CRYSTAL GROWTH & DESIGN
Controlled Synthesis of In2O3 Octahedrons and Nanowires Yufeng Hao, Guowen Meng,* Changhui Ye, and Lide Zhang Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, People’s Republic of China, and Graduate School of Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China
2005 VOL. 5, NO. 4 1617-1621
Received March 20, 2005
ABSTRACT: Indium oxide (In2O3) octahedrons and nanowires were synthesized by chemical vapor deposition (CVD) through selecting appropriate source materials and controlling proper external conditions. The growth mechanisms of In2O3 octahedrons and nanowires were analyzed in detail, based on which growth model of single-crystalline In2O3 nanostructures with different morphologies was proposed. The vapor pressures of different source materials, the temperature difference between the central heating zones and the product deposition zones, the reaction time, and the surface energies are all responsible for the final crystalline morphologies of the In2O3 nanostructures. The acquired results may facilitate not only the exploration of new approaches of preparing various nanostructures for potential technical application but also a deeper understanding of the fundamental physical and chemical processes of CVD methods. Introduction It is self-evident that there is a solid relationship between specific morphologies of nanostructures and their corresponding unique performance in chemistry and materials science,1,2 and the latter is the basis of their broad applications. Therefore, controllable growth and mechanism studies of functional nanostructures with different morphologies are important. Generally, the preparation of many anisotropic nanostructures (nanowires, nanorods, and nanobelts) by chemical vapor deposition (CVD) methods has focused on such crystal systems as hexagonal, monoclinic, and tetragonal systems, etc., in which the surface energies of various lowindex crystallographic planes differ significantly.3 However, the growth of anisotropic nanostructures of cubic crystalline materials may have other dominant factors because there is not much difference in surface energies among {100}, {110}, and {111} facets. In experiments, some external conditions can be used to modulate the growth processes of nanostructures. For example, the surface energy difference among several low-index crystallographic planes can be changed through manipulating some important external conditions, which then affect the final crystal morphologies. Therefore, in this article, we prepared single-crystalline cubic indium oxide (In2O3) nanowires and octahedrons by controlling appropriate external conditions. On the basis of the experimental results, we present a possible model to explain the growth mechanism of In2O3 nanostructures with different morphologies. As a very important n-type wide-band gap semiconductor, In2O3 has received considerable attention for its technological applications in optoelectronic devices4,5 and gas detectors6 owing to its high electric conductance, high transparency to visible light, and the strong interaction between certain poisonous gas molecules and In2O3 surfaces. Preparation of nanosized In2O3 struc* Corresponding author. Fax: +86-551-5591434. E-mail: gwmeng@ issp.ac.cn (G.M.),
[email protected] (Y.H.).
tures with specific morphologies is required to meet different scientific and technological needs, such as nanowires4,7,8 for optoelectronics, nanoparticles for toxicgas sensing,9 and single-crystalline In2O3 pyramids for efficient field emission.10 Therefore, much effort has been paid to the preparation of In2O3 nanowires,7,8,11,12 nanobelts,13,14 single-crystalline pyramids,10 nanotubes filled with metal indium (In),15 and nanocrystal chains.16 However, little has been reported about the controlled growth and formation mechanisms of In2O3 nanostructures with different morphologies. Therefore, we chose In2O3 as the research object not only to explore its growth mechanism but also to facilitate many practical applications. Experimental Section In2O3 octahedrons and nanowires were synthesized through directly heating metal indium (In) and In2O3 powders, respectively. In particles (purity: 99.99%, average particle size: 1 mm) or In2O3 powders (purity: 99.99%) were put into an alumina boat (length: 4 cm), and then the boat was positioned at the center of the alumina tube (inner diameter 19 mm, length 750 mm), which was placed in a horizontal tube furnace. Single-crystalline Si substrates were put downstream of the source material to collect the products. The system was heated to 950, 1050, 1200, 1300, and 1350 °C each time in about 10 min. The reaction durations are all 40 min under a constant flow of Ar (purity: 99.99%) at a flow rate of 50 stand cubic centimeters per minute (sccm). Afterward, the furnace was cooled to room temperature naturally. When metal In was the precursor, the gray-white powderlike products were collected on the Si substrates in the vicinity of the central heating zone. The temperature differences between the central heating zones and the downstream deposition zones are about 50-100 °C. When the heating temperatures are about 1350 °C and In2O3 powders are the precursors, the gray-white woollike products were collected downstream on the alumina tube surface. The temperature differences between the central heating zones and the deposition zones are about 400-500 °C. The products were characterized by X-ray diffraction (XRD, PHILIPS, X’ PERT Pro MPD), field emission scan electron microscopy (FESEM, JEOL, JEM 6700F), transmission electron microscopy (TEM,
10.1021/cg050103z CCC: $30.25 © 2005 American Chemical Society Published on Web 05/17/2005
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Hitachi H-800 at 200kv), and high-resolution transmission electron microscopy (HRTEM, JEOL 2010, at 200KV).
Results and Characterization Nano- and microsized In2O3 octahedrons were obtained when In was used as the raw material. FESEM observations (Figure 1a-d) reveal that all the products obtained at different reaction temperatures are almost perfect octahedrons. For an octahedral crystal, every facet is close to an equilateral triangle. All the surfaces are smooth, and there are no obviously defects. The distributions in size of these octahedrons are relatively uniform. The average sizes of the products are about 1 µm (950 °C), 500 nm (1050 °C), 300 nm (1200 °C), and 100 nm (1300 °C), respectively. Therefore, the average size of octahedrons decreases with the increase of reaction temperature. Additionally, there are more imperfect octahedrons when the reaction duration was lengthened to 2 h at 1200 °C in Figure 2. A FESEM image (Figure 3) of In2O3 nanowires obtained only when In2O3 powders were heated at least 1350 °C shows primarily wirelike structures with relatively uniform diameters of 80 nm with length on the order of 10 µm and often as large as 100 µm. The cross sections of these straight nanowires are rectangular (inset of Figure 3). That there are no particles attached on the ends of the nanowires indicates that they are formed via the vapor solid (VS) mechanism.17 XRD is employed to investigate the crystalline phases and structures. For both In2O3 octahedrons and nanowires, all the patterns in Figure 4 are consistent with those of the body centered cubic (bcc) structural In2O3 with lattice constant of a ) 10.11 Å (JCPD 89-4595). For octahedrons obtained on the Si (100) substrates from heating In at 1200 °C for 40 min, the diffraction pattern shows notably preferential orientation. The peak (222) is the most intensive because the octahedrons stand against the Si surface by their {111} facets. Other diffraction peaks almost disappear in Figure 4a1. The XRD patterns of other samples obtained at different reaction temperatures, such as 950, 1050, and 1300 °C, are almost the same. With the longer reaction duration of 2 h, the preferential orientation became weak. Consequently, other diffraction peaks were slightly strengthened, as displayed in Figure 4a2. However, for In2O3 nanowires, the XRD pattern is much different in Figure 4b. Although the In2O3 nanowires grow along 〈100〉, the diffraction pattern is similar to that of In2O3 powders owing to the collective effect of XRD. Figure 5a-c shows the TEM images of the octahedrons with various orientations when the electron beams are parallel to the 〈110〉, 〈100〉, and 〈111〉, respectively. All the morphologies further demonstrate their octahedral geometries and smooth surfaces. HRTEM was further employed to confirm the crystalline structures. It can be seen that the lattice fringes of (200) and (01 h 1) in Figure 6a. However, lattice fringes of planes (200) and (020) [or (002)] cannot easily be obtained simultaneously because it is less stable when octahedrons stand against only one corner. In addition, we can also see the lattice fringe of plane (011 h ) when the electron beam was vertical to {111} facets which correspond to the equilateral triangle facets of the octahedrons in Figure 6b. The plane spacing of (200) and (011)
Figure 1. FESEM images of In2O3 octahedrons. (a-d) Typical In2O3 octahedrons with reaction temperatures of 950, 1050, 1200, and 1300 °C respectively. Reaction durations are all 40 min.
is 0.506 and 0.715 nm, respectively. Furthermore, the selected area electron diffraction (SAED) patterns and the HRTEM images verify that the octahedrons are single-crystalline. The HRTEM images also demon-
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Figure 2. FESEM image of In2O3 octahedrons with a reaction temperature of 1200 °C and a reaction time of 2 h.
Figure 3. FESEM image of In2O3 nanowires. Inset is the rectangular-like cross section of one nanowire.
strate some fundamental crystallographic phenomena: for example, the corners of the octahedrons were “cut” slightly, and the lattice arrangement is not very ordered for the instability at the large curvature corners. Figure 7a,b shows the TEM and HRTEM images of In2O3 nanowires, respectively. We investigated the HRTEM images of more than 20 nanowires, and the results indicated that all the growth directions are 〈100〉. The lattice plane spacing of (200) is 0.506 nm. The surfaces of the nanowires are also {100} facets ascertained by the SAED. The surface is smooth and with no obviously structural defects. Mechanism Discussion For In2O3 with a bcc structure, the surface energy relationships among three low-index crystallographic planes should correspond to γ{111} < γ{100} < γ{110}. The crystal growth rates (r) perpendicular to different planes are proportional to their surface energies. When r{100}/r{111} ≈ 1.73, perfect octahedrons appeared; when 0.87 e r{100}/r{111} e 1.73, truncated octahedrons appeared.18 In addition, the change of crystal growth rate vertical to {110} facet will result in the formation of the edge-cut octahedrons. Imperfect octahedrons may be attributed to an environmental disturbance in alumina tubes and several structural defects, such as oxygen vacancies (VO), indium vacancies (VIn), indium interstitials (Ini), oxygen interstitials (Oi), and antisite defects (OIn). Perfect
Figure 4. XRD patterns of In2O3 nanostructures. The pattern in (a1) corresponds to the octahedrons prepared at 1200 °C for 40 min, and the pattern in (a2) corresponds to the octahedrons prepared at 1200 °C for 2 h. (b) The pattern corresponds to the In2O3 nanowires.
octahedrons were generally formed in relatively short reaction time spans (about 40 min, see Figure 1). However, when extending the reaction duration to about 2 h, more and more imperfect octahedrons, such as truncated octahedrons and edge-cut octahedrons, appeared, as shown in Figure 2. For our current CVD system, the oxygen may come from (1) the residue oxygen in the alumina tubes and the adsorbed oxygen on the surface of alumina boats because we did not vacuumize the alumina tubes in advance; (2) the leakage of the reaction system. Therefore, the oxygen could be slightly deficient when lengthening reaction time; In and O atoms may be antisited at relatively high synthesis temperatures. Additionally, some local physical states could be disturbed by some unstable external conditions. These factors may affect chemical potentials of different crystallographic planes, change the growth rate perpendicular to these planes, and then form different morphologies. With longer reaction durations, these inappropriate factors will take more effect on the product morphologies. The specific mechanisms of different morphologies by different source materials were described as follows (Scheme 1). Vapor pressures, supersaturation ratios, and surface energy are three important factors to the modulation of different morphology. For the precursor of metal In, the saturated vapor pressure of liquid In are much higher than those of the solid-state In2O3 at the same temperatures. Conse-
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Figure 5. TEM images of In2O3 octahedrons. (a-c) with the electron beams parallel to 〈110〉, 〈100〉, and 〈111〉, respectively.
Scheme 1. Possible Growth Processes of In2O3 Nanostructures with Different Morphologies by Specific Source Materials: (a) Metal In as Source Material and (b) In2O3 Powders as Source Material
Figure 6. HRTEM images of the In2O3 octahedrons. (a) The HRTEM image with the electron beam parallel to the 〈110〉 direction. (b) The beam is perpendicular to the {111} facet. On the right is the corresponding SAED pattern of the octahedron.
Figure 7. (a) TEM and (b) HRTEM images of In2O3 nanowires. Inset is the corresponding SAED pattern.
quently, this condition leads to a high supersaturation ratio of oxidized In vapor (ratio of factual oxidized In vapor to saturated oxidized In vapor at that temperature) even in the vicinity of central heating zones. Although the difference of surface energies among {110}, {100}, {111} facets can lead to their different growth rates, the high supersaturation ratio makes the effect of the surface energy difference on the growth very small. As a result, the crystal growth rates perpendicular to {111}, {100}, and {110} facets become quite close,
so that the octahedron growth model can be realized easily. It is relatively difficult to obtain In2O3 nanowires through directly heating In because most vapor contributes to the nucleation and growth of In2O3 octahedrons in the vicinity of central heating zones. When a much larger quantity of In precursor was employed, some nanowires grew downstream11b because some oxidized In does not feed the growth of octahedrons completely. In addition, we briefly explain the average size change of single-crystalline In2O3 octahedrons when metal In is used as precursor: the carrier gas flow rate, the size of the alumina tube, and the reaction time of 40 min are unaltered except for the elevated reaction temperatures in several experiments, and we suppose that the evaporated In vapor was oxidized completely by the residual oxygen in the systems. The saturated vapor pressure of liquid-state metal In increased remarkably, whereas the saturated vapor pressure of solid-state In2O3 increased inconspicuously during the elevated reaction temperatures. Therefore, the supersaturation ratio of the oxidized In vapor increased notably. The supersaturation ratio is inversely proportional to the critical core radius. The average sizes of octahedrons therefore decreased with elevated temperatures in similar growth circumstances. For the precursor of In2O3 powders, however, there is little product when the temperatures are below 1350 °C owing to its excessively low vapor pressure. The saturated vapor pressure of solid-state In2O3 around 1350 °C is also relatively low. In this case, most In2O3 vapor will be transported to the deposition zone by the carrier gas: the temperature difference between the central heating zone and the deposition zone is about 400-500 °C. In2O3 nanowires are observed in this zone. This is similar to the previously reported results.13,19 This growth mechanism can be explained as below:
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to a relatively high supersaturation ratio of oxidized In vapor when metal In was used as the source material, whereas the product from heating In2O3 powders is nanowires as a result of the relative surface energy variation among three low-index crystallographic planes at the low temperature deposition zones. The results would facilitate not only the understanding of the fundamental crystallization processes of this kind of nanostructure but also the exploration of new approaches of preparing various nanostructures for potential applications. Acknowledgment. This work was supported by the Key Project of National Fundamental Research: Nanomaterials and Nanostructures (Grant No. 1999064501) and the Natural Science Foundation of China (Grant No. 10374092). References
Figure 8. Different morphologies of octahedrons and the rectangular-like cross section of one nanowire are shown in FESEM images, and the corresponding sketches are illustrated.
under such a remarkable temperature difference, the surface energy difference among various low-index crystallographic planes becomes considerably obvious: γ{111} , γ{100}, and accordingly, r{100}:r{111} . 1.73. In other words, the nanowires grow along the 〈100〉 direction, which was corroborated by current characterization results in Figure 8. This is a general thermodynamics consequence that the surface energies of various crystal planes converge with each other with increasing temperatures and vice versa.20 On the other hand, the substrate temperature around 900 °C and low supersaturation ratio of In2O3 vapor also promote the effective diffusion of oxidized In clusters to high surface energy or high defect density planes (steps, ledges, and kinks) along the surface of formed In2O3 nanowires.21 Coordination of the two processes promotes one-dimensional incubation and continuous growth. Therefore, this kind of In2O3 nanowire, an extremely anisotropic nanostructure, can be formed in the relatively low supersaturation ratio of In2O3 vapor. Occasionally, a little In2O3 vapor nucleates in the vicinity of the central heating zone, then becomes octahedrons. Conclusion In summary, In2O3 octahedrons and nanowires are two related morphologies. Control of external conditions is an efficient means to synthesize the desired nanostructure. There are mainly In2O3 octahedrons owing
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