Calcination Syntheses of a Series of Potassium Titanates and Their

Continuous gradual calcination syntheses of a series of potassium titanates were achieved by replacing crystalline TiO2 reactants, such as anatase and...
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Calcination Syntheses of a Series of Potassium Titanates and Their Morphologic Evolution Ningzhong Bao,† Xin Feng,† Liming Shen,‡ and Xiaohua Lu*,† Department of Chemical Engineering and Department of Material Science and Engineering, Nanjing University of Chemical Technology, Nanjing 210009, People’s Republic of China

CRYSTAL GROWTH & DESIGN 2002 VOL. 2, NO. 5 437-442

Received June 18, 2002

ABSTRACT: In the present study, K2Ti2O5‚0.5H2O, K2Ti2O5, K2Ti4O9, K2Ti6O13(I), and K2Ti6O13(II) were synthesized by calcination from TiO2‚nH2O hydrate at 650, 820, 920, 1080, and 1123 °C, respectively. K2Ti2O5‚0.5H2O and K2Ti2O5 are composed of grain aggregates and single crystals, respectively. K2Ti4O9, K2Ti6O13(I), and K2Ti6O13(II) all have whisker morphology. The K2Ti4O9 whiskers and hydrosoluble surface wrappage occurred simultaneously in the K2Ti2O5 single crystals at 920 °C during calcination. Subsequently, K2Ti6O13(I) whiskers, coated by more hydrosoluble wrappage generated from both the K2Ti2O5 single crystals and the K2Ti4O9 whiskers, evolved in the K2Ti4O9 whiskers at 1080 °C. K2Ti6O13(I) whiskers converted into K2Ti6O13(II) whiskers through a crystal lattice deformation occurring at 1123 °C. The sinters of K2Ti4O9, K2Ti6O13(I), and K2Ti6O13(II) are all composed of the axial-array whisker sheaves. Introduction Potassium titanates,1-2 as well as K2Ti2O5, K2Ti4O9, and K2Ti6O13, are important functional materials1-3 and are of economic importance, are low cost, have wide applications, and assume layered or cage type structures.1-3 These whiskers are mainly used as photocatalysts,4 precursors for the synthesis of new titania-based materials by ion-exchange reaction5-7 or intercalation reaction,8-9 and reinforcing agents for the preparation of high-performance plastics10-11 and ceramics.12 Potassium titanates have been prepared from starting materials of anatase/rutile and potassium carbonate by the calcination method,1-3,13-15 which is simple and has achieved continuous industrial manufacture.3 Potassium carbonate is decomposed into potassium oxide that subsequently reacts with anatase/rutile, by which various types of potassium titanates were synthesized under corresponding synthesis conditions, such as temperature and TiO2/K2O molar ratio.13-16 Usually, a relatively high temperature is required for the formation of whiskers, and particles but no whisker products are synthesized below 1000 °C. We have fully studied the formation and growth of potassium titanate whiskers that were synthesized at high temperature from crystalline anatase/ rutile.14 The active TiO2‚nH2O hydrate is of both low crystalline and low reactivity energy. In large synthesis reactions, substituting crystalline reactants with amorphous reactants will change the reaction process. For the hydrothermal syntheses of some perovskites (e.g. sodium and potassium bismuth titanates,17 lead zirconate titanate,18 and lead titanate19) and zeolites (e.g. cobalt phosphate20), good kinetic conditions for optimum crystal growth and a low synthesis temperature were met when amorphous TiO2‚nH2O hydrate was used as reactant, which has also been proved by thermo* To whom correspondence should be addressed. E-mail: xhlu@ njuct.edu.cn; [email protected] (X. Lu). † Department of Chemical Engineering. ‡ Department of Material Science and Engineering.

dynamic calculations and the corresponding experimental results.17-20 If the TiO2‚nH2O hydrate is used to prepare potassium titanates by calcination, new results, similar to those of hydrothermal synthesis, could be found, which are of great use for the preparation of potassium titanates with high quality and good morphology. However, less attention was given to this point. In the present study, we investigate the influence of crystallinity of titania reactants on the calcination synthesis of potassium titanates. A series of potassium titanates, having different morphologies, are prepared from the TiO2‚nH2O hydrate, and the morphologic evolution of products, with an increase in reaction temperature, is observed. Experimental Section General Procedure. TiO2‚nH2O hydrate was prepared by hydrolyzing titanyl sulfate (reagent grade) in boiling water with vigorous stirring and then concentrated by centrifugation. K2CO3 (reagent grade) was mixed with the TiO2‚nH2O hydrate at a TiO2/K2O molar ratio of 3.0. The mixture was dried in an oven at 90 °C in vacuo for 10 h. The calcination of dried mixtures was performed in a muffle furnace. All samples were removed from the furnace at the corresponding calcination temperatures by the end of the holding time and cooled in air. Designed experiments are shown in Table 1. Two samples for TGA-DTA tests were prepared with (a) an anatase-K2CO3 mixture and (b) a TiO2‚nH2O-K2CO3 mixture, both having a TiO2/K2O molar ratio of 3.0. Measurements. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) experiments were performed from 20 °C up to 1200 °C in N2 at the heating rate of 20 °C min-1 on an analyzer (TGA-DTA, Model SDT 2960, TA Instruments, Inc.). X-ray powder diffraction patterns were obtained using a D8 Advance instrument from Bruker. Cu KR radiation with a nickel filter and a zero-background sample cell were used, operating at 40 kV and 20 mA. All samples were measured in the continuous-scan mode at 5-60° (2θ) with a scanning rate of 0.02° (2θ)/s. Peak positions and their relative intensities of crystal products were characterized by comparing to data from the ICDD (International Centre for Diffraction Data, Newtown Square, PA (formerly Joint Committee for Powder Diffraction Standards (JCPDS), Swarthmore, PA)).

10.1021/cg025541+ CCC: $22.00 © 2002 American Chemical Society Published on Web 08/13/2002

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Table 1. Reactant Composition and Reaction Conditionsa sample

calcination temp (°C)

calcination time (h)

A1 A2 B1 B2 C1 C2 D E

650 650 820 820 920 920 1080 1123

0.25 10 0.25 10 0.25 10 0.25 0.25

a The TiO /K O molar ratio was 3.0 and the heating rate was 5 2 2 °C min-1 for all samples.

Figure 1. TGA-DTA profiles of (a, s) the anatase-K2CO3 mixture and (b, ‚‚ - ‚‚) the TiO2‚nH2O-K2CO3 mixture, both with the same TiO2/K2O molar ratio of 3.0. Morphologies of products were observed via optical microscope (Model Galen III, Jiangnan Optical Instrument Co., Ltd., Nanjing, People’s Republic of China) and scanning electron microscope (SEM, JEOL JSM-6300, Tokyo, Japan).

Results and Discussions TGA-DTA Analysis. Parts a and b of Figure 1 show the TGA-DTA curves for an anatase-K2CO3 mixture and a TiO2‚nH2O-K2CO3 mixture, respectively, both with a TiO2/K2O molar ratio of 3.0. Two horizontal steps are visible on the TGA curve of Figure 1a, due to the dehydration of the free water and the decomposition of K2CO3, respectively. However, a long weight-loss curve covers the temperatures ranging from 100 to 650 °C, ending with one step on the TGA trace of Figure 1b, indicating that the dehydration of TiO2‚nH2O and the decomposition of K2CO3 are processed simultaneously. TGA traces both are horizontal at T > 800 °C. At T < 800 °C, two endothermic peaks appear at 120 and 720 °C on the DTA curve of Figure 1a, due to the evaporation of free water and the decomposition of K2CO3, respectively. Two endothermic peaks also appear at 150 and 600 °C on the DTA curve of Figure 1b, where one peak at 150 °C indicates the dehydration of free water and another peak at 600 °C does not indicate the decomposition of K2CO3, whose decomposition temperature should be around 700 °C.21 Furthermore, because the TGA curve types in parts a and b of Figure 1 are different, we thus deduce that the dehydration of TiO2‚ nH2O, the decomposition of K2CO3, and the reaction between them occur simultaneously at 150-600 °C, by which point amorphous potassium titanate is generated.

Figure 2. X-ray powder diffraction patterns of A1, A2, B1, B2, C1, C2, D, and E. The strongest characteristic peaks of different components are marked with 0 (K2Ti2O5‚0.5H2O), 9 (K2Ti2O5), O (K2Ti4O9), 4 (K2Ti6O13(I)), and 2 (K2Ti6O13(II)).

The amorphous potassium titanate converts into a type of potassium titanate by a crystallization reaction at 600-610 °C. Because there exists a small amount of weight loss at 600 °C < T < 700 °C on the TGA curve of Figure 1b, the crystal potassium titanate generated at 600-610 °C contains a small amount of crystallized water, which can be dehydrated totally at 700 °C. At T > 800 °C, three endothermic peaks appear at 860, 960, and 1150 °C on the DTA curve of Figure 1a and four endothermic peaks appear at 820, 920, 1080, and 1110 °C on the DTA curve of Figure 1b, both due to the generation of a series of potassium titanates. All of the endothermic peaks on the DTA trace of Figure 1b at T > 800 °C are weaker than the corresponding endothermic peaks on the DTA trace of Figure 1a, indicating that the intensities of the reactions and the phase changes are weaker when TiO2‚nH2O hydrate is used as reactant here. The reaction system in anataseK2CO3 with a TiO2/K2O molar ratio of 3.0 has been well studied experimentally by TGA-DTA, XRD, and SEM for products with a low whisker content.13-15 The present TGA-DTA traces for the anatase-K2CO3 mixture show good agreement with those earlier reports. Endothermic peaks at 860, 960, and 1150 °C on the DTA curve (Figure 1a) indicate the generation of K2Ti2O5, K2Ti4O9, and K2Ti6O13, respectively. However, in comparison with the TGA-DTA traces of the anatase-K2CO3 mixture, those of the TiO2‚nH2O-K2CO3 mixture show a continuously gradual reaction that provides the mild conditions suitable for attaining crystal growth. XRD. XRD patterns of all samples are shown in Figure 2.

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Figure 3. Micrographs of crystal products in A1-E imaged by an optical microscope in water. The scale bars all indicate 10 µm.

Peak positions are indexed and compared with the files of ICDD. Both A1 and A2 are K2Ti2O5‚0.5H2O, and the peak intensities of A2 are stronger than those of A1, indicating that the crystallinity of A2 is higher than that of A1. Both B1 and B2 are K2Ti2O5, containing a

small amount of K2Ti4O9. Peak intensities of B2 are stronger than those of B1, indicating that products generated at 820 °C are a mixture of K2Ti2O5 and K2Ti4O9. C1 is a mixture of K2Ti2O5 and K2Ti4O9, and C2 is pure K2Ti4O9, indicating that K2Ti2O5 entirely con-

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Figure 4. SEM images of C1 and D: (C1-1 and C1-2) K2Ti4O9 sinters; (D-1 and D-2) K2Ti6O13(I) sinters; (C1-3) K2Ti4O9 whiskers; (D-3) K2Ti6O13(I) whiskers. The scale bars of C1-1, C1-3, D-1, and D-3 indicate 10 µm. The scale bars of C1-2 and D-2 indicate 1 µm.

verted to K2Ti4O9 at 920 °C during 10 h of calcination. D is one type of potassium hexatitanate (K2Ti6O13(I), a ) 15.593 Å, b ) 3.796 Å, c ) 9.108 Å, β ) 99.78°), containing a small amount of K2Ti4O9. The peak intensity of K2Ti4O9 in D is weaker than that of C2, indicating that K2Ti4O9 converted to K2Ti6O13(I) at 1080 °C. E is another type of potassium hexatitanate (K2Ti6O13(II), a ) 15.6 Å, b ) 3.8 Å, c ) 9.13 Å, β ) 99.6°). XRD results verify that the endothermic peak at 600 °C on the DTA trace of Figure 1a indicates the generation of K2Ti2O5‚0.5H2O. The endothermic peak at 820 °C indicates a dehydration reaction from K2Ti2O5‚0.5H2O to K2Ti2O5. Furthermore, the endothermic peaks at 920, 1080, and 1110 °C indicate the generation of K2Ti4O9, K2Ti6O13(I), and K2Ti6O13(II), respectively. In our previous result,14 K2Ti2O5, K2Ti4O9, and K2Ti6O13(II) were synthesized at temperatures agreeing with the endothermic peaks at 860, 960, and 1150 °C on the DTA trace of Figure 1b. Both the synthesis results in experiments and the analysis of TGA-DTA curves verified that the TiO2‚nH2O hydrate lowers the phase change temperatures at which a series of potassium titanates are synthesized. Moreover, the weak intensity of each endothermic peak on the DTA curve of Figure 1b indicates that the use of amorphous TiO2‚nH2O hydrate provides a continuously gradual reaction that provides the mild conditions suitable for attaining crystal growth. Morphologic Evolution of Potassium Titanates. In Figure 3, “A1”, “A2”, “B1”, “B2”, “C1”, “C2”, “D”, and

“E” are the images of the crystal potassium titanate products in A1, A2, B1, B2, C1, C2, D, and E imaged by the optical microscope, respectively. Because the hydrous surface-coating product within the sinters of A1-E were leached in hot water for 2 h, the original places of potassium titanate products in the sinters are maintained and viewed in A1-E in Figure 3, respectively. In Figure 3, both A1 and A2 are K2Ti2O5‚0.5H2O grain agglomerations. The grains in A1 are smaller than those in A2, indicating that the crystal growth of K2Ti2O5‚ 0.5H2O lasts for 10 h at 650 °C. Both B1 and B2, having the same diameter of 10∼12 µm and the same length of 50∼100 µm, are K2Ti2O5 single crystals, indicating that the K2Ti2O5 single crystals are generated at 820 °C and no further crystal growth occurs during 10 h of calcination. A comparison of the morphologies of sinters in A1 with those in B1 shows that K2Ti2O5‚0.5H2O grain agglomerations converted to K2Ti2O5 single crystals at 820 °C. Both C1 and C2, having the same diameter of 1.0∼2 µm and the same length of 10∼50 µm, are K2Ti4O9 whisker bunches. We also observe that the undispersed K2Ti4O9 whisker bunches in C1 and C2 share a close size with that of the K2Ti2O5 single crystals in B1, indicating that the K2Ti4O9 whiskers evolve in the K2Ti2O5 single crystals. D and E are K2Ti6O13(I) whiskers and K2Ti6O13(II) whiskers, respectively, and both have the same diameter of 0.5∼1 µm and the same length of 10∼50 µm, indicating a crystal transformation

Calcination Syntheses of Potassium Titanates

Figure 5. Microstructure of D. A sole whisker partly separated from the bunch body in F-1 was immersed in ethanol and imaged as shown in F-2. The sinter structure in F-1 is observed as shown in F-3 after being leached in hot water for 2 h to remove the hydrosoluble wrappage.

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that occurred from K2Ti6O13(I) to K2Ti6O13(II), without affecting their whisker morphology. Formation and Growth of Potassium Titanate Whiskers. In Figure 4, both “C1-1” and “C1-2” are SEM images of the sinters of C1 (K2Ti4O9). Both “D-1” and “D-2” are SEM images of the sinters of D (K2Ti6O13(I)). “C1-3” and “D-3” are SEM images of the dispersed potassium titanate whiskers in the sinters of C1 (K2Ti4O9) and D (K2Ti6O13(I)). The K2Ti4O9 whisker aggregations within C1 were revealed, as shown by “C1-1” and “C1-2” in Figure 4. It can be found that the sinters of C1 are composed of the whiskers and the surface wrappage. The boiling water was used to disperse the K2Ti4O9 whiskers, as shown by “C1-3” in Figure 4. A comparison of the surface morphology of the sinters in C1-1 and C1-2 with the surface of the K2Ti4O9 whiskers in C1-3 shows that the surface wrappage in C1-1 and C1-2 is hydrosoluble. The surface wrappage and the K2Ti4O9 whiskers in C1 are all generated in the K2Ti2O5 single crystals, as shown in B1 in Figure 2. The same phenomena are shown for the K2Ti6O13 sinter in D-1 and D-2 and the K2Ti6O13 whiskers in D-3. Thus, both the K2Ti4O9 sinters in B1 and the K2Ti6O13 sinters in C1 were composed of the whisker products and hydrous surface wrappage. However, the K2Ti6O13 whiskers in the sinters of D-1 and D-2 are entirely coated by a large amount of the hydrosoluble wrappage, so that the whiskers in the sinters were not clearly observable. After the K2Ti6O13(I) sinters of D were treated in boiling water, the K2Ti6O13(I) whiskers of D-3 were clearly visible. The diameter of K2Ti6O13(I) in D-3 is smaller than that of K2Ti4O9 in C1-3, and the quantity of the surface wrappage in D is more than that of C1, indicating that the K2Ti6O13(I) whisker is formed in the K2Ti4O9(I) whiskers. Thus, a higher reaction temperature causes more surface wrappage Figure 5 shows a detailed study on the structure of D.

Figure 6. Schematic representation for the evolution of the morphology and the components of different types of potassium titanates.

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F-1 in Figure 5 shows that the K2Ti6O13(I) sinter is composed of the whisker bunches. A sole whisker partly separated from the bulk was immersed in ethanol to prevent its surface wrappage from dissolving, shown in F-2 in Figure 5. It can be observed that the sole whisker is composed of the shell of the surface wrappage and the whisker core. When the whisker bunches in F-1 in Figure 5 were leached in hot water for 2 h to dissolve the hydrosoluble surface wrappage, the whisker bunches are clearly visible (in F-3 in Figure 5). Scheme of Reactions and Morphologic Evolution of Potassium Titanates. Figure 6 shows a schematic representation for the evolution of the morphology and the component of the potassium titanates during calcination up to 1200 °C when the TiO2‚nH2O hydrate was used as reactant. The formation and the growth of different types of potassium titanate whiskers were also illustrated schematically, as well as by 600°C

dried mixture of TiO2‚nH2O and K2CO3 98

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Conclusions The present study addresses the reaction process and the product morphology in the calcination synthesis of a series of potassium titanates from the TiO2‚nH2O hydrate. Compared with the previous studies in which anatase and rutile are reactants, for the same crystal product, the generation temperature would decrease by 20-40 °C, and crystal growth would occur over a longer time and under mild and gradually changeable reaction conditions. With an increase in the temperature, a series of potassium titanates are synthesized into grain aggregations, single crystals, and whiskers. The hydrosoluble melt plays a critical role in the formation and the growth of potassium titanate whiskers, which results in the generation of potassium titanates with different morphologies and sizes. Acknowledgment. We thank the Outstanding Youth Fund of the National Natural Science Foundation of the People’s Republic of China (Grant No. 29925616). References

820°C

K2Ti2O5‚0.5H2O 98 K2Ti2O5 single crystals + 8 K2Ti4O9 whiskers + small amount of K2Ti4O9 9 920°C 1080°C

liquid melt 98 K2Ti6O13(I) whiskers + 1123°C

liquid melt98 K2Ti6O13(II) + liquid melt Here, the liquid melt converts to the hydrosouble wrappage coating on the whisker surface when the sinters are cooled in air. The liquid melt generated in the K2Ti2O5 single crystals splits the K2Ti2O5 single crystals into the K2Ti4O9 whiskers along the axial direction. When the K2Ti4O9 whiskers convert into the K2Ti6O13(I) whiskers, much more liquid melt is generated in K2Ti4O9 along the radial direction, which makes the diameter of the K2Ti6O13(I) whiskers smaller than that of the K2Ti4O9 whiskers. After the release of water and CO2 at T < 700 °C, the reaction system will change from starting materials of the TiO2‚nH2O-K2CO3 system to the TiO2-K2O system, both with the same TiO2/K2O molar ratio of 3.0. At the later different stages, K2Ti2O5, K2Ti4O9, K2Ti6O13(I), and K2Ti6O13(II) are synthesized, in which a different amount of noncrystalline K2O-rich melt is also generated. The average TiO2/K2O composition usually does not change. Although some K2O in the melt might be evaporated at high temperature, however, this does not change the chemical composition of crystal products of K2Ti2O5, K2Ti4O9, K2Ti6O13(I), and K2Ti6O13(II).

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