Metal-Free g-C₃N₄/Nanodiamond Heterostructures for Enhanced Photocatalytic Pollutant Removal and Bacteria Photoinactivation

Round 1

Reviewer 1 Report

The manuscript entitled: 'Metal-free g-C3N4/nanodiamond heterostructures for enhanced photocatalytic pollutant removal and bacteria photoinactivation' describe the use of a metal free photocatalyst for the degradation of methylene blue and enhanced reduction in bacterial growth. The manuscript results to be interesting and well written. However, some point should be addressed before consider it for publication.

Page 2

MB dye: MB is used for the first time, then, methylene blue should be added to the text

Page 3

Then, a certain mass of DNC (Table S1) was added to the mixture and sonicated for 30 min: Sonication condition should be reported

Page 5

MB was monitored through UV-Vis spectrophotometry at each 20 min up to 160 min.: wavelength should be reported

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DLS results confirmed the formation of non-agglomerated particles in solution: DLS analysis is usually used for spherical nanoparticles. However, in the manuscript the authors report nanosheets structures. This point should be explained. Compare size distribution obtained by DLS and size distribution obtained by SEM could help.

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Figure 2: figure 2 c) shows aggregates, this is in contrast with previous statement

The former peak is by the other side a signature of tri-s-triazine-based g-C3N4 formation by the in-plane structural packing motif at d = 0.681 nm [12,50]. This sentence should be rephrased

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Figure 5: In figure 5a) DNC after 40 min show a reduction of MB concentration of about 80% while, under irradiation the same material shows a reduction of MB concentration of 40%. This point should be explained

Author Response

Firstly, we would like to thank the Reviewer for the feedback and comments on our manuscript. We appreciate the suggestions and comments, which raised out important issues to be adjusted/improved for the final version of the manuscript.

1. Page 2. MB dye: MB is used for the first time, then, methylene blue should be added to the text.

Thank you for the suggestion. We corrected it in the revised version of the manuscript.

2. Page 3. Then, a certain mass of DNC (Table S1) was added to the mixture and sonicated for 30 min: Sonication condition should be reported.

Thank you for the observation. We have included the sonication condition in the revised version of the manuscript, as follows: “(UltraCleaner 1400A – model unique, 40 kHz - 315 W RMS)”

3. Page 5. MB was monitored through UV-Vis spectrophotometry at each 20 min up to 160 min.: wavelength should be reported.

Thank you for the observation. We have included the wavelength of maximum absorption of MB at 664 nm in the revised version of the manuscript.

4. Page 6. DLS results confirmed the formation of non-agglomerated particles in solution: DLS analysis is usually used for spherical nanoparticles. However, in the manuscript the authors report nanosheets structures. This point should be explained. Compare size distribution obtained by DLS and size distribution obtained by SEM could help.

Thank you for the comment. In our study, DLS was used only for the pristine DNC characterization in order to corroborate the SEM findings. From SEM results (Fig. 2c), we could identify that the DNC samples contain spherical like individual particles of about 100 – 200 nm. Then, by DLS analysis we confirmed that the samples were not agglomerated (see Fig. S4), determining their mean diameters of 125.4 ± 48.9 nm.  

5. Page 7. Figure 2: figure 2 c) shows aggregates, this is in contrast with previous statement

Thank you for the important observation. The statement “completely dispersed from each other” was replaced by “without formation of large aggregates” when describing Figure 2c. Although it is possible to observe some particles touch each other, large aggregates are not observed. Additionally, the proximity among the particles can be seen as an effect of solvent evaporation during the particles´ deposition at the substrate for SEM analysis. Therefore, DLS is better indicated for investigating the behavior of the particles in solutions, and our results show that no aggregates were overall observed.

6. The former peak is by the other side a signature of tri-s-triazine-based g-C3N4 formation by the in-plane structural packing motif at d = 0.681 nm [12,50]. This sentence should be rephrased.

We have simplified the sentence to “The former peak is by the other side a signature of tri-s-triazine-based g-C₃N₄ formation [12,50]” and hope it is better presented now.

6. Page 11. Figure 5: In figure 5a) DNC after 40 min show a reduction of MB concentration of about 80% while, under irradiation the same material shows a reduction of MB concentration of 40%. This point should be explained.

Thank you for the important observation. Figure 5a shows the adsorption of the MB dye in the dark for the studied catalyst materials. We repeated the MB adsorption experiment with DNC several times and it reached 71% of mean removal after 40 min under dark using the following experimental conditions: [MB] = 30 mg L^-1, 25 mg of photocatalyst, and 25 mL of total volume. Besides, we increased the adsorption time up to 120 min to observe the equilibrium achievement (see Supplementary Material, Fig. S6). After this adsorption step in dark, we created C/C₀ curves (Fig. 5b) that represent the temporal evolution of the concentration of MB at a time “t” (C) divided by the concentration of MB (C₀) (i.e., the concentration found straight before the illumination process). Therefore, for Fig. 5b the adsorption effect was already discounted, and the curves represent only the photocatalysis effect, i.e., removal of the dye due to light irradiation.

To avoid misinterpretations, we have changed Figure 5 caption as well as replaced C₀ in Fig. 5a with C_i, which now unequivocally represents the initial dye concentration. The following note was added to the figure caption. “Note: Fig. 5b was produced after the adsorption step in the dark (40 min for all samples, except DNC* (60 min)), and represents the concentration of MB at the time “t” divided by the concentration of MB at the moment that light was turned on (C₀).”  In addition, the following sentence was added to the experimental section. “Adsorption of the MB dye was investigated by C/Ci curves, where C is the concentration of MB at the time “t” and C_i is the initial concentration of MB. The photocatalytic effect was accounted for by the C/C₀ curves, where C₀ is the concentration of MB when the light was turned on.”

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors reported that the synthesis and characterization of g-C3N4/DNC heterojunctios. The g-C3N4/DNC-28 showed a remarkable photoactivity in MB degradation and bacteria photoinactivation. The results are interesting, however, there some issues need to be addressed before the further consideration.

1. TEM images of g-C3N4/DNC heterojunction samples should be incorporated.
2. Specific surface area of g-C3N4, DNC, g-C3N4/DNC heterojunction samples should be measured to clarify the observed various photoactivity come from the effects of surface area or charge separation.
3. Why the results of photocatalytic MB degradation of DNC showed a higher photoactivity at first 20 min than that at the rest of 100 min?
4. How to convert the potential vs Ag/AgCl into that vs RHE?
5. The current results cannot confirm the optimal composition of g-C3N4/DNC heterojunction and the best photoactivity, the sample with DNC containing over 28.3 should be incorporated.

Author Response

We would like to thank Reviewer #2 for the feedback and comments on our manuscript. We appreciate the suggestions and comments, which raised out important issues to be adjusted/improved for the final version of the manuscript.

1. TEM images of g-C3N4/DNC heterojunction samples should be incorporated.

As suggested, a representative TEM image of the g-C3N4/DNC-28 heterojunction was included in the revised version of the manuscript.

2. Specific surface area of g-C3N4, DNC, g-C3N4/DNC heterojunction samples should be measured to clarify the observed various photoactivity come from the effects of surface area or charge separation.

As suggested, specific surface areas were evaluated by the BET method and the results are described in the revised version of the manuscript. The following sentence was added to the main text: “Specific surface area (S_BET) was determined according to the Brunauer−Emmett−Teller (BET) method using nitrogen adsorption isotherms, and the results show the S_BET of the pristine DNC and g-C₃N₄ are 83.6 and 60.9 m² g^-1, respectively. All studied heterojunction showed higher S_BET than the pristine materials but with values very close to each other, where g-C₃N₄/DNC-11 presented the higher surface area of 111.7 m² g^-1, followed by g-C₃N₄/DNC-28 (106.9 m² g^-1) and g-C₃N₄/DNC-2 (104.1 m² g^-1). Therefore, as it can be seen the MB adsorption onto the photocatalysts does not follow perfectly the surface area trending.”

3. Why the results of photocatalytic MB degradation of DNC showed a higher photoactivity at first 20 min than that at the rest of 100 min?

Thank you for the valuable observation. We repeated the experiments for pristine DNC (under dark and light, increasing the adsorption time up to 120 min (Fig. S6). By revisiting these data, we now show that the photocatalysis effect on pure DNC is negligible as would be expected. Figure 5b was updated in the revised version of the manuscript, as can be seen below.

4. How to convert the potential vs Ag/AgCl into that vs RHE?

Thank you for the comment. Potentials vs. Ag/AgCl reference electrode can be converted to RHE by using the Nernst equation. A piece of additional information was inserted into the experimental section of the revised manuscript as follows: “Potentials versus the Ag/AgCl reference electrode (E_Ag/AgCl) were converted to RHE (E_RHE) by using the Nernst equation, where E_RHE = E_Ag/AgCl + (0.059 × pH) + 0.197 V.”

5. The current results cannot confirm the optimal composition of g-C3N4/DNC heterojunction and the best photoactivity, the sample with DNC containing over 28.3 should be incorporated.

Thank you for the comment and the observation. As pointed, we have limited our work to 3 compositions containing DNC, namely g-C3N4/DNC-2, g-C3N4/DNC-11, and g-C3N4/DNC-28, and two pristine materials (DNC and g-C3N4 nanosheets). To avoid any misinterpretation, we have changed the paragraph as follows: “In this context, among the studied photocatalysts, the g-C₃N₄/DNC-28 showed a higher photoactivity for the removal of the MB under simulated solar irradiation at 200 mW cm^-2 (Figure 5b).”

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors improved the manuscript, however an additional point needed to be clarified. 
Figure S6 shows the adsorption of MB with a sensible reduction of its concentration. 
How was the initial MB concentration restored? If not, the caption of figure 5 should be corrected. 

Author Response

Thank you very much for the comment.
Indeed, the initial MB concentration was not restored and the caption of Figure 5 was updated as suggested by the reviewer.
The new caption of Fig. 5 can be seen below.

"Figure 5. (a) C/C_i bar curves for MB adsorption under dark conditions (20 min under sonication followed by 20 min under magnetic stirring) at initial [MB] = 30 mg L^-1, 1 mg mL^-1 of photocatalyst, and 25 mL liquid volume. (b) photocatalytic removal of MB dye in water using pristine g-C₃N₄ and DNC, and g-C₃N₄/DNC heterojunction samples under simulated solar illumination (AM1.5G filter) at200 mW cm^-2. Note: Fig. 5b was produced immediately after the adsorption step in the dark (40 min for all samples, except DNC* who presented the highest adsorption ability where 60 min under dark was necessary to establish equilibrium – Fig S6), and represents the concentration of MB at the time “t” divided by the concentration of MB at the moment that light was turned on (C₀). The MB concentration was not restored to 30 mg L^-1 and error bars represent the mean standard deviation of the triplicate experiments."