Spontaneous Chemical Ordering in Bimetallic Nanoparticles - The

Aug 4, 2015 - In most experimental studies on bimetallic nanoparticles the homogeneous alloying is consistently reported or a priori assumed. It is ge...
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Spontaneous Chemical Ordering in Bimetallic Nanoparticles Aneta Januszewska,† Grzegorz Dercz,‡ Adam Lewera,† and Rafal Jurczakowski*,† †

Faculty of Chemistry, Biological and Chemical Research Centre, University of Warsaw, Zwirki i Wigury 101, PL-02-089 Warsaw, Poland ‡ Institute of Materials Science, University of Silesia, Pulku Piechoty 1A, 41-500 Chorzow, Poland S Supporting Information *

ABSTRACT: In most experimental studies on bimetallic nanoparticles the homogeneous alloying is consistently reported or a priori assumed. It is generally believed that at nanoscale, alloying is promoted even for otherwise immiscible systems. In this study we present evidence that Pd−Pt nanoalloys are much more susceptible to segregation than their bulk counterparts. The spontaneous segregation (chemical ordering) of Pd−Pt nanoalloys was evidenced in hydrogen absorption studies and additionally confirmed by using X-ray photoelectron spectroscopy. The results clearly show that phase segregation in bimetallic nanoparticles do occur despite the lack of the miscibility gap in the Pd−Pt phase diagram, negative heat of mixing and small lattice mismatch for both metals. Phase segregation for other bimetallic systems is also discussed. Hydrogen solubility in the investigated nanoparticles is enhanced nearly by 3 orders of magnitude with respect to H solubility in unsegregated Pd−Pt alloys. This superior solubility points out the importance of phase segregation, which is frequently overlooked, yet fundamental for systems designed for heterogeneous catalysis and hydrogen storage.

1. INTRODUCTION Palladium alloys are widely used in heterogeneous catalysis.1,2 In this group, palladium−platinum systems are of particular catalytic interest. They are used for oxygen reduction reaction (ORR),3,4 as anodes in low temperature fuel cells,5−7 and in hydrogenation of aromatic compounds in fuels.8 Pd−Pt catalysts often operate at elevated temperatures, for example, in catalytic converters for automobile exhausts.1,9,10 Under such conditions, segregation/alloying may be additionally promoted. Another set of applications of palladium-containing systems is related to hydrogen sorption and storage. They can be used in hydrogen sensing where fast and selective reaction in the presence of molecular hydrogen is required.11 In the literature little is known about the spatial distribution of segregated nanoparticle alloys. Regardless the complexity and a vast possibility of mixing arrangements resulting from the permutation of unlike atoms, four main categories of mixing patterns can be distinguished:12 (i) mixed nanoalloy, with alloy components uniformly distributed either in ordered or random fashion, the later indicates a homogeneous alloy particle or solid solution, (ii) subcluster segregated nanoalloys containing clusters of different chemical composition, for example, NPs of immiscible elements, (iii) core−shell segregated alloys, and (iv) multishell system with layered or onion-like structure. Segregated nanoparticles are usually regarded as core−shell type structures in which one component tends to accumulate in the outermost layer while the other in the nanoparticles nuclei. Currently there is a large discrepancy between theoretically explored nanoalloys’ architectures and experimental results, in which homogeneous alloying is readily reported or a priori © XXXX American Chemical Society

assumed. Recently, Barcaro et al. theoretically explored ordering in nanostructural Pd−Pt alloys. By means of DFT calculations, the authors predicted the existence of more complex segregation patterns having multishell structure, in which each shell is layered by “patches” of like atoms.13 Also, other segregation scenarios for Pd−Pt nanoparticles were theoretically predicted. 14,15 It should be stressed that segregation in multicomponent nanoparticles plays a fundamental role in hydrogen storage and catalysis.14 Despite the lack of the miscibility gap in the Pd−Pt phase diagram, negative heat of mixing and small lattice mismatch for both metals, it has been shown that the bulk Pd−Pt system can be subjected to a severe segregation induced by exposing to prolonged molecular hydrogen and heat treatment. However, no evidence for phase separation was found in the absence of dissolved hydrogen or short hydrogen and heat treatment. Specific conditions (hydrogen pressure and temperature) for phase segregation to occur were found to be strongly composition dependent. Segregation in bulk alloys of higher platinum content requires superior hydrogen pressures and more elevated temperatures. Prolonged hydrogen and heat treatment at the pressure 300 bar and temperature 673 K for 10 h were found to be insufficient to induce a considerable phase separation in Pd81Pt19. The latter became obvious when the hydrogen pressure was increased to 100 MPa.16,17 It should be emphasized that upon heating in vacuum at 673 K the alloy Received: May 19, 2015 Revised: August 4, 2015

A

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repeated 10 times, and the average value was used. To confirm that the mass did not change during experiments, the sample weight was confirmed after electrochemical measurements. No significant weight loss was detected (usually less than 5%). Platinum-rich nanoparticles were found to form more stable colloids in water, whereas nanoalloys with 40% at. platinum content or more tend to agglomerate, and a water suspension of these nanoalloys was much less stable. However, the colloids were re-established from flocculated precipitate by sonication prior to deposition at the electrode. Chemical composition of the nanoparticles was controlled by changing the molar ratio of the Pd and Pt precursors and verified in XPS and ICP-MS measurements. Platinum content determined by using ICP-MS was close to nominal concentration (Table 1), which indicates the effective control of the bulk nanoalloy composition. For convenience, throughout the paper we will refer to the nominal composition of the nanoalloys.

homogeneity is entirely recovered. Surprisingly, this process is even faster under low hydrogen pressure.18 The spatial distribution of atoms in Pd−Pt bulk materials was studied by Kirchheim and co-workers.19 Specific interactions of surfaces with monolayers of foreign adatoms (metals and nonmetals) are generally known as particularly sensitive surface probes,20 among which copper and hydrogen underpotential deposition (UPD) are the most widely used. However, by using this approach, little is known about the nanoparticle interior. In contrast, the chemical potential of absorbed hydrogen does provide deep insight into the homogeneity on the atomic scale since it is determined by intimate interactions between metal atoms and absorbed hydrogen occupying interstitials. For fcc metals, the immediate environment of the absorbed hydrogen is composed of 6 or 4 metal atoms in the octahedral or tetrahedral interstitial site, respectively.21 In this study we report electrochemical characterization combined with XRD and XPS analysis of ultrapure Pd−Pt nanoalloys obtained in surfactant-free polyol method recently developed in our laboratory.22 We also compare previously reported p-c-T data with electrosorption isotherms determined in this study. After careful evaluation of the absorption isotherms, the mixing/segregation at an atomic scale is critically assessed. Contrary to common general assumptions, we show that Pd−Pt nanoalloys are much more susceptible to segregation than their bulk Pd−Pt counterparts. We also report the highest hydrogen solubility in Pd80Pt20 exceeding H/(Pt + Pd) = 0.5 at the ambient temperature and hydrogen pressure, which additionally emphasizes the importance of both surface purity and system segregation on hydrogen storage.

Table 1. Nominal Platinum Content in Pd−Pt Nanoparticles and That Determined in ICP-MS Measurementsa nominal 20 40 60 80

ICP-MS 22.3 44.2 58.3 80.6

± ± ± ±

0.5 0.5 0.5 0.5

a

All concentrations expressed in atomic percentage (balance− palladium).

Apparatus. The X-ray diffraction experiments were performed on an X-Pert Philips PW 3040/60 diffractometer operating at 30 mA and 40 kV, which was equipped with a vertical goniometer and Eulerian cradle. The radiation wavelength (Cu Kα) was 1.54178 Å. For X-ray photoelectron spectroscopy (XPS) experiments, a PHI5700 spectrometer, made by Physical Electronics, was employed. Monochromatized X-ray Al Kα radiation of energy equal to 1486 eV was used. Spectra were recorded with 23.50 eV pass energy, 0.100 eV step, and 100 ms dwell time. The total energy resolution was about 0.35 eV. The measurements were performed in a vacuum of 10−10 Torr. Peak fitting was performed with CasaXPS 2.3.15 software.

2. EXPERIMENTAL SECTION Measurements were carried out in an aqueous 0.1 M sulfuric acid solution (99.999%, Aldrich). All solutions were prepared with triple-distilled water additionally purified by using Millipore filters (18.2 MΩ·cm). N 6.7 Ar gas (Air Products, BIP-PLUS) was used for solution deareation. All glassware was cleaned with sulfochromic acid, then with concentrated sulfuric acid, and with deionized water in a final step. Bare Pd−Pt nanoparticles were prepared by using a modified polyol method. PdCl2 and K2PtCl4 was dissolved in ethylene glycol (EG) and boiled (470 K) for 3 h under a reflux condenser. No other components were added. Electrochemical measurements were performed for nanoparticles supported on disposable Au (99.99%) substrates (Au foil). To ensure the highest possible cleanliness, the following procedure was followed: first, the Au substrate was cleaned in “piranha solution” and subsequently subjected to aqua regia for 60 s to increase the surface roughness, which helped to prevent detachment of the nanoparticles. Cleanliness of gold substrate was confirmed by using cyclic voltammetry. Subsequently, a small aliquot of nanoparticles in water suspension was deposited on the gold substrate, and the sample was left to dry in a UHP Ar atmosphere at room temperature. The usual weight of deposited material was about 30 μg. To prevent aggregation, the suspension was sonicated for 5 min prior to the deposition. In order to normalize observed catalytical currents per catalyst mass, the gold substrate was preweighed using ultramicrobalance from Sartortius (SE2, resolution 0.1 μg, repeatability 0.25 μg), and subsequently prepared electrode (substrate plus nanoparticles deposit) was weighed prior to any electrochemical experiments. As a rule, every weighing was

3. RESULT AND DISCUSSION 3.1. Nanoparticle Morphology. TEM images and size distributions of Pd−Pt nanoalloys are shown in Figure S1 (Supporting Information (SI)). The mean size of bimetallic nanoparticles have a diameter between 7 and 16 ± 2 nm, except palladium nanoparticles (50 ± 5 nm). X-ray diffraction patterns, shown in Figure 1, top, are similar for both palladium and platinum. One set of diffraction lines, at intermediate positions between those for pure Pd and Pt, were also observed for nanoalloys. Diffraction lines are shifted toward higher 2θ for pure palladium and palladium reach samples. Lattice parameters for nanoparticles were determined by using Rietveld refinement (see Figures S2−S8 in SI). The accuracy in their determination found using alumina plate SRM 1976 standard is ±0.015%. Lattice parameters follow a linear dependence in function of NPs composition (Figure 1, bottom). This behavior is unexpected since for homogeneous bulk Pd−Pt alloys slight but distinct negative deviation from Vegard’s was reported15,23,24 (see Figure 1, bottom (▲)). The reliable lattice parameters of pure bulk palladium and platinum taken from refs B

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about lattice parameters can be found in the literature for segregated Pd−Pt bulk alloys. The Pd−Pt sample compositions were investigated by using X-ray photoelectron spectroscopy (XPS). Survey and high resolution Pd 3d/Pt 4d and Pt 4f spectra were recorded for asreceived samples. Due to the overlapping between Pd 3d and Pt 4d signals, a specific deconvolution method was used. First, Pt 4f, Pt 4d, and Pd 3d high resolution spectra were fitted for pure Pt and Pd nanoparticles, respectively (Figures S9 and S13 in SI). Next, the experimentally determined peak parameters, such as line shape parameters, full width at half-maximum (fwhm), and tail factor, were determined for all components (Pt 4f 7/2, Pt 4f5/2, Pt 4d5/2, Pt 4d3/2, Pd 3d5/2, Pd 3d3/2), keeping the theoretical spin−orbit splitting and relative area ratio constraints.33 Shirley background34 and Lorentzian line shape with tail modifier was used to fit the data, due to the fact that Lorentzian line shape is well suited for peak area determination, as it has a finite integral between minus and plus infinity, which cannot be said for asymmetric functions sometimes used for the same purpose. All the spectra were fitted by using the fit parameters determined for pure elements (as above) but with varied intensity (peak area) of the whole Pt or Pd doublet (i.e., keeping the theoretical relative ratio of doublet’s components area constant). Sensitivity factors for Pt 4f and Pd 3d were taken from literature.35 Experimental Pt 4d sensitivity factor was calculated using Pt 4f sensitivity factor and the Pt 4d/Pt 4f area ratio determined from the survey scan for pure Pt nanoparticles. Quality of the fit can be judged from Figure 2, where the typical XP spectrum deconvolution for Pd−Pt nanoparticles, as used for quantitative analysis, is shown.

Figure 1. Top: X-ray diffraction pattern for Pt−Pd nanoparticles. Bottom: Lattice parameters in function of Pd−Pt nanoparticles’ composition, determined by ICP-MS measurements (colored points). Lattice parameters of pure bulk palladium and platinum metals from refs 25 and 26 indicated with black open circles (○); black continuous line (―) indicates Vegard’s law for Pd−Pt alloys. Lattice parameters for homogeneous bulk Pd−Pt alloys are from ref 24 (▲).

25 and 26 are shown in Figure 1 with open circles (○). As seen in this figure, for our Pd−Pt nanoparticles (colored solid circles), the Vegard’s law is nearly perfectly obeyed without any negative deviations. This behavior is frequently interpreted as ideal homogeneous alloying.27 However, the characterization of solid solutions by conventional X-ray diffraction has been critically assessed on the example of Cu−Co alloys in the form of multilayered and heterogeneous films.28,29 It has been shown on the example of this model system that for the layer thickness of about 10 nm, the X-ray diffraction patterns merge into a single set representing the average lattice parameters defined by the overall layers composition. By definition, this average lattice spacing must vary linearly between values for pure elements with Vegard’s law dependence when the overall composition is changed, yet this average lattice spacing is nowhere physically present in the heterogeneous sample. In fact, Vegard’s law is seldom obeyed, even for ideal solid solutions.30 As a consequence, the Vegard’s law dependence for nanostructures may actually be an indication for a severe departure from the homogeneity. Similar results were reported for various immiscible alloys reviewed in ref 31. Michaelsen concluded that coherent inhomogeneities in solid solutions must be larger than several nanometers before they can be detected by using conventional XRD in a straightforward fashion and that complementary measurements have to be performed in order to confirm the homo- or heterogeneity of an alloy.28 It should be stressed that segregated bulk Pd−Pt alloys also exhibited a single set of diffraction patterns.19,32 However, no information

Figure 2. XPS spectrum deconvolution for Pd−Pt nanoparticles with 60% at. Pt.

Results of XPS quantitative analysis is shown in Figure 3. Pt atomic content was determined from two data sets: first from Pt 4f and Pd 3d doublets, second one from Pt 4d and Pd 3d doublets. It is worth noting that in both cases there is more platinum registered than the nominal (and total, as determined from ICP-MS) composition, which suggests Pt segregation on nanoparticle surface, as XPS is a surface-sensitive technique. The difference in energy between Pt 4f and Pt 4d photoelectrons allowed us to vary the probing depth, as photoelectron escape depth (measured as Inelastic Mean Free Path, IMFP) depends on photoelectron energy. For metals, IMFP usually falls within the range 5−25 Å.36 The calculated IMFP values are given in Table 2; overall, Pt 4f signal comes from about 2−3 Å deeper than Pt 4d signal, which additionally confirms Pt segregation on nanoparticles surface, despite the C

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Figure 3. XPS quantitative analysis (Pt atomic percent) for Pd−Pt nanoparticles (ordinate) in function of platinum content determined by ICPMS. Two data sets are shown, one determined from Pt 4f and Pd 3d doublets (red circles), and another from Pt 4d and Pd 3d doublets (green squares). Increased Pt content in topmost atom layers can be observed.

Table 2. Photoelectron’s Inelastic Mean Free Path for Pt 4f 7/2, Pt 4d5/2, and Pd 3d5/2 in Pt and Pd photoelectron\material

IMFP in Pt (Å)

IMFP in Pd (Å)

Pt 4f 7/2 (KE = 1415 eV) Pt 4d5/2 (KE = 1172 eV) Pd 3d5/2 (KE = 1151 eV)

16.8 14.6 14.5

23.7 20.5 20.2

Figure 4. Voltammograms of (A) Pd20Pt80; (B) Pd40Pt60; (C) Pd60Pt40; (D) Pd80Pt20 nanoalloys recorded in 0.1 M H2SO4 at a scan rate of v = 5 mV·s−1. For comparison, voltammograms for Pt nanoparticles were included (gray lines).

larger determination error for Pt 4d/Pd 3d doublets pair as compared to the Pt 4f/Pd 3d pair due to overlapping between Pt 4d and Pd 3d signals (cf. Figures 2 and 3). The difference between composition data obtained for Pt 4d/Pd 3d and Pt 4f/ Pd 3d doublet pairs (Figure 3) was confirmed based on the t test on the 95% confidence level. Platinum enrichment in the outermost layer is rather surprising since theoretical and experimental studies show that in vacuum palladium tends to accumulate at the Pd−Pt surface, which is related to lower surface energy of this element.15,37 It is likely that platinum at the surface is favored by solvent interactions. Pt-enriched surface for Pd−Pt nanoalloys obtained by coreduction in EG was evidenced also by Liu et al. in FT-IR and EXAFS measurements.38 Pd surface oxides were observed in XPS experiments for Pd− Pt nanoalloys with 40 at. % and more (see Figures S9−S13 in SI) after inevitable contact of the investigated nanoparticles with atmosphere. No Pt oxides were detected in the same experiment; hence, the observed Pt segregation cannot be caused by interactions with atmospheric oxygen. The determined amount of Pd surface oxides was about 15 at. % for pure Pd nanoparticles, 12 at. % for Pt40Pd60, and 6 at. % for Pd40Pt60 and was undetectable for samples containing less than 40 at. % of Pd. This suggests that, for samples with less than 40 at. % of Pd, the Pd atoms are covered by Pt and, as such, cannot form surface oxides, which additionally suggest Pt surface segregation. 3.2. Voltammetric Behavior. Figure 4 shows voltammograms recorded in the hydrogen adsorption/absorption region for Pd−Pt nanoparticles of different compositions. Voltammogram recorded for pure Pt nanoparticles is also shown for comparison. For all samples, currents are normalized to the real surface calculated from the oxide charges. General shapes of the voltammograms are similar to those reported by Solla-Gullon et al. for Pd−Pt nanoparticles prepared in a water-in-oil microemulsion in the presence of surfactant and subjected to decontamination.39 In our studies, nanoparticles are pure

already in the as-received state without surface decontamination procedures.22 Voltammograms shown in Figure 4 were stable from the very first cycle. For pure Pt nanoparticles, the hydrogen absorption peaks, visible for potentials +0.238 and +0.130 V, are related to hydrogen adsorption at on-top and multifold surface adsorption sites, respectively.40 With an increase in the Pd content, the peak related to a multifold adsorption site gradually diminished. However, also the peak related to an on-top adsorption site nearly vanishes at moderate Pt concentrations. For platinum-rich samples, we have observed a sharp pair of peaks near the potential E = +0.04 V. The additional pair of peaks is absent for electrochemically deposited alloys at initial state.41 However, the formation of similar features has been reported by Guerin and Attard for electrochemically treated Pd−Pt alloys containing 38.67% of platinum and subjected to electrochemical oxidation/reduction cycles. The authors concluded that this pair of peaks (E = +0.04 V) is related to the formation of islands or a layer of pure palladium and concluded that the peaks are related to hydrogen absorption into these Pd clusters.42 The high cleanness of the Pd−Pt nanoalloys allowed us to investigate the hydrogen absorption characteristics in the “as synthesized” nanoparticles, which preserved their original structure formed during formation and were not distorted by cleaning events or surface oxidation. The latter was found to substantially change the surface state of both monocrystals and nanoparticles, mostly due to surface atoms reorganization.20 After oxidation, we have observed changes in relative intensities of strongly and weakly bound hydrogen (see Figure S14), whereas the total charge of the hydrogen UPD was similar for oxidized and unoxidized surfaces. The strongly bonded hydrogen adsorption peak was always higher for unoxidized surfaces. Similar changes in D

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currents related to CO-oxidation are marked with red lines. After CO oxidation with a distinct current peak, observed at potentials given in Figure 5, the voltammetric courses are the same as for oxidized nanoalloys (black lines). In the presence of CO adsorbed on the pure palladium nanoparticles (Figure 5A), hydrogen absorption is clearly visible although is suppressed because of a large kinetic barrier.45 Hydrogen electrosorption is markedly faster at platinum.46 For Pd80Pt20 alloy (Figure 5B) it can be observed that this sample combines both fast hydrogen absorption and vast hydrogen absorption capability. Surprisingly, for platinumrich alloys with Pt atomic percentage larger than 35%, the hydrogen absorption is still considerable and hydrogen absorption currents can be observed even for a Pd40Pt60 sample. Furthermore, the hydrogen absorption is nearly reversible for this nanoalloy since the current absorption/ desorption peaks are symmetrical (Figure 5C). For pure Pt nanoparticles poisoned with CO (Figure 5D), virtually no current was observed in the hydrogen region, which confirms that above-discussed cathodic current signals, observed for the CO-poisoned Pd−Pt samples, were indeed related to hydrogen absorption and not H2 evolution. For all samples, no current peaks related to hydrogen UPD were observed on the poisoned surfaces either in cathodic or anodic scans. HUPD peaks were re-established only after COads oxidation at higher potentials. Cathodic scan after CO oxidation is shown in Figure 5 (black lines); for all samples, HUPD peaks are visible, and the anodic part of HUPD is not shown for clarity (both are symmetrical (see Figure 4)). It is also interesting to examine the potentials at which the CO-stripping current peaks are formed, Ep, since these values are strongly dependent on the surface composition. Results are plotted in Figure 6. The highest potential of CO-stripping

voltammetric profiles were reported for decontaminated platinum nanoparticles.20 First, cyclic voltammograms were recorded in the hydrogen UPD region without surface oxidation (Figure 4). The characteristic feature on the I−E characteristics at E = +0.04 V, visible particularly for Pt-rich samples (see Figure 4A,B and Figure S14 in SI), is related to alloy segregation and the spontaneous formation of palladium-rich clusters during nanoparticles synthesis. It should be stressed that the voltammetric profiles shown in Figure 4 were stable and do not change during potential cycling within hydrogen absorption/desorption region. Therefore, the segregation cannot be related either to hydrogen absorption/desorption or to palladium dissolution/replating effects, as observed in ref 43. 3.3. CO Stripping. Considerable hydrogen absorption in palladium starts below potential +0.1 V versus RHE.44 In order to separate this process from molecular hydrogen evolution, occurring on platinum at similar potentials, we have performed electrochemical measurements in the presence of carbon monoxide, which strongly inhibits H2 evolution reaction (HER). Carbon monoxide was adsorbed at the onset of the hydrogen UPD at controlled potential E = 0.4 V. Next, the potential was scanned in the cathodic direction to E = −0.025 V, in order to examine the hydrogen absorption in the Pd−Ptrich alloys in the presence of adsorbed CO. Next, one cycle with anodic potential limit 1.25 V versus RHE was also recorded. Figure 5 shows exemplary curves recorded during these CO-stripping experiments for selected NP samples. A part of voltammetric courses related to hydrogen absorption in CO-covered electrodes is marked with green lines, whereas

Figure 6. CO oxidation peak potential vs atomic Pt concentration for Pd−Pt nanoparticles obtained in this work (197 °C; blue circles) and obtained by water-in-oil microemulsion (room temperature) from ref 39 (gray triangles).

current peak were observed for pure palladium nanoparticles, Ep = 0.890 V versus RHE, while for Pt containing samples, the potential of current peak decreases. CO-stripping was previously studied by Feliu and co-workers on Pd−Pt nanoalloys prepared by a water-in-oil microemulsion.39 COoxidation current peak potentials determined by this group are also shown in Figure 6. Carbon monoxide oxidation is a kinetically controlled process, and the stripping peak and the oxidation onset potential depend strongly on the sweep rate. Small differences, in the order of 25 mV, between current peak potentials for palladium and platinum nanoparticles recorded in

Figure 5. Comparison of voltammetric curves for the CO-stripping voltammetry on platinum, palladium nanoparticles, and Pd−Pt nanoalloys: H2SO4 (0.5 M), scan rate v = 5 mV·s−1. Hydrogen absorption/desorption in/from CO-poisoned nanoparticles are marked with green lines, currents related to CO oxidation are marked with red lines. E

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The Journal of Physical Chemistry C this work and ref 39 are related to different scan rates, 5 and 20 mV·s−1, respectively. Both sets of data shown in Figure 6 suggest possible segregation by unsymmetrical behavior around both concentration limits for pure elements on the CO oxidation potential (marked with continuous lines). Lower potentials are observed for Pd−Pt nanoalloys, which additionally support Pt segregation at the nanoalloys surface. Larger deviations observed in our results are most likely related to differences in the degree of segregation, in turn, related to the different method of preparation. Pd−Pt nanoparticles in this studies were synthesized at 197 °C, whereas those from ref 39 were obtained at room temperature. 3.4. Hydrogen Absorption Isotherms. Hydrogen absorption was determined according to a procedure described in detail elsewhere.47 In brief, the sample was equilibrated at a given potential. Next, the potential was scanned to E = 0.55 V (double layer region) at 10 mV·s−1, and hydrogen desorption current was recorded. Special attention has been devoted to avoid oxidation of molecular hydrogen evolved during electrode equilibration at potentials lower than E = +0.05 V and dissolved in the solution. For this purpose we used a continuous argon stream passing through the solution near the electrode surface.48,49 To verify whether this method is sufficient for nanoparticles, we recorded the desorption current for pure Pt NPs, which do not absorb hydrogen and are the most active toward dihydrogen evolution. Figure S15 in SI shows hydrogen desorption curves recorded for Pt and Pd60Pt40 nanoparticles. Platinum nanoparticles of 10 nm diameter do not exhibit hydrogen absorption; the measured charges are related to hydrogen desorption from the platinum surface only. From Figure S15(A) in SI it follows that during measurements the molecular hydrogen electrooxidation is eliminated up to potentials E = +0.025 V, and no excessive charge (greater than that related to hydrogen adsorption) was measured. For palladium−platinum alloy Pd60Pt40, much higher desorption currents were observed (see also Figure 4C). Below potential E = +0.075 V, hydrogen desorption charges exceeded Q = 600 μC·cm−2 (per real metallic surface), which indicates that hydrogen must be desorbed from the nanoparticle interior. This is a surprising finding since, for bulk Pd−Pt alloys, hydrogen absorption is virtually entirely suppressed already for the alloy with a platinum concentration equal to 32 at. %.41 In voltammograms recorded for nanostructured Pd−Pt alloys shown in Figure 4, a distinct current related to hydrogen absorption is also visible. Hydrogen absorption is obvious even for nanoalloys with 60 at. % of Pt (Figures 4B and 5C). The origin of this superior hydrogen solubility may be related to (i) mesoscopic effects and/or (ii) palladium and platinum atoms segregation and departure from a homogeneous state. Taking into account rather large sizes of the nanoparticles and only minor lattice distortions (see Table S2 in SI), the later effect is much more probable. Flanagan and co-workers have shown that departures from the homogeneous state can be preferably detected by the investigation of hydrogen absorption.16,18,50 Hence, it is interesting to compare the isotherms for Pd−Pt nanoalloys, bulk homogeneous and segregated Pd−Pt alloys. Figure 7 shows hydrogen absorption isotherms for Pd80Pt20 nanoalloy for the as-synthesized sample (blue points) and the electrochemically oxidized sample (red points). The hydrogen isotherms shown in Figure 7 include both hydrogen adsorbed and absorbed in the alloy. In the case of bulk alloy, the former is negligible, but for nanostructured samples, the influence from

Figure 7. Comparison of hydrogen sorption isotherms for Pd80Pt20 alloy obtained from gaseous and liquid environments. Relations of electrode potential (left ordinate) and molecular hydrogen pressure (right ordinate, as logarithm of the root of pressure) to hydrogen apparent concentration in the metal matrix. Nanostructured alloys (colored points): as prepared (blue circles), after surface reconstruction (red circles), and in the presence of crystal violet in the solution (green circles). For comparison bulk homogeneous alloy (gray points): 50 μm foil determined in 0.1 M H2SO4 in our previous studies48,49 (gray circles) and from gaseous environment from ref 16 (gray diamonds); Bulk segregated alloy (black triangles) reported by Flanagan et al. from ref 16; Sievert’s law (red dashed line).

hydrogen adsorption cannot be underestimated and it can even dominate the total charge at higher electrode potentials (lower H2 pressures). From the comparison of oxidized and unoxidized particles, it is visible how the low pressure part of the isotherm is affected by hydrogen adsorption. To estimate the contribution of adsorbed hydrogen to total electrosorption isotherm, we have measured the hydrogen solubility in the presence of crystal violet in the sulfuric acid solution. This surfactant partially blocks the hydrogen absorption sites and reduces the hydrogen coverage, particularly at higher potentials (lower hydrogen pressures); for example, hydrogen surface coverage at +0.1 V versus RHE is diminished by about 78%.44 For comparisons on the same figure, we have shown data for hydrogen solubility in bulk alloys, both homogeneous (gray points) and segregated (black points) Pd80Pt20 alloys. All isotherms for nanoparticle Pt20Pd80 samples exhibit a distinct plateau related to phase transition related to the formation of the hydride. The position of this plateau on the pressure/potential scale is 4.67 kPa/+39.3 mV that is close to that for Pd−Pt alloy with about 2.5% platinum content,51 which is about 10× smaller than the overall Pd80Pt20 composition. It should be emphasized that for Pd−Pt alloys the plateau pressure rapidly rises with the increase in the platinum content. For the alloy with 19% Pt, only so-called “ghost” phase transition occurs at a pressure of 10 bar. This manifests itself only as an inflection point on the p-c-T curve, and it is not related to a physical phase transition since for Pt19Pt81, the αphase is the only existing phase in the whole pressure range. The same is true for even higher platinum loadings, but the pressure necessary for achieving considerable hydrogen solubility drastically grows. For example, at the pressure 105 bar, the hydrogen concentration in Pd50Pt50 is r = H/(Pd + Pt) = 0.1, which is comparable to hydrogen solubility in bulk platinum.52 For our nanoparticles Pd80Pt20 at ambient hydrogen pressure, that is, at E = 0.0 V, the hydrogen concentration is r = H/(Pd + Pt) = 0.5. This concentration in homogeneous bulk Pd80Pt20 can be obtained only at hydrogen pressures exceeding F

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The Journal of Physical Chemistry C 100 bar.53 Such drastically enhanced hydrogen solubility in nano-Pd80Pt20 and the phase transition observed for virtually identical hydrogen pressures for both nanoparticles and segregated bulk alloys of the same composition clearly reveals a severe departure from the homogeneous state of the Pd−Pt NPs. Recently, similar hydrogen solubility enhancement was reported by Kobayashi et al. for Pd−Pt54 and Pd−Rh55 systems. The authors explained the solubility enhancement in terms of “nanosize-induced” effects for both systems, despite rather large nanoparticle sizes in the case of Pd−Pt alloys (∼7 nm). Alloy homogeneity “at the atomic scale level” was claimed based on X-ray powder diffraction and EDX imaging. Here we point out that the hydrogen absorption isotherm is, in fact, a fingerprint of the NPs fine structure since theoretical and experimental studies show that at least phase transition pressure is not size-dependent to a considerable extent.56 Theoretical investigation for the position of phase transition in Pd−H nanoparticles of various shapes also indicates that the phase transition occurs at similar chemical potentials than in bulk Pd.57 On the contrary, the hydrogen absorption isotherms for homogeneous and segregated alloys are drastically different.16,58,59 For spherical Pd and Pt nanoparticles, considerable lattice distortions are observed when the NPs diameter is smaller than ∼4 nm.60 Lattice contraction for larger nanoparticles are much less pronounced, and such systems usually exhibit physicochemical properties similar to bulk materials, yet with an advantageous surface to volume ratio for objects in the nanometer scale. The comparison of the isotherm reported for Pd79Pt21 in ref 54 with the isotherms from Figure 7 for Pd80Pt20 is shown in SI (Figure S16). Since for our much larger nanoparticles (10−16 nm), with low internal stress, we have observed qualitatively similar behavior, that is, the same pressure of phase transition, phase transition itself, and noticeable hydrogen solubility for nanoalloy of Pt content >30 at. %, in our opinion the “nanosizeinduced effect” reported in ref 55 is caused by the severe departure from the homogeneous state and points out the facilitated segregation of alloy nanoparticles with a comparison to bulk materials. The spatial distribution of atoms in segregated NPs is not known at the present stage; however, taking into account the large size of the NPs in our study, it is unlikely that the sample is segregated only at the surface. Considerable hydrogen solubility indicates phase separation also in the nanoparticle interior. Taking into account experimentally observed platinum and palladium distribution in an oscillatory fashion,37 it is plausible that the nanoparticles tend to lean toward the structure similar to multilayered shells. It should be noticed that we have additionally investigated both permeability of hydrogen through Pt monolayers and stability of platinum layers at the palladium surface in a separate study.61 Since the bulk Pd−Pt system exhibits low intrinsic tendency for spontaneous segregation, the results weaken the faith in a homogeneous distribution of constituents in many other alloyed nanoparticles recently readily reported as “alloyed on the atomic scale”, even for immiscible systems. The primary source of misconception may be caused by neglecting the characteristic length scales determined by experimental methods. Often XRD powder diffraction results are given to testify for system homogeneity. Here we show that not only the merging of diffraction patterns, but also the linear dependence

between lattice parameter and alloy composition do not imply homogeneity. On the contrary, the latter observation may be, in fact, an indication for a severe phase segregation, particularly for larger nanoparticles (∼4 nm or more) and systems that are known not to follow Vegard’s law.

4. CONCLUSIONS Ultrapure Pd−Pt nanoparticles prepared in a modified polyol method without surfactants additions were studied with XPS, ICP-MS, XRD, and classical electrochemical methods. The following conclusions can be drawn: 1. Pd−Pt nanoalloy segregation was evidenced by radically enhanced hydrogen solubility. Hydrogen absorption was observed, even for the sample containing 60% at. Pt at 293 K and ambient pressure. Hydrogen insertion into a homogeneous alloy of the same overall composition would require H2 pressures of the order of 105 bar.53 2. Platinum enrichment in the outermost layer was detected in XPS measurements. Carbon monoxide oxidation experiments also suggest possible surface segregation and increased Pt surface concentration with respect to the nanoparticles interior. 3. Nanoalloys exhibit phase transition (formation of the hydride) at the pressures/potentials virtually identical to those reported for bulk segregated alloys of the same overall composition, which suggest similar equilibrium structure at the atomic scale for bulk and nanostructured segregated materials. However, unlike for bulk materials, chemical ordering in nanomaterials appears at much lower temperatures and does not require a ternary component (hydrogen), which points out enhanced mobility of the platinum and palladium atoms in nanoalloys. 4. Nanoparticles were characterized by high surface purity in as-received state and do not required surface decontamination. In the hydrogen region, voltammograms (shown in Figure 4) were stable from the very first cycle, which indicates that the phase separation is a spontaneously occurring process taking place already during nanoparticles creation (at. 180 °C) and is not caused by hydrogen absorption/desorption cycles. Nanoparticle oxidation causes surface reconstruction similar to that reported for pure platinum.22 Surface reconstruction only has a minor effect on hydrogen absorption and mainly affects surface phenomena (adsorption). 5. The results obtained in this work were thoroughly compared with literature data obtained for bulk and nano Pd−Pt alloys obtained by different methods: polyol method at 197 °C, water-in-oil emulsion (obtained at room temperatures), as well as with annealed bulk Pd− Pt alloys. Results suggest that a homogeneous alloying in the Pd−Pt system is a metastable state and that this alloy spontaneously tend to segregate. The segregation is apparently facilitated at nanoscale. 6. Relative surface tension is the main driving force for the segregation to occur in Pd−Pt alloys.37 However, homogeneous alloying is favored for this system by other physicochemical factors such as negative heat of formation and lack of miscibility gap in Pd−Pt phase diagram. Strain due to the atomic size difference can also be neglected to a first approximation for this system G

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(8) Fujikawa, T.; Tsuji, K.; Mizuguchi, H.; Godo, H.; Idei, K.; Usui, K. EXAFS Characterization of Bimetallic Pt−Pd/SiO 2 −Al 2 O 3 Catalysts for Hydrogenation of Aromatics in Diesel Fuel. Catal. Lett. 1999, 63, 27−33. (9) Farrauto, R. J.; Heck, R. M. Catalytic Converters: State of the Art and Perspectives. Catal. Today 1999, 51, 351−360. (10) Chen, A.; Holt-Hindle, P. Platinum-Based Nanostructured Materials: Synthesis, Properties, and Applications. Chem. Rev. 2010, 110, 3767−3804. (11) Kumar, R.; Varandani, D.; Mehta, B. R.; Singh, V. N.; Wen, Z. H.; Feng, X. L.; Mullen, K. Fast Response and Recovery of Hydrogen Sensing in Pd-Pt Nanoparticle-Graphene Composite Layers. Nanotechnology 2011, 22, 275719. (12) Ferrando, R.; Jellinek, J.; Johnston, R. L. Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles. Chem. Rev. 2008, 108, 845−910. (13) Barcaro, G.; Fortunelli, A.; Polak, M.; Rubinovich, L. Patchy Multishell Segregation in Pd-Pt Alloy Nanoparticles. Nano Lett. 2011, 11, 1766−1769. (14) Tan, T. L.; Wang, L.-L.; Johnson, D. D.; Bai, K. A Comprehensive Search for Stable Pt−Pd Nanoalloy Configurations and Their Use as Tunable Catalysts. Nano Lett. 2012, 12, 4875−4880. (15) Yun, K.; Cho, Y. H.; Cha, P. R.; Lee, J.; Nam, H. S.; Oh, J. S.; Choi, J. H.; Lee, S. C. Monte Carlo Simulations of the Structure of PtBased Bimetallic Nanoparticles. Acta Mater. 2012, 60, 4908−4916. (16) Flanagan, T. B.; Clewley, J. D.; Noh, H.; Barker, J.; Sakamoto, Y. Hydrogen-Induced Lattice Migration in Pd-Pt Alloys. Acta Mater. 1998, 46, 2173−2183. (17) Flanagan, T. B.; Park, C.-N. Hydrogen-Induced Rearrangements in Pd-Rich Alloys. J. Alloys Compd. 1999, 293-295, 161−168. (18) Noh, H.; Clewley, J. D.; Flanagan, T. B. Hydrogen-Induced Lattice Migration in Pd-Pt Alloys. Scr. Mater. 1996, 34, 665−668. (19) Lüke, R.; Schmitz, G.; Flanagan, T.; Kirchheim, R. H-Induced Phase Separation in Pd−Pt Alloys as Studied by High Resolution Electron Microscopy. J. Alloys Compd. 2002, 330-332, 219−224. (20) Solla-Gullón, J.; Rodríguez, P.; Herrero, E.; Aldaz, A.; Feliu, J. M. Surface Characterization of Platinum Electrodes. Phys. Chem. Chem. Phys. 2008, 10, 1359−1373. (21) At ambient temperature, the octahedral sites are preferred. (22) Januszewska, A.; Dercz, G.; Piwowar, J.; Jurczakowski, R.; Lewera, A. Outstanding Catalytic Activity of Ultra-Pure Pt Nanoparticles. Chem. - Eur. J. 2013, 19, 17159−17164. (23) Kidron, A.; Machlin, E. Atomic Displacements in Palladium/ Platinum Solid Solutions. J. Mater. Sci. 1967, 2, 354−357. (24) Moysan, I.; Paul-Boncour, V.; Thiebaut, S.; Sciora, E.; Fournier, J. M.; Cortes, R.; Bourgeois, S.; Percheron-Guegan, A. Pd-Pt Alloys: Correlation between Electronic Structure and Hydrogenation Properties. J. Alloys Compd. 2001, 322, 14−20. (25) Arblaster, J. Crystallographic Properties of Platinum. Platinum Met. Rev. 1997, 41, 12−21. (26) Arblaster, J. W. Crystallographic Properties of Palladium Assessment of Properties from Absolute Zero to the Melting Point. Platinum Met. Rev. 2012, 56, 181−189. (27) Luo, J.; Maye, M. M.; Petkov, V.; Kariuki, N. N.; Wang, L. Y.; Njoki, P.; Mott, D.; Lln, Y.; Zhong, C. J. Phase Properties of CarbonSupported Gold-Platinum Nanoparticles with Different Bimetallic Compositions. Chem. Mater. 2005, 17, 3086−3091. (28) Michaelsen, C. On the Structure and Homogeneity of Solid Solutions: The Limits of Conventional X-Ray Diffraction. Philos. Mag. A 1995, 72, 813−828. (29) Takane, N.; Narita, H.; Kurogouchi, Y.; Arai, S. Crystal Structure of Co/Cu Multilayers Prepared by Pulse Potential Electrodeposition with Precisely Controlled Ultrathin Layer Thickness. AIP Adv. 2013, 3, 022119−1−022119−5. (30) Jacob, K.; Raj, S.; Rannesh, L. Vegard’s Law: A Fundamental Relation or an Approximation? Int. J. Mater. Res. 2007, 98, 776−779. (31) Ma, E. Alloys Created between Immiscible Elements. Prog. Mater. Sci. 2005, 50, 413−509.

(lattice mismatch 0.8%). It should be stressed that all these factors can additionally promote segregation for other bimetallic or multimetallic nanoparticles. Moreover, chemical ordering at the atomic level is difficult to evaluate and results of physicochemical characterization can be easily misinterpreted, as was shown in the case of classical powder diffraction. 7. We report the hydrogen sorption in Pd80Pt20 exceeding H/(Pd + Pt) = 0.5 at the ambient temperature and hydrogen pressure, which is the highest hydrogen sorption capacity for this system reported to date. This result additionally emphasizes that phase segregation is an extremely important factor that is frequently overlooked and, together with surface purity, is quite crucial in heterogeneous catalysis and hydrogen storage.

ASSOCIATED CONTENT

S Supporting Information *

Supplementary figures referred to in the main text. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04777. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: (+48) 22 55 26 550. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by the Ministry of Science and Higher Education (Poland) under Project No. N204275638. XPS measurements were performed at the Department of Physics, Silesia University, Katowice, Poland. We gratefully acknowledge J. Szade, M. Kulpa, E. Rowiński, A. Winiarski, and M. Pilch for the XPS measurements. Authors wish to thank Ms. Barbara Gralec for sample preparation. Helpful discussions with Dr Cezary Gumiński (University of Warsaw) is greatly acknowledged.

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REFERENCES

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