Sign In

A subscription to JoVE is required to view this content. Sign in or start your free trial.

In This Article

  • Summary
  • Abstract
  • Introduction
  • Protocol
  • Results
  • Discussion
  • Disclosures
  • Acknowledgements
  • Materials
  • References
  • Reprints and Permissions

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, the exploration of different synthesis of ZrP helps to establish fundamental understanding of discotic liquid crystal system.

The structure of ZrP was elucidated by Clearfield and Stynes in 19645. For the synthesis of layered crystals of ZrP, hydrothermal and reflux methods are commonly adopted6,7. Hydrothermal method gives a good control on size ranging from 400 to 1,500 nm and polydispersity within 25%6, while reflux method gives smaller crystals for the same duration time. Microwave heating has been proven to be a promising method for synthesis of nanomaterials8. However, there are no papers describing synthesis of ZrP based on microwave-assisted route. The effective control over size, aspect ratio, and mechanism of the crystal growth by hydrothermal method was systematically studied by our group6.

ZrP can be easily exfoliated into monolayers in aqueous suspensions, and the exfoliated ZrP have been well established as liquid crystal materials in Cheng's group3,9-13. So far, exfoliated ZrP nanodisks with various diameters, say different aspect ratios, have been studied to conclude that larger ZrP had the I (isotropic)-N (nematic) transition at lower concentration compared to smaller ZrP3. The polydispersity3, salt9 and temperature10,11 effects on the formation of nematic liquid crystal phase have been also considered. Moreover, other phases, such as sematic liquid crystal phase, have been investigated as well13,14.

In this article, we demonstrate experimental realization of such a colloidal ZrP nanodisks suspension. Layered ZrP crystals are synthesized via different methods, and then are exfoliated in aqueous media to obtain monolayer nanodisks. At the end, we show liquid crystal phase transitions exhibited by this system. A notable aspect of these disks is their highly anisotropic nature that the thickness to diameter ratio is in the range of 0.0007 to 0.05 depending on the size of disks3. The highly anisotropic monolayer nanodisks establish a model system to study phase transitions in the suspensions of nanodisks.

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.
  4. Place the hydrothermal autoclave into convection oven at 200 °C for 24 hr.
  5. After the reaction, allow the hydrothermal autoclave to cool down 8 hr to room temperature under ambient cooling.
  6. Collect α-ZrP disks in centrifuge tube after cooling using centrifuge at 2,500 x g for 10 min. Collect liquid portion in waste disposal container since the supernatant liquid contains unreacted phosphoric acid which is corrosive.
    1. Afterwards, add 40 ml of water to α-ZrP, vortex for 1 min and centrifuge at 2,500 x g for 10 min again. Repeat this step 3 times to ensure that all of the acid is washed away.
  7. Dry ZrP-water sticky mixture in oven at 65 °C for 8 hr and then grind it using a pestle and mortar.

2. Synthesis of α-ZrP by Reflux Method

  1. Mix 6 g of ZrOCl2·8H2O with 50 ml of 12 M phosphate acid in a 150 ml round bottom flask.
  2. The mixture prepared in step 2.1 is reflux in oil bath at 94 °C for 24 hr.
  3. Wash the product with DI water three times following same protocol in step 1.6, and then dried in the oven at 65 °C for 8 hr.
  4. Grind dried bulky sample into powder using a pestle and mortar, and stock for later use.

3. Synthesis of α-ZrP by Microwave-assisted Method

  1. Add 1 g of ZrOCl2·8H2O into 9 ml of 12 M phosphoric acid solution, and stir the resulting mixture well in a 20 ml scintillation vial.
  2. Pour 5 ml of the above mixture into a 10 ml glass vessel specified for microwave reactor.
  3. Set reaction temperature at 150 °C, pressure limit at 300 psi and allow the reaction to happen for 1 hr.
  4. After the reaction, let the glass vessel cool down for about 15 min and then follow the same procedure as in Steps 1.6-1.7 for acid washing and drying of α-ZrP crystals.

4. Exfoliation of Layered α-ZrP into Monolayers

  1. Disperse 1 g of α-ZrP into 10 ml of DI water in a 20 ml scintillation vial.
  2. Add 2.2 ml of TBAOH (40 wt. %) to it and vortex for at least 40 sec. Notice that molar ratio of Zr:TBAOH is kept as 1:1.
  3. Sonicate the resulting concentrated suspension for 1-2 hr and leave for 3 days to allow full intercalation of TBA+ ions and complete exfoliation of crystals. Optionally, concentrated suspension can be diluted (2 to 3 times dilution) with water to obtain better exfoliation.
  4. Centrifuge the exfoliated samples at high rotation speed (2,500 x g) for 1 hr to remove partially exfoliated crystals settled at the bottom. Collect the top part (exfoliated ZrP) in another container, and repeat the procedure until no sediment is found.

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 nanodisk...

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...

Disclosures

There is nothing to disclose.

Acknowledgements

This work is partially supported by NSF (DMR-1006870) and NASA (NASA-NNX13AQ60G). X. Z. Wang acknowledges support from the Mary Kay O'Connor Process Safety Center (MKOPSC) at Texas A&M University. We also thank Min Shuai for her guidance.

Materials

NameCompanyCatalog NumberComments
Material
Zirconyl Chloride OctahydrateFischer Scientific (Acros Organics)AC20837-500098% + 
o-Phosphoric AcidFischer ScientificA242-1>= 85 %
Tetra Butyl Ammonium HydroxideAcros Organics (Acros Organics)AC17661002540% wt. (1.5M)
NameCompanyCatalog NumberComments
Equipment
Reaction OvenFischer ScientificCL2 centrifugeIsotemperature Oven (Temperature Upto 350 C)
Centrifuge Thermo ScientificNot Available Rotation Speed : 100 - 4000 rpm
Microwave ReactorCEM CorporationDiscover and Explorer SPTemp. Upto 300oC, Power upto 300W, Pressure upto 30bar

References

  1. Usuki, A., Hasegawa, N., Kato, M. Polymer-clay nanocomposites. Adv Polym. 179, 135-195 (2005).
  2. Varoon, K., et al. Dispersible Exfoliated Zeolite Nanosheets and Their Application as a Selective Membrane. Science. 334, 72-75 (2011).
  3. Mejia, A. F., et al. Aspect ratio and polydispersity dependence of isotropic-nematic transition in discotic suspensions. Phys. Rev. E. 85, 061708 (2012).
  4. Bon, S. A. F., Colver, P. J. Pickering miniemulsion polymerization using Laponite clay as a stabilizer. Langmuir. 23, 8316-8322 (2007).
  5. Clearfield, A., Stynes, J. A. The preparation of crystalline zirconium phosphate and some observations on its ion exchange behaviour. J. Inorg. Nucl. Chem. 26, 117-129 (1964).
  6. Shuai, M., Mejia, A. F., Chang, Y. W., Cheng, Z. Hydrothermal synthesis of layered alpha-zirconium phosphate disks: control of aspect ratio and polydispersity for nano-architecture. Crystengcomm. 15, 1970-1977 (2013).
  7. Sun, L., Boo, W. J., Sue, H. -. J., Clearfield, A. Preparation of α-zirconium phosphate nanoplatelets with wide variations in aspect ratios. New J. Chem. 31, 39-43 (2007).
  8. Gawande, M. B., Shelke, S. N., Zboril, R., Varma, R. S. Microwave-sssisted chemistry: synthetic applications for rapid assembly of nanomaterials and organics. Accounts Chem. Res. 47, 1338-1348 (2014).
  9. Chang, Y. -. W., Mejia, A. F., Cheng, Z., Di, X., McKenna, G. B. Gelation via Ion Exchange in Discotic Suspensions. Phys. Rev. Lett. 108, 247802 (2012).
  10. Wang, X., et al. Thermo-sensitive discotic colloidal liquid crystals. Soft Matter. 10, 7692-7695 (2014).
  11. Li, H., Wang, X., Chen, Y., Cheng, Z. Temperature-dependent isotropic-to-nematic of charged nanoplates. Phys. Rev. E. 90, 020504 (2014).
  12. Chen, M., et al. Observation of isotropic-isotropic demixing in colloidal platelet-sphere mixtures. Soft Matter. 11 (28), 5775-5779 (2015).
  13. Sun, D., Sue, H. -. J., Cheng, Z., Martinez-Raton, Y., Velasco, E. Stable smectic phase in suspensions of polydisperse colloidal platelets with identical thickness. Phys. Rev. E. 80, 041704 (2009).
  14. Wong, M., et al. Large-scale self-assembled zirconium phosphate smectic layers via a simple spray-coating process. Nat. Commun. 5, 3589 (2014).
  15. Diaz, A., et al. Zirconium phosphate nano-platelets: a novel platform for drug delivery in cancer therapy. Chem. Commun. 48, 1754-1756 (2012).
  16. Kim, H. -. N., Keller, S. W., Mallouk, T. E., Schmitt, J., Decher, G. Characterization of zirconium phosphate/polycation thin films grown by sequential adsorption reactions. Chem. Mater. 9, 1414-1421 (1997).

Reprints and Permissions

Request permission to reuse the text or figures of this JoVE article

Request Permission

Explore More Articles

Zirconium PhosphateSynthesisExfoliationColloidal Liquid CrystalsNano PlateletsAspect RatioHydrothermal MethodReflux MethodSol gel MethodLiquid Crystal Model MaterialsPickling Emulsifiers

This article has been published

Video Coming Soon

JoVE Logo

Privacy

Terms of Use

Policies

Research

Education

ABOUT JoVE

Copyright © 2025 MyJoVE Corporation. All rights reserved