By slightly heating the configurations, we can get insight into the shape changes and melting processes of such congeries. In a heating process, the two NPs are drawn together quickly by the excess surface energy. Subsequently, two NPs constantly change their positions, looking for state in space in order to find a lowest energy configuration24. Figure 2 shows snapshots of the NP1 during the simulation process. The two NPs are initially separated without heating applied yet (in Fig. 2a), and the atoms arrangements of the (100) of Au and Ni are shown in Fig. 2b and Fig. 2c. Before the coalescence of the Au atoms of NP1, the particles undergo the shape convulsion which has been termed “quasimelting”, and then coalesce into the large one at a highly accelerated rate compared with its prior migration. The approach and rotation of the two NPs can be clearly seen in the Supporting Information, Video S1. The small Au particle start heating, Au atoms and Ni atoms of (100) face hold atomic arrangement FCC structure (Fig. 2(b) and (c)), During heating process, the pair distribution function (PDF) of Au atoms and Ni atoms are also employed to monitor particle structure evolutions during the simulation. As shown in Fig. 2(d), only first-nearest neighbor peaks of Au atomic arrangement can be seen in the PDF of the NP1 at 10K, and the crystalline peaks disappear, while the FCC structure is restored at 100K. When the temperature is less than 100K, Au NPs and Ni NPs did not contact. The Au NPs transform or rotate in space in order to find a low-energy configuration( Au NPs are not the FCC) and then Au and Ni NPs are drawn together quickly by the excess surface energy, which is similar to the experiment was observed25. Subsequently, when the nanoparticle is small, its atoms are incorporated into the structure until almost every atom of the nanoparticle has rearranged over the (100) surface of the Ni NPs. We have studied the same size of the single element Au without Ni conditions, in which the Au structure has not changed in the same temperature. That is to say, the coalesced clusters with larger Ni NPs form an FCC structure. Structural transformation was induced by NPs containing greater numbers of atoms26.
Au atoms are induced by Ni atoms lattice, and the cross section of Au atoms can also illustrate this point. As shown in the Fig. 3, when the temperature is 6 K, the Au atoms of NP1 are arranged in an orderly manner. When the temperature is 10K, the structure is deformed, and at a temperature of 100 K, Au atoms restore the ordered structure. We also found that in the process of merging, the interface defects disappeared in Fig. 3. When the temperature is 94K, Au and Ni start to contact, and soon Au atoms in accordance with Ni (100) arrangement, the defects were observed during the process. As the temperature increases, the defects are gradually shifted and eliminated. These results allowed us to identify the remaining defects are originated from the coalescence phenomenon mentioned previously, which can be eliminated by means of energy input into the system. The eliminate process can be clearly seen in the Supporting Information, Video S2. During the coalescence process, the surface atoms showed “flow” characteristics and “flowed” toward the neighboring Au NP because of the atomic interaction forces between the Au NPs27. On the other hand, the NPs tend toward a more stable structure, displacing defects and releasing internal stresses28. These findings suggested that thermal energy could be applied to reduce the number of coalescence defects, which might cause a poor performance. It is also found that, with the increasing of Au atoms, Au atoms and Ni atoms interface are still ordered. We believe that no matter how many the number of Au atoms, Ni content will induce Au in an orderly arrangement over the (100) surface due to Au atoms more fluidity.
To analyze the stability and structural transformation of Ni-Au NPs, the caloric curves of potential energy variation on temperature is illustrated in Fig. 4. The caloric curve of metallic nanomaterials is very different from that of bulk materials due to the decrease in the system size. The transition is no longer sharp but smooth and takes place over a finite temperature range29. The melting point corresponding to the temperature value is determined by monitoring the change where there is a jump in the slope of the caloric curves. The sudden increase in energy over a small temperature change indicates that the first order transition from solid to liquid phase is similar to previous work22. Figure 4a shows the caloric curves calculated from the NP1, NP2, NP3 and NP4. The NP1 shows a well-defined melting transition upon heating in the caloric curve. The change in energy of NP1 as the temperature increases is similar to that of monometallic NPs. For the NP2, NP3 and NP4, there is an irregular phenomenon of the melting that the energy decreases with the increase of the temperature. This is because that the Au atoms have lower surface energy, and the distribution of Au atoms on the surface layer can make the NPs in the state of low free energy. There is a competition between the decreased energy induced by the atomic segregation and the increased energy as the temperature increases. Then the atomic energy increases with the increase of the temperature. The sudden increase of the energy indicates that the total NPs melt. For NP5 in Fig. 4b, the irregular phenomenon occurs twice in the melting process. When the number of Au atoms increases, the Au atomic segregation leads to the decrease in the potential energy. After staying in a certain temperature, Au atoms will melt and release latent heat. When the energy increase in the temperature is more than the energy decrease induced by the atomic segregation, the first jump occurs. At this time Ni NPs has not melted yet, Au atoms continue to wrap Ni NPs, repeating the energy reduction process until Ni melts, then occurring a second jump. Though, relative to sample NP5,the first energy decrease of NP6, NP7 and NP8 is not obvious, still undergoing the same structure change process.
To better understand the nature of the energy change, we select the cross section of the particle at temperature 1173K in detail. As shown in Fig. 5a, Au atoms segregated to the surface of Ni particle is dominant during the continuous heating of NPs. The Au particles have melted and Ni particles are not melted. For NP1, NP2, NP3, Au atoms diffused along the surface of Ni. For NP4, NP5, NP6, Au and Ni form alloying at the interface besides the diffusion. This implies that Au and Ni can form alloy NPs although they are not mismatched in the bulk30,31. When Au is large enough (NP7, NP8), Au atoms encapsulate completely due to Au atoms segregation. With the Au atoms “flowing”, we can get the eccentric core-shell structure Au@Ni NPs. As shown in Fig. 5b and 5c, the PDF peaks reflect the local order of structure. The first PDF peak location on the curve corresponds to the nearest interatomic distances and the second peak corresponds to the lattice parameter. The Au atomic PDF of structure obtained at 1173 K are shown in Fig. 5b, and the corresponding PDF at 0.1 K are shown in Fig. S1a. There is only the first nearest-neighbor peak relative to the PDF at 0.1K, and the other primary peak of FCC structures disappear. This indicates that PDF of Au atoms displays the transition from ordered structure to amorphous structure. The Ni atomic PDFs at 1173 K and 0.1 K are shown in Fig. 5c and Fig. S1b. As shown in Fig. 5c, the second peak position on the curve corresponding to the Ni particle lattice parameter, is found to be 0.355–0.365 nm, which is more than that of 0.1 K (0.351nm). On the other hand, the width of the peak becomes wider. All characteristic peaks still occur. This illustrates that the Ni particles has similar characteristics (FCC structure) at 1173 K and 0.1 K. These observations also suggest that diverse melting modes occur during the continuous heating of Au-Ni NPs.
In order to obtain an intuitive size change of atomic diffusion in these Au-Ni NPs during the continuous heating, the concept of statistical radius was introduced into this work32. It can be seen from Fig. 6a and 6b that the statistical radii of Au and Ni NPs slowly increases with increasing temperature at low temperatures, which is usually attributed to the thermal expansion of lattice for the Au and Ni contents in the Au-Ni NPs. With the temperature further rising, the statistical radius abruptly increases with a bigger slope indicating the solid-liquid transition. Moreover, it can be noted from the curves that beyond the melting points, the statistical radii of Au are gradually decreased which is also caused by miscible between Au and Ni atoms. As shown in Fig. 6b, it can be noted that the statistical radii curves of Ni particles are obviously divided into two cases despite the same size of Ni particles. For NP1, NP2 and NP3, the radii of Ni particles become larger at high temperature, indicating that Ni particles require higher temperatures to deform. It is possible that the alloying only takes place on the surface because Au atoms only diffuse to the surface of Ni with a lower fraction of Au atoms in the NPs. Alloying between interior Ni atoms and Au atoms becomes almost impossible. For larger particle from NP4 to NP8, the transition is more apparent and occurs at low temperature. The reason could be that Au particles of larger size can simultaneously alloyed with Ni at the interface accompanied the diffusion process, which leads to the increase of the radius of Ni. To study the atomic arrangement of Au and Ni, volume increments of Ni and Au particles before and after melting are plotted in Fig. 6c. It can be seen that the ratio of Au and Ni atoms is 1:1, and the volume increment is the same. The reason could be that Au and Ni are miscible after melting. The mean square displacement (MSD) is a measure to describe the average distance that a particle travels in the sintering process. Figure 6d shows the MSD for the eight pairs of particles under the heating process. The MSD departs from the almost zero value and increases significantly, indicating the occurrence of atomic motion or melting. In general, when a small particle coalesces with a larger one, the temperature effect helps sintering where the smaller NPs tends to be incorporated into the larger ones33. The coalescence research of different sized gold NPs indicates that the melting occurs earlier when the size of the second particle is close to the first one34. The studies do not consider the weight of atoms due to the same element. However, as shown by the arrow in Fig. 6d, it is obvious that the atomic motion of Au-Ni NPs occurs earlier when the weights of Au and Ni particles are nearly the same according to our calculation (The NP4 being a typical representative). To perform an in-depth structural study, the MSD of Ni and Au atoms from NP1-NP8 are shown in the Fig. S2. The lighter particle tends to move towards heavier one. This illustrates that Au has a stronger affinity capacity relative to Ni when the size of Au particles is equal to that of Ni ones during the coalescence processes. The MSD of Ni and Au atoms almost experience a sharp rise simultaneously with the temperature increase, as can be seen from Fig. S2(d). Our atomic-scale observations clearly reveal that the weight plays a significant role during the process and the coalescence is driven by the surface diffusion and interface alloying. It can also be seen from Fig. S2 that the MSD from NP4 to NP8 provides direct evidence which indicates that Ni atoms are moved towards Au atoms.
To further investigate the change in morphology of the Au-Ni NPs in a quantitative way, the average distance Dave between the Au/Ni atoms and the NPs center of coordinate is
where ri is the position of the i atom and rC is the center of coordinate. The temperature dependence of this amount was obtained by calculating Dave at each temperature11. Dave is sensitive to the shape of the Au-Ni NPs, and has a maximum value when the Au NPs and Ni NPs are separated and decreased when the NPs coalesce. A minimum value of Dave is obtained for spherical Au-Ni NPs, which is the shape of some solid phase clusters or the liquid clusters. Here Dave is employed to describe the coalescence process. According to the segregation of Au and the melting of Ni, the evaluation of Dave of the NPs can be divided into three stages as shown in Fig. 7. Meanwhile, representative atomic morphology of the five representative temperatures of NP1-NP8 (in Table S2) is shown in Fig. S3. With respect to the behavior of the coalescence process, the dumbbell-like, Janus and eccentric core-shell spherical structure could be obtained during heating process.
The first stage corresponds to the process from separate to contact state (Am points, m = 1–8) between Au particles and Ni particles, the sample changes are similar from NP1 to NP8, and the Dave of NPs decreases sharply once the heating simulation starts. Then, the Dave slightly increases or remains almost unchanged. This is because when the sintering neck was growing, the barycenter of system keep stable in the low temperature. The NPs are mainly in the form of dumbbell.
After the coalesce process, the value of Dave decreases as stage 2 (Am to Cm, m = 1–8). The Dave of NP1 rapidly decreases at the point A1 due to the small number of Au atoms in NP1 and the rapid diffusion to the surface of Ni NPs. The Dave of NP2 and NP3 decreases at point A2 and A3, and then slowly rises, but after that drops to the lowest point C2 and C3. As shown in the Fig. 7b, the Dave of NP4, NP5 and NP6 continue to decrease from point A4, A5 and A6 to the lowest point C4, C5 and C6 respectively. This suggests that as the number of Au atoms increases, Au gradually diffuses to the Ni surface until a spherical structure is formed. And for NP7 and NP8, the most interesting phenomenon is that the Dave decreases suddenly from B7 and B8 points to C7 and C8 points respectively. This indicates that the number of Au atoms is sufficient, and Ni NPs can be suddenly adsorbed into the Au NPs.
The eccentric core shell NPs
The stage 3 is corresponding to the melting process of Ni NPs (Dm points to Em points (m = 1–8) in Fig. 7.). The Dave of NP1 to NP6 increases significantly with the rising temperature, which is corresponding to point C1, C2 ,C3, C4, C5 and C6 to point D1, D2, D3, D4 D5 and D6 respectively as shown in Fig. 7a and Fig. 7b. This is mainly due to the increasing temperature of the larger atomic spacing of Ni. In Fig. 7c, because Dave does not reach the minimum points C7 and C8, the Dave decreases slowly from points C7 and C8 points to D7 and D8 point respectively. This is due to the continuous wrapping process when Ni atoms are adsorbed inside Au particles. From Fig. S3, we can find that the NPs form a non-concentric structure for NP7 and NP8.
The morphology evolution in Fig. S3 conforms to the change of Dave (maximum value means separate state, and minimum value represents the formation of a sphere). As can be seen in Fig. S3, Au atoms preferentially diffuse to the Ni surface9,22, so Au and Ni combine to form dumbbell, Janus and eccentric core shell structure. The in-situ TEM experimental results of structure evolution behaviors of NiAu nanospindles shows that Au component wraps along Ni matrix to form a core–shell like structure at promoted temperature15. The conclusions directly verify our simulation results. The catalytic activity is highly dependent on the evolution of alloying and phase segregation35,36. The thermal control of the nanoscale alloying and phase segregation is of practical importance for its application as a catalyst and design of nanostructures.