Study On The Environmental Photocatalytic Performances And Reaction Mechanisms Of Two-dimensional Nanomaterials Regulated By Metal Site Modification

Currently,environmental pollution and energy crisis are two major global challenges.As an inexhaustible and green energy source,solar energy is one of the most precious wealth for the whole world.In the past decades,photocatalysis has received extensive research interests due to its capability of solar energy utilization,and it has been proposed as one of the most promising technologies to address the environmental and energy issues.During the photocatalytic process,solar energy can be converted into chemical energy,which is the driving force of oxidation and reduction reactions.Therefore,photocatalysis could be applied in the removal of environmental pollutants and also in the field of energy production and conversion.However,despite some progress has been achieved in the past studies,the utilization efficiency of solar energy is still far lower than the theoretical expectations.And its large-scale application in environmental pollution control and energy production has been rarely reported.Three basic steps are involved in a photocatalytic reaction:the light absorption of the photocatalysts,the separation and transfer of photoexcited charges and the surface reactions.An ideal photocatalyst should possess the catalytic reactivities in broad spectrum.In the meantime,the photoexcited charges over an ideal photocatalyst should be rapidly separated and efficiently transferred to the reactive sites.Those two mentioned issues are still the scientific bottlenecks that needed to be overcome.In this thesis,a series of two-dimension semiconductor photocatalysts were modulated via metal sites modification to realize catalytic reactivity over a broad spectrum and highly efficient photoexcited charges transportation process.The obtained catalysts were applied for degradation of organic pollutants and photocatalytic hydrogen evolution.By utilizing multidisciplinary spectroscopic characterizations and theoretical calculations,the relationship between the structural properties and the reactivity of the catalysts was investigated and analyzed.1.Cu-WO₃ samples with broad spectrum catalytic activities were prepared by atomic Cu decoration.The obtained Cu-WO₃ catalysts exhibited high tetracycline removal efficiency under the excitation of both visible light and near-infrared light.Under the excitation of visible light,the reaction rate constant of 1 wt%Cu-WO₃ was1.17×10^-2 min^-1,which was 3.1 times of that the pristine WO₃ sample(k=3.71×10^-3min^-1),and the mineralization rate reached 94.6%.Under the excitation of near-infrared light,the modulated WO₃ catalyst also showed excellent tetracycline degradation performance.The reaction rate constant(2.92×10^-3 min^-1)of 1 wt%Cu-WO₃ was 14.4times of that over the pristine WO₃(2.02×10^-4 min^-1),with a mineralization rate 92.4%,which is one of the best performances compared to the previously reported studies.The efficient mineralization of tetracycline under both visible and near-infrared light illustrates the efficient conversion and harmlessness of tetracycline.Detailed analysis of possible degradation pathways of tetracycline was also conducted by high performance liquid chromatography mass spectrometry.It was found that tetracycline molecules can be completely converted into carbon dioxide and water after multi-step oxidative decompositions,which was consistent with our experimental results.The structural properties analysis showed that the introduced Cu element existed as the highly dispersed species,without the formation of Cu nanoparticles.In the previously reported works,the Cu nanoparticles could facilitate the near-infrared driven catalysis due to its surface plasmonic resonance effect.However,in this study,the contribution of surface plasmonic resonance effect of Cu element could be excluded given its nature of atomic dispersion.The electronic structures and band alignments analysis were further performed to investigate the mechanism of boosted catalytic performance over Cu-WO₃ samples.Results showed that the introduction of Cu element induced the charge redistribution over WO₃ unit cells.More charges accumulated around O atoms over Cu-WO₃molecular structure,while the charge depletion was observed over W and Cu atoms.This result was consistent with the calculation results of density of states.Moreover,the evolved band alignment over Cu-WO₃ was favorable for the generation of highly reactive species such as hydroxyl radicals,thus stimulating the decomposition and mineralization of tetracycline.Luminescence spectral results reveal the upconversion capability of both WO₃ and Cu-WO₃ samples,which enabled the catalysts absorb the low-energy near-infrared photons and further convert it into high-energy visible photons.This is also one of the reasons why the Cu-WO₃ sample possess near-infrared light driven catalytic performance.The photoexcited charges transportation process between different samples were also studied through the combined characterizations like photoinduced luminescence spectra,time-resolved luminescence spectra,surface photovoltage spectra and etc.Compared with WO₃,the modulated Cu-WO₃ samples possessed strengthened internal electric field,facilitating the separation and transfer process of photoexcited carriers.This study provided a novel insight into the development of catalysts with broad spectrum catalytic performances.2.Highly reactive Pt-Au binary single site catalysts were obtained by constructing Pt-Au binary atomic sites over g-C₃N₄.The as-prepared samples exhibited a two-dimensional nanosheet morphology,which was similar to the g-C₃N₄ substrate.In addition,the loaded Pt and Au elements were identified as atomic sites.The atomic dispersion property of Pt and Au elements and related coordination structures over as-prepared samples were thoroughly examined and investigated by multiple characterizations such as high-angle annular dark-field scanning transmission electron microscopy,X-ray absorption spectroscopy,electron paramagnetic spectroscopy and X-ray photoelectron spectroscopy.Results showed that the coordination configuration of the binary single site catalyst Pt-Au SAC was Pt-N₄/Au-N₅（where Pt and Au atoms were stabilized by the six-fold cavities between adjacent heptazine subunits）.Pt and Au single atoms existed in the form of Pt-N₃ and Au-N₅ in unary single site catalyst Pt SAC and Au SAC,respectively.However,in the case of Pt SAC,the Pt single atoms were located at the C defective sites.And Au atoms in Au SAC were in the same location as that over Pt-Au SAC.Based on the proposed coordination structures,several possible molecular structures were established and related stabilities were evaluated.The theoretical simulation results showed that the Pt and Au atoms in Pt-Au SAC and Au SAC were better stabilized at the six-fold cavities between adjacent heptazine subunits in the form of Pt-N₄/Au-N₅,while the Pt atoms over Pt SAC were better stabilized at the C defective sites of g-C₃N₄ in the form of Pt-N₃.Those results were in accordance with the above spectroscopic analyses.3.Catalytic reactivities over as-prepared samples were evaluated using photocatalytic hydrogen evolution as a model reaction.The optimal Pt-Au binary site photocatalysts（0.25%loading）showed 4.9 and 2.3-folds enhancement of performance compared with their Pt and Au single-site counterparts,respectively,with a hydrogen evolution rate of 1.88 mmol g^-1 h^-1（apparent quantum efficiency of 1.72%at 365 nm）.The obtained catalysts exhibited similar band alignments,which excluded the major contribution of band alignments to the enhanced catalytic performances.Electronic structures were further examined.Results showed that the Pt-Au SAC exhibited much unbalanced charge distribution over Pt-Au binary sites,therefore the localized dipole moment over adjacent Pt-Au site was enhanced.And enhanced localized dipole moment is beneficial for the formation of stronger internal electric field.Combined analysis of Kelvin probe force spectroscopy and Zeta potential measurements revealed the strengthened internal electric field,which is favorable for the separation and transfer of photoexcited carriers.The highly efficient photoexcited charge transportation process was evidenced by the analyses of femtosecond-resolved transient absorption spectroscopy,time-resolved luminescence spectra and photoinduced luminescence spectra.