Electroless deposition of Pt on Ti

Journal of Electroanalytical Chemistry 511 (2001) 20–30

Electroless deposition of Pt on TiPart II. Catalytic activity for oxygen reduction

Georgios Kokkinidis a,*, Dimiter Stoychev b, Vesselin Lazarov b, Achilleas Papoutsis a,Alexander Milchev b

a Laboratory of Physical Chemistry, Department of Chemistry, Uni�ersity of Thessaloniki, 54006 Thessaloniki, Greeceb Institute of Physical Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonche� Street, bl. 11, 1113 Sofia, Bulgaria

Received 4 October 2000; received in revised form 5 April 2001; accepted 7 April 2001

Abstract

The reduction of oxygen on electroless deposited platinum on a freshly polished titanium electrode was studied in aqueous 0.1M HClO4 and 0.2 M NaOH solutions, utilising the rotating-disc electrode technique. TEM studies show that at short depositiontimes Pt is highly dispersed on titanium. At longer deposition times the degree of dispersion decreases due to coalescence andagglomeration processes and larger clusters are formed. The kinetics of oxygen reduction were affected substantially by the timeof Pt deposition on Ti at the open-circuit potential. A kinetic analysis of the current–potential curves was made in the kineticallycontrolled region in the form of Tafel plots and in the mixed kinetic-diffusion control region in the form of Koutecky–Levichplots. The catalytic activity of the Pt crystals decreases with increasing deposition time. The results were interpreted in terms ofthe effect of the Pt crystal size. The larger the Pt particles the lower the catalytic activity. The role of titanium oxide formationand the electronic effects were also discussed. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Platinum; Electroless deposition; Titanium substrate; Oxygen reduction; Catalytic activity; Crystal size effect

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1. Introduction

In a recent investigation [1] we found that platinumcan be deposited on a freshly polished titanium surfacefrom an aqueous solution of 0.1 M HClO4+2×10−3

M K2PtCl6 as a result of an electroless reaction takingplace at the open-circuit potential. The deposition reac-tion was very fast and led to fine dispersions of plat-inum crystals on the titanium surface. The hydrogenevolution reaction was used as a probe reaction fortesting both the catalytic activity and deposition condi-tions. The results provided some evidence that thekinetics of the hydrogen reaction are affected by themorphology and the size of the platinum crystals ob-tained. The smaller the crystals the higher the catalyticactivity was. However, the hydrogen evolution reactionon Pt in acidic solutions is a very fast reaction and we

are not quite sure that true kinetic rates were actuallymeasured in that study.

As a continuation of our previous work [1] in thispaper we report a study of the kinetics of oxygenelectroreduction on electroless deposited platinum ontitanium, utilising the rotating-disc electrode technique.The aim is to verify whether there is a distinct relation-ship between the morphology and the size of the elec-troless Pt crystals and their catalytic activity forelectrochemical reactions. The reduction of oxygen ap-pears to be a more suitable reaction for this purposesince its kinetics on Pt are much slower than that of thehydrogen reaction, and accurate kinetic rates can bemeasured both in acid and in alkaline solutions. Thiswork could also be considered as an attempt to con-tribute to the studies on oxygen reduction carried outduring the last two decades on supported Pt electrocat-alysts [2–24]. Till now, the most widely used supportsfor dispersing Pt (or other metals) were carbon andconducting polymers and occasionally other supports.Supported metal catalysts play a crucial role in many

* Corresponding author. Tel.: +30-31-997751; fax: +30-31-997709.

E-mail address: [email protected] (G. Kokkinidis).

0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 -0728 (01 )00505 -8

G. Kokkinidis et al. / Journal of Electroanalytical Chemistry 511 (2001) 20–30 21

industrial chemical and electrochemical reactions. In theparticular case of oxygen reduction the most notableneed for using supported Pt electrocatalysts is the devel-opment of cheap and efficient cathodes for oxygenreduction in fuel cells [2–10]. On the other hand, oxygenreduction is considered to be a very suitable reaction forstudying the crystal size effects mainly due to its complexmechanism. Such studies are aimed at a better under-standing of the structure and the composition of theelectrode surface in relation to its activity for thisreaction [11–24].

2. Experimental

2.1. Electrodes and electrolytes

Titanium discs were cut from a titanium wire (John-son Matthey, purity �99.99, 2 mm diameter) and fixedinto a teflon cylinder. The end surface was mechanicallypolished with 1500 grade emery paper. After polishingthe electrode was washed carefully with distilled water.Platinum crystals were deposited at the open-circuitpotential from an aqueous solution of 0.1 M HClO4

containing K2PtCl6 as described in Part I of this study[1]. The only difference was that in the present work, theconcentration of K2PtCl6 in the plating solution was2×10−4 M instead of 2×10−3 M and the solution was

stirred using a magnetic stirrer. Electrodes prepared inthis way were used as working electrodes for the oxygenreduction by adapting them to the Tacussel (EDI)rotator system.

The electronic set-up consisted of a Tacussel bipoten-tiostat BI-PAD, a triangular wave generator Bank Elec-tronics VSG 72 and a Yokogawa 3023 X,Y– t recorder.An aqueous Hg � Hg2SO4 � Na2SO4 (sat) electrode(MSE), connected to the working electrode compart-ment by a Luggin capillary and a platinum sheet servedas the reference and the counterelectrode, respectively.All potentials given in this paper are referred to the MSE(EMSE=0.65 V vs. SHE). The working electrolyte was0.1 M HClO4 or 0.2 M NaOH saturated with oxygen at1 atm. All measurements were made at constant temper-ature 25�0.2°C. Solutions were prepared using triplydistilled water and reagents from Merck (HClO4 andNaOH, suprapur) and Fluka (K2PtCl6, puriss).

2.2. Preparation of samples for TEM and XPS

Electroless platinum was deposited by immersing afreshly polished titanium plate (Johnson Matthey, purity99.7) in an aqueous solution of 0.1 M HClO4+2×10−4

M K2PtCl6 for deposition times td from 5 to 60 s. Afterthe deposition of platinum the specimens were washedimmediately and dried. Then, they were put in a vacuumevaporator (base pressure 10−6 Pa), where they werecoated with a relatively thick carbon layer. The C � Ptbilayer was removed from the titanium surface bychemical extraction (the so called ‘replica extractionmethod’) treating the specimens with an aqueous solu-tion of 0.045 M HF+0.02 M HNO3+0.02 MCH3COOH. The separated bilayer was rinsed with dis-tilled water up to a neutral pH of the washing water.After that, the C � Pt bilayer was transferred to a coppergrid for transmission electron microscopy (TEM). TheTEM study was performed in a JOEL 200 CX electronmicroscope operating at 160 kV. For every transmissionelectron micrograph the corresponding picture of thediffraction pattern was obtained.

The Pt deposits on the titanium plate were alsoexamined by X-ray photoelectron spectroscopy (XPS),comparing Pt 4f and Ti 2p peaks. The XPS analysis wascarried out in an Escalab II electron spectrometer (VGScientific) using Mg–K� radiation (h�=1253.6 eV) as anexcitation source. The base pressure in the spectrometerchamber was 10−8 Pa.

3. Results

3.1. Morphology of electroless deposited Pt on Ti

In addition to the SEM studies described in ourprevious work [1] the TEM images (Figs. 1 and 2) showthat the deposited platinum forms islands on the tita-

Fig. 1. TEM image and the corresponding diffraction pattern ofelectroless deposited Pt crystals on freshly polished Ti after immersingit in 0.1 M HClO4+2×10−4 M K2PtCl6 solution for 5 s.

G. Kokkinidis et al. / Journal of Electroanalytical Chemistry 511 (2001) 20–3022

Fig. 2. As in Fig. 1 with immersion time of 60 s.

nium substrate. The islands were most likely built upfrom Pt nanoparticles which were homogeneously dis-persed on the titanium surface. The amount of plat-inum deposited increases with increasing immersiontime in the working electrolyte. Due to coalescence andagglomeration processes the Pt particles form aggre-gates with linear sizes 30–40 nm and polycrystallinestructure (see the diffraction pattern in Fig. 2). At shortdeposition times (td=5 s) the diffraction patterns ob-tained are not that sharp (Fig. 1) because the platinumparticles have not reached the form and size of suffi-ciently large crystalline blocks needed for the registra-tion of a typical crystalline diffraction pattern. Suchstructures are known as ‘cryptocrystalline’ or ‘pseu-docrystalline’ and are considered by some authors as‘amorphous like’ structures [25]. As can be seen in Fig.1 the blocks which start forming aggregates have sizes�4–12 nm.

The XPS spectra (Fig. 3) of Ti � Pt samples preparedat three deposition times prove that the amount ofdeposited platinum increases with increasing depositiontime. The Pt/Ti ratio calculated from the Pt 4f and Ti2p peaks is indicated in each spectrum in Fig. 3. ThePt/Ti ratio can be considered as a qualitative criterionfor the total amount of Pt deposited rather than thetotal coverage surface area of Pt. The surface area of Ptclusters practically does not increase, as is shown byvoltammetry (see Table 1), despite the almost tenfoldincrease of the amount of deposited platinum between 5and 60 s deposition time. This is a somewhat unex-pected result. In our opinion this might be due tostrong coalescence effects during the growth of the Ptnanoclusters. That being the case the amount of de-posited Pt could increase much faster than its surfacearea. The indications for coalescence were presentfound in Part I of our study (Fig. 8 of Ref. [1]).

The deconvolution of the original experimental Pt 4fpeaks into platinum (A1 and B1) and platinum oxide(A2 and B2) peaks (Fig. 3) was possible only for thespectrum of the specimen obtained at td=60 s andprovided additional information on the chemical natureof the electroless platinum deposit. It turned out thatabout 20% of its surface area existed in the form of aplatinum oxide.

Fig. 3. XPS spectra for Pt 4f obtained from Ti � Pt samples preparedby immersing a freshly polished Ti plate in 0.1 M HClO4+2×10−4

M K2PtCl6 solution for: (1) 5 s; (2) 10 s; (3) 60 s. A and B: originalplatinum peaks; A1 and B1: deconvoluted platinum peaks; A2 and B2:deconvoluted pltainum oxide (PtO) peaks.

Table 1Active surface area of electroless deposited Pt crystals on a freshlypolished titanium electrode (STi=0.0314 cm2) from 0.1 M HClO4+2×10−4 K2PtCl6

Deposition time (td)/s Active area of Pt crystals 102 SPt/cm2

1.90515 2.0420 2.2835 2.1660 2.32


Fig. 4. Voltammograms for hydroquinone (2×10−3 M) oxidation onTi � Pt (td=15 s) rotating-disc electrode in 0.1 M HClO4 (dE/dt=20mV s−1). Rotation frequency f (Hz): (1) 8.33; (2) 12.5; (3) 18.33; (4)25; (5) 33.33. The inset shows plot of IL vs. �1/2 (�=2�f ). Thedotted curve corresponds to the Ti electrode without Pt at f=25 Hz.

3.2. Acti�e surface areas of the Pt crystals

Active surface areas of the Pt crystals (SPt) for differ-ent deposition times were calculated from the chargesof the hydrogen adsorption. The values obtained aregiven in Table 1. These values are in agreement withthose reported in our previous work [1] taking intoaccount the surfaces of the titanium substrate 7.85×10−3 cm2 in Ref. [1] and 0.0314 cm2 in the presentwork. As can be seen the exposed surface area of the Ptcrystals is practically constant within the experimentalerrors. The mean SPt is 0.021 cm2.

However, in the interpretation of the RDE datapresented below for the reduction of oxygen the areathat should be used in the Levich equation should notbe the electrochemically active area of the Pt crystalsbut the geometric area (0.0314 cm2) of the Ti-discelectrode. If a sufficient amount of Pt is distributedacross the radial dimension of the disc, the diffusionlimiting current would correspond to what the Levichequation predicts based on the surface area of 0.0314cm2 for a 2 mm diameter disc. Due to the overlappingof the spherical diffusion zones corresponding to the Ptmicroparticles, the diffusion on the Ti � Pt surfaceshould be equivalent to planar diffusion to the wholesurface, and therefore the limiting current should beindependent of the real surface area of the Pt particles.

In order to ascertain that the diffusion limiting cur-rents should be interpreted independently of the Ptloading, we studied the electrooxidation of hy-droquinone on the rotating Ti � Pt disc electrode. Itshould be noted that bare Ti is totally inactive for thisreaction. Fig. 4 shows, as an example, the rotating-discvoltammograms obtained in an aqueous 0.1 M HClO4

solution containing 2×10−3 M hydroquinone at theTi � Pt (td=15 s) electrode. As can be seen (inset of Fig.4) the limiting current (IL at +0.6 V) varies linearlywith �1/2. The slope dI/d�1/2=6.6×10−3 mA rad−1/2

s1/2 appears to be constant independently of the deposi-tion time of platinum and similar to that calculatedusing values of n=2, D=8.50×10−6 cm2 s−1 [29],�=8.929×10−3 cm2 s−1 and S=0.0314 cm2 for theelectrode surface area. This means that the area-term inthe Levich equation should not be the active area of thePt crystals (0.021 cm2) but the geometric area of theTi-disc electrode.

3.3. Oxygen reduction on electroless deposited Pt on Tiin acid solutions

Before discussing the results on oxygen reduction weshould emphasise that the rotating-disc voltam-mograms obtained on the Ti � Pt electrode may differsubstantially, even at a constant time of deposition ofelectroless Pt on Ti. As an example Fig. 5 shows a setof four voltammograms for td=15 s. They were

Fig. 5. Set of voltammograms for oxygen reduction on a Ti � Pt(td=15 s) rotating-disc electrode in O2-saturated 0.1 M HClO4

(dE/dt=20 mV s−1). Rotation frequency f=12.5 Hz. The solidcircles represent the average current. The dotted curve corresponds tothe Ti electrode without Pt.

Undoubtedly, an important point concerning theprocess of electroless platinum deposition on the tita-nium surface is the mode of growth of the three-dimen-sional (3D) platinum clusters. Excluding with certaintythe Frank-van der Merwe ‘layer by layer’ growth mech-anism [26] we are, unfortunately, not in a position tojudge whether the observed spherical platinum singlecrystals and polycrystalline aggregates (Fig. 2) wereformed directly on the bare titanium surface (Volmer–Weber mechanism [27]) or whether some preformedtwo-dimensional (2D) adsorbed platinum layer, or 2Dplatinum islands, play the role of ‘substrates’ or ‘activeregions’ for the 3D platinum deposition (Stranski–Krastanov mechanism [28]). More detailed investiga-tions, possibly including STM technique, should becarried out in order to elucidate this importantquestion.


recorded in the negative sweep direction after the elec-trode was kept at the starting potential Est= +0.2 Vfor 10 s to create standard conditions for oxidation ofthe titanium substrate. The observed scattering of thevoltammograms may be attributed to the random dis-tribution of the number and of the size of the Ptclusters deposited at the open-circuit potential on afreshly polished Ti surface. This is probably due toboth the random character of the electroless processitself and the differences in the energetic state of thetitanium surface, which had to be polished after eachexperiment in order to remove the platinum clustersand to prepare the electrode for a new experiment. Itshould be noted that similar behaviour was also ob-

served [30] for potentiostatic current transients recordedduring the nucleation and growth of Pt clusters on anoxidised titanium working electrode. Due to the scatter-ing at least four or five rotating-disc voltammogramswere recorded for each rotation rate, utilising a newlyprepared Ti � Pt electrode each time. The representativecurrent–potential curve (solid circles) is obtained byfinding the average value of the current for every 25mV.

Fig. 6 shows averaged current–potential curves forthe oxygen reduction in 0.1 M HClO4 on Ti � Pt elec-trodes corresponding to different deposition times ofplatinum. For comparison the current–potential curveon a smooth Pt rotating-disc electrode, having the samediameter (2 mm) as the Ti-disc electrode used for thedeposition of platinum, is also given in Fig. 6. Aver-aged current–potential curves were obtained over arange of rotation rates (500–2000 rpm) and one set ofsuch curves for the Ti � Pt (td=15 s) is shown in Fig. 7.The limiting current plateaus are not well expressedexcept for the Ti � Pt (td=5 s) electrode. There are twopossible reasons why the current decreases in the region−0.5 to −0.7 V versus MSE. One is that the upd ofhydrogen probably inhibits the dissociation of oxygen,thus shifting the O2 reduction pathway toward the 2e−

route [31]. The second could be that deposited Pt is notactive enough to catalyse the high O2 reduction currentsrequired to approach the limiting current for the 4e−

reaction pathway. Both these might explain why aslightly curved line is obtained when the maximumcurrents (i.e. I at −0.55 V) are plotted versus �1/2 (seethe inset diagram of Fig. 7). To prove which one of theabove explanations is valid rotating ring-disc experi-ments should be carried out.

The above results clearly show that the catalyticactivity of electroless deposited Pt crystals for the oxy-gen reduction is substantially affected by the time ofdeposition, with the activity decreasing with increasingdeposition time. Kinetic analyses of the current-poten-tial curves are presented in the kinetically controlledregion in the form of Tafel plots (Fig. 8) and in themixed kinetic-diffusion control region in the form ofKoutecky–Levich plots (Fig. 9). It should be notedagain that averaged currents from four or five indepen-dent experiments were used for tracing both Tafel andKoutecky–Levich plots. Tafel slopes at low currentswere around −140 mV dec−1. At higher currents theTafel slopes increased to around −290 mV dec−1.Exchange current densities (given in Table 2) wereobtained by extrapolation of the Tafel lines at lowcurrents to the equilibrium potential of O2/H2O (Eeq=+0.52 V vs. MSE in 0.1 M HClO4). For the evaluationof the exchange current densities the mean active sur-face areas SPt=0.021 cm2 was used. There is a ten-dency for the exchange current densities to decrease asthe time of deposition of Pt on Ti increases.

Fig. 6. Averaged current–potential curves for oxygen reduction on aTi � Pt (td/s) rotating-disc electrode in O2-saturated 0.1 M HClO4

(dE/dt=20 mV s−1). Rotation frequency f=18.33 Hz. Depositiontime td (s): (1) 5; (2) 15; (3) 20; (4) 35; (5) 60; (6) smooth Pt (0.0314cm2).

Fig. 7. Averaged current–potential curves for oxygen reduction on aTi � Pt (td=15 s) rotating-disc electrode in O2-saturated 0.1 M HClO4

(dE/dt=20 mV s−1). Rotation frequency f (Hz): (1) 8.33; (2) 12.5;(3) 18.33; (4) 25; (5) 33.33. The inset shows plot of Iat −0.55 V vs. �1/2

(�=2�f ).


Fig. 8. Tafel plots for oxygen reduction in O2-saturated 0.1 M HClO4

on a Ti � Pt electrode for different td (s): (1) 5; (2) 15; (3) 20; (4) 35;(5) 60. dE/dt=20 mV s−1. Rotation frequency f=12.5 Hz.

From the Koutecky–Levich plots (Fig. 9) kineticcurrents at two different potentials in the mixed kinetic-diffusion control region were calculated by means ofthe equation

1I=

1Ik

+1

b�1/2

where I is the measured current, � is the electroderotation rate, Ik is the kinetic current given by

Ik=nFSPtkccO2

and b is the Levich slope

b=0.62nFSPtcO2(DO2

)2/3�−1/6

Values of Ik and b were calculated at −0.35 and−0.40 V by means of the least-squares analysis of the1/I versus 1/�1/2 data. From the experimental values ofIk and b the cathodic reaction rate constant kc and thenumber n of the electrons transferred per O2 moleculewere evaluated by using values of cO2=1.1×10−6

mol cm−3 [24], DO2=1.8×10−6 cm2 s−1 [24] and �=8.929×10−3 cm2 s−1. The values of n and kc derivedare also given in Table 2. The number of electronstransferred are equal or very close to 4 and the rateconstant kc decreases by about a factor of 5–6 as thedeposition time of Pt crystals increases from 5 to 60 s.

3.4. Oxygen reduction on electroless deposited Pt on Tiin alkaline solutions

The kinetics of oxygen reduction on electroless de-posited Pt on Ti were also studied in aqueous 0.2 MNaOH solution. Figs. 10 and 11 show averaged cur-rent–potential curves for oxygen reduction on Ti � Ptelectrodes for three different deposition times at aconstant rotation rate and for one deposition time(td=15 s) as a function of the electrode rotation rate,respectively. For comparison the current–potentialcurve for O2 reduction on smooth Pt and the voltam-mograms for the hydrogen evolution reaction in theabsence of molecular oxygen in solution are also givenin Fig. 10.

As in acid solutions the catalytic activity of electro-less deposited Pt crystals or crystal aggregates for oxy-gen reduction decreases with increasing deposition time.The decrease of the catalytic activity results in the shiftof the current–potential curves toward more negativepotentials, but not as much as in acid solutions. Theanalyses of the currents in the mixed kinetic-diffusioncontrol region are shown in the form of Tafel plots(Fig. 12a) and Koutecky–Levich plots (Fig. 12b). Inorder to obtain Tafel plots in this region mass-transportcorrected currents were used. The mass-transport cor-rected or kinetic current (Ikin) was obtained using theequation [32]

Fig. 9. Koutecky–Levich plots for oxygen reduction in O2-saturated0.1 M HClO4 on a Ti � Pt electrode for different td (s): (1) 5; (2) 15;(3) 20; (4) 35; (5) 60. dE/dt=20 mV s−1. The solid circles correspondto smooth Pt with a surface area 3.14×10−2 cm2.


Ikin=ILI

IL−I

where IL and I are the limiting current and the observedcurrent, respectively. IL was taken as the limiting cur-rent measured for the Pt smooth electrode and notthose of the Ti � Pt electrodes.

The Tafel slope at low currents is between −125 and−150 mV dec−1 and at high currents between −155and −220 mV dec−1. The Tafel slopes are higher thanthe slopes −62 and −105 mV dec−1 obtained forsmooth platinum. Tafel slopes for O2 reduction −60and −120 mV dec−1 were reported for smooth Pt atlow and high currents, respectively, both in acid andalkaline solutions [33,34]. As a measure of the catalyticactivity the kinetic current (Ikin) at −0.8 V was takenarbitrarily instead of the exchange current that shouldstrictly be used, since extrapolation of the Tafel plots tothe equilibrium potential would result in an unaccept-able uncertainty. However, the choice of a different

potential within the mixed kinetic-diffusion control re-gion would not affect our conclusion. Kinetic data foroxygen reduction on Ti � Pt electrodes in 0.2 M NaOHare summarized in Table 3.

Finally, it is worth noting that the time of electrolessdeposition of Pt on Ti has a strong influence on thecatalytic activity of the Pt crystals for the hydrogenevolution reaction (see the voltammograms in Fig. 10obtained in the N2-saturated solution of 0.2 M NaOH).As we already mentioned in Section 1 in Part I of thisstudy [1] we reported that the catalytic activity ofelectroless Pt on Ti for the hydrogen evolution reactionin acid solutions decreases slightly with increasing de-position time. The variation in the activity was relatedto the size of the Pt crystals. In the alkaline solution theeffect is much more intense, but in the opposite direc-tion. The activity greatly increases with increasing de-position time. The Pt crystals obtained at td=60 sexhibit higher activity by more than one order of

Table 2Kinetic parameters for oxygen reduction on electroless deposited platinum crystals obtained on a freshly polished titanium electrode with a surfacearea STi=3.14×10−2 cm2 in 0.1 M HClO4 (T=298 K)

Koutecky–Levich analysisDeposition time (td)/s Tafel analysis

E=−0.35 V E=−0.40 VLow current High current

103 kc/cm s−1 n 103 kc/cm s−1 nb/mV dec−1 109 I0/A cm−2 b/mV dec−1

−310−135 29.2 3.9 39.0 4.013.35−290−139 18.3 3.5 22.4 3.810.815

3.915.93.312.120 −2808.9−1436.8 −270 9.3 3.2 11.5 3.735 −143

−144 6.660 2.76.1−2905.1 3.4

Fig. 10. (a) Averaged current–potential curves for oxygen reduction on Ti � Pt rotating-disc electrode in O2-saturated 0.2 M NaOH; and (b)voltammograms for hydrogen evolution in N2-saturated 0.2 M NaOH (dE/dt=10 mV s−1). Rotation frequency f=25 Hz. Deposition time td (s):(1) 5; (2) 15; (3) 20; (4) 35; (5) 60; (6) smooth Pt with a surface area 3.14×10−2 cm2. The dotted curve corresponds to the Ti electrode withoutPt both in O2-saturated and N2-saturated 0.2 M NaOH.


Fig. 11. Averaged current–potential curves for oxygen reduction onTi � Pt (td=15 s) rotating-disc electrode in O2-saturated 0.2 M NaOH(dE/dt=10 mV s−1). Rotation frequency f (Hz): (1) 8.33; (2) 12.5;(3) 18.33; (4) 25; (5) 33.33. The inset shows plot of Iat −1.15 V vs. �1/2

(�=2�f ).

this model the specific activity (rate per unit area)dependence can be ascribed to the change in the relativefraction of Pt surface atoms on the (111) and (100)faces of Pt particles, assuming cubo-octahedral geome-try. Markovic et al. [20] have recently postulated thatthe kinetics of oxygen reduction on Pt(hkl) surfaces inalkaline solutions and other nonadsorbing electrolytes(such as HClO4) increase in the order (100)� (110)�(111). Since the (111) face dominates in small Pt parti-cles they predict an increase, although not assignificant, of the specific activity with decreasing parti-cle size in these electrolytes. The opposite is expected insulphuric acid solutions. Due to the strong adsorptionof sulphate/bisulphate anions the Pt(111) face is muchless active than the (100) and (110) faces. Thus, thespecific activity of the crystals in this electrolyte isexpected to decrease with decreasing size of the Ptparticles.

Our experimental data on HClO4 and NaOH elec-trolytes confirm the prediction of Markovic et al. [20],

Fig. 12. (a) Mass-transport corrected Tafel plots; and (b) Koutecky–Levich plots for oxygen reduction in O2-saturated 0.2 M NaOH on aTi � Pt electrode for different td (s): (1) 5; (2) 15; (3) 35; (4) smooth Pt.dE/dt=10 mV s−1.

magnitude than those obtained at td=5 s. The hydro-gen evolution in alkaline solutions is known to be avery sensitive reaction to the structure of the Pt surface[35]. The observed substantial variation in the activity isprobably related to the differences in the morphologyand the size of the Pt crystallites, emphasizing the veryimportant role which these factors may play in electro-catalysis involving structure sensitive reactions like thehydrogen evolution reaction in alkaline solutions [35].

4. Discussion

Polarisation curves for oxygen reduction on electro-less deposited Pt on Ti in acid and alkaline solutionsshow that the kinetics of this reaction are affectedsubstantially by the time of Pt deposition. TEM obser-vations have revealed that at short deposition timesplatinum is highly dispersed on the titanium substrate.However, the degree of dispersion decreases at longerdeposition times. Coalescence and agglomeration dur-ing the cementation reaction result in a redistribution ofPt from smaller to larger particles. This, in turn, sug-gests that the kinetics of O2 reduction are probablyrelated to the size of the Pt crystallites.

The effect of Pt particle size on the kinetics of O2

reduction has been studied widely, but almost exclu-sively on carbon-supported Pt catalysts. Different andsometimes contradictory observations have been re-ported on these effects. The different views have beendiscussed in the literature [9,16–20] and need not berepeated here. Our results on the titanium-supported Ptcatalyst appear to agree with the model of the particlesize effect suggested by Kinoshita [15]. According to


Table 3Kinetic parameters for oxygen reduction on electroless deposited platinum crystals obtained on a freshly polished titanium electrode with a surfacearea STi=3.14×10−2 cm2 in 0.2 M NaOH (T=298 K)

Tafel analysis Koutecky–Levich analysisDeposition time (td)/sat −0.9 V

High current b/mV dec−1 103 Ikin, −0.8 V/A cm−2Low current b/mV dec−1 103 kc/cm s−1 n

−155 7.8 39.75 4.0−125−185 3.3−140 25.515 3.9

−15035 −220 2.6 19.4 3.7

Smooth Pt −105−62

since we obtained an increase (by a factor of 3–6) ofthe specific activity of electroless deposited Pt crystalswith decreasing deposition time, i.e. with decreasingsize of the Pt crystallites dispersed on the titaniumsupport. Further evidence in support of the modelproposed by Kinoshita [15] for the particle size effect,could be the experimental data on the hydrogen evolu-tion reaction in alkaline solutions (Fig. 10). Again,Markovic et al. [35] have reported that the kinetics ofthe hydrogen evolution reaction on Pt(hkl) surfaces inalkaline solutions depends strongly on the crystallo-graphic orientation, with the Pt(111) face being lessactive by about one order of magnitude than the (100)and (110) faces. Indeed, we found that the catalyticactivity of electroless deposited Pt crystals on Ti for thehydrogen evolution reaction increases, by more thanone order of magnitude, with increase of the depositiontime from 5 to 60 s. This can be attributed to the factthat at longer deposition times larger Pt crystals areformed, where the more active (100) face dominatesover the less active (111) face.

Another factor that should be considered is the influ-ence of electrocatalyst-support interactions on the cata-lytic activity of Pt particles for oxygen reduction.Tammeveski et al. [22] have studied the reduction ofoxygen on thin films of Pt sputtered onto a titaniumsubstrate. They demonstrated that the specific activitydecreased with decreasing Pt film thickness, and thiswas related directly to the dependence of the bindingenergy of bonding electrons on film thickness. Thebinding energy of Pt 4f electrons for thinner films isshifted toward higher energy values. According toTakasu et al. [18] the increase of the binding energyfacilitates the adsorption of oxygen on the Pt surfaceand this, in turn, decreases the activity for oxygenreduction. However, it should be noted that our XPSresults (Fig. 3) show that the binding energy of the Pt4f electrons is practically independent of the time ofdeposition of electroless Pt on Ti. Relatively thick filmsof sputtered Pt on Ti show sufficient stability for oxy-gen reduction and a voltammetric response, which issimilar to that of polycrystalline smooth platinum

[36,37]. The stability decreases as the Pt film thicknessdecreases [22]. Although sputtered Pt films are rathercompact, oxygen diffuses through the film and oxidisesthe titanium adhesive layer. The Ti � Pt interface isgradually replaced by the TiO(2−x) � Pt one, which haspoor adhesion.

In order to examine the role of titanium oxides thereduction of oxygen was studied on Pt clusters de-posited potentiostatically on oxidised titanium workingelectrodes from an aqueous 0.1 M HClO4 solutioncontaining K2PtCl6. After polishing the titanium elec-trode was polarised for 60 s at +0.4 V to createstandard conditions for titanium oxide formation.Then, the potential was stepped from +0.4 to −0.65V, where nucleation and growth of Pt clusters takesplace. Fig. 13 illustrates representative rotating-discvoltammograms for oxygen reduction in O2-saturated0.1 M HClO4 as a function of the pulse duration. Ascan be seen two reduction waves appear in the voltam-mograms. The reason for this result could be that there

Fig. 13. Voltammograms for oxygen reduction in O2-saturated 0.1 MHClO4 obtained on overpotentially deposited Pt on Ti at −0.65 Vvs. MSE for different pulse durations tp (s): (1) 13; (2) 30; (3) 60.dE/dt=20 mV s−1. Rotation frequency f=18.33 Hz. The insetshows the corresponding current transients for a potential step from+0.4 to −0.65 V vs. MSE in 0.1 M HClO4+2×10−3 M K2PtCl6.


exist Pt crystals exhibiting different catalytic activity foroxygen reduction. There is a small number of active Ptcrystals, which are probably in direct contact with baretitanium sites, and a greater (and continuously increas-ing with the deposition time) number of less active Ptcrystals, presumably located on the titanium surfacecovered by oxides. The catalytic activity of the moreactive Pt crystals decreases with continuous potentialcycling (�=20 mV s−1) in the range between +0.4 and−0.74 V. After four or five cycles the first reductionwave almost disappears and only one wave with E1/2

equal to the half-wave potential of the second reductionwave is obtained. This is likely due to the irreversibleoxidation of the whole titanium surface during poten-tial cycling. Analogous behaviour was observed withelectroless deposited Pt on Ti. The E1/2 of the polarisa-tion curves for the oxygen reduction, more or lessdepending on the deposition time, is shifted towardmore negative potentials and after some cycles closelyapproaches the E1/2 of the polarisation curves obtainedwith the Ti � Pt (td=60 s) electrode.

The formation of titanium oxide might cause alsoelectronic changes on the Pt clusters thus affecting theplatinum–oxygen interactions. The low activity of theTi � Pt compared to that of smooth Pt indicates apronounced electronic effect in this system, otherwisethe low activity of the Pt particles cannot be explained.

Finally, we would like to point out that Ta and Alwere also checked for electroless deposition of platinumat the open-circuit potential. Tantalum appears to be asuitable substrate for electroless deposition of Pt in theplating solution of 0.1 M HClO4 containing K2PtCl6.The Pt crystals obtained on Ta exhibit almost similarcatalytic behaviour for the hydrogen adsorption andhydrogen evolution reaction in 0.1 M HClO4 with thatdescribed previously [1] for the Pt crystals on Ti. Onaluminium, the plating of which is more significant andalso of practical interest, the electroless reaction pro-duces Pt black which catalyses the hydrogen evolution.The cementation reaction is immediately followed bythe very fast electroless reaction between aluminiumand hydrogen ions and, as a result, the aluminiumsample is completely dissolved in the plating solution.

5. Conclusions

To summarise the main points of this work we foundthat the reduction of oxygen on electroless deposited Pton freshly polished Ti is substantially affected by thetime of deposition. TEM observations revealed that atshort deposition times Pt is highly dispersed on tita-nium. At longer deposition times the degree of disper-sion decreases due to coalescence and agglomerationprocesses and larger Pt clusters are formed at theexpense of the smaller ones. The variation in catalytic

activity is correlated to the size of the Pt crystallites.The results confirm the prediction of Markovic et al.[20] that the specific activity of Pt particles for oxygenreduction in perchloric acid and in alkaline solutionsincreases with decreasing particle size which, in turn,supports the model proposed by Kinoshita [15] forparticle size effects on supported Pt electrocatalysts.Finally, the deposition of Pt by potential pulse experi-ments on oxidised titanium surfaces forms Pt crystalswith different activity for oxygen reduction. There exista small number of active Pt crystals probably in directcontact with bare Ti sites and a greater number of lessactive Pt crystals, which are located on the titaniumsurface covered by oxides.

Acknowledgements

The financial support of NATO, International Scien-tific Exchange Program ‘Linkage Grant’, project Ref.No. CRG LG 972954 is gratefully acknowledged.

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Electroless deposition of Pt on Ti