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A laser flash photolysis study of the photochemical activity of a synthesised ZrTiO4
June 1999
Materials Letters 39 Ž1999. 370–373 www.elsevier.comrlocatermatlet
A laser flash photolysis study of the photochemical activity of a synthesised ZrTiO4 Comparison with parent oxides, TiO 2 and ZrO 2 J.A. Navıo ´
a,)
, M.C. Hidalgo a , M. Roncel b, M.A. De la Rosa
b
a
Instituto de Ciencia de Materiales de SeÕilla, Centro Mixto CSIC-UniÕersidad de SeÕilla, Centro de InÕestigaciones Cientıficas ‘Isla de la ´ Cartuja’, AÕda. Americo Vespucio, s r n. 41092, SeÕilla, Spain ´ b Instituto de Bioquımica Vegetal y Fotosıntesis, Centro Mixto CSIC-UniÕersidad de SeÕilla, Centro de InÕestigaciones Cientıficas ‘Isla de ´ ´ ´ la Cartuja’, AÕda. Americo Vespucio, s r n. 41092, SeÕilla, Spain ´ Received 5 October 1998; accepted 22 December 1998
Abstract The photochemical activity of a prepared zirconium titanate, ZrTiO4 , sample has been studied by laser flash photolysis. A short duration laser pulse was used to produce electron–hole pairs; methyl viologen was employed as electron scavenger. The same procedure has been employed to estimate the photochemical activity of two home prepared single oxides, TiO 2 Žhp. and ZrO 2 Žhp. and for a commercially available TiO 2 ŽDegussa, P-25.. Our results further showed the potential of the laser flash photolysis technique to estimate the photosensitivity of semiconductor oxides and predict a poor photoactivity for the TiO 2 Žhp. sample in spite of this sample exhibiting Žby XRD technique. the anatase as the unique phase. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Laser flash photolysis; Photochemical; ZrTiO4
1. Introduction Certain materials Žoxides, sulphides, etc.., act as photoconductors on illumination with near-UV photons. If the two photoinduced charge carriers Želectrons and holes. do not recombine, they can reach the surface and react with chemisorbed species giving reductionroxidation reactions w1x. The mixed oxides TiO 2 –ZrO 2 provides efficient acid–base bifunctional catalysts w2–4x. Zirconium
) Corresponding author. Tel.: q34-5-4489550; Fax: q34-54460665
titanate, ZrTiO4 , which has been investigated as a high dielectric material, has been investigated in heterogeneous photocatalysis by some of us w5,6x. In a preliminary study, it has been shown that ZrTiO4 is photosensitive in the near-UV region and can give, to some extent, oxygen isotopic exchange w7x. It has also been shown that ZrTiO4 becomes a photoconductor on illumination w7x having correlated the photocatalytic activity and photoelectronic properties of ZrTiO4 in order to account for its photoactivity and for the differences observed with the two parent oxides, ZrO 2 and TiO 2 w8x. Using picosecond laser flash photolysis, Rothemberg et al. w9x have investigated the dynamics of
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 0 3 7 - 3
J.A. NaÕıo ´ et al.r Materials Letters 39 (1999) 370–373
charge trapping and recombination in TiO 2 colloidal particles. The mechanism of methyl viologen ŽMV 2q . reduction by the conduction band electrons of TiO 2 can be summarised as follows: TiO 2 q hn ™ TiOU2 Žey–hq . Žey–hq . ™ hnX q hV Žey–hq . ™ eyq hq ey Žbulk. ™ ey Žsurface. ey Žsurface. q MV 2q Žads. ™ MVqŽ ads.
exciton formation direct recombination exciton dissociation electron transport electron trapping Žmethyl viologen reduction.
Laser flash photolysis has been applied for the study of the photoactivity of undoped and iron-doped titania samples w10x. We now explore the potential use of this technique to estimate the inherent photocatalytic activity of a prepared zirconium titanate, ZrTiO4 , and its parent oxides ZrO 2 and TiO 2 . Results are briefly compared with those obtained by other techniques applied to the same kind of oxides w7,8x to predict their photocatalytic activities.
371
single oxides are referred to hereafter as ZrO 2 Žhp. and TiO 2 Žhp., respectively. Commercial titanium dioxide Žfrom Degussa, P25. was also used and before use, was calcined at 5008C for 24 h, exhibiting a molar fraction of anatase phase, XA s 0.58. This commercial titania sample will designated hereafter as TiO 2 ŽD.. The complete laser flash photolysis system for the recording of the transient absorbing species has been described previously w12x. All suspensions of powders were prepared at a final concentration of 62.5 mg ly1 in bidistilled water ŽpH s 5.5. and sonicated for 30 min before use. Methyl viologen ŽMV 2q . was chosen as electron acceptor because it undergoes reversible one-electron reduction with a well-defined and pH independent redox potential. In addition, the reduced form ŽMVq. can readily be identified by its characteristic absorption maximum w13x. UV–vis absorption spectra were recorded with a Shimadzu UV-2101PC spectrometer using barium sulphate as reference.
3. Results and discussion 2. Experimental details All the chemicals used in this work were of reagent grade and were used as supplied. Powdered zirconium titanate was processed following the sol–gel method described elsewhere w11x. The amorphous solid was precipitated by hydrolysis in an alcoholic solution containing equimolar amounts of TiCl 4 ŽMerck, 99.99%. and ZrOCl 2 ŽFluka, 43%, ZrO 2 . in the presence of an excess of hydrogen peroxide. The precipitate was washed, dried and calcined at 7008C for 2 h; the solid obtained had the structure of crystalline ZrTiO4 in the orthorhombic form. Zirconia and titania powders, were also independently prepared via hydrolysis of ZrOCl 2 or TiCl 4 using aqueous solution of ammonium hydroxide at pH ca. 11. After washing and drying, these amorphous single oxides were calcined for 2 h at 10008C Žzirconia. and 24 h at 5008C Žtitania.; the solid zirconia obtained had the structure of crystalline ZrO 2 in the monoclinic phase whereas the titania solid showed Žby XRD. the anatase as the unique crystalline phase. These two home prepared
The reduction of methyl viologen ŽMV 2q . by conduction band electrons of several photocatalysts ŽZrTiO4 and its parent oxides TiO 2 and ZrO 2 . has been studied, as a probe photoreaction, by laser flash photolysis. As shown in Fig. 1, the corresponding time courses of formation and subsequent decay of reduced methyl viologen ŽMVq. on laser excitation were recorded at 394 nm, a wavelength at which the MVq exhibits a clear absorption maximum. By comparison of Fig. 1ŽA., ŽB., ŽC., it can be seen that the initial MVq yield on ZrTiO4 , ZrO 2 Žhp. and TiO 2 Žhp. samples are more or less similar. However, by comparison of these figures and Fig. 1ŽD., it can be deduced that the initial MVq yield in the presence of TiO 2 ŽD. is larger than in the other samples. It should be noted that the previously described comparison of the MVq yields of the materials here reported is valid only when the surface areas of the four samples are similar; this situation appears to be true in our case since the surface areas of these samples are of the same order of magnitude: ZrTiO4 w39.5 m2 gy1 x, ZrO 2 Žhp. w23.5 m2 gy1 x, TiO 2 Žhp. w37 m2 gy1 x and TiO 2 ŽD. w46.5 m2 gy1 x.
372
J.A. NaÕıo ´ et al.r Materials Letters 39 (1999) 370–373
Fig. 1. Oscilloscope traces showing the time courses of MVq formation and decay at 394 nm in nitrogen-deaerated aqueous suspensions of the indicated samples. The methyl viologen ŽMV 2q . concentration was 50 mM. A ‘blank’ experiment was performed in 50 mM MV 2q solution in the absence of a photocatalyst. Each trace is the average of eight independent measurements.
Since the concentration of the scavenger MV 2q was kept constant in all the experiments, the differences observed between the MVq yields of the TiO 2 ŽD. and the other home prepared photocatalysts can be assigned to their different efficiencies of production of charge carriers Želectron–hole pairs. on laser excitation w14x. Therefore, in the TiO 2 ŽD. sample, the concentration of electron–hole pairs must be higher than the viologen concentration, and so the scavenger is quantitatively reduced. The opposite is true for the single and mixed home prepared samples, i.e., the concentration of Žey–hq . pairs should be lower than that of viologen.
The inherent photocatalytic activity of ZrTiO4 was previously determined and compared with those of the parent oxides, zirconia and titania w5x. The observed photocatalytic activity pattern TiO 2 ŽD. 4 TiO 2 Žhp. f ZrO 2 Žhp. ) ZrTiO4 , clearly indicates a ‘negative synergistic effect’, which produces an important decrease in activity for the solid resulting from the combination of two photoactive oxides. ZrTiO4 is a well-defined solid w11x, whose specific area Ž39.5 m2 gy1 . and band-gap energy Ž3.33 eV. w5x are close to those of titania and zirconia and cannot constitute textural and structural inhibiting factors. At the same time, the two home prepared
J.A. NaÕıo ´ et al.r Materials Letters 39 (1999) 370–373
373
estimate the photosensitivity of semiconductor oxide samples and predict a poor photoactivity for the TiO 2 Žhp. sample prepared by the above-described preparation procedure, in spite of this sample exhibiting, by XRD technique, the anatase as the unique phase, having been found that titania is the best photocatalyst, especially in its anatase form w1,16,17x.
Acknowledgements J.A.N. wishes to express his gratitude to the Spanish ‘Ministerio de Educacion ´ y Cultura’ for partial funding of this work within the framework of Project PB96-1346. Fig. 2. Optical absorption spectra for the following samples: Ža. TiO 2 ŽD.; Žb. TiO 2 Žhp.; Žc. ZrTiO4 ; Žd. ZrO 2 Žhp.; Žsee text for details..
References
single oxides have the crystalline structure of monoclinic ŽZrO 2 hp. and anatase ŽTiO 2 hp. phases exhibiting very similar UV–vis absorption spectra ŽFig. 2. although the absorption peak around 300 nm is higher for TiO 2 Žhp. than for ZrO 2 Žhp.. The large discrepancy in activity between TiO 2 ŽD. and ZrTiO4 has been correlated with the large discrepancy observed in their electrical photoconductivity in vacuum w8x and can be correlated initially with the large discrepancy observed here in their availability to reduces MV 2q by conduction band electrons of the photocatalysts. Under identical UV illumination, TiO 2 ŽD. can create more electron–hole pairs and consequently, more photoelectrons can reduces more MV 2q species. However, ZrTiO4 , and TiO 2 Žhp. have absorption spectra close to that exhibited by TiO 2 ŽD., Fig. 2. Therefore, it seems more probable that the better photocatalytic activity of TiO 2 ŽD. is related to a more efficient separation of the electron–hole pairs or to a substantially lower tendency of recombination of electrons and holes. It may be worth noting the different initial MVq yield values observed ŽFig. 1. for TiO 2 ŽD. Žmixture of anatase and rutile. and TiO 2 Žhp. Žanatase.. These differences may be caused by the structure and claim the explanation given by Bickley et al. w15x to explain the high photoactivity of TiO 2 ŽDegussa, P-25.. In summary, the present results further showed the potential of laser flash photolysis technique to
w1x N. Serpone, E. Pelizzetti ŽEds.., Photocatalysis. Fundamentals and Applications, Wiley, New York, 1989. w2x K. Tanabe, Solid Acids and Bases. Their Catalytic Activity, Academic Press, New York, 1970. w3x K. Tanabe, T. Suyimoshi, K. Shibata, K. Kiyoura, J. Kitagawa, Bull. Chem. Soc. Jpn. 47 Ž1974. 1064. w4x K. Araka, K. Tanabe, Bull. Chem. Soc. Jpn. 53 Ž1980. 299. w5x J.A. Navio, G. Colon, ´ in: V. Cortes Corberan, ´ S. Vic Bellon ´ ŽEds.., Proc. New Development in Selective Oxidation-II, Elsevier, Amsterdam, 1994, p. 721. w6x J.A. Navio, M. Garcia-Gomez, Ma. A. Pradera Adrian, J. ´ Fuentes Mota, Studies in Surface Science and Catalysis, Vol. 78, 1993, p. 431. w7x H. Courbon, J. Disdier, J.M. Herrmann, P. Pichat, J.A. Navio, Catal. Lett. 20 Ž1993. 251. w8x J.A. Navıo, ´ G. Colon, ´ J.M. Herrmann, J. Photochem. Photobiol. A: Chem. 108 Ž1997. 179. w9x G. Rothemberg, J. Moser, M. Gratzel, N. Serpone, D.K. ¨ Sharma, J. Am. Chem. Soc. 107 Ž1985. 8054. w10x J.A. Navio, F.J. Marchena, M. Roncel, M.A. De la Rosa, J. Photochem. Photobiol. A: Chem. 55 Ž1991. 319. w11x J.A. Navio, F.J. Marchena, M. Macias, P.J. Sanchez-Soto, P. ´ Pichat, J. Mater. Sci. 27 Ž1992. 2463. w12x J.A. Navarro, M. Roncel, M.A. De la Rosa, Photochem. Photobiol. 46 Ž1987. 965. w13x P.A. Trudinger, Anal. Biochem. 36 Ž1970. 221. w14x D. Duonghong, J. Ramsden, M. Gratzel, J. Am. Chem. Soc. ¨ 104 Ž1982. 2977. w15x R.I. Bickley, T. Gonzalez-Carreno, ˜ J.S. Lees, L. Palmisano, R.I. Tilley, J. Solid State Chem. 92 Ž1991. 178. w16x M. Schiavello ŽEd.., Photocatalysis and Environment: Trends and Applications, NATO-ASI Series C, Vol. 238, Kluwer Academic, London, 1987. w17x D.F. Ollis, H. Al-Ekabi ŽEds.., Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993.
Materials Letters 39 Ž1999. 370–373 www.elsevier.comrlocatermatlet
A laser flash photolysis study of the photochemical activity of a synthesised ZrTiO4 Comparison with parent oxides, TiO 2 and ZrO 2 J.A. Navıo ´
a,)
, M.C. Hidalgo a , M. Roncel b, M.A. De la Rosa
b
a
Instituto de Ciencia de Materiales de SeÕilla, Centro Mixto CSIC-UniÕersidad de SeÕilla, Centro de InÕestigaciones Cientıficas ‘Isla de la ´ Cartuja’, AÕda. Americo Vespucio, s r n. 41092, SeÕilla, Spain ´ b Instituto de Bioquımica Vegetal y Fotosıntesis, Centro Mixto CSIC-UniÕersidad de SeÕilla, Centro de InÕestigaciones Cientıficas ‘Isla de ´ ´ ´ la Cartuja’, AÕda. Americo Vespucio, s r n. 41092, SeÕilla, Spain ´ Received 5 October 1998; accepted 22 December 1998
Abstract The photochemical activity of a prepared zirconium titanate, ZrTiO4 , sample has been studied by laser flash photolysis. A short duration laser pulse was used to produce electron–hole pairs; methyl viologen was employed as electron scavenger. The same procedure has been employed to estimate the photochemical activity of two home prepared single oxides, TiO 2 Žhp. and ZrO 2 Žhp. and for a commercially available TiO 2 ŽDegussa, P-25.. Our results further showed the potential of the laser flash photolysis technique to estimate the photosensitivity of semiconductor oxides and predict a poor photoactivity for the TiO 2 Žhp. sample in spite of this sample exhibiting Žby XRD technique. the anatase as the unique phase. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Laser flash photolysis; Photochemical; ZrTiO4
1. Introduction Certain materials Žoxides, sulphides, etc.., act as photoconductors on illumination with near-UV photons. If the two photoinduced charge carriers Želectrons and holes. do not recombine, they can reach the surface and react with chemisorbed species giving reductionroxidation reactions w1x. The mixed oxides TiO 2 –ZrO 2 provides efficient acid–base bifunctional catalysts w2–4x. Zirconium
) Corresponding author. Tel.: q34-5-4489550; Fax: q34-54460665
titanate, ZrTiO4 , which has been investigated as a high dielectric material, has been investigated in heterogeneous photocatalysis by some of us w5,6x. In a preliminary study, it has been shown that ZrTiO4 is photosensitive in the near-UV region and can give, to some extent, oxygen isotopic exchange w7x. It has also been shown that ZrTiO4 becomes a photoconductor on illumination w7x having correlated the photocatalytic activity and photoelectronic properties of ZrTiO4 in order to account for its photoactivity and for the differences observed with the two parent oxides, ZrO 2 and TiO 2 w8x. Using picosecond laser flash photolysis, Rothemberg et al. w9x have investigated the dynamics of
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 0 3 7 - 3
J.A. NaÕıo ´ et al.r Materials Letters 39 (1999) 370–373
charge trapping and recombination in TiO 2 colloidal particles. The mechanism of methyl viologen ŽMV 2q . reduction by the conduction band electrons of TiO 2 can be summarised as follows: TiO 2 q hn ™ TiOU2 Žey–hq . Žey–hq . ™ hnX q hV Žey–hq . ™ eyq hq ey Žbulk. ™ ey Žsurface. ey Žsurface. q MV 2q Žads. ™ MVqŽ ads.
exciton formation direct recombination exciton dissociation electron transport electron trapping Žmethyl viologen reduction.
Laser flash photolysis has been applied for the study of the photoactivity of undoped and iron-doped titania samples w10x. We now explore the potential use of this technique to estimate the inherent photocatalytic activity of a prepared zirconium titanate, ZrTiO4 , and its parent oxides ZrO 2 and TiO 2 . Results are briefly compared with those obtained by other techniques applied to the same kind of oxides w7,8x to predict their photocatalytic activities.
371
single oxides are referred to hereafter as ZrO 2 Žhp. and TiO 2 Žhp., respectively. Commercial titanium dioxide Žfrom Degussa, P25. was also used and before use, was calcined at 5008C for 24 h, exhibiting a molar fraction of anatase phase, XA s 0.58. This commercial titania sample will designated hereafter as TiO 2 ŽD.. The complete laser flash photolysis system for the recording of the transient absorbing species has been described previously w12x. All suspensions of powders were prepared at a final concentration of 62.5 mg ly1 in bidistilled water ŽpH s 5.5. and sonicated for 30 min before use. Methyl viologen ŽMV 2q . was chosen as electron acceptor because it undergoes reversible one-electron reduction with a well-defined and pH independent redox potential. In addition, the reduced form ŽMVq. can readily be identified by its characteristic absorption maximum w13x. UV–vis absorption spectra were recorded with a Shimadzu UV-2101PC spectrometer using barium sulphate as reference.
3. Results and discussion 2. Experimental details All the chemicals used in this work were of reagent grade and were used as supplied. Powdered zirconium titanate was processed following the sol–gel method described elsewhere w11x. The amorphous solid was precipitated by hydrolysis in an alcoholic solution containing equimolar amounts of TiCl 4 ŽMerck, 99.99%. and ZrOCl 2 ŽFluka, 43%, ZrO 2 . in the presence of an excess of hydrogen peroxide. The precipitate was washed, dried and calcined at 7008C for 2 h; the solid obtained had the structure of crystalline ZrTiO4 in the orthorhombic form. Zirconia and titania powders, were also independently prepared via hydrolysis of ZrOCl 2 or TiCl 4 using aqueous solution of ammonium hydroxide at pH ca. 11. After washing and drying, these amorphous single oxides were calcined for 2 h at 10008C Žzirconia. and 24 h at 5008C Žtitania.; the solid zirconia obtained had the structure of crystalline ZrO 2 in the monoclinic phase whereas the titania solid showed Žby XRD. the anatase as the unique crystalline phase. These two home prepared
The reduction of methyl viologen ŽMV 2q . by conduction band electrons of several photocatalysts ŽZrTiO4 and its parent oxides TiO 2 and ZrO 2 . has been studied, as a probe photoreaction, by laser flash photolysis. As shown in Fig. 1, the corresponding time courses of formation and subsequent decay of reduced methyl viologen ŽMVq. on laser excitation were recorded at 394 nm, a wavelength at which the MVq exhibits a clear absorption maximum. By comparison of Fig. 1ŽA., ŽB., ŽC., it can be seen that the initial MVq yield on ZrTiO4 , ZrO 2 Žhp. and TiO 2 Žhp. samples are more or less similar. However, by comparison of these figures and Fig. 1ŽD., it can be deduced that the initial MVq yield in the presence of TiO 2 ŽD. is larger than in the other samples. It should be noted that the previously described comparison of the MVq yields of the materials here reported is valid only when the surface areas of the four samples are similar; this situation appears to be true in our case since the surface areas of these samples are of the same order of magnitude: ZrTiO4 w39.5 m2 gy1 x, ZrO 2 Žhp. w23.5 m2 gy1 x, TiO 2 Žhp. w37 m2 gy1 x and TiO 2 ŽD. w46.5 m2 gy1 x.
372
J.A. NaÕıo ´ et al.r Materials Letters 39 (1999) 370–373
Fig. 1. Oscilloscope traces showing the time courses of MVq formation and decay at 394 nm in nitrogen-deaerated aqueous suspensions of the indicated samples. The methyl viologen ŽMV 2q . concentration was 50 mM. A ‘blank’ experiment was performed in 50 mM MV 2q solution in the absence of a photocatalyst. Each trace is the average of eight independent measurements.
Since the concentration of the scavenger MV 2q was kept constant in all the experiments, the differences observed between the MVq yields of the TiO 2 ŽD. and the other home prepared photocatalysts can be assigned to their different efficiencies of production of charge carriers Želectron–hole pairs. on laser excitation w14x. Therefore, in the TiO 2 ŽD. sample, the concentration of electron–hole pairs must be higher than the viologen concentration, and so the scavenger is quantitatively reduced. The opposite is true for the single and mixed home prepared samples, i.e., the concentration of Žey–hq . pairs should be lower than that of viologen.
The inherent photocatalytic activity of ZrTiO4 was previously determined and compared with those of the parent oxides, zirconia and titania w5x. The observed photocatalytic activity pattern TiO 2 ŽD. 4 TiO 2 Žhp. f ZrO 2 Žhp. ) ZrTiO4 , clearly indicates a ‘negative synergistic effect’, which produces an important decrease in activity for the solid resulting from the combination of two photoactive oxides. ZrTiO4 is a well-defined solid w11x, whose specific area Ž39.5 m2 gy1 . and band-gap energy Ž3.33 eV. w5x are close to those of titania and zirconia and cannot constitute textural and structural inhibiting factors. At the same time, the two home prepared
J.A. NaÕıo ´ et al.r Materials Letters 39 (1999) 370–373
373
estimate the photosensitivity of semiconductor oxide samples and predict a poor photoactivity for the TiO 2 Žhp. sample prepared by the above-described preparation procedure, in spite of this sample exhibiting, by XRD technique, the anatase as the unique phase, having been found that titania is the best photocatalyst, especially in its anatase form w1,16,17x.
Acknowledgements J.A.N. wishes to express his gratitude to the Spanish ‘Ministerio de Educacion ´ y Cultura’ for partial funding of this work within the framework of Project PB96-1346. Fig. 2. Optical absorption spectra for the following samples: Ža. TiO 2 ŽD.; Žb. TiO 2 Žhp.; Žc. ZrTiO4 ; Žd. ZrO 2 Žhp.; Žsee text for details..
References
single oxides have the crystalline structure of monoclinic ŽZrO 2 hp. and anatase ŽTiO 2 hp. phases exhibiting very similar UV–vis absorption spectra ŽFig. 2. although the absorption peak around 300 nm is higher for TiO 2 Žhp. than for ZrO 2 Žhp.. The large discrepancy in activity between TiO 2 ŽD. and ZrTiO4 has been correlated with the large discrepancy observed in their electrical photoconductivity in vacuum w8x and can be correlated initially with the large discrepancy observed here in their availability to reduces MV 2q by conduction band electrons of the photocatalysts. Under identical UV illumination, TiO 2 ŽD. can create more electron–hole pairs and consequently, more photoelectrons can reduces more MV 2q species. However, ZrTiO4 , and TiO 2 Žhp. have absorption spectra close to that exhibited by TiO 2 ŽD., Fig. 2. Therefore, it seems more probable that the better photocatalytic activity of TiO 2 ŽD. is related to a more efficient separation of the electron–hole pairs or to a substantially lower tendency of recombination of electrons and holes. It may be worth noting the different initial MVq yield values observed ŽFig. 1. for TiO 2 ŽD. Žmixture of anatase and rutile. and TiO 2 Žhp. Žanatase.. These differences may be caused by the structure and claim the explanation given by Bickley et al. w15x to explain the high photoactivity of TiO 2 ŽDegussa, P-25.. In summary, the present results further showed the potential of laser flash photolysis technique to
w1x N. Serpone, E. Pelizzetti ŽEds.., Photocatalysis. Fundamentals and Applications, Wiley, New York, 1989. w2x K. Tanabe, Solid Acids and Bases. Their Catalytic Activity, Academic Press, New York, 1970. w3x K. Tanabe, T. Suyimoshi, K. Shibata, K. Kiyoura, J. Kitagawa, Bull. Chem. Soc. Jpn. 47 Ž1974. 1064. w4x K. Araka, K. Tanabe, Bull. Chem. Soc. Jpn. 53 Ž1980. 299. w5x J.A. Navio, G. Colon, ´ in: V. Cortes Corberan, ´ S. Vic Bellon ´ ŽEds.., Proc. New Development in Selective Oxidation-II, Elsevier, Amsterdam, 1994, p. 721. w6x J.A. Navio, M. Garcia-Gomez, Ma. A. Pradera Adrian, J. ´ Fuentes Mota, Studies in Surface Science and Catalysis, Vol. 78, 1993, p. 431. w7x H. Courbon, J. Disdier, J.M. Herrmann, P. Pichat, J.A. Navio, Catal. Lett. 20 Ž1993. 251. w8x J.A. Navıo, ´ G. Colon, ´ J.M. Herrmann, J. Photochem. Photobiol. A: Chem. 108 Ž1997. 179. w9x G. Rothemberg, J. Moser, M. Gratzel, N. Serpone, D.K. ¨ Sharma, J. Am. Chem. Soc. 107 Ž1985. 8054. w10x J.A. Navio, F.J. Marchena, M. Roncel, M.A. De la Rosa, J. Photochem. Photobiol. A: Chem. 55 Ž1991. 319. w11x J.A. Navio, F.J. Marchena, M. Macias, P.J. Sanchez-Soto, P. ´ Pichat, J. Mater. Sci. 27 Ž1992. 2463. w12x J.A. Navarro, M. Roncel, M.A. De la Rosa, Photochem. Photobiol. 46 Ž1987. 965. w13x P.A. Trudinger, Anal. Biochem. 36 Ž1970. 221. w14x D. Duonghong, J. Ramsden, M. Gratzel, J. Am. Chem. Soc. ¨ 104 Ž1982. 2977. w15x R.I. Bickley, T. Gonzalez-Carreno, ˜ J.S. Lees, L. Palmisano, R.I. Tilley, J. Solid State Chem. 92 Ž1991. 178. w16x M. Schiavello ŽEd.., Photocatalysis and Environment: Trends and Applications, NATO-ASI Series C, Vol. 238, Kluwer Academic, London, 1987. w17x D.F. Ollis, H. Al-Ekabi ŽEds.., Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993.