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2007, 111, 3211-3215 Published on Web 02/07/2007
MOCVD Template Approach to the Fabrication of Free-Standing Nickel(II) Oxide Nanotube Arrays: Structural, Morphological, and Optical Properties Characterization Graziella Malandrino,*,† Laura M. S. Perdicaro,† Ignazio L. Fragala` ,† Raffaella Lo Nigro,‡ Maria Losurdo,§ and Giovanni Bruno§ Dipartimento di Scienze Chimiche, UniVersita` di Catania, and INSTM, UdR Catania, V.le A. Doria 6, 95125 Catania, Italy, Istituto per la Microelettronica e Microsistemi, IMM-CNR Stradale Primosole 50, 95121 Catania, Italy, and Institute of Inorganic Methodologies and of Plasmas, IMIP-CNR, and INSTM, UdR Bari, Via Orabona, 4-70126 Bari, Italy ReceiVed: NoVember 20, 2006; In Final Form: January 25, 2007
Ordered homogeneous arrays of nickel (II) oxide nanotubes have been fabricated through an MOCVD template route using Ni(tta)2tmeda (Htta ) 2 thenoyl-trifluoroacetone, tmeda ) tetramethylendiamine) as a source. The X-ray diffraction pattern shows the formation of the NiO cubic phase. Scanning electron microscopy (SEM) images of the NiO nanotubes, after removing the template, indicate the formation of well ordered nanotube arrays. There is evidence, through transmission electron microscopy studies, that nanotubes are opened on both sides. The optical properties of NiO nanotubes, determined using spectroscopic ellipsometry, strongly depend on the polarization direction of the light electric field, indicating optical anisotropy that relates to the vertical alignment of nanotubes.
Introduction Nanotubes have attracted great interest in the past decade, because of the ability to control their properties by tuning their dimensions.1-4 Several possible applications of inorganic nanotubes can be foreseen. Due to their cylindrical geometry, these nanomaterials have a low mass density, a high porosity, and an extremely large surface to weight ratio. In fact, one of the effects of growing porous materials at the nanoscale is that the effective surface is dramatically increased. This is especially important in applications that involve an interaction between gases or liquids and the material surface, including gas sensors, solvent recovery, catalysis, solid-state batteries, and fuel cell electrodes. In addition, nanotubes possess several different areas of contact (borders, inner and outer surfaces, and structured tube walls) that in principle could be functionalized in several ways. Numerous growth techniques have been used for the synthesis of metal, oxide, halide, and chalcogenide nanotubes or nanowires. In some cases, the techniques use a “template” to synthesize well aligned nanostructures, and for this reason, they are usually reported as “template methods”. Pioneering studies in this area have been proposed by Martin et al. using different type of hard templates [track-etch polymeric, anodic aluminum oxide membranes, and carbon nanotubes (C-NTs)].5 As a matter of fact, the recent development and easy availability of ordered nanoporous alumina membranes through an easy and low-cost anodic process, have raised great interest in their application to fabricate nanostructured materials of metals or oxides. Template-directed synthesis represents a simple route to 1D nanostructures. The requirement to fabricate nanowires and * Corresponding author. E-mail:
[email protected]. † Universita ` di Catania. ‡ IMM-CNR. § IMIP-CNR.
10.1021/jp067696o CCC: $37.00
nanotubes through this route is that the material can be loaded into the pores of the template using a method based on vaporphase evaporation, liquid-phase injection, or solution-phase chemical or electrochemical deposition. Metal-organic chemical vapor deposition (MOCVD) offers the potential advantage of being a very reliable and reproducible method for the fast production of films with highly uniform thickness and composition over large areas. With regard to nanostructured materials, its potentiality in fabricating C-NTs,6 IrO2 NTs,7 In2O3 NTs,8 R-Fe2O3 NTs,9 and CuO NT arrays10 has been already demonstrated. Nickel oxide (NiO) is a very interesting materials for all of the above-mentioned applications, and to date, only solution routes, assisted by a template, have been reported to fabricate NiO nanotubes.11,12 In this paper, we report a simple and flexible fabrication method of dense homogeneous arrays of discrete aligned nickel(II) oxide nanotubes through an MOCVD template route using Ni(tta)2tmeda (Htta ) 2-thenoyl-trifluoroacetone, tmeda ) tetramethylethylendiamine)13 as a source. In addition, the optical properties of well-aligned vertical NiO nanotubes are studied using spectroscopic ellipsometry, which shows optical anisotropy due to the nanotubes alignment. Monitoring the optical properties may be of great interest in view of applications of these NiO nanosystems as optical gas sensors; that is, the reaction of the sensor with the gas causes changes of the optical properties. Experimental Methods The porous anodic alumina (AAO) membranes used as a template were purchased from Whatman. This template consists of vertical pore channel arrays with a hexagonal packing structure. The diameter of the self-ordered nanopores is 200 nm. © 2007 American Chemical Society
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In the fabrication of NiO nanotube arrays, the membrane was vertically positioned in a horizontal hot-wall reactor with the pores parallel to the gas flow and maintained at 500 °C using a 150 sccm Argon flow as carrier gas and 150 sccm oxygen flow as reaction gas. Depositions were carried out under reduced pressure (about 1-2 Torr) varying the deposition time from 60 to 120 min. X-ray diffraction patterns (XRD) were recorded on a θ-θ D5005 Bruker diffractometer using Cu KR radiation operating at 40 kV/30 mA over a 20° < 2θ < 70° angular range. The surface morphology of the arrays was examined by field emission scanning electron microscopy (FESEM) using a ZEISS SUPRA 55 VP. The chemical composition was assessed through energy dispersive X-ray analyses using the windowless IXRF detector. NiO nanotubes were characterized by transmission electron microscopy using a TEM JEOL 2010 F instrument operating at 200 KeV. Optical properties, i.e., spectra of the complex refractive index, N ) n + ik, where n is the refractive index and k is the extinction coefficient, of the NiO nanotube arrays supported on the alumina membrane were measured in the photon energy range 0.75-6.5 eV with an energy resolution of 10 meV using a phase modulated spectroscopic ellipsometer (Uvisel-Jobin Yvon). A linearly polarized light beam from a Xe lamp impinged on the top surface of the membrane (i.e., on the side facing the gas flow within the reactor) at various angles of incidence in the range 55-75° with an increment of 5°. Light reflected from the sample and becoming elliptically polarized was analyzed. In particular, ellipsometry measures the ratio, F, of the Fresnel reflection coefficient of the p-polarized (parallel to the plane of incidence of the linearly polarized light beam) and s-polarized (perpendicular to the plane of incidence) light reflected from the surface through the ellipsometric angles Ψ and ∆ defined by the equation
F ) tan Ψ exp(i∆) where tan Ψ ) |Epj|/|Es| and ∆ ) δp - δs represent the amplitude and phase variation of the electric field vector associated with the light electromagnetic wave. F and, hence, Ψ and ∆ are related to the film pseudodielectric function, 〈〉 ) 〈1〉 + i〈2〉, and complex refractive index, N ) (n + ik) (where n is the real refractive index and k is the extinction coefficient) through the equation
[
〈〉 ) 〈1〉 + i〈2〉 ) sin2 φ 1 + tan2 φ
]
(1 - F)2 (1 + F)2
) 〈N2〉
) 〈(n + ik)2〉 where φ is the angle of incidence. Experimental ellipsometric spectra were modeled using a two-layer model consisting of a bulk anisotropic layer with surface roughness (the latter described using the Bruggemann effective medium approximation of 50% bulk material and 50% voids), according to the approach described in ref 14. For the surface roughness layer thickness, data of the root-mean-square roughness (rms )45 nm) from atomic force microscopy measurements were considered. Results and Discussion Porous anodic alumina (AAO) membranes have been used as template materials because they have uniform pores of high
Figure 1. (a) Assembled NiO nanotube array in an AAO template. X-ray diffraction pattern indicating the formation of the cubic NiO phase. (b) Free-standing NiO nanotube arrays. EDX spectrum showing that no Al is present after the template removal.
density, are chemically stable during the process at least up to 800 °C, can be easily removed, and finally are commercially available. The nanotubes were obtained by depositing on vertically positioned membranes, in conditions optimized for the deposition of thin films, i.e., at deposition temperature of 500 °C, for 60 min under reduced pressure using oxygen as reaction gas and argon as carrier gas. The nature of fabricated nanotubes has been investigated by X-ray diffraction (XRD) before removing the AAO membrane. The XRD pattern (Figure 1a) shows more peaks at 2θ values of 37.21°, 43.22°, and 62.78° corresponding to 111, 200, and 220 reflections, respectively, of the NiO phase with a cubic lattice.15 The presence of all of these reflections clearly points to the polycrystalline nature of the nanotube arrays. Aluminum oxide has been removed using an alkaline solution. The nickel oxide nanotubes have been recovered after a treatment in NaOH (2M) solution for 2 h at room temperature, washed with distilled water and ethanol several times, and dried in air. Nanotube sheets have been recovered using transmission electron microscopy gold grids. The complete dissolution and elimination of the template has been confirmed using energy dispersive X-ray (EDX) analyses (Figure 1b). The nickel KR and Kβ peaks are observed at 7.460 and 8.250 keV; the L lines are spread in the 0.86-1.020 keV range. In addition, the windowless EDX detector allowed us to detect the O KR peak at 0.520 keV, and to exclude any F or S contamination, since no peaks are observed at 0.670 and at 2.307 keV, respectively. Note that no aluminum peak can be detected, thus confirming a complete removal of the AAO membrane. The morphology of the NiO nanotube arrays, obtained for a deposition time of 60 min, has been examined by field emission scanning electron microscopy (FE-SEM). In the low magnification image (Figure 2a) well ordered nanotube arrays, within the
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Figure 2. NiO nanotube arrays SEM images: (a) low magnification plan-view of the NiO nanotube sheet embedded in the AAO template. (b and c) High magnification plan-views of the free-standing NiO nanotube sheet.
membrane, are clearly visible. Upon removal of the template, there is evidence of the formation of very wide nanotube sheets. The nanotube arrays keep memory of the nanochannels alignment in the AAO membrane, even when the template is removed. This is a relevant key issue since fabrication of freestanding aligned nanotubes generally represents a difficult task. The high magnification SEM image (Figure 2, panels b and c) shows that nanotubes have an outer diameter of about 200 nm, whereas nanotube walls are about 20 nm thick. Nanotubes are about 1 µm in length. The diameter of nanotubes is controlled by the characteristic pore dimension of the template membrane, whereas their length depends on the process conditions. Nanotubes are opened on both sides and this feature becomes clearly discernible in the transmission electron microscopy (TEM) micrographs reported in Figure 3, panels a and b. The low resolution plan TEM micrographs show several nanotubes uniform in diameter of about 200 nm. The higher magnification micrograph indicates that the nanotubes have walls of very small thickness of about 15 nm (Figure 3b). Through performing a selected-area electron diffraction (SAED) analysis on the nanotube arrays, a clear electron diffraction pattern composed of several rings is obtained. At least five diffraction rings can
Figure 3. TEM micrographs of a free-standing NiO nanotube array: (a) low and (b) high magnification plan-views of the NiO nanotube sheet. (c) SAED pattern of the NiO nanotubes.
be distinguished, with average d spacings (distance between adjacent planes with any given Miller indices (hkl) within the crystals) of 2.42, 2.09, 1.48, 1.27, and 1.21 Å associated with the 111, 200, 220, 311, and 222 reflections, respectively. The SAED results, in accordance with XRD data, demonstrate that the nanotube arrays are polycrystalline of the bunsenite NiO phase. The optical properties of the NiO nanotubes supported on the membrane have been measured using spectroscopic ellipsometry. Specifically, the NiO-nanotube system has been modeled as an anisotropic layer with surface roughness. The Tauc-Lorentz16 dispersion equation has been used to parametrize the energy dependence of the optical properties. Figure 4 shows typical experimental spectra of the ellipsometric angles Ψ and ∆ acquired for a NiO nanotube array supported on the
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Figure 4. Spectra of the ellipsometric angles Ψ and ∆ acquired for a NiO nanotube array supported on the alumina membrane.
alumina membrane. The spectra are characterized by an interference system below approximately 3.5 eV, which originates from multiple reflections at the NiO/alumina interface, since below the NiO band gap the sample is transparent and the probing light propagates throughout the NiO nanotube array thickness. Above the NiO optical band gap, because of the NiO absorption, the penetration depth, d, being inversely related to the absorption coefficient, R, d ) 1/R, decreases with the increase of R, and at higher photon energy only the NiO outmost layers are probed. It is worth noting that the experimental spectra are not affected by scattering, yielding reliable results. Scattering only results in a decrease of the intensity of light reaching the detector; however, by increasing the voltage of the photomultiplier (detector), there is enough signal whose polarization state can be analyzed. The continuous line in Figure 4 represents the fit spectra obtained modeling the sample as alumina-support/anisotropic NiO nanotube array/surface roughness. The NiO nanotube assembly is considered anisotropic because of their cylindrical symmetry, and hence, their optical behavior is expected to strongly depend on the polarization direction of the electric field of the incident light beam. Therefore, an ordinary and extraordinary complex refractive index corresponding to components of the electric field of the incident light beam parallel and perpendicular to the vertical axial direction of nanotubes must be defined (see scheme in Figure 5). The extraordinary component may be strongly modified/affected by the presence of interfaces. The derived spectra of the ordinary and extraordinary refractive index (no, ne) and extinction coefficient (ko, ke) are reported in Figure 5. The optical functions of a NiO single crystal and of a NiO continuous thin film have been previously reported to be isotropic.17,18 These data showed that the refractive index and extinction coefficient of the film were lower than those of the single crystal because of the lower density of the thin film. NiO crystal and film were characterized by a sharp rise in the absorption at approximately photon energy of 4 eV, which has been attributed to transitions from 3d Ni valence electrons to 4s conduction band. In the present case, a highly anisotropic optical response has been observed. A reduction of the optical Tauc gap from 4 eV, previously observed for the NiO single crystal or continuous thin films, to 3.47 ( 0.24 and 2.19 ( 0.12 eV for the ordinary and extraordinary optical functions, respectively, is found and may be attributed to the so-called curvature effects19 due to the finite diameter of nanotubes. A similar behavior, i.e., a decrease of the band gap with the decrease of diameter, has been found
Figure 5. Spectra of the ordinary and extraordinary refractive index (no and ne) and extinction coefficient (ko and ke) for NiO vertically aligned nanotubes.
by calculations.20 The lower optical gap found for the extraordinary light ray can be explained by the fact that the extraordinary ray passes through the nanotube walls, where dangling bonds exist. The dangling bonds on nanotube walls cause defect states localized into the gap, resulting in a decrease of the edge of the fundamental absorption. Furthermore, the hollow geometry plays an important role in the optical properties of nanotubes; that is, the decrease of the volume fraction of solid part of nanotubes results in spectra of the refractive index and extinction coefficient lower than that reported for NiO single crystals and continuous thin films.18 Conclusions In summary, NiO nanotubes have been grown through a template-MOCVD route. This approach has the advantage of being a very simple method for the easy production of nanostructures such as free-standing NiO nanotube arrays on large-area. The synthesized nanotubes can be used either directly embedded in the template or in a free-standing mode since the template can be easily removed using a selective chemical etching. The optical properties of the NiO nanotubes have been determined using spectroscopic ellipsometry, and a strong optical anisotropy has been detected, which relates to the vertical alignment of nanotubes. Finally, note that the presently reported MOCVD-template route represents an actually challenging approach not only for the fabrication of simple oxides but also in view of the fabrication of nanoscale mixed, multielement oxides since the MOCVD technique has been widely and successfully applied to depositions of multielement oxide films. On the other hand, nanotube arrays of these complex systems could be hardly obtained with techniques such as sol-gel, electrochemistry, or evaporation, up to now essentially applied to single element oxides. Acknowledgment. This work has been partially supported by MIUR (Ministero dell’Istruzione, dell’Universita` e della
Letters Ricerca) within the PRIN 2005 project. The authors thank Mr. Corrado Bongiorno of the IMM CNR of Catania for assisting in TEM observation. Supporting Information Available: SEM images of the NiO nanotubes arrays obtained before the complete removal of the template in cross-section and as plan view. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (2) (a) Rao, C. N. R. Int. J. Nanosci. 2005, 4, 811. (b) Rao, C. N. R.; Manashi, N. Dalton Trans. 2003, 1. (c) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem. Int. Ed. 2002, 41, 2446. (3) Xiong, Y.; Mayers, B. T.; Xia, Y. Chem. Comm. 2005, 40, 5013. (4) Goldberger, J.; Fan, R.; Yang, P. Acc. Chem. Res. 2006, 39, 239. (5) (a) Martin, C. R. Science 1994, 266, 1961. (b) Martin, C. R. Chem. Mater. 1996, 8, 1739. (c) Cepak, V. M.; Hulteen, J. C.; Che, G.; Jirage, K. B.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R. J. Mater. Res. 1998, 13, 3072. (6) Terranova, M. L.; Sessa, V.; Rossi, M. Chem. Vap. Deposition 2006, 12, 315.
J. Phys. Chem. C, Vol. 111, No. 8, 2007 3215 (7) Chen, R.-S.; Huang, Y.-S.; Tsai, D.-S.; Chattopadhyay, S.; Wu, C.-T.; Lan, Z.-H.; Chen, K.-H. Chem. Mater. 2004, 16, 2457. (8) Shen, X.-P.; Liu, H.-J.; Fan, X.; Jiang, Y.; Hong, J.-M.; Xu, Z. J. Cryst. Growth 2005, 276, 471. (9) Shen, X.-P.; Liu, H.-J.; Pan, L.; Chen, K.-M.; Hong, J.-M.; Xu, Z. Chem. Lett. 2004, 33, 1128. (10) Malandrino, G.; Finocchiaro, S. T.; Lo, Nigro, R.; Bongiorno, C.; Spinella, C.; Fragala`, I. L. Chem. Mater. 2004, 16, 5559. (11) Liu, H.-J.; Peng, T.-Y.; Zhao, D.; Dai, K.; Peng, Z.-H. Mater. Chem. Phys. 2004, 87, 81. (12) Needham, S. A.; Wang, G. X.; Liu, H. K.; Yang, L. J. Nanosci. Nanotechnol. 2006, 6, 77. (13) Malandrino, G.; Perdicaro, L. M. S.; Condorelli, G.; Fragala`, I. L.; Rossi, P.; Dapporto P. Dalton Trans. 2006, 1101. (14) Tomkins, H. G. A user’s guide to Ellipsometry; Academic Press, Inc.: New York, 2005. (15) International Center of Diffraction Data (ICDD) No. 47-1049. (16) Jellison, G. E.; Modine, F. A. Appl. Phys. Lett. 1996, 69, 371. (17) Powell, R. J.; Spicer, W. E. Phys. ReV. B 1970, 2, 2182. (18) Franta, D.; Negulescu, B.; Thomas, L.; Dahoo, P. R.; Guyot, M.; Ohlidal, I.; Mistrik, J.; Yamaguchi, T. Appl. Surf. Sci. 2005, 244, 426. (19) Okada, S.; Saito, S.; Oshiyama, A. Phys. ReV. B 2002, 65, 165410. (20) Rubio, A.; Corkill, J. L.; Cohen, M. L. Phys. ReV. B 1994, 49, R5081.
