Room Temperature Resistive Volatile Organic Compound Sensing Materials Based on a Hybrid Structure of Vertically Aligned Carbon Nanotubes and Conformal oCVD/iCVD Polymer Coatings

Room temperature resistive volatile organic compound (VOC) sensing materials fabricated with vertically aligned-carbon nanotubes (VA-CNT) demonstrated 10-fold improved sensitivity upon application of a thin conformal layer of the conducting polymer coating ((poly(3,4-ethylenedioxythiophene) (PEDOT)). The PEDOT was directly synthesized on the VA-CNTs via oxidative chemical vapor deposition (oCVD). Conformal PEDOT coatings with thickness of 8 and 17 nm were easily achievable by oCVD. The hybrid VA-CNT/oCVD PEDOT sensing materials exhibited excellent response to low concentrations of analyte gases of different polarity. The projected detection limit for n-pentane is as low as similar to 50 ppm. A second polymer layer, nonconducting polystyrene (PS, similar to 6 nm), was further conformally coated on the VA-CNT/PEDOT via initiative chemical vapor deposition (iCVD) to enhance the gas selectivity. The iCVD PS enhanced the selectivity of n-pentane over methanol by 2.7-fold and toluene by 4.4-fold. Several unique advantages of these sensing materials include the following: (1) detection of nonpolar hydrocarbon molecule n-pentane at room temperature; (2) high signal quality (signal-to-noise ratio typically similar to 30 dB); (3) solvent-free facile fabrication method that preserves the accessible high-surface-area morphology of the VA-CNTs; (4) good reversibility and short response time (similar to 400 s). Our results indicate that both the polarity of the analyte molecule and the carrier transport regime of the PEDOT layer are important in sensing behavior. Furthermore, this versatile selective layer design is potentially useful for selectivity enhancement for other important target analytes.