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Novel Materials for Photocatalytic Water Splitting. 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.

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novel materials for photocatalytic water splitting

Novel Materials for Photocatalytic Water Splitting

Sagi Pasternak

Supervisor: Prof. Yaron Paz

RBNI Fall Symposium 2012

outline
Outline
  • Water splitting in general
  • Challenges and Approaches
  • Water splitting using bismuth titanate (BTO)
  • Summary
challenges
Challenges
  • Suitable band positions
  • Stability – corrosion by holes
  • Charge separation – co-catalysts
  • Visible light absorption – suitable band gap
co catalysts and sacrificial agents
Co-catalysts and Sacrificial Agents
  • Co-catalysts –assist in efficient H2evolution
  • Sacrificial agents – hole scavengers
bismuth titanate bto bi 2 ti 2 o 7
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

bto powder preparation
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.
characterization
Characterization

Typical morphology

Typical XRD spectrum

  • Bi2Ti2O7 phase is dominant
  • Homogeneous morphology – typical size of 100-150 nm

400 nm

niox co catalyst loading
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

h 2 evolution over ni treated bto
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%.
bto powder after reduction
BTO Powder After Reduction
  • BTO is reducedto Bi, Bi2O3 and (probably) amorphous TiO2.
  • Loss of original morphology

400 nm

co catalyst loading ideas
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
NiO-loaded BTO - XRD
  • BTO and NiO phases were obtained simultaneously
nio loaded bto sem
NiO-loaded BTO - SEM

400 nm

  • Typical BTO morphology is obtained
  • No differences seen using ESB - Good homogeneity
nio loaded bto sem eds
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

possible explanation
Possible Explanation

Troy K. Townsend et al.,Energy Environ. Sci., 2012

  • Hydrogen evolution does not take place on NiO
  • Metallic Ni is also needed
summary
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
slide19

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