Microstructure Dependence of Hydrogen Sensing Properties of Palladium Functionalized Tungsten Oxide Films
- PDF / 315,611 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 35 Downloads / 176 Views
Microstructure Dependence of Hydrogen Sensing Properties of Palladium Functionalized Tungsten Oxide Films Meng Zhao1,2, Jian-Xing Huang1 and Chung-Wo Ong1 1 Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hong Kong, P.R.C. 2 Department of Electronic Science and Technology, Changzhou University, Changzhou, P.R.C. ABSTRACT Tungsten oxide (WO3) films were prepared by using magnetron sputtering. Substrate temperature and sputtering pressure were adjusted to vary the microstructure. The films were found to contain nanoclusters; while their size L, and porosity θ and surface roughness zRMS of the film can be varied. After adding a palladium coating on the film surface, the hydrogen (H2) sensing properties of the films, including sensitivity of detection, response time and recovery time were measured. Their dependences on L, θ and zRMS were analyzed and interpreted. The information achieved is useful for improving H2 sensor technology. INTRODUCTION Hydrogen (H2) is widely used in industries [1,2]. Reliable H2 sensors are needed to monitor potential leakage of the gas to ensure its safe use. Tungsten oxide (WO3) films show excellent H2 sensing properties [3], and have been prepared by using various techniques, e.g. solgel process [4,5], sputtering [6,7], evaporation [8,9] and gas-phase deposition [10]. However, their performance indexes show great discrepancies, e.g. magnetron sputtered platinum coated WO3 film (Pt/WO3) shows a sensitivity S ≈ 0.89 defined as the ratio of resistance measured in air to that in H2 (denoted as Rair/ RH 2 ), and a response time tres of 3000 s during hydrogenation at room temperature (RT) [6]; Pt-doped WO3 film prepared by using sol-gel method has an S ≈ 106 in 1% H2-N2 admixture at RT, with a tres < 10 s [5]; and Pd-WO3 film prepared by using sol-gel method shows a high S ≈ 25000 and moderate tres ≈ 100 s in 1300 ppm H2-air at RT [4]. These great variations may be due to the differences in their structures. By further assuming that all these materials contain small WO3 inclusions which are critical in providing H2 sensing signals, the structural information relevant to the H2 properties should include the characteristic dimension L of the inclusions, porosity θ of the film structure and surface roughness zRMS. In the present study, Pd/WO3 films were prepared to have different L, θ and zRMS values in order to analyze and gain insights into how they affect the levels of sensitivity S, response time tres and recovery time trec. EXPERIMENTS Pd/WO3 films were prepared by applying magnetron sputtering. In this process, a WO3 layer was first sputtered from a W metal target (purity 99.95%) in an argon (Ar)-O2 atmosphere of a ratio of 2 to 8 stabilized with two electronic flow meters. The thicknesses of the film samples t were found to fall in the range of 100 – 185 nm (Table I), corresponding to coating rates varying from 5 – 9.3 nm min-1. A 1.15-nm thick Pd coating was then sputtered on the top surface. Substrate temperature Ts and sputtering pressure
Data Loading...
