Modulation Of Surface And Electronic Structure And Photocatalytic Performance Study Of Tin/Bismuth Based Photocatalysts

Recently,energy shortage and environmental deterioration were two majorproblems which have to be solved urgently by human.Heterogeneous photocatalysis based on semiconductors utilizing solar energy had been regarded as an effective and green strategy for environmental protection and energy saving,since TiO₂photocatalyst was applied to photoelectric splitting water reaction in the 1970s.However,the limited utilization of solar energy and the low photogenerated charge carrier separation efficiency of semiconductor photocatalysts were the culprits restricting the photocatalytic activity.Hence,the development and utilization of photocatalysts which had outstanding photocatalytic performance was the essential key to adopt solar energy into solving environmental issue.In this thesis,a series of tin and bismuth based semiconductor photocatalysts were developed with the aim to obtain catalysts with high photocatalytic activity via the regulation of surface properties and electronic structure.Furthermore,the influence of the composition,surface feature,electronic band structure and charge transfer kinetics on the photocatalytic properties were further discussed in this thesis.The main contents of this doctoral dissertation are summarized as follows:?1?Tin niobate photocatalysts with two different phase structure?SnNb₂O₆ and Sn₂Nb₂O₇?were successfully obtained with the aim to explore the effect of phase and electronic structure on the charge kinetics and photocatalytic activity.It is found that the variation of phase structure from SnNb₂O₆ to Sn₂Nb₂O₇ accompanied with the modulation of particle size and band edge potential,affecting the photocatalytic water splitting and methyl orange?MO?degradation activity.The experimental results demonstrated that the variation of electronic structure had great impacts on the charge separation and transfer rate of tin niobate catalysts and the subsequent photocatalytic performance.Photocatalytic testament suggested that SnNb₂O₆ had a higher catalytic activity toward MO degradation and H₂ evolution in compared with Sn₂Nb₂O₇,which might due to Sn⁴⁺was existed in Sn₂Nb₂O₇.Electron spin resonance?EPR?and trapping experiment results indicated that photogenerated holes,superoxide radical?·O₂^-?and hydroxyl radical?·OH?were the main active species in photodegradation of MO.?2?Disordered SnNb_xO_y with controlled structural distortion was developed in order to uncover the underlying origination for highly enhanced photocatalytic activity of the disordered materials.Synchrotron radiation wide-angle X-ray scattering?SR-WAXS?and transmission electron microscopy?TEM?results indicated that the as-prepared catalysts show disordered features with controlled short range ordering limits.In combination with optical diffuse reflectance spectra,valence band X-ray photoelectron spectroscopy?VB-XPS?and density functional theory?DFT?prediction,the disordered structure engineering can induce continuous band gap broadening and downward valence band edge,predicting enhanced oxidation driving force.Moreover,transient absorption spectroscopy?TAS?,EPR spectra suggested that the highly enhanced photocatalytic activity mainly originates from defect-assisted charge separation,which implies that abundant defective centers may serve as trap centers for efficient charge separation and improve charge transfer rate to suppress the charge recombination process.Photocatalytic test demonstrated that the optimal photocatalytic activity toward H₂ evolution of the disordered catalyst shows¹1 times improvement with respect to that of the crystalline counterpart.?3?A series of Bi₂Sn₂O₇ photocatalysts with oxygen vacancies?OVs?was developed in order to modulate defect chemistry and electronic structure by regulating the charge states of tin species for enhancing the photocatalytic selective oxidation of alcohols property.OVs were beneficial to promoting the adsorption and activation of molecular oxygen.Meanwhile,the valence band edge potential of Bi₂Sn₂O₇ with higher OVs content showed a distinct downshift,improving the thermodynamical driving force of photooxidation.Furthermore,OVs were conducive to facilitating the separation of photoinduced carriers,which can be confirmed by the surface photovoltage testament.By optimizing the electronic structure as well as defect chemistry,the optimal benzyl alcohol conversion efficiency was 76.0%with selectivity toward benzaldehyde being about 100%.It's found that photogenerated holes and·O₂^-active species played critical roles in photocatalytic aerobic oxidation.?4?The photocatalytic performance of graphite carbon nitride was optimized by regulating the morphology,electronic structure and surface properties via introducing cyano and urea groups.The insertion of defective groups in carbon nitride matrix led to a drastic downshift of band edge potentials in comparison to bulk g-C₃N₄,giving birth to a suitable band edge alignment between g-C₃N₄ and Sn₂Nb₂O₇.A traditional type-?Sn₂Nb₂O₇/g-C₃N₄ heterojunction was constructed in order to improving the photocatalytic activity towards oxytetracycline?OTC?degradation.X-ray photoelectron spectroscopy?XPS?investigated that a strong interaction was existed between Sn₂Nb₂O₇ and g-C₃N₄.The formation of Sn₂Nb₂O₇/g-C₃N₄heterojunction structure was beneficial to the separation and transfer of photogenerated electron-hole pairs.The optimal photocatalytic degradation efficiency was 1.6 and 2.19 times higher than that of pristine Sn₂Nb₂O₇ and g-C₃N₄.?5?A direct Z-scheme SnO₂/Bi₂Sn₂O₇ heterojunction photocatalysts were designed and prepared aiming to improve the photocatalytic activity of Bi₂Sn₂O₇ via boosting the separation of photo-induced electron and hole pairs.The photogenerated electrons and holes were retained in the conduction band of Bi₂Sn₂O₇ and valence band of SnO₂,respectively,contributing to preserving high photoredox ability.The higher photocatalytic performance of SnO₂/Bi₂Sn₂O₇ was mainly originated from the efficient spatial separation of photogenerated charge.Additionally,the phase structure,morphologies and the chemical state had unobvious change after three cycles,indicating that SnO₂/Bi₂Sn₂O₇ photocatalysts owned a high stability.?6?A novel direct Z-scheme CuBi₂O₄/BiOBr heterostructural photocatalyst was developed aiming to modulate the adsorption ability and photocatalytic degradation performance toward antibiotics.Tetracycline as the model antibiotics was found to show pH value and temperature dependent adsorption capacity over CuBi₂O₄/BiOBr.The adsorption process was fitted to Pseudo-second-order and Elovich kinetic models and adsorption isotherm was followed Freundich isotherm model.Adsorption thermodynamics analysis indicated that tetracycline adsorption by CuBi₂O₄/BiOBr was spontaneous and endothermic.The absorptive capability of CuBi₂O₄/BiOBr was about 6.3 and 4.2 times higher than that of pristine CuBi₂O₄ and BiOBr.Junction of CuBi₂O₄ and BiOBr can modulate the surface feature and interfacial interactions,leading to direct Z-scheme charge kinetics for improved photocatalytic activity.The maximal photocatalytic degradation efficiency of CuBi₂O₄/BiOBr was about 5-fold and 1.5 times higher than that of CuBi₂O₄ and BiOBr.The mineralization process and intermediates for tetracycline degradation were carefully identified by total organic carbon analysis and liquid chromatograph-tandem mass spectrometry.Plausible transformation pathway and photocatalytic mechanism were proposed.