Investigation On Surface Plasmon-modulated Photocatalytic Performance Of Au Nanoparticles Decorated Periodically Ordered ZnO Nanorod Arrays

Zinc oxide（ZnO）is considered as one of the promising semiconductor photocatalytic materials due to its advantages such as high chemical stability,good biocompatibility,large optical gain coefficient and so on.ZnO nanorod structure is expected to provide more photoactive surface sites under light exposure due to its big surface-volumn ratio and large length-diameter ratio,thus enhancing the photocatalytic activities.However,the bandgap of ZnO is～3.37 e V,which makes it be responsive only under irradiation of ultraviolet light.However,ultraviolet light only accounts for～5%of the solar energy.Plasmonic metal nanomaterials can excite surface plasmon（SP）effect under the irradiation of visible light.Particularly,Au nanoparticles（NPs）have been widely used in various areas,i.e.environmental purification,biomedicine,photocatalysis,etc.,due to its stable chemical property.If Au NPs are deposited on the surface of ZnO nanomaterials,the transfer of charge carriers between Au NPs and semiconductor can improve the lifetime of photogenerated electron-hole pairs due to the reduction of combination,thus enhancing photocatalytic performance.Nanomaterials with ordered structure can elevate the absorption and utilization of incident light,which is essential for the performance improvement of devices such as optical components,photocatalytic materials,photovoltaic cells,and so on.The periodically ordered ZnO nanorod arrays with deposited Au NPs can bring about the surface plasmon polaritons（SPP）on Au NPs due to SP effect.The SPP changes the propagation path of incident light to vertical direction after scattering on Au NPs.If the scattered light interfered with each other,the E-field intensity generated by SP on Au NPs can be increased,thus enhancing the photocatalytic performance.In this dissertation,periodically ordered ZnO nanorod arrays were fabricated on Ga N substrates by polystyrene（PS）nanosphere etching method and subsequent hydrothermal growth.By selecting PS nanospheres with diameters of 500 or 800 nm,ZnO nanorod arrays with different periodicity could be obtained.Au NPs were deposited on ZnO nanorods by sodium citrate reduction of chloroauric acid（HAu Cl₄）to form Au-ZnO nanorod arrays samples.P-aminothiophenol（PATP）was used as a pointer molecule,and the photocatalytic activities of different samples were carried out by surface enhanced Raman spectroscopy（SERS）under irradiation of 633 nm.It was found that the photocatalytic activities of the Au-ZnO nanorod array samples with a period of 800 nm were inferior relative to those of individual Au NPs.However,the photocatalytic activities of the Au-ZnO nanorod array samples with a period of 500 nm were superior relative to those of individual Au NPs.Since the work function of Au is larger than that of ZnO,which resulted in the transfer of electrons from Au NP to ZnO during the oscillation of free electrons generated by SP in Au NP,and thus it reduced the transfer of free electrons from Au NP to surrounding molecules,thus worsening the catalytic performance of Au-ZnO nanorod array sample with a period of 800 nm.Because the distance between neighbour ZnO rods of Au-ZnO nanorod array sample with a period of 500 nm was smaller than 633 nm,which is the wavelength of irradiation.After the incident light was scattered by the Au NPs on the neighbour ZnO rods,the scattered light interfered with each other,which increased the E-field intensity generated by SP on the surface of Au NP.Since the SP-mediated photocatalytic activity is proportional to the square value of local E-field intensity（E²）generated by SP,thus the photocatalytic activities on Au NPs were enhanced.The study in this dissertation displays how the periodicity of the ordered semiconductor arrays decorated with plasmonic metal NPs influences their SP-mediated photocatalytic activities.The main mechanism can be explained as follows.The SPP effect on plasmonic metal NPs changes the propagation path of incident light to vertical direction after scattering on them.If the wavelength of scattering light is larger than the period of nanostructure arrays,the scattering light will interfere with each other and elevate the E-field intensity generated by SP on plasmonic metal NPs,thus enhancing the photocatalytic performance.The main content of this dissertation is as follows.（1）A close-packed monolayer film of PS nanospheres with diameters of 500 or 800 nm was fabricated on water surface by Langgmuir-Blodgett（L-B）method.After removing the water,the film of PS nanospheres was deposited on a gallium nitride（Ga N）substrate.Then the size of nanospheres was reduced by reactive ion etching（RIE）,and subsequently silicon dioxide（Si O₂）was deposited as an isolation layer by magnetron sputtering.Finally,the PS nanospheres were removed by using tetrahydrofuran,and substrate with periodically ordered seed sites was obtained after annealed in vacuum.（2）Zinc nitrate hexahydrate（Zn（NO）₃·6H₂O）and hexamethylenetetramine（HMTA）were used as precursor solutions to fabricate periodically ordered ZnO nanorod arrays by hydrothermal method.And then Au NPs were deposited on ZnO nanorods by sodium citrate reduction of HAu Cl₄to form Au-ZnO nanorod arrays samples.（3）The properties of Au-ZnO nanorod arrays with different period,i.e.morphology,crystal structure,elemental component,etc.,were characterized by techniques such as scanning electron microscopy（SEM）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,and so on.（4）The photocatalytic activities of ordered Au-ZnO nanorod arrays samples with different periods were detected by SERS using the PATP to DMAB reaction.It was found that the periodicity of the ordered Au-ZnO nanorod arrays had a great influence on the SP-mediated photocatalytic activities.After the incident light is scattered by the Au NPs on the neighbour ZnO rods,the scattered light can interfere with each other on Au-ZnO nanorod arrays with proper period,which increases the E-field intensity generated by SP on the surface of Au NP and enhances the photocatalytic performance.