Comprehensive experiment on 2D/3D MoS2/HTCN heterojunction preparation and photocatalytic nitrogen fixation
[Objiectibe]Ammonia(NH3),a sustainable energy carrier and a vital industrial feedstock,plays a pivotal role in the progression of human civilization.Currently,ammonia is typically synthesized in factories using the Haber-Bosch method,which involves nitrogen(N2)and hydrogen(H2).This process consumes large amounts of energy and mostly gray hydrogen from fossil fuels,indicating that the Haber-Bosch method exerts enormous pressure on carbon emissions.To mitigate the growing environmental and energy challenges,investigating new methods for ammonia synthesis that are environmentally sustainable and have low energy consumption is crucial.Photocatalysis is an emerging catalytic technology that uses solar energy as an energy source and has the advantages of environmental friendliness and low energy consumption.Therefore,ammonia synthesis through photocatalysis has attracted the interest of many researchers.Graphite-phase carbon nitride(g-C3N4)with a narrow band gap(2.4-2.9 eV)has been widely used in photocatalytic reactions.However,the shortcomings of g-C3N4,such as low light absorption efficiency,poor effective charge-carrier separation,and weak N2 adsorption capacity,limit its further application in photocatalytic ammonia synthesis.[Methods]To overcome the disadvantages of g-C3N4,2D/3D MoS2/HTCN heterojunction photocatalysts were prepared,and the feasibility of their photocatalytic reactions for ammonia synthesis was investigated.First,hollow tubular g-C3N4(HTCN)was prepared to improve the light-absorbing ability of g-C3N4.Second,molybdenum disulfide(MoS2)nanosheets were loaded onto the HTCN surface to produce heterojunctions,enhancing the ability of the catalyst for carrier separation.In addition,the MoS2 nanosheets are superiorat absorbing N2,which aids in improving the N2 adsorption capacity of the MoS2/HTCN composites.Various tests were conducted to thoroughly investigate the properties of the synthesized catalysts.The structural and morphological features of samples were characterized using X-ray diffraction,X-ray photoelectron spectroscopy,scanning electron microscopy,and transmission electron microscopy.The opticaland electrical properties of the samples were assessed using ultraviolet-visible spectroscopy,diffuse reflectance spectroscopy,photoluminescence,transient photocurrent,and electrochemical impedance spectroscopy.Energy band data were determined using the Tauc curves and Mott-Schottky analysis.Performance evaluation involved designing photocatalytic ammonia synthesis experiments under simulated sunlight conditionsby employing controlled variables to validate the origin of the synthesized NH3 from the N2 reduction by the catalyst.In addition,catalyst stability was examined through cycling experiments.[Results]The following results were obtained through the various above mentioned tests:① 2D MoS2 nanosheets were successfully loaded onto the surface of 3D HTCN and exhibited a uniform distribution.② 3D-structured HTCN has stronger light absorption than the bulk g-C3N4,and the loading of 2D MoS2 nanosheets further enhanced the light absorption of 2D/3D MoS2/HTCN.③ 2D MoS2 nanosheets formed a heterojunction with 3D HTCN and the built-in electric field at its interface promoted the separation of photoelectrons and holes.④ In the photocatalytic nitrogen fixation experiment,the best performance was exhibited by 45％MoS2/HTCN,which is 9.5 times that of g-C3N4,4.3 times that of HTCN,and 4.8 times that of MoS2.After five cycles,45％MoS2/HTCN could maintain a photocatalytic nitrogen fixation efficiency of 81.63％.A Z-type 2D/3D MoS2/HTCN photocatalyst was synthesized via morphology modulation and heterojunction composites.[Conclusions]The hollow tubular structure of the catalyst enhanced the light absorption efficiency of g-C3N4.The heterojunction composites facilitated the separation of photogenerated carriers through the built-in electric field.In addition,the N2 adsorption capacity of the MoS2/HTCN composite materials improved because of the loading of 2D MoS2 nanosheets.Because of the aforementioned favorable conditions,the Z-type 2D/3D MoS2/HTCN photocatalyst exhibited a notable efficiency in reducing N2 to NH3,demonstrating an excellent photocatalytic ammonia synthesis capability.Characterized by comprehensiveness and novelty,this experimental study covers multiple aspects of catalyst preparation and testing,enhancing not only students'abilities in catalyst synthesis and characterization but also their scientific thinking and overall scientific literacy.
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