Fabrication of ZnO-Fe-MXene Based Nanocomposites for Efficient CO₂ Reduction

Round 1

Reviewer 1 Report

Kannan et al report the fabrication of ZnO-Fe-MXene nanocomposite for application as catalysts for CO₂ reduction. The ZnO-Fe-MXene nanocomposites were synthesized using a hydrothermal method followed by a subsequent calcination process. They found that ZnO-Fe-MXene nanocomposites showed the highest current density of CO₂ reduction as compared to other samples, which was due to the synergistic contribution from both electronic and geometric effects.

After careful consideration, the authors reported the data with a lack of discussion on the physicochemical properties of samples. The electrochemical results are also unclear with no detailed explanation of electrode preparation. The graphical presentation of data is poor resolution and needs improvement. Therefore, I do not recommend this manuscript to be published in Catalysts. Additional comments and suggestions are listed as follows.

1. The preparation of the ZnO-Fe-MXene composite was roughly explained with no details of the mixing ratio of each precursor.
2. What is the doping content of Fe in the ZnO-Fe and ZnO-Fe-MXene composite?
3. The resolution of all graphs should be improved.
4. In Figure 1a, the index Zn (hkl) should be revised to ZnO(hkl). Using Zn may be misleading to the crystalline phase of Zn.
5. From the XRD pattern of ZnO, small diffraction peaks beside ZnO(100) and ZnO(101) were also detected. These may be the impurity or residue organic compounds remaining in the product.
6. The detailed description of electrochemical measurements and electrode preparation should be provided in the Experimental section. What is the catalyst loading on the working electrode?
7. The detailed information of MXene obtained from Sigma Aldrich should also be provided.
8. The authors show the values of several structural parameters calculated from the XRD data (Table 2). However, there was no discussion regarding these data.
9. Which diffraction peak was used to determine the crystallite size of each sample?
10. For comparison of catalytic activity for CO₂ reduction, it seems that the author used MXene as-received, while the ZnO-Fe-MXene composite was exposed to hydrothermal at 180 ^oC for 12 h and calcined at 400 ^oC for 2 h. Therefore, the properties of MXene after thermal processes might be changed from as-received MXene, which may have a significant effect on catalytic activity. I think it would be better to prepare pure MXene using the same condition with the ZnO-Fe-MXene composite for comparison.
11. From the EDS analysis, the amount of each element was reported in wt% or atom%?
12. I recommend the authors showing the EDS mapping images on ZnO-Fe, ZnO-MXene, and ZnO-Fe-MXene nanocomposite, along with their EDS spectrum.
13. The scan rate of the LSV curves should be given.
14. The abbreviation “eCR” should be given for the first time mentioned in the manuscript.
15. I recommend the authors adding the CV curves in both N₂ and CO₂ conditions of other samples in the supporting information.
16. Why the potential range of LSV was scanned from 0.3 to -2.0 V, while the potential window of CV curves was between -1.0 and 0.2 V?
17. The catalytic stability for CO₂ reduction should be performed and discussed.

Author Response

Reviewer 1:

Kannan et al report the fabrication of ZnO-Fe-MXene nanocomposite for application as catalysts for CO2 reduction. The ZnO-Fe-MXene nanocomposites were synthesized using a hydrothermal method followed by a subsequent calcination process. They found that ZnO-Fe-MXene nanocomposites showed the highest current density of CO2 reduction as compared to other samples, which was due to the synergistic contribution from both electronic and geometric effects.

After careful consideration, the authors reported the data with a lack of discussion on the physicochemical properties of samples. The electrochemical results are also unclear with no detailed explanation of electrode preparation. The graphical presentation of data is poor resolution and needs improvement. Therefore, I do not recommend this manuscript to be published in Catalysts. Additional comments and suggestions are listed as follows.

1. The preparation of the ZnO-Fe-MXene composite was roughly explained with no details of the mixing ratio of each precursor.

We have extensivly modified the ZnO-Fe-MXene and others composites preparation content and included in manuscript as dicussed below:

Preparation of ZnO nanoparticles

In a distinctive process, 0.22g of Zn (CH3COO)2.2H2O was dissolved in 20mL of de-ionized (DI) water and stirred well by using magnetic stirrer. 1M of NaOH was mixed by using constant stirring for 2 h at 353 K. The blend was relocated into 100 mL of Teflon-lined stainless-steel autoclave (TLSSA), sealed, and maintained at 453 K for 12 h. After reaction, the autoclave was then usually cooled down to room temperature (RT). The attained precipitate was cleaned off and washed a number of times with DI water and ethanol, correspondingly, and dried at 353 K for for approximately 3 h in a hot air oven and next calcinated by using muffle furnace at 673 K for 2 h.

Preparation of ZnO-Fe nanoparticles

0.22g of Zn (CH3COO)2.2H2O, 0.05g of Fe (NO3)2.9H2O were dissolved in 40mL of DI water and stirred well by using magnetic stirrer. 2M of NaOH was mixed by using constant stirring for 2 h at 353 K. The blend was relocated into 100 mL of TLSSA, sealed, and preserved at 453 K for 12 h. After reaction, the TLSSA was then usually chilled down to RT. The attained precipitate was filtered off and washed a number of times with DI water and ethanol, correspondingly, and dried at 353 K for for approximately 3 h in a hot air oven and next calcinated by using muffle furnace at 673 K for 2 h.

Preparation of ZnO-Mxene composite

0.2g of Ti3C2 MXene was dispersed in ethanol by ultrasonication (20 mins) followed by the addition of 0.22g of Zn (CH3COO)2.2H2O and 2M of NaOH ino the above solution, and stirred well by using magnetic stirrering for 2 h at 353 K. The blend was relocated into 100 mL of TLSSA, sealed, and preserved at 453 K for 12 h. After reaction, the TLSSA was then usually chilled down to RT. The attained precipitate was filtered off and washed a number of times with DI water and ethanol, correspondingly, and dried at 353 K for for approximately 3 h in a hot air oven and next calcinated by using muffle furnace at 673 K for 2 h.

Preparation of the ZnO-Fe-MXene nanocomposite

0.2g of Ti3C2 MXene was dispersed in ethanol by ultrasonication (20 mins) followed by the addition of 0.22g of Zn (CH3COO)2.2H2O, 0.05g of Fe (NO3)2.9H2O, 2M of NaOH ino the above solution, and stirred well by using magnetic stirrering for 2 h at 353 K. The blend was relocated into 100 mL of TLSSA, sealed, and maintained at 453 K for 12 h. After reaction, the TLSSA was then naturally cooled down to RT. The attained precipitate was filtered off and washed a number of times with DI water and ethanol, correspondingly, and dried at 353 K for for approximately 3 h in a hot air oven and next calcinated by using muffle furnace at 673 K for 2 h.

1. What is the doping content of Fe in the ZnO-Fe and ZnO-Fe-MXene composite?

The composition of precursors is discussed in composite preparation section as follows:

“0.05g of Fe (NO3)2.9H2O was used for the preparation of ZnO-Fe and ZnO-Fe-MXene nanocompostie.”

As per our EDS analysis, we observed 3.55 (wt%) Fe in final composites as discussed in morphological analysis section.  

“The chemical composition data derived for the samples from the EDS analysis (wt %) is indicated as Zn (28.72%), Fe (3.55%), Ti (21.38%), Al (1.65%), C (18.26%) and O (26.44%) shown in Fig. 3”

1. The resolution of all graphs should be improved.

We have improved the resolution of all graphs.

1. In Figure 1a, the index Zn (hkl) should be revised to ZnO(hkl). Using Zn may be misleading to the crystalline phase of Zn.

We have changed the ZnO (hkl) in figure 1 based on the reviewer valuable suggestion.

1. From the XRD pattern of ZnO, small diffraction peaks beside ZnO(100) and ZnO(101) were also detected. These may be the impurity or residue organic compounds remaining in the product.

We have included this statement in XRD discussion based on the reviewer valuable suggestion.

1. The detailed description of electrochemical measurements and electrode preparation should be provided in the Experimental section. What is the catalyst loading on the working electrode?

Thanks to the reviewer for valuable comments. We have included detailed description of electrochemical measurements and electrode preparation in main manuscript as discussed below:

“Electrochemical experiments were executed with a Gamry electrochemical analyzer (reference 3000, Gamry Co., USA), using a standard 3-electrode system at RT. A Platinum wire, Ag/AgCl and glassy carbon (GC) with the diameter of 5 mm was used as counter, reference, and working electrodes correspondingly. 2 mg of prepared nanomaterials (catalyst) was scattered in a solution, which is a mixture of 200 μL of water and 5 μL of 5% Nafion solution, employing ultra-sonication technique for one hour to produce black ink with homogeneity. Proceeding to the sample filling, the GCE was well-polished with 0.05 μm of an Aluminum oxide powder and cleansed meticulously with distilled water. Then, 5 μL of ink was placed on the surface of GCE and dehydrated beneath an infrared lamp for 10 min to attain a catalyst sheet. Electrochemical measurements for CO-stripping, the CO were fizzed into the 0.5 M of NaOH solution for 15 min. Cyclic Voltammetry (CV) and electrochemical impedance spectroscopy (EIS) investigation were utilized. CV was conducted in the − 1.0 to + 0.4 V vs. E (V vs Ag/AgCl) under CO2 and N2 conditions, and the sweep rate of 50-200 mVs−1. EIS data were acquired in a frequency range of 0.2–100000 Hz with amplitude (10 mV).”

1. The detailed information of MXene obtained from Sigma Aldrich should also be provided.

We have given the detailed information of MXene in the materials section.

1. The authors show the values of several structural parameters calculated from the XRD data (Table 2). However, there was no discussion regarding these data.

We have extended our discussion for XRD analysis (table 1).  Following content has been included in the main manuscript.

“In this paper, iron was doped in the zinc oxide with MXene, and there are two valence states of iron. In the literature, the iron in zinc oxide is trivalent, and the radius of Fe3+ (0.078 nm), and the radius of the Zn2+ (0.074 nm) are close, so that the change of the lattice constant, crystallite size, dislocation density and lattice strain are small, and the ZnO material does not undergo significant lattice distortion. Figure 1a shows the XRD pattern of iron-doped zinc oxide. Compared with the ZnO (hexagonal), the structure of zinc oxide after Fe doping is a hexagonal structure, and the doping does not change the symmetry of the crystal structure. Pure, and Fe doped ZnO nanoparticles showed crystallite size (27.89 & 18.42 nm) from the table 1. Reason for decrease the crystallite size, the Fe atoms that do not shift onto the replacement sites produce crystallinity loss within the hexagonal crystal structure and diminish the crystallite size and are also accountable for enlargement of the peaks. The pointed peaks demonstrate the hexagonal crystalline nature of the synthesized hybrids. From the XRD of hybrids, it is apparent that the peaks are expanded and have lower intensity owing to the occurrence of etched MXene with Fe doped ZnO nanoparticles. All of the foremost peaks of ZnO and MXene are present in all composite materials, and this is an understandable confirmation of the flourishing creation of the hybrid composites.”

1. Which diffraction peak was used to determine the crystallite size of each sample?

We used all diffraction peaks for calculating the average crystallite size of all the prepared samples.

1. For comparison of catalytic activity for CO2 reduction, it seems that the author used MXene as-received, while the ZnO-Fe-MXene composite was exposed to hydrothermal at 180 ^oC for 12 h and calcined at 400 ^oC for 2 h. Therefore, the properties of MXene after thermal processes might be changed from as-received MXene, which may have a significant effect on catalytic activity. I think it would be better to prepare pure MXene using the same condition with the ZnO-Fe-MXene composite for comparison.

We thank the reviewer for the suggestion regarding comparison of catalytic activity for CO₂ reduction. This might be interesting to see the impact of thermal process. We will carry out this study in the future as it will be another study.

1. From the EDS analysis, the amount of each element was reported in wt% or atom%?

We have reported the amount of each element in wt% (EDS analysis).

1. I recommend the authors showing the EDS mapping images on ZnO-Fe, ZnO-MXene, and ZnO-Fe-MXene nanocomposite, along with their EDS spectrum.

 We thank the reviewer for the suggestion to improve the studies by EDS mapping analysis. The authors were aware about it. However, due to the unavailability of the facilities, we have done SEM with EDAX for morphological and composition analysis.

1. The scan rate of the LSV curves should be given.

We have added the scan rate of LSV curves in the manuscript.

1. The abbreviation “eCR” should be given for the first time mentioned in the manuscript.

We have abbreviated electrochemical reduction (eCR) in the manuscript.

1. I recommend the authors adding the CV curves in both N2 and CO2 conditions of other samples in the supporting information.

We have added the CV curves in both N₂ and CO₂ conditions of other samples in the SI.

1. Why the potential range of LSV was scanned from 0.3 to -2.0 V, while the potential window of CV curves was between -1.0 and 0.2 V?

LSV and CV must be different. In case of CV, the peak CO₂RR shows a shape peak due to it is included both CO₂RR current and capacitive current. In case of LSV, it may mainly current from CO₂RR (still existence a bit of capacitive current), so then LSV curves show lower E_1/2 compared to that of CV. The LSV from -0.3 ~ -0.6 V (mass transfer region) at low rotation rate of 0 - 400 rpm, they show broad peaks corresponding to limited mass transfer process, in which CO₂ gas is difficult to approach the active species due to your deposited catalyst layer on GCE may too thick or your active material may too much porosity. Other LSV from 625 - 1600 rpm look seem nice curves because the mass transfer was improved by electrode rotation.

1. The catalytic stability for CO2 reduction should be performed and discussed.

 

Reviewer 2 Report

In their manuscript, Kannan  et al. synthesized a novel ZnO-Fe-MXene nanocomposite and they claim this nanocomposite shows a high current density of 18.75 mA/cm2 under the CO2 and can be used in CO2 reduction. 

It would be good to have a reference for the optical bandgap of zinc oxide.

Schematic illustration of charge transfer and separation in the composite and schematic illustration of assembly may be useful.

Figure 5: the legends and labels of x and y axes of the inset plot are barely readable; please use one legend for the whole figure and do not change the color of the lines in the inset plot. 

I recommend the authors to compare the performance of their composite to other composite and report how much improvement they have achieved using their composite.

Author Response

 

 

Reviewer 2:

In their manuscript, Kannan et al. synthesized a novel ZnO-Fe-MXene nanocomposite and they claim this nanocomposite shows a high current density of 18.75 mA/cm2 under the CO2 and can be used in CO2 reduction. 

It would be good to have a reference for the optical bandgap of zinc oxide.

We have added the reference for the optical bandgap of ZnO in the manuscript.

“Among all, zinc oxide has (ZnO, n-type semiconductor, optical bandgap: 3.37 eV) [10]”.

Schematic illustration of charge transfer and separation in the composite and schematic illustration of assembly may be useful.

We have added the reaction mechanism of the eCR on ZnO-Fe-MXene hybrids (Fig. 6) in the manuscript.

Figure 5: the legends and labels of x and y axes of the inset plot are barely readable; please use one legend for the whole figure and do not change the color of the lines in the inset plot. 

As per reviewer suggestion, we have modified the graph.

I recommend the authors to compare the performance of their composite to other composite and report how much improvement they have achieved using their composite.

We have added the Comparison of eCR performance on various 2D metal oxide based catalysts (Table 3) in the manuscript.

Reviewer 3 Report

This work by Kannan et al. discuss the electrocatalytic activity of a composite of MXene and ZnO doped with Fe toward the CO₂ reduction reaction. Despite the material seems to exhibit promising performances, the paper needs extensive editing prior to be considered for publication. The following comments should be carefully addressed:

1. the nature of the 2D MXene materials must be disclosed (morphological analysis suggests that it is based on Ti, but this is never mentioned in the text);
2. the electrochemical setup for studying the electrocatalytic properties of the materials must be fully described, including the way the electrode was prepared starting from the MXene-ZnO-Fe material. This is necessary both for reproducibility reason and because different apparatus can give slightly different responses;
3. the origin of Cl and F peaks in Fig.3 must be discussed;
4. the electochemical performances in Figure 4a and 4b should be discussed in terms of capacitive current rather than reduction peaks;
5. in Table 2 it should be indicated that the current density was measured in CO2 saturated atmosphere;
6. the authors statement that bicarbonate is not participating and the reduction current is mainly due to dissolved CO2 must be proved;
7. the interpretation of the EIS is not clear, as the semicircle for the MXene-ZnO-Fe material seems larger than for ZnO. This must be clarified;
8. all abbreviations must be defined (e.g. FESEM, eCR, GCE, eCO2R)
9. most important, the language is very poor and makes it very difficult to read and understand the manuscript. Just to give a few examples: the first line skips many intermediate steps and must be split in a longer but better detailed argument; it is not clear that the numbers in  parenthesis close to methanol and ethanol are energy densities as this si explained only later; what is the meaning of "ionic nature in the visible region" (end of introduction)?. However, it is not only the introduction but the entire paper that must be better written;
10.  in the conclusion the authors refer to the lower cost of their material, however first we do not know what is the material, second they must clarify what is the costly material for comparison;
11. last, the introduction lacks of references to works that analyzed the catalytic activity of MXenes to CO2 reduction reaction (e.g. J. Mater. Chem. A, 2018,6, 21885-21890, ACS Nano 2017, 11, 11, 10825-10833) 

 

Author Response

Reviewer 3:

This work by Kannan et al. discuss the electrocatalytic activity of a composite of MXene and ZnO doped with Fe toward the CO2 reduction reaction. Despite the material seems to exhibit promising performances, the paper needs extensive editing prior to be considered for publication. The following comments should be carefully addressed:

1. the nature of the 2D MXene materials must be disclosed (morphological analysis suggests that it is based on Ti, but this is never mentioned in the text);

Thanks for the comment. We have mentioned Ti in the results and discussion part.

1. the electrochemical setup for studying the electrocatalytic properties of the materials must be fully described, including the way the electrode was prepared starting from the MXene-ZnO-Fe material. This is necessary both for reproducibility reason and because different apparatus can give slightly different responses;

Thanks for the comment. We have included detailed description of our experimental methods. The electrochemical setup for studying the electrocatalytic properties of the prepared nanocomposites has also been discussed in main manuscript as follow:

“  Electrochemical experiments were executed with a Gamry electrochemical analyzer (reference 3000, Gamry Co., USA), using a standard 3-electrode system at RT. A Platinum wire, Ag/AgCl and glassy carbon electrode (GCE) with the diameter of 5 mm were used as a counter, reference, and working electrodes correspondingly. 2 mg of prepared nanomaterials (catalyst) was scattered in a solution, which is a mixture of 200 μL of water and 5 μL of 5% Nafion solution, employing the ultra-sonication technique for one hour to produce black ink with homogeneity. Proceeding to the sample filling, the GCE was well-polished with 0.05 μm of an Aluminum oxide powder and cleansed meticulously with distilled water. Then, 5 μL of ink was placed on the surface of GCE and dehydrated beneath an infrared lamp for 10 min to attain a catalyst sheet. Electrochemical measurements for CO-stripping, the CO were fizzed into the 0.5 M of NaOH solution for 15 min. Cyclic Voltammetry (CV) and electrochemical impedance spectroscopy (EIS) investigation were utilized. CV was conducted in the − 1.0 to + 0.4 V vs. E (V vs Ag/AgCl) under CO₂ and N₂ conditions, and the sweep rate of 50-200 mVs⁻¹. ZnO, MXene, ZnO-Fe, ZnO-MXene, and ZnO-Fe-MXene nanocomposite Linear sweep voltammetry (LSV) was conducted in the − 2.0 to + 0.3 V vs. E (V vs Ag/AgCl) EIS data were acquired in a frequency range of 0.2–100000 Hz with amplitude (10 mV).”

 

1. the origin of Cl and F peaks in Fig.3 must be discussed;

We have discussed about the origin of Cl and F peaks in the fig.3.

1. the electochemical performances in Figure 4a and 4b should be discussed in terms of capacitive current rather than reduction peaks;

We have discussed electrochemical performance in terms of the capacitive current.

1. in Table 2 it should be indicated that the current density was measured in CO2 saturated atmosphere;

We have added CO2 saturated atmosphere in the table 2.

1. the authors statement that bicarbonate is not participating and the reduction current is mainly due to dissolved CO2 must be proved;

Thanks for the comment. Here it should be noted that this can be proved by two ways:1. By performing experiment using isotropic CO2 (C¹³), or comparing results in presence of N2 and CO2. In our case, we have discussed electrochemical results obtained in presence of N2 and CO2. If bicarbonate is participating in reducrion current, we should have obtained same current in presence of N2 and CO2. But this is not the case. Thus it is safe to say that bicarbonate is participation in reduction process.

1. the interpretation of the EIS is not clear, as the semicircle for the MXene-ZnO-Fe material seems larger than for ZnO. This must be clarified;

We have clearly discussed about the EIS studies in the manuscript.

1. all abbreviations must be defined (e.g. FESEM, eCR, GCE, eCO2R)

We have abbreviated the SEM, eCR, GCE, eCO2R in the manuscript.

1. most important, the language is very poor and makes it very difficult to read and understand the manuscript. Just to give a few examples: the first line skips many intermediate steps and must be split in a longer but better detailed argument; it is not clear that the numbers in parenthesis close to methanol and ethanol are energy densities as this si explained only later; what is the meaning of "ionic nature in the visible region" (end of introduction)?. However, it is not only the introduction but the entire paper that must be better written;

We have improved the English language in the whole manuscript.

1.  in the conclusion the authors refer to the lower cost of their material, however first we do not know what is the material, second they must clarify what is the costly material for comparison;

We have modified the conclusion part based on the reviewer valuable suggestion.

1. last, the introduction lacks of references to works that analyzed the catalytic activity of MXenes to CO2 reduction reaction (e.g. J. Mater. Chem. A, 2018,6, 21885-21890, ACS Nano 2017, 11, 11, 10825-10833) 

We have included the catalytic activity of MXenes to CO2­ reduction recent works added in the introduction part.

Reviewer 4 Report

The work performed in the manuscript titled "Fabrication of ZnO-Fe-MXene based nanocomposites for efficient CO2 reduction" is interesting

A few comments need to be addressed

Add ZnO deposition technique in the introduction.

Why did you choose hydrothermal when compared to other ZnO deposition techniques?

Cite the latest literature on ZnO deposition  "Single-pot ZnO nanostructure synthesis by chemical bath deposition and their applications"

 

Author Response

Reviewer 4:

The work performed in the manuscript titled "Fabrication of ZnO-Fe-MXene based nanocomposites for efficient CO2 reduction" is interesting

A few comments need to be addressed

Add ZnO deposition technique in the introduction.

We have added the ZnO deposition technique in the introduction part.

Why did you choose hydrothermal when compared to other ZnO deposition techniques?

Hydrothermal method extends eminent advantages including sample purity, low cost and eases to fabrication. Additionally, it has further reimbursement such as much superior surface area, controllable particle size, low temperature method, creation of particles with a narrow particle size distribution, elevated purity and also it needs reasonable resources compared to the physical and methods like melt mixing, laser ablation, electric arc deposition, chemical bath deposition and ion implantation etc.,

Cite the latest literature on ZnO deposition "Single-pot ZnO nanostructure synthesis by chemical bath deposition and their applications"

We have added the ZnO deposition techniques literature reports in the introduction part.

 

 

Round 2

Reviewer 1 Report

The authors adequately addressed all of my comments and suggestions. The manuscript was improved considerably by helpful comments from all reviewers.  This manuscript can be accepted for publication in Catalysts. 

Reviewer 3 Report

The authors kindly answered all questions and greatly improved the manuscript. The paper can be accepted for publication.