ARTICLE pubs.acs.org/crystal
Nanopipes in In2O3 Nanorods Grown by a Thermal Treatment David Maestre,*,†,‡ Dietrich H€aussler,‡ Ana Cremades,† Wolfgang J€ager,‡ and Javier Piqueras† † ‡
Departamento de Física de Materiales, Facultad de Física, Universidad Complutense de Madrid, 28040, Spain Mikrostrukturanalytik, Christian-Albrechts Universit€at zu Kiel, D-24143, Kiel, Germany ABSTRACT: In2O3 nanorods have been grown by a catalyst free evaporation-deposition method. Transmission electron microscopy (TEM) investigations reveal that the rods contain tubular cavities, herein referred to as “nanopipes”, along the total length of the nanorods. The nanopipe diameters are constant along the nanorod axis and appear to be independent of the nanorod thickness. In most of the investigated In2O3 nanorods, these nanopipes are centered within the nanorod. An average nanopipe diameter of (18.5 ( 0.7) nm has been determined from the TEM observations. Extended thermal treatments lead to nanorods with more complex morphologies, roughened interfaces, and formation of voids. The possibility that the nanopipes are related with a growth mechanism involving a dislocation along the growth axis is discussed.
’ INTRODUCTION During the last several years, considerable efforts are being invested in the synthesis and characterization of nanostructures with diverse morphologies, such as wires, rods, belts, or tubes in order to spread their applications in different fields of technology. Among them, different hollow nanostructures are considered in addition to the widely investigated carbon nanotubes as promising candidates in applications such as catalyst, energy storage, or biotechnology1,2 due to their unique geometry and high surfaceto-volume ratio. In recent years, different approaches have been developed to fabricate hollow nanostructures, as for instance, chemical etching,3 Kirkendall effect,4 galvanic replacement,5 and the template-mediated approach,6 being the latter the most commonly used one. In this work, we report the synthesis of In2O3 hollow nanorods via a thermal catalyst-free process. Indium oxide is a wide bandgap semiconductor with numerous important applications in optoelectronics, gas sensors, solar cells, or field emitters.7-9 Different routes have been used to synthesize In2O3 nanostructures10-12 with diverse morphologies. Some research referred to the synthesis of hollow nanospheres13 or microcubes,14 however less has been reported on the fabrication of hollow elongated nanostructures so far. Cheng et al.15 and Shen et al.16 describe the synthesis of polycrystalline In2O3 nanotubes by using a porous alumina template, whereas Li et al.17 report on the vapor-liquid-solid (VLS) synthesis of In2O3 nanotubes with the cavities filled by metallic In. This study reports on the original synthesis of single crystalline In2O3 nanorods with a narrow hollow core, herein referred to as “nanopipe”, fabricated via a vapor-solid thermal treatment that avoids the presence of catalyst, chemical etching, templates, or any external substrate. The study of these structures is a step toward the functionalization of different nano- and microdevices r 2011 American Chemical Society
as well as the understanding of the physical processes involved during the crystal growth. Nanopipes have been well referred in GaN and SiC thin films,18-20 where the mechanisms related to their formation and effects on the material properties have been discussed by means of TEM observations. In particular, the presence of a central nanopipe in the nanorods poses the question of the role of screw dislocations in their growth. Until now there is limited evidence of nanorod growth driven by dislocations in the core of the nanorods. In a study of PbS nanowire pine trees, Bierman et al.21 suggested that the screw component of an axial dislocation provides the self-perpetuating steps to enable the nanowire growth. Jacobs et al.22 observed nanopipes in GaN nanowires, suggesting them to represent hollow cores of screw dislocations. More recently Morin et al.23 have confirmed by means of transmission electron microscopy (TEM) observations that the growth of ZnO nanotubes, similar to the In2O3 nanopipes described in this study, is driven by axial screw dislocations. The observation of nanopipes in the In2O3 nanorods of this study suggests that a dislocation-driven nanorod growth mechanism is responsible for their formation, as in refs 21-23. The nanostructures have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), conventional and high-resolution TEM, selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDS) in TEM.
’ EXPERIMENTAL SECTION In2O3 nanorods were synthesized via a catalyst-free thermal treatment that has been previously used to grow nanostructures of other Received: October 13, 2010 Revised: January 11, 2011 Published: February 23, 2011 1117
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Figure 1. SEM images showing In2O3 nanorods and arrowlike nanostructures obtained by a two-step treatment.
Figure 3. (a) TEM image showing a single nanorod with a nanopipe at the core. (b) HRTEM image in correct alignment with the nanorod axis and (c) the corresponding SAED pattern.
Figure 2. XRD spectrum corresponding to the sample described in Figure 1. semiconducting oxides.24-26 As referred in a previous work,12 high purity (99.99%) InN powders have been used as precursors. The initial powders were pressed into pellets and then annealed under a controlled argon flow. The furnace was not sealed for high-vacuum conditions, so the thermal treatments were carried out at atmospheric pressure. The annealing consists of an initial step at 350 C for 5 h that favors the nitrogen desorption and the formation of In2O3, followed by a second step at 650 C during 5 or 10 h that leads to the complete oxidation and the growth of the In2O3 nanostructures in the surface of the pellet. The morphology of the grown structures was studied in a Leica 440 scanning electron microscope using accelerating voltages of 18 kV. XRD measurements were carried out in a Philips diffractometer. The microscopic structure of the nanorods was investigated under kinematical and dynamical bright field TEM imaging conditions, as well as by SAED. The voids and the nanopipes were investigated by defocus contrast experiments under kinematical imaging conditions. The elemental composition of the rods was investigated by spatially resolved EDS. For the TEM studies the nanostructures were carefully removed from the pellet and dispersed in isopropanol by ultrasounds. The solution containing the nanostructures was then dispersed onto TEM copper grids coated with a holey carbon film. The TEM characterizations were performed using a Philips CM30 TEM and a FEI-Tecnai F30 G2 TEM, both operated at 300 kV. For the X-ray microanalyses, the EDAX DX-4 system in combination with an UTW-Si (Li)-Detector attached to the FEI-Tecnai F30 G2 TEM was used.
’ RESULTS AND DISCUSSION Figure 1 shows typical rods and arrowlike structures obtained on the pellets by growth under the applied two-step treatments, as described in the Experimental Section. The SEM investigations show that the arrows consist of a rod with a pyramid at the
Figure 4. (a) Nanopipe contrast for defocused kinematical and (b) nanorod contrast due to lattice strain adjacent to the nanopipe for dynamical bright-field TEM imaging. Some defects causing strain contrast are visible as well.
top, as described in refs 12 and 27. X-ray diffraction patterns obtained from large areas of the pellet containing nanorods (Figure 2) confirm that the thermally treated samples are bodycentered cubic In2O3. Neither traces from the initial InN or metallic crystalline In were detected. The presence of extra phases could be excluded both by the XRD and by the SAED investigations in the TEM. The microscopic investigations by TEM were performed on rods with diameters ranging from 150 to 500 nm and lengths up to tens of micrometers. TEM investigations of nanorods grown by the thermal treatment with the shorter second step of 5 h (Experimental Section) reveal that all the investigated In2O3 nanorods contain empty tubular core regions, herein named as nanopipes, extending along the nanorod axis in some cases up to tens of micrometers. As an example, Figure 3a shows a straight nanopipe of constant diameter centered along the core of the rod and extending over its entire length. Figure 3b displays a representative high-resolution TEM (HRTEM) image of a single crystalline rod showing lattice parallel fringes with spacings of 0.505 nm, which correspond to the (200) interplanar distance of cubic In2O3. The SAED patterns taken from individual rods indicate the presence of an In2O3 bixbyite cubic structure. Figure 3c shows an example of an SAED pattern taken in the [001] zone axis. These TEM observations reveal that the nanorod axes extend along Æ100æ growth directions. The nature of the nanopipes has been deduced from the defocus contrast behavior under kinematical bright-field TEM imaging conditions (Figure 4a). Adjacent to the nanopipes, the nanorod lattice is strained, as indicated by the contrast behavior under dynamical bright-field TEM imaging (Figure 4b). TEM analyses of individual nanorods indicate that the nanopipe diameters are nearly 1118
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Figure 5. (a) Histogram showing the diameter distribution of the nanopipes as measured from their bright-field contrast of more than 100 rods. (b) EDS spectrum acquired from a single nanorod showing, besides Cu peaks from the TEM grid, only O and In peaks.
constant along the nanorod length and independent of the nanorod thickness. The average nanopipe diameter, statistically estimated from the analysis of more than a hundred of rods as shown in the histogram of Figure 5a, is (18.5 ( 0.7) nm. In these analyses, we included measurements taken at different positions along the length of individual nanorods as well as from nanorods with very different diameters up to few hundreds nanometers. The spatially resolved composition analyses of individual nanorods by EDS in TEM confirmed the presence of only In and O, besides Cu peaks from the TEM copper grid (Figure 5b). The presence of the nanopipe along the axis suggests the possibility of a dislocation driven growth mechanism of the nanorod, similarly as it was concluded for nanopipe formation in GaN nanorods22 or in the case of the screw dislocations observed on PbS nanowires.21 In our case, no dislocation contrast is observed in the micrographs, however the origin of the nanopipe can be discussed in the frame of the dislocation theory. According to the theory of Sears28,29 for crystal growth without catalyst and for low supersaturation conditions, as those used in our study, the growth of one-dimensional (1D) nano- and microstructures would require the presence of a dislocation with a substantial screw component located at or parallel to the axis. Furthermore, it was theoretically demonstrated by Frank30 that screw dislocations with a large Burgers vector accumulate high stress in the dislocation core. If the Burgers vector exceeds a critical value (∼10 Å), the creation of a pipe by removing the highly strained material around the dislocation is energetically favorable.30 Despite this accepted explanation, the screw dislocation driven growth mechanism as proposed by Sears has not been widely considered and discussed in connection with nanorod growth until recent years, when the presence of a screw dislocation in the axis of some nanorods has been confirmed by TEM.20,31 The formation mechanism of pipes as proposed by Frank does not refer only to the case of thin films but is applicable also to nanowires, as discussed by Avramov.32 His theoretical study of the kinetics of nanorod growth deals with the case that a nanopipe forms along the wire and the step of the screw dislocation on the nanowire top face favors the unidirectional growth. Dislocations with giant Burgers vectors, necessary to generate pipes, are expected for crystalline materials with large unit cells, as in the case of In2O3 (lattice parameter a = 10.11 Å). However, up to date the formation of nanopipes in In2O3 elongated nanostructures, as those described in this work, has not been reported to our knowledge. Our systematical TEM observations show that all investigated nanorods contain empty nanopipes in contrast to previous research17 where In2O3 nanotubes were partially filled with metallic In. According to the minimum free energy conditions as proposed by Frank,30 the
Figure 6. (a) Nanorod with a variable thickness showing a nanopipe with constant diameter at the core. (b) TEM bright-field image of a nanorod with nanopipe, showing parallel contrast striation perpendicular to the rod axis and surface ripples with an average period of around 5 nm. The corresponding SAED patterns have been included as insets.
nanopipe radius r depends on the Burgers vector b of the screw dislocation, the shear modulus G, and the surface energy γ of the nanorod material, as r = b2G/8π2γ. Applying this model to our experimental results, we are able to conclude that the fact that the radii of the nanopipes observed in our case remain nearly constant implies a constant magnitude of the Burgers vector b of the nanorod dislocations. No dependence of the nanopipe diameter on the diameter of the nanorods has been found. This is confirmed by observations of nanorods whose pipe diameters remain constant, even when eventually the nanorod diameter varies along their length (Figure 6a). The ratio of the projected nanorod radius R to the nanopipe radius r, R/r, as measured on a number of nanorods, varies between about 12 and 50. For some of the investigated nanorods contrast striation along the nanorod axis combined with surface ripples of =5 nm in length are observed (Figure 6b). Nanopipes are usually located in the center of symmetry of the rods (as shown in Figure 3a), however in some of the nanorods the nanopipe is located closer to the rod surface, as shown in Figure 7a. Nanotubes for which the position of the hollow channel deviates from the center position have been seldom reported, as in the case of Ga2O3 ribbon shaped tubes.33 Occasionally nanopipes are scarcely decorated with defects or small precipitates causing strain contrast, as shown in Figure 7b. The presence of these pipes has been also observed at the apex of the arrowlike nanorods, as shown in Figure 8. The pinholes observed on the final truncated plane of some pyramids (Figure 8a) present a cavity (pipe), which can be related to the open core screw dislocations observed by TEM in the nanorods (Figure 8b). A scheme describing the formation process of the arrowlike In2O3 nanorods with nanopipes has been displayed in Figure 8c. 1119
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Figure 7. TEM bright-field images of (a) a nanorod with a nanopipe out of the center position and (b) a nanorod with defects connected to the nanopipe. The defects cause localized strain in the surrounding nanorod lattice. The corresponding SAED patterns have been included as insets.
Figure 8. (a) SEM and (b) TEM images showing the presence of an open cavity at the apex of the pyramids in the arrowlike nanorods. (c) Schematic illustration describing the growth of the arrowlike nanorods with nanopipes.
TEM investigations of nanorods grown by the extended thermal treatment with the longer second step of 10 h (Experimental Section) result also in nanorod structures with nanopipes (Figures 9 and 10). However, as indicated by the selected examples of bright-field TEM images, their morphologies are more complex, their lattices contain defects, such as voids, and their interfaces are considerably roughened. Figure 9 shows examples of voids that are formed within the crystal matrix (marked with green arrows) and (in projection) linear or planar void arrangements extending perpendicular to the rod axis. In some sections of the nanorods, open nanochannels connected with the external surface (marked with blue arrows in Figure 9) are observed. The projected void extensions along the rod axes, between 2 and 5 nm width, separate the rod into regions and are connected to a rougher surface. The observations of these phenomena indicate that diffusion processes occur during the nanorod growth by extended thermal treatment, resulting in the formation of these defects and in the more complex rod morphology. Typical examples for nanorod morphologies obtained from the extended thermal treatment are the bamboolike appearance of some areas of the nanorods (Figures 9 and 10) and the formation of void arrangements that transform the rods into fragmented structures (Figure 10). The fact that also these nanorods fabricated by the extended thermal treatment contain
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Figure 9. TEM image from a In2O3 nanorod obtained for extended thermal treatment with complex morphology and roughened interfaces. Voids as well as empty interspaces perpendicular to the rod axis are marked in the image with green and blue arrows, respectively. The corresponding SAED pattern has been included as an inset.
Figure 10. TEM images of different complex nanostructures obtained for extended thermal treatment.
nanopipes with constant average diameter but, due to roughening processes, locally varying diameters along the nanorods, confirms our proposal that their formation is related to a dislocation-driven crystal growth of the nanostructures.
’ CONCLUSIONS To summarize, In2O3 nanorods grown by a catalyst-free evaporation-deposition and two different two-step thermal treatments were investigated by means of various methods of transmission electron microscopy. TEM investigations reveal that all rods from short treatments consist of single crystalline crystal lattices and contain nanopipes that extend along the total length of the nanorods with a constant average diameter of (18.5 ( 0.7) nm. The nanopipes have a regular tubular shape and most of them appear centered in the rod. The observation of the nanopipes suggests a mechanism for nanorod growth that is connected with a screw dislocation-driven mechanism where the nanopipe represents the empty dislocation core. This would agree with recent results relating nanowire growth of other materials with dislocations along the growth axis. Extended thermal treatments lead also to the formation of nanorods containing nanopipes. However, these nanorods show a more complex morphology with defects, voids, and roughened nanopipe and nanorod surfaces. These observations indicate that extended thermal treatment results in changes of the nanorod 1120
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’ AUTHOR INFORMATION Corresponding Author
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(28) Sears, G. W. Acta Metall. 1955, 3, 367–369. (29) Sears, G. W. J. Chem. Phys. 1956, 25, 637–642. (30) Frank, F. C. Acta Crystallogr. 1951, 4, 497–501. (31) Zhu, J.; Peng, H.; Marshall, A. F.; Barnett, D. M.; Nix, W. D.; Cui, Y. Nat. Nanotechnol. 2008, 3, 477–481. (32) Avramov, I. Nanoscale Res. Lett. 2007, 2, 235–239. (33) Hu, J.; Li, Q.; Zhan, J.; Jiao, Y.; Liu, Z.; Ringer, S. P.; Bando, Y.; Goldberg, D. ACS Nano 2008, 2, 107–112.
*E-mail: davidmaestre@fis.ucm.es.
’ ACKNOWLEDGMENT This work was supported by MEC (MAT2006-01259, MAT200907782, CSD 2009-00013) and BSCH-UCM (Group 910146) ’ REFERENCES (1) Lou, X. W.; Archer, L. A.; Yang, Z. Adv. Matter 2008, 20, 3987–4019. (2) Mor, G. K.; Kim, S.; Paulose, M.; Varghese, O. K.; Shankar, K.; Basham, J.; Grime, C. A. Nano Lett. 2009, 9, 4250–4257. (3) An, K.; Kwon, S. G.; Park, M.; Na, H. B.; Baik, S. I.; Yu, J. H.; Kim, D.; Son, J. S.; Kim, Y. W.; Song, I. C.; Moon, W. K.; Park, H. M.; Hyeon, T. Nano Lett. 2008, 8, 4252–4258. (4) Yin, Y.; Rioux, R.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711–714. (5) Chen, J.; Saeki, F.; Wiley, B. J.; Cang, H.; Cobb, M. J.; Li, Z. Y.; Au, L.; Zhang, H.; Kimmey, M. B.; Li, X.; Xiu, Y. Nano Lett. 2005, 5, 473–477. (6) Wang, Y.; Wu, K. J. Am. Chem. Soc. 2005, 127, 9686–9687. (7) Hsin, C. L.; He, J. H.; Lee, C. Y.; Wu, W. W.; Yeh, P. H.; Chen, L. J.; Wang, Z. L. Nano Lett. 2007, 7, 1799–1803. (8) Kar, S.; Chakrabarti, S.; Chaudhuri, S. Nanotechnology 2006, 17, 3058–3062. (9) Zhang, D. H.; Liu, Z. Q.; Li, C.; Tang, T.; Liu, X. L.; Han, S.; Lei, B.; Zhou, C. W. Nano Lett. 2004, 4, 1919–1924. (10) Singh, N.; Zhang, T.; Lee, P. S. Nanotechnology 2009, 20, 195605. (11) Li, C.; Zhang, D. M.; Han, S.; Liu, X. L.; Tang, T.; Zhou, C. W. Adv. Mater. 2003, 15, 143–146. (12) Magdas, D. A.; Cremades, A.; Piqueras, J. Appl. Phys. Lett. 2006, 88, 113107. (13) Guo, Z.; Liu, J. Y.; Jia, Y.; Chen, X.; Meng, F. L.; Li, M. Q.; Liu, J. H. Nanotechnology 2008, 19, 345704. (14) Liu, X. H.; Zhou, L. B.; Yi, R.; Zhang, N.; Shi, R. R.; Gao, G. H.; Qiu, G. Z. J. Phys. Chem. C 2008, 112, 18426–18430. (15) Cheng, B.; Samulski, E. T. J. Mater. Chem. 2001, 11, 2901–2902. (16) Shen, X. P.; Liu, H. J.; Fan, X.; Jiang, Y.; Hong, J. M.; Xu, Z. J. Cryst. Growth 2005, 276, 471–477. (17) Li, Y.; Bando, Y.; Goldberg, D. Adv. Mater. 2003, 15, 581–585. (18) Hawkridge, M. E.; Cherns, D. Appl. Phys. Lett. 2005, 87, 221903. (19) Strunk, H. P.; Dorsch, W.; Heindl, J. Adv. Eng. Mater. 2000, 2, 386–389. (20) Pirouz, P. Philos. Mag. A 1998, 78, 727–736. (21) Bierman, M. J.; Lau, Y. K. A.; Kvit, A. V.; Schmitt, A. L.; Jin, S. Science 2008, 320, 1060–1063. (22) Jacobs, B. W.; Crimp, M. A.; McElroy, K.; Ayres, V. M. Nano Lett. 2008, 8, 4353–4358. (23) Morin, S. A.; Bierman, M. J.; Tong, J.; Jin, S. Science 2010, 328, 476–480. (24) Hidalgo, P.; Liberti, E.; Rodriguez-Lazcano, Y.; Mendez, B.; Piqueras, J. J. Phys. Chem. C 2009, 113, 17200–17205. (25) Aleman, B.; Fernandez, P.; Piqueras, J. Appl. Phys. Lett. 2009, 95, 013111. (26) Maestre, D.; Cremades, A.; Piqueras, J. J. Appl. Phys. 2005, 97, 044316. (27) Magdas, D. A.; Cremades, A.; Piqueras, J. J. Appl. Phys. 2006, 100, 094320. 1121
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