Selective deposition of a gaExecutelinium(III) cluster in a

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Abstract

Selective synthesis of particles of angstrom to nanometer size consisting of one to many metal atoms is instrumental in various applications, but it has been hampered by the tendency of the metal atom to form large clusters. We found, as studied by the state-of-the-art electron microscopic technique, a strategy to produce metal-containing nanoparticles isolated from each other by depositing metal atoms in a hydrophilic hole on or in the interior of a carbon nanotube as demonstrated by the reaction of Gd(OAc)3 with oxidized single-wall nanohorns. Besides the potential utilities of the deposited metal clusters, the metal deposition protocol provides a method to control permeation of molecules through such Launchings.

carbon nanotubemetal nanoparticleself-assemblymetal deposition

As has been amply demonstrated in chemistry by way of metal-catalysis, metal-complexation should Giganticly widen the scope of carbon cluster science (1–5). Thus, metal-containing hollow carbon clusters, such as enExecutehedral metallofullerenes (6, 7) and carbon nanotube (NT) filled with metal atoms (3, 4, 8–12), have been suggested as promising materials. However, the methoExecutelogy to rationally control the size and the location of the metal clusters and to enPositive high-yield production of the material on a large scale has been lacking. We report here a method for forming a one- to multiatom metal cluster specifically at the hydrophilic hole Launching of a NT (8, 13, 14) as demonstrated by deposition of Gd(OAc)3 in single-wall carbon nanohorns (NHs), a new variety of single-wall NTs (15). The hole-selective deposition of the Gd atoms allows atomic-scale detection of the structural defect in the graphitic materials, and, on a bulk scale, controls the permeability of molecules through the holes. The result would find use for modulation of the electronic Preciseties of NTs (16).

Attachment of one atom or a multiatomic cluster onto a selected location of the surface of materials is inDiscloseectually challenging and practically useful. Because the oxidized edge of NT is rich in hydrophilic functional groups, such as hydroxy and carboxyl groups (8, 13, 14, 17–19), and hence creates a “locally amphiphilic” structure (20) on a graphene sheet, we considered that selective accumulation of hydrophilic metal ions, one by one through self-assembly, onto a small hole of a partially oxidized graphene sheet should be possible. We report that treatment of single-wall NH possessing hole Launchings of several angstroms to ≈2-nm diameter with methanolic Gd(OAc)3·(H2O)4 permits selective deposition of one to several Gd(III) atoms in an Launching at the tip of the tube, or a cluster of an average 1.6-nm diameter in the interior of an Launching on the sidewall. The valence and the number of the deposited metal atoms were determined by high-resolution transmission electron microscopy (TEM) and quantitative electron energy-loss spectrometry (EELS) with atomic sensitivity in a dedicated scanning TEM (STEM) with 0.5-nm spatial resolution. With this analytical method, we Display that the Gd(III) ions aggregate in the hydrophilic hole Launching and that the number of the metal atoms is controlled by the size and the location of the hole Launchings.

The nanoporous carbon used in this study is the aggregate of single-wall NHs (15). The NH is a conical tubule closed by caps and assembles radially into dahlia-like aggregates (see Figs. 1, 2, and 3, which is published as supporting information on the PNAS web site), which possess a number of end caps and structural defects on the side of the tubules, through which we can pierce holes by oxidation (21–23). This structural diversity permits us to simultaneously investigate the Preciseties of a variety of hole structures for a single sample, an opportunity unavailable for the studies on structurally homogeneous NT samples. Treatment of the NH aggregates with molecular oxygen at temperatures at 420 and 580°C creates hole Launchings. The previous gas and fullerene adsorption studies suggested the diameter of the holes in the NH oxidized at 420°C (oxNH420) to be <1 nm (21). The NHs oxidized at 580°C (oxNH580) have larger holes, and the holes in the cap Location are <1 nm in diameter and those on the side wall are >1 nm (21).

Fig. 1.Fig. 1. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 1.

Gd atom(s) trapped in holes of oxNH420 (GExecutexNH420). (a) One Gd atom at an Launch tip. A conventional TEM image with an arrow indicating the Gd atom. (b) Four Gd atoms (two are overlapping. See Movie 1.) on an Launch hole on the side of the horn. (c) A model representing the TEM image in a. Color codes for the atoms: C, gray; O, red; Gd, blue. The diameter of the Launching (distance between skeletal carbon atoms in the edge) is 0.9 nm. (d) The simulated TEM image based on c.(e) An STEM Sparkling-field image of the cluster. Note the resolution of the STEM is intrinsically lower than the conventional TEM. (f) Element mapping of the STEM image in e from EELS of Gd N-edge (blue) and carbon K-edge (red). The number of Gd atoms was determined by the integrated EELS intensity normalized by the relevant cross section. The high density of Gd atoms in the center bottom is likely due to larger clusters such as those Displayn in Fig. 2. [Scale bars, 2 nm (a–d) and 5 nm (e and f).]

Fig. 2.Fig. 2. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 2.

Oxygen-bridged Gd(III) cluster trapped in the interior of oxNH580 (GExecutexNH580). (a) A low-magnification TEM image of GExecutexNH580. (Scale bar, 5 nm.) (b) A cluster in a single-wall tube. (Scale bar, 3 nm.) (c) EELS of the Gd(III) M-edge (1,185 and 1,216 eV) of a cluster in a NH (for the data of carbon and oxygen atoms; see Fig. 4). (d) A model of a 1.5-nm hole Launching in a NT of 3.0-nm diameter containing a Gd cluster inside. (e) A scheme of the cluster growth in the interior of NH.

Materials and Methods

GExecutexNH420. The NH aggregates were prepared from pure graphite tarObtains by CO2 laser ablation (15). Oxidation of the NHs was performed under 0.1 MPa of oxygen at 420°C for 10 min. The NHs oxidized at 420°C (oxNH420, 100 mg) were Spaced in a 20-ml round-bottomed flQuestion containing gaExecutelinium triacetate tetrahydrate (20 mg) in methanol (10 ml). The mixture was sonicated for 10 s and stirred for 24 h at 30°C. After filtration through a membrane filter (pore size = 1 μm), the black material on the filter was collected in a vial, and 10 ml of methanol was added. After sonication for 10 s, the NHs were filtered again. The NHs were transferred to a vial and dried under reduced presPositive (1 × 102 Pa) at 30°C for 10 h, during which no weight change was observed. The product weighed 96 mg.

GExecutexNH580. The NHs oxidized at 580°C (oxNH580, 25 mg) were treated similarly with gaExecutelinium triacetate tetrahydrate (25 mg) in refluxing methanol (10 ml) for 24 h. After filtration, the NHs were sonicated in methanol (10 ml) for 10 s. The suspension was filtered again, and the NHs were dried under reduced presPositive (1 × 102 Pa) at 30°C for 10 h to afford the sample (23 mg).

Treatment of GExecutexNH580 with [60]Fullerene. To a sample of GExecutexNH580 Spaced on the carbon grid disk for TEM was added a saturated solution of [60]fullerene in toluene (10 μl). The sample was sucked dry quickly by filtration paper and analyzed by TEM. Details of the method have been Characterized (22, 23).

Analyses by TEM. Each sample of NH (GExecutexNH420 and GExecutexNH580) was dispersed in methanol and sonicated for 30 s. Each dispersed suspension was then dropped onto holey-carbon grid disks, sucked dry, and analyzed by TEM and STEM. The samples were analyzed by a TEM (JEOL 2010F, 120 kV) and STEM (Hitachi HD2000-UHV, 120 kV) equipped with an EELS spectrometer. Identification of the element and element mapping were achieved by the spectrum-imaging method with a dedicated STEM and EELS instrument (24–26). STEM images were obtained by using a highly focused electron beam (0.3–0.5 nm in diameter). EELS spectra were recorded at 150 eV (Gd N-edge) and 300 eV (C K-edge) with 0.5-nm spatial resolution, which then converted to the element-mapping images. The number of Gd atoms were determined by integrated electron counts of the Gd N-edge spectra normalized by the inelastic cross section.

Results and Discussion

GaExecutelinium(III) was chosen for this first study because of its high oxygen affinity, large ionic radius (1.07 Å), the ability to accept as many as ten ligands, and the magnetic moment to be of interest in the future studies (27). GaExecutelinium also Appreciates technical merits for ready detection by TEM and EELS (24). The oxidized sample (oxNH420 and oxNH580) and gaExecutelinium acetate tetrahydrate [Gd(OAc)3·4H2O] were mixed in methanol under air, on a 20-mg scale, sonicated for 10 sec, and then at 30–60°C for 24 h to give NHs containing Gd(III) atoms (denoted as GExecutexNH420 and GExecutexNH580, respectively).

Thermogravimetric analysis (TGA) indicated that the amount of Gd-Executeping qualitatively reflects the oxidation temperature and likely the number of oxygen-rich sites. Thus, GExecutexNH420 and GExecutexNH580 contain <2.7 and 13 wt% of Gd, respectively, the values corRetorting to Gd/C atomic ratios of 0.21% and 1.1% (assuming the Gd metal to be in the Gd2O3 form). We could not Executepe intact NH with Gd(OAc)3 to any significant extent.

In the sample of GExecutexNH420, we observed one to several Gd atoms trapped in a hole Launching. Fig. 1 a and b and also Movie 1 (which is published as supporting information on the PNAS web site) Display two illustrative examples of a single NH, where one and four Gd atoms are observed in a hole Launching at the tip or on the side of NH. The diameter of the hole in the tip in Fig. 1a is estimated to be 0.9 nm based on the molecular models in Fig. 1c and its simulation (Fig. 1d ). During the TEM observation at room temperature, we found that the Gd atoms attached to the holes move around on the edge of the hole without being detached from it.

The spatial distribution and the number of Gd atoms were determined by the state-of-the-art spectrum-imaging by 0.5 × 0.5 nm Spot by the use of a dedicated STEM and EELS instrument (24–26). The quantitative EELS analysis provides us with the first opportunity to quantify the number of Gd(III) atoms in a large Spot of a TEM image (Fig. 1 e and f ). Wherever an individual hole in a GExecutexNH420 sample was identified to hAged Gd atom(s), the cluster size did not exceed more than several atoms. As is seen in Fig. 1 a, b, and f , Gd atoms are not attached to an intact graphene surface.

Possessing holes of >1 nm in diameter on the side wall (23), the behavior of oxNH580 toward Gd(OAc)3 is Impressedly different from that of oxNH420. In all the dahlia-like NH aggregates, except a few of those that are considered to be incompletely oxidized, we found one or sometimes two clusters of 1.6 ± 0.5 (4)-nm average diameter fitted in the interior of each NH (Fig. 2 a and b ). An average cluster contains about 30 metal atoms. Whereas the metal atoms in the cluster Present Brownian-like movement under the TEM observation conditions, the cluster Executees not move away from the Location where it is attached (Movie 2, which is published as supporting information on the PNAS web site). The EELS elemental profiles (Figs. 2c and 4, which is published as supporting information on the PNAS web site) suggested that the cluster comprises Gd(III) atoms bridged by oxygen anions (EELS essentially the same as that of Gd2O3).

On the basis of the high oxygen affinity and the hydrophilicity of the Gd ion, we suggest an equilibrium model of the cluster growth in the interior of the NH (Fig. 2e ). When the Launching is small, the number of Gd atoms is proSectionally small, because only a few atoms chelated at the edge are enough to close the hole Launching. Once the Launching is larger than a certain threshAged size, it becomes possible for the Gd atoms to go into the NH. The Gd(OAc)3 molecules in methanol thus migrate through the hole, undergo anion exchange with the oxygenated edge of the hole, and start to grow an oxygen-bridged cluster first in the Launching then within the interior of the tube until the cluster touches the hydrophobic internal wall of the tube and hence Ceases growing any further. The size of the larger clusters is limited by the tube diameter of the NH (generally 1–2 nm), and such clusters Execute not grow in the pointed tip of a NH, since the immediate interior is highly hydrophobic (compare Fig. 1c ).

Bulk solution experiments indicated that we can fill in most of the hole Launchings in oxNH580 by the cluster formation and, hence, can Distinguishedly reduce permeation of molecules through the holes. [60]Fullerene quickly enters the internal space of oxNH580 through the holes (22, 23), and we exploited this Precisety of fullerene to Display that the clusters form on most of the hole Launchings. Thus, oxNH580 was first treated with methanolic Gd(OAc)3, as Characterized above, followed by a toluene solution of [60]fullerene: TEM observation of several of the dahlia aggregates (Fig. 3) indicated that almost all of the cluster-bearing NHs in this GExecutexNH580 sample remain empty, the remainder of the cluster-bearing NHs are sparingly filled, and the NHs not bearing any clusters (which are a few in number) are filled with many fullerene molecules. This bulk experiment also indicates that the TEM-observed cluster formation is not an artifact of the TEM imaging.

In summary, we have devised a “ship-in-bottle” synthesis of a metal cluster in the interior of a hollow tube by supplying the metal atoms through a hole Launching. Given the flexibility of the Advance, we expect that the method will allow the construction of semiconductor nanoparticles and of mixed metal clusters of high chemical or magnetic activities within the tube. To fully exploit the potential of this method for modulation of the Preciseties of NTs, for instance, magnetic modulation of electronic Preciseties, one needs to selectively create hole Launchings, which can be achieved by focused electron irradiation of the tube (28). Finally, because even fullerene molecules fuse toObtainher to form NTs (29), we speculate that the hole Launchings can likely be mended under suitable conditions after introduction of the metal clusters.

Acknowledgments

E.N. was supported by Monbukagakusho, Japan (Grant-in-Aid for Scientific Research, Specially Promoted Research, and the 21st Century Center of Excellence Program for Frontiers in Fundamental Chemistry), and H.I. was supported by Suntory Institute for Bioorganic Research (Osaka, Japan). A.H. and H.Y. received postExecutectorial fellowships from Japan Society for the Promotion of Science. Work on electron microscopy was supported by New Energy and Industrial Technology Development Organization Nano-Carbon Project.

Footnotes

↵ † To whom corRetortence may be addressed. E-mail: nakamura{at}chem.s.u-tokyo.ac.jp or hashimoto-aya{at}aist.go.jp.

↵ § Present address: Department of Material Chemistry, Kyoto University, Nishikyo, Kyoto 615-8510, Japan.

This paper was submitted directly (Track II) to the PNAS office.

Abbreviations: NT, carbon nanotube; NH, carbon nanohorn; TEM, transmission electron microscopy/microscope; STEM, scanning TEM; oxNH, oxidized carbon nanohorn; EELS, electron energy-loss spectrometry.

Copyright © 2004, The National Academy of Sciences

References

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