Surface free electrons endow 2D Alkali metal nitrides with extraordinary adsorption on volatile organic compounds

Surface free electrons endow 2D Alkali metal nitrides with extraordinary adsorption on volatile organic compounds | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Surface free electrons endow 2D Alkali metal nitrides with extraordinary adsorption on volatile organic compounds Jintian Wang, Shu Li, Xuan Zhang, Qian Chen, Miao Zhou This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8852905/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract Volatile organic compounds (VOCs) are a major cause of leukemia, carcinoma, and other diseases. However, removal technologies for VOCs cannot cope with the grim situation of VOC pollution. Two-dimensional (2D) alkali metal nitrides Ca 2 N, Sr 2 N, and Ba 2 N have great potential as a new adsorption material, and their application as a VOCs detector has not reported. Here, we study the sensing performance of Ca 2 N, Sr 2 N, and Ba 2 N on benzene, styrene, xylene, methylbenzene, ethylbenzene, butyl acetate, methanal, and n-undecane. We find that Ca 2 N, Sr 2 N, and Ba 2 N have outstanding adsorption capabilities for VOCs. This strong adsorption is attributed to the free-electron layer on their surface. When the isosurfaces are equal, there are more free electrons on the surface of Ca2N, followed by Sr2N, and finally Ba2N. Moreover, electric fields could effectively improve their adsorption performance. The perfect match of the electronic properties of VOCs with those of Ca2N, Sr2N, and Ba2N contributes to this extraordinary adsorption capability. Therefore, this study will lead to a new round of technological innovation in VOCs adsorption technology based on 2D alkali metal nitrides. Physical sciences/Chemistry Earth and environmental sciences/Environmental sciences Physical sciences/Materials science volatile organic compounds (VOCs) 2D Alkali metal nitrides Ca2N Sr2N Ba2N First principles calculations 2D electronic compounds Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Introduction Volatile organic compounds (VOCs) such as benzene, formaldehyde, and toluene usually accumulate indoors 1 . In national environmental health surveys conducted in the United States, Canada, and South Korea, over 90% participants had detected metabolites of benzene in their urine 2–4 . Long term exposure to high concentrations of VOCs causes various hazards to the human body. Skin and eye contact with methanal cause dryness and ulcers 5 . Long-term exposure to VOCs will not only damage the human respiratory 6 , hematopoietic, immune, digestive and reproductive systems 7 , but also lead to pneumonia, chronic obstructive pulmonary disease 8 , leukemia, lymphoma, bladder cancer and other diseases 9 . VOCs also increase the risk of depression and other mental diseases 10 ,11 . Besides, a negative correlation between VOCs exposure and adolescent growth indicators was found 12 . Moreover, elevated levels of VOCs in urine were significantly and positively correlated with an increase in the plasma atherogenic index 13 . The urinary metabolites of ethylbenzene and styrene were significantly associated with cardiovascular disease, respiratory disease, and cancer mortality 14 . In addition, the normal and pathological states of human physiological processes can be characterized by the VOCs released during respiration 15 – 17 . Therefore, the detection and removal of these pollutants is of great significance. Numerous VOC control technologies have emerged, such as incineration, condensation, biological degradation, absorption, adsorption, and catalytic oxidation, et al 18,19 . Among them, the adsorption technology has been recognized as an efficient and economical control strategy 20 – 22 . In current research on VOCs adsorption, traditional light absorption sensors 23 , heat conduction sensors 24 have insufficient adsorption capacity 24 . Carbon materials suffer from poor renewability, fire risks, high mass transfer resistance, pore blockage, and hygroscopicity 25 , 26 . Zeolites and metal organic frameworks materials are costly, with numerous voids leading to weak dispersion forces and insufficient open metal sites 27 ,28 . Organic polymers involve complex synthesis processes, making large-scale production challenging 29 . Adsorption and membrane separation processes have poor adsorption capacity and selectivity for VOCs 30 . The average specific surface area, pore volume and VOCs adsorption capacity of different adsorption materials are metal organic frameworks > activated carbons > hypercrosslinked polymeric resin > zeolites 31,32 . However, the focus of these studies is on the specific surface area, adsorption pore size, and number of functional groups of the adsorbent, without considering the electronic interaction between the adsorbent and VOCs molecules 33,34 . Therefore, it is vital to explore the enormous potential of two-dimensional (2D) electronic materials in the field of gas adsorption by utilizing their unique structures and surface properties. 2D electronic compounds own the advantages of a large specific surface area, high mechanical strength, as well as the high electron concentration, high active site density 35,36 , and low work function of electronic compounds 37–41 . Alkali metal nitrides Ca 2 N, Sr 2 N, and Ba 2 N are classical 2D electronic compounds that can be separated into single-layer structures in the laboratory 42 . They present M 2 N (M = Ca, Ba, Sr), in which conduction electrons are confined between the M layers 42 . The delocalized homogeneous electrons in the 2D interlayer space are responsible for the excellent transport characteristics, which are mainly conducted through the 2D space instead of the M 2 N layer 41–43 . However, the application of 2D electronic compounds in the sensing and detection of VOCs molecules has not been reported yet. In this work, we selected Ca 2 N, Sr 2 N, and Ba 2 N as substrate materials and benzene, styrene, xylene, methylbenzene, ethylbenzene, butyl acetate, methanal, and n-undecane as adsorption targets. Based on first principles study, the adsorption of VOCs on a single-layer 2D electronic compound substrate was calculate. The adsorption capacity was adjusted by applying an electric field. The surface free electron layer properties of Ca 2 N, Sr 2 N, and Ba 2 N endow them with powerful VOCs sensing capability and provide new technology for indoor air quality detection. Results and Discussion Geometric and Electronic Structures of Ca2N, Sr2N, and Ba2N To investigate the properties of single-layer 2D electronic compounds Ca 2 N, Sr 2 N, and Ba 2 N, we present the geometric and electronic structure of a single-layer 2D electronic compound in Fig. 1 . Figure 1 a shows the top and side views of a single-layer 2D electronic compound, where X represents Ca, Sr, and Ba 42,44 . The lattice structure of X 2 N electronic compounds is hexagonal, with optimized lattice constants of a = b=3.62 Å (Ca 2 N), a = b=3.81 Å (Sr 2 N), and a = b=3.98 Å (Ba 2 N). Each unit cell is composed of 2 X atoms and 1 N atom. From a top-down view, each X atom bonds to three adjacent N atoms, and each N atom is bonded to six adjacent X atoms. From a side view, X atoms construct the outer layers, and N atoms are in the middle layer. Figure 1 b shows the partial charge density (left) and band structure diagram (right) of Ca 2 N. In order to investigate the distribution of active electrons on the material surface, the Fermi level was selected and the partial charge density of Ca 2 N was calculated from − 1 eV to 1 eV, as shown in the left figure. The yellow areas (left) denote the charge density corresponding to the yellow energy range in the band diagram (right). Most of the charges near the Fermi level are distributed on the surface of the material. The distribution of free electrons on the material surface is mainly located between adjacent Ca atoms. The purple circle in the band diagram represents the projected energy band of free electrons in the surface space region of Ca 2 N. The energy of surface electrons of Ca 2 N is in the range of -1.2 eV to 1.5 eV. Similarly, Fig. 1 c & 1 d show the partial charge densities (left) and band structure diagrams (right) of Sr 2 N and Ba 2 N. The energy of surface electrons of Sr 2 N ranges from − 1.1 eV to 1.4 eV, and the energy of surface electrons of Ba 2 N ranges from − 0.8 eV to 1.5 eV. Comparing Fig. 1 b-d, the projection regions of their surface free electrons in the band are similar, all located in the band near the Fermi level. The distribution of free electrons between adjacent Ca atoms is relatively uniform, while the distribution of free electrons between adjacent Sr/Ba atoms tends to lean towards the Sr/Ba atom on one side. During molecular adsorption, the electron-rich regions attract more molecules and result in charge transfer and adsorption. When the isosurfaces are equal, there are more free electrons on the surface of Ca 2 N, followed by Sr 2 N, and finally Ba 2 N. These differences in their surface electrons result in their potential for molecular adsorption applications 45 . Collectively, Ca 2 N, Sr 2 N, and Ba 2 N all have a hexagonal lattice structure and surface free electron distribution characteristics near the Fermi level. The differences in these structures and electronic properties characterize their adsorption applications. Geometric and electronic structures of eight types of VOCs molecules To investigate the adsorption of VOCs molecules by single-layer 2D electronic compounds, we then explore the geometric and electronic structures. We use m-xylene as a representative to calculate the adsorption of xylene 46 . Figure 2 shows the molecular structure of eight types of VOCs, presented in a top view and a sideview. The molecular electrostatic potential of these eight molecules is shown in Fig. 3 47 .The blue area represents high energy and is prone to losing electrons. The red area indicates low energy and easy access to electrons. Both the red and blue areas represent more active areas on the surface of the molecule. In Fig. 3 a, there is a portion of red area on the surface of the benzene molecule, located on both sides of the six C atoms on the benzene ring structure. However, the color around the H atom is not obvious. This indicates that the most active position of the benzene is located inside the benzene ring and is easily accessible to electrons 48,49 . The red area on the surface of the styrene (Fig. 3 b) can be mainly divided into two parts. One part is located on both sides of the benzene ring structure, and the other part is around the C = C bond. Similarly, the alkyl functional groups in xylene (Fig. 3 c), methylbenzene (Fig. 3 d), and ethylbenzene (Fig. 3 e) are not active, and the most active part is the benzene ring structure, which is easy to obtain electrons. The region around the C = O bond in butyl acetate (Fig. 3 f) is the most active. In methanal (Fig. 3 g), both the O atom side and the H atom side are relatively active, with the O atom side easily gaining electrons and the H atom side easily losing electrons. The area around the H atom in n-undecane (Fig. 3 h) is the most active and easily loses electrons, which differs from other VOCs. Taken together, these active regions provide a theoretical basis for subsequent research on adsorption interactions. Adsorption of VOCs on Ca2N, Sr2N, and Ba2N Surface We then calculate the adsorption of molecules at different angles and sites and select the structure with the lowest energy as the most stable structure. Table 1 presents the quantitative results of the equilibrium distance and adsorption energy of VOCs molecules. We find that methanol and butyl acetate have the shortest equilibrium distance, followed by styrene, xylene, methylbenzene, and ethylbenzene. Benzene has a longer distance and a significant decrease compared to the initial distance. N-undecane has no significant change in equilibrium distance compared to the initial distance. Besides, methanol and styrene have the highest adsorption energy, followed by xylene, methylbenzene, ethylbenzene, and butyl acetate. However, benzene and n-undecane have the lowest adsorption energy. These data indicate that Ca 2 N substrates have the strongest adsorption effect on molecules containing C = O bonds. Molecules containing C = C, such as styrene, come next. Their adsorption energy is relatively high. Although benzene molecules have a benzene ring structure, the equilibrium distance does not change much from the initial distance. The n-undecane molecule shows no significant adsorption in terms of geometric structure, equilibrium distance, and adsorption energy. The H atoms on the surface of the molecule are prone to losing electrons, making it difficult to generate charge transfer with the free electrons on the Ca 2 N substrate surface. These data demonstrate that the Ca 2 N substrate exhibits the strongest adsorption of C = O bond-containing molecules and the weakest adsorption of n-undecane. Figure 4 shows a side view of the combined structure of eight VOCs molecules and three electronic compound substrates after structural relaxation. On the Ca 2 N substrate, benzene (Fig. 4 a), styrene (Fig. 4 b), xylene (Fig. 4 c), methylbenzene (Fig. 4 d), and ethylbenzene (Fig. 4 e) are adsorbed at a horizontal angle. The position of the C = C bond in the styrene molecule is closest to the substrate, followed by the benzene ring structure inside the molecule. These indicate that the C = C bond structure is mainly adsorbed. Among the other four molecules, however, the benzene ring structure is closest to the substrate, while the alkyl structure in the molecule is farther away. Hence, the benzene ring structure is mainly adsorbed. Butyl acetate (Fig. 4 f) is adsorbed at an inclined angle, with the O atoms that form the C = O bond closest to the substrate. The length of the C = O bond has been extended from the initial 1.22 Å to 1.44 Å, and some atoms are further away from the substrate than their initial position. This indicates that the substrate has a stronger adsorption effect on the O atom in the C = O bond and a weaker adsorption effect on the remaining atoms. The O atom in the methanal (Fig. 4 g) is closest to the substrate, and the C = O bond is no longer in the same plane as the C and H atoms, implying that the O atom is strongly adsorbed by the substrate, while the H atom is subjected to a repulsive force towards the outside of the substrate. The n-undecane molecule did not undergo any deformation or change in position during the reaction. The adsorption results of these VOCs molecules on Sr 2 N (Fig. 4 i-p) and Ba 2 N (Fig. 4 m-t) substrates are basically consistent with those on Ca 2 N substrates. All three substrates have the strongest adsorption ability for butyl acetate and methanol. Their adsorption for styrene and xylene is also quite strong. They show weak adsorption performance for benzene molecules and no adsorption effect on n-undecane molecules. Surprisingly, the molecular structure is completely destroyed due to the strong interaction with Ba 2 N substrate (Fig. 4 r). This significantly demonstrates that Ba 2 N is an ideal material for VOCs adsorption. In addition, the equilibrium distance for the adsorption of benzene and n-undecane molecules on the Ba2N substrate is larger than for the other two substrates. However, the adsorption energy for these two molecules on Ba2N is larger than on Ca 2 N and Sr 2 N. This may result from the higher mass of Ba atoms on the substrate surface. Collectively, these results demonstrate a clear functional-group specificity in adsorption configuration and a strong substrate-dependent variation in adsorption strength. Mechanism of VOCs adsorption on CaN, SrN, and BaN Surface Next, we calculate the electronic density of states (DOS) of the Ca 2 N substrate when adsorbing VOCs molecules. Figure 5 shows the changes in DOS of VOCs molecules before and after adsorption on a Ca 2 N substrate. Briefly, the electronic DOS of Ca 2 N has increased in energy compared to the initial energy. However, the DOS of VOCs molecules has decreased in energy compared to the initial state. This indicates that VOCs molecules gain electrons in the reaction, and the Fermi level is relatively higher. Meanwhile, the Ca 2 N substrate loses electrons in the reaction, and the Fermi level is relatively lower. Interestingly, the benzene and Ca 2 N substrate (Fig. 5 a) undergo hybridization in the energy range of -0.5 eV to 2 eV, and the DOS overlaps in this region. Styrene (Fig. 5 b), xylene (Fig. 5 c), methylbenzene (Fig. 5 d), and ethylbenzene (Fig. 5 e) exhibit similar orbital hybridization after adsorption on Ca 2 N substrates due to their similar structures. The hybridization all occurs in the energy range of -0.5 eV to 2.5 eV and − 4 eV to -3 eV, with some additional hybridization regions compared to benzene. The hybrid orbitals of butyl acetate (Fig. 5 f) and methanol (Fig. 5 g) adsorbed on a Ca 2 N substrate are located in the energy range of -4 eV to -3 eV below the Fermi level. The orbital hybridization of N-undecane (Fig. 5 h) after adsorption on the Ca 2 N substrate is not significant, indicating that the molecular effect is relatively weak. Besides, the degree of orbital hybridization of VOCs molecules adsorbed on the Ca 2 N substrate is consistent with the results of the adsorption energy analysis mentioned earlier. Moreover, the adsorption results of these VOCs molecules on Sr 2 N (Fig. 6 a-h) and Ba2N (Fig. 7 a-h) substrates are similar to those on Ca 2 N substrates. They are also consistent with the previous analysis results. Since the molecular structure is completely destroyed, we obtain no DOS for Styrene on Ba 2 N (Fig. 7 b). To investigate the charge transfer between VOCs molecules adsorbed on single-layer Ca 2 N, Sr 2 N, and Ba 2 N substrates, we calculate the differential charge density of the adsorbed structure and the amount of charge transferred during the reaction, which is summarized in Table 1. Figure 8 shows the differential charge density of the VOCs molecules adsorbed on a Ca 2 N substrate, including top and side views, with isosurfaces of 0.0015 e/Å for all charge densities. When benzene is adsorbed on a Ca 2 N substrate (Fig. 8 a), there is a small but significant charge transfer. The substrate part loses electrons on the surface, and the electron loss area is located directly opposite the Ca atom on the other side, slightly higher than the Ca atom at the interface. There is charge accumulation on both sides of the C atom in the benzene molecule, with more charge accumulation on the side closer to the substrate. Charge loss occurs around the H atom, with the lost electrons located on the side farther from the substrate, consistent with the active region in the molecular electrostatic potential. When styrene is adsorbed on Ca 2 N substrates (Fig. 8 b), there is a significant amount of charge transfer. The region where free electrons accumulate on the substrate surface loses electrons, consistent with the adsorption of benzene molecules. In styrene molecules, there is charge accumulation on both sides of the C atom and charge loss around the H atom. Specifically, the C atom on the C = C bond receives more electrons than the C atom on the benzene ring structure. This indicates that during the adsorption process, both structures interact with electrons on the Ca 2 N substrate surface. Among them, the C = C bond undergoes more charge transfer, which is consistent with the results of the geometric structure. Xylene (Fig. 8 c), methylbenzene (Fig. 8 d), and ethylbenzene (Fig. 8 e) all exhibit similar differential charge results when adsorbed on Ca 2 N substrates. Overall, they all transfer charges from the Ca 2 N substrate to VOCs molecules. Charge loss occurs at the locations where free electrons accumulate on the surface of the Ca 2 N substrate, with charge accumulation around Ca atoms at the substrate interface that adsorb xylene and ethylbenzene molecules on the side away from the interface. This arises from charge redistribution caused by charge transfer. The charge gain and loss in molecules mainly occur in the benzene ring structure region. Only a small amount occurs in the alkyl structure region. In addition, when xylene is adsorbed on the Ca 2 N substrate, the position of the benzene ring is consistent with that of the benzene molecule. However, for methylbenzene and ethylbenzene, the position of the benzene ring is located directly opposite the Ca atom at the interface. When butyl acetate (Fig. 8 f) and methanol (Fig. 8 g) are adsorbed on a Ca 2 N, there is a significant amount of charge transfer from the substrate to the molecule. The region where free electrons gather in the Ca 2 N substrate loses charge. The C and O atoms in the C = O bond of the molecule are charged, and other parts of the molecule also experience charge gain and loss due to charge redistribution. Charge accumulation occurs around Ca atoms on the Ca 2 N substrate surface that adsorbs methanal molecules. In the differential charge results of n-undecane adsorption (Fig. 8 h), no charge loss occurred at the substrate, indicating that there was no charge transfer between the molecule and the substrate. Collectively, there is no charge transfer between the n-undecane molecule and the Ca 2 N substrate. However, other gases have undergone significant charge transfer. Figure 9 shows the top and side views of the differential charge density after adsorbing VOCs molecules on Sr 2 N substrate, with an isosurface of 0.0015 e/Å. Overall, the adsorption results of Sr 2 N and Ca 2 N substrates are similar, with charges transferred from the substrate to VOCs molecules. The charge transfer amount of benzene (Fig. 9 a) and styrene (Fig. 9 b) adsorbed on the Sr2N substrate is reduced compared to the Ca2N substrate. In addition, the positions where charges are lost in the substrate are consistent. The charge transfer amount of xylene (Fig. 9 c) and methylbenzene (Fig. 9 d) molecules adsorbed on the Sr 2 N substrate shows no significant change compared to the Ca 2 N substrate. However, the benzene ring structure in the molecules is located directly opposite the Sr atom at the interface. Thus, the position where the charge is lost mainly centered around the Sr atom. The charge transfer amount of ethylbenzene adsorbed on the Sr 2 N substrate (Fig. 9 e) is significantly reduced compared to the Ca 2 N substrate. Besides, the adsorption site also changes. The charge transfer amount of butyl acetate (Fig. 9 f) and methanal (Fig. 9 g) adsorbed on Sr 2 N substrate is similar to that on Ca2N substrate, and the positions of gain and loss electrons are consistent. In Fig. 9 h, n-undecane shows no significant charge transfer on the Sr 2 N substrate. Concisely, except for changes in the adsorption sites of toluene, xylene, and ethylbenzene, the adsorption characteristics of other molecules on Sr 2 N substrates are basically consistent with those on Ca 2 N. Figure 10 shows the top and side views of the differential charge density after adsorbing VOCs molecules on a Ba 2 N substrate, with an iso surface of 0.0015 e/Å. Overall, the adsorption results of Ba 2 N and Ca 2 N substrates are similar, with charges transferred from the substrate to VOCs molecules. The charge transfer amount of benzene (Fig. 10 a), xylene (Fig. 10 c), methylbenzene (Fig. 10 d), and ethylbenzene (Fig. 10 e) adsorbed on Ba 2 N substrate is reduced compared to the Ca 2 N substrate. The adsorption sites of the four molecules are consistent, and the benzene ring structure is located directly above the Ba atoms. Surprisingly, there is a large amount of charge transfer distributed around the scattered atoms after the molecular structure is disrupted (Fig. 10 b). The charge transfer amount of butyl acetate (Fig. 10 f) and methanal (Fig. 10 g) adsorbed on Ba 2 N substrate is similar to that on Ca 2 N substrate. The n-undecane shows no significant charge transfer on the Ba 2 N substrate (Fig. 10 h). Collectively, the adsorption of other molecules on the Ba2N substrate is basically the same as that on Ca 2 N, except for the structural damage of styrene molecules during adsorption. Based on Table 1 and Figs. 8 , 9 , and 10 , the adsorption of n-undecane molecules on Ca 2 N, Sr 2 N, and Ba 2 N substrates is not significant. Molecules such as xylene, methylbenzene, and ethylbenzene, which are composed of a benzene ring and alkyl structures, have a stronger adsorption capacity. The adsorption of styrene is extremely strong, especially on Ba 2 N substrates, which have the ability to damage the molecular structure during the adsorption process. Electric field improves the adsorption of VOCs to 2D electronic compounds Given that Ca 2 N, Sr 2 N, and Ba 2 N exhibit weak adsorption toward benzene and n-undecane molecules, they are not readily applicable directly in sensing applications. Therefore, we next introduce regulatory strategies to enhance the adsorption performance of these three substrates. The poor adsorbability of n-undecane molecules arises from the tendency of their surface H atoms to lose electrons, which is incompatible with the abundant free and mobile electrons on the surface of single-layer 2D electronic compounds. For benzene molecules, their weak adsorption performance is attributed to the relatively stable internal structure of the benzene ring. In contrast, its analogous molecules possess additional functional groups that disrupt the internal stability of the benzene ring. This significantly improves the adsorption efficiency. Among the three substrate materials, Ba 2 N demonstrates the weakest overall adsorption performance. Thus, we select benzene as a representative molecule and Ba2N as the substrate to investigate the effect of external regulation on the adsorption capacity. Common strategies for regulating the adsorption performance of 2D materials include defect engineering, doping modification, electric field regulation, and pressure regulation 50 . Since electric fields can directly modulate the free electrons on the substrate surface, we employ an electric field applied along the z-axis for regulation. We simulate an external electric field and calculate the adsorption behavior of benzene molecules on the substrate under electric fields ranging from − 0.8 eV/Å to 0.8 eV/Å along the z-axis, with an interval of 0.2 eV/Å. Figure 11 a&b show the differential charge density maps of benzene adsorbed on the substrate under electric fields of varying intensities. Figure 11 c depicts the variation curves of charge transfer and equilibrium distance for benzene adsorption on the substrate under different electric field conditions. As shown in Fig. 11 b, when an electric field of − 0.2 to − 0.8 eV/Å is applied, the electrons in the system experience a downward electric field force, which hinders the transfer of free electrons from the substrate surface to the molecule and also impedes the redistribution of charge within the molecule, resulting in a reduction in charge transfer. When an electric field of 0.2 to 1.0 eV/Å is applied, the electrons in the system are subjected to an upward electric field force, which facilitates the transfer of free electrons from the substrate surface to the molecule. Meanwhile, the direction of charge redistribution within the molecule aligns with the direction of the electric field force, promoting the formation of a new equilibrium and thus increasing charge transfer. Figure 11 c visualizes the variation patterns of charge transfer and equilibrium distance. Within the range of 0.8 eV/Å, as the intensity of the electric field applied downward along the z-axis increases, the charge transfer increases while the equilibrium distance decreases. This indicates a linear relationship between the electric field and the sensing performance of the substrate. However, beyond 0.8 eV/Å, increasing the electric field intensity leads to a decrease in charge transfer and an increase in equilibrium distance. These results collectively indicate that the electric field modulates the adsorption interaction between VOCs molecules and the substrate material by altering the distribution and movement tendency of electrons. Conclusion We show that Ca 2 N, Sr 2 N, and Ba 2 N provide effective adsorption for VOCs. Ca 2 N, Sr 2 N, and Ba 2 N have adsorption effects on benzene, styrene, xylene, methylbenzene, ethylbenzene, butyl acetate, and methanal. All adsorption processes do not damage the structure of the substrate materials. Thus, the substrate can be reused. Surprisingly, the molecular structure of styrene adsorbs on Ba 2 N is destroyed due to strong interactions. Benzene and n-undecane adsorb mainly through van der Waals forces due to their low charge transfer. The adsorption energy and charge transfer amount of styrene containing a C = C bond and a benzene ring structure are the highest. Xylene, methylbenzene, and ethylbenzene, which contain benzene ring structures or C = O bonds, follow closely behind. The adsorption effect of benzene still exhibits significant adsorption energy and charge transfer. The adsorption effect of N-undecane is the worst. Especially, the application of an electric field in the z-direction significantly improves the ability of Ba 2 N to adsorb benzene. Our research can be used to effectively adsorb VOCs and control air pollution. In addition, the alkali metal nitrides used in the experiment and the method of applying an electric field provide new ideas for exploring other adsorption materials. Subsequent research can use doping, defects, and other methods to compare their adsorption effects, and it is worthwhile to make VOCs sensors based on Ca 2 N, Sr 2 N, and Ba 2 N. Methods DFT-based first-principles calculations were performed with a plane-wave basis set as implemented in the Vienna ab initio simulation package (VASP) 51 . The projector augmented wave method was employed to describe the ion–electron interaction 52 . The exchange correlation functional used in the calculation is PBE functional in GGA 53 , and the pseudopotential is PAW. The truncation energy based on plane waves is set to 400 eV. During the modeling process, a 25 Å vacuum layer is added to eliminate the mutual influence between periodic unit cells in the z-direction, in order to simulate the structure of a single layer. The initial structure is a 1 × 1 single-layer structure on the xy plane. In order to prevent mutual influence between adjacent unit cells in the plane and maintain sufficient distance between each molecule, the substrate structure was expanded to 3 × 3 × 1 when constructing the adsorption model. In the structural optimization, the total energy was set to converge to 10 − 5 eV, and the force on each atom converged to 0.01 eV/Å before stopping relaxation. The K-point sampling grid was set to 7 × 7 × 1. In electronic structure calculation, the accuracy of K-point sampling grid is improved to 9 × 9 × 1. In addition, in order to eliminate the influence of van der Waals forces in the calculation, the DFT-D3 method was used for van der Waals force correction in each step of the calculation. The adsorption energy (Ea) of different molecules on Ca2N, Sr2N, and Ba2N surfaces was computed, in which Ea can be expressed by, E a = E total − ( E m + E sur) where Etotal is the total energy of the adsorbed system. Em and Esur are the energies of relaxed molecules in the gas phase and on the Ca2N, Sr2N, and Ba2N surfaces, respectively. By this definition, larger negative values of Ea denote stronger interaction between the molecule and the surface. Declarations Competing interests All authors declare no financial or non-financial competing interests. Author Contribution JW performed the DFT study. XZ analyzed the data. QC & SL are contributors in writing the manuscript. MZ & QC design and supervise this study. All authors read and approved the final manuscript. Acknowledgements This work was supported by grants from the Natural Science Foundation of Chongqing (CSTB2023NSCQ-MSX0620) . Data Availability All data generated or analysed during this study are included in this published article. References 1 Feng, Y. L., Yang, C. & Cao, X. L. Intermediate volatile organic compounds in Canadian residential air in winter: Implication to indoor air quality. 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Phys Rev Lett 77 , 3865-3868 (1996). https://doi.org/10.1103/PhysRevLett.77.3865 Tables Table.1 Adsorption energy, charge transfer amount and equilibrium distance of benzene, styrene, xylene, methylbenzene, ethylbenzene, butyl acetate, methanal, and n-undecane adsorbed by Ca2N, Sr2N, and Ba2N. Ca 2 N Sr 2 N Ba 2 N d /Å △ E /eV △ q / e d /Å △ E /eV △ q / e d /Å △ E /eV △ q / e Benzene 2.66 -0.83 0.95 2.70 -0.71 0.77 2.93 -0.79 0.79 Styrene 1.78 -2.75 2.11 1.86 -2.53 1.80 0.87 2.17 5.11 Xylene 2.00 -1.54 1.81 1.85 -1.41 1.72 2.54 -0.89 0.91 Methylbenzene 1.89 -1.67 1.74 1.88 -1.36 1.70 2.89 -0.87 0.87 Ethylbenzene 1.90 -1.59 1.84 2.25 -0.83 1.00 2.51 -0.95 0.96 Butyl acetate 1.14 -2.07 1.73 1.25 -1.66 1.63 1.47 -1.25 1.38 Methanal 1.07 -3.21 1.60 1.14 -2.89 1.61 1.32 -2.52 1.48 N-undecane 2.97 -0.60 0.64 2.96 -0.62 0.67 2.96 -0.75 0.61 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 22 Apr, 2026 Reviews received at journal 19 Apr, 2026 Reviewers agreed at journal 14 Apr, 2026 Reviews received at journal 14 Apr, 2026 Reviewers agreed at journal 31 Mar, 2026 Reviewers invited by journal 26 Feb, 2026 Editor assigned by journal 21 Feb, 2026 Submission checks completed at journal 14 Feb, 2026 First submitted to journal 11 Feb, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8852905","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":599349958,"identity":"fc0d8ae4-4fa5-4af5-81c8-da2d0a723e68","order_by":0,"name":"Jintian Wang","email":"","orcid":"","institution":"Chongqing University","correspondingAuthor":false,"prefix":"","firstName":"Jintian","middleName":"","lastName":"Wang","suffix":""},{"id":599349960,"identity":"3c48361d-6bc8-439e-8fe9-c275fcdb1b7e","order_by":1,"name":"Shu Li","email":"","orcid":"","institution":"Army Medical University (Third Military Medical University)","correspondingAuthor":false,"prefix":"","firstName":"Shu","middleName":"","lastName":"Li","suffix":""},{"id":599349962,"identity":"7c5ac822-c5b5-4814-9260-fea6b8e74c7f","order_by":2,"name":"Xuan Zhang","email":"","orcid":"","institution":"Army Medical University (Third Military Medical University)","correspondingAuthor":false,"prefix":"","firstName":"Xuan","middleName":"","lastName":"Zhang","suffix":""},{"id":599349963,"identity":"31044b97-b02d-4cb0-9c12-7b8ddb2d8215","order_by":3,"name":"Qian Chen","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAuklEQVRIiWNgGAWjYBACAwhlA+WyEaWFGUSlSZCs5TAJWszZ+w8+eFNzvk532hkDhg9lhxn4Zzfg12LZc5jZcM6x2xJmt3MMGGecO8wgcecAAYfdSGaT5m2AaGHmbTvMYCCRQEDL/ccgLecgWv4SpeUGM0jLAYgWRqK0nEk2BvolWXLb7bSCgz3n0nkkbhDScvzgQ2CI2fGb3U7e+OBHmbUc/wwCWsCAB0ofQGITqWUUjIJRMApGAVYAAP3BQOvUQHJWAAAAAElFTkSuQmCC","orcid":"","institution":"Army Medical University (Third Military Medical University)","correspondingAuthor":true,"prefix":"","firstName":"Qian","middleName":"","lastName":"Chen","suffix":""},{"id":599349965,"identity":"8ea1998c-a62f-4d37-b54b-caf0362a467a","order_by":4,"name":"Miao Zhou","email":"","orcid":"","institution":"Chongqing University","correspondingAuthor":false,"prefix":"","firstName":"Miao","middleName":"","lastName":"Zhou","suffix":""}],"badges":[],"createdAt":"2026-02-11 14:39:22","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8852905/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8852905/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":103860163,"identity":"b2dbd2a5-3f17-4df2-961c-cc782835998e","added_by":"auto","created_at":"2026-03-03 19:41:54","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":471532,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Geometric structures of 2D electronic compounds Ca2N, Sr2N, and Ba2N.X represents Ca, Sr, and Ba. Each X atom is bonded to three adjacent N atoms, and each N atom is bonded to six adjacent X atoms. (b) Ca2N local charge density map and projected energy band. (c) Sr2N local charge density map and projected energy band. (d) Ba2N local charge density map and projected energy band. The purple circle in the band diagram represents the projected band of free electrons, and the yellow area represents the range corresponding to the local charge density map. The Fermi level is set to 0. The isosurface is set to 0.01e/A.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8852905/v1/e35160f208764847fe562bf7.png"},{"id":104401105,"identity":"9ae767ac-3d14-4c25-b6fd-180ab88f83f5","added_by":"auto","created_at":"2026-03-11 12:11:53","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":252005,"visible":true,"origin":"","legend":"\u003cp\u003e(a)~(h) Top view and side view of benzene, styrene, xylene, methylbenzene, ethylbenzene, butyl acetate, methanal, and n-undecane. The gray ball represents C, the red ball represents O, and the white ball represents H.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8852905/v1/03f529a22d7d4a065ae2486b.png"},{"id":104400874,"identity":"28a9e4e3-808c-44cc-9ad2-2ddb1666adf4","added_by":"auto","created_at":"2026-03-11 12:11:18","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":235314,"visible":true,"origin":"","legend":"\u003cp\u003eMolecular electrostatic potential of benzene (a), styrene (b), xylene (c), methylbenzene (d), ethylbenzene (e), butyl acetate (f), methanal (g), and n-undecane (h). The red area indicates low energy and easy access to electrons, while both the red and blue areas represent more active areas on the surface of the molecule.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8852905/v1/2dc4e6253d1deb3827d68dc3.png"},{"id":104400967,"identity":"3e52b90e-b151-4882-8e53-6826762c48b1","added_by":"auto","created_at":"2026-03-11 12:11:34","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":887375,"visible":true,"origin":"","legend":"\u003cp\u003eSide view of the combined structure of benzene, styrene, xylene, methylbenzene, ethylbenzene, butyl acetate, methanal, n-undecane with three electrides (Ca2N, Sr2N, Ba2N) substrates after structural relaxation. The gray ball represents C, the red ball represents O, and the white ball represents H. The light green, green and dark green balls represent Ca, Sr, and Ba, respectively. The blue ball represents N.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8852905/v1/617cfdbcbbaa6df3e6fd537e.png"},{"id":103860165,"identity":"bc95ed5d-febf-4e51-a257-3fb9f227e8d3","added_by":"auto","created_at":"2026-03-03 19:41:54","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":530687,"visible":true,"origin":"","legend":"\u003cp\u003eThe partial wave electron state density of the combination of benzene (a), styrene (b), xylene (c), methylbenzene (d), ethylbenzene (e), butyl acetate (f), methanal (g), and n-undecane (h) on the Ca\u003csub\u003e2\u003c/sub\u003eN substrate after structural relaxation. The gray shadow represents the state density of the Ca\u003csub\u003e2\u003c/sub\u003eN substrate after adsorption. The green line represents the state density of VOCs molecules after adsorption, and the purple line represents the state density of VOCs molecules before adsorption.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8852905/v1/7186c46c70efc1e8163116ee.png"},{"id":103860171,"identity":"9a4cce55-f6cf-49dc-9671-791d61ff8d8b","added_by":"auto","created_at":"2026-03-03 19:41:54","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":525275,"visible":true,"origin":"","legend":"\u003cp\u003eThe partial wave electron state density of the combination of benzene (a), styrene (b), xylene (c), methylbenzene (d), ethylbenzene (e), butyl acetate (f), methanal (g), and n-undecane (h) on Sr\u003csub\u003e2\u003c/sub\u003eN substrate after structural relaxation. The gray shadow represents the state density of the Sr\u003csub\u003e2\u003c/sub\u003eN substrate after adsorption. The green line represents the state density of VOCs molecules after adsorption, and the purple line represents the state density of VOCs molecules before adsorption.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8852905/v1/bbd99eac52ccc0dc66e9c72f.png"},{"id":105751799,"identity":"eea20c1f-d621-4a83-bf42-0041dc1986f6","added_by":"auto","created_at":"2026-03-30 15:44:17","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":456521,"visible":true,"origin":"","legend":"\u003cp\u003eThe partial wave electron state density of the combination of benzene (a), styrene (b), xylene (c), methylbenzene (d), ethylbenzene (e), butyl acetate (f), methanal (g), and n-undecane (h) on Ba\u003csub\u003e2\u003c/sub\u003eN substrate after structural relaxation. The gray shadow represents the state density of the Ba\u003csub\u003e2\u003c/sub\u003eN substrate after adsorption. The green line represents the state density of VOCs molecules after adsorption, and the purple line represents the state density of VOCs molecules before adsorption.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8852905/v1/7f02e1d6d60941b8a8385878.png"},{"id":104401134,"identity":"a4746749-c633-41d3-ab2d-0a1acfc5992e","added_by":"auto","created_at":"2026-03-11 12:11:57","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":641710,"visible":true,"origin":"","legend":"\u003cp\u003eTop view and side view of differential charge density diagram of benzene (a), styrene (b), xylene (c), methylbenzene (d), ethylbenzene (e), butyl acetate (f), methanal (g), and n-undecane (h) combined with Ca\u003csub\u003e2\u003c/sub\u003eN substrate. The isosurface is set to 0.0015e/A. The yellow area represents charge accumulation and the blue area represents charge depletion.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8852905/v1/c4b053499026f83bc06a2614.png"},{"id":103860167,"identity":"3fcd2350-8153-402e-9c47-200a9552ef21","added_by":"auto","created_at":"2026-03-03 19:41:54","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":661923,"visible":true,"origin":"","legend":"\u003cp\u003eTop view and side view of differential charge density diagram of benzene (a), styrene (b), xylene (c), methylbenzene (d), ethylbenzene (e), butyl acetate (f), methanal (g), and n-undecane (h) combined with Sr\u003csub\u003e2\u003c/sub\u003eN substrate. The isosurface is set to 0.0015e/A. The yellow area represents charge accumulation and the blue area represents charge depletion.\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8852905/v1/c868e85eabfbe28f192516da.png"},{"id":104401143,"identity":"240604c2-bef8-4ccc-b0c5-98bbcffff0ca","added_by":"auto","created_at":"2026-03-11 12:11:58","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":676231,"visible":true,"origin":"","legend":"\u003cp\u003eTop view and side view of differential charge density diagram of benzene (a), styrene (b), xylene (c), methylbenzene (d), ethylbenzene (e), butyl acetate (f), methanal (g), and n-undecane (h) combined with Ba\u003csub\u003e2\u003c/sub\u003eN substrate. The isosurface is set to 0.0015e/A. The yellow area represents charge accumulation and the blue area represents charge depletion.\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-8852905/v1/7bb5fce4d4354561b8bf9f9b.png"},{"id":103860170,"identity":"83bcc8f3-4994-46a4-a366-3b3a2b41ae40","added_by":"auto","created_at":"2026-03-03 19:41:54","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":456150,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The differential charge density maps of benzene molecules adsorbed on the Ba₂N substrate. (b) The differential charge density maps of benzene molecules adsorbed on the Ba₂N substrate after applying electric fields of varying intensities along the z-axis direction. The isosurface value is uniformly set to 0.0015 e/Å, where the yellow regions represent charge accumulation, and the blue regions correspond to charge depletion. (c)The variation of charge transfer and equilibrium distance with the magnitude of the applied electric field, in which the blue curve denotes charge transfer and the yellow curve represents the equilibrium distance.\u003c/p\u003e","description":"","filename":"floatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-8852905/v1/a684f719de720514057fac0e.png"},{"id":106993742,"identity":"8a3a6268-6ba5-4fed-819b-6be681de6045","added_by":"auto","created_at":"2026-04-15 14:50:09","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6653197,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8852905/v1/18a36d97-60db-4725-a743-6aae5d10948d.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Surface free electrons endow 2D Alkali metal nitrides with extraordinary adsorption on volatile organic compounds","fulltext":[{"header":"Introduction","content":"\u003cp\u003eVolatile organic compounds (VOCs) such as benzene, formaldehyde, and toluene usually accumulate indoors\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. In national environmental health surveys conducted in the United States, Canada, and South Korea, over 90% participants had detected metabolites of benzene in their urine\u003csup\u003e2\u0026ndash;4\u003c/sup\u003e. Long term exposure to high concentrations of VOCs causes various hazards to the human body. Skin and eye contact with methanal cause dryness and ulcers\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. Long-term exposure to VOCs will not only damage the human respiratory\u003csup\u003e6\u003c/sup\u003e, hematopoietic, immune, digestive and reproductive systems\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e, but also lead to pneumonia, chronic obstructive pulmonary disease\u003csup\u003e8\u003c/sup\u003e, leukemia, lymphoma, bladder cancer and other diseases\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. VOCs also increase the risk of depression and other mental diseases\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e10\u003c/span\u003e,11\u003c/sup\u003e. Besides, a negative correlation between VOCs exposure and adolescent growth indicators was found\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. Moreover, elevated levels of VOCs in urine were significantly and positively correlated with an increase in the plasma atherogenic index\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. The urinary metabolites of ethylbenzene and styrene were significantly associated with cardiovascular disease, respiratory disease, and cancer mortality\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. In addition, the normal and pathological states of human physiological processes can be characterized by the VOCs released during respiration\u003csup\u003e\u003cspan additionalcitationids=\"CR16\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. Therefore, the detection and removal of these pollutants is of great significance.\u003c/p\u003e \u003cp\u003eNumerous VOC control technologies have emerged, such as incineration, condensation, biological degradation, absorption, adsorption, and catalytic oxidation, et al\u003csup\u003e18,19\u003c/sup\u003e. Among them, the adsorption technology has been recognized as an efficient and economical control strategy\u003csup\u003e\u003cspan additionalcitationids=\"CR21\" citationid=\"CR14\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. In current research on VOCs adsorption, traditional light absorption sensors\u003csup\u003e23\u003c/sup\u003e, heat conduction sensors\u003csup\u003e24\u003c/sup\u003e have insufficient adsorption capacity\u003csup\u003e24\u003c/sup\u003e. Carbon materials suffer from poor renewability, fire risks, high mass transfer resistance, pore blockage, and hygroscopicity\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. Zeolites and metal organic frameworks materials are costly, with numerous voids leading to weak dispersion forces and insufficient open metal sites\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,28\u003c/sup\u003e. Organic polymers involve complex synthesis processes, making large-scale production challenging\u003csup\u003e29\u003c/sup\u003e. Adsorption and membrane separation processes have poor adsorption capacity and selectivity for VOCs\u003csup\u003e30\u003c/sup\u003e. The average specific surface area, pore volume and VOCs adsorption capacity of different adsorption materials are metal organic frameworks\u0026thinsp;\u0026gt;\u0026thinsp;activated carbons\u0026thinsp;\u0026gt;\u0026thinsp;hypercrosslinked polymeric resin\u0026thinsp;\u0026gt;\u0026thinsp;zeolites\u003csup\u003e31,32\u003c/sup\u003e. However, the focus of these studies is on the specific surface area, adsorption pore size, and number of functional groups of the adsorbent, without considering the electronic interaction between the adsorbent and VOCs molecules\u003csup\u003e33,34\u003c/sup\u003e. Therefore, it is vital to explore the enormous potential of two-dimensional (2D) electronic materials in the field of gas adsorption by utilizing their unique structures and surface properties.\u003c/p\u003e \u003cp\u003e2D electronic compounds own the advantages of a large specific surface area, high mechanical strength, as well as the high electron concentration, high active site density\u003csup\u003e35,36\u003c/sup\u003e, and low work function of electronic compounds\u003csup\u003e37\u0026ndash;41\u003c/sup\u003e. Alkali metal nitrides Ca\u003csub\u003e2\u003c/sub\u003eN, Sr\u003csub\u003e2\u003c/sub\u003eN, and Ba\u003csub\u003e2\u003c/sub\u003eN are classical 2D electronic compounds that can be separated into single-layer structures in the laboratory\u003csup\u003e42\u003c/sup\u003e. They present M\u003csub\u003e2\u003c/sub\u003eN (M\u0026thinsp;=\u0026thinsp;Ca, Ba, Sr), in which conduction electrons are confined between the M layers\u003csup\u003e42\u003c/sup\u003e. The delocalized homogeneous electrons in the 2D interlayer space are responsible for the excellent transport characteristics, which are mainly conducted through the 2D space instead of the M\u003csub\u003e2\u003c/sub\u003eN layer\u003csup\u003e41\u0026ndash;43\u003c/sup\u003e. However, the application of 2D electronic compounds in the sensing and detection of VOCs molecules has not been reported yet. In this work, we selected Ca\u003csub\u003e2\u003c/sub\u003eN, Sr\u003csub\u003e2\u003c/sub\u003eN, and Ba\u003csub\u003e2\u003c/sub\u003eN as substrate materials and benzene, styrene, xylene, methylbenzene, ethylbenzene, butyl acetate, methanal, and n-undecane as adsorption targets. Based on first principles study, the adsorption of VOCs on a single-layer 2D electronic compound substrate was calculate. The adsorption capacity was adjusted by applying an electric field. The surface free electron layer properties of Ca\u003csub\u003e2\u003c/sub\u003eN, Sr\u003csub\u003e2\u003c/sub\u003eN, and Ba\u003csub\u003e2\u003c/sub\u003eN endow them with powerful VOCs sensing capability and provide new technology for indoor air quality detection.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eGeometric and Electronic Structures of Ca2N, Sr2N, and Ba2N\u003c/h2\u003e \u003cp\u003eTo investigate the properties of single-layer 2D electronic compounds Ca\u003csub\u003e2\u003c/sub\u003eN, Sr\u003csub\u003e2\u003c/sub\u003eN, and Ba\u003csub\u003e2\u003c/sub\u003eN, we present the geometric and electronic structure of a single-layer 2D electronic compound in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea shows the top and side views of a single-layer 2D electronic compound, where X represents Ca, Sr, and Ba\u003csup\u003e42,44\u003c/sup\u003e. The lattice structure of X\u003csub\u003e2\u003c/sub\u003eN electronic compounds is hexagonal, with optimized lattice constants of a\u0026thinsp;=\u0026thinsp;b=3.62 \u0026Aring; (Ca\u003csub\u003e2\u003c/sub\u003eN), a\u0026thinsp;=\u0026thinsp;b=3.81 \u0026Aring; (Sr\u003csub\u003e2\u003c/sub\u003eN), and a\u0026thinsp;=\u0026thinsp;b=3.98 \u0026Aring; (Ba\u003csub\u003e2\u003c/sub\u003eN). Each unit cell is composed of 2 X atoms and 1 N atom. From a top-down view, each X atom bonds to three adjacent N atoms, and each N atom is bonded to six adjacent X atoms. From a side view, X atoms construct the outer layers, and N atoms are in the middle layer. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb shows the partial charge density (left) and band structure diagram (right) of Ca\u003csub\u003e2\u003c/sub\u003eN. In order to investigate the distribution of active electrons on the material surface, the Fermi level was selected and the partial charge density of Ca\u003csub\u003e2\u003c/sub\u003eN was calculated from \u0026minus;\u0026thinsp;1 eV to 1 eV, as shown in the left figure. The yellow areas (left) denote the charge density corresponding to the yellow energy range in the band diagram (right). Most of the charges near the Fermi level are distributed on the surface of the material. The distribution of free electrons on the material surface is mainly located between adjacent Ca atoms. The purple circle in the band diagram represents the projected energy band of free electrons in the surface space region of Ca\u003csub\u003e2\u003c/sub\u003eN. The energy of surface electrons of Ca\u003csub\u003e2\u003c/sub\u003eN is in the range of -1.2 eV to 1.5 eV. Similarly, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec \u0026amp; \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed show the partial charge densities (left) and band structure diagrams (right) of Sr\u003csub\u003e2\u003c/sub\u003eN and Ba\u003csub\u003e2\u003c/sub\u003eN. The energy of surface electrons of Sr\u003csub\u003e2\u003c/sub\u003eN ranges from \u0026minus;\u0026thinsp;1.1 eV to 1.4 eV, and the energy of surface electrons of Ba\u003csub\u003e2\u003c/sub\u003eN ranges from \u0026minus;\u0026thinsp;0.8 eV to 1.5 eV. Comparing Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb-d, the projection regions of their surface free electrons in the band are similar, all located in the band near the Fermi level. The distribution of free electrons between adjacent Ca atoms is relatively uniform, while the distribution of free electrons between adjacent Sr/Ba atoms tends to lean towards the Sr/Ba atom on one side. During molecular adsorption, the electron-rich regions attract more molecules and result in charge transfer and adsorption. When the isosurfaces are equal, there are more free electrons on the surface of Ca\u003csub\u003e2\u003c/sub\u003eN, followed by Sr\u003csub\u003e2\u003c/sub\u003eN, and finally Ba\u003csub\u003e2\u003c/sub\u003eN. These differences in their surface electrons result in their potential for molecular adsorption applications\u003csup\u003e45\u003c/sup\u003e. Collectively, Ca\u003csub\u003e2\u003c/sub\u003eN, Sr\u003csub\u003e2\u003c/sub\u003eN, and Ba\u003csub\u003e2\u003c/sub\u003eN all have a hexagonal lattice structure and surface free electron distribution characteristics near the Fermi level. The differences in these structures and electronic properties characterize their adsorption applications.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eGeometric and electronic structures of eight types of VOCs molecules\u003c/h3\u003e\n\u003cp\u003eTo investigate the adsorption of VOCs molecules by single-layer 2D electronic compounds, we then explore the geometric and electronic structures. We use m-xylene as a representative to calculate the adsorption of xylene\u003csup\u003e46\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the molecular structure of eight types of VOCs, presented in a top view and a sideview. The molecular electrostatic potential of these eight molecules is shown in Fig.\u0026nbsp;3\u003csup\u003e47\u003c/sup\u003e.The blue area represents high energy and is prone to losing electrons. The red area indicates low energy and easy access to electrons. Both the red and blue areas represent more active areas on the surface of the molecule. In Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, there is a portion of red area on the surface of the benzene molecule, located on both sides of the six C atoms on the benzene ring structure. However, the color around the H atom is not obvious. This indicates that the most active position of the benzene is located inside the benzene ring and is easily accessible to electrons\u003csup\u003e48,49\u003c/sup\u003e. The red area on the surface of the styrene (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb) can be mainly divided into two parts. One part is located on both sides of the benzene ring structure, and the other part is around the C\u0026thinsp;=\u0026thinsp;C bond. Similarly, the alkyl functional groups in xylene (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), methylbenzene (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed), and ethylbenzene (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee) are not active, and the most active part is the benzene ring structure, which is easy to obtain electrons. The region around the C\u0026thinsp;=\u0026thinsp;O bond in butyl acetate (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef) is the most active. In methanal (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg), both the O atom side and the H atom side are relatively active, with the O atom side easily gaining electrons and the H atom side easily losing electrons. The area around the H atom in n-undecane (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh) is the most active and easily loses electrons, which differs from other VOCs. Taken together, these active regions provide a theoretical basis for subsequent research on adsorption interactions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eAdsorption of VOCs on Ca2N, Sr2N, and Ba2N Surface\u003c/h3\u003e\n\u003cp\u003eWe then calculate the adsorption of molecules at different angles and sites and select the structure with the lowest energy as the most stable structure. Table\u0026nbsp;1 presents the quantitative results of the equilibrium distance and adsorption energy of VOCs molecules. We find that methanol and butyl acetate have the shortest equilibrium distance, followed by styrene, xylene, methylbenzene, and ethylbenzene. Benzene has a longer distance and a significant decrease compared to the initial distance. N-undecane has no significant change in equilibrium distance compared to the initial distance. Besides, methanol and styrene have the highest adsorption energy, followed by xylene, methylbenzene, ethylbenzene, and butyl acetate. However, benzene and n-undecane have the lowest adsorption energy. These data indicate that Ca\u003csub\u003e2\u003c/sub\u003eN substrates have the strongest adsorption effect on molecules containing C\u0026thinsp;=\u0026thinsp;O bonds. Molecules containing C\u0026thinsp;=\u0026thinsp;C, such as styrene, come next. Their adsorption energy is relatively high. Although benzene molecules have a benzene ring structure, the equilibrium distance does not change much from the initial distance. The n-undecane molecule shows no significant adsorption in terms of geometric structure, equilibrium distance, and adsorption energy. The H atoms on the surface of the molecule are prone to losing electrons, making it difficult to generate charge transfer with the free electrons on the Ca\u003csub\u003e2\u003c/sub\u003eN substrate surface. These data demonstrate that the Ca\u003csub\u003e2\u003c/sub\u003eN substrate exhibits the strongest adsorption of C\u0026thinsp;=\u0026thinsp;O bond-containing molecules and the weakest adsorption of n-undecane.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows a side view of the combined structure of eight VOCs molecules and three electronic compound substrates after structural relaxation. On the Ca\u003csub\u003e2\u003c/sub\u003eN substrate, benzene (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), styrene (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), xylene (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec), methylbenzene (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed), and ethylbenzene (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee) are adsorbed at a horizontal angle. The position of the C\u0026thinsp;=\u0026thinsp;C bond in the styrene molecule is closest to the substrate, followed by the benzene ring structure inside the molecule. These indicate that the C\u0026thinsp;=\u0026thinsp;C bond structure is mainly adsorbed. Among the other four molecules, however, the benzene ring structure is closest to the substrate, while the alkyl structure in the molecule is farther away. Hence, the benzene ring structure is mainly adsorbed. Butyl acetate (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef) is adsorbed at an inclined angle, with the O atoms that form the C\u0026thinsp;=\u0026thinsp;O bond closest to the substrate. The length of the C\u0026thinsp;=\u0026thinsp;O bond has been extended from the initial 1.22 \u0026Aring; to 1.44 \u0026Aring;, and some atoms are further away from the substrate than their initial position. This indicates that the substrate has a stronger adsorption effect on the O atom in the C\u0026thinsp;=\u0026thinsp;O bond and a weaker adsorption effect on the remaining atoms. The O atom in the methanal (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg) is closest to the substrate, and the C\u0026thinsp;=\u0026thinsp;O bond is no longer in the same plane as the C and H atoms, implying that the O atom is strongly adsorbed by the substrate, while the H atom is subjected to a repulsive force towards the outside of the substrate. The n-undecane molecule did not undergo any deformation or change in position during the reaction.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe adsorption results of these VOCs molecules on Sr\u003csub\u003e2\u003c/sub\u003eN (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ei-p) and Ba\u003csub\u003e2\u003c/sub\u003eN (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003em-t) substrates are basically consistent with those on Ca\u003csub\u003e2\u003c/sub\u003eN substrates. All three substrates have the strongest adsorption ability for butyl acetate and methanol. Their adsorption for styrene and xylene is also quite strong. They show weak adsorption performance for benzene molecules and no adsorption effect on n-undecane molecules. Surprisingly, the molecular structure is completely destroyed due to the strong interaction with Ba\u003csub\u003e2\u003c/sub\u003eN substrate (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003er). This significantly demonstrates that Ba\u003csub\u003e2\u003c/sub\u003eN is an ideal material for VOCs adsorption. In addition, the equilibrium distance for the adsorption of benzene and n-undecane molecules on the Ba2N substrate is larger than for the other two substrates. However, the adsorption energy for these two molecules on Ba2N is larger than on Ca\u003csub\u003e2\u003c/sub\u003eN and Sr\u003csub\u003e2\u003c/sub\u003eN. This may result from the higher mass of Ba atoms on the substrate surface. Collectively, these results demonstrate a clear functional-group specificity in adsorption configuration and a strong substrate-dependent variation in adsorption strength.\u003c/p\u003e\n\u003ch3\u003eMechanism of VOCs adsorption on CaN, SrN, and BaN Surface\u003c/h3\u003e\n\u003cp\u003eNext, we calculate the electronic density of states (DOS) of the Ca\u003csub\u003e2\u003c/sub\u003eN substrate when adsorbing VOCs molecules. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the changes in DOS of VOCs molecules before and after adsorption on a Ca\u003csub\u003e2\u003c/sub\u003eN substrate. Briefly, the electronic DOS of Ca\u003csub\u003e2\u003c/sub\u003eN has increased in energy compared to the initial energy. However, the DOS of VOCs molecules has decreased in energy compared to the initial state. This indicates that VOCs molecules gain electrons in the reaction, and the Fermi level is relatively higher. Meanwhile, the Ca\u003csub\u003e2\u003c/sub\u003eN substrate loses electrons in the reaction, and the Fermi level is relatively lower. Interestingly, the benzene and Ca\u003csub\u003e2\u003c/sub\u003eN substrate (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea) undergo hybridization in the energy range of -0.5 eV to 2 eV, and the DOS overlaps in this region. Styrene (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb), xylene (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec), methylbenzene (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed), and ethylbenzene (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee) exhibit similar orbital hybridization after adsorption on Ca\u003csub\u003e2\u003c/sub\u003eN substrates due to their similar structures. The hybridization all occurs in the energy range of -0.5 eV to 2.5 eV and \u0026minus;\u0026thinsp;4 eV to -3 eV, with some additional hybridization regions compared to benzene. The hybrid orbitals of butyl acetate (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef) and methanol (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg) adsorbed on a Ca\u003csub\u003e2\u003c/sub\u003eN substrate are located in the energy range of -4 eV to -3 eV below the Fermi level. The orbital hybridization of N-undecane (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eh) after adsorption on the Ca\u003csub\u003e2\u003c/sub\u003eN substrate is not significant, indicating that the molecular effect is relatively weak. Besides, the degree of orbital hybridization of VOCs molecules adsorbed on the Ca\u003csub\u003e2\u003c/sub\u003eN substrate is consistent with the results of the adsorption energy analysis mentioned earlier. Moreover, the adsorption results of these VOCs molecules on Sr\u003csub\u003e2\u003c/sub\u003eN (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea-h) and Ba2N (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea-h) substrates are similar to those on Ca\u003csub\u003e2\u003c/sub\u003eN substrates. They are also consistent with the previous analysis results. Since the molecular structure is completely destroyed, we obtain no DOS for Styrene on Ba\u003csub\u003e2\u003c/sub\u003eN (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo investigate the charge transfer between VOCs molecules adsorbed on single-layer Ca\u003csub\u003e2\u003c/sub\u003eN, Sr\u003csub\u003e2\u003c/sub\u003eN, and Ba\u003csub\u003e2\u003c/sub\u003eN substrates, we calculate the differential charge density of the adsorbed structure and the amount of charge transferred during the reaction, which is summarized in Table\u0026nbsp;1. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows the differential charge density of the VOCs molecules adsorbed on a Ca\u003csub\u003e2\u003c/sub\u003eN substrate, including top and side views, with isosurfaces of 0.0015\u0026nbsp;e/\u0026Aring; for all charge densities. When benzene is adsorbed on a Ca\u003csub\u003e2\u003c/sub\u003eN substrate (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea), there is a small but significant charge transfer. The substrate part loses electrons on the surface, and the electron loss area is located directly opposite the Ca atom on the other side, slightly higher than the Ca atom at the interface. There is charge accumulation on both sides of the C atom in the benzene molecule, with more charge accumulation on the side closer to the substrate. Charge loss occurs around the H atom, with the lost electrons located on the side farther from the substrate, consistent with the active region in the molecular electrostatic potential. When styrene is adsorbed on Ca\u003csub\u003e2\u003c/sub\u003eN substrates (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb), there is a significant amount of charge transfer. The region where free electrons accumulate on the substrate surface loses electrons, consistent with the adsorption of benzene molecules. In styrene molecules, there is charge accumulation on both sides of the C atom and charge loss around the H atom. Specifically, the C atom on the C\u0026thinsp;=\u0026thinsp;C bond receives more electrons than the C atom on the benzene ring structure. This indicates that during the adsorption process, both structures interact with electrons on the Ca\u003csub\u003e2\u003c/sub\u003eN substrate surface. Among them, the C\u0026thinsp;=\u0026thinsp;C bond undergoes more charge transfer, which is consistent with the results of the geometric structure. Xylene (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec), methylbenzene (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ed), and ethylbenzene (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ee) all exhibit similar differential charge results when adsorbed on Ca\u003csub\u003e2\u003c/sub\u003eN substrates. Overall, they all transfer charges from the Ca\u003csub\u003e2\u003c/sub\u003eN substrate to VOCs molecules. Charge loss occurs at the locations where free electrons accumulate on the surface of the Ca\u003csub\u003e2\u003c/sub\u003eN substrate, with charge accumulation around Ca atoms at the substrate interface that adsorb xylene and ethylbenzene molecules on the side away from the interface. This arises from charge redistribution caused by charge transfer. The charge gain and loss in molecules mainly occur in the benzene ring structure region. Only a small amount occurs in the alkyl structure region. In addition, when xylene is adsorbed on the Ca\u003csub\u003e2\u003c/sub\u003eN substrate, the position of the benzene ring is consistent with that of the benzene molecule. However, for methylbenzene and ethylbenzene, the position of the benzene ring is located directly opposite the Ca atom at the interface. When butyl acetate (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ef) and methanol (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eg) are adsorbed on a Ca\u003csub\u003e2\u003c/sub\u003eN, there is a significant amount of charge transfer from the substrate to the molecule. The region where free electrons gather in the Ca\u003csub\u003e2\u003c/sub\u003eN substrate loses charge. The C and O atoms in the C\u0026thinsp;=\u0026thinsp;O bond of the molecule are charged, and other parts of the molecule also experience charge gain and loss due to charge redistribution. Charge accumulation occurs around Ca atoms on the Ca\u003csub\u003e2\u003c/sub\u003eN substrate surface that adsorbs methanal molecules. In the differential charge results of n-undecane adsorption (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eh), no charge loss occurred at the substrate, indicating that there was no charge transfer between the molecule and the substrate. Collectively, there is no charge transfer between the n-undecane molecule and the Ca\u003csub\u003e2\u003c/sub\u003eN substrate. However, other gases have undergone significant charge transfer.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e shows the top and side views of the differential charge density after adsorbing VOCs molecules on Sr\u003csub\u003e2\u003c/sub\u003eN substrate, with an isosurface of 0.0015\u0026nbsp;e/\u0026Aring;. Overall, the adsorption results of Sr\u003csub\u003e2\u003c/sub\u003eN and Ca\u003csub\u003e2\u003c/sub\u003eN substrates are similar, with charges transferred from the substrate to VOCs molecules. The charge transfer amount of benzene (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea) and styrene (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb) adsorbed on the Sr2N substrate is reduced compared to the Ca2N substrate. In addition, the positions where charges are lost in the substrate are consistent. The charge transfer amount of xylene (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ec) and methylbenzene (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ed) molecules adsorbed on the Sr\u003csub\u003e2\u003c/sub\u003eN substrate shows no significant change compared to the Ca\u003csub\u003e2\u003c/sub\u003eN substrate. However, the benzene ring structure in the molecules is located directly opposite the Sr atom at the interface. Thus, the position where the charge is lost mainly centered around the Sr atom. The charge transfer amount of ethylbenzene adsorbed on the Sr\u003csub\u003e2\u003c/sub\u003eN substrate (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ee) is significantly reduced compared to the Ca\u003csub\u003e2\u003c/sub\u003eN substrate. Besides, the adsorption site also changes. The charge transfer amount of butyl acetate (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ef) and methanal (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eg) adsorbed on Sr\u003csub\u003e2\u003c/sub\u003eN substrate is similar to that on Ca2N substrate, and the positions of gain and loss electrons are consistent. In Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eh, n-undecane shows no significant charge transfer on the Sr\u003csub\u003e2\u003c/sub\u003eN substrate. Concisely, except for changes in the adsorption sites of toluene, xylene, and ethylbenzene, the adsorption characteristics of other molecules on Sr\u003csub\u003e2\u003c/sub\u003eN substrates are basically consistent with those on Ca\u003csub\u003e2\u003c/sub\u003eN.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e shows the top and side views of the differential charge density after adsorbing VOCs molecules on a Ba\u003csub\u003e2\u003c/sub\u003eN substrate, with an iso surface of 0.0015\u0026nbsp;e/\u0026Aring;. Overall, the adsorption results of Ba\u003csub\u003e2\u003c/sub\u003eN and Ca\u003csub\u003e2\u003c/sub\u003eN substrates are similar, with charges transferred from the substrate to VOCs molecules. The charge transfer amount of benzene (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ea), xylene (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ec), methylbenzene (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ed), and ethylbenzene (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ee) adsorbed on Ba\u003csub\u003e2\u003c/sub\u003eN substrate is reduced compared to the Ca\u003csub\u003e2\u003c/sub\u003eN substrate. The adsorption sites of the four molecules are consistent, and the benzene ring structure is located directly above the Ba atoms. Surprisingly, there is a large amount of charge transfer distributed around the scattered atoms after the molecular structure is disrupted (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eb). The charge transfer amount of butyl acetate (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ef) and methanal (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eg) adsorbed on Ba\u003csub\u003e2\u003c/sub\u003eN substrate is similar to that on Ca\u003csub\u003e2\u003c/sub\u003eN substrate. The n-undecane shows no significant charge transfer on the Ba\u003csub\u003e2\u003c/sub\u003eN substrate (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eh). Collectively, the adsorption of other molecules on the Ba2N substrate is basically the same as that on Ca\u003csub\u003e2\u003c/sub\u003eN, except for the structural damage of styrene molecules during adsorption. Based on Table\u0026nbsp;1 and Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, and \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e, the adsorption of n-undecane molecules on Ca\u003csub\u003e2\u003c/sub\u003eN, Sr\u003csub\u003e2\u003c/sub\u003eN, and Ba\u003csub\u003e2\u003c/sub\u003eN substrates is not significant. Molecules such as xylene, methylbenzene, and ethylbenzene, which are composed of a benzene ring and alkyl structures, have a stronger adsorption capacity. The adsorption of styrene is extremely strong, especially on Ba\u003csub\u003e2\u003c/sub\u003eN substrates, which have the ability to damage the molecular structure during the adsorption process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eElectric field improves the adsorption of VOCs to 2D electronic compounds\u003c/h3\u003e\n\u003cp\u003eGiven that Ca\u003csub\u003e2\u003c/sub\u003eN, Sr\u003csub\u003e2\u003c/sub\u003eN, and Ba\u003csub\u003e2\u003c/sub\u003eN exhibit weak adsorption toward benzene and n-undecane molecules, they are not readily applicable directly in sensing applications. Therefore, we next introduce regulatory strategies to enhance the adsorption performance of these three substrates. The poor adsorbability of n-undecane molecules arises from the tendency of their surface H atoms to lose electrons, which is incompatible with the abundant free and mobile electrons on the surface of single-layer 2D electronic compounds. For benzene molecules, their weak adsorption performance is attributed to the relatively stable internal structure of the benzene ring. In contrast, its analogous molecules possess additional functional groups that disrupt the internal stability of the benzene ring. This significantly improves the adsorption efficiency. Among the three substrate materials, Ba\u003csub\u003e2\u003c/sub\u003eN demonstrates the weakest overall adsorption performance. Thus, we select benzene as a representative molecule and Ba2N as the substrate to investigate the effect of external regulation on the adsorption capacity.\u003c/p\u003e \u003cp\u003eCommon strategies for regulating the adsorption performance of 2D materials include defect engineering, doping modification, electric field regulation, and pressure regulation\u003csup\u003e50\u003c/sup\u003e. Since electric fields can directly modulate the free electrons on the substrate surface, we employ an electric field applied along the z-axis for regulation. We simulate an external electric field and calculate the adsorption behavior of benzene molecules on the substrate under electric fields ranging from \u0026minus;\u0026thinsp;0.8 eV/\u0026Aring; to 0.8 eV/\u0026Aring; along the z-axis, with an interval of 0.2 eV/\u0026Aring;. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea\u0026amp;b show the differential charge density maps of benzene adsorbed on the substrate under electric fields of varying intensities. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ec depicts the variation curves of charge transfer and equilibrium distance for benzene adsorption on the substrate under different electric field conditions. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eb, when an electric field of \u0026minus;\u0026thinsp;0.2 to \u0026minus;\u0026thinsp;0.8 eV/\u0026Aring; is applied, the electrons in the system experience a downward electric field force, which hinders the transfer of free electrons from the substrate surface to the molecule and also impedes the redistribution of charge within the molecule, resulting in a reduction in charge transfer. When an electric field of 0.2 to 1.0 eV/\u0026Aring; is applied, the electrons in the system are subjected to an upward electric field force, which facilitates the transfer of free electrons from the substrate surface to the molecule. Meanwhile, the direction of charge redistribution within the molecule aligns with the direction of the electric field force, promoting the formation of a new equilibrium and thus increasing charge transfer. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ec visualizes the variation patterns of charge transfer and equilibrium distance. Within the range of 0.8 eV/\u0026Aring;, as the intensity of the electric field applied downward along the z-axis increases, the charge transfer increases while the equilibrium distance decreases. This indicates a linear relationship between the electric field and the sensing performance of the substrate. However, beyond 0.8 eV/\u0026Aring;, increasing the electric field intensity leads to a decrease in charge transfer and an increase in equilibrium distance. These results collectively indicate that the electric field modulates the adsorption interaction between VOCs molecules and the substrate material by altering the distribution and movement tendency of electrons.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eWe show that Ca\u003csub\u003e2\u003c/sub\u003eN, Sr\u003csub\u003e2\u003c/sub\u003eN, and Ba\u003csub\u003e2\u003c/sub\u003eN provide effective adsorption for VOCs. Ca\u003csub\u003e2\u003c/sub\u003eN, Sr\u003csub\u003e2\u003c/sub\u003eN, and Ba\u003csub\u003e2\u003c/sub\u003eN have adsorption effects on benzene, styrene, xylene, methylbenzene, ethylbenzene, butyl acetate, and methanal. All adsorption processes do not damage the structure of the substrate materials. Thus, the substrate can be reused. Surprisingly, the molecular structure of styrene adsorbs on Ba\u003csub\u003e2\u003c/sub\u003eN is destroyed due to strong interactions. Benzene and n-undecane adsorb mainly through van der Waals forces due to their low charge transfer. The adsorption energy and charge transfer amount of styrene containing a C\u0026thinsp;=\u0026thinsp;C bond and a benzene ring structure are the highest. Xylene, methylbenzene, and ethylbenzene, which contain benzene ring structures or C\u0026thinsp;=\u0026thinsp;O bonds, follow closely behind. The adsorption effect of benzene still exhibits significant adsorption energy and charge transfer. The adsorption effect of N-undecane is the worst. Especially, the application of an electric field in the z-direction significantly improves the ability of Ba\u003csub\u003e2\u003c/sub\u003eN to adsorb benzene. Our research can be used to effectively adsorb VOCs and control air pollution. In addition, the alkali metal nitrides used in the experiment and the method of applying an electric field provide new ideas for exploring other adsorption materials. Subsequent research can use doping, defects, and other methods to compare their adsorption effects, and it is worthwhile to make VOCs sensors based on Ca\u003csub\u003e2\u003c/sub\u003eN, Sr\u003csub\u003e2\u003c/sub\u003eN, and Ba\u003csub\u003e2\u003c/sub\u003eN.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eDFT-based first-principles calculations were performed with a plane-wave basis set as implemented in the Vienna ab initio simulation package (VASP)\u003csup\u003e51\u003c/sup\u003e. The projector augmented wave method was employed to describe the ion\u0026ndash;electron interaction\u003csup\u003e52\u003c/sup\u003e. The exchange correlation functional used in the calculation is PBE functional in GGA\u003csup\u003e53\u003c/sup\u003e, and the pseudopotential is PAW. The truncation energy based on plane waves is set to 400 eV. During the modeling process, a 25 \u0026Aring; vacuum layer is added to eliminate the mutual influence between periodic unit cells in the z-direction, in order to simulate the structure of a single layer. The initial structure is a 1 \u0026times; 1 single-layer structure on the xy plane. In order to prevent mutual influence between adjacent unit cells in the plane and maintain sufficient distance between each molecule, the substrate structure was expanded to 3 \u0026times; 3 \u0026times; 1 when constructing the adsorption model. In the structural optimization, the total energy was set to converge to 10\u0026thinsp;\u0026minus;\u0026thinsp;5 eV, and the force on each atom converged to 0.01 eV/\u0026Aring; before stopping relaxation. The K-point sampling grid was set to 7 \u0026times; 7 \u0026times; 1. In electronic structure calculation, the accuracy of K-point sampling grid is improved to 9 \u0026times; 9 \u0026times; 1. In addition, in order to eliminate the influence of van der Waals forces in the calculation, the DFT-D3 method was used for van der Waals force correction in each step of the calculation.\u003c/p\u003e \u003cp\u003eThe adsorption energy (Ea) of different molecules on Ca2N, Sr2N, and Ba2N surfaces was computed, in which Ea can be expressed by,\u003c/p\u003e \u003cp\u003e \u003cem\u003eE\u003c/em\u003ea\u0026thinsp;=\u0026thinsp;\u003cem\u003eE\u003c/em\u003etotal \u0026minus; (\u003cem\u003eE\u003c/em\u003em\u0026thinsp;+\u0026thinsp;\u003cem\u003eE\u003c/em\u003esur)\u003c/p\u003e \u003cp\u003ewhere Etotal is the total energy of the adsorbed system. Em and Esur are the\u003c/p\u003e \u003cp\u003eenergies of relaxed molecules in the gas phase and on the Ca2N, Sr2N, and Ba2N surfaces, respectively. By this definition, larger negative values of Ea denote stronger interaction between the molecule and the surface.\u003c/p\u003e\n"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eAll authors declare no financial or non-financial competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eJW performed the DFT study. XZ analyzed the data. QC \u0026amp; SL are contributors in writing the manuscript. MZ \u0026amp; QC design and supervise this study. All authors read and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis work was supported by grants from the Natural Science Foundation of Chongqing (CSTB2023NSCQ-MSX0620) .\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eAll data generated or analysed during this study are included in this published article.\u003c/p\u003e"},{"header":"References","content":"\u003cp\u003e1 Feng, Y. L., Yang, C. \u0026amp; Cao, X. L. Intermediate volatile organic compounds in Canadian residential air in winter: Implication to indoor air quality. \u003cem\u003eChemosphere\u003c/em\u003e \u003cstrong\u003e328\u003c/strong\u003e (2023). https://doi.org/ARTN 138567\u003c/p\u003e\n\u003cp\u003e10.1016/j.chemosphere.2023.138567\u003c/p\u003e\n\u003cp\u003e2 Corbasson, I., Hankinson, S. E., Stanek, E. J. \u0026amp; Reeves, K. W. 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Generalized Gradient Approximation Made Simple. \u003cem\u003ePhys Rev Lett\u003c/em\u003e \u003cstrong\u003e77\u003c/strong\u003e, 3865-3868 (1996). https://doi.org/10.1103/PhysRevLett.77.3865\u003c/p\u003e"},{"header":"Tables","content":"\u003cp\u003eTable.1 Adsorption energy, charge transfer amount and equilibrium distance of benzene, styrene, xylene, methylbenzene, ethylbenzene, butyl acetate, methanal, and n-undecane adsorbed by Ca2N, Sr2N, and Ba2N.\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"100%\" class=\"fr-table-selection-hover\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 22px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" style=\"width: 26px;\"\u003e\n \u003cp\u003eCa\u003csub\u003e2\u003c/sub\u003eN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" style=\"width: 26px;\"\u003e\n \u003cp\u003eSr\u003csub\u003e2\u003c/sub\u003eN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" style=\"width: 25px;\"\u003e\n \u003cp\u003eBa\u003csub\u003e2\u003c/sub\u003eN\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 22px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e\u003cem\u003ed\u003c/em\u003e/\u0026Aring;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e△\u003cem\u003eE\u003c/em\u003e/eV\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e△\u003cem\u003eq\u003c/em\u003e/\u003cem\u003ee\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e\u003cem\u003ed\u003c/em\u003e/\u0026Aring;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e△\u003cem\u003eE\u003c/em\u003e/eV\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e△\u003cem\u003eq\u003c/em\u003e/\u003cem\u003ee\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e\u003cem\u003ed\u003c/em\u003e/\u0026Aring;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e△\u003cem\u003eE\u003c/em\u003e/eV\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e△\u003cem\u003eq\u003c/em\u003e/\u003cem\u003ee\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 22px;\"\u003e\n \u003cp\u003eBenzene\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e2.66\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-0.83\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e0.95\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e2.70\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-0.71\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e0.77\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e2.93\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-0.79\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e0.79\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 22px;\"\u003e\n \u003cp\u003eStyrene\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.78\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-2.75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e2.11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.86\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-2.53\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.80\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e0.87\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e2.17\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e5.11\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 22px;\"\u003e\n \u003cp\u003eXylene\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e2.00\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-1.54\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.81\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.85\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-1.41\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.72\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e2.54\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-0.89\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e0.91\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 22px;\"\u003e\n \u003cp\u003eMethylbenzene\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.89\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-1.67\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.74\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.88\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-1.36\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.70\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e2.89\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-0.87\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e0.87\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 22px;\"\u003e\n \u003cp\u003eEthylbenzene\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.90\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-1.59\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.84\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e2.25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-0.83\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.00\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e2.51\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-0.95\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e0.96\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 22px;\"\u003e\n \u003cp\u003eButyl acetate\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-2.07\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.73\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-1.66\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.63\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.47\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-1.25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.38\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 22px;\"\u003e\n \u003cp\u003eMethanal\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.07\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-3.21\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.60\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-2.89\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.61\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.32\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-2.52\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e1.48\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 22px;\"\u003e\n \u003cp\u003eN-undecane\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e2.97\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-0.60\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e0.64\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e2.96\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-0.62\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e0.67\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e2.96\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e-0.75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 8px;\"\u003e\n \u003cp\u003e0.61\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"npj-computational-materials","isNatureJournal":false,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"npjcompumats","sideBox":"Learn more about [npj Computational Materials](http://www.nature.com/npjcompumats/)","snPcode":"41524","submissionUrl":"https://mts-npjcompumats.nature.com/","title":"npj Computational Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"volatile organic compounds (VOCs), 2D Alkali metal nitrides, Ca2N, Sr2N, Ba2N, First principles calculations, 2D electronic compounds","lastPublishedDoi":"10.21203/rs.3.rs-8852905/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8852905/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eVolatile organic compounds (VOCs) are a major cause of leukemia, carcinoma, and other diseases. However, removal technologies for VOCs cannot cope with the grim situation of VOC pollution. Two-dimensional (2D) alkali metal nitrides Ca\u003csub\u003e2\u003c/sub\u003eN, Sr\u003csub\u003e2\u003c/sub\u003eN, and Ba\u003csub\u003e2\u003c/sub\u003eN have great potential as a new adsorption material, and their application as a VOCs detector has not reported. Here, we study the sensing performance of Ca\u003csub\u003e2\u003c/sub\u003eN, Sr\u003csub\u003e2\u003c/sub\u003eN, and Ba\u003csub\u003e2\u003c/sub\u003eN on benzene, styrene, xylene, methylbenzene, ethylbenzene, butyl acetate, methanal, and n-undecane. We find that Ca\u003csub\u003e2\u003c/sub\u003eN, Sr\u003csub\u003e2\u003c/sub\u003eN, and Ba\u003csub\u003e2\u003c/sub\u003eN have outstanding adsorption capabilities for VOCs. This strong adsorption is attributed to the free-electron layer on their surface. When the isosurfaces are equal, there are more free electrons on the surface of Ca2N, followed by Sr2N, and finally Ba2N. Moreover, electric fields could effectively improve their adsorption performance. The perfect match of the electronic properties of VOCs with those of Ca2N, Sr2N, and Ba2N contributes to this extraordinary adsorption capability. Therefore, this study will lead to a new round of technological innovation in VOCs adsorption technology based on 2D alkali metal nitrides.\u003c/p\u003e","manuscriptTitle":"Surface free electrons endow 2D Alkali metal nitrides with extraordinary adsorption on volatile organic compounds","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-03 19:41:49","doi":"10.21203/rs.3.rs-8852905/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-04-22T15:18:22+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-19T08:03:42+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"51702739993986961552130257977621817621","date":"2026-04-15T01:19:08+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-14T15:04:55+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"147663715813810652970519742504156761148","date":"2026-03-31T13:33:24+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-26T08:07:32+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-22T00:46:14+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-02-14T17:57:59+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj Computational Materials","date":"2026-02-11T14:20:10+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-computational-materials","isNatureJournal":false,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"npjcompumats","sideBox":"Learn more about [npj Computational Materials](http://www.nature.com/npjcompumats/)","snPcode":"41524","submissionUrl":"https://mts-npjcompumats.nature.com/","title":"npj Computational Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"6b564ae7-b8a3-4124-a8ec-21cca6be3fc5","owner":[],"postedDate":"March 3rd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":63776122,"name":"Physical sciences/Chemistry"},{"id":63776123,"name":"Earth and environmental sciences/Environmental sciences"},{"id":63776124,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2026-05-05T14:53:24+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-03 19:41:49","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8852905","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8852905","identity":"rs-8852905","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}