J. Phys. Chem. B 2004, 108, 3817-3823
3817
Identification of Reactive Species in Photoexcited Nanocrystalline TiO2 Films by Wide-Wavelength-Range (400-2500 nm) Transient Absorption Spectroscopy Toshitada Yoshihara,†,‡ Ryuzi Katoh,*,†,‡ Akihiro Furube,†,‡ Yoshiaki Tamaki,†,‡,§ Miki Murai,†,‡ Kohjiro Hara,†,‡ Shigeo Murata,†,‡ Hironori Arakawa,†,‡ and M. Tachiya‡ Photoreaction Control Research Center (PCRC) and National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ReceiVed: December 11, 2003
Reactive species, holes, and electrons in photoexcited nanocrystalline TiO2 films were studied by transient absorption spectroscopy in the wavelength range from 400 to 2500 nm. The electron spectrum was obtained through a hole-scavenging reaction under steady-state light irradiation. The spectrum can be analyzed by a superposition of the free-electron and trapped-electron spectra. By subtracting the electron spectrum from the transient absorption spectrum, the spectrum of trapped holes was obtained. As a result, three reactive speciess trapped holes and free and trapped electronsswere identified in the transient absorption spectrum. The reactivity of these species was evaluated through transient absorption spectroscopy in the presence of hole- and electronscavenger molecules. The spectra indicate that trapped holes and electrons are localized at the surface of the particles and free electrons are distributed in the bulk.
Introduction Photocatalytic reactions using TiO2 have been attracting much interest because they can be applied to water splitting,1 the degradation of environmental pollutants,2,3 air purification,2,3 solar energy conversion,4,5 and so on. A lot of fundamental research has been carried out, and results from some of these studies have been successfully applied in industry. The primary reaction process is photoinduced electron transfer at the TiO2 surface, which suggests that a large surface area is required for a highly efficient reaction. Thus, ultrafine particles have been widely used for practical applications.2,3,6,7 Surface properties are important for achieving high photocatalytic activities, and there have been many attempts to modify the surface.6-12 Ultrafine TiO2 particles are useful and relatively easy to prepare. Because photocatalytic reactions are usually examined in solution, the ultrafine particles in slurries must be separated from the products after the reactions. Separation can be a crucial problem, especially in industrial applications. Recently, nanocrystalline semiconductor films were developed. They are prepared by the calcination of primary ultrafine particles so that the films have large surface areas, and they maintain the high reactivity of the particles. Such a film shape is useful for handling in applications. These nanocrystalline films can be used as transparent electrodes, and their use as catalysts has also been studied.13-15 In addition, the films are used as electrodes for dye-sensitized solar cells in which the films are covered with sensitizer dyes.4,5 The dynamics of photocatalytic reactions on TiO2 particles have been studied extensively with the goal of determining the reaction mechanisms and thus improving photocatalytic reactivity.16-60 Experimental techniques using pulsed lasers are useful for probing the behavior of photogenerated electrons and * Corresponding author. E-mail:
[email protected]. † PCRC. ‡ AIST. § NEDO Fellow.
holes. Transient absorption spectroscopy in the femtosecond to millisecond time range has been used to study the generation, relaxation, and recombination processes of electrons and holes.27-56 The presence of additional reactants in the solution makes it possible to probe the interfacial electron-transfer processes through this technique.27-35,39-44,47,49 For absorption spectroscopy, transparent samples are generally required. Therefore, colloidal nanoparticles have been investigated in many cases. However, the conditions surrounding the particles cannot be freely changed to prevent the aggregation and precipitation of the nanoparticles. Film samples are useful for preparing various experimental conditions. In general, to probe chemical reactions by absorption spectroscopy, it is necessary to identify the absorption bands of the reactive species. However, for TiO2 particles, a precise assignment of the absorptions of electrons and holes is rather difficult because their absorption spectra are very broad16-22,24-35,38,39,41-43,46-50 and the bands overlap each other in the visible wavelength region. Moreover, the spectra are very sensitive to the characteristics of the TiO2 particles, such as the crystal phase, particle size,20,22,38,46,48 and surface conditions, even with otherwise identical samples. This problem complicates comparisons between the obtained spectra. In addition, electrons and holes seem to be distributed in various kinds of trapping sites in which they exhibit different absorption spectra and reactivity. Therefore, transient absorption spectra frequently show complicated temporal changes. This complexity has limited our understanding of the reaction mechanism despite many previous efforts to make transient absorption measurements. Recently, transient absorption studies in the IR wavelength range were carried out to solve the above-mentioned problems.51-60 In the IR wavelength range, the strong absorption due to conducting electrons is distinct, and its shape can be analyzed by means of a simple theoretical model based on solidstate physics. Therefore, spectral assignments are more reliable than in the visible region, and reaction kinetics can be precisely
10.1021/jp031305d CCC: $27.50 © 2004 American Chemical Society Published on Web 02/28/2004
3818 J. Phys. Chem. B, Vol. 108, No. 12, 2004 probed by means of temporal changes in the conducting-electron signal. Using microsecond transient IR spectroscopy, Yamakata et al. investigated the kinetics of conducting electrons in the reaction of TiO2 particles with oxygen and water molecules.53 Hoffmann et al. also reported a similar absorption of conducting electrons.57-59 The vibrational bands of reactants also provide useful information about reaction processes.51,54,56-60 However, one disadvantage of this technique is the difficulty in carrying out experiments on TiO2 particles in media that strongly absorb in the IR region, such as water and organic solvents. In practice, most photocatalysis reactions occur under such conditions. Because of this limitation and because IR spectroscopy cannot be used to monitor hole dynamics, species such as holes and surface-trapped electrons must be assigned by means of other methods. The development of such methods would be very useful for future investigations of various kinds of photocatalytic reactions of TiO2 particles. We thought that it might be possible to make assignments by simultaneously observing the transient absorption band appearing in the visible range and the band for the conducting electrons in the IR range. We measured the transient absorption spectra of TiO2 nanocrystalline films over a wide spectral range from 400 to 2500 nm with a high degree of accuracy (∼10-4 optical density (OD)). The absorption bands of trapped holes and free and trapped electrons were clearly identified in the spectra and were compared with literature spectra. We also studied some typical reactions of these species with scavenger molecules such as methanol and oxygen. These results confirmed that trapped holes and electrons are localized at the surface of particles and free electrons are in the bulk. Experimental Section TiO2 nanoparticles were prepared by the method reported by Gra¨tzel and co-workers.61 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.62 When the sample was calcined, the opaque film became transparent enough for spectroscopic measurements. The mean diameter increased slightly to about 20 nm, as measured by scanning electron microscopy. The crystal phase was anatase. The TiO2 films were 1 cm2 (1 cm × 1 cm) in area and 2-5 µm thick. The nanocrystalline TiO2 films were placed in several solutions for steady-state and time-resolved spectroscopic experiments. Heavy water (D2O, Cambridge Isotope Laboratories, Inc., 99.8%) and methanol-d4 (CD3OD, Cambridge Isotope Laboratories, Inc., 99.8%) were used without further purification. For transient absorption measurements, the third harmonic (355 nm) of a Nd3+:YAG laser (HOYA Continuum, Surelite II) was employed for excitation. A Xe lamp (Hamamatsu, 150 W) and a halogen lamp (100 W) were used as probe light sources for the 400-800 and 800-2500 nm wavelength ranges, respectively. The probe light was introduced into a monochromator (ACTON, Spectra Pro-150) after passing through the sample specimen. The intensity of the probe light was detected by a Si photodiode (Hamamatsu, S-1722-02) in the 400-1000 nm range and a mercury cadmium telluride (MCT) photodetector (Dorotek, PDI-2TE-4) in the 900-2500 nm range. The photocurrent from the detectors was processed with an AC-coupled preamplifier (NF Electronic Instruments, SA-230F5) to extract only the transient signals, and the signals were magnified by an amplifier (NF Electronic Instruments, 5305). The amplified transient signals were recorded with a digital oscilloscope
Yoshihara et al.
Figure 1. Transient absorption spectrum of a nanocrystalline TiO2 film in N2-saturated D2O (b). The dashed line represents the theoretical curve λn, where n ) 1.7.
(Tektronix, TDS380C) and transferred to a personal computer for analysis. The DC offset of the photocurrent from the detector was subtracted using the preamplifier, and thus we could measure small absorbance changes (
