A Comprehensive Review on the Boosted Effects of Anion Vacancy in the Heterogeneous Photocatalytic Degradation, Part II: Focus on Oxygen Vacancy

Environmental problems, including the increasingly polluted water and the energy crisis, have led to a need to propose novel strategies/methodologies to contribute to sustainable progress and enhance human well-being. For these goals, heterogeneous semiconducting-based photocatalysis is introduced as a green, eco-friendly, cost-effective, and effective strategy. The introduction of anion vacancies in semiconductors has been well-known as an effective strategy for considerably enhancing the photocatalytic activity of such photocatalytic systems, giving them the advantages of promoting light harvesting, facilitating photogenerated electron-hole pair separation, optimizing the electronic structure, and enhancing the yield of reactive radicals. This Review will introduce the effects of anion vacancy-dominated photodegradation systems. Then, their mechanism will illustrate how an anion vacancy changes the photodegradation pathway to enhance the degradation efficiency toward pollutants and the overall photocatalytic performance. Specifically, the vacancy defect types and the methods of tailoring vacancies will be briefly illustrated, and this part of the Review will focus on the oxygen vacancy (OV) and its recent advances. The challenges and development issues for engineered vacancy defects in photocatalysts will also be discussed for practical applications and to provide a promising research direction. Finally, some prospects for this emerging field will be proposed and suggested. All permission numbers for adopted figures from the literature are summarized in a separate file for the Editor.

Construction of plasmonic Bi/Bismuth oxycarbonate/Zinc bismuth oxide ternary heterojunction for enhanced charge carrier separation and photocatalytic performances

In this work, a novel plasmonic ternary Bi/Bismuth oxycarbonate/Zinc bismuth oxide (Bi-Bi₂O₂CO₃-ZnBi₂O₄) is synthesized synergistically by a one-step hydrothermal method. The results show that the metallic Bi spheres and ZnBi₂O₄ nanoparticles are uniformly distributed on the surface of flower-like Bi₂O₂CO₃ layer. Compared with the bare ZnBi₂O₄ and Bi-Bi₂O₂CO₃, the ternary Bi-Bi₂O₂CO₃-ZnBi₂O₄ heterojunction displays a significantly improved solar energy harvesting efficiency and enhanced photocatalytic degradation activity for environmental organic pollutants. The degradation efficiency of organics reaches to 98.4% under simulated solar light illumination. The degradation kinetics indicates that the photocatalytic reaction rate constant of ternary system is about 4.4 and 29.5 times higher than that of pure ZnBi₂O₄ and Bi-Bi₂O₂CO₃, respectively. Moreover, O₂^- and h⁺ are the main active species in the photodegradation reaction. The improvement of the photocatalytic activity of the composites is attributed to the synergistic effect of ternary heterostructure and surface plasmon resonance (SPR), which promotes charge transfer and effectively inhibits the recombination of photogenerated carriers.

Bi2O2CO3/red phosphorus S-scheme heterojunction for H2 evolution and Cr(VI) reduction

Red phosphorus (RP) has a suitable energy band structure and excellent photocatalytic properties. However, there are some problems, such as low quantum efficiency and serious photogenerated electron-hole recombination. The S-scheme heterostructure shows great potential in facilitating the separation and transfer of photogenerated carriers and obtaining strong photo-redox ability. Herein, hydrothermally treated red phosphorus (HRP) was combined with Bi₂O₂CO₃ to construct Bi₂O₂CO₃/HRP S-scheme heterojunction composite. The Bi₂O₂CO₃ content was optimized, and the 5 %Bi₂O₂CO₃/HRP composite obtained at 5 %Bi₂O₂CO₃ mass fraction exhibited the strongest photoreduction ability. The Cr(VI) photoreduction and photolytic hydrogen production rates were as high as 0.22 min^-1 and 157.2 μmol •h^-1, which were 7.3 and 3.0 times higher than those of HRP, respectively. The promoted photocatalytic activity could be attributed to the formation of S-scheme heterojunctions, which accelerated the separation and transfer of useful photogenerated electron-hole pairs, while enhancing the recombination of relatively useless photogenerated electron-hole pairs, thereby resulting in the highest photocurrent density (17.3 μA/cm²) of the 5 %Bi₂O₂CO₃/HRP composite, which was 1.6 and 4.3 times higher than pure Bi₂O₂CO₃ (10.5 μA/cm²) and pure HRP (4.0 μA/cm²), respectively. This work would provide an advanced approach to enhance the photocatalytic activity of RP.

Oxygen Vacancy-Enhanced Ultrathin Bi2O3-Bi2WO6 Nanosheets' Photocatalytic Performances under Visible Light Irradiation

The oxygen vacancy caused by ultrathin structures would be introduced into the semiconductor photocatalyst to boost its photocatalytic activity. Herein, ultrathin Bi₂O₃-Bi₂WO₆ nanosheet composites have been successfully synthesized via a facile hydrothermal method. Compared to pure Bi₂WO₆ nanosheets, the Bi₂O₃-Bi₂WO₆ nanosheet composites possess abundant oxygen vacancies, which was confirmed by the positron annihilation spectra. The ultrathin Bi₂O₃-Bi₂WO₆ nanosheet composites exhibited remarkable photocatalytic degradation performance for oxytetracycline compared with that of pure Bi₂WO₆ nanosheets. The excellent photocatalytic activities of Bi₂O₃-Bi₂WO₆ composites could be attributed to the heterojunction structure and the oxygen vacancies caused by ultrathin structures.

n–n ZnO–Ag2CrO4 heterojunction photoelectrodes with enhanced visible-light photoelectrochemical properties

In this study, ZnO nanorods (NRs) were hydrothermally grown on an Au-coated glass substrate at a relatively low temperature (90 °C), followed by the deposition of Ag₂CrO₄ particles via a successive ionic layer adsorption and reaction (SILAR) route. The content of the Ag₂CrO₄ particles on ZnO NRs was controlled by changing the number of SILAR cycles. The fabricated ZnO–Ag₂CrO₄ heterojunction photoelectrodes were subjected to morphological, structural, compositional, and optical property analyses; their photoelectrochemical (PEC) properties were investigated under simulated solar light illumination. The photocurrent responses confirmed that the ability of the ZnO–Ag₂CrO₄ heterojunction photoelectrodes to separate the photo-generated electron–hole pairs is stronger than that of bare ZnO NRs. Impressively, the maximum photocurrent density of about 2.51 mA cm⁻² at 1.23 V (vs. Ag/AgCl) was measured for the prepared ZnO–Ag₂CrO₄ photoelectrode with 8 SILAR cycles (denoted as ZnO–Ag₂CrO₄-8), which exhibited about 3-fold photo-enhancement in the current density as compared to bare ZnO NRs (0.87 mA cm⁻²) under similar conditions. The improvement in photoactivity was attributed to the ideal band gap and high absorption coefficient of the Ag₂CrO₄ particles, which resulted in improved solar light absorption properties. Furthermore, an appropriate annealing treatment was proven to be an efficient process to increase the crystallinity of Ag₂CrO₄ particles deposited on ZnO NRs, which improved the charge transport characteristics of the ZnO–Ag₂CrO₄-8 photoelectrode annealed at 200 °C and increased the performance of the photoelectrode. The results achieved in the present work present new insights for designing n–n heterojunction photoelectrodes for efficient and cost-effective PEC applications and solar-to-fuel energy conversions.

ZnO NRs hydrothermally grown on Au coated glass substrate, followed by deposition of Ag₂CrO₄ particles via SILAR route. The content of the Ag₂CrO₄ particles on the ZnO NRs were controlled by changing the number of SILAR cycles.

Photophysical and Photocatalytic Properties of BiSnSbO₆ under Visible Light Irradiation

BiSnSbO₆ with strong photocatalytic activity was first fabricated by a high-temperature, solid-state sintering method. The resulting BiSnSbO₆ was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), UV-vis diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS). The results showed that BiSnSbO₆, with a pyrochlore structure and a cubic crystal system by a space group Fd3m, was well crystallized. The lattice parameter or the band gap of BiSnSbO₆ was 10.234594 Å or 2.83 eV. Compared with N-doped TiO₂, BiSnSbO₆ showed higher photocatalytic activity in the degradation of benzotriazole and rhodamine B. The apparent first-order rate constant for BiSnSbO₆ in the degradation of benzotriazole and rhodamine B was 0.0182 min^-1 and 0.0147 min^-1, respectively. On the basis of the scavenger experiment, during the photocatalytic process, the main active species were arranged in order of increasing photodegradation rate: •OH < •O₂^- < h⁺. The removal rate of benzotriazole or rhodamine B was approximately estimated to be 100% with BiSnSbO₆ as a photocatalyst after 200 min visible-light irradiation. Plentiful CO₂ produced by the experiment indicated that benzotriazole or rhodamine B was continuously mineralized during the photocatalytic process. Finally, the possible photodegradation pathways of benzotriazole and rhodamine B were deduced.

Surface crystal feature-dependent photoactivity of ZnO–ZnS composite rods via hydrothermal sulfidation

ZnO–ZnS core–shell composite rods were synthesized using a two-step facile hydrothermal methodology wherein different sulfidation durations were employed. The effects of sulfidation duration on the morphology and crystalline quality of ZnS shell layers on the surfaces of ZnO rods were investigated. A ZnS shell layer with visible granular features was obtained in the adequately controlled 3 h sulfidation process. A structural analysis demonstrated that the ZnS shell layers of ZnO–ZnS composite rods synthesized after 3 h sulfidation were in a well-defined crystalline cubic zinc blend phase. Moreover, optical properties revealed that these composite rods had a higher light-harvesting ability than those obtained after 1 and 2 h sulfidation. The density of surface crystal defects and the photoexcited charge separation efficiency of the composite rods were associated with changes in the microstructure of the synthesized ZnS shell layers. The optimal sulfidation duration of 3 h for the ZnO–ZnS composite rods resulted in the highest photocatalytic activity for the given photodegradation test conditions. The improved light harvesting and charge transport at the ZnO–ZnS heterointerface accounted for the enhanced photocatalytic activity of the ZnO–ZnS composite rods synthesized after 3 h sulfidation.

ZnO–ZnS core–shell composite rods were synthesized using a two-step facile hydrothermal methodology wherein different sulfidation durations were employed.

Controlled formation of a flower-like CdWO4–BiOCl–Bi2WO6 ternary hybrid photocatalyst with enhanced photocatalytic activity through one-pot hydrothermal reaction

Flower-like CdWO₄–BiOCl–Bi₂WO₆ ternary hybrid photocatalyst shows enhanced photocatalytic activity due to dye sensitization and its cascade structure.