Investigation On The Fabrication And Visible-light Photocatalytic Properties Of BaTiO₃ Deposited On Ag Nanocubes

The polarization in ferroelectric（FE）semiconductor materials can induce different types of charges on opposite surfaces,and then the charged surfaces create an internal electric field（E-field）,which can efficiently separate photogenerated charge carries in FE materials and increase the lifetime of photogenerated electron-hole pairs,thus consequently enhancing photochemical reactions.As a typical FE semiconductor material,BaTiO₃（BTO）,displays various advantages,such as high chemical stability,large FE polarization strength,good biosafety and so on,has been treated as one of the ideal photocatalytic materials.However,the bandgap of BTO is～3.2 e V,which makes it be responsive only under irradiation of ultraviolet light.While ultraviolet light only accounts for～5%of solar energy,thus the use of BTO in photocatalysis is limited.It has been reported that the application of external E-field on BTO surface could result in the appearance of surface polarization charges,which enhanced the photocatalytic performance of BTO.Generally,Ag nanoparticles（NPs）deposited on the surface of BTO can improve the photocatalytic performance of BTO in visible region.However,it is difficult to control the morphology of Ag NPs by using this method.Therefore,in this dissertation,Ag nanocubes（NCs）were covered by FE BTO to fabricate Ag-BTO core-shell structure（BTO@Ag）.Since the bandgap of BTO is～3.2 e V,visible light could pass through BTO layer to arrive at Ag core and excite surface plasmon（SP）effect.The E-field generated by SP due to oscillation of free electrons in Ag core could polarize the BTO layer and arouse polarization charges on the surface of BTO,thus enhancing the photocatalytic performance of BTO.Subsequently,surface-enhanced Raman scattering（SERS）was carried out both in ambient air and in ambient argon（Ar）to study the photocatalytic properties of BTO@Ag sample.It was found that the photocatalytic activities of BTO@Ag sample were superiour relative to those of Ag NCs.By using discrete dipole approximation（DDA）method,the contours of theoretical E-field generated by SP were obtained on both Ag NC and BTO@Ag sample.It was observed that the surface polarization of BTO layer caused by SP effect of Ag core resulted in the photocatalytic enhancement of BTO@Ag sample.The main content of this dissertation is as follows.（1）Ag NCs were fabricated by hydrothermal method.The precursors solution was composed of silver trifluoroacetic acid（CF₃COOAg）,sodium hydride sulfide（Na HS）,hydrochloric acid（HCl）,polyvinylpyrrolidone（PVP）and ethylene glycol（EG）.By adjusting the reaction temperature and concentration of precursors,Ag NCs with relatively uniform shape and size were synthesized.（2）A layer of BTO was deposited on the surface of Ag NCs by hydrothermal method.The cleaned Ag NCs were reacted with barium hydroxide（Ba（OH）₂?8H₂O）,tetrabutyl titanate and anhydrous ethanol（C₂H₅OH）to form BTO@Ag sample.The concentration of precursors could change the thickness of BTO layer.（3）The morphology,electronic structure and crystal structure of Ag NCs and BTO@Ag sample were analyzed by the characterization techniques such as scanning electron microscopy（SEM）,X-ray photoelectron spectroscopy（XPS）,transmission electron microscopy（TEM）and so on.（4）In this dissertation,p-aminothiophenol（PATP）was used as a pointer molecule.The reaction of PATP to p,p’-dimercaptoazobenzene（DMAB）was chosen to detect the photocatalytic performance under 633-nm excitation.The photocatalytic activities of Ag NCs and BTO@Ag sample both in ambient air and in ambient Ar were studied.（5）It has been proved that SP-mediated photocatalytic activity is proportional to the square value of local E-field intensity（E²）.Thus,contours of theoretical E-field intensities on a Ag NC and a BTO@Ag sample were carried out by discrete dipole approximation（DDA）method.According to the DDA results,BTO@Ag sample might exhibit inferior photocatalytic performance compared to that of Ag NC.However,it was contrary to the experimental results.It presented that the enhanced photocatalytic activities of BTO@Ag sample came from the FE polarization effect of BTO layer covered on Ag core.（6）By analyzing the microstructure and photocatalytic results of BTO@Ag sample,the mechanism can be explained as follows.Due to the wide bandgap of BTO（～3.2 e V）,visible light could pass through the BTO layer and excited the SP effect on the inside Ag core.The SP induced the oscillation of free electrons in Ag core,and then the E-filed generated by SP polarized the BTO layer and aroused appearance of surface polarization charge on BTO surface,which resulted in the enhanced photocatalytic properties of BTO.Thus,our work might provide a new method to improve the photocatalytic performance of wide bandgap FE semiconductor materials in visible light region.By depositing wide bandgap FE material on the surface of plasmonic metal nanostructures,the SP effect of plasmonic metal nanostructures can be utilized to polarize the FE material and result in the appearance of surface polarization charge,thus improving the photocatalytic performance of FE materials.