Water Encapsulation Control in Individual Single-Walled Carbon

Letter pubs.acs.org/JPCL

Water Encapsulation Control in Individual Single-Walled Carbon Nanotubes by Laser Irradiation Shohei Chiashi,*,†,‡ Tateki Hanashima,§ Ryota Mitobe,§ Kotaro Nagatsu,§ Takahiro Yamamoto,†,∥,⊥ and Yoshikazu Homma*,†,§ †

Nanocarbon Research Division, Research Institute for Science and Technology, Tokyo University of Science, Shinjuku, Tokyo 162-8601, Japan ‡ Department of Mechanical Engineering, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan § Department of Physics, Tokyo University of Science, Shinjuku, Tokyo 162-8601, Japan ∥ Department of Liberal Arts, Faculty of Engineering, Tokyo University of Science, Katsushika, Tokyo 125-8585, Japan ⊥ Department of Electrical Engineering, Graduate School of Engineering, Tokyo University of Science, Katsushika, Tokyo 125-8585, Japan S Supporting Information *

ABSTRACT: Owing to one-dimensionality, nanoscale curvature, and high chemical stability, single-walled carbon nanotubes (SWNTs) have unique surfaces for gas molecules: outer surface as adsorption (exohedral) site and inner surface that provides encapsulation (endohedral) space. Because as-grown SWNTs have different structure (chirality and diameter) and they are normally bundled, it is extremely difficult to investigate the intrinsic properties of SWNTs as adsorbent. Here we demonstrate controlling adsorption and encapsulation states of water in individual suspended SWNTs using laser irradiation with monitoring of their behavior by photoluminescence measurement and perform molecular dynamics simulation. The laser heating and the pressure control make water molecules encapsulated or ejected for SWNTs, which are individually oxidized and opened with laser heating. The precise control of oxidization makes it possible to observe the cluster formation of water molecules during the encapsulation process and to confine water molecules inside SWNTs even in vacuum. SECTION: Physical Processes in Nanomaterials and Nanostructures

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mainly reflect the chirality (n, m) of SWNTs,17 they are strongly influenced by the surrounding environment; this dependence has been attributed to changes of the dielectric constant.18,19 SWNTs suspended between microstructures were shown to be particularly sensitive to the atmosphere20 because gas molecules were easily adsorbed onto the surface and changed the optical transition energies. Recently, we reported the water adsorption and desorption phenomena on the outer surface of a suspended SWNT by using PL spectroscopy and Raman scattering spectroscopy.21 Water-filling in individual SWNTs has been reported using PL for SWNTs immobilized in aqueous agarose gels.22 Therefore, it is interesting to explore distinguishing and controlling the adsorption and encapsulation of water molecules for an individual suspended SWNT. In the present study, we analyzed the interaction between water molecules and suspended SWNTs by PL spectroscopy and controlled adsorption and encapsulation of water in individual suspended SWNTs using laser irradiation. Also, to elucidate the

ingle-walled carbon nanotubes (SWNTs)1 have both the outer surface with positive curvature and inner space in nanoscale. In the inner space, materials such as fullerenes,2 metal-nanowires,3 and organic molecules4,5 have been found to be encapsulated. In particular, the encapsulation of water (endohedral water) is of interest because unique transport properties,6 interesting phase-transition phenomena,7 and so on have been theoretically predicted, and it is important in various fields including biosciences. Moreover, the interaction between graphene and water molecules has attracted much attention, and it is under discussion.8 Liquid or solid water within SWNTs has been observed by techniques such as infrared absorption,9 neutron scattering,10 and X-ray diffraction.11,12 Furthermore, Raman scattering13 and photoluminescence (PL)14 spectra were shown to reflect the water encapsulation in SWNTs dispersed in water. Raman scattering spectroscopy has also revealed water adsorption on the outer surface of SWNTs.15 However, measurements of these water molecules were performed in bulk samples consisting of many SWNTs, which were generally bundled together, and the experimental results were averaged. By contrast, individual SWNTs suspended between microstructures can be examined by PL measurement using a laser.16 Although PL emission spectra © XXXX American Chemical Society

Received: November 22, 2013 Accepted: January 9, 2014

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Figure 1. SEM images of SWNTs suspended between micropillar structure. (a) Pairs of quartz pillars fabricated on quartz substrates. (b) High-magnification SEM image of SWNT suspended between a pair of pillars.

Figure 2. PL spectra from the same (9,8) SWNT measured under different conditions. (a) PL spectra from (I, II) as-grown (closed) and (III−V) oxidized (opened SWNT) measured in water vapor (I, III; 20 Torr) and in vacuum (II, IV, V). (b) PL map from oxidized (opened) SWNT measured in water vapor. The circle (blue), cross (red), and diamond (black) show the optical transition energies of suspended (closed) SWNTs in vacuum and water vapor and surfactant-wrapped SWNTs in solution,17,27 respectively.

spatial distribution of water molecules in SWNTs, we performed molecular dynamics (MD) simulation. Scanning electron microscopy (SEM) images of an SWNT suspended between a pair of quartz pillars are shown in Figure 1. Most SWNTs grew from the top of the quartz pillars: some were suspended between the pillars, whereas others grew downward.23 Only semiconducting SWNTs emit PL signal. However, the contact with metallic SWNTs quenches PL emission, and that with other semiconducting SWNTs decreases the optical transition energy.24 Additionally, defective SWNTs exhibit a satellite peak.25 In advance, we carefully select SWNTs with proper optical transition energies26 and without any additional peaks. PL spectra from a (9, 8) suspended SWNT were measured under different environmental conditions (Figure 2a). The spectra from the as-grown SWNT (I, II) and oxidized SWNT (III−V) were measured in water vapor (I, III; 20 Torr) and in vacuum (II, IV, V; 10−2 Torr), respectively. The PL spectra for the as-grown SWNTs showed two states (I, II). As shown in Figure 2a, the emission peak wavelength of spectrum (I) was larger (by 38 nm) than that in vacuum (II) and was equal to that of the suspended SWNTs in air.26 Water molecules were adsorbed on the SWNT surface in water vapor (I), while they were desorbed in vacuum (II).21 The adsorbed water molecules increased the dielectric constant surrounding the SWNTs and decreased the optical transition energies. After the oxidization of the same SWNT with an electric furnace (at 350 °C and for 5 min), the water vapor pressure dependence of the emission wavelength changed. The SWNTs were heated in air ambient from room temperature to 350 °C with a heating rate of 1 to 2 °C/min. After cooling in air ambient, they were transferred to the environmental chamber and PL emission spectra were measured. The emission peak wavelength of spectrum (IV) in vacuum was smaller (by 46 nm) than that of spectrum (III) in water vapor, owing to water adsorption (III) and desorption (IV) on the outer surface of the SWNT. The emission peaks of (III) and (IV) were redshifted in comparison with the as-grown SWNT (I) and (II), respectively. This can be understood by assuming an additional site for water adsorption after oxidization, that is, the inner surface of the SWNT. Oxidization caused holes, which were thought to be located at the tube ends, through which water molecules entered. Interstitial sites between the SWNTs could be a potential additional adsorption site for bundled SWNTs.28 However, we examined an isolated SWNT, and it was unlikely

that interstitial sites became available after oxidation treatment. The encapsulated water molecules as well as the adsorbed water molecules on the outer surface increased the dielectric constant surrounding the SWNT and then decreased the optical transition energy.14 PL spectrum (V) shown in Figure 2a was measured after the SWNT encapsulating water was heated (∼50 °C for a few minutes in vacuum) with a heating stage inside the environmental chamber. The emission peak blue-shifted, giving an identical result to spectrum (II), indicating that encapsulated water molecules were ejected from the SWNT by heating in vacuum. Because spectrum (II) was equivalent to spectrum (V) in the vacuum, the oxidization did not directly influence the optical transition energies or cause any additional transition such as that appeared for oxygen-doped SWNTs29 or atomichydrogen-exposed SWNTs,25 and the red shift after oxidization was caused by the encapsulation of water molecules. The oxidization opened the end caps of the SWNT, allowing the water molecules to move in and out. The PL map from oxidized SWNT in water vapor (20 Torr) shown in Figure 2b comprises PL emission spectra measured by sweeping the excitation laser wavelength. The circle (blue), cross (red), and diamond (black) correspond to the optical transition energies of (9, 8) (closed) SWNTs suspended in vacuum,30 in air26 and surfactant-wrapped SWNTs in solution,17,27 respectively. The PL peak from the opened SWNT was located in between those of closed SWNTs and surfactant-wrapped SWNTs. Encapsulated water molecules increased the dielectric constant surrounding the SWNT and red-shifted not only the emission peak but also the excitation peak. Next, the water-encapsulation process of SWNTs was monitored by in situ PL measurement. The PL spectra from an oxidized SWNT were measured after the introduction of air (the humidity was ∼70%) into the evacuated environmental chamber, as shown in Figure 3a. Water molecules in the air atmosphere were rapidly adsorbed onto the outer surface of the SWNT, and the PL peak immediately shifted from 1334 to 1370 nm. After that, a gradual red shift continued for 12.5 min before the peak wavelength achieved 1390 nm. The gradual red 409

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Figure 3. (a) Time variation of PL emission spectra from opened SWNTs after air exposure. Note that the intensity is absolute value. (b) Crosssectional snapshot of SWNT with adsorbed (exohedral) and encapsulated (endohedral) water molecules in MD simulation. The SWNT length is 8.52 nm and the number of water molecules is 42.

Figure 4. PL emission wavelength distribution of suspended (10,6) SWNT between pair of quartz pillars. The spacing was 7 μm. The PL spectra were measured at 0.5 μm intervals. The spectra were measured in vacuum (a,d,f) and water vapor (b,c,e). The red and blue dashed lines correspond to the emission wavelengths of (10,6) SWNT in water vapor and vacuum, respectively. The inserted illustrations represent the cap structure of SWNT (opened or closed) and the adsorption or encapsulation states of water. Note that the illustrations are not snapshots of MD simulation but just schematic images.

during the encapsulation process. In Figure 3a, all PL spectra were roughly fitted with two Lorentzian curves (blue dotted and green dashed curve lines). The two Lorentzian curves with lower and higher wavelengths were assumed to be the emission from the lower and higher water density regions, respectively. PL spectra suggested phase separation (empty or gas phase and condensed phase) of water molecules within the laser spot

shift and the stability of PL peak wavelength at 1390 nm were considered to be the gradual increase in the number of water molecules in the SWNT and the filling up with water molecules, respectively. The red shift was accompanied by significant peak broadening, and the peak width showed the maximum around 6 min. The peak broadening suggested that the density of water molecules was not uniform in SWNTs 410

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were left in the SWNT (Figure 4f). The encapsulated water molecules were independent of environmental conditions (that is, vacuum or water vapor) and were stably confined within the SWNT at room temperature. Moreover, it was possible to eject the water from the SWNT by laser heating in vacuum. Excess oxidization caused the potential barrier at the opened parts to vanish. Therefore, the precisely controlled oxidization with laser irradiation for individual SWNTs enables the confinement of water molecules. Although the detailed structure of the opened parts is not clear, the existence of potential barrier and its height should be important for confinement, which is affected by the oxidization process. In conclusion, a new method of encapsulation within SWNTs was explored. The laser heating technique not only opened the SWNTs but also made water molecules enter or be ejected from the inner space of SWNTs. These processes could be monitored by PL measurement. Thus, controlled encapsulation in individual SWNTs is possible using suspended SWNTs and laser irradiation. This technique will provide the basis for molecular-nanotube manipulation and functionalization at an individual SWNT level.

in the SWNT. To confirm the phase separation in the small area, we performed MD simulation. Figure 3b shows a typical snapshot of MD simulation of the (13, 0) SWNT (length 8.52 nm) with 42 water molecules inside at room temperature (T = 25 °C). Water molecules clearly formed a cluster structure, and they diffused as a cluster along the tube axis, while isolated water molecules were hardly observed. The MD result supports the interpretation of the experimental spectra, which was assumed to be composed of the emission from the lower and higher water density regions. The encapsulation rate of water differed among SWNTs: some SWNTs encapsulated water molecules immediately or gradually after water vapor or air was introduced into the chamber, while others did not encapsulate water molecules at all even after oxidization. The difference possibly comes from the opened edge structure. Although oxidization with an electric furnace is convenient for SWNT opening, some SWNTs were burned out and others were not opened. Therefore, more precise and individual control of oxidization temperature and time is required for efficient opening of SWNTs because SWNTs with various chirality, cap structure, and quality (defect density) are grown and their burning temperature is slightly different. Here we perfectly controlled and observed the adsorption and encapsulation states of water molecules for an SWNT by combining the laser heating technique and PL spectroscopy. Figure 4 shows the spatial distribution of the emission wavelengths. PL spectra were measured at different positions along the axis of the suspended SWNT, and the color counter indicates the PL intensity at each measurement point. PL spectra from SWNT were obtained between two pillars with 7 μm spacing, and the intensity gradually decreased nearer to the pillars because of the quenching effect of the pillars and the long diffusion length of exciton.31 The wavelength spatial distribution shown in Figure 4a was measured in vacuum from an as-grown (10, 6) SWNT. The wavelength was uniform along the tube axis and agreed with that of suspended SWNTs in vacuum, which indicated that no gas molecules were adsorbed onto the inner or outer surfaces of whole SWNT. Upon exposure to water vapor (20 Torr), the emission wavelength uniformly red-shifted (Figure 4b). Water molecules were uniformly adsorbed on the outer surface of the SWNT but did not enter the inside of closed as-grown SWNT. Then, the laser irradiation heated the SWNT in low-pressure air (0.1 Torr) and oxidized it. The SWNT temperature during the laser heating was estimated to be 300 °C. Additionally, the SWNT was moderately heated for a few minutes in water vapor by laser heating. The emission wavelength uniformly red-shifted (Figure 4c). The red-shift indicated that the laser irradiation opened the SWNT and water molecules were adsorbed and encapsulated. The emission wavelength distribution shown in Figure 4d was measured after the SWNTs encapsulating water molecules were heated by the laser in vacuum. The emission wavelength was uniform and was equal to that shown in Figure 4a. The oxidation treatment did not affect the optical properties of the suspended parts of the SWNT, which suggested that the end parts were selectively oxidized and opened. Water molecules adsorbed only onto the outer surface when the water vapor was introduced into the environmental chamber without laser heating (Figure 4e). After the encapsulation of water molecules, the environmental chamber was evacuated. Although the adsorbed water molecules on the outer surface were desorbed, water molecules

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ASSOCIATED CONTENT

S Supporting Information *

The experimental methods (fabrication of suspended SWNTs, the oxidization treatment, and PL measurement) and MD simulation methods. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by MEXT KAKENHI Grant Number 19054015 and by JSPS KAKENHI Grant Number 24310094. REFERENCES

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