Charge Recombination Dynamics in Sensitized SnO₂/TiO₂ Core/Shell Photoanodes

Studies have been conducted to examine the mechanisms of charge recombination in dye-sensitized SnO₂/TiO₂ core/shell films. Nanostructured SnO₂/TiO₂ core/shell films varying in TiO₂ shell thicknesses were prepared via atomic layer deposition and sensitized with a phosphonate-derivatized ruthenium chromophore [Ru­(bpy)₂(4,4′-(PO₃H₂)₂bpy)]²⁺. Transient absorption spectroscopy was used to study the interfacial charge recombination dynamics for these core/shell materials. Charge recombination for sensitized, as-deposited SnO₂/TiO₂ core/shell systems is dominated by a tunneling mechanism for shell thicknesses between 0 and 3.2 nm, with β = 0.25 Å^–1. For shell thicknesses greater than 3.2 nm, recombination primarily proceeds directly via electrons localized in the relatively thick TiO₂ shell. Annealing the SnO₂/TiO₂ core/shell structure at 450 °C affects the recombination dynamics substantially; charge recombination dynamics for the annealed films do not show a dependence on shell thickness and are comparable to ZrO₂/TiO₂ control samples, suggesting the annealing process perturbs the core/shell interface. This analysis of charge recombination dynamics indicates that there is an optimum shell thickness to maximize charge separation lifetimes in dye-sensitized core/shell photoanodes and that the nature of the core/shell interface influences the efficacy of these materials

Stabilizing Small Molecules on Metal Oxide Surfaces Using Atomic Layer Deposition

Device lifetimes and commercial viability of dye-sensitized solar cells (DSSCs) and dye-sensitized photoelectrosynthesis cells (DSPECs) are dependent on the stability of the surface bound molecular chromophores and catalysts. Maintaining the integrity of the solution-metal oxide interface is especially challenging in DSPECs for water oxidation where it is necessary to perform high numbers of turnovers, under irradiation in an aqueous environment. In this study, we describe the atomic layer deposition (ALD) of TiO₂ on nanocrystalline TiO₂ prefunctionalized with the dye molecule [Ru­(bpy)₂(4,4′-(PO₃H₂)­bpy)]²⁺ (RuP) as a strategy to stabilize surface bound molecules. The resulting films are over an order of magnitude more photostable than untreated films and the desorption rate constant exponentially decreases with increased thickness of ALD TiO₂ overlayers. However, the injection yield for TiO₂-RuP with ALD TiO₂ also decreases with increasing overlayer thickness. The combination of decreased injection yield and 95% quenched emission suggests that the ALD TiO₂ overlayer acts as a competitive electron acceptor from RuP*, effectively nonproductively quenching the excited state. The ALD TiO₂ also increases back electron transfer rates, relative to the untreated film, but is independent of overlayer thickness. The results for TiO₂-RuP with an ALD TiO₂ overlayer are compared with similar films having ALD Al₂O₃ overlayers

Atomic Layer Deposition of TiO₂ on Mesoporous nanoITO: Conductive Core–Shell Photoanodes for Dye-Sensitized Solar Cells

Core–shell structures consisting of thin shells of conformal TiO₂ deposited on high surface area, conductive Sn-doped In₂O₃ nanoparticle. Mesoscopic films were synthesized by atomic layer deposition and studied for application in dye-sensitized solar cells. Results obtained with the N719 dye show that short-circuit current densities, open-circuit voltages, and back electron transfer lifetimes all increased with increasing TiO₂ shell thickness up to 1.8–2.4 nm and then decline as the thickness was increased further. At higher shell thicknesses, back electron transfer to −Ru^III is increasingly competitive with transport to the nanoITO core resulting in decreased device efficiencies

Solution-Processed, Antimony-Doped Tin Oxide Colloid Films Enable High-Performance TiO₂ Photoanodes for Water Splitting

Photoelectrochemical (PEC) water splitting and solar fuels hold great promise for harvesting solar energy. TiO₂-based photoelectrodes for water splitting have been intensively investigated since 1972. However, solar-to-fuel conversion efficiencies of TiO₂ photoelectrodes are still far lower than theoretical values. This is partially due to the dilemma of a short minority carrier diffusion length, and long optical penetration depth, as well as inefficient electron collection. We report here the synthesis of TiO₂ PEC electrodes by coating solution-processed antimony-doped tin oxide nanoparticle films (nanoATO) on FTO glass with TiO₂ through atomic layer deposition. The conductive, porous nanoATO film-supported TiO₂ electrodes, yielded a highest photocurrent density of 0.58 mA/cm² under AM 1.5G simulated sunlight of 100 mW/cm². This is approximately 3× the maximum photocurrent density of planar TiO₂ PEC electrodes on FTO glass. The enhancement is ascribed to the conductive interconnected porous nanoATO film, which decouples the dimensions for light absorption and charge carrier diffusion while maintaining efficient electron collection. Transient photocurrent measurements showed that nanoATO films reduce charge recombination by accelerating transport of photoelectrons through the less defined conductive porous nanoATO network. Owing to the large band gap, scalable solution processed porous nanoATO films are promising as a framework to replace other conductive scaffolds for PEC electrodes

Characterization of Few-Layer 1T′ MoTe₂ by Polarization-Resolved Second Harmonic Generation and Raman Scattering

We study the crystal symmetry of few-layer 1T′ MoTe₂ using the polarization dependence of the second harmonic generation (SHG) and Raman scattering. Bulk 1T′ MoTe₂ is known to be inversion symmetric; however, we find that the inversion symmetry is broken for finite crystals with even numbers of layers, resulting in strong SHG comparable to other transition-metal dichalcogenides. Group theory analysis of the polarization dependence of the Raman signals allows for the definitive assignment of all the Raman modes in 1T′ MoTe₂ and clears up a discrepancy in the literature. The Raman results were also compared with density functional theory simulations and are in excellent agreement with the layer-dependent variations of the Raman modes. The experimental measurements also determine the relationship between the crystal axes and the polarization dependence of the SHG and Raman scattering, which now allows the anisotropy of polarized SHG or Raman signal to independently determine the crystal orientation

Visible Light Driven Benzyl Alcohol Dehydrogenation in a Dye-Sensitized Photoelectrosynthesis Cell

Light-driven dehydrogenation of benzyl alcohol (BnOH) to benzaldehyde and hydrogen has been shown to occur in a dye-sensitized photoelectrosynthesis cell (DSPEC). In the DSPEC, the photoanode consists of mesoporous films of TiO₂ nanoparticles or of core/shell nanoparticles with tin-doped In₂O₃ nanoparticle (nanoITO) cores and thin layers of TiO₂ deposited by atomic layer deposition (nanoITO/TiO₂). Metal oxide surfaces were coderivatized with both a ruthenium polypyridyl chromophore in excess and an oxidation catalyst. Chromophore excitation and electron injection were followed by cross-surface electron-transfer activation of the catalyst to −Ru^IVO²⁺, which then oxidizes benzyl alcohol to benzaldehyde. The injected electrons are transferred to a Pt electrode for H₂ production. The nanoITO/TiO₂ core/shell structure causes a decrease of up to 2 orders of magnitude in back electron-transfer rate compared to TiO₂. At the optimized shell thickness, sustained absorbed photon to current efficiency of 3.7% was achieved for BnOH dehydrogenation, an enhancement of ∼10 compared to TiO₂