Study on the Growth Mechanism of K₂Ti₄O₉ Crystal

Introduction

Potassium hexatitanate (K₂Ti₄O₉) is a two-dimensional layered compound, which is formed by the attachment of octahedral units of TiO₆ with oxygen bridging atoms. K₂Ti₄O₉ has a large layer spacing [1, 2] and high K⁺ activity between the layers, resulting in a poor chemical stability. However, the unique structure of K₂Ti₄O₉ endows it great applications in ion exchange, adsorption [3, 4], and photocatalysis. In 1958, E.I. Du Pont Company developed K₂Ti₆O₁₃ whiskers, mainly used in thermal insulation materials. Then, K₂Ti₄O₉ crystals were extensively studied, mainly in Japan. Many studies show that K₂Ti₄O₉ crystals exhibit excellent performance such as chemical stability, high strength, high modulus, high heat resistant, high infrared reflectivity, corrosion resistance, and other properties, and they are considered as a new type of functional material. Zhang et al. [5] synthesized titanate nanowires by microwave-assisted synthesis. Kang et al. [6] prepared K₂Ti₄O₉ nanostructures by a sol–gel–calcination process. Zhou et al. [7] prefabricated and characterized alkaline resistant porous ceramics from K₂Ti₄O₉ whiskers. K₂Ti₄O₉ whiskers have been prepared by many methods by Chen, Liu, Sun, Wang, and others [5, 8-13]. The studies on K₂Ti₄O₉ whiskers mainly focused on preparation methods, characteristics, and applications; however, the synthesis mechanism and growth pattern of K₂Ti₄O₉ whiskers have been rarely studied. Herein, we report a growth mechanism and growth model of K₂Ti₄O₉ whiskers in this paper.

Experimental

Results and discussion

Figure 1 shows the phase diagram of TiO₂–K₂O system. As shown in Figure 1, K₂Ti₂O₅, K₂Ti₄O₉, and K₂Ti₆O₁₃ exist stably in the phase diagram. When the ratio of TiO₂ and K₂O is in the range 2–4, the main crystal phases are K₂Ti₂O₅ and K₂Ti₄O₉. The resulting products are K₂Ti₄O₉ and K₂Ti₆O₁₃, whereas the ratio of TiO₂ and K₂O is 4–5.67. Because of the continuous loss of potassium source during the heating, to obtain the single crystal of K₂Ti₄O₉, the actual ingredients should be slightly deviated from the theoretical value. Figure 1 shows the approximate ratios 1 and 2 for the generation of the single phase of K₂Ti₄O₉ and K₂Ti₆O₁₃.

Figure 1:

Phase diagram of TiO₂–K₂O system.

Figure 2 shows the TG-DTA curves of mixture of TiO₂ and K₂CO₃. An obvious endothermic peak was observed at 155–225 °C corresponding to the loss of weight in the TG curve (~12 %), indicating the dehydration of the system. At 550–800 °C, the TG curve decreased rapidly, and the quality of the system was reduced (~9 %). This can be attributed to the decomposition of K₂CO₃ and continuous removal of CO₂. With the increase in temperature, K₂O and K₂CO₃ also began to melt and became volatile. After 800 °C, the TG curve did not show a decline, and the K₂CO₃ decomposition was complete. A broad exothermic peak was observed at 881 °C in the DTA curve; the corresponding TG curve did not change, indicating the nucleation and growth of K₂Ti₄O₉ crystals. Therefore, the reaction temperature should be controlled at 800 °C or above.

Figure 2:

TG–DTA curves of mixture.

Figure 3 shows the XRD patterns of samples with different T/K ratios calcined at 900 °C. The main crystal phase was K₂Ti₄O₉ when the T/K ratio was 3, almost consistent with the standard PDF card (32-0861). K₂Ti₂O₅ was obtained when the T/K ratio was 2.5, indicating that the unstable phase favored the fixation of the potassium source at a low temperature. When the T/K ratios were 3.5 and 4.5, a small amount of K₂Ti₂O₅ and K₂Ti₆O₁₃ was obtained, respectively, indicating that a T/K ratio of 3 was more conducive to the formation of K₂Ti₄O_9.

Figure 3:

XRD patterns of samples with different T/K ratios calcined at 900 °C.

Figure 4 shows the XRD patterns of the samples with the T/K ratio of 3 after sintering at different temperatures. The main crystal phase was K₂Ti₄O₉ when the calcination temperature was 800 °C or above. According to the composition of the samples at different temperatures, the DTA curve has a crest where water and K₂CO₃ exist stably with 1.5 crystal water during the drying process before the baking in the KDC method. During the drying, the K₂CO₃ water solution diffused to the surface, resulting in a slightly higher K₂CO₃ concentration on the surface of the dry material. On further heating, K₂CO₃ decomposed as follows:

Figure 4:

(a) XRD patterns of samples with T/K=3 at a low temperature. (b) XRD patterns of samples with T/K=3 at a high temperature.

At the same time, K₂CO₃ and K₂O react with TiO₂ as follows:

With the increase in temperature, a large amount of K₂CO₃ decomposes to K₂O. Because the amount of local TiO₂ material is not sufficient, K₂O reacts with TiO₂ as follows:

With the increase in temperature, Amor°A (l) reacts with TiO₂ to afford K₂Ti₈O₉,

K₂Ti₄O₉ (s) and K₂O (l) are obtained at 800 °C,

As shown in Figure 4(b), the main crystal growth planes of K₂Ti₄O₉, (200, 201, 004), (6ˉ03), and (205), are parallel to the (010) crystal zone axis; thus, the axial growth direction of K₂Ti₄O₉ was [010]. The (020) crystal plane was perpendicular to the direction of [010] and the end of K₂Ti₄O₉ crystal. The (200) plane, the main crystal growth face of K₃Ti₈O₁₇, was also parallel to the [010] direction; therefore, the production of K₃Ti₈O₁₇ was conducive to the directional diffusion of [K] and [O], promoting the rapid synthesis of K₂Ti₄O₉. The growth of K₂Ti₄O₉ is consistent with the LS mechanism [14, 15]. In the beginning, the gas (CO₂) formed by K₂CO₃ decomposition can form numerous microchannels in the reaction system, also conducive to the diffusion and volatilization of K₂O. A reaction zone would form during the diffusion of K₂O with TiO₂. The reaction in the microzone can promote the formation of K₂Ti₄O₉ crystal nucleus, resulting in a large probability of nucleation along the direction of K₂O easy to escape. With the decrease in the temperature in the interface region of crystal nucleus, nucleation growth occurred in the direction of temperature drop. The temperature of the calcined sample decreased from the surface to the interior, thus forming a K₂Ti₄O₉ fiber bundle from the outside.

The structure and morphology of the sample were analyzed by TEM. Typical TEM images of the K₂Ti₄O₉ whiskers are shown in Figure 5. The selected-area electron diffraction (SAED) pattern (inset of Figure 5) viewed along [1, 9, 2ˉ] the zone axis shows that the K₂Ti₄O₉ whiskers are single crystalline material, which can also be indexed using the K₂Ti₄O₉ crystal structure, and the K₂Ti₄O₉ whiskers grow along the [010] direction.

Figure 5:

TEM images of K₂Ti₄O₉ whiskers incinerated at 1050 °C.

Figure 6 shows the XRD patterns of samples with different holding times calcined at 800 °C. The main crystal phase was K₂Ti₄O₉ when the holding times were 1 h, 2 h and 3 h. K₃Ti₈O₁₇ was transformed into K₂Ti₄O₉ with the increase in holding time. Therefore, holding time has obvious influence on the crystal phase when the calcination temperature is lower.

Figure 6:

XRD patterns of samples with different holding times calcined at 800 °C.

Figure 7 shows the representative SEM micrographs of the crystal whisker of the K₂Ti₄O₉ samples after the calcination at different temperatures. Figure 7(a) shows that the raw material was dispersed into independent spherical small units with a size of <400 nm after the ball milling. The raw material was completely transformed into K₃Ti₈O₁₇ at 700 °C, and a thimbleful of K₃Ti₈O₁₇ was transformed into K₂Ti₄O₉. Figure 7(b) shows that the product still had a different granular shape, but some small whiskers with a diameter of 60 nm emerged. With the increase in temperature, K₂Ti₄O₉ whiskers arranged in a crisscross pattern were formed by the gradual disintegration of grain. Figure 7(c) shows that the diameter of the whisker is 142 nm, and the particle unit begins to appear in the liquid phase, causing a fuzzy boundary between the particles. After the heated preservation at 800 °C for 2 h, the diameter and length of the whisker changed to 280 nm and 0.6 μm, respectively, and small short rods of K₂Ti₄O₉ began to disperse, as shown in Figure 7(d). The measured value of the diameter of the whisker showed that the diameter increased exponentially, and nanowhiskers were formed in parallel. The EDS images shown in Figure 8(a) correspond to the sample shown in Figure 7(c). Figure 8(a) shows the local liquid enrichment of the sample, and the samples are composed of Ti, K, and O. This indicates that the potassium source began to melt, and the liquid–solid reaction progressed by the inclusion of TiO₂.

Figure 7:

Evolution process of K₂Ti₄O₉ morphology. (a) raw material, (b) 700 °C, heat preservation for 1 h, (c) 800 °C, heat preservation for 1 h (d) 800 °C, heat preservation for 2 h, (e) 950 °C, heat preservation for 2 h, (f) 1050 °C, heat preservation for 2 h.

Figure 8:

EDS images of K₂Ti₄O₉ crystals. (a) 800 °C, heat preservation for 1 h; (b) 950 °C, heat preservation for 2 h.

Figure 7(e) shows that the diameter of the K₂Ti₄O₉ whisker is ~360 nm, and the length of the whisker is ~30 μm. In contrast to Figure 7(d) and (e), the diameter changed slightly, but the length changed 50 times, indicating that the crystal whiskers undergo a series of rapid growth with the increase in energy. The small whiskers are connected in series to form medium whiskers, and the secondary whiskers are connected in series to form large whiskers. Figure 7(f) shows that the diameter of the K₂Ti₄O₉ whisker is ~690 nm, and the length of the whisker is ~80 μm. In contrast, Figure 7(e) and (f) show that the length of whisker increased by three times, and the diameter increased by one time, indicating that the whiskers are connected mainly in series, parallel to the auxiliary. At 950 °C, K₂Ti₄O₉ whisker is long enough and perplexing; the growth space is hindered. However, Figure 7(f) shows that the whisker length is similar, and the growth direction is consistent. This is because K₂Ti₄O₉ whisker has a certain degree of softening at a high temperature, and to grow further, the whisker coordinated creep and final growth orientation are consistent. Figure 8(b) shows a partial melting liquid phase in the sample at 950 °C. This phase is composed of K and O and shows that the rapid growth of K₂Ti₄O₉ whiskers in the anaphase is no longer due to the solid-solution reaction of TiO₂ with K₂O, but due to the series parallel effect of the crystal.

Conclusions

K₂O_˙nTiO₂ (n=3) crystal whiskers were prepared by the KDC method. Calcination of the sample from 500 °C to 1050 °C completely converted the mixture to K₂Ti₄O₉ crystals at 800 °C. Before 800 °C, different transition phases such as K₃Ti₈O₁₇, Amor°A (l), and K₂Ti₈O₉ appeared in the samples. The emergence of K₃Ti₈O₁₇ is conducive to the directional diffusion of K and O, thus promoting the rapid synthesis of K₂Ti₄O₉. Combined with the LS growth mechanism, K₂Ti₄O₉ whiskers mainly grow by parallel action at a low temperature, and the series effect is obvious with the increase in temperature.