Nanostructures: Controllable NaOAc

Oct 19, 2009 - Daniela Caruntu , Kun Yao , Zengxing Zhang , Tabitha Austin , Weilie Zhou and Charles J. O'Connor. The Journal of Physical Chemistry C ...
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J. Phys. Chem. C 2009, 113, 19493–19499

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In(OH)3 and In2O3 Micro/Nanostructures: Controllable NaOAc-Assisted Microemulsion Synthesis and Raman Properties Wenyan Yin,† Jing Su,† Minhua Cao,*,† Chaoying Ni,§ Sylvain G. Cloutier,| Zuogang Huang,§ Xin Ma,| Ling Ren,† Changwen Hu,*,† and Bingqing Wei*,‡ The Institute for Chemical Physics, Department of Chemistry, and State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, People’s Republic of China, Department of Mechanical Engineering, UniVersity of Delaware, Newark, Delaware 19716, Department of Materials Science and Engineering, UniVersity of Delaware, Newark, Delaware 19716, and Department of Electrical Computer Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: July 4, 2009; ReVised Manuscript ReceiVed: September 25, 2009

In(OH)3 micro/nanostructures, including nanorods, nanoellipses, microspheres, and microbricks, were successfully synthesized using a CH3COONa(NaOAc)-assisted method involving a cetyltrimethylammonium bromide (CTAB)/water/cyclohexane/n-pentanol microemulsion process. It was found that the experimental parameters, such as the molar ratio (w) between water and CTAB, reaction temperature, chelating ligand CH3COONa, and the concentration of reactants, played important roles in the morphological control of In(OH)3 structures. An oriented attachment or coalescence-based “self-assembly” mechanism was used to explain the morphology evolution from nanorods to nanoellipses, then to microspheres. When the In(OH)3 micro/ nanostructures were calcined at 500 °C in air, the resulting In2O3 crystals succeeded to similar morphologies and sizes as the corresponding In(OH)3 precursors. Raman vibrational dynamics modes of the In2O3 structures were analyzed in detail, where these Raman peaks exhibit shape- and size-dependent Raman properties. 1. Introduction Semiconductor nanomaterials with distinct shapes and sizes have attracted considerable attention due to their outstanding optical, electric, gas-sensor, and magnetic properties.1-5 Controllable synthesis of these nanomaterials with different morphologies and sizes is of great importance for achieving desirable properties. Among many semiconducting nanomaterials, a wide gap semiconductor In(OH)3 boasts a bandgap of 5.1 eV and demonstrates special semiconducting and optical properties.6 The conductivity of In(OH)3 films can be controlled from 10-7 to 10-3 S/cm2, depending on synthetic conditions.7 In2O3, a wide gap semiconductor with a bandgap of 3.6 eV, shows technologically important applications in optoelectronic devices, such as lasers, fluorescent lamps, display devices, and infrared reflectors, and has received considerable interest. In2O3 also has important applications in the microelectronic field as gas detectors,8-10 solar cells,11 and flat-panel display materials.12 Compared with bulk In2O3, nanoscale In2O3 with a specific morphology promises predominantly potential applications. For example, Nguyen et al.13 fabricated a depletion mode n-channel field effect transistor with an In2O3 nanowire as the active channel. Another key characteristic of the nanoscale In2O3 is its chemical sensor property for the detection of NO2,14 NH3,15 acetone,16 DNA,17,18 and for biosensing devices19 due to its small dimension and high surface-to-volume ratio. Various synthetic approaches have been demonstrated on the synthesis of nanosized In2O3, such as nanowires (including arrow-like, beadlike, bouquet, and comb* To whom correspondence should be addressed. E-mail: caomh@ bit.edu.cn (M.C.), [email protected] (C.H.), [email protected] (B.W.). † Beijing Institute of Technology. ‡ Department of Mechanical Engineering, University of Delaware. § Department of Materials Science and Engineering, University of Delaware. | Department of Electrical Computer Engineering, University of Delaware.

like nanowires),20,21 nanosheets,22 nanobelts,23 nanoparticles (including nanocubes, nanoflowers, octahedron, and tin-doped In2O3 nanocrystals),24-29 nanotubes,30 and In/In2O3 nanocables.31 The as-developed methods mainly include chemical vapor deposition (CVD) techniques,20-22 alumina or mesoporous silica template methods,32 and wet chemical methods (including sol-gel method, hydrolysis, alcoholysis, etc.).33-35 Among the wet chemical methods, the microemulsion process, which can be easily controlled, has been widely used to prepare nanomaterials with different morphologies and sizes in recent years. Many nanostructures, such as SrWO4 and Ni(OH)2, have been successfully synthesized using the microemulsion process.36,37 However, there are relatively few reports on the microemulsion synthesis of In(OH)3 nanostructures, which could be utilized as the precursor for nanostructured In2O3 fabrication. Lin et al. reported that In(OH)3 nanorod bundles and spheres were synthesized via a microemulsion-mediated hydrothermal method by adjusting the pH value at 140 °C.38 However, the reaction took a long time (24 h). In addition, besides the pH value, other reaction parameters, such as molar ratio (w) between water and CTAB, reaction temperature, and the concentration of reactants, were not discussed. Therefore, it is necessary to develop an effective process for the synthesis of In(OH)3 with controllable morphologies and sizes. Here, we report that we can fabricate In(OH)3 nanorods, nanoellipses, microspheres, and microbricks using a novel CH3COONa(NaOAc)-assisted method combined with a simple microemulsion process. The morphologies and sizes of In(OH)3 can be readily tuned by adjusting the reaction parameters, such as the w value between water and CTAB, reaction temperature, and the concentration of reactants of the microemulsion system. Especially, the NaOAc can control the nucleation and growth of the crystal growth process through slow dissociation of In(CH3COO)3 in the microemulsion system. An oriented at-

10.1021/jp906328z CCC: $40.75  2009 American Chemical Society Published on Web 10/19/2009

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TABLE 1: Products Obtained under Different Conditions and Their Textural Characterization reaction conditionsa

phase sample

structures

w

concentrations (mol/L)

temperature (°C)

[In ] ) 0.5 [Ac-] ) 3.0 [In3+] ) 0.5 [Ac-] ) 3.0 [In3+] ) 0.5 [Ac-] ) 3.0 [In3+] ) 0.5 [Ac-] ) 3.0 [In3+] ) 0.27 [Ac-] ) 1.62 [In3+] ) 0.14 [Ac-] ) 0.84 [In3+] ) 0.5 [Ac-] ) 3.0

160

nanorods with diameters of 160-180 nm and length of 500 nm

160

microbricks with edge lengths of 0.6-1.4 µm

160

microbricks with edge lengths of 1.6-1.9 µm

180

nanoellipses with diameters of 200-400 nm and length of 1.0 µm

180

ellipses and spheres coexist

180

microspheres with diameters of 0.8-1.0 µm

180

microbricks with edge length of 1.3 µm

1

In(OH)3

5

2

In(OH)3

10

3

In(OH)3

20

4

In(OH)3

10

5

In(OH)3

10

6

In(OH)3

10

7

In(OH)3

20

3+

morphologies and sizes

a Reaction conditions: t ) 10 h, T ) 160-180 °C, the molar ratio between water and CTAB (defined as w), and the concentration of the reactants (unit: mol/L).

tachment or coalescence-based “self-assembly” mechanism was used to explain the morphology evolution from nanorods to nanoellipses, then to microspheres. Subsequently, body-centered cubic phase In2O3 with similar morphologies and sizes were obtained by calcining the corresponding In(OH)3 precursors at 500 °C. The Raman vibrational dynamics modes of the In2O3 structures were studied in detail, according to the crystallographic structure model of body-centered cubic In2O3. Also, the Raman peaks display shape- and size-dependent Raman properties. 2. Experimental Section The starting solution of 0.50 M In3+ (pH < 2) was prepared by dissolving InCl3 · 4H2O (50 mmol, 15.0 g) into 0.4 M HCl (30 mL), transferring it to a volumetric flask, and diluting to 100 mL with distilled water. The In(OH)3 micro/nanostructures were fabricated by a reverse microemulsion system of cetyltrimethylammonium bromide (CTAB)/water/cyclohexane/n-pentanol, in combination with a complexing reagent, CH3COONa (NaOAc). In a typical synthesis, two identical solutions were prepared by dissolving CTAB (2.74 mmol, 1.00 g) in cyclohexane (25 mL) and n-pentanol (2.0 mL) and mechanically agitating for 10 min until they show transparence. The 0.50 M In3+ stock solution (0.25 mL) and 0.25 mL of 3.0 M NaOAc aqueous solution were then separately added into the above two solutions with vigorous stirring until two transparent solutions were formed. The molar ratio of In3+/Ac- was kept at 1:6, and the molar ratio of water/CTAB (defined as w) was 5. These newly formed two solutions were quickly mixed and stirred for another 10 min and then transferred into a Teflon-lined stainless autoclave (80 mL capacity) heated at 160 °C for 10 h. The final solution system, after the hydrothermal treatment, is optically transparent with white precipitates at the bottom of the solution. It is also found that the amount of white precipitates increases with the increasing of the w values. After centrifugation, white nanorods of In(OH)3 were collected, washed with ethanol and distilled water several times, and dried at room temperature in air. Using this method, but only adjusting the concentrations of [In3+] (the molar ratio of In3+ to Ac- was 1:6), reaction temperature, and w values, different In(OH)3 micro/nanostructures, including nanoellipses, microspheres, and microbricks, were also obtained. Yellow In2O3 samples were obtained by annealing the white In(OH)3 samples at 500 °C for 2 h in air with a heating rate of 8 °C/ min.

Figure 1. XRD patterns of the as-prepared In(OH)3 samples with four typical morphologies, as described in Table 1: (a) nanorods, (b) nanoellipses, (c) microspheres, and (d) microbricks.

The as-synthesized samples were characterized and analyzed by the X-ray powder diffraction (XRD) method using a SHIMADZU XRD-6000 diffractometer with Cu KR radiation (λ ) 1.54056 Å) at 40 kV and 30 mA. The 2θ range used was from 10° to 80° or 10° to 90° with a speed of 6°/min and a scanning step of 0.02. Field emission scanning electron microscopy (FE-SEM) images were obtained on a JSM-6700F microscope operating at an acceleration voltage of 5.0 kV to observe the morphology of the products. Transmission electron microscopy (TEM) images were captured using a JEM-2010F microscope. Samples for TEM were prepared by dispersing products on a filmy carbon-coated copper grid. Raman spectra were measured using a Jobin-Yvon fiber-coupled confocal Raman microscope at room temperature. The camera is the Synapse, and the grating is 1800 grooves/mm. A fiber-coupled TORUS 250 mW frequency-stabilized 532 nm DPSS laser was used. 3. Results and Discussion 3.1. XRD Analysis of the Samples. Samples obtained in the NaOAc-assisted microemulsion system under different reaction conditions are listed in Table 1. The crystal structure of the samples has been identified by XRD. Figure 1 shows the XRD patterns of four representative samples with typical morphologies: nanorods (sample 1), nanoellipses (sample 4), microspheres (sample 6), and microbricks (sample 2). All reflections of the XRD patterns could be perfectly indexed as the pure body-centered cubic phase [space group: Im3 (204)]

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Figure 2. XRD patterns of In2O3 (a) nanorods, (b) nanoellipses, (c) microspheres, and (d) microbricks.

Figure 4. FE-SEM images of In(OH)3 samples 4, 5, and 6 with different InCl3 concentrations of (a) 0.5 M, (b) 0.27 M, and (c) 0.14 M, respectively, and (d) sample 7 with an InCl3 concentration of 0.5 M and w value as high as 20 obtained at 180 °C for 10 h.

Figure 3. FE-SEM images of In(OH)3 samples 1, 2, and 3 obtained at 160 °C for 10 h with the same InCl3 concentration of 0.5 M and different w values: (a) w ) 5, (b) w ) 10, and (c) w ) 20.

In(OH)3 (JCPDS no. 73-1810), without other impurity phases detectable. The XRD patterns of the samples obtained by calcinations of the above four typical In(OH)3 samples at 500 °C for 2 h in air are shown in Figure 2. All reflections match well with the body-centered cubic phase crystal structure [space group: Ia3 (206)] In2O3 (JCPDS no. 06-0416). 3.2. Effect of the Reaction Conditions on the Morphology and Size of In(OH)3. The size and morphology of the as-prepared samples were characterized by FE-SEM and TEM. The molar ratio (w) between water and CTAB, the reaction temperature, and the concentration of the reactants (C) were found to have significant effects on the morphology of In(OH)3. Only the w value was adjusted for samples 1-3 in Table 1, and other reaction conditions were kept constant. Sample 1 was synthesized when the w value was kept at 5. It can be clearly seen that the sample is composed of uniform nanorods (Figure 3a) with diameters ranging from 160 to 180 nm and an average length of 500 nm. When the w value was increased to 10 and 20 (samples 2 and 3), the morphology changed from nanorods to microbricks with edge lengths of 0.6-1.4 µm for sample 2 and edge lengths of 1.6-1.9 µm for sample 3, as shown in Figure 3b,c, respectively. In Figure 3b, two types of microbricks exist in sample 2. One is rectangular-shaped with edge lengths of 0.6-1.4 µm and widths of 160-180 nm. The other exhibits a platelike shape with equal edge lengths of about 0.6-1.0 µm. For sample 3 (Figure 3c), the In(OH)3 microbricks have similar morphologies as sample 2. However, the edge lengths of the

two types of microbricks are longer than those of sample 2. These results suggest that as the w value increases from 5 to 10; both the morphology and the size of the products have dramatic changes. The size of the samples increased clearly from nanoscale to micrometerscale as the w value increases from 5, to 10, to 20. The effect of the temperature on the morphology and size of the samples with a constant w value (w ) 10) and concentration of [In3+] aqueous solution (0.5 M) was also shown in Table 1 for samples 2 and 4. The morphology changed from microbricks (sample 2) to nanoellipses (sample 4) with diameters of 200-400 nm and a length of 1.0 µm (Figure 4a) when the temperature increased from 160 to 180 °C. However, when the temperature was kept at 140 °C, only microbricks were obtained, regardless of the change of w from 5, to 10, to 20. It indicated that reaction temperature of the microemulsion system has great effect on the morphology and size of the products. Further study suggests that the morphology and size of In(OH)3 also strongly depend on the reactant concentration at 180 °C for 10 h (samples 4-6). When [In3+] was changed from 0.5 to 0.27 M and the w value was kept at 10, nanoellipses and microspheres coexisted in the final product, as shown in Figure 4b. The surfaces of the nanoellipses and microspheres are not smooth compared with that of the nanoellipses in Figure 4a. With the concentration further decreasing to 0.14 M, a number of microspheres with diameters of 0.8-1.0 µm were observed, as shown in Figure 4c. The In(OH)3 spheres are composed of rough striation nanostructures, and some nanoparticles have sizes ranging from 10 to 30 nm. The above observation indicates that, as the [In3+] concentration decreases, the morphology of the as-synthesized samples evolves from nanoellipses, a mixture of nanoellipses and microspheres, to microspheres, and the diameter of the structure has moderately increased as well. However, when [In3+] was kept at 0.5 M and the w value reached 20, In(OH)3 microbricks (i.e., sample 7 in Table 1) with average edge length 1.3 µm were observed to be dominant, together with a small fraction of nanoparticles of about 200 nm in diameter, as shown in Figure 4d. Further characterization for four different In(OH)3 micro/ nanostructures was performed with TEM. It can be seen that the In(OH)3 nanorods (Figure 5a for sample 1) consist of smaller

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Figure 6. FE-SEM images of In2O3 (a) nanorods and (b) nanoellipses, after calcination of corresponding In(OH)3 samples 1 and 4 in Table 1 at 500 °C, respectively. Low- magnification TEM images of an individual (c) nanorod bundle and (d, e) nanoellipse. The inset of (c) is the SAED pattern of the body of the nanorod bundle. (f) The SAED pattern of the body of the ellipse. Figure 5. Bright-field TEM mages of four typical In(OH)3 samples 1, 4, 6, and 2 in Table 1: (a) nanorods, (b, c) nanoellipses, (d, f) microspheres, and (g, h) microbricks. (e) Schematic representation of the formation process of the rods, ellipses, and spheres. The inset in (c) shows the magnified image of one side of a single ellipse. The inset in (f) shows the SAED pattern on the body of the microsphere. The inset in (h) shows the SAED pattern on the body of the microbrick.

parallel nanorods, each with a diameter of ca. 15 nm. Also, the parallel nanorods in the middle are longer than those of the two sides. Some nanorods are in spindle-like shapes. Figure 5b shows the TEM image of nanoellipses (sample 4). The average particle size of the nanoellipses increases markedly compared with sample 1. The diameter of the ellipses ranges from 200 to 400 nm and is consistent with the SEM result. Figure 5c shows a magnified image of one side of a single ellipse. It is revealed to be composed of highly parallel nanorods, exhibiting an ellipse-like structure. The inset in Figure 5c shows a highermagnification image of the ellipse (the white pane), which further proved the nanorod bundle structure of the ellipse. The TEM observations revealed that sample 6 (Figure 5d) mainly consisted of urchin-like spheres with diameters of 0.8-1.0 µm, and a few ellipses were occasionally observable. However, the morphology of the ellipses is different from that shown in Figure 5b. Figure 5f shows the magnified image of Figure 5d, in which the surfaces of the ellipses and spheres also exhibit an urchinlike shape. The coexistence of a few relatively small size ellipselike structures and the large number of microsphere-like structures provides an evidence that the microsphere-like structures are probably formed from the “self-assembly” of the nanoellipses through an attachment or coalescence mechanism, as is schematically illustrated in Figure 5e. As reported in ref 39, Wang’s group has recently investigated the size- and surface-determined phase and morphology transformation process of InOOH using the “oriented attachment process”, which is the process very similar to the formation of In(OH)3 nanorod bundles, nanoellipses, and microspheres in our experiments.

The mechanism in Figure 5e also explains the increased diameter of the three samples from nanosize to microsize through a morphology evolution from nanorods, to nanoellipses, to microspheres. More details about the structure of In(OH)3 samples were investigated by selected area electron diffraction (SAED). The inset in Figure 5f is an SAED pattern of the microspheres. The spotty rings are indexed as {200}, {220}, {310}, {321}, and {400} reflections of the body-centered cubic In(OH)3 phase and prove the polycrystalline nature of the In(OH)3 microspheres. Figure 5g shows the TEM image of the In(OH)3 microbricks (sample 2). It is clearly seen that two types of microbricks existed in the sample, as observed in the FE-SEM image. One has a length of about 1.3 µm and a width of about 180 nm; the other has a uniform edge length of about 600 nm. Figure 5h shows the corresponding HR-TEM image of the pane in Figure 5g and the SAED pattern (inset in Figure 5h). The lattice plane spacing between the adjacent lattice planes perpendicular to the preferential growth direction (marked with arrows) in Figure 5h is 0.386 nm, which is consistent with the {200} d-spacing of cubic In(OH)3. In addition, the [100] direction is parallel to the longitudinal axis of the brick, indicating that the growth direction of the brick is preferentially along the [100] direction. The SAED pattern further indicates that the In(OH)3 microbrick has a single-crystalline structure, representing the growth also along the [100] direction. Calcinations of the above four In(OH)3 samples 1, 4, 6, and 2 at 500 °C in air for 2 h resulted in In2O3 with similar morphologies and sizes as those of the precursors, as shown in Figures 6 and 7. The FE-SEM image in Figure 6a clearly demonstrates that the surface of the In2O3 nanorods is rough compared with Figure 3a. Some nanoparticles with a diameter of 15 nm can be clearly seen to cling to the surface of In2O3 nanorods due to the dehydration of In(OH)3 during heating. The low-magnification TEM image (Figure 6c) of an individual In2O3 nanorod also reveals the rod structure with a diameterof 160 nm, but it mainly consists of nanoparticles, as observed in Figure 6a, where the part of “gray” (the white panes) can be attributed to the interspaces among these nanoparticles’ inter-

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Figure 8. FE-SEM image (a) and XRD pattern (b) of the microcubes obtained by hydrolysis of In(OAc)3 aqueous solution at 180 °C for 10 h.

Figure 7. (a) FE-SEM image of In2O3 microspheres after calcination of In(OH)3 sample 6 in Table 1 at 500 °C. (b, c) Low-magnification TEM images of a single microsphere. The right-top inset in (c) shows the magnified edge of the sphere in the white pane. The right-bottom in (c) shows the SAED pattern of the body of the sphere. (d) HR-TEM image (the white pane in Figure 7c) of an edge of the single sphere. The left-bottom in (d) shows the HR-TEM image of the white pane in (d). (e) FE-SEM image and (f) TEM image of In2O3 microbricks after calcination of In(OH)3 sample 2 in Table 1 at 500 °C. The inset in (f) shows the SAED pattern on the body of the bricks.

connection areas. The SAED pattern (inset in Figure 6c) taken from the body of the nanorod in Figure 6c is consistent with strong polycrystalline diffraction rings of {110}, {200}, {211}, {220}, {013}, {222}, and {400} planes with a body-centered cubic phase. Figure 6b shows the FE-SEM image of the In2O3 nanoellipses, and small nanoparticles with a diameter of about 10 nm on the surface of the nanoellipses are clearly visible. The further characterization of a single In2O3 nanoellipse was achieved in detail using TEM (Figure 6d). The ellipse is 220 nm in diameter and 700 nm in length. The magnified TEM image in Figure 6e showed one end of the ellipse (the black pane in Figure 6d) that is composed of much smaller connected nanoparticles, which is different from the In(OH)3 ellipse precursor shown in the self-assembly processes of Figure 5b,c. The corresponding SAED pattern of the body of the In2O3 nanoellipse is shown in Figure 6f, indicating the singlecrystalline nature of the ellipse, which may be induced by the oriented, parallel bundles composed of the ellipse. Figure 7a shows the FE-SEM image of the In2O3 microspheres. It is noted that the surface of the spheres presents more obvious sticklike structures compared with the corresponding In(OH)3 spheres shown in Figure 4c. Also, the diameter of the In2O3 microspheres is almost the same as that of the In(OH)3 before calcination. Figure 7b is a low-magnification TEM image of a single microsphere with a diameter of 1.0 µm, which reveals the rough surface of the sphere. Figure 7c shows a magnified

TEM image of one edge of a typical In2O3 microsphere. It appears that the surface of the sphere is rough and is composed of some connected nanoparticles. The right-top inset in Figure 7c shows the magnified TEM image of the white pane area. These connected nanoparticles assemble into parallel rodlike nanostructures. The right-bottom inset in Figure 7c is the SAED pattern of the sphere body. The lattice planes of {200}, {220}, {222}, {332}, and {440} are indexed from SAED rings, indicating the polycrystalline nature of the In2O3 microsphere. Figure 7d shows the corresponding HR-TEM image of the white pane in Figure 7c. The lattice spacing is 0.293 nm, which corresponds to {222} crystal planes. The inset in Figure 7d shows the HR-TEM image of the white pane part of the microsphere. The {222} lattice planes of the structure can be clearly seen from the magnified HR-TEM image. It can also be seen that parallel bundle structures (nanorods, nanoellipses, microspheres) transformed into the connected nanoparticle structures after calcination (Figure 5e), which may be ascribed to the dehydration of the In(OH)3 precursors. However, some of the In(OH)3 microbricks collapsed, leaving some cracks after calcinations, as shown in the FE-SEM image in Figure 7e. The collapse for some of the In2O3 microbricks can also be clearly seen from the TEM image (Figure 7f). The inset in Figure 7f is the SAED pattern taken from the collapsed microbricks, which indicates the polycrystalline nature with body-centered cubic phase of the In2O3 bricks. The above observation suggests that the morphology and size of In(OH)3 precursors play an important role in the transformation of In(OH)3 to In2O3. Moreover, both the morphology and the size of In2O3 are conveniently tunable by preparing and adjusting corresponding In(OH)3 precursors. It is well-known that “water-in-oil” microemulsion is a transparent and isotropic liquid medium with nanosized water pools dispersed in a continuous oil phase and stabilized by surfactants and cosurfactant. The nanoscale water pools can provide ideal microreactors for the formation of nanoparticles.40 Therefore, the size of the water pool has a significant influence on the structure, size, and the phase formation of nanoparticles. Using a complex compound to control the nucleation and growth is also a simple and effective way. The microemulsion in our experiments, together with a transparent In(OAc)3 aqueous solution, was essential for the In(OH)3 micro/nanostructures’ development. To confirm this, two parallel experiments were carried out. When the microemulsion system was completely replaced by 25 mL of distilled water and 5 mL of 0.5 M In(OAc)3 aqueous solution without changing the total volume (30 mL), it was found that only uniform microcubes were formed (Figure 8a) by hydrolysis of In(OAc)3 aqueous solution at 180 °C for 10 h. The final pH of the aqueous solution is 6.5 after the reaction. The edge lengths of the microcubes were in the range of 0.8-1.6 µm. An XRD diffractogram of the sample

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shown in Figure 8b confirms that these microcubes have a pure In(OH)3 cubic phase structure (JCPDS no. 73-1810). The above observation suggests that the microemulsion system is a prerequisite for the morphology control of In(OH)3 micro/ nanostructures. Furthermore, from the experiment results, we know that the morphology changed from microbricks (sample 2) to nanoellipses with smaller sizes (sample 4) when the temperature increased from 160 to 180 °C. It can be concluded that the higher temperature under the hydrothermal conditions, in connection with the smaller water pools, may be the main factors that result in the formation of nanosized structures. When 2.5 or 5.0 mL of 0.5 M In3+ solution was used in the absence of NaOAc for the microemulsion system, no product was obtained at either 160 or 180 °C for 10 h, indicating that NaOAc plays a critical role in the nucleation and growth of In(OH)3 nanocrystals. Therefore, the reaction in the microemulsion system is proposed as follows: at the first stage, CH3COO-, acting as a complexing agent, reacts with In3+ to form In(CH3COO)3 coordination compound. The In(CH3COO)3 dissolves in the water/oil microemulsion system without the formation of In(OH)3 precipitation before heating. The reaction is illustrated in reaction 1 below:

In3+ + 3CH3COO- f In(CH3COO)3

(1)

At the second stage, In(CH3COO)3 slowly hydrolyzes in the microemulsion system after hydrothermal heating to form In(OH)3 (see reaction 2):

In(CH3COO)3 + 3H2O f In(OH)3 V +3CH3COOH

(2) At the same time, as the molar ratio of In3+ to Ac- was 1:6, the redundant CH3COO- can react with H2O to produce OH(reaction 3). The slow dissociation process of OH- in reaction 3, which promotes the formation of In(OH)3 precipitates in the solution during heating, can affect the nucleation and growth of In(OH)3 nanocrystals.

CH3COO- + H2O f CH3COOH + OH-

(3)

Finally, In2O3 is obtained by annealing treatment of In(OH)3 at 500 °C for 2 h in air, as shown in reaction 4.

2In(OH)3 ) In2O3 + 3H2Ov

(4)

3.3. Raman Properties of Four Typical In2O3 Micro/ Nanostructures. Raman scattering measurements have been carried out on four typical In2O3 micro/nanostructures, as described above, to test their sensitivity to the crystal potential fluctuations and local atomic arrangement in the nanostructured materials. As shown in Figure 9, the Raman spectra of In2O3 (a) microbricks, (b) microspheres, (c) nanoellipses, and (d) nanorods were recorded at room temperature using a confocal laser Raman spectrometer. In the range of 100-700 cm-1, five scattering peaks are observed in Figure 9, and their positions are approximately in agreement with those of previously reported body-centered cubic In2O3 Raman spectra (131,41 308, 365, 504, and 637 cm-142,43). In Figure 9, spectrum a, the five Raman peaks of the microbricks locate at 131, 308, 367, 497,

Figure 9. Raman spectra of the four typical In2O3 samples (a) microbricks, (b) microspheres, (c) nanoellipses, and (d) nanorods after calcination of the four corresponding In(OH)3 samples 2, 6, 4, and 1 in Table 1 at 500 °C for 2 h, respectively; The vertical black lines are a guide to the reader to indicate the peak shifts.

and 630 cm-1, consistent with the vibration modes of bodycentered cubic In2O3, in which vibration modes are predicted as 4Ag (Raman) + 4Eg (Raman) + 14Tg (Raman) + 5Au (inactive) + 5Eu (inactive) + 16Tu (infrared) modes.44 The vibrations with symmetry Ag, Eg, and Tg are Raman-active and infrared-inactive, whereas the Tu vibrations are infrared-active and Raman-inactive. The Au and Eu vibrations are inactive in both infrared and Raman measurements. The peak at 131 cm-1 is assigned to the In-O vibration of InO6 structure units.45 The peak at 308 cm-1 is assigned to the bending vibration of δ(InO6) octahedrons; the other two peaks 497 and 630 cm-1are attributed to the stretching vibrations of the same V(InO6) octahedrons,46 whereas the 367 cm-1 is assigned to the stretching vibrations of the In-O-In.47 However, compared with Figure 9, spectrum a, all the other four peaks besides 131 cm-1 in Figure 9, spectra b-d, shift to lower frequencies by 1, 2, 1, and 1 cm-1 (microspheres); 3, 5, 2, and 3 cm-1 (nanoellipses); and 6, 3, 4, and 5 cm-1 (nanorods), respectively, indicating a dimensional effect of In2O3 mincro/nanostructures on their Raman properties. The width and the intensity of the Raman peaks of the In2O3 nanorods in Figure 9, spectrum d, are relatively wide and weak compared with those of the In2O3 microbricks, microspheres, and nanoellipses in Figure 9, spectra a-c. As described in ref 41, the Raman peak position depends mainly on the number of atoms included in the nanocrystallites, whereas the width of the peak reflects both the shape and the dimension of the nanostructures. In this case, the relatively weak intensity of the nanorods is likely due to that the surface morphology of nanorods, which consists of many small In2O3 nanoparticles that facilitate scattering of the incident light. As shown in Table 1, the sizes of the four different types of structures from (a) microbricks to (d) nanorods decrease from microscale to nanoscale. The Raman peaks shift to lower frequencies as the corresponding In2O3 size decreases, which can be explained by the phonon confinement model.48 For a smaller particle size crystal, in particular, for small nanocrystals, as shown in Figure 6a, the phonon would be confined in a limited space. The smaller the crystal dimensions are, the stronger the finite size effects.49 Therefore, compared to the other three In2O3 morphologies with larger sizes, the Raman peaks shifted to low frequency is the most evident for the In2O3 nanorods with smaller size. Similar Raman shifts have also been reported for other nanoscale oxides, such as SnO2, Mn3O4, CaWO4, etc.50,51 due to the same phonon confinement effect.

In(OH)3 and In2O3 Micro/Nanostructures 4. Conclusions In(OH)3 micro/nanostructures with four different morphologies, nanorods, nanoellipses, microspheres, and microbricks, were controllably synthesized by dissociation of In(OAc)3 in the microemulsion system. Various comparison experiments showed that experimental parameters, such as the molar ratio (w) between water and CTAB, NaOAc, the concentration of reactants, and the reaction temperature played important roles in the morphological control of In(OH)3 micro/nanostructures. An oriented attachment or coalescence-based “self-assembly” mechanism was used to explain the morphology evolution from nanorods to nanoellipses, then to microspheres. In2O3 micro/ nanostructures were naturally obtained by calcination of corresponding In(OH)3 precursors at 500 °C in air. The Raman vibration modes of the In2O3 structures were studied in detail, and the Raman peaks display shape- and size-dependent Raman properties. Acknowledgment. This work was supported by the State Scholarship Fund of China Scholarship Council (CSC, File No. 2008603051), the Natural Science Foundation of China (NSFC, Nos. 20671011, 20731002, 10876002, 20771022, and 20871016), the 111 Project (B07012), Key Laboratory of Structural Chemistry Foundation (KLSCF, No. 060017), the Excellent Young Scholars Research Fund of Beijing Institute of Technology (No. 2006Y0715), and the Basic Research Fund of Beijing Institute of Technology (Nos. 20060742022 and 20070742010). References and Notes (1) Ng, H. T.; Han, J.; Yamada, T.; Nguyen, P.; Chen, Y. P.; Meyyappan, M. Nano Lett. 2004, 4, 1247. (2) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (3) Cao, M. H.; Liu, T. F.; Gao, S.; Sun, G. B.; Wu, X. L.; Hu, C. W.; Wang, Z. L. Angew. Chem., Int. Ed. 2005, 44, 4197. (4) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (5) Sysoev, V. V.; Goschnick, J.; Schneider, T.; Strelcov, E.; Kolmakov, A. Nano Lett. 2007, 7, 3182. (6) Pe´rez-Maqueda, L. A.; Wang, L. F.; Matijevic´, E. Langmuir 1998, 14, 4397. (7) Avivi, S.; Mastai, Y.; Gedanken, A. Chem. Mater. 2000, 12, 1229. (8) Gurlo, A.; Ivanovskaya, M.; Pfau, A.; Weimar, U.; Go¨pel, W. Thin Solid Films 1997, 307, 288. (9) Gurlo, A.; Barsan, N.; Ivanovskaya, M.; Weimar, U.; Go¨pel, W. Sens. Actuators, B 1998, 47, 92. (10) Atashbar, M. Z.; Gong, B.; Sun, H. T.; Wlodarski, W.; Lamb, R. Thin Solid Films 1999, 354, 222. (11) Granqvist, C. G. Appl. Phys. A: Mater. Sci. Process. 1993, 57, 19. (12) Murali, A.; Barve, A.; Leppert, V. J.; Risbud, S. H.; Kennedy, I. M.; Lee, H. W. H. Nano Lett. 2001, 1, 287. (13) Nguyen, P.; Ng, H. T.; Yamada, T.; Smith, M. K.; Li, J.; Han, J.; Meyyappan, M. Nano Lett. 2004, 4, 651. (14) 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. (15) Li, C.; Zhang, D.; Liu, X.; Han, S.; Tang, T.; Han, J.; Zhou, C. Appl. Phys. Lett. 2003, 82, 1613. (16) Vomiero, A.; Bianchi, S.; Comini, E.; Faglia, G.; Ferroni, M.; Sberveglieri, G. Cryst. Growth Des. 2007, 7, 2500.

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