Tuneable Phase, Morphology, and Performance of Bismuth Oxyhalide Photocatalysts via Microwave-Assisted Synthesis

pH-Triggered Phase Transitions, Coexposure of (001) and (110) Facets, and Oxygen Vacancies in BiOCl Photocatalysts

Bismuth oxychloride (BiOCl) is known for its unique layered microstructure, which plays a pivotal role in enhancing its photocatalytic properties. This study introduces a novel strategy for controlling the phase composition, facet orientation, and oxygen vacancy formation in BiOCl through precise pH adjustment during the synthesis. By employing a hydrothermal method, we systematically varied the pH to produce distinct BiOCl phases and conducted detailed structural and photocatalytic analyses. Remarkably, BiOCl synthesized at pH = 7 demonstrated superior photocatalytic activity on rhodamine B (RhB) degradation, which can be attributed to the coexposure of the (001) and (110) facets, as well as an increased concentration of oxygen vacancies. Density functional theory study also revealed that a high concentration of oxygen vacancies leads to enhanced charge separation, which is beneficial for photocatalytic activity. These results indicate that optimizing the pH during synthesis is a viable approach to enhancing the photocatalytic efficiency of BiOCl, offering significant potential for advanced applications in environmental remediation and solar energy conversion.

Elucidating Structural Disorder in Ultra-Thin Bi-Rich Bismuth Oxyhalide Photocatalysts

Advancing the field of photocatalysis requires the elucidation of structural properties that underpin the photocatalytic properties of promising materials. The focus of the present study is layered, Bi-rich bismuth oxyhalides, which are widely studied for photocatalytic applications yet poorly structurally understood, due to high levels of disorder, nano-sized domains, and the large number of structurally similar compounds. By connecting insights from multiple scattering techniques, utilizing electron-, X-ray- and neutron probes, the crystal phase of the synthesized materials is allocated as layered Bi24O31X10 (X = Cl, Br), albeit with significant deviation from the reported 3D crystalline model. The materials comprise anisotropic platelet-shaped crystalline domains, exhibiting significant in-plane ordering in two dimensions but disorder and an ultra-thin morphology in the layer stacking direction. Increased synthesis pH tailored larger, more ordered crystalline domains, leading to longer excited state lifetimes determined via femtosecond transient absorption spectroscopy (fs-TAS). Although this likely contributes to improved photocatalytic properties, assessed via the photooxidation of benzylamine, increasing the overall surface area facilitated the most significant improvement in photocatalytic performance. This study, therefore, enabled both phase allocation and a nuanced discussion of the structure-property relationship for complicated, ultra-thin photocatalysts.

Review on synthetic approaches and PEC activity performance of bismuth binary and mixed-anion compounds for potential applications in marine engineering

Photoelectrochemical (PEC) technology in marine engineering holds significant importance due to its potential to address various challenges in the marine environment. Currently, PEC-type applications in marine engineering offer numerous benefits, including sustainable energy generation, water desalination and treatment, photodetection, and communication. Finding novel efficient photoresponse semiconductors is of great significance for the development of PEC-type techniques in the marine space. Bismuth-based semiconductor materials possess suitable and tunable bandgap structures, high carrier mobility, low toxicity, and strong oxidation capacity, which gives them great potential for PEC-type applications in marine engineering. In this paper, the structure and properties of bismuth binary and mixed-anion semiconductors have been reviewed. Meanwhile, the recent progress and synthetic approaches were discussed from the point of view of the application prospects. Finally, the issues and challenges of bismuth binary and mixed-anion semiconductors in PEC-type photodetection and hydrogen generation are analyzed. Thus, this perspective will not only stimulate the further investigation and application of bismuth binary and mixed-anion semiconductors in marine engineering but also help related practitioners understand the recent progress and potential applications of bismuth binary and mixed-anion compounds.

Dipole Moment and Built-In Polarization Electric Field Induced by Oxygen Vacancies in BiOX for Boosting Piezoelectric-Photocatalytic Removal of Uranium(VI)

Piezoelectric-photocatalysis is distinguished by its piezoelectricity as an external force that induces deformation within the catalyst to engender a polarized electric field compared to conventional photocatalysis. Herein, the piezoelectric photocatalyst BiOBr has been expertly synthesized via a plasma process and applied for piezoelectric-photocatalysis removal of uranium(VI) for the first time. The abundant surface oxygen vacancies (OVs) could induce a dipole moment and built-in electric field, which endows BiOBr with excellent separation and transport efficiency of photogenerated charges to actuate more charges to participate in the piezoelectric-photocatalytic reduction process. Consequently, under visible light and ultrasound (150 W and 40 kHz), the removal rate constant of OVs-BiOBr-30 (0.0306 min-1) was 2.4, 30.6, and 6 times higher than those of BiOBr (0.01273 min-1), ultrasound, or photocatalysis, respectively. The piezoelectric-photocatalytic synergy is also universal for BiOX (X = Cl, Br, or I) to accelerate the reduction rate of uranium(VI). This work highlights the role of piezoelectric-photocatalysis in the treatment of uranium-containing wastewater, which is of great significance for resource conservation and environmental remediation.

Induced defect and ZnO nano-flower formation by N, N, dimethylformamide solvent for natural sunlight responsive floating photocatalytic advanced oxidation process

The increasing disposal of dyes and face-mask propel to hunt for a solution to fight water pollution while assisting sustainability. This research overcomes the key challenges associated with implementing photocatalytic water treatment by using natural sunlight active photocatalyst, changing slurry system, eliminating the use of external triggering sources, and reusing face-mask fabric coated with ZnO to act as a floating photocatalyst. Unique morphological structures-cauliflower, hydrangea, and petals-likes are obtained with the variation in synthesis medium (Diethylene glycol (DEG), N, N-dimethyl formamide (DMF), H₂O) and methods (precipitation, solvothermal) which are found to be dependent on the solvent properties. With the use of DMF having a higher dielectric constant and formation of dimethyl amine via hydrolysis, it influences in forming petals and flower-like morphologies, unlike DEG solvent. The ZnO-coated face-mask fabric is used as the floating photocatalyst under natural sunlight observing comparable 91% degradation efficiency in 100 min with that of 99% efficiency in the UV light-illuminated slurry system. The formation of petals-like structures, defects from the liberation of DMF molecules from the ZnO surface by calcination, larger pore sizes and pore volumes provided a synergistic effect on enhancing the degradation efficiency in these cases.

Oxygen vacancies and interfacial iron sites in hierarchical BiOCl nanosheet microflowers cooperatively promoting photo-Fenton

Controllable active site construction, crystal structure regulation and efficient charge separation are core issues in heterogeneous photo-Fenton. Herein, abundant oxygen vacancies and well-dispersed interfacial iron sites are simultaneously constructed in hierarchical nanosheet-assembled BiOCl microflowers. The composites exhibit superior performance in photo-Fenton oxidation of carbamazepine (10 mg L^-1) with a low H₂O₂ concentration (1.3 mM). The high performance highly depends on the synergistic effects between oxygen vacancies and iron species. Rather than modulating the valence band, the involvements of oxygen vacancies and iron species could modify the conduction band of BiOCl. The presence of oxygen vacancies promotes the migration of photo-generated electrons and accelerates the redox cycling of ≡Fe(III)/≡Fe(II) to boost the activation of H₂O₂ to generate hydroxyl radicals, and oxygen vacancies can be well preserved after cyclic use. This work provides understanding on efficient utilization of oxygen vacancies and interfacial iron sites to assist photo-Fenton and the underlying electron transfer mechanism.