First‑Principles Study of Small‑Molecule Adsorption on Penta‑Graphene for Gas‑Sensing Applications

Background

Detecting hazardous gases is vital for environmental, industrial, agricultural, and medical safety. Two‑dimensional materials, with their large surface area and high carrier mobility, are increasingly considered for gas‑sensing devices. Their electronic properties can be tuned by gas adsorption, which changes the material’s resistivity or conductivity. Graphene, while highly conductive, has a zero band gap and weak intrinsic gas adsorption, limiting its sensor performance. Functionalization or alternative 2D allotropes are therefore explored.

Penta‑graphene (PG) is a recently predicted carbon allotrope composed of carbon pentagons. It is an intrinsic quasi‑direct‑band‑gap semiconductor (1.52–4.48 eV) and possesses both sp³ and sp² bonds, giving it a corrugated surface that offers multiple adsorption sites. Previous studies have highlighted PG’s catalytic potential for CO oxidation and hydrogen storage, but its gas‑sensing properties remain largely unexplored.

To address this gap, we employed first‑principles calculations to investigate how CO, H₂O, H₂S, NH₃, SO₂, and NO interact with monolayer PG, providing insights into its suitability as a gas‑sensor platform.

Methods

Structural optimizations were performed using the Dmol³ code with the local‑density approximation (LDA) of Perdew‑Wang (PWC). Van der Waals interactions were included via the Ortmann–Bechstedt–Schmidt correction. A 2×2×1 Monkhorst–Pack grid sampled the Brillouin zone, and convergence thresholds were set to 1×10⁻⁵ Ha (energy), 0.002 Ha/Å (force), and 0.005 Å (displacement). Spin polarization was applied for NO adsorption.

A 3×3 supercell of PG (30 Å vacuum) was used, comprising two sp³ (C1) and four sp² (C2) atoms. Gas molecules were positioned horizontally at 3.5 Å above the surface and optimized from four candidate sites: top of C1 (T1), top of C2 (T2), groove center (T3), and opposite groove (T4). Adsorption energies (E_a), charge transfer (Q), and equilibrium distances (d₂) were calculated using GGA functionals PW91 and PBE for comparison with LDA results.

Adsorption energy is defined as:

E_a = E_(PG+mol) − E_PG − E_mol.
Charge transfer (Q) was obtained from Mulliken population analysis; a negative Q indicates electron donation from PG to the gas.

Results and discussion

Optimized PG lattice parameters (l₁ = 1.342 Å, l₂ = 1.551 Å, θ = 133.9°, d₁ = 0.612 Å) agree with prior reports. The most stable adsorption geometries for each gas are illustrated in Figure 2 and listed in Table 1.

Adsorption energies (E_a) for CO, H₂O, and NH₃ are −0.531, −0.900, and −1.069 eV, respectively—significantly higher than those on InSe or graphene, indicating strong physisorption. H₂S and SO₂ exhibit even larger E_a values (−1.345 and −1.212 eV), reflecting robust interaction. CO shows the weakest adsorption, while NO’s E_a is comparatively low because chemical bonding deforms PG, consuming additional energy.

All adsorbates except NO are physically bound, as d₂ values exceed the sum of covalent radii. NO’s d₂ (1.541 Å) lies within the covalent range, confirming chemical adsorption.

Charge transfer analysis reveals that CO, H₂O, H₂S, and NH₃ donate electrons to PG (Q = 0.023–0.169 e), whereas SO₂ and NO accept electrons (Q = −0.109 and −0.030 e). This substantial charge exchange surpasses values reported for InSe and graphene, underscoring PG’s sensitivity.

Density of states (DOS) calculations show significant contributions of the adsorbates near the Fermi level, especially for H₂S, NH₃, SO₂, and NO, indicating electronic structure modulation. Band‑gap reductions upon adsorption are observed: CO (2.15 eV), H₂O (2.02 eV), H₂S (1.86 eV), NH₃ (1.81 eV), SO₂ (1.61 eV), and NO (0 eV), confirming that PG’s semiconducting behavior is strongly altered by gas binding.

Projected DOS and electron localization function (ELF) analyses confirm chemical bonding between NO and PG, with orbital mixing between NO’s N p and O p and PG’s C s/p states. Other gases show no chemical bonds, reinforcing their physisorptive nature.

Conclusions

Our first‑principles study demonstrates that H₂O, H₂S, NH₃, and SO₂ are physically adsorbed on monolayer PG with appreciable adsorption energies and charge transfer, whereas CO shows weak physisorption. CO, H₂O, and H₂S, along with NH₃, donate electrons to PG, while SO₂ and NO withdraw electrons. The electronic structure of PG is significantly altered upon adsorption of all gases except CO, as evidenced by DOS and band‑gap changes. NO chemisorption suggests PG could serve as a sensitive detector or catalyst for NO. Overall, pristine PG exhibits promising characteristics for next‑generation gas‑sensing devices.

Abbreviations

2D:

Two‑dimensional

DFT:

Density functional theory

DOS:

Density of states

ELF:

Electron localization function

GGA:

Generalized gradient approximation

LDA:

Local‑density approximation

PBE:

Perdew–Burke–Ernzerh

PDOS:

Projected density of states

PG:

Penta‑graphene

PW91:

The Perdew‑Wang 1991

PWC:

Perdew–Wang correlational