Morphology-Controllable Synthesis Of Gold-Based Bimetallic Nanocrystals And Their Applications In Photo/Electrocatalysis

Solar/electro-drive conversion of greenhouse gas carbon dioxide（CO₂）or nitrogen（N₂）into the value-added fuels and feedstocks represents a green avenue to address the energy demands and climate chage issues.Owing to the thermodynamically stable and chemically inert properties of the CO₂ and N₂ molecules,one of the greatest challenges is to develop superior catalysts with capability of cleavage of the strong N≡N triple bond or C（28）O bonds.Metal catalytsts,such as copper（Cu）or ruthenium（Ru）,are the most promising materials for the reduction of CO₂or N₂due to the optimal adsorption energy.However,the poor selectivity and weak light-harvesting limit their practical applications.Gold nanocrystals（Au NCs）have attracted extraordinary interests in the fields of optics,biotechnology,and catalysis due to their unique localized surface plasmon resonance（LSPR）properties and well-controlled morphologies.The construction of bimetallic nanocrystals through the integration of Au NCs with another active metal catalyst holds great prospect in catalysis.In this thesis,we fabricated two types of Au-based bimetallic nanocrystals including Au-Cu Janus NCs（Au-Cu JNCs）and Au nanorod（Au NR）@ultrathin Ru nanocluster core@shell nanostructures,and investigated their applications in electrocatalytic reduction of CO₂and photocatalytic N₂fixation.The main results are as follows:（1）Symmetry-breaking synthsis of spatially separted Au-Cu JNCs and their tandem catalysis process in electrochemical CO₂reduction.Cu has attracted great interest as the catalyst for electrocatalytic CO₂reduction reaction（CO₂RR）due to its unique capability of converting CO₂into a wide spectrum of useful products,such as C₁（CO,CH₄,formate）and C₂（C₂H₄,C₂H₆）products,but poor C₂product selectivity is a major drawback.Au nanocrystals have recently shown great promise as catalysts for efficient CO₂reduction to CO with good selectivity.Since the adsorbed CO is critical for C-C bond coupling and the subsequent formation of C₂products,the integration of Au nanocrystals with Cu catalysts is expected to improve the C₂selectivity.In this thesis,we developed a facile and general strategy to synthesize the Au-Cu JNCs through the site-selective growth of Cu nanodomains on one side of Au nanobipyramids（Au NBPs）,which is directed by the substantial lattice mismatch between Au and Cu（～11.4%）,with the assistance of judicious manipulation of the growth kinetics.Control experiments confirmed that the concentration of cetyltrimethylammonium bromide（CTAB）and hexadecylamine（HDA）played crucial roles in the growth behavior of Cu nanodomains on Au NCs.More intriguingly,this strategy can work on Au nanocrystals with different architectures for the achievement of diverse asymmetric Au-Cu hybrid nanostructures,including 0 D Au nanospheres（Au NSs）,1 D long Au nanorods（Au NRs）,and 2 D hexagonal Au nanoplates（Au NPLs）.In addition,the spatial-separation design of the Au–Cu heterostructure possesses distinct advantages over the Cu NSs in the activity and selectivity of C₂production.The Faladay efficience（FE）and maximum partial current density are 4.1-fold and 6.4-fold higher than those of their monometallic Cu counterparts,respectively.The excellent electrocatalytic performances benefit from tandem catalysis process promoted by the hybrid nanostructure and the high-index facets on the Au NBPs.Finally,we proposed a tandem catalysis mechanism to explain the high C₂activity and selectivity of the Au–Cu heterostructures.In this tandem catalysis process,CO is generated on the Au catalyst through the reduction of CO₂,and subsequently migrates to the active sites of nearby Cu for further CO dimerization.This research provides an alternative strategy for the synthesis of spatially separated hybrid nanoblocks for efficient electrochemical reactions,and it providing a new avenue for pursuing high-efficiency catalysis through the manipulation of reaction pathways in CO₂RR.（2）The construction of antenna-reactor photocatalyst through the precisely taioring the Ru shell thickness on Au NRs for the application in N₂photofixation.Ru is known as the optimal metal catalyst for ammonia（NH₃）synthesis,but the poor light-harvesting capability restricts its application in photocatalysis.Plasmonic metal nanocrystals（e.g.,Au,Ag,and Cu）have garnered considerable interest due to their unique property of LSPR.They can not only harvest light from the ultraviolet to near-infrared regions but also generate hot carriers with high energy to drive chemical reaction.Accordingly,the coupling of plasmonic nanoantennas with active Ru catalysts facilitates efficient light harvesting for N₂photofixation.In this thesis,we construct an antenna-reactor nanostructure through the controllable growth of an ultrathin Ru nanocluster shell with desired catalytic activity on the plasmonic gold nanoantennas.The thickness of ultrathin Ru nanocluster shell can be varied from 1.6 nm to 6.2 nm by changing the CTAB concentration.The ultrathin Ru shell is discontinuous and is composed of nanoclusters because of the relatively large lattice mismatch（⁷.4%）.The scattered Ru nanoclusters rather than compact Ru nanoparticle shell is beneficial to N₂photofixation,because the discontinuous shell facilitates the accessible of photogenerated holes by the reactants and promotes the electron–hole separation.This antenna-reactor plasmonic photocatalyst exhibits shell-thickness dependent photocatalytic activity toward nitrogen photofixation under visible and near-infrared light illumination.When the thickness of Ru shell is 3.1±0.9 nm,the maximum ammonia generation rate is 105.1±5.5μmol×h^-1×g^-1.This merit benefits from the antenna-reactor nanostructures.In this scenario,Au NRs function as the plasmonic antennas for light harvesting and generate hot electrons for the N₂reduction,while Ru nanoclusters activate N₂molecules and convert them into ammonia product.This research demonstrates the genuine superiority of the antenna-reactor plasmonic photocatalyst and sheds new light on the design and construction of high-efficiency catalysts for photocatalytic applications.