Fabrication And Photocatalytic Nitrogen Fixation Performance Of Semiconductor Nanomaterials-based Bionic Hierarchical Assemblies

Ammonia（NH₃）is a critical basic ingredient for industrial and agricultural production,as well as for ensuring food security and energy supply.The Haber-Bosch technique is now the dominant ammonia synthesis technology,while its harsh reaction conditions have resulted in high energy consumption and contamination of the environment.Photocatalytic nitrogen reduction for ammonia synthesis is an effective alternative for green and sustainable development,but there are many obstacles and challenges to enhancing the efficiency of photocatalytic nitrogen fixation,and the slow mass transfer of reactant nitrogen in aqueous solution is one of the i barriers that cannot be overlooked.As a result,it is critical to integrate bionic principles to optimize nitrogen mass transfer and achieve effective photocatalytic ammonia synthesis.With the flourishing of the emerging interdisciplinary discipline of bionics,an increasing number of researchers are intentionally examining,learning,and emulating superb strategies in nature to design photocatalytic systems that accomplish effective photocatalytic performance.This paper meliorates the important problem of sluggish nitrogen mass transfer in the photocatalytic nitrogen fixation process.By imitating the structure and function of biological evolution and the process and mechanism of biological nitrogen fixation,we designed a series of bionic hierarchical assemblies based on semiconductor catalysts to construct the triphase system and the floatable triphase system to solve the nitrogen mass transfer problem in the photocatalytic nitrogen reduction reaction and achieve efficient photocatalytic ammonia synthesis.It also serves as a vital reference for other gas consumption processes to optimize gas mass transfer.（1）Inspired by the hierarchical structure mass transfer functions of plant vascular bundles,we have prepared a carbonized loofah sponge@BiOBr-OV/Au（CL@BiOBr-OV/Au）with a specifically optimized N₂ transport channel to improve the mass transfer of nitrogen.The hydrophobic CL sponge provides an optimized directional channel for N₂ diffuse to the triphase interface N₂（gas）-H₂O（liquid）-BiOBr-OV/Au（solid）under water,which replaces the slow diffusion of N₂through the liquid phase,increasing N₂concentration in the reaction activity center.The modification of Au NPs can broaden the photoresponse range of BiOBr-OV,increase oxygen vacancies concentration and enhance N₂adsorption and activation.CL@BiOBr-OV/Au shows an impressive in nitrogen fixation performance with 2.64 mmol·g_cat^-1·h^-1,that is 10.2 and 8.0 times compared to CL@BiOBr-OV and BiOBr-OV/Au,respectively.The CL@BiOBr-OV/Au exhibits efficient,highly selective,and well-stabilized photocatalytic nitrogen fixation performance.（2）A 3D-printed clay substrate@MoS₂-b/Au（CN@MoS₂-b/Au）with underwater gas trapping was prepared,which is inspired by the function of water spiders to trap gas underwater for aerobic respiration.Hydrophobic MoS₂-b/Au,can capture N₂ underwater and form a nitrogen layer on its surface to deliver N₂ for the reaction interface.The hierarchical structure of the hydrophilic bionetwork ensures effective water transport,forming an effective gas（N₂）-liquid（H₂O）-solid（MoS₂-b/Au）triphase coexistence microenvironment at the reaction interface.Au NPs extend the visible light absorption range of MoS₂-b,enhancing charge transfer and carrier separation.CN@MoS₂-b/Au shows an impressive in nitrogen fixation performance with 332.46μmol g_cat^-1 h^-1,which is 1.69 and 3.89 times greater than the relevant diphase system CN@MoS₂-h/Au and the powder system MoS₂-b/Au.CN@MoS₂-b/Au shows remarkable photocatalytic nitrogen fixation capability and cyclic stability.（3）A bionic floatable triphase system was prepared（CC@WO₃/CQDs@HCPs）by anchoring a bionic assembly of tungsten oxide/carbon quantum dots@supercrosslinked polymers（WO₃/CQDs@HCPs）side of carbon cloth（CC）,inspired by the floatable nature of Pistia and the underwater gas trapping function of water spiders.CC@WO₃/CQDs@HCPs can self-support on the water surface and be directly exposed to the N₂ atmosphere in the photocatalytic nitrogen fixation reaction,forming a floatale triphase system（N₂-H₂O-WO₃/CQDs@HCPs）.The hydrophobic and porous CC can transport N₂ to the surface of WO₃/CQDs@HCPs,while the HCPs can effectively adsorb N₂.We constructed two triphase system with different N₂ mass transfer modes by floating CC@WO₃/CQDs@HCPs on the water surface or submerging it in water to examine the influence of N₂ mass transfer on photocatalytic nitrogen fixation efficiency.The floatable triphase system CC@WO₃/CQDs@HCPs achieved more effective triphase contact,and the photocatalytic nitrogen fixation rate is 772.93μmol g^-1 h^-1 without sacrificial agent or precious metal catalyst assistance,and visible light irradiation,which is higher than the immersed triphase system by 1.26times and the diphase system WO₃/CQDs@HCPs by 1.38 times,4.31 times that of CC@WO₃/CQDs.This strategy provides new insights into the combination of organic polymers with the triphase system to achieve efficient photocatalytic performance.（4）The above-mentioned floatable triphase catalytic system can only react at the2D interface between the catalyst layer and the gas diffusion layer,resulting in just a limited quantity of catalyst contacting both gaseous and liquid reactants.Inspired by the Pistia floatable hierarchical structure and the effective gas and liquid mass transfer function,by anchoring Bi VO₄ on PDMS-modified Janus melamine sponge（MS@PDMS@Bi VO₄）,we have prepared a floatable system that forms a 3D triphase interface in the air phase,allowing nitrogen and water reach to the reaction active sites from separate diffusion pathways in different directions,effectively ensuring mass transfer of nitrogen and water simultaneously so that the performance improvement of the photocatalytic nitrogen fixation can be guaranteed in kinetics.Bi VO₄ is uniformly dispersed over the surface of the Janus MS macroporous structure forming the microreactor that can construct a reaction system with broad triphase interface area and exposing a large number of active sites in MS@PDMS@Bi VO₄.Furthermore,because the triphase interface is contrust above the water surface,sunlight no longer experiences a series of losses such as reflection and refraction from the water surface,but instead directly irradiates the reaction interface,improving light utilization.MS@PDMS@Bi VO₄ demonstrated an ammonia synthesis rate of 624.87μmol g_cat^-1 h^-1 with high selectivity and stability,which is approximately 1.67 times higher than the related diphase system（MS@Bi VO₄）.We employed the 3D triphase interface that forms in the gas phase to photocatalytic nitrogen fixation for the first time,effectively ensuring nitrogen and water mass transfer to improve photoactivity and selectivity and light utilization,offering a promising option in optimizing highly efficient mass transfer for gas-consuming reactions.