J. Phys. Chem. C 2009, 113, 19439–19444
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Synthesis of Nano-/Microsized KHCO3 Fibers via Quick Thermal Process and Its Toughness and Electron-Irradiating Degradation Xiang Qi†,‡ and Chunxu Pan*,†,§ Department of Physics and Key Laboratory of Acoustic and Photonic Materials and DeVices of Ministry of Education, Wuhan UniVersity, Wuhan 430072, People’s Republic of China, Department of Physics and Institute for Quantum Engineering and Micro-Nano Energy Technology, Xiangtan UniVersity, Xiangtan 411105, People’s Republic of China, and Center for Electron Microscopy, Wuhan UniVersity, Wuhan 430072, People’s Republic of China ReceiVed: May 14, 2009; ReVised Manuscript ReceiVed: September 8, 2009
High-density nano-/microsized KHCO3 fibers were first synthesized by using a quick thermal process. It was proposed that the thermal-treatment process, heat-treatment temperature, and stearate precursor played key roles for growing the KHCO3 fiber. It was found that the KHCO3 fiber exhibited sizable toughness due to its perfect single-crystal microstructure during bending experiment by using a nanomanipulator + SEM system. The electron-irradiating studies indicated that the monocrystalline microstructure of the KHCO3 fiber could be transformed into a polycrystalline K2CO3 phase under electron-beam illumination within TEM and SEM. Introduction
Experimental Methods
Fabrication of nano-/microsized inorganic materials with special size and morphology is of tremendous interest due to their importance in scientific fundamental research and potential technological applications.1-3 Recently, major research has focused on one-dimensional functional nanomaterials, such as metal oxides (ZnO,4 SnO25), metals (Au,6 Ag7), semiconductors (Si8), etc.9-12 However, the metal salt with nano-/microsized structure has been rarely studied. The potassium hydrogen carbonate (KHCO3) crystal is of a monoclinic (P21/a) structure with four molecules per unit cell (Z ) 4) at room temperature. Two CO3 groups are combined by two hydrogen bonds forming a centrosymmetric (HCO3)2 dimer. For these specific microstructures, KHCO3 is commonly represented as a nonferroelectric prototype for proton transfer across hydrogen bonds in centrosymmetric dimers.13 Presently, some researchers are studying the structural transition of KHCO3 crystal at different temperatures14 or different pressures,15 but a few works are focused on the synthesis of KHCO3 crystal with special dimensional microstructures. To our best knowledge, only limited microstructures of potassium hydrogen carbonate have been synthesized. For example, White and co-workers16 reported the self-assemble formation of glassy KHCO3 microsized fibers, which were obtained by exposing the glassy oligomerized films of poly[(aminopropyl)siloxane] to CO2 and H2O. But the fibers were still microsized and tubular microfibers filled with HCO2K. In the present paper, the novel KHCO3 nano-/microsized fibers are first synthesized via a quick thermal process. Their properties such as toughness and phase transformation during electron irradiation were studied.
All chemical products were of analytical grade and were purchased from Shanghai Chemical Reagents Company. The typical synthesis process includes the following: first, stearic acid (C17H35COOH, 0.01 mol) and an equimolar amount of KOH were mixed in 100 mL of deionized water and then dried in an oven at 100 °C for 1 h to obtain a C17H35COOK solution;17 then, the precursor powder was placed on a pure copper substrate and finally directly heat treated at 600 °C for 10 min in air. The microstructures and morphologies of the as-prepared products were characterized by using a scanning electron microscope (SEM, FEI SIRION, Netherlands) with an energy dispersive spectroscope (EDS), a transmission electron microscope (TEM, JEOL JEM 2010, Japan), and X-ray diffraction (XRD) (D8 Advanced XRD, Bruker AXS, Germany). Thermogravimetric analysis (TGA) was carried out with a NETZSCH STA 449C system with a heating rate of 10 deg/min in a flowing argon atmosphere. To investigate the effect of electron irradiation on the microstructure of KHCO3 fibers, the maximum operating voltage was 25 and 200 kV for SEM and TEM, respectively. The bending experiment was performed by using a nanomanipulator + SEM system (Kleindiek, Germany).18 Under observation with SEM, a tungsten tip was manipulated to contact and bend the selected fiber.
* To whom correspondence should be addressed at the Department of Physics, Wuhan University. Phone: +86-27-8721-4880. Fax: +86-27-68752569. E-mail:
[email protected]. † Department of Physics and Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University. ‡ Xiangtan University. § Center for Electron Microscopy, Wuhan University.
Results and Discussion Figure 1a illustrates the XRD pattern of the as-synthesized product. In addition to the diffraction lines of the copper substrate, such as signals for Cu and Cu2O, all the other diffraction peaks have been indexed as a monoclinic phase of KHCO3 (JCPDS Card 74-1846) with the lattice constants a ) 15.04 Å, b ) 5.51 Å, c ) 3.68 Å, and β ) 104.5°. The thermal decomposition of as-synthesized product was investigated with use of TGA measurements, and the result is shown in Figure 1b. Figure 1b indicates that the weight loss of the fiber started at ca. 200 °C and ended at ca. 300 °C, probably related to the decomposition of the KHCO3 crystal. Figure 2 shows the SEM morphologies and EDS map of the as-synthesized KHCO3 products. It was found that the straight
10.1021/jp904517p CCC: $40.75 2009 American Chemical Society Published on Web 10/19/2009
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Figure 1. (a) XRD pattern and (b) thermogravimetric curve of as-prepared products.
Figure 2. SEM images of the as-synthesized products: (a, b) low-magnification and (c, d) high-magnification. (e) EDS map taken from an ensemble of as-prepared products.
KHCO3 fibers were vigorously grown on the substrate with the diameter variation from tens of nanometers to hundreds of
nanometers and the length up to tens of micrometers. The EDS measurement indicates that the chemical compositions of the
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Figure 3. (a) TEM image and (b) SEAD pattern of as-synthesized KHCO3 fibers.
Figure 4. Schematic formation illustrations for the fiber growth from lamellar inorganic-surfactant mesostructures. The light balls represent inorganic components K ions, and the dark balls correspond to the head groups of the stearic cationic.
fibers contain K, C, and O elements, as shown in Figure 2e, in which the Cu peak was from the copper substrate. TEM observations and selected-area electron diffraction (SAED) patterns also confirm that the KHCO3 fiber is of a monoclinic KHCO3 crystalline with a [001] growth direction, as shown in Figure 3. We propose that the formation mechanism of the KHCO3 fibers should be discussed in two sections, that is, (i) obtaining the KHCO3 product and (ii) its orientated growth. In the present process, the chemical reaction is formulated as follows:
C17H35COOH + KOH f C17H35COOK + H2O
(1)
C17H35COOK + 26O2 f KHCO3 + 17CO2 + 17H2O (2) 2KHCO3 T K2CO3 + CO2 + H2O
(3)
If the C17H35COOH and KOH mixture is dried at 100 °C, the precursor powder C17H35COOK will be successfully synthesized.17 Therefore, the KHCO3 product can be obtained by thermally oxidizing precursor powder during the quick thermal process at 600 °C, following eq 2. The model for orientated growth of the KHCO3 fibers was proposed as shown in Figure 4. It has been known that the lamellar inorganic-surfactant intercalated mesostructures can be formed through a co-condensation mechanism of inorganic with ionic surfactant molecules.19,20 In the present case, the lamellar inorganic-surfactant intercalated compounds were composed of the stearic anionic surfactant and cationic inorganic species, as shown in Figure 4, illustration II. When the lamellar sheets were heated in the atmosphere, they tend to roll up and finally form many separate scrolls (Figure 4, illustration III). Then, the scroll was thermally oxidized for forming a KHCO3 crystal during heat treatment. During this process, the scroll serves as a microreactor, and plays a key role for growing KHCO3 fibers. At the same time, the eq 3 is favor to form the
KHCO3 material due to the rich decomposition of CO2 and H2O from C17H35COOK in eq 2. Further experiments reveal that the thermal-treatment process is a key parameter for the growth of KHCO3 fiber. That is, only particles instead of fibers were obtained if the sample was heat treated from room temperature and gradually increased to 600 °C at a rate of 100 deg per hour. The reasons are that the solvent (water) would be evaporated and result in a saturated concentration of the solution, which is a thermodynamic equilibrium state. Therefore, the surfactant intends to aggregate and form a KHCO3 sphere.21 Meantime, the control experiments with different thermaltreatment temperature were also studied. Fixing the treatment time at 10 min, the precursor powders were quickly heat treated under different temperatures from 200 to 800 °C. Figure 5 gives the SEM images of the as-synthesized products. It was found that the temperature range of 500-600 °C provided a desired condition for growing high-density KHCO3 fibers. However, when the temperature increased above 600 °C, the oxidized products were particles and few fibers. It is suggested that the water solvent was vaporize so faster that the solution rapidly reached saturated concentration, and led to the lamellar not being able to form. To evaluate the mechanical property of the KHCO3 fiber, a nanomanipulator + SEM system with a tungsten tip was used to contact and bend a single fiber, as shown in Figure 6. As the tungsten tip moved from the bottom to the top, the fiber was elastically stressed and still kept its initial configuration even at a 90° angle (Figure 6h), which demonstrated the fiber was of sizable toughness due to its perfect monocrystalline microstructure. During the SEM and TEM experiments, it was noted that the KHCO3 fiber is sensitive to electron-beam irradiation. That is to say, the spiky tips of the fibers were melted and changed to the “roundish” tips, as shown in Figure 7, when it was exposed in an electron beam for 30 s in SEM. Similarly, the fiber was damaged and the monocrystalline KHCO3 transformed into a polycrystalline K2CO3 phase in TEM with higher acceleration voltage, as shown in Figure 8. Generally, the
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Figure 5. SEM images of the as-synthesized products with different treatment temperatures: (a) 400, (b) 500, (c) 600, and (d) 700 °C.
Figure 6. SEM images of nanomanipulation, using a nanomanipulator + SEM system. When the tungsten tip is moved from the bottom to the top (a-i), the fiber is transformed from its straight shape into a curved configuration.
KHCO3 crystal can be decomposed into K2CO3, CO2, and H2O during heating above 200 °C. Therefore, in the present experiment, the damage indicates that the fiber exhibits a property of nonconductivity for electrons under high voltage electron beam illumination in TEM and SEM. To further understand the thermal stability of the KHCO3 fiber, the as-synthesized products were reheated at 600 °C for
1 min. It was found that the fiber-like morphology was retained, but it has been partly transformed into nanoparticles, as show in Figure 9. Integrated with the SEAD pattern in Figure 8b, it was suggested that the as-synthesized KHCO3 fibers were degraded into K2CO3 nanoparticles after long thermal treatment, which is consistent with the intrinsic properties of KHCO3 crystal.
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Figure 7. SEM images of the KHCO3 fibers: (a) before and (b) after electron-beam irradiation (“roundish” tips are marked by circle boxes; the insert shows the enlarged image inside the indexed box).
Figure 8. (a) TEM image and (b) SEAD pattern of KHCO3 fibers after electron-beam irradiation; (c, d) TEM images of KHCO3 fibers, whose microstructures were partially destructed by electron-beam illumination (the area after irradiation is inside the rectangular box).
Figure 9. SEM images of the KHCO3 fibers after reheat treatment: (a) low magnification and (b) high magnification.
Conclusions A novel KHCO3 nano-/microsized fiber has been synthesized by using a fast thermal process. It exhibits the properties of
sizable toughness and sensitivity to electron irradiation. It is expected to promote the study on KHCO3 crystal and its application in the future.
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Acknowledgment. We would like to thank Mr. Yaoyao Ren and Mr. Qiang Fu for their excellent technical assistance in TEM and SEM observations. This work was supported by the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20070486016), Ministry of Education, China, and National Basic Research Program of China (973 Program) (No. 2009CB939700). References and Notes (1) Moriarty, P. Rep. Prog. Phys. 2001, 64, 297. (2) Rao, C. N. R.; Cheetham, A. K. J. Mater. Chem. 2001, 11, 2887. (3) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Van Schalkwijk, W. Nat. Mater. 2005, 4, 366. (4) Wang, X. D.; Summers, C. J.; Wang, Z. L. Nano Lett. 2004, 4, 423. (5) Dai, Z. R.; Gole, J. L.; Stout, J. D.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 1274. (6) Zhang, X. Y.; Zhang, L. D.; Lei, Y.; Zhao, L. X.; Mao, Y. Q. J. Mater. Chem. 2001, 11, 1732. (7) Zong, R. L.; Zhou, J.; Li, Q.; Du, B.; Li, B.; Fu, M.; Qi, X. W.; Li, L. T.; Buddhudu, S. J. Phys. Chem. B 2004, 108, 16713.
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