Mechanistic insight into gold nanorod transformation in nanoscale confinement of ZIF-8

Core-shell hybrid nanomaterials have shown new properties and functions that are not attainable by their single counterparts.Nanoscale confinement effect by porous inorganic shells in the hybrid nanostructures plays an important role for chemical transformation of the core nanoparticles.However,metal-organic frameworks(MOFs)have been rarely applied for understanding mechanical insight into such nanoscale phenomena in confinement,although MOFs would provide a variety of properties for the confining environment than other inorganic shells such as silica and zeolite.Here,we examine chemical transformation of a gold nanorod core enclosed by a zeolitic imidazolate framework(ZIF)through chemical etching and regrowth,followed by quantitative analysis in the core dimension and curvature.We find the nanorod core shows template-effective behavior in its morphological transformation.In the etching event,the nanorod core is spherically carved from its tips.The regrowth on the spherically etched core inside the ZIF gives rise toformation of a raspberry-like branched nanostructure in contrast to the growth of an octahedral shape in bulk condition.We attribute the shell-directed regrowth to void space generated at the interfaces between the etched core and the ZIF shell,intercrystalline gaps in mult-domain ZIF shells,and local structural deformation from the acidic reaction conditions.

Nanoscale Cinematography of Soft Matter System under Liquid-Phase TEM

CONSPECTUS:One emergent theme in“soft matter”is to understand and manipulate the self-organization of synthetic materials and biological entities in space and time at the underexplored nanoscale.Encoded at this length scale can be a diversity of spatiotemporally fluctuating dynamics that are critical to function,from phase transition of nanoparticle self-assemblies as reconfigurable devices and morphology development of polymer membranes as separation layers for wastewater reclamation to the transformation of membrane proteins as the gatekeeper for mass and information flow in living cells.Extensive research efforts have thus been focused on resolving and understanding such dynamics that typically occur in a liquid medium.The proliferation of methods such as liquid-phase atomic force microscopy,cryogenic electron microscopy,and super-resolution optical microscopy has greatly expanded our knowledge in the structure or dynamics of soft matter at the nanoscale.However,these techniques do not offer direct real-space,real-time imaging of the structural and functional dynamics in a native liquid environment with nanometer resolution.This lack of experimental dataset also renders predictive modeling or computation difficult.As a result,how nanoscale morphology and interaction of the constituents affect the self-organization pathways or broadly collective structural evolution,such as interconversion among metastable states,as well as the involved energy measures remains poorly understood.In this Account,we present our recent efforts in adapting and using a nanoscopic cinematography method relatively new to the soft matter community,liquid-phase transmission electron microscopy(TEM),to study the self-organization pathways of nanoscale colloidal matter.Liquid-phase TEM has opened a new avenue to investigate materials chemistry questions,such as electrochemistry and catalysis,nanomaterial diffusion and growth,and nucleation of minerals and atomic crystals.Applying it to soft matter systems involves tackling complications,including the electron beam’s modification of nanoscale colloidal interaction and the substrate effect present in the liquid chamber,for both of which we highlight achievements of control.In addition,we discuss a series of first-time imaging of self-organization pathways of nanoparticle systems,accessible only by liquid-phase TEM.At low nanoparticle concentrations,chaining of nanoparticles occurs following quantitatively the kinetic laws of polymerization.This analogy originated from local collision and pairwise interaction,which can be directly mapped from trajectory sampling.At high nanoparticle concentrations,collective phase behaviors such as crystallization and coalescence are observed with single-particle resolution,allowing for the charting of phase coordinates and thermodynamic quantities based on statistical mechanics principles.We also discuss the general applicability of these methods.Lastly,toward taking live videos of organic soft matter at the nanoscale,we highlight recent instrumental developments,including machine learning based liquid-phase TEM video analysis to account for low signal-to-noise ratio data sets and low-dose electron tomography to resolve three-dimensional morphologies.We foresee that the examples,techniques,and understandings pinpoint the beginning of a paradigm shift in soft matter studies,where knowledge at the nanoscale can be derived from direct“seeing”.