Carbon nanotubes (CNTs) have attracted enormous technological and scientific interest since 1991 [1][2][3][4][5]. To optimize the use of CNTs in various applications, it is necessary to attach functional groups or other nanostructures to their surface [3][4][5]. The combination of CNTs with other nanocrystals is expected to be useful for applications in catalysts, sensors, nanoelectronic devices, data storage/processing devices, field emission displays, and polymer or ceramic reinforcement [6][7][8][9][10][11]. In past several years, a variety of metal oxide, such as V 2 O 5 , RuO 2 , ZrO 2 , Al 2 O 3 , RuO 2 , Eu 2 O 3 , SnO 2 , etc., have been bound to the surface of CNTs by us and other research groups [12][13][14][15][16][17][18][19].
CeO 2 is a technologically important rare earth material because of its wide applications as polishing materials, fuel cells, oxygen sensors, catalysts, and UV blockers [20][21][22][23][24]. More and more attractive nanometer-scale CeO 2 with well-defined morphologies and a narrow range of size distribution has been prepared by us and other research groups [25][26][27][28][29]. The hybrid of CNTs and CeO 2 shows exceptional properties, and has the potential to be widely used in the above-mentioned fields [11]. For example, Di et al. reported that CeO 2 /CNTs could be used as a novel adsorbent for Cr(VI) from drinking water, and the results show that CeO 2 /CNTs have great potential applications in environmental protection [30]. Recently, Wei et al. reported that CNTs refluxed in a mixture of concentrated nitric and sulfuric acids at 140 8C could be coated with CeO 2 nanoparticles [31]. Zhao et al. reported that ceria nanoparticles were supported on CNTs by supercritical hydrothermal methods [32]. However, only a few studies have been reported on the fabrication of CNT/CeO 2 composites due to those complex strategies. Therefore, a facile approach to synthesize CNTs/CeO 2 core-shell nanowires is still a challenge. No reports on the synthesis of CNT/CeO 2 core-shell nanowires without any chemical modification of CNTs or any assisted surfactant have been published to date.
Previously, we synthesized CNT/CeO 2 composites on a large scale by a simple ultrasonication method under ambient air [11]. Most recently, we synthesized necklace-like CNT/CeO 2 composites [33] and core-shell CNT/Eu 2 O 3 nanostructures [18] by a solvothermal method. Here, we report the first synthesis of CNTs/ CeO 2 core-shell nanowires by a novel and facile reflux method.
2. Methods
The CNTs were prepared by the catalytic decomposition of methane and purified in a diluted HNO 3 (20%) solution to remove the metal catalysts [34], and were $40 nm in diameter and 1-10 mm in length. Analytical grade (NH 4 ) 2 Ce(NO 3 ) 6 , Ce(NO 3 ) 3 , ethylene glycol, ethanol, and dimethylformamide (DMF) were purchased from Sinopharm Chemical Regent Company and were used without further purification. Deionized water was used throughout.