Direct Fabrication Of 2D Materials On Oxide Surface And Investigation Of The Interface Properties

The controlled synthesis of composite materials is of great importance for developing new materials as well as new functions,during which the serious control of the interface between different materials can play a key role in optimizing the overall properties of composite materials.Novel two-dimensional materials(2D-M)that have been developed recently,such as graphene(G)and molybdenum disulfide(MoS2),were often found able to enhance the photocatalytic activities of oxide materials(Ox)via interface electronic interactions when combined with the latter.However,during the conventional preparations for obtaining the 2D-M/Ox composites,such as the mostly applied physical mixing or hydrothermal synthesis methods,it is inevitable to leave a large number of intercalation molecules at the interface region,whose existence may seriously influence the properties of the entire composite materials.Being another routinely applied synthetic method,chemical vapor deposition(CVD)technique is usually performed with precisely controlled atmosphere and under high temperatures.Therefore,the formation of the intercalation molecules can be largely suppressed,hence the expected functionality of the 2D-M/Ox composite can be optimized.In this thesis,we have utilized CVD methods to directly synthesize graphene and MoS2 on the crystal surfaces of titanium dioxide(TiO2)and zinc oxide(ZnO).The strategy successfully ensures the fabrication of a number of contamination-free 2D-M/Ox heterojunctions with uniform interface contacts.For the obtained composite systems,we have carefully investigated the corresponding interfacial structures and electronic properties,as well as their influences on the performances of the composite systems.The achieved results are as follows:(1)Using C2H2 as the precursor of the CVD synthesis,polycrystalline graphene was successfully prepared on the atomically flat single crystal surfaces of rutile TiO2(110),(001)and(100).By optimizing the synthetic parameters,the terrace and step structures of the TiO2 surfaces were very well reserved,hence the clean and seamless interface of G and TiO2 were successfully constructed.As a comparison,conventional CVD synthesis using methane and hydrogen as precursors and performed at 1000 0C severely destructed the crystal surface of TiO2.For the fabricated model system,we have conducted a series characterizations including X-ray photoelectron spectroscopy(XPS)and UV-visible absorption spectroscopy(UV-vis),which clearly demonstrated the electron transfer from graphene to TiO2 at the ground state owing to the work function difference.(2)Through similar CVD process,we have successfully realized the direct growth of graphene on the surface of the well-shaped anatase TiO2(a-TiO2)nanosheets that were fabricated by hydrothermal method.It demonstrated that it is feasible to realize the clean and uniform G/a-TiO2 interface in the microscopic material system.With the series of characterizations including XPS and UV-vis spectroscopy,we found the interface interactions between G and a-TiO2 nanocrystals are very similar to those of G and r-TiO2 single crystals.For the nanocomposite system,the application of ultrafast optical spectroscopy can be applied,which unambiguously revealed the charge transfer dynamics at the G/a-TiO2 interface and the critical roles of graphene in prolonging the lifetime of the photo-induced charge carriers.We further checked the catalytic performance of the synthesized G/a-TiO2 composites in the photodegradation of methylene orange(MO)and found it gave much better activity than the catalysts prepared by physical mixing of graphene oxide and a-TiO2 nanocrystals.In addition,we found the double layer graphene exhibited the best performance.(3)We further expanded the oxide substrate to ZnO,and grew nanographene directly over the surface of ZnO nanocrystals via similar CVD strategy.The fabricated G/ZnO nanocomposite also present a clean and seamless interface.XPS measurements indicated that in the G/ZnO composite graphene donates electrons to ZnO,which is similar to the case of G/TiO2 composite system.The results of photo luminescence spectroscopy(PL)showed that the defect-mediated fluorescence of ZnO gradually decreases along with the increase of the graphene thickness until fully quenches,indicating a graphene-assisted behavior on electron-hole separation.To address the effect of interface control on tuning the properties of nanocomposite,the photodegradation of methylene blue(MB)was performed and the CVD-fabricated G/ZnO photocatalyst was found giving significantly better activity than that prepared by physical mixing of graphene oxide and ZnO nanoparticles.(4)As an alternative 2D material of graphene,we turned to grow MoS2 on the TiO2 substrate using molybdenum oxide and sulfur powder as solid precursors.The series of characterizations including atomic force microscopy(AFM),Raman spectroscopy(Raman),transmission electron microscopy(TEM),and scanning tunneling microscopy(STM)unambiguously demonstrated the formation of high-quality monolayer MoS2 on the atomically flat rutile TiO2(110)surface.XPS data show a stronger interface electric field of fabricated MoS2/TiO2(110)composite than the composite prepared by transferring MoS2 film to the r-TiO2(110)single crystal surface.Photoluminescence(PL)results indicated that the luminescence property of MoS2 was significantly tailored after the combination with TiO2.Tunneling AFM(TUNA)and transient photocurrent experiments directly provided the evidences of the photo-induced electron transfer at the MoS2/TiO2(110)interface at the microscopic and macroscopic levels,respectively.(5)In order to optimize the quality of MoS2,we further developed a two-step CVD strategy.In the first step we doped the TiO2(110)surface with Mo by using MoO3 as the precursor.Then the Mo-doped TiO2 crystal was vulcanized under the sulfur atmosphere.In this way,high quality MoS2 films of monolayer thickness can be achieved whose domain sizes can be precisely controlled by the vulcanization time.Synchrotron-based photoelectron spectroscopy measurements clearly revealed the valence states as well as the surface distribution of the Mo dopants in TiO2(110)substrate.Combining with other characterizations of a series of controlled samples,the formation mechanism of MoS2 on the TiO2 surface has been illustrated.The established two-step strategy can possibly be applied in the fabrication of high quality MoS2 on other substrates.In summary,the thesis has demonstrated that the CVD method can be feasibly applied in synthesizing 2D materials directly on the reducible oxide surfaces,and an optimized 2D-M/Ox interface free of contaminations can be achieved.The established several model systems,including G/TiO2,G/ZnO,MoS2/TiO2,have covered two different 2D materials and three kinds of oxides from both macroscopic and microscopic levels.The performed systematic characterizations and detailed analyses have revealed the specific interface properties of each model system,in particular the charge transfer behaviors under both the ground and photo-exited states.Moreover,for all the explored composite systems,a clean and uniform 2D-M/Ox interface has demonstrated the superior condition for optimizing the photo-responses of the composite materials.The established research methods in this thesis can be extended to constructing and investigating the interface properties of other 2D-M/Ox model systems composed of very different 2D materials and oxides,which will finally benefit the expansion of various oxide-based composite materials in practical applications.