J O U R N A L O F C H E M I S T R Y Materials Synthesis and photocatalytic properties of titania pillared H4Nb6O17 using titanyl acylate precursor M. Yanagisawa, S. Uchida, Y. Fujishiro and T. Sato* Institute for Chemical Reaction Science, Tohoku University, Sendai 980–8577, Japan. E-mail: tsusato@icrs.tohoku.ac.jp Received 27th July 1998, Accepted 14th September 1998 Titania pillared H4Nb6O17 has been synthesised by the intercalation of [Ti(OH)x(CH3CO2)y]z+ followed by photodecomposition with UV light irradiation.The incorporation of TiO2 in the interlayer of H4Nb6O17 has been confirmed by powder X-ray diVraction, DTA, UV-VIS reflectance and BET measurements. The incorporation of TiO2 in the interlayer of H4Nb6O17 results in enhanced water cleavage by band gap irradiation. The photocatalytic activity of TiO2 pillared H4Nb6O17 prepared using [Ti(OH)x(CH3CO2)y]z+ was higher than that of a sample prepared using a TiO2 sol solution.prepared by stepwise incorporation of TiO2 as follows. Introduction [Ti(OH)x(CH3CO2)y]z+ was prepared by modifying Kao and The pillaring of layered compounds by inorganic compounds Yang’s method10 starting with titanium isopropoxide, acetic is a promising method for fabricating unique porous materials acid and water in a 1516520 volume ratio.In a typical that possess some of the properties of zeolites. Semiconductor experiment, acetic acid (400 cm3) was mixed first with titanium pillars have attracted special attention from researchers isopropoxide (25 cm3) for 30 min, followed by the addition of because of their excellent photocatalytic activity.The photo- water (500 cm3), resulting in the formation of a white precipicatalytic activity of semiconductor pillars such as CdS–ZnS, tate, which was dissolved to give a clear solution upon Fe2O3 and TiO2 was much higher than those of unsupported continuous stirring for 5 h at room temperature. H4Nb6O17 catalysts.1–7 In the previous studies,5,7 we found that the (5 g) was converted to (C3H7NH3)4Nb6O17 by stirring in a 50 photoactivities of semiconductor pillars were dependent on vol.% C3H7NH2 aqueous solution (1 dm3) at 60°C for 3 days.the physico-chemical properties of the host layer (e.g. CdS–ZnS After separation by filtration, (C3H7NH3)4Nb6O17 (1 g) was pillars which are constructed in the interlayer of semi- added to a clear aqueous solution of [Ti(OH)x(CH3CO2)y]z+ conductors, such as H4Nb6O17 and H2Ti4O9) showed much and allowed to stand for 120 h at room temperature to allow higher photoactivities than those of insulators (such as intercalation of [Ti(OH)x(CH3CO2)y]z+. The obtained montmorillonite and layered double hydroxide), since electron sample, [Ti(OH)x(CH3CO2)y]z/4Nb6O17 after being filtered oV and hole recombination was eVectively depressed due and washed with water, was dispersed in water, and irradiated to charge transfer from the guest semiconductor to the host with UV light from a 450 W high pressure mercury lamp semiconductor layer.at room temperature for 12 h in order to decompose In general, semiconductor pillars are constructed by the [Ti(OH)x(CH3CO2)y]z+ in the interlayer. The resulting intercalation of soluble inorganic ion precursors, followed by material is designated H4Nb6O17/TiO2(c).the precipitation of the intercalated ions by chemical, thermal, and/or photochemical reactions. TiO2 pillars, however, have Synthesis of titania pillared H4Nb6O17 using a titania sol been made using TiO2 sol solutions because of the lack of solution water soluble titanium complex ions.8 By using TiO2 sols, TiO2 pillars were successfully constructed in clay minerals A titania pillar was constructed in the interlayer of H4Nb6O17 which swell in water8 such as smectite.However, it is not easy using titania sol, as reported previously.7 Titanium tetraisoto construct TiO2 pillars in semiconductor layer compounds propoxide (25 cm3) was added dropwise to vigorously stirred such as H4Nb6O17 and H2Ti4O9, since they do not easily swell 1 M HCl (250 cm3) so as to give a final molar ratio of alkoxide in water.A recent study found that a water soluble ionic to HCl of 0.25. The resulting slurry was peptized by further molecular precursor [the titanyl acylate complex, stirring for 3 h so as to give a clear TiO2 sol solution.Then Ti(OH)x(CH3CO2)y z+] can be obtained by the reaction of (C3H7NH3)4Nb6O17 (1 g) was added to the TiO2 sol solution, titanium isopropoxide, glacial acetic acid and water,9 and was and the suspension was continuously stirred for the desired used as a source of TiO2 in the preparation of a strontium time at room temperature in order to incorporate TiO2.After titanate ceramic.10 In the continuation of our studies on the being filtered oV and washed with water, the specimen was photocleavage of water, a series of tests were performed to dispersed in water and irradiated with UV light from a 450W evaluate the synthesis of titania pillared H4Nb6O17 by using high pressure mercury lamp at 60 °C for 12 h in order to titanyl acylate precursors.decompose the C3H7NH3+ remaining in the interlayer. The sample thus obtained is designated as H4Nb6O17/TiO2(s). Experimental Analysis Synthesis of titania pillared H4Nb6O17 using a titanyl acylate The crystalline phases of the products were identified by X-ray complex diVraction analysis (Rigaku Denki Geiger-flex 2013) using graphite-monochromatized Cu-Ka radiation. The chemical H4Nb6O17 was prepared by the ion-exchange reaction of K4Nb6O17 in 5 M HCl at 60 °C for 5 h, K4Nb6O17 being compositions of the products were determined by inductively coupled plasma–atomic emission spectroscopy (Seiko SPS- prepared by the calcination of K2CO3 and Nb2O3 in 253 molar ratio at 1200 °C for 20 min.H4Nb6O17/TiO2 was 1200A) after alkali fusion with Na2CO3 followed by dissolving J.Mater. Chem., 1998, 8, 2835–2838 2835the samples in 6 M HCl-15 wt.% H2O2. The band gap energies of the products were determined from the onset of the diVuse reflectance spectra of the powders measured using a Shimadzu Model UV-2000 UV–VIS spectrophotometer. The specific surface area was determined by the nitrogen gas adsorption method (Shibata SA-1000).Photocatalytic reactions Photocatalytic reactions were performed in a Pyrex reactor with a capacity of 1250 cm3 which was attached to an inner radiation type 450W high-pressure mercury lamp. The inner cell had thermostated water flowing through a jacket between the mercury lamp and the reaction chamber, and was constructed of quartz glass. The photoactivity of the catalyst was determined by measuring the total gas volume of hydrogen and oxygen evolved during the irradiation of the catalyst Fig. 2 XRD patterns of (A) H4Nb6O17, (B) suspensions in water with a gas burette after confirming the [Ti(OH)x(CH3CO2)y]z/4Nb6O17 prepared by the reaction of production of both hydrogen and oxygen by gas chromatogra- (C3H7NH3)4Nb6O17 and the titanyl acylate complex for 120 h at room temperature and (C) H4Nb6O17/TiO2(c) prepared by phy (Yanagimoto G2800) using a Molecular Sieve 13X (30–60 photodecomposition of (B).mesh) column. the Nb6O174- layer thicknesses of 0.56 nm) were 0.40, 0.62 Results and discussion and 0.52 nm, respectively. Synthesis of H4Nb6O17/TiO2 UV–VIS reflection spectra of (A) H4Nb6O17, (B) H4Nb6O17/TiO2(c) and (C) unsupported TiO2 are shown in DTA curves of (A) [ Ti(OH)x(CH3CO2)y]z/4Nb6O17 and (B) Fig. 3. From the onset of the spectra, the band gap energies H4Nb6O17/TiO2(c) (measured in air) are shown in Fig. 1. A of TiO2 gel, H4Nb6O17/TiO2(c) and H4Nb6O17 were deter- sharp exothermic peak at 291 °C (which corresponds to the mined as 3.0, 3.26 and 3.34 eV, respectively. Although the combustion of the titanyl acylate complex) was observed spectrum showed a red shift upon the incorporation of TiO2, for [Ti(OH)x(CH3CO2)y]z/4Nb6O17, but was absent in the band gap energy of the TiO2 pillar could not be determined H4Nb6O17/TiO2(c), indicating that it was photochemically by UV–VIS reflection spectra since H4Nb6O17/TiO2(c) did decomposed by UV-light irradiation.not show separate onsets corresponding to H4Nb6O17 and Fig. 2 depicts the powder X-ray diVraction patterns of (A) incorporated TiO2. H4Nb6O17, (B) [Ti(OH)x (CH3CO2)y]z/4Nb6O17 and (C) The time dependence of the amount of TiO2 incorporated H4Nb6O17/TiO2. The main peaks (which correspond to (040) using both (A) titanyl acylate complex and (B) TiO2 sol of H4Nb6O17 of samples (B) and (C)) shifted significantly solution is shown in Fig. 4. The amount of TiO2 pillar increased to lower 2h angles as compared to sample (A), indicating rapidly with time up to 50 h, then increased more gradually the expansion of the interlayer by the incorporation of and was almost constant after 120 h. The amount of TiO2 Ti(OH)x(CH3CO2)y z+ and TiO2. These results indicate that incorporated using the TiO2 sol was slightly larger than that the layer structure was still retained after the intercalation of using the titanyl acylate complex after 120 h.Ti(OH)x(CH3CO2)y z+ and after photochemical decompo- As seen in Fig. 2, H4Nb6O17/TiO2(c) (prepared using the sition of Ti(OH)x(CH3CO2)y z+ to TiO2 in the interlayer. The titanyl acylate precursor for 120 h) showed XRD diVraction gallery heights of H4Nb6O17, [Ti(OH)x(CH3CO2)y]z/4Nb6O17 peaks corresponding only to H4Nb6O17, indicating that TiO2 and H4Nb6O17/TiO2(c) (determined by XRD by subtracting was incorporated in the interlayer.On the other hand, diVraction peaks corresponding not only to H4Nb6O17 but also to rutile were observed for H4Nb6O17/TiO2 (s) as shown in Fig. 5: the diVraction peaks corresponding to rutile were not observed up to 6 h, but became noticeable after 24 h and increased with time.In general, TiO2 sols transform into rutile via anatase above 500 °C. The unusual formation of rutile at such a low temperature as 60 °C indicates that the H4Nb6O17 acts as nuclei to form rutile. Therefore, it appears that, when a TiO2 sol is used as a precursor, TiO2 is incorporated mainly in the Fig. 1 DTA patterns of [Ti(OH)x(CH3CO2)y]z/4Nb6O17 before (A) and after (B) UV irradiation, where [Ti(OH)x(CH3CO2)y]z/4Nb6O17 Fig. 3 DiVuse reflectance spectra of (A) H4Nb6O17, was prepared by the reaction of (C3H7NH3)4Nb6O17 and the titanyl acylate complex for 120 h at room temperature. (B) H4Nb6O17/TiO2(c) and (C) unsupported TiO2 sol. 2836 J. Mater. Chem., 1998, 8, 2835–2838Fig. 6 Cumulative amounts of hydrogen and oxygen gas produced from 1250 cm3 of water containing 1 g of dispersed catalysts at 60 °C exposed to irradiation from a 450 W mercury arc.(A) H4Nb6O17/TiO2(c), (B) H4Nb6O17/TiO2(s) intercalated for 6 h, (C) H4Nb6O17/TiO2(s) intercalated for 144 h and (D) mixture of Fig. 4 Time dependence of the amounts of TiO2 incorporated using H4Nb6O17 and TiO2 sol.(A) the titanyl acylate complex (&) and (B) a TiO2 sol ($). H4Nb6O17/TiO2(s) possessed gallery heights of 0.52 and 0.48 nm, indicating that the gallery height of the titania pillared H4Nb6O17 increased slightly when the titania pillar was prepared using the titanyl acylate precursor. Photocatalytic water cleavage The amounts of gas produced from 1250 cm3 of water containing 1 g of dispersed H4Nb6O17/TiO2(c), H4Nb6O17/ TiO2(6s), H4Nb6O17/TiO2(144s) and a mixture of 75 wt.% H4Nb6O17 and 25 wt.% TiO2 sol at 60 °C exposed to irradiation from a 450W mercury arc were measured, where H4Nb6O17/TiO2(6s) and H4Nb6O17/TiO2(144s) were prepared using TiO2 sol for 6 and 144 h, respectively. Significant gas evolution was observed in the presence of H4Nb6O17/TiO2(c), H4Nb6O17/TiO2(6s) and H4Nb6O17/TiO2(144s), but no noticeable gas was evolved for a mixture of H4Nb6O17 and TiO2 sol.Therefore, the titanium oxide pillar which was incorporated in the interlayer plays an important part in photocatalytic water cleavage, whereas TiO2 at the outerlayer does not possess photocatalytic activity for water cleavage. Previously7 we found that the charge injection from an excited TiO2 pillar Fig. 5 XRD diVraction patterns of H4Nb6O17/TiO2(s) prepared by into the conduction band of H4Nb6O17 occurs at a rate of reacting (C3H7NH3)4Nb6O17 and TiO2 sol solutions for (A) 6, (B) 24, 0.12×109 s-1; therefore, photogenerated electrons can quickly (C) 71 and (D) 120 h. be transferred from a TiO2 pillar into a H4Nb6O17 layer while the holes remain in the TiO2 pillar.Consequently, the recombi- interlayer of H4Nb6O17 in the initial stage, but a significant nation between the photoinduced charge carriers was eVec- amount of TiO2 is precipitated on the outerlayer after 24 h. tively depressed and the photocatalytic water cleavage was Consequently, the amount of TiO2 pillars constructed in the enhanced. Taking into acount a saturated water vapour press- interlayer can be increased using the titanyl acylate complex ure of 20.0 kPa at 60 °C and the molar ratio of hydro- as precursor.gen5oxygen evolved as 251, from the slope of the straight The gallery height, the amounts of TiO2 incorporated, and lines in Fig. 6 the rates of hydrogen evolution in the presence the specific surface area of H4Nb6O17, H4Nb6O17/TiO2(c) of H4Nb6O17/TiO2(c), H4Nb6O17/TiO2(6s) and H4Nb6O17/ and H4Nb6O17/TiO2(s) are listed in Table 1, where TiO2(144s) were determined as 0.0417, 0.0241 and H4Nb6O17/TiO2(c) and H4Nb6O17/TiO2(s) were prepared 0.0142 mmol h-1, i.e., the photocatalytic activity of using the titanyl acylate complex and TiO2 sol solution for H4Nb6O17/TiO2(c) was 1.7 times larger than that of 120 and 6 h, respectively.The amounts of TiO2 incorporated H4Nb6O17/TiO2(6s). This may be due to an increase in the in H4Nb6O17/TiO2(c) and H4Nb6O17/TiO2(s) were 26.7 and amount of TiO2 pillars. On the other hand, the photoactivity 7.3 wt.%, respectively. The specific surface area increased of H4Nb6O17/TiO2(144s) was about half that of greatly with increase in the amount of TiO2, indicating the H4Nb6O17/TiO2(6s), although the TiO2 content was 3.7 times construction of titania pillars.H4Nb6O17/TiO2(c) and larger. This may be attributed to the precipitation of TiO2 on the outerlayer, because such TiO2 does not possess photo- Table 1 Amounts of TiO2 incorporated, gallery heights and specific catalytic activity for water cleavage and may cut oV the light surface areas of the samples required to excite TiO2 pillars in the interlayer.Gallery TiO2 Specific height/ content surface Conclusion Sample nm (wt.%) area/m2 g-1 The conclusions from this study are: (i) titania pillared H4Nb6O17 0.40 0 16.1 H4Nb6O17 is first fabricated by the reactions of H4Nb6O17 H4Nb6O17/TiO2 (c) 0.52 26.7 125.6 with a titanyl acylate complex followed by UV light irradiation.H4Nb6O17/TiO2(s) 0.48 7.3 38.6 (ii) The amount of the titania pillar and the photocatalytic J. Mater. Chem., 1998, 8, 2835–2838 28373 H. Yoneyama, S. Haga and S. Yamanaka, J. Phys. Chem., 1989, activity of H4Nb6O17/TiO2 prepared using the titanyl acylate 93, 4833. precursor were larger than those fabricated using a TiO2 4 T. Sato, H. Okuyama, T. 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