双金属纳米团簇的热力学过程和并合行为对其微观结构演化的影响
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摘要
双金属纳米团簇具有许多奇特的物理和化学性能,而且这些特殊性能可以通过合金成分、原子分布和团簇尺寸等进行调节。因此,通过双金属团簇合成时原子尺度行为的控制有望得到具有特殊结构和性能的新材料。它们在异质催化、光电和磁装置甚至生物医学等领域有着广泛的应用前景。这也使得有关双金属团簇热力学过程及其相互作用行为的研究成为近年来的热点。但是由于该过程的复杂性,直接从实验上进行研究非常困难。计算机模拟,尤其是分子动力学模拟,成为揭示双金属团簇微观结构演化的关键途径。
本文首先构建了一个在较大的团簇尺寸和成分范围内有效的合金分子动力学模型。然后,用该模型研究了不同物性组元构成的双金属团簇的热力学过程和并合行为,揭示了双金属团簇合成过程控制对其微观结构演化的作用机理。研究工作取得了如下重要结论：
1.合金分子动力学模型的构建及其有效性验证
在分子动力学模拟中,原子间相互作用势的精度直接影响模拟结果的准确性。本文采用了基于密度泛函理论的普适的嵌入原子势。它使不同的元素具有相同的截断距离,可以用来模拟16种金属及其合金。然后,用该模型模拟了全部Ni成分的Cu-Ni块体和不同尺寸团簇的熔点组成的液相线,发现它与实验和理论模型给出的相图值基本一致。说明该模型可以准确描述团簇和块体中纯金属及合金原子间的相互作用。
2.热力学过程中双金属纳米团簇的微观结构演化
模拟了由不同物性参数的组元构成的双金属团簇在热力学过程中的微观结构演化。研究结果表明：(1)Ag-Co团簇结构演化与温度和Co含量密切相关。低温时Co的最稳定位置是团簇的亚表面层,随着温度的升高变为核心层。但是在亚表面层存在的能垒仍会使团簇形成Ag-Co-Ag的洋葱型结构。随着温度的继续升高,Co原子获得的能量能克服中间层的能垒时,团簇就会形成Co-Ag核壳层。(2)匀称的20面体结构转变与团簇的初始结构、原子数和组元分布密切相关。而且,这种相变可以通过不同物性组元的添加、含量和原子分布的改变进行控制。(3)团簇的凝固结构具有明显的尺寸效应。在小尺寸时Co团簇凝固成20面体,而在大尺寸时凝固成与块体相同的HCP结构。这是因为随着尺寸的增加团簇释放的能量减小,从而无法诱导结构转变。而在Co团簇中添加不同物性的Ni和Cu组元时会诱导团簇凝固成不同的结构。
3.并合行为对团簇微观结构演化的影响
不同的纯金属团簇在热力学过程中的微观结构演化不同,而并合行为的引入会导致其微观结构演化发生改变。本文首先考察了不同初始态(液态和固态)的各种团簇的并合。然后,又模拟了纯金属团簇、同质并合团簇以及异质并合团簇凝固时的微观结构演化。发现异质团簇并合行为可以诱导与纯金属团簇以及同质并合团簇不同的微观结构演化。尤其是在低温液-固并合时,低表面能的原子会在并合团簇表面形成一个壳层。这个壳层会抑制固态团簇由于过剩能量释放诱导的结构转变。如果未形成抑制壳层,那么易发生20面体转变的团簇会诱导并合团簇形成20面体。另外,还发现同质并合团簇凝固结构与纯金属团簇凝固结构相同。但是,异质并合团簇的凝固结构则有所不相同。其中,Co团簇由于易于凝固成20面体,其存在会诱导Co-Ni和Co-Cu团簇凝固成20面体。而Cu-Ni的凝固结构具有与Ni和Ni-Ni凝固后相似的十面体结构。由此可见,在合金纳米粒子形成时控制元素的添加和合成条件可以得到具有特定结构和形貌的纳米粒子。
4.双金属团簇冷却和并合过程诱导的微观结构演化
由于双金属纳米团簇的热力学过程和并合行为都会对团簇的结构产生影响。因此,研究冷却和并合冷却对合成物微观结构的影响,可以揭示出团簇形成时不同的控制行为诱导团簇微观结构演化的作用机理。通过模拟Cu-Co团簇冷却和并合冷却的微观结构发现：从高温冷却后的团簇具有HCP结构,而并合冷却后的团簇形成了类洋葱型的FCC结构。从低温冷却的团簇形成了Cu和Co均匀分布的FCC结构。而并合冷却团簇则形成了Co-Cu的核壳层的FCC与HCP孪生的结构。由此可见,特定结构和形貌的纳米团簇可以在合成过程中进行控制。
因此,通过对双金属团簇热力学过程和并合行为的控制可以得到具有特定形貌和结构的新型纳米团簇。而本文模拟方法在纳米尺度新材料的开发和设计中的应用不仅可以揭示出纳米材料特殊性能的来源以及如何通过合金成分、含量及其分布进行控制；又可以拓宽纳米技术在新材料研究领域的应用；同时,还可以用于指导实验研究和分析实验结果。
Particular physical and chemical properties are exhibited in many bimetallic nanoclusters and can be tuned by varying their composition, atomic distribution and size. Furthermore, new materials with exceptional structure and functionality may be created by controlling their synthesis processes and have extensive applications in heterogeneous catalysts and biodiagnostics, as well as optoelectronic and magnetic devices. Therefore, the research on their thermodynamics and interaction processes has become a hot topic recently. However, the understanding and predicting of these processes by using experimental studies are very difficult. Computer simulation methods, in particular molecular dynamics (MD), have become a powerful means of investigating the structural evolutions.
An alloy MD was constructed to apply in bimetallic clusters with a wide range of sizes and compositions. Then, the thermodynamics and coalescence processes of different clusters were studied by using this model. The influence mechanism of the synthesis processes on the structural evolutions of the bimetallic clusters was explored. The results showed as follows:
1. Construction of the alloy MD and the test of its accuracy
In MD simulation, the validity of the simulated results is strongly related to the accuracy of the interatomic potential. A general embedded atom method (EAM), which is based on the density functional theory, was used. This EAM can force the potentials to go smoothly to zero at the same cutoff distance for 16 metals and make the calculations of alloys able with their any combinations. The simulated liquidus curves of the Cu-Ni bulk and clusters with different sizes agree well with the experimental and theoretical phase diagrams. This indicates the model can be used to describe the atomic interactions in the pure and bimetallic clusters.
2. Influence of the thermodynamics on the structures of bimetallic clusters
The microstructural evolutions of the bimetallic clusters (constructed by the elements with different physical parameters) during the thermodynamics processes were studied. The results showed that:(1) The structural evolutions of the Ag-Co clusters are strongly related to the temperature and Co compositions. The most stable position for the Co atoms is the subsurface layer at lower temperature and changes to the core layer with the increase in temperature. But there is an energy barrier in the middle layer makes the cluster form an Ag-Co-Ag onion-like configuration. When the temperature is high enough, Co atoms can obtain enough energy to overcome the energy barrier to form a Co-Ag core-shell configuration. (2) The structural transformation of the symmetric icosahedron is strongly related to the initial configuration, atomic number and atomic distribution, and can be controlled by doping hetero atoms and changing their compositions and distributions. (3) The size effects of the cooling clusters are obvious. Icosahedron was formed during the freezing of the Co clusters with small size, while HCP structure was formed for the large-size Co clusters. This is because the released energy during the formation of the icosahedron decreases with the increase of the cluster size. The freezing structures are different after the Co clusters doping with Cu or Ni atoms.
3. Influence of the coalescence on the structural evolutions of the clusters
The structural evolutions are different for different pure metal clusters and can be changed by the coalescence. The coalescences of the clusters with different states (liquid and solid) were firstly studied. Then, the structural evolutions were investigated by simulating the freezing of the pure cluster, homocluster and heterocluster coalescences. The results indicate that the structural evolutions are different and can be induced by the coalescences. Especially for the liquid-solid coalescence at a low temperature, a shell layer will be formed by the atoms with low surface energy. And it can inhibit the structural transformation of the solid cluster. Otherwise, the icosahedral transformation will be induced in the coalesced cluster by the solid cluster, which can easily transform to the icosahedron. In addition, it also found that the freezing structure of the homocluster coalescence is same to that of the pure cluster. But, there are some differences for the freezing structures of the heterocluster coalescence. Co-Ni and Co-Cu coalesced to form an icosahedron because the Co cluster easily froze to form an icosahedron. However, the freezing structures of the Cu-Ni are decahedron and similar to those of Ni and Ni-Ni. Therefore, nanoparticles with special structure and morphology can be fabricated by controlling the doping of the hetero atoms and coalescence processes.
4. Structural evolutions induced by controlling the cooling and coalescence
Since the structures of the bimetallic clusters will be influenced by both the thermodynamics and coalescence processes. It is significant to understand the structural evolutions of the bimetallic clusters in the cooling and coalesced cooling processes. The structural evolutions of the Cu-Co clusters under different synthesis processes were studied. For the high temperature cooling cluster, there were obvious differences in the structures of the cooling (HCP) cluster and coalesced cooling (onion-like FCC) cluster. For the low temperature cooling cluster, the cluster formed an FCC structure. However, the coalesced cluster formed a Co-Cu core-shell morphology under the same condition. A twinned crystalline of the FCC and HCP structures formed. It is reasonable to conclude that nanoparticle with different structures and morphologies can be obtained by controlling the synthesis process.
In conclusion, the nanoparticles with special structure and morphology can be fabricated by controlling their synthesis processes. The model can be used to explore the functionality-structure relationship. It also can be used to design new materials and analyze the experimental results.
本文首先构建了一个在较大的团簇尺寸和成分范围内有效的合金分子动力学模型。然后,用该模型研究了不同物性组元构成的双金属团簇的热力学过程和并合行为,揭示了双金属团簇合成过程控制对其微观结构演化的作用机理。研究工作取得了如下重要结论：
1.合金分子动力学模型的构建及其有效性验证
在分子动力学模拟中,原子间相互作用势的精度直接影响模拟结果的准确性。本文采用了基于密度泛函理论的普适的嵌入原子势。它使不同的元素具有相同的截断距离,可以用来模拟16种金属及其合金。然后,用该模型模拟了全部Ni成分的Cu-Ni块体和不同尺寸团簇的熔点组成的液相线,发现它与实验和理论模型给出的相图值基本一致。说明该模型可以准确描述团簇和块体中纯金属及合金原子间的相互作用。
2.热力学过程中双金属纳米团簇的微观结构演化
模拟了由不同物性参数的组元构成的双金属团簇在热力学过程中的微观结构演化。研究结果表明：(1)Ag-Co团簇结构演化与温度和Co含量密切相关。低温时Co的最稳定位置是团簇的亚表面层,随着温度的升高变为核心层。但是在亚表面层存在的能垒仍会使团簇形成Ag-Co-Ag的洋葱型结构。随着温度的继续升高,Co原子获得的能量能克服中间层的能垒时,团簇就会形成Co-Ag核壳层。(2)匀称的20面体结构转变与团簇的初始结构、原子数和组元分布密切相关。而且,这种相变可以通过不同物性组元的添加、含量和原子分布的改变进行控制。(3)团簇的凝固结构具有明显的尺寸效应。在小尺寸时Co团簇凝固成20面体,而在大尺寸时凝固成与块体相同的HCP结构。这是因为随着尺寸的增加团簇释放的能量减小,从而无法诱导结构转变。而在Co团簇中添加不同物性的Ni和Cu组元时会诱导团簇凝固成不同的结构。
3.并合行为对团簇微观结构演化的影响
不同的纯金属团簇在热力学过程中的微观结构演化不同,而并合行为的引入会导致其微观结构演化发生改变。本文首先考察了不同初始态(液态和固态)的各种团簇的并合。然后,又模拟了纯金属团簇、同质并合团簇以及异质并合团簇凝固时的微观结构演化。发现异质团簇并合行为可以诱导与纯金属团簇以及同质并合团簇不同的微观结构演化。尤其是在低温液-固并合时,低表面能的原子会在并合团簇表面形成一个壳层。这个壳层会抑制固态团簇由于过剩能量释放诱导的结构转变。如果未形成抑制壳层,那么易发生20面体转变的团簇会诱导并合团簇形成20面体。另外,还发现同质并合团簇凝固结构与纯金属团簇凝固结构相同。但是,异质并合团簇的凝固结构则有所不相同。其中,Co团簇由于易于凝固成20面体,其存在会诱导Co-Ni和Co-Cu团簇凝固成20面体。而Cu-Ni的凝固结构具有与Ni和Ni-Ni凝固后相似的十面体结构。由此可见,在合金纳米粒子形成时控制元素的添加和合成条件可以得到具有特定结构和形貌的纳米粒子。
4.双金属团簇冷却和并合过程诱导的微观结构演化
由于双金属纳米团簇的热力学过程和并合行为都会对团簇的结构产生影响。因此,研究冷却和并合冷却对合成物微观结构的影响,可以揭示出团簇形成时不同的控制行为诱导团簇微观结构演化的作用机理。通过模拟Cu-Co团簇冷却和并合冷却的微观结构发现：从高温冷却后的团簇具有HCP结构,而并合冷却后的团簇形成了类洋葱型的FCC结构。从低温冷却的团簇形成了Cu和Co均匀分布的FCC结构。而并合冷却团簇则形成了Co-Cu的核壳层的FCC与HCP孪生的结构。由此可见,特定结构和形貌的纳米团簇可以在合成过程中进行控制。
因此,通过对双金属团簇热力学过程和并合行为的控制可以得到具有特定形貌和结构的新型纳米团簇。而本文模拟方法在纳米尺度新材料的开发和设计中的应用不仅可以揭示出纳米材料特殊性能的来源以及如何通过合金成分、含量及其分布进行控制；又可以拓宽纳米技术在新材料研究领域的应用；同时,还可以用于指导实验研究和分析实验结果。
Particular physical and chemical properties are exhibited in many bimetallic nanoclusters and can be tuned by varying their composition, atomic distribution and size. Furthermore, new materials with exceptional structure and functionality may be created by controlling their synthesis processes and have extensive applications in heterogeneous catalysts and biodiagnostics, as well as optoelectronic and magnetic devices. Therefore, the research on their thermodynamics and interaction processes has become a hot topic recently. However, the understanding and predicting of these processes by using experimental studies are very difficult. Computer simulation methods, in particular molecular dynamics (MD), have become a powerful means of investigating the structural evolutions.
An alloy MD was constructed to apply in bimetallic clusters with a wide range of sizes and compositions. Then, the thermodynamics and coalescence processes of different clusters were studied by using this model. The influence mechanism of the synthesis processes on the structural evolutions of the bimetallic clusters was explored. The results showed as follows:
1. Construction of the alloy MD and the test of its accuracy
In MD simulation, the validity of the simulated results is strongly related to the accuracy of the interatomic potential. A general embedded atom method (EAM), which is based on the density functional theory, was used. This EAM can force the potentials to go smoothly to zero at the same cutoff distance for 16 metals and make the calculations of alloys able with their any combinations. The simulated liquidus curves of the Cu-Ni bulk and clusters with different sizes agree well with the experimental and theoretical phase diagrams. This indicates the model can be used to describe the atomic interactions in the pure and bimetallic clusters.
2. Influence of the thermodynamics on the structures of bimetallic clusters
The microstructural evolutions of the bimetallic clusters (constructed by the elements with different physical parameters) during the thermodynamics processes were studied. The results showed that:(1) The structural evolutions of the Ag-Co clusters are strongly related to the temperature and Co compositions. The most stable position for the Co atoms is the subsurface layer at lower temperature and changes to the core layer with the increase in temperature. But there is an energy barrier in the middle layer makes the cluster form an Ag-Co-Ag onion-like configuration. When the temperature is high enough, Co atoms can obtain enough energy to overcome the energy barrier to form a Co-Ag core-shell configuration. (2) The structural transformation of the symmetric icosahedron is strongly related to the initial configuration, atomic number and atomic distribution, and can be controlled by doping hetero atoms and changing their compositions and distributions. (3) The size effects of the cooling clusters are obvious. Icosahedron was formed during the freezing of the Co clusters with small size, while HCP structure was formed for the large-size Co clusters. This is because the released energy during the formation of the icosahedron decreases with the increase of the cluster size. The freezing structures are different after the Co clusters doping with Cu or Ni atoms.
3. Influence of the coalescence on the structural evolutions of the clusters
The structural evolutions are different for different pure metal clusters and can be changed by the coalescence. The coalescences of the clusters with different states (liquid and solid) were firstly studied. Then, the structural evolutions were investigated by simulating the freezing of the pure cluster, homocluster and heterocluster coalescences. The results indicate that the structural evolutions are different and can be induced by the coalescences. Especially for the liquid-solid coalescence at a low temperature, a shell layer will be formed by the atoms with low surface energy. And it can inhibit the structural transformation of the solid cluster. Otherwise, the icosahedral transformation will be induced in the coalesced cluster by the solid cluster, which can easily transform to the icosahedron. In addition, it also found that the freezing structure of the homocluster coalescence is same to that of the pure cluster. But, there are some differences for the freezing structures of the heterocluster coalescence. Co-Ni and Co-Cu coalesced to form an icosahedron because the Co cluster easily froze to form an icosahedron. However, the freezing structures of the Cu-Ni are decahedron and similar to those of Ni and Ni-Ni. Therefore, nanoparticles with special structure and morphology can be fabricated by controlling the doping of the hetero atoms and coalescence processes.
4. Structural evolutions induced by controlling the cooling and coalescence
Since the structures of the bimetallic clusters will be influenced by both the thermodynamics and coalescence processes. It is significant to understand the structural evolutions of the bimetallic clusters in the cooling and coalesced cooling processes. The structural evolutions of the Cu-Co clusters under different synthesis processes were studied. For the high temperature cooling cluster, there were obvious differences in the structures of the cooling (HCP) cluster and coalesced cooling (onion-like FCC) cluster. For the low temperature cooling cluster, the cluster formed an FCC structure. However, the coalesced cluster formed a Co-Cu core-shell morphology under the same condition. A twinned crystalline of the FCC and HCP structures formed. It is reasonable to conclude that nanoparticle with different structures and morphologies can be obtained by controlling the synthesis process.
In conclusion, the nanoparticles with special structure and morphology can be fabricated by controlling their synthesis processes. The model can be used to explore the functionality-structure relationship. It also can be used to design new materials and analyze the experimental results.
引文
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[1]Wadley H N G, Zhou X, Johnson R A, et al. Mechanisms, models and methods of vapor deposition [J], Progress in Materials Science,2001,46:329-377.
[2]Chushak Y G, Bartell L S. Freezing of Ni-Al bimetallic nanoclusters in computer simulations [J], Journal of Physical Chemistry B,2003,107:3747-3751.
[3]Sankaranarayanan S K R S, Bhethannabotla V R, Joseph B. Molecular dynamics simulation study of the melting of Pd-Pt nanolcusters [J], Physical Review B,2005, 71(19):195415.
[4]Mottet C, Rossi G, Baletto F, et al. Single impurity effect on the melting of nanoclusters [J], Physical Review Letters,2005,95(3):035501.
[5]Jin Z H, Sheng H W, Lu K. Melting of Pb clusters without free surfaces [J], Physical Review B,1999,60(1):141-149.
[6]Kim D H, Kim H Y, Kim H G, et al. The solid-to-liquid transition region of an Ag-Pd bimetallic nanocluster [J], Journal of Physics:Condensed Matter,2008,20: 035208.
[7]Palacios F J, Iniguez M P. Fusion and fragmentation of colliding metal clusters [J], Nuclear Instruments and Methods in Physics Research B,2002,196:253-260.
[8]Mariscal M M, Dassie S A, Leiva E P M. Collision as a way of forming bimetallic nanoclusters of various structures and chemical compositions [J], Journal of Chemical Physics,2005,123(18):184505.
[9]Kim H Y, Kim H G, Ryu J H, Lee H M. Preferential segregation of Pd atoms in the Ag-Pd bimetallic cluster:density functional theory and molecular dynamics simulation [J], Physical Review B,2007,75(21):212105.
[10]Lewis L J, Jensen P, Barrat J L. Melting, freezing, and coalescence of gold nanoclusters [J], Physical Review B,1997,56(4):2248-2257.
[11]Hawa T, Zachariah M R. Coalescence kinetics of unequal sized nanoparticles [J], Journal of Aerosol Science,2006,37:1-5.
[12]Ding F, Rosen A, Bolton K. Size dependence of the coalescence and melting of iron clusters:a molecular-dynamics study [J], Physical Review B,2004,70(7):075416.
[13]Dorfbauer F, Schrefl T, Kirschner M, Hrkac G, Suess D, Ertl O, Fidler J. Structure calculation of CoAg core-shell clusters [J], Journal of Applied Physics,2006,99(8): 08G706.