Study On The Photoelectrochemical Cathodic Protection Performance Of The Modified Graphitic Carbon Nitride

Photoelectrochemical cathodic protection is a rising anti-corrosion technology.Making use of the photoelectric conversion effect of a semiconductor material,it can convert the solar energy to electric energy and transfer the photogenerated electrons to a coupled metal to provide cathodic protection.Therefore,this is an economic,efficient,green and energy saving technology which shows great potential in the anti-corrosion area.Graphitic carbon nitride(g-C3N4)is a potential material used in the area of the photoelectrochemical cathodic protection due to its excellent physicochemical properties and suitable band structure.However,the application of g-C3N4 in this area is restricted by its intrinsic defects such as fewer reactive sites,lower electron mobility,lower oxidation capacity of the photogenerated holes,and the amphoteric characteristics.In this article,the band structure of g-C3N4 is modulated by graphene modification and K&I element co-doping.The Fermi level is pulled and the electrical conductivity of g-C3N4 is improved,resulting in the enhanced photoelectrochemical properties and the photoelectrochemical cathodic protection in a NaCl solution for the coupled metals.Specifically,the g-C3N4 is modified by the reduced graphene oxide(rGO).An effective ?-? stacking interaction is formed between g-C3N4 and rGO due to their similar layered structure,which causes the formation of some chemical bonding between g-C3N4 and the specific sites of carbon.The rGO would act as the electronic conductive channels,which increases the electrical conductivity and facilitates the transfer of the photogenerated electrons.Additionally,the band structure of g-C3N4 is adjusted due to the modification of rGO.The conduction band position is adjusted to more negative direction and the modified g-C3N4 turns to show stronger "n-type"properties,which are desirable for the photoelectrochemical cathodic protection performance.The The R-rGO modified g-C3N4 photoelectrode can provide a photoinduced current density of approximately 17.8 ?A cm-2 to the coupled 304 stainless steel and the supplied electrons will achieve approximately 200 mV cathodic polarization for 304 stainless steel under white light illumination.A K&I co-doping method is adopted in order to further modulate the band structure of g-C3N4.The K doping would break the tri-s-triazine structure and combine with the residual N atom to form cyano groups at one apex of the melon structure of K&I co-doping g-C3N4.The changes in the molecular structure leads to the adjustment of its band structure.The bandgap is narrowed,leading to the expansion of the light absorption region.The electrical conductivity is increased,leading to the improvement of the separation efficiency of the photogenerated charge carriers.The Fermi level is pulled,bring about the n-type semiconductor properties.As a result,the K&I co-doping g-C3N4 in a NaCl solution can provide the photoelectrochemical cathodic protection for the coupled 316L stainless steel.It can provide a photoinduced current density of approximately 16.4 ?A cm-2 and a cathodic polarization of approximately 200 mV for the 316L stainless steel under white light illumination.In order to further improve the photoelectrochemical cathodic protection performance of g-C3N4 and explore the mechanism of the improved photoelectrochemical cathodic protection performance by the nano ordered structure and heterojunction system,WO3 nanoflower and ZnO nanorod arrays are prepared,on which CdS and In2S3 nanoparticles are then deposited to provide sensitization.The WO3 nanoflower and ZnO nanorod arrays can supply huge specific surface area and excellent unidirectional conductivity for the sensitizers.On one hand,the nano ordered structure can increase the reactive sites.On the other hand,the nano ordered structure can provide electronic conductive channels for the photogenerated electrons and promote the transfer and separation efficiency of the photogenerated charge carriers.The sensitizers can efficiently convert the light to chemical and electric energy.Additionally,the heterojunction electric field can be constructed at the interface between the ordered structure semiconductor materials and the loaded sensitizer materials by rationally selecting semiconductor materials with different band structures,which can further improve the separation efficiency of the photogenerated electrons and holes.Therefore,the combination of the nano ordered structure and heterojunction system can effectively improve the photoelectrochemical cathodic protection performance of semiconductor materials.The CdS/WO3 can provide current densities of 500 or 540?A cm-2 to the coupled 304 stainless steel or Q235 carbon steel,respectively,which cathodically polarize them to very negative potentials,-1.23 V(vs.Ag/AgCl)for 304 stainless steel and-1.09 V(vs.Ag/AgCl)for Q235 carbon steel,respectively.While the In2S3/ZnO can cathodically polarize the 304 stainless steel by approximately 300 mV and the photoinduced current density is approximately 280 ?A cm-2.The combination of the nano ordered structure and heterojunction system should also be applied to g-C3N4.In particular,WO3 possess an electron-storage capability,which endows it with the ability to store the redundant photogenerated electrons under light illumination and release them after the light is switched off,providing a time-delay cathodic protection for the coupled metals under dark.In summary,the research in this article not only improves the photoelectrochemical cathodic protection performance of g-C3N4,but also deepens the understanding on the mechanism and criteria of the photoelectrochemical cathodic protection technology.Especially the found that the band structure of g-C3N4 obviously influence its photoelectrochemical cathodic protection performance provides a theoretical principle for improving the photoelectrochemical cathode protection performance of a semiconductor and searching for more appropriate semiconductors.Therefore,this article laid a solid foundation for the practical application of the photoelectrochemical cathodic protection technology.