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Adsorption of molecules on C3N nanosheet: A first-principles calculations
Chemical Physics 526 (2019) 110442
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Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Adsorption of molecules on C3N nanosheet: A first-principles calculations A. Bafekry a b c
a,b,⁎
c,⁎
a
, M. Ghergherehchi , S. Farjami Shayesteh , F.M. Peeters
b
T
Department of Physics, University of Guilan, 41335-1914 Rasht, Iran Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium College of Electronic and Electrical Engineering, Sungkyun kwan University, Suwon, Republic of Korea
ARTICLE INFO
ABSTRACT
Keywords: C3 N nanosheet Molecule adsorption Electronic structure First-principle calculations
Using first-principles calculations we investigate the interaction of various molecules, including H2, N2, CO, CO2, H2 O, H2 S , NH3, CH4 with a C3N nanosheet. Due to the weaker interaction between H2, N2, CO, CO2, H2 O, H2 S , NH3 and CH4 molecules with C3N, the adsorption energy is small and does not yield any significant distortion of the C3N lattice and the molecules are physisorbed. Calculated charge transfer shows that these molecules act as weak donors. However, adsorption of O2, NO, NO2 and SO2 molecules are chemisorbed, they receive electrons from C3N and act as a strong acceptor. They interact strongly through hybridizing its frontier orbitals with the p-orbital of C3N, modifying the electronic structure of C3N. Our theoretical studies indicate that C3N-based sensor has a high potential for O2, NO, NO2 and SO2 molecules detection.
1. Introduction Due to the thin thickness and large surface-to-volume ratio of twodimensional materials (2DM), the electronic and magnetic properties can be tuned by adsorbate molecules. Furthermore, adsorption is a significant initial step in the activation of gas molecules. Detection of gas molecules is of great importance in both human health and environmental protection. [2,1] The various types of gas sensors may be useful for numerous applications, such as in medical diagnosis, environmental monitoring and surveillance, agriculture production, military safety and emission control [1,3]. The adsorption of molecules has been extensively studied by first-principle calculations on graphene [4–9] and other 2DM [10–12]. Recently, 2D polyaniline with stoichiometric formula C3N has been successfully synthesized [13]. C3N is predicted to be of importance in variety applications such as in solar cell devices, electrolyte gating and doping of transistors and anode material. [14–16] In addition, theoretical DFT calculations showed that the electronic properties of C3N can be tuned in different ways [17–21]. The single layer C3N, was first reported to be a semiconductor and three possible planar structures were suggested. [23,24] Understanding the interaction between C3N and the adsorbate gas molecules is important for the exploitation of C3N in gas sensors which has been reported in previous works [25,26]. In this paper, we study systemically the adsorption of numerous gas molecules such as, atmospheric gas molecules (including H2, O2, CO2
H2 O ) and and common polluted gases (including CO, NO, NO2 , SO2 , H2 S, NH3 and CH4 ) on C3N. These kinds of calculation results will help to expand our understanding of molecule-C3N interaction and could be of practical interest in different applications. 2. Method I this study we perform total energy and electronic structure calculations, within the generalized gradient approximation of the Perdew-Burke-Ernzerhof variant (GGA-PBE)[27] method, to deal with the exchange-correlation. In this work e use norm-conserving pseudopotentials [28], for carbon and nitrogen anhe d tforeign atoms. The wave functions were expanded in a linear combination of multiple pseudoatomic orbitals (LCPAOs) generated using a confinement scheme [29,30]. The k-points for sampling over the Brillouin zone (BZ) integration were generated using the Monkhorst-Pack scheme [31] and was sampled by k-mesh grid of 23 × 23 × 1 for primitive unit cell and scaled according to the size of the supercell. After convergence tests in OpenMX, an energy cutoff of 300 Ry was chosen so that the total-energies converge below 1.0 meV/atom. The geometries were fully relaxed until the force convergence acting on each atom was less than 1 meV/Å. The C3N were modeled as a periodic slab with a sufficiently large vacuum layer of 20 Åin order to avoid interaction between adjacent layers. The charge transfer was calculated using Mulliken charge analysis [32]. Simulated scanning tunneling microscopy (STM) images
⁎ Corresponding authors at: Department of Physics, University of Guilan, 41335-1914 Rasht, Iran (A. Bafekry). College of Electronic and Electrical Engineering, Sungkyun kwan University, Suwon, Korea (M. Ghergherehchi). E-mail addresses: [email protected] (A. Bafekry), [email protected] (M. Ghergherehchi).
https://doi.org/10.1016/j.chemphys.2019.110442 Received 5 July 2019; Accepted 7 July 2019 Available online 09 July 2019 0301-0104/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics 526 (2019) 110442
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Fig. 1. (a) Optimized structure of C3N, with its hexagonal primitive unit cell, indicated by a red parallelogram. (b) Total and difference charge density. The blue and yellow regions represent charge accumulation and depletion, respectively. (c) Simulated STM image of C3N. Overlay structure represents C3N repeating unit cell.
were obtained using the Tersoff-Hamann theory [33] for STM images, as supplied in OpenMX code with a bias of + 2.0 V (unoccupied states) and were plotted using WSxM software [34]. 3. Pristine C3N The atomic structure of C3N is a planar hexagonal lattice, as shown in Fig. 1(a). The lattice constant of C3N is 4.86 Å, which agrees well with the experimental value of 4.75 Å[13] and bond lengths are, 1.404 (C–C) and 1.403 (N–C) Å, which are consistent with previous theoretical studies [23,22,17]. With the structural optimization, all structures prefer to stay as a planar sheet, similar to graphene. The difference charge density was calculated by subtracting the charge densities of free C and N atoms from the charge density of C3N (see Fig. 1(b)). The total charge density shows high charge density around the N atom, indicating charge transfer from C to N atoms as shown in Fig. 1(b). Bader charge analysis was performed with Quantum Espresso code. The result shows that in C3N, each N atom gains about 0.6 electron from the adjacent C atoms. The charge redistributions is due to the different electro-negativities of C (2) and N (3) atoms. To provide visible guidance for experimental observation, first-principles DFT calculations were used to calculate the STM image as shown in Fig. 1(c). To produce the calculated STM image, the Kohn-Sham charge density was integrated at a voltage of +2.0 V. To correlate the STM image with the corresponding atomic structure, the carbon and nitrogen atoms are shown with gray and blue balls, respectively. We can see bright spots at the center of the hexagons. The orbital-projected band structure, DOS and PDOS of C3N, are as shown in Figs. 2(a, b). The GGA-PBE calculations demonstrate that C3N is an semiconductor with 0.4 eV indirect band gap between the valence band maximum (VBM) and the conduction band minimum (CBM) which are located at M and points, respectively (see Fig. 2(a)). Also orbital-projected electronic band structure was calculated (See Fig. 2(a)). Notice the VBM is dominated by N/C- pz orbitals and the Dirac-cone is still formed by C- pz orbitals. From DOS and PDOS of C3N in Fig. 2(b), we can see that the VBM is dominated by N/C- pz orbitals and the Dirac-point was formed by C- pz orbitals, whereas the CBM was dominated by C- pz orbitals.
Fig. 2. (a) Electronic structure with Orbital resolved band structure of C3N and (b) DOS and PDOS of C3N.. The zero of energy is set to the Fermi level energy (EF ) .
center of a hexagon with six C atoms (HCC ) , (2) the hollow site above the center of a hexagon with composed of both C and N atoms (HNC ) , (3) the bridge site above the middle of a C–C bond (BCC ) , (4) the bridge site above the middle of a N-C bond (BNC ) , (5) the top site above a C atom (TC ), and (6) the top site above a N atom (TN ) (see Fig. 3(a)). Schematic view of favorable adsorbed molecule orientation on C3N at stable sites, is shown in Fig. 3(b). In the first step, the molecule is placed above these adsorption sites, for each of which different orientations of molecules are assessed. Several typical orientations of the gas molecule either vertically or parallel to the C3N surface are examined, and relative rotation of gas molecule to the C3N surface is considered. For example, for the CO molecule, the molecular axis could be oriented parallel to the C3N surface or perpendicular with the C or O atom pointing to the C3N surface. While for the molecule SO2 , the molecular plane could be oriented parallel to the C3N surface: the S–O bonds are towards the C3N surface. Final structure of C3N adsorbed with various gas molecules corresponding to the most stable site are shown in Fig. 3(c). The adsorption energy of the molecule on C3N was determined as Ea = EC3 N + EMol EMol / C3 N , where EMol / C3 N , denote the total energy of the structure with the molecule adsorption on C3N, EC3 N denotes the total energy of the pristine C3N without molecule, and EMol
4. Adsorption of molecule Adsorption of gas molecules affects on the structural and electronic properties of 2DM. Schematic view of favorable adsorption sites of gas H2, N2, O2, CO, CO2 , NO , NO2 , SO2, H2 molecules including O, H2 S, NH3 O and CH4 O on C3N are shown in Fig. 3(a). After structural optimization, the most stable site configuration among the six different sites are determined where all atoms are relaxed in all direction. These six possible adsorption sites includes: (1) the hollow site above the 2
Chemical Physics 526 (2019) 110442
A. Bafekry, et al.
Fig. 3. Schematic view of favorable adsorption (a) sites and (b) orientation of molecules including H2, N2, O2, CO, CO2, NO, NO2 , SO2, H2 O, H2 S, NH3 and CH4 on the C3N. (c) Top view of the final structure of C3N adsorbed with various gas molecules corresponding to the stable sites. (d) Adsorption energy of molecules on C3N at most stable site.
denote the total energy of the energy isolated molecule in vacuum. The variation of adsorption for different molecules on C3N at most stable site are shown in Fig. 3(d). The configuration will be considered as physisorbed when the adsorption energy is low and the nearest atomic distance between the adsorbed molecule and the support is much larger than the sum of the covalent radii of the corresponding atoms. The adsorption energies of H2, corresponding gas molecules such as N2, O2, CO , CO2 , NO, H2 O , H2 S, NH3 and CH4 on C3N are quite small, as compared with 0.72 eV and 0.59 eV for NO2 and SO2 , respectively. The optimized structures with corresponding structural parameter for adsorption of H2, N2 , O2, CO, CO2 , NO , NO2 , SO2, H2 O , H2 S , NH3 and CH4 molecules on C3N at most stable site were are also in Fig. 4. It can be seen that the stable adsorption site for H2, N2 , O2, CO and NO molecules is at HCC -site, while CO2 is adsorbed in BNC -site. For H2 and N2 , the H and N atoms points towards the C3N surface and bond lengths are 0.757 (H–H) and 1.119 (N–N) Å. For O2 , O atom points toward the C3N surface and O–O bond length is 1.345 Å, while the CO points parallel to the C3N surface and C–O bond length is 1.345 Å. In CO2 , the C-O bond length is 1.195 Å. The N atom of NO molecule, points toward the surface and bond length of N–O is 1.221 Å. Also the closest distance between the lower atoms of the molecules with the C3N surface is in the range of 2.371 Å (NO)– 3.564 Å (CO2 ). The adsorption energy of H2, N2, O2, CO, CO2 and NO molecules is 0.13, 0.15, 0.29, 0.24, 0.21 and 0.24 eV respectively, which can be considered as physisorption which agrees with the relative large distance between these molecules and the C3N surface. The stable adsorption site for NO2 and SO2 is BNC -site, while the H2 O and H2 S molecules are adsorbed at HNC -site. For NO2 and SO2 , bond lengths were calculated as 1.270 Å (N–O) and 1.528 Å (S-O), while the bond angles are 122° (O–N–O) and 117° (O–S–O). The distance between the molecules and the C3N surface are 2.894 (NO2 ), 2.919 (SO2 ), 2.630 (H2 O ) and 2.962 Å (H2 S ). The NO2 and SO2 , have large adsorption energy of 0.72 eV and 0.59 eV, respectively, which can be considered as weak chemisorption. Whereas, H2 O and H2 S exhibit a small adsorption energy of, respectively, 0.17 and 0.21 eV and therefore can be considered as physisorption. It was found that the NH3 and CH4 molecules are located at stable adsorption site of HNC and TC , respectively. The N–H bond length is 1.304 Å and the H–N–H bond angle is 104°, while the distance between NH3 with the C3N surface is 2.837 Å. The closest distance of the most stable site between CH4 molecule with the C3N surface is 2.821 Å. The C–H bond lengths were 1.101 Å and the H-C–H bond angle is 110° . The NH3 and CH4 /C3 N , had adsorption energy of 0.23 eV and 0.19 eV, respectively. To know the
bonding character between molecules and C3N, the difference charge densities indicated, in the same panel in Fig. 4. The blue and yellow regions represent the charge accumulation and depletion, respectively. It can be seen that electrons were accumulated on the atoms of H2, N2, O2, CO, CO2 and NO molecules, whereas the depletion of electrons in its three nearest C neighbors of C3N, which shows that there are small charge transfers from C3N to these molecules. Difference charge density of these molecules indicates, that there is no formation of chemical bonds, hence we have physisorption rather than chemisorption. The charge transfer analysis showed weak electron acceptors by gaining electrons of 0.01e (H2 ), 0.01e (N2 ), 0.19e (O2 ), 0.14e (CO), 0.14e (NO), 0.01e (H2 O ) and 0.02e (H2 S ) from C3N. Such a subtle charge transfer indicates that these molecules were physisorbed on C3N through weak van der Waals interaction. The charge transfer calculations showed that NO2 and SO2 are strong electron acceptors by gaining 0.26 e and 0.23 e electrons from C3N to NO2 and SO2 , respectively. Such a large charge transfer shows that NO2 and SO2 is weak chemisorbed on C3N. Moreover, the electrons were depleted on the atoms of CO2 , NH3 and CH4 , whereas electrons are accumulated on C3N, which shows there is charge transfer from these molecules to C3N. Charge transfer analysis shows that CO2 , NH3 and CH4 act as weak electron donors by donating −0.002e, −0.002e and −0.004e to C3N. As shown in Fig. 5, H2, N2 , CO , CO2 , H2 O, H2 S, NH3 and CH4 /C3 N molecules can hardly change the band structure around EF . We found that CO/ C3 N becomes a semiconductor with band gap 0.4 eV. Furthermore, NO molecule causes modification to the electronic structure and result in induced impurity states around EF . These impurity states mainly come from states of NO molecule. It is also observed that the and spin states for NO / C3 N exhibit a dilute-magnetic semiconductor character and induce 1 µB magnetic moment. The O2 and SO2 molecules modify the electronic state around EF , giving rise to a vanishing band gap resulting in metallic characteristics with their Fermi levels crossing the electronic states. DOS of these structures is different from that of pristine C3N, showing an impurity state around EF . The NO2 paramagnetic molecule can induce impurity states around EF and these impurity states come mainly from states of NO2 . It becomes a ferromagnetic-metal and spin-splitting occurs in the and spin channels and with 0.6 µB magnetic moment. The very weak interaction of H2, N2 , CO , CO2 and NO molecules with C3N is also reflected by the sharp peaks in the PDOS (see Fig. 6). For these molecules, the HOMO and LUMO is fully filled and lies about 0.22 eV below and 0.21 eV above EF , respectively. When these molecules weakly interact with C3N, it occupies the HOMO state which 3
Chemical Physics 526 (2019) 110442
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Fig. 4. Optimized structures with corresponding structural parameter for adsorption of H2, N2, O2, CO, CO2, NO, NO2 , SO2, H2 O, H2 S, NH3 and CH4 molecules on C3N at the most stable site. Bothe, top and side views are shown. Difference charge density are indicated in the same panel. The blue and yellow regions represent charge accumulation and depletion, respectively.
with the DOS above EF which does not cause any adsorption therefore making HOMO a more important orbital and the charge transfer is consequently always to C3N. The interaction between O2 and C3N is also
moves below EF . Therefore, the net gain of electrons from C3N acts as an acceptor and the charge transfer are due to orbital overlap between the HOMO of these molecules with C3N. The LUMO will mostly interact
Fig. 5. Electronic band structure for adsorption of H2, N2, O2, CO, CO2, NO, NO2 , SO2, H2 O, H2 S, NH3 and CH4 molecules on C3N at the most stable site. The zero of energy is set at EF . 4
Chemical Physics 526 (2019) 110442
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Fig. 6. DOS and PDOS for adsorption of H2, N2, O2, CO, CO2, NO, NO2 , SO2, H2 O, H2 S, NH3 and CH4 molecules on C3N at the most stable site. Charge densities of HOMO and LUMO are shown in the insets. The blue and yellow regions represent charge accumulation and depletion, respectively.
reflected by the broad peaks in the PDOS. For O2 molecule, the HOMO and LUMO are 0.05 eV below and 0.004 eV above EF , respectively. These states induce a small charge transfer from C3N to O2 . When O2 molecule strongly interact with C3N, its occupied HOMO state moves below EF , increasing electrons in O2 molecule. Therefore, it makes an acceptor and the charge transfer are due to orbital overlap between the HOMO of O2 molecule and C3N. For NO2 molecule, the HOMO and LUMO of spin state are 0.097 eV below and 0.115 eV above EF , respectively and also HOMO and LUMO of spin channel are 0.733 eV below and 0.106 eV above the EF . For NO2 molecule, a spin-splitting orbital is located above EF that contributes to the LUMO state. NO2 molecule can accept electrons from C3N and the charge transfer occurs mainly through the orbital hybridization. From the difference spin density of NO and NO2 molecules, it can be concluded that the magnetism originates mainly from these molecules and its C and N atom neighbors of C3N. For H2 O and H2 S , HOMO state is completely located on the O atom, but the LUMO state is mostly located on the H atoms, while HOMO plays the prominent role and donates electrons through a
small mixing with C3N orbitals above EF . There is also a mixing with the orbitals below EF , because they are closer in energy. These states induce a small charge transfer from C3N to H2 O and H2 S . The HOMO and LUMO of NH3 and CH4 are located 0.21 eV below and 0.21 eV above EF and these states induce a small charge transfer from these molecules to C3N. The orbitals originally located above EF contribute to the LUMO state. It is found that the HOMO is the only orbital that can have a significant overlap with the C3N orbitals and thus can cause charge transfer. Therefore, these molecules will act as a donor. 5. Conclusion Two-dimensional polyaniline with structural unit C3N is a semiconductor, which has attracted a lot of interest because of its unusual electronic, optoelectronic, thermal and mechanical properties useful for various applications. Understanding the interaction between C3N and adsorbate gas molecules is important for the exploitation of C3N in e.g. gas sensors. In summary, using first principle calculations, the 5
Chemical Physics 526 (2019) 110442
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adsorption of common atmospheric (H2, N2 , O2, CO H2 O and H2 S ) and polluted (CO, CO2, NO, NO2, SO2, NH3 and CH4 ) gas molecules on C3N nanosheet was investigated. A detailed analysis of the optimized atomic structure and electronic properties of adsorption of different molecules was carried out. The results showed that O2 , NO, NO2 and SO2 are chemisorbed on C3N. It was also found that the corresponding electronic structure of C3N was modified. Adsorption of O2 and SO2 molecules, C3N becomes a metal, while under NO2 adsorption it turns into a dilute-magnetic semiconductor with 0.6 µB magnetic moment. Our computational results show that other molecules including H2, N2, CO , CO2 , H2 O, H2 S, NH3 and CH4 are physisorbed causing little distortion of C3N. The amount of charge transfer upon adsorption of these gas molecules are found to be small. Our theoretical studies indicate that C3N-based sensor has a high potential for O2 , NO, NO2 and SO2 detection due to the significant electronic structure changes with moderate adsorption energy.
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Adsorption of molecules on C3N nanosheet: A first-principles calculations A. Bafekry a b c
a,b,⁎
c,⁎
a
, M. Ghergherehchi , S. Farjami Shayesteh , F.M. Peeters
b
T
Department of Physics, University of Guilan, 41335-1914 Rasht, Iran Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium College of Electronic and Electrical Engineering, Sungkyun kwan University, Suwon, Republic of Korea
ARTICLE INFO
ABSTRACT
Keywords: C3 N nanosheet Molecule adsorption Electronic structure First-principle calculations
Using first-principles calculations we investigate the interaction of various molecules, including H2, N2, CO, CO2, H2 O, H2 S , NH3, CH4 with a C3N nanosheet. Due to the weaker interaction between H2, N2, CO, CO2, H2 O, H2 S , NH3 and CH4 molecules with C3N, the adsorption energy is small and does not yield any significant distortion of the C3N lattice and the molecules are physisorbed. Calculated charge transfer shows that these molecules act as weak donors. However, adsorption of O2, NO, NO2 and SO2 molecules are chemisorbed, they receive electrons from C3N and act as a strong acceptor. They interact strongly through hybridizing its frontier orbitals with the p-orbital of C3N, modifying the electronic structure of C3N. Our theoretical studies indicate that C3N-based sensor has a high potential for O2, NO, NO2 and SO2 molecules detection.
1. Introduction Due to the thin thickness and large surface-to-volume ratio of twodimensional materials (2DM), the electronic and magnetic properties can be tuned by adsorbate molecules. Furthermore, adsorption is a significant initial step in the activation of gas molecules. Detection of gas molecules is of great importance in both human health and environmental protection. [2,1] The various types of gas sensors may be useful for numerous applications, such as in medical diagnosis, environmental monitoring and surveillance, agriculture production, military safety and emission control [1,3]. The adsorption of molecules has been extensively studied by first-principle calculations on graphene [4–9] and other 2DM [10–12]. Recently, 2D polyaniline with stoichiometric formula C3N has been successfully synthesized [13]. C3N is predicted to be of importance in variety applications such as in solar cell devices, electrolyte gating and doping of transistors and anode material. [14–16] In addition, theoretical DFT calculations showed that the electronic properties of C3N can be tuned in different ways [17–21]. The single layer C3N, was first reported to be a semiconductor and three possible planar structures were suggested. [23,24] Understanding the interaction between C3N and the adsorbate gas molecules is important for the exploitation of C3N in gas sensors which has been reported in previous works [25,26]. In this paper, we study systemically the adsorption of numerous gas molecules such as, atmospheric gas molecules (including H2, O2, CO2
H2 O ) and and common polluted gases (including CO, NO, NO2 , SO2 , H2 S, NH3 and CH4 ) on C3N. These kinds of calculation results will help to expand our understanding of molecule-C3N interaction and could be of practical interest in different applications. 2. Method I this study we perform total energy and electronic structure calculations, within the generalized gradient approximation of the Perdew-Burke-Ernzerhof variant (GGA-PBE)[27] method, to deal with the exchange-correlation. In this work e use norm-conserving pseudopotentials [28], for carbon and nitrogen anhe d tforeign atoms. The wave functions were expanded in a linear combination of multiple pseudoatomic orbitals (LCPAOs) generated using a confinement scheme [29,30]. The k-points for sampling over the Brillouin zone (BZ) integration were generated using the Monkhorst-Pack scheme [31] and was sampled by k-mesh grid of 23 × 23 × 1 for primitive unit cell and scaled according to the size of the supercell. After convergence tests in OpenMX, an energy cutoff of 300 Ry was chosen so that the total-energies converge below 1.0 meV/atom. The geometries were fully relaxed until the force convergence acting on each atom was less than 1 meV/Å. The C3N were modeled as a periodic slab with a sufficiently large vacuum layer of 20 Åin order to avoid interaction between adjacent layers. The charge transfer was calculated using Mulliken charge analysis [32]. Simulated scanning tunneling microscopy (STM) images
⁎ Corresponding authors at: Department of Physics, University of Guilan, 41335-1914 Rasht, Iran (A. Bafekry). College of Electronic and Electrical Engineering, Sungkyun kwan University, Suwon, Korea (M. Ghergherehchi). E-mail addresses: [email protected] (A. Bafekry), [email protected] (M. Ghergherehchi).
https://doi.org/10.1016/j.chemphys.2019.110442 Received 5 July 2019; Accepted 7 July 2019 Available online 09 July 2019 0301-0104/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Optimized structure of C3N, with its hexagonal primitive unit cell, indicated by a red parallelogram. (b) Total and difference charge density. The blue and yellow regions represent charge accumulation and depletion, respectively. (c) Simulated STM image of C3N. Overlay structure represents C3N repeating unit cell.
were obtained using the Tersoff-Hamann theory [33] for STM images, as supplied in OpenMX code with a bias of + 2.0 V (unoccupied states) and were plotted using WSxM software [34]. 3. Pristine C3N The atomic structure of C3N is a planar hexagonal lattice, as shown in Fig. 1(a). The lattice constant of C3N is 4.86 Å, which agrees well with the experimental value of 4.75 Å[13] and bond lengths are, 1.404 (C–C) and 1.403 (N–C) Å, which are consistent with previous theoretical studies [23,22,17]. With the structural optimization, all structures prefer to stay as a planar sheet, similar to graphene. The difference charge density was calculated by subtracting the charge densities of free C and N atoms from the charge density of C3N (see Fig. 1(b)). The total charge density shows high charge density around the N atom, indicating charge transfer from C to N atoms as shown in Fig. 1(b). Bader charge analysis was performed with Quantum Espresso code. The result shows that in C3N, each N atom gains about 0.6 electron from the adjacent C atoms. The charge redistributions is due to the different electro-negativities of C (2) and N (3) atoms. To provide visible guidance for experimental observation, first-principles DFT calculations were used to calculate the STM image as shown in Fig. 1(c). To produce the calculated STM image, the Kohn-Sham charge density was integrated at a voltage of +2.0 V. To correlate the STM image with the corresponding atomic structure, the carbon and nitrogen atoms are shown with gray and blue balls, respectively. We can see bright spots at the center of the hexagons. The orbital-projected band structure, DOS and PDOS of C3N, are as shown in Figs. 2(a, b). The GGA-PBE calculations demonstrate that C3N is an semiconductor with 0.4 eV indirect band gap between the valence band maximum (VBM) and the conduction band minimum (CBM) which are located at M and points, respectively (see Fig. 2(a)). Also orbital-projected electronic band structure was calculated (See Fig. 2(a)). Notice the VBM is dominated by N/C- pz orbitals and the Dirac-cone is still formed by C- pz orbitals. From DOS and PDOS of C3N in Fig. 2(b), we can see that the VBM is dominated by N/C- pz orbitals and the Dirac-point was formed by C- pz orbitals, whereas the CBM was dominated by C- pz orbitals.
Fig. 2. (a) Electronic structure with Orbital resolved band structure of C3N and (b) DOS and PDOS of C3N.. The zero of energy is set to the Fermi level energy (EF ) .
center of a hexagon with six C atoms (HCC ) , (2) the hollow site above the center of a hexagon with composed of both C and N atoms (HNC ) , (3) the bridge site above the middle of a C–C bond (BCC ) , (4) the bridge site above the middle of a N-C bond (BNC ) , (5) the top site above a C atom (TC ), and (6) the top site above a N atom (TN ) (see Fig. 3(a)). Schematic view of favorable adsorbed molecule orientation on C3N at stable sites, is shown in Fig. 3(b). In the first step, the molecule is placed above these adsorption sites, for each of which different orientations of molecules are assessed. Several typical orientations of the gas molecule either vertically or parallel to the C3N surface are examined, and relative rotation of gas molecule to the C3N surface is considered. For example, for the CO molecule, the molecular axis could be oriented parallel to the C3N surface or perpendicular with the C or O atom pointing to the C3N surface. While for the molecule SO2 , the molecular plane could be oriented parallel to the C3N surface: the S–O bonds are towards the C3N surface. Final structure of C3N adsorbed with various gas molecules corresponding to the most stable site are shown in Fig. 3(c). The adsorption energy of the molecule on C3N was determined as Ea = EC3 N + EMol EMol / C3 N , where EMol / C3 N , denote the total energy of the structure with the molecule adsorption on C3N, EC3 N denotes the total energy of the pristine C3N without molecule, and EMol
4. Adsorption of molecule Adsorption of gas molecules affects on the structural and electronic properties of 2DM. Schematic view of favorable adsorption sites of gas H2, N2, O2, CO, CO2 , NO , NO2 , SO2, H2 molecules including O, H2 S, NH3 O and CH4 O on C3N are shown in Fig. 3(a). After structural optimization, the most stable site configuration among the six different sites are determined where all atoms are relaxed in all direction. These six possible adsorption sites includes: (1) the hollow site above the 2
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Fig. 3. Schematic view of favorable adsorption (a) sites and (b) orientation of molecules including H2, N2, O2, CO, CO2, NO, NO2 , SO2, H2 O, H2 S, NH3 and CH4 on the C3N. (c) Top view of the final structure of C3N adsorbed with various gas molecules corresponding to the stable sites. (d) Adsorption energy of molecules on C3N at most stable site.
denote the total energy of the energy isolated molecule in vacuum. The variation of adsorption for different molecules on C3N at most stable site are shown in Fig. 3(d). The configuration will be considered as physisorbed when the adsorption energy is low and the nearest atomic distance between the adsorbed molecule and the support is much larger than the sum of the covalent radii of the corresponding atoms. The adsorption energies of H2, corresponding gas molecules such as N2, O2, CO , CO2 , NO, H2 O , H2 S, NH3 and CH4 on C3N are quite small, as compared with 0.72 eV and 0.59 eV for NO2 and SO2 , respectively. The optimized structures with corresponding structural parameter for adsorption of H2, N2 , O2, CO, CO2 , NO , NO2 , SO2, H2 O , H2 S , NH3 and CH4 molecules on C3N at most stable site were are also in Fig. 4. It can be seen that the stable adsorption site for H2, N2 , O2, CO and NO molecules is at HCC -site, while CO2 is adsorbed in BNC -site. For H2 and N2 , the H and N atoms points towards the C3N surface and bond lengths are 0.757 (H–H) and 1.119 (N–N) Å. For O2 , O atom points toward the C3N surface and O–O bond length is 1.345 Å, while the CO points parallel to the C3N surface and C–O bond length is 1.345 Å. In CO2 , the C-O bond length is 1.195 Å. The N atom of NO molecule, points toward the surface and bond length of N–O is 1.221 Å. Also the closest distance between the lower atoms of the molecules with the C3N surface is in the range of 2.371 Å (NO)– 3.564 Å (CO2 ). The adsorption energy of H2, N2, O2, CO, CO2 and NO molecules is 0.13, 0.15, 0.29, 0.24, 0.21 and 0.24 eV respectively, which can be considered as physisorption which agrees with the relative large distance between these molecules and the C3N surface. The stable adsorption site for NO2 and SO2 is BNC -site, while the H2 O and H2 S molecules are adsorbed at HNC -site. For NO2 and SO2 , bond lengths were calculated as 1.270 Å (N–O) and 1.528 Å (S-O), while the bond angles are 122° (O–N–O) and 117° (O–S–O). The distance between the molecules and the C3N surface are 2.894 (NO2 ), 2.919 (SO2 ), 2.630 (H2 O ) and 2.962 Å (H2 S ). The NO2 and SO2 , have large adsorption energy of 0.72 eV and 0.59 eV, respectively, which can be considered as weak chemisorption. Whereas, H2 O and H2 S exhibit a small adsorption energy of, respectively, 0.17 and 0.21 eV and therefore can be considered as physisorption. It was found that the NH3 and CH4 molecules are located at stable adsorption site of HNC and TC , respectively. The N–H bond length is 1.304 Å and the H–N–H bond angle is 104°, while the distance between NH3 with the C3N surface is 2.837 Å. The closest distance of the most stable site between CH4 molecule with the C3N surface is 2.821 Å. The C–H bond lengths were 1.101 Å and the H-C–H bond angle is 110° . The NH3 and CH4 /C3 N , had adsorption energy of 0.23 eV and 0.19 eV, respectively. To know the
bonding character between molecules and C3N, the difference charge densities indicated, in the same panel in Fig. 4. The blue and yellow regions represent the charge accumulation and depletion, respectively. It can be seen that electrons were accumulated on the atoms of H2, N2, O2, CO, CO2 and NO molecules, whereas the depletion of electrons in its three nearest C neighbors of C3N, which shows that there are small charge transfers from C3N to these molecules. Difference charge density of these molecules indicates, that there is no formation of chemical bonds, hence we have physisorption rather than chemisorption. The charge transfer analysis showed weak electron acceptors by gaining electrons of 0.01e (H2 ), 0.01e (N2 ), 0.19e (O2 ), 0.14e (CO), 0.14e (NO), 0.01e (H2 O ) and 0.02e (H2 S ) from C3N. Such a subtle charge transfer indicates that these molecules were physisorbed on C3N through weak van der Waals interaction. The charge transfer calculations showed that NO2 and SO2 are strong electron acceptors by gaining 0.26 e and 0.23 e electrons from C3N to NO2 and SO2 , respectively. Such a large charge transfer shows that NO2 and SO2 is weak chemisorbed on C3N. Moreover, the electrons were depleted on the atoms of CO2 , NH3 and CH4 , whereas electrons are accumulated on C3N, which shows there is charge transfer from these molecules to C3N. Charge transfer analysis shows that CO2 , NH3 and CH4 act as weak electron donors by donating −0.002e, −0.002e and −0.004e to C3N. As shown in Fig. 5, H2, N2 , CO , CO2 , H2 O, H2 S, NH3 and CH4 /C3 N molecules can hardly change the band structure around EF . We found that CO/ C3 N becomes a semiconductor with band gap 0.4 eV. Furthermore, NO molecule causes modification to the electronic structure and result in induced impurity states around EF . These impurity states mainly come from states of NO molecule. It is also observed that the and spin states for NO / C3 N exhibit a dilute-magnetic semiconductor character and induce 1 µB magnetic moment. The O2 and SO2 molecules modify the electronic state around EF , giving rise to a vanishing band gap resulting in metallic characteristics with their Fermi levels crossing the electronic states. DOS of these structures is different from that of pristine C3N, showing an impurity state around EF . The NO2 paramagnetic molecule can induce impurity states around EF and these impurity states come mainly from states of NO2 . It becomes a ferromagnetic-metal and spin-splitting occurs in the and spin channels and with 0.6 µB magnetic moment. The very weak interaction of H2, N2 , CO , CO2 and NO molecules with C3N is also reflected by the sharp peaks in the PDOS (see Fig. 6). For these molecules, the HOMO and LUMO is fully filled and lies about 0.22 eV below and 0.21 eV above EF , respectively. When these molecules weakly interact with C3N, it occupies the HOMO state which 3
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Fig. 4. Optimized structures with corresponding structural parameter for adsorption of H2, N2, O2, CO, CO2, NO, NO2 , SO2, H2 O, H2 S, NH3 and CH4 molecules on C3N at the most stable site. Bothe, top and side views are shown. Difference charge density are indicated in the same panel. The blue and yellow regions represent charge accumulation and depletion, respectively.
with the DOS above EF which does not cause any adsorption therefore making HOMO a more important orbital and the charge transfer is consequently always to C3N. The interaction between O2 and C3N is also
moves below EF . Therefore, the net gain of electrons from C3N acts as an acceptor and the charge transfer are due to orbital overlap between the HOMO of these molecules with C3N. The LUMO will mostly interact
Fig. 5. Electronic band structure for adsorption of H2, N2, O2, CO, CO2, NO, NO2 , SO2, H2 O, H2 S, NH3 and CH4 molecules on C3N at the most stable site. The zero of energy is set at EF . 4
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Fig. 6. DOS and PDOS for adsorption of H2, N2, O2, CO, CO2, NO, NO2 , SO2, H2 O, H2 S, NH3 and CH4 molecules on C3N at the most stable site. Charge densities of HOMO and LUMO are shown in the insets. The blue and yellow regions represent charge accumulation and depletion, respectively.
reflected by the broad peaks in the PDOS. For O2 molecule, the HOMO and LUMO are 0.05 eV below and 0.004 eV above EF , respectively. These states induce a small charge transfer from C3N to O2 . When O2 molecule strongly interact with C3N, its occupied HOMO state moves below EF , increasing electrons in O2 molecule. Therefore, it makes an acceptor and the charge transfer are due to orbital overlap between the HOMO of O2 molecule and C3N. For NO2 molecule, the HOMO and LUMO of spin state are 0.097 eV below and 0.115 eV above EF , respectively and also HOMO and LUMO of spin channel are 0.733 eV below and 0.106 eV above the EF . For NO2 molecule, a spin-splitting orbital is located above EF that contributes to the LUMO state. NO2 molecule can accept electrons from C3N and the charge transfer occurs mainly through the orbital hybridization. From the difference spin density of NO and NO2 molecules, it can be concluded that the magnetism originates mainly from these molecules and its C and N atom neighbors of C3N. For H2 O and H2 S , HOMO state is completely located on the O atom, but the LUMO state is mostly located on the H atoms, while HOMO plays the prominent role and donates electrons through a
small mixing with C3N orbitals above EF . There is also a mixing with the orbitals below EF , because they are closer in energy. These states induce a small charge transfer from C3N to H2 O and H2 S . The HOMO and LUMO of NH3 and CH4 are located 0.21 eV below and 0.21 eV above EF and these states induce a small charge transfer from these molecules to C3N. The orbitals originally located above EF contribute to the LUMO state. It is found that the HOMO is the only orbital that can have a significant overlap with the C3N orbitals and thus can cause charge transfer. Therefore, these molecules will act as a donor. 5. Conclusion Two-dimensional polyaniline with structural unit C3N is a semiconductor, which has attracted a lot of interest because of its unusual electronic, optoelectronic, thermal and mechanical properties useful for various applications. Understanding the interaction between C3N and adsorbate gas molecules is important for the exploitation of C3N in e.g. gas sensors. In summary, using first principle calculations, the 5
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adsorption of common atmospheric (H2, N2 , O2, CO H2 O and H2 S ) and polluted (CO, CO2, NO, NO2, SO2, NH3 and CH4 ) gas molecules on C3N nanosheet was investigated. A detailed analysis of the optimized atomic structure and electronic properties of adsorption of different molecules was carried out. The results showed that O2 , NO, NO2 and SO2 are chemisorbed on C3N. It was also found that the corresponding electronic structure of C3N was modified. Adsorption of O2 and SO2 molecules, C3N becomes a metal, while under NO2 adsorption it turns into a dilute-magnetic semiconductor with 0.6 µB magnetic moment. Our computational results show that other molecules including H2, N2, CO , CO2 , H2 O, H2 S, NH3 and CH4 are physisorbed causing little distortion of C3N. The amount of charge transfer upon adsorption of these gas molecules are found to be small. Our theoretical studies indicate that C3N-based sensor has a high potential for O2 , NO, NO2 and SO2 detection due to the significant electronic structure changes with moderate adsorption energy.
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