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.