Enhanced CO2 Adsorptivity of Partially Charged Single Walled

Article pubs.acs.org/JPCC

Enhanced CO2 Adsorptivity of Partially Charged Single Walled Carbon Nanotubes by Methylene Blue Encapsulation Fitri Khoerunnisa,†,⊥ Toshihiko Fujimori,† Tsutomu Itoh,† Koki Urita,§ Takuya Hayashi,† Hirofumi Kanoh,‡ Tomonori Ohba,‡ Sang Young Hong,∥ Young Chul Choi,∥ Sri Juari Santosa,¶ Morinobu Endo,† and Katsumi Kaneko*,† †

Research Center for Exotic Nanocarbon, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan Department of Chemistry, Graduate School of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan § Department of Applied Chemistry, Nagasaki University, 1-14 Bunkyo, Nagasaki 852-8521, Japan ∥ Research and Development Center, Hanwha Nanotech, 423-1 Cheongcheon-dong, Bupyeong-gu, Incheon 403-030, Republic of Korea ⊥ Department of Chemistry Education, Indonesia University of Education, Bandung 40154, Indonesia ¶ Department of Chemistry, Gadjah Mada University, Jogjakarta 55281, Indonesia ‡

S Supporting Information *

ABSTRACT: We prepared a partially charged single walled carbon nanotube (SWCNT) by charge transfer-mediated encapsulation of methylene blue (MB) molecules, which enhances the CO2 adsorptivity. The liquid phase adsorption of MB molecules on SWCNT could give the MB-encapsulated SWCNT, which was evidenced by the remarkable depression of the X-ray diffraction intensity from the ordered bundle structure, the decrease of N2 and H2 adsorption in the internal tube spaces of SWCNT, and the high-resolution transmission electron microscopic observation. The molecular spectroscopic examination revealed the charge transfer interaction between the encapsulated MB molecules and SWCNT. The electrical conductivity increased by the encapsulation of MB suggested the electron transfer from SWCNT to MB molecules, giving rise to positively charged SWCNT. The enhancement of CO2 adsorption by the MB-encapsulation coincided with the positively charged SWCNT.

1. INTRODUCTION Single walled carbon nanotubes (SWCNT) are one of the most prospective materials because of nanometric dimension, unique structure, and unusual electronic, thermal, mechanical, and optical properties that can be applicable to create new sustainable technologies.1 The adjustment of SWCNT properties is essential to improve their performance, developing the intriguing applications. Several approaches to modify the properties of SWCNT have been proposed.2 As all carbon atoms of SWCNT are faced to internal and external tube interfaces, SWCNT should be understood as a surface solid. SWCNT can offer concave and convex interfaces, which have a promising application potential to a variety of chemistry fields. Hence, chemical and physical modifications of SWCNT walls could strengthen the application potential in chemistry. Chemical modification is an acceptable strategy for providing a new functionality to SWCNT. It is well-known that chemical modification improves the solubility of carbon nanotubes in water and organic solvents, facilitating a good processability.3 In addition, the selective interaction between chemical species and carbon nanotubes can lead to an efficient chirality-controlled separation of SWCNT.4 SWCNT can be chemically modified © 2012 American Chemical Society

through covalent interaction with the functional entities bound directly to the nanotubes surface or through noncovalent interaction where the functional molecules are only adsorbed on the nanotube surfaces without structural change of the tube frame.5 Particularly, modification of SWCNT using aromatic moieties through covalent or noncovalent interaction is very promising. Polycyclic aromatic hydrocarbons such as anthracene have successfully tuned the physical and chemical properties of carbon nanotubes owing to the charge transfer interaction.6 The charge transfer interaction can donate electrical charges on the SWCNT walls. Consequently, surface chemical properties of SWCNT such as molecular adsorption, catalysis, and dispersion in solution could be markedly influenced by the charge transfer interaction. The charge-transfer mediated encapsulation of aromatic molecules in the internal tube space of SWCNT is a promising route for controlling the interfacial chemistry of SWCNT. Social demand for solar Received: April 16, 2012 Revised: May 5, 2012 Published: May 7, 2012 11216

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Figure 1. Adsorption isotherm of MB on SWCNT at 298 K (a) and molecular structure of methylene blue (b).

concentration range of 0.2 to 60 mg L−1 for adsorption measurement. The dispersed SWCNT samples were filtered with a Millipore porous filter (0.45 μm) and washed by ethanol to remove any nonadsorbed MB molecules. The adsorption isotherm of MB on SWCNT was determined by measurement of the concentration change of MB solution using the maximum absorbance at 660 nm with an aid of UV-NIR spectroscopy (JASCO; V-670). The MB-adsorbed SWCNT samples were dried before characterization. 2.3. Characterizations. The X-ray diffraction patterns were measured at room temperature using the synchrotron X-ray with a radiation wavelength of 0.09976 nm at the Super Photon Ring (Spring-8, Hyogo, Japan); the powder samples were set in a Lindemann glass capillary (0.70 mm in an external diameter). The SEM and TEM images were taken by means of field emission scanning electron microscope (FE-SEM; JEOL, JSM6330F) under an emission current of 12 μA and high-resolution transmission electron microscopy (HR-TEM; JEOL, JEM2100F), respectively. The nanoporosity and adsorption activity change of SWCNT with MB adsorption were examined with N2 and H2 adsorption at 77 K and CO2 adsorption at 283 to 303 K using a volumetric apparatus (Quantachrome) after preheating at 423 K and 10−4 Pa for 2 h. The pore structure parameters were obtained by the subtracting pore effect (SPE) method.11 Infrared spectra over the range of 1000−4000 cm−1 were obtained by placing the sample on the KBr plate with the aid of a FTIR spectrometer (JASCO; FT/IR-410) to understand the adsorbed state of MB molecules. Each spectrum was 40 scans collected at a resolution of 2 cm−1. Two types of Raman spectrometer, i.e., JASCO (NRS-3100) equipped with a YAG laser (power 1.5 mW, wavelength 532 nm) and Renishaw (InVia Raman microscope) equipped with a diode laser (power 0.3 mW, wavelength 785 nm), were applied. Samples were exposed for 3 min with triple accumulations, and spectra were taken at three different places on the surface of each sample for better reproducibility. The fabrication of SWCNT and MBadsorbed SWCNT films were carried out by the spray-coating method for optical absorption and electrical conductivity measurement, where the SWCNT and MB-adsorbed SWCNT dispersions were coated on polyethylene terephthalate (PET) substrate. The optical absorption spectra were collected using a UV−vis-NIR spectrometer (JASCO; V-670). The DC electrical conductivity of SWCNT and MB-adsorbed SWCNT films about 0.1 μm in thickness was measured using the four probes apparatus (Mitsubishi Chemical Analytech; Loresta-GP; MCP-T610). The dispersion stability of SWCNT

energy-mediated chemistry needs fundamental understanding on the interaction between SWCNT and a dye molecule in order to utilize light over a wide wavelength range. Also a dye molecule has the π-conjugated electronic structure, and thereby, chemical modification of SWCNT with dye molecules is indispensable to develop carbon nanotube-associated solar chemistry. Current researches on application of dye molecules to electrochemical reaction and photosensitizer have been reported.7 The entrapment of the dye molecules in internal tube spaces of SWCNT could extend the surface chemical applicability of SWCNT. Methylene blue (MB) being a kind of basic dyes with planar structure (1.432 nm × 0.665 nm × 0.407 nm) belongs to the phenothiazine compound, as shown in Figure 1b. It exhibits an excellent photosensitive function and distinctive electrochemical properties. The molecular structure of methylene blue is close to polycyclic aromatic hydrocarbon with π-conjugated nature, being a candidate for specific interaction with SWCNT. Concentration and storage of CO2 by nanoporous materials are one of the key research targets for sustainable technology. We need to elucidate essential factors to enhance the adsorptivity of nanomaterials for CO2. As CO2 has a large quadrupole moment,8 modification of the SWCNT walls with surface charges should enhance the adsorptivity for CO2. Thus, encapsulation of methylene blue molecules in the internal tube spaces with charge transfer interaction could influence markedly the CO2 adsorptivity of SWCNT. This article describes encapsulation of methylene blue molecules in the tube spaces of SWCNT, enhancing CO2 adsorptivity.

2. EXPERIMENTAL SECTION 2.1. Materials. A cationic dye, MB, having molecular formula C16H18N3SCl·3H2O and molecular weight of 373.90 g mol−1 was purchased from Wako Pure Chemical Industries Co., Ltd. The MB was dissolved in ethanol to provide different concentration of MB concentration up to 60 mg L−1. We used SWCNT (ASP-100F) synthesized by the arc-discharge method (Hanwha Nanotech Co., Ltd.), which contains metal catalyst of 10 wt % and graphite impurities. Prior to the adsorption experiment, the SWCNT was oxidized under a mixed nitrogen−oxygen gas at 750 K for removing the amorphous carbons as well as their caps. The treated-SWCNT is denoted as SWCNT for further description. 2.2. Encapsulation of Methylene Blue in SWCNT. Encapsulation of MB molecules in SWCNT was carried out through liquid phase adsorption at 298 K. The SWCNT of 1 mg was dispersed ultrasonically in MB solution over the 11217

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distance of the bundle SWCNT is estimated to be 1.7 ± 0.1 nm. Unfortunately, this TEM observation cannot distinctly show the encapsulated MB molecules. The bird’s-eye view of the MB-adsorbed SWCNT indicates an ill-ordered bundle structure due to the partial unraveling. The partial unraveling of the SWCNT bundles also reduces the crystallinity of the ordered bundle structure, which is partially associated with the observed X-ray diffraction intensity decrease. The porosity change of SWCNT on MB adsorption was evaluated by nitrogen adsorption at 77 K. The adsorption isotherm of nitrogen on SWCNT is of IUPAC type II (Figure 4). The initial adsorption uptake stems from micropore filling of N2 in the internal tube spaces. The adsorption of nitrogen at higher P/Po derives from multilayer adsorption on the external surface of the bundles and in larger mesopores and macropores due to the interbundle gaps. The MB adsorption-treatment decreases the initial adsorption uptake remarkably and the adsorption isotherm of MB-adsorbed SWCNT just shifts downward, indicating that MB molecules are preferentially adsorbed in the internal tube spaces of micropores as the strongest adsorption sites in the SWCNT bundles. The pore structure parameters were evaluated with the subtracting pore effect (SPE) method using αs plots of nitrogen adsorption at 77 K.11 The pore structure parameters of SWCNT and MBadsorbed SWCNT are summarized in Table 1. The mesopore volume was obtained by subtracting the micropore volume from the total pore volume. The MB adsorption treatment decreases the micropore volume by 0.10 mL g−1. The X-ray diffraction examination showed that MB molecules cannot be adsorbed in the narrow interstitial space and MB molecules must be adsorbed in the internal tube spaces due to the X-ray diffraction intensity decrease. As the surface area decreases by the MB adsorption, the bundle size decrease by the partial unraveling is not the primary reason for the observed depression of the X-ray diffraction intensity. The possible excluded volume of adsorbed MB molecules corresponding to the 0.18 filling is evaluated to be 0.10 mL g−1, agreeing with the observed decrease in the micropore volume. Consequently, N2 adsorption data intensively support that MB molecules are selectively adsorbed only in the internal tube spaces. Accordingly adsorption of MB molecules on SWCNT can give the MB encapsulated SWCNT. Adsorption of supercritical H2 at 77 K also proves selective filling of smaller micropores with MB molecules because supercritical H2 can be preferentially adsorbed only in the smaller micropores having stronger interaction potential with an H2 molecule as shown in Figure 5.12 MB adsorption remarkably reduces the H2 adsorption amount on SWCNT almost by 57%. Using the assumption that density of H2 adsorbed in micropores is equal to the bulk liquid density (0.078 g mL−1) at 20 K, the adsorption depression due to the MB adsorption corresponds to 0.13 mL g−1, being close to that of N2 adsorption. Accordingly, N2 and H2 adsorption data support that MB molecules are adsorbed in the internal tube spaces of the SWCNT. The plausible encapsulation model of MB molecules in the internal tubes spaces of SWCNT is shown in Figure 6. It is well-known that bundled SWCNT provides the adsorption sites favorable for gas adsorption. In particular, the interstitial sites and internal tube spaces are preferential adsorption sites.13 Only the internal tubes space, however, can be available for adsorption of MB molecules due to the geometrical restriction of the interstitial sites. When MB molecules are introduced

and MB-adsorbed SWCNT was directly observed at 298 K at 12 h after dispersion treatment.

3. RESULTS AND DISCUSSION 3.1. Methylene Blue Encapsulated SWCNT. The adsorption isotherm of MB on SWCNT at 298 K is Langmuirian, as shown in Figure 1a, indicating highly siteselective adsorption. The MB adsorption is almost saturated around 25 mg L−1 of MB equilibrium concentration. The saturated adsorption amount is 110 mg g−1, being 0.18 of the fractional filling in pores, which was evaluated with the molecular complex model after the preceding articles.9 The amount of MB embedded in SWCNT can be also expressed by (SWCNT)0.996(MB)0.004; the embedded amount of MB in molar unit is quite small. The MB adsorption treated SWCNT having MB molecules of 0.18 filling was characterized after drying. The SEM image of SWCNT exhibits a highly bundled structure (Supporting Information, Figure S1). The bundles are randomly entangled in each other. The MB adsorption treatment of SWCNT induces a partial unraveling of bundle structure. The disentanglement of SWCNT bundles suggests the specific interaction between MB molecule and SWCNT. Figure 2 shows the characteristic X-ray diffraction pattern of SWCNT, which is assigned to the two-dimensional hexagonal

Figure 2. X-ray diffraction patterns of SWCNT (black line) and MBadsorbed SWCNT (red line).

lattice of the bundled structure and is dominated by the 10 peak. The other diffraction peaks at the scattering factor of 0.70, 0.85, and 1.10 Å−1 correspond to the ordered bundle structure of the hexagonal arrays, and the peak at around 1.90 Å−1 is attributed to the well-defined reflection from the graphite 002 lattice planes.10 The MB adsorption slightly shifts the peaks position and reduces their diffraction intensity especially the 10 peak, strongly indicating random encapsulation of MB molecules in the internal tube spaces and the decrease in the bundle size, as suggested by the SEM observation. Therefore, MB molecules are scarcely inserted in the interstitial spaces of the SWCNT bundles to vary the lattice parameters of the hexagonal bundle arrays. Encapsulation of MB in the SWCNT was examined by the high-resolution transmission electron microscopy (HR-TEM). Figure 3 shows the HR-TEM images of SWCNT and MB-adsorbed SWCNT. The bird’s-eye and cross-section views of SWCNT indicate the highly ordered and uniform bundle structure of nanotubes where the intertubes 11218

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Figure 3. HR-TEM images of SWCNT (a,c) and MB-adsorbed SWCNT (b,d).

Figure 4. N2 adsorption isotherms at 77 K on SWCNT and MB-adsorbed SWCNT. Linear (a) and logarithmic (b) plots: SWCNT (black square); MB-adsorbed SWCNT (red circle).

Table 1. Pore Structure Parameters of SWCNT Sample Determined by SPE Method microporosity

mesoporosity

sample

αt (m2 g−1)

Vt (mL g−1)

αmicro (m2g−1)

Vmicro (mL g−1)

αext (m2 g−1)

Vmeso (mL g−1)

SWCNT MB−SWCNT

1160 640

0.99 0.75

840 410

0.24 0.14

320 230

0.75 0.61

charges on the SWCNT should assist the unraveling of the bundle structure. 3.2. Molecular Spectroscopic Evidence on Encapsulation of MB Molecules. The FTIR spectrum of SWCNT (Supporting Information, Figure S2) shows the typical vibration modes of carbon nanotubes at around 3400, 2920,

inside tube spaces, it must interact with the internal tube wall of the negative curvature. The MB molecules having a πconjugated structures must give rise to a charge transfer interaction with the aromatic frame of nanotubes wall, resulting in the effective charges on the SWCNT walls. The partial 11219

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molecules and the inner wall of nanotubes, leading to the stiffen effect on the carbon−carbon bond and the perturbation of radial flexibility of the aromatic rings in the tubes structure16 owing to the weak charge transfer interaction (Supporting Information, Figure S3). This shrinkage of the tube structure on MB-adsorption is too small to detect by porosity measurement. The notable shift of the G′ band of SWCNT after MB adsorption discloses the structural change of SWCNT due to enhancement of defects since the G′ band derived from the first overtone of the D band. Regarding the modification of electronic structure, the RBM peak of SWCNT at low frequency corresponds to the metallic nanotubes according to the Kataura plot.17 These Raman spectroscopic examinations indicate the weak charge transfer between MB molecules and SWCNT (Supporting Information, Figure S4; Table S1). One can suggest that the π-stacking interaction between the aromatic ring of MB and SWCNT extends the π-conjugated system for the charge transfer interaction.18 It is well recognized that the optical response of SWCNT is dominated by the transition between pairs of van Hove singularities in the electron density of state.19 Chemical modification of SWCNT varies the electronic structure of carbon nanotubes.17 The electronic structure change of SWCNT with the MB encapsulation is also shown with optical absorption spectroscopy. Figure 7 shows the optical absorption

Figure 5. H2 adsorption isotherms at 77 K on SWCNT (black square) and MB-adsorbed SWCNT (red circle).

Figure 6. Plausible encapsulation model of MB molecules.

1600, 1380, and 1100 cm−1 corresponding to the stretching vibration modes of O−H, C−H of methylene group, CC of benzene ring, C−H of alkanes, and oxygen related functional groups, respectively.14 The MB adsorption on SWCNT induces the remarkable higher-frequency shifts, especially at 3448, 1636, and 1120 cm−1 corresponding to the stretching vibration modes of secondary amine, CC of benzene ring, and oxygen related functional group, respectively, suggesting that the interaction of MB molecules with carbon nanotubes predominantly involves the aromatic nanotube frame and secondary amine moieties. The Raman spectra summarized in Table 2 give an evidence of structural change of SWCNT induced by MB adsorption treatment. In particular, the radial breathing mode (RBM) frequency can be related to diameter of SWCNT by means of the relationship ωr = 248 nm cm−1/d (nm).15 The MBadsorbed SWCNT significantly shifts the RBM peak to higher frequency because of the specific interaction between MB

Figure 7. Optical absorption spectra of SWCNT (black line) and MBadsorbed SWCNT (red line).

spectra of SWCNT and MB-adsorbed SWCNT; the optical absorption spectrum of SWCNT has the absorption bands at 1750 nm (S11), 980 nm (S22), and 680 (M11), originating from semiconducting and metallic transition of van Hove singularities.20 The MB adsorption induces the notable shifts and hyperchromic effect in both the metallic and semiconducting

Table 2. Raman Spectral Parameters RBM (cm−1)

sample (λ = 785 nm) SWCNT MB−SWCNT (λ = 532 nm) SWCNT MB−SWCNT

D band (cm−1)

G band (cm−1)

152 159

166 170

306 319

1297 1300

1549 1550

1569 1568

170 178

1342 1342

1552 1553

1569 1565

1590 1589

20.9 35.1

11220

1591 1590

G/D

G′ band (cm−1)

13.9 14.2

2578 2584

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transitions of SWCNT due to a perturbation in the extended πnetwork of nanotubes, indicating modification of electronic structure of SWCNT in density of state (DOS) of van Hove singularities through the charge transfer interaction. Furthermore, fine structures of these absorption bands are specifically related to the distribution of diameter and chiralities of carbon nanotubes.21 The correlation between the transition energy and diameter can be evaluated using the simple tight-binding (STB) approximation (M11 = 6aγ/d, S22 = 4aγ/d, and S11 = 2aγ/d; where a, γ, and d, are carbon−carbon bond length (0.144 nm), the tight-binding nearest neighbor overlap integral (2.9 eV), and tube diameter, respectively.22 Then, the transition energy can be calculated using the tube diameter from the RBM frequency. The theoretical and observed values of band gap transitions are close to each other (Supporting Information, Table S2). 3.3. Unique Characteristics of SWCNT Induced by Encapsulation of MB Molecules. The above results strongly evidence the presence of the charge transfer interaction between MB molecules and SWCNT. As the charge transfer interaction induces charges on the SWCNT walls, an electrostatic repulsion between the MB-encapsulated SWCNT should stabilize the dispersion of the MB-adsorbed SWCNT in aqueous solution. The dispersion stability of SWCNT is highly improved by the MB encapsulation. This macroscopic evidence is given in Supporting Information Figure S5. Moreover, MB adsorption increases the electrical conductivity of SWCNT, indicating the charge transfer interaction between SWCNT and MB molecules as shown in Table 3. As major carriers of

Figure 8. CO2 adsorption isotherms on SWCNT and MB-adsorbed SWCNT denoted by black and red lines, respectively, at 283 K (circle), 293 K (square), and 303 K (triangle).

4. CONCLUSIONS We have shown the unique interfacial properties of SWCNT charged by methylene blue encapsulation. The MB molecules were effectively encapsulated in the internal tube spaces of SWCNT, which is proven by N2 and H2 adsorption uptake at 77 K. It is noteworthy that embedded MB can give a marked influence on various properties of SWCNT, although the molar ratio of the embedded MB is only 0.0035. The Raman scattering and optical absorption spectroscopic examinations revealed the presence of charge transfer interaction between MB molecules and nanotube walls. The enhancement of electrical conductivity and dispersion stability by the methylene blue encapsulation was evidently observed. Furthermore, a remarkable increase of CO2 adsorption capacity was shown due to the additional interaction of the quadrupole moment of CO2 with local electric field of the pore wall stemming from the charge transfer interaction.

Table 3. DC Electrical Conductivity of SWCNT with Filling Ratio of Pores with MB filling ratio

electrical conductivity (S·cm−1)

0 0.10 0.18

4.6 × 10−3 6.1 × 10−3 9.7 × 10−3

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SWCNT are holes, the DC electrical conductivity increase suggests the charge transfer from SWCNT to MB molecules. Then, the SWCNT walls should be positively charged. The effect of the positively charged walls is evidently shown in CO2 adsorption. The CO2 adsorption is an effective technique to provide the information on narrow micropores and to probe the specific interaction on the pore walls.23 In addition, Urita et al. reported that CO2 adsorption on pelletized SWCNH led to significant electrical conductivity response.24 Furthermore, adsorption of CO2 having a large quadrupole moment is sensitive to the surface electrical field coming from the pore wall charges. The adsorption isotherms of CO2 (Figure 8) shows the higher CO2 adsorption uptake on SWCNT at lower temperature because CO2 is physically adsorbed, and CO2 molecules even at 283 K easily diffuse into very narrow pores due to their larger kinetic energy. Interestingly, the encapsulation of MB significantly increases the CO2 adsorption capacity at 283 and 293 K, indicating the presence of an enhanced interaction between CO2 molecules and the pore walls, that is, the interaction between the quadrupole moment of CO2 and the local electrical field of the positively charged wall. Consequently, the addition of charges on the pore walls can produce a better CO2 adsorbent.

ASSOCIATED CONTENT

S Supporting Information *

FE-SEM, FTIR spectra, radial breathing modes, Raman spectra, and dispersion stability of SWCNT and MB-adsorbed SWCNT. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-(0)26-269-5743. Fax: +81-(0)26-269-5737. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The experimental part of this research was funded by a Grantin-Aid for Scientific Research A (21241026) from the Japanese government. K.K., T.F., T.I., and M.E., were supported by Exotic Nanocarbons, Japan Regional Innovation Strategy Program by the Excellent, JST. We thank the great support for F.K. from Global Center of Excellent (G-COE) Chiba University (G-03, MEXT) and scholarship by Graduate School of Science, Chiba University. 11221

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