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Manganese(II) Oxide Nanohexapods: Insight into Controlling the Form of Nanocrystals

Teyeb Ould-Ely,^† Dario Prieto-Centurion,^† A. Kumar,^† W. Guo,^‡ William V. Knowles,^§ Subashini Asokan,^§ Michael S. Wong,^†^§ I. Rusakova, Andreas Lüttge,^† and Kenton H. Whitmire*^†

Department of Chemistry, MS 60, Center for Biology and Environmental Nanotechnology, Department of Chemical and Biomolecular Engineering, MS362, and Department of Earth Science, MS 126, Rice University, 6100 Main Street, Houston, Texas 77005-1892, and Texas Center for Superconductivity, University of Houston, Houston, Texas 77204-5931

Chem. Mater., 2006, 18 (7), pp 1821–1829

DOI: 10.1021/cm052492q

Publication Date (Web): March 7, 2006

†

Department of Chemistry, Rice University.

‡

Center for Biology and Environmental Nanotechnology, Rice University.

Department of Chemical and Biomolecular Engineering, Rice University.

University of Houston.

Department of Earth Science, Rice University.

Corresponding author. Tel.: 713-348-5650. Fax: 713-348-51. E-mail: whitmir@rice.edu.

Abstract

Cross-shaped and octahedral nanoparticles (hexapods) of MnO in size, and fragments thereof, are created in an amine/carboxylic acid mixture from manganese formate at elevated temperatures in the presence of water. The nanocrosses have dimensions on the order of 100 nm, but with exposure to trace amounts of water during the synthesis process they can be prepared up to about 300 nm in size. Electron microscopy and X-ray diffraction results show that these complex shaped nanoparticles are single crystal face-centered cubic MnO. In the absence of water, the ratio of amine to carboxylic acid determines the nanocrystal size and morphology. Conventionally shaped rhomboehdral/square nanocrystals or hexagonal particles can be prepared by simply varying the ratio of tri-n-octylamine/oleic acid with sizes on the order of 35−40 nm in the absence of added water. If the metal salt is rigorously dried before the synthesis, then “flower-shaped” morphologies on the order of 50−60 nm across are observed. Conventional square-shaped nanocrystals with clearly discernible thickness fringes that also arise under conditions producing the nanocrosses mimic the morphology of the cross-shaped and octahedral nanocrystals and provide clues to the crystal growth mechanism(s), which agree with predictions of crystal growth theory from rough, negatively curved surfaces. The synthetic methodology appears to be general and promises to provide an entryway into other nanoparticle compositions.

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