Enhanced Ammonia Adsorption on Directly Deposited Nanofibrous Carbon Films

Abstract

The ammonia adsorption on the nanostructured carbon thin film was significantly influenced by the choice of deposition temperature and deposition time of thin film synthesis. The thin films were prepared on Si/SiO2 substrates by chemical vapour deposition in Ar/C2H2 gas mixture using iron catalytic nanoparticles. The analysis of the grown layer by the scanning and transmission electron microscopy showed the transition from long multiwalled nanotubes (MWCNTs) to bamboo-like hollow carbon nanofiber structure with the decrease of the deposition temperature from 700 to 600°C. Further, the material was analyzed by energy-dispersive X-ray spectroscopy and Raman spectroscopy confirmed the transition from graphitic sp2 structure to highly defective structure at lower deposition temperature. The resistance of the prepared layer strongly depends on deposition temperature (Td) and deposition time (td). High resistance layer, 38.6 kΩ, was formed at Td 600°C and td 10 min, while at Td 700°C and td 60 min, the resistance decreased to 860 ohms. Such behaviour is consistent with MWCNTs being responsible for the formation of the conductive network. Such system was studied using chemiresistor ammonia gas sensor configuration. The sensor resistance increased when exposed to ammonia in all the cases, but their response varied considerably. A decrease in deposition time, from 60 to 10 min, and the deposition temperature, from 700 to 600°C, led to the 10-fold increase in the sensor response. The measurements carried out at room temperature showed the higher sensor response than the measurements carried out at 200°C. This behaviour can be explained by the change in adsorption-desorption equilibrium at different temperatures. Analysis of dependence of the sensor response on the ammonia concentration proved that the underlying resistance change mechanism is chemisorption of ammonia molecules on the carbon network corresponding to the Langmuir isotherm.

Recent Advances in Ammonia Gas Sensors Based on Carbon Nanomaterials

“…In our consideration of sensors based on CNTs, we should take into account that there are also sensors based on carbon nanofibers (CNFs) [ 104 , 153 , 154 , 155 ]. Since CNFs are cheaper than CNTs and semiconducting metal oxides, they become excellent candidates for sensing applications [ 154 ].…”

“…In the majority of articles published on the impact of relative humidity on the ammonia sensing properties of carbon nanotubes [ 24 ], carbon nanofibers [ 153 ], graphite oxide, and graphene oxide [ 92 ], it was reported that the sensor response increases with an increasing RH.…”

“…Mainly, sensors based on carbon nanomaterials require recovery. This recovery can be carried out using heating [ 87 , 153 , 162 ], an increase in gas flow [ 40 ], a strong electric field [ 206 ], or irradiation by infrared light [ 207 ]. In [ 207 ], a sensor based on graphene decorated with Au nanoparticles was illuminated with infrared radiation (≈2 µW/mm 2 ), which made it possible to recover more than 90% of the original baseline level.…”

“…A reproducible complete recovery of ammonia molecules was achieved at 80 °C for 5 min (the flow rate of the carrier gas, nitrogen, was 1000 sccm). In [ 153 ], heating (100 °C, 3 min) was used for the 100% recovery of sensors based on nanofibrous carbon. In [ 162 ], the protocol of heat treatment for SWCNT/Al 2 O 3 ammonia gas sensors was 200 °C for 10 min in a nitrogen flow.…”

A review about the fluorination and oxyfluorination of carbon fibres

“…ammonia (NH3), at rates as low as possible. The example of NH3 was chosen as ammonia is amongst the most dangerous gases, both for human health and environment [126]. The electrical properties of carbonaceous materials, especially the nanostructured ones [126], change under gas absorption, making them ideal materials for gas sensing applications.…”