Detecting high concentration hydrogen with nanoporous palladium supported by anodic aluminum oxides

Abstract

Hydrogen-induced blistering of dense Pd films upon absorption of high concentration hydrogen is one of the big problems for hindering wide application of Pd-film hydrogen sensors fabricated on traditional wafers [1, 2]. Considerable stressing in the Pd film or stress mismatch at the interface between the Pd film and the supporting substrate is believed to cause such a failure in detecting high concentration hydrogen. In this work, we report hydrogen sensing properties of highly stable nanoporous Pd sensors fabricated on anodic aluminum oxides (AAOs). Aluminum film was deposited onto Ti-coated n-type Si wafers by e-beam evaporation. Through anodization of the Al film in 0.3 M oxalic acid, AAO substrate with pore diameters around 60 nm and pore lengths about 2.5 ,um was prepared. Nanoporous Pd films with a thickness of 45 nm or 5 nm were deposited, via r. f. sputtering, onto the AAO substrate by using Ni (2 nm in thickness) as a transition layer. The nanoporous Pd film sensors were put into a flask chamber. Resistive testing of the sensors under different concentrations of hydrogen gas was conducted with a Keithley 2000 multimeter. For comparison, dense Pd film sensors supported by silica wafers were also tested. Fig. 1 shows SEM morphologies of a dense Pd film and nanoporous Pd films. All of the sensors are sensitive to hydrogen gas at concentrations above 0.25% (Fig. 2). But the sensors made from the dense Pd films fail, by showing irreversible recovery (a sign of blistering) after switching off the hydrogen gas, at hydrogen concentrations above 1.5% for the 45 nm film and 3% for the 5 nm film. This once again proves that a blistering of dense Pd films will result in a failure to detect high concentration hydrogen. Whereas, the nanoporous Pd film sensors can detect much higher hydrogen concentrations up to 10%. At hydrogen concentrations above 1%, more than 20% of sensitivity (variation of film resistance upon absorption of H) can be obtained with the thicker nanoporous film (45 nm). At 12 concentrations above 2%, it only needs less than 30 seconds for the thicker nanoporous Pd film (45nm) to have a 10% variation of resistance (Fig. 3). With the film thickness being thinned down to 5 nm, the nanoporous film sensor has a much quicker response (Fig. 4). Typical response time of the thinner nanoporous Pd film (5nm) is less than 1 minute at 12 concentrations above 2%. And the response time decreases from 30 seconds at 4% 12 to 15 seconds at 10% 12. In comparison with dense Pd films deposited on traditional wafers, nanoporous Pd films shaped by the AAO nanotemplate can have a quick and reversible response to high concentration hydrogen without a blistering. Such a good mechanical stability indicates that the AAO substrate used here can help to stabilize the Pd films. Theoretically, a rough surface (including porous surface) can give an anchor effect to any deposited films and thus enhance the adhesion between deposited film and the substrate [3]. And a porous structure can have better resistance to stress-induced damage by showing less crack initiation and propagation [4,5]. Moreover, upon absorption of hydrogen, a volume expansion occurs in the Pd film deposited slightly below the upper edge of the AAO nanoholes, which results in favorable anchoring force. The above contributions from the AAO make it possible to fabricate highly stable nanoporous Pd film sensors that can detect high concentration hydrogen without a blistering. In conclusion, AAO substrate is an ideal substrate for fabrication of nanoporous Pd film sensors with quick and reversible response to hydrogen gas at room temperature. Unlike dense Pd film sensors, the nanoporous Pd sensors can detect high concentration hydrogen (up to 100% 2) without a blistering.