1. Lecture 16 Electrochemical Decomposition of Semiconductors Reference. 1.R. Memming, Semiconductor Electrochemistry, Wiley-VCH, 2000 (e-book) 2.A.J. Bard and L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, Wiley, 2001 3.J. O’M. Bockris, A.K.N. Reddy, and M. Gamboa-Aldeco, Modern Electrochemistry, Kluwer Academic/Plenum Publishers, 2000 Lecture note http://les.kaist.ac.kr/B_Lecture

2. 2 Turner et al, Science 280, 425 (1998) 24 hrs corrosion test @ 5 mA/cm 2 under 1sun World-best Water Splitting Cell

3. The Pourbaix diagram, or potential-pH diagram, is thermodynamic stability map of the given materials in the aqueous environment. The electrochemical equilibrium is given by the Nernst equation. (Example) 2H + + 2e = H 2   G= 0 at equilibrium. pH E 1.Above the line  Backward reaction is favorable. 2. Below the line  Forward reaction is favorable. H 2 is stable Water is stable Pourbaix Diagram

4. Construction of Pourbaix Diagram

5. Example: Thermodynamic Reaction of Pt in Aqueous Solution Species  o or  G o (kJ/molK) Pt (s) 0 PtO (s) - 48.15 PtO 2(s) - 83.67 Pt 2+ (aq) 229.27 H 2 O (l) - 237.18 Reactions between solid and ions 3.Pt 2+ + 2e = Pt 4.PtO + 2H + = Pt 2+ + H 2 O 5.PtO 2 + 4H 2+ + 2e = Pt 2+ + 2H 2 O Reactions between two solid 1.PtO + 2H + + 2e = Pt + H 2 O 2.PtO 2 + 2H + + 2e = PtO + H 2 O Water electrolysis a.2H + + 2e = H 2 b.1/2O 2 + 2H + +2e = H 2 O

6. Nernst Equations 1.PtO + 2H + + 2e = Pt + H 2 O 2.PtO 2 + 2H + + 2e = PtO + H 2 O

7. 3.Pt 2+ + 2e = Pt 4.PtO + 2H + = Pt 2+ + H 2 O

8. 5.PtO 2 + 4H 2+ + 2e = Pt 2+ + 2H 2 O a.2H + + 2e = H 2 b.1/2O 2 + 2H + +2e = H 2 O

9. Pourbaix Diagram of Pt

10. Corrosion Pourbaix Diagram of Pt

11. Examples – p-Cu 2 O Photocathode for HER

13. 1.Electrodeposition 2.Corrosion during HER

14. Requirements for a Protection Layer 1. Band edge alignment for charge injection

16. Requirements for a Protection Layer 1.Chemical stability in a solution with a given pH

21. Caution

24. Next Meeting Electrochemical Decomposition of Semiconductors