Towards Organized Hybrid Nanomaterials at the Air/Water Interface Based on Liquid‐Crystal/ZnO Nanocrystals

Abstract

The ability to self-assemble nanosized ligand-stabilized metal oxide or semiconductor materials offers an intriguing route to engineer nanomaterials with new tailored properties from the disparate components. We describe a novel one-pot two-step organometallic approach to prepare ZnO nanocrystals (NCs) coated with deprotonated 4-(dodecyloxy)benzoic acid (i.e., an X-type liquid-crystalline ligand) as a model LC system (termed ZnO-LC1 NCs). Langmuir and Langmuir-Blodgett films of the resulting hybrids are investigated. The observed behavior of the ZnO NCs at the air/water interface is rationalized by invoking a ZnO-interdigitation process mediated by the anchored liquid-crystalline shell. The ordered superstructures form according to mechanism based on a ZnO-interdigitation process mediated by liquid crystals (termed ZIP-LC). The external and directed force applied upon compression at the air/water interface and the packing of the ligands that stabilize the ZnO cores drives the formation of nanorods of ordered internal structure. To study the process in detail, we follow a nontraditional protocol of thin-film investigation. We collect the films from the air/water interface in powder form (ZnO-LC1 LB), resuspend the powder in organic solvents and utilize otherwise unavailable experimental techniques. The structural and physical properties of the resulting superlattices were studied by using electron microscopy, atomic force microscopy, X-ray studies, dynamic light scattering, thermogravimetric analysis, UV/Vis absorption, and photoluminescence spectroscopy.

‘Clickable’ ZnO nanocrystals: the superiority of a novel organometallic approach over the inorganic sol–gel procedure

“…Recently, our group developed a novel self-supporting organometallic approach for the synthesis of liquid crystalline carboxylate ligand-coated ZnO NCs which were effectively used for the preparation of highly-ordered hybrid nanomaterials at the air/water interface. 8 Simultaneously, we also demonstrated that ZnO NCs obtained by this organometallic procedure exhibit essentially a very low negative impact on mammalian cell lines. 9 We anticipated that the observed low cytotoxicity stems from the NC effective surface passivation and the impermeable organic shell.…”

“…In order to verify our hypothesis that the self-supporting organometallic procedure provides ZnO NCs with the ligand shell sufficiently shielding against Cu ions during the CuACC process, we selected ZnO NCs stabilized by deprotonated 10-undecynoic acid (una-H) for Cu(I)-mediated click chemistry. Initially, the una ligandpassivated ZnO NCs (hereafter denoted as una-ZnO NCs) were synthesized according to the self-supporting method, 8 which involves: (i) the equimolar reaction between Et 2 Zn and una-H in a donor solvent and (ii) the exposure of the in situ synthesized [EtZn(una)] precursor 10 to air resulting in its subsequent transformation to una-ZnO NCs, forming stable colloidal solutions in THF, CHCl 3 and DMSO (Scheme 1). Prior to further processing and after the purification (see the ESI †), the una-ZnO NCs were characterized using a wide library of analytical techniques.…”

Hydrolysis of organometallic and metal–amide precursors: synthesis routes to oxo-bridged heterometallic complexes, metal-oxo clusters and metal oxide nanoparticles

“…96, 173, 174, 176-179, 208, 209 Lewiński and co-workers employed a simple 'one-pot, two-step' strategy of exposing a THF solution of ligated precursor [ZnEtL]n (L = [RCO2] − , [R2PO2] − or aminoalcoholate) to air, producing small (~2-8 nm) ZnO@L NPs with a dense surface coverage of ligand. 176,178,179 This synthetic route yields ZnO NPs described as (ZnO)x(ZnL2)y(L)z (L = monoanionic ligand) with more than a single monolayer of ligands. This ligand shell is described as 'impermeable', and protects the ZnO core, which results in slow photoluminescence (electron-hole separation up to 2.2 μs) 180 , giving these NPs enhanced properties relative to ZnO from sol-gel techniques.…”

“…179 The process tolerates many different R groups upon the anionic ligand (Fig 24), for instance, the use of liquid crystalline carboxylates such as O2CC6H4OC12H25 allowed the preparation of ZnO NPs which self-assemble into free-standing thin films when compressed upon an air-water interface by the Langmuir method. 176,182 The films form via ligand interdigitation and retain the photoactivity of the individual ZnO NPs. Further studies showed that a Cu catalysed alkyne-azide cycloaddition reaction may be subsequently initiated on ZnO with alkyne terminated carboxylate ligands, and, unlike other NP preparation routes, these NPs remain luminescent after exposure to the Cu(I) catalyst, due to the as the impermeable surface layer of ligands.…”

“…Different ligand precursors lead to NPs with a variety of properties and applications. [176][177][178][179][180][181][182][183] TEM micrograph reproduced with permission from the RSC. 177 the amount of ligand is helpful in forming composite materials, where excess ligand would affect the material properties, e.g.…”