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Dynamics of Gold Nanoparticles on Carbon Nanostructures Driven by van der Waals and Electrostatic Interactions Alessandro La Torre, Maria del Carmen Gimenez-Lopez,* Michael W. Fay, Carlos Herreros Lucas, Paul D. Brown, and Andrei N. Khlobystov 1D and 2D carbon nanostructures, such as carbon nanotubes and graphene, have attracted a great deal of attention because of their outstanding structural and physical properties.[1,2] Decoration of nanocarbons with metallic nanoparticles (NP) facilitates the incorporating of additional functionalities for exploitation in future electronic, catalytic, and energy storage and conversion applications.[3–6] However, the utilization of composite metal–carbon nanostructures is presently room temperature limited, to preserve the NP size-dependent properties, as adsorbed metal nanoparticles are generally metastable and highly labile.[7,8] In this context, gaining improved understanding of the interactions between metal NP and carbon nanostructures is of paramount importance for controlling the size and position of adsorbed NP, in order to gain full control of the potential functionalities of these hybrid nanostructures.[9] The growth (otherwise termed sintering) of NP on various carbon nanostructures has been extensively studied,[10,11] but only recently has it been possible to shed some light on the key mechanisms involved in the growth of ≈2 nm gold nanoparticles (AuNP) on graphitized carbon nanofibers (GNFs).[12,13] In contrast to carbon nanotubes, GNFs exhibit corrugated interior surfaces dictated by their internal stackednanocone structures, with typical step-edge heights of ≈3 nm. The different internal and external surface morphologies Dr. A. La Torre, Dr. M. d. C. Gimenez-Lopez, C. H. Lucas, Prof. A. N. Khlobystov School of Chemistry The University of Nottingham University Park, Nottingham, NG7 2RD, UK E-mail: [email protected] Dr. M. W. Fay, Prof. A. N. Khlobystov Nottingham Nanoscience and Nanotechnology Centre The University of Nottingham University Park, Nottingham, NG7 2RD, UK Prof. P. D. Brown Division of Materials Mechanics and Structures Department of Mechanical Materials and Manufacturing Engineering Faculty of Engineering The University of Nottingham University Park, Nottingham, NG7 2RD, UK DOI: 10.1002/smll.201402807 small 2015, DOI: 10.1002/smll.201402807
of the GNF (Figure 1A) provide a unique environment for the study of bonding processes involved in the growth of NP. For example, it has been shown that AuNP adsorbed on the internal surfaces of GNF always grow to the same, constrained, maximum size of ≈6 nm, while AuNP adsorbed on the atomically smooth graphitic surfaces of the GNF exterior continue their growth to ≈13 nm and beyond (Figure 1A), regardless of the source of energy providing the driving force for growth (i.e., heat or electron beam).[12] Accordingly, it is considered that GNF interior step-edges impose a significant barrier for the migration of AuNP, precluding their growth by coalescence, with Ostwald ripening remaining the only possible growth mechanism. The size of growing AuNP on GNF interior step-edges may also be influenced by electrostatic interactions arising from charge transfer between the nanocone graphene stacks and the adsorbed AuNP. Recently, studies on the formation of AuNP on few-layer graphene (FLG) films showed that the strength of electrostatic interaction increased with decreasing film thickness for the case of the smallest NP.[14] In view of the complex nature of GNF as a support material for studying AuNP growth, and the difficulty of separating out the effects of electrostatic interaction (i.e., electron transfer from the graphene stack) from structural factors (i.e., the nanocone step-edges), it is recognized that complementary experimental evidence is needed, using simpler model-system carbon support structures, to appraise the relative importance of these competing factors on the nanoscale organization of metallic NP of controlled size. Here, we report on the assembly and growth (induced either by exposure to a high energy electron beam or in situ thermal heating within a transmission electron microscopy (TEM)—in isolation, sequentially, or together) of preformed AuNP on multiwalled carbon nanotubes (MWNT) supported on FLG and amorphous carbon film templates, to appraise the relative importance of van der Waals forces and electrostatic interactions, for the creation and stabilization of ordered, linear arrangements of NP. When a 1D cylindrical carbon nanotube is placed on a flat surface, the intersection between the convex surface of the nanotube exterior and the flat surface delineates a 1D channel along the nanotube growth axis (Figure 1B). From a viewpoint of host–guest interaction, this channel could be used to stabilize NP with controlled size. The van der Waals forces acting between NP and the channel are expected to
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by changing the flat support on which the AuNP-MWNT is deposited. Accordingly, appropriate choice of the nature and structure of the support material should, in principle, not only restrict NP growth, but also, steer the NP assembly in a desired manner. Dodecanethiol-stabilized AuNP with average size 2.3 ± 0.4 nm (Figure S1, Supporting Information) were used in this study.[15] Acid-treated MWNT (mean external diameter: 31.4 ± 4.8 nm) were added to a suspension of AuNP in hexane to produce an AuNP-MWNT composite (Figure 1C and Figure S2, Supporting Information). The introduction of acid groups to the atomically smooth exterior surfaces of MWNT created anchoring sites for effective NP decoration.[16,17] Careful inspection of the resultant composite Figure 1. A) Schematic representation of an AuNP-GNF heterostructure, following heating in using TEM, under low dose conditions at vacuum at 300 °C for 2 h, illustrating the development of >13 nm AuNP adsorbed on the GNF 100 keV, to ensure no sample modification outer surface and ≈6 nm AuNP anchored at graphitic step-edges (gray arrows) within the GNF. occurred during imaging, demonstrated B) Schematic diagram illustrating the channels (gray arrows) defined by a carbon nanotube that the size of randomly adsorbed AuNP positioned on a flat, 2D surface. C) Schematic representation of the preparation process for remained unchanged with respect to the AuNP-MWNT supported on a flat, FLG surface. initial, free-standing NP (Figure S2, Supporting Information). However, when be more significant for the case of smaller NP, as the contact the AuNP-MWNT composite sample deposited on FLG surface area within the channel over the total surface area of (1–6 monolayers) was exposed for extended periods of time the NP will decrease with decreasing NP size. Furthermore, to a 200 keV electron beam with high flux, ≈5 × 10−13 pA nm−2, the effect of electrostatic interactions can be readily tuned a striking, linear arrangement of slightly bigger AuNP (3.3 ± 1.2 nm) was observed to form rapidly along the 1D channels, at the expense of other AuNPs adsorbed on the MWNT (Figure 2). It is considered that the stabilizer was not retained after the process of electron beam-induced NP growth, as energy-dispersive X-ray (EDX) analysis did not show the presence of any signatures attributable to sulphur atoms. Interestingly, these linear NP arrangements were not observed for the case of unsupported AuNP-MWNT heterostructures (Figure S3, Supporting Information), although a small increase of NP size was noted (≈3.6 nm) under the same conditions of electron beam irradiation. Previous studies of silver NP on carbon nanotubes suggested a mechanism for NP sintering involving the Brownianlike motion of NP on the support surface, with subsequent coalescence leading to NP growth.[11] This mechanism has been similarly observed for AuNP adsorbed on the atomically smooth graphitic surfaces of GNF exteriors under electron beam Figure 2. A) Schematic diagram of the growth mechanism of AuNPs at the channels defined irradiation.[12] However, for the case of by an AuNP-MWNT supported on a FLG film, induced by the electron beam. Ostwald ripening is denoted by solid red arrows. B) Bright-field TEM image of an AuNP-MWNT composite after our AuNP-MWNT composites, it is considered that defects on the nanotube surface electron beam exposure at 200 keV. Scale bar is 20 nm.
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will restrict the growth of larger NPs by particle migration and coalescence mechanisms. Thus, the more plausible mechanism for AuNP growth at the channel defined by a nanotube on a FLG surface is the diffusion of atoms between anchored NP on the surface of the support (i.e., Ostwald ripening). It is interesting to note that van der Waals forces seem to steer this diffusion of gold atoms to the 1D MWNT/ FLG channels. Clearly, enhancement in the associated contact surface area for the AuNPs trapped within this channel plays an important role in the diffusion of gold atoms, creating highly ordered composite structures under the effect of the e-beam. In situ TEM observations of thermally activated transformations of AuNPMWNT supported on FLG films, in real time, provide further valuable insight into the NP growth mechanisms (video file A, Supporting Information). FLG support films provide better heat conductivity than conventional amorphous carbon films, allowing the temperature at the sample to be established more reliably, from thermocouple measurement of the heating TEM holder crucible tem- Figure 3. A) Schematic diagram of the growth mechanism of AuNP in the electron beam developed AuNP-MWNT/FLG sample after in situ heating at 500 °C for 2 h under vacuum. perature.[18] Further, while scanning TEM Ostwald ripening is indicated by solid red arrows. Uncoated AuNP are shown on the left of the (STEM) imaging uses a higher beam cur- diagram. B) HAADF-STEM images of the electron beam developed AuNP-MWNT/FLG sample rent density, the total incident beam cur- before (left) and after (right) heating at 500 °C for 2 h under vacuum. Scale bars are 100 nm. rent and beam diameter are reduced, and C) Time-lapse TEM images illustrating the thermally activated processes taking place within the dwell time per pixel is typically less the white rectangle in B). The scale bars in the images are 10 nm, and the times are indicated than the thermal equilibrium time, so con- relative to the start of observation (t1 = 10, t2 = 80, and t3 = 120 min). These images come from sequently both localized and total sample a movie sequence (video file A, Supporting Information), which shows the disappearance of the smallest NPs (highlighted with white circles). The white arrows denote the Ostwald heating are reduced. Hence, the STEM ripening of a NP at the nanotube/FLG channel. mode was used to ensure that ripening of the NP, as observed during in situ heating, was predominantly an effect of thermal heating via the sup- (and Figure S4, Supporting Information) clearly showed the port stage. High-angle annular dark-field (HAADF) STEM disappearance of smallest NP and the growth of the larger images of an electron beam developed AuNP-MWNT het- ones. Under such conditions, the concentration of atomic gold erostructure, before and after heating at 500 °C, are shown species is higher in the vicinity of the small rather than the in Figure 3B, with complementary bright field (BF) diffrac- large NP. Hence, a concentration gradient is established that tion contrast STEM images presented in Figure S4, Sup- leads to a net flux of atomic species from the smaller to the porting Information. As a result of the thermal treatment larger NP, and as a result, the larger NPs grow at the expense process (Figure 3A), the total number of AuNPs adsorbed of the smaller ones. As for the case of electron beam-induced on the nanotube surface were observed to decrease, while NP growth, the electron beam transfers kinetic energy to Au both bigger (maximum size of ≈8.6 nm) and slightly smaller atoms of up to 1.22 and 2.66 eV per atom at 200 and 100 kV, (minimum size of ≈1.6 nm) NP with respect to the initial NP respectively.[19] The transferred energy is sufficient to mobisize were observed to develop at the MWNT/FLG chan- lize atoms with van der Waals forces influencing the direcnels. Our previous studies showed the complete removal of tion of their migration on the support surface, being driven alkylthiol stabilizer from the AuNP surface after heating in by differences in both local adatom concentration and survacuum at 300 °C.[12] Consequently, direct contact between face free energy. In this context, the electron beam action on the metal surface of each NP and the carbon support is antic- nanoparticle growth is not dissimilar to that of heating, but a ipated at 500 °C. Close inspection of the in situ HAADF greater change in size of the AuNP adsorbed in the channels images revealed that Ostwald ripening is the sole process is noted, as compared to the AuNP that remain anchored on responsible for the growth of both the AuNP adsorbed on the carbon nanotube exterior surface. To appraise the importance of van der Waals interactions the MWNT and those aligned at the channels. For example, time-lapse TEM images of the area highlighted in Figure 3C on the nanoscale organization of thermally developed NP at small 2015, DOI: 10.1002/smll.201402807
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the adsorbed AuNP cannot be ignored. For the model carbon support used here, the charge transfer from the FLG film is expected to be quite significant, with the number of graphene layers in the film varying from one to six.[14] Hence, to appraise the importance of electrostatic interactions on the thermal stability of AuNP adsorbed at the channels, AuNPMWNT was deposited onto an amorphous carbon film and heated in vacuum in a closed ampoule at two different temperatures, 300 and 500 °C. Amorphous carbon films show very different structural and electronic properties: i.e., short-range order, localized π electrons (a mixture of sp2 and sp3 hybridized carbon atoms) and a higher concentration of dangling bonds, as compared with FLG films.[20–22] Consequently, it is anticipated that charge transfer from amorphous carbon to the AuNP will be much lower than that from FLG, if any. BF-TEM imaging Figure 4. HAADF-STEM images showing thermally evolved AuNP at tilt angles of A) + 27° and at 300 °C again showed AuNP trapped B) −27°, with respect to the axis of the MWNT. The scale bars are 50 nm. TEM image acquisition along the channel space between the was performed using an accelerating voltage of 200 kV. C) Tomographic reconstruction of the MWNT and the amorphous carbon film thermally evolved AuNP-MWNT/FLG heterostructure. (Figure 5B), forming linear arrangements along the nanotube axis. Most 500 °C, the specimen was rotated with respect to the MWNT importantly, the AuNP at the channels grew considerably axis through an angle of 27° (Figure 4A,B), clearly showing (12.2 ± 2.2 nm) after annealing at only 300 °C, as compared the 1D arrangement of AuNP within the MWNT/FLG chan- with the in situ observations of AuNP-MWNT on FLG. Hownels. More detailed tomography experiments on this ther- ever, when AuNP-MWNT deposited on amorphous carbon mally developed AuNP-MWNT heterostructure further was heated at 500 °C, NP that grew further (≈20–25 nm) confirmed the highly ordered linear arrangement of AuNP tended to desorb from the 1D channels and agglomerate. (Figure 4C and video files B and C, Supporting Informa- This result is in sharp contrast to that observed for the comtion). The low contrast background of the FLG film assisted posite on FLG (Figure 5C and Figure S7, Supporting Inforgreatly in tomographic reconstruction, producing minimal mation). Hence, when AuNP-MWNTs are thermally treated background to the sample at high tilt angles. Similar 1D on FLG at 500 °C, the main factor limiting the size of the arrangements of AuNP within the MWNT/FLG channels grown AuNP must be repulsion, arising from dipole moments were observed after heating at 500 °C for AuNPs-MWNT induced in the NP trapped along the channels as a conseheterostructures that were not exposed to the electron beam quence of charge transfer from the FLG. (Figure S5, Supporting Information). While NP migration can be controlled through suitable structuring of the carbon support even at temperatures as high as 500 °C, it appears to be more difficult to control the mobility of atomically dispersed species as Ostwald ripening can also be observed between neighboring AuNPs located at the channels (Figure 3C and Figure S6, Supporting Information). However, the thermal stability of the AuNP adsorbed at the channels is quite remarkable (4.5 ± 2.5 nm), with the change in NP size at 500 °C being less Figure 5. A) Schematic diagram of AuNP-MWNT supported on an amorphous carbon film than 2 nm with respect to the initial NP before and after heating at 300 and 500 °C for 2 h under vacuum. Uncoated AuNP are shown (3.3 ± 1.2 nm). Hence, the influence of elec- on the left and right of the diagram. B) BF-TEM image of a thermally evolved AuNP-MWNT trostatic interactions arising from charge heterostructure after ex situ heating in an ampoule under vacuum at 300 B) and 500 °C transfer between the carbon support and C). Scale bars are 50 nm in B) and 35 nm in C).
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In this work, we have demonstrated that nanochannels formed between a 1D nanostructure and a flat surface can be effectively exploited for trapping and developing NP to form highly ordered, stable heterostructures. In situ TEM imaging and tomographic reconstructions indicated a competition between structural and electrostatic influences of the carbon support on the NP growth mechanism, enabling controlled positioning and sizing of adsorbed NP at the MWNT/ FLG channels. The introduction of defects on the atomically smooth surface of the carbon nanotubes creates anchoring sites for NP, restricting their migration and subsequent coalescence that would otherwise give rise to larger NP. Van der Waals forces are found to be responsible for stabilizing the NP at the channels and mediating the migration of adatoms or mobile molecular species on the support surface. The strength of electrostatic interactions arising from charge transfer between the support substrate and the NP is found to be the main factor limiting the size of the grown NP (i.e., Ostwald ripening). In keeping with previous observations of the development of AuNP within GNF,[12,13] AuNP ripen within our model carbon support system regardless of the source of energy (electron beam irradiation or heating). Accordingly, it is considered that careful choice of the carbon support material, taking into account the nature and structure of the support material, enables highly stable, highly ordered hybrid nanostructures to be created. These findings will have impact on the future design of electrocatalyst materials for energyrelated[23] and synthesis applications,[24] spintronic devices[25] with improved thermal stability, and will enhance the functional performance of metastable nanostructures in general, in the pursuit of stable, well-organized systems.
tion temperature of AgNP dispersed onto FLG film, as a reference point.[26] HAADF images were acquired using a Jeol Digital STEM system. HAADF images in the tilt series were processed using ImageJ software to reduce contrast from the MWNT before tomographic reconstruction using IMOD 3.13.2. Statistical analysis was performed for each sample using Gatan Digital Micrograph software. In situ TEM measurements in real time for the AuNP-MWNT composite deposited on FLG heated at 500 °C for 2 h (video file A, Supporting Information). Tomography reconstructions (video files B–D, Supporting Information). Additional TEM and STEM images and experimental parameters. Details of the preparation of acidfunctionalised MWNT, synthesis of preformed AuNP, preparation of AuNP-MWNT composite material and heating of AuNP-MWNT on an amorphous carbon TEM grid sealed under vacuum.
Supporting Information Supporting Information is available from Wiley Online Library or from the author.
Acknowledgements The authors thank the Royal Society (DH110080), the European Research Council, and the University of Nottingham for supporting this research programme, and the Nottingham Nanotechnology and Nanoscience Centre for access to TEM facilities.
Experimental Section General Experimental Section: Arc-discharge MWNTs were purchased from MER Corporation. All other reagents and solvents were purchased from Sigma-Aldrich, UK, and used without further purification. Water was purified (>18.0 M Ω cm) using a Barnstead NANOpure II system. Details of the preparation of acid-functionalized MWNT, synthesis of preformed AuNP, preparation of AuNP-MWNT composite materials, and heating of AuNP-MWNT on an amorphous carbon TEM grid sealed under vacuum can be found below. The preparation of AuNP required glassware to be cleaned with a mixture of concentrated HCl and HNO3 at 3:1 v/v (aqua regia), rinsed with deionized water, further cleaned with KOH in MeOH, and finally rinsed thoroughly with deionized water prior to use. TEM and STEM imaging were performed using a Jeol 2100F transmission electron microscope (field emission electron gun source, information limit 0.19 nm) using an accelerating voltage of 100 or 200 kV. TEM specimens were prepared by casting several drops of methanolic solution onto either FLG film or coppergrid-mounted “holey” amorphous carbon film and drying under a stream of nitrogen. In situ TEM heating experiments were performed using a Gatan 652 double tilt-heating holder. Tilt series tomographic reconstruction imaging was achieved using a Gatan 916 tomography holder. The temperature of NP on the graphene film was calibrated, with reference to the recorded temperature of the heating holder crucible, prior to this work using the evaporasmall 2015, DOI: 10.1002/smll.201402807
[1] R. H. Baughman, A. A. Zakhidov, W. A. de Heer, Science 2002, 297, 787. [2] E. H. L. Falcao, F. Wudl, J. Chem. Technol. Biotechnol. 2007, 82, 524. [3] P. Serp, E. Castillejos, ChemCatChem 2010, 2, 41. [4] X. Pan, X. Bao, Acc. Chem. Res. 2011, 44, 553. [5] S. Datta, L. Marty, J. P. Cleuziou, C. Tilmaciu, B. Soula, E. Flahaut, W. Wernsdorfer, Phys. Rev. Lett. 2011, 107, 186804. [6] J. P. Cleuziou, W. Wernsdorfer, T. Ondarcuhu, M. Monthioux, ACS Nano 2011, 5, 2348. [7] T. W. Hansen, A. T. Delariva, S. R. Challa, A. K. Datye, Acc. Chem. Res. 2013, 46, 1720. [8] S. B. Simonsen, I. Chorkendorf, S. Dahi, J. Phys. C 2012, 116, 5646. [9] M. C. Gimenez-Lopez, A. La Torre, M. W. Fay, P. D. Brown, A. N. Khlobystov, Angew. Chem. Int. Ed. 2013, 52, 2051. [10] G. Rosenfeld, K. Morgenstern, I. Beckmann, W. Wulfhekel, E. Laegsgaard, F. Besenbacher, G. Comsa, Surf. Sci. 1998, 402, 401. [11] L. Zhu, G. Lu, S. Mao, J. Chen, J. Nanopart. Res. 2007, 2, 149. [12] A. La Torre, M. C. Gimenez-Lopez, M. W. Fay, G. A. Rance, W. A. Solomonsz, T. W. Chamberlain, P. D. Brown, A. N. Khlobystov, ACS Nano 2012, 6, 2000. [13] A. La Torre, M. W. Fay, G. A. Rance, M. C. Gimenez-Lopez, W. A. Solomonsz, P. D. Brown, A. N. Khlobystov, Small 2012, 8, 1222. [14] Z. Luo, L. A. Somers, Y. Dan, T. Ly, N. J. Kybert, E. J. Mele, A. T. Charlie Johson, Nano Lett. 2010, 10, 777.
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communications www.MaterialsViews.com [15] M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R. Whyman, Chem. Commun. 1994, 801. [16] L. Han, W. Wu, F. L. Kira, J. Luo, M. M. Maye, N. N. Kariuki, Y. Lin, C. Wang, C.-J. Zhong, Langmuir 2004, 20, 6019. [17] F. Stoffelbach, A. Aqil, C. Jerome, R. Jerome, C. Detrembleur, Chem. Commun. 2005, 4532. [18] M. A. Asoro, D. Kovar, P. J. Ferreira, ACS Nano 2013, 7, 7844. [19] R. F. Egerton, M. Malac, Micron 2004, 35, 399. [20] S. S. Datta, D. R. Strachan, E. J. Mele, A. T. Johson, Nano Lett. 2009, 9, 7. [21] F. Guinea, Phys. Rev. B: Condens. Mater. Phys. 2007, 75, 235433. [22] T. Ohta, A. Bostwick, T. Seyller, K. Horn, E. Rotenberg, Science 2006, 313, 951.
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[23] Y. Liang, Y. Li, H. Wang, H. Dai, J. Am. Chem. Soc. 2013, 135, 2013. [24] A. N. Khlobystov, ACS Nano 2011, 5, 9306. [25] R. O. Cherifi, V. Ivanovskaya, L. C. Phillips, A. Zobelli, I. C. Infante, E. Jacquet, V. Garcia, S. Fusil, P. R. Briddon, N. Guiblin, A. Mougin, A. A. Ünal, F. Kronast, S. Valencia, B. Dkhil, A. Barthélémy, M. Bibes, Nat. Mater. 2014, 13, 345. [26] M. W. Fay, A. La Torre, M. C. Giménez-López, C. Herreros Lucas, A. N. Khlobystov, P. D. Brown, J. Phys.: Conf. Ser. 2014, 522, 012073.
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Received: September 18, 2014 Revised: October 17, 2014 Published online:
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Nanostructures
Dynamics of Gold Nanoparticles on Carbon Nanostructures Driven by van der Waals and Electrostatic Interactions Alessandro La Torre, Maria del Carmen Gimenez-Lopez,* Michael W. Fay, Carlos Herreros Lucas, Paul D. Brown, and Andrei N. Khlobystov 1D and 2D carbon nanostructures, such as carbon nanotubes and graphene, have attracted a great deal of attention because of their outstanding structural and physical properties.[1,2] Decoration of nanocarbons with metallic nanoparticles (NP) facilitates the incorporating of additional functionalities for exploitation in future electronic, catalytic, and energy storage and conversion applications.[3–6] However, the utilization of composite metal–carbon nanostructures is presently room temperature limited, to preserve the NP size-dependent properties, as adsorbed metal nanoparticles are generally metastable and highly labile.[7,8] In this context, gaining improved understanding of the interactions between metal NP and carbon nanostructures is of paramount importance for controlling the size and position of adsorbed NP, in order to gain full control of the potential functionalities of these hybrid nanostructures.[9] The growth (otherwise termed sintering) of NP on various carbon nanostructures has been extensively studied,[10,11] but only recently has it been possible to shed some light on the key mechanisms involved in the growth of ≈2 nm gold nanoparticles (AuNP) on graphitized carbon nanofibers (GNFs).[12,13] In contrast to carbon nanotubes, GNFs exhibit corrugated interior surfaces dictated by their internal stackednanocone structures, with typical step-edge heights of ≈3 nm. The different internal and external surface morphologies Dr. A. La Torre, Dr. M. d. C. Gimenez-Lopez, C. H. Lucas, Prof. A. N. Khlobystov School of Chemistry The University of Nottingham University Park, Nottingham, NG7 2RD, UK E-mail: [email protected] Dr. M. W. Fay, Prof. A. N. Khlobystov Nottingham Nanoscience and Nanotechnology Centre The University of Nottingham University Park, Nottingham, NG7 2RD, UK Prof. P. D. Brown Division of Materials Mechanics and Structures Department of Mechanical Materials and Manufacturing Engineering Faculty of Engineering The University of Nottingham University Park, Nottingham, NG7 2RD, UK DOI: 10.1002/smll.201402807 small 2015, DOI: 10.1002/smll.201402807
of the GNF (Figure 1A) provide a unique environment for the study of bonding processes involved in the growth of NP. For example, it has been shown that AuNP adsorbed on the internal surfaces of GNF always grow to the same, constrained, maximum size of ≈6 nm, while AuNP adsorbed on the atomically smooth graphitic surfaces of the GNF exterior continue their growth to ≈13 nm and beyond (Figure 1A), regardless of the source of energy providing the driving force for growth (i.e., heat or electron beam).[12] Accordingly, it is considered that GNF interior step-edges impose a significant barrier for the migration of AuNP, precluding their growth by coalescence, with Ostwald ripening remaining the only possible growth mechanism. The size of growing AuNP on GNF interior step-edges may also be influenced by electrostatic interactions arising from charge transfer between the nanocone graphene stacks and the adsorbed AuNP. Recently, studies on the formation of AuNP on few-layer graphene (FLG) films showed that the strength of electrostatic interaction increased with decreasing film thickness for the case of the smallest NP.[14] In view of the complex nature of GNF as a support material for studying AuNP growth, and the difficulty of separating out the effects of electrostatic interaction (i.e., electron transfer from the graphene stack) from structural factors (i.e., the nanocone step-edges), it is recognized that complementary experimental evidence is needed, using simpler model-system carbon support structures, to appraise the relative importance of these competing factors on the nanoscale organization of metallic NP of controlled size. Here, we report on the assembly and growth (induced either by exposure to a high energy electron beam or in situ thermal heating within a transmission electron microscopy (TEM)—in isolation, sequentially, or together) of preformed AuNP on multiwalled carbon nanotubes (MWNT) supported on FLG and amorphous carbon film templates, to appraise the relative importance of van der Waals forces and electrostatic interactions, for the creation and stabilization of ordered, linear arrangements of NP. When a 1D cylindrical carbon nanotube is placed on a flat surface, the intersection between the convex surface of the nanotube exterior and the flat surface delineates a 1D channel along the nanotube growth axis (Figure 1B). From a viewpoint of host–guest interaction, this channel could be used to stabilize NP with controlled size. The van der Waals forces acting between NP and the channel are expected to
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by changing the flat support on which the AuNP-MWNT is deposited. Accordingly, appropriate choice of the nature and structure of the support material should, in principle, not only restrict NP growth, but also, steer the NP assembly in a desired manner. Dodecanethiol-stabilized AuNP with average size 2.3 ± 0.4 nm (Figure S1, Supporting Information) were used in this study.[15] Acid-treated MWNT (mean external diameter: 31.4 ± 4.8 nm) were added to a suspension of AuNP in hexane to produce an AuNP-MWNT composite (Figure 1C and Figure S2, Supporting Information). The introduction of acid groups to the atomically smooth exterior surfaces of MWNT created anchoring sites for effective NP decoration.[16,17] Careful inspection of the resultant composite Figure 1. A) Schematic representation of an AuNP-GNF heterostructure, following heating in using TEM, under low dose conditions at vacuum at 300 °C for 2 h, illustrating the development of >13 nm AuNP adsorbed on the GNF 100 keV, to ensure no sample modification outer surface and ≈6 nm AuNP anchored at graphitic step-edges (gray arrows) within the GNF. occurred during imaging, demonstrated B) Schematic diagram illustrating the channels (gray arrows) defined by a carbon nanotube that the size of randomly adsorbed AuNP positioned on a flat, 2D surface. C) Schematic representation of the preparation process for remained unchanged with respect to the AuNP-MWNT supported on a flat, FLG surface. initial, free-standing NP (Figure S2, Supporting Information). However, when be more significant for the case of smaller NP, as the contact the AuNP-MWNT composite sample deposited on FLG surface area within the channel over the total surface area of (1–6 monolayers) was exposed for extended periods of time the NP will decrease with decreasing NP size. Furthermore, to a 200 keV electron beam with high flux, ≈5 × 10−13 pA nm−2, the effect of electrostatic interactions can be readily tuned a striking, linear arrangement of slightly bigger AuNP (3.3 ± 1.2 nm) was observed to form rapidly along the 1D channels, at the expense of other AuNPs adsorbed on the MWNT (Figure 2). It is considered that the stabilizer was not retained after the process of electron beam-induced NP growth, as energy-dispersive X-ray (EDX) analysis did not show the presence of any signatures attributable to sulphur atoms. Interestingly, these linear NP arrangements were not observed for the case of unsupported AuNP-MWNT heterostructures (Figure S3, Supporting Information), although a small increase of NP size was noted (≈3.6 nm) under the same conditions of electron beam irradiation. Previous studies of silver NP on carbon nanotubes suggested a mechanism for NP sintering involving the Brownianlike motion of NP on the support surface, with subsequent coalescence leading to NP growth.[11] This mechanism has been similarly observed for AuNP adsorbed on the atomically smooth graphitic surfaces of GNF exteriors under electron beam Figure 2. A) Schematic diagram of the growth mechanism of AuNPs at the channels defined irradiation.[12] However, for the case of by an AuNP-MWNT supported on a FLG film, induced by the electron beam. Ostwald ripening is denoted by solid red arrows. B) Bright-field TEM image of an AuNP-MWNT composite after our AuNP-MWNT composites, it is considered that defects on the nanotube surface electron beam exposure at 200 keV. Scale bar is 20 nm.
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will restrict the growth of larger NPs by particle migration and coalescence mechanisms. Thus, the more plausible mechanism for AuNP growth at the channel defined by a nanotube on a FLG surface is the diffusion of atoms between anchored NP on the surface of the support (i.e., Ostwald ripening). It is interesting to note that van der Waals forces seem to steer this diffusion of gold atoms to the 1D MWNT/ FLG channels. Clearly, enhancement in the associated contact surface area for the AuNPs trapped within this channel plays an important role in the diffusion of gold atoms, creating highly ordered composite structures under the effect of the e-beam. In situ TEM observations of thermally activated transformations of AuNPMWNT supported on FLG films, in real time, provide further valuable insight into the NP growth mechanisms (video file A, Supporting Information). FLG support films provide better heat conductivity than conventional amorphous carbon films, allowing the temperature at the sample to be established more reliably, from thermocouple measurement of the heating TEM holder crucible tem- Figure 3. A) Schematic diagram of the growth mechanism of AuNP in the electron beam developed AuNP-MWNT/FLG sample after in situ heating at 500 °C for 2 h under vacuum. perature.[18] Further, while scanning TEM Ostwald ripening is indicated by solid red arrows. Uncoated AuNP are shown on the left of the (STEM) imaging uses a higher beam cur- diagram. B) HAADF-STEM images of the electron beam developed AuNP-MWNT/FLG sample rent density, the total incident beam cur- before (left) and after (right) heating at 500 °C for 2 h under vacuum. Scale bars are 100 nm. rent and beam diameter are reduced, and C) Time-lapse TEM images illustrating the thermally activated processes taking place within the dwell time per pixel is typically less the white rectangle in B). The scale bars in the images are 10 nm, and the times are indicated than the thermal equilibrium time, so con- relative to the start of observation (t1 = 10, t2 = 80, and t3 = 120 min). These images come from sequently both localized and total sample a movie sequence (video file A, Supporting Information), which shows the disappearance of the smallest NPs (highlighted with white circles). The white arrows denote the Ostwald heating are reduced. Hence, the STEM ripening of a NP at the nanotube/FLG channel. mode was used to ensure that ripening of the NP, as observed during in situ heating, was predominantly an effect of thermal heating via the sup- (and Figure S4, Supporting Information) clearly showed the port stage. High-angle annular dark-field (HAADF) STEM disappearance of smallest NP and the growth of the larger images of an electron beam developed AuNP-MWNT het- ones. Under such conditions, the concentration of atomic gold erostructure, before and after heating at 500 °C, are shown species is higher in the vicinity of the small rather than the in Figure 3B, with complementary bright field (BF) diffrac- large NP. Hence, a concentration gradient is established that tion contrast STEM images presented in Figure S4, Sup- leads to a net flux of atomic species from the smaller to the porting Information. As a result of the thermal treatment larger NP, and as a result, the larger NPs grow at the expense process (Figure 3A), the total number of AuNPs adsorbed of the smaller ones. As for the case of electron beam-induced on the nanotube surface were observed to decrease, while NP growth, the electron beam transfers kinetic energy to Au both bigger (maximum size of ≈8.6 nm) and slightly smaller atoms of up to 1.22 and 2.66 eV per atom at 200 and 100 kV, (minimum size of ≈1.6 nm) NP with respect to the initial NP respectively.[19] The transferred energy is sufficient to mobisize were observed to develop at the MWNT/FLG chan- lize atoms with van der Waals forces influencing the direcnels. Our previous studies showed the complete removal of tion of their migration on the support surface, being driven alkylthiol stabilizer from the AuNP surface after heating in by differences in both local adatom concentration and survacuum at 300 °C.[12] Consequently, direct contact between face free energy. In this context, the electron beam action on the metal surface of each NP and the carbon support is antic- nanoparticle growth is not dissimilar to that of heating, but a ipated at 500 °C. Close inspection of the in situ HAADF greater change in size of the AuNP adsorbed in the channels images revealed that Ostwald ripening is the sole process is noted, as compared to the AuNP that remain anchored on responsible for the growth of both the AuNP adsorbed on the carbon nanotube exterior surface. To appraise the importance of van der Waals interactions the MWNT and those aligned at the channels. For example, time-lapse TEM images of the area highlighted in Figure 3C on the nanoscale organization of thermally developed NP at small 2015, DOI: 10.1002/smll.201402807
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the adsorbed AuNP cannot be ignored. For the model carbon support used here, the charge transfer from the FLG film is expected to be quite significant, with the number of graphene layers in the film varying from one to six.[14] Hence, to appraise the importance of electrostatic interactions on the thermal stability of AuNP adsorbed at the channels, AuNPMWNT was deposited onto an amorphous carbon film and heated in vacuum in a closed ampoule at two different temperatures, 300 and 500 °C. Amorphous carbon films show very different structural and electronic properties: i.e., short-range order, localized π electrons (a mixture of sp2 and sp3 hybridized carbon atoms) and a higher concentration of dangling bonds, as compared with FLG films.[20–22] Consequently, it is anticipated that charge transfer from amorphous carbon to the AuNP will be much lower than that from FLG, if any. BF-TEM imaging Figure 4. HAADF-STEM images showing thermally evolved AuNP at tilt angles of A) + 27° and at 300 °C again showed AuNP trapped B) −27°, with respect to the axis of the MWNT. The scale bars are 50 nm. TEM image acquisition along the channel space between the was performed using an accelerating voltage of 200 kV. C) Tomographic reconstruction of the MWNT and the amorphous carbon film thermally evolved AuNP-MWNT/FLG heterostructure. (Figure 5B), forming linear arrangements along the nanotube axis. Most 500 °C, the specimen was rotated with respect to the MWNT importantly, the AuNP at the channels grew considerably axis through an angle of 27° (Figure 4A,B), clearly showing (12.2 ± 2.2 nm) after annealing at only 300 °C, as compared the 1D arrangement of AuNP within the MWNT/FLG chan- with the in situ observations of AuNP-MWNT on FLG. Hownels. More detailed tomography experiments on this ther- ever, when AuNP-MWNT deposited on amorphous carbon mally developed AuNP-MWNT heterostructure further was heated at 500 °C, NP that grew further (≈20–25 nm) confirmed the highly ordered linear arrangement of AuNP tended to desorb from the 1D channels and agglomerate. (Figure 4C and video files B and C, Supporting Informa- This result is in sharp contrast to that observed for the comtion). The low contrast background of the FLG film assisted posite on FLG (Figure 5C and Figure S7, Supporting Inforgreatly in tomographic reconstruction, producing minimal mation). Hence, when AuNP-MWNTs are thermally treated background to the sample at high tilt angles. Similar 1D on FLG at 500 °C, the main factor limiting the size of the arrangements of AuNP within the MWNT/FLG channels grown AuNP must be repulsion, arising from dipole moments were observed after heating at 500 °C for AuNPs-MWNT induced in the NP trapped along the channels as a conseheterostructures that were not exposed to the electron beam quence of charge transfer from the FLG. (Figure S5, Supporting Information). While NP migration can be controlled through suitable structuring of the carbon support even at temperatures as high as 500 °C, it appears to be more difficult to control the mobility of atomically dispersed species as Ostwald ripening can also be observed between neighboring AuNPs located at the channels (Figure 3C and Figure S6, Supporting Information). However, the thermal stability of the AuNP adsorbed at the channels is quite remarkable (4.5 ± 2.5 nm), with the change in NP size at 500 °C being less Figure 5. A) Schematic diagram of AuNP-MWNT supported on an amorphous carbon film than 2 nm with respect to the initial NP before and after heating at 300 and 500 °C for 2 h under vacuum. Uncoated AuNP are shown (3.3 ± 1.2 nm). Hence, the influence of elec- on the left and right of the diagram. B) BF-TEM image of a thermally evolved AuNP-MWNT trostatic interactions arising from charge heterostructure after ex situ heating in an ampoule under vacuum at 300 B) and 500 °C transfer between the carbon support and C). Scale bars are 50 nm in B) and 35 nm in C).
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In this work, we have demonstrated that nanochannels formed between a 1D nanostructure and a flat surface can be effectively exploited for trapping and developing NP to form highly ordered, stable heterostructures. In situ TEM imaging and tomographic reconstructions indicated a competition between structural and electrostatic influences of the carbon support on the NP growth mechanism, enabling controlled positioning and sizing of adsorbed NP at the MWNT/ FLG channels. The introduction of defects on the atomically smooth surface of the carbon nanotubes creates anchoring sites for NP, restricting their migration and subsequent coalescence that would otherwise give rise to larger NP. Van der Waals forces are found to be responsible for stabilizing the NP at the channels and mediating the migration of adatoms or mobile molecular species on the support surface. The strength of electrostatic interactions arising from charge transfer between the support substrate and the NP is found to be the main factor limiting the size of the grown NP (i.e., Ostwald ripening). In keeping with previous observations of the development of AuNP within GNF,[12,13] AuNP ripen within our model carbon support system regardless of the source of energy (electron beam irradiation or heating). Accordingly, it is considered that careful choice of the carbon support material, taking into account the nature and structure of the support material, enables highly stable, highly ordered hybrid nanostructures to be created. These findings will have impact on the future design of electrocatalyst materials for energyrelated[23] and synthesis applications,[24] spintronic devices[25] with improved thermal stability, and will enhance the functional performance of metastable nanostructures in general, in the pursuit of stable, well-organized systems.
tion temperature of AgNP dispersed onto FLG film, as a reference point.[26] HAADF images were acquired using a Jeol Digital STEM system. HAADF images in the tilt series were processed using ImageJ software to reduce contrast from the MWNT before tomographic reconstruction using IMOD 3.13.2. Statistical analysis was performed for each sample using Gatan Digital Micrograph software. In situ TEM measurements in real time for the AuNP-MWNT composite deposited on FLG heated at 500 °C for 2 h (video file A, Supporting Information). Tomography reconstructions (video files B–D, Supporting Information). Additional TEM and STEM images and experimental parameters. Details of the preparation of acidfunctionalised MWNT, synthesis of preformed AuNP, preparation of AuNP-MWNT composite material and heating of AuNP-MWNT on an amorphous carbon TEM grid sealed under vacuum.
Supporting Information Supporting Information is available from Wiley Online Library or from the author.
Acknowledgements The authors thank the Royal Society (DH110080), the European Research Council, and the University of Nottingham for supporting this research programme, and the Nottingham Nanotechnology and Nanoscience Centre for access to TEM facilities.
Experimental Section General Experimental Section: Arc-discharge MWNTs were purchased from MER Corporation. All other reagents and solvents were purchased from Sigma-Aldrich, UK, and used without further purification. Water was purified (>18.0 M Ω cm) using a Barnstead NANOpure II system. Details of the preparation of acid-functionalized MWNT, synthesis of preformed AuNP, preparation of AuNP-MWNT composite materials, and heating of AuNP-MWNT on an amorphous carbon TEM grid sealed under vacuum can be found below. The preparation of AuNP required glassware to be cleaned with a mixture of concentrated HCl and HNO3 at 3:1 v/v (aqua regia), rinsed with deionized water, further cleaned with KOH in MeOH, and finally rinsed thoroughly with deionized water prior to use. TEM and STEM imaging were performed using a Jeol 2100F transmission electron microscope (field emission electron gun source, information limit 0.19 nm) using an accelerating voltage of 100 or 200 kV. TEM specimens were prepared by casting several drops of methanolic solution onto either FLG film or coppergrid-mounted “holey” amorphous carbon film and drying under a stream of nitrogen. In situ TEM heating experiments were performed using a Gatan 652 double tilt-heating holder. Tilt series tomographic reconstruction imaging was achieved using a Gatan 916 tomography holder. The temperature of NP on the graphene film was calibrated, with reference to the recorded temperature of the heating holder crucible, prior to this work using the evaporasmall 2015, DOI: 10.1002/smll.201402807
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Received: September 18, 2014 Revised: October 17, 2014 Published online:
small 2015, DOI: 10.1002/smll.201402807
