3730 Chem. Mater. 2009, 21, 3730–3737 DOI:10.1021/cm901222j
Oxidation Conversion of Carbon-Encapsulated Metal Nanoparticles to Hollow Nanoparticles Jisheng Zhou,† Huaihe Song,*,† Xiaohong Chen,† Linjie Zhi,‡ Junping Huo,† and Bin Cheng† †
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029 P. R. China, and ‡Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Received May 5, 2009. Revised Manuscript Received June 26, 2009
We synthesized R-Fe2O3 hollow nanoparticles by directly oxidizing the carbon-encapsulated iron carbide (Fe3C@C) nanoparticles in air. In this paper, the conversion mechanism of Fe3C@C to hollow nanoparticles was deduced in detail by comparatively investigating the morphologies and compositions of the oxidized products at different oxidation stages using transmission electron microscope (TEM), high resolution TEM (HRTEM), energy-dispersive X-ray (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). It was found that both oxygen and carbon play important roles in the formation of hollow nanostructures, wherein oxygen is the driving force for the outward diffusion of core species and the carbon shell not only provides the diffusion vacancies but also effectively moderates the interdiffusion rates of metal core materials and oxygen. A growth model was proposed: during the oxidation process, three diffusion processes occur including the inward diffusion of oxygen along the carbon shell, outward diffusion of core materials, and inward diffusion of vacancies from carbon shell to core. The outward diffusion of core species involves two steps: the first step is the diffusion of Fe3C from core to carbon shells, which is only a physical change (single-crystal Fe3C was changed to multicrystal Fe3C); and the second one is the diffusion and chemical reactions of Fe3C in carbon shells with oxygen (the multicrystal Fe3C was oxidized to Fe3O4 and then to R-Fe2O3). The two-step diffusion is a theoretical extension to the nanoscale Kirkendall effect, which is expected to be valid in other diffusion couples and theoretical simulation. Introduction Hollow nanoparticles (HNPs), which represent a class of intriguing materials with unique structures and properties, have widespread potential applications in many areas including drug delivery, catalysis, lightweight structural materials, lithium-ion batteries, and sensors.1 The recently developed nanoscale Kirkendall effect-assisted method2 presents the potential power for template-free fabrication of hollow nanoparticles. This method was used recently by Yin and his co-workers to prepare successfully Co3S4 and CoO HNPs for the first time.3
Since then, many efforts have been focused on the synthesis of various hollow nanostructures, such as HNPs,2,4 hollow nanowires/tubes,5 nanoscale-dandelions,6 and other nanostructures7 via the nanoscale Kirkendall effect by the redox reactions between metal nanostructures and oxidants (O2, S, Se, etc.). On the basis of the nanoscale Kirkendall effect, the formation of the hollow nanostructures was generally attributed to that the outward-diffusion of cationic species is faster than the inward-diffusion of the anionic species.3 However, the factual formation process is quite complex and it is discriminating in different reaction systems. For the nanoparticles, the underlying growth
*Corresponding author. E-mail:
[email protected].
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(4) (a) Gao, H.; Qian, X.; Zai, J.; Yin, J.; Zhu, Z. Chem. Commun. 2006, 4548. (b) Henkes, A. E.; Vasquez, Y.; Schaak, R. E. J. Am. Chem. Soc. 2007, 129, 1896. (c) Chiang, R. K.; Chiang, R. T. Inorg. Chem. 2007, 46, 369. (d) Gao, J. H.; Zhang, B.; Zhang, X. X.; Xu, B. Angew. Chem., Int. Ed. 2006, 45, 1220. (e) Xie, L.; Zheng, J.; Li, Y.; Li, Y.; Li, X. Chem. Mater. 2008, 20, 282. (5) (a) Fan, H. J.; Knez, M.; Scholz, R.; Nielsch, K.; Pippel, E.; Hesse, :: D.; Zacharias, M.; Gosele, U. Nat. Mater. 2006, 5, 627. (b) Li, Q.; Penner, R. M. Nano. Lett. 2005, 5, 1720. (c) Chang, Y.; Lye, M. L.; Zeng, H. C. Langmuir 2005, 21, 3746. (d) Peng, H.; Xie, C.; Schoen, D. T.; Mcllwrath, K.; Zhang, X. F.; Cui, Y. Nano. Lett. 2007, 7, 3734. (6) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 16744. (7) (a) Yang, J.; Qi, L.; Lu, C.; Ma, J.; Cheng, H. Angew. Chem., Int. Ed. 2005, 44, 598. (b) Zhang, L. Z.; Yu, J. C.; Zheng, Z.; Leung, C. W. Chem. Commun. 2005, 2683.
Published on Web 07/16/2009
r 2009 American Chemical Society
Article
Chem. Mater., Vol. 21, No. 15, 2009
process can be classified into two categories: symmetry growth, where partial reaction leads to a metal island in the center and a gap between the remaining metal core and shell;3,8-10 and asymmetrical growth, where a single off-center void between a single off-center unreacted metal and shell can form on the uncompleted oxidized nanoparticles, just as demonstrated by Alivisatos11 and Nakamura et al.12 For the nanowires, Fan et al.5a also observed the surface diffusion during the ZnAl2O4 spinelforming reaction. Therefore, it is necessary to understand in detail the diffusion of various systems for constructing the rational growth model, which should be beneficial to control the shape and composition of the obtained hollow nanoparticles in further during the synthesis process. Although the formations of many hollow nanoparticles are attributed to the Nanoscale Kirkendall effect, there have been no enough investigations on the diffusion process including the morphology evolution of metal nanoparticles, and especially the phase transition of metal until now, partially because it is difficult to arrest the intermediates during the fast oxidation reaction for the bare metal nanoparticles. In our previous work, metal nanoparticles encapsulated with carbon shells (Fe3C@C nanoparticles) were prepared by a facile pyrolysis approach.13 The further oxidation indicates that the solid carbon-coated (metal@C) nanoparticles as the seeds can be transformed into the hollow nanostructures, such as the pure metal oxide hollow spheres (Fe2O3 hollow nanoparticles), and various hybrid hollow spheres containing carbon (Fe2O3@C hollow nanoparticles) by the direct oxidation in air.13d Herein, a further study on the formation mechanism of metal oxide hollow nanoparticles was developed. Interestingly, because of the encapsulation of carbon shells, the interdiffusion rates of metal core materials and oxygen were coordinated in the metal@C nanoparticles system. Thus, the morphologies and phases at the different growth stages were obtained more easily. Therefore, it can also provide us a great opportunity to find the abundant information and direct evidence to elucidate the formation mechanism and even the essence of diffusion based on the nanoscale Kirkendall effect by investigating the intermediates in detail. Experimental Procedures Synthesis of Carbon-Encapsulated Metal Nanoparticles. Carbon-encapsulated iron carbide (Fe3C@C) nanoparticles were synthesized by cocarbonization of an aromatic heavy oil (50 g) (8) Cabot, A.; Puntes, V. F.; Shevchenko, E.; Yin, Y.; Balcells, L.; Marcus, M. A.; Hughes, S. M.; Alivisatos, A. P. J. Am. Chem. Soc. 2007, 129, 10358. (9) Peng, S.; Sun, S. Angew. Chem., Int. Ed. 2007, 46, 4155. (10) Yin, Y.; Erdonmez, C. K.; Cabot, A.; Hughes, S.; Alivisatos, A. P. Adv. Funct. Mater. 2006, 16, 1389. (11) Cabot, A.; Smith, R. K.; Yin, Y.; Zheng, H.; Reinhard, B. M.; Liu, H.; Alivisatos, A. P. ACS Nano 2008, 2, 1452. (12) Nakamura, R.; Lee, J. G.; Mori, H.; Nakajima, H. Philos. Mag. 2008, 88, 257. (13) (a) Huo, J.; Song, H.; Chen, X. Carbon 2004, 42, 3177. (b) Song, H.; Chen, X. Chem. Phys. Lett. 2003, 374, 400. (c) Zhao, M.; Song, H.; Chen, X.; Lian, W. Acta Mater. 2007, 55, 6144. (d) Zhou, J.; Song, H.; Chen, X.; Zhi, L.; Yang, S.; Huo, J.; Yang, W. Chem. Mater. 2009, 21, 2935.
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and ferrocene (50 g) (analytical pure grade) at 450 °C under autogenous pressure and the subsequent solvent extraction. The details can be seen in refs 13a and 13b. Carbon-encapsulated Cu nanoparticles were synthesized by heating the mixture of phenolic resin, copper nitrate (Cu (NO3)3 3 3H2O, analytical pure grade), and cross-linker hexamethylenetetramine in the copper/ carbon atomic ratio of 1:16 at 700 °C for 6 h in a nitrogen gas atmosphere. The details can be seen in ref 13c. Oxidation of Carbon-Encapsulated Iron Carbide Nanoparticles. To make hollow Fe2O3 nanoparticles, the as-prepared Fe3C@C nanoparticles were heated in air by 5 °C /min from room temperature to 280 °C and maintained at 280 °C for 24 h. Then, the sample was cooled down to room temperature naturally. During the controlled oxidation process, the black sample turns to red, fine, and loose powder that is the final product. To investigate the evolution of R-Fe2O3 hollow nanoparticles, the as-prepared Fe3C@C nanoparticles were heated in air from room temperature to 200-280 °C, respectively, by 5 °C/min and maintained for various hours at the final temperatures. The samples heated to 200, 230, and 280 °C without soaking were denoted as Sample-200, Sample-230, and Sample-280, respectively, and the product obtained at 280 °C for 5 h was denoted as Sample-280-5. Characterization. Nanocrystal size, morphology, and structure were probed by TEM on a Hitachi H-800 transmission electron microscope operating at 200 kV. The HRTEM measurement was carried out with a JEOL JEM-3010 F microscope operating at 300 kV. EDX spectra were collected from an attached Oxford Link ISIS energy-dispersive spectrometer fixed on the JEM-3010 electron microscope operated at 200 kV. The samples for TEM and HRTEM measurements were prepared by dispersing the products in ethanol with an ultrasonic bath for 15 min and then a drop of the suspension was placed onto a carbon-coated copper grid at room temperature. XRD measurements were carried out with a Rigaku D/max2500B2þ/PCX system using Cu KR radiation (λ = 1.5406 A˚) over the range of 5-90° (2θ) at room temperature. XPS spectra were recorded using monochromatic AlK(1486.6 eV) X-ray sources with 30 eV pass energy in 0.5 eV step over an area of 650 μm 650 μm to the sample. Atomic concentrations were calculated using peak areas of elemental lines after Shirley background subtraction and taking account the sensitivity factors, the asymmetry parameters as well as the measured analyzer transmission function. XPS depth profiles were acquired by Ar ion beam sputtering (2 keV, current density 75 μA/cm2) followed by XPS acquisition of elemental lines. During sputtering the ion beam was rastered over an area of 2 mm 2 mm to the sample. Before XPS measurement, the sample is degassed under a high-vacuum condition (
