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Mesoscopic cage-like structured single-wall carbon nanotube cryogels
Journal Pre-proof Mesoscopic cage-like structured single wall carbon nanotube cryogels Yuito Kamijyou, Radovan Kukobat, Dragana Stevic, Koki Urita, Nurul Chotimah, Yoshiyuki Hattori, Ryusuke Futamura, Fernando Vallejos-Burgos, Isamu Moriguchi, Shigenori Utsumi, Toshio Sakai, Katsumi Kaneko PII:
S1387-1811(19)30671-7
DOI:
https://doi.org/10.1016/j.micromeso.2019.109814
Reference:
MICMAT 109814
To appear in:
Microporous and Mesoporous Materials
Received Date: 30 April 2019 Revised Date:
4 October 2019
Accepted Date: 18 October 2019
Please cite this article as: Y. Kamijyou, R. Kukobat, D. Stevic, K. Urita, N. Chotimah, Y. Hattori, R. Futamura, F. Vallejos-Burgos, I. Moriguchi, S. Utsumi, T. Sakai, K. Kaneko, Mesoscopic cage-like structured single wall carbon nanotube cryogels, Microporous and Mesoporous Materials (2019), doi: https://doi.org/10.1016/j.micromeso.2019.109814. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Mesoscopic cage-like structured single wall carbon nanotube cryogels Yuito Kamijyoua,b, Radovan Kukobatb, Dragana Stevicb, Koki Uritac, Nurul Chotimah b,†, Yoshiyuki Hattorid, Ryusuke Futamurae, Fernando Vallejos-Burgos
b, ††
, Isamu
Moriguchic, Shigenori Utsumif, Toshio Sakaia, Katsumi Kanekob, *
a
Department of Materials Chemistry, Faculty of Engineering, Shinshu University,
4-17-1 Wakasato, Nagano 380-8553, Japan b
Research Initiative for Supra-Materials, Shinshu University, 4-17-1 Wakasato, Nagano
380-8553, Japan c
Graduate School of Engineering, Nagasaki University, 1-14 Bunkyo, Nagasaki,
852-8521, Japan d
Department of Chemistry and Materials, Faculty of Textile Science and Technology,
Shinshu University, 3-15-1 Tsuneda, Ueda 386-8567, Japan e
Department of Science, Faculty of Science, Shinshu University, 3-1-1 Asahi,
Matsumoto, Nagano, 390-8621, Japan f
Department of Mechanical and Electrical Engineering, Faculty of Engineering, Suwa
University of Science, 5000-1 Toyohira, Chino 391-0292, Japan
1
†
Present address: Division of Chemistry, Faculty of Mathematics and Science,
Indonesia University of Education, Bandung 40522, Indonesia ††
Present address: Morgan Advanced Materials Carbon Science Centre of Excellence,
State College, PA 16803, USA *
Corresponding author. Tel: +81 (0)26 269 5743.
E-mail: [email protected]
ABSTRACT We fabricated mesoscopic cage-like structured Single Wall Carbon Nanotube (SWCNT) cryogels having ultramicroporous necks using highly concentrated SWCNT-water inks with the aid of Zn-Al sol-gel dispersant. High resolution transmission electron microscopy (TEM) image shows the presence of three-dimensional cage-like structures in the SWCNT cryogels, which are quite different from the bundle structures of pristine SWCNTs. Average bundle size of the SWCNT cryogels is 30 % smaller than that of pristine SWCNTs according to the TEM observation. Adsorption isotherms of N2 at 77 K and Ar at 87 K for SWCNT cryogels and pristine SWCNTs have explicit adsorption hysteresis in low and high relative pressure regions, indicating the presence of necked ultramicropores and mesopores. Ar adsorption isotherms show that the SWCNT cryogels have predominant low and high relative pressure hysteresis, suggesting
2
presence of mesopores with necked entrances of ultramicropores. Surface areas of SWCNT cryogels by Ar and N2 adsorption are about two times larger than those of pristine SWCNTs which is in agreement with geometrically evaluated surface area from SWCNT bundles. Mesopore volumes evaluated by adsorption of N2 and Ar are 3 and 2.3 times larger than those of pristine SWCNTs in the mesopore range above 15 nm. The marked increase in the mesoporosity of the SWCNT assemblies by freeze-drying supports the mesoscopic three-dimensional cage-like structures observed by TEM.
Keywords: Single wall carbon nanotube cryogels; Nitrogen adsorption; Argon adsorption; Necked pore structure
1.
Introduction Nanoporous carbons are indispensable to sustainable technology because of the
strong dispersion interactions with molecules [1,2], the high surface area [3], high electrical conductivity [4–6], and robustness in acidic and alkaline environments [7]. Hence researchers have actively challenged to develop new nanoporous carbons from various precursors [8–11]. Nanoporous carbons have unique hierarchical pore structures derived from inherent texture structures of precursors, which are required for
3
outstanding adsorption kinetics of target gases [12]; the hierarchical pore structures are often modified to obtain better accessibility of molecules and ions in order to get the solutions on greenhouse gases and water purification issues [10,13,14]. On the other hand, we have a bottom-up route for construction of a target structure of an appropriate porosity using nanoscale carbon units such as reduced graphene oxides [15] and/or single wall carbon nanotube(SWCNT)s [16]. Fabricating the nanoporous materials from nanoscale graphene and SWCNTs have advantage over activated carbons having the complex
frame-structure,
because
SWCNTs
and
nanoscale
graphenes
have
well-characterized structures for better designability [17]. Also precursors of well-defined texture structures encounter difficulties in controlling porosity. Understanding the relationship between the pore structure and adsorption properties for a target gas is crucial for developing the optimum adsorption technology with the merit of carbons such as high electrical and thermal conductivity, excellent mechanical and chemical robustness, and lightness [18]. The traditional activated carbon consists of nanometer-sized defective graphitic units which are combined with each other through sp3 bondings [19,20], which cannot provide sufficient electrical and thermal conductivity and structural robustness. Then, we need to use the sp3 bondings-free nanoscale carbon units to produce novel porous carbons having excellent properties
4
with the bottom-up route. The nanoscale graphene-based porous carbons with high surface area of > 2000 m2/g, the extreme lightness, and structural flexibility by different activation methods [21,22] and addition of electron donor or accepter molecules [23] are developed. Porosity in these
nanoporous
graphene-based
carbons
comes
from
accumulation
of
two-dimensional nanoscale graphene units having sp3 carbons at the edges [23]. Consequently, the nanoscale graphene-based porous carbon has many sp3 carbons at the edges [24]. On the other hand, the SWCNT of extremely high aspect ratio has little sp3 carbons at the pentagonal caps and defects [25]. Fabricating the nanoporous structure from one-dimensional SWCNT units of high specific surface area [26], mechanical [27] and chemical robustness, and high electrical and thermal conductivities [28,29] is promising. Addition of polymers or oxides for the fabrication of SWCNT decreases the surface area, suppressing the excellent physical properties of SWCNTs [30,31]; the bottom-up fabrication from pure SWCNTs is preferable. The SWCNT aerogels using surfactant-aided diluted SWCNT sols were prepared, giving a large mesoporosity [32,33]. It is well-known that the surfactants deteriorate the excellent properties of SWCNTs. Also the porosity evaluation of the SWCNT assemblies by N2 adsorption at 77 K in the previous studies [32,33] is not necessarily appropriate. We must understand
5
the essential structure-feature of the porosity of SWCNT-based porous materials with reliable porosity measurement. In present study, we fabricate porous SWCNT cryogels by freeze-drying treatment of aqueous SWCNT dispersant prepared with the inorganic Zn-Al sol-gel complex [34]. The structure analyses of the SWCNT cryogels with adsorption of N2 at 77 K and Ar at 87 K and TEM indicate the mesoscopic cage-like structure woven by fine SWCNT bundles. 2.
Materials and methods
2-1. Preparation of SWCNT cryogels Commercial SWCNTs (Meijo Nano Carbon Co., Ltd.) synthesized by chemical vapor deposition (CVD) method of average tube diameter of ~2 nm were used for preparing the SWCNT cryogels. Homogenizer tip (SONIC VS 750; 225 W, 20 kHz) and three-roll mill (AIMEX, BR 100 V III) dispersed the SWCNTs in aqueous media with aid of the Zn-Al sol-gel complex dispersant which was developed by our group [34]. 150 mg of SWCNTs and 1.50 g of Zn-Al sol-gel dispersant were mixed in 15 mL of water and sonicated for 30 min to form the SWCNT gels. After sonicating, we carried out three-roll mill treatment in order to improve homogeneity of the SWCNT gels. Three-roll milling of the SWCNT gels includes passing the gels between the rotating
6
rollers (the detailed procedure of the three-roll milling is given in Fig. S1). We froze the SWCNT gels in liquid nitrogen and dried using freeze-dryer (Eyela FDU-1200) to obtain the SWCNT cryogels; freeze drying lasted for 48 h at 220 K and 10 Pa. Finally, we obtained the SWCNT:Zn-Al complex mixed cryogels. Washing with 1 M HNO3 for 10 min removes the Zn-Al complex, giving pure SWCNT gels. The SWCNT gels were washed with dialysis using the cellulose membrane for 2 days and thereby we obtained acid-free SWCNT gels. The freeze-drying of the purified SWCNT gels gave highly porous SWCNT cryogels. 2-2. Characterization The TG profiles of SWCNT cryogels were measured with a thermogravimetric analyzer (TGA, Hitachi STA7200) at flow rate of 100 mL min-1 and heating rate of 5 K min-1 in an air atmosphere. We studied the assembly structure of SWCNT cryogels with optical microscopy (Olympus DP73), field emission scanning electron microscopy (FE-SEM; JSM-7000F, JEOL), and high-resolution scanning transmission electron microscopy (HR-STEM; ARM-200CF, JEOL). We evaluated the average bundle size using the HR-STEM micrographs. The crystallinity of the SWCNT cryogels was examined by Raman spectroscopy (Renishaw inVia, IAB 8303) at the laser excitation wavelengths of 532 nm and 785 nm which excite semiconducting and metallic
7
SWCNTs, respectively. 2-3. Nanoporosity measurements The adsorption isotherms of N2 at 77 K and Ar at 87 K for pristine SWCNTs and SWCNT cryogels were measured after pre-evacuation at 523 K with a volumetric equipment (Autosorb IQ-Quantachrome). We determined the total surface area by subtracting pore effect (SPE) method from high resolution αs–plot based on the nitrogen adsorption isotherms of nonporous acetylene black as a reference [20,35]. The SPE method gives more reliable surface area for microporous materials than the BET method; we also evaluated the BET surface area for comparison. We evaluated mesoporosity by Dollimore-Heal (DH) method [36] and micropore volume from Dubinin-Radushkevich (DR) method [37,38]. We obtained the pore size distribution (PSD) by application of the quenched solid density functional theory (QS-DFT) method for cylindrical pore model [39] to the N2 and Ar adsorption isotherms.
3.
Results and Discussion
3-1. Morphological structure and structural stability Optical and SEM micrographs show morphologies of pristine SWCNTs, SWCNT:Zn-Al complex cryogels and SWCNT cryogels (Fig. 1). Pristine SWCNTs
8
have a uniform felt like structure of tightly bound SWCNT bundles of several µm in width (Fig. 1(a0 – a2)). On the contrary, the SWCNT:Zn-Al complex mixed cryogels have an agglomerate-like structure in which fine SWCNT bundles are coated with the Zn-Al complex, forming the felt-like structure having macropores (Fig. 1(b0 – b2)). The SEM image of the SWCNT cryogels after removing the Zn-Al complex shows a fluffy layer structure consisting of fibrous SWCNTs (Fig. 1(c0 – c2)). An important structural difference between the pristine SWCNTs and the SWCNT cryogels stems from the bundle size. The SWCNT bundle size decreases significantly after removing the Zn-Al complex. [34] TEM micrographs show pore structures of pristine SWCNTs and SWCNT cryogels (Fig. 2). Pristine SWCNTs have an assembly structure of the straight SWCNT ropes (Fig. 2(a) and (b), Fig. S2), while the SWCNT cryogels have 3D semi-cage structures of curved SWCNTs (Fig. 2(c) and (d), Fig. S3). The average bundle size decreases from 27 ± 14 nm for pristine SWCNTs to 19 ± 10 nm for SWCNT cryogels according to the TEM observation. The morphological structure change of SWCNT on formation of the cryogels is associated with pore structural differences between the pristine SWCNT and the SWCNT cryogels from N2 and Ar adsorption, as shown later. The crystallinity change of the SWCNT cryogels through removal of Zn-Al complex
9
with the HNO3 treatment was negligibly small with the following Raman spectroscopic examination for metallic and semiconducting SWCNTs (Fig. 3). Raman spectra shows a distinctive band at 1588 cm-1 (G-band) coming from in-plane vibrations of sp2 carbons and a band at 1593 cm-1 (D-band) from sp3 defective carbons [40]. The D/G-band ratios of metallic and semiconducting pristine SWCNTs are 0.018 and 0.012, indicating highly crystalline state. The SWCNTs in SWCNT:Zn-Al complex cryogels show a blue shift of the G-band due to the charge transfer interactions from SWCNTs to the Zn-Al complex dispersant [34]. The G-band shifts from 1588 cm-1 to 1604 cm-1 for semiconducting SWCNTs (Fig. S4). The D/G ratio of the semiconducting SWCNTs in the SWCNT:Zn-Al complex cryogels increases to 0.041 from 0.012 due to slight damage on dispersion treatment. Further purification treatment of the SWCNT:Zn-Al complex cryogels with the diluted HNO3 and dialysis increases the D/G ratio to 0.054 from 0.041. The D/G ratio of the purified SWCNT cryogels is still small, guaranteeing the good crystallinity of semiconducting SWCNTs. Raman spectral analysis of metallic SWCNTs gave similar results (Table S1 and S2). Consequently, the well-crystalline state is preserved even after preparation of the SWCNT cryogels. We examined thermal stability of the SWCNT cryogels by TGA (Fig. 4). The pristine SWCNTs start to burn at 830 K and the SWCNT cryogels start to burn at 790 K, leading
10
to the constant weight loss of 7.0 wt. % and 6.1 wt. % above 1142 K, respectively. Decrease of the weight loss from 7.0 wt. % to 6.1 wt. % is attributed to removal of metallic impurities as the catalysts for production of SWCNT (see EDX data in Fig. S5). The SWCNT:Zn-Al complex decomposes below 600 K due to presence of oxygen and nitrogen species which catalyze incineration of the SWCNTs [34]. The SWCNT cryogels burn at higher temperature than the SWCNT:Zn-Al cryogels because of removing the Zn/Al complex which increases thermal stability of the SWCNT cryogels. We observe a slight weight loss below 316 K due to desorption of weakly adsorbed water in the SWCNT cryogels. The SWCNT cryogels show a marked weight loss of ~ 20 wt. % below 790 K due to desorption of water adsorbed on SWCNT cryogels. We measured water vapor adsorption isotherms of the pristine SWCNTs and the SWCNT cryogels (Fig. S6). Both of water vapor adsorption isotherms are of IUPAC Type V [41]; both samples are hydrophobic. However, the adsorption amount of SWCNT cryogels is about twice of that of pristine SWCNTs. The rising relative pressure of the adsorption isotherm of the SWCNT cryogels is smaller than that of pristine SWCNTs. The SWCNT cryogels adsorb 2.4 times more water than the pristine SWCNTs at the relative pressure of 0.7 which is close to the atmospheric condition. We measured TG profile of the SWCNT cryogels pre-treated at 573 K for 3 hours under an N2 flow (Fig. S7); the TG
11
profile of thus-treated SWCNT cryogels is similar to that of pristine SWCNTs. Therefore, the weight loss of ~20 wt. % mainly originates from water adsorbed on the SWCNT cryogels (the detailed explanation is given in Fig. S7). The SWCNT cryogels should have ultramicropores as strong adsorption sites and mesopores as a large water adsorption reservoir. 3-2. Nanoporosity of SWCNT cryogels from N2 adsorption Nanoporosity of SWCNT cryogels, SWCNT:Zn-Al complex cryogels, and pristine SWCNTs is evidenced by adsorption isotherms of N2 at 77 K (Fig. 5). The adsorption isotherm of SWCNT cryogels is of IUPAC type II [41], indicating a great contribution of the external surface to the porosity of cryogels (Fig. 5(a)). The adsorption amount of SWCNT:Zn-Al complex cryogels is quite small in comparison with that of SWCNT cryogels and pristine SWCNTs, meaning that the Zn-Al dispersant completely blocks porosity of the SWCNTs. Adsorption isotherms of pristine SWCNTs and SWCNT cryogels have adsorption hysteresis above P/P0 = 0.8 (Fig. 5(a)), indicating presence of the large mesopores due to the 3D jelly structure. The sharp increase in the adsorption below P/P0 = 0.1 for SWCNT cryogels indicates the presence of considerable amount of narrow micropores (Fig. 5(c)). Adsorption amount of N2 below P/P0 = 0.1 for SWCNT cryogels is two times larger than that of pristine SWCNTs. Adsorption isotherms of
12
pristine SWCNTs exhibit low pressure adsorption hysteresis which continues down to P/P0 = 10-2 at least (Fig. 5(c) and (d)). Low pressure adsorption hysteresis stems from the presence of ultramicropores where N2 molecules cannot easily access [42]. The low pressure hysteresis gaps at P/P0 = 0.1 are 29 mL(STP) g-1 for SWCNT cryogels and 17 mL(STP) g-1 for pristine SWCNTs, suggesting that the SWCNT cryogels have more necked ultramicropores than pristine SWCNTs. The surface area and pore volume are evaluated by the SPE method using high resolution αs-plot and BET method (Table 1). The BET method underestimates the surface area due to difficulty of forming bilayers of adsorbed N2 in the ultramicropores [43]; the BET surface area is smaller than surface area evaluated from the SPE method. The SPE surface area of SWCNT cryogels by N2 adsorption is two times larger than that of pristine SWCNTs, coinciding with the change of surface area which was geometrically evaluated from the bundle size difference observed by TEM. The observed increase of the surface area of SWCNT cryogels is caused by the marked decrease of the bundle size. Thin SWCNT bundles are associated with each other, forming the 3D cage-like structures which were shown in the TEM micrographs. The oxidized SWCNTs adsorb N2 two times more than SWCNT cryogels below P/P0 = 0.7, because oxidation removes caps, giving better accessibility to the opening the internal
13
porosity of SWCNT (Fig. S8). Importantly, adsorption isotherm of the oxidized SWCNT has no predominant low pressure adsorption hysteresis which are observed in the adsorption isotherms of SWCNT cryogels and pristine SWCNTs; major adsorption inside SWCNT of no blocking effect conceals the low pressure adsorption hysteresis of adsorption on the external surfaces of SWCNT bundles. Consequently, the observed increase in the porosity of SWCNT cryogels should not stem from opening SWCNTs during the dispersion and freeze-drying treatments. The micropore and mesopore volumes of SWCNT cryogels are about four and three times larger than those of the pristine SWCNTs, respectively. The low pressure adsorption hysteresis of the SWCNT cryogels is more predominant than that of the pristine SWCNTs. Therefore, the SWCNT cryogels should have pores necked with ultramicropores in addition to large micropores and mesopores. Adsorption of N2 at 77 K is not necessarily appropriate for evaluation of the ultramicroporosity due to the intensive pore blocking effect. Then, we must obtain porosity information from Ar adsorption at 87 K with less blocking effect at the ultramicropore necks. 3-3. Comparative pore analysis of SWCNT cryogels using adsorption of N2 and Ar Since N2 has the quadrupole moment and interact with the carbon pore through the electrostatic interaction in addition to the dispersion interaction [42], N2 molecules are
14
strongly adsorbed near the entrance of ultramicropores and block the further adsorption. A spherical Ar molecule has no quadrupole moment and the size of Ar (0.341 nm) is slightly smaller than that of N2 (0.375 nm). The boiling temperature of liquid Ar is 87 K, which is higher than that of liquid N2, being favorable for their intra-pore diffusion [44]. Consequently, Ar adsorption at 87 K has been recommended to evaluate precise microporosity by IUPAC [41]. The Ar adsorption isotherms of SWCNT samples at 87 K are compared with N2 adsorption isotherms at 77 K (Fig. 6). We observe a distinct discrepancy between adsorption isotherms of Ar and N2, although adsorption branches of Ar adsorption and N2 adsorption on SWCNT cryogels in the linear P/P0 expression are almost overlapped in the P/P0 range of 0.05 to 0.6 (Fig.6(a) and (b)). Magnified adsorption isotherms (Fig. 6(c) and (d)) explicitly show that N2 adsorption amount is much larger than Ar adsorption amount below P/P0 = 10-2; the discrepancy between adsorption amounts of N2 and Ar for SWCNT cryogels is larger than that of pristine SWCNTs. N2 molecules having the quadrupole moment are strongly adsorbed near the pore entrances in the very low P/P0 region (below P/P0 = 10-4) to block the further adsorption in the higher P/P0 region; the adsorption amount of Ar below P/P0 = 10-4 is much less than that of N2. However, adsorption amount of Ar on the adsorption branch becomes larger than that of N2 above P/P0 = 0.36; the difference between the adsorption
15
amounts of Ar and N2 of the desorption branches is more predominant than that on the adsorption branches. Thus, Ar adsorption isotherms give more evident low pressure adsorption hysteresis (Fig. 5(c) and (d)); the hysteresis gaps at P/P0 = 0.1 are 45 mL(STP) g-1 and 24 mL(STP) g-1 for SWCNT cryogels and pristine SWCNTs, respectively. The marked low pressure adsorption hysteresis gap for Ar adsorption indicates that even Ar molecules cannot access quickly to ultramicropores of these SWCNT samples on the adsorption branch. We need to use the desorption branch to obtain the better porosity, because the adsorption branch is more influenced by not sufficient accessibility at the ultramicropore entrances intervening the intrapore diffusion to larger pores. Ar adsorption provides more accurate porosity than N2 adsorption (see Table 1), as mentioned above. The total pore volume (2.55 cm3 g-1) of SWCNT cryogels is larger than that (2.01 cm3 g-1) from N2 adsorption by 0.54 cm3 g-1, being quite marked difference. The mesopore volume difference of 0.5 cm3 g-1 corresponds to 22 % of the mesopore volume of SWCNT cryogels (2.3 cm3 g-1), suggesting the presence of the abundant mesopores regulated by ultramicroporous necks. The should be associated with the 3D cage-like structures observed by TEM (Fig. 2(c) and (d)), whose size is in the mesopore range. The thin SWCNT bundles should bend to contact each other at the edges to keep such mesopore-cages and thereby some of the
16
contacting edges should work as ultramicropore entrance. Thus, the TEM and adsorption studies lead to the above pore structural image for SWCNT cryogels. The difference of the total pore volume for pristine SWCNTs is 0.36 cm3 g-1. Thus, N2 adsorption at 77 K underestimates the porosity by 21 % for SWCNT cryogels and 34 % for pristine SWCNTs at least. We must be cautious that even the porosity by Ar adsorption should be underestimated. The explicit difference in adsorption amounts of Ar and N2 is also observed for SWCNT:Zn-Al cryogels; although the N2 adsorption amount is quite small as mentioned above, Ar adsorption gives about three times larger surface area (Fig. S9). We compare the pore size distributions of SWCNT cryogels and pristine SWCNTs from Ar and N2 adsorption (Fig. 7). The clearest difference of the pore size distribution of SWCNT cryogels and pristine SWCNTs is in the mesopore region from 20 to 40 nm; SWCNT cryogels have a large distribution in the pore width range. The mesopores in SWCNT cryogels stem from the 3D-cage like structures of fine SWCNT bundles on the freeze-drying treatment, which are shown by TEM observation. Briefly speaking the pore size distributions from Ar and N2 adsorption are not so different from each other for SWCNT cryogels (Fig. 7(a)) and pristine SWCNTs (Fig.7(b)). The pore size distributions from Ar and N2 adsorption have micropores at 2 nm and small mesopores
17
of about 5 nm for both of SWCNT cryogels and pristine SWCNT; both distributions are not much different from each other. As we observed more distinct low pressure adsorption hysteresis in adsorption isotherms of Ar than that of N2, the ultramicropore size distribution should depend on the probe molecule of Ar or N2. However, even the QS-DFT cannot describe clearly the role of the nanoporous neck structures. The absolute pore volume of the ultramicroporous neck structures should be negligible in comparison with the mesopore volume even in comparison with the mesopore volumes in the SWCNT cryogels and pristine SWCNTs. In future we will challenge quantitative evaluation of the ultramicroporous neck structures and elucidate the physical properties of SWCNT cryogels. 4.
Conclusions We fabricated the SWCNT cryogels of mesoporous cage-like assembly structures
from
concentrated
SWCNT-water
inks,
being
different
from
the
previous
literatures[32,33] which used diluted SWCNT dispersants stabilized with surfactants. The Zn-Al sol-gel dispersant enables to prepare highly concentrated SWCNT-water inks. The Zn-Al complex can be more easily removed from the SWCNTs than the surfactants, leading to negligible deteriorations of SWCNTs. TEM observation and comparative pore structure analysis using adsorption of Ar at 87 K and N2 at 77 K support forming
18
the mesoscopic cage-like structural assemblies of SWCNTs in which many mesopores have necked ultramicropores, inducing explicit low pressure adsorption hysteresis. The presence of the necked ultramicropores attaching to the mesopores is supported by better accessibility of Ar at 87 K than that of N2 at 77 K. The 2.3-3 times-larger mesopore volume of the SWCNT cryogels than that of pristine SWCNTs is due to fine dispersion of SWCNTs by the Zn-Al complex. In the next stage, we will challenge preparing the films and fibers having mesoscopic cage-like structures.
Acknowledgements The work was partially supported by the Grant-in-Aid- for Scientific Research (B) (17H03039) and the JST OPERA Program (JPMJOP1722). KK is honorable to contribute to the special issue dedicated to professor Mietek Jaroniec.
19
Reference [1]
T. Fujimori, A. Morelos-Gómez, Z. Zhu, H. Muramatsu, R. Futamura, K. Urita, M. Terrones, T. Hayashi, M. Endo, S. Young Hong, Y. Chul Choi, D. Tománek, K. Kaneko, Conducting linear chains of sulphur inside carbon nanotubes, Nat. Commun. 4 (2013) 2162. doi:10.1038/ncomms3162.
[2]
D. Srivastava, E.E. Santiso, K.E. Gubbins, Pressure Enhancement in Confined Fluids: Effect of Molecular Shape and Fluid–Wall Interactions, Langmuir. 33 (2017) 11231–11245. doi:10.1021/acs.langmuir.7b02260.
20
[3]
M. Sevilla, G.A. Ferrero, N. Diez, A.B. Fuertes, One-step synthesis of ultra-high surface area nanoporous carbons and their application for electrochemical energy storage, Carbon. 131 (2018) 193–200. doi:10.1016/j.carbon.2018.02.021.
[4]
M. Salanne, B. Rotenberg, K. Naoi, K. Kaneko, P.L. Taberna, C.P. Grey, B. Dunn, P. Simon, Efficient storage mechanisms for building better supercapacitors, Nat. Energy. 1 (2016) 16070. doi:10.1038/nenergy.2016.70.
[5]
R. Futamura, T. Iiyama, Y. Takasaki, Y. Gogotsi, M.J. Biggs, M. Salanne, J. Ségalini, P. Simon, K. Kaneko, Partial breaking of the Coulombic ordering of ionic liquids confined in carbon nanopores, Nat. Mater. 16 (2017) 1225–1232. doi:10.1038/nmat4974.
[6]
A. Barroso-Bogeat, M. Alexandre-Franco, C. Fernández-González, A. Macías-García, V. Gómez-Serrano, Electrical conductivity of activated carbon– metal oxide nanocomposites under compression: a comparison study, Phys. Chem. Chem. Phys. 16 (2014) 25161–25175. doi:10.1039/C4CP03952A.
[7]
J.P. Chen, S. Wu, Acid/Base-treated activated carbons: characterization of functional groups and metal adsorptive properties., Langmuir. 20 (2004) 2233–42. doi:10.1021/la0348463.
[8]
Y. Yao, Q. Zhang, P. Liu, L. Yu, L. Huang, S.-Z. Zeng, L. Liu, X. Zeng, J. Zou,
21
Facile synthesis of high-surface-area nanoporous carbon from biomass resources and its application in supercapacitors, RSC Adv. 8 (2018) 1857–1865. doi:10.1039/C7RA12525A. [9]
F. Su, X.S. Zhao, L. Lv, Z. Zhou, Synthesis and characterization of microporous carbons templated by ammonium-form zeolite Y, Carbon. 42 (2004) 2821–2831. doi:10.1016/j.carbon.2004.06.028.
[10] Z. Geng, Q. Xiao, H. Lv, B. Li, H. Wu, Y. Lu, C. Zhang, One-Step Synthesis of Microporous Carbon Monoliths Derived from Biomass with High Nitrogen Doping Content for Highly Selective CO2 Capture, Sci. Rep. 6 (2016) 30049. doi:10.1038/srep30049. [11] A. Dasgupta, J. Matos, H. Muramatsu, Y. Ono, V. Gonzalez, H. Liu, C. Rotella, K. Fujisawa, R. Cruz-Silva, Y. Hashimoto, M. Endo, K. Kaneko, L.R. Radovic, M. Terrones, Nanostructured carbon materials for enhanced nitrobenzene adsorption: Physical vs. chemical surface properties, Carbon. 139 (2018) 833– 844. doi:10.1016/j.carbon.2018.07.045. [12] N.P. Wickramaratne, M. Jaroniec, Activated Carbon Spheres for CO 2 Adsorption, ACS Appl. Mater. Interfaces. 5 (2013) 1849–1855. doi:10.1021/am400112m.
22
[13] V.C. Srivastava, I.D. Mall, I.M. Mishra, Adsorption of toxic metal ions onto activated carbon, Chem. Eng. Process. Process Intensif. 47 (2008) 1269–1280. doi:10.1016/j.cep.2007.04.006. [14] S. Lei, J. Miyamoto, H. Kanoh, Y. Nakahigashi, K. Kaneko, Enhancement of the methylene blue adsorption rate for ultramicroporous carbon fiber by addition of mesopores, Carbon. 44 (2006) 1884–1890. doi:10.1016/j.carbon.2006.02.028. [15] Z. Fan, Q. Zhao, T. Li, J. Yan, Y. Ren, J. Feng, T. Wei, Easy synthesis of porous graphene nanosheets and their use in supercapacitors, Carbon. 50 (2012) 1699– 1703. doi:10.1016/j.carbon.2011.12.016. [16] F. Tristán-López, A. Morelos-Gómez, S.M. Vega-Díaz, M.L. García-Betancourt, N. Perea-López, A.L. Elías, H. Muramatsu, R. Cruz-Silva, S. Tsuruoka, Y.A. Kim, T. Hayahsi, K. Kaneko, M. Endo, M. Terrones, Large Area Films of Alternating Graphene–Carbon Nanotube Layers Processed in Water, ACS Nano. 7 (2013) 10788–10798. doi:10.1021/nn404022m. [17] V.T. Dang, D.D. Nguyen, T.T. Cao, P.H. Le, D.L. Tran, N.M. Phan, V.C. Nguyen, Recent trends in preparation and application of carbon nanotube– graphene hybrid thin films, Adv. Nat. Sci. Nanosci. Nanotechnol. 7 (2016) 033002. doi:10.1088/2043-6262/7/3/033002.
23
[18] Z. Fan, D.Z.Y. Tng, C.X.T. Lim, P. Liu, S.T. Nguyen, P. Xiao, A. Marconnet, C.Y.H. Lim, H.M. Duong, Thermal and electrical properties of graphene/carbon nanotube aerogels, Colloids Surfaces A Physicochem. Eng. Asp. 445 (2014) 48– 53. doi:10.1016/j.colsurfa.2013.12.083. [19] P.J.F. Harris, Z. Liu, K. Suenaga, Imaging the atomic structure of activated carbon, J. Phys. Condens. Matter. 20 (2008) 362201. doi:10.1088/0953-8984/20/36/362201. [20] K. Kaneko, C. Ishii, M. Ruike, H. Kuwabara, Origin of superhigh surface area and microcrystalline graphitic structures of activated carbons, Carbon. 30 (1992) 1075–1088. doi:10.1016/0008-6223(92)90139-N. [21] S. Wang, F. Tristan, D. Minami, T. Fujimori, R. Cruz-Silva, M. Terrones, K. Takeuchi, K. Teshima, F. Rodríguez-Reinoso, M. Endo, K. Kaneko, Activation routes for high surface area graphene monoliths from graphene oxide colloids, Carbon. 76 (2014) 220–231. doi:10.1016/j.carbon.2014.04.071. [22] N. Chotimah, A.D. Putri, Y. Ono, S. Kento, Y. Hattori, S. Wang, R. Futamura, K. Urita, F. Vallejos-Burgos, I. Moriguchi, M. Morimoto, R.T. Cimino, A. V. Neimark, T. Sakai, K. Kaneko, Nanoporosity Change on Elastic Relaxation of Partially Folded Graphene Monoliths, Langmuir. 33 (2017) 14565–14570.
24
doi:10.1021/acs.langmuir.7b03328. [23] A.D. Putri, N. Chotimah, S.K. Ujjain, S. Wang, R. Futamura, F. Vallejos-Burgos, F. Khoerunnisa, M. Morimoto, Z. Wang, Y. Hattori, T. Sakai, K. Kaneko, Charge-transfer mediated nanopore-controlled pyrene derivatives/graphene colloids, Carbon. 139 (2018) 512–521. doi:10.1016/j.carbon.2018.07.001. [24] B. Mortazavi, M.E. Madjet, M. Shahrokhi, S. Ahzi, X. Zhuang, T. Rabczuk, Nanoporous graphene: A 2D semiconductor with anisotropic mechanical, optical and thermal conduction properties, Carbon. 147 (2019) 377–384. doi:10.1016/j.carbon.2019.03.018. [25] S. Niyogi, M.A. Hamon, H. Hu, B. Zhao, P. Bhowmik, R. Sen, M.E. Itkis, R.C. Haddon, Chemistry of Single-Walled Carbon Nanotubes, Acc. Chem. Res. 35 (2002) 1105–1113. doi:10.1021/ar010155r. [26] A. Peigney, C. Laurent, E. Flahaut, R.R. Bacsa, A. Rousset, Specific surface area of carbon nanotubes and bundles of carbon nanotubes, Carbon. 39 (2001) 507– 514. doi:10.1016/S0008-6223(00)00155-X. [27] M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, Exceptionally high Young’s modulus observed for individual carbon nanotubes, Nature. 381 (1996) 678–680. doi:10.1038/381678a0.
25
[28] T.W. Ebbesen, H.J. Lezec, H. Hiura, J.W. Bennett, H.F. Ghaemi, T. Thio, Electrical conductivity of individual carbon nanotubes, Nature. 382 (1996) 54–56. doi:10.1038/382054a0. [29] S. Berber, Y.-K. Kwon, D. Tománek, Unusually High Thermal Conductivity of Carbon Nanotubes, Phys. Rev. Lett. 84 (2000) 4613–4616. doi:10.1103/PhysRevLett.84.4613. [30] S.A.C. Carabineiro, M.F. Pereira, J.N. Pereira, C. Caparros, V. Sencadas, S. Lanceros-Mendez, Effect of the carbon nanotube surface characteristics on the conductivity and dielectric constant of carbon nanotube/poly(vinylidene fluoride) composites, Nanoscale Res. Lett. 6 (2011) 302. doi:10.1186/1556-276X-6-302. [31] P. Sun, H. Yi, T. Peng, Y. Jing, R. Wang, H. Wang, X. Wang, Ultrathin MnO2 nanoflakes deposited on carbon nanotube networks for symmetrical supercapacitors with enhanced performance, J. Power Sources. 341 (2017) 27–35. doi:10.1016/j.jpowsour.2016.11.112. [32] K.H. Kim, Y. Oh, M.F. Islam, Mechanical and Thermal Management Characteristics of Ultrahigh Surface Area Single-Walled Carbon Nanotube Aerogels, Adv. Funct. Mater. 23 (2013) 377–383. doi:10.1002/adfm.201201055. [33] R. Du, Q. Zhao, N. Zhang, J. Zhang, Macroscopic Carbon Nanotube-based 3D
26
Monoliths, Small. 11 (2015) 3263–3289. doi:10.1002/smll.201403170. [34] R. Kukobat, D. Minami, T. Hayashi, Y. Hattori, T. Matsuda, M. Sunaga, B. Bharti, K. Asakura, K. Kaneko, Sol–gel chemistry mediated Zn/Al-based complex dispersant for SWCNT in water without foam formation, Carbon. 94 (2015) 518–523. doi:10.1016/j.carbon.2015.07.025. [35] K. Kaneko, C. Ishii, Superhigh surface area determination of microporous solids, Colloids and Surfaces. 67 (1992) 203–212. doi:10.1016/0166-6622(92)80299-H. [36] D. Dollimore, G.. Heal, Pore-size distribution in typical adsorbent systems, J. Colloid Interface Sci. 33 (1970) 508–519. doi:10.1016/0021-9797(70)90002-0. [37] H. Marsh, Adsorption methods to study microporosity in coals and carbons—a critique, Carbon. 25 (1987) 49–58. doi:10.1016/0008-6223(87)90039-X. [38] T. Ohba, K. Kaneko, GCMC Study on Relationship between DR Plot and Micropore Width Distribution of Carbon, Langmuir. 17 (2001) 3666–3670. doi:10.1021/la001463l. [39] A. V. Neimark, Y. Lin, P.I. Ravikovitch, M. Thommes, Quenched solid density functional theory and pore size analysis of micro-mesoporous carbons, Carbon. 47 (2009) 1617–1628. doi:10.1016/j.carbon.2009.01.050. [40] A. Jorio, M.A. Pimenta, A.G.S. Filho, R. Saito, G. Dresselhaus, M.S.
27
Dresselhaus, Characterizing carbon nanotube samples with resonance Raman scattering, New J. Phys. 5 (2003) 139.1-139.15. doi:10.1088/1367-2630/5/1/139. [41] M. Thommes, K. Kaneko, A. V. Neimark, J.P. Olivier, F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing, Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report), Pure Appl. Chem. 87 (2015) 1051–1069. doi:10.1515/pac-2014-1117. [42] F. Vallejos-Burgos, F.-X. Coudert, K. Kaneko, Air separation with graphene mediated by nanowindow-rim concerted motion, Nat. Commun. 9 (2018) 1812. doi:10.1038/s41467-018-04224-6. [43] N. Setoyama, T. Suzuki, K. Kaneko, Simulation study on the relationship between a high resolution αs-plot and the pore size distribution for activated carbon, Carbon. 36 (1998) 1459–1467. doi:10.1016/S0008-6223(98)00138-9. [44] S. Wang, D. Minami, K. Kaneko, Comparative pore structure analysis of highly porous graphene monoliths treated at different temperatures with adsorption of N2 at 77.4 K and of Ar at 87.3 K and 77.4 K, Microporous Mesoporous Mater. 209 (2015) 72–78. doi:10.1016/j.micromeso.2015.01.014.
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Tables Table 1. Surface area and pore volume from adsorption isotherms of N2 at 77 K and Ar at 87 K. N2 adsorption at 77 K
Ar adsorption at 87 K
Sαs
SBET
VMicro
VMeso
Sαs
SBET
VMicro
VMeso
m2/g
m2/g
cm3/g
cm3/g
m2/g
m2/g
cm3/g
cm3/g
Pristine SWCNTs
330
300
0.044
0.67
290
390
0.094
0.98
SWCNT:Zn-Al cryogels
100
88
0.035
0.013
295
230
0.094
0.061
SWCNT cryogels
570
540
0.21
1.8
600
490
0.25
2.3
29
Figure captions (Figure)
Fig. 1. Optical and FE-SEM micrographs of Pristine SWCNTs (a0 – a2), SWCNT:Zn-Al complex cryogels (b0 – b2), and SWCNT cryogels (c0 – c2). Fig. 2. TEM images of pristine SWCNTs (a, b) and SWCNT cryogels (c, d). Fig. 3. Raman spectra of SWCNT cryogels, SWCNT:Zn-Al complex cryogels, and pristine SWCNTs at the laser wavelength of (a) 785 nm and (b) 532 nm. Fig. 4. TG profiles of SWCNT cryogels, SWCNT:Zn-Al cryogels, pristine SWCNTs, and Zn-Al complex measured in the air atmosphere. Fig. 5. Adsorption isotherms of N2 at 77 K. ●, ○: SWCNT cryogels, ■, □:
30
SWCNT:Zn-Al cryogels, ▲, △: Pristine SWCNTs (solid and open symbols denote adsorption and desorption, respectively). (a) Liner scale of P/P0, (b) Logarithmic scale of P/P0, (c) Magnified fig. 5(a) in the lower P/P0 region, (d) Magnified fig. 5(b) in the lower P/P0 region. Fig. 6. Adsorption isotherms of Ar at 87 K and N2 at 77 K. SWCNT cryogels: Ar (●, ○), N2(■, □), Pristine SWCNTs: Ar(▲, △), N2(▼,▽). Solid and open symbols denote adsorption and desorption branches, respectively. (a) Liner scale of P/P0, (b) Logarithmic scale of P/P0, (c) Magnified fig. 6(a) in the lower P/P0 region, (d) Magnified fig. 6(b) in the lower P/P0 region. Fig. 7. Pore size distributions of (a) SWCNT cyogels and (b) pristine SWCNTs. (●: Ar and ■: N2 for SWCNT cryogels, ▲: Ar and ▼: N2 for Pristine SWCNTs)
31
Highlights Mesoscopic cage-like structured SWCNT cryogels are fabricated Mesoporous cage-like structures are completely different from bundle structures of SWCNT Mesoporous 3D-cage like structures consist of fine SWCNT bundles Adsorption of N2 and Ar shows the presence of ultramicropores attaching to mesopores. Even Ar cannot access to some of ultramicroprous neck structures.
1
S1387-1811(19)30671-7
DOI:
https://doi.org/10.1016/j.micromeso.2019.109814
Reference:
MICMAT 109814
To appear in:
Microporous and Mesoporous Materials
Received Date: 30 April 2019 Revised Date:
4 October 2019
Accepted Date: 18 October 2019
Please cite this article as: Y. Kamijyou, R. Kukobat, D. Stevic, K. Urita, N. Chotimah, Y. Hattori, R. Futamura, F. Vallejos-Burgos, I. Moriguchi, S. Utsumi, T. Sakai, K. Kaneko, Mesoscopic cage-like structured single wall carbon nanotube cryogels, Microporous and Mesoporous Materials (2019), doi: https://doi.org/10.1016/j.micromeso.2019.109814. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Mesoscopic cage-like structured single wall carbon nanotube cryogels Yuito Kamijyoua,b, Radovan Kukobatb, Dragana Stevicb, Koki Uritac, Nurul Chotimah b,†, Yoshiyuki Hattorid, Ryusuke Futamurae, Fernando Vallejos-Burgos
b, ††
, Isamu
Moriguchic, Shigenori Utsumif, Toshio Sakaia, Katsumi Kanekob, *
a
Department of Materials Chemistry, Faculty of Engineering, Shinshu University,
4-17-1 Wakasato, Nagano 380-8553, Japan b
Research Initiative for Supra-Materials, Shinshu University, 4-17-1 Wakasato, Nagano
380-8553, Japan c
Graduate School of Engineering, Nagasaki University, 1-14 Bunkyo, Nagasaki,
852-8521, Japan d
Department of Chemistry and Materials, Faculty of Textile Science and Technology,
Shinshu University, 3-15-1 Tsuneda, Ueda 386-8567, Japan e
Department of Science, Faculty of Science, Shinshu University, 3-1-1 Asahi,
Matsumoto, Nagano, 390-8621, Japan f
Department of Mechanical and Electrical Engineering, Faculty of Engineering, Suwa
University of Science, 5000-1 Toyohira, Chino 391-0292, Japan
1
†
Present address: Division of Chemistry, Faculty of Mathematics and Science,
Indonesia University of Education, Bandung 40522, Indonesia ††
Present address: Morgan Advanced Materials Carbon Science Centre of Excellence,
State College, PA 16803, USA *
Corresponding author. Tel: +81 (0)26 269 5743.
E-mail: [email protected]
ABSTRACT We fabricated mesoscopic cage-like structured Single Wall Carbon Nanotube (SWCNT) cryogels having ultramicroporous necks using highly concentrated SWCNT-water inks with the aid of Zn-Al sol-gel dispersant. High resolution transmission electron microscopy (TEM) image shows the presence of three-dimensional cage-like structures in the SWCNT cryogels, which are quite different from the bundle structures of pristine SWCNTs. Average bundle size of the SWCNT cryogels is 30 % smaller than that of pristine SWCNTs according to the TEM observation. Adsorption isotherms of N2 at 77 K and Ar at 87 K for SWCNT cryogels and pristine SWCNTs have explicit adsorption hysteresis in low and high relative pressure regions, indicating the presence of necked ultramicropores and mesopores. Ar adsorption isotherms show that the SWCNT cryogels have predominant low and high relative pressure hysteresis, suggesting
2
presence of mesopores with necked entrances of ultramicropores. Surface areas of SWCNT cryogels by Ar and N2 adsorption are about two times larger than those of pristine SWCNTs which is in agreement with geometrically evaluated surface area from SWCNT bundles. Mesopore volumes evaluated by adsorption of N2 and Ar are 3 and 2.3 times larger than those of pristine SWCNTs in the mesopore range above 15 nm. The marked increase in the mesoporosity of the SWCNT assemblies by freeze-drying supports the mesoscopic three-dimensional cage-like structures observed by TEM.
Keywords: Single wall carbon nanotube cryogels; Nitrogen adsorption; Argon adsorption; Necked pore structure
1.
Introduction Nanoporous carbons are indispensable to sustainable technology because of the
strong dispersion interactions with molecules [1,2], the high surface area [3], high electrical conductivity [4–6], and robustness in acidic and alkaline environments [7]. Hence researchers have actively challenged to develop new nanoporous carbons from various precursors [8–11]. Nanoporous carbons have unique hierarchical pore structures derived from inherent texture structures of precursors, which are required for
3
outstanding adsorption kinetics of target gases [12]; the hierarchical pore structures are often modified to obtain better accessibility of molecules and ions in order to get the solutions on greenhouse gases and water purification issues [10,13,14]. On the other hand, we have a bottom-up route for construction of a target structure of an appropriate porosity using nanoscale carbon units such as reduced graphene oxides [15] and/or single wall carbon nanotube(SWCNT)s [16]. Fabricating the nanoporous materials from nanoscale graphene and SWCNTs have advantage over activated carbons having the complex
frame-structure,
because
SWCNTs
and
nanoscale
graphenes
have
well-characterized structures for better designability [17]. Also precursors of well-defined texture structures encounter difficulties in controlling porosity. Understanding the relationship between the pore structure and adsorption properties for a target gas is crucial for developing the optimum adsorption technology with the merit of carbons such as high electrical and thermal conductivity, excellent mechanical and chemical robustness, and lightness [18]. The traditional activated carbon consists of nanometer-sized defective graphitic units which are combined with each other through sp3 bondings [19,20], which cannot provide sufficient electrical and thermal conductivity and structural robustness. Then, we need to use the sp3 bondings-free nanoscale carbon units to produce novel porous carbons having excellent properties
4
with the bottom-up route. The nanoscale graphene-based porous carbons with high surface area of > 2000 m2/g, the extreme lightness, and structural flexibility by different activation methods [21,22] and addition of electron donor or accepter molecules [23] are developed. Porosity in these
nanoporous
graphene-based
carbons
comes
from
accumulation
of
two-dimensional nanoscale graphene units having sp3 carbons at the edges [23]. Consequently, the nanoscale graphene-based porous carbon has many sp3 carbons at the edges [24]. On the other hand, the SWCNT of extremely high aspect ratio has little sp3 carbons at the pentagonal caps and defects [25]. Fabricating the nanoporous structure from one-dimensional SWCNT units of high specific surface area [26], mechanical [27] and chemical robustness, and high electrical and thermal conductivities [28,29] is promising. Addition of polymers or oxides for the fabrication of SWCNT decreases the surface area, suppressing the excellent physical properties of SWCNTs [30,31]; the bottom-up fabrication from pure SWCNTs is preferable. The SWCNT aerogels using surfactant-aided diluted SWCNT sols were prepared, giving a large mesoporosity [32,33]. It is well-known that the surfactants deteriorate the excellent properties of SWCNTs. Also the porosity evaluation of the SWCNT assemblies by N2 adsorption at 77 K in the previous studies [32,33] is not necessarily appropriate. We must understand
5
the essential structure-feature of the porosity of SWCNT-based porous materials with reliable porosity measurement. In present study, we fabricate porous SWCNT cryogels by freeze-drying treatment of aqueous SWCNT dispersant prepared with the inorganic Zn-Al sol-gel complex [34]. The structure analyses of the SWCNT cryogels with adsorption of N2 at 77 K and Ar at 87 K and TEM indicate the mesoscopic cage-like structure woven by fine SWCNT bundles. 2.
Materials and methods
2-1. Preparation of SWCNT cryogels Commercial SWCNTs (Meijo Nano Carbon Co., Ltd.) synthesized by chemical vapor deposition (CVD) method of average tube diameter of ~2 nm were used for preparing the SWCNT cryogels. Homogenizer tip (SONIC VS 750; 225 W, 20 kHz) and three-roll mill (AIMEX, BR 100 V III) dispersed the SWCNTs in aqueous media with aid of the Zn-Al sol-gel complex dispersant which was developed by our group [34]. 150 mg of SWCNTs and 1.50 g of Zn-Al sol-gel dispersant were mixed in 15 mL of water and sonicated for 30 min to form the SWCNT gels. After sonicating, we carried out three-roll mill treatment in order to improve homogeneity of the SWCNT gels. Three-roll milling of the SWCNT gels includes passing the gels between the rotating
6
rollers (the detailed procedure of the three-roll milling is given in Fig. S1). We froze the SWCNT gels in liquid nitrogen and dried using freeze-dryer (Eyela FDU-1200) to obtain the SWCNT cryogels; freeze drying lasted for 48 h at 220 K and 10 Pa. Finally, we obtained the SWCNT:Zn-Al complex mixed cryogels. Washing with 1 M HNO3 for 10 min removes the Zn-Al complex, giving pure SWCNT gels. The SWCNT gels were washed with dialysis using the cellulose membrane for 2 days and thereby we obtained acid-free SWCNT gels. The freeze-drying of the purified SWCNT gels gave highly porous SWCNT cryogels. 2-2. Characterization The TG profiles of SWCNT cryogels were measured with a thermogravimetric analyzer (TGA, Hitachi STA7200) at flow rate of 100 mL min-1 and heating rate of 5 K min-1 in an air atmosphere. We studied the assembly structure of SWCNT cryogels with optical microscopy (Olympus DP73), field emission scanning electron microscopy (FE-SEM; JSM-7000F, JEOL), and high-resolution scanning transmission electron microscopy (HR-STEM; ARM-200CF, JEOL). We evaluated the average bundle size using the HR-STEM micrographs. The crystallinity of the SWCNT cryogels was examined by Raman spectroscopy (Renishaw inVia, IAB 8303) at the laser excitation wavelengths of 532 nm and 785 nm which excite semiconducting and metallic
7
SWCNTs, respectively. 2-3. Nanoporosity measurements The adsorption isotherms of N2 at 77 K and Ar at 87 K for pristine SWCNTs and SWCNT cryogels were measured after pre-evacuation at 523 K with a volumetric equipment (Autosorb IQ-Quantachrome). We determined the total surface area by subtracting pore effect (SPE) method from high resolution αs–plot based on the nitrogen adsorption isotherms of nonporous acetylene black as a reference [20,35]. The SPE method gives more reliable surface area for microporous materials than the BET method; we also evaluated the BET surface area for comparison. We evaluated mesoporosity by Dollimore-Heal (DH) method [36] and micropore volume from Dubinin-Radushkevich (DR) method [37,38]. We obtained the pore size distribution (PSD) by application of the quenched solid density functional theory (QS-DFT) method for cylindrical pore model [39] to the N2 and Ar adsorption isotherms.
3.
Results and Discussion
3-1. Morphological structure and structural stability Optical and SEM micrographs show morphologies of pristine SWCNTs, SWCNT:Zn-Al complex cryogels and SWCNT cryogels (Fig. 1). Pristine SWCNTs
8
have a uniform felt like structure of tightly bound SWCNT bundles of several µm in width (Fig. 1(a0 – a2)). On the contrary, the SWCNT:Zn-Al complex mixed cryogels have an agglomerate-like structure in which fine SWCNT bundles are coated with the Zn-Al complex, forming the felt-like structure having macropores (Fig. 1(b0 – b2)). The SEM image of the SWCNT cryogels after removing the Zn-Al complex shows a fluffy layer structure consisting of fibrous SWCNTs (Fig. 1(c0 – c2)). An important structural difference between the pristine SWCNTs and the SWCNT cryogels stems from the bundle size. The SWCNT bundle size decreases significantly after removing the Zn-Al complex. [34] TEM micrographs show pore structures of pristine SWCNTs and SWCNT cryogels (Fig. 2). Pristine SWCNTs have an assembly structure of the straight SWCNT ropes (Fig. 2(a) and (b), Fig. S2), while the SWCNT cryogels have 3D semi-cage structures of curved SWCNTs (Fig. 2(c) and (d), Fig. S3). The average bundle size decreases from 27 ± 14 nm for pristine SWCNTs to 19 ± 10 nm for SWCNT cryogels according to the TEM observation. The morphological structure change of SWCNT on formation of the cryogels is associated with pore structural differences between the pristine SWCNT and the SWCNT cryogels from N2 and Ar adsorption, as shown later. The crystallinity change of the SWCNT cryogels through removal of Zn-Al complex
9
with the HNO3 treatment was negligibly small with the following Raman spectroscopic examination for metallic and semiconducting SWCNTs (Fig. 3). Raman spectra shows a distinctive band at 1588 cm-1 (G-band) coming from in-plane vibrations of sp2 carbons and a band at 1593 cm-1 (D-band) from sp3 defective carbons [40]. The D/G-band ratios of metallic and semiconducting pristine SWCNTs are 0.018 and 0.012, indicating highly crystalline state. The SWCNTs in SWCNT:Zn-Al complex cryogels show a blue shift of the G-band due to the charge transfer interactions from SWCNTs to the Zn-Al complex dispersant [34]. The G-band shifts from 1588 cm-1 to 1604 cm-1 for semiconducting SWCNTs (Fig. S4). The D/G ratio of the semiconducting SWCNTs in the SWCNT:Zn-Al complex cryogels increases to 0.041 from 0.012 due to slight damage on dispersion treatment. Further purification treatment of the SWCNT:Zn-Al complex cryogels with the diluted HNO3 and dialysis increases the D/G ratio to 0.054 from 0.041. The D/G ratio of the purified SWCNT cryogels is still small, guaranteeing the good crystallinity of semiconducting SWCNTs. Raman spectral analysis of metallic SWCNTs gave similar results (Table S1 and S2). Consequently, the well-crystalline state is preserved even after preparation of the SWCNT cryogels. We examined thermal stability of the SWCNT cryogels by TGA (Fig. 4). The pristine SWCNTs start to burn at 830 K and the SWCNT cryogels start to burn at 790 K, leading
10
to the constant weight loss of 7.0 wt. % and 6.1 wt. % above 1142 K, respectively. Decrease of the weight loss from 7.0 wt. % to 6.1 wt. % is attributed to removal of metallic impurities as the catalysts for production of SWCNT (see EDX data in Fig. S5). The SWCNT:Zn-Al complex decomposes below 600 K due to presence of oxygen and nitrogen species which catalyze incineration of the SWCNTs [34]. The SWCNT cryogels burn at higher temperature than the SWCNT:Zn-Al cryogels because of removing the Zn/Al complex which increases thermal stability of the SWCNT cryogels. We observe a slight weight loss below 316 K due to desorption of weakly adsorbed water in the SWCNT cryogels. The SWCNT cryogels show a marked weight loss of ~ 20 wt. % below 790 K due to desorption of water adsorbed on SWCNT cryogels. We measured water vapor adsorption isotherms of the pristine SWCNTs and the SWCNT cryogels (Fig. S6). Both of water vapor adsorption isotherms are of IUPAC Type V [41]; both samples are hydrophobic. However, the adsorption amount of SWCNT cryogels is about twice of that of pristine SWCNTs. The rising relative pressure of the adsorption isotherm of the SWCNT cryogels is smaller than that of pristine SWCNTs. The SWCNT cryogels adsorb 2.4 times more water than the pristine SWCNTs at the relative pressure of 0.7 which is close to the atmospheric condition. We measured TG profile of the SWCNT cryogels pre-treated at 573 K for 3 hours under an N2 flow (Fig. S7); the TG
11
profile of thus-treated SWCNT cryogels is similar to that of pristine SWCNTs. Therefore, the weight loss of ~20 wt. % mainly originates from water adsorbed on the SWCNT cryogels (the detailed explanation is given in Fig. S7). The SWCNT cryogels should have ultramicropores as strong adsorption sites and mesopores as a large water adsorption reservoir. 3-2. Nanoporosity of SWCNT cryogels from N2 adsorption Nanoporosity of SWCNT cryogels, SWCNT:Zn-Al complex cryogels, and pristine SWCNTs is evidenced by adsorption isotherms of N2 at 77 K (Fig. 5). The adsorption isotherm of SWCNT cryogels is of IUPAC type II [41], indicating a great contribution of the external surface to the porosity of cryogels (Fig. 5(a)). The adsorption amount of SWCNT:Zn-Al complex cryogels is quite small in comparison with that of SWCNT cryogels and pristine SWCNTs, meaning that the Zn-Al dispersant completely blocks porosity of the SWCNTs. Adsorption isotherms of pristine SWCNTs and SWCNT cryogels have adsorption hysteresis above P/P0 = 0.8 (Fig. 5(a)), indicating presence of the large mesopores due to the 3D jelly structure. The sharp increase in the adsorption below P/P0 = 0.1 for SWCNT cryogels indicates the presence of considerable amount of narrow micropores (Fig. 5(c)). Adsorption amount of N2 below P/P0 = 0.1 for SWCNT cryogels is two times larger than that of pristine SWCNTs. Adsorption isotherms of
12
pristine SWCNTs exhibit low pressure adsorption hysteresis which continues down to P/P0 = 10-2 at least (Fig. 5(c) and (d)). Low pressure adsorption hysteresis stems from the presence of ultramicropores where N2 molecules cannot easily access [42]. The low pressure hysteresis gaps at P/P0 = 0.1 are 29 mL(STP) g-1 for SWCNT cryogels and 17 mL(STP) g-1 for pristine SWCNTs, suggesting that the SWCNT cryogels have more necked ultramicropores than pristine SWCNTs. The surface area and pore volume are evaluated by the SPE method using high resolution αs-plot and BET method (Table 1). The BET method underestimates the surface area due to difficulty of forming bilayers of adsorbed N2 in the ultramicropores [43]; the BET surface area is smaller than surface area evaluated from the SPE method. The SPE surface area of SWCNT cryogels by N2 adsorption is two times larger than that of pristine SWCNTs, coinciding with the change of surface area which was geometrically evaluated from the bundle size difference observed by TEM. The observed increase of the surface area of SWCNT cryogels is caused by the marked decrease of the bundle size. Thin SWCNT bundles are associated with each other, forming the 3D cage-like structures which were shown in the TEM micrographs. The oxidized SWCNTs adsorb N2 two times more than SWCNT cryogels below P/P0 = 0.7, because oxidation removes caps, giving better accessibility to the opening the internal
13
porosity of SWCNT (Fig. S8). Importantly, adsorption isotherm of the oxidized SWCNT has no predominant low pressure adsorption hysteresis which are observed in the adsorption isotherms of SWCNT cryogels and pristine SWCNTs; major adsorption inside SWCNT of no blocking effect conceals the low pressure adsorption hysteresis of adsorption on the external surfaces of SWCNT bundles. Consequently, the observed increase in the porosity of SWCNT cryogels should not stem from opening SWCNTs during the dispersion and freeze-drying treatments. The micropore and mesopore volumes of SWCNT cryogels are about four and three times larger than those of the pristine SWCNTs, respectively. The low pressure adsorption hysteresis of the SWCNT cryogels is more predominant than that of the pristine SWCNTs. Therefore, the SWCNT cryogels should have pores necked with ultramicropores in addition to large micropores and mesopores. Adsorption of N2 at 77 K is not necessarily appropriate for evaluation of the ultramicroporosity due to the intensive pore blocking effect. Then, we must obtain porosity information from Ar adsorption at 87 K with less blocking effect at the ultramicropore necks. 3-3. Comparative pore analysis of SWCNT cryogels using adsorption of N2 and Ar Since N2 has the quadrupole moment and interact with the carbon pore through the electrostatic interaction in addition to the dispersion interaction [42], N2 molecules are
14
strongly adsorbed near the entrance of ultramicropores and block the further adsorption. A spherical Ar molecule has no quadrupole moment and the size of Ar (0.341 nm) is slightly smaller than that of N2 (0.375 nm). The boiling temperature of liquid Ar is 87 K, which is higher than that of liquid N2, being favorable for their intra-pore diffusion [44]. Consequently, Ar adsorption at 87 K has been recommended to evaluate precise microporosity by IUPAC [41]. The Ar adsorption isotherms of SWCNT samples at 87 K are compared with N2 adsorption isotherms at 77 K (Fig. 6). We observe a distinct discrepancy between adsorption isotherms of Ar and N2, although adsorption branches of Ar adsorption and N2 adsorption on SWCNT cryogels in the linear P/P0 expression are almost overlapped in the P/P0 range of 0.05 to 0.6 (Fig.6(a) and (b)). Magnified adsorption isotherms (Fig. 6(c) and (d)) explicitly show that N2 adsorption amount is much larger than Ar adsorption amount below P/P0 = 10-2; the discrepancy between adsorption amounts of N2 and Ar for SWCNT cryogels is larger than that of pristine SWCNTs. N2 molecules having the quadrupole moment are strongly adsorbed near the pore entrances in the very low P/P0 region (below P/P0 = 10-4) to block the further adsorption in the higher P/P0 region; the adsorption amount of Ar below P/P0 = 10-4 is much less than that of N2. However, adsorption amount of Ar on the adsorption branch becomes larger than that of N2 above P/P0 = 0.36; the difference between the adsorption
15
amounts of Ar and N2 of the desorption branches is more predominant than that on the adsorption branches. Thus, Ar adsorption isotherms give more evident low pressure adsorption hysteresis (Fig. 5(c) and (d)); the hysteresis gaps at P/P0 = 0.1 are 45 mL(STP) g-1 and 24 mL(STP) g-1 for SWCNT cryogels and pristine SWCNTs, respectively. The marked low pressure adsorption hysteresis gap for Ar adsorption indicates that even Ar molecules cannot access quickly to ultramicropores of these SWCNT samples on the adsorption branch. We need to use the desorption branch to obtain the better porosity, because the adsorption branch is more influenced by not sufficient accessibility at the ultramicropore entrances intervening the intrapore diffusion to larger pores. Ar adsorption provides more accurate porosity than N2 adsorption (see Table 1), as mentioned above. The total pore volume (2.55 cm3 g-1) of SWCNT cryogels is larger than that (2.01 cm3 g-1) from N2 adsorption by 0.54 cm3 g-1, being quite marked difference. The mesopore volume difference of 0.5 cm3 g-1 corresponds to 22 % of the mesopore volume of SWCNT cryogels (2.3 cm3 g-1), suggesting the presence of the abundant mesopores regulated by ultramicroporous necks. The should be associated with the 3D cage-like structures observed by TEM (Fig. 2(c) and (d)), whose size is in the mesopore range. The thin SWCNT bundles should bend to contact each other at the edges to keep such mesopore-cages and thereby some of the
16
contacting edges should work as ultramicropore entrance. Thus, the TEM and adsorption studies lead to the above pore structural image for SWCNT cryogels. The difference of the total pore volume for pristine SWCNTs is 0.36 cm3 g-1. Thus, N2 adsorption at 77 K underestimates the porosity by 21 % for SWCNT cryogels and 34 % for pristine SWCNTs at least. We must be cautious that even the porosity by Ar adsorption should be underestimated. The explicit difference in adsorption amounts of Ar and N2 is also observed for SWCNT:Zn-Al cryogels; although the N2 adsorption amount is quite small as mentioned above, Ar adsorption gives about three times larger surface area (Fig. S9). We compare the pore size distributions of SWCNT cryogels and pristine SWCNTs from Ar and N2 adsorption (Fig. 7). The clearest difference of the pore size distribution of SWCNT cryogels and pristine SWCNTs is in the mesopore region from 20 to 40 nm; SWCNT cryogels have a large distribution in the pore width range. The mesopores in SWCNT cryogels stem from the 3D-cage like structures of fine SWCNT bundles on the freeze-drying treatment, which are shown by TEM observation. Briefly speaking the pore size distributions from Ar and N2 adsorption are not so different from each other for SWCNT cryogels (Fig. 7(a)) and pristine SWCNTs (Fig.7(b)). The pore size distributions from Ar and N2 adsorption have micropores at 2 nm and small mesopores
17
of about 5 nm for both of SWCNT cryogels and pristine SWCNT; both distributions are not much different from each other. As we observed more distinct low pressure adsorption hysteresis in adsorption isotherms of Ar than that of N2, the ultramicropore size distribution should depend on the probe molecule of Ar or N2. However, even the QS-DFT cannot describe clearly the role of the nanoporous neck structures. The absolute pore volume of the ultramicroporous neck structures should be negligible in comparison with the mesopore volume even in comparison with the mesopore volumes in the SWCNT cryogels and pristine SWCNTs. In future we will challenge quantitative evaluation of the ultramicroporous neck structures and elucidate the physical properties of SWCNT cryogels. 4.
Conclusions We fabricated the SWCNT cryogels of mesoporous cage-like assembly structures
from
concentrated
SWCNT-water
inks,
being
different
from
the
previous
literatures[32,33] which used diluted SWCNT dispersants stabilized with surfactants. The Zn-Al sol-gel dispersant enables to prepare highly concentrated SWCNT-water inks. The Zn-Al complex can be more easily removed from the SWCNTs than the surfactants, leading to negligible deteriorations of SWCNTs. TEM observation and comparative pore structure analysis using adsorption of Ar at 87 K and N2 at 77 K support forming
18
the mesoscopic cage-like structural assemblies of SWCNTs in which many mesopores have necked ultramicropores, inducing explicit low pressure adsorption hysteresis. The presence of the necked ultramicropores attaching to the mesopores is supported by better accessibility of Ar at 87 K than that of N2 at 77 K. The 2.3-3 times-larger mesopore volume of the SWCNT cryogels than that of pristine SWCNTs is due to fine dispersion of SWCNTs by the Zn-Al complex. In the next stage, we will challenge preparing the films and fibers having mesoscopic cage-like structures.
Acknowledgements The work was partially supported by the Grant-in-Aid- for Scientific Research (B) (17H03039) and the JST OPERA Program (JPMJOP1722). KK is honorable to contribute to the special issue dedicated to professor Mietek Jaroniec.
19
Reference [1]
T. Fujimori, A. Morelos-Gómez, Z. Zhu, H. Muramatsu, R. Futamura, K. Urita, M. Terrones, T. Hayashi, M. Endo, S. Young Hong, Y. Chul Choi, D. Tománek, K. Kaneko, Conducting linear chains of sulphur inside carbon nanotubes, Nat. Commun. 4 (2013) 2162. doi:10.1038/ncomms3162.
[2]
D. Srivastava, E.E. Santiso, K.E. Gubbins, Pressure Enhancement in Confined Fluids: Effect of Molecular Shape and Fluid–Wall Interactions, Langmuir. 33 (2017) 11231–11245. doi:10.1021/acs.langmuir.7b02260.
20
[3]
M. Sevilla, G.A. Ferrero, N. Diez, A.B. Fuertes, One-step synthesis of ultra-high surface area nanoporous carbons and their application for electrochemical energy storage, Carbon. 131 (2018) 193–200. doi:10.1016/j.carbon.2018.02.021.
[4]
M. Salanne, B. Rotenberg, K. Naoi, K. Kaneko, P.L. Taberna, C.P. Grey, B. Dunn, P. Simon, Efficient storage mechanisms for building better supercapacitors, Nat. Energy. 1 (2016) 16070. doi:10.1038/nenergy.2016.70.
[5]
R. Futamura, T. Iiyama, Y. Takasaki, Y. Gogotsi, M.J. Biggs, M. Salanne, J. Ségalini, P. Simon, K. Kaneko, Partial breaking of the Coulombic ordering of ionic liquids confined in carbon nanopores, Nat. Mater. 16 (2017) 1225–1232. doi:10.1038/nmat4974.
[6]
A. Barroso-Bogeat, M. Alexandre-Franco, C. Fernández-González, A. Macías-García, V. Gómez-Serrano, Electrical conductivity of activated carbon– metal oxide nanocomposites under compression: a comparison study, Phys. Chem. Chem. Phys. 16 (2014) 25161–25175. doi:10.1039/C4CP03952A.
[7]
J.P. Chen, S. Wu, Acid/Base-treated activated carbons: characterization of functional groups and metal adsorptive properties., Langmuir. 20 (2004) 2233–42. doi:10.1021/la0348463.
[8]
Y. Yao, Q. Zhang, P. Liu, L. Yu, L. Huang, S.-Z. Zeng, L. Liu, X. Zeng, J. Zou,
21
Facile synthesis of high-surface-area nanoporous carbon from biomass resources and its application in supercapacitors, RSC Adv. 8 (2018) 1857–1865. doi:10.1039/C7RA12525A. [9]
F. Su, X.S. Zhao, L. Lv, Z. Zhou, Synthesis and characterization of microporous carbons templated by ammonium-form zeolite Y, Carbon. 42 (2004) 2821–2831. doi:10.1016/j.carbon.2004.06.028.
[10] Z. Geng, Q. Xiao, H. Lv, B. Li, H. Wu, Y. Lu, C. Zhang, One-Step Synthesis of Microporous Carbon Monoliths Derived from Biomass with High Nitrogen Doping Content for Highly Selective CO2 Capture, Sci. Rep. 6 (2016) 30049. doi:10.1038/srep30049. [11] A. Dasgupta, J. Matos, H. Muramatsu, Y. Ono, V. Gonzalez, H. Liu, C. Rotella, K. Fujisawa, R. Cruz-Silva, Y. Hashimoto, M. Endo, K. Kaneko, L.R. Radovic, M. Terrones, Nanostructured carbon materials for enhanced nitrobenzene adsorption: Physical vs. chemical surface properties, Carbon. 139 (2018) 833– 844. doi:10.1016/j.carbon.2018.07.045. [12] N.P. Wickramaratne, M. Jaroniec, Activated Carbon Spheres for CO 2 Adsorption, ACS Appl. Mater. Interfaces. 5 (2013) 1849–1855. doi:10.1021/am400112m.
22
[13] V.C. Srivastava, I.D. Mall, I.M. Mishra, Adsorption of toxic metal ions onto activated carbon, Chem. Eng. Process. Process Intensif. 47 (2008) 1269–1280. doi:10.1016/j.cep.2007.04.006. [14] S. Lei, J. Miyamoto, H. Kanoh, Y. Nakahigashi, K. Kaneko, Enhancement of the methylene blue adsorption rate for ultramicroporous carbon fiber by addition of mesopores, Carbon. 44 (2006) 1884–1890. doi:10.1016/j.carbon.2006.02.028. [15] Z. Fan, Q. Zhao, T. Li, J. Yan, Y. Ren, J. Feng, T. Wei, Easy synthesis of porous graphene nanosheets and their use in supercapacitors, Carbon. 50 (2012) 1699– 1703. doi:10.1016/j.carbon.2011.12.016. [16] F. Tristán-López, A. Morelos-Gómez, S.M. Vega-Díaz, M.L. García-Betancourt, N. Perea-López, A.L. Elías, H. Muramatsu, R. Cruz-Silva, S. Tsuruoka, Y.A. Kim, T. Hayahsi, K. Kaneko, M. Endo, M. Terrones, Large Area Films of Alternating Graphene–Carbon Nanotube Layers Processed in Water, ACS Nano. 7 (2013) 10788–10798. doi:10.1021/nn404022m. [17] V.T. Dang, D.D. Nguyen, T.T. Cao, P.H. Le, D.L. Tran, N.M. Phan, V.C. Nguyen, Recent trends in preparation and application of carbon nanotube– graphene hybrid thin films, Adv. Nat. Sci. Nanosci. Nanotechnol. 7 (2016) 033002. doi:10.1088/2043-6262/7/3/033002.
23
[18] Z. Fan, D.Z.Y. Tng, C.X.T. Lim, P. Liu, S.T. Nguyen, P. Xiao, A. Marconnet, C.Y.H. Lim, H.M. Duong, Thermal and electrical properties of graphene/carbon nanotube aerogels, Colloids Surfaces A Physicochem. Eng. Asp. 445 (2014) 48– 53. doi:10.1016/j.colsurfa.2013.12.083. [19] P.J.F. Harris, Z. Liu, K. Suenaga, Imaging the atomic structure of activated carbon, J. Phys. Condens. Matter. 20 (2008) 362201. doi:10.1088/0953-8984/20/36/362201. [20] K. Kaneko, C. Ishii, M. Ruike, H. Kuwabara, Origin of superhigh surface area and microcrystalline graphitic structures of activated carbons, Carbon. 30 (1992) 1075–1088. doi:10.1016/0008-6223(92)90139-N. [21] S. Wang, F. Tristan, D. Minami, T. Fujimori, R. Cruz-Silva, M. Terrones, K. Takeuchi, K. Teshima, F. Rodríguez-Reinoso, M. Endo, K. Kaneko, Activation routes for high surface area graphene monoliths from graphene oxide colloids, Carbon. 76 (2014) 220–231. doi:10.1016/j.carbon.2014.04.071. [22] N. Chotimah, A.D. Putri, Y. Ono, S. Kento, Y. Hattori, S. Wang, R. Futamura, K. Urita, F. Vallejos-Burgos, I. Moriguchi, M. Morimoto, R.T. Cimino, A. V. Neimark, T. Sakai, K. Kaneko, Nanoporosity Change on Elastic Relaxation of Partially Folded Graphene Monoliths, Langmuir. 33 (2017) 14565–14570.
24
doi:10.1021/acs.langmuir.7b03328. [23] A.D. Putri, N. Chotimah, S.K. Ujjain, S. Wang, R. Futamura, F. Vallejos-Burgos, F. Khoerunnisa, M. Morimoto, Z. Wang, Y. Hattori, T. Sakai, K. Kaneko, Charge-transfer mediated nanopore-controlled pyrene derivatives/graphene colloids, Carbon. 139 (2018) 512–521. doi:10.1016/j.carbon.2018.07.001. [24] B. Mortazavi, M.E. Madjet, M. Shahrokhi, S. Ahzi, X. Zhuang, T. Rabczuk, Nanoporous graphene: A 2D semiconductor with anisotropic mechanical, optical and thermal conduction properties, Carbon. 147 (2019) 377–384. doi:10.1016/j.carbon.2019.03.018. [25] S. Niyogi, M.A. Hamon, H. Hu, B. Zhao, P. Bhowmik, R. Sen, M.E. Itkis, R.C. Haddon, Chemistry of Single-Walled Carbon Nanotubes, Acc. Chem. Res. 35 (2002) 1105–1113. doi:10.1021/ar010155r. [26] A. Peigney, C. Laurent, E. Flahaut, R.R. Bacsa, A. Rousset, Specific surface area of carbon nanotubes and bundles of carbon nanotubes, Carbon. 39 (2001) 507– 514. doi:10.1016/S0008-6223(00)00155-X. [27] M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, Exceptionally high Young’s modulus observed for individual carbon nanotubes, Nature. 381 (1996) 678–680. doi:10.1038/381678a0.
25
[28] T.W. Ebbesen, H.J. Lezec, H. Hiura, J.W. Bennett, H.F. Ghaemi, T. Thio, Electrical conductivity of individual carbon nanotubes, Nature. 382 (1996) 54–56. doi:10.1038/382054a0. [29] S. Berber, Y.-K. Kwon, D. Tománek, Unusually High Thermal Conductivity of Carbon Nanotubes, Phys. Rev. Lett. 84 (2000) 4613–4616. doi:10.1103/PhysRevLett.84.4613. [30] S.A.C. Carabineiro, M.F. Pereira, J.N. Pereira, C. Caparros, V. Sencadas, S. Lanceros-Mendez, Effect of the carbon nanotube surface characteristics on the conductivity and dielectric constant of carbon nanotube/poly(vinylidene fluoride) composites, Nanoscale Res. Lett. 6 (2011) 302. doi:10.1186/1556-276X-6-302. [31] P. Sun, H. Yi, T. Peng, Y. Jing, R. Wang, H. Wang, X. Wang, Ultrathin MnO2 nanoflakes deposited on carbon nanotube networks for symmetrical supercapacitors with enhanced performance, J. Power Sources. 341 (2017) 27–35. doi:10.1016/j.jpowsour.2016.11.112. [32] K.H. Kim, Y. Oh, M.F. Islam, Mechanical and Thermal Management Characteristics of Ultrahigh Surface Area Single-Walled Carbon Nanotube Aerogels, Adv. Funct. Mater. 23 (2013) 377–383. doi:10.1002/adfm.201201055. [33] R. Du, Q. Zhao, N. Zhang, J. Zhang, Macroscopic Carbon Nanotube-based 3D
26
Monoliths, Small. 11 (2015) 3263–3289. doi:10.1002/smll.201403170. [34] R. Kukobat, D. Minami, T. Hayashi, Y. Hattori, T. Matsuda, M. Sunaga, B. Bharti, K. Asakura, K. Kaneko, Sol–gel chemistry mediated Zn/Al-based complex dispersant for SWCNT in water without foam formation, Carbon. 94 (2015) 518–523. doi:10.1016/j.carbon.2015.07.025. [35] K. Kaneko, C. Ishii, Superhigh surface area determination of microporous solids, Colloids and Surfaces. 67 (1992) 203–212. doi:10.1016/0166-6622(92)80299-H. [36] D. Dollimore, G.. Heal, Pore-size distribution in typical adsorbent systems, J. Colloid Interface Sci. 33 (1970) 508–519. doi:10.1016/0021-9797(70)90002-0. [37] H. Marsh, Adsorption methods to study microporosity in coals and carbons—a critique, Carbon. 25 (1987) 49–58. doi:10.1016/0008-6223(87)90039-X. [38] T. Ohba, K. Kaneko, GCMC Study on Relationship between DR Plot and Micropore Width Distribution of Carbon, Langmuir. 17 (2001) 3666–3670. doi:10.1021/la001463l. [39] A. V. Neimark, Y. Lin, P.I. Ravikovitch, M. Thommes, Quenched solid density functional theory and pore size analysis of micro-mesoporous carbons, Carbon. 47 (2009) 1617–1628. doi:10.1016/j.carbon.2009.01.050. [40] A. Jorio, M.A. Pimenta, A.G.S. Filho, R. Saito, G. Dresselhaus, M.S.
27
Dresselhaus, Characterizing carbon nanotube samples with resonance Raman scattering, New J. Phys. 5 (2003) 139.1-139.15. doi:10.1088/1367-2630/5/1/139. [41] M. Thommes, K. Kaneko, A. V. Neimark, J.P. Olivier, F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing, Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report), Pure Appl. Chem. 87 (2015) 1051–1069. doi:10.1515/pac-2014-1117. [42] F. Vallejos-Burgos, F.-X. Coudert, K. Kaneko, Air separation with graphene mediated by nanowindow-rim concerted motion, Nat. Commun. 9 (2018) 1812. doi:10.1038/s41467-018-04224-6. [43] N. Setoyama, T. Suzuki, K. Kaneko, Simulation study on the relationship between a high resolution αs-plot and the pore size distribution for activated carbon, Carbon. 36 (1998) 1459–1467. doi:10.1016/S0008-6223(98)00138-9. [44] S. Wang, D. Minami, K. Kaneko, Comparative pore structure analysis of highly porous graphene monoliths treated at different temperatures with adsorption of N2 at 77.4 K and of Ar at 87.3 K and 77.4 K, Microporous Mesoporous Mater. 209 (2015) 72–78. doi:10.1016/j.micromeso.2015.01.014.
28
Tables Table 1. Surface area and pore volume from adsorption isotherms of N2 at 77 K and Ar at 87 K. N2 adsorption at 77 K
Ar adsorption at 87 K
Sαs
SBET
VMicro
VMeso
Sαs
SBET
VMicro
VMeso
m2/g
m2/g
cm3/g
cm3/g
m2/g
m2/g
cm3/g
cm3/g
Pristine SWCNTs
330
300
0.044
0.67
290
390
0.094
0.98
SWCNT:Zn-Al cryogels
100
88
0.035
0.013
295
230
0.094
0.061
SWCNT cryogels
570
540
0.21
1.8
600
490
0.25
2.3
29
Figure captions (Figure)
Fig. 1. Optical and FE-SEM micrographs of Pristine SWCNTs (a0 – a2), SWCNT:Zn-Al complex cryogels (b0 – b2), and SWCNT cryogels (c0 – c2). Fig. 2. TEM images of pristine SWCNTs (a, b) and SWCNT cryogels (c, d). Fig. 3. Raman spectra of SWCNT cryogels, SWCNT:Zn-Al complex cryogels, and pristine SWCNTs at the laser wavelength of (a) 785 nm and (b) 532 nm. Fig. 4. TG profiles of SWCNT cryogels, SWCNT:Zn-Al cryogels, pristine SWCNTs, and Zn-Al complex measured in the air atmosphere. Fig. 5. Adsorption isotherms of N2 at 77 K. ●, ○: SWCNT cryogels, ■, □:
30
SWCNT:Zn-Al cryogels, ▲, △: Pristine SWCNTs (solid and open symbols denote adsorption and desorption, respectively). (a) Liner scale of P/P0, (b) Logarithmic scale of P/P0, (c) Magnified fig. 5(a) in the lower P/P0 region, (d) Magnified fig. 5(b) in the lower P/P0 region. Fig. 6. Adsorption isotherms of Ar at 87 K and N2 at 77 K. SWCNT cryogels: Ar (●, ○), N2(■, □), Pristine SWCNTs: Ar(▲, △), N2(▼,▽). Solid and open symbols denote adsorption and desorption branches, respectively. (a) Liner scale of P/P0, (b) Logarithmic scale of P/P0, (c) Magnified fig. 6(a) in the lower P/P0 region, (d) Magnified fig. 6(b) in the lower P/P0 region. Fig. 7. Pore size distributions of (a) SWCNT cyogels and (b) pristine SWCNTs. (●: Ar and ■: N2 for SWCNT cryogels, ▲: Ar and ▼: N2 for Pristine SWCNTs)
31
Highlights Mesoscopic cage-like structured SWCNT cryogels are fabricated Mesoporous cage-like structures are completely different from bundle structures of SWCNT Mesoporous 3D-cage like structures consist of fine SWCNT bundles Adsorption of N2 and Ar shows the presence of ultramicropores attaching to mesopores. Even Ar cannot access to some of ultramicroprous neck structures.
1