Band Structure Engineering and Optical Properties of Pristine and Doped Monoclinic Zirconia (m-ZrO2): Density Functional Theory Theoretical Prospective

Recently, monoclinic ZrO2 has received great technological importance because of its remarkable dielectric properties, high chemical stability, and high melting point. Herein, we introduce first-principles calculations using the Hubbard approach (DFT + U) to study the effects of doping with Nb and W on the electronic and optical properties of pristine ZrO2. The introduction of dopant atoms into the pristine crystal structure led to the displacement of the bandgap edges and reallocation of the Fermi level. The valence band maximum (VBM) shifted upward, resulting in band gap tightening from 5.79 to 0.89 for ZrO2: Nb and to 1.33 eV for ZrO2: W. The optical absorption of doped crystals extended into the visible and near-infrared regions. Partial density of states (PDOS) calculations showed valence band dependency on the O 2p orbital energy, with the conduction band predominantly composed of Nb 4d and W 5d. For pristine ZrO2, the results obtained for the imaginary and real parts of the dielectric function, the refractive index, and the reflectivity show good agreement with the available experimental and theoretical results. For ZrO2:W, we checked the dopant location effect, and the obtained results showed no significant effect on the calculated values of the band gap with a maximum difference of 0.17 eV. Significant band gap tightening and optical properties of our systems indicate that these systems could be promising candidates for photoelectrochemical energy conversion (PEC) applications.

Photocatalytic Properties of Bi2-xTi2O7-1.5x (x = 0, 0.5) Pyrochlores: Hybrid DFT Calculations and Experimental Study

The photocatalytic properties of Bi_2-xTi₂O_7-1.5x (x = 0, 0.5) pyrochlores are examined via ab initio calculations and experiments. A coprecipitation method is applied for the synthesis of nanopowder pyrochlores. The pyrochlore phase formation starts at 500 °C (Bi₂Ti₂O₇) and 550 °C (Bi_1.5Ti₂O_6.25). Nanopowders are found to be a metastable character of pyrochlore phases. The presence of bismuth and oxygen vacancies enhances the thermal stability of the Bi_1.5Ti₂O_6.25 phase in comparison with the Bi₂Ti₂O₇ phase. The estimated crystallite size is 30-40 nm with noticeable agglomerates of about 100-300 nm according to scanning electron microscopy (SEM) and with the formation of particles (510-580 nm) in the aqueous medium. The isoelectric points of the nanopowders seem to be shifted to the strongly acidic region, resulting in the formation of negative surface particle charges of -33 mV (Bi₂Ti₂O₇) and -27 mV (Bi_1.5Ti₂O_6.25) at pH 5.88 in distilled water. The specific surface area is 11.5 m²/g (Bi₂Ti₂O₇) and 12.00 m²/g (Bi_1.5Ti₂O_6.25). The use of the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional allows achieving an excellent agreement between theoretical and experimental structural parameters. The screened Coulomb hybrid HSE03 functional is the most appropriate for describing the optoelectronic properties. Bismuth titanate pyrochlores are wide-gap semiconductors with strong abilities to be active photocatalysts under visible irradiation. The optical E_g values for direct/indirect transition according to the experiment, 3.19/2.94 eV (x = 0) and 3.24/3.03 eV (x = 0.5), and the DFT/HSE03 calculations, 2.92/2.87 (x = 0) and 3.42/- eV (x = 0.5), are in the visible light region and are close. The calculated low effective masses of the charge carriers and suitable band edge positions confirm the ability of the pyrochlores to act as photocatalysts. The photocatalytic activity has been evaluated through the decomposition of rhodamine B under visible irradiation. Bi₂Ti₂O₇ shows the highest activity in comparison with Bi_1.5Ti₂O_6.25, which is in good agreement with theoretically predicted and experimentally revealed characteristics.