Optical And Catalytic Properties Of Novel Gold-based Nanorods Heterostructures Via Selective Etching

Hybrid or heterostructure nanoparticles constructed from more than one structure orcomponent phase have been attracting increasing interest due to theiradditionalfunctionalities.Gold nanorods heterostructue have some unforeseen developments in some areas, such asmodulation of the optical response or combination of optical and catalytic properties. Here, wethrough oxidative etching obtained some new gold nanorods heterostructures. UV-Visabsorption spectroscopy and TEM microscopy are powerful tool to identify reaction processand the composition and structure of intermediates. The details are as follow:（1） A number of dumbbell-like Ag-tipped gold nanorods were obtained by lateral etchingof the core-shell Au@Agnanorods with FeCl₃at room temperature.A series of well-designedcontrol experiments revealthat thepreferential etching at theside of the core-shell nanorods isocaettf tryibtlhtureit medre etshtouytltlhiaenm ig mncreased reactivity of the side facets, due to less surface passivation ofnoanniourmo dbsr odmeicdree.aDseu e aton dt heth per eafesrpeenctti alr aetticoh iningc mreaaksee,ss oth et heef eccotirvree sdpioanmdeitnegrLSPR（longitudinal surface plasmon resonance） peak shifts to a longer wavelength.The similarsynthetic strategy has also been applied to the Au@Pd core-shell nanorods, indicating thegreatpotential of this selective etching. The Ag-tipped Au nanorods exhibit ahigher catalytic activitytrthhoaeon m AtAge-mu tippnpeareandto urAroedu, snw ahaninocdrh o dccsoo.ureld-s bheeallt trAibuu@teAd gto nthaen oerloedctsrionn itch ee freecdtu acntido nth eo fu npiq-nuietr sotpruhcetnuorle iant（2）Core-shell Au@AuAg nanorods reacted with Fe（NO3）3at room temperature obtainedthe gold nanorods in a porous shell of AuAg alloy. The formation of the tiny pores in the shellresults in the huge red-shift, sharp decreasing and drastic broadening of longitudinal surfaceplasmon resonance absorption. The continuous removal of silver from the porous nanorodsleads to the breakage of the tiny pores and leaves a rough surface on the nanorods. The roughsurface gradually becomes smooth in the subsequent dealloying process. These surfacestructures of the intermediates are correlated with their absorption spectra and catalyticactivities for the catalytic reduction of p-nitrophenol. The porous nanorods show a highercatalytic efficiency than those of the gold nanorods, the core-shell nanorods and the roughnanorods. The results indicate that the dealloying of anisotropic bimetal nanomaterials not onlyprovides an effective pathway to carve the structures on the nanoscale, but also offers numerousopportunities to observe novel optical properties and enhanced catalysis performances. （3）A simple one-pot method with gold nano-rods as a template has been successfullydeveloped for novel polymetallic hollow nanostructures in a one-dimensional shape, like thehollow dumbbell-like nanorods of Au@PtAg and Au@RhAg. The formation of the hollownanostructures involves the growth of a metallic shell and then the etching of the gold nanorods.Since the growth of the metal shell is before the etching process, its component and interactionwith surface ligands greatly affect the subsequent etching of the gold nanorods. The presence ofa higher cohesive-energy metal in the shell than gold, would prefer an island-like growth andthen benefit the formation of hollow structures. The existence of silver in the shell wouldweaken the strong protection of CTAB to the side facets and release them to the etchingreaction, resulting in a middle-preferential etching. The underlying reactions behind the etchingare disclosed to be the oxidation of metallic gold induced by oxygen with the assistance ofascorbic acid. These insightful understandings about the underlying reactions and the formationmechanism of polymetallic hollow nanostructures in a one-dimensional shape would provideuseful guidance for the preparation of hollow nanostructures in the other anisotropicmorphologies. The hollow dumbbell-like nanorods of Au@PtAg effectively improves thedirect oxidation of formic acid to CO2via the dehydrogenation pathway, which could beattributed to the hollow structure, one-dimensional shape, highly conductive surface, and the“foreign-atom” effect.（4） Silve ions were introduced to guide the lateral etching of gold nanorods.Silver（I）bromide complex （CTA Br Ag⁺） acts as a face-specifc cappingagent adsorbed on the sidefaces of the gold nanorods, which is make for the lateral etching of the gold nanorods. The rolesof silver ions for the chemical etching and anisotropicgrowth of gold nanorods are completelydifferent.