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Bottom-up synthesis of titanate nanosheets and their morphology change by the addition of organic ligands and dialysis Takayuki Ban, Takuya Nakagawa, and Yutaka Ohya Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg501852a • Publication Date (Web): 16 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Crystal Growth & Design

Bottom-up synthesis of titanate nanosheets in aqueous sols and their morphology change by the addition of organic ligands and dialysis Takayuki BAN *, Takuya NAKAGAWA, and Yutaka OHYA Department of Chemistry and Biomolecular Science, Gifu University, Yanagido 1-1, Gifu 5011193, Japan Abstract The size, shape and stacking of the titanate nanosheets synthesized by heating aqueous mixtures of titanium complexes and tetrabutylammonium hydroxide (TBAOH) were dependent on the type and amount of organic ligands and the amount of TBA+. When titanium isopropoxide was used as the titanium source, most titanate crystals were smaller than 10 nm in lateral size. However, the use of titanium complexes provided a lateral size of about 100 nm. The titanate crystals were synthesized by the acid-base reaction of titanic acid and TBAOH. The decrease in titanic acid concentration by titanium complex formation led to larger titanate crystals by the inhibition of crystal nucleation. Moreover, the use of triethanolamine and lactic acid as an organic ligand provided round and hexagonal titanate crystals, respectively. However, when the concentration of organic ligands was decreased by the dialysis of the titanate sols, the form of titanate crystals was changed to a rhombic shape. Thus, the adsorption of organic ligands on the titanate crystals probably influenced the shape of the titanate crystals. Furthermore, the decrease in TBA+ concentration by the dialysis of the sols inhibited the stacking of the titanate nanosheets during the evaporation of the sols.

Corresponding Author [*] Takayuki Ban Department of Chemistry and Biomolecular Science, Gifu University, Yanagido 1-1, Gifu 5011193, Japan Tel: +81 58 293 2585, Fax: +81 58 293 2794, e-mail: [email protected]

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Bottom-up synthesis of titanate nanosheets in aqueous sols and their morphology change by the addition of organic ligands and dialysis Takayuki BAN *, Takuya NAKAGAWA, and Yutaka OHYA Department of Chemistry and Biomolecular Science, Gifu University, Yanagido 1-1, Gifu 5011193, Japan Corresponding Author [*]: e-mail [email protected]

Abstract

The size, shape and stacking of the titanate nanosheets synthesized by heating aqueous mixtures of titanium complexes and tetrabutylammonium hydroxide (TBAOH) were dependent on the type and amount of organic ligands and the amount of TBA+. When titanium isopropoxide was used as the titanium source, most titanate crystals were smaller than 10 nm in lateral size. However, the use of titanium complexes provided a lateral size of about 100 nm. The titanate crystals were synthesized by the acid-base reaction of titanic acid and TBAOH. The decrease in titanic acid concentration by titanium complex formation led to larger titanate crystals by the retardation of crystal nucleation. Moreover, the use of triethanolamine and lactic acid as an organic ligand provided round and hexagonal titanate crystals, respectively. However, when the


2

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concentration of organic ligands was decreased by the dialysis of the titanate sols, the form of titanate crystals was changed to a rhombic shape. Thus, the adsorption of organic ligands on the titanate crystals probably influenced the shape of the titanate crystals. Furthermore, the decrease in TBA+ concentration by the dialysis of the sols inhibited the stacking of the titanate nanosheets during the evaporation of the sols.

Introduction Two-dimensional nanomaterials have highly anisotropic shapes. There are possibilities that novel chemical and physical properties emerge, taking advantage of the morphologies.1−5 Metalate nanosheets are one of two-dimensional nanomaterials. So far, epitaxial growth of metal oxide thin films using metalate nanosheets as a seed layer 6−9 and robust high-κ properties of perovskite nanosheets

10

were reported. Recently, C. Wang et al.11 reported all-metalate

nanosheet ultrathin capacitors. M. Osada et al.12 reported that titanate nanosheets have so high dielectric constant. When titanate nanosheets are used as dielectric thin films, the decrease of titanate nanosheet boundaries, which can cause leakage current across the thin films, is required. Thus, the use of large titanate nanosheets is important for preventing leakage current by decrease of nanosheet boundaries. Metalate nanosheets have been conventionally synthesized by the following method: 2−5 First, layered metalates with alkaline or alkaline earth ions in the interlayers are synthesized by solid state reaction at high temperature. Next, the interlayer cations are ion-exchanged for H+ by acid treatment. Then, bulky cations, such as tetrabutylammonium ion (TBA+), are intercalated into the interlayers by the acid-base reaction between interlayer H+ and TBAOH. Finally, the interlayers


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are swollen in water, resulting in the formation of nanosheets. The lateral size of the nanoheets thus obtained is dependent on the size of the starting layered metalates. For example, T. Tanaka et al.13 prepared titanate nanosheets with lateral size of about 100 µm from millimeter-sized titanate single crystals synthesized by the flux method. Until now, we

14−16

reported that

transparent aqueous sols were prepared by mixing alkoxides of Ti, Nb or Ta with tetraalkylammonium hydroxides (TAAOH). Layered metalate crystals with TAA+ ions in the interlayer were prepared by evaporating the sols. Layered tungstate crystals with tetramethylammonium (TMA+) ion as an interlayer cation were also synthesized by mixing tungstic acid (H2WO4) and tetramethylammonium hydroxide (TMAOH) in water under the limited condition.17 Since layered metalates with bulky interlayer cations are swollen in water and provide metalate nanosheets, there were possibilities that this aqueous solution process can be applied to bottom-up synthesis of metalate nanosheets. K. Kai et al.18,19 also reported the synthesis of layered manganates and manganate nanosheets in a similar way. For our aqueous solution process, layered metalate crystals are synthesized by the acid-base reactions between metallic acids, which are formed by the hydrolysis of metal alkoxides, and TAAOH. Since the metallic acids and TAAOH are mixed at so high concentrations, the nucleation of layered metalate crystals occurs so fast. Thereby, the layered metalate crystals synthesized by this aqueous solution process were so small. As mentioned above, bottom-up synthesis of large layered metalate crystals and nanosheets are desirable, although the application to monolayer nanodots is also interesting taking advantage of the small size of the metalate nanosheets 20. E.L. Tae et al.21 reported that the lateral size of the titanate nanosheets synthesized by mixing titanium isopropoxide (TIP) and TMAOH in water was enlarged to about 20 nm by refluxing the titanate nanosheet sols.


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For the synthesis of larger single crystal, crystal nucleation is required to occur more slowly. The use of metal complexes as a metal source is one of synthesis methods of large single crystals for bottom-up synthesis of oxides in aqueous solutions. For example, J.F. Charnell 22 reported the hydrothermal synthesis of large crystals of A-type and X-type zeolites by adding triethanolamine (teaH3) to the reaction gels. The added teaH3 is reacted with Al(OH)4− ion to form complexes.23−26 Thereby, the Al(OH)4− concentration is decreased, resulting in slow zeolite nucleation. Although Al(OH)4− is consumed by the zeolite crystallization, a small amount of Al(OH)4− continues to be supplied by the equilibrium between the complexes and Al(OH)4−, providing large zeolite crystals by further crystal growth.24−27 This synthesis method of large single crystals using organic ligands has been applied to other types of zeolites.28−31 Moreover, we

32,33

reported the preparation of transparent aqueous sols containing Ti species by using

various organic ligands, until now. Since titanium complexes are present as highly dispersible colloidal particles in these sols, we envisaged that there are possibilities that these aqueous sols of titanium complexes can be used for bottom-up synthesis of large titanate nanosheets. In this study, the crystallization of layered titanates and titanate nanosheets by the reaction between titanic acid and TBAOH was retarded at room temperature by using titanium complexes as Ti source, and was caused by heating the aqueous mixtures of the titanium complexes and TBAOH. The influence of the use of titanium complexes on the size and shape of layered titanate crystals or titanate nanosheets was examined. Moreover, the influence of TBA+ concentration in the sols on the stacking of titanate nanosheets was also examined by dialyzing the titanate nanosheet sols.

Experimental Section


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Materials. First, TIP (20 mmol) was mixed with teaH3 (N(C2H4OH)3, 20 – 60 mmol). The mixtures were stirred for a period longer than 1 day under the ambient condition, and then diluted with distilled water to 20 mL. The transparent aqueous sols containing colloidal particles of titanium complexes were obtained. TBAOH aqueous solution (40%) was added to the aqueous sols at a molar ratio TBAOH/Ti of 0.5. The sols were diluted with distilled water to Ti concentration of 0.5 M. Moreover, the sols using lactic acid (lacH2) as an organic ligand were prepared in a similar way to the ones using teaH3. The Ti concentration of the sols using lacH2 was 0.1 M. The mixing ratio was TIP : lacH2 : TBAOH = 1 : 2 : 3. Next, the sols thus obtained were heated for the crystallization of layered titanates or titanate nanosheets. The sols were transferred to a Teflon-lined steel vessel, and then heated at 80 °C for 1 – 7 days. Then, the heated sols were dialyzed for examining the influence of the amount of organic ligands and TBA+ on the morphology and stacking of the titanate nanosheets. The sols (about 5 mL) were transferred to a dialysis tube (20 µm thick) with a molecular weight cut off of 12,000 − 14,000. The dialysis tube was placed in 400 mL distilled water, which was stirred. The water was replaced with fresh water after dialysis for 1 h and 1day. The total dialysis period was from 1 h to 2 days.

Characterization. X-ray diffraction (XRD) measurements were performed on a Rigaku Ultima IV diffractometer with a monochromatic CuKα irradiation. XRD patterns were recorded at a scan rate of 2° min−1 in the 2θ range of 2 to 70°. XRD measurements were conducted for the thin films prepared by evaporating about 200 µL of sols on a glass substrate (1 cm × 4 cm, Corning #1737). For several


6

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samples, XRD measurements of the powder obtained by drying the sols using a rotary evaporator under the reduced pressure (about 4 kPa) were also conducted. Transmission electron microscopy (TEM) images were captured using a JEOL JEM-2100 model at an accelerating voltage of 200 kV. The samples were prepared by evaporating a drop of the diluted sols with Ti concentration of 10 mM on a Cu grid supported with a Formvar thin film. Hydrophilic treatment was conducted for the Cu grid before use. Atomic force microscopic (AFM) images were captured on Seiko instruments SPI3800 model and Hitachi AFM5400L model in a tapping mode using a silicon tip cantilever (16 N m−1). The samples were prepared by dropping the diluted sols with Ti concentration of 5 µM on a mica substrate and then drying at 100 °C for 10 min. The scanning area was 0.5 µm × 0.5 µm to 5 µm × 5 µm. The scanning frequency was 0.5 to 2 Hz. 1

H liquid-state nuclear magnetic resonance (NMR) spectra were recorded on a JEOL ECX-

400P model at the external magnetic field of 9.39 T. Single pulse sequence with a pulse width of 6.3 µs (45° pulse) was used. The data accumulation was made by repeating the pulse sequence 8 times with delay time of 7 – 60 s. The samples were prepared by mixing the sols and deuterium oxide in the same volume. 4,4-Dimethyl-4-silapentane-1-sulfonic acid (DSS) was added to the samples as an internal standard.

Results and Discussion Influence of the addition of organic ligands on the crystallization of layered titanate. It was examined if the crystallization of titanates by the reaction between Ti species and TBAOH at room temperature was hindered by using Ti complexes as the Ti species. First, triethanolamine (teaH3) was used as the organic ligand for Ti complex formation. We previously


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reported that aqueous mixtures of TIP and teaH3 became transparent sols.33 The transparent sols including teaH3 were mixed with TBAOH aqueous solution at TBAOH/Ti molar ratio of 0.5. Figure 1a shows the appearance of the sols prepared at teaH3/TIP = 2. The sols thus obtained exhibited Tyndall phenomenon, indicating the preparation of colloids. XRD measurements were made for the samples prepared by evaporating the sols on a glass substrate. The XRD patterns of the samples without the addition of organic ligands exhibited peaks with d-spacings of 1.76, 0.88, and 0.59 nm. The their relation of 1 : 1/2 : 1/3 indicated the formation of layered titanates. Upon adding teaH3 at teaH3/TIP = 1, the d-spacing of the peak at the smallest angle, that is, the basal spacing of the layered titanates was changed from 1.76 to 1.89 nm. It is likely that this change in basal spacing was attributed to the chemical modification of teaH3 on the titanate layers. The peak intensity of layered titanate crystals became weaker with increasing the amount of added teaH3, and at teaH3/TIP ≥ 1.5, no peaks of the layered titanate appeared (Figure 1b). Thus, the complex formation of Ti species and teaH3 inhibited the crystallization of the layered titanate by the reaction with TBAOH at room temperature. Moreover, 1H NMR measurements were made for the sol prepared at teaH3/TIP = 2. The peaks assigned to 2-propanol, free teaH3 molecules, free TBA+ ions, and water were observed, while no peaks assigned to the tea ligands bound to Ti4+ appeared, because the Ti complexes were present as colloidal particles. On the basis of their peak intensities, their molar ratio was estimated as 2-propanol : free teaH3 : free TBA+ = 4.00 : 1.70 : 0.50 (Table 1). The difference between the amounts of the added teaH3 and the free teaH3 in the sols was probably attributed to the tea species in the Ti complexes. Since the sols were prepared at teaH3/TIP = 2, it was inferred that the Ti complexes with a chemical composition of Ti : tea = 1 : 0.3 (= 2.00 – 1.70), that is Ti/tea ≈ 3.3, were formed.


8

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Figure 1 (a) Appearance of the aqueous sols prepared by mixing TIP, TBAOH, and teaH3 at TIP / TBAOH / teaH3 = 1 / 0.5 / 2. (b) XRD patterns of the thin films prepared by evaporating the aqueous mixture of TIP and TBAOH without organic ligands and with the addition of teaH3 at different teaH3 / TIP ratios.

Table 1 Change in molar ratio of 2-PrOH, free organic ligand, and TBA+ in the sols by heating at 80 ºC. These molar ratios were estimated using 1H NMR spectra. 1 TIP : 2 teaH3 : 0.5 TBAOH

1 TIP : 2 lacH2 : 3 TBAOH

2-PrOH

teaH3

TBA+

2-PrOH

lacH2

TBA+

Before heating

4.00

1.70

0.50

4.00

1.24

2.99

After heating

4.00

1.90

0.48

4.00

1.60

2.99

Next, titanium complex sols were also prepared using lactic acid (lacH2) as an organic ligand. The mixing ratio was TIP : lacH2 : TBAOH = 1 : 2 : 3. Since strong basicity is required for the formation of layered titanate, a larger amount of TBAOH was added in order to neutralize an acidic ligand, lacH2. For the case using teaH3, the molar ratio of TBAOH/Ti was 0.5, whereas for the case using lacH2, the TBAOH/Ti ratio was 3. The pH values of the sols were above 13. The XRD patterns of the samples prepared by evaporating the sols showed that no crystallization of


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layered titanate occurred. Thus, irrespective of the type of organic ligands, the use of Ti complexes inhibited the reaction of Ti species with TBAOH at room temperature. Moreover, the 1

H NMR spectra of the sol showed that the molar ratio of 2-propanol : free lacH2 : free TBA+

was 4.00 : 1.24 : 2.99. Based on this ratio, it was inferred that the Ti complexes with a chemical composition of Ti : lac = 4 : 3 (lac/Ti = 0.76) were formed. We 32 previously reported that the complexes prepared by mixing TIP and lacH2 in water at lacH2/TIP = 2 had a chemical composition of lac/Ti = 0.8. The Ti complexes obtained in this study was likely the same as the ones obtained in our previous study. The aqueous sols including Ti complexes and TBAOH were heated for the crystallization of layered titanate. When the sols prepared at teaH3/Ti molar ratio of 2 were heated at 80 °C for different periods, the sols became yellowish (Figure 2a), indicating that a slight amount of teaH3 was probably degraded. Moreover, the XRD patterns of the samples prepared by evaporating the sols showed that the heating for a period longer than 4 days caused the crystallization of layered titanate (Figure 2b). Further heating increased the amount of the formed layered titanate. Thus, the heating of aqueous mixtures of Ti complexes and TBAOH led to slow crystallization of the layered titanate with a long induction period. The sols containing lacH2 were also heated at 80 °C. A slight amount of layered titanate crystals were formed upon heating for 1 day. With increasing heating period, the peaks of layered titanate crystals became stronger (Figure S1 in the Supporting Information (SI)). Thus, irrespective of the type of organic ligands, the heating at 80 °C caused the crystallization of layered titanate. Furthermore, the basal spacing was 2.24 − 2.41 nm and 1.86 − 1.94 nm for the samples using teaH3 and lacH2, respectively, and was larger than that of the samples without organic ligands (Fig. 1b). This suggests that the organic ligands chemically modified the titanium atoms in the layered titanate crystals, resulting in the increase


10

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in the interval between the titanate layers. Moreover, the basal spacing of the layered titanate became a little shorter, with increasing heating period. The reason for this decrease in basal spacing was not known; however, we think that there are two possibilities: (1) The uptake of organic ligands into the interlayers during the evaporation was dependent on the size of titanate nanosheets, or (2) it decreased with the ratio of free organic ligands to titanate nanosheets.

Figure 2 (a) Appearance of the aqueous sols obtained by heating aqueous mixture with a molar ratio of TIP / TBAOH / teaH3 = 1 / 0.5 / 2 at 80 ºC for 1 − 7 days. (b) XRD patterns of the thin films prepared by evaporating their aqueous sols. (c) Powder XRD pattern of the powder obtained from the sols heated for 7days.


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Consequently, the crystallization of layered titanates formed by the reaction between titanic acid and TBAOH could be controlled by the addition of organic ligands and the heating of the sols. Powder XRD measurements were made for the samples obtained by heating the sols with a molar ratio of teaH3/TIP = 2 for 7 days (Figure 2c). The sample was obtained by drying the heated sols under the reduced pressure. Other than the diffractions from the lattice planes parallel to the titanate layers, two peaks appeared at 2θ = 47.9º and 62.4º. They were assignable to the diffractions from the (200) and (002) planes of the layered titanate with the lepidocrocite-type structure 34, respectively. We35 and E.L. Tae et al.21 previously reported that the layered titanate synthesized by mixing TIP and TMAOH in water had the lepidocrocite-type structure with the orthorhombic system, in which titanate layers are stacked along the b-axis direction. Thus, this XRD pattern suggests that the layered titanate obtained in this study also had the lepidocrocitetype structure. Moreover, since the titanate with the lepidocrocite-type structure has a chemical composition of (TBA, H)0.7Ti1.825O4 xH2O

34

, titanate nanosheets with chemical composition of

Ti1.825O40.7− are probably obtained by exfoliation.

Influence of the addition of organic ligands on the morphology of layered titanate crystals. The morphology of the formed titanate was observed by TEM (Figure 3). When the titanate sols without the addition of organic ligands were heated at 80 °C for 7 days, the obtained titanate crystals were so small (Figure 3a). The lateral size of most crystals was smaller than 10 nm. Even for the largest titanate crystals, the lateral size was about 25 nm. The addition of organic ligands to the titanate sols enlarged the size of the titanate crystals obtained by heating at 80 °C for 7 days. At teaH3/Ti = 2, titanate crystals with a lateral size of about 100 nm were formed,


12

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Figure 3 TEM and HRTEM images of the layered titanate nanocrystals obtained by heating aqueous sols (a) without organic ligands and with the addition of organic ligands of (b, e) teaH3 / TIP = 2, (c) teaH3 / TIP = 3, and (d, f) lacH2 / TIP = 2 at 80 ºC for 7 days.

although small crystals were co-present (Figure 3b). The outlines of the obtained titanate crystals were round. By increasing the amount of teaH3 to teaH3/Ti = 3, the number of titanate crystals with a lateral size larger than 100 nm became larger, while the number of small crystals became much smaller (Figure 3c). Moreover, on the basis of the contrast of the TEM images, it is inferred that the large titanate crystals observed by TEM consisted of several titanate nanosheet layers, but were not a single layer nanosheet. However, when AFM measurements were made for the sample prepared by evaporating the heated sols on a mica substrate, the height profile showed that the thickness of the titanate crystals was about 1.3 nm, indicating the formation of titanate nanosheets (Figure 4). The samples for AFM measurements were prepared from the sols with a Ti concentration of 5 µM, while the sols with a Ti concentration of 10 mM were used for


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the preparation of TEM samples. As described later, the titanate nanosheets were exfoliated in the sols. Thus, the extent of the stacking of the titanate nanosheets during evaporation might be larger for more concentrated sols. Furthermore, the use of lacH2 as an organic ligand also provided layered titanate crystals with a lateral size larger than 100 nm (Figure 3d). The outlines of many titanate crystals were a hexagon. Thus, the shape of the formed titanate crystals was dependent on the type of organic ligands. It is likely that this is attributed to the adsorption of the organic ligands on the layered titanate crystals, as mentioned above.

Figure 4 AFM images and height profile of the titanate nanosheets obtained by heating aqueous sols with a molar ratio of TIP / TBAOH / teaH3 = 1 / 0.5 / 2 at 80 ºC for 7 days.

The crystal structure of the formed layered titanate was investigated by selected area electron diffraction (SAED) and HRTEM images. On the basis of the lepidocrocite structure 34, the SAED patterns and HRTEM images were analyzed. However, for the samples synthesized with the addition of teaH3 and lacH2, the spots observed in the SAED were assigned as shown in Figures S2b and S3b in the SI. Moreover, the lattice fringes observed in Figure 3e and f were assigned to


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the (200) planes. Furthermore, the titanate crystals synthesized with the addition of lacH2 were surrounded with the {101} and {100} planes (Figure S3a in the SI).

Formation mechanism of the titanate nanosheets. The role of organic ligands in the formation of relatively large layered titanate crystals was examined by 1H NMR measurements of the sols before and after the heating. By comparing peak intensity between free organic ligands, which were not bound to Ti4+, and 2-PrOH, the amount of free organic ligands was estimated (Table 1). For the sols using teaH3 (teaH3/Ti = 2), 85% and 95% of the added teaH3 was present as free organic ligands in the sols before and after the heating at 80 °C, respectively. For the sols using lacH2, 62% and 80% of the added lacH2 was present as free organic ligands before and after the heating, respectively. Thus, the heating increased the amount of free organic ligands in the sols. When Ti(OH)4 was consumed by the crystallization of layered titanate, the following equilibrium reaction of complex formation was probably shifted toward the left side, resulting in the increase in the amount of free organic ligands; 3 Ti(OH)4 + teaH3 ⇌ Ti3On(OH)9‒2n(tea) + (n+3) H2O. Furthermore, since the concentration of Ti(OH)4 was kept low by this equilibrium reaction, relatively large layered titanate crystals were probably formed by the inhibition of crystallization. Moreover, the amount of free TBA+ in the sols was estimated by comparing peak intensity between free TBA+ and 2-PrOH. In both cases using teaH3 and lacH2, the amount of free TBA+ did not changed by the heating (Table 1), and almost all added TBA+ was present as free TBA+. If layered titanate crystals consisting of stacking titanate layers existed in the heated sols, the heating would decrease the peak intensity of free TBA+ by the formation of layered titanates,


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because TBA+ ions in the interlayer of the layered titanate crystals exhibit no peaks. Thus, this indicates that the formed titanate was present as an exfoliated titanate nanosheet in the heated sols.

Table 2 Change in pH and molar ratio of 2-PrOH, free organic ligand, and TBA+ in the sols by dialysis. The molar ratios were estimated using 1H NMR spectra. 1 TIP : 2 teaH3 : 0.5 TBAOH Dialysis time

1 TIP : 2 lacH2 : 3 TBAOH

pH

2-PrOH

teaH3

TBA+

pH

2-PrOH

lacH2

TBA+

Before dialysis

> 13

8.3

4.0

1

> 13

1.3

0.5

1

1h

11.0

4.4

3.5

1

10.8

0.7

0.7

1

1 day

10.1

0.4

0

1

7.9

0.2

0

1

2 days

8.4

0.05

0

1

－

－

－

－

Morphology change of the titanate nanosheets by dialysis of the sols. On the basis of the fact that the titanate crystals in the TEM images consisted of several titanate layers, it is inferred that the titanate nanosheets are prone to stacking during the evaporation of the sols. Moreover, the shape of the titanate crystals was dependent on the type of organic ligands. So, the influence of the amount of TBA+ and organic ligands on the stacking and shape of titanate crystals was examined by dialyzing the heated sols for the removal of TBA+ and the organic ligands. The molar ratio of free TBA+ : free organic ligands : 2-PrOH in the sols dialyzed for different periods were estimated by 1H NMR measurement. For the sols using teaH3, their molar ratios and the pH values of the dialyzed sols are listed in Tables 2. The pH values of the sols were decreased from > 13 to 11 by the dialysis for 1 h. Further dialysis for 2 days


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significantly decreased pH values to 8.4. Since the major counter cation of OH− was TBA+, it is certain that the TBA+ concentration was decreased with pH values. Moreover, the dialysis for 1 h decreased the molar ratios of 2-propanol / TBA+ and free teaH3 / TBA+ from 8.3 to 4.4 and from 4.0 to 3.5, respectively. After dialysis for 1 day, free teaH3 and 2-propanol were so slightly present in the sols. Thus, the removal rates of the organic species in the sols were in the following order; 2-PrOH > teaH3 > TBA+. Furthermore, XRD measurements were made for the samples prepared by evaporating the dialyzed sols on a glass substrate (Figure 5). The basal spacing of the layered titanate prepared by the evaporation was decreased from 2.23 to 1.85 nm by the dialysis for 1 day. The dialysis for 2 days broadened the XRD peaks. It is likely that the shift and broadening of the XRD peaks had some relation to the decrease in TBA+ and ligand concentrations by the dialysis. Also for the dialysis of the sols using lacH2, similar results were obtained (Table 2 and Figure S4 in the SI).

Figure 5 XRD patterns of the thin films prepared by evaporating the sols dialyzed for 1h to 2 days. The sols prepared by heating aqueous mixtures with a molar ratio of TIP / TBAOH / teaH3 = 1 / 0.5 / 2 at 80 ºC for 7 days were used for the dialysis.


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Figure 6 TEM images of the titanate nanosheets obtained by dialyzing the sols for 1 h. The sols prepared by heating aqueous mixtures with molar ratios of (a) TIP / TBAOH / teaH3 = 1 / 0.5 / 3 and (b) TIP / TBAOH / lacH2 = 1 / 3 / 2 at 80 ºC for 7 days were used for the dialysis.

The TEM and AFM images of the samples prepared from the sols dialyzed for 1 h are shown in Figures 6 and 7, respectively. Irrespective of the type and amount of the organic ligands used, the titanate crystals had a lateral size of about 100 nm and became rhombic shapes. The SAED of the samples after the dialysis showed that the titanate had the lepidocrocite structure and were surrounded with {101} planes. E.L. Tae et al.21 reported that the titanate nanosheets synthesized from TIP and TMAOH had a rhombic shape. Thus, it is likely that the round or hexagonal figures of the layered titanate crystals before the dialysis were attributed to the adsorption of the organic ligands on the titanate layers, and the desorption of the organic ligands by dialysis provided the rhombic shape by dissolution-reprecipitation process. As mentioned above, a little larger basal spacing of layered titanates synthesized using organic ligands also suggested the adsorption of organic ligands to the titanate layers. Furthermore, the small contrast of the TEM images showed that the titanate crystals were so thin, suggesting the formation of titanate nanosheets. The AFM images (Fig. 7) showed the aggregation of the titanate crystals in the lateral directions; however, the height profile showed that the titanate crystals were about 1 nm thick, confirming the formation of titanate nanosheets. Thus, the decrease in organic species content inhibited the stacking of the titanate nanosheets. Consequently, the size, shape and


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stacking of the titanate nanosheets were controlled by organic ligands and the amount of TBA+ ion.

Figure 7 AFM images of the aggregates of the titanate nanosheets obtained by dialyzing the sols for 1 h. The sols prepared by heating aqueous mixtures with molar ratios of TIP / TBAOH / teaH3 = 1 / 0.5 / 3 at 80 ºC for 7 days were used for the dialysis.

Conclusions Until now, we reported that layered titanate crystals or titanate nanosheets were synthesized by mixing TIP and TBAOH in water.14 However, since this aqueous solution process provided so small titanate crystals, highly anisotropic titanate nanosheets could be obtained. In this study, bottom-up synthesis of large and highly anisotropic titanate nanosheets was examined by the aqueous solution process using titanium complexes instead of TIP. For this aqueous solution


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process, titanates are synthesized by the acid-base reactions between titanic acid and TBAOH. The concentration of titanic acid in the sols was kept low by the equilibrium between Ti complexes and titanic acid. Thereby, the crystal nucleation of titantes was inhibited, resulting in the formation of larger titanate nanosheets, which had a lateral size of about 100 nm. (Figure 8a) Moreover, the organic ligands added for the formation of Ti complexes were chemically adsorbed on the formed titanate nanosheets. Thereby, the shapes of the titanate nanosheets were dependent on the type of the organic ligands used. Furthermore, the titanate nanosheets obtained by this aqueous solution process were prone to stacking; however, their stacking could be inhibited by decreasing TBA+ concentration in the sols by dialysis. (Figure 8b)

Figure 8 Schematic illustration (a) of the effect of the use of Ti complexes on the size of the titanate nanosheets and (b) of the effect of the dialysis of titanate nanosheet sols on nanosheet stacking.

Bottom-up synthesis of metalate nanosheets would be useful for the fabrication of monolayer nanosheet thin films by the deposition of nanosheets on substrates placed in the reaction sols or


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solutions. The inhibition of nanosheet stacking and the control of nanosheet growth are required for the fabrication of homogeneous thin films. Thus, when nanosheets are used in the thin film form, the results obtained in this study would provide important information about morphology of the thin films.

Acknowledgements This study was supported by KAKENHI (Grant-in-Aid for Scientific Research (C) 26410237) from Japan Society for the Promotion of Science (JSPS).

Supporting Information Available: Change in appearance and XRD patterns of the sols including lacH2 by the heating, TEM images and SAED patterns of the layered titanate crystals synthesized by heating the sols including teaH3 or lacH2, and XRD patterns of the thin films fabricated by evaporating the dialyzed sols in the case using lacH2. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutiérrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F.; Johnston-Halperin, E.; Kuno, M.; Plashnitsa, V. V.; Robinson, R. D.; Ruoff, R. S.; Salahuddin, S.; Shan, J.; Shi, L.; Spencer, M. G.; Terrones, M.; Windl, W.; Goldberger, J. E. ACS Nano 2013, 7, 2898-2926.


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Table of Contents Use Only

Bottom-up synthesis of titanate nanosheets in aqueous sols and their morphology change by the addition of organic ligands and dialysis Takayuki BAN *, Takuya NAKAGAWA, and Yutaka OHYA

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Synopsis Titanate nanosheets were synthesized by reacting Ti species and NBu4OH in water. The use of Ti complexes as the Ti species provided relatively large nanosheets. The shape of the nanosheets was changed by the adsorption of the organic ligands used for the formation of the Ti complexes. Moreover, the dialysis of nanosheet sols hindered the stacking of the nanosheets.


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Bottom-Up Synthesis of Titanate Nanosheets in ... - ACS Publications

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