Artificial Photosynthesis - Use of a Ferroelectric Photocatalyst
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Artificial Photosynthesis - Use of a Ferroelectric Photocatalyst Steve Dunn a and Matt Stockb a
School and Engineering and Materials, Queen Mary University of London, E1 4NS, UK
b
Cranfield University, Cranfield, MK43 0AL
Email: [email protected] ABSTRACT The solid-gas phase photoassisted reduction of carbon dioxide (artificial photosynthesis) was performed using ferroelectric lithium niobate and titanium dioxide as photocatalysts. Illumination with a high pressure mercury lamp and visible sunlight showed lithium niobate achieved unexpectedly high conversion of CO2 to products despite the low levels of band gap light available and outperformed titanium dioxide under the conditions used. The high reaction efficiency of lithium niobate is explained due to its strong remnant polarization (70 μC/cm2) thought to allow longer lifetime of photo induced carriers as well as an alternative reaction pathway. INTRODUCTION The threat of climate change due to rising levels of green house gases from human activity has made the need for renewable sources of energy a priority. One promising area of providing green energy is artificial photosynthesis[1], a process that mimics mechanisms of nature. A semiconductor can be used as a photocatalyst absorbing light and using the energy to chemically convert CO2 and water into fuel stock in a carbon neutral mechanism. In this work we describe artificial photosynthesis taking place on ferroelectric lithium niobate (LiNbO3) producing formic acid and formaldehyde at efficiencies exceeding those of other unmodified metal oxide photocatalysts such as titanium dioxide (TiO2). Artificial photosynthesis has been investigated using a variety of approaches[2] including replication of the chemical reactions that take place in plants[3] and concerted efforts using ruthenium complexes and structures as a dye[4] to sensitise wide band gap semiconductors to visible light. Metal oxide semiconductor systems including WO3, Fe2O3 and ZnO have also proven effective as photoactive surfaces for water purification[5], alternative electron transport materials in nanostructured photovoltaics[6] and as a means to sensitise titanium dioxide to longer wavelength radiation through doping. One aspect of surface photochemistry that has not been extensively addressed is to use materials that sustain a dipole to separate the photoinduced electrons and holes - ferroelectric materials. Early work by [7] showed selective oxidation and reduction reactions take place on BaTiO3. Subsequent work on other ferroelectric systems PbZr0.3Ti0.7O3[8; 9] and LiNbO3[10] indicated that the dipole in the ferroelectric material determines the space charge layer structure. The surface charge has also been shown to interact with the species producing a tightly bound
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double layer[11]. There has been discussion that such a tightly bound layer can alter the nature of bonding in physisorbed materials[12]. LiNbO3 was selected due to remnant polarization of 70 μC/cm2 [13] as compared to 30 μC/cm2 [14] (KNbO3) and 25 μC/cm2 [15] (PZT). Li
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