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Fabrication of hollow gold nanoparticles by dewetting, dealloying and coarsening
Acta Materialia 102 (2016) 108e115
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Fabrication of hollow gold nanoparticles by dewetting, dealloying and coarsening Anna Kosinova a, Dong Wang b, Peter Schaaf b, Oleg Kovalenko a, Leonid Klinger a, Eugen Rabkin a, * a b
Department of Materials Science and Engineering, Technion e Israel Institute of Technology, 32000 Haifa, Israel Chair Materials for Electronics, Institute of Materials Science and Engineering, TU Ilmenau, 98693 Ilmenau, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 June 2015 Received in revised form 29 August 2015 Accepted 13 September 2015 Available online xxx
A sequence of diffusion-controlled processes, namely, thin film dewetting, dealloying and thermal coarsening was employed for producing of hollow gold nanoparticles. The porous gold nanoparticles on silica substrate were obtained by solid state dewetting of AueAg bi-layers followed by selective dealloying of Ag. We studied in detail the last stage of the proposed method enabling the transformation of porous nanoparticles into hollow ones due to curvature-driven surface diffusion. The porous nanoparticles were annealed at 350 C in air and in vacuum and characterized by high-resolution scanning electron microscopy. The microstructure evolution of the particles during thermal treatment was studied by in-situ X-ray diffraction. The observed decrease of compressive strain in the particles during coarsening was discussed in terms of WeissmüllereCahn model. Annealing in ambient air resulted in faster coarsening of gold nanoparticles compared to annealing in vacuum. The isolated pores trapped in the particle bulk as well as the pores located at the particle/substrate interface were observed in the particles annealed in air. A qualitative model illustrating the observed coarsening behavior is proposed. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Surface Diffusion Coarsening Nanoporous Hollow nanoparticles
1. Introduction Hollow and porous metallic nanostructures attract great attention due to their high surface-to-volume ratio and resulting from it tunable optical [1,2], electrocatalytic [3] and mechanical [4] properties. Hollow nanoparticles are considered as promising candidates for a number of biomedical applications. For example, hollow gold nanoparticles are much more efficient than their solid counterparts at heat generation when exposed to near-infrared radiation, which makes them attractive for cancer theranostic applications [5]. The most widespread methods for the synthesis of hollow nanostructures are based on the Kirkendall effect [6] and on galvanic replacement reactions [2,7,8]. The latter method allows reproducible synthesis of hollow nanoparticles, but they often exhibit polycrystalline microstructure and high density of defects [8]. The hollowing process induced by the Kirkendall effect refers to the interdiffusion in the coreeshell nanostructures accompanied by
* Corresponding author. E-mail address: [email protected] (E. Rabkin). http://dx.doi.org/10.1016/j.actamat.2015.09.024 1359-6454/© 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
the flux of vacancies from the low diffusivity shell into the high diffusivity core. Ultimately, vacancies accumulate and collapse into a void. The Kirkendall effect-based methods have been applied for manufacturing different nanostructured systems with various compositions and shapes, such as partially oxidized Ni(core)/ NiO(shell) nanoparticles [9], spinel ZnAl2O4 nanotubes [10], hollow CoS nanocrystals [11] etc. The hollow nanoparticles of AgeAu and AgePd alloys were successfully synthesized via the bulk Kirkendall effect from the hemispherical Ag/Au [12] and Ag/Pd [13] coreeshell nanoparticles. It is worth noting that high temperatures required for Kirkendall hollowing based on bulk interdiffusion result in capillary instability [14] and transformation of hollow nanostructure into a porous [15] or a solid one. Here we propose an alternative route for producing hollow metal nanoparticles based on surfaceeand interface diffusionassisted processes of dewetting, dealloying and thermal coarsening. The thin film dewetting process, or formation of isolated particles from thin solid films exposed to high temperature, is driven by surface/interface energy minimization [16]. The next stage, dealloying, allows obtaining the nanoparticles with interconnected open porosity via selective dissolution of the less noble alloy constituent, accompanied by rearrangement of the more
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noble atoms into stable clusters [17]. This rearrangement during dealloying is controlled by surface diffusion of the more noble atoms, while the role of bulk diffusion is negligible [18]. Asdealloyed nanoparticles exhibit three-dimensional bi-continuous structure with pore dimensions in the range of tens of nanometers. Finally, during the coarsening stage the open porosity transforms into the metastable closed one by means of surface diffusion. Thermal treatments are widely employed to modify nanoporous structure and achieve various ligament sizes of nanoporous metals [19e21]. Ligament coarsening by means of surface diffusion occurs at relatively low homological temperatures. The studies of pore morphology evolution in palladium nanoparticles [22] showed that pore shrinkage is governed by surface diffusion at temperatures up to 200 C, and above 200 C grain boundary diffusion becomes essential. Low temperature coarsening of nanoporous gold was observed by Qian et al. [21] during annealing at 200 C for 2 h in ambient air, and by Viswanath and coworkers [23] after thermal treatment for a longer time (6 h) at lower temperature (85 C). Positron lifetime experiments showed that onset of thermal instability is related to the temperature at which the vacancies become mobile [23]. We suggest that surface diffusion in porous nanoparticles should not only result in pores coarsening, but also lead to the closure of porosity in the near-surface region of the nanoparticles and formation of hollow nanoparticles. The possibility of formation of closed pores (“bubbles”) during coarsening of porous gold nanoparticles (PGNs) at elevated temperatures was demonstrated in kinetic Monte Carlo simulations of Erlebacher [24]. These bubbles were comparable in size to the initial pores in the as-dealloyed microstructure, and their formation was related to the Rayleigh instability of hollow cylinders. The first experimental evidence of enclosed voids in the as-dealloyed microstructure of €sner et al. [25]. The voids bulk nanoporous gold was provided by Ro were extremely small, 1e5 nm in diameter, and they could not form by surface diffusion alone, since during the dealloying the latter €sner et al. proposed that such requires the access of electrolyte. Ro pores may form as a result of condensation of excess vacancies produced by dealloying [25,26]. This study is focused on thermal coarsening of PGNs aimed at fabrication of the hollow nanoparticles. The thermal instability of PGNs is characterized as a function of coarsening time (30e420 min) and annealing atmosphere (vacuum and air). 2. Experimental 2.1. Sample preparation Porous gold nanoparticles were prepared from thin Au/Ag bilayer films (10 nm Au/20 nm Ag) deposited on SiO2/Si substrates by electron beam evaporation. 200 nm thermal SiO2 were grown on the Si substrates prior to deposition in order to prevent the reaction between the substrate and the subsequently deposited films. Firstly, the dewetting annealing of the bi-layer films was performed at 900 C in Ar for 15 min. Isolated AueAg alloy nanoparticles formed as a result of dewetting are shown in Fig. 1. Afterwards, silver was selectively removed from the alloy nanoparticles in a nitric acid. The samples were submerged in a 65 wt.% HNO3 aqueous solution at 21 C for 5 min. As-prepared porous gold nanoparticles samples were annealed in a mullite tube resistance furnace at 350 C in ambient air and in vacuum (106 Torr). Three consecutive heat treatments, 30 min each, were performed. 2.2. Sample characterization As-dealloyed and annealed samples were characterized by highresolution scanning electron microscope (HR-SEM, Zeiss Ultra
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Fig. 1. SEM micrograph of as-dewetted AueAg nanoparticles.
Plus), operating at an acceleration voltage of 3 kV. Cross-section samples were prepared in a dual-beam focused ion beam (FIB; FEI Strata 400-S). To characterize the structural evolution during thermal treatment we used in-situ heating X-ray diffraction (XRD) system (Rigaku SmartLab) equipped with a high-temperature heating stage. The measurements were performed in a parallel beam configuration, and monochromatization of the beam was performed using a 2-bounce Ge(220) monochromator. In-situ XRD data were collected before and after annealing, and at the temperature of 350 C and annealing times ranging from 30 to 420 min. The measurements were carried out in low-vacuum mode (101 Torr). The diffraction patterns were recorded in qe2q scanning mode using Cu Ka radiation (l ¼ 1.5406 Å) in the range of 2q between 37.5 and 38.5 . Data collection time was 90 s per scan. 3. Experimental results PGNs with a diameter ranging from 100 to 500 nm were prepared on SiO2/Si substrates using dewetting process followed by dealloying. The scanning electron microscopy (SEM) micrographs in Fig. 2a, b shows the typical structure of as-dealloyed PGNs, which consists of interconnected ligaments and pores. The existence of small closed pores inside the individual ligaments (similar to those €sner et al. [25] in the bulk nanoporous gold) cannot observed by Ro be excluded based on the cross-sectional SEM micrograph in Fig. 2b. The coarsening temperature 350 C was selected in order to ensure that surface self-diffusion of Au is fast enough for morphology changes, whereas the bulk self-diffusion of Au is too slow for total closing of porosity and for the formation of solid nanoparticles with the shape corresponding to thermodynamic equilibrium. Taking into account that surface adsorbates may change the surface diffusion rate, we studied the effect of annealing atmosphere by performing heat treatment in vacuum and in ambient air. The microstructural evolution of PGNs annealed in vacuum, is shown in Fig. 2c, d. It is seen that heat treatment caused an increase in the ligament width from 10 nm for as-dealloyed sample to 35 nm for the sample annealed in vacuum at 350 C for 30 min. An average value of ligament width was measured at the narrowest point of each ligament and was estimated from measurements of about 100 separate ligaments in SEM and cross-sectional SEM images. After the heat treatment for 60 and 90 min, the ligament width remained unchanged with a value of 37 ± 2 nm. The morphology changes of PGNs after two consecutive
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Fig. 2. (a, c) General view and (b, d) cross-sectional SEM images of PGNs: (a, b) as-dealloyed and (c, d) annealed in vacuum for 30 min.
annealings in vacuum and in air are shown in Fig. 3b, c and Fig. 3e, f, respectively. In both cases, one and the same region of the substrate is shown and it is evident that thermal coarsening occurs after the
first annealing. The curvature-driven surface diffusion transports the material from the regions of positive curvature to the junctions between the ligaments. Thus, the mean curvature of the ligaments
Fig. 3. SEM images of PGNs: (a, d) as-dealloyed; annealed in vacuum for (b) 30 min, (c) 60 min; annealed in air for (e) 30 min, (f) 60 min. 60 min is the cumulative annealing time after two annealing stages of 30 min each.
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diminishes and the total volume of the pores decreases. Initial small pores collapse into bigger ones. During the following stages of the heat treatment, the samples were relatively stable, with little changes in their morphology. As seen in Fig. 3, coarsening occurred much faster during annealing in air compared to that in vacuum. The near-surface ligaments merged into a continuous layer. Similar results have been obtained by Sun at al [20]. It was found that surface diffusion in nanoporous gold proceeds faster in nitrogen than in vacuum, which may be explained by the easier generation of vacancies and gold adatoms in the presence of adsorbates. The effect of annealing atmosphere on coarsening kinetics of nanoporous gold was also studied in the recent works of Chen et al. [27] and KuwanoeNakatani et al. [28]. Both studies demonstrated the low thermal stability of thin nanoporous gold films in oxidizing atmosphere. In Ref. [28] the activation energy for the thermal coarsening of nanoporous gold in air was shown to be only half of that in vacuum. We revealed that after heat treatment in air, some particles develop large single through holes (Fig. 4). Yet, the vast majority of nanoparticles exhibits a rough surface with multiple concavities (Fig. 3e, f) but without open pores. We performed the in-situ SEM heating of the PGNs, which allowed us to observe the kinetics of thermal coarsening in vacuum (105 Torr). The experimental conditions were close to those in the annealing furnace. The onset of coarsening was observed at the temperature of about 160 C. At higher temperatures (up to 500 C) the kinetics of morphological changes was slow. Since the e-beam induced coating of carbonaceous material on the surface of PGNs which is inevitable in the SEM chamber may inhibit the surface self-diffusion on Au, we performed the in-situ XRD annealing experiments. Because of the strong [111] texture of the as-deposited Au and Ag films inherited by the single crystal PGNs, tracking the position of the (111) Bragg reflection provides accurate information on the average strain in the particles. Fig. 5 shows the evolution of the Au (111) Bragg reflection as a function of temperature and annealing time. Heating from room temperature to 350 C results in a significant shift of the diffraction peak to the lower values of 2q due to thermal expansion of the crystal lattice. Within the time span of the annealing process from 30 to 420 min only small, insignificant changes in the position of the (111) diffraction peak are observed. After cooling the sample to room temperature, the diffraction peak position shifts back to the higher values of 2q (smaller d-spacing). However, the fact of the particular importance is the new position of the peak, which does not coincide with the one before annealing. The additional diffraction maximum at 37.7 is attributed to the response from aluminum nitride holder of the
Fig. 4. SEM image of a through hole formed after annealing of PGNs in air for 30 min.
111
Fig. 5. The in-situ X-ray diffraction spectra of PGNs. The red and the black spectra refer to PGNs prior and after the heat treatment, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
heating stage (see Fig. S1.1 in the Supplementary Information), which becomes apparent in case of a small sample size. 4. Discussion Considering the rough surface of PGNs annealed in air (Fig. 3e, f) we supposed that the particles which are fully closed from outside still may contain pores inside. We observed both the pores encapsulated in the particle volume and the pores located at the particle/substrate interface (Fig. 6). The SEM images in Fig. 7e, j shows the FIB cross-sections of PGNs annealed in air for 60 min. It is obvious that thermal treatment results in closing of porosity that occurs faster at the surface than inside the particle. Based on the results of kinetic Monte Carlo simulations, Erlebacher attributed formation of small closed pores during coarsening of PGNs to the Rayleigh instability of cylindrical cavities [24]. Once formed, such closed pores become stagnant since they are cut off from the fast surface diffusion paths. They can be only eliminated by the pinch-off events of the nearby ligaments, or the ligaments they reside in [24]. Yet this scenario implies that the closed
Fig. 6. Cross-sectional SEM image of PGNs annealed in air at 350 C for 60 min.
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Fig. 7. Schematic illustration of morphology evolution of Au nanoparticle and corresponding cross-sectional SEM images: (aee) formation of closed pore inside the particle, (fej) formation of pore at the particle/substrate interface. Cross-sections of the particles are taken after annealing the sample in air at 350 C for 60 min. The annealing times are (a,f) 0; 4 kT (b) 15.2t0; (c) 15.4 t0; (d) 19.9 t0; (g) 2.6 t0; (h) 14.4 t0; (i) 63.5 t0, with t0 ¼ ð0:1LÞ . 2 nDU g s
s
s
pores should be homogeneously distributed through the volume of the nanoparticles. Indeed, many pores residing close to the nanoparticle surface were observed in simulations [24]. On the contrary, in the present study the heat treatments in air resulted in full closure of the near-surface porosity (see Fig. 3e, f), with concomitant development of the internal closed pores of the sizes exceeding the initial average ligament size (see Figs. 6 and 7e, j). This behavior can be understood in terms of the contribution of bulk diffusion, and the topology of the PGNs. While the contribution of bulk diffusion to the development of porous microstructure is negligible during the dealloying at room temperature [18], its importance increases with increasing temperature because the activation energy for bulk self-diffusion is significantly higher than that of surface self-diffusion. We performed an approximate estimate of the dissolution time of the closed pore trapped inside the nanoparticle or the individual ligament, shrinking by the volume diffusion mechanism (see Supplementary Information). The corresponding estimate for the internal pore of 20 nm in radius located at the distance of 60 nm
from the surface (see Fig. 7e) indicates that the time of pore dissolution by bulk diffusion at 350 C is about 92 h, much longer than the annealing times employed in the present study. At the same time, a small pore of 2.5 nm in radius residing at the distance of 5 nm from the ligament surface (the pores of this type were observed both in the as-dealloyed nanoporous gold [25], and in simulations of Erlebacher [24]) will fully dissolve after only 7 min of annealing, shorter than the shortest annealing time employed in this study. Thus, we can conclude that only relatively large pores formed at the late stages of coarsening can survive the heat treatments at 350 C. At the late stages of coarsening the characteristic dimensions of the ligaments and channels become comparable with the overall dimensions of the particle (see Fig. 2c, d). Therefore, we believe that at these stages a deterministic process of surface self-diffusion, rather than perturbation-controlled Rayleigh instability of long perfect cylinders is responsible for the development of closed pores. The formation of closed porosity is associated with the fact that average surface curvature at the surface of PGNs is more
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positive than inside the particles. Thus, pores shrinkage is faster close to the surface, which results in gradual closing of the porosity and trapping of isolated pores inside the particles, or at the particle/ substrate interface. A similar idea has recently guided Salvalaglio et al. to design a method of producing suspended Ge thin film on Si substrate [29]. They have grown an ordered, dense array of Ge pillars on the Si substrate. Annealing of these arrays at 450 C resulted in closure of the gaps between the pillars at their outer ends and formation of continuous Ge film attached to the substrate by the lower parts of the pillars. Also in this case, the higher average surface curvature of the pillars at their outer ends as compared with their bottom parts resulted in formation of continuous film at the pillars top. In what follows, we will illustrate this coarsening behavior with the aid of a simple two-dimensional model, assuming the rectangular form of channels in the nanoparticle. We will consider the late stages of coarsening, at which the characteristic size of the individual channel is comparable with the nanoparticle outer dimensions. The detailed description of the model is given in the Supplementary Information, while here only the main assumptions and the results of simulations will be presented. We suppose that kinetics of the process is determined by the self-diffusion of Au on the surface and along the particle/substrate interface only (bulk self-diffusion is negligible). The corresponding diffusion fluxes are driven by the gradients of chemical potential of Au atoms. Divergence of the diffusion flux leads to accretion of Au atoms on the surface and at the interface. The capillary-related part of the chemical potential of surface atoms scales with a local curvature, ks, of the surface. The corresponding Mullins equation [30] describes a normal velocity, Vn, of the surface
Vn ¼ Bs
v2 ks vs2
(1)
where s is the arc length along the surface,
Bs ¼
ns Ds U2 gs kT
(2)
is the Mullins coefficient for the surface, and ns, Ds, gs, and U are the area density of the mobile surface atoms, the surface self-diffusion coefficient of Au, the surface energy of Au and its atomic volume, respectively. The factor kT has its usual thermodynamic meaning. The self-diffusion of Au along the rigid particle/substrate interface cannot be described in a similar way. The accretion of Au atoms at the rigid interface leads to rigid-body translation (normal to the substrate) and rotation of the particle with respect to the substrate. It was shown by Klinger and Rabkin that in this case the chemical potential of mobile Au atoms at the interface is a non-local function depending on the interface shape and on boundary conditions at the interface termini [31,32]. For the flat interface it is a cubical parabola with respect to the coordinate, s, along the interface:
mðsÞ ¼ a0 þ a1 s þ a2 s2 þ a3 s3
(3)
The coefficients of this polynomial can be found by calculating the variations of the total surface/interface energy of the particle caused by infinitesimal rigid normal translation (dy) and rotation (dq) of the particle with respect to the substrate. The rigid translation and rotation rates (Vy and Vq, respectively) scale with the selfdiffusion coefficient of Au atoms along the particle/substrate interface (see details in Refs. [31,32]). Thus the total displacement of the free surfaces of the particle (including the internal surfaces of the pores) is a sum of the local normal displacement due to surface diffusion (Vn), and rigid translation and rotation (Vy and Vq). To
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preserve the integrity of the particle (in the two-dimensional model an open channel reaching to the substrate separates the particle into two disconnected sub-particles), we imposed an additional limitation that the two sub-particles move as a single rigid solid (see Supplementary Information). The values for the surface and interface energies employed in the model were taken from the literature: the surface free energy for pure gold is 1.36 J/m2 [33], the value for Au/SiO2 interfacial energy is 1.17 J/m2 [34], and the corresponding energy for silica substrate is 0.2 J/m2 [35]. Moreover, we assumed that the self-diffusion coefficients of Au on the surface and along the particle/substrate interface are identical. Fig. 7aed illustrates that closing of the open channel that does not reach to the substrate results in a pore trapped in the bulk of the particle. On the contrary, closing of the channel which initially reaches to the substrate results in a pore at the particle/substrate interface (Fig. 7fei), and in some cases may leave both the interface and bulk pores (not shown here). In both cases, the rapid closure of the open channel is associated with a high surface curvature of the channel near the surface of the particle. While the bulk pore shrinks by a slow process of bulk self-diffusion, the shrinking of the interface pore is controlled by the Au atoms self-diffusion along the particle/substrate interface, which is much faster than the bulk selfdiffusion [36,37]. A combination of high contact angle of solid Au on silica and of the open nature of porosity in the as-dealloyed PGNs (see Fig. 2a, b) inevitably means that a large number of interface pores should form at the initial stages of coarsening. The fact that after prolonged annealing these pores disappear in a number of particles (see. Fig. 7e) indicates that the rate of Au-self diffusion along the Au/silica interface is comparable to the rate of surface self-diffusion on Au. If several interface pores coalesce into bigger ones (Fig. 6), the dissolution time of the formed pore increases. Also, Fig. 7d, i clearly illustrate that surface concavities of the particles are associated with the closure of the channels in the nearsurface region. Once the channel is closed, the surface curvature decreases and the rate of the shape change becomes very slow. The ridges on the SiO2 substrate in the vicinity of the nanoparticles annealed in air (see Figs. 3e, f, and 4) clearly indicate that the nanoparticles shrink due to the increase of their average density. It is interesting that such ridges are less developed in the case of the nanoparticles annealed in vacuum, again indicating the slower pace of densification of the latter. We would like to emphasize that in all experiments, most of coarsening has occurred at the onset of annealing, and then the process came to a nearly complete standstill. An obvious reason for this slow-down would be a well-known fact that the rate of surface shape change by surface diffusion scales with the characteristic size of the system, l, as l4 [30]. This means that the increase of the typical ligament size from 10 to 35 nm after the first anneal means the decrease of the coarsening rate by a factor of 150. Yet our experimental data provide some indications that the actual slowdown of coarsening is even stronger than dictated by the l4 law. Indeed, Fig. 3d, e, demonstrate a partial merging of two adjacent PGNs after the first 30 min annealing. The sharp protrusion with a highly curved tip (radius of curvature is 17 nm) formed as a result of this merge remained nearly unchanged after the second anneal for 30 min (Fig. 3f). In our opinion, there are two possible reasons for this increase in thermal stability of the partially coarsened particles: (i) Surface faceting, which is known to reduce the rate of capillary-driven shape evolution due to the difficulties of step nucleation on atomically flat surfaces [38e40]; (ii) Segregation of residual impurities in PGNs (most notably, S and C) at the particle surface and concomitant decrease of the surface self-diffusion coefficient [41]. Such segregation
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can also explain faster coarsening rates in air as compared with the annealing in vacuum: the sulfur atoms segregated on particle surface may react with oxygen forming the volatile SO2 and thus reduce surface coverage by the segregant. It should be noted that both mentioned reasons for slow-down of coarsening should increase the thermal stability of the pores trapped inside the coarsened particles well beyond that dictated by bulk diffusion [42]. This is because atomically flat or contaminated internal surfaces of the pore are unlikely to serve as sources of vacancies. Our in-situ heating experiments in SEM confirm the stability of PGNs at the temperatures at which bulk diffusion is significant. The same factors are also responsible for the high stability of the “doughnut-like” nanoparticles with a single through hole (Fig. 4). The in-situ XRD annealing experiments revealed shift of the Au (111) Bragg reflection (Dd/d ¼ 1.5 103) after cooling the sample relatively to its position before annealing. This fact is apparently related to the decrease in capillary-induced compressive stresses in the particles upon pores coarsening. We employed the WeissmüllereCahn equation for estimating the surface stress of gold based on the observed decrease of the average compressive stress in the particles [43,44]:
P¼
2 fa 3
coarsening of PGNs controlled by surface self-diffusion. The model illustrates that the closure of the open channels in the PGNs is associated with the higher positive average surface curvature in the near-surface regions of the particles, as compared with the particles interior. The advantage of the hollow nanoparticles fabrication method proposed in the present work is that it involves a transition from the open to close porosity in the course of PGNs annealing. Thus, the connectivity of the internal porosity with the outside environment can be controlled by a proper selection of annealing conditions. Therefore, the hollow nanoparticles produced by the proposed method can be “fine-tuned” for specific tasks, i.e. in drug delivery or theranostics. Acknowledgment Partial support of the Russell Berrie Nanotechnology Institute at the Technion and of the Deutsche Forschungsgemeinschaft (DFG Scha 632/20) is heartily acknowledged. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.actamat.2015.09.024.
(4) References
where P is the average compressive pressure inside the particle, f is the surface stress, and a is the volume-specific surface area of the PGNs. The value of the compressive pressure can be derived from the relative change of crystal lattice parameter
Da 1 P ¼ a 3K
(5)
where K ¼ 220 GPa is the bulk modulus of Au. Depending on the employed model of porosity [44], we obtained the value of f ¼ 1.2 J/ m2 for the array of spherical pores, and f ¼ 1.8 J/m2 for cylindrical pores. These values are in a reasonable agreement with the experimentally determined surface stress of small gold nuclei (1.14 J/m2 at 50 C) [45]. Though no direct comparison of surface stress and surface energy is possible, it should be noted that the values of surface stress obtained in the present work are in a good agreement with the value of 1.36 J/m2 for the surface energy of Au [2,33]. We would like to emphasize that while the compressive nature of the capillary-induced stress is obvious for the solid nanoparticles [45], this is not the case for the PGNs exhibiting both convex and concave regions of internal porosity. The application of WeissmüllereCahn model [43,44] is essential in this case.
5. Conclusions In this work, we proposed a new fabrication method of hollow and doughnut-like gold nanoparticles relying on diffusioncontrolled processes of dewetting, dealloying and thermal coarsening. The optimal coarsening conditions for fabrication of hollow nanoparticles imply annealing of PGNs at 350 C for 30e60 min in ambient air. It is shown that thermal coarsening is accompanied by the decrease of the capillary induced compressive stresses in the PGNs. The value of surface stress of Au estimated employing the WeissmüllereCahn equation was 1.2e1.8 J/m2. We also demonstrated higher thermal stability of PGNs during annealing in vacuum as compared with annealing in air. We proposed a qualitative two-dimensional model for thermal
[1] A.Y. Chen, S.S. Shi, Y.D. Qiu, X.F. Xie, H.H. Ruan, J.F. Gu, D. Pan, Pore-size tuning and optical performances of nanoporous gold films, Micropor. Mesopor. Mat. 202 (2015) 50e56. [2] A.M. Schwartzberg, T.Y. Olson, C.E. Talley, J.Z. Zhang, Synthesis, characterization, and tunable optical properties of hollow gold nanospheres, J. Phys. Chem. B 110 (2006) 19935e19944. [3] M. Shao, K. Shoemaker, A. Peles, K. Kaneko, L. Protsailo, Pt monolayer on porous PdeCu alloys as oxygen reduction electrocatalysts, J. Am. Chem. Soc. 132 (2010) 9253e9255. [4] A.M. Hodge, J.R. Hayes, J.A. Caro, J. Biener, A.V. Hamza, Characterization and mechanical behavior of nanoporous gold, Adv. Eng. Mater. 8 (2006) 853e857. [5] H. Shen, J. You, G. Zhang, A. Ziemys, Q. Li, L. Bai, X. Deng, D.R. Erm, X. Liu, C. Li, M. Ferrari, Cooperative, nanoparticle-enabled thermal therapy of breast cancer, Adv. Healthc. Mater. 1 (2012) 84e89. €sele, M. Zacharias, Formation of nanotubes and hollow nano[6] H.J. Fan, U. Go particles based on Kirkendall and diffusion processes: a review, Small 3 (2007) 1660e1671. [7] Y. Sun, Y. Xia, Shape-controlled synthesis of gold and silver nanoparticles, Science 298 (2002) 2176e2179. [8] S. Preciado-Flores, D. Wang, D.A. Wheeler, R. Newhouse, J.K. Hensel, A. Schwartzberg, L. Wang, J. Zhu, M. Barboza-Flores, J.Z. Zhang, Highly reproducible synthesis of hollow gold nanospheres with near infrared surface plasmon absorption using PVP as stabilizing agent, J. Mater. Chem. 21 (2011) 2344e2350. [9] J.G. Railsback, A.C. Johnston-Peck, J. Wang, J.B. Tracy, Size-dependent nanoscale Kirkendall effect during the oxidation of nickel nanoparticles, ACS Nano 4 (2010) 1913e1920. [10] H.J. Fan, M. Knez, R. Scholz, K. Nielsch, E. Pippel, D. Hesse, M. Zacharias, € sele, Monocrystalline spinel nanotube fabrication based on the KirkenU. Go dall effect, Nat. Mater. 5 (2006) 627e631. [11] Y. Yin, R.M. Rioux, C.K. Erdonmez, S. Hughes, G.A. Somorjai, A.P. Alivisatos, Formation of hollow nanocrystals through the nanoscale Kirkendall effect, Science 304 (2004) 711e714. n, C. Cserha ti, I. Beszeda, D.L. Beke, Production of hollow hemisphere [12] G. Gloda shells by pure Kirkendall porosity formation in Au/Ag system, Appl. Phys. Lett. 97 (2010) 113109. n, C. Cserh [13] G. Gloda ati, D.L. Beke, Temperature-dependent formation and shrinkage of hollow shells in hemispherical Ag/Pd nanoparticles, Philos. Mag. 92 (2012) 3806e3812. €sele, Hollow nanostructures based on the Kirkendall effect: [14] K.N. Tu, U. Go design and stability considerations, Appl. Phys. Lett. 86 (2005) 093111. €sele, [15] Y. Yang, D.S. Kim, M. Knez, R. Scholz, A. Berger, E. Pippel, D. Hesse, U. Go M. Zacharias, Influence of temperature on evolution of coaxial ZnO/Al2O3 onedimensional heterostructures: from core - shell nanowires to spinel nanotubes and porous nanowires, J. Phys. Chem. C 112 (2008) 4068e4074. [16] C.V. Thompson, Solid-state dewetting of thin films, Annu. Rev. Mater. Res. 42 (2012) 9.1e9.36. [17] J. Erlebacher, M.J. Aziz, A. Karma, N. Dimitrov, K. Sieradzki, Evolution of nanoporosity in dealloying, Nature 410 (2001) 450e453.
A. Kosinova et al. / Acta Materialia 102 (2016) 108e115 [18] A. Forty, Corrosion micromorphology of noble metal alloys and depletion gilding, Nature 282 (1979) 597e598. [19] I.C. Cheng, A.M. Hodge, High temperature morphology and stability of nanoporous Ag foams, J. Porous Mater. 21 (2014) 467e474. [20] Ye Sun, S.A. Burger, T.J. Balk, Controlled ligament coarsening in nanoporous gold by annealing in vacuum versus nitrogen, Philos. Mag. 94 (2014) 1001e1011. [21] L.H. Qian, X.Q. Yan, T. Fujita, A. Inoue, M.W. Chen, Surface enhanced Raman scattering of nanoporous gold: smaller pore sizes stronger enhancements, Appl. Phys. Lett. 90 (2007) 153120. [22] M.P. Klein, B.W. Jacobs, M.D. Ong, S.J. Fares, D.B. Robinson, V. Stavila, G.J. Wagner, I. Arslan, Three-dimensional pore evolution of nanoporous metal particles for energy storage, J. Am. Chem. Soc. 133 (2011) 9144e9147. [23] R.N. Viswanath, V.A. Chirayath, R. Rajaraman, G. Amarendra, C.S. Sundar, Ligament coarsening in nanoporous gold: insights from positron annihilation study, Appl. Phys. Lett. 102 (2013) 253101. [24] J. Erlebacher, Mechanism of coarsening and bubble formation in high-genus nanoporous metals, Phys. Rev. Lett. 106 (2011) 225504. €sner, S. Parida, D. Kramer, C.A. Volkert, J. Weissmüller, Reconstructing a [25] H. Ro nanoporous metal in three dimensions: an electron tomography study of dealloyed gold leaf, Adv. Eng. Mater 9 (2007) 535e541. [26] L. Klinger, I. Gotman, E. Rabkin, A model of interfacial cavitation induced by vacancy supersaturation on the surface, Scr. Mater. 67 (2012) 352e355. [27] A.Y. Chen, S.S. Shi, F. Liu, Y. Wang, X. Li, J.F. Gu, X.F. Xie, Effect of annealing atmosphere on the thermal coarsening of nanoporous gold films, Appl. Surf. Sci. 355 (2015) 133e138. [28] S. Kuwano-Nakatani, T. Fujita, K. Uchisawa, D. Umetsu, Yu Kase, Yu Kowata, K. Chiba, T. Tokunaga, Sh Arai, Yu Yamamoto, N. Tanaka, M. Chen, Environment-sensitive thermal coarsening of nanoporous gold, Mater. Trans. 56 (2015) 468e472. [29] M. Salvalaglio, R. Bergamaschini, F. Isa, A. Scaccabarozzi, G. Isella, R. Backofen, A. Voigt, F. Montalenti, G. Capellini, T. Schroeder, H. von K€ anel, L. Miglio, Engineered coalescence by annealing of 3D Ge microstructures into highquality suspended layer on Si, ACS Appl. Mater. Interfaces 7 (2015) 19219e19225. [30] W.W. Mullins, Theory of thermal grooving, J. Appl. Phys. 28 (1957) 333e339.
115
[31] L. Klinger, E. Rabkin, Shape evolution by surface and interface diffusion with rigid body rotations, Acta Mater. 59 (2011) 6691e6699. [32] L. Klinger, E. Rabkin, Capillary-driven motion of nanoparticles attached to curved rigid substrates, Acta Mater. 60 (2012) 6065e6075. € bel, R. von Blanckenhagen, A study of surface diffusion on gold with an [33] H. Go atomic force microscope, Surf. Sci. 331e333 (1995) 885e890. [34] L.J. de Vreede, A. van den Berg, J.C.T. Eijkel, Nanopore fabrication by heating Au particles on ceramic substrates, Nano Lett. 15 (2015) 727e731. [35] S. Brunauer, D.L. Kantro, C.H. Weise, The surface energies of amorphous silica and hydrous amorphous silica, Can. J. Chem. 34 (1956) 1483e1496. [36] D. Amram, L. Klinger, N. Gazit, H. Gluska, E. Rabkin, Grain boundary grooving in thin films revisited: the role of interface diffusion, Acta Mater. 69 (2014) 386e396. [37] A. Kosinova, O. Kovalenko, L. Klinger, E. Rabkin, Mechanisms of solid-state dewetting of thin Au films in different annealing atmospheres, Acta Mater. 83 (2015) 91e101. [38] W.W. Mullins, G.S. Rohrer, Nucleation barrier for volume-conserving shape changes of faceted crystals, J. Am. Ceram. Soc. 83 (2000) 214e216. [39] O. Malyi, E. Rabkin, The effect of evaporation on size and shape evolution of faceted gold nanoparticles on sapphire, Acta Mater. 60 (2012) 261e268. [40] Y.K. Chen-Wiegart, S. Wang, Y.S. Chu, W. Liu, I. McNulty, P.W. Voorhees, D.C. Dunand, Structural evolution of nanoporous gold during thermal coarsening, Acta Mater. 60 (2012) 4972e4981. [41] D. Chatain, V. Ghetta, P. Wynblatt, Equilibrium shape of copper crystals grown on sapphire, Interface Sci. 12 (2004) 7e18. [42] P. Wynblatt, D. Chatain, Effects of anisotropy on the equilibrium shape of nanoscale pores at grain boundaries, Acta Mater. 61 (2013) 4572e4580. [43] J. Weissmüller, J.W. Cahn, Mean stresses in microstructures due to interface stresses: a generalization of a capillary equation for solids, Acta Mater. 45 (1997) 1899e1906. [44] J. Weissmüller, H.-L. Duan, D. Farkas, Deformation of solids with nanoscale pores by the action of capillary forces, Acta Mater. 58 (2010) 1e13. [45] J.S. Vermaak, K. Kuhlmann-Wilsdorf, Measurement of the average surface stress of gold as a function of temperature in the temperature range 50-985 C, J. Phys. Chem. 72 (1968) 4150e4154.
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Acta Materialia journal homepage: www.elsevier.com/locate/actamat
Fabrication of hollow gold nanoparticles by dewetting, dealloying and coarsening Anna Kosinova a, Dong Wang b, Peter Schaaf b, Oleg Kovalenko a, Leonid Klinger a, Eugen Rabkin a, * a b
Department of Materials Science and Engineering, Technion e Israel Institute of Technology, 32000 Haifa, Israel Chair Materials for Electronics, Institute of Materials Science and Engineering, TU Ilmenau, 98693 Ilmenau, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 June 2015 Received in revised form 29 August 2015 Accepted 13 September 2015 Available online xxx
A sequence of diffusion-controlled processes, namely, thin film dewetting, dealloying and thermal coarsening was employed for producing of hollow gold nanoparticles. The porous gold nanoparticles on silica substrate were obtained by solid state dewetting of AueAg bi-layers followed by selective dealloying of Ag. We studied in detail the last stage of the proposed method enabling the transformation of porous nanoparticles into hollow ones due to curvature-driven surface diffusion. The porous nanoparticles were annealed at 350 C in air and in vacuum and characterized by high-resolution scanning electron microscopy. The microstructure evolution of the particles during thermal treatment was studied by in-situ X-ray diffraction. The observed decrease of compressive strain in the particles during coarsening was discussed in terms of WeissmüllereCahn model. Annealing in ambient air resulted in faster coarsening of gold nanoparticles compared to annealing in vacuum. The isolated pores trapped in the particle bulk as well as the pores located at the particle/substrate interface were observed in the particles annealed in air. A qualitative model illustrating the observed coarsening behavior is proposed. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Surface Diffusion Coarsening Nanoporous Hollow nanoparticles
1. Introduction Hollow and porous metallic nanostructures attract great attention due to their high surface-to-volume ratio and resulting from it tunable optical [1,2], electrocatalytic [3] and mechanical [4] properties. Hollow nanoparticles are considered as promising candidates for a number of biomedical applications. For example, hollow gold nanoparticles are much more efficient than their solid counterparts at heat generation when exposed to near-infrared radiation, which makes them attractive for cancer theranostic applications [5]. The most widespread methods for the synthesis of hollow nanostructures are based on the Kirkendall effect [6] and on galvanic replacement reactions [2,7,8]. The latter method allows reproducible synthesis of hollow nanoparticles, but they often exhibit polycrystalline microstructure and high density of defects [8]. The hollowing process induced by the Kirkendall effect refers to the interdiffusion in the coreeshell nanostructures accompanied by
* Corresponding author. E-mail address: [email protected] (E. Rabkin). http://dx.doi.org/10.1016/j.actamat.2015.09.024 1359-6454/© 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
the flux of vacancies from the low diffusivity shell into the high diffusivity core. Ultimately, vacancies accumulate and collapse into a void. The Kirkendall effect-based methods have been applied for manufacturing different nanostructured systems with various compositions and shapes, such as partially oxidized Ni(core)/ NiO(shell) nanoparticles [9], spinel ZnAl2O4 nanotubes [10], hollow CoS nanocrystals [11] etc. The hollow nanoparticles of AgeAu and AgePd alloys were successfully synthesized via the bulk Kirkendall effect from the hemispherical Ag/Au [12] and Ag/Pd [13] coreeshell nanoparticles. It is worth noting that high temperatures required for Kirkendall hollowing based on bulk interdiffusion result in capillary instability [14] and transformation of hollow nanostructure into a porous [15] or a solid one. Here we propose an alternative route for producing hollow metal nanoparticles based on surfaceeand interface diffusionassisted processes of dewetting, dealloying and thermal coarsening. The thin film dewetting process, or formation of isolated particles from thin solid films exposed to high temperature, is driven by surface/interface energy minimization [16]. The next stage, dealloying, allows obtaining the nanoparticles with interconnected open porosity via selective dissolution of the less noble alloy constituent, accompanied by rearrangement of the more
A. Kosinova et al. / Acta Materialia 102 (2016) 108e115
noble atoms into stable clusters [17]. This rearrangement during dealloying is controlled by surface diffusion of the more noble atoms, while the role of bulk diffusion is negligible [18]. Asdealloyed nanoparticles exhibit three-dimensional bi-continuous structure with pore dimensions in the range of tens of nanometers. Finally, during the coarsening stage the open porosity transforms into the metastable closed one by means of surface diffusion. Thermal treatments are widely employed to modify nanoporous structure and achieve various ligament sizes of nanoporous metals [19e21]. Ligament coarsening by means of surface diffusion occurs at relatively low homological temperatures. The studies of pore morphology evolution in palladium nanoparticles [22] showed that pore shrinkage is governed by surface diffusion at temperatures up to 200 C, and above 200 C grain boundary diffusion becomes essential. Low temperature coarsening of nanoporous gold was observed by Qian et al. [21] during annealing at 200 C for 2 h in ambient air, and by Viswanath and coworkers [23] after thermal treatment for a longer time (6 h) at lower temperature (85 C). Positron lifetime experiments showed that onset of thermal instability is related to the temperature at which the vacancies become mobile [23]. We suggest that surface diffusion in porous nanoparticles should not only result in pores coarsening, but also lead to the closure of porosity in the near-surface region of the nanoparticles and formation of hollow nanoparticles. The possibility of formation of closed pores (“bubbles”) during coarsening of porous gold nanoparticles (PGNs) at elevated temperatures was demonstrated in kinetic Monte Carlo simulations of Erlebacher [24]. These bubbles were comparable in size to the initial pores in the as-dealloyed microstructure, and their formation was related to the Rayleigh instability of hollow cylinders. The first experimental evidence of enclosed voids in the as-dealloyed microstructure of €sner et al. [25]. The voids bulk nanoporous gold was provided by Ro were extremely small, 1e5 nm in diameter, and they could not form by surface diffusion alone, since during the dealloying the latter €sner et al. proposed that such requires the access of electrolyte. Ro pores may form as a result of condensation of excess vacancies produced by dealloying [25,26]. This study is focused on thermal coarsening of PGNs aimed at fabrication of the hollow nanoparticles. The thermal instability of PGNs is characterized as a function of coarsening time (30e420 min) and annealing atmosphere (vacuum and air). 2. Experimental 2.1. Sample preparation Porous gold nanoparticles were prepared from thin Au/Ag bilayer films (10 nm Au/20 nm Ag) deposited on SiO2/Si substrates by electron beam evaporation. 200 nm thermal SiO2 were grown on the Si substrates prior to deposition in order to prevent the reaction between the substrate and the subsequently deposited films. Firstly, the dewetting annealing of the bi-layer films was performed at 900 C in Ar for 15 min. Isolated AueAg alloy nanoparticles formed as a result of dewetting are shown in Fig. 1. Afterwards, silver was selectively removed from the alloy nanoparticles in a nitric acid. The samples were submerged in a 65 wt.% HNO3 aqueous solution at 21 C for 5 min. As-prepared porous gold nanoparticles samples were annealed in a mullite tube resistance furnace at 350 C in ambient air and in vacuum (106 Torr). Three consecutive heat treatments, 30 min each, were performed. 2.2. Sample characterization As-dealloyed and annealed samples were characterized by highresolution scanning electron microscope (HR-SEM, Zeiss Ultra
109
Fig. 1. SEM micrograph of as-dewetted AueAg nanoparticles.
Plus), operating at an acceleration voltage of 3 kV. Cross-section samples were prepared in a dual-beam focused ion beam (FIB; FEI Strata 400-S). To characterize the structural evolution during thermal treatment we used in-situ heating X-ray diffraction (XRD) system (Rigaku SmartLab) equipped with a high-temperature heating stage. The measurements were performed in a parallel beam configuration, and monochromatization of the beam was performed using a 2-bounce Ge(220) monochromator. In-situ XRD data were collected before and after annealing, and at the temperature of 350 C and annealing times ranging from 30 to 420 min. The measurements were carried out in low-vacuum mode (101 Torr). The diffraction patterns were recorded in qe2q scanning mode using Cu Ka radiation (l ¼ 1.5406 Å) in the range of 2q between 37.5 and 38.5 . Data collection time was 90 s per scan. 3. Experimental results PGNs with a diameter ranging from 100 to 500 nm were prepared on SiO2/Si substrates using dewetting process followed by dealloying. The scanning electron microscopy (SEM) micrographs in Fig. 2a, b shows the typical structure of as-dealloyed PGNs, which consists of interconnected ligaments and pores. The existence of small closed pores inside the individual ligaments (similar to those €sner et al. [25] in the bulk nanoporous gold) cannot observed by Ro be excluded based on the cross-sectional SEM micrograph in Fig. 2b. The coarsening temperature 350 C was selected in order to ensure that surface self-diffusion of Au is fast enough for morphology changes, whereas the bulk self-diffusion of Au is too slow for total closing of porosity and for the formation of solid nanoparticles with the shape corresponding to thermodynamic equilibrium. Taking into account that surface adsorbates may change the surface diffusion rate, we studied the effect of annealing atmosphere by performing heat treatment in vacuum and in ambient air. The microstructural evolution of PGNs annealed in vacuum, is shown in Fig. 2c, d. It is seen that heat treatment caused an increase in the ligament width from 10 nm for as-dealloyed sample to 35 nm for the sample annealed in vacuum at 350 C for 30 min. An average value of ligament width was measured at the narrowest point of each ligament and was estimated from measurements of about 100 separate ligaments in SEM and cross-sectional SEM images. After the heat treatment for 60 and 90 min, the ligament width remained unchanged with a value of 37 ± 2 nm. The morphology changes of PGNs after two consecutive
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Fig. 2. (a, c) General view and (b, d) cross-sectional SEM images of PGNs: (a, b) as-dealloyed and (c, d) annealed in vacuum for 30 min.
annealings in vacuum and in air are shown in Fig. 3b, c and Fig. 3e, f, respectively. In both cases, one and the same region of the substrate is shown and it is evident that thermal coarsening occurs after the
first annealing. The curvature-driven surface diffusion transports the material from the regions of positive curvature to the junctions between the ligaments. Thus, the mean curvature of the ligaments
Fig. 3. SEM images of PGNs: (a, d) as-dealloyed; annealed in vacuum for (b) 30 min, (c) 60 min; annealed in air for (e) 30 min, (f) 60 min. 60 min is the cumulative annealing time after two annealing stages of 30 min each.
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diminishes and the total volume of the pores decreases. Initial small pores collapse into bigger ones. During the following stages of the heat treatment, the samples were relatively stable, with little changes in their morphology. As seen in Fig. 3, coarsening occurred much faster during annealing in air compared to that in vacuum. The near-surface ligaments merged into a continuous layer. Similar results have been obtained by Sun at al [20]. It was found that surface diffusion in nanoporous gold proceeds faster in nitrogen than in vacuum, which may be explained by the easier generation of vacancies and gold adatoms in the presence of adsorbates. The effect of annealing atmosphere on coarsening kinetics of nanoporous gold was also studied in the recent works of Chen et al. [27] and KuwanoeNakatani et al. [28]. Both studies demonstrated the low thermal stability of thin nanoporous gold films in oxidizing atmosphere. In Ref. [28] the activation energy for the thermal coarsening of nanoporous gold in air was shown to be only half of that in vacuum. We revealed that after heat treatment in air, some particles develop large single through holes (Fig. 4). Yet, the vast majority of nanoparticles exhibits a rough surface with multiple concavities (Fig. 3e, f) but without open pores. We performed the in-situ SEM heating of the PGNs, which allowed us to observe the kinetics of thermal coarsening in vacuum (105 Torr). The experimental conditions were close to those in the annealing furnace. The onset of coarsening was observed at the temperature of about 160 C. At higher temperatures (up to 500 C) the kinetics of morphological changes was slow. Since the e-beam induced coating of carbonaceous material on the surface of PGNs which is inevitable in the SEM chamber may inhibit the surface self-diffusion on Au, we performed the in-situ XRD annealing experiments. Because of the strong [111] texture of the as-deposited Au and Ag films inherited by the single crystal PGNs, tracking the position of the (111) Bragg reflection provides accurate information on the average strain in the particles. Fig. 5 shows the evolution of the Au (111) Bragg reflection as a function of temperature and annealing time. Heating from room temperature to 350 C results in a significant shift of the diffraction peak to the lower values of 2q due to thermal expansion of the crystal lattice. Within the time span of the annealing process from 30 to 420 min only small, insignificant changes in the position of the (111) diffraction peak are observed. After cooling the sample to room temperature, the diffraction peak position shifts back to the higher values of 2q (smaller d-spacing). However, the fact of the particular importance is the new position of the peak, which does not coincide with the one before annealing. The additional diffraction maximum at 37.7 is attributed to the response from aluminum nitride holder of the
Fig. 4. SEM image of a through hole formed after annealing of PGNs in air for 30 min.
111
Fig. 5. The in-situ X-ray diffraction spectra of PGNs. The red and the black spectra refer to PGNs prior and after the heat treatment, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
heating stage (see Fig. S1.1 in the Supplementary Information), which becomes apparent in case of a small sample size. 4. Discussion Considering the rough surface of PGNs annealed in air (Fig. 3e, f) we supposed that the particles which are fully closed from outside still may contain pores inside. We observed both the pores encapsulated in the particle volume and the pores located at the particle/substrate interface (Fig. 6). The SEM images in Fig. 7e, j shows the FIB cross-sections of PGNs annealed in air for 60 min. It is obvious that thermal treatment results in closing of porosity that occurs faster at the surface than inside the particle. Based on the results of kinetic Monte Carlo simulations, Erlebacher attributed formation of small closed pores during coarsening of PGNs to the Rayleigh instability of cylindrical cavities [24]. Once formed, such closed pores become stagnant since they are cut off from the fast surface diffusion paths. They can be only eliminated by the pinch-off events of the nearby ligaments, or the ligaments they reside in [24]. Yet this scenario implies that the closed
Fig. 6. Cross-sectional SEM image of PGNs annealed in air at 350 C for 60 min.
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Fig. 7. Schematic illustration of morphology evolution of Au nanoparticle and corresponding cross-sectional SEM images: (aee) formation of closed pore inside the particle, (fej) formation of pore at the particle/substrate interface. Cross-sections of the particles are taken after annealing the sample in air at 350 C for 60 min. The annealing times are (a,f) 0; 4 kT (b) 15.2t0; (c) 15.4 t0; (d) 19.9 t0; (g) 2.6 t0; (h) 14.4 t0; (i) 63.5 t0, with t0 ¼ ð0:1LÞ . 2 nDU g s
s
s
pores should be homogeneously distributed through the volume of the nanoparticles. Indeed, many pores residing close to the nanoparticle surface were observed in simulations [24]. On the contrary, in the present study the heat treatments in air resulted in full closure of the near-surface porosity (see Fig. 3e, f), with concomitant development of the internal closed pores of the sizes exceeding the initial average ligament size (see Figs. 6 and 7e, j). This behavior can be understood in terms of the contribution of bulk diffusion, and the topology of the PGNs. While the contribution of bulk diffusion to the development of porous microstructure is negligible during the dealloying at room temperature [18], its importance increases with increasing temperature because the activation energy for bulk self-diffusion is significantly higher than that of surface self-diffusion. We performed an approximate estimate of the dissolution time of the closed pore trapped inside the nanoparticle or the individual ligament, shrinking by the volume diffusion mechanism (see Supplementary Information). The corresponding estimate for the internal pore of 20 nm in radius located at the distance of 60 nm
from the surface (see Fig. 7e) indicates that the time of pore dissolution by bulk diffusion at 350 C is about 92 h, much longer than the annealing times employed in the present study. At the same time, a small pore of 2.5 nm in radius residing at the distance of 5 nm from the ligament surface (the pores of this type were observed both in the as-dealloyed nanoporous gold [25], and in simulations of Erlebacher [24]) will fully dissolve after only 7 min of annealing, shorter than the shortest annealing time employed in this study. Thus, we can conclude that only relatively large pores formed at the late stages of coarsening can survive the heat treatments at 350 C. At the late stages of coarsening the characteristic dimensions of the ligaments and channels become comparable with the overall dimensions of the particle (see Fig. 2c, d). Therefore, we believe that at these stages a deterministic process of surface self-diffusion, rather than perturbation-controlled Rayleigh instability of long perfect cylinders is responsible for the development of closed pores. The formation of closed porosity is associated with the fact that average surface curvature at the surface of PGNs is more
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positive than inside the particles. Thus, pores shrinkage is faster close to the surface, which results in gradual closing of the porosity and trapping of isolated pores inside the particles, or at the particle/ substrate interface. A similar idea has recently guided Salvalaglio et al. to design a method of producing suspended Ge thin film on Si substrate [29]. They have grown an ordered, dense array of Ge pillars on the Si substrate. Annealing of these arrays at 450 C resulted in closure of the gaps between the pillars at their outer ends and formation of continuous Ge film attached to the substrate by the lower parts of the pillars. Also in this case, the higher average surface curvature of the pillars at their outer ends as compared with their bottom parts resulted in formation of continuous film at the pillars top. In what follows, we will illustrate this coarsening behavior with the aid of a simple two-dimensional model, assuming the rectangular form of channels in the nanoparticle. We will consider the late stages of coarsening, at which the characteristic size of the individual channel is comparable with the nanoparticle outer dimensions. The detailed description of the model is given in the Supplementary Information, while here only the main assumptions and the results of simulations will be presented. We suppose that kinetics of the process is determined by the self-diffusion of Au on the surface and along the particle/substrate interface only (bulk self-diffusion is negligible). The corresponding diffusion fluxes are driven by the gradients of chemical potential of Au atoms. Divergence of the diffusion flux leads to accretion of Au atoms on the surface and at the interface. The capillary-related part of the chemical potential of surface atoms scales with a local curvature, ks, of the surface. The corresponding Mullins equation [30] describes a normal velocity, Vn, of the surface
Vn ¼ Bs
v2 ks vs2
(1)
where s is the arc length along the surface,
Bs ¼
ns Ds U2 gs kT
(2)
is the Mullins coefficient for the surface, and ns, Ds, gs, and U are the area density of the mobile surface atoms, the surface self-diffusion coefficient of Au, the surface energy of Au and its atomic volume, respectively. The factor kT has its usual thermodynamic meaning. The self-diffusion of Au along the rigid particle/substrate interface cannot be described in a similar way. The accretion of Au atoms at the rigid interface leads to rigid-body translation (normal to the substrate) and rotation of the particle with respect to the substrate. It was shown by Klinger and Rabkin that in this case the chemical potential of mobile Au atoms at the interface is a non-local function depending on the interface shape and on boundary conditions at the interface termini [31,32]. For the flat interface it is a cubical parabola with respect to the coordinate, s, along the interface:
mðsÞ ¼ a0 þ a1 s þ a2 s2 þ a3 s3
(3)
The coefficients of this polynomial can be found by calculating the variations of the total surface/interface energy of the particle caused by infinitesimal rigid normal translation (dy) and rotation (dq) of the particle with respect to the substrate. The rigid translation and rotation rates (Vy and Vq, respectively) scale with the selfdiffusion coefficient of Au atoms along the particle/substrate interface (see details in Refs. [31,32]). Thus the total displacement of the free surfaces of the particle (including the internal surfaces of the pores) is a sum of the local normal displacement due to surface diffusion (Vn), and rigid translation and rotation (Vy and Vq). To
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preserve the integrity of the particle (in the two-dimensional model an open channel reaching to the substrate separates the particle into two disconnected sub-particles), we imposed an additional limitation that the two sub-particles move as a single rigid solid (see Supplementary Information). The values for the surface and interface energies employed in the model were taken from the literature: the surface free energy for pure gold is 1.36 J/m2 [33], the value for Au/SiO2 interfacial energy is 1.17 J/m2 [34], and the corresponding energy for silica substrate is 0.2 J/m2 [35]. Moreover, we assumed that the self-diffusion coefficients of Au on the surface and along the particle/substrate interface are identical. Fig. 7aed illustrates that closing of the open channel that does not reach to the substrate results in a pore trapped in the bulk of the particle. On the contrary, closing of the channel which initially reaches to the substrate results in a pore at the particle/substrate interface (Fig. 7fei), and in some cases may leave both the interface and bulk pores (not shown here). In both cases, the rapid closure of the open channel is associated with a high surface curvature of the channel near the surface of the particle. While the bulk pore shrinks by a slow process of bulk self-diffusion, the shrinking of the interface pore is controlled by the Au atoms self-diffusion along the particle/substrate interface, which is much faster than the bulk selfdiffusion [36,37]. A combination of high contact angle of solid Au on silica and of the open nature of porosity in the as-dealloyed PGNs (see Fig. 2a, b) inevitably means that a large number of interface pores should form at the initial stages of coarsening. The fact that after prolonged annealing these pores disappear in a number of particles (see. Fig. 7e) indicates that the rate of Au-self diffusion along the Au/silica interface is comparable to the rate of surface self-diffusion on Au. If several interface pores coalesce into bigger ones (Fig. 6), the dissolution time of the formed pore increases. Also, Fig. 7d, i clearly illustrate that surface concavities of the particles are associated with the closure of the channels in the nearsurface region. Once the channel is closed, the surface curvature decreases and the rate of the shape change becomes very slow. The ridges on the SiO2 substrate in the vicinity of the nanoparticles annealed in air (see Figs. 3e, f, and 4) clearly indicate that the nanoparticles shrink due to the increase of their average density. It is interesting that such ridges are less developed in the case of the nanoparticles annealed in vacuum, again indicating the slower pace of densification of the latter. We would like to emphasize that in all experiments, most of coarsening has occurred at the onset of annealing, and then the process came to a nearly complete standstill. An obvious reason for this slow-down would be a well-known fact that the rate of surface shape change by surface diffusion scales with the characteristic size of the system, l, as l4 [30]. This means that the increase of the typical ligament size from 10 to 35 nm after the first anneal means the decrease of the coarsening rate by a factor of 150. Yet our experimental data provide some indications that the actual slowdown of coarsening is even stronger than dictated by the l4 law. Indeed, Fig. 3d, e, demonstrate a partial merging of two adjacent PGNs after the first 30 min annealing. The sharp protrusion with a highly curved tip (radius of curvature is 17 nm) formed as a result of this merge remained nearly unchanged after the second anneal for 30 min (Fig. 3f). In our opinion, there are two possible reasons for this increase in thermal stability of the partially coarsened particles: (i) Surface faceting, which is known to reduce the rate of capillary-driven shape evolution due to the difficulties of step nucleation on atomically flat surfaces [38e40]; (ii) Segregation of residual impurities in PGNs (most notably, S and C) at the particle surface and concomitant decrease of the surface self-diffusion coefficient [41]. Such segregation
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can also explain faster coarsening rates in air as compared with the annealing in vacuum: the sulfur atoms segregated on particle surface may react with oxygen forming the volatile SO2 and thus reduce surface coverage by the segregant. It should be noted that both mentioned reasons for slow-down of coarsening should increase the thermal stability of the pores trapped inside the coarsened particles well beyond that dictated by bulk diffusion [42]. This is because atomically flat or contaminated internal surfaces of the pore are unlikely to serve as sources of vacancies. Our in-situ heating experiments in SEM confirm the stability of PGNs at the temperatures at which bulk diffusion is significant. The same factors are also responsible for the high stability of the “doughnut-like” nanoparticles with a single through hole (Fig. 4). The in-situ XRD annealing experiments revealed shift of the Au (111) Bragg reflection (Dd/d ¼ 1.5 103) after cooling the sample relatively to its position before annealing. This fact is apparently related to the decrease in capillary-induced compressive stresses in the particles upon pores coarsening. We employed the WeissmüllereCahn equation for estimating the surface stress of gold based on the observed decrease of the average compressive stress in the particles [43,44]:
P¼
2 fa 3
coarsening of PGNs controlled by surface self-diffusion. The model illustrates that the closure of the open channels in the PGNs is associated with the higher positive average surface curvature in the near-surface regions of the particles, as compared with the particles interior. The advantage of the hollow nanoparticles fabrication method proposed in the present work is that it involves a transition from the open to close porosity in the course of PGNs annealing. Thus, the connectivity of the internal porosity with the outside environment can be controlled by a proper selection of annealing conditions. Therefore, the hollow nanoparticles produced by the proposed method can be “fine-tuned” for specific tasks, i.e. in drug delivery or theranostics. Acknowledgment Partial support of the Russell Berrie Nanotechnology Institute at the Technion and of the Deutsche Forschungsgemeinschaft (DFG Scha 632/20) is heartily acknowledged. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.actamat.2015.09.024.
(4) References
where P is the average compressive pressure inside the particle, f is the surface stress, and a is the volume-specific surface area of the PGNs. The value of the compressive pressure can be derived from the relative change of crystal lattice parameter
Da 1 P ¼ a 3K
(5)
where K ¼ 220 GPa is the bulk modulus of Au. Depending on the employed model of porosity [44], we obtained the value of f ¼ 1.2 J/ m2 for the array of spherical pores, and f ¼ 1.8 J/m2 for cylindrical pores. These values are in a reasonable agreement with the experimentally determined surface stress of small gold nuclei (1.14 J/m2 at 50 C) [45]. Though no direct comparison of surface stress and surface energy is possible, it should be noted that the values of surface stress obtained in the present work are in a good agreement with the value of 1.36 J/m2 for the surface energy of Au [2,33]. We would like to emphasize that while the compressive nature of the capillary-induced stress is obvious for the solid nanoparticles [45], this is not the case for the PGNs exhibiting both convex and concave regions of internal porosity. The application of WeissmüllereCahn model [43,44] is essential in this case.
5. Conclusions In this work, we proposed a new fabrication method of hollow and doughnut-like gold nanoparticles relying on diffusioncontrolled processes of dewetting, dealloying and thermal coarsening. The optimal coarsening conditions for fabrication of hollow nanoparticles imply annealing of PGNs at 350 C for 30e60 min in ambient air. It is shown that thermal coarsening is accompanied by the decrease of the capillary induced compressive stresses in the PGNs. The value of surface stress of Au estimated employing the WeissmüllereCahn equation was 1.2e1.8 J/m2. We also demonstrated higher thermal stability of PGNs during annealing in vacuum as compared with annealing in air. We proposed a qualitative two-dimensional model for thermal
[1] A.Y. Chen, S.S. Shi, Y.D. Qiu, X.F. Xie, H.H. Ruan, J.F. Gu, D. Pan, Pore-size tuning and optical performances of nanoporous gold films, Micropor. Mesopor. Mat. 202 (2015) 50e56. [2] A.M. Schwartzberg, T.Y. Olson, C.E. Talley, J.Z. Zhang, Synthesis, characterization, and tunable optical properties of hollow gold nanospheres, J. Phys. Chem. B 110 (2006) 19935e19944. [3] M. Shao, K. Shoemaker, A. Peles, K. Kaneko, L. Protsailo, Pt monolayer on porous PdeCu alloys as oxygen reduction electrocatalysts, J. Am. Chem. Soc. 132 (2010) 9253e9255. [4] A.M. Hodge, J.R. Hayes, J.A. Caro, J. Biener, A.V. Hamza, Characterization and mechanical behavior of nanoporous gold, Adv. Eng. Mater. 8 (2006) 853e857. [5] H. Shen, J. You, G. Zhang, A. Ziemys, Q. Li, L. Bai, X. Deng, D.R. Erm, X. Liu, C. Li, M. Ferrari, Cooperative, nanoparticle-enabled thermal therapy of breast cancer, Adv. Healthc. Mater. 1 (2012) 84e89. €sele, M. Zacharias, Formation of nanotubes and hollow nano[6] H.J. Fan, U. Go particles based on Kirkendall and diffusion processes: a review, Small 3 (2007) 1660e1671. [7] Y. Sun, Y. Xia, Shape-controlled synthesis of gold and silver nanoparticles, Science 298 (2002) 2176e2179. [8] S. Preciado-Flores, D. Wang, D.A. Wheeler, R. Newhouse, J.K. Hensel, A. Schwartzberg, L. Wang, J. Zhu, M. Barboza-Flores, J.Z. Zhang, Highly reproducible synthesis of hollow gold nanospheres with near infrared surface plasmon absorption using PVP as stabilizing agent, J. Mater. Chem. 21 (2011) 2344e2350. [9] J.G. Railsback, A.C. Johnston-Peck, J. Wang, J.B. Tracy, Size-dependent nanoscale Kirkendall effect during the oxidation of nickel nanoparticles, ACS Nano 4 (2010) 1913e1920. [10] H.J. Fan, M. Knez, R. Scholz, K. Nielsch, E. Pippel, D. Hesse, M. Zacharias, € sele, Monocrystalline spinel nanotube fabrication based on the KirkenU. Go dall effect, Nat. Mater. 5 (2006) 627e631. [11] Y. Yin, R.M. Rioux, C.K. Erdonmez, S. Hughes, G.A. Somorjai, A.P. Alivisatos, Formation of hollow nanocrystals through the nanoscale Kirkendall effect, Science 304 (2004) 711e714. n, C. Cserha ti, I. Beszeda, D.L. Beke, Production of hollow hemisphere [12] G. Gloda shells by pure Kirkendall porosity formation in Au/Ag system, Appl. Phys. Lett. 97 (2010) 113109. n, C. Cserh [13] G. Gloda ati, D.L. Beke, Temperature-dependent formation and shrinkage of hollow shells in hemispherical Ag/Pd nanoparticles, Philos. Mag. 92 (2012) 3806e3812. €sele, Hollow nanostructures based on the Kirkendall effect: [14] K.N. Tu, U. Go design and stability considerations, Appl. Phys. Lett. 86 (2005) 093111. €sele, [15] Y. Yang, D.S. Kim, M. Knez, R. Scholz, A. Berger, E. Pippel, D. Hesse, U. Go M. Zacharias, Influence of temperature on evolution of coaxial ZnO/Al2O3 onedimensional heterostructures: from core - shell nanowires to spinel nanotubes and porous nanowires, J. Phys. Chem. C 112 (2008) 4068e4074. [16] C.V. Thompson, Solid-state dewetting of thin films, Annu. Rev. Mater. Res. 42 (2012) 9.1e9.36. [17] J. Erlebacher, M.J. Aziz, A. Karma, N. Dimitrov, K. Sieradzki, Evolution of nanoporosity in dealloying, Nature 410 (2001) 450e453.
A. Kosinova et al. / Acta Materialia 102 (2016) 108e115 [18] A. Forty, Corrosion micromorphology of noble metal alloys and depletion gilding, Nature 282 (1979) 597e598. [19] I.C. Cheng, A.M. Hodge, High temperature morphology and stability of nanoporous Ag foams, J. Porous Mater. 21 (2014) 467e474. [20] Ye Sun, S.A. Burger, T.J. Balk, Controlled ligament coarsening in nanoporous gold by annealing in vacuum versus nitrogen, Philos. Mag. 94 (2014) 1001e1011. [21] L.H. Qian, X.Q. Yan, T. Fujita, A. Inoue, M.W. Chen, Surface enhanced Raman scattering of nanoporous gold: smaller pore sizes stronger enhancements, Appl. Phys. Lett. 90 (2007) 153120. [22] M.P. Klein, B.W. Jacobs, M.D. Ong, S.J. Fares, D.B. Robinson, V. Stavila, G.J. Wagner, I. Arslan, Three-dimensional pore evolution of nanoporous metal particles for energy storage, J. Am. Chem. Soc. 133 (2011) 9144e9147. [23] R.N. Viswanath, V.A. Chirayath, R. Rajaraman, G. Amarendra, C.S. Sundar, Ligament coarsening in nanoporous gold: insights from positron annihilation study, Appl. Phys. Lett. 102 (2013) 253101. [24] J. Erlebacher, Mechanism of coarsening and bubble formation in high-genus nanoporous metals, Phys. Rev. Lett. 106 (2011) 225504. €sner, S. Parida, D. Kramer, C.A. Volkert, J. Weissmüller, Reconstructing a [25] H. Ro nanoporous metal in three dimensions: an electron tomography study of dealloyed gold leaf, Adv. Eng. Mater 9 (2007) 535e541. [26] L. Klinger, I. Gotman, E. Rabkin, A model of interfacial cavitation induced by vacancy supersaturation on the surface, Scr. Mater. 67 (2012) 352e355. [27] A.Y. Chen, S.S. Shi, F. Liu, Y. Wang, X. Li, J.F. Gu, X.F. Xie, Effect of annealing atmosphere on the thermal coarsening of nanoporous gold films, Appl. Surf. Sci. 355 (2015) 133e138. [28] S. Kuwano-Nakatani, T. Fujita, K. Uchisawa, D. Umetsu, Yu Kase, Yu Kowata, K. Chiba, T. Tokunaga, Sh Arai, Yu Yamamoto, N. Tanaka, M. Chen, Environment-sensitive thermal coarsening of nanoporous gold, Mater. Trans. 56 (2015) 468e472. [29] M. Salvalaglio, R. Bergamaschini, F. Isa, A. Scaccabarozzi, G. Isella, R. Backofen, A. Voigt, F. Montalenti, G. Capellini, T. Schroeder, H. von K€ anel, L. Miglio, Engineered coalescence by annealing of 3D Ge microstructures into highquality suspended layer on Si, ACS Appl. Mater. Interfaces 7 (2015) 19219e19225. [30] W.W. Mullins, Theory of thermal grooving, J. Appl. Phys. 28 (1957) 333e339.
115
[31] L. Klinger, E. Rabkin, Shape evolution by surface and interface diffusion with rigid body rotations, Acta Mater. 59 (2011) 6691e6699. [32] L. Klinger, E. Rabkin, Capillary-driven motion of nanoparticles attached to curved rigid substrates, Acta Mater. 60 (2012) 6065e6075. € bel, R. von Blanckenhagen, A study of surface diffusion on gold with an [33] H. Go atomic force microscope, Surf. Sci. 331e333 (1995) 885e890. [34] L.J. de Vreede, A. van den Berg, J.C.T. Eijkel, Nanopore fabrication by heating Au particles on ceramic substrates, Nano Lett. 15 (2015) 727e731. [35] S. Brunauer, D.L. Kantro, C.H. Weise, The surface energies of amorphous silica and hydrous amorphous silica, Can. J. Chem. 34 (1956) 1483e1496. [36] D. Amram, L. Klinger, N. Gazit, H. Gluska, E. Rabkin, Grain boundary grooving in thin films revisited: the role of interface diffusion, Acta Mater. 69 (2014) 386e396. [37] A. Kosinova, O. Kovalenko, L. Klinger, E. Rabkin, Mechanisms of solid-state dewetting of thin Au films in different annealing atmospheres, Acta Mater. 83 (2015) 91e101. [38] W.W. Mullins, G.S. Rohrer, Nucleation barrier for volume-conserving shape changes of faceted crystals, J. Am. Ceram. Soc. 83 (2000) 214e216. [39] O. Malyi, E. Rabkin, The effect of evaporation on size and shape evolution of faceted gold nanoparticles on sapphire, Acta Mater. 60 (2012) 261e268. [40] Y.K. Chen-Wiegart, S. Wang, Y.S. Chu, W. Liu, I. McNulty, P.W. Voorhees, D.C. Dunand, Structural evolution of nanoporous gold during thermal coarsening, Acta Mater. 60 (2012) 4972e4981. [41] D. Chatain, V. Ghetta, P. Wynblatt, Equilibrium shape of copper crystals grown on sapphire, Interface Sci. 12 (2004) 7e18. [42] P. Wynblatt, D. Chatain, Effects of anisotropy on the equilibrium shape of nanoscale pores at grain boundaries, Acta Mater. 61 (2013) 4572e4580. [43] J. Weissmüller, J.W. Cahn, Mean stresses in microstructures due to interface stresses: a generalization of a capillary equation for solids, Acta Mater. 45 (1997) 1899e1906. [44] J. Weissmüller, H.-L. Duan, D. Farkas, Deformation of solids with nanoscale pores by the action of capillary forces, Acta Mater. 58 (2010) 1e13. [45] J.S. Vermaak, K. Kuhlmann-Wilsdorf, Measurement of the average surface stress of gold as a function of temperature in the temperature range 50-985 C, J. Phys. Chem. 72 (1968) 4150e4154.