Modification Of Narrowband Gap Semiconductor Photocatalysts By Ultrathin MOF For Photocatalytic CO₂ Reduction

Solar-driven CO2 reduction is one of the effective ways to mitigate global warming and achieve carbon neutrality.Therefore,the research and development of high activity and high selectivity of CO2 reduction photocatalyst has important scientific significance and broad application prospects.In many semiconductors photocatalytic materials,2D narrow-band-gap semiconductor,such as organic polymer graphite carbon nitride(g-C3N4)and inorganic oxide bismuth vanadate(BiVO4)with visible light response,such as shorter carrier transmission distance and larger specific surface area,especially after morphology control for ultrathin 2D structure,can be used as a potential CO2 conversion of photocatalytic materials.However,these narrow-band-gap semiconductors still have some problems,such as unreasonable band structure,poor photogenerated charge separation and lack of catalytic sites,resulting in unsatisfactory photocatalytic activity and selectivity.To solve the above problems,it is expected to achieve high activity and selectivity of CO2 conversion process by selecting 2D narrow-bandgap semiconductor with matched energy band and abundant catalytic sites to construct heterojunction photocatalyst with close interface linkage.Metal-organic frameworks(MOF)are a class of porous crystalline phase materials with periodic structure self-assembled by metal ions and organic ligands.Its uniformly dispersed unsaturated metal catalytic center can enhance the adsorption and activation of CO2.The function of ligand provides suitable band position for MOF to match the energy level of narrow-band-gap semiconductor and improve the charge separation ability.The large surface area and high porosity of MOF can realize the effective diffusion of the reaction substrate and product.Therefore,MOF is expected to be an ideal material for the modification of narrow-band-gap semiconductors and the construction of new heterojunction photocatalysts.However,there are still the following scientific problems in MOF modified narrow-band-gap semiconductor photocatalysts:(1)how to regulate the charge separation and transport of MOF in heterojunctions and improve the interface binding.(2)how to regulate the catalytic properties and selectivity of metals in MOF;(3)how to further expand the visible light response of narrowbandgap semiconductors.Aiming at the above three scientific problems,this paper carried out the following research works:Firstly,on the surface of 2D ultrathin g-C3N4,a 2D high-energy electronic platform NiMOF with catalytic function was in situ grown with ultrasonic assistance,which could prolong the lifetime of g-C3N4 high-energy electrons and realize the dual function of central metal catalysis.An ultrathin 2D heterojunction photocatalytic system with matching dimensions was constructed.The yield of CO and CH4 was 46.8 μmol·g-1 and 7.7 μmol·g-1,respectively,in the 4NiMOF/CN-AA nanocomposite catalyzed by UV and visible light for 4 hours.P-aminobenzoic acid(AA)was used to functionalize g-C3N4,improve the charge transport between NiMOF and g-C3N4,and expose more photocatalytic active centers.Second,a 2D ultrathin TiMOF/BiVO4 S-scheme heterojunction photocatalytic system with dimensionally matched and catalytic center was synthesized by hydrogen bond-induced assembly.The yield of CO and CH4 was 55.6 μmol·g-1 and 1.5 μmol·g-1,respectively,when the optimal amount of 6TMNS/BVNS composite photocatalytic reduction of CO2 for 4 hours.The selectivity of CO reached 97.4%.In situ technology was used to prove that the S-scheme charge-transfer mechanism between TiMOF and BiVO4 was consistent,and the electrons accepted by TiMOF ligand could be further transferred to the metal oxygen cluster,thus realizing the photocatalytic reduction of CO2.Thirdly,the synthesis strategy of anchoring a high amount of single atoms using the oxygen-cup microenvironment formed by Ti(μ-OC=O)3(μ-OOH)3 in TiMOF was developed,and an efficient 2D S-scheme Ni@TMNS/BVNS photocatalytic system was constructed by improving the kinetic process of electron reduction reaction.The yield of CO was 178 μmol·g-1 and the selectivity was 99.2%in the 4 hour UV-visible light catalytic CO2 reduction reaction of the 1Ni@6TMNS/BVNS nanocomposite with the best modified ratio.Experimental and theoretical calculations were used to study the charge transfer and injection processes during the reaction.It was confirmed that the improved photocatalytic activity was due to the MOF-anchored single atom Ni as the catalytic center,which could significantly promote the S-scheme charge transfer between TiMOF and BiVO4.On the other hand,the localized microenvironment(LME)composed of the single atom Ni(II)site and the adjacent carboxyl group can synergistically catalyze the CO2 conversion efficiently and selectively.The conversion path of CO2 to CO involved by LME was revealed by theoretical calculation combined with in situ infrared,and the main reason for the enhancement of reactivity and selectivity in the process of photocatalytic reduction of CO2 conversion was explained.The research in this paper provides a new idea for the design and synthesis of MOF modified narrow bandgap semiconductor heterojunctions for the photocatalytic reduction of CO2 with high activity and high selectivity.