Article pubs.acs.org/crystal
Perfect, Sectorial, Branched Sb2O3 Microstructures Consisting of Prolate Microtubes: Controllable Seeded Growth Synthesis and Optical Properties Limin Song,*,† Shujuan Zhang,*,‡,# and Qingwu Wei† †
College of Environment and Chemical Engineering & State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin Polytechnic University, Tianjin 300160, P. R. China ‡ College of Science, Tianjin University of Science & Technology, Tianjin, 300457, P. R. China ABSTRACT: Perfect, sectorial, and branched Sb2O3 microstructures (st-Sb2O3) were synthesized using a hydrothermal method based on a seeded growth procedure. The structure and composition of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), UV−vis diffusive reflectance spectroscopy (UV−vis), and photoluminescence (PL) spectroscopy. SEM images show that the st-Sb2O3 microstructures consist of several prolate microtubes, and the perimeter of the sectors are around 10−40 μm. The prolate microtubes are approximately 30 μm long and 10 μm wide. The growth of st-Sb2O3 may be controlled by changing the different experimental conditions. The introduction of the seed during the growth process plays an important role in the formation of the microstructures. In addition, the PL spectrum of the st-Sb2O3 microstructures shows a strong fluorescence. The formation mechanism of the st-Sb2O3 microstructures is discussed in detail.
1. INTRODUCTION Sb2O3 is a transparent semiconducting material with an indirect band gap of 3.3 eV. It has several applications in catalysis, fire retardants, glasses, and sensors, and serves as the anode material for Li-ion batteries.1 Morphology has an important effect on the chemistry and properties of micro- and nanomaterials.2 A number of reports on the synthesis of Sb2O3 with different morphologies, such as micro- and nanoparticles, hollow spheres, wires, belts, rods, and so on,3−11 are available. Extended and oriented micro- and nanostructures are desirable for a number of applications such as in the microelectronic, chemical, optical, and biological fields.12−15 In the present paper, we successfully prepared perfect, sectorial, branched Sb2O3 microstructures consisting of prolate microtubes. In the past decades, a number of oriented micro- and nanostructures have been prepared through various routes, including high-temperature vacuum and electrochemical deposition, hydrothermal techniques, and so on. Although oriented micro- and nanostructures have attracted considerable attention, the direct assembly of complex structures with a controlled crystalline morphology, orientation, and surface architecture remains a serious challenge. In the present report, crystal seeds were used as directing agents in the synthesis of stSb2O3 microstructures in the solution phase. Experimental results show that the crystal seeds play a crucial role in the formation of st-Sb2O3 microstructures. © 2011 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Chemicals. All of the chemicals were of analytical grade and were used without further purification. Distilled water was used throughout. 2.2. Synthesis of st-Sb2O3 Microstructures. The preparation process is divided into three steps: (1) Treatment of glass slides. In order to remove the impurities on the surface, the glass slide was soaked in H2O2 (30 wt %) and H2SO4 (98 wt %) solutions for 1 h. The glass slide was then washed with deionized water to neutral pH and dried in air. (2) Introduction of crystal seeds on the glass slide. SbCl3 (0.001 mol, 0.23 g) was dissolved in 20 mL of anhydrous ethanol under intense stirring. The glass slide was soaked in the solution for 10 min, and then pulled out at a rate of 2 cm/min. The procedure was repeated three times. After drying the glass slide in air, it was calcined for 1 h in a furnace at 300 °C under static air. (3) Preparation of st-Sb2O3 microstructures. SbCl3 (0.0025 mol, 0.57 g) and C6H12N4 (0.0025 mol, 0.35 g) were each dissolved in 50 mL of deionized water, and the two solutions were mixed under continued stirring. The glass slide containing the crystal seeds was placed in the mixed solution to obtain the desired samples. The solution was heated at 70 °C for 12 h. After the reaction, the resulting solution was cooled naturally to room temperature, and the glass slide was washed several times with absolute ethanol and distilled water and dried in air at room temperature. 2.3. Characterization of st-Sb2O3 Microstructures. The asprepared samples were characterized by X-ray powder diffraction Received: August 23, 2011 Revised: December 3, 2011 Published: December 21, 2011 764
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Figure 1. FESEM (a−g) and TEM (h) images of typical st-Sb2O3 microstructures at 70 °C for 12 h (0.025 mol/L SbCl3 and 0.025 mol/L C6H12N4). (XRD) analysis on a Rigaku X-ray diffractometer with CuKα radiation (1.5406 Å). The X-ray tube was operated at 45 kV and 25 mA. Samples were scanned from 10° up to 60°. The morphologies and sizes of the as-obtained products were observed by transmission
electron microscopy (TEM, Hitachi H-7650) and field-emission scanning electron microscopy (FESEM, Hitachi S-4800). The groups on the samples were studied by infrared absorption spectroscopy using a Bruker Tensor37 Fourier transform infrared spectrometer (FT-IR). 765
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Figure 2. X-ray diffraction (A), EDS (B), and XPS (C, D) patterns of typical st-Sb2O3 microstructure at 70 °C for 12 h (0.025 mol/L SbCl3 and 0.025 mol/L C6H12N4). The UV−vis diffusive reflectance spectrum was performed in a HP8453 spectrophotometer at room temperature using barium sulfate as the reference. The ability of emission excitation of st-Sb2O3 microsturctures was investigated using Cary Eclipse photoluminescence (PL) analyzer. The binding energy (BE) was measured by an Xray photoelectron spectrometer (XPS, Pekin-Elmer PHI5300). In the XPS analysis, the calibration of BE is the standard peak of adventitious carbon (C1s). The BE has been calibrated according to the standard peak of carbon (C1s) in the manuscript.
ends, further confirming the characteristic shape of the st-Sb2O3 microstructure. The prolate microtubes in the sectorial arrays are approximately 25−30 μm in length and 3−5 μm in width, as estimated from Figure 1c,f. The thickness of the prolate microtubes ranges between 0.5 and 1 μm, as estimated from Figure 1g. The surfaces of the prolate microtubes are rough, their ends are irregular, and several small pits are present. A typical TEM image of a st-Sb2O3 microstructure is shown in Figure 1h. The morphology of the samples is consistent with the images from the SEM. The phase composition of the as-synthesized st-Sb2O3 microstructures were analyzed using XRD. In Figure 2A, the main diffraction peaks are attributed to the orthorhombic phase Sb2O3 with lattice constants of a = 4.93 Å, b = 12.48 Å, and c = 5.43 Å, in accordance with the standard (JCPDF Card No. 030530). The diffraction intensity in Figure 2A is very strong, indicating good crystallization for the obtained st-Sb2O3 microstructures. However, very weak peaks still remain at 27.68°, 32.08°, and 46.00°, which are attributed to cubic Sb2O3 (JCPDF No. 05-0534). The energy dispersive spectrum (EDS) of the st-Sb2O3 microstructures confirms the presence of Sb and O (Figure 2B). Elemental C, Si, Al, and Au in the EDS of the sample are assigned to the holder. The atomic percent ratio of O and Sb in the as-prepared sample is 1.07:1, which indicates that a large number of oxygen vacancy defects exist on the surface of the as-obtained Sb2O3.
3. RESULTS AND DISCUSSION 3.1. Characterization of st-Sb2O3 Microstructures. Figure 1a−g depicts the low- and high-magnification scanning electron microscopy (SEM) images of a typical st-Sb2O3 microstructure. The mixture containing SbCl3 (0.025 mol/L) and C6H12N4 (0.025 mol/L) was treated at 70 °C for 12 h. In Figure 1a−c, a large number of well-defined branched st-Sb2O3 microstructures exhibiting a perfect sectorial morphology are evenly distributed on the glass slide, implying that the microstructures can be prepared using the current method. The st-Sb2O3 microstructures observed in Figure 1d−g consist of orderly and sectorial arrays of prolate microtubes. The sectorial arrays consist of various numbers of prolate microtubes with an open end at one side. The number of sectorial arrays varies from several ones to tens, and the perimeters are approximately 10−40 μm. As shown in Figure 1a−c, the microtubes in the arrays radiate from the center to the other 766
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Figure 3. FESEM images of the as-prepared products obtained with different concentrations, temperatures as well as without crystal seeds for 24 h. (a) 0.0125 mol/L SbCl3 and 0.0125 mol C6H12N4, (b) 0.05 mol/L SbCl3 and 0.05 mol/L C6H12N4, (c) 50 °C, (d) 90 °C, (e) without crystal seeds.
Figure 2C,D show the X-ray photoelectron spectra of the asprepared st-Sb2O3 microstructures. The Sb3d3/2 and Sb3d5/2 binding energies (539.35 and 529.99 eV, respectively) result from the charge-transfer screening and are solely attributed to the presence of Sb3+ cations from Sb2O3 (Figure 2C). The peak at 530.00 eV corresponds to the O1s binding energy (Figure 2D). The result indicates the presence of elemental Sb and O. The ratio of the atomic concentrations calculated from the O1s and Sb3d peak areas slightly deviates from the theoretical value
of 1.5 because of the presence of several oxygen defects on the surface of the st-Sb2O3 microstructures. The solution concentration and reaction temperature are key factors in the formation and growth of the st-Sb 2 O 3 microstructures in the current experiments. Figure 3a shows the SEM images of the as-synthesized Sb2O3 product in a 100 mL aqueous solution containing 0.0125 mol/L SbCl3 and 0.0125 mol/L C6H12N4, with the other conditions unchanged. Sb2O3 consists of accumulated microbelts. Many of the branched microstructures were formed when the concen767
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Figure 4. FESEM images of the as-prepared products obtained at 180 °C for different reaction times (0.025 mol/L SbCl3 and 0.025 mol/L C6H12N4) (a) 1 h, (b) 3 h, (c) 6 h, and (d) 9 h.
trations of SbCl3 and C6H12N4 were increased to 0.05 mol/L (Figure 3b). The lower concentration produced a small quantity of the crystal seeds. The Sb2O3 crystal seeds grew along the fastest growth orientation to form microbelts. The higher concentration resulted in a great quantity of the crystal seeds. The little crystal seeds dissolved gradually and moved to the surface of large particles to accumulate and form layered as well as claw-like microstructures along the fastest growth orientation because Gibbs free energy of the crystal seeds is higher than those of large particles. When the concentrations remained the same, the reaction temperature changed to 50 or 90 °C, and the morphology of Sb2O3 changed to long-branched belts or microparticles, respectively (Figure 3c,d). The particles were able to grow easily at higher temperature. Therefore, the microparticles with wide diameter were obtained at 90 °C. However, the slow and different growth makes particles directly become microbelts with narrow width at lower temperature. Therefore, the particles showed different shapes at distinct temperatures. Thus, the appropriate concentration and reaction temperature are key factors in the preparation of st-Sb2O3 microstructures. 3.2. Formation Mechanism of the st-Sb2O3 Microstructures. According to the above experimental process, the chemical reaction equation is the following:
SbCl3 (on the dried glass) → Sb2O3crystal seeds (calcined in air)
(1)
C6H12N4 + 6H2O → 6HCHO + 4NH3
(2)
NH3 + H2O → NH3·H2O
(3)
NH3·H2O → NH4 + + OH−
(4)
Sb3 + + 3OH− → Sb(OH)3 → Sb2O3
(5)
To further investigate the growth mechanism of the st-Sb2O3 microstructures, the effect of the crystal seeds and the reaction time on the morphology of the Sb2O3 product was investigated. Figure 3e shows the SEM images of the Sb2O3 structures without oriented seeds. From the figure, the st-Sb 2 O 3 microstructures and the bundles of long-branched belts are absent. Figure 4 shows the SEM images of the as-synthesized microstructures at 70 °C with 1, 3, 6, and 9 h reaction times. Oval, claw-like, and layered microstructures were obtained when a shorter reaction time (1 h) was used (Figure 4a). When the reaction time was increased to 3 h, the petals of the oval microstructures partly changed to rectangular formations (Figure 4b). When the reaction time was 6 h, the layered microstructures at both ends of the rectangles partly spread out radially (Figure 4c). When the reaction time was prolonged to 9 h (Figure 4d), the claw-like and layered microstructures 768
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Chart 1. The Growth Process of the st-Sb2O3 Microstructures
completely changed to urchin-like formations. When the same reaction was carried out for 12 h, only st-Sb2O3 microstructures were obtained (Figure 1a). Therefore, the appropriate reaction time may be a key factor in the formation of st-Sb2O3 microstructures. On the basis of the experimental results above, we hypothesize that the formation of st-Sb2O3 microstructures is divided into five steps: (1) a small number of Sb2O3 crystal seeds (nuclei) are deposited on the glass slide; (2) Sb3+ cations adsorbed on the surface of Sb2O3 crystal nuclei and with a random orientation grow to form oval, claw-like, layered microstructures; (3) the oval shape gradually changes to a rectangular appearance; (4) both ends of the rectangular, layered microstructures radially spread out; and (5) the directly oriented growth of crystals form the st-Sb2O3 microstructures after an extended reaction (Chart 1). According to the above observations, we are inclined to describe the formation process of the st-Sb2O3 microstructures as involving a sequence of dissolution, recrystallization, and oriented self-assembly of the layered microstructures into the st-Sb2O3 microstructures, this being a thermodynamically driven process. First, the glass slide soaked in Sb3+ cations in an ethanol solution was taken out and dried in air. A small amount of Sb3+ cations adhered to the glass surface and reacted with O2 in the air via calcination to produce Sb2O3 crystal seeds on the surface of the glass slide. When the glass slide was soaked in a mixed solution of SbCl3 and C6H12N4, the Sb3+ cations were adsorbed on the surface of the seeds and began to grow. Small particles dissolved gradually and diffused to the surface of large particles to grow again because Gibbs free energy of small particles is higher than those of large particles. In the early stages of growth, the Sb2O3 crystals grew along the fastest growth orientation to form oval, claw-like, and layered microstructures because of a weak degree of crystallization. The early oval, claw-like, and layered microstructures gradually changed to a rectangular and layered appearance with increasing time because the as-prepared Sb2O3 showed an orthorhombic phase. However, the crystal growth direction became physically limited with increasing reaction time, and growth along the initial, fast-growth orientation slowed. At the same time, both ends of the rectangular, layered microstructures began to grow rapidly and to spread out radially.
Finally, perfect st-Sb2O3 microstructures were formed because the layered microstructures can form a tubal conformation. 3.3. Optical Performances. Figure 5 shows the UV−vis diffusive reflectance spectra of commercial powders and the st-
Figure 5. UV−vis diffusive reflectance spectra of samples.
Sb2O3 microstructures. No obvious difference is observed between the two spectra, except for a slight shift of the absorption edge to shorter wavelengths for the st-Sb2O3 microstructures. Both samples show obvious absorption in the 250−300 nm range, which are very consistent with the other reference,16 in which the reported maximum absorption wavelengths are shown from 250 to 300 nm. However, the stSb2O3 microstructures exhibit weak absorption in the visiblelight region, which possibly results from the different morphologies and size of the two samples. The effect indicated that the particle size became larger compared with concentration of commercial powders.17 Figure 6 shows the FT-IR spectra of the commercial powders and st-Sb2O3 microstructures. The IR peaks of the two samples are very similar. Both show two absorption peaks at 1637 and 3439 cm−1, which are attributed to the O−H bending vibration and stretching of adsorbed water molecules, respectively. The peaks at 2036 and 2348 cm−1 are clearly observed in the two 769
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two peaks at around 376 and 440 nm, an additional broad peak, which appears as a doublet with a peak maxima at around 545 and 592 nm, is also observed. These two peaks can be attributed to the oxygen vacancies, surface dangling bonds, or surface defects.18
4. CONCLUSION In summary, a facile, seeded-controlled self-assembly route was demonstrated for the synthesis of perfect, sectorial, branched Sb2O3 microstructures. Further investigation of other metal oxides may lead to an extension of the current technique to the assembly of other materials.
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AUTHOR INFORMATION
Corresponding Author
*Tel./Fax: +86-22-60600658. E-mail: (L.S.)
[email protected]; (S.Z.)
[email protected]. Author Contributions
Figure 6. Infrared (IR) spectrum of samples.
#
Co-first author.
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−1
samples. The peak at 2036 cm corresponds to the bending vibration mode of the C−O band from CO32−, which may have been formed by the adsorption of CO2 molecules in air on the sample surfaces. Several defects are clearly present in the crystal structure. The CO2 molecules may have been absorbed on the surface defects, which shifted the peak position of the bending vibration mode of the CO32− C−O band to long wave. Therefore, the peaks at 2348 cm−1 correspond to the CO32− C−O band. In addition, the peak at 697 cm−1 is consistent with the bending vibration mode of Sb−O in Sb2O3. Figure 7 shows the PL spectrum of the st-Sb2O3 microstructures. A strong, sharp UV emission at 376 nm and another
ACKNOWLEDGMENTS This work was supported by Tianjin Science & Technology Project of Research Fundation (Grant 20100206) and the National Natural Science Foundation of China (Grant 21103122).
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REFERENCES
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Figure 7. PL of a typical as-synthesized st-Sb2O3 microstructure at 70 °C for 12 h (0.025 mol/L SbCl3 and 0.025 mol/L C6H12N4).
strong, broad, blue emission between 390 and 500 nm are observed. The UV emission at 376 nm may be assigned to the band-edge emission of the well-crystallized Sb2O3 crystals. The conclusion is according to the result of UV−vis reflectance spectra. The band-edge emission of the st-Sb2O3 microstructures is about 376 nm in Figure 5. The broad blue emission from 390 to 500 nm can be explained by the oxygen vacancy-related defect emission. Oxygen vacancy-related defects are introduced in the products during oriented growth-assisted self-assembly processes. In addition to the 770
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