Crucial Role of Oxidation Debris of Carbon Nanotubes in

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Crucial Role of Oxidation Debris of Carbon Nanotubes in Subsequent End-Use Applications of Carbon Nanotubes Yern Seung Kim, Jun Young Oh, Jae Ho Kim, Minho Shin, Yo Chan Jeong, Sae Jin Sung, Jisoo Park, Seung Jae Yang, and Chong Rae Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 2, 2017

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ACS Applied Materials & Interfaces

Crucial Role of Oxidation Debris of Carbon Nanotubes in Subsequent End-Use Applications of Carbon Nanotubes AUTHOR NAMES Yern Seung Kima†, Jun Young Oha,b†, Jae Ho Kima, Min Ho Shina, Yo Chan Jeonga, Sae Jin Sunga, Jisoo Parka, Seung Jae Yangb*, Chong Rae Parka∗ †

These authors contributed equally to this work.

AUTHOR ADDRESS a

Carbon Nanomaterials Design Laboratory, Research Institute of Advanced Materials,

Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Republic of Korea b

Advanced Nanohybrids Lab. Department of Applied Organic Materials Engineering, Inha

University, Incheon 22212, Republic of Korea

KEYWORDS Carbon nanotubes, Acidic carbonaceous compound, pKa distribution, Hansen solubility parameter, Buckypaper

1 ACS Paragon Plus Environment


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ABSTRACT

A facile purification method for oxidized carbon nanotubes (CNTs) is developed to preserve acidic carbon compounds (ACCs) for achieving high-quality dispersion of CNTs. The remaining ACCs, which originated from the surface destruction of CNTs during the oxidation process, are considered to play a crucial role in the dispersion of CNTs in water and various polar protic solvents. To elucidate the concrete role of ACCs, a direct titration method is applied to quantitatively investigate the degree of ionization of both CNTs and ACCs in their aqueous dispersions. While the ACCs with strong carboxylic groups (pKa around 2.9) are easily removed by the neutral or base washing of oxidized CNTs, which is common in the purification process, ACC selective purification using acid washing preserves the ACCs attached to CNTs, thereby effectively stabilizing CNT dispersions in aqueous solutions. Additionally, the Hansen solubility parameters (HSPs) of ACC-preserved and ACCremoved CNTs were determined by the inverse gas chromatography (IGC) method to estimate their miscibility in various solvents. The preserved ACCs significantly influenced the dispersibility of CNTs in polar protic solvents, which may widen the possible application of CNTs. Specifically, the ACC-preserved high-quality CNT dispersion produces high performance CNT buckypaper with densely-packed nanostructures. The Young’s modulus and tensile strength of these buckypapers reaches up to 12.0 GPa and 91.0 MPa, respectively, which exceed those of ACC-removed CNTs in previous reports.


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1. Introduction Carbon nanotubes (CNTs) are widely recognized as potential candidates for novel structural materials due to their extraordinary stiffness and strength with superior electrical conductivity.

1-2

CNTs have been applied in two types of applications: solid-type applications based on high strength buckypaper and yarn for reinforcing fillers, flexible actuators, and filter membranes; and solution-type applications based on CNT ink for thin film transistors and electrochemical sensors.

3-6

The

performance of the CNT-assembled structures is generally determined by alignment, packing density, and interactions between CNTs. Thus, it is crucial to develop tailored CNT nano-networks to make high performance CNT-based materials, thus maximizing their potential applications. Buckypaper is a freestanding CNT-assembled macrostructure fabricated in two-dimensions and can be used for various applications, such as flexible actuators, sensors, filter membranes, and electronic devices 7-14. The general procedure to fabricate buckypaper includes purification and functionalization of CNTs to disperse them into aqueous or organic solvents. 7, 14 The desirable formation of a denselypacked network for making robust buckypaper is highly dependent on the quality of the CNT dispersion.

15-16

The CNTs should be monodispersed in the dispersion without any aggregates, which

might disturb the close-packing state of CNTs and potentially act as a crack initiator, limiting the mechanical performance of the resulting buckypaper.

15, 17-18

To make CNTs disperse well in polar or

hydrophilic solvents, functionalization via acid treatment has been widely used as a standard method. 7, 19-24

These studies concentrated on establishing the dispersion mechanisms of CNTs based on the

functional groups directlty attacted on the surface of CNTs, and tried to find the relationship between the solubility parameters of the functionalized CNTs and their dispersion behaviors. 21 However, recent studies have reported that not only the covalently bonded surface functional groups but also the acidic carbon compounds (ACCs) are generated from the sidewalls of CNTs during the acid treatment and play an important role in dispersing CNTs due to their aromatic structure with numerous acidic functional groups.

19, 25-29

Therefore, in addition to the surface functional groups

directly attached on the surface of CNTs, 21 non-covalently attached ACCs should be also considered for improvement of the dispersion behaviors of CNTs. To the best of our knowledge, the fundamental



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investigation describing the concrete influence of ACCs on the CNTs’ dispersion behaviors, along with optimized methods to utilize them in various ways, have rarely been discussed. The lack of understanding of the genuine roles of ACCs in the CNT dispersions limits the huge potential for engineering CNT assemblies. In this work, we investigate the effect of ACCs on stabilizing a dispersion of CNTs in an aqueous system by studying the ionization behaviors of both CNTs and ACCs in water using an acid-base direct titration method. It has been reported that ACCs are readily removable from the surface of CNTs when dissolved in basic solution. However, we found that the ACCs are inevitably dissipated even by the conventional purification of oxidized CNTs with neutral water. A new method for ACC selective purification is developed as a strategy to fully preserve the ACCs on CNTs, thereby facilitating monodispersed CNTs in aqueous solution. Furthermore, the effects of preserved ACCs on the affinity of CNTs with surrounding media including polar protic solvents were investigated by determination of the Hanssen solubility parameters (HSPs) of CNT samples by employing an inverse gas chromatography (IGC) method. The buckypapers fabricated from ACC-preserved CNT dispersions exhibited approximately a three-fold increase in tensile strength and Young's modulus compared to that of ACC-removed CNT dispersions. The suggested protocol based on systematic analysis along with high-performance buckypaper is believed to elucidate the stabilization of CNT dispersions and offer possibilities for diverse CNT engineering. Moreover, acidic or basic components or fragments which resemble the ACCs of CNTs have been generally decorated on the various materials including nanoclays 30, quantum dots nanoparticles

32

31

and even metal

to adjust their surface natures. This implies both qualitative and quantitative method

to adjust the ACCs developed in this study can be generally and extensively applied to modulate the surface nature of various kinds of materials.

2. Methods 2.1. Preparation of oxidized CNTs


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All chemicals, including nitric acid (70%, HNO3), sulfuric acid (99%, H2SO4), hydrochloric acid (HCl), and sodium hydroxide (NaOH), were purchased from Daejung, Korea. For the preparation of oxidized CNTs, 1 g of multi-walled CNTs (CM250, Hanhwa Chemicals) were dispersed in a mixture of HNO3 (75 mL) and H2SO4 (225 mL) at 60 °C for 3 hours. After the acid treatment, the dispersion was cooled to room temperature and filtered through a 0.2 µm PTFE membrane filter (Adventec).

2.2 ACC selective purification process for ACC-preserved and ACC-removed CNTs For preparation of oxidized MWCNTs, 1 g of MWCNTs (CM250, Hanhwa Chemicals) were put into the mixture of HNO3 (75 mL) and H2SO4 (225 mL) and dispersed by magnetic stirring (300 rpm) at 60 °C for 3 hours. After the acid treatment the dispersion was poured into the 2 L of D.I. water to terminate the oxidation and cooled into room temperature until the oxidized MWCNTs precipitated. The precipitated CNTs were then firstly filtered through the PTFE filter paper (0.2 µm of pore size). After the first purification, the subsequent purification process of the black-colored CNT filter cakes was conducted with a pH-controlled aqueous solution. To avoid potential removal of the ACCs

22, 28-

and prepare ACC-preserved CNTs, the CNT filter cake was refluxed in a 0.1 N HCl solution, followed by filtering and washing with a 0.1 N HCl solution three times. The resulting filter cakes were dried on a heating stage at 60 °C for 3 hours for evaporation of residual HCl and water. After preliminary drying, CNTs were put into a 60 °C vacuum oven overnight for complete removal of HCl and water. This procedure, purifying the mixed acid-oxidized CNTs with a HCl solution, was defined as the acid washing process, and the resultant CNTs were denoted as CNTAW. As a comparison, to prepare the ACC-removed CNTs, acid-oxidized CNTs were washed with deionized water and a 0.01 N NaOH solution following the same procedure as acid washing. The CNTs purified with a 0.01 N NaOH solution were then dispersed in a 0.01 N HCl solution for the acidification of the deprotonated functional groups, followed by filtration and washing with deionized water until the pH of the filtrate was neutral. This acidification and washing process also eliminate the Na and OH ions which can affect the surface properties of CNTs. The reaming volatile HCl was then



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evaporated during the drying process. The ACC-removed CNTs prepared by the neutral wash and base wash were denoted as CNTNW and CNTBW, respectively.

2.3 Preparation of CNT dispersion and buckypaper 40 mg of each CNT sample was dispersed in 40 mL of water by a horn sonication instrument (Sonoplus, Bandelin electronic) for 20 minutes. The CNT dispersion was carefully decanted to the vacuum filtration system with a 0.2 µm PTFE membrane. The filtration was finished within an hour, but the prepared buckypaper remained in the filtration system for the compete removal of water molecules for 5 hours. The dried buckypaper was easily detached from the filter membrane and subjected to additional drying in a vacuum oven at 60 °C overnight.

2.4. Characterization The quantity of ACCs dissolved in an aqueous solution with various pH was measured by a UV-VisNIR absorption spectrometer (Cray 5000, Varian). For the characterization of the functional groups on the CNT samples, X-ray photoelectron spectroscopy (XPS) was conducted by an AXIS-His (Kratos Analytical). To examine thermal degradation behaviors of the CNT samples, each sample was heated to 900 °C at 10 °C/minute after the isothermal process at 60 °C for 1 hour to desorb the physically attached water molecules on the surface of CNTs using a thermogravimetric analysis (TGA) instrument (SDT Q600, TA Instruments). The acidity distribution for investigation of the ionization behaviors of the CNT samples was analyzed by a direct titration method.

33

In this method, 20 mg of CNT samples were agitated in 20

mL of a 0.01 N HCl solution, and 15 mL of the mixture was titrated with a 0.01 N NaOH solution as a titrant at 25 °C using a Titrando 888 (Metrohm). The titration system was completely sealed and purged with N2 gas. To guarantee the equilibrium between functional groups on CNTs and NaOH, additional titrant was dosed only when the pH change rate was lower than 0.02 pH unit/minute. The titration curves were then converted to the proton dissociation isotherms θ(pH), indicating the concentration [meq/g] of proton dissociated sites of the functional groups by applying the modified


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Henderson-Hasselbalch equation developed in our previous reports

34-35

. Then, the pKa distribution

f(pKa), which is an indication of the population of acidic groups in the specific pKa range

33

, was

obtained using the first-order approximation 36 from the proton dissociation isotherms as

 ∂θ (pH)  f (pK a ) ≅  .   ∂pH  pH = pK a

(1)

The detailed procedures for obtaining θ(pH) and f(pKa) are described in our previous report. 37 For determination of the dispersion state of CNTs in water, the zeta potential of the 1/10 times diluted CNT dispersion was measured by a zeta potentiometer (ELSZ-1000, Otsuka Electronics) to define the effective charges and dispersion states of CNTs. Additionally, absorbance of visible light (λ = 550 nm) of the CNT dispersions was measured after centrifugation (Heraeus megafuge 16, Thermo Scientific) of the dispersions at 3,000 rpm for 20 minutes followed by a 1/20 times dilution of the supernatants with water. 38 IGC measurement were conducted by a Clarus 600 gas chromatograph (Perkin Elmer, USA) equipped with a thermal conductivity detector. Helium was used as the carrier gas. The solubility parameters δ of each probe molecule were used to calculate the Hansen solubility distance R. 39

R = α (δ d ,Solute − δ d ,Solvent ) 2 + (δ p , Solute − δ p , Solvent ) 2 + (δ h, Solute − δ h , Solvent ) 2

(2)

Here, d, p, and, h indicate the dispersion, polar, and hydrogen bonding components of HSPs, respectively. Solute and Solvent represent the HSPs of each molecule. α represents the proportionality constant, which reflects the strong influence of the dispersion component that was empirically observed and generally taken as 4. CNT samples were packed into a stainless steel tube to prepare the columns. The columns were stabilized under a carrier gas flow over 24 hours to eliminate any preadsorbed volatile impurities. Measurements were carried out at the temperature range of 30 – 120 °C.



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The flow rate of the carrier gas was 30 ml/minute. The probe molecule was injected into the column using GC syringes (SGE, Australia and Hamilton, USA). More than five injections were performed for each probe molecule retention time measurement. The general experimental procedures and systematic analysis of the IGC results were described in detail in our previous publication. 38 Surface morphology of the buckypapers was observed via scanning electron microcopy (SEM; JSM6700F, JEOL). For the mechanical property measurements of the buckypapers, each buckypaper was cut into rectangular strips 2.5 mm in width and 30 mm in length. The mechanical tensile test was performed using a universal tensile machine (Inston-5543, Instron). At least ten samples were tested with a 10 mm length and a 10%/minute strain rate. The electrical properties of the buckypapers were determined from the resistance measurements of the buckypaper strips utilizing a Keithley 2643B (Keithley Instruments) before the tensile tests. 14

3. Results and discussion 3.1. Preservation and removal of ACCs from acid-oxidized CNTs


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Figure 1. Graphical representations of (a) pristine CNT, (b) ACC-functionalized CNT after oxidation, and (c) ACC-removed CNT by washing.

The generation and removal process of ACCs on the sidewalls of CNTs during oxidation and purification are graphically represented in Figure 1. The ACCs, which have carboxyl and hydroxyl/epoxy groups on the edge of polyaromatic cores, are physically adsorbed on the surface of CNTs after acid treatment, which plays an important role in stabilizing CNTs in solvents. 24 However, a few functional groups are left on the CNTs after the ACCs are desorbed by washing. The possibility of the dissolution of ACCs in basic or even neutral conditions has never been considered to date due to the lack of investigation of the ionization behaviors of ACCs adsorbed on CNTs. Typically, the



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acid treated CNTs are purified by washing and filtration of the CNTs with deionized water, which might cause desorption of the ACCs. 19, 25-27 To examine the effect of the type of washing solution on the preservation of ACCs adsorbed on acid-oxidized CNTs, the solubility of ACCs in the presence of CNTs depending on various pH conditions was investigated. For this purpose, the CNT samples without the removal of ACCs (CNTAW) were dispersed in the pH-controlled aqueous solutions ranging from 1 to 14 by stirring for 24 hours followed by filtration. Indeed, the brown color appeared in the filtrates, implying that the ACCs dissolved not only from the basic (pH 10 – 14)

19, 26

but also

from the neutral (pH 7) or acidic (pH 2 – 4) environments (Figure 2a). Figures 2b and 2c show the absorbance of visible light of the diluted filtrates, indicating the relative concentrations of ACCs in the filtrates. The concentration of ACCs gradually increased with increasing pH until it reached 7, followed by a drastic increase in the basic region (pH 7 – 12). However, when the pH became higher than 12, the concentration slightly decreased because the ionic strength of the solution was high enough to make the attraction force dominant between CNTs and ACCs.

19, 40

To further characterize

the physio-chemical properties of ACCs and the solubilizing behaviors of ACC-preserved and ACCremoved CNTs in detail, washing and filtration to selectively separate ACCs was thoroughly repeated at least 10 times until each filtrate became colorless, as shown in Figure 2d.


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Figure 2. (a) The filtrates of acid-oxidized CNTs dissolved in the pH-controlled aqueous solution with HCl or NaOH, (b) absorbance spectra, and (c) absorbance at λ = 550 nm of the diluted filtrates acquired by UV-vis spectroscopy. (d) Absorbance at λ = 550 nm of the filtrates after repeated washing using neutral deionized water (pH 7) and NaOH aqueous solution (pH 12).



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3.2. Surface identification of ACC-preserved and ACC-removed CNTs

Figure 3. XPS C1s spectra of (a) CNTBW, (b) CNTNW, and (c) CNTAW, and (d) their quantitative analysis. (e) TGA, (f) differential thermogravimetric (DTG) profiles, and (g) Raman spectra of CNTBW, CNTNW, CNTAW, and ACC.


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The preservation of ACCs was confirmed by chemical investigation of CNT samples with XPS, TGA, and the direct titration method. The wide scan XPS (Figure S1) show that the oxygen content decreased from 15.9 atomic % (CNTAW) to 11.5 (CNTNW) and 9.0 atomic % (CNTBW) after the removal process of ACCs by neutral water and NaOH solution, respectively. Additionally, no sodium (1071 eV) nor chlorine (200 eV) XPS 1s peak was detected in the spectra showing the perfect elimination of sodium or chlorine ions which possibly remained in the samples during the sample preparation process. The atomic composition of the carbon atom in the oxidized state of the CNT samples was further investigated by XPS C1s spectra. All of the samples showed the typical peaks exhibited in functionalized carbon materials originating from carbon single (C-C)/double bonds (C=C), oxygen containing functional groups such as epoxy/hydroxyl (C-OH/C-O-C), ketone (C=O), and carboxylic (COO) groups, and π-π interactions (π-π).

24, 41-43

Quantitative analysis shows that the

number of oxidized carbon decreased from 30.7 atomic % (CNTAW) to 28.5 atomic % (CNTNW) and 22.9 atomic % (CNTBW), indicating that the ACCs that contain a lot of oxygen containing groups were removed from CNTs by washing NaOH solution or even by neutral water.

Table 1. Weight percent (W) at 800 °C and the resultant estimation of the weight fraction (wf) of CNTs and ACCs in the CNT samples and ACCs.

W at 800 °C (%)

wfCNT

wfACC

CNTBW

84.2

1

0

CNTNW

78.1

0.89

0.11

CNTAW

72.3

0.78

0.22

ACCs

29.5

0

1

The evidence for modulation of ACCs on the surface of CNTs was also demonstrated in TGA results. Figures 3e - 3f present the thermal degradation behavior of CNT samples and isolated ACCs. Compared to the CNTs, ACCs were less thermally stable due to the lower molecular weights and higher amount of oxidized carbon functional groups. The gradual weight loss of ACCs during the



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temperature ramping process is due to thermal degradation of the functional groups dominated by carboxylic or phenolic groups, which is also shown in the case of ACC-removed CNTBW.

44-46

Meanwhile, the distinct weight drop at 450 °C was possibly attributed to the boiling of the lowmolecular-weight components or breakdown of the aromatics structures on the ACCs. 46-47 Because of the lower thermal stability of ACCs, the thermal degradation rates became faster as a higher amount of ACCs was attached on the surface of the CNTs (CNTAW, CNTNW). Since the ACCs might be uniformly distributed on the outermost wall of CNTs, the peak of ACCs around 450 °C partially or completely disappeared in CNTAW and CNTNW, respectively, and these components of ACCs attached on CNTs were gradually vaporized during the thermal treatments. The TGA results were applied for the estimation of the weight fraction (wf) of ACCs on CNTAW or CNTNW, assuming the remaining weight (W) of the ACC-preserved CNTs is the linear summation of those of CNTs and ACCs as shown in Equation (3.1) in the case of CNTAW. 45

WMCNTAW = WCNT ⋅ wf CNT + WACC ⋅ wf ACC = WCNTBW ⋅ wf CNT + WACC ⋅ (1 − wf CNT )

(3.1)

Here, WA and wfA are the weight loss percentage and weight percent of component A, respectively.

WCNT is supposed to be identical to W C N TBW on the assumption that ACCs were completely removed by repeated washing of oxidized CNTs with 0.01 N NaOH solutions. Therefore, the weight fraction of CNT (WCNT) can be readily obtained as followed by reassignment of Equation (3.1):

wf CNT =

WCNTAW − WACC WCNTBW − WACC

(3.2)

for CNTAW and


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wf CNT =

WCNTNW − WACC WCNTBW − WACC

(3.3)

for CNTNW. W at 800 °C, at which point the thermal degradation rate became stable for the whole sample, was chosen for the calculation of wfCNT from Equations (3.2) and (3.3). The estimation of the weigh fraction of CNTs and ACCs in each sample is summarized in Table 1. The table shows that the weight fraction of ACCs became half the amount when CNTAW is washed and purified by neutral water (CNTNW). Along with the XPS investigations, the TGA results also verified that the typical purification process inevitably removes the ACCs from the surface of CNTs, which can be potentially useful in CNT engineering. The presence and removal of ACCs were also revealed in Raman spectra of the CNT and ACC samples. The upper three Raman spectra (blue, black, and red lines) presented in Figure 3g shows the D- (1343 cm-1) and G-band (1575 cm-1) of MWCNTs which typically represent the degree of disordered and graphitic carbon, respectively.

48

Besides, the D- and G band of Raman spectra of

ACC sample (grey line) were broader and blue shifted to 10 - 15 cm-1 comparing to those of CNT samples, which are observed in those of highly disordered carbon quantum dots or graphene oxides. 49-53

The ratio of the peak intensity of D-band to that of G-band (ID/IG) is normally recognized as the

degree of disorder of carbonaceous materials. Indeed, ID/IG of the ACC sample (1.13) was the highest among the analyzed samples due to the a large fraction of the fictionalized and defected parts of carbon bonds. In the case of CNT samples, CNTAW with the largest amount of ACCs shows the highest ID/IG (1.09). This ratio become lower as the amount of ACC was reduced and become 0.92 for CNTBW. It is widely established that the surface nature and dispersion properties of CNTs are strongly affected by the acidic surface functional groups, including carboxylic, lactonic, and phenolic groups.

25

The

ACCs, which are enriched by these varieties of acidic groups, make CNT more dispersible in numerous aqueous or organic solvents, and thereby widen the possible applications of CNTs.

19, 26

Using the titration method, previous reports only showed the difference between the amounts of



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functional groups on the oxidized CNTs before and after the removal of ACCs in order to demonstrate the effects of ACCs on various properties of CNTs.

26

However, the details of the mechanism

describing how ACCs promote the dispersion of CNTs are rarely established, and therefore, the engineering of ACCs on the surface of CNTs to boost the applications of CNTs was hardly available. Here, we investigated and compared the acidic properties of both CNTs and ACCs, which is one of the keys for the dispersion of carbon nanomaterials, by analyzing a recently developed titration method for CNTBW, CNTNW, and CNTAW. This methodology converts the pKa distribution function (f(pKa)) from direct titration results into useful information, such as the ionization behaviors of the functional groups, or the concentrations of practical functional groups like carboxylic, lactonic, or phenolic groups. 34-35, 53

Figure 4. (a) Proton dissociation isotherms (θ), (b) pKa distribution functions (f) of CNTBW, CNTNW, and CNTAW. (c) pKa distribution functions (f) of ACCs on CNTAW and CNTNW. (d) Proton dissociated f(pKa) of ACCs on CNTAW (1 mg/mL) in the pH-controlled aqueous solutions, and (e) the number and percent degree of ionized functional groups on the ACCs as a function of the initial pH (pHi) of the aqueous solutions.


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Figure 4a presents the proton dissociation isotherms (θ(pH)) of the CNT samples showing the concentration of dissociated protons from the acidic groups at specific pH points during the titrations. Then, pKa distribution functions (f(pKa)) (Figure 4b) of the CNTs were obtained via first-order differentiation of proton dissociation isotherms at pH = pKa using Equation (1). 36 Due to the presence of ACCs on CNTs, the concentration and pKa range of the acidic groups of CNTAW (3.21 meq/g) were much larger and wider than those of CNTNW (1.91 meq/g) and CNTBW (1.06 meq/g), respectively. The peaks for pKa around 5 – 6 and 9 – 9.5 displayed in all CNT samples implied the existence of carboxylic and phenolic groups, which were mostly formed during the mixed acid treatment of CNTs. 36, 54-55

The additional peak at pKa = 2.9 in the distribution function of CNTAW indicated that more

acidic carboxylic groups were attached to ACCs in this sample. 46 For the calculation of f(pKa) for ACCs (fACC) dissociated from CNT (fCNT), we assume that the pKa distribution function for ACC-preserved CNTs (CNTAW or CNTNW) is the linear summation of that of CNT (fCNT) and ACC, and that no additional ACCs are attached on CNTBW. Equation (4) shows the calculation process of fACC.

f CNTAW = f CNT ⋅ wfCNT + f ACC ⋅ wf ACC = f CNTBW ⋅ wf CNT + f ACC ⋅ (1 − wf CNT ) ∴ f ACC =

(4)

f CNTAW − f CNTBW ⋅ wf CNT 1 − wf CNT

Here, fA is the pKa distribution function of component A, and wfCNT and wfACC are the weight fractions of CNT and ACC in CNTAW, respectively, estimated from the TGA results shown in Table 1. The same logic was adopted for the calculation of fACC on CNTNW by substitution of f CNTAW into f CNTNW in Equation (4). Figure 4c presents f(pKa) of ACC for CNTAW (red solid line) and CNTNW (blue dashdotted line). These two curves resembled each other in the displayed pKa range except for the peak at pKa = 2.9 shown in fACC for CNTAW. This difference implied that the typical washing process of CNTs



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by neutral water after acid oxidation completely removes the ACCs with strong carboxylic groups, which play a crucial role in the function of CNTs in various applications. The ionization behaviors of ACCs extracted from acid-oxidized CNTs using pH-controlled aqueous solution (Figure 2) was then analyzed based on the titration method. 37 The concentration of ACCs in the filtrates of this removal process is strongly related to the number of the ionized functional groups on ACCs, which will be estimated by the following procedures. First, the pH of the CNT-dissolved NaOH or HCl solution at the equilibrium state (pHe) was numerically calculated via numerical resolution of Equation (S-1) in the supplementary document and our previous report. 37 After calculating the pHe value for the CNT-dispersed HCl or NaOH solution, the population of the proton dissociated functional group, defined as the proton dissociated f(pKa) of ACCs, was obtained as follows:

 K  , proton dissociated f (pK a,i ) =  + a ,i f (pK a,i )   [H ] + K a ,i  pH=pHe

(5)

where the f(pKa) applied in this equation is that of ACCs on CNTAW drawn as the red solid line in Figure 4c. Subsequently, the concentrations of the ionized functional groups on ACCs can be obtained from the area under the curve of the proton dissociated f(pKa). Figure 4d shows proton dissociated f(pKa) of ACCs in the mixture of CNTAW and the aqueous solution with pH values of 2, 3, 7, 11, and 12. Indeed, the ACCs tended to ionize even in the acidic condition (pH 2 and 3) to a certain extent (4.6% and 13.4% of the degree of ionization, respectively), resulting in the dissociation of ACCs from the surface of CNTs by filtration, as shown in Figure 2. The ionization degree was raised to 19.1% and the majority of stronger carboxylic groups around pKa = 2.9 were ionized as the pH increased to 7, the neutral condition. Indeed, these groups were absent in f(pKa) of ACCs on CNTs purified by neutral water (CNTNW), as shown in Figure 4c (blue dash-dotted line) and also verified in the XPS analysis (Figure 3d). As the pH increased to 12 (basic condition), the acidic groups in the whole pKa window are deprotonated, which resulted in the complete removal of the ACCs from the surface of


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CNTs after the filtration process. It was also confirmed in the XPS results that the population of all kinds of oxygen-containing functional groups diminished after washing with NaOH solution (Figure 3d). The area under the curve of the proton dissociated f(pKa), indicating the number of functionalized groups on ACCs, was plotted as a function of the initial pH (pHi) of the HCl or NaOH solution in Figure 4e. The degree of ionization gradually increased until the pH increased from 1 to 4, followed by the plateau region for pH 4 – 10. After this region, the ionization degree drastically increased until the degree reached 100% at pH 12. It is noteworthy that the shape of this plot is similar to that of Figure 2c, exhibiting the relative concentration of ACCs in the filtrates for the ACC-removal process. It should be noted that the concentration drop after pH 12 shown in Figure 2c is due to the enhanced attraction between CNTs and ACCs as the ionic strength of the solution becomes stronger.40

3.3. Dispersion properties of CNTs in aqueous solution The dispersion properties of CNTs are critically affected by the ionization behaviors of the acidic functional groups either attached directly on their surfaces or indirectly on ACCs.

16, 26

Based on the

titration results analyzed in Figure 4, the ionization tendencies of both CNTs and ACCs when dissolved in the neutral water were estimated and displayed in Figure 5. The proton dissociated f(pKa) of CNTBW (CNTs without ACC) and ACCs of CNTNW and CNTAW when the CNT samples are dissolved in deionized neutral water (pH 7) is obtained using Equations (S-1), (S-2), and (5).



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Figure 5. Proton dissociated f(pKa) of CNTs and ACCs normalized by weight of the CNTs showing the ionization behavior of (a) CNTBW, (b) CNTNW, and (c) CNTAW dispersed in neutral water (1 mg/mL).

Figure 5a shows that 10.2% (0.109 meq/g) of the acidic groups on CNTs were ionized when CNTs without ACCs (CNTBW) were dispersed in water. In this case, none of the acidic groups on ACCs were supposed to be ionized since ACCs were completely removed by the base wash. However, the ionization degree of CNTs decreased to 8.3% (0.091 meq/g) and 5.5% (0.050 meq/g) as ACCs were attached onto the surface of CNTs for CNTNW and CNTAW, respectively. In the dispersion of CNTNW (Figure 5b), the addition of ACCs provided protons to the functional groups on CNTs by the Le Chatelier’s principle, inducing a slight decrease in the ionization of the functional groups of CNTs. In contrast, only half of these functional groups from CNTAW were deprotonated because of the presence of the stronger acidic groups in the pKa region lower than 3, possibly originating from the strong carboxylic groups.

54

In this case, the acidified CNTs relatively acted as a weak base, significantly

lowering the degree of ionization of CNTs. Additionally the strong hydrogen bonding between the


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hydroxyl and carboxylic groups of CNTs and ACCs can further decrease the possibility of acidic groups on the CNTs to disprotonate. 27 However, as less functional groups were ionized from CNTs with the addition of ACCs, much more acidities were activated from the ACCs. A nearly equal quantity (0.092 meq/g) of acidic groups and a quantity that was 11 times higher (0.583 meq/g) were ionized from ACCs of CNTNW and CNTAW, respectively. These ionized ACCs (defined as active ACCs) can function as efficient dispersion stabilizers by attaching themselves on the surface of CNTs by π-π interaction, thereby making CNTs dispersible in aqueous solution. 29 The protonation of CNTs due to the strong acidity of ACCs (Figure 5) might decrease the repulsion between CNTs and ACCs, and add an effective negative charge on CNTs in the dispersions.

Figure 6. Concentration-normalized absorbance of visible light (λ = 550 nm) after centrifugation (red dashed line) and zeta potential (black solid line) of the CNT aqueous dispersions as a function of the number of active ACCs on CNTs.

Indeed, the amplitude of zeta potential (|ξ|), indicating the effective charge on the colloidal particles, 36 increased as more ACCs became active (see black circles in Figure 6). The zeta potential of the CNTBW dispersion (-40.2 mV), where no ACCs were included, originated from the negatively charged functional groups on CNTs, as indicated in Figure 5a. |ξ| of the CNTNW and CNTAW dispersions



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became 1.3 (51.4 mV) and 1.5 (59.9 mV) times higher, respectively, in spite of the decrease in charges on the CNTs themselves (Figures 5b and 5c). This paradoxical increase in |ξ| certainly implied that the ionized ACCs were efficiently attached on the surface of the CNT. Additionally, this increase in |ξ| also implied that the dispersion state and stability of CNTs in water became much better, and the ACCs had an effective role as the dispersion agents of CNTs. 36, 56 Additionally, the quality and the stability of dispersions of the CNT samples were estimated by measuring the absorbance of visible light (λ = 550 nm), showing the relative concentration of stable CNTs after centrifugation at 3,000 rpm.

19, 38

The red circles in Figure 6 show the absorbance data of

the CNT dispersions normalized by their concentrations to show the relative solubility of the CNTs in water. 22 As expected from ionization behaviors and zeta potential results, the solubility of CNTs from the acid wash process (CNTAW) was more significantly increased since the ACCs generated additional negative charges on the surface of CNTs to make them more stable in the aqueous dispersion even in the high gravity environment as compared to CNTs washed by neutral water (CNTNW) or base (CNTBW). These observations imply that the newly suggested purification method of oxidized CNTs with HCl solution certainly preserves the strongly carboxylated ACCs on the surface of CNTs, which performed a significant role in the stabilization of the CNT dispersion.

Table 2. Hansen solubility parameter (HSP) components of CNTs determined by IGC (unit: MPa1/2) CNT

δD

δP

δH

Pristine CNTs

21.3 ± 0.1

6.8 ± 1.0

3.2 ± 0.6

CNTBW

20.6 ± 0.2

9.8 ± 1.6

7.7 ± 0.9

CNTAW

17.8 ± 0.3

12.2 ± 1.9

16.1 ± 1.1


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Figure 7. Plot of the solubility parameter distance of (a) ACC-removed CNTs (CNTBW) and (b) ACCpreserved CNTs (CNTAW) versus the absorbance at a wavelength of 550 nm for the CNT solutions.

The effects of ACCs on the dispersion behaviors of water and the other polar protic solvents were further investigated by determination of HSPs adopting an IGC technique. This information on the surface characteristics can provide guidelines for selecting a proper solvent to disperse CNTs for their various applications. The experimental and analytical details for the investigation of HSPs using the IGC method were demonstrated in our previous report. 38 In this work, the HSPs of the pristine CNTs and oxidized CNTs with (CNTBW) and without (CNTAW) the removal of ACCs were determined using IGC and summarized in Table 2. As shown in the table, the polar (δP) and hydrogen (δH) bonding components highly increased after oxidation due to the introduction of functional groups on CNTs. However, the dispersion solubility parameter (δD) of the



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Page 24 of 46

ACC-removed CNTs by base wash (CNTBW) was the same as that of the pristine CNTs. Meanwhile, the ACC-preserved CNTs by acid wash (CNTAW) had a more significantly decreased dispersion (δD) and increased polar (δP) and hydrogen (δH) bonding components of HSPs due to the presence of ACCs on the surface of CNTs, which enable them to more easily disperse in common polar solvents. The validity of these assumptions on HSPs were further verified by plotting the solubility parameter distance (R) (calculated from Equation (2)) against the absorbance at a wavelength of 550 nm of each CNT dispersion, as shown in Figure 7. While the pristine CNTs had low R values in non-polar solvents, such as o-dichlorobenzene as shown in our previous publication,38 the oxidized CNTAW and CNTBW showed the highest absorbance in N-methyl-2-pyrrolidone and N, N-dimethyl formamide. Interestingly, the absorbance of CNTAW, with the highest polar and hydrogen bonding components, increased in the polar protic solvents such as isopropyl alcohol and ethanol. This result demonstrated that the preserved ACCs in CNTAW enabled the CNTs to disperse even in the polar protic solvents.38

3.4. Morphologies and properties of aligned buckypaper using the ACC-preserved CNT dispersion The enhanced dispersibility of CNTs in polar protic solvent with the addition of ACCs, especially for CNTAW, gives the potential to enlarge the application fields of CNTs with environment-friendly processes. As one of the possible applications, we demonstrate the assembly of highly aligned buckypaper with high strength using the ACC-preserved CNT dispersion. Recently, we reported that filtering a concentrated CNT suspension facilitated the self-alignment behaviors of CNTs.

587

The

dispersion state of CNTs in a suspension was a critical prerequisite to achieve an isotropic to nematic transition, thereby making highly aligned CNT buckypaper. The suspension containing ACCpreserved CNT (CNTAW) and ACC-removed CNT (CNTBW and CNTNW) dispersions were filtered and dried to prepare the two-dimensional assembly of CNTs. Figure 8 shows the physical properties of the resultant buckypapers fabricated from CNTBW, CNTNW, and CNTAW. As shown in the SEM images, the CNTs were assembled into randomly oriented and entangled nanostructures in CNTBW and CNTNW buckypapers (Figures 8a and 8b, respectively), which have been commonly observed in


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previously reported buckypapers.

57

Meanwhile, the CNT nanostructure with uniform orientation and

dense packing was seen when CNTAW was used (Figure 8c). The highly aligned nanostructure of CNTAW buckypaper was attributed to the highly dispersed state of the CNTs, as we confirmed in the previous report.

57

The repulsive forces of CNTs based on highly carboxylated ACCs surpassed the

attractive forces to form a thermodynamically stable nematic phase in the concentrated suspension described in Onsager’s theory. Specifically, during the filtration process for preparation of buckypaper, the local concentration of CNTs near the filter rapidly increase. When the dispersibility of CNTs in the dispersion is high enough, the phase of lyotropic rigid rod-like CNTs change from isotropic to nematic when the local concentration exceed the critical concentration. At this point, the individual CNTs in the dispersion tend to assembled to form densely packed structure by entropydriven alignment. 57 The buckypaper (CNTAW) with a highly aligned nanostructure had a silvery smooth surface, unlike the buckypapers from CNTBW and CNTNW (Figures 8d - 8f). The compact nanostructure further influenced the pore characteristics of the buckypapers. The nitrogen sorption isotherms (Figures 8g 8h) were measured to quantitatively analyze the pore characteristics, which might reflect the density and compactness of the resulting buckypapers. In the SEM results, the pore structure formed in the buckypaper of CNTBW and CNTNW was hardly found in that of CNTAW. As the packing density of the buckypaper increased, the overall porosity was significantly reduced. Interestingly, while CNTBW and CNTNW showed typical type IV isotherms, CNTAW had a clear hysteresis, which might be attributed to reorganized small-sized mesopores.

57

Indeed, the pore size distribution presented in the

Supplementary Materials (Figure S2) certainly shows that the pore volume of CNTBW buckypaper (0.71 cm3/g) was much larger with bigger pore size than CNTNW (0.23 cm3/g) and CNTAW buckypaper (0.10 cm3/g). Accordingly, the apparent CNT density of CNTAW buckypaper (1.19 g/cm3) also became much higher than CNTBW (1.00 g/cm3) and CNTNW (0.74 g/cm3) buckypapers (Table 3).



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Figure 8. (a, b, c) SEM images, (d, e, f) digital photos, and (g, h, i) N2 adsorption isotherms (at 77K) of the buckypapers fabricated from CNTBW, CNTNW, and CNTAW, respectively.

The compact nanostructure of CNTAW buckypaper leads to superior mechanical properties. Notably, both the tensile strength and Young’s modulus of the CNTAW buckypaper were much higher than those of ACC-removed CNTNW and CNTBW exhibiting similar results to previous works (Figure 9, Table 3). The increments in the mechanical properties were certainly attributed to the densely-packed structures, which potentially maximized the contact area between CNTs to enhance the π-π interactions and hydrogen bonding of functional groups. 22 In addition, the pore structure (Figure S2) also attribute to the mechanical properties of the buckypapers as the pore in the buckypaper mainly role as the stress concentration center during the tensile test. Therefore, the buckypaper with smaller size and lower volume fraction of pores (CNTAW) has much lower possibility to initiate and propagate


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the cracks at the equivalent stress resulting in the improved mechanical strength than opposite cases (CNTBW). The multiple cracks formed in the CNTBW buckypaper during the tensile test make the cross-section rough and bumpy (Figure S3a, and c) after the fracture, while the fracture cross-section. Moreover, the electrical conductivity of the CNTAW buckypaper was also improved compared to the ACC-removed CNT (CNTNW, CNTBW) buckypapers, possibly owing to this densely-packed structure despite the presence of the electrically insulating ACCs (Table 3). It is noticeable that although the oxidation condition for the surface functionalization of CNTs is the same, the physical and mechanical properties of the resultant materials significantly depended on the presence of ACCs formed during the acid treatment. The mechanical properties exceed or are comparable to the previously reported benchmarks

7, 10-14

due to the preparation of highly water-

dispersible CNTs in the presence of the ACCs possessing strong carboxylic groups. Our findings imply that the suggested ACC-selective purification method is a practically useful technique for CNT applications, which is achieved by improving their dispersion states with possible additional postprocesses, such as heterogeneous hybridization, ACC-carbonization, and cross-linking of CNTs. Moreover, the titration method successfully estimated the origin of high dispersibility of ACCpreserved CNTs, suggesting this methodology can be extensively applied to investigate the surface properties of not only carbonaceous nanomaterials but also the other various sorts of materials for their proper utilizations.

Figure 9. Representative stress–strain curves collected from the buckypapers of CNTBW, CNTNW, and



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Page 28 of 46

CNTAW.

Table 3. Comparison between the properties of freestanding CNT buckypapers in this work and those reported previously. Apparent Young's

Tensile

Electrical

modulus

strength

conductivity

(GPa)

(MPa)

(S/cm)

1.19

12.0

91.1

57

This

± 0.05

± 0.74

± 10.9

±2

work

1.00

8.70

48.5

38

This

± 0.06

± 0.83

± 6.3

±1

work

0.74

4.06

27.3

37

This

± 0.04

± 0.20

± 2.6

±1

work

5.0

74 120

7

± 0.2

±2

0.98

1.54

7.80 -

10

± 0.09

± 0.39

± 2.06

-

12 ~ 13

35 - 80

294

11

0.64

2.5

18

-

12

-

4

35

-

14

density CNT functionalization

Solvent

Ref.

of CNTs (g/cm3) HNO3/H2SO4 Water (Acid wash, CNTAW) HNO3/H2SO4 Water (Neutral wash, CNTNW) HNO3/H2SO4 Water (Base wash, CNTBW)

HNO3

Aniline

4-Ethoxybenzoic acid

Water

-

NMP

Water SDS

None /water Triton None

X-100 /water


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4. Conclusion Numerous studies have focused on improvement of the dispersion state of CNTs to synthesize wellorganized buckypapers. The side-walls of CNTs have been directly functionalized with acidic media (HNO3, SOCl2), organic reagents (esters, amines), or non-covalent functionalization utilizing surfactants (Triton X-100, sodium dodecyl sulfate [SDS]). Our approach for highly dispersible CNTs in solvents takes advantage of both functionalization principles. In addition to the covalently attached acidic groups on the surface of CNTs, ACCs generated on the sidewalls of the CNTs during the oxidation process played significant roles on the stabilization of CNTs in polar protic solvents and buckypaper formation. For this purpose, ACCs were maximally preserved on the CNTs by an ACCselective purification method for oxidized CNTs. Additionally, the ionization behaviors of ACCs in the CNT dispersions were systematically analyzed by applying a titration method. Furthermore, the HSPs and degree of dispersion of ACC-preserved and ACC-removed CNTs were obtained by IGC and UV-vis spectroscopy. The ACCs preserved by acid wash enhanced the dispersibility of CNTs in polar protic solvents. The results showed that the preservation of strongly carboxylated ACCs readily removable by conventional neutral wash of the oxidized CNTs was the key for the dispersion of CNTs in water, and hence the formation of highly packed buckypapers.

ASSOCIATED CONTENT The detailed description on the calculation of equilibrium pH (pHe) of the CNTs dispersed in aqueous solution and supporting data (XPS, pore size distributions SEM images) are presented in Supplementary Information.

AUTHOR INFORMATION

Corresponding Author 29 ACS Paragon Plus Environment


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Page 30 of 46

*S.J. Yang, [email protected] *C.R.Park, [email protected]

Present Addresses a

Carbon Nanomaterials Design Laboratory, Research Institute of Advanced Materials,

Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Republic of Korea b

Advanced Nanohybrids Lab. Department of Applied Organic Materials Engineering, Inha

University, Incheon 22212, Republic of Korea

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

FUNDING SOURCES This work was supported by the National Research Foundation of Korea(NRF) grant funded by

the

Korea

government(MSIP)

(No.

NRF-2016R1CB2010772

and

NRF-

2016R1EA2A0193982)

ABBREVIATIONS CNT, carbon nanotube; ACC, acidic carbonaceous compound; IGC, inverse gas chromatography; HSP, Hanssen solubility parameter; XPS, X-ray photoelectron spectroscopy (XPS); TGA, thermogravimetric analysis


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Supporting Information Available : The details on calculation of pHe of the aqueous CNT dispersion and supporting data (XPS, pore size distributions SEM images)

TABLE OF CONTENTS

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Figure 1. Graphical representations of (a) pristine CNT, (b) ACC-functionalized CNT after oxidation, and (c) ACC-removed CNT by washing. 110x166mm (96 x 96 DPI)

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Figure 2. (a) The filtrates of acid-oxidized CNTs dissolved in the pH-controlled aqueous solution with HCl or NaOH, (b) absorbance spectra, and (c) absorbance at λ = 550 nm of the diluted filtrates acquired by UV-vis spectroscopy. (d) Absorbance at λ = 550 nm of the filtrates after repeated washing using neutral deionized water (pH 7) and NaOH aqueous solution (pH 12).

330x219mm (85 x 78 DPI)


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Figure 3. XPS C1s spectra of (a) CNTBW, (b) CNTNW, and (c) CNTAW, and (d) their quantitative analysis. (e) TGA, (f) differential thermogravimetric (DTG) profiles, and (g) Raman spectra of CNTBW, CNTNW, CNTAW, and ACC. 309x147mm (97 x 101 DPI)



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Figure 4. (a) Proton dissociation isotherms (θ), (b) pKa distribution functions (f) of CNTBW, CNTNW, and CNTAW. (c) pKa distribution functions (f) of ACCs on CNTAW and CNTNW. (d) Proton dissociated f(pKa) of ACCs on CNTAW (1 mg/mL) in the pH-controlled aqueous solutions, and (e) the number and percent degree of ionized functional groups on the ACCs as a function of the initial pH (pHi) of the aqueous solutions. 466x243mm (150 x 150 DPI)


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Figure 5. Proton dissociated f(pKa) of CNTs and ACCs normalized by weight of the CNTs showing the ionization behavior of (a) CNTBW, (b) CNTNW, and (c) CNTAW dispersed in neutral water (1 mg/mL). 247x148mm (150 x 150 DPI)



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Figure 6. (a) Concentration-normalized absorbance of visible light (λ = 550 nm) after centrifugation (red dashed line) and zeta potential (black solid line) of the CNT aqueous dispersions as a function of the number of active ACCs on CNTs. Plot of the solubility parameter distance of (b) CNTBW and (c) CNTAW versus the absorbance at a wavelength of 550 nm for the CNT solutions. 736x217mm (150 x 150 DPI)


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Figure 7. (a, b, c) SEM images, (d, e, f) digital photos, and (g, h, i) N2 adsorption isotherms (at 77K) of the buckypapers fabricated from CNTBW, CNTNW, and CNTAW, respectively. 266x208mm (150 x 150 DPI)



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Figure 8. (a) Representative stress–strain curves collected from the buckypapers of CNTBW, CNTNW, and CNTAW. (b) Comparison between the mechanical properties of CNTAW and the functionalized CNT buckypapers reported in the previous works. 235x395mm (150 x 150 DPI)


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