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Comment on ‘Low-temperature synthesis of α-Fe2O3 nanoparticles with a closed cage structure’ by X. Wang et al. [Chem. Phys. Lett. 384 (2004) 391]
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Chemical Physics Letters 451 (2008) 68–69 www.elsevier.com/locate/cplett
Comment
Comment on ‘Low-temperature synthesis of a-Fe2O3 nanoparticles with a closed cage structure’ by X. Wang et al. [Chem. Phys. Lett. 384 (2004) 391] Jane Y. Howe a,*, James Bentley a, Wei Wang b a
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA b Environmental Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Received 12 June 2007; in final form 20 November 2007
Abstract A Letter by Wang et al. reported synthesis of a-Fe2O3 nanoparticles with a closed cage structure. The authors mistook thickness fringes in the transmission electron micrographs (TEM) as evidence for ‘layers’ of ‘a closed cage structure’. We prepared a-Fe2O3 nanoparticles using procedures described by Wang and recorded a series of TEM images of the particles at 0.5° increments in specimen tilt angle. In this Letter, we first explain the origin of thickness fringes, and then present a series of images to prove that these particles do not have a closed cage structure. Published by Elsevier B.V.
In 2004, Wang et al. published a Letter reporting synthesis of a-Fe2O3 nanoparticles with a closed cage structure [1]. In that Letter, the particles were synthesized by hydrothermal hydrolysis of FeCl3 in the presence of cetyltrimethylammonium bromide (CTAB), and then characterized using transmission electron microscopy (TEM) and other techniques. Fig. 4a, c and d are three TEM micrographs in which dark and lighter shaded bands appeared at the edge of some particles. The authors attributed the origin of these bands to a ‘closed cage structure’. They claimed that ‘the closed cage structure’ with five layers can be seen in Fig. 4d. We have repeated the synthesis according to the procedure reported by Wang et al. and found that the interpretation of ‘a closed cage structure’ is incorrect. These aFe2O3 nanoparticles are not hollow inside. Instead, the dark and lighter shaded bands observed at the edge of the particles are thickness fringes, which result from a diffraction effect. A detailed explanation of thickness fringes
*
Corresponding author. Fax: +1 865 576 5413. E-mail address: [email protected] (J.Y. Howe).
0009-2614/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.cplett.2007.11.062
can be found in many classic TEM textbooks, for example, in Edington [2] and Williams and Carter [3]. Given here is a brief description: in a perfect crystal, the transmitted beam intensity oscillates with depth with a periodicity known as the extinction distance, ng [3]. ng ¼
pV c cos hB kF g
where Vc is the volume of the unit cell, hB is the Bragg angle, Fg is the structure factor for reflection g,and k is the electron wavelength. An edge of a crystal exhibits dark fringes at thicknesses of 0.5ng, 1.5ng, 2.5ng, etc. However, with an increase in thickness, the fringes are damped out, especially above 5 fringes. With typical values in the range of 10– 100 nm, the extinction distance ng is dictated by both the operative reflection and the deviation from the exact Bragg position. This suggests that the position and number of fringes will change if a crystal is tilted to a different angle, e.g. by 0.5°. In order to prove that the bands observed by Wang et al. are merely thickness fringes, we have taken TEM micrographs of a-Fe2O3 nanoparticles of similar size and shape
J.Y. Howe et al. / Chemical Physics Letters 451 (2008) 68–69
69
Fig. 1. A series of micrographs that were recorded at 0.5° tilt increments reveal the disappearance and reappearance of the thickness fringes.
using an FEI Tecnai 20 TEM at 200 kV. A series of micrographs taken at 0.5° tilting increments are presented in Fig. 1. The contrast of the particles A, B, and C, varies as a function of the tilting angle. Thickness fringes appear at certain angles but disappear after tilting 0.5°. Clearly, these fringes do not indicate that the particles have a cage-like structure. More than 20 micrographs were taken at higher magnifications from several batches of particles. They show that all the contrast is consistent with singlephase, single-crystal particles. Nor there is phase contrast evidence which might suggest a b-FeOOH core with an a-Fe2O3 shell. Likewise, the dark fringes in the micrographs of the a-Fe2O3 nanoparticles in the Letter by Wang et al. originates from a diffraction effect, not from a closed cage morphology.
Research was conducted through the High Temperature Materials Laboratory Program and the SHaRE User Facility at the Oak Ridge National Laboratory, sponsored by the Office of FreedomCAR and Vehicle Technologies, and Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with UTBattelle, LLC. References [1] X. Wang, X. Chen, X. Ma, H. Zheng, M. Ji, Z. Zheng, Chem. Phys. Lett. 384 (2004) 391. [2] J.W. Edington, in: Philips (Ed.), Practical Electron Microscopy in Materials Science, Interpretation of Transmission Electron Micrographs, Monograph 3, Eindhoven, 1975, p. 3. [3] D.B. Williams, C.B. Carter, Transmission Electron Microscopy, Plenum, New York, 1996, p. 205 and p. 369.
Chemical Physics Letters 451 (2008) 68–69 www.elsevier.com/locate/cplett
Comment
Comment on ‘Low-temperature synthesis of a-Fe2O3 nanoparticles with a closed cage structure’ by X. Wang et al. [Chem. Phys. Lett. 384 (2004) 391] Jane Y. Howe a,*, James Bentley a, Wei Wang b a
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA b Environmental Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Received 12 June 2007; in final form 20 November 2007
Abstract A Letter by Wang et al. reported synthesis of a-Fe2O3 nanoparticles with a closed cage structure. The authors mistook thickness fringes in the transmission electron micrographs (TEM) as evidence for ‘layers’ of ‘a closed cage structure’. We prepared a-Fe2O3 nanoparticles using procedures described by Wang and recorded a series of TEM images of the particles at 0.5° increments in specimen tilt angle. In this Letter, we first explain the origin of thickness fringes, and then present a series of images to prove that these particles do not have a closed cage structure. Published by Elsevier B.V.
In 2004, Wang et al. published a Letter reporting synthesis of a-Fe2O3 nanoparticles with a closed cage structure [1]. In that Letter, the particles were synthesized by hydrothermal hydrolysis of FeCl3 in the presence of cetyltrimethylammonium bromide (CTAB), and then characterized using transmission electron microscopy (TEM) and other techniques. Fig. 4a, c and d are three TEM micrographs in which dark and lighter shaded bands appeared at the edge of some particles. The authors attributed the origin of these bands to a ‘closed cage structure’. They claimed that ‘the closed cage structure’ with five layers can be seen in Fig. 4d. We have repeated the synthesis according to the procedure reported by Wang et al. and found that the interpretation of ‘a closed cage structure’ is incorrect. These aFe2O3 nanoparticles are not hollow inside. Instead, the dark and lighter shaded bands observed at the edge of the particles are thickness fringes, which result from a diffraction effect. A detailed explanation of thickness fringes
*
Corresponding author. Fax: +1 865 576 5413. E-mail address: [email protected] (J.Y. Howe).
0009-2614/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.cplett.2007.11.062
can be found in many classic TEM textbooks, for example, in Edington [2] and Williams and Carter [3]. Given here is a brief description: in a perfect crystal, the transmitted beam intensity oscillates with depth with a periodicity known as the extinction distance, ng [3]. ng ¼
pV c cos hB kF g
where Vc is the volume of the unit cell, hB is the Bragg angle, Fg is the structure factor for reflection g,and k is the electron wavelength. An edge of a crystal exhibits dark fringes at thicknesses of 0.5ng, 1.5ng, 2.5ng, etc. However, with an increase in thickness, the fringes are damped out, especially above 5 fringes. With typical values in the range of 10– 100 nm, the extinction distance ng is dictated by both the operative reflection and the deviation from the exact Bragg position. This suggests that the position and number of fringes will change if a crystal is tilted to a different angle, e.g. by 0.5°. In order to prove that the bands observed by Wang et al. are merely thickness fringes, we have taken TEM micrographs of a-Fe2O3 nanoparticles of similar size and shape
J.Y. Howe et al. / Chemical Physics Letters 451 (2008) 68–69
69
Fig. 1. A series of micrographs that were recorded at 0.5° tilt increments reveal the disappearance and reappearance of the thickness fringes.
using an FEI Tecnai 20 TEM at 200 kV. A series of micrographs taken at 0.5° tilting increments are presented in Fig. 1. The contrast of the particles A, B, and C, varies as a function of the tilting angle. Thickness fringes appear at certain angles but disappear after tilting 0.5°. Clearly, these fringes do not indicate that the particles have a cage-like structure. More than 20 micrographs were taken at higher magnifications from several batches of particles. They show that all the contrast is consistent with singlephase, single-crystal particles. Nor there is phase contrast evidence which might suggest a b-FeOOH core with an a-Fe2O3 shell. Likewise, the dark fringes in the micrographs of the a-Fe2O3 nanoparticles in the Letter by Wang et al. originates from a diffraction effect, not from a closed cage morphology.
Research was conducted through the High Temperature Materials Laboratory Program and the SHaRE User Facility at the Oak Ridge National Laboratory, sponsored by the Office of FreedomCAR and Vehicle Technologies, and Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with UTBattelle, LLC. References [1] X. Wang, X. Chen, X. Ma, H. Zheng, M. Ji, Z. Zheng, Chem. Phys. Lett. 384 (2004) 391. [2] J.W. Edington, in: Philips (Ed.), Practical Electron Microscopy in Materials Science, Interpretation of Transmission Electron Micrographs, Monograph 3, Eindhoven, 1975, p. 3. [3] D.B. Williams, C.B. Carter, Transmission Electron Microscopy, Plenum, New York, 1996, p. 205 and p. 369.