Synthesis And Photo/electrocatalytic Water Splitting Performance Of Two-dimensional Metal Sulfides And Phosphides

Hydrogen has been advocated as alternative fuel in order to solve the energy crisis and environmental problems in future because of its superior efficiency,zero environmental pollution and good sustainability.Water splitting is a promising approach to obtain hydrogen,and the key problem is to design high-performance and low-cost catalysts.Currently,noble-metal-based catalysts perform best,for example,Pt-based materials for HER and Ir/Ru-based for OER.However,large-scale application is limited by the high cost and low reserves of these materials.Recently,many researches have been focused on transition metal compounds as potential candidates of catalysts owing to their unique d-orbital electronic structure,low cost,high chemical stability,and excellent catalytic properties.Moreover,two-dimensional materials with ultrathin thickness and large surface area would provide convenient approach for charge transfer and more active sites for the catalysis.Consequently,this thesis aims to explore 2D metal compounds,and improve their catalytic ability by appropriate structure design and modification.In this work,we prepared Sn-doped Ni3S2 nanosheet arrays with the thickness of 5-9 nm on Ni foam via a simple solvothermal procedure.The introduction of Sn element has a significant effect on the structure and morphology of the materials.Moderate doping content of Sn element would induce nanorods to transform into nanosheets structure.3D network structures comprised by ultrathin nanosheets were formed and provided more active sites.More important,the Sn doping could optimize the adsorption and desorption process of hydrogen atoms.Electrochemical measurement results show that Sn-doped Ni3S2 nanosheets possess excellent electrocatalytic activity and good durability toward HER both in acidic and alkaline media.We also discussed the possible enhancement mechanism of electrocatalytic ability caused by doped Sn element.We examined the effect of carbon composite on the electrocatalytic performance.G@Ni3S2 and C@Ni8P3 were synthesized by solvothermal method,and NG/NiSe2/NF hybrid was obtained via electrodeposition technique.The introduction of carton material would enhance the electrical conductivity of initial materials,expose more active sites,as well as serve as a protection layer to prevent dissolution and stabilize electrode to some degree.The synergistic chemical coupling effect between carton material and active material significantly enhanced the electrocatalytic abilities.For example,for the HER process in alkaline solution,Ni3S2 and G@Ni3S2 need the overpotentials of 134 m V and 109 m V vs.RHE to afford 10 m A cm-2 current density.Ni8P3 and C@Ni8P3 require the overpotentials of 194 m V and 144 m V vs.RHE to afford 10 m A cm-2 current density.During the OER process,the surface of catalysts would be partially oxidized,and in-situ-produced oxides shell act as new active sites to promote the OER reaction.Ternary nickel cobalt phosphides with different nickel to cobalt ratios were fabricated via an electrodeposition technique.The films have a triple-layered and hierarchical morphology,consisted by nanosheets in the bottom layer,~90–120 nm nanospheres in the middle layer,and larger spherical particles on the top layer.Moreover,the surface of spherical particles are covered with ultrathin nanosheets,exposing more active sites for catalytic reactions.The binding energies of Ni,Co and P exhibited strong dependence on the Ni/Co ratio,indicating the composition-dependent electronic structure.Compared with other Ni CoP,Ni0.51Co0.49 P has the maximum energy shifts,with the highest partial positive(Ni and Co)and negative(P)charge.The positive Ni and Co centers and the negative P centers function as hydride-acceptor and proton-acceptor sites respectively during the electrocatalysis process.The negative shift of P binding energy reveals the increased electron occupation,resulting in enhanced electron-donating ability.The positive shifts of Ni and Co indicate improved electron transfer ability,benefiting the catalytic reaction.Ni0.51Co0.49 P exhibited efficient HER and OER performances in basic condition,with the overpotentials of 82 m V and 239 m V vs.RHE to deliver 10 m A cm-2,respectively.Furthermore,Ni0.51Co0.49 P has the best state-of-the-art performance with a voltage of 1.57 V to afford 10 m A cm-2 in a two-electrode electrolyzer.Monodisperse SnS2 nanosheets with the diameters of ca.0.8-1 μm and thickness of ca.22 nm were successfully prepared by a simple solvothermal procedure in the presence of polyvinyl pyrrolidone(PVP).Large PVP molecules absorbing on(001)facets of Sn S2 would inhibit crystal growth along [001] orientation and protect the product from agglomeration.The formation mechanism was proposed to undergo a nucleation-dissolution-recrystallization-growth procedure.SnS2 nanosheets demonstrate excellent photocatalytic water splitting performance.The average H2 production rate was detected to be 1.06 mmol h-1 g-1,which is much higher than that of Sn S2 with different morphologies and commercial P25-Ti O2.The huge contact area and thin thickness enable them to harvest signally increased visible light and help electron-hole pairs to transfer faster,which could reduce the recombination rate of photogenerated electrons and holes.The optical properties of Sn S2 could be efficiently tuned by Se doping and the band gaps can be discretely modulated from 2.23 to 1.29 e V with the increase of Se content.The photoelectrochemical measurements indicate that the performance of ternary Sn S2-xSex alloys depends on their band structures and morphology characteristics.