1. Hydrogen: the Fuel of our Future Marie Mayer Stanford Synchrotron Radiation Laboratory University of Illinois at Urbana-Champaign

2. Overview • Environmental relevance • Photoelectrochemical hydrogen production • X-Ray Photoelectron Spectroscopy (XPS) • Results

3. Carbon-free Energy • Sunlight that hits earth for 1 hour = planet’s power for 1 year • Equivalent of 1.3 trillion barrels of oil from sun in 1 year • How do we capture it? • Solar cells • Hydrogen production • Storage of solar energy

4. current Photoelectrochemical Hydrogen Production h O2 H2 Pt Anode Conduction Band H2O/H2 2H2O + 2e- 2OH- + H2 Eg 2H2O + 4h+  4H+ + O2 H2O/O2 Valence Band Hydrogen Electrode (Semiconductor Cathode) Thank you, Theanne, for providing me with most of this slide.

5. ExperimentalGaPxN1-x for PEC cells • Dilute (x < 0.02) nitrogen decreases PEC corrosion ~20 times1 • As grown & corroded samples • GaP • GaP0.98N0.02 • GaP0.998N0.002 Pt Counter Electrode Ag/AgCl Reference Electrode H2SO4 electrolyte GaP(N) sample Optical lamp: 100 mW-cm-2 (1 sun) 1. T. Deutsch. Sunlight, Water and III-V Nitrides for Fueling the Future, PhD Thesis, Boulder, Colorado, 2006. Thank you to NREL for lending the cell.

6. Yay! I’ve escaped! e- Detector 3. Detector records kinetic energies (depths) of escaped electrons—radius of curvature depends on velocity 2. Core electron is excited to free electron 4. Identify using chemical binding energy e- e- e- e- e- e- e- Synchrotron XPS Theory:the watered-down version 1. Synchrotron radiation as X-rays

7. Synchrotron XPS (BL 5-1) • Chemical Environment • Tunable h! • Pick kinetic energy by picking h • Depth profile: Ga3d, Ga3p, P2p, N1s, O1s, C1s

8. More dramatic with N • Possible Mechanism: • GaP + 3H+ +  Ga3+ + PH3 • Electrolyte analysis Result 1: Ga surface depletion during corrosion DEPTH

9. 396 394 392 390 388 386 BE/eV Result 2: Bulk nitrogen migration to surface • N + XO  NOx • N  surface during corrosion • Oxidizes with air contact N (bulk) KE=900 hν=1310 NOx, NHx Intensity [arb. Units] N N N N N N N Nitrogen-containing sample

10. Possible formation of GaN (before oxidizing)? • Corrosion stability of wider band gap • GaP: 2.26 eV • GaN: 3.2 eV Result 2: Bulk nitrogen migration to surface (2)

11. Recap: GaP1-xNx • Observed that N content assists H2 production • Formation of oxides during H2 production • Surface Ga depletion • Bulk N to surface • Future work • Mechanisms • Electrochemical measurements • SEM

12. A moment of gratitude… • Theanne Schiros • Jen Leisch • Lars-Åke Näslund • Hirohito Ogasawara • Anton Nikitin • Anders Nilsson • Dennis Nordlund • Mike Toney • Robert Kirby, Surface & Materials Science • Sarah Kurtz, National Renewable Energy Laboratory (NREL) • Friendly supply room fellows • Anyone who even so much as made me smile.

13. Questions?

14. Yes, my friends, I believe that water will one day serve as our fuel, that the hydrogen and oxygen which compose it will supply an inexhaustible source of heat and light. I believe that when the coal mines have been exhausted, we will both heat and be heated with water. Water is the coal of the future.” -Jules Verne The Mysterious Island

15. Fossil fuels produce CO2… Data from Energy Information Administration Database. 2006, US Department of Energy, www.eia.doe.gov.