Growth Mechanism and Photoluminescence Properties of In2O3

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DOI: 10.1021/cg9011839

Growth Mechanism and Photoluminescence Properties of In2O3 Nanotowers

2010, Vol. 10 2104–2110

Sen-Tsun Jean and Yung-Chiun Her* Department of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan 40254, Republic of China Received September 25, 2009; Revised Manuscript Received January 7, 2010

ABSTRACT: To elucidate the growth mechanism of In2O3 nanotowers synthesized via a Au-catalyzed vapor transport process, the structural evolution of In2O3 nanotowers was carefully examined during the synthesis process. It was found that Au catalysts only play a role at the initial stage, where they facilitate the formation of In2O3 nanoparticles and nanorods. After the Au atoms are consumed by the formation of Au-In compound(s), the liquid In droplets will form on the tips of In2O3 nanoparticles or nanorods, and the self-catalytic vapor-liquid-solid (VLS) growth mechanism will dominate the subsequent one-dimensional (1D) growth of In2O3 nanopillars. Since the supply of In2O may not be sufficient for the continuous 1D growth, the lateral growth of In2O3 nanopillars governed by the vapor-solid (VS) mechanism will occur. The periodical axial and continuous lateral growth leads to the formation of In2O3 nanotowers with a truncated octahedron structure of 4-fold symmetry {111} accumulated planes along the [100] direction. The photoluminescence (PL) spectrum of In2O3 nanotowers exhibited an intense green-yellow luminescence at the wavelength of 580 nm, which can be ascribed to the possible recombination of electrons on singly ionized oxygen vacancies and holes on the valence band or doubly ionized oxygen vacancies.

Introduction The syntheses of one-dimensional (1D) nanostructures have stimulated intensive research activities because of their contribution to the understanding of basic concepts and potential technological applications. A broad range of 1D nanostructural oxides materials, such as Ga2O3,1 MgO,2 SnO2,3 ZnO,4 and In2O3,5 have been successfully synthesized by various methods including laser ablation,6 template-assisted growth,7 arc discharge,1 vapor-phase transportation,8 hydrothermal process,9 and solution-liquid-solid growth.10 Semiconducting indium oxide (In2O3) with a wide direct bandgap of ∼3.6 eV has attracted much attention in recent years, owing to its distinctive optical, chemical, and electronic properties and its applications in solar cells,11 field-emission displays,12 lithium-ion batteries,13 nanoscale biosensors,14 gas sensors,15 optoelectronics,16 and photocatalysis.17 Since the performance of those applications is expected to be improved by lowering the dimension or increasing the surface-tovolume ratio of the material, considerable efforts have thus been devoted to the syntheses of 1D In2O3 nanostructures. Up to now, many simple In2O3 nanostructures, such as nanotubes,18,19 nanowires,20 nanorods,21 nanobelts,22 nanoflowers,23 nanosheets,24 nanocubes,25 nanosheres,26 and nanopyramids,27 have been successfully synthesized. In contrast, the investigations on the complex In2O3 nanostructures such as nanotowers28 are limited. It is well-known that the properties of nanostructures strongly depend on their morphologies so that nanostructures with different morphologies have special applications. In2O3 nanotowers, with a wide base and a sharp tip, are expected to be helpful for efficient field emission applications. So far, the growth mechanism of In2O3 nanotowers is still not clear. Although Yan et al. have proposed a periodical 1D and persistent 0D growth to explain *Corresponding author. Phone: þ886-4-22859112. Fax: þ886-4-22857017. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 04/12/2010

the formation of the tapered In2O3 nanotowers,28 they did not provide the direct evidence of the structural evolution of In2O3 nanotowers during the synthesis process. In this paper, we synthesized large-scale In2O3 nanotowers by a Au-catalyzed vapor transport process and carefully investigated the growth mechanism of In2O3 nanotowers by examining the structural evolution of In2O3 nanotowers during the synthesis process. The experimental results showed that the growth of In2O3 nanotowers is governed by a Au-catalytic vapor-liquid-solid (VLS) mechanism at the initial stage and then by self-catalytic VLS and vapor-solid (VS). The photoluminescence (PL) spectrum at room temperature of the assynthesized In2O3 nanotowers was analyzed, and exhibited an intense green-yellow luminescence at the wavelength of 580 nm (∼2.1 eV). The possible mechanism behind yellow-red luminescence was also discussed. Experimental Section Quasi-1D In2O3 nanotowers were synthesized by a Au-catalyzed VLS process. High-purity In powders (99.99%) were loaded into an alumina boat and positioned at the center of a quartz tube inside a horizontal tube furnace with three heating zones. Au films with a thickness of 10 nm were deposited on (100) Si substrate. The substrate was placed at the downside of the In powder source. Before heating, the quartz tube was evacuated to 10-2 Torr by a vacuum pump, and then filled with argon. The quartz tube was then heated to 860 C at a heating rate of 20 C/min and kept at this temperature for different growth times under the constant flows of argon and oxygen at rates of 100 and 25 sccm, respectively. During the growth process, the pressure inside the tube remained at ∼1 Torr. The growth temperature was chosen to be 860 C as no 1D nanostructure except In2O3 islands could be found when the growth temperature was kept at 960 or 760 C. In order to understand the growth mechanism of In2O3 nanotowers, the synthesis process was terminated at different growth times of 2, 3, 5, 6, 8, 10, and 30 min to examine the evolution of morphologies and microstructures of the resultant products. When the setting growth time was up, the power of the furnace and the Ar and O2 gas flow controllers were turned off immediately. The split-hinge design of the r 2010 American Chemical Society

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Figure 1. FE-SEM images of as-synthesized nanostructures at different growth times of (a) 2 min, (b) 3 min, (c) 5 min, (d) 6 min, (e) 8 min, and (f) 10 min. furnace allows us to easily open the furnace and let the quartz tube be cooled by the fresh air to ensure fast cooling. The same synthesis processes at different growth times was conducted three times to confirm the reproducibility of the growth phenomena. After the quartz tube was cooled down to room temperature in the air, the yellow resultant products were collected. The morphologies, crystalline structures, and compositions of the as-synthesized products were characterized by field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) using Cu KR radiation, high resolution transmission electron microscopy (HRTEM, JEOL TEM-3010), and X-ray photoelectron spectroscopy (XPS) using Mg radiation. The chemical compositions of the resultant products were determined by an energy dispersive spectroscope (EDS) equipped with the same transmission electron microscope. The photoluminescence spectra were measured at room temperature in the spectral range of 350-800 nm using a He-Cd laser with a wavelength of 325 nm as the excitation source.

Results and Discussion Figure 1a-f shows the typical FE-SEM images of the assynthesized products prepared at 860 C for 2, 3, 5, 6, 8, and 10 min. The surface morphologies of the resultant films were found to change progressively as the growth time was increased. After 10 min, the substrate was fully covered by In2O3 nanotowers. At a growth time of 2 min, irregular nanoparticles

with sizes ranging from 20 to 80 nm were formed. As the growth time was increased to 3 min, the irregular nanoparticles would transform to granulated nanoparticles with diameters of about 300-400 nm mixed with a trace amount of nanorods. As the growth time was increased to 5 min, the granulated nanoparticles would disappear and were completely replaced by bowling pin-like nanostructures. The base sizes of these bowling pin-like nanostructures were approximately 400 nm and the lengths were approximately 0.5-1 μm. When the growth time was further increased to 6 min, the bowling pin-like nanostructures would transform to pin-like nanostructures (nanopillars) where the lower part has a rectangular shape and the upper part has a round shape. Many horizontal stripes could also be observed on the surfaces of nanopillars at lower parts, as shown in the inset of Figure 1d. The widths and diameters of the nanopillars at lower and upper parts were found to be ∼400 and 250 nm, respectively, while the total lengths were about 1-1.5 μm. When the growth time reached 8 min, the nanopillars would further transform to tower-like nanostructures (nanotowers). Each single nanotower with a typical length of 1.5-2 μm consists of many rectangular layers parallel to each other, and the widths gradually become smaller along the axial direction

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Figure 2. XRD spectra of as-synthesized nanostructures at different growth times.

from the bottom to the top. Such structures have widths of about 500 nm at the bottom and about 200 nm at the top. As the growth time was further increased to 10 min, the typical lengths of nanotowers would increase to 3-4 μm and the widths of the nanotowers would increase to 800 nm at the bottoms and shrink to 30 nm at the tops. Moreover, a tiny particle can be observed on the tip of each nanotower, as shown in the inset of Figure 1f, which implies that the growth of the nanotowers may be governed by a Au- or self-catalytic VLS mechanism. As the growth time was extended to 30 min, the morphology of In2O3 nanotowers was found to remain unchanged. It should be noted that the reproducibility of the growth phenomena was good after we repeated the same synthesis processes at different growth times for three times. The crystalline structures of the nanotowers at different growth times were determined by XRD, as shown in Figure 2. It was found that the as-synthesized products compose of two components at a growth time of 2 min: one is the AuIn2 facecentered cubic (fcc) structure with a lattice plane of a = 6.515 A˚ (JCPDS card no. 65-2993) and the other is the In2O3 bodycentered cubic (bcc) structure with a lattice plane of a = 10.11 A˚ (JCPDS card no. 89-4595). The presence of AuIn2 phases implies the formation of eutectic Au-In liquid alloy during the growth process at the initial stage. When the growth time was increased to 3 min, the AuIn2 phase became hardly detected, and the metallic In phase was observed in addition to the preexisting In2O3 phase. The absence of AuIn2 phases suggests that the Au catalysts may no longer play a role in the growth of In2O3 nanostructures. From the XRD patterns, we also can find that the primary growth planes of the In2O3 nanostructures were (100) planes when the growth time was shorter than 5 min. As the growth time was extended above 6 min, the main growth transferred to the (111) plane, while the (100) plane continued growing. The structural evolution of the In2O3 nanotowers was further characterized by TEM, HRTEM, and EDS, as shown in Figure 3a-h. An individual In2O3 nanopillar with a lateral dimension of ∼400 nm was observed at a growth time of 6 min, as shown in Figure 3a. The HRTEM image taken from the edge of the nanopillar and the corresponding selected-area electron diffraction (SAED) patterns confirm that the nanopillar is In2O3 with a single-crystalline bcc structure. The interplanar spacing of the lattice fringes in the longitudinal

Jean and Her

and transversal directions was measured to be 0.506 and 0.715 nm, respectively, corresponding to the d values of the (200) and (011) planes. From the SAED patterns and the lattice image, the growth direction of the In2O3 nanopillars can be determined to be along the [100] direction. The elemental distribution along the growth direction of the assynthesized nanopillar was analyzed by EDS. The EDS spectra at the top and bottom of the nanopillar were shown in Figure 3, panels b and c, respectively. It was found that the as-synthesized nanopillar contains 48 at% In and 52 at% O at the bottom, while contains 80 at% In and 20 at% O at the top. No appreciable amount of Au atoms was detected at the top of the nanopillar. The decrease of concentration of oxygen from the bottom to the top of the indium oxide nanopillar may be contributed to the consumption of O2 during the growth process. Meanwhile, the abnormally high concentration of indium without the presence of Au atoms at the top of the nanopillar implies that the growth of the indium oxide nanopillar is dominated by the self-catalyst VLS method. Figure 3d shows a typical TEM image of a single nanotower, in which the layered structures can be observed. The magnified TEM image at the edge of a nanotower is shown in Figure 3e, from which the angle between two edge planes can be determined to be 110, and the angle between the normal of the edge plane and the [100] growth direction is determined to be 55. Since the included angle between (100) and (111) planes is 54.7, we can conclude that the edge planes of the In2O3 nanotowers should be {111} planes. The HRTEM image taken from the tip of a single nanotower and the corresponding SAED patterns, as shown in Figure 3f, confirm that the tiny particle on the tip of the nanotower is In2O3 with a single-crystal bcc structure and the growth direction of the In2O3 nanotower is along the [100] direction. The EDS spectra at the tip and bottom of the nanotower are shown in Figure 3, panels g and h, respectively. Unlike the obvious decrease of O and increase of In from the bottom to the top of the nanopillar, the concentrations of oxygen and indium of the whole nanotower remained at ∼56 and 44 at%, respectively, which suggests the completion of the self-catalyst VLS growth process. On the basis of the structural evolution during the synthesis process, the growth of In2O3 nanotowers could be ascribed to three different mechanisms: the Au-catalytic VLS followed by self-catalytic VLS and VS mechanisms. For a typical thermal transportation process, the metallic vapor phase(s) should be first generated from the powder source, transported in a flow reactor, and finally react(s) with oxygen and condense(s) to form the desired product(s). Considering the free energy change (ΔG1) and saturated vapor pressure of the evaporation of liquid indium as a function of temperature, expressed by eqs 1 and 2, the free energy change and saturated vapor pressure of the evaporation of liquid indium at 860 C are calculated to be ΔG1 = 29 ( 0.5 kcal/mol and 1.67  10-3 Torr, respectively. Inð1Þ f InðgÞ ΔG1 ¼ 58840 þ 3:2T log T - 35:82T ( 500 cal=mol ð1Þ PðInÞ ¼ exp½ - ΔG1 o ðT Þ=RT

ð2Þ

The positive value of ΔG1 indicates that this reaction is not spontaneous. In addition, the saturated vapor pressure is much lower than the actual pressures during the heating and growth processes. Accordingly, it is impossible to transform

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Figure 3. (a) HRTEM images and SAED pattern of a nanopillar at a growth time of 6 min, (b) and (c) EDS spectrum at the top and bottom of a nanopillar, respectively, (d) low-magnification and (e) high-magnification TEM images of a nanotower at a growth time of 10 min, (f) HRTEM images and SAED pattern of a nanotower at a growth time of 10 min, (g) and (h) EDS spectrum at the tip and bottom of a nanotower, respectively.

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Figure 4. (a-c) Schematic illustration for the growth mechanisms of In2O3 nanotowers.

liquid In into gas In at 860 C. Therefore, the phase transformation of the In powder source to In2O3 in an oxygen atmosphere may occur through intermediate reactions. It is known that a number of reactions involving In and O2 in the source region at a source temperature of 860 C may be possible, which are listed below29 2Inð1Þ þ 1 =2 O2ðgÞ f In2 OðgÞ

ð3Þ

In2 OðgÞ þ O2ðgÞ f In2 O3ðsÞ

ð4Þ

3In2 OðgÞ f 4lnð1Þ þ In2 O3ðsÞ

ð5Þ

Inð1Þ þ 3 =2 O2ðgÞ f In2 O3ðsÞ

ð6Þ

Because of the low melting point of In (430 K at 1 atm) and low boiling point of In2O (800 K at 1 atm), In is in the liquid phase and In2O is in its vapor phase.29 Apparently, the dominating evaporating component is In2O. Once the In2O vapor was transported downstream to the substrate, it would be captured by the catalytic droplets on the substrate and then decompose to liquid In and solid In2O3. Since the substrate was covered with numerous Au clusters, the resultant liquid In would react with Au clusters to form liquid eutectic Au-In alloy droplets at the initial stage. As the liquid surface has a large sticking coefficient, the eutectic Au-In alloy droplets were therefore the preferred absorption sites for incoming In2O vapor. After the liquid alloy became supersaturated with In, the In solid phase would precipitate at the solid-liquid interface and react with O2 to form irregular In2O3 nanoparticles. As the presence of AuIn2 phases at the growth time of 2 min provides the indirect evidence of the formation of eutectic Au-In liquid alloy at 860 C, the growth of In2O3 nanotowers at the initial stage could be confirmed to be governed by the Au-catalytic VLS mechanism. Figure 4a schematically illustrates the growth mechanism of In2O3 nanotowers at the initial stage.

As the growth process was carried on, In2O vapor would be continuously transported to the substrate and absorbed by the eutectic Au-In alloy droplets on the tops of In2O3 nanoparticles. Since the Au atoms can easily react with In atoms to form Au-In compound(s) which may reside in In2O3, the growth of In2O3 nanoparticles to nanorods would be accompanied by the gradual consumption of Au atoms. When all the Au atoms were completely consumed, the liquid In droplets decomposed from In2O would form on the tips of In2O3 nanoparticles or nanorods, which is consistent with our previous XRD results. Afterward, the dominant growth mechanism of In2O3 nanotowers would transfer from the Au-catalytic VLS to self-catalytic VLS. The main difference between Au- and self-catalytic VLS processes is that the growth of the 1D In2O3 nanostructure is facilitated by eutectic Au-In liquid alloy in the Au-catalytic VLS process, while by In liquid droplets in the self-catalytic VLS process. Similarly, the In droplets with high surface energy would become the preferred absorption sites for incoming In2O vapor. The In solid phase would precipitate at the solid-liquid interface and react with O2 to form In2O3, leading to the 1D growth of In2O3 to form bowling pin-like nanostructures and nanopillars, as shown in Figure 4b. To maintain continuous 1D growth, the supply of In2O vapor to the In liquid droplet on the tip of the In2O3 nanopillar should be sufficiently high. Since the growth temperature may not be high enough to continuously supply sufficient In2O vapor to In liquid droplets, In2O vapor could also be absorbed on the lateral surfaces of the growing nanopillars, leading to the lateral growth of In2O3 nanopillars by the VS mechanism. The periodical axial growth controlled by a self-catalytic VLS mechanism and continuous lateral growth controlled by a VS mechanism explains the growth of tapered In2O3 nanotowers, as shown in Figure 4c. A similar mechanism has been proposed by Yan et al. for the growth of In2O3 nanotowers.28 The difference is they proposed that the periodical 1D (axial) growth is controlled by the Au-catalytic VLS mechanism, but we confirm the periodical 1D (axial)

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Figure 5. XPS spectra of (a) In(3d5/2) and In(3d3/2) and (b) O(1s) for the as-synthesized In2O3 nanotowers.

growth is controlled by the self-catalytic VLS mechanism. As mentioned earlier, many horizontal stripes composed of the accumulated {110} planes could be observed on the lateral surfaces of the In2O3 nanopillars at lower parts. For In2O3 with a bcc structure, it is known that the surface energy relationships among three low-index crystallographic planes should correspond to γ{111} < γ{100} < γ{110}, so the growth rates of three growth directions have such relationships: rÆ111æ < rÆ100æ < rÆ110æ.30 The {110} plane that went along with a higher surface energy brought about a faster growth speed. In the end, it would form a truncated octahedron structure of 4-fold symmetry {111} accumulated planes along the [100] direction, which is consistent with our previous TEM results. The constituent phases of the In2O3 nanotower can be further confirmed by the core-level spectra of In 3d and O 1s, as shown in Figure 5a,b. All peak positions were corrected by the C (1s) peak at 284.5 eV, which was due to the adventitious contamination. The In (3d5/2) and O (1s) peaks were located at around 443.8 and 529.4 eV, respectively, indicating that the major constituent phase of the as-synthesized nanotowers is In2O3.31,32 It was found that the peak positions for In (3d5/2) and O (1s) in our In2O3 nanotowers are a little bit lower than those data published in the handbook of XPS. This may be caused by the existence of oxygen vacancies in the nanotowers which would result in the nonstoichiometric compound of In2O3-X. The stoichiometric ratio (Sij) can be calculated from the XPS spectra using the following equation,33 Si , j ¼

Ci Ii =ASFi ¼ Cj Ij =ASFj

ð7Þ

where Ci and Cj are the concentrations, Ii and Ij are the background corrected intensities of the photoelectron emission lines, and ASFi and ASFj are the atomic sensitivity factors for photoionization of the ith and jth elements. In this study, ASFO1s = 0.733 and ASFIn3d5/2 = 4.530. Accordingly, the atomic concentration ratio of In to oxygen can be calculated to be about 1:1.19, indicating that a certain amount of oxygen vacancies does exist in the as-synthesized nanotowers. Photoluminescence measurements of the as-synthesized In2O3 nanotowers were performed at room temperature, using a He-Cd laser line of 325 nm as the excitation source, as shown in Figure 6. It is well-known that the bulk In2O3 cannot emit light at room temperature.34 However, a strong PL emission at 580 nm (2.13 eV) was detected in the as-synthesized In2O3 nanotowers. Many PL peaks in the visible range have been observed in different In2O3 nanostructures,5,22,27,35-37 and most of them were attributed to the deep level or trap state emissions due to the presence of oxygen vacancies. In this work, oxygen vacancies can be generated by the incomplete oxidation of In

Figure 6. Room-temperature PL spectrum for the as-synthesized In2O3 nanotowers.

liquid during the synthesis process. Normally, oxygen vacancies in oxides can occur in three different charge states: (1) the VOX state which has captured two electrons and is neutral relative to the lattice, (2) the singly ionized VO 3 state which has captured one electrons and is singly positively charged with respect to the lattice, and (3) the doubly ionized VO 3 3 state which did not trap any electron and is doubly positively charged. As VOX is very shallow donor, it is expected that most oxygen vacancies will be in their paramagnetic VO 3 state under the flat band condition.38 As a result, the PL emission at 580 nm from In2O3 nanotowers can be ascribed to the possible recombination of electrons on singly ionized oxygen vacancies and holes on the valence band or doubly ionized oxygen vacancies. Conclusions Large-scale In2O3 nanotowers having typical lengths of 3-4 μm and widths of 800 nm at the bottoms and 30 nm at the tops were synthesized by a Au-catalyzed vapor transport process. The vertical and lateral growth directions of the In2O3 nanotowers are along [100] and [111], respectively. The experimental results showed that the growth of In2O3 nanotowers is governed by the Au-catalytic VLS mechanism at the initial stage, and then by self-catalytic VLS and VS processes. A strong greenyellow PL emission at 580 nm was detected in the In2O3 nanotowers, which can be ascribed to the possible recombination of electrons on singly ionized oxygen vacancies and holes on the valence band or doubly ionized oxygen vacancies. Acknowledgment. This work was sponsored mainly by the National Science Council of the Republic of China under

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Grant No. NSC97-2221-E005-014 and supported in part by the Ministry of Education under the ATU plan.

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