Feature Article

Electrodeposition of Lead at Boron-Doped Diamond FilmElectrodes: Effect of TemperatureCe¬sar Prado,a Shelley J. Wilkins,a Peter Gr¸ndler,b Frank Marken,c Richard G. Compton*a

a Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK* e-mail: compton@chemistry.ox.ac.ukb Universit‰t Rostock, FB Chemie, Abt. f¸r Analytische, Technische und Umweltchemie, Albert Einstein Str. 3a D-18051 Rostock,Germany

c Department of Chemistry. Loughborough University.Loughborough, LE11 3TU, UK

Received: July 26, 2002Final version: October 3, 2002

AbstractThe electrodeposition of lead on boron-doped diamond has been studied with a view to identifying the fundamentalparameters controlling the sensitivity and lower detection limit in anodic stripping voltammetry. Chronoampero-metric transients are used to explore the deposition, indicating a progressive growth mechanism confirmed by ex situAFM images. Linear sweep ASV experiments show a threshold concentration of ca 10�6 M below which no lead isdetected; this is attributed to the need for nucleation of the solid phase on the electrode. Experiments with variabletemperature show that this threshold can be usefully lowered at elevated temperatures.

Keywords: Boron-doped diamond, Lead, Electrodeposition, Temperature, Nucleation and growth

1. Introduction

Lead has been noted as one of the most dangerous heavymetals due to its long-term toxicity. A number of electro-chemical techniques have been proposed for its directdetection in aqueous solutions of environmental samples,based on electrodeposition of the reducedmetal [1 ± 4] or itsoxides [3, 5, 6]. These have proved very suitable for traceanalysis and are compatible with portable equipment [7]such as is desirable in environmentalmonitoring.The cost ofthe equipment is typically low compared to other techniquesused such as atomic absorbance spectroscopy or ICP-MS. Inthis article we investigate the electrodeposition of lead onboron-doped diamond electrodes from the point of view ofidentifying the factors controlling the sensitivity of anodicstripping voltammetry for the detection and determinationof lead. In particular we note that while a good linearrelationship is seen between stripping charge and concen-tration at ™high∫ levels, at lower values there exists a™threshold concentration∫ below which no lead can bedetected. This is attributed to the need for a lead phase tonucleate on the electrode surface and presents a fundamen-tal limitation to the sensitivity to the ASVexperiment. Thefactors controlling this threshold are explored in this article.Highly boron-doped diamond (BDD) has been success-

fully employed recently for the direct electrochemicaldetection of lead [1, 3 ±5]. BDD is a relatively new electrodematerial, which has gained in popularity recently as a higheffective substrate for a number of electrochemical applica-tions [8 ±12]. BDD is both structurally and chemically robust[13]. and suffers from very low levels of backgroundinterference [14] allowing it to be useful over awide potential

window (ca. 3.5 V) in aqueous media. It is for these reasonsthat interest in thematerial for use in various electrochemicalprocesses has been growing steadily since the middle of the1980s [15]. These unique properties of BDD make it ideallysuited for stripping voltammetry analysis of Pb, Mn, Cd, CuandAg [2, 3, 5, 6, 13, 16] particularly in termsof eliminationofbackground current interferences.

2. Experimental

A standard solution containing 0.1 M Pb2� in 0.1 M HNO3

was prepared from its nitrate analytical-reagent grade saltsupplied by Aldrich and used without any further purifica-tion. Solutions were prepared using UHQ grade water, ofresistivity not less than 18 M� cm (Elgastat, High Wy-combe, UK). Pure nitrogen was bubbled into the cell inorder to deoxygenate the solution, and the latter wasblanketed with N2 during experiments.A two-arm glass-bodied 40 mL electrochemical cell, with

a screw-thread removable base to allow insertion of theworking electrode, was used for all electrochemical experi-ments. The temperature of the cell was kept constant with acoil connected to a thermostated bath, giving a controlledtemperature with an error of � 1 �C. A polished BDD film(5� 5� 0.535 mm3, resistivity 0.4 � cm) grown by chemicalvapor deposition (CVD), was supplied by DeBeers Indus-trial DiamondDivision (Ascot, UK), and housed in a Teflonmounting with an electrical connection to the rear side via abrass rod attached using silver epoxy resin (RS). The rear ofthe electrode assemblywas enclosed using a sealantwax andthe unit was placed at the bottom of the cell.

1011

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All electrochemical experiments were performed using acomputer controlled �-AUTOLAB potentiostat (Eco-Chemie, Utrecht, Netherlands), with a standard threeelectrode system consisting of a BDD film working elec-trode, a spiral-wound platinum wire as counter electrode(flame annealed before every experiment). The referenceelectrode used was a saturated calomel electrode, SCE(Radiometer, Copenhagen), kept at 20 �C and connected tothe cell via a salt bridge. A pretreatment was applied to theBDDelectrode consisting on polishing using diamond paste1 �m and then activation by cycling at 0.1 V s�1 between 0.0and 5.0 Vunder vigorous stirring in 1 MHNO3 solution untila stable signal was obtained [17].A Topometrix TMX 2010 Discoverer atomic force

microscope operating in contact mode, was employed toimage the BDD surface and lead deposits, with SFM probestype 1520-00 and 75 �m scanner type 5590-00.

3. Results and Discussion

Cyclic voltammetry was applied to determine the electro-chemical behavior of lead ions in 0.1M HNO3 solution at aBDD film electrode. Cyclic voltammograms were recordedover the range of potentials between � 1.0 and � 1.75 Vat ascan rate of 50 mV s�1. As shown in Figure 1, cyclicvoltammetry of 10�4 M Pb2� solutions reveals that thedeposition current commences near � 0.75 Von scanning inthe negative direction, but continues flowing at less negativepotentials when scanning anodically. A current crossover istherefore observed. Such voltammetric behavior is charac-teristic of a nucleation and growth mechanism [3, 15, 18] inwhich the initial formation of Pb nuclei on the diamondsurface serves to increase the rate of further deposition.Thus on the return scan the cathodic current flows at lessnegative potentials than these required to initiate deposi-tion. Continued scanning in the positive direction yields astripping peak (oxidation) peak at � 0.475 V (vs. SCE).

Next the temperature was raised to 60 �C and analogouscyclic voltammograms were recorded while maintaining thereference electrode at 20 �C (see Section 2). The effect of thetemperature on the cyclic voltammograms is shown inFigure 1: although qualitatively similar in shape, the curvesreveal that the beginning of the deposition current is shiftedpositively to near � 0.70 V and the deposition currentincreases. The subsequent current crossover on the returnscan is found to be also shifted positively with respect to thelower temperature: from � 0.515 to � 0.500 V (vs. SCEmaintained at 20 �C) and, finally, the stripping signal is foundto be substantially increased by approximately a factor often.The cross-over point in voltammograms such as those in

Figure 1 is directly related to the formal reduction potentialE�f [18] of lead and can therefore be used as ameasure of theshift of the latter with temperature, as shown in Table 1.Knowledge of the formal reduction potential found in thisway as a function of temperature allows us to apply acontrolled or fixed overpotential (potential relative to theformal potential E�f) during subsequent electrochemicalmeasurements, for example in comparing chrono-ampero-metric transients at two different temperatures.We next considered the effect of temperature on the

stripping peak. Quantification of the ion was performed byfirst reducing and depositing the metal from a 40 cm3

volume of nitrogen-degassed solution at 20 �C, at a fixedoverpotential of 500 mV more negative than the formalpotential for the reduction of lead (the crossing pointpotential) over a period of 300 s. The potential was thenscannedpositive at a rate of 50 mVs�1 to obtain the strippingpeak. The area below the anodic stripping peak wascalculated and, as can be observed in Figure 2, above ca10�4 M a linear dependence of the charge with concentra-tion up to 10�3 M was found. Moreover, a ™thresholdconcentration∫ is required before any stripping is seen. Sucha threshold places a clear limit on the sensitivity of theanalytical uses of ASV via linear sweep voltammetry. Wenote however that the use of differential pulse [4] or squarewave techniques can reduce the limitation in analyticalapplications; for example a detection limit of 4� 10�7 M isnoted in [4]. However, in the present article we focus on thelinear sweep approach for fundamental insights.Figure 2 shows how the sensitivity of anodic stripping

detection of lead is favored at higher temperature. Whenincreasing the temperature from 20 �C to 60 �C and applyinga constant overpotential of 500 mV, a ten-fold increase inthe amount of lead deposited and a reduction in thedetection limit, measured as 3 times the standard deviation,from 3.0� 10�6 to 1.6� 10�6 M were found. Also, the rise intemperature led to a considerable decrease in the linear

Fig. 1. Cyclic voltammograms of a 10�4 M solution of Pb2� in0.1 M HNO3 at a BDD electrode (25 mm2), at 20 ( ) and 60 �C(- - -). Scan rate 50 mV s�1.

Table 1. Variation of the cross-over potential (vs. SCE) (at 20 �C)with temperature.

T (�C� 1 �C) 20 40 60Eo

f (mV� 2 mV) � 515 � 512 � 505

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Electroanalysis 2003, 15, No. 12

sweep ASV threshold concentration from 1.2� 10�6 to6.5� 10�7 M.Next the topology of the metallic deposits was studied ex

situ by AFM for qualitative insight with the electrodeposi-tion mechanism noting the possibility of contact AFMleading to the removal or redistribution of deposits duringthe imaging process and of surface contamination arisingfrom the ex-situ imaging. In Figure 3a and 3b the imagesobtained respectively at 20 and 60 �C are shown for a 10�6 MPb2� solution after deposition at a potential 500 mV morenegative than the formal potential over 300 s. Isolateddeposits of different sizes could be observed: the measurednumber of nuclei in both images is similar (ca. 200 within20� 20 �m) but there is a small but significant variation inthe size of thesewith temperature. These results suggest thata progressive nucleation and growth mechanism of electro-deposition of Pb on diamond is involved. The maximumheight of the lead deposits changes from1.95 to 2.85 nm, andthemean height from 0.80 to 1.20 nmwhen the temperatureis raised from20 to 60 �C. Similar studieswere performed fora 2.5� 10�6 M and 10�5 M concentration of Pb2� and shownin Figures 3c ± f. After applying an overpotential of 500 mVduring different deposition times it could be observed thatisolated deposits were present either for 2.5� 10�6 (Fig. 3c)and 10�5 M (Fig. 3e). However, after a deposition time of300 s the whole electrodic BDD surface appears to becovered by a layer of lead because of the coalescence of the

nuclei, incipient for the 2.5� 10�6 M (Fig. 3d) and completein the case of the higher concentration 10�5 M (Fig. 3f). Thissuggests that further reduction and deposition of Pb2� ionstakes place on lead and not on diamond substrate, which isconsistent with the difference in deposition rate observedabove or below the concentration of 10�5 M as apparent inFigure 2.To gain insight into the nature of the electrodeposition

process and hence both the temperature and thresholdeffects, current-time responses were next measured [19 ±32]. Specifically, chronoamperometric transients were reg-istered after a step potential perturbation from 0.0 V (vs.SCE), where no reduction current flows, to overpotentialsbetween 200and500 mV(every 50 mV), in the regionwherethe metal cations are reduced and deposited. In Figure 4transients for 10�6, 10�5 and 10�4 M Pb2� solutions at 20 �Cand 10�4 M Pb2� at 40 �C are shown. The transients showdifferent sections: First of all an initial decay which in somecase extends over ca. 10 s, and ultimately decayes to an off-set steady value can be observed. This is likely related toearly stages of hydrogen evolution on the electrodicdiamond surface at negative potentials. Figure 4a showeda concentration of 10�6 M, near the threshold below whichthe stripping signal does not appear, and no measurablechange could be observed with respect to the chronoam-perometric signal of the blank. With increasing concentra-tion a rise in the intensity of the current due to electro-deposition is observed. Finally, when the concentration wasincreased up to 10�4 M the curves showed a shapewith a fastrise and the presence of amaximumwhich is better resolvedat higher overpotentials. This shape is typical of nucleationand growth mechanisms. When the temperature wasincreased, a rise in the intensity of the current measuredwas observed. In the case of the 10�4 M concentration a shiftin the position of the maximum to shorter times was seen.These transients were analyzed on the basis of the models

that describe the differentmechanisms of electrodeposition,so that the different behaviors could be explained. For highoverpotentials such as those used for the deposition step inASVanalysis, the electrodeposition can be explained simplyas a diffusion controlled nucleation and growth process. Theeffect of coalescence (overlapping of the nuclei) wasintroduced by Gunawardema [25, 29 ± 33] and the conceptwas lately developed by Scharifker and Mostany [27, 34].who deduced the expression for the transient at constantoverpotential. The current due to the electrodeposition isdefined by theCottrell equation for a planar diffusive flux toan electrode of area �:

i(t)� zFcD1/2(�t)�1/2� (1)

where the area covered, �, by the diffusion zones corre-sponding to the different growing nuclei, including over-lapping, comes defined by:

�� 1� exp{�N0�kD[t� (1� e�Ar]} (2)

and therefore

Fig. 2. Stripping peak area vs. concentration plot for Pb2�.Anodic scans beginning at � 1.005 V (vs. SCE) (�� 500 mV) at50 mV/s, after depositing during 300 s at a constant overpotentialof 500 mV at 20 �C (�), 40 �C (�) and 60 �C (�). Inset: Strippingsignal vs. Pb2� concentration at 20 �C in the same conditions,extended up to 10�3 M.

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Electroanalysis 2003, 15, No. 12

Fig. 3. AFM images at a constant over-potential of 500 mV of a 10�6 M solution ofPb2� after 300 s deposition at a) 20 �C andb) 60 �C. A 2.5� 10�6 M solution of Pb2� at20 �C after c) 100 and d) 300 s, and a 10�5 Msolution at 20 �C after e) 100 and f) 300 s.For the cases of a) and b) the distributionsof heights are also shown.

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Electroanalysis 2003, 15, No. 12

i t� � � a 1��t

� 1� exp �� At � 1� e�At� �� �� � �3�

where

a� zFcD1/2��1/2 (4)

is the diffusive flux to the electrode, in A s1/2 cm�2 , A is thenucleation rate per active site in s�1 and

��N0�kD/A (5)

is a dimensionless parameter including both the nucleationrate, A, and k� (8�cM/�)1/2 the dimensionless constantaffecting growth rate of diffusion zones, along with N0 thenumber density of active sites in cm�2. z stands for ioniccharge,F the Faraday constant, C mol�1, c the concentrationof the deposited ion in the bulk, mol cm�3,D is the diffusioncoefficient, cm2 s�1 and � the density in g cm�3.Figure 5 shows the fitting of the experimental data of a

10�4 M concentration of Pb2� at an overpotential of 450 mVat 20 and 40 �C, and good agreement can be observed (R�0.999). The parameters obtained from the fitting are shownin Table 2. Avalue of 9.6� 10�6 cm2 s�1 [35] for the diffusioncoefficient of lead in aqueous solution at 20 �Cwas used. Thediffusion coefficient for the different temperatures wascalculated on the basis of simple Arrhenius behavior:

Fig. 4. Chronoamperometric transients after a step potentialperturbation from 0.0 V to overpotentials from 200 (lower) to450 mV (higher), step potential 50 mV, for a) 10�6 M Pb2� at 20 �C,b) 10�5 M Pb2� at 20 �C, c) 10�4 M Pb2� at 20 �C and d) 10�4M Pb2�

at 40 �C.

Fig. 5. The chronoamperometric transients were recorded using a potentiostatic step corresponding to an overpotential of 500 mVagainst E�f and Equation 3 was used for the fitting. 10�4 M solution at a) 20 and b) 40 �C.

Table 2. Nucleation Rate, A, and N0 values for 10�4 M solution atan overpotential of 500 mV.

T (�C) A (s�1) N0 (cm�2)

18 0.48 1.3� 107

20 0.81 1.4� 107

30 0.89 1.3� 107

35 1.1 1.4� 107

40 1.7 1.4� 107

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Electroanalysis 2003, 15, No. 12

DT � D293 exp�EAR

1293

� 1T

� ��6�

with a diffusion activation energy of 22.5 kJ mol�1 [36, 37].Taking this into account, the results of the analysis are showninTable 2. It canbeobserved that there is a small but evidentincrease in the nucleation rate with temperature. AnArrhenius plot relating lnA and T�1 was linear and givesthe activation energy of 42 kJ mol�1. On the other hand theincreasing temperature does not lead any significant changeinN0, consistent withwhat is observed in theAFM images inFigure 3, taken after 300 s deposition when the active siteshave all been covered by nuclei. It follows that the majoreffect induced by the rise in temperature is on the masstransport, that is, in the diffusion rate which almost doublesits value. A second effect is on the rate of nucleation. Theseincreases lead to a significant rise of the amount of metaldeposited, in agreement with the electrochemical resultsobtained for the increase in the stripping signal.

4. Conclusions

Temperature has a substantial effect on the electrodeposi-tion of metallic lead and its subsequent anodic stripping,increasing the amount of metal cations reduced anddeposited. Although the detection limit is lowered signifi-cantly, at higher temperatures a threshold concentrationbelow which no linear sweep ASV can be observed is stillfound.AFM images of the lead deposits at different temper-

atures show that the number of nuclei does not changesignificantly but at higher temperature the size of thesenuclei is bigger and coalescence of the clusters can beobserved. Increasing concentration leads to a smoothsurface completely covered with Pb.Two limiting regimes can be depicted from the combined

AFM and current transients: at the lower concentrationslead ions are reduced and deposited directly on the carbonsubstrate on isolated clusters in a progressive growthprocess. Meanwhile, at higher concentrations coalescenceof the deposits occurs and the diamond surface is coveredcompletely by lead, so that further deposition occurs not ondiamond substrate but on lead. The differences in thedeposition rate of these two regimes provide an explanationfor the appearance of two linear regimes in the ASV peakarea vs. concentration plots, above the threshold concen-tration.

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