Hyperbranched Anatase TiO2 Nanocrystals: Nonaqueous Synthesis, Growth Mechanism, and Exploitation in Dye-Sensitized Solar Cells

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• 作者：Raffaella Buonsanti ; Elvio Carlino ; Cinzia Giannini ; Davide Altamura ; Luisa De Marco ; Roberto Giannuzzi ; Michele Manca ; Giuseppe Gigli ; P. Davide Cozzoli
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：November 30, 2011
• 年：2011
• 卷：133
• 期：47
• 页码：19216-19239
• 全文大小：2368K
• 年卷期：v.133,no.47(November 30, 2011)
• ISSN：1520-5126

文摘

A colloidal crystal-splitting growth regime has been accessed, in which TiO₂ nanocrystals, selectively trapped in the metastable anatase phase, can evolve to anisotropic shapes with tunable hyperbranched topologies over a broad size interval. The synthetic strategy relies on a nonaqueous sol鈥揼el route involving programmed activation of aminolysis and pyrolysis of titanium carboxylate complexes in hot surfactant media via a simple multi-injection reactant delivery technique. Detailed investigations indicate that the branched objects initially formed upon the aminolysis reaction possess a strained monocrystalline skeleton, while their corresponding larger derivatives grown in the subsequent pyrolysis stage accommodate additional arms crystallographically decoupled from the lattice underneath. The complex evolution of the nanoarchitectures is rationalized within the frame of complementary mechanistic arguments. Thermodynamic pathways, determined by the shape-directing effect of the anatase structure and free-energy changes accompanying branching and anisotropic development, are considered to interplay with kinetic processes, related to diffusion-limited, spatially inhomogeneous monomer fluxes, lattice symmetry breaking at transient Ti₅O₅ domains, and surfactant-induced stabilization. Finally, as a proof of functionality, the fabrication of dye-sensitized solar cells based on thin-film photoelectrodes that incorporate networked branched nanocrystals with intact crystal structure and geometric features is demonstrated. An energy conversion efficiency of 6.2% has been achieved with standard device configuration, which significantly overcomes the best performance ever approached with previously documented prototypes of split TiO₂ nanostructures. Analysis of the relevant photovoltaic parameters reveals that the utilized branched building blocks indeed offer light-harvesting and charge-collecting properties that can overwhelm detrimental electron losses due to recombination and trapping events.