Experimental Information on the Adsorbed Phase of Water Formed in

Jan 12, 2016 - Thus far, nobody has successfully obtained the accurate information on the properties of the adsorbed phases of gases or vapors formed ...
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Experimental Information on the Adsorbed Phase of Water Formed in the Inner Pore of Single-Walled Carbon Nanotube Itself Masayasu Nishi,† Takahiro Ohkubo,*,† Koki Urita,‡ Isamu Moriguchi,‡ and Yasushige Kuroda† †

Department of Chemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan ‡ Division of Chemistry and Materials Science, Graduate School of Engineering, Nagasaki University, 1-14 Bunkyomachi, Nagasaki 852-8521, Japan S Supporting Information *

ABSTRACT: Thus far, nobody has successfully obtained the accurate information on the properties of the adsorbed phases of gases or vapors formed inside a cylindrical micropore of single-walled carbon nanotube (SWCNT) itself based on the experimental procedure. In this work, we succeeded in analyzing experimentally the properties of adsorbed nitrogen and water confined in the inner pore of SWCNT itself by opening the pore composed of close-ended SWCNT without any changes in the surface state and also by applying the unique method for characterization; both the amounts, as well as properties, of surface functional groups and the bundle structure are the same even after the treatments for introducing an open-ended structure to a close-ended one. As a result, the average pore sizes, as well as characteristic adsorption behavior, on the two types of sample were available from the analysis of respective difference adsorption isotherms of nitrogen measured at 77 K between the adsorbed amounts on the open-ended SWCNT and that on the close-ended one. The evaluated pore sizes well coincide with the results estimated by Raman data. These results strongly support that we could analyze the adsorbed phases formed only in the inner pore of SWCNTs by applying the present method. Furthermore, we could analyze the adsorbed phase of water formed inside the cylindrical micropore of SWCNTs, showing the difference in the densities of adsorbed water depending on the pore sizes from the value of bulk water; the densities of the adsorbed water were evaluated to be 0.62 and 0.71 g mL−1 for SWCNTs having average pore sizes of 1.3 and 1.7 nm, respectively, which were in harmony with those obtained by the theoretical calculations reported by other researchers. The proposed analysis method makes it possible to recognize the focused states of the adsorbed water formed inside the cylindrical micropore of SWCNT more precisely and correctly. The method proposed will shed light on the discussion related to the detailed nature of various adsorbed gases into SWCNT, to the detailed role of adsorbed species formed inside pore in various phenomena, and to the designing the useful materials based on the gained knowledge.

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INTRODUCTION Microporous carbons producing hydrophobic nanospaces have been studied from the aspects of specific adsorption nature of molecules because they can store huge amounts of molecules by their strong potential well. Since the adsorbed phase of molecules in the micropore is obviously different from that in the bulk phase, a lot of researchers have been tried to elucidate unique structure and properties of adsorbed phases formed inside micropores. For instance, the adsorbed phase of water in the micropore of activated carbons (ACs) has been studied from both experimental and theoretical techniques for a long time.1−7 However, the micropore structure of ACs is generally far from the ideally uniform structure for which it is hard to © 2016 American Chemical Society

analyze the experimental data with theoretical results obtained by using perfectly uniform micropore models. Single-walled carbon nanotube (SWCNT) is, on the other hand, a candidate to give us ideally uniform cylindrical micropores.8 Actually, the adsorbed phase formed in the cylindrical micropore using nitrogen,9,10 argon,11 hydrogen,12,13 methane,14 and water15−17 has been investigated by both experimental and theoretical techniques. However, SWCNTs generally involve not only internal micropores formed inside each tube but also interstitial Received: November 16, 2015 Revised: January 6, 2016 Published: January 12, 2016 1058

DOI: 10.1021/acs.langmuir.5b04222 Langmuir 2016, 32, 1058−1064

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Langmuir

nonlinear optical crystal inside the laser cavity) was used for excitation at a power level of 100 mW. The spectrometer was equipped with a CCD camera detector. XRD profiles were corrected using a diffractometer (MiniFlexII; Rigaku Ltd.) with Cu Kα radiation (λ = 0.154 18 nm) under 30 kV and 15 mA at room temperature. The inner spaces are confirmed by using a transmission electron microscope (JEM2010; JEOL) and high-resolution scanning transmission electron microscope (ARM200CF; JEOL) operating with at 120 kV in TEM mode. All of the samples for TEM measurements were dispersed in ethanol and then fixed on a copper grid coated with holey carbon. Nitrogen adsorption and desorption isotherms at 77 K and water adsorption and desorption isotherms at 303 K were measured using BELSORP-max (MicrotracBEL Corp.). The pore structures such as specific surface area and pore volume were determined by the subtracting pore effect (SPE) method for the high-resolution αs plot.20,21 Total acidic SFGs on SWCNTs were determined by the Boehm titration method.22 50 mg of each SWCNT was dispersed in aqueous solution of sodium hydroxide (0.1 mol L−1, 50 mL) for 48 h at 298 K followed by filtration. The filtrate was titrated using 0.1 mol L−1 hydrochloric acid.

space which can be formed between tubes leading to disagreement with the results using an ideal model. Recently, Al-zubaidi et al. have reported the properties on electric doublelayer capacitor using SWCNT where they used an open-ended SWCNT as well as a close-ended one prepared by an appropriate heat treatment in vacuum conditions.18 The method to prepare both close-ended and open-ended SWCNTs with the same surface chemistries and bundle structure can give us a chance to study the adsorbed phase formed only inside the internal pore. The properties of adsorbed phase of water restricted in hydrophobic micropores has been gathering much attention for a long time, since adsorption process of water into the hydrophobic micropores such as carbon micropores involves the formation of water cluster through which water can condense even in a hydrophobic environment. Although there are a lot of examples to analyze water adsorption phenomena into hydrophobic spaces using molecular simulation techniques as an ideal micropore model, it lacks any reports discussing the adsorbed phase of water in an ideal micropore from experimental points of view. The experimental results to discuss the water adsorption phenomena into an ideally uniform micropore can give us more appropriate understanding for the adsorption phenomena of molecules including water into hydrophobic micropores. In this paper, we report experimental information on adsorbed phase formed only in the inner pore of SWCNTs where we can obtain actual densities of adsorbed water formed inside the cylindrical micropore itself. Here, we could prepare close-ended and open-ended SWCNTs and obtained the adsorption isotherms of nitrogen and water for only internal pore of each SWCNT by the subtraction of adsorbed amounts on close-ended SWCNT from those of open-ended one. Moreover, we have succeeded in analyzing the subtracted isotherm to discuss the adsorbed phase only inside internal pore of SWCNT.

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RESULTS AND DISCUSSION Pore-Opening Procedure of the Close-Ended SWCNT. The fundamental characteristics of respective carbon nanotube samples were first explored by applying various types of methods, such as Raman spectroscopy, XRD, and TEM measurements. Figure 1 shows the Raman spectra of close-

EXPERIMENTAL SECTION

We used two kinds of SWCNTs (SO and EC type) purchased from Meijo Nano Carbon Co. Ltd. SO- and EC-type SWCNTs are synthesized by arc discharge and enhanced direct injection pyrolytic synthesis (eDIPS) methods,19 respectively; each SWCNT is denoted as Arc-SWCNT and Dip-SWCNT in the following discussion. Here, we need both open-ended and close-ended SWCNTs without any differences in tubular structure or surface properties to analyze the adsorbed phase formed in the inner micropore. Since Al-zubaidi et al. have succeeded in preparing close-ended SWCNT by the heat treatment under the evacuated condition,18 we applied the same procedure to synthesize close-ended SWCNT. Actually, close-ended Arc-SWCNT (named close-Arc-SWCNT) was prepared at 1473 K for 10 h under the evacuated condition ( 0.6) for close-ended and openended SWCNTs. Recently, Morishige et al. reported that the condensation of water into the mesopores of carbons with ordered size of mesopore whose pore size range is of 6.2−9.2 nm occurs at pressures close to or equal to saturated vapor pressure (i.e., P/P0 = 1).30 Therefore, the result of the isotherm found in the higher pressure region could be attributed to adsorption into larger micropore or smaller mesopore. The subtraction of adsorbed amounts of water on closeended SWCNTs from those of open-ended ones could provide the adsorption data focused on the internal pore of SWCNT. As mentioned above, the amounts of SFGs are negligible or the same even after the pore-opening and annealing processes. Therefore, the adsorbed amount onto SFGs in subtracted isotherm must be negligible. In subtracted isotherm of ArcSWCNT, tiny uptake and hysteresis loop were still observed at P/P0 = 0.6. Although the position of the tiny uptake on subtracted isotherm for Arc-SWCNT is almost the same as the position of Dip-SWCNT, we could not confirm the existence of SWCNTs having larger micropores in Arc-SWCNT from TEM and Raman spectra, as described in the previous part. Therefore, the tiny uptake at P/P0 = 0.6 for the subtracting data on Arc-SWCNT could be assigned to adsorption on smaller mesopore on carbon impurities as shown in Figure S1. Thus, we could assume that all water molecules must be fully filled in the cylindrical pore of Arc-SWCNT until P/P0 = 0.6. With the discussion mentioned above in mind, we calculate the 1062

DOI: 10.1021/acs.langmuir.5b04222 Langmuir 2016, 32, 1058−1064

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Society for the Promotion of Science (JSPS), the Kurata Grant awarded by the Kurata Memorial Hitachi Science and Technology Foundation, and the Kyoto Technoscience Center.

densities of adsorbed water inside internal pore of Arc-SWCNT and Dip-SWCNT to be 0.62 and 0.71 g mL−1, respectively, by taking account of both adsorbed amounts of water inside the micropore and micropore volumes. As regards these results, da Silva recently reported theoretically the relative densities to be 0.65 and 0.70 for the adsorbed water enclosed inside the (12,12) and (16,16) SWCNTs having individual diameters of 1.31 and 1.86 nm, respectively.31 Guse and Hentschke also reported that the density of adsorbed water confined in SWCNTs with the size of inside diameter between 1.1 and 2.5 nm decreases more than 20% in comparison to that of bulk water.32 From the viewpoint of the experimental data, Maniwa et al. and Paineau et al. indicated independently that the amounts of water into SWCNT having pore size of 1.4 nm gives the values between 13.6 and 15.5 wt %.24,33 These findings are in accord with our results obtained in the present experiment. As a result, it cannot be emphasized too strongly that we are successfully extracted the density exclusively focused on water confined inside the cylindrical micropore of SWCNT by taking advantage of both close-ended and openended SWCNTs samples prepared by the method developed in this work.

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CONCLUSION We investigated the adsorbed phase of nitrogen and water only inside internal pore of SWCNT. The adsorption−desorption isotherms onto internal pore of SWCNT could be obtained by subtraction of adsorption amounts of close-ended SWCNTs from those of open-ended ones. The subtracted isotherms of nitrogen exhibited the typical features of the type I(a) or I(b), and the diameters of SWCNTs obtained from isotherms agreed well with the results of Raman spectra, indicating that we could extract and analyze the internal pore of SWCNT selectively. Moreover, we could clarify the appropriate water density formed inside internal pore of SWCNT. The calculated densities of adsorbed water estimated in the present study were much lower than that in the bulk; the densities in the cylindrical micropores whose pore sizes are 1.3 and 1.7 nm are 0.62 and 0.71 g mL−1, respectively, coinciding with the theoretical results using the ideal model pores. We are convinced that our study must be applicable to various analyses using adsorption−desorption isotherms and develop an appropriate understanding of structure or properties of confined species in internal micropores of carbon materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04222. TEM image(PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel & Fax +81-86-251-7843 (T.O.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grant-in-Aid for Scientific Research (Nos. 15K05645 and 15J0859017) from Japan 1063

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DOI: 10.1021/acs.langmuir.5b04222 Langmuir 2016, 32, 1058−1064