Bismuth-based photocatalytic semiconductors: Introduction, challenges and possible approaches

Preparation, Characterization, and Performance Analysis of S-Doped Bi2MoO6 Nanosheets

S-doped Bi₂MoO₆ nanosheets were successfully synthesized by a simple hydrothermal method. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), N₂ adsorption-desorption isotherms, Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), elemental mapping spectroscopy, photoluminescence spectra (PL), X-ray photoelectron spectroscopy (XPS), and UV-visible diffused reflectance spectra (UV-vis DRS). The photo-electrochemical performance of the samples was investigated via an electrochemical workstation. The S-doped Bi₂MoO₆ nanosheets exhibited enhanced photocatalytic activity under visible light irradiation. The photo-degradation rate of Rhodamine B (RhB) by S-doped Bi₂MoO₆ (1 wt%) reached 97% after 60 min, which was higher than that of the pure Bi₂MoO₆ and other S-doped products. The degradation rate of the recovered S-doped Bi₂MoO₆ (1 wt%) was still nearly 90% in the third cycle, indicating an excellent stability of the catalyst. The radical-capture experiments confirmed that superoxide radicals (·O^2-) and holes (h⁺) were the main active substances in the photocatalytic degradation of RhB by S-doped Bi₂MoO₆.

All-Solid Z-Scheme Bi-BiOCl/AgCl Heterojunction Microspheres for Improved Electron-Hole Separation and Enhanced Visible Light-Driven Photocatalytic Performance

All-solid Z-scheme Bi-BiOCl/AgCl heterojunction microspheres are successfully prepared via hydrothermal, NaBH₄ reduction and chemical deposition strategy. They are tested by various characterization methods, and they show that metal Bi is present after reduction and AgCl nanoparticles are successfully compounded onto BiOCl. Bi plays the role of a bridge connecting the two semiconductors of BiOCl and AgCl. All-solid Z-scheme heterojunction structures are formed successfully. The narrow band gap of the Z-scheme Bi-BiOCl/AgCl heterojunction microspheres is about 2.17 eV, which can expand the optical response range. Moreover, the photocatalytic hydrogen production rate still reaches 198.2 μmol h^-1 g^-1, extends the electron transport life, inhibits the recombination of electron hole pairs, and improves the photocatalytic activity.

Bismuth titanate-based UV filters embedded mesoporous silica nanoparticles: Role of bismuth concentration in the self-sealing process

The development of new safe inorganic UV filters to effectively protect the skin from ultraviolet (UV) radiation effects is an emerging issue. Bismuth titanate-based UV filters embedded into mesoporous silica nanoparticles (MSN) represent a new class of inorganic sunscreens, with excellent UVA and UVB shielding properties. In addition, the presence of bismuth ions promotes a self-sealing process, allowing (i) the entrapment of the active phases in the deepest core of the system and (ii) the formation of an external glassy silica layer with a consequent suppression of the photocatalytic activity. In this work, aimed at studying in detail the self-sealing mechanism and accessing the role of bismuth ions in the formation of the system, a series of samples impregnated with a different amount of bismuth were investigated. The self-sealing process already occurs at the lowest content of bismuth and the mechanism is demonstrated to be triggered by the ability of Bi to work as a low-melting point agent for silica. Finally, a sunscreen formulation containing the new UV filter was prepared and the Sun Protection Factor (SPF), the pH and the viscosity were measured, demonstrating the potential of the proposed material for large-scale applications.

Multivariate comparison of photocatalytic properties of thirteen nanostructured metal oxides for water purification

Although the environmental photocatalysis is being developed for many years, the relationships between simple metal oxides have not been explored so far. In this study a multivariate comparison of thirteen nanostructured metal oxides (Bi₂O₃, CeO₂, Co₃O₄, Fe₂O₃, NiO, Pr₆O₁₁, SnO₂, SrTiO₃, TiO₂, WO₃, ZnFe₂O₄, ZnO and ZrO₂) was performed. The solution containing twenty-six psychotropic pharmaceuticals was used as a model mixture. In order to ensure the influence of the dissolved organic matter on the process, all the experiments were conducted in the river water. Simulated solar radiation was applied as the most environmentally relevant. The high-resolution LC-MS profiles, obtained for the photocatalytic samples after 1 h of irradiation, were then submitted to the multivariate chemometric analysis. Graphical representations of principal component analysis and hierarchical cluster analysis enabled visualization of the relationships between the studied oxides. The registered degradation profiles were compared qualitatively and discussed.

Electronic, Optical, Mechanical and Lattice Dynamical Properties of MgBi2O6: A First-Principles Study

Electronic structure, optical, mechanical, and lattice dynamical properties of the tetragonal MgBi2O6 are studied using a first-principles method. The band gap of MgBi2O6 calculated from the PBE0 hybrid functional method is about 1.62 eV and agrees well with the experimental value. The calculations on elastic constants show that MgBi2O6 exhibits mechanical stability and strong elastic anisotropy. The detailed analysis of calculated optical parameters and effective masses clearly indicate that MgBi2O6 has strong optical response in the visible light region and high separation efficiency of photoinduced electrons and holes.

Overview of Photocatalytic Membrane Reactors in Organic Synthesis, Energy Storage and Environmental Applications

This paper presents an overview of recent reports on photocatalytic membrane reactors (PMRs) in organic synthesis as well as water and wastewater treatment. A brief introduction to slurry PMRs and the systems equipped with photocatalytic membranes (PMs) is given. The methods of PM production are also presented. Moreover, the process parameters affecting the performance of PMRs are characterized. The applications of PMRs in organic synthesis are discussed, including photocatalytic conversion of CO2, synthesis of KA oil by photocatalytic oxidation, conversion of acetophenone to phenylethanol, synthesis of vanillin and phenol, as well as hydrogen production. Furthermore, the configurations and applications of PMRs for removal of organic contaminants from model solutions, natural water and municipal or industrial wastewater are described. It was concluded that PMRs represent a promising green technology; however, before the application in industry, additional studies are still required. These should be aimed at improvement of process efficiency, mainly by development and application of visible light active photocatalysts and novel membranes resistant to the harsh conditions prevailing in these systems.

Design and fabrication of polyaniline/Bi2MoO6 nanocomposites for enhanced visible-light-driven photocatalysis

PANI/Bi₂MoO₆ composites with improved photoelectrochemical performance and accessible interfacial active sites were fabricated for enhanced visible-light-driven photocatalytic degradation of RhB.

Application of metal oxide semiconductors in light-driven organic transformations

Herein, we provide an up-to-date overview of metal oxide semiconductors (MOS) as versatile and inexpensive photocatalysts to enable light-driven organic transformations.

Laboratory based synthesis of the pure form of gananite (BiF3) nanoparticles: a potential material for electrochemical supercapacitor application

Gananite is a rare bismuth based mineral with a general formula of bismuth trifluoride (BiF₃).

Uniform flower-like BiOBr/BiOI prepared by a new method: visible-light photocatalytic degradation, influencing factors and degradation mechanism

BiOBr/BiOI photocatalyst with different molar ratios was synthesized via a simple one-step solvothermal method. The uniform flower-like BiOBr/BiOI (3 : 1) owns high photocatalytic degradation efficiency, excellent recyclability and stability.

Dual-functional persistent luminescent nanoparticles with enhanced persistent luminescence and photocatalytic activity

NIR persistent luminescence and photocatalytic activity of the PLNPs were significantly and simultaneously improved via additional doping of Bi³⁺.

Fabrication of surface hydroxyl modified g-C3N4with enhanced photocatalytic oxidation activity

Photocatalytic activity of C₃N₄in the decomposition of phenolic compounds in water was significantly improved with hydroxyl surface modification.

Novel hierarchical Sn3O4/BiOX (X = Cl, Br, I) p–n heterostructures with enhanced photocatalytic activity under simulated solar light irradiation

Sn₃O₄/BiOX (X = Cl, Br, I), a series of p–n-heterojunction-based photocatalysts, were prepared by a combination of an ultrasonic-assisted precipitation–deposition method and hydrothermal method.

Room-temperature fabrication of bismuth oxybromide/oxyiodide photocatalyst and efficient degradation of phenolic pollutants under visible light

Bismuth oxybromide/oxyiodide (Bi₄O₅Br_xI_2-x) photocatalysts were successfully fabricated using a facile homogeneous precipitation method at room temperature. The obtained Bi₄O₅Br_xI_2-x demonstrated highly enhanced visible-light performances compared with Bi₄O₅Br₂ and Bi₄O₅I₂. The poducts were characterized by X-ray diffraction (XRD), UV-vis diffuse reflectance spectra (DRS), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and transmission electron microscope (TEM). The DRS analysis shows that the band gap structures of Bi₄O₅Br_xI_2-x have been gradually modulated by changing the Br/I molar ratio. The obtained Bi₄O₅Br_xI_2-x samples have exhibited efficient photocatalytic activities in decomposing resorcinol, o-phenylphenol, and 4-tert-butylphenol. The Br/I molar ratio has great influence on the activity of the photocatalysts, and Bi₄O₅Br_0.6I_1.4 exhibited the best activity which was about 2.77 and 1.80 times higher than that of Bi₄O₅Br₂ and Bi₄O₅I₂, respectively. The degradation intermediates were identified by liquid chromatography-mass spectrometry (LCMS), and the possible degradation pathway of resorcinol over Bi₄O₅Br_xI_2-x photocatalysts was proposed. The strong visible light absorption, high charge separation efficiency, and proper band potentials should be responsible for the excellent activity of Bi₄O₅Br_xI_2-x photocatalyst. Trapping experiments using radical scavengers confirmed the generation of O₂^-, OH, and h⁺, but only O₂^- and h⁺ have played the chief role in removing organic pollutants from water.

Review on nanoscale Bi-based photocatalysts

Nanoscale Bi-based photocatalysts are promising candidates for visible-light-driven photocatalytic environmental remediation and energy conversion. However, the performance of bulk bismuthal semiconductors is unsatisfactory. Increasing efforts have been focused on enhancing the performance of this photocatalyst family. Many studies have reported on component adjustment, morphology control, heterojunction construction, and surface modification. Herein, recent topics in these fields, including doping, changing stoichiometry, solid solutions, ultrathin nanosheets, hierarchical and hollow architectures, conventional heterojunctions, direct Z-scheme junctions, and surface modification of conductive materials and semiconductors, are reviewed. The progress in the enhancement mechanism involving light absorption, band structure tailoring, and separation and utilization of excited carriers, is also introduced. The challenges and tendencies in the studies of nanoscale Bi-based photocatalysts are discussed and summarized.

Graphene Oxide/BiOCl Nanocomposite Films as Efficient Visible Light Photocatalysts

A novel graphene oxide/BiOCl (GO/BiOCl) nanocomposite film was prepared via a spread coating method. In visible-light photocatalytically degrading Rhodamine B (RhB) experiments, 2 wt% GO/BiOCl could degrade 99% of RhB within 1.5 h and the rate constant was 12.2 times higher than that of pure BiOCl. The degradation efficiency still kept at 80% even after 4 recycles, evidencing the relatively good recyclability. The enhancement was attributed to the improvement of visible light adsorption and charge separation. Holes and superoxide radicals· O2- played a major role as reactive species. The values of conduction band and valence band for GO and BiOCl were calculated and a new photocatalytic mechanism of GO/BiOCl nanocomposite was proposed.

Palladium nanoparticles and rGO co-modified BiVO4 with greatly improved visible light-induced photocatalytic activity

A ternary composite, Pd-rGO-BiVO₄, was fabricated with reduced graphene oxide (rGO) and palladium nanoparticles decorated on the surface of BiVO₄. As-prepared samples were tested for the photocatalytic degradation of phenol under visible light irradiation. Enhancement was observed for the ternary structure, merits of which may be as follows: 1) rGO wrapped BiVO₄ facilitated the photogenerated electrons transfer, 2) palladium nanoparticles served as electron acceptors, 3) palladium nanoparticles on the surface were capable of absorbing visible light photons. The uptake of photogenerated charge carriers would improve their separation and more oxidative species may be produced that can participate in the degradation of organics. Due to the SPR effect of palladium nanoparticles on the surface, the harvesting capacity of the photocatalyst to absorb visible light photons was increased, and thus its photocatalytic activity was improved. It should be noted that phenol was more easily adsorbed by rGO due to the π-π interaction between rGO and phenol, which also contributed to the enhancement in the photocatalytic activity. This work provides new evidence to confirm the advances of ternary structures applied in the photocatalytic removal of phenolic compounds in water under visible light irradiation.

Photocatalytic degradation of bisphenol A in aqueous media: A review

Bisphenol A (BPA) is known to be an emerging pollutant in various environmental compartments. Human exposure to BPA occurs widely because it is commonly used as the raw material in a variety of industrial processes (e.g., the preparation of epoxy and polycarbonate resins). In this review, a brief survey was carried out to cover a range of photocatalytic materials (e.g., titania, zinc, silver, carbon, and bismuth) and their modified forms as an effective means to treat water systems contaminated with BPA. The overall efficiency and limitations of these catalysts are described for the photocatalytic treatment of BPA.

BiOX (X = Cl, Br, I) photocatalytic nanomaterials: Applications for fuels and environmental management

Energy and environmental issues are the major concerns in our contemporary "risk society". As a green technique, photocatalysis has been identified as a promising solution for above-mentioned problems. In recent decade, BiOX (X = Cl, Br, I) photocatalytic nanomaterials have sparked numerous interest as economical and efficient photocatalysts for energy conversion and environmental management. The distinctive physicochemical properties of BiOX nanomaterials, especially their energy band structures and levels as well as relaxed layered nanostructures, should be responsible for the visible-light-driven photocatalytic performance improvement, which could be utilized in dealing with the global energy and environmental challenges. In this review, recent advances for the enhancement of BiOX photocatalytic activity are detailedly summarized. Furthermore, the applications of BiOX photocatalysts in water splitting and refractory organic pollutants removal are highlighted to offer guidelines for better development in photocatalysis. Particularly, no relative reports in previous studies were documented in CO₂ reduction as well as heavy metals and air pollutants removal, thus this review presented as a considerable research value. Challenges in the construction of high-performance BiOX-based photocatalytic systems are also discussed. With the exponential growth of studies on BiOX photocatalytic nanomaterials, this review provides unique and comprehensive perspectives to design BiOX-based photocatalytic systems with superior visible light photocatalytic activity. The knowledge of both the merits and demerits of BiOX photocatalysts are updated and provided as a reference.