Design,Synthesis And Properties Of WO_3-x Based Photocatalysts With Wide Spectral Response

Photocatalysis has broad application prospects in energy production and management,environmental protection,chemical synthesis and other areas.In process of photocatalysis,solar energy utilization plays an important role in determining photocatalytic efficiency.Currently,most active photocatalysts can only absorb and utilize ultraviolet and visible light in the whole solar spectrum.Therefore,the efficiency of these photocatalytic systems falls far short of expectations.In order to improve the efficiency of photocatalysts,it is urgent to design and develop photocatalysts with a wide spectral response to achieve the effective utilization of a whole solar spectrum including near-infrared light.Based on the above objective,this thesis mainly focuses on broadening the range of solar energy absorption in semiconductor photocatalysts.Regarding tungsten oxide nano wires as a research object,a photocatalytic system capable of collecting near-infrared photon energy is constructed by introducing up-conversion/plasmon components into semiconductor photocatalytic systems,or creating intermediate states by constructing vacancies.In such a way,it can broaden the response range of the catalysts to solar light,improve the photocatalytic activity of the catalysts,and realize the high-efficiency conversion of hydrogen production by ammonia-borane hydrolysis and CO₂ photoreduction reaction.The composition and structure of the catalysts are investigated by a variety of characterization methods.And 3D finite element simulations,theoretical calculations,transient absorption spectroscopy and photoelectric tests are combined to investigate the light absorption,separation and transfer of photoexcited charge carriers as well as the photocatalytic reaction mechanism in the photocatalytic process in depth.The specific research contents and conclusions are as follows:（1）With NaYF₄:Yb-Tm nanoparticles as upconversion luminescent layer and W₁₈O₄₉ nanowires grown on an FTO glass substrate as plasmonic layer,NaYF₄:Yb-Tm/W₁₈O₄₉ composite film was prepared by a simple solvothermal method combined with self-assembly method,and the upconversion luminescence intensity was measured under 980 nm laser excitation.The results showed that the luminescence intensity of upconversion nanoparticles in the NaYF₄:Yb-Tm/W₁₈O₄₉ composite film was enhanced by about two orders of magnitude compared with the pure NaYF₄:Yb-Tm film,and it was proposed that NIR-excited surface plasmon resonance（SPR）of W₁₈O₄₉ was the primary reason for the enhanced upconversion luminescence of NaYF₄:Yb-Tm nanoparticles.Meanwhile,this plasmon-enhanced upconversion luminescence could be absorbed by W₁₈O₄₉ to re-excite its higher energy SPR,thus achieving a more efficient NIR up-conversion plasmon energy to enhance the activity of the catalyst.Based on this process of plasmonic energy transfer,we synthesized NaYF₄:YbTm@W₁₈O₄₉ and NaYF₄:Yb-Er@W₁₈O₄₉ heterostructure photocatalysts.Under 980 nm light irradiation,Na YF₄:Yb-Tm@W₁₈O₄₉ and NaYF₄:Yb-Er@W₁₈O₄₉ exhibited excellent photocatalytic activity in the hydrolysis of NH₃BH₃ for H₂ evolution,with H₂ production rates of 1.1 μmol h^-1 and 4.74 μmol h^-1,which were 2 and 7.6 times higher than those of W₁₈O₄₉ respectively,broadening of the photo-response range of the semiconductor through the nonmetallic plasmon-induced energy transfer up-conversion process,and thus enhancing its photocatalytic reaction activity.（2）Ag/W₁₈O₄₉ heterostructure photocatalyst was successfully prepared by a simple solvothermal method combined with a self-assembly synthesis method.It was reported that the strong plasmonic coupling driven by infrared light enhanced photocatalytic activity in a metallic/nonmetallic heterostructure system.Through the 3D-finite element simulation,we demonstrated that the plasmonic coupling between Ag and W₁₈O₄₉ significantly enhanced the localized electric field at the "hot spots".This increased the intensity of incident light by 101 to 104 times,Uhus leading to the promoted generation of plasmonic "hot electrons".Moreover,the resonance excitation of plasmonic coupling on the heterostructures not only induced ultra-fast electron transfer from W₁₈O₄₉ to Ag hetero-components,but also produced a photothermal effect that increased the localized temperature.Under infrared light irradiation,the Ag/W₁₈O₄₉ heterostructure film showed a remarkable enhancement of the photo/thermal catalytic activity for the production of H₂ from NH₃BH₃ hydrolysis,The H₂ yield was 0.18 μmol min^-1,which was about 9 times higher than that of W₁₈O₄₉ and Ag.Under natural sunlight irradiation,the reaction rate of H₂ produced by NH₃BH₃ hydrolysis over the Ag/W₁₈O₄₉ heterostructure film could reach 2.76 μmol h^-1.（3）A novel defective WO_3-x/MoO_3-x heterojunction photocatalyst was successfully prepared by a simple in-situ solvothermal synthesis.Due to the well-matched energy band structure and the strong interfacial coupling between WO_3-x and MoO_3-x,the resultant WO3x/MoO_3-x heterojunction could not only extend the optical response to overlap the near-infrared region,but also significantly promote the separation and transport efficiency of photogenerated charge carriers.Meanwhile,the improvement of specific surface area and the creation of surface oxygen vacancies endowed WO_3-x/MoO_3-x heterojunction with enhanced CO₂ adsorption and activation capacities.Therefore,WO_3-x/MoO_3-x heterojunction photocatalyst exhibited an excellent photocatalytic activity for the CO₂ photoreduction reaction under UV-Vis-NIR light irradiation.And the yields of the products CO and CH₄ were 40.2 μmol·h^-1·g^-1 and 3.1μmol·h1·g^-1 respectively,which were increased by 9.5-fold and 8.2-fold compared with the MoO_3-x catalyst.This thesis provides new ideas for the design of photocatalysts with broad spectral response and the improvement of photocatalytic performance,and also offers great potential for exploring the applications of wide photo-responsive materials in solar cells as well as photovoltaic devices.