Electrochemical Characterization of PbS Quantum Dots Capped With Oleic Acid and PbS t

ISSN 1068�3755, Surface Engineering and Applied Electrochemistry, 2012, Vol. 48, No. 3, pp. 193–211. © Allerton Press, Inc., 2012.

193

1 INTRODUCTION

Semiconductor nanocrystals or quantum dots(QDs) exhibit electronic, optical, photochemical andphotophysical properties greatly differing from thoseobserved in corresponding bulk materials due to quan�tum size effects [1]. Transition metal chalcogenidesare important semiconductor materials, especially onthe nanoscale level because of their excellent photo�electron transformation properties and potentialapplication in physics, chemistry, biology, materialsscience, etc. [2–7].

Among conventional semiconductors, PbS is par�ticularly important because of its direct and narrowband gap (0.41 eV at 300 K), a large exciton Bohrradius of 20 nm, diverse morphologies [1] and almostequal effective masses of electrons, , and holes, [8]. The band gap blueshifts to the spectral region of700–1500 nm (1.77–0.82 eV) upon the decrease in thenanocrystal diameter below the size of the excitonicBohr radius due to quantum confinement effect.These properties of PbS quantum dots (Q�PbS) con�dition their use in a wide variety of photonic devices,electrochromic films, including light�emitting diodes

1 The article is published in the original.

me* mh*

[3, 9], infrared photodetectors [4–6] and photovoltaicdevices [7].

The advantage of electrochemical studies of QDsconsists in the fact that charge transfer events can beregistered and characterized in terms of their energet�ics (peak potentials) and intensity (currents). Theabove mentioned advanced applications of QDsinvolve charge transfer to/from nanoparticles from/toa metal electrode or a conducting polymer or otherenvironment. Hence a preliminary study of this phe�nomenon is needed.

There are numerous reports on investigation andprobing semiconductor properties of various QDs byelectrochemistry (see reviews [10, 11]). The absolutepositions (that is, relative to the vacuum scale) of con�duction and valence band edges as well as defect statesare necessary for photovoltaic application wherein thematch between the QD energy levels and those of thesurrounding conducting polymers or other supports iscrucial [12, 13]. Photoluminescence (PL) and absorp�tion spectroscopy provide only the difference betweenthe band edges whereas the absolute positions can beaccessed by X�ray photoelectron spectroscopy (XPS)and the ultraviolet photoelectron spectroscopy [10].However, the respective equipment is costly. Cyclic

Electrochemical Characterization of PbS Quantum Dots Capped with Oleic Acid and PbS Thin Films – a Comparative Study1

A. S. Cuharuc, L. L. Kulyuk, R. I. Lascova, A. A. Mitioglu, and A. I. DikusarInstitute of Applied Physics, Academy of Sciences of Moldova, 5 Academiei str., Chisinau, MD�2028, Republic of Moldova

e�mail: [email protected] August 2, 2011

Abstract—This research aims at probing electrochemical response of oleic acid capped PbS quantum dotsdeposited on a Pt electrode in 0.1 M aqueous sodium hydroxide solution by cyclic voltammetry and chrono�amperometry. Quantum dots were also characterized by photoluminescence and IR spectroscopy. Cyclic vol�tammetry of bulk PbS thin films obtained via chemical bath deposition is investigated in order to interpret thedata on PbS quantum dots. It was found that the two materials exhibit essentially similar voltammetric behav�ior; however, oxidation of PbS quantum dots tends to start at slightly more cathodic potentials than that ofbulk PbS films. This effect is attributed to the influence of capping oleic acid that binds lead ions into an insol�uble lead oleate thereby causing the cathodic shift of the formal redox potential. The procedures designed topartially remove oleic acid from PbS quantum dots result in the anodic shift of the PbS quantum dots oxida�tion towards the values characteristic for the bulk material. A possibility of determining absolute positions ofconduction and valence bands in quantum dots by cyclic voltammetry is discussed but the influence of theenergy level structure of PbS quantum dots on their voltammetric response was not revealed under the con�ditions of the present study. However, PbS quantum dots can withstand multiple redox cycles whereas bulkPbS films dissolve readily upon the first oxidation. The effect was attributed to the oleic acid layers on the PbSquantum dots surface, those layers preventing soluble oxidation products from diffusing into the bulk of solu�tion. Certain PbS quantum dots samples showed remarkable stability against oxidation typical for PbS andstarting at –0.2 V vs Ag/AgCl (sat.) and a stable response at oxidation and reduction at higher (0.55 V) andlower (–0.8 V) potentials, respectively.

DOI: 10.3103/S1068375512030040

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CUHARUC et al.

voltammetry (CV) offers a very tempting opportunityof measuring the absolute energy levels in semicon�ductor QDs as the experimental arrangement is verysimple and the cost of equipment is not very high.Although all measurements are carried out withrespect to a reference electrode, the absolute valuescan always be obtained from estimation of the poten�tial of the normal hydrogen electrode with respect tothe vacuum scale (–4.35 eV [14]) and, certainly, thepotential of the reference electrode in use.

Many authors report on a successful correlationbetween QD energy levels determined by CV and theirdifference, the energy gap, found from absorption orPL measurements [10, 15–19]. In our opinion thiscorrelation is not always unambiguous, possibly due tosmall ranges of QD sizes and/or insufficient amount ofthe sizes tested. Most probably, a very thorough andconvincing investigation was performed by Inamdarwith co�authors [18] wherein optical band gaps of Q�CdSe were compared with quasi�particle band gapsdetermined from the difference between the oxidationand reduction peaks corresponding to the valence andconduction bands of QDs, respectively.

Regardless of the fact whether the correlationbetween optical band gaps and electrochemical dataare consistent or not, one thing is obvious from all thestudies: the medium where measurements are per�formed critically affects the voltammetric response.Organic solvents are preferred due to their broadpotential window available as compared to aqueousmedia; however, the use of ionic liquids with a phasetransfer detergent accounts for very informative volta�mmograms, with band edges and defect states clearlydefined (Q�CdSe in [10]).

In electrochemical experiments QDs are often pre�pared as a thin film on a working electrode. The con�ductivity of such a film may be very poor as large cap�ping organic molecules that normally constitute theprotective/passivating layer hinder charge transferand, therefore, no useful data in voltammograms willbe acquired as indicated by Guyot�Sionnest [20].Those molecules can be substituted for shorter ones bysimple immersion of a dried film into a solution ofshort�chain dithiols [21] or diamines and dithiols [22].Such molecules serve as cross�links for nanocrystals,decreasing the distance between the nanocrystals.

In some cases cyclic voltammograms may not reg�ister charge transfer events in the form of peaks as isnormally expected. A practically featureless voltam�mogram was acquired in [21] for Q�PbSe capped with1,6�hexanedithiol. Nonetheless, the electron and holeinjection into the QDs under study was clearly regis�tered by changes in the IR absorption spectra duringapplication of potential, and respective electron tran�sitions were assigned to spectral bands. Therefore,electrochemical techniques alone may not be suffi�cient to track the events occurring in nanocrystals.

The reports devoted to electrochemical studies ofQ�PbS are not very numerous [13, 23–26], withChen’s et al. work being probably the most cited[11, 23]. CV of Q�PbS of several sizes was presented in[13], where the authors demonstrated the shift of thereduction peak that correlated to the QD size, withlarger particles being reduced at less negative poten�tials. However, a systematic study of voltammogramsand electrochemistry of QDs was not done, probablybecause the CV analysis was not the main objective ofthe last cited research. Other researchers dealt withQ�PbS of only one certain size [23, 25, 26]; and if onecompares the voltammograms presented therein, thequalitative difference will be found although the testedparticles were of roughly the same size (3–4 nm).Reproducibility of voltammetric data was also a chal�lenge in the present research as will be shown further.

Herein we report on CV of the synthesized Q�PbScapped with oleic acid (OA) and compare the acquireddata with CV of the PbS film deposited onto a Pt elec�trode as a representative of the bulk material. A simpleexplanation of the voltammetric data is attemptedbased on the electrochemistry of semiconductors. Thequantum confinement effect in the synthesized prod�uct is confirmed by PL spectra. We emphasizepostsynthesis purification of Q�PbS and demonstrateits relevance for the voltammetric behaviour of the QDfilms. FT IR spectroscopy provides evidence on thestructure of a QD as a core�shell system.

EXPERIMENTAL

Reagents

Lead (II) oxide (99.999%), oleic acid (OA, techni�cal grade, >90%), octadecene�1 (ODE, technicalgrade, > 90%), bis(trimethylsilyl)sulphur ((TMS)2S)p.p.a., sodium citrate tribasic dehydrate (ACSreagent), lead (II) acetate trihydrate (puriss. p.a.) andthiourea (puriss.) were purchased form Aldrich andchloroform (HPLC grade), sodium hydroxide (puriss.p.a.), methanol (puriss. p.a.), sulfuric acid (puriss.p.a.) and acetone (puriss.) – from Riedel�de Haen andused without further purification. All aqueous solu�tions were prepared with triple�distilled water.

Synthesis and Purification of Q�PbS

The synthesis procedure described below basicallyfollows that in [27]. In a three�neck flask equippedwith a thermometer, an inert gas supply tube and amagnetic bar, 0.49 g of PbO were combined with 5.0 gof OA under Ar atmosphere. The flask was heated andmaintained at 150°C for 1 hour. These conditionsallow removing some water formed as the reaction by�product. Afterwards, the flask was allowed to cooldown to a certain temperature (indicated further in thetext), depending on the desired size of the QDs inquestion, and the solution of (TMS)2S in ODE (0.22 g


ELECTROCHEMICAL CHARACTERIZATION OF PbS QUANTUM DOTS 195

of (TMS)2S and 2.6 g of ODE) was injected into theflask with a syringe through a rubber septum, with thereaction mixture being intensively stirred. In 10 s anice bath was placed under the flask to quench the reac�tion. The reaction products were dissolved in a mini�mum amount of chloroform.

Purification of the target product was performedvia precipitation with methanol and re�dispersion inchloroform. Specifically, a quantity of methanolneeded to cause the QDs coagulation was added to thechloroformic solution followed by centrifugation. Theprecipitate was separated, washed with methanol andredispersed in a minimum amount of chloroform.This cycle was repeated four times. It was noted that anincreasingly larger amount of methanol was requiredto cause the precipitation. This procedure will bereferred to in the main text as ‘basic purification’.

Further purification is based on treatment with ace�tone and NaOH solution. Specifically, acetone wasadded to cloroformic solution of the QDs studied (4 : 1)to cause the coagulation and the mixture was refluxedunder Ar atmosphere for 1/2 hour. The precipitate wasremoved by centrifugation and re�dispersed in chloro�form and stored under Ar. The acetone extract being thepurification by�product was preserved for IR analysis.

Then treatment in NaOH followed. Substitution ofhexane for chloroform: from a certain amount ofQ�PbS treated in acetone, chloroform was evaporatedin vacuum and nanocrystals were re�dispersed in hex�ane since it is more volatile and does not react withNaOH. The solution in hexane was added to aqueous0.5 M NaOH solution preheated to 50°C under Aratmosphere. The mixture was gently stirred for 15 min,cooled down and centrifuged. The precipitate wasthoroughly washed with acetone, dried and mixedwith chloroform. The prepared product turned outinsoluble in organic solvents, yielding unstable sus�pensions. The alkaline extract was acidified withdiluted H2SO4, and the matter poorly soluble in aque�ous medium was extracted twice with chloroform andpreserved for the IR analysis. It will be referred to as“chloroform extract” in the main text.

The sample notation given below indicates the syn�thesis temperature, and purification procedure andwill be used throughout the text for Q�PbS designationprepared under various conditions.

Prefix Synthesis tem�perature, °C

Purification procedure Example

QD� 60, 100, 120 B � basic QD�120AC

AC � in acetone

SH � in sodium hydrox�ide

Synthesis of Lead (II) Oleate

0.4 g of PbO and 1.1 g of OA were mixed under inertatmosphere and kept at 150°C until all PbO dissolved.The reaction mixture was cooled down and dissolvedin chloroform. Lead oleate was precipitated withmethanol, filtered off, washed with methanol twiceand dried at 60°C in a moderate vacuum.

Chemical Bath Deposition (CBD) of PbS Films

In the synthesis procedure we largely departed fromthat in [28–30]. The deposited films demonstrateproperties of the bulk lead sulphide. According to [30],from which we basically borrow the experimentalparameters, the particle size in the deposits variedbetween 100–300 nm. The deposition time was notclearly revealed in that work; however, one can con�clude that the authors carried out the deposition untilthe full completion of the reaction. Belova et al. [28]report that some 30 min suffice for lead (II) to quit thesolution and deposit as PbS under the following con�ditions: pH = 12.5, citrate – 0.02 M, lead (II) – 0.005M and T = 323 K.

In the present study, the solution prepared for theCBD was 0.025 M in sodium citrate, 0.01 M in leadacetate and 0.05 M in thiourea. By adding NaOH,pH was adjusted to 11–12. Then the solution wasdeaerated and thermostated at 50°C for 1/2 hour. Thedeposition started when thiourea was added to thesolution of other components; at the same time, the Ptworking electrode was immersed in the reactionmedium. After 30–45 min a dark�grey mirror�likelayer of PbS formed on the electrode that was furtherremoved, thoroughly washed and transferred into thecell for electrochemical measurements with 0.1 MNaOH deaerated beforehand.

Electrochemical Measurements

A conventional one�compartment three�electrodecell connected to the potentiostat Parstat 2273 (Prince�ton Applied Research) was employed for the measure�ments. Pt wire electrodes served as working (apparentarea – 0.063, true area – 0.108 cm2, as determined byhydrogen adsorption [31]) and counter electrodes(apparent area 0.12 cm2); Ag/AgCl/KCl (sat.) wasused as a reference electrode. All the potentials cited inthe present study are referred to this electrode unlessotherwise stated. Potential sweeps reported hereinwere done at the rate of 25 mV/s unless otherwisestated. Both the cell made of Duran glass and elec�trodes were cleaned with hot sulphuric acid mixedwith concentrated H2O2. The working electrode wasfinally cleaned by cycling potential between –0.2 and+1.2 V in 0.5M H2SO4 at 400 mV/s for 5 min, with thecycling always finished at the double layer region onPt. The appearance of a tiny peak in the region of theoxidation of adsorbed hydrogen (voltammograms

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recorded at 50 mV/s), in addition to two other promi�nent peaks, served as the evidence of a very clean Ptsurface [32].

Colloidal solutions or suspensions of Q�PbS inchloroform were drop�cast onto the working electrodeto make up a noticeable layer and the solvent was evap�orated in a moderate vacuum. Unfortunately, theamount of the solution dried on the wire electrodecould not be determined or controlled due to the elec�trode shape. This circumstance did not allow a directcomparison of currents for different QD film deposi�tions. All the measurements with QD films and bulkPbS were carried out in aqueous 0.1 M NaOH deaer�ated with Ar (4–7 L/h) for at least 30 min prior theexperiment.2

IR Measurements

IR spectra were acquired with the Perkin ElmerFT�IR spectrometer Spectrum�100. All samples(including the extracts just mentioned) were preparedas films on a KBr glass. Pure KBr glass was used forbackground measurement.

Photoluminescence Measurements

PL measurements at room temperature were per�formed with a grating monochromator coupled to a

2 In separate experiments, we established that this time is suffi�cient to diminish oxygen content by a factor of 30. Purging forlonger times did not improve this value. The residual oxygencontent in our experiments (ca. 0.2 mg/mL) was almost thesame as reported in a special research [36]. PbS surface gradu�ally oxidizes in the presence of dissolved oxygen; this conditionsthe OCP values of the electrode made of bulk PbS [37].

cooled photomultiplier (PM) with S1 photocathodeor to an InGaAs photodiode using standard lock�indetection techniques. The optical excitation was pro�vided by the frequency�doubled YAG : Nd3+ laser (λL2

= 532 nm).

RESULTS AND DISCUSSION

Photoluminescence

The PL spectra of PbS nanocrystals prepared atvarious synthesis temperatures are given in Fig. 1. As isevident, PbS nanoparticles exhibit a bright band�edgeluminescence spectra with clear exciton peaks at roomtemperature located in the near�IR spectral range.According to [33], these relatively narrow spectralbands are attributed to the radiative recombination ofexcitons in QD. The spectral PL maximum of PbSnanoparticles shifts from about 800 to 1300 nm as thesynthesis temperature is increased, which indicatesthe growth of Q�PbS.

Assuming parabolic bands to the first approxima�tion (the effective mass approximation) for a semicon�ductor nanocrystal with the radius R, the band gapenergy, ΔEg, due to quantization effects in the conduc�tion and valence bands can be estimated through thetheoretical equation derived by Wang [34]:

(1)

where R is in nm, Eg = 0.41 eV and m* = 0.089 me isthe effective mass for PbS (me is an electron mass).

Formula (1) includes the basic physical principlesunderlying the increase in the band gap when the

ΔEg Eg2 2ћEg

m*��⎝ ⎠⎛ ⎞ π

R��⎝ ⎠⎛ ⎞

2

+1/2

=

01.0 1.2 1.4 1.6

0.2

0.4

0.6

0.8

1.0

15001400 900 80013001200 1100 1000Wavelength, nm

Inte

nsi

ty, a

.u.

Energy, eV

a b c a'

Fig. 1. Photoluminescence spectra of Q�PbS synthesized at different temperatures: (a) at 120°C; (a') the same sample as in a butrecorded with the PM detector; (b) at 100°C, the effect of purification procedure: basic purification (bold line), acetone treat�ment (dashed line) and NaOH treatment (dashed�dotted line); (c) at 60°C.



nanocrystalline size decreases but is not quantitativelycorrect for dimensions smaller than 10 nm. Using thedata from Fig. 1, we estimated a nanocrsytalline radiusof 4.4 nm, corresponding to hν = 1.56 eV, 5.5 nm forhν = 1.29 eV, 6.5 nm for hν = 1.109 eV and 7.8 nm forhν = 0.95 eV, respectively. According to (1) the bandgap energy rises for nanocrystals of smaller radii.However, it usually overestimates ΔEg because the max�imum of the PL spectra are shifted to lower energies rel�ative to the absorption spectra – the shift known asStokes shift [35]. An adjustable band gap of PbS opens away to a new class of applications [3–7, 9].

IR Spectroscopy

The study in this section was undertaken for threereasons. First, there was an idea that bulk material canbe admixed to Q�PbS as a result of coagulation duringpurification procedure. Second, there was a hope todraw certain conclusions concerning the ligand state.Third, the present investigation provides proof for

assumptions we initially made regarding the effect ofthe purification procedure.

Since bulk PbS band gap amounts to 0.41 eV [1], itsabsorption edge is expected to occur near 3300 cm–1.However, the spectrum of the QDs studied (Fig. 2)shows no edge absorption in that region, hence nobulk PbS is presented in the sample and the electro�chemical signals to be discussed below are entirely dueto Q�PbS.

The IR spectra of the QDs in our research can beeasy understood by comparing them to the spectra ofOA and lead oleate. Inspection of the region ca. 2800–3000 cm–1 reveals that the signal is due to the hydrocar�bon chain of OA and virtually identical to that of leadoleate. It is of importance that the broad line presentedin the spectrum of OA in the region 2400–3400 cm–1

caused by hydrogen bonding [38] (presumably inOA dimmers) is not observed in the QD and lead ole�ate spectra. Therefore, OA exists in the QDs investi�gated as a salt. Moreover, there are characteristicbands 1399 and 1428 cm–1 in the lead oleate spectrum,

500 1000 1500 2000 2500 3000 3500 4000Frequency, cm–1

500 1000 1500 2000 2500 3000 3500 4000

T, %25

722

13991429

v(s)CH2 2852

3038

2959 v(a)CH3

2925 v(a)CH2v(s) C=O in asalt

lead oleate

chloroformextract

r R(CH2)n,n > 3

ν RCH=CHR' trans

OA

QD�100AC

QD�100SH+

938d OH 1286

ν C–O1465d(s) CH2d(a) CH3

1712ν C=O

H�bonds

PbS

Fig. 2. IR transmission spectra of lead oleate, OA, the chloroform extract and two QD samples purified in acetone, QD�100AC,and in sodium hydroxide solution (enhanced treatment), QD�100SH+. The inset shows the proposed orientation of OA mole�cules on the PbS nanocrystal surface.

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CUHARUC et al.

assigned to symmetrical valence vibrations of C=Obond of a carboxylic group anion in a salt, and thesebands are also strong in the QD spectrum. In fact, theQD spectrum essentially coincides with that of leadoleate, with some lines attenuated. Based on thesedata, we consider that the carboxylic group of OA facesthe nanocrystal surface at the positions of lead atoms,thus forming a surface compound, lead oleate, andcompensating “dangling” valence of surface leadatoms (see the inset in Fig. 2). Such a conclusion wasalso made in [39] where Q�PbS capped with OA andTOPO were studied by XPS. Finally the signal(absorption edge) from PbS nanocrystals per se wasnot observed before 7800 cm–1 as expected for the syn�thesized QDs.

There are indications that ligands surrounding dis�solved QDs are in a certain equilibrium with the sol�vent [20]. To make up a complete picture on that issueone would have to undertake a separate research. Herewe can only qualitatively support the idea that under�lies the purification procedure. Namely, the treatmentof the QDs described in acetone and sodium hydrox�ide should reduce the amount of OA that either stilldrags after synthesis as an admixture or is chemicallycoordinated to the nanocrystal surface. The IR spec�trum of the QD sample treated with acetone(Q�100AC) is presented in Fig. 2 and was discussedabove. The IR spectrum of the sample Q�100SH+,treated with NaOH as described in the Experimentalsection but at a larger concentration of NaOH and

higher temperature, shows only very weak signals inthe region of valence and deformation vibrations ofthe hydrocarbon tail. This suggests that the treatmentwith NaOH removes OA from the QDs, thus bringingthem into a merger with each other. This can also beproved by the spectrum of chloroform extract (seeExperimental) which albeit distorted is essentially thatof OA (compare “OA” and “chloroform extract” tracesin Fig. 2). Practically the same spectrum was recordedfor the acetone extract (not shown), hence the samematter was extracted by both NaOH and acetone, andit is OA or its sodium salt.

ELECTROCHEMISTRY

Generally, the electrochemical research we carriedout revealed several aspects of Q�PbS electrochemistryin the conditions employed. Firstly, Q�PbS films showvery similar response to that of the bulk PbS thin films,in terms of energetics of the reactions � peak potentialson voltammetric curves. Secondly, certain differences(again, compared to the bulk PbS film) in kinetics ofthe main oxidation peak (see A1 in Fig. 3) have beenestablished for certain QD samples. Thirdly, this oxi�dation peak diminishes gradually and surprisingly evenincreases during subsequent potential cycles for theQD samples whereas it drastically drops with everynext cycle for the bulk PbS film. Fourthly, the oxida�tion and reduction peaks at the extremes of the poten�tial range available emerge as very sharp and stableones and appear independently of A1 for the QD sam�

–6

–0.2

–1.0 –0.5 0 0.5

0 0.2 0.4–0.5 0 0.5

–4–2

024

i, μA

A1

A3 A4 A2

C1

C2C3

C4C5

E, V

E, V

potential sweep:

–0.25→–0.9→+0.75→–0.9→+0.25V

Fig. 3. Schematic presentation of cyclic voltammograms of studied bulk PbS thin films. Potential sweep starts from –0.25 V (usuallocation of OCP) and then follows the direction given in the figure. The dashed line is the initial part of the second cycle. Theright inset shows the diversity of real experimental profiles of peak A1 and the left one � the cyclic voltammogram of polycrystal�line Pt in 0.1 MNaOH.



ples only. The discussion of the experimental data forbulk PbS films and the QDs will be given further in twoseparate sections.

The question whether CV can provide absolutepositions of band edges of Q�PbS was the main stimu�lus for the present research. We found out in the avail�able literature that this question cannot be answeredunequivocally. As mentioned in the Introduction,there are publications that demonstrated the power ofCV for probing band edges and surface states of QDs(CdS [15], CdSe in [10, 16, 18]). Other authors reporton the potential range of 1.7–1.8 eV in which Q�PbSgave no voltammetric signals in organic solvents, itbeing due to the band gap of Q�PbS, which wasexpected based on spectroscopic data [23, 25, 26]. Theapplication of CV for HOMO and LUMO determina�tion (ionization potential and electron affinity, as theyare also referred to) in polymer molecules and conju�gated organic compounds was reported by a number ofauthors. Some of them claim that absolute values ofthese energy levels and the band gap can be found fromthe positions of the first reduction (for LUMO) andoxidation (for HOMO) peak potentials [40–42]whereas other authors are more cautious in data inter�pretation [43, 44] or state that only a certain correla�tion between the oxidation and reduction peaks, onone hand, and HOMO and LUMO, on the otherhand, can be claimed [45, 46].

Bulk PbS Film Electrochemistry

We are convinced that electrochemistry of bulkPbS film will serve a basis for understanding phenom�ena in Q�PbS. Figure 3 schematically depicts thecyclic voltammogram of a PbS thin film, which wascomposed of the data from multiple experiments so asto make a whole picture of this very complex process.Any real voltammogram would miss some of the peakssince the signals are either very weak relative to otherones or undetectable at all. The reliability of thisscheme is based on the fact that each peak was regis�tered 3–11 times, and Table 1 provides some statisticaldata and comments for every peak. The picture also

reflects the right succession of peaks during potentialsweep starting from the open circuit potential (OCP)in the cathodic direction.

To ensure that the peaks in question are all related toPbS electrochemistry, we provide the cyclic voltammo�gram of the blank: a pure polycrystalline Pt electrode indeaerated 0.1M NaOH (Fig. 3, left inset). The voltam�mogram is very similar to the one reported in [47], fea�turing reductive hydrogen adsorption in the far cathodicregion, the formation of platinum oxide commencing at–0.2 V and reduction of the oxide on the reverse scanwith the peak potential at –0.2 V. In the present study,the platinum oxide formation and reduction are impor�tant as they appear in the voltammograms of the QDfilms and bulk PbS thin films usually at later cycleswhen Pt surface becomes exposed (see below).

Peak C1 appeares in all voltammograms as a veryweak and broad signal and can be assigned to the reduc�tion of oxidized PbS surface, i.e., elemental sulphur(during the synthesis procedure we minimized exposureof PbS to the atmosphere but it could not be completelyeliminated). In [48], galena (natural PbS) was exposedto oxygen�saturated alkaline solution for different timesand a broad peak was observed during the cathodicsweep between ~–0.9…–0.4 V vs Ag/AgCl (sat),depending on the time of exposure, and it was attributedto the reduction of S0 on the electrode surface. A similarpeak near –0.5 V vs SCE was also observed in the deox�ygenated alkaline solution during the cathodic sweep ongalena electrode following brief oxidation of it by excur�sion of potential in the anodic direction [37].

Reduction of PbS depends on pH and commencesat about –1.2 V vs SCE in 0.1 M NaOH [49, 50], shift�ing in the anodic direction, with decreasing pH. Thecathodic wave C3, as we observed, does not really fitthese data, starting at appreciably more anodic poten�tials; however, the overall picture in the cathodicregion may suggest that nevertheless the reductiontakes place. Specifically, in [37] and [51] the anodicwave ~–0.65 V vs SCE was observed after a sweepreversal to the anodic direction. Our voltammogramsalso possess this peak, A3, and frequently another peak

Table 1. Voltammetric signals of bulk PbS thin films

Peak Peak potential, V Comments

C1 –0.64– –0.44 Weak signal (~1 μA) detected during cathodic excursions of potential, not detected after anodic one

C2 –0.81 ± 0.02 Detected only after anodic excursion, rather strong (25 μA)C3 (wave) starts at

–0.7 – –0.6Not a peak but a rising cathodic current becoming much stronger after anodic sweep

C4 0.20 ± 0.04 Always detected, currents 1–20 μAC5 0.04 ± 0.03 Very weak and rarely detectableA1 0.00–0.11 Always detected, diverse in shapes, 140–400 μA during first cycleA2 0.57 ± 0.02 Detected very frequently , currents 8–80 μAA3 –0.77– –0.48 Weak peaks (<≈1 μA), observed frequently and frequently together, i.e., sometimes

only one of them detectedA4 –0.65– –0.37

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A4 can be observed in that region of potentials. Salo�niemi et al. assigned the anodic wave to the dissolutionof elemental lead previously deposited on PbS [51].Hence it can be concluded that C3 represents PbSreduction, yielding Pb0 and SH–. As for this signal C2after a far anodic excursion (see dashed line of the sec�ond cycle in Fig. 3), it is not only reduction of PbS;starting at ~–0.6 V, it largely represents the reductionof matter generated by electrochemical PbS oxidation.The importance of processes in this potential range forPbS restoration will be discussed later.

The most prominent feature of PbS electrochemis�try is the anodic peak A1. According to the availableliterature, in strong alkaline medium (pH ≥ 13), thereshould be not a peak but a monotonously increasingsignal starting at ca. –0.2 V vs SCE. Formation of sol�uble products that are not deposited on the electrodesurface accounts for the absence of a peak or pre�peak[48, 52]; at a lower, but not acidic pH, partial passiva�tion of the surface occurs due to the formation of S0

and PbO [48]. High pH values favour the formation ofplumbite ions; and increasingly large amounts of thio�sulfate along with S0 are also formed as Gardener andWoods argue [48]. According to their calculations, nearly

60% of PbS yield S0 and 40% – S2 at pH = 13.Hence we can write down main oxidation reactions asfollows:

PbS + 3OH– – 2e → HPb + H2O + S, (2)

2PbS + 12OH– – 8e → 2HPb + S2 + 5H2O, (3)

The formal potential, at pH = 13 for reaction

can be calculated based on the value reported in[48] for reaction (4)

PbS – 2e → Pb2+ (aq) + S, = +0.1V vs SCE (4)

= + 0.827 – 0.0885 pH

O32–

O2– E1

0,

O2– O3

2– E20.

E10 '

,

E30

E30

E10' E3

0

thus giving = –0.22 V vs SCE (a shift of –0.32 V),which is consistent with the initial potentials reported[48, 52] and confirmed in the present study (–0.25…–0.20 V). We should stress that the formal potentialcan become close to the initial one, being always lessthan Emax, if the constant, k0, in Batler�Volmer equa�tion is sufficiently small, which is generally true forirreversible redox reactions and therefore is justifiedfor use in our argumentation.3

It can be demonstrated that the formal potential,

of reaction (3) shifts from its standard potential,

= 0.614 V vs SHE [48], by –0.96 V at pH = 13, so

that = –0.346 V vs SHE or ~ –0.57 V vs Ag/AgCl(sat). Therefore reaction (3) is thermodynamically evenmore favourable than reaction (2). Nonetheless, theexperiment is consistent with reaction (2), which can beattributed to the kinetic hindrance of reaction (3).

Our findings show that oxidation of PbS is morecomplex than it can seem and reactions (2) and (3) arenot completely consistent with experimental observa�tions. We have to acknowledge the formation of insol�uble lead species on the fact that oxidation peak A1and most of other peaks resume in the second and sub�sequent cycles, with the magnitude largely reducingwith every next cycle. Restoration of PbS electro�chemistry after the first cycle is possible only if a partof both sulphur and lead species are deposited on theelectrode surface. No doubt, a significant portion oflead sulphide dissolves during oxidation and diffusesinto the bulk of solution. A specially designed experi�ment showed that PbS restores from adsorbed speciesat potentials <~–0.6 V but not from those surroundingthe electrode as one might think. Freshly preparedbulk PbS film underwent the potential cycle –0.4 →+0.75 → –0.4 → +0.75 V (Fig. 4a). Oxidation peak

3 See e.g. the work by Hubbard [58] that is discussed below inmore detail.

E10'

E20',

E20

E20'

–0.4

0

–0.2 0 0.2 0.4 0.6

1

2

3

–4

–0.5 0 0.5

–2

0

2

(a) (b)i × 105, A

E, V

i × 103, A

E, V

Fig. 4. Cyclic voltammograms of freshly prepared bulk PbS thin film. (a) Potential sweep –0.4 → 0.75 → –0.4 → 0.75. Contin�uous curve shows the first cycle. (b) Potential sweep 0.75 → –0.9 → 0.75 followed after the sweep given in (a) and accompaniedby stirring the cell solution with argon.



A1 appeared, as expected, during the first sweep butnot during the second one, thus suggesting that re�for�mation of PbS occurs at more negative potentials. Thenext cycle, +0.75 → –0.9 → +0.75 V, was accompa�nied by bubbling Ar through the solution (Fig. 4b). Wedid not observe any distortions on a voltammetriccurve due to uneven stirring, and PbS oxidation wasobserved again; hence, only adsorbed species partici�pate in all the processes involved. Taking into accountthe literature just discussed, we propose another reac�tion for the oxidation process, which simply states that

only a part of lead diffuses away as HPb and theother part is PbO deposited on the electrode:

PbS + 2OH– – 2e → PbO + H2O + S. (5)

The fact that we observed a peak rather than amonotonously rising current can be explained by twofactors: (1) a limited amount of PbS thin film availableon the electrode (unlike the cases when the workingelectrode was made of massive PbS piece and, hence,the matter was quasi�infinite); (2) the PbS surface isbeing passivated. These two factors do not excludeeach other and can operate simultaneously. Further,

O2–

the peak, as it is depicted in the scheme, is not alwaysobserved; instead, it can be either broader, or lose thepeak shape at all, or appear as multiple successivepeaks (see the right inset in Fig. 3). It was noticed thatprocesses with higher currents tend to deviate from the“ideal” shape depicted and shift anodically. One can

assume that this is conditioned by the shift of dueto its dependency on pH. When reaction (2) proceedsit consumes 3 moles of OH– per each mole of PbSwhile reaction (3) – 12 moles of OH–, resulting in adecrease of pH near the electrode and, as a conse�

quence, the anodic shift of Further we will discusssome kinetics aspects of process A1.

The reasons behind the distortion of the peak A1shape are beyond our comprehension at the momentbut simpler cases when process A1 is represented as asingle “ideal” peak can be characterized to someextent. Several experimental facts have been estab�lished and summarized in Table 2 and Fig. 5. In thetable, the Tafel�type linearization data for peaks A1 arepresented and in the Appendix we substantiate how

E10'

E10'.

Table 2. The data on the linearization of peak A1 for the bulk PbS films and peak A1 potentials

Cycle number* Intercept Slope Slope, mV/decade Emax, V

c2 3.52 8.63 116 0.041

c3 –4.22 7.91 126 0.044

c1 –3.63 8.36 120 0.049

c2 –4.37 9.49 105 0.11

c4 –4.39 9.65 104 0.067

c5 –4.53 9.08 110 0.054

c2 –4.21 8.61 116 0.0252

Note: *) for a given bulk PbS film, several potential cycles were carried out and are indicated. The curly brackets unite the cycles of thesame film.

–0.2–6.0

0–0.1 0 0.1 0.2

–5.5

–5.0

–4.5

–4.0

–3.5

–3.0

50 100 150 200

200

400

600

800

1000

1200

E, V imax, μA

qtot, μCi

γPbS��log

(a) (b)

Fig. 5. Linearization of peak A1 for the bulk PbS films (a) and the plot of total peak charge vs peak current (b).

202


CUHARUC et al.

potentiodynamic curves can be used to obtain theTafel constant for adsorbed species. As is evident, theTafel constants vary in a narrow range, correspondingto the Tafel slopes 104–126 mV/decade. Furthermorepeak current�to�charge ratio is constant q/imax = 5.32(see Fig. 5b) for all the data. This suggests that the pro�cesses represented by the “ideal” peak shape areessentially the same in terms of their nature and kinet�ics. The differing peak currents are only due to theamount of PbS available. Peak potentials vary in therange 0–0.11 V, with the majority of data confined to0.04–0.08 V. There is no correlation between peakpotentials and peak charges or currents; therefore peakpotentials are not conditioned by the amount of PbSreacted or its kinetics and depend on an unknown fac�tor. This implies that, unfortunately, the bulk PbS filmoxidation cannot be characterized by a certain peakpotential.

The Tafel linearization mentioned above is pre�sented in Fig. 5a. Deviations from the linear behaviourare characteristic and always observed in the post�max�imum region. This can be regarded as an “intervention”of another process that results in a significant decreaseof the rate of PbS oxidation, which can be due to thepassivation of PbS surface, but not to limited amountsof PbS on the electrode. The oxidative dissolution ofPbS can be interpreted in terms of dissolution of abinary bivalent ionic semiconducting compound anddescribed by ion transport kinetics [14]:

(6)

(7)

where i is current, e is elemental charge, pb is bulk con�centration of holes, k is the Boltzmann constant, T is

absolute temperature, and are rate constants,[S]* and [Pb]* are surface concentrations of sulphur

and lead ions; A is the electrode area, is the trans�fer coefficient of lead ions, ΔφSC and ΔφH are potentialdrops across a semiconductor space charge layer andthe Helmholtz layer, respectively. Formulas (6) and (7)apply when the band edge level pinning (BLP) or theFermi level pinning (FLP) take place, respectively.The Tafel slope for BLP, implied in (6), at 298 K, is30 mV/decade, which is not consistent with our datathat show 114 mV/decade, on average. Therefore dis�solution occurs with FLP – the change of potentialdrop applied is entirely in the Helmholtz layer – and

the transfer coefficient is = 0.52. The Tafel slopesof 57–76 mV/decade were reported in [53] for galenadissolution in acidic medium, higher values related tosurface states of the galena samples.

Next, we attempt to present basic redox reactionsof the bulk PbS film from the standpoint of semicon�

BLP i 2ekS+A S[ ]*pb

2 2eΔφSC

kT�� ,exp=

FLP i 2ekPb+ A Pb[ ]*

αPb+ eΔφH

kT��exp=

kS+ kPb

+

αPb+

αPb+

ductor electrochemistry. Richardson and O’Dellfound a polarizable space charge layer of their galenasamples in the potential range ca. –0.6 – –0.2 V vsSCE with the flat�band potential close to –0.6 V or⎯0.2 V for highly n� or p�type samples, respectively[37]. Therefore one can admit that these potentialsdetermine the positions of conduction (CB) andvalence bands (VB) of PbS. High anodic currentsobserved for our samples in the present study suggestthat they are of p�type and the Fermi level in PbS,εF(SC), is located close to VB (Fig. 6a). Our experimentsalso showed that anodic polarization from OCPimmediately results in the anodic current of peak A1.This implies that the Fermi level in PbS becomes lowerthan the redox Fermi level, εF(dec, p), corresponding top�type decomposition of PbS – a thermodynamiccondition for oxidative decomposition of a semicon�ductor [14, 54]:

εF(SC) < εF(dec, p) (8)

or in terms of potentials on electrochemical scaleEF(SC) > EF(dec, p). (9)

Indeed, if EVB ~ –0.2 V and EF(dec, p) = = –0.22 V,then condition (9) is met in p�type PbS as soon as theelectrode is polarized anodically, +eη (Figs. 6b and 6d).

Under cathodic polarization, εF(SC) moves cathodi�cally, –eη, charging surface states, SS, correspondingto the oxidized PbS surface as discussed above, andwhen conditions (10) and (11) are met

εF(SC) > εF(dec, n) (10)EF(SC) < EF(dec, n) (11)

PbS undergoes reductive decomposition (Figs. 6c and6d), if we accept that EF(dec, n) is ~–1.2 V as establishedin publications cited above. As is evident, the band gapand absolute positions of VB and CB are not involvedin this process; conditions (8)–(11) result only fromthermodynamic reasons and are not related to thestructure of energy levels.

As for other peaks, C4 and C5, the nature of reac�tions involved has not been clarified in the presentstudy; however, they will be mentioned in the follow�ing section.

Electrochemistry of Q�PbS

The first important aspect of Q�PbS electrochemis�try under the conditions of study consists in proximityof redox reaction potentials of Q�PbS samples and ofbulk PbS thin films. Figure 7a demonstrates 15 consec�utive cycles of potential for the sample QD�120AC.Most of the peaks detected for a bulk PbS film appear inthis voltammogram as well. Note that the cathodic peakat –0.3 …–0.2 V on this and other voltammogramsrepresents reduction of Pt oxide formed during theanodic potential excursion beyond –0.1 V. Peakpotentials for reactions denoted A2, C4 and C5 on thefirst cycle are 0.598, 0.209 and 0.063 V, respectively.

E10'



These values fit (within the standard deviation (s.d.))those presented in Table 1 for the same redox reactionsof a bulk PbS film. In the second cycle, process A2 ispresented by an additional peak at 0.52 V. Thecathodic reaction C3 appears as a rather well�definedpeak unlike the wave observed for a bulk PbS film.Peak A1 is shifted towards negative potentials; it ishighly reproducible with an average value –0.05 V. Ifcompared to peak potentials for bulk PbS films, onecan conclude that Q�PbS undergoes oxidation easier.However, most remarkable features of oxidation thatmake the difference between the two materials are (1)gradual decrease of peak currents during cycling, (2)temporal upsurge of peak currents during cycling, and(3) unexpected reproducibility of peak currents bothwithin a given experiment and between replicateexperiments.

A gradual decrease of peaks A1 in subsequent cyclesis characteristic of the QD samples only. We assumethat OA forms “a bag” surrounding nanocrystals,hence preventing soluble oxidation products from dif�fusing into the bulk of solution. Alternatively, lead ole�ate rather than plumbite can be admitted as one of theoxidation products; this compound is insoluble in anaqueous medium and keeps lead ions available for the

following reaction with S2– forming during thecathodic potential sweep.

In Fig. 7a, peaks A1 of the first 7 cycles are markedwith numbers to show the effect just mentioned.Under the conditions of the present study, one cannotdeposit a reproducible amount of Q�PbS on the elec�trode, hence, peak currents are expected to be differ�ent with every new deposition of Q�PbS. However, thevalues that peak currents assumed, although vary, formfour groups or clusters.4 Figure 7b exemplifies howpeak currents change with a number of cycles for threedifferent PbS depositions. The gray bands show limitsfor every current group as mean ±s.d. Peak chargesalso follow grouping which is not so pronounced asthat of currents.5 The hierarchical analysis yields threeclusters instead of four, with the charge values corre�sponding to the two first current groups being mergedinto one. Yet, if one sets to find four clusters and plots

4 Hierarchical clustering of data (20 values) was performed bymeans Mathematica and Silhouette statistics was employed forthe significance test.

5 The peak charges undergo greater dispersion since they are moresensitive to baseline subtraction than peak currents. In the caseof Q�PbS, baseline for peak A1 cannot be taken as a straight linethus making peak treatment less accurate.

–4.4

0

–3.4

–3.6

–3.8

–4.0

–4.2

–0.2

–1.2

–1.0

–0.8

–0.6

–0.4

i

E, V

ε, eV

CB

VB

SS

EF(dec, n)

EF(dec, p)

εF(SC)

εF(SC)

εF(SC)

CB

VB

VB

CB

+eη

–eη

vs. Ag/AlCl (sat)

(a) (b) (c) (d)

Fig. 6. Energy levels of bulk PbS films under OCP (a), anodic polarization (b), cathodic polarization (c). Schematic presentationof the respective i�E profile, redox Fermi levels of oxidative, EF(dec, p), and reductive EF(dec, n) decomposition of PbS and surfacestates, SS (d). (Note that in this schematic picture, the bands are not depicted curved for simplicity.).

204


CUHARUC et al.

the group mean values for the charges versus groupmean values for the currents, a straight line results withthe slope (q/imax ratio) equal to 7.56 (Fig. 7c). Thisquantity is smaller for a bulk PbS film (q/imax = 5.31)possibly because Q�PbS gives a rather symmetric peakA1. Kinetics of oxidation is quite similar to that estab�lished for a bulk PbS film. Specifically, if one attemptsto linearize i�E profile in a manner given above, theresulting line is generally not straight. The Tafel slopesfor the data demonstrating a reasonably linear behav�iour are within 100–115 mV/decade.

The explanation of unusual reproducibility of peakcurrents, that we propose, consists in layer�by�layerdissolution of Q�PbS. Apparently, a certain integernumber of layers reacts during the potential sweep. Itis not possible to count the number of layers preciselybased on the peak charge as the true area of Q�PbScoating and packing density of nanocrystals in the lay�ers are unknown. To the first approximation, one canestimate that thickness of the PbS layer correspondingto the first charge group (average 43 μC) is about0.65 nm, which is close to the lattice constant of

galena 5.936 Å [55], assuming that Q�PbS layer has thesame area as the true electrode area.

The cathodic shift of oxidation peak A1 can beexplained by the change in the formal potential of

reaction (2), in alkaline media since equilibrium

concentration [Pb2+] decreases due to the formation

of HPb In the case of Q�PbS, OA plays a role of a

ligand, additionally decreasing [Pb2+] and thereby

moving further in the cathodic region. Unfortu�nately, the data on the solubility product of lead oleateare not available in literature to date, thus it is not pos�sible to provide quantitative basis for this supposition.The given explanation implies that the size of nanocrys�tals does not affect their redox properties. More experi�mental evidences will follow later in the text in supportof this idea.

The effect of the developed purification procedurecan be appreciated from voltammograms of the sam�ple QD�100 that underwent basic purification withmethanol and chloroform (Fig. 8a), further treatment

E10',

O2–

.

E10'

–1.0

–2

10

–0.5 0 0.5

–1

0

1

2

i × 105, A

E, V

C5 C4

C3

A1 A23, 4

1, 2, 5

67

2 3 4 5 6 7

5

10

15

20

imax, μA

Cycle #

05 10 15 20

50

100

150

q, μC

imax , μA

(a)

(b) (c)

Fig. 7. (a) Cyclic voltammograms of QD�120AC: cycle 1 (bold continuous line), cycle 2 (bold dashed line), cycles 3–15 (thinshort�dashed line). The first seven cycles are marked with numbers. (b) The plot of peak A1 currents vs cycle number. (c) The plotof group mean peak A1 charge vs group mean peak A1 current.



in acetone (Fig. 8b) and further treatment in thesodium hydroxide solution (Fig. 8c). The reduction ofnoise and qualitative improvement of voltammetricresponse can be seen on the i�E profiles of the samplesof higher purity. Since in purification OA is removed,as discussed in the ‘IR spectroscopy’ section, one caninfer that excessive OA imposes insulating propertieson the QD layers.

Figure 8 also demonstrates that peak potential,Emax, for peak A1, depends on the environment, i.e.,amount of capping OA. For the sample purified in ace�

tone, Emax ~ –0.08 V whereas for the sample treated inNaOH, it is ca. 0.02 V. The PbS nanocrystals were notsignificantly affected by this treatment since the PLmaximum did not change (see curves b in Fig. 1).Another confirmation of the environment effect onthe peak potential was obtained through an experi�ment with samples QD�60B, which yielded twoanodic peaks Emax = –0.22 and –0.02 V; after thetreatment in acetone, a single peak appeared at~0.02 V; an enhanced treatment in NaOH (0.5 MNaOH and 100°C) yielded a sample (QD60�SH+, see

–0.30

–0.5 0 0.5

10

0 0.3

–0.5 0 0.5

–0.5 0 0.5

5 μA

5 μA

20 μA

i, μA

E, V

i

1

3

11

1

3

1

(a)

(b)

(c)

Fig. 8. The effect of purification procedure on the voltammetric response of the QD samples. (a) Sample QD�100B obtained withonly basic purification procedure; (b) QD�100AC � after additional acetone treatment; (c) QD�100SH � after additional treat�ment in NaOH. The numbers indicate potential cycle sequence number for a given deposition. The inset shows anodic currentsin the region –0.4–0.3 V for sample QD�100SH.

206


CUHARUC et al.

its IR spectrum in Fig. 2) with a very pronounced peakA1 with Emax = 0.05 V, which is very close to Emax, typ�ically observed for bulk PbS (data not presented). It isworth noting that PL spectrum for QD60�SH+ differssignificantly from that of QD�60B and QD60�AC: thePL maximum (1250 nm) is red shifted by 200 nm andthe peak is very broad, which is indicative of partialmerging of nanocrystals. Yet the quantum confine�ment effect was still there. All said above signifies thatthe quantum confinement effect and, therefore, energystructure of PbS are not manifested in these experiments.As mentioned in the Introduction, the medium inwhich measurements are carried out critically affectsthe voltammetric response. Therefore we do not gen�eralize this conclusion and admit that the use of a dif�ferent medium might yield more informative voltam�mograms of Q�PbS.

The QD samples studied by us show certain irre�producibility between each other. On the one hand,the sample QD�120AC has a pronounced oxidationpeak A1 with its unusual properties discussed earlier.On the other hand, the samples QD�100AC andQD�60AC show a very moderate peak A1 or theabsence of that. Removing much of OA from QD�60AC gives an intensive peak A1 which, althoughdecreased gradually during potential cycling asexpected for QDs, does not result in a temporalincrease of the peak current. Apparently, the amountof OA has a very subtle effect on the electrochemicalresponse of the QDs investigated besides the shift ofEmax, and the experimental protocols presented herein

do not always result in reproducible amounts of cap�ping OA in Q�PbS.

The presence of OA and ODE does not distort theinterpretation of QD voltammograms. Indeed, in aux�iliary experiments, OA or ODE were deposited ontothe working electrode and the potential was swept like�wise in the QD experiments. No peak currents butonly rising anodic and cathodic currents commencingat E > 0.0 and < –0.8 V were registered, respectively.

In the final part of the paper, we present experi�mental data for QD�100SH. This sample as well asQD�60SH demonstrates stable electrochemicalresponse without notable oxidation A1 and with inten�sive (apparently irreversible) oxidation and reductionprocesses, A2 and C3, at the extremes of potentialwindow (Fig. 9).

As mentioned before, we associate process C3 withPbS reduction. The nature of process A2 remainsmostly unknown. Both processes are formed after pro�longed potential cycling in the interval –0.9–0.75 V.Specifically, after 7 consecutive cycles, the workingelectrode with the QDs was taken out of the cell,washed with water, acetone, dried in Ar stream andinserted in the cell again. The following cycles 8–10and 15–17 are shown in the same figure. As is evident,virtually identical i–E profiles are produced at cycles15–17. Further experiments revealed that washing anddrying the working electrode is not important inobtaining a stable response; suffice to leave the elec�trode in the solution for some time and then resumepotential cycling. Apparently some soluble reactionproducts are gradually removed from the electrodesurface during the time of rest.

The stability of voltammograms allowed us toextract quantitative information from peak currents,imax, as well as peak potentials, Emax, for a single depo�sition. Firstly, imax values show an irregular trend toincrease at faster scan rates, if potential is cycled in therange –0.9–0.75 V. However, imax demonstrates lineardependency on the scan rate, s, if potential cycling isconfined within the range –0.3–0.85 V (see Figs. 10aand 10b). A reversible depolarization of absorbed spe�cies results in such a dependency and the potentiody�namic curve represents a symmetrical bell�shape pro�file, as is generally known [56, 57]. Obviously, thereaction in question is not reversible. It can be shownthat irreversible and quasi�reversible depolarization ofadsorbed species gives the same result. The theory ofthese electrochemical reactions complying with theButler�Volmer kinetics was first presented by Hubbard[58]. Besides the peak currents given by formula (12),the shift of the peak potential with the scan rate givenby formula (13)6 also brings important information

6 Formula (13) also deduced by the authors differs from the onegiven in [58] by the ratio V/A omitted under logarithm functionsince Hubbard considered thin layer cells electrochemistry.

E, V

i, μA

–0.5 0 0.5

8

9

10

15–17

Fig. 9. Cyclic voltammograms of sample QD�100SH.Numbers show the cycle number. The bars are all 50 μA.Scan direction: OCP → –0.9 → 0.75 → OCP.



and can be used to verify the compliance of the irre�versible model, Red � ne → Ox, with the data.

(12)

(13)

where α is the transfer coefficient from the Butler�Volmer equation, n0 is the number of electrons trans�ferred in an elementary step, n is the number of elec�trons transferred in an overall reaction, F is the Fara�day constant, k0 is the oxidation reaction rate coeffi�cient independent on the potential, [Red]0 is the initialamount of reductant (mole) adsorbed on the electrodesurface, s is the scan rate (V/s), R is universal gas con�stant.

According to (13), the plot of Emax vs. con�tains the reciprocal of the Tafel constant, (1 – α)n0F /RT, and the interception presents a linear relationshipbetween E0' and ln[k0] thereby the two quantities can�not be determined from this experiment. Our datayield a straight line in the coordinates (Fig. 10c) andthe Tafel constant of 16.18 or 142 mV/decade and

imax

1 α–( )n0F

2.71RT��nF Red[ ]0s=

Emax E0 ' 2.303RT1 α–( )n0F

��1 α–( )n0Fs

k0RT��log+=

s[ ]log

(1 – α)n0 = 0.42. On the other hand, the Tafel slopemight be estimated from (12). Indeed, plotting imax vs. sgives a straight line with the slope of 0.610 (Fig. 10b).However, the slope includes the product nF[Red]0

which is the peak charge, qA2. Unfortunately, peak A2cannot be reliably separated from the other signal fol�lowing it and, therefore, the peak charge is unavail�able. Inversely, qA2 can be estimated from that slope ifthe Tafel constant is known; simple calculation givesqA2 = 102 μC. This value agrees with the expected onefor the irreversible model. The charge values of peakA2 before Emax are reproducible at various scan ratesand vary between 58 and 61 μC. Since i�E profile of anirreversible reaction is asymmetric, with most of thecharge passed before the peak potential, qA2 must be�120 μC, which is in agreement with the anticipatedpeak shape. This value is used further to obtain theTafel constant from linearization of the potentiody�namic curves A2. Indeed, the plot of log[i/γRed] vs. Eyields a fairly good linear dependency (the inset inFig. 10a) but the slopes (the Tafel constants) diminishwith the scan rate (see Table 3). This contradicts a littlethe result just mentioned. Namely, the Tafel constantfound according to (13) has a certain value and onewould expect the same value from the linearization.

–0.2

–50

0.3

–14

–12

–10

–8

0.4 0.5 0.6 0.7

E, V

0 0.2 0.4 0.6 0.8

0

50

100

150

200

E, V

i, μAln i

γPbS

��

0

50

100

150

200

100 200 300s, mV/s

3

0.55

0.60

0.65

0.70

4 5 6ln[s], s in mV/s

R2 = 0.9993 R2 = 0.9958

imax, μA Emax, V

(a)

(b) (c)

Fig. 10. Cyclic voltammograms of sample QD�100SH at scan rates 25, 50, 100, 200 and 300 mV/s with the potential sweep⎯0.3 → (0.75–0.9) → –0.3 V. The inset demonstrates the linearization of peak A2 for scan rates 25, 100 and 300 mV/s. The arrowspoint the change in voltammograms with increasing scan rate (a). Peak current as a function of the scan rate (b) and peak potentialas a function of logarithm of the scan rate (c) based on the data shown in (a).

208


CUHARUC et al.

The mean value for the slopes obtained from the lin�earization amounts to 134 mV/decade, which is ratherclose to the value found from (13). Apparently, for�mula (13) yields an averaged Tafel constant that, infact, varies in the experiments performed at differentscan rates. We are convinced that process A2 is morecomplex than a one�stage irreversible oxidation reac�tion and the regularities found experimentally are cov�ered by this model only in part. Besides, the processcan be complicated by diffusion especially at earlierstages.

Chronoamperometry experiments revealed theformation of a new phase in the A2 region. The poten�tial was stepped from OCP (~0.0 V) to 0.65 V, giving acharacteristic curve, i(t), with a maximum (Fig. 11a).It is not possible to determine which type of nucle�ation operates in this case. The dimentionless plot ofi/imax vs t/tmax for the experimental data, given inFigs. 11b and 11c, is not adequate at higher times withany of the four models of progressive (2D, 3D) orinstantaneous (2D, 3D) nucleation [59–61]. Proba�bly, a passive film grows on the QD surface and bulkPbS thin films beyond a monolayer coverage (2D) butblocking the surface prevents further 3D growth.

An attempt to investigate an oxidation process ofgalena at far anodic potentials (apparently A2) atpH 12 was also made in [52]. The nature of chemicalreactions involved remained unclear. The authors onlysuggested that passivating film that had formed at ear�lier stages of galena oxidation dissolved and anotherone formed.

As is obvious, the electrochemistry of PbS is verycomplex and probably only spectroscopic and chemi�cal analyses of PbS electrode surface during or after apotential sweep may shed light on the chemistry of thereactions involved.

0

20

60

100

140

1 2 3 4 5t, s

i, μA (a)

0

0.2

0.4

0.6

1.0

1 2 3 4 5t/tmax

i/imax (b)

0.8

0

0.2

0.4

0.6

1.0

1 2 3 4 5t/tmax

i2/i2max (c)

0.8

Fig. 11. Potential step experiment: from E1 = OCP (~0.0 V) to E2 = 0.65 V (a). The plots of the data from (a) in dimensionlesscoordinates (—), constructed according to instantaneous (� � �) and progressive (– – –) growth models in the case of 2D (b) and3D (c) deposits.

Table 3. The data on the linearization of peak A2 for QD�100SH for different scan rates

Scan rate, mV/s Intercept Slope Slope,

mV/decade R2

25 –22.08 9.71 103 0.9952

50 –20.75 8.49 118 0.9749

100 –19.15 7.23 138 0.9892

200 –18.55 6.65 150 0.9849

300 –17.84 6.28 159 0.9858



CONCLUSIONS

(1) The synthesized Q�PbS showed PL maxima inthe range 800–1300 nm, indicating a quantum con�finement effect; the radius of nanocrystals was esti�mated to vary between 4–8 nm.

(2) By means of IR spectroscopy, the OA moleculeswere found to face the nanocrystal surface with theircarboxyl groups, forming a surface compound – leadoleate. Refluxing Q�PbS in acetone or heating it withNaOH, being a purification procedure, resulted in apartial removal of OA from the synthesized product.The purification improved voltammetric response ofQ�PbS.

(3) Cyclic voltammograms of Q�PbS depositedonto a working electrode essentially followed that ofbulk PbS thin films obtained by chemical bath deposi�tion. The slight cathodic shift of the oxidation peak isaccounted for by the change in the formal potential ofthe oxidation reaction. The oxidation and reduction ofQ�PbS are governed solely by the thermodynamics ofthe respective redox reactions, and electronic struc�ture of those QDs is not manifested, as follows fromthe experimental data. A simple explanation of PbSreduction and oxidation is formulated from the view�point of electrochemistry of semiconductors.

(4) The peculiarities of Q�PbS electrochemistryconsisted in (a) the ability of Q�PbS to participate inmultiple redox cycles whereas bulk PbS films dissolvedfast upon oxidation, (b) the layer�by�layer oxidativedissolution that was manifested as reproducibility ofpeak currents and charges between various experi�ments, and (c) stabilizing voltammetric response aftermultiple redox cycles. The presence of capping OAlayers which prevent the redox products from escapingin the solution bulk is suggested to account for theseeffects.

(5) Kinetics of two oxidation peaks A1 and A2 wasinvestigated by the linearization of potentiodynamiccurves, suggested in the present study. Kinetics of pro�cess A1 for bulk PbS films is consistent with the disso�lution of a binary ionic semiconductor; the transfer

coefficient was determined = 0.52. Process A1 forthe QDs turned out to be more complex: the lineariza�tion was not as effective as it was for bulk PbS. The oxi�dation occurs slightly faster. The chemical nature ofprocess A2 remained unclear. Our data only suggestedthe involvement of adsorbed species and the growth ofa new phase.

APPENDIX

Normally the Tafel constants are not extractedfrom potentiodynamic curves due to the involvementof diffusion of redox species. Here we show that theTafel constants can be obtained from a single potentio�dynamic curve if 1) the redox species are pre�adsorbedonto an electrode surface, 2) they do not continue to

αPb+

adsorb from or diffuse away into the solution duringthe sweep of potential, 3) the electron transfer reactionis irreversible, 4) no chemical reactions precede theelectron transfer and 5) the redox reaction follows theButler–Volmer kinetics. Then one can write the But�ler–Volmer equation [62] for the oxidation reactionRed – ne → Ox with the constant of backward reac�tion, kb = 0, as follows:

(A1)

If we denote relative amount of the reductant as

[Red]/[Red]0 = γRed (A2)

where [Red]0 is the initial amount of Red, then γRed

can be found from experimental data as follows:

γRed = 1 – [Ox]/[Ox]∞

= 1 – q/qtot (A3)

where [Ox]∞

is the final amount of the oxidant, q is thepeak charge at any instant and qtot is the charge underthe whole peak corresponding to the redox reactionconsidered. Using (A2) and the total charge expressedas qtot = nF[Red]0, (A3) can be re�written

(A4)

Plotting ln[i/γRed] vs potential E results in a straightline with the slope being a Tafel constant and the inter�ception as given below:

slope = (1 – α)nF/RT (A5)

interception = ln[k0qtot] – slope E0' (A6)

If one knows E0' then k0 can be determined from asingle experiment. The approach presented holds ifthe redox process follows the Tafel�like kinetics as isthe case of the dissolution of an ionic semiconductor,according to (A5) and (A6); but the slope and theintercept have a different interpretation.

ACKNOWLEDGMENTS

We are thankful to our colleagues Maria Rusu andDiana Shepel from the Institute of Chemistry of theAcademy of Sciences of Moldova for FT�IR measure�ments. This work was supported by the state grants inthe framework of institutional projects of the Instituteof Applied Physics of the Academy of Sciences ofMoldova 11.817.05.03A and 11.817.05.05A.

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Electrochemical Characterization of PbS Quantum Dots Capped With Oleic Acid and PbS t

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