J. Phys. Chem. C 2009, 113, 11741–11746
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Femtosecond Visible-to-IR Spectroscopy of TiO2 Nanocrystalline Films: Elucidation of the Electron Mobility before Deep Trapping† Yoshiaki Tamaki,‡ Kohjiro Hara, Ryuzi Katoh, M. Tachiya, and Akihiro Furube* National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ReceiVed: February 27, 2009; ReVised Manuscript ReceiVed: April 22, 2009
The transient absorption of nanocrystalline TiO2 films in the visible-to-IR wavelength region was measured under UV excitation at 266 nm in order to purposely generate plural electron-hole pairs in single nanoparticles. Trapped holes, trapped electrons, and bulk electrons were observed as in our previous transient absorption studies, where the electron-hole density reduction to be as low as second-order recombination did not occur, namely, the number of the electron hole pairs was less than unity per TiO2 nanoparticle. The kinetics of the second-order electron-hole recombination, induced in a controlled manner, was analyzed to estimate the mobility of the electrons before their deep trapping, which was found to occur with a time constant of 500 ps in our previous report (Tamaki, Y.; et al. Phys. Chem. Chem. Phys. 2007, 9, 1453-1460000). Electron-hole dynamics in the TiO2 nanoparticle from 100 fs to 1 ns has been understood in detail, and its relation to the photocatalytic nature of TiO2 nanoparticles is discussed. 1. Introduction Understanding primary processes of charged carriers in photocatalytic nanoparticles is important. The most representative photocatalyst TiO2 has been extensively studied more than several decades because of their high reactivity in decomposing environmental pollutants and bacteria and in splitting water for hydrogen generation.1-6 Although recently visible responsible photocatalytic materials such as nitrogen-doped TiO2,7-9 a solid solution of GaN and ZnO,10 and tungsten oxide11 are attracting much attention for the sake of more efficient utilization of sunlight than TiO2, which absorbs only UV light, comprehensive knowledge of TiO2 photocatalyst is useful to design future materials overcoming TiO2. Photogenerated electrons and holes in TiO2 react with molecules at the surface (Figure 1). Hence, generation and relaxation processes of electrons and holes are important in determining the efficiency of photocatalytic reactions. To understand these primary processes, transient absorption spectroscopy is one of the most powerful techniques because it provides us with very high temporal resolutions (∼100 fs) and allows identification of transient chemical species. Transient absorption spectroscopy has been used extensively to study the ultrafast trapping dynamics of electrons and holes12-15 and the oxidation and reduction of adsorbed molecules in terms of the rate16,17 and efficiency18 of photocatalytic reaction of TiO2. Absorption spectra of electrons and holes in TiO2 are very broad, from the UV to the IR region.19-26 Assignments of electrons and holes were made by measuring transient absorption spectra in the presence of electron or hole scavengers. Using nanocrystalline films in our previous studies, transient absorption in a wide wavelength region from the visible to IR has been †
Part of the “Hiroshi Masuhara Festschrift”. * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +81-29-861-2953. Fax: +81-29-8615301. ‡ Present address: Department of Chemistry, Biology, and Marine Sciences, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 9030213, Japan.
Figure 1. Illustration of primary processes of the photocatalytic reactions in a TiO2 particle.
assigned to surface-trapped holes, surface-trapped electrons, and bulk electrons.20 Through these studies, the spectroscopic features of electrons and holes in TiO2 have been well understood. On the basis of the assigned absorption bands of the electrons and holes in TiO2, their reaction dynamics has recently been studied by observing the temporal evolution of transient absorption. For understanding charged carrier dynamics in photocatalysis experiments in the usual light irradiation, it is important to measure the transient absorption under the weakexcitation condition.20,27-29 Excitation pulse intensity must be sufficiently low so that second-order electron-hole recombination does not take place. Under this condition, a single electron-hole pair is generated within a particle and undergoes geminate recombination. Such a weak-excitation condition enables us to correlate transient absorption spectroscopy results with the activity of a photocatalytic reaction under stationary illumination. We have realized such a weak excitation condition for nanosecond and even for femtosecond transient absorption spectroscopy using a very weak 355 nm (close to the band edge energy) excitation laser as well as very sensitive transient absorption detection.18,27,30,31 We successfully elucidated the reaction dynamics of hole transfer from photoexcited TiO2 to adsorbed alcohols (methanol, ethanol, and
10.1021/jp901833j CCC: $40.75 2009 American Chemical Society Published on Web 05/06/2009
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Figure 2. A steady-state absorption spectrum of the TiO2 nanocrystalline film studied. Arrows indicate excitation wavelengths in the femtosecond transient absorption measurements.
2-propanol) without interference of electron-hole recombination.18 Following this study, we also elucidated electron-hole dynamics within TiO2 nanoparticles and succeeded to reveal ultrafast trapping dynamics of electrons and holes in the subpicosecond time region and slow electron relaxation in the hundreds picosecond time region.30 It turned out that the holes were trapped at trap sites near the surface of TiO2 nanoparticles within 170 fs. On the other hand, a portion of electrons was first trapped at shallow sites near the surface, and the remaining electrons were distributed over the bulk of the nanoparticles. All of these electrons were equilibrated migrating over a nanoparticle and then relaxed into deeper trapping sites in the bulk with a common time constant of 500 ps. This behavior is included in the following illustration to show all revealed dynamics (Figure 7). Before our studies, many studies using femtosecond pulsed lasers to elucidate the ultrafast process of photogenerated carriers were carried out.12,13,19,32 For example, the initial trapping times of free electrons and free holes in TiO2 nanoparticles are reported to be a few hundred femtoseconds.12,13 In these studies, the second-order electron-hole recombination dominates the charge carrier relaxation because of a large carrier density generated by the ultrafast laser excitation. The second-order recombination rate constants were evaluated from decay of transient absorption signals of trapped charge carriers, not knowing such charge distribution and relaxation in TiO2 nanoparticles. In this paper, we did use an intense 266 nm excitation laser to generate a large density of charge carriers in TiO2 nanocrystalline films, so that second-order electron-hole recombination took place efficiently. Since the holes are trapped near the surface immediately after laser excitation, it is expected that the second-order recombination is governed only by electron diffusion. Transient absorption spectra of TiO2 nanocrystalline films in the visible to IR region were measured, and the constitutive absorption bands were assigned based on previously obtained knowledge for the same TiO2 nanocrystalline film. In the subpicosecond region, retardation of the ultrafast charge trapping process due to the presence of excess energy compared with the band gap energy was clarified. From the analysis of second-order electron-hole recombination, the mobility of shallowly trapped bulk electrons that exist only in early photochemical stages of TiO2 catalysis was evaluated. 2. Experimental Section 2.1. Samples. The TiO2 nanocrystalline film was prepared by a method similar to Gra¨tzel’s method.33 The mean diameter of the primary nanoparticles was 10-15 nm. The organic paste containing the semiconductor nanoparticles was printed on a glass substrate by a screen-printing technique. When the sample was calcined, the opaque film became transparent enough for spectroscopic measurements. The mean diameter increased
Tamaki et al. slightly to about 20 nm, as measured by scanning electron microscopy. The crystal phase was anatase, as confirmed by X-ray diffraction data. The TiO2 films were 1 cm2 (1 cm × 1 cm) in area and 1.5 µm thick. 2.2. Femtosecond Transient Absorption Spectrometer. The light source for transient absorption spectroscopy was a femtosecond titanium sapphire laser with a regenerative amplifier (Hurricane, Spectra Physics, 800 nm, 130 fs, 1 mJ/pulse, 1 kHz). The fundamental output of the laser was divided into two beams. One of the beams was used for the pump pulse at 266 or 355 nm, and the other was used for the probe pulse. To obtain 266 nm light, a beam was frequency-tripled using BBO crystals. To obtain 355 nm light, a beam was introduced into an optical parametric amplifier (OPA) (TOPAS, Quantronix), and the output (signal light) was frequency-quadrupled. The instrument responses using 266 and 355 nm pump beams were 170 and 220 fs, respectively, which were evaluated by two-photon absorption in TiO2, as in our previous studies.15,30 The pump beam was chopped to decrease the repetition rate to half of the fundamental. The pump beam radius on the film surface was about 0.5-1.0 mm as observed with a CCD camera. The probe beam was a white light continuum generated by focusing the fundamental into a sapphire plate (450-1200 nm) or the signal, idler, or DFG beam of the OPA (1300-5000 nm). The probe beam was divided into two beams; one beam passed through the sample, and the other beam was a reference for compensating the intensity fluctuation of each pulse. The intensities of the pulses were measured after passing through monochromators with a Si photodetector, an InGaAs photodetector, or a mercury-cadmium-telluride (MCT) photodetector, depending on the probe wavelength (400-800, 800-1500, or 1500-5000 nm, respectively). The signals from the detectors were gated and acquired. Transient absorption was calculated from the pulse intensity of the probe with and without excitation, typically using thousands of pulses. The TiO2 film was placed in air for the transient absorption measurements at 295 K. 3. Results and Discussion 3.1. Assignment of Transient Absorption Spectra under 266 nm Excitation. Figure 2 shows the steady-state UV-visible absorption spectrum of the TiO2 film investigated. The onset of the absorbance is at around 380 nm. The excitation wavelengths chosen in this study, 266 and 355 nm, are indicated by arrows. At 355 nm, the absorption is weak, ∼0.3, and therefore, a relatively uniform charge carrier density along the film thickness can be expected. On the other hand, at 266 nm, the absorption is very strong with the optical density larger than 3; therefore, charge carriers are expected to exist only near the irradiated side of the film. Figure 3 shows transient absorption spectra in the spectral range from 500 to 5000 nm of TiO2 film excited by several microjoule pump lasers at 266 nm. The optical delay times were 1, 10, 20, 100, and 280 ps. A very broad absorption band extends over the whole spectral range, and it can be divided into two parts; one is located in the visible and near-IR regions (500-1500 nm), having a peak between 700 and 1100 nm, and the other has increasing intensity with increasing wavelength in the IR spectral region over 1500 nm. Both absorptions decay with time in the tens of picosecond time range. In our previous study using nanosecond transient absorption spectroscopy,20 we assigned the transient absorption of the TiO2 nanocrystalline film to three kinds of charge carriers, as shown in Figure 4a. Trapped holes have an absorption band with a peak at 500 nm, and trapped electrons have a band at 800 nm.
Femtosecond Visible-to-IR Spectroscopy of TiO2
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Figure 3. Transient absorption spectra of TiO2 nanocrystalline film excited at a 266 nm wavelength.
Figure 4. (a) Absorption bands for the trapped holes, the trapped electrons, and the bulk electrons in TiO2 nanocrystalline film. (b) Comparison between the transient absorption at 1 ps under femtosecond excitation at 266 nm and that at 1 µs under the nanosecond transient absorption experiment (ref 20).
Figure 5. Rise of the transient absorption at 700 nm of a TiO2 nanocrystalline film excited at 266 (bottom) and 355 (top) nm. Simulation of rise responses of the rise times between 150 and 250 fs is also presented.
Also, bulk electrons show increasing absorption in the IR region toward the longer wavelength. These are ascribed based on experiments using a hole scavenger molecule, methanol, and an electron scavenger, oxygen molecule, and on a theoretical analysis. Electrons in the conduction band and shallow trap levels show absorbance which can be fitted to a function, Aλn,
where A is a constant and λ is a wavelength. Details are described in ref 20. The transient absorption of the TiO2 film excited by the 266 nm femtosecond laser can be assigned using the three abovementioned species, trapped holes, trapped electrons, and bulk electrons, as we have already assigned the transient absorption under 355 nm excitation.30 For strict comparison, the transient absorption spectrum at 1 ps of the femtosecond 266 nm excited TiO2 film is overlaid on the 1 µs transient absorption spectrum measured by nanosecond spectroscopy (Figure 4b). The peak of the femtosecond 266 nm excited TiO2 film seems to be located at the longer wavelength than that of the nanosecond transient absorption. This can be explained by the pronounced 800 nm trapped electron absorption band in the early time region, which was shown in our previous study using weak 355 nm excitation laser.30 In the experiment, we found that both absorptions of the trapped electrons and the bulk electrons decay with a common decay time of 500 ps. Therefore, we considered that the trapped electrons at the surface and the bulk electrons were in equilibrium because of their migration between the surface and the bulk and that they relaxed simultaneously to deep trapping sites in the bulk. During this relaxation, 800 nm absorption of the trapped electrons decays due to population decrease, while the IR absorption of the bulk electrons did so because of the absorption coefficient decrease. The latter phenomenon has been observed for injected electrons in the TiO2 conduction band from surface-adsorbed dye molecules upon visible light excitation.34 Similar dynamics of the trapped electrons seem to be taking place under the 266 nm excitation, judging from the similarity in the transient absorption spectra between the 266 and 355 nm excitation conditions. 3.2. Intraband Charge Carrier Dynamics. In the visible region, the observed species under 266 nm excitation can be dominantly ascribed to trapped charge carriers, although separation between trapped electrons and trapped holes is not easy due to their spectral overlap. In our previous study using a weak 355 nm excitation laser, it was shown that the charge trapping times both for electrons and holes were within ∼200 fs (time resolution of the apparatus).30 Figure 5 indicates a comparison of transient absorption rise profiles at 700 nm between 266 and 355 nm excitation conditions. For 355 nm excitation, the rise profile agreed with the instrumental response, as expected from our previous study. Actually, simulation of several rise times revealed that the trapping time could be evaluated to be less than 100 fs. On the other hand, the rise profile under 266 nm excitation showed a clearly slower rise than the response function using the 266 nm beam. The rise profile under 266 nm excitation was between the simulation curves using a 150 and a 250 fs rise time. Judging from the transient absorption spectral shape at 1 ps, which is similar to the trapped electron
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A ) aC/(kCt + 1)
Figure 6. Transient absorption decay kinetics at 2500 nm of a TiO2 nanocrystalline film after excitation at 266 nm with the pulse energy of 4, 2, and 1 µJ. Also, analysis results based on the second-order recombination are displayed. The inset graph shows the early time range up to 15 ps for clear view of the fast decay.
absorption rather than the trapped hole one, the retardation of the trapping process reflects intraband relaxation of free electrons in the conduction band. According to Enright and co-workers,35 the diffusion coefficient of valence band holes is 4 × 10-5 m2 s-1, which predicts that the transit time of free holes from the center to the surface of a nanoparticle with a radius of 10 nm should be 250 fs. This estimate does not conflict with our experimental results. Also, Yang and Tamai reported a trapping time of
