Synthesis and magnetic properties of single-crystalline Na2- x Mn8O16 nanorods
© Lan et al; licensee Springer. 2011
Received: 15 October 2010
Accepted: 11 February 2011
Published: 11 February 2011
The synthesis of single-crystalline hollandite-type manganese oxides Na2- x Mn8O16 nanorods by a simple molten salt method is reported for the first time. The nanorods were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and a superconducting quantum interference device magnetometer. The magnetic measurements indicated that the nanorods showed spin glass behavior and exchange bias effect at low temperatures. The low-temperature magnetic behaviors can be explained by the uncompensated spins on the surface of the nanorods.
One dimensional (1D) nanostructures including nanobelts, nanotubes, nanowires, and nanorods have attracted much attention due to their fascinating physical and chemical properties and their potential applications in nanodevices [1, 2]. Manganese oxides have a wide range of applications such as catalysts , ion sieves , and battery materials . Much effort has been made to prepare low dimensional manganese oxides nanomaterials with various polymorphs [6–8]. As a novel Mn3+-Mn4+ mixed valence system, hollandite-type compounds with chemical formula A x Mn8O16 (A = K, Rb, Ba, or Pb, etc. and x ≤ 2) have been enthusiastically pursued for their applications in fast ionic conductors, solid state electrolytes, oxidation catalysts, and stable host materials for radioactive ions from nuclear wastes [9–12]. The crystal structure of the hollandite-type material is very porous, including 1D 2 × 2 tunnels among rigid MnO2 framework composed of edge-shared MnO6 octahedra [4, 10, 13]. The A ions occupy in the tunnels as guest cations and they are easily replaced by other ions . Due to the special crystal structure and the mixed valence properties of Mn, these compounds show interesting magnetic and electric properties [13–16]. The formation of KxMn8O16 with hollandite-type structure is very easy, since the K+ cation is of the ideal dimension to fit in the 2 × 2 tunnels. But the Na+ cation is on the small side to stabilize the 2 × 2 tunnels, thus hollandite Na-Mn-O compound is hard to be obtained . Na2- x Mn8O16 is known to have hollandite structure with unit-cell parameters a = 9.91 Å, b = 2.86 Å, c = 9.62 Å and β = 90.93° (JCPDS No. 42-1347), and the ion tunnel of which is along b-axis. To the best of our knowledge little information about this compound has been reported. Here, we report the synthesis of Na2- x Mn8O16 nanorods by a very simple molten salt method for the first time.
Exchange bias (EB) effect is observed in the materials with good ferromagnetic (FM)/antiferromagnetic (AFM) interface, such as Ni80Fe20/Ir20Mn80 system . The EB effect originates from the interfacial interaction between FM and AFM materials . Recently, it was reported that 1D pure phase AFM nanomaterials exhibited EB effect at low temperatures, such as Co3O4 nanorods , SrMn3O6-δ nanobelts , CuO nanowires . Since there is no FM layer in those materials, the EB effect in pure 1D AFM nanomaterials is probably related to the surface layer of the nanomaterials, which is due to the changes in the atomic coordination form a layer of disordered spins (i. e. spin glass layer) . As a kind of 1D magnetic nanomaterials, the Na2- x Mn8O16 nanorods may show novel magnetic properties. Thus the magnetic properties of Na2- x Mn8O16 nanorods are explored and we find that the as-synthesized nanorods exhibit spin glass behavior and EB effect at low temperatures.
Results and discussion
The spin-glass-like behavior of the surface can also be clearly observed for the opening in the upper right side of the FC hysteresis loop, which is shown in the upper left inset of Figure 4b. This indicates that we have a loss of magnetization during one hysteresis cycle. A similar phenomenon has been observed in Co3O4 nanowires . This striking experimental feature is observed here because of the large amount of measured material and due to the absence of additional ferromagnetic materials which could mask the observation of the interfacial spins behavior . The EB effect induced by surface effects of the nanorods suggests that Na2- x Mn8O16 nanorods may find potential application in multifunctional spintronic devices .
In summary, single-crystalline Na2-xMn8O16 nanorods were synthesized by a simple molten salt method for the first time. SEM and TEM images show that the nanorods are about 200 nm in width and several micrometers in length. HRTEM and SAED indicate the single-crystalline of the nanorods. The growth direction of the nanorods is along the tunnel direction of the hollandite structure. The chemical formula of the nanorods can be written as Na1.74Mn8O16 calculated from EDS result. Magnetic measurements indicate that the nanorods show spin glass behavior and EB effect at low temperatures. The low-temperature magnetic behaviors can be explained by the uncompensated surface spins of the nanorods.
In a typical procedure, MnCl2•4H2O and NaOH (1:2 in molar) were dissolved in distilled water, respectively. Then NaOH aqueous solution was added to MnCl2 aqueous solution slowly with constant magnetic stirring. The precipitation was filtered and washed several times and then dried at 90°C for 24 h. After being dried, black powder was obtained. 0.1 g of the obtained black powder was mixed with 5 g NaNO3 and ground for 20 min in an agate mortar by hand. The mixture was then placed in a corundum crucible and annealed at 550°C for 6 h. The product was collected after naturally cooling the furnace to room temperature and then washed several times with distilled water to remove residual NaNO3. The obtained black powder was dried at 90°C for 24 h.
XRD patterns were collected using a Philips X'Pert diffractometer with Cu Kα irradiation at room temperature. For the SEM characterization, the product was pasted on a Cu sheet with conductive adhesive. A thin layer of Pt was sputtered on the sample to enhance its conductivity for the facility of SEM measurements. SEM and EDS pattern were carried out in a Hitachi-S-3400N II instrument. In further characterization, TEM images, HRTEM images, and SAED were obtained in a Philips Tecnai F20 instrument, operating at 200 kV. Magnetic properties were obtained in a superconducting quantum interference device magnetometer.
This work was supported by National Natural Science Foundation of China (10774068), Program for New Century Excellent Talents in University (07-0430) and National Basic Research Program of China (2009CB929501).
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