Potassium intercalation of carbon onions ‘opened’ by carbon dioxide treatment

CARBON

4 6 ( 20 0 8 ) 1 1 3 3–11 4 0

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Potassium intercalation of carbon onions ‘opened’ by carbon dioxide treatment Yu.V. Butenkoa,*, Amit K. Chakrabortyb,c,1, N. Peltekisa, S. Krishnamurthya, V.R. Dhanakd,e, M.R.C. Huntb, L. Sˇillera,* a

School of Chemical Engineering and Advanced Materials, The University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, UK Department of Physics, Durham University, Durham, DH1 3LE, UK c School of Physics and Astronomy, The University of Nottingham, Nottingham, NG7 2RD, UK d CCLRC, Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, UK e Physics Department, University of Liverpool, Liverpool, L69 3BX, UK b

A R T I C L E I N F O

A B S T R A C T

Article history:

The potassium intercalation of onion-like carbon (OLC) samples consisting of aggregates of

Received 13 November 2007

carbon onions is studied with photoemission spectroscopy. OLC samples were initially pre-

Accepted 7 April 2008

pared by annealing nanodiamonds (3–20 nm in diameter) at 1800 K in vacuum. The result-

Available online 12 April 2008

ing OLC consists of closed fullerene-like shells. The ‘closed’ OLC was subsequently treated with carbon dioxide at 1020 K in order to open the carbon shells by partial oxidation to create ‘opened’ OLC. Core level and valence band photoelectron spectroscopy have been employed in characterizing the changes in electronic structure of the samples. Upon intercalation of the closed OLC with K the C1s core level and valence band features shift to higher binding energies and the density of states at the Fermi level increases, while this effect is significantly smaller for intercalated opened OLC. These results indicate that opening the shells of carbon onions allows potassium to penetrate inside the particles and thus opens up a possible route to fill carbon onions with desired substances and their application as nanocapsules.  2008 Elsevier Ltd. All rights reserved.

1.

Introduction

Synthesis of carbon onions filled with different substances is of interest for several applications. In recent years there have been a number of publications demonstrating that magnetic nanoparticles encapsulated into graphite-like layers may find applications as components of magnetic recording systems [1,2], magnetic fluids [1,2], electromagnetic shielding materials [3] or magnetic resonance imaging agents [4]. Carbon onions can even serve as nanocapsules for high pressure experiments [5–7]. External graphite layers in such materials

provide protection to the inner substances [8] and can serve for attachment of desirable functional groups [9] for further applications, for example, as drug delivery agents as in case of carbon nanotubes [10]. Existing methods of synthesis of filled carbon onions such as arc-discharge [5,11–15], annealing [2,16,17], electron bombardment [11,18] of carbonaceous material in the presence of catalytic metal particles, and chemical vapour deposition techniques [19] require the presence of all substances in the reaction media at high temperature. Therefore the substances which can be encapsulated into carbon onions are limited to metals, metal oxides and

* Corresponding authors: E-mail addresses: [email protected] (Yu.V. Butenko), [email protected] (L. Sˇiller). 1 Present address: Department of Chemistry, Durham University, Durham, DH1 3LE, UK. 0008-6223/$ - see front matter  2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.04.012

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CARBON

4 6 ( 2 0 0 8 ) 1 1 3 3 –1 1 4 0

carbides. Another disadvantage of these techniques is that the size and number of shells of the filled carbon onions depend on the properties of the encapsulated substances. This can restrict the synthesis of filled carbon onions with structures needed for a particular application. Another approach to the synthesis of filled carbon onions is to use a two stage process: initial opening of the carbon onions and subsequent filling of the opened onions with desired substances. This type of approach has been successfully used for carbon nanotubes. It has been shown that opening of initially closed carbon nanotubes can be achieved by treatment with carbon dioxide [20], nitric acid [21] and oxygen [22]. Opened carbon nanotubes have been successfully filled with different substances [22–26]. Such a two stage process would also allow one to have a larger choice of carbon onions, enabling selection of material with particular structures (particle size, number of shells, size of inner space), which better satisfy final applications. Tsang et al. used high resolution transmission electron microscopy (HRTEM) to study changes in multi-walled carbon nanotube structures brought about by their partial oxidation with carbon dioxide [20]. It was demonstrated that this treatment results in the partial or complete destruction of the tube caps, i.e. the formation of holes at the nanotube ends. However, it is rather more difficult to observe the formation of such holes in the shells of the carbon onions by HRTEM, since these holes have lower contrast on HRTEM images. In this paper we employ carbon dioxide treatment to open up the carbon shells of closed carbon onion structures, such as those present in onion-like carbon (OLC), and use potassium intercalation as a means to probe this opening. Suzuki et al. showed that in case of closed single-walled carbon nanotubes potassium intercalates between the individual carbon nanotubes within their bundles [27]; whereas potassium penetrates inside opened carbon nanotubes [25]. In our previous studies we demonstrated, by means of photoelectron spectroscopy, that potassium intercalation of closed OLC results in core level and valence band states shifting to higher binding energies, and an increase of the density of states at the Fermi level, both of which are associated with charge transfer from potassium to the OLC [28]. Since the electronic properties of potassium intercalated graphitic structures strongly depend on the location of the potassium atoms (whether on external surfaces and/or inside the graphitic structures) [25,27,29–31], the electronic states of intercalated ‘closed’ and ‘opened’ OLC have been investigated using photoelectron spectroscopy to test the efficacy of our opening and filling procedure.

2.

Experimental

Samples of OLC were prepared by heat treatment of nanodiamonds (diamond crystallites with diameters of 3–20 nm) at 1800 K under a vacuum of 105 mbar for 1 h: details of the preparation methods are described elsewhere [32]. Studies using HRTEM [32,33], density measurements [32], and X-ray photoelectron spectroscopy [34] have shown that annealing nanodiamond in this way results in complete transformation of diamond nanocrystallites into graphitic OLC containing

carbon onions with closed outer fullerene-like shells. HRTEM images showed that the sample consisted of OLC carbon particles mainly composed of 3–10 fullerene-like spherical shells and extended curved graphitic layers between them, binding the carbon onions together. The OLC sample containing closed carbon onions is referred to throughout this paper as ‘closed OLC’. Opening the carbon onions in the closed OLC sample was achieved by treatment in a flow of carbon dioxide as follows: (1) the closed OLC sample was placed in a ceramic boat and inserted into a ceramic tube; (2) the sample was heated to 1020 K under nitrogen flow (40 cm3/min); (3) after the sample temperature reached 1020 K the nitrogen was replaced with carbon dioxide; (4) the sample was maintained at 1020 K under carbon dioxide flow (40 cm3/min) for 1 h at atmospheric pressure; (5) the sample was cooled down to room temperature under nitrogen flow. This sample is referred to throughout the paper as ‘opened OLC’. Potassium intercalation of the two types of samples and characterization by X-ray and ultraviolet photoelectron spectroscopies were carried out at Beamline 4.1 of the Synchrotron Radiation Source (SRS), Daresbury Laboratory, UK. Both samples (closed and opened OLC) were ultrasonicated in isopropanol and drops of the resulting suspension were deposited onto silicon substrates with a native oxide layer until a macroscopically thick film was produced. Once dried, the samples were mounted in a sample holder using tantalum retaining clips in good electrical contact with the OLC films and introduced into an ultra-high vacuum (UHV) chamber with a base pressure below 2 · 1010 mbar. Before potassium intercalation the samples were annealed at 1270 K for 10 min to remove adsorbates such as condensed water, traces of isopropanol and any oxygen-containing groups bound to the OLC. Potassium intercalation was performed by evaporating potassium onto the OLC films from commercial (SAES) ‘getter’ sources while keeping the samples at room temperature. For each sample of closed and opened OLC a new getter source was used. Valence band photoelectron spectra were measured using a SCIENTA SES-200 analyzer at a fixed pass energy of 40 eV and a photon energy of 40 eV. The binding energy scales of the valence-band spectra were calibrated by measuring the position of the Fermi edge obtained from a platinum foil in good electrical contact with the sample. For the valence band data an overall energy resolution of 0.15 eV was determined from the width of the Fermi cutoff. The C1s and K2p core-level spectra were acquired with a VG Scientific CLAM2 analyzer and Mg Ka radiation (1253.6 eV) from a conventional X-ray source. The C1s binding energy of a highly oriented pyrolytic graphite reference sample (284.4 eV) was utilized for calibration of the core-level photoelectron spectra. An overall energy resolution of 0.75 eV was determined from the Gaussian width of a Au 4f line from a gold foil in good electrical contact with the samples. The C1s photoemission spectra of sp2 bound carbon were fitted using a Doniac–Sˇunjic´ lineshape [35] convoluted with a Gaussian broadening. The p plasmon peak of graphitic carbon was fitted with a Lorentzian peak convoluted with the same Gaussian employed for broadening the sp2-related peak. The background photoelectron intensity was subtracted by the Shirley method [36].

CARBON

3.

4 6 ( 20 0 8 ) 1 1 3 3–11 4 0

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Results and discussion

Fig. 1A shows core level photoelectron spectra of the closed OLC sample after successive treatments. The spectra were fitted with a component for sp2 carbon and a component associated with p–p* transitions in graphitic materials (p plasmon) [37]. The fitting curves are presented only for the as-introduced sample, as shown in Fig. 1A (a). The fitting parameters of the sp2 component for all spectra are presented in Table 1. Fitting the C1s spectrum of the as-introduced sample shows (Fig. 1A (a)) the absence, within an experimental error of 0.5%, of components at higher binding energies related to sp3 carbon and carbon atoms bound to oxygen-containing groups. Typical binding energies of C1s electrons in sp3 hybridized carbon lie in the range 285.7–286.1 eV and for oxygen-containing groups binding energies lie between 286.5 eV and 287.7 eV [34]. Analysis of the O1s photoelectron spectrum (not shown) of the as-introduced closed OLC also indicated negligible oxygen concentration (less than 0.5%). Fig. 2A presents valence band photoelectron spectra of the closed OLC after different successive treatments of annealing and potassium intercalation. The key features of the valence band spectrum of the as-introduced sample (Fig. 2A (a)) are two prominent peaks at 2.9 ± 0.1 eV and 7.9 ± 0.1 eV, and a shoulder at 4.2 ± 0.2 eV. The peaks at 2.9 ± 0.1 eV, and 7.9 ± 0.1 eV are related to p bonding and r bonding states in graphite, respectively [38,39] while the shoulder at 4.2 ± 0.2 eV is associated with mixed r–p states [39]. We believe that the broad peaks at approximately 14.1 and 16.6 eV are associated with r bonding states of carbon atoms in the sample. The slight increase in intensity of the peak at 14.1 eV after annealing at 1270 K for 10 min is related to the desorption of water and residual solvent from sample surfaces. Also of interest is the region in the vicinity of the Fermi level where an increase in the density of states (DOS) as a function of potassium dose is clearly visible. For clarity this region is plotted in a separate graph in Fig. 3A which shows the near Fermi level valence band spectra of the closed OLC after different successive treatments of annealing and potassium intercalation. These results also show that annealing the as-introduced closed OLC at 1270 K for 10 min in UHV does not significantly affect the C1s and valance band spectra (see Table 1 and compare Fig. 1A (a) and (b), and Fig. 2A (a) with (b)). Intercalation of the sample by potassium results in the appearance of new peaks at binding energies of 294.6 ± 0.1 eV and 297.4 ± 0.1 eV, which become most prominent after the highest potassium dose (see inset in Fig. 1A). These peaks are assigned to the K2p doublet [40]. The atomic percentages of potassium in the sample after the different potassium doses were determined from the C1s and K2p peak areas taking into account the corresponding sensitivity factors, and are presented in Table 1. For adsorption of potassium on HOPG the K2p3/2 peak was previously observed at the same binding energy of 294.6 ± 0.1 eV and was attributed to ‘‘elemental’’, not oxidized, potassium [40]. Therefore, we can conclude that the potassium deposited on the OLC sample is not oxidized. In the valence band spectra the presence of potassium in the OLC sample as a result of potassium dose becomes evident by the emergence of a rather broad feature at 20.4 ± 0.2 eV

Fig. 1 – (A) Evolution of C1s core-level spectra of closed OLC as a function of successive treatments: (a) as-introduced; the points are experimental data and solid lines are the fit components into which the spectra were decomposed; the resulting fit is superimposed on the data as a solid line, (b) after annealing at 1270 K for 10 min. After potassium deposition for: (c) 90 min, (d) 115 min, (e) 140 min, (f) 190 min; (g) after final annealing at 1270 K for 10 min. The inset shows the K2p doublet (2p3/2 = 294.6 eV; D = 2.8 eV) of the sample after 190 min of potassium deposition and the p plasmon peak (shake-up satellite) at a binding energy of 291.0 ± 0.1 eV expanded from spectrum (f)). (B) Evolution of C1s core-level spectra of opened OLC as a function of various successive steps of annealing and potassium doses: (a) as-introduced; the points are experimental data and the solid lines underneath are the fit components into which the spectra were decomposed; the resulting fit is superimposed on the data as a solid line; (b) after annealing at 1270 K for 10 min; after potassium deposition for: (c) 90 min, (d) 115 min, (e) 140 min, (f) 190 min, (g) 250 min; and (h) after final annealing at 1270 K for 10 min. The inset shows the K2p doublet (2p3/2 = 294.6 eV; D = 2.8 eV) of the sample after 250 min of potassium deposition and the p plasmon peak (shake-up satellite) at a binding energy of 291.0 ± 0.1 eV expanded from spectrum (g). The spectra were recorded using a photon energy of 1253.6 eV in normal emission geometry.

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CARBON

4 6 ( 2 0 0 8 ) 1 1 3 3 –1 1 4 0

Table 1 – Fitting parameters for the sp2-related C1s photoemission line (Doniac–Sˇunjic´ lineshape) of closed OLC (Fig. 1A) and apparent percentage of potassium in the sample after different potassium doses deduced from the photoemission spectra Fitting parameters for sp2 components of C 1 s spectra presented in Fig. 1A

Different successive stages of the treatment of closed OLC Binding energy, eV (a) Initial (as-introduced) sample (b) Annealing at 1270 K for 10 min (c) 90 min of K deposition (d) 115 min of K deposition (e) 140 min of K deposition (f) 190 min of K deposition (g) Final annealing at 1270 K for 10 min

Lorentzian width, eV

FWHMa, eV

Percentage of potassium,%

0.26 ± 0.01 0.26 ± 0.01 0.29 ± 0.01 0.29 ± 0.01 0.30 ± 0.01 0.32 ± 0.01 0.27 ± 0.01

1.23 ± 0.01 1.22 ± 0.01 1.28 ± 0.01 1.32 ± 0.01 1.34 ± 0.01 1.35 ± 0.01 1.25 ± 0.01

0 0 0.3 ± 0.2 0.4 ± 0.2 0.6 ± 0.2 0.7 ± 0.2 0

284.40 ± 0.03 284.40 ± 0.03 284.49 ± 0.03 284.51 ± 0.03 284.52 ± 0.03 284.55 ± 0.03 284.40 ± 0.03

Gaussian broadening, representing instrumental broadening and sample inhomogeneity, is fixed for all samples at 0.75 eV. The singularity index is fixed at 0.15 [34]. The position of the shakeup peak for all spectra is 291.0 ± 0.1 eV. a Full width at half maximum.

(Fig. 2A (c)–(f)) associated with K3p electrons [28]. The position of this peak is higher than the reported value of 19.3 eV for adsorption of potassium on graphite [30] and is closer to the value of 19.8 eV observed for this peak in potassium intercalated OLC produced by annealing nanodiamonds at a different temperature (2140 K) to the present sample [28]. A closer inspection of Fig. 1A reveals that upon potassium intercalation the C1s peak gradually shifts towards higher binding energies and becomes broader (see Table 1). The maximum shift of 0.15 ± 0.03 eV is registered after 190 min of potassium deposition (see Table 1 and Fig. 1A (f)). Similar shifts of main spectral features towards higher binding energies upon potassium intercalation are also observed in the valence band spectra (Fig. 2A), but are of larger magnitude. At the maximum potassium dose of 190 min the valence band shift reaches 0.30 ± 0.05 eV (compare spectra (b) and (f) in Fig. 2A). The upward shift of the spectral features is also accompanied by a significant increase in the density of states (DOS) at the Fermi level (see Figs. 2A and 3A) upon potassium deposition. The increase of DOS at the Fermi level and upward shift of features in photoelectron spectra upon potassium intercalation has been previously reported for graphite [29,31,40,41], carbon nanotubes [25,42] and OLC produced by annealing nanodiamonds at 2140 K [28]. This behaviour is explained by the charge transfer from the donor potassium atoms into unoccupied states of graphitic samples. Filling of the unoccupied states near Fermi level results in a continuous movement of the Fermi level ‘‘up’’ the DOS [28]. Since the binding energies of the photoelectrons are presented with reference to the Fermi level, the upward movement of the Fermi level causes shifts in the position of photoemission peaks towards higher binding energies. In Fig. 3A this is manifested by an apparent change in curvature of the rising intensity on the high binding energy side of the Fermi level. The upward shift of the Fermi level is not visible because the binding energy scale is referenced to this level (Fig. 3A). Desorption of potassium upon final annealing of the closed OLC at 1270 K for 10 min results in the vanishing of charge transfer and in the recovery of core level and valence band spectra, as indicated by the reduction of DOS at the Fer-

mi level to its original intensity, a loss of the K3p peak in the valence band spectra (compare Fig. 2A (g) with (b) and Fig. 3A (g) with (b)), and a loss of the K2p peak and associated shifts in the C1s core level spectra (compare Fig. 1A (g) with (b)). Fig. 1B presents the C1s photoelectron spectra of opened OLC sample after different successive treatments of annealing and potassium doses. The spectra were again fitted with a component for sp2 carbon and a component accounting for the p plasmon peak. The fitting parameters for the sp2 component are presented in Table 2. Treatment of the closed OLC sample with carbon dioxide resulted in a shift of the sp2 peak by 0.05 ± 0.03 eV to higher binding energies (compare positions for the non-intercalated samples in Tables 1 and 2). Taking into account that the experimental error of 0.03 eV is very close to the observed shift, we repeated this experiment with three different closed OLC samples treated with CO2 and found that this shift is reproducible. Analysis of C1s (Fig. 1B) and O1s (not shown) spectra of the sample revealed no significant presence of oxygen-containing groups on its surfaces. To explain this we note that the reaction of carbon with carbon dioxide at 1070 K at atmospheric pressure results in oxidation of carbon according to the following reaction [43]: Csolid þ CO2; gas () 2COgas : As a result of the above reaction some carbon atoms are removed from the graphite layers. The observed shift towards higher binding energy of the sp2 component can be explained by the appearance of carbon atoms which have lost their neighbours. These carbon atoms can be present in defects such as the edges of voids produced in graphite-like layers as a result of the CO2 treatment. It is notable that for a single-wall carbon nanotube sample subjected to Ar ion bombardment the sp2-related component of the C1s peak shifted to lower binding energy and there were strong changes in the valence band shape near the Fermi level [44]. Unlike the case of high fluence Ar ion bombardment, no evidence of any feature associated with sp3 hybridized carbon was found for the opened OLC. The difference between the results of Ar ion bombardment of graphitic material and oxidation with carbon dioxide suggest a qualitative difference

CARBON

4 6 ( 20 0 8 ) 1 1 3 3–11 4 0

Fig. 2 – (A) Evolution of valence band spectra of closed OLC as a function of successive steps of annealing and potassium dose: (a) as-introduced; (b) after annealing at 1270 K for 10 min; after potassium deposition for: (c) 90 min, (d) 115 min, (e) 140 min, (f) 190 min; and (g) after final annealing at 1270 K for 10 min. (B) Evolution of valence band spectra of opened OLC as a function of successive steps of annealing and potassium dose: (a) after annealing at 1270 K for 10 min; after potassium deposition for: (b) 90 min, (c) 115 min, (d) 140 min, (e) 190 mins, (f) 250 min; and (g) after final annealing at 1270 K for 30 min. Spectrum (a) is replotted as (h) for direct comparison with (g). Spectra were recorded using a photon energy of 40 eV in normal emission geometry. between the nature of the defects formed in the two processes, and that a low level of overall damage results from OLC opening. To explain the negligible presence of oxygen-containing groups in the opened OLC sample we note that after CO2 treatment the sample was cooled under nitrogen flow from 1020 K to room temperature. Therefore, if during CO2 oxidation any oxygen-containing groups were formed on sample surfaces they would have been removed by the subsequent heating in inert nitrogen atmosphere. We also note that before insertion into the UHV chamber the opened OLC sample was exposed for approximately 1 h to atmosphere at room temperature. However, the apparent absence of oxygen-con-

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Fig. 3 – (A) Evolution of valence band spectra of closed OLC in the near Fermi level region as a function of successive steps of annealing and potassium dose: (a) as-introduced; (b) after annealing at 1270 K for 10 min; after potassium deposition for: (c) 90 min, (d) 115 min, (e) 140 min, (f) 190 min; and (g) after final annealing at 1270 K for 10 min. (B) Evolution of valence band spectra of opened OLC in the near Fermi level region as a function of successive steps of annealing and potassium dose: (a) after annealing at 1270 K for 10 min; after potassium deposition for: (b) 190 min, (c) 250 min; and (d) after final annealing at 1270 K for 10 min. The dashed line marks the Fermi level position. Spectra were recorded using a photon energy of 40 eV in normal emission geometry.

taining groups (as revealed by photoemission data) in the opened OLC sample indicates that carbon atoms occurring in different defect structures in graphite-like layers are relatively inert and do not react with atmospheric oxygen and water at room temperature. Further annealing of the opened OLC at 1270 K for 10 min in UHV resulted in only a slight narrowing of the main sp2 component of the C1s spectrum, while its position remained the same, within experimental error, compared with the sample before annealing (see Table 2). Valence band spectra of the opened OLC are presented in Fig. 2B. The spectrum of the opened OLC sample after annealing in vacuum at 1270 K (Fig. 2B (a)) contains the same features as the spectrum of the closed OLC after annealing under the same conditions (Fig. 2B (b)). The similarity between the spectra from opened and closed OLC (compare

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CARBON

4 6 ( 2 0 0 8 ) 1 1 3 3 –1 1 4 0

Table 2 – Fitting parameters for the sp2-related C1s photoemission line (Doniac–Sˇunjic´ lineshape) of opened OLC (Fig. 1B) and apparent percentages of potassium in the sample after different potassium doses Different successive stages of the treatment of opened OLC (a) Initial (as-introduced) sample (b) After annealing at 1270 K for 10 min (c) 90 min of K deposition (d) 115 min of K deposition (e) 140 min of K deposition (f) 190 min of K deposition (g) 250 min of K deposition (h) Final annealing at 1270 K for 10 min

Fitting parameters for sp2 components of C 1s spectra presented in Fig. 1B Binding energy, eV 284.45 ± 0.03 284.46 ± 0.03 284.55 ± 0.03 284.54 ± 0.03 284.55 ± 0.03 284.56 ± 0.03 284.57 ± 0.03 284.44 ± 0.03

Lorentzian width, eV

FWHM, eV

Percentage of potassium,%

0.27 ± 0.01 0.25 ± 0.01 0.28 ± 0.01 0.29 ± 0.01 0.29 ± 0.01 0.29 ± 0.01 0.30 ± 0.01 0.27 ± 0.01

1.26 ± 0.01 1.22 ± 0.01 1.28 ± 0.01 1.31 ± 0.01 1.31 ± 0.01 1.31 ± 0.01 1.34 ± 0.01 1.25 ± 0.01

0 0 0.3 ± 0.2 0.3 ± 0.2 0.4 ± 0.2 0.4 ± 0.2 1.4 ± 0.2 0

Gaussian broadening, representing instrumental broadening and sample inhomogeneity, is fixed for all samples at 0.75 eV. The singularity index is fixed at 0.15 [34]. The position of the shakeup peak for all spectra is 291.0 ± 0.1 e V.

Fig. 2A (b) and Fig. 2 B (a)) supports the assertion that the treatment with CO2 did not significantly damage the sample. To compare the influence of potassium intercalation on the electronic structure of closed and opened OLC we deposited potassium on the sample of opened OLC to the same doses as the sample of closed OLC. As with the closed OLC sample, potassium intercalation of the opened OLC results in the appearance of a K2p doublet, with components at 294.7 ± 0.1 eV and 297.5 ± 0.1 eV (see inset in Fig. 1B), the position of which, within experimental error, is the same as that for the potassium intercalated closed OLC (see inset in Fig. 1A). However, the atomic percentages of potassium acquired from the relative K2p and C1s peak areas in the sample after different potassium doses is lower than that after the approximately same potassium doses on closed OLC (see Table 2). For example, the atomic percentage of potassium in the sample of opened OLC after the dosing for 190 min is 0.4 ± 0.2% which is lower than that after the approximately same potassium dose in closed OLC, which is 0.7 ± 0.2% (see Table 1). We believe that this can be explained by penetration of potassium deep inside the carbon onions in the opened OLC sample and therefore a reduction in sensitivity to potassium atoms situated deeper in the carbon onions than the ˚ [45]). Further potassium photoelectrons escape depth (15 A dosing for a total of 250 min resulted in an increase of the potassium concentration to 1.4 ± 0.2%. Comparison of the positions of K3p peaks in the valence band spectra of the opened and closed OLC after potassium deposition revealed a rather surprising result: the binding energy of the K3p peak of the opened OLC was found to be approximately 19.4 eV (see Fig. 2B (b)–(f)). This value of the binding energy is lower than that for the K3p peak observed in closed OLC (see Fig. 2A (c)–(f)) and is almost the same as was reported for potassium intercalated graphite [30]. Further differences between closed and opened OLC can be found when one compares the spectral features in the photoemission spectra of closed and opened OLC upon potassium intercalation of an approximately equal dose of 190 min. A smaller shift of the C 1s level in the photoemission spectrum of the opened OLC (Fig. 1B (f)) is observed compared with that for the closed OLC sample (Fig. 1A (f)): for closed OLC this shift is 0.15 ± 0.03 eV, while for opened OLC this shift is only 0.10 ± 0.03 eV (see Tables 1 and 2). Even an additional potassium dose for 60 min on the opened OLC did not produce sig-

nificant changes in the C1s spectrum (Fig. 1B (g), Table 2). As can be seen from Fig. 2A and B, the valence band spectral features of the opened OLC are also less affected by potassium intercalation compared with those for the closed OLC. While 190 min potassium dose results in a shift of 0.30 ± 0.05 eV towards higher binding energies for the valence band features (Fig. 2A (f)) of closed OLC, the corresponding shift at an even larger potassium dose (250 min) for the opened OLC is only 0.11 ± 0.05 eV (compare Fig. 2B (a) and (f)). Inspection of the valence band spectra near Fermi level of the potassium intercalated opened OLC showed a very minor rise of the DOS when compared with that for the potassium intercalated closed OLC (see Fig. 3A and B). To explain the observed differences in the response of the valence band and core level features of opened and closed OLC we have to take into account that at room temperature potassium atoms are quite mobile [30] and penetrate not only deep into the film of OLC (between OLC particles) but also, in the case of the opened OLC, inside the individual carbon onions to occupy their interior space and intercalate between graphite-like layers [31,41]. In the latter case, at the same potassium doses potassium coverage of the external graphite-like layer will be less for opened OLC compared with that for closed OLC. The difference in potassium concentration between closed and opened OLC was found in our X-ray photoelectron measurements, see Tables 1 and 2. As a result of this difference of potassium concentration, the amount of transferred charge per carbon atom in the graphite layers from the intercalated potassium is different in the two samples. Bennich et al. [30] showed that for 0.1 monolayer potassium coverage on a graphite surface the amount of transferred charge from potassium atoms to carbon atoms in different graphite layers decreases from the first layer down to the bulk layers. As amount of the transferred charge decreases, the shift of photoemission spectral features for carbon atoms in these graphite layers also decreases [30]. If we assume that in our experiments the amount of transferred charge per carbon atom in a graphite-like layer is proportional to the amount of the potassium present on that particular graphite-like layer, we can expect that the amount of charge transfer, and thus the shift of core level and valence band features, will be less for carbon atoms in the external layers of opened OLC compared with those of closed OLC, due to the smaller density of potassium present on the exter-

CARBON

4 6 ( 20 0 8 ) 1 1 3 3–11 4 0

nal shells of opened OLC. Photoelectron spectroscopy is only sensitive to the outermost surface layers of a sample, with most sensitivity for the external surface layer. For example, at an excitation energy of 1253.6 eV, the kinetic energy of a C1s photoelectron is around 970 eV – for this kinetic energy ˚ [45], which correthe inelastic mean free path (IMFP) is 15 A sponds to 4–5 graphite-like layers. At the excitation energy of 40 eV the kinetic energy of photoelectrons originating from valence band is around 12–37 eV. The IMFP for these electrons is even lower than that for photoelectrons with kinetic energy of 970 eV. Therefore, the observed lower shifts of the photoemission spectral features and almost unaltered DOS near Fermi level for opened OLC upon potassium intercalation can be simply explained by a smaller amount of potassium present in the external surface layers of the opened OLC compared with that for closed OLC, causing a reduction of charge transfer to surface carbon atoms in the former sample. We believe that this potassium deficiency on the external layers arises from the penetration of potassium atoms deep inside the carbon onions through holes made by oxidation of the OLC by carbon dioxide. Finally, it is interesting to note that the desorption of potassium from opened OLC at 1270 K does not lead to a complete recovery of the valence band spectrum: the DOS near Fermi level for the opened OLC after potassium desorption (Fig. 3B (d)) is higher than that for the initial sample (Fig. 3B (a)). This behavior is different to that observed for closed OLC (Fig. 3A) where we observed a complete recovery of all spectral features after the final annealing step at 1270 K (compare Fig. 3A (b) and (g)). A similar increase of DOS near Fermi level was observed by photoelectron spectroscopy for singlewall carbon nanotubes irradiated by argon ions [44]. This indicates a possible irreversible structural disorder, which may have been introduced into the opened carbon onions upon potassium intercalation inside them.

4.

Conclusions

Core level and valence band photoelectron spectroscopy have been employed to study the effect of potassium intercalation on the electronic structure of closed OLC, prepared by annealing of nanodiamonds at 1800 K, and opened OLC, prepared by carbon dioxide treatment of closed OLC. The results show that, for a given potassium dose, core level and valence band features of opened OLC are less affected by potassium intercalation than those of closed OLC. This difference in behavior of OLC samples upon potassium intercalation has been explained by penetration of potassium inside carbon onions in the opened OLC. The diffusion of potassium inside the carbon onions results in a reduction of the amount of transferred charge to carbon atoms in external graphite-like layers, and therefore in smaller shifts of core level and valence band features in photoemission spectra of the opened OLC. Diffusion of potassium inside OLC is indicative of their successful opening by carbon dioxide oxidation. Thus, we envisage that the opening of carbon onions can be used as a method to prepare nanocapsules for different substances. Since the process of the opening of carbon onions is performed separately from

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their filling, the range of these substances can be increased almost indefinitely.

Acknowledgments We thank George Miller for his valuable technical support throughout the experiment. We also wish to thank Ray Jones and Chris Corrigan at the Materials Science Laboratory (SRS, Daresbury Laboratory, UK) for their help in preparation of the samples. YVB is grateful to the European Community’s Sixth Framework Programme for award of a Marie Curie Incoming International Fellowship (MIF1-CT-2005-021528). AKC thanks the University of Nottingham, and EPSRC for the financial support in the form of a Ph.D. studentship. This work has also received support from the Royal Society and the Council for the Science and Technology Facilities Council (STFC), UK.

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