Photodeposition of Metal Sulfide Quantum Dots on Titanium(IV) Dioxide and its Applications
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Photodeposition of Metal Sulfide Quantum Dots on Titanium(IV) Dioxide and its Applications Hiroaki Tada Department of Applied Chemistry, School of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan ABSTRACT In situ photodeposition techniques taking advantage of the TiO2 photocatalysis have been developed for coupling metal sulfide quantum dots (QDs) and TiO2 at a nonoscale. The coupled metal sulfide-TiO2 systems possess the following characteristics: (I) a large amount of metal sulfides can be directly formed on TiO2 during a fairly short period with excellent reproducibility, (II) the band energies of metal sulfides QDs are widely tunable by irradiation time, (III) metal sulfide QDs can be deposited on not only the external surfaces but also the inner ones of mesoporous TiO2 nanocrystalline films without pore-blocking, (IV) the simple solutionbased technique at low temperature enables the low-cost production, (V) this technique has a wide possibility for coupling TiO2 and narrow gap metal sulfides. These unique features produce the excellent performances of the resulting heteronanojunaction systems as the photoanodes for QD-sensitized solar cells. INTRODUCTION Narrow gap semiconductor represented by metal sulfide-TiO2 coupling systems have attract much interest due to the possible applications to photocatalysts [1,2] and photovoltaics [3,4], where the common crucial process is the visible light-induced interfacial electron transfer (IET) from metal sulfide QDs to TiO2. The self-assembled monolayer (SAM) technique using bifunctional coupling molecules is frequently used for the loading of metal sulfide QDs on the mesoporous TiO2 nanocrystalline films (mp-TiO2) [5,6], which is the key material of the dyeand QD-sensitized solar cells [7]. While this method allows us to precisely control the particle size, the loading level of QDs is limited below monolayer coverage, and thus the amount of light absorbed becomes low. Also, the direct contact of a large fraction of the TiO2 surface with the electrolyte solution permits the back electron transfer from TiO2 to the oxidant in the electrolyte solution. Further, the insulating molecules intervening between QD and TiO2 at the junction could retard the IET [8,9]. Presently, the successive ionic layer adsorption and reaction (SILAR) technique [10] is believed to be the best way to prepare the metal sulfide QD-loaded TiO2 films. On the other hand, since the discovery of the photodeposition of Pt on TiO2 by Kraeutler and Bard [11], the photocatalytic synthesis has been developed for preparing TiO2-based nanocomposites with metal complexes [12-14], polymers [15-17], and metal oxides [18-22]. Currently, it is being revealed to have a wide possibility of constructing the metal chalcogenideTiO2 coupled particle and film systems [23-32]. In the heterojunction systems, the efficient IET may be expected because the photoinduced redox reactions on the TiO2 surface are taken advantage of for the preparation. Here I summarizes the recent develop
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