Behavior and Energy States of Photogenerated Charge Carriers

Transcription

1 S1 Behavior and Energy States of Photogenerated Charge Carriers on Pt- or CoOx-loaded LaTiO2N Photocatalysts: Time-resolved Visible to mid-ir Absorption Study Akira Yamakata, 1,2* Masayuki Kawaguchi, 1 Naoyuki Nishimura, 3 Tsutomu Minegishi, 3 Jun Kubota, 3 Kazunari Domen 3* 1. Graduate School of Engineering, Toyota Technological Institute, Hisakata, Tempaku, Nagoya , Japan. 2. Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Honcho Kawaguchi, Saitama , Japan. 3. Department of Chemical System Engineering, the University of Tokyo, Hongo, Bunkyo-ku, Tokyo , Japan.

2 S2 1. Time-resolved spectrometers The behaviors of electrons and holes were investigated by home-build femtosecond to second time-resolved spectrometers as shown in Figures S2 and 3. In the femtosecond to nanosecond region, experiments were performed with a conventional pump-probe method described in Figure 2. In brief, a Ti:sapphire regenerative amplifier (Spectra Physics, Solstice: 9 fs, 8 nm, repetition rate 1 khz, 3.5 W) was used as the primary light source. The output laser pulse was divided into three beams, and the two visible pulses and one mid-ir pulse were obtained with optical parametric amplifiers (Spectra Physics, TOPAS prime) and a non-collinear difference frequency generation unit (Spectra Physics, NDFG). For the TR-Vis measurements, the visible-probe pulses diffuse-reflected from the sample were dispersed by a spectrometer (JASCO, CT25) and detected by a photomultiplier (Hamamatsu). For the TR-IR measurements, the mid-ir probe pulses transmitted through the sample were dispersed by a TOPAS nm NDFG 4-8 cm -1 MCT array polychromator (Horiba, ihr32) and detected with a 128-channel linear MCT array detector (Infrared Systems Development, FPAS). To obtain the absorbance change with a better Spectra Physics Solstice 9 fs, 1 khz, 8 nm, TOPAS nm optical delay TOPAS nm optical delay Figure S1. Femtosecond time-resolved absorption spectrometer. ND shutter chopper ND sample fiber polychromator spectrometer pol PMT signal-to-noise ratio, pump pulses were IR: 8 ~ 1 cm-1 (1.3 ~ 1µm) sample IR source chopped with a 5 Hz optical grating Spectrometer f = 3 mm chopper, and the signal acquisition was Amplifier MCT Continuum Surelite II 6 ns, 1 Hz Surelite OPO 355~25 nm carried out on a shot-by-shot basis at a rate of 1 khz. The decay curves were Oscilloscope PC Si-PD or InGaAs grating Spectrometer f = 3 mm sample obtained at.1 or 5 ps intervals and averaged over 2 scans for one Visible ~ NIR: 25 ~ 6 cm-1 (4 ~ 17 nm) Halogen lamp Figure S2. Home-built microsecond time-resolved visible, NIR and IR absorption spectrometers.

3 point. The time-resolution of this spectrometer was ~ 9 fs, limited by the temporal width of the laser pulse. S3 Microsecond time-resolved absorption measurements in visible to mid-ir region were performed by using home-built spectrometers as shown in Figure 3. 1, 2 Briefly, in the mid-ir region, the probe light emitted from MoSi 2 coil was focused on the sample and the transmitted light was introduced into a spectrometer (Acton, SP-23i). The monochromatized light from the spectrometer was detected by a photovoltaic MCT (Kolmar), and temporal profiles of the signal intensity were recorded by a digital oscilloscope (Iwatsu, DS-4262). In the case of visible to NIR region, the experiments were performed in the diffuse-reflection mode. A halogen lamp (5W) and a Si-photodiode (Hamamatsu) or an InGaAs detector (Hamamatsu) were used as a light source and detectors, respectively. The powder photocatalyst was fixed on a CaF 2 plate with a density of ca. 1 mg cm -2, and thus the obtained sample plate was placed in a stainless-steel IR cell. The spectra were obtained at 2 cm -1 intervals and averaged over 3 scans for a spectrum by irradiating the sample using UV laser pulse from Nd:YAG laser (Continuum, Surelite-II, 6 ns, 355 nm, repetition rate 5~.1 Hz). The time-resolution of this spectrometer in NIR and IR regions was 1 ~ 2 µs, limited by the time-constant of an ac-coupled amplifier (Stanford Research Systems, SR56), and that in Vis region was ~3 µs, limited by the stray light of the pump pulse and/or short-lived strong emission from the sample. 2. Diffuse reflectance absorption spectra of LaTiO 2 N at the ground state Diffuse reflectance absorption spectra of the LaTiO 2 N samples without the pump laser pulse irradiations are shown in Figure S3. The UV-Vis-NIR spectra were collected using a JASCO V-67 spectrometer equipped with an integrating sphere in the diffuse reflectance mode. In this measurement, the powder photocatalysts fixed on the CaF 2 plate (density of the sample ~ 1 mg cm -2, the same sample plates for the transient measurements) were used.

4 (a) S4 1.2 Absorbance.8.4. (b) Wavenumber / nm 1.2 Absorbance Wavenumber / nm Figure S3 Diffuse reflectance absorption spectra of Pt or CoO x loaded LaTiO 2 N (a), and Pt and CoO x co-loaded LaTiO 2 N (b). 3. Femtosecond time-resolved IR absorption spectra Transient IR spectra of LaTiO 2 N photocatalysts were measured by using femtosecond 2 (A) bare LaTiO 2 N 2 (B) Pt.5wt% 2 (C) CoO x 2wt% Absorbance / ps 1 ps 1 ps 1 ps Absorbance / ps 1 ps 1 ps 1 ps Absorbance / ps 1 ps 1 ps 1 ps Wavenumber / cm Wavenumber / cm Wavenumber / cm-1 Figure S4 Femtosecond time-resolved IR spectra of (A) bare LaTiO 2 N, (B).5 wt% Pt loaded LaTiO 2 N, and (C) 2 wt% CoO x loaded LaTiO 2 N irradiated by 5 nm pump pulse (9 fs duration, 6 µj, 5 Hz ) in air.

5 time-resolved IR absorption spectrometer as shown in Figure S4. 4. Laser power dependence on the electron decay kinetics Laser power dependence on the electron Normalized Absorbance 1. CoOx 2 wt% 2µJ 4µJ 6µJ 1µJ 2µJ S5 decay kinetics in ps region is shown in Figure S5. The number of photogenerated electrons increases with increasing the laser power, therefore the intensity at ps was normalized. The decay of electrons becomes faster as the intensity of pump Time delay / ps 4 5 Figure S5 Normalized decay curves of transient absorption at 2 cm -1 on 2 wt% CoO x loaded LaTiO 2 N measured in air. The 5-nm pump laser power was changed from 2 to 2 µj pulse -1. pulse increases. This result suggests that the second order recombination process is involved in the picosecond region. The pump power and pump wavelength dependences on the electron decay kinetics in microsecond region are examined as shown in Figure S6. It is clearly shown that Vis pump pulse up to 65 nm excite electrons. The number of photogenerated electrons depends on the wavelength of the pump pulse, but the decay curves are almost parallel: the decay kinetics of the electrons does not Figure S6 Decay curves of transient absorption at 2 cm -1 on bare LaTiO 2 N. 355 nm (.2 mj), 5 nm (1.4 mj), 6 nm (2. mj), 65 nm (.9 mj),.1 depend on the number of photogenerated charge carriers. This result exhibits that the recombination follows the 1 st order kinetics. Absorbance.1 CoOx 2 wt% Bare Pt 2 wt% 5. Change of the decay kinetics of trapped electrons Effects of Pt or CoO x loading on the decay kinetics of trapped electrons were examined by observing the intensity change at 6 cm -1 (Figure S7). The S/N ratio is 1 2 Time delay / ps 3 Figure S7 Decay curves of transient absorption at 6 cm -1 on 2 wt% Pt and CoO x loaded LaTiO 2 N. The sample was excited by 5-nm laser pulses (9-fs duration, 6 µj, 5 Hz).

6 S6 not good because the transmittance of the IR light at 6 cm -1 is low and the sensitivity of MCT is also not sufficient. The decay curves of bare and Pt 2wt% loaded samples are almost identical. CoO x loading decelerates the decay of the trapped electrons. 6. Photocatalytic Reactions for Water oxidation and Water Redaction Reactions were carried out in a Pyrex top-irradiation-type reaction vessel connected to a glass closed gas circulation system. For the water oxidation, 2 ml aqueous solution containing.2 g of photocatalyst and.5 M AgNO 3 was used. The ph environment of the reaction solution was adjusted to ca. 8.5 by a buffer solution containing.2 g La 2 O 3.Pn the other hand, for the reduction of water, 1 vol% MeOH aqueous solution was used. Prior to reaction, the solution was evacuated several times to completely remove any air, and then irradiated from the top side with a 3 W Xenon lamp (λ > 42 nm) using a filtration mirror. A flow of cooling water was used to maintain the reactant solution at room temperatures. The evolved gases were analyzed by gas chromatography. References (1) Yamakata, A.; Ishibashi, T.; Onishi, H., Time-Resolved Infrared Absorption Spectroscopy of Photogenerated Electrons in Platinized TiO 2 Particles. Chem. Phys. Lett. 21, 333, (2) Yamakata, A.; Soeta, E.; Ishiyama, T.; Osawa, M.; Morita, A., Real-Time Observation of the Destruction of Hydration Shells under Electrochemical Force. J. Am. Chem. Soc. 213, 135,