An analysis of microstructural variations in carbon black modified by oxidation or ultrasound

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An analysis of microstructural variations in carbon black modified by oxidation or ultrasound Pengfei Xue, Jie Gao, Yubin Bao, Jibin Wang, Qiuying Li, Chifei Wu

*

School of Material Science and Engineering, East China University of Science and Technology, 130 Meilong Rd., Shanghai 200237, China

A R T I C L E I N F O

A B S T R A C T

Article history:

The microstructure and electronic structure of modified carbon black (CB) were investi-

Received 28 January 2011

gated by Raman spectroscopy, transmission electron microscopy, electron energy loss spec-

Accepted 14 April 2011

troscopy and ultraviolet spectroscopy. The modified CB samples include oxidised CB and

Available online 20 April 2011

ultrasound-treated CB under different modification conditions. Typical parameters, such as graphene layer size, the ratio of sp2/sp3-hybridised carbon atoms, energy gap (Eg), and p–p* band position, provide information on the microstructure and electronic structure, and these parameters also allow discrimination between different modified CB samples to achieve a desired structure. Oxidation conditions could be carefully chosen to prevent excessive corrosion and form an ordered structure. However, ultrasound has a reverse effect; the graphite layers of the CB samples were exfoliated, and a disordered microstructure was visible. The results indicate that increasing sp2-island size in CB samples decreases the optical gap and increases ultraviolet absorption. Ó 2011 Elsevier Ltd. All rights reserved.

1.

Introduction

Carbon materials are very attractive because they can be prepared relatively cheaply from a wide variety of low-cost precursors; these materials are typically biocompatible and quite chemically stable under nonoxidising conditions. In addition, they are also characterised by low density, high thermal conductivity, good electrical conductivity, mechanical stability and ease of handling [1–3]. Carbon materials, which possess a high specific surface-area material, can be used as effective sorbents, catalyst supports, filters, and electrodes for a range of applications [4]. The utility of carbon materials can be further enhanced via functionalisation to fine-tune their interactions with guest molecules and to optimise the bulk and interfacial properties of the materials. Although researchers have built a significant base of knowledge concerning the functionalisation of many forms of carbon, including activated carbon, carbon black (CB), graphite, glassy carbon electrodes, and carbon nanotubes [1,3,5,6], new carbon systems occasionally present

unique challenges that are related to microstructural issues, significant surface variations, changes in physical and chemical properties, and potential fields of application. Methods of carbon functionalisation have not yet been perfectly adapted to the level of large-scale application, but they are rapidly gaining momentum. Characterisation of modified carbon samples presents additional challenges that are also being addressed in current research. The modification of carbon materials for various applications has frequently employed well-defined materials, such as glassy carbon [7] and carbon nanotubes [8]. However, these materials are still relatively expensive. Graphite is a significantly cheaper alternative; however, a poor surface-to-mass ratio inevitably requires large amounts of graphite [6]. As a possible alternative, CB is a cheap, easily obtainable material consisting of relatively small particles with a quasi-graphitic microstructure [9]. The established fine structure of CB is comprised of coexisting crystalline carbon (graphite) and amorphous carbon. CB particles consist of layers of graphene plane

* Corresponding author: Fax: +86 21 64251844. E-mail address: [email protected] (C. Wu). 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.04.040

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segments, 1–5 nm in length that can be grouped in stacks (building up crystallites). These stacks, often termed basic structural units [10], are typically oriented concentrically around the particle centre, are significantly bent, and do not exhibit any order with respect to each other [11]. CB particles have a considerable number of graphitic bonds but also contain sp3 bonds in addition to sp2 bonds [12]. Herein, we have studied the micromorphology, spectral behaviour and electronic structure of modified CB. The modified CB samples under investigation include oxidised CB (OCB) and ultrasound-treated CB (UCB). These materials were chosen to illustrate the microstructural variety among different modified CB samples. Our aim was to investigate the microstructure and electronic structure of carbon materials after controlled modification, and special emphasis was placed on examining the relationship between the microstructure and physical properties. Additional information concerning the carbon hybridisation in the modified samples was also obtained. Methods of characterising the functionalised carbon materials and describing the benefits of carbon functionalisation with a designed architecture for potential applications are highlighted as well.

2.

Experimental

2.1.

Materials

CB, in the form of MogulâL, was obtained from Cabot Co. (USA) and was used without further purification. All other chemicals were obtained from commercial sources and used as received without further purification.

2.2.

Modification via oxidation of CB particles

Two separate CB samples (150 mg) were refluxed at 90 °C in 150 ml of 3 M nitric acid with stirring for 4 h and 12 h, respectively. The oxidised CB samples were centrifuged, extensively washed with deionised water until the washings tested as neutral, and then dried under vacuum at 80 °C. To simplify the following discussion, the oxidised CB samples treated for 4 h and 12 h are abbreviated as OCB-1 and OCB-2, respectively.

2.3.

Modification via ultrasound of CB particles

Two separate CB samples were transferred into beakers with a solution of ethanol (99.5%, Sinopharm Chemical Reagent Co. Ltd.) and subjected to an ultrasonic processor at a power of 300 W/20 kHz for 1 h and 3 h, respectively. The 1-h and 3-h ultrasound-treated CB samples were named UCB-1 and UCB-2, respectively. The ultrasonic density was 1.2 W/ml. The ultrasonic irradiation instrument was a VCX 750 (Sonic & Material Co.) equipped with a titanium horn with a diameter of 13 mm and an adjustable power output. A cooling bath was utilised to maintain the temperature at 25 °C.

2.4.

Characterisation techniques

Raman spectra were recorded using an inVia+Reflex from Renishaw, UK. A He–Ne laser at an excitation wavelength of

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633 nm was used. To accurately profile the contributions of different bands, Raman spectra were fitted to identify the D band (1340 cm1), G band (1600 cm1) and A band (1550 cm1) [13]. Generally, the D band is related to the breathing mode of the sixfold aromatic ring near the edge [14]. The G band is considered to be an in-plane bond-stretching motion of sp2-hybridised C atoms [14]. The A band is ascribed to amorphous carbon [15,16]. A linear background was subtracted. Then, the Lorentzian-shaped D and G bands and the broad, Gaussian-shaped A band were used to fit the spectra [15,16]. The ratio of the D band to the G band can be used to approximate the microcrystalline size (La) of the CB particles [17]. According to the work of Matthews et al. [18], the equation for the approximation can be described as: La = C(kL)IG/ID, where IG/ID is the band intensity ratio and C(kL)  C0 + kLC1 in which C0 = 12.6 nm and C1 = 0.033, and this term is valid for 400 nm < kL < 700 nm. An X-ray diffractometer (RIGAKU, D/MAX 2550 VB/PC) was also used to investigate the CB, OCB and UCB samples. The BET specific surface area was determined using a specific surface area and porosity analyser (ASAP 2010N, Micromeritics). Each test was repeated for at least three replicates. A Philips TEM CM 200 FEG transmission electron microscope (Philips, Netherlands) equipped with a field-emission gun was used to study the morphology and microstructure of the samples. The acceleration voltage was set to 200 kV. ˚ and 1.4 A ˚ , respectively, Point and spatial resolutions of 2.1 A are obtainable with this technique. The CB particles were ultrasonically dispersed in chloroform, and then a drop of the solution was deposited on a holey C/Cu TEM grid for high-resolution transmission electron microscopy (HRTEM) characterisation. Electron energy loss spectra (EELS) were recorded with a Gatan Image Filter (GIF Tridiem; Gatan, USA) mounted below the Philips CM 200 FEG. The spectra were acquired under magic angle conditions [19] to determine the sp2/sp3 ratio of carbon bonds. For every sample, 50 spectra were recorded at 10 different, arbitrary selected positions. Each spectrum was background subtracted and corrected for multiple scattering, and an average of the spectra was calculated. Acquisition of EELS spectra was accomplished using the diffraction mode. Highly ordered pyrolytic graphite (HOPG) was used as a reference sample with 100% sp2 bonds. The absorption spectra of the CB samples were measured in the 200–2000 nm wavelength range using a commercial Varian-Cary 500 UV/Vis/NIR spectrometer. Spectroscopic analyses were carried out on particles deposited onto CaF2 substrates. The advantage of using this kind of substrate is that they are transparent in a wide range of the electromagnetic spectrum.

3.

Results and discussion

3.1.

Raman spectroscopy

Fig. 1 shows the Raman spectra acquired for CB, OCB and UCB samples. Each spectrum corresponds to the average of five spectra; thus, it is highly representative for each sample. Compared to untreated CB, which exhibits broad and overlapping

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Fig. 1 – Raman spectra for CB, OCB-1, OCB-2, UCB-1 and UCB2. The spectra are offset for clarity.

peaks, the Raman spectra for OCBs (OCB-1 and OCB-2) exhibit more separated G and D peaks, indicating the presence of a more homogeneous structure with a smaller molecular carbon content. However, the UCB spectra retained largely overlapping peaks. The different behaviour of the OCB samples and the UCB samples can be explained by differences in their structures. This assumption is in good agreement with HRTEM analyses of untreated CB, OCB, and UCB samples, which are discussed later. An example spectrum for OCB-1 with three band fits used for quantitative spectra analysis is shown in Fig. 2. The deconvoluted Raman spectra for CB, OCB and UCB samples are shown in Fig. SI 1 (see Supporting Information), and the fitting results are summarised in Table 1. The full width at half maximum (FWHM) of the D band and the A band intensity are the spectroscopic parameters that can provide information about the relative abundance and structural order of graphic and molecular carbon and yield the greatest

Fig. 2 – Example spectrum for OCB-1 with three band fits.

amount of information about the chemical structure [20]. Moreover, the D bandwidth has been found to exhibit a nearly linear negative correlation to the amount of apparent elemental carbon in different types of carbonaceous materials [20]. Compared to untreated CB, the G peaks of both OCB samples exhibit a blue shift after oxidation, and the positions of the G bands are shifted higher while the corresponding FWHMs decrease, which indicates that the microcrystalline structure of the OCB samples assumes a more ordered arrangement [14,15]. In addition, the relative intensity of the A band at 1540 cm1 decreases, which is responsible for the strong overlap of the G and D peaks, and the two observed Raman peaks become more separated during oxidation. The intensity of the D peak also decreases with a longer oxidation time while the position stays constant, and the FWHFs of the D and G bands become progressively narrower, indicating a decrease in the chemical heterogeneity and an increase in structural order, which are consistent with studies reported previously [20,21]. OCB-1 and OCB-2 present different spectra. After oxidation for 12 h, the relative intensity and the FWHM of the G and D peak of OCB-2 are altered. The two Raman peaks of OCB-2 are less separated, and the FWHM of the band is generally larger. Compared to OCB-1, the slight changes upon oxidation for OCB-2 suggest the presence of a more disordered structure. According to a previous study [22], CB with an undeveloped crystallographic structure was easily oxidised at an early oxidation stage. As the oxidation proceeded, the carbon crystallites and layers were broken up; thus, it can be concluded that this result is consistent with a less ordered arrangement of the microcrystalline structure formed in OCB-2. When ultrasonic treatment is applied to the CB samples, more profound effects are observed. As shown in Table 1, the FWHM of the D band is consistently increased. This result indicates that the degree of disorder of the microcrystalline structure consistently increased during ultrasonic treatment. The position of the G band can also be used to characterise the structural evolution in the samples. As shown in Fig. SI 1 (see Supporting Information), the G bands of UCB-1 and UCB-2 are both around 1600 cm1, corresponding to an ideal graphitic lattice vibration. This mode does not require the presence of sixfold rings, and therefore, it occurs for all sp2 sites, not only for those in rings. The FWHM of the G band is broadened due to the lower degree of structural order. Generally, a large number of sp3-like defects form under the impact of ultrasound waves [23]. Consequently, ultrasonic treatment is considered to play an important role in modifying the CB structure, and an increase in the time of ultrasound treatment increases the degree of disorder in the microcrystalline structure. The crystallite diameter data obtained from X-ray analyses (see Supporting Information) provide further understanding concerning the structural evolution. As shown in Table SI 1, disordered carbon materials modified using ultrasound exhibit lower La values while oxidised CB samples exhibit higher La values than untreated CB. La values can also be obtained from Raman measurements, but Ferrari and Robertson argued that Tuinstra and Koenig’s correlation should not be valid at very small La values [14], because the D band strength for small La values is proportional to the probability of finding

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Table 1 – The results of the fitting of the Raman spectra for the CB, OCB and U-CB samples. G band/cm1

CB OCB-1 OCB-2 UCB-1 UCB-2

D band/cm1

Position

FWHM

Position

FWHM

Position

FWHM

1585 ± 3 1592 ± 1 1595 ± 1 1597 ± 1 1600 ± 2

85 ± 3 75 ± 5 80 ± 1 83 ± 3 89 ± 6

1348 ± 2 1347 ± 4 1349 ± 3 1351 ± 3 1356 ± 5

241 ± 7 223 ± 6 229 ± 4 247 ± 5 256 ± 11

1540 ± 6 1519 ± 3 1523 ± 2 1540 ± 9 1551 ± 5

155 ± 5 128 ± 2 144 ± 7 130 ± 3 101 ± 8

a sixfold aromatic ring in the carbon cluster, which is proportional to the cluster area (ffi L2a ). However, this argument is based on the assumption that the D band is strictly associated with aromatic rings, which might be questionable in view of recent theoretical work on the origin of the D band [24]. Because Ferrari and Robertson [14] presented no experimental data to support their predictions, their correlation is proposed to be an oversimplification and the pre-factor C’ is not dependent only on the laser wavelength [25]. Therefore, direct measurements using XRD are recommended.

3.2.

A band/cm1

HRTEM analysis

HRTEM investigations were carried out to deduce the microstructure of the different CB samples. We focused on comparable structural analysis of the CB, OCB and UCB samples. High-resolution micrographs of CB (Fig. 3) show a structure similar to a model proposed by Hess and Herd [9], in which paracrystalline, curved, and disturbed graphene layers are stacked together to form a particle. The graphene segments are bent and connected to long chain-like agglomerates [25]. Structured domains (crystallites) were not observed in some limited regions of the CB samples, which appeared rather unstructured on the atomic scale. This feature can be seen in some areas of CBs, and we attribute them to a disordered, amorphous carbon that had not assembled into crystallites. Although the destruction of the crystallites (structured areas) on the CB surface is observed in some areas, this characteristic was observed in virtually all of the images recorded for the OCB samples in Fig. 4, implying that from a structural point of view, selective oxidation leads to a general and rather uniform increase in the long-range order compared to

untreated CB [26]. On the surface of the CB particles, poorly ordered carbon is present that is preferentially removed during the oxidation process. The preferential oxidation of the highly disordered fraction should lead to an apparent increase in overall order if its volume fraction was significant in the deep parts of the secondary structure. This characteristic suggests that oxidation results in the removal of these minor molecular layers, which should slightly modify the overall order and graphitisation of the CB. The micrographs of the OCB-2 sample that was oxidised for 12 h (Fig. 4c and d) show that there is less chain-like orientation and elimination of crystallite boundaries. The particles seem disorderly and less connected than those in the OCB-1 sample, suggesting that there is a decrease in structural order with progressive oxidation. This decrease also indicates that the CB particles underwent coagulation of the crystallites and the carbon layers were broken up by progressive oxidation [22]. Typical HRTEM images of the samples that were subjected to ultrasound treatment are displayed in Fig. 5. Many curved graphitic sheets present in the original CB samples disappeared, and a novel sheet-like structure composed of amorphous and crystalline phases was formed under ultrasonic treatment. Short-range order is observed in Fig. 5b, and this characteristic is likely due to the local alignment of the graphite layers under the intense shock waves occurring during the sonochemical process; however, this effect does not occur on a macroscopic scale, i.e., no long-range order is present. In Fig. 5a (the overview HRTEM image of UCB-1), medium-range disorder becomes prominent, and basic structural units with plane, randomly oriented graphitic structures are observed. After 3 h of ultrasound treatment, the medium-range disorder becomes even more pronounced in Fig. 5c and d. These

Fig. 3 – Overview (a) and enlarged (b) HRTEM images of untreated CB.

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Fig. 4 – Overview (left) and enlarged (right) HRTEM images of OCB-1 (a, b) and OCB-2 (c, d).

Fig. 5 – Overview (left) and enlarged (right) HRTEM images of UCB-1(a, b) and UCB-2(c, d).

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Table 2 – BET specific surface area of the samples. SBET (m2/g) CB OCB-1 OCB-2 UCB-1 UCB-2

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121.1 136.0 110.4 134.8 149.3

observations are evidence that the interlayer spacing of the crystal was smaller than that of the microcrystalline structure in CB. It has been suggested that the graphitic basal plane of CB is disturbed by ultrasonic treatment. The resultant carbon fragments then condense to form a highly disordered structure [23]. A few similar modifications using medium intensity ultrasound on sheets of graphite have also been reported [27,28]. The BET value was determined to detect the CB surface area changes. The BET values are shown in Table 2. For OCB samples, the BET surface area initially increased and then decreased. The initial increase was likely due to be the generation of micropores [3]. However, longer oxidation time caused partial structural collapse, which resulted in the observed drop in surface area. During ultrasound treatment, the BET value consistently increased from 121.1 (CB) and 134.8 (UCB-1) to 149.3 m2/g (UCB-2) as the time under ultrasonic treatment increased. The increased BET area could be attributed to the decreased La and the increased disorder of the microcrystalline structure.

The spectra of the analysed CB samples were then fitted with three Gaussian curves [29] to determine the areas under the curves, which were used to quantify the ratio of sp2 to sp3 bonds in the samples. Fig. 6 displays the EEL spectra of the investigated samples acquired under magic angle conditions. The features visible in the carbon K-edge represent different components of unoccupied electronic states. The peak at an energy loss of 285 eV arises from transitions of C 1s electrons to unoccupied p* orbits, whereas the r* feature at 291 eV reflects a transition to an unoccupied r* state. The p* feature is typical for a sp2-hybridised carbon. The percentage of sp2-bonded carbon in these samples was calculated according to Eq. (1), and the resulting ratio values are displayed in Table 3. h i  sp2 ¼ h

aeraðp Þ aeraðp þr Þ

i

aeraðp Þ aeraðp þr Þ

sample

ð1Þ

100%sp2 -reference

The results for untreated CB indicate that a good correlation is obtained between HRTEM and EELS data. The poor parallel alignment of the graphene units visible in Fig. 3 is consistent with the low abundance of sp2-bonded carbon in the sample derived from the results of EELS. OCB-1 undergoes a change in its microstructure during the 4 h oxidation process that leads to a substantial increase of the degree of graphitisation. The abundance of sp2-bonded carbon increases from an initial value of 77% to a final value of 89% in the treated sample. According to the literature [30], an oxidative episode may also destroy surface irregularities, which leaves

Table 3 – Quantification of sp2/sp3 hybridisation.

3.3.

Electron energy loss spectroscopy

EELS was applied to CB and modified samples to reveal information about the average electronic structures of the samples. To determine the hybridisation of the carbon bonds and their changes upon modification, the spectra were obtained under magic angle conditions [29]. The EEL spectrum of HOPG was acquired as the standard for sp2-bonded carbon.

Nsp2 (%) CB OCB-1 OCB-2 UCB-1 UCB-2 HOPG

77 89 81 64 55 100

Nsp3 (%) 23 11 19 36 45 0

Fig. 6 – (a) EEL spectra recorded under magic angle conditions for CB, OCB, UCB and HOPG samples. (b) Fit of HOPG spectrum to deduce the sp2/sp3 hybridisation ratio.

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behind the graphitic layer structure consisting mainly of sp2hybridised carbon in the shell part of the primary particles. After 12 h of oxidation, the OCB-2 sample shows a decrease in hybridisation (81%) compared to OCB-1, which is consistent with the Raman data results that indicates that this decrease in hybridisation is due to the removal of ordered carbon crystallites and layers as the oxidation process proceeds. Both UCB-1 and UCB-2 exhibit a large decrease in hybridisation after ultrasound treatment (64% and 55%, respectively), which is in good agreement with the HRTEM data. The decrease in hybridisation is attributed to the disordered structure that results from the cavitation phenomenon and the intense shock waves. Generally, the flattening of the graphene layers during the oxidation treatment correlates with the increase in the fraction of sp2-hybridised carbon atoms in the sample. In contrast to the oxidation treatment, ultrasound treatment exerts a reverse effect on the fraction of sp2-hybridised carbon atoms.

3.4.

UV–Vis spectroscopy

As presented in Fig. 7, UV–Vis spectra of the all samples were collected, and these spectra can be interpreted in terms of the electronic band structure of the material. The absorption decreased gradually as the wavelength increased. Electronic transitions from the bonding p orbit to the antibonding p* orbit give rise to a band with a maximum in the UV between 180 and 260 nm (p–p* excitations). The position of the (p–p*) transition depends on the internal structure of the carbon material. According to Robertson’s postulate [31], the formation of plane graphene layers due to the optimal overlap of the pz orbits, and the formation of six-membered aromatic rings should be preferred. Furthermore, already existing clusters tend to grow, resulting in the formation of isolated sp3- and sp2-hybridised areas. The optical gap is the energy required for an electron to reach the first excited state. Photons of higher energy may cause electronic transitions; photons with lower energy cannot cause transitions and are not absorbed. The optical gap concept has been used to explain the electronic behaviour of carbon containing many different types of electrons [32,33]. The absorption coefficient a follows the Tauc relation [34].

pffiffiffiffiffiffi aE ¼ BðE  Eg Þ

ð2Þ

where E is the energy of the incident radiation, B is a constant, and Eg is the optical gap. In practice, Eg is determined by plotting aE as a function of E and extrapolating to zero absorption. Although Eq. (2) was originally derived to express the interaction of radiation with a compact film of semiconductor material, the Tauc relation is often used in the laboratory for the determination of the Eg in samples prepared by particle deposition onto substrates [35–37]. The UV absorption should be determined by the size of the graphitic clusters, e.g., the number of condensed benzene rings in these domains [31], which is correlated with the gap energy Eg by the following rule: Eg ¼ 2jbjM0:5 ¼ 7:7=La ðAÞ

ð3Þ

where M stands for the number of condensed rings in the graphene layers. The factor b is 2.9 eV and represents a calculated quantum chemical overlapping energy between neighbouring pz orbits referred to as the Fermi energy of graphite; the value of 7.7 is an empirical value [30]. The gap energy influences the position of the UV band of carbon materials and is extremely sensitive to many different factors including the internal electronic structure of the solid, the band width of the peak, and the agglomeration state of the particles [36]. Fig. 7 and Table 4 display the UV–Vis spectra and results for all of the CB samples. HOPG was used as a reference sample exhibiting the highest absorption. CB presents a maximum absorption at 253 nm. Oxidation of CB may destroy surface irregularities, which leads to an increase of its sp2 character, and the p–p* transitions are strengthened and shift to a longer wavelength. Thus, the absorbance maximum of OCB-1 is shifted to 260 nm and that of OCB-2 is shifted to 256 nm. HRTEM investigations of both UCB-1 and UCB-2 showed the disappearance of bent graphitic subunits, which form by direct bending of the flat sp2 sheets. The sp3-like line defects in sp2 sheets and ultrasound wave impact produce an increase in the sp3 hybridisation and a decrease in the size of the graphitic clusters or graphene layers, which corresponds to an increase in the gap energy. In principle, the p bonding becomes more electron poor, and the hybridisation state increases. This effect is correlated with an increase in the gap energy and a shift of the p–p* transition to shorter wavelengths. Thus, the UCB-1 and UCB-2 samples exhibit absorption maxima at 251 nm and 249 nm, respectively. Compared to untreated CB, UCB samples exhibit a weaker UV absorption. In contrast, stronger UV absorptions are observed for OCB samples. As shown in Fig. 7, the CB samples

Table 4 – Results from the UV–Vis spectral analysis.a p–p* (nm) CB OCB-1 OCB-2 UCB-1 UCB-2 Fig. 7 – UV–Vis spectra for CB, OCB, U-CB and HOPG samples.

a

253 260 256 251 249

Eg (eV) 0.61 0.55 0.58 0.63 0.66

Eg, the gap energy, and M, the number of condensed rings.

M 90 111 100 85 77

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Fig. 8 – Changes in the UV absorbance at 250 nm as correlated to the sp2 ratio.

modified by oxidation have narrower band widths and higher UV absorption maxima as compared to the CB materials modified by ultrasound. This difference arises because the intensity of an absorption process is determined by the transition probability of the electronic transition that corresponds to the electronic density of states in the solid. In addition, the energy bands of carbon particles modified by ultrasound are widened due to a stronger structural variety when compared to the energy bands of particles modified by oxidation. In Fig. 8, the UV absorbance values at 250 nm are plotted as a function of the Nsp2 (%). As the Nsp2 value increases, the UV absorbance also increases. This overall effect on UV absorbance is enhanced by the extent of sp2 hybridisation. Although the methods of modification are different, when the main spectral data are compared, we find that the primary contributions to the high UV absorbance arise from structural variety, more specifically the amount of sp2 hybridisation. Generally, the presence of Nsp2 correlates with the amount of defects in the graphene structure. Apparently, the carbon materials modified by ultrasound incorporate more defects than those modified by oxidation. HRTEM measurements support these results. The OCB-1 possesses a primarily graphitic structure (89%) and displays the highest UV absorbance, whereas UCB-2 contains a large number of defects (55%) and shows the lowest UV absorbance. Both theory and experimental studies indicate that the electronic properties are controlled by the medium-range order in the CB sample, specifically the number of sp2-bonded rings that are adjacent or clustered together [38]. The finding that the number of sp2-islands governs the optical or electronic properties is an important one, and this proposal has been confirmed in several clever and detailed experiments for other systems [39– 42]. In conclusion, increasing the sp2-island size of the CB particles decreases the optical gap and increases UV absorption.

4.

Conclusions

While tremendous progress has been made in the modification of carbon materials with new morphologies, exciting

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opportunities remain for functionalising such materials to optimise their properties for specific applications. Controlled oxidation is used to introduce oxygen-containing groups on the carbon surface for subsequent modification using chemical interactions. Furthermore, careful oxidation increases the fraction of micropores and the surface area of the CB, and this process also enhances the electrochemical capacitance of the carbon materials [43]. Based on the results of this study, we conclude that oxidation conditions must be carefully chosen to prevent excessive corrosion and structural breakdown of the carbon skeleton. Structural degradation was observed after excessive oxidation, and the degree of disorder increased. However, CB was found to be more robust following carefully controlled oxidation. This discrepancy could be explained by the difference in oxidation conditions. The initial increase in order was considered to be the result of the disappearance of amorphous carbon. However, longer periods of oxidation caused partial structural collapse, which resulted in the observed drop in ordered surface area and Nsp2. The aim of the controlled oxidation is to produce an ordered structure, on the other hand the macroporous structure is retained even when the surface is etched, producing additional microporosity for applications of mesoporous carbons. The modification of carbon materials by ultrasound is an interesting method to consider. Under the appropriate conditions, ultrasound can be used to directly functionalise the surfaces and structures of carbon materials. In fact, ultrasound can completely fracture carbon materials, e.g. the preparation of CB nanosheets [44]. In this paper, this functionalisation resulted in visible changes to the CB structure that became more pronounced with increased sonication time. The graphite crystal layers of CB are held together by weak Van der Waals interactions. When CB was exposed to medium intensity ultrasound, the graphite layers were exfoliated. The resultant carbon fragments (e.g., atomic and molecular carbon species and graphene particulates) then condensed to form a highly disordered structure. In some quite simple and eloquent demonstrations, Wang and coworkers treated the graphite oxide with high intensity ultrasound to exfoliate the layers [27]. Li et al. also reported the preparation of carbon nanosheet via ultrasound irradiation of carbon black under ambient conditions [44]. It was suggested that the carbon fragments formed from the disruption of CB by ultrasound mainly were disordered exfoliated graphite. Conveniently, the degree of order can be controlled by the time of ultrasonic treatment, allowing for a balance of the various desired properties. Overall, carbon materials with designed structures offer tremendous benefits, particularly for prospective applications.

Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No. 50733001).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2011.04.040.

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R E F E R E N C E S

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