Preparation Of Low-dimensional Layered Nanomaterials And Their Photocurrent Response Properties

The world's energy consumption is predicted to grow by 56%by the year 2040,from 520 quadrillion Btu p.a.in 2010 to 820 quadrillion Btu/p.a.,as reported by the U.S.Energy Information Administration in 2013.Our continuously growing energy demands and sustainable development have forced scientists to need to find clean,renewable and environmentally friendly energy.Solar energy as a kind of clean and renewable energy therefore has received the widespread attention of scientists.In recent years,two-dimensional(2D)materials,such as graphene because of its unique physical and chemical properties has been made preliminary applications in solar cells,photoelectrochemical energy storage,and so on,but still in the exploratory stage.The main factor which limits the development is how to make the photo-generated electron-hole pairs efficiently separation,transportation,and utilization.The mechanism of the photo-generated electron-hole pairs separation,transportation,and utilization is needed to be carefully discussed.Moreover,design and construction of two-dimensional heterojunctions aiming at effectively improving the photo-generated electron-hole pairs separation,transportation,and utilization should be addressed and researched.In this dissertation,MoS2 and g-C3N4,respectively as a typical inorganic and organic layered nanomaterials in energy research are mainly discussed in order to provide some useful reference and guidance in solar cells and photoelectrochemical energy storage.In this dissertation,the preparation of bulk and 2D MoS2 and g-C3N4 nanosheets and the construction of two-dimensional heterojunctions were addressed.The formation mechanism,optical,electrical,and photocurrent response properties of the prepared nanosheets and heterojunctions were carefully discussed.Moreover,the preparation and photocurrent response of low-dimensional TiO2 nanostructured films were also addressed and carefully discussed.The innovative research results are as follows:(1)Ti-MoS2 nanosheets thin films were successfully prepared by Ti foil assisted hydrothermal synthesis method.The upside and downside of the prepared films showed different novel morphologies with flowerlike microspheres and nanosheets arranged regularly with the thickness of ca.30 nm and good single crystal structure.Subsequently,2D MoS2 nanosheets were further prepared via liquid exfoliation in the mixed solution of ethanol and water.The prepared 2D nanosheets showed p-type semiconductor property and their thin films showed a prominent transient photocurrent response.The ratio of transient and steady photocurrent response increased with the increased thickness of the thin films,which was mainly due to the synergistic effect of photo-induced bias and inherent potential difference on the semiconductor/electrolyte interface.(2)2D g-C3N4 nanosheets were successfully prepared by two processes:acid treatment and liquid exfoliation.The thickness of the nanosheets was ca.4.54 nm containing ca.13 C-N layers.The acid treatment process before liquid exfoliation for bulk g-C3N4 could effectively destroy the in-plane periodicity of the aromatic systems and made the bulk easily exfoliated.The prepared 2D nanosheets showed p-type semiconductor property and their thin films showed steady photocurrent response.The photocurrent response decreased with the increased thickness of the thin films,which was mainly due to the low electroconductivity and short distance of the carriers transportation.CNTs functionalized thin films could effectively improve the photocurrent response compared to the pure thin films.(3)2D g-C3N4/MoS2 nano heterojunction thin films were successfully prepared by liquid mixing and vacuum filtration self-assembled method.The photocurrent response of the 2D g-C3N4 thin film could be effectively improved from ca.0.30 ?A/cm2 to ca.0.66?A/cm2 by constructing 2D-2D g-C3N4/MoS2 nano heterojunction(p-g-C3N4/p-MoS2,a type-? junction)thin film.This type-? band alignment could effectively promote the photo-generated electron-hole pairs separation and transportation.In addition,CNTs and TiO2 were introduced to couple with 2D-2D g-C3N4/MoS2 nano heterojunction to further improve the photocurrent response.Particularly,TiO2 interfacial coupled nano heterojunction thin film could change the photocurrent response from cathode photocurrent to anodic photocurrent as a result of TiO2 band gap energy and its n-type seminconductor property.(4)Ti-TiO2 nanostructured films were successfully prepared by Ti foil assisted hydrothermal synthesis method.The formation mechanism and photocurrent response of the prepared films were addressed and discussed.The formation process of the TiO2 nanostructured films without/with acid treatment before hydrothermal treatment process could be showed as follows:Ti/TiCl3?NaxH20xTi3O7?H2Ti3O7?TiO2(Anatase).As the reaction proceeding,the concentration of sodium hydroxide gradually decreased.Consequently,more protons tended to intercalate between the stacked polyanion sheets.The removal of Na+ leaded to the weakened interaction between neighboring TiO6 layers,making titanate nanotubes no longer stable.So the loosening may promote the possibility of un-scrolling nanotubes back to flat nanosheets.It was suggested that a relatively low hydrothermal treatment temperature and short hydrothermal treatment time were preferred for preparing excellent photocurrent response films.Moreover,TiO2 nanotubes were better for electrons transportation,since nanotubes can provide direct pathways for electrons transportation from the point of injection to the Ti foil electrode.