The Effect of the Metal Impurities on the Stability, Chemical, and Sensing Properties of MoSe₂ Surfaces

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Reviewer Comments

“The effect of the metal impurities on the stability, chemical, and sensing properties of the MoSe₂ surface”

The manuscript presents a comprehensive and methodically detailed theoretical investigation of the effects of metal impurities on the stability, electronic structure, adsorption properties, and sensing and catalytic behaviors of MoSe₂. The work is grounded in density functional theory (DFT) calculations and includes enthalpy and free energy analyses, temperature and concentration effects, and simulations of gas adsorption and CO₂ reduction reactions.

Before final acceptance the following points should be explained:

The Introduction section requires supplementation.
The authors do not explain the rationale behind the selection of metals introduced into the MoSe₂ structure. If the intention was to include first-row transition metals, why were Mn and Zn excluded? Similarly, what was the justification for including W, Pd, and Pt from other rows?

It is unclear whether the metals were introduced into the system as atoms or as cations. Throughout the text, both notations (e.g., "Ti" and "Ti⁴⁺") are used interchangeably, which causes confusion. Clarification is needed regarding the chemical form of the dopants in the simulations.

The sentence “For Ti, V, Cr, W the 4⁺ oxidation state is usual” is scientifically questionable. Basic chemical knowledge suggests otherwise: vanadium typically exhibits a 5⁺ state, tungsten a 6⁺ state, and chromium most often 3⁺ or 6⁺. Only titanium commonly exists in the 4⁺ state in compounds. The authors should reconsider and rephrase this statement to reflect established oxidation state trends.

Figure 2 lacks clarity.
It is unclear why the partial density of states (pDOS) for Ti is missing. For consistency and clarity, the pDOS of Mo should also be shown in panels 2b and 2c, not just 2a. This would allow for easier comparison between doped and undoped MoSe₂.

Table 1 are overly dense and difficult to interpret.
Consider reorganizing the table to improve readability - perhaps by separating ΔG values and vacancy formation energies into distinct columns or sections.
If the current format is retained, the caption must be corrected. The authors state that Se-vacancy formation energies are shown in the right column, when they actually appear in the left.

The origin of Figure 3 and the methodology behind the presented adsorption probability calculations are not clear.
The supplementary materials should include at least one worked-out example of these calculations for a single analyte. Scientific articles should also serve an educational function, and this detail is currently missing. The authors write vaguely "in the presence of other competitors." At least one example should clearly specify which gases were used for the calculations.

The inclusion of Section 3.3 “Catalytic properties” is insufficiently justified.
The authors must explain why Cu:MoSe₂ and Pt:MoSe₂ were selected for catalytic performance evaluation. What criteria (electronic structure parameters or specific energy values) led to their selection? Additionally, the source and numerical values of the calculated energies are missing. Figure 5 should clearly display the energy barriers associated with the reaction steps.

The authors correctly note that there is no clear correlation between electronic structure and sensing selectivity.
However, this important observation deserves more in-depth discussion, perhaps with the proposal of a hypothesis or future machine-learning-based exploration, as briefly mentioned.

Summary
In its current form, the manuscript appears to be a compilation of isolated computational results without a clearly defined methodological framework or overarching conclusions. The study would benefit from a more systematic approach, clearer justification of modelling choices, and greater emphasis on the interpretability and applicability of the findings.

Author Response

Thank you for the careful reading and high evaluation of our manuscript. All your comments have been taken into account, and the necessary changes have been made in the revised manuscript. Nine new references were added to support our additions. The English language was checked and improved. Below is the line-by-line response to your questions, along with the additional changes in the revised manuscript. 

 

Reviewer Comments

Before final acceptance the following points should be explained:

The Introduction section requires supplementation.
The authors do not explain the rationale behind the selection of metals introduced into the MoSe₂ structure. If the intention was to include first-row transition metals, why were Mn and Zn excluded? Similarly, what was the justification for including W, Pd, and Pt from other rows?

Answer:

Thank you, the choice of the dopant was justified in the revised introductory section:

“Manganese and zinc were excluded due to difficulties in fabricating MoSe₂ doped with these species [34, 35] and a lack of comprehensive experimental data on the properties of these systems. Tungsten has been chosen as a dopant because it belongs to the same periodic table group as molybdenum, and tungsten dichalcogenides exhibit similar physical and chemical properties as MoSe₂ [36,37]. Platinum and palladium were considered to investigate how the ionic radii difference between impurities and molybdenum atoms [38] affects the electronic structure and sensing properties.” (p. 2)

 

Reviewer:

It is unclear whether the metals were introduced into the system as atoms or as cations. Throughout the text, both notations (e.g., "Ti" and "Ti⁴⁺") are used interchangeably, which causes confusion. Clarification is needed regarding the chemical form of the dopants in the simulations.

Answer:

Since we considered only substitutional defects, the formation of chemical bonds between impurities and ligands results in the formation of cations. The type of impurities was pointed out in the abstract of the revised manuscript. For the sake of clarity, we also pointed this out in the introductory section:

“In this work, we present the results of simulations examining the effects of doping MoSe₂ with substitutional impurities such as various 3d transition metals (Ti, V, Cr, Fe, Co, Ni, Cu) and other transition metals (W, Pd, Pt)…” (p. 3)

“The effects of embedded substitutional impurities on electronic structures can be divided into three groups. The first group consists of Ti, V, Cr, and W.” (p. 3)

 

Reviewer:

The sentence “For Ti, V, Cr, W the 4⁺ oxidation state is usual” is scientifically questionable. Basic chemical knowledge suggests otherwise: vanadium typically exhibits a 5⁺ state, tungsten a 6⁺ state, and chromium most often 3⁺ or 6⁺. Only titanium commonly exists in the 4⁺ state in compounds. The authors should reconsider and rephrase this statement to reflect established oxidation state trends.

Answer:

In general, the Reviewer is correct. However, in similar compounds (diselenides), these atoms have a 4+ oxidation state. We clarified this in the revised manuscript text and provided additional references. 

“The difference in the electronic structure of these impurities can be attributed to the variations in the typical oxidation states of the impurities. For Ti, V, Cr, and W, the 4+ oxidation state is usual in similar layered compounds  [45-47].” (p.4)

 

Reviewer:

Figure 2 lacks clarity.
It is unclear why the partial density of states (pDOS) for Ti is missing. For consistency and clarity, the pDOS of Mo should also be shown in panels 2b and 2c, not just 2a. This would allow for easier comparison between doped and undoped MoSe₂.

Answer:

The omission of Ti-pDOS was justified in the submitted manuscript:

“(see Fig. 2a, since Ti⁴⁺ have almost zero occupancy of the valence band, we omitted its on these picture).”

A partial DOS for Mo in undoped MoSe2 was added to panels (b) and (c) as requested by the reviewer.

 

Reviewer:

Table 1 are overly dense and difficult to interpret.
Consider reorganizing the table to improve readability - perhaps by separating ΔG values and vacancy formation energies into distinct columns or sections.
If the current format is retained, the caption must be corrected. The authors state that Se-vacancy formation energies are shown in the right column, when they actually appear in the left.

Answer:

We added the vacancy formation energy changes for Se as a separate column. The caption of the figure was changed accordingly. 

 

Reviewer:

The origin of Figure 3 and the methodology behind the presented adsorption probability calculations are not clear.
The supplementary materials should include at least one worked-out example of these calculations for a single analyte. Scientific articles should also serve an educational function, and this detail is currently missing. The authors write vaguely "in the presence of other competitors." At least one example should clearly specify which gases were used for the calculations.

Answer:

The method for calculating Maxwell-Boltzmann probabilities was described in section 2. In the revised manuscript, we pointed out the technique in the appropriate part of Section 3.2. The list of the compounds were also specified:

“The Maxwell-Boltzmann probability of the analyte's adsorption on active sites in the presence of all other competitors listed in Fig. 3 was calculated using equation (5).” (p.7)

 

Reviewer:

The inclusion of Section 3.3 “Catalytic properties” is insufficiently justified.
The authors must explain why Cu:MoSe₂ and Pt:MoSe₂ were selected for catalytic performance evaluation. What criteria (electronic structure parameters or specific energy values) led to their selection? 

Answer:

To clarify this point, in the revised manuscript, we extended the justification of the choice of substrates:

“In contrast to gas sensing in actual air, a mix of reactants consists only of a few species (usually solvents and reactants). Thus, contention for the active sites occurs only between chemical species and molecules in solvents. For the reaction in the liquid media, the contribution from the entropy plays a minor role (see section 2), and, therefore, the lower enthalpy of the adsorption of species is crucial for the simulation of the catalytic properties. Results reported in Table 1 demonstrate that adsorption of CO₂ is substantially favorable on copper and platinum sites in the MoSe₂ matrix. Therefore, these substrates were taken to evaluate the possible catalytic performance of doped MoSe₂.

Fig. 5 illustrates the reaction steps for the gradual hydrogenation of carbon dioxide in the liquid media by protons on both substrates. Both Cu- and Pt-doped substrates demonstrate a moderate (below 100 kJ mol^-1) energy cost for the first step.  Considering semi-empirical relationships between calculated energies and reaction temperatures, this reaction requires temperatures below 100 °C [58]. The energy costs of the following steps do not exceed this value. Thus, both copper and platinum-doped MoSe₂ are prospective catalysts for CO₂ conversion. ” (p. 9)

 

Reviewer:

Additionally, the source and numerical values of the calculated energies are missing. Figure 5 should clearly display the energy barriers associated with the reaction steps.

Answer:

We clarified this point in Section 2 of the revised manuscript:

“The energies of CO₂ conversion are defined as the differences between the total energies of products and reactants. Since the hydrogenation of CO₂ has been realized by hydrogen atoms from solution, each reaction step consists of a gradual barrierless approach of the proton to the active site.” (p.3)

The chemical formulas of the intermediates were added in updated Fig. 5.

 

Reviewer:

The authors correctly note that there is no clear correlation between electronic structure and sensing selectivity.
However, this important observation deserves more in-depth discussion, perhaps with the proposal of a hypothesis or future machine-learning-based exploration, as briefly mentioned.

Answer:

The possible usage of the results for machine learning was discussed in the revised manuscript:

“This conclusion suggests that collecting a larger number of experimental and theoretical results for further application of machine learning-based methods could be a solution for revealing the connection between sensing-related properties of impurities and features in electronic structure, as was done in recent works [54,55].” (p. 7)

 

Reviewer:

Summary
In its current form, the manuscript appears to be a compilation of isolated computational results without a clearly defined methodological framework or overarching conclusions. The study would benefit from a more systematic approach, clearer justification of modelling choices, and greater emphasis on the interpretability and applicability of the findings.

Answer:

The justification for model choices was added to the sensing and catalytic sections of the manuscript. The direct description of key conclusions from the findings was substantially extended. 

Reviewer 2 Report

Comments and Suggestions for Authors

The authors report on sensing properties of MoSe2 with transition metal (TM) substitutions, focusing on formaldehyde and ethanol analytes and environmental molecules (O2, CO2, H2O, NO2, etc). The investigation reveals the modification in the density of states after incorporating the TM impurities, the absorption energies and explicitly accounts for temperature effects and the competition amongst the different molecules for absorption. Also, the catalytic properties of MoSe2 for catalytic reduction processes were evaluated.

Tuning 2D materials for a rapid organic molecule detection is a topic of high interest. The authors indicate a good selectivity of two setups towards NO2 and ethanol. The paper deserves consideration for publication, pending some clarifications:

1. Given the relatively small changes of electronic properties, can the authors estimate the observable modifications in the resistance of the sensing device ?   

2. How can the spin-polarized regime induced by the magnetic impurities be exploited in a sensing device ?

 

Author Response

Thank you for the careful reading and high evaluation of our manuscript. All your comments have been taken into account, and the necessary changes have been made in the revised manuscript. Below is the line-by-line response to your questions, along with the additional changes in the revised manuscript. 

 

Reviewer:

The paper deserves consideration for publication, pending some clarifications:

1. Given the relatively small changes of electronic properties, can the authors estimate the observable modifications in the resistance of the sensing device ?   

Answer:

The change in resistivity is inversely proportional to the density of charge carriers. Based on the literature, the density of charge carriers in MoSe2 varies over a wide range: 10^15 cm^-3 for n-type and 10^18 cm^-3 for p-type. In addition, the number of transferred charges per unit of square depends on the concentration of active centers. Thus, for the calculation of exact changes of resistivity, the concentration of dopants and the initial density of the charge carriers should be known. The number of transferred electrons, multiplied by the saturation of active sites at a given temperature, can then be used to calculate the change in resistivity. We pointed out this opportunity in the revised manuscript. 

“The reported number of transferred electrons can be used to calculate the change in resistivity for a given concentration of impurities and charge density in the samples.” (Page 8).

Reviewer:

2. How can the spin-polarized regime induced by the magnetic impurities be exploited in a sensing device ?

Answer:

Thank you, we checked the change of the magnetic moments after adsorption of NO2 on magnetic impurities and discussed it in the revised manuscript:

“This significant charge transfer leads to a decrease in the magnetic moment of Cr, Fe, Co, and Ni impurities, which are 0.09, 0.19, 0.16, and 0.13 μ_B, respectively. Thus, magnetic impurities in the MoSe₂ matrix can be utilized to create highly sensitive sensors.” (Page 7).

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Review of the Revised Manuscript

 

In the revised version of the manuscript, the Authors have clarified some ambiguities and addressed several of the comments I raised in my review. However, many important issues remain unaddressed. In my opinion, these issues are critical enough that I cannot recommend the article for publication without further revisions.

 

Omitted issues:

1. Based on the information added to the manuscript, including literature references, it appears that the dopants introduced into the MoSe₂ system are assumed to substitute Mo atoms. However, this remains an assumption, as this key information is not explicitly stated anywhere (in the abstract, introduction, or results sections).
   I would like to emphasize that in layered materials, dopants can occupy interlayer positions or interstitial sites within the lattice. The Authors must clearly state and justify the assumed location of the dopants.
   In anticipation of a possible response: Figure 1b does not in any way demonstrate the location of Cu atoms, particularly since the figure caption does not specify which color represents Cu.

 

1. The data presented in Table 1 remain difficult to interpret. I repeat my earlier suggestion that the data would be much easier to analyze if the differentiating parameter were the substrates rather than the dopants. However, since this is merely a suggestion from me as a reviewer, it may be disregarded at the Authors discretion.

 

1. The choice of Cu:MoSe₂ and Pt:MoSe₂ for catalytic tests still has not been adequately justified. The Authors wrote: “Results reported in Table 1 demonstrate that adsorption of CO₂ is substantially favorable on copper and platinum sites in the MoSe₂ matrix. Therefore, these substrates were taken to evaluate the possible catalytic performance of doped MoSe₂.”
   However, if I have interpreted the data correctly (which is admittedly difficult due to the structure of the table) I ask the Authors to clarify on what basis this statement was made. While the selection of Pt appears justified and requires no further explanation, it is unclear what supports the claim that "adsorption of CO₂ is substantially favorable on copper."

 

	DH	DG 400C	DH 800C
MoSe2	-157.1	-139.8	-93.4
Ti	-241.2	-223.9	-177.5
V	-48.0	-30.7	15.7
Cr	-176.4	-159.1	-112.7
Fe	-39.9	-22.6	23.8
Co	-38.6	-21.3	25.1
Ni	-43.8	-26.5	19.9
Cu	-89.3	-72.0	-25.6
W	-51.7	-34.4	12
Pd	-95.4	-78.1	-31.7
Pt	-222.33	-205	-158.6

 

I would also like to point out that in Table 1, the Authors state that the listed values refer to the adsorption of substrates on Se vacancies, not on the introduced metallic dopants. This raises the question: on what basis is the claim of adsorption on Cu or Pt sites made? What, then, is the actual adsorption site in the studied systems?

 

1. Neither Figure 5 (whose caption once again lacks annotations regarding the colors used to represent the introduced dopants or the depicted substrates) nor the main text of the publication provides any numerical data, which undermines the credibility of the calculations and negatively affects the overall quality and clarity of the presented information.

 

1. I still believe that example calculations demonstrating the generation of at least one of the presented figures should be included in the Supplementary Information for the sake of clarity and transparency of the publication, however, if the Editor does not see the need, my remark can be disregarded."

 

 

 

While the manuscript has improved, several key issues, particularly regarding clarity of dopant location, interpretation of adsorption results, and justification of material selection, remain unresolved. I therefore recommend a major revision before the manuscript can be considered for publication.

 

Comments for author File: Comments.pdf

Author Response

Thank you for pointing out a few important issues in the revised manuscript. 

Below is the line-by-line answer to the questions.

 

Reviewer:

Omitted issues:

Based on the information added to the manuscript, including literature references, it appears that the dopants introduced into the MoSe₂ system are assumed to substitute Mo atoms. However, this remains an assumption, as this key information is not explicitly stated anywhere (in the abstract, introduction, or results sections).
I would like to emphasize that in layered materials, dopants can occupy interlayer positions or interstitial sites within the lattice. The Authors must clearly state and justify the assumed location of the dopants.

 

Answer:

We are surprised by this statement of the reviewer since the word “substitutional” was already used in the first sentence of the abstract:

“In this study, we present a comprehensive theoretical analysis of the modifications in the physical and chemical properties of MoSe₂ upon the introduction of substitutional transition metal impurities,..”

However, we stressed a substitutional type of discussed impurities in the introductory section:

“Notably, many investigations report that the sensing capabilities of MoSe₂ can be significantly enhanced through the incorporation of substitutional impurities.”

“These experimental findings have motivated density functional theory (DFT)-based simulations to investigate the effects of substitutional impurities on the sensing behavior of MoSe₂.”

 

 

Reviewer:
In anticipation of a possible response: Figure 1b does not in any way demonstrate the location of Cu atoms, particularly since the figure caption does not specify which color represents Cu.

 

Answer:

Thank you, we fixed Fig. 1.

 

Reviewer:

The data presented in Table 1 remain difficult to interpret. I repeat my earlier suggestion that the data would be much easier to analyze if the differentiating parameter were the substrates rather than the dopants. However, since this is merely a suggestion from me as a reviewer, it may be disregarded at the Authors discretion.

 

Answer:

Sorry, but we do not understand the expression “differentiating parameter was the substrates rather than the dopants.” Because we considered only one type of substrate (the surface of MoSe2) with and without dopants, we suppose that the author uses the word “substrate” to denote “analytes”.

Note that one of the key aims of our work is the simulation of competitive adsorption of different molecules that are present in actual air. We clarify this issue in the first sentence of section 3.2:

“The adsorption of different gases was simulated to evaluate the effect of impurities on the sensing properties of MoSe₂ in real-life conditions when water, some hydrogen, various volatile organic compounds (VOCs), carbon monoxide, and dioxide are competing for the adsorption on the active site.” (p.7).

The main takeaway from the table was already articulated in the first paragraph in section 3.2:

“Calculated enthalpies of the physical adsorption (ΔH), summarized in Table 1, demonstrate that the dopants affect the energetics of the adsorption only quantitatively, without a significant switch of the adsorption patterns. However, considering the contribution from entropy using equation (2), these quantitative changes are transformed into qualitative changes even at 400 °C. A further increase in temperature corresponds to a switch in the sign of the free energies for most analytes.”

We stressed the importance of these words in the revised manuscript:

“Thus, the large magnitude of the adsorption enthalpy (at zero temperature) obtained in DFT-based calculations does not correspond with the stable adsorption at working temperatures in the presence of other molecules in actual air.” (p.7)

 

Reviewer: 

The choice of Cu:MoSe₂ and Pt:MoSe₂ for catalytic tests still has not been adequately justified. The Authors wrote: “Results reported in Table 1 demonstrate that adsorption of CO₂ is substantially favorable on copper and platinum sites in the MoSe₂ matrix. Therefore, these substrates were taken to evaluate the possible catalytic performance of doped MoSe₂.”
However, if I have interpreted the data correctly (which is admittedly difficult due to the structure of the table) I ask the Authors to clarify on what basis this statement was made. While the selection of Pt appears justified and requires no further explanation, it is unclear what supports the claim that "adsorption of CO₂ is substantially favorable on copper."

 

Answer:

Thank you. We clarified the conditions for choosing of the substrates on page 10 of the revised manuscript:

“Therefore, only if the enthalpy of the adsorption of species is lower than the enthalpy of the adsorption of water, the substrate can be proposed as potentially suitable for the catalysis. Results reported in Table 1 demonstrate that adsorption of CO₂ is substantially more favorable than water on copper and platinum sites in the MoSe₂ matrix. Therefore, these substrates were taken to evaluate the possible catalytic performance of doped MoSe₂.”

 

Reviewer:

I would also like to point out that in Table 1, the Authors state that the listed values refer to the adsorption of substrates on Se vacancies, not on the introduced metallic dopants. This raises the question: on what basis is the claim of adsorption on Cu or Pt sites made? What, then, is the actual adsorption site in the studied systems?

Answer:

First, we did not understand the words “adsorption of substrates on Se vacancies”. Perhaps authors use the word “substrate” to denote “analytes”. Second, in the first paragraph of Section 3.1, we provide a detailed discussion of the stabilization of the MoSe2 lattice through the incorporation of substitutional impurities. We stressed this point at the end of the section in the revised manuscript:

“Thus we excluded consideration of the combination of substitutional defects and anionic vacancies from further consideration.” (p. 4). 

Regarding the actual 

 

Reviewer: 

Neither Figure 5 (whose caption once again lacks annotations regarding the colors used to represent the introduced dopants or the depicted substrates) nor the main text of the publication provides any numerical data, which undermines the credibility of the calculations and negatively affects the overall quality and clarity of the presented information.

 

Answer:

The key numerical value was already reported in the manuscript. To improve the clarity of the manuscript, we pointed out that the mentioned value corresponds with the rate-determining step of the reaction:

“Fig. 5 illustrates the reaction steps for the gradual hydrogenation of carbon dioxide in the liquid media by protons on both substrates. Both Cu- and Pt-doped substrates demonstrate a moderate (below 100 kJ mol^-1) energy cost for the rate-determining step.”

 

Reviewer:

I still believe that example calculations demonstrating the generation of at least one of the presented figures should be included in the Supplementary Information for the sake of clarity and transparency of the publication, however, if the Editor does not see the need, my remark can be disregarded."

 

Answer:

The coordinates of the atomic structures used for Fig. 5 were added to the file of the Supplementary Information.