Controlled Colloidal Preparation And Properties Of Transition Mental Dichalcogenides/Phosphide As Electrocatalyst For The Hydrogen Evolution Reaction

Many materials of transition metal chalcogenides and transition metal phosphides,considered as earth-abundant materials,have been attracting considerable attention because of their unique material properties,differing from their bulk counterpart.Although the preparation and properties of transition metal chalcogenides and transition metal phosphides have been well studied,it remains challenge to develop a facile strategy for the precise control of the lateral sizes and layer numbers of TMD nanosheets.Based on the preparation methods,such as mechanical cleavage,sonication in various solvents,a nd gas phase chemical vapor depos ition,these methods may create large monolayers of micrometer lateral size but isintrinsically limited by the number of monolayers being produced or lead to a polydisperse distribution of flakes without any control over their lateral sizes and layer numbers.Even with such synthetic advances,much effort is currently being made to address issues such as controllability over lateral size,thickness and compoistion,which will be of importance to study the physical and chemic al properties of small sized TMD nanosheets with vaious composition,and further reals the relation of structure v.s.properties of these materaisls.Based on these information,we detailed investigate colloidal preparation and properties of transition tental dichalcogenides/phosphides as electrocatalyst for the hydrogen evolution reaction in this theses.The main results obtained are as follows:（1）We demonstrate colloidal synthesis for single-and multi-layer MoS₂ nanosheets by exclusively controlling their anisotropic growth as characterized by transmission electron microscopy,X-ray diffraction,UV–vis absorption spectra and X-ray photoelectron spectroscopy.The typical peaks of XRD parttens of MoS₂ nanosheets were in good agreementwith the standard pattern of hexagonal MoS₂.With the increase in the number of layers,the frequencies of the Raman modes gradually shifted to the values of bulk MoS₂.Exciton peak positions,including A,B and C,were only weakly dependent on the thickness of the nanosheets.These ultra-small sized nanosheets,especially the single-layer ones,showed excellent electrocatalytic activity towards the HER with quite good stability.The overpotential of single-layered MoS₂ nanosheets was ^-99.5 m V,and the Tafel slop was 57 m V/decade.We also demonstrate colloidal synthesis for MoS₂ QDs and Mo Se₂ nanoflowers by changing sulfur precursors for S-OLA and Se-ODE SUS,respectively.（2）We demonstrate colloidal synthesis for single-and multi-layer WS₂ nanosheets by changing the reaction temperature.We also demonstrate colloidal synthesis for WS₂ QDs and WSe₂ nanoflowers by changing sulfur precursors for S-OLA and Se-ODE SUS,respectively.The samples were characterized by TEM,SEM,EDS,XRD,UV,XPS,and CHI662 E potentiostat.XRD patterns of the nanosheets confirmed the identity of the hexagonal 2H-WS₂ phase and a gradual decline of the（002）reflection of the c-axis took place with the thickness of the WS₂ nanosheets decreased.In the Raman spectra,the frequency of the E12 g vibration softened（red shifted）,while the A1 g vibration stiffened（blue shifted）with the increasing number of layers.In addition,the single-layer WS₂ nanosheets showed excellent electrocatalytic activity towards the HER with quite good stability.The overpotential of single-layered WS₂ nanosheets was ^-77 m V,and the Tafel slop was 50 m V/decade.（3）We demonstrate colloidal synthesis for Mo-W-S and Mo-S-Se nanosheets with tunable composition as characterized by TEM,XRD,UV,and XPS.In X-ray diffraction patterns of each alloy,all of them can be assigned to hexagonal 2H-molybdenum dichalcogenide with no discernible impurities and the continuous downshift of Mo-S-Se nanosheet was caused by the increasing of the selenium content in alloys.Raman spectra of Mo-W-S and Mo-S-Se alloy exhibitted two sets of composition-dependent vibration modes.The X-ray photoelectron spectroscopy of Mo-S-Se showed that a gradual displacement of Mo 3d peak to lower binding energy from 229.2 e V to 228.9 e V with the introduction of selenium.In addition,the Mo SSe nanosheets showed excellent electrocatalytic activity towards the HER with quite good stability.The overpotential of Mo SSe nanosheets was ^-81.5 m V,and the Tafel slop was 47.3 m V/decade.（4）We demonstrate colloidal synthesis for Mo P nanoflowers,N i2 P nanoflowers,and N i2 P nanoparticles.With controlling the size and morphology of N i2 P nanostructures,we reveal the morphology evolution of N i2 P nanomaterials,and discuss the influence of the ligands in the preparation of N i2 P nanostructures.The 5.4 nm N i2 P nanoparticles exhibitted the optimal HER performance,with an HER overpotential of approximately-78 m V and a Tafel slope of 41.4 m V per decade.In this report,we demonstrate rapid colloidal synthesis for transition metal chalcogenides and transition metal phosphides nanostructures,such as MX2（M = Mo and W;X = S and Se）,Mo-S-Se,Mo-W-S,Mo P,and Ni₂ P,with tunable layer numbers,size,and composition.We provided the data of the structural and optical properties of nanomaterials with the change of size and composition.And the electrocatalytic properties of MoS₂,WS₂ and Ni₂ P nanomaterials were improved effectively.Thus,this work opened a new avenue toward the control of the size and thickness of other TMDs and the realization of their unique properties for theoretical study and practical applications.