Formation of Bi₂WO₆ Bipyramids with Vacancy Pairs for Enhanced Solar‐Driven Photoactivity

Abstract

3726 wileyonlinelibrary.com of the physical structure of TiO 2 crystals to enhancing its photoactivity. [ 6,7 ] TiO 2 has a wide bandgap of 3.2 eV, leading to only utilization of the UV region in solar resource, which limits its wide applications. [8][9][10] Therefore, the development of effi cient solar-photoactive catalysts has become an active pursuit by many researchers aiming at harvesting solar energy effectively. Bismuth tungstate (Bi 2 WO 6 ), stable, nontoxic, and with a relatively small bandgap of 2.6 eV has been suggested an ideal photocatalyst for applications in visible-lightdriven photocatalysis. [11][12][13][14][15] To enhance the separation effi ciency of the photoexcited charge carriers in Bi 2 WO 6 , great efforts have been made by adjusting the chemical composition or manipulating the physical structure. [16][17][18][19][20] There have studies on improving photoeffi ciency by chemical modifi cation such as by partially substituting the M sites in the A x M y O z structure with alkalis or earth alkalis. [21][22][23] However, these ions tend to decrease the thermal stability and increase the recombination of charge centers of Bi 2 WO 6 . [ 24 ] Therefore, modifying the physical morphology, to achieve more high energy surfaces in Bi 2 WO 6 , becomes an important alternative strategy for enhancing its photoactivity. [ 25 ] By exposing high-energy surfaces can enhance the photoelectric and photocatalytic properties of a catalyst. [26][27][28][29] Specifi cally, by increasingly exposing the high-energy facets in Bi 2 WO 6 can improve the effective separation of electron-hole pairs, thereby arriving at the fabrication of more effi cient catalysts. Moreover, to deduce the relationship between the ratio of high energy surface and the enhanced effect, the overwhelming majority of researchers have only fi xed their attention on the strength of the interaction of the reactive surfaces with reactant molecules. [ 30 ] However, this explanation does not fully explain the enhancement. Recently, there are reports that these reactive facets are accompanied always by various types of surface defects. [31][32][33] Furthermore, with recent advances in scanning tunneling microscopy (STM), electron paramagnetic resonance spectroscopy (EPR), and positron annihilation spectroscopy (PAS), [34][35][36] the role of defects in photocatalysis now can be thoroughly studied. In contrast to bulk defects, which only act as trap sites for charge carriers, surface defects can be adsorption sites where charge carriers can be transferred more easily to the adsorbed species, thereby impeding their recombination and enhancing the photoeffi ciency. Therefore, it is hypothesized that surface defects on the reactive surfaces of a crystal can play an important role in controlling the photocatalytic effi ciency.In order to improve the photoactivity, many attempts have focused on increasing the exposure of highly reactive surfaces on crystals. However, the connection between the reactive surfaces and enhancement is still elusive. Herein, ...

Heterojunction photoanodes for solar water splitting using chemical-bath-deposited In 2 O 3 micro-cubes and electro-sprayed Bi 2 WO 6 textured nanopillars

“…The bandgap energy values of the pristine Bi 2 WO 6 and In 2 O 3 lms were extrapolated from the Tauc plot, which yielded 2.7 and 2.5 eV, respectively. 27,28 The bandgap energy of the In24-Bi80 bilayer lm was 2.4 eV, which is in agreement with the value reported by Liu et al 21 This reduced bandgap is favorable to absorb the increased range of the light spectrum. Note that the conduction band (E cb ) and valance band (E vb ) edge positions can be determined by using the empirical formula such as, E cb ¼ E vb À E g and E vb ¼ X À 4.5 + 0.5E g , where X is the absolute electronegativity, 4.5 eV is the energy of free electrons on a hydrogen scale.…”

Three-in-one synergistic therapy for tooth whitening and biofilm eradication using Cu-doped Bi 2 WO 6

“…Compared with that of Bi 2 WO 6 the percentage of W 5+ in the 0.5CBWO sample increases from 45.7% to 49.7%, while that of the W 6+ chemical state decreases from 54.3% to 50.3%. Similar results were also reported by Huo 35 and Zhang 36 et al The generation of oxygen vacancies will cause the surrounding electron density of W to increase, which will promote the low valence state of W 5+ increasing to keep electric neutrality.…”

Fe Substitutions Improve Spectral Response of Bi2WO6-Based Photoanodes

“…In comparison, the addition of Fe 3+ ions shifts the binding energy of W 6+ 4f 7/2 and 4f 5/2 to 35.66 and 37.8 eV, respectively, which is 0.3 eV lower than that of pure Bi 2 WO 6 . A comparable shift in W 4f peaks was also reported in Bi 2 WO 6 photocatalysts containing defects such as oxygen vacancies. − This relatively large shift of W 4f peaks upon incorporation of Fe implicates the modification of the W environment in Bi 2 WO 6 :Fe, either due to the formation of neighboring oxygen vacancies with a high electron-attracting effect and/or nearby Fe substitutions on one of the cation sites. Oxygen vacancy-mediated substitution of Fe on W sites has been previously proposed for Fe-modified Bi 2 WO 6 , which is contrary to other demonstrations that Fe incorporation into Bi 2 WO 6 is limited to Fe 3+ substitutions on Bi 3+ sites. , …”