Ultrafast XAFS Studies on the Photoabsorption Processe

Ultrafast XAFS Studies on the Photoabsorption Processe Kiyotaka Asakura 1, Yohei Uemura 2 and Toshihiko Yokoyama 3, 1 Institute for Catalyst, Hokkaido University, 2Utrecht University 3Institute for Molecular Science.

Acknowledgements Institute for Molecular Science Dr. A. Koide SACLA Dr. M. Yabashi Dr. T. Katayama DR. T. Togashi Photon Factory Prof. S. Adachi Ass. Prof S. Nozawa Ass. Prof. K. Ichiyanagi Dr. R. Fukaya Mr. Y. Niwa ICAT, Hokkaido University Ass. Prof. S. Takakusagi Dr. Y. Wakisaka Mr. D. Kido Prof. B. Otani Tokyo University of Science Prof. A. Kudo Dr. A. Iwase Toyama University Ass. Prof. K. Hatada Funding. the polymer membrane fuel cell project from NEDO and YU supported by JSPS.

Time resolved EXAFS 3 QEXAFS Rapid rotation of Monochromator 0.1 s 60 s Not only transmission but also fluorescence mode measurement Is possible. DEXAFS Energy dispersion is made by polychromator q 1 µ-1 s Only transmission mode is available.

A Pump-Probe method for much higher resolution Slow Measurement of fs repeatable phenomena 4 X-ray Pump : Pulse Laser Probe : Pulse X-ray SR (ps-ns) or XFEL(fs-ps)

5 XFEL(X-ray Free Electron Laser) (http://www.xfel.net/en/ ) LCLS http://www-ssrl.slac.stanford.edu/lcls/ SACLAhttp://xfel.riken.jp/i ndex.html PAL-XFEL http://pal.postech.ac.kr/pal eng/ China, Swaziland, Germany,Italy.. Many other XFELs are ready or will be ready. European XFEL (Germany) https://lcls.slac.stanford.edu/overview SASE XFEL SASE=Self-Amplified Spontaneous Emission

Difficulty of Pump-probe in solid and surface material 6 Optical length of excited light and probe X- ray should be the same. Excite the sample as much as possible. Damage of the sample. Too strong pump laser damages the sample

Difficulty of Pump-probe in solid and surface material 7 Optical length of excited light and probe X- ray should be the same. Excite the sample as much as possible. Strong power of pump laser Dilute the sample Damage of the sample. Too strong pump laser damages the sample Use the colloidal particle and flow the suspension.

Fundamentals of photocatalysts and water splitting activity Decomposition of H 2 O 2H 2 + O 2 Evolution of O 2 Time / min Band structure E conduction (W 5d) electro n H-/H 2 Carrier dynamics > 2.4 ev hole valence (O 2p) H 2 O/O 2 P. V. Kamat, E. Selli et al ACS Energy Lett. 2017 2 1362-1367

Unsolved problems: reaction mechanism Band structure Questions What is the local structure and electronic state change of metals after the photo absorption? conduction (W 5d) electro n > 2.4 ev E H-/H 2 What is the rate of change? Time-resolvedXAFS is useful hole valence (O 2p) H 2 O/O 2 electronic state local structure

WO 3 is one of Z-scheme photocatalysts 10 TaON Redox potential of O 2 /H 2 O WO 3 Two-step photoabsorption like photosynthesis to use visible light R. Abe, et. al. J. Am. Chem. Soc. 2008, 130, 7780-7781

SACLA : X-ray Free Electron Laser - Faster process Experiments at SACLA SPring-8 ~30 fs SACLA ~50 fs Sample: 4 mm WO 3 suspension pump laser: 520 mj/cm 2 @15 Hz X-ray pulse width: 30 fs(fwhm) Time resolution: 700 fs

Photoexcitation of WO 3 : XFEL Three distinct peaks were found in the differential spectra. peaks A,B, C : decrease of absorption from e g orbitals ~ 200 ps peak B which was not found in the previous experiments was observed.

13 First derivative and reference compound. Photoabsorption is accompanied by the edge shift because of O2p to W 5d electron transfer occurs. e - W 6+ -O 2- W 5+ -O -

Comparison of derivative and difference spectra Red Shift and d filling by photoabsorption 14 W 5+ is created during the photoabsorption derivative Due to edge shift Difference spectra 700 fs d-state is filled by electrons e - W 6+ -O 2- W 5+ -O - Due to edge shift

15 Structure Change = more distorted structure. Fig. 2 Changes of absorption intensities of W LIII XANES at peak A, peak B and peak C A*exp(-k1t) + B*exp(-k2t). k1 =0.007(1) ps -1 K2=0.00056(5) ps -1 FT after 150 ps delay Transient L1 XANES Structure Change

16 Scheme after the photoabsorption. Shorter W-O distance

Pump-Probe XAFS for photocatalysts 17 WO 3 has been studied. 1. XAFS is sensitive for local structure and electronic change if we use L 3 edge absorption because dipole transition. 2. In the WO 3 case, Electron transition, followed by structure change 3. Pump-probe method requires a long measurement time. 4. We can follow the repeatable phenomena.

Photoexcited XAFS of BiVO 4 in the time scale of ps A. Kudo and Y. Miseki, Chemical Society Reviews, 2009, 38, 253-278 To study electronic states and local structure of BiVO 4, pumpprobe XAFS method employed. SEM Band structure excitation BI6s, O2p electron to V3d orbital. BI is oxidized? BiVO 4 structure O 2 H 2 O

Photoexcited XAFS of BiVO 4 in the time scale of ps Y. Uemura, D. Kido, A. Koide, Y. Wakisaka, Y. Niwa, S. Nozawa, K. Ichiyanagi, R. Fukaya, S. Adachi, T. Katayama, T. Togashi, S. Owada, M. Yabashi, K. Hatada, A. Iwase, A. Kudo, S. Takakusagi, T. Yokoyamaa, K. Asakura, Chemical Communications 2017, 53, 7314-7317 10.1039/c7cc02201h. Bi 6s V3d Bi L3 edge shift should be positively shifted. Red is difference Blue is negative shift Green is positive shift

Origin of changes in XANES of BiVO 4 XANES can be modified by changing local structures around Bi. Simulated Bi L3 XANES by FEFF The shorter Bi-O bonding causes the change in Bi L3 XANES. J. Nan, C. H. S. John, J. Phys.: Cond. Matt. 2006, 18, 8029.

Photoexcitation process of BiVO 4 τ ~ 14 ps τ < 500 fs τ ~ 40 ns ground state Uemura et al. Chem. Commun. 2017, 53, 7314-7317

Pump-Probe XAFS for photocatalysts 28 WO 3 has been studied. 1. XAFS is sensitive for local structure and electronic change if we use L 3 edge absorption because dipole transition. 2. We can see the electronic state change and structure change 3. In the WO 3 case, Electron transition, followed by structure change 5. Pomp-probe method requires a long measurement time. 6. We can follow the repeatable phenomena. 7. No one-time transient phenomena were possible.

How about much faster processes?? Real timing jitters of XFEL Arrival Timing Monitor Katayama et al. Struct. Dyn. 3, 034301 (2016)

Arrival timing monitor can give precise pictures around time 0 Peak A Peak C d-orbitals are less filled in less than 500 fs the first transition state might not be the d state. ( W 6s or 6p then relaxed to 5d state. ) Or coherent vibration and rapid dephasing

31 Scheme after the photoabsorption. 6s or 6p other than 5d or dephasing >1ps -1 Shorter W-O distance

Photoexcitation process of BiVO 4 τ ~ 14 ps τ < 500 fs τ ~ 40 ns ground state Uemura et al. Chem. Commun. 2017, 53, 7314-7317