1. Basic Principles of Time-resolved Spectroscopy and Their Application in Photo(electro)catalytic Mechanisms Xiuli Wang State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences August 20, 2016

2. Outline  Introduction of time-resolved spectroscopy  Principles of time-resolved photoluminescence (PL) spectroscopy  Principles of transient absorption spectroscopy (TAS)  Application of time-resolved spectroscopy in photo(electro)catalytic mechanisms

3. What is “time-resolved”?-Dynamic process Chemical kinetics and reaction dynamics: Chemical kinetics deals with the rates of chemical reactions, factors which influence the rates and the explanation of the rates in terms of the reaction mechanisms of chemical processes. In chemical kinetics, the time variable is introduced and rate of change of concentration of reactants or products with respect to time is followed. Santosh K. Upadhyay, Chemical kinetics and reaction dynamics, Springer, 2006

4. What is time-resolved spectroscopy? In physics and physical chemistry, time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic techniques. ——From Wikipedia In principle, time-resolved spectroscopy can be applied to any process that leads to a change in properties of a material. (Time scale: second-hour) Taixing Wu et al. J. Phys. Chem. B 1998, 102, 5845-5851 Photo degradation of Rhodamine B in aqueous TiO 2 dispersions UV-Vis absorption spectroscopy is used to monitor the reaction process.

5. Most often, time-resolved spectroscopy are used to study the processes after the illumination of a material occurs. With the help of pulsed lasers, it is possible to study processes that occur on time scales as short as 10 −16 s. What is time-resolved spectroscopy? Transient Concentration Gate pulse ‘on’ Time Intensity Pump Probe tt Effect of pulsed laser irradiation: Which state is excited? Does it form reaction products? Does it form intermediates? Is it stable? What is its relaxation dynamics? … Different time-resolved spectroscopy with different time resolution

6. Different types of time-resolved spectroscopy Pump-Probe Techniques  Time-resolved photoluminescence spectroscopy (TRPL)  Transient absorption spectroscopy (TAS)  Time-resolved infrared spectroscopy (TRIR)  Time-resolved microwave conductivity (TRMC)  Time-resolved terahertz spectroscopy  Other multiple-pulse techniques: e.g. Four-wave mixing

7. Valence Band Conduction Band Energy e-e- h+h+ hνhν Wannier exciton Exciton emission band gap Band edge emission hole traps electron traps Trap-related emission PL processes in semiconductor materials PL spectroscopy can give rich information on impurities and defects in semiconductor materials.

8. Steady-state PL spectra: Excited by continuous-wave light source (lamp or laser) Steady-state and time-resolved PL spectra Time-resolved PL spectra: Excited by pulsed light source (flash lamp or pulsed laser) Relationship between steady-state and time-resolved PL: Transient Concentration Time Intensity Pump  t2  t1 Gate pulse ‘on’  t3 t=0  t1  t2  t3 Steady-state PL 600 nm 500 nm 700 nm 400 nm

9. Time-resolved PL spectra—Exponential decay Lifetime ： log(I)—t or S0S0 S1S1 S1S1 Relaxation (10 -12 s) k nr Γ hν A hν F A simplified Jablonski diagram Quantum Yield:

10. Principle of transient absorption spectroscopy Transient Concentration Gate pulse ‘on’ Time Intensity Pump Probe tt Pump laser t=0 Probe laser t= △ t Sample Pump-probe technique: non-equilibrium state (excited state) Pump: pulsed laser Probe: UV-Vis light, IR light, etc.

11. W lamp (probe) monochromator Oscilloscope & DAQ card computer detector Laser (pump) Principle of transient absorption spectroscopy sample Relationship between UV-Vis absorption spectroscopy and transient absorption spectroscopy (TAS):

12. Transient absorption spectra and decays t<0: normal UV-Vis absorption spectra t=  t: UV-Vis absorption spectra at  t after pump excitation t<0 t=  t Optical Density (OD) TAS:  OD=OD 2 -OD 1 =log(I 1 /I 2 (t)) TA spectrum TA decays

13. What can we get from PL and TAS?  PL spectra: PL band: band position, band shape PL decay kinetics:  Transient absorption spectra Bleach: ground state Absorption: excited state or intermediate  Transient absorption decays Bleach recovery kinetics Absorption decay kinetics Photophysical dynamics Photochemical dynamics: photocatalysis

14. Relationship between time-resolved spectroscopy and photo(electro)catalytic process Solar energy conversion: Photocatalytic processes: suspension system: No bias H2H2 O2O2 N. S. Lewis, Nature, 2001, 414, 589 Solar light is needed as light source in photo(electro)catalytic process, while time-resolved spectroscopy is a powerful technique to study the effect of light excitation.

15. Time scales of various processes 1. Bulk carrier dynamics: trapping, recombination, etc. 2. Interfacial charge transfer: 3. Surface reaction processes: Time scales: fs, ps, ns, μs, ms Time scales: fs, ps, ns Time scales: μs, ms, s Time-resolved spectroscopy + D D+D+ A A-A- Photooxidation Photoreduction Cocatalyst + + + + + + Surface and Volume Recombination

16. Assignments of electrons and holes in TiO 2 Toshitada Yoshihara et al. J. Phys. Chem. B 2004, 108, 3817-3823 Trapped holes and electrons had absorption peaks at 520 and 770 nm, respectively. The absorbance of free electrons increased with increasing wavelength, and it was strong in the IR range. Effect of methanol Effect of O 2 100 ns 1  s 30 min in N 2 free electron trapped electron Bulk carrier dynamics

17. Trapping dynamics of holes in TiO 2 film Yoshiaki Tamaki et al. C. R. Chimie 9 (2006) 268–274 The formation rate of deeply trapped holes was estimated to be 200 ± 50 fs. The relaxation of trapped holes from shallow sites to deep ones was occurring more than 100 ps after photoexcitation. Bulk carrier dynamics

18. Wavelength/nm 540 850 anatase rutile 600 800 1000 1200 Recombination processes studied with TRPL Time/μs 0 100 200 300 400 IRF ex =340 nm em =520 nm 100 200 400 10 1 10 3 10 2 Log time α=2.86 77 K anatase Time/μs ex =390 nm em =820 nm 0 100 200 300 τ≈40 μs 77 K rutile Jianying Shi, Can Li et al. J. Phys. Chem. C., 2007, 111, 693 Xiuli Wang, Can Li et al. Phys. Chem. Chem. Phys., 2010, 12, 7083 Bulk carrier dynamics Anatase TiO 2 shows a visible emission at about 540 nm, while rutile TiO 2 displays a near-infrared emission at about 850 nm. The different PL decay kinetics of anatase and rutile indicate different recombination processes.

19. anatase TiO 2 rutile TiO 2 Xiuli Wang, Can Li et al. Phys. Chem. Chem. Phys., 2010, 12, 7083  The trapped carriers may participate in photocatalytic reactions and the slow decay processes may be benificial for the photocatalytic reactions.  The trapped electrons in rutile TiO 2 may be hindered to participate in photocatalytic raction processes. Roles of trap states in photocatalytic processes Bulk carrier dynamics

20. Interfacial charge transfer Electron transfer dynamics from TiO 2 to CoP Anna Reynal et al. Energy Environ. Sci., 2013, 6, 3291-3300 The first reduction of the catalyst from CoIII to CoII can proceed efficiently. In contrast, the second reduction from CoII to CoI, appears to be at least 10 5 slower.

21. Interfacial charge transfer Ultrafast charge separation and long-lived charge separated state in CdS−Pt heterostructures Kaifeng Wu et al. J. Am. Chem. Soc. 2012, 134, 10337−10340 26.9 ± 2.1 nm Pt CdS (2.3 ± 0.2 nm) CdS nanorod CdS-Pt nanorod heterostructures XA1: hot excitons XB: state filling of electron level PA: trapped holes Electron transfer: ∼ 3.4 ps Charge recombination: 1.2 ± 0.6 μs

22. Yoshiaki Tamaki et al. J. Am. Chem. Soc., 2006, 128, 416-417. Reaction of photoinduced hole with scavengers Rates and yields of oxidation reactions of absorbed alcohols were successfully evaluated by measuring absorption of reactive trapped holes in nanocrystalline TiO 2 films. Interfacial charge transfer and surface reaction processes

23. Mechanism of water oxidation by PEC Interfacial charge transfer and surface reaction processes Correlating long-lived photogenerated hole populations with photocurrent densities in hematite water oxidation photoanodes S.R. Pendlebury et al. Energy Environ. Sci., 2012, 5, 6304–6312 S.R. Pendlebury et al. J. Am. Chem. Soc. 2014, 136, 9854−9857

24. Multihole catalysis of water oxidation by hematite Florian Le Formal et al. J. Am. Chem. Soc. 2015, 137, 6629−6637 These results reveal a transition from a slow, first order reaction at low accumulated hole density to a faster, third order mechanism once the surface hole density is sufficient to enable the oxidation of nearest neighbor metal atoms. Interfacial charge transfer and surface reaction processes

25. Conclusion and Prospect Basic principles of time-resolved spectroscopy: steady-state and time-resolved PL spectroscopy transient absorption spectra Time-resolved spectroscopy has proven to be a indispensable technique to study photo(electro)catalytic mechanisms: bulk carrier dynamics interfacial charge transfer surface reaction mechanism Thank you for your attention!