Electroabsorption as a Probe of Electrons Injected into Nanocrystalline SnO2/TiO2 Core/Shell Thin Films

Abstract
Core/shell nanoparticles comprised of a SnO₂ core and a TiO₂ shell, SnO₂/TiO₂, present in a mesoporous thin film, are of practical interest for applications in dye-sensitized water splitting as they outperform sensitized materials based on SnO₂ or TiO₂ (anatase or rutile polymorphs) for the water oxidation half-reaction. Here we report electroabsorption studies designed to quantify the surface electric field and the ability of the shell to screen an electric field from a surface anchored dye. The metal-to-ligand charge transfer (MLCT) absorbance of [Ru(bpy)₂(4,4′-(PO₃H₂)₂-2,2′-bipyridine)](PF₆)₂, abbreviated RuP, was indeed sensitive to electron injection into the oxide support. When a fixed charge of 1 mC was injected, the electric field magnitude (in MV/cm) increased in the order 0.15 TiO₂ (rutile) < 0.35 TiO₂ (anatase) ≪ 1.4 SnO₂ (rutile). This order tracks the reported bulk dielectric constants of the oxide materials. Comparative studies with SnO₂/TiO₂ core/shell materials, fabricated by atomic layer deposition (ALD) of a TiO₂ shell on a mesoporous SnO₂ thin film, diminished the field strength by about a factor of 4 to values less than or about equal to that measured for anatase TiO₂. Heat treating some SnO₂/TiO₂ core/shell materials at 450 °C resulted in a crystalline rutile TiO₂ shell that screened the field most effectively, more so even than rutile TiO₂ alone. Taken together, the data indicate that injected electrons reside within the SnO₂ core region.

Citation
Electroabsorption as a Probe of Electrons Injected into Nanocrystalline SnO2/TiO2 Core/Shell Thin Films Erica M. James, Marc T. Bennett, and Gerald J. Meyer The Journal of Physical Chemistry C 2023 127 (12), 5904-5910 DOI: 10.1021/acs.jpcc.3c00638