A Review of Carbon Nanotubes-Based Gas Sensors

Table 1

Equilibrium tube-molecule distance (𝑑), adsorption energy (𝐸_𝑎), and charge transfer (𝑄) of various molecules on (10,0), (17,0), and (5,5) individual SWNTs^a. The optimal adsorption sites are given in the table: T (top of a carbon atom), B (top of the centre of the C–C bond), C (top of the centre of carbon hexagon) [27]. (a)Tube-molecule distance d is defined as the nearest distance between atoms on the molecule and the nanotube for T site, or the distance between the centre of the gas molecule and the centre of the carbon hexagon (carbon-carbon bond) for the C (B) site. The adsorption energy 𝐸_𝑎(𝑑) is defined as the total energy gained by molecule adsorption at equilibrium distance: 𝐸_𝑎(𝑑)=𝐸_tot(tube+molecule)−𝐸_tot(tube)−𝐸_tot(molecule). Charge transfer (𝑄)denotes the total Mulliken charge number on the molecules, positive (𝑄) means charge transfer from molecule to tube.

	NO2	O2	H2O	NH3	CH4	CO2	H2	N2	Ar
(10,0) SWNT
𝑑 (Å)	1.93	2.32	2.69	2.99	3.17	3.20	2.81	3.23	3.32
𝐸𝑎 (meV)	797	509	143	149	190	97	113	164	57
𝑄 (e)	−0.061	−0.128	0.035	0.031	0.027	0.016	0.014	0.008	0.01
Site	T	B	T	T	C	C	C	C	C
(5,5) SWNT
𝑑 (Å)	2.16	2.46	2.68	2.99	3.33	3.54	3.19	3.23	3.58
𝐸𝑎 (meV)	427	306	128	162	122	109	84	123	82
𝑄 (e)	−0.071	−0.142	−0.033	0.033	0.022	0.014	0.016	0.011	0.011
Site	T	B	T	T	C	C	C	C	C
(17,0) SWNT
𝑑 (Å)	2.07	2.50	2.69	3.00	3.19	3.23	2.55	3.13	3.34
𝐸𝑎 (meV)	687	487	127	133	72	89	49	157	82
𝑄 (e)	−0.089	−0.096	0.033	0.027	0.025	0.015	0.012	0.006	0.01
Site	T	B	T	T	C	C	C	C	C