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Retardation of Liquid Indium Flow in Indium Oxide Nanotubes Mukesh Kumar, Vidya N. Singh, Bodh R. Mehta,* and Jitendra P. Singh* Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India ReceiVed: October 27, 2009; ReVised Manuscript ReceiVed: January 13, 2010
High-resolution transmission electron microscopy and energy-dispersive X-ray analysis carried out on indium oxide nanotubes grown by a chemical vapor deposition technique show the presence of indium metal segments along the indium oxide (IO) nanotube axis having one end closed. A real-time HRTEM video in continuous mode imaging has been carried out to study the directional flow of liquid indium. Electron-beam-induced heating results in the increase in indium vapor pressure and desorption of gases at the closed end of the IO nanotubes. This buildup of differential pressure between open and closed columns leads to the flow of indium away from the closed end of the IO nanotube. Interestingly, the indium flow rate was observed to decrease from 2.8 to 0.3 nm/s with a corresponding decrease in the nanotubes’ diameter from 138 to 38 nm. This study indicates that the wetting properties of the liquid-host nanotube interface critically decides the fluid dynamics at nanoscale, and depending upon the interfacial properties, enhancement or retardation of flow can be observed on the reduction of the nanotube diameter. 1. Introduction Size dependence of liquid flow in nanotubes is of current research interest due to its potential application in future nanofluidic devices, such as nanothermometers and nanorobotic spot welding.1-9 Recent reports have shown that the behavior of fluids in nanochannels differs drastically from that of micrometer-sized channels.2,3,10-12 The capillary forces can facilitate filling of inner cavities of carbon nanotubes (CNTs), provided the filling liquid has a surface tension below the cutoff value of 100-200 mN/m. When a droplet of water with a surface tension value of 73 mN/m is placed on a CNT surface, it is readily sucked into the capillary nanochannels, eventually leaving the top surface dry.13 CNTs with an inner diameter less than 4 nm cannot be filled with molten silver nitrate due to the size lowering of the nanotube-salt interface energy with increasing curvature of the nanotube walls.14 Most of the studies on this topic are restricted to the flow of liquids in CNTs. Oxide nanotubes have completely different surface and bulk atomic arrangement in comparison with CNT’s. In a number of studies, indium-filled SiO2 nanotubes, Ga-filled MgO, and Au-filled Ga2O3 have been synthesized using different techniques.15-17 Despite its importance and potential applications,18-22 there is only one report of liquid indium flow in indium oxide (IO) nanotubes.23 In this study, electron-beam-induced thermal expansion of indium inside the IO nanotubes, resulting in a nanothermometer-like effect, was reported.16,17,23 In the present work, continuous flow of liquid in indium IO nanotubes having wide ranging diameters from 38 to 138 nm has been investigated using real-time high-resolution transmission electron microscopy (HRTEM) investigations. The objective of the present study is to understand how interfacial wetting properties of the liquid-host surface influence the flow in nanotube structures. * To whom correspondence should be addressed. E-mail: brmehta@ physics.iitd.ac.in (B.R.M.),
[email protected] (J.P.S.). Phone: +9111-2659-1323. Fax: +91-2658-1114.
2. Experimental Section 2.1. Growth of Indium-Filled IO Tubular Nanostructures. Indium-filled IO closed nanotubes were synthesized by a vapor phase evaporation method in a horizontal tube furnace maintained at a temperature of 1000 °C and 1 atm. The mixture of IO and carbon (1:1) powder was used as a precursor. The mixture was placed in an alumina boat and inserted into the central heating zone of the furnace. The system was heated to 1000 °C at a rate of 20 °C/min and maintained at this temperature for 1 h. The bare Si(100) substrates were placed downstream at a temperature of 960 °C. A small reservoir (5-10 mL) of ethanol was placed in a low-temperature region (∼65 °C) in the upstream direction during the growth. The constant flow of Ar gas at the rate of 200 mL/min was maintained during the growth process. 2.2. Characterization. Glancing angle X-ray diffraction (GAXRD) (Phillips X’Pert, PRO-PW 3040) with a glancing angle of 1° was used for structural characterization of grown samples. The HRTEM (Tecnai G20-Stwin at 200 kV) equipped with EDX was used to characterize IO tubular nanostructures. The electron-beam irradiation induced mass flow was performed in situ in a TEM chamber having a base pressure in the range of 5 × 10-8 Torr. The indium flow inside the IO nanotubes was accomplished by setting the electron-beam intensity in the range of 450-600 pA/cm2 (measured on a phosphor screen), which is nearly 1 order of magnitude higher than the normal imaging current. A high-quality Gatan camera was set to acquire the images of indium flow in IO tubular nanostructures in continuous mode with an acquisition time interval of 10 s and exposure time of 0.4 s. 3. Results and Discussion Figure 1a shows TEM micrographs of IO tubular nanostructures. The inset in Figure 1a shows the HRTEM of closed and open ends of the tip region of the nanostructures. The HRTEM image in Figure 1b reveals the tubular nature of IO nanotubes and the presence of a cavity at the center of the structure. The interplanar spacing in IO present along the nanotube is 0.506 nm, which corresponds to the (200) plane of cubic IO. The
10.1021/jp910252f 2010 American Chemical Society Published on Web 01/29/2010
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Figure 1. (a) TEM of a typical IO tubular nanostructure. The inset reveals the HRTEM of an arrow-like closed structure at one end and an open tube at other end. (b) HRTEM images confirming the presence of a cavity at the center of the IO tubular nanostructure with a cavity diameter of 12 nm. The HRTEM images show the growth of the nanotube along the 〈100〉 direction. (c) The elemental composition of indium and oxygen across indium-filled IO tubular nanostructures at the marked position from A to B on the nanotube is shown along with the STEM micrograph. The EDX spectra reveal the positions A and B at the x axis and intensity of the In/O signal at the y axis. The EDX spectra confirming the presence of indium segments filled in the IO tubular nanostructure.
Kumar et al. growth of the tubular nanostructures was found to occur along the 〈100〉 direction. The compositional analysis of In and O along the radial direction of IO tubular nanostructures is determined using scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDX) analysis, as shown in Figure 1c, along with EDX spectra imposed on the structure. The results confirm the tubular nature of the IO nanostructure cavity partially filled with indium metal segments. The spot EDX at the center of the tubular nanostructure (not included in the figure) further confirms that the IO tubular nanostructure is filled with indium. Near the closed end, a relatively large indium bulblike structure can be clearly observed. The GAXRD study (not shown here) of these nanotube samples has confirmed the presence of the cubic In2O3 phase with a lattice constant a ) 1.011 nm. The growth mechanism of indium-filled IO tubular nanostructures is discussed in detail elsewhere.24 During synthesis, In2O3 gets carbothermally reduced into InxO (x ) 1, 2) at 1000 °C and reaction species get further reduced to indium-rich vapors in the presence of ethanol, resulting in indium-rich growth ambient, which leads to the growth of indium-filled IO nanotubes. On account of the low melting point of indium, the condensation and re-evaporation during growth may simultaneously occur and result in the formation of partially indiumfilled IO nanotubes.25 The diameter of various IO nanotubes investigated in this study varies from 35 to 150 nm.25 Different stages of electron-beam irradiation induced flow of indium in two IO nanotubes having cavity diameters of 38 and 138 nm are shown in Figure 2a,b. The arrows marked on the nanotubes show the movement of indium segment at time intervals of 0, 50, 100, and 200 s. A real-time TEM video showing the flow of liquid indium in an IO nanotube is shown in the Supporting Information. By carrying out repeated experiments on different nanotubes, it was concluded that the flow of indium takes place in the direction away from the closed nanotube ends, as shown in Figure 2a,b. For the transport of liquid metal in CNTs, different mechanisms have been proposed in the literature, such as (i) electricfield-induced surface electromigration, (ii) electron-beaminduced electric field and temperature gradient along the nanotubes, and (iii) thermal expansion.7,15,26-29 In an interesting study, Regan et al. have reported the transport of indium on the CNT surface by applying an electric field between the two ends of the nanotube.28 In was found to flow in the direction of the electric field on the CNT surface. In another study on Fefilled CNTs, Fe was reported to flow in the direction opposite to the applied electric field.27 Goldberg et al. have reported the melting, expansion, sudden shrinkage, and ultrafast birth and ripening of tiny indium in silica nanotubes during in situ electron-beam irradiation.15 It was assumed that electron-beam irradiation results in a blast of indium balls filled in silica nanotubes, though no directional flow of indium was reported. All these results were explained within the framework of surface electromigration. It is important to notice that, in the present study, no external electric field was applied along the nanotube ends. It is quite unlikely that electron-beam-induced irregular charge distribution will result in any directional electric charging capable of influencing or resulting in continuous and directional flow of indium in IO nanotubes. Lee et al. have explained the indium flow in IO nanotubes due to nonsynchronized thermal expansion and contraction of the IO wall and liquid indium along the tube axis.23 The thermal expansion or thermal gradient along indium-filled IO nanotubes may also not be the possible driving force in our case, as the displacement of liquid indium
Retardation of Liquid Indium Flow in IO Nanotubes
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Figure 2. TEM images of indium-filled IO tubular nanostructures having a diameter of (a) 38 nm and (b) 138 nm recorded at different time intervals during electron-beam irradiation. During the investigation, the images were recorded in continuous mode with the interval of 10 s. The arrows show the flow of indium segments in IO tubular nanostructures with time. The scale bar is 200 nm.
Figure 3. Schematic of the flow of the indium segment in IO tubular nanostructures. The PF is the pressure calculated by the sum of indium vapor pressure plus pressure of adsorbed gas due to electron-beam-induced heating. PV is the pressure inside the TEM chamber. The pressure difference (PF - PV) is responsible for the flow of the indium segment. Symbols in the schematic represent indium liquid [solid circles], indium vapors [small dots], and IO [clusters of dark and light circles].
is much more than the thermal expansion (0.116 × 10-3 per °C) of indium at possible electron-beam-induced temperatures (200-300 °C).30-32 To understand the continuous and directional mass flow of indium in IO nanotubes, we have carefully analyzed the real-time TEM images. It is important to note that the flow of indium always took place in the direction away from the closed ends of the IO nanotubes, as shown in Figure 2a,b. Indium segments present along the IO nanotube axis can be considered as separating the tube into two columns. The flow of indium is due to the pressure difference between the columns at the closed end in comparison to that of the open end of the IO nanotubes. The electron-beam-induced melting or evaporation of indium in nanotubes has been reported in literature.15,33 It is important to note that local heating of the indium with an electron beam elevates both temperature and pressure and the complex phase of indium liquid and indium vapor may coexist in the closed column.6,34 The solubility of different gases in liquid metals has been reported by researchers.35,36 Specifically, there are reports on the solubility and diffusivity of oxygen in liquid indium.35 We have also observed that the growth under reducing ambient (ethanol) results in the hydrogen incorporation in IO tubular nanostructures.37 On electron-beam-induced heating, the closed column at one end of the nanotubes may act as a miniaturized pressure vessel in comparison to low pressure (∼10-8 Torr) at the open end of the nanotube. This increased gas pressure is responsible for the continuous and outward moment of the indium segment, as shown pictorially in a schematic diagram in Figure 3. This explains the indium flow from closed end to open end. This also explains the observation of parallel movement of the different indium segments present along the IO nanotube length. The pressure difference between
Figure 4. Flow rate of the indium liquid segment versus diameter (D) of IO tubular nanostructures. The error bars represent the highest and lowest values of the flow rate measured for different indium liquid segments in one nanotube.
the open and closed column on opposite sides of the indium segment solely depends on the temperature increase on account of electron-beam irradiation and is expected to be the same in IO nanotubes having the same diameters. The flow of indium segments was studied for different IO nanotubes having cavity diameters varying from about 38 to 138 nm. The flow rate of indium segments versus nanotube cavity diameter is shown in Figure 4. It is interesting to notice that the indium segment flow rate in IO nanotubes decreases with the decrease of the cavity diameter. The flow rate decreases from 2.8 to about 0.3 nm/s for a corresponding decrease in the cavity diameter from 138 to 38 nm. The value of flow rate is
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Kumar et al. or polytetrafluoroethylene to create a hydrophilic or hydrophilic surface has been achieved.38 Metal oxide surfaces also can be easily functionalized.39,40 It may thus be possible to modify the flow rate of the liquid by conveniently modifying and functionalizing the nanotube surface. 4. Conclusions
Figure 5. Flow rate of the indium liquid segment with time in different diameter IO nanotubes.
an average over the values determined for the different indium liquid segments in a single IO nanotube. The flow rate of the indium segment in each nanotube is found to be higher at the closed end in comparison with the value near to the open end. For example, in an IO nanotube with a cavity diameter of 138 nm, the flow rate of the indium segment near the closed end is observed to be ∼3.3 nm/s and the value decreases to ∼2.2 nm/s near the open end, while in the center of the nanotube, the flow rate comes out to be 2.8 nm/s. This observation further strengthens our proposed mechanism based on the pressure difference induced flow of liquid indium segments in IO nanotubes. The flow rate of indium segments with time in different IO nanotubes is determined and is shown in Figure 5. The flow rate remains nearly constant with time. The effect of size on the flow of liquid indium observed in IO nanotubes is opposite to the observation of the flow of water in CNTs. About a 45 times enhancement in the flow rate of water in CNTs was observed than that predicted by conventional fluid mechanics.2 Chen and co-workers have simulated the flow of water in CNTs by a nonequilibrium molecular dynamic calculation.1 The hydrophobic nature of CNTs and hydrogen bond formation of water with a graphitic surface in CNTs provide almost frictionless mass transport, which results in enhancement of water flow by 3-4 orders of magnitude as the tube diameter decreases from 44 to 3.5 nm.1,2 The linear decrease in the flow rate of indium with the decrease in the IO nanotube cavity diameter (Figure 4) may be explained on the basis of the nature of indium liquid interaction with the IO wall surface. Indium is known to have a good wetting nature with an IO surface.23 As the nanotube cavity diameter decreases, the surface area increases and, hence, there is more resistance toward the flow of the indium segment. It is quite possible that wetting properties of an In-IO interface are also enhanced due to change in the curvature of the IO nanotube surface at lower nanotube diameters. Both factors possibly make the indium flow slower at a smaller diameter. The comparison of the results of the present study with the above observation indicates that the nature of the liquid and nanotube surface is the most important factor. The dependence of flow on the nanotube diameter originates due to the increased surface area.Dependingonthewettingpropertiesbetweenliquid-nanotube surfaces, the effect of decrease in nanotube diameter can be retardation or enhancement of flow. The results of this present study indicate that the enhancement of flow in nanotubes at a lower diameter is not universal, as may appear from the reported flow of water in CNT. Functionalization of the nanotube surface can be carried out to tailor the surface properties of the nanotube surface and thus liquid-surface interaction. For example, functionalization of a CNT wall surface with a carboxyl group
This study reports the size dependence on liquid indium flow in IO nanotubes having diameters varying from 38 to 138 nm. The electron-beam irradiation induced heating causes an increase in the vapor pressure due to indium evaporation and desorption of dissolved gases. This builds a pressure difference between closed and open columns along the tube, which leads to the flow of indium segments. The flow rate of indium segments was observed to decrease linearly from 2.8 to 0.3 nm/s with a decrease in the nanotube cavity diameter from 138 to 38 nm, respectively. Due to the wetting properties of liquid indium with the IO wall surface and the increase in the surface area, the flow rate is reduced in the case of smaller diameter nanotubes. The present study reveals that enhancement in flow rate in nanotubes is not universal and critically depends on the nature of the nanotube surface and its wetting properties. It is conjectured that functionalization of nanotube surfaces can be used to control the liquid flow in nanotubular structures. Acknowledgment. M.K. is thankful to IIT Delhi for providing senior research fellowship. Supporting Information Available: The HRTEM real-time movie showing the continuous flow of liquid indium in a 38 nm diameter indium oxide nanotube. The pressure buildup at the closed end of the nanotubes is responsible for continuous flow of liquid indium toward the open end. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Chen, X.; Cao, G.; Han, A.; Punyamurtula, V. K.; Liu, L.; Culligan, P. J.; Kim, T.; Qiao, Y. Nano Lett. 2008, 8, 2988. (2) Whitby, M.; Cagnon, L.; Thanou, M.; Quirke, N. Nano Lett. 2008, 8, 2632. (3) Majumder, M.; Chopra, N.; Andrews, R.; Hinds, B. J. Nature 2005, 438, 44. (4) Burns, M. A.; Johnson, B. N.; Brahmasandra, S. N.; Handique, K.; Webster, J. R.; Krishnan, M.; Sammarco, T. S.; Man, P. M.; Jones, D.; Heldsinger, D.; Mastrangelo, C. H.; Burke, D. T. Science 1998, 282, 484. (5) Hong, M. H.; Kim, K. H.; Bae, L.; Jheb, M. Appl. Phys. Lett. 2000, 77, 2604. (6) Rossi, M. P.; Ye, H.; Gogotsi, Y.; Babu, S.; Ndungu, P.; Bradley, J. C. Nano Lett. 2004, 4, 989. (7) Gao, Y.; Bando, Y. Nature 2002, 415, 599. (8) Dong, L.; Tao, X.; Zhang, L.; Zhang, X.; Nelson, B. J. Nano Lett. 2007, 7, 58. (9) Mani, R. C.; Li, X.; Sunkara, M. K.; Rajan, K. Nano Lett. 2003, 3, 671. (10) Holt, J. K.; Park, H. G.; Wang, Y.; Stadermann, M.; Artyukhin, A. B.; Grigoropoulos, C. P.; Noy, A.; Bakajin, O. Science 2006, 312, 1034. (11) Hummer, G.; Rasaiah, J. C.; Noworyta, J. P. Nature 2001, 414, 188. (12) Tuzun, R. E.; Noid, D. W.; Sumpter, B. G.; Merkle, R. C. Nanotechnology 1996, 7, 241. (13) Ebbesen, T. W. Annu. ReV. Mater. Sci. 1994, 24, 235. (14) Ugarte, D.; Chatelain, A.; DeHeer, W. A. Science 1996, 274, 1897. (15) Goldbeg, D.; Li, Y. B.; Mitome, M.; Bando, Y. Chem. Phys. Lett. 2005, 409, 75. (16) Li, Y. B.; Bando, Y.; Goldberg, D.; Liu, Z. W. Appl. Phys. Lett. 2003, 83, 999. (17) Gong, N. W.; Lu, M. Y.; Chen, C. Y.; Chen, L. J. Appl. Phys. Lett. 2008, 92, 073101. (18) Chopra, K. L.; Major, S.; Pandya, D. K. Thin Solid Films 1983, 102, 1.
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