and CO Gas Sensors - American Chemical Society

Article pubs.acs.org/JPCC

DFT Study of Polyaniline NH3, CO2, and CO Gas Sensors: Comparison with Recent Experimental Data Habib Ullah,† Anwar-ul-Haq Ali Shah,*,† Salma Bilal,‡ and Khurshid Ayub*,§ †

Institute of Chemical Sciences, University of Peshawar, 25120 Peshawar, Pakistan National Centre of Excellence in Physical Chemistry, University of Peshawar, 25120 Peshawar, Pakistan § Department of Chemistry, COMSATS Institute of Information Technology, University Road, Tobe Camp, 22060 Abbottabad, Pakistan ‡

S Supporting Information *

ABSTRACT: Density functional theory studies (DFT) have been carried out to evaluate the ability of polyaniline emeraldine salt (PANI ES) from 2 to 8 phenyl rings as sensor for NH3, CO2, and CO. The sensitivity and selectivity of nPANI ES among NH3, CO2, and CO are studied at UB3LYP/ 6-31G(d) level of theory. Interaction of nPANI ES with CO is studied from both O (CO(1)) and C (CO(2)) sides of CO. Interaction energy, NBO, and Mulliken charge analysis were used to evaluate the sensing ability of PANI ES for different analytes. Interaction energies are calculated and corrected for BSSE. Large forces of attraction in nPANI ES-NH3 complexes are observed compared to nPANI ES−CO2, nPANI ESCO(1), and nPANI ES-CO(2) complexes. The inertness of +CO− in nPANI ES-CO(1) and nPANI ES-CO(2) complexes are also discussed. Frontier molecular orbitals and energies indicate that NH3 changes the orbital energy of nPANI ES to a greater extent compared to CO2, CO(1), and CO(2). Peaks in UV−vis and UV−vis−near-IR spectra of nPANI ES are blue-shifted upon doping with NH3, CO2, CO(1), and CO(2) which illustrates dedoping of PANI ES to PANI emeraldine base (PANI EB). Finally, it is concluded that PANI ES has greater response selectivity toward NH3 compared to CO2 and CO and it is consistent with the experimental observations. protection,15 electro-optic16 and electrochromic devices,17 solar cells,18 and fuel cells.19 The ability of conducting polymer to interact with various analytes with concomitant changes in the electronic and geometric properties of polymers makes CPs as potential sensor for a range of toxic molecules. The sensor behavior of CPs is quite important because several gases in the atmosphere are quite toxic and their concentration at ppm and/or ppb level can be detrimental for the human health. For example, ammonia, although a vital constituent in many industries such as fertilizers, plastic, petrochemical, textiles, pesticides, dyes, and related industries,20,21 is quite hazardous for human health beyond a concentration of 25 ppm in air. Moreover, ammonium hydroxide, produced from hydrolysis of ammonia causes irritation in eyes and respiratory tract. Similarly, CO2 is used in the carbonated beverage industry, production of carbonates, carbon monoxide, carboxylic acids, petroleum operations, and urea. However, its concentration approximately 50 000 ppm causes respiratory problems along with a number of other symptoms, such as headache, breathing difficulty,

1. INTRODUCTION One of the famous quotes of Alan G. MacDiarmid that “there are as many dif ferent types of polyaniline as there are people who synthesize it”1 is an inspiration for future upcoming scientists. The versatile applications of polyaniline (PANI) in various fields of life is because of the doping/dedoping chemistry,2 stability,3 cost effectiveness, ease of synthesis,4 and processing.5 Based on the synthesis procedure (either chemical or electrochemical), dopants, and supporting electrolytes, PANI has three different oxidation states: PANI leucoemeraldine base (LB), pernigraniline base (PNB), emeraldine base (EB), and the most promising one emeraldine salt (ES). A brief schematic presentation for the synthesis of PANI in different oxidation states is given in Scheme 1.6 Among the four different kinds of PANI, PANI ES is electrically conducting and this nature can be easily tuned by controlling pH, dopant concentration, interaction with atmospheric oxygen (under nitrogen atmosphere), and voltage (in the electrochemical method).4 Other forms of PANI such as LB, EB, and PNB are reduced, semioxidized, and fully oxidized, respectively.7 This tunable nature of PANI ES has made it a promising material in a wide range of technological applications such as in batteries,8,9 actuators,10 electromagnetic shielding,11 chemical sensors,12 drug delivery,13 catalysis,2 antistatic coating,14 corrosion © 2013 American Chemical Society

Received: July 18, 2013 Revised: October 5, 2013 Published: October 14, 2013 23701

dx.doi.org/10.1021/jp407132c | J. Phys. Chem. C 2013, 117, 23701−23711

The Journal of Physical Chemistry C

Article

Scheme 1. Synthesis of PANI and Its Different Forms

from both sides, nPANI ES-OC (CO(1)) and nPANI ES-CO (CO(2).

palpitation, dizziness, and weakness. CO2 is also well-known for its severe greenhouse effects.22 CO is a poisonous gas as well and its presence in atmosphere can interact with heme of the hemoglobin in human body which causes decline in oxygen transfer activity and ultimately death.23 About 5 ppm concentration of CO in atmosphere is dangerous for human health.24 Sensors are therefore required to detect the presence of poisonous gases at ppb/ppm level. A number of highly effective and selective sensors based on different mechanisms are proposed for these analytes.25−27 Sensors are notable and crucial for health, safety, and sustainable life. Sensors based on CP25,28−30 are more imperative as compared to metal oxide semiconductors, quartz crystal microbalance,31 surface acoustic wave,32 and metal oxide semiconductor field effect transistor.33 The literature reveals a numbers of experimental reports on the response study of CP at different concentrations (ppm) of organic and inorganic analytes. Since 1980s, CPs such as polypyrrole (PPy), polythiophene (PT) poly(3,4-ethylenedioxythiophene) (PEDOT), and PANI are used in the field of sensors.25 CP-based sensors are superior over other sensors because of their high sensitivities, short response time, good mechanical properties, and room temperature operation.34,35 Among CPs, PANI is the most intriguing due to its ease of synthesis and good response to guest molecules even at room temperature.4,36−38 Several experimental studies on PANI ES as NH3,39 CO2,25 HCl,4 cigarette smoke,40 and CO41 sensors are reported. The literature reveals that PANI ES is selective to NH3 among CO2, CO, and NH3. Although experimentally explored, theoretical study on the sensor ability and selectivity of PANI ES for NH3 among a variety of analytes such as NH3, CO2, and CO is not reported. The aspiration of the present theoretical work is to elucidate the response mechanism and selectivity of PANI ES at NH3, CO2, and CO through quantum mechanical calculation. The general reaction mechanisms through which PANI ES shows response to analytes X (X = NH3, CO2, and CO) are given in reactions I and II. PANI ES oligomers with 2−8 phenyl rings are used as sensing material for NH3, CO2, and CO. CO has stable resonance structure (−CO+), which means both sides have equal reactivity. Therefore, its sensor study is performed

PANI ES + X 0 → PANI EB + X +

(I)

PANI EB + X + → PANI ES + X 0

(II)

2. METHODS Density functional theory (DFT) at hybrid functional [Becke 3parameter (Exchange), Lee, Yang, and Parr both local and nonlocal (correlation; DFT)] (B3LYP)42 with a well-accepted basis set, 6-31G(d) using unrestricted formalism is used (DFTUB3LYP/6-31G(d).43−51 The DFT method has been preferred because of the accuracy associated. Calculations were performed on Gaussian 0952 and the results are visualized with Gabedit53 and Avogadro54 computer programs. nPANI ES oligomers are used, where n represents number of phenyl rings (n = 2, 4, 6, 8. All isolated oligomers of nPANI ES and their complexes with analytes, nPANI ES-X, are positively charged species and have doublet spin (charge = 1 and doublet state). Optimized geometries were obtained by gradient minimization at DFT55,56 method without any symmetry constraints and were considered complete when stationary point was located. The stationary points were further confirmed as true minima through frequency calculations (no imaginary frequency). Geometries, intermolecular interaction energies, excited-state properties such as UV−vis spectra, ionization potential (IP), electron affinities (EA), electronic properties (Mulliken charge analysis, natural bond orbital (NBO)), energy of highest occupied molecular orbital (HOMO), energy of lowest unoccupied molecular orbital (LUMO), and band gap are simulated at the above-mentioned level of theory, although NBO and Mulliken population analysis are basis set dependent and hence are considered not very reliable. We have simulated Mulliken and NBO charge analysis on different types of basis sets and concluded that they are basis set dependent properties. But, If we use the same type of level of theory for different complexes such as B3LYP/6-31G(d) or B3LYP/6-311++G(d,p), then the results will be fruitful. ΔSCF (TD-UB3LYP) and UB3LYP methods are used for band gap simulation. The interaction energies are counterpoise corrected (BSSE). Infinite polymer properties such as binding energies with analyte, IP, 23702



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6, and from 6 to 8PANI ES-NH3 complexes. Increase of dX6···H5 distance with chain length elongation illustrates decrease in interaction with chain elongation, which is believed due to the bulkiness of nPANI ES. The intramolecular distance dN2−H5 increases when nPANI-ES interacts with ammonia (Table 2). This bond elongation is believed to be due to creation of ion− dipole electrostatic interaction between nPANI ES and NH3 molecules. Another important geometric parameter in nPANI ES and nPANI ES-X is the bridging angle (∠C1N2C3) which reduces when nPANI ES interacts with NH3. The decrease in angle of about 2.27° in 2PANI ES-NH3, 1.81° in 4PANI ESNH3, 1.41° in 6PANI ES-NH3, and 1.19° in 8PANI ES-NH3 complexes compared to their corresponding noncomplexed oligomers is observed. The larger the decrease in angle, the stronger will be the electrostatic attraction between nPANI ES and NH3. From the analysis of the decrease in bond angles, one can infer that the interaction of nPANI ES with NH3 decreases with increase in chain length elongation, consistent with the inference from the bond length analysis (vide supra). This decrease in interaction with increase in chain length elongation can also be seen from interaction energy (Table 2). Another important intermolecular geometric parameter is the intermolecular angle (∠N2H5X6) which increases from about 130° (in nPANI ES) to about 180° in all nPANI ES-X complexes. The 180° angle (∠N2H5X6) means both reacting species became parallel in complex formation (nPANI ES-X). Intramolecular dihedral angle such as ∠C1C2N3C4 is a parameter for the coplanarity between adjacent phenyl rings (sensor) and analytes, and it increases in nPANI ES from 22.53° (2PANI ES) to 25.24° (8PANI ES) with increase in oligomer size (2 to 8PANI ES). This increase in dihedral angle ∠C1C2N3C4 is more pronounced on going from 2PANI ES to 4PANI ES compared to the one observed on going from 6PANI ES to 8 PANI ES. Large difference in this angle causes coplanarity in phenyl rings adjacent to analyte. Coplanarity between phenyl rings causes an easy electron transfer from NH3 to nPANI ES, and thereby results in the formation of nPANI EB (conductance decline), as shown in Scheme 1. Similar but less pronounced effects are observed in nPANI ES-CO2, nPANI ES-CO(1), and nPANI ES-CO(2) complexes compared on PANI ES-NH3 (Table 2). In nPANI ES-CO2 complexes, dX6···H5 distance decreases about 0.05 Å on going from 2 to 4PANI ES-CO2, 0.03 Å from 4 to 6PANI ES-CO2, and 0.02 Å from 6 to 8PANI ES-CO2 complex. This bond is 0.2 Å larger in all PANI-CO2 complexes compared to the corresponding ammonia complexes. Intramolecular bond distance dN2−H5 is about 0.024 Å smaller compared to nPANI ES-NH3 complexes. Both of these facts correspond to relatively weak interaction in nPANI ES-CO2 compared to nPANI ESNH3. A decrease in bond lengths would be expected for CO2 complexes with PANI, provided the interaction is of comparable strength because oxygen is smaller in size compared to nitrogen. Moreover, the bridging angle (∠C1N2C3), intermolecular angle (∠N2H5X6), and dihedral angle ∠C1C2N3C4) also support the less effective binding of nPANI ES to CO2 than NH3. These differences also have effect on the nature of interaction which is further elaborated in interaction energy calculation. This difference in interaction of PANI ES with ammonia and CO2 may be attributed to the nonpolar nature of the latter. Interaction of CO2 with PANI ES is dipole−induced dipole interaction whereas the interaction in the case of ammonia is ion−dipole interaction

EA, HOMO, and LUMO are obtained by extrapolating oligomers properties through second degree polynomial fit equation. Conductivity measurements (band gap) were estimated from the difference of LUMO and HOMO energies. The negative of HOMO and LUMO is estimated as IP57,58 and EA,42,59 respectively.

3. RESULTS AND DISCUSSION 3.1. Optimized Geometric Parameters. Individual geometries of PANI ES, analytes and PANI ES-X complexes were optimized at UB3LYP/6-31G*. Optimized geometry of 2PANI ES is taken as reference for other long-chain oligomer species (shown in Figure 1), and the optimized geometrical parameters

Figure 1. Optimized geometric structure of 2PANI ES.

such as bond lengths, bond angles, and dihedral angles of nPANIES (where n = 2, 4, 6, and 8) are shown in Table 1. It Table 1. Optimized Geometric Parametersa of nPANI ES (n = 2, 4, 6, and 8) Using B3LYP/6-31G(d) Level of Theory (Atomic Labels Are with Reference to Figure 1)

a

n

dN2−H5

∠C1N2C3

∠C1C2N3C4

2 4 6 8

1.015 1.013 1.011 1.011

130.89 130.62 129.95 129.67

22.53 23.009 24.085 25.244

dN2−H5 in Å, ∠C1N2C3 and ∠C1C2N3C4 Angles in deg.

can be seen that intramolecular bond distance (dN2−H5, see Figure 1 for numbering) slightly decreases with chain length elongation (Table 1). Bridging angle (∠C1N2C3) also decreases with increase of oligomeric size; 4PANI ES, 6PANI ES, and 8PANI ES have 0.27°, 0.94°, and 1.22° bridging angles, respectively, smaller than the corresponding angle in the uncomplexed polymer 2PANI ES. The decrease in bridging angle in larger oligomers is because of zigzag geometrical structure, compared to the planar geometry in smaller oligomers. Geometries of the optimized complexes deliver important information about the response mechanism of conducting polymers toward different analytes and therefore we have optimized the geometries nPANI ES-X (X = NH3, CO2, CO(1), and CO(2)) complexes. Optimized geometry of 4PANI ES-X is shown in Figure 2, while the rest of the optimized geometric structures are given in Figures S1−S3 of the Supporting Information). The important inter- and intramolecular optimized geometric parameters such as dX6···H5, dN2−H5 (Å), ∠C1N2C3, ∠N2H5X6, and dihedral angle ∠C1C2N3C4 (degrees) for the selected complexes are given in Table 2. Intermolecular distance (dX6···H5) shows remarkable changes with different analytes, and with size of the oligomers nPANI ES as well. In the case of nPANI ES-NH3 complexes, dX6···H5 distance increases gradually about 0.03 Å from 2 to 4 and 4 to 23703



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Figure 2. Optimized geometric structures of 4PANI ES-X, where X = NH3, CO2, CO(1), and CO(2).

CO) at UB3LYP/6-31G(d) level of theory. Two methods are employed for interaction energy measurements; simple interaction energy calculation (eq 1) and counterpoisecorrected energy method (BSSE, eq 2). With the use of finite basis set, basis set superposition errors (BSSE) occur (basis functions of atoms overlap) when atoms of interacting molecules approach one another. Counterpoise method corrects this energy as shown in eq 2.

Although CO has dipole moment (0.06 D see Table 3) compared to CO2, surprisingly its interaction with nPANI ES is weak compared to CO2. CO has two resonance structures: a carbene like and a dipolar one. We have considered both structures to study interaction of PANI ES with CO. The carbene like and dipolar structures are denoted as CO(1) and CO(2), respectively. The inertness of both CO(1) and CO(2) for PANI ES is quite evident from the analysis of dX6···H5, dN2−H5, ∠C1N2C3, ∠N2H5X6, and dihedral angle ∠C1C2N3C4. The intermolecular distance dX6···H5 of nPANI ES-CO(1) is identical to that of nPANI ES-CO2 complexes (Table 2); however, nPANI ES-CO(2) complexes have about 0.22 Å large intermolecular distance (dX6···H5) compared to nPANI ES-CO2. Large intermolecular distances are clear illustration of the fact that the reacting molecules have small attraction. CO2 has oxygen atoms available for interaction with PANI ES and, when compared with NH3, one should expect smaller bond distances if similar interactions are present because oxygen has a smaller radii than a nitrogen atom. However the calculated values are smaller in nPANI ES-NH3 compared to nPANI ES-CO2. 3.2. Interaction Energy and Counterpoise-Corrected Energy (BSSE). We have determined the interaction energy between nPANI ES and analyte molecules (NH3, CO2, and

ΔE int = Ereactant1 + Ereactant2 − Eproduct

(1)

ΔE int ,CP = ΔE int − E BSSE

(2)

Binding energies of nPANI ES-X complexes are listed in Table 2; 2PANI ES-NH3 complex has interaction energy of −15.75 kcal mol−1 and counterpoise-corrected energy is −15.06 kcal mol−1. The binding energy decreases with increase in the oligomer size, while this binding energy decreases to −13.42 kcal mol−1 (BSSE −12.73 kcal mol−1) in 4PANI ESNH3, −11.92 kcal mol−1 (BSSE −11.23 kcal mol−1) in 6PANI ES-NH3, and −10.91 kcal mol−1 (BSSE −10.29 kcal mol−1) in 8PANI ES-NH3 complexes. The interaction is believed to be ion−dipole in nature based on the interaction energy. An average difference of 0.7 kcal mol−1 is observed between the 23704



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Table 2. Optimized Geometric Parameters,a ΔEint, and ΔEint,CP of nPANI ES-X (n = 2, 4, 6, and 8) and QNBO/QMulliken of Analytes in nPANI ES-X Using UB3LYP/6-31G(d) Level of Theory (Atomic Labels Are with Reference to Figure 2)b species

dH5···X6

dN2−H5

∠C1N2C3

∠N2H5X6

∠C1C2N3C4

ΔEint

ΔEint,CP

QNBO

QMulliken

2PANI ES-NH3 2PANI ES CO2 2PANI ES CO(1) 2PANI ES CO(2) 4PANI ES-NH3 4PANI ES CO2 4PANI ES CO(1) 4PANI ES CO(2) 6PANI ES-NH3 6PANI ES CO2 6PANI ES CO(1) 6PANI ES CO(2) 8PANI ES-NH3 8PANI ES CO2 8PANI ES CO(1) 8PANI ES CO(2) ∞PANI ES-NH3 ∞PANI ES CO2 ∞PANI ES CO(1) ∞PANI ES CO(2)

1.89 2.09 2.19 2.33 1.92 2.14 2.24 2.36 1.95 2.17 2.27 2.39 1.97 2.19 2.29 2.41 − − − −

1.044 1.016 1.016 1.016 1.038 1.014 1.014 1.016 1.034 1.013 1.012 1.014 1.032 1.012 1.011 1.013 − − − −

128.62 130.72 130.62 130.07 128.81 130.58 130.36 129.92 128.54 129.98 129.92 129.49 128.48 129.73 129.60 129.24 − − − −

179.27 179.95 179.83 179.94 178.85 179.89 179.01 179.23 178.36 179.80 179.81 179.78 178.11 178.20 178.91 179.27 − − − −

24.74 21.97 22.47 23.17 23.92 21.75 22.58 23.38 25.57 23.49 24.11 23.98 24.33 24.45 24.86 25.19 − − − −

−15.75 −4.51 −2.88 −4.32 −13.42 −3.57 −2.38 −3.63 −11.92 −3.07 −2.07 −3.20 −10.91 −2.76 −1.88 −2.94 −7.63 −1.76 −1.23 −2.04

−15.06 −4.01 −2.51 −3.76 −12.73 −3.07 −2.00 −3.13 −11.23 −2.57 −1.69 −2.69 −10.29 −2.32 −1.56 −1.63 −7.10 −1.39 −0.99 0.315

0.068 0.014 0.012 0.029 0.060 0.012 0.010 0.022 0.055 0.011 0.008 0.022 0.051 0.009 0.008 0.020 − − − −

0.078 0.033 0.029 0.053 0.069 0.028 0.025 0.041 0.063 0.025 0.022 0.042 0.059 0.023 0.021 0.038 − − − −

a dN6···X5 and dN2−H5 in Å, ∠C1N2C3, ∠N2H5X6, and ∠C1C2N3C4 Angles in deg. bX denotes the atom of the analyte which reacts directly with nPANI ES, N in NH3, and O in CO2 and CO(1), and C in CO(2).

CO(1) is simulated to be −2.88 kcal mol−1 and its counterpoise-corrected energy is −2.51 kcal mol−1. Quite similar to PANI ES and ammonia or CO2 complexes, the binding energy of PANI ES with CO also decreases chain length elongation of oligomers. For example, binding energy of PANI ES is −2.38 kcal mol−1 (BSSE −2.00 kcal mol−1) in 4PANI ES-CO(1), −2.07 kcal mol−1 (BSSE −1.69 kcal mol−1) in 6PANI ES-CO(1), and −1.88 kcal mol−1 (BSSE −1.56 kcal mol−1) in 8PANI ES-CO(1). The interaction energies are indicative of weak dipole−dipole interactions. Binding energy of ∞PANI ES-CO(1) complex is −1.23 and −0.99 kcal mol−1 based on ΔEint and ΔEint,CP analysis, respectively. Intermolecular interaction energies of nPANI ES-CO(2) complexes reveal opposite results compared to their geometrical parameters. Interaction energy in 2PANI ES-CO(2) is −4.32 kcal mol−1 and its counterpoise-corrected energy is −3.76 kcal mol−1. Again, the same trends of decrease in binding energy in longer oligomer of PANI ES are observed, i.e., −3.63 kcal mol−1 (BSSE −3.13 kcal mol−1) in 4PANI ES-CO(2), −3.20 kcal mol−1 (BSSE −2.69 kcal mol−1) in 6PANI ESCO(2), and −2.94 kcal mol−1 (BSSE −1.63 kcal mol−1) in 8PANI ES-CO(2). These interaction energies clarify that CO can react via C side instead of O, and this weak electrostatic force is greater than that of nPANI ES-CO(1) complexes. Binding energy of ∞PANI ES-CO(2) complex is −2.04 and 0.315 kcal mol−1 based on ΔEint and ΔEint,CP analysis, respectively. Comparative analysis of ΔEint and ΔEint,CP in nPANI ES-X complexes (X = NH3, CO2, CO(1) and CO(2)) reveals that nPANI ES has greater response (selectivity) to NH3 than that of CO2, CO(1), and CO(2). A relatively large difference of about ±10 kcal mol−1 of interaction energy between nPANI ES-NH3 and nPANI ES-CO2 can be explained on the basis of electronegativity difference, high p character (sp3 hybridization), and high dipole moment in NH3 molecule compare to CO2 (see Table 3). On the other hand, nPANI ES has

Table 3. HOMO, LUMO, and of NH3, CO2, and CO analyte

HOMO

LUMO

dipole moment (D)

electronegativity

NH3 CO2 CO

−6.86 −10.06 −10.10

2.13 0.81 −0.60

1.91 0.00 0.06

−2.36 −4.62 −5.35

interaction energy and the counterpoise-corrected energy in nPANI ES-NH3 complexes. Binding energy of infinite PANI ES (polymer) with NH3, obtained through second-order polynomial fit equation, is calculated to be −7.63 and −7.10 kcal mol−1 (BSSE) (see Supporting Information).The electrostatic interaction energy of the 2PANI ES-CO2 complex is −4.51 kcal mol−1 and its counterpoise energy is −4.01 kcal mol−1, which is much lower than the binding energies for the 2PANI ES-NH3 complex. Like nPANI ES-NH3 complexes, the binding energy of nPANI ES-CO2 complexes also decreases with increase in the size of oligomers. 4PANI ES-CO2 has interaction energy of −3.57 kcal mol−1 (BSSE −3.07 kcal mol−1), 6PANI ES-CO2 has −3.07 kcal mol−1 (BSSE −2.57 kcal mol−1), and 8PANI ESCO2 has −2.76 kcal mol−1 (BSSE −2.32 kcal mol−1). The values of interaction energies indicate that instead of ion− dipole, a weak dipole−induced dipole interaction may exist for nPANI ES-CO2 complexes. In nPANI ES-CO2 complexes a difference of 0.5 kcal mol−1 of energy is observed between the simple energy and BSSE-corrected energies. Interaction energies between the infinite polymer and CO2 molecule are estimated from the second-order polynomial fit equation and these energies are −1.76 and −1.39 kcal mol−1 based on ΔEint and ΔEint,CP charge analysis, respectively. Interaction energy of nPANI ES-CO(1) complexes also reflects the inertness of CO toward nPANI ES. This behavior is due to its stable resonance structure −CO+ (unavailability of lone pair of electrons of oxygen to PANI ES). This is consistent with experimental results that nPANI ES has low sensor ability for CO.25 Intermolecular interaction energy in 2PANI ES23705



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Table 4. NBO Charge Analysis (in Unit of electrons) of nPANI ES and nPANI ES-X at UB3LYP/6-31G(d) Level of Theory species 2PANI 2PANI 2PANI 2PANI 2PANI 4PANI 4PANI 4PANI 4PANI 4PANI 6PANI 6PANI 6PANI 6PANI 6PANI 8PANI 8PANI 8PANI 8PANI 8PANI

ES ES-NH3 ES-CO2 ES-CO(1) ES-CO(2) ES ES-NH3 ES-CO2 ES-CO(1) ES-CO(2) ES ES-NH3 ES-CO2 ES-CO(1) ES-CO(2) ES ES-NH3 ES-CO2 ES-CO(1) ES-CO(2)

ring 1

ring 2

ring 3

ring 4

ring 5

ring 6

ring 7

ring 8

0.422 0.393 0.412 0.415 0.726 0.383 0.264 0.277 0.280 0.279 0.247 0.242 0.242 0.242 0.242 0.229 0.216 0.224 0.227 0.226

0.422 0.393 0.412 0.415 0.726 0.359 0.356 0.360 0.357 0.359 0.299 0.295 0.295 0.295 0.294 0.274 0.257 0.268 0.270 0.269

0.359 0.356 0.360 0.357 0.359 0.308 0.307 0.311 0.309 0.308 0.282 0.275 0.281 0.278 0.280

0.283 0.264 0.277 0.280 0.279 0.308 0.307 0.311 0.309 0.308 0.277 0.286 0.280 0.279 0.278

0.299 0.30 0.295 0.297 0.296 0.277 0.290 0.280 0.279 0.278

0.247 0.24 0.242 0.243 0.242 0.279 0.272 0.277 0.277 0.276

0.272 0.253 0.266 0.268 0.277

0.226 0.214 0.223 0.224 0.223

greater response (based on ΔEint and ΔEint,CP) to CO2 than CO(1), which is due to the stable resonance structure of CO, although CO2 has no dipole moment (Table 3). Large force of attraction established between the cationic form of nPANI ES and electronegative part of NH3 molecule indicates an ion− dipole/electrostatic force to exist there. These interactions are also supported by inter- and intramolecular bond distances of dN6···X5 and dN2−H5, respectively (vide supra). Interaction energy calculations also lead us to conclude that the dominant structure is CO(2) in the case of CO detection by nPANI ES. 3.3. NBO Charge Analysis. The amount of charge transfer between nPANI ES (sensors) and analytes is simulated at the UB3LYP/6-31G(d) level of theory. Analytes change the electronic properties of nPANI ES through charge transfer. This charge transfer will alter the resistance, band gap, and λmax of nPANI ES which are in fact measurements of sensitivity of interaction. Mulliken and NBO charge analysis of isolated and analyte (NH3, CO2, CO(1), and CO(2)) bound nPANI ES complexes are given in Tables 2 and 4. Interaction in nPANI ES-X complexes is established due to the transferring of electrons from analytes to nPANI ES, especially near the adjoining phenyl rings and bridging NH groups of nPANI ES as can be seen from Table 4 (bold numbers) including some exceptional. In 2PANI ES-NH3 complex, ammonia loses about 0.068 e− charge to nPANI ES based on NBO, and 0.078 e− based on Mulliken charge analysis, and leaves both rings of 2PANI ES to 0.393 e− from 0.422 e− based on NBO analysis (Table 4). It means that before reacting with NH3 the phenyl rings have 0.422 e− and when reacting with NH3 the charges on these rings are reduced to 0.393 e−. Charge transfer in nPANI ESNH3 complexes also decreases with chain length elongation; this phenomenon is also consistent with the results from geometric and energetic analysis (vide supra). For example, ammonia loses 0.060 e− (based on NBO) and 0.069 e− (based on QMulliken) in 4 PANI ES, 0.055 e− (based on NBO) and 0.063 e− (based on QMulliken) in 6PANI ES-NH3, and 0.051 e− (based on NBO) and 0.059 e− based on Mulliken charge analysis in 8PANI ES-NH3 complexes.

The charge-transferring ability of CO2 to nPANI ES is calculated to study the potential of nPANI ES as CO2 gas sensor. The amount of charge transfer from CO2 to 2PANI ES is 0.014 e− (based on NBO) and 0.033 e− (based on QMulliken). The charge transfer from CO2 to PANI ES is much smaller than the charge transfer from ammonia to PANI ES, on sensing (compare 0.068 e− for ammonia with 0.014 e− for CO2). This charge transfer becomes smaller in the case of 4PANI ES-CO2 complex, 0.012 e− (based on NBO) and 0.028 e− (based on QMulliken). In the case of 6PANI ES-CO2, CO2 loses 0.011 e− (based on NBO) and 0.025 e− (based on QMulliken), while this amount further decreases in a larger repeating unit such as 8PANI ES. For infinite polymer, it reduces to 0.009 e− (based on NBO) and 0.023 (based on QMulliken). The charge transfer from analyte to nPANI ES is much lower for CO2 compared to NH3 and one can conclude that the sensitivity of the polymer is lower for CO2 than for NH3. The sensing capability of nPANI ES at CO(1) is also analyzed by NBO and Mulliken charge analysis, given in Tables 2 and 4. From these tables it is evident that nPANI ES has much (low/high) reducing ability for interaction at CO(1) and this is consistent with geometric and thermodynamic analysis. This statement is further supported when CO(1) transfers 0.012 e− (based on NBO) and 0.033 e− (based on QMulliken) to 2PANI ES, which is about 0.056 and 0.002 e− (based on NBO) smaller from 2PANI ES-NH3 and 2PANI ES-CO2 complexes, respectively. In the 4PANI ES-CO(1) complex, CO(1) loses 0.01 e− (based on NBO) and 0.025 e− (based on QMulliken), while it loses about 0.008 e− (based on NBO) and 0.022 e− (based on QMulliken) in 6PANI ES-CO(1) and 8PANI ESCO(1) complexes. Charge analyses of nPANI ES-CO(2) complexes produce quite different results compared to that of nPANI ES-CO(1). CO(2) has 0.017 e− (based on NBO) and 0.036 e− (based on QMulliken) more charge-transferring ability compared to CO(2) which gives 0.022 e− (based on NBO) and 0.041 e− (based on QMulliken) to 4PANI ES. In 6PANI ES-CO(2) complex, CO(2) loses 0.22 e− (based on NBO) and 0.042 e− (based on 23706



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Figure 3. Frontier orbitals of 2PANI ES-X, where X = NH3, CO2, CO(1), and CO(2).

QMulliken), while it loses about 0.020 e− (based on NBO) and 0.038 e− (based on QMulliken) in 8PANI ES-CO(2) complexes Molecular Orbital Analysis. Quantum mechanically, the interaction/reaction between two reactants takes place because of interaction of frontier molecular orbitals.60 The type of interaction (bonding) between sensor and dopant can be best explained in terms of HOMO and LUMO energies. HOMO has the ability to donate electrons, while LUMO has the similar effect as that of EA. If a molecule has high HOMO energy then it will be more reactive (unstable) and vice versa. HOMO and LUMO of 2PANI ES before and after reacting with analytes (NH3, CO2, CO(1), and CO(2)) are given in Figure 3, while HOMO and LUMO of NH3, CO2, CO, nPANI ES, and nPANI ES-X are given in Figures S4−S7 of the

Supporting Information. Frontier molecular orbital energies of nPANI ES and nPANI ES-X are given in Table 5. A decrease of 0.39 eV of energy is simulated in the HOMO of 2PANI ES, 0.17 in 4PANI ES, 0.11 in 6PANI ES, and 0.10 eV in 8PANI ES on sensing NH3. The corresponding HOMO and LUMO energies of nPANI ES decrease on sensing (Table 5) and this indicates that electrons are added from analytes to polymer. The lower HOMO value of nPANI ES is also supportive to the notion that electrons are gained from NH3 (and other analytes as well) A weak interaction is observed in nPANI ES-CO2 complexes, compared to those of nPANI ES-NH3 (see Table 5). The HOMO of 2PANI ES gets electronic charge clouds from CO2 of about 0.15 eV which is 0.24 eV lower than that of the 2PANI 23707



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Table 5. HOMO, LUMO, and Band Gap of nPANI ES and nPANI ES-X Complexes, Where n = 2−8 and ∞ and X = NH3, CO2, CO(1), and CO(2) species

HOMOs (eV)

LUMOs (eV)

band gap (eV)

2PANI ES 2PANI ES-NH3 2PANI ES-CO2 2PANI ES-CO(1) 2PANI ES-CO(2) 4PANI ES 4PANI ES-NH3 4PANI ES-CO2 4PANI ES-CO(1) 4PANI ES-CO(2) 6PANI ES 6PANI ES-NH3 6PANI ES-CO2 6PANI ES-CO(1) 6PANI ES-CO(2) 8PANI ES 8PANI ES-NH3 8PANI ES-CO2 8PANI ES-CO(1) 8PANI ES-CO(2) ∞PANI ES ∞PANI ES-NH3 ∞PANI ES-CO2 ∞PANI ES-CO(1) ∞PANI ES-CO(2)

−8.28 −7.89 −8.13 −8.19 −8.17 −7.07 −6.90 −7.01 −7.04 −7.02 −6.41 −6.30 −6.37 −6.39 −6.38 −6.02 −5.92 −5.99 −6.00 −5.99 −4.71 −4.66 −4.70 −4.69 −4.72

−3.71 −3.40 −3.58 −3.64 −3.91 −3.01 −2.84 −2.94 −3.12 −3.43 −2.56 −2.48 −2.53 −2.82 −3.10 −2.23 −2.21 −2.22 −2.57 −2.83 −1.21 −1.38 −1.28 −1.87 −2.03

4.57 4.49 4.55 4.55 4.26 4.06 4.06 4.05 3.92 3.59 3.85 3.82 3.84 3.57 3.28 3.79 3.71 3.77 3.43 3.16 3.49 3.27 3.46 2.81 2.69

of 2PANI ES decrease to about 0.09 and 0.07 eV, respectively, when the polymer senses CO(1). HOMO energy is perturbed from −7.07 to −7.04 eV for 4PANI ES, −6.41 to −6.39 eV for 8PANI ES, and −6.02 to −6.00 eV for 8PANI ES. It is clear from Table 5 that, with chain length elongation, interaction decreases due to bulkiness of the PANI ES. A relatively pronounced orbitals effect is observed in nPANI ES-CO(2) compared to nPANI ES-CO(1) complexes (about 0.02 eV lesser HOMO energy see Table 5). The HOMO energy of 2PANI ES decreases to about 0.11 eV when it senses CO(2). HOMO energy of 4PANI ES changes from −7.07 to −7.02 eV and from −6.41 to −6.38 eV in 6PANI ES. A decrease of about 0.01 eV is observed in 8PANI ES on reacting with CO(2). Analyses of frontier orbitals and energies of nPANI ES-X before and after sensing lead us to this conclusion that NH3 changes the orbital energy of nPANI ES to a greater extent compared to CO2, CO(1), and CO(2). This statement shows consistency with the recent experimental data.4,39−41 So, on the basis of orbitals study, we can say that nPANI ES is selective to sense NH3 , which also corroborates the experimental observations. UV−Vis−Near-IR Spectroscopic Study. MacDiarmid and Xia et al.12 have found that PANI ES has two conformers in its fully doped form, coil-like (PANI ES in CHCl3 solvent) and expanded coil-like structures (PANI ES in m-cresol solvent). The coil-like structure is referred to the localized polaronic conformation (shorter conjugation length) and the latter in which m-cresol expands the polymer is consistent with the delocalized polaronic form (longer conjugated polymeric body). This statement was further supported by in situ UV− vis−near IR spectroscopic study.12 UV−vis−near-IR spectra are simulated with different methods and level of theories such as ZINDO, TD-HF, TDB3LYP/6-31G(d), TD-B3LYP/6-31 +G(d,p), and TD-B3LYP/ 6-31G(d) in chloroform solvent. We report here that TDUB3LYP/6-31G(d) can reliably calculate optical properties of PANI ES. UV−vis−near-IR spectra of nPANI ES and nPANI ES-X (where X = NH3, CO2, CO(1), and CO(2) and n = 2, 4, 6, 8) at TD-UB3LYP/6-31G(d) level of theory are given in Figures 4−7. Calculated excitation energies, oscillator strengths, and molecular orbitals of the first allowed singlet transition involved in the excitation for nPANI ES and nPANI ES-X complexes are given in Table 6. The effects of CO(2) on UV− vis−near-IR spectra are negligible. Binding of CO from either end CO(1) and CO(2) produces comparable spectra as can be seen from Figures 4−7. In all these four species, three distinct peaks are observed in 2PANI ES and two distinct peaks in

ES-NH3 complex. This electronic clouds transfer from the HOMO of CO2 to LUMO of nPANI ES gives rise to a weak type of attractive force, dipole−induced dipole in nPANI ESCO2 complexes (vide supra), instead of ion−dipole electrostatic interaction with ammonia. The small electronic cloud transfer from analyte to polymer in 2PANI ES-CO2 compared to 2PANI ES-NH3 complex manifests itself in weak sensing of CO2 by the polymer. The 4, 6, and 8PANI ES-CO2 complexes have 0.11 and 0.07 eV smaller electronic cloud attraction, respectively, as compared to its counterpart nPANI ES-NH3 complexes. For an infinite polymer, electronic cloud transfer from NH3 to PANI ES is 0.05 eV, whereas for CO2 it is only 0.01 eV. Complexes of nPANI ES-CO(1) are observed to have the weakest electrostatic attractive forces compared to nPANI ESNH3 and nPANI ES-CO2 complexes (Table 5). The HOMO energies of nPANI ES are slightly affected when CO(1) approaches nPANI ES. The HOMO−LUMO orbitals energies

Figure 4. UV−vis spectra of 2PANI ES and 2PANI ES-X complexes, where X = NH3, CO2, CO(1) and CO(2). 23708



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reported data.4 As can be seen from Figure 4, NH3 has high blue-shifting ability in 2PANI ES compared to that of CO2, CO(1), and CO(2). The same trend is observed in 4PANI ES and 4PANI ES-X as that of 2PANI ES, but here two distinct peaks are present (see Figure 5). In 4PANI ES, λmax (1340 nm) which is present in the near-IR region is blue-shifted to 1209, 1293, and 1315 nm on being bound with NH3, CO2, CO(1), and CO(2), respectively. Two band peaks in 6PANI ES are peaked at 414 and 2071 nm (Figure 6). The longer wavelength peak (2071 nm) shows blue shifting on being bound with NH3, CO2, CO(1), and CO(2), and it is because of the dedoping chemistry of the polymer. NH3 blue shifts the 2071 nm of 6PANI ES to 1847 nm, CO2 shifts to 1993 nm, and both CO(1) and CO(2), the weakest analyte, shift it to 2024 nm. Almost the same results are concluded in 8PANI ES and analyte-bound complexes as can be seen from Figure 7. The low-frequency band peak at 2983 nm (near-IR region) of 8PANI ES is simultaneously blue-shifted to 2671, 2861, and 2908 nm on reacting with NH3, CO2, and either CO(1) or CO(2). Our simulated UV−vis−near-IR spectra of short oligomer (2PANI ES) have three distinctive peaks, which correlate well with the earlier reported results and lead us to deduce that 2PANI ES has localized polaronic structure. Longer oligomers from 4PANI ES onward give two distinct peaks (except 8PANI ES because of nonlinear structure) and validate the delocalized polaronic conformation. 8PANI ES gives four peaks instead of two which is due to the nonlinear geometric structure as shown in Figure S3 in the Supporting Information.

Table 6. Calculated Excitation Energies, Oscillator Strengths, and Molecular Orbitals (MOs) of the First Allowed Singlet Transition Involved in the Excitation for nPANI ES and nPANI ES-X Complexes, Where n = 2−8 and X = NH3, CO2, CO(1), and CO(2) at the TD-UB3LYP/631G(d) Level of Theory energy (eV)

wavelength (nm)

oscillator strength

MOs

coeff

2PANI ES 2PANI ESNH3 2PANI ESCO2 2PANI ESCO(1) 2PANI ESCO(2) 4PANI ES

1.6534 1.7187

749.87 721.39

0.3633 0.3416

52→53 57→58

0.85536 0.87017

1.6810

737.55

0.3590

63→64

0.98952

1.6690

742.85

0.3590

59→60

0.99030

1.6694

742.69

0.3545

59→60

0.99026

0.9248

1340.72

0.7778

1.03089

4PANI ESNH3 4PANI ESCO2 4PANI ESCO(1) 4PANI ESCO(2) 6PANI ES

1.0250

1209.55

0.7562

0.9587

1293.23

0.7776

0.9423

1315.75

0.7771

0.9463

1310.21

0.7721

0.5986

2071.33

0.9960

6PANI ESNH3 6PANI ESCO2 6PANI ESCO(1) 6PANI ESCO(2) 8PANI ES

0.6713

1846.94

1.0152

0.6221

1993.00

1.0209

0.6125

2024.36

1.0102

0.6156

2013.90

1.0094

0.4154

2984.92

0.8356

8PANI ESNH3 8PANI ESCO2 8PANI ESCO(1) 8PANI ESCO(2)

0.4632

2676.56

0.9075

0.4327

2865.19

0.8795

0.4256

2913.25

0.8625

0.4282

2895.25

0.8686

100→ 101 105→ 106 111→ 112 107→ 108 107→ 108 148→ 149 153→ 154 159→ 160 155→ 156 155→ 156 196→ 197 201→ 202 207→ 208 203→ 204 203→ 204

species

1.01320 1.02374 1.02695 1.02571 1.09763 1.04636 1.07896 1.08585

4. CONCLUSIONS DFT studies of nPANI ES and nPANI ES-X (where n = 2, 4, 6, 8 and X = NH3, CO2, CO(1), CO(2)) were caried out to analyze the sensitivity and selectivity of nPANI ES for different anayltes. Optimized geometric pararmeters were evaluated, and it was observed that in the case of good interaction/sensing, intermolecular bond distance (dX6···H5) decreases and dN2−H5 increases. In the case of NH3 sensing these parameters are more prominent compared to those for CO2, CO(1) and CO(2) sensing. NBO and Mulliken charge analysis also go side by side with the results from optimized geometric parameters analysis except for nPANI ES-CO(2) complexes. NH3 loses electron clouds about 3 times as those of CO2, CO(1), and CO(2) upon interaction with PANI ES. The dX6···H5 bond energy is calculated with simple energy and counterpoise-corrected energy calculation. This bond energy is simulated to be in the range of −15.75 to −10.29 kcal mol−1 in the case of nPANI ES-NH3 complexes (n = 2, 4, 6, and 8). Large forces of

1.08256 1.18921 1.09174 1.15554 1.16929 1.16321

longer oligomers except 8PANI ES. The lower energy band peak of 2PANI ES (749 nm) is blue-shifted to 721, 737, and 742 nm upon reacting with NH3, CO2, CO(1), and CO(2), respectively. This blue shifting also confirms the dedoping process of 2PANI ES, which is consistent with the experimental

Figure 5. UV−vis−near-IR spectra of 4PANI ES and 4PANI ES-X complexes, where X = NH3, CO2, CO(1), and CO(2). 23709



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Figure 6. UV−vis spectra of 6PANI ES and 6PANI ES-X complexes, where X = NH3, CO2, CO(1), and CO(2).

Figure 7. UV−vis spectra of 8PANI ES and 8PANI ES-X complexes, where X = NH3, CO2, CO(1), and CO(2).

nPANI ES compared to that CO2, CO(1), and CO(2). Finally, it is concluded that PANI ES has greater response selectivity at NH3, compared to that at CO2, CO(2), and CO(1).

attraction established between the cationic form of nPANI and electronegative part of NH3 molecule mean that ion−dipole/ electrostatic forces operate, whereas this (dX6···H5) binding energy is found to be weak electrostatic or hydrogen bond type bonding in nPANI ES-CO2, nPANI ES-CO(1), and nPANI ESCO(2) complexes. Interaction energy of nPANI ES-CO(1) complexes indicates the inertness of CO(1) at nPANI ES, which is due to its stable resonance structure −CO+ (unavailability of lone pair of electrons of oxygen to PANI ES). However, nPANI ES has shown response consistence with the experimental results.25 CO interaction to nPANI ES from the carbon atom of CO is more prominent compared to the interaction with the oxygen atom. Comparative results of ΔEint and ΔEint,CP conclude that in nPANI ES-X complexes (X = NH3, CO2, CO(1), and CO(2)), nPANI ES has greater response (selectivity) to NH3 as that of CO2, CO(2), and CO(1). The relatively large difference in energy of about ±10 kcal/mol can be explained on the basis of electronegativity difference, high p character (sp3 hybridization), and high dipole moment in NH3 molecule compared to its counterpart CO2, CO(2), and CO(1). nPANI ES is described to show greater response to CO2 than CO(2) which is due to stable resonance structure of CO, although CO2 has even no dipole moment. Moreover, nPANI ES shows greater response to CO(2) than CO(1). Frontier molecular orbitals and energies of nPANI ESX before and after sensing confirm that NH3 changes the orbital energy of nPANI ES to a greater extent in contrast to CO2, CO(2), and CO(1). So, on the basis of orbitals study we can say that nPANI ES is very selective to sense NH3, which also corroborates well the experimental findings. UV−vis and UV− vis−near-IR spectra of short and longer oligomers of PANI ES are simulated, respectively. In all these three kinds of sensing study, blue shifting is observed in nPANI ES, upon doping with NH3, CO2, CO(1), and CO(2), which confirms the dedoping chemistry of PANI ES. NH3 has high blue-shifting ability in

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ASSOCIATED CONTENT

S Supporting Information *

Optimized geometric structures, contours of HOMO and LUMO, tables of IP and EA of nPANI ES and nPANI ES-X complexes, and extrapolating data plots through second-order polynomial fit equation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +92 91 9216652. Fax: +92 91 9216652. E-mail: shah@ s2004.tu-chemnitz.de, [email protected]. *Phone: +92-992-383591. Fax: +92-992-383441. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully thank Professor Dr. Ulrike Salzner for helpful discussions. We also acknowledge the Institute of Chemical Sciences, University of Peshawar, and Department of Chemistry, COMSATS, Abbottabad, Pakistan. We highly acknowledge financial support to the project in the form of research grant from Higher Education Commission (HEC) of Pakistan. Further support from the University of Peshawar and COMSATS is acknowledged.

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ABBREVIATIONS PANI, polyaniline; PANI ES, polyaniline emeraldine salt; CPs, conducting polymers; DFT, density functional theory; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied 23710



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molecular orbital; IP, ionization potential; EA, electron affinities; NBO, natural bonding orbital; BSSE, basis set superposition error

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