Study Of TiO₂-based Plasmonic Photocatalysts:Microstructure Modulation And Photocatalytic Activity

Energy and environmental issues are the biggest challenges in the 21th century.Semiconductor photocatalytic materials exhibit great potentials in environmental protection and solar energy conversion.The heterogeneous photocatalyst TiO₂ has been extensively researched for decades because it can eliminate environmental pollutants,purificate air,and produce clean hydrogen and oxygen by efficiently utilizing solar energy.Nevertheless,the applications of pure TiO₂ remain quite limited,mainly due to the rapid recombination rate of the photogenerated electron–hole pairs within the TiO₂ particles and the limited UV-light response.Recently,surface plasmon resonance?SPR?of noble metal nanoparticles has been recognized as a promising strategy to boost the performance of photocatalysts.Such an enhancement is probably caused by two reasons:?1?the Schottky barrier formed on the interface of metal and semiconductor,and acted as the“electron trap”,which improving charge migration and separation.?2?the SPR-induced charge transfer from the photoexcited metal to the semiconductor and/or electromagnetic fields near the plasmonic nanostructure.In this dissertation,we have carried out in-depth research work based on modified titanium dioxide photocatalytic materials,through the micro-structure control,doping and recombination of plasma photocatalysts,aiming at the preparation of efficient visible light catalyst and its performance in photocatalytic degradation of pollutants and photocatalytic selective oxidation applications,the main content could be summarized as follow:Firstly,a series of gold-promoted TiO₂/SBA-15 catalysts were prepared by a facile,rapid microwave-assisted alcohol reduction method,exhibiting visible-light response for the photocatalytic selective oxidation of alcohols.Comprehensive experimental measurements were conducted using these catalysts for correlating the catalytic activity with the corresponding loading amount and size of Au NPs.As compared with that of benchmark P25,the surface area of anatase TiO₂ nanoparticles and absorption ability dramatically increased through interaction with these porous supports as the key parameter affecting photocatalytic activity.In addition,small domains of monodisperse mesoporous silica resulted in the homogeneous deposition of Au NPs inside the pores of the support,resulting in small Au NPs with a mean diameter of 4.8 nm for a Au loading at 2 wt%.And the gold nanoparticles deposited onto TiO₂ can greatly improve the photocatalytic performance of alcohol oxidation.This enhancement effect may be attributed to the increase in the formation rate of electron–hole pairs induced by SPR of Au NPs and higher efficiency of electron transfer due to the Schottky barrier at the Au–TiO₂ interface.The simple strategy and materials developed here dramatically improved photocatalytic activity and can also be applied to other traditional catalytic reactions.Secondly,Ti³⁺doped TiO₂ nanocrystals with a hollow structure were prepared via a microwave-assisted ionic liquid solvothermal approach,followed by a vacuum-activation method.The predominant advantages of this method are that it is fast,simple and relatively green.Moreover,the synthesized samples have been demonstrated to be highly photocatalytically active toward the selective oxidation of benzyl alcohol and the photodegradation of MB under visible light irradiation???400 nm?.The highest photocatalytic activity is found for the sample prepared at the molar ratio of Rf=2,which is more active than either P25 or other reduced TiO_2-x-x synthesized under the same conditions.Based on TEM,STEM,UV-DRS,PL,XPS,EPR,and Raman spectra analyses,it was found that the ionic liquid[Bmim][BF₄]plays a unique and critical role in obtaining TiO₂ nanocrystals and leading to wormhole structures,which are expected to be favorable for the diffusion and adsorption of reactant molecules,shorten the bulk diffusion length of charge carriers,and inhibit the photoelectron–hole recombination.Furthermore,the subsequent vacuum-activation process incorporates Ti³⁺ions into the TiO₂ lattice,which narrow the energy band gap of TiO₂ via forming intermediate energy levels,thereby enhancing visible-light absorption.Moreover,the unique disordered core/ordered shell protects the TiO_2-x nanoparticle core from further oxidation and effectively blocks oxidation between the Ti³⁺and dissolved oxygen in the solvent,thereby allowing long-term recycling stability.The present study demonstrates a simple and economical method for synthesizing Ti³⁺self-doped TiO₂ with a hollow structure and develops a highly active photocatalyst under visible-light irradiation.It is hoped that our work can provide useful information and guide ongoing efforts for synthesizing defective semiconductor and defective semiconductor-based nanocomposite photocatalysts for solar energy conversion in heterogeneous photocatalysis.Thirdly,noble-metal nanoparticles?Au,Ag,Pt,and Pd?were successively deposited on the TiO₂ nanocrystal with hollow structures by photodeposition.As compared to bare TiO_2-H,the resulting M–TiO₂?M=Au,Ag,Pt,and Pd?nanocomposites exhibited enhanced photoactivity toward the selective oxidation of benzyl alcohol under visible-light irradiation.The enhanced photoactivity is predominantly attributed to the contribution from noble-metal nanoparticles in increasing visible-light absorbance and creating a Schottky barrier at the heterojunction,which in provide additional energetic photo-electrons and facilitate the transferring of electrons from metal to anatase.Meanwhile,direct correlation was observed between light intensity,light wavelength,reaction temperature,and photocatalytic activity of prepared photocatalysts,suggesting that the rate of alcohol oxidation is enhanced by increasing the illumination intensity or reaction temperature or by adjusting irradiation wavelength to the most suitable range.In particular,Pt₂–TiO₂,??2 nm?when supported on TiO₂,behaves as an efficient visible-light-driven catalyst for aerobic oxidation at room temperature.The visible-light absorption of Pt nanoparticles produces a large number of hot electrons via the interband transition of their 5d electrons,which cross over the Schottky barrier at the interface between the metal and semiconductor and activate O₂,as well as produce a large number of surperoxide species,which behave as key active species for oxidation.The Pt₂–TiO₂ catalysts promote the aerobic oxidation of alcohols with an apparent quantum yield of 5.58%?400 nm?,which is almost 14 times as high as that exhibited by the TiO_2-H catalyst?0.4%?.The investigation of the kinetic isotope effect?KIE?for the oxidation of benzyl alcohol suggested that the breaking the C–H bond of benzyl alcohol is the rate determining step.The reaction mechanism of the photocatalytic reaction over Pt₂–TiO₂ is proposed by using different radical scavengers and ESR analysis.Finally,the results obtained by changing reaction temperature indicated that platinum and palladium NPs also effectively couple thermal and light energy sources to more efficiently drive chemical transformation.Fourthly,in order to further enhance the synergetic effect of surface plasmon resonance and Schottky barrier,the Au₂Pt₂/N-TiO₂ plasmonic photocatalyst was synthesized by in situ photoreduction.The selective oxidation of benzyl alcohol was studied.The photocatalytic activity of Au₂Pt₂/N-TiO₂ plasmonic photocatalyst under simulated sunlight?wavelength>460 nm?was investigated.The experimental results show that Au₂Pt₂/N-TiO₂ has high visible light photocatalytic activity and a higher quantum yield of 5.86%in the 500 nm,which is 24 times of that of titanium dioxide.The experimental results show that the catalytic activity of Au₂Pt₂/N-TiO₂ is derived from three aspects:First,N-doped into the titanium dioxide lattice will form a new hybrid orbital in the N2p orbit and O2p orbit.Secondly,the absorption capacity of the visible-light is enhanced by the plasma resonance effect of Au nanoparticles.Thirdly,the Schottky barrier formed by Pt-TiO₂ interface promotes the transfer of photogenerated electrons and suppresses the recombination probability of photogenerated carriers.The expriment results show that interfacial effects of Pt and titania play a major role in the promotion of catalytic efficiency.FDTD simulation experiments show that both hot electron injection mechanism and local electric field enhancement mechanism of Au nanoparticles in Au₂Pt₂/N-TiO₂ catalyst play an important role in improving the catalytic efficiency.