Controlled Synthesis of Nanocrystals for Photocatalytic and Photoelectrochemical Application

Author:Wu Yu Xuan

Supervisor:liu li min


Degree Year:2019





Nano-catalysis is a science based on the surface structural design at atomic or molecular scale,which aims at studying the relationship between structure and catalytic performance.It brings us a new idea to solve the current energy and environmental problems.Solar energy is undoubtedly inexhaustible currently.When coupled with solar energy,photocatalysis or photoelectrocatalysis is definitely the most economic and efficient plan.Photocatalytic reaction is generally recognized as the following three processes:(1)photogeneration of electrons and holes;(2)transfer of electrons and holes;(3)redox reactions involved surface holes and electrons.The catalytic performance is highly effected by the kinetic behaviors of holes and electrons,which can be modulated by the structural and interface design of nano-catalysis.As for the structural design of nanocrystals,it is important to develop reliable synthetic methods with precisely shape and size control.While for the interface design,more efforts must be paid for the transfer of carriers at interface and the corresponding mechanism.This dissertation focuses on the structural design for semiconductor nanocatalysis.On the basis of structural control of nanocrystals by colloidal method,and optimized heterostructure in the existence of facet nanocrystals and defect,the realationship between structure and catalytic performance is studied.The dissertation is organized as follows:The first chapter briefly introduces the research process of colloidal synthesis.The basic theory for nucleation and growth was introduced firstly.In the following part,several strategies,including shape control,strain and large-scale production,were given.The novel performance of colloidal nanocrystals for photocatalytic and photoelectrochemical performance were also introduced.Every research starts from the step of materials preparation.To fulfill the research goal for structure-property,a simple and reliable method must be present.Among all synthetic methods,colloidal synthesis may be the most powerful strategies in nano-synthesis,especially in the aspect of shape control.In chaper 2,a general colloidal synthetic strategy to obtain metal sulfides was introduced.By using the bio-molecular L-cysteine as sulfur source,the method can be applied to synthesis PbS,Cu7S4,Ag2S,CdS,Bi2S3 and SnS.By this method,taking cadmium sulfide as an example,the size and shape can also be easily controlled by changing the molar ratio of precursor,reaction temperature and cosurfactant.Such CdS nanorods,bipods,nano-twintower and nanoparticles were obtained under above experimental parameters.Among all these CdS nanocrystals,CdS nanorods showed the best performance for photocatalytic hydrogen evolution.The nanostructure with anisotropic characteristic often shows the good charge separation and transfer ability.Accordingly,the dimensional control of semiconductor nanocrystals offers new opportunity to improve their photocatalytic performance.In chapter 3,colloidal strategy is used to synthesis SnS nanocrystals from OD to 2D.The homogeneous OD SnS nanoparticles were synthesized via i"hot-injection" route.While the "cool-injection" strategy,which is based on the seed-mediated growth process,was used to synthesis 1D SnS nanorods and 2D SnS nanoplates.Due to the low resistance and high concentration of carriers,1D SnS showed best PEC performance among all.Apart from the effects of macroscopic morphology,the very essential atomic arrangement also determines the photocatalytic properties.The deviation of atomic arrangement from their bulk counterparts generates strain.It is believed that the electronic and physical properties greatly changed in the existence of strain.As a result,the strategy to introduce strain pave a new route to tailor the photocatalytic performance.In charpter 4,self-strained SnSe/SnS bending nanoplates were synthesized via hot-injection method.Based on the careful calculation on the lattice mismatch for rocksalt SnS and orthorhombic SnSe,it is theoretically reliable to epitaxial growth SnSe onto SnS for bending structure.While the normal or homogeneous growth process results in SnSe nanorods.The two samples were used for PEC performance,due to the existence of strain,SnSe bending nanoplates show the photocurrent density of 75uA/cm2(0.8V.vs Ag/AgCl),which is 3 folds larger than the strain-free counterparts.The construction of heterojunction is another useful stradegy to facilitate the charge transfer and seperation at the interface of two type of semiconductors.Furthermore,this process can be promoted by introducing defects at interface.In charpter 5,the TiO2 nanocrystals with different dominant facet were used to couple with C3N4.The charge transfer mechanism was fully investigated.When 101-TiO2 was integrated with g-C3N4,it facilitated the Z-scheme recombination of electrons in the CB of 101-TiO2 and holes in the VB of g-C3N4.And the oxygen defect in 101-TiO2 will promote the Z-scheme transfer of electrons.Differently.001-facets of TiO2 were prone to accept electrons from the CB of g-C3N4,forming the conventional Type Ⅱ junction with poor photoreduction ability.For 001-TiO2/g-C3N4 heterojunctions,nitrogen vacancies in g-C3N4 presented more important role in separating charge carriers.