2003.01.24 Dong Won Hwang , Jae Sung Lee Catalysis & Ecofriendly Science Lab.

1. 제2회 광촉매 연구모임 The Electronic Structure of Layered Perovskites and Its Photocatalytic Water Splitting Activity 2003.01.24 Dong Won Hwang, Jae Sung Lee Catalysis & Ecofriendly Science Lab. Chemical Engineering, POSTECH

2. Alternative Energy -Types and Amounts (108 kcal/sec) Solar (420,000) Wind (880) Hydro (4.8) Biomass (250) Geothermal (77) Ocean (Thermal, Tidal) (7.2) etc. (50) -Driving Forces Sustainability Renewability Pollution Reduction Total Energy Consumed at 1989; 1.0  1017 kcal  The amount of solar radiationfor 60 min

3. Cat 1 2H+ CB - C.B. : more negative than the reduction potential H+/H2 H2 hv EG>> 2.43 eV Cat 2 OH-/O2 VB V.B. : more positive than the oxidation potential + O2 + 2H+ Electrochemical Requirements of photocatalyst for water splitting 2OH- Photocatalytic Water Splitting

4. Factors which have been considered in Photocatalysis of Water Splitting • Band gap and band position • Band Bending • Cocatalysts; Junction structure • Surface area • Defects in crystal structure and composition • Solution pH; External additive • Stability against photocorrosion

5. bulk structure Photocatalytic Water Splitting Activity defect/surface electronic structure Factors to Photocatalysis in Water Splitting

6. bulk structure Ni/SrTiO3 << Layered Ni/K4Nb6O17

7. Electronic Structure of Layered Perovskite Photocatalytic Activity of Various Perovskites 1All the samples were prepared by solid-state reaction at 1273 K for10 h. 2At room temperature; catalyst 0.3 g; water 500 cc; 450 W medium-pressure Hg lamp. All of the layered perovskites do not have good photocatalytic activity!

8. How does the electronic structure of layered perovskite work? Elucidation of Relationship between Electronic Structure of Layered Perovskites and Its Photocatalytic Activity Ln2Ti2O7 (Ln = La, Pr, Nd)

9. La2O3, Pr6O11, Nd2O3 + TiO2 Catalyst Preparation Macromixing Grinding Calcination (Air, 1150 oC) Ln2Ti2O7 Ni-Nitrate Reduction (H2, 500 oC) Oxidation (Air, 200 oC) NiO/Ln2Ti2O7

10. La2Ti2O7 with Layered Structure c = 13.01Å a = 7.81Å b = 5.544Å

11. 3 nm 200nm SEM & TEM image of La2Ti2O7 10 nm Ni Particle La2Ti2O7 sintered at 1423K for 10 h

12. Crystal Structure of Ln2Ti2O7 monoclinic P21 All samples were calcined at 1423K for 10h.

13. local spin density approximation DFT Kohn-Sham equation I + FLAPW Method II basis set Calculation of Electronic Structure Wien97

14. Electronic Structure of La2Ti2O7 Conduction Band direct band gap Eg = 3.2eV Valence Band wave vector

15. defect/surface Summary Until Present Study • Dependence of photocatalytic activity on the electronic band structure as well as bulk structure of photocatalyst materials • The valence band consists of mainly O2p, which is hybridized a little with Ti3d, while the conduction band consists of mainly Ti3d. • The Ln4f level in Ln2Ti2O7 was shifted to lower energy as the number of Ln4f electron increased. • Both bulk structure and electronic structure was not sufficient to determine the photocatalytic activity. Therefore, other factors such as crystal defect and surface property should be considered, as shown in Ba-doped La2Ti2O7.