Magnetic single atom catalyst in C2N to induce adsorption selectivity toward oxidizing gases

Density functional theory (DFT) method is used to study the effect of single-atom catalyst (SAC) of Mn embedded in C2N nanoribbon (C2N-NR) on the adsorption properties as an attempt to achieve selectivity. Many gases (e.g., CO, CO2, H2, H2O, H2S, N2 and O2) of interest to energy and environmental applications were tested. The results show that SAC-Mn alters chemisorption processes with all gas molecules except N2. Clear adsorption selectivity is obtained towards oxidizing CO, CO2 and O2 molecules as evidenced by the enhancements in binding energy and charge transfer and the reduction in magnetization. While the SAC-Mn contributes predominantly to Fermi-energy region with spin-down states, the strong binding to oxidizing molecules introduces there more spin-up states to compromise and reduce the magnetization. Hence, C2N-NR:Mn is proposed to be used as platform for gas sensor (if combined with magnetic sensor) to yield high selectivity toward these latter gases.

www.nature.com/scientificreports/ reactions (e.g., CO x , NO x and SO x ) of environmental concern 29-31 . The successful syntheses of SAC and DAC in C 2 N also inspired and triggered many theoretical efforts to study the corresponding reaction mechanisms [32][33][34] .
On the computational side, many researchers reported the importance of utilization of transition metal (TM) atoms as catalysts for gas-sensing applications. For instance, Zhao and co-workers reported DFT results of the effect of copper dimer (Cu 2 ) embedded in C 2 N on the reduction of CO 2 to hydrocarbons 35 . Ma and co-workers used DFT to test the embedment of five TM atoms (i.e., Sc, Ti, V, Cr and Mn) in C 2 N for CO and O 2 oxidation reactions. They concluded that the best catalyst among these to be Cr and Mn 36 . In a related work, Liu and coworkers presented a combination of electrochemical measurements and DFT calculations to study and compare the catalytic activity of MnN 4 to FeN 4 embedded in graphene in inducing the oxygen reduction reaction (ORR). They clearly demonstrated the superiority of MnN 4 catalyst 37 . In a different study based on DFT, Impeng and co-workers investigated the correlation between magnetic moment and strength of chemisorption of 13 gases on MnN 4 -embedded graphene 38 . Only five gases (i.e., NO, NO 2 , O 2 , CO, and SO 2 ) exhibited strong chemisorption with large binding energies (-2.30, -1.42, -1.32, -1.11, and -0.51 eV, respectively). Furthermore, only three gases (i.e., NO, CO and NO 2 ) were able to reduce the magnetization from 3.01 μ B to 0.13, 1.04, and 2.01 μ B , respectively. The estimated recovery times for these three gases were large except CO gas to have 1.7 s. So, the authors proposed MnN 4 -embedded graphene as a platform for a promising magnetic sensor for CO detection 38 . Regarding the magnetism of TM-atom embedded in C 2 N, Du and co-workers presented DFT study of magnetization due the embedment of 3d-TM atoms into the pore of C 2 N. They reported the existence of ferromagnetic ground state for TM atoms: Sc, Ti, V, Cr, Mn, Fe, Co and Ni, and paramagnetic state for Cu and Zn 39 . Wang and co-workers presented DFT study of magnetization in C 2 N nanoribbons (C 2 N-NRs) under the effect of diversifying the passivation of the dangling bonds of the edge atoms 40 . They reported that armchair-edged C 2 N-NR is a non-magnetic semiconductor with direct gap. Whereas zigzag-edged C 2 N-NR is magnetic either semiconductor with indirect band-gap or metallic. Besides the co-saturation of edges with (H and O) or (F and O) can induce a large magnetization into the system to make the system an interesting material for spintronic devices 40 .
The scope of the present work is to study the effect of a magnetic SAC-Mn embedded in C 2 N nanoribbon on the adsorption of seven gases of energy and environmental interest (e.g., CO, CO 2 , H 2 , H 2 O, H 2 S, N 2 and O 2 ) using the state-of-the-art DFT based on VASP. The aim is the attainment of selectivity and analyzing the reasons behind it or its origins, with hypothesis to include among the arguments the inspection of variation of magnetization and charge transfer (i.e., type of molecule whether oxidizing or reducing). The paper is organized as follows: Last section gives details about the model and the computational method. Section 2 gives elaboration of discussing the results. The concluding section summarizes our main findings.

Results and discussion
Atomic relaxations. As first stage, the adsorptions of gas molecules on pristine C 2 N nanoribbons were attempted. Atomic relaxations of gas molecules (e.g., CO, CO 2 , H 2 , H 2 O, H 2 S, N 2 and O 2 ) were included in our study. All molecules are found to exhibit physisorption processes on pristine C 2 N-NR and prefer to stabilize above the main pore of the C 2 N-NR at a height of about 2.12, 2.39, 1.61, 1.25, 1.42, 2.30, and 2.0 Å, respectively. In case of angular molecules (e.g., H 2 O and H 2 S) the reported distances are even lower (1.25 and 1.42 Å, respectively) as these molecules are found to stabilize by having their arms directed toward the surface; as likely due to H atoms being electropositive and getting attracted to the electronegative nitrogen atoms of the pore via van der Waals interactions. In the physisorption states of these latter 2 molecules, their angles are found decreased to 101° and 90°; while in the cases of free molecules the angles used to be about 104° and 92°, respectively. Such reductions in angular parameters might be attributed to the transfer of charge from surface to molecules; especially as being oxidizing molecules. However, for the other linear molecules (CO, CO 2 , H 2 , N 2 , O 2 ), they seem to stabilize at distances more than 2.0 Å above the surface and in horizontal position parallel to the surface with molecular atoms being close to N-atoms along the direction of the diagonal of the pore.
In second stage, the adsorptions of the same gas molecules were attempted on Mn-embedded C 2 N-NRs (i.e., C 2 N-NR:Mn). The results of atomic relaxations are shown in Fig. 1. Chemisorption processes seem to be the destiny to occur for all molecules except N 2 , whose triplet covalent bond seems strong and not dissociable to persist in yielding physisorption process. The N 2 molecule stabilizes at a distance 3.10 Å above Mn-atom. Table 1 shows the adsorption energy in case of N 2 to be as low as E ads = -0.123 eV and the molecule to be oxidizing the Mn catalyst with mimic charge transfer of about -0.0178 e. The effect on magnetization is, in turn, very mimic of about 0.027 μ B reduction to corroborate the weak interaction between N 2 molecule and Mn-catalyst.
All other studied molecules are found to act as oxidizing the Mn-catalyst. Three molecules (O 2 , CO, and CO 2 ) seem to bond strongly to the Mn catalyst causing huge effect on its magnetization more than any other molecule, with reductions of about 1.011 μ B , 0.303 μ B and 0.913 μ B , respectively. Also, the charge exchanges are drastically large of about -0.779 e, -0.173 e and -0.570 e, respectively. These two indications are signals of potential occurrence of selective gas-sensing of C 2 N-NR:Mn towards these three molecules. It should be emphasized that the chemisorption of O 2 occurs with partial molecular dissociation as the molecule breaks its π-bond to pave the way for the two oxygen atoms to form new bonds with Mn-catalyst. In case of CO-molecule chemisorption, the bond is formed between Mn-C as carbon atom has ability for four valency. Again, in chemisorption of CO 2 -molecule, the two π-bonds between C-O breaks to pave the way for C-atom to form covalent bond with Mn-catalyst. Consequently, the angle of CO 2 molecule changes from 180° to 144°.
Relatively weaker chemisorption processes of different style of bonding occur between H 2 -based molecules and the Mn-catalyst embedded in the pore of C 2 N-NR.   Table-1 and was explained in details in sub-Sect. 3a in discussing results of Figure-1. Their corresponding band structures (see Panels 2b, 2c and 2 h, respectively) show the evidence of enormous changes to occur in the spin-down band structures. For more details, one must deal with density-of-states perspectives (e.g., see next sub-sections). Nevertheless, these modifications happening in the spin-down band-structures might be originated from the huge reductions in magnetization (see Table- . In order to further investigate the origin of the discrepancy (i.e., ΔM), one needs to dig into the spin-polarized partial and orbital density of states that we do next.   www.nature.com/scientificreports/ contributions whose discrepancy should account for the formation of magnetization. First, one should start describing the case of the substrate or the platform to be used for the gas detection or sensing.
Panel 4(a) shows the case of C 2 N-NR:Mn before the arrival of any gas molecule. Mn-catalyst's PDOS (in red color) is shown to have clearly huge asymmetry with large contribution from spin-down states. Mn is located at the pore center and that should be considered to be the center of the magnetization. Mn-catalyst would induce some magnetization to neighboring atoms (i.e., so called "spin relaxation"). So, N-atoms' PDOS (blue color), in turn, shows some asymmetry to reveal its participation to the collective magnetization. C-atoms' PDOS (in black www.nature.com/scientificreports/ color) is the lease affected but does show some asymmetry in its overall profile especially above Fermi level. Now, one should discuss the effects of interactions of lattice (platform) with various gas molecules. In panel 4(b): CO molecule's PDOS (in green color) show great asymmetry contributions between spin-up and spin-down states. Near Fermi level, one can notice more contribution from spin-up states in response to the bonding of C-atom to Mn-catalyst due to the occurrence of chemisorption. So, one should expect some reduction of magnetization (i.e., ΔM = -0.303 μ B ). In panels 4(c,h): CO 2 and O 2 molecules' PDOSs (in green color) show even stronger effect than CO-molecule. Large contribution is due to spin-up states and causing large reductions in magnetization (i.e., ΔM = -0.913 μ B and -1.011 μ B , respectively). In panels 4(d,e,f): H 2 , H 2 O and H 2 S molecules' PDOSs (in green color) are basically overlapped to follow the lattice profile. So, the asymmetry is not pronounced as in previous case and consequently the effect on magnetization is mimic (i.e., |ΔM|= 0.03-0.09 μ B ). The last panel to discuss is panel 4(g): N 2 molecule exhibits physisorption process so that its PDOS shows discrete molecular levels (i.e., see 2 spin-up states at energies E = + 1.2 eV and + 1.3 eV, and one spin-down state at energy E = + 1.3 eV). Yet somehow, small magnetic moment got induced into the N 2 molecule by its being close above Mn-catalyst at a distance of about 3.32 Å (i.e., with ΔM = -0.027 μ B ). Figure 5 shows the results of ODOS on the same systems studied in previous figure. This figure should investigate the original atomic orbital contributing to magnetization or causing effect on it. Panel 5(a) shows the case of our substrate/platform (i.e., C 2 N-NR:Mn). The valence electrons in Mn-catalyst reside at s and d-states. Panel 5(a) shows that d-states have huge contribution to magnetization by having huge discrepancy between spin-up and spin-down ODOSs. N and C atoms are forming the lattice with planar hybridization sp 2 while their P z -orbitals populate the states at Fermi level and are accessible to be perturbed under the effect of magnetization; especially if this latter is attributed to z-based d-orbitals (i.e., d yz , d zx , d z 2 ). So, we decided to present only P-orbitals of C, N, S and O atoms to assess their contributions to magnetization (with PDOS in light-blue color). Panels 5(b,c,h) show the cases of strong chemisorption processes with CO, CO 2 and O 2 molecules. It seems that the formation of an axial σ-bond between the molecule and Mn-catalyst would impose on the molecule to get its electronic contributions from spin-up states because Mn-catalyst has mostly spin-down states near Fermi level. This is to fulfill the requirements of Pauli-exclusion principle. In doing such act and having strong covalent bond would reduce the discrepancy in electronic population between spin-up and spin-down. Consequently, such Pauli exclusion principle effect would cause large reductions in magnetization. Panels 5(d,e,f) correspond to the weak chemisorption processes of H 2 , H 2 O and H 2 S molecules, respectively. The chemical bonds here are weaker and Pauli-exclusion principle is less effective. So, the discrepancy between spin-down and spin-up PDOSs are less pronounced than those of the previous group of molecules. So, these latter chemisorption processes cause less effect on magnetization. Last case is in Panel 5(g) corresponding to the physisorption of N 2 molecule. The molecular states are discrete and having less discrepancy between spin-up and spin-down states. So, the effect on magnetization is the least one.
Charge transfer. Figure 6 displays the difference in charge density plot between the cases after the adsorption and before the adsorption for 7 molecules adsorbed on C 2 N-NR:Mn substrate in the following order: (a) CO, (b) CO 2 , (c) H 2 , (d) H 2 O, (e) H 2 S, (f) O 2 , and (g) N 2 molecules. To study the charge-transfer process between molecule and substrate/platform, it is customary to explore one of three methods: (i) Compare the charge contour plots before and after adsorption; or (ii) Compare the plots of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) before and after adsorption; or (iii) Do everything in one plot illustrating the charge difference/transfer such as the one shown in Fig. 6, where yellow (cyan) color indicates charge depletion (accumulation).
In all cases, the 7 molecules are oxidizing the Mn-catalyst. In the 6 cases of chemisorption, the molecule is attached to Mn-catalyst via covalent bond with partial ionic character. Panels 6 (a,b) show that the cases of chemisorption of CO and CO 2 molecules to be associated with the depletion of charge from vicinity of Mn and its shifting toward C and O in the molecule as well as the 6 N atoms neighboring Mn. The discrepancy in electronegativity characters between the atoms play a major role in the charge distribution along the bond. For instance, χ O = 3.44 > χ N = 3.04 > χ C = 2.55 > χ Mn = 1.55 (in units of Pauling) can be used to justify the accumulation of charge at the sites of anion atoms of higher electronegativity. Panel 6(c) shows the case of chemisorption of H 2 to be accompanied with the transfer of charge toward H atoms because χ H = 2.20 > χ Mn = 1.55 (in Pauling units). Furthermore, the charge distribution clearly shows the H-atoms to be well separated confirming the occurrence of chemisorption associated with a weak molecular dissociation. Panels 6(d,e) show that the H 2 O and H 2 S molecule maintain their molecular structures while the anion atom is making somehow weak covalent bonding with Mn-catalyst (i.e., so less ΔM occurs). Panel 6(f) demonstrates a kind of strong chemisorption with O 2 molecule with strong charge transfer to it. Last panel 6(g) shows the case of N 2 physisorption. It is clearly shown almost inexistence of charge transfer between molecule and substrate. Figure 7 summarizes our findings and displays the results of Table-1 concerning the absolute values of adsorption energy, charge transfer, and change in magnetization. Clearly, one can distinguish two groups of molecules in the perspective of their interactions with the SAC-Mn embedded in C 2 N-NR: (i) A first group with strong and well-pronounced oxidizing character (e.g., CO, CO 2 and O 2 ) with strong chemisorption: These molecules have strong adsorption energy. They make strong covalent bonds with SAC-Mn and thus introduce spin-up states to TDOS in the Fermi-energy region and consequently reduce the discrepancy between spin-up and spin-down TDOS. Thus, they are able to reduce enormously the net magnetization. (ii) A second group with vanishing oxidization or rather known to have reducing character (e.g., N 2 , H 2 , H 2 O, and H 2 S) with either physisorption or weak chemisorption: These molecules have weak interactions with SAC-Mn. Thus, they contribute mostly with spin-down states to TDOS in the Fermi-energy region of same kind as the magnetic Mn-atom. So, the discrepancy between spin-up and spin-down TDOS remains about unchanged and the effect on changing the www.nature.com/scientificreports/ magnetization would be vanishingly small (i.e., the magnetization is maintained robust or conserved). So, one can conclude that C 2 N:Mn would be a good candidate for platform in the fabrication of magnetic sensor with promising selectivity to detect the oxidizing gas molecules (CO, CO 2 and O 2 ).

Conclusions
Aiming for Gas-sensing selectivity, we presented a theoretical study of functionalized C 2 N nanoribbons based on the DFT-package of VASP. Magnetic atom such as Mn is used as SAC embedded in C 2 N pore to test the effect on the adsorption properties of various gases of interest in energy and environmental sciences (e.g., CO, CO 2 , H 2 , H 2 O, H 2 S, N 2 and O 2 ). The results of atomic relaxations show that pristine C 2 N-NR always alters physisorption processes with these gases while C 2 N-NR:Mn has the full ability to alter chemisorption processes with all gas molecules except N 2 . The results of molecular chemisorption can be categorized into two groups: (a) Strong chemisorption with oxidizing CO, CO 2 , and O 2 gas molecules is evidenced by large binding energy and charge transfer. Mn-catalyst seems very active in interacting with the oxidizing gas molecules. Furthermore, SAC-Mn induces large magnetization into the system by contributing enormously with spin-down states to the electronic band-structure at Fermi-energy region, while its interaction with these mentioned molecules would introduce spin-up states at Fermi level to compromise and rather reduce the magnetization. Actually, such phenomenon occurs as a consequence of the Pauli-exclusion principle; (b) Weak chemisorption with reducing H 2 , H 2 O, and H 2 S gas molecules is observed with low binding energy and charge transfer. The interaction does not affect the asymmetry in spin-up and spin-down statistics and the discrepancy remains about the same between the states of spin-up and spin-down. Consequently, these adsorptions have mimic effects on magnetization (i.e., M persists to remain constant).
The results suggest that C 2 N-NR:Mn is a promising platform for gas sensing of oxidizing CO, CO 2 and O 2 gas molecules with high sensitivity and selectivity. The efficiency of the sensor could be further enhanced if it is combined with magnetic sensor to detect the change in magnetization and the system would be of great importance in environmental applications.
Computational methodology. Density functional theory (DFT) has been well established to be the most reliable to predict the ground state properties of materials, including the adsorption properties. Perhaps, the worldwide most popular and reliable package is the Vienna Ab-initio Simulation Package (VASP) 41 , which masters to incorporate all basic interactions, such as spin-polarization, magnetic (Hubbard U) and dipole-dipole long-range (i.e., van der Waals) interactions. Furthermore, the package is competent to deal with challenges in our system which involves magnetic dopants (e.g., manganese "Mn") and its binding as embedded in pore of C 2 N as well as its interaction with gas molecules in order to reliably predict the occurrence of either physisorption or chemisorption. Our calculations include atomic relaxation to study the adsorption of various gas molecules, known to be either oxidizing (e.g., CO, CO 2 , O 2 ) or reducing (e.g., H 2 , H 2 O, H 2 O, N 2 ), spin-polarized band structures and partial as well as orbital densities of states, difference of charge density plots, Bader-charge analysis, and magnetic moments. Targeting to use such system of Mn-embedded in C 2 N as a platform of electrical gas sensor, we decided to study the system in the nanoribbon form 40 .
As a model, we have designed a nanoribbon shape with zigzag edges and made the dangling bonds be saturated at the two edges by bonding them with hydrogen atoms. Actually, such a structure was used before us and exists in literature 40,41 . We made sure that it is thermodynamically stable and it yields metallic behavior (i.e., Eg = 0), to make it suitable for gas-sensing applications. The designed C 2 N-NR structures are displayed in Figure-1. We emphasize that the primitive cell of pristine C 2 N-NR is composed of 50 atoms (i.e., 32 C + 10 N + 8   (1) Pristine C 2 N-NR, or (2) SAC-Mn embedded in the central pore of C 2 N-NR; and as adsorbate one gas molecule at a time among the 7 gases mentioned above. As a method, the calculation employs plane-wave basis set (with cutoff energy of 500 eV). Within the framework of the projected-augmented plane-waves (PAW) method, the electronic exchange-correlation was treated using the generalized gradient approximation (GGA + U) 43 , where + U stands for the Hubbard parameter which is usually added in case of highly-correlated spin systems such as transition metal, and especially those which are ferromagnetic such as in our case "Mn". The Hubbard parameter U = 3.5 eV was taken for Mn 3d-states was due to reference 43. Furthermore, we included in our calculations DFT-D3 technique 44,45 to take care of the van der Waals interactions, which are important in the study of adsorption. In the geometry optimization, all the atoms in the supercell were allowed to relax until the Hellmann-Feynman force on each atom became smaller than 0.01 eV Å −1 ; whereas the tolerance for the total energy convergence was set to 10 -4 eV. The sampling of the Brillouin zone was performed using 25 × 1 × 1 Monkhorst-Pack technique 46 for total energy calculations. However, the density of states' calculations were performed with a relatively denser grid of 50 × 1 × 1 k-mesh. The adsorption energy of any gas molecule on C 2 N-NR substrate was evaluated using the formula: where E C 2 N−NR+gas , E C 2 N−NR , and E gas are the total energies of the system of pristine (or Mn-embedded) C 2 N-NR with and without gas molecule and the gas molecule, respectively. Furthermore, we emphasize that the charge transfer was estimated using the Bader-charge analysis 47 .
In order to address the magnetic coupling, and to find the most stable magnetic state, a dimer of Mn is embedded in the pore of C 2 N-NR (i.e., C 2 N-NR:2Mn) and the system is relaxed under ferromagnetic (FM) and anti-ferromagnetic (AFM) spin configurations. The obtained results of total energy and magnetization are summarized in Table 2.
The results were in favor of anti-ferromagnetic state as having lower total energy by ΔE = E tot (FM)-E tot (AF M) = + 0.595 eV value. Nevertheless, in our present investigation, the focus will not be upon the dimer-atomcatalyst (DAC) case but the full focus will be upon the single-atom-catalyst (SAC) case. So, we study single Mn atom embedded in C 2 N-NR and assess its interaction with various gas molecules. Definitely, we expect non-zero magnetic moment and rather large effect of magnetization on the electronic structures as will be demonstrated by the spin-relaxation effects. (1) E ads = E C 2 N−NR+gas − E C 2 N−NR − E gas ,