Indium Hydroxide and Indium Oxide Nanospheres, Nanoflowers, Microcubes, and Nanorods: Synthesis and Optical Properties Jimin Du,† Mino Yang,‡ Seung Nam Cha,‡ Danielle Rhen,† Miwon Kang,† and Dae Joon Kang*,†
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2312–2317
BK 21 Physics Research DiVision, Institute of Basic Science, SKKU AdVanced Institute of Nanotechnology, and Center for Nanotubes and Nanostructured Composites, Sungkyunkwan UniVersity, Suwon 440-746, South Korea, and SAMSUNG AdVanced Institute of Technology, P.O. Box 111, Suwon 440-600, South Korea ReceiVed October 28, 2007; ReVised Manuscript ReceiVed February 13, 2008
ABSTRACT: This paper reports the facile synthesis of indium hydroxide nanospheres, nanoflowers, microcubes, and nanorods using a solvothermal method at 240 °C for 18 h in an ethanol solution of indium acetate with the directing surfactants of ethylenediamine, acetic acid, and oleic acid. After calcination of as-synthesized indium hydroxide products at 500 °C for 4 h, corresponding indium oxide nanostructures were also obtained with sizes and morphologies similar to the indium hydroxide products. The phase compositions and morphologies of the resulting samples were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and selected area electron diffraction. On the basis of our findings, we propose a surfactant-assisted self-assembly formation mechanism to account for their formation processes. Room temperature photoluminescence was further carried out on the indium oxide samples to investigate their optical properties.
1. Introduction
2. Experimental Section
Over the past several years, nanomaterials have attracted considerable attention due to their size and shape-dependent properties1 and potential applications such as miniaturized connectors,2 gas sensors,3 catalysts,4 and so on.5 Among nanomaterials, indium hydroxide, with a wide band gap of 5.1 eV, has drawn much attention because of its special semiconducting and optical properties.6 The conductivity of indium hydroxide films can be controlled from 10-7 to 10-3 S cm-1, depending on the synthetic conditions.7 Furthermore, indium oxide is a very important n-type semiconductor with a direct band gap of 3.6 eV and an indirect band gap of 2.6 eV.8 Much endeavor has been exerted on the development of novel optoelectronic devices9 and gas sensors10 with indium oxide owing to its high electric conductance and the strong interaction with certain gas molecules.11 For example, Zhang et al. successfully fabricated an In2O3 nanowire transistor with an on/ off ratio as high as 104.12 And In2O3 nanowire based chemical sensors for detecting NH3 and NO2 exhibited significant sensing behavior in comparison with existing solid-state sensors in many aspects, for example, sensitivity, selectivity, and lowest detectable concentration.13 To continue to exploit possible applications of indium hydroxide and indium oxides, it is essential to control the sizes and morphologies of In(OH)3 and In2O3 nanomaterials in order to achieve tailored properties.14 Therefore, the controlled synthesis of In(OH)3 and In2O3 nanostructures has been widely investigated by many methods such as the sol-gel route,15 precipitation in homogeneous solution,16 double-jet precipitation,17 forced hydrolysis,18 alcoholysis,19 and sonohydrolysis.20 In the present work, we applied the solvothermal route to synthesize In(OH)3 nanospheres, nanoflowers, microcubes, and nanorods. Then, In2O3 nanomaterials were prepared by calcinating In(OH)3 at 500 °C and retaining the size shape and morphology because of the high-energy barrier of the morphological variation.
2.1. Materials. All chemicals, indium acetate (In(AC)3, 99.99% purity), ethylenediamine (99%, purity), acetic acid (99% purity), oleic acid (90% purity), and ethanol (98.5% purity), were used as received without further purification. 2.2. Synthesis. In a typical synthesis of In(OH)3 nanospheres, 0.25 mmol of In(AC)3 was dissolved in 10 mL of an ethanol solution with 3 mL of ethylenediamine and treated by ultrasonication for 30 min. The reaction solution was then transferred into a stainless steel autoclave and heated to 240 °C for 18 h in an oven followed by natural cooling to room temperature. The white precipitates were separated by centrifugation (4000 rpm) and rinsed three times with copious deionized water and ethanol, respectively. For the sake of investigating the controlled synthesis of In(OH)3, experiments were also carried out with 1 mL of ethylenediamine, 3 mL of acetic acid, and 3 mL of oleic acid while keeping other reaction conditions constant, respectively. Finally, the In(OH)3 samples were calcined in a furnace at 500 °C for 4 h to form In2O3 products. 2.3. Characterization. Powder X-ray diffraction (XRD) spectra of the samples were obtained using a powder X-ray diffractometer (D8 FOCUS 2200 V, Bruker AXS), using KR radiation (λ ) 1.5418 Å). The scanning electron microscopy (SEM) images were taken using FESEM (JEOL JSM-7401F). Transmission electron microscopy (TEM) images at low and high magnification as well as selected areas electron diffraction (SAED) were performed using TEM (JEOL, JEM 2010) to observe the microstructure of the composites. Further, to investigate the In2O3 structures, micro-Raman spectroscopy was carried out with the 514 nm line of an Ar+ laser (Invia Basic Renishaw). Photoluminescence (PL) spectra were recorded using an MFP-3D Bio - Asylum Research with He-Cd laser excitation at 325 nm to study their optical properties.
* To whom correspondence should be addressed. E-mail:
[email protected]. † Sungkyunkwan University. ‡ SAMSUNG Advanced Institute of Technology.
3. Results and Discussion 3.1. X-Ray Diffraction Patterns. The formation of In(OH)3 can be described through the well-known alcoholysis ester elimination reaction at high temperature as shown in Scheme 1.21 The reaction processes first carried out the nucleophilic attack of the hydroxyl group of ethanol on the carbonyl carbon atom of indium acetate to form the derivatives. Then, the decomposition of the derivative favorably occurred due to its
10.1021/cg701058v CCC: $40.75 2008 American Chemical Society Published on Web 06/10/2008
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Scheme 1. Alcoholysis Ester Elimination Reaction between Indium Acetate and Ethanol
unstable characteristics, leading to the formation of indium hydroxide and ester. The crystallinity of as-prepared samples by the solvothermal method was determined with XRD as shown in Figure 1. All the diffraction peaks can be assigned to the (200), (220), (310), (222), (321), (400), (420), (422) and (440) bcc In(OH)3 products with the calculated a constant 0.797 nm (space group: Im3j (204), JCPDS card No. 85-1338).10 3.2. Morphologies. In(OH)3 nanospheres with a diameter of approximately 120 nm, as confirmed by SEM at low magnification and SEM at high magnification (Figure 2a,b), were synthesized in the presence of 3 mL of ethylenediamine. Figure 2b shows that the In(OH)3 nanospheres surfaces were quite rough, possibly owing to a self-assembly mechanism in which smaller nanoparticles congregate into nanospheres. The TEM at low magnification image in Figure 2c confirms that these products are nanospheres with some interstices in each In(OH)3 nanospheres that are comprised of much smaller nanoparticles, which is quite a common phenomenon in self-assembly processes.21 The TEM at high magnification image (Figure 2d) indicates that the small nanoparticles have clear lattice fringes with a lattice spacing of 0.29 nm, corresponding to the spacing of the (220) plane of bcc In(OH)3 crystals. 23 The SAED pattern (inset) displays the typical ring patterns of (200), (220), (310), (321), (400), (420), and (440) planes of polycrystalline crystal structures. Interestingly, when the quantity of ethylenediamine was decreased from 3 to 1 mL, while keeping the other reaction conditions the same, the obtained In(OH)3 structures have flower morphologies of about 90 nm in diameter as confirmed with SEM at low magnification (Figure 3a). The SEM at high magnification image given in Figure 3b shows that flower structures consist of smaller nanoparticles, each with a diameter of ∼30 nm. TEM at low magnification (Figure 3c) also reveals
Figure 2. (a) SEM at low magnification, (b) SEM at high magnification, (c) TEM at low magnification, and (d) TEM at high magnification images of In(OH)3 nanospheres. Inset of (d) shows SAED patterns.
Figure 3. (a) SEM at low magnification, (b) SEM at high magnification, (c) TEM at low magnification, and (d) TEM at high magnification images of In(OH)3 nanoflowers. Inset of (d) is the SAED patterns.
Figure 1. XRD patterns of In(OH)3 (a) nanospheres, (b) nanoflowers, (c) microcubes, and (d) nanorods yielded by the solvothermal method.
the flowered structure where the “pale” and “light” parts can be attributed to the interspaces among their connection areas. The TEM at high magnification image shown in Figure 3d shows a clear lattice fringe with d spacing of 0.29 nm, which is in agreement with the (220) crystal facet of the cubic structures.24 The SAED patterns are consistent with bcc phase with strong ring patterns of the (200), (220), (310), (321), (400), (420), and (440) planes.
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Figure 4. (a) SEM at low magnification, (b) SEM at high magnification, (c) TEM at low magnification, and (d) TEM at high magnification images of In(OH)3 microcubes. Inset of (d) is the SAED patterns.
Figure 5. (a) SEM at low magnification, (b) SEM at high magnification, (c) TEM at low magnification, and (d) TEM at high magnification images of In(OH)3 nanorods. Inset of (d) is the SAED patterns.
For the controlled synthesis of In(OH)3 with various morphologies in our experiments, the control experiment was also conducted under the same conditions except that 3 mL of acetic acid was used instead of 3 mL of ethylenediamine. The products were regular In(OH)3 microcubes with edge length ranging from 600 to 700 nm in high yield, as measured by the SEM images at low magnification image and at high magnification shown in Figure 4a and b, respectively. The TEM at low magnification image, as presented in Figure 4c, reveals that most of the microcubes are connected via the cubic corners, possibly due to their self-assembly growth mechanism.25 Detailed analysis from TEM at high magnification (Figure 4d) clearly indicates that In(OH)3 microcubes are of polycrystalline structures with a lattice-spacing distance of approximately 0.39 nm, consistent with a (200) facet distance of the bcc In(OH)3.26 The inset in Figure 4d depicts the corresponding SAED pattern, which clarifies that diffraction spots obtained with an electron beam along the [001] direction can be indexed to the (200), (220), and (420) crystalline facets of the cubic bulk materials. Moreover, when acetic acid (3 mL) was replaced with oleic acid (3 mL), In(OH)3 nanorods were synthesized with a diameter of ∼40 nm and a length of more than 120 nm, as confirmed by SEM at low magnification image (Figure 5a). From careful observation, one can see that some In(OH)3 nanorods exhibit curved characteristics along their longitudinal direction (Figure 5b), which may originate from a two-step growth processes as discussed in the following section. The TEM at low magnification image (Figure 5c) shows that individual In(OH)3 nanorods are pointed like a sword at the terminal section, demonstrating that the tip is the active growth part compared with other sections. Lattice spacing of 0.29 nm between adjacent planes corresponds to the lattice spacing of the (220) planes as measured with the TEM at high magnification (Figure 5d).27 The SAED spots (inset in Figure 5d) taken from the In2O3 nanorods in Figure 5c constitute of diffraction rings, which can
be indexed to the (200), (220), (310), (400), and (411) planes of the cubic crystal. 3.3. Possible Formation Mechanism. On the basis of our experimental results, the surfactant-assisted self-assembly mechanism is proposed to account for the formation of the In(OH)3 nanospheres, nanoflowers, microcubes, and nanorods. Regarding this growth mechanism, the directing surfactants, which can coordinate with indium ions, favorably facilitate controlling the nucleation and growth to form indium hydroxide with various morphologies, well consistent with the oriented aggregation mediated growth mechanism suggested by Yang, et al.28 In our reaction system, when the ethylenediamine was added into the reaction solution, the nuclei and growth proceeded slowly owing to its coordinated behavior with indium ions.29 Once small particles are formed in the reaction solution, they are active and proceed to the self-assembly process to form larger nanocrystals to minimize the surface energies.30 When the ethylenediamine content was 3 mL in the reaction solution, the excess surfactants completely cover the In(OH)3 nanoparticles because In(OH)3 is an ionic compound, leading to the homogeneous interactions between the nanocrystals. Thus the homogeneous interactions facilitate the nanocrystals to conduct density stacking to form In(OH)3 nanospheres. However, when only 1 mL of ethylenediamine was added into the reaction solution, the insufficient coverage of the In(OH)3 leads to the existence of some high surface energy nanocrystals. This condition is favorable for oriented attachment at the active corners of the primary nanocrystals to form In(OH)3 nanoflowers. As mentioned above, In(OH)3 microcubes were produced by the same experimental procedures except that the ethylenediamine was replaced with 3 mL of acetic acid. To the best of our knowledge, acetic acid only has weak binding to the surface of crystals. Therefore, the intrinsic structure of bcc phase is strongly responsible for the formation of In(OH)3 microcubes. Nevertheless, the solubility of the starting material, In(AC)3, was enhanced under the assisted effect of acetic acid in the
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Figure 6. XRD patterns of In2O3 (a) nanospheres, (b) nanoflowers, (c) microcubes, and (d) nanorods yielded by calcination of In(OH)3 formed by the solvothermal method at 500 °C for 4 h.
Figure 8. SEM images of In2O3 (a) nanospheres, (b) nanoflowers, (c) microcubes, and (d) nanorods obtained by calcination of In(OH)3 precursors at 500 °C for 4 h.
Figure 7. Raman spectra of In2O3 (a) nanospheres, (b) nanoflowers, (c) microcubes, and (d) nanorods obtained by annealing In(OH)3 at 500 °C for 4 h.
ethanol solution, leading to a preferable condition to form large cubes of final products through a self-assembly attachment process. When 3 mL of oleic acid was used in our experiments, In(OH)3 nanorods were formed via anisotropic evolution. Oleic acid serves as a face-inhibiting functional surfactant driving the In(OH)3 nuclei to carry out oriented assembly to form In(OH)3 nanorods.31 For the curved nanorods the newly formed atoms tended to spontaneously transfer onto the surfaces of the existing particles, which served as seed sites for further growth. As the reaction continued, the reaction rate decreased because the concentration of atoms was not high enough for the former spherical particles to grow from their circumferences; this resulted in undersaturation around the existing particles. Therefore, the continuous addition of atoms to the as-formed particle surface would preferentially occur at the active sites of the circumferential edges along another direction to lead to the curved shapes. 3.4. Transformation into In2O3 Nanostructures. It is known that indium hydroxide can be dehydrated by heat treatment to yield indium oxide with a similar morphologies and sizes under heat treatment.32 Therefore, the In(OH)3 products were heat-treated at 500 °C for 4 h to change into In2O3 products. After calcination, all the recorded peaks in the XRD spectra pattern were indexed to the bcc In2O3 crystalline phase with a lattice constant of a ) 10.1 Å (JCPD 89-4595) as shown in Figure 6.33
Figure 9. Room-temperature PL emission spectra of (a) nanospheres, (b) nanoflowers, (c) nanorods, and (d) microcubes with He-Cd laser excitation at 325 nm.
To further investigate the structures of our annealed samples, Raman spectroscopy was utilized to examine the structures of the annealed samples. According to the crystallography data, the bcc In2O3 crystal structure belongs to the Ia3j space group with the point group Th.34 According to group theory analysis, the 52 optical modes have the irreducible representation as below:
Γopt ) 5A1g + 5E1g + 5E2g + 17Tg + 20Tu The Ag, Eg1, Eg2, and Tg are Raman active, and Tu modes are infrared active. Therefore, 32 active modes are expected in the Raman spectra of bcc In2O3. In fact, only four modes were observed for our samples, as can be seen in Figure 7. The modes centered at 306 (E1g), 390 (E2g), 489 (A1g), and 580 (E2g) cm-1 can be ascribed to the typical modes of bcc In2O3.35 The intensity of the Raman modes of In2O3 nanospheres and
2316 Crystal Growth & Design, Vol. 8, No. 7, 2008 Table 1. In(OH)3 and In2O3 Morphologies, Synthesis Parameters, and PL Properties
samples
morphologies
In(OH)3
nanospheres nanoflowers microcubes nanorods
In2O3
nanospheres nanoflowers microcubes nanorods
synthesis parameters In(AC)3 (0.25 mmol) + ethanol (10 mL) at 240 °C for 18 h ethylenediamine (3 mL) ethylenediamine (1 mL) acetic acid (3 mL) oleic acid (3 mL) calcination of corresponding In(OH)3 nanostructures at 450 °C for 5 h
PL emission wavelength (nm) no no no no at ∼480 at ∼480 at ∼460 weak and abroad emission at ∼475
nanoflowers are weak in contrast with those of In2O3 microcubes and nanorods. This is likely due to the nanosphere and nanoflower construction being the self-assembly of many small In2O3 nanoparticles that facilitates scattering of the incident light.36 It is evident from our study that the Raman shifts of the as-synthesized In2O3 products are not identical because their thermal expansion, strain, or anharmonic couplings to other phonons are different.37 After calcination of In(OH)3 products, the In2O3 nanospheres with a mean diameter of ∼120 nm, nanoflowers with average sizes of ∼90 nm, microcubes with sizes of several micrometers, and nanocubes with a diameter of ∼40 nm and length up to 120 nm were produced correspondingly, which was confirmed by SEM (Figure 8). However, after annealing of In(OH)3 microcubes, most of them collapsed with some fractures due to the elimination of water (Figure 8c). By close observation, the surfaces of In2O3 nanorods also display some defects due to the dehydration as shown in Figure 8d. Hence, the morphologies and sizes of In(OH)3 precursors play an important role in the transformation from indium hydroxide to indium oxide via calcination.38 3.5. Optical Properties of In2O3 Nanospheres, Nanoflowers, and Microcubes, and Nanorods. The PL spectra of the In2O3 nanospheres, nanoflowers, nanorods, and microcubes were measured at room temperature as shown in Figure 9, panels a, b, c, and d, respectively. Figure 9 indicates that In2O3 samples have a broad emission peak (Figure 9) in the blue ranges, which may originate from oxygen vacancies.39 Oxygen deficiencies should be produced since oxygen was effused from the bulk samples during the heat treatment stage. Thus, the oxygen deficiencies would cause the formation of shallow energy levels in the band gap. For In2O3 nanospheres and nanoflowers, the excited electrons in the valence band can combine with holes formed due to the oxygen deficiencies, which result in the emission peak centered at ∼480 nm. However, the In2O3 nanorods exhibit a blue-shifted emission peak positioned at 460 nm (Figure 9c) in comparison with In2O3 nanospheres and nanoflowers.40 The different emission peaks are originated from the varied energy levels yielded by oxygen vacancies.41 The In2O3 microcubes have such a weak emission peak (Figure 9d), which is consistent with weak emission peaks of bulk In2O3 materials.42
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4. Conclusions In summary, we have successfully synthesized In(OH)3 nanospheres, nanoflowers, microcubes, and nanorods by the solvothermal method. In our experiments, the surfactants and their quantity have an important influence in the formation, nucleation, and growth of indium hydroxide. On the basis of our experimental results, the surfactant-assisted self-assembly formation mechanism is proposed to account for the growth of the various In(OH)3morphologies. Moreover, In2O3 nanostructures with similar sizes and morphologies were produced by calcination of In(OH)3 precursors at 500 °C for 4 h in air. And indium oxide samples display shape-dependent optic properties, which may be related to the varied energy levels yielded by oxygen vacancies. Table 1 shows the reaction conditions for as-synthesized In(OH)3 and In2O3 with different morphologies and optical properties, respectively. It is anticipated that various sizes and morphologies of indium complexes should enable potential applications in optoelectronic nanodevices and gas sensors. Acknowledgment. This work is supported in part by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2005-005-J11903) and also, in part by the SRC program (Center for Nanotubes and Nanostructured Composites) of Ministry of Science and Technology of Korea/Korea Science and Engineering Foundation. D.R. also wishes to thank the Korean Government through Korea Research Foundation (MOEHRD) (KRF-2005-211-D00296) for financial support.
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