Dawei Gong

Titanium oxide nanotube arrays prepared by anodic oxidation

Titanium oxide nanotubes were fabricated by anodic oxidation of a pure titanium sheet in an aqueous solution containing 0.5 to 3.5 wt% hydrofluoric acid. These tubes are well aligned and organized into high-density uniform arrays. While the tops of the tubes are open, the bottoms of the tubes are closed, forming a barrier layer structure similar to that of porous alumina. The average tube diameter, ranging in size from 25 to 65 nm, was found to increase with increasing anodizing voltage, while the length of the tube was found independent of anodization time. A possible growth mechanism is presented.

Crystallization and high-temperature structural stability of titanium oxide nanotube arrays

The stability of titanium oxide nanotube arrays at elevated temperatures was studied in dry oxygen as well as dry and humid argon environments. The tubes crystallized in the anatase phase at a temperature of about 280 °C irrespective of the ambient. Anatase crystallites formed inside the tube walls and transformed completely to rutile at about 620 °C in dry environments and 570 °C in humid argon. No discernible changes in the dimensions of the tubes were found when the heat treatment was performed in oxygen. However, variations of 10% and 20% in average inner diameter and wall thickness, respectively, were observed when annealing in a dry argon atmosphere at 580 °C for 3 h. Pore shrinkage was even more pronounced in humid argon environments. In all cases the nanotube architecture was found to be stable up to approximately 580 °C, above which oxidation and grain growth in the titanium support disrupted the overlying nanotube array.

Gas sensing characteristics of multi-wall carbon nanotubes

Abstract Impedance spectroscopy was used to study the gas sensing behavior of both capacitance and resistance based sensors employing multi-wall carbon nanotubes (MWNTs) as the active sensing element. Studies revealed the chemisorption of reducing gases upon the surface of the MWNTs. Increasing sensor impedance was observed with increasing humidity or partial pressures of ammonia, carbon monoxide, and carbon dioxide. The impedance changes are attributed to p-type conductivity in semiconducting MWNTs, and the formation of Schottky barriers between the metallic and semiconducting nanotubes. Reversible behavior is demonstrated for the MWNT sensors in response to humidity, carbon monoxide and carbon dioxide. The MWNT sensors strongly respond to ammonia behaving as dosimeters.

Controlled Molecular Release Using Nanoporous Alumina Capsules

The use of mechanically robust nanoporous alumina capsules, with highly uniform pores of 25 nm to 55 nm, for controled drug delivery is demonstrated. The nanoporous alumina capsules were fabricated by anodization of an aluminum tube, resulting in a highly uniform, large surface area, relatively inexpensive device suitable for biofiltration applications. Characterization of diffusion from the nanoporous capsules using fluorescein isothiocyanate and dextran conjugates of varying molecular weight, showed that molecular transport could be readily controlled by selection of capsule pore size. A branched membrane structure, with a stepwise change in pore size from large to small, is used to provide small pore-sized membranes with sufficient mechanical strength for handling.

Highly Ordered Nanoporous Alumina Films: Effect of Pore Size and Uniformity on Sensing Performance

The effect of pore size and uniformity on the humidity response of nanoporous alumina, formed on aluminum thick films through an anodization process, is reported. Pore sizes examined range from approximately 13 to 45 nm, with a pore size standard deviations ranging from 2.6 to 7.8 nm. The response of the material to humidity is a strong function of pore size and operating frequency. At 5 kHz an alumina sensor with an average pore size of 13.6 nm (standard deviation 2.6 nm) exhibits a well-behaved change in impedance magnitude of 10 3 over 20% to 90% relative humidity. Increasing pore size decreases the humidity range over which the sensors have high sensitivity and shifts the operating range to higher humidity values. Cole-Cole plots of 5 to 13 MHz measured impedance spectra, modeled using equivalent circuits, are used to resolve the effects of water adsorption and ion migration within the adsorbed water layer. The presence of impurity ions within the highly ordered nano-dimensional pores, accumulated during the anodization process, appear highly beneficial for obtaining a substantial variation in measured impedance over a wide range of humidity values.