Changes in Structural, Morphological and Optical Features of Differently Synthetized C3N4-ZnO Heterostructures: An Experimental Approach
, Maria Cristina Paganini 1,* and Erik Cerrato 2Round 1
Reviewer 1 Report
The authors applied different synthesis techniques to prepare bare C3N4 and combined C3N4/ZnO mixed systems. The role of the precursors used in the preparation of the materials was investigated. It was found that the use of the supramolecular complex for the preparation of C3N4 leads to a higher stringently relation between the two phases at the heterojunction, resulting in a much higher visible light harvesting. The manuscript looks like a report for different synthetic methods of C3N4-ZnO heterojunctions. The obtained materials are only simply characterized by XRD, DRS-UV spectra, and SEM. It is regretted that significance and necessity of this work is very unclear from a scientific point of view. Thus, this work cannot be accepted. Some obvious issues are shown below:
1. In the last paragraph of the Introduction section, the very long expression of “The introduction should briefly place the study in a broad context and highlight why it is important…References should be numbered in order of appearance and indicated by a numeral or numerals in square brackets—e.g., [1] or [2,3], or [4–6]. See the end of the document for further details on references” seems like some comments, which is irrelevant to the manuscript. Why the authors write this part here?
2. If the authors really want to compare the difference of C3N4-ZnO heterojunctions prepared by different methods, some other characterizations should also be done.
3. The conclusion section is too long, which should be more concise to highlight the key findings.
Author Response
We would like to thank the referee for the work and for the suggestions in improving our work. We answered to all their remarks and we added all the corrections in the text in red.
Q1. In the last paragraph of the Introduction section, the very long expression of “The introduction should briefly place the study in a broad context and highlight why it is important…References should be numbered in order of appearance and indicated by a numeral or numerals in square brackets—e.g., [1] or [2,3], or [4–6]. See the end of the document for further details on references” seems like some comments, which is irrelevant to the manuscript. Why the authors write this part here?
R1. The reviewer is right we uploaded not the last version of the manuscript but a previous one in which there were still some sentences to be deleted. These sentences, not related to the paper, have been removed.
Q2. If the authors really want to compare the difference of C3N4-ZnO heterojunctions prepared by different methods, some other characterizations should also be done.
R2. The comment is correct, indeed this is a preliminary work, we started with the study of the effects of synthesis procedures but the final goal is to prepare photocatalyst, so for sure other characterization techniques are necessary and also some tests for the evaluation of the photocatalityc activity.
Q3. The conclusion section is too long, which should be more concise to highlight the key findings.
R3. The conclusions have been shortened and some bullet points have been added to simplify and clarify the paragraph.
Reviewer 2 Report
The manuscript deals with the synthesis of C3N4/ZnO composites using 3 different methods. The obtained composites were characterized by XRD, FESEM and DRS. It was shown that the synthesis method used affects the structural and optical properties of the composite. Before publication, the manuscript needs a minor revision.
The XRD pattern of melamine should be inserted in Figure 1.
The XRD pattern of C3N4-CME and C3N4-CMW indicates a porous structure for these materials. I recommend performing N2 sorption measurements with these samples to confirm this assumption.
Which temperature was used for synthesis of bare C3N4 (500°C page 8 line 313 or 550°C given in table 1)?
What was the content of ZnO (or C3N4) in the synthesized materials? Is the low amount of C3N4 the reason for the absence of C3N4 reflections in the XRD powder pattern?
Does the synthesis method used affects the primary ZnO crystallite size?
The samples studied with FESEM should be mentioned in the caption of the Figure 3, 4 and 5.
Is it possible that nitrogen or carbon atoms can diffuse into the ZnO lattice during the calcination step, especially for samples DEP11-DEP31?
Due to the similar color, the UV/Vis spectra in Figure 4b are difficult to assign to the individual samples.
Add the mass of melamine and cyanuric acid dispersed in 80 mL of solvent (page 8, line 316).
Add the mass of ZnO and melamine used to make DEP11, DEP21, and DEP31 to the text.
What amount of C3N4 was used for the synthesis of US31 and US21?
Author Response
We would like to thank the referee for the work and for the suggestions in improving our work. We answered to all their remarks and we added all the corrections in the text in red.
Q1. The XRD pattern of melamine should be inserted in Figure 1.
R1. The XRD pattern of melamine has been added to fig 1
Q2. The XRD pattern of C3N4-CME and C3N4-CMW indicates a porous structure for these materials. I recommend performing N2 sorption measurements with these samples to confirm this assumption.
R2. The analysis of the surface area through the BET method showed that the samples C3N4-CME and C3N4-CMW present a surface area of about 30 m2/g. We will soon perform a measurement of the porosity.
Q3. Which temperature was used for synthesis of bare C3N4 (500°C page 8 line 313 or 550°C given in table 1)?
R3. The calcination temperature was always 550°C and the mistakes have been corrected
Q4. What was the content of ZnO (or C3N4) in the synthesized materials? Is the low amount of C3N4 the reason for the absence of C3N4 reflections in the XRD powder pattern?
R4. The amount of ZnO and C3N4 has been reported in molar ratio in the text. The FESEM images of sample DEP31 shows the presence of thin layers composed by low-atomic number elements, therefore we suppose this could be representative of C3N4. If it is so, such thin layers are not sufficient to generate reflections in the diffractogram; indeed, the amorphous structure of this phase prevents the observation of its weak and broad XRD reflections when mixed with the highly crystalline structure of hydrothermally prepared ZnO.
Q5. Does the synthesis method used affects the primary ZnO crystallite size?
R5. DEP e US samples use as base hydrothermal ZnO. The crystallite size reveals a size around 55 nm according to the crystalline domain calculated by the Debye-Scherrer equation with a variation in the dimensions spanning from 3 % to 15 % among different batches of ZnO-H. Similar variations can be obtained for co-synthetised heterojunctions ZnO-M and ZnO-CMW, while a smaller size (~30%) was obtained for ZnO-CME respect to ZnO-H.
Q6. The samples studied with FESEM should be mentioned in the caption of the Figure 3
R6. The caption in figure 3 has been corrected (3e ZnO-CME, 3f ZnO-CMW), 4 (sample DEP 31) and 5 (5c US31).
Q7. Is it possible that nitrogen or carbon atoms can diffuse into the ZnO lattice during the calcination step, especially for samples DEP11-DEP31?
R7. Nitrogen and Carbon are very small atoms so it is possible that a fraction of them enter into the structure. Moreover, the doping of ZnO with non-metals, forming a p-type semiconductor is not an easy issue. It has been demonstrated that N can enter into the ZnO lattice in particular conditions [1]. However, previously preliminary studies focused on the photoactivity of the mixed material prepared by a co-precipitation method and carried on by means EPR spectroscopy (thus a very sensitive tool) didn’t evidence the presence of both C and N. In any case, the various synthetic routs adopted in this study bring to a different organization at the phase interfaces that could potentially allow a partial N and C diffusion. this is for sure a topic that could be better explored in the incoming investigation and optimization of this interfaced material.
Q8. Due to the similar color, the UV/Vis spectra in Figure 4b are difficult to assign to the individual samples.
R8. The color of the spectra has been changed
Q9. Add the mass of melamine and cyanuric acid dispersed in 80 mL of solvent (page 8, line 316).
R9. The mass of melamine is 0.977 g and of cyanuric acid is 1.023 g. These values have been added in the manuscript
Q10. Add the mass of ZnO and melamine used to make DEP11, DEP21, and DEP31 to the text.
R10. These values have been added in the manuscript: DEP11: 1.00 g ZnO and 1.56 g melamine; DEP21: 1.00 g ZnO and 0.76 g of melamine; DEP 31: 1.00 g of ZnO and 0.5 g melamine.
Q11. What amount of C3N4 was used for the synthesis of US31 and US21?
R11. US31 0.40 g melamine; US21 0.55 g melamine. These values have been added in the text.
Reviewer 3 Report
This article is recommended for publication in Inorganics after major revision. Please find below some detailed comments.
1. Abstract: The authors should clarify the novelty and significance of this paper clearly. The current version is too general.
2. Figure 3c and 4c: The colors of different curves are too similar to be distinguished easily. To make the data more readable, the authors had better reset the line colors used.
3. Line 278-280 “Indeed...recombination”: References are needed for this statement. Some suggested references: (1) Journal of Materiomics 2021, 7, 929-939; (2) Inorganics 2019, 7, 120; etc.
4. To demonstrate the formation of heterojunction between C3N4 and ZnO, the authors should provide experimental evidence like photocurrent/PL change, Mott–Schottky plots, etc. Based on this, the charge transfer/separation process should be discussed in more detail. Some suggested references: (1) Nanomaterials 2022, 12, 904; (2) Chemical Engineering Journal 2021, 424, 130444; (3) Sensors and Actuators B: Chemical 2019, 290, 233-241; etc.
5. Overall, there seems no obvious change in the bandgap width among the different photocatalysts prepared by the authors, which means no big difference in their light absorption properties. Is this contradictory to the original intention of this work?
Author Response
We would like to thank the referee for the work and for the suggestions in improving our work. We answered to all their remarks and we added all the corrections in the text in red.
Q1. Abstract: The authors should clarify the novelty and significance of this paper clearly. The current version is too general.
R1. The abstract has been revised
Q2. Figure 3c and 4c: The colors of different curves are too similar to be distinguished easily. To make the data more readable, the authors had better reset the line colors used.
R2. The colors of the spectra have been changed
Q3. Line 278-280 “Indeed...recombination”: References are needed for this statement. Some suggested references: (1) Journal of Materiomics 2021, 7, 929-939; (2) Inorganics 2019, 7, 120; etc.
R3. The recommended references have been added
Q4. To demonstrate the formation of heterojunction between C3N4 and ZnO, the authors should provide experimental evidence like photocurrent/PL change, Mott–Schottky plots, etc. Based on this, the charge transfer/separation process should be discussed in more detail. Some suggested references: (1) Nanomaterials 2022, 12, 904; (2) Chemical Engineering Journal 2021, 424, 130444; (3) Sensors and Actuators B: Chemical 2019, 290, 233-241; etc.
R4. The suggestion of the referee is correct, and we will test these features in the future, anyway the goal of this work was to compare different synthesis techniques and different precursors to evaluate the best conditions for the preparation of these samples.
Q5. Overall, there seems no obvious change in the bandgap width among the different photocatalysts prepared by the authors, which means no big difference in their light absorption properties. Is this contradictory to the original intention of this work?
R5. The original goal of the work was to individuate a simple, facile and green synthesis way to prepare photocatalysts able to work with visible light in the dregadation of organic pollutants and eventually in the water splitting reaction. At the moment we are at the first step of this complex and not completed aim. Since now what we can say is that the precursor and the preparation route can affect the morphology and in some case also the band gap of the prepared systems. Finally, we partially reached the goal since we were able to synthetized C3N4 with an acceptable exposed surface area, where the low surface area represents one of the most limiting drawback for this material photocatalytic application.
As demonstrated in the figure we compared samples obtained with different methods, actually the band gap is quite similar among three of them (except for the bare C3N4) but it is different the presence of absorption shoulders that are present in the region of visible light. As demonstrated by our groups years ago studying N doped TiO2 the presence of these shoulders are due to the formation of intraband gap states that are responsible for the photoactivity of this material. [2]
SEE figure in the file uploded
- Cerrato, E.; Privitera, A.; Chiesa, M.; Salvadori, E.; Paganini, M.C. Nitrogen-Doped Zinc Oxide for Photo-Driven Molecular Hydrogen Production. Int J Mol Sci 2022, 23, doi:10.3390/ijms23095222.
- Barolo, G.; Livraghi, S.; Chiesa, M.; Paganini, M.C.; Giamello, E. Mechanism of the Photoactivity under Visible Light of N-Doped Titanium Dioxide. Charge Carriers Migration in Irradiated N-TiO2 Investigated by Electron Paramagnetic Resonance. J. Phys. Chem. C 2012, 116, 20887-20894, doi:10.1021/jp306123d.
Author Response File: 
Author Response.docx
Round 2
Reviewer 1 Report
The quality of the revised mansucript is much better, and can be accepted.
Reviewer 3 Report
Since the authors have well addressed all my comments, I recommend its acceptance by Inorganics in present form.
