The Investigation of Adsorption Behavior of Gas Molecules on FeN3-Doped Graphene
Herein, we have investigated the adsorption behavior of gas molecules, including C2H2, H2S, SO2, SO3, and O2, on FeN3-doped graphene (FeN3-gra). The change of geometric stability, electric structure, and magnetic properties is discussed comprehensively. The results have demonstrated that the stability of the substrate is enhanced by the hybridization between Fe and N atoms in FeN3-gra. Besides, the Fe dopant can enhance the adsorption ability of gases on graphene. The gas molecules all exhibit high binding strength on FeN3-gra especially for SO3 with the adsorption energy of -3.30 eV. The mechanism of interaction between gases and substrate is investigated based on the charge density difference and density of states, which can clarify the distribution of electrons and magnetic moments. Moreover, the high stability and sensitivity of FeN3-gra are promising characters for gas detection. Our research has paved the way for the application of the graphene-based material in gas sensor and electronic instrument.
The air pollution caused by the emission of poison gases has attracted global attention in recent years. For example, the gases, such as C2H2, H2S, SO2, SO3, and O2, can harm the health of human beings in our daily life. Among these gases, SO2 and SO3 are the general inducement of acid rain ; H2S is a kind of combustible gas with no colour ; C2H2 is a gas with a little poison for cells in the human body ; the gas sensor can be used to detect inflammable and poison gases and has been applied extensively into areas including industry and fire control . Based on this basic model, many works have also been performed on the gas capture and catalysis [2, 3, 5–8]. The commonly used adsorbates mainly include metals, metal oxides, and graphene-based materials. The graphene-based single-atom catalysts (SAC-gra) have been extensively applied into gas sensor due to the merits including low cost, high conductivity, perfect chemical stability, big specific surface area, and high mechanical strength [9–11]. Most importantly, the electric properties of substrates can be changed under the influence of molecule adsorption, which mainly originates from the charge transfer between molecules and the substrates . Thus, graphene is a promising candidate in gas sensor [13–15]. In some experimental works, the graphene has been made into some devices of gas sensors with high sensitivity [16, 17], which has great significance for establishing early warning systems.
The transition metals (TMs) and N atoms are commonly used dopants in graphene-based SAC, which play an important role in catalytic reactivity [18–23]. The Fe SAC based on N-doped graphene has been proved to possess remarkable reactivity in pioneering works . Thus, the N-coordinated SAC-gra has been extensively investigated by scientists to find out the effective candidates for electric catalysts [25–27]. For example, the first-principle investigation of N2 adsorption on FeN3-doped graphene (FeN3-gra) has been performed and the results demonstrate that the catalyst can activate N to NH with high efficiency . Liu et al. have also investigated the degradation of CH2O to H2O and CO2 on FeN3-gra, indicating that the desorption process of the reaction products is advantageous in this system . However, the reports on the gases, including C2H2, H2S, SO2, SO3, and O2, are limited on FeN3-gra.
Herein, we have performed calculations based on density functional theory (DFT) on FeN3-gra, including geometric configurations, stability, and origin of magnetic. In addition, the adsorption of gas molecules, including C2H2, H2S, SO2, SO3, and O2, has also been studied on FeN3-gra. The calculations are mainly focused on the change of configurations, charge density, and spin states influenced by the interaction between gas molecules and substrates. The intrinsic mechanism has also been analyzed via density of states (DOS). Our results may pave the way for development of gas sensor on both experimental and theoretical aspects.
2. Computational Methods
In this work, the first-principle calculations are carried out based on density functional theory (DFT) in the Vienna ab initio simulation package (VASP). Exchange and correlation potential is illustrated by the projector augmented wave (PAW) [30, 31] and the generalized gradient approximation (GGA) in format [32–34]. The interaction of the van der Waals (vdW) is employed with DFT-D3 . Other geometric and physical parameters are described in detail in our previous work . The charge transfer characteristics between FeN3-gra and the catalysts are evaluated by the Bader charge .
The binding energy of single Fe atom in Fe/N3-gra is expressed as where the , , and denote total energy of Fe/N3-gra, Fe/N3-gra with Fe vacancy, and single Fe atom in vacuum, respectively.
The adsorption energy of gas molecules on Fe/N3-gra is expressed as where the , , and denote total energy of gas adsorbed Fe/N3-gra, Fe/N3-gra surface, and single gas molecules in vacuum, respectively.
In addition, the electron density difference is visualized via VESTA3 .
3. Results and Discussion
3.1. The Configuration and Electric Properties of FeN3-gra
The configuration of FeN3-gra after full optimization is shown in Figure 1. The length of Fe-N bonds is 1.87 Å. The uplift height of Fe atom above graphene plane is about 1.47 Å, which can serve as an appropriate adsorption site . In order to verify the stability of the configuration, the binding energy of Fe atom in the FeN3-gra is also calculated with a value of 4.97 eV, which is larger than the cohesive energy of Fe. This may indicate that the N atoms can bind Fe tightly enough to prohibit the aggregation of Fe atoms into clusters. Therefore, the FeN3-gra possesses strong geometric stability, which is beneficial for application in gas adsorbate. Table S1 has summarized the data of bond length, the uplift height of Fe, charge, binding energies, and magnetic moments. The values calculated generally agree with the pioneering works, indicating the accuracy of our results .
It is worth noting that the charge transfer (CT) between Fe and N atoms in FeN3-gra is 0.96 e (Bader charge). However, the CT between Fe and adjacent C atoms in Fe-doped graphene with no N doping (Fe-gra) is only 0.69 e . Thus, N doping can lead to the formation of covalent bonds between Fe and the substrate. Besides, the N doping can also increase the magnetic moment from 0 (Fe-gra) to 3.17 μB (FeN3-gra), which further leads to a half-occupied state of the system [41, 42]. In order to verify the magnetic character of FeN3-gra, the DOS has been calculated, showing an asymmetrical distribution between up and down spin, leading to a high magnetic moment (see Figure 2). There exists a strong overlap between Fe-3d states and total dos (TDOS) of FeN3-gra. This may indicate that the magnetic moments are mainly contributed by the 3d states, which can be enhanced via the hybridization between Fe and N atoms. In order to further clarify the character of magnetism, we have also displayed the density of spin in Figures 1(c) and 1(d). The distribution of spin mainly concentrates on the Fe site, which agrees with the DOS.
3.2. The Adsorption Behavior of C2H2 and H2S
The optimal adsorption configurations of gas molecules are confirmed after comprehensive calculation of different adsorption orientations (see Figure S1).
We first focus our study on the adsorption of C2H2 and H2S on FeN3-gra. For C2H2 adsorption, the C-C bond distributes nearly parallel to the FeN3-gra surface (see Figure S2). The bond angle and bond length of C2H2 have changed dramatically after adsorption, which is mainly caused by the strain around Fe dopant. The adsorption energy and adsorption distance are 2.23 eV and 1.93 Å (see Table S2), which agrees with the situation on Mn-doped graphene . The Fe doping can also promote the charge transfer between gas molecules and substrates (see Table S2): the C2H2 and H2S all act as electron acceptor and are negatively charged with 0.43 e and 0.07 e, respectively (see Figure S3). For H2S adsorption, H2S tends to adsorb on the top site of FeN3-gra with an adsorption energy of 1.19 eV, which agrees with the pioneering works [22, 40]. The bond length and bond angle of H2S are similar as adsorbed on FeN3-gra and Fe-gra. Therefore, the N doping has nearly no influence on the adsorption behavior of H2S (see Table S3) .
In order to clarify the adsorption behavior of C2H2 and H2S on FeN3-gra, we have also calculated the partial density of state (PDOS). As shown in Figure 3(a), the distribution of Fe-3d states has become delocalized after C2H2 adsorption and hybridized with C2H2’s orbitals dramatically near Fermi level. In addition, the spin states have also changed significantly with an increased magnetic moment of 3.02 μB (see Table S2), which agrees with the spin density (see Figure S1 (a)). Therefore, the regulation of DOS near Fermi level can significantly change the electric and magnetic properties of FeN3-gra, which is beneficial for the application in spintronic devices.
In comparison, the hybridization between H2S and FeN3-gra is much less than C2H2 situation, leading to a weaker adsorption ability of H2S. The Fe in FeN3-gra can form chemical bonds with H2S (see Figure S3 (b)), which agrees with PDOS analysis. Similar to C2H2, the spin states of H2S/FeN3-gra complex has changed significantly for both up and down spins compared to bare FeN3-gra (see Figure 3(b)), leading to a total magnetic moment of 3.24 μB. The increased spin state originates from the charge transfer from FeN3-gra to H2S, leading to a decreased electric energy of H2S. The spin states are strongly localized on Fe site in FeN3-gra (Figure S1 (b)). The changed band gap is particularly interesting, which is beneficial for the application in the gas sensor for H2S.
3.3. The Adsorption Properties of SO2 and SO3
The adsorption configurations of SO2 and SO3 after full optimization are displayed in Figures S2 (c) and S2 (d). Table S1 displays the adsorption energies, charge transfer, adsorption distance between gas molecules and FeN3-gra, adsorption height, and magnetic moments. For SO2 adsorption, SO2 tends to bind Fe atom with O atoms (see Figure S2 (c)), which agrees with the situation of Ti-doped graphene [44, 45]. Unlike H2S, the N doping can effectively enhance the adsorption of SO2 with an increased adsorption energy of 0.25 eV. The length of Fe-O bond is 1.97 Å, and the angle of SO2 has decreased to 98.1° after adsorption. Compared to free SO2 molecule, the S-O bond has enlarged about 0.11 Å, indicating a decreased density of S-O bonds. In comparison, SO3 exhibits an enhanced adsorption strength than SO2 with an adsorption energy of 3.30 eV (). This adsorption strength is much larger than the results reported in the pioneering works on Fe-gra [40, 45]. Thus, the enhancement effect of N doping exhibits much favorable effect for SO3 compared to SO2 situation. The SO3 binds FeN3-gra bidentately with two O atoms with Fe, and the corresponding Fe-O bond length is 1.88 Å. One O-S bond in SO3 distributes parallel to FeN3-gra surface. Compared to free SO3, the bond angle (length) of O-S-O (S-O) in SO3 has decreased (increased) from 120° (1.44 Å) to 44.2° and 108.7° (1.63 Å and 1.46 Å). These results have demonstrated that the N doping can effectively enhance the adsorption of SO2 and SO3.
In order to clarify the adsorption of SO2 and SO3 on FeN3-gra, we have also analyzed electric properties including the charge transfer, DOS, and density of spin during the interaction between FeN3-gra and SO2/SO3 (see Table S2). The SO2 and SO3 are negatively charged with 0.73 e and 1.09 e, respectively. The SO2 and SO3 act as the electron acceptor, which agrees with the pioneering works [1, 22, 40]. In order to describe the process of charge transfer visually, we have also calculated the charge density difference (CDD) (see Figures S3 (c) and S3 (d)). The CDD demonstrated that the charge mainly accumulates on O atoms in SO2/SO3 and Fe atom in FeN3-gra. Larger charge transfer may cause significant change of conductivity, which is beneficial for improved sensitivity for gas sensing [46, 47].
In order to clarify the change of electric properties in detail, we have analyzed the DOS to investigate the adsorption properties of SO2 and SO3 (see Figure 4). The DOS of gas molecules has experienced dramatic change caused by the strong interaction between FeN3-gra and SO2/SO3. The DOS of SO2 ranging from -1.5 to 0 eV has become delocalized after adsorption on FeN3-gra. In addition, the magnetic moment of FeN3-gra has decreased from 3.17 to -1 μB due to the no magnetic character of SO2 and SO3. The spin mainly concentrates on SO2 and Fe atom with identical spin direction (see Figure S1 (c)). There exists a significant overlap between Fe-3d states and SO2 at energy range of -1~0 eV and 1~1.5 eV (Figure 4(a)). This can also be reflected from the charge overlap in Figure S3 (c), indicating the strong chemical interaction between Fe-3d and SO2. These characters can also illustrate that a delocalized DOS distribution of SO2 may contribute to enhance the adsorption strength. About SO3, the TDOS ranging from -3 to 0 eV has increased and shifted to lower energy level after SO3 adsorption. Similar to SO2, the adsorption of SO3 can also induce decreased magnetic moments (see Table S2). The formation of spin states of SO2 near Fermi level can hybridize dramatically with Fe. The spin states mainly concentrate on Fe site as shown in Figure S1 (d). As shown in Table S3, the occupied states in SO3 can lead to increased length of S-O bond, which is verified from charge transfer in Figures S3 (c) and S3 (d). Above all, the analysis of electric and magnetic properties can pave the way for FeN3-gra application in gas sensor and spintronic device.
3.4. The Adsorption Properties of O2
In addition, O2 is also an important gas that is frequently present in the atmosphere. We thus investigated the adsorption behavior of O2 at the end of this article. The most stable configuration of O2 adsorption after optimization is shown in Figure S4. Other information, including the adsorption height, binding distance, charge transfer, adsorption energy, and magnetic moment, is displayed in Table S2. As shown in Figure S4, O2 can adsorb on FeN3-gra parallelly with an adsorption energy of -3.03 eV, which is similar to pioneering research . This result has proceeded about 1.5 times of the adsorption energy on FeSV-gra . In addition, the FeN3-gra substrate can activate O2 more effectively than FeSV-gra with a bond length of 1.42 Å of O-O bond, compared to the 1.39 Å on FeSV-gra. This character may originate from the enhanced interaction between O2 and FeN3-gra.
In order to clarify the O2 adsorption on FeN3-gra, we have also calculated the electric properties, including charge transfer, DOS, and spin density. The O2 has been negatively charged with -0.77 e by the substrate, indicating the strong interaction between O2 and FeN3-gra. In other words, O2 can act as an electron acceptor. These characters can also be reflected from the charge density difference (Figure S4 (c)): There exists a strong overlap between the charge distribution of O2 and FeN3-gra, indicating a covalent bond. In addition, O2 has been magnetized after adsorption on FeN3-gra as shown in Figure 5: The spin density distributes both on O2 and FeN3-gra. This may derive from the significant hybridization between O2 and Fe-3d orbitals (Figure 5(a)).
In conclusion, we have investigated a series of gas molecules, including C2H2, H2S, SO2, SO3, and O2, on FeN3-gra. These molecules can interact strongly with FeN3-gra. Particularly, FeN3-gra can adsorb SO2/SO3 and O2 much strongly than Fe-gra, which mainly originates from N doping. The strong interaction may induce dramatic change of electric conductivity of FeN3-gra, leading to a high sensitivity for gas sensing. Besides, the adsorbed gas molecules can modulate the magnetic property of FeN3-gra effectively. Thus, our results can provide theoretical basement for applications of gas sensing and spintronic devices.
The data underlying the results presented in the study are available within the manuscript and supplementary materials.
Conflicts of Interest
All the authors declare no conflict of interest.
This research was supported by the Transformation of Scientific and Technological Achievements Programs of Higher Education Institutions in Shanxi (No. 2020CG032), Cultivation Plan of Young Scientific Researchers in Higher Education Institutions of Shanxi Province, and the Fund for Shanxi “1331 Project”.
The supporting information is available free of charge on the Hindawi website publications at DOI. Table S1: the comparison of parameters between our work and the pioneering studies for FeN3-gra substrate: the length of Fe-N bond (/Å), the uplift height of Fe (/Å), binding energy of Fe ([Fe]/eV), charge transfer between Fe and graphene (CT/e), and the magnetic moment of the system (/μB). Table S2: the parameters of gas molecules, including C2H2, H2S, SO2, and SO3, adsorbed FeN3-gra: the length of Fe-N bond (/Å), the uplift height of Fe (/Å), the binding energy of Fe (Eb[Fe]/eV), the adsorption energy of gas molecules (/eV), the charge transfer between gas molecules and FeN3-gra (CT/e), and the magnetic moment of the system (/μB). Table S3: the geometric configurations of different systems: the length (/Å) and angle (/°) of various bonds in the gas molecules in separated and adsorbed states. Figure S1: the spin density of gas molecules, including (a) C2H2, (b) H2S, (c) SO2, and (d) SO3, on FeN3-gra (spin up (yellow) and spin down (cyan); isosurface value: 0.01 e/Bohr3). Figure S2: the adsorption configurations of gas molecules, including (a) C2H2, (b) H2S, (c) SO2, and (d) SO3, on FeN3-gra. Figure S3: the charge density difference (CDD) of (a) C2H2-FeN3-gra, (b) H2S-FeN3-gra, (c) SO2-FeN3-gra, and (d) SO3-FeN3-gra. GN. The accumulation and depletion of electrons are represented by the yellow and cyan regions, respectively (isosurface value: 0.003 e/Bohr3). Figure S4: (a) the adsorption configuration of O2 on FeN3-gra. (b) The spin density of FeN3-gra with O2 adsorbed on it (spin up (yellow) and spin down (cyan); isosurface value: 0.01 e/Bohr3). (c) The charge density difference of FeN3-gra with O2 adsorbed on it. The accumulation and depletion of electrons are represented by the yellow and cyan regions, respectively (isosurface value: 0.003 e/Bohr3). (Supplementary Materials)
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