A Superficial Intramolecular Alignment of Carbon Nitride through Conjugated Monomer for Optimized Photocatalytic CO₂ Reduction

Round 1

Reviewer 1 Report

The authors report the preparation of a new carbon nitrite (CN)-based material to which a porphyrin is covalently bound. The material has been characterized through different techniques and used for the enhancement of CO2, and in particular for its reduction from CO2 to CO. The photocatalytic activity of the material containing porphyrin is much more active than non-derived carbonititre. The authors reported that porphyrin is covalently linked, but in my opinion this is not demonstrated by any of the experiments reported by the authors. The only difference that is seen is a slight increase in fluorescence to wavelength above 400nm (Figure 3, please y-values on both the absorbance graph and the fluorescence graph) and a different morphology of the material through surface area and SEM measurement. Since the preparation of the material is done by heating to 600°C in the air, the first thing that comes to think is that all the organic material is burned and does not covalently bond to CN. This favors the formation of pores with subsequent increase of the surface area which is solely responsible for the increase in catalytic activity. The text has several critical issues and it seems. First of all it is not been written and must be carefully checked: some graphs are incomplete due to the lack of y-values. Some references are not consistent with the topic covered such as 42 in the main text and 1 in supporting information. The caption of figure 3d is quite curious: it is not clear how absorption above 420nm should affect elementary analysis. These are only three of critical issues. In my opinion, the manuscript cannot be accepted in this form unless thoroughly revised. It is not possible to say that porphyrin has covalently linked to CN because nothing proves this. The material actually turns out to have increased photocatalytic activity but this, as already mentioned, can only be due to a different morphology of the material and not to a different chemical composition.

Author Response

The authors report the preparation of a new carbon nitrite (CN)-based material to which a porphyrin is covalently bound. The material has been characterized through different techniques and used for the enhancement of CO2, and in particular for its reduction from CO2 to CO.

Q.1. The photocatalytic activity of the material containing porphyrin is much more active than non-derived carbonititre. The authors reported that porphyrin is covalently linked, but in my opinion this is not demonstrated by any of the experiments reported by the authors.

Ans: Thanks for your experience question. Dear teacher, first of all there was a small mistake in the copolymerization process of TDP within CN and has been updated successful. Please have a look of scheme 1 again.

Secondly, paper composed of a lot of characterizations that give the evidence of molecular doping (copolymerization) of organic monomer TDP within the skeleton of CN. First of all, we can see the surface area BET that increase almost triple compared of parental sample. Secondly we can see NMR that extra peak demonstrates the contents of extra carbon or sulfur after copolymerization process. Thirdly, EPR explain well this process that happen successful. Fourthly, EAS demonstrate each contents of materials are more in superior sample than pure CN. We can also see the results of DRS that band gap decrease and same photoluminescence spectra (Pl). the most important is the photocatalytic activity that boost well of CO2 reduction as compared of parental CN. Please check the whole paper. Thanks.

Secondly, the decomposition rate of CN under urea precursor is start from 700-900 °C. we give a moderate temperature that the decomposition rate not happen. We almost done three four experiments that we keep solitary organic monomers such porphyrin, furan, pyradazine, quinone etc. as in the crucible and keep in the air furnace and extend the temperature from 600 to 1000 °C. As a result, the color of monomer slightly changes its position but not decomposed. More importantly, if we see our paper and synthesis, then each researcher can suggest that molecular engineering of TDP happen successfully because of above techniques I mention. At high temperature, Porphyrin is not decomposed but present that can boost a lot of techniques and enhance photocatalytic activity. Thank you so much.

Q.2. The only difference that is seen is a slight increase in fluorescence to wavelength above 400 nm (Figure 3, please y-values on both the absorbance graph and the fluorescence graph) and a different morphology of the material through surface area and SEM measurement.

Ans: Thanks for your experience question. Dear teacher, we can see the surface area BET that increase almost triple compared of parental sample. Secondly we can see NMR that extra peak demonstrates the contents of extra carbon or sulfur after copolymerization process. Thirdly, EPR explain well this process that happen successful. Fourthly, EAS demonstrate each contents of materials are more in superior sample than pure CN. We can also see the results of DRS that band gap decrease and same photoluminescence spectra (Pl). the most important is the photocatalytic activity that boost well of CO2 reduction as compared of parental CN.

Secondly, all figures of paper were again plotted in same format and x-axis and y-axis values has been drawn where necessary. Please have a look. Thanks again.

Q.3. Since the preparation of the material is done by heating to 600 °C in the air, the first thing that comes to think is that all the organic material is burned and does not covalently bond to CN. This favors the formation of pores with subsequent increase of the surface area which is solely responsible for the increase in catalytic activity.

 Ans: Thanks for your experience question. Dear teacher, the decomposition rate of CN under urea precursor is start from 700-900 °C. we give a moderate temperature that the decomposition rate not happen. We almost done three four experiments that we keep solitary organic monomers such porphyrin, furan, pyradazine, quinone etc. as in the crucible and keep in the air furnace and extend the temperature from 600 to 1000 °C. As a result, the color of monomer slightly changes its position but not decomposed. More importantly, if we see our paper and synthesis, then each researcher can suggest that molecular engineering of TDP happen successfully because of above techniques I mention. At high temperature, Porphyrin is not decomposed but present that can boost a lot of techniques and enhance photocatalytic activity. Thank you so much.

Q.4. The text has several critical issues and it seems. First of all it is not been written and must be carefully checked: some graphs are incomplete due to the lack of y-values. Some references are not consistent with the topic covered such as 42 in the main text and 1 in supporting information. The caption of figure 3d is quite curious: it is not clear how absorption above 420nm should affect elementary analysis. These are only three of critical issues. In my opinion, the manuscript cannot be accepted in this form unless thoroughly revised. It is not possible to say that porphyrin has covalently linked to CN because nothing proves this. The material actually turns out to have increased photocatalytic activity but this, as already mentioned, can only be due to a different morphology of the material and not to a different chemical composition.

Ans: Thanks for your experience question. Dear teacher, we almost checked whole paper and removed a small single mistake and fluency of English and removed grammar mistake. All graphs have been designed again and removed all mistake as you suggest. Thank you.

We give new related references and update whole paper. Thank you

Figure 3.d you can see now. It can be easily study now and explain well in text. Thank you

Proposed scheme has been repeated and manifested in main paper. Please have a look. Thank you

 

Author Response File: Author Response.pdf

Reviewer 2 Report

As a series of author’s studies incorporating various functional organic compounds into carbon nitride, this study uses the porphyrin. It is interesting that there would be covalent bonds between porphyrin and carbon nitride by synthesizing in a bottom-up manner using a monomer having a partial skeleton of carbon nitride. Further, the activity is greatly improved by the hybridization of the porphyrin. However, the following questions remain regarding the most important porphyrins. Accordingly, my opinion is that the manuscript may be publishable after the major revision of the following points: 1. Evidence for the existence of the porphyrin skeleton in the catalyst The catalyst was prepared by the calcination of the raw materials at 600 oC under aerobic conditions. Does not the porphyrin skeleton decompose? The evidence for the porphyrin skeleton in the catalyst should be required, such as solid-state NMR. 2. The role of the porphyrin in the photocatalysis While authors described the mechanism of the photocatalysis in Figure 8, the role of the porphyrin is unclear. 3. Other points a) Why is the ESR signal observed in the closed shell compound (not radical)? b) The structure of the porphyrin in Scheme 1 would be mistaken. Porphyrin has only two pyrrole protons inner the ring.

Author Response

As a series of author’s studies incorporating various functional organic compounds into carbon nitride, this study uses the porphyrin. It is interesting that there would be covalent bonds between porphyrin and carbon nitride by synthesizing in a bottom-up manner using a monomer having a partial skeleton of carbon nitride. Further, the activity is greatly improved by the hybridization of the porphyrin. However, the following questions remain regarding the most important porphyrins. Accordingly, my opinion is that the manuscript may be publishable after the major revision of the following points:

Q.1. Evidence for the existence of the porphyrin skeleton in the catalyst The catalyst was prepared by the calcination of the raw materials at 600 ^oC under aerobic conditions. Does not the porphyrin skeleton decompose?

Ans: Thanks for your experience question. Dear teacher, actually paper composed of a lot of characterization that give the evidence of molecular doping (copolymerization) of organic monomer TDF within the skeleton of CN. First of all, we can see the surface area BET that increase almost triple compared of parental sample. Secondly we can see NMR that extra peak demonstrates the contents of extra carbon or sulfur after copolymerization process. Thirdly, EPR explain well this process that happen successful. Fourthly, EAS demonstrate each contents of materials are more in superior sample than pure CN. We can also see the results of DRS that band gap decrease and same photoluminescence spectra (Pl). the most important is the photocatalytic activity that boost well of CO2 reduction as compared of parental CN. Please check the whole paper. Thanks.

Secondly, the decomposition rate of CN under urea precursor is start from 700-900 °C. we give a moderate temperature that the decomposition rate not happen. We almost done three four experiments that we keep solitary organic monomers such porphyrin, furan, pyradazin, quinone etc. as in the crucible and keep in the air furnace and extend the temperature from 600 to 1000 °C. As a result, the color of monomer slightly changes its position but not decomposed. More importantly, if we see our paper and synthesis, then each researcher can suggest that molecular engineering of TDP happen successfully because of above techniques I mention. At high temperature, Porphyrin is not decomposed but present that can boost a lot of techniques and enhance photocatalytic activity. Thank you so much.

Q.2.The evidence for the porphyrin skeleton in the catalyst should be required, such as solid-state NMR.

Ans: Thanks for your experience question. Dear teacher, the solid state NMR has been conducted between parental sample over superior sample and explained in main text and plotted in the supporting information Figure. S2. All corrected points have been highlight as yellow for your convenience. Please have a look. Thank you.

[More crucially, the solid-state 13C NMR techniques was utilized to analyzed the quantity of carbon-containing compounds involved in CN after molecular doping. The results showed that the spectrum acquired for Pure CN and CN-TDP_7.0 materials were essentially identical, although there was a new additional peak discovered for CN-TDP_7.0 at 20.26 ppm, as shown in Figure S2. This indicates that a novel species, such as carbon contents was created by the copolymerization interaction of TDP monomer with CN].

Q.3. The role of the porphyrin in the photocatalysis While authors described the mechanism of the photocatalysis in Figure 8, the role of the porphyrin is unclear.

Ans: Thanks for your experience question. Dear teacher, graphical abstract (Figure 8) has been modified and discussed in the mechanism section. We also well explain the conjugation of monomer porphyrin in main paper (end of introduction portion). All corrected points have been highlight as yellow for your convenience. Please have a look. Thank you.

Q.3. Other points

1. a) Why is the ESR signal observed in the closed shell compound (not radical)?

Ans: thanks for your experience question. Dear teacher, I not understand well on your question. I think you are talking about electrochemical impedance spectroscopy (EIS) Nyquist plot demonstrating that the combination of TDP monomer in the framework of CN increased its electronic conductivity and thereby improved its charge isolation as shown in Figure 7b. thanks again.

1. b) The structure of the porphyrin in Scheme 1 would be mistaken. Porphyrin has only two pyrrole protons inner the ring.

Ans: Thanks for your experience question. Dear teacher, scheme.1 in revised paper has been corrected accordingly. Two hydrogen has been skipped in the scheme. Please have a look.  It has been highlight as yellow for your convenience. Thanks again.

 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Despite the authors' response, still no experimental evidence shows that a covalent bond has formed between carbon nitride and porphyrin: the C/N ratio goes from 0.64 to 0.65 (how much is the standard deviation?) and the presence of a carbon signal in the alkyl zone of the solid-state 13C-NMR spectra is not sufficient to demonstrate that porphyrin is still intact and covalently bound to carbon nitrite. Certainly, the new material obtained using porphyrin in its preparation, has higher catalytic properties than that obtained using only urea, but no evidence shows that a covalent bond with porphyrin has been formed and the porphyrin ring has remained intact. It is necessary to make a thermogravimetric analysis of the pristine TDP that shows that at 590°C it does not degrade. Alternatively, it is necessary to avoid writing that the covalent bond is formed (there is no evidence) and to write that with the new procedure a more porous material is obtained with better catalytic properties (most likely due to an increase in the surface area).

Author Response

Reviewer: 1

Comments and Suggestions for Authors

Despite the authors' response, still no experimental evidence shows that a covalent bond has formed between carbon nitride and porphyrin: The C/N ratio goes from 0.64 to 0.65 (how much is the standard deviation?) and the presence of a carbon signal in the alkyl zone of the solid-state 13C-NMR spectra is not sufficient to demonstrate that porphyrin is still intact and covalently bound to carbon nitrite. Certainly, the new material obtained using porphyrin in its preparation, has higher catalytic properties than that obtained using only urea, but no evidence shows that a covalent bond with porphyrin has been formed and the porphyrin ring has remained intact. It is necessary to make a thermogravimetric analysis of the pristine TDP that shows that at 590°C it does not degrade. Alternatively, it is necessary to avoid writing that the covalent bond is formed (there is no evidence) and to write that with the new procedure a more porous material is obtained with better catalytic properties (most likely due to an increase in the surface area).

 

 

 

 

 

 

 

 

Author response:

Q1. The C/N ratio goes from 0.64 to 0.65 (how much is the standard deviation?

Ans: In statistics, the standard deviation is a measure of the amount of variation or dispersion of a set of values. A low standard deviation indicates that the values tend to be close to the mean (also called the expected value) of the set, while a high standard deviation indicates that the values are spread out over a wider range.

 

Where, σ = Population standard deviation. N = the size of the population

x_i = each value from the population. μ = population mean

Therefore, by applying above formula we obtained the standard deviation value of about

σ = 0.005

Q2. Evidence for the covalent bonding between carbon nitride and porphyrin.

Ans: Respected sir, thank you very much for your valuable query. First of all, we have modified our scheme according to your valuable suggestion, this time we have shown the correct covalent bond formation between carbon nitride and porphyrin (previously by mistakenly we shown wrong bonding between urea and porphyrin).

 

 

 

Scheme 1: Modified scheme

Based on previously reported articles on covalent bonding interaction between carbon nitride and porphyrin, we have tried our level best to provide you the valid evidence

1. Tian, S. Chen, X. Ren, Y. Hu, H. Hu, J. Sun, F. Bai, Nano Research, 13 (2020) 2665-2672.
2. Ong,L. Tan, Y. H. Ng, S. Yong, S. Chai, Chemical Review, 116 (2016), 7159−7329.
3. Zhao, H. Pang, G. Liu P. Li, H. Liu, H. Zhang, L. Shi, J. Ye. Applied Catalysis B: Environmental, 200 (2017), 141-149.
4. Chen, X. Liu, L. Hou, X. Guo, R. Fu, J. Sun. Chemical Engineering Journal, 383 (2020), 123132.

Porphyrins are another great photocatalyst photosensitizer because of their remarkable chromophore activities throughout a wide range of solar photon flux and their efficient electron donor due to a huge system of p-electrons. The flow of electrons between photocatalysts and porphyrins can be substantially enhanced through the channels created by covalent bonds, resulting in the delocalization of porphyrin p* orbitals. Nonetheless, the covalent connection between photocatalysts and porphyrins is difficult, resulting in porphyrin absorption on the photocatalyst surface being restricted.

Because g-C₃N₄ has layer structures with -conjugated electrons, it is expected that g-C₃N₄ will be coupled to porphyrins via covalent bonding and electrostatic interactions. The strong covalent C-N bonds in the tri-s-triazine units were more resistant to rupture than hydrogen bonding and van der Waals forces when the g-C3N4 was subjected to post-thermal treatment, resulting in the retention of short-range atomic order.

1. UV-Visible Spectra:

It can be seen from the UV-visible spectrum in Figure 3b that the optical edges of the CN-TDP_7.0 material clearly experienced a shift to 430 nm from 495 nm for CN. The thick-ness and smaller size of the particles might be the main reason for such a shift that results in the quantum confinement effect. With the integration of varying amounts of TDP, the absorption range spread up to 500 nm owing to the superior optical absorption performance for the TDP. Further, optical band gaps were found to be shifted from 2.95 for CN to 2.71-2.54 eV for varying the amount of TDP. The induction of conjugated TDP into the skeleton of CN significantly reduces the p-electron delocalization within the con-jugated system of the CN by generating photogenerated electrons-hole pairs, resulting in a simple decline in the related band gap as described in the DRS section

Hence with the incorporation of TDP, there was a shift in the absorption band edge, which was accredited to the presence of a covalent bonds formed between TDP and CN during the thermal annealing process, as confirmed by the XPS analysis.

2. Photoluminescence (PL):

The PL signals moved from shorter to extended wavelength as the dopant (TDP) concentration was increased, as shown in Figure 3a. The result indicates that the CN-TDP nanosheets displayed fluorescence quenching, suggesting a lower re-combination rate for the photogenerated charges and enhanced separation efficiency. Such a quenching might be due to the synergistic effect of the CN-TDP nanosheet’s de-creased thickness and the creation of molecular covalent bonding heterojunction between TDP and CN components, forcing the charge carriers to move to the surface.

3. Electron paramagnetic resonance (EPR):

Furthermore, the EPR spectrum was utilized to determine the origin of electronic band assembly for both CN and TDP integrated CNs, as shown in Figure 3c. Compared to CN, the strength of the Lorentzian line attributed to carbon atom unpaired electrons in CN-TDP_7.0 (g =2.003) was clearly increased. It was observed that once TDP was embedded in the skeleton of the CN at room temperature (RT) in the dark condition, the productivity of excited electrons separation and the thickness of the electron signals increased considerably

Therefore, an enhanced photocatalytic performance of CN-TDP_7.0 was attributed to the covalent bonding heterojunction's prolonged visible light absorption, restricted photo-induced charge carrier recombination, fast charge carrier transfer, and increased lifespan of active charge carriers.

 

4. ¹³C NMR spectral analysis

The solid-state ¹³C NMR technique was utilized to analyze the quantity of carbon-containing compounds involved in CN after molecular doping. The results showed that the spectrum acquired for Pure CN and CN-TDP7.0 materials were essentially identical, although a new additional peak discovered for CN-TDP7.0 at 20.26 ppm, as shown in Figure S2. This indicates that a novel species, such as carbon contents, was created by the copolymerization interaction of TDP monomer with CN.

 

Q3. Thermogravimetric analysis

Ans: Thanks for your good question. Dear teacher, we conducted TGA spectra and manisfested in supporting information file Figure.S3 and also discussed in result and discussion. For convenience, we keep it yellow. Thanks again

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript has been improved according to the comments.

Accordingly, I would like to recommend that the manuscript is publishing.

Author Response

Reviewer: 2

Comments and Suggestions for Authors

The manuscript has been improved according to the comments. Accordingly, I would like to recommend that the manuscript is publishing.

Response: Thanks Professor.

Author Response File: Author Response.docx