Common Wet Chemical Agents for Purifying Multiwalled Carbon Nanotubes

Purification and functionalization of multiwalled carbon nanotubes (MWCNTs) are challenging but vital for their effective applications in various fields including water purification technologies, optoelectronics, biosensors, fuel cells, and electrode arrays. The currently available purification techniques, often complicated and time consuming, yielded shortened and curled MWCNTs that are not suitable for applications in certain fields such as membrane technologies, hybrid catalysis, optoelectronics, and sensor developments. Here we described the H 2 O 2 synergy on the actions of HCl and KOH in purifying and functionalizing pristine MWCNTs. The method (HCl/H 2 O 2 ) showed 100% purification yield as compared to HCl and KOH/H 2 O 2 with purification yields 93.46 and 3.92%, respectively. We probed the findings using transmission electron microscope, energy dispersive X-ray spectroscope, attenuated total reflectance infrared spectroscope, Raman spectroscope, thermal gravimetric analysis, and X-ray powder diffraction. The study is a new avenue for simple, rapid, low cost, and scalable purification of pristine MWCNTs for application in versatile fields.

CNTs are often received or synthesized with other generic impurities such as nonnanotube or amorphous carbons, ash, and metal catalysts with extreme hydrophobicity [19].The existing CNT synthesis methods added the impurities such as metal catalysts to increase the yield and reduce cost [20].
The level of these unwanted matters depends on the specific method used for CNT synthesis.Whatever might be the method of choice, impurities often hinder CNTs performances and confuse the understanding of their original functionalities, limiting their applications in many important fields.For instance, this triggered CNT aggregation in various polymers [21], which destabilizes polymer's mechanical strength and electrical conductivity [21].
In order to purify pristine CNTs, three classes of CNT purification methods such as chemical, physical, and a combination of both were developed [19].Chemical purification method was effective because of its selectivity, sensitivity, faster rate kinetics of the oxidation of carbonaceous impurities, and metal catalysts dissolution from the pristine CNTs [22].In addition, chemical agents are widely available and cost-effective and need simple laboratory settings.In contrast, physical method was involved in removing graphitic sheets and carbon nanospheres from CNT [19].The method was not effective to remove carbon impurities, often complicated and time consuming [19].Therefore, based on pristine CNT impurities, one can select chemical, physical, or a combination of both in order to get pure CNT with desired properties.
HCl, H 2 O 2 , and KOH are easily available simple wet chemicals that are commonly found in most of the ordinary laboratories.Here we treated MWCNTs with HCl [23,24] as a reference.But HCl is often incapable of completing removal of nonnanotube impurities [25].Pristine few walled carbon nanotubes (FWCNTs) were oxidized and purified by H 2 O 2 [26], but the method thus far is not extensively studied for multiwalled carbon nanotubes (MWCNTs) purification except few [22,27].Although single-walled carbon nanotubes (SWCNTs) were purified by a mixture of HCl and H 2 O 2 [25,[28][29][30][31], no study has yet been adopted for MWCNT purification by using that mixture.Therefore, we studied here the synergistic effects of HCl/H Transmission electron microscope (TEM) was used to study the morphological changes of MWCNTs.Energy dispersive X-ray spectroscope (EDX) was used for the analysis of elemental composition and state of impurities.Attenuated total reflectance infrared (ATR-IR) spectroscope was performed to estimate the degree and type of functionalization.Raman spectroscope was conducted to measure the defects and character of graphite bands, and thermal gravimetric analysis (TGA) was used to record the mass profile of pristine and treated MWCNTs.Finally, X-ray powder diffraction (XRD) was used to define crystallinity, in-plane regularity, and lattice profiles.

Experimental Section
2.1.Materials.Pristine MWCNTs of 12 ± 5 and 4 nm in outer and inner diameters and >1 m in length were bought from Bayer MaterialScience AG (Germany).The tubes were prepared by catalytic chemical vapor deposition (CCVD) and contained >95% carbon by weight and were used as received.Hydrochloric acid and hydrogen peroxide were purchased from Merck Sdn Bhd.(Malaysia).Potassium hydroxide, ethanol, and acetone were purchased from Sigma-Aldrich Sdn Bhd.(Malaysia).The purity of all reagents was ≥99% except hydrochloric acid (37%), hydrogen peroxide (30%), and ethanol (70%) in water.

Instrumentation
. TEM (Hitachi-HT7700, Japan) was used for the morphological characterizations of the MWC-NTs.It was performed at 120 kV.An EDX coupled with a FE-SEM (QUANTA FEG 450, FEI, USA) was used for elemental analysis.An X-Max Silicon Drift detector (Oxford, UK) of 80 mm 2 was used to identify the elements and energy and relative intensity of emitted X-rays were analyzed at 10 Kev.ATR-IR spectra were recorded on a KBr using an IR spectrometer (IFS 66 v/S, Bruker, Germany).Raman spectra were acquired for 10 min at a laser power of 100 on Ar + laser (514 nm) focused (50X objective) on a spot size of about 1.5-2.0m (Renishaw inVia, UK).TGA (TGA/SDTA 851, Mettler Toledo, USA) was performed under air flow (50 mL) between 25 and 1000 ∘ C at 10 ∘ C/min.XRD diffracted patterns were collected at Ni filtered Cu K radiation (40 kV, 40 mA,  = 1.5401Å) (XRD/D8, Bruker, Germany).

Wet Chemical Treatments of MWCNTs.
Three wet chemical treatments were performed to purify and oxidize the asobtained pristine MWCNTs.

Removal of the Residual Impurities. All treated MWCNTs
were extracted from the residual acids, bases, metallic byproducts, and carbonaceous impurities by repeated cycle of dilution and centrifugation (dissolved in 1 L of deionized water and centrifuged (Eppendorf-5430R; Germany) at 7000 rpm for 30 min).The supernatant was carefully collected when the MWCNTs were precipitated at the bottom of the polyethylene centrifuge tube.The procedure was repeated 5-6 times until the resistivity of the supernatant was greater than 0.5 MΩ⋅cm and pH was ∼7.0.The treated MWCNTs were then rinsed with ethanol (70 wt%) and dried overnight in a vacuum oven at 100 ∘ C.

Predicted Chemical Reactions of HCl, H 2 O 2 , and KOH
with MWCNTs.To get pure MWCNTs, agents such as HCl, HCl/H 2 O 2 , and KOH/H 2 O 2 were found to be promising (Figure 1).The method (HCl/H 2 O 2 ) can purify MWCNTs through different routes.The metals that are usually present in pristine CNTs act as catalysts to produce hydroxyl radical (OH o ) through Fenton's chemistry [33] (Figure 1(a)), which is stronger oxidizing agent than H 2 O 2 :  The radical (OH o ) is then reacted with amorphous carbon impurities of pristine CNTs [22] and converted them into CO 2 [25] (Figure 1(b)): The oxidized metals and other impurities are then dissolved into HCl (Figure 1(c)), which are subsequently removed through filtering and washings.However, a mixture of KOH and H 2 O 2 was unable to complete amorphous carbon oxidation and removal of metal impurities from MWCNT.This is because of the chemical reactions between KOH and H 2 O 2 (Figures 1(d), 1(e), and 1(f)).KOH decreases the availability of H 2 O 2 in the system , so there is the least chance to produce free radicals and other etching agents.The ultimate products of the reaction were KOH and O 2 .Oxygen was evaporated while KOH may have some chemical interactions with amorphous carbons, which might be negligible to remove MWCNT core impurities.

TEM Analysis.
TEM microscope was used to closely examine the contents of amorphous carbon and location of metal catalysts trapped into the tubular interstitial spaces of pristine and treated MWCNTs (Figure 2).Pristine MWCNTs reflected clumped, cloudy, and amorphous carbon containing MWCNTs (Figure 2(a)).The impure carbonaceous particles were found to be wrapped around the nanotube structures, and metal catalysts were trapped into the MWCNTs.Although the overall amorphous carbons and metals were removed from the nanotube surface after HCl treatment, some MWCNTs were thick suggesting small percentages of nonnanotube carbonaceous agents and impurities may present on MWCNT surfaces (Figure 2(b)).The HCl/H 2 O 2 treatment produced much cleaner, fresh, and complete amorphous carbon and metals-free MWCNTs and the nanotubes appeared in thin and loosely connected bundles (Figure 2(c)).Although the oxidizing strength of H 2 O 2 is high (Pka 11.6), it did not produce vigorous CNT fragmentations upon the purification process in presence of HCl.In the KOH/H 2 O 2 treatment, some of the amorphous carbons from MWCNT surfaces were removed.However, the effects were local since the presence of some nonnanotube carbon impurities was globally obvious (Figure 2(d)) and the nanotubes appeared more flattened and thick than those of HCl (Figure 2(b)) and HCl/H 2 O 2 (Figure 2(c)) treated MWCNTs.

EDX Analysis.
EDX is a significant characterization tool for measuring the extent of CNT oxidation and elemental composition [22].EDX findings of the pristine and treated MWCNTs are shown in Figure 3 and representative analysis is listed in Table 1.Purification yield of pristine and treated MWCNTs was calculated based on the following: where  0 is the metal content of the pristine MWCNT (%) and   is the metal content of purified MWCNT (%).
As we observed in Table 1, by far, the largest element in the as-received pristine MWCNT is carbon (either graphitic  or amorphous), with some extent of oxygen (Figure 3(a)).However, pristine MWCNTs were highly contaminated with metal impurities such as Co, Mn, Al, and Mg (Figure 3  substituted aromatic structures [36].Some weak peaks that appeared in 2500-3500 cm −1 region in pristine MWCNTs (shown by asterisks) (Figure 4 3.5.Raman Spectroscopy.Typically, MWCNT represents two significant high frequency bands called D-and G-bands at 1330 and 1585 cm −1 for CNT structural defects and graphite in-plane vibration, respectively [39,40].In this study, D-and G-bands appeared at ∼1349 and ∼1588 cm −1 both in treated and pristine MWCNTs, respectively (Figure 5).
The intensity of the D-band which is induced by nonzero center phonon mode usually depends on the presence of disordered carbon atomic networks [31,41].However, Figure 5 shows the D-band intensities were practically constant in both pristine and treated MWCNTs.This indicates that MWCNTs were purified with less defects density.This might be due to the milder reaction conditions among graphitic   The etching properties of OH o , which was generated by Fenton's chemistry [33], may have direct affinity to oxidize amorphous carbons due to the presence of many active sites on it [32] (Figure 1(b)) rather than oxidizing graphitic layer's carbon atoms.On the other hand, KOH/H 2 O 2 was unable to directly react with graphitic skeleton, since most of the amorphous carbons were wrapped around the pristine MWCNTs (Figures 1(d  impurities and metal catalysts and generated well graphitic MWCNT structure [41] compared to HCl and KOH/H 2 O 2 while maintaining intact MWCNT integrity (Figure 5).
Finally, the purity states of the pristine and treated MWCNTs were compared from the intensity ratio of the G ( G ) and D-bands ( D ) [32].The highest ratio (0.91) of  G / D was found for HCl/H 2 O 2 treated MWCNTs, suggesting the better efficiency of HCl/H 2 O 2 in removing amorphous and carbonaceous materials from MWCNTs [26].The  G / D ratios for HCl (0.81) and KOH/H 2 O 2 (0.73) treated MWCNTs were less effective in complete removal of nonnanotube carbon impurities and metal catalysts from pristine MWCNT surfaces (Figure 5).

TGA Analysis.
TGA was performed to measure the amorphous carbons, oxidation defects, and overall quality of purified MWCNTs.TGA of pristine and treated MWCNTs (down) with their derivative spectra (up) are presented in Figure 6.By oxidation temperature, herein, we mean the temperature where MWCNTs lose their weight and thus show the highest derivative weight curve.This can define the stability of MWCNTs at a given temperature.At first pristine and KOH/H 2 O 2 treated MWCNTs showed lowest decomposition temperatures at around 100 ∘ C and lost their weights of about 1 and 70%, respectively, which correspond to the pyrolytic evolution of hydroxyl and/or water [32].Typically, amorphous carbons oxidized at around 500 ∘ C [42] due to their lower activation energy and the presence of many heat sensitive active sites [32].TGA of pristine and KOH/H 2 O 2 treated MWCNTs showed highest decomposition temperatures at 550 ∘ C and loss of their weights of about 5 and 75%, respectively (Figure 6).However, pure and well graphitic carbon skeletons are commonly reacted at relatively

XRD Analysis.
The two characteristic XRD peaks of MWCNTs for two important phases such as 002 and 100 in the range of 2 = (10-60 ∘ ) (Figure 7) were followed in this study [44].The peak characterizing the interlayer spacing (002) of CNT tubular walls was observed at 2 = 26.08 and 26.03 ∘ (mean) for pristine (Figure 7

Conclusions
The purification of MWCNTs using three common wet chemical agents (HCl, HCl/H s k e le t o n CNT Less pure CNT Here, R: alkyl or aryl and so forth.
(a)) disappeared following chemical treatments (Figures 4(b), 4(c), and 4(d)).It clarifies the presence of some minor functional groups of the pristine MWCNTs anchored by amorphous carbons and other carbonaceous fragments which were successfully removed by wet chemicals processing.

Figure 6 :
Figure 6: TGA curves of pristine and treated MWCNTs (down) and their derivative spectra (up).
2 O 2 mixture for improving the carbon yield and getting purified well graphitic layer of MWCNTs.A basic treatment involving NH 4 OH/H 2 O 2 was effective in purifying MWCNTs [32].We developed and applied another basic wet oxidizing technique involving a mixture of KOH/H 2 O 2 in purifying MWCNTs.The methods (HCl, HCl/H 2 O 2 , and KOH/H 2 O 2 ) were effective to get oxidized MWCNTs, but high purification yield was observed for HCl/H 2 O 2 compared with HCl and KOH/H 2 O 2 .We focused on MWCNTs because of their low preparation costs and availability in large quantities.

Table 1 :
Elemental analysis of before and after MWCNT treatments in this study.
2 O 2 , and KOH/H 2 O 2 ) is presented.Journal of NanomaterialsThe HCl/H 2 O 2 mixture produced more cleanly and well orderly MWCNT skeleton compared with that of ordinary HCl.Although considerable removal of amorphous carbon is possible with KOH/H 2 O 2 treatment, complete MWCNT purification is out of place.Thus the MWCNT purities depend on the nature of wet chemical agents and impurities present in the overall MWCNT structure.