Tuning the Aspect Ratio of Ceria Nanorods and Nanodumbbells by a Face-Specific Growth and Dissolution Process Anwar Ahniyaz, †,‡ Yasuhiro Sakamoto, § and Lennart Bergstro ¨m* ,† Materials Chemistry Research Group, Department of Physical, Inorganic and Structural Chemistry, and DiVision of Structural Chemistry, Arrhenius Laboratory, Stockholm UniVersity, 10691 Stockholm, Sweden ReceiVed January 14, 2008; ReVised Manuscript ReceiVed May 4, 2008 ABSTRACT: Fine tuning of the axial ratio of ceria nanorods by a time-dependent, face-specific growth and dissolution process has been demonstrated for the first time. Nearly monodisperse ceria nanorods can be synthesized by thermal decomposition of a ceria complex in the presence of only one type of surfactant (oleic acid). Strikingly, we find that the ceria nanorods are slowly and selectively shortened with time without any change of the rod diameter. The growth and subsequent dissolution along the [100] direction has been related to the high surface energy and the strong tendency for the {100} surface to reconstruct to reduce the dipolar moment. The addition of a second surfactant, octadecylamine, to the reaction mixture promotes the growth rate in other crystallographic directions and thus yields ceria nanodumbbells and nanospheres. Inorganic nanocrystals with tailored morphologies, especially one-dimensional nanostructures, display shape-dependent electronic, optical, magnetic, and mechanical properties with potential ap- plications in, for example, catalysis, optoelectronics, and energy conversion. 1 Cerium oxide possesses an interesting combination of unique properties, e.g., a high oxygen storage capacity, high ionic conductivity, relatively high mechanical strength, and strong adsorption and photoluminescence in the UV-vis range. 2 The unique combination of properties with applications in, for example, catalysis, fuel cells, solar cells, and sensors are related to the ease of forming oxygen vacancies and changing the valency of Ce(III or IV) while preserving the fluorite structure. 2,3 Synthesis of ceria nanocrystals with well-defined crystalline faces and morphologies has been the focus of a significant research effort. 4–7 Aqueous routes are attractive but most studies using hydrothermal or precipitation methods result in intermediate products (e.g., Ce(OH) 3 or Ce(OH)CO 3 ) that yield CeO 2 nanoc- rystals only after subsequent drying or calcinations. 5b,f,h,6 Recently, it was found that well-crystalline ceria nanocrystals can be produced by hydrothermal synthesis at a temperature (400 °C) within the supercritical range. 7a,b In contrast, nonhydrolytic methods can directly produce ceria nanocrystals at ambient pressures and substantially lower temperatures. 5a,d,f We report here how ceria nanorods and nanodumbbells with a tuneable aspect ratio can be produced without byproduct by a face- specific growth and dissolution process. The CeO 2 nanorods are produced by thermal decomposition at 200 °C of a ceria-oleate complex in a high-boiling organic solvent at extended reaction times. 8 The as-prepared nanorods can be easily separated from the reaction system by centrifugation and redispersed in nonpolar solvents (e.g., toluene), where they display a long-term stability. 8 The most striking feature of the synthesis route is that the ceria nanorods are slowly and selectively shortened with time. Hence, because the diameter does not change, it is possible to simply tune the aspect ratio by the reaction time. The shape of the ceria nanocrystals can also be controlled by the addition of a second surfactant, octadecylamine, to the reaction mixture resulting in nanodumbbells and nanospheres. Figure 1 gives examples of ceria nanorods produced at different reaction times at 200 °C. The nanorods obtain a length of about 25 nm after about 2 days. A small number of nanorods grow to a maximum length of about 35 nm but it proved difficult to grow the nanorods any further (Figure 1a). Instead, prolonged reaction times result in a shortening of the nanorods that yield nanorods with an average length of 15 nm after 10 days (Figure 1b) and 8 nm long nanorods after 14 days (Figure 1c). Both the SAED patterns (Figure 1a-c) and the high resolution TEM (Figure 1d) images show that all the nanorods are highly crystalline with the fluorite structure (Fm3 j m, a ) 5.41134 Å, JCPDS card 43-1002). The nanorods have the long axis along one of the 〈100〉 directions (Figure 1d) regardless of the aspect ratio. The elongated crystal shape contrasts with the symmetrical equivalence of all three 〈100〉 directions ([100], [010], and [001]) in a cubic system. The very strong and sharp 200 and 400 reflections in the SAED patterns * Corresponding author. Phone: 46-8-162368. Fax: 46-8-152187. E-mail: lennartb@inorg.su.se. † Department of Physical, Inorganic and Structural Chemistry, Stockholm University. ‡ Current address: YKI, Institute for Surface Chemistry, Stockholm, Sweden. § Division of Structural Chemistry, Stockholm University. Figure 1. (a-c) TEM images of ceria nanorods with a diameter of 2 nm and average lengths of (a) 25 ( 5, (b) 15 ( 3, and (c) 8 ( 2 nm; the insets are SAED patterns. (d) HRTEM images of the ceria nanorods with different lengths. Scale bars for images a-c are 20 nm and the scale bar for the individual high-resolution image in d is 2 nm. CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 6 1798–1800 10.1021/cg800051s CCC: $40.75  2008 American Chemical Society Published on Web 05/20/2008