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Femtosecond Transient Absorption Spectroscopy on Photocatalysts: K4Nb6O17 and Ru(bpy)32+-Intercalated K4Nb6O17 Thin Films Akihiro Furube,†,‡,§ Toshihiko Shiozawa,† Akio Ishikawa,† Akihide Wada,† Kazunari Domen,*,†,‡ and Chiaki Hirose† Chemical Resources Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan, and Core Research for EVolutional Science and Technology (CREST), Japan Science and Technology, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan ReceiVed: March 22, 2001; In Final Form: NoVember 13, 2001
Transient absorption spectra of the layered compound K4Nb6O17, which is known as a photocatalyst for splitting water into H2 and O2, were measured by the femtosecond UV pump and visible probe technique. Under band-gap excitation of a K4Nb6O17 single crystal and a spin-coated K4Nb6O17 film, a broad transient absorption extending over the whole visible region, which could be ascribed to photogenerated carriers in K4Nb6O17, was observed, and its decay behavior was analyzed on the basis of second-order kinetics. Dynamics of photoexcited Ru(bpy)32+ intercalated at K4Nb6O17 interlayers was also investigated. Intercalation of Ru(bpy)32+ in K4Nb6O17 is known to give visible light response for photocatalytic chemical reaction. Observed transient bleaching of Ru(bpy)32+ band showed fast and nonexponential decay differing from that of Ru(bpy)32+ in water. The decay mechanism is discussed in terms of electron transfer between Ru(bpy)32+ and K4Nb6O17.
Introduction A photocatalytic material, K4Nb6O17, has a layer structure and is expected to become a noble photocatalyst for splitting water into oxygen and hydrogen molecules by virtue of a large area for redox reactions and appropriate location of conduction and valence bands. Extensive studies of structural and photochemical properties of K4Nb6O17 have been performed by UVvis and Raman spectroscopies, XRD, AFM, SEM, TEM, and photocatalytic reaction.1-20 For further understanding of the mechanism of the photocatalytic reaction, it is essential to know ultrafast primary processes initiated by absorption of photons. Thus, we have investigated ultrafast dynamics of photogenerated carriers in K4Nb6O17 single crystal and spin-coated film in the time range from femtosecond to nanosecond by means of femtosecond transient absorption measurements for the first time. The single-crystal sample is expected to have fewer defects, impurities, and boundaries with its larger areas of a single two-dimensional layer than the spin-coated film. The single-crystal sample is thus suited for the investigation of the intrinsic properties of K4Nb6O17. The spin-coated film is prepared from powder particles that are practically used for photocatalysis, so that primary events of photogenerated carriers in almost actual photochemical reactions will be monitored. The intercalation of some dye molecules in K4Nb6O17 is known to render visible light response to the photocatalytic reaction.21,22 The visible light response is important from the viewpoint of the application to the photocatalysis by the irradiation of sunlight. We investigated the dynamics of photoexcited Ru(bpy)32+ intercalated between the interlayers of K4Nb6O17 as the Ru(bpy)32+/K4Nb6O17 system is known to show * Corresponding author. Fax: +81-45-924-5282. E-mail: kdomen@ res.titech.ac.jp. † Chemical Resources Laboratory. ‡ Core Research for Evolutional Science and Technology (CREST). § Current address: National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, Tsukuba, Ibaraki 305-8565, Japan.
photocatalytic reactivity to produce H2 under the illumination of visible light. The reactivity suggests that photoexcited Ru(bpy)32+ ejects an electron into K4Nb6O17 and the ejected electron is transferred to H+ to form H2. We used femtosecond time-resolved spectroscopy to reveal, for one thing, that the excitation lifetime of the intercalated Ru(bpy)32+ is much shorter than that of Ru(bpy)32+ in water. The decay mechanism is discussed on the basis of electron-transfer process between Ru(bpy) 32+ and K4Nb6O17. Experimental Section Apparatus. For transient absorption experiments, an excitation light source consists of a cw self-mode-locked Ti:sapphire laser (Mira 900, Coherent) pumped by an Ar+ laser (Innova 300, Coherent) and a Ti:sapphire regenerative amplifier system (Alpha-1000, B. M. Industries) pumped by a Q-switched YLF laser (AL.502 C. B. M. Industries). The fundamental output from the regenerative amplifier (800 nm, 0.7 mJ/pulse, 120 fs fwhm, 1 kHz) was frequency-doubled to 400 nm by a BBO crystal for the use as excitation pulse. The 400 nm pulse was mixed with the fundamental pulse to generate THG (266 nm) with a BBO crystal to obtain the excitation pulse at shorter wavelength. The residual of the fundamental output was focused into a quartz cell (1 cm path length) containing H2O to generate a whitelight continuum and use as the probe pulse. The excitation beam was modulated by an optical chopper at the frequency of 500 Hz. Beam sizes of pump and probe light on the sample surface were about 1 mm and 0.5 mm, respectively, and the incident angles were about 0° and 5°, respectively. When the 266 nm light was used as the pump beam, the polarizations of the pump and probe beams were parallel to each other, while they were perpendicular when the 400 nm light was used. The probe light transmitted through the sample was led to a monochromator and detected by a photomultiplier tube. The pump-induced change of signal intensity, ∆T, was extracted by a boxcar integrator and a lock-in amplifier and the magnitude of the
10.1021/jp011083o CCC: $22.00 © 2002 American Chemical Society Published on Web 03/05/2002
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Figure 1. UV-visible absorption spectra of a K4Nb6O17 spin-coated film (- -), K4Nb6O17 single crystal (‚ ‚), K4Nb6O17 film intercalated with Ru(bpy)32+ (thick line), and aqueous solution of Ru(bpy)32+ (thin line) are presented. The band-gap edge of K4Nb6O17 is located around 300350 nm. Absorption peak around 450 nm is ascribed to an MLCT (metal-ligand charge transfer) band of Ru(bpy)32+. The inset shows schematic illustration for the structure of K4Nb6O17 (see text for detail).
transient absorption was derived as -log((T + ∆T)/T), where T is the intensity of the light transmitted through the sample without the photoexcitation and it was obtained by an averaged signal output from the boxcar integrator without excitation. For the measurement of transient absorption spectra the wavelength of the probe light was scanned over the range from about 430 to 750 nm, while the measurement for the temporal decay of the transient absorption was performed by scanning the delay time to about 1 ns using a translation stage. Time resolution of the pump-probe measurement was about 250 fs. To measure photoluminescence lifetimes, a streak camera (HAMAMATSU STREAK SCOPE C4334) was used. In this case, the excitation light source was the second harmonic light of the mode-locked Ti:sapphire laser output (800 nm, 76 MHz) and its intensity at the sample position is typically about 1 nJ/ pulse. Time resolution of the measurements is usually 30-50 ps. Samples. K4Nb6O17 Single Crystal. The crystal structure of K4Nb6O17 has been reported previously.23,24 Schematic illustration for the structure of K4Nb6O17 is displayed in the inset of Figure 1. It consists of octahedral units of NbO6 forming a twodimensional layered structure through bridging oxygen atoms. Two types of interlayer exist alternately, in which K+ ions compensate for the negative charge of the layer itself. It is reported that optical transition at about 320 nm has been attributed to a charge transfer from O2- to the empty 4d orbital of the Nb(V) ion.25 K4Nb6O17 is easily hydrated by H2O molecules in ambient atmosphere and exists as K4Nb6O17•3H2O. This hydrated substance is the material of the present investigation and will be abbreviated as K4Nb6O17 in this paper. A K4Nb6O17 single crystal with a few mm thickness and about 1 × 1 cm area was placed for more than several hours in water to obtain a thinner crystal by exfoliation. Planer crystals with micrometer thickness were obtained and a piece of them was held between two quartz plates for spectroscopic measurements. K4Nb6O17 Film. K4Nb6O17 powder was prepared according to the procedure described in previous report.20 Wet grinding of K4Nb6O17 was carried out as follows: the suspension of K4Nb6O17 in distilled water was put into a centrifugal planetary ball mill with zirconia beads and vigorously stirred for 2 h at room temperature. After the milling, the slurry was filtered to
Furube et al. remove zirconia beads, washed with distilled water, and centrifuged to remove the precipitate. The centrifuged slurry containing fine particles of K4Nb6O17 was used for the preparation of thin films. A quartz plate (2 × 2 cm) was fixed on a spin-coater and rotated at 1000-3000 rpm, and a couple of droplets of the slurry were dropped to the plate. The plate was rotated until the slurry spread homogeneously. The obtained spin-coated film was calcined at 800 °C for 1 h in air so that good crystallization was achieved. It has been shown by SEM and TEM measurements that this prepared film was layered by the K4Nb6O17 sheets stacked parallel to the substrate surface and that the size of a single sheet was about 1 µm in diameter.19,20 Intercalation of Ru(bpy)32+. Methods to intercalate some dye molecules, metal ions, and metal particles into K4Nb6O17 have been well-established.18,20 The intercalation of relatively large molecule, Ru(bpy)32+, was carried out in two steps: first K+ ions between the layers were replaced by n-hexylammonium ions in order to increase the interlayer distance, and then the n-hexylammonium ions were replaced by Ru(bpy)32+ by leaving the film in the aqueous solution of Ru(bpy)32+ for 1 day. The interlayer distances have been evaluated by XRD as 8.2, 20.8, and 16.7 Å for normal, n-hexylammonium ion exchanged, and Ru(bpy)32+ exchanged K4Nb6O17, respectively.18 Results and Discussion UV-Vis Absorption Spectra. UV-visible absorption spectra of single crystal and spin-coated film of K4Nb6O17 are shown in Figure 1 by thin dotted and thick dotted curves, respectively. The spectrum of the film sample after intercalation by Ru(bpy)32+ is shown by the thick solid curve, and the spectrum of the aqueous solution of Ru(bpy)32+ by the thin solid curve. It is seen from the figure that the band-gap edge of the spin-coated film at around 300 nm is located at shorter wavelengths compared with that of the single crystal at around 350 nm. The blue shift of the band-gap edge for the spin-coated film may be ascribed to the fact that the film was prepared from suspension containing micrometer sized exfoliated K4Nb6O17 sheets. Actually, the absorption spectrum of the K4Nb6O17 suspension solution was very similar to that of the K4Nb6O17 spin-coated film. The K4Nb6O17 film intercalated with Ru(bpy)32+ clearly shows the metal-ligand charge transfer (MLCT) band of Ru(bpy)32+ with very similar spectral shape to that of Ru(bpy)32+ in water, except for a slight red shift of the band position. The small red shift of the band position for the film sample will be mainly due to the effect of a different dielectric constant of K4Nb6O17 from that of water. Transient Absorption Spectra and Decay of a Single Crystal. Figure 2a shows transient absorption spectra of the K4Nb6O17 single crystal under excitation by a 266 nm femtosecond pulse. The polarization of the pulse is parallel to the c-axis. The solid and dotted lines indicate the spectra obtained at the delay time of 10 and 500 ps, respectively, and open diamonds are the 2.8 times magnification of the latter. It is seen that a very broad absorption band with a maximum at around 620 nm is observed over the measured wavelength region with the spectral profile remaining unchanged. Rather distinct-looking sidebands changed in their position and sharpness by both the used sample and the position of irradiation and we ascribed them to the optical artifact arising from the interference effect of probe light inside the film-shaped sample. Figure 2b shows the temporal behaviors of the transient absorption intensities at the wavelength of 500 nm (filled triangles) and 700 nm (open circles) in the delay time region
Intercalated K4Nb6O17 Thin Films
Figure 2. (a) Transient absorption spectra of a K4Nb6O17 single crystal at 10 ps (thick line) and 500 ps (dotted line) delay times after excitation by a 266 nm femtosecond pulse. An ()) indicate a spectrum at a 500 ps delay time whose intensity was multiplied by 2.8. (b) Time profiles of the transient absorption at 550 nm (2) and 700 nm (O).
from -0.7 ps to 1.0 ps. Both time profiles show quick rise and there is no difference between them under the time resolution (250 fs) of our experimental system. The results in Figure 2a,b combined suggest that the spectral profile of the transient absorption does not change by the delay time from a few hundred femtoseconds to about 500 ps. We have no information on either the electronic structure in the conduction band or the nature and whereabouts of defects in the K4Nb6O17 sheets at present, but it is hard to consider that free electrons stay in the conduction band for the period of over hundreds picoseconds. Thus the observed transient absorption should have originated from the electrons trapped in exciton, self-trapped exciton, or defect states. Since it is expected that the conduction band consists of d-electrons of Nb4+, the observed absorption band with a peak at 620 nm (2.0 eV photon energy) might have originated from the d-d transition. However its detailed nature such as delocalization of electrons, their interaction with holes or phonons, and so on, are not unknown at present. Also, the transition may be from defect states below the d-bands to the d-bands. A familiar photocatalytic metal oxide, TiO2, has been well investigated by transient absorption spectroscopy,26-30 and a very broad transient absorption band observed in the picosecond time region has been assigned to trapped electrons generated by rapid trapping of the free electrons into defect Ti4+ sites in the bulk or on the surface of TiO2.31,32 It is necessary to discuss about the possibility that photogenerated holes are observed in the obtained transient absorption spectrum of K4Nb6O17. However, it is difficult to consider that the photogenerated holes are responsible for the broad transient absorption from the following reason. Optical transition near the band-gap energy is mainly due to CT transition from O atom to Nb atom. Though which part of oxygen in the octahedral unit has a large contribution to the transition has not been specified yet to our knowledge, it is likely that the π-electron at double bond of NbdO group sticking out into the interlayer as seen in the inset of Figure 1 contributes to a lower CT transition energy than σ-electrons at other Nb-O single bonds. Therefore our excitation pulse at 266 nm is considered to cause dominantly CT transition at the NbdO group and generate
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Figure 3. Excitation intensity dependence of the transient absorption decay at 600 nm of a K4Nb6O17 single crystal. Excitation intensity of the 266 nm pump pulse is 9 µJ and 4 µJ. Solid lines indicate results of curve fitting (see text). The fitting parameters are listed in Table 1.
localized holes at the oxygen atom. Then the absorption band of the localized hole would be not so broad. By this reason we here ascribe the broad transient absorption band to electrons trapped somewhere in K4Nb6O17 crystal. Confirmation of this assignment requires either theoretical calculation to verify the conduction band structure and simulate the transient absorption spectral shape or an experimental study using an electron acceptor such as methyl viologen which in its reduced form should give a strong absorption band in the visible region33,34 and let the broad absorption band of K4Nb6O17 disappear. The relaxation process of the trapped electrons was investigated by measuring the excitation intensity dependence of the decay behavior of the transient absorption. The results are shown in Figure 3, where closed circles and open triangles denote the time profiles of transient absorption at 600 nm obtained at the excitation intensity of 9 and 4 µJ, respectively, of the 266 nm pump pulse. When the excitation intensity was decreased from 9 to 4 µJ, slower decay was observed suggesting that the kinetics is dependent on the carrier concentration. The decay behavior was analyzed on the basis of second-order kinetics in which the time dependent absorbance change, A(t), is expressed as follows:30
A(t) )
1 + C'1, k′t + 1/C′0
(1)
where k′ ) k/R, C′0 ) RC0, and C′1 ) RC1 with R being associated with absorption coefficient and optical path length. k is the second-order rate constant, and C0 is the initial concentration of the trapped electrons. C1 represents a long lifetime component which is assumed to be constant within the measured delay time. Solid lines in Figure 3 are the results of fitting to eq 1, where we used the parameters k′, C′0, and C′1 as fitting parameters because the value of R is independent of the excitation intensity and it is sufficient to know the relative change of parameter values by the change of excitation intensity. The values obtained by fitting are listed in the Table 1. It is seen from the table that the values of C′0 and C′1 vary by the excitation intensity while that of k′ remains constant. Note that this k′ for the 4 µJ pump intensity was fixed to the value determined by the fitting for the 9 µJ pump intensity, since for 4 µJ pump intensity the fitting error was large due to the data scatter. The result of fitting indicates that the second-order
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Figure 4. Transient absorption decay profiles of a K4Nb6O17 single crystal (pump polarization parallel to the c-axis (2), pump polarization parallel to the a-axis (9)) and a K4Nb6O17 spin-coated film (O). Excitation is at 266 nm with 15 µJ intensity and probe is at 650 nm. Solid lines are guide for eyes.
TABLE 1: Fitting Parameters Used in the Curve Fitting of Figure 3: See Text for Definition of the Parameters excitation intensity
k′
C0′
C1′
9 µJ 4 µJ
6.5 6.5
0.013 0.006
0.013 0.010
process is dominant in the relaxation of the short-lived trapped electrons. A faster decay for larger pump intensity means that the photogenerated electrons and hole do not undergo geminate recombination but can migrate in the niobate layer sheet. The origin of the long lifetime component is not clear. However, the delay time independent spectral shape means that the trapped electrons do not change their distribution feature in the trapping states. If trapped electrons were to relax into deep trapping states with the lifetime on the order of of picoseconds, the spectral shape should depend on the delay time. The timeindependent spectral shape of the trapped electrons suggests that the long lifetime component is caused not by the observed electrons themselves but by the holes, such as those at the oxygen atoms of NbdO where electron-hole recombination is inhibited due to their localization. Further investigation is important to understand the mechanism of photocatalytic reaction, because this long lifetime species is expected to play a significant role in photocatalytic reactions. The decay behavior of transient absorption was changed by the 90° rotation of the sample about the surface normal. Shown in Figure 4 are the decay behaviors at 650 nm of the single-
crystal sample for the case of pump polarization being parallel to c- and a-axis, respectively. The observed polarization dependence may be due to the difference in the initial concentration of photogenerated carriers as the absorption coefficient is dependent on the polarization of UV light. It has been reported that hydrated K4Nb6O17 crystal (K4Nb6O17•3H2O) belongs to the space group of orthorhombic P2212123,35 with the b-axis perpendicular to the two-dimensional niobate sheet, and the absorption coefficient of the crystal for the light polarized along a-axis should be different from that for the light polarized along c-axis. Transient Absorption Spectra and Decay of the Film Sample. The pump-probe experiments were also carried out for the spin-coated K4Nb6O17 film. Transient absorption spectra at 10, 100, and 200 ps delay times are shown in Figure 5. With the exception of the structure of transient bands observed for the single-crystal sample, the observed transient absorption spectra were very similar to that of the single crystal in both the spectral feature and their invariance by the delay time. The suppression of the structure will be due to the film thickness of about 200 nm. Concerning the time profiles of transient absorption, the rise time is again close to the time resolution of measurements and the excitation intensity dependence of the transient absorption decay also showed similar behavior to that of the single crystal. Comparison between Crystal and Film Samples. We expected that the small area of individual two-dimensional sheets on the film sample causes the lifetime of the transient species to differ from that in the single crystal via effects as the confinement of excitons within the 1 µm-sized two-dimensional niobate sheet and the enhancement of electron-hole recombination due to defects at the edges. We have shown above that the decay profile is dependent on the excitation intensity, so that the transient absorption decay profiles measured under exactly the same excitation condition were compared of the two samples. Shown by open circles in Figure 4 is decay profile at 650 nm wavelength for the spin-coated film. It is seen that the decay of the spin-coated film proceeds between two curves of the single crystal, implying that the lifetimes of trapped electrons in both samples are comparable. Effect of the exciton confinement can be ruled out, since transient absorption spectral shape of both samples is same. The absence of the sheet boundary effect can be explained by locating the photogenerated holes at the oxygen atom of NbdO, because only photogenerated
Figure 5. Transient absorption spectra of a K4Nb6O17 spin-coated film under excitation of a 266 nm fs pulse. The delay times are 10 (0), 100 (3), and 200 ps ([).
Intercalated K4Nb6O17 Thin Films
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Figure 6. A transient absorption spectrum of Ru(bpy)32+ aqueous solution at a delay time of a few picosecond after excitation at 400 nm with intensity of 20 µJ. The inset shows transient absorption spectrum obtained with 100 µJ intensity. The corresponding ground-state absorption spectrum is shown by a dotted line.
electrons, but not holes, can then migrate to the recombination sites at the edge of the sheet and no enhancement of the electron-hole recombination process occurs. From the standpoint of application to photocatalysis, K4Nb6O17 samples composed of small fragments of the sheets are advantageous by giving large reaction area available so that reactants in solution can easily access to the surface of the layer. The unchanged lifetime without any edge effects of photogenerated carriers in µm-sized K4Nb6O17 fragments is encouraging for the design of catalytic materials. Transient Absorption Spectra of Ru(bpy)32+ in Water. The transient absorption spectrum of Ru(bpy)32+ aqueous solution was measured as reference for the Ru(bpy)32+ intercalated K4Nb6O17 sample. The spectrum at the delay time of a few picoseconds after excitation of a 400 nm femtosecond pulse with about 20 µJ intensity is presented by filled circles in Figure 6. The ground-state absorption spectrum is also shown by a dotted line for comparison. The inset shows transient absorption spectrum obtained with 100 µJ excitation intensity to magnify the weak absorption in longer wavelength region. It is seen that the main feature of the transient spectrum is the bleaching of the ground-state spectrum, but a very weak transient absorption was present in longer wavelength region as shown in the inset. This very weak transient absorption could be the tail of the spectrum reported for the excited state of Ru(bpy)32+ (triplet state) peaked at around 370 nm.36,37 The lifetime of the transient bleaching was much longer than our time range of the measurement (1 ns). This is consistent with reported result,36 in which the lifetime was about 670 ns. Transient Absorption Spectra and Decay of the Ru(bpy)32+ Intercalated Film. Plotted by filled circles in Figure 7a is the transient absorption spectrum of the spin-coated film into which Ru(bpy)32+ was intercalated. The spectrum was obtained at a delay of few picosecond after the 400 nm excitation with 20 µJ pulse energy. The ground-state absorption spectrum is also shown by solid line. Multiphoton band-gap excitation of the K4Nb6O17 is negligible in the present excitation condition, because no transient signal was observed on the same but Ru(bpy)32+-undoped film when excited by the 400 nm pulse with the pulse energy of as high as 100 µJ. Therefore, we concluded that the intercalated Ru(bpy)32+ was selectively excited. Comparison with Figure 6a reveals that the present results are very similar to those for Ru(bpy)32+ in water in that the bleachingof the ground state absorption was mainly observed. No temporal change was observed on the spectral shape of transient bleaching
Figure 7. (a) Transient absorption spectra of the Ru(bpy)32+ intercalated K4Nb6O17 film excited at 400 nm with laser intensity of about 20 µJ. The corresponding ground-state absorption spectrum is also shown by a solid line. (b) Spectral evolution after several ps in the wavelength region from 450 to 600 nm. The excitation intensity is about 20 µJ. (c) Spectral evolution in a longer wavelength region from 480 to 720 nm. The excitation intensity is about 90 µJ.
except for the magnitude of the bleaching up to 500 ps delay time. Figure 7c shows the transient spectra in the longer wavelength region obtained by the excitation with 90 µJ pulse energy. We again observed a very weak transient absorption as in the aqueous solution. Temporal profiles of the transient absorption at 700 nm (Figure 8a,b) and bleaching at 480 nm (Figure 8c,d) are shown in Figure 8, where Figure 8a,c indicate the profiles up to 1.5 ps delay time and Figure 8b,d indicate the profiles up to 1000 ps delay time. It is seen that both of the transient absorption and the bleaching generate as fast as the time resolution of the measurements and then decay gradually up to 1 ns. This decay much faster than that of Ru(bpy)32+ in water suggests the presence of interaction between excited Ru(bpy)32+ and K4Nb6O17. Possible mechanisms based on the interaction which can result in such a short lifetime are (i) enhanced radiative relaxation from the excited state to the ground state of Ru(bpy)32+ due to electronic interaction between Ru(bpy)32+ and K4Nb6O17, (ii) enhanced nonradiative relaxation from the excited state to the ground state of Ru(bpy)32+, and (iii) electron transfer from photoexcited Ru(bpy)32+ to K4Nb6O17 followed by back electron transfer from K4Nb6O17 to Ru(bpy)33+. Mechanism (i) is unlikely because the ground-state absorption spectra of Ru(bpy)32+ in water and in K4Nb6O17 are very similar to each other as seen in Figure 1. Mechanism (ii) cannot be denied since the number of phonon modes responsible for nonradiative relaxation of excited Ru(bpy)32+ will be larger in the solid layer structure than in water solution. Mechanism (iii) is significantly possible, because K4Nb6O17 intercalated with Ru(bpy)32+ has high photocatalytic reactivity for H2 generation under the irradiation
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Figure 8. Temporal profiles of transient absorption at 700 (a, b) and 470 (c, d) nm of the Ru(bpy)32+ intercalated K4Nb6O17 film excited at 400 nm.
of visible light,21 and therefore, an electron should be ejected from the photoexcited Ru(bpy)32+ into K4Nb6O17. Transient absorptions by Ru(bpy)33+ and electrons injected into K4Nb6O17 should be observed for the mechanism (iii). However, it was difficult to observe the distinct transient absorption bands attributed to Ru(bpy)33+ or injected electrons: namely, the molar absorption coefficient of the MLCT band of Ru(bpy)32+ is relatively large being on the order of 104 (M-1 cm-1) at the peak wavelength36 and the weak absorption band of Ru(bpy)33+, which should be observed at
