In Situ Atomic-Scale Observation of Surface-Tension-Induced

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In Situ Atomic-Scale Observation of Surface Tension Induced Structural Transformation of Ag-NiPx Core-Shell Nanocrystals Xing Huang, Zhongqiang Liu, Marie-Mathilde Millet, Jichen Dong, Milivoj Plodinec, Feng Ding, Robert Schloegl, and Marc-Georg Willinger ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b03106 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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In Situ Atomic-Scale Observation of Surface Tension Induced Structural Transformation of AgNiPx Core-Shell Nanocrystals Xing Huang,*a,b,δ Zhongqiang Liu,d,e,δ Marie-Mathilde Millet,b Jichen Dong,e Milivoj Plodine,b,g Feng Ding,*e,f Robert Schlögl,a,b and Marc-Georg Willinger*b,c a

Department of Heterogeneous Reactions, Max Planck Institute for Chemical Energy Conversion, 45470 Mülheim an der Ruhr, Germany

b

Department of Inorganic Chemistry, Fritz Haber Institute of Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany c

Scientific Center for Optical and Electron Microscopy, ETH Zürich, Auguste-Piccard-Hof 1, 8093 Zürich, Switzerland d

e

Department of physics, Qufu Normal University, Qufu, 273165, P. R. China

Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea f

Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

ACS Paragon Plus Environment

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Division of Material Physics, Rudjer Boskovic Institute, Bijenicka 54, 10000 Zagreb, Croatia

*Email: [email protected], [email protected]; [email protected]; [email protected] δ

These author contributed equally to the work.

ABSTRACT

The properties of nanocrystals are highly depending on their morphology, composition and structure. Tailored synthesis over these parameters is successfully applied for the production of nanocrystals with desired properties for specific applications. However, in order to get a full control over the properties, the behavior of nanocrystals under external stimuli and application conditions needs to be understood. Herein, using Ag-NiPx nanocrystals as a model system, we investigate the structural evolution upon thermal-treatment by in situ aberration-corrected scanning transmission electron microscopy (STEM). A combination of real-time imaging with elemental analysis enables the observation of the transformation from a Ag-NiPx core-shell configuration to a Janus structure at the atomic scale. The transformation occurs through dewetting and crystallization of the NiPx shell and is accompanied by surface segregation of Ag. Further temperature increase leads to a complete sublimation of Ag, and formation of individual Ni12P5 nanocrystals. The transformation is rationalized by theoretical modelling based on density functional theory (DFT) calculations. Our model suggests that the transformation is driven by changes of the surface energy of NiPx and the interfacial energy between NiPx and Ag. The here presented direct observation of atomistic dynamics during thermal-treatment induced structural


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modification will help to understand more complex transformations that are induced by ageing over time or the interaction with a reactive gas phase in applications such as catalysis.

KEYWORDS in situ STEM, Ag-NiPx nanocrystal, atomic-scale observation, structural transformation, surface energy, theoretical modelling Progress in chemical synthesis has enabled controlled fabrication of colloidal nanocrystals including metals, semiconductors, dielectric and magnetic materials with desired properties for a wide range of applications.1-3 Amongst them, multicomponent nanocrystals, which combine two or more functional materials, are of particular interest.4,5 The close interaction between the nanosized constituents can give rise to significantly improved functionalities and/or the emergence of new properties that cannot be obtained from individual single-phase nanocrystals.69

In view of actual applications, there is still a concern about the stability of beautifully designed

structures. This is partially related to the fact that some structures in multicomponent nanocrystals are stabilized by kinetic hindrance.10 In order to get better understanding of their structural stability, a mechanistic understanding of external stimulus induced transformation is required. So far, the understanding is mostly based on post and ex situ characterizations, which is largely limited due to the missing atomistic details of the transformation. This could lead to oversimplified or even wrong interpretations of the underlying mechanisms. Aberration-corrected transmission electron microscopy (TEM) is a powerful tool for direct visualization of the atomic arrangement in nanomaterials. The combination of state-of-the-art TEM with recently developed Micro-Electro-Mechanical System (MEMS) based in situ holders enables the in situ observation of atomic-scale structural dynamics induced by external


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stimulus.11-19 The knowledge gap left by post and ex situ characterizations can thus be bridged by in situ TEM investigations, leading to an improved understanding of the stability limits of complex heterostructures and the transformation mechanisms. Herein, we use Ag-NiPx nanocrystals as a model system to study the dynamic process of structural evolution upon thermal-treatment by in situ aberration-corrected STEM. Due to their small size, defined structure and composition, the chosen system is ideally suited for the investigation of atomistic dynamics driven by external stimulus and processes related to diffusion, reconstruction, and interface modification. By combining real-time structural imaging with elemental analysis, we are able to follow the transformation of a core-shell arrangement to a Janus type configuration. Finally, the driving force for the temperature induced transformation is rationalized by modelling based on density functional theory (DFT) calculations.

RESULTS AND DISCUSSION

Structural and compositional characterizations of Ag-NiPx core-shell nanocrystals The Ag-NiPx nanocrystals were synthesized by thermal decomposition of Ni and Ag precursors in presence of reducing agents as reported elsewhere.20 The detailed synthetic procedure can be found in the experimental section. An aberration-corrected transmission electron microscope (JEOL, ARM-200F) was used to characterize the morphology, composition and structure of as-prepared nanocrystals. The annular bright-field and simultaneously recorded high angle annular dark-field scanning transmission electron microscopy (ABF- and HAADFSTEM) images reveal that the nanocrystals, with an average size of about 9 nm, have a welldefined core-shell configuration with a distinct imaging contrast (Figure 1a, b and Figure S1a,b). The high brightness of the core due to stronger scattering is Ag, which is confirmed by analyses


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of high resolution STEM and fast Fourier transform (FFT). The lattice fringes with measured dspacing of 2.35 Å coincide well with (111) planes of cubic Ag (Figure 1c). The shell with weak contrast is thus Ni or Ni containing compound. Interestingly, the shell exhibits an ill-defined, amorphous-like structure. It is also noted from ABF-STEM image (Figure 1a) that there are some materials of light contrast on the surface of core-shell nanocrystals due to surfactant that was used to enhance the dispersion and stability of the nanocrystals. In order to characterize the composition and elemental distribution of the nanocrystals, we carried out detailed analysis by energy-dispersive X-ray spectrometry (EDX). As shown from EDX-line scan (Figure 1d,e) and elemental maps (Figure 1f-i), the core area of the nanocrystal mainly shows signal of Ag, in line with the structural analysis. In contrast, the shell exhibits both Ni and P elements, suggesting a composition of NiPx. According to EDX quantitative analysis, the Ni to P ratio is about 2.1 (Figure S2a). It is also worth to mention that in addition to Ni and P, the shell is likely to contain traces of C, O, and H, considering its amorphous state and the synthetic conditions. However, due to the presence of surfactant (on nanocrystals) which contains similar elements, it is difficult to demonstrate directly the presence of those elements in the NiPx shell, especially when their contents are extremely low. In situ observation of structural transformation of Ag-NiPx core-shell nanocrystals The heating experiment was performed inside the column of aberration-corrected TEM using MEMS-based in situ heating holder (Protochips Fusion). The Ag-NiPx core-shell nanocrystals were loaded on the heating chip serving as both the heat element and the sample support (Figure S3). It contains a patterned ceramic SiC membrane and is coated with a thin layer of electrontransparent SiNx (20 nm). In order to better visualize the atomic process of structural evolution, the experiment was conducted in HAADF-STEM mode, which provides good contrast for the


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relatively heavy Ni and Ag atoms. The temperature profile of the heating experiment is shown in Figure S4. It is found that the core-shell structure remains stable below 200 ˚C. No obvious changes are observed except that in the temperature range of 100-150 ˚C, carbon contamination builds up during the imaging process as a result of interaction between the surfactant and the electron beam. Fortunately, the carbon contamination becomes negligible when the temperature is beyond 200 ˚C, which is considerably due to decomposition of the surfactant. With a heating rate of 5 ˚C/min, the temperature was increased to 250 ˚C. The reconstruction of nanocrystals takes place during the temperature ramp. It is observed that the Ag gradually diffuses from the core to the surface. Simultaneously, the NiPx de-wets and segregates to the opposite side of the Ag, forming a Ag-NiPx Janus structure with a sharp hetero-interface in between, as shown in Figure 2a-h and Movie 1. It is also noted, that at around 212. 4 ˚C a small crystalline nucleus shows up in the amorphous NiPx (Figure 2f). It grows and spreads to the whole NiPx particle during transformation process, giving rise to the formation of crystalline NiPx (Figure 2h). Both Ag and NiPx remain compositional pure during the reconstruction, and no signs of inter-diffusion are detected. The EDX line profile (Figure 2i,j) and the EDX elemental mapping (Figure 2k-n) clearly demonstrate that the half sphere of the Janus nanocrystal with high brightness contains only Ag whereas the other half with reduced brightness is NiPx. In situ observation of Ag sublimation and structural characterization of remaining NiPx nanocrystals The Janus structure is stable up to 450 ˚C. There is no further significant structural reconstruction observed except the minor shift of interfacial boundary towards the Ag part due to slight sublimation of Ag (Figure S5, Movie 2). Additionally, the interface, which was flat and sharp before, bends towards the Ag counterpart. However, the Ag sublimation is strongly


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promoted when the temperature is increased from 450 to 500 ˚C. It leads to a complete sublimation of Ag from Janus structure (Figure 3a-f and Movies 3, 4, 5). In situ observation of Ag sublimation upon isothermal heating (at 500 ˚C) reveals clearly the effect of size on the sublimation (Figure S6, Movie 5). The average radius of Ag as a function of time shows two distinct linear regions with the second region (showing larger slope) setting in when the radius decreases to 2.2 nm (Figure S6b). The enhanced degree of sublimation in the second region is considerably due to the size shrinkage that develops a high density of kinks and steps serving as the active sites for sublimation.21 On the other hand, the NiPx part remains and shows improved crystallinity and structural ordering. As confirmed by STEM-EDX, it only contains Ni with a homogeneous distribution of P (Figure 3g-i). In order to identify the phase of resultant NiPx, high-resolution HAADF-STEM images were recorded and compared with simulated images (Figure 4a and Figure S7a). On the basis of the structural analysis, the nanocrystal can be assigned to Ni12P5 of a tetragonal structure (Space group: I4/M).22 The lattice fringes with measured d-spacing of 2.6 and 1.9 Å correspond to the (002) and (4-20) planes of Ni12P5 (Figure 4a). In order to verify our structural assignment, we carried out image simulation using the “JEMS” software23 based on the parameters used for image recording. The simulated HAADFSTEM image and electron diffraction are shown in Figure 4d and e and agree very well with the experimental data shown in Figure 4b and c. Another set of experimental and simulated data is shown in Figure S7, which is in consistent to the above statement. Additionally, EDX quantitative analysis (Figure S2b) reveals a Ni to P ratio of about 2.4 (12:5), in good agreement with the stoichiometry attained from the structural analysis. Interestingly, this value is slightly larger than that obtained from the initial NiPx in the core-shell structure, suggesting a slight loss of P during the annealing process. We also performed electron energy loss spectroscopy (EELS)


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measurement of the specimens at room temperature and 500 ˚C. A comparison of electronic structure of Ni in the initial and post-reaction nanocrystals shows a slight shift of Ni L edge to lower energy by 0.35 eV, which can be associated to a decreased oxidation state of Ni.24 This result agrees with the reduction of the amount of P in the nickel phosphide during annealing (Figure S8). Electron beam effect on the structural transformation of Ag-NiPx core-shell nanocrystals In electron microscopy, one has to be aware, that the electron beam of TEM, while enabling imaging of specimen, can also transfer energy to it, inducing beam effects.25-27 To verify the degree to which the beam influences on our experimental result, regions that were not preirradiated by the beam were investigated. The HAADF-STEM images are shown in Figure S9. One can see that at 250 ˚C the nanocrystals are in their process to but not yet finish the transformation from core-shell to Janus structure (Figure S9a). In contrast, the transformation has been complete in the region that was observed under permanent electron beam irradiation (Figure 2). This observation confirms that the beam effect acts by accelerating the transformation process. In fact, it has been reported previously that irradiation of isolated nanoparticles can cause a substantial increase of temperature if the heat flow from the illuminated particle to the support is restricted.27,28 The beam heating depends on parameters such as particle size, dose rate, and contact geometry between the particle and the support. Nevertheless, at 400 and 500 ˚C, the states of nanocrystals (Figure S9b,c) agree well to those recorded under electron beam irradiation at the same temperatures during in situ observation (Figure 2h,3h). The beam only accelerates the kinetics of the transformation by increasing the local temperature without changing the pathway of the transformation. Structural transformation mechanism of Ag-NiPx core-shell nanocrystals


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To understand the structural transformation of Ag-NiPx from core-shell nanocrystal to Janus structure and gain some fundamental insights into the transformation mechanism, we performed theoretical modelling through combination of thermodynamics theory with DFT calculations. It has been reported that a sub-10-nm Ag nanoparticles can be easily deformed like a liquid droplet yet still remain in high crystallinity.29 Therefore, to simply the theoretical model yet without losing the key features of the system, we use the liquid droplets to represent the nanocrystals in the following calculations. Our structural and compositional analyses demonstrate that the P to Ni ratio in NiPx keeps decreasing upon annealing, while at the same time, crystallization of NiPx is observed. It is reasonable to assume that the structural transformation of Ag-NiPx from core-shell to Janus structure is related to the continuous change of the surface tension of NiPx induced by the crystallization of NiPx and its changing stoichiometry. Generally speaking, there are three possible structures for Ag-NiPx nanocrystals, namely Ag-NiPx core-shell, NiPx-Ag core-shell, and Ag-NiPx Janus structures, respectively (see Figure 5). The most stable structure is determined by the total energy of the system, which is a summation of surface energies of NiPx (γ SI ), Ag (γAg SII ) and interfacial energy between them (γAg SIII ). Here γ , γAg ,

γAg and SI , SII , SIII are the surface tension coefficients and their corresponding surface

(interface) areas (see Figure 5), respectively. By using the DFT method, γAg is calculated as 6.542 eV/nm2 (see Figure S10). However, for NiPx, owing to continuous change of ratio in NiPx during the transformation process, calculation of its surface tension coefficient is very challenging. To simplify the model, we use the surface tension coefficient between Ag and Ni (γAgNi =1.099 eV/nm2) to represent the surface tension coefficient between Ag and NiPx, and consider the surface tension coefficient of NiPx as a variable. Combining these values and


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possible structural models of Ag-NiPx nanocrystals, we can therefore explore the structural transformation by numerical calculations. One can see that the total surface energies of structural models 1 and 2 (Figure 5a and b) contain two components, as given in Eq. (S1) and (S2). In contrast, the total energy of model 3 has three components arising from two surfaces and one interface (see Figure 5c), as described in Eq. (S8). For model 3, we have to first determine the steady structures of Ag-NiPx nanocrystals by solving a series of equations of four variables (R1, R2, R3, and d12), where R1, R2 and R3 represent the curvature radii of three interfaces I, II and III (Figure S11), respectively, and d12 is the distance between centers of two sphere segments I and II. The relationships among R1, R2, R3, and d12 are given by conditions of mechanical equilibrium in bulk Eq. (S3) and at the triple phase contact line Eq. (S4), the conservation of volumes Eq. (S5) and (S6). Substituting the values of γAg , γ and γAg- into Eqs. (S3)-(S7), we can then obtain R1, R2, R3, and d12

numerically. Without losing generality, we set VAg = V (in units of 10-6 µm3), γAg- /γAg =

γAg- /γAg =0.168. According to Eqs. (S1), (S2) and (S8), the surface energies of different models as functions of γ /γAg are plotted (see Figure 6). It needs to be mentioned that the surface

energy of model 3 (Es3) can be calculated only when γ is in the range from γAg − γAg- to

γAg + γAg- (i.e., 0.832 < γ /γAg < 1.168). As shown in Figure 6, increasing the interfacial

energy of NiPx leads to a transition from Ag-NiPx core-shell structure to a Janus structure and further, to a modified Janus structure with interface between NiPx and Ag bending towards the Ag, which perfectly reproduces the scenarios observed experimentally (Figure 2c-h, Figure 3ad). The good agreement between the experimental observation and the theoretical model implies that the evolution of the Ag-NiPx nanocrystal follows an energetically favorable pathway. Our


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calculations also suggest formation of NiPx-Ag core-shell structure from Ag-NiPx Janus structure. It was however not observed experimentally, likely due to reason that the γ cannot

practically reach such a high value to trigger this transformation. Indeed, in situ experiment shows that the Ag starts to sublimate at relative low temperature and is completely sublimated at 500 ˚C, which makes the transformation unable to be detected even if γ can fulfill the

criterion. Theoretical modelling was made by assuming that Ag and NiPx have the same volume, which is not always the case according to our experimental observation (see HAADF-STEM images inserted in Figure 7). Moreover, the loss of Ag due to the sublimation was ignored in the calculations for simplicity. In order to be more close to our experimental parameters, the volume effect is taken into account in the following calculations. Figure 7 illustrates that regardless of changes in volume ratio ( / ), the evolution from core-shell structure into Janus structure is predicted at the same γ /γ ratio. The HAADF-STEM images of Ag-NiPx nanocrystals recorded at different stages are inserted in Figure 7, which are consistent well with the results of calculations. Besides, our calculations are able to qualitatively speculate how Ag-NiPx core-shell structure evolves as a function of the decreased volume of Ag and increased surface tension coefficient ratio (γ /γ ). One of the possible pathways is indicated in Figure 7 by the dotted black arrows. It schematically illustrates the variation of the Janus structure when sublimation of Ag occurs and fits well to our experimental result shown in Figure S12. Overall, the theoretical calculations provide important insight into understanding the mechanism of structural transformation from Ag-NiPx core-shell structure to Janus structure under heating. It is proposed that the crystallization and stoichiometry change of NiPx lead to the variation of its surface


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energy coefficient, which drives the Ag-NiPx nanocrystals to change their appearances based on the criterion of total surface energy minimization.

CONCLUSIONS

We have investigated the stability of Ag-NiPx core-shell nanocrystals upon thermal treatment by a combination of in situ STEM imaging and DFT based theoretical modelling. Real-time atomic-scale imaging reveals that Ag migrates from the core to the exterior upon heating from 200 to 250 ˚C. The transformation is driven by energy minimization and results in the formation of a Ag-NiPx Janus structure with well-distinguished hetero-interface between the Ag and the NiPx. Accompanied by the structural transformation is the crystallization of NiPx. The newly generated Ag-NiPx Janus structure is relatively stable in the temperature range between 250 and 450 ˚C, where only a slight sublimation of Ag is observed. However, further temperature increase from 450 to 500 ˚C leads to complete sublimation of Ag, leaving individual Ni12P5 nanocrystals. Our theoretical calculations confirm that the structural transformation stems from the surface tension coefficient change of NiPx. It drives the Ag-NiPx nanocrystals to alter their structure in order to minimize the surface energy. The alternation of surface tension coefficient is considered to be induced by the crystallization and stoichiometry change of NiPx upon annealing. The removal of the surfactant and associated C, O and H species in the amorphous NiPx shell during heating reduces the kinetic barrier towards crystallization, which is finally overcome due to the supplied thermal energy. Similar mechanisms are at work in catalytic processes. In order to stabilize the core-shell structure (if that is the desired one) requires a careful balance of the temperature and chemical potential of the gas phase. On the other hand, this process can also be used for the fabrication of defined Janus-like nanocrystals. Therefore, this work may shed some


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light on structural engineering of multicomponent nanocrystals for optimized catalytic applications. Additionally, the work demonstrates that in situ observation of atomistic processes offers relevant insight for our understanding of complex structural evolutions in heterostructured materials.

METHODS

Synthesis of Ag-NiPx core-shell nanocrystals The Ag-NiPx core-shell nanocrystals were prepared via a slightly adapted version of the one pot synthesis from Guo and co-authors.20 The synthesis is based on the thermal decomposition of Ni and Ag precursors in presence of a reducing agent (oleyamine). In a double neck round bottom flask equipped with a stopcock filled with 6ml of oleyamine, were added: 0.1mml of Triphenylphsophine (TPP), 0.4mmol of Ni(acac)2 and 0.1mmol of AgNO3. The flask was equipped with a Dimroth condenser and a gas bubbler, and the whole installation was set inside a fume hood. The mixture was first kept under an argon flow of high purity argon (99,999%) at room temperature for 30min. Then, the mixture was slowly heated to 80°C under strong magnetic stirring and kept 15min at this temperature. After that, the mixture was heated up to 195 °C, and kept at that temperature for 1hour. At 150 °C the solution took a yellowish color, and got black during holding at 195 °C for 1 hour. After cooling, 15ml of acetone were added to the mixture, resulting in the formation of a black precipitate that was separated from the organic solvents by centrifugation (500rpm for 10min). The precipitate was then washed successively with a 50/50, 75/25 and then a 25/75 mixture of hexane and acetone, before being dried in vacuum and re-dispersed in toluene. Ex situ and in situ TEM Characterizations


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TEM sample was prepared by drop-casting the sample suspension onto carbon coated Cu TEM grid. The sample was examined at room temperature by aberration-corrected JEOL JEM-ARM 200CF transmission electron microscope operated at 200 kV. The elemental analysis was performed using a high angle silicon drift EDX detector (solid angle up to 0.98 steradians with a detection area of 100mm2) equipped to the TEM. EELS measurement was performed on JEMARM 200F using a GIF Quantum energy filter operated in dual EELS mode which allows recording of both low loss and high loss spectra simultaneously. For in situ experiment, the sample was loaded on MEMS-based E-chip which serves as both the heater and the sample support. It is composed of a thin ceramic SiC membrane supporting an electron transparent SiNx layer. After the sample was loaded, the E-chip was assembled into the heating holder (Protochips Fusion) which was inserted subsequently into the TEM for observation. The sample was stepwise heated up to 500 °C with the structural transformation monitored by atomic scale HAADF-STEM combined with EDX elemental analysis. DFT calculations of the surface tension coefficients Spin-polarized density functional theory (DFT) calculations are performed to calculate γAg , γ and γAg- with projector-augmented-wave (PAW) pseudopotentials30,31 for the core–

electron interaction and the Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA) for the exchange-correlation functional_ENREF_2832-33 as implemented in the Vienna Ab initio Simulation Package (VASP). The kinetic energy cutoff of the plane wave basis is 400 eV, and all calculations use a convergence criterion of 10−4 eV. To ensure that all interactions between neighboring images are negligible, the vacuum space in the perpendicular direction is set to 10 Å. The matching number between Ag and Ni lattices is set to 5:6 with mismatch less


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than 0.05. The Brillouin zone is sampled only at the Γ point in the reciprocal space integrations due to the large unit cells. As mentioned in the main text, we use the surface tension coefficient γAgNi to estimate the value γAg- for qualitative calculations. Under this circumstance, the metal slabs and their

energies in Figure S10 are adopted to estimate surface tension coefficients. The surface tension coefficient can be considered as the surface energy per unit area, i.e., γ = , / , where represents Ag, Ni or Ag-Ni, = , =1.47 nm. The surface energies of Ag, Ni and their interface are, respectively, defined as , = − ⁄2, , = − ⁄2, , , = − ( + )⁄2. Inserting the corresponding energy values given in Figure S10f into the above equations, one can calculate the values of γAgNi , γAg , γNi and γAgNi /γAg , which are 1.099, 6.542, 14.203 eV/nm2, and 0.168, respectively.

ASSOCIATED CONTENT

Supporting information. HAADF-STEM images of Ag-NiPx core/shell structure; EDX analysis on NiPx before and after reaction; Optical images of MEMS based heating chip loaded with sample; Temperature profile for in situ heating experiment; Sequential high-resolution HAADF-STEM images of Ag-NiPx Janus nanoparticles recorded at 300, 350, 400, and 450 ˚C; Kinetic analysis of Ag sublimation; Structural simulation; STEM-EELS; HAADF-STEM images recorded under different temperature (250, 400 and 500 ˚C) from regions where no electron beam was irradiated before; Calculation of surface energy for different structural models of Ag-


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NiPx nanocrystals; Schematic picture of Ag-NiPx Janus structure with the Neumann’s triangle; Structural evolution of Ag-NiPx nanocrystal in consideration of volume shrinkage of Ag. AUTHOR INFORMATION Corresponding Authors Email: [email protected]; [email protected] Email: [email protected] Email: [email protected] Author contributions X.H. conceived the idea, carried out the in situ STEM experiments, data analysis and wrote the manuscript with input from all other authors. Z.L. performed theoretical calculations and wrote the calculation part under F.D.’s supervision. J.D. contributed to the discussion on the theoretical calculations. M.M. synthesized the material. M.D. assisted with in situ STEM experiments. M.W. and R.S. co-supervised the project, made important comments and suggestions on the manuscript.

ACKNOWLEDGEMENTS

Dr. Xing Huang would like to thank the financial support from Department of Heterogeneous Reactions at Max Planck Institute for Chemical Energy Conversion and Department of Inorganic Chemistry at Fritz Haber Institute of Max Planck Society.

REFERENCES


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Dehydrogenation Using Ag-Ni Binary Nanoparticles. ACS Catal. 2017, 7, 2294-2302.


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Semiconductor" FePt-PbS and FePt-PbSe Nanostructures: Magnetic Properties, Charge Transport, and Magnetoresistance. J. Am. Chem. Soc. 2010, 132, 6382-6391. 10. Grzelczak, M.; Vermant, J.; Furst, E. M.; Liz-Marzán, L. M. Directed Self-Assembly of Nanoparticles. ACS Nano 2010, 4, 3591-3605. 11. Huang, X.; Jones, T.; Fan, H.; Willinger, M. G. Atomic-Scale Observation of IrradiationInduced Surface Oxidation by in Situ Transmission Electron Microscopy. Adv. Mater. Interfaces 2016, 3, 1600751. 12. Jiang, Y.; Li, H. B.; Wu, Z. M.; Ye, W. Y.; Zhang, H.; Wang, Y.; Sun, C. H.; Zhang, Z. In Situ Observation of Hydrogen-Induced Surface Faceting for Palladium-Copper Nanocrystals at Atmospheric Pressure. Angew. Chem.-Int. Edit. 2016, 55, 12427-12430. 13. He, Y.; Zhong, L.; Fan, F. F.; Wang, C. M.; Zhu, T.; Mao, S. X. In Situ Observation of Shear-Driven Amorphization in Silicon Crystals. Nat. Nanotechnol. 2016, 11, 866-871. 14. LaGrow, A. P.; Ward, M. R.; Lloyd, D. C.; Gai, P. L.; Boyes, E. D. Visualizing the Cu/Cu2O Interface Transition in Nanoparticles with Environmental Scanning Transmission Electron Microscopy. J. Am. Chem. Soc. 2017, 139, 179-185. 15. Dai, S.; Hou, Y. S.; Onoue, M.; Zhang, S. Y.; Gao, W. P.; Yan, X. X.; Graham, G. W.; Wu, R. Q.; Pan, X. Q. Revealing Surface Elemental Composition and Dynamic Processes Involved in Facet-Dependent Oxidation of Pt3Co Nanoparticles via in Situ Transmission Electron Microscopy. Nano Lett. 2017, 17, 4683-4688.


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16. Lehnert, T.; Kinyanjui, M. K.; Ladenburger, A.; Rommel, D.; Worle, K.; Borrnert, F.; Leopold, K.; Kaiser, U. In Situ Crystallization of the Insoluble Anhydrite All Phase in Graphene Pockets. ACS Nano 2017, 11, 7967-7973. 17. Liu, X.; Zhang, C. H.; Li, Y. W.; Niemantsverdriet, J. W.; Wagner, J. B.; Hansen, T. W. Environmental Transmission Electron Microscopy (ETEM) Studies of Single Iron Nanoparticle Carburization in Synthesis Gas. ACS Catal. 2017, 7, 4867-4875. 18. Yalcin, A. O.; Fan, Z. C.; Goris, B.; Li, W. F.; Koster, R. S.; Fang, C. M.; van Blaaderen, A.; Casavola, M.; Tichelaar, F. D.; Bals, S.; Van Tendeloo, G.; Vlugt, T. J. H.; Vanmaekelbergh, D.; Zandbergen, H. W.; van Huis, M. A. Atomic Resolution Monitoring of Cation Exchange in CdSe-PbSe Heteronanocrystals During Epitaxial Solid-Solid-Vapor Growth. Nano Lett. 2014, 14, 3661-3667. 19. Zhang, Q. B.; Yin, K. B.; Dong, H.; Zhou, Y. L.; Tan, X. D.; Yu, K. H.; Hu, X. H.; Xu, T.; Zhu, C.; Xia, W. W.; Xu, F.; Zheng, H. M.; Sun, L. T. Electrically Driven Cation Exchange for In Situ Fabrication of Individual Nanostructures. Nat. Commun. 2017, 8, 14889. 20. Guo, H. Z.; Chen, Y. Z.; Chen, X. Z.; Wen, R. T.; Yue, G. H.; Peng, D. L. Facile Synthesis of Near-Monodisperse Ag@Ni Core-Shell Nanoparticles and Their Application for Catalytic Generation of Hydrogen. Nanotechnology 2011, 22, 195604. 21. Yim, J. W. L.; Xiang, B.; Wu, J. Q. Sublimation of GeTe Nanowires and Evidence of Its Size Effect Studied by in Situ TEM. J. Am. Chem. Soc. 2009, 131, 14526-14530. 22. Yupko, L. M.; Svirid, A. A.; Muchnik, S. V. Phase-Equilibria in Nickel Phosphorus and Nickel Phosphorus Carbon Systems. Sov. Powder Metall. Met. Ceram. 1986, 25, 768-773.


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23. Stadelmann, P. JEMS-EMS Java Version. http://www.jems-saas.ch. 24. Caporali, M.; Serrano-Ruiz, M.; Telesio, F.; Heun, S.; Nicotra, G.; Spinella, C.; Peruzzini, M. Decoration of Exfoliated Black Phosphorus with Nickel Nanoparticles and Its Application in Catalysis. Chem. Commun. 2017, 53, 10946-10949. 25. Dang, Z. Y.; Shamsi, J.; Palazon, F.; Imran, M.; Akkerman, Q. A.; Park, S.; Bertoni, G.; Prato, M.; Brescia, R.; Manna, L. In Situ Transmission Electron Microscopy Study of Electron Beam-Induced Transformations in Colloidal Cesium Lead Halide Perovskite Nanocrystals. ACS Nano 2017, 11, 2124-2132. 26. Sutter, E.; Huang, Y.; Komsa, H. P.; Ghorbani-Asl, M.; Krasheninnikov, A. V.; Sutter, P. Electron-Beam Induced Transformations of Layered Tin Dichalcogenides. Nano Lett. 2016, 16, 4410-4416. 27. Asoro, M. A.; Kovar, D.; Ferreira, P. J. In Situ Transmission Electron Microscopy Observations of Sublimation in Silver Nanoparticles. ACS Nano 2013, 7, 7844-7852. 28. Gryaznov, V. G.; Kaprelov, A. M.; Belov, A. Y. Real Temperature of Nanoparticles in Electron-Microscope Beams. Philos. Mag. Lett. 1991, 63, 275-279. 29. Sun, J.; He, L. B.; Lo, Y. C.; Xu, T.; Bi, H. C.; Sun, L. T.; Zhang, Z.; Mao, S. X.; Li, J. Liquid-Like Pseudoelasticity of Sub-10-Nm Crystalline Silver Particles. Nat. Mater. 2014, 13, 1007-1012. 30. Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector AugmentedWave Method. Phys. Rev. B 1999, 59, 1758-1775.


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Figures and captions

Figure 1. structural characterization and EDX analysis of Ag-NiPx nanocrystals. (a-b) ABFSTEM and corresponding HAADF-STEM images of Ag-NiPx core-shell nanocrystals. (c) Highresolution HAADF-STEM image of a single nanocrystal composed of a crystalline Ag core and an amorphous NiPx shell. (d) HAADF-STEM image and (e) EDX-line profile of a core-shell nanocrystal. (f) HAADF-STEM image and corresponding elemental maps of (g) Ag, (h) Ni, (i) P.


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Figure 2. In situ observation of structural transformation from core-shell to Janus structure and EDX elemental analysis. (a-d) A series of HAADF-STEM images showing structural transformation of Ag-NiPx nanocrystals from core-shell to Janus structure during annealing from 200 to 250 ˚C. (e-h) Enlarged portions of (a-d). (i) HAADF-STEM image and (j) corresponding EDX line profile of formed Ag-NiPx Janus structure recoded at 250 ˚C. (k-n) HAADF-STEM image and corresponding elemental maps of Ag, Ni, and P recoded at 250 ˚C.


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Figure 3. In situ observations of Ag sublimation and EDX elemental analysis. (a-h) Sequential atomically resolved HAADF-STEM images of sublimation of Ag from the Ag-NiPx Janus structure during temperature increase from 450 to 500 ˚C. (j) HAADF-STEM image and (k-m) EDX elemental maps of Ni, P and superposition of Ni and P.


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Figure 4. Structural characterization and simulation of Ni12P5 nanocrystal. (a) High resolution HAADF-STEM image of a Ni12P5 nanocrystal in [120] zone axis. inset shows the atomic structure model of Ni12P5. (b) Magnified high resolution HAADF-STEM image of Ni12P5. (c) FFT. (d,e) Simulated HAADF-STEM image and electron diffraction in [120] zone axis.


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Figure 5. Illustrations of three possible structures of Ag-NiPx nanocrystals. (a) Model 1: AgNiPx core-shell structure. (b) Model 2: NiPx-Ag core-shell structure. (c) Model 3: Ag-NiPx Janus structure. The white, dark gray and light gray zones represent Ag, NiPx and their overlapping areas, respectively. The surface tension coefficients of Ag, NiPx and Ag-NiPx interface are illustrated by γAg , γ , and γAg respectively.


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Figure 6. Surface energy of Ag-NiPx nanocrystal vs surface tension coefficient ratio (γ /γAg )

for three possible configurations: Ag-NiPx core-shell (model 1), NiPx-Ag core-shell (model 2), and Ag-NiPx Janus (model 3) structures. The value below each inserted structure is the value of γ /γ . Here, we set VAg = V , and γAg- /γAg is estimated by calculating γAg- /γAg ,

which is 0.168.


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Figure 7. Evolution diagram of Ag-NiPx nanocrystals on the plane of surface tension coefficient ratio (γ /γ ) and volume ratio ( / ). Two vertical parallel blue solid lines divide the phase diagram into three zones: (I) Ag-NiPx core-shell structure, (II) Ag-NiPx Janus structure and (III) NiPx-Ag core-shell structure. The schematic pictures of structural transformation for four volume ratios (a 1:2, b 1:1, c 2:1, d 4:1) are plotted from the bottom to top, respectively. Inserts give the HAADF-STEM image series of Ag-NiPx nanocrystals with different NiPx to Ag volume ratios (see numbers on arrows) recorded at different temperatures. The dotted black arrows point out a possible evolution pathway of Ag-NiPx nanocrystals, in which the volume of Ag is shrinking due to the sublimation.


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44x24mm (300 x 300 DPI)


In Situ Atomic-Scale Observation of Surface-Tension-Induced

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