Fabrication of Densely Packed AlN Nanowires by a Chemical Conversion of Al2O3Nanowires Based on Porous Anodic Alumina Film
Porous alumina film on aluminum with gel-like pore wall was prepared by a two-step anodization of aluminum, and the corresponding gel-like porous film was etched in diluted NaOH solution to produce alumina nanowires in the form of densely packed alignment. The resultant alumina nanowires were reacted with NH3and evaporated aluminum at an elevated temperature to be converted into densely packed aluminum nitride (AlN) nanowires. The AlN nanowires have a diameter of 15–20 nm larger than that of the alumina nanowires due to the supplement of the additional evaporated aluminum. The results suggest that it might be possible to prepare other aluminum compound nanowires through similar process.
KeywordsAluminum nitride Alumina Nanowire Chemical converting Porous film
In past few decades, aluminum nitride (AlN) has attracted considerable interests because of its exceptional mechanical, thermal, electrical, and optical properties [1, 2, 3, 4]. For example, its good thermal conductivity, low dielectric constant, and high electrical resistance as well as thermal expansion coefficient matching to that of silicon offer an application in semiconductor devices as passivation and dielectric layers, and electric substrates [5, 6, 7, 8, 9, 10]. Also, as one of wide band-gap semiconductors, aluminum nitride has a promising candidate for optoelectronic and field-emission materials [11, 12]. Furthermore, AlN fibers and nanowires are considered to be able to optimize these properties and applications. To date, AlN nanowires have been synthesized by some methods including silica-assisted catalytic growth , dc-arc plasma process , extended vapor–liquid–solid growth technique , and chemical vapor deposition method . However, the developed methods seem to have technologic and economic limits for mass-produced AlN nanowires. Here, we demonstrate a low-cost approach for fabricating densely packed AlN nanowires via a chemical conversion of Al2O3 nanowires produced by a chemical etching of the porous anodic alumina film.
In our work, the fabrication of AlN nanowires mainly involves three steps: preparation of porous anodic alumina film, formation of Al2O3 nanowires, and conversion of the nanowires into AlN nanowires. First, the porous anodic alumina film was prepared by a two-step anodization of aluminum as described elsewhere [17, 18]. Briefly, the cleaned and electropolished high-purity aluminum sheet (99.999%) was anodized in oxalic acid at 40 V for 4–6 h to form porous alumina layer on the aluminum surface. After the alumina layer was removed in a mixture of phosphoric acid (6% wt) and chromic acid (1.8% wt), the Al sheet was again anodized under identical conditions to those of the first anodization for 30 min, which results in forming porous alumina film on the aluminum surface. Then, the as-prepared porous film was etched in diluted NaOH solution to produce Al2O3 nanowires. Finally, the chemical conversion of the alumina nanowires into AlN nanowires was carried out at 1,300–1,400 °C for 2 h in ammonia atmosphere with a flow of ~150 SCCM. The samples were observed by field-emission scanning electron microscopy (FE-SEM; JEOL JSM-6700F). XRD measurement was taken using a Rigaku D/MAX-2500 X-ray diffractometer with CuKα incident radiation.
Results and Discussion
As a result, the Al2O3nanowires are chemically converted into the AlN nanowires. Furthermore, the converting reaction from Al2O3to AlN nanowires is involved with the supplement of the additional evaporated aluminum, and thus gives a larger diameter of the AlN nanowires than that of the Al2O3ones.
Summary and Conclusions
We have developed a convenient method for the preparation of densely packed AlN nanowires based on a new strategy. In this strategy, the porous anodic alumina film on aluminum is etched into the alumina nanowires, and then the alumina nanowires are chemically converted into the AlN ones. In principle, the successful preparation of the aluminum nitride nanowires suggests that the strategy could be applied to prepare a range of aluminum compound nanowires (i.e., sulfide, carbide, chloride, etc.), which is of great fundamental and practical significance.
The authors would like to thank Prof. S. S. Fan, Prof. Y. D. Li, Prof. L. J. Bie, and Mr. Y. C. Sun for their help and valuable discussions. This work was supported by the National Natural Science Foundation of China under Grant No. 20671070, the Key Project of Chinese Ministry of Education under Grant No. 207008, the Science and Technology Programs of Tianjin (China) under Grant No. 06YFGZGX02900 and 06TXTJJC14602, and Tianjin Key Subject for Materials Physics and Chemistry.
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