Ever since the discovery of the all-carbon fullerenes and their linear cousins the carbon nanotubes, researchers have focused on unravelling their intriguing properties and finding applications. But, inorganic nanotubes could have just as many diverse properties and applications as their carbon counterparts. The synthesis of the first single-crystalline magnetite (Fe3O4) nanotubes could open up a whole new area of materials science.
Electrical engineer Chongwu Zhou of the University of Southern California and his colleagues Zuqin Liu, Daihua Zhang, Song Han, Chao Li, and Bo Lei and researchers at the University of New Orleans, Weigang Lu and Jiye Fang, started their nanotube synthesis by growing magnesium oxide wires just a few nanometres in diameter (typically around 10nm). These nanowires would act as a template on which the nanotubes could be made. They then applied a coating of magnetite using pulsed laser deposition. This produced so-called core-shell nanowires of MgO/Fe3O4. The last step involved removing the inner magnesium oxide core by wet etching it out with a corrosive solution of ammonium sulphate. This left behind the hollow magnetite nanotubes with a diameter of 30 nanometres. The crystal structure and chemical composition of magnetite nanotubes were confirmed using transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS).
Chongwu Zhou
Zhou has pointed out that inorganic materials have a wide range of useful properties that might be exploited in nanotube and nanowire form, including high temperature superconductivity for low-loss electrical power delivery, enormous magnetoresistivity for information storage, and ferroelectric and ferromagnetic properties for quantum computing and spintronics applications. He adds that the current magnetite nanotubes might also be used as nanofluidic channels for nanoscale chemical reactors. The researchers can easily change the dimensions of their nanotubes, i.e. length and inner diameter by fine-tuning the length and diameter of the magnesium oxide cores. By controlling the deposition time and rate of the shell they can also alter the thickness of the walls of their nanotubes.
The researchers believe that their synthetic strategy could readily be extended to other metal oxides, such as YBCO (the high temperature superconductor yttrium-barium-copper-oxide), PZT (the ferroelectric material lead zirconate titanate), and LCMO (the perovskite lanthanum calcium manganese oxide), and offer the opportunity for studying superconductivity, piezoelectricity, and ferromagnetism in nanoscale structures.
Schematic showing how the nanotubes are made
Inorganic nanotubes could become just as useful as their carbon cousins
Further reading
J. Am. Chem. Soc., 2005, 127, 6
http://dx.doi.org/10.1021/ja0445239
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