Effects Of Cold Plasma Treatment On Au/TiO₂ Surface Property And Photocatalytic Activity Under Visible Light

Titanium dioxide is a typical semiconductor photocatalyst.However,TiO₂ can only absorb and utilize ultraviolet light owing to the wide band gap,which is as high as 3.2 eV for the commonly used anatase.At present,plasmonic metals are used to expand the light absorption in the visible region.And it is a key issue for the practical application of TiO₂ photocatalysis to extend the wavelength response region to visible light.In this paper,Au/P25 was prepared using P25 as the support,and the as-prepared Au/P25 was treated by plate dielectric barrier discharge plasma with atmospheric oxygen,hydrogen,nitrogen or argon to improve its visible-light photocatalytic activity.The photocatalytic activity of Au/P25 samples were evaluated in a continuous flow reactor by formaldehyde oxidation in synthetic air under visible light emitted from white LED.The experimental results show that the photocatalytic activity of oxygen,hydrogen,nitrogen plasma-treated Au/P25 are higher in comparison with fresh and conventional thermal-treated Au/P25.Au/P25 samples were characterized by UV-vis diffuse reflectance spectroscopy,X-ray photoelectron spectroscopy and Photoluminescence spectroscopy.XPS spectra characteristic showes that the sample activated by O₂ plasma achieves surface oxygen species content of 30%,which is higher than others,while metallic Au content of 72%,which is lower than other samples.In contrast,the surface Au⁺over P25 was completely reduced to Au⁰ after the hydrogen plasma treatment or thermal treatment.In the UV-vis diffuse reflectance spectra,we can clearly observe that there is an obvious peak at 575 nm because of local surface plasmon resonance effect?LSPR?for plasma-treated Au/P25 and thermal-treated Au/P25.Meanwhile,their peaks in photoluminescence spectra are lower than others indicating lower recombination rate of photogenerated charge carriers.Particularly,O₂ plasma-treated Au/P25 presents an enhancement of visible-light absorption and surface charge transfer efficiency after HCHO photocatalytic oxidation process.Furthermore,surface species of Au/P25 samples and the mechanism of plasma treatment influencing formaldehyde photocatalytic oxidation were studied by in-situ diffuse reflectance infrared Fourier transform?DRIFT?spectroscopy.It shows that plasma-treated Au/P25 has larger adsorption capacity of formaldehyde and higher conversion rate of formaldehyde adsorbates towards formate and carbonate,remarkably.Besides,Au/P25 treated by O₂ plasma presents plenty of surface oxygen,which accelerates hot electron transfer on the surface of the Au/P25 under visible-light irradiation with moisture.As a consequence,it promotes formate oxidation to carbonate,carbonate decomposition and thus HCHO oxidation,remarkably.