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Electrochimica Acta 55 (2010) 2859–2864

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

odification of vertically aligned carbon nanotubes with RuO2 for a solid-stateH sensor

in Xu, Wei-De Zhang ∗

ano Science Research Center, School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, Guangdong, PR China

r t i c l e i n f o

rticle history:eceived 30 November 2009eceived in revised form4 December 2009ccepted 28 December 2009

a b s t r a c t

In this work, a novel type electrode based on RuO2 nanoparticles-modified vertically aligned carbonnanotubes (RuO2/MWCNTs) was prepared by magnetron sputtering deposition. This RuO2/MWCNTs elec-trode not only shows a high capacity nature, but also possesses a good response to the pH value. The pHsensor based on the RuO2/MWCNTs nanocomposite electrode exhibits some advantages over the con-ventional pH sensors. It shows good reproducibility, long-term storage stability (over 1 month) and linear

vailable online 11 January 2010

eywords:H sensorarbon nanotubesuthenium oxide

response in the whole pH range (2–12) of Britton–Robinson (B–R) buffer solutions with near-Nernstianresponse (about −55 mV/pH). The hysteretic widths of the nanocomposite electrode are 6.4 mV, 5.1 mVand 10.2 mV in pH 7–4–7–10–7, pH 7–10–7–4–7 and pH 2–8–12–8–2 loop cycles, respectively. Moreover,the RuO2/MWCNTs electrode displays an excellent anti-interference property and fast response time (lessthan 40 s). According to the electrochemical impedance measurements, the pH sensing properties of the

were

anocompositelectrochemical impedance spectroscopy

RuO2/MWCNTs electrode

. Introduction

Determination of pH is indispensable in a wide range of indus-rial production processes as well as in clinic, environmentalontrol and biological systems [1]. Efforts have been made on devel-ping new approaches for the determination of pH, most of whichtilize potentiometric electrodes. Among the various methods, these of glass electrode has been widely adopted due to its goodensitivity, selectivity, stability and long lifetime. However, the lim-tations of glass electrode, such as acid and alkaline error, highmpedance, high temperature instability and mechanical fragilityestrict its further applications in certain circumstances [2]. As aesult, non-glass pH electrodes, especially solid-state pH sensorsased on metal oxides, have gained considerable concern in termsf developing potential alternatives to glass electrode for minia-urized systems, because metal oxides are mechanically robust,ess sensitive to cation interference [3]. Up to now, various metalxides have been used to develop pH sensors such as PtO2, OsO2,a2O5, TiO2, PdO, SnO2, ZrO2 [3], IrO2 [4–6], RuO2 [7], molybdenumronzes [8], Co2O3 [9], WO3 [10], and PbO2 [11]. Among these metal

xides, RuO2 is one of the most promising materials and has beenore widely used in pH sensors [12,13], biosensors [14] and super-

apacitors [15] due to its chemical stability and high conductivityhich inhibits the space charge accumulation.

∗ Corresponding author. Tel.: +86 20 87114099; fax: +86 20 87112053.E-mail address: zhangwd@scut.edu.cn (W.-D. Zhang).

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2009.12.099

also discussed.© 2010 Elsevier Ltd. All rights reserved.

On the other hand, the discovery of carbon nanotubes(CNTs) provides a new material for electrode with high per-formance because of their good electrical conductivity, largesurface area, surface chemical flexibility, high mechanical strengthand one-dimensional nanostructure [16]. Modification of CNTswith functional materials will enhance their properties or endowthem with novel properties. For instance, the capacitance of theCNT electrode in 1.0 M H2SO4 was significantly increased from0.35 mF/cm2 to 16.94 mF/cm2 by modification with RuO2 [17].Electrodeposition of TiO2 or MnO2 on CNTs promoted the electro-catalytic activity towards electrochemical oxidation of hydrogenperoxide and glucose, respectively [18,19]. However, to our knowl-edge, few articles on pH sensor based on metal oxide-modifiedMWCNTs electrodes have been reported previously. In the pre-liminary work [20], we demonstrated a pH sensor based onWO3/MWCNTs electrode with high mechanical strength, repro-ducibility, stability and selectivity. However, the sensitivity ofthis sensor is lower than the theoretical value. In order toimprove the sensitivity of the pH sensor, ruthenium oxide (RuO2)was selected to modify multi-walled carbon nanotubes (MWC-NTs) arrays (RuO2/MWCNTs) for pH sensor. Compared with theWO3/MWCNTs electrode, the RuO2/MWCNTs electrode presenteda near-Nernstian response (−55 mV/pH) and shorter response

time (less than 40 s) besides the advantages of WO3/MWCNTselectrode. The enhancement mechanism of pH sensing prop-erty of the RuO2/MWCNTs electrode is exhaustively discussedin this paper. This approach not only provides a possibility forthe miniaturization of solid-state pH sensors for specific applica-

2 himica Acta 55 (2010) 2859–2864

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ions, but also promotes the application of CNTs in electrochemicalensors.

. Experimental section

Phosphoric acid, acetic acid, boric acid, sodium hydroxide andther reagents used in this experiment are all of analytical grade.ll solutions were prepared with high quality deionized water

18.4 M�/cm).Well-aligned MWCNTs were grown on Ta substrates [21–23],

hich is facile for the fabrication of an MWCNTs electrode [24,25]r for further modification for composite electrodes [26,27]. Inhis work, coating of ruthenium oxide on the vertically aligned

WCNTs was achieved by magnetron sputtering deposition withruthenium target at a power of 200 W for 5 min in an Ar/O2

3:1) atmosphere. During this procedure, the sputtered Ru was oxi-ized in oxygen and deposited on the MWCNTs. The Ta plate withWCNTs modified by RuO2 was connected to the surface of a Cu

lectrode using conductive silver paint (Structure probe, Inc., USA).he edge of the Ta plate and Cu electrode was insulated by pastingith nail enamel.

A field-emission scanning electron microscope (SEM) (JEOL JSM700F) was used to observe the RuO2-modified MWCNTs. Open-ircuit potential of the RuO2/MWCNTs electrode was measureds a function of pH value of the sample solutions by using CHI60C electrochemical workstation (Shanghai Chenhua, China). Ahree-electrode system was employed with RuO2/MWCNTs elec-rode as working electrode, an Ag/AgCl (3 M KCl) electrode andplatinum wire served as reference electrode and counter elec-

rode, respectively. All potentials were referred to Ag/AgCl (3 MCl) electrode. Electrochemical impedance spectroscopy (EIS) mea-urements were carried out with a frequency response analyzerPGSTAT 30, Autolab, Eco-Chemie, the Netherlands) using thebove three-electrode cell. Measurements were performed withmplitude of 5 mV and frequency ranged between 100 kHz and00 mHz. Non-linear least-squares analysis was used to simulatehe impedance plane plot.

. Results and discussion

Fig. 1A shows the overall morphology of the MWCNTs mod-fied by RuO2. Enlarged observation (inset in Fig. 1A) on theuO2/MWCNTs indicates the modified MWCNTs with a larger tubu-

ar diameter near the tips due to the presence of sputtering coating,nd the underneath parts of the tubes are clear. During overheadputtering deposition, the ruthenium oxide was directly coated onhe top of the sample. Meanwhile, the coated tips blocked the lowerarts of the tubes from coating with the oxide. From the TEM image,ne can observe the coating layer on an MWCNT, as displayed inig. 1B. A magnified TEM image has been added as an inset in Fig. 1Bo clearly indicate the particles of RuO2. The deposition of RuO2 onhe MWCNTs was further confirmed by energy-dispersive X-raypectrometer, as depicted in Fig. 1C. The strong peaks of Ta in EDXrofile came from the metallic substrate, where the MWCNTs werettached.

Performance of pH sensors is usually characterized by mea-uring the open-circuit potential of the electrodes in solutionsith various pH values [28]. Fog and Buck reported the generalechanism of metal oxides for pH sensing and suggested that pH

esponse could be due to ion exchange in a surface layer containingH group [5]. Zoubov et al. described only one redox equilib-

ium between insoluble ruthenium oxides [29]. So, the generalensing mechanism of RuO2-based pH sensor can be expressed asollows:

uO2 · nH2O + H+ + e− ⇔ (n − 1)H2O + Ru(OH)3 (1)

Fig. 1. (A) SEM, (B) TEM images and (C) EDS of the RuO2/MWCNTs nanocomposite.

According to Nernst equation for the equilibrium, the electrodepotential can be stated:

ERuO2·nH2O/Ru(OH)3= E0

RuO2·nH2O/Ru(OH)3− RT

zFln

a(RuO2·nH2O)

a(Ru(OH)3)

− RT

Fln

1a(H+)

= E0′RuO2·nH2O/Ru(OH)3

− 2.303 RT

FpH = E0′

RuO2·nH2O/Ru(OH)3+ m pH

(2)

where E is the measured potential, E0′is the conditional standard

potential, R is the gas constant (8.314 J/K mol), T is the absolutetemperature (K), z is the signed ionic charge and F is the Fara-day constant (96487.3415 C/mol). At room temperature (T = 298 K),

B. Xu, W.-D. Zhang / Electrochimica

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electrodes have wide response voltage range and a near-Nernstian

ig. 2. pH response curves of the RuO2/MWCNTs electrode in B–R buffer solutionsA) with different immersion time in pH 7 buffer solution, (B) the effect of time onotential and sensitivity and (C) with N2 or O2-saturated.

he slope m should be −59.1 mV/pH. As shown in Fig. 2A, theuO2/MWCNTs electrode displays linear characteristics over a wideH range from 2 to 12. A slope of about −55 mV/pH is determinedor this nanocomposite electrode. Even though the sensitivity is ait lower than theory value (−59.1 mV/pH), it is much higher thanhe WO3/MWCNTs electrode (−41 mV/pH) [19] and nearly consis-ent with the other RuO2 film composite electrode [13,30,31].

The decrease in E0′of the freshly prepared electrode after

mmersing in pH 7 buffer solution for 24 h is the typical aginghenomenon that affects metal oxide electrodes [32]. Potentialrifts exceeding 200 mV have been reported for freshly prepared

rO2 electrodes [33,34]. Fig. 2A shows the pH-potential response

Acta 55 (2010) 2859–2864 2861

curves of the RuO2/MWCNTs electrode. After being immersed ina pH 7 buffer solution for 24 h, the E0′

drifted about 79 mV, whilethe sensitivity drifted about 2.3 mV/pH. For 48 h immersion, thethree calibration curves of the electrode almost coincided with oneanother. Fig. 2B more directly shows the effect of immersing timeon E0′

and sensitivity of the RuO2/MWCNTs electrode. After immer-sion of 48 h, the E0′

and sensitivity are the same as those with 24 himmersion of the electrode, which indicates the electrode becom-ing stable. In our studies, all the RuO2/MWCNTs electrodes wereimmersed in pH 7 buffer solution for 48 h prior to use. The agingphenomenon has long been considered to be caused by the progressof hydration reactions at the electrode surface [32,34], the redoxprocesses involving atmospheric oxygen [33] and the initial pres-ence of RuO3 [35]. As we all know, ruthenium has three oxidationstates: RuO2, RuO3 and RuO4. RuO3 and RuO4 are more volatileand have relatively low melting and boiling points [36]. Accord-ing to Bell and Tagami [37], the formation of RuO3 and RuO4 atstandard state or reduced oxygen pressure is thermodynamicallyunfavorable due to the standard state free energy (�G0

298) andequilibrium partial pressures (p) are 191.81 kJ/mol, 101.77 kJ/moland 2.39 × 10−34 atm, 1.44 × 10−18 atm, respectively. So, it is almostimpossible for RuO3 to be present in RuO2/MWCNTs electrodesunder the fabrication procedure and storage conditions. It also canbe seen from Fig. 2C, when the electrode was placed in the nitrogensaturated and oxygen saturated buffer solutions, there was almostno difference between the calibration curves. In addition, duringthe CV test in 0.5 M H2SO4, the immersed electrode encircled largerarea and had larger capacitance than the freshly prepared electrode,which could be attributed to the hydration of RuO2 [15] (data notshown). Therefore, it is reasonable to conclude that the slow sur-face hydration of RuO2 gives rise to a drift in E0′

and sensitivity,which is in line with the response mechanism (Eq. (1)). However,this conclusion is different from the work reported in Ref. [13],which ascribes the emf drift at pH measurements to the H+ dif-fusion in RuO2 film. The difference in conclusions may result fromthe different composition of electrode materials.

Hysteresis is a common phenomenon for glass pH electrode ormetal oxide pH electrodes. When the electrode is measured manytimes in the same pH buffer solution, different output voltagesoccurred. This phenomenon is called memory effect or hystere-sis. According to the depiction by Bousse et al., the hysteresis ofhydrogen ion selective electrodes could be regarded as a delay ofthe pH response [38]. The hysteretic widths of the RuO2/MWCNTselectrode were valued by successively measuring the open-circuitpotentials of different pH buffer solutions in each cycle. As shownin Fig. 3, the hysteretic widths are 6.4 mV, 5.1 mV and 10.2 mV inpH 7–4–7–10–7, pH 7–10–7–4–7 and pH 2–8–12–8–2 loop cycles,respectively. These hysteretic widths of RuO2/MWCNTs electrodeare smaller than those of WO3 film electrode and glass pH elec-trode [39], which indicates the RuO2/MWCNTs electrode has a goodresponse to pH value. So far, there is little explanation to hystere-sis phenomenon. According to the experimental results, the slowdiffusion of the H+ between inner and outer surface and the slowadjustment of the hydration situation of the electrode surface aremost likely to cause this phenomenon. The open-circuit potentialdetermination in different pH solutions is quickly carried out, sothe electrode surface fails to adjust instantaneously with the cor-responding solution.

Reproducibility and stability are critically important to pH sen-sors. In this study, four samples of RuO2/MWCNTs electrode werefabricated and measured. As shown in Fig. 4A, all of the sample

response (about −55 mV/pH). Three electrodes show almost thesame E0′

(about 640 mV), while sample 3 electrode shows a little bithigher E0′

. When sample 1 electrode was selected to be alternatelymeasured in buffer solutions of pH 2, 4, 6, 8, 10 and 12 separately

2862 B. Xu, W.-D. Zhang / Electrochimica Acta 55 (2010) 2859–2864

F(

(fi4lteso

ig. 3. Hysteresis width of the RuO2/MWCNTs electrode with different loop cycles:A) pH 7–4–7–10–7; (B) pH 7–10–7–4–7; (C) pH 2–8–12–8–2.

shown in Table 1), the relative standard deviation determined fromve measurements was about 0.06%, 0.07%, 0.19%, 0.47%, 1.47% and.70%, respectively. The potential drifts for all measurements were

ess than 3 mV, which shows this pH sensor could effectively avoidhe acid and alkaline error. After being stored in air and testingveryday for 1 month, no significant change was observed in mea-uring pH value of the B–R buffer solutions, as depicted in Fig. 4B. Allf the experimental results illustrate that the RuO2/MWCNTs elec-

Fig. 4. pH response curves of (A) different electrodes in B–R buffer solutions, (B)under different testing time for sample 1 electrode and (C) potential response inB–R buffer solutions with different pH value.

trode holds good reproducibility, stability and long-term storagestability.

Response time is also an important factor for sensors. Tradi-tionally, it is defined as the time required for electrode to reach95% of the equilibrium [40]. Fig. 4C shows the response of theRuO2/MWCNTs electrode in pH 4, 8 and 12 buffer solutions, respec-

B. Xu, W.-D. Zhang / Electrochimica Acta 55 (2010) 2859–2864 2863

Table 1Open-circuit potential of the RuO2/MWCNTs electrode in solutions with various pH values.

pH value (B–R buffer solution) Measured value of OCP for five times (mV) Averaged value of OCP (mV) RSD%

1 2 3 4 5

2 540.2 540.6 540.4 540.0 539.8 540.2 0.064 421.3 421.2 420.8 420.8 420.6 420.9 0.07

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with the expansion of the response time upon the increase of pH.The RuO2/MWCNTs electrode has also been employed to

measure real samples. It was applied to pH determination of var-ious soft drinks and solutions, such as Fanta, Sprite and NaOHsolution (0.001 M), the pH values were 2.86 (RSD = 1.32%), 3.37

6 304.0 302.5 302.88 195.9 193.7 194.8

10 92.3 90.8 92.312 −23.5 −23.8 −22.6

ively. The response time is less than 40 s in the buffer solutions ofH from 2 to 12 and it is affected by the pH value of solutions. In

ow pH solutions the response time is shorter than that in the highH ones, which may result from the fact that the dynamic responsef the electrode is related to the H+ diffusion [41].

The selectivity of the electrode was established by studying theffect of some common ions (Cl−, NO3

−, SO42−, F−, I−, Ca2+, K+) on

otentiometric response. According to two solutions method (TSM)42] sponsored by IUPAC [43], the selectivity coefficient (Kij) can bealculated using the extended Nernstian equation as follows [44]:

i,j = ai{exp(zjF �E/RT) − 1}(aj)

zi/zj(3)

here ai and aj are the activities of the primary ion whose charge isi and the interfering ion whose charge is zj, respectively, �E is theifference between the electrode potentials in the solutions con-aining both ‘i’ and ‘j’ ions, and only ‘i’ ion, while ai remains the samen both solutions. In our studies, the selectivity coefficients (Ki,j) ofhis electrode for H+ with respect to Cl−, NO3

−, SO42−, F−, Ca2+, K+

nd I− were calculated based on Eq. (3) by considering hydrion ashe primary cation with 1.0 × 10−1 mol dm−3 of the correspondingnterfering ion. As shown in Table 2, the largest selectivity coeffi-ient (Ki,j) comes from I−, which is less than 1.0 × 10−5, while theelectivity coefficients are all less than 1.0 × 10−10 from other testedons. This is because the oxidation of I− was involved in the redoxf RuO2. The results indicate that the interference from the mostommon ions for pH measurement was negligible. Especially forhe F−, which can cause great interference on glass pH sensor, haso interference on the RuO2/MWCNTs electrode.

The RuO2/MWCNTs electrode was also evaluated by elec-rochemical impedance spectroscopy (EIS) in an attempt tonderstand its pH sensing characteristics. EIS is an effectivepproach for investigating the electron transfer across the elec-rolyte and the surface of electrode. The Nyquist plot for theuO2/MWCNTs electrode in a buffer solution of pH 8 is presented

n Fig. 5. One can see only one time-constant from the plot and it isypical capacitance behavior. These demonstrated that the reactionf the electrode was under kinetic control [45]. Simulation of thexperimental data was carried out via Boukamp non-linear least-

quares program provided by the FRA software (Version 4.9.007).he values resulting from the fit are in good agreement with thexperimental data giving the �2 < 10−2. The equivalent circuit ofhe RuO2/MWCNTs (the inset of Fig. 5) consisted of a charge trans-

able 2on selective coefficients of the RuO2/MWCNTs electrode.

Interfering ion Concentration (mol/L) Selectivity coefficient (Ki,j)

K+ 0.1 <10−10

Cl− 0.1 <10−10

Ca2+ 0.1 <10−10

SO42− 0.1 <10−11

F− 0.1 <10−10

NO3− 0.1 <10−10

I− 0.1 <10−5

303.0 303.1 303.8 0.19195.4 195.9 195.1 0.47

93.6 94.4 92.7 1.49−21.4 −21.7 −22.6 4.70

fer resistance (RCT), a constant phase element (CPE) in parallel,the electrolyte resistance (Rs) and the T diffusion element (ZT).The constant phase element, defined as ZCPE = Z0(jω)−n, where Z0and ω are constants, j = (−1)1/2, and 0 ≤ n ≤ 1, is used instead of acapacitance to describe a non-ideal capacitive response because ofsurface inhomogeneities [46]. The T diffusion element (ZT) is char-acteristic of another type of film which contains a fixed amountof electroactive substance. It is a useful model for diffusion whenthe finite diffusion (a thin film) is involved and defined as ZT =Z0(jω)−0.5 coth[B(jω)0.5]. So, it is also called the “bounded War-burg”. Batteries or supercapacitors often share this behavior [47].The finite diffusion region of EIS (Fig. 5) showed a slope slightlybelow 90◦ and a constant phase element can be used to simulateit. This anomalous behavior was already observed by other authors[48,49] and explained as a result of both electrode porosity androughness at the blocking interface. The charge transfer resistance(RCT) is assigned to the impedance related to charge transport atthe Pt counter electrode and the surface of the RuO2/MWCNTselectrode.

The impedance behaviors in different pH solutions were quitesimilar to that presented in Fig. 5 for pH 8. For all applied pHsolutions, the same type of equivalent circuit was obtained. The dif-ference lied in the values of the RCT. The charge transfer resistant,RCT, was calculated and found to increase gradually from 1.80 k�to 10.50 k� with the pH values from pH 2 to 12, indicating fasterreaction kinetics at higher proton concentration, as listed in Table 3.It is believed that this phenomenon was due to the decrease ofhydrogen concentration difference between the electrolyte and thesurface of electrode with the increase of pH which halted the trans-fer of H+ and caused the RCT increase. This result was well consistent

Fig. 5. Electrochemical impedance spectra of the RuO2/MWCNTs electrode in B–Rbuffer solution (pH = 8). Inset is the equivalent circuit.

2864 B. Xu, W.-D. Zhang / Electrochimica

Table 3The fitting results of Rs and RCT at various pH values.

pH Rs (k�) RCT (k�)

2 0.303 1.804 0.572 2.33

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R

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6 0.319 3.578 0.232 4.92

10 0.221 9.6612 0.189 10.5

RSD = 0.17%), and 11.08 (RSD = 0.14%), respectively. These resultsere almost consistent with the values of 2.74, 3.32 and 11.03etermined by the conventional glass pH electrode, which fullyhowed the RuO2/MWCNTs electrode could be applied in reality.

. Conclusion

A novel solid-state pH sensor based on RuO2-modified MWCNTsas been successfully fabricated. Structure and composition anal-sis elucidated the successful deposition of RuO2 thin film on theertically aligned MWCNTs by magnetron sputtering. The MWC-Ts not only served as support, but also played as a conductor asther metals do in metal/metal oxide pH sensors. In pH measure-ents, the RuO2/MWCNTs electrode demonstrates high stability

over a month), good reproducibility (RSD <5%), fast response<40 s), favorable anti-interference property, and a high sensitiv-ty of about −55 mV/pH from pH 2 to 12. Moreover, the methodlso makes it possible to miniaturize pH sensors. With verticallyligned nanoscale pH electrode, the determination of the pH valuen vitro and in vivo intracellular can be achieved. For example, bynserting one carbon nanotube or a bundle of carbon nanotubeslectrode modified with RuO2 into kidney cell, the intracellular pHould be determined, which is our further research.

cknowledgements

The authors thank Natural Science Foundation of China (No.0773041) and the Research Fund for the Doctoral Program ofigher Education (No. 20070561008) for financial support.

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