Hollow carbon spheres and a hollow carbon sphere/polyvinylpyrrolidone composite as ammonia sensors

Abstract

This study reports on the ammonia vapour sensing behaviour of hollow carbon spheres (HCSs), a hollow carbon sphere–polyvinylpyrrolidone (HCS/PVP) composite and annealed hollow carbon spheres (in a humid environment). For device fabrication, a surfactant assisted method was used to homogeneously disperse the hollow carbon spheres onto an interdigitated electrode. Conductance measurements (sensor response and recovery time) were performed at 20 °C and 40 °C. The sensor response was investigated by varying both ammonia concentration and relative humidity. The presence of amorphous domains and oxygenated groups on the pristine hollow carbon spheres resulted in a high relative humidity response. However, the detection of ammonia at high relative humidity using the pristine hollow carbon spheres was found to be negligible due to the inhibition of ammonia adsorption sites by the high concentration of water molecules. In contrast, a decline in conductivity at high relative humidity was recorded in the HCS/PVP sensors due to polymer swelling and plasticization. Annealing of the hollow carbon spheres resulted in a decrease in the amorphous domains in the carbon structure and a subsequent increase in the surface area. The topology of the response was determined as a function of these two variables (NH₃ and H₂O concentration) and analysed by applying a generalized tristimulus analysis to allow the ammonia concentration to be determined independently of the relative humidity. The pristine HCS, HCS/PVP and annealed HCS sensor responses to 74 ppm NH₃ at ambient humidity were 6%, 86% and 196%, respectively. The ammonia sensitivity values (% per ppm) of the pristine HCSs, HCS/PVP and annealed HCSs were 0.08, 4 and 1.6, respectively. The annealed HCSs exhibited a good ammonia sensitivity to NH₃ concentration (74–295 ppm) over a broad range of relative humidity (10–97%); indeed the values measured were higher than those reported for other nanomaterial based sensors. This study demonstrates the critical role played by humidity and surface chemistry in the ammonia sensing properties of hollow carbon spheres.