Thermotropic Liquid Crystals as Templates for Anisotropic Growth of Nanoparticles

Abstract

Control over size, shape, and composition of nanomaterials is one of the major concerns in the field of nanoscience today. This task has induced a tremendous amount of work, in which methods based on chemical or physical processes were developed to synthesize such nanosystems. Among the various strategies, organic molecules and macromolecules, exhibiting an anisotropic shape or a particular organization, have been used as templates for controlling the shape and size of inorganic materials.[1] Literature reveals particularly interesting attempts in synthesizing nanoparticles (NPs) within mesophases of liquid crystals (LCs).[2] For example, ZnSe nanomaterials have been synthesized in lyotropic systems based on amphiphilic triblock copolymers.[3] Depending on the liquid-crystalline state, quantum dots, nanodisks, or even nanowires could be obtained. Nanoporous materials (generally silica) have also been fabricated by true liquid crystal templating.[4] Whereas the large majority of such research employs lyotropic LCs, very few publications deal with the elaboration of nanomaterials within thermotropic ones. [5][6][7][8] Indeed, the development of an in situ procedure to generate NPs within an LC medium has proved to be quite a challenging task. Most studies involve the in situ reduction of metal precursors through oxidation of the LC medium in order to obtain the desired NPs. For example, the formation of CuCl nanostructures inside a mixture of an ionic liquid and a derivative of ascorbic acid has been reported. This approach resulted in the formation of CuCl nanoplatelets with a relatively uniform thickness of about 220 nm and in-plane sizes of 5-50 mm.[5a-c] Glass-forming liquid-crystalline materials acting as a reducing agent were also used to obtain Au NPs, the size and shape of which depended on both the amount of precursor content and the LC state.[5d] Isotropic NPs of gold or silver have also been synthesized by heating LC materials doped with the corresponding metal salts.[6] In other cases sputtering [7] or electrodeposition [8] techniques were used to form NPs in thermotropic systems. However, none of the examples above have shown a direct relation between the structure of the LCs and the morphology of the synthesized NPs. For the few examples that report an anisotropic growth, structures in most cases have been outside the nanometer range. To improve such results, we suggest that three criteria should be fulfilled in order to tailor the NP morphology using an LC phase; 1) the chemical reaction leading to the NPs should not disrupt the LC organization. Thus the LC molecules should not play the role of reactants (side products should be avoided as much as possible throughout the NP formation). 2) Interactions between the LC molecules, the NP precursor, and eventually the synthesized NPs should favor the templating effect of the LC phase.3) The use of relatively high viscosity LCs should prevent a fast disruption of the organization during the NP formation.We have previously described a very simple organometa...

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

“…Usually the aggregation of semiconductor NCs results in the formation of photoinactive structures due to the abolitiono fq uantum confinement within particles larger than over ad ozen nanometers. In contrast to the previously reported data, [10,11] we faced an apparent paradox, that is, the photoactivity was preserved despite the formation of aggregates. Twop ossible explanations were considered:1 )the NCs were squeezed/interdigitate together during compression to form aggregates, but without coalescence of the cores;2 )water molecules hydrolyzed the bonds between the ZnO cores and ligand molecules.W ithoutt he organic shell,t he ZnO cores coalescedd ue to high surface energy.T he observedt urbidityo f the film appeared upon the formation of macroscopicZ nO, which was not photoactive and did not influence the emission spectra.Because the process was restricted by the geometry of the system not all the NCs aggregated.…”

“…[9] Only very recently,K ahn and co-workers reported the first successful hybridization of liquid-crystalline molecules with small-sizedZ nO NCs [10] and the use of aL C system as at emplating agent for the preparation of ZnO NCs. [11] Notably,t he passivating LC systemsi na ll the abovementioned cases act as aL -type neutrall igand. [12] To the best of our knowledge,t here is only one example of an effective modification of ZnO NC surfaces by aX -type liquid-crystalline ligand,w hich was obtained by using al igand-exchange reaction andg raftingm esogenic units aroundt he ZnO NC cores.…”

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

“…In contrast, small (<6 nm) isotropic ZnO NPs are formed at higher temperatures, as local order is disturbed throughout the liquid crystal. 216 Considering reactions in air, it is useful to determine the oxygen source, which could be H2O or O2. While hydrolysis of MLx (L = R or NR2) cleanly generates M−OHx and L−H (x equiv.…”

One‐Step Synthesis of Hybrid Liquid–Crystal ZnO Nanoparticles: Existence of a Critical Temperature Associated with the Anisotropy of the Nanoparticles

“…[9,[12][13][14] For severaly ears, we have developed the preparation of metalo xide NPs taking advantage of the exothermic hydrolysis reactiono fo rganometallic precursors to get, in one step and underm ild conditions (room temperature and atmospheric pressure), well-definedc rystalline metal oxide NPs. [19,20] Herein, to gain further insights aboutt he NPs growth mechanism withouts olventi nt he presence of fatty amine, we have investigated the effect of the temperature and composition upon the morphologyo fZ nO NPs and upon the structural and optical properties of the obtained hybrid materials.A ss tated above, the synthesiso fZ nO NPs studied herei sv ery simple and involved just fattya mines. eitheri sotropic NPs or nanorods) has been achievedb yv arying the experimental conditions:i sotropic NPs are obtainedi nt he presence of alkyl chain amine ligandsi na no rganic solvent while nanorods are obtained under the same experimental conditions but in the absence of solvent.…”

“…[17,18] Moreover,a tam acroscopic scale, we demonstrated that the use of an ematic phaseo btained from at hermotropicl iquid-crystalline compound can serve as template for anisotropic growth of ZnO NPs and that hybrid NPs/LC materials can be obtained. [19,20] Herein, to gain further insights aboutt he NPs growth mechanism withouts olventi nt he presence of fatty amine, we have investigated the effect of the temperature and composition upon the morphologyo fZ nO NPs and upon the structural and optical properties of the obtained hybrid materials.A ss tated above, the synthesiso fZ nO NPs studied herei sv ery simple and involved just fattya mines. [20] In at ypical experiment, the dicyclohexyl zinc precursor,[ Zn(cy) 2 ], is mixeda tr oom temperature with two equivalents of hexadecylamine( HDA)a st he ligand; no solvent was used.…”