First-Principles Insights into Highly Sensitive and Reusable MoS2 Monolayers for Heavy Metal Detection

This study explores the potential of MoS2 monolayers as heavy metal sensors for As, Cd, Hg, and Pb using density functional theory (DFT) and Non-Equilibrium Green’s Function (NEGF) simulations. Our findings reveal that As and Pb adsorption significantly alters the surface structure and electronic properties of MoS2, introducing impurity levels and reducing the band gap. Conversely, Cd and Hg exhibit weaker interactions with the MoS2 surface. The MoS2 monolayer sensors demonstrate exceptional sensitivity for all four target heavy metals, with values reaching 126,452.28% for As, 1862.67% for Cd, 427.71% for Hg, and 83,438.90% for Pb. Additionally, the sensors demonstrate selectivity for As and Pb through distinct response peaks at specific bias voltages. As and Pb adsorption also induces magnetism in the MoS2 system, potentially enabling magnetic sensing applications. The MoS2 monolayer’s moderate adsorption energy facilitates rapid sensor recovery at room temperature for As, Hg, and Cd. Notably, Pb recovery time can be significantly reduced at elevated temperatures, highlighting the reusability of the sensor. These results underscore the potential of MoS2 monolayers as highly sensitive, selective, and regenerable sensors for real-time heavy metal detection.


Introduction
Heavy metals such as arsenic (As), cadmium (Cd), mercury (Hg), and lead (Pb) are known for their acute toxicity and carcinogenic properties, even at minute concentrations.Furthermore, they resist biodegradation in the environment.Found in water bodies, air, and soil, these pervasive environmental contaminants enter the human body through various pathways, including inhalation, the food chain, and absorption through the skin and gastrointestinal tract.Upon entry, heavy metals react with organic molecules in the body to form metal complexes, altering the original biochemical functions of substances and thus posing significant health risks.
In the environment, heavy metals exist in various forms, including ions, particle-bound (HMp), and elemental (HM0).Traditional detection methods have primarily focused on the ionic and HMp states.However, the elemental form, HM0, presents a challenge due to its low solubility in water or electrolyte solutions, rendering conventional detection methods ineffective [1].For instance, elemental mercury (Hg0), emitted from coal-fired boiler flue gases, and gaseous cadmium (Cd0), from solid waste incineration, are difficult to measure and capture due to their low solubility and surface affinity, respectively [2,3].Similarly, elemental lead (Pb0), released into the fuel flue gases with low solubility in water, poses a challenge for purify using wet flue gas desulfurization systems [4].Elemental arsenic (As0) is also one of the predominant forms in which arsenic exists [5].Therefore, there is an urgent need to develop sensors capable of real-time detection of HM0.
Recent studies have highlighted the potential of two-dimensional materials in heavy metal adsorption and sensing.M. Ghashghaee et al. discovered that black phosphorene monolayers could adsorb Cd, Pb, Hg, and As, effectively removing these heavy metals from the environment [6].M. A. Hamed et al. reported that graphene-structured ZnO exhibited adsorption capabilities for Cd [7].Carbon-based two-dimensional materials have also been employed for heavy metal adsorption.C. Zhao et al. demonstrated that pyridinic nitrogen-doped graphene modified with Pd clusters could effectively adsorb Hg and As [8].I. Shtepliuk et al. utilized the selectivity of graphene/SiC Schottky diodes for highly sensitive detection of lead atoms [9].J. Liao et al. observed that single vacancy defects significantly enhanced the adsorption of Cd on graphene [10].Research by M. Srivastava et al. revealed that boron-modified graphene and copper-modified nitrogendoped graphene could detect As in water [11].Furthermore, J. Zou et al. modified glassy carbon electrodes with sulfur-doped graphitic carbon nitride nanosheets, which proved to be sensitive and selective for Pb detection [12].
To address this knowledge gap, our study employs density functional theory (DFT) to explore the heavy metal sensing capabilities of MoS 2 -based monolayer sensors.The adsorption energies, charge transfer, density of states (DOS), band structure, and Bader analysis of As, Cd, Hg, and Pb on MoS 2 monolayers have been investigated.The desorption process is predicted to be governed by the recovery time.This paper also measures the current voltage characteristics and sensitivity of the MoS 2 monolayer when detecting heavy metals.Our findings could pave the way for the development of novel nanosensors for environmental monitoring and facilitate the rapid, selective detection of heavy metal contaminants.

Materials and Methods
Calculations were conducted using the Vienna Ab initio Simulation Package (VASP) 5.4, which implements spin-polarized density functional theory (DFT) [24,25].The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) was employed for the exchange-correlation functional [26].To account for van der Waals interactions within the MoS 2 monolayer, the DFT-D3 method was utilized [27].Geometry optimization continued until the forces on all atoms converged below a threshold of 0.02 eV/Å.The total energy convergence criterion was set to 10 −5 eV.A plane wave energy cutoff of 400 eV was used, and a vacuum layer of 20 Å was applied perpendicular to the 2H-MoS 2 monolayer to minimize interlayer interactions.Brillouin zone integration was performed using a 4 × 4 × 1 k-point Monkhorst-Pack mesh.
The adsorption energy E ad was computed using the relation: where E total is the total energy of the fully relaxed adsorbed system consisting of the MoS 2 monolayer and the adsorbed metal atom, E MoS 2 is the energy of the pristine MoS 2 monolayer, and E metal is the energy of the isolated heavy metal atom.Additionally, quantum transport calculations were conducted using the TRANSIESTA code within the SIESTA 4.1.5framework, utilizing the non-equilibrium Green's function method [28].The Landauer-Büttiker formula was applied to calculate the current-voltage (I-V) characteristics of the MoS 2 monolayer both before and following heavy metal adsorption under [29]: where e and h are the electron charge and Planck's constant, respectively.I is the current through the central scattering region.µ and f (E − µ) are the electrochemical potentials and the Fermi-Dirac distribution function, where the subscript L/R is employed to differentiate between the left and right electrodes, respectively.By employing Green's function formalism, the transmission coefficient T(E, V b ) at energy E and bias voltage V b was calculated.
For the visualization of crystal structures and differential charge density, the software VESTA 3 was employed [30].Data post-processing was performed using the tool vaspkit 1.3.3[31].

Structures
As depicted in Figure 1, the MoS 2 monolayer supercell exhibits a distinct structural arrangement, where each Mo atom is coordinated with six S atoms, and each S atom is coordinated with three Mo atoms.The Mo-S bond length is measured at 2.417 Å, aligning remarkably with previous studies, which reported bond lengths of 2.424 Å [19].The bond angles within the MoS 2 lattice, specifically the S-Mo-S and Mo-S-Mo angles, are 82.60 • , forming a hexagonal configuration.
Additionally, quantum transport calculations were conducted using the TRANSI-ESTA code within the SIESTA 4.1.5framework, utilizing the non-equilibrium Green's function method [28].The Landauer-Büttiker formula was applied to calculate the current-voltage (I-V) characteristics of the MoS2 monolayer both before and following heavy metal adsorption under [29]: where e and h are the electron charge and Planck's constant, respectively.I is the current through the central scattering region.μ and   −  are the electrochemical potentials and the Fermi-Dirac distribution function, where the subscript L/R is employed to differentiate between the left and right electrodes, respectively.By employing Green's function formalism, the transmission coefficient  ,  at energy E and bias voltage Vb was calculated.
For the visualization of crystal structures and differential charge density, the software VESTA 3 was employed [30].Data post-processing was performed using the tool vaspkit 1.3.3[31].

Structures
As depicted in Figure 1, the MoS2 monolayer supercell exhibits a distinct structural arrangement, where each Mo atom is coordinated with six S atoms, and each S atom is coordinated with three Mo atoms.The Mo-S bond length is measured at 2.417 Å, aligning remarkably with previous studies, which reported bond lengths of 2.424 Å [19].The bond angles within the MoS2 lattice, specifically the S-Mo-S and Mo-S-Mo angles, are 82.60°,forming a hexagonal configuration.In this study, the energies of heavy metal atoms at four distinct sites on the MoS2 surface have been computed: the hexagonal center (H site), atop the Mo atom (TMo site), atop the S atom (TS site), and the bridge site of the Mo-S bond (B site).These calculations In this study, the energies of heavy metal atoms at four distinct sites on the MoS 2 surface have been computed: the hexagonal center (H site), atop the Mo atom (T Mo site), atop the S atom (T S site), and the bridge site of the Mo-S bond (B site).These calculations aim to ascertain the most energetically favorable adsorption sites for heavy metals on the MoS 2 monolayer surface.
Figure 2 presents the most stable adsorption configurations of heavy metals-As, Pb, Cd, and Hg-on the MoS 2 monolayer.Geometric optimization results indicate a predilection for As and Pb to adsorb at the T S site, where they effectively engage with the MoS 2 surface, achieving the lowest adsorption energies.In contrast, Cd and Hg show a preference for adsorbing on the T Mo site.
Figure 2 presents the most stable adsorption configurations of heavy metals-As, Pb, Cd, and Hg-on the MoS2 monolayer.Geometric optimization results indicate a predilection for As and Pb to adsorb at the TS site, where they effectively engage with the MoS2 surface, achieving the lowest adsorption energies.In contrast, Cd and Hg show a preference for adsorbing on the TMo site.During the adsorption process, the heavy metals cause alterations in the underlying bond angles of the MoS2 structure.Specifically, following the adsorption of Hg and Cd, the S-Mo-S bond angles increase to 83.15° and 83.36°, respectively.Adsorption of As and Pb results in an increase in the Mo-S-Mo bond angles to 83.52° and 83.74°, respectively.These changes in bond angles indicate the extent of structural perturbation imposed by different heavy metals on the MoS2 surface, with Pb exerting the most significant impact, followed by As, Cd, and Hg, in that order.The adsorption distance data show that As and During the adsorption process, the heavy metals cause alterations in the underlying bond angles of the MoS 2 structure.Specifically, following the adsorption of Hg and Cd, the S-Mo-S bond angles increase to 83.15 • and 83.36 • , respectively.Adsorption of As and Pb results in an increase in the Mo-S-Mo bond angles to 83.52 • and 83.74 • , respectively.These changes in bond angles indicate the extent of structural perturbation imposed by different heavy metals on the MoS 2 surface, with Pb exerting the most significant impact, followed by As, Cd, and Hg, in that order.The adsorption distance data show that As and Pb are closer to the MoS 2 surface, at 2.175 Å and 2.625 Å, respectively, compared to the greater distances for Cd (3.358 Å) and Hg (3.526 Å).This suggests a tighter interaction and stronger interfacial forces between As and Pb with the MoS 2 surface, leading to more pronounced structural distortions.Conversely, the longer adsorption distances for Cd and Hg suggest relatively weaker interactions and a diminished interfacial coupling effect.
A detailed analysis of the adsorption characteristics of different heavy metal ions on the MoS 2 monolayer surface has been performed, and the key parameters, including adsorption energy, adsorption distance, Bader charge transfer, and the corresponding changes in the material's bandgap, are summarized in Table 1.Among the listed data, the heavy metal elements Cd and Hg exhibit longer adsorption distances, lower corresponding adsorption energies, and smaller charge transfers.This indicates relatively weaker interactions between Cd and Hg with the MoS 2 monolayer, which are not conducive to forming stable chemisorption states.Conversely, As and Pb demonstrate significantly enhanced adsorption characteristics on the MoS 2 monolayer.Not only is their adsorption energy significantly reduced, indicating more stable adsorption, but their adsorption distances are also substantially shorter.This implies that the interactions between As and Pb with the MoS 2 monolayer are more intense, leading to more efficient surface adsorption reactions.

Eletronic Properties
In the exploration of monolayer MoS 2 as a sensing material, the analysis of its band structure is crucial, particularly the characteristics of the bands near the Fermi level (from −2 eV to 2 eV) which directly influence the material's conductivity changes and sens- ing response sensitivity.The theoretical band gap of pristine monolayer MoS 2 has been calculated to be 1.585 eV.
Upon adsorption of As atoms on the MoS 2 monolayer surface, a significant alteration in the band structure is observed.As depicted in Figure 3, four primary impurity levels originating from the As atoms emerge between the valence band maximum and the conduction band minimum.The introduction of these impurity levels substantially narrows the original band gap, reducing the new band gap of MoS 2 post-As adsorption to 0.242 eV.This represents an approximate 84.7% decrease compared to the baseline state, suggesting a marked enhancement in the electrical conductivity of the MoS 2 -As adsorption system, thereby improving its efficiency as a heavy metal sensor.
On the other hand, the adsorption of Cd or Pb on the MoS 2 monolayer introduces a new impurity level below the Fermi energy, resulting in band gaps of the MoS 2 -Cd and MoS 2 -Pb adsorption systems being reduced to 1.14 eV and 0.326 eV, respectively.Compared to the baseline state, the band gaps have been reduced by 28.1% and 79.4%, respectively.While adsorption in both cases promotes an increase in conductivity, the extent is not as significant as with As adsorption.In contrast, Hg atom adsorption on the MoS 2 monolayer does not induce noticeable impurity level formation, implying that post-Hg adsorption, the band gap of the MoS 2 monolayer decreases by only 3.2%.This suggests that Hg adsorption has a minimal impact on the change in conductivity of the MoS 2 monolayer, and compared to the adsorption of other elements, it does not significantly alter its conductive response characteristics as a sensor.
monolayer, and compared to the adsorption of other elements, it does not significantly alter its conductive response characteristics as a sensor.To further elucidate the adsorption characteristics of MoS2 towards heavy metals, the impact of heavy metal adsorption on the electronic properties of MoS2 was investigated by analyzing the total density of states (TDOS) and partial density of states (PDOS) as shown in Figure 4.
After As adsorption on MoS2, four new hybridized energy levels were generated near the Fermi level.These levels correspond to spin-up peaks at −0.809 eV and −0.292 eV, a spin-down peak at −0.070 eV, and a sharp, narrow peak just above the Fermi level at 0.152 eV.The p-orbitals of As play a dominant role in the formation of these hybridized levels, simultaneously inducing peak states in the density of states for Mo's d-orbitals and S's porbitals at corresponding energy positions.These results demonstrate strong orbital hybridization between As and the surface atoms of MoS2, and due to the asymmetry of the DOS curves for spin-up and spin-down, it is evident that As adsorption imparts magnetism to the MoS2 system, with a calculated magnetic moment of 1 µB.
For Cd adsorption on MoS2, a DOS peak is observed at −0.121 eV, resulting from the hybridization of Cd's s-orbital electrons with Mo's d-orbital electrons and S's p-orbital electrons.However, Cd adsorption does not lead to significant spin polarization, as the DOS curves remain symmetrical for spin-up and spin-down, indicating that the Cd adsorption system is non-magnetic.
After Pb adsorption on MoS2, a DOS peak is observed at −0.121 eV, primarily attributed to the cooperative effect of Pb's p-orbital electrons with Mo's d-orbital electrons and S's p-orbital electrons.Similar to As adsorption, the system post-Pb adsorption exhibits asymmetry in the DOS curves for spin-up and spin-down, resulting in a magnetic moment of 2 µB, confirming the magnetic nature of the Pb adsorption system.To further elucidate the adsorption characteristics of MoS 2 towards heavy metals, the impact of heavy metal adsorption on the electronic properties of MoS 2 was investigated by analyzing the total density of states (TDOS) and partial density of states (PDOS) as shown in Figure 4.
After As adsorption on MoS 2 , four new hybridized energy levels were generated near the Fermi level.These levels correspond to spin-up peaks at −0.809 eV and −0.292 eV, a spin-down peak at −0.070 eV, and a sharp, narrow peak just above the Fermi level at 0.152 eV.The p-orbitals of As play a dominant role in the formation of these hybridized levels, simultaneously inducing peak states in the density of states for Mo's d-orbitals and S's p-orbitals at corresponding energy positions.These results demonstrate strong orbital hybridization between As and the surface atoms of MoS 2 , and due to the asymmetry of the DOS curves for spin-up and spin-down, it is evident that As adsorption imparts magnetism to the MoS 2 system, with a calculated magnetic moment of 1 µB.
For Cd adsorption on MoS 2 , a DOS peak is observed at −0.121 eV, resulting from the hybridization of Cd's s-orbital electrons with Mo's d-orbital electrons and S's p-orbital electrons.However, Cd adsorption does not lead to significant spin polarization, as the DOS curves remain symmetrical for spin-up and spin-down, indicating that the Cd adsorption system is non-magnetic.
After Pb adsorption on MoS 2 , a DOS peak is observed at −0.121 eV, primarily attributed to the cooperative effect of Pb's p-orbital electrons with Mo's d-orbital electrons and S's p-orbital electrons.Similar to As adsorption, the system post-Pb adsorption exhibits asymmetry in the DOS curves for spin-up and spin-down, resulting in a magnetic moment of 2 µB, confirming the magnetic nature of the Pb adsorption system.As a result, As, Cd, and Pb all exhibit a certain degree of orbital hybridization effects during their interaction with MoS2.The adsorption of As and Pb leads to a transition of the MoS2 system to a magnetic state, suggesting that MoS2 holds promise as a magnetic sensing material for As and Pb detection, while the detection of Cd and Hg does not depend on a magnetic response mechanism.As a result, As, Cd, and Pb all exhibit a certain degree of orbital hybridization effects during their interaction with MoS 2 .The adsorption of As and Pb leads to a transition of the MoS 2 system to a magnetic state, suggesting that MoS 2 holds promise as a magnetic sensing material for As and Pb detection, while the detection of Cd and Hg does not depend on a magnetic response mechanism.
To gain deeper insight into the bonding characteristics between the adsorbed metal atoms and the MoS 2 monolayer surface, we analyzed the differential charge density.This quantity is calculated using Equation (3): where ∆ρ represents the charge density difference, ρ total is the charge density of the combined adsorption system, ρ MoS 2 is the charge density of the pristine MoS 2 monolayer, and ρ metal is the charge density of the isolated adsorbate atom.Figure 5 illustrates the differential charge density, with regions of electron accumulation depicted in yellow and areas of electron depletion shown in blue.
Micromachines 2024, 15, x FOR PEER REVIEW 8 of 15 To gain deeper insight into the bonding characteristics between the adsorbed metal atoms and the MoS2 monolayer surface, we analyzed the differential charge density.This quantity is calculated using Equation (3): where ∆ρ represents the charge density difference, ρ is the charge density of the combined adsorption system, ρ is the charge density of the pristine MoS2 monolayer, and ρ is the charge density of the isolated adsorbate atom.Figure 5 illustrates the differential charge density, with regions of electron accumulation depicted in yellow and areas of electron depletion shown in blue.Through the study of As and Pb adsorption on the MoS2 surface, significant regions of charge exchange were identified within these adsorption systems, as further confirmed by Bader charge analysis depicted in Table 1.Notably, As and Pb exhibited substantial charge transfers to the MoS2, with values of −0.243 e and −0.410 e, respectively.In contrast, Through the study of As and Pb adsorption on the MoS 2 surface, significant regions of charge exchange were identified within these adsorption systems, as further confirmed by Bader charge analysis depicted in Table 1.Notably, As and Pb exhibited substantial charge transfers to the MoS 2 , with values of −0.243 e and −0.410 e, respectively.In contrast, Cd exhibits a relatively weaker charge exchange with the MoS 2 surface, with a value of −0.115 e.The charge transfer between Hg and MoS 2 is even more negligible, amounting to only −0.023 e.

Recovery Time
The recovery time is a crucial property for sensor performance as it determines the speed and repeatability of the sensor's response.The recovery time (τ) represents the time needed for the sensor surface to restore its initial adsorption capacity after becoming saturated with adsorbates.The recovery time, denoted by (τ), is deduced from Equation (4), which is an extrapolation from classical transition state theory references: where ω signifies the attempt frequency, postulated to be 10 13 s −1 [32].E ad represents the adsorption energy, k B is the Boltzmann constant, and T denotes the temperature.After adsorption of various heavy metals on MoS 2 , the calculated recovery times within the 298-498 K range are presented in Table S1.As delineated in Figure 6, the recovery times for heavy metals on the MoS 2 monolayer are inversely proportional to the temperature increment.At 298 K, the desorption times for As, cadmium (Cd), and Hg from the MoS 2 monolayer surface are 9.67 s, 3.54 × 10 −6 s, and 1.56 × 10 −7 s, respectively, indicating a swift recovery performance at ambient conditions.For Pb, the recovery time on a MoS 2 monolayer reduces to 2.21 × 10 10 s at 298 K, and further to 9.32 s at an elevated temperature of 498 K.This observation suggests that the MoS 2 monolayer, when employed as a Pb sensor, exhibits reusability at the heightened temperature of 498 K. Consequently, the MoS 2 monolayer emerges as a promising candidate for sensing As, Cd, Hg, and Pb, attributable to its expedited recovery dynamics.
Micromachines 2024, 15, x FOR PEER REVIEW 9 of 15 Cd exhibits a relatively weaker charge exchange with the MoS2 surface, with a value of −0.115 e.The charge transfer between Hg and MoS2 is even more negligible, amounting to only −0.023 e.

Recovery Time
The recovery time is a crucial property for sensor performance as it determines the speed and repeatability of the sensor's response.The recovery time (τ) represents the time needed for the sensor surface to restore its initial adsorption capacity after becoming saturated with adsorbates.The recovery time, denoted by (τ), is deduced from Equation (4), which is an extrapolation from classical transition state theory references: where ω signifies the attempt frequency, postulated to be 10 13 s −1 [32].Ead represents the adsorption energy, kB is the Boltzmann constant, and T denotes the temperature.After adsorption of various heavy metals on MoS2, the calculated recovery times within the 298-498 K range are presented in Table S1.As delineated in Figure 6, the recovery times for heavy metals on the MoS2 monolayer are inversely proportional to the temperature increment.At 298 K, the desorption times for As, cadmium (Cd), and Hg from the MoS2 monolayer surface are 9.67 s, 3.54 × 10 −6 s, and 1.56 × 10 −7 s, respectively, indicating a swift recovery performance at ambient conditions.For Pb, the recovery time on a MoS2 monolayer reduces to 2.21 × 10 10 s at 298 K, and further to 9.32 s at an elevated temperature of 498 K.This observation suggests that the MoS2 monolayer, when employed as a Pb sensor, exhibits reusability at the heightened temperature of 498 K. Consequently, the MoS2 monolayer emerges as a promising candidate for sensing As, Cd, Hg, and Pb, attributable to its expedited recovery dynamics.

Sensitivity
The electrical transport properties of the MoS 2 monolayer upon heavy metal adsorption were investigated by constructing a double-probe model, comprising two electrodes with an intervening scattering region, as depicted in Figure 7.A bias voltage was applied across the electrodes.This quantum transport problem was solved using the NEGF-DFT method under open boundary conditions and in the presence of external electric fields.

Sensitivity
The electrical transport properties of the MoS2 monolayer upon heavy metal adsorption were investigated by constructing a double-probe model, comprising two electrodes with an intervening scattering region, as depicted in Figure 7.A bias voltage was applied across the electrodes.This quantum transport problem was solved using the NEGF-DFT method under open boundary conditions and in the presence of external electric fields.
Figure 7 also delineates the current-voltage (I-V) characteristics of MoS2 monolayers, both in pristine form and following the adsorption of heavy metals.At a bias voltage of 0.2 V, the MoS2 monolayer with adsorbed As demonstrated a significantly enhanced current, reaching 1.75 × 10 −8 µA, which is markedly higher than the basal current of intrinsic MoS2 at 1.39 × 10 −11 µA.Comparatively, the currents observed for MoS2 monolayers with adsorbed Cd, Hg, and Pb were 8.92 × 10 −12 µA, 9.01 × 10 −12 µA, and 1.35 × 10 −10 µA, respectively, indicating a pronounced selectivity for arsenic.For Pb adsorption, the current at low bias voltages (≤0.6 V) was markedly higher than that of intrinsic MoS2, while at higher bias voltages (≥0.8 V), the disparity was considerably reduced.Notably, at a bias voltage of 0.4 V, the MoS2 monolayer with adsorbed arsenic (As) manifested a current of 2.54 × 10 −8 µA, which is considerably higher than the currents measured for MoS2 monolayers with adsorbed As The MoS2 monolayer with Hg adsorption showed a relatively minor difference in current from intrinsic MoS2, except in the 0.5-0.6V range.The current for Cd-adsorbed MoS2 was generally lower than that for As, but higher than for Hg.
The sensitivity (S) of MoS2 monolayers was also evaluated using Equation ( 5): Figure 7 also delineates the current-voltage (I-V) characteristics of MoS 2 monolayers, both in pristine form and following the adsorption of heavy metals.At a bias voltage of 0.2 V, the MoS 2 monolayer with adsorbed As demonstrated a significantly enhanced current, reaching 1.75 × 10 −8 µA, which is markedly higher than the basal current of intrinsic MoS 2 at 1.39 × 10 −11 µA.Comparatively, the currents observed for MoS 2 monolayers with adsorbed Cd, Hg, and Pb were 8.92 × 10 −12 µA, 9.01 × 10 −12 µA, and 1.35 × 10 −10 µA, respectively, indicating a pronounced selectivity for arsenic.
The MoS 2 monolayer with Hg adsorption showed a relatively minor difference in current from intrinsic MoS 2 , except in the 0.5-0.6V range.The current for Cd-adsorbed MoS 2 was generally lower than that for As, but higher than for Hg.
The sensitivity (S) of MoS 2 monolayers was also evaluated using Equation ( 5): where I represents the current measured across the MoS 2 monolayer upon heavy metal adsorption, and I 0 denotes the intrinsic current in the absence of heavy metal contaminants.
As illustrated in Figure 8, the sensitivity of the MoS 2 -based monolayer sensors can be quantitatively estimated.Upon adsorption of As, the MoS 2 sensor exhibits a remarkable sensitivity, reaching 126,452.28% at a bias voltage of 0.2 V.This sensitivity is significantly higher compared to other heavy metals, indicating a pronounced selectivity for As.Moreover, the MoS 2 sensor maintains a sensitivity above 200% across the entire voltage range below 1.2 V after As adsorption, demonstrating exceptional sensitivity and superior sensing performance.where I represents the current measured across the MoS2 monolayer upon heavy metal adsorption, and I0 denotes the intrinsic current in the absence of heavy metal contaminants.
As illustrated in Figure 8, the sensitivity of the MoS2-based monolayer sensors can be quantitatively estimated.Upon adsorption of As, the MoS2 sensor exhibits a remarkable sensitivity, reaching 126,452.28% at a bias voltage of 0.2 V.This sensitivity is significantly higher compared to other heavy metals, indicating a pronounced selectivity for As.Moreover, the MoS2 sensor maintains a sensitivity above 200% across the entire voltage range below 1.2 V after As adsorption, demonstrating exceptional sensitivity and superior sensing performance.In the case of Pb, the MoS2 sensor achieves a sensitivity of 83,438.90% at 0.4 V, which is considerably higher than that for other heavy metals, suggesting good selectivity at this voltage.Within a voltage range of ≤0.7 V, the sensitivity for Pb remains above 200%.
For Cd, the peak sensitivity of 1862.67% is reached at 0.8 V, with the sensitivity exceeding 200% at bias voltages of 0.1 V, 0.3 V, and 0.6-0.8V. Conversely, the sensitivity for Hg is relatively lower, peaking at 427.71% at 0.5 V and surpassing 200% within the range of 0.5-0.6V.
The limit of detection (LOD) signifies the lowest analyte concentration a sensor can reliably detect.Determined via signal-to-noise ratio (SNR), LOD compares the analyte signal to a blank signal to establish the minimum detectable concentration.For MoS2 sensors, current serves as the output signal.Baseline current (I₀) represents the MoS2 current without heavy metal adsorption, while current (I) reflects the post-adsorption state.
Conventionally, LOD corresponds to an SNR of 3:1, which also translates to a sensitivity of 200%.
To quantify heavy metal adsorption on MoS2 monolayers, concentration is expressed as the atomic count per area.The calculations above employed a 25.54 Å × 22.12 Å scattering region (564.94Å 2 ).A single heavy metal atom within this area (concentration: 1/564.94In the case of Pb, the MoS 2 sensor achieves a sensitivity of 83,438.90% at 0.4 V, which is considerably higher than that for other heavy metals, suggesting good selectivity at this voltage.Within a voltage range of ≤0.7 V, the sensitivity for Pb remains above 200%.
For Cd, the peak sensitivity of 1862.67% is reached at 0.8 V, with the sensitivity exceeding 200% at bias voltages of 0.1 V, 0.3 V, and 0.6-0.8V. Conversely, the sensitivity for Hg is relatively lower, peaking at 427.71% at 0.5 V and surpassing 200% within the range of 0.5-0.6V.
The limit of detection (LOD) signifies the lowest analyte concentration a sensor can reliably detect.Determined via signal-to-noise ratio (SNR), LOD compares the analyte signal to a blank signal to establish the minimum detectable concentration.For MoS 2 sensors, current serves as the output signal.Baseline current (I 0 ) represents the MoS 2 current without heavy metal adsorption, while current (I) reflects the post-adsorption state.
Conventionally, LOD corresponds to an SNR of 3:1, which also translates to a sensitivity of 200%.
These findings underscore the high sensitivity and selectivity of MoS 2 -based monolayer sensors towards heavy metals, positioning them as promising candidates for future environmental monitoring applications.
Doubling the scattering region while maintaining a single metal atom reduces concentration to 1/1129.88Å 2 .Under these conditions, the sensitivities of MoS2 towards As, Cd, Hg, and Pb are 132.33%,215.42%, 122.38%, and 828.19%, respectively.Here, MoS₂ sensitivity significantly exceeds the LOD for Pb, marginally surpasses it for Cd, but falls below for As and Hg.Consequently, MoS₂ detects Pb at concentrations below 1/1129.88Å 2 , Cd near 1/1129.88Å 2 , and As and Hg between 1/1129.88Å 2 and 1/564.94Å 2 .
These findings underscore the high sensitivity and selectivity of MoS2-based monolayer sensors towards heavy metals, positioning them as promising candidates for future environmental monitoring applications.
A critical advantage of MoS 2 monolayer sensors over other 2D material-based heavy metal sensors lies in their ability to selectively detect specific heavy metals in complex samples.Unlike other 2D materials, such as tin disulfide (SnS) and tin selenide (SnSe), where multiple heavy metals exhibit similar response peaks, MoS 2 displays unique response peaks for each heavy metal.
As illustrated in Table S2, SnS exhibits response peaks for Cd, Hg, and Pb at the same voltage (0.5 V), leading to potential interference issues.In samples containing multiple heavy metals, the signal of more sensitive metals like Pb, Cd, and Hg can be masked by the stronger response of Pb.Similarly, SnSe exhibits response peaks for As, Cd, Hg, and Pb at distinct bias voltages (0.2 V, 0.1 V, 0.1 V, and 0.2 V, respectively).However, in mixed heavy metal samples, Cd and Hg, as well as As and Pb, can still interfere with each other.
In contrast, MoS 2 demonstrates remarkable selectivity, with distinct response peaks for different heavy metals at varying bias voltages.For instance, As, Cd, Hg, and Pb exhibit response peaks at 0.2 V, 0.8 V, 0.5 V, and 0.4 V, respectively.This unique characteristic allows for the selective detection of specific heavy metals by adjusting the working voltage applied across the MoS 2 monolayer.

Conclusions
The sensing performance of the MoS 2 monolayer on heavy metals (As, Cd, Hg, and Pb) is investigated using DFT and NEGF simulations.As and Pb adsorption onto MoS 2 monolayers causes significant surface structural modifications and intensive electrical interactions.These interactions introduce impurity energy levels within the adsorption system, leading to significant band gap alterations.Conversely, the MoS 2 surface is less affected by Cd and Hg, as evidenced by a weaker electronic exchange.The sensitivities of MoS 2 monolayers to As, Cd, Hg, and Pb are 126,452.28%,1862.67%,427.71%, and 83,438.90%,respectively.For As or Pb, the MoS 2 monolayer sensors exhibit response peaks at different bias voltages that are significantly higher than those for other heavy metals, demonstrating a high degree of selectivity.When As or Pb are adsorbed on MoS 2 monolayers, they acquire a non-zero magnetic moment of 1 µB or 2 µB, respectively, indicating that they may find use as magnetic sensors.The MoS 2 monolayer's moderate adsorption energy facilitates rapid sensor recovery for As, Hg, and Cd at room temperature.Furthermore, the recovery time for Pb can be remarkably reduced to just 9.32 s at 498 K, highlighting the MoS 2 monolayer's efficacy as a reusable sensor.The results of this study show that MoS 2 monolayer has the potential to be a high sensitivity, selectivity, and reusability heavy metal sensing material that can be used to make heavy metal detectors that operate in real-time.

Supplementary Materials:
The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/mi15080978/s1,Table S1: Temperature-dependent recovery time of MoS 2 for heavy metal adsorption; Table S2: Comparison between this work and previous reports.

Figure 1 .
Figure 1.(a) The top side view of MoS2 monolayer with the position sites (H, B, TS, and TMo); (b) the side view of MoS2 monolayer.

Figure 1 .
Figure 1.(a) The top side view of MoS 2 monolayer with the position sites (H, B, T S , and T Mo ); (b) the side view of MoS 2 monolayer.

Figure 2 .
Figure 2. The most stable configuration and Bader charge analysis for (a) As, (b) Cd, (c) Hg, and (d) Pb on the MoS2 monolayer.Negative numbers indicate loss of electrons, positive numbers indicate gain of electrons.

Figure 2 .
Figure 2. The most stable configuration and Bader charge analysis for (a) As, (b) Cd, (c) Hg, and (d) Pb on the MoS 2 monolayer.Negative numbers indicate loss of electrons, positive numbers indicate gain of electrons.

Figure 6 .
Figure 6.The recovery time (τ) for As, Cd, Hg, and Pb adsorbed on MoS2 monolayers at different temperatures (T).

Figure 6 .
Figure 6.The recovery time (τ) for As, Cd, Hg, and Pb adsorbed on MoS 2 monolayers at different temperatures (T).

Figure 7 .
Figure 7.The current-voltage characteristics of MoS2 monolayers with As, Cd, Hg, and Pb adsorption.

Figure 7 .
Figure 7.The current-voltage characteristics of MoS 2 monolayers with As, Cd, Hg, and Pb adsorption.

Figure 8 .
Figure 8.The sensitivity for As, Cd, Hg, and Pb adsorbed on MoS2 monolayers.

Figure 8 .
Figure 8.The sensitivity for As, Cd, Hg, and Pb adsorbed on MoS 2 monolayers.

Figure 9 .
Figure 9.The comparison of sensitivity for (a) As, (b) Cd, (c) Hg, and (d) Pb sensing materials.

Author Contributions:
Conceptualization, J.W. (Jiayin Wu); methodology, J.W. (Jiayin Wu); software, Z.L.; investigation, J.W. (Jiayin Wu); resources, T.L.; writing-original draft preparation, J.W. (Jiayin Wu); writing-review and editing, Q.M., T.L., J.W. (Jingting Wei) and B.L.; supervision, Z.L. and X.X.; funding acquisition, Q.M., T.L., Z.L. and B.L. All authors have read and agreed to the published version of the manuscript.Funding: This research was funded by the Foundation Research Project of Kaili University, grant number 2022ZD05; National Natural Science Foundation of China, grant number 22269019; the Key Research Platform and Research Project of General Colleges and Universities in Guangdong

Table 1 .
Adsorption energy, adsorption distance, charge transfer, and energy gap.The adsorption distance is defined as the closest contact distance between the adsorbed atom and the nearest atom on the MoS 2 surface.