Impact of Photosensitizing Multilayered Structure on Ruthenium(II)-Dye-Sensitized TiO2-Nanoparticle Photocatalysts

Photoredox Cascade Catalysts for Solar Hydrogen Production From Sustainable Hydrogen Sources

Visible-light-driven photocatalytic hydrogen (H2) production has been extensively studied as a clean and sustainable energy resource. Although sacrificial electron donors (SEDs) are commonly used to evaluate photocatalytic activity, their irreversible decomposition forces charge separation, which disrupts the inherent dual productivity of photocatalysis, that is, the formation of both the reduction and oxidation products. To achieve highly efficient photoinduced charge separation without SED decomposition, the layer-by-layer assembly of redox-active photosensitizing dyes and electron mediators through Zr4+-phosphonate bonds has been extensively studied as an artificial mimic of the electron transport chain in natural photosynthesis. This concept paper presents an overview of photoredox cascade catalytic (PRCC) systems comprising multiple Ru(II)-trisbipyridine-type dyes and mediator layers on Pt-loaded TiO2 nanoparticles for H2 production from redox reversible electron donors (RREDs). The PRCC structure-activity relationship for photocatalytic H2 production is briefly discussed in terms of layer thickness, surface structure and modification, and cooperativity with molecular oxidation catalysts. Finally, new insights into the design of efficient dual-production photocatalysts based on the PRCC structure are presented.

Enhanced Visible Light-Driven Photocatalytic Water-Splitting Reaction of Titanate Nanotubes Sensitised with Ru(II) Bipyridyl Complex

The ion exchange of Na+ cations was used to photosensitise titanates nanotubes (Ti-NTs) with tris(2,2'-bipyridine)ruthenium(II) cations (Ru(bpy)32+); this yielded a light-sensitised Ti-NTs composite denoted as (Ru(bpy)3)Ti-NTs, exhibiting the characteristic absorption of Ru(bpy)32+ in visible light. Incident photon-to-current efficiency (IPCE) measurements and the photocatalytic reduction of methyl viologen reaction confirmed that in the photosensitisation of the (Ru(bpy)3)Ti-NTs composite, charge transfer and charge separation occur upon excitation by ultraviolet and visible light irradiation. The photocatalytic potential of titanate nanotubes was tested in the water-splitting reaction and the H2 evolution reaction using a sacrificial agent and showed photocatalytic activity under various light sources, including xenon-mercury lamp, simulated sunlight, and visible light. Notably, in the conditions of the H2 evolution reaction when (Ru(bpy)3)Ti-NTs were submitted to simulated sunlight, they exceeded the photocatalytic activity of pristine Ti-NTs and TiO2 by a factor of 3 and 3.5 times, respectively. Also, (Ru(bpy)3)Ti-NTs achieved the photocatalytic water-splitting reaction under simulated sunlight and visible light, producing, after 4 h, 199 and 282 μmol×H2×gcat-1. These results confirm the effective electron transfer of Ru(bpy)3 to titanate nanotubes. The stability of the photocatalyst was evaluated by a reuse test of four cycles of 24 h reactions without considerable loss of catalytic activity and crystallinity.

Impact of the bromination of carbazole-based D-π-A organic dyes on their optical and electrochemical properties and visible-light-driven hydrogen evolution

Brominated dyes, 2C-n (n = 1-5), 3C-4 and 4C-4, were prepared through bromination of three carbazole-based D-π-A dyes, 2C, 3C and 4C with N-bromosuccinimide (NBS). The detailed structures of the brominated dyes were confirmed by ¹H NMR spectroscopy and mass spectrometry (MS). The introduction of the Br atom on the 1,8-positon of carbazole moieties led to blueshifted UV-vis and photoluminescence (PL) spectra, increased initial oxidation potentials and enlarged dihedral angles, indicating bromination enhanced non-planarity of the dye molecules. In the hydrogen production experiments, with the increase of the Br content in brominated dyes, the photocatalytic activity increased continuously (except 2C-1). The dye-sensitized Pt/TiO_2, 2C-4@T, 3C-4@T and 4C-4@T, exhibited high hydrogen production efficiencies of 655.4, 877.9 and 905.6 μmol h^-1 g^-1, respectively, which were 4-6-fold higher than those of 2C@T, 3C@T and 4C@T. The enhanced performance of photocatalytic hydrogen evolution was attributed to decreased dye aggregation resulting from the highly non-planar molecular structures of the brominated dyes.

Photocatalyst-Mediator Interface Modification by Surface-Metal Cations of a Dye-Sensitized H2 Evolution Photocatalyst

To develop highly active H₂ evolving dye-sensitized photocatalysts (DSPs) applicable for Z-scheme water splitting, we synthesized a series of Ru(II)-dye-double-layered DSPs, X'-RuCP⁶-Zr-RuP⁶@Pt-TiO₂ (X'-DSP) with different surface-bound metal cations (X' = Fe²⁺, Y³⁺, Zr⁴⁺, Hf⁴⁺, and Bi³⁺). In 0.5 M KI aqueous solution, the photocatalytic H₂ evolution activity under blue light irradiation (λ = 460 ± 15 nm) increased in the following order: nonmetal-modified DSP, H⁺-DSP (turn over number for 6 h irradiation = 35.2) < Fe²⁺-DSP (54.9) ≈ Bi³⁺-DSP (55.2) < Hf⁴⁺-DSP (65.5) ≈ Zr⁴⁺-DSP (68.3) ≈ Y³⁺-DSP (71.5), suggesting that the redox-inactive and highly charged metal cations tend to improve the electron donation from the iodide electron mediator. On the other hand, DSPs having heavy metal cations, Hf⁴⁺-DSP (18.4) and Bi³⁺-DSP (16.6), exhibited better activity under green light irradiation (λ = 530 ± 15 nm) than Zr⁴⁺-DSP (15.7) and H⁺-DSP (7.80), implying the contribution of a heavy atom effect of the surface-bound metal cation to partially allow the spin-forbidden metal-to-ligand charge-transfer excitation.

Phosphate Group-Derivated Bipyridine-Ruthenium Complex and Titanium Dioxide Nanoparticles for Electrochemical Sensing of Protein Kinase Activity

Monitoring of protein kinase activity is of significance for fundamentals of biochemistry, biomedical diagnose, and drug screening. To reduce the usage of a relatively complicated bio-labeled signal probe, the phosphate group-derivated bipyridine-ruthenium (Pbpy-Ru) complex and titanium dioxide nanoparticles (TiO₂ NPs) were employed as signal probes to develop an electrochemical sensor for evaluating the protein kinase A (PKA) activity. Through the specific interaction between the phosphate groups and TiO₂ NPs, the preparation of a Pbpy-Ru-TiO₂ NP signal probe and its linkage with the phosphorylated PKA substrate peptides could be performed in a simple and effective way. The tethering of Pbpy-Ru onto the TiO₂ NP surface does not degrade the electrochemical property of the complex. The Pbpy-Ru-TiO₂ NP probe exhibits well-defined redox signals at about 1.0 V versus Ag/AgCl reference and notably has about fivefold current response than that of the TiO₂ NPs with physically adsorbed tris-(bipyridine)-Ru. The PKA activity evaluation was realized by measuring the electrochemical response of the Pbpy-Ru-TiO₂ NPs at the phosphorylated peptide-assembled electrode. Operating at optimal conditions, the cathodic signals at the potential of 1.03 V exhibit a good linearity with the PKA concentrations of 0.5-40 U mL^-1. The electrochemical sensor shows good selectivity, low detection limit (0.2 U mL^-1, signal/noise = 3), qualified reproducibility, and satisfactory applicability for PKA determination in the cell lysate. The Pbpy-Ru-TiO₂ NPs/electrode system would be an excellent electrochemical platform for protein phosphorylation monitoring and sensing.

Two Excited State Collaboration of Heteroleptic Ir(III)-Coumarin Complexes for H2 Evolution Dye-Sensitized Photocatalysts

Interfacial electron injection from a photoexcited surface-immobilized dye to a semiconductor substrate is a key reaction for dye-sensitized photocatalysts. We previously reported that the molecular orientation of heteroleptic Ir(III) photosensitizer on the TiO2 nanoparticle surface was important for efficient interfacial electron injection. In this work, to overcome the weak light absorption ability of heteroleptic Ir(III) photosensitizer and to improve the photoinduced charge-separation efficiency at the dye–semiconductor interface, we synthesized two heteroleptic Ir(III) complexes with different coumarin dyes, [Ir(C6)2(H4CPbpy)]Cl and [Ir(C30)2(H4CPbpy)]Cl [Ir-CX; X = 6 or 30; HC6 = 3-(2-enzothiazolyl)-7-(diethylamino)coumarin, HC30 = 3-(2-N-methylbenzimidazolyl)-7-N,N-diethylaminocoumarin, H4CPbpy = 4,4′-bis(methylphosphonic acid)-2,2′-bipyridine], as the cyclometalated ligands and immobilized them on the surface of Pt-cocatalyst-loaded TiO2 nanoparticles. Ultraviolet-visible absorption and emission spectroscopy revealed that the singlet ligand-centered (1LC) absorption and triplet 3LC emission bands of Ir-C30 occurred at shorter wavelengths than those of Ir-C6, while time-dependent density-functional-theory data suggested that the ligand-to-ligand charge transfer (LLCT) excited states of the two complexes were comparable. The photocatalytic H2 evolution activity of the Ir-C6-sensitized Pt-TiO2 nanoparticles (Ir-C6@Pt-TiO2) under visible light irradiation (λ > 420 nm) was higher than that of Ir-C30@Pt-TiO2. In contrast, their activities were comparable under irradiation with monochromatic light (λ = 450 ± 10 nm), which is absorbed comparably by both Ir-CX complexes. These results suggest that the internal conversion from the higher-lying LC state to the LLCT state effectively occurs in both Ir-CX complexes to trigger electron injection to TiO2.