In2S3 Micropompons and Their Conversion to In2O3 Nanobipyramids

CRYSTAL GROWTH & DESIGN

In2S3 Micropompons and Their Conversion to In2O3 Nanobipyramids: Simple Synthesis Approaches and Characterization

2007 VOL. 7, NO. 1 163-169

Anuja Datta,† Subhendu K. Panda,† Dibyendu Ganguli,† Pratima Mishra,‡ and Subhadra Chaudhuri*,† Department of Materials Science and DST unit of Nanoscience, Indian Association for the CultiVation of Science, Kolkata 700 032, India, and AdVanced Materials Technology DiVision, Regional Research Laboratory, Bhubaneswar 760 013, India ReceiVed October 9, 2006; ReVised Manuscript ReceiVed NoVember 9, 2006

ABSTRACT: In2S3 micropompons composed of randomly oriented flakes of ∼10 nm thickness were synthesized in high yield by a facile hydrothermal process. When the product was thermally oxidized at 600 °C, single crystalline well-faceted body-centered cubic (bcc) In2O3 bipyramids of sizes between 50 and 300 nm were obtained. It was proposed that the growth of the In2S3 nanoflakes was initially controlled by the Ostwald ripening process, and finally the aggregation of the flakes led to the development of the micropompons. The growth of the In2O3 bipyramids from the In2S3 micropompons started with the formation of In2O3 nulclei at the surface of the micropompons causing the destruction of the flakelike structure to produce In2O3 bipyramids. The micropompons showed good optical properties due to a strong quantum confinement effect. In2O3 bipyramids showed near-band-edge (NBE) UV emission, which was mainly attributed to the high crystal quality of the bipyramids. The study may provide guidance for the morphology controllable synthesis of different nanostructures and may help in exploring the crystal growth process. Introduction Semiconductor nanomaterials with interesting shapes and structures have long been of great fundamental and technological interest, since they often act as model systems for the study of nanoscale shape-dependent properties.1-4 Assembly of 2D components into 3D novel structures has always been a challenging task, particularly when the superstructures are obtained in a template-free process utilizing a simple chemical route. Tetragonal indium sulfide (In2S3), an n-type III-VI semiconductor, because of its defect structure, has shown many important optoelectronic,5-7 photoconductive,8 and optical properties,5,9,10 which subsequently inspired applications in the preparation of phosphors for color televisions11,12 and heterojunctions for photovoltaic electric generators.13 Morphologically distinct nanocrystals of In2S3 have been prepared by various methods including the solvent reduction technique,14 oxidationsulfidation route,15 microwave synthesis route,16 and solvothermal technique from an organometallic precursor.17 All of them were found to show shape-dependent properties, which inspired us to synthesize In2S3 utilizing a simple process. We employed a template-free hydrothermal route and prepared high-quality 3D In2S3 micropompons, which were formed by the aggregation of 2D nanometer-thick flakes, rather randomly. A hydrothermal process was chosen for the synthesis because of its economic and very easy processing steps along with a high degree of compositional control. It is a heterogeneous reaction in aqueous media carried out above room temperature and has been gathering interest for synthesis of a wide varieties of materials, such as ceramics,18,19 composites,20 and above all nanocrystals21 with desired shapes. Hydrothermal technology has also been used earlier for the synthesis of In2S3, but only nanocrystalline powder was prepared.22 Furthermore, In2S3 microspheres were prepared by a two-step method from the breakdown of prepared * Corresponding author. Telephone number: +9133 24734971, ext 377. Fax number: +9133 24732805. E-mail: [email protected]. † Indian Association for the Cultivation of Science. ‡ Regional Research Laboratory, Bhubaneswar.

complex precursor.17 In this paper, we present the hydrothermal growth of In2S3 micropompons in a single step without using any complex precursor form and also studied their optical properties. Meanwhile, In2O3, another important member of the III-VI group of oxide compounds has been the center of study owing to its low resistivity, low absorbance rate in the visible region, and high infrared light reflectivity.23,24 Several methods, like the vapor-solid method,25 vapor-liquid-solid method,26,27 laser ablation technique,28 physical evaporation technique, etc.,29 have been exploited to synthesize various nanostructures of In2O3 including 1D rods and wires, 2D columns, and 3D tetrahedrons. Previously, our group successfully synthesized In2O3 bipyramids and nanocolumns, which were reported to exhibit good optical properties, utilizing a vapor-solid-liquid process.27 It was further shown, that In2O3 nanoforms were suitable candidates for field emission because of their relatively low electron affinity, convenience of n-type doping, high chemical inertness, and sputter resistance.30 In the present work, we employed a noncatalyzed calcination route to synthesize In2O3 bipyramids at a relatively lower temperature. The hydrothermally obtained In2S3 micropompons were thermally oxidized in a controlled oxygen atmosphere, giving rise to In2O3 bipyramids (octahedrons). The synthesized bipyramids were found to show intense UV emission at room temperature. The current investigations indicate that a conversion of indium sulfide to indium oxide involves not only total transformation of phase but also interesting microstructural appearance. Possible growth mechanisms have been suggested from the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies. The present study shows that these effective and low cost fabrication processes have good potential for scale up. Experimental Section Preparation of Indium Sulfide Micropompons. In2S3 micropompons were prepared via a simple hydrothermal technique in a Teflonlined steel chamber. Initially 2.7 × 10-3 M indium chloride was dissolved in 7.5 mL of deionized water, and after 15 min, thioacetamide

10.1021/cg0606895 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/14/2006

164 Crystal Growth & Design, Vol. 7, No. 1, 2007

Figure 1. XRD patterns of (a) In2S3 micropompons, (b) intermediate product, and (c) In2O3 nanobipyramids. equal to 2.5 times the molar concentration of indium chloride was slowly added to the solution. The mixture was continuously stirred for 3 h and was then transferred to a Teflon container of 85 mL capacity. The pH of the solution was obtained to be ∼2.5 after addition of thioacetamide (H+ released by the protonation of H2S generated due to the breakdown of thioacetamide). The container was filled with deionized water to 80% volume and maintained at 200 °C for 12 h. The resulting reddish orange precipitate was separated by centrifugation and washed repeatedly with water and ethanol to remove the unreacted chemicals. Preparation of Indium Oxide Nanobipyramids. Indium oxide (In2O3) nanobipyramids were prepared by thermally oxidizing the indium sulfide (In2S3) micropompons in a flowing oxygen atmosphere. The oxidation was carried out inside a quartz tube inserted in a horizontal tube furnace at a previously maintained temperature of 400 and 600 °C for 6 h. After the desired period, the furnace was gradually cooled to room temperature to yield a reddish white or an off-white product, depending on the oxidation temperature. The oxygen flow rate was adjusted at 20 cm3/min during the oxidation process to facilitate a slow and steady rate of conversion. Characterization. Microstructural, morphological characterizations of In2S3 micropompons and In2O3 nanobipyramids were carried out by scanning electron microscopy (SEM, HITACHI S-2300), field emission scanning electron microscopy (FESEM, JEOL, JSM- 6700F), and transmission electron microscopy (TEM, JEOL 2010). Phase determination and elemental composition analysis were carried out by X-ray diffraction (XRD, Seifert 3000P) and energy dispersive X-ray analysis (EDAX, JEOL, JSM- 6700F), respectively. Optical absorption spectra within the range 350-800 nm were recorded using spectroscopic grade isopropyl 2-propanol as the reference (UV-vis-NIR spectrophotometer, Hitachi U-3410). Room-temperature photoluminescence (PL) studies were completed by fluorescence spectrophotometer (Hitachi -2500). Raman spectra were acquired with the Renishaw in via Raman spectrometer, which is amenable to measure samples in the micrometer range. The excitation source was a 514 nm argon ion laser with 100% power. The resolution of all the spectra was within (2 cm-1. All the spectra were analyzed with the software supplied by Renishaw, and background corrections were made when needed.

Results and Discussion X-ray diffraction spectrum (Figure 1a) of the In2S3 micropompons synthesized hydrothermally can be indexed to bodycentered tetragonal structure with lattice parameters of a ) 7.628 Å and c ) 32.375 Å, which are close to the reported values (JCPDS card no. 25-0390). The X-ray pattern indicated that the micropompons were well crystallized and phase pure. Figure 1b shows the X-ray spectrum of the product obtained by oxidizing In2S3 micropompons at 400 °C for 6 h, where peaks

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corresponding to both In2S3 and In2O3 phases are present. This indicates that at 400 °C, the phase conversion from In2S3 to In2O3 was partial. The temperature for oxidation was then carefully chosen to ensure complete phase conversion, and phase-pure In2O3 was obtained after oxidation of In2S3 powder at 600 °C, keeping the time constant at 6 h. The XRD spectrum (Figure 1c) shows that the product is devoid of any detectable impurities indicating complete conversion of In2S3 to In2O3. All the peaks can clearly be indexed to the cubic system with space group Ia3 (206). The lattice parameter a ) 10.12 Å is in good agreement with the reported value of a ) 10.118 Å (JCPDS card no. 06-0416), indicating a simple cubic structure for In2O3 nanobipyramids. SEM and FESEM observations indicated that In2S3 micropompons were composed of randomly ordered flakes with thickness of ∼10-15 nm and length of ∼1 µm. The micropompons have nearly uniform size distribution with diameter around 2-3 µm (Figure 2a). These structures were very stable, as the morphology was not destroyed even after a long time (∼1 h) ultrasonication treatment. A high-resolution FESEM image (Figure 2b) of the micropompons reveals that the flakes are curved with corrugated margins and are interconnected with each other. A closer view is shown in the inset of Figure 2b, which reveals that the structure is highly porous. The abundance of micropompons was above 90% as calculated from the SEM and FESEM images. Figure 2c,d contains the TEM images showing the structure of the flakes. The flakes have well-defined triangular and polygonal boundaries interpenetrating each other (Figure 2c). A triangular flake has been shown in Figure 2d, which indicates that the flakes are nearly transparent particularly at the margins due to their very small thickness. A HRTEM image of the center of the flake reveals the growth direction of three faces of the triangular flake. Lattice fringe patterns of the planes (109), (2212) and (116) corresponding to the d-spacings of 0.324, 1.905, and 0.381 nm, respectively, constitute the three arms (planes) of the triangle. The flakes are well crystalline and also show periodic Moire´ fringes as a result of the difference in the flake curvature (Figure 2e). The selected area electron diffraction (SAED) pattern also shows the well crystalline nature of the flakes (inset of Figure 2e). A representative EDAX spectrum (Figure 2f) recorded from different places of the flakes revealed an In/S atomic percentage ratio equal to the mean value of 2:2.95, indicating the product to be slightly sulfur deficient, which may be due to evaporation of sulfur as H2S during the preparation. When the above In2S3 micropomopons were oxidized in an oxygen atmosphere, the structures were destroyed. Figure 3a is the SEM image of the product oxidized at 400 °C for 6 h. The microsructure of the product revealed the loss of the flakelike structures giving rise to a particulate morphology. However, the products still maintained the spherical shapes. Figure 3, panels b and c, show the SEM and FESEM images of the completely oxidized products at 600 °C. The figures clearly indicated the development of a large amount of well-faceted In2O3 nanobipyramids of ∼50-300 nm sizes. It also appeared that the individual nanobipyramids had a common square plane. The composition of the bipyramids was confirmed by EDAX, which indicated an In/O atomic ratio equal to 2:2.78, showing that the In2O3 bipyramidal structures were slightly oxygen deficient (Figure 3d). The rate of flow of oxygen was also found to play an important role in controlling the final morphology of the calcined product. Under the conditions of high oxygen flow rate (>20 cm3/min), no regular shape was found in the

In2S3 Micropompon Conversion to In2O3 Nanobipyramids

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Figure 2. (a) SEM image showing a cluster of spherical micropompons, (b) FESEM image of a single micropompon with inset showing a magnified view, (c) TEM image showing aggregation of flakes, (d) TEM image of an individual nanoflake, (e) HRTEM image showing the lattice planes in a flake with inset showing the SAED pattern, and (f) EDAX spectrum of In2S3 micropompons.

final oxidized product (not shown here). Even the conversion time critically controlled the morphology of the product. When the time of calcinations was less than 6 h, only particles were obtained. Hence, in our designed procedure, critical estimation of the time, temperature, and flow rate of the gas were necessary in order to obtain pure In2O3 of well-shaped bipyramids. Figure 4a shows the bright field TEM image of the cluster of several nanobipyramids, and Figure 4b shows an individual bipyramidal structure with eight well-defined facets. Figure 4c shows the steps at the edges of the bipyramidal structure, which threw light on the growth process of the structures. SAED (inset of Figure 4a) and FFT (inset of Figure 4c) patterns indicate that the bipyramids are single crystalline in nature. From the HRTEM image (Figure 4d), the d-spacing was calculated to be ∼0.413 nm indicating that the eight facets of a single bipyramid were constructed of {211} planes. Bipyramid structure with the planes has been schematically shown in the inset of Figure 4b. UV-vis absorption studies, room-temperature photoluminescence, and Raman studies were carried out to reveal the optical properties of the synthesized In2S3 micropompons and In2O3 nanobipyramids. The optical absorption spectra of In2S3 micropompons and In2O3 bipyramids have been shown in Figure 5. For In2S3, the band gap was calculated to be ∼4.43 eV, which

is very high compared with the reported bulk value.31,32 In2S3 has a large Bohr exciton radius of 33.8 nm, and in the present case, the In2S3 micropompons were composed of an average 10 nm thick flakes. The large blue shift in the band gap was due to the quantum confinement effect. For In2O3 bipyramids, which are quite large in size (50-300 nm) compared with the Bohr exciton radius of In2O3 (∼2.14 nm),33 did not show any size confinement effect. The band gap value calculated for In2O3 bipyramids is ∼3.89 eV, which is close to the bulk value.23 Figure 6a shows the emission spectra from the In2S3 micropompons. Compared with the nonluminescent behavior of bulk In2S3,34 the micropompons composed of thin nanoflakes showed good luminescence at room temperature. Following excitation at 300 nm, a sharp and strong emission at ∼472 nm and a weak emission at ∼539 nm were observed. The peak at 472 nm was accompanied by two shoulders at 484 and 495 nm, respectively. The emissions can be attributed to the presence of several deep trap states or defects in the structure. The weak green luminescence at ∼540 nm may be an emission from the indium interstitial sites. Chen et al. reported green emission from In2S3 nanoparticles and also correlated it with the presence of indium interstitial sites.35


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Figure 3. (a) SEM image of the intermediate product with inset showing the magnified view, (b) SEM image of the In2O3 bipyramids, (c) FESEM image of the bipyramids, and (d) EDAX spectrum of the In2O3 bipyramids.

Figure 4. TEM images showing (a) cluster of bipyramids (inset shows the SAED pattern), (b) a single bipyramid (inset shows the schematic view), and (c) a bipyramid perpendicular to the basal plane (inset shows the FFT pattern indicating single crystalline nature) and (d) HRTEM image indicating lattice fringes.

The In2O3 nanobipyramids, on the other hand, showed a stable and strong emission band at ∼342 nm when excited at a

wavelength of 300 nm (Figure 6b). It is known that bulk In2O3 cannot emit light at room temperature.36,37 Quantum confinement


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Figure 5. Optical absorbance spectra of (a) In2S3 micropompons and (b) In2O3 nanobipyramids.

Figure 7. Raman spectra of (a) In2S3 micropompons and (b) In2O3 nanobipyramids.

Figure 6. Room-temperature PL spectra of (a) In2S3 micropompons and (b) In2O3 nanobipyramids.

effect and high crystal quality are the two essential factors that favor this UV emission at room temperature.37 The sharp UV emission in our sample can be correlated to the near-band-edge (NBE) transitions, attributed by the high crystal quality of the prepared product. The annealing helped to decrease the impurities and structural defects, resulting in NBE emission. Chen et

al.17 again explained the origin of the UV luminescence in In2O3 nanoparticles due to the existence of oxygen vacancies. Singly ionized oxygen vacancies incorporated in the In2O3 bipyramidal structures during the crystallization can induce shallow energy levels in the band gap. The low content of oxygen in the In2O3 bipyramids is evident from the EDAX analysis. Therefore, radiative recombination of a photoexcited hole with an electron occupying the vacancy site may also be a reason for the UV emission.38,39 Raman-active modes of the In2S3 micropompons and In2O3 bipyramids are shown in Figure 7, which revealed the high crystalline quality of the products. Eight normal modes of vibrations were obtained for In2S3 micropompons, which corresponded to those studied by other workers.15,40 A slight deviation from the bulk phonon modes may be ascribed to the effect of quantum confinement of the flakes in the micropompons. The Raman spectrum for the In2O3 bipyramids indicated the presence of four phonon modes at 202.4, 296.9, 378.4, and 485.1 cm-1, of which the vibration mode at 296.9 cm-1 is the strongest. The bands are due to In-O vibrations of InO6 structural units of the body-centered cubic (bcc) In2O3 structure.41,42 In a surfactant/polymer/chelating agent mediated synthesis process, the growth of a 3D microstructure constituted of 2D components can be explained by a self-aggregation and oriented attachment mechanism.43,44 In the present case however, absence of any kind of structure-binding molecule rules out the possibility of self-assembly of individual flakelike components toward the development of the 3D micropompon structure. An Ostwald ripening process, proposed to explain the anisotropic lateral growth, probably is a more likely phenomenon in this case, where the flakes grew at the expense of smaller nuclei formed initially.45,46 The spherical diffusion model proposed by Liu et al. emphasizes that the Ostwald ripening may be responsible for lateral growth in absence of active agents. Ostwald ripening results in the regular increase in the primary nuclei giving rise to the flakelike morphologies. In the subsequent growth process, these flakes act as the building blocks. We assume that the nucleation of In2S3 started during the solution stage, prior to the hydrothermal heat treatment. Thio-


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acetamide (C2H5NS) molecules undergo slow dissociation at room temperature resulting in the low degree of supersaturation, which prefers the occurrence of heterogeneous nucleation above homogeneous nucleation in the solution.47-49 High-energy planes then grew out at ease, thus developing the flakelike crystal morphology. The lateral growth of the nanoflakes took place in the hydrothermal stage where it was guided by the mass transport process. The solute was transported from the bulk of the solution to the crystal/solution interface and gradually became incorporated into the crystal by a surface kinetics process. After the formation of In2S3 nanoflakes, micropomponlike structures developed due to the aggregation of the flakes, supposedly in a random manner. The reduction of the number of nuclei after the formation of the nanoflakes induced more homogeneity in the solution, and spherical micropompon-like structures were produced by the aggregation of the flakes. Xu et al. described a similar process to be responsible for the formationofLa2(MoO4)3 micropomponsconstructedofnanoflakes.45 In the formation of In2O3 bipyramidal morphology after oxidation of micropompon structures, the elemental substitution mechanism may be involved for the transformation of indium sulfide to phase-pure indium oxide at the expense of sulfur atoms. The evaporation of indium sulfide molecules under constant flow of oxygen led to the destruction of flakelike structure and lifting of small cubic indium oxide nuclei. Though we have continuously supplied oxygen during the pyrolysis, SO2 was not found to be generated. We carried out pyrolysis in a quartz tube inserted in a horizontal furnace. After each pyrolysis reaction, the quartz tube contained a yellow ring at the cooler zone of the tube, which was characterized as sulfur. The oxidation reaction may therefore be written as

2In2S3 + 3O2 f 2In2O3 + 6S

(1)

Thermodynamic stability of the oxidized products at two different temperatures (400 and 600 °C) were calculated in terms of Gibbs free energy, using the Gibbs-Helmholtz reaction,50

∑∆G°r(products/reactants) ) ∑∆H°f (products/ reactants) - ∑T∆S°f(products/ reactants)

(2)

Gibbs free energy of the oxidation reaction 2In2S3 + 3O2 f 2In2O3 + 6S at different oxidation temperatures can be calculated from the expression

∆G°r(reaction) )

∑∆G°f(products) - ∑∆G°f(reactants)(3)

Thermodynamic calculations revealed that at both 400 and 600 °C, ∆G°f(reaction) is negative; that is, the phase transformation reaction from In2S3 to In2O3 is spontaneous. Figure 8 shows the variation of ∆G°f with temperature, which proves that the increase in the temperature causes a drop in the negative value of Gibbs free energy, indicating gradual approach to the equilibrium temperature (equilibrium temperature is defined as the temperature at which ∆G°f(reaction) is zero). The equilibrium temperature calculated theoretically was found to follow the same trend as our experimental values showed. The detail analysis of the thermodynamics of the system is on the way. Cubes and bipyramids (octahedrons) are duals of each other, which is just a reflexive operation consisting of exchange of faces and vertices. An important property of a pair of dual polyhedra is that both possess the same symmetry. In2O3 crystallizing in cubic symmetry thus leads to the bipyramidal structure by proper face cumulation.51 With a view to under-

Figure 8. ∆G°f vs temperature (K) graph of the conversion reaction from In2S3 micropompons to In2O3 bipyramids.

standing the growth mechanism of the bcc In2O3 bipyramids accurately, investigation of the morphology of the intermediate products was performed. A partially oxidized sample revealed that the microspherical structure of the In2S3 micropompons was not destroyed, but the flakes were replaced by particles of ∼100 nm in diameter, comparable to the dimension of the bipyramids. At high temperature (600 °C), slow growth rate under the controlled oxygen flow gradually leads to the formation of a tip on either side, giving rise to the formation of the bipyramids. The randomness in the size and the orientation suggests that each nanobipyramid is isolated, formed separately. Step like appearance at the edges of the nanobipyramidal crystals suggests the growth initiation to be from the edges (Figure 4c).52 According to Wang,53 the surface energies associated with the different facets actually play a controlling role in the growth of a facet relative to another. He deduced that surface energies of different planes hold the general sequence γ{111} < γ{100} < γ{110} and an increase in the area ratio of {111} to {110} results in the evolution of a particle shape from cuboid to bipyramid. Particles with growth ratio of {111} plane to {110} plane of ∼1.73 may turn out to be bipyramidal. In our case, the bipyramids are bounded by eight {211} planes, which are high index planes (confirmed by HRTEM studies). The growth process of the bipyramids starting from an evaporated In2S3 precursor actually makes the effect of the surface energy difference on the growth of In2O3 bipyramids limited. As a result, the growth rates normal to the low index planes became nearly equal and higher index planes grew giving rise to the bipyramidal In2O3. Conclusion In2S3 micropompons were successfully synthesized using a simple hydrothermal technique. The micropompons were composed of very thin (∼10 nm) triangular or polygonal flakes, randomly oriented on the surface, giving rise to its typical flower-like appearance. The micropompons showed a strong quantum confinement and were found to show a broad blue and a short green luminescence. In2O3 bipyramids were synthesized by a thermal conversion route from In2S3 micropompons under a flowing oxygen environment at 600 °C. In2O3 bipyramids crystallized in a bcc structure had eight well-developed faces, and the dimension ranged between 50 and 300 nm. Strong UV luminescence, correlated to NBE emission, was attributed to the high-crystal quality of the product prepared by annealing. The basic principle for the conversion is to control the crystallization kinetics by


controlling the calcination temperature and oxygen gas flow rate so as to facilitate a slow growth rate. The synthesized In2S3 micropompons and In2O3 bipyramids with short-wavelength PL emission properties may have potential application in optoelectronic devices. The current synthesis approaches are alternative strategies for the morphology control of other inorganic substances. Acknowledgment. Authors thank Mr. K. K. Das of IACS for recording the SEM micrographs. One of the authors (A.D.) expresses her sincere gratitude to the Council of Scientific and Industrial Research (CSIR, Government of India) for sanction of a research fellowship during the tenure of the work. References (1) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (2) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (3) Jun, Y.-W.; Lee, J.-H.; Choi, J.-S.; Cheon, J. J. Phys. Chem. B 2005, 109, 14795. (4) Zhao, N.; Qi, L. AdV. Mater. 2006, 18, 359. (5) Kim, W. T.; Kim, C. D. J. Appl. Phys. 1986, 60, 2631. (6) Nomura, R.; Inazawa, S.; Kanaya, K.; Matsuda, H. Appl. Organomet. Chem. 1989, 3,195. (7) Asikainen, T.; Ritala, M.; Leskela, M. Appl. Surf. Sci. 1994, 82/ 83,122. (8) Dalas, E.; Kobotiatis, L. J. Mater. Sci. 1993, 28, 5456. (9) Diehl, R.; Nitsche, R. J. Cryst. Growth 1975, 28, 306. (10) Choe, S. H.; Band, T. H.; Kim, N. O.; Kim, H. G.; Lee, C. I.; Jin, M. S.; Oh, W. T.; Kim, W. T. Semicond. Sci. Technol. 2001, 16, 98. (11) Takeshi, T.; Susumu, M.; Toshio, N.; Nobuo, I. Japanese Patent Application; Chem. Abstr. 1979, 91, 67384a. Patent Application No.: 77/139,889. Date of Application: November 24, 1977. (12) Toshiba Corp. Japanese Patent Application; Chem. Abstr. 1979, 96, 113316h. Patent Application No.: 80/57764. Date of Application: May 4, 1980. (13) Dalas, E.; Sakkopoulos, S.; Vitoratos, E.; Maroulis, G. J. Mater. Sci. 1993, 28, 5456. (14) Xiong, Y.; Xie, Y.; Du, G.; Tian, Y. J. Mater. Chem. 2002, 12, 98. (15) Xiong, Y.; Xie, Y.; Du, G.; Tian, X.; Qian, Y. J. Solid State Chem. 2002, 166, 336. (16) Patra, C. R.; Patra, S.; Gabashvili, A.; Mastai, Y.; Koltypin, Y.; Gedanken, A.; Palchik, V.; Slifkin, M. A. J. Nanosci. Nanotechnol. 2006, 6, 845. (17) Chen, X.; Zhang, Z.; Zhang, X.; Liu, J.; Qian, Y. Chem. Phys. Lett. 2005, 407, 482. (18) Yoshimura, M.; Kikugawa, S.; Somiya, S. J. Jpn. Soc. Powder Powder Metall. 1983, 30, 207. (19) Shackelford, J. F. Mater. Sci. Forum 1999, 293, 99. (20) Tenhuisen, K. S.; Brown, P. W.; Reed, C. S.; Allcock, H. R. J. Mater. Sci. Mater. Med. 1996, 7, 673. (21) Fujishiro, Y.; Yabuki, H.; Kawamura, K.; Sato, T.; Okuwaki, A. J. Chem. Technol. Biotechnol. 1993, 57, 349. (22) Yu, S.; Shu, L.; Qian, Y.; Yang, J.; Yang, L. Mater. Res. Bull. 1998, 33, 717.

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In2S3 Micropompons and Their Conversion to In2O3 Nanobipyramids

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