Photoelectrochemical Properties of the Zn‐Ti‐Fe Spinel Oxides

Abstract

The electronic and photoelectrochemical properties of the n‐type spinel oxides consisting of Zn‐Ti‐Fe are studied. The composition systems that give the single phase of the spinel structure are Znx2+Fe1−x3+false[Fe1−x2+Fe1+x3+false]O4 and Zn2+false[Zn0.5x2+Ti0.5x4+Fe2−x3+false]O4 . The conductivities of the former oxides are governed by the hopping mechanism between Fe2+ and Fe3+ at the octahedral site, while those of the latter oxides will be governed by normal band conduction. The difference between the Fermi level and the bottom energy of the conduction band is determined from the activation energies of the conductivity and the Seebeck coefficient. Photocurrents are observed for almost all the samples but disappear in the composition region at x<0.8 in the Znx2+Fe1−x3+false[Fe1−x2+Fe1+x3+false]O4 system. This is caused by the disappearance of the space charge layer developed by the high carrier density in the hopping level. The flatband potential and the bandgap depend on the cations in the octahedral site, but not on those in the tetrahedral site. The flatband potential shifts to positive and the bandgap decreases when Fe increases in the octahedral site. Zn2TiO4 is a useful anode material for the photoelectrolysis of water without bias, but it requires UV light. On the other hand, ZnFe2O4 has a relatively small bandgap and a little more negative flatband potential than that of a reversible hydrogen electrode, but its onset potential of the anodic photocurrent is more positive. The band structures of the spinel oxides used in this study are also discussed.

Optical interband transitions in Zn2TiO4 single crystals

“…The exponent n can take values of 2, 1/2, 2/3, and 1/3 for the allowed direct, allowed indirect, forbidden direct and the forbidden indirect transitions, respectively. The previous reports on polycrystalline samples gave the Zn 2 TiO 4 bandgap by estimating the absorption edge and measuring the conductivity, respectively 12, 23–25. However, none of the above authors pointed out the types of interband transition, and no theoretical calculation study has been carried out.…”

“…The T sites are occupied by half of the divalent Zn cations and the O sites are filled by a stoichiometric mixing of the divalent Zn and tetravalent Ti cations randomly. The band structure of Zn 2 TiO 4 should be estimated from the interaction between the cations and the oxygen in the octahedral site, and it is presumed that the valence band of Zn 2 TiO 4 is O 2− 2p orbitals, and the conduction band is Ti 4+ t 2g orbitals or Zn 2+ 4s orbitals 12. Our results indicate that Zn 2 TiO 4 is an indirect bandgap material, and the bandgap is 3.29 eV.…”

“…Due to its excellent dielectric property, Zn 2 TiO 4 is also applied in microwave devices. Recently, many studies have been focused on its electrical, optical and photoelectrochemical properties in different conditions to investigate its potential applications 1, 8–12.…”

“…Since then there has been no report on the crystal growth of Zn 2 TiO 4 . Thus, all the measured properties of Zn 2 TiO 4 , particularly the bandgap and interband transitions process, have been made only on polycrystalline samples 12, 23–25. Recently, we reported the growth of high‐quality Zn 2 TiO 4 crystals by the optical floating zone method 26, 27.…”

Adsorption and degradation performance of Rhodamine B over BiOBr under monochromatic 532nm pulsed laser exposure

“…To date, several methods have been adopted for the solid-based band/electronic structure measurement and speculation, such as ultraviolet photoelectron spectroscopy (UPS) [28], flatband-potential-determination based photoeletrochemical techniques [29][30][31], atom's Mulliken electronegativity theory [6,27,[32][33][34][35][36][37][38] and density functional theory (DFT) calculation based on plane wave pseudo potential method [39][40][41]. The band-gap width, CB/VB of the catalysts series BiOX (X = F, Cl, Br, I) semiconductor were calculated by means of DFT method [42].…”