Synthesis of large monodisperse ETS-10 crystals and observation of quantum confinement effects from very long titanate quantum wires

Synthetic conditions to produce highly crystalline monodisperse ETS-10 crystals with the length along the [1 1 0] direction (L[1 1 0]) between 17 and 55 μm and the length along the [0 0 1] direction between 15 and 80 μm were developed. This was achieved by varying the amounts of KF in the gel consisting of Na2SiO3, titanium isopropoxide, HCl, NaCl, KF, and H2O. The UV–vis spectra of the ETS-10 crystals revealed that the absorption band red-shifts with increasing L[1 1 0]. The plot of band gap energies (Eg values) with respect to L[1 1 0] values revealed a progressively decaying relationship. The mixing of the newly observed data set with that reported previously from the ETS-10 crystals with L[1 1 0] between 0.3 and 20 μm gave a smooth exponential decay relationship, Eg = 0.2578 exp(−L[1 1 0]/13.751) + 3.7667, as if they came from a single data set. Analysis shows that the quantum confinement effect still operates in TiO32− quantum wires with the estimated length of 368 nm. The μxy and μz values are 15.2 and 0.0006 me, respectively. A monotonous quadratic relationship exists between L[1 1 0] and l. Thus the length of the titanate quantum wire increases more than the degree the crystal size increases, indicating that the degree of defect per unit volume decreases as the crystal size increases.

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► Synthesis of a series of monodisperse large ETS-10 crystals from 15 to 80 μm under a mild condition.
► The mole ratio of KF in the gel is the key to control the size of ETS-10.
► Elucidation of quantum confinement effects from very long titanate quantum wires.