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, J. O’M. Bockris, A.K.N. Reddy, and M. Gamboa-Aldeco, Modern Electrochemistry, Kluwer Academic/Plenum Publishers, 2000 Lecture note

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

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

Construction of Pourbaix Diagram

Example: Thermodynamic Reaction of Pt in Aqueous Solution Species  o or  G o (kJ/molK) Pt (s) 0 PtO (s) PtO 2(s) Pt 2+ (aq) H 2 O (l) Reactions between solid and ions 3.Pt e = Pt 4.PtO + 2H + = Pt 2+ + H 2 O 5.PtO 2 + 4H e = Pt H 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

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

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

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

Pourbaix Diagram of Pt

Corrosion Pourbaix Diagram of Pt

Examples – p-Cu 2 O Photocathode for HER

1.Electrodeposition 2.Corrosion during HER

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

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

Caution

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