Fabrication Of G-C₃N₄ Quantum Dots Heterojunction Photoelectrode For Degrading Phenols

The semiconductor photocatalyst has long attracted much more attention for the application prospect in the fields of energy crisis and environmental pollution.Among all the semiconductor photocatalyst,TiO₂ semiconductor material has been widely researched.However,most of the commonly used photocatalyst often suffered from some serious problems,such as the narrow response range and the high recombination rate of photo-generated hole-electron pairs,which become the main obstacles that hinder the development of photocatalysis technology.Silicon is another commonly used photocatalyst.It is abundant in the earth and its processing technology is mature.Nevertheless,silicon material still faces a big problem,namely once Si is exposed to moist air or in aqueous condition,an oxide layer will form to obstruct the charge transfer from Si to aqueous solution.As a result,its ability for photoenergy conversion is turned off.To solve the problems mentioned above,we prepared the graphite carbon nitride quantum dot?g-C₃N₄ QDs?in this paper,and expect to use it as a protective coating material to modify TiO₂ nanotubes?TNTAs?and silicon nanowires?SiNWs?materials.As we all known,g-C₃N₄ is a metal-free semiconductor photocatalyst.It is visible light responsed and has a strong ability of electron transfer.Besides,g-C₃N₄ quantum dots have small size,high chemical stability and the quantum confined effect.When coating on the surface of other semiconductor,the new fabricated composite can obtain an expanded visible light absorption,a reduced photo-corrosion and an enhanced the photo-generated charge separation rate.As a result,the photoelectrocatalytic performance is improved.In this thesis,several works have been carried out as follows:?1?The g-C₃N₄ was prepared by directly thermal polymerization of melamine.Then the g-C₃N₄ QDs were fabricated by the strategy of chemical stripping,hydrothermal treatment and ultrasonication.Highly ordered TNTAs were prepared by anodization oxidation.And then the g-C₃N₄ QDs/TNTAs heterojunction was successfully prepared using a facile dipping method.Observed by TEM,the average size of quantum dots ranged from 2 to 6 nm,mainly around 3 nm.Typical SEM top-view image of TNTAs displayed perfectly aligned nanotubes.The UV-DRS results displayed that the optical absorption intensities of g-C₃N₄ QDs/TNTAs enhanced distinctly in both UV and visible light regions.Besides,the optimal condition of preparing g-C₃N₄ QDs/TNTAs heterojunction was found as 60 min of dipping duration and 0.2 mg m L-1 of g-C₃N₄ QDs dipping solution.And the photocurrent generated by the optimal g-C₃N₄ QDs/TNTAs photoanode was 0.64 mA cm-2 at 0.6 V?vs.SCE?,which was 4.3 times than that of pristine TNTAs.It implied that the fabricated g-C₃N₄ QDs/TNTAs heterojunction showed a better separation capability of photo-generated charges comparing to TNTAs.Besides,the g-C₃N₄ QDs/TNTAs heterojunction also exhibited superior photoelectrocatalytic activities in degradation of phenol.98.6% of phenol was successfully degraded in 120 min,compared to the 59.3% degradation of phenol using TNTAs photoanode.And the pseudo-first-order kinetic constant of phenol degradation using g-C₃N₄ QDs/TNTAs photoanode was 0.032 min-1,which was 4.6 times than that of pristine TNTAs?0.007 min-1?.?2?SiNWs were prepared by Ag-assisted electroless chemical etching method.And then the as-prepared g-C₃N₄ QDs were deposited onto SiNWs via a dipping process to fabricated SiNWs@ g-C₃N₄ QDs.Observed by SEM images,SiNWs were vertically oriented on the Si substrate,and the length of SiNWs prolonged with the increase of the etching duration.TEM images confirmed that dispersive g-C₃N₄ QDs anchored on the surface of SiNW,and some nanopores also appeared on the surface of SiNWs@g-C₃N₄ QDs.The XPS result indicated that the SiNWs@g-C₃N₄ QDs was composed of Si,O,C and N elements.Photoelectrochemical performance indicated that the SiNWs-2 showed favorable chemical stability and the most excellent PEC performance.The photocurrent of SiNWs@g-C₃N₄ QDs reached 6.7 mA cm-2 at-1.5 V?vs.SCE?which was 1.6 times higher than that of pristine SiNWs?4.2 mA cm-2?.Taking 4-chlorophenol as a target pollutant to investigate the photoelectrocatalytic capability of the SiNWs@g-C₃N₄ QDs,more than 85.1% of 4-chlorophenol was successfully removed in 120 min,while the value for SiNWs was only 52.0%.The pseudo-first-order kinetic constant of 4-chlorophenol degradation on SiNWs@ g-C₃N₄ QDs was 0.870 h-1,which was 2.3 times as great as that on pristine SiNWs?0.372 h-1?.Repeated photoelectrocatalytic degradation experiments were also carried out to degrade 4-chlorophenol under the same condition,indicating the high stability of the SiNWs@g-C₃N₄ QDs in the photoelectrocatalytic process.The above results showed that the g-C₃N₄ QD with excellent photoelectrochemical properties is an ideal decoration material.And the coating of g-C₃N₄ QDs on the surface of TiO₂ nanotube arrays and SiNWs could successfully improve their photoelectrocatalytic activity.This research will provide a new sight for the modification of Ti O₂ nanotube arrays and SiNWs,which is benefit to their further application and development in the field of environmental pollution.