g-C₃N₄/CoN₄ heterojunction as a sensor for detecting volatile organic compounds: a density functional study

Abstract

Utilizing density functional theory (DFT) calculations, we investigated the adsorption behavior of key exhaled breath biomarkers – benzene, toluene, aniline, and o-toluidine – on both pristine g-C₃N₄ and g-C₃N₄/CoN₄ composite surfaces. By analyzing the key parameters such as adsorption energy, electronic density of states (DOS), band structures, charge density difference, conductivity and work function we gained critical insight into the interaction mechanisms between the gas molecules and the sensor surfaces. Our results reveal that the incorporation of CoN₄ significantly enhances the chemical reactivity and stability of the g-C₃N₄ substrate, further improving upon gas adsorption. Notably, benzene, toluene, and aniline exhibit reversible adsorption behavior on g-C₃N₄/CoN₄, highlighting their suitability for reusable gas sensor applications. In contrast, o-toluidine shows irreversible binding, potentially limiting its reusability. Aniline exhibits the lowest band gap and highest conductivity, highest sensitivity and strongest orbital hybridization, confirming that the g-C₃N₄/CoN₄ substrate acts as the best sensor towards aniline gas molecules, among the investigated molecules. Band structure analysis of aniline adsorbed on the g-C₃N₄/CoN₄ heterostructure further confirms that the composite exhibits improved electrical conductivity, even at room temperature, reinforcing its potential for biomarker sensing in human breath analysis. These theoretical findings establish g-C₃N₄/CoN₄ as a promising candidate material and provide predictive guidelines for future experimental investigations into selective and sensitive detection of benzene-related VOCs.

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1 Introduction

Lung cancer is a major global health issue, ranking second in terms of prevalence and a leading cause of cancer-related deaths worldwide.¹ Despite significant strides in diagnostic techniques, therapeutic approaches, and modern medication, the overall success rate in curing lung cancer remains unsatisfactory. However, early detection through screening programs for high-risk individuals can substantially enhance survival rates by enabling diagnosis at earlier stages of the disease.¹

Changes in bioelectric signals have been associated with various cancers, including those affecting the breast, lung, liver, brain, pancreas, and prostate.² These bioelectric alterations could potentially serve as early indicators for cancer detection. Moreover, each type of cancer is linked to specific metabolic and biochemical processes, resulting in the production of unique volatile organic compound (VOC) profiles. These distinct VOC patterns, associated with different cancers and affected organs, can be used to differentiate between healthy individuals and those with cancer, as well as to identify the specific type of cancer.² Detecting these biomarkers from exhaled breath samples offers a simple, affordable, and non-invasive method, potentially playing a crucial role in the early detection of lung cancer.³

Among the reported exhaled VOCs, benzene, aniline, toluene, and o-toluidine are recognized as important lung cancer biomarkers. These compounds have been frequently detected in breath samples of lung cancer patients and, even at trace concentrations, provide valuable diagnostic information. Hence, the development of highly sensitive and selective sensing platforms for their detection is of great clinical significance. In this context, Gregis et al.⁴ found that activated carbon microspheres (W₅) and zeolite NaY are the most effective materials for adsorbing toluene gas at very low concentrations. Ayad et al.⁵ developed a sensor using the quartz crystal microbalance (QCM) technique to detect various primary aliphatic alcohols, including ethanol, methanol, 1-propanol, and 2-propanol vapors. The detection mechanism relies on a thin, sensitive polyaniline film coated onto the QCM electrode, enabling effective identification of these alcohol vapors. Luo et al.⁶ reported that Co-doped ZnO exhibited a strong response to low concentrations of isopropanol, while Co₃O₄–ZnO showed better performance in detecting acetone. Panigrahi et al.⁷ proposed a nanosensor based on titanium carbide MXenes (Ti₃C₂T_x) to detect specific volatile organic compounds (VOCs) present in human breath. A set of representative VOCs, including 2,3,4-trimethylhexane, aniline, ethylbenzene, isoprene, and o-toluidine, were selected, and their adsorption mechanisms with Ti₃C₂T_x were thoroughly studied using first-principles DFT calculations.

Nanomaterials have emerged as promising candidates for early lung cancer detection due to their high surface-to-volume ratio, enhanced sensitivity, and ability to selectively interact with specific biomarkers.⁸ Nanosensors offer non-invasive, rapid, and cost-effective diagnostic tools, contributing to early detection and improved patient outcomes. In 2004, Páez et al.⁹ conducted both experimental and theoretical studies on the gas sensing capabilities of CNx nanotubes for ethanol, acetone, and ammonia. They found that CNx nanotubes were more effective than pure carbon nanotubes due to the presence of highly reactive pyridine-like sites. Karlický et al.¹⁰ investigated the adsorption of various small organic molecules, including acetone, ethanol, dichloromethane, benzene, and toluene, on graphene surfaces using both theoretical and experimental approaches. Aasi et al.¹¹ employed DFT to study the effects of individual toluene molecules on the structural, electronic, and magnetic properties of pristine and metal-decorated Single-Walled Carbon Nanotubes (SWCNTs). Their findings suggested that metal-decorated SWCNT molecular sensors hold promise for the detection of toluene and other lung cancer biomarkers in exhaled breath. Mosahebfard et al.¹² studied MoSe₂ monolayer as a potential biosensor for early lung cancer diagnosis by examining the adsorption behavior, bandgap modulation, and charge transfer of MoSe₂ upon adsorption of biomarkers hexanal, nonanal, and p-cresol. Their findings suggest that MoSe₂ exhibits promising physisorption properties and reasonable recovery times, making it a potential candidate for further development as a lung cancer biosensor.

In that context, graphitic carbon nitride (g-C₃N₄) is a promising material for gas sensor applications due to its unique properties.¹³ It possesses a favorable electronic band structure, high biocompatibility, and easy synthesis. Its large surface area with tunable electronic structures and excellent thermal stability make it suitable for gas sensing.¹⁴ However, the sensing performance of g-C₃N₄ can be further improved through strategies like metal doping, metal oxide/g-C₃N₄ composites, and surface functionalization. g-C₃N₄ exhibits intrinsic defects that act as active sites for gas adsorption. Self-doping with carbon atoms can improve the electrical conductivity of g-C₃N₄ by creating delocalized π-bonds, leading to enhanced electron transfer and reduced bandgap.¹⁵ These combined properties make g-C₃N₄ a promising candidate for gas sensing applications. Bicheng Zhu and co-workers¹⁶ studied CO₂ adsorption on g-C₃N₄ using first-principles calculations. Parthasarathy Srinivasan et al.¹⁷ presented a detailed sensing mechanism of g-C₃N₄ as a chemiresistive gas sensor for volatile organic compounds (VOCs), along with the specific rationales for selective detection. They found that optimizing the composition, dopant concentration, loading percentage, and functionalization ratio of g-C₃N₄ with metals and metal oxides can lead to enhanced selectivity, faster response times, and lower detection limits.

Currently, there is increasing research into the sensor capabilities of heterostructures^18–21 and the reasons are many-fold. First of all, the nanoscale effects of heterostructures can significantly influence the band alignment and carrier concentration within the heterostructure, resulting in effective electron–hole separation at the heterojunction. This characteristic is crucial for their potential use in chemical and biological sensors. Secondly, heterostructures allow two different materials to interact with target molecules, enhancing their sensing properties synergistically. By stacking materials with distinct band structures and work functions, it is possible to achieve controlled band alignment at the interface, leading to improved sensing performance and increased selectivity. This enhanced selectivity can be achieved by exploiting the molecular adsorption properties of each component. In view of this, Cao et al.¹⁸ created a cocoon-like hybrid nanocomposite of ZnO and graphitic carbon nitride using a simple hydrothermal process and found that the g-C₃N₄/ZnO nanocomposite sensor demonstrated exceptional gas sensing performance for ethanol. It featured a low optimal operating temperature and maintained strong linearity across a broad range of gas concentrations. Wang et al.¹⁹ developed a highly efficient acetone sensor using g-C₃N₄/WO₃ nanocomposites and demonstrated its superior performance compared to unmodified WO₃ nanostructures. Zhang et al.²⁰ created g-C₃N₄/SnO₂ composites using a sonication mixing technique, which enhanced the gas sensing properties compared to pure SnO₂. They discovered that the g-C₃N₄/SnO₂ material shows promise as a sensor for acetic acid. Li et al.²¹ successfully prepared flower-like NiO and g-C₃N₄-decorated NiO composites (Ni/CN) using a straightforward one-step hydrothermal method. They found that the 10 wt% g-C₃N₄(Ni/CN-10) sensor outperformed the pure NiO sensor, showing a nearly threefold increase in response to triethylamine at an optimal temperature of 280 °C. Additionally, the Ni/CN sensor demonstrated improved selectivity and stability, largely due to the incorporation of g-C₃N₄.

Transition metal-based g-C₃N₄ has been reported as a promising material for gas sensing applications. Zhang et al.²² prepared a-Fe₂O₃/g-C₃N₄ nanocomposites using a hydrothermal and pyrolysis method. The formation of heterojunctions in these nanocomposites resulted in improved gas sensing performance compared to pure a-Fe₂O₃ and g-C₃N₄. Gong et al.²³ synthesized a ball-flower-like Co₃O₄ composite decorated with carbon nitride using a simple hydrothermal method. The carbon nitride-decorated Co₃O₄ composite demonstrated significantly enhanced ethanol sensing properties, with a response 1.6 times higher than pure Co₃O₄. Additionally, the composite exhibited high selectivity and stability towards ethanol detection. Dong et al.²⁴ developed a Schottky heterojunction sensor modified with g-C₃N₄ quantum dots (g-C₃N₄QDs) and reduced graphene oxide (rGO) deposited on TiO₂ nanotube (TNT) arrays. This sensor demonstrated a high response, exceptionally fast response and recovery times, and outstanding sensitivity for detecting NO₂ at ppb levels at room temperature. Zhang et al.²⁵ successfully synthesized g-C₃N₄-decorated magnesium ferrite (MgFe₂O₄) porous microsphere composites using a one-step solvothermal method. The gas-sensing performance of these composites was measured and compared with a sensor based on pure MgFe₂O₄. Notably, the MgFe₂O₄/g-C₃N₄ composite sensor with 10 wt% g-C₃N₄ content showed a response to acetone that was enhanced by approximately 145 times, while the optimal operating temperature was reduced by 60 °C. Wang et al.²⁶ developed a one-step hydrothermal method to synthesize a CuO–ZnO/g-C₃N₄ ternary composite. The resulting CuO–ZnO/g-C₃N₄ displayed significantly enhanced ethanol-sensing properties, with responses 1.34 times and 2.17 times higher than those of CuO–ZnO and CuO alone, respectively. This improvement in gas-sensing performance was attributed to the effective p–n junctions between CuO and ZnO, along with the excellent substrate effect of the g-C₃N₄ nanosheets. Sneha et al.²⁷ reported that Co-doping significantly enhanced the gas sensing performance of g-C₃N₄ toward lung cancer biomarkers such as acetone, ethanol, and aniline. Co-g-C₃N₄ exhibited a reduced band gap, improved conductivity, and shorter recovery times—particularly for aniline due to its physisorption nature. These results established Co-g-C₃N₄ as a promising reusable sensor for exhaled breath analysis.

Very recently utilising the DFT calculations, V. N et al.^28,29 modelled g-C₃N₄/CoN₄ heterojunction and found that it can act as a type I photocatalyst for water-splitting reactions. Furthermore, in another work, they reported³⁰ that the same g-C₃N₄/CoN₄ heterojunction while exposed under strain and electric field perturbation switches from type I to type II photocatalyst, which was attributed by the crossover between valence and conduction band in presence of external perturbation. In a more recent work, V. N et al.³¹ showed that boron and fluorine co-doping of the g-C₃N₄/CoN₄ heterojunction transforms its electronic structure from type-I to a direct Z-scheme, which enhances charge carrier separation and redox ability. This Z-scheme heterojunction is proposed as an efficient photocatalyst for overall water splitting reaction, highlighting the tunability of g-C₃N₄/CoN₄-based systems for energy applications.³²

In this work, density functional theory (DFT) was employed to systematically investigate the adsorption behavior and electronic response of key aromatic biomarkers—benzene, toluene, aniline, and o-toluidine—on both pristine g-C₃N₄ and g-C₃N₄/CoN₄ surfaces. Although g-C₃N₄ had been widely studied as a photocatalyst, its potential for selective gas sensing, particularly when modified with transition metal complexes in the form of a heretojunction is still less explored. Adsorption energetics, electronic structures, charge transfer characteristics, and work function variations were evaluated to gain insights into how functional groups influenced sensor performance. It was revealed that the g-C₃N₄/CoN₄ heterostructure exhibited enhanced sensitivity and selectivity toward aniline, indicating its promise as a potential candidate for real-time detection of breath biomarkers. The in silico observation of g-C₃N₄/CoN₄ heterostructure for selective and sensitive detection of aniline vapors is unknown to date.

1.1 Computational methodology

Spin-polarized density functional theory (DFT), as implemented in the Quantum Espresso package, was employed for calculating both the structural optimization and electronic properties³⁰ of the systems under investigation. The exchange-correlation effects were incorporated using the Perdew–Burke–Ernzerhof (PBE) functional within the Generalized Gradient Approximation (GGA).³³ To address the limitations of GGA functional in handling partially filled d-orbitals and to better account for the localization of d-electrons via Coulomb and exchange corrections, the GGA + U method was applied, with a U value of 3.4 eV for Co atoms, as supported by several studies.³⁴ Ultrasoft pseudopotentials were utilized with a plane-wave energy cutoff of 145 Ry, and a vacuum gap of 30 Å was introduced to avoid spurious interactions in the non-periodic z-direction. Structural optimization was carried out using the Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithm, alongside a dispersion-corrected GGA-PBE functional employing the Grimme-D2 scheme.³⁵ Lattice parameters and atomic positions were optimized until all forces were below 10⁻³ Ry. Phonon spectra of CoN₄ bulk and CoN₄ slab were computed to confirm their lattice-dynamic stability. The unit cell of g-C₃N₄ and CoN₄ were relaxed with k-mesh values of 12 × 12 × 1 and 8 × 8 × 8, respectively.³⁴ A 2 × 2 × 1 heptazine and √3×√3 × 1 CoN₄ (111) slab with minimal lattice strain and lowest lattice mismatch (δ) were used to model the g-C₃N₄/CoN₄ heterojunction.³⁰ To understand the chemical sensing of the g-C₃N₄/CoN₄ heterojunction, the gas molecules such as benzene (bz), toluene (bz-CH₃), aniline (bz-NH₂), and o-toluidine (NH₂-bz-CH₃) were initially placed approximately at a distance of 3 Å from the surfaces. After completing geometry relaxations, the adsorption energy (E_ads), related with each gas adsorption process were calculated using the following formula:³⁶

E_ads = E_total − [E_{Heterojunction} − E_{gas molecule}] (1)

Here, E_total is the energy of the g-C₃N₄/CoN₄ heterojunction after adsorption. E_{gas molecule} and E_{Heterojunction} are the energy of the individual gas molecules and energy of g-C₃N₄/CoN₄ heterojunction before adsorption, respectively. The charge density difference of the materials under study was examined using GGA-PBE exchange-correlation potential developed in VASP (license number: 22-0418) and DFT-D approach was incorporated for handling van der Waals (vdW) interactions.³⁷ Bader charge analysis was performed to evaluate the charge transfer (Q_T) between the g-C₃N₄/CoN₄ and each adsorbed biomarker (benzene, toluene, aniline and o-toluidine). A negative charge value indicates, transfer of charge from gas molecules to g-C₃N₄/CoN₄ heterojunction. Band structure and projected density of states (PDOS) calculations were done to understand the change in electronic properties after adsorption of biomarkers. To assess the real time application of g-C₃N₄/CoN₄ towards the sensing of biomarkers, work function, sensitivity and recovery time were analysed. Although hybrid functionals such as HSE06 are known to yield more accurate band gaps, their computational cost becomes prohibitive for large heterojunction systems, particularly when considering adsorbed molecules. Therefore, in this work, we employed the GGA + U approach, which is widely recognized as a reliable and computationally efficient alternative to mitigate the band-gap underestimation of conventional GGA.

2 Results and discussion

2.1 Structural details of g-C₃N₄, CoN₄, g-C₃N₄/CoN₄ heterojunction and gas molecules

Heptazine-based g-C₃N₄ consists of heptazine (C₆N₇) units arranged in a 2D lattice, where each unit is connected to the others via C–N bonds, forming a hexagonal network. The unit cell of this hexagonal lattice comprises 14 atoms, including 8 nitrogen and 6 carbon atoms (Fig. 1(a)). There are two types of carbon atoms, labelled as C1 and C2, each binding with three neighboring nitrogen atoms. The nitrogen atoms are categorized into three types: six N1 atoms with two neighboring carbon atoms, one N2 atom, and one N3 atom, both of which have three neighboring carbon atoms.³⁸ The optimized lattice parameters for g-C₃N₄ are a = b = 7.2 Å, α = β = 90°, and γ = 120°, which closely match both XRD experimental values³⁹ (a = b = 7.11 Å) and theoretical predictions⁴⁰ (a = b = 7.2 Å). The bond lengths for C1–N1, C1–N3, C2–N1, and C2–N2 are 1.33 Å, 1.4 Å and 1.49 Å, respectively, aligning well with previously reported values (1.33, 1.39, and 1.48 Å).⁴⁰ The distance between the two nitride pores is 6.91 Å, which is consistent with experimental findings of 6.81 Å.³⁹ These results validate our computational methods, allowing us to proceed with further investigations.⁴¹

Fig. 1 Optimized geometries of (a) g-C₃N₄ primitive unit cell (b) CoN₄ bulk and (c) g-C₃N₄/CoN₄ heterojunction.

Next, we considered the diamond-like crystal structure of bulk CoN₄ (Fig. 1(b)), which was optimized using the Generalized Gradient Approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional. The primitive unit cell of CoN₄ was derived from this optimized bulk structure. To confirm the thermodynamic stability of bulk CoN₄, we calculated the formation energy (E_formation) using the equation:⁴²

where E_CoN4, E_Co, and E_N2 are the total energies of CoN₄, Co crystal, and N₂ crystal, respectively. The calculated formation energy of CoN₄ is −0.2064 eV, closely matching the reported value of −0.1992 eV,⁴² thus confirming the stability of the CoN₄ system. To check the magnetism associated with CoN₄, the total energies of anti-ferromagnetic (AFM), ferromagnetic (FM) and nonmagnetic (NM) states were calculated to determine the ground magnetic state of CoN₄. The energy values tabulated in Table 1 show that the FM ground state of CoN₄ is more stabilized compared to the AFM state by an amount of 0.5 eV. The stability of the FM state of CoN₄ over the AFM state is agreed with the previous report.⁴² We considered the stable FM state for all related calculations.

Table 1 Calculated energies of different magnetic states of CoN₄ bulk

System	Non magnetic state	Ferromagnetic state	Anti-ferromagnetic state
Energy (eV)	0	−2.0	−1.5

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The computed lattice parameters for CoN₄ are a = b = c = 11.4 Å (Fig. 1(b)), in good agreement with theoretical values (a = b = c = 11.235 Å).⁴⁵ The bond lengths for Co–N and N–N are 1.89 Å and 1.15 Å, respectively, in line with reported theoretical values (Co–N: 1.84 Å, N–N: 1.18 Å),⁴² affirming the reliability of our results.

To construct a realistic heterojunction model combining g-C₃N₄ and CoN₄, we first calculated the surface energy (γ) for the (100), (110), and (111) CoN₄ slabs using the equation:⁴³

where E_slab and E_bulk represent the energies of the CoN₄ slab and bulk, respectively, and A is the slab area. The CoN₄ slabs were created by cleaving the primitive unit cell into (100), (010), and (001) planes, separated by a 30 Å vacuum. The surface energies for the (100), (110), and (111) slabs were found to be 0.36, 0.37, and 0.11 J m⁻², respectively, indicating that the (111) slab is the most stable, and was thus selected for the heterojunction formation and subsequent calculations.⁴³

The compatibility of the lattice interface (Fig. 1(c)) was ensured by matching a 2 × 2 × 1 heptazine with a √3×√3 × 1 CoN₄ (111) slab. The lattice mismatch (δ) was calculated using the equation:

where a(111) and a(g) are the relaxed lattice parameters of the CoN₄ (111) slab and g-C₃N₄ monolayer, respectively. A mismatch of 3% was observed, which was within acceptable limits, consistent with previous reports.⁴⁴ The strain in the 2 × 2 × 1 heptazine system was calculated as 0.05%, and in the √3×√3 × 1 CoN₄ (111) slab, it was 2.7%.

After while phonon spectra analysis was performed to check the dynamic stability of CoN₄(111) slab and presented in Fig. 2. Absence of negative frequencies in the phonon spectra in Fig. 2(a) and (b) confirmed the lattice-dynamic stability of CoN₄ bulk and slab. Fig. 2(a) represents the phonon spectra of CoN₄ bulk, which matched with the reported literature and thereby confirms the stability of the system as well as reliability of our calculations.⁴²

Fig. 2 Phonon spectra of CoN₄ (a) bulk and (b) (111) slab.

To assess the thermodynamic stability of the heterojunction and the interaction between g-C₃N₄ and CoN₄ (111), the adhesion energy (E_ad) was calculated using the equation:⁴⁵

E_ad = E_g-C3N4/CoN4 − E_gC3N4 − E_CoN4 (5)

where E_gC3N4/CoN4, E_gC3N4, and E_CoN4 represent the total energies of the relaxed g-C₃N₄/CoN₄ heterostructure, g-C₃N₄ monolayer, and CoN₄ (111) slab, respectively. The calculated negative adhesion energy of −4.7396 eV indicates that the adsorption process is thermodynamically stable, making the adhesion of CoN₄ onto the g-C₃N₄ surface exothermic and energetically favorable.²⁸

The C–C bond length in benzene (Fig. S1(a)), the C1–C2 and C2–C3 bond length in toluene (Fig. S1(b)), the C–N bond length in aniline (Fig. S1(c)) and the C5–C6 bond length in o-toluidine (Fig. S1(d)) were measured as 1.399 Å, 1.403 Å,1.398 Å, 1.395 Å, and 1.396 Å respectively. The C–C–C bond angle in benzene, C6–C1–C2 in toluene, N–C–C in aniline, and N–C1–C6 in o-toluidine were found to be, 119°54′,119.40°, 120.00° and 118.9° respectively. These calculated bond length and bond angle of aniline align with the earlier study by Vaschetto et al.⁴⁶ where reported C–N bond length and N–C–C bond angle were of 1.392 Å and 120°. Similarly, the geometrical parameters of benzene are in good agreement with earlier study by Koichi Tamagawa et al.⁴⁷ and E. G. Cox,⁴⁸ where they reported a C–C bond length of 1.399 Å and C–C–C bond angle of 119°28′. The bond length and bond angle values for toluene are consistent with the study by David R Borst et al.,⁴⁹ they noted a C1–C2 bond length of 1.397 Å, C2–C3 bond length of 1.394 Å and C6–C1–C2 bond angle of 119.40°. The parameters of o-toluidine are aligned with the study by V. K. Bel'skii et al.⁵⁰ which reported a C6–C5 bond length of 1.389 Å and a N–C1–C6 bond angle of 120.3°. The bond lengths and bond parameters are presented Table S1.

2.2 The adsorption of selected gas molecules on the g-C₃N₄

Before exploring the adsorption of gas molecules on the g-C₃N₄/CoN₄ heterojunction, we first calculated the adsorption energies of gas molecules on pristine g-C₃N₄. The adsorption of biomarkers such as benzene, toluene, aniline, and o-toluidine was examined and the resulting adsorption energies are presented in Table 2. The most stable configurations for g-C₃N₄/benzene, g-C₃N₄/aniline, g-C₃N₄/o-toluene, and g-C₃N₄/o-toluidine are shown in Fig. 3. The adsorption energies for g-C₃N₄ interacting with various aromatic molecules – benzene, toluene, aniline, and o-toluidine – are −1.51 eV, −1.87 eV, −0.98 eV, and −2.03 eV, respectively. Notably, the adsorption energy of g-C₃N₄ with o-toluidine is the highest among these, indicating a stronger interaction. Aniline is physisorbed on the g-C₃N₄ surface, whereas the other gas molecules are chemisorbed on the g-C₃N₄ surface. The intermolecular distances between the gas molecules and g-C₃N₄ are illustrated in the Fig. 3 The distance between g-C₃N₄ and benzene is 3.10 Å. Due to the presence of a methyl group, the distance between toluene and g-C₃N₄ decreases to 2.67 Å compared to benzene. However, the nitrogen in aniline causes a slight repulsion with g-C₃N₄, increasing the distance to 3.12 Å. In contrast, the additional methyl group in o-toluidine results in a shorter distance from g-C₃N₄ (2.95 Å) compared to aniline.

Table 2 Adsorption energy of benzene, toluene, aniline and o-toluidine biomarkers adsorbed on g-C₃N₄ unit

System	g-C3N4/benzene	g-C3N4/toluene	g-C3N4/aniline	g-C3N4/o-toluidine
Adsorption Energy (eV)	−1.51	−1.87	−0.98	−2.03

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Fig. 3 Optimised structures of biomarkers (a) benzene, (b) toluene (c) aniline and (d) o-toluidine adsorbed on the g-C₃N₄ surface.

After calculating the adsorption energy, the sensing potential of pristine g-C₃N₄ was evaluated, by calculating the recovery times of four biomarkers on the g-C₃N₄ surface using the following equation:⁴⁶

where ν_o represents the attempt frequency (a constant of 10¹²) for visible light, T is the temperature, and k_B is the Boltzmann constant. The calculated recovery times for benzene, toluene, aniline, and o-toluidine gas molecules on the pristine g-C₃N₄ surface are 3.52 × 10¹³ s, 4.33 × 10¹⁹ s, 3.80 × 10⁴ s, and 2.58 × 10²² s, respectively. A close examination of these values reveals that the recovery times are excessively long, making it impractical to desorb these gas molecules at room temperature.

In that context, research has shown that incorporating metal atoms can significantly enhance the sensing capabilities of materials.^47,48 Therefore, to improve the sensing ability of g-C₃N₄, it was combined with CoN₄, and a detailed investigation of the adsorption behavior and sensor activity of the g-C₃N₄/CoN₄ heterojunction was conducted.

2.3 Optical band gap of g-C₃N₄ and g-C₃N₄/CoN₄

The optical band gap of g-C₃N₄ was calculated using Tauc's method,⁴⁹ as described by the equation:where α is the absorption coefficient, hν is the energy of the incident photon, E^opt_g represents the optical band gap, and A₀ is a constant that is related to the transition probability. The optical band gap is presented in Fig. 4. From Fig. 4, the calculated optical band gap of g-C₃N₄ is determined to be 2.58 eV, which is in close agreement with the experimentally reported value of 2.65 eV.⁵⁰ Similarly, the optical band gap of g-C₃N₄/CoN₄ was found as 2.06 eV (see Fig. 4). The optical band gaps of g-C₃N₄ and g-C₃N₄/CoN₄ have been plotted in Fig. 4. The optical absorption maxima of g-C₃N₄ and g-C₃N₄/CoN₄ are 480 nm and 602 nm,^28,29 respectively (see the vertical line in Fig. 4). The red shift in the optical absorption maximum of the g-C₃N₄/CoN₄ heterojunction compared to g-C₃N₄ indicates enhanced light absorption and more effective sensing ability compared to pristine g-C₃N₄. The red-shifted absorption maximum of the heterojunction and higher recovery time of the gas molecules on pristine g-C₃N₄, motivated to investigate the sensor activity of the g-C₃N₄/CoN₄ heterojunction.

Fig. 4 Optical band gap of pristine g-C₃N₄ and g-C₃N₄/CoN₄.

2.4 The adsorption of selected gas molecules on the g-C₃N₄/CoN₄

To simulate the adsorption process, we positioned each gas molecule under investigation such as benzene, toluene, aniline, and o-toluidine on various potential sites of g-C₃N₄/CoN₄ heterosurface. At the very first point the most favorable adsorption points for each gas molecule on the 2D system was determined. The g-C₃N₄/CoN₄ + gas molecules in their ground states were optimized considering all probable possibilities, and the most stable configurations was identified based on the adsorption energies calculated using eqn (1). These stable structures of g-C₃N₄/CoN₄ heterojunction with gas molecules are presented in Fig. 5 and the adsorption energies are summarized in Table 3. The adsorption sites on the g-C₃N₄/CoN₄ heterojunction was found to be varied depending on the gas molecules involved, with each molecule occupying a distinct site. Notably, all gas molecules prefer to adsorb absolutely on the g-C₃N₄ layer of the heterojunction. For benzene, the adsorption energy was −0.67 eV, and it preferred mostly to align on void of the g-C₃N₄. Toluene adsorbs with an energy of −0.84 eV, positioning its CH₃ group projecting into the g-C₃N₄ void. Aniline binds the heterojunction with an adsorption energy of −0.97 eV, with its NH₂ group oriented into the void. Similarly, o-toluidine adsorbs with an energy of −1.08 eV, positioning both its CH₃ and NH₂ groups within the g-C₃N₄ void. Notably, the adsorption energy in case of physisorption, generally ranges from −0.5 eV to −1.0 eV. Based on these values, benzene, toluene, and aniline display physisorption behavior on the g-C₃N₄/CoN₄ heterojunction, while o-toluidine lies on the borderline between physisorption and chemisorption. This major advantage of physisorption is that – it enables the g-C₃N₄/CoN₄ heterojunction to function as a reusable sensor. Interestingly, while the ring electron density of the gas molecules follows the order benzene < toluene < aniline < o-toluidine.

Fig. 5 Optimised structures of biomarkers (a) benzene, (b) toluene (c) aniline and (d) o-toluidine adsorbed on the g-C₃N₄/CoN₄ heterojunction.

Table 3 Adsorption energy of benzene on g-C₃N₄/CoN₄, toluene on g-C₃N₄/CoN₄, aniline on g-C₃N₄/CoN₄ and o-toluidine on g-C₃N₄/CoN₄

System	g-C3N4/CoN4-benzene	g-C3N4/CoN4-toluene	g-C3N4/CoN4-aniline	g-C3N4/CoN4-o-toluidine
Adsorption Energy (eV)	−0.67	−0.84	−0.97	−1.08

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The adsorption energies tabulated in Table 3 clearly reveals that benzene showed weak adsorption while o-toluidine adsorbed strongly on the g-C₃N₄/CoN₄ surface. Consequently, the adsorption energy of the gas molecules followed an order as benzene < toluene < aniline < o-toluidine. This can be attributed by the electron density of the gas molecules: benzene, being the least electron-rich, has a lower tendency to donate electrons to the g-C₃N₄/CoN₄ heterojunction, resulting in a lower adsorption energy. In contrast, o-toluidine is having the highest electron-density due to the presence of two electron-donating groups (CH₃ and NH₂), which enhanced its electron-donating capability and lead to a higher adsorption energy towards heterojunction surface. Toluene and aniline are positioned in between the two terminals as their electron density are higher compared to benzene but lowered compared to o-toluidine. These calculated adsorption energies vis-à-vis the direction and magnitude of charge transfer has been discussed in the next section.

The prime criteria of a gas sensor to achieve optimal sensing performance is that the interactions between the adsorbent and the gas molecules must be sufficiently strong to generate measurable signals for detection. However, the gas molecules should also be able to desorb from the sensor without altering its properties, allowing for quick recovery. It is important to highlight that the adsorption energy of gas molecules is relatively high, ensuring stability against thermal fluctuations at room temperature (k_BT = 0.025 eV).⁴⁹

The calculated adsorption energy for benzene is −0.67 eV, which lies within the optimal range of −0.60 to −1.0 eV for reversible gas capture. Although this value is less negative than the literature value reported by Yaseen et al.⁵² (−0.93 eV) for benzene adsorbed on a Pt-cluster modified g-C₃N₄ nanosheet, the observed trend remains consistent, indicating moderate interaction strength suitable for sensor applications. For toluene, our computed adsorption energy is −0.84 eV, falling within the optimal window. However, Aasi, A. et al.⁵³ reported a significantly stronger adsorption energy of −1.3 eV for toluene on a Pd-decorated carbon nanotube (CNT) surface. In the case of aniline, our result of −0.97 eV compares reasonably with those reported by Panigrahi, P. et al.,⁵⁴ who studied aniline adsorption on various Ti₃C₂ MXene-based surfaces: Ti₃C₂F₂ (−0.663 eV), Ti₃C₂(OH)₂ (−1.052 eV) and Ti₃C₂O₂ (−1.031 eV).

For o-toluidine, the calculated adsorption energy of −1.08 eV lies just outside the ideal window, indicating relatively strong adsorption. Panigrahi, P. et al.⁵⁴ report a range of values on the same Ti₃C₂-based surfaces: Ti₃C₂F₂ (−0.691 eV), Ti₃C₂(OH)₂ (−0.942 eV) and Ti₃C₂O₂ (−1.158 eV) Overall, the adsorption energies of benzene, toluene, aniline and o-toluidine biomarkers on the g-C₃N₄/CoN₄ heterojunction demonstrate strong agreement with literature values, confirming their relevance for reversible gas capture applications.

2.5 Charge density difference

It is commonly understood that the sensing mechanism in 2D materials is primarily driven by charge transfer, which plays a key role in altering the resistance across sensing channels during gas adsorption. To analyze the distribution of electrons at the gas-surface interface, we employ charge density difference (CDD) calculations using the given formula.⁵⁵

(Δρ = ρ_{gC3N4/CoN4_gas} − ρ_gC3N4/CoN4 − ρ_gas) (8)

Here Δρ is the charge density difference. ρ_{gC3N4/CoN4_gas}, ρ_gC3N4/CoN4and ρ_gas are the electronic charge of gC₃N₄/CoN₄_gas, gC₃N₄/CoN₄ and gas molecules (of benzene, toluene, aniline and o-toluidine) respectively.

Fig. 6 represents 3D charge density difference plots (Δρ) along the z-axis. The iso-surface value for all targeted gas molecules was set to 0.0002e Å⁻³ in the 3D plots to maintain the consistency which ensures the graphs are on the same scale. This allows us to easily observe the regions of charge accumulation and depletion, and provide valuable insights into the charge transfer process. Generally, a higher or lower iso-surface value indicates areas with greater or lower electron density, respectively. The figure illustrates distinct electron-rich (yellow) and electron-deficient (cyan) regions. Notably, in all cases, the gas molecules exhibit a predominance of electron-deficient areas, while the heterojunctions show a dominance of electron-rich areas. This suggests a potential electron transfer mechanism from the gas molecules to the heterojunction. The electron density of the gas molecules is directly proportional to the charge transfer, which is clearly visible in the charge density difference plot. In all cases, both electron-rich and electron-deficient regions are present in both the gas molecules and the heterojunctions. Fig. 6 consequently, indicates that the high electron density of o-toluidine leads to a significant charge transfer within the system. To quantify the extent of electron transfer, we performed Bader charge analysis. The calculated charge transfers for benzene, toluene, aniline, and o-toluidine were found to be 0.0260|e|, 0.0237|e|, 0.0238|e|, and 0.0610|e| respectively. Notably, a clear direct relationship exists between the amount of charge transfer and the electron density of the gas molecules. Benzene, being the highest electron deficient, exhibits the lowest charge transfer compared to the other molecules. This reduced electron donation propensity is attributed to the lower electron density of benzene, which hinders electron transfer to the heterojunction. Due to the presence of two electron donating groups o-toluidine shows an exceptionally high amount of charge transfer. The highest electron transfer of o-Toluidine may be due to the higher adsorption energy of o-Toluidine on the heterojunction.

Fig. 6 Charge density difference plots of biomarkers (a) methanol, (b) ethanol (c) acetone and (d) aniline adsorbed on the g-C₃N₄/CoN₄ heterojunction.

2.6 Band structure

To understand the influence of gas molecule adsorption on the electronic properties of the g-C₃N₄/CoN₄ heterojunction, we have calculated the electronic properties, including the band structure, total density of states (TDOS), and partial density of states (PDOS) of both the heterojunction and the heterojunction with adsorbed molecules.

Fig. 7 presents the band structure of g-C₃N₄/CoN₄ heterojunction and the same with adsorbed gas molecules. As already reported g-C₃N₄/CoN₄ is half metallic in nature^27,28 where spin up channel (purple in Fig. S2 in SI file) behaves as semiconductor and spin down channel acting as a metal. The g-C₃N₄/CoN₄ heterojunction exhibits a direct bandgap of approximately 1.81 eV, from the Γ point of conduction band minimum (CBM) to the Γ point valence band maximum (VBM) (see Fig. 7), which aligns with previously reported values.³⁰ This bandgap energy of g-C₃N₄/CoN₄ heterojunction falls within the visible light spectrum, indicating that visible light exposure could enhance the VOC sensing efficiency, similar to organic molecules and ZnO gas sensors functionalized with organic compounds.^56,57

Fig. 7 Band structure (spin up), DOS (spin up) and PDOS (spin up) of biomarkers (a) benzene, (b) toluene (c) aniline and (d) o-toluidine adsorbed on the g-C₃N₄/CoN₄ heterojunction.

To further understand the gas adsorption behavior on g-C₃N₄/CoN₄, electronic band structures were obtained for the gas-adsorbed systems, as illustrated in Fig. 7. Band structures of all the gas-adsorbed systems are half metallic in nature. Gas adsorption was found to influence the electronic properties of the g-C₃N₄/CoN₄ heterojunction. The pristine g-C₃N₄/CoN₄ has an energy gap of about 1.81 eV, which changes upon gas adsorption. The band gap values after the adsorption of benzene, toluene, aniline, and o-toluidine were 1.50, 1.12, 0.61 and 1.11 eV, respectively, indicating variations of 0.31, 0.69, 1.20 and 0.70 eV for each gas. This suggests that the adsorption of benzene has minimal impact on the band structure near the Fermi level. However, the adsorption of aniline has the greatest impact on the band structure near the Fermi level. Despite these changes, the direct bandgap nature of g-C₃N₄/CoN₄ remains intact. The change in the band gap due to benzene adsorption is notably lower compared to other gas molecules. This is because benzene has a lower electron density than the other gases, resulting in a reduced tendency to adsorb on the surface of g-C₃N₄/CoN₄ heterojunction. Consequently, benzene's weak adsorption impacts the heterojunction's band gap minimal.

The calculated band gap change of 0.31 eV for benzene indicates a strong interaction between the adsorbate and the material surface, suggesting its suitability for gas sensing applications. This shift is significantly higher than the 0.21 eV reported by Jyothi et al.⁵⁸ for benzene on 4–8 hBN NT, and the 0.005 eV reported by Phung et al.⁵⁹ for benzene on graphene, implying enhanced sensitivity in the present system. For toluene, a notable band gap change of 0.69 eV is observed, indicating a strong interaction with the heterostructure. This value substantially exceeds the literature-reported shifts of 0.016 eV and 0.024 eV by Aasi et al.,⁵³ corresponding to toluene adsorption on Pt-SWCNT and Pd-SWCNT, respectively, thereby demonstrating superior sensing potential. In the case of aniline, the observed band gap modulation of 1.2 eV reflects an exceptionally strong electronic interaction, which could result in significant changes in electrical properties even at low analyte concentrations. This is considerably larger than the 0.11 eV reported by Zhang et al.⁶⁰ for aniline on Pd–ZnO, highlighting improved sensing characteristics. For o-toluidine, the computed band gap change of 0.7 eV further supports the material's enhanced sensing ability. This is significantly higher than the 0.185 eV reported by Wan et al.⁶¹ for o-toluidine adsorption on Ir-doped CNTs, reinforcing the improved response in the current study. Due to the highest observed band gap change, the g-C₃N₄/CON₄ heterostructure exhibits superior sensitivity towards aniline, making it a highly promising candidate for selective aniline gas sensing compared to other analyte molecules. Overall, the larger band gap shifts observed in this work compared to literature values underscore the stronger adsorbate-material interactions and suggest that the studied heterostructure is a promising platform for high-performance gas sensor applications.

To gain a clear understanding of the electronic properties of the systems under investigation, the total density of states (TDOS) and partial density of states (PDOS) of the systems were calculated and presented in Fig. 7. The band gap reduction of the heterojunction in the presence of gas molecules is clearly visible in the TDOS plots. By analyzing the PDOS of the g-C₃N₄/CoN₄ heterojunction, it can be concluded that the valence band maximum (VBM) of the heterojunction is primarily composed of C 2p and N 2p orbitals, while the conduction band minimum (CBM) is dominated by Co 3d and N 2p orbitals.^28,29 In the cases of g-C₃N₄/CoN₄-benzene, g-C₃N₄/CoN₄-toluene, and g-C₃N₄/CoN₄-o-toluidine, the valence band maximum (VBM) and conduction band minimum (CBM) remain consistent with those of the g-C₃N₄/CoN₄ heterojunction. However, for g-C₃N₄/CoN₄-aniline, while the VBM aligns with that of the g-C₃N₄/CoN₄ heterojunction, the CBM is primarily composed of N 2p and C 2p orbitals from the aniline molecule. Due to the presence of aniline orbitals interposed between those of the heterojunction, the band gap of g-C₃N₄/CoN₄-aniline is significantly reduced compared to the other systems. When gas molecules adsorb onto the g-C₃N₄/CoN₄ heterojunction, their orbitals hybridize with those of the heterojunction, facilitating strong interactions. For example, when benzene is adsorbed, the C 2p orbitals of benzene overlap with the C 2p orbitals of the heterojunction at the valence band maximum (VBM), while the H 1s orbitals of benzene interact with the Co 3d orbitals at the conduction band minimum (CBM). Similarly, in the case of toluene, both the C 2p and H 1s orbitals of toluene hybridize with the C 2p orbitals of the heterojunction at the VBM, and the H 1s orbitals of toluene overlap with the Co 3d orbitals at the CBM. Additionally, when o-toluidine adsorbs onto the heterojunction, the C 2p orbitals of o-toluidine hybridize with those of the heterojunction at the VBM, while the N 2p orbitals of o-toluidine interact with the C 2p orbitals of the heterojunction at the CBM. There is significant overlap between the PDOS peaks of aniline (especially C-2p and N-2p of aniline) and the g-C₃N₄/CON₄ substrate (C-2p, N-2p, Co-3d), indicating strong electronic coupling and orbital hybridization. This pronounced interaction suggests that the g-C₃N₄/CON₄ heterostructure is particularly well-suited for selective aniline detection, outperforming its sensing performance for other gas molecules. These orbital interactions significantly enhance the adsorption properties and electronic characteristics of the g-C₃N₄/CoN₄ heterojunction, making it a promising material for applications in gas sensing and catalysis.

2.7 Gas sensing device evaluation

2.7.1 Sensitivity. The development of an effective gas-sensing device requires meeting several key criteria, such as high sensitivity, reliable selectivity, and fast response and recovery times. For this reason, a gas sensor based on the g-C₃N₄/CoN₄ heterojunction must exhibit strong adsorption energies for potential target gases. Additionally, the sensor should demonstrate ideal charge transfer to influence electrical conductivity. Our calculations suggest that the g-C₃N₄/CoN₄ heterostructure has sufficient adsorption energy for benzene, toluene, aniline and o-toluidine, preventing spontaneous desorption at room temperature. Electronic calculations revealed that all adsorbate-covered surfaces showed significantly lower band gap energies compared to the pristine heterostructure. This finding suggests that adsorption has a pronounced impact on the electronic properties of the heterostructure. As a result, gas molecules can be identified by analyzing the changes in electrical conductivity observed before and after the adsorption process. Sensitivity, a crucial aspect of gas sensing, can be addressed by evaluating changes in electrical conductivity using the equation,⁶²

σ ∼ exp[−E_g/2k_BT] (9)

where σ, E_g, T and k_B are the electrical conductivity, band gap, temperature, and Boltzmann constant (8.62 × 10⁵ eV K⁻¹), respectively. At room temperature of 298 K, the pristine g-C₃N₄/CoN₄ heterojunction has an electrical conductivity of 6.3 × 10⁻¹⁶. After adsorption, the conductivity values for benzene, toluene, aniline and o-toluidine gases were determined and shown in Table 4. The electrical conductivities of benzene, toluene, aniline and o-toluidine are 2.08 × 10⁻¹³, 3.40 × 10⁻¹⁰, 6.94 × 10⁻⁶ and 4.13 × 10⁻¹⁰ respectively. Based on these calculations, we found that the conductivity of the adsorbate systems was higher than that of the pristine g-C₃N₄/CoN₄ heterostructure, with the most significant change in conductivity occurring in the case of aniline (6.94 × 10⁻⁶). Combined with the higher band gap change and strong orbital hybridization observed in the PDOS analysis, this suggests that the g-C₃N₄/CON₄ heterojunction can act as an excellent sensor for aniline detection. This reduction in conductivity may be attributed to benzene's lower electron ring density, which weakens its interaction with the heterojunction and thereby limits charge transfer (0.0260|e), leading to lower conductivity.

Table 4 Recovery time and conductivity of benzene, toluene, aniline and o-toluidine while adsorbed on g-C₃N₄/CoN₄

Biomarkers	Recovery time (s) at 298K	Conductivity
Benzene	0.216	2.08 × 10^−13
Toluene	162.71	3.40 × 10^−10
Aniline	3.18 × 10^3	6.94 × 10^−6
o- oluidine	1.86 × 10^6	4.13 × 10^−10

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2.7.2 Recovery time. To know the reusability of the sensor the recovery time was calculated in the next step. Recovery time is the minimum time required for a sensor to desorb the adsorbed gas from its surface. Higher adsorption energy leads to difficulty in desorption process thereby prolonging the recovery time. It can be obtained from the transition state theory and van't Hoff Arrhenius equation which is given in eqn (8).

The recovery time for each species at room temperature (T = 298 K), has been presented in Table 4. The foremost criterion of reversible gas capture is that – optimal adsorption energies should be typically fall within the range of −0.60 to −1.0 eV. A higher adsorption energy (E_ads) makes gas desorption more challenging, though an increase in temperature would greatly speed up the process. The recovery time, ranging from milliseconds to a few seconds, indicates the reusability of the gas sensor. However, if the recovery time is too short, it may imply insufficient adsorption capability, which hinders effective gas detection. The adsorption energies of benzene, toluene, aniline, and o-toluidine on the heterojunction range from −0.60 to −1.0 eV. The recovery times for benzene, toluene, aniline and o-toluidine were calculated as 0.216 s, 162.71 s, 3.18 × 10³ s, and 1.86 × 10⁶ s, respectively. Therefore, the heterojunction undoubtedly functions as an efficient reusable sensor for the detection of benzene, toluene, aniline and o-toluidine. The recovery time of the g-C₃N₄/CoN₄ heterojunction is significantly shortened when benzene is adsorbed onto it. This faster recovery may be attributed to benzene's lower ring electron density, which weakens its interaction with the heterojunction, thereby reducing charge transfer and facilitating a quicker desorption process. Consequently, the limited interaction results in a lower impact on conductivity, allowing the system to return to its baseline state more efficiently.

The computed recovery time of 0.216 s for benzene adsorption on the g-C₃N₄/CON₄ heterostructure is remarkably shorter than the 5.32 × 10³ s reported by Yaseen et al.⁵² for benzene on Pt-g-C₃N₄, showcasing a significant advancement in gas sensing performance with rapid sensor reset capabilities. For toluene, the calculated recovery time of 162.71 s is substantially lower than the 5.85 × 10⁶ s reported by Nagarajan et al.⁶³ for toluene detection using ring-silicon-doped twisted bilayer arsenene nanosheets (ASNS), revealing a drastic enhancement in sensing efficiency. The improved desorption behavior underlines the heterostructure's potential for high-throughput gas sensing applications. Rahman et al.⁶⁴ reported that the recovery time of aniline on AINNT (aluminium nitride nanotube) is in the order of 10³⁵, while that on AIPNT (aluminium phosphide nanotube) is in the order of 10²³. In contrast, our calculated recovery time for aniline on the g-C₃N₄/CoN₄ system is on the order of 10³, which is significantly lower than the values reported by Rahman et al.⁶⁴ This clearly indicates that our material can act as an efficient sensor for aniline. Overall, the considerable reduction in recovery times compared to previously reported materials confirms that the g-C₃N₄/CON₄ heterojunction offers exceptional performance, making it a promising candidate for sensitive, fast, and reusable gas sensors suited for real-world applications.

2.7.3 Work function. To know the effect of adsorption on the work function of the heterojunction we have calculated the work functions of the heterojunction and adsorbed systems. The work function (Φ) represents the minimum energy needed to remove an electron from the surface of material to vacuum and given by:⁵¹

∅ = E_vac − E_F (10)

Here, E_vac is the vacuum level energy and E_F is the Fermi level energy. The work function, therefore, being a surface property could also be used to understand the sensing capability of sensor materials. The variations in work function upon adsorption was studied by Gilani and coworkers, which shows that DFT calculations can reasonably predict the changes in work function induced by physisorbed gases.⁵⁵ The adsorbed gas molecules in the present study may alter the work function of the g-C₃N₄/CoN₄. In order to get deeper insight about the sensing capacity, the work function of g-C₃N₄/CoN₄ after adsorption of benzene, toluene, aniline and o-toluidine gas molecules was calculated. The work function of isolated g-C₃N₄/CoN₄ as well as the same after adsorption of gas molecules is given in Table 5. Our calculations predict that the work function of the isolated g-C₃N₄/CoN₄ heterostructure is 4.56 eV, slightly higher than that of graphene (4.5 eV). As shown in Table 5, the adsorption of toluene and o-toluidine increases the work function by 0.03 eV and 0.06 eV respectively, indicating that electron transfer to the vacuum level is hindered by adsorption. In contrast, the adsorption of other molecules, such as benzene (by 0.32 eV) and aniline (by 0.22 eV), reduces the work function. Thus, the work function of the heterostructure can be effectively modified by the presence of these gas molecules.

Table 5 Work functions of benzene, toluene, aniline and o-toluidine on g-C₃N₄/CoN₄

Gas molecule	Work function(Φ)	Change in work function (ΔΦ)
g-C3N4/CoN4	4.56
Benzene	4.24	−0.32
Toluene	4.59	+0.03
Aniline	4.34	−0.22
o-Toluidine	4.62	+0.06

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3 Conclusion

In this study, we employed density functional theory (DFT) calculations to investigate the adsorption behavior of lung cancer–related volatile organic compounds (VOCs)—benzene, toluene, aniline, and o-toluidine—on pristine g-C₃N₄ and its CoN₄-modified composite (g-C₃N₄/CoN₄ heterojunction). Our results reveal that CoN₄ incorporation substantially enhances the chemical reactivity of g-C₃N₄, leading to stronger interactions and improved electronic responses toward the target molecules. While benzene, toluene, and aniline exhibit reversible adsorption on g-C₃N₄/CoN₄ heterojunction, enabling potential sensor reusability, o-toluidine binds irreversibly. Among the studied VOCs, aniline demonstrated the most pronounced electronic and conductivity changes, establishing g-C₃N₄/CoN₄ as a highly selective and sensitive candidate for aniline detection.

The key take-home message of this work is that rational surface engineering of g-C₃N₄ with transition-metal sites such as CoN₄ can transform an otherwise moderate adsorbent (such as g-C₃N₄) into a highly promising material (by forming g-C₃N₄/CoN₄ heterojunction) platform for breath-based biomarker sensing. These insights not only highlight aniline as a reliable target VOC for theoretical screening but also provide predictive guidelines for experimental validation and device development. Future efforts should focus on integrating g-C₃N₄/CoN₄ composites into real sensor architectures and benchmarking their performance under realistic conditions, thereby advancing non-invasive diagnostic tools for lung cancer.

Conflicts of interest

There are no conflict of interest to declare.

Data availability

All data used in the research article can be obtained from the corresponding author upon reasonable request. Data for this article are available at https://zenodo.org/records/17707981.

Supplementary information (SI): optimised structures of biomarkers, bond parameters of gas molecules and the band structure of g-C₃N₄/CoN₄ heterojunction. See DOI: https://doi.org/10.1039/d5ra04202j.

Acknowledgements

M. Chattopadhyaya acknowledges the SERB POWER GRANT (Fileno. SPG/2021/004752) for providing the funding of the project. Dhilshada. V. N acknowledges the UGC for providing the fellowship grant. Shilpa M acknowledges NIT Calicut for financial support. The author M. Chattopadhyaya also acknowledges the center for Computational Modelling and Simulation (CCMS), NIT Calicut, for computational support.

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