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Appl Phys A 2011 102 333 337 DOI 10 1007 s00339 010 5971 y Comparative investigation on decorating carbon nanotubes with different transition metals J Y Guo C X Xu Received 12 November 2009 Accepted 6 April 2010 Published online 31 July 2010 Springer Verlag 2010 Abstract The diffusion dynamics and structure evolvement of the transition metal TM Ni Cu Au and Pt atoms decorating carbon nanotubes CNTs with differences have been systematically studied by Monte Carlo MC simula tion The studies show that TM atoms can be encapsulated inside aggregated and even wrapped on the surface of the CNT which depend on the interactions among TM TM and TM C during the spontaneous diffusion process The deco rating effect is greatly infl uenced by the diameters of CNTs TM atoms tend to be encapsulated inside the tube in the rel atively large CNTs while they are inclined to stack on the surface for the small ones More interestingly Au and Pt atoms would wrap around the smaller CNT whereas Ni and Cu atoms are still clustering outside of the CNTs with the increase of the number of TM atoms Simulation results in dicate that Pt and Au possess a better wetting effect with CNT than Ni and Cu 1 Introduction Due to their extraordinary electronic properties intrinsic structure large chemically active surface and stability at high temperatures CNTs have been shown promise for re placing silicon in future generation electronic devices and have been used as a support material for encapsulation dis persion and stabilization of metal nanoclusters 1 3 These J Y Guo C X Xu State Key Laboratory of Bioelectronics Advanced Photonics Center Southeast University Nanjing 210096 P R China e mail xcxseu J Y Guo College of Mathematics and Physics Jiangsu University of Science and Technology Zhenjiang 212001 P R China hybrid materials have been drawing a great deal of atten tion in fundamental research and are widely explored for manyphysical chemicalandbiologicalapplications includ ing nanoscale composite materials nanoelectronics 4 6 hydrogen 7 and lithium storage 8 catalysis 9 sen sors 10 nanoscale pharmaceutical drug delivery devices etc 11 To guide fabrication of the materials it is important to understand the formation mechanism of decorating CNTs with metal atoms Materials composed of CNTs and metals such as Ni Cu Au Pt Pd and Ag 12 15 have been stud ied experimentally by means of surface imaging techniques such as scanning tunneling microscopy atomic force mi croscopy and transmission electron microscopy Despite the use of highly developed experimental techniques the task of gaining a detailed understanding for the dynamic properties of metal atoms such as diffusion and aggregation is diffi cult because imaging techniques only provide temporal dis crete images during diffusion processes On the other hand atomistic computer simulations have played important roles in understanding the decorating processes for the properties of practicability In recent years the molecular dynamics method has been employed to simulate structure 16 so lidifi cation of the encapsulated nanoparticles 17 buckling behaviors of CNTs fi lled with metal atoms 18 19 and the diffusion of nanoclusters on CNTs 20 The fi lling process of Ni atoms into CNTs has been investigated by the MC method 21 Other methods such as the ab initio method have also been introduced to show that the metal decorated CNTs were good candidates for the hydrogen storage ma terial only if the metal atom aggregation can be prevented effi ciently 22 and the fi rst principles calculations have ex plored both the structures and electromagnetic properties of metal fi lled CNTs 23 334J Y Guo C X Xu Many theoretical studies have been performed on the dy namic properties of CNT supported metal materials how ever few studies on the decorating effects of CNTs with dif ferent TM atoms at the same conditions have been reported and systematical investigation is still required In this work MC simulation is carried out to investigate the decorating effects when a certain quantities of different TM atoms dif fuse to CNTs at the same conditions The dynamic proper ties of the spontaneous diffusion and the atomic encapsula tion aggregationandwrappedeffectsarediscussedindetail The interesting structures in nanoscale are presented which are generated by the naked TM atoms diffusing directly to CNTs The simulation processes provide a simple approach to compare the wetting effects between the TM and CNT It is helpful to design some nanocomposite materials and de vices such as optimization for catalytic properties of CNT supported hybrid nanomaterials or selecting TM which has the better wetting effect for TM based nanoscale intercon nects with CNTs 2 Models and simulation details The model can essentially be divided into two parts cover ing the TM TM and TM C interactions The effective po tential of the Sutton Chen S C potential model is selected to reliably describe the static and dynamic properties of TM 24 26 and it has been applied to optimize the structure and search for the global minima of TM clusters using the MC method 27 The interactions of TM C are represented by the Lennard Jones L J pair potential All the parameters employed in this work are listed in 24 25 In 22 Kras nov et al have shown that the potential between the TM and CNT was related to the tube diameter by an ab initio all electron calculation but it is unaffordable for our large sys tem and so classical potentials are still used here MC simulation was carried out in canonical ensemble conditions A series of open ended zigzag n 0 single walledCNTsarefi rstlygenerated where n issetintherange from 25 to 7 which corresponds to the diameter changing from 19 75 to 5 48 the lengths of CNTs are 34 08 and the numbers of the TM atoms are set to N 150 In our simulation the axis of CNT is assigned as the z axis and the center of CNT is set as the origin of the reference frame The entire MC simulation boxes are 4 nm 4 nm 10 nm with out periodic boundary conditions 25 and TM atoms can migrate freely in the model while CNTs are rigid In the ini tial confi guration a specifi ed number of TM atoms are ran domly dispersed in the box and outside the CNT only one CNT and one kind of TM atoms exist in the box every time MC steps are used to represent the simulation time one MC step is defi ned as the duration in which all TM atoms have an attempt to migrate and a standard Metropolis method is Fig 1 End on views of 150 TM atoms diffusing to CNTs with the same length of 34 08 and different diameters From top to bottom four TM atoms are Ni Cu Pt and Au respectively adopted The simulation temperature is chosen from 2100 K to 300 K with a decrement step of 100 K Free relaxation is carried out for enough time to obtain an initial equilibrium structure and then an annealing procedure is added for each temperature The simulation results are shown in Fig 1 on the CPK scale of 0 5 and Figs 3 4 6 and 7 on the CPK scale of 1 0 3 Results and discussion 3 1 Encapsulation in CNT The encapsulating effect of CNTs favorably exists in the bigger tube Figure 1 shows an end on view of the struc tures of TM atoms in different sized CNTs It is found that all the 150 Ni or Cu atoms are complete encapsulated in each CNT to form one nanowire and exhibit a coaxial cylin drical multi shell structure inside the CNT Appearances of these encapsulating effects had been ascribable to the tem plate effect of CNT 18 21 When the simulation is car ried out for Pt and Au the different the diffusion effect is observed in the same size of the above CNTs as shown in Figs 1 c and 1 d These two kinds of TM atom are diffi cult to be completely encapsulated in these CNTs and some atoms form small clusters outside the tube Our re sults in Fig 1 have a few differences with previous theoret ical studies 17 19 in those cases Pt or Au metal atoms were inserted into the CNT fi rstly and then relaxed freely for enough time to get equilibrium structure which was easy to be encapsulated due to the effective confi nement In the present case the migration of Pt or Au atoms is based on Comparative investigation on decorating carbon nanotubes with different transition metals335 Fig 2 Radial atom distributed profi les for the case of TM diffuse to CNT which corresponds to the structure of 25 0 in Fig 1 R0 9 785 the free diffusion outside of CNTs In Fig 2 the radial atom distributed profi les RADPs for the encapsulating effect are shown and as expected the peaks denote different layers and R0is the radius of CNT 25 0 For a more comparative analysis the different diffusion effects among these four TM atoms may be attributed to the competition between the TM TM interaction and the TM C interaction According to values of indices of S C potential model 24 25 the transition metals of Ni Cu Pt and Au can be classifi ed into two groups One group is Ni and Cu and the other is Pt and Au In the periodic table considering the atomic numbers of Ni No 28 Cu No 29 Pt No 78 and Au No 79 the quantitative analysis demonstrates that the intensity of the TM TM interaction is Ni Ni Cu Cu Pt Pt Au Au and the intensity of the TM C interac tion is Ni C Cu C Pt C Au C These intensity orders are the essential reason for the different decorating effects 3 2 Aggregation on CNT As the diameter of the CNT decreases more and more TM atoms form nanoclusters outside the tube Figure 3 shows the aggregation trend of four kinds of TM atoms on CNTs from left to right in Fig 3 more and more TM atoms distrib ute outside to form bigger and bigger cluster while less and less TM atoms stay inside of CNT In the bigger CNT the pullingforceandtheinteractionbetweentheclustersandthe CNT are dominant TM atoms are encapsulated in the CNT When CNT gets small the TM TM interaction slowly be comes dominant and then CNT supported nanocluster ap pears The decorating effects of the complete aggregation on CNTs are laid out in Fig 4 These hybrid materials of metal nanocluster decorated CNTs have been synthesized through many methods in experiments 1 2 The simulation shows that naked nanoclusters are deposited directly onto the sur face of CNT Nanocluster CNT interfaces are different the Fig 3 Typical structures of 150 metal atoms diffusing to CNTs with thesamelengthof34 08 anddifferentdiameters Fromtoptobottom the four transition metals are Ni Cu Pt and Au respectively Fig 4 Structures of metal atoms aggregating completely on small CNTs From top to bottom metals of Ni Cu Pt and Au respectively strong TM TM interaction tended to fl atten the bottom layer of the metal nanocluster The fl attest bottom layer of Ni nan ocluster is just due to the strongest interaction of Ni Ni and the weakest Au Au interaction indicates the curviest bottom layer of Au nanocluster To illustrate these effects angled atom distributed profi les AADPs through counting all the atoms around the axis of the CNT are introduced In Fig 5 the RADPs and AADPs for the same CNT are shown The peaks of Pt or Au are higher at the RADPs indicating these atoms are more apt to stay closer around CNT and from the AADPs Pt atoms have the larger angled distributed range 15 255 especially Au atoms distribute all angled di rection that correspond to the wrapped effect which will be 336J Y Guo C X Xu Fig 5 a Radial atom distributed profi les for the case of TM dif fusing to CNT which corresponds to the structure of 7 0 in Fig 4 R0 2 74 b Angled atom distributed profi les for the case of TM diffusing to CNT which corresponds to the structure of 7 0 in Fig 4 Fig 6 Structures of different number of Pt and Au atoms wrapping around the CNT of 8 0 respectively introduced below All these observations indicate that Pt and Au have the better wetting effect with CNT than Ni and Cu 3 3 Wrapped around CNT When the same number of Au atoms N 150 are em ployed to decorate the smaller CNTs of 8 0 and 7 0 the interesting wrapped effects are observed as shown in Figs 4 and 6 A hoop composed of Au atoms encircle on the sur face of CNT This simulation is consistent with the TEM images in 13 For the 150 Pt atoms the wrapping effect Fig 7 Snapshots of 300 Ni and Cu atoms forming nanoclusters on the CNT of 8 0 and 7 0 Fig 8 a Radial atom distributedprofi les for the case of TM diffusing to CNT which corresponds to the structure of 8 0 in Figs 6 and 7 R0 3 13 b Angled atom distributed profi les for the case of TM diffuse to CNT which corresponds to the structure of 8 0 in Figs 6 and 7 has not emerged on the CNT of 8 0 since the Pt Pt inter action is slightly stronger than the Au Au interaction 20 When more metal atoms such as N 200 and N 300 are added a Pt nanocluster wraps around the CNT of 8 0 as exhibited in Fig 6 a and simulation results for Au atoms with the same number are shown in Fig 6 b However the wrappingeffecthasnotemergedwhenmoreNiorCuatoms such as N 300 in Fig 7 are added they still form nan oclusters on the CNT of 8 0 and 7 0 Such results fur ther elucidate that Ni Ni interaction and Cu Cu interaction Comparative investigation on decorating carbon nanotubes with different transition metals337 are stronger than the Pt Pt interaction or Au Au interac tion At the same time it also elucidates that Pt and Au have better wetting with CNT than Ni and Cu which is favor able for fabricating a CNT fi eld effect transistor 28 29 The RADPs and AADPs corresponding to Figs 6 and 7 are shown in Fig 8 in the AADPs Pt and Au atoms show the better distribution for all angles which explainsthe wrapped effect and illustrates the wetting effect with CNT 4 Conclusions In summary we have presented the MC simulation results fora certainnumberof different TM atomsdiffused freely to CNTs with the same conditions and we considered the sup port of CNTs with different diameters With decrease of the diameter the TM atoms are observed to be encapsulated in side the nanotube aggregated on the nanotube and wrapped around the nanotube The different interaction strengths of the TM TM and the TM C play important roles to the dec orating effects of CNT Our simulation results show that Pt and Au have a better wetting effect than Ni and Cu The present simulation results provided some insight into the specifi c CNT supported hybrid nanomaterials and provided a simple method for picking out the better wetting effect TM for CNT which is important for interconnecting in nanoar chitectures AcknowledgementsThis work was supported by the NSFC Grant Nos 60725413and10674023 973Program GrantNo 2007CB936300 MOE Grant No 309015 and NSFJ Grant No BK2008285 References 1 G Vasilios G Dimitrios T Vasilios P Lucia M G Dirk P Mau rizio J Mater Chem 17 2679 2007 2 X G Hu S J Dong J Mater Chem 18 1279 2008 3 J P Tessonnier O Ersen G Weinberg P H Cuong D S Su R Schlogl Acs Nano 3 2081 2009 4 A Naeemi J D Meindl Annu Rev Mater Res 39 255 2009 5 O Hjortstam P Isberg S S derholm H Dai Appl Phys A 78 1175 2004 6 Y F Li R Hatakeyama T Kaneko Appl Phys A 88 745 2007 7 A K S