Article pubs.acs.org/JPCC
Abundant Oxygenated Groups at Hole Edges of Carbon Nanotubules Increase the Quantity of Materials Confined by Thermal Hole Closing Michiko Irie,† Maki Nakamura,† Ryota Yuge,‡ Sumio Iijima,†,‡,§ and Masako Yudasaka*,† †
National Institute of Advanced Science and Technology, Nanotube Research Center, Central 5-2, 1-1-1 Higashi, Tsukuba, 305-8565, Japan ‡ Green Innovation Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, 305-8501, Japan § Meijo University, Faculty of Science and Technology, Tenpaku-ku, Nagoya, 468-8502, Japan ABSTRACT: Holes at the tips of graphene-based nanotubules such as singlewalled carbon nanotubes and nanohorns can be thermally closed, and this enables materials to be confined inside the nanospaces. Herein, we show that increasing the number of carboxylic groups at the edge of the hole prevents them from reopening during the heat treatment. This means more material can be confined inside the nanospaces, which is demonstrated by the encapsulation of Gd oxide nanoparticles inside the carbon nanohorns.
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INTRODUCTION The inner nanospaces of graphene-based nanotubules such as single-walled carbon nanotubes (SWNTs)1 and single-walled carbon nanohorns (CNHs)2 are useful for materials storage.3 These graphene-based nanotubules are closed in an as-grown state, and holes in them are opened up for materials to enter by oxidation. Holes at the tips of tubes can be closed by heating in an inert atmosphere, 4,5 and this ability is useful for encapsulating materials inside the nanospaces. For example, Gd2O3-encapsulated CNHs can be obtained by incorporation of Gd acetate in CNHs and subsequent closure by a heat treatment at 1200 °C in Ar.6 The thermal hole-closing mechanism has not been well studied, but it is known that oxygenated groups at the hole’s edge play a key role in closing the hole, even causing close− open−close evolutions during the heating depending on the hole size.7 Herein, we report that increasing the number of carboxylic groups at the hole edge enabled an encapsulation of a larger quantity of materials within the nanospaces of the CNHs by the thermal hole closing. CNH is a single-graphene tubule with diameters of 2−5 nm and lengths of 40−50 nm, and about 2000 of them assemble to form a spherical aggregate with diameters of about 100 nm.2
exposed to m-xylene vapor at room temperature in a closed container for one hour, and this was followed by a thermogravimetric analysis performed in He (TGA(He)) by using a TGA Q500 (TA Instruments). The amount of adsorbed m-xylene was evaluated from the weight loss below 300 °C in TGA(He).9 The amounts of oxygenated hoxCNH groups and their species were estimated by performing thermogravimetric-mass spectroscopy (TG-MS) in He (using a Thermo plus TG8120, Rigaku Corporation). The method of encapsulating Gd2O3 nanoparticles inside the CNHs was previously reported.6 Briefly, Gd acetate (Gd(CH3COO)3·4H2O, 50 mg) was dissolved in ethanol (20 mL) and mixed with hoxCNH (50 mg), followed by filtration, washing with ethanol, and drying in nitrogen gas at room temperature.10 The Gd acetate incorporated hoxCNH was heat treated at 1200 °C for 3 h in Ar gas (pressure 760 Torr, flow rate 0.3 mL/min) and washed with HCl, after which Gd2O3@ CNH was obtained as a result.6 The quantity of Gd2O3 encapsulated inside the CNH was estimated from the amount of TGA(O2) residue at 1000 °C. The quantity of Gd acetate incorporated in hoxCNH was calculated considering that the residue was Gd2O3 and that hoxCNH had oxygenated groups. The quantity of oxygenated groups of hoxCNH was estimated from TG-MS performed in He. Structural observations using high-resolution transmission election microscopy (TEM) were carried out at 120 kV by using a Topcon EM-002B (Topcon Techno House Corporation). To determine the effect of the oxygenated groups in hoxCNH on the hole closure behavior, similar measurements
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EXPERIMENTAL SECTION The CNHs were produced by CO2 laser ablation of graphite.2,8 Holes were opened by treating the CNHs with hydrogen peroxide at 105 °C for 3 h. The hydrogen peroxide-treated CNHs (hoxCNH) were washed with water and dried in a vacuum (0.001 Torr) at 100 °C for one day. To close the holes, hoxCNHs were heat treated at various temperatures (600, 800, 1000, and 1200 °C) for three hours.7 To study the hole-closing process, hoxCNHs were heat treated for various periods (18−198 min) at 1200 °C.7 The degree of closure was evaluated by measuring the amount of adsorbed m-xylene. The heat-treated hoxCNHs were © 2012 American Chemical Society
Received: March 1, 2012 Revised: May 7, 2012 Published: May 16, 2012 12886
dx.doi.org/10.1021/jp3020222 | J. Phys. Chem. C 2012, 116, 12886−12889
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were performed on CNH with fewer oxygenated groups. In this case, the CNHs were oxidized by slow combustion.11 They were heated in dry air by increasing the temperature 1 °C/min from room temperature to various target temperatures ranging from 300 to 500 °C. These samples are designated as airCNH or airCNH(450 °C) for the target temperature of 450 °C.
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RESULTS AND DISCUSSION Effect of Heat Treatment Temperature on Hole Closure Behavior. The TEM image of hoxCNH in Figure 1a indicates that the spherical shape of the nanohorn aggregate
Figure 3. Xylene quantities adsorbed by hoxCNH and air CNH (oxidation target temperatures: 300−500 °C), depending on the period (0−196 min) of heat treatment at 1200 °C. Data of airCNHs are taken from ref 7.
wherein small holes closed quickly (airCNH(300 °C)), whereas large ones closed slowly (airCNH(500 °C)). The mediumsized holes showed a close−open−close evolution (airCNH(350, 400, 450 °C)),7 which can be explained by the oxygenated groups changing and forming C−O−C bridges over the holes early in the heat treatment; the temporary closure is however unstable and is broken up upon further heating, occurring subsequent slower closing.7 The time course of the hole closing of hoxCNH by the heat treatment was quite different from those of airCNHs. There were a few similarities in the time courses of hoxCNH and airCNH(450 °C) (Figure 3), but the holes of hoxCNH did not reopen. The holes of airCNH(500 °C) and hoxCNH started to close monotonically, however, the airCNH(500 °C) holes did not close as quickly, and the degree of closure after the HT was smaller (Figure 3). For the purpose of increasing the quantity of materials that are encapsulated inside CNH by exploiting the thermal holeclosing phenomenon, hoxCNH seemed to have advantages over airCNHs. Indeed, the large inner nanospace is suitable for preincorporation of materials. In addition, the characteristics of initial quick closing, no reopening, and high degree of closure should prevent materials inside CNHs from escaping by diffusion or vaporization during the heat treatment. Numerous Carboxylic Groups in hoxCNH. Let us discuss why the behavior of hoxCNH was different from that of airCNH on the basis of the TG-MS and TGA(He) data. The TG-MS data show that hoxCNH had carboxylic groups apparent from CO2 emissions below 400 °C (Figure 4a).12 The quantity was about 2.5%, as estimated from TGA (He) (Figure 4c). The 2.5% of weight loss caused by the CO2 emission means that one hoxCNH tubule (diameter 3−4 nm and length 40−50 nm,2 10 000−20 000 carbon atoms) has 70− 140 carboxylic groups. This number of carboxylic groups is enough to fully modify the edges of one or two holes with typical sizes of 1.5−2 nm (45−60 edge carbons) and other holes with different sizes opened in one hoxCNH tubule.13 According to our previous study, the emission of CO2 finished within 18 min since the heating at 1200 °C started.7 The airCNH(450 °C) and airCNH(500 °C) had almost no carboxylic groups (Figures 4a and 4c). CO was similarly emitted from the three specimens above 500 °C (Figure 4b). It is likely that carboxylic groups at the edges of hoxCNH caused
Figure 1. TEM image of the hoxCNH aggregate.
was not changed by the H2O2 treatment. The holes in the hoxCNHs were well opened because the adsorbed xylene was 0.37 g/g (Figure 2, red arrow), which is almost the highest
Figure 2. Xylene quantities adsorbed by hoxCNH and air CNH (oxidation target temperatures: 350−500 °C) depending on the heat treatment temperature (rt−1200 °C) for 3 h. Data of airCNHs are taken from ref 7.
value ever obtained, 0.39 g/g, for open nanohorns with a specific surface area of 1300−1400 m2/g and pore volume 0.8− 0.9 m3/g.11 Adsorbed xylene decreased, namely, the degree of closure increased, with the increase of the heat treatment (HT) temperature (Figure 2, red line). By the heat treatment at 1200 °C, it became 0.136 g/g. This value was still a little larger than 0.09 g/g of as-grown CNH with few holes,11 indicating that some holes, mostly those on the sidewalls, remained open.5 In Figure 2, the hole-closure profile of hoxCNH is similar to those of airCNHs, especially that of airCNH(450 °C). Note that the profiles of airCNHs in Figure 2 are from ref 7. HT temperature dependence did not reveal much information about the closing mechanism, but the time course of the closure did, as shown in the following. Time Course of Hole Closing Behavior Influenced by Oxygenated Groups. Figure 3 shows the results from ref 7 12887
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Figure 5. Thermogravimetric analysis of Gd2O3@CNH prepared using hoxCNH and airCNH(450 or 500 °C) (a) and TEM images of Gd2O3@CNH prepared by using hoxCNH (b, c). The Gd2O3 particles are near the center of the aggregates in (b) and CNH sheaths in (c). Thermogravimetric analysis of Gd acetate-incorporated hoxCNH and Gd acetate-incorporated airCNH(450 or 500 °C) (d).
Figure 4. TG-MS measurement results for m/e 44 (CO2) (a) and 28 (CO) (b) emitted from hoxCNH and airCNH(450 or 500 °C) and weight change measured by TGA in a He atmosphere (c).
the holes of hoxCNH to close differently from those of airCNHs. The above discussion on the hole-closing mechanism and effect of oxygenated groups enables us to infer that the numerous carboxylic groups at the edges of hoxCNH holes thermally change and form multiple bridges that were made of carbon and oxygen such as the C−O−C bridges over the holes at an early stage of the heating. Perhaps because of the multiple bridges, the break-up of the bridges was difficult to occur. Indeed the reopening of holes was unlikely to happen. Encapsulation of Gd2O3 inside CNH by Thermal Hole Closure. To confirm that hoxCNH is more appropriate than airCNH for the material encapsulation purposes, Gd2O3 nanoparticles were encapsulated and their quantities compared. Gd2O3 particles in Gd2O3@CNH prepared by using hoxCNH appeared as black spots in the TEM images (Figures 5b and 5c). Gd2O3 was 16% when hoxCNH was used; the estimate was made from the amount of residue at 1000 °C measured by TGA in oxygen gas. The percentage was significantly smaller, about 7−9%, when airCNH(450 °C) or airCNH(500 °C) was used (Figure 5a). Indeed, the quantities of Gd acetate initially incorporated in hoxCNH, airCNH(450 °C), and airCNH(500 °C) were similar, 22−26%, which is reasonable because the amounts of adsorbed xylene on hoxCNH and airCNH(450, 500 °C) before the heat treatments were similar; namely, their pore volumes were similar (Figures 2 and 3). Thus, it has been shown that the hoxCNH having more oxygenated groups at the hole edges than airCNHs can encapsulate more materials
within their inner hollow nanospaces by the thermal hole closing.
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CONCLUSION We found that the number of carboxylic groups around the edges of holes of CNHs influenced the thermal hole closure process. Having more carboxylic groups around the edge enables more materials to be thermally confined inside CNH because the holes close quickly and do not reopen when there are many groups. In addition, the CNHs so treated have a higher degree of closure. We believe that oxygenated groups affect the closure process of single-wall carbon nanotubes and other graphene-based nanocontainers; thus, the results reported here would be useful for controlling the properties of graphenebased nanocontainers by confining materials inside them.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Ms. Jing Fan for oxidizing CNH with hydrogen peroxide. 12888
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