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 Sn1–x Ti x O2 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.

A dye sensitized photosynthesis cell for stable water oxidation mediated by photo-generated bromine

“…2 arise from electron injection, surface oxidation, and the dynamic local electric field effect in the double layer. 15,23,27 Notably, the absence of the photocurrent spike for FTO|SnO 2 /TiO 2 -RuP 2+ under the same conditions indicates no significant buildup of charge at the interface attributed to rapid photo-generated charge separation and catalysis of Br − . As documented in other reports, the improved photocurrent response for the core/shell structure arises from its impact on local interfacial dynamics.…”

“…In order to boost interfacial photo-generated carrier transfer, great efforts have been devoted to surface molecular assembly modification, 1,3,5 such as changing the arrangement of the chromophore and catalyst by chromophore-catalyst assembly [6][7][8][9] or electropolymerization, [10][11][12] or optimizing the structure of the semiconduction by a core-shell strategy. [13][14][15][16] However, the solar conversion efficiency is still low and remains far from practical use.…”