Nanogold: A Quantitative Phase Map Amanda S. Barnard, †, * Neil P. Young, ‡ Angus I. Kirkland, ‡ Marijn A. van Huis, § and Huifang Xu  † CSIRO Materials Science & Engineering, Clayton, 3168, Australia, ‡ Department of Materials, University of Oxford Parks Road, Oxford, OX1 3PH, U.K., § Kavli Institute of Nanoscience, Delft University of Technology Lorentzweg 1, 2628 CJ Delft, The Netherlands, and  Department of Geology and Geophysics, and Materials Science Program University of WisconsinOMadison, Madison, Wisconsin 53706 N anoparticles of gold are currently attracting considerable attention for use in biomedical applications including drug delivering, heating, sensing, 1 and in nanocatalysis. 2 However, our ability to control the properties upon which these applications are based is still intrinsically linked to the nanomorphology of individual particles. Most theoretical models predict that, in general, the more perfect the nanoparti- cles, the better they perform. However, real nanoparticles are rarely crystallographically ideal, and planar defects such as contact twins and intrinsic or extrinsic stacking faults, form during growth in materials with low stacking fault or twin boundary en- ergy, and surface energy anisotropy. 3 Gold features in this group, often exhibiting structural and morphological modifications including single or multiple (parallel, con- tact) twinning 4 and cyclic twinning result- ing in decahedral 5 and truncated decahe- dral structures 6 (Figure 1). The observation of decahedral structures, often referred to as multiply twinned particles or MTPs, are particularly interesting, due to their un- usual, crystallographically forbidden 5-fold (pentagonal) symmetry and concomitant lattice strain at small sizes. 7 These are dis- tinct from larger MTPs characterized by large fcc domains with a dislocation core. The relative stability between these struc- tural motifs has been the focus of much at- tention, 8 but from a technological stand- point the key question is what the stable structure of a gold nanoparticle will be postsynthesis in realistic engineering and natural environments. An exceptional way of capturing the es- sential information about the structural sta- bility of gold nanoparticles under a range of conditions is to consult a nanoscale phase diagram, which provides a powerful, predictive map of chemical equilibrium. In general, a phase diagram (or map) identifies the thermodynamically stable structure of a material at a given temperature (T), com- position (C), and/or pressure (P), but the de- velopment of nanoscale phase diagrams with an additional dimension representing the critical diameter (D) has been slow. Ide- ally in addition to size, a useful quantitative nanoscale phase map for gold should dis- tinguish between structural motifs and shapes, as above, since the shape is funda- mentally linked to important properties in- cluding reactivity 2 and surface plasmon resonances. 9 The high temperature region of the nanogold phase diagram has been explored experimentally by Koga et al., but only the relationship between structural motif and the melting transition was firmly established. 10 Using classical molecular dy- namics on a limited set of small structures, a qualitative phase map for gold nanoparti- cle has been previously suggested by Kuo and Clancy, 11 but is in disagreement with other earlier empirical attempts. 12 A phase map of this type could also be generated by *Address correspondence to amanda.barnard@csiro.au. Received for review March 5, 2009 and accepted May 13, 2009. Published online June 2, 2009. 10.1021/nn900220k CCC: $40.75 © 2009 American Chemical Society ABSTRACT The development of the next generation of nanotechnologies requires precise control of the size, shape, and structure of individual components in a variety of chemical and engineering environments. This includes synthesis, storage, operational environments and, since these products will ultimately be discarded, their interaction with natural ecosystems. Much of the important information that determines these properties is contained within nanoscale phase diagrams, but quantitative phase maps that include surface effects and critical diameter (along with temperature and pressure) remain elusive. Here we present the first quantitative equilibrium phase map for gold nanoparticles together with experimental verification, based on relativistic ab initio thermodynamics and in situ high-resolution electron microscopy at elevated temperatures. KEYWORDS: gold · nanoparticles · shape · phase diagram · thermodynamics · modeling ARTICLE www.acsnano.org VOL. 3 ▪ NO. 6 ▪ 1431–1436 ▪ 2009 1431 Downloaded by CSIRO INFO & TECH SERVICES on August 22, 2009 Published on June 2, 2009 on http://pubs.acs.org | doi: 10.1021/nn900220k