J O U R N A L O F C H E M I S T R Y Materials Communication EVects of RuO2 on activity for water decomposition of a RuO2/Na2Ti3O7 photocatalyst with a zigzag layer structure Shuji Ogura, Mitsuru Kohno, Kazunori Sato and Yasunobu Inoue* Department of Chemistry, Nagaoka University of Technology, Nagaoka, 940-2188, Japan Received 6th July 1998, Accepted 26th August 1998 Sodium trititanate, Na2Ti3O7, with a zigzag layer structure photocatalyst when ruthenium oxide is dispersed as small particles on the titanate.produces a surface lattice O- radical upon UV irradiation and has the ability to decompose water to hydrogen and Na2Ti3O7 was prepared by calcining an equimolar ratio of TiO2 (high purity grade, Soekawa Chemical Co.) and Na2CO3 oxygen when ruthenium oxide is highly dispersed on the titanate.(high purity grade, Soekawa Chemical Co.) in air at 1073 K for 16 h. The formation of these titanates was confirmed by X-ray diVraction. For measurements of EPR signals, about Barium tetratitanate, BaTi4O9, with a pentagonal-prism tunnel structure and alkaline metal hexatitanates, M2Ti6O13 (M= 300 mg of the titanates were placed in a quartz cell and degassed at 573 K in high vacuum.The g values were calibrated Na, K, Rb), with a rectangular tunnel act as good photocatalysts to decompose water to oxygen and hydrogen in combi- by Mn2+ in MgO, the error of which was within ±0.001. For the preparation of a photocatalyst, Na2Ti3O7 was impregnated nation with ruthenium oxide.1–3 The common features of these titanates are that both have tunnel structures and give rise to with the two kinds of ruthenium compounds: one consisted of RuCl3 aqueous solutions which were the same as used in the surface lattice O- species in the presence of gases at 77 K upon UV irradiation.4–6 These results lead to a view that previous procedures,1–3 and the new other was dodecacarbonyltriruthenium, Ru3(CO)12, dissolved in tetrahydrofuran.photocatalysis by the titanates is closely related to the surface lattice O- radical generated by UV irradiation. The Ru metal loading was 0.7 wt%, unless otherwise specified. The impregnated titanate was dried at 353 K and then sub- In a series of titanates with the chemical formula Na2TinO2n+1 to which Na2Ti6O13 belongs, Na2Ti3O7 has a jected to reduction at 673 K in a H2 atmosphere for 4 h, followed by oxidation in air at 623 K.Ruthenium oxides zigzag layer structure.7 Fig. 1 shows a schematic structure of Na2Ti3O7, together with the rectangular tunnel structure of prepared using RuCl3 and Ru3(CO)12 are referred to as RuO2(CL) and RuO2(CB), respectively. A powdered photo- Na2Ti6O13. When RuO2 was supported on Na2Ti3O7 by the same conventional impregnation method using RuCl3 aqueous catalyst (250 mg) was dispersed in a quartz reaction cell filled with distilled and deionized pure water (20 cm3), stirred by solutions as employed for BaTi4O9 and M2Ti6O13 (M=Na, K, Rb), poor photocatalytic performance for water decompo- bubbling with Ar gas, and irradiated through a water filter with a Xe lamp operated at 400 W.Hydrogen and oxygen sition has been known to occur.It is of particular importance to clarify the reasons for the photocatalytic diVerences between produced were analyzed by a gas chromatograph directly connected to the reaction system. the tunnel and the layer titanate and to confirm whether or not a correlation between photocatalytic activity and the The HRTEM images of the RuO2-dispersed titanates were obtained with a JEOL 2010 transmission electron microscope surface lattice O- radical formation holds for the layer titanate, for better understanding of the photocatalysis mechanism.operated at 200 kV. The energy-dispersive X-ray (EDS) spectra were collected for nanometer-sized areas of the samples Since the photocatalytic processes in water decomposition on RuO2-deposited titanates are composed of photoexcited with a Voyager energy dispersive analyzer (Noran Instruments) installed on the microscope. charge formation in the titanates and charge transfer to the surface reactants through RuO2, each step has to be examined Fig. 2 shows the EPR signals of Na2Ti6O13 and Na2Ti3O7 obtained at 77 K in 4 kPa O2 under UV irradiation by a 500 W separately in order to shed light on the diVerences between the two types of titanates.In the present study, the ability of photoexcited charge formation was examined by an electron paramagnetic resonance (EPR) method, and that of charge transfer by changing the eVects of RuO2 deposited on the titanates and then by high resolution transmission electron microscopic (HRTEM) observation. We have discovered that a layered titanate of Na2Ti3O7 has a high ability for photoexcited charge formation and the eVects of RuO2 dispersion are important for photocatalysis: Na2Ti3O7 is a promising Fig. 2 Comparison of EPR signals of Na2Ti6O13 (a) and Na2Ti3O7 (b) with UV irradiation. The signals were recorded at 77 K in the Fig. 1 A schematic representation of Na2Ti3O7 with a zigzag layer (a) presence of 4 kPa oxygen.The signal of Na2Ti6O13 was the same as reported previously (cf. ref. 4). and Na2Ti6O13 with a rectangular tunnel structure (b). J. Mater. Chem., 1998, 8(11), 2335–2337 2335Fig. 3 Production of hydrogen and oxygen from water by a RuO2(CB) and RuO2(CL)/Na2Ti3O7 photocatalyst with a zigzag layer structure. $, H2; #, O2 for RuO2(CB)/Na2Ti3O7; &, H2; %, O2 for RuO2(CL)/Na2Ti3O7.high pressure mercury lamp. As reported previously, the tunnel structure Na2Ti6O13 provided a strong signal with g=2.020, g=2.018 and g=2.004, which is assigned to a surface lattice O- radical.4,6 For Na2Ti3O7, nearly the same signal with g= 2.021, g=2.018, and g=2.004 was observed. The close similarity clearly indicates that Na2Ti3O7 is able to produce the surface lattice O- radical.Since the formation of the radical is associated with highly eYcient photoexcited charge separation in BaTi4O9 and Na2Ti6O13,8 Na2Ti3O7 is concluded to have the ability of photoexcited charge formation. Fig. 3 shows water decomposition on a RuO2(CB)-deposited Na2Ti3O7 [referred to as RuO2(CB)/Na2Ti3O7] photocatalyst, together with a RuO2(CL)/Na2Ti3O7 photocatalyst.In addition to hydrogen, the evolution of oxygen occurred from Fig. 4 HRTEM images of RuO2(CL)/Na2Ti3O7 (a) and RuO2(CB)/ an initial stage and continued at a constant rate as long as the Na2Ti3O7 (b). sample was irradiated. Note that a RuO2(CL)/Na2Ti3O7 photocatalyst which underwent the same reduction and oxidation resulted in little evolution of oxygen, although a small amount of hydrogen was produced.Fig. 4 shows HRTEM images of RuO2 deposited Na2Ti3O7. For RuO2(CL)/Na2Ti3O7, large egg-shaped black spots, whose sizes were around 20–30 nm, were observed. On the other hand, for RuO2(CB)/Na2Ti3O7 spherical dark spots of 2–4 nm in diameter were mostly distributed uniformly on the regular lattice image of Na2Ti3O7. EDS analysis showed that the dark egg-like and spherical spots are composed of ruthenium.Note that RuO2(CB) produces smaller, better distributed RuO2 particles than does RuO2(CL). In order to compare the roles of RuO2(CL) and RuO2(CB) in photocatalysis, these Ru oxides were supported on Na2Ti6O13 with a rectangular tunnel structure. Fig. 5 shows the photocatalytic activities of RuO2(CL)/ and RuO2(CB)/Na2Ti6O13.RuO2 (CL)/Na2Ti6O13 produced Fig. 5 Photocatalytic activities of RuO2(CL)/ and RuO2(CB)/ hydrogen and oxygen in nearly the stoichiometric ratio. These Na2Ti6O13. Ru content: 1 wt%. results exclude the possibility that the poor performance of the RuO2(CL)/Na2Ti3O7 photocatalyst is due to a Cl residue which might remain on the surface. Interestingly, the photo- diVerences in RuO2 particle sizes and shapes between RuO2(CL) and RuO2(CB), as shown in Fig. 4. catalytic activity was higher by a factor of 2.2 for RuO2(CB)/Na2Ti6O13 than for RuO2(CL)/Na2Ti6O13. This It is likely that the larger RuO2 particles produce an oxygen-deficient state in the interior of the particles, because indicates that a larger number of active sites are produced in the former, thus suggesting that RuO2(CB) is superior to of the diYculty of complete oxidation, and/or weak interactions at interface between RuO2 and the titanate surface, RuO2(CL) in the formation of smaller RuO2 particles.The surface geometric eVect of a pentagonal-prism tunnel structure which has unfavorable influences on the photoexcited charge transfer in photocatalysis. These considerations lead to the of BaTi4O9 has been previously described as playing the role of a ‘nest’ in the accommodation of Ru oxides, which presents view that the good photocatalytic performance of H2 and O2 production observed for RuO2(CB)/Na2Ti3O7 is due to the a barrier for the aggregation and growth of RuO2 particles and keeps Ru oxide particles small.1 Thus, the diVerences in presence of smaller RuO2 particles.In conclusion, there is no intrinsic diVerence in photo- particle sizes between RuO2(CL) and RuO2(CB) are considered to be rather small in Na2Ti6O13 in view of the tunnel catalysis between the tunnel and layer structures: a Na2Ti3O7 titanate with a zigzag layer structure makes a good photocata- structure. For a layered titanate of Na2Ti3O7 which has no tunnel space, there is little geometric eVect to suppress the lyst which produces H2 and O2 when RuO2 is highly dispersed.The view that the ability to form O- surface radicals upon aggregation and growth of RuO2. This leads to significant 2336 J. Mater. Chem., 1998, 8(11), 2335–23373 Y. Inoue, T. Niiyama and K. Sato, Top. Catal., 1994, 1, 137. UV irradiation is correlated with the photocatalytic activity 4 S.Ogura, M. Kohno, K. Sato and Y. Inoue, Appl. Surf. Sci., 1997, may be also applicable to the layered titanate. For the design 121/122, 521. of eYcient photocatalysts, in addition to the choice of titanates 5 M. Kohno, S. Ogura, K. Sato and Y. Inoue, J. Chem. Soc., Faraday which permit the formation of O- radicals upon UV irradia- Trans., 1994, 93, 2433. tion, it is also important to have well dispersed small RuO2 6 M. Kohno, S. Ogura, K. Sato and Y. Inoue, Stud. Surf. Sci. Catal., particles. 1996, 101, 143. 7 S. Andersson and A. D. Wadsley, Acta Crystallogr., 1961, 14, 1245; 1962, 15, 194. Notes and references 8 M. Kohno, S. Ogura, K. Sato and Y. Inoue, Chem. Phys. Lett., 1997, 267, 72. 1 Y. Inoue, Y. Asai and K. Sato, J. Chem. Soc., Faraday Trans., 1994, 90, 797. 2 Y. Inoue, T. Kubokawa and K. Sato, J. Phys. Chem., 1991, 95, Communication 8/05172K 4059. J. Mater. Chem., 1998, 8(11), 2335–2337 2337