Abstract
The prominent role of oxygen vacancies in the photocatalytic performance of bismuth tungsten oxides is well recognized, while the underlying formation mechanisms remain poorly understood. Here, we use the transmission electron microscopy to investigate the formation of oxygen vacancies and the structural evolution of Bi2WO6 under in situ electron irradiation. Our experimental results reveal that under 200 keV electron irradiation, the breaking of relatively weak Bi–O bonds leads to the formation of oxygen vacancies in Bi2WO6. With prolonged electron irradiation, the reduced Bi cations tend to form Bi clusters on the nanoflake surfaces, and the oxygen atoms are released from the nanoflakes, while the W–O networks reconstruct to form WO3. A possible mechanism that accounts for the observed processes of Bi cluster formation and oxygen release under energetic electron irradiation is also discussed.
Highlights
Bi2WO6 has drawn great interest regarding its physical properties such as the piezoelectric effect and ferroelectricity with large spontaneous polarization and high Curie temperature [1,2,3], and pyroelectric and non-linear optical properties [4,5]
Our results reveal that Bi2WO6 nanoflakes can be decomposed into Bi precipitates and WO3 nanosheets after the generation of oxygen vacancies
Our detailed high-resolution TEM (HRTEM) and Selected-area electron diffraction (SAED) analyses have revealed that energetic electrons break the relatively weak Bi–O bonds in Bi2WO6 crystals forming oxygen vacancies
Summary
Bi2WO6 has drawn great interest regarding its physical properties such as the piezoelectric effect and ferroelectricity with large spontaneous polarization and high Curie temperature [1,2,3], and pyroelectric and non-linear optical properties [4,5]. Bi2WO6 has shown good performance in the degradation of organic compounds [6,7,8], and photocatalytic oxygen evolution [9,10] and CO2 reduction [11,12,13] under visible-light irradiation. Bi2WO6 is the simplest member of the Aurivillius phases, perovskites with the general formula of (Bi2O2)(Am−1BmO3m+1) and a crystal structure of tilted WO6. The W–O layers in the Bi2WO6 crystal transfer electrons to the surface of catalysts, and Bi–O layers act as insulating layers that self-adapt to keep the balance of space charges. The formation mechanisms of Bi/O defects are discussed in detail by combining the HRTEM imaging of defects and the calculation of the electrostatic site potentials of Bi2WO6
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