Activated carbon-based gas sensors: effects of surface features on the sensing mechanism

Abstract

Ammonia sensing capability of a commercial wood-based activated carbon (BAX) and its oxidized counterpart (BAX-O) was studied. The materials were exposed to continuous cycles of various ammonia concentrations, followed by purging with air, and changes in a normalized resistance were analyzed. When initially exposed to the reducing gas, chemical interactions between ammonia and the carbons generated an irreversible signal change, which was stronger in the case of the initial carbon. After equilibration and reactive adsorption the sensors were further tested with ammonia. During the reversible sensing, the two carbons exhibited opposite trends in their signal. This indicates that oxidation changed the electrical properties of the carbon sample, leading to a different sensing mechanism. For the initial material holes play the predominant role in a current transport, while their depletion upon the exposure to the reducing gas leads to a decrease in the conductivity. In the case of the oxidized sample, the presence of electron donating nitro groups causes a hole annihilation in the carbon itself converting it to a predominantly n-type material. Here the exposure to ammonia increases the conductivity of the chip owing to the electron donating properties of the gas. Physical interactions, such as hydrogen bonding, polar interactions with surface functional groups, and dispersive interactions of ammonia with the carbon matrix, lead to the pore filling, which governs the extent of the response signal. The sensor signal changes linearly with the ammonia concentration.

Toxic gas sensing on nanoporous carbons

“…The latter sample was further oxidized with 50% of HNO 3 for 5 h. The sample obtained in this way is referred to as BAX-M-O. The details on these carbons preparation are described elsewhere (Travlou et al 2015(Travlou et al , 2016c.…”

“…The objective of this paper is to present the summary of the results obtained so far in our laboratory on the ammonia sensing capability of nanoporous carbons containing O-, S-and N-heteroatoms, and to show how their surface chemical features and pore structure afect the electrical response, sensitivity and selectivity (Travlou et al 2015(Travlou et al , 2016b. Considering that according to the Occupational Safety and Health Administration (OSHA) 50 ppm of NH 3 is the permissible exposure limit over an 8-hours work-day, and 500 ppm of NH 3 is the maximum short exposure tolerance (OSHA 2016), we decided to carry out the sensing tests at this speciic concentration range.…”

“…The mechanism governing the reversible sensing involved several processes, including speciic interactions between NH 3 and surface functional groups, pore-illing with NH 3 /NO 2 , and electron-hole conductivity. The structural and chemical features of the of gas molecules with surface functional groups, and the physical adsorption of these molecules in a pore system Travlou et al 2015Travlou et al , 2016b. The incorporation of speciic heteroatoms to a carbon matrix is an efective way of altering carbons' chemical and structural features, and controlling their electronic properties (Kiciński et al 2014).…”

“…Therefore various functionalization processes such as high cost metal/metal oxide doping or polymer coating, are usually required (Huang et al 2012;Nguyen et al 2013;Xiang et al 2015). The complexity of surface modiications targeting an improved selectivity directed our attention to the application of nanoporous (microporous) carbons for the irst time as toxic gas sensors Travlou et al 2015). They may have a limited conductivity compared to the above-mentioned carbon-based materials, however, they exhibit certain advantages such as an extensive surface area, developed internal porous structure, and rich surface chemistry, which favor their application as gas sensors (Gonçalves et al 2011;Luo et al 2015;Travlou et al 2015).…”

“…The complexity of surface modiications targeting an improved selectivity directed our attention to the application of nanoporous (microporous) carbons for the irst time as toxic gas sensors Travlou et al 2015). They may have a limited conductivity compared to the above-mentioned carbon-based materials, however, they exhibit certain advantages such as an extensive surface area, developed internal porous structure, and rich surface chemistry, which favor their application as gas sensors (Gonçalves et al 2011;Luo et al 2015;Travlou et al 2015). Especially the latter feature allows a better selectivity without the need for further functionalization/doping processes.…”