摘要:

IntroductionThe transformation of structure between stable or metastable phases reveals basic thermodynamical information about the state of matter. Consequently, investigations into the phase stability of nanoparticles have given some of the clearest demonstrations of the differences between nanoscale and bulk material of the same stoichiometry. For example, a reversal in phase stability at small particle size is observed in some systems;1–4melting temperatures are generally lower in nanoparticles;5transition temperatures to high temperature phases are often lower,6while transition pressures to high pressure phases can be higher7–9or lower.10By now, it is clear that the phase diagrams for materials can be size dependent.11Using the concepts of classical thermodynamics, observed size dependencies can be associated with the large surface area present in nanoscale materials. In particular, the excess energy of a nanoparticles relative to that of the bulk material (normalized by the surface area) is defined as the surface energy (usually in J m−2). Many of the observed data can be explained once surface energy contributions are considered. Yet, this quantity is difficult to measure accurately, and depends on the details of both surface and interior structure. The excess energy need not be confined to the surface: the effect of finite size on interior structure is currently a topic of interest.12,13In the field of nanoscience, it is generally difficult to translate specific experimental observations and thermodynamic principles into a precise quantitative theory of energy and structure. Structural and thermodynamic experiments are complicated by the fact that nanoparticles are metastable with respect to macroscopic crystals: ultimately, extremes of temperature or pressure, and aggregation, will lead to coarsening. An additional consideration is that conditions on both sides of the nanoparticle/environment interface are significant.In this review, we show how classical thermodynamics can be combined with microscopic predictions from molecular dynamics (MD) simulations to consider the energetics and phase stability of two nanoparticle systems. In addition, non-equilibrium kinetic theory allows growth and phase transformation processes to be understood, revealing novel transformation mechanisms. We principally review work performed by this group on titanium dioxide (TiO2) and zinc sulfide (ZnS), environmentally significant model systems that are also relevant due to the utility of these materials as fine metal oxide catalysts and semiconducting electrooptic materials, respectively.We first consider nanoparticle growth, followed by examples of phase transitions accompanying growth. The final topic is structural transition without growth, stimulated by changes in surface environment. Overall, this review illustrates the significance of the surface in considerations of thermodynamic phase stability, and highlights the significance of particle–particle contacts for interpretation of the kinetics of transformation and growth.

ISSN:1467-4866    

DOI:10.1039/b309073f

出版商:RSC    

年代:2003

数据来源: RSC