Photocatalytic Properties of Bi_2–xTi₂O_7–1.5x (x = 0, 0.5) Pyrochlores: Hybrid DFT Calculations and Experimental Study

Abstract

The photocatalytic properties of Bi2–x Ti2O7–1.5x (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 (Bi1.5Ti2O6.25). Nanopowders are found to be a metastable character of pyrochlore phases. The presence of bismuth and oxygen vacancies enhances the thermal stability of the Bi1.5Ti2O6.25 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 (Bi1.5Ti2O6.25) at pH 5.88 in distilled water. The specific surface area is 11.5 m2/g (Bi2Ti2O7) and 12.00 m2/g (Bi1.5Ti2O6.25). 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 Bi1.5Ti2O6.25, which is in good agreement with theoretically predicted and experimentally revealed characteristics.

Effect of Li and Li-RE co-doping on structure, stability, optical and electrical properties of bismuth magnesium niobate pyrochlore

“…The DFT-HSE03 calculated and experimental band gap values for the (Bi 1.5 Li 0.5 )(Nb 1.5 Mg 0.5 )O 7 pyrochlore are found to be identical (Table 5). This shows once more that the screened Coulomb hybrid HSE03 exchange-correlation functional is an excellent way to describe the electronic structure of pyrochlores [37,55]. Moreover, the determined band gap values of the BMg 0.9-x Li x NO (x = 0.25, 0.4) pyrochlores agree with the main tendency of the values for the Mg, Na-codoped pyrochlores [37].…”

“…We applied the screened Coulomb hybrid HSE03 exchange-correlation functional [53,54] to accurately predict the electronic properties of (co-)doped bismuth niobate pyrochlores. This functional was successfully used in our previous works, where excellent agreement with the experiment was achieved [37,55]. A Monkhorst-Pack 6 × 6 × 6 k-point sampling together with the Gaussian smearing method was employed for DFT-HSE03 calculation.…”

Exploring the effect of partial B-site Al3+–Mg2+ dual substitution on optoelectronic, surface, and photocatalytic properties of BaTaO2N

“…The effective masses of electrons and holes were calculated using the Sumo- 11 bandstats program through the parabolic fitting to the conduction band minimum (CBM) and valence band maximum (VBM). 41 The band edge positions were calculated according to the empirical formula: [42][43][44][45] E CB = , E e -0.5E g (1)…”

Versatile Titanates: Classification, Property, Preparation, and Sustainable Energy Catalysis

“…(b) Reproduced with permission. [ 79 ] Copyright 2020, American Chemical Society. (c) Reproduced with permission.…”