Ultrathin Tin-Doped Titanium Oxide by Atomic Layer Deposition on a Mesoporous Substrate: Physical/Electronic Structure, Spectroelectrochemistry, and Interfacial Charge Transfer

Abstract

Ternary atomic layer deposition (ALD) of tetrakisdimethylamido tin(IV) and titanium(IV) with water was used to fabricate ultrathin (<5 nm) Sn-doped TiOx (Sn:TiOx) coatings on a transparent, insulating, and mesoporous ZrO2 substrate. Annealed Sn:TiOx coatings were fabricated to make comparisons to annealed SnO2/TiOx core/shell materials, which have applications as an anode for dye-sensitized photoelectrosynthesis cells (DSPECs) and are also fabricated using ALD. It has been hypothesized that the annealed SnO2/TiOx core/shell material possesses an interfacial Sn1–xTixO2 material that impacts charge transfer in functioning DSPEC devices. The incorporation of Sn into TiOx by ternary ALD was corroborated by X-ray photoelectron spectroscopy and elemental mapping by energy-dispersive spectroscopy (EDS). The physical structure of annealed Sn:TiOx coatings on the ZrO2 substrate was investigated by high-resolution transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, elemental mapping by EDS, and Raman spectroscopy. The microscopic imaging techniques demonstrate that Sn:TiOx coatings are relatively conformal and smooth. The data from Raman spectroscopy and the microscopic imaging suggest that the annealed TiOx crystallizes into the tetragonal anatase polymorph, but coatings become amorphous with the incorporation of Sn dopants. We demonstrate that the insulating ZrO2 substrate allows for the (spectro)electrochemical analysis of ultrathin ALD coatings to assess the electronic structure as a function of composition. Reductive electrochemistry of the Sn:TiOx coatings reveals that both the shallow exponential and deep trap state distributions are enhanced with increasing Sn incorporation. Spectroelectrochemistry of Sn:TiOx demonstrates that two reduction processes occur, which we argue is the reduction of Sn4+ ions to Sn2+ and electrons localizing in the TiOx. Using the results from the spectroelectrochemistry of Sn:TiOx materials, we propose that for annealed SnO2/TiOx core/shell materials electrons localize in a shell that is best described as a Sn:TiOx material. Finally, transient absorption spectroscopy was used to investigate interfacial charge recombination between the Sn:TiOx coatings and a surface-anchored [Ru(bpy)2(4,4′-(PO3H2)2bpy)]2+ (bpy = 2,2′-bipyridine; 4,4′-(PO3H2)2bpy = 4,4′-bis(phosphonic acid)-2,2′-bipyridine) dye. The results by transient absorption spectroscopy suggest that charge recombination in annealed SnO2/TiOx and Sn:TiOx is rate-limited by the TiOx with the same mechanism of thermally activated Ti3+ → Ti4+ hopping, despite large changes to the shallow exponential and deep trap state distributions as a function of the composition.



Citation

Ultrathin Tin-Doped Titanium Oxide by Atomic Layer Deposition on a Mesoporous Substrate: Physical/Electronic Structure, Spectroelectrochemistry, and Interfacial Charge Transfer

Michael J. Mortelliti, Milan Y. Patel, and Jillian L. Dempsey
The Journal of Physical Chemistry C 2022 126 (11), 5265-5282

DOI: 10.1021/acs.jpcc.1c10412