Design,Preparation And Photo/Electrocatalytic Performance Of Metallic Phase Molybdenum Disulfide Based Catalysts

Hydrogen(H₂)is thought to be a promising clean energy owing to its high energy density,environmental-friendly and renewability.The application of hydrogen energy is considered to be an ideal solution to alleviate the current global fossil energy crisis and environmental issues including boosting pollutants(SO_x,NO_x and fine particles)and greenhouse gas(CO₂,CH₄,etc.)emissions.Compared with the traditional H₂ production by reforming fossil fuels,photo/electrocatalytic water splitting is considered as sustainable and environment-friendly H₂ production technology.As the core of photo/electrocatalytic water splitting,the design and preparation of low-cost,high-efficiency and stable catalyst materials is the research hotspot.As a typical transition metal disulfide(TMDs),molybdenum disulfide(MoS₂)with multiple crystal types(metallic 1T(1T?)phase,semiconductor 2H phase)is considered to be the ideal component of high-efficiency photo/electrocatalysts due to its suitable hydrogen adsorption free energy,adjustable microscopic morphology and low preparation cost.It is worth noting that metallic 1T(1T?)phase has the advantages of higher electron mobility,more abundant active sites,and better hydrophilicity than semiconductor 2H phase,and thus is recommended as the MoS₂ crystal form with the highest catalytic performance.This thesis aims at designing and preparing a variety of high-activity,high-stability,and practical-oriented photo/electrocatalysts based on metallic phase MoS₂,and then studying their catalytic activity for water spillting to produce H₂.Details are as follows:(1)A facile one-step hydrothermal method was reported to synthesize high-percentage metallic 1T-MoS₂ quantum dots at a reaction temperature of180°C(1T-MoS₂ QDs-180).Such prepared 1T-MoS₂ QDs-180 possesses well-dispersed ultrasmall size of³.3 nm and high-percentage 1T-phase composition over 82%.The 1T-MoS₂ QDs-180 exhibits a large number of exposed active sites and low electron transport impedance.Impressively,the 1T-MoS₂-C photocatalyst prepared by loading 1T-MoS₂ QDs-180 on Cd S nanorods shows excellent visible-light(?>420 nm)photocatalytic HER performance and long-term stability.Specifically,the HER rate of the optimized 3 wt.%1T-MoS₂-C reaches climbing up to 131.7 mmol h^-1 g^-1,over 65-fold the rate of pure Cd S(2.0 mmol h^-1 g^-1)and appropriately 2-fold the rate of precious-metal Pt loaded Cd S(67.0 mmol h^-1 g^-1).This work offers an effective protocol to fabricate 1T-MoS₂ quantum dots with high-density active sites and high conductivity,which is not only appropriate in the photocatalytic HER but also extendable to electrocatalysis and biological applications.(2)For solution-processing 1T-MoS₂,the intercalated reducible-NH and-NH₂ species play a key role in stabilizing its crystal structure.Based on this,a novel H₂O₂ post-treatment strategy was reported to modulate the amount of reducible-NH and-NH₂ species to weaken the interlayer Van der Waals force,and thus reduce crystallinity and disorder the stack of 1T-MoS₂ layers along the c-axis direction without degrading to 2H-MoS₂.By delicately optimizing H₂O₂concentration and reaction time,the resultant 1T-MoS₂(M60)exhibits larger specific surface area(up by 1.6 times)and higher electron mobility compared to the pristine MoS₂.Consequently,M60 shows a low overpotential of 165 m V at the current density of 10 m A cm^-2 and a low Tafel slope of 44.0 m V dec^-1(lower than 87.6 m V dec^-1 of MoS₂)in acidic medium.In addition,M60/Cd S composite photocatalyst prepared by ultrasonic electrostatic self-assembly shows a very high photocatalytic HER activity(235.0 mmol h^-1 g^-1)under visible-light(?>420 nm),which is much better than MoS₂/Cd S(124.7 mmol h^-1 g^-1)and Pt/Cd S(104.5 mmol h^-1 g^-1).This work not only presents a new understanding of the role of the intercalated reducible nitrogen-hydrogen species in the crystal structure of metastable 1T-MoS₂,but also offer an effective structural fine-tuning approach to promote the catalytic activity of metastable1T-TMDs.(3)Based on the overall design of the composite photocatalyst,metallic1T-MoS₂ cocatalyst was in-situ grown at the tip of Cd S nanorods by a hydrothermal method,and thus the MoS₂-tip-Cd S composite photocatalyst with a special“dumbbell structure”was successfully prepared.Compared with the MoS₂/Cd S photocatalyst prepared by ultrasonic mixing,MoS₂-tip-Cd S possess closer interface coupling effect and uniformly microstructure.More importantly,the special“dumbbell structure”separates the reduction and oxidation reaction sites of electrons and holes,which is conducive to the effective separation of carriers in MoS₂-tip-Cd S.Finally,owing to the closely coupled interface,the“dumbbell structure”and the favorable metallic 1T-MoS₂,MoS₂-tip-Cd S photocatalyst achieves a high and stable HER efficiency(138.2 mmol h^-1 g^-1)under visible-light(?>420 nm),which is much higher than that of MoS₂/Cd S photocatalyst(10.3 mmol h^-1 g^-1).This work expands the application of metallic phase MoS₂ in the composite photocatalysts and is conducive to the development of photocatalysts with high efficiency and stability.(4)Abundant electrochemical active sites,high electronic conductivity as well as stable structure and electrochemical activity are indispensable but usually incompatible to the ideal electrocatalysts for HER.For these favorable features,PDDA and DMSO were adopted to pretreat the Ti₃C₂ MXene,and then several-layers thick,edge-oriented and high-percentage 1T?-phase(85.0%)MoS₂ nanosheets were well grown on the surface and interlayers of Ti₃C₂MXene substrate by a carefully designed one-step hydrothermal method.The anchored MoS₂ nanosheets can shield dissolved oxygen,and thus avoid the oxidation of unstable Ti₃C₂ MXene during the hydrothermal proces.The structure and phase engineering of MoS₂ as well as the fully surface-shielding Ti₃C₂ MXene endow the as-synthesized MoS₂-Ti₃C₂ MXene electrocatalyst with a wealth of accessible active sites and high electrical conductivity for HER together with excellent structure stability.Finally,MoS₂-Ti₃C₂ MXene exhibits remarkable electrocatalytic HER performance in acidic electrolyte(low overpotential of 98 m V at 10 m A cm^-2 current density and low Tafel slope of 45m V dec^-1).This work provides new insights into the development of efficient metallic phase MoS₂-based catalysts and structural-stable MXene-based functional materials.