Swiss User Group Surfaces & Interfaces 23 January 2008, Université de Fribourg - Institut de Physique Pérolles
Photo-anode material for solar hydrogen generation Materials Science & Technology Pulsed laser deposited WO3/TiO2 model films and porous sol gel WO3 films A. Braun1, R. Solarska1, S. Erat1,2, AnaIsabel Borras1, T. Graule1, X. Zhang3, Z. Liu3, J. Augustynski4, A. Barranco5, S.M. Mao3 1 Empa
– Laboratory for High Performance Ceramics, Dübendorf 2 ETH Zürich, Department of Non-Metallic Materials, Zürich 4 University of Warsaw, Department of Chemistry, Warsaw, Poland
WO3 porous films for photo anodes
1 Empa
– Laboratory for nanotech @ surfaces, Thun 3 Lawrence Berkeley National Laboratory, Berkeley, California 5 Instituto de Ciencia de Materiales de Sevilla (CICIC), Sevilla, Spain
Wetting behavior and Contact angle
WO3 satisfies relevant criteria for solar-electrochemical H2 generation from H2O • sol gel film technology on fluorene tin oxide • transparency, stability, homogeneity • high corrosion resistance • optimum thickness for efficient hν absorption • high crystallinity → small h+e- recombination • superior incident –photon-to-current efficiency • band gap 2.5 eV
TiO2 Substr. (001)
81 ± 3 (100) 63 ± 1
500 nm
(110)
Photoelectrochemical Reactions for Hydrogen Evolution Cathode
Electrolyte
WO3 Photoanode
2H+ + 2e- → H2 ↑
CH3HSO3 (aq.)
H2O + 2h+ → ½ O2 + H+
2H+ + 2e- → H2↑
2H+ + 2e- → H2 ↑
NaCl (aq.) sea water
organic pollutants in NaHSO4 (aq.)
θc [°]
H2O + 2h+ → ½ O2 + H+ and 2Cl- + 2h+ → Cl2 ca. 20% Cl2 from 0.5 M NaCl HCHO + 2 H2O + 4 h+ → CO2 + 4H+
Highly oriented growth of WO3/TiO2
71 ± 3 75 ± 1.5
WO3/TiO2 (001) 10 nm 100 nm (100) 10 nm 100 nm (110) 10 nm 100 nm
θc [°] 66.5 ± 1.5 73 ± 5 71.5 ± 1.5 70 ± 2 75.5 ± 1 76 ± 5
Photo anodes are exposed to liquids or humidity. The coupling of anode surface and aqueous environment has not really been in the center of attention of photocatalyst researchers.
Molecular & Electronic Structure Synchrotron core level x-ray spectroscopy probes unoccupied density of states and helps understand the functionality of photocatalysts on a molecular basis, and allows to probe the cation/anion super-exchange units with element specific sensitivity. K-edges of O,N,S permit access to the valence band. H2
-1.0
- - 0.0
E° (H2/H2O)
H2O
- - -
1.6 eV
CB of WO3 1.0
E° H2O/O2)
+ + + Dye-sensitized TiO2
2.0
2.6 eV
Photoelectrochemical water decomposition by A tandem mechanism cell O2
3.0
+ + + VB of WO3
H2O
Energy band situation and x-ray core level absorption spectra at the K-edges of N,O, and L-edge of Ti. Different synthesis conditions (right) have influence on the N-Ti-O orbital hybridization, as indicated by the t2g/eg peak height ratios in the right panel with oxygen spectra from three Ti-O-N powders.
Future and Planned Activities
• Optical solid/liquid interface and solvation dynamics studies • Electronic structure of the water molecule at catalyst surfaces • Structure of novel hydrogenase; development of Fe/S/O nanoparticles • Activities in nanoarchitecture for PEC applications How does the lattice mismatch between TiO2 (a=b=4.59 Å, c=2.96 Å) and WO3 (a=b=5.28 Å, c=7.85 Å) impact the band structure and the optical and corrosive properties of the WO3 ?
Contact: +41 44 823 4850 •
[email protected]
