Energy shortage and environmental pollution are two major global challenges faced by humanity today. Air pollution, which is closely related to human health, has attracted considerable attention in the past few years. Indoor air quality is particularly important for human health because humans spend 70%–90% of their life indoors [1]. Carbonyl compounds, in particular formaldehyde (HCHO), are among the most common indoor air pollutants, mainly released from decorating materials, plywood, fiberboard, particleboard, and other artificial boards [2-5]. Long-term exposure to high concentrations of HCHO may cause irritation, allergies, decreased immune function [6], and even nasopharyngeal carcinoma or leukemia [7, 8]. Therefore, the removal of HCHO is a necessary measure to reduce air pollution and protect human health.

Various techniques have been reported for dealing with indoor HCHO, and the most common ones are adsorption and catalytic oxidation. The conventional adsorption methods usually employ porous adsorbents such as activated carbon [9, 10], zeolites [9, 11], and molecular sieves [12], which have been widely used due to their advantages including simplicity, low cost, and room-temperature operation. However, the limited adsorption capacity and adsorbent regeneration make these systems inadequate to meet present demands [13-16]. Catalytic degradation is another widely studied process to perform HCHO oxidation [17-22]. However, it also has some shortcomings in practical applications. For example, TiO₂-based catalysts have been widely investigated, but the relatively wide band gap of TiO₂ restricts its light energy utilization; therefore, the degradation efficiency requirements are often unsatisfied [23-25]. Therefore, new catalysts for the effective removal of HCHO are still urgently needed.

Recently, a new type of carbon nitrides, the holey C₂N with two-dimensional (2D) structure, has been synthesized through a simple wet-chemical reaction [26]. As a new type of graphene-like porous structure with well-distributed pores, this material has attracted significant attention because of its ultra-large surface area and appropriate band gap value. For example, Zhu et al. [27] prepared C₂N for the adsorption of He atoms. Liu et al. [28] reported that Ca-embedded C₂N is an efficient adsorbent for CO₂ capture, while Qin et al. [29] employed charge and electric field effects to control CO₂ capture and gas separation on a C₂N monolayer. Furthermore, in the context of targeting gas pollution, Bhattachayya et al. [30] showed that a C₂N monolayer can efficiently trap HF, HCN, and H₂S pollutants, causing different changes in the I-V characteristics of the monolayer, which is a promising feature for the detection of the corresponding gases. These studies show that C₂N may be a promising candidate for the adsorption and degradation of HCHO molecules. However, pristine 2D materials cannot always satisfy the demands of practical applications, and functionalization is required in some cases. Previous studies have shown that modification with Al atoms can in some cases lead to enhanced adsorption capacity. For example, our previous studies have shown that Al doping of graphene can significant enhance the adsorption of CO [31] and H₂ [32, 33]. In addition, Al-modified MoS₂ exhibits strong adsorption capacity toward NO₂ and NH₃ [34], while the enhanced adsorption of HCHO [35], H₂ [36], and CO₂ [37] has been observed on Al-modified BN systems. Hence, modification with Al atoms can be considered as a promising approach for improving the adsorption ability of 2D materials.

Herein, the HCHO adsorption on both pristine and Al-decorated C₂N monolayer is modeled using the density functional theory (DFT) method. Moreover, the adsorption capacity toward H₂O and O₂, which are sources of active hydroxyl and superoxide radicals, is investigated for the subsequent catalytic degradation of HCHO. Furthermore, the adsorption enhancement mechanism is analyzed by calculating the partial density of states (PDOS), Mulliken atomic charges, and electron density distribution.

All calculations reported in this study were carried out using the DFT-based DMol³ [38, 39] module of the Materials Studio software, which includes unique methodologies and an highly efficient optimization for dealing with electrostatic problems. The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional within the generalized gradient approximation (GGA) was employed in the calculations [40]. It is essential to take into account van der Waals forces to describe the decorated metal atoms and the adsorption of gas molecules; hence, the Tkatchenko-Scheffler (TS) method for the DFT-D correction was used for this purpose. In order to take relativity effects into account, all electrons were included in the calculations, to obtain more accurate results without any special treatment of core electrons. Double numerical plus polarization (DNP) functions were adopted as the basis set. A 24 Å vacuum gap was included above the monolayer to minimize interlayer interactions. A 7×7×1 Monkhorst-Pack grid of k-points was used, and all atoms were allowed to relax until the residual forces on individual atoms were smaller than 10^–5 Ha (1 Ha = 27.211 eV). The maximum force and displacement were set to 0.002 Ha/Å and 0.005 Å, respectively.

For examining the structural stability of Al atoms decorated on the C₂N monolayer, the binding energy E_b-Al was determined by the relation:

(1)

where E_Al+C2N, E_Al, and E_C2N represent the total energies of the Al-decorated C₂N, an independent Al atom in the slab, and the pristine C₂N monolayer, respectively. Moreover, to probe the adsorption of gas molecules on the pristine/Al-decorated C₂N systems, the adsorption energy E_ad-gas was determined as:

(2)

where E_C2N+gas, E_C2N, and E_gas are the total energies of a gas molecule adsorbed on the C₂N layer with or without Al atoms, the isolated C₂N monolayer, and the corresponding isolated gas molecule in the slab, respectively.

The optimized structure of the C₂N monolayer unit cell is shown in Fig. 1(a), and consists of 12 C and 6 N atoms (solid black line), with four unit cells forming a uniform hole. The lattice parameters calculated in this study are a = b = 8.325 Å, which are consistent with recently reported results (a = b = 8.33 Å) [41, 42]. Moreover, the C–N, C–C(1), and C–C(2) bond lengths are 1.337, 1.428, and 1.470 Å, respectively, also in good agreement with the recent theoretical values of 1.337, 1.429, and 1.470 Å [42]. Then, we examined the electronic properties of the pristine C₂N monolayer, whose calculated band structure is shown in Fig. 1(b). The figure shows that C₂N is a semiconductor with a direct band gap of 1.67 eV, in good agreement with the reported theoretical [43] and experimental data (1.66 and 1.96 eV, respectively) [26].

The decoration of Al atoms on the C₂N monolayer was then considered. Five different adsorption sites were examined based on the high-symmetry structure shown in Fig. 1(a). The structure of Al-decorated C₂N after full geometry optimization is shown in Fig. 2(a). As can be seen in the figure, the Al atom prefers to move close to the two nitrogen atoms of the C₂N monolayer near the vacancy (Ⅴ) site, even though its initial position is on the top of a C or N atom. A similar conclusion has been reported for of Li [43] and transition metal (Pt, Co, Ni, Cu) [44] atoms decorated on a C₂N layer. The deposition of one Al atom on the C₂N monolayer causes negligible structural changes in the system, and all atoms remain in the same plane without strong deformation. In addition, the decorated Al atom forms strong chemical bonds with the two N atoms, with a bond length of 1.97 Å. It should be noted that this bond length is longer than both the C–C and C–N bonds in the C₂N monolayer, which reveals that the Al–N bonds are easily broken in a chemical reaction, compared with the pristine C₂N system. Previous research has shown that a high concentration of metal atoms leads to their aggregation; in order to verify the structural stability of the Al-decorated C₂N system, the binding energy (E_b) was calculated based on Eq. (1). Table S1 (Supporting Information) shows the calculated binding energy of the decorated Al atom at different positions on the pristine C₂N monolayer. The results show that the V site has the highest binding energy (E_b = –4.57 eV), which exceeds the cohesive energy of bulk Al (–3.39 eV/atom) [45]. Thus, the aggregation of Al atoms in the C₂N monolayer can be neglected, confirming the stability of Al-decorated C₂N. It is worth mentioning that the decorated Al atom leads to a sharp decrease in the band gap from 1.69 to 0.22 eV, as shown in Fig. 2(b), and a similar phenomenon has been also reported by Hashmi et al. [43] for a Li-decorated C₂N monolayer. A 3×3 C₂N supercell was constructed to assess the effect of the loading amount of Al on the adsorption of both Al atoms and HCHO molecules. After geometry optimization, the structures of the 3×3 Al-decorated C₂N monolayer with and without adsorbed HCHO are shown in Fig. S1. Compared with the 2×2 C₂N monolayer supercell (Al loading amount ~2.9 wt%), the calculated binding energy of the Al atom on the 3×3 C₂N monolayer supercell (Al loading amount ~1.3 wt%) slightly increases to –4.87 eV. In addition, the adsorption energy of the HCHO molecule on the Al-decorated C₂N also shows a slight change, to –2.33 eV. Therefore, the Al loading amount has a negligible effect on the HCHO adsorption; the maximum Al loading amount can reach ~10.6 wt% with one Al atom at each hollow site of the C₂N monolayer. Higher Al loadings were also considered, with two or three Al atoms decorated on the same hollow site; however, Al aggregation occurs in these cases, which would negatively affect the adsorption of HCHO molecules and also the possible catalytic degradation performance.

To further understand the strong binding between the decorated Al atom and the C₂N monolayer, we carried out an electron population analysis using the Mulliken method. Because the decorated Al atom mainly affects the charge distribution in the nearby region, to better describe the electron variation we analyzed the atomic charges of the Al atom and of 14 atoms (sites 1–14 in Fig. 2) close to it. The results are listed in Table 1. As shown in the table, the electrons show good dispersion on the pristine C₂N monolayer, where the C atoms have charges of 0.133–0.135 e, whereas the charges of the N atoms vary between –0.267 and –0.270 e. After decorating an Al atom, the electron distribution shows a marked variation. The Al atom transfers electrons to the atoms near the vacancy, and carries a +0.843 e charge. In particular, for the N atoms at sites 4 and 7, which are directly connected with the Al atom, the electronic charges change from –0.267 and –0.268 e to –0.630 and –0.616 e, respectively. For the N atoms close to the vacancy at sites 1, 10, 11, and 12, the charges are –0.414, –0.410, –0.318, and –0.314 e, respectively, showing a decreasing ability to attract electrons, which also reflects the distances between the N atoms and the Al atom. The N atoms at sites 13 and 14, close to another vacancy, have electronic charges of –0.270 and –0.272 e, respectively, similar to those in the pristine C₂N system. This indicates that the decorated Al atom mainly affects the electron distribution close to the vacancy, and a large number of electrons transfer from the Al atom, leading to a strong binding between the Al atom and the C₂N layer, as well as achieving a high stability of the decorated structure.

The adsorption of HCHO molecules on the C₂N monolayer was then investigated. Based on the high-symmetry structure, five possible adsorption sites were investigated, denoted as V, C, N, H1, and H2 in Fig. 1(a). After structural relaxation, the most stable adsorption configuration is observed at the V site, with an adsorption energy E_ad-gas of –0.583 eV, calculated from Eq. (2). The structure of this configuration is shown in Fig. 3(a), and the corresponding structural parameters are listed in Table 2. The figure shows that the HCHO molecule prefers to sit on the vacancy site, with the oxygen atom up and the two hydrogen atoms pointing down; the molecule is located 1.132 Å away from the C₂N monolayer and the adsorption energy is –0.583 eV. This short distance is in the range of chemical bonds; however, as can be seen in the figure, the adsorbed HCHO molecule is embedded in the vacancy site of C₂N, and no chemical bonds are formed. In addition, the adsorption causes negligible structural changes in both the HCHO molecule and C₂N layer, denoting a physisorption behavior. After adsorption, the C–O and C–H lengths in the HCHO molecule change from 1.212 and 1.117 Å to 1.218 and 1.113 Å, respectively. All atoms in the C₂N layer maintain a flat arrangement. Table S2 shows the calculated adsorption energies at different adsorption sites, which further confirm that the HCHO molecule adsorbed on the pristine C₂N monolayer undergoes weak physisorption.

Then, we investigated the adsorption of the HCHO molecule on the Al-decorated C₂N monolayer, focusing on six possible sites (V, H1, H2, H3, C1, and C2) besides the decorated Al atom, as shown in Fig. 2(a). The corresponding calculated adsorption energies at these six sites are listed in Table S3. The results show that the preferred adsorption structure involves the HCHO molecule lying on the V site, with an adsorption energy of –2.585 eV, as shown in Fig. 3(b). The figure shows the presence of two chemical bonds (Al–O and C–N) between the HCHO molecule and the C₂N layer, and the corresponding bond lengths are 1.706 Å (Al–O bond) and 1.524 Å (C–H bond), respectively. As also shown in the figure, the strong adsorption causes the structural distortion of the C₂N layer, which exhibits a slight curvature. In particular, the Al and N atoms connected with the adsorbed HCHO molecule move toward it, attracting its C atom toward the C₂N layer. The C–O and C–H bond lengths of the HCHO molecule change from 1.212 and 1.117 Å to 1.383 and 1.100 Å, respectively, exhibiting a different adsorption behavior compared with the pristine system. It is worth mentioning that the Al-decorated C₂N monolayer shows a better performance for HCHO adsorption compared with that previously reported for Fe-doped graphene (E_ad-gas = –1.45 eV) [46] and Ti-doped graphene (E_ad-gas = –2.26 eV) [47], revealing the potential application for HCHO concentration, use, and treatment.

To further understand the mechanism behind the enhanced adsorption of HCHO molecules on the Al-decorated C₂N, the PDOS plots of HCHO and pristine/Al-decorated C₂N were calculated and displayed in Fig. 4. The overlap between bands in the PDOS is representative of the adsorption strength. For adsorption on the Al-decorated C₂N monolayer, electron interactions can be clearly observed between the adsorbed molecule, the decorated Al atom, and the C₂N monolayer, as indicated by the blue dashed area in Fig. 4(b). However, in the pristine system, these interactions only involve the adsorbed molecule and the C₂N monolayer, as indicated by the red dashed area. In other words, in the Al-decorated system, the electrons of the adsorbed molecule interact not only with the electrons in the C₂N monolayer, but also with those of the decorated Al atom. Thus, the Al atom acts as a bridge connecting the HCHO molecule and the C₂N monolayer, which strengthens the interaction between electrons and enhances the adsorption. In addition, the PDOS curve of the Al-decorated system is shifted to the left (i.e., toward lower energies) compared to that of the pristine system, indicating a more stable adsorption.

Furthermore, the Mulliken population analysis was performed to obtain the specific charges of the HCHO molecule and Al atoms. The results are displayed in Table 3. A considerably stronger electron transfer is observed in the Al-decorated system, indicating a stronger interaction between the HCHO molecule and the C₂N layer. To analyze the distribution of electrons in more detail, we calculated the electron density distribution of the HCHO molecule adsorbed on Al-decorated C₂N, shown in Fig. 5(a). The electron cloud extends over the decorated Al atom, the C₂N monolayer, and the adsorbed HCHO molecule, which is consistent with the band overlap in the PDOS shown in Fig. 4(a), and induces the strong adsorption of gas molecules. In addition, a relatively higher density of the electron cloud can be observed among the decorated Al atom, the C₂N monolayer, and the adsorbed HCHO molecule. These results further confirm that the Al atom acts as a bridge connecting the HCHO molecule and the C₂N layer. This indicates that the decorated Al atom alters the electron distribution and lead to the enhanced adsorption of HCHO molecules, which is in good agreement with the other results discussed above.

As discussed above, the HCHO molecule can be strongly adsorbed on the Al-decorated C₂N, which provides an excellent precondition for the subsequent treatment. Different techniques for the degradation of HCHO molecules are available, such as electro-Fenton [48], electro-oxidation [49, 50], and photocatalysis processes [51-53]. Among these degradation methods, the generation of the hydroxyl (•OH) and superoxide (O₂^•–) radicals usually plays a crucial role in the oxidation processes [54-61]. As these two radicals are generally derived from H₂O and O₂, it is also necessary to investigate the adsorption behavior of these two molecules. Therefore, we also investigated the adsorption of H₂O and O₂ molecules on pristine and Al-decorated C₂N monolayer. Similar to the adsorption of volatile organic compounds (VOCs) discussed above, we considered five possible adsorption sites (V, C, N, H1, and H2 as shown in Fig. 1(a)) on the pristine C₂N monolayer and six possible sites (V, H1, H2, H3, C1, and C2 as shown in Fig. 2(a)), besides the decorated Al atom, on the Al-decorated C₂N monolayer. The calculated adsorption energies are listed in Table S2 and Table S3, respectively.

The most stable configurations for a water molecule adsorbed on the pristine and Al-decorated C₂N monolayer are shown in Figs. 6(a) and 6(b), respectively, and the corresponding structural parameters are listed in Table 2. In the pristine system, Fig. 6(a) shows that the H₂O molecule prefers to lie above the V site of the C₂N monolayer, with the oxygen atom up and the two hydrogen atoms pointing downward. The calculated adsorption energy is –0.743 eV. The distance between the H₂O molecule and the C₂N monolayer is 0.986 Å, and no chemical bonds are formed. The two O–H bond lengths of the water molecule increase from 0.968 Å in the isolated H₂O molecule to 0.978 Å in the adsorbed state. After Al atom decoration on the C₂N layer, the most stable adsorption configuration is shown in Fig. 6(b). The obtained adsorption energy is –3.177 eV, which is around four times higher than that measured in the pristine system. This strong adsorption causes slight structural changes in the Al-decorated C₂N sheet as well as induces the dissociation of the H₂O molecule into an –OH group and a H atom, which is attached to the decorated Al atom with Al–O and N–H bond lengths of 1.709 and 1.049 Å, respectively. This indicates a very strong interaction between the H₂O molecule and the Al-decorated C₂N.

In the case of O₂ adsorption on pristine and Al-decorated C₂N monolayer, we found that the O₂ molecule also prefers to adsorb on the V site; the most stable structure is shown in Fig. 7(a). As shown in the figure, in the pristine C₂N system the O₂ molecule lies perpendicular to the center of the vacancy site, with an adsorption energy of –0.206 eV. This relatively weak adsorption has negligible impact of the C₂N monolayer. The O–O bond length of the O₂ molecule increases from 1.225 Å in an isolated O₂ molecule to 1.288 Å after adsorption. In addition, the O₂ molecule is nearly 1.5 Å away from the C₂N layer, which is large than the distance observed for the HCHO molecule (around 0.9 Å), and the corresponding adsorption energy is also lower than that of the HCHO molecule (–0.743 eV). However, in the Al-decorated system, regardless of its initial position, the O₂ molecule prefers to move to the V site after geometry optimization; the final structure is shown in Fig. 7(b). The O₂ molecule prefers to lie near the vacancy site (E_ad-gas = –2.767 eV); as the O=O bond breaks, both O atoms form chemical bonds with the Al atom, as shown in the side view of Fig. 5(b). The two Al–O bond lengths are 1.791 and 1.765 Å, while the O–O bond length increases to 1.516 Å, which is much longer than that of an isolated O₂ molecule (1.225 Å) and of the O₂ molecule adsorbed on the pristine C₂N monolayer (1.288 Å). Furthermore, the Al-decorated C₂N monolayer exhibits slight distortion with the Al atom shifted toward the adsorbed O₂ molecule.

Therefore, the decoration of Al atoms on C₂N can significantly enhance the adsorption of H₂O and O₂ molecules as well. In particular, the hydroxyl group can be formed directly upon H₂O adsorption, which greatly benefits the generation of hydroxyl radicals. On the other hand, the chemisorbed O₂ molecules also have a much higher ability to attract additional electrons, with a high potential to be activated into superoxide radicals. Therefore, the Al-decorated C₂N could be a promising novel material for concentrating VOCs, as well as for their degradation through radical routes.

Similar to the case of HCHO molecules on the Al-decorated C₂N monolayer, the PDOS of the H₂O and O₂ molecules on the pristine/Al-decorated C₂N systems were also calculated and are displayed in Fig. 4. Based on the band overlap, the electron interactions between the H₂O or O₂ molecule, the decorated Al atom, and the C₂N monolayer (indicated as blue dashed areas in Figs. 6(d) and 6(f)) are considerably stronger than the interaction in the pristine system, indicated by the red dashed band overlap in Figs. 6(c) and 6(e). Therefore, a similar adsorption enhancement mechanism can be achieved, with the Al atom acting as a bridge to connect the H₂O or O₂ molecule and the C₂N monolayer, strengthening the interaction between electrons and enhancing the adsorption. In addition, Mulliken atomic charges were calculated for the H₂O and O₂ molecules on the pristine and Al-decorated C₂N systems, and the results are listed in Table 3, while the corresponding electron density distribution is shown in Figs. 5(b) and 5(c). Similar to HCHO, the electron transfer is considerably stronger in the Al-decorated system, and the electron cloud extends over the adsorbed HCHO, the decorated Al atom, and the C₂N monolayer, inducing the strong adsorption of the gas molecules.

The adsorption behavior of the HCHO molecule and the potential catalytic degradation ability of pristine and Al-decorated C₂N monolayer were investigated using first-principles calculations. Because the adsorption of the HCHO molecule on the pristine C₂N monolayer is relatively weak (E_ad-gas = –0.583 eV) and not suitable for its concentration and subsequent treatment, Al atoms were decorated on the C₂N layer, resulting in a high HCHO adsorption energy of –2.586 eV. Moreover, the H₂O and O₂ molecules were strongly adsorbed on the Al-decorated C₂N monolayer, providing a favorable precondition for the possible generation of hydroxyl and superoxide radicals. In particular, the H₂O molecule dissociated spontaneously and generated a hydroxyl group connected with the Al atom on the C₂N surface, revealing its promising potential for the generation of hydroxyl radicals. Furthermore, we investigated the adsorption enhancement mechanism for HCHO, H₂O, and O₂ molecules. The results showed that the decorated Al atom can alter the electron distribution and thus the chemical and physical properties in its vicinity, acting as a bridge between the HCHO molecule and the C₂N layer, and strengthening the adsorption. Hence, the Al-decorated C₂N monolayer proved to be an excellent material for the adsorption and possible catalytic degradation of HCHO molecules.