Preparation And Photoelectric Properties Of Au?Pt?-loaded TiO₂ Nanotubes On FTO Substrate

Well-aligned TiO₂ nanotubes?TNT?array film fabricated by electrochemical anodization has been regarded as a promising photoanode for applications in water purification,hydrogen generation,and solar cells fields,owing to its large specific surface area,excellent chemical stability,and facile synthesis process.Currently,highly transparent TNT array film,which could be applied in some specific fields,has attracted considerable interest of scientists.Nevertheless,TNT array film is far from being a perfect photocatalyst,largely due to the limitation of its large band gap,which results in very poor efficiency under visible light irradiation.One of the promising strategies to enhance the photocatalytic activity of TNT array film is loading of noble metals on its surface.It is considered that major roles of the noble metal nanoparticles present on the TNT surface are promotion of charge-separation by forming Schottky junction,and enhancement of the local electric field through its localized surface plasma resonance effect.In this paper,the FTO glass was used to be the substrate,and Au,Pt were adopted to decorate the TNT films.Highly transparent TNT array film and noble metal-loaded TNT array films were fabricated by magnetron sputtering and anodization.The contents of this thesis are listed as follows:?1?Highly transparent TNT array film was directly fabricated on FTO glass.The Ti film was firstly deposited on FTO glass using magnetron sputtering method,and then was completely anodized in self-controlled double electrodes cell.Finally,transparent TNT array film was attained after annealing.Through the composition characterization of samples,it was found that there was no residual Ti in the transparent TNT array film,and a little of residual Ti in the non-transparent TNT array film.By comparing the photoelectric performance of two films under visible light irradiation,it was observed that the photoelectric conversion efficiency of transparent TNT film was higher than that of non-transparent TNT film,while the photocatalytic activity of transparent TNT film was lower than that of non-transparent TNT film.A Schottky barrier was formed on the interface between the Ti and the n-type semiconductor TiO₂ because the Fermi levels of Ti and TiO₂ were different.This barrier could prevent the photogenerated electrons flowing to the external circuit,so the photocurrent of non-transparent TNT array was lower than that of transparent TNT array film.From the open-circuit voltage decay curves,the electrons lifetime of non-transparent TNT array film was longer than that of transparent TNT array film.Moreover,under visible light irradiation,in the non-transparent TNT array film,except TiO₂,Ti nanoparticles also produced photogenerated electrons which could participate in the redox reaction in the photocatalytic experiments.So the total number of photogenerated electrons of non-transparent TNT array film was larger than that of transparent TNT array film.Thus the non-transparent TNT array film had higher photocatalytic efficiency in comparison with transparent TNT array film.Therefore,the oxidation degree of the sputtered Ti film is crucial to the transparency and photoelectric properties of TNT array film.?2?Semi-transparent TNT-Au and TNT-Pt films were prepared by sputtering noble metal nanoparticles on highly transparent TNT film.Compared with transparent TNT film,the prepared TNT-Au and TNT-Pt films possessed excellent absorption ability,high photocurrent response,and improved photocatalytic activity under visible-light irradiation,indicating that the deposition of noble metal nanoparticles by sputtering was effective for modifying TNT array films.Owing to the semi-transparency of TNT-Au and TNT-Pt films,the visible light irradiated the films in two ways:front-side illumination and back-side illumination.It was found that the photoelectric properties with back-side illumination were superior to those with front-side illumination.Both samples showed the asymmetric photoelectric properties,which was analyzed in detail from the perspective of interface effect in this paper.The result was that the interface effects between TiO₂ and noble metal nanoparticles under back-side illumination of the visible light occurred more than that under front-side illumination.Moreover,the condition for the generation of interface effects was summarized.Only the interface satisfying the formula???????>0 could produce the interface effect,which might provide a reference for the selection of the incident light direction for the transparent TNT array film.?3?By comparing the photoelectric properties of TNT-Au and TNT-Pt,Au/TNT and Pt/TNT films,it was discovered that Pt nanoparticles were more advantageous than Au to enhance the photoelectric conversion efficiency of TNT array film,and Au nanoparticles were more beneficial than Pt to improve the photocatalytic activity.Because the work function of Pt is higher than that of Au,the potential of Schottky barrier between Pt and TiO₂ is higher than that between Au and TiO₂.The higher potential can accelerate the migration rate of photogenerated carriers,resulting in the improvement of the photocurrent value.So the photoelectric conversion efficiency of Pt-loaded TNT films was higher than that of Au-loaded TNT films.However,there are impurities in the TNT-Pt and Pt/TNT films,leading to their dark current not zero,which is not conducive to the separation of photoelectron-hole pairs.Therefore,the photocatalytic activity of Pt-loaded TNT films was lower than that of Au-loaded TNT films.?4?A pre-doping method was designed in this paper to fabricate noble metal-loaded TNT film,which could avoid the problem of noble metal nanoparticles blocking the nanotubes top.Firstly,the noble metal nanoparticles film and Ti film were successively deposited on FTO substrate by magnetron sputtering.Then the as-prepared noble metal/Ti film was anodized to noble metal/TNT film.The Au/TNT and Pt/TNT films were fabricated by the pre-doping method.Because the noble metal nanoparticles could promote the separation of photoelectron-hole pairs,it was found that the photoelectric properties of Au/TNT and Pt/TNT films were superior to those of non-transparent TNT film.However,this data was far from the expected value.The reason might be that in pre-doping method,the Ti film was not directly sputtered on the FTO substrate,which not only influenced the adhesion between the Ti film and the substrate,but also changed the density of the Ti film.This is a deficiency of the novel pre-doping method.Therefore,it is considered to change the magnetron sputtering parameters of the Ti film in subsequent experiments,such as raising the substrate temperature,increasing the sputtering current,and so on.?5?In pre-doping experiment,noble metal nanoparticles were evidently observed at the bottom of Au/Ti and Pt/Ti films,but were not seen at the bottom of Au/TNT and Pt/TNT films after anodization and annealing.Through analyzing the ingredient of Au/TNT and Pt/TNT films on the surface and cross-section,it was discovered that the noble metal nanoparticles had diffused from the bottom into the upper TNT array films,indicating that the pre-doping method could realize the effective doping of noble metal nanoparticles on TNT,which would be used as a reference on preparation of noble metal-loaded TNT film.