author's personal copy - ntu singapore nw_tsf2010.pdf · ta-c:n lm electrodes in the nacl...

This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution

and sharing with colleagues.

Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party

websites are prohibited.

In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information

regarding Elsevier’s archiving and manuscript policies areencouraged to visit:

http://www.elsevier.com/copyright

http://www.elsevier.com/copyright

Author's personal copy

Stripping voltammetric behavior of nitrogen-doped tetrahedral amorphous carbonthin film electrodes in NaCl solutions

N.W. Khun, E. Liu ⁎School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

a b s t r a c ta r t i c l e i n f o

Article history:Received 26 August 2009Received in revised form 30 November 2009Accepted 9 December 2009Available online 16 December 2009

Keywords:Tetrahedral amorphous carbonNitrogen dopingFiltered cathodic vacuum arcMetal tracingSodium chloride electrolyte

Conductive nitrogen-doped tetrahedral amorphous carbon (ta-C:N) thin films fabricated with a filteredcathodic vacuum arc technique were investigated for their ability to detect multiple trace heavy metals suchas mercury, copper and lead by linear sweep anodic stripping voltammetry in sodium chloride aqueoussolutions. The ta-C:N film electrodes exhibited a significant stripping response for determination ofindividual elements (Pb2+ and Hg2+) and multiple elements (Pb2++Hg2+ and Cu2++Hg2+), indicatingthat these electrodes have a great potential for simultaneously tracing multiple heavy metals in aqueoussolutions.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Contamination, over-use and mismanagement of water resourcescause pollution in water including a wide spectrum of chemicals,pathogens, and physical chemistry or sensory changes [1]. Thepresence of toxic metals such as mercury (Hg), copper (Cu) andlead (Pb) in the aquatic ecosystem directly or indirectly affects biotaand human beings. Lead and copper are detrimental to children withpossible development of neurological problems and adverse chroniceffects [1]. Mercury occurs in deposits throughout the world and ispoisonous in soluble forms such as mercuric chloride or methylmer-cury. Mercury can enter the environment from natural sources such asvolcanoes, stationary combustion such as power plants, production ofmaterials, non-ferrous metals, cement, caustic soda, pig iron, steel,mercury itself, and waste disposals including municipal and hazard-ous waste, crematoria, and the disposal of certain used products suchas auto parts, batteries, fluorescent bulbs, medical products, thermo-meters, and thermostats [2]. Many studies have shown some effectssuch as tremors, impaired cognitive skills, and sleep disturbance inhuman beings with chronic exposure to mercury of concentrations aslow as mM levels [3–5]. Therefore, tracing and determination of theseheavy metals in aqueous solutions are of major importance inelectrochemical analysis.

In the past, Hg was the first metal extensively used for electroan-alytical purposes in the form of dropping mercury electrode because it

can avoid many problems pertaining to electrode poisoning in acomplex matrix. However, a drawback of a Hg-based electrode is thatonly reduction reactions can be investigated due to the self oxidation ofHg. Thus, thedetection limit and toxicitywith aHg-based electrodehavecaused some experimental manipulation difficulties. Therefore, carbonbased solid electrode materials such as glassy carbon, graphite, carbonpaste and carbon fiber have been developed in order to compensate thelimitations of mercury electrodes.

Though glassy carbon is one of the most widely used electrodematerials in electroanalytical applications for its qualities such asrobust, smooth surface and large electrochemical window, irrepro-ducible background contributions and gradual loss of surface activityfrequently affect its electroanalytical performance.

Boron doped diamond films prepared by chemical vapor deposi-tion can have remarkable properties such aswide potential window inaqueous and non-aqueousmedia, low and stable capacitance chargingbackground current, weak adsorption of polar molecules leading to animproved resistance to electrode deactivation and fouling, long-termresponse stability and superbmicrostructural and morphologicalstability at high temperatures [6,7]. However, synthesis of diamondfilms demands a high substrate temperature, which has imposed asevere constraint on fabrication of semiconductors and microelec-trode arrays [8].

Nowadays, much interest has been focused on conductive diamond-like carbon (DLC) thin films for electrochemical analysis [9]. DLC filmscan be fabricated at low temperature and made electrically conductivebydopingnitrogen or somemetals. It was reported that nitrogen-dopedDLC (DLC:N) films prepared by filtered cathodic vacuum arc (FCVA)were also chemically stable and could achieve good mechanical

Thin Solid Films 518 (2010) 4003–4009

⁎ Corresponding author. Tel.: +65 67905504; fax: +65 67924062.E-mail address: [email protected] (E. Liu).

0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2009.12.084

Contents lists available at ScienceDirect

Thin Solid Films

j ourna l homepage: www.e lsev ie r.com/ locate / ts f


properties with a high sp3 content in the films. Moreover, these filmsmay achieve similar chemical characteristics to those of diamondmaterials, such as excellent chemical inertness, high fouling resistanceand large potential window [10]. Khun et al. [11] reported that DLC:Nfilms deposited by FCVA showed high corrosion resistance and long-time stability in corrosive media.

Though electrochemical stripping voltammetric measurements aresimple, quick and cheap ways of tracing metals, the sensitivities of suchmeasurements change with electrode properties (electrical conductivity,surface roughness, surface cleanliness), operation parameters (scan rate,deposition potential and time, voltammetricmethod) and environmentalparameters (current flow, pH value, metal ion concentration, metalsolubility). Liu and Liu reported [12] that DLC:N films could trace singlemetals such as Hg2+, Pb2+ and Cu2+ and multi-metals (Pb2++Cu2+).Zeng et al. also [10] reported that DLC:N films deposited with DCmagnetron sputtering could detect multiple heavy metals (Cd2+, Pb2+,Cu2+) simultaneously. Anodic stripping voltammetry has been widelyused for detection of heavy metals in various samples because of itsremarkably low detection limits [13]. In a linear sweep voltammetricmeasurement, current response is plotted as a function of voltage, withwhich well defined and selective peaks can be obtained due to littleinterference of species that can be adsorbed by working electrode [14].

It was reported that nitrogen-doped tetrahedral amorphouscarbon (ta-C:N) thin films prepared with FCVA could achieve widepotential windows in acidic, neutral, and hydroxide solutions [15].The detection of single elements such as Zn2+, Pb2+, Cu2+ and Hg2+,and simultaneous detection of multiple elements such as Pb2+ andHg2+ at μM concentrations in KCl solutions using ta-C:N filmelectrodes were also reported [16], which the concentrations of theelements were reasonably lower than the critical amounts tolerable tothe health of the human body. However, the effects of electrochemicaldeposition parameters such as scan rate, pH value and solutionconcentration on the stripping performance of ta-C:N film electrodesstill need a systematic understanding. This paper aims to investigatethese effects using linear sweep anodic stripping voltammetry(LSASV) and examine the applicability of LSASV to ta-C:N filmelectrodes for direct detection and determination of single- andmulti-elements, namely, Pb, Hg and Cu in sodium chloride (NaCl) solutions.

2. Experimental details

Ta-C:N thin films were deposited on highly conductive p-Si (111)substrates (1–6×10−3 Ω cm) using a filtered cathodic vacuum arc(FCVA) deposition method (Nanofilms). The substrates were cleanedfirst with a detergent solution and then de-ionized water using anultrasonic machine before being placed in the vacuum chamber. Priorto the film deposition, the substrate surfaces were etched by Ar+ ionsin the vacuum chamber to remove the oxide layers. A pure graphitetarget (≥99.9%C) was used as the carbon source and nitrogen gasintroduced by a mass flow controller at 5 sccm was used as thenitrogen source for doping. The substrate bias was maintained at1500 V (pulse). The thickness of the ta-C:N films was approximately100 nm.

A linear sweep anodic stripping voltammetric (LSASV) technique(Bionano LK6200) was used for heavy metal tracing, which had apotential resolution of 0.1 mV and a current resolution of better than0.1 pA, equipped with a three-electrode flat cell kit. Deaerated andunstirred 0.1 M NaCl solutions of different pH values were employedand the pH values of the solutions were compensated by HCl. For allthe electrochemical tests, the ta-C:N coated samples were cut into2 cm×2 cm square pieces and a gold layer was deposited on thebacksides of the Si substrates to make the samples in a good electricalconnection. The testing area on the films was a circle of 1 cm indiameter. Potentials were measured with respect to a standardsaturated calomel reference electrode (SCE) in a saturated KClsolution (244 mV vs. standard hydrogen electrode (SHE) at 25 °C)

and a Pt mesh was used as the counter electrode. After each strippingexperiment, the working electrode was held at 0.5 V vs. SCE for 2 minto remove the deposits on the electrode and then the cell and all theelectrodes were washed with distilled water.

The voltammogram responses to single elements (Pb2+, Hg2+)and multiple elements (Pb2++Hg2+, Cu2++Hg2+) measured at theta-C:N film electrodes in the NaCl solutions were studied with linearsweep anodic stripping voltammetry (LSASV) in terms of depositionpotential and time, scan rate, pH value of solutions, and concentra-tions of metal ions and electrolytes in the solutions. All the chemicalsemployed in this study were of analytical reagent grades.

3. Results and discussion

In Fig. 1a–c, an influence of deposition time over the range of 0–200 s on the stripping voltammograms of Pb was investigated in the1×10−4 M Pb2++0.1 M NaCl solutions in terms of pH value of thesolutions and potential scan rate, where the deposition potential usedis −1.2 V. The ta-C:N film electrodes show the well-defined strippingpeaks of Pb even with near zero deposition time, indicating their highsensitivity to the Pb2+ ions and the stronger Pb stripping peaks withincreasing deposition time for all the pH values and scan rates areobserved in Fig. 1a–c. A near linear increase of the Pb anodic strippingpeak current with increased deposition time is found in Fig. 1dbecause the amount of Pb preconcentrated on the ta-C:N filmelectrode surface is proportional to the deposition time [10]. Theshift of the stripping potentials of Pb to lower negative values withincreased deposition time (Fig. 1e) can be correlated to the increasedamount of stripped Pb2+ ions during stripping according to the Nernstequation [16],

E = E0−ðRT = nFÞ ln½Pb2 + �

where E0, R, T, n, and F are the standard half cell potential of Pb, gasconstant, absolute temperature, number of electrons transferred inthe half-reaction, and Faraday constant, respectively.

It is found that the Pb stripping peaks are more pronounced whenmeasured in amore acidic solution (pH 1) than in a less acidic one (pH 3)under the same scan rate of about 36 mV/s as shown in Fig. 1a and b,which indicates an influence of pH value on the sensitivity of the ta-C:Nfilm electrodes to Pb as depicted in Fig. 1d and is attributed to therestricted transport of the Pb2+ ions induced by formation of moreinsoluble lead hydroxide in the less acidic solution [10]. In addition,unbalancedH+andOH− ions in the solutionsalsoaffect thekineticsof theredox reactions of Pb during the stripping analysis, whichmeans that theincreased H+ ions with decreased pH value of the solution decreases thesolution resistivity, leading to an increased mobility of the Pb2+ ionsduring accumulation. The increased strippingpeak currents of Pbwith theincrease of deposition time show a more pronounced trend in the moreacidic solution compared to that for the less acidic one, which furtherreveals a lower restriction in transport of the Pb2+ ions in themore acidicsolution as shown in Fig. 1d.

The measured standard deviation of stripping peak currents forfive stripping cycles of Pb2+ (deposition potential=−1.2 V, deposi-tion time=120 s, pH=3, scan rate≈36 mV/s) is about 1.9%, that isconfirmed by the repeated two hour tests, demonstrating goodmeasurement repeatability and stability of the ta-C:N film electrodes.

Dependence of the stripping potentials of Pb on the pH values ofthe solutions is illustrated in Fig. 1e, where it is clearly indicated that alower pH value shifts the stripping potential of Pb to more negativevalues in accordance with the Nernst equation, e.g. from −486 to−429 mV with pH 1 compared to −488 to −415 mV with pH 3,especially at longer deposition durations. This reveals a strongerinfluence of the amount of H+ ions on the stripping potentials of Pbthan that of the deposited Pb.

4004 N.W. Khun, E. Liu / Thin Solid Films 518 (2010) 4003–4009


Fig. 1b and c shows an influence of scan rate on the strippingvoltammograms of Pb measured in the same solution with the samepH value (pH 1). The stripping peaks measured at the faster scan rate(about 97 mV/s) have larger intensities and widths than the onesmeasured at the slower scan rate (36 mV/s) as shown in Fig. 1c, whichcan be explained by taking into account the size of diffusion layer andthe time taken to record a scan. A linear sweep anodic strippingvoltammetric (LSASV) technique measures an exact form of voltam-mograms that can be rationalised by voltage and mass transporteffect, though the stripping voltammograms of Pb take a longer timeto record at the lower scan rate (36 mV/s) at which the diffusion layergrows much thicker than the one measured at the faster scan rate(97 mV/s). Consequently, the flux (electrochemically active speciesfrom the bulk solution) to the film electrode surface is considerablysmaller at the slower scan rate (36 mV/s) than that at the faster rate(97 mV/s). As a stripping current is proportional to a flux towards theelectrode, the stripping peak currents of Pb are thus significantlyhigher with the scan rate of 97 mV/s than those measured with36 mV/s as shown in Fig. 1d.

It is found that the stripping potentials of Pb also change with thescan rate. A faster scan rate (97 mV/s) shifts the stripping potentials ofPb to less negative values (−474 to −385 mV) than those (−488 to−415 mV) obtained at a lower scan rate (36 mV/s) as shown in Fig. 1band c, which can be explained by the kinetic limitation of electrode

reactions that causes an oxidation of Pb at less negative potentials[17]. If the kinetic of the reactions is lower corresponding to 97 mV/sthan that corresponding to 36 mV/s, it means that the strippingcurrent of Pb takes more time to respond to the applied voltage at afaster scan rate, resulting in the stripping potentials of Pb shifted toless negative values.

The stripping voltammograms of Pb obtained from the ta-C:N filmelectrodes in the same solutions as used from Fig. 1 as a function ofdeposition potential are shown in Fig. 2a and b, where more negativedeposition potentials from −1.0 to −1.6 V give rise to apparentlystronger stripping peaks of Pb with both pH values. More negativedeposition potentials result in an increase of ion mobility from thebulk solution to the film electrode surface, leading to a proportionalincrease in the amount of Pb deposited on the electrode surface.Furthermore, when the pH of the solution is reduced to unity, thestripping peaks of Pb becomemore pronounced (Fig. 2b) as well as thesensitivity of the ta-C:N film electrodes to the Pb2+ ions issignificantly enhanced (Fig. 2c). However, a falling stripping peakcurrent of Pb (Fig. 2c) is found in the more acidic solution (pH 1)when the deposition potential is more negative than−1.4 V, which isprobably due to a concomitant hydrogen evolution occurring earlierin the more acidic solution during accumulation [12,18].

In Fig. 2d, the stripping potentials of Pb shift to less negative values(−0.446 to−0.441 V for pH 1 and−0.436 to−0.434 V for pH 3) with

Fig. 1. Influence of pH value and scan rate on stripping voltammograms (a–c), stripping peak currents (d), and stripping peak potentials (e) obtained from ta-C:N film electrodes in1×10−4 M Pb2++0.1 M NaCl solutions with respect to deposition time. The deposition potential is −1.2 V.

4005N.W. Khun, E. Liu / Thin Solid Films 518 (2010) 4003–4009


more negative deposition potentials from −1 to −1.2 V, which hasshown a quasi-reversible reaction of the Pb2+ ions on the ta-C:N filmelectrode surface. It is known that the concentration of redox speciesat the interface region depends on the transport of these species fromthe bulk solution to the electrode surface. When the flux to the filmelectrode surface is much slower than the kinetics of the electrodereactions, an equilibrium between the oxidized and reduced speciesinvolved in the electrode reactions is established at the film electrodesurface, indicating a much slower transport of the metal ions [17].Therefore, the observed quasi-reversible reaction of Pb indicates thatthe concentration of Pb2+ ions (0.1 mM) used in this study is highenough so that the kinetics of the electrode reactions is not muchfaster than the transport of the Pb2+ ions caused by a largeconcentration gradient between the interface region and the bulksolution. When the deposition potential is further shifted to morenegative values up to−1.6 V, it is found that the stripping potential ofPb is shifted to more negative values from−0.441 to−0.445 V for pH1 and from −0.434 to −0.437 V for pH 3, revealing that greaternegative deposition potentials result in the reversible reactions of thePb2+ ions on the ta-C:N film electrode surface. It is known that thelarger the negative value of a deposition potential, the closer thehydrogen evolution is. Therefore, when the deposition potential is morenegative than−1.2 V, the developed hydrogen evolution near the filmelectrode surface disturbs the flux to the surface so that the kinetics ofthe electrode reaction on the film electrode surface becomes faster thanthe transport of the Pb2+ ions causing downward shifts of the strippingpotentials of Pb as shown in Fig. 2. In terms of pH value, it is found thatthe stripping potentials of Pb as a function of deposition potential aremorenegative in themoreacidic solution (pH1) than thosemeasured inthe less acidic one (pH 3), which can be correlated to the increasedconcentration of H+ ions with decreased pH by applying the Nernstequation.

The stripping voltammograms of Hg obtained from the ta-C:N filmelectrodes in the solutions containing1×10−5 MHg2+anddifferentNaCl

concentrations as a function of deposition time are shown in Fig. 3a and b,where the pHvalues of the solutions are not compensated byHCl in orderto make the solutions near neutral (pH 7). The deposition potential andscan rate are−1.2 V and 36 mV/s, respectively. It is clearly seen that thestripping peaks of Hgmeasured in the 0.2 MNaCl solution are apparentlypronounced compared to those measured in the 0.15 M NaCl solution.However, all the stripping peaks measured in both solutions are stillaffected by the restricted mobility of the Hg2+ ions due to a low ionicconductivity of theneutral solutions. Inaddition, the reactionsof theHg2+

ions with the OH− ions existing in the solution adjusted to the neutralityshould be taken into account because of a high probability to forminsoluble mercury (II) hydroxide that in turn affects the strippingvoltammograms of Hg. That increasing concentration of NaCl in thesolution from 0.15 to 0.2 M gives rise to stronger and sharper strippingpeaks of Hg (Fig. 3a) due to a promoted ionic conductivity of the 0.2 Msolution, which makes the mobility of the Hg2+ ions faster.

The concentration of electrochemically active species (EAS) in thesolutions such as H+ and Cl− ions mainly influence the strippingcurrents of Hg during the stripping because the number of electronsreleased by dissolving the deposited Hg atoms is proportional to theconcentration of the EAS [17]. When the solution has a limited supplyof EAS at a large pH value and a low NaCl concentration, the transportof the EAS can restrict the access of EAS to the ta-C:N film electrodesurface due to their low concentration gradient between theinterfacial region and the bulk solution, resulting in diffusioncontrolled stripping peaks of Hg that are shown in Fig. 3a and b asthe fluctuating stripping peaks [19]. Therefore, the replotted strippingpeak currents of Hg vs. deposition time are consistently higher in thehigher NaCl concentrated solution than those measured in the lowerNaCl one (Fig. 3c).

Though a shift of stripping potential of Hg tomore positive values (98to 128 mV) with increased deposition time according to the Nernstequation is observed in the 0.2 M NaCl solution (Fig. 3a and d), acorrelation between the stripping potential and deposition time is not

Fig. 2. Influence of pH value on stripping voltammograms (a and b), stripping peak currents (c) and stripping peak potentials (d) obtained from ta-C:N film electrodes in the samesolutions as for Fig. 1 with respect to deposition potential. The deposition time and scan rate are 120 s and 36 mV/s, respectively.



found in the0.15 MNaCl solutionup to120 s (Fig. 3b)as adeposition timeless than 120 smay not be enough for amore dilute solution to follow theNernst equation. Therefore, it is suggested that a longer deposition time isrequired to correlate the stripping potential of Hg with the depositiontime for the solution with a lower NaCl concentration.

Fig. 4a and b displays the stripping voltammograms of Hg at theconcentrations of 1×10−4 M and 1.1×10−7 M in the 0.1 M NaClsolutions as a function of deposition time at a scan rate of 36 mV/s and

a deposition potential of −1.2 V. It is found that a higher concentra-tion of Hg2+ ions (1×10−4 M) gives rise to much better-definedstripping voltammograms of Hg as shown in Fig. 4a. A possible reasonis that an increased concentration gradient of Hg2+ ions can facilitatethe transport of the Hg2+ ions. However, a much lower concentrationof Hg2+ ions (1.1×10−7 M) can also generate the smooth strippingpeaks of Hg in the 0.1 M NaCl solution (Fig. 4b) compared to thestripping peaks of Hg obtained from the solutions having 1×10−5 M

Fig. 3. Effect of electrolyte concentration on stripping voltammograms (a and b), stripping peak currents (c) and stripping peak potentials (d) obtained from ta-C:N film electrodes inNaCl solution (pH 7) containing 1×10−5 M Hg2+ vs. deposition time. The preconcentration potential and scan rate are −1.2 V and 36 mV/s, respectively.

Fig. 4. Stripping voltammograms (a and b), stripping peak currents (c), and stripping peak potentials (d) obtained from ta-C:N film electrodes in 0.1 M NaCl solutions containing1×10−4 M Hg2+ and 1.1×10−7 M Hg2+ (pH 7) with respect to deposition time. The deposition potential and scan rate are −1.2 V and 36 mV/s, respectively.



and higher (0.2 and 0.15 M) NaCl concentrations as shown in Fig. 3aand b. It may be supposed that the concentration of the EAS existing inthe 0.1 M NaCl solution is sufficient for stripping the Hg depositresulted from the very low concentration of 1.1×10−7 M Hg2+. Aproportional increase of the stripping peak current of Hg withdeposition time for both concentrations is found in Fig. 4a–c. Suchexperimental results point out that the ta-C:N film electrodes canhave a very low detection limit of Hg2+ ions in near neutral solutionscompared to the stripping performance of similar ta-C:N filmelectrodes as reported in Ref. [12].

The Hg stripping potentials continuously shift to more positivevalues with longer deposition durations as shown in Fig. 4d, exceptthe deposition time of 200 s for 1.1×10−7 M Hg2+. The lowerconcentration of Hg2+ ions (1.1×10−7 M) results in the strippingpotentials of Hg at lower values (77 to 100 mV) compared to those(154 to 246 mV) that resulted from the higher concentration of Hg2+

ions (1×10−4 M). As mentioned above, herein an increased amountof Hg deposit with increased deposition time causes a shift of thestripping potentials of Hg to more positive values according to theNernst equation.

A simultaneous tracing of multiple elements is carried out in the1×10−4 M Pb2++9×10−5 M Hg2++0.1 M NaCl solution (pH 1) as afunction of deposition time with the resulted stripping voltammo-grams of Pb and Hg shown in Fig. 5a in which the Hg stripping peaksare pronounced than the Pb ones. The deposition potential and scanrate used are −1.2 V and 36 mV/s, respectively. A near linearrelationship between the stripping peak current and depositiontime for both Pb and Hg is observed in Fig. 5b. Though theconcentration of Pb2+ ions is larger than that of Hg2+ ions, thestripping peak currents of Hg vs. deposition time are consistentlyhigher than those of Pb (Fig. 5b), which implies that the ta-C:N filmelectrode used has a higher sensitivity to Hg2+ than to Pb2+ in theNaCl solution because Pb has a smaller deposition overpotentialcompared to that of mercury at the same deposition potential of

−1.2 V [20]. Mercury's tendency towards oxidization would havecontributed to the sensitivity of the ta-C:N film electrode to Hg2+

during the multi-element tracing. Because different elements havedifferent redox potentials, their stripping potentials are also expectedto be different, e.g., the stripping potentials of Pb and Hg are foundfrom −454 to −411 mV and from 133 to 175 mV, respectively,depending on the deposition time as shown in Fig. 5c. A simultaneousdetection of Pb and Hg also shows the same phenomenon mentionedabove that increased deposition time shifts the stripping potentials ofboth metals to more positive values.

Fig. 6 shows the well-defined stripping voltammograms obtainedfrom a simultaneous detection of Cu and Hg in the 1×10−4 M Cu2++9.3×10−5 M Hg2++0.1 M NaCl solution (pH 1) as a function ofdeposition time, where the deposition potential and scan rate usedare−1.2 V and 36 mV/s, respectively. Although a higher concentration ofCu2+ ions than that of Hg2+ ions is used during accumulation, the

Fig. 5. Stripping voltammograms (a), stripping peak currents (b), and stripping peak potentials (c) obtained from ta-C:N film electrodes in 1×10−4 M Pb2++9×10−5 M Hg2++0.1 MNaCl solution (pH 1) vs. deposition time. The deposition potential and scan rate are −1.2 V and 36 mV/s, respectively.

Fig. 6. Stripping voltammograms obtained from ta-C:N film electrodes in 1×10−4 MCu2++9.3×10−5 M Hg2++0.1 M NaCl solution (pH 1) against deposition time. Thedeposition potential and scan rate are −1.2 V and 36 mV/s, respectively.



sensitivity of the ta-C:N film electrode to the Cu2+ ions is much lowerthan that to the Hg2+ ions due to stabilization of Cu(I) by forming CuCl−2

through the following electrochemical mechanism [21]:

Cu0↔Cu

2þ þ 2e− [22]

Cu2þ þ 2Cl

−↔CuCl−2 [23].

In addition, a larger deposition overpotential and self oxidizationof Hg are themain reasons contributing to the higher sensitivity of theta-C:N film electrode to the Hg2+ ions.

The different redox potentials of Cu and Hg result in the strippingpotentials of Cu at more negative values (−0.114 V to −0.072 V)compared to those of Hg (0.147 to 0.179 V) (Fig. 6). The strippingvoltammograms recorded from the simultaneous analysis of multi-elements (Cu and Hg) show the third peaks that are the shoulders ofthe Hg peaks and might have resulted from the formation of theamalgamated Cu–Hg [24].

4. Conclusions

The experimental results showed that the ta-C:N film electrodesweresuitable for the determination of single elements such as Pb2+ and Hg2+

and simultaneousmulti-elements suchasPb2++Hg2+andCu2++Hg2+.It was observed that the sensitivities of the ta-C:N film electrodes to theseelements were apparently influenced by deposition time, depositionpotential, concentrations of both trace metal ions and electrolytes insolutions, pHvalue of solutions, and scan rate. The simultaneous tracing ofPb2++Hg2+ and Cu2++Hg2+ using linear sweep anodic strippingvoltammetry produced well-defined stripping peaks with good peakseparations. Some other trace heavy metals such as Cd, Fe, Ni, As, Ag, andAu in aqueous solutions will be investigated with the ta-C:N filmelectrodes developed in this study in future works.

Acknowledgements

This work was supported by the research grant (EWI-0601-IRIS-035-00) from the Environment &Water Industry Development Council(EWI), Singapore. N.W. Khun is grateful for the PhDscholarship fromtheNanyang Technological University (NTU), Singapore.

References

[1] O.R. Awofolu, Z. Mbolekwa, V. Mtshemla, O.S. Fatoki, Water SA. 31 (2005) 87.[2] E.G. Pacyna, J.M. Pacyna, F. Steenhuisen, S. Wilson, Atmos. Environ. 40 (2006)

4048.[3] C.H. Ngim, S.C. Foo, K.W. Boey, J. Jeyaratnam, Brit. J. Ind. Med. 49 (1992) 782.[4] Y.X. Liang, R.K. Sun, Z.Q. Chen, L.H. Li, Environ. Res. 60 (1992) 320.[5] R.B. McFarland, H. Reigel, J. Occup. Med. 20 (1978) 532.[6] M. Hupert, A. Muck, J. Wang, J. Stotter, Z. Cvackova, S. Haymond, Y. Show, G.M.

Swain, Diam. Relat. Mater. 12 (2003) 1940.[7] J.W. Strojek, M.C. Granger, G.M. Swain, T. Dallas, M.W. Holtz, Anal. Chem. 405

(1996) 95.[8] N. Mizuochi, M. Ogura, H. Watanabe, J. Isoya, H. Okushi, S. Yamasaki, Diam. Relat.

Mater. 11 (2004) 2096.[9] A. Fujishima, C. Terashima, K. Honda, B.V. Sarada, T.N. Rao, New Diam. Front. C.

Tech. 12 (2002) 73.[10] A. Zeng, E. Liu, S.N. Tan, S. Zhang, J. Gao, Electroanal 18 (2002) 1294.[11] N.W. Khun, E. Liu, X.T. Zeng, Corros. Sci. 51 (2009) 2158.[12] L.X. Liu, E. Liu, Surf. Coat. Tech. 198 (2005) 189.[13] J. Wang, P.A.M. Farias, J.S. Mahmoud, Anal. Chim. Acta 171 (1985) 215.[14] H.B.P. Ferreira, T.A.F. De Lima, P.J.S. Barbeira, Electroanal. 20 (2008) 390.[15] N.W. Khun, E. Liu, H.W. Guo, Electroanal. 17 (2008) 1851.[16] N.W. Khun, E. Liu, Electrochim. Acta 54 (2009) 2890.[17] C.M.A. Brett, A.M. Oliveira Brett, Electroanalysis, Oxford University Press, 2005.[18] M.A. Baldo, C. Bragato, G.A.Mazzocchin, S. Daniele, Electrochim.Acta 43 (1998) 3413.[19] W.S. Tait, An Introduction to Electrochemical Corrosion Testing for Practicing

Engineers and Scientists, PairODacs Publications, USA, 1994.[20] T. Lee, K.C. Chung, J. Park, Electroanal. 14 (2002) 833.[21] K. Wikiel, J. Osteryoung, Electrochim. Acta. 15 (1993) 2291.[22] M.L. Tercier, J. Buffle, Electroanal. 5 (1993) 187.[23] K.R. Wehmeyer, R.M. Wightman, Anal. Chem. 57 (1985) 1989.[24] J.M. Zen, H.H. Chung, A.S. Kumar, Anal. Chim. Acta. 421 (2000) 189.


author's personal copy - ntu singapore nw_tsf2010.pdf · ta-c:n lm electrodes in the nacl...

Documents