Study On Photocatalytic And Electrocatalytic Water Splitting Of Transition Metal Sulfides

With the development of the industrial revolution,energy crises and environmental pollution problems have caused by the extensive use of fossil energy.While solar energy has attracted widespread attention because of its cleanliness and vastness.Solar energy can be used for generating solar power and producing hydrogen through photocatalytic decomposition of water.The clever twist here is that,electricity and hydrogen energy can be used as secondary energy sources for people.However,electric energy has the disadvantage that it cannot be stored easily,so it is also plays an important role in breaking the water down,producing hydrogen by electrocatalytic.The key to photocatalytic water splitting lies in the selection of semiconductor catalysts.A suitable optical catalyst must have a stable structure as well as a suitable band gap to satisfy the band edge position that absorbs more visible light and satisfies the redox potential of photocatalytic water-splitting.It can effectively inhibit electron-hole recombination.A suitable electrocatalyst should have a higher conductivity and a lower ionization potential.At present,there are few photocatalysts that can meet these requirements at the same time.The preparation of ideal photocatalysts and electrocatalysts is the focus of research on water splitting right now.For example,non-noble metal materials such as transition metal sulfides and oxides show excellent photocatalytic activity,electrocatalytic activity and high conductivity have attracted the attention of researchers.This paper is based on first-principles calculations and density functional theory research methods:the stability and energy band structure of molybdenum disulfide（MoS2）and nickel sulfide（Ni₃S₂）are studied through surface molecular modification and transition metal doping.By adjusting these methods,realizing their application in photocatalytic hydrogen production and electrocatalytic water splitting.The main contents are as follows:（1）The adsorption of polar molecules on the surface adjusts the energy band structure of the monolayer MoS2 to meet its photocatalytic water decomposition needs.Monolayer molybdenum disulfide is a direct band gap semiconductor,the band gap value is about 1.8 eV.However,because the bottom position of the conduction band is lower than the hydrogen production reduction potential which limit the ability of hydrogen production of MoS2 in photocatalytic water splitting.In recent years,our research has found that polar molecules adsorbed on the surface can effectively adjust the edge position of MoS2 to meet the needs of photocatalytic water splitting to produce hydrogen.This is mainly due to the electric dipole moment of polar molecules and interface electric dipole moment induced by surface adsorption.At the same time,the position of the MoS2 band edge will also be affected by the surface molecular coverage and the interaction of the substrate.Among them,the heterostructure of D3/MoS2/graphene can meet with the requirements of photocatalytic water splitting that’s why it is considered to be an ideal photocatalyst.（2）Doping transition metals to improve the electrocatalytic performance of Ni₃S₂ has achieved the optimization of its OER performance in electrocatalytic water decomposition.Ni₃S₂ has good electrical conductiv ity which is similar with traditional TMDs.The base surface with a large percentage of surface area of Ni₃S₂ has poor catalytic activity,which limits its application in electrocatalytic decomposition of water.We use density functional theory calculations to show that the catalytic activity of the Ni₃S₂ base surface can be effectively activated by doping with transition metals TM（Mo,Mn,Fe,Co）.Our results show that doping with TM can not only significantly reduce the Gibbs free energy（ΔG）of the OER process of Ni₃S₂ by adjusting the adsorption energy of H2O on the Ni₃S₂ surface,but also can expose more active sites and unsaturated electrons.When the voltage of Mo-doped Ni₃S₂ was increased to 1.56 V,the overall reaction showed a downward trend.The Mo-Ni₃S₂ exhibited the highest catalytic activity than that doped with Mn,Fe and Co.It may become the most suitable alternative catalyst for the most common noble metal oxides in OER reactions.Our findings will help guide the future design of new OER catalysts based on TMDs.