Surface termination modulation for superior S-Scheme Bi2WO6/BiOI heterojunction photocatalyst: a hybrid density functional study.

The prevailing theoretical frameworks indicate that depending on the growth conditions, the Bi2WO6(001) surface can manifest in three distinct terminations-DL-O-Bi (DL: double layers), O-Bi, and O-W. In this study, we conduct a comprehensive examination of the interplay between these terminations on Bi2WO6(001) and the 1I-terminated BiOI(001) facet, especially focusing on their impact on the photocatalytic activity of Bi2WO6/BiOI heterostructure, applying hybrid functional calculations. The models formulated for this research are designated as Bi2WO6(O-Bi)/BiOI(1I), Bi2WO6(DL-O-Bi)/BiOI(1I), and Bi2WO6(O-W)/BiOI(1I). Our findings reveal that Bi2WO6(O-Bi)/BiOI(1I) shows a type II band alignment, which facilitates the spatial separation of photo-generated electrons and holes. Notably, the Bi2WO6(DL-O-Bi)/BiOI(1I) configuration has the lowest binding energy and results in an S-scheme (or Step-scheme) heterostructure. In contrast to the type II heterostructure, this particular configuration demonstrates enhanced photocatalytic efficiency due to improved photo-generated carrier separation, augmented oxidation capability, and better visible-light absorption. Conversely, Bi2WO6(O-W)/BiOI(1I) presents a type I projected band structure, which is less conducive for the separation of photo-generated electron-hole pairs. In summation, this investigation points out that one could significantly refine the photocatalytic efficacy of not only Bi2WO6/BiOI but also other heterostructure photocatalysts by modulating the coupling of different terminations via precise crystal synthesis or growth conditions.

The direct Z-scheme character and roles of S vacancy in BiOCl/Bi2S3-(001) heterostructures for superior photocatalytic activity: a hybrid density functional investigation.

Given some current speculations and controversies regarding the type of BiOCl/Bi2S3-(001) heterostructure in experiments, it is of great importance to clarify these controversies and further explain the relevant experimental results. In this work, based on first-principles hybrid density functional calculations, it is verified that the BiOCl/Bi2S3-(001) heterostructure is a direct Z-scheme photocatalyst with high photo-generated carrier separation efficiency and strong redox ability that can react with O2 and OH- to produce photocatalytic active species of superoxide ions (˙O2-) and hydroxyl radicals (˙OH), respectively. This is consistent with the experimental findings and explains the excellent photocatalytic performance of the BiOCl/Bi2S3-(001) heterostructure in experiments. Besides, excitingly, it is found that the optical absorption, built-in electric field intensity, interlayer recombination probability, hydrogen evolution reaction ability, and the difference in electron-hole mobility are further enhanced via S vacancy introduction in BiOCl/Bi2S3-(001). Therefore, the significant roles of S vacancy in further improving the photocatalytic properties of the BiOCl/Bi2S3-(001) heterostructure are profoundly revealed. This work can provide valuable theoretical insights for designing the superior direct Z-scheme BiOCl/VS-Bi2S3-(001) heterostructure with promising photocatalytic properties.

Exploring the effects of different crystal facet combinations and I-doping in the BiOCl/BiOI heterostructure on photocatalytic properties: a hybrid density functional investigation.

This study uses hybrid functional calculations to investigate the effects of various crystal facet combinations in BiOCl and BiOI on the photocatalytic activity of the BiOCl/BiOI heterostructure. The results show that the separation efficiencies of photo-generated electron-hole pairs in BiOCl(010)/BiOI(001) and BiOCl(010)/BiOI(010) are constrained by type I band alignments in principle. In contrast, BiOCl(001)/BiOI(001) and BiOCl(001)/BiOI(010) heterostructures, which operate under the direct Z-scheme type, exhibit an enhanced photo-generated charge separation efficiency, superior redox capacity, and enhanced visible light absorption. Specifically, BiOCl(001)/BiOI(010) exhibits a more remarkable reduction ability that can reduce O2 to ˙O2-. Furthermore, our investigations demonstrate that targeted I element doping in BiOCl(001)/BiOI(010) can reduce the band gap of the BiOCl(001) sheet, enhance visible light absorption, and maintain the direct Z-scheme characteristics, thereby further improving the photocatalytic performance. Additionally, we discovered that I doping can transform the BiOCl(010)/BiOI(001) heterostructure from type I into a direct Z-scheme heterostructure, resulting in a substantial enhancement in the separation efficiency and reduction ability of photo-generated carriers as well as visible light absorption with increasing I doping concentration. Considering the excellent charge injection efficiency observed in experiments with the BiOCl(010)/BiOI(001) heterostructure, I-BiOCl(010)/BiOI(001) may represent a superior photocatalyst. Thus, this study highlights the crucial and substantial roles of engineering specific crystal facet combinations and I doping in enhancing the photocatalytic performance of the BiOCl/BiOI heterostructure. This theoretical study contributes to the comprehension of related experimental findings and offers valuable insights for the development of novel BiOCl/BiOI heterostructures with superior photocatalytic activity.

Intrinsic point defects and the n- and p-type dopability in α- and ß-Bi2O3 photocatalysts.

In this work, all kinds of intrinsic point defects, unintentional N and H impurities and possible complex defects between impurities and native defects in α- and ß-Bi2O3 with different growth conditions are systematically investigated using hybrid density functional calculations. And then, the n- or p-type doping mechanisms in α- and ß-Bi2O3 are explored and discussed. It is found that α-Bi2O3 presents the n-type conductivity under O-poor conditions. The unintentional H interstitials as the shallow donors should be majorly responsible for the n-type conductivity character. While under O-rich conditions, α-Bi2O3 displays the p-type conductivity, and the unintentional complex defects VBi1 + 2H as the shallow acceptors should be the primary origins of the p-type conductivity. The hydrogenation of the Bi vacancy in α-Bi2O3 not only significantly lowers the formation energy of the Bi vacancy but also markedly decreases its acceptor transition level. This well explains the experimental observation that α-Bi2O3 changes from n-type to p-type conductivity with increasing O partial pressure. Compared to α-Bi2O3, ß-Bi2O3 always presents the n-type conductivity behaviour regardless of the growth conditions. The native O1 vacancies (VO1) and unintentional H interstitials in ß-Bi2O3 are shallow and excellent donors. They are responsible for the n-type conductivity and further perfectly explain the observed unintentional n-type conductivity character in ß-Bi2O3 experiments. Understanding the defect physics in α- and ß-Bi2O3 could inspire more significant studies on developing Bi2O3-based photocatalysts.

Evidence of direct Z-scheme triazine-based g-C3N4/BiOI (001) heterostructures: a hybrid density functional investigation.

In this work, we systematically investigate the photocatalytic mechanism of g-C3N4/BiOI (001) through hybrid functional calculations based on first-principles theory. The staggered band structure is observed in the g-C3N4/BiOI (001); meanwhile, a built-in electric field exists from the g-C3N4 monolayer to the BiOI surface at the interface. BiOI has lower band edges, which bend downward at the interface; whereas g-C3N4 has higher band edges, which bend upward. With Coulomb interaction and the built-in electric field, photo-generated electrons in the conduction bands (CB) of BiOI recombine with photo-generated holes in the valence bands (VB) of g-C3N4. Meanwhile, the stronger reduction capacity for photo-excited electrons in the g-C3N4's CB and the stronger oxidation capacity for photo-generated holes in the BiOI (001)'s VB are retained. Therefore, a direct Z-scheme heterostructure character is presented. As a result, the electrons and holes generated by the photons can be separated and migrate highly effectively at the interface. The separated electrons and holes can effectively participate in the redox reactions with water/pollutants to produce the photocatalytically reactive species superoxide ions (˙O2-) and hydroxyl radicals (˙OH), respectively. This is consistent with the experimental results. It is also worth noting that the g-C3N4/BiOI (001) heterostructure shows a larger difference in the effective mass of carriers. Therefore, the direct Z-scheme charge transfer and separation mechanism and the larger effective mass difference of carriers lead to the superior photocatalytic activity of the g-C3N4/BiOI (001) in experiments. A few speculations and controversies that arose from the experiments are clarified.