Evolution of Oxyhalide Crystals under Electron Beam Irradiation: An in Situ Method To Understand the Origin of Structural Instability

Bismuth oxychloride as avan der Waalsdielectric for 2D electronics

Two-dimensional (2D) semiconductors have received a lot of attention as the channel material for the next generation of transistors and electronic devices. On the other hand, insulating 2D gate dielectrics, as possible materials for gate dielectrics in transistors, have received little attention. We performed an experimental study on bismuth oxychloride, which is theoretically proposed to have good dielectric properties. High-quality bismuth oxychloride single crystals have been synthesized, and their high single crystallinity and spatial homogeneity have been thoroughly evidenced by x-ray diffraction, Raman spectroscopy, x-ray photoelectron spectroscopy, transmission electron microscopy (TEM), and scanning TEM studies. We then mechanically exfoliated high-quality BiOCl crystals to fabricate metal-insulator-metal (MIM) capacitors and measured the dielectric properties at various frequencies and different thicknesses. We found that BiOCl exhibits an out-of-plane static dielectric constant up to 11.6, which is 3 times higher than 2D hexagonal boron nitride making it a suitable candidate for 2D dielectrics. We also carried out cross-section TEM studies to look into the MIM interface and provide some future directions for their integration with metal-dielectric interfaces and possibly with other 2D devices.

Dual-scale modified BiOBr with enhanced structural self-transformation at wide pH for bifunctional treatment of Cr(VI)

Hexavalent chromium (Cr(VI)) poses threat to both ecosystems and human health. Complex pollution conditions, particularly the pH levels, significantly influence the treatment process of Cr(VI). In this study, BiOBr materials were synthesized with exposed (110) facets and Bi vacancies through dual modifications at both grain and atomic scales. The modified material exhibits a unique pH-adaptive structural transformation capability, enhancing the treatment of Cr(VI) under variable pH conditions. This structural adaptability notably enhances the bifunctional utility of modified BiOBr under visible light. Specifically, under acidic conditions, the material's surface undergoes restructuring, progressively exposing the (001) facets in conjunction with the original (110) facets to form a heterojunction, thereby enhancing reduction capacity. Under neutral to alkaline conditions, visible-light induced surface defects increase the availability of CrO42- ion-exchange channels, enhancing intercalation adsorption capacity. Additionally, facet and vacancy modifications improve the separation of charge carriers and expose more intercalation channels, enhance the pH adaptability and bifunctionality of modified BiOBr. Ultimately, the system demonstrates superior Cr(VI) removal efficiencies exceeding 90 % from pH 2-10. This innovative bifunctional BiOBr not only redefines the reaction mechanism in the Cr(VI) treatment system but also lays the groundwork for designing pH-adaptive catalysts aimed at environmental remediation.

Insights into the nucleation and growth of BiOCl nanoparticles by in situ X-ray pair distribution function analysis and in situ liquid cell TEM

The synthesis of bismuth oxyhalides as defined nanostructures is hindered by their fast nucleation and growth in aqueous solutions. Using our recently developed single-source precursor, the formation of bismuth oxychloride in such solutions can be slowed significantly. As reported herein, this advance enables BiOCl formation to be investigated by in situ X-ray total scattering and in situ liquid cell transmission electron microscopy. In situ pair distribution function analysis of X-ray total scattering data reveals the local order of atomic structures throughout the synthesis, while in situ liquid cell transmission electron microscopy allows for tracking the growth of individual nanoparticles. Through this work, the precursor complex is shown to give rise to BiOCl upon heating in solution without the observation of structurally distinct intermediates. The emerging nanoparticles have a widened interlayer spacing, which moderately decreases as the particles grow. Mechanistic insights into the formation of bismuth oxyhalide nanoparticles, including the absence of distinct intermediates within the available time resolution, will help facilitate future design of controlled BiOX nanostructures.

Time-resolved transmission electron microscopy for nanoscale chemical dynamics

The ability of transmission electron microscopy (TEM) to image a structure ranging from millimetres to Ångströms has made it an indispensable component of the toolkit of modern chemists. TEM has enabled unprecedented understanding of the atomic structures of materials and how structure relates to properties and functions. Recent developments in TEM have advanced the technique beyond static material characterization to probing structural evolution on the nanoscale in real time. Accompanying advances in data collection have pushed the temporal resolution into the microsecond regime with the use of direct-electron detectors and down to the femtosecond regime with pump-probe microscopy. Consequently, studies have deftly applied TEM for understanding nanoscale dynamics, often in operando. In this Review, time-resolved in situ TEM techniques and their applications for probing chemical and physical processes are discussed, along with emerging directions in the TEM field.

Facile ammonium oxidation to nitrogen gas in acid wastewater by in situ photogenerated chlorine radicals

The treatment of low-concentration ammonium (e.g., <50 mg L^-1) in highly acidic wastewaters through traditional biological nitrification, physical separation, or chemical stripping remains a huge challenge. Herein, we report that photocatalytic ammonium oxidation using bismuth oxychloride (BiOCl) can successfully occur in Cl^--laden solutions within a pH range of 1.0-6.0. All reactions follow pseudo-zero-order kinetics (with rate constants of 0.27-0.32 mg L^-1 min^-1 at pH 2.0-6.0 and 0.14 mg L^-1 min^-1 at pH 1.0), indicating the saturation of reactive species by the reactants. The interlayer is self-oxidized by the valence band holes (VB h⁺), resulting in the formation of Cl^• and subsequently HClO, which is excited upon UV irradiation to provoke consecutive photoreactions for chlorine radical generation. Compared to the free chlorine, HO^•, Cl^•, and Cl₂^•-, the ClO^• produced using the UV/BiOCl system plays a predominant role in oxidizing ammonium under acidic conditions. BiOCl exhibits good stability because of the compensation of Cl^- from solution and maintains high activity under different conditions (i.e., different cations and co-existing anions, temperatures, and initial substrate concentrations). The successful removal of ammonium from real wastewater using the UV/BiOCl system suggests that this is a promising method for treating diluted ammonium under highly acidic conditions.

Recent advances in photocatalytic decomposition of water and pollutants for sustainable application

Photoinduced reduction and oxidation, the important processes in photocatalytic water splitting and organic degradation, have generated increasing interest to address the energy and environmental issues. In this review, the recent developments in bandgap and interfacial engineering for enhanced light absorption, efficient charge separation and interfacial reaction are focused toward the applications in photocatalytic water splitting and organic degradation. In photoinduced reduction for hydrogen evolution, three major strategies are discussed: cocatalysts, sacrificial agents and heterojunctions. In photoinduced oxidation for organic degradation, three types of emerging pollutants of current concerns are highlighted: organic dyes, pharmaceuticals and volatile organic compounds. The key challenges of promising photocatalysts are discussed for future development and practical application.

Engineering Z-scheme TiO2-OV-BiOCl via oxygen vacancy for enhanced photocatalytic degradation of imidacloprid

The development and application of photocatalysts with strong redox ability to degrade refractory pesticides is the key to eliminating pesticide contamination. In this work, we develop a facile, time-saving, and surfactant-assisted method to fabricate a new Z-scheme heterojunction based on TiO2/BiOCl. This photocatalyst is rich in oxygen vacancy defects (TiO2-OV-BiOCl), and displays an excellent photocatalytic degradation performance for imidacloprid (IMD), and a possible degradation pathway of IMD is provided. The surfactant F127 plays an essential role in regulating the oxygen vacancy defects (OVDs) of TiO2-OV-BiOCl, where the OVD mainly exists in 5 layer BiOCl ultrathin nanosheets. Free radical trapping experiments demonstrate that the introduction of an OVD in BiOCl as a 'charge mediator' changes the charge-transfer mode from a type-II mechanism to a Z-scheme mechanism. The formation of a Z-scheme heterojunction leads to an excellent light utilization and higher separation efficiency of photogenerated charge carriers with a prolonged lifetime compared to those of BiOCl and TiO2/BiOCl. This work highlights the critical role of an OVD in the construction of a Z-scheme heterojunction of TiO2/BiOCl, and it can be applied to construct efficient photocatalytic systems for pesticide degradation.

Tuning the Active Sites of Atomically Thin Defective Bi12O17Cl2 via Incorporation of Subnanometer Clusters

The introduction of subnanometer clusters as active sites on the surface of photocatalysts for efficiently tuning the selectivity and activity of the photocatalyts is still a challenge. Herein, the subnanometer Ag/AgCl clusters were incorporated on atomically thin defective Bi₁₂O₁₇Cl₂ nanosheets via rebinding with unsaturated Cl atoms. Benefiting from the surficial Bi vacancies (V_Bi) and Bi-O vacancies (V_Bi-O) in this atomically thin architecture, the local atomic arrangement was tuned so that the subnanometer Ag/AgCl clusters were successfully incorporated. An enhancement of photocatalytic activity for NO removal was achieved in which the activity is 3 times higher than that of Bi₁₂O₁₇Cl₂ and 1.8 times higher than that of defective Bi₁₂O₁₇Cl₂. The substitution of the active sites from surficial V_Bi and V_Bi-O to be subnanometer Ag/AgCl clusters enables a tunable redox potential and different reaction mechanisms in NO removal. Moreover, the selectivity of the photoinduced redox reaction on NO oxidation and CO₂ reduction was achieved via introducing an extra energy level.

Atomically Thin Oxyhalide Solar-Blind Photodetectors

2D wide-bandgap semiconductors demonstrate great potential in fabricating solar-blind ultraviolet (SBUV) photodetectors. However, the low responsivity of 2D solar-blind photodetectors still limits their practical applications. Here, high-responsivity solar-blind photodetectors are achieved based on 2D bismuth oxychloride (BiOCl) flakes. The 2D BiOCl photodetectors exhibit a responsivity up to 35.7 A W^-1 and a specific detectivity of 2.2 × 10¹⁰ Jones under 250 nm illumination with 17.8 µW cm^-2 power density. In particular, the enhanced photodetective performances are demonstrated in BiOCl photodetectors with increasing ambient temperature. Surprisingly, their responsivity can reach 2060 A W^-1 at 450 K under solar-blind light illumination, maybe owing to the formation of defective BiOCl grains evidenced by in situ transmission electron microscopy. The high responsivity throughout the solar-blind range indicates that 2D BiOCl is a promising candidate for SBUV detection.

Regulating the phase transition of monoclinic Bi4O5Br2 through the synergistic effect of “drag force” and facet recognition by branched polyethyleneimine

The synergistic effect of the “drag force” and facet recognition by BPEI was the driving force for the phase transition of BiOBr to Bi₄O₅Br₂.

Regulation of surface plasmon resonance and oxygen vacancy defects in chlorine doped Bi–BiO2−x for imidacloprid photocatalytic degradation

In this work, chlorine doped Bi–BiO_2−x (Cl–Bi–BiO_2−x) was prepared by a one-step solvothermal method using sodium bismuthate (NaBiO₃·2H₂O) as a Bi source without the addition of a surfactant.