First-Principles Insight into a B₄C₃ Monolayer as a Promising Biosensor for Exhaled Breath Analysis

Abstract

Nanomaterial-based room temperature gas sensors are used as a screening tool for diagnosing various diseases through breath analysis. The stable planar structure of boron carbide (B₄C₃) is utilized as a base material for adsorption of human breath exhaled VOCs, namely formaldehyde, methanol, acetone, toluene along, with interfering gases of carbon dioxide and water. The adsorption energy, charge density, density of states, energy band gap variation, recovery time, sensitivity, and work function of adsorbed molecules on pristine B₄C₃ are analyzed by density functional theory. The computed adsorption energies of VOC are in the range of − 0.176 to − 0.238 eV, and a larger interaction distance validate the physisorption behavior of these VOCs biomarkers on pristine boron carbide monolayer. Minute changes are determined from the electronic band structure of all adsorbed systems conserving the semiconducting nature of the B₄C₃ monolayer. The band gap variation upon adsorption of VOCs and interfering gases is examined between 0.05 and 0.52%. The 13.63 × 10^–9 s recovery time of methanol is slower among VOCs, and 0.556 × 10^–9 s of carbon dioxide (CO₂) is faster for desorption. The results reveal that boron carbide can be utilized as a biosensor at room temperature for the analysis of exhaled VOCs from human breath.

Graphical abstract

Introduction

Following the severe consequences of Covid-19, researchers are pursuing novel methods for early disease detection. Existing capabilities for early disease detection through efficient, simple, low-cost and noninvasive mechanisms are desperately required. Human exhaled breath offers a reasonable, inexpensive, and rapid revealing practice.^1,2 Exhaled breath of human comprises of a mixture of water, carbon dioxide, oxygen, nitrogen and other traces of volatile organic compounds.^3,4 Volatile organic compounds (VOCs) identification has become an important objective.^5–8 The higher concentration of VOCs in indoor air is detrimental to human health. VOCs having low boiling points and higher vapor pressure, such as methanol, ethanol, formaldehyde, acetone, and ammonia, are the primary causes of indoor air pollution.^9–12 Exhaled human breath can be used to examine these molecules as biomarkers for a variety of disorders, including new coronavirus, liver cirrhosis, diabetes, lung cancer, and tuberculosis.^13–18 Around 870 VOCs carry information about dysfunction in the human body.^3,19 More than 3500 VOCs are identified in parts per million (ppm) or parts per trillion (ppt) concentration as a part of human breathing out.²⁰ Gas chromatography-mass spectroscopy used in clinical trials can detect VOCs concentration between 1 and 5000 parts per billion (ppb). As a screening tool, this technology requires more time and expensive equipment.^21–24 So enormous efforts are being made to develop simple and efficient sensors for detecting the lower concentration of VOCs at parts per billion.

Purposefully, semiconductor gas sensors²⁵ are extensively employed for the detection of VOCs with alteration of electronic conductivity after and before gas adsorption.^26–28 To boost the sensitivity of semiconductor-based gas sensors, low dimensionality and easily adjustable features of two-dimensional (2D) materials^29–31 have enchanted intensive research in inorganic^32–35 and organic^36–38 gas identification. 2D materials for example graphene,^39–43 phosphorene,^26,44,45 stanene,^46–48 arsenene,⁴⁹ silicone,^50,51 borophene,^52,53 molybdenum disulfide (MoS₂),^13,54–60 tungsten disulfide (WS₂)⁶¹ monolayers have been investigated for the adsorption of small gas molecules and VOCs as well. Numerous compounds, elemental crystals, and atomically thin materials with exceptional electronic and mechanical properties are formed by lighter elements such as carbon (C), boron (B), and nitrogen (N).⁶² Experimentally synthesized a chemically inert material, boron carbide sheets with boron as a dopant possess the same structure as graphene but exorbitantly changed characteristics to utilize in electronic devices.^63–65 Boron carbide (BC₃) monolayer serves as a gas sensor for harmful carbonaceous pollutants^66–69 and is highly selective and sensitive to acetone.⁷⁰ Another 2D monolayer, BC₆N, has been recently examined for human breath analysis⁷¹ and proved to be a potential candidate for ethanol.¹⁹ Among other 2D B-C compounds, a highly stable semiconducting B₄C₃ monolayer⁶² evolved from a hypothetically designed B4C3 cluster in 2000.⁷² The planar structure of B4C3 has been recently investigated as a toxic gas sensor for carbon monoxide (CO), sulfur dioxide (SO2), nitrogen oxide (NO), ammonia (NH3), and hydrogen sulfide (H2S).⁷³

Inspired by the thermal and dynamic stability of the B4C3 monolayer,⁷⁴ a comprehensive analysis is presented on the adsorption of various volatile organic compounds (VOCs) such as methanol (CH3OH), formaldehyde (H2CO), toluene (C7H8) and acetone (C3H6O) on B4C3 monolayer as familiar breath biomarkers. These molecules belong to aldehydes, ketones, and alcohols of different functional groups of hydrocarbons. Carbon dioxide (CO2) and water (H2O) are used as interfering gases expected to create hindrance in the detection of biomarkers. Density functional theory (DFT) is employed to explore the adsorption capabilities of VOCs. This research aims at early detection of disease by VOCs identification on B₄C₃ monolayer. The results explain the superior sensing properties of B₄C₃ adsorption energies, charge analysis, band structure, density of states, and recovery time.

Computational Methods

Based on DFT, Vienna Ab-initio Simulation Package (VASP) is adapted to carry out all calculations. The projector augmented wave method⁷⁵ is employed for ion–electron interactions. Perdew-Burke-Ernzerhof with generalized gradient approximation (PBE-GGA),^76,77 is implemented for exchange–correlation potential. Grimme’s DFT-D3 correction⁷⁸ is being applied for the adsorption properties of VOCs. During geometry optimization, the Brillouin zones are sampled using a 3 × 3 × 1 k-point grid Monkhorst–Pack scheme.³⁰ The plane wave basis is fixed at 500 eV. Using the quasi-Newton approach of Broyden-Fletcher-Goldfarb-Shanno (LBFGS), all of the structures with full relaxation of stress and force on each atom are evaluated to be less than 0.001 eV/ų and 0.01 eV/Å, respectively. The supercell size of 2 × 2 × 1 is being preserved for B₄C₃ monolayer.

For band structure computations, Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional⁷⁹ for band is being executed. The interaction between the adsorbent monolayer and adsorbed molecules is quantitatively described by the van der Waals (vdW) corrected adsorption energy and can be expressed as:

$$E_{\mathit{ads}}=E_{\mathit{total}}-E_{\mathit{monolayer}}-E_{\mathit{molecule}}$$ 1

where E_ads is the adsorption energy, E_total is the combined energy of monolayer and molecule. E_monolayer and E_molecule are the adsorption energies of an individual monolayer and the isolated molecule. The criteria used to determine the physical adsorption of VOCs on the monolayer is exothermic adsorption (< 0 eV). E_ads < 0.5 eV is an indicator of physisorption.⁸⁰ Stronger gas adsorption is indicated by the larger negative value of E_ads. Furthermore, the Bader charge method is obtained to analyze the behavior of charge transfer upon molecule adsorption.⁸¹ The charge density difference upon the adsorption of gas molecule is computed by the following expression.

$$\mathrm{\Delta }\rho ={\rho }_{\mathit{total}}-{\rho }_{\mathit{monolayer}}-{\rho }_{\mathit{molecule}}$$ 2

whereas ∆_ρ is the charge density difference. Total charge density of combined system is referred as ρ_total. The individual charge densities of monolayer and isolated molecule are represented by ρ_monolayer and ρ_molecule.⁸²

Results and Discussion

Pristine B₄C₃

The monolayer of B₄C₃ is a network of B₄C₃ ordered patterns that are arranged to each other structurally. Each B₄C₃ unit accompanying a B atom at the center is enclosed by hollow B₃C₃ rings, and the entire unit is coupled to the nearby B₄C₃ array by B-C bonds, as depicted in Fig. 1a. The central boron atoms of all B₄C₃ formations are positioned at ca. 0.3 Å above the hexagonal rings, giving the structure a buckled appearance. Two types of coordination exist in the boron atoms (B₁ and B₂). One of the B atoms (B₁) is placed in the middle of the hexa coordinated ring consisting of three C and three B₂ atoms. While the other atom, B₂ is positioned at the corner with tetra coordination comprising one B₁ and three C atoms. So the structure displays the B₁-B₂ bond distance as 1.69 Å closer to the B₄C₃H₆ cluster (1.77 Å) and 2D borophene (1.70 Å). Due to hyperconjugation and different geometrical positions, three types of B-C bonds appear; the first bond B₂-C (1.52 Å) connects the neighboring B₄C₃ cluster, and another bond B₂-C (1.55 Å) exists at the edge points of B₃C₃ ring, third bond B₁-C has the bond length of 1.59 Å. These bond lengths are in accordance with the BC₃ structure (1.58 Å). Additionally, the B-B bond is consistent with 2D boron. These observations suggest a strong connection between carbon and boron and carbon atoms with tetra coordination bridges B₄C₃ cluster. So the planar hyper coordinated (phC) carbon bonds comprising 2D materials exhibit exceptional physiochemical properties.^73,74

Fig. 1.

(a) The top and side views of B4C3 monolayer. (b) Band structure along with PDOS of pristine B4C3 monolayer. Grey color represents carbon and pink color represents boron (Color figure online).

Electronic Band Structure of Pristine B₄C₃

The B₄C₃ monolayer is a direct band gap semiconductor with a band gap of 1.915 eV. The band structure and partial density of states (PDOS) of B₄C₃ are illustrated in Fig. 1b. The valence band maximum (VBM) and conduction band minimum (CBM) lie at Γ point.⁷³ Fermi level lies at 0 eV. The electronic configurations of B and C are [He] 2s²2p¹ and [He] 2s²2p². It is clear from the electronic configuration that the p orbitals of B and C form conduction and valence bands, which is consistent with PDOS of B₄C₃.

This work investigates the adsorption geometries of VOCs, including C₃H₆O, CH₃OH, H₂CO and C₇H_8, along with CO₂ and H₂O. Each molecule is deposited on the B₄C₃ monolayer with a different orientation. Adsorption energies, charge analysis, total density of states, band gap, recovery time of adsorbed gases, and work function are analyzed for each adsorbed system on B₁ site and are enlisted in Table I. All the molecules show physisorption. B₄C₃ monolayer proves to be a good sensor for all physisorbed molecules.

Table I.

Absorption energy (E_ads) in eV, band gap (E_g) in eV and charge transfer of adsorbed molecules on B₄C₃ (Q) and recovery time (τ) in sec and work function (Φ) in eV

System	Eads (eV)	Eg (eV)	Q (e)	Recovery time τ (10–9 s)	Work function Φ (eV)	Nature of molecule
Pristine B4C3	–	1.915	–	–	4.961	–
H2CO-B4C3	− 0.194	1.916	− 0.023	2.345	4.946	Acceptor
CH3OH-B4C3	− 0.238	1.913	− 0.021	13.630	4.95	Acceptor
C3H6O-B4C3	− 0.176	1.917	− 0.004	1.141	4.885	Acceptor
C7H8-B4C3	− 0.21	1.914	− 0.005	4.447	4.806	Acceptor
CO2-B4C3	− 0.158	1.913	− 0.015	0.556	4.769	Acceptor
H2O-B4C3	− 0.196	1.925	− 0.013	2.540	5.071	Acceptor

A supercell of 3 × 3 is assumed to study the adsorption of gas molecules on the pristine B₄C₃ monolayer, comprising 32 B atoms and 23 C atoms. Each gas molecule prefers a specific molecular orientation. The alignment of molecules can be in a parallel or perpendicular direction with respect to B₄C₃ sheet.

VOCs Adsorption on B₄C₃ Monolayer

The most stable adsorption geometries of all adsorbed structures are being studied and illustrated in Fig. 2. Formaldehyde has a planar structure with one O atom and two H atoms connected by a central C atom⁸³.Figure 2a displays an optimized structure of H₂CO on the B₄C₃ monolayer taking a parallel tilted orientation of molecule with O atom away from the monolayer and H atoms facing towards the monolayer.

Fig. 2.

The optimized structures (top and side view) of (a) formaldehyde, (b) methanol, (c) acetone, (d) toluene, (e) carbon dioxide and (f) water molecules on pristine B₄C₃ monolayer. Grey, pink, green, and red colors represent Carbon, Boron, Hydrogen and Oxygen (Color figure online).

The preferable position of CH₃OH molecule on the B₄C₃ surface is parallel but with a tilted axis. The H of the OH group is facing towards the monolayer while methyl group (CH₃) is away from the surface, as presented in the top and side view of optimized structure in Fig. 2b. C₃H₆O molecule in Fig. 2c is oriented perpendicularly but tilted in such a way that one atom of H of the methyl (CH₃) group gets closer to the C of B₄C₃ substrate, and the other CH₃ group is away from the substrate. The carbonyl (C = O) group is at a distance to B₂ atom and is not parallel to the surface. C₇H₈ prefers to adsorb in a perpendicular position with carbon hexagon skewed towards the right and away from B₄C₃ monolayer and H of CH₃ being closer towards the surface as depicted in Fig. 2d.The position of CO₂ molecule is parallel to the B₄C₃ surface (Fig. 2e). However, one of the O atoms is slightly tilted towards the surface. H₂O prefers to adsorb on the B₄C₃ monolayer in the parallel orientation. O atom is pointing the monolayer surface outwards with the H atom facing toward the surface, as illustrated by the optimized structure in Fig. 2f.

For the development of efficient gas sensors, strong as well as weak interactions between the gas molecule and the substrate must exist. The strong interaction is required to retain the gas molecule on the substrate surface, and weak interaction is necessary for removing of gas molecules from the substrate without damaging the substrate’s properties.^19,54

Adsorption energy provides information about the strength/extent of interaction between the sensing substrate and the adsorbed gas molecule. The E_ads values of adsorption of gas molecules shLi et al., 2021ould be between − 0.40 eV (strong physisorption) to − 1.00 eV (weak chemisorption) for an efficient sensing mechanism.⁶⁹ A direct relationship exists between the adsorption energy and the sensitivity of the material. A larger value of adsorption energy leads to stronger interaction between the monolayer and gas molecules, which will enhance the sensitivity of the material. On the other hand, strong interaction makes desorption of gas molecules difficult from the material for a reusable gas sensors.^84–86

The adsorption energies of H₂CO, CH₃OH, C₃H₆O, C₇H₈, CO₂ and H₂O are computed to be − 0.194 eV, − 0.238 eV, − 0.176 eV, − 0.21 eV, − 0.158 eV and − 0.196 eV. These values confirm weak physical adsorption of all gas molecules onto the pristine B₄C₃ monolayer. These adsorption energies are lower than those reported for H₂CO (− 0.228 eV), CH₃OH (− 0.316), C₃H₆O (− 0.381 eV), C₇H₈ (− 0.910 eV), CO₂ (− 0.253 eV) and H₂O (0.263 eV) on pristine BC₆N nanosheet.¹⁹ The values of adsorption energies of C₃H₆O (− 0.320 eV) and C₇H₈ (− 0.432 eV) on black phosphorus⁸⁴ are also greater than calculated pristine B₄C₃. The interaction of H₂CO (− 0.091 eV) ,⁸⁷ CO₂ (− 0.03 eV), and H₂O (0.05 eV)⁸⁸ on graphene are found to be weaker than those accomplished for pristine B₄C₃ monolayer. The adsorption energy of H₂CO is − 0.556 eV on pristine carbon nitride (C₂N) monolayer.⁸⁹ The estimated adsorption energy values (i.e., − 0.18 eV, − 0.33 eV, − 0.40 eV, and − 0.60 eV) demonstrate subsistent interaction of H₂CO, CH₃OH, C₃H₆O, and C₇H₈ on pristine oxygen-terminated titanium carbide MXenes (Ti₂CO₂).⁹⁰ Adsorption of alcohol molecules on germanene⁹¹ and stanene⁹² nanosheets is prominent and occurs due to the presence of the OH group. In this scenario, the reported values of adsorption energy are − 1.76 eV for H₂CO, − 1.95 eV and − 1.65 eV for CH₃OH. The adsorption energies of CH₃OH (− 0.47 eV), C₃H₆O (− 0.69 eV), and C₇H₈ (− 1 eV) on oxidized molybdenum carbide MXene (Mo₂CO₂)⁹³ are greater than B₄C₃. It shows that B₄C₃ monolayer performs better than pristine graphene but is less sensitive than other 2D materials.

The small value of adsorption energy and the large value of the distance between the molecule and the substrate reveal physical adsorption due to weak van der Waals interaction.^94–96

After adsorption of gas molecules, B₁-C bond length slightly shortens up to 1.57 Å from 1.59 Å and B₂-C bond lengths slightly increase from 1.52 Å to 1.53 Å, and from 1.55 Å to 1.58 Å. The interaction distance between all the gas molecules and B₄C₃ monolayer is computed to be greater than 3 Å exhibiting weak physisorption of gas molecules.

Charge Density Analysis

Due to physisorption, gas molecules and monolayers exchange a minute amount of charge.¹⁹ The charge transfer is computed in terms of Bader charge analysis in order to understand the adsorption better. The Bader charge (Q) analysis determines whether the base material functions as a donor or acceptor. The charge transfer calculated between B₄C₃ monolayer and H₂CO, CH₃OH, C₃H₆O, C₇H₈, CO₂ and H₂O is − 0.023 e, − 0.021 e, − 0.004 e, − 0.005 e, − 0.015 e, and − 0.013 e, respectively.

The differential charge density for all six adsorbed molecules is also depicted in Fig. 3a, b, c, d, e and f. From Fig. 3, it is confirmed that all molecules acquire charge from B₄C₃ monolayer upon adsorption, acting as electron acceptors. The cyan color shows the depletion of charge, and the yellow color depicts the accumulation of charge. B transfers electrons to C of B₄C₃ framework indicating the chemical bonding between them. There also exists an ionic bonding in the framework. The cyan color in B₄C₃ framework shows the substrate is losing charge near the region of interaction, and the yellow color around the molecule indicates charge accumulation. O, the most electronegative atom, gains more charge from the molecule and the substrate in O containing molecules. In C₇H₈, C, with the electronegativity of 2.5, has greater power to attract electrons as compared to B (2.04) and H (2.20). B₄C₃ is an electron donor.

Fig. 3.

Differential charge density (top and side view) of (a) formaldehyde, (b) methanol, (c) acetone, (d) toluene, (e) carbon dioxide and (f) water molecules on pristine B₄C₃ monolayer. Cyan and yellow colors shows the charge depletion and accumulation. Colors represent Carbon (grey), Boron (pink), Hydrogen (green) and Oxygen (red) (Color figure online).

Density of States and Band Structure Analysis

Adsorption of molecules changes the electronic properties of the sensor.^19,97 For this purpose, the electronic band structure and partial density of states (PDOS) of pristine and gas molecules adsorbed on B₄C₃ monolayer are displayed in Fig. 4. The energy band gap of pristine B₄C₃ is computed as 1.915 eV. Slight change in band structure is observed in the presence of H₂CO, CH₃OH, C₃H₆O, C₇H₈, CO₂ and H₂O with band gaps of 1.916 eV, 1.913 eV, 1.917 eV, 1.914 eV, 1.913 eV, and 1.925 eV. These values are in close proximity with the small adsorption energy values of adsorbed systems.

Fig. 4.

Energy band structures and density of states of (a) pristine B₄C₃ monolayer, (b) formaldehyde, (c) methanol, (d) acetone, (e) toluene, (f) Carbon dioxide and (g) water molecules on pristine B₄C₃ monolayer. The dashed line indicates the Fermi level. Grey (carbon), Pink (boron), Green (hydrogen) and Red (oxygen), (h) Comparison of work function of pristine and adsorbed gas molecules on B₄C₃ monolayer, P: Pristine B₄C₃ monolayer, F: formaldehyde, M: methanol, A: acetone, T: toluene, C: carbon dioxide and W: water pristine B₄C₃ monolayer (Color figure online).

In Fig. 4, the Fermi level is represented by 0 eV, and the density of states spans from − 5 to 5 eV. B₂-p orbitals and C-p orbitals contribute to the VBM and CBM.⁷⁴ Above 0 eV, the peaks of p states of B are more pronounced in the conduction band. The contribution of O-p states (red color) are seen below the Fermi level in the case of H₂CO, CH₃OH, C₃H₆O, CO₂ and H₂O (Fig. 4b, c, d, e, f and g).The p orbital of C and s orbital of H of CH₃ interact with the hybridized orbitals of B₄C₃. The contribution of H states can be seen above 3 eV (above Fermi level) and below − 3 eV in the density of states (DOS) as depicted in Fig. 4e.

It shows that the adsorbed molecules have a minor impact on the electronic properties of pristine B₄C₃ monolayer. Although the adsorbed molecules contribute to the conduction and valence bands of pristine monolayer, they make no significant changes to the electronic characteristics of pristine B₄C₃ monolayer. The monolayer show semiconducting behavior after weak physisorption of molecules.¹⁹ Owing to larger distance and physical adsorption of molecules, orbital hybridization favors charge transfer between monolayer and gas molecules.⁹⁸

In summary, generally, more adsorption energy and a greater amount of charge transfer enhance the sensitivity of a sensor. The physical adsorption perturbs the electronic states of the gas molecule and the sensor to a minimum level.⁹⁹ Theoretically, sensing involves orbital interaction and charge transfer. However, from a practical perspective, more negative adsorption energy is not beneficial since it will be difficult to desorb the biomolecule for subsequent usage of the sensor.¹⁰⁰

Sensing Explanation of the B₄C₃ Monolayer

The energy band gap variation for all absorbed molecules is calculated from the following expression⁴⁹;

$$E_g^a\%=\left(\frac{E_{g(pristine)}-E_{g(adsorbed)}}{E_{g(pristine)}}\right)\times 100$$ 3

where $E_g^a$ is the average band gap energy, E_g(pristine) and E_g(adsorbed) represent band gap energies of pristine and adsorbed B₄C₃ systems. The adsorption of H₂CO, CH₃OH, C₃H₆O, C₇H₈, CO₂ and H₂O slightly alter the band gap energy. The energy band gap variation for CH₃OH, C₃H₆O and CO₂ is noticed as 0.10% and 0.05% for H₂CO and C₇H₈, while for H₂O, it is 0.52%. The variation supports the physisorption behavior of adsorbed molecules. Alteration in band gap results in the conductivity of the system, which is negligible in this case. The pristine B₄C₃ monolayer distinguishes the specific type of VOC based on the sensitivity. Sensitivity of the monolayer can be estimated by adsorption energy, charge transfer and band gap.^19,101 The B₄C₃ monolayer shows moderate sensitivity towards methanol. Moreover, surface functionalization^13,102 of monolayer and introduction of molecular sieve¹⁰³ will be utilized to avoid the interference between target VOCs and interfering gases.

For a reusable gas sensor, the gas molecules are capable enough to desorb themselves from the sensing material after detection.³¹ So the performance of a gas sensor can be evaluated by its recovery time (τ).¹⁰⁴ Using transition state theory, τ can be expressed as;

$$\tau ={\nu }_o^{-1}{e}^{(-E_{\mathit{ads}}/kT)}$$ 4

In the above expression, ν_o (10¹² Hz) is the attempt frequency for visible light, k is Boltzmann constant, T is the room temperature at 300 K, and E_ads is the adsorption energy of adsorbed systems. From this equation, a longer recovery time is expected for a large (more negative) value of E_ads, which will slow down the desorption process of gas molecules.¹⁹ The recovery time for all molecules is listed in Table I. The calculated recovery time for H₂CO, CH₃OH, C₃H₆O, C₇H₈, CO₂ and H₂O are 2.345 × 10^–9 s, 13.630 × 10^–9 s, 1.141 × 10^–9 s, 4.447 × 10^–9 s, 0.556 × 10^–9 s and 2.540 × 10^–9 s. It shows that the CH₃OH (τ = 13.630 × 10^–9 s) and CO₂ (τ = 0.556 × 10^–9 s) possess the highest and lowest recovery time at a constant temperature. The Desorption process of CH₃OH is slower, while the Desorption of CO₂ is faster than other molecules. The reversibility of the sensor can be better achieved as the VOCs are physically adsorbed on the B₄C₃ monolayer with ease of desorption process.^18,98

Work Function

The electrical conductivity of a system is related to its work function alteration.⁶⁹ The adsorption of gases on the sensing material creates surface charge redistribution, which will alter the material’s work function (Φ). The energy required to remove an electron from Fermi level to infinity is known as the work function. It is calculated by the following expression;

$$\mathrm{\Phi }=V\left(\mathrm{\Phi }\right)-E_f$$ 5

Here, V (Φ) represents the electrostatic potential at the vacuum level, and E_f denotes the potential at the Fermi energy level. Φ is sensitive to any change that occur at the surface due to gas adsorption.¹⁰⁵ The transmission of charge between the sensing material and gas molecules establishes a dipole moment in the physisorption process. There will be a variation in the work function, which may lead to more changes in the sensing properties of an adsorbed system. The values of work function are summarized in the Table I. The work function for pristine B₄C₃ monolayer is 4.961 eV. The adsorbed gas molecules of H₂CO, CH₃OH, C₃H₆O, C₇H₈ and CO₂ show a minor decreasing trend from 4.95 eV to 4.769 eV as displayed in Fig. 4h. The work function for H₂O adsorbed system is computed to be 5.071 eV, slightly greater than 4.961 eV of pristine B₄C₃. The greater value of work function of H₂O from pristine B₄C₃ monolayer means the electron overflow from Fermi to vacuum level in pristine monolayer is obstructed by H₂O adsorption. However the reduction in work function exhibits greater electron mobility.¹⁰⁶ The results reveal an almost negligible change in the work function of all systems compared to pristine monolayer. This indicates a very weak physisorption of all gas molecules. Nevertheless the change in work function shows that B₄C₃ monolayer is favorable for gas sensing.

Conclusion

Gleaned from the framework of DFT, this proposed study computes the structural and electronic properties of human breath exhaled VOCs adsorbed onto boron carbide monolayer. The adsorption energies, charge analysis and band structure, recovery time, and work function of VOCs, including formaldehyde, methanol, acetone, Toluene and interfering gases of CO₂ and H₂O are being studied. When VOCs adsorb on B₁ site of pristine B₄C₃ monolayer, their adsorption energies fall in between 0.158 eV and 0.238 eV. The results reveal physisorption nature of all molecules. Among all other molecules, methanol shows the highest adsorption energy. O functionalized VOCs display less adsorption energies. Due to very weak physisorption and large interaction distance between molecules and monolayer, less charge has transferred from boron carbide monolayer to adsorbed molecules. So, VOCs along with CO₂ and H₂O are exposed to be charge acceptors. The band gap of the adsorbed systems does not change and lies in the range of 1.913–1.925 eV. Less apparent changes are discovered (0.05% to 0.52%) in the electronic properties of boron carbide layer after the interaction with VOCs. The calculated work function for H₂O is greater (5.071 eV) and lower for CO₂ (4.769 eV) compared to pristine B₄C₃. The recovery time for CO₂ is greater than all other considered molecules. The results conclude that pristine B₄C₃ monolayer can be used as a potential candidate for sensing VOCs for breath analysis.

Declarations

Conflict of interest

The authors declare no conflict of interest.