Electronic Structures and Electrical Properties of Cr(2+)-, Cu(2+)-, Ni(2+)-, and Zn(2+)-Doped Sodium Titanate Nanotubes

[Image: see text] Sodium titanate nanotubes (Na-TNTs) and their metal-doped derivatives were prepared using simple hydrothermal and metal ion-exchange methods, respectively. The as-prepared doped materials were characterized by X-ray powder diffraction, thermal gravimetric analysis, high-resolution transmission electron microscopy, field-emission scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The dielectric constant, the loss tangent, and the AC electrical conductivity of NaM-TNTs (where M = Cr, Cu, Ni, or Zn) were measured at selected frequencies (400 kHz and 2 MHz) as a function of temperature. The activation energy was calculated and reported at 400 kHz. All samples showed mixed ionic electronic conductivity. Some of the materials were used as examples for theoretically exploring structures and optoelectronic properties (density of states, reflectivity, absorption curve, refractive index, dielectric function, optical conductivity, and loss function) using density functional theory calculations. The band gaps of the materials were found to vary from 2.4 to 3.17 eV, which makes them suitable for many optoelectronic applications.