Observation and Characterization of Fragile Organometallic Molecules Encapsulated in Single-Wall Carbon Nanotubes

1 Department of Chemistry, Nagoya University, Nagoya 464-8602, Japan 2Department of Chemistry, The University of Tokyo, Tokyo 113-0033, Japan 3Nanotube Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8565, Japan 4Department of Information and Biological Science, Nagoya City University, Nagoya 467-8501, Japan 5Division of Physics, Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan 6 Japan Synchrotron Radiation Research Institute, Hyogo 679-5198, Japan

In 1998, it was found that single-wall carbon nanotubes (SWCNTs) are able to encapsulate C 60 fullerenes in their inner hollow space [12].Since the discovery, various materials have been encapsulated in SWCNTs, which has produced a wide variety of low-dimensional hybrid nanomaterials; SWCNTs have provided ideal nanospace and protective walls for encapsulated materials [13][14][15][16][17].In addition to their specific structure, these newly formed hybrid nanomaterials have shown interesting properties [18][19][20].
The encapsulation of organometallic complex molecules, such as Co(C 5 H 5 ) 2 [21] and Fe(C 5 H 5 ) 2 [22][23][24][25], in the SWCNTs has attracted wide attention due to an expected control of SWCNT's electronic properties through charge transfer between encapsulates and SWCNT.Furthermore, organometallics can act as a precursor to form metal atomic wires in SWCNTs via the so-called nanotemplate reaction, which may lead to the formation of novel metal atomic wires.
Encapsulation of organometallics in SWCNTs, however, has so far been difficult, mainly because of the fact that, under atmospheric condition or at high temperature, such materials normally decompose during the encapsulation process.Here, we have focused on the development of a versatile method to encapsulate fragile organometallic complexes in SWCNTs.To confirm the encapsulation, we have employed a structure determination procedure that is based on high-resolution transmission electron microscope (HR-TEM) observations and HR-TEM image simulation by the multislice method [26].

Experimental
SWCNTs were synthesized by the so-called extended direct injection pyrolytic synthesis (e-DIPS) [27].As-produced e-DIPS SWCNTs (a-CNTs) were annealed at 1200 ∘ C in vacuum (∼10 −5 Pa) for 14 hours in order to remove remaining Fe catalyst nanoparticles and amorphous carbon impurities [28].Before the encapsulation reaction, the purified e-DIPS SWCNTs (p-CNTs) were heated under dry air flow at 600 ∘ C for 30 min in order to open the endcap of SWCNTs.
ErCp 3 is an air-and moisture-sensitive material and decomposes easily upon exposure to the atmosphere [29].ErCp 3 was thus carefully handled and purified by sublimation at 200 ∘ C under high vacuum (∼10 −4 Pa) [29] and was kept in an anaerobic glove box.In the following experiments, all the sample preparation was performed using the purified ErCp 3 under anaerobic condition to avoid any undesired degradation of ErCp 3 during the preparation.Open-ended p-CNTs (o-CNTs) were vacuum sealed (∼10 −4 Pa) in a Pyrex tube with the purified ErCp 3 , and the sealed Pyrex tube was heated at 250 ∘ C for 72 hours.At this temperature and pressure, ErCp 3 was sublimed and encapsulated in the hollow space of the SWCNTs.The as-prepared ErCp 3 @o-CNTs were washed with anhydrous tetrahydrofuran in order to remove any ErCp 3 molecules physically adsorbed on the outer surface of the SWCNTs.The final products were dried at 80 ∘ C for 12 hours.As a control experiment, we also prepared a sample, ErCp 3 @p-CNTs, by performing the encapsulation process of ErCp 3 on p-CNTs.

Results and Discussion
Figure 1 shows a thermal gravimetric analysis (TGA) trace of a-CNTs and p-CNTs.Two shoulders were clearly seen at around 400 ∘ C and 600 ∘ C on the TGA curve of a-CNTs.The former and the latter shoulders correspond to the oxidation of amorphous carbon and the oxidation of SWCNTs and graphite, respectively.In contrast, only one sharp drop at 500 ∘ C and a smaller amount of residual material (3.84 wt.%) were observed in the TGA curve of p-CNTs, which indicates higher purity of p-CNTs than that of a-CNTs.
Figures 2(a) and 2(b) are HR-TEM images of a-CNTs and p-CNTs, respectively.As seen in Figure 2(a), dark contrasts of Fe catalyst nanoparticles and amorphous impurity attached outer surface of SWCNTs are clearly observed.The amount of Fe catalyst nanoparticles and amorphous impurity greatly decreases after purification, as shown in Figure 2(b).In Figures S1(a The sharp drop in sample weight at higher temperature and the smaller amount of residual material show higher quality of purified SWCNTs than that of as-produced ones. The observed interval of 0.94 nm is almost the same with the molecular size of the ErCp 3 molecule; the molecular size was estimated considering van der Waals radius of constituent atoms.Moreover, the difference between the diameter of SWCNTs (i.e., 1.30 nm) and the intervals of dot-like contrasts (i.e., 0.94 nm) observed is nearly equal to the van der Waals radii of the inner wall of SWCNTs (∼0.34 nm) [30,31].
When the diameter of SWCNTs is larger than 1.30 nm, ErCp 3 molecules aggregate to form clusters (Figure 3(b)).An EDX spectrum observed in the same area shows strong peaks that can be attributed to Er   ,   , and   (Figure S1(c)).The dark spots were not observed in o-CNTs prior to the ErCp 3 encapsulation, and no EDX peaks from Er atoms were observed.Therefore, we conclude that observed contrasts arise from encapsulated ErCp 3 molecules forming one-dimensional regular array in hollow space of SWCNTs.The estimated filling ratio from the HR-TEM images is ca.30% or more, which is substantially higher than that previously reported for organometallic complexes molecules encapsulated in SWCNTs [21,22].
To confirm the high filling ratio of ErCp 3 in o-CNTs, we have measured X-ray diffraction (XRD) patterns.Figures S2(a) and (b) show the XRD patterns of o-CNTs, ErCp 3 @o-CNTs, p-CNTs, and ErCp 3 @p-CNTs.As shown in Figures S2(a) and (b), the number of diffraction peaks is limited, so that it is difficult to determine precise filling ratio by pattern fitting.Filling of ErCp 3 molecules, however, can be confirmed by changes in intensity of (10) peak.As clearly seen in Figure S2(a), the intensity of (10) peak of ErCp 3 @o-CNTs is much smaller than that of o-CNTs.This is a clear indication that the inner space of o-CNTs is filled with guest materials (i.e., ErCp 3 ) [14,32,33].The intensity of (10) peak of ErCp 3 @p-CNTs is comparable to that of p-CNTs, suggesting that the observed intensity drop in ErCp 3 @o-CNTs can be attributed to encapsulation of ErCp 3 molecules (Figure S2(b)).The high filling ratio is caused by (1) purification and anaerobic handling of ErCp 3 , (2) an improved preparation method of o-CNTs (i.e., high quality, proper diameter, and optimized cap-opening conditions), and (3) performing the encapsulation reaction under high vacuum at optimized conditions.
Based on the observed HR-TEM images of ErCp 3 @SWCNT, we have constructed a structure model.Image simulations by the multislice method (at a defocus of 600, 650, 700, 750, and 800 nm) have been carried out based on the structure model constructed.To attain satisfactory agreements between the observed and simulated HR-TEM images, we have superimposed simulated HR-TEM images of ErCp 3 @SWCNT with different molecular orientations, in which simulated images based on fixed molecular orientations do not match with the observed images.
As illustrated in inset of Figure 3(a), the final simulated HR-TEM image well reproduces both the observed dark ellipsoids and their intensities.Hence, encapsulated ErCp 3 molecules may be rotating much faster than the time scale of HR-TEM observation (the typical exposure time is several seconds).This suggests that the interaction between encapsulated ErCp 3 and SWCNTs should fairly be weak.Since the ionization energy of ErCp 3 (7∼8 eV) is much larger than the threshold under which charge transfer interaction between SWCNTs occurs [14]; the interaction between ErCp 3 and SWCNTs should not be significant.Here the ionization energy of ErCp 3 was estimated from the ionization energy of other molecules, that is, LaCp 3 (7.9eV), PrCp 3 (7.68 eV), and TmCp 3 (7.43eV), respectively [34].
To further investigate the electronic structure of ErCp 3 , X-ray absorption spectrum (XAS) measurements at the Er  5 absorption edge were performed, which indicates trivalency of Er ions (i.e., Er 3+ ) (Figure 4).The spectrum is almost identical to that of Er 2 O 3 , suggesting that encapsulated ErCp 3 molecules do not transform into any clusters or aggregates as such.SWCNTs act only as a template that restricts the space where ErCp 3 molecules are encapsulated.The restricted space of SWCNTs is, therefore, well suited to stabilize unstable metal-containing complexes and to create low-dimensional alignment of various metal complexes including unstable organometallics.

Conclusions
We have successfully fabricated novel low-dimensional crystalline ErCp 3 nanowires encapsulated in SWCNTs with filling yield of ∼30% and characterized their structural properties.Encapsulation reactions carried out under high temperature and high vacuum conditions using high quality SWCNTs are necessary in order to obtain ErCp 3 @SWCNTs.A structure determination method based on the simulated annealing method and HR-TEM image simulation has been shown to be useful in characterizing the crystal structure of metal complex nanowires formed in SWCNTs.The present study may lead to future fabrication of various low-dimensional metal complexes in SWCNTs in high yield.

Figure 1 :
Figure1shows a thermal gravimetric analysis (TGA) trace of a-CNTs and p-CNTs.Two shoulders were clearly seen at around 400 ∘ C and 600 ∘ C on the TGA curve of a-CNTs.The former and the latter shoulders correspond to the oxidation of amorphous carbon and the oxidation of SWCNTs and graphite, respectively.In contrast, only one sharp drop at 500 ∘ C and a smaller amount of residual material (3.84 wt.%) were observed in the TGA curve of p-CNTs, which indicates higher purity of p-CNTs than that of a-CNTs.Figures2(a) and 2(b) are HR-TEM images of a-CNTs and p-CNTs, respectively.As seen in Figure2(a), dark contrasts of Fe catalyst nanoparticles and amorphous impurity attached outer surface of SWCNTs are clearly observed.The amount of Fe catalyst nanoparticles and amorphous impurity greatly decreases after purification, as shown in Figure2(b).In FiguresS1(a) and (b) (see FiguresS1(a) and S1(b) in Supplementary Material available online at http://dx.doi.org/10.1155/2014/539295), the corresponding energy dispersive X-ray (EDX) spectra are shown.Obtained spectra clearly show that the amount of residual Fe had been greatly reduced by purification.These HR-TEM images are consistent with the TGA results.Figures3(a) and 3(b) show HR-TEM images of ErCp 3 @o-CNTs having different tube diameters.As seen in Figure3(a), dot-like contrasts align in one-dimensional fashion at intervals of 0.94 nm inside SWCNTs whose diameter is 1.30 nm.
Figure 4: X-ray absorption spectra of ErCp 3 @o-CNTs and Er 2 O 3 , confirming the presence of Er 3+ in both species.