Enhancing the adsorption-photocatalytic efficiency of BiOBr for Congo red degradation by tuning the surface charge and bandgap via an Y3+-I- co-doping strategy

Metal-ion doping and halogen substitution have been largely applied to tune the bandgap of bismuth oxybromide (BiOBr) to upgrade its photodegradation capacity. In this work, the adsorption capacity and photocatalytic behavior of solvothermally synthesized BiOBr photocatalysts can be optimized via the synergistic effect of Y3+- and I--doping. After an adsorption reaction in the dark and exposure for another 80 min to visible light, pure BiOBr can remove 46.5% of Congo red (CR) from water with an initial CR concentration of 50 mg L-1. Meanwhile, Bi0.8Y0.20OBr0.97I0.03, the co-doped catalyst, displays total degradation rates exceeding 98% and 92% with CR dosages of 50 and 100 mg L-1, respectively, demonstrating a doubled degradation capacity. With the co-doping solution, the negative charges on the catalysts reduce, more oxygen vacancies are generated, the bandgap remarkably narrows, and the photoabsorption range broadens for derivation of photoinduced electron-hole pairs. The mechanism for optimized photodegradation behavior and dramatically increased adsorption capacity are discussed based on analyses of the structural evolution, surface properties including the chemical state and surface charge, electrochemical performance and the yield/type of photogenerated species. Density functional theory (DFT) simulations were conducted to investigate the structural state, density of states (DOS) and electrostatic potential.

Potential application of bismuth oxyiodide (BiOI) when it meets light

Bismuth oxyiodide (BiOI) is a kind of typical two-dimensional (2D) material that has been increasingly developed alongside other 2D materials such as graphene, MXenes, and transition-metal dichalcogenide. However, its potential applications have not been widely whispered compared to those of other 2D materials. Using its distinctive properties, BiOI can be used for various applications, especially when it meets sunlight and other light-related electromagnetic waves. In this present review, we discuss the developments of BiOI and challenges in the applications for photodetector and light-assisted sensors, photovoltaic devices, optoelectronic logic devices, as well as photocatalysts. We start the discussion with a basic understanding and development of BiOI, crystal structure, and its properties. The synthesis and further development, such as green synthesis and its challenges in the synthesis-suited industry, as well as device integration, are also explained together with a plausible strategy to enhance the feasibility of BiOI for those various applications. We believe that the provided discussion and perspectives will not only promote BiOI to be one of the highly considered 2D materials but can also assist recent graduates in any materials science discipline and inform the senior scientists and industrial-based stakeholders of the latest advances in bismuth oxide and mixed-anion compounds.

A multi-factor adjustable PtSe2/GaN van der Waals heterostructure with enhanced photocatalytic performance

Two-dimensional van der Waals heterostructures with multiple tunable approaches in electronic and optical properties are highly superior for photocatalysis and novel devices. By applying first-principles calculations, we systematically studied the electronic structure, optical absorption, carrier mobility and solar-to-hydrogen efficiency of PtSe2/GaN heterostructures, which are affected by different thicknesses, varying directions of polarization of the GaN nanosheets and applied mechanical strain of the whole system. The results indicate that these heterostructures exhibit thermodynamic stability at room-temperature (300 K), and most configurations have type-II band alignment, among which the heterostructure consisting of GaN trilayers and PtSe2 shows high visible-light absorption (1.71 × 105 cm-1) and ultra-wide range of pH values (pH = 0-14) for the photocatalytic water splitting reaction and exceedingly high overpotential for the hydrogen evolution reaction (3.375 eV). Simultaneously, being two of the most significant parameters of photocatalysis and devices, the carrier mobility and solar-to-hydrogen efficiency have also been calculated, respectively, reaching up to 1601 cm2 V-1 s-1 and 17.2%. Moreover, the photoelectrical properties can be highly tuned through further biaxial strain engineering; especially, the visible-light absorption can be enhanced to 2.85 × 105 cm-1 by applying 6% compression strain. Thus, the PtSe2/GaN heterostructure we proposed shows a broad prospect for photocatalytic water splitting.

Optically Induced Coherent Phonons in Bismuth Oxyiodide (BiOI) Nanoplatelets

Bismuth oxyiodide (BiOI) is a promising material for photocatalysis combining intriguing optical and structural properties. We show that excitation by a femtosecond laser pulse creates coherent phonons inducing a time-variant oscillating modulation of the optical density. We find that the two underlying frequencies originate from lattice vibrations along the [001] crystallographic axis, the stacking direction of oppositely charged layers in BiOI. This is consistent with a subpicosecond charge separation driven by a built-in dipolar field. This partially screens the field, launching coherent phonons. Further, we determine the two major dephasing mechanisms that lead to the loss of vibronic coherence: (i) the anharmonic decay of an optical phonon into two acoustic phonons and (ii) phonon-carrier scattering. Our results provide a direct demonstration of the presence of an electric field in BiOI along the [001] axis and show its role in efficient charge separation that is crucial for photocatalytic applications of BiOI.

Thermochemical stability, and electronic and dielectric properties of Janus bismuth oxyhalide BiOX (X = Cl, Br, I) monolayers

Ultrathin monolayers of bismuth oxyhalide materials BiOX (X = Cl, Br, I) grown along 〈001〉 are studied using first-principles density functional theory. Both pristine BiOX and Janus (X, X' = Cl, Br, I) monolayers are investigated by analyzing their structural stability using formation enthalpy and phonon density of states. On the other hand, their thermochemical reactivity is understood from their surface energy trends in symmetric and asymmetric terminations. The theoretically measured optical band gaps and fundamental band gaps of these Janus monolayers are compared with their pristine counterparts BiOX and BiOX' as well as to the known experimental measurements. All of the possible Janus monolayers possess structural, electronic and optical properties intermediate to the corresponding properties of the two associated pristine BiOX and BiOX' monolayers. According to the formation enthalpy, stabilization is equally favorable for all the monolayers, whereas the lowest surface energy is found for BiOCl_0.5Br_0.5, leading to excellent thermochemical reactivity which is consistent with recent experimental measurements. The frequency dependent dielectric functions are simulated in the density functional perturbation theory limit, and the optical band gaps are estimated from the absorption and reflectance spectra, and are in excellent agreement with the known experimentally measured values. High frequency dielectric constants of these materials with 2D symmetry are estimated from G ₀ W ₀ calculations including local field and spin-orbit effects. The larger dielectric constants and wider differences in the charge carriers' effective masses also provide proof that this new class of 2D materials has potential in photo-electrochemical applications. Thus, fabricating Janus monolayers of these oxyhalide compounds would open up a rational design strategy for tailoring their optoelectronic properties, which may offer guidance for the design of highly efficient optoelectronic materials for catalysis, valleytronic, and sensing applications.

First-principles investigation of nonmetal doped single-layer BiOBr as a potential photocatalyst with a low recombination rate

Nonmetal doping is an effective approach to modify the electronic band structure and enhance the photocatalytic performance of bismuth oxyhalides.

Bismuth-Based Photocatalysts for Solar Photocatalytic Carbon Dioxide Conversion

Photocatalytic CO₂ conversion into solar fuels is an effective means for simultaneously solving both the greenhouse effect and energy crisis. In the past ten years, bismuth-based photocatalysts for environmental remediation have experienced a golden period of development. However, solar photocatalytic CO₂ conversion has only been developed over the past five years and, until now, no reviews have been published on bismuth-based photocatalysts for the photocatalytic conversion of CO₂ . For the first time, solar photocatalytic CO₂ conversion systems are reviewed herein. Synthetic methods and photocatalytic CO₂ performances of bismuth-based photocatalysts, including Sillén-structured BiOX (X=Cl, Br, I); Aurivillius-structured Bi₂ MO₆ (M=Mo, W); and Scheelite-structured BiVO₄ , Bi₂ S₃ , BiYO₃ , and BiOIO₃ , are summarized. In addition, activity-enhancing strategies for this photocatalyst family, including oxygen vacancies, bismuth-rich strategy, facet control, conventional type II heterojunction, Z-scheme heterojunction, and cocatalyst deposition, are reviewed. Finally, the main mechanistic research methods, such as in situ FTIR spectroscopy and theoretical calculations, are presented. Challenges and research trends reported in studies of bismuth-based photocatalysts for photocatalytic CO₂ conversion are discussed and summarized.

Composite Magnetic Photocatalyst Bi₅O₇I/MnxZn1-xFe₂O₄: Hydrothermal-Roasting Preparation and Excellent Photocatalytic Activity

A new composite magnetic photocatalyst, Bi₅O₇I/Mn_xZn_1-xFe₂O₄, prepared by a hydrothermal-roasting method was studied. The photocatalytic properties of Bi₅O₇I/Mn_xZn_1-xFe₂O₄ were evaluated by degradation of Rhodamine B (RhB) under simulated sunlight irradiation, and the structures and properties were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet-visible light (UV-Vis) diffuse reflectance spectra (DRS), and a vibrating sample magnetometer (VSM). The results indicated that Bi₅O₇I/Mn_xZn_1-xFe₂O₄ was an orthorhombic crystal, which was similar to that observed for Bi₅O₇I. Bi₅O₇I/Mn_xZn_1-xFe₂O₄ consisted of irregularly shaped nanosheets that were 40⁻60 nm thick. The most probable pore size was 24.1 nm and the specific surface area was 7.07 m²/g. Bi₅O₇I/Mn_xZn_1-xFe₂O₄ could absorb both ultraviolet and visible light, and the energy gap value was 3.22 eV. The saturation magnetization, coercivity and residual magnetization of Bi₅O₇I/Mn_xZn_1-xFe₂O₄ were 3.9 emu/g, 126.6 Oe, and 0.7 emu/g respectively, which could help Bi₅O₇I/Mn_xZn_1-xFe₂O₄ be separated and recycled from wastewater under the action of an external magnetic field. The recycling experiments revealed that the average recovery rate of the photocatalyst was 90.1%, and the photocatalytic activity was still more than 81.1% after five cycles.

Heavy pnictogen chalcohalides: the synthesis, structure and properties of these rediscovered semiconductors

Heavy pnictogen chalcohalides offer various shades from the same palette, like “Paysage” by Nicolas de Staël. Their versatility and tunability lead to a new world of possible applications.