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 TiO_x (Sn:TiO_x) coatings on a transparent, insulating, and mesoporous ZrO₂ substrate. Annealed Sn:TiO_x coatings were fabricated to make comparisons to annealed SnO₂/TiO_x 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/TiO_x core/shell material possesses an interfacial Sn_1–xTi_xO₂ material that impacts charge transfer in functioning DSPEC devices. The incorporation of Sn into TiO_x by ternary ALD was corroborated by X-ray photoelectron spectroscopy and elemental mapping by energy-dispersive spectroscopy (EDS). The physical structure of annealed Sn:TiO_x 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:TiO_x coatings are relatively conformal and smooth. The data from Raman spectroscopy and the microscopic imaging suggest that the annealed TiO_x 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:TiO_x coatings reveals that both the shallow exponential and deep trap state distributions are enhanced with increasing Sn incorporation. Spectroelectrochemistry of Sn:TiO_x demonstrates that two reduction processes occur, which we argue is the reduction of Sn⁴⁺ ions to Sn²⁺ and electrons localizing in the TiO_x. Using the results from the spectroelectrochemistry of Sn:TiO_x materials, we propose that for annealed SnO₂/TiO_x core/shell materials electrons localize in a shell that is best described as a Sn:TiO_x material. Finally, transient absorption spectroscopy was used to investigate interfacial charge recombination between the Sn:TiO_x 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/TiO_x and Sn:TiO_x is rate-limited by the TiO_x 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.

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

DOI: 10.1021/acs.jpcc.1c10412