Synthesis and Characterization of N-Doped SiC Powder with Enhanced Photocatalytic and Photoelectrochemical Performance

Round 1

Reviewer 1 Report

The manuscript id catalysts-839065 deals with the synthesis and characterization of N-SiC doped powder. The SiC material is prepared by a combustion method, and the doping is performed in a horizontal furnace heated at 1550°C using N2 as nitrogen sources. Even if a lot of work has been done by the authors, some aspects have to be reconsidered. Moreover, there are too many typography mistakes. 

For these reasons, I recommend major revisions for this manuscript. 

My significant comments are listed below. I hope that they will help authors to improve the quality of their manuscript. 

• There are too many typography mistakes. For instance, H2 instead of H₂ or Si3N4 instead of Si₃N₄, sic instead of SiC, etc. all of them did not allow the reader to focus on the content of the manuscript. Thus, the authors have to read the manuscript carefully and to correct the mistakes and also to homogenized the sample names.  
• Keywords are missing.
• In the XPS part, binding energy values from the literature are missing. I suggest to the authors to add a summary-table-gathering binding energy values, attributions, and references.  
• For other attributions, references are also missing (e.g., for lattice spacing attribution)
• Fig. 4: the band-gap is determined by the extrapolation of the linear portion of the curve to the x-axis. However, as the y-axis starts at 1.2 and not 0, the determined value will not reflect reality. The authors should consider another way to determine them, e.g., the intersection of the straight lines in the linear region and the baseline. 
• Fig. 4: what happens if we consider direct-band-gap (indices should be 2) instead of indirect one (indices is 1/2)? In other terms, did the authors check if their materials have a direct or indirect band-gap, and if the doping affects the behavior of SiC?
• Page 8: Fig 5a is mentioned within the text but not the Fig 5b. Is it an oversight? 
• The discussion about the reaction mechanism should be introduced before in the different parts. As an example, Fig 7b in mentioned in page 7, but it is located on page 9. 
• There is a mistake in Fig7 caption. 
• A discussion about the results obtained in comparison to the ones from literature is missing.

Author Response

Dear Reviewers：

On behalf of my co-authors, we thank you very much for giving us an opportunity to revise our manuscript, we appreciate editor and reviewers very much for their valuable and detailed comments and suggestions on our manuscript for revising and improving our paper. We have studied comments carefully and have made correction which we hope meet with approval.

I’m sorry for not reply the comments and finish the revised version in time.

Response to Reviewer 1 Comments

Pint 1：

• There are too many typography mistakes. For instance, H2 instead of H2 or Si3N4 instead of Si3N4, sic instead of SiC, etc. all of them did not allow the reader to focus on the content of the manuscript. Thus, the authors have to read the manuscript carefully and to correct the mistakes and also to homogenized the sample names.  

• For other attributions, references are also missing (e.g., for lattice spacing attribution)

• Keywords are missing.
• The discussion about the reaction mechanism should be introduced before in the different parts. As an example, Fig 7b in mentioned in page 7, but it is located on page 9.
• There is a mistake in Fig7 caption.
• A discussion about the results obtained in comparison to the ones from literature is missing.

Response 1：

I have checked and corrected the typography mistakes, and added the keywords.

Keywords: N-doped, SiC, Photocatalysis, Hydrogen production, Phase transformation

The references for lattice spacing was added.

As shown in Fig. 3e, the clear lattice fringes with an interplanar lattice spacing of 0.25 nm was correspond to (111) facet of SiC [1]whereas the lattice fringes with d spacing of about 0.33nm can be indexed as the (004) plane of carbon[2].

The discussion about the reaction mechanism was moved to the fig 4.

The caption of Fig 7 was corrected：Figure 7. Photocatalytic process of H₂ formation in N-doped SiC.

All the correction has been updated in the revision.

Pint 2：

• In the XPS part, binding energy values from the literature are missing. I suggest to the authors to add a summary-table-gathering binding energy values, attributions, and references.

• 4: the band-gap is determined by the extrapolation of the linear portion of the curve to the x-axis. However, as the y-axis starts at 1.2 and not 0, the determined value will not reflect reality. The authors should consider another way to determine them, e.g., the intersection of the straight lines in the linear region and the baseline.
• 4: what happens if we consider direct-band-gap (indices should be 2) instead of indirect one (indices is 1/2)? In other terms, did the authors check if their materials have a direct or indirect band-gap, and if the doping affects the behavior of SiC?

Response 2：

The value of binding energy in the literature and obtained from the test was listed in the table 2

Table 2. results of chemical status of SiC-N₂

As we all known, the band gap of un-doped SiC-Ar was indirect , but its absorption edge was not evident in the plot, so the intersection of the straight lines in the linear region and the baseline was treated to be the approximation of band gap, based on the suggestion offered in comments by the reviewer. The value of the estimated data was pointed out in the Fig4.For the discussion about band gap energy, a new UV-vis and Mott-Schottky measurement was carried out, and the result was updated in the paper. Considering the reviewer’s suggestion, I consulted more reference and calculated the value by two ways. Assuming that the band gap of SiC-N₂ was direct band gap, the value measured from the absorbance and αhν=A(hν-Eg)^1/2 can match well. The type of n-doped SiC would transform into direct transition with band gap shrinkage caused by occupation of N on C site, which also supported and explained in the the reference.

Pint 3:

• Page 8: Fig 5a is mentioned within the text but not the Fig 5b. Is it an oversight?

Response 3：

The description about Fig 5b was : “ No apparent decline of hydrogen evolution was observed in Fig 5b after three repeated run over 15h, indicating SiC-N₂ exhibited satisfactory stability and cyclability during photocatalytic hydrogen production process.” , the picture number was not mentioned before the revision.

Pint 4:

• A discussion about the results obtained in comparison to the ones from literature is missing.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors showed N doped SiC for photoelectrochemical reaction.  The reviewer believe the author needs to change some minor correction then the paper must be improved. 

1. I see there is some shift in XRD. Is there any electronic interaction when N doped in SiC? In Figure 1b, what is compound of figure 1b. clarify it. 
2. Provide all thermodynamic parameter of equation (1-3). Why intermediate product is shown in there? 
3. XPS Analysis needs to be chaged.- did you guys do calibration? If you want to show the amount of chemical state, then you should use the surface amount*deconvoluted area and show as a graph that the Si-N amount is growing. Otherwise, it's hard to believe from one spot of surface XPS technique.
4. If you guys claiming the interface is catalytic active sites, then can you guys quantity the interface? or change N doped amount and show how much improved it. Non- or N-doped one is hard to believe it. There would be so many parameters to improve N-doped one. 
5. Can you change NHE value as RHE? value?
6. Tafel slope should be improved and show how much value did you get from there. (Show in the graph)
7. Seems like bandgap changed down in SiN one but why Tafel slope is same? show the number of Tafel slope. Don't leave it vague. 
8.  

Author Response

Dear Reviewers：

On behalf of my co-authors, we thank you very much for giving us an opportunity to revise our manuscript, we appreciate editor and reviewers very much for their valuable and detailed comments and suggestions on our manuscript for revising and improving our paper. We have studied comments carefully and have made correction which we hope meet with approval.

I’m sorry for not reply the comments and finishi the revised version in time.

The main corrections in the paper and the responds to the reviewer’s comments are as flowing:

Pint 1：I see there is some shift in XRD. Is there any electronic interaction when N doped in SiC? In Figure 1b, what is compound of figure 1b. clarify it.

Response 1：

When the N atom replaces the C atom in β-SiC, after four N electron form bond with four Si atom, another excess electron in N atom is left. This electron will be captured by Si 3p and form defects with negative electricity.

Pint 2：Provide all thermodynamic parameter of equation (1-3). Why intermediate product is shown in there? 

Response 2：When carbon was sufficient under an inert atmosphere above the boundary temperature 1450℃, conversion from Si₃N₄ to SiC would happen. The carbothermal reduction of a mixture of Si3N4 and graphite powder was regarded as another way for producting SiC. so the intermediate product is shown in there.

Pint 3：XPS Analysis needs to be chaged.- did you guys do calibration? If you want to show the amount of chemical state, then you should use the surface amount*deconvoluted area and show as a graph that the Si-N amount is growing. Otherwise, it's hard to believe from one spot of surface XPS technique.

Response 3：I corrected the XPS spectrum, the increase of Si-N amount can be evidenced by the change of atomic% after N-doping

Pint 4：If you guys claiming the interface is catalytic active sites, then can you guys quantity the interface? or change N doped amount and show how much improved it. Non- or N-doped one is hard to believe it. There would be so many parameters to improve N-doped one. 

Response 4：I can't demonstrate these question. I used to try to control the N-doped amount by changing the temperature of synthesis, and the pattern of XRD maybe can tell the differences, and the H₂ production was measured as shown in the table below

Pint 5：Can you change NHE value as RHE? value?

Response 5：SiC-N2: -0.49+0.1976+0.0592*7=0.122

                   SiC-Ar：-0.67+0.1976+0.0592*7=-0.058

Pint 6,7 ：Tafel slope should be improved and show how much value did you get from there. (Show in the graph)

Seems like bandgap changed down in SiN one but why Tafel slope is same? show the number of Tafel slope. Don't leave it vague. 

Response 6,7:  a new UV-vis and Mott-Schottky measurement was carried out, and the Fig 4 was updated

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors gave answers to my comments and made almost all the needed modifications in their manuscript. However, table 1 and Figure 3 (and the associated text) have to be modified. For Table 1, the values, units, or labels have to be checked (just as a reminder: V is a potential unit, and eV is an energy unit.). Fig 3 has to show the plots with n=1 and n=4 for both samples. Then the choice between direct or indirect transition should be based on figure 3 and discussed within the text . (see p6816 of the reference DOI: 10.1021/acs.jpclett.8b02892; for SiC-N2 Eg is higher than 2.34 eV.) Additionally, there are still a few typing mistakes. For instance, Introduction last paragraph: spaces missing “SiC-Ar(120.1…” and “photocatalysts(166 µL…”

Author Response

On behalf of my co-authors, we thank you very much for giving us an opportunity to revise our manuscript, we appreciate editor and reviewers very much for their valuable and detailed comments and suggestions on our manuscript for revising and improving our paper. We have studied comments carefully and have made correction which we hope meet with approval.

I have checked and corrected the typography mistakes. The units of E_fb in table 1 has been modified to be potential unit V. The discussion about Fig 3 was rewriten and the plots with n=1 and n=4 for both samples was added whose value of Eg was also caclulated on two conditions, respectively.

Considering the reviewer’s suggestion, the indirect band gap semiconductor was calculated by the intersection between the extrapolated linear portion and the baseline as exampled in the reference, because of its absorption was not steep. In order to obtain the similar value of E_g with the result acquired from the absorbance，the value of direct band gap was gained from the intersection with x axis. By matching the Eg value with the result concluded from the edge of absorbance, the characteristics of transition for SiC-Ar was proved to be indirect, as well as SiC-N₂ had direct band gap.

Reviewer 2 Report

I am happy to see the revised one and ask Editor to accept this paper. 

Author Response

On behalf of my co-authors, we thank you very much for approval to accept this paper.