Reducing Reaction of Fe 3 O 4 in Nanoscopic Reactors of a-CNTs Fangyu Cao, ² Kaifu Zhong, ² Aimei Gao, ‡ Changle Chen, § Quanxin Li, ‡ and Qianwang Chen* ,² Hefei National Laboratory for Physical Sciences at Microscale and Department of Materials Science & Engineering, UniVersity of Science & Technology of China, Hefei 230026, P. R. China ReceiVed: September 18, 2006; In Final Form: December 14, 2006 A reduction of Fe 3 O 4 nanowires in nanoscopic reactors of amorphous C:H nanotubes (a-CNTs) was taken to understand features of the chemical reaction mechanism in nanoscale reactors. Fe 3 O 4 nanowires encapsulated in a-CNTs were reduced into iron at a relatively low temperature of 570 °C, producing iron nanoparticles encapsulated in CNTs accompanied by the crystallization of the a-CNT shell. It was found that carbon in the a-CNT shell rather than hydrogen (5.5 wt % in it) reduced Fe 3 O 4 , showing features different from those in a macroscopic system. The possible mechanisms behind this phenomenon are discussed. Introduction Nanoscopic reactions have gained wide interest because of their mechanism and high activity different from macroscopic reactions. A carbon nanotube (CNT), like a nanometer-sized test tube, could be used as a nanoscopic reactor with motion restricted to a quasi-one-dimensional environment. 1 Several successful applications have been reported, including synthesis of nanostructured materials in the cavity of CNTs under mild conditions (e.g., SiC, 2-4 -zeolite, 5 CoFe 2 O 4 , 5 GaN, 6 SiC-SiOx biaxial nanowires, 7 Si-B-C-N nanocables, 8 and heterostruc- tures of CNTs and carbide nanorods 9 ), focusing on making different kinds of nanometer-sized heterostructures by filling the cavity of CNTs with heterogenities or by decorating the outside surfaces of them. Some chemical reactions confined within these nanometer-sized channels exhibited higher activi- ties. 4,5,10 However, chemical reactions that take place in the filled CNTs or heterostructures of them with other materials have rarely been reported. As an example, Bao et.al. 11 found that R-Fe 2 O 3 nanoparticles in the cavity of CNTs were reduced by them at 600 °C, and those adhered at the outside wall were reduced at 800 °C, while R-Fe 2 O 3 powder was reduced by graphite when the temperature was higher than 1000 °C, implying that the reactions exhibited higher activities. Recently, we developed a one-pot method for the synthesis of Fe 3 O 4 /amorphous C:H nanotube (a-CNT) coaxial nanocables by pyrolysis of ferrocene in Sc-CO 2 at 400 °C. 12 In the as- prepared products, single-crystal Fe 3 O 4 nanowires with a diameter of around 40 nm are continuously covered by a-CNT shell. The a-CNT shell is rich in hydrogen (elemental analysis shows that H% in the a-CNTs’ shell is as high as 5.5 wt %.), which releases in the form of molecular H 2 from 400 °C. This cable-like nanostructure provides a natural nanometer-sized a-CNT test tube filled with Fe 3 O 4 nanowires. a-CNTs with a hollow cavity can also be obtained from the Fe 3 O 4 /a-CNT nanocables by acid disposal. In this work, we present experi- mental evidence of facile reduction of Fe 3 O 4 nanowires located in the cavities of a-CNTs to Fe nanoparticles at around 570 °C, vs reduction of powdered Fe 3 O 4 by a-CNTs mechanically mixed together at higher temperature (>700 °C), as an example of chemical reactions of obtaining CNT-confined metallic nano- particles through direct reduction of incorporated oxide nanow- ires in nanometer-sized reactors. Experimental Section Heat treatment of the Fe 3 O 4 /a-CNT nanocables was taken in quartz boat heating by a tube furnace in N 2 stream, with a flow rate of 100 mL/min. The rate of heat increase of the system was 10 °C/min, and the temperature was held for 10 min when it came to certain temperature points. Gas effluents were collected and injected into a Finnigan Trace 2000-MS Gas Chromatography/Mass Spectrometer for analysis, and the solid products were saved for further characterizations. In control experiments, a-CNTs and a mixture of 0.06 g of a-CNTs and 0.04 g of commercial Fe 3 O 4 (CP) powder were also treated with high temperature by a tube furnace in N 2 stream, respectively. Heat treatment of a-CNTs was taken in temperature-programmed desorption (TPD) with heating rate of 10 °C/min. In the heating process, gas effluents such as H 2 (m/e ) 2), H 2 O(m/e ) 18), and CO 2 (m/e ) 44) were continuously monitored by a Blazers GSD300 Omnistar quadrupole mass spectrometer. The powder X-ray diffraction (XRD) analyses were performed on a Rigaku D/MAX-γA X-ray diffractometer equipped with Cu KR radia- tion (λ ) 1.542 Å) over the 2θ range of 10-70×bc. Transmis- sion electron microscopy (TEM) analyses were performed on a Hitachi H-800 transmission electron microscope with electron diffraction, and the accelerating potential is 200 kV. High- resolution TEM (HRTEM) images were taken on a JEOL-2010 with an accelerating voltage of 200 kV. Magnetic measurements were carried out on a Riken BHV-55 vibrating sample magne- trometer (VSM). Results and Discussion Heat treatment of the Fe 3 O 4 /a-CNT nanocables at various temperatures was performed under high-purity N 2 stream. From XRD patterns of the solid products (Figure 1), it can be found that the main solid components changed from Fe 3 O 4 and a-CNTs * To whom correspondence should be addressed. E-mail: cqw@ustc.edu.cn. Fax: (+86)551-3631760. ² Hefei National Laboratory for Physical Sciences at Microscale and Department of Materials Science & Engineering, University of Science & Technology of China. ‡ Department of Chemical Physics, Lab of Biomass Clean Energy, University of Science & Technology of China. § Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois. 1724 J. Phys. Chem. B 2007, 111, 1724-1728 10.1021/jp0661037 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/01/2007