Abstract
Bi4Ti3O12−x nanosheet photocatalysts with abundant oxygen vacancies are fabricated by a facile solid-state chemical reduction method for the first time. This method is simple in operation, has short reaction time, and can be conducted at mild temperatures (300~400 °C). The electron paramagnetic resonance, thermogravimetric analysis, X-ray photoelectron spectrometer, and positron annihilation lifetime spectra results indicate that oxygen vacancies are produced in Bi4Ti3O12−x, and they can be adjusted by tuning the reduction reaction conditions. Control experiments show that the reduction time and temperature have great influences on the photocatalytic activities of Bi4Ti3O12−x. The optimal Bi4Ti3O12−x is the sample undergoing the reduction treatment at 350 °C for 60 min and it affords a hydrogen evolution rate of 129 μmol·g−1·h−1 under visible-light irradiation, which is about 3.4 times that of the pristine Bi4Ti3O12. The Bi4Ti3O12−x photocatalysts have good reusability and storage stability and can be used to decompose formaldehyde and formic acid for hydrogen production. The surface oxygen vacancies states result in the broadening of the valence band and the narrowing of the band gap. Such energy level structure variation helps promote the separation of photo-generated electron-hole pairs thus leading to enhancement in the visible-light photocatalytic hydrogen evolution. Meanwhile, the narrowing of the band gap leads to a broader visible light absorption of Bi4Ti3O12−x.
Highlights
The development of green energy has become one of the most prominent research fields
When the reaction temperature rises to 400 ◦C, black Bi4Ti3O12−x can be synthesized in 60 min, suggesting that the solid-state chemical reduction process modifies the surface features of the Bi4Ti3O12 nanosheets
The hydrogen production rate reaches up to 129 μmol·g−1·h−1 under visible-light irradiation for the optimal Bi4Ti3O12−x photocatalyst, which is about 3.4 times that of the pristine Bi4Ti3O12
Summary
The development of green energy has become one of the most prominent research fields. Various types of Bi4Ti3O12 photocatalysts, such as nanofibers [8,11], particles [12,13], platelets [14], and films [15] have been developed for processes involving alternative energy development or destruction of different pollutants These Bi4Ti3O12 nanomaterials are not very efficient as visible-light photocatalysts because of the high recombination rate of photo-induced electron-hole pairs [16,17,18]. The energy level variation induced by oxygen vacancy can facilitate the separation efficiency of the photo-generated electron-hole pairs, which contributes significantly to the improvement of the photocatalytic performance of Bi4Ti3O12−x. We propose a mechanism for the decrease of the band gap of Bi4Ti3O12−x and its photocatalytic activity improvement
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