Fabrication And Photoconversion Ability Of Functional Nano-heteojunctions, And Their Performances In Degradation Of Organic Pollutants And Inactivation Of Bacteria In Aqueous Solution

Photocatalysis,one of the most attractive techniques for environmental pollution control, has been widely investigated.However,the photocatalytic oxidation technology always suffers from the difficulties of separating suspended photocatalyst particles from aqueous solution. Meanwhile,TiO₂ can not absorb visible light effectively as well as the low quantum yield. These disadvantages limit the development of photocatalysis.In order to increase the TiO₂ photocatalytic efficiency,attempts have been made to load TiO₂ on substrates for separation purpose,to assist by bias potential for increasing the quantum yield,to sensitize TiO₂ by dyes or dope TiO₂ with other ions for improving light absorption.However,these methods could overcome single disadvantage.Heterojunction is a key concept in semiconductor physics, whose inner electric field can provide a driving force for the separation of photogenerated charge carriers.Heterojunction,constructed by coating wide-gap semiconductor on narrow-gap semiconductor,can utilize photoresponse ability of both semiconductors.Therefore,based on the theory of heterojunction and photocatalysis,the following several parts of work have been done in this dissertation:In order to enhance quantum efficiency of TiO₂,carbon nanotube(CNT)/TiO₂ heterojunction arrays on Ti substrate were fabricated by a two-step thermal chemical vapor deposition(CVD) method.The thickness of TiO₂ layer could affect the photoresponse ability of heterojunction remarkably and about 100 nm thickness of the TiO₂ layer proved to be best for efficient charge separation and transfer ability among the tested samples.This CNT/TiO₂ exhibited rectification behavior,meaning the exiting of junction.And they displayed enhanced photocurrent(about 6 times of that of TiO₂ nanotube) under zero bias potential.Phenol was selected as a probe in the photocatalytic experiments,and CNT/TiO₂ heterojunction arrays and a platinum foil were connected by a voltaic wire to transfer the photogenerated electrons from the photoanode to the cathode.The kinetic constants and total organic carbon(TOC) removal of phenol degradation with CNT/TiO₂ heterojunction arrays were 1.92 and 3.4 times of that with TiO₂ nanotube arrays.These results demonstrated the super capability of the CNT/TiO₂ heterojunction arrays in photocatalysis with comparison to TiO₂-only nanomaterials.In order to utilize visible light,an electrodeposition method was developed to fabricate ZnIn₂S₄ film.The photoresponse of film could be affected by annealing temperature.The film showed the highest response to visible light when the annealing temperature was 500℃. Photocatalytic ability of ZnIn₂S₄ was investigated by performing inactivation of E.coli with the initial concentration～3×10⁸ colony forming units per milliliter.All E.coli were killed within 60 min of inactivation with ZnIn₂S₄ film under visible light and 0.6 V positive potential.Then this ZnIn₂S₄ was deposited onto silicon nanowire(SiNW) to fabricate SiNW/ZnIn₂S₄ heterojunction array.Because of the high visible-light response of ZnIn₂S₄,this heterojunction array would enhance the photocatalytic efficiency under visible light.In order to extend usable range of solar light(includingλ＞600 nm),SiNW/TiO₂ heterojunction arrays were fabricated by covering TiO₂ on SiNW.SiNW was prepared by a chemical etching method.Various types of heterojunction such as Schottky,n-n and p-n junction could be obtained by modulating the conductive type of SiNW.The n-n junction could superpose the UV-light response of TiO₂ and visible-light response of SiNW,its open circle potential reached -0.9 V in 0.01 M Na₂SO₄ solution under Xe lamp irradiation.In the phenol degradation experiment,TOC removal using n-n junction was 2.0 times that using TiO₂ film on Si wafer.Schottky junction could enhance UV-light activity of TiO₂ markedly, and its photocurrent was 2.4 times of that of TiO₂ film on Si wafer.The p-n junction as cathode could utilize visible light effectively.The space-charge-layer thickness of p-n junction could be controlled by charging carrier concentration of SiNW,and the highest photocurrent of p-n junction was observed when hole concentration of SiNW was about 10¹⁵ cm^-3.In the phenol degradation experiment under -2.0 V bias potential and visible light,the kinetic constants with p-n junction was 0.983 h^-1,which was 18.7 times of that with TiO₂ film.In order to investigate the photoconversion ability of Schottky junction composed of p type semiconductor and metal,the graphene/porous Si heterojunctions were fabricated.The highest surface photovoltage and photocurrent can be harvested by the graphene/porous Si junction with 10 s deposition of graphene,and its photocurrent at the -2.0 V bias potential is 2.21 mA cm^-2,which is 1.83 times as great as the value of porous Si.These results indicate that grapheme/porous Si heterjunction can enhanced the photoconversion ability of porous Si due to the assisting of Schottky junction.The above results illuminated heterojunction can not only enhance quantum efficiency but also extend usable wavelength,therefore heterojunction can improve the photocatalytic ability of photocatalysts.The effects of heterojunction type,thickness of top layer semiconductor and carrier concentration on photoresponse were also investigated,which was hoped to help optimize the photocatalytic ability of heterojunction and develop heterojunction theory in pollution controlling.