Fast and Selective Room-Temperature Ammonia Sensors Using Silver Nanocrystal-Functionalized Carbon Nanotubes

We report a selective, room-temperature NH3 gas-sensing platform with enhanced sensitivity, superfast response and recovery, and good stability, using Ag nanocrystal-functionalized multiwalled carbon nanotubes (Ag NC-MWCNTs). Ag NCs were synthesized by a simple mini-arc plasma method and directly assembled on MWCNTs using an electrostatic force-directed assembly process. The nanotubes were assembled onto gold electrodes with both ends in Ohmic contact. The addition of Ag NCs on MWCNTs resulted in dramatically improved sensitivity toward NH3. Upon exposure to 1% NH3 at room temperature, Ag NC-MWCNTs showed enhanced sensitivity (similar to 9%), very fast response (similar to 7 s), and full recovery within several minutes in air. Through density functional theory calculations, we found that the fully oxidized Ag surface plays a critical role in the sensor response. Ammonia molecules are adsorbed at Ag hollow sites on the AgO surface with H pointing toward Ag. A net charge transfer from NH3 to the Ag NC-MWCNTs hybrid leads to the conductance change in the hybrid.