Water Adsorption on MgO Surfaces: A Vibrational Analysis

Round 1

Reviewer 1 Report

The manuscript presents a systematic investigation of the adsorption of water on MgO surfaces using DFT calculations and Raman spectroscopy. The authors provide a robust methodology and the conclusions of the study add valuable insights into the surface science of MgO. The use of DFT calculations is well justified and provides compelling evidence for the arguments put forth in the study. Therefore, I think that this work can be accepted in this journal after some minor revisions that may help to improve this version:

1. The use of Crystal Software for DFT calculations is appropriate and the hybrid B3LYP Hamiltonian is a widely accepted tool for such calculations. However, a more detailed explanation of why the B3LYP functional was specifically chosen for this study would strengthen the methodology, given the wide array of functionals available.

2. While the manuscript provides detailed methodology, the rationale behind selecting specific surfaces ((001), (110), and (211)) of the face centered cubic (fcc) magnesium oxide for Raman spectrum calculation could be clarified. Also, it would be beneficial to elaborate more on the choice of four and eight layers of MgO for these calculations.

3. The authors are recommended to include a more in-depth discussion on the implications of their findings. For instance, how might the high reactivity of MgO surfaces with water impact its usage in practical applications?

4. The author mentioned many properties of nanomaterials in the introduction, and I suggest that the author briefly introduce some applications of nanomaterials in the introduction, such as catalysis [Nanoscale (10.1039/d2nr06665c)], batteries [Adv. Energy Mater. (10.1002/aenm.202300790)] could be mentioned.

5. The author mentions in Line 23 of the first page that 'The high surface area and the lower coordination sites of the nanoparticles make them particularly efficient when it comes to catalytic activity.' There is a lack of reference support here, and related studies [ACS Appl. Mater. Interfaces (10.1021/acsami.2c14041), Chem. Phys. (10.1016/j.chemphys.2020.110953)] on the high activity brought about by the high specific surface area and low coordination sites of nanoparticles could be mentioned.

Minor editing of English language required

Author Response

Comment 1. The use of Crystal Software for DFT calculations is appropriate and the hybrid B3LYP Hamiltonian is a widely accepted tool for such calculations. However, a more detailed explanation of why the B3LYP functional was specifically chosen for this study would strengthen the methodology, given the wide array of functionals available.

Answer 1. Generally speaking, hybrid functionals as B3LYP provide ‘good’ gap (not too small as with pure DFT hamiltonians, nor too large as with Hartree-Fock), then good response properties as polarizability and then, Raman intensity. We have added this complementary information in section Methods: ”We have used the hybrid B3LYP Hamiltonian^17,18 known as providing a good optical gap necessary for accurate polarizability and then Raman intensity calculations.”

Comment 2. While the manuscript provides detailed methodology, the rationale behind selecting specific surfaces ((001), (110), and (211)) of the face centered cubic (fcc) magnesium oxide for Raman spectrum calculation could be clarified. Also, it would be beneficial to elaborate more on the choice of four and eight layers of MgO for these calculations.

Answer 2. In our work we have effectively chosen three stable slabs of four layers only, with small Miller indices of the cubic MgO bulk, as a first Raman spectrum study of H2O-adsorbed MgO. Certainly, a larger set of surfaces could be studied in the future, with an increasing number of layers of the MgO slab in the numerical simulation.

Comment 3. The authors are recommended to include a more in-depth discussion on the implications of their findings. For instance, how might the high reactivity of MgO surfaces with water impact its usage in practical applications?

Answer 3. As a conclusion of our article, we added the following paragraph:

As MgO is extensively used for various applications (see the introduction part), its surface reactivity, demonstrated here with water, should be systematically considered. When it is stored in ambient air, MgO reacts rapidly with water to form various surface species. In the specific case of nanoparticles, for which the surface to volume ratio is very high, those reactions leads to the formation of brucite. Of course the chemical properties of the formed species are different from the standard MgO surface and will affect its efficiency, in particular for catalytic applications. To avoid this problem we strongly recommend paying attention to the storage conditions of MgO, ideally in dry air, in particular in its nanoparticles form.   

Common 4. The author mentioned many properties of nanomaterials in the introduction, and I suggest that the author briefly introduce some applications of nanomaterials in the introduction, such as catalysis [Nanoscale (10.1039/d2nr06665c)], batteries [Adv. Energy Mater. (10.1002/aenm.202300790)] could be mentioned.

Answer 4. Those references have been added in the bibliography.

Common 5. The author mentions in Line 23 of the first page that 'The high surface area and the lower coordination sites of the nanoparticles make them particularly efficient when it comes to catalytic activity.' There is a lack of reference support here, and related studies [ACS Appl. Mater. Interfaces (10.1021/acsami.2c14041), Chem. Phys. (10.1016/j.chemphys.2020.110953)] on the high activity brought about by the high specific surface area and low coordination sites of nanoparticles could be mentioned.

Answer 5. Those references have been added in the bibliography.

Reviewer 2 Report

In this study, Rerat et al. investigated on the interaction between different MgO surfaces and water using computed Raman Spectroscopy. Their aim was to elucidate the origin of four Raman peaks observed experimentally through a computational approach. The researchers constructed various intermediate structures and calculated their Raman spectra to match the experimental results. They proposed that the experimental Raman spectra could be explained by the dissociation of water on MgO surfaces, leading to the formation of brucite (Mg(OH)2), which contributed to an intense peak in the experimental spectra. The authors also argued that all peaks above 3650 cm-1 originated from OH chemisorbed intermediates. The paper is well-written and the DFT methods are relevant.

Based on my assessment I recommend this paper to be published after a major revision.

1.     It would be beneficial for the readers if the author included the Raman spectrum of free water in the inset of Figure 1. This addition would facilitate a direct comparison of peak shifts.

2.     In the case where water dissociation is not spontaneous and water adsorbs through H-bonding structures that are not at the minimum potential energy surface, the author should provide the reaction energy for water dissociation on the MgO(011) and MgO(112) surfaces.

3.     It would be interesting if the authors could compute the Raman spectra of a Brucite-like structure where Mg is attached to two -OH groups, and observe how the peak shifts. If two water molecules dissociate on the same Mg, this intermediate structure might be present. However, this analysis depends on the relative stability of the intermediates, which can be easily understood from the DFT energies.

4.     Regarding Figure 1, it is still unclear what the origin of the peak at 3670 cm-1 is. Is it from Figure 5 (112), as claimed, but masked by the intense brucite peak?

5.     In Figure 5, there is an intense peak observed at 3460 cm-1 on the MgO(011) surface. Is this peak also observed experimentally? If not, the author should investigate the favorability of water dissociation on the MgO(011) surface.

6.     The authors may consider providing the optimized coordinates for anyone who wants to reuse these results for their purpose.

Author Response

Comment 1. It would be beneficial for the readers if the author included the Raman spectrum of free water in the inset of Figure 1. This addition would facilitate a direct comparison of peak shifts.

Answer 1. The Raman spectrum of water as well as the brucite one are now discussed in the text and reported in Table 1, at the harmonic and anharmonic levels of calculation for the two atomic basis sets used in this work.

In addition, the experimental Raman spectrum of free water has been added in figure 1.

Comment 2. In the case where water dissociation is not spontaneous and water adsorbs through H-bonding structures that are not at the minimum potential energy surface, the author should provide the reaction energy for water dissociation on the MgO(011) and MgO(112) surfaces.

Answer 2. We have calculated the following energies:

The adsorption energy of water for the different surfaces is rather different. It appears that the (211) surface is less stable, compared to the (110). Its reactivity towards water is thus much higher.  

Comment 3. It would be interesting if the authors could compute the Raman spectra of a Brucite-like structure where Mg is attached to two -OH groups, and observe how the peak shifts. If two water molecules dissociate on the same Mg, this intermediate structure might be present. However, this analysis depends on the relative stability of the intermediates, which can be easily understood from the DFT energies.

Answer 3. In order to better compare the theoretical and experimental spectra, we have reported the anharmonic results with the richest basis set, in a new figure, for H2O/MgO(011) and MgO/(112), adding also the brucite peak. The obtained results for brucite are now reported in Table 1.

Comment 4. Regarding Figure 1, it is still unclear what the origin of the peak at 3670 cm-1 is. Is it from Figure 5 (112), as claimed, but masked by the intense brucite peak?

This peak is attributed to the chemical species formed on the (112) surface. It is now explicitly mentioned in the article.

Comment 5. In Figure 5, there is an intense peak observed at 3460 cm-1 on the MgO(011) surface. Is this peak also observed experimentally? If not, the author should investigate the favorability of water dissociation on the MgO(011) surface.

This peak is not experimentally observed, which is not surprising as the MgO(011) surface is rather stable compared to the (112) (see the energy calculations).   

Comment 6. The authors may consider providing the optimized coordinates for anyone who wants to reuse these results for their purpose.

Answer 6. The optimized atomic coordinates of the studied structures have been now reported in Supplementary Information.

Round 2

Reviewer 2 Report

The author has duly taken into account my concerns and effectively incorporated all the requested revisions into the manuscript. At this point, the manuscript is prepared and suitable for publication in Crystals.

English is fine. Minor editing is required.