Formation of Free-Standing Supercrystals from the Assembly of Polyhedral Gold Nanocrystals by Surfactant Diffusion in the Solution

Abstract

Gold supercrystals with polyhedral morphologies can be prepared from the ordered packing of octahedral and rhombic dodecahedral nanocrystals in the presence of a sufficient amount of surfactant by slow water droplet evaporation. The whole supercrystal formation process has been video-recorded using a specially designed chamber to enclose a substrate containing the nanocrystal droplet in a moist environment. Supercrystal growth from the assembly of octahedra is completed within a shorter time. The presence of cetyltrimethylammonium chloride (CTAC) within the supercrystals has been confirmed by small-angle X-ray diffraction analysis. Transmission electron microscopy examination reveals the tendency of two gold octahedra with face contact to fuse, a process frequently observed in the formation of octahedron-assembled supercrystals. Remarkably, we have developed a diffusional surfactant transport approach to make freestanding supercrystals in bulk aqueous solution by adding a concentrated CTAC solution to a concentrated particle solution with a lower CTAC concentration in an Eppendorf tube. Gradual diffusion of CTAC to the lower nanocrystal solution promotes the growth of polyhedral supercrystals. A solution with a sufficiently high surfactant concentration has been shown to be necessary for particle aggregation and supercrystal formation. This method allows the deposition of dense but evenly distributed supercrystals on a substrate. Supercrystals were also used to make a modified electrode for electro-oxidation of glucose. This simple and organic solvent-free approach to making a large quantity of supercrystals allows an ample supply of supercrystals for studies of densely assembled nanocrystal systems and for biomedical applications.

Spontaneous Self‐Assembly of Perovskite Nanocrystals into Electronically Coupled Supercrystals: Toward Filling the Green Gap

“…The self‐assembly process of the SCs in the dispersions is likely governed by the van der Waals attraction between the hydrophobic ligands and the perovskite NC cores under high colloidal concentration. Such spontaneous self‐assembly processes have been previously observed in the formation of free‐standing SCs of polyhedral gold NCs …”

“…In fact this process has been widely enforced to obtain large‐area 2D and 3D self‐assemblies of metal and semiconductor NCs on solid surfaces. By contrast, the spontaneous self‐assembly of NCs into well‐defined SCs in colloidal dispersions has only been rarely observed . The self‐assembly process of the SCs in the dispersions is likely governed by the van der Waals attraction between the hydrophobic ligands and the perovskite NC cores under high colloidal concentration.…”

Ligand Versatility in Supercrystal Formation

“…In a following study, Yang et al used the same method and examined the SC formation process by video recorded optical microscopy. In a continuous process, the first SCs were observed shortly after the beginning while the majority reached an observable size after 20 minutes.…”

“…In the following minutes, the accretion of NPs from the dispersion led to a quick growth of the SCs into structures of about 3 µm. In a second approach, the SC formation has been initiated by a gradual increase of the surfactant concentration . Therefore, an Eppendorf tube was filled with the NP dispersion and a CTAC solution was added on top leading to a slow diffusion of the surfactant into the nanoparticle dispersion ( Figure ).…”

Self-Assembly of Shaped Nanoparticles into Free-Standing 2D and 3D Superlattices

“…[15][16][17] Capping ligands such as small molecular surfactants, peptides, and oligonucleotides have been commonly used to control the lattice parameters including interparticle spacing of NP superlattices. [18][19][20][21][22][23][24][25][26] For instance, Luo's group [ 27 ] and Jaeger's group [ 28 ] have pioneered the self-assembly of NPs decorated with dodecanethiol or DNA tethers into free-standing elastic membranes of NP superlattices. [ 27,28 ] Despite great efforts being made in this fi eld, only few examples have been demonstrated for the self-assembly of shaped NPs into closely packed superlattices using grafted polymers.…”