ARTICLE pubs.acs.org/JPCC
Stability and Transformations of Heated Gold Nanorods G. Opletal,†,* G. Grochola,† Yu Hang Chui,‡ I. K. Snook,† and S. P. Russo† † ‡
Department of Applied Physics, School of Applied Sciences, RMIT University, G.P.O Box 2476 V, Melbourne, Victoria 3001, Australia Institute of Physics, Joohannes-Gutenberg University, D-55099 Staudinger Weg 7, Germany ABSTRACT: We have simulated the heating process of gold nanorods, elucidating a mechanism by which nanorods alter their aspect ratio at higher temperatures. We also studied the relative stabilities of nanorods by constructing nanorods with varying ratios of {110} to {100} exposed surfaces along the body of the nanorod. The least stable nanorod was found to be the nanorod with the largest {110} surfaces, followed by the nanorod with the largest {100} surfaces, while the nanorod with approximately equal surface areas of {100} and {110} surface was found to be the most stable. It was also found that the addition of surface disorder increased the stability of nanorods with large {110} surfaces, while paradoxically decreasing the stability of nanorods with large {100} surfaces. The reasons for this are elucidated and compared to experimental laser-induced gold nanorod transformation studies.
’ INTRODUCTION With their large surface to volume ratio, gold nanorods have distinctive structural behavior differentiating them from their bulk counterparts. Their size and shape have been linked to applicable phenomena such as the enhancement of surface Raman scattering1 and increased fluorescence.2 Of essential importance is the understanding of the thermal stability of these nanorods and information on the structural defects and deformations that occur once they are heated sufficiently. Experimentally, various laser-induced melting studies have found that high laser fluences tend to fragment the nanoparticles and, as the energy is reduced, nanorods are found to transform into spherical nanoparticles and at lower energies still can either partially melt or deform to reduce their aspect ratio.3-5 Such laser pulses can easily heat the gold nanorods past the melting and boiling point of bulk gold. Transmission electron microscopy studies of these deformed nanorods found the presence of multiple twins and stacking faults.4,6 The molecular dynamics simulations and analysis of Wang et al.7 represents a significant attempt to model the dynamics of these deformations. In their work, they suggest that it is the roughening of the {110} facets and their conversion to {111} facets that then results in a bulk realignment with these new {111} surfaces. This process involves the appearance of hexagonally closed packed (HCP) planes among the initially facecentered cubic (FCC) bulk as observed experimentally. In more recent simulations,8 Wang et al. looked at three nanorods dominated by either {111}, {110}, or {100} surface facets and found that the structural deformation occurs in the latter two by a surface-driven sliding of the internal {111} planes at higher temperature. The stability of their {111} nanorods can be understood from experimental and theoretical results illustrating that the {110} surface has a melting temperature and stability (high surface free energy) lower than that of the {111} surface.9,10 r 2011 American Chemical Society
Unfortunately, the Glue potential11 used by Wang et al. is known to contain various inaccuracies including a zero stacking fault energy and a zero energy difference between the HCP and FCC lattice.12 This results in stability issues at low temperatures as illustrated in the significant structural changes, observed even at room temperature, of their modeled nanorods. Because of the uncertainties contained in the Glue potential, the exact processes via which these nanorods change structure during heating remain unclear. In this work, we employ a relatively new ‘force matching’ embedded atom method (EAM) gold potential fitted by Grochola et al.12 which rectifies the majority of these issues. This potential has been successfully used to study the formation process of various gold nanoparticles formed during vapor growth. The studies show the critical role played by the hexagonally reconstructed gold {100} surface, explaining a number of processes during growth including the formation mechanism of the “pancake” m-Dh even though it is not energetically stable,13,14 the instability of the TOh FCC structured nanoparticle during vapor growth,14 nanorod growth,15 population statistics of gold nanoparticles formed by vapor growth16 and aggregation,17 and the surfacedriven crystallization of icosahedral clusters.18 A comparative study using the Glue and the new EAM potentials looking at the crystallization of icosahedral clusters illustrates significantly more visible surface defects when using the Glue potential.19 Because the new potential has been used successfully to explain many phenomena, we apply it here to a study of gold nanorods. In this study, we investigated the stability of various nanorods with differing ratios of {110} to {100} exposed surfaces. Further, we also investigate the effects on stability by Received: August 9, 2010 Revised: January 28, 2011 Published: February 28, 2011 4375
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introducing surface disorder onto these nanorods prior to heating, which represents a more realistic state nanorods would be in. Finally, an analysis of the structural deformations is made and an explanation of these processes is outlined.
’ METHODS A. Simulation Details. Three pristine nanorods were constructed with a principal axis oriented in the [001] direction as shown in Figure 1. These are labeled P110, P100, and Peven, designating the pristine nanorods dominated respectively by {110} surfaces, {100} surfaces, and even areas. All the nanorods were constructed to have approximately the same physical size (13 nm 4 nm 4 nm) and as a result differed in the number of atoms they contained with P110, P100, and Peven containing 8533, 9593, and 10557 atoms, respectively. A further three surface-disordered nanorods were created by simulating vacuum, depositing gold vapor atoms upon the pristine nanorods using the method of Grochola et al.13 in which atoms are periodically introduced into an MD box and then the system is allowed to equilibrate before more insertions. This resulted in the appearance of ad-atoms on top of the pristine nanorods. These are labeled as D110, D100, and Deven using analogous labeling as for the pristine rods. The choice of surface facet types was motivated by experiment4,6 and more recent molecular dynamics simulations.7,8 All nanorods when initially constructed were cooled to 10 K and then equilibrated to remove any longitudinal oscillations as a result of using bulk lattice constants. The nanorods were then heated to 400 K and equilibrated further to remove any oscillations as a result of this heating. These equilibrated nanorods were then used in the heating simulations. We used a Broughton thermostat20 to generate an NVT phase space in between insertion events and the Verlet algorithm to integrate Newton’s equations of motion, with an MD time step of 1.8 fs (fs). The recently proposed ‘force matching’ empirical embedded atom method (EAM) intermolecular potential12 was used to represent the Au-Au interactions. The six nanorods were heated linearly in time from 400 K to 1000 K over two different time intervals of 0.9 and 9 ns corresponding to heating rates of 6.7 1011K/s and 6.7 1010K/s. For convenience, the results and illustrations of the longer simulations are used because the conclusions are similar for both sets of simulations. All simulations were performed in vacuum, as this would be the most realistic medium. While experimental nanorods are suspended in a fluid, studies have shown that during heating, nanorods can be very briefly heated to and past the melting point of the fluid, and indeed sometimes past the boiling point of bulk gold. In such instances, any surrounding fluid would be temporarily boiled off the nanorod surface, creating a temporary gas vapor bubble around the nanorod which primarily has an effect on the heat dissipation behavior of the nanorod.6,21 B. Structure Characterization. Bulk atom characterization of FCC and HCP local ordering was accomplished using previously described ring size methodology.22 Briefly, within a close packed environment, for any particular atom, the bonding between its first nearest neighbors produces a bond network which can be analyzed for populations of various ring sizes. A bond here is defined geometrically by being within a maximum separation defined by the minimum in between the first and second peaks in the radial distribution function. The number of three- and four-membered rings and their arrangement within the bond
Figure 1. Surface cross-sections of the three initial pristine [001]oriented nanorod configurations (labeled P110, P100, and Peven from top to bottom). Yellow represents {111} surfaces, blue {100} surfaces, red {110} facets. A further three nanorods were prepared by simulating atomic deposition upon the pristine rods to introduce surface disorder.
network is sufficient to differentiate between the various closed packed environments. For surface classification, the surface layer of each nanorod was determined. To determine whether an atom is a surface atom, a radial shell of equidistant points was placed around the atom at a radius near the bond distance cutoff. At each of these many points, a test atom was placed and distance checks were made to see if any overlap occurred between these test atoms and any other real atoms in the system. If no overlap occurred, the atom was classified as a surface atom. 4376
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Figure 3. Temperature at which HCP plane defects appear inside nanorods at the two different heating rates.
Figure 2. Average center of mass to surface distance and bulk fraction of HCP ordered atoms as a function of temperature for the six nanorods in the 9 ns simulations.
Surface classification of atoms with {111}, {100}, and {110} surface geometry within the surface layer was determined via an application to surfaces of a Monte Carlo (MC)-based signature cell-fitting method original developed by Shetty et al.23 Within this method, ‘signature cells’ appropriate to the system under investigation are overlaid over each atom and via rotation and scaling MC moves of the cell, a fictitious potential is minimized which has its minimum at maximum overlap between the cell and the real bonding environment around an atom. The signature cells used within this work are the surface first nearest neighbors of a typical atom found on {111}, {100}, and {110} surface types. A {111} signature cell contains six atoms around the origin in a hexagon geometry while a {100} cell has four atoms in a square arrangement around the origin. A {110} cell simply has two atoms in a line passing through the origin (only the top layer is picked up by the surface-finding methodology).
’ RESULTS AND DISCUSSION The appearance of HCP planes inside the bulk was used as an indicator of the onset of structural instability because their appearance was associated with a physical shape deformation and a change in aspect ratio. This relationship is illustrated in Figure 2 where the mean center of mass to surface distance is shown along with the fraction of HCP ordered atoms within the bulk. It can be seen that the spikes in the HCP fraction are associated with a change in the mean distance. The pristine nanorods tend to have more abrupt single event transformations while the disordered nanorods are generally transformed over multiple stages. Figure 3 shows the temperatures at which HCP plane defects appeared inside the nanorods. The appearance of HCP planes
(and nanorod deformation) always began 30 K to 60 K lower in the longer simulations. The overall pattern of structural stability among the nanorods was however similar for both heating rates. A. Nanorod Stability. Overall the most unstable nanorods were those dominated by {110} facets followed by nanorods dominated by {100} facets. The most stable were nanorods with an approximate balance ratio of {110} to {100} surface facets; see Figures 2 and 3. When surface disorder was introduced, the {110} nanorods greatly increased their stability while the {100} nanorods were decreased in stability. The even surface nanorods slightly decreased in stability with the introduction of surface disorder. Overall surface disorder did not alter the stability ordering of the various nanorods. An increased stability of the disordered {110} nanorods is expected because an excess of vapor atoms allows the {110} surface to reconstruct to the more energetically favorable (1 2) surface as observed experimentally.24 The reduction in stability for the {100} surface nanorod is interesting because it goes against surface stability arguments, namely that surface disorder induces hexagonal reconstructions which are known to be more stable, yet we obtain a reduced stability. As we shall discuss further, we hypothesize that this is due to the hexagonal reconstructions reducing the free energy barrier to the deformation process. Shown in Figure 4 are the surfaces of all the nanorods that were heated for 9 ns which from the top row to the bottom are P110, D110, P100, D100, Peven, and Deven. The starting configurations (400 K) are on the left, and the final configurations (1000 K) are on the right. The middle column configurations show each nanorod at the beginning of each deformation at the first appearance of the HCP slip planes (shown in green). All nanorods have been oriented in such a way as to illustrate the common direction of the slip. B. Deformation Processes. Initially during heating for the disordered nanorods, the {100} surface area increases and almost all disorder is removed. The mechanism via which ad-atoms move across a facet boundary, where one of the surfaces involved is the {100} surface, has been observed previously.13 In that previous work, ad-atoms from the relatively lower energy {111} surface were observed to move to the higher energy {100} surface. Here the mechanism is similar except that ad-atoms move from the relatively lower energy {100} surface to the higher energy {110} surface. First, ad-atoms on the {100} surfaces become embedded into the surface, reconstructing a part of the surface into a hexagonal structure. The surface then deconstructs, and the excess atoms are pushed out onto an adjacent {110} surface; this process extends the {100} surface. The general trend is for the flux of ad-atoms to go from the more stable closed surface to the less stable open surface. This is because ad-atoms on a closed surface are less stable than ad-atoms on an open surface 4377
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Figure 4. Nanorods from the 9 ns heating simulations. From the top row to the bottom: P110, D110, P100, D100, Peven, and Deven. The left column shows the starting configurations at 400 K while the right column shows the nanorod postdeformations at 1000 K. The middle row shows each nanorod at the beginning of the transformation. Yellow is {111}, blue {100}, red {110}, and green internal bulk HCP.
Interestingly, the hexagonal reconstruction on our nanorod {100} surface, while sometimes observed, is not preferred. We know that the GRS potential does not prefer this reconstruction,12 although it is not completely unfavorable as sometimes observed. While it is known that the (1 5) hexagonal reconstruction is preferred for the Au{100} surface, we speculate that a small experimental gold nanorod would not contain this reconstruction because of two reasons. First, we know that a surrounding fluid would tend to deconstruct the surface by contributing bonding electron density to the surface atoms; second, the {100} surface on a small nanorod is only sometimes commensurate with a (1 5) periodicity. Hence, we believe experimentally that this reconstruction would also be only temporarily observed. As nanorods continue to heat, a stacking fault develops, initiating the deformation. The observed orientation of this stacking fault and the resulting crystallographic transformation was the same for all nanorods. We observed that this stacking fault is initiated by a spontaneous hexagonal reconstruction on a {100} surface bordering a top {110} surface, while at the same time this planar slip causes the top {110} surface to transform to the {100} surface.
Figures 5 and 6 illustrate this process in detail over a sequence of four frames taken from the Peven nanorod. A stacking fault shows up as two neighboring layers of HCP packing in an otherwise pure FCC bulk, as the ordering sequence is changed only for those two layers. Inspection of the molecular dynamics snapshots illustrate these HCP planes originating from the {100} surfaces. Although surface disorder obscures the surface classification in the transforming regions of the early frames in Figure 5, the surface transformations can still clearly be seen as highlighted in the figure. The transformation propagates by converting a layer adjacent to the stacking fault into the same crystallographic orientation. This propagates the conversion of the top and bottom {110} surfaces to {100}, the bordering {100} to {111}, and the side {110} to a {110} surface with a different crystallographic orientation. The transformation is effectively a 45° downward crystallographic rotation of the FCC structure in the x-plane. Interestingly, the propagation of this transformation through the nanorod also results in a reduction in the internal coordination defects. As the nanorods were heated, a progressive build-up of 11- and 13-fold coordinated atoms occurred. The transition and the realignment to a new crystallographic 4378
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Figure 5. Deformation of nanorods initiated by the appearance of {111} atoms on the {100} surface from which propagates a slip plane resulting into two internal HCP planes. The {110} surface also sees a conversion to {100} ordering highlighted by a red box. Blue {100}, red {110}, yellow {111}, green (HCP), dark gray (surface atoms), light gray (below surface bulk).
Figure 6. Progression of the deformation mechanism of a nanorod. The red box illustrates the propagation of the slip planes (HCP atoms) originating from the new {111} surface formed on the former {100} surface. Blue {100}, red {110}, yellow {111}, green (HCP), dark gray (surface atoms), light gray (below surface bulk).
orientation resulted in a significant reduction of these defects during the course of the transformation. As a comparison, Wang et al.7 observed a very strong preference for {110} surfaces to transform into the {111} surface
in their nanorod simulations. We however believe this to be due to the inaccuracies contained in the Glue potential and different observed behavior. The transforming nanorods we observed resulted in a cross-section shown in Figure 7 if the transformation 4379
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’ ACKNOWLEDGMENT This work was supported by the National Computational Infrastructure (NCI) National Facility, Victorian Partnership of Advanced Computing (VPAC), the Australian Research Council (ARC), and the RMIT Platform Technology Research Institute. ’ REFERENCES Figure 7. P110 initial (left) and final (right) nanorod morphology. All six nanorods resulted in a similar transformation of surfaces with only their areas differing in size.
converted the remaining layers. Two of the initial {110} facets are converted into {100} faces while all of initial {100} facets are converted to {111} facets. Our result provide an explanation that could possibily explain the experimental results of Link et al.4 who observed a stepwise decrease in nanorod length of just under one-half, consistent with our observations, after exposure to a low energy femtosecond laser pulse of 0.001 J cm-2 fluence. While they attribute the changes in the nanorod aspect ratio to “gentle surface melting”, we observe the development of internal stacking faults below the point at which the surface melts. The TEM picture in Figure 1c of the Links et al.6 manuscript shows a nanorod with the same distinctive stacking faults as observed in our simulations. In that work, Link et al.6 show other transformations and state that internal defects cause internal twining planes; however, from direct observations here, it seems more likely that these mostly originate from the {100} surface. Although these experimental nanorods were significantly larger, it is reasonable that the surface-mediated transformation mechanism dominating our simulated nanorods could provide a possible explanation to these observations.
4. CONCLUSION We have carried out molecular dynamics simulations of the heating of gold nanorods to investigate the effects on their relative stability by varying the ratios of {110} to {100} exposed surfaces and introducing surface disorder. The most stable nanorod was found to be the one with an equal ratio of {110} to {100}, followed by the large {100} surface nanorod. The large {110} surface nanorod was found to be the least stable. The introduction of surface disorder increased the stability of the {110} nanorods but interestingly decreased the stability of the large {100} nanorod. We hypothesize that this decrease is due to the resulting hexagonal reconstructions on the {100} surfaces which reduced the free energy barrier to the deformation process. The deformation process which alters the aspect ratio of the nanorods was found to be initiated on the {100} surfaces by a hexagonal reconstruction of that surface into a {111} surface. This produces a slip plane which also converts two of the four {110} surfaces into {100} surfaces. The transformation propagates from its initial location, converting all four of the initial {100} surfaces into {111}. The changes in aspect ratio and observed slip plane directions, which are detected as HCP planes in an otherwise FCC bulk within our simulations, are consistent with various laser pulse irradiation studies of gold nanorods.
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’ AUTHOR INFORMATION Corresponding Author
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