Nanoporous naphthalene diimide surface enhances humidity and ammonia sensing at room temperature

Abstract We report fabrication and characterisation of humidity and ammonia sensors based on a nanoporous naphthalene diimide (NDI) thin layer, the active material coded as NDI-1, operating at room temperature. A thin layer of NDI-1 as an organic semiconducting sensing material was deposited onto the interdigitated electrodes using the spin-coating technique. Surface morphology investigations of the thin film using atomic force microscopy and scanning electronic microscopy revealed a uniform nanoporous surface. Structural and thermal behaviour studies of the NDI-1 using X-ray diffraction and thermogravimetric analyses confirmed the crystallinity and thermal stability, respectively. The capacitive type NDI-1 humidity sensor displayed a sensitivity of 10.13 pF/%RH, quick response and recovery (20.1/6.28 s), low hysteresis (0.72%), long-term stability and more importantly, a wide working range of relative humidity (10–95%) at 250 Hz. Interestingly, the developed sensor was sensitive enough to monitor respiration rate and demonstrated an excellent non-contact skin humidity sensing performance. The amperometric response of the sensor towards ammonia was also investigated. An increase in the sensor current was recorded when exposed to different ammonia concentrations (25, 37.5 and 50 ppm). The sensor showed high selectivity and good sensitivity (27.7%) towards 50 ppm ammonia with 200 s and 350 s response and recovery times, respectively.