Experimental And Theoretical Studies On SnO₂-TiO₂ Nanotubes Composite Photoelectrodefor Photocathodic Protection

In recent years,researchers all over the world have developed a series of methods and technologies to inhibit the corrosion problem.Recently,photogenerated cathode protection has beenconsidered as a novel technique to protect metals from corrosion bythephotoelectric response characteristics of semiconductors which are attracting more and more attentions.In the process of protection,the electrons generated by the illumination of the photoanode are transferred to the linked metal toformthe cathodic protection witha more negativepotentialto suppress the corrosion.TiO₂ ismostwidely studied for photoelectrochemistryand related applications owing to itssuperior chemical stability,low cost and low toxicity.The TiO₂ nanotube arrays（TiO₂NTs）withlarger surface areaand higher regularity of the channel structure of the nanoarrays,have also beenused to protect stainless steel（304SS）and other metal materials from corrosion by means of the photogenerated cathode protection.This isdue to theirhigher photocatalytic and photoelectrochemical（PEC）properties.However,there are still two serious problems that restrict the applications of bare TiO₂-NTs in practice.One is thewide energy gap of TiO₂（Eg=3-3.2 eV）,due to which it can only absorb UV-light and thus seriously limits its applications.The other is its rapid recombination of photogenerated electron-hole pairs due to which the photoelectric protection efficiency is seriously affected,making the stainless steel hard to be protectedin the dark.In this study,titanium dioxide nanotube arrays（TiO₂NTs）were prepared on titanium plates through a two-step anodization method and then the SnO₂nanoparticles were deposited onto TiO₂NTs by electrodeposition method.The electrodeposition time has a great influence on the photoeletrochemical（PEC）performance of SnO₂-TiO₂NTs composite photoelectrode.The photocurrent density of SnO₂-TiO₂NTs electrode increased to 4 times as compared topristine TiO₂-NTs photoelectrode.Furthermore,open circuit potential and tafel curves were measured to evaluate the photocathodic protection properties.The corrosion potential dropped to-730mV and the corrosioncurrent density reached to 259.8μA/cm^-2.SnO₂is also a good semiconductor materialandhas beenprovedto havea certain charge storage capacity.Thus SnO₂-TiO₂ composite materials could be able to maintain photogenerated cathodic protection afterthe ultraviolet light irradiation。The results indicate that SnO₂deposition into the TiO₂NTs photo-electrode is a simple and effective way to protectstainless steel（304SS）from corrosion.The structure of TiO₂NTs and SnO₂-TiO₂NTs was optimized by VASP（5.20）software package,and electron density difference and Bader charge were calculated to prove the electron transfer process in SnO₂-TiO₂NTs.The calculation of the density of states also verified that SnO₂-TiO₂NTs composite electrode has better photocatalytic activity,which is in good agreement with the experimental results.