Phase and Morphology in Mixed CuO-WO 3 Films for Chemical Sensing
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Phase and Morphology in Mixed CuO-WO3 Films for Chemical Sensing A. El Madi,1,3 B. Meulendyk,2,3 R. S. Pilling1,3 G. Bernhardt,2,3 R. J. Lad,2,3 and B. G. Frederick1,3 1 Dept. of Chemistry, 2Dept. of Physics, and 3Laboratory for Surface Science and Technology (LASST), University of Maine, Orono, ME 04469 ABSTRACT Semiconducting metal oxide (SMO) chemiresistive sensors are highly sensitive toward a broad range of hydrocarbons. To develop a gas phase sensor with selectivity toward organophosphorus compounds, such as chemical warfare agents and pesticides, we have developed dosimeters based upon a poisoning mechanism. Here, we report the growth and characterization of WO3 thin films, modified with Cu2O. XPS data show that exposure to phosphonate compounds leads to accumulation of phosphate on the surface, together with dramatic changes in the surface segregation of copper. We present XRD and XPS results to characterize the phase changes following growth, annealing, and exposure to phosphonate compounds. The correlation between sensor response and phosphorous accumulation shows that the highest activity occurs at intermediate coverages of Cu2O, in the15-25 Å range, on 500 Å WO3 films. INTRODUCTION We have shown that WO3 based chemiresistive sensors respond to part-per-billion levels of organophosphonate compounds, such as dimethyl methyl phosphonate (DMMP), that are commonly used as simulants for chemical warfare agents.[1] Like other semiconducting metal oxides, WO3-based sensors have the advantage of being relatively inexpensive devices for a broad range of chemical sensing applications. The disadvantage is an inherent lack of selectivity. Various approaches are used to improve the selectivity of the system, including sensor arrays with signal processing [2, 3] and pre-filtration of the gas stream. The decomposition of organophosphonates has been studied on a number of oxides, [4-7] and generally leads to stable, phosphate or alkyl-phosphate intermediates. In the search for materials to decompose chemical warfare agents, copper oxide based systems have been found to be effective. But, they have a limited lifetime due to poisoning by the phosphate species formed.[1] In a separate paper,[8] we have shown that this poisoning effect can be exploited for detection of organophosphonates. We used a probe molecule, methanol, to measure the activity of the sensor element; when exposed to phosphonates under certain conditions, the response toward methanol decreases, indicating the presence of volatile phosphonate compounds. Although little is known about the levels of volatile phosphonate compounds in buildings,[9] our results indicate that background levels are low. This approach is not likely to be compound specific, but the ability to detect organophosphorous compounds in general, and perhaps other relatively toxic compound classes such as sulfur compounds, may be an advantage for surveillance applications. In a preliminary study, we used XPS to analyze the phosphorous coverage of a set of WO3based sensors that had
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