Preparation of BaTi,09 by a sol-gel method and its photocatalytic activity for water decomposition Mitsuru Kohno, Shuji Ogura and Yasunobu Inoue* Analysis Center & Department of Chemistry, Nagaoka University of Technology, Nagaoka, Niigata 940-21, Japan In the development of photocatalyst materials, barium tetratitanate, BaTi,O,, with a pentagonal-prism tunnel structure was prepared by a sol-gel method and calcined in air at various temperatures from 873 to 1273 K. The changes in the structures were investigated using TG-DTA, X-ray diffraction, Raman and FTIR spectroscopies, and the photoinduced properties were examined by EPR spectroscopy. X-Ray diffraction patterns showed that crystallization occurs by calcination above 973 K. A characteristic Raman peak at 860 cm-', assigned to the stretching vibration of short Ti-0 bonds, increased in intensity with increasing calcination temperatures.EPR signals with g= 2.018 and g=2.004 were produced for BaTi,O, calcined above 1173 K with UV irradiation at 77 K in the presence of oxygen. These signals are associated with an 0'-radical. Calcined BaTi,09 was combined with RuO,, and its photocatalytic activity for water decomposition increased with increasing calcination temperatures of BaTi,O,. From the findings that the formation of the 0'-species and the photocatalytic activity are the results of high efficiency for the separation of photoexcited charges and are closely associated with crystallization of BaTi,O,, it is proposed that the crystallization develops the pentagonal-prism tunnel structure of BaTi,O, and enhances the role of the polarization fields present in the TiO, octahedra which facilitates the separation of photoexcited charges.In view of the current interest in photocatalysis by metal oxides, the design and preparation of efficient photocatalyst materials have been among the most important subjects. Recently, we have found that barium tetratitanate, BaTi,09, having a unique pentagonal-prism tunnel structure, acts as an excellent photocatalyst for the decomposition of water' when the titanate is combined with RuO,. In the development of barium titanate-based photocatalysts applicable to various gaseous and liquid reactions, further research into the prep- aration of BaTi409 and the investigation of its photocatalytic properties are necessary.In the present work, we have employed a sol-gel method, since the method has the advan- tage of producing complex metal oxides with different morpho- logies such as thin films and ultrafine powders as a material. The sol-gel method has been applied for the preparation of Ti0,,2-5 BaTiO, ,6*7 Ba2Ti9020 and BaTi5Ol1 ,' but there have been few studies on BaTi,O,. In the present study, BaTi,O, powder was prepared from barium acetate and titanium isopropoxide. Since the photocat- alytic activities of metal oxides are closely associated with their structures, the prepared BaTi,O, was subjected to heat treat- ment at various temperatures and characterized by TG-DTA, X-ray diffraction, FTIR and Raman spectroscopies.For the evaluation of photocatalytic features, the efficient production of photoexcited charges is an important factor. Thus, the ability to produce surface radicals in the presence of oxygen with UV irradiation was measured by an EPR method. Furthermore, the prepared BaTi,O, was combined with RuO, and the photocatalytic activity of the resulting mixture for water decomposition was examined. On the basis of the obtained results, the relation between the structure of BaTi,09 and the photocatalytic activity is discussed. Experimental In the sol-gel method for the preparation of barium tetratita- nate, BaTi,O, (referred to as BTO), acetic acid anhydride solutions of barium acetate, Ba(CH,COO), (Extra pure grade, Nakarai Tesque Inc.), and titanium isopropoxide, Ti(OC3H7), (Extra pure grade, Nakarai Tesque Inc.), were mixed in a 1: 1 molar ratio in a nitrogen-purged glove box, and water was added dropwise at room temperature.The resulting viscous sol solution was dried at 473 K for 5 h and then calcined in air at various temperatures from 873 to 1273 K for 20h. Powder samples were obtained which are here referred to as BTO (calcination temp./K): for example, a powder obtained by calcination at 873 K is denoted as BTO( 873). TG-DTA curves were recorded on a Rigaku TG 8110 thermal analyser. The temperature was increased from 473 to 1473 K at a heating rate of 10 K min-'. Al,O, powder was used as a reference. The structures of the prepared samples were determined using a Rigaku RADII1 X-ray diffractometer.The degree of crystallinity was evaluated from the intensity of the diffraction peaks due to the (121) and (031) planes of BTO As a reference for the evaluation, a well sintered BTO sample, which was prepared from a mixture of BaCO, and TiO, by calcination at 1273 K, was employed. The surface area of BTO was measured via the BET method using nitrogen at 77 K. Raman spectra of BTO were measured at room temperature with a JASCO NR-1100 spectrometer in the wavelength region from 300 to 1000 cm-'. For measurements of FTIR spectra, BTO was mixed with a fine KBr crystal and then pressed into a disc. The FTIR spectra were obtained with a JASCO FTIR-500 spectrometer. For measurements of EPR signals, ca.300mg of sample was placed in an X-band quartz cell. After degassing at 573 K and then introduction of 30 Torr of oxygen at room temperature, the sample was cooled to 77 IS. The EPR signals were recorded on a JEOL JES-RE2X instru- ment with irradiation by a 500 W low-pressure mercury lamp. The g values were calibrated using Mn2+ in MgO. The photocatalysts were prepared by impregnation of BTO with an RuC13 aqueous solution at 353 K, which was then dried in air and then reduced in a hydrogen atmosphere at 673 K for 2 h, and oxidized in an oxygen atmosphere at 773 K for 2 h. The ruthenium loading was 1mass% (metal content). The powder Ru0,-BTO photocatalyst was placed in a quartz cell and filled with pure water.The decomposition of water was carried out in an Ar atmosphere of 40Torr using a gas circulation system under irradiation by an Xe lamp operated at 400 W. Hydrogen and oxygen evolved in the gas phase were analysed by a gas chromatograph. Details of the appar- atus and procedure for the photocatalytic reaction have been described elsewhere.' J. Muter. Chem., 1996, 6(12), 1921-1924 1921 I -1 92 90 473 673 873 1073 1273 1473 TIK Fig. 1 TG and DTA curves of BTO dried at 473 K Results Fig 1 shows the TG and DTA curves of a dried sol at 473 K In the TG curve, a ca 5% mass loss occurred in the temperature region 573-723 K, followed by a gradual mass loss at higher temperatures The total mass loss reached ca 7% at 1473 K In the DTA curve, a strong exothermic peak was observed at ca 630 K, which corresponded to the significant mass loss Two small exothermic peaks appeared at ca 980 and 1095 K without noticeable mass loss Fig 2 shows X-ray diffraction patterns of BTO treated at calcination temperatures between 873 and 1273 K Table 1 lists the degrees of crystallinity and BET surface areas of the BTO samples prepared BTO(873) was amorphous, as seen in a largely broadened X-ray diffraction peak With increasing calcination temperatures, crystallization proceeded and reached 83% at 1273 K The X-ray diffraction pattern of BTO( 1273) was exactly the same as that of BaTi,O, reported previously The specific surface area decreased with increasing calcination temperature it was 37 m2 g-' for BT0(873), 20 m2 g-' for BT0(973), 1 m2 g-' for both BTO(1073) and BTO( 1173), and c1 m2 g-' for BTO( 1273) Fig 3 shows the Raman spectra of the prepared samples In the spectrum of BT0(873), broad peaks appeared in the I frC a cc 20 30 40 50 60 70 80 90 28/degrees (Cu-Ka) Fig.2 X-Ray diffraction patterns of (a) BT0(873), (b) BT0(973), (c) BTO( 1073), (d) BTO( 1173) and (e) BTO( 1273) Table 1 Crystallinity and surface areas of the prepared BTO samples BTO calcination temperature/K crystallinity (%) surface area /m2 g-' BTO( 873) BTO(973) BTO( 1073) BTO( 1173) BTO( 1273) 873 973 1073 1173 1273 0 14 35 70 83 37 20 1 1 <1 1922 J Muter Chem, 1996, 6(12), 1921-1924 x10 1 I 1 10 800 600 400 wavenumbedcm-' Fig.3 Raman spectra of (a) BT0(873), (b) BT0(973), (c) BTO( 1073), (d) BTO(1173) and (e) BTO(1273) following four regions 430-450, 520, 590-650 and 860 cm-' Upon calcination at 973 K, the peak at 520 cm-' disappeared completely, whereas considerable increases in the intensities of the remaining peaks were observed Further calcination at 1173 K resulted in a remarkable growth of these three peaks The spectral patterns of BTO( 1173) and BTO( 1273) are nearly the same Fig 4 shows the FTIR spectra of BTO in the range 2700-3100 cm-' The spectrum of BTO(873) shows two peaks at 2850 and 2930 cm-' with a shoulder peak at 2955 cm-' which are assigned to symmetric and asymmetric C-H stretch-ing vibrational modes, respectively The two peaks became considerably smaller when the BTO was heated at 973 K, and completely disappeared at temperatures > 1173 K Fig 5 shows the EPR spectra of BTO measured at 77 K in the presence of gaseous oxygen Two signals appearing at g= 2034 and g= 1981 were due to Mn2+ which was used to calibrate the g values No characteristic EPR signal was observed upon UV irradiation of BTO(873) For BTO calcined at 1173 K, two very strong peaks appeared at g=2018 and g=2004 These signals were not observed prior to UV 1 1001 3100 3000 2900 2800 2700 wavenumberkm-' Fig.4 FTIR spectra of (a) BT0(873), (b) BT0(973), (c) BT0(1073), (d) BTO( 1173) and (e) BTO( 1273) ~~2.018-3 I I L 3200 3250 3300 magnetic field/G Fig.5 EPR spectra of (a) BT0(873), (b) BT0(973), (c) BT0(1073), (d) BTO( 1173) and (e) BTO( 1273).All the EPR spectra were obtained at 77 K with UV irradiation after evacuation at 573 K for 1 h followed by the introduction of O2 at a pressure of 30 Torr. irradiation, and they remained stable until the UV irradiation was turned off. The peaks increased further for BTO( 1273). BTO samples treated at various calcination temperatures were combined with RuO, and employed as photocatalysts for the decomposition of water. Fig. 6 shows the photocatalytic activity for the production of hydrogen and oxygen. The activity of Ru0,-BTO( 873) is quite low: neither hydrogen nor oxygen was produced to a noticeable extent. The activity became higher with increasing calcination temperature of BTO: both hydrogen and oxygen were produced for Ru0,-BTO( 1073).The activity increased significantly for Ru02-BT0(1173), but it decreased by 20% for Ru0,-BTO(1273). The ratio of reaction rate for hydrogen prod- uction to that for oxygen production was different from the expected stoichiometric ratio of 2: it was 4 for a catalyst using BT0(973), 2.9 for BTO( 1073), 2.7 for BTO( 1173) and 2.6 for BTO(1273). This shows that the production of oxygen is lowered for BTO calcined at lower temperatures, but it is improved with increasing calcination temperatures. 10 t Fig, 6 Water decomposition on Ru0,-combined BTO. (a) BTO( 873), (b) BT0(973), (c) BTO( 1073), (d) BTO( 1173) and (e) BTO( 1273). U, H2; 0,02.Discussion A large mass loss appearing at temperatures between 573 and 673 K in the TG curve accompanies a large exothermic peak. In the reaction of barium acetate with titanium isopropoxide in the~presence of water, hydrolysis and polymerization pro- duce acetic acid and propanol as products. Thus, the changes correspond to the evaporation of these products. Two small peaks were observed at 980 and at 1095 K in the DTA curve, which were accompanied by no significant mass loss, and it appears that these are associated with phase changes. BTO( 873) showed no characteristic X-ray diffraction peaks and provided broad and weak Raman peaks in the four regions observed. Since the strong peaks due to the C-H vibration were present in the FTIR spectra, these results indicate that the structure of the titanate is amorphous and contains hydro- carbon residues.The X-ray diffraction pattern and Raman spectrum of BTO(973) show the development of definite peaks, followed by a decrease in the intensity of the C-H peaks. These findings mean that crystallization takes place at ca. 973 K, removing the hydrocarbon residues. Drastic increases in the intensities of both the diffraction and Raman peaks for BTO( 1173) show that crystallization proceeded intensively above 1173 K. At these temperatures, the hydrocarbon residues were eliminated so that they were not detected in the FTIR spectra. The Raman spectrum of BTO( 1273) exhibited major peaks in three regions: 430-450, 590-650 and 860cm-'.The peak appearing at the highest wavenumber is interesting, since it represents a feature of Ti-0 bonding. Gratzel and Rotzinger" showed that TiO(C10J2 gives rise to a peak at 970cmP1, which is assigned to the stretching vibration of short Ti-0 bonds. Dehnickell also pointed out that the peak of short Ti-0 bonds appeared at 836 cm-'. Therefore, the 860 cm-' peak of BTO( 1273) is associated with the stretching vibration of Ti-0 with a small separation. This assignment is supported by the results obtained from the X-ray diffraction of a BTO crystal: the TiO, octahedra of BTO contain short Ti-0 bonds, with distances of 0.17-0.18 nm. Because of the presence of the short Ti-0 bonds, the TiO, octahedra are distorted so significantly that the position of the Ti ion deviates from the centre of gravity of the surrounding six oxygens so producing a dipole moment.There was no characteristic EPR peak for BTO(1073), whereas for BT0(1173), EPR signals with g=2.018 and g= 2.004 were obtained in the presence of oxygen at 77 K upon UV irradiation [Fig. 5(d)]. The signal did not appear in the dark in an oxygen atmosphere, or in the absence of oxygen under UV irradiation: note that both oxygen and UV light are essential for the generation of the signal. Lunsford et ~1.'~ showed that the adsorption of N20 on ZnO gives rise to EPR signals with g =2.021 and 2.0026, which are assigned to surface 0-species. Shvets and Kazansky13 also assigned the EPR signals with g =2.020 and g= 2.006 for N,O adsorption on Mo/Si02 to 0-.The g values and the shape of the signal observed here are in good agreement with those values associ- ated with an oxygen radical, 0'-.The formation of the EPR- active species indicates that the photoexcitation and separation of the excited charges occurs efficiently for BTO calcined at higher temperatures. The calcination temperature at which the 0'-peak appeared is in agreement with that for the crystalliz- ation of BTO. These findings imply that crystallization of BTO leads to an increased ability for the separation of photoex- cited charges. The photocatalytic activity for water decomposition increased with increasing calcination temperature of BTO. Fig. 7 shows the relation between the crystallinity of BTO and the photocatalytic activity. This indicates that the well defined crystal structure is preferred to the amorphous structures.As shown in the Raman spectra, BTO provides distorted TiO, J. Mater. Chem., 1996, 6( 12), 1921-1924 1923 crystallinity (%) Fig. 7 Dependence of photocatalytic activity on crystallinity of BTO octahedra such that a dipole moment is generated in the distorted Ti0, octahedra In previous studies, we proposed that the dipole moment is useful for the charge separation in photoexcitation l4 The distorted structure played a much more important role in the rigid crystal structure than in the amorphous structure, since the distorted structure is more stably preserved for the crystal phases This explains the relation between the high photocatalytic activity and the crystal structure of BTO The decrease in the photocatalytic activity for Ru02-BT0(1273) is possibly due to the growth of RuO, particles deposited on BTO, since the surface area of BTO is significantly smaller This makes it difficult to deposit a small particle of Ru02 on the BTO surface, which lowers the activity It is proposed that the formation of a well defined pentagonal-prismatic tunnel structure due to the crystallization of BTO is important for the effective separation of photoex- cited charges The H2/02 ratios were different from the expected stoichio- metric ratio of 2 it was 4 for a catalyst using BT0(973), 29 for BTO( 1073), 2 7 for BTO( 1173) and 2 6 for BTO( 1273) The evolution of oxygen increased with increasing crystallinity of BTO One of the reasons for lower oxygen production with less crystallized BTO may be the consumption of oxygen by the reaction with hydrocarbon residues within the BTO The generation of 0 -species upon UV irradiation occurs at a calcination temperature of 1173 K, whereas the photocatalyst became active at lower calcination temperature, 1073 K The difference is attributed to the surface-bulk properties for EPR measurements, bulk phenomena are involved, while the photo- catalytic activity is based on the surface region Since the removal of hydrocarbon residues occurs preferentially at the surface, it follows that the photocatalytic activity is generated at calcination temperatures lower than the generation of 0 -species In conclusion, BaTi409 synthesized by a sol-gel process exhibits photocatalytic activity for water decomposition The preparation of BTO films coated on porous supports is feasible by this method, which is able to broaden the application of this material to practical photocatalytic reactions such as the purification of water and air This work was supported by a Grant-in-Aid for priority areas from the Ministry of Education, Culture, Sports and Science References 1 Y Inoue, Y Asai and K Sato, J Chem Soc Faraday Trans, 1994, 90,797 2 C Anderson and A J Bard, J Phys Chem ,1995,99,9882 3 I Moriguchi, H Maeda and S Kagawa, J Am Chem Soc 1995, 117,1139 4 T Yoko, A Yuasa and K Kamiya, J Electrochem Soc, 1991, 138,2279 5 N Negishi, T Iyoda, K Hashimoto and A Fujishima, Chem Lett, 1995,841 6 T Yoko, K Kamiya and K Tanaka, J Mater Sci ,1990,25,3922 7 Y Nosaka, M Jimbo, M Aizawa, N Fujii and R Igarashi, J Mater Scz Lett, 1991,10,406 8 H Lu, L E Burkhart and G L Schrader, J Am Ceram SOC, 1991,74,968 9 K Lukaszewicz, Rocz Chem, 1957,31,1111 10 M Gratzel and F P Rotzinger, Znorg Chem ,1985,24,2320 11 K Dehnicke, 2 Anorg Allg Chem ,1961,309,266 12 N B Wong, Y B Taant and J H Lunsford, J Chem Phys, 1974, 60,2148 13 V A Shvets and V B Kazansky, J Catal, 1972,25,123 14 M Kohno, S Ogura and Y Inoue, manuscript in preparation Paper 6/043561, Received 24th June, 1996 1924 J Mater Chem, 1996, 6(12), 1921-1924