Electronic Structure Studies and Photocatalytic Properties of Cubic Bi_1.5ZnNb_1.5O₇

Abstract

The photocatalytic ability of cubic Bi 1.5 ZnNb 1.5 O 7 (BZN) pyrochlore for the decolorization of an acid orange 7 (AO7) azo dye in aqueous solution under ultraviolet (UV) irradiation has been investigated for the first time. BZN catalyst powders prepared using low temperature sol-gel and higher temperature solid-state methods have been evaluated and their reaction rates have been compared. The experimental band gap energy has been estimated from the optical absorption edge and has been used as reference for theoretical calculations. The electronic band structure of BZN has been investigated using first-principles density functional theory (DFT) calculations for random, completely and partially ordered solid solutions of Zn cations in both the and sites of the pyrochlore structure. The nature of the orbitals in the valence band (VB) and the conduction band (CB) has been identified and the theoretical band gap energy has been discussed in terms of the DFT model approximations.

Computational prediction and experimental confirmation of rhombohedral structures in Bi1.5CdM1.5O7 (M = Nb, Ta) pyrochlores

“…27 Our recent investigations on the electronic band structures of cubic-BZN and cubic-BZT have shown that the use of the (widely accepted) CIFs as inputs for Density Functional Theory (DFT) calculations, result in complete absence of electronic band gaps when using the Full Plane Wave Functions of the Cambridge Serial Total Energy Package (CASTEP) of Materials Studio. 28,29 These results contradict the reported experimental observations of optical band gaps of 3.0-3.2 eV. 30,31 A band gap is obtained for fractional occupancy of cations in BZT using the Linear Muffin Tin Potential.…”

“…28,29 The DFT calculations on BZN and BZT also predict small modications from the cubic structure, when complete substitution order is assumed, which appears favoured as attested by the signicantly more negative enthalpy of formation. 28,29 However, such favourably calculated modied structures could not be clearly detected by standard X-ray diffraction (XRD) or transmission electron microscopy (TEM) characterizations. An overall cubic structure, predicted by the DFT calculation, could be reconciled by assuming partial order, with order just on the B-sites, while the A-sites remained randomly substituted.…”

“…28,29 In contrast, when the substitutions are considered completely or partially ordered (that is assuming all or part of the substitutions at preferential sites), a remarkable match of calculated electronic band gaps to experimental measurements can be obtained. 28,29 The DFT calculations on BZN and BZT also predict small modications from the cubic structure, when complete substitution order is assumed, which appears favoured as attested by the signicantly more negative enthalpy of formation. 28,29 However, such favourably calculated modied structures could not be clearly detected by standard X-ray diffraction (XRD) or transmission electron microscopy (TEM) characterizations.…”

“…32 However, the calculated band gap is much lower than experiment and our calculated results for BZT. 29 According to the CIF for BZN and BZT used in these studies, both A and B sites are randomly occupied by Bi/Zn and Nb/Zn (or Ta/Zn) fractional mixtures, respectively. 28,29 In contrast, when the substitutions are considered completely or partially ordered (that is assuming all or part of the substitutions at preferential sites), a remarkable match of calculated electronic band gaps to experimental measurements can be obtained.…”

“…29 According to the CIF for BZN and BZT used in these studies, both A and B sites are randomly occupied by Bi/Zn and Nb/Zn (or Ta/Zn) fractional mixtures, respectively. 28,29 In contrast, when the substitutions are considered completely or partially ordered (that is assuming all or part of the substitutions at preferential sites), a remarkable match of calculated electronic band gaps to experimental measurements can be obtained. 28,29 The DFT calculations on BZN and BZT also predict small modications from the cubic structure, when complete substitution order is assumed, which appears favoured as attested by the signicantly more negative enthalpy of formation.…”