hole to indium filled io nanotubes

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hole to indium filled io nanotubes

  1. 1. Article pubs.acs.org/JPCCTunable Growth of Indium Oxide from Nanoflute to Metal-FilledNanotubesMukesh Kumar,*,†,‡ V. N. Singh,† B. R. Mehta,† and J. P. Singh*,††Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India‡Department of Electrical Engineering, South Dakota State University, EECS Building, Box 2222, Brookings, South Dakota 57007,United States ABSTRACT: Here, we report a tunable morphological transformation of indium oxide (IO) nanoflute, that is, nanowires embedded with discrete and hollow nanocavities to indium-filled IO nanotubes by simply controlling the In/O partial pressure in a thermal chemical vapor deposition method. The growth of nanoflute is observed as a prestage for the growth of indium-filled IO nanotubes. The out- diffusion of indium metal at lower In/O partial pressure results in the growth of discrete and hexagonal-shaped nanocavities enclosed by minimum surface energy planes, {111}, which get connected together for the formation of continuous and indium-filled IO nanotubes at higher In/O partial pressures. The tunable and controlled growth of indium-filled IO nanotubes may have potential applications in future nanodevices.I. INTRODUCTION of IO nanowires having hexagonal nanocavities, whileOne-dimensional metal oxide nanostructures have attracted comparatively higher indium partial pressure results in thegreat attention in recent years because of their potential use in growth of In-filled IO nanotubes.scientific and technological applications. These nanostructures II. EXPERIMENTAL DETAILSoffer an opportunity to investigate the effect of dimensionalityand size on the material’s physical properties.1−5 Among them, The vapor phase transport and condensation in the presence ofone of the important metal oxides, indium oxide (IO), has been reducing reagents ethanol have been used for the growth of IOsynthesized in different morphologies due to their wide band nanowires having hexagonal nanocavities and indium-filled IOgap (∼3.7 eV) and high transparency in the visible region (80− nanotubes on the Si(100) substrate. The In2O3+C (1:1) mixed90%), which makes them suitable for a number of applications powder as a precursor was placed in an alumina boat andin nanoelectronics and optoelectronic devices.6−8 Besides the inserted into the heating zone of quartz tube open at both ends.advantage of IO pyramids, nanowires, and nanorods in a The system was heated to 1000 °C at a rate of 20 °C/min.number of applications,7,9−12 particularly nanotubes have their Silicon substrates coated with about ∼5 nm gold film wereown advantage and may offer unique applications.13,14 In the placed downstream at a temperature of 960 °C. A smallpast few years, much effort has been paid in developing hollow reservoir (5−10 mL) of ethanol was placed in the lownanostructures including nanotubes and hollow nanoparticles temperature region (∼65 °C) in the upstream direction duringowing due to their unique morphology, which shows their the growth. The partial pressure of reaction species, In and O,applications in delivery vehicle system, nanofilters, nano- during growth was varied by controlling the flow rate of thethermometer, and mass sensor with an atomic accu- carrier argon gas at (i) 30 sccm at a deposition pressure of 1racy.1,13,15−21 The tubular nanostructures with nanoporosity/ Torr (set I) and (ii) 200 sccm at a deposition pressure of 760nanocavities were reported to have entirely different properties Torr (set II). The deposition time was 1 h, and argon gas flowthan their regular shape and have shown unique enhanced was stopped after the deposition. The furnace was switched offcatalytic and superior optical absorption applications.22 Besides and cooled to room temperature.the vast applications of hollow nanostructures, there is no clear The morphological and structural details of the grown IOmechanism for the growth of metal oxide nanotubes except for nanostructures were studied using scanning electron micros-a few attempts reported in the literature.23−25 For example, copy (SEM) (Zeiss EVO 50) and high-resolution transmissionMorin et al. recently proposed a screw dislocation-driven electron microscopy (HRTEM, Tecnai G20-Stwin at 200 kV).growth mechanism for single-crystalline ZnO nanotubes.23 The scanning transmission electron microscopy (STEM) in In the present study, we propose a unified growth model forthe nanotube formation. We argued that In/O partial pressure Received: December 4, 2011during the growth determines the final IO tubular morphology. Revised: February 11, 2012The low partial pressure of indium vapors results in the growth Published: February 13, 2012 © 2012 American Chemical Society 5450 dx.doi.org/10.1021/jp211658a | J. Phys. Chem. C 2012, 116, 5450−5455
  2. 2. The Journal of Physical Chemistry C Articlenanoprobe mode with energy dispersive X-ray (EDX) facilitywas used to determine the In/O stoichiometry across thenanostructures. For precise measurements, these one-dimen-sional nanostructures were scanned after each EDX datacollection, and the experiment was repeated three times. Themorphology of IO nanostructures changes dramatically for set Iand set II as the carrier gas flow rate and pressure increase. Forgas sensing measurement, the interdigitated electrodes of goldwere thermally deposited on top of the nanostructures andmounted on the PID controlled heater with the temperatureaccuracy of ±1 °C (Excel Instruments). The gas sensingmeasurements was performed at 142, 221, and 300 °C with1000 ppm ethanol. Prior to gas sensing measurement, thesample was heated at 350 °C for 3 h in cyclic flow of ethanoland air, respectively, to eliminate the effect of humidity and forthe stability of the sensor. After each cycle, the dry air waspassed over the sensor to avoid the moisture from the ambientas well as remove the ethanol gas from the chamber. The sensor Figure 2. (a) TEM image of nearly equally spaced nanocavity in IOresponse for ethanol gas is calculated as the ratio of resistance nanowire and (b) TEM image of In-filled IO nanotube. Inset of (a)in air to that in the ethanol gas (Rair/Rgas), where Rair is the and (b) shows higher magnified TEM micrograph of single nanocavityresistance of sensor in air and Rgas is the resistance in ethanol and In-filled cavity in IO nanotube, respectively. (c) HRTEM ofgas ambient, while the response time is taken as the time nanocavity shows lattice fringes of spacing 0.41 nm, which corresponds to the (211) plane of IO, and the inset reveals the HRTEM thatrequired to reach 90% of the saturated value of resistance in the confirms the growth direction of nanowires along the ⟨100⟩ direction.ethanol gas. (d) HRTEM of In-filled nanotube showing side walls and In-filled nanocavity. The HRTEM confirms the growth of IO nanotubes alongIII. RESULTS AND DISCUSSION the ⟨100⟩ direction. A. Growth of Nanoflute and Indium-Filled IO Nano-tubes. Figure 1 shows the TEM micrographs of set I and set II and are nearly equally spaced along the nanowire length. Interestingly, the nanocavity has a hexagonal shape, as shown in the inset of Figure 2a. The size of the nanocavity was measured over more than 100 nanowires and found to vary from 10 to 30 nm with an peak average size of 10−15 nm. Figure 2b shows the TEM micrograph of IO nanostructure grown with comparatively higher argon flow rate and at an atmospheric pressure that resulted in an arrow-like tubular nanostructure. The structure has a uniform cylindrical base with a typical diameter of 95 nm and an octahedron tip having the radius of curvature as low as 5 nm at the end. The cavity and wall thickness of various IO tubular structures vary from 10 to 60Figure 1. (a) TEM micrographs of nanocavity decorated IO nanowires nm and 35 to 100 nm, respectively. The HRTEM micrographsand (b) indium-filled IO nanotubes. The inset shows the of the tip of the IO nanostructures are shown as insets in Figurecorresponding SEM images. The lower In partial pressure during 2b. The images reveal the tubular nature of the structure, whichgrowth resulted in the growth of nanocavities decorated IO nanowires, is filled with indium metal up to the tip end, which is laterwhile the higher In partial pressure ended with the In-filled IO confirmed with STEM-EDX measurements. A high-resolutionnanotubes. TEM image of a single nanowire with nanocavity is shown insamples along with the SEM images in the inset. Set I samples Figure 2c, which shows the lattice planes. This confirms thatshow nanowires grown with hexagonal nanocavities, Figure 1a, the nanocavity resides within the nanowire body but not on the surface of the nanowire as in the case of titania nanorods aswhereas set II samples show a completely different morphology reported in literature.22 The interplanar spacing of 0.41 nm inof indium-filled IO tubular nanostructures, Figure 1b. The nanowire corresponds to the (211) plane of cubic IO. HRTEMdense and uniform growth of IO nanostructures in sets I and II of IO nanowire, inset of Figure 2c, reveals that the latticeis clearly observed in SEM images. The nanowires and fringes correspond to the (200) plane of IO and clearly showsnanotubes were observed to have nearly the same average the ⟨100⟩ growth direction of nanowires with nanocavitylength of ∼5−12 μm, but their diameters vary from 25 to 90 structures. Figure 2d shows the HRTEM for IO nanotube. Thenm for set I samples and from 40 to 250 nm for set II samples, lattice fringes of IO shell planes and interplanar spacing of tworespectively. The TEM micrograph of set I sample in Figure 1a orthogonal planes as 0.506 and 0.71 nm correspond to (200)clearly shows the presence of nanocavities within the nanowire and (110) planes of IO nanotube, respectively. Interestingly,structures, while the TEM micrograph of set II reveals the both nanostructures show the same growth direction oftubular nanostructures. ⟨100⟩.26,27 The presence of nanocavity in IO nanowire and A TEM image of a single nanowire having the nanocavities is indium-filled nature of IO nanotubes is confirmed usingclearly shown in Figure 2a along with the high-resolution TEM scanning transmission electron microscopy and energyimage of the single nanocavity as in the inset. The TEM dispersive X-ray analysis (STEM-EDX) study, which is shownmicrograph reveals that these nanocavities are uniform in size in Figure 3a,b. Figure 3 a shows the EDX line profile with an 5451 dx.doi.org/10.1021/jp211658a | J. Phys. Chem. C 2012, 116, 5450−5455
  3. 3. The Journal of Physical Chemistry C ArticleFigure 3. STEM EDX along the radial direction of (a) nanocavities decorated IO nanowires and (b) In-filled IO nanotube along with STEM imageas an inset. The variation of In and O across the nanostructure confirms the presence of nanocavity in nanowire and the In-filled nature of IOnanotube, respectively.Figure 4. (a,b) Precursor stage, nucleation stage, growth stage, and final morphology stage for the growth of nanocavities decorated IO nanowiresand In-filled IO nanotube. The different filled spheres in the precursor stage are the symbolic representation of different reaction species. [●]indium; [gray ●] oxygen; [one gray, two black cluster] oxygen deficient IO; [three gray, two black cluster] IO.inset showing a line through a single nanocavity. The variation substrate. As the adsorption of reaction species continues, theof In and O across the nanocavity clearly confirms the presence growth of thermodynamically allowed crystallographic planesof nanocavity in IO nanowire. The STEM-EDX measurement takes place. Hao et al. have mentioned that the growth rate (r),on IO tubular structure along its radial direction is shown in perpendicular to three different low-indexed crystallographicFigure 3b along with the STEM image as the inset. The higher planes in IO, is proportional to the surface energies, γ{111} <In/O ratio in the central region confirms that the IO nanotube γ{100} < γ{110}.26 So, the growth rate perpendicular to theis filled with In metal. {111} planes, that is, r{111}, is comparatively slower, and the B. Growth Mechanism. It was reported that the two {111} planes have a tendency to appear as facets on the fastdifferent growth mechanisms, vapor−solid (VS) or vapor− growing {110} and {100} planes.26,30 Therefore, as theliquid−solid (VLS), for the growth of nanostructures are deposition continues, different planes grow according to theirstrongly influenced by the reaction kinetics.28,29 In our previous surface energy and lead to the formation 1D IO nanostructuresstudy, we have established that the partial pressure of incoming with growth direction along ⟨100⟩, which is confirmed byreaction species changes the growth mechanism from VLS to HRTEM images as shown in Figure 2c and d for bothVS in a conventional vapor transport method.30 Therefore, the nanostructures. The lower partial pressure of In metal in thesustainability of VLS growth is affected by the partial pressure growth reduced the possibility of In-rich core during theof the incoming reaction species. In the present case, IO+C precursor state. On the other hand, the indium species that arecarbothermally reduced to oxygen-deficient In2O3−x, and the incorporated and embedded during the growth of nanowiresreducing ambient, ethanol, further makes the growth ambient have a tendency to out-diffusion and contributes in the growthindium rich. In set II growth conditions, the higher carrier gas of IO at the surface of IO nanowires and leave the vacanciesflow rate at 200 sccm and atmospheric growth pressure, 760 behind, which is shown in the nucleation stage in Figure 4a.Torr, may increase the partial pressure of In/O vapors in The low growth pressure, nearly 3 orders of magnitude lesscontrast to set I growth conditions of gas flow rate at 30 sccm than the atmospheric pressure in set II growth conditions, mayand nearly 3 orders lower growth pressure of 1 Torr. The also promote the out-diffusion of In. The out-diffusion of metalchange in the ratio of In/O partial pressures along with the in the formation of the nanocavity has already been discusseddifference in the pressure of growth chamber may be for the growth of cobalt oxide and iron oxide hollowresponsible for this structural transformation. nanoparticles and nanocrystals.19,31 The out-diffusion of the The different stages such as precursor, nucleation, growth, In metal during growth may be sustained and supported by theand final morphology stages are shown in Figure 4. In the lower growth pressure of 1 Torr. As the out-diffusion continues,nucleation stages, the reaction species condense on the silicon the numbers of vacancies increase and no longer separate, butsubstrate and form the nuclei of oxygen-deficient IO on silicon they coalesce inside IO nanowire bulk and start to form a 5452 dx.doi.org/10.1021/jp211658a | J. Phys. Chem. C 2012, 116, 5450−5455
  4. 4. The Journal of Physical Chemistry C Articlenanovoid structure, as shown in the growth stage in Figure 4a. of the above-mentioned factors result in the formation of aThese nanovoids combine and form a nanocavity within IO bottom-truncated octahedron. As the reaction proceeds further,nanowires bulk; the final morphology of nanocavities decorated the reagent species get adsorbed directly at the bottom and onIO nanowires is shown in the morphology stage in Figure 4a. the facet planes of the growing structure. Because the facetThe self-catalytic grown nanowires consist of nanocavities with planes of the octahedrons are the minimum surface energythe cavity size varying from 10 to 30 nm and have hexagonal planes and have low binding energy and high desorptionappearance, as shown in Figure 2a. More specifically, the probability of incoming reaction species, therefore, the growthhexagonal appeared nanocavity may be the projection of three- along these plane is minimum.35 It is quite probable that the In-dimensional octahedron cavities inside the IO nanowire along rich reaction species initially adsorbed on the facets of thethe [111] direction.32 The octahedron cavity is truncated in octahedrons are transferred to the bottom of the structure. Asnature as is clearly revealed from the inset of Figure 2a. The the growth continues, the excess In may phase separate fromgrowth of truncated octahedron nanocavity has also been In2O3 and get encapsulated inside the octahedron structures.reported in Si at SiC thin film/Si substrate by annealing at 1050 Indium inside the core is sucked from the base of the growth,°C.32 Some of the nanocavity in IO nanowires gives a circular due to the good wetting properties between In and IO surfacesappearance, but that could be due to the more complex nature and the capillary effect in the nanosize cavity.25 Figure 4b showsof polygon nanocavity in IO nanowires. The growth of IO the growth stage of In-filled IO nanotube. Because of the lownanowires with nanocavity is governed by self-catalytic vapor− melting point of indium metal filled inside the IO nanosizesolid growth followed by the out-diffusion of indium-rich cavity, the large vapor pressure at growth temperature mayspecies from IO nanowires and the formation of nanovoids, the exert a continuous thrust in upward growth direction. TheKirkendall effect.29,31−34 The hexagonal shape of the nanocavity vapor pressure of macroscopic indium heated at 923 °C is 1may be considered as due to the cubic symmetry of IO crystal. Pa.36 It has been reported that the vapor pressure has an inverseIt is understandable that the minimum energy planes like (111) dependence on the diameter of the cavity of nanotubemay form the facets for the growth of IO octahedron structures.37 Therefore, the large upward thrust of meltedstructures.26−30 The growth of nanocavity may be considered indium in the nanosize cavity may result in the growth ofsimilar to the growth of IO octahedron structures having the indium-filled IO nanotube.{111} facets but with no IO atoms.32 In 3D, nanocavity may In the case of nanocavities decorated IO nanowires where thevisualize as the truncated octahedron structures with no IO growth pressure is nearly 3 orders of magnitude smaller thanatoms in IO nanowires. The angle between {111} and the other the atmospheric pressure, the initial In embedded may be atoccurred planes is ∼35°, which indicates the possibility of nanoscale. Figure 5a reveals the observed initial diameter of In{110}, which is another higher surface energy plane in IO. So,the truncated octahedron nanocavity is enclosed with {111}planes and {110} planes. The (110) plane is not the minimumenergy plane, but the occurrence of these planes may beenergetically possible because it may reduce the entire surfaceenergy of the nanocavity.32 The free energy per unit volumefavors to enlarge the nanocavity, but the surface energy ofsurrounded cogrowth IO planes disfavors that. So, the totalminimization energy determines the size of the nanocavity inIO nanowires. It is important to note that the size of differentnanocavities in IO nanowires has the maximum size in therange of 10−15 nm in IO nanowires. The formation of Figure 5. (a) Initial stage of nanovoids formation for the final growthhexagonal shape voids has also been reported in Si(111).32 In of nanocavities decorated IO nanowires. (b) Partially In-filled IOSi(111), the presence of higher surface energy planes was nanotube.attributed to the higher substrate temperature during growth(1050 °C) in comparison to the 850 °C, which results in the embedded during the growth of nanocavities embedded IOformation of triangular voids enclosed within (111) planes. In nanowires and confirmed the proposed growth mechanism,our case, the growth temperature is 960 °C, which is higher while the presence of indium in IO nanotubes was confirmedthan the 850 °C and, that may promote the appearance of using STEM-EDX elsewhere.38 Nearly 2−4 nm voids at growthhigher surface energy planes to enclose the nanocavity in IO temperature may exert an order of magnitude higher pressurenanowires and reduce the total surface energy of nanosize than nanocavity of 10−60 nm in In-filled IO nanotubes andcavity. As In/O partial pressure increases by increasing the gas results in the In out-diffusion and hence the formation offlow rate, 200 sccm, and working pressure, 760 Torr, the nanoscale voids (Figure 5a). These voids further coalesce andgrowth ambient becomes In-rich and supports the growth of form the hexagonal nanocavity inside the IO nanowire. Some ofindium-filled IO nanotubes. The different stages during the the nanotubes grown with set II were observed partially filledgrowth of indium-filled IO nanotube are shown in Figure 4b. with In, Figure 5b, indicating the possibility of In out-diffusionThe precursor stage in Figure 4b describes the formation of In- from the thin IO shell during the growth.38rich nuclei on the silicon substrate. With time, the excess Finally, the representative gas sensing response of nanocavityindium has a tendency to re-evaporate and/or oxidize except at decorated IO nanowires is shown in Figure 6a. The inset showsthe vicinity of substrate. The bottom of the growing structure is the electrodes used to measure the gas sensing. The effect ofin contact with the substrate and does not get sufficient oxygen substrate temperature on gas sensor response and responseto form the IO. So, initially the growth along this plane is time for both IO nanoflute and IO nanotube structures isrelatively slower. Other IO planes are expected to grow shown in Figure 6b. The sensor response of IO nanoflute andaccording to the surface energy value: γ{111}< γ{100}< γ{110}.26 All indium-filled IO nanotubes increases from 2.5 to 18 and from 5453 dx.doi.org/10.1021/jp211658a | J. Phys. Chem. C 2012, 116, 5450−5455
  5. 5. The Journal of Physical Chemistry C ArticleFigure 6. (a) The representative sensing response of nanocavity decorated nanowires measured with gold interdigitated electrodes at 300 °C for1000 ppm ethanol and (b) sensor response and response time for nanocavity decorated nanowires and indium-filled IO nanotubes as a function oftemperature.1.1 to 3.7, while the response time decreases from 16 to 3 s and pressure than atmosphere in the growth chamber induced the50 to 20 s, respectively, as temperature increases from 142 to out-diffusion of indium metal and results in the formation of300 °C. The interaction of ethanol gas and chemisorbed oxygen indium voids in the growing IO nanowire. As the growthon the surface of IO nanostructures is responsible for the continues, these nanovoids coalesce, and this results in thechange in the resistance of sensor. The possible reaction growth of discrete and hexagonal-shaped nanocavities enclosedbetween ethanol and chemisorbed oxygen is as follows:39 by minimum surface energy planes of IO, {111}. The atmospheric growth pressure and nearly 1 order of magnitude 6O− + C2H5OH(g) → 3H2O(g) + 2CO2 (g) + 6e− higher gas flow rate result in a higher In/O partial pressure and cause the formation of continuous and indium-filled IO The reactions produce more electrons and thus increase the nanotubes. ■conductivity of n-type IO nanostructures upon exposure toethanol. The temperature-dependent sensor response can beunderstand on the basis of temperature-dependent interaction AUTHOR INFORMATIONof ethanol and chemisorbed oxygen on the surface of Corresponding Authornanowires. The surface of oxide nanostructures can be covered *E-mail: mukesh.iitdelhi@gmail.com (M.K.); jpsingh@physics.by chemisorbed oxygen in the form of O2−, O−, or O2− iitd.ac.in (J.P.S.).depending on the sample temperature. The O2− is the mostunstable due to its high charge.40 Therefore, the possibility of Notesits occurrence at the surface is very small. The O2− is the The authors declare no competing financial interest. ■dominant species near room temperature, but the surfacecoverage due to this species is low. Also, the O2− has much REFERENCESlesser reactivity than O−.39 This explains the low sensor (1) Iijima, S. Nature 1991, 354, 56.response and large response time of IO nanostructures at 142 (2) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.;°C. With increase in temperature, adsorption of O− becomes Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353.the dominant species and results in increased sensitivity and (3) Cui, Y.; Lieber, C. M. Science 2001, 291, 851.reduced response time at 300 °C. The higher sensing of (4) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Naturenanocavity decorated IO nanowires than indium-filled IO 2001, 409, 66.nanotubes may be due to the presence of nanocavities in (5) Wu, Y.; Messer, B.; Yang, P. Adv. Mater. 2001, 13, 1487.nanowires, which increases the surface area and hence the (6) O’Dwyer, C.; Szachowicz, M.; Visimberga, G.; Lavayen, V.;sensing properties. Newcomb, S. B.; Sotomayor Torres, C. M. Nat. Nanotechnol. 2009, 4, 239. (7) Nguyen, P.; Ng, H. T.; Yamada, T.; Smith, M. K.; Li, J.; Han, J.;IV. CONCLUSIONS Meyyappan, M. Nano Lett. 2004, 4, 651.In summary, the mechanism of IO nanotube growth is (8) Wan, Q.; Dattoli, E. N.; Fung, W. Y.; Guo, W.; Chen, Y.; Pan, X.;proposed on the basis of morphological transformation from Lu, W. Nano Lett. 2006, 6, 2909.IO nanowire embedded with discrete and hollow nanocavities (9) Du, N.; Zhang, H.; Chen, B.; Ma, X.; Liu, Z.; Wu, J.; Yang, D.to continuous hollow and indium-filled IO nanotubes. Our Adv. Mater. 2007, 19, 1641. (10) Jia, H. B.; Zhang, Y.; Chen, X. H.; Shu, J.; Luo, X. H.; Zhang, Z.;results suggest that In/O vapor pressure is an important Yu, D. Appl. Phys. Lett. 2003, 82, 4146.controlling parameter for morphological transformation to (11) Li, C.; Zhang, D.; Liu, X.; Han, S.; Tang, T.; Han, J.; Zhou, C.nanotubes growth. The comparative lower growth pressure and Appl. Phys. Lett. 2003, 82, 1613.lower gas flow rate results in the growth of 2−4 nm nanovoids (12) Li, C.; Fan, W.; Lei, B.; Zhang, D.; Han, S.; Tang, T.; Liu, X.;in IO nanowires. The high In vapor pressure in nanovoids at Liu, Z.; Asano, S.; Meyyappan, M.; Han, J.; Zhou, C. Appl. Phys. Lett.growth temperature and nearly 3 orders of magnitude lower 2004, 84, 1949. 5454 dx.doi.org/10.1021/jp211658a | J. Phys. Chem. C 2012, 116, 5450−5455
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