Decorating carbon nanotubes with nickel nanoparticles

Transcription

1 Chemical Physics Letters 436 (2007) Decorating carbon nanotubes with nickel nanoparticles C. Bittencourt a, *, A. Felten a, J. Ghijsen a, J.J. Pireaux a, W. Drube b, R. Erni c, G. Van Tendeloo c a LISE, University of Namur, 61 rue de Bruxelles, Namur, Belgium b DESY, Deutsches Elektronen-Synchrotron, 85 Notkestrasse, Hamburg, Germany c EMAT, University of Antwerp, 171 Groenenborgerlaan, Antwerp, Belgium Received 30 September 2006; in final form 19 January 2007 Available online 27 January 2007 Abstract Carbon nanotubes (CNTs) were decorated with Ni clusters by thermal evaporation. It is shown that the CNT decoration with Ni varies from well-organized clusters to the complete coverage of the surface resulting in electronic charge transfer and formation of Ni C bonds at the interface between these two materials, depending on the evaporated amount of Ni. The Ni CNTs interaction induces an increase in the metallicity of the system suggesting that CNTs decorated with metal atoms can be a good candidate as template for production of metal nanowires. Ó 2007 Published by Elsevier B.V. 1. Introduction Electronic band line-up and relative chemical potentials between adjacent materials govern electronic transport through a junction. At the interface, electronic band discontinuities lead to a potential barrier that must be overcome for electronic transport to occur. While theoretical prediction of the electronic transport through a perfect junction is already a difficult task, in actual junctions the complexity is increased by the presence of defects [1]. Thus, predictions of the electronic transport are mostly based on experimental results. Modern electronic devices based on carbon nanotubes (CNTs) have shown that their performance is influenced by a potential barrier, existing at the metal CNT contact, that governs the electron injection into the carbon nanotubes [2]. To achieve low-resistance ohmic contacts with nanotubes and thus their integration in new nanodevices, the study of the metal CNT interaction is essential. In this context, metal nanoparticles supported on CNTs are * Corresponding author. address: Carla.Bittencourt@MATERIANOVA.BE (C. Bittencourt). important prototypes for understanding the nature of the metal CNT interaction. Furthermore, these hybrid systems formed by two interacting structures whose electronic properties are affected by their dimension, open a vast and very exciting field for basic research. It is well established that the electronic structure of CNTs is remarkably sensitive to the nanotube chirality and diameter. Moreover, the electronic properties of metal clusters are well known to be influenced by their dimension, i.e. by change in the atomic coordination. The combination of these finite-size effects may have strong impact on metal cluster CNT interaction, having outstanding influence for future nanoscale devices. Low resistance metal-nanotube ohmic contacts have been reported in the case of Ti, Pt, Ni and Pd [3,4]. Among these metals Ni appears as an important one; besides its contact nature there is also its important role as a catalyst in the growth of carbon nanotubes [5]. The available studies deal mostly with the interaction of Ni atoms with graphite [6 10]. Although these can be used to support some findings on the study of the Ni CNT surface interaction, it is important to remember that in graphite the carbon 2p p electron density is symmetrically distributed with respect to the flat graphene plane, whereas the curvature /$ - see front matter Ó 2007 Published by Elsevier B.V. doi: /j.cplett

2 C. Bittencourt et al. / Chemical Physics Letters 436 (2007) of the CNTs induces the re-distribution of the electron density, with most of the wave function being outside of the curved graphene layers [11]. Thus, due to the difference in the surface electron density, interactions at the CNT surface are expected to be different from interaction at a flat graphene layer. Theoretical studies have already demonstrated that the metal CNT surface interaction depends on the CNT diameter and that the metal CNT surface atomic registry affects the contact resistance [12,13]. Different techniques such as self-assembly of nanoparticles at the CNT surface, heterogeneous coagulation and direct hydrolysis, inorganic reactions in supercritical CO 2 -methanol solutions and physical vapour deposition have been used to decorate the CNT surface with metals [14 20]. In this work we study the Ni CNT interaction by analyzing MWCNTs decorated with clusters produced by thermal evaporation of Ni atoms. It is shown that the MWCNT decoration with Ni varies from well-organized clusters to the complete coverage of the surface resulting in electronic charge transfer and formation of Ni C bonds at the interface between these two materials, depending on the evaporated amount of Ni. In accordance with reported theoretical results it will be shown that the Ni CNT interaction induces the MWCNT Fermi level shift due to charge transfer from Ni to C [21]. 2. Experimental section The samples were prepared using commercially available MWCNTs powder synthesized by arc-discharge without use of catalysts (Mercorp) [22]. XPS measurements were performed at BW2 beamline Hasylab (Hamburg) using a photon energy of 3300 ev. The overall resolution of the system (source + analyser) was 0.8 ev [23]. The Au 4f 7/2 peak at ev, recorded on a reference sample was used for calibration of the binding energy scale. In order to check for any possible photon energy drift during the measurements reference spectra were measured before and after each core levels data sets recorded on all the samples. To perform the XPS measurements, the MWCNT powder was pressed on a conductive adhesive tape suitable for ultra high vacuum. The thickness and the homogeneity of the obtained CNT-layer were checked to assure no interference of the tape on the measurements. Transmission electron microscopy (TEM) was used to determine the evolution of the cluster size for increasing evaporation time. For this study, the MWCNT powder was dispersed in ethanol, and a drop was deposited onto a honeycomb carbon film supported by a copper grid. Transmission electron microscopy was carried out on a Philips Tecnai 10 microscope operating at 80 kv, while high resolution TEM was performed using a Jeol 4000EX microscope at 400 kv. Ni was thermally evaporated from a Ni wire. A quartz balance was used to calibrate the evaporation rate. The accuracy of this calibration is equal to 0.2 Å/min. The rate used for metal evaporation (1 Å/min) was the same for all samples and the evaporation time was varied in order to increase the particle size. 3. Results and discussion The evolution of the overlayer morphology for increasing amount of evaporated Ni can be seen in the TEM images shown in Fig. 1 (the amount of evaporated Ni calibrated with a quartz microbalance is indicated on the left hand side of the figure). For 10 nm of Ni deposition the morphology can be described as a non-uniform layer covering the nanotube surface completely. A closer study of this image suggests that the layer is formed by clusters that were formed independently before connecting. For 4 nm of Ni deposition discrete dispersed particles are observed at the CNT surface (Fig. 1). As metal deposition is further reduced, the particle size is also reduced. HREM analysis performed first focusing on the CNT walls (Fig. 2a) reveals that their graphitic-like disposition is preserved after the decoration with Ni. The presence of features (evidenced in the image by a darker grey overlayer covering the CNT surface) at the CNT surface can be observed. When the focus is moved to these features atomic plans are revealed (Fig. 2b), the presence of these well-defined atomic planes indicates a tendency for an epitaxial relationship between the Ni clusters planes and the CNT surface. Theoretical studies suggest that Ni atoms on CNT surface interact mainly with vacancies; the energy for substitution of a C atom by a Ni atom in a graphene sheet was estimated to be 9.5 ev while Ni atoms interact with vacancies with a strong adsorption bond of 4.7 ev (minimum energy gain associated with the saturation of the three dangling bonds in graphite by a Ni atom when filling an existing vacancy) [21]. Fig. 1. TEM images of different Ni amounts on MWCNT: the equivalent thickness as measured with a quartz microbalance.

3 370 C. Bittencourt et al. / Chemical Physics Letters 436 (2007) Fig. 2. HREM image of Ni coating on MWCNT (a) focus on the graphitic-like walls and (b) focus on the Ni clusters. Based on the results presented above it can be suggested that Ni decoration of MWCNTs by thermal evaporation involves first the adsorption of Ni atoms onto the surface, followed by diffusion of these adatoms across the surface until nucleation of islands occurs when diffusing adatoms form a stable nucleus. After the formation of stable nuclei at nucleation centres, the next incoming adatoms can either attach to an existing nucleus or diffuse on the surface until they encounter another adatom to form a new stable nucleus. The density of nucleation centers depends on the interaction between adatoms and the substrate; considering the strong adsorption bond between Ni atoms and C vacancies, such defects will be the principal nucleation sites. Ni CNT interaction can be studied by X-ray photoelectron spectroscopy. If there is a chemical reaction at the interface, then the new chemical environment of the atoms at the interface in the XPS spectra leads to the appearance of new features. The evolution of the C 1s peak for a sequence of Ni evaporations is shown on Fig. 3; clearly an additional feature peaking in the low-binding energy region of the C 1s singlet peak is observed. This new structure is associated to the formation of Ni C bonds [24]. It can thus be claimed that a chemical reaction has taken place. The Ni C bond length is (about 0.20 nm) larger than Fig. 3. Carbon 1s core level spectra recorded for increasing amount of Ni evaporation (a) pristine, (b) 1 Å, (c) 5 Å, (d) 35 Å, and (e) 200 Å. the C C bond (0.14 nm) in a graphene sheet [21], thus one could expect that the formation of Ni C bonds perturbs the graphene lattice and affects the distance between the tubes under the Ni cluster. However the HREM images (Fig. 2) show that the CNT structure is preserved after the bond formation. This result is in agreement with theoretical studies that showed that a graphene lattice is not much perturbed by the presence of a substitutional Ni atom if it can expand and relax laterally, as in the case of a MWCNT outmost layer; the Ni atom was predicted to be displaced out of the graphite plane by 0.1 nm and the C Ni C angle to be 98 [21]. The inset of Fig. 3 shows the direct comparison between the C 1s core-level photoemission before and after evaporation of 1 Å of Ni. It is observed that beyond the appearance of the new feature related to the Ni C bond formation, the asymmetry of the C 1s peak increased; this asymmetry is associated with the many-electron response to the sudden creation of a photohole [25]. The potential created between the photohole and the remaining electrons after the photoemission induces the promotion of electrons near the Fermi level to empty states just above it. The associated singularity index a describes the electron-hole interaction, whose strength is reflected in the magnitude of a [25,26]. The value of the singularity index a can be obtained by fitting the asymmetric C 1s peak with the Doniach Sunjic (DS) function [26]. Using the Doniach Sunjic function to reproduce the recorded C 1s spectra, an increase of 0.1 up to 0.3 was measured in the a values for successive Ni deposition. Within the Born approximation, it is shown that a is proportional to the square of r(e f ), the density of states (DOS) near the Fermi level E f, as well as to the effective charge of the photohole seen by the conduction electron, v q [26]. Hence, changes in a following the Ni deposition, suggest changes in v q and r(e f ) due to Ni CNT interaction. Consequently, the variation observed in a after nominal evaporation of 1Å of Ni indicates that this small amount of metal is enough to affect the screening process of the C1s core hole by perturbing the DOS near E f. This demonstrates that the Ni CNT interaction strongly perturbs the DOS of the MWCNTs. In fact it was reported that when one Ni atom replaces one C atom of a graphene sheet, a few electronic

4 C. Bittencourt et al. / Chemical Physics Letters 436 (2007) levels resulting from the Ni atom interaction with the sheet appear close to the Fermi energy, which suggests a metallic behaviour [21]. The increased metallicity of this system (when compared to a graphene sheet) mainly due to the action of the Ni atom acting as an electron donor, explains the increase of the a parameter. In addition, based on these results, it can be suggested that the small shift to higher binding energy observed for the C 1s peak following Ni evaporation (Fig. 3) corresponds to an upward displacement of the Fermi level and consequently a rigid shift of the electronic states [21,27]. The Ni 2p 3/2 core level binding energy shift relative to the bulk value for a sequence of evaporations is displayed in Fig. 4. It shows an increase in the Ni 2p 3/2 binding energy with decreasing coverage, i.e, cluster size. Three factors can contribute to the core-electron binding energy: initial-state effects associated to changes in the electronic structure (valence-electron configuration), final-state effects due to changes in the relaxation processes (extra-atomic response to the positively charge photohole), and cluster charging [28]. Core-electron photoemission from a positively charged cluster (due to photoemission from a previous electron) will have a higher core-level binding energy as a result of the Coulomb attraction barrier [29]. The weight of each of these factors on the core level binding energy will depend on the nature of the cluster (i.e., the chemical composition, shape, size and area of interaction with the support) and on the nature of the support. For supported clusters the binding energy shift is expected to be smaller than for non-supported ones as charge can be transferred to screen the final state. Moreover, valence band narrowing as the size of the clusters decreases (initial-state effect) can be compensated by hybridization of the cluster electronic states with electronic states of the support [28]. Watson and Perlman calculated a core level shift of 5.5 ev for the 3d 8 4s 2! 3d 9 4s 1 configuration change between atomic and bulk metallic nickel [30]. Conversely, smaller values were found in the current study (Fig. 4), in agreement with values of about 0.6 ev reported for Ni clusters supported on amorphous carbon [27]. Small binding energy shifts are expected for clusters that strongly interact with the support [28]. In effect, Ni C bond formation testifies that the Ni clusters strongly interacts with the CNT surface: then the initial state effects on the binding energy are small whereas the final-states effects assume a great relative importance [28,31]. Based on these results it can be suggested that the relative shift observed on the Ni 2p 3/2 is mainly due to final-state effects. In summary, it is demonstrated that Ni atoms evaporated onto the MWCNT surface form clusters with well-defined atomic planes. The nucleation sites are suggested to be vacancies. Chemical reaction occurs at the cluster CNT interface with formation of Ni C bonds. The lattice of the external tube is not much perturbed by the formation of the Ni C bonds. The C 1s shift toward high binding energy is associated with the rigid shift accompanying the reallocation of the Fermi level; increasing shift is observed until 2 Å of Ni evaporation and then no additional shift was detected for increasing amounts of Ni evaporation up to 350 Å, whereas, the shift on the Ni 2p 3/2 decreases until a coverage of 200 Å has been reached, before leveling off with the Ni 2p 3/2 bulk value is mainly due to cluster size effects. 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