Electrochimica Acta 46 (2001) 4197–4204

The effect of modification of carbon electrodes with hybridinorganic/organic monolayers on morphology and

electrocatalytic activity of platinum deposits

David Martel a, Alexander Kuhn a,*, Pawel J. Kulesza b,*,Mariusz T. Galkowski c, Marcin A. Malik c

a Laboratoire d’Analyse Chimique par Reconnaissance Moleulaire,Ecole Nationale Superieure de Chimie et de Physique de Bordeaux, 16, a�enue Pey Berland, 33607 Pessac, France

b Department of Chemistry, Uni�ersity of Warsaw, Pasteura 1, 02-093 Warsaw, Polandc Di�ision of Chemistry, Department of Metallurgy and Materials Engineering, Technical Uni�ersity of Czestochowa,

Armii Krajowej 19, 42-200 Czestochowa, Poland

Received 27 November 2000; received in revised form 6 February 2001

Abstract

Carbon electrodes which were modified with a monolayer of polyoxometalate (P2Mo18O626−) or with a hybrid film

of P2Mo18O626− and protonated 1,12-diaminododecane (NH2�(CH2)12�NH2), were used as substrates for the galvano-

static electrodeposition of highly dispersed platinum microparticles. For comparison, an unmodified (bare) carbonelectrode was also considered. Significant differences in the morphology (dispersion and size of Pt deposits) wereobserved as a function of the pretreatment of the carbon substrate. Reactivity of the obtained electrocatalytic surfaceswas probed by monitoring currents corresponding to proton reduction and arsenic(III) oxidation. It was found thatthe respective voltammetric currents varied up to an order of magnitude despite the fact that, in a first-orderapproximation, equal amounts of Pt were deposited at the electrode surfaces in all cases. The electrocatalyticarsenic(III) oxidation current was correlated with the specific surface area of platinum particles deposited on the threetypes of electrodes studied. The existence of positively charged amino groups within the layer of polyoxometalateseems to have a profound effect on the distribution and morphology of platinum centers. Electrodeposition ofmetallic platinum in a form of submicro- or even nanoparticles is expected. © 2001 Elsevier Science Ltd. All rightsreserved.

Keywords: Modified electrodes; Polyoxometalate; Platinum particles; Electrostatic interactions; Electrocatalysis

www.elsevier.com/locate/electacta

1. Introduction

Micro- and nanoparticles of noble metals are of greatinterest due to their optical, electronic or magneticproperties and their catalytic activity. They can begenerated by various techniques like microwave dielec-

tric heating [1] or sonochemical reduction [2]. In elec-trochemistry, the preparation of electrodes covered withmetallic nanoparticles is also widely studied. Typicalexamples include the preparation of spatially dis-tributed noble metal particles deposited on activatedcarbon supports [3–5], high surface area metal oxides[6,7] or in polymeric films on electrodes [8–15] includ-ing those fabricated on passivating metal surfaces[16,17]. This type of electrodes offers improved catalyticactivities for various technologically important reac-

* Corresponding authors. Tel.: +33-5-5684-6573; fax: +33-5-5684-2717 (A.K.).

E-mail address: kuhn@enscpb.u-bordeaux.fr (A. Kuhn).

0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0013 -4686 (01 )00686 -7

D. Martel et al. / Electrochimica Acta 46 (2001) 4197–42044198

tions such as hydrogen evolution, methanol oxidationand the four-electron reduction of oxygen to water.

Based on our and others’ work concerning the irre-versible adsorption of polyoxometalate monolayers andmultilayers on different electrode materials [18–24], aswell as on the recent observations implying that thepresence of polyoxometalates in solution has an impor-tant influence on the morphology and the catalyticactivity of electrodeposited metal clusters [25], we havedecided to address the question whether the presence ofcharged, preadsorbed, layers of polyoxometalates mayinduce similar effects. Our approach has also beenprompted by the well-known electrochemical [26–30],photochemical [31] and catalytic [32–35] properties ofnanostructures of polyoxometalates and related tung-sten, vanadium and molybdenum oxides. The ultimategoal is to prepare ultrathin films, self-assembled frompolyoxometalate-based hybrid monolayers with catalyticmetal submicrocenters that are dispersed, physicallyseparated and exhibiting mutually activating metal–sup-port interactions.

In the present work, highly dispersed platinum hasbeen generated from solutions of PtCl6

2− by applying aconstant current to the electrode substrate, i.e. to eitherbare (unmodified) carbon, to carbon modified with amonolayer of P2Mo18O62

6−, or to a carbon substratecovered with a hybrid film of P2Mo18O62

6− and 1,12-di-aminododecane, i.e. NH2�(CH2)12�NH2, abbreviated asDAD. The latter compound is positively charged at lowpH, and therefore its introduction into P2Mo18O62

6−

allows us to control, namely to invert the interfacialcharge when compared to the case where the electrodesurface is covered with the polyoxometalate monolayeronly.

The chronopotentiometric signals, which have beenmonitored during the galvanostatic electrodeposition ofPt, are compared for the three types of samples describedabove, and their corresponding surface morphologies areanalyzed using scanning electron microscopy (SEM).Finally, the system’s reactivity is characterized withrespect to the hydrogen evolution reaction, and thesurface area of dispersed platinum is probed [35] bymonitoring the electrocatalytic arsenic(III) oxidationcurrents.

2. Experimental

The polyoxometalate, (NH4)6P2Mo18O62, abbreviatedas Mo18, was prepared according to the proceduredescribed in a previous work [18]. All other chemicalswere reagent grade and were used as received. Solutionswere prepared from ultrapure water that had been passedthrough a purification train (Milli-Q Plus 185, Millipore).Carbon rods of spectrographic purity (�=0.3 cm, lessthan 2 ppm impurities) were employed as working

electrodes (Carbone Lorraine, France). They were usedas received, without any chemical or electrochemicalpretreatment, implying a quite small density of surfacegroups that could act as nucleation centers.

Three types of carbon electrode surfaces were consid-ered: (a) bare, i.e. unmodified, (b) modified with amonolayer of Mo18, and (c) with a hybrid layer of Mo18

and DAD. Electrode modification for case (b) wascarried out by dipping 2 cm of the carbon rod for 10 minin a solution of 0.5 mM Mo18 in 0.5 M H2SO4. In case(c) the above modification step was followed by dippingin a saturated solution of DAD in 0.5 M H2SO4. As arule, after each modification step, the electrode wasrinsed for 5 min in ultrapure water. The characteristicsof the resulting electrode surfaces, including their cyclicvoltammetric responses, have been reported earlier [18].

On each type of electrode surface, platinum particleswere generated galvanostatically by applying a constantcurrent of −0.5 mA for 300 s to the electrode immersedin a solution of 0.2 g H2PtCl6 (37.5%) in 20 ml water (nosupporting electrolyte added). The galvanostatic deposi-tion mode has been chosen in order to illustrate in theclearest way that the modification with Mo18 or withMo18+DAD has an important influence on the thermo-dynamics of the metal deposition process. Voltammetricprobing of platinum particles via electrocatalytic oxida-tion of arsenic(III) was carried out in a 7.5 mM solutionof As2O3 in 2 M H2SO4. Details of the concept andprocedure are described elsewhere [35].

Standard electrode surfaces containing various plat-inum loadings were obtained by electrodeposition fromchloroplatinate solutions as described previously [35,36].To produce fairly well defined, separated and sphericalmicroparticles, which are three-dimensionally dispersedin the film covering the electrode, platinum was elec-trodeposited into a tungsten oxide matrix [36].

Experiments were performed at ambient temperature(20�2 °C) under nitrogen atmosphere in a single com-partment cell (electrodeposition of Pt) or in a three-com-partment cell (cyclic voltammetry) using an AUTOLAB(Netherlands) PGSTAT 30 potentiostat or CH Instru-ments (Austin, TX, USA) model 660 analyzer. Allpotentials are given with respect to a saturated calomel(SCE) reference electrode; and a platinum wire was usedas a counter electrode. The scanning electron microscopic(SEM) examinations were performed with a JEOL 5200(Japan) microscope.

3. Results and discussion

3.1. Chronopotentiometric monitoring of depositionprocess

Electrodeposition of metallic platinum on three dif-ferent types of electrode surfaces— (a) bare carbon, (b)

D. Martel et al. / Electrochimica Acta 46 (2001) 4197–4204 4199

carbon modified with Mo18, and (c) carbon modifiedwith a hybrid film of Mo18 and DAD—was achievedby application of a constant current for the prescribedamount of time (300 s). The first information about theinfluence of the surface modification on the formationof platinum particles can be obtained from chronopo-tentiometry by monitoring changes of the electrodepotential with electrodeposition time. A potentiostaticmode, as frequently used in the literature [37], mostlikely would lead to analogous results but the influenceof the surface modification on the deposition overpo-tentials is more difficult to extract from the resultingchronoamperograms. Fig. 1 shows the variation ofpotential necessary to ensure a constant current of−0.5 mA for the three different electrode surfacesexposed to the unstirred solution of PtCl6

2− that inten-tionally contained no supporting electrolyte. The latterfactor excluded the possibility of undesired electrostaticinteractions between electroinactive ions and thecharged electrode surface. For example, under suchconditions, the repulsion forces between the negativelycharged Mo18 layer and the anionic PtCl6

2− could beconsidered as a major electrostatic factor in case (b). Asthe overall transport of the chloroplatinate species wasidentical in all three experiments, any differences in thepotential versus time curves (Fig. 1) must have hadtheir origin in the nature of interactions occurring atthe interfaces and reflected the different states of theelectrode surfaces. In all three cases, the final potentialrecorded during electrodeposition (Fig. 1) tended toreach the same value which was comparable to thatfound for a platinum electrode platinized underanalogous conditions. The result is also consistent withthe view that PtCl6

2− was reduced to Pt0. However, thecurrent efficiency for this process depends on the poten-tial and for some potentials the reduction mechanismcan go through a Pt(II) complex [37]. At lower overpo-tentials the intermediate formation of a Pt(II) complexmight be favored because of kinetic reasons. From a

thermodynamic point of view the reduction of PtCl62−

and PtCl42− to metallic platinum occurs at almost iden-

tical potentials (0.74 and 0.73 V, respectively). Thismeans that, if Pt(II) is formed in an intermediate step,it should be immediately reduced further to Pt0 at thepotentials reported in Fig. 1. However, this reduction ofPt(II) to Pt0 might be inhibited, for example by smallamounts of Cl− coming from the decomposition of thePt complexes during reduction. It is therefore possiblethat current efficiencies as low as 20% are obtained (seebelow). Finally, the fact that the observed potentialvalues lie in the range from 0.1 to 0.3 V (Fig. 1) implythat no side reactions (e.g. hydrogen evolution) occur.

Upon careful examination of the chronopotentiomet-ric responses summarized in Fig. 1, the following obser-vations can be made. First, in the case ofelectrodeposition on bare carbon (curve a), the poten-tial decreases at the beginning, and it reaches a mini-mum value after several seconds. This decreasepresumably corresponds to the overpotential that oc-curs during formation of Pt0 from a solution containingthe PtCl6

2− anions. Electrostatic repulsion between theelectron rich electrode and the negatively charged spe-cies probably hinders the approach of the latter ones tothe surface. After passing the minimum, the potentialincreases gradually and reaches the plateau of ca. 0.250V after 300 s. For the carbon electrode modified withMO18 (curve b), the overall shape of the chronopoten-tiometric curve is the same, but the potential minimumis steeper and it reaches 0.080 V. This feature is likelyto reflect the relatively higher density of negativecharges on the electrode surface modified with Mo18

anions, when compared to bare carbon. This phe-nomenon is expected to result in electrostatic repulsionof PtCl6

2− (by Mo18) and, consequently, in a higheractivation energy for its reduction and nucleation. Thecoulombic charge consumed by the Mo18 present in thiscase on the surface can be estimated to be 0.075 mC,supposing that it is adsorbed as a monolayer. Thisvalue can be neglected with respect to the total chargedelivered to the system during the electrodepositionstep (150 mC), and therefore any difference in the finalamount and in the structure of platinum cannot resultfrom this effect.

Curve c corresponds to the signal of the electrodemodified with a hybrid layer of Mo18 and DAD. Thepattern of the potential evolution is completely differentfrom that characteristic for the preceding two cases.The starting potential is significantly more positive, andthere is no potential minimum at all. Such behaviorindicates that the deposition process is favored by thepresence of the positively charged DAD within theMo18 layer on the electrode surface. Regardless of theactual structure of the hybrid layer, the fact, that thenegative charge of Mo18 is neutralized by the positive

Fig. 1. Chronopotentiometric curves recorded using a constantcurrent of −0.5 mA in an aqueous solution of H2PtCl6 at: (a)a bare carbon electrode, (b) a carbon electrode modified witha monolayer of Mo18, and (c) a carbon electrode modified witha monolayer of Mo18 and a monolayer of DAD.

D. Martel et al. / Electrochimica Acta 46 (2001) 4197–42044200

Fig. 2. (A) Cyclic voltammetric responses (scan rate: 100mV/s) related to hydrogen evolution (in 0.5 M H2SO4) occur-ring at modified electrodes which were platinized as in Fig. 1.Electrode substrates: (a) bare carbon, (b) carbon modified withMo18, and (c) carbon modified with the hybrid Mo18/DAD.(B) Cyclic voltammograms (scan rate: 20 mV/s) showing elec-trocatalytic oxidation of As(III) at the platinized electrodes (asin Fig. 1). Measurements performed in the solution of 7.5 mMAs2O3 in 2 M H2SO4. Curve (a) corresponds to the back-ground signal of bare carbon in this solution. Curves (b)– (d)show As(III) oxidation peaks at the surfaces produced byplatinizing bare carbon, carbon modified with Mo18, andcarbon modified with hybrid Mo18/DAD, respectively. Beforerecording each voltammogram, the potential was held for 30 sat −0.2 V.

activity of the three types of electrodes with respect tothe hydrogen evolution reaction and the oxidation ofarsenic(III). Fig. 2A shows the voltammograms corre-sponding to proton discharge (hydrogen evolution) inpure 0.5 M H2SO4. The voltammetric responses ob-tained for platinum deposited on bare carbon (curve a)and on Mo18-premodified carbon (curve b) are quitesimilar. Also, the respective chronopotentiometric pat-terns recorded during platinum deposition have beenqualitatively similar (Fig. 1, curves a and b). In the caseof the platinized hybrid inorganic/organic film, a signifi-cantly greater activity towards proton reduction is ob-served (Fig. 2, curve c). This means that the actualsurface area of the reactive platinum centers is thehighest in case (c).

A quite analogous result was obtained when, insteadof the proton reduction, another diagnostic reaction,this time based on electrocatalytic oxidation, which isknown to be highly sensitive to the presence of traces ofplatinum, was considered. The voltammetric oxidationof arsenic(III), namely H3AsO3 in 2 M H2SO4, wassuch a model reaction for probing dispersed Pt [35]. Itshould be noted that the method based on electrocata-lytic probing of platinum was historically developed inorder to characterize electrode surfaces containing ex-tremely low (ultratrace) Pt loadings which cannot beeasily detected using common spectroscopic surfaceanalysis approaches such as Auger, XPS, SIMS andenergy dispersion X-ray methods [35]. Fig. 2B showsthat the respective cyclic voltammetric responses werestrongly dependent on the choice of the electrode sur-face. Curve a, which was obtained for bare carbon,indicated that no oxidation was possible in the absenceof platinum metal. The results of Fig. 2B (curve b andc) indicate the presence of voltammetric peaks at about1.0 V that shall be attributed to the oxidation of As(III)[38] at platinized bare carbon and Mo18-modified car-bon. By analogy with the hydrogen evolution patterns(Fig. 2A, curves a and b), the respective voltammetricpeaks of Fig. 2B (curve b and c) were also quite similar.Finally, as it is apparent from Fig. 2B (curve d), themost pronounced voltammetric peak for the arsenic(III)oxidation was observed for the system in which plat-inum was generated on top of a Mo18/DAD modifiedelectrode. The fact that the oxidation of As(III) tookplace at slightly less positive potentials, and the obser-vation that a more defined peak was obtained in curved, can be explained, in terms of differences in morphol-ogy or interfacial reactivity of platinum centers immo-bilized in hybrid Mo18/DAD film. It shall beremembered that both, the chronopotentiometric pat-tern recorded during generation of platinum particles(Fig. 1, curve c) and the voltammetric response for thehydrogen evolution reaction (Fig. 2A, curve c) weremarkedly different in the case of the hybrid Mo18/DADsystem. The superior electrocatalytic behavior of the

charge of DAD, eliminates the previously mentionedrepulsion forces existing in case (b). Further, the ab-sence of a potential minimum in curve c may imply theexcess of positive charge due to the presence of DAD atthe interface thus being capable of attracting negativelycharged chloroplatinate. It should be remembered thatcontrolled spatial distribution of noble metal catalystswas historically achieved by ion exchange with surfacegroups on carbon fiber supports [3,4]. In the presentcase, it is even possible that true chemical interactionalso exists, which can be understood in terms of thewell-known high affinity of Pt(IV) for amino groups.

3.2. Electrocatalytic characteristics of the surfaces

It is plausible to expect that the existence of initiallydifferent overpotentials, related to the current flowingduring electrodeposition at different electrode surfaces(Fig. 1), has an impact on the nucleation and the finalmorphology of the obtained platinum particles. Beforeperforming the examination of the electrode surfaces bySEM, we have decided to probe the electrocatalytic

D. Martel et al. / Electrochimica Acta 46 (2001) 4197–4204 4201

platinized Mo18/DAD modified electrode most proba-bly corresponds to the higher dispersion of platinum inthe hybrid system. This conclusion is even strengthenedwhen taking into account that for the deposition ofplatinum on the hybrid layer lower overpotentials havebeen observed. With respect to the above discussionabout inhibition, this means that the current efficiencyfor generating Pt should be the smallest in this casecompared with the other two samples. Despite theresulting lower absolute quantity of deposit a highercatalytic activity is observed, meaning that its activesurface/dispersion has to be higher.

3.3. Examination of dispersed platinum

In order to get more direct information about theactual dispersion of the platinum metal deposits, wehave studied their morphologies by scanning electronmicroscopy (SEM). Fig. 3 shows SEM micrographscorresponding to the galvanostatic electrodeposition ofplatinum that has been performed for 300 s on barecarbon (A), carbon modified by Mo18 (B), and carbonmodified by Mo18 and DAD (C), respectively. It isapparent from the first two images (Fig. 3A and B) thatplatinum particles are clearly visible as white spots ofapproximately 0.15 �m radius. The fact, that no signifi-cant differences in the morphologies of particles gener-ated at the first two electrode surfaces (Fig. 1, curves aand b) are visible in Fig. 3A and B confirms our viewabout the comparable electrochemical characteristics ofboth surfaces. Fig. 3C does not show, however, anyplatinum particles at all, at least at the magnificationlevel available in the present study. On the basis of ourelectrochemical observations, namely electrocatalyticresults (Fig. 2A and B), it can be stated that platinumis not only present on the surface of the hybrid systembut also in a quite active form. It can be hypothesizedthat the metal is highly dispersed on the surfacemodified by Mo18 and DAD, and most likely formsparticles of diameters much smaller than 0.1 �m (sincethey are not visible as white spots). A possible explana-tion is that the existence of the positively chargedamino groups on the surface favors generation of alarge number of nucleation sites for the formation ofplatinum submicro particles.

Since neither the amount nor the active area ofplatinum electrodeposited on the hybrid Mo18/DADlayer can be assessed from SEM data, we have referredto the previously reported idea of estimation of theplatinum coverage on a carbon substrate by using theAs(III) oxidation reaction as an electrocatalytic probefor the detection of traces of platinum [35]. The conceptwas based on the following observations and assump-tions: (1) the electrocatalytic oxidation of As(III) is avery fast and specific process, and the respectivevoltammetric responses (recorded at a sufficiently lowscan rate) of the platinized electrode can be described in

Fig. 3. SEM micrographs (acceleration voltage: 15 kV) ofelectrode surfaces obtained following platinum electrodeposi-tion (performed as in Fig. 1) onto (a) bare carbon, (b) carbonmodified with Mo18, and (c) carbon modified with hybridMo18/DAD.

D. Martel et al. / Electrochimica Acta 46 (2001) 4197–42044202

Fig. 4. Working curve that correlates the voltammetric analyt-ical (peak) currents for the electrocatalytic oxidation of As(III)(as in Fig. 2B) with different platinum loadings expressed asspecific surface areas of Pt, i.e. as the ratios of SPt/Selectrode

where SPt is the estimated active area of dispersed platinumand Selectrode is the geometric surface of the whole electrode.

reactivity of Pt particles), the SPt/Selectrode valuesfound should be considered just as estimates. Never-theless, the determined sequence of increasing specificsurface areas of deposited platinum upon going frombare substrate via Mo18-modified carbon to the hy-brid Mo18/DAD system is justified. What is evenmore important is that the results of electrocatalyticprobing of platinized surfaces clearly imply the exis-tence of sizable amounts of metallic Pt centers de-posited on the hybrid Mo18/DAD layer.

In the case of Pt deposition on bare carbon orcarbon modified with Mo18, the obtained particles arebig enough to allow a direct comparison of informa-tion extracted from the images and from the cyclicvoltamograms. First of all it is possible to calculateapproximately the current efficiency during metal for-mation. Referring to picture A of Fig. 3 we can makethe following estimation:

The typical diameter of the particles is around 0.3�m and there are roughly 25 particles on this part ofthe surface. Supposing spherical geometry, one parti-cle has a volume of 0.014 �m3 and therefore the totalexperimental Pt volume on this fraction of the surfaceis 0.35 �m3. On the other hand, 150 mC have beendelivered to the system, corresponding to 7.5×10−5

g of Pt or a volume of 3.5× l0−6 cm3. With the totalelectrode surface one can calculate that on the part ofthe electrode corresponding to the SEM image oneshould find 1.7 �m3 of Pt. Comparing this to the 0.35�m3 obtained above, the current efficiency seems tobe around 20%, a value that is in agreement withdata recently reported in the literature [40].

Furthermore we can verify independently theamount of Pt determined by SEM. This is usuallydone by H or O upd [40] but a rough estimate shouldalso be available from diffusional currents.

In Fig. 2B (curve b) the measured current at 1 V is2 mA. Using the Randles–Sevcik equation and as-suming a typical diffusion coefficient of around 1×10−5 cm2/s we obtain, together with the otherexperimental data, an active Pt surface area of 0.128cm2.

On the other hand, based on the SEM observation,we can calculate the surface area of the visible plat-inum particles. As above, we use a mean diameter of0.3 �m and 25 particles on the picture visualized bySEM, leading to a total platinum surface in contactwith the solution of 0.134 cm2. Despite the severeapproximations, like assuming linear diffusion, onecan conclude that there is an agreement between bothvalues and the visible Pt centers seem to be responsi-ble for the arsenic oxidation current. The ratio ofSPt/Selectrode can be recalculated from this value andleads to 0.068, which is quite close to the result ob-tained with the above-mentioned working curve.

diffusional terms according to the Randles–Sevcikequation in which peak current is proportional to theactive area; (2) each dispersed platinum particle isapproximately spherical and its whole surface is reac-tive; (3) the surface area can be estimated from geo-metric parameters (number of Pt particles and theirradii) which are available from SEM micrographs; (4)the resulting active surface area of platinum, SPt, canbe normalized by dividing SPt by the geometric elec-trode surface (Selectrode), i.e. by calculating the ratio ofSPt/Selectrode that will be further termed specific sur-face area of Pt. In order to correlate the specific sur-face area of Pt with the related electrocatalyticAs(III) oxidation current, a series of standard elec-trode surfaces with different platinum loadings wereprepared as described in Section 2. To obtain a prac-tical ‘working curve’ (Fig. 4), we have consideredstandard electrode surfaces with moderate loadings ofdispersed platinum that are characterized by welldefined, separated and circular white spots on SEMmicrographs.

Using the working curve presented in Fig. 4 andthe voltammetric peak currents (background sub-tracted) from Fig. 2B, we have attempted to evaluatethe specific surface areas (ratios of SPt/Selectrode) fordispersed platinum particles that exist at the threeelectrocatalytic surfaces considered in this work. Theresults of SPt/Selectrode can be summarized as follows:0.06, 0.1, and 0.5 for platinized carbon, carbonmodified with Mo18, and carbon modified with thehybrid Mo18/DAD layer, respectively. Due to uncer-tainties in the determination of voltammetric analyti-cal currents and in the assumptions related to theconcept applied (possibility of some contribution fromspherical diffusion to submicro Pt particles, nonide-ally spherical shape of particles, or incomplete surface

D. Martel et al. / Electrochimica Acta 46 (2001) 4197–4204 4203

4. Conclusions

On the basis of the present experiment, we are unableto make unequivocal judgment about the morphologyand mechanism of generation of platinum particles onthe hybrid Mo18/DAD layer. The fact that the reduc-tion of protons and the electrooxidation of arsenic(III)(Fig. 2B, curve d) need Pt0 [38] supports our view aboutthe existence of metallic platinum centers as a result oftheir galvanostatic fabrication (Fig. 1, curve c). Al-though we do not have direct evidence, all our resultsare consistent with the presence of metallic, rather thannot completely reduced, platinum species. Having inmind recent important findings [37,39] concerning plat-inum electrodeposition at low overvoltages, one canexpect that the process of growth of Pt particles is akinetically controlled nucleation (what could be ex-pected from the data of Fig. 1) and, mechanistically, itmay be complex and involve disproportionation ofPt(II) chloride complexes (or some other products ofincomplete reduction of [PtCl6]2−) during the interme-diate step. Regardless the actual mechanism, the pres-ence of positively charged amino groups from DADhas a profound effect on the final morphology ofdispersed Pt. It is likely that platinum is produced in aform of submicro- or even nanoparticles. Further re-search along this line that involves scanning probemicroscopic techniques (STM, AFM, SECM) is re-quired to assess the actual size and distribution of Ptdeposits on the Mo18/DAD layer as well as their mor-phological changes with time. At present, we can statethat the system’s electrocatalytic response (Fig. 2B,curve d) has been found to be reproducible duringrepetitive measurements performed on a time scale of afew days.

Acknowledgements

This work was supported by the French–Polish Col-laboration Program ‘Polonium’. D.M. and A.K. aregrateful for the support from Egide under contract No.1388UC. The support from the State Committee forScientific Research Poland (KBN under contract No. 3T09A 031 20) is acknowledged by P.J.K. and M.A.M.

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