Theoretical Studies Of Structure Design And Performance Regulation On Two-Dimensional Photocatalytic Systems

Exploring renewable energy has drawn extensive attentions due to the everincreasing energy demand,serious environmental problems and excessive consumption of traditional fossil fuels.As an inexhaustible clean energy,solar energy has been considered as one of the most valuable energy sources.The photocatalytic conversion of solar energy into hydrogen,hydrocarbons,ammonia and other chemical energy(fuels)is recognized as an ideal solution to these issues.Two-dimensional(2D)materials and their heterostructures have shown great possibility in the field of photocatalysis due to their unique atomic structures,highly adjustable optical and electronic properties.However,the low solar energy conversion efficiency induced by the rapid recombination of photogenerated carriers remains the main obstacle for its large-scale application.Although the traditional type-II heterostructure can effectively promote the separation of photogenerated carriers,the sacrificial agents or co-catalysts are always needed to drive the reaction processes because of the reduced redox capacity.Therefore,developing a new photocatalytic system has become an important topic in this field.In this dissertation,based on the density functional theory(DFT),we investigated a series of 2D structures,such as transition metal sulfide heterojunctions,intrinsically polarized materials,porous carbon-based materials,etc,aiming at the potentials in photocatalytic overall water splitting,photocatalytic reduction of carbon dioxide and photocatalytic nitrogen fixation.We reveal the transfer and separation mechanism of photogenerated carriers,the construction of Z-scheme photocatalytic systems and the evaluation of energy conversion efficiency in these processes.Besides,the photocatalytic mechanism based on 2D polarized bilayers,regulation of the catalytic activity based on defects and built-in electric field are also be discussed.These studies provide theoretical insight for the further experimental researches.1.It remains a challenge to achieve visible light response and efficient photocatalytic overall water splitting because of the need for both suitable energy band structures,low reaction overpotentials and spatially separated photogenerated carriers.Our theoretical research shows that,as Z-scheme photocatalytic systems,the twodimensional MoSe2/SnSe2 and WSe2/SnSe2 heterostructures have high photogenerated carrier separation efficiencies and strong redox capabilities.The overall water splitting can be achieved without any external assistance.Introducting Se vacancy on the MoSe2(WSe2)monolayer can greatly reduce the overpotential and obtains the synergistic effects on visible light absorption,catalytic active sites and photogenerated charge separation.According to the characteristics of Z-scheme photocatalytic systems,we proposed the evaluation standard of energy conversion efficiency.The predicted upper limit of conversion efficiency can reach 10.5%,which meets the basic condition for commercial HER application.Above results provide a deep theoretical insight for the construction,regulation and evaluation of efficient Z-scheme photocatalytic systems.2.Polarity is an important means to promote separation of photogenerated carriers.Based on the 2D polarized bilayer,we propose a new type-? photocatalytic mechanism.Under the built-in electric field,the band structures between different layers are splitted to type-II band alignments,which effectively separate the photogenerated carriers.In addition,the built-in electric field changes the positions of the standard hydrogen electrode on the upper and lower layers of the catalyst,which greatly improves the oxidation-reduction capability of the systems.Moreover,the built-in electric field breaks the limitation of band gap for photocatalyst(>1.23 eV),extending the light absorption range to infrared spectral region.Based on first-principles calculations,we further verified this photocatalytic mechanism for overall water splitting by investigating polarized LiMX2(M=Al,Ga,In;X=S,Se,Te)materials and Janus M2XY(M=Ga,In;X=S,Se,Te)multilayers.The predicted upper limit of energy conversion efficiency can reach up to 38.5%,which is close to the theoretical limit of 47%.This result provides a new solution for obtaining highly efficient photocatalysts.3.High reaction potential barrier and low product selectivity seriously restrict the practical application of photocatalytic CO2 reduction.We extended the Z-scheme photocatalytic mechanism and the polarized type-? photocatalytic mechanism found in photocatalytic water splitting to the photocatalytic CO2 reduction.Our theoretical calculations show that the experimentally-synthesized C2N/aza-CMP heterostructure is available for achieving the above Z-scheme photocatalytic mechanism.After anchored by two Cu atoms,C2N/aza-CMP heterobilayer can effectively inhibit the competition of HER and achieve high selection toward photocatalytic CO2 reduction to CH4.In addition,the ferroelectricity of the CuInP2S6 multilayer enable it to be an efficient photocatalyst for CO2 reduction followed the polarized type-? photocatalytic mechanism.The theoretical upper of energy conversion efficiency can reach 32.57%.These excellent performances further verify the advantages of the two photocatalytic mechanisms and open up a new way for designing efficient photocatalytic CO2 reduction systems.4.High overpotential of N2 activation and low solar-to-chemical conversion efficiency are the two obsticals in photocatalytic nitrogen reduction.Here,we demonstrate that introducing sp-hybridized boron atom into metal-free triphenylenegraphdiyne monolayer(B@TP-GDY)would be a promising solution to achieve highly efficient nitrogen reduction reaction(NRR).We disclose that spin-polarization on sphybridized B site play a key role in activating the N2 molecule through the"acceptancedonation" concept,making the NRR process can be driven effectively by the photogenerated electrons.Besides,B@TP-GDY monolayer exhibit a high selectivity towards the NH3 production.Moreover,the defective state induced by B atom not only enhance the light absorption ability,but also significantly suppress the recombination of photogenerated carriers.Our finding broadens the understanding of B-based photocatalytic mechanism for N2 fixation and provides a feasible route to design metalfree NRR photocatalysts.