Nondestructive covalent functionalization of carbon

Applied Surface Science 346 (2015) 520–527

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Nondestructive covalent functionalization of carbon nanotubes by selective oxidation of the original defects with K2 FeO4 Zhao-yang Zhang, Xue-cheng Xu ∗ Department of Physics, East China Normal University, 500 Dong Chuan Road, Shanghai 200241, China

a r t i c l e

i n f o

Article history: Received 3 January 2015 Received in revised form 11 March 2015 Accepted 3 April 2015 Available online 11 April 2015 Keywords: Carbon nanotubes (CNTs) Nondestructive functionalization Covalent functionalization Oxidation Potassium ferrate

a b s t r a c t Chemical oxidation is still the major approach to the covalent functionalization of carbon nanotubes (CNTs). Theoretically, the defects on CNTs are more reactive than skeletal hexagons and should be preferentially oxidized, but conventional oxidation methods, e.g., HNO3 /H2 SO4 treatment, have poor reaction selectivity and inevitably consume the C C bonds in the hexagonal lattices, leading to structural damage, ␲-electrons loss and weight decrease. In this work, we realized the nondestructive covalent functionalization of CNTs by selective oxidation of the defects. In our method, potassium ferrate K2 FeVI O4 was employed as an oxidant for CNTs in H2 SO4 medium. The CNT samples, before and after K2 FeO4 /H2 SO4 treatment, were characterized with colloid dispersibility, IR, Raman spectroscopy, FESEM and XPS. The results indicated that (i) CNTs could be effectively oxidized by Fe (VI) under mild condition (60 ◦ C, 3 h), and hydrophilic CNTs with abundant surface COOH groups were produced; and (ii) Fe (VI) oxidation of CNTs followed a defect-specific oxidation process, that is, only the sp3 -hybridized carbon atoms on CNT surface were oxidized while the C C bonds remained unaffected. This selective/nondestructive oxidation afforded oxidized CNTs in yields of above 100 wt%. This paper shows that K2 FeO4 /H2 SO4 is an effective, nondestructive and green oxidation system for oxidative functionalization of CNTs and probably other carbon materials as well. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Carbon nanotubes (CNTs) are a special kind of carbon allotrope comprised of rolled-up graphene sheets with extraordinary structural, mechanical and electronic properties [1,2]. These intrinsic properties and many potential applications demand both structural integrity and chemical functionality [3,4]. In this context, functionalization of CNTs without structural damage has been pursued consistently. Noncovalent functionalization is known as a strategy to preserve the conjugated skeleton, but the covalent approach, which shows obvious advantages in the chemistry and applications of CNTs, is suffering from the drawback of structural damage [5–7]. Currently, the methods for covalent functionalization of CNTs have been well-developed, but the nondestructive covalent functionalization is still challenging. The covalent chemistry of CNTs, according to its reaction mechanism, can be classified into two types [5–8]: (i) organic additions including halogenation, cycloadditions, radical-, nucleophilic- and electrophilic additions, etc., and (ii) oxidation reactions with

∗ Corresponding author. Tel.: +86 130 2318 4440. E-mail address: [email protected] (X.-c. Xu). http://dx.doi.org/10.1016/j.apsusc.2015.04.026 0169-4332/© 2015 Elsevier B.V. All rights reserved.

inorganic oxidants such as O3 [9], O2 [10], HNO3 [11] or HNO3 /H2 SO4 [12], KMnO4 [13] or KMnO4 /H2 SO4 [14], K2 Cr2 O7 /H2 SO4 [14], piranha solution (H2 O2 /H2 SO4 ) [15] and Fenton agent (Fe2+ /H2 O2 ) [16]. Apparently, the organic additions occur exclusively at the sp2 -hybridized carbon atoms (sp2 -C). Once one functional moiety is bonded onto CNT sidewall, two ␲ electrons are removed [17]; in this way the ␲ system of CNTs is inevitably destroyed. In the case of chemical oxidation, on the other hand, it is generally believed that oxidants preferentially react with the original defective sites [10,15,18–23]. Indeed, realistic CNTs contain many vacancies, adatoms, dislocations, holes, cracks, etc. that involve dangling bonds, as well as topological defects such as pentagons, heptagons and Stone-Wales defects with strained C C bonds [24–26]; both experimental results and theoretical calculations indicate that these defects are energetically unstable and more reactive toward oxidation than the perfect hexagonal lattices [20–23,27,28]. Hence, theoretically speaking, there seems some hope that oxidation of CNTs occurs only at the original defects and does not affect the hexagons. Unfortunately, all those oxidants attempted to oxidize CNTs have been shown to disrupt the structural integrity and induce new defects, as convinced by Raman spectra and electron microscope results [9–16]. They can also shorten nanotubes and degrade

Z.-y. Zhang, X.-c. Xu / Applied Surface Science 346 (2015) 520–527

them into amorphous carbon [10–13,29]. As a result, many desirable intrinsic properties of CNTs are impaired and even destroyed. Another annoying problem is the severe weight loss [11,15,30], which is also originated from the structural damage. To minimize the structural damage during CNT oxidation, many efforts are dedicated to the optimization of reaction conditions and the innovation of treatment process [31–40]. However, to the best of our knowledge, the nondestructive oxidation, namely selectively oxidizing the original defects without consuming the hexagons, has not been achieved. In this paper, we present the nondestructive (defect-specific) oxidation of CNTs. Potassium ferrate K2 FeVI O4 , a hexavalent iron salt, was utilized as an oxidant and the treatment was conducted in H2 SO4 medium at 60 ◦ C. The defect degree, morphology, oxidation degree and surface chemistry of the CNT samples, before and after oxidation for different time, were investigated by Raman spectra, FESEM, XPS, etc. All the results consistently showed that Fe (VI) oxidation occurred selectively at the sp3 -C while the C C bonds remained intact. To show the essential differences between the nondestructive and the destructive (nonselective) oxidation, the prevailing HNO3 /H2 SO4 treatment, known as an effective but destructive oxidation method, was taken as a comparison. 2. Experimental 2.1. Material and chemicals Multiwalled CNTs prepared by CVD were obtained from Chengdu Organic Chemicals Co., Ltd., China. According to the manufacturer, the CNT product has a purity of more than 95%. The mean outer diameter is examined to be 15–20 nm. K2 FeO4 with a purity of more than 93% was bought from Shanghai Zhenpin Chemical Co., Ltd., China, and the major impurities are ferric oxide and chlorides. The other reagents were analytically pure. Deionized water was used throughout the experiments. 2.2. Oxidation treatment Fig. 1 shows the reagents used and the reaction conditions for CNT treatment. Each treatment was repeated for 3 times. The procedures of the conventional H2 SO4 and HNO3 /H2 SO4 treatment can be found in Supplementary data. K2 FeO4 /H2 SO4 treatment is described below in detail. 3.0 g of potassium ferrate (black powder) was slowly added to 20.0 mL of cold sulfuric acid (95–98%) under magnetic stirring in a 50 mL PTFE beaker. One can see that potassium ferrate was immediately dissolved in sulfuric acid to form a milky solution (Figure S1), into which 100.0 mg of raw CNTs were added and dispersed under stirring and sonication for 10 min. Then the dispersion was transferred into a PTFE flask and stirred at 60 ± 1 ◦ C for 3 or 6 h. The resulting mixture was diluted in 500 mL of cold water in a 500 mL PTFE beaker, and settled for 2 h to sediment the solid prior to filtration with 0.22 ␮m membrane. After filtration, the solid was

521

redispersed in 100 mL of 1 mol/L HCl under brief sonication and stored overnight for desorption of Fe3+ on oxidized CNTs, followed by filtration. The obtained solid was washed at least 10 times with 50 mL of water each time. The products were dried at 60 ◦ C, collected, weighed, and stored in a desiccator. 2.3. Characterization Tyndall effect was observed using a 532 nm YAG green laser. Fourier transform IR spectra were recorded on a Nicolet 6700 spectrometer (Thermo Fisher Scientific) equipped with a smart collector and a MCT detector. Raman spectra were carried out by a Jobin-Yvon LabRAM HR 800 UV spectrometer with a 633 nm line of a He–Ne laser as the exciting source. FESEM images were obtained with a JSM6700F (JEOL) field emission scanning electron microscope; for FESEM characterization, CNT samples were first dispersed in ethanol, then a few drops of the dispersion were deposited onto copper foils, followed by evaporation of the solvent. The CNT diameter was measured from FESEM images using Nano Measurer software; at least 150 tubes were counted to get the diameter distribution histograms. The surface chemistry of the samples was detected by XPS on a RBD upgraded PHI-5000 C ESCA system (Perkin Elmer); all spectra were calibrated with respect to the position of sp2 -hybridized component peak of C 1s spectrum at 284.6 eV after the subtraction of Shirley-type background and peak decomposition by using XPS Peak 4.1 software. 3. Results Among the various oxidation methods for CNTs, HNO3 and KMnO4 treatments in H2 SO4 medium are the most effective, and they have already been used in industrial production1 . However, these oxidation methods cause environmental problems that cannot be ignored, especially in large-scale production. HNO3 treatment produces NO2 gas, which is a prominent air pollutant; oxidation with KMnO4 releases toxic Mn2+ into water body. On the other hand, K2 FeO4 is acclaimed as a green oxidant because its reduction product Fe (III) is non-toxic [41,42]. With a redox potential of as high as 2.2 V, this green oxidant has attracted special attention due to its effective oxidation of various organic and inorganic compounds [43,44], but the oxidation effectiveness of this oxidant for CNTs remains unknown. Therefore, the green oxidation of CNTs with this novel oxidant is the primitive motivation of this study. Fig. 1 shows the treatment conditions. All treatments were conducted in H2 SO4 medium at a relatively low temperature of 60 ◦ C. The green oxidant K2 FeO4 was attempted to produce oxidized CNTs for the first time, with H2 SO4 treatment as a blank experiment. For comparative purposes, the prevailing HNO3 /H2 SO4 oxidation was also conducted. 3.1. Colloid dispersibility Dispersibility test of treated CNTs in polar solvents is a convenient way to qualitatively evaluate the effectiveness of oxidation [31,45]. As shown in Fig. 2a, raw CNTs cannot be dispersed in water; neither can those treated with H2 SO4 at 60 ◦ C for 3 h (s3), due to their unmodified hydrophobic surface. Note that H2 SO4 is not able to oxidize CNTs even at 100 ◦ C up to 24 h, as reported previously [46], thus it is a safe media for functionalization. In contrast, after

Fig. 1. Treatment conditions for raw CNTs with different oxidants.

1 HNO3 /H2 SO4 treatment, NanoLab Inc., UK, http://www.nano-lab.com/coohfunctionalized-nanotubes.html. KMnO4 /H2 SO4 treatment, Chengdu Organic Chemicals Co., Ltd., China, http://www.timesnano.com/view.php?prt=3,29,50,81.

522

Z.-y. Zhang, X.-c. Xu / Applied Surface Science 346 (2015) 520–527

such as -COOH, which showed a peak at 1718 cm−1 [12]. Also, the peaks at 1045 and 1579 cm−1 came from C O bonds and the C C bonds located near the oxygenated groups [47], respectively. On the other hand, raw CNTs and s3 exhibited the similar featureless IR absorption, which is in accordance with the colloid stability result. 3.3. Raman spectra

Fig. 2. Photographs of (a) 0.5 g/L CNT samples in H2 O that have been sonicated for 5 min and settled for 1 month, and (b) Tyndall effect of the dispersions containing 10 mg/L f 3 in water, ethanol and N-methyl-2-pyrrolidone (NMP).

Fig. 3. IR spectra of the CNT samples before and after 3 h of treatment with H2 SO4 and K2 FeO4 /H2 SO4 .

K2 FeO4 /H2 SO4 treatment at 60 ◦ C for 3 h, the CNTs (f3) can be easily dispersed in water after brief sonication, giving rise to stable ink-like dispersion. From this result, it is clear that Fe (VI) is able to successfully oxidize CNTs. Moreover, f3 dispersion exhibited Tyndall effect in various polar solvents (Fig. 2b), which indicated that the oxidation-produced oxygenated groups on CNT surface were abundant enough to overcome the van der Waals attractions within CNT bundles. 3.2. IR spectra Fig. 3 shows IR spectra of the CNT samples. The IR spectrum of f3 confirmed the presence of the surface oxygenated groups,

Raman spectroscopy is one of the most powerful methods to probe the defect degree of carbon materials by quantifying the relative intensity of defect band (D band) and graphite band (G band). The G band corresponds to the symmetric E2g mode of the hexagonal lattices, while the non-hexagonal structures, namely the defects, contribute to the D band [48]. The Raman spectra of raw CNTs and Fe (VI) oxidized CNTs are shown in Fig. 4a; all the spectra have been normalized with respect to the D band. The intensities of the G bands did not show noticeable variation after 3 and even 6 h of oxidation, which was somewhat surprising because the remarkable increase in D/G intensity ratio (ID /IG ) is always considered as an indicator for successful covalent functionalization including oxidation [49,50]. To measure the ID /IG accurately, the spectra were decomposed into three Lorentzian peaks [51,52] at ∼1320, ∼1575 and ∼1600 cm−1 , corresponding to D, G and D band, respectively (see Supplementary data for detail). The results of HNO3 /H2 SO4 oxidized CNTs followed the same treatment conditions (60 ◦ C, 3 or 6 h) were also examined. As expected, in the case of HNO3 /H2 SO4 oxidation, ID /IG remarkably increased with the treatment duration (Fig. 4b), because HNO3 /H2 SO4 oxidation led to structural damage and generated new defects. However, the CNTs before and after Fe (VI) oxidation remained relatively constant ID /IG : from 2.06 to 2.10 and 2.00 after oxidation for 3 and 6 h, respectively, implying that Fe (VI) was not good at producing new defects during CNT oxidation. The data fluctuation in a narrow range was considered normal in view of the heterogeneity of CNT samples. 3.4. FESEM Typical FESEM images of oxidized CNTs are shown in Fig. 5. Here we only discussed the surface morphology of f6 and n6 because they were subjected to longer treatments and could display more distinct effects of oxidation on the structural integrity of CNTs. FESEM images of raw CNTs, f3, n3, and additional images of f6 and n6 were provided in Figure S3. f6 consisted of long individual tubes with clear and smooth surface, and no adverse effects of Fe (VI) oxidation on the structural integrity were observed. On the

Fig. 4. (a) Raman spectra of the CNT samples before and after Fe (VI) oxidation for 3 and 6 h. Inset is a magnified view of the spectra at 1500–1650 cm−1 . (b) Comparison of the changes in ID /IG values of the CNT samples after oxidation with K2 FeO4 /H2 SO4 and HNO3 /H2 SO4 .

Z.-y. Zhang, X.-c. Xu / Applied Surface Science 346 (2015) 520–527

523

Fig. 5. FESEM images of the CNT samples after 6 h of oxidation with K2 FeO4 /H2 SO4 and HNO3 /H2 SO4 . Red arrows point to some very thin tubes.

contrary, HNO3 /H2 SO4 oxidation resulted in shortened and curled tubes, indicating high defect degree of n6 and structural degradation. Another visible difference was that f6 contained many very thin tubes (diameter < 10 nm) but they disappeared in the n6 network. The diameter distribution (see histograms in Figure S4) also indicated the elimination of the thin tubes with diameters below 15 nm by HNO3 /H2 SO4 treatment. It is well known that CNTs with small diameter have high-degree curvature and thus are vulnerable toward covalent functionalization; hence the very thin tubes cannot survive destructive oxidation, e.g., the HNO3 /H2 SO4 treatment. Accordingly, the presence of the very thin tubes with long and straight morphology in the f6 network might serve as an evidence for nondestructive oxidation by Fe (VI). The morphology observed by FESEM was consistent with the Raman spectra results.

3.5. XPS: elemental analysis and surface chemistry Surface elemental analysis was performed to investigate the oxidation degree of the CNT samples. The general XPS spectra for elemental analysis are shown in Figure S5, and the atomic concentration results are listed in Table 1. Raw CNTs contains 6.1% oxygen, which may arise during the CVD growth and purification process. H2 SO4 treatment made little contribution to the surface oxygen simply because H2 SO4 is a safe media, as aforementioned. After Fe (VI) treatment for 3 h, CNTs showed a significant increase in the O/C ratio, resulting from the effective oxidation effect of Fe (VI) on CNTs. Interestingly, the Fe (VI) oxidation, considered to be effective, was totally finished within 3 h since treatment for another 3 h did not cause any increase in the O/C ratio. In contrast, HNO3 /H2 SO4 treatment provided persistent oxidation and reached much higher oxidation degree.

Table 1 Elemental analysis from XPS and fitting results of the XPS C 1s spectra. All values are given in %. CNT sample

Raw CNTs s3 f3 f6 n3 n6

Elemental analysis

Carbon atoms in different bonding structures

C

O

O/C ratio

C C

sp3 -C

C O

C O

O C O

93.9 93.7 86.3 86.4 82.3 81.3

6.1 6.3 13.7 13.6 17.7 18.7

6.5 6.7 15.9 15.7 21.5 23.0

58.1 57.5 57.2 57.8 48.8 38.6

22.0 20.9 4.8 4.5 8.5 12.4

11.4 12.6 22.3 23.7 28.1 30.0

2.6 2.8 3.6 2.0 0.8 2.8

5.9 6.2 12.1 12.0 13.8 16.2

To gain insights into the evolution of the surface chemistry during oxidation process, high-resolution C 1s spectra were detected. As shown in Fig. 6, all the spectra were deconvolved into the same set of six components, and the proportions of the corresponding carbon species were listed in Table 1. Peaks at 284.6 eV corresponded to the sp2 -C in C C bonds, while the peaks located at 285.3 eV arose from the sp3 -C at the defective sites. Peaks centered at 286.3, 287.7 and 288.9 eV were assigned to the oxygenated carbon atoms in C O, C O and O C O bonds, respectively. The small shake-up peaks at above 291.0 eV came from ␲→␲* transition and were not included in the quantification of carbon species in different bonding structures [53,54]. Raw CNTs contain as many as 22.0% of sp3 -C (Table 1), implying high density of surface defects. These sp3 -C have higher reactivity and are assumed to be preferentially oxidized. In the classical HNO3 /H2 SO4 oxidation, ∼60% of the original sp3 -C were oxidized and converted into oxygen-containing groups for the first 3 h, but we also observed a considerable decrease in the content of C C bonds. Another 3 h of oxidation witnessed further consumption of the C C bonds; meanwhile, sp3 -C were regenerated, accompanied with the introduction of more oxygenated groups. Fe (VI) oxidation also led to remarkable increase in C O and O C O in 3 h of oxidation. Although f3 showed less oxygen-containing groups and lower oxidation degree than n3, during Fe (VI) oxidation the C C bonds remained intact and only the original sp3 -C were oxidized, with a ∼80% conversion. Moreover, f6 exhibited almost the same surface chemistry with f3, although the former had suffered prolonged treatment with Fe (VI). This result, together with the elemental analysis above, revealed that the reaction spontaneously finished within 3 h. 3.6. Yields Yields of the treated CNT samples are shown in Table 2. Each treatment was repeated for three times with 100.0 mg starting raw CNTs. A small weight loss was inevitable during the treatment process, therefore 2–3 mg of CNTs were lost after H2 SO4 treatment in our experiments. Undoubtedly, in the case of destructive oxidation, the weight loss depends on the oxidation duration or the extent of structural damage. To obtain n3 and n6, ∼14 and ∼31 wt% CNTs were lost, respectively. Surprisingly, Fe (VI) oxidized CNTs reached an average yield of ∼102.7 wt% with reproducible results. The concern about Fe3+ adsorption-induced weight increase should be ruled out because the content of Fe detected by XPS in f3 and f6 were less than 0.05%; the unshaped Fe 2p spectra (Figure S6) also indicated that the iron residue was negligible. Thus, the increase in

524

Z.-y. Zhang, X.-c. Xu / Applied Surface Science 346 (2015) 520–527

Fig. 6. Curve fitting of the XPS C 1s spectra.

Table 2 Yields of the CNT samples obtained from different treatment methods. CNT sample s3 n3 n6 f3 f6

Raw CNT dosage (mg) 100.0 100.0 100.0 100.0 100.0

Production (mg)

Average yield (%)

Rep. 1

Rep. 2

Rep. 3

97.1 86.4 72.3 103.1 102.4

97.5 87.8 71.2 101.6 103.1

98.8 84.1 69.3 103.5 102.4

97.8 86.1 70.9 102.7 102.6

weight can be only attributed to the introduction of oxygen atoms, specifically the conversion of sp3 -C into C O, O C O groups, without consuming the C C bonds. 4. Discussion 4.1. Effective oxidation of CNTs with Fe (VI) Fe (VI) treatment produced hydrophilic CNTs that could be stably dispersed in various polar solvents (Fig. 2). The surface hydrophilicity was attributed to the oxidation-produced oxygenated groups (e.g., COOH), as confirmed by IR spectra (Fig. 3). The oxidation effect was also quantitatively evaluated by XPS (Table 1), which showed that the O/C ratio was considerably increased and that CNTs were abundant in oxygenated groups after Fe (VI) oxidation, although the treatment was conducted at 60 ◦ C for only 3 h. Therefore, Fe (VI) is an effective oxidant for CNTs. 4.2. Nondestructive oxidation by selectively oxidizing the defects Nowadays, HNO3 /H2 SO4 oxidation is the most widely used method, mainly because of its high effectiveness in producing oxygenated groups. However, it has been well-documented that this oxidation method always results in severe structural damage; typical consequences include shortened/curved tubes (Fig. 5) and weight loss (Table 2). Destructive oxidation is relentless and will not terminate until CNTs are completely digested into CO2 /CO,

thus the yield of HNO3 /H2 SO4 oxidized CNTs decreases with the treatment duration. The structural damage originates from the breakage/consumption of C C bonds in the hexagonal lattices, as demonstrated by the pronounced increase in ID /IG (Fig. 4b) and the remarkable decrease in the content of C C bonds (Table 1) after HNO3 /H2 SO4 oxidation. Both defects and hexagons can be oxidized. Only by selectively oxidizing the defects without attacking the hexagons can we realize the nondestructive oxidation. According to the experimental results, we believe that Fe (VI) oxidation is selective and nondestructive. The evidences were summarized below:

(1) The defect degree of the CNT samples, measured by ID /IG , kept relatively unchanged after Fe (VI) oxidation. In other words, Fe (VI) oxidation did not produce new defects and thus only the original defects were converted into oxygenated groups; (2) The content of the C C bonds, calculated from the C 1s spectra, remained constant during oxidation, namely the C C bonds were not consumed. On the other hand, the content of the original sp3 -C was largely decreased during oxidation, and not coincidentally, the number of the newly produced oxygenated groups was equivalent to that of the decreased sp3 -C. Hence, Fe (VI) oxidation must have occurred selectively at the defects, specifically the sp3 -C; (3) Although effective, Fe (VI) oxidation spontaneously finished within 3 h. This result is not surprising if we consider that only the original defects on CNTs can be oxidized without generation of new defects. For defect-specific oxidation, once the original defects have been consumed up, further oxidation is impossible (a small number of sp3 -C were still left on f6, probably due to these sp3 -C detected by XPS were distributed at inner tubes but not the accessible outermost walls and they had no chance to be oxidized by Fe (VI) molecules); (4) FESEM images of the Fe (VI) oxidized CNTs showed long tubes with smooth clear surface and contained many very thin tubes with long and straight morphology, which is consistent with the nondestructive oxidation;

Z.-y. Zhang, X.-c. Xu / Applied Surface Science 346 (2015) 520–527

525

Fig. 7. Comparison between the nondestructive Fe (VI) oxidation and the destructive HNO3 oxidation.

(5) The yields of the Fe (VI) oxidized CNTs were beyond 100 wt%. The weight increase strongly supports the viewpoint that abundant oxygenated groups were attached on CNT surface without any degradation of CNT skeleton. K2 FeO4 /H2 SO4 and HNO3 /H2 SO4 treatment respectively represents the nondestructive and destructive oxidation. Their essential differences are illustrated in Fig. 7. First, nondestructive oxidation is characterized with constant content of sp2 -C and ID /IG ratio, while destructive oxidation leads to remarkable decrease in sp2 -C and increase in ID /IG . Second, the original surface sp3 -C atoms are the only oxidizable carbon specie during nondestructive oxidation, thus the increment of oxygenated carbon species is equal to the decreased amount of sp3 -C, otherwise the sp2 -C have been oxidized, as in the case of destructive oxidation. Third, different with the destructive oxidation during which the oxidation degree depends on reaction duration and CNTs can be completely consumed, the reaction extent of the nondestructive oxidation is limited by the finite quantity of surface sp3 -C, thus oxidation reaction comes to an end when all the surface sp3 -C are oxidized. All of the realistic CNTs are defective, and our results suggest that the oxygenated groups converted from the original defects are already sufficient enough to endow CNTs with good dispersity and abundant chemical functionalities. We have to admit that HNO3 /H2 SO4 as an oxidation system can produce more oxygenated groups, but the higher oxidation degree is at the expense of severe structural damage, which is unreasonable for many practical applications of CNTs. 4.3. Distinguishing the two different kinds of defects for oxidation Here, it is necessary to distinguish the two essentially different kinds of defective carbon atoms for oxidation: (i) sp3 -C in C H or C C dangling bonds, which are the most reactive [20,26,55], and (ii) topologically defective (non-hexagonal) carbon atoms in strained C C bonds. We have concluded that Fe (VI) oxidation occurred selectively at the defects, but did Fe (VI) oxidize both of them or only oxidize the sp3 -C? As we know, the non-hexagonal carbon atoms are mainly sp2 bonded but contain a small sp3 component, similar with the carbon atoms in C60 which exhibit sp2.28 hybridization [56]; the sp3 component may explain why they have a higher reactivity than the hexagonal sp2 -C. However, the non-hexagonal sp2 -C presumably have a similar C 1s binding energy with the hexagonal sp2 -C, owing to their only slight difference in the hybridization state. Therefore, if the non-hexagonal sp2 -C have been oxidized, there should be a conspicuous decrease in the content of C C bonds. In fact, the decrease in C C bonds after

Fe (VI) oxidation was rather negligible (Table 1), which suggested that Fe (VI) could highly selectively oxidize the sp3 -C and were not capable of oxidizing the sp2 -C in either hexagons or non-hexagons. Although non-hexagons are regarded as structural defects, they represent an integral part of the CNT skeleton and they also contain ␲ electrons. For the structural integrity and the preservation of the ␲ system, it would be better to keep the non-hexagons intact as well. 4.4. On the oxidation mechanism and Fe (VI) chemistry in H2 SO4 Although defects are more reactive toward oxidation, defectspecific oxidation has seldom been achieved. The selectivity should primarily depend on the property of an oxidant. For example, HNO3 /H2 SO4 oxidation occurs via the formation of nitronium ion (NO2 + ), which is a very powerful electrophile and readily attacks the C C bonds [18,57,58], resulting in poor selectivity between NO2 + and the defects. However, Fe (VI) seems not aggressive enough to break the C C bonds and only the sp3 -C are oxidized. Delaude and Laszlo [59] reported that the C H bonds of hydrocarbons were efficiently activated by Fe (VI) to produce alcohols and aldehydes/ketones, which shares similarity with our results. More interestingly, many researchers observed that some unsaturated alcohols were oxidized by Fe (VI) into the corresponding aldehydes/ketones and acids without affecting the C C bonds in alkyl chains [60–63]; these results also indicated the selectivity in Fe (VI) oxidation. Fe (VI) ion is unstable in aqueous solutions, especially in acidic conditions, because it can rapidly react with water molecules and decompose to Fe (III), which greatly hampers the exertion of Fe (VI) oxidative ability. To address this problem, Fe (VI) oxidation is always conducted in organic solvents or alkaline aqueous solutions. It is also worth mentioning that K2 FeO4 has already been used to treat carbon materials in two studies, both in alkaline solutions. The first study was by Mao et al. [64], who reported that treating natural graphite in K2 FeO4 solution (pH 9.3, 80 ◦ C, 8 h) could eliminate the surface disorder at the edge planes of graphite without attacking the basal planes; however, the content of the generated oxygenated groups, the most important parameter for assessing the oxidation effectiveness, was unknown. In the other study, K2 FeO4 was used as a precursor to deposit iron oxides on carbon nanofoam substrate; this reaction was conducted in 9 M KOH solution for 40 h and some oxygenated functionalities seemed produced [65]. In our experiments, both neutral and alkaline solutions containing Fe (VI) have been attempted to oxidize CNTs, but with unsatisfactory results (bad colloid stability). Fortunately, using concentrated H2 SO4 as reaction medium not

526

Z.-y. Zhang, X.-c. Xu / Applied Surface Science 346 (2015) 520–527

only avoided the instability of K2 FeO4 in water but also achieved effective oxidation. Thus, H2 SO4 may be an excellent solvent for harnessing the redox potential of K2 FeO4 . The black K2 FeO4 powder can be quickly dissolved in H2 SO4 with high exothermicity, which reflects the strong solute-solvent interaction. Moreover, the milk-white color of the resulting solution (Figure S1) strongly indicates that Fe (VI) in H2 SO4 does not exist as the classical four species (FeO4 2− , HFeO4 − , H2 FeO4 and H3 FeO4 + ) as in aqueous solutions [41,42]. It has been well known that K2 Cr2 O7 and KMnO4 change their color when dissolved in H2 SO4 and turn into CrO3 and Mn2 O7 , respectively. Analogically, K2 FeO4 might transform into FeO3 that contains three Fe O bonds, following the reaction: K2 FeO4 + conc. H2 SO4 → FeO3 + K2 SO4 + H2 O + heat black

(1)

white

The detailed chemistry might be described as follows: First, K2 FeO4 molecules are solvated by H2 SO4 and form H2 FeO4 (Eq. (2)), and then H2 FeO4 become dehydrated by concentrated H2 SO4 to be FeO3 (Eq. (3)). The FeO3 might be the factual Fe (VI) specie that participated into the oxidation of CNTs (Eq. (4)). K2 FeO4 + conc. H2 SO4 → H2 FeO4 + K2 SO4 H2 FeO4

conc. H2 SO4

−→

CNTs + FeO3

FeO3 + H2 O

conc. H2 SO4

−→

oxidized CNTs + Fe2 O3

(2) (3)

Finally, it is exciting to mention that, as was just reported by Gao’s group [66], K2 FeO4 /H2 SO4 was also a very successful oxidation system for producing graphene oxide via oxidation and exfoliation of graphite. This approach was demonstrated as fast (in 1 h), green (due to the usage of K2 FeO4 and the recycling of H2 SO4 ) and cost-efficient (large-scale production in high yields). Given the superior performance of K2 FeO4 /H2 SO4 in oxidizing both CNTs and graphite, this oxidation system is highly recommended for various carbon materials. Acknowledgements We sincerely thank Prof. Zhi-gao Hu and Dr. Kai Jiang (Department of Electronic Engineering) for providing the Raman tests. Zhao-yang Zhang is grateful to Dr. Hao-miao Xu (Shanghai Jiao Tong University) for performing the IR tests. Dr. Ri Xu is gratefully acknowledged for her valuable comments on manuscript preparation. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2015.04. 026

(4)

The identification of Fe (VI) species in H2 SO4 and their properties will be addressed in the near future. The Fe (VI) chemistry in H2 SO4 , such as the oxidation selectivity and its application in organic syntheses, is an entirely new topic for future investigation. 5. Conclusions and prospects A green oxidant K2 FeO4 was employed to oxidize CNTs for the first time. The oxidation was proved effective under mild condition (60 ◦ C, 3 h) in H2 SO4 medium, producing hydrophilic CNTs with abundant surface COOH groups. Fe (VI) oxidation of CNTs occurs selectively at the original defects, specifically the sp3 -C, without affecting the C C bonds in either hexagons or non-hexagonal rings. During the oxidation process, the sp3 -C are converted into oxygenated groups; when all the accessible sp3 -C have been oxidized, the reaction spontaneously stops (within 3 h); prolonged treatment exerts no additional oxidation effect, let alone consumes the C C bonds. Thus, the annoying drawbacks of structural damage and ␲-electrons loss during the conventional oxidation (e.g., HNO3 /H2 SO4 treatment) can be overcome by this defect-specific oxidation. As a result of the nondestructive oxidation, oxidized CNT products can be obtained in gratifying yields of above 100 wt%, which is very economical in the industrial production. The covalent functionalization of CNTs is characterized by structural damage [6,8], but Fe (VI) oxidation provides an approach to realizing the nondestructive covalent functionalization. The structural integrity can be combined with desired surface chemistry by the well-established methods of chemical derivatization of the surface COOH groups [5,8]. Nowadays, oxidized CNTs are extensively used in the fabrication of functional materials; considering the advantages of nondestructive oxidation, the utilization of the Fe (VI) oxidized CNTs may result in materials with better performance. Interestingly, CNTs are not directly oxidized by K2 FeO4 but react with a unique Fe (VI) specie, possibly FeO3 , formed in situ when K2 FeO4 is dissolved in H2 SO4 . Obviously, the oxidation selectivity and mechanism for Fe (VI) oxidation of CNTs is mainly dominated by the properties of this Fe (VI) specie, but Fe (VI) chemistry in H2 SO4 is totally unknown. Addressing these issues remains an open area of research.

References [1] M.S. Dresselhaus, G. Dresselhaus, A. Jorio, Unusual properties and structure of carbon nanotubes, Annu. Rev. Mater. Res. 34 (2004) 247–278. [2] R.H. Baughman, Carbon nanotubes – the route toward applications, Science 297 (2002) 787–792. [3] C.A. Dyke, J.M. Tour, Covalent functionalization of Single-Walled carbon nanotubes for materials applications, J. Phys. Chem. A 108 (2004) 11151–11159. [4] T.J. Simmons, J. Bult, D.P. Hashim, R.J. Linhardt, P.M. Ajayan, Noncovalent functionalization as an alternative to oxidative acid treatment of single wall carbon nanotubes with applications for polymer composites, ACS Nano 3 (2009) 865–870. [5] S. Banerjee, T. Hemraj-Benny, S.S. Wong, Covalent surface chemistry of SingleWalled carbon nanotubes, Adv. Mater. 17 (2005) 17–29. [6] M. Melchionna, M. Prato, Functionalizing carbon nanotubes: an indispensible step towards applications, ECS J. Solid State Sci. Technol. 2 (2013) M3040–M3045. [7] E. Del Canto, K. Flavin, D. Movia, C. Navio, C. Bittencourt, S. Giordani, Critical investigation of defect site functionalization on Single-Walled carbon nanotubes, Chem. Mater. 23 (2010) 67–74. [8] N. Karousis, N. Tagmatarchis, D. Tasis, Current progress on the chemical modification of carbon nanotubes, Chem. Rev. 110 (2010) 5366–5397. [9] L.T. Cai, J.L. Bahr, Y.X. Yao, J.M. Tour, Ozonation of Single-Walled carbon nanotubes and their assemblies on rigid self-assembled monolayers, Chem. Mater. 14 (2002) 4235–4241. [10] G.S.B. McKee, J.S. Flowers, K.S. Vecchio, Length and the oxidation kinetics of chemical-vapor-deposition-generated multiwalled carbon nanotubes, J. Phys. Chem. C 112 (2008) 10108–10113. [11] I.D. Rosca, F. Watari, M. Uo, T. Akasaka, Oxidation of multiwalled carbon nanotubes by nitric acid, Carbon 43 (2005) 3124–3131. [12] Y. Wang, Z. Iqbal, S. Mitra, Rapidly functionalized, water-dispersed carbon nanotubes at high concentration, J. Am. Chem. Soc. 128 (2006) 95–99. [13] T.J. Aitchison, M. Ginic-Markovic, J.G. Matisons, G.P. Simon, P.M. Fredericks, Purification, cutting, and sidewall functionalization of multiwalled carbon nanotubes using potassium permanganate solutions, J. Phys. Chem. C 111 (2007) 2440–2446. [14] T. Bortolamiol, P. Lukanov, A. Galibert, B. Soula, P. Lonchambon, L. Datas, E. Flahaut, Double-walled carbon nanotubes: quantitative purification assessment, balance between purification and degradation and solution filling as an evidence of opening, Carbon 78 (2014) 79–90. [15] K.J. Ziegler, Z. Gu, H. Peng, E.L. Flor, R.H. Hauge, R.E. Smalley, Controlled oxidative cutting of Single-Walled carbon nanotubes, J. Am. Chem. Soc. 127 (2005) 1541–1547. [16] W. Li, Y. Bai, Y.K. Zhang, M.L. Sun, R.M. Cheng, X.C. Xu, Y.W. Chen, Y.J. Mo, Effect of hydroxyl radical on the structure of multi-walled carbon nanotubes, Synth. Met. 155 (2005) 509–515. [17] D.M. Guldi, H. Taieb, G. Rahman, N. Tagmatarchis, M. Prato, Novel photoactive Single-Walled carbon nanotube-porphyrin polymer wraps: efficient and longlived intracomplex charge separation, Adv. Mater. 17 (2005) 871–875. [18] J. Zhang, H. Zou, Q. Qing, Y. Yang, Q. Li, Z. Liu, X. Guo, Z. Du, Effect of chemical oxidation on the structure of Single-Walled carbon nanotubes, J. Phys. Chem. B 107 (2003) 3712–3718.

Z.-y. Zhang, X.-c. Xu / Applied Surface Science 346 (2015) 520–527 [19] Y. Chiang, W. Lin, Y. Chang, The influence of treatment duration on multi-walled carbon nanotubes functionalized by H2 SO4 /HNO3 oxidation, Appl. Surf. Sci. 257 (2011) 2401–2410. [20] T. Shimada, H. Yanase, K. Morishita, J. Hayashi, T. Chiba, Points of onset of gasification in a multi-walled carbon nanotube having an imperfect structure, Carbon 42 (2004) 1635–1639. [21] D. Bom, R. Andrews, D. Jacques, J. Anthony, B. Chen, M.S. Meier, J.P. Selegue, Thermogravimetric analysis of the oxidation of multiwalled carbon nanotubes: evidence for the role of defect sites in carbon nanotube chemistry, Nano Lett. 2 (2002) 615–619. [22] X. Lu, K.D. Ausman, R.D. Piner, R.S. Ruoff, Scanning electron microscopy study of carbon nanotubes heated at high temperatures in air, J. Appl. Phys. 86 (1999) 186–189. [23] C. Wang, G. Zhou, H. Liu, J. Wu, Y. Qiu, B. Gu, W. Duan, Chemical functionalization of carbon nanotubes by carboxyl groups on Stone-Wales defects: a density functional theory study, J. Phys. Chem. B 110 (2006) 10266–10271. [24] J.C. Charlier, Defects in carbon nanotubes, Acc. Chem. Res. 35 (2002) 1063–1069. [25] F. Ding, Theoretical study of the stability of defects in Single-Walled carbon nanotubes as a function of their distance from the nanotube end, Phys. Rev. B 72 (2005) 245409. [26] H. He, B. Pan, Studies on structural defects in carbon nanotubes, Front. Phys. China 4 (2009) 297–306. [27] S.M. Lee, Y.H. Lee, Y.G. Hwang, J.R. Hahn, H. Kang, Defect-induced oxidation of graphite, Phys. Rev. Lett. 82 (1999) 217–220. [28] M. Grujicic, G. Cao, A.M. Rao, T.M. Tritt, S. Nayak, UV-light enhanced oxidation of carbon nanotubes, Appl. Surf. Sci. 214 (2003) 289–303. [29] H. Hu, B. Zhao, M.E. Itkis, R.C. Haddon, Nitric acid purification of Single-Walled carbon nanotubes, J. Phys. Chem. B 107 (2003) 13838–13842. [30] S. Yeong-Tarng, L. Gin-Lung, W. Hou-Hsi, L. Chung-Chieh, Effects of polarity and pH on the solubility of acid-treated carbon nanotubes in different media, Carbon 45 (2007) 1880–1890. [31] F. Aviles, J.V. Cauich-Rodriguez, L. Moo-Tah, A. May-Pat, R. Vargas-Coronado, Evaluation of mild acid oxidation treatments for MWCNT functionalization, Carbon 47 (2009) 2970–2975. [32] G. Mercier, J. Gleize, J. Ghanbaja, J. Marêché, B. Vigolo, Soft oxidation of Single-Walled carbon nanotube samples, J. Phys. Chem. C 117 (2013) 8522– 8529. [33] A.H. Brozena, J. Moskowitz, B.Y. Shao, S.L. Deng, H.W. Liao, K.J. Gaskell, Y.H. Wang, Outer wall selectively oxidized, water-soluble double-walled carbon nanotubes, J. Am. Chem. Soc. 132 (2010) 3932–3938. [34] S. Osswald, M. Havel, Y. Gogotsi, Monitoring oxidation of multiwalled carbon nanotubes by Raman spectroscopy, J. Raman Spectrosc. 38 (2007) 728–736. [35] Y.C. Xing, L. Li, C.C. Chusuei, R.V. Hull, Sonochemical oxidation of multiwalled carbon nanotubes, Langmuir 21 (2005) 4185–4190. [36] J. Ming, Y.Q. Wu, Y.C. Yu, F.Y. Zhao, Steaming multiwalled carbon nanotubes via acid vapour for controllable nanoengineering and the fabrication of carbon nanoflutes, Chem. Commun. 47 (2011) 5223–5225. [37] N.Y. Zhang, J. Me, V.K. Varadan, Functionalization of carbon nanotubes by potassium permanganate assisted with phase transfer catalyst, Smart Mater. Struct. 11 (2002) 962–965. [38] N. Chopra, M. Majumder, B.J. Hinds, Bifunctional carbon nanotubes by sidewall protection, Adv. Funct. Mater. 15 (2005) 858–864. [39] W. Tseng, C. Tseng, P. Chuang, A. Lo, C. Kuo, A high efficiency surface modification process for multiwalled carbon nanotubes by electron cyclotron resonance plasma, J. Phys. Chem. C 112 (2008) 18431–18436. [40] K. Flavin, I. Kopf, E. Del Canto, C. Navio, C. Bittencourt, S. Giordani, Controlled carboxylic acid introduction: a route to highly purified oxidised Single-Walled carbon nanotubes, J. Mater. Chem. 21 (2011) 17881–17887. [41] Y. Lee, M. Cho, J.Y. Kim, J. Yoon, Chemistry of ferrate (Fe(VI)) in aqueous solution and its applications as a green chemical, J. Ind. Eng. Chem. 10 (2004) 161–171. [42] V.K. Sharma, Potassium ferrate(VI): an environmentally friendly oxidant, Adv. Environ. Res. 6 (2002) 143–156. [43] V.K. Sharma, Ferrate(VI) and ferrate(V) oxidation of organic compounds: kinetics and mechanism, Coord. Chem. Rev. 257 (2013) 495–510.

527

[44] V.K. Sharma, Oxidation of inorganic contaminants by ferrates (VI, V, and IV)kinetics and mechanisms: a review, J. Environ. Manag. 92 (2011) 1051–1073. [45] V. Datsyuk, M. Kalyva, K. Papagelis, J. Parthenios, D. Tasis, A. Siokou, I. Kallitsis, C. Galiotis, Chemical oxidation of multiwalled carbon nanotubes, Carbon 46 (2008) 833–840. [46] Y. Shin, I. Jeon, J. Baek, Stability of multi-walled carbon nanotubes in commonly used acidic media, Carbon 50 (2012) 1465–1476. [47] D.B. Mawhinney, V. Naumenko, A. Kuznetsova, J.T. Yates, J. Liu, R.E. Smalley, Infrared spectral evidence for the etching of carbon nanotubes: ozone oxidation at 298 k, J. Am. Chem. Soc. 122 (2000) 2383–2384. [48] M.S. Dresselhaus, A. Jorio, M. Hofmann, G. Dresselhaus, R. Saito, Perspectives on carbon nanotubes and graphene raman spectroscopy, Nano Lett. 10 (2010) 751–758. [49] C.A. Dyke, J.M. Tour, Solvent-free functionalization of carbon nanotubes, J. Am. Chem. Soc. 125 (2003) 1156–1157. [50] H. Kitamura, M. Sekido, H. Takeuchi, M. Ohno, The method for surface functionalization of Single-Walled carbon nanotubes with fuming nitric acid, Carbon 49 (2011) 3851–3856. [51] S.A. Curran, J.A. Talla, D. Zhang, D.L. Carroll, Defect-induced vibrational response of multi-walled carbon nanotubes using resonance Raman spectroscopy, J. Mater. Res. 20 (2005) 3368–3373. [52] Y. Peng, H. Liu, Effects of oxidation by hydrogen peroxide on the structures of multiwalled carbon nanotubes, Ind. Eng. Chem. Res. 45 (2006) 6483–6488. [53] S. Banerjee, S.S. Wong, Rational sidewall functionalization and purification of Single-Walled carbon nanotubes by solution-phase ozonolysis, J. Phys. Chem. B 106 (2002) 12144–12151. [54] M.H. Liu, Y.L. Yang, T. Zhu, Z.F. Liu, Chemical modification of Single-Walled carbon nanotubes with peroxytrifluoroacetic acid, Carbon 43 (2005) 1470–1478. [55] A. Galano, M. Francisco-Marquez, A. Martinez, Influence of point defects on the free-radical scavenging capability of Single-Walled carbon nanotubes, J. Phys. Chem. C 114 (2010) 8302–8308. [56] R.C. Haddon, L.E. Brus, K. Raghavachari, Electronic structure and bonding in icosahedral C60 , Chem. Phys. Lett. 125 (1986) 459–464. [57] M.L. Toebes, J.M.P. van Heeswijk, J.H. Bitter, A. Jos Van Dillen, K.P. de Jong, The influence of oxidation on the texture and the number of oxygen-containing surface groups of carbon nanofibers, Carbon 42 (2004) 307–315. [58] C. Yang, J.S. Park, K.H. An, S.C. Lim, K. Seo, B. Kim, K.A. Park, S. Han, C.Y. Park, Y.H. Lee, Selective removal of metallic Single-Walled carbon nanotubes with small diameters by using nitric and sulfuric acids, J. Phys. Chem. B 109 (2005) 19242–19248. [59] L. Delaude, P. Laszlo, A novel oxidizing reagent based on potassium ferrate(VI), J. Org. Chem. 61 (1996) 6360–6370. [60] K.S. Kim, Y.K. Chang, S.K. Bae, C.S. Hahn, Selective oxidation of allylic and benzylic alcohols using potassium ferrate under Phase-Transfer catalysis condition, Synthesis 1984 (1984) 866–868. [61] K.S. Kim, Y.H. Song, N.H. Lee, C.S. Hahn, Selective oxidation of alcohols by K2 FeO4 –Al2 O3 –CuSO4 ·5H2 O, Tetrahedron Lett. 27 (1986) 2875–2878. [62] H. Firouzabadi, D. Mohajer, M.E. Moghaddam, Silver ferrate Ag2 FeO4 , an efficient and selective oxidizing agent for the oxidation of benzylic and allylic alcohols to their corresponding carbonyl compounds in aprotic organic solvents, Synth. Commun. 16 (1986) 211–223. [63] M.A.K. Zarchi, M. Taefi, Oxidation of alcohols mediated by a polymer supported potassium ferrate as an effective mild oxidant, J. Appl. Polym. Sci. 119 (2011) 3462–3466. [64] W.Q. Mao, J.M. Wang, Z.H. Xu, Z.X. Niu, J.Q. Zhang, Effects of the oxidation treatment with K2 FeO4 on the physical properties and electrochemical performance of a natural graphite as electrode material for lithium ion batteries, Electrochem. Commun. 8 (2006) 1326–1330. [65] M.B. Sassin, A.N. Mansour, K.A. Pettigrew, D.R. Rolison, J.W. Long, Electroless deposition of conformal nanoscale iron oxide on carbon nanoarchitectures for electrochemical charge storage, ACS Nano 4 (2010) 4505–4514. [66] L. Peng, Z. Xu, Z. Liu, Y. Wei, H. Sun, Z. Li, X. Zhao, C. Gao, An iron-based green approach to 1-h production of single-layer graphene oxide, Nat. Commun. 6 (2015) 5716.