J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 797-802 Photocatalysts with Tunnel Structures for Decomposition of Water Part 1.-BaTi,O, ,a Pentagonal Prism Tunnel Structure, and its Combination with various Promoters Yasunobu Inoue," Yoshihiro Asai and Kazunori Sat0 Department of Chemistry, Nagaoka University of Technology, Nagaoka, Niigata 940-21,Japan Photocatalysts have been prepared by impregnating barium tetratitanate, BaTi,O, , which has a pentagonal-prism tunnel structure, with aqueous solutions of CoCI, , Ni(NO,), , In(NO,), , RuCI, , H,lrCI, or H,PtCI, , and then activating with either reduction or oxidation. For decomposition of water under irradiation with light from a Xe lamp, reduction caused low photocatalytic activities. Oxidation of RuCI,-impregnated BaTi,O, at 848 K led to active photocatalysts which produce H, and 0, in the correct stoichiometric ratio.The photocatalytic activity increased with an increase in the amount of loaded Ru until it reached 1 wt.%, and remained nearly constant up to 3 wt.%. The X-ray photoelectron spectra showed that the active Ru was in a tetravalent state, forming RuO, on the BaTi,O, surface. UV diffuse reflectance spectra showed that BaTi,O, had a threshold of absorption of light at around 410 nm which reached a maximum at 320 nm. From high-resolution transmission electron microscopy and microanalysis the spherical RuO, particles of 1.4-3.0 nm in diameter were found to be dis- persed uniformly on the regular lattice of BaTi,O, . It is concluded that the pentagonal-prism tunnel structure of BaTi,O, has a 'nest' effect and is responsible for the high dispersion of RuO, particles, which leads to the high photocatalytic activity.In the development of photocatalysts using transition-metal oxides with high efficiency for the decomposition of water, it is important to design photocatalysts which promote the for- Experimental mation of photoexcited electrons and holes and the transfer Barium tetratitanate, BaTi,O, (BTO), was prepared by cal- of the charges to the adsorbed reactants. In this respect, it is cining a mixture of BaCO, and TiO, in air. Barium carbon- required that the oxides producing photoexcited charges are ate of reagent grade was obtained from Nakarai Chemicals combined suitably with metals or metal oxides which can act Ltd.Three kinds of TiO, were used: one from Junsei Chemi- as promoters. cal Co. Ltd., and the others were TYP-0511 and TYP-1011 In previous studies,'*2 we have shown that sodium hexa- from Nippon Soda Co. Ltd. which were in the form of titanate, Na2Ti601 3, becomes active for photodecomposition spheres and had narrow distributions with an average par- of water on combination with ruthenium oxide. The hexa- ticle size of 0.5 pm and 1 pm, respectively. Barium tetra- titanate is one of the Wadsley-Andersson type oxides3 in titanates prepared by using these forms of TiO, are referred which the octahedra share an edge at one level in linear to as BTO(J), BTO(T5) and BTO(T10), respectively. In order groups of three, giving a tunnel structure characterized by a to obtain BTO suitable for photocatalysts, the temperature of wide space corresponding to three octahedra.We have calcination was varied. The formation of BTO was confirmed pointed out that the photocatalytic activity is closely associ- by X-ray powder diffraction patterns obtained with a Rigaku ated with the presence of the tunnel structure. Recently, we Denki RAD I11 diffractometer. The UV diffuse reflectance have found4 that barium tetratitanate, BaTi,O,, is a prom- spectra were recorded on a JASCO UNIDEC 600 spectrom-ising oxide for development as a photocatalyst for the decom- eter. position of water. This oxide has an orthorhombic structure Barium tetratitanate was impregnated to incipient in which the coordination octahedra around the titanium wetness with aqueous solutions of CoCl, (Nakarai), atoms are not oriented parallel to each other, in contrast to Ni(NO,), (Nakarai), In(N03), (Junsei), RuCl, (Tanaka Na,Ti60, 3, such that a pentagonal-prism tunnel structure is Kikinzoku Kogyo, Ltd.), H,IrC16 (Tanaka Kikinzoku) or f~rrned.~.~ H,PtCI, (Soekawa Chemical Co.Ltd.). The impregnated The present study was aimed at establishing an efficient BTO was dried at 363 K and then subjected to either photocatalyst system for the decomposition of water using reduction or oxidation at various temperatures between 553 BaTi,O,. For this, optimum conditions for the synthesis of and 923 K. The amounts of promoters loaded were described BaTi,Og were investigated, and then the combinations with in terms of wt.% of metal and were maintained in the range various metals (Co, Ni, In, Ru, Ir and Pt) and their oxides 0-3.0 wt.%.which could be expected to have promoting effects were For the characterization of the photocatalysts, X-ray examined. The combination of BaTi,O, with oxidized ruthe- photoelectron spectra were recorded on a JEOL JPS-1OOSX nium was found to produce a promising photocatalytic spectrometer with an. Mg-Ka source. High-resolution elec- system, and the photocatalysts with different amounts of tron microscopic images of BTO and the photocatalysts were loaded Ru were prepared and subjected to different activa- obtained at 200 kV with a JEOL JEM-2010 electron trans- tion conditions. The effects of wavelength of light on the mission microscope. The distributions and sizes of promoters photocatalytic activity and photocurrents were investigated.loaded on BTO surfaces were measured, together with ele- In an attempt to reveal relationships between the structures mental analysis for the constituent elements at definite areas of the photocatalysts and the photocatalytic acitivity, the of the microscopic image. photocatalysts were characterized by X-ray powder diffraction, A closed gas-circulation system was used for the photo- X-ray photoelectron spectroscopy, UV diffuse reflection spec- catalytic reaction. The powdered photocatalyst (about 250 troscopy, and high-resolution transmission electron micros- mg) in a quartz reaction cell filled with 20 cm3 of distilled and COPY.deionized pure water was irradiated through a water filter J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 with an Xe lamp operated at 400 W. The reaction conditions were almost the same as those described previously.2 Hydro- gen and oxygen evolved in the gas phase were analysed by a gas chromatograph which was directly connected to the reac- tion system. Quantum yield is an important parameter to evaluate the efficiency of photocatalysts, but it is difficult to determine accurately the number of photons absorbed by photo- catalysts which are in the form of powders dispersed in water. In the present work, the ratio of H atoms produced to photons introduced to water from an Xe lamp was taken as a measure of photocatalytic activity and was represented by a term of catalytic efficiency, Q,.Monochromatic light, of dif- ferent wavelengths, was produced using a Ritsu MC-1ON monochromator, and the rate of hydrogen evolution was monitored as a function of wavelength. The number of photons emitted from an Xe lamp under the same conditions was determined by a chemical actinometer using K3[Fe(C,O&I ' 3H20.' Results Fig. 1 shows the photocatalytic activity of H,PtCl,-impregnated BTO(T10) which was subjected to reduction in H, at 773 K for 2 h. Initially, both H, and 0, were produced but the activity decreased remarkably. Fig. 2 shows the activ- ity of an RuCl,-impregnated BTO(T 10) photocatalyst which was activated either by reduction at 773 K for 2 h or by oxidation at 848 K for 7 h.Reduction brought about pro- duction of H, and O,, but the amount of 0, was about 30% less than that required by stoichiometry. Oxidation caused not only about a ten-fold larger production of H,, but also an improvement in the H, and 0, production ratio; the value off, defined as the ratio of (2 times the amount of 0,) to the amount of H, produced, was 0.96, which is quite close to unity. Fig. 3 compares the effects of various promoters (Co, Ni, In, Ru, Ir and Pt), combined with BTO upon either reduction or oxidation, on the photocatalytic activity. For reduction, which produces a metallic state, the photocatalytic activities were quite low, although 0, was evolved in addition to H, for Ru-, Ir-, or Ni-deposited BTO.Thefvalues were as low as 0.6-0.8. With oxidation, about six-fold higher activity was obtained for Ir, whereas there were no significant increase in activity with other metals apart from Ru. These results indi- cate that the combination of BTO with oxidized Ru and also oxidized Ir may result in a useful photocatalytic system. 150 -E =L 100 3U 2 n + c 3 0 50 0 2 4 6 8 time/h Fig. 2 Water decomposition on photocatalysts prepared by reduction or oxidation of RuC1,-impregnated BTO(T10). Reduction at 773 K for 2 h in a hydrogen atmosphere: H, (0);0, (y).Oxida-tion at 848 K for 7 h in air: H, (0);0, (a).1 wt.% loading as Ru metal. BTO was prepared at different calcination temperatures and employed as a photocatalyst after combination with oxi- dized Ru.As shown in Fig. 4, when BTO(T10) was used as starting material, the activity was nearly constant for calcina- tion temperatures between 1223 and 1373 K, and then " Co Ni In Ru Ir Pt (b) -20 --15--r 05s-10-. 5-0 L-U-0 2 4 6 8 Co Ni In Ru time/h Fig. 3 Effects of promoters on photocatalytic activity: H, (0);0, Fig. 1 Photoassisted water decomposition on H,PtCl,-impregrated (D). After impregnation, photocatalysts were activated by (a) BTO(T10). Pt was reduced at 773 K in an H, atmosphere: H, (0);reduction at 773 K for 2 h in a hydrogen atmosphere and (b) oxida-0,(0)-tion at 848 K for 7 h in air. 1 wt.% loading as metal. J. CHEM. SOC.FARADAY TRANS., 1994, VOL,. 90 \\-\ T,K Fig. 4 Effect of calcination temperatures of BTO on photocatalytic activity. BTO(T10); H, (01,0, (0):BTO(J); H, (A), 0, (A).BTO was impregnated with RuCI, (aq) and oxidized at 848 K; 1 wt.O/oRLI. decreased to approximately half at 1473 K. A similar change tiioc nhcnrxinA in thn coon nF RTT\(T\ In the X-ray powder diffraction patterns of BTO(T10), the but a small extra peak was observed at 20 = 31.85" with cal- cination temperatures between 1223 and 1273 K. This peak was assigned to the (1 10) plane of BaTiO, and it disappeared upon calcination above 1373 K. The surface area decreased with increasing calcination temperature, with values of 2.1 (1273), 1.1 (1373) and 0.9 m2 g-' (1473 K) for BTO(J) and 0.57 (1273) and 0.59 m2 g-' (1473 K) for BTO(T10), and 17 (1273 K) for BTO(T5). The scanning electron microscopic observation of BTO calcined at 1273 K showed that BTO(T10) consisted of rugged spherical particles (average size of 1-3 pm), whereas BTO(T5) had smaller particles of 0.5-1 pm with quite smooth surfaces.BTO(J), BTO(T10) and BTO(T5) calcined at 1273 K were combined with oxidized Ru and the photocatalytic activity compared; this was 20-30% higher for BTO(T10) than for BTO(J) and BTO(T5). Calcination at 1473 K resulted in the disappearance of the rugged structures of the BTO(T10) particles, and, as shown in Fig. 4, the photocatalytic activity decreased at this tem-perature. Fig. 5 shows the UV diffuse reflectance spectra of BTO.The absorption had a threshold wavelength at around 310 nm and reached a maximum at 320 nm. A shoulder was observed at around 380 nm for each oxide, which was signifi- cant for BTO(T10). A sample of BTO(T10) was pressed into a disc (20 mm in diameter and 0.5 mm thick), and a pair of the transparent thin Au electrodes were attached in parallel on the surface. On applying a dc voltage of 5 V, the surface photocurrents were measured in air as a function of wave-length of light from an Xe lamp. The photocurrents were gen- erated initially at 400 nm, increased markedly at wavelengths shorter than 400 nm and attained a constant level at around 310 nm. This change corresponded to that observed in the light-absorption characteristics.Fig. 6 shows changes in the photocatalytic activity Kith oxidation temperatures of RuC1,-impregnated BTO(TI 0). The threshold temperature for activation was 500 K. The activity increased with increasing temperature, reached a maximum at 848 K, and then decreased with higher oxida- tion temperatures. The production of hydrogen and oxygen 200 300 400 500 600 wavelength 'nm Fig. 5 UV diffuse reflectance spectra of BTO: BTO(J) (-), BTO(T10) (---) and BTO(T5) (- -1 was observed over the oxidation temperatures examined; the fvalue was 0.8 below 653 K, attained the stoichiometric ratio between 753 and 848 K, and then decreased to 0.8 above 953 K. Note that H, and 0, production was facilitated by oxidation and reduction was not required in the present photocatalytic system.For the active photocatalyst prepared by RuC1,-impregnation and then oxidation at 848 K, X-ray photoelec- tron spectra showed that a peak due to an Ru 3d,,, level appeared at 280.6 eV. This binding energy was the same as that found for the RuO,/Na,Ti,O, catalyst activated by Oxidation at 773 K after R~Cl,-impregnation.~ This value uas comparable with 280.7 eV for commercially available R u0, ,thus indicating that the oxidation state of Ru was +4 and the active photocatalyst was composed of a combination of BTO and RuO,. A small peak due to the C1 2p level was observed. As for the stability of RuO,/BTO(TlO) photocatalyst, there uas little deterioration of the photocatalytic activity with 600 800 1000 TiK Fig.6 Effects of oxidation temperature of RuC1,-impregnated BTO(T10) on photocatalytic activity: H, (O),0, (0);1 wt.% Ru irradiation over 30 h. A photocatalyst employed for the cata- lytic reaction was stored in water without irradiation. After three months, the photocatalytic acitivity was examined in fresh water and it was found to be unchanged. After impregnation of BTO(T10) with different amounts of Ru and then oxidation at 848 K, the photocatalytic activities were examined as a function of the amount of Ru. As shown in Fig. 7, the activity of Ru02/BTO(T10) increased with increasing amounts of Ru, levelled off at 1 wt.% and remained constant up to 3 wt.%. The stoichiometric pro- duction of H, and 0, was observed over the entire range examined. The figure also shows the activity changes with amount of Ru on BTO(J).There were similarities in the activ- ity changes between BTO(T10) and BTO(J), although the activity of the former was higher on average by a factor of 1.3. The effect of the amount of Ir deposited on BTO(T10) upon the photocatalytic activity is shown in Fig. 8. The activ- ity increased up to 1 wt.%, passed through a maximum, and then decreased considerably. High-resolution electron transmission microscopic obser- vations of BTO(T10) gave lattice images arrayed regularly up to the topmost surface. Fig. 9 shows the transmission micro- scopic image of a 1 wt.% RuO,/BTO(TlO) photocatalyst which was activated by oxidation at 848 K.Well distributed spherical dark spots of 1.4-3.0 nm in diameter could be seen on the regular BTO lattice image. An energy analysis of the characteristic X-ray peak was carried out for a small region of the dark spots, and it was demonstrated by microanalysis that they were composed of elemental ruthenium. Fig. 10 shows the catalytic efficiency, Q,, for a 1 wt.% Ru02/BTO(T10) photocatalyst as a function of wavelength of incident irradiation. The value of Q, at 360 nm was ca. 1% and increased with shorter wavelength. Measurements were repeated for photocatalysts prepared in different batches; the values obtained were 5-6% at 340 nm and 10 & 3% at 330 nm. The change in Q, with wavelength corresponded to that in absorption characteristics observed in the UV diffuse reflectance spectra.0-1 2 3 RU (wt.%) Fig. 7 Photocatalytic activity as a function of amounts of Ru deposited. BTO(T10): H, (O),0, (a);BTO(J):H, (A), 0, (A).Oxi-dation at 848 K for 7 h in air. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 20! A c I r-0 E, 102 0 1 2 Ir (wt.Y0) Fig. 8 Photocatalytic activity of BTO(T10) as a function of amounts of Ir deposited: H, (O),0, (0).1 wt.% Ir; oxidation at 848 K in air after impregnation with H,IrCl, (aq). Fig. 9 A high-resolution transmission electron microscopic image of RuO,/BTO(TlO). A white arrow shows one example of an RuO, particle. 1 wt.% Ru; oxidation at 848 K in air after impregnation with RuC1, (aq). 15 0 -2 10 me-v 0" -5 I I0 I 300 320 340 360 wavelengthlnm Fig.10 Values of catalytic efficiency Q, for RuO,/BTO(TlO) as a function of wavelength. 1 wt.% Ru; oxidation at 848 K in air after impregnation with RuCl, (as). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Discussion The X-ray diffraction patterns show that a small fraction of BaTiO, existed in BTO(T10) prepared by calcination at tem-peratures between 1223 and 1273 K and disappeared with calcination above 1373 K. The photocatalytic activities were almost the same for the photocatalysts using BTO(T10) pre- pared at 1223-1373 K, as shown in Fig. 4. Invariance in the photocatalytic activities was also observed for the BTO(J) photocatalysts prepared between 1273 and 1373 K. Thus, BaTiO, can be considered to have a negligible effect on photocatalysis. The BTO(T10) photocatalyst was 20-30% active, compared to those of BTO(T5) and BTO(J).As demonstrated by scanning electron microscopic images, the activity differences seem to be closely correlated with the presence of the rugged surface structures on BTO(Tl0). As for the effects of the promoters, Ni, Ru and Ir were able to produce oxygen not only on reduction but also on oxida- tion, whereas no oxygen was evolved for Go, In and Pt irre- spective of the treatment. For Ni-, Ru- and Ir-deposited photocatalysts which were activated by reduction, the activity was significantly lower and the f values were also lower than the stoichiometric ratio. These characteristics were improved by oxidation, in particular, for Ru and Ir metals.The X-ray photoelectron spectra showed the presence of tetravalent Ru ions by oxidation of Ru at 848 K, which indicates the forma- tion of RuO, . In the electrochemical decomposition of water, the overpotential for oxygen production was 0.22 V for ruthenium oxide electrodes and 0.28 V for iridium oxide elec- trodes in 1 mol dm- KOH solutions. These values are lower than 0.66 V for platinized Pt and 1.32 V for Pt electrode^.^ Thus, the hypothesis that the contribution of the metals and oxides deposited on BTO should decrease the overpotential for oxygen production accounts for the trend of the promoter effects shown in Fig. 3. The role of RuO, in 0, evolution has been reported by several whereas the evolu- tion of hydrogen from an RuO, surface has also been demon- ~trated.’~*’’It is a matter of controversy whether or not RuO, acts as an oxidation site.However, from the role of RuO, in decreasing the overpotential for oxygen production and the fact that the appearance of the high photocatalytic activity is constantly accompanied by the oxygen evolution, it is likely that the RuO, surface becomes an active site for the transfer of holes to adsorbed OH-species. It is of particular interest that photocatalytic activity of Ru0,JBTO occurred without any reduction. The fact that deterioration of the activity was hardly observed is related to activation in an oxidising atmosphere. Thus, it is unlikely that oxygen atoms produced in the decomposition of water are incorporated into the lattice of BTO surface, which has been thought to be one of the reasons for the lack of oxygen as a product in conventional Pt-TiO, photocatalysts.The total number of hydrogen atoms produced after 30 h irradia- tion is 550 times larger than that of the exposed Ti ions which was calculated from the surface area and the unit cell dimensions. The catalytic efficiency Q,of 10 f3% at 330 nm was higher by a factor of 2.5 than that of the RU02/Na2Ti601 photocatalyst obtained in our previous study., The value is similar to that (at 330 nm) obtained by Domen et al. for the initial stage of water decomposition on the NiO(O.1 wt.%)/Rb4Nb60, photocatalyst’6 (ca. 10%) and much higher than that for NiO(O.1 wt.%)/K,Nb,O, l7 (3.5%).Thus it is evident that the combination of RuO, and BTO has high potential for photocatalysis. As shown in the high-resolution transmission electron microscopic image, the spherical RuO, particles of 1.4-3.0 nm in diameter were uniformly distributed over the BTO surface. The dependence of photocatalytic activity on the amount of Ru deposited exhibited a nearly linear increase in 801 the low concentration of Ru until levelling off occurred at 1 wt.% Ru. These results suggest that an increase in number rather than growth of the RuO, particles is responsible for the activity increase below 1 wt.% Ru. There was a maximum for the dependence of the activity on the amount of Ir. It is not yet clear why there is a drastic decrease in the activity above 1 wt.%, but is seems likely that the complete oxidation of Ir to form IrOz at 848 K becomes dificult with increasing Ir, which lowers the effects of the promoter.The main feature of the structure of BTO is the presence of the pentagonal-prism tunnel structure, as shown in Fig. 1 l(a). Although most of the RuO, particles were considerably larger than the space of one pentagonal-prism tunnel, it is considered that the uniform distribution of the small RuO, particles is strongly associated with the characteristic struc- ture of the tunnel. The pentagonal-prism space provides the form of a ‘nest’, i.e., a concave site with a ridge. This unique structure substantially prevents RuO, particles from aggre- gating and growing into larger particles.An example of nest- egg type incorporation which accommodates a 1.4 nm particle in the tunnel space is shown in Fig. ll(b). In this model, it is anticipated that strong interactions between the small RuO, particle and the surrounding TiO, octahedra are produced, because of larger interfacial contact. This facilitates the formation of photoexcited electrons and holes and then the transfer to the adsorbed species. It is certain that the pres- ence of the tunnel structure causes the distortion of Ti06 octahedra, which appears to be closely associated with effi- cient production of photoexcited charges. A study to eluci- date a relationship between photoexcitation and the tunnel structure is in progress.=L X 0 0 Fig. 11 Schematic projection of BaTi,O, (a) and a model (b) of a ‘nest’ effect of a pentagonal-prism tunnel in the RuO,/BTO photo-catalyst. 802 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 This work was supported by the Kajima Foundation’s Research Grant and a Grant-in-Aid for Scientific Research on Priority Area for the Ministry of Education, Science and Culture of Japan. We thank the EM Application Laboratory of JEOL Ltd. for helpful observation of high-resolution transmission electron microscopic images. 9 10 11 12 M. Morita, C. Iwakura and H.Tamura, Electrochim. Acta, 1978, 23, 331 and ref. therein. J. M. Lehn, J. P. Sauvage and R. Ziessel, Noun J. Chim., 1979, 3, 423. T. Kawai and T. Sakata, Chem.Phys. Lett., 1980,72,87. D. Duonghong, E. Borgarello and M. Gratzel J. Am. Chem. SOC., 1981,103,4685. References Y. Inoue, T. Kubokawa and K. Sato, J. Chem. SOC., Chem. Commun., 1990,1298. Y. Inoue, T. Kubokawa and K. Sato, J. Phys. Chem., 1991, 95, 13 14 15 G. Blondeel, A. Harriman, G. Porter, D. Urwin and J. Kiwi, J. Phys. Chem., 1983,87,2629. E. Amouyal, P. Keller and A. Moradpour, J. Chem. SOC., Chem. Commun., 1980,1019. T. Sakata, K. 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