Film Structure and Conductometric Hydrogen-Gas-Sensing Characteristics of Ultrathin Platinum Films Sanjay V. Patel, § John L. Gland, †,‡ and Johannes W. Schwank* ,† Departments of Chemical Engineering and Chemistry, University of Michigan, Ann Arbor, Michigan 48109-2136 Received July 28, 1998. In Final Form: February 10, 1999 The structure and hydrogen-induced conductometric response of thin nanoparticulate platinum films has been investigated. Platinum films with a nominal thickness of 35 Å were deposited on silicon oxide and stabilized by thermal treatment in air at 673 K, resulting in an average platinum particle diameter of approximately 30 nm. These platinum films exhibited a positive temperature coefficient of resistance and resistance values intermediate between ultrathin and thick films. Exposure to ppm levels of hydrogen in the presence of 5% oxygen with nitrogen as the carrier gas caused decreases in electrical resistance. In the temperature range between 300 and 570 K, the relative response became more pronounced with increasing temperature, indicating that the response mechanism for hydrogen sensing is an activated process. In the temperature range of 370-470 K, the hydrogen concentration dependence of resistance changes can be divided into two nearly linear regimes. From 10 to 200 ppm of hydrogen, the response (ΔR/R0/CH 2 ) is 0.36/1000 ppm H2, while above 200 ppm, the response is 0.01/1000 ppm. The decreased response at higher hydrogen concentrations can be attributed to saturation of the active sites for hydrogen oxidation. The response is significantly decreased in the absence of oxygen, suggesting that the surface- catalyzed hydrogen-oxygen reaction plays an important role in the sensing mechanism. 1. Introduction In the past, the conduction mechanisms of metal films on insulating substrates have been studied with an eye on film thickness, morphology, and electric field effects. The resistivity and temperature coefficient of resistance (TCR) of thin continuous films (<200 Å) have been found to deviate from Fuchs theory. 1 These deviations are attributed to the shortened mean free path of electrons as the film thickness becomes smaller. 1,2 The conduction mechanisms in ultrathin discontinuous films (<50 Å) have been found to depend on the particle diameter and the gap size between metal particles. For example, for large particles with a large gap spacing, the conduction mech- anism is attributed to thermionic emission or bulk conduction in the substrate. However, as the particle spacing decreases, electron tunneling becomes more important. These types of films can have either positive or negative TCR values, depending on the deposition parameters and film morphology. 3-5 There are several other factors that can affect the conduction through thin films, including the substrate composition, film deposition rate, and substrate temperature. The resistance values of ultrathin platinum films on insulating supports have been found to vary, depending on the pressure and composition of the gas environment. Nowroozi-Esfahani and Maclay 6 have suggested that through-substrate conduction between platinum particles supported on SiO 2 can be increased by the addition of hydrogen or carbon monoxide to the gas phase. Adsorbed hydrogen may change the surface potential of the SiO 2 support by forming a dipole at the platinum/SiO 2 interface. This capability of certain gases to change the resistance of thin films can be exploited for gas-sensing applica- tions. 7,8 Sensors for combustible gases are important for many applications. For example, the automobile industry is developing new sensors to be used for air-to-fuel ratio control and catalytic converter diagnostics in engine exhaust streams. Sensors in this type of environment may be required to sense gases in the concentration range of a hundred parts per million (ppm) in a stream of oxygen, nitrogen, and other interfering gases. Several conductance- based gas sensors have been developed for sensing reducible gases; most notable are the tin dioxide (SnO 2 ) sensors that are sometimes doped with platinum or palladium. 9-11 Some very selective hydrogen sensors made from Ga 2 O 3 have been developed by Meixner and co- workers; 12,13 however, these sensors require heating to over 573 K. When doped with platinum, these films respond with greater sensitivity but also are affected by interfering gases such as CO or CH 4 . Others have produced low temperature (353 K) hydrogen sensors using pal- ladium and palladium-nickel films on microelectronic gas sensors, which are aided by the use of an array of sensing materials and by sophisticated software to enhance selectivity. 14 * To whom correspondence should be addressed. † Department of Chemical Engineering. ‡ Department of Chemistry. § Currently with Sandia National Laboratories, Albuquerque, NM 87185. (1) Hoffman, H.; Fischer, G. Thin Solid Films 1976, 36, 25. (2) Neugebauer, C. A.; Webb, M. B. J. Appl. Phys. 1962, 33, 74. (3) Hill, R. M. Proc. Royal Soc. A 1969, 309, 377. (4) Morris, J. E.; Coutts, T. J. Thin Solid Films 1977, 47, 3. (5) Williams, J. L.; Stone, I. L Thin Solid Films 1972, 11, 329. (6) Nowroozi-Esfahani, R.; Maclay, G. J. J. Vac. Sci. Technol. A 1990, 8, 3591. (7) Johnson, C. L.; Schwank, J. W.; Wise, K. D. Sensors Actuators B 1994, 20, 55. (8) Johnson, C. L.; Schwank, J. W.; Wise, K. D. U.S. Patent 4,953,387, 1990. (9) Malyshev, V. 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