The Electrocatalystic Properties And CVD Growth Of Low Dimensional Materials:First-Principles Studies

The development of bifunctional catalysts for oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)are highly desirable for fuel cells and rechargeable metal–air batteries.Up to now,the most effective and widely used catalysts still are precious metals and metal oxide-based materials,such as Pt,Ru,Ru O₂,etc.But,their limited natural resources,high costs,and poor stabilities seriously hamper the large-scale commercial applications of these renewable energy technologies.Therefore,it is very important to develop effective catalysts that are cheap and environmental friendly in place of precious metals.Since the first successful exfoliation of graphene,more and more two dimensional layered materials have been prepared,such as black phosphorus(BP),transition metal dichalcogenide,h-BN,etc.Benefitting from their excellent structural and electronic properties,people are attempting to select effective ORR and OER electrocatalysts from these two dimensional materials.Many experiments have suggested that nitrogen(N)and phosphorus(P)co-doped graphene can achieve effective bifunctional catalytic activity for ORR and OER.In addition,it was recently reported that a BP nanosheet can act as a promising electrocatalyst for OER.Its excellent electrochemical activity is equivalent to those of previous metal-based electrocatalysts,such as Ru O₂ and Pt/C.However,the atomic-scale understanding of the catalytic mechanism and catalytic active sites of N-P co-doped graphene and BP nanosheet is still lacking.At the same time,it is also critical for improving catalytic performance to grow certain two dimensional material surfaces that expose more active sites.Recently,people have successfully prepared vertically oriented Mo S₂ sheets by chemical vapor deposition(CVD).Because of more exposed active sites,vertically oriented Mo S₂ sheets exhibits excellent catalytic properties.Therefore,the atomic-scale understanding of the vertical growth mechanism is also important for achieving controlled growth on surface with more active sites.First,in this paper,we investigate the ORR and OER catalytic activities of N-P co-doped graphene.Based on first principle calculations,we find that the N and P atoms prefer to bond with each other,forming embedded N-P clusters in graphene.The catalytic performances of the N-P clusters are sensitive to their geometries,especially the N:P ratios.The N:P ratio of?2 is optimal for OER,while?3 is optimal for ORR.Through evaluating the ORR/OER potential gaps,we found that the N-P cluster designated asN_C² P_C¹ shows both the high performances of ORR and OER,responsible for the unique bifunctionality in the N,P co-doped graphene.Next,we investigate the catalytic mechanism of a single-layer BP sheet(phosphorene)and the influences of oxidation and various defect.We find that pristine phosphorene shows poor ORR/OER performances.Introducing various vacant defect and heteroatoms also cannot improve catalytic activity of phosphorene.However,oxidation can effectively tune the adsorption strength of reactive intermediates and thus change its OER/ORR electrocatalytic performance.For OER,the higher the local oxidation degree(D^O_local)of phosphorene,the better the OER activity.Therefore,the oxidized phosphorene site with highestD^O_localshows the best OER catalytic property.In contrast,there exists an optimum D^O_local for ORR.Then,we systematically investigate synergistic effects of tellurium and intrinsic defects on the ORR and OER catalytic acticity of phosphorene.The results show that doped Te atoms prefer to bond with each other to form clusters in phosphorene and can be further stabilized by various intrinsic defects(Stone-Wales,single vacancy defects and zigzag nanoribbon).Benefiting from the reduced binding strength of O*,Te dopants and intrinsic defects synergistically boost the catalytic activity of phosphorene.The best OER catalytic activity could be realized in clusterSW2-Te_P¹(Stone-Wales defect decorated by one Te atom).For ORR,the clusterPri-Te_P³(pristine phosphorene decorated by three Te atoms)exhibits optimal catalytic activity.Calculated ORR/OER potential gaps indicate that theSW2-Te_P³ cluster most likely acts as the efficient bifunctional catalytic sites for both ORR and OER.Chemical vapor deposition(CVD)is the highly preferred method for mass production of transition metal dichalcogenide(TMD)layers,yet the atomic-scale knowledge is still lacking about the nucleation and growth.By using first-principles calculations,we then study the vertical growth mechanism of Mo S₂ on Au(111),focusing on the initial nucleation process.The results show that on Au(111)surface,one-dimensional(1D)Mo_xS_y chains are firstly formed by coalescing of smaller feeding species and are energetically favored at the early stage of nucleation.Two-dimensional(2D)layers can be stabilized only after the number of Mo atoms exceeds?12.A vertical growth mode is revealed which accomplishes the structural transformation from the 1D chains to the 2D layers for the clusters while growing.The competition between intralayer and interlayer interactions is the key.In conclusion,on the basic of first principle methods,we investigate ORR and OER catalytic property of N-P co-doped graphene and various phosphorene-based matreials decorated by heteroatoms and defects,and initial nucleation of Mo_xS_y clusters.We have a deeper understanding of the catalytic mechanism and catalytic active center of N-P co-doped graphene and phosphorene at the atomic scale,and synergistic effects of tellurium and intrinsic defects on catalytic acticity of phosphorene.In addition,we also propose a new vertical growth mode of the initial nucleation of Mo S₂.These findings serve as new insights for better understanding and improving the catalytic performances of low-dimensional materials related electrocatalysts,and also help people to better understand the atomic mechanism of surface nucleation growth of TMD,and further simulate more theoretical and experimental research in catalytic field and controlled growth of low-dimensional materials.