Fabrication of Indium Sulfide Hollow Spheres and Their Conversion to

Aug 15, 2007 - Department of Chemistry, Huazhong University of Science and ... and Material Science of the State Ethnic Affairs Commission and Ministr...
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J. Phys. Chem. C 2007, 111, 12890-12897

ARTICLES Fabrication of Indium Sulfide Hollow Spheres and Their Conversion to Indium Oxide Hollow Spheres Consisting of Multipore Nanoflakes Pingtang Zhao,† Tao Huang,‡ and Kaixun Huang*,† Department of Chemistry, Huazhong UniVersity of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China, and Key Laboratory of Catalysis and Material Science of the State Ethnic Affairs Commission and Ministry of Education, Hubei ProVince, South-Central UniVersity for Nationalities, Wuhan 430074, People’s Republic of China ReceiVed: May 3, 2007; In Final Form: June 30, 2007

Indium sulfide hollow spheres consisting of nanoflakes with the thickness of about 20 nm were successfully prepared by dodecanethiol-assisted hydrothermal process at 180 °C for 12 h, employing indium chloride tetrahydrate and L-cysteine as precursors. The diameter of In2S3 hollow spheres is 3-5 µm. More interesting, some hollow spheres hold multipore shells. And indium sulfide hollow spheres can be converted to indium oxide hollow spheres consisting of multipore sheets when In2S3 hollow spheres were oxidized in atmosphere in Muffle at 600 °C for 6 h. The synthesized product was characterized by X-ray diffraction, energy-dispersive X-ray spectroscopy, field emission scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, electron diffraction, UV-vis, and fluorescence spectrophotometer. The effects of the reaction conditions on morphologies of In2S3 structures were investigated. The results show that temperature, sulfur source, and dodecanethiol play key roles on the formation of the morphology of In2S3 crystal. The optical properties were also investigated. And the formation mechanism of In2S3 hollow spheres is discussed.

1. Introduction The properties of semiconductor nanostructures depend not only on their chemical composition but also on their shape and size.1,2 And their properties also depend on their synthetic and processing methods.3,4 The exploration of various shapecontrolling synthetic methods and studies on their unusual properties will inevitably drive the progress in nanotechnology. It is well-known that In2S3, as a typical III-VI main-group chalcogenide, exists in three phases: a defective cubic structure R-In2S3, a defect spinel structure β-In2S3, and a higher temperature layered structure γ-In2S3. Almost all properties of indium sulfide rest with its defect structure. In2S3 holds a lot of promising properties, such as optoelectronic properties, optical properties, electronic properties, acoustic properties, semiconductor sensitization, and cancer diagnosis.5-10 And β-In2S3 shows a stable temperature up to 693 K, which seems to be an ideal candidate to substitute toxic CdS as the buffer layer in CuInSe2- and CuInS2-based solar cells.11,12 In addition, it has also been used for the preparation of green and red phosphors and the manufacture of picture tubes for color televisions.13 Due to the morphology-dependent properties of nanomaterials and good physical properties of In2S3,14 it should be fabricated into regular nanostructures in order to improve its properties. Up to now, various modern methods for the preparation of In2S3 nanoand microstructures have been reported, such as a sonochemical * Corresponding author. Telephone: +86-27-87543133. Fax: +86-2787543632. E-mail: [email protected]. † Huazhong University of Science and Technology. ‡ South-Central University for Nationalities.

route,15 an oxidization-sulfidation growth route,16 a metalorganic chemical vapor deposition approach,17 a hydrothermal process,18 microwave,19 and a solvothermal route.20,21 And monodisperse hexagonal β-In2S3 nanoplates of 0.76 nm thickness have been obtained in organic media at high temperature by using the arrested-precipitation method.22 Among the above methods of preparation of nanostructures, the hydrothermal method is advantageous as it is economical and has very easy processing steps along with a high degree of compositional control.23 In2O3 is an important transparent conductive oxide with a wide band gap of 3.55-3.75 eV closing to GaN, which holds promising applications in gas sensors, solar cells, flat-panel displays, and so forth.24-27 Various morphologies of In2O3, such as nanowires, nanowire arrays, nanocubes, octahedrons, hollow microspheres, and nanorod bundles and spheres, have been synthesized via electrodeposition and oxidizing method,28 chemical vapor deposition,28 and annealing as-prepared In(OH)3 precursors with different morphologies.30-32 Recently, our group focuses on fabricating semiconductor nanostructures.33,34 In this report, indium sulfide hollow spheres consisting of nanoflakes were successfully prepared by dodecanethiol-assisted hydrothermal process. And hollow spheres with multipore shell were obtained in this system, which is obviously different from hollow spheres prepared by the doublesulfur-source method.35 In2S3 hollow spheres can also be converted to In2O3 hollow spheres consisting of multipore sheets when In2S3 hollow spheres were oxidized in atmosphere in Muffle at 600 °C for 6 h. The optical properties of In2S3 and

10.1021/jp073390l CCC: $37.00 © 2007 American Chemical Society Published on Web 08/15/2007

Fabrication of In2S3 and In2O3 Hollow Spheres

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Figure 1. (a) XRD pattern and (b) EDS spectrum of as-prepared In2S3 hollow spheres at 180 °C for 12 h.

In2O3 hollow spheres were investigated. And the formation mechanism of In2S3 hollow spheres is discussed. 2. Experimental Section All the chemicals were of analytical grade and purchased from Shanghai Chemical Reagent Co. In a typical procedure, equivalent molar amounts (1.4 mmol) of indium chloride tetrahydrate (InCl3‚4H2O), L-cysteine, and 0.4 g of dodecanethiol were dissolved into 20, 30, and 20 mL of distilled water under ultrasonic treatment of 20 min, respectively. InCl3‚4H2O solution was added into dodecanethiol and given a 20 min ultrasonic treatment. Then the resulting solution was mixed with L-cysteine solution. The final mixed solution was sealed into an 80 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 12 h in an electric oven. The autoclave was cooled to room temperature naturally when the reaction time was finished. The product was collected by centrifugation at 9000 rpm for 10 min and washed with distilled water and absolute ethanol several times to remove the excessive reactants and byproducts, followed by drying in an oven at 60 °C for 6 h. The product was collected for characterization. In2O3 hollow spheres were prepared by thermally oxidizing In2S3 hollow spheres in atmosphere in Muffle at 600 °C for 6 h. The oxidation was carried out inside a boat crucible inserted in Muffle. Muffle is heated to 600 °C with a ramping rate of 10 °C min-1, and the temperature was held for 6 h under ambient pressure. After the desired period, the Muffle was gradually cooled to room temperature to yield an off-white product. Scanning electron microscopy (SEM) images were obtained on an FEI Sirion 200 field emission scanning electron microscope (FESEM), with energy-dispersive X-ray spectroscopy (EDS) attached to FESEM. The X-ray diffraction (XRD) pattern of the products was recorded by employing a PANalytical B. V. (Philips) χ’Pert PRO XRD with Cu KR radiation at a scanning rate of 0.02° s-1 in a 2θ range of 10-90°. A small amount of products was dispersed in ethanol by ultrasonic treatment for 10 min; then, one drop of the resulting solution was placed onto a carbon-coated copper grid and dried at room temperature for high-resolution transmission electron microscope (HRTEM) visualization. The electron diffraction (ED) patterns and HRTEM images were carried out on a JEM-2010FEF transmission electron microscope at an acceleration voltage of 200 kV. Transmission electron microscopy (TEM) images were observed on a Tecnai G220 transmission electron microscope at an acceleration voltage of 200 kV. UV-vis absorption spectra were recorded on a SHIMADZU UV-2550 UV-vis spectrom-

Figure 2. SEM images of In2S3 hollow spheres fabricated by hydrothermal process at 180 °C for 12 h, with concentrations of both InCl3‚4H2O and L-cysteine being 0.02 mol/L and dodecanethiol being 0.4 g in 70 mL solution: (a) low-magnification SEM image; (b) higher magnification of In2S3 hollow spheres; (c) high-magnification SEM image of the surface of an individual In2S3 hollow sphere with multipore; (d) high-magnification SEM image of an individual open In2S3 hollow sphere.

eter. Photoluminescence spectra were measured on a SHIMADZU RF-5301 PC fluorescence spectrophotometer. 3. Results and Discussion The phase and purity of the sample were confirmed by XRD pattern. A typical XRD pattern of as-synthesized In2S3 hollow spheres at 180 °C for 12 h is shown in Figure 1a. All the diffraction peaks can be indexed to cubic structured In2S3 with a lattice constant a ) 10.7712 Å, which is in good agreement with the standard data from JCPDS card No. 03-065-0459 (a )10.7740 Å). No impurity phase can be detected. The strong and sharp diffraction peaks indicate that the as-obtained product is well-crystalline. The relative intensity of the peaks corresponding to the (440)/(311) plane (99.55:100) varied distinctly from the standard data (57.8:100) of JCPDS card No. 03-0650459, implying that the orientational growth plane of nanoflakes of In2S3 hollow spheres connects with {110} planes. The spectrum of EDS from an In2S3 hollow sphere is shown in Figure 1b. The analysis of EDS demonstrates that the crystal consists of In and S. And the molar ratio of S to In is about 2.83:2. Figure 2 shows SEM images of In2S3 microcrystals obtained under the typical condition at 180 °C for 12 h; the concentrations of InCl3‚4H2O and L-cysteine are both 0.02 mol/L, and dodecanethiol is 0.4 g in 70 mL solution. A low-magnification

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Figure 4. (a) SEM image of as-prepared spherelike In2S3 crystals without dodecanethiol under the same conditions. The inset is one broken spherelike In2S3 crystal. (b) High-magnification SEM of the surface of one spherelike In2S3 crystal.

Figure 3. (a) TEM image of two In2S3 hollow spheres; (b) TEM image of one In2S3 hollow sphere with multi-cavity; (c) ED pattern of a whole In2S3 hollow sphere marked by 1 in a; (d) HRTEM image of the nanoflakes of one In2S3 hollow sphere marked by 1 in a. An inset is the FFT pattern of the selected area marked by a square in d.

image is shown in Figure 2a, clearly exhibiting that the assynthesized products are spherelike crystals with the diameter of 3-5 µm. And many spherelike crystals hold a multipore surface, which has not been observed for In2S3 nanostructures previously. The high-magnification SEM image further indicates that many In2S3 hollow spheres hold a multipore surface (Figure 2b). In2S3 hollow spheres with multipore consist of nanoflakes (Figure 2c). The pores are circularity with the diameter of about 200-500 nm. For clarifying the structures, the high-magnification image of an open sphere is shown in Figure 2d, which more clearly indicates that the product is a hollow sphere consisting of nanoflakes with the thickness of about 20 nm, with widths and lengths in the range of 200-300 nm. And the nanoflakes are clean and smooth. The structure of these In2S3 hollow spheres was further investigated by TEM. A bright contrast (dark/bright) between the boundary and the center of the spheres is shown in Figure 3a, confirming that these spheres are hollow spheres. Figure 3b shows the existence of the In2S3 hollow sphere with multicavity. More details of the nanostructures were investigated by HRTEM and ED. Figure 3c is the ED pattern of a whole In2S3 hollow sphere marked by 1 in Figure 3a. The annular ED pattern indicates that In2S3 hollow sphere is a polycrystal. The HRTEM image (Figure 3d) indicates that the lattice distance is 0.618 nm, almost in accordance with the {111} lattice distance (0.622 nm) of the cubic In2S3 crystal. The HRTEM image exhibits clear lattice planes, and the In2S3 nanoflake is a perfect single crystal. To study the effect of dodecanethiol, the experiment without dodecanethiol was carried out under the same conditions. Figure 4 is the SEM images of as-prepared In2S3 crystals without dodecanethiol under the same conditions. Figure 4a shows that the product is a spherelike crystal with the diameter of about 2-5 µm. And the spherelike crystal is composed of nanoflakes with the thickness of about 40 nm (Figure 4b). The width and length of nanoflakes are in the range of 400-500 nm. However, the SEM image of a broken spherelike In2S3 crystal shows that the spherelike crystals are not hollow (an inset in Figure 4a). The result indicates that dodecanethiol plays a key role in the formation of the In2S3 hollow sphere. On the basis of comparing Figure 4b with Figure 3d, dodecanethiol can decrease the thickness, width, and length of nanoflakes. Dodecanethiol can adsorb on nuclei, which inhibit crystal growing. Thus smaller

nanoflakes were formed. When the amount of dodecanethiol is decreased to 0.1 or 0.2 g under the same conditions, the number of hollow spheres with multipore surface is decreased (Supporting information Figure 1a,b). However no distinct difference was observed when the amount of dodecanethiol was increased to 0.5 or 0.7 g under the same conditions (Supporting information Figure 1c,d). When the experiment without L-cysteine was carried out under the same conditions, no In2S3 crystal was obtained. So the sulfur element of In2S3 comes from the L-cysteine, not the dodecanethiol under our system. Figure 5a with an inset and Figure 5b show that In2S3 hollow spheres consisting of nanosheets were also gotten when L-cysteine was replaced by thioacetamide. The shell of the In2S3 hollow sphere is looser (Figure 5b). That can be attributed to the S2- release of thioacetamide being faster than that of L-cysteine. That is unfavorable to formation of a compact shell of a hollow sphere. And no multipore In2S3 hollow spheres were observed, implying L-cysteine plays a key role in the formation of the multipore In2S3 hollow sphere. The molar ratio of L-cysteine to indium sulfide tetrahydrate can affect the thickness of nanoflakes of indium sulfide hollow spheres. When the molar ratio of L-cysteine to indium sulfide tetrahydrate (0.02 mol/L) is decreased to 1:2 from 3:2, the thickness of the nanoflakes of the hollow spheres is increased to about 40 nm (Figure 5d). And no hollow sphere with multipore surface was observed (Figure 5c). The nuclei size and growth speed of the crystal increase when the amount of L-cysteine complexing agent is decreased. So the thickness of nanoflakes is increased. The thickness of nanoflakes is decreased to about 10 nm when the molar ratio of L-cysteine to indium sulfide tetrahydrate (0.02 mol/L) is increased to 5:2 from 3:2 (Figure 5f). It is attributed to the nuclei size and growth speed of crystal being reduced when the amount of L-cysteine complexing agent is increased. In the meantime the number of hollow spheres with multipores is markedly decreased (Figure 5e). Parts a and b of Figure 6 are the SEM images of as-prepared product when the reaction time is reduced to 1.5 h under the same other condition. Figure 6a exhibits that the product is a spherelike crystal with the diameter of about 1.5-3 µm. Some spherelike crystals are an aggregation of smaller spherelike crystals. And some spherelike crystals hold half-sphere concaves. Figure 6b indicates that the surface of spherelike crystals displays a netted structure. The EDS of the crystals shows the existence of C and O elements (Figure 6c). And the molar ratio of In to S is 2:3.162. The S element of spherelike crystals is over the stoichiometric value of In2S3. These results indicate the existence of cysteine and dodecanethiol in spherelike crystals when the reaction time is 1.5 h. Figure 6d indicates that the major product is spherelike crystals with multipores when the reaction time is 3 h, which shows that spherelike crystals with multipores come from spherelike crystals without pores. And the surface of spherelike crystals with multipores is still the

Fabrication of In2S3 and In2O3 Hollow Spheres

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Figure 5. (a) SEM image of as-prepared In2S3 hollow spheres using thioacetamide as the sulfur source in place of L-cysteine. The inset is the SEM image of an as-prepared one open In2S3 hollow sphere. (b) High-magnification SEM image of the surface of one open In2S3 hollow sphere in a. (c, d) SEM images of as-prepared In2S3 hollow spheres when the molar ratio of L-cysteine to indium chloride tetrahydrate (0.02 mol/L) is 1:2. (e) SEM image of as-prepared In2S3 hollow spheres when the molar ratio of L-cysteine to indium chloride tetrahydrate (0.02 mol/L) is 5:2. The inset is an open hollow sphere. (f) High-magnification SEM image of the surface of an open hollow sphere in the inset in e.

Figure 6. (a) SEM image of as-prepared product when the reaction time is reduced to 1.5 h under the same conditions. (b) High-magnification SEM image of as-prepared one spherelike crystal with half-sphere concave for 1.5 h. (c) EDS of one spherelike crystal in b. (d) SEM image of as-prepared product when the reaction time is 3 h under the same conditions. The inset is the high-magnification SEM image of the surface of one as-prepared spherelike crystal. (e) SEM image of as-prepared product when the reaction time is 5 h under the same conditions. (f) High-magnification SEM image of the surface of one as-prepared spherelike crystal with multipore surface in e.

netted structure (an inset in Figure 6d). If the reaction time was further prolonged to 5 h (Figure 6e,f), the spherelike crystal consisting of smaller nanoflakes was obtained. And many spherelike crystals still hold the multipore surface. The FT-IR spectra of pure L-cysteine and the product prepared for different reaction time are shown in Figure 7. In the FT-IR spectrum of the product obtained after 1 h, the peaks at 3484, 2917.5, and 1097.1 cm-1 are the vibration modes of N-H, C-H, and C-N bonds, respectively. The presence of a carboxylate group, νas(COO-) (asymmetric stretching) at 1634.5 cm-1 and νs(COO-) (symmetric stretching) at 1465.3 cm-1, is observed.36 The result indicates that as-obtained product for 1.5 h contains some organic compound, which is consistent with the EDS result (Figure 6c). And the strength of all peaks is weakened when the reaction time is prolonged to 3 h. The characteristic signal of -SH at about 2521 cm-1 disappeared after 1.5 h (Figure 7b) compared with the IR spectrum of pure ,-cysteine (Figure 7a), indicating that the S component of the product originates from the -SH group of ,-cysteine.

On the basis of the above results, we propose the following growth mechanism for the In2S3 hollow sphere. At first, dodecanethiol single-layer micelles or/and multilayer micelles are formed under the assistance of an ultrasonic wave. -SH can coordinate with inorganic cations.37 Subsequently In3+ ion is absorbed on the surface of dodecanethiol micelles through -SH coordination. And there are several functional groups in the ,-cysteine molecule, such as -NH2, -COOH, and -SH, which have a strong tendency to coordinate with inorganic cations. Some metal sulfide nanostructures have been prepared in the presence of cysteine, where cysteine can act not only as a complexing agent but also as a sulfur source and structuredirecting molecule.37-39 Thus it is rational to speculate that indium ions of the surface of dodecanethiol micelle can also coordinate with cysteine to form initial precursor complexes when L-cysteine solution is mixed with the solution of dodecanethiol and indium chloride in this system. Since the Lcysteine molecule has several functional groups, one L-cysteine molecule may coordinate with two indium cations coming from

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Figure 7. (a) FT-IR spectrum of pure L-cysteine; (b) FT-IR spectra of as-prepared products for different reaction times.

different surfaces of dodecanethiol micelles. Different dodecanethiol micelles can be bridged together under the assistance of L-cysteine. The existence of the aggregation of smaller spherelike crystals (Figure 6a), spherelike crystals with halfsphere concave (Figure 6b), and In2S3 hollow sphere with multicavity (Figure 3b) all demonstrate that different dodecanethiol micelles can be bridged together. The growth rate of the connected sites of bridged dodecanethiol micelles is inhibited due to space obstructions. Upon the high temperature and pressure in the hydrothermal process, the S-C bond of the cysteine-indium complexes is broken and In2S3 nuclei are formed. Nanoflakes are further formed on the surface of micelle through the Ostwald ripening process. Finally, the hollow spheres crystals consisting of nanoflakes are formed. It is wellknown that dodecanethiol is flammable. The forming gases of dodecanethiol inflammation swell and lead to spurt out from the thinner sites or gaps of the spherelike crystals under the high temperature and pressure of the hydrothermal process. In the end, open or multipore In2S3 hollow spheres can be obtained. Comparing Figure 6a with Figure 6b, it is also speculated that some pores were formed by the blow up of spherelike crystals and some pores were gotten by the Ostwald ripening process. The formation process of the In2S3 hollow sphere and multipore In2S3 hollow spheres is sketched in Figure 8. When L-cysteine was replaced by thioacetamide, S2- release of thioacetamide is faster than that of L-cysteine. That is favorable to form the loose shell with more gaps of In2S3 hollow sphere (Figure 5b). The forming gases of dodecanethiol inflammation can release gradually. The inner pressure of In2S3 hollow sphere is not enough to blow up. So it is difficult to form multipore In2S3 hollow spheres. When the amount of dodecanethiol is reduced, the lower concentration of the micelle is unfavorable to the bridging of different micelles. So the number of spherelike crystals with multipores is reduced. When the amount of L-cysteine is reduced, the growth speed of the crystal is increased. So the pores can be quickly covered. No pore is obtained (Figure 5c). The influence of reaction temperature on the formation of In2S3 hollow spheres was also investigated, at 100, 120, and 220 °C. When the temperature is decreased to 120 °C, the SEM images (Figure 9a-c) show that the In2S3 crystals are also hollow spheres consisting of smaller nanoflakes. The average diameter of the hollow sphere is about 3 µm, and the number of In2S3 multipore hollow spheres is decreased. It is attributed to low temperature leading to low pressure, which is unfavorable to the blowing up of hollow spheres. In the meantime, the growth rate of In2S3 crystals decreases. So, smaller nanoflakes, bigger pore size, and smaller diameter of hollow spheres resulted. When the temperature was further decreased to 100 °C (Figure 9d,e), the netted spherelike In2S3 crystals with the average diameter of about 2.5 µm and some spherelike crystals consisting of particles were obtained. No open sphere and multipore sphere were observed. The temperature and

Zhao et al. pressure are too low to blow up spherelike In2S3 crystals at 100 °C. When the temperature is increased to 220 °C (Figure 9g,h), In2S3 hollow spheres consisting of nanoflakes also result, but the number of multipore In2S3 hollow spheres is also decreased. And the diameter of some hollow spheres is up to 8 µm, because the growth speed is raised under higher reaction temperature. The higher temperature leads to the higher diffusion rate of micelles, which is also unfavorable to bridge between different micelles. So, the number of multipore In2S3 hollow spheres is decreased obviously. These results exhibit reaction temperature playing a key role in the formation of multipore hollow spheres. When the as-prepared In2S3 hollow spheres at 120 °C were oxidized in atmosphere in Muffle at 600 °C for 6 h, the product still maintained hollow spheres with multipores (Figure 10a,b). However, this kind of hollow sphere was composed of multipore nanosheets. And multipore nanosheets consist of nanoparticles (Figure 10c). Figure 10d is the XRD pattern of the oxidized product. All the diffraction peaks can be indexed to bodycentered cubic (bcc) structured In2O3 with a lattice constant a ) 10.132 Å, which is nearly in agreement with the standard data from JCPDS card No. 03-065-3170 (a )10.140 Å). No impurity phase can be detected. The strong and sharp diffraction peaks indicate that the as-obtained products are well-crystalline. In the formation process of In2O3 hollow spheres, the elemental substitution may be involved in the transformation of indium sulfide to phase-pure indium oxide at the exhaustion of sulfur atoms. The evaporation of sulfur dioxide molecules results in the destruction of flakelike structure and formation of small indium oxide nanoparticles.23 These kinds of as-prepared In2O3 hollow spheres by oxidized In2S3 hollow spheres have not reported previously. Figure 11a is the TEM image of oxidized hollow spheres by atmosphere in Muffle at 600 °C for 6 h. A visible dark-bright contrast between the boundary and the center of the spheres also proves that these spheres are hollow spheres. An inset in Figure 11a is the ED pattern of one In2O3 hollow sphere marked by an arrow in Figure 11a. The annular ED pattern indicates that the In2O3 hollow sphere is polycrystalline. The highmagnification TEM image (Figure 11b) of the shell of one In2O3 hollow sphere shows that the nanosheets are multipore, which is consistent with the result of the SEM image. The HRTEM image (Figure 11c) of the nanosheet indicates that the lattice distance of 0.4130 nm is almost in accordance with the {211} lattice distance (0.4196 nm) of a body-centered cubic In2O3 crystal. And it shows that the frame of the multipore nanosheet is made of nanoparticles. The UV-vis absorption spectra of as-prepared In2S3 and In2O3 hollow spheres are shown in Figure 12a. Figure 12a(1) is the UV-vis absorption spectrum of the as-prepared In2S3 hollow spheres, showing a peak at 251 nm, which obviously reveals their blue shift compared with the reported data of 620.6 nm for In2S3 bulk materials.40 In2S3 has a large Bohr exciton radius of 33.8 nm. And, in our product, the In2S3 hollow spheres consist of nanoflakes with the thickness of about 20 nm, which is smaller than the Bohr exciton radius of 33.8 nm. Consequently, the absorption for the products is obviously shifted toward shorter wavelengths, which is due to the quantum confinement effect. For In2O3 hollow spheres, the UV-vis absorption spectrum (Figure 12a(2)) has a broad absorption peak between 239 and 371 nm, which is around the reported data 338 nm for In2O3 bulk materials.41 For In2O3 hollow spheres consisting of multipore nanosheets, because the size of multipore

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Figure 8. Formation process of In2S3 hollow spheres and multipore In2S3 hollow spheres.

Figure 9. SEM images of as-prepared In2S3 microcrystals at different reaction temperatures under the same conditions: (a) low- and (b) highmagnification SEM image of as-prepared In2S3 hollow spheres at 120 °C; (c) high-magnification SEM image of the surface of one as-prepared In2S3 hollow sphere at 120 °C; (d) low-magnification SEM image of as-prepared In2S3 crystals at 100 °C; (e) high-magnification SEM image of as-prepared one netted spherelike In2S3 crystal at 100 °C; (f) high-magnification SEM image of the surface of as-prepared one netted spherelike In2S3 crystal at 100 °C; (g) low-magnification SEM image of as-prepared In2S3 hollow spheres at 220 °C; (h) high-magnification SEM image of as-prepared one In2S3 hollow sphere with multipores at 220 °C.

sheets is bigger than the Bohr exciton radius of In2O3 (∼2.14 nm).42 The absorption does not show any size confinement effect. Compared with the nonluminescent property of bulk In2S3,43 the hollow spheres consisting of nanoflakes have a property of luminescence at room temperature. Figure 12b is the photolu-

minescence spectrum of as-prepared In2S3 hollow spheres at 180 °C for 12 h, which has three peaks at about 428, 403, and a weak peak at 381 nm with the excitated wave of 356 nm. Datta et al. also reported the emission peak at 472 nm was accompanied by two shoulders at 484 and 495 nm from the In2S3 micropompons.23 They believe that the emissions can be

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Figure 10. (a) SEM image of oxidized hollow spheres in atmosphere in Muffle at 600 °C for 6 h; (b) SEM image of an individual oxidized hollow sphere; (c) high-magnification SEM image of the surface of an individual oxidized hollow sphere in b; (d) XRD pattern of oxidized hollow spheres.

Figure 11. (a) TEM image of the In2O3 hollow sphere; (b) highmagnification TEM image of the shell of one In2O3 hollow sphere; (c) HRTEM image of a multipore nanosheet of In2O3 hollow sphere. The inset is the FFT pattern of the selected area marked by a square in c.

attributed to the presence of several deep trap states or defects in the structure. On the basis of the photoluminescence spectrum of as-prepared In2S3 hollow spheres at 180 °C for 24 h (Figure 12c), the peaks at 381 and 403 nm are probably defect emission peaks. When the reaction time is prolonged, the crystallized degree becomes perfect gradually and the defect photoluminescence peak is weakened or disappears. Another peak at 428 nm is probably due to the presence of a deep trap state. On the other hand, the photoluminescence spectrum of In2O3 hollow spheres is at 425 nm with the excitated wave of 330 nm (Figure 12d). It is known that the perfect crystals of bulk In2O3 cannot emit light at room temperature.44,45 In previous studies, the photoluminescence spectra around 416 and 439 nm from In2O3 nanorod bundles and spheres,32 at 450 nm from In2O3 nanocubes,30 at 480 and 520 nm from In2O3 nanoparticles,46 and at 378, 398, and 420 nm from In2O3 nanofibers47 have been reported. Those are mainly attributed to the oxygen vacancies in the In2O3 nano- or microstructures during the annealing process. The photoluminescence peak at 425 nm of In2O3 hollow spheres with multipore sheets is basically in the same spectral region as those in the above reports, which also should be attributed to the oxygen vacancies.

Figure 12. (a) UV-vis absorption spectra of as-prepared In2S3 and In2O3 hollow spheres: (1) UV-vis absorption spectrum of as-prepared In2S3 hollow spheres; (2) UV-vis absorption spectrum of as-prepared In2O3 hollow spheres. (b) Photoluminescence spectrum of as-prepared In2S3 hollow spheres at 180 °C for 12 h. (c) Photoluminescence spectrum of as-prepared In2S3 hollow spheres at 180 °C for 24 h. (d) Photoluminescence spectrum of as-prepared In2O3 hollow spheres.

4. Conclusion In summary, indium sulfide hollow spheres consisting of nanoflakes were successfully fabricated by dodecanethiolassisted hydrothermal process, in which many hollow spheres hold a multipore surface. The temperature, sulfur source, dodecanethiol, and the molar ratio of precursors play key roles on the final morphologies formation of In2S3 crystals. Indium sulfide hollow spheres could be converted to indium oxide hollow spheres consisting of multipore sheets when In2S3 hollow spheres were oxidized in atmosphere in Muffle at 600 °C for 6 h. The UV-vis absorption spectrum of In2S3 hollow spheres shows an absorption sharp peak centered at 251 nm, which obviously reveals the blue shift compared with the reported data of 620.6 nm for In2S3 bulk materials39 due to the quantum confinement effect. For In2O3 hollow spheres, the UV-vis absorption spectrum has a broad absorption peak between 239 and 371 nm. The absorption does not show any size confinement

Fabrication of In2S3 and In2O3 Hollow Spheres effect due to the size of the multipore sheet being bigger than the Bohr exciton radius of In2O3 (∼2.14 nm). The fluorescence spectrum of as-prepared indium sulfide hollow spheres at 180 °C for 12 h displays emission peaks at 403, 428, and a weak peak 381 nm. However, when the reaction time is prolonged to 24 h, the fluorescence spectrum of as-prepared indium sulfide hollow spheres shows an emission peak at 428 nm with a weak shoulder at 403 nm. The fluorescence spectrum of indium oxide hollow spheres consisting of multipore sheets has an emission peak at 425 nm. It is speculated that the formation mechanism of In2S3 hollow spheres may be involved with dodecanethiol micelles as template and the Ostwald ripening process. These In2S3 and In2O3 hollow spheres maybe have potential application in nanodevices, such as solar cells, gas sensors, and flat-panel displays, etc. Acknowledgment. We thank the faculty from the Analysis and Test Center of Huazhong University of Science and Technology and Professor Zhenya Sun from the Center for Test and Analysis of Wuhan University of technology for the technical assistance on characterization. This research was supportedbytheMOST973program(ProjectNo.2006CB705606a) and by the Open Foundation of Hubei Key Laboratory for Catalysis and Material Science. Supporting Information Available: SEM images of asprepared products at different amounts of dodecanethiol under the same conditions, with (a) 0.1, (b) 0.2, (c) 0.5, and (d) 0.7 g of dodecanethiol. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353. (2) Xiao, Z. L.; Han, C. Y.; Kwok, W. K.; Wang, H. H.; Welp, U.; Wang, J.; Crabtree, G. W. J. Am. Chem. Soc. 2004, 126, 2316. (3) Williams, F.; Nozik, A. J. Nature 1984, 312, 21. (4) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; Elsayed, M. A. Science 1996, 272, 1924. (5) Kim, W. T.; Kim, C. D. J. Appl. Phys. 1986, 60, 2631. (6) Kamoun, N.; Belgacem, S.; Amlouk, M.; Bennaceur, R.; Bonnet, J.; Touhari, F.; Nouaoura, M.; Lassabatere, L. J. Appl. Phys. 2001, 89, 2766. (7) Nomura, R.; Inazawa, S.; Kanaya, K.; Matsuda, H. Appl. Organomet. Chem. 1989, 3, 195. (8) Choe, S. H.; Bang, T. H.; Kim, N. O.; Kim, H. G.; Lee, C. I.; Jin, M. S.; Oh, S. K.; Kim, W. T. Semicond. Sci. Technol. 2001, 16, 98. (9) Amlouk, M.; Ben Said, M. A.; Kamoun, N.; Belgacem, S.; Brunet, N.; Barjon, D. Jpn. J. Appl. Phys. 1999, 38, 26. (10) Nagesha, D. K.; Liang, X.; Mamedov, A. A.; Gainer, G.; Eastman, M. A.; Giersig, M.; Song, J.-J.; Ni, T.; Kotov, N. A. J. Phys. Chem. B 2001, 105, 7490. (11) Kumaresan, R.; Ichimura, M.; Sato, N.; Ramasamy, P. Mater. Sci. Eng. B 2002, 96, 37. (12) Mane, R. S.; Lokhande, C. D. Mater. Chem. Phys. 2002, 78 15. (13) Yu, S.; Shu, L.; Qian, Y.; Xie, Y.; Yang, J.; Yang, L. Mater. Res. Bull. 1998, 33, 717.

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