pubs.acs.org/cm Published on Web 12/09/2009 r 2009 American Chemical Society Chem. Mater. 2010, 22, 149–154 149 DOI:10.1021/cm902734e Spontaneous Growth of ZnCO 3 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 CO 2 ) 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 CO 2 to form amorphous ZnCO 3 thin films and nanowires. The factors that specially affect the corrosion of ZnO nanowires in a laboratory environment include CO 2 , humidity, and substrates. Our results suggest that a CO 2 - and/or moisture-free environment are required in order for optimal applications of ZnO nanowires. Introduction ZnO nanowires are probably the single most investi- gated nanomaterials after carbon nanotubes because of their unique optical properties and wide potential applica- tions 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 CO 2 and water molecules coexist, because of a phenomenon called atmo- spheric corrosion. 7-10 The atmospheric corrosion of ZnO was recognized in the study of Zn-coated hot-dip galva- nized steels. The ZnO powder layer formed on the galva- nized coating surface at the early weathering stage is very unstable in atmosphere because of the presence of airborne moisture, CO 2 , and/or other gaseous pollutants, such as SO 2 , NO 2 , 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 CO 2 to form a stable and passive ZnCO 3 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 CO 2 in the atmo- spheric 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 ob- served on ZnO combs 11 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 CO 2 to form amor- phous ZnCO 3 thin films and nanowires. A CO 2 - 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 SiO 2 thin layer, polycrystalline alumina plates, and sapphire wafers. *Corresponding author. E-mail: panz@uga.edu. (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. 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