Letter Cite This: Nano Lett. 2017, 17, 7892−7896
pubs.acs.org/NanoLett
Local Chemical Ordering and Negative Thermal Expansion in PtNi Alloy Nanoparticles Qiang Li,† He Zhu,† Lirong Zheng,‡ Longlong Fan,† Na Wang,† Yangchun Rong,† Yang Ren,§ Jun Chen,† Jinxia Deng,† and Xianran Xing*,† †
Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100039, China § X-Ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ‡
S Supporting Information *
ABSTRACT: An atomic insight into the local chemical ordering and lattice strain is particular interesting to recent emerging bimetallic nanocatalysts such as PtNi alloys. Here, we reported the atomic distribution, chemical environment, and lattice thermal evolution in full-scale structural description of PtNi alloy nanoparticles (NPs). The different segregation of elements in the well-faceted PtNi nanoparticles is convinced by extended X-ray absorption fine structure (EXAFS). Atomic pair distribution function (PDF) study evidences the coexistence of the face-centered cubic and tetragonal ordering parts in the local environment of PtNi nanoparticles. Further reverse Monte Carlo (RMC) simulation with PDF data obviously exposed the segregation as Ni and Pt in the centers of {111} and {001} facets, respectively. Layer-by-layer statistical analysis up to 6 nm for the local atomic pairs revealed the distribution of local tetragonal ordering on the surface. This local coordination environment facilitates the distribution of heteroatomic Pt−Ni pairs, which plays an important role in the negative thermal expansion of Pt41Ni59 NPs. The present study on PtNi alloy NPs from local short-range coordination to long-range average lattice provides a new perspective on tailoring physical properties in nanomaterials. KEYWORDS: PtNi alloy nanoparticles, extended X-ray absorption fine structure, atomic pair distribution function, negative thermal expansion, local ordering
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distribution and local structure in PtNi alloy nanoparticles. Simulations based on Monte Carlo and molecular dynamics have been performed to predict the existence of local chemical ordering on the outmost surface of PtNi alloy particle.19,20 Nevertheless, the exponential growth of computing consumption with the increase of atoms becomes a barrier in the real nanocatalysts above 5 nm. Meanwhile, on account of the variation of synthetic and dynamic condition, theoretical simulations stemming from thermodynamics equilibrium leaves the gap to the real structure of PtNi alloy NPs in practical application. Recently, Auger electron spectroscopy (AES), lowenergy electron diffraction (LEED) and low-energy ion scattering (LEIS) had been combined to reveal the composition oscillation and structural reconstruction in the first few atomic layers of Pt3Ni (111) surface region.21 However, the rigorous requirement on the surface flatness are desired, which makes it difficult for nanoparticle samples. Methods based on transmission electron microscope (TEM) can determine the element segregation into an intuitional picture;22,23 however, the characteristic of projection in some cases conceals the 3D
t-based bimetallic nanoparticles, especially PtNi alloy nanoparticles, have been paid much attention for the excellent catalytic performances and reduced usage of noble metal in electrocatalytic reactions.1−4 Compositional modification, segregation, and morphology-tailoring give rise to both electronic and lattice strain effects resulting in the shift of the dband center position and thermal stress during the catalytic process. As one of the crucial criteria in selection of catalytic and base material, thermomechanical properties keep control of the operational environment and service life of catalytic.5 Even so, matching performance of thermal expansion between supported Pt-based bimetallic nanoparticles and base materials6,7 has drawn little concern for the difficulties in elaborate probing of atomic-scale lattice strain and element distribution in the whole particles. Lattice thermal expansion behaviors in nanoparticles differ greatly from the analogues of the bulk state.8−11 Inner lattice defects and distortion,12,13 surface structural imperfection,14−16 and stress17 bring plenty of novelty of local structure and lattice strain in nanosized particles. Artificial tailoring for the thermal expansion of nanocatalysts puts forward an urgent demand for the understanding of local structure from short-range scale to entire average scale.18 Therefore, some preliminary efforts have been dedicated into the theoretical and experimental investigations of elemental © 2017 American Chemical Society
Received: September 30, 2017 Revised: November 10, 2017 Published: November 21, 2017 7892
DOI: 10.1021/acs.nanolett.7b04219 Nano Lett. 2017, 17, 7892−7896
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Nano Letters structural information and spatial distribution of fine local structure. In this work, a comprehensive method combining X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), atomic pair distribution function (PDF), and the corresponding simulated techniques were adopted to study the composition distribution and local ordering in PtNi NPs. EXAFS, as an ideal technique to identify the nearest coordination shell and local structural distortion, has started to be an effective tool to inspect the short-range structure in PtNi alloy NPs.24−26 Here, the average coordination numbers and contributions of nearest bonding length on the thermal expansion extracted by EXAFS helped to clarify the different roles of heteroatomic and homoatomic pairs. Atomic pair distribution function (PDF) measurements provided complete structural information from local coordination to long-range lattice strain giving a visualized insight into the relation between average thermal expansion and local ordering distribution, lending further and full-scale understanding to the nanosize effect on the elemental distribution, lattice strain, and thermal performance. PtNi alloy nanoparticles were synthesized by a modified solvothermal reactions 27 using N,N-dimethylformamide (DMF) as solvent and reductant without any capping agents (the details of synthesis can be seen in Table S1.). With the increase of Pt component, PtNi alloy nanoparticles undergo a transition from well-faceted octahedron to cube suggesting a preferred facet from {111} to {100} (see TEM images in Figure 1 and the energy-dispersive spectrometer (EDS) in Figure S1e.). Average particle diameter of the uniform PtNi alloy NPs is 10 nm (Figure S1a−d). Selected area electron diffraction (SAED) images display as a typical face-centered cubic structure (FCC) with diffraction rings of {111}, {200}, {220}, {311}, and {222}, indicating the random distribution of Pt or Ni atoms on the lattice sites in a long-range scale (Figure 1c,f,i,l). High-resolution TEM images (HRTEM, Figure 1b,e,h,k) for all alloy particles show the good crystallinity and single-phase characteristic of prepared PtNi nanoparticles. However, local structure information along the direction of electron beam was overwhelmed by the projection feature of HRTEM. With more Pt components, the lattice parameter shows an obvious increase in PtNi NPs (see Figure 2a). Extracted data from the Lebail fitting of X-ray diffraction (XRD) demonstrate positive thermal expansion (PTE) behaviors in PtNi alloys from room temperature to 250 °C except for Pt41Ni59 (see Figure S2). In the latter, however, a remarkable transition from positive to negative thermal expansion (NTE) is observed at 150 °C. This thermal expansion performance was confirmed by the second thermal cycle run, ruling out the side effects such as the desorption of adsorbate and lattice atom rearrangement.28 The abnormal NTE may be ascribed to local ordering and segregation in PtNi NPs. Therefore, EXAFS was utilized to probe the local coordination information from the average FCC structure revealed by XRD and SAED for its highly sensitivity of short-range orderings.29,30 As revealed in Pt41Ni59 and Pt83Ni17, EXAFS measurements of the Pt L3-edge and Ni Kedge demonstrate the quite closed first coordination shell from absorbing atom to Ni or Pt in R space (Figure 2b,c). Even so, wavelet transformation (WT) helped to distinguish the two nearest coordination for the different response area of Ni and Pt atoms in k space.31 With the increase of Pt component, WT identified an enhancement of oscillation in the high-k part of Pt
Figure 1. (a−l) TEM, HRTEM, and SAED images of synthesized PtNi alloy nanoparticles under different reactant ratios. Insets are the crystal shapes of alloy particles. Diameter-distribution histograms counting about 300 particles are listed in Figure S1a−d.
L3-edge suggesting more Pt−Pt coordination (Figure S3). Unlike the stable Pt−Pt bond length, increase of Pt component results in the obvious relaxation of Pt−Ni bond length. R-space fitting for first coordination shell was conducted based on these two scattering paths from absorbing atoms to Ni and Pt at room temperature. According to the method developed by Hwang et al.,32 alloying extent (JPt or JNi) representing the ratio of observed heteroatomic coordination proportion to perfect random analogue, revealed the average composition segregation and short-range ordering (Figure 2d). As for Ni-rich PtNi alloy nanoparticles like Pt41Ni59, both JPt and JNi are a little lower than 100, confirming the existence of slight segregation of Ni and Pt. On account of more Ni component and lower JPt, Ni atoms are suggested as more segregation and distribution on the surface with an accompanying Pt-rich core. Increase of Pt component yields a progressive growing of Pt segregation on the surface of PtNi NPs. Average bond lengths of Pt41Ni59 alloy NPs demonstrate the different contributions of homoatomic and heteroatomic scattering pathes in the thermal behaviors (Figures 2e, S4, and S5). Pt−Ni pairs make a key contribution to the NTE of lattice. Relatively, Pt−Pt and Ni−Ni coordinations perform slight changes to lattice parameters in the investigated temperature range. Due to the same synthetic conditions except for the ratio of Pt and Ni sources, the transformation of the particle shape in Figure 1 is only dominated by the chemical composition in PtNi alloy nanoparticles. During nucleation and growth, difference of local coordination environment due to different chemical 7893
DOI: 10.1021/acs.nanolett.7b04219 Nano Lett. 2017, 17, 7892−7896
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Figure 2. (a) Temperature dependences of unit cell parameters of PtNi alloy nanoparticles (NPs). (b,c) Fourier transformations of k3-weighted Pt L3-edge and Ni K-edge EXAFS spectra and the fitting results of Pt83Ni17 and Pt41Ni59 alloy NPs at room temperature. (d) Alloying extent estimated from coordinations of EXAFS according to Hwang’s method32 and the schematic diagrams of PtNi alloy NPs. Alloying extent JA or JB is estimated as JA = Pobserved/Prandom × 100%, in which Pobserved = NA−B/∑NA−i and Prandom is evaluated by the atomic ratio. (e) Temperature dependence of first coordination shell in Pt41Ni59 according to the fitting of EXAFS.
or blue arrows in Figure 3b. Although more Ni−Ni pairs were demonstrated in EXAFS of Pt41Ni59 NPs as Figure 2c, which may generate the asymmetry in atomic pair peaks, such multiple-peaks characteristic from more atomic pairs have not manifest in other coordination peaks except for the second shell. Multipeak fittings based on Gaussian peak showed the single-peak feature under 5 Å for the second shell as in Figure S6, which confirms the main contribution of local tetragonal distortion on the second shell region. For Ni-rich samples, the asymmetric coordination peak mainly presents in second shell region, representing the local tetragonal distortion including two Gaussian peaks along different axes and the existence of layer-ordered part in such PtNi nanoparticles. With the increase of Pt component, tetragonal distortion gradually faded out, suggesting the transition to totally disordered FCC structure. These PDF spectra of Pt41Ni59 alloy NPs were analyzed according to the fitting strategies with a single FCC phase and two phases of FCC and tetragonal ordered phase (Figure 4a). As is the same with results of EXAFS, local atomic pairs in PtNi alloy NPs include Ni−Ni, Ni−Pt, and Pt−Pt pairs. Difference of bond length between homoatomic and heteroatomic pairs in a particle with local ordering cannot be depicted well by a single FCC structure. However, the coexistence of FCC and local tetragonal ordering part makes a great progress for fitting the first two shells and goodness of a fit (Rw). Next, an improved matching only using FCC structure for Pt-rich samples demonstrates the disappearance of local tetragonal distortion in Pt-rich PtNi alloy NPs (Figure S7). A similar layered ordering structure had been predicted by Monte Carlo simulations19 and evidenced by LEED on the outmost surface of PtNi single-crystal surface.36 Our present results from PDF described the coexistence of local ordering under the average disordering structure in Ni-rich PtNi alloy NPs. To gain the elaborate atomic insight of local ordering and bond length distribution instead of an average structural information in the PtNi alloy NPs, the reverse Monte Carlo (RMC) method for the PDF data of Pt41Ni59 NPs with a higher Q = 27 Å−1 was carried out. Starting from a disordered wellfacet octahedron with the composition probed by EDS, a new PtNi alloy structure was achieved after RMC fitting (Figure 4b). Obvious component segregations of Ni in the centers of
composition gives rise to quite different exposed crystal faces and then the particle shape. Shape effect33 on the lattice thermal expansion can be ultimately ascribed to local atomic distribution. NTE of heterogeneous atomic pairs is the feature of L10 ordered phase of the Pt−3d alloy as the layered distribution of the Pt and 3d elements (Figure 3b).34,35 With thermal
Figure 3. (a) Atomic pair distribution function (PDF) for PtNi alloy nanoparticles at room temperature. (b) Structural model of disordered FCC and ordered tetragonal phase as L10 phase. Red arrows stand for the first coordination shells, and yellow and blue arrows indicate the second shells.
activation to a certain temperature, relaxation of magnetic interaction between the heterogeneous atomic layers prompts the weakening of tetragonal distortion, resulting in the NTE of heteroatomic pairs. For the further identification of the atomicscale local coordination structure and local ordering distribution in PtNi alloy NPs with a longer-range structure information, atomic pair distribution function (PDF) was applied (see Figure 3a). For all PtNi alloy nanoparticle samples with different Pt content, a face-centered cubic atomic pair structure is shown, except for a shift to higher distance with more Pt component as XRD results (see Figure 2a). A remarkable difference was observed in the second shell peak, which corresponds to the axial atomic pairs as the yellow 7894
DOI: 10.1021/acs.nanolett.7b04219 Nano Lett. 2017, 17, 7892−7896
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Figure 4. (a) Fitting results of atomic pair distribution function (PDF) for Pt41Ni59 alloy NPs at room temperature using disordered FCC structure, combined tetragonal ordering, and RMC. (b) Nanoparticle model from RMC fitting. Blue balls stand for Pt atoms and green balls for Ni atoms. (c,d) Layer-by-layer composition statistics along the or directions. Dotted line is marked as the average composition.
Performances dominated by local interaction including bond length and thermal expansion will display the same behaviors. Key role of heteroatomic pairs in the NTE can be reserved and magnified in nanosized PtNi particles.29 With the increase of Pt content, the completely disordered FCC state appears convinced by full-range fitting in PDF spectra, and it then presents a positive thermal-expansion behavior (Figure 1a). In summary, a systematic investigation on the thermal performance and local lattice strain was conducted in PtNi alloy nanoparticles. Short-range coordination structure revealed by extended X-ray absorption fine structure confirmed the dominated role of heteroatomic pair as Pt−Ni in the transition from positive thermal expansion to negative thermal expansion of Ni-rich NPs. Fittings based on average structure adopting the strategies with a single FCC phase and combined ordered tetragonal phase evidenced the existence of hidden local tetragonal distortion in Pt41Ni59 alloy NPs. Combining with the results of reverse Monte Carlo, an atomic insight of composition distribution as Ni in the centers of {111} facets and Pt in the edge parts was identified. Layer-by-layer statistic analysis for the local atomic pairs indicated the main distribution of local tetragonal ordering on the surface. The similarity of local coordination environment as a totally ordered L10 phase retained the key character of Pt−Ni pairs to the negative thermal expansion, which then was amplified in the nanosized PtNi alloy particles. This comprehensive method provides a clear picture of the atomic-scale distribution of local chemical environment and ordering in PtNi alloy nanoparticles and can served as a significant guideline for the design of nanocatalytic material.
{111} facets and Pt in the edge parts are perfectly consistent with previous electron energy-loss spectroscopy (EELS) element maps and ADF-STEM determinations.23,37 The clearer and more visualized atomic-resolution picture of the PtNi alloy nanoparticle makes it possible to analyze the local structure and its distribution. Layer-by-layer composition statistics, whether along or , divide the octahedral alloy nanoparticle into three parts (Figure 4c,d). Surface part (part A) demonstrates as a Pt-rich shell than the average composition with plenty of alternant-sandwich local component ordering along direction, while a smoother transition was found along the with a Ni-rich outmost layer. Then inner part (part B) shows a stable Ni-rich composition with the thickness of about 2 to 3 nm. Core part (part C) is Pt-rich agreeing with the preferential nucleation of Pt in the weak reductive solution of DMF. Corresponding to the transition from PTE to NTE in Pt41Ni59 nanoparticles, statistic analysis for the nearest atomic pairs (Figure S8−10) shows that the significant contribution for the negative thermal expansion mainly occurring on the surface as part A, especially the role of Pt−Ni pairs. Although surface part in Pt41Ni59 NPs did not suffer the perfect stoicheiometry as an ordered L10 phase. Local thermal expansion behaviors of atomic pairs can also emerge out in the average result of XRD (as shown in Figure 2a). According to the first-principles calculations on the ordered surface models of (001) surfaces with Ni termination, Pt termination, and (111) surface (Figure S11), heteroatoms in the layers can induce the formation of local ferromagnetic cluster consisting of heteroatom and nearby homoatomic layers. Ferromagnetic cluster including Ni atoms and magnetized Pt atoms in PtNi alloy leads to a contraction of homoatomic pair. Local interaction in the ferromagnetic cluster reinforces stability of homoatomic pairs under thermal disturbance. Meanwhile, for Pt41Ni59 alloy NPs, distribution of local tetragonal ordering on the surface forms a similar local coordination environment as the perfectly ordered phase. 7895
DOI: 10.1021/acs.nanolett.7b04219 Nano Lett. 2017, 17, 7892−7896
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b04219. Additional experimental details. A table showing synthesis conditions. Figures showing histograms, X-ray diffraction and refinement results, wavelet transformations, EXAFS results, local coordination shells, fitting results, statistic analysis, and surface views. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jun Chen: 0000-0002-7330-8976 Xianran Xing: 0000-0003-0704-8886 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant nos. 21590793, 21731001, and 21231001), the Program for Changjiang Scholars, the Innovative Research Team in University (grant no. IRT1207), and the Program of Introducing Talents of Discipline to Universities (grant no. B14003). We are thankful to Prof. Dr. Reinhard Neder from Friedrich-AlexanderUniversität Erlangen-Nürnberg (FAU) for his kind and careful help in reverse Monte Carlo. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357.
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DOI: 10.1021/acs.nanolett.7b04219 Nano Lett. 2017, 17, 7892−7896
