There is a great deal of interest in TiO2 nanoparticles, nanowires and nanotubes due to their potential advantages (safety, rate) as anodes replacing graphite in a new generation of rechargeable lithium batteries. Here we report the synthesis of mesoporous anatase with an ordered 3D pore structure, using a hard template, and investigate its properties as a lithium intercalation host. It exhibits a hierarchical pore structure. Despite being composed of micrometer sized particles, the ordered mesoporous morphology inside the particles results in a high Li storage capacity and high rates of intercalation, with the material exhibiting an energy density between 30 and 200% higher than the best high rate performance reported to date for any titanate (6 nm nanoparticle anatase). It has been proposed that the reason nanoparticles such as anatase and LiFePO4 exhibit facile Li insertion is the ability of such particles to transform spontaneously for one phase to the other, i.e., a particle is either phase A or B but not both. The micrometer sized mesoporous particles cannot do so but still show facile intercalation. This is related to the ease with which the strain of transforming between the anatase (phase A) and the orthorhombic Li0.59TiO2 structures (phase B) is accommodated within the thin (6.5 nm) walls on intercalation. Mesoporous anatase with an ordered 3D pore structure was synthesized using the silica KIT-6 as a hard template (see experimental section). The ordered pore structure is evident in the TEM data (Figure 1A,B) and replicates that of the KIT-6 hard template with space group Ia3d. An a0 lattice parameter for the mesostructure of 23.3 nm was extracted from the data. The mesoporous structure is preserved throughout as demonstrated by examining many particles. The walls (6.5 nm) are composed of anatase crystallites. A lattice spacing of 0.350 nm was observed in HRTEM (Figure 1B), in good agreement with the d-spacing of 0.352 nm associated with the (101) direction of anatase (ICDD 00-0015062). The low and wide-angle PXRD data are shown in Figure 2. The low-angle diffraction patterns exhibit one relatively sharp peak below 18, which could be indexed as the 211 reflection in the Ia3d space group, corresponding to an a0 lattice parameter of 23.5 nm in good agreement with the TEM data. The broad peaks in the wide-angle PXRD for the as-prepared mesoporous material are in good agreement with those for anatase nanoparticles, AK-1 (Bayer) (Figure 2A). The mesoporous anatase peak widths are greater than those of the nanoparticles in accord with the walls being thinner than the diameter of the nanoparticles (15 nm). The mesostructures were investigated further by N2 sorption measurements. Typical type IV isotherms exhibiting H2 hysteresis were observed (Supporting Information, Figure S1a), consistent with the mesoporosity evident in the TEM and low-angle PXRD data. BJH pore size distributions exhibit at least three peaks, demonstrating a hierarchical pore structure. Awell resolved narrow peak centered at 5 nm, a peak at approximately 11 nm and a third broad peak at ca. 50 nm (Figure S1b). The first peak corresponds to the mesopores observed by TEM in Figure 1. The second arises because KIT-6 has two interpenetrating sets of pores connected by microporous bridges. In regions where the bridges are complete, both pores will be filled and the replica TiO2 is composed of the 5 nm pores. Where the bridges are incomplete and only one set of KIT-6 pores are filled, the replica exhibits 11 nm pores (Figure S2a). Such a phenomenon had been discussed previously for other KIT-6 templated materials. The third peak corresponds to interparticle voids. The 11 Figure 1. TEM and HRTEM data for ordered mesoporous anatase: A,B) as-prepared; C,D) after 1000 cycles (12000 mAg ).
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Jul 31, 2012
Kamp-Du Lee + 2
Open Access
