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Photocatalytic Water Oxidation Under Visible Light Irradiation By A Pyrochlore Oxide : Rhodium Substitution Into Yttrium Titanate
| Content Provider | Semantic Scholar |
|---|---|
| Author | Kiss, Borbala Didier, Christophe R. Johnson, Timothy C. Manning, Troy D. Dyer, Matthew S. Alexander, Justin Cowan Claridge, John B. Darwent, James R. Rosseinsky, Matthew J. |
| Copyright Year | 2017 |
| Abstract | A stable visible light-driven photocatalyst ( 450 nm) for water oxidation is reported. Rhodium substitution into the pyrochlore Y2Ti2O7 is demonstrated through observation of Vegard’s law evolution of the unit cell parameters with Rh content, to a maximum content of 3% dopant. Substitution renders the solid solutions visible light active. The overall rate of oxygen evolution is comparable to WO3 but with superior light harvesting and surface area normalised turnover rates, making Y2Ti1.94Rh0.06O7 an excellent candidate for use in a Z-scheme water splitting system. The photocatalytic splitting of water by visible light using semiconductor materials has been proposed as a route to sustainable hydrogen generation. Of the systems available, Zscheme photocatalysis offers several advantages over single particle photocatalysis or photoelectrocatalysis. The Z-scheme system is inspired by natural photosynthesis in which two separate photocatalysts are linked by a shuttle redox mediator. In an artificial Z-scheme, each of the water splitting halfreactions is performed separately on each of the catalysts. Zscheme systems lower the energy required for photocatalysis as the valence and conduction band edges of the individual photocatalysts do not have to straddle the reduction potentials of H/H2 and O2/H2O as they do for a single particle photocatalyst. In this way a wider range of visible light can be used. Importantly using the Z-scheme approach, water splitting can take place in a simple reactor without the requirement for potentially costly electrode connections and transparent conducting supports. The production of oxygen is a critical step in solar fuels production and is a requirement for any photocatalytic water splitting system. This half of the water splitting process is more challenging since it typically involves a four-electron process and powerful oxidizing species which can lead to breakdown of the photocatalyst. It is also essential for photo-electrolysis. Whilst significant effort has been expended on developing visible light active hydrogen generating photocatalysts, only a very limited number of stable, visible light active oxygen evolving photocatalysts are known. Monoclinic WO3 has been extensively studied as an oxygen generating photocatalyst and is generally used as the standard comparative material due to its commercial availability and reasonable performance. However WO3 has a relatively large band gap (2.6 eV) limiting its absorption of visible light to < 480 nm. Nanostructured BiVO4 (Eg ~2.4 eV) [6] has been shown to photocatalytically generate oxygen under visible light irradiation and has a similar activity to WO3. Oxynitride perovskites have also displayed photocatalytic oxygen generation but are less stable to photocorrosion than oxide materials unless OH is removed in situ by the addition of La2O3. Recently Ag3PO4 has been demonstrated as a highly efficient oxygen evolving photocatalyst though questions remain over its stability under reaction conditions. Therefore to enable the development of an efficient Z-scheme for water splitting new stable, efficient, visible light active oxygen evolving photocatalysts are urgently required as highlighted in the recent review by Ma et al. The first reported photocatalyst for water splitting under ultraviolet (UV) radiation was TiO2. As a consequence, considerable attention has been given to titanium oxides, including the perovskite SrTiO3 which also possesses photocatalytic activity for water splitting under UV irradiation. Several substitutions for Ti have been evaluated in order to render these materials active under visible light, with Rh reported as one of the most effective; SrTi1-xRhxO3 with Pt cocatalyst in an aqueous CH3OH solution is a highly-active hydrogen evolving photocatalyst under visible light, and has been used with BiVO4 or with WO3 in a Z-scheme to split water with visible light. Calculations suggest that Rh Eg states are located within the conduction band in SrTi1-xRhxO3, allowing visible light excitation from the Rh T2g to generate delocalized electrons which evolve hydrogen, however the formation of Rh inter-gap states, that act as a recombination center for photoexcited carriers, can inhibit hydrogen evolution. The A2B2O7 pyrochlore structure (Figure 1a) consists of corner-sharing BO6 octahedra with A cations in a 2+6 environment; there is one B-O-B angle which is around 135°. The rare-earth R2Ti2O7 pyrochlores thus have some structural similarities with perovskite SrTiO3, which also features a cornersharing TiO6 octahedral network but with angles much closer to 180°. Pyrochlore is an important ternary oxide type which sustains a range of functions and is extensively tuneable by substitution. Abe et al. have shown that pyrochlore Y2Ti2O7 with NiOx co-catalyst can be successfully used as a photocatalyst for water splitting under UV light, with a band gap of 3.5 eV. As this gap is much larger than that of SrTiO3, (3.2 eV), the Rh-derived Eg states in Y2Ti2-xRhxO7 may sit just below the conduction band edge and thus become localised, potentially acting as recombination states or becoming photocatalytically inactive to hydrogen generation. The extensive chemical tuneability of pyrochlore suggests that suitable substitutions would enhance its photocatalytic properties and [a] Miss B. Kiss, Dr. C. Didier, Mr. T. Johnson, Dr. T. D. Manning, Dr. M. S. Dyer, Dr. J. B. Claridge, Prof. J. R. Darwent, Prof. M. J. Rosseinsky Department of Chemistry University of Liverpool Liverpool, L69 7ZD, UK E-mail: M.J.Rosseinsky@liverpool.ac.uk [b] Dr. A. J. Cowan Stephenson Institute for Renewable Energy University of Liverpool Liverpool, L69 7ZF Supporting information for this article is given via a link at the end of the document. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://livrepository.liverpool.ac.uk/3006505/1/Resubmission_ACIE-201407179_highlighted_noendnote_accepted%20version_greyscale%20figures.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
