1 Introduction

Ecosystem stability and human security are facing a serious threat due to global warming (Smith and Fang 2010; Han et al. 2022). The global average temperature from 2020 to 2021 has increased by 1.1℃ compared with 1850–1900 and will continue to rise in the future (IPCC, 2021). The major inducement of global warming is the sharp enhancement of atmospheric concentrations of greenhouse gases (GHGs) [for instance, methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2)] (Wang et al. 2019; Zhou et al. 2022). According to statistics in 2015, the accumulation concentrations of CH4, N2O, and CO2 in the air increased by 150%, 20%, and 40%, respectively (Stocker et al. 2014; Tian et al. 2015). Paddy fields have been deemed as the primary emission source of CH4 in agricultural fields, contributing 10% of the total CH4 emissions (Cao et al. 2021; Shin et al. 2021). Furthermore, agricultural fields contribute approximately 66.7% of the total N2O emissions (Akiyama et al. 2005; Van Groenigen et al. 2011; Yang and Silver 2016). Superfluous addition of nitrogen (N) fertilization and agricultural intensification increases the conversion from carbon (C) and N in soils to GHGs and their release (Wu et al. 2013; Pajares and Bohannan 2016). Multiple measures have been adopted for reducing the emission of GHGs in agricultural fields such as moisture management and the addition of amendment materials (Yan et al. 2005; Feng et al. 2013).

Biochar possesses substantial pores and abundant functional groups, which can conduce to the restriction of GHGs, and has been considered as a potential candidate for the reduction of GHGs in agricultural fields (Wu et al. 2013; Godlewska et al. 2017; Cui et al. 2021; Shakoor et al. 2021; Shin et al. 2021). The main restriction mechanisms of biochar for GHGs include: (1) biochar with abundant porosity could reduce the oxygen deficit in soils, which is helpful to retard the production of methanogens and inhibit the conversion of CH4 (Karhu et al. 2011; Nan et al. 2021); (2) biochar can enhance the abundance of methanotrophic proteobacterial and reduce the CH4 emission (Feng et al. 2012); (3) biochar may anchor the inorganic N in soils and avoid the conversion of inorganic N to N2O (Cayuela et al. 2014; Dong et al. 2020); (4) biochar can lower soil acidity and control the process of nitrification and denitrification (Sohi et al. 2010; Cayuela et al. 2013); and (5) biochar is an ‘electron shuttle’ for electron transfer of denitrification in soils which can lead the reduction of N2O to N2 (Cayuela et al. 2013; Yuan et al. 2022). However, unlike for the N2O and CO2, researchers have different standpoints on the mitigation effect of biochar for CH4. (Karhu et al. 2011; Wu et al. 2013; Bruun et al. 2016). For example, Jeffery et al. found that biochar could reduce the CH4 emissions from soils (Jeffery et al. 2016), but Zhang et al. considered that biochar could improve the total CH4 emissions from soils (Zhang et al. 2010). Thus, it is necessary to develop biochar-based modification materials for mitigating the emissions of GHGs in paddy fields. Recently, single atomic catalyst (SAC) doped carbon materials have gathered widespread interest in the environmental field due to their low cost, excellent chemical characteristics, superior atom utilization efficiency, and abundant specific surface area (Qiao et al. 2011; Yuan and Shi 2013; Wang et al. 2015; Zhou et al. 2019). Among them, FeN3-doped carbon materials have shown excellent performance in previous studies, which will be beneficial to improve the current issues (such as heavy metal pollution and GHG emission) in agricultural fields. For example, Gao et al. proved that FeN3-embedded carbon possesses a good mercury adsorption ability via the first-principles calculation (Gao et al. 2019); Xie et al. found that FeN3-doped graphene could adsorb the gas molecules (C2H2, H2S, SO2, SO3, and O2) (Xie et al. 2022). Thus, it is an efficient method to use the FeN3-doped biochar as the sorbent in paddy fields which are the main emission source for GHGs. However, the adsorption effect and mechanism of FeN3-doped biochar for mitigation of GHG emissions in paddy fields are unclear.

In this study, due to the various components in biochar, FeN3 may possess diverse sites on biochar. Herein, according to previous studies (Okamoto 2009; Gao et al. 2019; Li et al. 2021), the first-principles calculations were carried out using graphene (G) as a simplified model to explore the inhibition effect and mechanism of G and its FeN3-doped for GHGs. At first, the electron characteristics of G and the FeN3-doped G (FG) were evaluated. The adsorption ability and stability between the different substrates and CH4, N2O, or CO2 were then compared. Finally, the electronic properties of adsorption systems were investigated and the inhibition mechanism of CH4, N2O, and CO2 adsorption on FeN3-C were revealed. These results can provide insights that will aid in a better understanding of the effect and mechanism of SAC-doped carbon materials for GHGs mitigation in paddy fields, and the effective selection of materials for GHG mitigation.

2 Computational methodology

All the calculations were based on the first principles and carried out by the Vienna Ab Simulation Package (VASP). The exchange-correlation function has been described by the Perdew-Burke-Ernzerhof (PBE) type and the interaction between electrons and ions was considered through the projector augmented wave (PAW) method (Hohenberg and Kohn 1964; Kresse and Furthmüller 1996a, b). The construction of FG model referred to the previous study. In brief, four carbon sites were replaced by three nitrogen (N) atoms and a Fe atom site. Among them, Fe was confined in the central of three N atoms (Gao et al. 2019). The DFT-D3 was set to describe the van der Waals interaction in all the calculations for adsorption systems (Perdew et al. 1996; Zhang et al. 2020). The localized d electrons of the Fe element have been described via the DFT + U method, with the U value set to 4 eV (Zhou and Sun 2011; Wang et al. 2022a). The cutoff energy, convergence standard of energy, and force were set to 500 eV, 10− 5 eV, and 0.02 eV Å−1, respectively (Zhou et al. 2022). The k mesh of the structure optimization and density of state (DOS) calculations were set to 3 × 3 × 1 and 11 × 11 × 1 with the Gamma-centered grids (Froyen 1989; Liu et al. 2021). To avoid the interplay of the adjacent unit cells, the 15 Å vacuum layer was instituted. The formulas of adsorption energy (Ea) and differential charge density (Δρ) are as follows:

$${\text{E}}_{{\text{a}}} = {\text{ E}}_{{{\text{gas}} - {\text{substrate}}}} - \left( {{\text{E}}_{{{\text{gas}}}} + {\text{E}}_{{{\text{substrate}}}} } \right),$$

where Egas−substrate, Egas, and Esubstrate represent the total energy after the gas molecule adsorbed on the substrate, single energy of the different gases, and different substrates, respectively (Luo et al. 2019).

$$\Delta \rho = \rho _{{{\text{gas}} - {\text{substrate}}}} - \rho _{{{\text{gas}}}} - \rho _{{{\text{substrate}}}} ,$$

where ρgas−substrate, ρgas, and ρsubstrate represent representthe charge density of the adsorption configuration of the gas molecule and substrate, charge density of the different gas molecules, and different substrates, respectively. The differential charge density and electron localization function (ELF) images were drawn by VESTA (Momma and Izumi 2011).

3 Results and discussion

We first studied the structural stability of the graphene substrate doped with FeN3 molecules (Fig. 1). As shown in Fig. 1a, b, the structure of FG was not planar. This phenomenon is attributed to the larger size of Fe atoms compared to the C atom, which renders Fe non-planar; further, this property may aid in providing efficient adsorption sites (Miao et al. 2021). The electronic properties of G and FG were then explained by the electron localization function (ELF) and band calculation. As the ELF plots shown, there were conspicuous electron transformations in FG, which could change the adsorption performance of G (Fig. 1c, d and Additional file 1: Fig. S1). Furthermore, band calculation was carried out to study the electron transfer ability and electronic characters. The computed results of the band showed that G and FG exhibited metallic properties and facilitated electron exchange for GHG adsorption (Fig. 1e, f). Contrastingly, a distinct orbital spin in FG was observed, which is attributed to the introduction of Fe atoms, rendering can render electron transfer easy for the adsorption of GHGs (Légaré et al. 2018).

Fig. 1
figure 1

Schematics of G (a) and FG (b), ELF diagrams of G (c) and FG (d), band structures of G (e) and FG (f)

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Next, the bond angle and length were used to compare the adsorption difference of G and FG for GHGs (Fig. 2a–f). Compared with the GHGs-G adsorption systems, the bond angles (H-C-H, O-C-O, N-O-N) of the GHGs-FG adsorption systems were 104.75°, 148.89°, and 99.34°, respectively, which were all lower than those of GHGs-G adsorption systems (109.26°, 179.76°, and 179.31°). These results imply the excellent anchoring ability of FG for GHGs, which can aid in reduction of GHG emissions from soil to air(Wang et al. 2022a). Besides, the CO2 and N2O on FG exhibit a longer bond length (1.19 Å and 1.39 Å) than those adsorbed on G (1.18 Å and 1.20 Å), which is attributed to the strong adsorption ability of FG for GHGs. However, there was no noticeable difference in bond length (1.10 Å) for CH4 on G and FG, which may be due to the weaker electron exchangeability for FG adsorbing CH4 than adsorbing CO2 and N2O. The calculations of adsorption energy were further used to analyze the anchoring performance of G and FG for GHGs. As the previous studies proposed, the small adsorption energy means the weak anchoring ability (Hou et al. 2016; Wang et al. 2022b). In contrast with the adsorption energies of CH4, CO2, and N2O adsorbed on G (–0.95, −0.99, and −1.00 eV), the adsorption energies of FG for CH4, CO2, and N2O all improved (–1.37, −1.54, and −2.91 eV), demonstrating the greater anchoring ability of FG for GHGs (Fig. 2g).

Fig. 2
figure 2

Schematics of CH4 on G (a) and FG (d), the CO2 on G (b) and FG (e), N2O on G (c) and FG (f), Adsorption energies of GHGs on G and FG (g)

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To comprehend the adsorption reaction mechanism, the electronic performances of the adsorption systems were computed. At the outset, the differences in the electronic charge density of GHGs adsorbed on different substrates are displayed in Fig. 3 and Additional file 1: Fig. S2, to explain the status of charge transfer and redistribution. As shown in Fig. 3d–f, there were distinct existing accumulation and depletion regions of the charge in GHGs-FG adsorption systems. It is worth noting that the inferior performance of charge redistribution was observed in GHGs-G adsorption systems, despite their larger isosurface value (Fig. 3a–c). The main reason for the above differences is that the adsorption of GHGs by FG was physical chemisorption, while G only exhibited physical adsorption behavior.

Fig. 3
figure 3

Charge density difference of the front views of CH4 on G (a) and FG (d), CO2 on G (b) and FG (e), N2O on G (c) and FG (f) The isosurface is set to 6 × 10− 5 e Å-3 in GHGs-G and set to 4 × 10− 4 e Å-3 in GHGs-FG. Note that red and yellow correspond to charge depletion and charge accumulation

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The ELF plots further prove the behavior of electron charge transfer of GHGs-G or -FG adsorption systems. As shown in Fig. 4a–c, there were apparent improvements in both GHG molecules and substrates upon adsorption of GHGs on FG, with more noticeable changes displayed in neighboring atoms, especially at the direct adsorption sites. However, these performances were difficult to find in GHGs-G adsorption systems (Additional file 1: Fig. S3). The results of the electron charge density difference and ELF plots proved that the distribution of the charges changed while the GHGs adsorbed on FG. The Bader charge analysis was computed to further evaluate the charge transfer ability of GHGs-G or -FG adsorption systems. As the previous study confirmed, the greater the charge transformation, the stronger the ability of molecule restriction (Wang et al. 2022b). Combined with the low electron transfer numbers of G for CH4, CO2, and N2O (0.00, 0.01, and 0.02 e), the FG showed satisfactory results for GHG adsorption, wherein the number of electron transfers for CH4, CO2, and N2O increased to 0.03, 0.42, and 0.83 e, respectively. Additionally, as shown in Additional file 1: Table S1, the number of electron changes mainly occurred in Fe atoms and GHG molecules, with a small difference in the number of electron changes in C atoms, which is consistent with the implications of Figs. 3 and 4.

Fig. 4
figure 4

ELF diagrams of CH4 (a), CO2 (b) and N2O (c) on FG. d The number of charge transfer of the GHGs on G and FG

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The density of state (DOS) and partial density of state (PDOS) analyses were further used for an in-depth evaluation of the mechanism of electron transfer and orbital contribution for adsorption systems. As shown in Fig. 5a–c, the energy shift could be seen after CH4, CO2, or N2O was adsorbed on FG. These results illustrate that the electron state has been transformed in GHGs-FG adsorption systems and prove that the FG can inhibit GHGs via physico-chemical adsorption. However, there is no evidence that the energy shift occurred during the CH4, CO2, or N2O adsorbing on G, implying the presence of only physical adsorption behavior in GHGs-G adsorption systems. In addition, the PDOS analyses of GHGs-FG were used to study the orbital contribution. As shown in Fig. 5d–f and Additional file 1: Fig. S4, there was a clear exhibition that a drastic energy up- or down-shift of the electron of FG (Fe d and N p orbital) and GHGs (C p, O p, and N p orbital) occured when CH4, CO2, or N2O adsorbed on FG, which may be the major induction for the electron energy shift in GHGs-FG adsorption systems. However, there was no noticeable electron energy shift electron in GHGs-G adsorption systems, which provided evidence that G could restrict CH4, CO2, or N2O by physical adsorption alone (Additional file 1: Fig. S5).

Fig. 5
figure 5

The density of states of CH4 (a), CO2 (b) and N2O (c) on G and FG. The partial density of states of CH4 (d), CO2 (e) and N2O (f) on FG

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4 Conclusions

In summary, we have proposed an effectively modified biochar material, FeN3-doped biochar, to reduce the emissions of GHGs in paddy fields and revealed the adsorption effect and mechanism via first principles calculations. Comparing with pure biochar, the FeN3-doped biochar exhibited satisfactory adsorption ability for various GHGs in paddy fields (CH4, CO2, and N2O), which not only changed the structure of GHG molecules, but also improved the adsorption energies to −1.37, −1.54, and −2.91 eV, respectively. The electron charge density difference, ELF, and Bader charge analyses further confirmed that the FeN3-doped biochar displayed physico-chemical adsorption ability, which was different from that of pure biochar. In addition, the DOS and PDOS analyses revealed that the factor responsible for FeN3-doped biochar to exhibit excellent adsorption ability is occurrence of the drastic energy up- or down-shift of the electron for Fe d, C p, O p, or N p orbitals upon adsorption of CH4, CO2, or N2O.

Consequently, our study suggested an advanced modified biochar material for reducing the GHG emissions in paddy fields and investigated the adsorption property and mechanism of FeN3-doped biochar for GHG mitigations, which provided a strategy for the exploration of biochar modification and efficient emission reduction materials.