Abstract. Titanium dioxide (TiO2) polymorphs are widely used in many energy-related applications due to their peculiar electronic and physicochemical properties. The electronic structures of brookite TiO 2 surfaces doped with transition metal ruthenium have been investigated by ab initio band calculations based on the density functional theory with the planewave ultrasoft pseudopotential method. The generalized gradient approximation (GGA) was used in the scheme of Perdew-Burke-Ernzerhof (PBE) to describe the exchange-correlation functional. All calculations were carried out with CASTEP (Cambridge Sequential Total Energy Package) code in Materials Studio of Accelrys Inc. The surface structures of Ru doped TiO 2 were constructed by cleaving the 1 × 1 × 1 optimized bulk structure of brookite TiO2. The results indicate that Ru doping can narrow the band gap of TiO2, leading to the improvement in the photoreactivity of TiO2, and simultaneously maintain strong redox potential. The theoretical calculations could provide meaningful guide to develop more active photocatalysts with visible light response.
IntroductionIn the last decades, properties for titanium dioxide (TiO 2 ) polymorphs have been the subject of many experimental and computational studies, the most common polymorphs being the minerals rutile, anatase, and brookite [1]. A large number of applications of TiO 2 in materials science is, almost without exception, ultimately a result of the facile electron-transfer processes that occur at the interface between the semiconductor and adsorbed molecules [1][2][3][4][5][6][7]. When photons are excited, TiO 2 is a good electron and hole donor and can, therefore promote photocatalytic processes at its interface [3]. Its photocatalytic properties, in addition to its abundance, low cost, stability, and low toxicity, are the basis for its use in solar cells [4]. However, brookite is the rarest of the natural occurring TiO 2 polymorphs, and it is the most difficult phase to prepare in the laboratory [1]. As a result, the properties of pure brookite are poorly known. TiO 2 can only show photocatalytic activity under ultraviolet (UV) light irradiation (λ < 387.5 nm) that accounts for only a small portion of solar energy (approximately 5%), in contrast to visible light for a major part of solar energy (approximately 45%), but can be photosensitized by the adsorption of chromophores that, when excited, inject electrons into the TiO 2 conduction band [8]. In spite of a large number of publications on pure and doped TiO 2 many aspects of
The theoretical and computational studies of dye sensitized solar cells (DSSCs) can contribute to a deeper understanding of these types of solar cells. The DSSCs are the novel design of solar cells which could be used as power producing windows or skylights. They represent a particular promising approach to a direct conversion of sunlight into electrical energy at low cost and with high efficiency. The light adsorption occurs in dye molecules adsorbed on a highly porous structure of TiO2 film. Despite the progress in the efficiency and stability of these solar cells, there is still a room of research on some of their operational aspects that are still not understood. One process, for which there is limited information, is the time taken to upload the dye on the TiO2 nanoporous film. The processes followed experimentally for dye uptake is by dipping the TiO2 semiconductor electrode into the dye solution for periods of several hours to several days. However, such long dipping times are not economical for industrial production of DSSCs. The factors controlling this process are not yet fully investigated. We propose a simple model based on the Langmuir isotherms to study and understand the diffusion and adsorption of the dye molecules in TiO2 films. Our computational modelling results show that the adsorption of dye into the TiO2 nanotubes film is controlled by the diffusion coefficient, the adsorptiondesorption ratio and the initial dye concentration. Our results show that the initial dye concentration plays an important role on the surface coverage. It is also noted that for a higher concentration shorter immersion time is needed for the sufficient surface coverage. Furthermore, it is observed that for the large values of the adsorption-desorption ratio there is a delay in the diffusion of dye molecules on the surface.
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