Film structure and conductometric hydrogen-gas-sensing characteristics of ultrathin platinum films

The structure and hydrogen-induced conductometric response of thin nanopart iculate platinum films has been investigated. Platinum films with a nominal thickness of 35 Angstrom were deposited on silicon oxide and stabilized by thermal treatment in air at 673 K, resulting in an average platinum partic le diameter of approximately 30 nm. These platinum films exhibited a positi ve temperature coefficient of resistance and resistance values intermediate between ultrathin and thick films. Exposure to ppm levels of hydrogen in t he presence of 5% oxygen with nitrogen as the carrier gas caused decreases in electrical resistance. In the temperature range between 300 and 570 K, t he relative response became more pronounced with increasing temperature, in dicating that the response mechanism for hydrogen sensing is an activated p rocess. In the temperature range of 370-470 K, the hydrogen concentration d ependence of resistance changes can be divided into two nearly linear regim es. From 10 to 200 ppm of hydrogen, the response (Delta R/R-0/C-H2) is 0.36 /1000 ppm H-2, while above 200 ppm, the response is 0.01/1000 ppm. The decr eased response at higher hydrogen concentrations can be attributed to satur ation of the active sites for hydrogen oxidation. The response is significa ntly decreased in the absence of oxygen, suggesting that the surface-cataly zed hydrogen-oxygen reaction plays an important role in the sensing mechani sm.