Template-Free, Surfactantless Route to Fabricate In(OH)3

Template-Free, Surfactantless Route to Fabricate In(OH)3 Monocrystalline Nanoarchitectures and Their Conversion to In2O3 ... E-mail: [email protected]...
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Template-Free, Surfactantless Route to Fabricate In(OH)3 Monocrystalline Nanoarchitectures and Their Conversion to In2O3 Hui Zhu,†,‡ Xiaolei Wang,†,‡ Fan Yang,† and Xiurong Yang*,† State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin, 130022, China, and Graduate School of the Chinese Academy of Sciences, Beijing, 100039, China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 3 950–956

ReceiVed September 6, 2007; ReVised Manuscript ReceiVed NoVember 22, 2007

ABSTRACT: In this work, a one-dimensional microrod-based three-dimensional flowerlike indium hydroxide (In(OH)3) structure was fabricated, without any templates or surfactants, using a well-known hydrothermal approach at a non-high temperature. In2O3 with similar morphology was formed by annealing In(OH)3 precursors and was characterized by Raman spectrum and photoluminescence (PL) spectrum in detail. The properties of the obtained In(OH)3 composites were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and thermogravimetric analysis. The effects of the experimental parameters such as the alkaline source selection, the concentration of urea, and the temperature on the morphology are discussed. These results indicate that the aligned superstructure originated from the oriented attachment of small particles. Introduction Recently, in order to endow nanomaterials with additional properties or potential applications, much effort has been focused on the synthesis of inorganic nanostructures with well-defined morphologies.1 Up to now, a variety of methods, including vapor–liquid–solid (VLS), solution-liquid–solid (SLS)3 phase catalytic growth, and anisotropic growth of crystalline nanomaterials using proper capping agents4 have been introduced to fabricate nanostructures with the desired architectures. Another synthetic strategy is based on some templates, such as carbon nanotubes,5 porous alumina,6 mesoporous materials,7 polymers,8 surfactants,9 and biomolecules.10 These templates could physically confine the growth of nanostructures within the framework of the model. However, this template-assisted method is also restricted by the template materials. Subsequent removal of the template is tedious, and this postprocessing often destroys the desired product structure. Therefore, the development of template-free synthesis is seriously considered.11 Unfortunately, some current synthetic routes are still limited in many ways. Nanostructure formation in a solution typically involves the fast nucleation of primary particles and the subsequent growth by two mechanisms: Ostwald ripening and aggregation.12,13 Ostwald ripening refers to the processes where larger particles grow at the expense of smaller ones, and it is generally believed to be a dominant path for crystal growth. However, recent studies on nanocrystal growth have indicated that such a model could not provide a reasonable explanation in many systems. For instance, a special aggregation-based growth model was proposed to introduce natural iron oxyhydroxide by Banfield et al.14 They found that the adjacent 2-3 nm particles could spontaneously aggregate and further grow into bulk crystals in a process called “oriented attachment” (OA). In addition to natural minerals, the OA mechanism has been observed to play a key role in the preparation of several kinds of nanomaterials including TiO2,15 ZnO,16 Cu2O,17 CdSe,18 ZnTe,19 PbSe,20 and so forth. * Corresponding author. Tel: +86-431-85682056. Fax: +86-431-85689278. E-mail: [email protected] † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.

Indium hydroxide (In(OH)3), an important semiconductor with a wide band gap (Eg ) 5.15 eV),21 has gained great attention because of its electronic and optical properties.21b Indium oxide (In2O3) is also a good n-type semiconductor with band gaps of 3.55–3.75 eV and has been widely used for nonlinear optics research,22 nanoelectronics,23a,b gas sensors,23c,27 and biosensors.23d Up to now, various In(OH)3 and In2O3 morphologies have been synthesized via different methods. For example, In2O3 nanobelts through thermal evaporation,24 tindoped In2O3 whiskers formed by the electron shower photo voltage decay (PVD) method,25 In2O3 nanowires by using the VLS technique,2d In2O3 nanowire arrays6b or nanorods26 induced by template-assisted growth, etc. Beyond these physical methods, there are also a few reports about the solution synthesis routes to prepare them with novel superstructures. For instance, Qian’s group has prepared In(OH)3 nanocubes28 via hydrothermal methods, quasi-monodisperse In2O3 nanocrystals could be obtained through an organic solution synthetic route,29 Yang et al. has successfully synthesized In(OH)3 nanorod bundles and sphere agglomerates by a microemulsion-mediated hydrothermal route,30a and metastable corundum type In2O3 nanotubes have been obtained in ethanol assisted by sodium dodecylbenzenesulfonate.30b However, those solution routes could not be accomplished without high temperature (140 °C or even above), and some surfactants or organic ligands are also required in such approaches. In 2004, a hydrothermal method using NaOH and NH3 · H2O as an alkaline source was reported to prepare In(OH)3 nanorods and micron cube-shaped structures.31 However, the reaction took place at a high temperature (200 °C) for a long time period (20 h), which was time-consuming and energy-intensive. Other reaction parameters were not discussed in their research. So it is quite necessary to develop a effective approach to synthesis In(OH)3 with novel structures at a non-high temperature. In this article, urea was chosen as the alkaline source to fabricate onedimensional (1D) rod-based three-dimensional (3D) flowerlike In(OH)3 frameworks at 130 °C without any templates or surfactants. Reaction parameters such as alkaline source, concentration of the alkali, and reaction temperature were also systematically investigated. An oriented attachment-based assembly mechanism was put forward to explain their formation.

10.1021/cg700850e CCC: $40.75  2008 American Chemical Society Published on Web 02/06/2008

Template-Free Route to In(OH)3 Nanoarchitectures

Crystal Growth & Design, Vol. 8, No. 3, 2008 951 speed of 4°/min in the range from 10° to 80°. Scanning electron microscopy (SEM) images were taken using Philips XL 30 and a JEOL JSM-6700F microscope. Transmission electron microscopy (TEM) measurements were carried out with JEM-2000 FX microscope operating at 160 kV accelerating voltage. Thermogravimetric analysis (TGA) of the as-synthesized sample was carried out on a Shimadzu TA-50 thermal analyzer at a heating rate of 10 K min-1 from room temperature to 700 °C in air. A FT-Raman spectrometer (Thermo Nicolet 960) equipped with an InGaAs detector and a Nd/VO4 laser at 1064 nm was used to characterize the In2O3 nanoframeworks. Photoluminescence (PL) emission measurements were performed using a LS55 luminescence spectrometer (Perkin-Elmer).

Results and Discussion Figure 1. X-ray diffraction pattern of the In(OH)3 sample (130 °C for 12 h).

In addition, Raman spectroscopy and photoluminescence (PL) spectroscopy were utilized to characterize the In2O3 obtained from the calcinations of In(OH)3 precursors. Experimental Section All chemicals used in this study were of analytical reagent grade and were purchased from Beijing Fine Chemical Company. In a typical synthesis of multipod flowerlike In(OH)3 nanoframeworks, 0.4 mmol of InCl3 · 4H2O and 4 mmol of urea were added to 24 mL distilled water under constant strong stirring for 30 min. Then the transparent solution was transferred to a 30 mL Telfon lined stainless steel autoclave. The autoclaves were heated at 130 °C for 12 h and cooled to room temperature naturally. The precipitation was collected, washed with distilled water several times, and then dried at 60 °C. To investigate the effect of the alkaline source on the morphology, urea was replaced by NaOH and NH3 · H2O as the alkaline source, while other conditions were fixed. The conversion of In(OH)3 constructions to In2O3 frameworks was carried out in an oven in air at 500 °C for 2 h. X-ray diffraction (XRD) patterns of the prepared samples were recorded on a Rigaku-Dmax 2500 diffractometer equipped with graphite monochromatized Cu KR (λ ) 0.15405 nm) radiation at a scanning

Composition of the Products. XRD was performed to investigate the crystal structure of the sample. As shown in Figure 1, according to the diffraction peak locations, the sample could be easily indexed to a cubic lattice [space group Im3(204)] of pure In(OH)3. The calculated lattice constant, a ) 0.7920 nm, is in good agreement with the standard JCPDS card (No. 85-1338, a ) 0.7979 nm). Moreover, we also found that the ratio between the maximum intensities of the (200) and the (220) diffraction lines was obviously larger than the value in JCPDF card (3.26 versus 1.1). Similar results were also observed for the ratios between the intensities of the (200) and other diffraction lines. These results indicated that our product was abundant in (200) facets and thus led to relatively greater accelerated growth along the directions. Influence of Urea on the Morphology. To investigate the influence of urea in the synthetic route, different quantities of urea were used in the preparation, and the products were characterized by SEM. As shown in Figure 2, the quantity of urea played an important role in determining the morphology of In(OH)3. When 0.8 mmol of urea was utilized in the reaction, the sample mainly showed cubical shapes with 2–3 µm width and smooth faces (Figure 2a). The flowerlike superstructure

Figure 2. FE-SEM images of the In(OH)3 products obtained in the presence of different quantities of urea: (a) 0.8 mmol, (b) 2 mmol, (c) 4 mmol, (d) 8 mmol.

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Figure 3. FE-SEM images of the In(OH)3 products prepared in different alkaline sources with the same concentration: (a) NaOH, (b) NH3 · H2O.

Figure 4. FE-SEM images of the In(OH)3 products prepared at different temperatures for 12 h: (a) 100 °C, (b) 140 °C, (c) 160 °C, (d) 180 °C.

began to appear when the concentration of urea was increased to 2.0 mmol (Figure 2b). When the urea concentration was increased to 4 mmol, the microrod-based flowerlike architectures developed further and became the dominant product (Figure 2c). All these rods exhibited a diameter of 450 nm and a length of 2–3 µm, and attached together to fabricate 3D multipod flowerlike architectures. If the concentration of urea was increased to 8 mmol, as shown in Figure 2d, the morphology was also flowerlike; however, the length of the rod subunit was shortened to 1 µm. Besides the influence of urea concentration, the differences among alkaline sources were also compared. When the alkaline source was supplied by NaOH and NH3 · H2O, respectively, instead of urea, the products obtained showed different morphologies. As shown in Figure 3a, when NaOH acted as the alkaline source, the sample exhibited a cubic structure, and their width was in a quite nonuniform size range. When NH3 · H2O was chosen as the alkaline source, a comparative uniform cubic structure with a width of 50–60 nm was found in the reaction.

From the above data, we can draw the conclusion that not only the alkaline source but also the concentration of urea played important roles in the fabrication of the multipod flowerlike superstructure. Urea is an important biological molecule that can decompose and release NH3 molecules in an indirect manner. In addition, it also can coordinate with metal ions such as Ag+, Co2+, Cu2+, Fe2+, Fe3+, and In3+ 32 to form complex structures. In the growth of In(OH)3 multipod flowerlike structures, urea plays at least two roles. First, due to the difficulty of release of NH3 from urea, a transparent solution was formed at room temperature. During the chemical reaction at a high temperature, OH- was supplied in a homogeneous gradual process, and the generation rate of In(OH)3 nanoparticles was slow in solution. The relative slow generation rate of In(OH)3 is favorable for the subsequent growth of 1D and 3D nanostructures along the determined direction. Second, as a Lewis base, urea can coordinate with In3+ to form indium-urea complexes, and keeping the free indium ion concentration low. Urea may also coordinate to the In(OH)3 crystal, hindering the

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Figure 5. (a) TEM image of the In(OH)3 samples prepared in 130 °C for 30 min; (b, c) TEM images and SAED patterns of the typical 1D rods and their crosslike structures; (d, e, f) FE-SEM images for rod, cross, hexadic pod shaped products, respectively.

Scheme 1. Schematic Illustration of the Morphological Change with Temperature and Concentrationa

a Ostwald ripening (at high temperature and in low quantity of urea) and growth by oriented attachment (at low temperature and in high quantity of urea).

growth of certain surfaces. With additional systematic studies, several of these mechanisms may be responsible for the urea shape control. Because of this, the multipod flowerlike In(OH)3 appeared at only a high concentration of urea. Influence of the Reaction Temperature on the Architecture. We also found that the morphology of the products was strongly affected by the reaction temperature. The reaction using 4 mmol of urea and 0.4 mmol of InCl3 was selected as a model. The obtained products prepared at different temperatures for 12 h are shown in Figure 4. When the reaction was carried

out at 100 °C (Figure 4a), both the 1D microrod structures and the 1D microrod-based 3D flowerlike structures were observed in the image, but most of them were irregular in shape and size. When the reaction temperature was higher than 130 °C, as shown in Figure 4b, besides the 1D and 1D based 3D structures, some irregular aggregates assembled from nanocubes began to appear. When the temperature was increased to 160 °C, the products exhibited two kinds of morphologies. One was bunchy 1D microrod assembled by wire-like subunits with a coarse face. The other has a cubic morphology with smooth face (Figure 4c). When the reaction took place at 180 °C, the cubic and cubebased architectures formed the main part of the sample. It has been proven that the cubical shape is consistent with the cubic crystal structure of In(OH)3.28 A high hydrothermal treatment temperature is beneficial for the fabrication of cubic shaped In(OH)3. In our study, a control experiment showed that the rod-based flowerlike In(OH)3 superstructure could only be obtained at a relative low temperature (T e 130 °C), which was favorable for the 1D directional assembly and growth. If the temperature was increased further (T > 140 °C), thermodynamic growth began to be the dominating growth step, and finally the cubical shaped In(OH)3 was formed. Growth Mechanism of In (OH)3 Multipod Structure. To clearly understand the growth mechanism of the In(OH)3 multipod-based flowerlike structure, the embryo stage and the detailed structures were systematically investigated by TEM,

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Figure 6. (a) TG/DTA curves of the In(OH)3 sample; (b) XRD pattern of the In2O3 products annealed from In(OH)3 superstructure; (c, d) FE-SEM images for the In2O3 samples.

Figure 7. (a) Raman spectra of the above In2O3 sample; (b) PL emission spectra (excited at 280 nm) of the above In2O3 sample.

SEM, and SAED characterization. From Figure 5a, it can be seen that at the early stage when the reaction occurred at 130 °C for 30 min, the products showed irregular floccules-like particles with a size of a few nanometers. It is intriguing to note that some nanoparticles began to assemble to form 1D rod structures in some regions. After the reaction was maintained for 1 h, 1D rod based morphology came to be the primary shape (data not shown). Typical TEM images for the 1D rods and their superstructures are shown in Figure 5b,c. Arc-shaped electron diffraction spots (shown in the insets) proved the monocrystalline nature and indicated that they were oriented assembled along the direction, which was consistent with XRD data. The magnified SEM images of typical In(OH)3 structures are shown in Figure 5d-f. It is shown that these 1D based structures had a rough surface and were composed of many small well-aligned crystal nanostructures. It is an interesting phenomenon that the nanostructures can self-assemble into a 1D rod-like hierarchical architecture along a certain direction in the absence of any specific additives or templates. In classic colloidal models, crystal growth in solution can be categorized as either kinetically or thermodynamically controlled and is subject to Ostwald ripening during the synthetic courses. Recently, a view of crystal growth, oriented attachment, has emerged from experiments on fine particles under hydrothermal

treatment.11,14–17 The impetus for the aggregation of particles in this case is still to reduce the total surface energy through the elimination of the higher surface energy faces. On the basis of the SEM and TEM results in our experiment, we ascribe this oriented attachment mechanism to the formation of 1D and 1D based flowerlike architectures. The anisotropic growth of the nanostructures in a template-free method is generally related to the different surface energies of the corresponding crystal planes. Those planes with a high surface energy have a strong tendency to capture monomers from the reaction solution and assemble together to reduce their surface energy. This leads to growth along those planes and eventually producing crystals with anisotropic morphology. Besides this, as a polar molecule, the long-range ordered dipole–dipole interactions between In(OH)3 monomers may also play an important role. For the formation mechanism of these novel morphologies (T e 130 °C), two steps should be involved: (1) nucleation and growth; (2) self-assembly by oriented attachment. The second step should be the key step for the construction of the nanostructures. It was noticed that the In(OH)3 nanoparticles can coalesce under hydrothermal conditions in a way we call oriented attachment. In such formed aggregates, the crystalline lattice planes may be almost perfectly aligned. In the current study, the impetus for the aggregation may be the reduction of the total surface

Template-Free Route to In(OH)3 Nanoarchitectures

energy. This growth mechanism includes the irreversible and specifically oriented self-assembly of primary nanoparticles and results in the formation of these superstructures. At such a suitable medium temperature (for example 130 °C), the gradually oriented aggregation took place, while at higher temperature (such as 180 °C), it is the Ostwald ripening that is beneficial for the formation of the cubical shaped In(OH)3. A schematic image of the proposed growth mechanism is shown in Scheme 1. Conversion to In2O3 Superstructure and Their Spectra. Like other metal hydroxides, indium hydroxide can dehydrate to form indium oxides with the same morphology upon heating. The conversion process of the In(OH)3 sample during calcination in air was studied via thermogravimetric/differential thermal analysis (TG/DTA), as shown in Figure 6a. The TGA curve can be mainly divided into two weight-loss steps. The first step between 30 and 210 °C was attributed to physical water evaporation from the In(OH)3 sample, and the weight loss of this step was 3%. The second step between 210 and 700 °C, was ascribed to chemical dehydration (2In(OH)3 f In2O3 + 3H2O), and the DTA curve indicated an endothermic process in this step whose weight loss was about 17%, consistent with the theoretical analysis (16.27%) within the error. Therefore, In2O3 architecture can be obtained by calcination of the as-made In(OH)3 precursors. Figure 6b shows the XRD pattern of the product obtained after annealing treatment at 500 °C in air for 2 h. All of the peaks can be indexed to the pure cubic phase of In2O3 (JCPDS No. 71-2194), indicating that the pure phase of In2O3 can be obtained by annealing the In(OH)3 precursors directly. The morphologies of the In2O3 samples were shown in Figure 6c,d. As a result, similar morphologies were observed as their In(OH)3 precursors. The Raman spectrum of flowerlike In2O3 shown in Figure 7a was characterized at room temperature. It is noticed that there are five obvious photon modes at 131.5, 305.5, 364.7, 494.8, and 627.4 cm-1 for the In2O3 samples. Among them, the vibration mode at 305.5 cm-1 is the strongest one. Besides the harmonic electron–phonon contribution several roles begin to play anharmonic interactions. These bands are due to In-O vibrations of InO6 structural units of the body-centered cubic (bcc) In2O3 structure.33 Since In2O3 is a promising transparent conducting oxide material, we also studied the PL characteristics of the products at room temperature (Figure 7b). A strong luminescence band is centered at 428 nm (blue emission), and two marked shoulders appeared at 451 and 486 nm. Generally, the blue luminescence emission mechanism of In2O3 is mainly attributed to the existence of oxygen vacancies.28,30a These oxygen vacancies normally act as deep defect donors in semiconductors and would induce the formation of new energy levels in the band gap. The blue emission thus results from the radioactive recombination of a photo excited hole and an electron occupying the oxygen vacancies. Conclusions In summary, 1D microrod-based 3D flowerlike In(OH)3 architectures have been successfully synthesized by a simple hydrothermal method at relatively low temperature without using any surfactants or templates. In this case, both the alkaline source selected and the concentration of urea played important roles in governing the morphologies of In(OH)3. In addition, it was intriguing to note that the reaction temperature was also a key parameter for the preparations. A possible oriented attachment mechanism for these products was proposed based on their shape evolutions. The In2O3 novel architectures have been obtained

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by annealing the In(OH)3 template precursor with the same morphology. The obtained In2O3 exhibits a strong blue emission, which allows it to be considered as a promising material for quantum electronics. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20475052) and the Project of Chinese Academy of Sciences (KJCX2. YW. H09).

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