltrimethoxysilane, and0.025 ml hydrochloric acid (11 M) was mixed ultrason-ically for 2 min, then 1.875 g graphite powder wasadded and shaken on a vortex agitator for an addi-tional 3 min. The homogenized mixture was used topack 3 mm i.d. silicate glass tubes to a length of 0.8 cmfrom one of their ends. In addition, a little extramixture was needed to be retained on the top of theelectrodes, and the mixture in the tubes was pressedlightly on smooth plastic paper with a copper stickthrough the back. After gelation at 30°C for 48 h, theelectrodes were polished with 600-grit emery paper toremove extra composite material. A copper stick wasinserted through the opposite end, and electric contactwas made by silver powder.

3. Results and discussion

3.1. Fabrication of the a2-K7P2W17VO62-modifiedelectrodes

In this work, propyltrimethoxysilane was used as theprecursor for the sol-gel polymerization and Eq. (1)shows the overall reaction which proceeds via hydroly-sis (Eq. (2)) and condensation (Eq. (3)) reactions [35]:

CH3CH2CH2Si(OCH3)3+ (3−x)H2O

�CH3CH2CH2SiOx(OCH3)3−2x+2xCH3OH (1)

�Si�OCH3+H2O��Si�OH+CH3OH (2)

�Si�OCH3+HO�Si���Si�O�Si�+CH3OH (3)

P. Wang et al. / Journal of Electroanalytical Chemistry 495 (2000) 51–56 53

Additional water for the hydrolysis step is providedby air humidity [17]. Base (NaOH or NH4OH) orneutral (NH4F) catalysts failed to produce rigid mono-liths and yielded only silica-covered graphite powder.Large dosages of hydrogen chloride and propyl-trimethoxysilane resulted in rapid gelation before addi-tion of the graphite powder, and poor conductivity ofthe composite [36], respectively. The Si–C and C–Cbonds remain unchanged during the hydrolysis andpolymerization, and the propyl group remains exposedat the surface of the porous silicate network. In theprocess of fabrication of the a2-K7P2W17VO62-modified

electrodes, a little extra mixture was needed to beretained on top of the electrodes in order to obtainconveniently an intact and uniform surface when theywere first polished. In addition, the composite materialbecame fragile and thus it was difficult to obtainsmooth electrode surfaces if the gelation temperaturewas higher than 60°C. The modified electrode surfacewas mirror like and homogeneous after simple mechan-ical polishing on 600-grit emery paper. A drop of waterdeposited on the surfaces did not spread, indicating theapparent hydrophobic nature of the surfaces.

3.2. Electrochemical beha6ior of thea2-K7P2W17VO62-modified electrode

Fig. 1 presents comparative cyclic voltammogramsfor (a) a glassy carbon electrode in 2 mM a2-K7P2W17VO62+0.1 M Na2SO4+0.5 M H2SO4 solutionand (b) a a2-K7P2W17VO62-modified graphite organoce-ramic electrode in 0.1 M Na2SO4+0.5 M H2SO4 solu-tion. The redox peaks correspond to reduction andoxidation through a one-electron process for the cou-ples of a2-P2W17V(V)O62

7− and a2-P2W17V(IV)O628− [37–

39]. Although peak potential differences (DEp) for thea2-K7P2W17VO62 redox reaction at a glassy carbonelectrode and doped in the graphite organoceramicmatrix are 100 and 50 mV, respectively, the formalpotentials (E0%) are the same (+0.460 V), indicatingthat the electrochemical behavior of a2-K7P2W17VO62

was retained after being doped in the graphiteorganoceramic matrix. Fig. 2 shows typical cyclicvoltammograms for an a2-K7P2W17VO62-modified elec-trode at different scan rates and the inset is the corre-sponding dependence of the peak current on scan rate.The good linear relationship observed indicates that thepeak currents are surface confined. However, DEp islarger than the theoretical value (0 mV) expected for areversible surface redox process [40] and increases alongwith increasing scan rate, suggesting a quasi-reversibleelectrochemical process.

3.3. pH-dependent electrochemical beha6ior of thea2-K7P2W17VO62-modified electrode

In order to study the pH-dependent electrochemicalbehavior of the a2-K7P2W17VO62-modified electrode,square-wave voltammetry with excellent sensitivity wasadopted to measure accurately the formal potentials(E0%). Fig. 3 shows Osteryoung square-wave voltam-mograms for the modified electrode in 0.1 M Na2SO4

aqueous solutions at different pH. It can be seen clearlythat with increasing pH, the redox potential graduallyshifts in the negative potential direction and the peakcurrent also decreases in the pH range from 0.00 to2.02. Reduction of a2-K7P2W17VO62 immobilized in thegraphite organoceramic matrix is accompanied by theevolution of cations from solution to the wetting of the

Fig. 1. Comparative cyclic voltammograms for (a) a glassy carbonelectrode in 2 mM a2-K7P2W17VO62+0.1 M Na2SO4+0.5 M H2SO4

solution and (b) an a2-K7P2W17VO62-modified graphite organoce-ramic electrode in 0.1 M Na2SO4+0.5 M H2SO4 solution. Scan rate20 mV s−1.

Fig. 2. Cyclic voltammograms for an a2-K7P2W17VO62-modifiedgraphite organoceramic electrode in 0.1 M Na2SO4+0.5 M H2SO4

solution at different scan rates (from inner to outer: 20, 40, 60, 80,100, 150 and 200 mV s−1). The inset is the dependence of peakcurrent on scan rate.

P. Wang et al. / Journal of Electroanalytical Chemistry 495 (2000) 51–5654

Fig. 3. Square-wave voltammograms for an a2-K7P2W17VO62-modified graphite organoceramic electrode in 0.1 M Na2SO4 solu-tions at different pH: (a) 0.00, (b) 0.52, (c) 1.03, (d) 1.52, (e) 2.02 and(f) 2.49. The inset is the dependence of peak potential on pH.Amplitude 25 mV; frequency 15 Hz; scan rate 60 mV s−1.

hydrogen peroxide reduction. Fig. 4 shows the electro-catalytic reduction of hydrogen peroxide at an a2-K7P2W17VO62-modified electrode. With the addition ofhydrogen peroxide, the reduction peak current in-creased while the corresponding oxidation peak currentdecreased, suggesting that hydrogen peroxide was re-duced by a2-P2W17VO62

8−. Our experiments showed thatthe voltammetric behavior of the a2-K7P2W17VO62-modified electrode did not change after it had beenused for electrocatalytic reduction of hydrogen perox-ide and was then washed with 0.5 M H2SO4, suggestingthat the stability of a2-P2W17VO62

8− in the graphiteorganoceramic matrix was not affected by hydrogenperoxide. In addition, we have noted that the catalyticactivity of the electrode for hydrogen peroxide reduc-tion is moderate, unlike other POM systems with highercatalytic activity [31,33,48,49]. However, the catalyticreduction wave at the a2-K7P2W17VO62-modified elec-trode arises at a relatively positive potential. The cata-lytic process is regarded as an EC catalytic mechanismand can be expressed as follows:

3.5. Amperometric hydrogen peroxide sensing andinterference

On the basis of the voltammetric results describedabove, it is possible to use the a2-K7P2W17VO62-modified electrode as an amperometric hydrogen perox-ide sensor. According to the potential dependence ofthe hydrogen peroxide electrocatalytic current under

electrode to maintain charge neutrality. As shown inthe inset of Fig. 3, the gradually decreasing slope alongwith increasing pH suggests that not only protons butalso sodium cations participate in the redox process. Atlow pH, the effect of protons is dominant, but that ofsodium ions is dominant at high pH. In addition, whenthe pH is higher than 2.02, the redox potential isunchanged. Along with increasing pH, the graduallydecreasing current and the more negative reductionpotentials can be elucidated by Fick’s first law [41] andthe Nernst equation [42], respectively. As is known,a2-P2W17VO62

7− is stable in neutral aqueous solution[37,38]; the a2-K7P2W17VO62 modified electrode stillshowed well defined electrochemical behavior in 0.1 MNa2SO4 solution, unlike other POM-modified elec-trodes [43–46], although the peak current becamesmaller and DEp became a little larger.

3.4. Electrocatalytic reduction of hydrogen peroxide atthe modified electrode

The direct electroreduction of hydrogen peroxide re-quires a large overpotential at most bare electrodesurfaces, including the glassy carbon electrodes and theunmodified graphite organoceramic electrodes[31,33,47]. Anson and coworkers [48] and Dong andLiu [49] studied the electrocatalytic activity of[H2OFeIIISiW11O39]5− and [H2OFeIIIP2W17O61]7− an-ions toward hydrogen peroxide reduction in acidic solu-tions. Electrocatalytic reduction of hydrogen peroxideat sol-gel-derived electrodes containing PMo12O40

3− andP2W17VO62

7− has also been reported [31,33]. In thepresent work, we found that the a2-K7P2W17VO62-modified electrode also showed catalytic activity toward

Fig. 4. Cyclic voltammograms of an a2-K7P2W17VO62-modifiedgraphite organoceramic electrode in 0.1 M Na2SO4+0.5 M H2SO4

solutions containing (a) 0, (b) 2.5, (c) 4.0, (d) 6.5, and (e) 8.0 mMH2O2. Scan rate 20 mV s−1.

P. Wang et al. / Journal of Electroanalytical Chemistry 495 (2000) 51–56 55

steady-state conditions, the optimum polarizing poten-tial was selected as +0.435 V versus the Ag � AgClelectrode in order to obtain constant and high sensitiv-ity. Typical hydrodynamic amperometry (Fig. 5) wasobtained by adding hydrogen peroxide successively tocontinuously stirred 0.1 M Na2SO4+0.5 M H2SO4

aqueous solution. The electrode response time was lessthan 5 s. The fast response is attributed to the ultrathinwetting section controlled by the propyl group and theshort penetration depth of hydrogen peroxide. Theinset of Fig. 5 shows the calibration graph for hydrogenperoxide reduction at the modified electrode. The elec-trode response was linear for hydrogen peroxide withinthe concentration range 1×10−4–2×10−2 M, and the

sensitivity was 0.753 mA mM−1 (correlation coefficientof 0.998). The detection limit was 4×10−5 M when thesignal-to-noise ratio was 3.

In our experiments, we found that a large amount ofNa+, F−, Cl−, SO4

2−, HPO42−, H2PO4

−, CO32−, Br−,

HCO3−, Mg2+, Ca2+, and Ba2+, and 200-fold Pb2+

and Zn2+ did not interfere in the detection of hydrogenperoxide.

3.6. Renewal repeatability and long-term stability

Compared with POM-modified film electrodes fabri-cated by conventional methods, a a2-K7P2W17VO62-modified electrode based on the sol-gel technique hascertain advantages. One of the main attractions ofusing the modified electrode to electroreduce hydrogenperoxide is that the electrode surface can be renewedafter every use. The bulk modified electrode can bepolished using emery paper and a fresh surface exposedwhenever needed. This is especially useful for electro-catalytic study since the catalytic activity is known todecrease when the electrode is fouled. Indeed, ten suc-cessive polishings resulted in a relative standard devia-tion (RSD) of 6.8% for an a2-K7P2W17VO62-modifiedelectrode. The stabilities of two a2-K7P2W17VO62-modified electrodes were tested by cyclic voltammetricmeasurements in 0.1 M Na2SO4+0.5 M H2SO4

aqueous solution containing 0 and 4 mM H2O2, respec-tively, over a period of 3 months. When the electrodeswere not in use, they were stored under dry conditionsat room temperature. Fig. 6 shows the response of themodified electrode as a function of storage time. Noobvious response decrease was found over this period.We think that the high stability of the a2-K7P2W17VO62-modified electrode is related to the sta-bility of the silicate matrix, the limited wetting sectioncontrolled by the propyl group, and possible interac-tions between the doped a2-K7P2W17VO62 and silanolgroups.

4. Conclusions

A surface-renewable graphite organoceramic amper-ometric hydrogen peroxide sensor has been developedusing propyltrimethoxysilane as the precursor in thesol-gel polymerization and a2-K7P2W17VO62 as the cat-alyst. The unhydrolyzed Si–CH2CH2CH3 groups in thesilicate network reject water and control only a thinactive layer at the outermost section of the sensorexposed to aqueous solution. The sensor exhibits goodchemical and mechanical stability, and the distinct ad-vantage of surface-renewal by mechanical polishing inthe event of surface fouling. Although the new concepthas been presented in the context of a2-K7P2W17VO62-modified graphite organoceramic electrodes, it could be

Fig. 5. Amperometric response of an a2-K7P2W17VO62-modifiedgraphite organoceramic electrode on successive addition of 0.1 mMH2O2 into 0.1 M Na2SO4+0.5 M H2SO4 solution. The inset is thesteady-state calibration curve for current versus H2O2 concentration.Applied potential +0.435 V; stirring speed 800 rpm.

Fig. 6. Stability of the a2-K7P2W17VO62-modified graphite organoce-ramic electrodes stored under dry conditions at room temperature.Obtained by cyclic voltammetric measurements in 0.1 M Na2SO4+0.5 M H2SO4 aqueous solution containing (a) 0 and (b) 4 mM H2O2,respectively.

P. Wang et al. / Journal of Electroanalytical Chemistry 495 (2000) 51–5656

extended readily to the construction of other polishablethree-dimensional POM-modified graphite organoce-ramic electrodes for electroanalytical and electrolyticapplications.

Acknowledgements

The authors thank the Ministry of Science and Tech-nology of China for the financial support. Fruitfuldiscussions with Professor Yuanhua Shao and Dr YiYuan are also acknowledged.

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