Tunable Carrier Transfer of Polymeric Carbon Nitride with Charge-Conducting CoV₂O₆∙2H₂O for Photocatalytic O₂ Evolution

Round 1

Reviewer 1 Report

This review concerns the manuscript nanomaterials-1730411 titled "Tunable Carrier Transfer of Polymeric Carbon Nitride with charge-conducting CoV₂O₆∙₂H₂O for photocatalytic O₂ evolution." The authors describe a composite material consisting of CoV₂O₆ nanostructures onto polymeric carbon nitride, where the former is added to facilitate the kinetics of water oxidation reactions. Cobalt vanadate materials have been reported as suitable electrocatalysts for O₂ evolution, and the authors speculated that its use could show potential in photocatalytic water splitting to O₂. Although the creation of supported metals or metal oxides to improve photocatalytic properties has no novelty, the authors clearly explained why performing studies with this system is essential. The paper is well-written and easy to follow, but a few things are missing and needed to improve the discussion. Therefore, this referee finds the manuscript suitable for publication in MDPI Nanomaterials upon minor to major revisions. A major revision will be requested because these comments are necessary to address to improve the discussion. Still, this referee finds it overall minor because comments will be easy to address (no additional experiments needed).

Comments:
1) The TEM analysis is local and may not be a good representation of the overall material. Try to use the XRD to determine the crystalline size of the CoVO phase using the most intense peak (@22.4?). This should give a qualitative analysis to the least. Sometimes line broadening correction helps with accuracy in data, and it is easy to do. 

2) The relative intensity between the (101) and (022) planes drastically changed from CoVO*0.5 and 10% CoVO/PCN. This indicates preferential crystal growth, and the TEM images seem to show that too. I highly recommend this discussion as different planes may have different photocatalytic activities. Perhaps comment one can apply here when considering both diffraction planes. The following reference is to help discuss this comment only: Identification of preferentially exposed crystal facets by X-ray diffraction (https://doi.org/10.1039/D0RA00769B).

3) Lines 182 and 183: "the analysis of XPS once again demonstrated the successful complex of CoVO combined with PCN." That isn't true. The discussion shows the characteristics of the CoVO and PCN but not a combination of both. In other words, you provided a discussion establishing the physical presence of each element but not a chemical connection between the two.

Properly addressing these three comments might better explain why an optimal performance was obtained with 3% loading. It will give a scientific explanation instead of saying the system was optimized (by trial and error).

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript describes the synthesis of graphite-like carbon nitride doped with cobalt compounds for the photocatalytic production of oxygen. In principle, the problem is interesting, but most of the experiments were carried out carelessly, additional experiments are needed.
Main comments:

1. XRD patterns. All peaks should be clearly labeled, Miller indices should be shown.
2. HRTEM mapping should be provided for the composite sample. The TEMs available in the article are absolutely not informative.
3. I do not understand why the rate of oxygen production changes with time and a plateau is observed. In addition, one cannot call eight-hour experiments long-term.
4. I would like to see the electronic diagram of the composite photocatalyst and the mechanism of the emerging heterojunctions. Without a clear consideration of the mechanism of charge transfer, the work looks unfinished.

Author Response

Comment 1: XRD patterns. All peaks should be clearly labeled, Miller indices should be shown.

Response 1: Thanks so much for the good suggestion! We have added the Miller indices in the XRD image in the revised manuscript (Figure 2a, marked in yellow color).

Figure R2. Powder XRD patterns of x wt% CoVO/PCN samples.

Comment 2: HRTEM mapping should be provided for the composite sample. The TEMs available in the article are absolutely not informative.

Response 2: We thank the referee for the professional comments. HRTEM mapping was shown in Figure R3.

It has been added in the revised manuscript (Figure 3e, marked in yellow color).

Figure R3. TEM mapping of CoVO/PCN sample.

Comment 3: I do not understand why the rate of oxygen production changes with time and a plateau is observed. In addition, one cannot call eight-hour experiments long-term.

Response 3: This is a good question. In the photocatalytic water splitting to O₂ reaction, AgNO₃ was added to act as an electron scavenger. With irradiation time increasing, the amount of electron scavenger AgNO₃ reduced, that is why oxygen production changes with time. Moreover, Ag⁺ was reduced to Ag and loaded on the surface of PCN in the reaction, which hinders the light absorption of samples.

The details have added in the revised manuscript (marked in yellow color).

Comment 4: I would like to see the electronic diagram of the composite photocatalyst and the mechanism of the emerging heterojunctions. Without a clear consideration of the mechanism of charge transfer, the work looks unfinished.

Figure R4. (a) Mott–Schottky plots of CoVO; (b) electronic band structure of the PCN and CoVO.

Scheme R1. Schematic illustration of the between PCN and CoVO.

Response 4: We thank the referee for the professional comments. The charge carrier transfer route between PCN and CoVO was presented in Scheme R1. The band structure of CoVO was determined by Mott–Schottky plots and DRS spectra (Figure R). Based on experimental analysis, a possible photocatalytic mechanism of Co-VO/PCN samples in WOR was proposed in Scheme R1. Irradiated by solar light, the photo-excited electrons generated from conduction band of CoVO transfer to PCN side and are trapped by Ag⁺. Meanwhile the holes generated at valence band of PCN moved to CoVO side and reacted to O₂. Therefore, construction of the CoVO/PCN heterojunction cannot only conducive to charge transfer but also restrain charge recombination and finally improve the photoactivity in WOR.

A concise discussion of this phenomenon has been added into the revised manuscript (Scheme 1, marked in yellow color).

Author Response File: Author Response.pdf

Reviewer 3 Report

The present study concerns fabrication of CoVO/PCN photocatalyst and its application in water oxidation.

The introduction provides a clear background for the study, including an explanation of the subject and a brief summary of the recent developments concerning the use of transition metal vanadates in catalysis (supported by several examples).

My comments are as follows:

• Lines 69-79: This part looks more like an abstract or conclusions than the aim of the study, which usually is stated at the end of the ‘Introduction’ section. Therefore, the Authors should clearly describe the goals of the study (what questions did the Authors want to answer?).
• Line 81: Where were the materials purchased?
• In my opinion the Authors should reconsider putting the sections ‘Results’ and ‘Discussion’ into one, as both sections contain experimental results.
• Lines 150-152: I don’t think that the TEM photographs are a proof of ‘good electronic contact’, as they simply show the morphology of the materials and not their electronic or catalytic properties.
• Line 192: what do you mean by ‘UV light (λ > 300 nm)’? UV-Vis or far UV only (350-400 nm)? Is this in accordance with Figure 5b? The legend says ‘UV-Vis’, not only UV. Please specify.
• Lines 188-189 (‘AgNO₃ acts as an electron scavenger to short-cut the reduction side’) and lines 202-205 (‘the reduction of Ag⁺ loaded on the surface of PCN in the reaction, which hinders the light absorption). The first sentence says that the role of AgNO₃ is to be an electron scavenger, whereas the other one – that Ag⁺ on the surface of PCN is beneficial for the light absorption. I don’t think that this issue is clear, please explain.

Moreover, the photocatalytic water oxidation tests were conducted in distilled water. Can other compounds be adsorbed on the surface of the composite and impede the catalytic process?

• Conclusions: the statement about the charge transfer is repeated three times: ‘fast charge transfer efficiency’, ‘good charge-conducting property’, ‘accelerates the transmission and separation of photogenerated electron-hole pairs’. In my opinion these phrases express the same information.
• In my opinion the manuscript lacks more discussion with available literature, as well as more information on the stability of the materials – for example: are the bonds, shown in the XPS analysis, stable during the photocatalytic process? Is it possible to separate and recycle the photocatalyst?
• The manuscript should be carefully checked to correct grammatical errors, spelling, and interpunction. Please choose either present or past tense when describing experiments, correct the use of singular and plural verb form and passive voice use.

Author Response

Reviewer: The present study concerns fabrication of CoVO/PCN photocatalyst and its application in water oxidation.

The introduction provides a clear background for the study, including an explanation of the subject and a brief summary of the recent developments concerning the use of transition metal vanadates in catalysis (supported by several examples).

Comment 1: Lines 69-79: This part looks more like an abstract or conclusions than the aim of the study, which usually is stated at the end of the ‘Introduction’ section. Therefore, the Authors should clearly describe the goals of the study (what questions did the Authors want to answer?).

Response 1: We thank the referee for the professional comments. We have change it to “These reports reveals that cobalt vanadates has excellent charge transfer performance which organic polymers are lacking. Herein, we report a CoV₂O_6∙2H₂O (CoVO) hybriding with PCN by a facile immersion strategy to enhance the activity of photocatalytic water splitting to O₂.”

This discussion has been added into the revised manuscript (Line 64-72, marked in yellow color).

Comment 2: Line 81: Where were the materials purchased?

Response 2: We thanks the referee for reminding. We have added the related information in the revised manuscript (Line 84-87, marked with yellow color).

Urea was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.. NH₄VO₃, TMCl_x·nH₂O (Fe, Co, Ni, Cu, Mn) were purchased from Sinopharm Chemical Reagent Co., Ltd.. All the reagents were of analytical grade and were used without further purification.

Comment 3: In my opinion the Authors should reconsider putting the sections ‘Results’ and ‘Discussion’ into one, as both sections contain experimental results.

Response 3: As suggested! We have recomposed to “Results and Discussion” in the revised manuscript (Line 131, marked in yellow color).

Comment 4: Lines 150-152: I don’t think that the TEM photographs are a proof of ‘good electronic contact’, as they simply show the morphology of the materials and not their electronic or catalytic properties.

Response 4: We thank the referee for the professional comments. We means that good contact is beneficial for charge transfer. The expression had been reminded to “Since the surface of the graphitic carbon nitride was fully intact with cobalt vanadate, the charge transfer between vanadate semiconductor and polymeric carbon nitride will be unobstructed.” It was added in the revised manuscript (Line 157-159, marked in yellow color).

Comment 5: Line 192: what do you mean by ‘UV light (λ > 300 nm)’, UV-Vis or far UV only (350-400 nm)? Is this in accordance with Figure 5b? The legend says ‘UV-Vis’, not only UV. Please specify.

Response 5: We thank the referee for the good suggestion. UV (λ > 300 nm) and visible light (λ > 420 nm) irradiation had been added as Figure R1.

This section had been added in the revised manuscript (Figure 5b, marked in yellow color).

Comment 6: Lines 188-189 (‘AgNO₃ acts as an electron scavenger to short-cut the reduction side’) and lines 202-205 (‘the reduction of Ag⁺ loaded on the surface of PCN in the reaction, which hinders the light absorption’). The first sentence says that the role of AgNO₃ is to be an electron scavenger, whereas the other one----that Ag⁺ on the surface of PCN is beneficial for the light absorption. I don’t think that this issue is clear, please explain.

Moreover, the photocatalytic water oxidation tests were conducted in distilled water. Can other compounds be adsorbed on the surface of the composite and impede the catalytic process?

Response 6: We thank the referee for the good question. The photo-excited electrons on the hybrids was trapped by Ag⁺, Ag⁺+electron--Ag. The reaction product “Ag” loaded on the surface of PCN, which hinders the light absorption of photocatalysts. In the system, La₂O₃ is pH stabilizer, AgNO₃ is an electron trapper. When the CoVO/PCN catalysts, or La₂O₃, or AgNO₃ were not added to the system, there was merely no O₂ gas can be detected. As shown in Figure R2, the experimental results meant that other compounds in the system played a great role in the catalytic process.

Figure R2. Photocatalytic O₂ evolution curves of 3 wt% CoVO/PCN samples under UV irradiation (λ > 300 nm).

Comment 7: Conclusions: the statement about the charge transfer is repeated three times: ‘fast charge transfer efficiency’ ‘good charge-conducting property’ ‘accelerates the transmission and separation of photogenerated electron-hole pairs’. In my opinion these phrases express the same information.

Response 7: We thank the referee for the nice suggestion. We have amended it in the revised manuscript as follows (Line 274-279, marked in yellow color):

In summary, the CoVO/PCN was constructed by a simple immersion method and performed a significant improved photoactivity for water oxidation reaction. With the introduction of CoVO, the visible light absorption of PCN are broadened. Furthermore, the good charge-conducting property of CoVO not only accelerates the transmission and separation of photogenerated electron-hole pairs, but also restrains the recombination of electron-hole pairs in the hybrids.

Comment 8: In my opinion the manuscript lacks more discussion with available literature, as well as more information on the stability of the materials-for example: the bonds shown in the XPS analysis, stable during the photocatalytic process? Is it possible to separate and recycle the photocatalyst?

Response 8: We thank the referee for the professional comments. The stability of the CoVO/PCN could be confirmed by the XRD and DRS spectra of the recycled sample in Figure R3. The major structure of CoVO/PCN sample was no obvious change except the appearance of Ag peaks which illustrated that the hybrids were stable during the photocatalytic process. The activity of the recycled sample shows a slightly reduced rate (Figure R4), which is mainly due to the loading of Ag nanoparticles on the surface of the substrate material.

We have added the analysis of the XRD data of the samples into the revised manuscript as follow (Figure S5 and S6, marked in yellow color).

Comment 9: The manuscript should be carefully checked to correct grammatical errors, spelling, and interpunction. Please choose either present or past tense when describing experiments, correct the use of singular and plural verb form and passive voice use.

Response 9: We thank the referee for the careful examination. The writing of this paper has been checked carefully and the mistakes have been corrected: Line 28, delete ‘as’; Line 30, ‘lead’ reminds to ‘leads’; Line 33, delete ‘that’, ‘of’ was changed to ‘for’ and ‘the’ was deleted; Line 40, ‘is’ was changed to ‘are’.

And some other mistakes were also corrected and marked and marked in yellow color.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I'm satisfied with the review.

Reviewer 3 Report

The manuscript is well revised