A Pyrolytic, Carbon-Stabilized, Nanoporous Pd Film for Wide-Range H2 Sensing**

With the increasing relevance of H2 gas in modern industry, high-performance H2 sensors (sensors that exhibit, e.g., a fast response, good stability, sensitivity, etc.) are becoming increasingly important in detecting and monitoring H2. Because of its noteworthy ability to absorb a large quantity of H2 and its highly selective response to H2, Pd has found wide applicability in various H2 sensors including Pd-film-based resistive sensors, metal oxide semiconductor-type sensors, Schottkydiode-type sensors, and optical sensors. Among these H2 sensors, resistive sensors based on dense Pd films have attracted much attention because of their simple device structure and fabrication process. However, several drawbacks, including a narrow detection range (dense-film resistive sensors are only sensitive to medium-level concentrations of H2 gas), a slow response time, and instability in higher concentrations of H2 usually hinder the widespread applicability of resistive sensors based on dense films. To improve H2-sensing performance, nanostructured Pd materials have been developed, and they have demonstrated a much quicker response to H2 gas. Break-junctioned Pd nanowires reported by Favier et al. have shown the fastest response because of a volume-expansion-induced resistance decrease upon the absorption of H2. [8] However, these Pd nanowires are only sensitive to high concentrations of H2. Based on the same sensing mechanism, Xu et al. have used self-assembled monolayers to promote the formation of break-junctioned Pd nanoclusters to realize a fast H2 response. Recently, we have fabricated nanoporous Pd film sensors by using anodic aluminum oxide (AAO) as a substrate. It was shown that the nanoporous Pd film demonstrated an increased resistance upon absorption of H2. The nanoporous Pd sensor had a quick response to low concentrations of H2 gas (less than 1 % H2). For higher H2 concentrations, the nanoporous Pd films directly supported by AAOs lost their sensing stability because of stress-induced blistering of the film. Here, we report a simple but effective method to fabricate a nanoporous Pd film sensor that has a remarkable sensing performance and is able to detect both dilute and high concentrations of H2 gas (ranging from 250 ppm to 10 % H2) with quick response times at room temperature, through the use of pyrolytic carbon as a transition layer between the Pd film and the AAO template/substrate. A mechanism based on nanoporous structure-enhanced H2 sensing is proposed. Briefly, an aluminum film deposited onto a highly doped n-type Si wafer was anodized in 0.3 M oxalic acid to obtain the AAO template. A film of amorphous carbon only several nanometers thick was deposited onto the AAO template by using the chemical vapor deposition (CVD) pyrolysis of acetylene. Detailed information about the anodization and CVD processes can be found elsewhere. A Pd film was then deposited, via sputtering, onto the upper surface of the carbon-coated AAO substrate. A resistive sensor based on a Pd film was then fabricated and tested under various concentrations of H2 in nitrogen (Fig. 1).