Novel Materials for Photocatalytic Water Splitting

1. Novel Materials for Photocatalytic Water Splitting Sagi Pasternak Supervisor: Prof. Yaron Paz RBNI Fall Symposium 2012

2. Outline • Water splitting in general • Challenges and Approaches • Water splitting using bismuth titanate (BTO) • Summary

3. Water Splitting – At a Glance

4. Challenges • Suitable band positions • Stability – corrosion by holes • Charge separation – co-catalysts • Visible light absorption – suitable band gap

5. Co-catalysts and Sacrificial Agents • Co-catalysts –assist in efficient H2evolution • Sacrificial agents – hole scavengers

6. Bismuth Titanate (BTO) – Bi2Ti2O7 • Recent theoretical studies suggest applicability for water splitting • Reduced bandgap – Bi 6s orbitals shift valence band upward Murugesan, S.; Huda, M. N.; Yan, Y.; Al-Jassim, M. M.; Subramanian, V. The Journal of Physical Chemistry C 2010, 114, 10598-10605

7. BTO Powder Preparation • Bi(NO3)3∙5H2O is dissolved in acetic acid. • 2-methoxyethanol is added to adjust viscosity and surface tension. • Acetylacetone is added as a stabilizer. • Solution is added to tetrabutyl titanate liquid. • Solution is filtered through 0.22 μm filter to obtain a clear yellowish sol. • Calcination yields the BTO powder.

8. Characterization Typical morphology Typical XRD spectrum • Bi2Ti2O7 phase is dominant • Homogeneous morphology – typical size of 100-150 nm 400 nm

9. NiOx Co-Catalyst Loading Dispersed BTO powder • Calcination at 300⁰C in air  Formation of Ni oxides and hydroxides • Calcination at 500⁰C under 5% H2/N2 Reduction to Ni • Re-oxidation step to obtain NiOx Aqueous solution containing Ni(NO3)2 Heat Domen, K., Naito, S., Soma, M., Onishi, T., and Tamaru, K., J. Phys. Chem. 86, 3657

10. H2 Evolution over Ni-treated BTO • Stable evolution of H2 from Methanol-Water solution under UV irradiation. Average rate of 1 μmol/hr. • Quantum efficiency – about 1%.

11. BTO Powder After Reduction • BTO is reducedto Bi, Bi2O3 and (probably) amorphous TiO2. • Loss of original morphology 400 nm

12. Co-Catalyst Loading – Ideas • Reduction and re-oxidation steps – crude and damaging to BTO structure itself • Separate preparation of NiO nanoparticles 400 nm

13. NiO-loaded BTO - XRD • BTO and NiO phases were obtained simultaneously

14. NiO-loaded BTO - SEM 400 nm • Typical BTO morphology is obtained • No differences seen using ESB - Good homogeneity

15. NiO-loaded BTO – SEM-EDS • Atomic ratios of Bi:Ti:O of 1:1.1:4.5 (Bi2Ti2O7) • Amount of Ni – about 2 wt.% • O2 ratio– larger than expected due to low vacuum 25 μm

16. Possible Explanation Troy K. Townsend et al.,Energy Environ. Sci., 2012 • Hydrogen evolution does not take place on NiO • Metallic Ni is also needed

17. Summary • H2 evolution rate of 1 μmol/hr over NiOx-loaded reduced BTO • NiO-loaded BTO showed no activity for H2 evolution Current Work • Loading of metallic Ni in addition to NiO

18. Acknowledgements

19. Thank you for listening…