Electrodeposition of Platinum on Highly Oriented Pyrolytic Graphite

7998

J. Phys. Chem. B 2005, 109, 7998-8007

Electrodeposition of Platinum on Highly Oriented Pyrolytic Graphite. Part I: Electrochemical Characterization Guojin Lu† and Giovanni Zangari*,‡ Department of Metallurgical and Materials Engineering, The UniVersity of Alabama, Tuscaloosa, Alabama 35487, and Department of Materials Science and Engineering and Center for Electrochemical Science and Engineering, UniVersity of Virginia, CharlottesVille, Virginia 22904 ReceiVed: NoVember 30, 2004; In Final Form: February 11, 2005

The electrochemical deposition of Pt on highly oriented pyrolytic graphite (HOPG) from H2PtCl6 solutions was investigated by cyclic voltammetry and chronoamperometry. The effects of deposition overpotential, H2PtCl6 concentration, supporting electrolyte, and anion additions on the deposition process were evaluated. Addition of chloride inhibits Pt deposition due to adsorption on the substrate and blocking of reduction sites, while SO42- and ClO4- slightly promote Pt reduction. By comparing potentiostatic current-time transients with the Scharifker-Hills model, a transition from progressive to instantaneous nucleation was observed when increasing the deposition overpotential. Following addition of chloride anions the fit of experimental transients with the instantaneous nucleation mode improves, while the addition of SO42- induces only small changes. Chloride anions strongly inhibit the reduction process, which is shifted in the cathodic direction. The above results indicate that the most appropriate conditions for growing Pt nanoparticles on HOPG with narrow size distribution are to use an H2PtCl6 solution with HCl as supporting electrolyte and to apply a high cathodic overpotential.

1. Introduction Nanostructured platinum particles dispersed onto carbon electrodes are widely used as electrocatalyst materials in lowtemperature fuel cells, including polymer electrolyte membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC).1,2 In these electrode assemblies, optimization of the performance by maximization of the catalytic activity while limiting the noble metal loading is of the utmost importance in order to lower the cost of the finished component. To this end, it is thus necessary to closely control particle size, narrow size distribution, and optimize dispersion. In particular, particle size, shape, and dispersion are important variables since a dependence of the catalytic activity on these parameters has been demonstrated.3-11 Electrochemical deposition (ECD) on solid surfaces with low surface energy is a low-cost, simple, and efficient method to prepare metal nanoparticles. The Volmer-Weber threedimensional island growth of separate islands which eventually coalesce into a continuous film is in fact favored on these surfaces over the two-dimensional layer-by-layer growth or the more complex layer followed by island growth, leading to nanoparticle formation if growth is stopped before coalescence of the islands.12 In this respect, ECD presents the advantage that the driving force for metal formation (the substrate potential) can be controlled more precisely and on a much shorter time scale than in alternative processessfor example, chemical reductionsallowing the required control of the growth process. These features have enabled the synthesis of metal nanoparticle * Corresponding author. Tel: 434-243-5474. Fax: 434-982-5799. Email: [email protected]. † The University of Alabama. ‡ University of Virginia.

arrays with narrow particle size distribution.13,14 Direct ECD on carbon electrodes would constitute a very advantageous method for the fabrication of electrode assemblies. However, any evaluation of the performance of such electrodes would require a detailed characterization of the morphology of metal nanoparticles, which would be very difficult to accomplish due to the complicated morphology and chemistry of conventional carbon supports. To achieve a detailed understanding of the correlation between catalytic performance and particle size, distribution, and loading, it is important to use model electrodes that are very well defined and characterized. One such model system is highly oriented pyrolytic graphite (HOPG), a layered material composed of single-crystal graphite sheets. HOPG not only provides a material with a chemistry similar to that of carbon electrodes, but alsosdue to the atomic smoothness of its surfacesenables imaging by relatively straightforward techniques such as atomic force or scanning tunneling microscopy. Platinum ECD has been mainly studied using electrolytes that contain Pt as chloroplatinic acid (H2PtCl6). According to Feltham et al.,15 electrodeposition from chloroplatinic acid electrolytes involves three steps (the corresponding standard redox potentials are taken from ref 16):

PtCl62- + 2e- f PtCl42- + 2Cl- E0 ) 0.726 VSHE (1) PtCl62- + 4e- f Pt + 6Cl- E0 ) 0.744 VSHE

(2)

PtCl42- +2e- f Pt + 4Cl- E0 ) 0.758 VSHE

(3)

and/or

10.1021/jp0407324 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/26/2005

Electrodeposition of Pt on Pyrolytic Graphite

J. Phys. Chem. B, Vol. 109, No. 16, 2005 7999

Figure 1. Cyclic voltammograms recorded on HOPG at a scan rate of 20 mV/s for 1 mM H2PtCl6. The solid line is for the first cycle, and the dash line is for the second cycle, respectively. Figure 4. Cyclic voltammograms for H2PtCl6 solutions of various concentrations at 20 mV/s. (a) 0.5 mM; (b) 1 mM; (c) 2 mM; (d) 5 mM; (e) 10 mM. Note: the scale for current density is different.

Figure 2. Cyclic voltammograms recorded on HOPG at various scan rates ν for 1 mM H2PtCl6. The inset plot shows a linear relationship between the peak current density ip for Pt reduction and ν1/2.

undergoes fast reduction or disproportionation at low chloride concentration, particularly when H2SO4 is used as the supporting electrolyte. In this case, clear 2-electron reduction processes could be observed.17 On the contrary, at high chloride concentration the reduction of Pt(II) to Pt(0) is strongly cathodically polarized, indicating inhibition of the reduction process. In acid environments, the hydrolysis of PtCl62- (Pt(IV)) can be neglected, making this the major species present in solution. In this paper we perform a detailed characterization of the electrochemistry of Pt electrodeposition from H2PtCl6-based electrolytes onto HOPG, focusing in particular on the effect of Pt concentration, the supporting electrolyte, and complexation effects by chlorides on the kinetics and mechanism of Pt electrocrystallization. These results will be used in the second part of this article to determine the optimum conditions for the growth of Pt nanoparticles with uniform dispersion and narrow size distribution. 2. Experimental Section

Figure 3. Linear sweep voltammetry curve for 1 mM H2PtCl6, scan rate: 2 mV/s.

These reactions involve the exchange of several electrons and the breaking of several Cl bonds. Consequently, they are expected to possess low exchange current densities and to display a strong irreversibility. The PtCl42- (Pt(II)) species

Pt electrodeposition was carried out from 0.5-16 mM H2PtCl6 solutions onto HOPG substrates, grade 2 (average terrace distance 500 nm) with dimensions 10 × 10 × 1 mm, supplied by Structure Probe, Inc. In some cases, dilute HCl or H2SO4 was added to the solutions as a supporting electrolyte. The surfaces were prepared by cleaving and detaching a small number of graphitic planes using a piece of adhesive tape, just prior to each experiment. The deposition mode was potentiostatic. One short potential pulse with duration 10-3 to 1 s was applied to limit particle growth to the nanometer scale and avoid coalescence. The electrolyte was never stirred during the experiments. The deposition process was studied by cyclic voltammetry and analyzed by current-time (i-t) transients up to 100 s. The potentials applied during recording of the i-t transients were carefully chosen to avoid hydrogen evolution. It was thus not necessary to correct these transients for the hydrogen evolution current. A conventional three-compartment Pyrex cell and an EG & G potentiostat (model 273A) with a PC were used to perform electrodeposition and electrochemical experiments at room temperature. A defined sample area of 1 cm2 was exposed to the electrolyte, and all the currents were normalized to this apparent geometric surface area. A Pt foil

8000 J. Phys. Chem. B, Vol. 109, No. 16, 2005

Figure 5. Cyclic voltammograms for 1 mM H2PtCl6 solutions with various concentrations of (a) HCl or (b) NaCl as supporting electrolyte at 20 mV/s, the vertical line indicates the Pt reduction peak potential for 1 mM bare H2PtCl6, the arrow in (b) points to the position of peak B for the solution with 0.1 M NaCl.

was used as the counter electrode; the reference electrode was a saturated calomel electrode (SCE), which was placed directly into the main electrochemical cell, using a Luggin capillary to minimize the effect of solution resistance. In the following, all the electrode potentials are referred to a SCE if not otherwise stated. All solutions were prepared from reagent grade chemicals and deionized water (resistivity > 18 MΩ cm). Prior to each electrochemical experiment, the working solution was deaerated with bubbling N2 for 15 min. No N2 was bubbled during the experiments. To reduce the extent of spontaneous reduction of platinum onto incompletely oxidized functionalities on the HOPG surface,13 the potential perturbation was applied immediately after the graphite surface was exposed to the working solution. After deposition, the electrodes were removed immediately from the deposition solution and rinsed thoroughly with deionized water. 3. Results and Discussion 3.1. Cyclic Voltammetry and Potentiodynamic Sweeping Study. The cyclic voltammogram (CV) from a 1 mM H2PtCl6

Lu and Zangari

Figure 6. Cyclic voltammograms for 1 mM H2PtCl6 with various concentrations of (a) H2SO4 or (b) HClO4 as supporting electrolyte at 20 mV/s, the vertical line indicates the Pt reduction peak potential for 1 mM bare H2PtCl6.

solution (no supporting electrolyte, pH 2.76) on freshly cleaved HOPG recorded at a potential scan rate of 20 mV/s is reported in Figure 1. The shape of this voltammogram is similar to those reported in the literature.18,19 The solid and dash lines are for the first and second cycle, respectively. The plateau A starting at 0.18 V can be assigned to the reduction of Pt(IV) to Pt(II) (eq 1), while the peak B at -0.13 V is attributed to the reduction of Pt(IV) (eq 2) or Pt(II) (eq 3) to Pt(0).18-22 The shoulder observed between 0.0 and -0.05 V is more obvious at lower scan rates (see Figure 2) and represents the Pt reduction reaction at defect sites, where it is characterized by a lower nucleation overvoltage.19 It is noted that a cathodic current is already present at potentials anodic to peak A. One of the possible sources of this current is Pt reduction on nuclei already formed by spontaneous deposition at open circuit potential.13 The shallow peak C around -0.32 V has been related to the reduction of hydrogen ions to hydrogen adatoms.18-20,22 At more cathodic potentials the surface is saturated and hydrogen recombination with bulk hydrogen evolution occurs on alreadyformed Pt nanoparticles. Peak D on the returning branch of the CV is probably associated with the oxidation of some H2 that has not yet diffused away from the Pt electrode after being formed. This peak represents an anodic process, but the overall current is cathodic because this oxidation occurs while Pt



Figure 7. (a) Potentiostatic i-t transients for the deposition of Pt on HOPG from 1 mM H2PtCl6 after stepping the electrode potential from their open-circuit potential (about 0.55 V) to potentials between -0.3 and 0.1 V, (b) i-t-1/2 curves for the transients in (a) at long times.

reduction is still taking place. Finally, peak E can be associated with the desorption and electro-oxidation of strongly adsorbed hydrogen. Figure 2 shows CVs for the same unsupported 1 mM H2PtCl6 solution, recorded at different scan rates. The inset demonstrates a linear relationship between the maximum current density at peak B and the square root of the sweep rate, suggesting that the reduction process Pt(IV)/Pt(II) to Pt(0) is diffusion-controlled. Peaks A and B shift cathodically with increasing sweep rate, indicating that both reduction reactions of the Pt complexes are highly irreversible, as expected.15 As a consequence of the partial transformation of the carbon electrode to a platinum one, the second CV cycle in Figure 1

exhibits a coalescence of the two reduction peaks due to a strong anodic shift of peak B and also a decrease of the overpotential for hydrogen adsorption and evolution. The peaks for hydrogen adsorption and desorption are now better defined. Linear sweep voltammetry (LSV) was performed on the 1 mM H2PtCl6 solution at a scan rate of 2 mV/s, as shown in Figure 3. Three well-defined potential plateaus can be observed, indicating the limiting current for the Pt reduction processes identified above: the Pt(IV) to Pt(II) reduction (eq 1) between 0.25 and 0.18 V, the reduction to Pt(0) at defect sites from 0.11 to 0.03 V, and the reduction to Pt(0) (eqs 2, 3) at nondefect sites between 0 and -0.4 V. In this region, a contribution to the current from the discharge of H3O+ to hydrogen atoms cannot be ruled out. The limiting current for the third plateau is 0.168 mA/cm2. This value can be utilized to estimate an upper limit for the diffusivity of the reducible species (which is mainly PtCl62- at low pH value15) of 2.17 × 10-5 cm2/s, where the diffusion layer thickness of the unstirred solution was assumed to be 0.05 cm.23 Reported values for the diffusion coefficient vary between about 3.4 × 10-6 (ref 24) and about 1.3 × 10-5 (see later in this paper), depending on the method used in its determination and the details of Pt complexation in the various electrolytes used. Our estimate above is thus slightly higher than literature values, in agreement with the fact that the measured limiting current includes spurious processes at the electrode, beyond Pt reduction. Figure 4 depicts CVs from 0.5 to 10 mM H2PtCl6 solutions onto HOPG, obtained at a scan rate of 20 mV/s. No qualitative differences are observed among the various voltammograms; however, the amplitude of all peaks increases with metal complex concentration [PtCl62-] (note the different scales), and their positions shift anodically as a consequence of the increasing activity term in the Nernst equation. On the other hand, the position of peak C (H adsorption) shifts only to a limited extent, due to the small change in pH of the solutions (about 2 for 16 mM Pt to about 3 for 0.5 mM Pt complex). The redox potential for the hydrogen evolution reaction (HER) in this pH range varies between -0.3007 and -0.3303 VSCE, values which roughly agree with the experimental data. The addition of supporting electrolyte to the above Pt solutions can influence faradaic processes, and in particular the nucleation and growth kinetics, through various means. For example, any supporting electrolyte would increase electrolyte conductivity and thus compress the double layer and shorten the double-layer charging transient. This could influence the interface conditions at the start of nucleation and possibly the nucleation and growth kinetics. On the other hand, the use of different counterions could alter reduction kinetics, nucleation, and growth by varying the extent of complexation of the Pt ions and also by selective adsorption at the interface. In the

TABLE 1: Characteristic Parameters im, tm Obtained from Nondimensional Current-Time Transients for Pt Electrodeposition on HOPG and Calculated Diffusion Coefficients electrolyte

1 mM H2PtCl6

0.5 mM H2PtCl6 1 mM H2PtCl6 + 0.05 M HCl 1 mM H2PtCl6 + 0.1 M HCl 1 mM H2PtCl6 + 0.05 M H2SO4 1 mM H2PtCl6 + 0.5 M H2SO4 a

potential/V

tm/s

im/mA cm-2

im2tm/mA s cm-2

105 D/cm2 s-1

0 -0.1 -0.15 -0.2 -0.025 -0.3 -0.1 -0.1 -0.1 -0.1 -0.1

21.2 5.0 3.0 1.75 1.275 0.7 9.75 6.01 17.8 3.75 1.625

0.2 0.335 0.405 0.522 0.6 0.756 0.455 0.26 0.135 0.3775 0.606

0.848 0.5611 0.492 0.477 0.459 0.4 0.2064 0.4063 0.3244 0.5344 0.597

2.19 1.449a-2.311b 1.271a-2.027b 1.965 1.891 1.648 0.85 1.674 1.336 1.381a-2.202b 1.542a-2.459b

Based on progressive nucleation mode. b Based on instantaneous nucleation mode.


Lu and Zangari

Figure 8. Nondimensional i2/im2 vs t/tm plots of the data in Figure 7a. The thicker dash and dot lines correspond to instantaneous and progressive nucleation in the Scharifker-Hills model, respectively.

following section we will investigate the effect of the addition of Cl-, ClO4-, and SO42- anions on the electrochemistry of Pt reduction. The effect of the addition of 0.01-0.2 M HCl to 1 mM H2PtCl6 is presented in the CV plots of Figure 5a. The increase of [HCl] has the effect to weaken and gradually eliminate peak A, while peak B is also attenuated and shifted cathodically. These changes indicate inhibition of Pt electroreduction, which may be due to the decrease in pH by 2.5 units consequent to the addition of acid, or to changes in the extent and chemistry of Pt complexation due to the increase in [Cl-]. To differentiate between these two effects, NaCl instead of HCl was added to the chloroplatinic acid solution (Figure 5b). The addition of 0.1 M NaCl decreases the solution pH from 2.76 to 2.62, thus the change in pH could be considered negligible. The intensity of peak B correspondingly decreases and its position shifts cathodically with increasing [Cl-], which indicates this time

that the presence of chloride is mainly responsible for the inhibition of Pt deposition. Lau and Hubbard25 have explained this effect in terms of inhibition of the reduction of the Pt(II) complex due to the surface coverage by Cl-. Pt(II) would require the availability of several adjacent sites due to its planar geometry, while Pt(IV) reduction would not be inhibited to the same extent due to its octahedral shape and the consequent requirement for the availability of only one site.26 The effect of conductivity, pH, and anion additions on Pt reduction was further investigated by adding supporting electrolytes containing weakly adsorbing anions, i.e., H2SO4 and HClO4. CVs from supported solutions of H2PtCl6 containing SO42and ClO4- anions are displayed in Figure 6, parts a and b, respectively. An increase in [SO42-] masks peak A, shifts the potential for the reduction process corresponding to peak B toward more positive values, and increases its amplitude. The


Figure 9. Current transients for various concentrations (from 0.5 to 16 mM) of H2PtCl6 after stepping the HOPG electrode potential from their open-circuit potential to -0.1 V.

Figure 10. Potentiostatic i-t transients for the deposition of Pt on HOPG from 1 mM H2PtCl6 with 0.1 M HCl after stepping the electrode potential from their open-circuit potential to potentials between -0.3 and 0.1 V.

same effects are observed with the addition of ClO4-. In both cases, the decrease of pH consequent to the addition of an acid shifts the hydrogen evolution peak toward more noble potentials. It should be noted in particular that the shift of peak B is opposite to that observed in the case of chloride addition. This relatively small change can be interpreted in terms of the variation of potential at the outer Hemholtz plane (OHP) as a consequence of the compression of the double layer due to the addition of supporting electrolyte (Frumkin effect).27 In other terms, the potential drop across the OHP decreases when its thickness diminishes, decreasing the overpotential of the discharging anionic species Pt(IV) or Pt(II) and effectively increasing the rate constant for the reduction reaction. 3.2. Chronoamperometry and Nucleation Modes. SEM and AFM observations show that Pt electrodeposition onto HOPG is characterized by Volmer-Weber island growth. Assuming a fixed number of nucleation sites with similar activities, the density of growing nuclei should saturate exponentially with time:

N(t) ) N0[1 - exp(-At)] where N0 is the density of available nucleation sites, and A is the nucleation rate constant.28 The model proposed by Scharifker

J. Phys. Chem. B, Vol. 109, No. 16, 2005 8003 and Hills29 (S-H model) assumes either an instantaneous (At . 1, N(t) ) N0) or progressive (At , 1, N(t) ) AN0t) nucleation and derives diffusion-limited growth rate laws for these two limiting conditions that can be plotted using reduced variables and easily compared with experimental data. Despite the controversies and the approximations involved in this theory (see, for example, refs 28 and 30]), the S-H theory provides a straightforward and widely used method for the analysis of the nucleation mode using current-time transients obtained by potential step techniques, and will be used in this section to characterize the nucleation mode of Pt. Figure 7a shows a series of current density vs time profiles obtained from an unsupported 1 mM H2PtCl6 solution after stepping the HOPG electrode potential from its open circuit potential (about 0.55 V) to potentials between 0.1 and -0.3 V. These values were selected from potentials around peak B, which represents the reduction of Pt(IV) or Pt(II) to Pt(0) (see Figure 1). The transients are characterized by an initial, large cathodic current and quite a long charging time for the double layer (up to 5 s, due to the low density of charge carriers in the unsupported electrolyte), during which current steadily decreases, followed by an increase in cathodic current, a maximum and a successive decay at longer times, when the diffusion limiting current iL is approached. The time tm at which the maximum in current im is observed corresponds to the overlapping of diffusion zones around single nuclei. This value decreases as the applied potential becomes more cathodic. These transients are typical of three-dimensional growth controlled by the diffusion of electroactive species toward the electrode surface.28,31 Under these conditions, at long times the current evolves according to the Cottrell equation,32 and the nominal concentration of Pt(IV) can be used to estimate the diffusion coefficient from the slope of the linear part of i versus t-1/2 plot. The corresponding i vs t-1/2 plots calculated for t > tm from Figure 7a are shown in Figure 7b. The Pearson’s correlation coefficient for each linear regression fit is also reported in the figure. The average diffusion coefficient thus calculated for 1 mM H2PtCl6 is about 1.276 × 10-5 cm2/s. This value is 1.7 times lower than that obtained above from LSV measurements, but is close to the value of 1.2 × 10-5 cm2/s obtained by electrochemical measurements on carbon fiber microelectrodes.33 On the other hand, this same value is 2-3 times larger than the values of the diffusion coefficient calculated using the same method and reported previously: that is, 4.27 × 10-6 cm2/s,34 4.5 × 10-6 cm2/s,21 and 5.89 × 10-6 cm2/s.13 The effective diffusion coefficient is bound to depend strongly on the solution chemistry (in particular, the Cottrell equation is strictly applicable only when there is presence of an excess base electrolyte32) and the details of Pt complexation; it should be taken mostly as a semiquantitative means to determine limiting reduction rates, not as diffusivities of a determined species in solution. At very low overpotential, the maximum in current is not observed and at long times the current remains below the limiting current iL, indicating that the reaction is not under diffusion control. The experimental i-t transients can be plotted in terms of the reduced variables i/im and t/tm, where im and tm are the current density and time at which the maximum in current is observed, respectively, and compared to theoretical transients (Figure 8). Except for short times (where double layer charging is occurring) and for the curve at low overpotential, the transients follow quite closely the theoretical behavior for the instantaneous nucleation process. The current transient at 0 V follows closely the theoretical curve for progressive nucleation at short times,


Lu and Zangari

Figure 11. Nondimensional i2/im2 vs t/tm plots of the data in Figure 10. The thicker dash and dot lines correspond to instantaneous and progressive nucleation in the Scharifker-Hills model, respectively.

but later shows a current that decreases more slowly than predicted by both nucleation and growth modes. This phenomenon has been observed by others during the electrodeposition of cobalt onto HOPG and has been explained in terms of enhanced diffusion and rapid replacement of the discharging ion through hemispherical diffusion to growth centers.35 To summarize, the potentiostatic transients from 1 mM H2PtCl6 solutions exhibit a transition from progressive to instantaneous nucleation with increasing deposition overpotential, as observed by others.35,36 The diffusion coefficient for the species being reduced can also be estimated from characteristic parameters (im and tm) obtained from i-t curves. When instantaneous nucleation occurs, the diffusion coefficient, D, can be estimated according to the formula im2tm ) 0.1629D(nFc)2, where c is the concentration of the diffusing species, n its charge, and F the Faraday constant. For progressive nucleation, the formula is im2tm )

0.2598D(nFc)2.29,37 The characteristic parameters obtained from Figures 7 and 8 and the calculated diffusion coefficients are collected and presented in Table 1. It is noted that the diffusion coefficients for 1 mM H2PtCl6 calculated by this method are around 2 × 10-5 cm2/s except for the smaller value calculated from the data at -0.3 V, which could be affected by the possible concomitant hydrogen evolution.24,38 These values are a little larger than the value calculated for the same system using the Cottrell equation, while the limiting current (0.1544 mA/cm2) calculated from this diffusion coefficient agrees very well with the diffusion limit (0.155 mA/cm2) in the i-t transient, indicating that this method is a reliable one to obtain the diffusion coefficient for this Pt deposition system. In the literature, the diffusion coefficients calculated by using the S-H model for H2PtCl6 solutions differ widely (e.g., 3.4 × 10-6 cm2/ s,24 1.0 - 1.5 × 10-5 cm2/s,25 and 5.47 × 10-5 cm2/ s 39), and the value calculated here is in this range.


Figure 12. Current transients for 1 mM H2PtCl6 in the presence of HCl at various concentrations, after stepping the HOPG electrode potential from their open-circuit potential to -0.1 V.

The effect of [PtCl62-] on the nucleation and growth mode was examined by recording i-t transients at -0.1 V from unsupported electrolytes containing 0.5-16 mM H2PtCl6 as presented in Figure 9. A maximum in current is barely visible only for the 0.5 and 1 mM solutions. Transients obtained for 0.5 and 1 mM solutions follow closely the curve for an instantaneous nucleation process, while the current transients obtained with larger [PtCl62-] first show a slow decay of current and then an increase in current with time, which does not reach a stable value even after 100 s. It is not possible to fit the curves obtained at large [PtCl62-] with the S-H theory; this is probably an indication that an increasingly important inhibition of the


Figure 14. Current transients for 1 mM H2PtCl6 in the presence of various concentrations of H2SO4 after stepping the HOPG electrode potential from their open-circuit potential to -0.1 V.

reduction process is occurring and that discharge of the Pt complex becomes very sluggish. Another possible explanation could be that dendritic growth is occurring, such that the surface area of the Pt deposit increases more than what would be expected for semi-hemispherical particles. The effect of the supporting electrolyte was studied by recording and analyzing transients from 1 mM H2PtCl6 solutions containing 0.1 M HCl, as shown in Figure 10. Transients similar to those of unsupported electrolytes (Figure 7a) are obtained, but the absolute values of the currents are smaller. The reduced variable plots and the corresponding comparisons with the S-H models are reported in Figure 11. These time transients at all



Lu and Zangari


potentials follow closely the expected trends for instantaneous nucleation. The diffusion coefficient was again calculated using the Cottrell equation for the solution containing 1 mM H2PtCl6 in the presence of 0.1 M HCl. D for this solution is about 6.38 × 10-6 cm2/s and is very close to the value (5.89 × 10-6 cm2/s) reported by Zoval et al. for the same bath.13 The calculation of D from the parameters im and tm gives a value of 1.336 × 10-5 cm2/s as shown in Table 1. Both values calculated using different methods are smaller than the corresponding values for 1 mM H2PtCl6 in the absence of HCl. This decrease is easily explained considering the possibility for the formation of larger Pt-Cl complexes in the presence of high Cl- concentrations.40 An increase in the concentration of such complexes would decrease the effective rate of arrival of the reducible species to the electrode and thus the effective diffusion coefficient. Transients obtained at -0.1 V from solutions containing 1 mM H2PtCl6 with varying amounts of HCl (Figure 12) evidence a decrease and a flattening in the current maximum, with a corresponding increase in the transition time. For sufficiently large [HCl], the transition from growth of isolated nuclei to overlapping of the diffusion zones is not observed, suggesting that such diffusion limitations do not occur in the presence of a sufficiently high concentration of chloride. The limiting current also decreases with increasing [HCl], which indirectly indicates that an increasing fraction of [PtCl62-] is reduced to PtCl42but not further, leaving a fraction of the Pt in solution in complexed form. Analysis of these data in terms of the S-H model yields close fitting to the instantaneous nucleation mode (Figure 13). Again, the diffusion coefficient calculated from im and tm in Figures 12 and 13 for 1 mM H2PtCl6 decreases with increasing HCl concentration, as shown in Table 1.

The effect of H2SO4 additions to 1 mM H2PtCl6 on the transients is opposite to that of HCl, as shown by transients obtained at -0.1 V from H2SO4-supported electrolytes, Figure 14. Namely, the current increases and the transition time decreases with increasing [SO42-]. These changes can be interpreted in terms of the anodic shift for the Pt reduction peak in the presence of H2SO4. A constant deposition potential actually implies a higher overpotential applied in the H2SO4supported solution, which leads to an increase in current and a decrease in the transition time. The plot of reduced variables in Figure 15 reveals that the addition of H2SO4 has limited effect on the nucleation mode. Diffusion coefficients for the solution with H2SO4 calculated from the S-H model are almost the same as those for the solution without H2SO4 as supporting electrolyte. In summary, the addition of HCl to H2PtCl6 solutions decreases the currents observed at equivalent potentials, shifts the transition time to very long times, and inhibits the observation of diffusion-limiting conditions. These effects clearly indicate a slowing down of the reduction kinetics in the presence of chlorides. On the contrary, diffusion effects are observed at more anodic potentials in H2SO4-supported electrolytes, as a consequence of the compression of the double layer due to the increase in conductivity. In both cases, an instantaneous nucleation mode is inferred from the S-H theory, which is advantageous for the prospective growth of nanoparticles with narrow size distribution. The possibility to grow the nanoparticles at larger cathodic potentials and using smaller currents in HCl-supported solutions is further advantageous from the point of view of controlling the particle size to very small diameters, as required for the electrocatalysis of hydrogen oxidation and oxygen reduction.

Electrodeposition of Pt on Pyrolytic Graphite 4. Conclusions The reduction, nucleation, and growth of Pt from chloroplatinic acid electrolytes onto HOPG were investigated by electrochemical methods. It is confirmed that the presence of chlorides in the electrolyte inhibits the Pt reduction process, reportedly due to the formation of adsorbed chloride films which occupy available sites for the discharge of the Pt complexes. On the contrary, the addition of perchlorate or sulfate anions slightly promotes reduction processes, due to a compression of the double layer consequent to the increased electrolyte conductivity. Fitting of potentiostatic current-time transients in 1 mM H2PtCl6 electrolytes with the Scharifker-Hills model shows a transition from progressive to instantaneous nucleation with increasing overpotential. Addition of chlorides slows the reduction kinetics strongly by polarizing the reduction processes. An increase of electrolyte conductivity with chloride, sulfate, or perchlorate improves the instantaneous character of nucleation. The possibility of instantaneous nucleation at large overpotential by using electrolytes with large chloride concentration is advantageous for the growth of arrays of small, well-dispersed nanoparticles, optimum in principle for electrocatalysis. Acknowledgment. This work was funded in part by the United States Department of Energy (US DOE) EPSCoR Implementation, Grant No. DE-FG02-ER45867, and by NSF through Grant NSF-DMR-0093154. References and Notes (1) Liu, Z.; Ling, X. Y.; Su, X.; Lee, J. Y. J. Phys. Chem. B 2004, 108, 8234. (2) Antoine, O.; Durand, R. Electrochem. Solid-State Lett. 2001, 4, A55. (3) Gloaguen, F.; Leger, J.; Lamy, C. J. Appl. Electrochem. 1997, 27, 1052. (4) Howells, A.; Harris, T.; Sashikata, K.; Chottiner, G. S.; Scherson, D. A. Solid State Ionics 1997, 94, 115. (5) Kucernak, A. R.; Chowdhury, P. B.; Wilde, C. P.; Kelsall, G. H.; Zhu Y. Y.; Williams, D. E. Electrochim. Acta 2000, 45, 4483. (6) Kabbabi, A.; Gloaguen, F.; Andolfato, F.; Durand, R. J. Electroanal. Chem. 1994, 373, 251. (7) Bregoli, L. J. Electrochim. Acta 1978, 23, 489. (8) Satter, M. L.; Ross, P. N. Ultramicroscopy 1986, 20, 21. (9) Watanabe, M.; Sei, H.; Stonehart, P. J. Electroanal. Chem. 1989, 261, 375.

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Electrodeposition of Platinum on Highly Oriented Pyrolytic Graphite

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