An unexplored role of the CrOx shell in an elaborated Rh/CrOx core–shell cocatalyst for photocatalytic water splitting: a selective electron transport pathway from semiconductors to core metals, boosting charge separation and H₂ evolution

A core–shell structured Rh/CrOx cocatalyst has endowed various semiconductors with high efficiency in water-splitting photocatalysis, where thin CrOx layers on Rh have been assumed to be physical blockers of O₂ to the metal surface to suppress unfavorable reverse reactions (e.g., catalytic H₂O formation from H₂ and O₂). Herein, we propose another unexplored but favorable function of CrOx layers: a selective electron transport pathway from photocatalysts to the Rh core boosting charge separation and H₂ production. The subsequent loading of CrOx layers onto Rh increased the rate of visible light H₂ evolution of a Bi₄NbO₈Cl photocatalyst, even in a half reaction with a hole scavenger where O₂ does not evolve. Transient absorption spectroscopy revealed that the CrOx layer increases the electron path from Bi₄NbO₈Cl to Rh. Importantly, the highest H₂-evolution activity was obtained by simultaneous photodeposition using CrIII and RhIII precursors, which had not yet been examined. In this sample, Rh nanoparticles were enclosed by an amorphous CrOx shell, where Rh particles were less directly attached to the semiconductor. Therein, CrOx inserted between Bi₄NbO₈Cl and Rh effectively suppresses undesirable hole transfer from Bi₄NbO₈Cl to Rh, while such hole transfer partially occurs when they are in direct contact. These results indicated that CrOx functions as a selective electron transport pathway and improves the H₂ evolution activity. Although the development strategy of cocatalysts has so far focused on surface redox reactions, this study offers a new approach for the design of highly efficient cocatalysts based on the carrier transfer process, especially at semiconductor–cocatalyst interfaces

Manipulation of charge carrier flow in Bi₄NbO₈Cl nanoplate photocatalyst with metal loading

Separation of photoexcited charge carriers in semiconductors is important for efficient solar energy conversion and yet the control strategies and underlying mechanisms are not fully established. Although layered compounds have been widely studied as photocatalysts, spatial separation between oxidation and reduction reaction sites is a challenging issue due to the parallel flow of photoexcited carriers along the layers. Here we demonstrate orthogonal carrier flow in layered Bi₄NbO₈Cl by depositing a Rh cocatalyst at the edges of nanoplates, resulting in spatial charge separation and significant enhancement of the photocatalytic activity. Combined experimental and theoretical studies revealed that lighter photogenerated electrons, due to a greater in-plane dispersion of the conduction band (vs. valence band), can travel along the plane and are readily trapped by the cocatalyst, whereas the remaining holes hop perpendicular to the plane because of the anisotropic crystal geometry. Our results propose manipulating carrier flow via cocatalyst deposition to achieve desirable carrier dynamics for photocatalytic reactions in layered compounds

高効率太陽光エネルギー変換に向けたBi系層状酸ハロゲン化物光触媒の設計

京都大学新制・課程博士博士(工学)甲第23911号工博第4998号新制||工||1780(附属図書館)京都大学大学院工学研究科物質エネルギー化学専攻(主査)教授 阿部 竜, 教授 陰山 洋, 教授 藤田 晃司学位規則第4条第1項該当Doctor of Philosophy (Engineering)Kyoto UniversityDFA

Theoretical and Experimental Studies on the Near‐Infrared Photoreaction Mechanism of a Silicon Phthalocyanine Photoimmunotherapy Dye: Photoinduced Hydrolysis by Radical Anion Generation

Ligand release from IR700, a silicon phthalocyanine dye used in near-infrared (NIR) photoimmunotherapy, initiates cancer cell death after NIR absorption, although its photochemical mechanism has remained unclear. This theoretical study reveals that the direct Si-ligand dissociation by NIR light is difficult to activate because of the high dissociation energy even in excited states, i. e., >1.30 eV. Instead, irradiation generates the IR700 radical anion, leading to acid-base reactions with nearby water molecules (i. e., calculated pK(b) for the radical anion is 7.7) to produce hydrophobic ligand-released dyes. This suggests two possibilities: (1) water molecules participate in ligand release and (2) light is not required for Si-ligand dissociation as formation of the IR700 radical anion is sufficient. Experimental evidence confirmed possibility (1) by using O-18-labeled water as the solvent, while (2) is supported by the pH dependence of ligand exchange, providing a complete description of the Si-ligand bond dissociation mechanism

Development of a red-shifted photosensitizer for near-infrared photoimmunotherapy of cancer

Near-infrared photoimmunotherapy (NIR-PIT) is a recently described method for cancer treatment that utilizes an antibody-conjugated phthalocyanine photosensitizer and NIR light. In NIR-PIT, light of 690 nm wavelength is used to activate a photosensitizer, IR700, while longer-wavelength light penetrates deeper into tissues. Thus, more effective NIR-PIT would be achieved by using photosensitizers that are activated by longer-wavelength light. The absorption wavelength would be red-shifted by destabilizing the highest occupied molecular orbital (HOMO) energy level by introducing electron donating groups at the α positions of a phthalocyanine ring. In this study, we developed a red-shifted photosensitizer for NIR-PIT, KA800, whose absorption wavelength was red-shifted by the introduction of ethoxy groups to IR700. As intended, the absorption maximum of KA800 was red-shifted compared to IR700 by 84 nm. Although phototoxicity of the antibody-KA800 (Ab-KA800) conjugate was observed in cultured cancer cells, no therapeutic effect was observed in mice. This is because the cytotoxicity of Ab-KA800 was mainly due to singlet oxygen, which can be quenched by abundant antioxidants in vivo. KA800 had low reactivity with respect to axial ligand cleavage required for inducing cell death via aggregate formation, a unique cytotoxic mechanism in NIR-PIT. The axial ligand cleavage proceeds via the anion radical formation of the photosensitizer, and KA800 was found to be less likely to receive an electron than IR700. This may be due to the destabilization of the HOMO energy level of KA800. Therefore, our findings suggest that stabilizing the lowest unoccupied molecular orbital (LUMO) energy level would be better than destabilizing the HOMO energy level for developing a red-shifted photosensitizer for NIR-PIT

Theoretical and Experimental Studies on the Near-Infrared Photoreaction Mechanism of a Silicon Phthalocyanine Photoimmunotherapy Dye : Photoinduced Hydrolysis by Radical Anion Generation

Ligand release from IR700, a silicon phthalocyanine dye used in near-infrared (NIR) photoimmunotherapy, initiates cancer cell death after NIR absorption, although its photochemical mechanism has remained unclear. This theoretical study reveals that the direct Si-ligand dissociation by NIR light is difficult to activate because of the high dissociation energy even in excited states, i. e., >1.30 eV. Instead, irradiation generates the IR700 radical anion, leading to acid-base reactions with nearby water molecules (i. e., calculated pK(b) for the radical anion is 7.7) to produce hydrophobic ligand-released dyes. This suggests two possibilities: (1) water molecules participate in ligand release and (2) light is not required for Si-ligand dissociation as formation of the IR700 radical anion is sufficient. Experimental evidence confirmed possibility (1) by using O-18-labeled water as the solvent, while (2) is supported by the pH dependence of ligand exchange, providing a complete description of the Si-ligand bond dissociation mechanism