Nanotube-based gas sensors – Role of structural defects

Abstract

Existing theoretical literature suggests that defect-free, pristine carbon nanotubes (CNTs) interact weakly with many gas molecules like H 2 O, CO, NH 3 , H 2 , and so on. The case of NH 3 is particularly intriguing because this is in disagreement with experimentally observed changes in electrical conductance of CNTs upon exposure to these gases. In order to explain such discrepancy, we have carried out Density Functional Theory (DFT) investigations of the role of common atomistic defects in CNT (Stone-Wales, monovacancy, and interstitial) on the chemisorption of NH 3 . Computed binding energies, charge transfer, dissociation barriers, and vibrational modes are compared with existing experimental results on electrical conductance, thermal desorption and infrared spectroscopy. PACS: 61.46.+w, 85.35 [2,[9][10][11]. This is in disagreement not only with the observed drop in electrical conductance at room temperature, but also with recent temperature-programmed-desorption (TPD) measurements [12], which shows that most of the gas desorbs only at elevated temperatures (500-700 K).A logical resolution to the above problem would be to note that SWNTs are likely to have some defects incorporated either thermally or during high-temperature growth conditions. Also, they are not in isolation, and surrounding environment (oxygen, water vapor), or the substrate, or metal contacts at the ends might directly or indirectly provide a mechanism of binding of the gas molecules. Limited 2 investigations have been made on the effect of metal contacts, especially in connection with O 2 adsorption [13,14]. However, NH 3 adsorption appears to occur along the whole length of the tube [1], which suggests either topological defects on the tube itself, or an indirect mechanism due to involvement of solvents or the substrate.In this Letter, we investigate the role of three types of topological defects commonly considered for SWNTs and graphene sheet, i.e., a Stone-Wales (SW) defect [15] (i.e. a 5775 defect formed by bondrotation), a monovacancy (i.e. missing a C-atom), and an insterstitial (i.e. an extra bridging C). The presence of such geometric defects has recently been confirmed experimentally [16]. As a concrete model of s-SWNT, we chose the (8, 0) SWNT. All the defect and chemisorbed structures were created by suitably modifying the original defect-free tube (Fig. 1a). In order to speed up our calculations the initial structures were minimized with the COMPASS forcefield [17], followed by further relaxation with the DFT code (Fig. 1d); (ii) SW2, a chiral Stone-Wales defect (Fig. 1f); (iii) a monovacancy (V) (Fig. 1b); and (iv) an interstitial (I) (Fig. 1c). It also considers the situation in which an O 2 molecule has dissociated over a SW1 (Fig. 1e) [24]. Table 1 lists the respective DMol 3 -computed formation energies of these defects for supercells of two sizes, i.e., two and four times the smallest periodic segment of the (8, Table 1, we note that SW1 is energetically more stable than SW2, and is therefore the only SW d...

Adsorption of Gas Molecules on Graphene Nanoribbons and Its Implication for Nanoscale Molecule Sensor

“…From and O 2 are relatively strong electron-withdrawing molecules while NH 3 is relatively strong electron-donating molecule in other carbon-based materials [4,5,15,27,29]. Comparing with gas adsorption on the graphene (GNR) surface [27], the values of the charge transfer are much larger.…”

“…It is well known that DB defects around the vacancy sites or at the tips play a very important role in CNTs gas sensors because they are very chemically reactive [28,29,30]. Similar to CNTs, when there are DB defects at GNRs edges, covalent attachment of chemical groups and molecules also significantly influences their electronic properties [22,24,25].…”

Density functional theory molecular modelling and experimental particle kinetics for CO2–char gasification

“…The Boudouard reaction [31,93] mechanism was examined by application of TS theory, based on a generalized synchronous transit scheme, which Ereac and Eb [94][95][96] from DFT modelling of the char-CO2 gasification reaction using the 3x3 model. Transition state theory enables consideration of transition rates in diffusion reactions and also energy profiles between reactants and products, as discussed in [73,[97][98][99]; the TS for a reaction is the geometry corresponding to the highest energy along the MEP, which gives the activation energy.…”

Comparison of Gas Sensors Based on Oxygen Plasma-Treated Carbon Nanotube Network Films with Different Semiconducting Contents

“…These functional groups formed on the CNT surface are known to significantly increase the interaction of CNTs with gas molecules. [15][16][17][18] CNT surfaces have been modified by changing their chemical composition using chemical and plasma treatments. [19][20][21] Although both treatments introduce oxygenated functional groups (-COOH, C=O, C-O, etc.)…”