CRYSTAL GROWTH & DESIGN
Environmentally Friendly Growth of Highly Crystalline Photocatalytic Na2Ti6O13 Whiskers from a NaCl Flux
2008 VOL. 8, NO. 2 465–469
Katsuya Teshima,*,† Kunio Yubuta,‡ Tomoyuki Shimodaira,† Takaomi Suzuki,†,§ Morinobu Endo,§,| Toetsu Shishido,‡ and Shuji Oishi†,§ Department of EnVironmental Science and Technology, Faculty of Engineering, Shinshu UniVersity, 4-17-1 Wakasato, Nagano 380-8553, Japan, Institute for Materials Research, Tohoku UniVersity, 2-2-1 Katahira, Aoba-ku, Sendai 980-8577, Japan, Institute of Carbon Science and Technology, Shinshu UniVersity, 4-17-1 Wakasato, Nagano 380-8553, Japan, and Department of Electrical and Electronic Engineering, Faculty of Engineering, Shinshu UniVersity, 4-17-1 Wakasato, Nagano 380-8553, Japan ReceiVed April 7, 2007; ReVised Manuscript ReceiVed September 28, 2007
ABSTRACT: Idiomorphic and high quality Na2Ti6O13 whiskers were easily grown by the rapid cooling of a NaCl flux. Environmentally friendly whisker growth was conducted by heating a mixture of solute (Na2CO3 + TiO2) and flux (NaCl) at 1100 °C and holding at this temperature for 10 h. After that, the mixture was cooled at various rates of 100, 200, >9000 (quenching in air), or >120000 °C h-1 (quenching into water). The obtained whiskers, which had average sizes of up to 101.5 µm × 0.7 µm and aspect ratios of up to 145, were colorless and transparent. The grown whiskers tended to form aggregates, that looked just like a lustered cotton. The whisker forms and sizes obviously depended on the cooling rates of the high-temperature solutions. The aspect ratio of the whiskers increased with the increasing cooling rate and decreasing solute concentration. Transmission electron microscope (TEM) images indicated that the grown whiskers were of a very good crystallinity. The major constituents, that is, sodium, titanium, and oxygen, were homogeneously distributed throughout the whiskers, however, chlorine from the flux and platinum from the crucible were not detected. Furthermore, highly crystalline, ultralong, and flexible Na2Ti6O13 whiskers were successfully grown by the slow cooling of an NaCl flux by the use of the nonstoichiometric mixture of Na2O and TiO2 (2Na2O-3TiO2). The grown whiskers exhibited good photocatalytic activity under ultraviolet (UV) light irradiation. It is considerably reasonable to suppose that NaCl is adequate to synthesize high quality and well-developed titanate whiskers in an environmentally friendly process of crystal growth. Introduction The fabrication of one-dimensional (1D) materials including tubes, fibers, rods, and whiskers appropriate for a wide variety of leading edge technologies has recently been of great interest.1–7 Among them, whiskers are a needle-shaped single crystal with mostly theoretical strength due to their perfect geometry. In particular, 1D nanomaterials exhibit the unique physical and chemical properties correlated with the 1D structural confinements in the nanoscale.1,2 Our research motivation is to produce highly efficient materials for use as photocatalysts for degradation of toxic substances and for decomposition of pure water, or photoactive electrodes in dyesensitized solar cells (DSSCs). We recently first reported on the growth of the high quality, hexagonal prismatic sodium hexatitanate (Na2Ti6O13) whiskers with aspect ratios of up to 50 by the slow cooling of a NaCl flux.5 The grown whiskers of Na2Ti6O13 exhibited a good photocatalytic activity under UV light illumination. Sodium hexatitanate 1D nanostructures with high aspect ratios are expected to facilitate electron transport and to enhance dye adsorption and interpenetration of hole transport materials.8 Herein, we report on the fabrication of Na2Ti6O13 whiskers with high aspect ratios by cooling of a NaCl flux. Furthermore, the effects of the cooling rates and the starting compositions of the high-temperature solutions on the whisker forms were studied. In the case of the rapid cooling method, compared with the previous slow cooling method,5 the period * Corresponding author: E-mail:
[email protected]. † Department of Environmental Science and Technology, Shinshu University. ‡ Tohoku University. § Institute of Carbon Science and Technology, Shinshu University. | Department of Electrical and Electronic Engineering, Shinshu University.
of whisker growth can be shortened by approximately 1/5, and environmental load can also be reduced. In addition, NaCl has some advantages for flux growth of Na2Ti6O13 whiskers. The melting point of NaCl (801 °C) is relatively low and easily soluble in warm water. It has a common cation (Na+) with the solute. It is abundant in nature and is also harmless to human beings and the environment. Experimental Section Sodium hexatitanate whiskers were grown by a cooling method of the NaCl flux. The reagent-grade Na2CO3 (Wako Pure Chemical Industries, Ltd.), TiO2 (anatase, Wako Pure Chemical Industries, Ltd.), and NaCl (Wako Pure Chemical Industries, Ltd.) were used for the growth of Na2Ti6O13 whiskers. First, a stoichiometric mixture of reagentgrade Na2CO3 and TiO2 powders and NaCl were respectively used as a solute and the flux. The typical growth conditions are given in Table 1. The solute concentration was varied from 0.5 to 20 mol % of the NaCl flux. These masses of the reagents were kept at approximately 20 g for all growth runs (runs 1–12). Each of the mixtures was put into platinum crucibles of 30 cm3 capacity. After the lids were loosely closed, the crucibles were placed in an electric furnace with silicon carbide heating elements. The crucibles were heated to 1100 °C at a rate of 45 °C h-1 and held at this temperature for 10 h. After that, they were cooled to 500 °C at a rate of 100, 200, >9000 (air quenching), or >120000 °C h-1 (water quenching). The cooling rates of 100 and 200 °C h-1 were controlled by the cooling program, and the crucibles were allowed to cool to room temperature in the furnace. The cooling rates of >9000 or >120000 °C h-1 were not able to be attained by the cooling program in the furnace. In the case of >9000 °C h-1 (quenching in air), the crucibles were held at the holding temperature of 1100 °C for 10 h, and then removed from the furnace and allowed to cool rapidly to room temperature. For >120000 °C h-1 (quenching in water), after they were removed from the high-temperature furnace, the crucibles were immersed into water and cooled extremely rapidly. These two cooling rates, >9000 and >120000 °C h-1, are an approximate estimate
10.1021/cg070341p CCC: $40.75 2008 American Chemical Society Published on Web 01/04/2008
466 Crystal Growth & Design, Vol. 8, No. 2, 2008
Teshima et al.
Table 1. Typical Growth Conditions of the Na2Ti6O13 Whiskers
run no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14
solute concentration (mol %) 0.5 0.5 0.5 0.5 5 5 5 5 20 20 20 20
solute
flux
Na2CO3 (g)
TiO2 (g)
NaCl (g)
cooling rate (°C h-1)
0.174 0.174 0.174 0.174 1.283 1.283 1.283 1.283 2.735 2.735 2.735 2.735 0.348 0.348
0.788 0.788 0.788 0.788 5.804 5.804 5.804 5.804 12.368 12.368 12.368 12.368 0.394 0.394
19.111 19.111 19.111 19.111 13.446 13.446 13.446 13.446 6.033 6.033 6.033 6.033 19.111 19.111
100 200 >9000 >120000 100 200 >9000 >120000 100 200 >9000 >120000 5 >120000
value. The crystal products were separated from the remaining flux in warm water. Next, a nonstoichiometric mixture of reagent-grade Na2CO3 and TiO2 powders (2Na2O-3TiO2) and NaCl was used (Table 1, runs 13 and 14). The experimental procedures were basically the same as mentioned above. The obtained crystals were observed by use of an optical microscope and field emission scanning electron microscope (FE-SEM, JEOL, JSM7000F). Phases and elongated directions of the crystals were studied by X-ray diffraction (XRD, Shimadzu, XRD-6000). An energydispersive X-ray spectrometer (EDS, Horiba, EMAX-5770Q) was used to study any variation in the concentration of the major constituents in the grown crystals. The high-resolution transmission electron microscopy (HRTEM) and electron diffraction observations were carried out on JEM-2010 (JEOL) and JEM-2000EXII (JEOL) instruments operated at 200 kV to analyze the crystallinity and elongated direction of the grown crystals. The presence of impurities from the NaCl flux and platinum crucible was also observed. The length (L, parallel to the 〈010〉 directions) and width (W, perpendicular to the 〈010〉 directions) of relatively large Na2Ti6O13 whiskers (100 samples) were measured, and their average sizes (Lav and Wav) and aspect ratios (Lav/Wav) were calculated for each growth run. The ultraviolet-visible (UV-vis) light diffuse reflectance spectra of the grown crystals were obtained on a spectrophotometer (Shimadzu, UV3150). To simply examine its photocatalytic property, photodecolorization examination of aqueous organic dyes was carried out.
Results and Discussion Idiomorphic Na2Ti6O13 whiskers were successfully grown from all growth runs. First, concerning the use of stoichiometric mixtures (runs 1–12), a very small amount of byproduct crystallites (TiO2) was also grown from mixtures containing solute of 5 and 20 mol % by air and water quenchings (runs 7, 8, 11, and 12). Typical TiO2 crystallites were gray, and their form was a bulk. The grown Na2Ti6O13 whiskers were colorless and transparent. Figure 1 shows typical Na2Ti6O13 whiskers (Figure 1a: run 9 and Figure 1b: run 4) grown from a NaCl flux. For a relatively high solute concentration (e.g., 20 mol
Figure 1. Optical micrographs showing Na2Ti6O13 whiskers grown at a solute concentration and cooling rate of (a) 20 mol % and 100 °C h-1 and (b) 0.5 mol % and >120000 °C h-1 (quenching in water), respectively.
Figure 2. SEM images showing typical Na2Ti6O13 whiskers grown from a NaCl flux: cooling rate (a) 100, (b) 200, (c) >9000 (air quenching), and (d) >120000 °C h-1 (water quenching). The solute concentration was fixed at 0.5 mol %.
Figure 3. SEM images showing typical Na2Ti6O13 whiskers grown by the water quenching of a NaCl flux: solute concentration (a) 5 and (b) 20 mol %.
%) and low cooling rate (100 °C h-1), the grown whiskers tended to become comparatively thick and short as shown in Figure 1a. On the other hand, in the case of a relatively low solute concentration (e.g., 0.5 mol %) and high cooling rate (e.g., quenching), the whiskers were ordinarily obtained as aggregates that looked like cotton and had a silky luster, as shown in Figure 1b. Relatively long whiskers were much greater than 200 µm in length. Figure 2 shows SEM images of Na2Ti6O13 whiskers grown at various cooling rates ((a) 100, (b) 200 °C h-1, (c) air quenching, and (d) water quenching). The solute concentration was fixed at 0.5 mol %. The tendency of aspect ratios of the grown whiskers to increase with an increasing cooling rate is clearly observed in Figure 2. It seems that the widths of the grown whiskers became small although the lengths are almost the same. Figure 3 shows SEM images of whiskers grown at various solute concentrations (cooling; water quenching). The solute concentration was, respectively, 5 (Figure 3a) and 20 mol % (Figure 3b). In comparing Figures 2d (0.5 mol %), 3a (5 mol %), and 3b (20 mol %), the aspect ratios and lengths of the whiskers drastically decreased with an increasing solute concentration. Only the whiskers of Na2Ti6O13 were observed in Figures 2 and 3; however, no byproduct crystals (TiO2) were clearly observed. A typical whisker top (run 3) is shown in Figure 4. The basic form of the grown whiskers was a hexagonal cylinder, and their surfaces were very flat. Figure 5 shows the variation in the aspect ratio of the Na2Ti6O13 whiskers with the cooling rate. When the solute concentration and cooling rate were, respectively, 0.5 mol % and 100 °C h-1, the aspect ratio was about 89.5 (Lav ) 125.3 µm and Wav ) 1.4 µm). When the solute concentration was
Crystalline Photocatalytic Na2Ti6O13 Whiskers
Figure 4. SEM image showing a typical tip of a Na2Ti6O13 whisker grown by the rapid cooling of a NaCl flux (solute concentration of 0.5 mol % and air quenching).
Figure 5. Variation in the aspect ratio of Na2Ti6O13 whiskers with cooling rates and solute concentrations: solute concentration ([) 0.5, (9) 5, and (O) 20 mol %.
fixed at 0.5 mol %, the aspect ratio gradually increased with increasing cooling rate, reaching about 145 (Lav ) 101.5 µm and Wav ) 0.7 µm) at water quenching. In the case of the other concentrations, however, the aspect ratios were almost the same (solute conc.; 20 mol %) or increased inappreciably (solute conc.; 5 mol %) with an increase in the cooling rate. On the other hand, when the cooling rate was fixed, the aspect ratios drastically increased with a decrease in solute concentration. For example, when the cooling rate was >9000 °C h-1 (air quenching), the aspect ratios (Lav/Wav) were, respectively, 141.8 (113.4 µm /0.8 µm, 0.5 mol %), 39.7 (27.8 µm /0.7 µm, 5 mol %), and 20.3 (8.1 µm /0.4 µm, 20 mol %). Figure 6 shows XRD profiles of data for the transparent-colorless whiskers (Figure 6a), pulverized crystallites (Figure 6b-d), and Na2Ti6O13 ICDD PDF9 (Figure 6e). Figure 6b-d were, respectively, XRD profiles of the whiskers grown at 0.5 mol % and 100 °C h-1 (run 1), 0.5 mol % and water quenching (run 4), and 20 mol % and water quenching (run 12). All whiskers were identified as Na2Ti6O13 by their powder XRD patterns (Figure 6b-d), using data given in the literature (Figure 6e).9 The diffraction patterns attributed to byproducts were not detected in Figures 6b-d. The XRD pattern for oriented whiskers indicated that the diffraction intensities of the (200), (2j01), (2j03), (402), (6j01), (4j04), and (602) planes were dominant. The XRD analysis made it clear that the whiskers grown by the rapid cooling had almost the same crystallinity as those grown by the slow cooling.5 Figure 7 parts a and b show a bright field TEM image and the
Crystal Growth & Design, Vol. 8, No. 2, 2008 467
Figure 6. X-ray diffraction patterns (Cu KR) of Na2Ti6O13 whiskers. (a) Whiskers of well-developed faces were laid in parallel with the holder plate, (b) Pulverized crystallites (solute concentration ) 0.5 mol % and cooling rate ) 100 °C h-1). (c) Pulverized crystallites (0.5 mol % and water quenching). (d) Pulverized crystallites (20 mol % and water quenching). (e) Na2Ti6O13 ICDD PDF.9
Figure 7. (a) TEM micrograph, (b) diffraction pattern, and (c) lattice image of a typical Na2Ti6O13 whisker (run 4, water quenching).
corresponding selected area diffraction pattern (SAD) of a typical whisker (run 4, water quenching). The SAD indicated that the lattice parameters of the whisker are b ) 0.373 and c ) 0.918 nm, which are in good agreement with those found in the previous study.5,9 Since the incident electron beam is parallel to the [100] direction, the lattice parameter a cannot be measured by only this incident beam. The lattice image obtained from a Na2Ti6O13 whisker is shown in Figure 7c, taken with the incident beam along the [100] direction. The whisker was of very good crystallinity because no defects were observed in this image. From these results of XRD and TEM, the elongated direction clearly corresponded to the 〈010〉 directions. Considering the lattice parameters, it seems reasonable to elongate to the 〈010〉 directions, and the elongated direction was the same result as our previous study.5 Additionally, the EDS data showed that sodium, titanium, and oxygen atoms were homogeneously distributed in the whiskers. Chlorine atoms from the flux were not detected in the whiskers. The UV–vis diffuse reflectance spectra of Na2Ti6O13 whiskers grown at various cooling rates are shown in Figure 8. The UV–vis diffuse reflectance spectra of the grown Na2Ti6O13 whiskers are basically the same. The spectra show the onset wavelength of absorption at around 360–380 (100 °C h-1), 360–420 (air quenching), and 380–440 nm (water quenching) and a maximum at around 315–325 nm. For the air and water quenching (Figure 8b and c), the onset
468 Crystal Growth & Design, Vol. 8, No. 2, 2008
Teshima et al.
Figure 8. UV–vis light diffuse reflectance spectra of (a-c) Na2Ti6O13 whiskers, (d) TiO2 powder (anatase), and (e) TiO2 powder (rutile): (a) 100 °C h-1, run 1; (b) air quenching, run 3; (c) water quenching, run 4. (The solute concentration was fixed at 0.5 mol %.)
wavelengths tended to move from the region of ultraviolet light (about 380 nm) to visible light (420–440 nm). The exact cause has not been completely explained. Typical absorption spectra of TiO2 (anatase, Figure 8d) and TiO2 (rutile, Figure 8e) powders shift, respectively, to longer wavelength by about 20 and 50 nm at the onset. From these spectra, there is a fair possibility that all of the grown Na2Ti6O13 whiskers reveal a photocatalytic property under UV (500 (for slender whiskers), as shown in Figure 9. Typical ultralong and flexible whiskers were also of a very good crystallinity (Figure 10) because no defects were observed in their lattice image (Figure 10a). Infrequently, ultralong and flexible whiskers including stacking faults were observed. In the case of 1D materials, a variety of defects would be generally included. For example, in general, carbon fibers grown by chemical vapor deposition and glass fibers synthesized by rapid cooling were of a relatively low crystallinity and have imperfections and defects in their lattice, and therefore, they were not whiskers but fibers. The elongated direction clearly corresponded to the 〈010〉 direction (Figure 10b and c). The morphology was similar to that of the thick and short Na2Ti6O13 whisker grown from a stoichiometric Na2O-6TiO2 mixture. Moreover, the ultralong and flexible whiskers exhibited a high capability for organic dye photodegradation.
Figure 9. SEM image showing typical ultralong and flexible Na2Ti6O13 whiskers grown from a NaCl flux.
Figure 10. (a) Lattice image, (b) TEM micrograph, and (c) diffraction pattern of a typical ultralong and flexible Na2Ti6O13 whisker (run 13).
Finally, it is very important to reduce environmental load in order to make environmentally functional materials. The rapid cooling growth is more environmentally friendly than the slow cooling method.5 In particular, the growth period of the ultrarapid cooling (water quenching) is only about 35 h and is much shorter than that of slow cooling (5 °C h-1), that is, 155 h, reaching about 1/5. Furthermore, since ultralong and flexible Na2Ti6O13 whiskers grown by our environmentally friendly technique have high crystallinity and their characteristic properties, they will be favorable materials for various technological applications such as novel electronic materials, asbestos alternatives, composites, and environmental functional materials (photocatalyst, adsorbents, etc.). Conclusions High quality and well formed whiskers of Na2Ti6O13, which were transparent and colorless, were successfully grown by the cooling of the NaCl flux. In the case of the stoichiometric mixtures, the Na2Ti6O13 whiskers were up to 101.5 µm × 0.7 µm in average size and 145 in aspect ratio. The whiskers were usually produced as shiny and cottony aggregates. Their basic form was a hexagonal rod with relatively smooth surfaces. The whisker size and aspect ratio were dependent on the cooling rate and solute concentration. They increased drastically (solute conc., 0.5 mol %) or gradually (solute conc., 5 and 20 mol %)
Crystalline Photocatalytic Na2Ti6O13 Whiskers
with an increasing cooling rate. The crystallinity of the grown whiskers was almost the same, independent of cooling rates. The obtained whiskers had no defects, and major components were homogeneously distributed in the whiskers. They exhibited good photocatalytic properties under UV light irradiation. For the use of the nonstoichiometric mixture, ultralong and flexible photocatalytic whiskers of Na2Ti6O13, having a high aspect ratio >500, were successfully grown from the NaCl flux. Our whisker growth technique is one of the environmentally friendly processes and can be applied to grow functional oxide crystals. Finally, we can recognize from this research that NaCl is adequate for an environmentally friendly growth of Na2Ti6O13 whiskers. Acknowledgment. This research was partially supported by the CLUSTER of the Ministry of Education, Culture, Sports, Science and Technology. A part of this work was supported by the TEPCO Research Foundation and The Salt Science Research Foundation, No.0706. A part of this work was performed under
Crystal Growth & Design, Vol. 8, No. 2, 2008 469
the interuniversity cooperative research program of Advanced Research Center Metallic Glasses, Institute for Materials Research, Tohoku University.
References (1) Iijima, S. Nature 1991, 354, 56. (2) Endo, M.; Muramatsu, H.; Hayashi, T.; Kim, Y. A.; Terrones, M.; Dresselhaus, M. S. Nature 2005, 433, 476. (3) Teshima, K.; Yubuta, K.; Sugiura, S.; Fujita, Y.; Suzuki, T.; Endo, M.; Shishido, T.; Oishi, S. Cryst. Growth Des. 2006, 6, 1598. (4) Teshima, K.; Yubuta, K.; Ooi, S.; Suzuki, T.; Shishido, T.; Oishi, S. Cryst. Growth Des. 2006, 6, 2538. (5) Teshima, K.; Yubuta, K.; Sugiura, S.; Suzuki, T.; Shishido, T.; Oishi, S. Bull. Chem. Soc. Jpn. 2006, 79, 1725. (6) Koyama, T.; Endo, M. Jpn. J. Appl. Phys 1974, 3, 1175. (7) Nelson, J. A.; Wagner, M. J. J. Am. Chem. Soc. 2003, 125, 332. (8) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (9) ICDD PDF 73-1398. (10) ICDD PDF 37-951.
CG070341P
