Nanoporosity Change on Elastic Relaxation of Partially Folded

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Nanoporosity Change on Elastic Relaxation of Partially Folded Graphene Monoliths Nurul Chotimah, Austina Dwi Putri, Yuji Ono, Kento Sagisaka, Yoshiyuki Hattori, Shuwen Wang, Ryusuke Futamura, Koki Urita, Fernando Vallejos-Burgos, Isamu Moriguchi, Masafumi Morimoto, Richard Tyler Cimino, Alexander V. Neimark, Toshio Sakai, and Katsumi Kaneko Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03328 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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Nanoporosity Change on Elastic Relaxation of Partially Folded Graphene Monoliths Nurul Chotimaha,b, Austina D. Putria,b, Yuji Onoa,b, Sagisaka Kentoc, Yoshiyuki Hattoric, Shuwen Wanga, Ryusuke Futamuraa, Koki Uritad, Fernando Vallejos-Burgosa, Isamu Moriguchid, Masafumi Morimotoe, Richard T. Ciminof, Alexander V. Neimarkf, Toshio Sakaib, Katsumi Kanekoa* a. Center for Energy and Environmental Science, Shinshu University, Wakasato, Nagano, 3808553, Japan b. Faculty of Material and Chemistry Engineering, Shinshu University, Wakasato, Nagano, 380-8553, Japan c. Division of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567, Japan d. Department of Applied Chemistry, Faculty of Engineering, Nagasaki University, Bunkyo, Nagasaki, 852-8521, Japan e. Quantachrome Instruments Corporation, Kawasaki, Kanagawa 213-0012, Japan f. Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, NJ 08854-8058, USA *To whom correspondence should be addressed. E-mail: [email protected]

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ABSTRACT Fabrication of nanographene can show a promising route for production of designed porous carbons, which is indispensable for high-efficient molecular separation and energy storage applications. This process requires a better understanding of the mechanical properties of nanographene in their aggregated structure. We studied the structural and mechanical properties of nanographene monoliths compressed at 43 MPa over different time from 3 to 25 h. While in monoliths compressed over shorter time, adsorption isotherms of Ar at 87 K or N2 at 77 K exhibit a prominent hysteresis due to presence of predominant mesopores, the hysteresis becomes almost nil after the compression for 25 h. On the other hand, compression for 25 h increases the microporosity evaluated by Ar adsorption, not by N2 adsorption, indicating that 25 h compression rearranges the nanographene stacking structure to produce ultramicropores that can be accessible only for Ar. TEM, X-ray diffraction, and Raman spectroscopic studies indicated that the compression for 25 h unfolds double-bent like structures, relaxing the unstable nanographene stacked structure formed on the initial compression without nanographene sheets collapse. This behavior stems from the highly elastic nature of the nanographenes.


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INTRODUCTION Nanoporous carbons whose pore width, w, is smaller than 100 nm have been extensively applied to various technologies1. The adsorption properties of nanoporous carbons sensitively depend on the pore width. Briefly speaking, microporous carbons (w < 2 nm) are suitable for gas phase adsorption2, although mesopores (2 nm < w < 50 nm) accelerate adsorption in micropores through rapid diffusion. On the other hand, mesoporous carbons have a merit for liquid phase adsorption. Macropores enable rapid flow of gas and liquid to both micropores and mesopores; structurally-ordered macropores guarantee uniform mass transfer, which is indispensable in chemical engineering processes. Therefore, tuning microporosity and mesoporosity but preserving macroporosity is indispensable to develop high performance carbon adsorbents or membranes for target molecules and ions. Recent studies on nanographene3-7 oxides have opened new routes for production of nanographene-based porous carbons of new application potentials such as electrodes8,9, adsorbents10, membranes11, catalysts12, solar cells13 and sensors14. High surface area-graphene has been produced using KOH activation. Ruoff et al. showed that highly porous graphene produced by KOH activation exhibits unusually high capacitance15. In the preceding studies, we applied KOH activation16, 17 and ice-templating method18 to develop high surface area graphene monolith with honeycomb structured macroporosity. This has a great advantage for chemical engineering applications; having surface area of 2150 m2g-1 from subtracting pore effect (SPE) method, which can effectively remove the overestimation of the monolayer adsorption amount induced by the overlapped potential effect in the micropores19, and a wide range of pore size distribution from micro- to macropores. Tuning of the microporosity and mesoporosity of high surface area graphene monoliths is requested to obtain high performance adsorbents and


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membranes. In particular, development of highly microporous graphene monoliths with reduced mesoporosity is essential to excellent filters and collectors owing to a satisfactory mass transfer through the honeycomb structured macroporosity. High resolution transmission electron microscopic observation unveiled that the breadth of unit graphene walls of nanographene monoliths is about 10-times wider than conventional activated carbons20. Consequently, the nanoporous graphenes should have distorted sheet structure, leading to partially necked pore structures. The combined pore analysis using small angle X-ray scattering with adsorption of Ar at 87 K and N2 at 77 K evidenced predominantly inaccessible pores whose surface area is 735 m2/g, which is caused by the flexible nature of the widely developed graphene sheets21,22. Hence, the nanopore structure of the nanoporous graphene monolith (NGM) must be highly sensitive to mechanical compression. In particular, mesopores arising from a random aggregation of nanoscale graphenes should be converted to micropores by mechanical compression. Also it is well known that graphene has an excellent elastic properties23-25. We need to pay attention to elastic properties of even nanographenes on the nanopore structure control through the compression. In this study, we compressed the NGM to obtain micropore-dominant porous graphene monoliths with a minor mesoporosity. We characterized the compression NGMs with electron microscopic observation, Raman spectroscopy, X-ray diffraction, and electrical resistivity measurements. The porosity evaluation was carefully carried out for the compressed NGMs, because of partially blocked micropores between flexible graphene sheets. Hence, we evaluated the porosity change of the NGMs on compression with comparative nanoporosimetry using adsorption of Ar at 87 K and N2 at 77 K21. This is because Ar porosimetry at 87 K has the following advantages over N2 porosimetry at 77 K: The Ar probe is smaller than the N2 molecule


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(the Lennard-Jones size parameter of Ar and N2 are 0.341 nm and 0.375 nm, respectively); the measuring temperature of Ar adsorption is higher than that of N2 adsorption by 10 K, having a higher thermal energy for the intrapore diffusion barriers; Ar is a one-center molecule without quadrupole-moment, giving no underestimation of micropore size, as observed in N2. EXPERIMENTAL SECTION Production and purification of GOs GOs were prepared from natural graphite (Bay carbon graphite from Michigan U.S.A. and Madagascar graphite from Madagascar) by an improved Hummers’ method18,26,27. In a typical experiment, 5 g natural graphite was added together with 200 mL H2SO4 (96%, Wako) and 25 mL H3PO4 (85%, Wako) into a 1 L beaker followed by an addition of KMnO4 (25 g, Wako). All the operations are carried out very slowly in a fume hood and the mixture was cooled by an icewater bath. Consequently, the mixture was controlled at 311 (± 2) K and kept stirring at 200 rpm for 2 h. Afterwards, 500 mL distilled water was added slowly into the mixture followed by a 10% H2O2 solution (100 mL). The product was then repeatedly washed by 1 M HCl for 3 times through centrifugation followed by same washing process with distilled water. The obtained GO suspension was kept in an opaque bottle for further use. Preparation of NGMs The graphene monolith was prepared from a mixture of two types of GO colloid produced from Bay carbon graphite and Madagascar graphite at mass ratio of 1 to 1. GO suspension from Madagascar graphite guarantees a good mechanical property of the produced monoliths, while that from Bay carbon graphite can provide a high carbon density18. The GO colloid was then mixed with KOH at a weight ratio of KOH/C equal to 10. The graphene monoliths were produced with unidirectional freeze drying method27,28. Thereafter it was heated under pure Ar


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flow (300 mL min-1) from room temperature to the target temperature (1073 K) at 10 K min-1. Then the samples were kept at the target temperature for 1 h before cooling down to room temperature to reduce and activate the GO. The products were washed by distilled water until neutral pH (7.0 ± 0.3). Finally, the products were dried at 348 K for 24 h to obtain NGMs. Pore structure control In order to increase microporosity of NGMs, pores sizes were controlled by mechanical compression. 10-15 mg of NGMs were compressed under vacuum at 43 MPa and room temperature with various compression time (3, 9, 15 and 25 h), obtaining a compact pellet NGMs. Treated sample is expressed by NGM-β h for compressed for β h. Characterizations Porosity analysis of all activated graphene monoliths were carried out by Ar adsorption at 87 K and N2 adsorption at 77 K on a volumetric adsorber (Autosorb-IQ, Quantachrome) after pre-evacuation at 473 K for 3 h. The total surface area was determined by SPE method using high resolution αs-plot of the adsorption branch of the isotherm29 and by Brunauer-EmmettTeller (BET) method30,31 for comparison. Micropore volume (VMicro) was evaluated by using Dubinin-Radushkevich (DR) equation32,33 and mesopore volume (Vmeso) is evaluated by using Dollimore-Heal (DH) method34. Total pore volume (Vtotal) is obtained from the summation of VMicro and Vmeso. The percentage of micropore volume was determined from the ratio of VMicro to VTotal. Meanwhile, pore size distributions (PSDs) were determined using the quenched solid density functional theory (QSDFT)35 method using slit-shaped model kernel. X-ray diffraction (XRD) patterns of porous graphene samples were measured using CuKα radiation at 40 kV and 30 mA (XRD SmartLab, Rigaku Co.). Raman spectra of samples were measured by a Renishaw Raman spectrometer using 532 nm laser excitation. The nanoscale structure of the samples is


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observed by using a field emission scanning electron microscope (FE-SEM; JSM-7000F, JEOL) and a transmission electron microscopy (TEM; JEOL JEM-4000), respectively. X-ray photoelectron spectra (XPS) were measured by using a monochromatized AlKα X-ray 12 kV source (AXIS-ULTRA, Kratos). Inter-nanographene connectivity was examined by surface electrical conductivity by the four-point probe method at room temperature. RESULTS AND DISCUSSION Morphological changes and surface composition The NGM samples have distorted honeycomb structure about 10 µm in size whose walls consist of ruffled sheets. The compression orientates the ruffled sheets, as shown in Figure 1a and 1b. The compression for 25 h deforms the cylindrical macropores without distinct change in the stacked graphene walls, as shown in Figure 1c and 1d. The morphology conversion of NGM at nanoscale are shown in Figure 2. Thin graphene sheets have folded structures, which are observed as dark images, as shown in Figure 2a. The folded structure disappears after compression (Figure 2c and 2d), although there are slightly folded structures at the edges (Figure 2c); the compression extends the folded structure. TEM images show that compression can tune the porosity by rearranging nanographene structures without great damage and preserve the fundamental structure. More SEM and TEM images of the nanographene monolith before and after the compression treatment are provided in Figure S5 and Figure S6. The surface composition of samples was determined from the XPS intensities of C1s and O1s. The atomic percentages of carbon and oxygen for samples are as follows: C: 66 ± 1 % and O: 34 ± 1 % for GO, C: 95 ± 1 % and O: 5 ± 2 % for NGM-none, C: 94 ± 1 % and O: 6 ± 2 % for NGM-9h, and C: 94 ± 1 % and O: 6 ± 2 % for NGM-25h. NGM-none and compressed NGM samples have similar carbon and oxygen compositions. GO has the larger oxygen content, being


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greatly different from others. Consequently, the compression treatment does not change the surface chemistry.

Figure 1. SEM images of NGM samples. (a, b): non-compressed, (c, d): NGM-25h.

Figure 2. TEM images of NGM samples. (a, b): non-compressed, (c, d): NGM-25h.


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Pore structural changes Since Ar adsorption at 87 K is more appropriate for characterization of micropores than N2 adsorption at 77 K, we compared both adsorption data on the pore structure-tuned NGMs. The compression decreases the N2 adsorbed amount (See Figure 3), in particular, in the higher P/P0 region, shrinking the adsorption hysteresis loop above P/P0 = 0.4. This indicates a marked reduction of mesopores by compression. Compression during 9-25 h results in an adsorption isotherm close to IUPAC type 1a1, indicating the presence of predominant micropores. The longer the compression time, the smaller the adsorption amount, although the compression for 25 h leads to an evident inversion effect in the adsorption amount; the adsorption amount for 25 h is larger than that for 15 h. This indicates the relaxation of the illstacking structure due to the enforcedly bent nanographenes, which will be discussed later. At the same time, compression gives rise to a marked low-pressure adsorption hysteresis which is ascribed to restricted diffusion in ultramicropores whose pore width is smaller than 0.7 nm. Accordingly, the compression increases the ultramicropores. 1200

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Figure 3. N2 adsorption isotherms at 77 K of NGMs compressed for different times. Closed and open markers indicate adsorption and desorption branches, respectively.


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The first compression must enforcedly orientate or bend nanographene sheets to induce illstacked nanographene structures. The general tendency of the change in the Ar adsorption isotherm by the compression is similar to that in the N2 adsorption isotherm except for change in the adsorbed amount against compression time over 15 h (see Figure 4). Compression during 25 h increases the Ar adsorbed amount by 13 % against the amount of NGM compressed during 15 h above P/P0 = 0.8. The initial compression should form ill-stacked nanographene units such as doubly bent graphene sheets. The inversion of Ar adsorbed amount between 15 h and 25 h should be associated with relaxation of unstable stacking structures of elastic nanographenes, producing ultramicropores where N2 molecules are not accessed at 77 K, but Ar at 87 K. The comparative adsorption study with Ar at 87 K and N2 at 77 K gives essential information on the elastic nature of nanographenes and relaxation process of ill-stacked nanographenes under compression.

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Figure 4. Ar adsorption isotherms at 87 K of NGMs compressed for different times. Closed and open markers indicate adsorption and desorption branches, respectively.


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Here, we must obtain information on the interaction between a probe molecule and pores from their low pressure adsorption isotherms of NGM compressed for 25 h and non-compressed NGM in terms of log P/P0 (see Figure 5). As the N2 molecule has a quadrupole moment which interacts with surface functional groups, N2 adsorption below P/P0 = 10-3 is much larger than Ar adsorption at these range of pressures. There is a crossing point at P/P0 = 2x10-3 in Ar and N2 adsorption isotherms of NGM-25 h. Similar crossing is observed for NGM-15 h (see Figure S1). This crossing arises from the quadrupole-moment mediated N2-carbon interaction and better accessibility of Ar in ultramicropores, as will be discussed later. This quadrupole moment effect of N2 brings about the adsorbed amount larger than Ar, leading to underestimation of the micropore size by N2 adsorption. Comparing nanoporosimetry using adsorption of Ar at 87 K and of N2 at 77 K is indispensable to evaluate accurately the nanoporosity, as discussed below. 1200 NGM-none (Ar) NGM-none (N2) NGM-25h (Ar) NGM-25h (N2)

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Figure 5. N2 and Ar adsorption isotherms of NGM-none and NGM-25h. Closed and open markers denote adsorption and desorption branches, respectively.


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Importance of comparative porosimetry in elastic nanographene-based nanopores We constructed high resolution αs-plots for adsorption isotherms of Ar and N2 on all samples (see Figure 6). αs-plots have two upward swings below and above αs = 0.5, indicating a wide distribution of micropores. The SPE surface area (Sαs) was determined by these αs-plots. The BET surface area, SBET, was also evaluated for comparison, although the SBET is overestimated for micropores larger than ~ 0.7 nm35. Briefly speaking, longer compression decreases Sαs and increases the micropore volume percent (see Figure 7). Ar adsorption gives a different tendency from N2 adsorption for both of Sαs and pore volume percent of NGM-25h. This should come from the presence of ultramicropores where N2 cannot access, but Ar can.

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800 600 400 NGM-none (Ar) NGM-none (N2) NGM-25h (Ar) NGM-25h (N2)

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Figure 6. αs -plots from Ar and N2 adsorption isotherms of NGM-none and NGM-25h. Compression for 25 h should relax the ill-stacked structure of partially bent nanographenes into a considerably well-stacked one having small micropores accessible only for Ar. This fact demonstrates the crucial necessity of the comparative porosimetry using combined Ar and N2 adsorption for NGM samples having elastic pore walls22. These parameters are summarized in


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Table 1, in which the Sαs is lower than the SBET and VMicro increases more evidently than VMeso after the compression.

Figure 7. Plots of surface area and micropore volume of NGMs samples against compression time from Ar and N2 adsorption isotherms. 1.0 NGM-none (Ar) NGM-none (N2) NGM-25h (Ar) NGM-25h (N2)

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Figure 8. PSD from Ar and N2 adsorption isotherms of NGM-none and NGM-25h. Basically all samples have both micropores of around 0.8 nm and small mesopores of less than 4 nm; compression decreases intensively the mesoporosity but not so much the microporosity (See Figure 8). These PSD profiles show that the compression efficiently reduces the contribution of small mesopores. We must note the importance of Ar adsorption. The peak positions of micropores and mesopores are almost similar from Ar and N2 adsorption, but the


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pore volumes from Ar adsorption are much larger than those of from N2 adsorption even for nontreated NGM. Table 1. Surface area determined from αs-plot method (Sαs) and BET method (SBET), micropore volume (VMicro), and mesopore volume (VMeso), determined by using Ar adsorption isotherms at 87 K and N2 adsorption isotherms at 77 K.

Ar adsorption at 87 K Sample NGM-none NGM-3h NGM-9h NGM-15h NGM-25h

N2 adsorption at 77 K

Sαs

SBET

VMicro

VMeso

Sαs

SBET

VMicro

VMeso

/m2 g-1

/m2 g-1

/cm3 g-1

/cm3 g-1

/m2 g-1

/m2 g-1

/cm3 g-1

/cm3 g-1

1760 1290 1230 1200 1380

2040 1380 1330 1260 1480

0.76 0.55 0.52 0.48 0.55

0.70 0.36 0.28 0.25 0.30

1730 1540 1300 1230 1220

2050 1830 1440 1340 1370

0.74 0.66 0.51 0.48 0.47

0.55 0.27 0.23 0.20 0.19

Elasticity-mediated relaxation of ill stacked nanographenes The above comparative porosimetry with adsorption of Ar at 87 K and N2 at 77 K gives essential information on the relaxation mechanism of ill-stacked nanographene structure stemmed from the initial compression. Some of nanographene sheets of the non-treated NGM should form randomly and highly strained stacked structures20. The interstices in the randomly aggregated nanographenes mainly offer mesopores and the intergraphene spaces provide micropores; the mesoporosity is much predominant compared with the microporosity. The initial compression leads to locally bent nanographene sheets, provides mesopores and highly necked ultramicropore structures. Compression during 25 h relaxes the enforcedly bent nanographene sheets to give mutually stacked layers, as suggested by XRD and Raman spectroscopy (see Figure S2 and Figure S3). NGM-25h has the best crystallinity of


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nanographenes according to XRD. Also, Raman spectroscopy shows that the D/G band intensity ratio of NGM-none and NGM-25 is 1.1, indicating that compression does not collapse the nanographene structures. The subsidiary compression experiment for 40 h supports the abovementioned mechanism (Figure S9 and S10, and Table S5). The sheet electrical resistivity of the pelletized NGMs decreases markedly from 1000 Ω/sq to 30 Ω/sq (Figure S4) with compression time, supporting better stacking structure of nanographenes by the compression for a longer time. Thus, long time compression relaxes the defective nanographene stacking structures having bent nanographenes without serious damages. Figure 9 shows a relaxation model of stacked nanographenes. The initial compression forms a doubly bent layer of enough elasticity between distorted nanographenes, providing mesopores; the longer compression induces restoring of the bent layer to a better stacked structure having ultramicropores in which N2 cannot, but only Ar can be accessible. Thus, the observed Ar and N2 adsorption behaviors stem from the high elasticity nanographenes.

Figure 9. Relaxation model of stacked nanographenes containing doubly bent nanographene with compression. CONCLUSIONS The initial compression of porous nanographene monoliths provides mesopore-rich porosity due to the presence of the bent structure of nanographenes. However, the nanographene has enough elasticity to restore the bent layer to the stretched graphene sheet under a long


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compression for 25 h. The restoring of the bent layers in the nanographene aggregates decreases the mesoporosity, whereas it increases the ultramicroporosity in which N2 cannot access at 77 K, but Ar does at 87 K because of the well-stacked nanographene structure. The relaxation model of the unstable stacked nanographene structure containing the doubly bent nanographene sheet is suggested on the basis of integrated information from TEM, Raman spectroscopy, X-ray diffraction, and electrical resistivity measurement in addition to the comparative micro- and meso-porosimetry using adsorption of Ar at 87 K and N2 at 77 K. ACKNOWLEDGMENTS This work was partial supported by JST CREST “Creation of Innovative Functional Materials with Advanced Properties by Hyper-nano-space Design” and the grant by the Center of Innovation Program from JST. N. C. and A. D. P. were supported by scholarship provided by Quantachrome Co. REFERENCES 1.

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Nanoporosity Change on Elastic Relaxation of Partially Folded

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