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Enhanced gas sensing performance of polyaniline incorporated with graphene: A first-principles study
Physics Letters A 383 (2019) 2751–2754
Contents lists available at ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
Enhanced gas sensing performance of polyaniline incorporated with graphene: A first-principles study Zhi Guo, Ningbo Liao ∗ , Miao Zhang ∗ , Aixin Feng College of Mechanical & Electrical Engineering, Wenzhou University, Wenzhou, 325035, PR China
a r t i c l e
i n f o
Article history: Received 17 November 2018 Received in revised form 26 February 2019 Accepted 30 March 2019 Available online 29 May 2019 Communicated by R. Wu Keywords: First principles calculation Polyaniline-graphene composite Gas sensor
a b s t r a c t Conducting polymers such as polyaniline (PANI) are potential sensing materials for ammonia due to their fast response and low cost, however, the corresponding sensing mechanism needs to be studied further. In this paper, molecular dynamic and first-principles simulations are carried out to investigate ammonia sensing mechanisms of polyaniline/graphene heterostructure. The adsorption of ammonia at different locations of pure polyaniline and graphene/PANI composites is analyzed. The band gap for graphene/PANI system shows more significant change after adsorption than that of pure polyaniline, which indicates higher sensitivity for detecting ammonia, and the results of sorption isotherm imply that graphene/PANI system exhibits more adsorption capacity for ammonia. Moreover, diffusion coefficient of ammonia in polyaniline/graphene system is much large than that of pure polyaniline, demonstrating that gas diffusion occurs more easily in the heterostructure. The results are verified by experimental data, and the proposed computational frame can be used to evaluate and design nanocomposite materials for gas sensor. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Ammonia (NH3 ) is a colorless gas used in environment, automation and medical applications, which may become toxic and affect the health of humans and animals [1,2]. Current NH3 sensors are mainly based on inorganic oxide/dioxide and conducting polymers, which correspond high cost and low sensitivity [3–11]. Conducting polymer such as polyaniline (PANI) is potential sensing material for ammonia, however, pure polyaniline shows low chemical stability in normal conditions [12–14]. Nanostructure polyaniline-based gas sensors show excellent performance with their large surface areas and high porosity of nanostructured polyaniline [15–18]. In particular, polyaniline incorporated with graphene presents enhanced ammonia sensitivity [19,20], however, the corresponding sensing mechanism needs to be studied further. First-principles simulations can be used to explore varied materials properties [21–24], and were applied to model and simulate carbon-based materials [25–27] and evaluate gas sensing performance [28–31]. In this work, atomistic simulations are used to study diffusion and adsorption of ammonia in pure PANI and graphene/PANI heterostructure. The computational details for MD
*
Corresponding authors. E-mail addresses: [email protected], [email protected] (N. Liao).
https://doi.org/10.1016/j.physleta.2019.03.045 0375-9601/© 2019 Elsevier B.V. All rights reserved.
and first-principles calculations are presented in section 2, and section 3 are the simulation results and discussions. 2. Computational details The unit cell contains twenty monomers for isotherm of sorption and forty monomers for diffusion. The temperature and pressure were controlled by the Berendsen’s method using a half-life for decay to the target temperature of 0.1 ps and 0.1 ps for the pressure scaling constant. The non-bonded electrostatic and van der Waals forces were controlled by Ewald method with a cutoff value of 10 Å. Exchange correlation interaction of electron is described using generalized gradient approximation (GGA) [32]. Ultra-soft pseudo-potentials is used to treat core electron. Special points sampling integration over the Brillouin zone were employed using Monkhorst-Pack schemes with a 3 × 3 × 3 k-point mesh. A cut off energy of 340 eV is used for plane wave expansion. All the calculations were implemented by commercial software Materials Studio. Adsorption energy between the composite and gas is calculated by:
Eads = EGraphene/PANI +gas − (EGraphene/PANI + Egas )
(1)
where EGraphene/PANI+gas , Egas and EGraphene/PANI are total potential energies of gas adsorbed system, gas molecules and isolated composite system.
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Fig. 1. The adsorption sites of ammonia on polyaniline/graphene and pure polyaniline. A and B represent the sensing sites for the NH3 molecules diffused into the interface of grapheme and PANI, C and D represent the sites of adsorption in polyaniline/graphene, E and F represent the sites of adsorption in pure PANI. The N, C and H atoms are presented as blue, gray and white colors respectively. (For interpretation of the colors in the figure(s), the reader is referred to the web version of this article.) Table 1 Adsorption energies, band gaps and Mulliken charges of NH3 on various configurations. Configuration
Eads
Q
Eg (before adsorption)
Eg (after adsorption)
A B C D E F
-2.8 -0.18 -0.15 -0.20 -0.40 -0.20
0.83 0.09 0.03 0.02 0.10 0.06
0.37 0.37 0.37 0.37 1.16 1.16
0.02 0.21 0.32 0.28 1.08 1.15
3. Results and discussions Fig. 1 (C-F) shows the sites of adsorption for pure PANI and graphene/PANI composite. As the gas molecules may diffuse into the interface between PANI and graphene, the models of NH3 sandwiched between graphene and polyaniline are also considered, as shown in Fig. 1 (A-B). The adsorption energy, charge transfer and energy gaps for different adsorption sites are listed in Table 1. Site A and site E show higher adsorption energy and more charge transfer, which is in accordance to its highest value of adsorption energy and lowest adsorbent-adsorbate distance, and present better performance for ammonia sensing. With the same adsorption location, the structure with incorporation of graphene generally shows a better performance than pure PANI structure. Based on the Mulliken charge for sites C and D, electron transfer from NH3 to graphene is small. The band gap (the energy difference between the top of valence band and the bottom of conduction band) for configuration A decreases from 0.37 eV to 0.02 eV, the variation is much larger than those of the other configurations. The electron transfer for site A is much larger than that of site E, it means the incorporation of graphene significantly promotes the sensing performance of polyaniline, which agrees with the experimental results [19]. The density of states (DOSs) for most stable adsorption sites A and E in graphene/PANI composite and pure PANI are calculated further, as shown in Fig. 2. After adsorption, compared with pure PANI, a significant difference on DOS can be found for polyani-
line/graphene composite due to the strong interaction between gas and composite. The change on DOS around Fermi level indicates large change in electric conductivity. Furthermore, the appearance of a new strong peak means intense electronic activities between NH3 and nanocomposite. It is concluded that the graphene/PANI system is more suitable for sensing of NH3 , which consists with the experimental results [19]. Further evidences for sensing performance are provided by N-N pair distribution function (PDF) of graphene/PANI and PANI after adsorption, as shown in Fig. 3. The N-N distribution is determined by the distance of NH3 molecules. The peaks at 5.2 Å and 5.6 Å represent the distance of uniformly distributed NH3 in PANI matrix, however, an additional peak appears for the NH3 -graphene/PANI system at around 3 Å, which corresponds to a smaller distance of N-N for the NH3 molecules near interface of graphene and PANI. The smaller N-N distance indicates a significant change in bonding state, which could influence the electronic properties of the material and thus improve the sensitivity. The sorption isotherms of NH3 in two systems are shown in Fig. 4. The amount of gas adsorbed by two kinds of adsorbent increased gradually with the increase of pressure, approaching an upper limit where the sorption amount can no longer go up even if pressure increases further. The amount of NH3 corresponds to similar value at low pressure (P<400 KPa) for the two systems. At higher pressure (P>400 KPa), the graphene/PANI system presents a higher adsorption quantity and a faster sorption ratio than pure PANI. The average adsorption heats of NH3 are 8.9 and 6.8 kcal/mol for graphene/PANI and PANI respectively. These results are in agreement with above first principles calculations. The calculated mean square displacements (MSD) of NH3 in the two systems are shown in Fig. 5. The slopes of MSD for NH3 are 0.11 and 0.04 for graphene/PANI and pure PANI respectively. The coefficient of NH3 diffusion for polyaniline/graphene composite and pure PANI are 1.83 × 10−10 and 0.67 × 10−10 m2 /s respectively, meaning diffusion of ammonia is much easier in the composite. This is in agreement with the large surface area observed in graphene/PANI by experiment [19], as surface area is favorable for gases diffusion. Thus, ammonia response of polyaniline/graphene
Z. Guo et al. / Physics Letters A 383 (2019) 2751–2754
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Fig. 4. Sorption isotherm of NH3 in graphene/PANI and pure PANI at 298 K. The graphene/PANI system presents a higher adsorption quantity and a faster sorption ratio than pure PANI.
Fig. 2. The density of states of the graphene/PANI composite and pure PANI before and after NH3 adsorption. The increasing of DOS around Fermi level for graphene/PANI composite indicates large change in electric conductivity.
Fig. 5. The mean square displacements (MSDs) of NH3 in graphene/PANI(black) and pure PANI (red) diffusion. The dot line is original data and the solid line is the linear fit. The slope of MSD for NH3 graphene/PANI is larger than that of pure PANI, meaning diffusion of ammonia is much easier in the composite.
shown in Fig. 6, diffusion is anomalous between 0 and 135 ps with a slope of 0.58, and the slope increases to 0.99 after 100 ps, implying the presentence of Einstein diffusion. For pure PANI, although the slope is only 0.40, it keeps constant in diffusion process and corresponds a coefficient of determination of R2 = 0.998. The results mean the above calculated diffusion coefficients are reliable. 4. Conclusion
Fig. 3. The computed pair distribution functions g (r ) of graphene/PANI and PANI during sorption and diffusion of NH3 . The smaller N-N distance of graphene/PANI indicates a significant change in bonding state.
is faster than that of pure polyaniline, consisting with experiment [19]. To determine whether Einstein diffusion occurs, the slope of log(MSD) versus log(t) curve is obtained, which is linear when diffusion reaches an equilibrium. For polyaniline/graphene system, as
By combining first-principles and MD calculations, electronic and thermodynamic properties of graphene/PANI and PANI for ammonia sensing are investigated. The adsorption energy, Mulliken charge and DOS are sensitive to adsorption sites. Based on the calculated band gap and adsorption isotherm, graphene/PANI exhibits much higher ammonia sensitivity and adsorption capacity than pure PANI. Pair distribution function of N-N indicated that the incorporation of graphene makes it easier for gas accumulating, which is also demonstrated by the calculated diffusion coefficient, both means the graphene/PANI system is more favorable for ammonia detection. By analyzing Einstein diffusion, it proves the calculated diffusion coefficients are reliable.
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References
Fig. 6. The relationship between log(MSD) and log(t) for NH3 diffusion in graphene/PANI and pure PANI systems. The results mean the above calculated diffusion coefficients are reliable.
Acknowledgements The authors would like to acknowledge the supports of the National Natural Science Foundation of China (51675384).
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Contents lists available at ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
Enhanced gas sensing performance of polyaniline incorporated with graphene: A first-principles study Zhi Guo, Ningbo Liao ∗ , Miao Zhang ∗ , Aixin Feng College of Mechanical & Electrical Engineering, Wenzhou University, Wenzhou, 325035, PR China
a r t i c l e
i n f o
Article history: Received 17 November 2018 Received in revised form 26 February 2019 Accepted 30 March 2019 Available online 29 May 2019 Communicated by R. Wu Keywords: First principles calculation Polyaniline-graphene composite Gas sensor
a b s t r a c t Conducting polymers such as polyaniline (PANI) are potential sensing materials for ammonia due to their fast response and low cost, however, the corresponding sensing mechanism needs to be studied further. In this paper, molecular dynamic and first-principles simulations are carried out to investigate ammonia sensing mechanisms of polyaniline/graphene heterostructure. The adsorption of ammonia at different locations of pure polyaniline and graphene/PANI composites is analyzed. The band gap for graphene/PANI system shows more significant change after adsorption than that of pure polyaniline, which indicates higher sensitivity for detecting ammonia, and the results of sorption isotherm imply that graphene/PANI system exhibits more adsorption capacity for ammonia. Moreover, diffusion coefficient of ammonia in polyaniline/graphene system is much large than that of pure polyaniline, demonstrating that gas diffusion occurs more easily in the heterostructure. The results are verified by experimental data, and the proposed computational frame can be used to evaluate and design nanocomposite materials for gas sensor. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Ammonia (NH3 ) is a colorless gas used in environment, automation and medical applications, which may become toxic and affect the health of humans and animals [1,2]. Current NH3 sensors are mainly based on inorganic oxide/dioxide and conducting polymers, which correspond high cost and low sensitivity [3–11]. Conducting polymer such as polyaniline (PANI) is potential sensing material for ammonia, however, pure polyaniline shows low chemical stability in normal conditions [12–14]. Nanostructure polyaniline-based gas sensors show excellent performance with their large surface areas and high porosity of nanostructured polyaniline [15–18]. In particular, polyaniline incorporated with graphene presents enhanced ammonia sensitivity [19,20], however, the corresponding sensing mechanism needs to be studied further. First-principles simulations can be used to explore varied materials properties [21–24], and were applied to model and simulate carbon-based materials [25–27] and evaluate gas sensing performance [28–31]. In this work, atomistic simulations are used to study diffusion and adsorption of ammonia in pure PANI and graphene/PANI heterostructure. The computational details for MD
*
Corresponding authors. E-mail addresses: [email protected], [email protected] (N. Liao).
https://doi.org/10.1016/j.physleta.2019.03.045 0375-9601/© 2019 Elsevier B.V. All rights reserved.
and first-principles calculations are presented in section 2, and section 3 are the simulation results and discussions. 2. Computational details The unit cell contains twenty monomers for isotherm of sorption and forty monomers for diffusion. The temperature and pressure were controlled by the Berendsen’s method using a half-life for decay to the target temperature of 0.1 ps and 0.1 ps for the pressure scaling constant. The non-bonded electrostatic and van der Waals forces were controlled by Ewald method with a cutoff value of 10 Å. Exchange correlation interaction of electron is described using generalized gradient approximation (GGA) [32]. Ultra-soft pseudo-potentials is used to treat core electron. Special points sampling integration over the Brillouin zone were employed using Monkhorst-Pack schemes with a 3 × 3 × 3 k-point mesh. A cut off energy of 340 eV is used for plane wave expansion. All the calculations were implemented by commercial software Materials Studio. Adsorption energy between the composite and gas is calculated by:
Eads = EGraphene/PANI +gas − (EGraphene/PANI + Egas )
(1)
where EGraphene/PANI+gas , Egas and EGraphene/PANI are total potential energies of gas adsorbed system, gas molecules and isolated composite system.
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Fig. 1. The adsorption sites of ammonia on polyaniline/graphene and pure polyaniline. A and B represent the sensing sites for the NH3 molecules diffused into the interface of grapheme and PANI, C and D represent the sites of adsorption in polyaniline/graphene, E and F represent the sites of adsorption in pure PANI. The N, C and H atoms are presented as blue, gray and white colors respectively. (For interpretation of the colors in the figure(s), the reader is referred to the web version of this article.) Table 1 Adsorption energies, band gaps and Mulliken charges of NH3 on various configurations. Configuration
Eads
Q
Eg (before adsorption)
Eg (after adsorption)
A B C D E F
-2.8 -0.18 -0.15 -0.20 -0.40 -0.20
0.83 0.09 0.03 0.02 0.10 0.06
0.37 0.37 0.37 0.37 1.16 1.16
0.02 0.21 0.32 0.28 1.08 1.15
3. Results and discussions Fig. 1 (C-F) shows the sites of adsorption for pure PANI and graphene/PANI composite. As the gas molecules may diffuse into the interface between PANI and graphene, the models of NH3 sandwiched between graphene and polyaniline are also considered, as shown in Fig. 1 (A-B). The adsorption energy, charge transfer and energy gaps for different adsorption sites are listed in Table 1. Site A and site E show higher adsorption energy and more charge transfer, which is in accordance to its highest value of adsorption energy and lowest adsorbent-adsorbate distance, and present better performance for ammonia sensing. With the same adsorption location, the structure with incorporation of graphene generally shows a better performance than pure PANI structure. Based on the Mulliken charge for sites C and D, electron transfer from NH3 to graphene is small. The band gap (the energy difference between the top of valence band and the bottom of conduction band) for configuration A decreases from 0.37 eV to 0.02 eV, the variation is much larger than those of the other configurations. The electron transfer for site A is much larger than that of site E, it means the incorporation of graphene significantly promotes the sensing performance of polyaniline, which agrees with the experimental results [19]. The density of states (DOSs) for most stable adsorption sites A and E in graphene/PANI composite and pure PANI are calculated further, as shown in Fig. 2. After adsorption, compared with pure PANI, a significant difference on DOS can be found for polyani-
line/graphene composite due to the strong interaction between gas and composite. The change on DOS around Fermi level indicates large change in electric conductivity. Furthermore, the appearance of a new strong peak means intense electronic activities between NH3 and nanocomposite. It is concluded that the graphene/PANI system is more suitable for sensing of NH3 , which consists with the experimental results [19]. Further evidences for sensing performance are provided by N-N pair distribution function (PDF) of graphene/PANI and PANI after adsorption, as shown in Fig. 3. The N-N distribution is determined by the distance of NH3 molecules. The peaks at 5.2 Å and 5.6 Å represent the distance of uniformly distributed NH3 in PANI matrix, however, an additional peak appears for the NH3 -graphene/PANI system at around 3 Å, which corresponds to a smaller distance of N-N for the NH3 molecules near interface of graphene and PANI. The smaller N-N distance indicates a significant change in bonding state, which could influence the electronic properties of the material and thus improve the sensitivity. The sorption isotherms of NH3 in two systems are shown in Fig. 4. The amount of gas adsorbed by two kinds of adsorbent increased gradually with the increase of pressure, approaching an upper limit where the sorption amount can no longer go up even if pressure increases further. The amount of NH3 corresponds to similar value at low pressure (P<400 KPa) for the two systems. At higher pressure (P>400 KPa), the graphene/PANI system presents a higher adsorption quantity and a faster sorption ratio than pure PANI. The average adsorption heats of NH3 are 8.9 and 6.8 kcal/mol for graphene/PANI and PANI respectively. These results are in agreement with above first principles calculations. The calculated mean square displacements (MSD) of NH3 in the two systems are shown in Fig. 5. The slopes of MSD for NH3 are 0.11 and 0.04 for graphene/PANI and pure PANI respectively. The coefficient of NH3 diffusion for polyaniline/graphene composite and pure PANI are 1.83 × 10−10 and 0.67 × 10−10 m2 /s respectively, meaning diffusion of ammonia is much easier in the composite. This is in agreement with the large surface area observed in graphene/PANI by experiment [19], as surface area is favorable for gases diffusion. Thus, ammonia response of polyaniline/graphene
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Fig. 4. Sorption isotherm of NH3 in graphene/PANI and pure PANI at 298 K. The graphene/PANI system presents a higher adsorption quantity and a faster sorption ratio than pure PANI.
Fig. 2. The density of states of the graphene/PANI composite and pure PANI before and after NH3 adsorption. The increasing of DOS around Fermi level for graphene/PANI composite indicates large change in electric conductivity.
Fig. 5. The mean square displacements (MSDs) of NH3 in graphene/PANI(black) and pure PANI (red) diffusion. The dot line is original data and the solid line is the linear fit. The slope of MSD for NH3 graphene/PANI is larger than that of pure PANI, meaning diffusion of ammonia is much easier in the composite.
shown in Fig. 6, diffusion is anomalous between 0 and 135 ps with a slope of 0.58, and the slope increases to 0.99 after 100 ps, implying the presentence of Einstein diffusion. For pure PANI, although the slope is only 0.40, it keeps constant in diffusion process and corresponds a coefficient of determination of R2 = 0.998. The results mean the above calculated diffusion coefficients are reliable. 4. Conclusion
Fig. 3. The computed pair distribution functions g (r ) of graphene/PANI and PANI during sorption and diffusion of NH3 . The smaller N-N distance of graphene/PANI indicates a significant change in bonding state.
is faster than that of pure polyaniline, consisting with experiment [19]. To determine whether Einstein diffusion occurs, the slope of log(MSD) versus log(t) curve is obtained, which is linear when diffusion reaches an equilibrium. For polyaniline/graphene system, as
By combining first-principles and MD calculations, electronic and thermodynamic properties of graphene/PANI and PANI for ammonia sensing are investigated. The adsorption energy, Mulliken charge and DOS are sensitive to adsorption sites. Based on the calculated band gap and adsorption isotherm, graphene/PANI exhibits much higher ammonia sensitivity and adsorption capacity than pure PANI. Pair distribution function of N-N indicated that the incorporation of graphene makes it easier for gas accumulating, which is also demonstrated by the calculated diffusion coefficient, both means the graphene/PANI system is more favorable for ammonia detection. By analyzing Einstein diffusion, it proves the calculated diffusion coefficients are reliable.
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References
Fig. 6. The relationship between log(MSD) and log(t) for NH3 diffusion in graphene/PANI and pure PANI systems. The results mean the above calculated diffusion coefficients are reliable.
Acknowledgements The authors would like to acknowledge the supports of the National Natural Science Foundation of China (51675384).
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