Article pubs.acs.org/IECR
Preparation of Finite Particles of Layered Niobate (KCa2Nb3O10) for Improved Materials Performance Shota Igarashi,† Soh Sato,† Tadashi Takashima,‡ and Makoto Ogawa*,†,§ †
Graduate School of Creative Science and Engineering, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan Tokyo Nano-Tech Engineering Center, Yoshida Kikai Co., Ltd., No. 307, 3-28-1, Yaguchi, Ota-ku, Tokyo 146-0093, Japan § Department of Earth Sciences, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan ‡
ABSTRACT: A layered calcium niobate, KCa2Nb3O10, with submicrometer sized particle was prepared by the solid-state reaction of potassium carbonate, calcium oxide, and niobium pentoxide in air at the temperatures of 900 °C, which is lower than the reported value (1100 °C). The lower temperature synthesis led to better results in the photocatalytic decomposition of methyl orange and the preparation of an aqueous suspension and thin film. The question here is very simple that “is the solid-state reaction at lower temperature good for the preparation of KCa2Nb3O10?”. The lower temperature synthesis has an economical advantage, in addition to the possible modification of the products’ properties. In the present investigation, our interest is mainly focused on the particle size of the product, and the particle size variation of KCa2Nb3O10 is expected to affect the dispersion in solvents, transparency and stability of the suspension, ease of film fabrication, and so on.
1. INTRODUCTION Layered materials and their intercalation compounds have extensively been investigated from both basic scientific viewpoints on the synthesis, structure, composition, and characterization as well as from the viewpoints of their practical applications in the fields of environment, energy, electronic, photonics, and health and life problems.1 In addition, by means of host−guest interactions, intercalation compounds have been prepared so far for possible materials applications.2,3 In order to meet the specific requirements, one can select and use host and guest, composition of them, synthetic methods for the desired production scale, properties, and processing. We have been interested in the particle morphology of layered materials as a key issue to optimize the performance and reported morphosyntheses of a layered material, layered double hydroxides.4 In the present study, we synthesized a layered niobate, KCa2Nb3O10, with different particle size by simply changing the reaction temperature in the solid-state reactions in order to examine the effects of the particle size on the properties. The Dion−Jacobsen phase AB2Nb3O10 (where A = Li, Na, K, H, Rb, Cs; B = Ca, Sr) are ion exchangeable layered solids, consisting of stacked negatively charged perovskite sheets and interlayer exchangeable cation.5 The group of materials have been investigated from the basic scientific viewpoints as well as for the advanced materials applications.6−10 The modification of the structures and surface properties by using host−guest chemistry has been conducted so far.6,11−19 Ion exchange with the tetrabutylammonium (TBA) ion has been done to obtain an aqueous suspension, which has been used to prepare a film by layer-by-layer deposition.20−24 Grafting of organic functionality and pillaring with oxide nanoparticle have been reported to modify the surface properties.25−31 Heteroelements doping has been done to design the electronic and optical properties.32,33 Thus, the preparation, fabrication, and characterization of KCa2Nb3O10 and their derivatives have been done by many researchers in different fields. On the other hand, the synthesis has been done based on the original contribution by Dion et al., where solid-state reaction of potassium carbonate, calcium oxide, and niobium pentoxide in air at 1100 °C was developed.5 © 2013 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials. Potassium carbonate (K2CO3 99.95%), calcium oxide (CaO 97.0%), niobium pentoxide (Nb2O5 99.95%), and methyl orange (MO) were purchased from Kanto Chemical Co., Ltd. All the reagents were used without further purification. 2.2. Sample Preparation. KCa2Nb3O10 was synthesized according to the previous report5 with the modification of the heating temperature. The starting materials (K2CO3, CaO, and Nb2O5) were mixed manually with an agate mortar and a pestle for 2 h, and the mixture was calcined in air at 800 °C for 12 h. An excess of K2CO3 was added to compensate for the loss due to volatilization. After the cooling, the sample was mixed again with an agate mortar and a pestle for another 2 h, and the mixture was heated again at desired temperatures (800, 900, and 1100 °C) for 24 h. The products were designated as KCa2Nb3O10 (T), where T indicates the temperature for the solid-state reactions in the second step. 2.3. Photocatalytic Reaction. Photocatalytic decomposition of methyl orange was carried out in an inner irradiation type quartz vessel. The KCa2Nb3O10 (100 mg) was added to deionized water (100 mL) and dispersed by ultrasonic irradiation for 10 min. Then, the aqueous suspension of the KCa2Nb3O10 was mixed with a 10 ppm methyl orange aqueous solution (100 mL). The catalyst/solution ratio was 100 mg/200 Received: Revised: Accepted: Published: 3329
November 23, 2012 February 4, 2013 February 12, 2013 February 21, 2013 dx.doi.org/10.1021/ie303243h | Ind. Eng. Chem. Res. 2013, 52, 3329−3333
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mL, and the initial concentration of methyl orange in the solution was 5 ppm. The adsorption of methyl orange onto the catalyst reached equilibrium within 120 min, and then the catalyst containing methyl orange aqueous solution was irradiated with a 20 W-mercury lamp for 7 h under magnetic stirring. A portion of the solution was collected and centrifuged (4000 rpm, 20 min) to obtain supernatant for the determination of the concentration of residual methyl orange was determined by the Lambert−Beer law. A blank experiment was carried out that the MO was irradiated by the mercury lamp in the absence of catalyst while keeping all other parameters constant. 2.4. Proton Exchange and Ion Exchange with Tetrabutylammonium Ion. Proton exchange was conducted by the method used in the previous report.34 The obtained powder was magnetically stirred in 200 mL of an aqueous HNO3 solution (6 mol/L) for 3 days to replace K+ by H3O+. After the centrifugation, the solid was washed with distilled water and then dried in air. The proton-exchanged products were designated as HCa2Nb3O10 (T), where T indicates the temperature for the solid-state reactions in the second step. HCa2Nb3O10 powder (50 mg) was dispersed in 50 mL of an aqueous solution of tetrabutylammonium hydroxide (TBA+OH−), where the molar ratio of TBAOH/HCa2Nb3O10 was 1:2. The mixture was then shaken vigorously at room temperature for 4 days and then was kept without shaking to separate the well-dispersed portion. 2.5. Dispersion of HCa2Nb3O10 Using a Wet Type Super Atomizer. The powder sample was dispersed using a wet type super atomizer (Yoshida Kikai Co., Ltd., NanoVater, C-ES). An aqueous mixture of HCa2Nb3O10 (900) (50 mg in 100 mL of water) was vigorously agitated with the wet type super atomizer. The discharge pressure and pass times were conducted at 200 MPa and 5 passes, respectively. Then, the suspension was allowed to stand for 2 days and the supernatant was cast on a glass substrate and dried in air overnight. 2.6. Characterizations. X-ray powder diffraction patterns of the products were recorded on a Rigaku RAD IB powder diffractometer equipped with monochromatic Cu Kα radiation operated at 20 mA and 40 kV. The size of the crystallites was determined using the Scherrer’s equation based on the full width at half maxima of the (002) reflection. Scanning electron micrographs (SEM) were obtained on a Hitachi S-2380N scanning electron microscope. Prior to the measurements, the samples were coated with 20 nm gold layer. UV−vis absorption spectra of liquid samples were recorded on a Shimadzu UV3100PC spectrometer using deionized water as a reference.
Figure 1. XRD patterns of the products obtained by the solid-state reactions at (a) 800, (b) 900, and (c) 1100 °C.
pattern (Figure 1, trace c) together with that of KCa2Nb3O10. Thus, the single phase KCa2Nb3O10 preparation was possible by the solid-state reaction at the temperature as lows as 900 °C and, for simple comparison, KCa 2 Nb 3 O 10 (900) and KCa2Nb3O10 (1100) were used for further investigation. As far as we know, this is the lowest temperature reported for the preparation of KCa2Nb3O10. The lower temperature affords the low cost synthesis, consequently, the application of the strategy to other layered solids is worth investigating. The scanning electron micrographs of KCa2Nb3O10 (900) and KCa2Nb3O10 (1100) are shown in Figure 2, where plated particle with different particle size are seen (200 nm−1 μm and 2−7 μm for KCa2Nb3O10 (900) and KCa2Nb3O10 (1100),
3. RESULTS AND DISCUSSION 3.1. Solid-State Syntheses at Lower Temperature. The X-ray diffraction (XRD) patterns of KCa2Nb3O10 (900) and KCa2Nb3O10 (1100) are shown in Figure 1. All the diffraction peaks observed for the XRD patterns of KCa2Nb3O10 (900) and KCa2Nb3O10 (1100) were ascribed to KCa2Nb3O10, while the peaks intensity and sharpness depend on the reaction temperature, lower temperature resulted in the broader and weaker diffractions. The assignment was based on the previous report.5 The crystallite size, which was calculated using the Scherrer’s equation from the (002) reflection of XRD was 40 and 63 nm for KCa2Nb3O10 (900) and KCa2Nb3O10 (1100), respectively. When the solid-state reaction was conducted at 800 °C, the diffraction peaks ascribable to the original materials as well as the unknown phase were detected in the XRD
Figure 2. SEM images of the products obtained by the solid-state reactions at (a) 800, (b) 900, and (c) 1100 °C. 3330
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respectively). The particles seen in SEM are thought to be aggregates of primary particles. Thus, KCa2Nb3O10 with different particle sizes was obtained by changing the synthetic temperatures. It should be noted here that the particle size is substantially modified by lowering the synthesis temperature while the achieved size is not so small to exhibit quantum size effects. The particle size variation affected various fundamental characteristics relevant to their materials applications. When the samples were dispersed in water, a clear difference was seen in the states (transparency and stability) of the suspension. Figure 3 shows the photographs of the aqueous suspensions
Figure 4. Change in the concentration of MO by UV irradiation in the presence of KCa2Nb3O10 (900), (b) KCa2Nb3O10 (1100), and (c) a blank experiment in the absence of catalyst.
other layered niobates and titanates) with TBA resulted in the stable suspension, from which films have been fabricated by layer-by-layer assembly technique.20−24 Proton exchange of KCa2Nb3O10 (900) was done, and subsequently the exchange with TBA was conducted as reported previously.20−24 The XRD patterns of the proton-exchanged products, which were prepared by acid treatment with aqueous HNO3, are shown in Figure 5. The change in the basal spacing is consistent with the reported proton exchange reaction (from 1.47 to 1.62 nm).34 Stable suspension was obtained after the addition of TBA as shown in the photograph (Figure 6). The suspension was allowed to stand to isolate a well-dispersed portion, and the
Figure 3. Photographs of the aqueous solution just after sonication and after the suspension stood without agitation for 24 h, (right) KCa2Nb3O10 (900) and (left) KCa2Nb3O10 (1100).
containing the same amount of KCa2Nb3O10 (10 mg/50 mL) just after the sonication and after the suspension stood without agitation for 24 h. The suspension of KCa2Nb3O10 (900) is stable compared to that of KCa2Nb3O10 (1100). We examined a rough estimation of the sedimentation rate based on the Stokes equation. The sedimentation rate of KCa2Nb3O10 (900) and KCa2Nb3O10 (1100) is 0.9 and 27.8 cm/s, respectively. Thus, KCa2Nb3O10 (900) with different particle size were successfully obtained, and as expected, the materials characteristics are shown to be dependent on the particle size. 3.2. Photocatalytic Decomposition of Methyl Orange. The photocatalytic decomposition of MO was examined using KCa2Nb3O10 (900) and KCa2Nb3O10 (1100) as a photocatalyst by UV irradiation. The change in the concentration of MO (the initial concentration of MO was 5 ppm), which was determined by the visible absorption spectra, is shown in Figure 4. Since MO is an anionic dye, no adsorption onto KCa2Nb3O10 was observed. The decrease of MO concentration as the result of the decomposition was observed by UV irradiation, and the decomposition rate was enhanced when KCa2Nb3O10 (900) was used if compared with the reaction when KCa2Nb3O10 (1100) was used as a photocatalyst. This difference was thought to reflect the particle size from the following points: (i) difference in the transparency of the suspension, which corresponds to the efficient use of incident light, (ii) the access of the substrate (MO molecules) to the surface of KCa2Nb3O10, larger surface area for the smaller particles lead to efficient contact of MO to the catalytically active site. Further systematic study on the photocatalytic ability of the finite KCa2Nb3O10 particle will be reported subsequently. 3.3. Ion Exchange and Preparation of the Cast Film. Another advantageous aspect of KCa2Nb3O10 (900) is seen by the ion exchange with tetrabutylammonium (TBA) ion. It has been reported that the ion exchange of KCa2Nb3O10 (as well as
Figure 5. XRD patterns of the proton-exchanged products (a and b) and its pristine forms (c and d). 3331
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Figure 6. Photographs of stable suspension after the ion exchange with tetrabutylammonium (TBA) ion, (right) HCa2Nb3O10 (900) and (left) HCa2Nb3O10 (900).
yield of the stable dispersion was determined by subtracting the amount of precipitated KCa2Nb3O10 from the used amount. The yields of suspension after standing for 4 days were 87 and 57 wt % for KCa2Nb3O10 (900) and KCa2Nb3O10 (1100), respectively, indicating that lower temperature synthesis led to better yield. KCa2Nb3O10 (900) gave relatively stable suspension in its pristine form as shown in Figure 3, while the stability of the suspension is a problem to be modified further. Accordingly, a wet type super atomizer (Yoshida Kikai Co., Ltd., NanoVater, C-ES; Figure 7) was used to prepare an aqueous suspension of
Figure 8. Photograph of the HCa2Nb3O10 (900) suspension obtained with a wet type super atomizer after standing without agitation for 2 days.
Figure 9. Scanning electron micrograph and photograph of the cast film of HCa2Nb3O10 (900).
well as for other layered materials. The application of the wet type super atomizer to prepare stable suspension for a wide range of application is being done in our laboratory. Figure 7. Schematic diagram of wet type super atomizer.
4. CONCLUSIONS In summary, it was shown that the solid-state reaction of the starting materials (K2CO3, CaO, and Nb2O5) at 900 °C was good enough to obtain single phase KCa2Nb3O10. The lower temperature synthesis resulted in the product with smaller particle size if compared with that of the product prepared by reported synthetic conditions. The use of a wet type super atomizer is also helpful to disperse the powder. As a result, the sample showed better dispersion in water, and the stable suspension was used for thin films fabrication. All these characteristics are ideal for the photocatalyst applications with lower light scattering and easier access of the substrates (as suggested by the larger surface area), and this idea was experimentally supported confirmed by the efficient photocatalytic decomposition of methyl orange by UV irradiation. The particle size effects are also worth investigating in the applications as adsorbent, fillers, and so on.
HCa2Nb3O10 (900). Thus, it was shown that the use of a wet type super atomizer is a possible option to obtain a suspension with improved stability. It should be noted here that the crystallinity was not changed by the procedure as confirmed by XRD and SEM. Furthermore, starting from the suspension thus obtained, a fraction of well-defined particles was obtained. The suspension was allowed to stand for 2 days to see the separation into three layers as shown in Figure 8. Then, a film composed of smaller and uniform-sized particles was obtained by casting the top layer supernatant. The photograph and the SEM image of the HCa2Nb3O10 (900) cast film are shown in Figure 9, where several hundreds of relatively uniform nanometer sized particle are seen in the SEM image. The combination of lower temperature synthesis and the vigorous mixing using a wet type super atomizer is shown to be a useful way for KCa2Nb3O10 as 3332
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Thus, the solid-state reaction at lower temperature led several advantageous properties of KCa2Nb3O10. The applications of present KCa2Nb3O10 (900) is worth conducting. The present lower temperature synthesis seems to be applied to other materials where a solid-state reaction was used. Further study on the lower temperature preparation of another type of Dion−Jacobsen phase AB2Nb3O10 as well as other layered solids are being done in our laboratory, and the results will be reported subsequently.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by the Global COE Program of MEXT and Waseda University Grant for Special Research Projects (Grant 2011A-604). Nippon Sheet Glass Foundation supported us financially.
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REFERENCES
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