Reverse Structuring And Photocatalytic Water Splitting Property Of CdZnS Quantum Dots-Based Composites

As one of the most promising alternatives to conventional fossil fuels in 21 century,hydrogen has many advantages such as:light weight,ideal heating value,good combustion performance,non-toxic and no secondary pollution,hydrogen-water-hydrogen recycling.We can solve the problem of energy crisis and environmental pollution,and achieve the goal of low cost hydrogen production by using the endless solar energy to decompose water into hydrogen by semiconductor.Therefore,it is one of the hot spots for researches to seek a semiconductor material which can effectively utilize solar to hydrogen.In the many photocatalytic semiconductor materials,CdxZn1-xS has became an ideal catalyst for its controllable band gap and wide range of visible light response.However,due to the low surface active sites and low electron mobility,and the hydrogen generation efficiency can be improved by the introduction of co-catalysts.Most of the current research are loading the precious or non-precious metal catalysts onto the surface of CdZnS/CdS nanoparticles or nanorods,which has improved the photocatalytic hydrogen production efficiency in a certain extent,but considering the hydrogen evolution reaction occurs in the interface of materials and electrolyte,it will further improve the hydrogen production efficiency by constructing a composite structure with high specific surface area and active sites.Therefore,in this thesis,the Cd0.5Zn0.5S quantum dot with size 6-7nm was used as the base absorbance material,and the two non-precious metal nanosheets were introduced as co-catalysts.Construct a reverse structure composite via one-step hot solution method by loading Cd0.5Zn0.5S quantum dots(QDs)onto the surface of the two co-catalysts.Using the superior visible light absorption of Cd0.5Zn0.5S QDs and the good electronic transport of Ni2P and MoS2,and its high HER property to build more co-catalyst/Cd0.5Zn0.5S heterojunction interface to speed up the electronic transport,and the dynamical process of hydrogen evolution reaction(HER),thus improve the efficiency.The detailed study are as follows:The one part is the study on the reverse structure and photocatalytic water splitting property by loading the Cd0.5Zn0.5S QDs onto the Ni2P nanosheets.First,NiOOH was synthesized by hydrothermal synthesis,and after annealing with P source,Ni2P nanosheets were formed.Then disperse it in ethylene glycol solution,and add the Cd2+,Zn2+ and S2+ ions by hot solution method.Finally,the Cd0.5Zn0.5S QDs were grown onto the surface of the Ni2P nanosheets.We have obtained a series of Ni2P-Cd0.5Zn0.5S composites by adjusting the ratio of Ni2P,and the hydrogen production rate of these samples was tested.The experimental results showed that when the proportion of Ni2P in the composite nanostructure was 1.5wt%,the best photocatalytic hydrogenation efficiency was obtained(43.3 ?/h/mg).And its optical and electrochemical performance were tested and studied,the electrochemical impedance spectroscopy(EIS)and the photoluminescence(PL)spectra show that when the content of Ni2P in the composite sample was 1.5 wt%,the recombination rate of photogenic electron-hole can be effectively suppressed,and accelerate the transport and transfer of electrons in hydrogen production to reach to best hydrogen production efficiency(43.3?mol/h/mg).The optimal hydrogen production efficiency is approximately 3.4 times higher than that of the pure Cd0.5Zn0.5S QDs.The second part is the study on the reverse structure and photocatalytic efficiency by loading the Cd0.5Zn0.5S QDs onto the MoS2 nanosheets.First,the petal-like MoS2 nanoparticles were synthesized by a simple hydrothermal method with sodium molybdate as raw material,and the crystallization was enhanced by high temperature annealing.Then the as-prepared MoS2 powder was dispersed in ethanol,by a long time repeated ultrasonic stripping and centrifugal purification process,ultrathin and monolayer MoS2 nanosheets were obtained by controlling the ultrasonic time and the centrifugal rate.Finally,the Cd0.5Zn0.5S QDs were grown onto the surface of the monolayer MoS2 nanosheets.A series of MoS2-Cd0.5Zn0.5S composite nanostructures were synthesized by adjusting the ratio of MoS2,and the hydrogen generation rate of these samples was tested.The results show that the single layer MoS2 can be combined with Cd0.5Zn0.5S QDs to form chemical bonds,which can reduce the reaction potential barrier and accelerate the transfer of photon-generated carrier.And when the content of MoS2 reached 2wt%in the composite samples,it can get optimal hydrogen production rate(8863 ?mol/h/g),compared with the pure Cd0.5Zn0.5S QDs,this anti-type of composite structure can improve photocatalytic production efficiency about 3.2 times.To sum up,in this paper,we constructs an anti-type structure on the surface of Ni2P and MoS2,which is different from the traditional structure(catalyst on the surface of photosensitizer).The photocatalytic water splitting performance of Cd0.5Zn0.5S has been improved,and the corresponding charge transfer mechanism model is analyzed,which provides a research foundation for further design a high efficiency photocatalytic catalyst.