Functionalization of Multiwall Carbon Nanotubes by Ozone at Basic

Article pubs.acs.org/JPCC

Functionalization of Multiwall Carbon Nanotubes by Ozone at Basic pH. Comparison with Oxygen Plasma and Ozone in Gas Phase Francisco Morales-Lara,† Manuel J. Pérez-Mendoza,† Deisi Altmajer-Vaz,‡ Miguel García-Román,‡ Manuel Melguizo,§ F. Javier López-Garzón,*,† and María Domingo-García† †

Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain Departamento de Ingeniería Química, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain § Departamento de Química Inorgánica y Orgánica, Facultad de Ciencias Experimentales, Universidad de Jaén, 23071 Jaén, Spain ‡

S Supporting Information *

ABSTRACT: The selective functionalization of carbon nanotube surfaces is crucial for many potential applications of these materials. For this purpose several oxidants, among other substances, are used. The aim is to reach a large degree of functionalization which depends on the oxidant character of the reagent. For this reason the functionalization of multiwall carbon nanotubes (MWCNTs) by treatment with ozone in basic solution is studied. At basic pHs, ozone results into hydroxyl radicals whose reduction potential is very high (E° = 3.06 V). The results have been compared to those obtained by ozone in gas phase and with cold oxygen plasma. The oxidation with ozone in basic solution seems to be kinetically restricted. As a consequence, the degree of oxidation in this medium is smaller than this of ozone gas, in spite of the larger oxidation capacity of the former. The oxygen-containing groups fixed by these two treatments are mainly attached to defects of the nanotubes. Moreover, no modification of the graphene layers and no porosity result from these treatments. The oxygen plasma treatment stands out in the content of oxygen groups fixed to the MWCNTs, as it is by far more effective, although some of these groups have relatively low thermal stability. Nevertheless, this treatment mainly fixes the oxygen groups on the walls of the nanotubes.

1. INTRODUCTION Large amounts of envisaged applications of carbon nanotubes (CNTs) depend on their functionalization. This is due to the hydrophobicity and inertness of pristine CNTs, which limit their potential use. Fixing chemical functions on their surface is a way to overcome this drawback. There are two general procedures for this purpose: the noncovalent and covalent functionalization.1 The first one is usually carried out by adsorption of different molecules that in many cases is produced by nonspecific interactions, although in some other occasions π−π stacking forces are prevalent. The electronic properties of the CNTs are scarcely modified when nonspecific interactions control the adsorption process, while they are certainly changed if π−π stacking forces are involved, and even more relevant changes of these properties are produced when covalent functionalization is produced. The covalent functionalization of CNTs usually consists on the fixation of heteroatoms or molecules on the nanotube surface. Among the former, oxygen, halogens, or nitrogen are very frequent. The characteristics of these atoms, more electronegative than carbon, alter the electronic properties of the CNTs. For instance, covalent fixation of fluorine or nitrogen results in the change of the semiconducting properties.2,3 Moreover, these heteroatoms are frequently used as primary functions which behave as intermediates for further covalent fixation of more complex compounds.4,5 The © XXXX American Chemical Society

aim of this secondary functionalization is usually to enhance the CNT properties. For example, in CNT/polymer composites, large improvement on the dispersion of the CNTs into the polymer matrix is reached after a secondary covalent functionalization.6−8 Similarly CNT dispersions can be prepared by functionalization with biocompatible molecules, which in turn can be very promising for biomedical applications.9−11 In the case of covalent anchorage of complex molecules, the primary functionalization is a key factor, being oxygencontaining groups the most frequently used. For this purpose CNTs are usually treated with several oxidants in aqueous solutions or in gas phase.12−14 Amid these oxidants, ozone and oxygen plasma are excellent candidates for an effective functionalization due to their high oxidative capacity. The reaction of ozone with the carbon materials is usually carried out as a gas−solid process, although in some cases the oxidation is also produced by bubbling ozone into aqueous CNT suspensions.12,15−22 The reaction in aqueous solution can progress through a direct or an indirect path. In the first one the reactive species is the O3 molecule, and it can consist of different processes such as a redox reaction, the Criegee Received: February 18, 2013 Revised: April 9, 2013

A

dx.doi.org/10.1021/jp4017097 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Effect of the ozone treatments at basic pH on the oxidation of the MWCNTs: (a) TG weight loss vs pH; (b) XPS oxygen surface concentration vs pH.

obtained by monitoring their masses in a mass spectrometer (Omnistar QMG 220, Pfeiffer Vacuum) while the samples were heat-treated in a helium flow (100 mL min−1) at 10 K min−1 up to 900 °C. The surface oxygen-containing groups were analyzed by XPS using a Kratos Axis Ultra-DLD equipment. Monochromatic Al Kα radiation of twin anode, in the constant analyzer energy mode, was used. The binding energies were determined by setting up the C 1s transition to 284.3 eV. After background correction, the high-resolution spectra were fitted to Lorentzian and Gaussian curves by using XPS CASA software. Raman spectra were obtained in a JASCO NRS-5100 apparatus using an excitation wavelength of 532 nm. Powder Xray diagrams were obtained in a Bruker system by using the Cu Kα radiation. The textural characteristics of the samples were determined from nitrogen adsorption experiments at 77 K. The isotherms were obtained in a commercial equipment (ASAP 2020) from Micromeritics. The experimental nitrogen adsorption data were used to determine the micropore volumes by applying the Dubinin−Radushkevich (DR) equation.

mechanism, and the electrophilic or nucleophilic substitution depending on the experimental conditions. The indirect path results from the decomposition of ozone into several species. Among them, in aqueous solution it can decompose into the hydroxyl radical, which is one of the strongest oxidants (standard reduction potential E° = 3.06 V) and indeed more oxidant than the ozone molecule (E°A = 2.06 V). The transformation of ozone into hydroxyl radicals depends on the pH, and it is catalyzed at basic pHs by the hydroxyl ion.23 Based on these facts, the aim of this work is to study the primary functionalization of multiwalled carbon nanotubes (MWCNTs) by ozone in basic aqueous solution. This allows us to analyze the effect of the pH on the characteristics and degree of functionalization. The results have been compared to those obtained by treatments with O3 in gas phase and with oxygen plasma which is also a strong oxidant.24

2. EXPERIMENTAL SECTION Commercial MWCNTs (Nanocyl-3100, 0.61% ash content) produced by Nanocyl have been used. The treatments with ozone were carried out by flowing an oxygen/ozone gas mixture (333.3 mL min−1) containing 6.5 × 10−2 g mL−1 of ozone through a suspension of 500 mg of MWCNTs in 200 mL of water. The suspension was sonicated for 30 min before the treatment. The pH of the reaction medium was set up in the 7−11 range, and two treatment times (15 min and 1 h) were used. The temperature of the suspension was kept at 25 °C. After the treatments the samples were washed with distilled water. The treatment with ozone gas was carried out by flowing the same oxygen/ozone mixture for two different times (15 min and 1 h) through a reactor containing 0.5 g of MWCNTs at room temperature. The treatment with oxygen cold plasma was carried out using a device from Europlasma (Europlasma Junior Advanced SS). The equipment produces plasma by using a 2.45 GHz microwave source. The MWCNTs, once in the device chamber, were outgassed to a residual pressure of 50 mTorr before the treatments. Prior to the treatment, oxygen was flowed during 5 min (12 mL min−1) up to a stable pressure of 200 mTorr. The source power was set to 300 W. Three different times of treatment (2, 10, and 30 min) were used. The characterization of the samples was carried out by using thermogravimetric analysis (TG), thermal programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD), and nitrogen gas adsorption. A Shimadzu TGA-504 thermogravimetric analyzer was used for TG measurements. The samples were heat-treated at 10 K min−1 in nitrogen flow (50 mL min−1) up to 950 °C. The carbon dioxide and carbon monoxide TPD profiles were

3. RESULTS AND DISCUSSION The TG weight losses are indirect evidence of the oxygen attachment resulting from the oxidative treatments, as the oxygen containing groups are desorbed during the thermal treatment. Thus, Figure 1a shows the TG weight loss of the samples obtained by treatment with ozone in basic solutions versus the pH. It is seen that larger amounts of oxygen are fixed after 1 h of treatment, although the differences between both treatment times are, in general, not very high. Moreover, there is a clear maximum for both periods of treatment. As already commented, when ozone is at basic pH, the main expected reactive species are hydroxyl radicals, whose production is highly dependent on the pH.23,25 Therefore, the increase in the oxygen content up to pH = 9 (Figure 1a) is probably due to a higher production of these radicals as the pH rises. Nevertheless, at pH over 9 the amounts of fixed oxygen decrease, which is likely related to the mechanism by which ozone is transformed into hydroxyl radicals at basic pH. The decomposition of ozone in basic aqueous solution is a complex process that consists of several steps. Besides, it has been reported26 that, in the presence of carbon materials, the first step involves the adsorption of hydroxide ions, OH− on the surface of the material: OH− + C* ⇄ C*−OH− B

(1)



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Figure 2. XPS survey spectra of (a) original MWCNTs, (b) sample obtained with ozone at pH = 9 for 1 h, and (c) sample obtained by plasma treatment for 30 min.

Figure 3. High-resolution C 1s XPS spectra of (a) sample obtained by ozone treatment at pH = 9 for 1 h and (b) samples obtained by plasma treatments.

spectra (Figure 2a,b). The values obtained from those spectra are plotted versus the pH of the treatment in Figure 1b. The comparison of Figures 1a and 1b clearly shows smaller amounts of oxygen as obtained from XPS (Figure 1b) than the weight losses measured by TG (Figure 1a). There are two reasons that explain these differences. The first one is that while XPS only analyses the external surface of the tubes (up to about 7 nm in depth), TG renders information on the bulk. As a consequence, the oxygen-containing groups inside the samples are not “seen” by XPS. The second is that the oxygencontaining groups evolve in TG as CO and CO2 which results in larger weight losses than if these groups evolve as molecular oxygen. As in Figure 1a, larger oxygen concentrations after 1 h of treatment are observed. Moreover, a decrease of these concentrations with the pH is also seen for both periods of treatment, although it is more intense in samples obtained after 15 min. The decrease of the oxygen content even in the 7−9 pH range contrasts with the trend in weight loss in Figure 1a.

where C* is a carbon adsorption center. Then, the ozone reacts with the adsorbed hydroxide ions rendering the hydroxyl radicals, OH•: C*−OH− + O3 ⇄ C*−O3− + OH•

(2)

After equilibrium 2, there are several other steps which eventually result in the decomposition of the C*−O 3− functions, rendering less reactive species.23−25 The adsorption of hydroxyl groups on the carbon centers (equilibrium 1) is probably the factor explaining the decrease of oxygen fixation at very basic pH. Thus, at very high pH values, a large amount of carbon adsorption centers are occupied by hydroxyl ions owing to their high concentration. The consequence is that, although hydroxyl radicals are produced (equilibrium 2), their reaction with carbon centers is kinetically restricted because most of these centers are already occupied by hydroxyl ions. More information about the oxygen fixed on the surface by the reaction with ozone at basic pHs can be obtained from the XPS C



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Figure 4. XPS relative amount of oxygen-containing groups of samples obtained by treatment with ozone in basic medium vs pH: (a) 15 min and (b) 1 h.

Figure 5. Data in (a−c) have been obtained from XPS high-resolution spectra. The ordinate values in (c) have been deduced from Raman spectra. Variation of (a) the graphitic carbon, (b) the nonconjugated carbon, (c) the π−π* shakeup satellite, and (d) the ID/IG ratio versus the surface oxygen content. The square symbols correspond to the original sample.

Taking into account the surface nature of the XPS analysis, the trend is consistent with the kinetic restriction already suggested. In fact, Figure 1b means that a large amount of active external carbon atoms are preferentially occupied by the adsorbed OH−, even at not very basic pH, thus avoiding the further oxidation of the external carbon surface. In other words, the more external the carbon atoms, the more intense are the kinetic restrictions to the oxidation. Therefore, the increase in the basic pH largely avoids the oxidation by the hydroxyl radicals. The deconvolution of the high-resolution C 1s and O 1s XPS spectra allows us to determine the nature of the oxygencontaining groups. Figure 3a shows an example of peak fitting and of the deconvolution of the C 1s peak of the sample obtained by ozone treatment at pH = 9 for 1 h. The assignments have been carried out according to data already reported.27 Thus, the C 1s main component at 284.3 eV is assigned to graphitic carbon. The shoulder of the mean peak consists of four components which are assigned to nonconjugated carbon (284.9 eV), C−O of phenol, ether or alcoholic hydroxyls (286 eV), CO of carbonyl or quinone (287 eV), and OC−O of carboxyl, anhydride, or ester (288.5 eV). In addition, the C 1s peaks also contain a band at 290.5 eV, which is due to π−π* shakeup feature.

Figure 4 shows the evolution of the relative amounts of oxygen-containing groups, obtained from the high-resolution XPS spectra, versus the pH of treatment. In all cases C−O are the most abundant groups, which is probably due to these groups are the first stage of the MWCNTs oxidation. An important aspect to be considered is the nature of the carbon sites where the oxygen-containing groups are fixed. This can be inferred by analyzing the evolution of the graphitic, the nonconjugated, and the π−π* shakeup components of the high-resolution XPS spectra. It is worth mentioning that the nonconjugated carbon fraction seems to be mainly related to carbon atoms in defects of the MWCNT surface. This statement is based on the results from the MWCNT TG experiment in air, which does not show any significant gasification below 520 °C,28,29 so suggesting that the amount of amorphous carbon present in the MWCNTs is small. Figure 5 shows the variation of these three components (Figures 5a− c) in all the samples obtained at 15 min and 1 h, with the amount of oxygen on the carbon nanotubes surface. It is seen (Figure 5a) a very small decrease of the graphitic component and a concomitant increase (Figure 5b) of the nonconjugated carbon as the oxygen content increases. These trends, together with the nearly negligible decrease of the π−π* shakeup D



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Figure 6. Raman spectra of the original MWCNTs and of some selected samples obtained by treatment with ozone at basic pHs.

(Figure 5c), suggest that the graphitic structure of the graphene layers is scarcely modified. In other words, this means that most of the oxygen-containing groups have been fixed in irregularities or borders already present in the MWCNTs, without a significant alteration of the main graphitic structure. Additional information on this aspect can be obtained from the Raman spectra. The first-order spectrum of the original sample has two bands: at 1340 cm−1 and in the range 1570− 1610 cm−1 (Figure 6). The first one is the so-called D line, which is a disorder-induced peak.30−36 The origin of this band stems from the sp3 carbon atoms at defect sites (nonconjugated carbon), amorphous carbon, or impurities.32 The second one, the G line, is due to sp2 carbon networks. The ID/IG intensity ratio of the D and G bands is frequently used as a measure of the defects at the carbon nanotube structures. Figure 5d shows the plot of the ID/IG ratio versus the surface oxygen content as determined from XPS. It is seen that the ID/IG ratio has a very small rise as the oxygen content increases. This fact together with the trend of the graphitic and nonconjugated carbon and of the π−π* shakeup signal (Figures 5a−c) suggests that the ozone treatment on basic solution scarcely changes the sp2 carbon network, as mentioned above. Despite this small influence produced by the ozone treatments in the graphitic structure, it is relevant determining any other possible modifications of the textural characteristics of the MWCNTs. X-ray diffraction diagrams and nitrogen adsorption isotherms were obtained with this aim. The X-ray diffractogram of the original MWCNTs has, among others, a main strong and well-defined peak at 2θ = 25.7° which can be indexed as the (002) plane diffraction. The interplanar value, d002, obtained by Bragg’s law is 0.345 nm, which corresponds to the interlaminar distance of graphitic structures. No peaks related to metal phases are detected. The X-ray diffractograms of the samples obtained by basic ozonation do not show any shifting of the peak position or any modification of the crystallinity. This suggests that the oxygen atoms are not located subsurface37,38 (forming interlaminar ether structures), at least to a significant extent, and also that the graphene layers are scarcely damaged, in agreement with the above comment on the XPS and Raman data. The nitrogen adsorption isotherms in Figure 7 show almost no difference between the samples. The shape of the isotherms is type II, but the adsorption at very low relative pressures (see inset), although it is not large, quickly increases as in the type I

Figure 7. Nitrogen adsorption isotherms of the original MWCNTs and of samples obtained by treatment with ozone at basic pHs.

isotherms. This increase means that the samples have some microporosity, and therefore the volume of micropores can be obtained by applying the Dubinin−Radushkevich (DR) equation. The calculated volume is 0.093 cm3 g−1 for the original sample while it ranges between 0.081 and 0.104 cm3 g−1 in the ozonized samples, suggesting no modification of the textural characteristics. To sum up: all the above data point out that the basic ozonation of the MWCNTs results in fixation of oxygencontaining groups (mainly C−O) in defects or borders with almost no modification of the graphene layers and no creation of porosity. The data obtained by ozone treatment in basic aqueous solution have been compared to those resulting from the treatments with ozone gas and plasma. Table 1 contains the results obtained from XPS and Raman analysis of the samples from the different treatments, together with those of the pristine MWCNTs. The sample treated with ozone at pH = 8 for 1 h was selected for comparison, as this sample has the largest amount of surface oxygen, as determined by XPS, among those prepared in aqueous solution. Despite the higher oxidation character of the ozone basic solutions in comparison with ozone gas (as explained above), the treatment with the gas for 15 min already introduces a similar amount of oxygen, while E



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Table 1. Chemical Characteristics of the Samples As Obtained from XPS Spectra and Values of the ID/IG Ratio from Raman Spectra MWCNTs pH = 8, 1 h O3 gas, 15 min O3 gas, 1 h plasma, 2 min plasma, 10 min plasma, 30 min

O (%)

C−O (%)

CO (%)

OC−O (%)

graphitic carbon (%)

π−π*

ID/IG

1.2 3.1 3.4 5.1 9.1 14.0 17.3

6.2 8.1 6.7 5.7 10.6 5.4 6.7

5.1 5.2 5.9 6.4 9.3 16.5 20.0

1.2 1.5 2.5 2.9 3.6 5.8 7.6

61.9 59.1 58.4 57.6 54.1 48.7 44.1

13.2 12.8 12.5 12.7 11.0 8.9 7.7

1.40 1.46 1.42 1.35 1.38 1.48 1.48

Figure 8. TPD profiles of some selected samples obtained by the three treatments: (a) carbon dioxide and (b) carbon monoxide.

ozone gas for 1 h is able of fixing a larger amount than the wet treatment for the same time. These data support that the reaction with ozone in basic aqueous solution is kinetically restricted, as commented before. The data in Table 1 also show that the plasma treatment stands out in the total amounts of fixed oxygen, as in a short time of 2 min is able of almost doubling the amount fixed by ozone gas for 1 h. The effect of the plasma is also evident in the XPS survey spectrum of the sample (Figure 2c) obtained after 30 min. Moreover, the amount of oxygen increases with the time of plasma treatment, as observed in Table 1, and it can also be deduced from the increase in the intensity of the shoulder of the high-resolution C 1s spectra in Figure 3b. It is also seen in Figure 3b a decrease of the C 1s main component that it is probably due to the partial transformation of sp2 carbon atoms to sp3 (this aspect will be further addressed when XPS values of the π−π* shakeup component are considered). Table 1 also shows the percentages of the three oxygen functionalities obtained by deconvolution of the high-resolution spectra. Ozone gas results in close values of C−O and CO groups at both 15 min and 1 h of treatment, and the same is also found for the 2 min oxygen plasma treatment. Nevertheless, the plasma treatments for 10 and 30 min largely increase the CO contents. The trend in the oxygencontaining groups of the samples obtained by plasma treatments seems to support an already reported mechanism accounting for the generation of these groups under oxygen plasma conditions.39 According to this, C−O groups are the primary functions produced by oxygen plasma. A further treatment results in the transformation of these primary functions onto CO and later on into OC−O. The data in Table 1 of the samples obtained by plasma show that C−O groups are prevalent at 2 min of treatment. Nonetheless, CO

are the most abundant at 10 and 30 min, which is likely due to the further oxidation of the parent C−O. Nevertheless, the amounts of O−CO groups detected are smaller. This could probably be related to the small amounts of hydrogen present in the original MWCNTs (0.3 wt %), as hydrogen is necessary to stabilize the carboxylic groups. In any case, it is evident that the amounts of OC−O groups increase for longer treatment times, which seems to agree with the reported mechanism. In the case of the samples obtained by treatment with ozone at basic pHs (Table 1 and Figure 4) the prevalent groups are C− O. Thus, it seems that the further oxidation to CO and O C−O groups is constrained which again points out to kinetic restrictions to oxidation. In the ozone gas treatments C−O and CO groups are the most abundant, although OC−O have the highest relative increase. Therefore, although there is no experimental evidence to support a reaction mechanism, it seems that the oxidation progresses in three different paths depending on the reactive and on the experimental conditions. Figure 8 shows the qualitative CO2 and CO TPD profiles of the samples obtained by aqueous ozone at pH = 8, by ozone gas (1 h), and by plasma (30 min) treatments. The desorption profiles of the original MWCNTs (not plotted in Figure 8) show almost negligible amounts of both CO2 and CO. The sample obtained by plasma shows in the CO2 profile a welldefined maximum at low temperature (175 °C) and a long tail, which extends up to high temperatures. The samples obtained at pH = 8 and by oxygen gas show wide bands which range from around 175−225 to 800 °C. The CO2 evolutions at low temperatures are usually assigned to carboxylic groups, whereas those at higher temperatures are to lactones or anhydrides.40 The fact that the CO2 profiles extend in a wide range of temperatures means the chemical functions have a large diversity of stabilization energies. F



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Figure 9. Modification of the high-resolution XPS spectra of the sample obtained by plasma (30 min) after being heat-treated (see the text): (a) C 1s and (b) O 1s.

related to the concomitant partial elimination of some amorphous carbon during the treatment. Regarding possible modifications of the MWCNT texture, the data from nitrogen gas adsorption do not suggest any formation of porosity or irregularities for none of the samples obtained by ozone gas or plasma treatment, similarly to that commented before for the samples obtained by the ozone wet treatments. The nitrogen adsorption isotherms are almost coincident with that of the original MWCNTs. Moreover the application of the DR equation to the adsorption data provides a micropore volume range between 0.087 and 0.100 cm3 g−1, which are very close to the value obtained for the original MWCNTs (0.093 cm3 g−1). Similarly, the interlayer spaces as determined by X-ray diffraction are also similar (0.345 nm) to the interlayer spaces of the parent MWCNTs, which accounts for no oxygen intercalation between the graphene layers.

In the case of the CO profiles (Figure 8b) the sample obtained by plasma has two maxima at low and high temperatures, respectively. Moreover, the peak at low temperature coincides (175 °C) with the maximum of the CO2 profile (Figure 8a), which is probably due to the condensation between several adjacent oxygen-containing groups.41,42 The CO profiles of the other two samples only show desorption at relatively high temperatures, usually assigned to phenol and hydroquinones. A general conclusion of these data is that the plasma treatments fix the largest amounts of oxygen-containing groups (Table 1), but some of them seem to have relatively low thermal stability. To check this fact, the sample obtained by plasma at 30 min was heat-treated in air at 125 °C for 24 h and subsequently analyzed by XPS. The survey spectrum shows a large decrease of the O 1s peak, reducing the surface oxygen content from 17.3% (Table 1) to 9%. This decrease is also evident in Figure 9, where the C 1s and O 1s high-resolution spectra of the sample before and after the heat treatment are depicted. In spite of this, the oxygen content after the thermal treatment is still much larger than those resulting from both ozone treatments. The values of the XPS graphitic component and of the π−π* shakeup of the sample obtained by ozone gas treatments are similar to these of the original MWCNTs (Table 1). This means that this treatment scarcely modifies the nanotube walls; i.e., the oxygen-containing groups are fixed in irregularities or borders, not affecting the π cloud of the graphene sheets. This behavior is similar to that already commented about the bonded oxygen by ozone at basic pHs. Nevertheless, the values of the graphitic and the π−π* shakeup components of the samples obtained by plasma treatment clearly decrease with the treatment time, suggesting that this procedure is mainly bonding the oxygen-containing groups on the nanotube walls. This trend is in agreement with the decrease in the C 1s main component of Figure 3b already commented. The ID/IG values in Table 1 show almost no variation with the different treatments, which is in agreement with the above statement about the oxygen is mainly bonded in irregularities and borders of the nanotubes in the cases of ozone aqueous solution and of gas phase treatments. In the case of the plasma treatments, it should be expected an increase of this ratio owing to oxygen is mainly fixed on the walls of the MWCNTs, according to the above conclusion on the modification of the C 1s peak. The fact that almost no change is shown is probably

4. CONCLUSIONS The efficiency of ozone in basic aqueous solution to attach oxygen groups is smaller than this of ozone gas due to the former is kinetically restricted. Oxygen plasma is the most efficient, among the three procedures, in the covalent attachment of oxygen-containing groups on MWCNTs. Although a significant amount of these groups are relatively labile, as it evolves under mild heating conditions (

Functionalization of Multiwall Carbon Nanotubes by Ozone at Basic

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