Array-to-Array Transfer of Microspheric Sensors Stable over Time
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Figure. Crystal faces of hexagonal ZnO microrods and microtubes. Reprinted with permission from Chem. Mater., December 17, 2001, 13, 4395–4398. Copyright 2001 American Chemical Society.
Physical Chemistry at the University of Uppsala, Sweden, have prepared highly ordered arrays of hollow crystalline ZnO microtubes 1–2 µm in diameter and 10 µm in length. The preparation, carried out by Lionel Vayssieres and co-workers, consisted of the growth of oriented crystalline ZnO microrods by controlled precipitation of ZnO, and the selective etching of these rods to produce microtubes. The one-step synthesis was carried out in aqueous solution using common reagents and without the use of a template. This synthetic method allows for a simple, safe, and inexpensive production of purposebuilt materials on a variety of substrates. According to the researchers, similar microrod arrays have demonstrated high-UV photoresponse and excellent electron-transfer properties with potential applications as catalysts, sensors, and other photovoltaic devices. As reported in the December 17, 2001, issue of Chemistry of Materials, the researchers prepared the microtube arrays by thermal decomposition of a Zn 2+ methenamino complex precursor in aqueous solution to produce ZnO. The arrays were grown on polycrystalline F-SnO2 glass, silicon wafers, and indium-tinoxide-coated polyester substrates. The substrates were covered with the precursor solution, and the reaction was kept at 90°C for two days. The precursor solution was prepared by dissolving Zn(NO3)2* 4H2O and methenamine (both 0.1 M) in MilliQ + and water. Following the synthesis, the product materials were washed with water to remove any unreacted complex and salts. Microtube arrays covering several tens of square centimeters may be produced by this method. MRS BULLETIN/JANUARY 2002
The formation of the microtubes in the arrays exploits the chemical and structural metastability of the (001) face of the ZnO wurtzite structure. A ZnO microrod consists of a hexagonal single crystal elongated along the [001] direction (Figure, left). This elongation is the result of preferential growth of the (001) face during conditions of high precursor concentration on the first day of the synthesis. When the precursor concentration drops, the resultant Ostwald ripening, combined with the preferential dissolution of the (001) face, results in a net transfer of material from the (001) face to the more stable crystal faces, producing a hollow structure (Figure, right side). Scanning electron microscopy of the product materials showed a uniform array of hexagonal microtubes 1–2 µm in diameter and 10 µm in length, oriented perpendicular to the substrate. X-ray and electron-diffraction studies confirm that the microtubes are crystalline and exhibit the wurtzite structure and lattice spacing of bulk ZnO. The crystallinity of the arrays allows the use of flexible and temperaturesensitive substrates by eliminating the need for high-temperature annealing normally re
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