Design Of Bismuth Molybdate-Based Materials With Enhanced Photocatalytic Performance

The Industrial Revolution promotes the rapid development of science and technology.Human society has entered a new era.With the acceleration of industrialization and urbanization,environmental pollution has posed a serious threat to ecosystems and human health.At the same time,the global population boom has led to the continuous reduction of fossil fuel reserves.The lack of energy supply has become a major problem that mankind will face.Photocatalytic technology which centered on semiconductor materials provides new ideas for alleviating environmental degradation and energy demand.In this paper,aiming at the problems of bismuth molybdate（Bi₄Mo O₉）photocatalyst such as poor photogenerated charge separation efficiency,low utilization of visible light and weak redox ability of photogenerated charges.The size of Bi₄Mo O₉ material was optimized to achieve controllable modulation of its energy band structure.On this basis,combined with the design of surface defect,heterostructure construction,and surface modification,further regulated the charge separation efficiency and the redox ability of photogenerated charges.X-ray powder diffraction,transmission electron microscopy,fourier transform infrared spectroscopy,UV-Vis diffuse reflectance spectroscopy and photoelectric characterizations were carried out to confirm the composition,morphological characteristics and optical properties of the prepared Bi₄Mo O₉-based materials.The performance of photocatalytic degradation towards organic pollutants and photocatalytic CO₂ reduction over the Bi₄Mo O₉-based materials were evaluated.The specific research contents of this paper are as follows:1.Bi₄Mo O₉ photocatalytic material with rich oxygen vacancy（Bi₄Mo O₉-OVR）was synthesized by one-step solvothermal method with precise p H control.As a comparison,Bi₄Mo O₉ photocatalytic material with low oxygen vacancy（Bi₄Mo O₉-OVL）has been prepared by changing the reaction solvent.Electron paramagnetic resonance and X-ray photoelectron spectroscopy spectra confirmed the existence of oxygen vacancy in as-prepared Bi₄Mo O₉-OVR and Bi₄Mo O₉-OVL samples.The results of photocatalytic degradation experiment verified that the higher concentration of oxygen vacancy significantly facilitated the performance of photocatalytic degradation of Bi₄Mo O₉ materials.Under visible light（λ>400 nm）irradiation for225 min,88.7%tetracycline was degraded by Bi₄Mo O₉-OVR,which was 4.7 times higher than that of Bi₄Mo O₉-OVL material.Photoelectric characterizations proved that introduction of rich oxygen vacancy in the Bi₄Mo O₉ could boost the separation and migration of electron and hole pairs,ultimately increase the utilization efficiency of carriers in the whole photodegradation process.Moreover,electron spin resonance technology and free radical capture experiments clarified the active species in the photodegradation process was superoxide radicals.2.Bi₂WO₆/Bi₄Mo O₉ direct Z-scheme heterojunction composites was prepared by using Bi₄Mo O₉ nanoparticles as growth substrate.Ciprofloxacin have been used for exploring the photocatalytic activity of Bi₄Mo O₉ nanoparticles and Bi₂WO₆/Bi₄Mo O₉ composites under visible light irradiation.The results of photocatalytic experiments exhibited that Bi₂WO₆/Bi₄Mo O₉-2composite material showd the highest photocatalytic activity.Under visible light irradiation for180 min,nearly 81.7%ciprofloxacin was degraded by Bi₂WO₆/Bi₄Mo O₉-2 composite material,which was 4.1 times higher than that Bi₄Mo O₉ nanoparticles.The results of photoelectrochemical characterization confirmed that the formation of in-situ Z-type heterojunction could greatly reduce the recombination of photogenerated carriers.At the same time,the matched energy band structure of Bi₂WO₆ and Bi₄Mo O₉ material maximized the oxidation ability of holes on the valence band of Bi₂WO₆ material and the reduction ability of high-energy electrons on the conduction band of Bi₄Mo O₉ material.The electron spin resonance and radical trapping experiments determined that holes,superoxide radicals and hydroxyl radicals were the main reactive species during the photocatalytic degradation reaction.3.N-CQDs were introduced in situ onto the surface of Bi₄Mo O₉ nanoparticles by electrostatic adsorption.The introduced N-CQDs realized higher charge separation efficiency and greatly increased electron density around the sites active for CO₂ adsorption.Impressively,after5 h of light irradiation,the N-CQDs/Bi₄Mo O₉-2 composite showed a maximum CO yield rate of3.2μmol g^-1 h^-1in pure water without any sacrificial reagents,which was 3.2 times higher than that of Bi₄Mo O₉ nanoparticles(1.00μmol g^-1 h^-1).The mechanism of photocatalytic CO₂conversion over the N-CQDs/Bi₄Mo O₉ composite was revealed by in situ Fourier-transform infrared spectroscopy.