NANO LETTERS
Environmental Scanning Electron Microscopy Study of Water in Carbon Nanopipes
2004 Vol. 4, No. 5 989-993
M. Pı´a Rossi,† Haihui Ye,† Yury Gogotsi,*,† Sundar Babu,‡ Patrick Ndungu,‡ and Jean-Claude Bradley‡ Department of Materials Science and Engineering and A. J. Drexel Nanotechnology Institute, and Department of Chemistry, Drexel UniVersity, Philadelphia, PennsylVania 19104 Received February 26, 2004; Revised Manuscript Received March 25, 2004
ABSTRACT The ability of the ESEM to condense and evaporate liquids has enabled the in situ dynamic study of condensation, evaporation and transport of water inside carbon nanotubes. It has been possible to see liquid menisci inside straight, CVD-fabricated carbon nanotubes (CNTs) having disordered walls. From the measured contact angles, it is clear that these CNTs are hydrophilic. Complex meniscus shapes and slow liquid dynamics due to water confinement and strong interaction with tube walls have been observed.
Introduction. Since the discovery of carbon nanotubes (CNTs),1 the scientific community has been fascinated by their outstanding properties and potential applications,2,3 including use in nanofluidic devices and composites.4-6 The behavior of water inside CNT channels is of interest for a number of reasons. First, a fundamental understanding of liquid flow in nanochannels is still lacking.5 Additionally, CNTs may be used as nanopipes in nanofluidic devices, and control of liquid transport through CNTs is needed to develop functional devices.5,7 Finally, there is an ongoing discussion in the literature regarding the wetting of CNT surfaces. Mostly hydrophobic behavior is anticipated,7,8 but hydrophilic behavior has also been shown for CNTs under special conditions.9 The in situ study of liquids of practical interest (aqueous solutions and biofluids) inside CNTs is therefore extremely important. Transmission electron microscopy (TEM) studies have shown that closed CNTs can retain fluids trapped during synthesis.4,9-11 However, TEM is limited to imaging closed CNTs only, and the autoclave treatment used for filling the CNTs in these studies restricts the choice of liquids. Recent advances in environmental scanning electron microscopy (ESEM) have enabled scientists to image hydrated materials at low pressures.12-14 Unlike the conventional scanning electron microscope (SEM), the ESEM operates by circulating a gas, generally water vapor or nitrogen, through the chamber while imaging. Furthermore, by means of several * Corresponding author. † Department of Materials Science and Engineering and A. J. Drexel Nanotechnology Institute. ‡ Department of Chemistry. 10.1021/nl049688u CCC: $27.50 Published on Web 04/15/2004
© 2004 American Chemical Society
pressure-limiting apertures, the ESEM can be used at pressures of up to 20 Torr,15 which are much higher than the pressures allowed by the conventional SEM. With the ability to manipulate the pressure and temperature by means of a Peltier cooling stage within the ESEM chamber, it is possible to induce the condensation and evaporation of organic and aqueous fluids. In the case of distilled water, if the temperature is maintained constant at approximately 4 °C, water droplet formation will initiate around 5.8 Torr.15 Interactions of CNT and carbon nanofibers with fluids have been observed with the ESEM.15 However, due to the low depth of penetration of the electron beam, these studies were limited to surface interactions only. The penetration depth of electrons depends on the atomic weight and density of the material, as well as the accelerating voltage. A higher voltage causes a deeper penetration of the electrons. Thus, thin walls and relatively high accelerating voltages are required to see through a CNT. The objective of this work was to conduct in situ fluidic experiments at the nanoscale in the ESEM, by using the ability to see through the CNT walls, and to study the liquid flow, condensation, and evaporation inside CNT. Experimental Section. The CNTs used in these experiments were obtained by chemical vapor deposition (CVD) of carbon in alumina membranes16 with pores ∼250 nm in diameter from a 30% ethylene/helium gas mixture at 670 °C for 6 h in a process described in ref 17. Sonication in 1 M NaOH for 90 min was used to dissolve the alumina and release the CNTs. Tubes of 200-300 nm in diameter (Figure 1) have been fabricated. These carbon nanopipes have wide
Figure 1. SEM (a, b) and TEM (c-e) images of CVD carbon nanopipes. (a, c) Individual tubes. (b) A bundle of tubes after dissolution of the membrane shows an asymmetric shape and polygonization due to thin and flexible walls. (d) Open tip of a tube. The tapered morphology of the tip indicates it is the as-grown end of the nanotube, not the broken end. (e) High-resolution TEM image showing that the tube wall has a disordered structure. The thickness of the tube wall is 12 nm.
channels and very thin, smooth, and straight walls, similar to water pipes but orders of magnitude smaller, as shown in Figures 1a and 1c. A great majority had at least one end open, as shown in Figure 1d, and more than 50% of the tubes were open on both sides after release from the membrane. High resolution TEM imaging shows that the CNTs have disordered walls (Figure 1e). It is also likely that graphene sheets in the walls are hydrogen-terminated because of the synthesis from the C-H environment. Due to the CNTs’ thin walls (12-15 nm), they are transparent under the ESEM electron beam at electron voltages as low as 10 kV, though optimal imaging was done at 25 kV or higher. To observe the CNTs under the ESEM in environmental mode, they were placed on a type-303 stainless steel sample holder surface, which was in turn positioned on a Peltier cooling stage. A gaseous secondary electron detector was employed for imaging. There are two possible ways to induce condensation of water within the chamber: either a droplet of water can be placed on the sample holder surface and/or the cooling stage, or the moisture from the water vapor within the chamber can be used. However, it is easier to control the liquid when it is placed within the chamber initially than if the moisture from the chamber is condensed. The chamber was evacuated at an intermediate pressure (around 5 Torr) in order to evaporate the water droplet initially placed on the cooling stage. As a result, the CNT were initially dry. The temperature was set constant, generally to 4 or 5 °C. The pressure was then increased at a rate of about 0.1 Torr for approximately every 15 s. Since condensation of water within the ESEM chamber occurs suddenly, this slow pressure increase rate allowed for better control of liquid phase formation. The contact angle for water on the sample holder was measured optically in air. The contact angle for water in 990
Figure 2. ESEM images showing the condensation of water inside a carbon tube (a) and underneath or around the tubes (b, c). Water initially condenses inside open tubes when they are placed on a steel substrate. Condensation and formation of a water bridge may occur between a tube and the substrate when the tube is slightly elevated above the substrate (b) or between two tubes which are close to each other (c). Framed areas 3 and 4 show the sections of the tube presented in Figures 3 and 4, respectively.
nanotubes was measured from SEM images and the estimates were done with understanding that the tube surface was not flat and that the environment was a low-pressure water vapor rather than air.
Results and Discussion Since the principles of image forming and contrasts in ESEM are different from the TEM18 or the conventional SEM,19 interpretation of images is not as straightforward. In TEM, the liquid is an obstacle to electron transmission and liquid plugs in nanotubes appear dark. In the field emission ESEM, the vapor phase may appear darker (Figure 2a) because there is less obstruction of the electron beam, the metal sample holder surface underneath is more visible. Conversely, liquids look lighter (Figure 2a) because they impede electron penetration and the sample holder surface underneath is less discernible. Furthermore, analysis of liquid motion inside the tube, which was recorded on video (see Supporting Information), clearly indicates that the liquid is the lighter phase inside the CNT. A thin (tens of nanometers) layer of water is transparent to the electron beam at the maximum voltage used and some tube features can be observed through the water film. Before the first indication of water condensation on the sample holder surface (marked by the formation of small droplets), menisci began to become evident within the CNTs as pressure increased above 5 Torr at 4 °C, as seen in Figure 2a. This shows that the carbon tubes were more hydrophilic than steel (contact angle ∼55°) and attracted water vapor from the environment. In addition to the hydrophilic walls, surface tension and thermocapillarity are the forces that may cause a liquid to become drawn into a CNT.20 When water pressure increases further and the open tubes get fully or partially filled, condensation of water begins around the tubes, especially when several tubes are in close contact as shown in Figure 2b,c. Condensation experiments on branched, Nano Lett., Vol. 4, No. 5, 2004
Figure 3. ESEM images showing the condensation of water at the closed end of the tube shown in Figure 2a. Condensation starts at the walls of the tube (a) and the volume of liquid grows with increasing time or pressure (b, c). Two bubbles can be seen trapped inside the liquid.
closed CNTs showed no meniscus formed because water did not penetrate inside the tube. A sequence of ESEM images taken under different pressures from the closed end of the tube shown in Figure 2a shows condensation of water inside the tube (Figure 3). Surface tension after the formation of a thin film of water inside the tube applies a force to the walls that is sufficient to change the tube shape slightly (Figure 3a). Small bubbles may eventually be trapped in the liquid (Figure 3b,c). The process of condensation and evaporation of water inside the tubes is fully reversible and can be repeated multiple times. Figure 4 shows the change in meniscus shape and size in response to pressure at a constant temperature of 4 °C. The dynamics of this process for the water plug with a volume of approximately 10-17 liters (10 attoliters) was recorded on video (Supporting Information) and can be quantitatively analyzed. Measured contact angles of the water with the CNT internal walls shown in Figure 4a are comparable to the ones predicted for H-terminated graphite.21 Figure 4 was obtained at a scan rate of 33.2-66.4 ms for image clarity. As a result, there is a lag in the imaging of these dynamic processes, and the liquid phase seems to decrease as pressure increases. In addition, although the cooling stage is maintained at a constant temperature of 4-5 °C, CNTs have a high thermal conductivity and are quickly heated by the electron beam under the prolonged, continuous observation at high magnification and slow scan rates in the ESEM. Because of CNT heating under the electron beam, it is possible that water evaporation can occur even when the conditions dictate condensation. It is evident in Figure 4 that the menisci observed within the CNT are asymmetric. This could be attributed to the fact that the pores in the Al2O3 membrane are not perfectly circular, as shown in Figure 1b.17 This gives the CNT a nonuniform cross-sectional shape and could be the reason for the distorted shape of the meniscus. The slow scan rate at which the image was taken could also contribute to the distortion of the meniscus. However, the distinct difference in the meniscus shape on the left and right sides, which is reproducible from cycle to cycle (compare Figure 4a and e, which are almost identical), suggests that the closed end of the tube and presence of liquid at the tube tip (Figure 3b,c) led to different pressures on different sides of the water plug. Pinning of the plug to the hydrophilic tube walls has been Nano Lett., Vol. 4, No. 5, 2004
Figure 4. (a-e) ESEM images showing the dynamic behavior of a water plug close to the open end of the tube shown in Figure 2a. The meniscus shape changes when, at a constant stage temperature, the vapor pressure of water in the chamber is changed (a) 5.5 Torr, (b) 5.8 Torr, (c) 6.0 Torr, (d) 5.8 Torr and (e) 5.7 Torr, where the meniscus returns to the shape seen in (a). The asymmetrical shape of the meniscus, especially the complex shape of the meniscus on the right side in (a, e), is a result of the difference in the vapor pressure caused by the open left end and closed right end of the tube. (f) TEM image showing a similar plug shape in a closed CNT under pressure.
demonstrated for hydrothermal nanotubes4 and can explain why the plug deforms instead of just moving under the pressure gradient. In the case of hydrothermal nanotubes, the rapid heating of liquid inclusions by the electron beam caused the formation of a complex interface between the liquid and the gas inside the nanotube. Movement of this interface occurred as a result of bubble formation in the bulk liquid. Basically, liquid inclusions were pinched-off between the gas bubbles, creating a wavelike pattern within the nanotubes. In addition, interface deformation by peeling of the graphite layers on the inner wall of the nanotubes was observed. This behavior was most likely caused by the strong attraction between the hydrophilic walls of the hydrothermal nanotubes and the 991
liquid inclusion. A complex shape of the meniscus was also observed by Tas et al.22 for a water plug in a hydrophilic nanochannel between two plates. According to this study, the meniscus shape is attributed to the negative pressure induced inside the nanochannel as a result of tensile capillary forces. It has been argued that the results obtained on hydrothermal nanotubes4,9 were inherent to that specific system used, and pure water in a nanopipe may behave differently.23 However, the interfaces observed in ESEM are similar to the complex liquid-gas interfaces recorded in the experiments in hydrothermally produced tubes (50-100 nm inner diameter) with a H2O/CO/CH4 fluid at high pressures as shown in Figure 4f. The kinetics of membrane pinch-off and liquid propagation along the walls is also similar in order of magnitude (compare Figure 4b,c and ref 4). The fact that liquid inclusions in two distinct types of CNT (highly ordered hydrothermally produced tubes having a conical scroll structure and disordered CVD tubes) under significantly different conditions (pressure above 760 Torr in TEM experiments (Figure 4f) and 4-6 Torr in ESEM experiments (Figure 4e)) behave in such a similar fashion strongly suggests that such a complex interface is intrinsic to the behavior of water in nanochannels, at least those with hydrophilic walls. It is believed that the forces responsible for liquid becoming drawn into a hydrophilic nanotube are surface tension and thermocapillarity.11,20 Studies on capillary condensation of a fluid in a cylindrical pore, which have been done by the scientific community throughout the past thirty years, could serve to explain some of the behavior reported here. In 1972, Everett and Haynes24 first proposed their theory, which has been confirmed recently by Vishnyakov and Neimark.25 According to the Everett-Haynes scenario, a metastable, vapor-like fluid in a cylindrical pore with attractive walls will form a bump or undulation that will grow into a liquid-like bridge. As the liquid-like phase grows, the vapor-like bubble begins to contract until the entire cylinder is filled with condensed liquid. This behavior follows the Laplace equation -1
Pcap ) -γ (r1
-1
+ r2 ) ) constant
where γ is the surface tension of the liquid and r1 and r2 are the main radii of curvature. The observed behavior indicates an attraction between the CNT inner walls and the water, which demonstrates that the CNTs are hydrophilic. An explanation for the behavior seen in Figure 4 c-e can be explored by using this model. In parallel, Cole and Saam studied the adsorption of liquid films in cylindrical pores.26 In this theory, the condensation of a liquid film in a cylindrical cavity does not form the liquid bump between the vapor phases described in ref 25. In their argument, Cole and Saam state that a liquid bridge collapses so that only meniscus behavior similar to that observed in Figure 4b-d occurs. In this case, the behavior of the liquid/gas interface follows the equation 992
pv ) pl + γ/a where pv and pl are the equilibrium vapor and liquid pressures, a is the film radius, and γ is surface tension. The behavior observed in Figure 4 has not been clearly hypothesized by one particular theory, since what has been observed varies from what is expected from different individual theories. However, it is also evident, by comparing the observations in Figures 4e and f, that this behavior is innate of hydrophilic cylindrical pores irrespective of parameters such as pore diameter or external pressure. The next step in studies such as those presented here, therefore, is to carefully analyze theories of capillary condensation presented thus far to explain the observed phenomena. Conclusions. Dynamic fluidic experiments with pure water inside the CNTs show that interface dynamics and complex shapes are controlled by the small tube diameter rather than wall structure, fluid composition, or pressure. Complex interfaces previously reported from the TEM study on hydrothermal nanotubes containing pressurized fluid have been observed for pure water in CVD tubes at low pressure. This work shows that CVD CNTs with disordered walls are hydrophilic and form contact angles with water from 5 to 20 degrees. It is also shown that water condenses preferentially inside the CNT, which demonstrates that such tubes will easily imbibe aqueous fluids. This could indicate that CNTs can be used as probes to guide fluids to and from specific locations, such as cells, and that they could provide an efficient way to collect attoliter to picoliter amounts of liquids. Acknowledgment. This work was supported by the NSF Grant number CTS-0235234 and the Commonwealth of Pennsylvania’s Ben Franklin Technology Development Authority through the Ben Franklin Technology Partners of Southeastern Pennsylvania and the Nanotechnology Institute. The ESEM purchase was supported by NSF Grant number BES-0216343. M.P.R. also acknowledges the Department of Education GAANN fellowship for additional support. Helpful discussions with Prof. C. Megaridis are appreciated. Supporting Information Available: Windows Media Player movie of liquid motion in CNT. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Iijima, S. Nature 1991, 345, 56. (2) Baughman, R. H.; Zhakidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (3) Harris, P. J. F. Carbon Nanotubes and Related Structures; Cambridge University Press: Cambridge, 1999. (4) Gogotsi, Y.; Libera, J. A.; Guvenc-Yazicioglu, A.; Megaridis, C. M. Appl. Phys. Lett. 2001, 79, 1021. (5) Supple, S.; Quirke, N. Phys. ReV. Lett. 2003, 90, 214501-1. (6) Riegelman, M.; Liu, H.; Evoy, S.; Bau, H. H. Nanofabrication of Carbon Nanotube (CNT) Based Fluidic Devices. In Proceedings of NATO-ASI Nanoengineered Nanofibrous Materials; Guceri, S., Kutznetsov, V., Gogotsi, Y., Eds.; Kluwer: The Netherlands, 2004; pp 407-414. (7) Werder, T.; Walther, J. H.; Jaffe, R. L.; Halicioglu, T.; Noca, F.; Koumoutsakos, P. Nano Lett. 2001, 1, 697. (8) Walther, J. H.; Jaffe, R.; Halicioglu, T.; Koumoutsakos, P. J. Phys. Chem. B 2001, 105, 9980. Nano Lett., Vol. 4, No. 5, 2004
(9) Gogotsi, Y.; Naguib, N.; Libera, J. Chem. Phys. Lett. 2002, 365, 354. (10) Gogotsi, Y.; Libera, J. A.; Yoshimura, M. J. Mater. Res. 2000, 15, 2591. (11) Megaridis, C. M.; Yazicioglu, A. G.; Libera, J. A.; Gogotsi, Y. Phys. Fluids 2002, 14, L5. (12) Stokes, D. J.; Thiel, B. L.; Donald, A. M. Langmuir 1998, 14, 4402. (13) Tai, S. W.; Tang, X. M. Scanning 2001, 23, 267. (14) Donald, A. M. Nature Mater. 2003, 2, 511. (15) Rossi, M. P.; Gogotsi, Y. Microscopy and Analysis 2004, 18, issue 4. (16) Che, G.; Lakshmi, B. B.; Martin, C. R.; Fisher, E. R. Chem. Mater. 1998, 10, 260. (17) Bradley, J. C.; Babu, S.; Ndungu, P.; Nikitin, A.; Gogotsi, Y. Chemistry Preprint Server CPS:030302 2003. (18) Williams, D. B.; Carter, C. B. Transmission Electron Microscopy: Imaging III; Plenum Press: New York, 1996.
Nano Lett., Vol. 4, No. 5, 2004
(19) Newbury, D. E.; Joy, D. C.; Echlin, P.; Fiori, C. E.; Goldstein, J. E. AdVanced Scanning Electron Microscopy and X-ray Microanalysis; Kluwer Academic/Plenum Publishers: New York, 1986. (20) Ye, H.; Naguib, N.; Gogotsi, Y.; Yazicioglu, A. G.; Megaridis, C. M. Nanotechnology 2004, 15, 232. (21) Werder, T.; Walther, J. H.; Jaffe, R. L.; Halicioglu, T.; Koumoutsakos, P. J. Phys. Chem. B 2003, 107, 1345. (22) Tas, N. R.; Mela, P.; Kramer, T.; Berenschot, J. W.; van der Berg, A. Nano Lett. 2003, 3, 1537. (23) Rivera, J. L.; McCabe, C.; Cummings, P. T. Nano Lett. 2002, 2, 1427. (24) Everett, D. H.; Haynes, J. M. J. Colloid Interface Sci. 1972, 38, 125. (25) Vishnyakov, A.; Neimark, A. V. J. Chem. Phys. 2003, 119, 9755. (26) Cole, M. W.; Saam, W. F. Phys. ReV. Lett. 1974, 32, 985.
NL049688U
993
