Photosensitive titanium oxide sols and gels for solar energy conversion and storage

author: Mireille Richard, Institute of Materials (IMN), University of Nantes
published: Jan. 18, 2008,   recorded: October 2007,   views: 5476

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Due to the potential applications in the field of environmental protection, the photochemistry of TiO2 is a fast growing area both in terms of research and commercial activity. Beside to the white pigment properties of rutile and anatase (e.g. paints and cosmetic products), titanium dioxide is used in heterogeneous catalysis and photocatalysis (water purification, air cleaning), in photoelectrochemical solar cells for the production of hydrogen and electricity, as an active layer in the design of electrochromic devices, as a gas sensor, as a corrosion-protective coating, in ceramics and in electric devices such as varistors, to name few. In such applications, the performance of titanium oxide could be optimized with specific nanostructural control over the morphology of the material. Under UV irradiation, an electron-hole pair is generated in TiO2 then technological devices are based on chemical reactions or induced electron transfers. Due to the large band gap of TiO2 (3.2eV), only 10% of the solar spectrum is used. A major objective for future work is the development of a semiconductor photocatalyst film which is able to utilize visible as well as UV light. Nanoscience has the potential to provide entirely new classes of materials with capabilities that transcend these limitations and generate the performance breakthroughs required for a viable economy based on sustainable energy. Recent investigations in this area allowed us to synthesize novel photo-sensitive titanium oxide sols and gels by controlling the condensation of titanium species in non aqueous solvents. Depending on different parameters such as concentration in Ti4+, ageing or thermal treatment, to emphasise few, the structuration of the inorganic framework leads to various layered structure. The adsorbed organic species control the growth of the nano-objects present in the sol or gel. Due to the enhanced surface area to volume ratio, these nanostructured sols and gels produce singular photo-electrochemical properties that are drastically different from their bulk counterparts. When irradiated, these materials can absorb photons with a lower energy than the bandgap energy of the original semiconductor, and thus a significant increase in the limiting efficiency of conventional solar cells is expected. These new materials are principally characterized by a partially occupied intermediate band, isolated from the valence and conduction bands. Our purpose is to use these intermediate band materials as sensitizers in both third generation photoelectrochemical solar cells and photo-batteries (ultracapacitors).

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