Gold Nanorod Complexes For Plasmon-Enhanced Near-Infrared Photothermal Catalysis

Nowadays,about 95%chemical products come from catalytic pathways,but many of the catalytic reactions rely on the energy provided by fossil fuels,and the subsequent large amount of carbon emissions will cause a series of climate problems.Using light energy,a renewable energy source,to drive the catalytic reactions and realize the conversion of light energy into chemical energy is considered to be a mild and environmentally friendly solution.Near-infrared（NIR）light makes up 44%of the solar spectrum,and the research on NIR photothermal catalysis is still in its infancy,which is also related to the case that the low energy of NIR light cannot effectively drive most chemical reactions.The efficient light-to-heat conversion in the NIR region provides new opportunities for the development of NIR photothermal catalysts.At present,the research on NIR photothermal catalysts mostly focuses on semiconductor catalysts or organic composite catalysts.For the further development of NIR photothermal catalysis,it is urgent to explore other types of catalysts.Considering that plasmonic metals have a wide photo-response range,combining them with semiconductors can broaden the range of light response,which helps the utilization of light energy.In addition,under the excitation of light,the plasmonic effect of the metal generates heat and releases high-energy electrons,further enhancing the activity of photothermal catalysis.Therefore,the development of a plasmon-enhanced NIR photothermal catalysis system is of great significance for NIR photothermal catalysis and improving the utilization rate of light energy.By adjusting the size of gold nanorods,the longitudinal plasmonic absorption band can be moved to the NIR region,and high-efficiency photothermal conversion can be achieved under the excitation of NIR light.The plasmon-enhanced photothermal catalysis effect of gold nanorod composites driven by NIR light is of great value for expanding the applications of plasmonic metals in the field of NIR photothermal catalysis.Based on the foregoing,in this thesis,a series of gold nanorod composites were designed and synthesized.After studying the photothermal conversion ability and stability of the composites,they were incorporated into the NIR photothermal catalytic system to achieve plasmon enhancement.First,the phase transfer agent ofβ-cyclodextrin（β-CD）and photothermal agent of gold nanorod（AuNR）were combined together by covalent bonding to develop the NIR photothermal phase transfer agent（CD-AuNR）,which effectively improves the conversion and selectivity of benzene hydroxylation with Fenton’s reagent as catalyst in aqueous solution.Benefiting from the CD components modified on the surface of AuNR,CD-AuNR can be well dispersed in aqueous solution,and gold nanorods act as nanoheater.The NIR photothermal conversion efficiency of the composite reaches 51.2%.More importantly,the hydrophobic cavity of CD helps to anchor the substrates in the local heating center,while improving the reaction conversion rate.The host-guest selectivity avoids the excessive oxidation of phenol,the conversion rate of benzene hydroxylation catalyzed by Fenton’s reagent was increased to 50.2%with a selectivity of 90.3%,which greatly improved the potential of Fenton’s reaction in an environmentally friendly process.Supported by CD-AuNR,Fenton’s reagent catalyzes the direct conversion of benzene to phenol with almost the highest catalytic efficiency among iron-based catalysts so far.The research provides a new idea for the design of two-phase reaction catalysts.Second,based on the former work,the catalyst was directly modified on the surface of gold nanorods,so that the photothermal center and the catalytic center were integrated,and the plasmon-enhanced NIR photothermal catalysis was realized.Based on the high catalytic activity and polyanion properties,polyoxometalates（POMs）were modified onto the surface of cationic ligand-coated gold nanorods（M-AuNR）through electrostatic interactions to obtain a water-soluble complex（M-AuNR@POM）.Due to the electric repulsion and volume effects between anion clusters,POMs are decorated on the surface of gold nanorods in a monolayer structure,which is confirmed by transmission electron microscopy,isothermal titration calorimetry,and theoretical calculations of the model.The light-to-heat conversion efficiency of M-AuNR@POM in the aqueous phase is above60%.Using M-AuNR@POM as a plasmon-enhanced photothermal catalyst,it can efficiently catalyze the selective oxidation of thioether substrates in the aqueous phase.The kinetic constant of the photothermal reaction(K=4.24 h^-1)is 5 times more than the external heating reaction(0.79 h^-1).The TOF value is higher than other reported POM composites that have been used to catalyze the oxidation of thioether,which confirms that the photothermal catalyst integrating the catalytic center and the NIR photothermal center can achieve efficient enhancement of the catalytic effect.The efficient transformation of organic substrates in aqueous phase under relatively mild NIR light conditions is of great value and significance to green environmental chemistry and biochemistry.Third,since the combination of gold nanorods and POMs has a good plasmon-enhanced photothermal catalysis effect,in this part,a two-dimensional ionic framework composite（HG-POM）based on POMs was synthesized,and AuNP@HG-POM and AuNR@HG-POM were obtained by loading gold nanoparticles and gold nanorods into the framework.Considering the different photoresponse ranges of plasmonic gold nanocrystals,the photocatalytic activity for nitrogen reduction under excitation of visible light and NIR light was studied respectively.The yield of NH₃ in the N₂ reduction reaction catalyzed by AuNR@HG-PWV(PWV:PW₁₁VO₄₀^4-)under NIR light excitation was 70.4μmol L^-1 h^-1,which was higher than the catalytic activity of AuNP@HG-PWV under visible light irradiation(53.7μmol L^-1 h^-1).In addition,the TOF value of NH₃ production in the N₂ reduction reaction catalyzed by AuNR@HG-PMo V(PWV:PMo₁₁VO₄₀^4-)under NIR photothermal conditions is as high as 869.1 mmol M^-1 h^-1,which proves that NIR photothermal catalysis is effective in the N₂ reduction reaction.Gold nanocrystal-supported HG-POM realizes the photocatalysis of N₂ immobilization driven by visible light and NIR light under mild conditions.The design of this catalyst provides an important reference for the synthesis of photothermal catalysts and further expands the application of plasmonic photothermal catalysis,providing a new strategy for the design of high-activity photocatalysts.The research results of this thesis show that a series of NIR photothermal composites can be acquired by combining gold nanorods with organic phase transfer agents or inorganic catalysts through covalent and electrostatic forces,and realize the plasmon-enhanced photothermal catalysis.Modifying water-soluble functional organic molecules or inorganic nanoclusters on the surface of gold nanorods can increase the stability of nanorods in aqueous phase,and the obtain gold nanorod composites show high photothermal conversion efficiency.The components are located in the photothermal center,and the local high temperature effectively promotes the catalytic reaction.In addition to being released into the surrounding environment in the form of heat,the high-energy hot electrons released by the plasmon resonance effect of gold nanorods can be directly transferred to the modified inorganic nanoclusters,realizing the catalytic reaction of nonpolar molecules under mild conditions and further improving the utilization of light energy.The research in this thesis explores the application of gold nanorod composites in plasmon-enhanced NIR photothermal catalysis,further develops the application prospects of gold nanorods in the field of catalysis,and also provides new ideas for the design of NIR photothermal catalysts.The role of gold nanorod composites played in catalysis can also be extended to other plasmonic metals,thus promoting the development of plasmonic-enhanced photothermal catalysis.