Cu-Doped MoSe2 Monolayer: A Novel Candidate for Dissolved Gas Analysis in Transformer Oil.

Dissolved gas analysis (DGA) in transformer oil is a workable approach to evaluate the operation status of transformers. In this paper, we proposed a Cu-doped Se-vacancy MoSe2 (Cu-MoSe2) monolayer as a promising sensing material for DGA based on first-principles theory. Three typical dissolved gases, namely, CO, C2H2, and C2H4, are the representatives to investigate the potential of the Cu-MoSe2 monolayer upon their adsorption and sensing. Our results indicate that Cu-doping causes strong n-doping for the Se-vacancy MoSe2 monolayer, and the Cu-MoSe2 monolayer exhibits strong chemisorption the three gas molecules, with a calculated adsorption energy (E ad) of -1.25, -1.06, and -1.16 eV, respectively. Such strong interactions lead to remarkable changes in the electrical conductivity of the Cu-MoSe2 monolayer, allowing its application as a resistance-type sensor. Besides, work function (WF) analysis shows the potential of the Cu-MoSe2 monolayer as a promising field-effect transistor sensor as well. It is our hope that our work can stimulate more leading-edge studies of the TM-doped MoSe2 monolayer for sensing applications in many fields.

Introduction

Chemical gas sensors based on two-dimensional (2D) nanomaterials have been applied in many fields including equipment diagnosis, environmental monitoring, and industrial manufacturing.[1−3] For the past few years, transition-metal dichalcogenides (TMDs) have been successfully isolated and been regarded as the promising candidates for the next generation of electric devices following the upsurge of graphene.[4−6] In the field of gas sensing, TMDs are widely studied and have become the burgeoning 2D materials for detection of gaseous species given their favorable specific surface area and strong chemical reactivity in gas interactions.[7,8]

Among various TMDs, the MoSe2 monolayer is one of the most intriguing and widely investigated materials, which has been demonstrated with desirable sensitivity and reversibility upon gas species comparable to graphene.[9] Besides, transition-metal (TM) doping on the MoSe2 monolayer has been studied to further enhance its adsorption performance and sensing response. For example, Cui et al.[10] reported the Rh-doped MoSe2 monolayer upon four toxic gases and found that the Rh dopant can significantly facilitate the interaction strength between the MoSe2 monolayer and various gas molecules. However, some studies show that chalcogen vacancies are the most common defect in TMDs (here is a Se vacancy in the MoSe2 monolayer), which can create strong accepting states and behave as efficient electron traps, leading to stronger chemical reactivity compared to the perfect sites upon impurities around the surroundings.[11] Therefore, the study of TM doping on the Se-vacancy can be interesting and essential to further understand the TM-doping behavior on the MoSe2 surface. Also, for gas sensing, the TM dopant(s) in the TM-doped system plays the dominant role to promote the sensing performance for the nanosurfaces given their excellent electron mobility and catalytic behavior upon gas interactions.[12−14] From this regard, studying the sensing performance of the TM-doped Se-vacancy MoSe2 monolayer upon gaseous species would be meaningful and beneficial to further broaden its potential as gas sensors with higher performance.

Transformers in the field of electrical engineering are the most expensive equipment to transmit and distribute the electricity, which in general are filled with oil to guarantee the insulation status.[15] In a long run, the oil will decompose under the insulation defects such as partial discharge and partial overheat, forming several gases and dissolving into the oil.[16] Thereby, dissolved gas analysis is proposed to evaluate the decomposing behavior of the oil and operation status of transformers.[17] The main gases in the oil as reported include H2, CO, CH4, C2H2, and C2H4, and using chemical gas sensors to realize their detection has been deemed as an effective and simple manner with rapid response and low cost.[18]

In this paper, we propose a Cu-doped MoSe2 (Cu-MoSe2) monolayer for sensing three typical carbide gases of partial discharge in oil (namely, CO, C2H2, and C2H4) using first-principles theory to explore its potential as a chemical gas sensor for application in the field of electrical engineering. Also, the adsorptions of other typical gases in transformer oil, including H2 and CH4, are also studied for comparison and to highlight the main purpose of this work. The Cu metal with strong catalytic behavior is frequently used as a dopant for surface doping to enhance the gas interactions of the nanosurfaces.[19,20] This theoretical study mainly highlights two points: (i) analyzing the Cu-doping behavior on the geometric and electronic properties of the Se-vacancy MoSe2 surface and (ii) studying the sensing mechanism of the Cu-MoSe2 monolayer upon three typical dissolved gases in the transformer oil so as to exploit its potential as a chemical gas sensor. Our calculations can stimulate the further investigation of the TM-doped MoSe2 monolayer in terms of their physicochemical properties and their potential as gas sensors for use in many fields.

Results and Discussion

Analysis of the Cu-MoSe2 Monolayer

The optimized structure of the Se-vacancy MoSe2 monolayer is first obtained to initiate the Cu-doping process on the Se-vacancy site, and the charge density difference (CDD) of the Cu-MoSe2 monolayer is also calculated to illustrate the charge-transferring behavior of Cu-doping. The whole process is plotted in Figure .

Figure 1

Cu-doping process on the Se-vacancy MoSe2 monolayer. (a) Se-vacancy MoSe2 monolayer, (b) Cu-MoSe2 monolayer, and (c) CDD of the Cu-MoSe2 monolayer. The black value is the Cu–Mo bond length. In CDD, the green area is electron accumulation while the rosy area is electron depletion, with an isosurface of 0.005 e/Å3.

Figure a describes the optimized configuration of the Se-vacancy MoSe2 monolayer. One can see that because of the existence of a Se vacancy in the MoSe2 monolayer, its geometric structure around the Se vacancy suffers slight deformation compared with the pristine counterpart, wherein the Mo–Se bonds are shortened by ∼1.2%. On the other hand, in the Cu-MoSe2 system, as depicted in Figure b, the Mo–Se bonds recover to their original length of 2.54 Å. These findings indicate the deterioration in the geometric structure of the MoSe2 monolayer caused by the existence of the Se vacancy and the recovery of its geometric structure by repairing the vacancy with certain impurity atoms, as proved in the MoS2 system.[11] Besides, the Cu on the Se-vacancy site is bonded with three Mo atoms with a length of 2.63 Å. To verify the Eb of the Cu-MoSe2 monolayer, its total energy under the magnetic and nonmagnetic property is studied, and we find that the ground state for the Cu-MoSe2 monolayer should be the one without magnetic property which has lower total energy indicating its better stability. Based on this, Eb is calculated as −4.71 eV, which suggests the strong binding force between the Cu dopant and the Se vacancy of the MoSe2 monolayer. Besides, the Eb of the Cu-MoSe2 system is much larger than the cohesive energy of a single Cu atom (3.49 eV), which implies the stable configuration of Cu-doping on the Se vacancy. Furthermore, the Cu diffusion process on the Se-vacancy MoSe2 monolayer is conducted to convince the stable Cu-doping on this site, as plotted in Figure S1. One can see that the energy barrier for Cu diffusion to the nearest possible doping site is quite large (1.88 eV), much larger than the critical energy of 0.95 eV that allows the reaction to occur at room temperature. In other words, the Cu dopant can be stably anchored in the Se-vacancy site of the MoSe2 surface without the diffusion issue.[21]

Based on the Hirshfeld analysis, the Cu dopant is positively charged by 0.130 e, which shows its electron-releasing property transferring charge to the Se-vacancy MoSe2 monolayer. At the same time, this result confirms the strong electron-accepting state for the Se vacancy, which is similar to that in the MoS2 and SnS2 systems.[11,22] From Figure c, where the CDD of the Cu-MoSe2 monolayer is shown, one can see that the electron depletion is mainly localized on the Cu dopant, whereas the electron accumulation is mainly on the Cu–Mo bonds, which not only agrees with the Hirshfeld analysis upon the electron-releasing property of the Cu dopant but also illustrates the strong electron hybridization on the Cu–Mo bonds where the electron localization occurs.[23]

Figure a shows the band structure (BS) of the pristine MoSe2 monolayer, wherein the band gap is obtained as 1.546 eV, quite close to the previous report of 1.55 eV[9] indicating the good accuracy of our basic set in terms of electronic calculations. Figure b displays the BS of the Se-vacancy MoSe2 monolayer. One can see that there exist several novel states within the band gap of the pristine MoSe2 system giving rise to the narrowed band gap by 0.533–1.013 eV accordingly. This finding suggests that the Se vacancy induces some impurity states within the band gap of the pristine MoSe2 system and thus reduces the band gap,[24] similar as that in the SnS2 system.[25] Moreover, the states in the Se-vacancy MoSe2 system become denser suggesting the enhanced carrier mobility here, which would be beneficial to the chemical reactivity toward the surroundings.

Figure 2

(a–c) BS of pristine, Se vacancy, and Cu-doped MoSe2 monolayer and (d) orbital DOS of Cu and Mo atoms wherein the dash line is the Fermi level. The values in the BS are related band gaps.

From Figure c, the band gap of the Cu-MoSe2 monolayer is obtained as 0.898 eV, and the Fermi level approaches to the bottom of the conduction band rather than the top of the valence band in the Se-vacancy MoSe2 system. These manifest that Cu-doping tunes the p-type semiconducting property of the Se-vacancy MoSe2 monolayer, making the Cu-MoSe2 monolayer behave as an n-type semiconductor instead.[26] In other words, n-doping could be identified for Cu-doping on the Se-vacancy MoSe2 monolayer. However, the semiconducting behavior is not changed in the Cu-MoSe2 system. Figure d plots the orbital DOS of Cu and Mo atoms to show the orbital interaction in the Cu–Mo bonds. It can be observed that the Cu 3d orbital is hybrid with the Mo 4d orbital at around −4.5∼−0.8 and 0.5 eV, which indicates their strong electron hybridization in the formation of chemical bonds and verifies the strong binding force between the Cu dopant and the Mo atoms.

Adsorption Behavior of the Cu-MoSe2 Monolayer

With the obtaining of the configuration of the Cu-MoSe2 monolayer, its adsorption behavior upon the dissolved gases is investigated, including H2, CH4, CO, C2H2, and C2H4. The gas molecule is approaching the Cu dopant with an initial distance of approximately 2.5 Å in various configurations to perform the adsorption. The most stable configuration (MSC) for gas adsorption is determined by the lowest energy (Ead), calculated aswhere ECu-MoSe is the total energy of the adsorbed system, while ECu-MoSe and Egas are the total energy of the isolated Cu-MoTe2 monolayer and gas molecule, respectively.

Figure plots the MSC and CDD of three dissolved gas adsorption on the Cu-MoSe2 monolayer, namely, CO, C2H2, and C2H4. For H2 and CH4 adsorption, the MSC and detailed information could be found in Figure S2. Because of the weak interactions in the H2 and CH4 system (physisorption), they would not be the competing gases for sensing in the work environment, thus we would not put much analysis about them in the main body.

Figure 3

MSC and CDD of [a(1–3)] CO, [b(1–3)] C2H2, and [c(1–3)] C2H4 adsorption on the Cu-MoSe2 monolayer. In CDD, the sets are the same as Figure .

From Figure , one can see that all three gases can be trapped by the Cu dopant, forming novel Cu–C bonds on the right top of the Cu atom. Specifically, the CO molecule is standing vertical to the MoSe2 plane through the C-end position, with the Cu–C bond measured as 1.85 Å, while the C2H2 and C2H4 molecules are parallel with the MoSe2 plane forming the Cu–C bonds measured as 2.08 and 2.16 Å, respectively. Meanwhile, the Cu–Mo bonds are elongated to 2.74, 2.75, and 2.76 Å for the CO, C2H2, and C2H4 systems, respectively. Thus, we assume that the order of bond lengths in the Cu–C and bond deformations in Cu–Mo may be to some extent related to the molecular size of three gases which is in order CO < C2H2 < C2H4. Also, the gas molecules are afflicted with somewhat deformation after adsorption. For example, the C≡O bond in the CO molecule is elongated to 1.15 Å from that of 1.14 Å in its gas phase and the C=C and C≡C bonds in the C2H2 and C2H4 molecules are prolonged to 1.24 and 1.37 Å from their isolated phases of 1.21 and 1.34 Å, respectively. Moreover, the linear molecule of C2H2 and plane molecule of C2H4 are slightly distorted, making them bent in the adsorbed systems.

According to the definition, Ead is calculated to be −1.25, −1.06, and −1.16 eV for CO, C2H2, and C2H4 adsorption on the Cu-MoSe2 monolayer, respectively. Thus, the interactions between three gas molecules and the Cu-MoSe2 monolayer could be regarded as chemisorption given their larger Ead than the critical value of 0.8 eV.[27] Even though the Cu-MoSe2 monolayer conducts the strongest performance upon CO adsorption, its adsorption strength upon three gases is in the order CO > C2H4 > C2H2. Based on the Hirshfeld analysis, CO and C2H2 are positively charged by 0.013 and 0.034 e, respectively, while C2H4 is negatively charged by 0.018 e. These findings imply the electron-donating property of CO and C2H2 molecules as well as the electron-accepting property of the C2H4 molecule when interacting with the Cu-MoSe2 monolayer. Moreover, in the CDD, strong electron accumulations could be found localizing on the newly formed Cu–C bonds, suggesting the strong orbital hybridization and binding force between the gas molecules and Cu dopant during the formation of new bonds.

Electronic Behavior of the Cu-MoSe2 Monolayer upon Gas Adsorption

With the adsorption of gas molecules, the electronic behavior of the Cu-MoSe2 monolayer is supposed to be tuned, which could further provide some evidence for its exploration as a gas sensor using certain electronic devices. Figure exhibits the BS and DOS of the Cu-MoSe2 monolayer upon gas adsorptions to expound this issue.

Figure 4

BS and DOS of [a(1–3)] CO system, [b(1–3)] C2H2 system, and [c(1–3)] C2H4 system. The figures from left to right are in the order BS, molecular DOS, and orbital DOS. In BS, the black values are related band gaps, and in DOS, the dash line is the Fermi level.

First, focusing on the BS distribution in three gas systems, it is found that the band gap of the Cu-MoSe2 monolayer is narrowed to different levels after adsorption of CO, C2H2, and C2H4 molecules, leading to the band gap of 0.819, 0.769, and 0.805 eV, respectively. That is, the electrical conductivity of the Cu-MoSe2 monolayer would be increased to different levels in the environment of the typical gases. Interestingly, the decreasing order of the band gap in three systems is C2H2 > C2H4 > CO, which is exactly reverse to the order to Ead CO > C2H4 > C2H2. From this regard, we assume that the larger Ead resulted from the stronger binding force of Cu–C bonds is also attributed to the size of the gas molecule. Because three gases are all trapped by the Cu dopant through the Cu–C bond, the smaller gas molecule would have more advantage to be captured around the Cu center and cause larger Ead. Also, one can infer that Eg may not have the direct relationship with Ead in the gas adsorption systems.

At the same time, the electronic behavior of the gas molecules would be impacted as well after interactions. In the first place, one can see that the DOS states of the gas molecules after adsorption split into several small states shifting to the areas below the Fermi level. These state deformations are ascribed to the electron hybridization between the Cu-MoSe2 monolayer and the gases that lead to the orbital activation of the gas molecules. Moreover, the orbital interaction could be found from the orbital DOS of bonded atoms. In three gas adsorption systems, the orbital interactions all occur between the Cu dopant and the C atom of related molecules. From the orbital DOS between Cu 3d and C 2p orbitals, the hybridizations are mainly localized at 0 and 1.0 eV in the CO system, at −4.4, −2.1, and 0 eV in the C2H2 system, and at −6.7, −4.3, and 0 eV in the C2H4 system. These state overlaps manifest the orbital hybridization in the formation of Cu–C bonds with strong binding force. The abovementioned analyses about BS and DOS put forward not only the electronic behavior of the Cu-MoSe2 monolayer upon gas adsorption but also the good interactions between the Cu dopant and the gas molecules.

Resistance-Type Sensor Exploration

The abovementioned section analyzes the electronic behavior of the Cu-MoSe2 monolayer upon gas adsorption, and the change in the electronic behavior will lead to the change in its electrical conductivity, which could be evaluated by the value of band gap. Formula shows the relationship between electrical conductivity (σ) of materials to its band gap,[28] and formula shows the sensing response (S) of the Cu-MoSe2 monolayer upon various gaseswherein λ is a constant, Bg is the band gap, k is the Boltzmann constant, and T is temperature, while σgas and σpure signify the electrical conductivity of the Cu-MoSe2 monolayer after and before gas adsorption, respectively. From formula , one can find that the larger band gap will lead to smaller electrical conductivity. In this work, the band gap of the Cu-MoSe2 monolayer is reduced by 0.079 eV after adsorption of the CO molecule and is reduced by 0.129 and 0.093 eV after adsorption of C2H2 and C2H4 molecules, respectively. Furthermore, it could be calculated based on formula that the S of the Cu-MoSe2 monolayer for detection of CO, C2H2, and C2H4 gases is −78.5, −91.9, and −82.7%, respectively. Such high responses in the electrical conductivity of the Cu-MoSe2 monolayer guarantee the sensitive detection using a certain device working at the resistance detection.[29] Thus, the Cu-MoSe2 monolayer is full of potential to be explored as a resistance-type gas sensor for the detection of three dissolved gases in transformer oil, thus evaluating the operation status of the transformers. However, the selective detection of three gases might not be realized using such a sensor.

Work Function (WF) Analysis

WF denotes the minimum required energy for the analyzed material to dislodge an electron to the vacuum level,[30] and a higher WF means the much difficulty to release electron out of its surface. Figure exhibits the WF of the Se-vacancy MoSe2 monolayer, Cu-MoSe2 monolayer, and those gas adsorbed systems. One can see that the WF of the Se-vacancy MoSe2 monolayer is obtained as 5.36 eV, and after Cu-doping, the WF in the Cu-MoSe2 systems reduces to 5.31 eV. These results reveal that Cu-doping enhances the electron mobility of the Se-vacancy MoSe2 monolayer, thus lowering the difficulty for electron overflow from its surface.[31] Although the adsorption of CO molecules slightly enlarges the WF of the Cu-MoSe2 monolayer to 5.36 eV, the adsorption of C2H2 and C2H4 drops its WF by 0.09 and 0.06 eV, giving rise to the WF of 5.22 and 5.25 eV, respectively, for two systems. Because the WF reflects the charge-transfer behavior of the sensing material upon gas adsorption, it could be presumed that the Cu-MoSe2 monolayer is promising for exploration of a field-effect transistor sensor[32] selectively detecting three dissolved gases into two groups, one for CO and another for C2H2 and C2H4. On the other hand, the selective detection of C2H2 and C2H4 could not be realized as well given their similar values of WF using the Cu-MoSe2 monolayer as the sensing pioneer.

Figure 5

WF of various systems.

Conclusions

In this work, using first-principles theory, we studied the Cu-doping behavior on the Se-vacancy MoSe2 monolayer and analyzed the adsorption performance of the Cu-MoSe2 monolayer toward three dissolved gases, namely, CO, C2H2, and C2H4, in order to explore its potential as a novel sensing material for DGA in transformer oil. The obtained conclusions are as follows:

Cu-doping causes strong n-doping for the Se-vacancy MoSe2 monolayer, making the Cu-MoSe2 system behave like an n-type semiconductor;

Cu-MoSe2 monolayer exhibits chemisorption upon CO, C2H2, and C2H4 molecules, leading to a different level of changes in its electrical conductivity and allowing its exploration as a resistance-type gas sensor;

The WF analysis shows that the Cu-MoSe2 monolayer is a promising field-effect transistor sensor selectively detecting three dissolved gases into two groups, one for CO and another for C2H2 and C2H4.

All these findings manifest the strong potential of the Cu-MoTe2 monolayer for DGA in transformer oil with detectable electrical response. Our calculations can put forward the physicochemical property of the Cu-MoSe2 monolayer and stimulate more pioneer research on the TM-MoSe2 monolayer upon gas-sensing application.

Computational Details

The whole first-principle calculations reported below adopted the Perdew–Burke–Ernzerhof (PBE) function, and the generalized gradient approximation (GGA) was selected to treat the exchange and correlation of electrons.[33] All the results were spin-polarized, in which the double numerical plus polarization (DNP) was used as the basis atomic orbital set.[34] The DFT semicore pseudopotential (DSSP) method was employed to dissolve the relativistic effect of the Cu atoms.[35] The DFT-D2 method developed by Grimme was employed to better understand the van der Waals force and long-range interactions.[36] We determined Monkhorst–Pack grid k-point mesh of 7 × 7 × 1 for both geometric optimization and electronic structure calculations.[37] The self-consistent loop energy of 10–6 Ha, global orbital cutoff radius of 5.0 Å, and smearing of 0.005 Ha were utilized for the static calculations to ensure the accurate results of total energy.[38]

A large-enough 4 × 4 × 1 supercell of the MoSe2 monolayer was built with a vacuum region of 15 Å to conduct the gas adsorption process for prevention of the interaction between adjacent units.[39] A Se atom is removed from the pristine MoSe2 surface to form the Se-vacancy MoSe2 monolayer, and the Cu-MoSe2 monolayer is defined as the adsorption of a single Cu atom in the Se-vacancy of the MoSe2 surface. To evaluate the chemical stability of the Cu-MoSe2 monolayer, binding force (Eb) is defined to clarify the interaction between the Cu dopant and the Se-vacancy MoSe2 monolayer, calculated aswhere ECu-MoSe, ECu, and Evac-MoSe signify the total energies of the Cu-MoSe2 monolayer, isolated Cu atom, and Se-vacancy MoSe2 monolayer, respectively. Besides, the Hirshfeld method is considered to analyze the atomic charge of the Cu dopant (QCu) and molecule charge transfer (QT) in gas adsorptions because it cannot be impacted by the basic set of the calculation and guarantee the good accuracy of the charge-transfer results.[40,41] According to the definition, a positive value implies the electron-releasing property of the Cu dopant or the gas molecule.