Substrate Structural Effects on the Synthesis and Electrochemical

J. Phys. Chem. C 2010, 114, 18439–18448

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Substrate Structural Effects on the Synthesis and Electrochemical Properties of Platinum Nanoparticles on Highly Oriented Pyrolytic Graphite Maryam Bayati,* Jose M. Abad,† Richard J. Nichols, and David J. Schiffrin Department of Chemistry, UniVersity of LiVerpool, LiVerpool L69 7ZD, United Kingdom ReceiVed: August 5, 2010; ReVised Manuscript ReceiVed: September 15, 2010

Platinum nanoparticles have been prepared by potentiostatic multipulse electrodeposition with controlled nucleation and growth on freshly cleaved and electrochemically oxidized highly oriented pyrolytic graphite. The influence of the applied potential sequence on the size distribution was investigated. For short electrolysis times, the deposition of nanoparticles takes place via a progressive nucleation mechanism. A narrow size distribution was obtained by controlling independently the nucleation and growth steps, and particles with heights between 52 and 1.4 nm could be prepared by altering the pulse parameters. Anodic oxidation of the substrate had a large influence on the particle size, resulting in the preparation of particles 1.4 nm in height. XPS demonstrated that Pt particles of small size were readily oxidized. The rate of electrochemical methanol oxidation showed a dependence on the particle size, and no oxidation of methanol could be observed for the smaller sizes investigated. 1. Introduction There is currently an intense interest in the study of the electrochemical properties of metal clusters,1,2 in particular for fuel cell applications. Their investigation presents particular challenges for preparing materials of uniform size, controllable geometry, and good electrical connectivity to an inert conducting substrate while displaying, at the same time, long-term stability. A pronounced particle size, morphology, and coverage dependence on the electrocatalytic activity and reaction mechanism has been observed for reactions such as methanol oxidation,3-5 oxygen reduction,6,7 and CO oxidation.2,5,8,9 A variety of techniques have been used for preparing supported Pt nanoparticles.9,10 A commonly used method involves immobilization onto the electrode surface from their colloidal dispersion.11 Although monodisperse preparations with a variety of shapes can be conveniently synthesized, the need to use stabilizing ligands can be a disadvantage due to blocking of active sites, and these preparations require appropriate annealing procedures. High-vacuum metal evaporation onto a surface12,13 and impregnation with a metal salt and subsequent reduction by hydrogen13a have also been used. Although particles without attached ligands can be synthesized, the materials prepared do not exhibit good monodispersity. Colloidal lithography is another method to fabricate uniformly sized and shaped nanostructures with controllable geometry, but their size is limited to tens of nanometers.14 Chemical reduction of precious metal ions coordinated to organic monolayers attached to carbon substrates9 or encapsulated within dendrimers15 yields nanoparticles with good monodispersity, allowing good control of the particle size by repeating the precursor attachment and growth procedures. Although this method has been used successfully to study size effects,9 the presence of a metal-organic tether-substrate construct can give rise to uncertainties regarding the state and cleanliness of the nanostructured surface. An * To whom correspondence should be addressed. Phone: +44 151 794 2274. Fax: +44 151 794 3588. E-mail: [email protected]. † Current address: Departamento de Quı´mica Analı´tica y Ana´lisis Instrumental, Facultad de Ciencias, Universidad Auto´noma de Madrid, Cantoblanco, Madrid 28049, Spain.

attractive alternative that avoids the use of linker molecules is direct metal electrodeposition, which can provide ligand-free and monodisperse preparations. Importantly, a wide size range and good electrical contact to the substrate can be achieved.12 Therefore, this method has been further explored here. Early attempts at electrodeposition of Pt particles on carbon resulted in agglomerated polydisperse materials.16 Lee et al.17,18 used single-pulse chronoamperometry to prepare different particle sizes by applying a constant deposition potential for different times. Although 20-100 nm Pt crystals could be prepared, a wide size distribution was observed. By applying different pulse potentials and times, Penner et al.19-21 discussed, for the first time, the conditions under which general ideas of nucleation and growth in the electrodeposition of metals could be transposed to the controlled synthesis of nanostructures attached to highly oriented pyrolytic graphite (HOPG) by employing pulse techniques. In addition, it was demonstrated that diffusion coupling between growing particles leads to a higher size dispersity.20,21 This problem can be avoided by the use of a double-pulse method.22 Bru¨lle et al.23 also demonstrated that well-separated particles with heights in the range from 2.5 ( 0.9 to 7.2 ( 2.7 nm and from 1.3 ( 0.7 to 10 ( 2.5 nm could be grown on HOPG.19 Aggregation and broadening of the height distribution were observed, however, when larger particles were being prepared. In the present work we further explore the use of multipulse methods by controlling, not only nucleation and growth but, importantly, the properties of the substrate, thereby demonstrating that changes in the hybridization of graphene sheets have a profound effect on the kinetics of nucleation. HOPG was the preferred substrate for these studies due to its ease of cleavage, chemical stability, and surface flatness, which makes it a suitable surface for STM and AFM studies. Nucleation of Pt on HOPG begins at intrinsic defects and edges or at extrinsic defects created by oxidation in air,24 plasma treatment,25 or electrochemical oxidation.18 The effect of the latter on the cluster number density and particle height has also been investigated in the present work.

10.1021/jp107371z  2010 American Chemical Society Published on Web 10/08/2010

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This paper is divided into three main sections. The nucleation and growth characteristics of Pt are presented first, then the influence of substrate oxidation is discussed, and finally, XPS results and the size dependence of the electrocatalytic properties of the electrodeposited clusters for methanol oxidation are presented. 2. Experimental Section 2.1. Materials and Instruments. Noncontact AFM measurements were performed ex situ using a Nanoscope III (Digital Instruments) microscope equipped with a standard silicon nitride tip with a force constant of 21 N/m and a resonance frequency of ∼300 kHz. Tungsten STM tips were freshly etched from 0.25 mm wire in 0.1 M KOH. HOPG was supplied by Agar Scientific (ZYH grade with a mosaic spread of 3.5 ( 1.5°). All the electrochemical experiments were carried out in a threeelectrode cell previously described.9 A 0.28 cm2 circular area of the HOPG surface bound by a Kalrez O-ring (E-A-P International Ltd.) was exposed to the electrolyte. The electrode surface was renewed by freshly cleaving surface layers with adhesive tape at least twice before use. The terraces on the surfaces thus prepared had an average width of 500-700 nm as shown in Figure SI1 (Supporting Information). Methanol (BDH, AnalaR), potassium hydroxide (BDH, AnalaR), perchloric acid (Aldrich, 99.99%), sulfuric acid (Aldrich, 99.999%), and potassium tetrachloroplatinate(II) (Aldrich, 99.9+%) were used as received. All solutions were prepared with Milli-Q water and purged with nitrogen (BOC, 99.998%) for 20 min before every experiment. Potentials were measured and are reported with respect to the saturated calomel electrode (SCE). The electrochemical measurements were carried out with an Autolab potentiostat, PGSTAT 10, The Netherlands. Photoelectron spectra were recorded with a VG ESCA 300 spectrometer employing an Al KR X-ray source. All measurements were carried out at room temperature, 22 ( 2 °C. 2.2. Platinum Electrodeposition on HOPG. In nucleation experiments, care has to be taken to ensure the absence of unwanted metal centers. For graphite and freshly cleaved HOPG electrodes in contact with H2PtCl6 solutions, electroless platinum deposition can occur spontaneously,26 preferentially at step edges since their local surface energy is greater than that of terraces.19 Spontaneous deposition leads to an inhomogeneous distribution of nanoparticles on freshly peeled or thermally oxidized HOPG.19,22,26 This was prevented by anodic protection before the nucleation step.27,28 The influence of the metal salt concentration on the nucleation mechanism and therefore on the size distribution of the deposit has been extensively studied by Gloaguen et al.26 and Lu et al.29 for H2PtCl6. Increasing the salt concentration from 2 to 10 mM changes the nucleation mechanism from progressive to instantaneous. Therefore, a concentration of 10 mM K2PtCl4 was chosen for the present work in the expectation that progressive nucleation could be avoided. A transition from progressive to instantaneous nucleation has been observed by changing the electrolyte from perchlorate or sulfate to chloride. The latter, however, strongly inhibits Pt deposition as a result of competitive adsorption of chloride compared with the platinum chloro complexes and the consequent blocking of reduction sites.29,30 For these reasons, sulfuric acid has been used here as the supporting electrolyte. Electrodeposition of Pt was carried out through a protection, nucleation, and growth cycle as shown in Figure 1. The first short potential step (potential E1, pulse length t1) formed nuclei which then grew during the second or growth step (E1 < E2), for which the critical radius was much larger than that of the

Figure 1. Schematic representation of the double potential step profile employed.

Figure 2. Cyclic voltammograms for Pt electrodeposition from 10 mM K2PtCl4 + 0.5 M H2SO4 (sweep rate 20 mV s-1): solid line, first scan; dashed line, second scan.

first pulse. Therefore, only nuclei with a radius greater than the critical radius at E2 could grow.31 Growth was continued at E2 for time t2. Depending on the value of E2, additional nucleation during the growth phase could be suppressed. This strategy provides independent control of the nucleation and growth processes, leading to a more homogeneous and narrower particle size distribution than would be obtained with a single potential step where nucleation and growth can take place simultaneously.20,32 It was expected that the number density and size of the particles could be altered by changing these parameters separately. Electrodeposition was carried out from 10 mM K2PtCl4 in 0.5 M H2SO4, either on freshly cleaved or on electrochemically oxidized HOPG. The substrate was oxidized either in this solution or from 3 M H2SO4. For the latter, the electrode was treated by cycling the potential twice between 0.4 and 1.0 or 1.2 V at a sweep rate of 10 mV s-1. 3. Results and Discussion 3.1. Pt Electrodeposition on Freshly Cleaved HOPG. 3.1.1. Cyclic Voltammetry. Typical cyclic voltammograms for the electrodeposition of Pt are shown in Figure 2. The first scan shows a broad reduction wave at potentials more negative than 0.15 V that can be attributed to Pt nucleation and growth.29,33 The voltammetric wave observed at -0.18/-0.12 V corresponds to hydrogen underpotential deposition (upd) occurring concurrently with Pt electrodeposition.16,29,34,35 The potential scan was reversed before the onset of hydrogen evolution. The second scan shows a broad peak with an onset potential at the more positive potential of 0.45 V, corresponding to Pt electrodeposition on nuclei deposited during the first scan.33 The value of the onset potential demonstrates that the protection potential

Synthesis and Properties of Pt Nanoparticles on HOPG

Figure 3. Potentiostatic I-t transients for the deposition of Pt from 10 mM K2PtCl4 + 0.5 M H2SO4 on freshly cleaved HOPG after stepping the potential from +0.6 to (1) 0.0, (2) -0.03, (3) -0.06, (4) -0.09, (5) -0.14, (6) -0.18, and (7) -0.20 V. The surface was prepared as described in section 2.1.

used of 0.6 V is sufficiently positive to avoid electroless platinum deposition.19,27 Anodic dissolution of the metal deposit was not observed due to the absence of ligands for Pt in solution (note that very small traces of chloride will be in solution after electrodeposition from the Pt-chloride complex), and therefore, particle growth during the second scan occurs at much more positive potentials compared with the results in the first scan (see Figure 2). To avoid particle size dispersion during growth, crystal seed formation has to occur instantaneously or, if nucleation is progressive, during a very short time. Also, the growth phase has to be conducted at low overpotentials to avoid further nucleation so that only clusters of a critical size greater than those corresponding to the potential E2 can grow.

J. Phys. Chem. C, Vol. 114, No. 43, 2010 18441 3.1.2. Single Potential Step Experiments. Figure 3 shows chronoamperometry transients when the potential was stepped from 0.6 V to increasingly negative values. These are very similar to many previous observations for 3D electrocrystallization of metals under diffusion control.34 An initial decaying charging current is observed followed by an increase in cathodic current that reaches a maximum value, Im, at a time tm. This is followed by a decay when individual spherical diffusion zones around the growing nuclei overlap and the diffusional geometry changes from spherical to planar.36 By comparing Figures 2 and 3, it can be concluded that there is no inhibition of platinum deposition caused by Hupd, and the expected trend for 3D nucleation and growth is observed. The nucleation mechanism was examined by comparing the dependence of the reduced current (I/Im) on the reduced time (t/tm) with theoretical predictions from the Scharifker-Hills model for a range of potentials,37 and this analysis is given as Supporting Information. A continuous nucleation mechanism was found indicating that a single potential step method is unlikely to lead to a deposit of nanoparticles with a narrow size distribution. For this reason, a multistep potential profile including a very short prenucleation pulse was investigated. 3.1.3. Double Potential Step Electrodeposition. The nucleation and growth potentials were independently controlled to separate these two processes. Since progressive nucleation kinetics prevails, the length of the nucleation pulse was very short (0.1 ms) to produce a narrow size distribution. For comparison purposes the growth time t2 was kept constant at 3 s in these experiments. Figures 4 and 5 show representative morphology of three deposits obtained at different nucleation and growth potentials to illustrate their influence on the particle size and number density. Figure 4A shows a typical AFM image of the resulting nanoparticle deposit obtained for E1 ) -0.2 V and E2 ) -0.02 V, where metal particles with an average height and number density of 26 ( 3 nm and 58 µm-2, respectively, can be observed.

Figure 4. AFM images of Pt nanoparticles prepared using the potential pulse sequence shown below the respective images. The average particle height is 26 ( 3 and 42 ( 8 nm for parts A and B, respectively. The inset shows very small individual particles formed on the substrate along with 42 nm coagulated particles. The lower scale corresponds to the inset.

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Figure 5. AFM image of Pt nanoparticle growth on HOPG using the potential profile shown. The average particle height is 16 ( 2 nm.

Figure 4B shows an image of the deposit obtained under the same conditions as in Figure 4A but in the absence of a nucleation step to investigate the effect of the nucleation prepulse on the height and particle distribution. Particles with an average height and with a number density of 42 ( 8 nm and 15 µm-2, respectively, can be seen. The image and the inset show that every particle is a group of smaller ones, clearly a consequence of relying only on the growth pulse alone to produce the nuclei for subsequent growth (the inset is an area imaged at higher vertical resolution highlighting the very small individual particles as well as the large coagulated particles). In this case, progressive nucleation prevails. This type of deposit has been observed before26,27 and results from the coalescence of particles grown from nuclei formed around surface defects and their subsequent aggregation. The inset also reveals the presence of very small clusters of 2 nm average height. These images show that the number density of particles decreases and their size increases in the absence of a nucleation prepulse and clearly demonstrate that the inclusion of the latter leads to a greater uniformity in the particles electrodeposited by providing a higher number of active nucleation sites. The growth phase at -0.02 V in the above experiments was accompanied by further nucleation. To achieve good separation between nucleation from growth, the growth potential was set to the more positive value of 0.37 V, where no nucleation is observed (see Figure 2). In addition, the nucleation potential was set to 0.00 V to decrease the number density of nuclei. Figure 5 shows that the decrease in the number of nuclei and in the growth potential results in a decrease of the particle height to 16 ( 2 nm and of the surface number density to 18 µm-2. A comparison of Figures 4A and 5 clearly shows that the number density of particles is diminished by these changes. Importantly, the formation of well-separated nanoparticles (Figure 5) is observed. From the regularity of the deposit, there is little evidence of secondary nucleation taking place at this growth potential, and hence, growth only occurs on the nuclei formed during the nucleation phase. 3.2. Pt Electrodeposition on Oxidized HOPG. The above results demonstrate that the particle height and coverage can be controlled from the electrochemical parameters. The electrode substrate can have a major influence on nucleation by changing the defect number density or, as will be discussed further on, by altering its structure. The chemistry of carbon electrode surfaces can be altered by electrochemical oxidation, and the resulting changes have been investigated. Pt nanoparticles deposited on HOPG can exhibit high mobility,16,19,25,28 and therefore, the possibility of establishing a stronger interaction

between the nanoparticles and the substrate was investigated by oxidizing the HOPG surface before electrodeposition. This leads to the formation of defects and functionalities such as carboxyl and OH groups that not only enhance adsorption of metal ions and nucleation of deposits but also increase their adhesion.25,38 The formation of surface defects by ion beam irradiation and oxygen plasma and Ar bombardment12,25,27 or chemically, for example, by intercalation,39 has been shown to increase the surface roughness. Increasing the defect density also increases the density of states (DOS) and improves the reactivity of HOPG for the formation of nuclei.38,39 An additional advantage of surface oxidation is to decrease the degree of adventitious electroless deposition, as observed for electrochemically preoxidized glassy carbon40 and HOPG.28,40 It has been proposed that the driving force for electroless deposition of Pt is the reaction between Pt(II) and incompletely oxidized functionalities on the carbon surface, but the origin of this effect is unclear.19 For these reasons, the influence of the electrochemical oxidation of HOPG on Pt electrodeposition was investigated. Two different oxidation procedures were studied, using the same electroplating solution employed above and oxidation in 3 M H2SO4 where anion intercalation in HOPG can take place. The former method was intensively employed for step edge decoration and preparation of nanowires.41 3.2.1. HOPG Oxidation in 10 mM K2PtCl4 + 0.5 M H2SO4. Electrodeposition of Pt clusters on freshly cleaved HOPG was carried out using a triple potential step sequence involving oxidation, nucleation, and growth steps as indicated in Figure 6A. Surface oxidation was carried out for 1 s at 2 V followed by nucleation and growth potential steps. An AFM image of the resulting surface is shown in Figure 6A, where the formation of particles with a height of 5.5 ( 1.5 nm is observed. The difference between the experiments described in Figures 5 and 6A is the inclusion of an oxidation step, which increases coverage by forming a high number of surface defects onto which Pt clusters can grow. Formation of Pt(IV) species at the oxidation potential is unlikely to alter the nucleation and growth behavior of the system.42 The substrate undergoes severe structural changes on oxidation, leading to an increase in the number of nucleation sites and the formation of blisters, as previously observed.39,43-45 These blisters have different areas and heights and are randomly distributed over the surface. Parts B and C of Figure 6 compare cross-sections of a surface containing only electrodeposited crystals with another one showing the presence of a blister onto which metal particles have grown.

Synthesis and Properties of Pt Nanoparticles on HOPG

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Figure 6. (A) AFM image of Pt nanoparticles prepared using the triple pulse potential technique shown. (B) AFM cross-section of the sample from a place with no blisters. (C) Cross-section of the sample showing a blister with nanoparticles grown on its surface. The short arrows are markers for particle height measurements.

3.2.2. HOPG Oxidation in 3 M H2SO4. From Raman spectroscopy measurements, McCreery et al.45 demonstrated that the behavior of HOPG on oxidation in dilute acids (1 M) is strongly dependent on the acid anion. Lattice damage was observed when both intercalation and graphite oxide formation took place. To achieve better control of the state of the surface and to avoid blistering, oxidation and nanoparticle deposition reactions were carried out separately. Beck et al. demonstrated that the critical potential of HSO4- intercalation decreases linearly and the anodic intercalation current increases steeply with anion concentration.46 For 3 M H2SO4, an intercalation current efficiency of approximately 60% was observed.46 This concentration represented a good compromise for attempting surface restructuring by HSO4- intercalation within the graphene sheets without the severe surface disruption that occurs for lower acid concentrations at higher potentials, e.g., blister formation and uncontrolled surface disruption.39,44,45,47 A typical oxidation voltammogram is shown in Figure 7. The increase in current for potentials more positive than approximately 0.8 V is due to graphite oxidation accompanied by anion intercalation. For potentials more positive than 1.0 V, oxidation of HOPG occurs simultaneously with oxygen evolution with formation of microbubbles and surface disruption, leading to the pattern of sharp small current peaks observed in Figure 7 for E > 1.0 V. The surfaces resulting from oxidation at 1.0 and 1.2 V were imaged before and after electrochemical oxidation by AFM and in some cases by STM to assess the morphological changes that occur on oxidation. Anodic oxidation disrupted the surface when the potential was cycled up to 1.0 and 1.2 V (Figure 8). The surface oxidized at the lower potential was imaged by STM to obtain better

Figure 7. Electrochemical oxidation of HOPG in 3 M H2SO4 (sweep rate 10 mV s-1).

resolution. This surface (Figure 8A) shows unusual features. The steps separating the terraces that can be observed in a freshly cleaved HOPG surface (Figure SI1, Supporting Information) are still clearly visible, but the terraces now show a characteristic corrugation with a repetition rate of approximately 5 nm between lines. The appearance of corrugations has been previously observed for HOPG electrochemically oxidized in dilute acid solutions.46,39 The origin of the corrugation is not associated with the intercalation of the HSO4- anion since this is a reversible process, which does not lead to permanent structural changes.44 It is proposed that the formation of surface periodic motifs is related to the partial oxidation of the graphene sheets. The structural changes are due to the consequent changes in bonding of the surface carbon atoms, from sp2 to sp3, thus

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Figure 8. HOPG oxidation in 3 M H2SO4: (A) STM image of HOPG electrochemically oxidized, obtained by cycling between 0.4 and 1.0 V (the z range is 5 nm), (B) AFM image of electrochemically oxidized HOPG, obtained by cycling between 0.4 and 1.2 V (the z range is 15 nm).

Figure 9. Cyclic voltammogram of platinum deposition on HOPG previously oxidized by cycling up to 1.0 V and in contact with 10 mM K2PtCl4 in 0.5 M H2SO4 (sweep rate 20 mV s-1): solid line, first cycle; dashed line, second cycle.

inducing stresses in the surface layers as a result of mismatch in the surface structure with that of the bulk. Oxidation at 1.2 V (Figure 8B) severely disrupts and roughens the electrode, producing a very heterogeneous surface with a high density of defects, including the formation of nanometersized pits. These pits are distributed nonuniformly on the surface and have a depth of up to 3 nm. The pits are formed by fracturing of the blistered outer layer of HOPG due to anion intercalation between the graphene sheets.43,39,48 Applying a high oxidation potential leads also to gas evolution and further blistering due to the resulting mechanical stresses followed by fracturing, further roughening, and formation of pits (Figure 8B). 3.2.3. Pt Electrodeposition on Oxidized HOPG. Blistering was not observed at an oxidation potential of 1.0 V, although the surface was roughened. Therefore, the nucleation and growth of Pt on this substrate was further investigated for comparison with untreated HOPG. Figure 9 shows a typical cyclic voltammogram for Pt electrodeposition on this oxidized surface. An irreversible reduction peak is observed at -0.18 V during the first cycle, with a potential onset of approximately 0.04 V. The comparison of Figures 2 and 9 shows important differences for platinum electrodeposition. Surface oxidation removes the plateau observed at 0 V in Figure 2 for freshly cleaved HOPG, and importantly, the nucleation potential is shifted by approximately 200 mV to more negatiVe values. Such a large shift would indicate that oxidation provides a surface with nucleation

sites where the interaction energy between the nuclei and the substrate is much lower compared with that of native HOPG. This result is counterintuitive since oxidation creates a large coverage of oxygenated surface species, which would be expected to produce a highly polar surface onto which nucleation should be facilitated. The nucleation potential, however, is the result of a complex interplay between the surface energy of the deposited metal phase and the interaction energy of the growing nuclei with the substrate. The results in Figure 9 indicate that oxidation substantially decreases particle-substrate interactions. Although surface oxidation may lead to the formation of groups that impede the adsorption and consequently reduction of PtCl42-, in acid solutions this is unlikely to be the only effect. Recent XRD results have clearly demonstrated that functionalization of an HOPG electrode changes the bonding of the surface carbon atoms from sp2 to sp3.9 The same changes are expected to occur if oxidation results in the reconstruction of the graphene structure at the surface. The presence of an extended delocalized π electron system in carbon species such as fullerenes is known to lead to a very strong interaction with metals.49 Oxidation, howeVer, destroys the sp2 domains and hence significantly decreases the interaction energy between the surface and the metal nuclei formed, with a corresponding increase in the nucleation oVerpotential. Another example of the influence of the interaction of the growing nuclei with the surface environment on the nucleation potential is the comparison of the growth of gold nanoparticles on a glassy carbon electrode and on the same electrode modified with nitrosophenyl groups.50 For the latter, the nucleation potential is shifted to more positiVe values due to the decrease in energy of the critical radius nuclei. In addition to the change in surface hybridization from sp2 to sp3 described above, other changes in the surface chemistry of carbon are known to occur. Shimazu et al.51 observed that electrochemically preoxidized glassy carbon shows less reactivity toward electroless deposition of platinum, and Shen et al.28 reported that this could even be prevented for HOPG by its electrochemical oxidation. It was shown that polarizing HOPG for 2 min at 0.8 V vs SCE deactivated the surface for Pt spontaneous nucleation.27 All the above observations show that electrochemical oxidation of HOPG decreases the activity of the surface for Pt nucleation. For both freshly cleaved and oxidized HOPG surfaces, the second voltammetric cycle shows a broad peak with a potential onset at ∼0.42 V, which is attributed to the reduction of Pt(II) on the platinum clusters grown during the first cycle. Since Pt deposition occurs in this case on a surface containing metallic

Synthesis and Properties of Pt Nanoparticles on HOPG

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Figure 10. AFM image of electrodeposited nanoparticles on electro-oxidized HOPG. The electrodeposition nucleation and growth potential pulses were 0 and 0.37 V, respectively, and lasted for 0.1 ms and 5 s.

TABLE 1: Effect of Nucleation, Growth, and Substrate Preoxidation on the Particle Height and Number Density nucleation potential (V)/ nucleation time (ms)

growth potential (V)/ growth time (s)

HOPG oxidation pretreatment

particle number density × 1010 (cm-2)

particle height (nm)

0.2/0.1

-0.02/3 -0.02/3 0.37/3 0.37/5 0.37/3

no no no yes yes

0.58 0.15 0.2 4.2 3.9

26 ( 3 42 ( 8 16 ( 2 1.7 ( 0.5 1.4 ( 0.5

0.0/0.1 0.0/0.1 0.0/0.1

platinum, the features related to the state of the carbon surface are not observed. The peaks at -0.18 and -0.11 V correspond to hydrogen adsorption (Hupd) on a surface continuously modified by Pt electrodeposition. The study of the kinetics of this reaction was beyond the scope of the present work. For all the results described below, the HOPG electrode was first cycled between 0.4 and 1.0 V in 3 M H2SO4 and then used as the substrate for the electrodeposition of Pt nanoparticles for a longer growth time than that for the triple pulse technique described before (Figure 6A). Electrodeposition on this surface resulted in the formation of particles 1.7 ( 0.5 nm in height as shown in Figure 10. Another image obtained for shorter growth times is shown in Figure SI4, Supporting Information. It is noteworthy that these nanoparticles are well separated and homogeneously distributed over the large surface area imaged of 9.0 × 9.0 µm2 (Figure SI5, Supporting Information). Thus, Pt electrodeposition on the oxidized HOPG surface resulted in a good surface distribution of particles with a surface concentration of 4.2 × 1010 cm-2. The number density of particles deposited on freshly cleaved HOPG (Figure 5) under the same nucleation pulse condition was 0.2 × 1010 cm-2, which demonstrates the importance of electrode pretreatment. A collection of the results obtained is given in Table 1. Surface oxidation not only changes the nucleation potential but also increases the number density of sites available for nucleation due to the disruption of the surface. The results shown in Table 1 illustrate these two effects. Whereas the decrease in substrate-metal particle interactions shifts the nucleation potential to more negative values, the number of particles that can be grown depends on the number of sites that, once the appropriate nucleation potential is reached, can lead to nucleus growth. This sharply increases during the oxidation pretreatment. An important consequence of this is related to the exclusion

zone of metal ions generated during particle growth. This has been extensively discussed by Penner et al.20 and shown to lead to growth inhibition in the neighborhood of growing nuclei (“interparticle diffusion coupling” or “IDC” following Penner’s terminology). Thus, as shown in the last three entries in Table 1, for the same nucleation potential the number density of nuclei is much higher for the oxidized surfaces (note also that for a longer growth time, from 3 to 5 s, the average particle height increases, as would be expected for electrochemically grown nuclei). Considering that the IDC regions have a dimension (diameter) approximately 10 times the particle size20 and taking the height as a measure of the particle radius, it can be seen from the last three entries in Table 1 that an increase of particle coverage by a factor of 20 is closely matched by the corresponding decrease in particle height. 3.3. Properties of the Electrodeposited Pt Nanoparticles. 3.3.1. XPS. An XPS spectrum in the Pt4f region of the sample prepared as described in Figure 6 is shown in Figure 11A. The spectrum could be deconvoluted in two doublets with area ratios of 4:3 for the expected Pt4f7/2 and Pt4f5/2 peaks. The binding energies (BEs) of the Pt4f7/2 peaks were found at 71.2 (peak a) and 73.3 (peak b) eV. The peak at 71.2 eV is due to metallic platinum,52 and the peak at 73.3 eV is very close to that observed by Zhang et al. for PtOx formed on Pt evaporated on Ar-treated HOPG.52,53 Kim et al. showed that electrochemical oxidation of platinum foil leads to the formation of PtO and PtO2 with BEs at 73.3 and 75 eV, respectively.54 The broad platinum oxide peak indicates a wide range of environments for oxides and hydroxides on the surface of nanoparticles as has been previously reported.43,54,55,9 These results demonstrate that the features observed in the AFM images correspond to electrodeposited Pt, although, as previously observed for small size particles, surface oxidation is in evidence. When the size of the deposit is decreased to 1.7 nm, the Pt(0) features are absent (Figure

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Figure 12. Cyclic voltammograms for methanol oxidation for five electrodes in 0.1 M HClO4 + 1 M methanol for surfaces modified with Pt particles of different heights (sweep rate 20 mV s-1): (A) 1.4 ( 0.5, (B) 1.7 ( 0.5, (C) 16 ( 2, (D) heterogeneous sample showing partial aggregation (the aggregate height was approximately 40 nm, and the height of the individual particles in the aggregates was approximately 10 nm), (E) 26 ( 3 nm.

Figure 11. (A) XPS spectrum of Pt nanoparticles of 5.5 nm average height. Peaks a and a′ correspond to the contributions due to Pt(0). The bands with binding energies b and b′ correspond to the additional contribution to the spectrum due to platinum oxides/hydroxides. (B) XPS spectrum of platinum clusters with a mean height of 1.7 nm grown on electrochemically oxidized HOPG.

11B). The double peaks observed at 75.4 and 78.8 eV are characteristic of platinum oxide.54,54 Since no peaks were found in the binding energy range of metallic platinum, it can be concluded that these small nanoparticles are completely oxidized at the protection potential of 0.6 V at the end of the nucleation and growth sequence or that they are oxidized in air. It has been previously reported that the oxygen affinity of Pt clusters increases with decreasing cluster size,56 and Wang et al.57 found stable PtO2 at steps of metallic platinum at room temperature. The smaller nanoparticles show stronger bonding and hence higher coverage by adsorbed OH than observed for larger particles.6 This can be attributed to the larger number density of edges and vertices, which becomes more pronounced with decreasing size, especially for particles of less than 5 nm diameter.58 Thermogravimetric studies of platinum nanoparticles on aluminum oxide supports confirmed that smaller nanoparticles are more readily oxidized,59 and recently, Christensen et al.60 used X-ray absorption techniques to demonstrate the spontaneous oxidation of small Pt particles. For example, it was found that freshly deposited platinum particles of 0.7 nm crystallite size contained 90% platinum oxide and even for 2.7 nm particles 43% of the total Pt content was present as PtOx.60 3.3.2. Electrochemical ReactiWity of Electrodeposited Pt Nanoparticles. The oxidation of methanol was employed as a probe for the influence of nanoparticle size on electrochemical reactivity. The aim of this part of the work was not to carry out an extensive study on electrocatalysis but to demonstrate that the size of electrochemically synthesized nanoparticles, as measured by AFM, does correlate with previously observed size

effects.9 The influence of particle size on reactivity has been extensively discussed.9 These effects were investigated for the oxidation of methanol for the various electrocatalytic surfaces prepared. The methanol oxidation reaction was studied in 0.1 M HClO4 + 1 M methanol, and a comparison of voltammetric data for the platinum nanoparticles reported in Table 1 is shown in Figure 12. Typical methanol oxidation features are observed in voltammograms C, D, and E, which correspond to HOPGsupported nanoparticles of 16 ( 2 nm height and greater. The peak position is located at approximately 0.58 V for this size range. The peak current decreases with decreasing nanoparticle dimensions when the surface number density is considered. This leads to a decrease in specific activity for methanol oxidation for smaller particles, in agreement with previous results.9 In situ X-ray absorption studies of Mukerjee et al.61 have shown that, for particle sizes below 5 nm, the adsorption of OH and CO is greatly enhanced, thus leading to a decrease in electrocatalytic activity for MeOH oxidation. This size effect has been related to the decrease in the number density of Pt(100) facets and contiguous Pt terraces62 since the presence of three adjacent bare Pt surface atoms is essential for this reaction.63 A decrease of current is observed at potentials more positive than the peak potential due to the formation of platinum oxide. The electrochemical oxidation of methanol shows clear nanoparticle size dependence. The voltammograms for 1.7 ( 0.5 nm and smaller particles do not show any electrocatalytic activity. Zoval et al.19 and Lu et al.23 also observed that electrodeposited Pt nanoparticles smaller than 3 nm do not show hydrogen upd adsorption/desorption waves. The strong stabilization of surface platinum oxides and adsorption of OH observed by Christensen et al.60 and Mukerjee et al.,61 coupled to the demonstration by Savinova et al.2,8 that decreasing the particle size greatly hinders the rate of surface diffusion of adsorbed CO, could be the origin of the reduced activity for the methanol oxidation reaction observed for very small particles. These observations could explain the inactivity of particles smaller than 1.7 nm for methanol oxidation, and indeed, the intriguing high stability of oxidized Pt particles might find applications for the synthesis of Adam’s catalysts.55 The above considerations are not in disagreement with the well-founded CO oxidation mechanism proposed by Savinova et al.,2,8 where the extensive work by these authors and others was rationalized by considering that adsorbed OH groups occupy fixed positions on the nanoparticle surface and the rate of CO oxidation is determined by its surface mobility. A difference between the materials

Synthesis and Properties of Pt Nanoparticles on HOPG studied and commercial catalysts is the thermal treatment of the latter, which may lead to a better electrical Pt-carbon contact. The role of the support and of the electrocatalyst-carbon interaction requires further investigation. 4. Conclusions Double- and triple-pulse chronoamperometry methods have been described for the preparation of platinum nanoparticles on HOPG. These methods attempted to separate nucleation and growth steps and control the surface coverage and nanoparticle dimensions. Prior to electrodeposition, the substrate was protected anodically to prevent electroless deposition of the metal. It is demonstrated that the electrochemical oxidation of the substrate increases the number density of nucleation sites for nanoparticle formation. Decreasing the nucleation potential increases the number density of clusters while making the growth potential more positive, resulting in smaller clusters. In addition, the electrochemical surface oxidation leads to an increase in the Pt nucleation potential, and it is proposed that this is due to the surface hybridization changing from sp2 to sp3. By employing these pulse methods, a wide range of nanoparticle heights, from 52 to 1.4 nm, were prepared. XPS measurements showed that nanoparticles of 5.5 nm height have a metallic core with PtOx present on their outer layer whereas the 1.7 nm nanoparticles synthesized were oxidized to platinum oxide. The smaller nanoparticles were readily oxidized either during the protection step or later on in the air, resulting in the formation of platinum oxide, and did not show any reactivity for methanol oxidation. Acknowledgment. We thank Dr. Graham Beamson, from the U.K. National Centre for Electron Spectroscopy and Surface Analysis (NCESS), Daresbury, U.K., for help and advice in the analysis of the XPS data. M.B. acknowledges financial support from the Ministry of Science, Research and Education of Iran. J.M.A. acknowledges research funding by a “Ramon y Cajal” contract from the Spanish Ministry of Science and Innovation. Partial support from the European Union, FP6 program, NENA project (Contract Number NMP3-CT-2004-505906), is gratefully acknowledged. Supporting Information Available: AFM image of freshly cleaved HOPG, analysis of chronoamperometric experiments, image of electrodeposited nanoparticles on HOPG electrooxidized at 1.0 V, and AFM image of nanoparticles as shown in Figure 10B at a larger scale of 9.0 × 9.0 µm2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Chao Wang, C.; Daimon, H.; Sun, S. Nano Lett. 2009, 9, 1493. (b) Murray, R. W. Chem. ReV. 2008, 108, 2688. (c) Rhee, C. K.; Kim, B. J.; Ham, H.; Kim, W. J.; Song, K.; Kwon, K. Langmuir 2009, 25, 7140. (2) (a) Maillard, F.; Schreier, S.; Hanzlik, M.; Savinova, E. R.; Weinkauf, S.; Stimming, U. Phys. Chem. Chem. Phys. 2005, 7, 385. (b) Maillard, F.; Eikerling, M.; Cherstiouk, O. V.; Schreier, S.; Savinova, E.; Stimming, U. Faraday Discuss. 2004, 125, 357. (c) Maillard, F.; Savinova, E. R.; Stimming, U. J. Electroanal. Chem. 2007, 599, 221. (3) Bergamaski, K.; Pinheiro, A. L. N.; Teixeira-Neto, E.; Nart, F. C. J. Phys. Chem. B 2006, 110, 19271. (4) Cherstiouk, O. V.; Gavrilov, A. N.; Plyasova, L. M.; Molina, I. Yu.; Tsirlina, G. A.; Savinova., E. R. J Solid State Electrochem. 2008, 12, 497. (5) Lee, S. W.; Chen, S.; Sheng, W.; Yabuuchi, N.; Kim, Y. T.; Mitani, T.; Vescovo, E.; Shao-Horn, Y. J. Am. Chem. Soc. 2009, 131, 15669. (6) Mayrhofer, K. J. J.; Blizanac, B. B.; Arenz, M.; Stamenkovic, V. R.; Ross, P. N.; Markovic, N. M. J. Phys. Chem. B 2005, 109, 14433–14440. (7) Watanabe, M.; Sei, H.; Stonehart, P. J. Electroanal. Chem. 1989, 261, 375.

J. Phys. Chem. C, Vol. 114, No. 43, 2010 18447 (8) Andreaus, B.; Maillard, F.; Kocylo, J.; Savinova, E. R.; Eikerling, M. J. Phys. Chem. B 2006, 110, 21028–21040. (9) Bayati, M.; Abad, J. M.; Bridges, C. A.; Rossinsky, M. J.; Schiffrin, D. J. J. Electroanal. Chem. 2008, 623, 19. (10) (a) Roth, C.; Hussain, I.; Bayati, M.; Nichols, R. J.; Schiffrin, D. J. Chem. Commun. 2004, 13, 1532. (b) Horswell, S. L.; O’Neil, I. A.; Schiffrin, D. J. J. Phys. Chem. B 2003, 107, 4844. (11) Prabhuram, J.; Wang, X.; Hui, C. L.; Hsing, I. M. J. Phys. Chem. B 2003, 107, 11057. (12) Howells, A. R.; Hung, L.; Chottiner, G. S.; Scherson, D. A. Solid State Ionics 2002, 150, 53. (13) (a) Yeung, K. L.; Wolf, E. E. J. Vac. Sci. Technol., A 1992, 10, 651. (b) Sarapuu, A.; Kasikov, A.; Laaksonen, T.; Kontturi, K.; Tammeveski, K. Electrochim. Acta 2008, 53, 5873. (14) Lindstro¨ma, R. W.; Seidel, Y. E.; Jusys, Z.; Gustavsson, M.; Wickman, B.; Kasemo, B.; Behm, R. J. J. Electroanal. Chem. 2010, 644, 90. (15) Ye, H.; Crooks, R. M. J. Am. Chem. Soc. 2005, 127, 4930. (16) Zubimendi, J. L.; Vazquez, L.; Ocon, P.; Vara, J. M.; Triaca, W. E.; Salvarezza, R. C.; Arvia, A. J. J. Phys. Chem. 1993, 97, 5095. (17) Lee, I.; Chan, K. Y.; Phillips, D. L. Ultramicroscopy 1998, 75, 69. (18) Lee, I.; Chan, K. Y.; Phillips, D. L. Appl. Surf. Sci. 1998, 136, 321. (19) Zoval, J. V.; Lee, J.; Gorer, S.; Penner, R. M. J. Phys. Chem. B 1998, 102, 1166. (20) Penner, R. G. J. Phys. Chem. B 2002, 106, 3339. (21) (a) Liu, H.; Favier, F.; Ng, k.; Zach, M. P.; Penner, R. M. Electrochim. Acta 2001, 47, 671. (b) Liu, H.; Penner, R. M. J. Phys. Chem. B 2000, 104, 10223. (22) Ueda, M.; Dietz, H.; Anders, A.; Kneppe, H.; Meixner, A.; Plieth, W. Electrochim. Acta 2002, 48, 377. (23) Bru¨lle, T.; Stimming, U. J. Electroanal. Chem. 2001, 636, 10. (24) Chang, H. P.; Bard, A. J. J. Am. Chem. Soc. 1990, 112, 4598. (25) Yang, D. Q.; Sacher, E. Chem. Mater. 2006, 18, 1811. (26) Gloaguen, F.; Leger, J. M.; Lamy, C.; Marmann, A.; Stimming, U.; Vogel, R. Electrochim. Acta 1999, 44, 1805. (27) Lu, G. J.; Zangari, G. Electrochim. Acta 2006, 51, 2531. (28) Shen, P. K.; Chi, N.; Chan, K. Y.; Phillips, D. L. Appl. Surf. Sci. 2001, 172, 159. (29) Lu, G. J.; Zangari, G. J. Phys. Chem. B 2005, 109, 7998. (30) Lau, L. Y.; Hubbard, A. T. J. Electroanal. Chem. 1970, 24, 237. (31) Southampton Electrochemistry Group. Instrumental Methods in Electrochemistry; Ellis Horwood: Chichester, U.K., 1985. (32) Ueda, M.; Dietz, H.; Anders, A.; Kneppe, H.; Meixner, A.; Plieth, W. Electrochim. Acta 2002, 48, 377. (33) Gregory, A. J.; Levason, W.; Noftle, R. E.; Le Penven, R.; Pletcher, D. J. Electroanal. Chem. 1995, 399, 105. (34) (a) Dominguez-Dominguez, S.; Pardilla, J. A.; Murcia, A. B.; Morallon, E.; Cazorla-Amoros, D. J. J. Appl. Electrochem. 2008, 38, 259. (b) Correira, A. N.; Machado, S. A. S.; Avaca, L. A. J. Electroanal. Chem. 2000, 488, 110. (35) Lin-Cai, J.; Pletcher, D. J. Electroanal. Chem. 1983, 149, 237. (36) Avrami, A. J. Chem. Phys. 1941, 9, 177. (37) Scharifker, B.; Hills, G. Electrochim. Acta 1983, 28, 879. (38) Hovel, H.; Becker, T.; Bettac, A.; Reihl, B.; Tschudy, M.; Williams, E. J. J. Appl. Phys. 1997, 81, 154. (39) Zhang, B. L.; Wang, E. K. Electrochim. Acta 1995, 40, 2627. (40) McCreery, R. L.; Cline, K. K.; McDermott, C. A.; McDermott, M. T. Colloids Surf., A 1994, 93, 211. (41) Walters, E. C.; Murray, B. J.; Favier, F.; Kaltenpoth, G.; Grunze, M.; Penner, R. M. J. Phys. Chem. B 2000, 106, 11407. (42) Xu, L.; Li, F.; Dong, S. J. Electroanal. Chem. 1995, 383, 133. (43) Goss, C. A.; Brumfield, J. C.; Irene, E. A.; Murray, R. W. Anal. Chem. 1993, 65, 1378. (44) Alliata, D.; Kotz, R.; Haas, O.; Siegenthaler, H. Langmuir 1999, 15, 8483. (45) Alsmeyer, D. C.; McCreery, R. L. Anal. Chem. 1992, 64, 1528. (46) Beck, F.; Junge, H.; Krohn, H. Electrochim. Acta 1981, 26, 799. (47) Gewirth, A. G.; Bard, A. J. J. Phys. Chem. 1988, 92, 5563. (48) Hathcock, K. V.; Brumfield, J. C.; Goss, C. A.; Irene, E. A.; Murray, R. W. Anal. Chem. 1995, 67, 2201. (49) Kuzume, A.; Herrero, E.; Feliu, J. M.; Nichols, R. J.; Schiffrin, D. J. Phys. Chem. Chem. Phys. 2004, 6, 619. (50) Mirkhalaf, F.; Tammeveski, K.; Schiffrin, D. J. Phys. Chem. Chem. Phys. 2009, 11, 3463. (51) Shimazu, K.; Weisshaar, D.; Kuwana, T. J. Electroanal. Chem. 1987, 223, 223. (52) NIST X-ray Photoelectron Spectroscopy Database, NIST Standard Reference Database 20, version 3.4. (53) Zhang, G.; Yang, D.; Sacher, E. J. Phys. Chem. C 2007, 111, 565. (54) Briggs, D.; Seah, M. P. Practical Surface Analysis: Ion and Neutral Spectroscopy; John Wiley& Sons Ltd.: New York, 1996; Vol. 2.

18448

J. Phys. Chem. C, Vol. 114, No. 43, 2010

(55) (a) Reetz, M. T.; Koch, M. G. J. Am. Chem. Soc. 1999, 121, 7933. (b) Bancroft, G. M.; Adams, I.; Coatsworth, L. L.; Bennewitz, C. D.; Brown, J. D.; Westwood, W. D. Anal. Chem. 1975, 47, 586. (56) Xu, Y.; Shelton, W. A.; Schneider, W. F. J. Phys. Chem. B 2006, 110, 16591. (57) Wang, J. G.; Li, W. X.; Borg, M.; Gustafson, J.; Mikkelsen, A.; Pedersen, T. M.; Lundgren, E.; Weissenrieder, J.; Klikovits, J.; Schmid, M.; Hammer, B.; Andersen, J. N. Phys. ReV. Lett. 2005, 95, 256102. (58) Benfield, R. E. J. Chem. Soc., Faraday Trans. 1992, 88, 1107. (59) Wang, C. B.; Yeh, C. T. J. Catal. 1998, 178, 450.

Bayati et al. (60) Christensen, S. T.; Elam, J. W.; Rabuffetti, F. A.; Ma, Q.; Weigand, S. J.; Lee, B.; Seifert, S.; Stair, P. C.; Poeppelmeier, K. R.; Hersam, M. C.; Bedzyk, M. J. Small 2009, 5, 750. (61) Mukerjee, J.; McBreen, J. J. Electroanal. Chem. 1998, 448, 163. (62) (a) Takasu, Y.; Iwazaki, T.; Sugimoto, W.; Murakami, Y. Electrochem. Commun. 2000, 2, 671. (b) Park, S.; Xie, Y.; Weaver, M. J. Langmuir 2002, 18, 5792. (63) Cuesta, A. J. Am. Chem. Soc. 2006, 128, 13332.

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