Tuning the Conduction Band for Interfacial Electron Transfer: Dye-Sensitized SnxTi1–xO2 Photoanodes for Water Splitting

Spies, Jacob A.; Swierk, John R.; Kelly, H. Ray; Capobianco, Matt D.; Regan, Kevin P.; Batista, Víctor S.; Brudvig, Gary W.; Schmuttenmaer, Charles A.

doi:10.1021/acsaem.1c00305

Cited by 6 publications

(8 citation statements)

References 73 publications

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“…Changes to the electronic structure can, in turn, affect charge-transfer processes observed by (spectro)electrochemistry or transient absorption spectroscopy. Importantly, changes to E gap have been used in previous reports to support the incorporation of Sn into the lattice of a-TiO 2 and r-TiO 2 materials. − Values of E gap were quantified by the Tauc analysis, which is demonstrated through the use of eq where α is the absorption coefficient, h is the Planck constant, ν is the frequency of light, C is a constant, and n is equal to 2 for an indirect band-gap transition. A plot of (α h ν) 1/2 versus h ν should yield a linear region that can be fit to extrapolate to the x-intercept, which represents E gap .…”

Section: Resultsmentioning

confidence: 99%

“…While this report is narrowly focused on comparisons of Sn-doped TiO 2 to SnO 2 /TiO 2 core/shell materials, broader implications, interest, and influence can be derived from the work presented. SnO 2 –TiO 2 mixed oxides and interfaces are ubiquitous in a number of fields including dye-sensitized solar cells and DSPECs, ,− perovskite solar cells, batteries, , photocatalysis, ,− and gas-sensing technologies. , The TiO 2 interface with fluorine-doped tin oxide is especially prevalent in the aforementioned areas of study. The diffusion of Sn into TiO 2 has been demonstrated to occur from heat treatments during normal sample processing, − and thin interfacial mixed tin–titanium oxides should be of interest to researchers assessing charge transfer and separation at any general SnO 2 –TiO 2 interface.…”

Section: Discussionmentioning

confidence: 99%

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

2022

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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.

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Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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

2022

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“…For RuP adsorbed on SnO 2 /TiO 2 core/shell electrodes, ΔG( 3 MLCT) is 2.46 eV and ΔG( 1 MLCT) is 2.68 eV, according to a previous study. 36 Assuming that the excitation energy of the molecule is the same in different solvents, the excited state potentials can be calculated. The driving forces for electron injection are calculated according to eq 7. and the results are listed in Table 2.…”

Section: ■ Discussionmentioning

confidence: 99%

“…Samples in electrolytes were prepared by sealing the sample using a second quartz substrate with a 60 μm Surlyn spacer (Solaronix). The samples were then vacuum backfilled with electrolyte as previously described …”

Section: Methodsmentioning

confidence: 99%

Electron Transfer Dynamics at Dye-Sensitized SnO₂/TiO₂ Core/Shell Electrodes in Aqueous/Nonaqueous Electrolyte Mixtures

Xiao,

Spies,

Sheehan

et al. 2024

J. Am. Chem. Soc.

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The dynamics of photoinduced electron transfer were measured at dye-sensitized photoanodes in aqueous (acetate buffer), nonaqueous (acetonitrile), and mixed solvent electrolytes by nanosecond transient absorption spectroscopy (TAS) and ultrafast optical-pump terahertz-probe spectroscopy (OPTP). Higher injection efficiencies were found in mixed solvent electrolytes for dye-sensitized SnO 2 /TiO 2 core/shell electrodes, whereas the injection efficiency of dye-sensitized TiO 2 electrodes decreased with the increasing acetonitrile concentration. The trend in injection efficiency for the TiO 2 electrodes was consistent with the solventdependent trend in the semiconductor flat band potential. Photoinduced electron injection in core/shell electrodes has been understood as a two-step process involving ultrafast electron trapping in the TiO 2 shell followed by slower electron transfer to the SnO 2 core. The driving force for shell-to-core electron transfer increases as the flat band potential of TiO 2 shifts negatively with increasing concentrations of acetonitrile. In acetonitrile-rich electrolytes, electron injection is suppressed due to the very negative flat band potential of the TiO 2 shell. Interestingly, a net negative photoconductivity in the SnO 2 core is observed in mixed solvent electrolytes by OPTP. We hypothesize that an electric field is formed across the TiO 2 shell from the oxidized dye molecules after injection. Conduction band electrons in SnO 2 are trapped at the core/shell interface by the electric field, resulting in a negative photoconductivity transient. The overall electron injection efficiency of the dye-sensitized SnO 2 /TiO 2 core/shell photoanodes is optimized in mixed solvents. The ultrafast transient conductivity data illustrate the crucial role of the electrolyte in regulating the driving forces for electron injection and charge separation at dyesensitized semiconductor interfaces.

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“…Indeed, recombination for TiO 2 is by far the slowest of the common metal oxides, behavior that is likely responsible for its prevalence in gold standard regenerative dye-sensitized solar cells. , On the other hand, recombination for dye-sensitized SnO 2 interfaces is rapid and competes kinetically with iodide oxidation, resulting in a lower efficiency solar cell. , The data here reveal that fast recombination results from a small barrier for non-adiabatic interfacial electron transfer and indicate that a larger SnO 2 -sensitizer distance is necessary to increase the kinetic barrier and slow interfacial electron transfer. Indeed, the Marcus parameters reported here are likely to be most relevant to tin oxide-based solar cells that continue to be under active investigation. − …”

Section: Discussionmentioning

confidence: 99%

Reorganization Energies for Interfacial Electron Transfer across Phenylene Ethynylene Rigid-Rod Bridges

Heidari

Loague

Bangle

et al. 2022

ACS Appl. Mater. Interfaces

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A family of three ruthenium bipyridyl rigid-rod compounds of the general form [Ru(bpy) 2 (LL)](PF 6 ) 2 were anchored to mesoporous thin films of tindoped indium oxide (ITO) nanocrystals. Here, LL is a 4-substituted 2,2-bipyridine (bpy) ligand with varying numbers of conjugated phenylenethynylene bridge units between the bipyridine ring and anchoring group consisting of a bis-carboxylated isophthalic group. The visible absorption spectra and the formal potentials, E o (Ru III/II ), of the surface anchored rigid-rods were insensitive to the presence of the phenylene ethynylene bridge units in 0.1 M tetrabutyl ammonium perchlorate acetonitrile solutions (TBAClO 4 /CH 3 CN). The conductive nature of the ITO enabled potentiostatic control of the Fermi level and hence a means to tune the Gibbs free energy change, −ΔG°, for electron transfer from the ITO to the rigid-rods. Pseudo-rate constants for this electron transfer reaction increased as the number of bridge units decreased at a fixed −ΔG°. With the assumption that the reorganization energy, λ, and the electronic coupling matrix element, H ab , were independent of the applied potential, rate constants measured as a function of −ΔG°and analyzed through Marcus− Gerischer theory provided estimates of H ab and λ. In rough accordance with the dielectric continuum theory, λ was found to increase from 0.61 to 0.80 eV as the number of bridge units was increased. In contrast, H ab decreased markedly with distance from 0.54 to 0.11 cm −1 , consistent with non-adiabatic electron transfer. Comparative analysis with previously published studies of bridges with an sp 3 -hybridized carbon indicated that the phenylene ethynylene bridge does not enhance electronic coupling between the oxide and the rigid-rod acceptor. The implications of these findings for practical applications in solar energy conversion are specifically discussed.

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Tuning the Conduction Band for Interfacial Electron Transfer: Dye-Sensitized Sn_xTi_1–xO₂ Photoanodes for Water Splitting

Cited by 6 publications

References 73 publications

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

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

Electron Transfer Dynamics at Dye-Sensitized SnO₂/TiO₂ Core/Shell Electrodes in Aqueous/Nonaqueous Electrolyte Mixtures

Reorganization Energies for Interfacial Electron Transfer across Phenylene Ethynylene Rigid-Rod Bridges

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Tuning the Conduction Band for Interfacial Electron Transfer: Dye-Sensitized SnxTi1–xO2 Photoanodes for Water Splitting

Cited by 6 publications

References 73 publications

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

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

Electron Transfer Dynamics at Dye-Sensitized SnO2/TiO2 Core/Shell Electrodes in Aqueous/Nonaqueous Electrolyte Mixtures

Reorganization Energies for Interfacial Electron Transfer across Phenylene Ethynylene Rigid-Rod Bridges

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Tuning the Conduction Band for Interfacial Electron Transfer: Dye-Sensitized Sn_xTi_1–xO₂ Photoanodes for Water Splitting

Electron Transfer Dynamics at Dye-Sensitized SnO₂/TiO₂ Core/Shell Electrodes in Aqueous/Nonaqueous Electrolyte Mixtures