In-situ SAXS study of aqueous clay suspensions submitted to alternating current electric fields

Mesostructured Materials with Controllable Long-Range Orientational Ordering and Anisotropic Properties

Inorganic-organic mesophase materials provide a wide range of tunable properties, which are often highly dependent on their nano-, micro-, or meso-scale compositions and structures. Among these are macroscopic orientational order and corresponding anisotropic material properties, the adjustability of which are difficult to achieve. This is due to the complicated transient and coupled transport, chemical reaction, and surface processes that occur during material syntheses. By understanding such processes, general criteria are established and used to prepare diverse mesostructured materials with highly aligned channels with uniform nanometer dimensions and controllable directionalities over macroscopic dimensions and thicknesses. This is achieved by using a micropatterned semipermeable poly(dimethylsiloxane) stamp to manage the rates, directions, and surfaces at which self-assembling phases nucleate and the directions that they grow. This enables mesostructured surfactant-directed silica and titania composites, including with functional guest species, and mesoporous carbons to be prepared with high degrees of hexagonal order, as well as controllable orthogonal macroscopic orientational order. The resulting materials exhibit novel anisotropic properties, as demonstrated by the example of direction-dependent photocurrent generation, and are promising for enhancing the functionality of inorganic-organic nanocomposite materials in separations, catalysis, and energy conversion applications.

Multiresponsive Polymer Nanocomposite Liquid Crystals Having Heterogeneous Phase Transitions for Battery-Free Temperature Maintenance Indicators

Multiresponsive functional materials that respond to more than one external stimulus are promising for novel photonic, electronic, and biomedical applications. However, the design or synthesis of new multiresponsive materials is challenging. Here, this work reports a facile method to prepare a multiresponsive colloidal material by mixing a liquid-crystalline 2D nanocolloid and a functional polymer colloid. For this purpose, electrically sensitive exfoliated α-ZrP 2D nanocolloids and thermosensitive block copolymer colloids that are dispersed well in water are mixed. In the liquid-crystalline nanocomposite, nematic, antinematic, or isotropic assemblies of α-ZrP, nanoparticles can be electrically and selectively obtained by applying electric fields with different frequencies; furthermore, their rheology is thermally and reversibly controlled through thesol-gel-sol transition. The nanocomposite exhibits a solid gel phase within a predesigned gel temperature range and a liquid sol phase outside this range. These properties facilitate the design of a simple display device in which information can be electrically written and thermally stabilized or erased, and using the device, a battery-free temperature maintenance indication function is demonstrated. The proposed polymer nanocomposite method can enrich the physical properties of 2D nanocolloidal liquid crystals and create new opportunities for eco-friendly, reusable, battery-free electro-optical devices.

A temperature-controlled electric field sample environment for small-angle neutron scattering experiments

A new sample environment is introduced for the study of soft matter samples in electric fields using small-angle neutron scattering instruments. The sample environment is temperature controlled and features external electrodes, allowing standard quartz cuvettes to be used and conducting samples or samples containing ions to be investigated without the risk of electrochemical reactions occurring at the electrodes. For standard 12.5 mm quartz cuvettes, the maximum applied field is 8 kV/cm, and the applied field may be static or alternating (up to 10 kHz for 8 kV/cm and up to 60 kHz for 4 kV/cm). The electric fields within the sample are calculated and simulated under a number of different conditions, and the capabilities of the setup are demonstrated using a variety of liquid crystalline samples. Measurements were performed as a function of temperature and time spent in the electric field. Finally, the advantages, drawbacks, and potential optimization of the sample environment are discussed with reference to applications in the fields of complex soft matter, biology, and electrorheology.

Probing permanent dipoles in CdSe nanoplatelets with transient electric birefringence

Zinc-blende CdSe semiconducting nanoplatelets (NPL) show outstanding quantum confinement properties thanks to their small, atomically-controlled, thickness. For example, they display extremely sharp absorption peaks and ultra-fast recombination rates that make them very interesting objects for optoelectronic applications. However, the presence of a ground-state electric dipole for these nanoparticles has not yet been investigated. We therefore used transient electric birefringence (TEB) to probe the electric dipole of 5-monolayer thick zinc blende CdSe NPL with a parallelepipedic shape. We studied a dilute dispersion of isolated NPL coated with branched ligands and we measured, as a function of time, the birefringence induced by DC and AC field pulses. The electro-optic behavior proves the presence of a large dipolar moment (>245 D) oriented along the length of the platelets. We then induced the slow face-to-face stacking of the NPL by adding oleic acid. In these stacks, the in-plane dipole components of consecutive NPL cancel whereas their normal components add. Moreover, interestingly, the excess polarizability tensor of the NPL stacks gives rise to an electro-optic contribution opposite to that of the electric dipole. By monitoring the TEB signal of the slowly-growing stacks over up to a year, we extracted the evolution of their average length with time and we showed that their electro-optic response can be explained by the presence of a 80 D dipolar component parallel to their normal. In spite of the 4[combining macron]3m space group of bulk zinc blende CdSe, these NPL thus bear an important ground-state dipole whose magnitude per unit volume is twice that found for wurtzite CdSe nanorods. We discuss the possible origin of this electric dipole, its consequences for the optical properties of these nanoparticles, and how it could explain their strong stacking propensity that severely hampers their colloidal stability.

Liquid-Crystalline Suspensions of Photosensitive Paramagnetic CeF3 Nanodiscs

The design of high-performance energy-converting materials is an essential step for the development of sensors, but the production of the bulk materials currently used remains costly and difficult. Therefore, a different approach based on the self-assembly of nanoparticles has been explored. We report on the preparation by solvothermal synthesis of highly crystalline CeF₃ nanodiscs. Their surface modification by bisphosphonate ligands led to stable, highly concentrated, colloidal suspensions in water. Despite the low aspect ratio of the nanodiscs (∼6), a liquid-crystalline nematic phase spontaneously appeared in these colloidal suspensions. Thanks to the paramagnetic character of the nanodiscs, the nematic phase was easily aligned by a weak (0.5 T) magnetic field, which provides a simple and convenient way of orienting all of the nanodiscs in suspension in the same direction. Moreover, the more dilute, isotropic, suspensions displayed strong (electric and magnetic) field-induced orientation of the nanodiscs (Kerr and Cotton-Mouton effects), with fast enough response times to make them suitable for use in electro-optic devices. Furthermore, an emission study showed a direct relation between the luminescence intensity and magnetic-field-induced orientation of the colloids. Finally, with their fast radiative recombination decay rates, the nanodiscs show luminescence properties that compare quite favorably with those of bulk CeF₃. Therefore, these CeF₃ nanodiscs are very promising building blocks for the development and processing of photosensitive materials for sensor applications.

Electric field induced gelation in aqueous nanoclay suspensions

Aqueous colloidal LAPONITE® clay suspensions transform spontaneously to a soft solid-like arrested state as its aging or waiting time increases. This article reports the rapid transformation of aqueous LAPONITE® suspensions into soft solids due to the application of a DC electric field. A substantial increase in the speed of solidification at higher electric field strengths is also observed. The electric field is applied across two parallel brass plates immersed in the LAPONITE® suspension. The subsequent solidification that takes place on the surface of the positive electrode is attributed to the dominant negative surface charges on the LAPONITE® particles and the associated electrokinetic phenomena. With increasing electric field strength, a dramatic increase is recorded in the elastic moduli of the samples. These electric field induced LAPONITE® soft solids demonstrate all the typical rheological characteristics of soft glassy materials. They also exhibit a two-step shear melting process similar to that observed in attractive soft glasses. The microstructures of the samples, studied using cryo-scanning electron microscopy (SEM), are seen to consist of percolated network gel-like structures, with the connectivity of the gel network increasing with increasing electric field strengths. In comparison with salt induced gels, the electric field induced gels studied here are mechanically stronger and more stable over longer periods of time.

Isotropic, nematic, and lamellar phases in colloidal suspensions of nanosheets

The phase diagram of colloidal suspensions of electrically charged nanosheets, such as clays, despite their many industrial uses, is not yet understood either experimentally or theoretically. When the nanosheet diameter is very large (∼100 nm to 1 µm), it is quite challenging to distinguish the lamellar liquid-crystalline phase from a nematic phase with strong stacking local order, often called "columnar" nematic. We show here that newly upgraded small-angle X-ray scattering beamlines at synchrotron radiation facilities provide high-resolution measurements which allow us to identify both phases unambiguously, provided that single domains can be obtained. We investigated dilute aqueous suspensions of synthetic Sb₃P₂O₁₄^3- nanosheets that self-organize into two distinct liquid-crystalline phases, sometimes coexisting in the same sample. Close examination of their X-ray reflection profiles in the directions perpendicular to the director demonstrates that these two mesophases are a columnar nematic and a lamellar phase. In the latter, the domain size reaches up to ∼20 µm, which means that each layer is made of >600 nanosheets. Because the lamellar phase was only rarely predicted in suspensions of charged disks, our results show that these systems should be revisited by theory or simulations. The unexpected stability of the lamellar phase also suggests that the rims and faces of Sb₃P₂O₁₄^3- nanosheets may have different properties, giving them a patchy particle character.

Graphene and Other 2D Colloids: Liquid Crystals and Macroscopic Fibers

Two-dimensional colloidal nanomaterials are running into renaissance after the enlightening researches of graphene. Macroscopic one-dimensional fiber is an optimal ordered structural form to express the in-plane merits of 2D nanomaterials, and the formation of liquid crystals (LCs) allows the creation of continuous fibers. In the correlated system from LCs to fibers, understanding their macroscopic organizing behavior and transforming them into new solid fibers is greatly significant for applications. Herein, we retrospect the history of 2D colloids and discuss about the concept of 2D nanomaterial fibers in the context of LCs, elaborating the motivation, principle and possible strategies of fabrication. Then we highlight the creation, development and typical applications of graphene fibers. Additionally, the latest advances of other 2D nanomaterial fibers are also summarized. Finally, conclusions, challenges and perspectives are provided to show great expectations of better and more fibrous materials of 2D nanomaterials. This review gives a comprehensive retrospect of the past century-long effort about the whole development of 2D colloids, and plots a clear roadmap - "lamellar solid - LCs - macroscopic fibers - flexible devices", which will certainly open a new era of structural-multifunctional application for the conventional 2D colloids.

Electric field induced birefringence in non-aqueous dispersions of mineral nanorods

Lanthanum phosphate (LaPO4) nanorods dispersed in the non-aqueous solvent of ethylene glycol form a system exhibiting large intrinsic birefringence, high colloidal stability and the ability to self-organize into liquid crystalline phases. In order to probe the electro-optical response of these rod dispersions we study here the electric-field-induced birefringence, also called Kerr effect, for a concentrated isotropic liquid state with an in-plane a.c. sinusoidal electric field, in conditions of directly applied (electrodes in contact with the sample) or externally applied (electrodes outside the sample cell) fields. Performing an analysis of the electric polarizability of our rod-like particles in the framework of Maxwell-Wagner-O'Konski theory, we account quantitatively for the coupling between the induced steady-state birefringence and the electric field as a function of the voltage frequency for both sample geometries. The switching time of this non-aqueous transparent system has been measured, and combined with its high Kerr coefficients and its features of optically isotropic "off-state" and athermal phase behavior, this represents a promising proof-of-concept for the integration of anisotropic nanoparticle suspensions into a new generation of electro-optical devices.

Strain-controlled fluorescence polarization in a CdSe nanoplatelet–block copolymer composite

Composite materials obtained by doping a SBS thermoplastic elastomer matrix with CdSe nanoplatelets show reversible platelets alignment upon stretching.

The formation of a structural framework in gelled Wyoming bentonite: direct observation in aqueous solutions

HYPOTHESIS

Particle space arrangement is a very important factor that determines the physico-mechanical properties of soil. Formations of three-dimensional (3D) structured networks within gelled or flocculated suspension may prevent clay particles and aggregates from settling under gravity force and by encapsulate water within such a network, lead to poor sludge dewatering. To better understand this phenomenon, a microstructural investigation of a smectite clay (SWy2) suspension was conducted.

EXPERIMENTS

SWy-2 was diluted in water and a moderately salty aqueous solution and was studied with the aid of a synchrotron-powered transmission X-ray microscope (TXM) and cryogenic transmission electron microscope (Cryo-TEM). Observations of mutual particle arrangement in 3D spaces were conducted within a natural water environment after vitrification without drying.

FINDINGS

A new type of micro-architecture in particle space arrangement was observed. Smectite flakes were mostly in edge-to-edge (EE) contact and formed a 3D network, confirming a "net of flakes" structural model. Clay particles form a complex and multi-hierarchic flocculated structure with characteristic cellular chained networking. Chained aggregates build cellular elements, encapsulating water inside closed voids. Increasing ionic strength results in the development of multi-hierarchic voids categories, with most water retained within nano-pores.