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The swelling transition of lepidocrocite-type protonated layered titanates into anatase under hydrothermal treatment

The common facets of anatase crystals are the (001) and (101) planes. However, the phase transformation from lepidocrocite-type titanate into anatase by hydrothermal processing yields an anatase microstructure with high concentration of exposed (010) planes. The phase transformation of a lepidocrocite-type protonated layered titanate (HTO) into anatase was studied using XRD, TEM, FTIR, and measurement of pH and zeta potential. It was found that HTO is proton-deficient. The phase transformation process begins after uptake of a sufficient number of protons into the lepidocrocite-type structure. With the uptake of protons new hydroxyl groups form on the internal surfaces of the layered titanate and result in a bilayer state of HTO. The phase transformation reaction is a topotactic dehydration reaction in which anatase forms and water is expelled by syneresis.

The formation mechanism of anatase has been considered to involve an in situ topotactic transformation reaction in the crystal bulk of the layered titanate H 1.07 Ti 1.73 O 4 ?nH 2 O 12 . It has been proposed that a distorted layered structure is formed during hydrothermal treatment, which is subsequently transformed into a distorted anatase structure, and subsequently into the normal anatase structure 13 . However, details of the topotactic phase transformation mechanism are lacking. Such information could help in preparing new materials by this approach. We studied the phase transformation from lepidocrocite-type titanates to anatase with H 1.07 Ti 1.73 O 4 ?nH 2 O (HTO) as model compound because of its simple structure among the topotactic reactants 11,13,14 . HTO can easily be obtained from K 0.8 [Ti 1.73 Li 0.27 O 4 ] (KLTO) by an ion exchange reaction 15,16 . KLTO is composed of corrugated host layers of edge-sharing TiO 6 octahedra and interlayer K 1 ions compensating for the negative charge of the TiO 6 octahedral layers, wherein Li 1 ions occupy 13.5% of the Ti octahedral sites in the host layers, as shown in Figure 1. Only 40% of the potassium positions shown in Figure 1 are occupied 15 . After protonation a monolayer of water and H 3 O 1 occupies the interlayer space instead of alkali ions 15 .
In the present paper the behavior of lepidocrocite-type protonated layered titanates derived from KLTO under hydrothermal treatment is reported. We present details of the topotactic reaction of lepidocrocite-type titanates to anatase during hydrothermal treatment. Our results show that the process first goes through a stage of protons uptake, which causes swelling of the layered material. Then, with increasing concentration of protons at a suitable temperature the topotactic phase transformation reaction occurs. It is noted that this work focuses on the phase evolution prior to and in the early stages of transformation rather than on the stage after the dehydration reaction. Details of the phase transformation after topotactic reaction can be found in the work of Wen et al. 13

Results
The uptake of protons and swelling of the layered structure. We performed a series of experiments to investigate the influence of hydrothermal treatment temperature on HTO pre-dried at room temperature (further denoted as HTO RT) in aqueous solutions at 80-120uC. Figure 2a shows the XRD pattern of HTO RT. It is consistent with a protonated titanate with one hydrous layer 15,17 . The peak at 2h 5 9.59u indicates that the basal spacing between the layers is 0.92 nm. Figure 2b shows that after HTO RT was hydrothermally treated at 80uC, a small peak corresponding with a basal spacing of 1.15 nm was observed at 2h 5 7.72u, suggesting an expansion of the interlayer distance between the titanate planes of HTO. The increase of ,0.20 nm is approximately equal to the molecular size of water (,0.25 nm) 18 . At a hydrothermal treatment temperature of 120uC, the peak intensity increased by a factor of ,16 compared to the peak intensity of a sample prepared at 80uC (Figure 2c). This indicates that the swollen phase was the dominant phase at 120uC. The shoulder between the peaks at 2h 5 7.72u and 9.59u implies that the new phase was derived from HTO RT. Also traces of the anatase phase can be observed in Figure 2c. Figure 2d shows the XRD pattern of HTO RT after repetitive hydrothermal treatment at 120uC, which is discussed in more detail below.
The pH change after 24 h of hydrothermal treatment as a function of treatment temperature is shown in Figure 3. It is noted that no further pH changes occurred when the hydrothermal treatment process was prolonged beyond 24 h. The data revealed an increasing pH with increasing hydrothermal treatment temperature, with an anomaly around 100uC. Repeated hydrothermal experiments at 100uC resulted in final pH values of 7.2, 7.3, 7.6 and 7.7. Hence, the change of pH below 100uC was limited to around pH 5 8, although the trend would predict a higher pH. The final pH increased further at temperatures above 100uC. The trend in pH indicates a decreasing proton concentration in the solution with increasing treatment temperature. The reduced proton concentration may have been caused by a competitive uptake of protons by the surface O 2 groups of the titanate planes of HTO. This agrees with the XRD data, which suggest an increased spacing between the HTO layers. This swelling process is most likely caused by the effect of the uptake of H 3 O 1 ions.
From lepidocrocite-type titanate to anatase. One sample was first given a single hydrothermal treatment at 120uC for 24 h, after which it was titrated with 2 M HNO 3 acid solution to adjust the pH of the solution to 2.8. Then a second hydrothermal treatment was applied. It was observed that no more protons were absorbed after repetitive hydrothermal treatment at 120uC because the pH remained constant. The FTIR spectrum of the supernatant after hydrothermal treatment shows that no additional products were observed in the solution besides water and nitrate ions. The XRD pattern of the final state of the powder product is presented in Figure 2d. The diffractogram indicates that an anatase-type TiO 2 phase has formed. The broad and low intensity peaks indicate a low degree of crystallinity.
The microscopic structure of HTO-RT after repetitive hydrothermal treatment at 120uC was further investigated by transmission electron microscopy (TEM). A typical high resolution image of the   as-formed powder in Figure 4a indicates that the product was polycrystalline and the average grain size was about 10 nm. The small grain size is in agreement with the broad peaks observed with XRD as shown in Figure 2d. The crystal structure of each grain was analyzed by performing a fast Fourier transform (FFT) on the high resolution TEM (HRTEM) image. Two kinds of crystalline planes, i.e. (010) and (001), were observed in the TEM images. Figure 4b shows an example of a (010) oriented anatase nanocrystal. The plane distances were determined to be d (002) 5 0.49 nm and d (101) 5 0.36 nm, indicating an (010) oriented plane 13 . The FFT at the left top panel of Figure 4b also revealed that the zone axis is [010] and thus the plane which is vertical to the TEM electron beam is (010). The HRTEM image of a (001) oriented nanocrystal is shown in Figure 4c. The FFT revealed a lattice structure with d (200) 5 d (020) 5 0.19 nm, which is the same as the bulk value. In our TEM investigation, we found that most of the external surface could be identified as (010) surface plane, and only a minor fraction of the surface was found to be (001) plane. Therefore, the nanocrystals preferentially exhibit their (010) facets.
The mechanism of phase transformation. The zeta potential of HTO RT as function of pH is shown in Figure 5. The zeta potential of anatase is shown for the sake of comparison. The zeta potential is related to surface charge characteristics of the external HTO surface. It can be seen that the point of zero charge (pzc) of HTO RT was 3.1. In comparison, the pzc of anatase with particle sizes of 15 nm ( Figure 5) is approximately 6.0 19 . Hence, HTO is much more acidic than anatase. At pH . 3 the external surface of the HTO grains is proton-deficient. Remarkably, we observed substantial uptake of protons by HTO from solutions with a starting pH of 2.8 upon hydrothermal treatment, with a resulting increase in pH up to values of ,10. Since the external grain surfaces of HTO are protondeficient at pH value of 10, it must be the inner surface planes of HTO RT that absorb protons strongly. The proton exchange behavior of the external HTO surfaces and the internal HTO planes is therefore completely different. Casarin et al. investigated multilayer structures of protonic titanate nanosheets H 2 Ti 3 O 7 with a step structure by first principles calculations 20 . They found that two ribbon oxygen atoms on adjacent inner surfaces of H 2 T 3 O 7 , which have a similar bond configuration as the ribbon oxygen atoms in HTO, share a hydrogen atom in the form of a hydroxyl bond and a hydrogen bond. The results of our study are in agreement with their conclusions.
For the sake of comparison HTO dried at 80uC (further denoted as HTO HT) was also hydrothermally treated. HTO HT has a smaller interlayer spacing than HTO RT because it contains less interlayer water 21 . Several reports concluded that dehydrated layered materials cannot be intercalated by chemical means because of the strong electrostatic force between the titanate layers and the proton counter ions [21][22][23] . Figure 6 shows the corresponding XRD data. The peak of HTO RT at 2h 5 9.59u shifted to 2h 5 12.42u in HTO HT (Figure 6a), indicating that the basal spacing of 0.92 nm of HTO RT decreased to 0.71 nm upon dehydration (HTO HT). The decrease of 0.21 nm is close to the ,0.25 nm thickness of a single molecular layer of water 18 . The XRD patterns of HTO HT after hydrothermal treatment at 80uC and 120uC are shown in Figure 6b and Figure 6c, respectively. A similar peak appeared at 2h 5 7.72u (d 5 1.15 nm) as for HTO RT after hydrothermal treatment at 80-120uC (Figure 2b, c). The correspondence indicates that a structure with the same large plane   spacing formed upon hydrothermal treatment, irrespective of whether the initial sample was HTO RT or HTO HT. However, while the major part of the HTO RT material appears to transform to the state with a large lattice spacing of 1.15 nm, only a minor part of the HTO HT material phase appears to do that. The larger part of the HTO HT material appears to remain in a swollen state of the original dry HTO HT structure with a lattice spacing of 0.71 nm. As shown in Figure 6, the original peak of dry HTO HT at 2h 5 12.42u shifted to slightly lower angles and broadened so that a shoulder formed at lower 2h. This is a very strong indication of swelling caused by interaction with water. Apparently, the ability of the HTO HT structure to swell is smaller than that of HTO RT. This is probably due to the presence of a barrier to swelling in the HTO HT samples due to strong electrostatic forces [21][22][23] . The initial pH of 8.3 of a dispersion of HTO HT in water increased to 8.8 and 9.5 after hydrothermal treatment at 80uC and 120uC, respectively, which indicates that HTO HT is proton-deficient and absorbs protons during hydrothermal treatment just like HTO RT. Nevertheless, the swelling of HTO HT demonstrates that the driving force for the uptake of protons is large, which implies that the process involves a chemical reaction between host layers and protons.
Based on the discussion above, we propose that the bonding configuration changes upon hydrothermal treatment: 1) Before hydrothermal treatment of HTO RT the two oxygen atoms on an adjacent titanate sheet share one hydrogen atom, which forms a hydroxyl bond with one oxygen atom and a hydrogen bond with the other oxygen; 2) During hydrothermal treatment absorbed protons form hydroxyl groups with the hydrogen-bond oxygen atoms on the inner surfaces of titanate layers. A bilayer hydroxyl bond configuration forms when all the oxygen atoms on the inner surfaces of titanate layers are bonded to their own hydrogen atom.
To validate this hypothesis, FTIR was used to characterize the bonding configuration. Figure 7 shows the FTIR results of HTO RT before and after hydrothermal treatment at 120uC. The FTIR spectra of anatase derived from HTO RT are also shown. The data indicate that two broad bands around 3400 and 1640 cm 21 , which are assigned to stretching and bending vibrations of H 2 O and/or H 3 O 1 , respectively 24 , did not change intensity upon hydrothermal treatment. On the other hand, the peak at 920 cm 21 shifted to 880 cm 21 21 , which was attributed to the bending mode of hydroxyl groups 15 . Based on that interpretation, Sasaki et al. 15,16,26 concluded that no hydroxyl groups are present in lepidocrocite-type protonated layered titanates since no peaks are observed in that range of 950-1000 cm 21 . However, it is noteworthy that the present material has a planar rather than a zigzag structure. The basicity of the oxygen atoms on the planar structure is lower than the nonshared oxygen atoms at the stepping corner in the zigzag structure 16 . The reduced basicity results in a weaker O-H bond, thus reducing the force constant of the O-H vibration. This implies that the peak that can be assigned to the hydroxyl group is present at a lower wave number than in the zigzag structures. This agrees with our observation of a peak at 880 cm 21 . We recorded the FTIR spectrum of HTO-derived anatase to rule out the possibility that the peak at 880 cm 21 is associated with anatase. Hence, the peak at 920 cm 21 in Figure 7 may be assigned to hydroxyl groups. Upon hydrothermal treatment the intensity of the peak increased significantly, and the interpretation agrees well with the observed pH change and the XRD data. Furthermore, the peak shift from 920 cm 21 to 880 cm 21 which indicates that the bending strength of the O-H groups weakened upon hydrothermal treatment. To our best knowledge, the stability of fully hydroxylated lepidocrocite-type titanates has not been reported till date. However, it is known that bilayer (001)-oriented anatase films can be converted through a barrierless pathway into a lepidocrocite nanosheet 25,26 , which implies that the (001) surface of anatase has a similar chemistry as the (001) surface of lepidocrocitetype titanate. Arrouvel et al. found that the fully hydroxyl-covered surface state is less stable than the partially hydroxyl-covered surface state on the TiO 2 (001) surface 27 . So our results indicate that protons were absorbed and participated in the formation of hydroxyl groups, i.e. the surface of the inner HTO layers became hydroxylated. The formation of hydroxyl groups provides an explanation why HTO HT swells under hydrothermal treatment. It is noteworthy that this intermediate state is interesting for the preparation of 2-dimensional HTO nanosheets, since the charges between the interlayers are neutralized, thus facilitating the loss of coherence between HTO planes. However, since the hydroxyl groups formed on the surface of the titanate layers are not stable, they can easily participate in the reaction of hydroxyl groups on adjacent titanate layers once the surface OH concentration is saturated, following the net reaction 2 -OH R -O-1 H 2 O.

Discussion
The phase transformation from lepidocrocite-type titanate to anatase under hydrothermal treatment has been observed not only in layered titanates and exfoliated nanosheets, but also in titania nanotubes [1][2][3][4][28][29][30][31][32] . The common conditions in those preparation procedures are that: 1) the crystal structure of the host titanate is lepidocrocite-type; 2) protons are present in the solution. Wen et al. observed that the phase transformation from lepidocrocite-type nanosheets to anatase occurred preferably under low basic, neutral, or acidic conditions 11 . Gao et al. observed that the electrical conductivity that was associated with the presence of interlayer H 1 ions decreased at the temperature at which the phase transformation from lepidocrocite-type nanotubes to anatase occurred 31 . These findings suggest that the concentration of surface hydroxyl groups which results from uptake of protons is a key factor for phase transformation of lepidocrocite-type titanate to anatase.
Based on our results the mechanism of phase transformation from HTO to anatase is thought to involve two steps. Firstly, protons are absorbed by the layered structure of HTO. They form hydroxyl groups of the (internal) surface of the titanate host layer of HTO. The data in figure 3 suggest that hydroxyl formation starts at low temperatures. Secondly, after the hydroxyl groups are formed, the topotactic reaction from layered HTO to anatase occurs. Adjacent hydroxyl groups located on two neighboring layers react with each other to form an oxo (-O-) bond between the layers, generating H 2 O as byproduct, and yielding anatase upon expulsion of water ( Figure 2). The schematic diagram of phase transformation is shown in Figure 8.
In summary, we found that the inner surface of dry HTO is proton deficient. Our results suggest that the protons in HTO involve a hydrogen bond configuration in which the hydrogen atom forms a hydroxyl bond with one oxygen atom and a hydrogen bond with the other. The phase transformation from lepidocrocite-type titanate to anatase is a topotactic dehydration reaction between hydroxyl groups that are located on adjacent titanate layers. The phase evolution involves 1) proton uptake into the proton-deficient layered structure of HTO and formation of hydroxyl bonds on the inner layer of HTO, and 2) reaction between adjacent hydroxyl groups located on different layers, forming anatase-type TiO 2 under expulsion of H 2 O.
Preparation of lepidocrocite-type protonated layered titanates. The K 0.8 [Ti 1.73 Li 0.27 O 4 ] precursor was obtained with a flux method as described by Tanaka et al. 17 The resulting KLTO powder was washed 3 times in 250 mL water to remove K 2 MoO 4 . Then 5.5 g KLTO powder was dispersed in a 2 mol/L HNO 3 solution (250 mL) at room temperature while stirring. The acidic solution was replaced daily by a fresh one by decantation. After treatment for 3 days to completely remove all alkali metal ions 15 , the acid-exchanged crystals H 1.07 Ti 1.73 O 4 ?nH 2 O (HTO) were collected by filtration and washed with a copious quantity of pure water, then air dried, and finally slightly ground to get HTO powder (HTO RT). Part of the HTO RT powder was dried at 80uC to release all intercalated water in the interlayer space, so that finally a dried HTO powder was obtained (HTO HT).
Hydrothermal treatment of HTO. 0.1 g of HTO RT was weighted and mixed with 30 mL water without any additives and placed in a glass bottle for hydrothermal experiments. The closed glass bottles were placed in an oven at temperatures ranging from 60 to 140uC for 24 h. Another part of the HTO RT powder with 30 mL water was hydrothermally treated at 120uC and then titrated with HNO 3 solution (2 mol/L) to a pH of 2.8. Then, this powder received hydrothermal treatment again.
Characterization. Powder X-ray diffraction (XRD) data were acquired on a Bruker D2 PHASER (Cu K a radiation with a wavelength of 0.15405 nm). Measurements on wet state samples were conducted after vacuum filtration (glass microfiber filter, maximum pore size is limited to 1 mm) without further drying, and measurements on other samples were conducted after vacuum filtration with 24 h room temperature drying unless mentioned otherwise. Transmission electron microscopy (TEM) observation was performed on a Philips CM300ST-FEG at 300 kV, and the sample was supported on a carbon microgrid. The pH was measured by a Fisher Scientific Accumet Research AR15 pH meter. The Fourier transform infrared (FTIR) spectra were recorded by a Bruker Fourier Transform Infrared Spectrometer Model TENSOR 27 with a Pike GladiATR. The zeta potential data were measured by a Malvern Zetasizer Nano ZS. The sample was ultrasonicated for 10 min to get a nicely dispersed solution and the pH of the samples was titrated manually with HCl aqueous solution (0.2 mol/L).