The Behavior of Carbon Dots in Catalytic Reactions

Round 1

Reviewer 1 Report

In this review, the authors partially sort out the catalytic aspects of carbon dots. The logic of the article is clear, the topic selection scale is appropriate, and it has academic reference value, but there are still shortcomings such as lack of induction and failure to reveal universal laws, and major modification are required to be accepted by catalysts. Specific comments are as follows:

1. Pay attention to the unity of abbreviations in the whole text, and the abbreviation of carbon dots are mixed to CD and CDs which makes the article messy. Abbreviations that appear for the first time need to indicate the full name, such as EXAFS, etc., and authors need to check article carefully to avoid such simple writing mistakes.

2. There is a serious lack of discussion about the surface properties of CDs, for example, how many categories should the authors think surface properties should be divided into? What is the relationship between the surface properties of CDs and the catalytic performance?

3. Section 2.3 lacks an introduction to stability. In fact, the entire section is devoted to describing the chemistry of carbon dots as catalysts, and I suggest to correspond the description of the surface properties with stability.

4. On page 6, it is stated that "As will be seen in these reviews, the focus has mainly been on their use, rather than on the morphology of the CD after the reaction." Not reasonable. Because the relevant system is based on "CDs as catalysts" as the main research subject, this is the work that must be done in related research. In fact, the author provides an overview of this aspect on the middle page below. I suggest further refinement.

5. The title of section 3.1 is confusing. The logical framework is incorrect and inconsistent with the content.

6. The author has reviewed a large number of carbon dots-metal materials as catalysts, but in fact, there are many cases of carbon dots themselves as catalysts and non-metallic doped carbon dots as catalysts, and the authors must complete corresponding supplements.

7. The synthesis requires a further summary of work in this area. For example, the relationship between the structure (raw material) of carbon dots and catalytic performance, what kind of structure is more suitable for catalyzing what kind of system? What are the advantages of carbon dots catalysis over traditional catalysts?

8. As a review, the authors' outlook on relevant areas is too sketchy. Does the counterion of the carbon dots affect the catalytic performance? Is there any relationship between the luminescent properties of carbon dots and catalytic performance? Is it possible to visualize catalytic processes using carbon dot luminescence?

9. Representative work related to carbon dots, especially with regard to the surface properties of carbon dots, needs to be cited:J. Phys. Chem. Lett. 2023, 14, 1088.; J. Phys. Chem. C 2019, 123, 22447.; J. Mater. Chem. C 2020, 8, 8980.; J. Mater. Chem. C, 2017, 5, 4951-4958.; Acta Phys.-Chim. Sin. 2020, 2005004.; Dyes Pigment., 2021, 190, 109287. Colloids and Surfaces A 2017, 529, 38–44.

Author Response

Dr. Duangkamon Phuakkhaw

MDPI Catalysts Editorial Office

St. Alban-Anlage 66,

4052 Basel,

Switzerland

26^th July 2023

Dear Dr Phuakkhaw

We thank the referees for their comments and we have responded to all the points raised. The changes have added to our original version. A revised version indicates the changes made in yellow highlight. The specific additions relate to the comments raised by the referees as well as some changes we have also made to enhance readability. We have added two more figures that will assist with the data discussed in the review.

We trust that the responses have addressed the questions raised.

Regards

Prof Neil Coville

REFEREE 1

• lines 171-172:  While CDs have a small size, their surface area tends to vary and can be lower than expected. - can you please specify, why "the surface area can be lower than expected"?

REPLY: We have modified this ‘The surface area can vary between 16.4 m² g^-1 [51] and 1690 m² g⁻¹[52].’  The reason for the statement: surface areas should be very large as the CDs are small. However, in some reports they are listed as being very small. This could be due to the presence of surface groups/insufficient drying/loss of micropores/packing etc. We are not sure as to why the range is so large. We merely wish to point out the observation at this stage.

• line 406: it is not clear from the text, where did Ag ions came from?

REPLY: We have modified the sentences ‘Similar studies were performed on Cu/CD and Ag/CD catalysts to  ‘Similar studies were performed on Cu/CD and Ag/CD catalysts made from Ag or Cu ions, and CD mixtures’. We checked the SI and the paper but have not seen mention of the actual salts that were used; they appear not to have been reported? We assume that nitrates were used.

• Can authors discuss the reason why in different experiments CD structure is retained/is lost? What are the reason for such a difference and can it be predictable whether structure changes or not? 

REPLY: Thanks for this comment, which is indeed central to the review. We have now added more information to explain this issue. This includes the addition of an Ellingham diagram (Figure 7; p 14) and comments on this diagram.

We have also added a summary to the review outlining the different possibilities.

Summary of CD conversion reactions

A consideration of papers published in the area leads to some generalisations on metal-CD mixtures that have can be made and the role this will have in catalysis.

1. CDs can act as carbocatalysts and their reactions, as is known, will be dependent on their functional groups.
2. CDs can react with each other, either through physical bonds or through chemical bonds, to create CD assemblies. These assemblies could then be used as carbocatalysts.

• CDs can react at temperatures of ca 200 ^oC to convert to carbon sheet like structures and the graphicity of the sheets have been shown to increase with temperature. This will modify the catalytic property of the carbon

1. CDs can lose many of their surface functional groups at temperatures > 200 ^oC and this will impact on their chemical properties
2. In the presence of easily reducible metal oxide catalysts, CDs can be converted to carbon sheet like materials at temperatures < 100 ^o The ability to achieve this will be dependent on the metal reducibility with directions given by Ellingham diagrams.
3. Ellingham diagrams have been determined for bulk metals but metal nanoparticles are expected to have similar temperature values for reactions with carbon

• The chemical process by which CDs are converted to a sheet-like carbon is not known.

• Figure 6 seems to be pasted incorrectly

REPLY: Thanks. We have moved the words ‘Figure 6’ to 2 lines later viz. ‘…..for use in the Fischer-Tropsch (FT) reaction (Figure 6).’

• line 23: as as well and line 369 photocatlytic

Corrections made

REFEREE 2

1. Pay attention to the unity of abbreviations in the whole text, and the abbreviation of carbon dots are mixed to CD and CDs which makes the article messy. Abbreviations that appear for the first time need to indicate the full name, such as EXAFS, etc., and authors need to check article carefully to avoid such simple writing mistakes.

REPLY: Thanks for the comment. We have now gone through and given full names for abbreviations. The comment on the nomenclature to use for CDs is a problem and we are aware of this. To address this we have now done the following

1. Made a statement that we will use the abbreviation for all carbon dots p5 “In this review the term CD will be used for all carbon dots shown in Figure 1.”
2. The issue of the CD and metal combination is problematic. In addition, in many papers the nomenclature used is not correct. To avoid the issue we have called all CD and metal combinations as CD/metal to indicate that the metal particle is larger than or the same size as the CD.

• The issue of calling the complex as CD@metal is also problematic. This nomenclature implies a core@shell structure and in most cases, this has not been seen. We have avoided its use unless it is clear that this has occurred ( see old Fig 10 with TiO₂ covered by a graphene layer)

2. There is a serious lack of discussion about the surface properties of CDs, for example, how many categories should the authors think surface properties should be divided into? What is the relationship between the surface properties of CDs and the catalytic performance?

REPLY:  Thanks for this comment. We have now added more to this section. We have now specifically mentioned the change in surface charge on the CDs achieved by CD surface reactions. This has been done under the topic of carbocatalysis. We have commented on their reactions with metal ions or metal particles.

3. Section 2.3 lacks an introduction to stability. In fact, the entire section is devoted to describing the chemistry of carbon dots as catalysts, and I suggest to correspond the description of the surface properties with stability.

REPLY: We have renamed this section, as the issue of stability was not the key point in these few paragraphs.

The issue of stability is central to the review and we have now added more to the other sections on the topic. Indeed, we have added a summary of the results of the review, which addresses the issue

4. On page 6, it is stated that "As will be seen in these reviews, the focus has mainly been on their use, rather than on the morphology of the CD after the reaction." Not reasonable. Because the relevant system is based on "CDs as catalysts" as the main research subject, this is the work that must be done in related research. In fact, the author provides an overview of this aspect on the middle page below. I suggest further refinement.

REPLY: Thanks. We agree that this needed a restatement. We have modified our statement on this.

5. The title of section 3.1 is confusing. The logical framework is incorrect and inconsistent with the content.

REPLY: Thanks. The title from section 3.1 was changed from ‘Carbon dots (CDs) as catalysts (carbocatalysis)’ to ‘Applications of carbon dots-based catalysts’. We have had to make some changes to ensure our category on carbocatalysis is more clear. We did not want to cover this section in extensive detail, as there are many excellent reviews on the topic from many perspectives.

6. The author has reviewed a large number of carbon dots-metal materials as catalysts, but in fact, there are many cases of carbon dots themselves as catalysts and non-metallic doped carbon dots as catalysts, and the authors must complete corresponding supplements.

REPLY: Correct. This is covered in our section on carbocatalysts. I hope the response to (5) above resolves this.

7. The synthesis requires a further summary of work in this area. For example, the relationship between the structure (raw material) of carbon dots and catalytic performance, what kind of structure is more suitable for catalyzing what kind of system? What are the advantages of carbon dots catalysis over traditional catalysts?

RELPY: We have now added more to the section on carbocatalysis to address this issue. The review has not focussed on carbocatalysis as such but on the role of temperature and metals on changing the behaviour/morphology of the CDs that have been used in catalysis

A line has been added to the introduction on the advantages of CDs, versus other catalysts

8. As a review, the authors' outlook on relevant areas is too sketchy. Does the counterion of the carbon dots affect the catalytic performance? Is there any relationship between the luminescent properties of carbon dots and catalytic performance? Is it possible to visualize catalytic processes using carbon dot luminescence?

REPLY: WE have added information to address the different issues i) on the charge on the CD and  ii) the general decrease in PL as functional groups are modified/coordinated with metals.

On the issue of monitoring a catalytic reaction with PL:  We did not address this issue in the review. It is known that modification of the CD surface will modify the cD PL properties. Indeed this is how metal ions can be detected in solutions containing appropriate CDs. One could thus monitor a gas phase reaction by following the changes in concentration of products/reactants by UV-vis studies, that could be affected by changes in the CD surface chemistry (PL data) over time.

9. Representative work related to carbon dots, especially with regard to the surface properties of carbon dots, needs to be cited:J. Phys. Chem. Lett. 2023, 14, 1088.; J. Phys. Chem. C 2019, 123, 22447.; J. Mater. Chem. C 2020, 8, 8980.; J. Mater. Chem. C, 2017, 5, 4951-4958.; Acta Phys.-Chim. Sin. 2020, 2005004.; Dyes Pigment., 2021, 190, 109287. Colloids and Surfaces A 2017, 529, 38–44.

REPLY: Thanks for the useful references that have now been added. We have added other 2023 references, to bolster the review. We have been unable to access the ref: Acta Phys.-Chim. Sin. 2020, 2005004.

Author Response File: Author Response.pdf

Reviewer 2 Report

The review by Mokoloko et. al. is devoted to the use of carbon dots (CDs) in catalytic reactions. The review is well written and, what is the most important for review, it has a paragraph about future directions of studies. Nevertheless, the manuscript can be improved. Below, you'll see my comments:

lines 171-172:  While CDs have a small size, their surface area tends to vary and can be lower than expected. - can you please specify, why "the surface area can be lower than expected"?

line 406: it is not clear from the text, where did Ag ions came from

Can authors discuss the reason why in different experiments CD structure is retained/is lost? What are the reason for such a difference and can it be predictable whether structure changes or not? 

Minor correction:

 Figure 6 seems to be pasted incorrectly

line 23: as as well

line 369 photocatlytic

Author Response

Dr. Duangkamon Phuakkhaw

MDPI Catalysts Editorial Office

St. Alban-Anlage 66,

4052 Basel,

Switzerland

26^th July 2023

Dear Dr Phuakkhaw

We thank the referees for their comments and we have responded to all the points raised. The changes have added to our original version. A revised version indicates the changes made in yellow highlight. The specific additions relate to the comments raised by the referees as well as some changes we have also made to enhance readability. We have added two more figures that will assist with the data discussed in the review.

We trust that the responses have addressed the questions raised.

Regards

Prof Neil Coville

REFEREE 1

• lines 171-172:  While CDs have a small size, their surface area tends to vary and can be lower than expected. - can you please specify, why "the surface area can be lower than expected"?

REPLY: We have modified this ‘The surface area can vary between 16.4 m² g^-1 [51] and 1690 m² g⁻¹[52].’  The reason for the statement: surface areas should be very large as the CDs are small. However, in some reports they are listed as being very small. This could be due to the presence of surface groups/insufficient drying/loss of micropores/packing etc. We are not sure as to why the range is so large. We merely wish to point out the observation at this stage.

• line 406: it is not clear from the text, where did Ag ions came from?

REPLY: We have modified the sentences ‘Similar studies were performed on Cu/CD and Ag/CD catalysts to  ‘Similar studies were performed on Cu/CD and Ag/CD catalysts made from Ag or Cu ions, and CD mixtures’. We checked the SI and the paper but have not seen mention of the actual salts that were used; they appear not to have been reported? We assume that nitrates were used.

• Can authors discuss the reason why in different experiments CD structure is retained/is lost? What are the reason for such a difference and can it be predictable whether structure changes or not? 

REPLY: Thanks for this comment, which is indeed central to the review. We have now added more information to explain this issue. This includes the addition of an Ellingham diagram (Figure 7; p 14) and comments on this diagram.

We have also added a summary to the review outlining the different possibilities.

Summary of CD conversion reactions

A consideration of papers published in the area leads to some generalisations on metal-CD mixtures that have can be made and the role this will have in catalysis.

1. CDs can act as carbocatalysts and their reactions, as is known, will be dependent on their functional groups.
2. CDs can react with each other, either through physical bonds or through chemical bonds, to create CD assemblies. These assemblies could then be used as carbocatalysts.

• CDs can react at temperatures of ca 200 ^oC to convert to carbon sheet like structures and the graphicity of the sheets have been shown to increase with temperature. This will modify the catalytic property of the carbon

1. CDs can lose many of their surface functional groups at temperatures > 200 ^oC and this will impact on their chemical properties
2. In the presence of easily reducible metal oxide catalysts, CDs can be converted to carbon sheet like materials at temperatures < 100 ^o The ability to achieve this will be dependent on the metal reducibility with directions given by Ellingham diagrams.
3. Ellingham diagrams have been determined for bulk metals but metal nanoparticles are expected to have similar temperature values for reactions with carbon

• The chemical process by which CDs are converted to a sheet-like carbon is not known.

• Figure 6 seems to be pasted incorrectly

REPLY: Thanks. We have moved the words ‘Figure 6’ to 2 lines later viz. ‘…..for use in the Fischer-Tropsch (FT) reaction (Figure 6).’

• line 23: as as well and line 369 photocatlytic

Corrections made

REFEREE 2

1. Pay attention to the unity of abbreviations in the whole text, and the abbreviation of carbon dots are mixed to CD and CDs which makes the article messy. Abbreviations that appear for the first time need to indicate the full name, such as EXAFS, etc., and authors need to check article carefully to avoid such simple writing mistakes.

REPLY: Thanks for the comment. We have now gone through and given full names for abbreviations. The comment on the nomenclature to use for CDs is a problem and we are aware of this. To address this we have now done the following

1. Made a statement that we will use the abbreviation for all carbon dots p5 “In this review the term CD will be used for all carbon dots shown in Figure 1.”
2. The issue of the CD and metal combination is problematic. In addition, in many papers the nomenclature used is not correct. To avoid the issue we have called all CD and metal combinations as CD/metal to indicate that the metal particle is larger than or the same size as the CD.

• The issue of calling the complex as CD@metal is also problematic. This nomenclature implies a core@shell structure and in most cases, this has not been seen. We have avoided its use unless it is clear that this has occurred ( see old Fig 10 with TiO₂ covered by a graphene layer)

2. There is a serious lack of discussion about the surface properties of CDs, for example, how many categories should the authors think surface properties should be divided into? What is the relationship between the surface properties of CDs and the catalytic performance?

REPLY:  Thanks for this comment. We have now added more to this section. We have now specifically mentioned the change in surface charge on the CDs achieved by CD surface reactions. This has been done under the topic of carbocatalysis. We have commented on their reactions with metal ions or metal particles.

3. Section 2.3 lacks an introduction to stability. In fact, the entire section is devoted to describing the chemistry of carbon dots as catalysts, and I suggest to correspond the description of the surface properties with stability.

REPLY: We have renamed this section, as the issue of stability was not the key point in these few paragraphs.

The issue of stability is central to the review and we have now added more to the other sections on the topic. Indeed, we have added a summary of the results of the review, which addresses the issue

4. On page 6, it is stated that "As will be seen in these reviews, the focus has mainly been on their use, rather than on the morphology of the CD after the reaction." Not reasonable. Because the relevant system is based on "CDs as catalysts" as the main research subject, this is the work that must be done in related research. In fact, the author provides an overview of this aspect on the middle page below. I suggest further refinement.

REPLY: Thanks. We agree that this needed a restatement. We have modified our statement on this.

5. The title of section 3.1 is confusing. The logical framework is incorrect and inconsistent with the content.

REPLY: Thanks. The title from section 3.1 was changed from ‘Carbon dots (CDs) as catalysts (carbocatalysis)’ to ‘Applications of carbon dots-based catalysts’. We have had to make some changes to ensure our category on carbocatalysis is more clear. We did not want to cover this section in extensive detail, as there are many excellent reviews on the topic from many perspectives.

6. The author has reviewed a large number of carbon dots-metal materials as catalysts, but in fact, there are many cases of carbon dots themselves as catalysts and non-metallic doped carbon dots as catalysts, and the authors must complete corresponding supplements.

REPLY: Correct. This is covered in our section on carbocatalysts. I hope the response to (5) above resolves this.

7. The synthesis requires a further summary of work in this area. For example, the relationship between the structure (raw material) of carbon dots and catalytic performance, what kind of structure is more suitable for catalyzing what kind of system? What are the advantages of carbon dots catalysis over traditional catalysts?

RELPY: We have now added more to the section on carbocatalysis to address this issue. The review has not focussed on carbocatalysis as such but on the role of temperature and metals on changing the behaviour/morphology of the CDs that have been used in catalysis

A line has been added to the introduction on the advantages of CDs, versus other catalysts

8. As a review, the authors' outlook on relevant areas is too sketchy. Does the counterion of the carbon dots affect the catalytic performance? Is there any relationship between the luminescent properties of carbon dots and catalytic performance? Is it possible to visualize catalytic processes using carbon dot luminescence?

REPLY: WE have added information to address the different issues i) on the charge on the CD and  ii) the general decrease in PL as functional groups are modified/coordinated with metals.

On the issue of monitoring a catalytic reaction with PL:  We did not address this issue in the review. It is known that modification of the CD surface will modify the cD PL properties. Indeed this is how metal ions can be detected in solutions containing appropriate CDs. One could thus monitor a gas phase reaction by following the changes in concentration of products/reactants by UV-vis studies, that could be affected by changes in the CD surface chemistry (PL data) over time.

9. Representative work related to carbon dots, especially with regard to the surface properties of carbon dots, needs to be cited:J. Phys. Chem. Lett. 2023, 14, 1088.; J. Phys. Chem. C 2019, 123, 22447.; J. Mater. Chem. C 2020, 8, 8980.; J. Mater. Chem. C, 2017, 5, 4951-4958.; Acta Phys.-Chim. Sin. 2020, 2005004.; Dyes Pigment., 2021, 190, 109287. Colloids and Surfaces A 2017, 529, 38–44.

REPLY: Thanks for the useful references that have now been added. We have added other 2023 references, to bolster the review. We have been unable to access the ref: Acta Phys.-Chim. Sin. 2020, 2005004.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The author has revised the MS carefully. I sugget it  can be accpeted in its present form.

Reviewer 2 Report

Dear authors!

Thank you for your work on the manuscript. I do think that now it can be accepted for publication.

Best regards!