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      Insights into the Mechanism of a Covalently Linked Organic Dye–Cobaloxime Catalyst System for Dye‐Sensitized Solar Fuel Devices

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          Abstract

          A covalently linked organic dye–cobaloxime catalyst system based on mesoporous NiO is synthesized by a facile click reaction for mechanistic studies and application in a dye‐sensitized solar fuel device. The system is systematically investigated by photoelectrochemical measurements, density functional theory, time‐resolved fluorescence, transient absorption spectroscopy, and photoelectron spectroscopy. The results show that irradiation of the dye–catalyst on NiO leads to ultrafast hole injection into NiO from the excited dye, followed by a fast electron transfer process to reduce the catalyst. Moreover, the dye adopts different structures with different excited state energies, and excitation energy transfer occurs between neighboring molecules on the semiconductor surface. The photoelectrochemical experiments also show hydrogen production by this system. The axial chloride ligands of the catalyst are released during photocatalysis to create the active sites for proton reduction. A working mechanism of the dye–catalyst system on the photocathode is proposed on the basis of this study.

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          Most cited references55

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          Photochemical and photoelectrochemical reduction of CO2.

          The recent literature on photochemical and photoelectrochemical reductions of CO(2) is reviewed. The different methods of achieving light absorption, electron-hole separation, and electrochemical reduction of CO(2) are considered. Energy gap matching for reduction of CO(2) to different products, including CO, formic acid, and methanol, is used to identify the most promising systems. Different approaches to lowering overpotentials and achieving high chemical selectivities by employing catalysts are described and compared.
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            Photoassisted overall water splitting in a visible light-absorbing dye-sensitized photoelectrochemical cell.

            Iridium oxide nanoparticles stabilized by a heteroleptic ruthenium tris(bipyridyl) dye were used as sensitizers in photoelectrochemical cells consisting of a nanocrystalline anatase anode and a Pt cathode. The dye coordinated the IrO(2) x nH(2)O nanoparticles through a malonate group and the porous TiO(2) electrode through phosphonate groups. Under visible illumination (lambda > 410 nm) in pH 5.75 aqueous buffer, oxygen was generated at anode potentials positive of -325 mV vs Ag/AgCl and hydrogen was generated at the cathode. The internal quantum yield for photocurrent generation was ca. 0.9%. Steady-state luminescence and time-resolved flash photolysis/transient absorbance experiments were done to measure the rates of forward and back electron transfer. The low quantum yield for overall water splitting in this system can be attributed to slow electron transfer (approximately 2.2 ms) from IrO(2) x nH(2)O to the oxidized dye. Forward electron transfer does not compete effectively with the back electron transfer reaction from TiO(2) to the oxidized dye, which occurred on a time scale of 0.37 ms.
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              Molecular engineering of a cobalt-based electrocatalytic nanomaterial for H₂ evolution under fully aqueous conditions.

              The viability of a hydrogen economy depends on the design of efficient catalytic systems based on earth-abundant elements. Innovative breakthroughs for hydrogen evolution based on molecular tetraimine cobalt compounds have appeared in the past decade. Here we show that such a diimine-dioxime cobalt catalyst can be grafted to the surface of a carbon nanotube electrode. The resulting electrocatalytic cathode material mediates H(2) generation (55,000 turnovers in seven hours) from fully aqueous solutions at low-to-medium overpotentials. This material is remarkably stable, which allows extensive cycling with preservation of the grafted molecular complex, as shown by electrochemical studies, X-ray photoelectron spectroscopy and scanning electron microscopy. This clearly indicates that grafting provides an increased stability to these cobalt catalysts, and suggests the possible application of these materials in the development of technological devices.
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                Author and article information

                Contributors
                haining.tian@kemi.uu.se
                Journal
                ChemSusChem
                ChemSusChem
                10.1002/(ISSN)1864-564X
                CSSC
                Chemsuschem
                John Wiley and Sons Inc. (Hoboken )
                1864-5631
                1864-564X
                03 May 2017
                09 June 2017
                : 10
                : 11 ( doiID: 10.1002/cssc.v10.11 )
                : 2480-2495
                Affiliations
                [ 1 ] Department of Chemistry-Ångström Laboratory Uppsala University Box 523 SE 751 20 Uppsala Sweden
                [ 2 ] Department of Physics and Astronomy Uppsala University Box 516 SE 751 20 Uppsala Sweden
                Author information
                http://orcid.org/0000-0002-4665-3520
                http://orcid.org/0000-0001-6897-2808
                Article
                CSSC201700285
                10.1002/cssc.201700285
                5488223
                28338295
                d12d4cc4-f41a-4e54-a26a-a2351b3aa0ce
                © 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

                This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

                History
                : 14 February 2017
                : 23 March 2017
                Page count
                Figures: 20, Tables: 2, References: 62, Pages: 16, Words: 0
                Funding
                Funded by: Energimyndigheten
                Funded by: Knut och Alice Wallenbergs Stiftelse
                Funded by: Stiftelsen Olle Engkvist Byggmästare
                Funded by: Vetenskapsrådet
                Funded by: Göran Gustafssons Stiftelse för Naturvetenskaplig och Medicinsk Forskning
                Funded by: Stiftelsen Åforsk
                Categories
                Full Paper
                Full Papers
                Custom metadata
                2.0
                cssc201700285
                June 9, 2017
                Converter:WILEY_ML3GV2_TO_NLMPMC version:5.1.2 mode:remove_FC converted:06.07.2017

                Sustainable & Green chemistry
                click chemistry,dye-sensitized solar cells,hydrogen,photocatalysis,reaction mechanisms

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