School of Materials Science and Engineering, Central South University , Changsha, Hunan 410083, China.
Acc Chem Res. 2016 Sep 20;49(9):1587-95. doi: 10.1021/acs.accounts.6b00205. Epub 2016 Jun 29.
Conventional thermodynamics for bulk substances encounters challenges when one considers materials on the nanometer scale. Quantities such as entropy, enthalpy, free energy, melting temperature, ordering temperature, Debye temperature, and specific heat no longer remain constant but change with the crystal dimension, size, and morphology. Often, one phenomenon is associated with a variety of theories from different perspectives. Still, a model that can reconcile the size and shape dependence of the thermal properties of the nanoscaled substances remains one of the goals of nanoscience and nanotechnology. This Account highlights the nanoscopic thermodynamics for nanoparticles, nanowires, and nanofilms, with particular emphasis on the bond energy model. The central idea is that the atomic cohesive energy determines the thermodynamic performance of a substance and the cohesive energy varies with the atomic coordination environment. It is the cohesive energy difference between the core and the shell that dictates the nanoscopic thermodynamics. This bond energy model rationalizes the following: (i) how the surface dangling bonds depress the melting temperature, entropy, and enthalpy; (ii) how the order-disorder transition of the nanoparticles depends on particle size and how their stability may vary when they are embedded in an appropriate matrix; (iii) predictions of the existence of face-centered cubic structures of Ti, Zr, and Hf at small size; (iv) how two elements that are immiscible in the bulk can form an alloy on the nanoscale, where the critical size can be predicted. The model has enabled us to reproduce the size and shape dependence of a number of physical properties, such as melting temperature, melting entropy, melting enthalpy, ordering temperature, Gibbs free energy, and formation heat, among others, for materials such as Pd, Au, Ag, Cu, Ni, Sn, Pb, In, Bi, Al, Ti, Zr, Hf, In-Al, Ag-Ni, Co-Pt, Cu-Ag, Cu-Ni, Au-Ni, Ag-Pt, and Au-Pt on the nanometer scale. Furthermore, this model predicts the phenomena of the thermal stability of metal particles on graphene, the superheating of embedded nanoparticles, the order-disorder transition of nanoalloys, the size-temperature phase diagram for low-dimensional solids and the alloying ability on the nanoscale. Extensions of this model may lead to the design of new functional nanomaterials.
当考虑到纳米尺度的材料时，传统的体相热力学会遇到挑战。熵、焓、自由能、熔点、有序温度、德拜温度和比热容等量不再保持常数，而是随晶体尺寸、大小和形态而变化。通常，一种现象与来自不同视角的多种理论相关联。然而，能够协调纳米尺度物质热性质的尺寸和形状依赖性的模型仍然是纳米科学和纳米技术的目标之一。本综述重点介绍了纳米颗粒、纳米线和纳米薄膜的纳米尺度热力学，特别强调了键能模型。其核心思想是原子的内聚能决定物质的热力学性能，而内聚能随原子配位环境而变化。正是核心和壳层之间的内聚能差决定了纳米尺度的热力学。该键能模型可以合理地解释以下现象：（i）表面悬空键如何降低熔点、熵和焓；（ii）纳米粒子的有序-无序转变如何取决于粒径，以及当它们嵌入适当的基质中时，它们的稳定性如何变化；（iii）预测在小尺寸下 Ti、Zr 和 Hf 具有面心立方结构的存在；（iv）两种在体相不可混溶的元素如何在纳米尺度上形成合金，以及可以预测临界尺寸。该模型使我们能够再现许多物理性质的尺寸和形状依赖性，例如 Pd、Au、Ag、Cu、Ni、Sn、Pb、In、Bi、Al、Ti、Zr、Hf、In-Al、Ag-Ni、Co-Pt、Cu-Ag、Cu-Ni、Au-Ni、Ag-Pt 和 Au-Pt 等材料在纳米尺度上的熔点、熔化熵、熔化焓、有序温度、吉布斯自由能和形成热等。此外，该模型还预测了金属颗粒在石墨烯上的热稳定性、嵌入纳米粒子的过热、纳米合金的有序-无序转变、低维固体的尺寸-温度相图以及纳米尺度上的合金化能力等现象。该模型的扩展可能会导致新型功能纳米材料的设计。
