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      Phase-controllable synthesis of MOF-templated maghemite–carbonaceous composites for efficient photocatalytic hydrogen production

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          Abstract

          A facile approach for the phase-controllable synthesis of maghemite–carbonaceous composites and their application for improved photocatalytic H 2 production have been realized.

          Abstract

          Solar water splitting to produce H 2 represents a solution with high potential for the current severe energy and environmental issues. Iron oxides are earth-abundant and nontoxic, with narrow bandgaps and suitable valence band positions for the visible-light-driven water oxidation reaction; however, an energy limitation for hydrogen generation is encountered due to the improper conduction band level. Herein, we demonstrated that this limitation could be overcome by the incorporation of small amounts of GO in the metal–organic framework (MOF)-templated synthesis of iron oxide, affording a uniform and highly ordered ferrite octahedral nanostructure embedded on graphene nanosheets. Such structural superiorities would result in a highly synergistic effect between Fe 2O 3 and reduced graphene oxide (rGO), affording an elevated flat band potential and a promoted photogenerated charge carrier separation and transportation. As a consequence, the resulting maghemite–carbonaceous composite exhibited a high photocatalytic H 2 evolution rate of 318.0 μmol h −1 g −1 in the absence of noble metal cocatalysts and external bias. This work provides for the first time an ideal pathway for the utilization of Fe 2O 3 as the dominant component of a nanocomposite in efficient photocatalytic H 2 production, as well as the prospect of developing highly active photocatalysts for overall water splitting. In addition, different phases of iron oxide, including maghemite (γ-Fe 2O 3), hematite (α-Fe 2O 3) and magnetite (Fe 3O 4), and their carbonaceous composites can be obtained through the cautiously selected thermolysis of Fe-MOF and MOF/GO composites. The maghemite phase could be maintained with high saturation magnetization values under a large temperature gradient, implying a great potential for applications in magnetism and biomedicine-related research fields.

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          Most cited references 49

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          Metal-organic frameworks (MOFs).

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            Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials

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              Roles of cocatalysts in photocatalysis and photoelectrocatalysis.

              Since the 1970s, splitting water using solar energy has been a focus of great attention as a possible means for converting solar energy to chemical energy in the form of clean and renewable hydrogen fuel. Approaches to solar water splitting include photocatalytic water splitting with homogeneous or heterogeneous photocatalysts, photoelectrochemical or photoelectrocatalytic (PEC) water splitting with a PEC cell, and electrolysis of water with photovoltaic cells coupled to electrocatalysts. Though many materials are capable of photocatalytically producing hydrogen and/or oxygen, the overall energy conversion efficiency is still low and far from practical application. This is mainly due to the fact that the three crucial steps for the water splitting reaction: solar light harvesting, charge separation and transportation, and the catalytic reduction and oxidation reactions, are not efficient enough or simultaneously. Water splitting is a thermodynamically uphill reaction, requiring transfer of multiple electrons, making it one of the most challenging reactions in chemistry. This Account describes the important roles of cocatalysts in photocatalytic and PEC water splitting reactions. For semiconductor-based photocatalytic and PEC systems, we show that loading proper cocatalysts, especially dual cocatalysts for reduction and oxidation, on semiconductors (as light harvesters) can significantly enhance the activities of photocatalytic and PEC water splitting reactions. Loading oxidation and/or reduction cocatalysts on semiconductors can facilitate oxidation and reduction reactions by providing the active sites/reaction sites while suppressing the charge recombination and reverse reactions. In a PEC water splitting system, the water oxidation and reduction reactions occur at opposite electrodes, so cocatalysts loaded on the electrode materials mainly act as active sites/reaction sites spatially separated as natural photosynthesis does. In both cases, the nature of the loaded cocatalysts and their interaction with the semiconductor through the interface/junction are important. The cocatalyst can provide trapping sites for the photogenerated charges and promote the charge separation, thus enhancing the quantum efficiency; the cocatalysts could improve the photostability of the catalysts by timely consuming of the photogenerated charges, particularly the holes; most importantly, the cocatalysts catalyze the reactions by lowering the activation energy. Our research shows that loading suitable dual cocatalysts on semiconductors can significantly increase the photocatalytic activities of hydrogen and oxygen evolution reactions, and even make the overall water splitting reaction possible. All of these findings suggest that dual cocatalysts are necessary for developing highly efficient photocatalysts for water splitting reactions.
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                Author and article information

                Journal
                JMCAET
                Journal of Materials Chemistry A
                J. Mater. Chem. A
                Royal Society of Chemistry (RSC)
                2050-7488
                2050-7496
                2018
                2018
                : 6
                : 8
                : 3571-3582
                Affiliations
                [1 ]State Key Laboratory of Pulp and Paper Engineering
                [2 ]School of Chemistry and Chemical Engineering
                [3 ]South China University of Technology
                [4 ]Guangzhou 510640
                [5 ]China
                Article
                10.1039/C7TA10284D
                © 2018
                Product
                Self URI (article page): http://xlink.rsc.org/?DOI=C7TA10284D

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