One-Step Synthesis of Nearly Monodisperse, Variable-Shaped

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One-Step Synthesis of Nearly Monodisperse, Variable-Shaped In2O3 Nanocrystals in Long Chain Alcohol Solutions Daniela Caruntu,* Kun Yao, Zengxing Zhang, Tabitha Austin, Weilie Zhou, and Charles J. O’Connor AdVanced Materials Research Institute, UniVersity of New Orleans, New Orleans, Louisiana ReceiVed: December 2, 2009; ReVised Manuscript ReceiVed: February 14, 2010

Nearly monodisperse, variable-shaped In2O3 NCs have been synthesized from different indium-organic precursors in oleyl alcohol solutions at 320 °C under both nitrogen and air atmospheres. When indium(III) acetate (In(ac)3) was employed as the metal precursor, spheroidal In2O3 NCs with an average diameter of 9.2 nm were obtained in a nitrogen atmosphere, whereas nanoparticles with an incompletely developed morphology were obtained under air. Cubic In2O3 NCs with sizes of about 7 and 20 nm were produced in an air atmosphere when indium(III) acetylacetonate (In(acac)3) and indium(III) isopropoxide (In(OiPr)3), respectively, were used as precursors. For the In2O3 NCs synthesized from In(ac)3 and In(OiPr)3, the influence of oxygen (air) on the growth kinetics as well as on the morphology of the resulting nanoparticles was also investigated. On the basis of the FT-IR studies of the postreaction solutions, different reaction mechanisms, depending on the nature of the metal precursor, were proposed. The variable-shaped In2O3 NCs showed relatively strong PL emissions at room temperature in the UV region. The spheroidal In2O3 NCs were used for the design of gas sensor devices that were able to detect concentrations of H2S as low as 30 ppb at room temperature. 1. Introduction Indium oxide is a versatile material that has been extensively used in the design of solar cells,1 UV lasers and light emitting diodes,2 window heaters,3 flat panel displays,4 and photocatalysts.5 The technological importance of In2O3 originates primarily from its wide band gap (3.6 eV) and high transparency to visible light. However, as in other n-type semiconductors, it can change its electrical conductance upon exposure to various gases, being very suitable for the design of ultrasensitive gas sensors in the detection of H2, Cl2, NO2, CO2, ethanol, formaldehyde, and O3.6–11 For application in gas-sensing devices, the oxide material should be able to detect ultralow (ppb) levels of analyte, be selective to different gases, and have a complete and fast recovery of the base resistance and long-term stability against microstructural changes when operating at high temperatures. Because it is well-known that gas sensing is a surface phenomenon and the surface-to-volume ratio of the particles increases significantly with decreasing their sizes, the fabrication of In2O3 nanostructures with controllable size, shape, and surface characteristics is essential to achieving reliable performance in gas sensing. Various methods, both physical and chemical, have been developed for the synthesis of 0-D, 1-D, and 2-D In2O3 nanostructures. For example, thin films (2-D) were produced by chemical vapor deposition (CVD),12,13 sputtering,4,14,15 and spray pyrolysis,16,17 whereas nanorods, nanobelts, nanotubes, nanofibers and nanowires (1-D) were obtained by the vaporliquid-solid technique,18 microemulsion/calcination process,19 thermal evaporation,20 and template-assisted growth.21–24 In a similar fashion, nanocrystals (0-D), such as dots, spheres, nanoporous clusters, flowers, lotus roots, pyramids or cubes, have been synthesized by solution-based routes, including thermolysis,25–27 the polyol method,28 the hot-injection technique,29 and the solvothermal/hydrothermal method.30–32 * To whom correspondence should be addressed. Tel: +1-(504)-2801384. Fax: +1-(504)-280-3185. E-mail: [email protected].

Among the solution-based synthetic routes, the thermolysis of metal-organic precursors is the one of the most versatile in producing In2O3 nanoparticles with a controllable size and shape. In a typical synthesis, a metal source is decomposed by heating/ injecting the precursor into a high boiling point organic solvent. The decomposition of the metal salt generates nuclei that subsequently grow into well-defined nanoparticles. The growth process is usually controlled by surfactant molecules. These molecules passivate the surface of the nanoparticles and prevent their aggregation, thereby rendering them dispersible in solvents with similar polarity. The binding properties of the individual nanocrystals combined with the presence of organic stabilizers and their uniformity in size and shape can promote a selfassembly process, yielding highly ordered hierarchical architectures with new collective properties useful to certain applications. Seo and co-workers prepared nearly monodisperse In2O3 spherical nanoparticles by the thermal decomposition of In(acac)3 at 250 °C in oleylamine.25 The size of the nanocrystals was varied from 4 to 8 nm by changing the molar ratio between the metal precursor and the amine, which serves as both a solvent and a capping agent. Fang et al. have adopted a similar route by decomposing In(ac)3 in a mixture of oleylamine/oleic acid and hexadecane at 290 °C.26,27 The resulting oleic acidcapped In2O3 nanocrystals are monodisperse and have octahedral shapes. The size can be varied between 11.5 and 20 nm upon using a multiple-injection procedure, and by controlling the evaporation rate of the solvent, the nanoparticles can be assembled into highly ordered superlattices. Although significant progress has been made in the preparation of monodisperse In2O3 nanoparticles by nonhydrolytic routes, the fine control of the size and shape, especially for nanoparticles smaller than 10 nm, still remains a challenge. Long chain aliphatic and aromatic alcohols are versatile organic solvents in the nonhydrolytic synthesis of metal oxide nanoparticles. This is because they are inexpensive, have high boiling points (>300 °C), and can play multiple roles in the synthesis:

10.1021/jp911427b  2010 American Chemical Society Published on Web 03/02/2010

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solvent for the precursors as well as reaction medium and stabilizing agent for the growing nanocrystals, which simplifies the synthetic protocol considerably. Despite these advantages, the use of alcohols in the nonhydrolytic synthesis of metal oxide nanoparticles remains sparse. Niederberger and co-workers proposed an interesting route that uses benzyl alcohol as both a solvent and a capping agent for the solvothermal preparation of various nanostructured materials, including simple (Fe3O4, In2O3, WO3)33–35 and complex metal oxides (BaTiO3, SrTiO3, (Ba, Sr)TiO3, BaZrO3, LiNbO3).36,37 Recently, O’Brien’s group has used oleyl alcohol as a capping ligand and a coordinating agent in the synthesis of BaTiO3 nanoparticles.38 They decomposed a metal-organic Ba precursor obtained by dissolving metallic barium in oleyl alcohol or a mixture of benzyl alcohol/ oleyl alcohol via an alcoholysis process in the presence of Ti isopropoxide at 320 °C. The resulting nanoparticles are smaller than those obtained by using pure benzyl alcohol due to the increasing coordination power of the oleyl alcohol, as well as its long chain. Narayanaswany et al. prepared In2O3 nanodots and nanoflowers by an alcoholysis-esterification technique involving a metal carboxylate salt in 1-octadecene in the presence of a fatty acid at elevated temperatures.39 Zhong and co-workers synthesized ZnO nanocrystals by a nonhydrolytic alcoholysis route involving the reaction between Zn(ac)2 and oleyl alcohol in the presence of oleic acid at 325 °C. The nanocrystals present a complex morphology, varying from dendritic fractal structures to tetrapod-like structures as a function of the surfactant/solvent ratio.40 Herein, we report a nonhydrolytic synthetic route for the preparation of variable-shaped In2O3 nanoparticles from different indium-organic precursors in oleyl alcohol solutions at 320 °C. This method offers a convenient way to produce highly crystalline colloidal In2O3 particles with relatively uniform morphologies because it employs a simple reaction system containing only a metal precursor and a long chain unsaturated alcohol, which can function as both a solvent and a capping ligand. 2. Experimental Section 2.1. Chemicals. The reagents and solvents were purchased from the following suppliers: indium(III) acetate (In(ac)3, 99.99% (metals basis)), indium(III) 2,4-pentanedionate (In(acac)3, 98%), indium(III) isopropoxide (In(OiPr)3, 99.9% (metals basis)), and oleyl alcohol (OLOH, technical, 80-85%) were from Alfa Aesar, toluene (99.5%) from Merck, absolute ethyl alcohol from AAper Alcohol and Chemical Co. Chemicals and solvents were used without further purification. 2.2. Synthesis of Spherically Shaped c-In2O3 Nanocrystals. In a typical experimental procedure, 1 mmol (0.292 g) of In(ac)3 was mixed with 25 mL of OLOH at room temperature in a 50 mL three-neck round-bottom flask equipped with a reflux condenser under nitrogen flow. The reaction mixture was heated during 75 min to 320 °C and then kept at this temperature for 1.5 h. The obtained white-yellowish solid was isolated by cooling the reaction mixture to room temperature and centrifuging. The solid product was washed three times with ethanol and then dispersed in toluene. The resulting toluene dispersion was centrifuged for 5 min (3.5 × 103 rpm) in order to remove any insoluble fraction. Similar experiments were performed under both an open atmosphere and flowing air while keeping all the other reaction parameters unchanged. When the experiments were carried out in open air, the flask was closed when the temperature of the reaction mixture reached 300 °C; then the heating was continued to 320 °C, where it was maintained

Caruntu et al. for 1.5 h. Similarly, when the syntheses were conducted under flowing air, the flow was turned off at 300 °C; then the temperature was increased to 320 °C, where it was kept constant for 1.5 h. 2.3. Synthesis of Cubic-Shaped c-In2O3 Nanocrystals. 2.3.1. Synthesis of c-In2O3 Nanocubes from Indium(III) 2,4-Pentanedionate Precursor. In a typical synthesis, 1 mmol (0.412 g) of In(acac)3 was mixed with 25 mL of OLOH at room temperature in a 50 mL three-neck round-bottom flask under flowing air. The reaction mixture was heated for 85 min to 320 °C and then kept at this temperature for 1.5 h. The obtained solid was isolated by cooling the reaction mixture to room temperature and centrifuging. The solid product was washed three times with ethanol and then dispersed in toluene. The resulting toluene dispersion was centrifuged for 5 min (3.5 × 103 rpm) in order to remove any insoluble fraction. For the similar experiments performed in open air, the flask was closed at 300 °C and the heating was continued to 320 °C and it was kept constant for 1.5 h at this temperature. 2.3.2. Synthesis of c-In2O3 Nanocubes from Indium(III) Isopropoxide Precursor. In a typical experimental procedure, 1 mmol (0.292 g) of In(OiPr)3 was mixed with 25 mL of OLOH at room temperature in a 50 mL three-neck round-bottom flask under flowing air. The reaction mixture was heated during 40 min to 320 °C and then kept at this temperature for 1.5 h. The obtained white-greenish solid was isolated by cooling the reaction mixture to room temperature and centrifuging. The solid product was washed three times with ethanol and then dispersed in toluene. The resulting toluene dispersion was centrifuged for 5 min (3.5 × 103 rpm) in order to separate any insoluble solid fraction. For the similar experiments performed in open air, the flask was closed at 300 °C and the heating was continued to 320 °C, where it was maintained for 1.5 h. 2.4. Characterization. A Phillips X’pert system equipped with a graphite monochromator (Cu KR radiation, λ ) 1.54056 Å) was used to determine the structure and the phase purity of the as-prepared nanopowders. The nanoparticles’ morphology was investigated by transmission electron microscopy (TEM). Additionally, high-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns were taken in order to determine the nanocrystal structure. The samples for TEM measurements were prepared by placing a drop of In2O3 nanoparticle-containing toluene solution on a carbon-coated copper grid (Ted Pella, Inc.). The measurements were performed with a JEOL JEM 2010 electron microscope at an accelerating voltage of 200 kV. Selected area electron diffraction (SAED) patterns were obtained with a camera length of 80 cm. Photoluminescence (PL) measurements were performed with a PerkinElmer LS 55 spectrometer. The cross section of the nanoparticle film-based sensor was examined by a field emission scanning electron microscope (LEO 1530 VP FESEM). 3. Results and Discussion Structure of In2O3 Nanocrystals. The structure and phase purity of In2O3 nanocrystals prepared from the three indiumorganic precursors were investigated by powder X-ray diffraction (XRD). Figure 1 shows the XRD patterns of the nanopowders obtained at 320 °C from In(ac)3 (pattern a), In(acac)3 (pattern b), and In(OiPr)3 (pattern c) in oleyl alcohol solutions under flowing nitrogen. Pattern (a) in Figure 1 matches well with that of cubic bulk In2O3 (bixbyte structure, space group ) Ia3, JPCDS file no. 006-0416) with no other secondary crystalline phases. The peaks are broadened, indicating the

Nearly Monodisperse, Variable-Shaped In2O3 NCs

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Figure 1. Powder X-ray diffraction patterns of the nanopowders obtained from the reaction of (a) In(ac)3, (b) In(acac)3, and (c) In(OiPr)3 with oleyl alcohol in a nitrogen atmosphere; (d) standard bulk indium oxide; and (e) standard metallic indium.

Figure 2. Powder X-ray diffraction patterns of the nanopowders obtained from the reaction of (a) In(ac)3, (b) In(acac)3, and (c) In(OiPr)3 with oleyl alcohol under flowing air and of (d) standard bulk indium oxide.

nanocrystalline nature of the resulting In2O3 powders. The average crystallite size determined from the fwhm of the four most intense reflections (2θ ) 30.57°, 35.20°, 51.14°, and 60.73°) after correction for the instrument broadening by using Debye-Scherrer’s equation41 was found to be 10.4 nm. The refined value of the lattice parameter, a ) 10.1325 ( 0.009(4) Å, is in good agreement with that reported for bulk In2O3 (a ) 10.118 Å). Whereas the acetate precursor yields single-phase In2O3 nanopowders, In(acac)3 produces a mixture of two phases when reacted with oleyl alcohol at 320 °C under a nitrogen atmosphere, which could be indexed to cubic bulk In2O3 (JPCDS file no. 006-0416) and tetragonal metallic In (JPCDS file no. 001-1042) (Figure 1, pattern b). In the case of the In(OiPr)3 precursor, the XRD analysis showed that the product is metallic In. However, a small peak centered at 2θ ) 30.61°, corresponding to the most intense reflection of In2O3, was also observed (Figure 1, pattern c). Regardless of the nature of the indium

precursor, the nanopowders obtained when the reaction is performed under flowing air were identified as single-phase In2O3 (Figure 2, patterns a-c). In all cases, the XRD patterns showed peaks consistent with those of cubic bulk In2O3 (JPCDS file no.006-0416). The average crystallite sizes estimated from the (222), (400), (440), and (622) reflections of each diffractogram were found to be 10, 6.9, and 23.8 nm, respectively. The refined lattice parameters, a ) 10.1218 ( 0.0021 Å, a ) 10.1373 ( 0.0037 Å, and a ) 10.1427 ( 0.0082 Å, agree well with that corresponding to crystalline bulk In2O3. Morphology of the In2O3 Nanocrystals. Information about the crystal structure and morphology of In2O3 nanoparticles obtained from the reaction of indium-organic precursors with oleyl alcohol at 320 °C under both nitrogen and air were provided by transmission electron microscopy (TEM) measurements. Figure 3a shows the bright-field TEM image of In2O3 nanocrystals synthesized from the In(ac)3 precursor in a nitrogen atmosphere. The as-prepared nanoparticles have an average

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Figure 3. (a) TEM micrograph of In2O3 nanoparticles obtained from the reaction of the In(ac)3 precursor with oleyl alcohol in a nitrogen atmosphere. (b) HRTEM image of a 9.8 nm sized In2O3 spheroidal particle. (c) SAED pattern taken from a monolayer of spherically shaped In2O3. (d) TEM image of In2O3 nanoparticles prepared from the In(ac)3-oleyl alcohol system under flowing air (turned off at 300 °C).

TABLE 1: Interplanar Spacings, d (Å), Deduced from the Analysis of SAED and XRD Patterns and the Values Corresponding to Standard Bulk Indium Oxide diffraction ring

1

2

3

4

5

4.365 3.138 2.751 1.940 1.653 d(SAED) (Å) d(XRD) (Å) 4.127 2.917 2.526 1.787 1.523 d(XRD) (Å) (standard 4.130 2.921 2.529 1.788 1.525 bulk In2O3) hkl 211 222 400 440 622

diameter of 9.2 nm with a relative standard deviation (RSD) of 13%. This value, calculated from the TEM data by taking into account 100 particles, is in good agreement with the one estimated from the line broadening of the most intense Bragg reflections in the X-ray diffraction pattern, indicating that In2O3 nanoparticles are single-crystalline. The HRTEM image of a 10.2 nm spheroidal particle presented in Figure 3b confirms the single-crystalline nature of these nanoparticles. The nanocrystal exhibits crossed lattice fringes with interplanar distances of around 0.292 nm, corresponding to the (222) planes of cubic phase In2O3.42 Figure 3c shows a typical selected area electron diffraction (SAED) pattern taken from a monolayer of In2O3 nanospheres. The pattern reveals five well-defined diffraction rings with a spotted appearance, which is indicative of the high crystallinity of the as-prepared In2O3 nanoparticles. Table 1 lists the interplanar spacings (dhkl) calculated from both the electron diffraction pattern and the X-ray diffraction data. The obtained values are in good agreement with the ones corresponding to the standard bulk In2O3 with a cubic structure (JPCDS file no. 006-0416). When the reaction of the In(ac)3 precursor with oleyl alcohol was performed under flowing air (turned off at 300 °C), particles with a less uniform morphology were obtained (Figure 3d). TEM analysis reveals shapeless particles with sizes varying between 9 and 16 nm as well as a relatively small fraction of 4-7 nm spherically shaped nanocrystallites. The appearance of most of

Figure 4. TEM images of In2O3 nanocrystals prepared from the reaction of In(ac)3 with oleyl alcohol in open air: (a) flask kept open all the time during the experiment (320 °C/1.5 h), (b) HRTEM image of a nanocluster, (c) flask closed at 300 °C, and (d) flask closed at 235 °C.

these shapeless nanoparticles indicates that they formed through the aggregation of several small nanocrystallites that were generated in the initial stage of the reaction. Similar TEM results (Figure 4c) were also observed for the In2O3 particles synthesized in open air (flask closed at 300 °C). To understand the influence of the oxygen/air on the growth kinetics of In2O3 nanoparticles, additional experiments were conducted in open air in which the flask was closed at different temperatures during the heating process. When the flask was closed at 235 °C and the solution was heated to 320 °C, where it was maintained constant for 1.5 h, nanoparticles with a roughly spherical shape and sizes varying between 7 and 19 nm were obtained (Figure 4d). Although not very uniform in size and shape (the largest particles are faceted), these nanoparticles have a better defined morphology compared with the ones synthesized when the flask was kept open up to 300 °C (Figure 4c). The sample produced when the flask was closed at 265 °C (Supporting Information, Figure SI 1) consists of a mixture of irregularly shaped particles with sizes ranging from 10 to 19 nm and a small number of spherical crystallites (4-8 nm). The overall morphology of these nanoparticles resembles that of the particles shown in Figure 4c (flask closed at 300 °C). If the flask is left open all the time during the synthesis (for 1.5 h at 320 °C), clusters containing 2-10 nanocrystallites as well as individual crystallites (6-9 nm) were observed by TEM measurements (Figure 4a). These clusters are monocrystalline in nature, as illustrated by the HRTEM image shown in Figure 4b. It can be clearly seen that the lattice fringes of the four neighboring nanocrystallites are perfectly aligned over the whole cluster. For the experiment in which the flask was left open all the time during the synthesis, the course of the reaction was monitored visually by taking aliquots of the reaction mixture at different temperatures during the heating process. It was observed that the In(ac)3 precursor was completely dissolved at 245 °C and the solution remained clear until a very slight turbidity, indicating the formation of In2O3 nuclei, was noticed at 255 °C. Therefore, in those experiments in which the flask was closed at a temperature lower


Figure 5. Bright-field TEM images of cubic-shaped In2O3 nanoparticles obtained from the reaction of the In(acac)3 precursor with oleyl alcohol in open air (flask closed at 300 °C) with an average edge length of (a) 7 nm (inset: HRTEM image of a 7 nm sized In2O3 cube) and (b) 14.2 nm. (c) HRTEM micrograph of a 3D superlattice of 7 nm sized In2O3 cubes. (d) TEM image of In2O3 nanocubes synthesized under flowing air (turned off at 300 °C).

than 255 °C, In2O3 nanocrystals with completely developed morphologies (Figure 4d) were produced after aging the reaction mixture for 1.5 h at 320 °C. When the flask was closed at a temperature higher than 255 °C, particles with irregular shapes or a cluster-like appearance (Figures 4(a-c) and SI 1, Supporting Information) were obtained, depending on the duration of exposure of the reaction mixture containing the growing nanocrystals to oxygen (air). As the In2O3 nanocrystals form by the oriented attachment of small particles initially generated in the system, a prolonged exposure of the reaction mixture to oxygen/air affects the growth of the nanoparticles by slowing down the aggregation/recrystallization processes. The effect of oxygen/air on the growth kinetics becomes more evident when comparing the morphology of the nanoparticles synthesized in open air (flask kept open all the time during synthesis) with that of the nanoparticles prepared under flowing nitrogen. Despite the fact that all the reaction parameters were the same, the TEM analysis showed that the nanoparticles obtained under flowing N2 have well-defined spheroidal shapes, whereas those produced in open air possess a cluster-like morphology (Figures 3a and 4a). Unlike the acetate precursor, the reaction of In(acac)3 with oleyl alcohol at 320 °C under open air (flask closed at 300 °C) produces nanoparticles with a well-developed morphology. The corresponding TEM micrograph (Figure 5a) reveals cubically shaped particles with an average edge length of 7.0 nm (RSD of 9.4%) and aspect ratios ranging from 1.0 to 1.2. The HRTEM image of a single 7 nm sized cube (inset of Figure 5a) shows crossed lattice fringes with fringe separations of 2.92 Å, which can be assigned to the (222) planes of cubic phase In2O3. In addition to the small cubes, a relatively small fraction (∼5 to 10%) of large cubes with an average edge length of 14.2 nm (RSD of 9.6%) and aspect ratios of 1.0-1.45 was also identified (Figure 5b). These differently sized nanocubes are systematically segregated on the TEM copper grids, and the small nanoparticles

J. Phys. Chem. C, Vol. 114, No. 11, 2010 4879 form close-packed 2-D and 3-D assemblies even in the absence of a size selection process. The HRTEM image of a highly ordered 3-D array of 7 nm sized cubes shown in Figure 5c demonstrates the morphological uniformity of the small nanocubes as well as the attachment of the oleyl alcohol molecules to the surface of the nanocrystals. Similar experiments were carried out under flowing air (turned off at 300 °C), and the morphology of the obtained nanoparticles was found to closely resemble that of the nanocrystalline products synthesized in an open atmosphere (Figure 5d). Both small- and large-sized cubes were identified in the In2O3 samples produced under flowing air with the small nanocubes having an average edge length of 6.8 nm (RSD of 9.3%) and a aspect ratio of 1.0-1.2. Nanocrystals with a cubic morphology were also obtained when the In(OiPr)3 precursor was heated in the presence of oleyl alcohol at 320 °C under flowing air (turned off at 300 °C). The resulting In2O3 nanocubes have round corners and an average edge length of 19.9 nm (RSD of 11% and aspect ratios of 1.0-1.15). Besides the nanocubes, a small amount of smaller (∼16 nm) spherically shaped nanoparticles can also be observed by TEM analysis (Figure 6a). The monocrystalline behavior of these nanoparticles is revealed by the HRTEM image of a single 20 nm sized cube presented in Figure 6b. The lattice spacings of 2.92 and 4.12 Å are consistent with the (222) and (211) planes of cubic phase In2O3, respectively. As in the case of In(ac)3 and In(acac)3, the morphology of the In2O3 nanocrystals produced in an open atmosphere (flask closed at 300 °C) is very similar to that of the nanoparticles synthesized under flowing air (Figure 6a,c). Although the presence of oxygen is necessary for obtaining single-phase In2O3 nanopowders when In(OiPr)3 is used as the metal precursor, the prolonged exposure of the reaction mixture to air (oxygen) during the heating process was found to affect the growth kinetics and, consequently, the morphology of the resulting nanoparticles. A similar behavior was observed for the In2O3 nanoparticles synthesized from In(ac)3, but it was not evident for those obtained from In(acac)3. To study the influence of oxygen on the growth kinetics as well as on the morphology of the In2O3 nanoparticles, several experiments were carried out in which the air flow was turned off at different temperatures during the heating process. In all cases, after the air flow was turned off, the solution was heated to 320 °C and kept constant for 1.5 h at this temperature. The experiments in which the air flow was turned off at 220 °C yielded spherically shaped nanocrystals with sizes ranging from 15 to 22 nm. Although most of the nanoparticles are spherical, some cubically shaped nanocrystals can also be observed (Figure 7a). As shown earlier, when the air flow was turned off at 300 °C, nanocubes (average edge length of ∼20 nm and RSD of 11%) with round corners and smooth surfaces were produced (Figure 7b). Although these nanoparticles have well-defined shapes, the In2O3 nanocrystals synthesized when the air flow was turned off at 320 °C/(t ) 0 min) showed incompletely developed morphologies. TEM analysis reveals nanocubes with rough facets and a relatively broad size distribution (edge length varying between 20 and 50 nm) as well as a small fraction of ∼10 nm spherically shaped nanoparticles (Figure 7c).The HRTEM micrograph (inset of Figure 7c) indicates that the nanocubes are single-crystalline and they most likely formed by the fusion of the smaller spheroidal particles. For the experiments in which the air flow was turned off at 300 °C, aliquots of the reaction mixture were taken at different temperatures during the heating process. Because a stronger turbidity of the reaction solution was observed above 300 °C, only the samples collected at 310 and 320 °C at different time

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Figure 6. TEM images of In2O3 nanoparticles obtained from the reaction of the In(OiPr)3 precursor with oleyl alcohol under different atmospheres: (a) flowing air (turned off at 300 °C), (b) HRTEM image of an ∼20 nm In2O3 cubic particle prepared under flowing air, and (c) open air (flask closed at 300 °C).

Figure 7. TEM images of In2O3 nanocrystals resulting from the reaction of In(OiPr)3 with oleyl alcohol under flowing air: (a) turned off at 220 °C, (b) turned off at 300 °C, and (c) turned off at 320 °C/(t ) 0 min).

intervals (0 min, 30 min, 1 h, and 1.5 h) were investigated by transmission electron microscopy (TEM). The experimental results showed that the solid isolated at 310 °C contains two types of nanoparticles: small (2-8 nm) spheroidal nanoparticles as well as a small fraction of large (>15 nm) particles with roughly spherical and cubic shapes (Figure SI 2a, Supporting Information). The appearance of the large-sized particles suggests that they formed by the aggregation of the small, spherically shaped nanocrystals. These small-sized particles (“primary particles”) are generated in the reaction mixture through nucleation and diffusional growth processes and eventually coalesce into aggregates with poorly defined shapes.43–45 If the constituent particles of each aggregate attach to one another in such a way that there is a continuity of their crystal lattices at the shared interfaces (“oriented aggregation”), single-crystalline aggregates are obtained.46–48 The sample collected at 320 °C/(t ) 0 min) consists of cubic nanocrystals (∼20 nm) with round corners and smooth surfaces and a small amount of spherically shaped nanoparticles (∼16 nm). Although most of the cubic particles have well-defined shapes, a relatively small fraction of nanocubes with rough facets was identified (Figure SI 2b, Supporting Information). Examination of the TEM data clearly suggests that, within the 10 min of heating from 310 to 320 °C/(t ) 0 min) in the absence of the air flow, the smallsized particles initially generated in the reaction mixture formed aggregates most of which were subsequently converted into well-defined cubic nanocrystals (aspect ratios close to 1.0) with round corners and smooth surfaces. Thus, in the absence of the air flow, the “primary particles” quickly coalesce into aggregates with less-defined shapes. Coalescence is followed by a slower recrystallization process when the aggregates are converted into “secondary particles” with completely developed morphologies. According to the TEM data shown in Figure SI 2c in the Supporting Information, the morphology of the nanoparticles isolated after 30 min and 1 h at 320 °C is quite similar to that of the nanoparticles collected immediately after the solution

reached 320 °C (t ) 0 min). Longer aging at 320 °C in the absence of the air flow will further improve the morphology and crystallinity of the In2O3 nanocrystals. As seen in the TEM micrograph of the sample collected after 1.5 h of heating at 320 °C, the fraction of the nanocubes with rough facets decreased substantially and almost all of the cubic nanoparticles have a well-defined shape (Figure 7b). The above experimental data suggest that the formation of In2O3 nanoparticles from In(OiPr)3 involves aggregation/recrystallization processes that most likely occur above 300 °C. When the air flow is turned off at 300 °C, less air/oxygen is present in the flask during the aggregation/recrystallization processes and, therefore, a large fraction of ∼20 nm nanocubes with round corners and smooth surfaces is observed even at 320 °C/(t ) 0 min). The experimental results showed that only 10 min of heating from 310 to 320 °C/(t ) 0 min) in the absence of the air flow is sufficient to convert the mixture of “primary particles” and aggregates (Figure SI 2a, Supporting Information) into cubic-like nanoparticles with a completely developed morphology (Figure SI 2b, Supporting Information). We also noted that the heating of the reaction mixture for 1.5 h at 320 °C will further improve the morphology of the In2O3 nanocubes (Figure 7b). Turning off the air flow at higher temperatures (320 °C/t ) 0 min) allows more oxygen in the flask during the aggregation/recrystallization processes. In this case, nanocubes with an incompletely developed morphology as well as a small fraction of “primary particles” are observed even after heating for 1.5 h at 320 °C (Figure 7c). On the basis of the above analysis, we can reasonably conclude that the oxygen plays a critical role in the growth kinetics of the In2O3 nanoparticles; that is, its presence slows down considerably the aggregation/recrystallization processes. Mechanism of Formation of the In2O3 Nanocrystals. To gain insight into the mechanism of formation of In2O3 nanoparticles from the three different precursors in oleyl alcohol solutions, the postreaction solutions obtained after centrifuging


Figure 8. Infrared spectra of the solutions obtained after removal of the solid products resulting from the reaction of In(ac)3 (1), In(acac)3 (2), and In(OiPr)3 (3) with oleyl alcohol under (a) nitrogen and (b) flowing air; spectrum (4) in (a) and b represents the IR spectrum of oleyl alcohol (85%).

the solid products were analyzed by Fourier transform infrared (FT-IR) spectroscopy. Figure 8a shows the IR spectra of the solutions resulting from the reactions of In(ac)3 (curve 1), In(acac)3 (curve 2), and In(OiPr)3 (curve 3) with oleyl alcohol under flowing nitrogen. When the starting material is In(OiPr)3, the IR spectrum of the postreaction solution (curve 3) closely resembles that of oleyl alcohol (85%, technical grade), represented by curve 4. Both spectra show bands at 3355 and 1056 cm-1, corresponding to O-H and C-OH stretching vibrations, respectively. The strong bands appearing at 2927 and 2853 cm-1 can be attributed to the asymmetric and symmetric stretching modes of the -CH2- group, and the weak band at 3006 cm-1 can be ascribed to the stretching vibration of the dCH- group in oleyl alcohol. The IR spectra also reveal two strong bands at 1466 and 720 cm-1, corresponding to C-H scissoring and C-H rocking modes, respectively. The weak band at 1657 cm-1 is observed in both IR spectra and can be associated with the stretching vibration of the CdC bond. The FT-IR spectra of the solutions obtained when In(ac)3 and In(acac)3 are used as precursors show similar characteristics with an additional band at 1739 cm-1 (curve 1) and 1744 cm-1 (curve 2), which can be assigned to the stretching vibration of the CdO group in esters.49–51 Similarly, the ester band is also observed in the FTIR spectra of the postreaction solutions corresponding to In(ac)3 and In(acac)3 when the experiments were performed under both flowing air (Figure 8b) and an open atmosphere (Figure SI 3, Supporting Information). These observations strongly suggest that the formation of In2O3 nanoparticles from oleyl alcohol solutions occurs through a pathway that is closely dependent on the nature of the metal-organic precursor: whereas for

J. Phys. Chem. C, Vol. 114, No. 11, 2010 4881 In(ac)3 and In(acac)3, the metal oxide forms in solution as a result of an esterification reaction, in the case of In(OiPr)3, the In2O3 nanoparticles are precipitated in solution as a result of a thermolysis process. The formation of metal oxide nanoparticles by an ester elimination reaction was well-studied in many systems.26,52–54 In these reactions, the hydroxyl group of the alcohol initiates a nucleophilic attack on the carbon atom of the carbonyl group of the acetate precursor. The resulting metal hydroxide undergoes a condensation reaction at elevated temperatures, yielding the corresponding metal oxide nanoparticles (Scheme 1). Because the XRD pattern of the product resulting from the reaction of In(acac)3 with oleyl alcohol in a nitrogen atmosphere revealed the existence of both In2O3 and metallic indium (Figure 1, pattern b), we assume that, in addition to the esterification reaction, the acetylacetonate precursor thermally decomposes with the formation of metal and acetylacetone. Recently, Franzman and co-workers reported a new solution-based method to synthesize In2O3 nanoparticles from In(acac)3 at relatively low temperatures (120-180 °C) in the presence of organic peroxides.42 A detailed investigation of the reaction pathway by FT-IR spectroscopy indicated that In(acac)3 decomposes in solution with the release of acetylacetone regardless of the presence or absence of organic peroxides. However, their presence is critical for the formation of In2O3 nanocrystals at low temperatures, which was explained by an oxygen transfer from the reactive species resulting from decomposition of peroxides to the indium precursor. In our case, by performing the reaction between In(acac)3 and oleyl alcohol under air, single-phase In2O3 nanopowders were produced (Figure 2, pattern b). The absence of the ester peak in the FTIR spectra of the postreaction solutions corresponding to In(OiPr)3 indicates that, most likely, the precursor undergoes a thermal decomposition reaction, leading to the formation of metallic In when the experiments are performed under nitrogen and In2O3 when the syntheses are carried out under air (Figures 1, pattern c, and 2, pattern c). Photoluminescence Properties of the In2O3 Nanoparticles. Although the optical properties of the In2O3 nanostructures have been extensively studied, a limited number of papers reported the existence of PL bands only in the UV region.25,26,55,56 In many of the previous studies, In2O3 nanostructures with different dimensionalities and morphologies, including nanowires,57–59 nanorod bundles,60 nanoporous nanocrystal clusters,61 microparticles,62 and nanocrystals,63 showed emission bands in both UV and visible regions. Whereas the UV peaks are associated with the near-band-edge emissions, the PL bands in the visible domain originate from deep-level or trap-state emissions. Cubic In2O3 crystallizes into an oxygen-deficient fluorite structure with a unit cell twice that of the fluorite and one-quarter of the oxygen ions missing in an ordered fashion.8,9,64 In nanostructures, the oxygen vacancies can coexist with other point defects (indium vacancies, indium interstitial, and oxygen antisite) as well as other structural defects, such as dislocations and stacking faults.65–68 In general, the PL of various In2O3 nanostructures in the visible domain was ascribed to a higher concentration of defects in the crystal lattice, especially oxygen vacancies, which act as deep defect donors and generate localized states in the band gap.26,29,40,54,59,64,69–73 On the other hand, for high-quality nanostructures, that is, those with a lower concentration of vacancies, impurities, and imperfections, the ratio between the near-band-edge and deep-level emissions increases significantly and strong PL peaks are observed in the UV domain.74–78 Besides the high crystal quality of the material, the quantum confinement effect is another important factor responsible for

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SCHEME 1: Reaction Pathway for the Alcoholysis between a Metal Carboxylate and a Long Chain Alcohol with Elimination of an Ester

the UV emission in various nanostructures.25,55,62,79 In Figure 9 are shown the room-temperature photoluminescence spectra (excitation wavelength of 290 nm) of the In2O3 nanocrystals obtained in oleyl alcohol solutions from the In(ac)3, In(acac)3, and In(OiPr)3 precursors. All samples display a relatively strong UV emission peak that is centered at 320 nm (E ) 3.88 eV) for the 7 nm cubic In2O3 nanoparticles and shifts to 353 nm (E ) 3.51 eV) for the 19.9 nm In2O3 cubes and to 363 nm (E ) 3.42 eV) for the 9.2 nm spheroidal In2O3 nanoparticles. The optical absorption spectra (Figure SI 4, Supporting Information) show a peak maximum at around 290 nm (4.28 eV) for all the samples investigated. The blue shift (610 meV) with respect to the band gap (3.67 eV, 338 nm) of the bulk In2O3 indicates a weak quantum confinement effect in all In2O3 samples, regardless of their size, shape, atmosphere in which they were produced, and the reaction pathway.25,55,61 A similar blue shift (210 meV) with respect to the bulk band gap can be observed in the PL spectrum of 7 nm cubic In2O3 nanoparticles synthesized from the In(acac)3 precursor under air (Figure 9). Although the average size of the nanocrystals is slightly larger than the exciton Bohr diameter (whose values typically range between 2.6 and 5 nm in the case of In2O3), the UV emission band centered at 320 nm (3.88 eV) can be attributed to a quantum confinement effect.4,80 Few other papers also reported that the origin of the UV emission in In2O3 nanoparticles with sizes larger than the exciton Bohr diameter is the quantum confinement effect. For example, Lee et al. showed that the 6.6 nm sized In2O3 particles obtained by pulsed laser ablation exhibited a PL emission peak centered at 328 nm (3.28 eV), which was ascribed to the quantum confinement effect.55 Also, Park and co-workers observed a weak size dependence of the UV emission wavelength in the case of 4, 6, and 8 nm In2O3 nanocrystals obtained by thermolysis of In(acac)3 in oleylamine.

Figure 9. Photoluminescence spectra of the In2O3 nanoparticles obtained from different precursors: (a) In(acac)3, (b) In(OiPr)3, and (c) In(ac)3.

They attributed the UV emission bands located at 325, 330, and 332 nm to the quantum confinement effect.25 In the case of 19.9 nm cubic particles obtained under air from the In(OiPr)3 precursor, the UV emission band appears at a lower energy (3.51 eV, 353 nm) as compared with 3.67 eV of bulk In2O3. Because the larger nanocubes are formed by oriented aggregation, the observed red shift of 160 meV can be presumably associated with the existence of a low concentration of defects in the crystal lattice (oxygen vacancies, indium vacancies and interstitials, dislocations, stacking faults) that are likely to form during the growth process.71 A larger red shift of the peak energy (250 meV) was observed for the 9.2 nm spheroidal particles obtained from the In(ac)3 precursor in a nitrogen atmosphere. A possible explanation for this larger red shift is the higher concentration of oxygen vacancies in the spherical nanoparticles as compared with the large nanocubes due to the lack of oxygen during growth by oriented attachment. The different red shift values observed for the PL peaks of the spherical and cubic nanoparticles with respect to the bulk In2O3 may also originate from the different reaction mechanisms. Cubic In2O3 nanoparticles form via an esterification process, whereas the spherical nanocrystals are obtained by thermal decomposition, and this can potentially induce defects of different types and concentrations in the two samples.60,63 Gas-Sensing Properties. The spherically shaped In2O3 nanoparticles obtained from the reaction of In(ac)3 with oleyl alcohol under a nitrogen atmosphere were tested toward both reducing (H2S and CO) and oxidizing (NO2) gases at room temperature.81 The sensing device was fabricated by placing a colloidal toluene solution containing the In2O3 nanoparticles on a silicon wafer substrate covered with a 600 nm thick thermal oxide film. Because of the relatively good size uniformity of the spherically shaped In2O3 nanocrystals (average size of 9.2 nm, RSD of 13%) and the passivation of their surfaces with oleyl alcohol molecules, they can self-assemble into short-range ordered arrays onto the silicon substrate upon solvent evaporation at room temperature. The silicon wafer substrate containing the nanoparticle film was subsequently heat-treated for 1 h at 400 °C in air. This is necessary in order to remove the capping ligand and to ensure good contact between the adjacent nanoparticles without altering their morphology. Two gold pads with a thickness of about 400 nm were then sputter-coated on the nanoparticle film at a distance of about 100 µm. Figure 10a shows the cross-sectional field emission scanning electron microscope (FESEM) image of the In2O3 nanoparticle-based sensing device. The FESEM micrograph indicates that the thickness of the In2O3 nanoparticle film is about 500 nm. Because the average size of the spherically shaped In2O3 nanocrystals is 9.2 nm, we estimate that the film consists of about 53 layers of particles. It can also be observed that the surface of the nanoparticle film is relatively flat and smooth and lies nearly parallel to the surface of the substrate.


Figure 10. (a) SEM image of the cross section of the In2O3 nanoparticle device. (b) Time-dependent current curve when the air flow is turned “on” and “off” (applied voltage ) 0.5 V).

The current response of the In2O3 nanoparticle-based sensor to the air under flowing conditions was initially investigated. The measurement was performed at room temperature at a fixed bias voltage of 0.5 V. Figure 10b shows the current variation of the sensor device in response to turning “on” and “off” the air flow. When the air flow is turned “on” (point A), the sensor current starts to decrease gradually; once the current is stabilized, the air flow is turned “off” (point B) and the sensor recovers slowly. These measurements were repeated several times and found to be highly reproducible. The variation of the current when the sensor is exposed to the flowing air is presumably due to the change of the thickness of the depletion region on the surface of the In2O3 nanoparticles. As previously reported in the literature,82–85 ambient oxygen molecules can adsorb onto the surface of n-type semiconducting metal oxides as anionic species (O2- and O22-) by capturing the conduction electrons. This results in the formation of a depletion region near the surface of the metal oxides where the conductivity is suppressed. When the sensor device is exposed to the flowing air, more oxygen molecules are likely to be adsorbed onto the surface of the In2O3 nanoparticles of the outermost layers of the film, leading to an increase of the thickness of the depletion region and, subsequently, to a decrease of the current. Also, the oxygen molecules are likely to reach to the inner layers of the In2O3 film and adsorb onto the surface of the particles, causing a further decrease of the current. When the air flow is stopped, the oxygen molecules gradually desorb from the surface of the nanoparticles, the depletion region is reduced, and the current increases.

J. Phys. Chem. C, Vol. 114, No. 11, 2010 4883 The gas-sensing performance of the In2O3 nanoparticle-based sensor toward H2S was also investigated. Figure 11a shows the current response of the sensor to different concentrations of H2S/ air (0, 30, 50, 100, 200, and 500 ppb) at room temperature with the bias fixed at 0.5 V. As previously shown, when the air flow (0 ppb) is introduced into the chamber (point A), the sensor current starts to decrease gradually from ∼2.8 nA to a value lower than 1 nA. Once the current is stabilized, the air flow is turned off (point B) and the sensor starts to recover slowly. At the point C, where the current is recovered to about 1 nA, 30 ppb of H2S/air is introduced into the chamber and the current goes up to ∼3.8 nA during 15 min as the reducing gas molecules (H2S) adsorb onto the surface of the nanoparticles, react with the chemisorbed oxygen species (O2- and O22-), and release the electrons back to the metal oxide.86 This process (from A to C) was repeated for different concentrations of H2S/air, and it was observed that the sensor current increases as the concentration of the reducing gas increases. Figure 11b shows the current-voltage (I-V) curves of the nanoparticle device measured for different concentrations (ppb level) of H2S/air. It can be clearly seen that the conductivity of the device increases with increasing the concentration of H2S/air. It was also found that the curves are perfectly linear, indicating a good Ohmic contact between the spherically shaped In2O3 nanoparticles and the electrodes. The In2O3 nanoparticle-based device shows an increased sensitivity to ppm levels of H2S/air. Figure 11c,d shows the results corresponding to different concentrations of H2S/air ranging from 0.5 to 10 ppm. As seen in Figure 11c, when 2 ppm of H2S/air was introduced into the chamber, the sensor current went up to around 30 nA during 4 min, whereas for a higher concentration of 10 ppm of H2S/air, the current increased to more than 250 nA. In addition to H2S, other gases, such as CO and NO2, were tested at room temperature in order to obtain information on the selectivity of the In2O3 nanoparticle-based sensor. The comparison of the sensor behavior toward the reducing gases (CO and H2S) is illustrated in Figure 12a. It can be seen that there is almost no current response when the sensor is exposed to high concentrations of 1000 ppm of CO/air, but the current increases immediately to over 25 nA when 500 ppb of H2S/air is introduced into the chamber. In the case of oxidizing gas, the nanoparticle sensor detected concentrations as low as 100 ppb of NO2/air at room temperature (Figure 12b). However, the sensor response is opposite to that recorded for the reducing gases, showing a decrease of the current once the oxidizing gas NO2 is introduced into the chamber. The obtained results indicate a certain selectivity of the In2O3 nanoparticle-based sensor at room temperature toward H2S over the other testing gases (CO and NO2). 4. Conclusions In summary, we have synthesized highly crystalline, variableshaped In2O3 nanoparticles by the heat treatment of different indium-organic precursors in oleyl alcohol solutions. The morphological features and composition of the resulting nanopowders can be tuned upon changing the metal precursor and the atmosphere in which the reactions are conducted. Under a nitrogen atmosphere, In(ac)3 yields spherically shaped In2O3 nanocrystals with an average size of 9.2 nm, whereas in air, the acetate precursor leads to the formation of nanoparticles with an incompletely developed morphology. Unlike the acetate precursor, both In(acac)3 and In(OiPr)3 produce well-defined, cubic In2O3 nanoparticles when heated under air. Whereas the In2O3 nanocubes obtained from In(acac)3 have well-defined

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Figure 11. (a) Time-dependent current curve corresponding to different concentrations (ppb) of H2S/air (applied voltage ) 0.5 V). (b) I-V curves corresponding to different concentrations (ppb) of H2S/air (the inset is the enlarged image of the circled part). (c) Time-dependent current curve corresponding to different concentrations (ppm) of H2S/airS. (d) I-V curves corresponding to different concentrations (ppm) of H2S/air.

Figure 12. Time-dependent current curve of an In2O3 nanoparticle-based device tested in: (a) CO/air and H2S/air and (b) NO2/air (applied voltage ) 0.5 V).

edges and an average size of 7 nm, those obtained from In(OiPr)3 possess round corners and an edge length close to 20 nm. TEM studies showed that the In2O3 nanoparticles form through an oriented attachment mechanism when In(ac)3 and In(OiPr)3 are used as metal precursors. However, it was found that oxygen (air) plays a critical role in the growth kinetics of the metal oxide nanoparticles; that is, its presence slows down considerably the aggregation/recrystallization processes. Regardless of

their morphology, the colloidal In2O3 nanocrystals present a strong luminescence in the UV domain. Whereas for the 7 nm sized cubes, the PL peak was ascribed to a weak quantum confinement effect, in the case of both the 19.9 nm cubes and the 9.2 nm spherical nanoparticles, the red shift observed in the PL spectra was ascribed to a combined effect between weak quantum confinement and the existence of oxygen defects, whose concentration increases in the case of In2O3 nanospheres

Nearly Monodisperse, Variable-Shaped In2O3 NCs formed by oriented attachment under an inert atmosphere. Spherically shaped In2O3 nanocrystals were used for the design of gas sensors that showed superior sensing characteristics in the detection of H2S at room temperature. Acknowledgment. Financial support from DARPA (Grant No. HR0011-07-1-0032) is gratefully acknowledged. We thank Dr. Cuikun Lin for assistance during PL measurements and for helpful discussions. Supporting Information Available: Figure SI 1: TEM images of In2O3 nanocrystals resulting from the reaction of In(ac)3 with oleyl alcohol in open air (flask closed at 265 °C). Figure SI 2: TEM images of In2O3 nanocrystals obtained from the reaction of In(OiPr)3 with oleyl alcohol under flowing air (turned off at 300 °C); (a) aliquot taken at 310 °C, (b) aliquot taken at 320 °C/(t ) 0 min), and (c) aliquot taken at 320 °C/ (t ) 30 min). Figure SI 3: infrared spectra of the solutions obtained after removal of the solid products resulting from the reaction of In(ac)3 (1), In(acac)3 (2), and In(OiPr)3 (3) with oleyl alcohol under open air; curve 4 represents the IR spectra of oleyl alcohol (85%). Figure SI 4: UV-vis absorption spectra of the In2O3 nanoparticles obtained from different precursors; (a) In(acac)3, (b) In(ac)3, and (c) In(OiPr)3. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Puetz, J.; Al-Dahoudi, N.; Aegerter, M. A. AdV. Eng. Mater. 2004, 6, 733. (2) Qadri, S. B.; Kim, H. J. Appl. Phys. 2002, 92, 227–229. (3) Naghavi, N.; Marcel, C.; Dupont, L.; Rougier, A.; Leriche, J.-B.; Guery, C. J. Mater. Chem. 2000, 10, 2315–2319. (4) Hamberg, I.; Granqvist, C. G. J. Appl. Phys. 1986, 60, R123. (5) Reyes-Gil, K.; Reyes-Garcia, E.; Raftery, D. J. Phys. Chem. C 2007, 111, 14579. (6) Gagaoudakis, E.; Bender, M.; Douloufakis, E.; Kataarakis, N.; Natsakou, N.; Cimalla, V.; Kiriakids, G. Sens. Actuators, B 2001, 80, 155. (7) Chung, W. Y.; Sakai, G.; Shimanoe, K.; Miura, N.; Lee, D. D.; Yamazoe, N. Sens. Actuators, B 1998, 46, 139. (8) Liess, M. Thin Solid Films 2002, 410, 183. (9) Tamaki, J.; Naruo, C.; Yamanoto, Y.; Matsuoka, M. Sens. Actuators, B 2002, 83, 190. (10) Du, N.; Zhang, H.; Chen, B. D.; Ma, X. Y.; Liu, Z. H.; Wu, J. B.; Yang, D. R. AdV. Mater. 2007, 19, 1641. (11) Gurlo, A.; Ivanovskaya, M.; Barsan, N.; Schweizer-Berberich, M.; Weimar, U.; Gopel, W.; Dieguez, A. Sens. Actuators, B 1997, 44, 327. (12) Ginley, D. S.; Bright, C. Mater. Res. Soc. Bull. 2000, 25, 15. (13) Chopra, K. L.; Major, S.; Pandya, D. K. Thin Solid Films 1983, 102, 1. (14) Dawar, A. L.; Joshi, J. C. J. Mater. Sci. 1984, 19, 1. (15) Granqvist, C. G. Appl. Phys. A: Solids Surf. 1993, 57, 19. (16) Li, X.; Wan-lass, M. W.; Gessert, T. A.; Emery, K. A.; Coutts, T. J. Appl. Phys. Lett. 1989, 54, 2674. (17) Shigesato, Y.; Takaki, S.; Haranoh, T. J. Appl. Phys. 1992, 71, 3356. (18) Li, C.; Zhang, D.; Han, S.; Liu, X.; Tang, T.; Zhou, C. AdV. Mater. 2003, 15, 143. (19) Yin, W.; Su, J.; Cao, M.; Ni, C.; Cloutier, S. G.; Huang, Z.; Ma, X.; Ren, L.; Hu, C.; Wei, B. J. Phys. Chem. C 2009, 113, 19493. (20) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (21) Zheng, M. J.; Zhang, L. D.; Li, G. H. Appl. Phys. Lett. 2001, 79, 839. (22) Zheng, M. J.; Zhang, L. D.; Zhang, X. Y.; Zhang, J.; Li, G. H. Chem. Phys. Lett. 2001, 334, 298. (23) Kuo, C. Y.; Lu, S. Y.; Wei, T. Y. J. Cryst. Growth 2005, 285, 400. (24) Chang, S. C.; Huang, M. J. Phys. Chem. C 2008, 112, 2304. (25) Seo, W. S.; Jo, H. H.; Lee, K.; Park, J. T. AdV. Mater. 2003, 15, 795. (26) Liu, Q. S.; Lu, W. G.; Ma, A. H.; Tang, J. K.; Lin, J.; Fang, J. Y. J. Am. Chem. Soc. 2005, 127, 5276. (27) Lu, W. G.; Liu, Q.; Sun, Z.; He, J.; Ezeolu, C.; Fang, J. Y. J. Am. Chem. Soc. 2008, 130, 6983. (28) Yang, J.; Li, C.; Quan, Z.; Kong, D.; Zhang, X.; Yang, P.; Lin, J. Cryst. Growth Des. 2008, 8, 695.

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