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    Öğe
    Enhanced Photocatalytic Hydrogen Evolution by Star-Shaped Viologen-Sensitized TiO2 Nanoparticles
    (Amer Chemical Soc, 2023) Turgut, Kubra; Altinisik, Sinem; Yanalak, Gizem; Koyuncu, Sermet; Hatay Patir, Imren
    Viologens are well-suited for serving as electron-transfer mediators in redox systems due to their low reduction potential and ability to form stable radical cations. Because of this feature, viologens can play a key role in modifying semiconductors toward enhanced photocatalytic performance. Herein, a series of hybrid photocatalysts composed of TiO2 nanoparticles and star-shaped viologen derivatives with different alkyl chains [TPCBP-X_TiO2; X: ethyl (E), butyl (B), hexyl (H) and octyl (O)] were prepared for the photocatalytic hydrogen evolution from water under visible-light irradiation. The TPCBP-X molecules not only provide photosensitization of TiO2 nanoparticles in the visible-light region but also act as an efficient electron-transfer mediator for the transfer of photoinduced electrons to TiO2 and Pt. Among these photocatalysts, TPCBP-E_TiO2 exhibited a 1.013 mmol g(-1) h(-1) H-2 evolution rate with an apparent quantum yield (AQY) of 20.15% (470 nm), which dramatically improved hydrogen evolution activity among the other structures [TPCBP-X_TiO2 (X; B, H, O)] due to the more porous and uniform surface, resulting in its low barrier effect for electron transfer. In addition, in the presence of a Pt cocatalyst, TPCBP-E_TiO2 yielded a H-2 evolution rate of 17.7 mmol g(-1), which is about 2.2 times higher than that of pure TPCBP-E_TiO2 (8.1 mmol g(-1)) after 8 h of visible-light illumination.
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    Öğe
    Viologen-Based Covalent Organic Frameworks toward Metal-Free Highly Efficient Photocatalytic Hydrogen Evolution
    (American Chemical Society, 2023) Altınışık, Sinem; Yanalak, Gizem; Hatay Patır, İmren; Koyuncu, Sermet
    Covalent organic frameworks (COFs) have shown promise in the field of photocatalysts for hydrogen evolution. Many studies have been carried out using various electroactive and photoactive moieties such as triazine, imide, and porphyrin to produce COFs with different geometric structures and units. Electron transfer mediators like viologen and their derivatives can accelerate the transfer of electrons from photosensitizers to active sites. Herein, the combination of a biphenyl-bridged dicarbazole electroactive donor skeleton with a viologen acceptor structure is reported for the photocatalytic hydrogen evolution of novel COF structures with various alkyl linkers {TPCBP X-COF [X = ethyl (E), butyl (B), and hexyl (H)]}. The structures became more flexible and exhibited less crystal behavior as the length of the alkyl chain increased according to scanning and transmission electron microscopy images, X-ray diffraction analyses, and theoretical three-dimensional geometric optimization. In comparison, the H2 evolution rate of the TPCBP B-COF (12.276 mmol g-1) is 2.15 and 2.38 times higher than those of the TPCBP H-COF (5.697 mmol h-1) and TPCBP E-COF (5.165 mmol h-1), respectively, under visible light illumination for 8 h. The TPCBP B-COF structure is one of the best-performing catalysts for the corresponding photocatalytic hydrogen evolution in the literature, producing 1.029 mmol g-1 h-1 with a high apparent quantum efficiency of 79.69% at 470 nm. Our strategy provides new aspects for the design of novel COFs with respect to future metal-free hydrogen evolution by using solar energy conversion.