Sensors and Actuators B 124 (2007) 12–17 Hydrogen sensors based on aligned carbon nanotubes in an anodic aluminum oxide template with palladium as a top electrode Dongyan Ding, Zhi Chen ∗ , Suresh Rajaputra, Vijay Singh Department of Electrical and Computer Engineering, University of Kentucky, Lexington, KY 40506, USA Received 25 July 2006; received in revised form 22 November 2006; accepted 22 November 2006 Available online 29 December 2006 Abstract Robust carbon nanotube (CNT)-based hydrogen sensors were fabricated from aligned CNTs grown in an anodic aluminum oxide (AAO) template. Without Pd modification, an Au-electrode CNT sensor shows no response to hydrogen gas. On the other hand, a Pd-electrode CNT sensor is sensitive to medium concentration hydrogen gas (from 0.1% to 1.5% H 2 ) at room temperature. The aligned CNTs in the AAO template are also a good support for the nanoporous Pd film sensor, which is sensitive to hydrogen gas at concentrations ranging from 100 ppm to 1.5% H 2 . © 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen sensor; Carbon nanotube; Palladium; Resistance 1. Introduction Since its discovery in 1991 carbon nanotube (CNT) has attracted much attention due to its noteworthy physical and chemical properties. The application of carbon nanotubes in various gas sensing has been widely investigated. Multi-walled carbon nanotubes (MWNTs) are shown to be promising for detection of ammonia and water vapor [1]. Single-walled carbon nanotubes (SWNTs) can detect oxidizing gases (oxygen, nitro- gen dioxide) and reductive gases (ammonia) as well as volatile organic compound at room temperature [2–6]. With regard to hydrogen sensing, development of high performance hydrogen sensors is of great importance to meet increasing applications of hydrogen gas in recent years [7–9]. Pd-modified SWNTs in either individual or film form have been demonstrated to be sen- sitive to H 2 gas at room temperature [10–12]. Nanostructured Pt (and Pd) functionalized MWNTs in a film form were also found to have reversible response to hydrogen gas [13]. How- ever, fabrication processes for the CNT hydrogen sensors based on individual nanotube or films of entangled nanotubes are often complicated and costly. And these CNT-based hydrogen sensors lack device robustness and process reproducibility because of the difficulty in integrating randomly dispersed CNTs. Thus, it ∗ Corresponding author. Tel.: +1 859 257 2300; fax: +1 859 257 3092. E-mail address: zhichen@engr.uky.edu (Z. Chen). is of great interest to fabricate robust CNT hydrogen sensors with controllable processes. To date, hydrogen sensing properties of aligned CNTs grown in anodic aluminum oxide (AAO) template have not been reported. Considering the robustness of the AAO template on top of silicon wafers and the easiness of growing aligned CNTs in the template, we report here the fabrication of some robust and resistive hydrogen sensors based on aligned CNTs in the AAO template. Hydrogen sensing properties of these CNT-based devices are investigated. 2. Experimental A titanium layer (100 nm in thickness) and aluminum film (2.5 m in thickness) were deposited, in turn, onto highly doped n-type Si wafers by e-beam evaporation. The Al film was anodized in a 0.3 M oxalic acid through a two-step method, and pore-widened in a 5 wt.% H 3 PO 4 solution to obtain the AAO template with pore diameters around 60–70 nm and pore lengths about 1 m. Co-catalysts were electrodeposited at the bottom of the AAO template. Aligned CNTs with partially crystallized (near amorphous) structures were grown in the AAO template through pyrolysis of 10% acetylene in argon at 650 ◦ C. Detailed anodization, CVD processes and structural characterization of the aligned CNTs can be found elsewhere [14–16]. The samples were cut into small pieces. 0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.11.034