Langmuir 2008, 24, 14195-14202
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Transmission Electron Microscope-Induced Structural Evolution in Amorphous Fe, Co, and Ni Oxide Nanoparticles Andrew H. Latham and Mary Elizabeth Williams* Department of Chemistry, The PennsylVania State UniVersity, 104 Chemistry Building, UniVersity Park, PennsylVania 16802 ReceiVed NoVember 13, 2007. ReVised Manuscript ReceiVed May 8, 2008 The high-energy electron beams in transmission electron microscopes (TEM) are known to cause structural changes and damage in some materials. In this paper, we describe unique and reproducible TEM-induced changes to the morphology of amorphous metal oxide (Fe, Co, and Ni) nanoparticles. The studied particles were synthesized via literature methods and fully characterized by X-ray powder diffraction and time-resolved, low-dose TEM. As a result of electron beam irradiation, we observe these particles to transform from initially solid spheres to core/void/shell structures and eventually to hollow nanoparticles. The rate of these transformations depends on the size and composition of the particles but is not unique to the Fe oxide we previously reported. These data suggest that structural analysis of nanoparticles by TEM must consider the impact of the high-energy electron beam and use low-dose imaging.
Introduction Nanomaterials comprised of a wide range of metallic,1 magnetic,2 polymer,3 and semiconductor materials4 in many shapes and sizes can now be readily prepared by solution-based synthetic methods. The surface chemistries of these materials are modified postsynthesis5 making them useful for biomedical imaging and drug delivery,6 for surface enhanced Raman spectroscopy (SERS),7 and in ultrasensitive DNA detection devices.8 This library of nanoscale particles has further grown to encompass heterocomposites,9 core/shell structures,10 and hollow particles.11,12 Hollow nanoparticles are particularly interesting for applications in drug delivery,11a plasmonics,11b * Corresponding author. Phone: (814) 865-8859. Fax: (814) 863-8403. E-mail:
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and catalysis.11c It is possible to prepare hollow nanoparticles using a variety of methods, most commonly with template-based approaches that typically involve coating the surface of a particle with a layer(s) of a second material followed by selective removal of the template. These templates can take the form of liquid droplets,11d polymer micelles,11e surfactant vesicles,11f or other nanoparticles. For example, Au nanoshells have been synthesized by coating silica spheres with a thin layer of Au and have been shown to be useful for imaging and cancer therapy.11g Conversely, polymer nanocapsules have been prepared by first coating Au particles with polymer and then selective removal of the Au core with cyanide.11h,i An alternative method for forming hollow structures uses galvanic replacement reactions on the surfaces of particles. Recent reports using this method have produced hollow Au octahedra11j and nanoboxes.11k Many additional methods and examples exist for the preparation of hollow nanomaterials.12 Electron microscopy is arguably the most common method for the characterization of nanomaterials. For examination of nanostructures with dimensions smaller than 30 nm, transmission (10) (a) Lyon, J. L.; Fleming, D. A.; Stone, M. B.; Schiffer, P.; Williams, M. E. Nano Lett. 2004, 4, 719–723. (b) Lai, J.; Shafi, K. V. P. M.; Ulman, A.; Loos, K.; Popovitz-Biro, R.; Lee, Y.; Vogt, T.; Estourne`s, C. J. Am. Chem. Soc. 2005, 127, 5730–5731. (c) Teng, X.; Black, D.; Watkins, N. J.; Gao, Y.; Yang, H. Nano Lett. 2003, 3, 261–264. (d) Wang, L.; Luo, J.; Fan, Q.; Suzuki, M.; Suzuki, I. S.; Engelhard, M. H.; Lin, Y.; Kim, N.; Wang, J. Q.; Zhong, C.-J. J. Phys. Chem. B 2005, 109, 21593–21601. (e) Kobayashi, Y.; Horie, M.; Konno, M.; Rodrı´guezGonza´lez, B.; Liz-Marza´n, L. M. J. Phys. Chem. B 2003, 107, 7420–7425. (f) Xu, Z.; Hou, Y.; Sun, S. J. Am. Chem. Soc. 2007, 129, 8699–8700. (11) (a) Bergbreiter, D. E. Angew. Chem., Int. Ed. 1999, 38, 2870–2872. (b) Oldenberg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243–247. (c) Kim, S.-W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642–7643. (d) Tartaj, P.; Gonza´lez-Carren˜o, T.; Serna, C. J. AdV. Mater. 2001, 13, 1620–1624. (e) Liu, T.; Xie, Y.; Chu, B. Langmuir 2000, 16, 9015–9022. (f) Schmidt, H. T.; Ostafin, A. E. AdV. Mater. 2002, 14, 532–535. (g) Pham, T.; Jackson, J. B.; Halas, N. J.; Lee, T. R. Langmuir 2002, 18, 4915–4920. (h) Marinakos, S. M.; Anderson, M. F.; Ryan, J. A.; Martin, L. D.; Feldheim, D. L. J. Phys. Chem. B 2001, 105, 8872–8876. (i) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C.; House, A. B.; Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518–8522. (j) Yin, Y.; Erdonmez, C.; Aloni, S.; Alivisatos, A. P. J. Am. Chem. Soc. 2006, 128, 12671–12673. (k) Sun, Y.; Mayers, B. T.; Xia, Y. Nano Lett. 2002, 2, 481–485. (12) (a) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711–714. (b) Kim, D.; Park, J.; An, K.; Yang, N.-K.; Park, J.-G.; Hyeon, T. J. Am. Chem. Soc. 2007, 129, 5812–5813. (c) Liu, Q.; Liu, H.; Han, M.; Zhu, J.; Liang, Y.; Xu, Z.; Song, Y. AdV. Mater. 2005, 17, 1995–1999. (d) Vasquez, Y.; Sra, A. K.; Schaak, R. E. J. Am. Chem. Soc. 2005, 127, 12504–12505. (e) Lv, J.-Q.; Feng, Y.-L.; Zhang, S.-X.; Guo, J.-Z. Mater. Lett. 2005, 59, 3109–3111.
10.1021/la7035423 CCC: $40.75 2008 American Chemical Society Published on Web 06/10/2008
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electron microscopy (TEM) is typically used.13 Using this method, high-resolution imaging and elemental and crystal structure analysis can be simultaneously performed on single nanoparticles while irradiating with high-energy electrons. To confirm the preparation of a hollow or core/shell structure, it is typical to report observed variations in contrast between the inner core and outer shell. Scanning transmission electron microscopes equipped with electron energy loss spectrometers (STEM-EELS) can further confirm particle structures. However, these specialized methods are less common in the literature than standard electron microscopy due to their high cost and lack of available facilities. Although TEM is used to observe and confirm particle structure,13 it is also known to impact their structure and chemistry.14 One of the first reports of electron beam induced effects involved Au nanoparticles.14a Small Au clusters (
