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This article was downloaded by: [Mollaamin, Fatemeh]On: 15 January 2010Access details: Access Details: [subscription number 918635969]Publisher Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Fullerenes, Nanotubes and Carbon NanostructuresPublication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t713597253
Thermodynamic Investigation of Enol↔Keto Tautomerism for AlcoholSensors Based on Carbon Nanotubes as Chemical SensorsM. Monajjemi a; L. Mahdavian b; F. Mollaamin c; B. Honarparvar a
a Department of Chemistry, Science & Research Branch, Islamic Azad University, Tehran, Iran b
Department of Chemistry, Islamic Azad University, Doroud, Iran c Department of Chemistry, QomBranch, Islamic Azad University, Qom, Iran
Online publication date: 15 January 2010
To cite this Article Monajjemi, M., Mahdavian, L., Mollaamin, F. and Honarparvar, B.(2010) 'ThermodynamicInvestigation of Enol↔Keto Tautomerism for Alcohol Sensors Based on Carbon Nanotubes as Chemical Sensors',Fullerenes, Nanotubes and Carbon Nanostructures, 18: 1, 45 — 55To link to this Article: DOI: 10.1080/15363830903291564URL: http://dx.doi.org/10.1080/15363830903291564
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Thermodynamic Investigation of Enol «KetoTautomerism for Alcohol Sensors Based on Carbon
Nanotubes as Chemical Sensors
M. Monajjemi,1 L. Mahdavian,2 F. Mollaamin,3 and B. Honarparvar1
1Department of Chemistry, Science & Research Branch, Islamic Azad University,
Tehran, Iran2Department of Chemistry, Islamic Azad University, Doroud Branch, Doroud,
Iran3Department of Chemistry, Qom Branch, Islamic Azad University, Qom, Iran
Abstract: Single-walled carbon nanotubes (SWNT) are molecular-scale wires with
high mechanical stiffness and strength. The faster response and substantially
higher sensitivity of chemical nanotubes sensors over existing solid-state sensors,
which was due to exposure of gaseous molecules such as C2H5OH, served as the
basis to consider gas sensors based on SWNT. These factors also focused on the
ethanol sensing effects of the sensors on the acetaldehyde and ethyl methyl ketone
tautomerism at room temperature, which have been formed within alcohol
dehydrogenation and condensation reactions involved in chain growth pathways
on SWNT.
Since temperature has a strong effect on structure and stability, we were
interested in calculating some specific thermo chemical properties to justify the
structural stability of existing isomers in keto«enol tautomerism. So, the
thermodynamic and electric properties of produced ethyl-methyl ketone involved
in keto«enol tautomerism have been investigated using semi-empirical methods.
The obtained results showed that the keto structure is more stable because of
intramolecular hydrogen bond formation due to strong C5O bond existence in
the keto tautomers. Our finding implies that the semi-empirical quantum
chemical calculations can be suggested as proper methods to study the interaction
of different chemical compounds with SWNT in a reliable way.
Keywords: Acetaldehyde, Electrical resistance, Ethanol sensor, Ethyl methyl
ketone, keto and enol tautomerism, Single-walled carbon nanotube, Thermo-
dynamic functions
Address correspondence to Dr. Fatemeh Mollaamin, Department of Chemistry,
Qom Branch, Islamic Azad University, Qom, Iran. E-mail: [email protected]
Fullerenes, Nanotubes and Carbon Nanostructures, 18: 45–55, 2010
Copyright # Taylor & Francis Group, LLC
ISSN 1536-383X print/1536-4046 online
DOI: 10.1080/15363830903291564
45
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1. INTRODUCTION
Sensors continue to make a significant impact on everyday life. There has
been a strong demand for producing highly selective, sensitive,
responsive, and cost-effective sensors (1).
Carbon nanotubes (CNTs) have many distinct properties that may
be exploited to develop the next generation of sensors. Utilization of
these properties has led to applications of individual nanotubes or
ensembles of nanotubes as scanning probes (2, 3), electron field emission
sources (4), actuators (5), and nanoelectronic devices (6).
Here we report the realization of individual semiconducting single-
walled carbon nanotube (S-SWNT)-based chemical sensors capable of
detecting small concentrations of alcohol gas molecules through quantum
mechanical calculations of thermochemical and other electronic struc-
tural parameters. The nanotube sensors exhibit a fast response and a
substantially higher sensitivity than that of existing solid-state sensors at
room temperature. Sensor reversibility is achieved by slow recovery
under ambient conditions or by heating to high temperatures (7). The
interactions between molecular species and SWNTs and the mechanisms
of molecular sensing with nanotube molecular wires have been of great
interest to the authors.
Ethanol is the most popular member of the alcohol family because
of its rare blend of useful and harmful effects. This is the only alcohol
most people encounter in their daily lives. Detection and control of
ethanol is necessary for the well being of society. Ethanol sensors are
useful in various areas such as the control of drunken driving and the
monitoring of fermentation and other processes in chemical industries.
Development of ethanol sensors based on thin film technology offers
the advantages of greater sensitivity, shorter response time and lower
costs.
The keto«enol tautomerism is one of the most common investigated
subjects of isomerism that occurs for carbonyl compounds (8–12).
Sensors are devices that detect or measure physical and chemical
quantities such as temperature, pressure, sound and concentration. The
measured are converted into an electrical signal. The main requirements
of a good sensor are high sensitivity, fast response, low cost, high volume
production and high reliability (see Figure 1).
In this work, gas sensors based on SWNT have been considered, and
we focused on the ethanol sensing effects of the sensors on the
acetaldehyde and ethyl methyl ketone tautomerism at room temperature.
Generally, the keto form is more stable than the enol form for neutral
systems. The performed calculations illustrated the effects of the metallic
additives and of the adsorbed molecule on the grain binding strength and
its conductance.
46 M. Monajjemi et al.
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2. COMPUTATIONAL METHODS
Semi-empirical calculations are set up with the same general structure as
HF calculations in that the former have a Hamiltonian and a wave
function. Within this framework, certain pieces of information are
approximated or completely omitted. Usually, the core electrons are not
included in the calculation, and only a minimal basis set is used. Also,
some of the two-electron integrals are omitted. To correct for the errors
introduced by omitting part of the calculation, the method is
parameterized (12).
The advantage of semi-empirical calculations is that they are much
faster then ab initio calculations. The modified neglect of diatomic
overlap (MNDO) method has been found to give reasonable qualitative
results for many organic systems (12).
Transport properties can be expressed in terms of the submatrix S12
describing transmission through the wire. For example, the electrical
conductance G is given by the Landauer formula (13) as:
G~2e2
h
ðdE
{Lf Eð ÞLE
Tr S{12 Eð ÞS12 Eð Þ
n oð1Þ
where f(E) is the Fermi–Dirac distribution function and S(E) is the
scattering matrix. Similarly, thermodynamic properties can be expressed
in terms of the scattering matrix through the electronic density of states
(DOS) D(E) (13) as:
D Eð Þ~ 1
2piTr S{ Eð Þ LS
LE{
LS{
LES Eð Þ
� �ð2Þ
from which the relevant thermodynamic potential for an open system,
namely, the grand canonical potential V, is obtained as (13):
V~{kBT
ðdED Eð Þ ln 1ze
{E{mð ÞkBT
h ið3Þ
Figure 1. Cross sectional structure of the SWNT field effect transistors (FET-
based sensor) and the experimental geometry (7).
Alcohol Sensors Based on Carbon Nanotubes 47
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where kB is the Boltzmann constant, T is the temperature and m the
chemical potential specified by the macroscopic electrodes.The geometry optimizations were performed with semi-empirical
calculations using the Gaussian A7 package (14). The accuracy of the
semi-empirical quantum mechanics method depends on the database
used to parameterize the method. We can use the information obtained
from semi-empirical calculations to investigate many thermodynamic and
kinetic aspects of chemical processes. Energies and geometries of
molecules have clear relationships to chemical phenomena (15, 16).
3. RESULTS AND DISCUSSION
Semiconducting sensors offer an inexpensive and simple method for
monitoring gases. The change of the electrical conductivity of semi-
conducting materials upon exposure to reducing gas C2H5OH has been
used for gas detection. Therefore, CNTs have been used as sensingmaterials in pressure, flow, thermal, gas, optical, mass, position, stress,
strain, chemical and biological sensors (17–19). The sensor was based on
the principle that the electronic properties of CNTs change when
subjected to strains. The isotropic nature of CNT films helps in
measuring strains in multiple locations and in different directions. This
study demonstrated using first-principle quantum transport calculations,
semi-empirical with MNDO method that hydrostatic pressure can induce
radial deformation and, therefore, electrical transition of SWNTs. Themeasurements were carried out in a humid air atmosphere, which is the
same condition of general operation of a sensor.
3.1. Interaction of Ethanol with SWNT
We have investigated the formation of the two products observed most
commonly in significant amounts, namely, acetaldehyde, Ethyl methylketone as follows:
The reaction between the reducing gases led to the decrease of the
carrier-hole density in the surface charge layer and thus the increase in
the SWNT resistance. Also, the reduction in tube length and diameter
enhances the sensitivities of sensors (20), in this study, we have
considered SWNT armchair (4,4).
3.2. Acetaldehyde Formation
The characteristics of sensor have been investigated by changing of
resistance in ethanol with different products in air. Formation of
48 M. Monajjemi et al.
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acetaldehyde by the oxidative dehydrogenation of ethanol depends upon
the reaction step that requires the oxide surface to acquire a negative
charge; in Figure 2, the ball stick model shows this reaction. It is well
accepted that upon adsorption the O–H bond of the alcohol dissociates
hydrolytically to yield an ethoxide and a hydrogen molecule as follows
(20):
When ethanol is injected into the micro-reactor containing either pure
SWNT compound a large fraction of ethanol that was found to be
transformation to acetaldehyde. It then may be adsorbed in the form of
ethoxide (CH3CH2O), producing an adsorbed hydrogen atom. This
transformed to acetaldehyde (CH3CHO) forming an adsorbed hydrogen
atom (20).
CH3CH2OH gð Þ?CH3CH2OadszHads
CH3CH2Oads?CH3CHOadszHads
HadszHads?2Hzadsz2e
The adsorbed hydrogen atoms then loose electrons to the conduction
band of SWNT that electrical transition of SWNTs were increased. The
generated protons (H+) join together and eliminate H2. During the above
dehydrogenation process, one net electron is released into the conduction
band of SWNT, reducing its resistance. The graph of electrical resistance
(V) of ethanol and acetaldehyde formation using SWNT is displayed in
Figure 2. Schematic diagram of the CNT array ethanol, acetaldehyde formation.
Alcohol Sensors Based on Carbon Nanotubes 49
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Figure 3. As can be seen, the electrical resistance (V) of ethanol and
acetaldehyde formation on SWNT has been increased from 20.1 to 2.0.2
in time intervals from 0 to 2.5. This increasing trend is due to the
generated electrons from the adsorbed hydrogen atoms to the conduction
band of SWNT that led to increase in electrical transition of SWNTs.
After that it remains constant for some time and then decreased in
reference to product formation of considered reaction.
The thermodynamic properties related to this interaction between the
molecules of ethanol and carbon nanotubes have been calculated through
quantum mechanical calculations of IR vibrational frequencies imple-
mented in G98 program using semi-impractical method as listed in
Table 1. The heat formation energy for this interaction is exothermic,
increasing to 6.20 MJ/mol and then decreasing to 5.90 MJ/mol and
yielding the lowest value.
3.3. Ethyl Methyl Ketone Formation
In the environment of reaction, there are ethanol and acetaldehyde where
ethyl methyl ketone represents the second most important reaction
product in our calculations. This reaction requires proton transfer from
one adsorbed acetaldehyde, which becomes oxidized, to another
adsorbed ethanol, which is reduced to alkoxide. The thermodynamic
parameters of interaction between ethanol (ethanol and acetaldehyde on
SWNT (4,4)-based sensor to ethyl methyl ketone) and acetaldehyde using
SWNT (4,4) have been reported in Table 2. According to the results,
Figure 3. The graph of electrical resistance (V) of ethanol and acetaldehyde
formation versus time (s) using SWNT.
50 M. Monajjemi et al.
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along with transition from step 1 up to step 15, the more negative values
have been yielded for E total and Gelec which shows the exotermic
characteristic of considered reaction.
This process may form a complex in a transition state that requires
participation of the surface oxygen: where an adsorbed acetaldehyde
molecule with the participation of a surface oxygen anion transfers a
hydride to another adsorbed ethanol molecule, which has been shown in
Figure 4. In the sensor, we have set the sensor signals based on the
relative changes of the electrical resistance (V) so we had to convert
calculated data (see Table 2) to the electrical resistance (V) shown in
Figure 5.
Table 1. The thermodynamic properties of interaction between ethanol (ethanol
on SWNT (4,4)-based sensor to acetaldehyde) with SWNT (4,4)
Steps E total
(MJ/mol)
Eelec
(V)
E bin
(MJ/mol)
H
(MJ/mol)
Gelec
(MJ/mol)
E nuc
(MJ/mol)
1 21046.14 29168.12 250.87 7.05 215604.09 14557.95
2 21109.01 29769.32 252.16 6.26 216627.33 15518.32
3 21173.11 210576.2 255.20 6.21 218000.66 16827.56
4 21173.23 210578.8 255.33 6.09 218005.10 16831.87
5 21173.12 210690.5 255.22 6.20 218195.23 17022.10
6 21173.40 210684.9 255.50 5.92 218185.71 17012.31
7 21173.42 210654.3 255.52 5.90 218133.50 16960.08
Table 2. The thermodynamic properties of interaction between ethanol
(Ethanol and acetaldehyde on SWNT (4,4)-based sensor to ethyl methyl
ketone) and acetaldehyde using SWNT (4,4)
Steps E total
(MJ/mol)
Eelec
(V)
E bin
(MJ/mol)
H
(MJ/mol)
Gelec
(MJ/mol)
E nuc
(MJ/mol)
1 21046.14 29168.12 250.87 7.05 215604.09 14557.95
2 21109.01 29769.32 252.16 6.26 216627.33 15518.32
3 21147.88 210456.58 232.81 30.70 217797.05 16649.17
4 21173.57 210777.76 258.50 4.98 218343.70 17170.14
5 21172.66 210836.47 257.59 5.89 218443.62 17270.96
6 21173.39 210910.52 258.32 5.15 218569.65 17396.26
7 21171.85 210949.36 256.78 6.70 218635.75 17463.9
8 21173.40 210907.19 258.33 5.15 218563.98 17390.58
9 21172.13 210972.07 257.06 6.41 218674.41 17502.28
10 21174.27 210952.52 259.20 4.28 218641.13 17466.87
11 21173.70 210998.95 258.63 4.85 218720.17 17546.47
12 21172.15 210934.97 257.09 6.39 218611.28 17439.12
13 21172.01 210847.56 256.94 6.54 218462.49 17290.49
14 21172.31 210607.50 257.25 6.23 218053.92 16881.61
15 21172.65 210613.79 257.59 5.89 218064.62 16891.96
Alcohol Sensors Based on Carbon Nanotubes 51
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A current versus voltage curve recorded with a SWNT sample after
time exposure to ethanol showed upfold conductance depletion.
Exposure to ethanol molecules increased the conductance of the SWNT
sample. The SWNT is a hole-doped semiconductor, as can be gleaned
from the current versus gate voltage curve shown in Figure 5 (middle
curve), where the resistance of the SWNT is observed to be decreased.
Figure 4. Schematic diagram of the CNT array ethanol, ethyl methyl ketone
formation.
Figure 5. Electrical resistance (V) of from ethanol with SWNT and ethyl methyl
ketone formation.
52 M. Monajjemi et al.
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Tautomerism refers to the equilibrium between two different
structures of the same compound. Usually the tautomer forms differ in
the point of attachment of a hydrogen atom. One of the most common
examples of a tautomeric system is the equilibrium between a ketone and
its enol form due to flexibility of the C5C—O chain in the enol form and
convergence of the C—C5O chain in the keto:
This result revealed a weakly formed enol species in proximity of an
ethoxide species in an unstable configuration. Both species react together
and yielded an ethyl methyl ketone molecule. Therefore, it can be
deduced that ethyl methyl ketone has a more active surface reaction than
ethanol on SWNT and results in higher sensitivity and a quicker response
time.
3.4. The Other Thermodynamic Properties
Thermodynamic equilibrium constants (K) for these interactions were
calculated by the related standard Gibbs free energy difference (DGelec):
K~exp {DGelec=RTtrð Þ ð1Þ
where Ttr is the transition temperature and R is gas constant. According
to our results, the keto structure is the most stable form in ethyl methyl
ketone, but the enol structure is the most stable form for it keto « enol
tautomerism (20). The equilibrium constant can be defined as follows:
K~keto½ �enol½ � ð2Þ
where [enol] and [keto] represent the molar concentration of the enol and
keto tautomers, respectively. The entropy difference (DS) at the phase
transitions may be given by:
DS~DG
Ttr
ð3Þ
where DH is the electronic enthalpy difference (20, 21). The thermo-
dynamic properties of interaction of ethanol on SWNT (4,4)-based
Alcohol Sensors Based on Carbon Nanotubes 53
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sensor have been listed in Table 3. Based on these obtained thermo-
dynamic parameters, the conformational preference or the relative
stability of tautomers can be distinguished. According to the reported
data, it can be inferred that the equilibrium constant (K) of the second
reaction (CH3CH2COCH3 + H2O), which contains the keto form is much
higher than other cases (K57.99 61046). In addition, the most negative
free Gibs energy (DG5 2267.57 MJ/mol) and the most negative enthalpy
(DH5 224.77MJ/mol) as well as the most positive entropy (DS5
0.81MJ/mol) was related to this case.
4. CONCLUSION
The current research sets a goal to investigate the interactions between
the molecules of ethanol and carbon nanotubes. In this work, gas sensors
based on SWNT-based chemical sensors capable of detecting small
concentrations of alcohol gas molecules have been considered. We
focused on the ethanol sensing effects of the sensors on the acetaldehyde
and ethyl methyl ketone tautomerism through using semi-empirical
calculations of thermochemical and other electronic structural para-
meters of produced ethyl-methyl ketone involved in keto«enol
tautomerism. These calculations have been carried out to find the most
preferable tautomer in keto«enol tautomerism which fulfills the
structural stability.
The sensitivity of carbon nanotubes is affected by ethanol
concentration at specific room temperature. The sensitivity increases
with increasing ethanol gas concentration. The response and sensitivity of
the alcohol-treated sensor were found to be very high compared with the
air-treated sensor.
Development of ethanol sensors based on thin film technology offers
the advantages of greater sensitivity, shorter response time and lower
costs. The sensors developed with alcohols were found to have good
selectivity and sensitivity and were unaffected by various environmental
conditions.
Table 3. The thermodynamic properties of interaction of ethanol on SWNT (4,
4)-based sensor
Products DG0elec
(MJ/mol)
K DH
(MJ/mol)
DS
(MJ/mol)
CH3CHO+H2 904.13 3.36102159 5.61 23.02
CH3CH2COCH3+H2O 2267.57 7.9961046 224.77 0.81
enol«keto tautomerism 263.13 1.1761011 20.02 0.21
54 M. Monajjemi et al.
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