Novel Interface in CuAg Nanostructure Induced by Size Effect - The

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

A Novel Interface in CuAg Nanostructure Induced by Size-Effect Luping Tang, Wei Wu, Longbing He, Kaihao Yu, Tao Xu, Qiubo Zhang, Lei Zhang, and Litao Sun J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00484 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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The Journal of Physical Chemistry Letters

A Novel Interface in CuAg Nanostructure Induced by SizeEffect Luping Tang1, Wei Wu2, Longbing He*,1,3, Kaihao Yu1, Tao Xu1,3, Qiubo Zhang1,4, Lei Zhang1 and Litao Sun*,1,3 1SEU-FEI

Nano-Pico Center, Key Lab of MEMS of Ministry of Education, Southeast

University, Nanjing 210096, China 2Laboratory

of Printable Functional Nanomaterials and Printed Electronics, School of Printing

and Packaging, Wuhan University, Wuhan 430072, China 3Southeast

University-Monash University Joint Research Institute, Suzhou 215123, China

4Materials

Science Division, Lawrence Berkeley National Laboratory, Berkeley, California

94720, United States Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].

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ABSTRACT: Bimetallic Janus nanostructures (JNs) have been revealed to be valuable materials in having unique intermetallic interfaces that enable their potential use in a range of applications. However, with the increasing miniaturization of electronic devices, particle sizes influence the structure and orientation of these heterointerfaces, which plays a significant role in their application. Our in situ annealing experiments in high-resolution transmission electron microscopy have shown that for particle sizes in the sub-10 nm range, CuAg JNs preferentially show a Cu(100)/Ag(100) interface, differing from the larger CuAg JNs, where the Cu(111)/Ag(111) interface is favored. We have discussed a feasible atomic motion mechanism to explain the effect of particle size on the formation of different heterointerfaces. Our results reveal the presence of a novel sub-10 nm heterostructure with a unique Cu(100)/Ag(100) interface, and also provide crucial insights into understanding the role of particle size in interfacial evolution during thermal annealing of heterostructures.

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Bimetallic nanocrystals have attracted increasing attention due to their fascinating catalytic,1,2 optical,3 magnetic4 and electronic5,6 properties. The two elements in a bimetallic crystal can be phase-separated like in core-shell structures,7 chemically ordered structures,8 or Janus structures (asymmetric structures with totally different properties of the two parts or two phases of the bimetallic structure),9,10 or disordered solid solutions where the two species are randomly mixed.11 Not surprisingly, the properties of such structures depend strongly not only on the elemental composition and particle morphology, but also on how the two elements are organized in the alloy structure.12 As is known, because of the unique interface effect, the bimetallic Janus nanostructures (JNs) have been considered as one of the most promising materials.13-17 For instance, CuAg JNs display excellent catalytic performance in the water-gas shift reaction due to their interface effect, which endows them with enhanced antioxidant ability.15 Compared with the Ag/N (N = Pd, Ni, Cu, and W) interfaces, the Ag(111)/Ni(111) interface own the largest quantum conductivity, making it to be the best candidate for the interface of nano-multilayer electrical contact material.16 Furthermore, the improved catalytic of AuPt JNs has been attributed to the concentration difference across the asymmetric bimetallic interface due to the different heterogeneous electron-transfer rate constants of Pt and Au.17 Thus, a comprehensive characterization of the heterointerface is extremely important when studying the kinetics of interface strength and interface failure mechanisms.18-20 Moreover, interface between different materials play a vital role in device,21,22 e.g. the Au/ZnO interface leading to Au/ZnO heterostructures as photoanodes exhibited enhanced photoelectrochemical properties.23 Faced with the increasing need for miniaturization in the electronics industry, it is increasingly important to understand and control the nature of intermetallic interfaces at the nanoscale to improve device performance.


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Interface energy is found to be a key parameter in determining the nucleation barrier and the preferential orientation relationship of precipitates.24 It is generally assumed, based on theoretical calculations, that the interface is formed along the lowest-energy facets, e.g. Cu(111)/Ag(111),25 but often these don't consider surface effects, which is extremely important for small nanoparticles (NPs).26 For particle size down to sub-10 nm, surface and interface energies begin to play an important role in determining the main energetic landscape.27 For example, in our previous work, we found that due to surface diffusion, sub-10 nm Ag NPs can be deformed akin to a liquid droplet but remain highly crystalline in the interior at room temperature.28 Nanoelectronic devices with feature sizes down to 14 nm are currently commercially available, and future devices may be in the sub-10 nm regime. It is therefore indispensable to examine crystallographic orientation at the interface and internal structure stability of sub-10 nm bimetallic heterostructure components caused by size-effect (i.e. surface effect). Although there already exist some annealing studies of bimetallic NPs inducing segregation to forming JNs,29-32 size-effect on segregation and further investigation at the atomic scale of the heterointerfaces of the JNs is still unclear. In the present study, disordered CuAg NPs with different sizes were chosen as model systems to investigate the effect of particle size on the segregation and the evolution of the heterointerface between the two metals under in situ thermal heating in an aberration-corrected transmission electron microscope (TEM; FEI Titan operating at 300 kV) combined with a low-drift heating holder. It is demonstrated that at 500 °C, growth of JNs is favored compared to other types of alloy structures. Detailed atomic investigation of the heterointerfaces of the JNs indicated that the preferred crystallographic orientations at the interface are different for sub-10 nm CuAg NPs compared with traditional Cu(111)/Ag(111) interface of large CuAg NPs. Moreover, statistical analyses of the observed relationships


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between the orientations of the heterointerfaces, demonstrated a high conversion rate. Finally, a mechanism for the size-effect based on the dynamic evolution of the interface is proposed. We report results on the effect of crystalline size on thermal segregation in CuAg NPs during in situ thermal treatment experiments performed in Titan 80-300 transmission electron microscope (TEM) operated at 300 kV under a vacuum of 1.8×10−5 Pa. Scanning TEM (STEM) mode was used for the direct identification of Cu and Ag elements based on high-angle annular dark-field (HAADF) Z-contrast imaging; in this imaging mode, we made use of the increased contrast of Ag relative to Cu due to its higher atomic number.33 Figure 1a shows a representative STEM-HAADF image of CuAg NPs before annealing; the average diameter of 9.4 nm for the NPs was calculated based on a statistical average of several measurements, as shown in the histogram (Figure S1a). The magnified STEM-HAADF image and the energy-dispersive X-ray (EDX) mapping reveal that a majority of NPs contain both Cu and Ag and present a mixed configuration (Figure 1c, the Ag and Cu elemental maps were shown in Figure S2, respectively). After annealing at 500 °C for 10 min, the NP density decreases indicating that a coalescence between the partial particles occurred, as shown in Figure 1b. It is interesting to note that this particle coalescence to form larger particles completely alters the initial structure to form a typical Janus bimetallic structure, with the segregation of Ag on one side of the NP and Cu on the other. In most cases, the Cu and Ag hemispheres are separated by a planar interface crossing the particle (Figure 1d). The interfaces could also show an angle, as observed for the particle shown in the middle of Figure 1d. According to the line profiles across the planar interfaces from the EDX maps shown at the bottom of Figure 1d, a total segregation is confirmed, with the Janus structure of the NPs consisting of two distinct parts of nearly pure Cu and Ag.


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For relatively larger sized NPs, we raised the temperature of the sample to 500 °C followed by a dwell time of 30 min, to enable all the particles in the NP assembly to evolve into their equilibrium structures. Figure 1e and 1f show, respectively, the representative STEM-HAADF images of the initial and final configurations of the large-sized CuAg NPs. Images before heat treatment reveal that Ag and Cu atoms are mixture distribution in the NP to form an alloy and no clear interface is seen. The histogram (Figure S1b) obtained from particle size measurements displays that the NPs have an average diameter of 39.2 nm. The nanostructure of the alloy was further identified from the EDX elemental maps that showed the random distribution of Ag and Cu in a single particle (Figure 1g, the Ag and Cu elemental maps were shown in Figure S3, respectively). Figure 1f and the corresponding EDX elemental map (Figure 1h) clearly prove that heterointerfaces between the Cu and Ag elements appear after annealing, similar to that observed in small CuAg NPs. The EDX line profiles across the NPs marked in Figure 1g and 1h confirm that Ag atoms are mostly segregated on one side of the particle and Cu on the other after annealing.


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Figure 1. (a, b) High-angle annular dark field (HAADF) scanning transmission electron microscope (STEM) images of CuAg nanoparticles (NPs) with average size of 9.4 nm before and after annealing under vacuum at 500 °C for 10 min. The corresponding high magnification STEM-HAADF images and EDX elemental maps are shown in (c) and (d), respectively. (e, f) STEM-HAADF images of CuAg NPs with average size of 39.2 nm before and after annealing under vacuum at 500 °C for 30 min; the corresponding EDX elemental maps are shown in (g) and (h), respectively. The corresponding line scan intensity of the map along the dashed arrows indicated on the EDX elemental maps of NPs are shown below each figure. Cu and Ag have a relatively high size mismatch with respect to their atomic radii (rAg/rCu = 1.13), leading to a non-negligible stress at the interface.34 Moreover, the strain relaxation is found to be the driving force for CuAg system to adopt the lowest energy configuration.35


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Generally, transformations that a system undergoes following alloy formation must correspond to a free-energy decrease. When the free-energy differences between CuAg nanostructures with different chemical ordering were calculated by the Bennett’s method, it was found that the Janus structure is the most stable structural arrangement for this system.36 For a canonical ensemble (temperature, number-density and volume), the Helmholtz free energy F can be expressed on the base of the configurational integral Q as,36 1 F = ― In Q β where β = (𝑘BT) -1 and Q=

∫e

-βU(qN)dqN

Ω

The value of Q is obtained by integration over the volume Ω of the coordinate space occupied by the N particles of the system.Where kB, T, U and q are the Boltzmann constant, temperature, potential energy, and a potential parameter referring to the interplay between pairs of atoms of Cu and Ag, respectively. The free-energy difference (∆F) between two states 1 and 0 can be expressed as ∆F = F1 - F0 = -kBTln

Q1 Q0

where Q1 and Q0 having the same integrated volume of the phase space. Here we compare four configurations for the same atomic positions but organized in different ways, namely, Ag-Cu core-shell, Cu-Ag core-shell, perfectly mixed Ag and Cu and CuAg JN. For simplicity, these four cases will be here after written as C, A, M and J particles. The four structures display different potential energies and the free-energy differences ∆F values versus the particle radius R between J and C, ∆FCJ = FC - FJ , J and M, ∆FMJ = FM - FJ, and J and A, ∆FAJ = FA - FJ, structures


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are presented in Figure S2. It is interesting to note here that all the values of ∆FAJ, ∆FCJ and ∆FMJ in Figure S4 are positive, demonstrating that the CuAg JN always exhibits the lowest free energy. Therefore, from the thermodynamic stand point, the CuAg JN is favored over Ag-Cu core-shell, Cu-Ag core-shell and perfectly mixed Ag and Cu structures.

Figure 2. High resolution TEM (HRTEM) images showing heterointerfaces (marked by translucent red dashed lines) with different preferential orientation relationship of sub-10 nm (ac) and large (d-f) CuAg NPs after annealing at 500 °C; both Cu and Ag crystals are present and


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their zone axes are identified. In the bottom row, the corresponding Fourier transforms of the images marked by the green and yellow boxes in (d-f) are shown. To further investigate the size-effect on the CuAg interface structure and its orientation relationship in annealed NPs, various high-resolution transmission electron microscopy (HRTEM) images were obtained. As displayed in Figure 2a-c, it appears to be an epitaxial relationship between the Cu and Ag domains for sub-10 nm NPs after annealing at 500 °C for 10 min, which is in sharp different with the atomic arrangement of the NPs before annealing (shown in Figure S5a). The interplanar spacing of 2.1 Å measured in the HRTEM image in Figure 2a-c is assigned to the (200) plane of face-centered cubic (fcc) Ag and the value of 1.8 Å corresponds to the (200) plane of fcc Cu. Obviously, in nearly all cases, the Cu(200) and Ag(200) atomic planes are parallel to the interface (the method to determine the position of the interface was show in supporting information). Moreover, the angle between the interface and the Ag(111) facet in Figure 2a is nearly 56.7°, which is approximately equal to the angle between Ag(200) and Ag(111) in the pure crystal (54.7°). Although there is a 2 degree difference which we believe can be treated as experimental error, Ag(200) facets are considered to be parallel to the interface. Therefore, we conclude that the sub-10 nm NPs preferentially form Cu(100)/Ag(100) heterointerfaces; this tendency has not been identified before in Janus structures. However, it should be stressed that the heterointerface for large NPs is different from that of the small ones, as shown in Figure 2d-f. The Fourier transforms obtained from the selected areas marked in the image are shown below. The measured interplanar spacing of 2.4 Å can be assigned to the (111) plane of fcc Ag and the value of 2.1 Å is attributed to the (111) plane of the fcc Cu. Thus, for the representative heterostructures displayed in Figure 2d-f, a preferential orientation relationship of Cu(111)/Ag(111) can be inferred.


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Figure 3. (a, b) Schematic diagrams of the Cu(100)/Ag(100) and Cu(111)/Ag(111) heterointerface, respectively. (c, d) Schematic diagram of the first and second layer atoms of Cu(100)/Ag(100) and Cu(111)/Ag(111) heterointerface, respectively. The period of the atomic layer arrangement is two layers. The D1 and D2 are the distance between Ag atom and two adjacent Cu atoms, respectively. (e) Statistics of the formation probability of Cu(100)/Ag(100) heterointerface. The green columns represent the total number of CuAg NPs (here random fiftysix sub-10 nm NPs are analyzed) and the orange columns represent the number of CuAg NPs,


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the interface of which turn to Cu(100)/Ag(100) after annealing, indicating a high conversion rate up to 82.1% at least. (f) Statistics of the formation probability of Cu(111)/Ag(111) heterointerface. Green represents the total number of CuAg NPs (here random seventy large NPs are analyzed) and orange represents the number of CuAg NPs, the interface of which turn to Cu(111)/Ag(111) after annealing, indicating a high conversion rate up to 85.7% at least. The schematic diagrams of the Cu(100)/Ag(100) and Cu(111)/Ag(111) heterointerfaces are illustrated in Figure 3a and 3b, respectively. In order to distinguish the two interfaces more clearly, schematic diagrams of the first and second layer atoms of Cu(100)/Ag(100) and Cu(111)/Ag(111) heterointerface are shown in Figure 3c and 3d, respectively. The period of the atomic layer arrangement is two layers. It still remains challenging to control the interfacial heteroepitaxial growth for metals possessing similar crystalline structures with larger lattice mismatches.37 From the images of the interface, the lattice mismatch (m) can be calculated as the absolute difference between two lattice spacings (d1 and d2) relative to the average of the two lattice spacings:38 m = 2 × |d1 - d2|/(d1 + d2) Using this relation, for the fcc phase, the lattice mismatch between Ag and Cu can be calculated to be more than 10%. Accordingly, although Cu and Ag have a large lattice mismatch, CuAg JNs with controlled interface properties can be produced by thermal segregation due to reduction in the total free energy of the particle and generate synergy in addition to inheriting their individual functions. Furthermore, a statistical analysis of the HRTEM of the JNs formed by annealing sub-10 nm CuAg NPs shown in Figure 3e indicates that nearly all the interfaces show preferential Cu(100)/Ag(100). Randomly chosen 56 sub-10 nm CuAg NPs were analyzed here, among


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which, at least 46 NPs converted to heterostructures with a Cu(100)/Ag(100) interface, demonstrating a high conversion rate up to 82.1%. Similarly, as shown in Figure 3f, 70 randomly selected large CuAg NPs were also analyzed, among which, at least 60 NPs converted to a heterostructure with a Cu(111)/Ag(111) interface, also demonstrating a high conversion rate up to 85.7%. The remaining 14.3% and 17.9% NPs presented Janus structure after annealing, but the interfacial orientation relationship could hardly be identified (examples are shown in Figure S7). Note that in the histogram the particle sizes before annealing are used to calibrate each NP and the particles occurring coalescence didn't counted. In addition, there was no significant effect of Ag:Cu component ration in the particle (0.5

Novel Interface in CuAg Nanostructure Induced by Size Effect - The

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