Synthesis and Characterization of ZnO Nanoparticles Within Diblock Copolymers

Robert F. Mulligan^*, Agis A. Iliadis⁺, Peter Kofinas^*

* Depatment of Materials and Nuclear Engineering ,

⁺Department of Electrical and Computer Engineering

University of Maryland, College Park, MD 20742

Introduction

Nanoscale electronic, optical, and sensing devices have been recognized as a major technological goal of the 21^st century. Ultimately, the properties of such devices will be controlled by novel engineered materials. Composite polymer and metal, metal oxide, or metal sulfide materials are ideal candidates for research and development in these nanotechnology applications.

This research reports on the synthesis of ZnO nanoclusters contained within the microphase separated domains of (Norbornene)-(Norbornene-dicarboxylic acid) diblock copolymers. Microphase separation in block copolymers is a well documented phenomenon (1,2), giving rise to a variety of morphologies. Polymer-ZnO nanocomposites are of great interest because ZnO is a piezoelectric material used in many acoustoelectric applications (3). The synthesis scheme is similar to previously reported reaction schemes for other metal oxides (4,5,6); however, the inorganic wet chemistry aspects have been optimized for conversion methods with the ZnO system.

Experimental

Polymer Synthesis Diblock copolymers of norbornene (NOR) and norbornene-dicarboxylic acid (NORCOOH) were synthesized in tetrahydrofuran (THF) via Ring Opening Metathesis Polymerization (ROMP) using Grubbs' catalyst, a Ruthenium based organometallic catalyst. Copolymers with block ratios of [NOR]_m[NORCOOH]_n m/n = 400/50, 400/150, and 200/75 repeat units were produced.

Zn Doping and Film Formation Diblock copolymer solutions of 0.6 wt% in THF were doped with a stoichiometric amounts of Zn⁺² to –COOH groups in the form of .5M ZnCl₂ in THF, and stirred overnight to allow association of the cations with the highly electronegative carboxylic acid groups. Films were static cast slowly by evaporation of the solvent over several days onto a Teflon coated substrate. Following removal of residual solvent under vacuum, the freestanding films were treated with a weak base and deionized water to convert the ZnCl₂ to ZnO.

ZnO Nanoparticle Characterization Gel Permeation Chromatography (GPC) was used to determine the molecular weight distribution of the synthesized diblock copolymers. X-Ray Photoelectron Spectroscopy (XPS) was employed to identify the inorganic species contained within the nanoclusters as ZnO by comparing experimental binding energies to those reported in the literature for ZnO.

Results and Discussion

Polymer synthesis was carried out with great reproducibility. Choice of solvent, concentration, and reaction time were parameters that were optimized to optimize the diblock copolymer molecular weight and molecular weight distribution. GPC demonstrated that a significantly greater amount of time was needed to polymerize the second block of the copolymer in order to obtain an uni-modal distribution due to the bulkiness and steric hindrance exhibited by the second block.

A number of different treatment techniques were used to convert the ZnCl₂ to ZnO contained within the nanoclusters, all of which were evaluated using XPS on the doped polymer films. When the doped films were treated with a strong base such as NaOH or KOH, XPS results demonstrated that the binding interactions of the Zn and carboxylic groups were not strong enough to prevent replacement by the more electropositive ion; however, when a weak base was used, such as NH₄OH, the carboxylic groups favored association with Zn, and the metal ions remained bound during treatment. Treatment with H₂O₂, and NH₄OH followed by H₂O₂ proved to be unsuccessful methods of oxidation of the Zn metal.

The Zn doped films treated with NH₄OH had excellent mechanical properties. They were very flexible, did not break when folded, and did not buckle during treatment with the weak base. When treated with a strong base, the films had undesirable mechanical properties, being very brittle and curling up onto themselves.

Binding energies determined by XPS experiments indicate that conversion of ZnCl₂ to ZnO was successful. The 2p³ electron binding energy was found to shift to a higher energy after treatment with NH₄OH. Although this shift is slightly higher than what is reported in literature, it may be due to the quantum confinement of the nanoparticles or perhaps a crystal structure other than wurtzite for ZnO. This result is currently under investigation.

The doped films underwent a color change from green/black to orange/brown following treatment with NH₄OH. In the bulk state, ZnO is a white solid. These color changes associated with the doped films are indicative of novel optical properties of nanoparticle ZnCl₂ and ZnO confined within the microphase separated diblock copolymer domains.

Conclusions

The synthesis of ZnO nanoparticles within the microphase-separated domains of a diblock copolymer by wet chemical methods is presented here for the first time. This is an exciting result, and holds a very promising future in the construction of nanoscale electronic devices. Experiments determining optical and piezoelectric properties of this material are presently being conducted, as well as the incorporation of this technology into existing silicon integrated circuit technology.

References

1. Bates, Frank S. Science Feb. 1991, pp251-905.

2. Thomas, E.L.; Lescanec, R.L. Phil. Trans. R. Soc. Lond. A 1994, 348, pp149-166.

3. Duffy, M.T. in "Heteroepitaxial Semiconductors for Electronic Devices," 1977, G.W. Cullen and C.C. Wang Editors, Chapter 4, p150, Springer-Verlag, New York.

4. Clay, R.T.; Cohen R.E. Supramolecular Science 1995, 2, pp183-191.

5. Sohn B.H.; Cohen R.E. Chem. Mater. 1997, 9, pp264-269.

6. Ciebien, J.F.; Clay, R.T., Sohn, B.H.; Cohen, R.E. New J. Chem. 1998, pp685-691.

Acknowledgements

This research has been supported by the National Science Foundation ,, Grant # ECS-9980794.