P. Ricci, A. Rossi, A. Kelly, O. Gregory
University of Rhode Island,
Keywords: electrospinning, nanowires, advanced surface area, TATP, DNT
Summary:There is a growing need for a continuous monitoring system that can detect explosives such as triacetone triperoxide (TATP) in the vapor phase at trace levels. TATP is an explosive commonly used by terrorists in improvised explosive devices both as the initiator and the energetic material itself. TATP is still going largely undetected in many densely-populated venues such as train stations and airports. No electronic trace detection system currently exists that is capable of continuously monitoring TATP, or its precursors, in the vapor phase. Recently, we developed a thermodynamic sensor that can detect TATP and 2,4 DNT at the ppb level. These low-mass sensors show excellent selectivity and sensitivity that relies on a unique sensing mechanism. This mechanism relies on specific oxidation-reduction reactions between the energetic molecules and metal oxide thin films deposited onto the surface of nickel micorheaters. These sensors measure the heat effect associated with the redox reactions taking place on the metal oxide catalyst surface. Based on this mechanism, enhanced sensitivity and selectivity were achieved by employing high surface area catalyst supports. Preparation and characterization of these supports was based on the select metal oxide required for the detection of TATP and 2,4 DNT. High surface area was achieved using electrospinning methods as well as nanowire growth employing various metal precursors and heat-treating in atmospheres containing very low partial pressures of oxygen. The effect of catalyst surface area on sensor response was initially investigated via deposition of high surface area nickel catalyst supports via electrospinning. Electrospinning time was optimized to produced larger responses without significantly affecting thermal mass of the sensor. Electrospinning times of 2.5, 5, and 7 minutes were investigated and SEM analysis showed complete surface coverage with nickel fibers after 7 minutes of electrospinning. The electrospun fibers provided strong adhesion to the nickel microheaters and significantly increased surface area. Upon deposition of the fibers, the microheaters were then sputter coated with SnO. Due to the increase in catalyst surface area, the microheaters with electrospun catalyst supports exhibitied a 60% greater response than a similar SnO film deposited directly onto the surface of the microheater. Sensor response using CuO nanowires as a catalyst support was also investigated. CuO nanowires were “grown” directly on metallic copper surfaces using heat treatment in a reduced oxygen atmosphere. Nanowire growth was confirmed using SEM imaging in which CuO based surface structures were observed. Surface area was substantially increased when compared to as-sputtered CuO catalyst surfaces. Sensors with the higher surface area catalysts showed a much larger response to TATP and 2,4 DNT. Overall, the implimentation of high-surface area catalyst supports showed siginifcant improvments in sensor response and selectivity to peroxide and non peroxide based explosives at trace levels.