Sagi Pasternak Supervisor: Prof. Yaron Paz RBNI Fall Symposium 2012

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

Water Splitting – At a Glance

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

Co-catalysts and Sacrificial Agents Sacrificial agents – hole scavengers Co-catalysts –assist in efficient H 2 evolution

Bismuth Titanate (BTO) – Bi 2 Ti 2 O 7 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,

BTO Powder Preparation Bi(NO 3 ) 3 ∙5H 2 O 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.

Characterization Bi 2 Ti 2 O 7 phase is dominant Homogeneous morphology – typical size of nm Typical XRD spectrum Typical morphology 400 nm

NiOx Co-Catalyst Loading Calcination at 300 ⁰ C in air  Formation of Ni oxides and hydroxides Calcination at 500 ⁰ C under 5% H 2 /N 2  Reduction to Ni Re-oxidation step to obtain NiOx Dispersed BTO powder Aqueous solution containing Ni(NO 3 ) 2 Heat Domen, K., Naito, S., Soma, M., Onishi, T., and Tamaru, K., J. Phys. Chem. 86, 3657

H 2 Evolution over Ni-treated BTO Stable evolution of H 2 from Methanol-Water solution under UV irradiation. Average rate of 1 μmol/hr. Quantum efficiency – about 1%.

BTO Powder After Reduction BTO is reduced to Bi, Bi 2 O 3 and (probably) amorphous TiO 2. Loss of original morphology 400 nm

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

NiO-loaded BTO - XRD BTO and NiO phases were obtained simultaneously

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

NiO-loaded BTO – SEM-EDS Atomic ratios of Bi:Ti:O of 1:1.1:4.5 (Bi 2 Ti 2 O 7 ) Amount of Ni – about 2 wt.% O 2 ratio– larger than expected due to low vacuum 25 μ m

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

Summary H 2 evolution rate of 1 μmol/hr over NiOx-loaded reduced BTO NiO-loaded BTO showed no activity for H 2 evolution Current Work Loading of metallic Ni in addition to NiO

Acknowledgements