In2O3 Nanorod Bundles Derived from a Novel Precursor and In2O3

Dec 17, 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 2010, 114, 65–73

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In2O3 Nanorod Bundles Derived from a Novel Precursor and In2O3 Nanoaggregates: Controllable Synthesis, Characterization, and Property Studies Wenyan Yin,† Jing Su,† Minghua Cao,*,† Chaoying Ni,‡ 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 Materials Science and Engineering, UniVersity of Delaware, Newark, Delaware 19716, and Department of Mechanical Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: August 27, 2009; ReVised Manuscript ReceiVed: NoVember 4, 2009

An indium-ethylene glycol-ethylenediamine (In-EG-EDA) complex precursor with a nanorod bundle structure was controllably synthesized in EG solvent via an EDA-assisted solvothermal process at 180 °C. The morphologies and phase structures of the samples were found to depend strongly on the pH values and alkaline reagents. This complex precursor was subsequently calcined at 640 °C in air, resulting in the formation of cubic phase In2O3 nanorod bundles without changing its one-dimensional morphology. In addition, In2O3 nanoaggregates with a diameter of 20 nm can be directly obtained using ammonia as alkaline reagent. A possible mechanism was proposed to explain the formation of In-EG-EDA nanorod bundles and In2O3 nanoaggregates. Both the In2O3 nanorod bundles and the nanoaggregates showed wide emission peaks from the ultraviolet to visible region in photoluminescence spectra. 1. Introduction Semiconductor nanomaterials with distinct shapes and sizes have drawn much attention because of their exceptional physical and chemical properties and potential applications in various fields, such as electronic and optoelectronic nanodevices.1,2 Indium oxide (In2O3) is a kind of transparent semiconductor with a bandgap of 3.6 eV. Compared with bulk In2O3, nanoscaled In2O3 structures often possess a shape and size dependence of physical and chemical properties that are of great predominance in scientific studies and applications as gas detectors for NO2, NH3, and acetone;3-6 biosensors;7,8 solar cells;9 flat-panel display materials; window heaters;10,11 fieldeffect transistors;12 etc. However, the great possibility in applications of the In2O3 nanostructures depends on controllable synthesis of them with specific morphologies and sizes. Until now, various In2O3 nanostructures, such as nanowires,13-17 nanosheets,18 nanobelts,19 nanoparticles,20,21 nanaocubes,22,23 nanotubes,24 In/In2O3 nanocables,25 lotus-root-like In2O3 nanorods,26 nanocrystal chains,27 and hollow spheres,28 have been prepared by a variety of methods, mainly including vapor-phase methods (chemical vapor deposition (CVD), physical thermal evaporation and condensation, in situ thermal oxidation method),13-18 “soft” or “hard” template methods,29-31 and solution-phase methods (including sol-gel method, alcoholysis, etc.).21,32 Although high-quality nanostructures can be synthesized using vapor-phase methods, the required high temperature (800-1500 °C), rigorous vacuum condition, carrier flow, and sophisticated equipment apply limitations in terms of costeffectiveness and production yield. The template methods, such * To whom correspondence should be addressed. E-mail: [email protected] (B.W.), [email protected] (M.C.), [email protected] (C.H.). † The Institute for Chemical Physics, Department of Chemistry, and State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology. ‡ Department of Materials Science and Engineering, University of Delaware. § Department of Mechanical Engineering, University of Delaware.

as emulsion droplets and alumina or mesoporous silica acting as templates, also suffer from disadvantages of high cost and complicated synthetic procedures, which may have some effects on properties and large-scale applications of the products. Moreover, these templates are not easily removed from the products. Among all the methods mentioned above, the solutionphase methods have been considered to be attractive toward the preparation of In2O3 nanostructures as they could avoid the issues mentioned above. However, the solution-phase methods often employ environmentally harmful organic precursors or surfactants to control the morphology and size of final products. For example, monodispersed In2O3 nanocrystals can be prepared by thermolysis of indium acetate under elevated temperatures20 or by hydrolysis and alcoholysis of indium carboxylates.21 With growing emphasis on green chemistry principles and great possibility in applications of nanoscale In2O3, it is becoming increasingly important to develop a more environmentally friendly and facile solution-phase method for controllable synthesis of In2O3 nanostructures with different morphologies and sizes. Moreover, there are relatively few reports on the synthesis of In2O3 nanostructures using a novel precursor,32 and the development of a new precursor to obtain corresponding In2O3 nanostructures may be a promising route. Herein, we report a controllable method to synthesize In2O3 nanorod bundles and In2O3 nanoaggregates using ethylene glycol (EG) as solvent in the absence of any surfactants. In2O3 nanorod bundles were obtained by calcining the In-EG-EDA nanorod bundle precursor (EDA ) ethylenediamine), which were synthesized via the EDA-assisted solvothermal reaction in EG solvent. During this calcination-transformation process, the obtained In2O3 nanostructures almost maintained the nanorod bundle morphology from the precursor. In2O3 nanoaggregates with an average diameter of 20 nm can be directly synthesized in the EG solvent under solvothermal conditions using ammonia as alkaline reagent instead of EDA. To the best of our knowledge, this is the first report about controllable synthesis of In2O3 nanostructures derived from calcination of the novel

10.1021/jp908298y  2010 American Chemical Society Published on Web 12/17/2009

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TABLE 1: Summary of the Samples Obtained under Different Reaction Conditions and Textural Characterization reaction conditionsa

a

sample

phase structures

pH value

alkaline reagent

1

In-EG-EDA complex

9.31

EDA

2

In-EG-EDA complex

10.26

EDA

3

In-EG-EDA complex

10.90-11.14

EDA

4

In-EG-EDA complex

12.00

EDA

5

In2O3

10.17

ammonia

morphologies and sizes nanoparticles with diameters of 140 nm nanoparticles with diameters of 450 nm nanorod bundles with diameters of 200 nm and a length of 3 µm bouquet-like nanorods with diameters of 200 nm and a length of 4.2-4.5 µm regular nanoaggregates with diameters of 20 nm

Reaction conditions: t ) 10 h, T ) 180 °C.

complex precursor In-EG-EDA. The solvothermal process can be performed at relatively low temperature (180 °C) without using environmentally harmful organic compounds, templates, and/or surfactants. A “self-assembly” mechanism was proposed to explain the formation of In-EG-EDA nanorod bundles and In2O3 nanoaggregates. The photoluminescence spectra of the In2O3 nanorod bundles and nanoaggregates showed wide emission peaks from the ultraviolet to visible region (blue-green for nanorod bundles; blue-green and orange for nanoaggregates). These In2O3 nanomaterials have potential applications in optoelectronic nanodevices, such as lasers, fluorescent lamps, and display devices. 2. Experimental Section 2.1. Synthesis. All of the reagents were of analytical grade and used as received without any further purification. In-EG-EDA Nanorod Bundle Precursor. InCl3 · 4H2O (0.056 g, 0.2 mmol) was dissolved in 30 mL of ethylene glycol under mechanical agitation to form a transparent solution. Ethylenediamine (0.25 mL) was added into the above solution. The pH value of the mixture solution was about 10.90. After stirring for 10 min, the obtained homogeneous solution was transferred and sealed into an 80 mL Teflon-lined autoclave, heated at 180 °C for 10 h, and then cooled to room temperature. The nanorod bundles of the as-prepared In-complex precursor were collected by centrifugation, washed several times with distilled water and absolute ethanol, and dried in air at room temperature. In2O3 Nanorod Bundles. In2O3 nanorod bundles with a yellow color were obtained by annealing the above In-EG-EDA nanorod bundle precursor in air at a temperature of 640 °C with a heating rate of 1 °C/min for 2 h and then cooled to room temperature. One-Step Synthesis of In2O3 Nanoaggregates. When EDA was replaced by 0.25 mL of ammonia in the synthesis process of the In-EG-EDA nanorod bundle precursor while keeping other reaction conditions unchanged, In2O3 nanoaggregates were directly obtained during the solvothermal process. 2.2. Characterization. The as-synthesized products were characterized by X-ray powder diffraction (XRD) spectra using a SHIMADZU XRD-6000 diffractometer with Cu KR radiation (λ ) 1.54056 Å) at 40 kV and 50 mA. Field emission scanning electron microscopy (FE-SEM) images were obtained on a JEOL S-4800 field emission microscope. An energy-dispersive X-ray spectroscopy (EDS) facility attached to the FE-SEM was employed to analyze the chemical composition of the Incomplex precursor. Transmission electron microscopy (TEM) images were captured using a JEOL JEM-2010 transmission

electron microscope. A Fourier transform infrared spectrum (FTIR) was obtained (as KBr pressed pellets) using a Nicolet 170SXFT/IR spectrometer. Thermal behavior of the In-complex precursor was studied by thermogravimetric analysis (TGA) at a heating rate of 10 °C/min under a constant flow of air using an SDT Q600 system to determine the appropriate temperature for thermal conversion of the precursor to the In2O3 structure. Photoluminescence (PL) spectra were examined using a fluorescence spectrophotometer (Edinburgh FLS920) with a Xe lamp at room temperature. The elemental analysis was conducted on a Vario EI elemental analyzer, and inductively coupled plasma (ICP) analysis was performed on a JY Ultima (French) spectrometer. 3. Results and Discussion 3.1. In-EG-EDA Nanorod Bundle Precursor and In2O3 Nanorod Bundles. Samples obtained in the solvothermal system under different reaction conditions are listed in Table 1. When EDA was used in the reaction system, the In-complex precursor was obtained (samples 1-4 in Table 1). Figure 1a shows the XRD pattern of an In-complex precursor synthesized at 180 °C for 10 h in the EG-EDA system (sample 3). It can be seen that no obvious diffraction peak was observed in the pattern, indicating an amorphous feature of the In-complex. As reported in ref 33, EG tends to produce glycolates with indium due to its appropriate velocity and coordination ability. In addition, EDA used here is also a strong bidentate ligand. Many metallic elements or cations can coordinate with EDA to form relatively stable complexes,34,35 including chemical bonding with indium.36 Therefore, we speculated that the sample was an amorphous compound that resulted from the reaction of InCl3 with EG and EDA during the solvothermal process. To confirm the above hypothesis, FT-IR spectroscopy was carried out. Figure 1b shows the FT-IR spectrum of sample 3. The bands at 1460, 2846, and 2927 cm-1 correspond to asymmetric bending, symmetric stretching, and asymmetric stretching vibrations of the -CH2, respectively. The splitting of the -CH2 bands at 2927 and 2846 cm-1, which was also found with Ni(OCH2-CH2O) or In(OCH2-CH2O) synthesized in EG system, was attributed to two different protons (equatorial and axial) present in this compound.33,37,38 Moreover, the band at ∼1040 cm-1, which is characteristic of C-O stretching vibration,39 shifts to a lower wavenumber and clearly widens compared with the IR spectrum of pure EG,37 indicating that the C-O in EG coordinated to In3+. The strong band at ∼3450 cm-1, the bands at 1631 and 671 cm-1, and the band at 2360 cm-1 are characteristics of the NH2 stretching vibration, N-H deformation vibration,36 and the NH2+ stretching vibration,

In2O3 Nanorod Bundles

Figure 1. (a) XRD pattern and (b) FT-IR spectrum of the In-EG-EDA complex precursor synthesized at 180 °C for 10 h under the EG-EDA system (sample 3).

respectively, whereas the bands at ∼3450 and 1631 cm-1 also shift [compared with free -NH2 stretching vibration (∼3400 cm-1) and -NH (1560 cm-1) deformation vibration], confirming the formation of hydrogen bonding between N-H and In3+. The band at ∼3450 cm-1 corresponds to the stretching mode of hydroxyls of adsorbed water. Therefore, FT-IR results in Figure 1b revealed that InCl3 not only coordinated with EG but also with EDA during the solvothermal process. We named the In-complex precursor as In-EG-EDA. The morphology and size of the as-prepared In-EG-EGA sample were characterized by FE-SEM and TEM. Figure 2a shows the FE-SEM image of sample 3. It can be clearly seen that the sample consists of uniform nanorod bundles with an average diameter of 200 nm and a length of 3 µm. A highmagnification TEM image in Figure 2b reveals that an individual nanorod bundle is composed of several nanorods with a diameter of ∼15 nm, aggregating into a thicker nanorod bundle with a rough surface. A selected area electron diffraction (SAED) pattern (inset in Figure 2b) and a high-resolution TEM (HRTEM) image (Figure 2c) of the individual nanorod bundle indicate that the In-EG-EDA nanorod bundle exhibits an amorphous structure, consistent with the XRD result (Figure 1a). In addition, the HRTEM image clearly showed that the nanorod bundle was formed by aggregating the single nanorod, where the “gray” areas (the white panes) can be attributed to the interspaces among these nanorod interconnection areas. To determine the appropriate thermal treatment temperature of the In-EG-EDA nanorod bundle precursor, the thermal behavior of the precursor was investigated with TGA measurement (Figure 2d for sample 3). The TGA curve clearly shows that the sample undergoes a two-step weight-loss process. The first step is a major weight loss of 27.41% between 100 and 440 °C and the second is a weak weight loss of 3.17% between

J. Phys. Chem. C, Vol. 114, No. 1, 2010 67 440 and 640 °C. The weight loss of 10.0% from 30 to 100 °C is attributed to the removal of the absorbed water, as confirmed by the FT-IR spectrum in Figure 1b. There is no obvious weight loss when the temperature is higher than 640 °C, indicating that the product is thermally stable above 640 °C. Furthermore, the product is still stable even at as high as 900 °C. Therefore, it can be reasonably deduced that the final residue (68.91% at 900 °C) may be ascribed to the thermodynamically crystallographic phase of In2O3. On the basis of the TGA result, the heating temperature of 640 °C was applied to ensure a complete decomposition of the precursor. The chemical composition of the In-EG-EDA nanorod bundles was verified by an EDS facility attached to FE-SEM. The EDS spectrum (Figure 2e) indicated that the sample consists of In, O, C, and N elements, in which N may come from EDA, O from EG, and C from EG and EDA. In fact, the In-EG-EDA complex also includes hydrogen, but it cannot be measured by EDS because of its low atomic number. Elemental analysis was employed to further determine the quantitative composition of the precursor. The elemental analysis showed that the In-EG-EDA precursor contains 4.18 wt % C, 3.06 wt % H, and 1.39 wt % N. The ICP analysis result showed that the In element content is 38.2 wt %. In addition, according to the TGA result in Figure 2d, we inferred that the absorption water of the precursor is 1.12 mol. The molar ratio of In/C/N/H is 1.0/1.4/0.4/10 in the precursor, and therefore, the composition of the as-synthesized InEG-EDA precursor was determined approximately as (In1.0C1.4N0.4H10O7.4) · 1.12 H2O. Figure 3a shows a typical XRD pattern of In2O3 prepared by thermal decomposition of the precursor at 640 °C for 2 h. All diffraction peaks of the sample can be indexed to a bodycentered cubic (bcc) phase In2O3 (JCPDS card no. 06-0416). The peaks are sharp and intense, indicating good crystallinity of the sample. No other impurity phase is detectable, indicating a complete conversion of the precursor into In2O3. When the decomposition temperature is decreased to 440 or 500 °C, cubic phase In2O3 can still be obtained, indicating that the weak weight loss of 3.170% between 440 and 640 °C could be attributed to the lost of the organic component of the precursor. Figure 3b clearly shows the low-magnification FE-SEM image of In2O3 after calcination of sample 3. The obtained In2O3 almost completely maintains the morphology and diameter of the precursor. However, the length of the In2O3 decreased from ∼3 µm before calcination to ∼800 nm after calcination. The surface of the In2O3 nanorod bundles is rough compared with the surface of the precursor (Figure 2a). A high-magnification FE-SEM image (inset in Figure 3b) shows that the In2O3 nanorod bundles are composed of individual nanorods with a diameter of ∼15 nm. Moreover, these individual nanorods (chainlike structure) consisted of subunit nanoparticles with a diameter of ∼15 nm. These nanoparticles may be resulted from the decomposition of the In-EG-EDA precursor during calcination. Further structural characterizations for the In2O3 nanorod bundles presented were performed with TEM, HRTEM, and SAED (Figure 3c-e). Figure 3c showed the magnified TEM image of a nanorod bundle. It can be clearly seen that the nanorod bundle consists of individual nanorods with a diameter of ∼15 nm aggregated by interconnected nanocrystallites of ∼15 nm in size. In addition, it can be clearly seen that the nanorod bundle shows an interconnected porous structure after the calcination. The sizes of these pores are about 2-3 nm. Figure 3d is a typical SAED pattern of a single nanorod bundle, displaying four diffraction rings. These diffraction rings were indexed as {200}, {220}, {332}, and {431} reflection planes

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Figure 2. (a) FE-SEM image of In-EG-EDA complex nanorod bundles and (b) high-magnification TEM image of a single In-EG-EDA complex nanorod bundle (sample 3). The inset in (b) is the SAED pattern of the single In-EG-EDA nanorod bundle. (c) HRTEM image of the single In-EG-EDA nanorod bundle. (d) TGA curve of the In-EG-EDA nanorod bundle complex decomposition at a heating rate of 10 °C/min in air. (e) EDS spectrum of the In-EG-EDA nanorod bundles.

of bcc phase In2O3, respectively, confirming the polycrystalline structure of the In2O3 nanorod bundle. The HRTEM image in Figure 3e showed clear lattice fringes with a d spacing of 0.295 nm, which is in agreement with the (222) crystal facet of the cubic In2O3. In addition, the HRTEM image clearly presents the aggregated nanocrystallite structure of the nanorod bundle. The white panes areas can be attributed to the pore structures among these aggregated nanocrystallite interconnection areas. It has been reported that pH is an important factor in morphological and dimensional control for nanomaterials.29 Our experimental studies showed that pH values of the EG-EDA system have significant effects on the morphology and size of the as-prepared samples. Figure 4a shows the FE-SEM image of the In-EG-EDA complex when EDA was added dropwise into EG solution until the pH value was 9.31 (sample 1). The complex is composed of aggregated nanoparticles with an average diameter of 140 nm. When the pH value was increased

to 10.26 (sample 2) and other reaction conditions were kept the same as those for sample 1, the morphology of the complex was still nanoparticles but with an increased average diameter of 450 nm (Figure 4b). When the pH value was 10.90, nanorod bundles formed (Figure 2a, sample 3). If the pH value was further increased to 12.00, bouquet-like nanorods with a diameter of 200 nm and a length of 4.2-4.5 µm were obtained (Figure 4c, sample 4). On the basis of our systematic experimental results, we concluded that an optimum pH range for controllable synthesis of In-EG-EDA nanorod bundles is 10.90-11.14 (sample 3 in Table 1). 3.2. One-Step Synthesis of In2O3 Nanoaggregates. It is interesting to note that, when EDA was substituted by 0.25 mL of 28% (w/w) ammonia to adjust the pH value of the EG system to 10.90, while keeping other reaction conditions as the same as those for the synthesis of In-EG-EDA nanorod bundles, In2O3 nanoparticles with an average diameter of 20 nm (sample

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Figure 3. (a) XRD pattern taken from the calcined In-EG-EDA nanorod bundles (sample 3) at 640 °C for 2 h. (b) FE-SEM image of In2O3 nanorod bundles after calcination of the In-EG-EDA complex of sample 3. The inset in (b) is the high-magnification FE-SEM image of one edge of a single nanorod. (c) TEM image and (d) a typical SAED pattern of a single In2O3 nanorod bundle. (e) HRTEM images of the single In2O3 nanorod bundle.

Figure 4. FE-SEM images of In-EG-EDA complex samples obtained at 180 °C for 10 h under the EG-EDA system with different pH values: (a) pH ) 9.31, (b) pH ) 10.26, and (c) pH ) 12.00.

5) were directly obtained, as revealed by the XRD (Figure 5a) and FE-SEM image (Figure 5b). Figure 5a shows the XRD

pattern of sample 5. All of the diffraction peaks can be indexed to bcc phase In2O3, which are in good agreement with the

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Figure 5. (a) XRD pattern, (b) FE-SEM image, and (c) low-magnification TEM image of the In2O3 nanoparticles under the EG-ammonia system (sample 5). The inset in (c) is the TEM image of a single In2O3 nanoaggregate. (d) HRTEM image of the single In2O3 nanoaggregate. The right-top inset in (d) shows the SAED image of the single In2O3 nanoaggregate. The right-bottom inset in (d) shows the magnified TEM image of a plane lattice of the single nanoparticle (the white pane in (d)).

literature values (JCPDS card no. 06-0416). No other impurity phase is detected. A representative TEM image of the welldispersed In2O3 nanoparticles is shown in Figure 5c. The In2O3 nanoparticles were formed by aggregating many primary subunit nanoparticles with an average diameter of 2-3 nm. The inset in Figure 5c shows a magnified TEM image of a typical nanoparticle aggregate with a diameter of 25 nm. The HRTEM image (Figure 5d) clearly shows that each particle has identical lattices fringes with d spacing of 0.292 nm, which is in agreement with the (222) crystal facet of cubic phase In2O3 (JCPDS card no. 06-0416), indicating that an individual subunit nanoparticle is a single-crystal structure. A typical SAED pattern (inset in Figure 5d.) shows that all the diffraction rings were indexed as {200}, {220}, {222}, {332}, and {440} reflection planes of cubic phase In2O3, proving the polycrystalline structure of the In2O3 nanoaggregates. 3.3. Possible Formation Mechanism. A possible schematic depiction for the formation of In-EG-EDA complex nanorod bundles using an assembly mechanism was proposed (Figure 6a, steps 1-3). The formation of In-EG-EDA nanorod bundles may be resulted from the combined roles of EDA and EG under the appropriate reaction conditions, as described in Table 1. Without EDA, no precipitate was formed in the In-EG system while other reaction conditions were kept the same. Only when the pH value of the In-EG system is higher than 9.31 with the addition of EDA can the precipitates with different morphologies be formed (samples 1-3). EG is a key factor to prepare the nanorod bundle structures, which is confirmed by the following

experiment result. If ethanol was used instead of EG and the pH value of the system was kept at 10.17 with the addition of EDA, a large quantity of aggregated spherical particles with an average diameter of 15 nm were observed, as shown in the FESEM image (Figure 6b). However, the XRD result of this sample shows that it is pure orthorhombic phase of InOOH (Figure 6c, JCPDS card no. 73-1592). Thus, it can be assumed that EG and EDA both act as the morphology-directing reagents for the assembly of the In-EGEDA nanorods into bundles. On the basis of the above analysis, the following process for the formation of In-EG-EDA nanorod bundles was proposed: First, small In-EG-EDA complex nuclei are formed in the reaction solution (Figure 6a, step 1) and they are highly active and begin to form a single nanorod because of high surface energy of the nuclei and strong bidentate ligand of EDA, which plays important roles in forming a rodlike morphology, as reported in the literature35 (Figure 6a, step 2). EG may also act as a nucleophilic solvent in this process. EG and EDA could act as synergy effects that reacted with In-Cl groups to form linear coordination complexes. A onedimensional long-chain In-EG-EDA complex precursor with a diameter of ∼15 nm, in which the In-Cl group was bonded and anchored to hydroxyls (-OH) from the ED unit and amino (-NH2) groups from the EDA unit, was formed, at least on the surface of the precursor. Because of the limits of the long chain, the growth of the In-EG-EDA is not random. The In-EG-EDA linear, rodlike complexes could then aggregate into ordered (-In-EG-EDA-)n nanorod bundles through “self-assembly”

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Figure 6. (a) Schematic depiction of the formation of In-EG-EDA complex nanorod bundles and In2O3 nanoaggregates in the whole synthetic process (EG, ethylene glycol; EDA, ethylenediamine). (b) FE-SEM image and (c) XRD pattern of the obtained InOOH nanoparticles in the presence of ethyl alcohol and EDA at the pH value of 10.17.

SCHEME 1

of van der Waals interactions (Figure 6a, step 3). Second, In2O3 nanorod bundles could be obtained by calcining the (-InEG-EDA-)n long-chain ligand bundles at 640 °C for 2 h in air. The possible formation mechanism of the In2O3 nanoaggregates was also demonstrated in Figure 6a (steps 1′-3′) as well. First, the In-EG complex dissociates slowly into In3+ cations due to the existence of a little water in the EG-ammonia system compared with the EG-EDA system (water-deficiency system) (see Scheme 1, reaction 1). At the same time, the ammonia (NH3 · H2O) dissociate slowly into OH- ions (Scheme 1, reaction 2). The weak ionization tendency of NH3 · H2O can then kinetically control reaction 1 (Scheme 1) through controlling the growth rate. Therefore, the whole growth process could be controlled by reaction 1 (Scheme 1)because of the weak dissociation tendency of the In-EG complex. Second, the OHions subsequently react with the In3+ to form small In2O3 intermediate nuclei at the pH value of 10.17 (Figure 6a, step 1′). Finally, the resulting In2O3 nuclei grew into larger In2O3 nanocrystals (Scheme 1, reaction 3). Among the above three reactions, reaction 3 (Scheme 1), which continuously consumes OH- ions, accelerates the decomposition of the In-EG complex in reaction 1 and the ionization of NH3 · H2O in reaction 2 until

all In3+ cations were used up. Once the small In2O3 nuclei are formed in the reaction solution, they are active because of their high surface energy and tend to spontaneously attach together rapidly (Figure 6a, step 2′), leading to the formation of larger aggregates to minimize the surface energies. The formed In2O3 primary nanoparticles rearranged to minimize the surface energy, followed by further crystallization, and became compact crystals through fast and “self-assembly” aggregation growth (Figure 6a, step 3′). 3.4. PL Properties of In2O3 Nanostructures. It is known that the bulk In2O3 does not exhibit PL properties at room temperature.24 However, in the recent 10 years, Qian, Lee, and Guha et al. have observed the PL peaks centered at 360, 450, and 470 nm from In2O3 nanocubes, nanofibers, and octahedrons, respectively.40-43 Zhang and Zhou et al. reported 460 and 480 nm PL peaks from In2O3 nanowires and nanoparticles.44,45 The PL peaks located at 468, 551, and 632 nm (from blue to orange region) of lotus-root-like In2O3 nanostructures have been reported by Chen and Xiao et al.24 Nanoscaled In2O3 structures show PL emission at different wavelengths, indicating that the PL properties of In2O3 might be affected by its dimensionality, shape, and size. Figure 7 displays the room-temperature PL spectra of In2O3 nanorod bundles (Figure 7a) (as shown in Figure 3b) obtained by calcinations of the In-EG-EDA precursor at 640 °C for 2 h and nanoaggregates (Figure 7b,c). For the In2O3 nanorod bundles, four PL peaks appeared when excited at 291 nm (Figure 7a), two broad luminescence bands centered at 336 and 383 nm in the UV region and another two peaks at 423 and 468 nm in the blue-green region. The strongest peak of the four peaks is centered at 468 nm, and a mark shoulder appeared at 423 nm. For the In2O3 nanoaggregates, eight PL peaks were observed in the UV and blue-green regions with a 291 nm excitation wavelength (Figure 7b). Two stronger peaks are centered at 334 and 378 nm in the UV region, and the other six peaks are centered at 422, 449, 465, 469, 480, and 489 nm in the blue-

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Yin et al. near-band-edge (NBE) emission of In2O3 nanostructures with a wide band gap. In addition, a highly crystalline quality was found to be an important factor favoring the increase of UV emission at room temperature.46 Generally, the blue-green luminescence emission mechanism of nanoscaled In2O3 is mainly attributed to the existence of oxygen vacancies.42,43 We all know that bcc In2O3 belongs to an n-type semiconductor. There are oxygen vacancies in the In2O3 crystal, an oxygendeficient fluorite structure with twice the unit-cell edge of the corresponding fluorite cell and with 1/4 of the anions missing in an ordered way.24 These oxygen vacancies normally act as deep defect donors in semiconductors and would induce the formation of new energy levels in the band gap. The blue-green emissions thus result from the radioactive recombination of a photoexcited hole and an electron occupying the oxygen vacancies.24 In addition, the aggregation growth and assembly process (Figure 6a) of the two nanostructures as well as the calcination of In-EG-EDA precursor might induce different oxygen vacancies and different PL emission peaks. Therefore, we believe that the different emission peaks from the blue-green to orange region in the visible region are related to the energy levels induced by the oxygen vacancies caused in the preparation of the structures and the morphology of products. The wide UV-visible photoluminescence properties of the two In2O3 nanostructures could have potential applications in optoelectronic devices, such as lasers, fluorescent lamps, and display devices. 4. Conclusions A simple and template-free solvothermal process was successfully employed to controllably synthesize In-EG-EDA complex precursor nanorod bundles and In2O3 nanoaggregates in an EG solvent. When EDA was used as an alkaline reagent, an In-EG-EDA complex precursor was obtained with a nanorod bundle structure. After the In-EG-EDA precursor was calcined, it was converted into cubic phase In2O3 without altering its one-dimensional morphology. The morphology and phase structure of the products were found to depend strongly on the pH value and the alkaline reagent. Dispersed, cubic phase In2O3 nanoaggregates can be directly obtained in the EG solvent using ammonia as alkaline reagent. A possible formation mechanism is proposed. The photoluminescence spectra of both the In2O3 nanorod bundles and the nanoaggregates show wide emission peaks from the ultraviolet to visible region.

Figure 7. PL emission spectra of In2O3 (a) nanorod bundles obtained at calcinations of the In-EG-EDA precursor at 640 °C for 2 h (excited at 291 nm) and (b, c) nanoaggregates (excited at 291 and 319 nm) obtained in the EG-ammonia system.

green region. Among the six peaks, two relatively stronger peaks are centered at 465 and 469 nm. To further identify the peaks on the PL spectra, the PL property of In2O nanoaggregates was investigated by another excitation wavelength of 319 nm (Figure 7c). The sample exhibited different broad-band emission peaks ranging from 400 to 600 nm (422, 451, 467, 482, 492, 504, 557, and 572 nm). The stronger peaks centered at 422, 467, and 504 nm locate in the blue-green region and the weaker peaks at 557 and 572 nm in the orange region. When Figure 7b is compared with Figure 7c, the peak intensity of 422 nm becomes the highest in Figure 7c and other four peaks at 449, 465, 480, and 489 nm in Figure 7b shifted to 451, 467, 482, and 492 nm, respectively, with the excitation wavelength changing from 291 to 319 nm. The UV emission could be explained mainly by the

Acknowledgment. We thank Tao Dong for TGA measurement. This work was supported by the State Scholarship Fund of China Scholarship Council (CSC, File No. 2008603051), the National Science Foundation of China (NSFC, Nos. 20671011, 20731002, 10876002, 20771022, 20973023, and 20871016), the 111 Project (B07012), the Key Laboratory of Structural Chemistry Foundation (KLSCF, No. 060017), the Excellent Young Scholars Research Fund of Beijing Institute of Technology (No. 2006Y0715), the Basic Research Fund of Beijing Institute of Technology (Nos. 20060742022 and 20070742010), and the Program for New Century Excellent Talents in University. References and Notes (1) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Nature 2001, 409, 66. (2) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (3) Waitz, T.; Wagner, T.; Sauerwald, T.; Kohl, C.; Tiemann, M. AdV. Funct. Mater. 2009, 19, 653. (4) 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.

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