MnO Octahedral Nanocrystals and MnO@C Core- Shell Composites: Synthesis,Characterization, and...

MnO Octahedral Nanocrystals and MnO@C Core-Shell Composites: Synthesis,Characterization, and

Electrocatalytic Properties

Sangaraju Shanmugam and Aharon Gedanken*

Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan UniVersity Center forAdVanced Materials and Nanotechnology, Bar-

Ilan UniVersity, Ramat-Gan, 52900, Israel

Advisor ：王聖璋 副教授 Advisee ：許祐元

J. Phys. Chem. B 2006, 110, 24486-24491

Outline

Introduction Experimental Results and Discussion Conclusion

Introduction

Nanocrystals with unique size-dependent electrical, optical, magnetic, and chemical properties are of interest to both fundamental science and technological applications. For future applications, tunable synthesis of nanocrystals with uniform shapes and sizes is of key importance.

For manganese oxides, a wealth of chemical and physical methods have been developed for the synthesis of MnO nanostructures with well-controlled shapes, including spheres, quasi-cubes, octahedra,crosslike particles, wires, rods, dumbbells, and hexapods.

We present a novel method for preparing octahedral MnO and MnO@C core-shell nanoparticles by the direct pyrolysis of a composite gel of potassium permanganate (KMnO4) and cetyltrimethylammomium bromide (C16H33N(CH3)3- Br, CTAB) in a specially made Let-lock union. The shapecontrolled synthesis was carried out by varying the reaction temperature and the duration.

ExperimentalKMnO40.1 M

CTAB0.1 M

cetyltrimethylammonium permanganate

(CTAP)

stirring

A purple gel was formed andwas aged in air overnight

A purple solild(0.3 g)

then filtered and washed with water several times

Furnace

was heated at 700°C for 3 h

XRD HRTEMRaman

spectrometer

Result and discussion

Figure 1. XRD pattern of the MnO product synthesized at 700 °C for3 h.

Figure 2. (a) TEM image shows anisotropic MnO nanocrystals, arrowsindicate the carbon shell sorrounding the MnO core, (b) selected areadiffraction pattern of sample, indexed to cubic rock salt structure, and(c) an individual MnO octahedron and carbon replica alike are shownwith arrows.

Figure 3. (a) TEM image of an individual octahedral MnO crystal,(b) tilted nanocrystal, edge outlines are depicted in the insets, (c)MnO@C core-shell nanoparticles, arrow shows the thickness of thecarbon shell, and (d) HRTEM image of an edge of an MnO nanocrystalshows resolved lattice fringes of the (200) plane of MnO.

Figure 4. Raman spectrum of product obtained at 700 °C, showingthe presence of disorder graphitic carbon.

I(D)/I(G)=0.79

D (Disorder) band G (Graphitic) band

Figure 5. TEM images of product synthesized at different temperatures(a) 600 °C for 6 h, (b) 700 °C for 12 h, and (c) 800 °C for 3 h. Arrowsshow the carbon shell on the MnO core in (a) and (b)

Figure 6. XRD patterns of products obtained at (a) 600 °C, 6 h, (b)700 °C, 12 h, and (c) 800 °C, 3 h.

SCHEME 1: Schematic Representation of theFormation of MnO Nanocrystals and MnO@C Core-Shell Particles

Conclusion In summary, octahedral MnO nanocrystals and core-shell nanoparticles were

synthesized by a simple and facile single step. The formation of octahedral MnO nanocrystals is assisted by the presence of cetyltrimethylammonium cation.

The product mainly consists of truncated cubes, cubes, hexagons, spheres, and tetrahedra. The formation of octahedral MnO is accompanied by imprinted carbon replicas.

When the MnO crystal size is small, a shell of carbon is present, giving rise to coreshell nanocrystals. As the crystal size of MnO increases, it separated from the surrounding shell, giving rise to the imprinting carbon hollow cubelike structures.

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