Abstract
The chemical modification of multiwalled carbon nanotubes (MWCNTs) with a long chain mercapto acid is reported as a way to improve sensitivity and response time of gas sensors for detecting alcohols, acetone and toxic gases such as DMMP. We have developed sensors employing MWCNTs decorated with gold nanoparticles and modified with a 16-mercaptohexadecanoic acid (MHDA) monolayer. Morphological and compositional analysis by Transmission Electron Microscopy (TEM), Fourier Transform Infra-red Spectroscopy (FTIR) and X-ray photoelectron spectroscopy were performed to characterize the gold nanoparticles and to check the bonding of the thiol monolayer. The detection of aromatic and non-aromatic volatiles and DMMP vapors by MWCNT/Au and MWCNT/Au/MHDA shows that the presence of the self-assembled layer increases sensitivity and selectivity towards non-aromatics. Furthermore, it ameliorates response dynamics, and significantly reduces nitrogen dioxide and moisture cross-sensitivity.
Highlights
The monitoring of the environment requires devices that must be fast, sensitive, stable and selective to detect the pollutants and toxic gases/vapors in a simple and efficient way[1,2,3]
In this paper we develop resistive gas sensors employing films of gold nanoparticle decorated, multiwalled carbon nanotubes (MWCNT/Au) functionalized with a self-assembled monolayer of 16 Mercaptohexadecanoic acid (MHDA), designated as MWCNT/Au/MHDA
Samples of the Au-decorated MWCNT films were mechanically removed from the alumina sensor substrate and deposited on standard copper Transmission electron microscopy (TEM) grids
Summary
In non-functionalized nanotubes (i.e. MWCNT/Au), both stronger binding between VOCs and surface defects generated on the outer wall of CNTs by the oxygen plasma treatment and direct adsorption of VOCs on Au NPs occurs[51], which explains the slower response and recovery kinetics This is the case for MWCNT/ Au sensors detecting aromatic volatiles in which stronger non-covalent interactions (i.e., π-π) exist between the Figure 6. The baseline resistance of this sensor is not heavily affected when the relative humidity changes between 50% and 80% This is possibly due to the formation, at humidity levels below 50%, of a monolayer of water molecules, adsorbed at the hydrophilic terminals of the MHDA molecules grafted to CNTs. To further study the effect of moisture on sensor response, the detection of 20 ppm of ethanol in air humidified at 50% R.H was measured. A stronger binding between DMMP and the surface of the MWCNT/Au sensor takes place because the presence of baseline drift can be observed, together with slower response kinetics
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