Investigating the Electronic and Molecular Adsorption Properties of Ti/Co-Doped Boron Carbon Nitride

Abstract

Two-dimensional (2D) hexagonal boron carbon nitride (h-B_xC_yN_z) has garnered a lot of interest in the last two decades because of its remarkable physical and chemical characteristics. Because of the carbon atoms, it has a smaller gap than its cousin, boron nitride, and is hence more appropriate for a wider range of applications. In the frame of density functional theory, we discuss the structural, electronic, and magnetic properties of mono Ti-doped and Co-doped BC₆N (Ti/Co-BC₆N) at different sites of substitutional doping (Ti/Co) in the BC₆N monolayer. The mono substitutional doping at the B (Ti_B/Co_B), N (Ti_N/Co_N), and two different C (C1 (Ti_C1/Co_C1), C2 (Ti_C2/Co_C2)) sites, are investigated. The position of the Ti/Co dopant is an important parameter that changes the electronic state, magnetic moment, and adsorption activity of the pristine BC₆N nanosheet. We find that the adsorption of the gases NO, NO₂, CO₂, NH₃, N₂, and O₂ is significantly improved on the doped sheet at all doped positions compared to the adsorption on the pristine structure. The Ti/Co-BC₆N can adsorb NO and NO₂ better than CO₂ and NH₃. Ti_C1-BC₆N and Ti_B-BC₆N are the best doped sheets for adsorbing NO and NO₂, respectively. The CO₂ and the N₂ molecules are moderately adsorbed at all doped positions as compared to the other adsorbed molecules. Ti-doped sheets can adsorb the CO₂, NH₃, and O₂ better than the corresponding Co-doped sheets. We also study the adsorption of molecular hydrogen on our single-atom Ti/Co-doped systems, as well as on 4-atom Ti and Co clusters embedded in the BC₆N sheets. We show that the cluster-embedded sheets can adsorb up to four H₂ molecules. These novel findings are important for many applications of BC₆N, including spintronics, gas filtration, molecular sensing, and hydrogen storage.

1. Introduction

Two-dimensional (2D) nanostructures have attracted much theoretical and experimental attention since the preparation of graphene in 2004 [1]. The low dimensionality of these materials and the diversity of their composition and structural characteristics may have significant potential for technological use. The development of experimental techniques has increased interest in this field since it raises the possibility of creating novel 2D structures with particular chemical or physical characteristics.

Graphene, which has special electrical, mechanical, and optical characteristics that make it suitable for a wide range of technological applications [2], is arguably the most well-known two-dimensional (2D) material. Nevertheless, several drawbacks of graphene, such as the absence of an electronic gap, have prompted scientists to look for other graphene-like structures [3,4]. Hexagonal boron Nitride (h-BN) [5], Germanene [6], Silicene [7], MXenes [8,9], transition metal dichalcogenides (TMDs) [3,10], and Mo(W)Si₂N₄ [11,12,13] are samples of 2D structures that have attracted a lot of attention lately due to their several possible applications: gas sensing [14,15,16], field-effect transistors [17], hydrogen storage [18,19], sustainable ammonia production [20], and photocatalysis [21,22,23].

Since carbon, nitrogen, and boron are neighbor atoms in the periodic table, they can form bonds that result in a variety of (h-B_xC_yN_z) compounds [24,25]. This has attracted a lot of attention [26] because substituting carbon for boron or nitrogen may lead to a spectrum of interesting properties without significantly distorting the lattice structure. It has been demonstrated that the graphene lattice can be changed from a semimetal to a semiconductor by adding BN grains, with a band gap created at the vanishing Dirac points [27,28,29]. Therefore, one expects that a wide range of physical characteristics might result from varying the relative compositions of the three elements in h-B_xC_yN_z.

Various techniques, including the solvothermal approach, chemical vapor coating, and the chemical reaction approach [30], have been used to synthesize several ternary B-C-N structures (such as BCN, BC₂N, BC₄N, BC₆N, and B₂CN). Because BC₂N is predicted to be more difficult than c-BN [27,31] and more chemically and thermally inert than diamond, it has additionally attracted interest from researchers. The applications of BCN are many and include the oxidation of pollutants and colorants, the extraction of hydrogen from water, the creation of transparent photovoltaic cells, UV absorption, optoelectronics, and catalysis for a variety of chemical interactions [32,33].

Implanting graphene quantum dots with atoms of boron and nitrogen gives BC₆N 2D quantum dots [34], which were found to be semiconducting with a band gap of 1.2–1.3 eV [35,36]. BC₆N has some of the physical and mechanical features of graphene, including high stiffness and high thermal conductivity [24,36], due to its graphene-like structure. Intensive research is being conducted to customize the characteristics of BC₆N for a variety of uses. By substitutional and adsorption doping of BC₆N [35,36], magnetic semiconductor behavior may become metallic, half-metallic, or diluted. Moreover, the BC₆N electrostatic landscape can be changed by defects such as vacancies, making it more reactive to certain gases [35,36].

Due to their electronic activity, metals can be used to enhance the adsorption and catalytic properties of 2D nanomaterials. Among the different doping elements, titanium stands out due to its diverse chemical reactivity [37]. Ti-doped 2D materials open many applications in various fields. For example, in graphene, Ti has shown enhanced catalytic activity for oxygen reduction, a potential candidate for fuel cells and other microelectronic devices [37]. In TMDs, Ti has been found to improve the performance of semiconductor devices and enhance their gas-sensing properties [38].

Co-doping has also generated much interest in materials research. Co-doped 2D MoSe₂ [39] can create active sites that enhance electrocatalysts for hydrogen evolution reduction reactions (HER). Doping MoS₂ with Co forms a more stable 1T phase with much better catalytic activity [40]. The charge–discharge efficiency of black phosphorus nanosheets can reach more than 90% after doping with Co, which promotes its potential for electrochemical hydrogen storage [41]. The Co dopant improves the absorption of SWCNT for SOF₂ and SO₂, which makes Co-SWCNT a promising gas sensor for them [42]. The Co-MOF-5-synthesized materials demonstrate greater abilities to adsorb H₂, CO₂, and CH₄ under high pressure than their Co-free homologues [43].

Although studies have been conducted on pristine BC₆N, further research has to be carried out on structures doped with other atoms to improve the physical and chemical properties of the BC₆N sheet. In this work, we investigate the electronic and molecular adsorption characteristics of Ti-doped and Co-doped BC₆N in the frame of density functional theory. A 3 × 3 supercell of BC₆N (consisting of 72 atoms) is used, and the effect of substitutional Ti-doping and Co-doping on electronic and magnetic properties, as well as the adsorption activity of these systems, is investigated. The unit cell of the studied structure has four symmetrically inequivalent sites: two C atoms (C1 and C2), B, and N. We consider the adsorption of four gases (NO, CO₂, NO₂, and NH₃) on the pristine and doped systems.

2. Results and Discussion

To study the effect of doping on the structural, electronic, magnetic, and adsorption properties of BC₆N, we use a 3 × 3 supercell. The optimized atomic structure of a monolayer is shown in Figure 1a with the bond lengths of C-C, C-B, and C-N being 1.41 Å, 1.47 Å, and 1.46 Å, respectively, which agree with previous studies [35,36,44,45]. The four distinct sites of the BC₆N unit cell are shown: two are at the C1 and C2 sites, and two are at the N and B sites (all marked with red circles).

Figure 1b illustrates the density of states (DOS)/projected DOS (PDOS) of the pristine monolayer. It is semiconducting with a band gap of 1.3 eV, which is in good agreement with published work [35,36,44,45]. The band gap value of BC₆N indicates that it may be a potential candidate structure for optoelectronic devices. The C 2p states are the main contributors to the valence band (VB) and conduction band (CB), with little contribution from the N 2p states in the VB, and B 2p states in the CB. Charge analysis shows that N and C1 gain some charge, while B and C2 lose some charge.

2.1. Ti-Doped and Co-Doped BC₆N

The relaxed Ti-doped and Co-doped BC₆N (Ti/Co-BC₆N) sheets are shown in Figure 2. Systems with doping at the B ((Ti_B/Co_B)), C1 (Ti_C1//Co_C1), C2 (Ti_C2/Co_C2), and N (Ti_N/Co_N) sites exhibit some distortion around the Ti/Co dopant atom, with average nearest doping distances of 1.95 Å/1.76 Å, 1.98 Å/1.8 Å, 1.9 Å/1.8 Å, and 2 Å/1.8 Å respectively. The dopants protrude out of the plane by 1.70 Å/1.45 Å for the first three sites and by 1.8 Å/1.5 Å for the N site (

Table 1. Ti/Co-BC₆N: The charge transfer ($\Delta Q$ (e)), formation energy ($E_f$ (meV)), magnetic moment (Mag (${\mu }_{\mathrm{B}}$)), band gap ($E_g$ up/down (dn) (eV)), average distance between the dopant and its nearest neighbors, $\overline{r}$, and average angle between the dopant and its nearest neighbors, $\overline{\theta }$.

Systems	$\Delta \mathit{Q}$	${\mathit{E}}_{\mathit{f}}$	Mag	${\mathit{E}}_{\mathit{g}}$ (up)	${\mathit{E}}_{\mathit{g}}$ (dn)	$\overline{\mathit{r}}$	$\overline{\mathit{\theta }}$
BC6N	-	-	0.0	1.3	1.3	-	-
TiB/CoB-BC6N	0.78/0.13	77/52	0.9/0.0	-/1.2	-/1.2	1.95/1.76	82.92/93.79
TiC1/CoC1-BC6N	0.82/0.03	88/72	−0.1/1	0.9/0.6	0.9/1.1	1.98/1.8	80.19/91.09
TiC2/CoC2-BC6N	1.03/0.18	58/65	0.0/1	1.1/0.4	1.1/0.3	1.9/1.79	94.42/91.08
TiN/CoN-BC6N	0.98/0.17	46/21	0.9/0.0	-/1.2	1.2/1.2	2/1.8	91.32/102.38

). The average angles made at the dopants by their nearest neighbors are also shown. The formation energies indicate that Ti_N/Co_N-BC₆N has slightly smaller energy compared to other doped nanosheets. All doped systems have thermal stabilities compared to that of the pristine sheet (Table 1). The charge transfer ($\Delta Q$) between the dopant and the sheet of all the considered structures is presented in Table 1.

We now inspect the DOS of the Ti/Co-doped systems (Figure 2e–h). Ti_B-BC₆N has an asymmetric DOS. The CB has contributions from the 4d Ti states right below the Fermi energy and up to ∼0.6 eV. The C states dominate the VB as in the pristine structure. The structure is metallic as there is no gap in the two spin channels, and the asymmetry of those channels indicates that the sheet is magnetic with 0.9 μ_B. In the case of Ti_C1- and Ti_C2-BC₆N, the structure is non-magnetic, as is obvious from the identical channels of the DOS. The band gaps decrease to 0.9 eV and 1.1 eV, compared to pristine BC₆N because of the contribution of Ti $4d$ states, as in the CB, The effect of substitutional doping of Ti_C1 is more significant in the band gap value than the corresponding effect for Ti_C2. For Ti_N-BC₆N, the structure is a half-metal with no spin-up gap and a spin-down gap of 1.2 eV. The Fermi energy is shifted to the CB. The structure is magnetic with a moment of 0.92 μ_B, as illustrated by the asymmetrical spin-up and spin-down components.

For the DOS of Co_B- and Co_N-BC₆N and as compared to the pristine, many states are created above the VB with a dominated Co $3d$ states from −1.2 eV to −0.2 eV for Co_B-doping and from −1.5 eV to −1.2 eV for Co_N-doping. The contribution of Co-states in the VB is larger than in the CB, which contrasts with the effect in the case of Ti_B. The DOS of Co_N-BC₆N is disturbed more than the Co_B-BC₆N. The structures are semiconducting with a band gap of 1.2 eV for both structures. The symmetrical effect of Co-states in both spin directions refers to the zero magnetic moment.

Both Co_C1- and Co_C2-BC₆N monolayers are dilute magnetic semiconductors (DMSCs) due to the significant contributions of Co $3d$ states with a spin-up (down) band gap of 0.60 eV (1.1 eV) for Co_C1-doping and 0.4 eV (0.3 eV) for Co_C2-doping. The spectra of the Co_C doped structures are more disturbed than the spectra of systems doped at the B and N positions. The Co $3d$ contributions are noticed at the top of the VB, in the gap, and at the bottom of the CB. Doping at the C positions changes the magnetic moment to 1 μ_B due to the asymmetry of the DOS of the spin-up and spin-down components.

Experimental techniques such as UV-Vis absorption spectroscopy can be used to estimate the band gaps, while X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy can provide information on the valence and conduction band positions. In addition, scanning tunneling spectroscopy can offer a direct measurement of the electronic structure. These methods have been widely used in the literature to investigate the band structure of 2D materials [46], including boron–carbon–nitride-based systems.

2.2. Adsorption on Ti-/Co-BC₆N Nanosheets

Now we address the gas-adsorption properties of our structures. We consider six gases NO, NO₂, CO₂, NH₃, O₂, and N₂. The initial positions of each gas above the considered sheets are (@_B), (@_N), (@_H1), and (@_H2) for pristine BC₆N, and (@_B, @_C1, @_C2, and @_N) for the doped Ti/Co-BC₆N sheets. The gas-sheet systems are structurally relaxed. We then compute the adsorption energies and the electronic structures of the investigated nanosheets compared to the adsorption results on pristine structures.

2.2.1. NO Gas Adsorption

The bond length of N-O is 1.16 Å, which agrees well with previous publications [8]. The NO gas is adsorbed very weakly (${\mathrm{E}}_{ad}<0.2\mathrm{eV}$) above the pristine sheet at all adsorption positions [47] (

Table 2. NO@pristine [47] and Ti/Co-BC₆N monolayers: The closest distance (Å), charge transfer ($\Delta Q$ (e)), nearest atom (X), adsorption energy ($E_{ad}$ (eV)), magnetic moment (Mag (μ_B)), band gap ($E_g$ up/down (dn) (eV)).

Systems	d	X	$\mathbf{\Delta }\mathit{Q}$	${\mathit{E}}_{\mathbf{ad}}$	Mag	${\mathit{E}}_{\mathit{g}}$ (up)	${\mathit{E}}_{\mathit{g}}$ (dn)
@BBC6N	2.8	N	0.0	0.2	1.0	0.4	1.1
@NBC6N	3.1	N	0.0	0.1	1.0	1.1	0.4
@H1BC6N	3.1	N	0.0	0.1	1.0	0.3	1.2
@H2BC6N	2.9	N	−0.1	0.2	1.0	0.5	1.3
@TiB/CoB-BC6N	2.0/1.7	N/N	−0.4/−0.1	2.9/2.3	2/1	1.0/0.5	1.3/1.0
@TiC1/CoC1-BC6N	1.9/1.7	N/N	−0.3/−0.1	3.4/1.9	1/0	1.2/1.1	0.6/1.1
@TiC2/CoC2-BC6N	1.9/1.7	N/N	−0.6/−0.1	2.4/3.2	1/0	1.1/0.4	0.9/0.4
@TiN/CoN-BC6N	2.0/1.8	N/N	−0.3/−0.1	2.6/2.7	2/1	1.0/0.4	1.2/0.9

). Therefore the NO gas adsorption on the sheet has negligible effects on the DOS of the pristine system.

When we use the Ti/Co-doped structures, the results are completely different. The Ti/Co dopant’s closest distance from the gas’s N atom is 2.0 Å/1.7 Å, 1.9 Å/1.7 Å, 1.9 Å/ 1.7 Å and 2.0 Å/1.8 Å for NO@Ti_B-/Co_B-, NO@Ti_C1-/Co_C1-, NO@Ti_C2-/Co_C2- and NO@Ti_N-/Co_N-BC₆N, respectively. This means that the interaction between the NO gas and the doped sheets is stronger than that of the pristine sheet because the distances are much closer for the doped sheet compared to the pristine structure.

The distance between the N atom and the Ti-BC₆N is larger than the corresponding distance in the case of Co-BC₆N. Furthermore, the N-O length increases to 1.2 Å/1.2 Å for NO@Ti_B/Co_B, NO@Ti_C1/Co_C1 and @Ti_N/Co_N, and 1.3 Å/1.2 Å for NO@Ti_C2/Co_C2. The charge transfer for the Ti-doped system is higher than the corresponding charge transfer for the Co-doped system at the same position, and with higher adsorption energies: 2.9 eV/ 2.3 eV, 3.4 eV/1.9 eV, 2.4 eV/3.2 eV, and 2.6 eV/2.7 eV for Ti_B-/Co_B-, Ti_C1-/Co_C1, Ti_C2-/Co_C2, and Ti_N-/Co_N-BC₆N sheets. The Ti dopant is more effective in adsorbing the NO at Ti_B and Ti_C1 than the corresponding cases using Co dopant. However, the Co dopant can adsorb the NO at Co_C2 and Co_N more than the corresponding Ti cases.

Figure 3e–h show the DOS/PDOS of NO@Ti_B and NO@Ti_N-BC₆N. The top/bottom of the VB/CB is disturbed due to the contribution of the N and O $2p$ states, such as for the spin-up/spin-down channel for the top/bottom of the VB/CB. The structures are DMSC with a band gap up (down) of 1.0 eV (1.3 eV) for NO@Ti_B-BC₆N and 1.0 eV (1.2 eV) for NO@Ti_N-BC₆N, and with a magnetic moment of 2 μ_B. The adsorption of NO gas on the Ti_C1 and Ti_C2 sites creates states at 0.4 eV and 0.6 eV, respectively. The NO adsorbed gas converts the Ti_C1-BC₆N and Ti_C2-BC₆N from a SC state to a DMSC state and nonmagnetic to magnetic with 1 μ_B. The spin-up (down) band gaps for Ti_C1-BC₆N and Ti_C2-BC₆N sheets are 1.2 eV (0.6 eV) and 1.1 eV (0.9 eV).

For adsorbed NO gas on the Co-BC₆N at different adsorption positions, the contribution of N and O $2p$ states has a significant effect around the Fermi energy of the Co-BC₆N sheet. The adsorbed NO molecule converts the SC state of Co_B- and Co_N-BC₆N to a DMSC with spin-up (down) band gaps of 0.5 eV (1.0 eV) and 0.4 eV (0.9 eV), respectively. In addition, the NO molecule converts the nonmagnetic sheet to a magnetic sheet by 1 μ_B for Co_B- and Co_N-BC₆N. However, the molecule converts the Co_C1- and Co_C2-BC₆N to an SC with a band gap of 1.1 eV and 0.4 eV, respectively. The NO molecule converts the magnetic sheet to a nonmagnetic sheet for Co_B- and Co_N-BC₆N.

2.2.2. NO₂ Gas Adsorption

We now discuss the adsorption NO₂ adsorption results. In the NO₂ isolated molecule, the angle and the bond length are 134° and 1.2 Å, respectively [8]. The adsorption of the molecule is extremely weak (0.1 eV) at all sites of the pristine system [47]. Due to that weak adsorption, its influence on the electronic properties of the pristine sheet is very small [47].

For the adsorption of NO₂ on the Ti/Co-doped monolayers, in all structures, the bond length of N-O is ∼1.3 Å. We also find that the O-N-O angle decreases to 118.8 °/111.8°, 120.9°/123.7°, 110.8°/108.9°, and 121.1°/110.7° for Ti_B/Co_B-, Ti_C1/Co_C1-, Ti_C2/Co_C2-, and Ti_N/Co_N-BC₆N, respectively. The significant interaction between Ti/Co-doped sheets and adsorbed NO₂ is reflected in the change in angle. This is further supported by the charge transfer between the sheets and the gas (∼0.5e/0.4e) and the reduced distance between NO₂ and the Ti/Co atom (

Table 3. NO₂@pristine [47] and Ti/Co-BC₆N: The closest distance (Å), O-N-O angle (θ°), charge transfer ($\Delta Q$ (e)), nearest atom (X), adsorption energy ($E_{ad}$ (eV)), magnetic moment (Mag (${\mu }_B$)), and band gap ($E_g$ up/down (dn) (eV)).

Systems	d	θ°	X	$\mathbf{\Delta }\mathit{Q}$	${\mathit{E}}_{\mathbf{ad}}$	Mag	${\mathit{E}}_{\mathit{g}}$ (up)	${\mathit{E}}_{\mathit{g}}$ (dn)
@BBC6N	2.9	126.7	O	−0.1	0.1	0.9	1.4	-
@NBC6N	3.0	126.9	O	−0.2	0.1	0.9	1.4	-
@H1BC6N	3.0	126.9	O	−0.1	0.1	−0.9	-	1.3
@H2BC6N	2.8	128.6	N	−0.1	0.1	0.9	0.3	1.3
@TiB/CoB-BC6N	2.0/1.8	118.8/111.8	O/O	−0.5/−0.4	3.8/2.1	0/1	1.0/1.2	1.0/0.4
@TiC1/CoC1-BC6N	1.9/1.8	120.9/123.7	O/N	−0.5/−0.3	3.7/2.0	1/0	0.6/1.2	0.9/1.2
@TiC2/CoC2-BC6N	2.1/2.0	110.8/108.9	O/O	−0.5/−0.3	2.9/3.4	0.3/0	-/0.5	-/0.5
@TiN/CoN-BC6N	2.1/2.0	121.1/110.7	O/O	−0.5/−0.4	3.6/2.8	0/0	1.0/-	1.0/-

). Similar to the adsorption of NO gas, the Ti_B-, Ti_N- and Ti_C1-BC₆N can adsorb the NO₂ molecule better than the Co_B-, Co_N- and Co_C1-BC₆N. On the other hand, Co_C2-BC₆N can adsorb NO₂ better than Ti_C2-BC₆N.

Figure 4e–h show the effect of NO₂ on the DOS/PDOS of the Ti-BC₆N sheets. The contribution of the O $2p$ states is more significant than the N $2p$ states of the gas. Adsorption changes the magnetic properties and the electronic state of Ti-BC₆N, from metal to SC for NO₂@Ti_B and from half-metal to SC for NO₂@Ti_N with a band gap of 1 eV for both structures, from SC to DMSC for NO₂@Ti_C1 with spin-up/down band gap of 0.6 eV/ 0.9 eV with a magnetic moment of 1 μ_B, and from SC to metal for NO₂@Ti_C2. Regarding the DOS/PDOS of NO₂ on the Co-doped nanosheets (Figure 4), the NO₂ gas converts the Co_B- and Co_N-BC₆N from SC to DMSC (with spin-up/down band gap of 1.2 eV/0.4 eV for Co_B-BC₆N and magnetic moment of 1 μ_B) and metal state, respectively. However, the NO₂ adsorption changes the state of the Co_C1- and Co_C2-BC₆N systems from DMSC to SC with a gap of 1.2 eV and 0.5 eV, respectively.

2.2.3. CO₂ Gas Adsorption

The last triatomic gas we examine is CO₂. Adsorption on the pristine system is extremely weak (0.2 eV) [47]. The story in the Ti/Co-BC₆N is completely different. The doped monolayer adsorbs the CO₂ at a distance of ∼2 Å/2.1 Å, and the O-C-O angle decreases significantly. The closest atom of the molecule to the sheet is the O atom except for Co_B- and Co_C1-BC₆N sheets. The charge transfer and the adsorption energy of CO₂ on Ti-doped sheets are larger than that of Co-doped sheets (

Table 4. CO₂@pristine [47] and Ti/Co-BC₆N structures: The closest distance (Å), O-C-O angle (θ°), charge transfer ($\Delta Q$ (e)), nearest atom (X), adsorption energy ($E_{ad}$ (eV)), magnetic moment (Mag (μ_B)), and band gap ($E_g$ up/down (dn) (eV)).

Systems	d	θ°	X	$\mathbf{\Delta }\mathit{Q}$	${\mathit{E}}_{\mathbf{ad}}$	Mag	${\mathit{E}}_{\mathit{g}}$ (up)	${\mathit{E}}_{\mathit{g}}$ (dn)
@BBC6N	3.3	179.3	C	−0.02	0.2	0.0	1.3	1.3
@NBC6N	3.2	179.8	C	−0.01	0.2	0.0	1.3	1.3
@H1BC6N	3.2	179.3	C	−0.02	0.2	0.0	1.3	1.3
@H2BC6N	3.2	179.3	C	−0.02	0.2	0.0	1.3	1.3
@TiB/CoB-BC6N	2.0/2.0	139/151.9	O/C	−0.5/−0.3	1.1/0.4	1/0	1.1/1.1	0.4/1.1
@TiC1/CoC1-BC6N	1.9/1.9	128.1/145.4	O/C	−0.5/−0.3	2.2/0.4	0/1	1.2/0.9	1.2/0.9
@TiC2/CoC2-BC6N	1.9/2.2	126.6/177.9	O/O	−0.5/0.1	1.2/0.8	0/1	1.1/0.6	1.1/0.4
@TiN/CoN-BC6N	2.1/2.2	144.7/157.2	O/O	−0.4/−0.2	0.7/0.4	1/0	1.1/1.1	0.7/1.1

).

The PDOS of the CO₂@Ti_B- and CO₂@Ti_N-BC₆N systems show that they are DMSCs with spin-up/down and gap of 1.1 eV/0.4 eV and 1.1 eV/0.7 eV, respectively, while CO₂@Ti_C1- and CO₂@Ti_C2-BC₆N sheets are SCs with gap of 1.2 eV and 1.1 eV, respectively. On the other hand, CO₂@Co_B- and CO₂@Co_N-BC₆N are SCs with the same band gap of 1.1 eV and CO₂@Co_C1- and CO₂@Co_C2-BC₆N are DMSCs with spin-up/down band gap of 0.9 eV/0.9 eV and 0.6 eV/0.4 eV, respectively (Figure 5e–h).

2.2.4. NH₃ Gas Adsorption

NH₃ is adsorbed very weakly on the pristine system (0.2 eV) [47]. For the Ti/Co-doped sheets, the closest atom of the sheets is the N atom, and the distance is smaller than the corresponding distance in the pristine sheet (

Table 5. NH₃@pristine and Ti/Co-BC₆N nanosheets: The closest distance (Å), H-N-H angle (θ°), charge transfer ($\Delta Q$ (e)), nearest atom (X), adsorption energy ($E_{ad}$ (eV)), magnetic moment (Mag (μ_B)), and band gap ($E_g$ up/down (dn) (eV)).

Systems	d	θ°	X	$\mathbf{\Delta }\mathit{Q}$	${\mathit{E}}_{\mathbf{ad}}$	Mag	${\mathit{E}}_{\mathit{g}}$ (up)	${\mathit{E}}_{\mathit{g}}$ (dn)
@BBC6N	2.7	102.2	H	−0.01	0.2	0.0	1.3	1.3
@NBC6N	2.7	106.4	H	−0.01	0.2	0.0	1.3	1.3
@H1BC6N	2.7	106.2	H	−0.01	0.2	0.0	1.3	1.3
@H2BC6N	2.8	106.2	H	−0.01	0.2	0.0	1.3	1.3
@TiB/CoB-BC6N	2.3/2.1	106.6/107.4	N/N	0.1/0.2	1.3/1.2	−0.73/0	-/1	-/1
@TiC1/CoC1-BC6N	2.2/2	108.5/107.6	N/N	0.2/0.2	1.6/1.2	0/1	0.8/0.5	0.8/1.2
@TiC2/CoC2-BC6N	2.3/2	107.0/108.2	N/N	0.2/0.2	1.5/1.4	0/1	1.1/0.7	1.1/0.6
@TiN/CoN-BC6N	2.3/2.1	107.4/107.4	N/N	0.2/0.2	1.3/1.4	0/0	-/1.3	-/1.3

). As with the previous systems, the Ti/Co-doped system greatly enhances the charge transfer and the adsorption of NH₃. It is chemisorbed with an average energy of ∼1.4 eV/1.3 eV. The electronic structure becomes metal for NH₃@Ti_B- and NH₃@Ti_N-BC₆N, SC with a band gap of 0.8 eV and 1.1 eV in both spin directions for NH₃@Ti_C1-BC₆N and NH₃@Ti_C2-BC₆N, respectively (Figure 6). For Co-doping, NH₃@Co_B-, NH₃@Co_N-BC₆N are semiconductors with a band gap of 1.0 eV and 1.3 eV, respectively. NH₃@Co_C1-, NH₃@Co_C2-BC₆N are DMSC with spin-up/down band gap of 0.5 eV/1.2 eV and 0.7 eV (0.6 eV), respectively (Figure 6e–h).

2.2.5. N₂ Gas Adsorption

The fifth molecule that we study is nitrogen (N₂). The N-N bond length is 1.11. N₂ gas is adsorbed very weakly by 0.09 eV above the pristine surface at all adsorption positions, which is in agreement with recent studies of N₂ adsorption on similar systems [48]. As a result, N₂ gas adsorption on the sheet has a negligible effect on the DOS of a pristine system.

In the doped systems, the distance between the N atom of the gas and the dopant atom ranges from 2.0 Å to 2.9 Å for the Ti systems and from 1.6Å to 2.0 Å for the Co systems (Figure 7a–d). The adsorption energy ranges between ∼0.4 eV and ∼1.3 for Ti-doped systems, and slightly lower for the Co-doped systems (

Table 6. N₂@Ti/Co-BC₆N monolayers: The closest distance (Å), charge transfer ($\Delta Q$ (e)), nearest atom (X), adsorption energy ($E_{ad}$ (eV)), magnetic moment (Mag (μ_B)), band gap ($E_g$ up/down (dn) (eV)).

Systems	d	X	$\mathbf{\Delta }\mathit{Q}$	${\mathit{E}}_{\mathbf{ad}}$	Mag	${\mathit{E}}_{\mathit{g}}$ (up)	${\mathit{E}}_{\mathit{g}}$ (dn)
@TBBC6N	3.29	N	0.01	0.09	0.0	1.2	1.2
@TiB/CoB-BC6N	2.4/1.84	N/N	−0.34/−0.17	0.94/0.86	1/0	0.6/1.1	1.1/1.1
@TiC1/CoC1-BC6N	2.04/1.69	N/N	−0.34/−0.27	0.44/0.49	0/0	0.9/-	0.9/-
@TiC2/CoC2-BC6N	2.89/1.97	N/N	−0.17/−0.27	1.29/0.51	0/1.08	0.4/0.4	0.4/0.2
@TiN/CoN-BC6N	2.23/1.87	N/N	−0.29/−0.11	0.39/0.66	1/0	0.4/1.1	0.8/1.1

). The N₂ adsorption causes some changes in the DOS/PDOS of Ti/Co-BC₆N sheets (Figure 7a–d). The adsorption changes the metal Ti_B-BC₆N and half-metal Ti_N-BC₆N into DMSCs with spin-up (down) gaps of 0.6 (1.1) and 0.4 (0.8), respectively. However, the corresponding Co-doped structures remain SC. At the C1 and C2 positions, the Ti-doped structures are SC after adsorption, although with a smaller band gap. The N₂@Co_C1-BC₆N flips from a DMSC to a metal, while the N₂@Co_C2-BC₆N system remains a DMSC.

2.2.6. O₂ Gas Adsorption

The last molecule that we study is O₂. Oxygen has a bond length of 1.23 Å. Our calculations show that the O₂ adsorption on the pristine BC₆N is weak (∼0.1 eV), consistent with previous studies [49]. The distance between the O atom and the dopant atom ranges from 1.78 Å to 1.85 Å for the Ti systems, and from 1.81 Å to 1.94 Å for the Co systems (Figure 8a–d), indicating a stronger interaction than that of the sister molecule N₂. This is also reflected in the energies, which indicate that O₂ is chemisorbed to the dopants (

Table 7. O₂@Ti/Co-BC₆N monolayers: The closest distance (Å), charge transfer ($\Delta Q$ (e)), nearest atom (X), adsorption energy ($E_{ad}$ (eV)), magnetic moment (Mag (μ_B)), band gap ($E_g$ up/down (dn) (eV)).

Systems	d	X	$\mathbf{\Delta }\mathit{Q}$	${\mathit{E}}_{\mathbf{ad}}$	Mag	${\mathit{E}}_{\mathit{g}}$ (up)	${\mathit{E}}_{\mathit{g}}$ (dn)
@TBBC6N	2.8	O	0.17	0.114	0.0	-	-
@TiB/CoB-BC6N	1.85/1.87	O/O	−0.72/−0.65	4.43/2.36	0.9/1.04	1.1/-	-/-
@TiC1/CoC1-BC6N	1.82/1.85	O/O	−0.67/−0.58	5.06/3.66	0/−1.02	0.9/0.5	0.9/1
@TiC2/CoC2-BC6N	1.83/1.81	O/O	−0.63/−0.54	4.63/3.14	0/0.87	1/-	1/-
@TiN/CoN-BC6N	1.78/1.94	O/O	−0.54/−0.55	3.81/1.96	0.7/−1.7	-/-	-/-

).

Figure 8e–h show the influence of O₂ on the DOS/PDOS of Ti/Co-BC₆N systems. Adsorption of O₂ on Ti_B-/Co_B-BC₆N and Ti_N-/Co_N-BC₆N shifted Fermi energy to the VB, resulting in an asymmetry between spin-up/down spectra with a greater contribution of the O₂ $2p$ states in the VB, with the exception of @Co_N, which has the O₂ $2p$ states mainly located in the spin-up gap region. O₂ adsorption converts the Ti_B structure from metal to half-metal, and vice versa for Ti_N. On the other hand, the structure was changed from SC to metal in both O₂@Co_B and Co_N. The adsorption of O₂@Ti_C1 and Ti_C2 leaves the two spectra unchanged (spin-symmetric and SC), with band gaps of 0.9 eV and 1 eV. As for the Co systems, O₂@Co_C1-BC₆N remains SC, unlike O₂@Co_C2-BC₆N which changes from SC to metal.

2.2.7. H₂ Gas Adsorption

We now explore the potential of molecular hydrogen adsorption on our doped BC₆N systems. We place the one and two H₂ molecules consecutively close to the Ti/Co site at the 4 different doping locations. Figure 9 shows two of our systems, Ti_C1/Co_C1, with 1 and 2 adsorbed H₂ molecules. As we see in

Table 8. Average adsorption energies and number (storage) of hydrogen molecules on the Ti/Co-doped sheets.

System	${\mathit{E}}_{\mathit{a}\mathit{d}}^{\mathit{a}\mathit{v}\mathit{e}}$ Ti/Co (eV)
1H2 @TiB/CoB-BC6N	0.15/0.39
2H2 @TiB/CoB-BC6N	0.2/0.40
1H2 @TiC1/CoC1-BC6N	1.21/0.58
2H2 @TiC1/CoC1-BC6N	0.79/0.39
1H2 @TiC2/CoC2-BC6N	0.002/0.42
2H2 @TiC2/CoC2-BC6N	0.1/0.24
1H2 @TiN/CoN-BC6N	0.05/0.27
2H2 @TiN/CoN-BC6N	0.16/−0.58

, the adsorption strength is intermediate, except for Ti/Co systems at the C1 site. We also notice that the Co-doped systems are slightly better for hydrogen storage than the Ti-doped systems. The relatively small adsorption energy may be attributed to the large saturation of the dopants with the neighboring lattice sites, which leaves little room for the charge transfer necessary for a strong H₂ adsorption.

Now that we see that the single-atom doped systems may not be ideal for hydrogen storage, we explore the potential of other Ti- and Co-doped BC₆N systems. Figure 10 shows our Ti-doped systems, where 4 Ti atoms are implanted in the BC₆N skeleton, which may be experimentally realized in an implantation scheme. Given the symmetry of our BC₆N pristine structure, there are two inequivalent locations where the 4-atom cluster can land: one centered around the B site, and the other around the C2 site (Figure 10 shows the systems with the cluster centered around the former). Centering the clusters around the N and C1 sites yields the same systems. We systematically add up to four H₂ molecules, each time structurally relaxing our systems and obtaining the average H₂ adsorption energies. The average is taken over the number of H₂ molecules, according to:

$$E_{\mathrm{ad}}=\frac{E_{\mathrm{n}\mathrm{H}2+4\mathrm{Ti}/\mathrm{Co}@\mathrm{BC}6\mathrm{N}}-E_{4\mathrm{Ti}/\mathrm{Co}@\mathrm{BC}6\mathrm{N}}-n{E}_{\mathrm{H}2}}{n},$$

(1)

where $E_{\mathrm{Ad}}$ is the average adsorption energy per H₂ molecule for n adsorbed molecules, $E_{\mathrm{n}\mathrm{H}2+4\mathrm{Ti}/\mathrm{Co}@\mathrm{BC}6\mathrm{N}}$ is the total energy of the 4Ti-BC₆N sheet with n H₂ molecules adsorbed, $E_{4\mathrm{Ti}/\mathrm{Co}@\mathrm{BC}6\mathrm{N}}$ is the total energy of the 4Ti/Co-BC₆N sheet, and $E_{H2}$ is the total energy of a single H₂ molecule. The H₂ molecules can initially be placed in many locations around the cluster. The variability in the location may result in adsorption energy of the $n^{\mathrm{th}}$ H₂ that is higher than that of the preceding molecule. This may also be caused by a dynamic reorientation of the cluster or sheet atoms, resulting in higher adsorption energy. The relaxation process partially addresses the variability in the location by determining the atomic positions of a (possibly local) energy minimum. Therefore, the average adsorption energy per H₂ molecule is preferred over the adsorption energy of a specific $n^{\mathrm{th}}$ H₂ addition, as the average accounts for the variability in the location of the hydrogen molecule around the cluster as well as any reorientation of the cluster atoms as more molecules are added.

As we see in Figure 10, the 4-atom Ti cluster protrudes out of the BC₆N plane, with the BC₆N sheet becoming slightly non-planar, with a vertical spread of about 1.53 Å. The sheet does not significantly change its shape as H₂ is added. The average distances of the H₂ molecules from the closest Ti atom are 1.61 Å, 2.07 Å, 2.07 Å, and 2.4 Å. The average adsorption energies are 0.48 eV, 0.36 eV, 0.85 eV, and 0.72 eV. For the systems where the Ti cluster is centered around the C2 site, the average H-Ti distances are slightly lower, whereas the average energies are higher (

Table 9. Average adsorption energies of hydrogen molecules on the Ti/Co 4-atom cluster-doped systems.

System	${\mathit{E}}_{\mathit{a}\mathit{d}}^{\mathit{a}\mathit{v}\mathit{e}}$ Ti/Co-BC6N (eV)
1H2 @4TiB/4CoB-BC6N	0.48/0.22
2H2 @4TiB/4CoB-BC6N	0.36/0.98
3H2 @4TiB/4CoB-BC6N	0.85/0.74
4H2 @4TiB/4CoB-BC6N	0.72/0.60
1H2 @4TiC2/4CoC2-BC6N	2.47/0.57
2H2 @4TiC2/4CoC2-BC6N	1.33/1.18
3H2 @4TiC2/4CoC2-BC6N	1.27/0.94
4H2 @4TiC2/4CoC2-BC6N	1.03/0.79

).

The corresponding Co-doped systems are shown in Figure 11. One striking difference from the Ti case is that the Co cluster takes a pyramidal shape, with one Co atom at the top. The sheet suffers some deformation, with a vertical spread of 1.32 Å. The average distances of the H₂ molecules from the closest Co atom are 1.61 Å, 1.57 Å, 1.59 Å, and 1.54 Å. The average adsorption energies are 0.22 eV, 0.98 eV, 0.73 eV, and 0.60 eV. For the systems where the Co cluster is centered around the C2 site, the average H-Co distance and the average adsorption energies are highly similar (Table 9).

Our results of storing hydrogen on Ti- and Co-doped BC₆N compares with previous work, which has shown that Ti-decorated h-BN monolayers can adsorb up to 5 H₂ molecules per Ti atom, with an adsorption energy range from 0.68 eV for one molecule and up to 0.22 eV for five molecules [50]. Ti-decorated carbon-doped h-BN can store hydrogen at room temperature and mild pressure, with an average adsorption energy of 0.58 eV per molecule [51]. Li-decorated BC₆N significantly enhances hydrogen storage capacity compared to pristine structures, as Double-sided Li-decorated BC₆N can adsorb up to eight hydrogen molecules, with adsorption energies of 0.23–0.29 eV [24]. Further research demonstrated that 8Li-decorated BC₆N could adsorb up to 32 H₂ molecules [52].

One can determine the Ti/Co-doped BC₆N’s potential for gas adsorption/filtration by calculating the average adsorption energy of each gas on the various adsorption positions.

Table 10. Average adsorption energies ($E_{ad}^{ave}$ (eV)) on the Ti/Co-doped systems. The average is taken over the 4 different adsorption sites in each system. The lower 4 rows show the hydrogen adsorption on the systems doped with the 4-atom Ti/Co clusters, and the average is taken over the two equivalent cluster centers.

Gas	${\mathit{E}}_{\mathit{a}\mathit{d}}^{\mathit{a}\mathit{v}\mathit{e}}$ Ti/Co-BC6N (eV)
NO2	2.8/2.5
NO	3.5/2.6
NH3	1.4/1.3
CO2	1.3/0.5
N2	0.77/0.63
O2	4.49/2.78
1H2	0.36/0.41
2H2	0.31/0.11
1H2@4Ti/Co-BC6N	1.48/0.40
2H2@4Ti/Co-BC6N	0.85/1.18
3H2@4Ti/Co-BC6N	1.06/0.84
4H2@4Ti/Co-BC6N	0.88/0.70

illustrates that NH₃ and CO₂ are physisorbed on the Ti/Co-doped systems, whereas NO₂ and NO are chemisorbed. This shows that our Ti/Co-doped BC₆N-based sensors would work well for filtering NO₂ and NO, and to a lesser degree for NH₃ and CO₂. Additionally, we have observed that the adsorption of NO₂, NO, NH₃, and CO₂ alters our Ti/Co-doped systems’ band gaps in certain ways that could be used to create sensors for those gases. Consequently, the Ti/Co-BC₆N systems investigated in this work represent promising candidate materials for NO₂, No, NH₃, and CO₂ filtration and sensing. In addition, the Ti/Co cluster-doped BC₆N systems show great potential for the adsorption of up to 4 H₂ molecules.

The Ti- and Co-doped BC₆N system exhibits promising adsorption characteristics for various gases, suggesting its applicability in gas-sensing, capture, and catalytic processes. The high adsorption energies observed for NO (2.0–3.4 eV) and NO₂ (2.0–3.4 eV) indicate strong binding, making the material suitable for gas sensing and environmental monitoring of these toxic pollutants. Such sensors could be used to detect nitrogen oxides in industrial emissions and urban environments, improving air quality-control efforts [53]. The moderate adsorption energies for CO₂ (0.4–2.2 eV) suggest potential applications in carbon-capture and storage (CCS) technologies, where reversible adsorption is crucial for efficient gas separation and sequestration [54].

Additionally, the material’s affinity for NH₃ (1.2–1.5 eV) and N₂ (0.5–1.5 eV) suggests it could serve as a selective gas filter or membrane for ammonia removal in industrial processes, such as fertilizer production and waste treatment [55]. The exceptionally high adsorption energy for O₂ (3.0–5.0 eV) indicates potential applications in oxygen-storage and -release systems, which are critical for medical and industrial applications [56]. These results highlight the versatility of BC₆N-based materials for next-generation adsorption technologies, combining tunable gas interactions with structural stability.

Experimental studies show that B_xC_yN_z materials exhibit significantly higher gas uptake than graphene, with CO₂ and CH₄ adsorption increasing exponentially with surface area, unlike the linear trend in graphene. These findings, supported by theoretical calculations, highlight the superior adsorption capabilities of B_xC_yN_z [57]. Similarly, another study demonstrates that vertically aligned MoS₂ exhibits enhanced gas adsorption at its edge sites due to a high density of exposed edges, leading to stronger NO₂ binding [58].

To the best of our knowledge, no experimental measurements have been reported for gas adsorption on Ti- and Co-doped boron carbon nitride systems. However, studies on similar Ti-doped 2D materials may provide useful insights. It has been found that NO gas molecules adsorb on Ti-doped graphene with an energiy of 1.72 eV [59]. Additionally, Ti-doped 2D materials exhibit strong adsorption capabilities for HCHO, CO, and SO₂ [59], and acetone [60], with adsorption energies reaching up to 0.8 eV. These findings suggest that Ti- and Co-doped boron carbon nitride could also demonstrate promising gas-adsorption properties, which warrants experimental validation.

3. Computational Methods

We utilize the spin-polarized first-principles calculations using the Quantum Espresso software (V6.5) [61]. The generalized gradient approximation in the framework of the Perdew–Burke–Ernzerhof functional [62] is used to describe the exchange–correlation interaction. We use a 50 Ry energy cut-off. Our pristine system is a 3 × 3 supercell of BC₆N monolayer with 72 atoms. The doped monolayers are created by substituting a dopant atom for C (2 positions), B, and N. We take a 20 Å vacuum spacing along the z-axis to suppress interactions between neighboring images. A minimum force of 0.001 Ry/Bohr is used to obtain the optimized atomic positions and supercell volume of the considered structures. Van der Waals correction [63,64] is considered in our calculations. Our choice of the k-point grid is determined by performing convergence tests on the total energy and electronic structure. A Monkhorst–Pack grid of 12 × 12 × 1 is used for the density of states calculations. Charge transfer from/to the BC₆N sheets is calculated by the Löwdin method, and we complement this by showing the charge density difference maps (Supplementary Materials). In order to investigate the structural stability of different Ti-BC₆N and Co-BC₆N systems, we compute the formation energy per atom, $E_f$, as:

$$E_f=\frac{E(\mathrm{Ti}/\mathrm{Co}\text{-}{\mathrm{BC}}_6\mathrm{N})+\mathit{E}\left(\mathrm{Y}\right)-\mathit{E}\left({\mathrm{BC}}_6\mathrm{N}\right)-\mathit{E}(\mathrm{Ti}/\mathrm{Co})}{n},$$

(2)

where $E(\mathrm{Ti}/\mathrm{Co})$, $E\left({\mathrm{BC}}_6\mathrm{N}\right)$, $E\left(\mathrm{Y}\right)$, and $E(\mathrm{Ti}/\mathrm{CO}\text{-}{\mathrm{BC}}_6\mathrm{N})$ are the total energies of the isolated Ti/Co atom, the pristine sheet, the isolated removed Y atom (Y = C1, C2, B, or N), and the doped sheet, respectively, and n is the number of atoms in the supercell. The adsorption energy of a molecule is calculated by

$$E_{ad}=\mathit{E}\left(\mathrm{sheet}\right)+\mathit{E}\left(\mathrm{molecule}\right)-\mathrm{E}(\mathrm{sheet}+\mathrm{molecule}).$$

(3)

where $E(\mathrm{sheet}+\mathrm{molecule})$, $E\left(\mathrm{sheet}\right)$, and $E\left(\mathrm{molecule}\right)$ are the energies of the sheet with the adsorbed molecule, the sheet without the adsorbed molecule, and the isolated molecule, respectively.

4. Conclusions

This study employs first-principles calculations to investigate the structural, electronic, and molecular adsorption properties of Ti/Co-doped BC₆N monolayers, focusing on various substitutional doping sites (B, N, and two distinct C positions). The stability of these doped structures is assessed via formation energy calculations. The findings indicate that Ti/Co doping significantly alters the electronic states and band gaps of BC₆N nanosheets. The adsorption behaviors of NO, NO₂, CO₂, NH₃, N₂, and O₂ molecules on both pristine and doped BC₆N structures are analyzed. Doping improves the adsorption activity, particularly for NO, NO₂, and O₂ while CO₂ exhibits weaker adsorption across all doped configurations. Adsorption also influences the electronic and magnetic properties of the doped sheets, potentially transitioning them between metallic, semiconducting, and diluted magnetic semiconducting states, or modifying their band gaps. Furthermore, the study explores H₂ adsorption on Ti/Co-doped BC₆N and on structures embedded with 4-atom Ti and Co clusters. Although single-atom doping results in relatively weak H₂ adsorption, cluster-doped configurations can strongly adsorb up to four H₂ molecules. These results suggest that Ti/Co-doped BC₆N monolayers hold promise for applications in gas filtration, hydrogen storage, molecular sensing, and spintronics.