Chem. Mater. 2010, 22, 149–154 149 DOI:10.1021/cm902734e
Spontaneous Growth of ZnCO3 Nanowires on ZnO Nanostructures in Normal Ambient Environment: Unstable ZnO Nanostructures Zhengwei Pan,*,† Jing Tao,‡ Yimei Zhu,‡ Jing-Fang Huang,§ and M. Parans Paranthaman^ †
Faculty of Engineering, Department of Physics and Astronomy, University of Georgia, Athens, Georgia 30602, ‡Condensed Matter Physics & Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, §Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, and ^Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received September 2, 2009. Revised Manuscript Received November 14, 2009
ZnO nanowires, one of the most investigated nanostructures that promise numerous applications in nanophotonics, opto-electronics, and energy, are generally thought to be highly stable under ambient conditions because of their oxide nature. Here, we report that ZnO nanowires are actually extremely unstable even in normal ambient environment (70% RH, and ∼350 ppm CO2) because of atmospheric corrosion. When placed on an oxide substrate (e.g., glass slide) and exposed in air, ZnO nanowires tend to react with airborne moisture and CO2 to form amorphous ZnCO3 thin films and nanowires. The factors that specially affect the corrosion of ZnO nanowires in a laboratory environment include CO2, humidity, and substrates. Our results suggest that a CO2- and/or moisture-free environment are required in order for optimal applications of ZnO nanowires.
Introduction ZnO nanowires are probably the single most investigated nanomaterials after carbon nanotubes because of their unique optical properties and wide potential applications in lasing,1 opto-electronics,2 transistors,3 sensing,4,5 and energy.6 Because of the oxide nature of ZnO, the stability of these ZnO nanowire-based devices under ambient conditions is usually considered unproblematic. Actually, ZnO in the powder form has been found to be very unstable in environments where CO2 and water molecules coexist, because of a phenomenon called atmospheric corrosion.7-10 The atmospheric corrosion of ZnO was recognized in the study of Zn-coated hot-dip galvanized steels. The ZnO powder layer formed on the galvanized coating surface at the early weathering stage is very unstable in atmosphere because of the presence of airborne *Corresponding author. E-mail:
[email protected].
(1) Huang, M.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (2) Law, M.; Sirbuly, D.; Johnson, J.; Goldberger, J.; Saykally, R.; Yang, P. D. Science 2004, 305, 1269. (3) Arnold, M. S.; Avouris, P.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2003, 107, 659. (4) Fan, Z. Y.; Lu, J. G. IEEE Trans. Nanotechnol. 2006, 5, 834. (5) Hsueh, T. J.; Chen, Y. W.; Chang, S. J.; Wang, S. F.; Hsu, C. L.; Lin, Y. R.; Lin, T. S.; Chen, I. C. Sens. Actuators B 2007, 125, 498. (6) Law, M.; Green, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 455. (7) Falk, T.; Svensson, J. E.; Johansson, L. G. J. Electrochem. Soc. 1998, 145, 39. (8) Falk, T.; Svensson, J. E.; Johansson, L. G. J. Electrochem. Soc. 1998, 145, 2993. (9) Lindstrom, R.; Svensson, J. E.; Johansson, L. G. J. Electrochem. Soc. 2000, 147, 1751. (10) Rahrig, P. G. Powder Coating 2004, 15, 25. r 2009 American Chemical Society
moisture, CO2, and/or other gaseous pollutants, such as SO2, NO2, and chloride.7-10 In the simplest scenario of pure air, for example, ZnO powders react with water molecules in the air to form zinc hydroxide, followed by the reaction with CO2 to form a stable and passive ZnCO3 patina film that is tightly bound to the galvanized coating and protects the inside steel from being corroded.10 In spite of the obvious effects of humidity and CO2 in the atmospheric corrosion of ZnO powders, however, there are no studies addressing the stability and atmospheric corrosion of the extensively studied ZnO nanowires. In this work, we report the atmospheric corrosion observed on ZnO combs11 in normal ambient environment. When placed on an oxide substrate (e.g., glass slide and sapphire wafer) and exposed in air, the teeth (nanowires) as well as the base bone (mircoribbon) of the ZnO combs tend to react with airborne moisture and CO2 to form amorphous ZnCO3 thin films and nanowires. A CO2- and/or moisture-free environment is required in order for optimal applications of ZnO nanowires. Experimental Section The ZnO combs were synthesized by thermal evaporation of ZnO powder in a high temperature furnace, as that described in ref 11. The as-synthesized ZnO combs were placed on several kinds of oxide substrates including Corning soda lime glass slides, Si wafer with a native SiO2 thin layer, polycrystalline alumina plates, and sapphire wafers.
(11) Pan, Z. W.; Mahurin, S. M.; Dai, S.; Lowndes, D. H. Nano Lett. 2005, 5, 723.
Published on Web 12/09/2009
pubs.acs.org/cm
150
Chem. Mater., Vol. 22, No. 1, 2010
Pan et al.
Figure 1. SEM images of ZnO combs before and after atmospheric corrosion. (a) An uncorroded ZnO comb showing aligned and clean teeth. (b, c) ZnCO3 nanowires, (d) ZnCO3 nanorods, and (e) ZnCO3 cubes grown on the teeth and base bones of ZnO combs. The white arrows in (e) indicate the broken teeth caused by corrosion.
Corrosion in Air. The ZnO combs together with the oxide substrates were exposed in normal laboratory environment (70% RH and ∼350 ppm CO2) for up to 2 years. The samples were checked once a week in the first two months and then once a month later on with a high-magnification (∼1000) optical microscope to monitor the changes in morphology and luster of the originally shining ZnO crystals. Corrosion in Various Environments. The corrosion experiments were also conducted in a vacuum chamber, a desiccator, a chamber filled with Ar and 1% CO2, and a chamber filled with Ar, 1% CO2, and a small bottle of water (∼1 mL) for up to 6 months. The samples were checked once a half month in the first two months and then once a month later on with an optical microscope. Materials Characterization. The samples were characterized and analyzed by scanning electron microscope (SEM, FEI Inspect F FEG-SEM), transmission electron microscopes (TEM, Hitachi HF-2000 FEG STEM at 200 KeV and JEOL JEM-3000F FEG TEM at 300 KeV), energy-dispersive X-ray spectroscopy (EDS) attached to the SEM and TEMs, and electron energy loss spectroscopy (EELS) attached to the JEOL JEM-3000F TEM. The TEM samples were prepared by directly mounting the corroded ZnO combs on a Cu folding TEM grid without carbon supporting film.
Results and Discussion Our finding of the atmospheric corrosion of ZnO nanowires was by accident. When we placed ZnO combs on a glass slide (Corning soda lime glass) for optical property study,11 we surprisingly found through a highmagnification optical microscope that new nanowires started to grow from both the teeth and base bone of the combs after one month exposure to the atmosphere, and the amount and length of the new nanowires increased with the exposure time. SEM observations show
that after two months of exposure in normal laboratory environment, the original perfect ZnO combs (Figure 1a and the Supporting Information, Figure S1) were totally destroyed by growing a large amount of new nanowires (Figure 1b, c), nanorods (Figure 1d) or nanocubes (Figure 1e) on both the teeth and base bones of the combs (more SEM images in the Supporting Information, Figure S2). Composition analysis using the X-ray energy-dispersive spectrometer attached to the SEM revealed that besides zinc and oxygen the new nanowires also contained a significant amount of carbon, which was not present originally in the samples. This indicates that the new nanowires are zinc carbonate. The corrosion process proceeded so severe that some ZnO nanowires were completely covered by dense zinc carbonate nanowires (Figure 1c) or nanorods (Figure 1d), and some nanowires were even etched to break (indicated by the white arrow heads in Figure 1e). The length of the zinc carbonate nanowires after 2 months of growth are typically in the range of several tens to several hundreds of micrometers; some of them even have lengths on the order of millimeters. The morphology, microstructure, and composition of the zinc carbonate nanowires were further studied using TEM. Figure 2a-e show the typical TEM images of the zinc carbonate nanowires and nanorods grown on the teeth of ZnO combs (more TEM images in the Supporting Information, Figure S3). The nanowires are thin (10100 nm in diameter) and tend to connect together (Figure 2b) to form a nanowire network (Figure 2a), while the nanorods are short (
