Introduction

H2S, HF and H2 are toxic and colorless gases, and they are common industrial and environmental pollutants, and also became extensive range sources of health effects1,2,3. Additionally, due to rapid industrialization, universally use of chemicals, transportation of automobiles has led to increasing global environmental pollution and diminution of the ozone layer, which has expected a great hazard and major threat to the human health4,5,6. However, the arena of gas sensing has always concentrated on the research in control of environmental pollution, monitoring of industrial activities, health diagnostics and noxious gas sensing’s sake of public safety7,8,9. In recent years, numerous nanomaterials have been scrutinized for gas sensing applications. In spotlight, the two-dimensional (2D) nanomaterials have castoff to sense a low concentration of gas molecules in sensing applications because of their compensations like, high stability, novel structures, large surface area/volume ratio, excellent adsorption abilities and low cost10,11,12,13,14,15,16,17,18,19. For example, graphene20, Phosphorene21, carbon nitride compound22, B4C3 monolayer23, InN monolayer24 and transition-metal dichalcogenides (TMDs) like MoSe2, PtSe2, SnS2 and HfS2, etc.25,25,26,27,28,30, have become the burgeoning materials as toxic gases sensor.

Moreover, the key foundations of (i) H2S gas are from volcanic gas, fuel, sewage plants, sulfur deposits and ammonia synthesis from hydrocarbon feedstock31,31,32,33,34,35,37. Although, very recently, a study shows B4C3 nanosheet, as excellent gas sensor for CO, NO, NH3, SO2 and H2S23 and Gao et al.38, testified the strong adsorption capacity of H2S and CH4 with the modified 2D graphene sheet. Then, (ii) HF gas is also eminent toxic gas it is mainly originate from industrial processes at high temperature and the combustion of products containing fluoride, which is harmfully affect to not even only human but also flora and fauna. The high exposure of HF gas causes, muscle spasms and result may even in fatality at extreme cases2.

Bhattacharya et al.38, also studied the sensing of HF, HCN and H2S with nitrogenated holey carbon (C2N-h2D) monolayer. Next, (iii) H2 a tasteless, odourless and flammable gas. Even, by lacking O2 concentration in enclosed area, only 4% concentration of H2 gas could be reasons to burnt and suffocation39. Li et al.40 also explored Ru-doped PtSe2 monolayer as good sensor of H2 gas and Pandey et al.41 also suggested that MoS/WS monolayer is potential substrates for gas sensing of H2, NO and CO gases.

Apart from aforementioned two-dimensional (2D) materials gas sensor, the newly discovered pentagonal structures had been also showed to design gas sensor due to its superior quality such as high surface carrier mobility, large surface area, more adsorption sites and great optical properties over the traditional unit hexagon structure42,43. Additionally, in past, several 2D pentagon unit structure has also been reported for the promising candidates for innovative applications in the gas sensor. Like, Wei et al.44 predicted that penta-BCN monolayer is good sensor of CO, H2S, NH3 and NO gas molecules and stated that the targeted gases have moderate adsorption energies in the range of − 0.797 eV to − 1.186 eV and proved it as potential applicant for said gas molecules sensors applications. Lakhani et al.45 shows the dissociation of air pollutants on the uniform surface of pentagonal BeP2 monolayer using first principles study. Xia et al.46 also explored that penta-PdSe2 monolayer has meaningful and promising material to be applied in FET type gas sensors as detection of NO2 gas. Then, Tang et al.47 revealed the new sensor material P-SiC2 monolayer and concluded that after the contact with NO2 gas the electronic resistance of P-SiC2 monolayer is decreased significantly, which indicating the ultrahigh sensitivity towards NO2 sensing. Hereafter, inspired by these Penta two-dimensional (2D) gas sensing research work owing to their robust adsorption capabilities and sensitivity, herein we are first time examined H2S, HF and H2 gas sensing capabilities on the newly discovered novel penta-PdAs2 monolayer, because modelling new gas sensing resources have paramount bid to probe the pollutant molecules in the current era. Viz, H2S, HF and H2 gases. And finding an effective material to detecting and adsorbing these toxic (H2S and HF) and highly flammable (H2) gases are great worth to protect our living environment for modern civilization. Even though, the outstanding electronic properties, transport, and optical properties of pristine penta-PdAs2 monolayer is previously done by our research group48.

In this work, the capacity of monolayer penta-PdAs2 as gas sensor (of H2S, HF and H2 gases) capture is discovered by first-principles calculations. As density functional theory calculation is a widespread and feasible approach for simulation of gas adsorption on the novel nano-surfaces. Henceforth, the adsorption process of H2S, HF and H2 gases on penta-PdAs2 monolayer was studied and compared by evaluating differences in adsorption energy (Ead), appearance change (e), charge transfer (Q), work function (Φ), charge density difference (CDD), electronic band structure, density of states/eV (DOS) and partial density of states/eV (PDOS) in this research note. These observations deliver quest of sensor materials with high sensitivity and selectivity towards pollutant gases such like H2S, HF and H2.

Simulation details

Density Functional theory (DFT) calculations were executed using SIESTA code49 to examine the optimization, electronic and adsorption (of targeted gases) properties of penta-PdAs2 nanosheet. To treat the exchange–correlation (XC) effect, the generalized gradient approximation (GGA) of the Perdew–Burke–Ernzerhof (PBE)50 functional was employed. We incorporated the DRSLL dispersion correction method to treat the long range vdWs forces51. The Kohn–Sham one electron states were expanded in a plane-wave basis set. The plane wave kinetic energy cut-off is considered as 450 Ry. The doubled zeta plus (DZP) basis set was used with an energy of 0.01 Ry to expand the Kohn–Sham orbital. The Brillouin zone sampled using a 5 × 5 × 1 Monkhorst–Pack k-point mesh during structural optimization52. The k-point mesh was then increased to 15 × 15 × 1 to attain more accurate results for electronic structure simulations. The optimized structures of 3 × 3 × 1 supercell penta-PdAs2 monolayer is shown in Fig. 1a. As aforementioned, potential properties of penta-PdAs2 monolayer such as structural, electronic, transport and optical properties have already reported by our group48 and proven that it is dynamically stable via the positive phonon frequencies of penta-PdAs2. Hence, using same substrate of penta-PdAs2, herein we reported the sensing potential in the penta-PdAs2 monolayer for H2S, HF and H2 gases. All the optimized structures were relaxed without given any geometric constraint (GC) on the sheet. The atomic force is specified as 0.01 eV/Å to relaxing the systems. A vacuum of 15 Å have been considered along the perpendicular direction of the penta-PdAs2 surface to avoid the interaction of periodic boundary conditions as employed in XY-plane. The adsorption energy is determined by the following equation53:

$$E_{ads} = E_{{{\text{nanosheet}} + {\text{Gas}}}} {-}E_{{{\text{nanosheet}}}} - E_{{{\text{Gas}}}} ,$$
(1)

where, Enanosheet+Gas represents the total energy of the penta-PdAs2 nanosheet with the gas molecules absorbed on, while Enanosheet and EGas are the total energy of penta-PdAs2 sheet without the gas molecules and the isolated gas molecules, respectively. From the adsorption energy definition, a negative Eads specifies an exothermic process and thermodynamically favourable for molecular gas sensing. All the initial distances between sheet and gas molecules are set to 2.0 Å. The Hirshfeld atomic population analysis method was used to estimate the charge transfer (Q) as defined in SIESTA and negative value of Q means that electron transfer sheet to gas molecules. The charge density difference (CDD) Δρ between the adsorbed and isolated gases is calculated by:

$$\Delta \rho = \rho_{{{\text{nanosheet}} + {\text{Gas}}}} - \rho_{{{\text{nanosheet}}}} - \rho_{{{\text{Gas}}}} ,$$
(2)

where, ρnanosheet+Gas, ρnanosheet and ρGas are represent the charge densities of penta-PdAs2 monolayer with, without the adsorption of gas and charge density of gas molecules, respectively. This CDD has been imagined with utility of VESTA software54. The recovery time (τ) is the crucial parameter for gas sensor, which can be measured experimentally ranging from second to several minutes. Based on activated-complex theory55, recovery time has been defined by:

$${\uptau = \omega }^{-1}\mathrm{exp}\left(-\frac{{E}_{ads}}{{K}_{B}T}\right),$$
(3)

where, ω is the attempt frequency (~ 1012 s−1), Eads is adsorption energy, KB is Boltzmann’ Constant and T is temperature. At last, based on non-equilibrium Green’s function (NEGF), the IV response was examined by TRANSIESTA package56. The current (I) through the scattering region is calculated by exploring the Landauer-Buttiker formalism,

$$I\left({V}_{bias}\right)={G}_{0}{\int }_{{\mu }_{L}}^{{\mu }_{R}} T\left(E,{V}_{bias}\right) \left[f \left(\mathrm{\rm E}-{\mu }_{L}\right)-f \left(\mathrm{\rm E}-{\mu }_{R}\right)\right] dE,$$
(4)

where, G0 is quantum conductance and T (E, Vb) is the transmission probability of an electron incident at an energy E through the device under the bias voltage Vbias, \({\mu }_{L}\) and \({\mu }_{R}\) are the electrostatic potentials of left and right electrodes.

Figure 1
figure 1

Side and top view of the (a) Bare penta-PdAs2 monolayer and after gas adsorption of (b) H2 (c) H2S and (d) HF. The Pd, As, H, S and F atoms are indicated in navy blue, pink, water blue, red and yellow colors, respectively.

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Results and discussions

Adsorption of H2, H2S and HF gases on penta-PdAs2 monolayer

In the first step, we have systematically examined the geometries of pristine penta-PdAs2 monolayer. The optimized structure of penta-PdAs2 is displayed in Fig. 1a. The bond length between Pd–As and As–As are 2.51 Å and 2.32 Å, respectively, which are comparable with previous reported studies57. To study the electronic properties such as the electronic band structure, DOS and PDOS were calculated and the obtained electronic band gap (Eg) of penta-PdAs2 is 0.34 eV48. In the second step, we have optimized the gas molecules of H2S, HF and H2 on the surface of penta-PdAs2 sheet. We have placed the gas molecules at all possible parking sites on the sheet and initially molecules are fixed at the (i) Hollow (ii) Top of Pd atom (iii) Top of As atom (iv) Bridge of Pd-As and (v) Bridge of As-As atoms. The adsorption energy of each case is reported in ESI (Table S1, ESI) and highest adsorption energy site is considered for the further calculations. Therefore, the most favorable sites of H2S and H2 gases are perceived at bridge of Pd-As with adsorption distance of 2.91 Å and 2.70 Å, respectively. While HF gas is relaxed on the top of Pd atom with distance of 2.34 Å. Although, after the relaxation of that sites, the gas molecules are shifted on the top of Pd atom in all cases, which is shown in Fig. 1b–d. After the stable adsorption, the bond length between H–S, H–H, and H–F are 1.37, 0.77 and 0.94 Å, respectively and the bond angle of non-linear H2S is 92.142°, which is agreed with previous reported work58.

Table 1 presents the adsorption energy (Eads), adsorption distance (d) and relaxation time (τ) and type of interaction for considered H2S, HF and H2 gas molecules on the surface of penta-PdAs2 monolayer. As shown in table, the calculated adsorption energy of H2S, HF and H2 gases are − 0.49, − 0.39 and − 0.16 eV, respectively. The Eads values of H2S, HF and H2 were negative, suggesting that the adsorption process was all energetically favorable on the penta-PdAs2 monolayer and all in the physisorption range. Among three gases, H2S (HF) has strongest with 0.49 (0.39) eV compared to H2, it has the weakest adsorption strength about Eads ~ 0.16 eV. Although, the obtained adsorption energy of H2 is 18.75% higher compared to adsorbed on phosphorene (0.13 eV) nanosheet59. Additionally, Majidi et al. reported the HF and H2S detection on the twin graphene and Ti-embedded twin graphene with quoted adsorption energy of − 0.16 and − 0.22, respectively58. Which means, present results are quite good compared to them as H2S and HF gas sensor on penta-PdAs2 sheet. On other side, the obtained adsorption energy of H2S gas on penta-PdAs2 monolayer is − 0.49 eV, which is smaller than penta-BCN monolayer (− 0.797 eV)44, but relatively larger than functionalized graphene58 and phosphorene (− 0.41 eV)21. Further, it also be noted that the recovery time (τ) plays a crucial role in sensing application, τ is also summarized in the Table 1. The obtained sequence of the recovery time (τ) for three gases are as follow: H2 < HF < H2S. Although, the recovery time of H2S is very long owing to the strong adsorption capacity of PdAs2 + H2S (0.49 eV). Hence, the low value of recovery time strongly specified that PdAs2 + H2S configuration have high selectivity and can be good choice as reversible sensors of H2S.

Table 1 The adsorption energy (Eads), adsorption distance (d), relaxation time (τ) and interaction type.
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Moreover, the charge transfer mechanism was studied by Hirshfeld atomic population analysis and results are summarized in Table 2. The value of Q < 0 indicates that H2S (− 0.137 e) gas molecules accept the electrons from the PdAs2 sheet. whereas Q > 0, in HF (+ 0.111) and H2 (+ 0.037) indicates that both gas molecules are donating electrons to the PdAs2 sheet as result of that holes are expected to stay in the gas molecules as depicted in Fig. 2 by the blue arrow. Figure 2a–c, shows the charge density difference (Δρ), which is obtained by the Eq. (2). From that, it can be observed that, charge is accumulated at the negative charge region (shown as green color) and charge is depleted in the positive charge region (shown as orange color) due to the reformed of the effective interface dipoles and polarization of electrons. Where adsorption energy (Eads) is taking place to form the redistribution of electrons at the interface region. Here, among the three gases, the H2S has higher charge transfer between the adsorbing PdAs2 sheet suggesting that H2S has effective and strong detection with high selectivity towards the penta-PdAs2 nanosheet compared to the HF and H2 gases.

Table 2 The electronic band gap (Eg), charge transfer (Q), work function (Φ) and style.
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Figure 2
figure 2

The charge density difference (CDD) and charge transfer of (a) H2, (b) H2S and (c) HF molecules after absorbed. Green and orange color represent accumulation and depletion of charge, respectively. The isovalue used for plotting is 0.03 e/Å3.

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The surface sensitivity of the 2D nanosheet could also approached by determining the differences in work function parameter (Φ)60 as illustrate:

$$\Phi \, = \, V_{\infty } - \, E_{F} ,$$
(5)

where, V∞ and EF are the electrostatic potential and the fermi energy level, respectively. Basically, the work function describes the minimum energy needed to dislodge an electron from the surface of penta-PdAs2 monolayer. The calculated work function of penta-PdAs2 with H2S, HF and H2 lies in the range of 4.63 to 4.70 eV, as tabulated in Table 2 and electrostatic potential energy level is presented in in Fig. 3a–d. The red and black dashed line indicates the fermi energy level (EF) and energy vacuum level (V∞), respectively and the black double head arrow gives the work function values. The value of Φ of bare graphene is 4.5 eV which shows it as an ideal material for where work function optimization is vital61. As shown in Table 2, the Φ of bare penta-PdAs2 monolayer is 4.63 eV that is slightly larger than the graphene61. The nature of the Φ is symmetric for H2S, HF and H2 gas molecules with Φ value of 4.573 eV, 4.708 eV and 4.634 eV, respetively. Because of the adsorption of gases, the Φ is increase in all the cases compared to bare PdAs2 monolayer demonstrating that the electrons are transferring to the vacuum level is obstructed. Therefore, the Φ can be effectively affected by the adsorption of H2S, HF and H2 gas molecules on PdAs2 surface, which implies that Schottky barrier height can be adjusted by the H2S, HF and H2 gas molecules.

Figure 3
figure 3

The planar average of the electrostatic potential of (a) bare monolayer and adsorbed with (b) H2, (c) H2S and (d) HF gas molecule.

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Electronic properties

In order to deeper elucidate the gas sensing behavior of penta-PdAs2 monolayer towards H2S, HF and H2 gases, we analyzes the total density of states/eV (TDOS), projected density of states/eV (PDOS) and electronic band structure of gas molecules absorbed along that of bare PdAs2 monolayer system. Figure 4a–d, shows the density of state before and after the adsorption of H2S, HF and H2 gases on the PdAs2 surface. For comparison, the electronic band struture and TDOS and PDOS also demostrated in the ESI (Figs. S1, S2, ESI).

Figure 4
figure 4

Total density of states/eV of (a) penta-PdAs2 with adsorbed (b) H2, (c) H2S and (d) HF gas molecules.

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Visibly, it is confirmations no significant change either the electronic conduction or valence bands in the band structure after the adsorption of the gas molecules but the electronic band gap (Eg) is decreases from the bare counterparts. As tabulated in Table 2, the Eg decreases to 0.28 and 0.30 as a result of H2S and HF(H2) adsorption on pristine PdAs2 monolayer, respectively. The trivial impact of H2S, HF and H2 molecules in the unoccupied states of the PdAs2 monolayer above the fermi level is highly responsible to decreases the electronic band gap due to π to π* transition at S-point after the adsorption of the gases. Additionally, the valance bands (VBs) and conduction bands (CBs) have new sub-bands are outcome from the adsorption of H2S, HF and H2 gases on PdAs2 monolayer. The presence of new sub-bands justifies the variations of the electronic energy levels due to the adsorption of three gases. Therefore, TDOS/eV and PDOS/eV in further utilized to investigate the energy levels of bare PdAs2 prior and after adsorption of H2 H2S and HF as shown in Fig. 4a–d. It can be observe from Fig. 4a–d, the significant contribution in the DOS comes from the Pd-d and As-p orbitals with a small contribution from H, S and F atoms at the fermi level. From the DOS of the H2 adsorption (Fig. 4b), one can see that PdAs2 monolayer after adsorbing upholds the semiconducting feature with electronic band gap of 0.30 eV and H2 gas molecules acts as electron donor. However, there is no more influence seen at fermi level between PdAs2 + H2 and bare PdAs2 monolayer. But, the 1s orbital of H atom in the H2 is mainly hybridizied with the 4p orbital of the As atom and localized between 5 to 6 eV in the conduction bands as shown in Fig. S2a. While in case of H2S absorption (Fig. 4c), the highest occupied molecules orbital (HOMO) was mainly dominated by the interrelation of the H-1s and S-3p orbital of the H2S gas molecules and induces distinct states around 3.5 to 4.5 eV in the CBs. The same trend also follows in the adsorbing of H2S gas molecules on the CoOOH sheet surface62. The relatively higher adsoprtion energy and charge transfer of H2S on the PdAs2 sheet surface was due to the hybridyzation between the Pd-4d and S-3p states near the fermi level in the VBs as depicted in Fig. S2b. Similarly, after the adsorption of the HF gas molecules (Fig. 4d), the energy around 4 to 6 eV is arised due to the mixing of the H-1s and F-2p orbital in the HOMO region. While strong contribution of the hybridization of the F-2p (− 4 to − 2 eV) orbitals and Pd-4d are responsible for the next higher adsorption energy of the PdAs2 + HF system and acts as the electron donor near the fermi energy level as viewed in Fig. S2c. Likewise, very recently, Kaur et al. also reported the 2D janus WSSe monolayers as efficient nanosensor towards toxic HF gas molecules and the PDOS of adsorbed HF gas molecules is also contributed as same as here presented results63. Therefore, the remarkable ability of penta-PdAs2 monolayer makes a potential candidate for the H2S gas sensing applicatons.

Electric transport properties of adsorbed H2S, HF and H2 gases on penta-PdAs2 monolayer

To quantitatively probe the gas sensing properties of the PdAs2 being an efficient nano sensor, non-equilibrium Green’s function formalism (NEGF) was employed to analyze the current (I)-voltage (V) response64. The structural schematic model of sensing device set up are shown in Fig. 5. Where, the shaded region shows the electrodes which were simulated within the a 3 × 3 supercell of bare PdAs2 monolayer. The central or scattering region exemplifies the device area, where gas adsorption take place. Figure 6a,b, exhibits the I–V characteristics of PdAs2 before and after gas molecules exposure.

Figure 5
figure 5

Schematic structural model of a gas sensor based on penta-PdAs2 + H2S with two electrodes. The Pd, As, H and S atoms are indicated in navy blue, pink, water blue and red colors, respectively.

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Figure 6
figure 6

The current–voltage charateristics of adsorbed (a) H2, H2S and HF gas molecules, (b) bare PdAs2 monolayer.

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Although the adsorption of H2S, HF and H2 gas molecules does not affect much the electronic structure pointedly, but the charge transfer (Q) after their exposure is expected to vary due to doping of electrons or holes during the electron transportation. And these effects are revealed to be possible indication for gas sensors. As shown in Fig. 6b, In the case of bare PdAs2, we found that the current response in the range of micro-ampere (µA). Further, the junction remains switched off at the 0 V but seems switched on with the increasing of bias voltage. Then, noticeably above the 0 V, all the absorbed gas systems have a relevant effect on the transport from the bare one, which making all systems for gas sensing applications.

As seen in Fig. 6a, after the gas exposure the current is reducing from the bare PdAs2 and gives response in order of nA. When bias over 0 V, the current starts to increase simultaneously up to 1.6 V for all cases. Interestingly, after the bias of 1.6 V the current response trends change slightly for all absorbed gas molecules. At V = 1.8 V, the highest current sensitivity (current of 0.32 nA) with current is observed for the PdAs2 + H2S system compared to HF and H2 system. Which could be due to acceptor nature of the H2S gas molecules. Further, in the H2 adsorption (Fig. 6a), the current is significantly enhanced and reached up to 0.58 nA at the bias of 2 V. While it calibrate, for H2S (with current of 0.35 nA) and HF (with current of 0.33 nA) gas exposure a current reduction is about 60% and 56.89%, respectively compared to H2 gas exposure under the bias of 2 V as summarized in the Table 3. Overall, all the results indicated that the penta-PdAs2 monolayer could be better applicants for achieving the sensitivity and selectivity towards H2S compare to HF and H2 gas molecules.

Table 3 The current ratio and current variation of H2, H2S and HF absorbed PdAs2 nanosheet at the bias voltage of 2.0 V.
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Conclusion

In summary, the noxious H2S, HF and H2 gas molecules on a penta-PdAs2 nanosheet are first time investigated within the density functional level of theory including van der Waals corrections. We systemically discovered the most favorable binding site of each molecule based on the five different adsorption positions. The adsorption energy (Eads), charge transfer (Q), recovery time (τ) and work function (Φ) were investigated to understand performance and behavior of adsorption of the pollutant on PdAs2 surface. The gas sensitivity order of pollutant gases was predicted as follow: H2S > HF > H2 on the penta-PdAs2 nanosheet. The obtained recovery time (τ) of the sensor to all target gases are less than 3 min. The adsorption of H2S and HF results in an obvious increasing in DOS near EF, which is not observed by the adsorption of H2. Eventually, the IV response were carried out to study the responses of bare and absorbed PdAs2 nanosheet and the sharp rises seen at V = 1.8 V in order of H2S > HF > H2 that shows the authenticated potential as efficient gas sensor. These outcome acclaim the exciting prospects of developing penta-PdAs2 monolayer for the ultrahigh-sensitivity gas sensing nano-devices.