Photoelectrochemical Studies on Metal-Doped Graphitic Carbon Nitride Nanostructures under Visible-Light Illumination
Round 1
Reviewer 1 Report
- Please provide description for C2H6O2 and C3H8O (page 2) since there are many compounds that fit these formulas.
- Quality of most figures (especially fig.3) should be improved, one can barely read captions on figures itself.
- Page 7: "As the dopant concentration increases, the optical bandgap of g-C3N4 gradually decreases and shifts toward lower energies, thereby indicating that the Ni dopant is effectively incorporated into the g-C3N4 host structures." According authors' data the optical bandgap of 0.5%-doped g-C3N4 is higher comparing 0.4%-doped g-C3N4. Please explain that fact.
Author Response
Manuscript ID: catalysts-897370
Title: Photoelectrochemical studies on metal-doped graphitic carbon nitride nanostructures under visible-light illumination
Reviewer comments
Note: All the modification are as follows:
The reviewer 1 suggested modifications are kept in GREEN color
Reviewer 1
Comment 1: Please provide description for C2H6O2 and C3H8O (page 2) since there are many compounds that fit these formulas.
Reply: As per reviewer suggestions the above comments has been modified in the revised MS. (Line: 87; line: 91)
Comment 2: Quality of most figures (especially fig.3) should be improved, one can barely read captions on figures itself.
Reply: As per reviewer suggestions the quality of the images has been increased in the revised MS.
Comment 3: Page 7: "As the dopant concentration increases, the optical bandgap of g-C3N4 gradually decreases and shifts toward lower energies, thereby indicating that the Ni dopant is effectively incorporated into the g-C3N4 host structures." According authors' data the optical bandgap of 0.5%-doped g-C3N4 is higher comparing 0.4%-doped g-C3N4. Please explain that fact.
Reply: Respected honorable reviewer, thanks for your valuable comment. As per reviewer suggestion the above comment as follows: The Burstein–Moss (B–M) effect, which suggests that the optical band gap of degenerately doped semiconductors increases when all states close to the conduction band get populated due to shifting of an absorption edge to higher energy. (Line: 212-215)
Author Response File: 
Author Response.docx
Reviewer 2 Report
Reviewer’s Report
Photoelectrochemical studies on metal-doped graphitic carbon nitride nanostructures under visible light illumination
Manuscript ID: catalysts-897370
In this manuscript, the authors have investigated the systematic variation of the Ni dopant effect on the structural, morphological, and optical properties of g-C3N4, as well as its PEC activity. They reported that the optimized 0.4 mol% Ni-doped g-C3N4 photoelectrode showed a noticeably improved PEC water-splitting activity. The results presented in the manuscript can be publishable after the following revisions.
1) Calculate the crystallite size and lattice parameter of the Ni-doped C3N4 materials.
2) The authors can also utilize solid-state absorption technique (Kubelka–Munk, where the transmittance can be converted to absorbance, for example see papers: Chemistry of Materials 28 (2016), 5406-5414; Catalysis Communications 113 (2018) 1-5) which would provide a linear onset from which one can estimate the band gap accurately.
3) Provide good quality figures for XPS results.
4) It would be great if authors can provide the EDS mappings for the Ni-doped C3N4 materials.
Author Response
Manuscript ID: catalysts-897370
Title: Photoelectrochemical studies on metal-doped graphitic carbon nitride nanostructures under visible-light illumination
Reviewer comments
Note: All the modification are as follows:
The reviewer 2 suggested modifications are kept in LIGHT BLUE color
Reviewer 2
Comment 1: Calculate the crystallite size and lattice parameter of the Ni-doped C3N4 materials.
Reply: Respected honorable reviewer, thanks for your valuable comment. We are very sorry. We couldn’t able to give that calculation data because of only one reflected peak was observed in XRD analysis. SO, to calculate the crystallite size and lattice parameter of the Ni-doped C3N4, it must need at least 3 minimum reflection peaks to avoid unknown errors. For calculating lattice parameters, minimum 3-5 peaks are required for Unitcell Software. So, please allow this article for further process.
Comment 2: The authors can also utilize solid-state absorption technique (Kubelka–Munk, where the transmittance can be converted to absorbance, for example see papers: Chemistry of Materials 28 (2016), 5406-5414; Catalysis Communications 113 (2018) 1-5) which would provide a linear onset from which one can estimate the band gap accurately.
Reply: Respected honorable reviewer, thanks for your valuable comment. In the present study, the optical properties were carried out by UV–vis diffuse reflectance spectra which was already used the reviewer for their work, published in Catalysis Communications 113 (2018) 1-5. So, please allow this article for further process.
Comment 3: Provide good quality figures for XPS results.
Reply: As per reviewer suggestions, the quality of the images has been increased in the revised MS.
Comment 4: It would be great if authors can provide the EDS mappings for the Ni-doped C3N4 materials.
Reply: As per reviewer suggestions the EDS images has been included in the revised MS. (Line: 141-145)
Author Response File: Author Response.docx
Reviewer 3 Report
In this paper, the authors investigate the systematic variation of Ni dopant effect on the structural, morphological and optical properties of g-C3N4, and its PEC activity. They report that the optimized photoelectrode (0.4 mol% Ni-doped C3N4) exhibits a noticeably improved six-fold photocurrent density by comparison with pure g-C3N4.
Overall, the article is rather significant. However some appropriate details should be included and different points need to be clarified before publication. It is therefore recommended for publication in Catalysts but only after the authors have considered the following corrections and suggestions:
1) The authors claim that these Ni-doped C3N4photoelectrodes are good candidates for PEC water-splitting applications. They observed a decrease in optical bandgap value and an increase in photocurrent with increasing Ni-dopant concentration, but they do not determine the position of conduction and valence bands of these components. As the consequence, they are not able to determine if these modifications are suitable for PEC water-splitting applications.
For this, the authors should determine the flat band potentials of the sample. They explain that “X-ray photoelectron spectroscopy (XPS) analysis was performed to investigate the chemical states, flat band potentials, and electronic structures of the samples” (Line 100). However the flat band potential was never determined in this study. The authors should determine it by XPS or by Mott Schottky (electrochemical measurements) and discuss about it and about band positions by comparison with redox potential of water.
2) The authors do not explain which kind of light source they used? (with filter or not, etc.).
3) Could the authors explain the sentence: “Meanwhile, the decrement in the interplanar positions could enable the passage of carriers between the stacked crystal planes [29]” (Line 136).
Indeed, by contrast with the reference [29], the (002) diffraction peak of Ni-doped g-C3N4 does not shift to larger angle, which could indicate in [29] that Ni doping has a certain effect on the interlayer spacing. In [29], the reduction in the interplanar spacing would facilitate the transport of charge carriers between the crystal layers. But there are no evidences of this phenomenon in the present work. The authors should clarify this point.
4) The quality of all the figures is fairly (very) low.
5) To discuss about “the possibly in the form of Ni-N bonds” (Line 122), the authors could acquire FTIR measurements on the samples. For instance in [30], the Ni-doped samples show 2 very small peaks which appear at 2924 and 2846 cm-1. The peaks are attributed to the bonding between the dopant Ni atoms with the host g-C3N4 matrix.
6) Line 147: what is “fatal”? (= “terminal” or “at the N atom on the edge of the tri-s-triazine unit”?).
Line 313, 348 and 394: what is “g-400”?
7) The authors should determine the repartition of Ni atoms in the sample: are they homogeneously distributed? Are they present as nanoclusters which are too small to be observed by XRD?
They should provide a STEM-EDS mapping of nanosheets? (or at least, EDS local measurements).
8) Figure 4: a slight peak in the absorbance spectra seems to be present in Ni-doped samples at about 400 nm. Could the authors discuss about this peak? Could it be due to SPR from Ni nanoparticles?
9) Figure 11c: Y axis is not “photocurrent” but “current density”.
Figure 11d: the authors should indicate the potential used to obtain the chronoamperometry curves.
10) The authors explained that: “Generally, the radius of the semicircle in the Nyquist plot at higher frequencies represents the charge transfer resistance of the photoelectrodes” (Line 268), and they first discussed about this arc radius which very difficult to determine on figure 11a. Then, subsequently, they developed a more complex model with 3 resistances and 2 capacitances where R3 is assigned to the charge transfer resistance. In conclusion, the authors should discuss only about the value of R3 and the discussion about the arc radius should be deleted.
11) In table 2: how are determined the photocurrent densities (ΔJ values): for which applied potential? For instance, we can see that these values are very different from the values observed on figure 11d! Why?
12) In order to confirm the stability of Ni-doped C3N4 photoelectrodes, stability measurements should be determined for longer period of time (several hours).
Author Response
Manuscript ID: catalysts-897370
Title: Photoelectrochemical studies on metal-doped graphitic carbon nitride nanostructures under visible-light illumination
Reviewer comments
Note: All the modification are as follows:
The reviewer 3 suggested modifications are kept in PURPLE color
Reviewer 3
Comment 1: The authors claim that these Ni-doped C3N4photoelectrodes are good candidates for PEC water-splitting applications. They observed a decrease in optical bandgap value and an increase in photocurrent with increasing Ni-dopant concentration, but they do not determine the position of conduction and valence bands of these components. As the consequence, they are not able to determine if these modifications are suitable for PEC water-splitting applications.
For this, the authors should determine the flat band potentials of the sample. They explain that “X-ray photoelectron spectroscopy (XPS) analysis was performed to investigate the chemical states, flat band potentials, and electronic structures of the samples” (Line 100). However the flat band potential was never determined in this study. The authors should determine it by XPS or by Mott Schottky (electrochemical measurements) and discuss about it and about band positions by comparison with redox potential of water.
Reply: Respected honorable reviewer, thanks for your valuable comment. The above comment has been addressed in the revised MS. (Line: 300-309)
Comment 2: The authors do not explain which kind of light source they used? (with filter or not, etc.).
Reply: Respected honorable reviewer, thanks for your valuable comment. The above comment has been included in the revised MS. (Line: 109-111)
Comment 3: Could the authors explain the sentence: “Meanwhile, the decrement in the interplanar positions could enable the passage of carriers between the stacked crystal planes [29]” (Line 136).
Indeed, by contrast with the reference [29], the (002) diffraction peak of Ni-doped g-C3N4 does not shift to larger angle, which could indicate in [29] that Ni doping has a certain effect on the interlayer spacing. In [29], the reduction in the interplanar spacing would facilitate the transport of charge carriers between the crystal layers. But there are no evidences of this phenomenon in the present work. The authors should clarify this point.
Reply: Respected honorable reviewer, thanks for your valuable comment. In the meantime, the reduction in the interplanar sites could make easy to transport of charge carriers between the stacked crystal planes. Therefore, the generated charge carrier may move freely in the electrode. So that only, we assumed that the high photocurrents are achieved for doped samples may be due this phenomenon. Hence, we stated or mentioned this phenomena or a reference in our present work thinking it could happened in the doped samples because of the shift occurred in doped samples compare to undoped sample.
Comment 4: The quality of all the figures is fairly (very) low.
Reply: As per reviewer suggestions the quality the images has been increased in the revised MS.
Comment 5: To discuss about “the possibly in the form of Ni-N bonds” (Line 122), the authors could acquire FTIR measurements on the samples. For instance in [30], the Ni-doped samples show 2 very small peaks which appear at 2924 and 2846 cm-1. The peaks are attributed to the bonding between the dopant Ni atoms with the host g-C3N4 matrix.
Reply: Respected honorable reviewer, thanks for your valuable comment. As per reviewer suggestions the FTIR analysis has been included in the revised MS. (Line: 239-247)
Comment 6: Line 147: what is “fatal”? (= “terminal” or “at the N atom on the edge of the tri-s-triazine unit”?).
Line 313, 348 and 394: what is “g-400”?
Reply: Respected honorable reviewer, thanks for your valuable comment. Fatal refers to “N atom on the edge of the tri-s-triazine unit”. (Line: 158)
We are very sorry for the typo mistakes occurred in line 313, 348 and 394, there are modified in the revised MS. (Line: 331, 393, and 410)
Comment 7: The authors should determine the repartition of Ni atoms in the sample: are they homogeneously distributed? Are they present as nanoclusters which are too small to be observed by XRD?
They should provide a STEM-EDS mapping of nanosheets? (or at least, EDS local measurements).
Reply: As per reviewer suggestions, the STEM images of optimized sample has been included in the revised MS.
Comment 8: Figure 4: a slight peak in the absorbance spectra seems to be present in Ni-doped samples at about 400 nm. Could the authors discuss about this peak? Could it be due to SPR from Ni nanoparticles?
Reply: Respected honorable reviewer, thanks for your valuable comment. The origin of that particular peak has been stated in the revised MS. (Line: 191-193)
Comment 9: Figure 11c: Y axis is not “photocurrent” but “current density”.
Figure 11d: the authors should indicate the potential used to obtain the chronoamperometry curves.
Reply: Respected honorable reviewer, thanks for your valuable comment. The potential used to obtain the chronoamperometry has been mentioned in the revised figure.
Comment 10: The authors explained that: “Generally, the radius of the semicircle in the Nyquist plot at higher frequencies represents the charge transfer resistance of the photoelectrodes” (Line 268), and they first discussed about this arc radius which very difficult to determine on figure 11a. Then, subsequently, they developed a more complex model with 3 resistances and 2 capacitances where R3 is assigned to the charge transfer resistance. In conclusion, the authors should discuss only about the value of R3 and the discussion about the arc radius should be deleted.
Reply: Respected honorable reviewer, thanks for your valuable comment. As per reviewer suggestion the discussion about the radius of the semicircle in the Nyquist plot has been deleted.
Comment 11: In table 2: how are determined the photocurrent densities (ΔJ values): for which applied potential? For instance, we can see that these values are very different from the values observed on figure 11d! Why?
Reply: Respected honorable reviewer, thanks for your valuable comment. The ΔJ values evaluated from the variations in photocurrent under dark and illuminated states, mentioned in Line: 407. These ΔJ values are acquired from Fig. 13c (as per new). That’s way these values are different that of Fig. 13d (as per new).
Comment 12: In order to confirm the stability of Ni-doped C3N4 photoelectrodes, stability measurements should be determined for longer period of time (several hours).
Reply: Respected honorable reviewer, thanks for your valuable comment. We could not able to provide long run for several hours because of non-availability of long run measurement facility setup. However, we tried to establish this to fulfil the review comment but we failed due to COVID-19.
Author Response File: Author Response.docx
Reviewer 4 Report
I. Neelakanta Reddy et al. reported a metal-doped effect on the Ni-doped g-C3N4 used as a photoanode. Although the characteristics were performed, several issues should be discussed. It is necessary to clarify the relationship between the materials feature and PEC performance. For the above reasons, I do not recommend this manuscript accepted in this version. The specific suggestions were shown below:
(1) In Fig 1, although the author mentioned the (002) reflection peaks of various Ni-doped g-C3N4 did not shift with increasing the doping concentration and only lower the crystallinity.
These peaks shifting to the low angle were observed compared to pristine g-C3N4. The authors should study carefully. Also, the typical diffraction peaks of g-C3N4 should be indexed, and the wavelength of X-ray should be provided.
(2) From the results of the optical properties, both of absorption spectra and PL spectra indicate that 0.4% Ni show superior behavior. Please give some comments.
(3) From the XPS results, the corresponding concentration of Ni dopant is suggested to provide.
(4) In the PEC measurement (Fig. 11), all of the plots are not clear and hard to read. The sample named g-400 was not defined. In the LSV measurement (Fig 11(c)), the curves of g-C3N4-light and 0.1%Ni-light show different from others. Please give some comments. Also, during the cycle test for I-T measurement, the current density revealed a significant decrease. Please give some comments.
(5) What is the main reason for the highest performance of 0.4% Ni in PEC?
(6) The X-axis of the title and unit in Fig. 1 should be corrected.
Author Response
Manuscript ID: catalysts-897370
Title: Photoelectrochemical studies on metal-doped graphitic carbon nitride nanostructures under visible-light illumination
Reviewer comments
Note: All the modification are as follows:
The reviewer 4 suggested modifications are kept in ORANGE color
Reviewer 4
Comment 1: In Fig 1, although the author mentioned the (002) reflection peaks of various Ni-doped g-C3N4 did not shift with increasing the doping concentration and only lower the crystallinity.
Reply: Respected honorable reviewer, thanks for your valuable comment. The above statement has been corrected in the revised MS. (Line: 133-136)
Comment 2: These peaks shifting to the low angle were observed compared to pristine g-C3N4. The authors should study carefully. Also, the typical diffraction peaks of g-C3N4 should be indexed, and the wavelength of X-ray should be provided.
Reply: Respected honorable reviewer, thanks for your valuable comment. The above statement has been corrected in the revised MS. (Line: 116)
Comment 3: From the results of the optical properties, both of absorption spectra and PL spectra indicate that 0.4% Ni show superior behavior. Please give some comments.
Reply: Respected honorable reviewer, thanks for your valuable comment. The optimized 0.4% Ni showed superior properties compare to others photocatalysts due to may be crystalline structure, morphology, SPR effect of Ni, and optimal dopant which can be alter interior band structure of host material.
Comment 4: From the XPS results, the corresponding concentration of Ni dopant is suggested to provide.
Reply: Respected honorable reviewer, thanks for your valuable comment. The concentration of Ni dopant has been provided in the revised MS. (Line: 283)
Comment 5: In the PEC measurement (Fig. 11), all of the plots are not clear and hard to read. The sample named g-400 was not defined. In the LSV measurement (Fig 11(c)), the curves of g-C3N4-light and 0.1%Ni-light show different from others. Please give some comments. Also, during the cycle test for I-T measurement, the current density revealed a significant decrease. Please give some comments.
Reply: Respected honorable reviewer, thanks for your valuable comment. We are very sorry for the Typo mistakes, this has been taken care in the revised MS. The LSV curves of g-C3N4-light and 0.1%Ni-light show different from others may be due to fast recombination rate, high charge transfer resistance and low light absorption activity. The significant decrease of current density in I-t graphs may be due lowering the active sites with respect to time or may increase the surface rigidity of the electrode, which may block the easy electron transfer evidenced by EIS spectra by the increment in charge transfer resistance.
Comment 6: What is the main reason for the highest performance of 0.4% Ni in PEC?
Reply: Respected honorable reviewer, thanks for your valuable comment. The highest performance of 0.4% Ni in PEC is due to low charge transfer resistance, surface morphology, low tafel slopes and absorption properties, which are enhanced the PEC activity of 0.4% photoelectrode.
Comment 7: The X-axis of the title and unit in Fig. 1 should be corrected.
Reply: Respected honorable reviewer, thanks for your valuable comment. We are very sorry for the typo mistakes occurred in Fig. 1 has been corrected in the revised MS.
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
The authors have satisfactorily addressed comments and concerns about the original manuscript. It is therefore recommended for publication in Catalysts.
Author Response
Done
