1、收 稿日期 ：2008-08-08。 收修改稿日期 ：2008-10-27。国家自然科学项目 (No.20435010，20503012，20521503)和国家重点基础研究发展计划 (973 Program，2007CB936302)。通讯联系人 。 E-mail：；Tel：+86-25-83596523第一作者 ：蒋璐芸 ，女 ，21 岁 ；研究方向 ：纳米材料的分子动力学模拟 。研究简报金纳米球热力学稳定性的理论研究蒋璐芸 尹 星 赵健伟(生命分析化学教育部重点实验室 ，南京大学化学化工学院 ，南京 210093)关键词 ： 分子动力学模拟 ； 金纳米空心球 ； 坍塌中图分类号 ： O5
2、52.5 文献标识码 ： A 文章编号 ： 1001-4861(2009)01-0176-04Theoretical Investigation on the Thermal Stability of Au Hollow Nano-ParticleJIANG Lu-Yun YIN Xing ZHAO Jian-Wei(Key Lab of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering,Nanjing University, Nanjing 210093)Abstract:
3、Due to the significant importance in many applications, the thermal stability of hollow gold nano-particles has been studied theoretically by using ultra-large molecular dynamics simulation. By changing thenano-particle size while keeping the aspect ratio constant, it was found that the large gold n
4、ano-particle hasbetter thermal stability than the small ones.Key words: molecular dynamics simulation; hollow gold nano-particles; collapsed0 IntroductionHollow metal nano-particles have captured thegreat attention recently. The increased surface area, lowdensity, saving of material and concomitant
5、reduction incost coupled with the interesting optical properties ofsuch structures make these materials promising inmany fields such as catalysis1, chemo-sensing2, enca-psulation3, drug delivery4and cancer hyperthermia5.Hollow metallic nano-particles may be prepared bycoating the surface of latex pa
6、rticles6, silica beans7,8,gold and silver nano-particles by the desired materialsand then etching away the core while retention of thecore leads to the formation of thin metallic shells9.Among these systems, the gold hollow nano-particlesare very promising, and being studied intensivelyrecently10. D
7、ue to predict indeed the occurrence,moleculat dynamics was used11. Such as Chui et al. hasfocused on theoretical study of nanoparticles usingmolecular dynamics12. One vital factor predominatingthe utility of these materials in practice is the thermalstability. In order to tune the electronic propert
8、ies, theshell was synthesized as thin as possible. Under thiscircumstance, the thermal stability is especiallyconcerned. In order to investigate this issue and give aninstruction in the materials synthesis, in the presentwork, we will study theoretically the effect of aspectratio, an important facto
9、r governing the nano-particlethermal stability, by using an ultra-large moleculardynamics (MD) simulations13.第 25 卷第 1 期2009 年 1 月Vol25 No1176179无 机 化 学 学 报CHINESE JOURNAL OF INORGANIC CHEMISTRY第 1 期 蒋璐芸等 ：金纳米球热力学稳定性的理论研究1 Caculation methodThe gold hollow nano-particles was constructedwith FCC (Face
10、-Centered Cubic) gold crystal withcutting the concentric sphares, as shown in Fig.1. Theratio of out radius over inter radius is defined as theaspect ratio and denote as Rout-Rint, where R is in crystalcell corresponding to 0.408 nm per cell. 9 sampleswhose out radius, Rout, ranges from 2.04 to 20.4
11、 nm butwith the same aspect ratio Rout/Rint=1.25 were studied inthis paper. MD calculations were performed with theself-developed software NanoMD13. A time step of 1.6fs is used for all simulations. The temperature of thesystem is kept constant at 300 K during the wholeprocess using a velocity resca
12、ling method13,14. Theinteractions between gold atoms were described byMorse potnetial15and EAM potential16. It has beenproven that these potentials can provide a relevantdescription of the surface effect and defect properties ofthe transition metals with FCC structure10.2 Results and discussionBefor
13、e the relaxation of atomic configuration, thegold hollow ball display a perfect crystalline form, asshown in Fig.1a and the cross sectional view in Fig.1b.The crystallographic features can also be identifiedfrom the radius distribution function (RDF) analysis(the vertical solid lines in Fig.1d), fro
14、m which we mayfind very shape peaks at 0.7 and 1.0 of lattice constantcorresponding to the first and second nearest neighborsof gold atoms. The relaxation may optimize the relativeposition of the nearest atoms lowering the interatomicinteraction. This may reflected from the potential dropcurve as sh
15、own in Fig.1c. Unlike the most dynamicprocesses, the reconstruction of the hollow ball followsa different mechanism as evidenced by the insertedsemi-logarithmic plot. Three unique steps can beidentified, in terms of the slow variation at beginning,followed by a sudden drop of potential, and the reac
16、hingof an energy plateau finally. At the first thousand steps,the hollow ball remains in a stage of accumulatingpower; in particular, the transformation from a perfectcrystalline form needs some initiators. Therefore, at thisstage, defects and dislocations increase gradually.When the hollow ball has
17、 adequate amount of defectsand dislocations, an avalanche-like decrease ofpotential takes place. This indicates that the atomicstructure of the hollow gold nano-particle is quicklycollapsed. This character is of particular importance forthe MD simulation. Without this avalanche-likecollapse, the MD
18、simulation needs too long time toreach the stable energy plateau. When the high-energyhollow structure reaches a relative stable configuration,it may impede the further collapse. In the exponentialdecay range, a half-life of 8.6 ps indicates that therelaxation to a stable crystallographic structure
19、is veryfast. This result also inferred that the normalcharacterization technique, such as TEM andspectroscopy couldnt track this process.A sequential four representative RDF analyseswere compared in Fig.1d. At first stage (1000 steps) thefirst peak position is located at 0.718 far from the idealcrys
20、talline form, indicating that all atoms are located inan active position. With increasing the relaxation time,this peak position shows an obvious left shift. When therelaxation time is longer than 20 000 steps, the peakposition becomes stable, locating at 0.70 that is close tothe ideal value. This r
21、esult means that after adequaterelaxation, the hollow ball may reach a relatively goodcrystalline form.Fig.1 (a) Atomic configuration of the hollow gold nano-particles of model 4050; (b) Cross section of themodel system 4050; (c) Potential as a functionof the relaxation time; (d) Radius distribution
22、function before and after adequate relaxation asmarked in the potential curve177第 25 卷无 机 化 学 学 报System*Thickness ofshellFinal hollowradiusProperty45 0.204 0.0 Collapsed810 0.408 0.01215 0.612 0.01620 0.816 1.80 Semi-collapsed2025 1.020 2.752430 1.224 4.90 Stable2835 1.428 5.713240 1.632 6.534050 2.
23、040 8.16The final atomic configuration reached isdependent on the systems as demonstrated by the seriesof simulations in present work. Interestingly, the largesystem looks more stable as compared with those smallones, when they have the same aspect ratio. Fig.2presents the cross-sectional view of th
24、e systemsconcerned with various radii. From the figure we mayfind the small systems, such as 810 and 1215, havecompletely collapsed, forming a solid ball. Due to suchsignificant transformation, the final atomic structurelooks more amorphous. The systems with moderate size,such as 1620 and 2025, have
25、 experienced partialcollapse. Although the hollow remains in thesemoderate systems, we can observe the shrink of theradii of both out sphere and inter sphere. Unlike thesmall systems, they display some crystal domains, butmany defects and dislocations are visualized from thesectional view. For the l
26、arge size of the hollow ball, 2430 and 28 35 for example, their structure remainsalmost unchangeable. Very few dislocations can beobserved. This is probably due to the thick shell of thesystem, in which the dislocation is hardly generated.A detailed comparison of the thermal stability ofthe series o
27、f hollow gold nano-particles has been givenin Table 1. Even all systems have the same aspect ratio;the final atomic configuration is dependent on thethickness of the shell. There are two critical thicknessvalues. One is between 0.612 and 0.816 nm and theother between 1.020 and 1.224 nm. Below the fi
28、rstcritical thickness, the metal atoms may have adequatethermal energy to overcome the motion barrier togenerate dislocation. Then the sliding along (111) facettakes place till the complete collapse. The samplesbetween the first and second critical thickness showonly partial reconstruction around th
29、e weak connectionsdue to the discrete feature of the crystal. However, thisfeature is vanished as the hollow ball size increases.Specifically, when the size is larger than the 24 30system, the hollow ball tends to be symmetric, and theshell is thick enough to impede the generation ofdefects and prop
30、agation of dislocation. In this work, thethinnest thickness of the stable gold hollow nano-particles is estimated to be around 1.8 nm, which isreasonable as compared with those reported inliteratures. Theoretical prediction is a little smaller,because the gold hollow ball consists of perfect singlec
31、rystal in simulation. On the contrary, a mean thicknessis reported experimentally, which may have afluctuation in thickness. The stability of the real systemis dependent on the thinnest part of the shell.3 ConclusionsIn summary, the theoretical molecular dynamicssimulations were performed for the se
32、ries of hollow goldnano-particles with the same aspect ratio. The resultsconfirmed that the shell thickness is an important factorgoverning the thermal stability of the hollow ball. Whenthe shell thickness is comparable to the crystal celllattice, the discrete crystallography results in a dynamicevo
33、lution of the sample. Defects and dislocations weregenerated and propagated quickly till a completecollapse. When a thick shell is applied, the sampleFig.2 Cross sectional view of the systems afteradequate relaxationTable 1 Summary of the stability of the hollow goldnano-particles with different rad
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