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Unveiling the Inner Structure of PtPd Nanoparticles Vagner Zeizer Carvalho Paes, Marcus Vinicius Castegnaro, Daniel L. Baptista, Pedro L. Grande, and Jonder Morais J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05472 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 12, 2017

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The Journal of Physical Chemistry

Unveiling the Inner Structure of PtPd Nanoparticles Vagner Z. C. Paes, Marcus V. Castegnaro, Daniel L. Baptista, Pedro L. Grande, Jonder Morais* Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Avenida Bento Gonçalves, 9500, 91501-970, Porto Alegre, RS, Brazil. Corresponding Author *[email protected] ABSTRACT: Despite of all efforts to explore the structural properties of bimetallic nanoparticles, there is still a constraint of proper tools to successfully probe their composition and atomic arrangement. In this work, bimetallic PtPd nanoparticles with approximately 5 nm mean diameter were synthesized to achieve distinct atomic distributions: nanoalloys or core@shell. The samples were probed by Medium Energy Ion Scattering (MEIS) and spaceresolved elemental analysis via energy dispersive X-ray (EDX) spectroscopy in STEM (Scanning Transmission Electron Microscope) mode. The complementary association of STEMEDX profiling with MEIS, which simultaneously surveys millions of nanoparticles, becomes a powerful tool for a statistically representative structural analysis. As result, the measurements provided key details such as core size, shell thickness and composition, and even distinguished core@shell from core@alloy structures. PtPd nanoalloys and Pd-core structures were successfully obtained while the attempt to produce Pt-core NPs actually resulted in a mixture of

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nanoalloy and core@alloy structures (core = Pt or Pd). Moreover, MEIS sensitivity to the NPs’ shell enable to quantify its most plausible alloy composition. Introduction The growing interest on nanoparticles (NPs) is owed to their distinctive electronic, optical and catalytic properties, a direct outcome of their reduced size that originates dissimilar performances than equivalent bulk materials 1. At the same time, the characterization of NPs is itself a challenging task 2, requiring many associated experimental techniques, in a selfconsistent way, to unveil their electronic and structural properties

3,4

. In the last decades,

bimetallic NPs have drawn even more attention in catalysis due to their even more unique properties, distinct from the monometallic counterparts. Surface science research has invested on studying model bimetallic catalysts, which provided relevant insights on reaction mechanisms 5,6. Nevertheless, there is still the need of a better understanding on the structural information of the active sites in a real catalyst. Pt-based catalysts are among the most industrially relevant materials in heterogeneous catalysis

7-9

, however, the high cost and limited supply of Pt propelled a search for alloying.

Such bimetallic (Pt-M; M=transition metals) nanoalloys 10,11 reveal higher catalytic performance than their respective monometallic systems potential in catalysis

2,14-17

12,13

. Particularly, Pt-Pd nanoalloys have promising

with consolidated applications in hydroisomerization, hydrocracking

and hydrotreatment reactions 18. Compared to monometallic Pt or Pd NPs, it has been shown that the controlled addition of the second metal enables to tune the catalytic activity, selectivity, and poisoning resistance through cooperative effects

19,20

. Such findings inspired several

investigations that explored the chemical and structural properties of PtPd NPs 21-26.


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Another variable that plays an essential role in the final catalytic properties is the elemental distribution within the bimetallic NPs. Particularly, the atomic arrangements in Pt-Pd NPs have been extensively investigated via several experimental techniques, such as, Cscorrected scanning transmission electron microscopy (STEM) 22, extended X-ray absorption fine structure (EXAFS) 21,23-25 and X-ray photoelectron spectroscopy (XPS) 26. Although many papers embraced the challenge of correlating the electronic, morphological and structural properties of PtPd NPs with their reactivity, the number of techniques that can provide a detailed analysis of their surface composition and internal structure is scarce. Consequently, it is mandatory the determination of the NPs surface and inner composition aiming to have insights of its effects on the final catalytic properties 18. In a recent work, we have reported a clear structure-dependent reactivity of Pt-Pd catalysts during NO reduction by in situ EXAFS and XPS

21

. The PtPd NPs produced with either core-

shell or random alloy structures provided distinct catalytic behavior. In all cases, the results revealed a migration of Pd atoms towards the NPs’ surface, forming a Pd-rich shell, which contributed to modify their catalytic performance and tendency toward oxygen poisoning. Although the catalytic effects were quite noticeable, it was not possible to fully determine the atomic arrangement of the NPs, especially the composition of the shell in a core@shell case. Such information is crucial to comprehend their distinct catalytic performance and to produce more efficient catalysts. Medium Ion Energy Scattering (MEIS) is an ion beam technique with high surface sensitivity and high depth resolution, which is suitable to elucidate nanostructures 27-34. MEIS is particularly appropriate to investigate the PtPd NPs due to the large contrast in mass, which allows distinguishing the signal from each element. The technique presents an additional advantage


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since it probes millions of NPs simultaneously, providing a structural analysis that is statistically representative of the investigated sample.

Methods In the present work, we address a comprehensive structural characterization of PtPd NPs using MEIS and Scanning Transmission Electron Microscope (STEM) associated with space-resolved energy dispersive X-ray (EDX) spectroscopy. The investigation of the inner structure of several NPs with STEM-EDX profiling is a difficult task, thus the complementary association of these techniques becomes a powerful tool. Furthermore, X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), HRTEM (High Resolution Transmission Electron Microscopy), and Rutherford Backscattering Spectrometry (RBS) were also applied. Three different types of PtPd NPs, named PtPd1, PtPd2 and PtPd3, were prepared using different variations of a method previously published

21,35

, aiming to achieve different elemental

arrangements within the NPs. The synthesis of PtPd1 was designed in order to obtain a random PtPd nanoalloy, while the remaining two were prepared using a seeded growth method to produce core@shell structures, Pd@Pt (PtPd2) or Pt@Pd (PtPd3). To allow ion beam analysis of these NPs, thin layers of the colloids containing the different types of PtPd NPs, were deposited on silicon substrates by spin coating and later dried in vacuum. During this step we aimed to achieve a monolayer of NPs on top of the Si substrate.

Results and Discussion The experimental 2D MEIS maps of ion scattering intensities obtained for the three samples are displayed in Figure 1. In order to verify the correct atomic distribution within the NPs, a


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series of structural models were considered (some are shown in Figure 2) in our MEIS simulations using the PowerMEIS package 28. The 2D MEIS simulations that better described the experimental results are also shown in Figure 1, and will be discussed below. For instance, simulated energy spectra (1D MEIS plots) for some of the tested NP models are displayed in Figure 3 for 128o backscattering angle, and compared with the experimental results. The results for other scattering angles can be found in the Supporting Information, in Figures S4, S5 and S6.

Figure1. Experimental (left) and simulated (right) MEIS 2D maps for samples (a) PtPd1, (b) PtPd2, and (c) PtPd3. The dashed red lines in the experimental maps depict the ion backscattering angles used in the 1-D plots, while the red arrows point the contribution of Pt and Pd signals.


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Figure2. Selected NP structures models used in this work: (a) Pt-Pd alloy, (b) Pt@Pd, (c) Pd@Pt, (d) Pt@Pt-Pd, and (e) Pd@Pt-Pd structures.

The criterion to select the best structural model is not only the minimum of the χ2 function, as defined in ref. 36, but also when a good description of the Pt and Pd peaks shape in the 1-D MEIS spectra, at three different scattering angles, was achieved (see Figures S4-S6). For instance, the Pt peak for sample PtPd1 (see Figure 3a) is somewhat shifted to lower scattering energies indicating a low concentration of Pt at the NP surface or shell. For each sample, the structure, average diameter (including its error), core size and shell thickness, as well as stoichiometries, can be obtained. The simulations show that sample PtPd1 has a characteristic model consisting of a Pt0.6Pd0.4 alloy, which properly describes the experimental data. For sample PtPd2, two structural models are possible, a Pt0.5Pd0.5 alloy and a [email protected] core@alloy. Sample PtPd3 MEIS spectra can be modeled by both a Pt0.6Pd0.4 alloy as well as a [email protected] core@alloy structure. As a following step, the NPs diameter for each model was optimized. Figure S7 displays

χ2 as a function of diameter for all the samples in view of their best structures previously determined. A numerical interpolation of

χ2 curves was employed to obtain the best mean

diameter of each sample. The best structural parameters and stoichiometries are shown in Table S1. The MEIS simulation for sample PtPd3 considering a monolayer of PtPd NPs on Si agrees


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with the mean diameter obtained by TEM (Table 1). On the other hand, the NPs diameter for samples PtPd1 and PtPd2 as determined by MEIS present slightly larger values. MEIS simulations that were calculated with the TEM mean diameter values (Figures S8 and S9) corroborate that fact. We believe that such discrepancy is due to the sample preparation by spin coating, which may privilege larger NPs to remain on the Si surface. The simulated 2D-MEIS patterns shown in Figure 1 correspond to the best fitting results and reproduce, apart from the background, the experimental data.

Figure 3. MEIS simulations at 128° backscattering angle for samples (a) PtPd1, (b) PtPd2, and (c) PtPd3 considering some atomic arrangements.


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Table 1. Structural information extracted from MEIS simulation for samples PtPd1, PtPd2 and PtPd3: The simulated structures; core radius and shell size (for core@shell strutures); NPs mean diameters obtained by MEIS and TEM.

SAMPLE

Structures

Core radius (nm)

PtPd1

Pt0.6Pd0.4 alloy

-

-

Pt0.5Pd0.5 alloy

-

-

[email protected]

1.5

1.1

Pt0.6Pd0.4 alloy

-

-

[email protected]

1.6

1.1

PtPd2 PtPd3

Shell (nm)

NPs Diameter NPs Diameter by MEIS (nm) by TEM (nm) 6.6 ± 0.6

4.1 ± 1.1

5.4 ± 0.6

3.9 ± 0.7

5.4 ± 0.5

5.0 ± 1.4

In attempt to deepen our understanding on the MEIS limitations, simulations were performed for samples PtPd1 and PtPd2 considering NPs clustering on the Si surface. For this sake, a pyramidal structure composed of three layers of PtPd NPs (Figure S10), instead of a regular NP monolayer, was used in the MEIS simulations (Figures S11 and S12). The results clearly show that stacking leads to fitting improvement and also to a deterioration of the spatial resolution. Certainly, clustering of NPs is indeed possible

37,38

, but has to be avoided. For that reason, a

careful sample preparation for MEIS analysis is critical since agglomeration/clustering eclipses MEIS capability of discriminating different NP structures. Several spin coating conditions were exhaustively tested until we reached the required MEIS sample. Indeed, the use of MEIS along with another imaging technique, such as TEM and HAAFF-STEM, is of major importance in order to ascertain the structure interpretation.


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STEM-EDX spectrum profiling is a well-established experimental technique that can probe the inner composition of NPs

22,39,40

using sub-nm electron beam that raster across the one NP.

Interactions between the energetic electrons and the sample atoms produce characteristic X-rays, which reach an energy dispersive X-ray (EDX) spectrometer to build the signal level at any position, providing the intensity (and presence of a specific element) as a function of a location in the image. The intensity of the signal is proportional to the amount of that atom. It therefore generates compositional profiles of a particular NP image with nanometric resolution. Highangle annular dark-field (HAADF)-STEM images and EDX elemental profiles of our bimetallic samples have been acquired, and the representative results are shown in Figure 4. The PdPt1 sample presents a homogenous Pd:Pt profile typically observed in alloy bimetallic NPs. The correspondent Z-contrast HAADF image also support such characteristic. On the other hand, the HAADF image of the PdPt2 sample noticeably reveals a Pd@Pt core-shell structure. The high-Z Pt shell results in a brighter contrast surrounding a darker Pd-core in the HAADF image. The respective EDX profile was also acquired supporting the observation of a Pd@Pt core-shell NP. Finally, STEM analyses of the PdPt3 sample indicate the presence of mixed structures. The results suggest the coexistence of alloy, Pt and Pd-core NPs formation. MEIS best structural models for each case were scaled properly and used to plot estimated compositional profiles. More details on how these profiles were determined are found in Section S3 of the Supporting Information. First, the PtPd nanoalloy phase observed by HAADF-STEM for sample PtPd1 matches the estimated composition profile, as shown in Figure S14.


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Figure 4. High-angle annular dark-field (HAADF)-STEM images and EDX elemental profiles of the PtPd samples. Additionally, the experimental EDX profile of sample PtPd2 evidencing a Pd-core (Figures 5 and S15) is compared with profile simulations for a Pd@Pt (Figure 5b) and Pd@PtPd structures (Figure 5c). It clearly indicates the shell consists of an alloy, supporting MEIS results. It is worth reporting that the inclusion of a Pd-core structure contribution in the MEIS simulations for sample PtPd3, with both PtPd alloy and Pt@PtPd phases, actually improved the fitting.


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Figure 5. (a) HAADF-STEM image of sample PtPd2 superimposed with its EDX composition profile (red line for Pt and blue for Pd), and estimated profiles considering the spherical structures (b) Pt@Pd and (c) [email protected]. The MEIS results made possible to find the best structural models for the PtPd NPs produced in each synthesis method, which were corroborated by the STEM/EDX profiling analysis. For sample PtPd1 both techniques confirmed a nanoalloy structure, and equally indicated the coexistence of different structures for sample PtPd3 (Pt-seed). STEM/EDX profiling clearly determined the occurrence of a Pd core for PtPd2 (Pd-seed), a structure that was not excluded by MEIS findings. A recent work 41 predicts that a phase coexistence of Pt-core and PtPd structures may occur in the sub-10 nm regime. However, their model does not sustain the formation of a Pd-core structure. It is noteworthy that, to our knowledge, it is the first time that the synthesis of sub-10 nm Pd-core NPs via a simple wet chemical reduction is reported. Pt-core or a PtPd alloy NPs are more favorable since Pt has larger surface and cohesive energies 41,42. Nevertheless, Ref.


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41 reports an energy difference of only 0.1 eV/atom between the formation of Pt-core and PtPd alloy structures from that of a Pd-core structure. Perhaps, that is the case where further calculations should be carried out to explain the formation of 5 nm Pd-core structure in our synthesis. MEIS sensitivity to the shell-composition of NPs is hardly obtained by other techniques, such as XPS

26,43

. Indeed, XPS peak area ratio can be used to estimate core and shell sizes for

core-shell bimetallic NPs. Although its methodology was proposed and successfully applied for similar PtPd NPs,

26

it always assumes a perfect core-shell structure. In another case

43

, XPS

analysis of Rh@Fe2O3 distinguished cases where Rh atoms were present only at the core from that of Rh in both core and shell. Nevertheless, it did not provide core or shell dimensions, shell stoichiometry and particle size distribution. Here, STEM/EDX profiling analysis was also able to distinguish Pd@Pt and Pd@PtPd structures, corroborating MEIS simulation results.

Conclusions In summary, it has been demonstrated that the association of MEIS and STEM/EDX reveals the inner structure of bimetallic nanoparticles. The synthesis method provided PtPd NPs with distinct atomic arrangements, from nanoalloys from core@shell structures, which were successfully characterized. For instance, the synthesis of NPs with a Pd core was achieved as revealed by STEM/EDX measurements and not ruled out by MEIS. PtPd NPs having a Pd core under the 10 nm regime is unexpected theoretically 41. In contrast, the attempt to produce Pt core NPs actually resulted in a mixture of nanoalloy and core@shell structures (core = Pt or Pd). Such result is probably due to our particular synthesis conditions, which appear to form NPs within a border region of the cohesive energy 41, where the critical size and composition provide stability


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for the simultaneous formation of all structures. New in situ XAS experiments, as previously applied for monometallic NPs

44,45

, are planned in order to clarify such multiple structural

synthesis. All synthesized core@shell NPs were proven to consist of core@alloy systems, and MEIS sensitivity to the NPs’ shell enable to quantify the most plausible alloy composition. Such information is of fundamental importance to understand the catalytic activity of bimetallic NPs, since it is highly dependent on the NPs surface composition.

AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge: PDF file that contains results and details of the experimental procedures involving XRD, TEM, STEM, RBS and MEIS. It also includes details on the MEIS and STEM simulations.


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ACKNOWLEDGMENTS The authors would like to thank the Brazilian funding agencies CNPq, CAPES, PRONEX/FAPERGS and INCT/INES for their financial support.

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Unveiling the Inner Structure of PtPd Nanoparticles - The Journal of

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