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Effect of Size on the Electrochemical Stability of Pt Nanoparticles Deposited on Gold Substrate S. Garbarino, A. Pereira,§ C. Hamel, E´. Irissou, M. Chaker, and D. Guay* INRS-E´nergie, Mate´riaux et Te´le´communications, 1650 BouleVard Lionel-Boulet, C.P. 1020, Varennes, Que´bec Canada J3X 1S2 ReceiVed: September 9, 2009; ReVised Manuscript ReceiVed: NoVember 24, 2009
Pulsed laser deposition was used to prepare Pt nanoparticles of various sizes ranging from 1.8 to 6.0 nm. These nanoparticles were deposited on highly oriented pyrolytic graphite (HOPG) and gold substrates. For Pt deposited on HOPG and Au, the size of the nanoparticles was established by scanning tunneling microscopy (STM) and calculated from the electrochemically active surface area (EASA) obtained through measurements of the hydrogen underpotential deposited charges, QHupd. The diameters determined from these two sets of measurements agreed with each other to within 60%. X-ray photoelectron spectroscopy (XPS) was used to assess the size of the nanoparticles before and after an electrochemical treatment that involved potential cycling in 0.5 M H2SO4. The upper potential limit was progressively increased from 1.15 to 1.40 V in steps of 0.05 V, and the EASA was continuously monitored. The EASA decreased with cycle number and with increasing upper potential limit to 1.35 V vs RHE. This effect was found to be larger for the smaller Pt nanoparticles (50% decrease for φ ) 1.8 nm) than for the larger ones (20% decrease for φ ) 3.0 nm). It was found by XPS that the diameter of the smaller Pt nanoparticles increased from φ ) 1.8 nm to φ ) 6.5 nm as a result of the electrochemical treatment, whereas the diameter of the larger nanoparticles (φ ) 3.0 nm) remained constant. In this potential range, the corrosion of the gold substrate is minimal, and this observation can be explained by an increase of the equilibrium soluble Pt concentration with decreasing Pt nanoparticle diameter. Introduction In heterogeneous catalysis, the use of nanoparticles ensures a large contact area between the active material and the reactant phase, allowing for a decrease of the total catalyst content without a penalty in the performance of the process. Moreover, the properties of a given particular catalyst can be dramatically influenced by the particle size, so one of the technical challenges associated with the use of nanoparticles is to control their composition and structure. In polymer electrolyte membrane fuel cells (PEMFCs), one of the key issues is to increase the specific activity and the durability of the noble-metal catalyst electrodes that catalyze both the oxidation of hydrogen (or alcohols) and the reduction of oxygen. In today’s PEMFCs, platinum-based nanoparticles anchored on high-surface-area carbon supports are mostly used to perform these reactions.1 However, catalyst durability under operating conditions remains a key challenge to meet long-term performance degradation requirements. For this reason, current research is focused on elucidating mechanisms of catalyst degradation. Performance degradation in PEMFCs under steady-state2–4 or voltage-cycling conditions5,6 has been attributed to a loss of the electrochemically active surface area (EASA) of the platinum catalyst. Different processes can be involved in the apparent loss of platinum surface area, including (i) platinum dissolution and redeposition (Ostwald ripening process), (ii) coalescence of platinum nanoparticles occurring as a result of particle * Corresponding author. E-mail:
[email protected]. § Permanent address: Laboratoire de Physico-Chimie de Materiaux Luminescents, Universite´ de Lyon, CNRS UMR 5620 - 69622 Villeurbanne, France.
migration on the carbon support, (iii) platinum nanoparticle agglomeration triggered by corrosion of the carbon support, (iv) platinum dissolution and migration of the dissolved Pt species to the interior of the polymer electrolyte membrane where they can react (be reduced) with H2 gas diffusing from the anode to the cathode, and (v) platinum dissolution and migration of the dissolved Pt species to water collected from the reactant gas exiting the cell. Also, it should be stressed that the relative importance of these processes might vary with the size of the Pt nanoparticles. Most of the studies dealing with the stability of Pt nanoparticles have been performed with Pt deposited on carbon electrocatalyst samples that were obtained from commercially available sources. In most instances, the diameter of the Pt nanoparticles was fixed, although it might vary from one source to the other. Thus, from these samples, it is difficult to assess the effect of the particle diameter on the electrochemical stability of Pt. Moreover, because Pt is most often deposited on carbon, coalescence of platinum nanoparticles occurring as a result of particle migration on the carbon support or triggered by corrosion of the carbon support can mask the importance of other processes. The aim of the present work was therefore to study the effect of the diameter of Pt nanoparticles on their electrochemical stability. To achieve this end, gold-supported Pt nanoparticles with diameters ranging from 1.8 to 3.0 nm were prepared by pulsed laser ablation, and their stability was assessed by potential cycling in H2SO4. In this study, gold was chosen as the substrate because of its high corrosion resistance in the studied potential range. The electrode operating conditions involved are not the same as those encountered in PEM fuel cells, but the present
10.1021/jp908724k 2010 American Chemical Society Published on Web 01/28/2010
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work can be regarded as an accelerated nanoparticle rearrangement (and consequent activity decay) test under PEMFC operating conditions. Experimental Section Synthesis. Platinum was deposited by pulsed laser deposition (PLD) at room temperature in a helium-gas atmosphere. The Pt target (purity > 99.9%) was ablated with a focused KrF laser beam (wavelength λlaser ) 248 nm, laser pulse duration τlaser ) 17 ns) operating at 20 Hz. The laser fluence was set to 4 J/cm2 (laser spot size ) 1.5 mm2). The deposition was performed on freshly cleaved highly oriented pyrolytic graphite (HOPG) and gold substrates held at room temperature and positioned 4 cm from the target. The kinetic energy (Ek) of the ablated species was determined by time-of-flight emission spectroscopy using the experimental setup described elsewhere.7,8 The value of Ek was held constant at 35 eV/atom by adjusting the He background pressure to 0.2 Torr. The amount of material deposited was monitored by a quartz crystal microbalance and was varied by changing the number of laser pulses on the target. Physical Characterization. The size and distribution of Pt nanoparticles deposited on HOPG was investigated by scanning tunneling microscopy (STM) using a Nanoscope III microscope (Digital Instruments) operated at room temperature in ambient air. X-ray photoelectron spectroscopy (XPS) measurements were made for Pt nanoparticles deposited on both HOPG and Au substrates. XPS core-level spectra were obtained using a VG Escalab 220i-XL spectrometer system equipped with a polychromatic Mg X-ray source (hυ ) 1253.6 eV) and a hemispherical analyzer with a multichannel detector. The analyzer pass energy was set at 20 eV, and the photoelectron takeoff angle was held at 90°. The binding energies were determined with an accuracy of ∼0.1 eV and were systematically corrected by positioning the C 1s core-level peak of adventitious carbon contamination at 284.6 eV. Electrochemical Characterization. Pt nanoparticles deposited on Au substrates were used as working electrodes in a standard three-compartment glass electrochemical cell. Gold was selected as a substrate because its electrochemical response is relatively featureless over the potential range of interest [from 0.0 to 1.3 V vs a reversible hydrogen electrode (RHE)]. Prior to their utilization, the glassware and the electrochemical cell were cleaned according to a well-established method.9 Cyclic voltammograms (CVs) were recorded by immersing the electrode (0.2 cm2) in deaerated (argon N5.0, Praxair) 0.5 mol dm-3 ultrapure H2SO4 (Omnitrace ultra, Fisher Scientific). The electrodes were first cycled from 0.05 to 1.10 V at 50 mV s-1. The upper potential limit was chosen to minimize reorganization of the surface.10 Up to 100 CVs were collected, and the electrochemical response was usually stable after the 75th cycle. No dramatic change in the CVs occurred during this period, apart from an increase of the electrochemically active surface area by a factor ∼2 (see Supporting Information). Data were acquired with a potentiostat (Pine Instruments, model AFCBP1) connected to a computer equipped with data acquisition software (Pinechem voltammetry software). The counter and reference electrodes were a platinum wire and a reversible hydrogen electrode (RHE), respectively. All potentials are referred to the RHE scale. Results Platinum was deposited onto a quartz crystal (0.2 cm2), and the mass change, estimated from the change in resonant frequency (∆f) using the Sauerbrey equation,11 was determined
Figure 1. Variation of the mass of Pt as a function of the number of laser pulses.
Figure 2. Variation of the average Pt nanoparticle size with respect to the number of laser pulses. The average nanoparticle size was determined from STM analysis of ∼100 nanoparticles deposited on HOPG substrates.
as a function of the number of laser pulses. As seen in Figure 1, the mass of Pt increased linearly with the number of laser pulses. The slope of the linear curve was found to be 1.21 ng pulse-1. A linear mass increase was observed over the whole range of laser pulses, and an accurate determination of the amount of Pt deposited on a substrate could be obtained from the number of laser pulses used to prepare the deposit. Pt was first deposited on HOPG substrates to get an accurate determination of the average nanoparticle size, dn, as a function of the number of laser pulses, n. For this purpose, STM was used. In each case, Pt was deposited on freshly cleaved HOPG surfaces, and the average nanoparticle size was obtained from STM images by computing a histogram based on a population of ∼100 nanoparticles. As can be seen in Figure 2, the average nanoparticle size increased rapidly from ca. 1 to 4 nm as n increased from 10 to 500. In that regime, Pt exists as isolated nanoparticles at the surface of the substrate. For larger values of n, the average nanoparticle size tended to level off and reached 6 nm for n ) 5000. In that case, isolated Pt nanoparticles no longer exist at the surface of the substrate. The data of Figure 2 were fitted with a generic curve that was used in the subsequent analysis to determine the average Pt nanoparticle size obtained from a predetermined number of laser pulses. No
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Figure 3. Cyclic voltammograms of isolated Pt nanoparticles recorded in 0.5 M H2SO4 at room temperature. The hatched regions show how QHupd was calculated.
particular physical meaning is attributed to the curve shown in Figure 2. A detailed study of the mechanisms responsible for the growth of Pt nanoparticles as a function of n and Ek was presented elsewhere.12 Platinum nanoparticles were then deposited on gold substrates to perform the electrochemical measurements. The typical cyclic voltammetric responses of Pt nanoparticles prepared with n ) 50 and 250 laser pulses, corresponding to 1.8- and 3.0-nmdiameter nanoparticles, respectively, are shown in Figure 3. Similar voltammograms were obtained for all samples prepared in this study. These CVs exhibit three different regions: (i) a hydrogen sorption region from 0.05 to 0.35 V, (ii) a doublelayer region from 0.35 to 0.80 V, and (iii) a region for the formation/removal of platinum oxides at potentials greater than 0.80 V. These CVs bear a close resemblance to those of polycrystalline Pt wire, but there are also some noticeable differences. This is especially true in the platinum oxide formation/removal potential region, where the absence of a welldefined reduction peak is most particularly evident for the smallest Pt nanoparticles. This is in accordance with many studies, as already mentioned, that the oxophilicity of Pt nanoparticles increases with decreasing particle size; that is, a stronger bonding of oxygen to Pt nanoparticles is observed compared to oxygen bonding to extended surfaces.13–17 A recent combinatorial screening of Pt nanoparticles in 0.5 M H2SO4 revealed a strikingly similar potential shift of the platinum oxide reduction peak from ca. 0.75 to ca. 0.72 V for 3.0- and 1.8nm-diameter Pt nanoparticles, respectively;18 in the case of the 1.8-nm-diameter Pt nanoparticles, the platinum oxide reduction peak was also poorly resolved. A more complete study of the phenomena underlying these observations is underway. The hydrogen sorption properties of platinum in 0.5 mol dm-3 H2SO4 aqueous media allow a precise determination of the electrochemically active surface area (EASA) of platinum through measurements of the hydrogen underpotential deposited charge, QHupd, which is measured by integrating the current values in the reverse sweep, between 0.40 and 0.05 V. The hatched areas of Figure 3 correspond to the charge QHupd of the two electrodes. In these calculations, the double-layer charge was taken into account and was estimated from the minimum current recorded in the region close to 0.40 V. In Figure 4, QHupd charges recorded on Pt nanoparticles are plotted as a function of the mass of the Pt loading that was estimated from the number of laser pulses. For m < 0.15 µg, QHupd increased almost linearly with the loading. For m > 0.15 µg, QHupd continued to increase
Figure 4. Variation of the hydrogen underpotential deposition charge with respect to the mass of Pt deposited on gold. The hydrogen underpotential deposition charge was calculated from the hatched regions shown in Figure 3.
but the slope of the curve was less steep than at lower masses. For m > 0.70 µg, QHupd reached a plateau value of ∼50 µC. Because the geometric area of the substrate was 0.2 cm2, this corresponds to 250 µC cm-2. Assuming that 210 µC cm-2 is the charge needed to adsorb one hydrogen monolayer on 1 cm2 of Pt,19 this would indicate that the roughness factor of the thicker deposit is ∼1.2. This value is consistent with the roughness factor estimated by STM on thick Pt films deposited at the same kinetic energy.20 It is also consistent with the fact that the density of thick Pt films prepared under the same deposition conditions is 19 g cm-3,20 close to the value expected for bulk platinum, which is 21.45 g cm-3. We now explain how QHupd values can be used to estimate the diameter of isolated Pt nanoparticles. The total mass of Pt deposited at the surface of the substrate, mPt, can be expressed as
mPt ) NPartmPart
(1)
where Npart is the total number of Pt nanoparticles and mpart is the mass of a single nanoparticle. The mass of a nanoparticle, assuming that it has a spherical shape with diameter dn, can be expressed as the product of its volume and the bulk density of Pt, FPt ) 21.45 g cm-3
mPart ) VPartFPt ) (4πdn3 /24)FPt
(2)
mPt ) NPart(4πdn3 /24)FPt
(3)
and so
Rearrangement of eq 3 gives
NPart )
mPt (4πdn3 /24)FPt
The total platinum surface area, SPt, can be expressed as
(4)
Effect of Size on the Stability of Pt NPs on Au
SPt ) NPartSPart
J. Phys. Chem. C, Vol. 114, No. 7, 2010 2983
(5)
where Spart is the surface of a single nanoparticle and is given by
SPart ) πdn2
(6)
Therefore, combining eqs 4-6 gives
SPt )
mPt (4πdn3 /24)FPt
(πdn2)
(7)
TABLE 1: Diameters of Pt Nanoparticles Deposited on Highly Oriented Pyrolytic Graphite and Gold Substrates number of laser pulses
diameter from STM measurementsa (nm)
initial diameter from QHupd measurements b (nm)
50 75 100 250 500
1.8 2.0 2.2 3.0 3.5
2.6 2.7 2.8 4.8 7.5
a From Pt deposited on highly oriented pyrolytic graphite substrate (see Figure 2. b From Pt deposited on gold substrate.
and
SPt ) (6mPt)/(dnFPt)
(8)
Hence, from eq 8, one can estimate the diameter of Pt nanoparticles, dn, once the mass and the total surface area of Pt are known. In this study, the mass of Pt is given by the number of laser pulses (Figure 1), and SPt can be estimated from QHupd through the relation
SPt ) QHupd /(210 µC cm-2)
(9)
with 210 µC cm-2 being the theoretical charge required to adsorb one hydrogen monolayer on 1 cm2 of polycrystalline platinum atoms.19 Thus, combining eqs 8 and 9 gives
(10)
Figure 5. Cyclic voltammograms of 3.0-nm-diameter Pt nanoparticles recorded in 0.5 M H2SO4 at room temperature. The upper potential limit was progressively increased from 1.10 to 1.40 V. In each case, 75 cycles were performed before the upper potential limit of the scan was increased to the next value.
where dn,el is the diameter of Pt nanoparticles estimated from the QHupd values. It can be shown that eq 10 is valid even if Pt atoms are organized in hemispherical nanoparticles. In that case, the mass of each nanoparticle is one-half that of a spherical nanoparticle, but there is twice as much nanoparticle on the substrate. Thus, the size of Pt nanoparticles deposited at the surface of gold substrates can be inferred from electrochemical measurements. The hydrogen underpotential deposited charge, QHupd, was used to evaluate the diameter of Pt nanoparticles deposited on gold substrate. As shown in Table 1, for the smallest nanoparticles (
