New insights into ETS-10 and titanate quantum wire: A comprehensive characterization

The titanate quantum wires in ETS-10 crystals remain intact during ion exchange of the pristine cations (Na⁺_0.47 + K ⁺_0.53) with Mⁿ⁺ ions (Mn⁺) Na ⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺, Cd²⁺, Zn²⁺) and during reverse exchange of the newly exchanged cations with Na⁺. The binding energies of O(1s) and Ti(2p) decrease as the electronegativity of the cation decreases, and they are inversely proportional to the negative partial charge of the framework oxygen [-δ(O_f)]. At least five different oxygen species were identified, and their binding energies (526.1-531.9 eV) indicate that the titanate-forming oxides are much more basic than those of aluminosilicate zeolites (530.2-533.3 eV), which explains the vulnerability of the quantum wire to acids and oxidants. The chemical shifts of the five NMR-spectroscopically nonequivalent Si sites, δ(I_A), δ(I_B), δ(II_A), δ(II_B), and δ(III), shift downfield as -δ(O_f) increases, with slopes of 2.5, 18.6, 133.5, 216.3, and 93.8 ppm/[-δ(O_f)], respectively. The nonuniform responses of the chemical shifts to -δ(O _f) arise from the phenomenon that the cations in the 12-membered-ring channels shift to the interiors of the cages surrounded by four seven-membered-ring windows. On the basis of the above, we assign δ(I _A), δ(I_B), δ(II_A), and δ(II_B) to the chemical shifts arising from Si(12,12), Si(12,7), Si(7,12), and Si(7,7) atoms, respectively. The frequency of the longitudinal stretching vibration of the titanate quantum wire increases linearly and the bandwidth decreases nonlinearly with increasing -δ(O_f), indicating that the titanate quantum wire resembles a metallic carbon nanotube. As the degree of hydration increases, the vibrational frequency shifts linearly to higher frequencies while the bandwidth decreases. We identified another normal mode of vibration of the quantum wire, which vibrates in the region of 274-280 cm^-1. In the dehydrated state, the band-gap energy and the first absorption maximum shift to lower energies as -δ(O_f) increases, indicating the oxide-to-titanium(IV) chargetransfer nature of the transitions.