Structural characterization of exfoliated graphite nanofibers

Carbon 45 (2007) 751–759 www.elsevier.com/locate/carbon

Structural characterization of exfoliated graphite nanofibers Angela D. Lueking a

a,b,*

, Humberto R. Gutierrez c, Dania A. Fonseca b, Elizabeth Dickey

d

Department of Energy and Geo-Environmental Engineering, The Pennsylvania State University, University Park, PA 16802, United States b The Energy Institute, The Pennsylvania State University, United States c Department of Physics, The Pennsylvania State University, United States d Material Science and Engineering, The Pennsylvania State University, United States Received 5 October 2006; accepted 24 November 2006 Available online 8 January 2007

Abstract Structural characterization of exfoliated graphite nanofibers (EGNFs) with transmission electron microscopy (TEM) and high-angle annular dark-field–scanning TEM (HAADF–STEM) indicates exfoliation has led to structural expansion along the fiber axis, with dis˚ . Image contrast in HAADF–STEM demonstrates that crete domains of graphitic nanocones separated by gaps ranging from 50 to 500 A structural expansion dominates over chemical etching. Raman spectroscopy indicates the EGNF is more graphitic than the precursor, and the disappearance of the characteristic defect (D) peak with multi-wavelength excitation is inconsistent with the presence of amorphous carbon. The highly expanded EGNF structure oxidizes at two distinct rates at 750 C in CO2, leading to a highly-disordered graphitic fiber, with apparent collapse of the expanded structure as no gaps or discrete graphitic domains are observed after oxidation. Variation in the heat input per intercalant mass during thermal shock leads to changes in fiber morphology, including the extent of fiber expansion, the number of defects and pores observable within the fiber via TEM, and the surface area measured by nitrogen adsorption.  2006 Elsevier Ltd. All rights reserved.

1. Introduction Lueking et al. have recently reported the synthesis of exfoliated graphite nanofibers (EGNFs) [1]. EGNFs are closely related to exfoliated (macro-scale) graphite and (micro-scale) carbon fibers, and the preparation process is similar. EGNF are an order of magnitude smaller than previous reports of exfoliated carbon fibers, and the resulting EGNF structure is highly regular. This is in contrast to previous reports of exfoliated carbon fibers that served to create a flowery bundle of eye-shaped pores within the cross-section of the fiber with highly irregular structures [2–4]. It is also quite different than exfoliation of fibrous carbon fiber bundles and single-wall nanotube bundles, in which exfoliation is used to denote separation of the bundles into individual strands or tubes. Exfoliation of graph* Corresponding author. Address: Department of Energy and GeoEnvironmental Engineering, The Pennsylvania State University, University Park, PA 16802, United States. Fax: +1 814 865 3248. E-mail address: [email protected] (A.D. Lueking).

0008-6223/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.11.023

ite is a phase transition and expands the space between the graphene sheets (along the c-axis of graphite) [5]. The first step in exfoliation is to form graphite intercalation compounds (GICs), in which a foreign species becomes incorporated into the graphite lattice. GICs are known to have highly regular structures with stoichiometric relationships between the carbon and the intercalant. It is more thermodynamically feasible for a complete intercalant layer to form prior to starting a second intercalant layer [6]. Similarly, thermodynamic considerations result in regular intervals of filled versus non-filled intercalant layers, and this is known as staging in graphite intercalation compounds. A thermal shock vaporizes the intercalant in the GIC, providing the necessary force to overcome the van der Waals interactions that bind the graphene layers. It was previously reported that exfoliation was limited to a maximum stacking height, such that the material was thin with respect to the graphitic c-axis, such that the vaporization force could overcome the cumulative binding force at play in the graphitic structure [7]. For similar reasoning, it was also previously reported that exfoliation was limited to

752

A.D. Lueking et al. / Carbon 45 (2007) 751–759

Fig. 1. TEM analysis of EGNF-1000. Dark and light regions are observed in the fibers (a and b) after exfoliation. Certain fiber regions (arrows) with large expansion indicate fiber rupture. (c) SAED pattern of the nanofiber in (b), (0 0 2), (1 0 0) and (1 1 0) diffraction features can be observed. The inset is a schematic model of the cup-stacked crystalline structure of the fiber after exfoliation; those segments of the fiber keeping the graphite spacing between cups ˚ ) are responsible for the (0 0 2) diffraction spots observed in the SAED pattern. (3.35 A

graphitic particles that exceeded a certain width (i.e. 75 lm) along the ab-plane such that sizable intercalant island sizes provided the necessary expansion force [5]. Thus graphite exfoliation was previously thought to be limited to fibers with a low aspect ratio of height to width, and graphitic materials with micro-, rather than nano-, dimensions [5]. Our previously reported EGNF had a diameter in the range of 60–400 nm, giving clear indication that exfoliation of graphite layers is not limited to particles with widths of 75 lm. In other words, our previous report of EGNFs indicates that exfoliation of materials with a high aspect ratio is possible. The resulting EGNF material after a 1000 C anneal (EGNF-1000) had a unique structure, with TEM indicating the bands alternated between regions of light and dark contrast [1] (see also Fig. 1). These light and dark regions will be referred to as ‘‘gap’’ and ‘‘band’’ regions, respectively. In our original report, we stated that this alternating contrast may be due to alternating regions of amorphous and graphitic carbon as opposed to gaps in the graphitic carbon. This statement was based on the analysis of the contrast and order in high resolution TEM (HRTEM) images as well as qualitative analysis of preliminary electron energy loss spectroscopy (EELS) data.1 However, the results of these two techniques can be affected by the angle between the analyzed structure and the electron beam. An alternate explanation for the gap and band regions is a change in the local apparent thickness of the nanofiber after expansion. Distinguishing between these two possible scenarios has important implications for use of EGNF-1000 in various applications that rely on pore size and graphitic structure, such as adsorption, separation,

1

Pan, L. Unpublished electron energy loss spectroscopy (EELS) data.

catalysis, and electronics. This paper provides an extensive analysis and characterization including TEM, HAADF– STEM, selected area electron diffraction (SAED), multiwavelength Raman spectroscopy, and Fourier transform infrared (FTIR) spectroscopy; in order to conclusively distinguish between these two possible scenarios. Lastly, we report interesting findings after oxidation of EGNF-1000 in carbon dioxide and structural variations in the EGNF1000 material after variations in the synthesis procedure. These latter results suggest a means to systematically control the structural properties of the EGNF materials.

2. Methods 2.1. Synthesis Synthesis of EGNF was described previously [1]. In brief, the precursor GNF appeared herringbone in HRTEM, with a high degree of graphitic order and was purchased from Catalytic Materials, Ltd; with a metal content of less than 1% as reported by the manufacturer. A 1:1 mixture of concentrated sulfuric (95–98%) and nitric acids (70%) were used to intercalate the GNF. The EGNF-1000 discussed in this work was prepared by first subjecting the intercalated GNF to a 600–700 C2 thermal shock for 2 min (EGNF-700), followed by a thermal anneal for 36 h at 1000 C in flowing Argon (EGNF-1000). EGNF-1000 was further exposed to 20 bar hydrogen prior to analysis. In this report, we further oxidize the resulting EGNF-1000 in 100 ml/min CO2 for 4 h at 750 C to obtain EGNF-CO2. We also explore variations in the standard preparation conditions of EGNF-1000 by simple variation in the amount of EGNF subjected to the thermal shock: (referred to as EGNF-1000’) while keeping the thermal shock time constant. EGNF-1000 was prepared with 100 mg and EGNF-1000b was prepared with 500 mg; with both being subjected to 600–700 C for 2 min in an atmospheric drop tube furnace. This variation in synthesis alters the heat input per unit intercalant. The EGNF-1000b 2 A drop tube furnace was used in this initial preparation and thus the temperature is only an estimate, as it varied significantly during the 2 min.

A.D. Lueking et al. / Carbon 45 (2007) 751–759 precursor was a much wetter material prior to the thermal anneal at 1000 C, due to differences in filtration and the heat input per intercalant mass of the initial thermal shock.

2.2. Characterization Materials were characterized using standard BET methods with nitrogen at 77 K (Micromeritics 2020) after a 150 C outgas for 4 h. Temperature programmed desorption (TPD) was performed at 10 C/min using a Perkin–Elmer Thermo Gravimetric Analyzer 7 (TGA) in 100 ml/min CO2 flow (Coleman grade, 99.99%). HRTEM (JEOL 2010F) was used to characterize the crystalline structure of the material, graphitic defects and lattice expansion. The JEOL 2010F was also used in scanning mode (STEM) in order to obtain HAADF–STEM images of the fibers. STEM images were taken with 1 nm spot size and 2.4 cm camera length, using a high-angle annular dark-field Gatan detector. Under these conditions the inner angle is 34 mrad. The dark-field STEM images are inverted in contrast compared to conventional TEM since the detector senses only the scattered electrons (the scattering angle range is selected by changing the camera length). Low-magnification TEM images, selective area diffraction and diffraction contrast (dark-field) images were obtained in a Philips 420 (tungsten-based 120 keV). Raman spectroscopy was done on a Renishaw inVia spectrometer with a confocal Leica DM LM microscope and a Peltier cooled RnCam ddCCD with a 633 nm HeNe ion laser. Additional spectra for select samples were obtained with a 514 nm argon ion laser and a frequency doubled argon laser to obtain 257 nm. Analysis of peak height and full-width half maxima (FWHM) was performed after subtracting the baseline from the spectra (drawn as dotted lines in the figure), using a custom curve-fitting software. The transmission infrared data was collected on a Nexus 670 FTIR spectrometer. Pellets were prepared mixing a small amount of the material with potassium bromide, pressing it at 5000 psi for 5 min in vacuum. All the spectra were baseline corrected by using a KBr pellet as reference. The FTIR data has been smoothed with baseline subtraction. The relative intensities in IR data discussed below are based on intensity of the peaks with baseline correction.

3. Results and discussion 3.1. Microstructure of EGNF-1000 3.1.1. TEM and SAED characterization Previously, we have reported that the light regions of the ˚ , compared to the EGNF-1000 may vary from 50 to >500 A ˚ spacing of the lattice in the graphite nanofiber 3.35 A (GNF) precursor [1]. Low-resolution TEM images of EGNF-1000 show the frequency of the expanded EGNF1000 fiber (Fig. 1). Both SEM (see Fig. 4 of [1]) and TEM (see inset in Fig. 1) images reveal a circular cross-section of the nanofibers. Combined with the orientation of the graphene sheets observed by HRTEM, the circular cross-section can be used to distinguish stacked-cup structures from herring-bone carbon fiber structures, as discussed in [8,9]. The circular cross-section thus suggests both the GNF precursor and correspondingly the EGNF consist of stacked-cup carbon fibers. The SAED pattern of the EGNF-1000 (Fig. 1c, corresponding to the region of 1b) is similar to that observed in the precursor (data not shown). The (0 0 2) reflections are four elongated spots consistent with the fish bone-like projection of stackedcups GNF (see inset), and are present in both the GNF pre-

753

cursor and EGNF-1000. The (1 0 0) and (1 1 0) diffraction rings arise from the sections of the graphene cup (cone) that are perpendicular to the electron beam (i.e. the center of the projected fiber). The fact that the (1 0 0) and (1 1 0) diffraction rings are continuous indicates that the chiralities of the cones are random. Included as an inset to Fig. 1 is a cartoon of the structure indicating the expanded, conical, cup-shape of EGNF-1000. At high resolution, the light gap region appeared to be amorphous, as it showed no defined graphene planes [1]. However, if the only contribution to the phase-contrast image (HRTEM image) of the light region comes from the curved surface of the graphene cone which is not oriented (parallel or perpendicular) to the electron beam, then the resultant contrast will be almost similar to that obtained from an amorphous material. Therefore, an alternate explanation for the light regions could be gaps (pores) generated by the expansion of the nanofibers in these points. For a stacked-cup, or herringbone, GNF that was expanded periodically along the fiber axis, the expanded regions have a lower apparent thickness of the fiber. Such an expanded region would appear to be lighter in contrast and the projected image would not have the characteristic parallel graphene layers. Based on the above discussion, HRTEM alone does not give conclusive information about the structure and composition of the light regions. 3.1.2. STEM characterization HAADF–STEM was used to further differentiate between regions of expansion (empty space) and those of amorphous carbon. Image contrast in HAADF–STEM measurements (at 34 mrad, as described in Section 2) can be used to differentiate the presence of elements with different atomic number (z), thickness and/or density variations. For materials of constant composition, as is the case in the carbon fibers, differences in contrast denote variations in thickness and/or density. However, as graphite and amorphous carbon have very similar density [10–12], only variations in apparent thickness along the nanofiber will produce noticeable contrast under the imaging conditions used in HAADF–STEM here [13]. Fig. 2 shows three examples of EGNF-1000 images in HAADF–STEM mode. As described above, image contrast is inverted relative to conventional TEM images, thus the dark contrast regions in HAADF–STEM indicate empty or thinner regions distributed along the fiber. Exfoliation should produce an even separation of neighboring graphene layers, thus these gaps should be symmetric with respect to the nanofiber axis and oriented parallel to the conical graphene layers. Gaps of this type are indicated by arrows in Fig. 2a and c. In certain locations, a larger gap is observed in the fibers (e.g. Figs. 1 and 2a), such that a break is observed in the fiber. STEM measurements also allow differentiation between regions of fiber expansion (exfoliation) and those produced by chemical etching, as chemical etching is expected to produce gaps that are irregular in shape without symmetry with respect to the orientation of the graphene layers.

754

A.D. Lueking et al. / Carbon 45 (2007) 751–759

Fig. 2. HAADF–STEM of EGNF-1000: (a–c) are different examples of individual fibers. (d) Higher magnification of (b). Straight gaps (dark contrast) symmetrically distributed around the fiber axis (center) suggest expansion (arrows in (a) and (b)) and gaps with asymmetric shapes suggest chemical etching (arrows in (d)). The appearance of bands in HAADF–STEM imaging is inconsistent with the presence of amorphous carbon.

Examples of irregular gaps are shown in Fig. 2d (arrows). Irregular gaps were observed less often than symmetric gaps that paralleled the graphene layers. The STEM results suggest that EGNF-1000 consists of exfoliated graphene layers with some regions of chemical etching and structural damage. It is interesting to note that exfoliation in graphite occurs due to an expansion of the structure along the c-axis (0 0 1 crystalline direction) [5]. The term exfoliation has been used to denote separation of SWNT bundles into individual strands or tubes [14]. Expansion along the c-axis is not possible for multiwalled carbon nanotubes due to the closed, rigid, cylindrical structure of the graphene sheets which disallows expansion along the c-axis in the radial direction. The stacked-cup nanofiber is an intermediary state between graphite exfoliation and (hypothetical) MWNT exfoliation: expansion does not occur along the c-axis but along the fiber axis. Expansion along the c-axis in the case of GNF would imply a change in the inner angle of the cones and this is not possible without destroying this rigid, closed graphene geometry. 3.1.3. Multi-wavelength (MW) Raman spectroscopy MW Raman was selected to more fully characterize EGNF-1000. MW Raman, including ultraviolet excitation, has the advantage over a single excitation wavelength, in that different carbon structures may resonate at different excitations. As the excitation energy is varied, different carbon structures may resonate; thus, tracking the shift in a Raman peak with changing excitation frequency may provide further insight into the carbon structure [15–17]. Wang et al. [18] describe MW Raman applied to graphitic carbon and Ferrari and Robertson [16] describe MW Raman applied to carbons that are more amorphous in nature. In brief, the Raman spectra for all carbons show several common features in the 800–2000 cm 1 region, the so-called G

(graphite) and D (defect) peaks, which lie at around 1560 and 1360 cm 1 for visible excitation, and the T peak, seen for UV excitation at around 1060 cm 1. Both the G and D peaks are due to sp2 hybridized carbons and are seen for graphitic and amorphous carbons. For graphite, the characteristic G peak is associated with the E2g vibrational mode—vibrations of the carbon atoms within the hexagonal sp2 network of the graphene layer [18,19]. In amorphous carbon, there is no defined hexagonal sp2 graphene layer, thus the G peak has been associated with bond stretching of all pairs of sp2 atoms in both rings and chains [20]. For graphite, the origin of the D peak has been heavily debated, as discussed by Wang et al. [18] and more recently by Brown et al. [21]. Wang et al. demonstrate that variations in D peak behavior with excitation wavelength are consistent with disruptions of the symmetry of the carbon atoms in the graphene layer yet inconsistent with resonance effects due to variations in microcrystalline domain [18]. As the origin of the D peak for graphitic carbons is still debated, the origin of the D peak for amorphous carbons is also likely still in question. Ferrari and Robertson attribute the amorphous carbon D peak to the breathing modes of sp2 atoms in rings [16]. In a comparative study of several carbons with varying degrees of order, Ferrari and Robertson observe that disappearance of the D peak in UV excitation is characteristic of graphitic (rather than amorphous) materials [21]. As the laser energy is increased, the D peak of EGNF1000 disappears (Fig. 3, Table 1). Thus, MW Raman of EGNF-1000 is typical for nanocrystalline graphite with a high degree of order and inconsistent with the presence of significant amounts of amorphous carbon. The location of the EGNF-1000 G peak is fairly insensitive to excitation wavelength (Fig. 3, Table 1); behavior which has previously been associated with graphite [18], nanocrystalline graphite [16], carbon blacks [22], and glassy carbon [18]. G peak

Intensity

A.D. Lueking et al. / Carbon 45 (2007) 751–759

755

`

a

b c 1000

1200

1400

1600

1800

Wave Number (cm-1) Fig. 3. Multi-wavelength Raman of EGNF-1000 with excitation wavelength of (a) 633 nm; (b) 514 nm; and (c) 257 nm. Horizontal bars denote full-width half-maxima for the peaks, and the dotted line is drawn to guide the eye for the background fit that was used in the calculation of the peak intensity. The data are plotted without normalization to illustrate the change in intensity of the Raman as a function of laser excitation see Table 1 for quantification of the Raman data.

Table 1 Characterization of EGNF materials Sample

EGNF EGNF EGNF EGNF GNF0

N2 BET surface area (m2/g) 1000 1000 1000 1000’

550 362 47

Raman analysis Excitation wavelength (nm)

· @Gmax (cm 1)

FWHM (G)

I(G)

· @Dmax (cm 1)

FWHM (D)

I(D)

I(D)/I(G)

257 514 633 633 633

1584 1583 1586 1585 1583

41 91 94 69 63

868 996 2102 4475 4157

NA 1346 1329 1329 1328

NA 130 140 64 61

0 768 2296 9345 8999

0.00 0.77 1.09 2.09 2.16

dispersion—or variation in position with excitation wavelength—is expected for disordered carbon materials, due to resonant selection of various sp2 clusters at a given wavelength. The amount of G peak dispersion is proportional to the cluster size of graphitic regions in amorphous carbon materials [20]; thus, the lack of G peak dispersion for EGNF-1000 is indicative of a constant graphitic cluster size. Increasing D peak dispersion is proportional to order in amorphous carbon, and is maximum for microcrystalline and nanocrystalline graphite [16]. The observed shift in the D peak location from 1329 cm 1 to 1346 cm 1 for EGNF-1000, as the laser excitation is varied from 633 nm to 514 nm, is consistent with nanocrystalline graphite. The third observation from the MW Raman of EGNF1000 is the dispersion of the I(D)/I(G) ratio: As the laser frequency is varied from 514 to 633 nm, the ratio varies from 0.78 to 1.1. In other words, the D peak is greater than the G peak at 633 nm and less than the G peak at 514 nm. This behavior has also been observed for sputtered amorphous carbon [16], and various forms of tetrahedral amorphous carbon after thermal annealing to increase the order

[16]. As a material becomes more ordered, the dispersion of the I(D)/I(G) ratio has been observed to increase, and thus may be thought of as an indication of graphitic cluster size [16,20,23]. Wang et al. [18] show D peak dispersion to be independent of carbon type, and thus state this effect is due to resonance enhancement of different populations of phonons. For these reasons, the observed dispersion of EGNF-1000 cannot be conclusively related to fiber diameter. What can be conclusively stated is that the MW Raman behavior of EGNF-1000 is not consistent with the presence of amorphous carbon. 3.1.4. Infrared spectroscopy FTIR spectroscopy was used to ascertain the presence of functional groups on the surface of the EGNF-1000 sample (Fig. 4b), relative the GNF precursor (Fig. 4a). Peaks are assigned as follows: 880 cm 1 to isolated aromatic CAH out-of-plane bending mode [24]; 1150 cm 1 to CAO stretching vibrations in carboxylic acids [25] or CAO stretching and OAH bending vibrations [24,25]; 1382 cm 1 to nitrate [24]; 1585–1600 cm 1 to C@C

756

A.D. Lueking et al. / Carbon 45 (2007) 751–759

fibers [30]; it is difficult to comment under what conditions hydroxyl groups are prominent in IR spectra, as typical FTIR spectra of carbon materials are reported only to 3000 cm 1. The changes in FTIR spectra for EGNF1000 relative to the GNF precursor are subtle: the band correlated to conjugation of the carbon double bond vibration and C@O stretching (1550–1710 cm 1) is less broad in EGNF-1000, there is a slight decrease in the peak attributable to nitrate (at 1382 cm 1), and the peak at 3450 cm 1 (–OH groups) with respect to the peak at 1740 cm 1 (C@O) is more intense in EGNF-1000. This indicates that a larger amount of –OH groups is present in the exfoliated sample, despite the high temperature (1000 C) thermal anneal used after the exfoliation process.

ν(-OH)

Absorbance (a.u.)

Hydroxyl groups

ν(-OH) Carboxyl groups ν (C=C)

(b)

ν (C-O) ν (C-H)

ν (C=O)

NO3-

(a)

1000

1500

2000

2500

3000

3500

4000

3.2. Oxidation of EGNF-1000

Wave Number (cm-1)

EGNF-1000 was oxidized in CO2 (750 C, 4 h) in an attempt to remove any closed loop or outer graphitic layer evident in HRTEM [1] and determine the accessibility of the ‘‘pores’’ of the gap region. The selected CO2 oxidation was drawn from the SWNT literature: oxidation in CO2 between 700 and 800 C has been shown to remove the reactive tips of the SWNT while leaving the length of the tube intact [31]. Monitoring the oxidation on the TGA indicates two distinct oxidation rates at 750 C: initially, the sample loses mass at 2 lg/min, and after approximately 3 h, the rate increases to 20 lg/min (Fig. 5). The two distinct oxidation rates correspond to oxidation of carbons of different reactivity. The increased oxidation rate for EGNF-1000 may be due to its increased surface area (555 m2/g, as reported in [1]) relative to GNF (47 m2/g [1]). The expanded EGNF-1000 structure is also likely more susceptible to oxidation with greater access of oxygen to carbon sites. The difference in oxidation rates does not, however, seem

Fig. 4. FTIR spectra of: (a) the GNF precursor and (b) EGNF-1000.

aromatic stretching [24,25]; 1740 cm 1 to C@O vibrations of carboxylic acid or ketone groups [25–27]; bands in the region of 2890–2960 cm 1 to CH2/CH3 stretching [24,28,29]; 3290 cm 1 to OAH stretching in carboxylic acid groups; 3450 cm 1 to OAH stretching in hydroxyl groups [27,29]. The OAH stretching in carboxylic acid groups (at 3290 cm 1) overlaps with the OAH stretching in hydroxyl groups (at 3450 cm 1) [27,29]. Conjugation of C@C with C@O bonds is thought to be between 1585 and 1660 cm 1 [27]. However, a band at 1630 cm 1 has been also associated with adsorbed water [24]. Carboxyl and hydroxyl groups are the prominent feature in both GNF and EGNF-1000, likely due to the high number of carbon edge sites along the stacked cup fiber structure. A high intensity of hydroxyl groups have been previously reported for activated carbon fibers and treated 110%

800

105%

Mass fraction

slope = -3.7E-04 2

R = 0.99

600

95%

EGNF-1000 90%

400

slope = -1.9E-03x 2

R = 0.99

85% 80%

slope = -2.3E-02

Temperature (C)

GNF

100%

200

2

R = 0.97

75% 70% 0

50

100

150

200

250

0 300

Time (min) Fig. 5. Mild oxidation of EGNF-1000 in CO2 had two distinct oxidation rates. EGNF-CO2 discussed in the text is at the conclusion of 4 h of CO2 oxidation.

A.D. Lueking et al. / Carbon 45 (2007) 751–759

due to difference in chemical functional groups as FTIR spectra of the two materials are very similar (Fig. 4). TEM analysis of the resulting EGNF-CO2 indicates the CO2 oxidation led to significant structural modifications (Fig. 6). No band-like structures of the EGNF-1000 were observed (see Fig. 6a), suggesting the CO2 oxidation led to either structure collapse or the banded structures were more susceptible to oxidation via CO2. HRTEM images from several nanofibers suggest they are highly-disordered: Fig. 6b shows a representative HRTEM and its corresponding Fourier Transform (FT, inset). The FT intensities at the frequencies corresponding to the (0 0 2) planes are slightly brighter but in general the FT spectrum is mostly an amorphous-like disk, typical of a material with very low crystalline order.

757

3.3. Variations in exfoliation conditions A slight variation in the synthesis procedure—changing the thermal shock procedure–led to changes in the EGNF structure. EGNF-1000, discussed in Section 1, resulted from a thermal anneal of EGNF-700 for 36 h in argon [1]. A second preparation of EGNF-1000 (denoted as EGNF-1000b) used more carbon material without altering the thermal shock conditions. EGNF-1000b can thus be thought of as a variation of EGNF-1000 with less heat transfer per unit intercalant. The alternating band and gap structure for EGNF-1000b is much less pronounced (Fig. 7). There are some indications that the band structure has started to form (e.g. Fig. 7, arrows), with the light regions observable in EGNF-1000b more apparent than the GNF precursor. These light (gap) regions of EGNF1000b generally vary in width from 1 to 4 nm. HRTEM of the nanofiber surface (Fig. 7c) shows a highly-disordered structure with no correlation between graphene layers. Other characterizations show structural changes in EGNF-1000b (Table 1): the BET surface area of EGNF1000b has increased to 362 m2/g, relative to the BET surface area of 47 m2/g for the GNF precursor, which is consistent with the observation of gaps in TEM (Fig. 7). The surface area of EGNF-1000b is less than the 550 m2/g surface area of EGNF-1000 (Table 1). Visible (633 nm) Raman spectroscopy3 indicates subtle variations in the spectra of EGNF-1000b. At 633 nm excitation, the D/G ratio for the GNF precursor is 2.16, EGNF-1000b has a D/G ratio of 2.09, and the D:G ratio for EGNF-1000 is 1.1 (Table 1). In carbon fibers, an increasing D:G indicates increasing structural flaws within the graphitic lattice [32]. These changes are consistent with the structural variations seen in HRTEM. However, the variations in the D/G ratio between EGNF-1000b and GNF are small. Also observed is a broadening of the G and D peaks upon exfoliation, as evidenced by changes in the FWHM (Table 1). The data in Table 1 suggest that the location and FWHM of the G peak may be a better—or additional—means to detect structural variations brought about by exfoliation. As discussed by Ferrari and Robertson, the breadth of the D and G peak reflect variations in cluster size of the types of carbons selected at a particular wavelength. A systematic study of the variations of EGNF properties with varying thermal shock conditions is the subject of a forthcoming paper [33].

4. Summary and conclusions

Fig. 6. (a) TEM of EGNF-CO2 at different locations in the sample, no gaps from the fiber exfoliation were observed in the nanofibers. (b) Representative HRTEM of this material shows a highly-disordered structure. Fourier transform (inset): even though the frequencies corresponding to the (0 0 2) planes are slightly brighter, the FT is mostly composed by a disk typical of disordered carbon.

The microstructure of EGNF-1000 is a result of expansion along the nanofiber axis, leading to discrete domains of graphite stacked cups/nanocones, separated by gaps ˚ . The HAADF–STEM concluon the order of 50 to >500 A sively demonstrates the gaps arising due to exfoliation (i.e. 3 Raman data was collected only at 633 nm excitation for EGNF-1000b. Thus, Raman data for EGNF-1000b is presented in tabular form only.

758

A.D. Lueking et al. / Carbon 45 (2007) 751–759

Fig. 7. TEM of EGNF-1000b, prepared with a variation of heat transfer per intercalant mass. (a–b) are images from two locations in the sample, and represent fibers of different diameter. (c) HRTEM indicates a region of the sample with a highly irregular exterior surface of the fiber.

expansion) dominate in EGNF-1000 compared to chemical etching. The majority of the gaps are thus ‘‘empty’’ in nature. Raman spectroscopy further corroborates this point, as the dispersion and decrease in the D peak as the excitation energy is changed is consistent with graphitic, rather than amorphous, carbon. FTIR of EGNF-1000 shows only very subtle changes in the presence of functional groups on EGNF-1000 relative to the GNF precursor. The presence of discrete domains of graphite stacked cups/nanocones, separated by empty gap regions on the ˚ suggests the EGNF-1000 material may order of 50–500 A have applications in separation and adsorption. Mild oxidation of EGNF-1000 in CO2 shows two distinct rates of oxidation, behavior which was not observed for the GNF precursor. After CO2 oxidation, the observed GNF structure is highly-disordered. Altering the EGNF1000 synthesis route by changing the heat input per intercalant mass and omitting the hydrogen exposure leads to variations in the fiber structure. HRTEM of EGNF1000b shows the onset of formation of the expanded band structure, increased BET surface area, and variations in the intensity and width of characteristic peaks in Raman spectra. Variations in synthesis also led to variations in the number of defects within the fiber, BET surface area, and fiber morphology. The different properties of EGNF-CO2 and EGNF-1000b relative to EGNF-1000 suggest a means to create fibers with different morphologies, defects, surface area, and functional groups. Acknowledgments Deepa Narayanan and Sabil Huda assisted in material preparation. Dirk Van Essandelft obtained the Raman data with direction from Jacob Caulkins. The work was funded by the Pennsylvania State University, including the Material Research Institute, the Energy Institute, and the Penn State Institutes for the Environment.

References [1] Lueking AD, Pan L, Narayanan D, Burgess-Clifford CE. Effect of expanded graphite lattice in exfoliated graphite nanofibers on hydrogen storage. J Phys Chem B 2005;109(26):12710–7. [2] Yoshida A, Hishiyama Y, Inagaki M. Exfoliation of vapor-grown graphite fibers as studied by scanning electron-microscope. Carbon 1990;28(4):539–43. [3] Toyoda M, Kaburagi Y, Yoshida A, Inagaki M. Acceleration of graphitization in carbon fibers through exfoliation. Carbon 2004; 42(12–13):2567–72. [4] Inagaki M, Kang F, Toyoda M. Exfoliation of graphite via intercalation compounds. In: Radovic LR, editor. Chemistry and physics of carbon. New York: Marcel Dekker; 2004. p. 1–69. [5] Chung DDL. Exfoliation of graphite. J Mater Sci 1987;22(12): 4190–8. [6] Dresselhaus MS, Dresselhaus G. Intercalation compounds of graphite. Adv Phys 2002;51(1):1–186. [7] Dowell MB. Exfoliation of intercalated graphites, Part I, effect of graphite crystallinity. In: 12th biennial conference on carbon. Pittsburgh, PA: Pittsburgh The American Carbon Society & The University of Pittsburgh; 1975. p. 31–3. [8] Kim Y-A, Hayashi T, Naokawa S, Yanagisawa T, Endo M. Comparative study of herringbone and stacked-cup carbon nanofibers. Carbon 2005;43(14):3005–8. [9] Eksßioglu B, Nadarajah A. Structural analysis of conical carbon nanofibers. Carbon 2006;44(2):360–73. [10] Wang CZ, Ho KM. Structural trends in amorphous–carbon. Phys Rev B 1994;50(17):12429–36. [11] Ferrari AC, Libassi A, Tanner BK, Stolojan V, Yuan J, Brown LM, et al. Density, sp(3) fraction, and cross-sectional structure of amorphous carbon films determined by X-ray reflectivity and electron energy-loss spectroscopy. Phys Rev B 2000;62(16): 11089–103. [12] Mathioudakis C, Kopidakis G, Kelires PC, Wang CZ, Ho KM. Physical trends in amorphous carbon: a tight-binding moleculardynamics study. Phys Rev B 2004;70(12):125202. [13] Williams DB, Carter CB. Transmission electron microscopy: a textbook for materials science. New York: Plenum Press; 1996. [14] Kovtyukhova NI, Mallouk TE, Pan L, Dickey EC. Individual singlewalled nanotubes and hydrogels made by oxidative exfoliation of carbon nanotube ropes. J Am Chem Soc 2003;125(32):9761–9. [15] Ferrari AC, Rodil SE, Robertson J. Resonant Raman spectra of amorphous carbon nitrides: the G peak dispersion. Diam Relat Mater 2003;12(3–7):905–10.

A.D. Lueking et al. / Carbon 45 (2007) 751–759 [16] Ferrari AC, Robertson J. Resonant Raman spectroscopy of disordered, amorphous, and diamondlike carbon. Phys Rev B 2001;64(7): 075414. [17] Ferrari AC, Robertson J. Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond. Philos T Roy Soc A 2004;362(1824):2477–512. [18] Wang Y, Alsmeyer DC, Mccreery RL. Raman-spectroscopy of carbon materials – structural basis of observed spectra. Chem Mater 1990;2(5):557–63. [19] Tuinstra F, Koenig JL. Raman spectrum of graphite. J Chem Phys 1970;53(3):1126. [20] Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B 2000;61(20): 14095–107. [21] Brown SDM, Jorio A, Dresselhaus MS, Dresselhaus G. Observations of the D-band feature in the Raman spectra of carbon nanotubes. Phys Rev B 2001;64:073403. [22] Mernagh TP, Cooney RP, Johnson RA. Raman spectra of graphon carbon black. Carbon 1984;22(1):39–42. [23] Ferrari AC, Kleinsorge B, Adamopoulos G, Robertson J, Milne WI, Stolojan V, et al. Determination of bonding in amorphous carbons by electron energy loss spectroscopy, Raman scattering and X-ray reflectivity. J Non-Cryst Solids 2000;266:765–8. [24] Ros TG, Van Dillen AJ, Geus JW, Koningsberger DC. Surface oxidation of carbon nanofibres. Chem Eur J 2002;8(5):1151–62. ´ rfa˜o JJM. Modifica[25] Figueiredo JL, Pereira MFR, Freitas MMA, O tion of the surface chemistry of activated carbons. Carbon 1999;37(9):1379–89.

759

[26] Feng X, Matranga C, Vidic R, Borguet E. A vibrational spectroscopic study of the fate of oxygen-containing functional groups and trapped CO2 in single-walled carbon nanotubes during thermal treatment. J Phys Chem B 2004;108(52):19949–54. [27] Kim UJ, Furtado CA, Liu X, Chen G, Eklund PC. Raman and IR spectroscopy of chemically processed single-walled carbon nanotube. J Am Chem Soc 2005;127(44):15437–45. [28] Mawhinney DB, Yates JT. FTIR study of the oxidation of amorphous carbon by ozone at 300 K – direct COOH formation. Carbon 2001;39(8):1167–73. [29] Vovchenko L, Matzui L, Zakharenko M, Babich M, Brusilovetz A. J Phys Chem Solid 2004;65:171–5. [30] Park S-J, Kim Y-M, Shin J-S. Cu(II) adsorption of activated carbon fibers produced by radiation-induced graft polymerization. J Porous Mater 2005;12:41–6. [31] Tsang SC, Chen YK, Harris PJF, Green MLH. A simple chemical method of opening and filling carbon nanotubes. Nature 1994;372 (6502):159–62. [32] Dresselhaus MS, Dresselhaus G, Pimenta MA, Eklund PC. Raman scattering in carbon materials. In: Pelletier MJ, editor. Analytical applications of Raman spectroscopy. Ann Arbor: Blackwell Science; 1999. p. 367–434. [33] Fonseca DA, Gutierrez HR, Lueking AD. Influence of thermal treatment on the structure of exfoliated graphite nanofibers. Prepr Pap-Am Chem Soc, Div Fuel Chem 2006;51(2).