Synthesis and Exfoliation of Discotic Zirconium Phosphates to Obtain Colloidal Liquid Crystals

Summary

A two dimensional model material of discotic zirconium phosphate was developed. The inorganic crystal with lamellar structure was synthesized by hydrothermal, reflux, and microwave-assisted methods. On exfoliation with organic molecules, layered crystals can be converted to monolayers, and nematic liquid crystal phase was formed at sufficient concentration of monolayers.

Abstract

Due to their abundance in natural clay and potential applications in advanced materials, discotic nanoparticles are of interest to scientists and engineers. Growth of such anisotropic nanocrystals through a simple chemical method is a challenging task. In this study, we fabricate discotic nanodisks of zirconium phosphate [Zr(HPO₄)₂·H₂O] as a model material using hydrothermal, reflux and microwave-assisted methods. Growth of crystals is controlled by duration time, temperature, and concentration of reacting species. The novelty of the adopted methods is that discotic crystals of size ranging from hundred nanometers to few micrometers can be obtained while keeping the polydispersity well within control. The layered discotic crystals are converted to monolayers by exfoliation with tetra-(n)-butyl ammonium hydroxide [(C₄H₉)₄NOH, TBAOH]. Exfoliated disks show isotropic and nematic liquid crystal phases. Size and polydispersity of disk suspensions is highly important in deciding their phase behavior.

Introduction

Discotic colloids are naturally abundant in the form of clay, asphaltene, red blood cells, and nacre. A range of applications in many engineered systems, including polymer nanocomposites¹, biomimetic materials, functional membranes², discotic liquid crystal studies³ and Pickering emulsion stabilizers⁴ are developed based on discotic colloidal nanodisks. Nanodisks with uniformity and low polydispersity is important for studying phases and transformations of liquid crystals. Zirconium phosphate (ZrP) is a synthetic nanodisks with well-ordered layered structure and controllable aspect ratio (thickness over diameter). Therefore,....

Protocol

1. Synthesis of α-ZrP Using Hydrothermal Method

1. Dissolve 6 g of zirconyl chloride octahydrate (ZrOCl₂·8H₂O) in 3.75 ml deionized (DI) water in a 150 ml round bottom flask.
2. Add 48 ml of 15 M phosphoric acid (H₃PO₄) dropwise to the ZrOCl₂ solution prepared in step 1.1 followed by adding 8.25 ml  deionized (DI) water under vigorous stirring.
3. Pour resulting gel-like mixture into Teflon-lined pressure vessel of 80 ml volume. Place the vessel into hydrothermal autoclave composed of stainless steel shell and lid, pressure plate and then tighten well.
....

Results

Figure 1a-c show SEM images of α-ZrP nanodisks obtained from hydrothermal, reflux, and microwave-assisted methods, respectively. It was observed that α-ZrP nanodisks show hexagonal in shape and different thickness depending on synthesis conditions and prepared methods. A previously reported study from our group⁶ suggests that for the crystal growth time 48 hours or above, the edge of the disks become sharper. Usually, the reflux method yields nanodisks smaller .......

Discussion

The reflux method is a good option for making a smaller size of α-ZrP with a uniform diameter and thickness. Similar to the hydrothermal method, the reflux method is limited by the preparation time. In general, it takes longer time for the crystals to grow.

The longer reaction time required for reflux method may result in nanodisks with a larger size. The average size of exfoliated nanodisks is measured by dynamic light scattering (DLS). In this study, the size of exfoliated ZrP nanodisks.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Material
Zirconyl Chloride Octahydrate	Fischer Scientific (Acros Organics)	AC20837-5000	98% +
o-Phosphoric Acid	Fischer Scientific	A242-1	≥ 85%
Tetra Butyl Ammonium Hydroxide	Acros Organics (Acros Organics)	AC176610025	40% wt. (1.5 M)
Equipment
Reaction Oven	Fischer Scientific	CL2 centrifuge	Isotemperature Oven (Temperature up to 350 °C)
Centrifuge	Thermo Scientific	Not Available	Rotation Speed: 100 - 4,000 rpm
Microwave Reactor	CEM Corporation	Discover and Explorer SP	Temp. up to 300 °C, power up to 300 W, pressure up to 30 bar