Solvothermal Synthesis of g-C3N4/TiO2 Hybrid Photocatalyst with a Broaden Activation Spectrum
Round 1
Reviewer 1 Report
Composite of graphitic C3N4 and TiO2 were prepared by a solvothermal method and the resulted materials have been tested in photocatalytic degradation of acid green-25 (AG25) under irradiation, which consists of both UV and visible light components. Enhanced degradation rate over the composite was observed in comparison to that of over TiO2 and C3N4. Nevertheless, it is not clear what the added value of the present manuscript is in comparison to the related art. Heterojunction of C3N4 and TiO2 and as a result improved photocatalytic activity has already been published. Does the hydrothermal synthesis contribute to formation of heterojunctions more effective than published previously? If it is so, why? There are additional questions listed below:
1) Abstract
I have to emphasize that BET is not a characterization method. Nitrogen adsorption and recording isotherms is the method, which are processed by BET model.
Reactive oxidative species (ROS) were not identified at all, although this was promised in the abstract.
2) Introduction
A better introduction should be written, clearly highlighting the lack of scientific knowledge in the area covered by the study.
3) Results and discussion
line 58: “XRD diffractometer” – It is XR diffractometer of simply XRD.
Still in the line 58 the sharp peaks of the diffraction pattern was attributed to high purity of the product. It is important to note that the sharp diffraction pattern simply means that the products crystallinity is high, i.e. not amorphous and there are large crystallites. Anyway, the crystallite sizes should have been collected in a table.
The surface area of 15.43 m2/g obtained for graphitic C3N4 seems to be low in photocatalysis. Anyway, it is recommended to compare the surface areas of TiO2, graphitic C3N4 and the composite in a table.
4) SEM images
SEM images shown in Figure 2 tell nothing about nanosize scale. Hundreds of nanometers is its resolution. I have doubt that mesoporosity can be revealed from images as stated in line 78. I recommend removal of these pictures from the manuscript.
However, EDX analysis has conclusively shown that the sheet-shaped particles can be composed of C3N4, while the spherical particles can be composed of TiO2. Although SEM cannot account for chemical analysis, Figure 2 can be part of the paper if it is connected to EDX analysis.
5) UV-vis and DRS analysis
According to the generally accepted terminology, it is recommended to use the "absorption spectrum" instead of the "UV-vis spectrum".
A figure demonstrating Z scheme charge transfer route with exact position of valance and conduction bands of the two participating solids would be recommended and helpful for general understanding.
On Figure S8, spectrum of parent Titania should be shown as well.
The interpretation given for Figure S8 is very confusing. It is not clear how the bandgap of the composite would be reduced and in comparison to what. Anyway, a simpler interpretation based on the Z scheme would not require changing the bandgap of the participating materials.
6) Photocatalytic degradation of AG-25
Line 147: There is no Figure 5.
Figure 4 was not referenced in the text.
7) Materials and methods
Line 177-183: The editorial instructions remained in the manuscript.
8) Figures
Legends should be moved into the captions. Experimental conditions are missing from the captions. The captions must be worded in such a way that they can be understood even without reading the text.
The manuscript can be published after major revision.
Author Response
MDPI-Catalyst: Solvothermal Synthesis Of g-C3N4 – TiO2 Hybrid Photocatalyst With a Broaden Activation Spectrum
Reviewer comments – author answers
- Abstract
Comment: I have to emphasize that BET is not a characterization method. Nitrogen adsorption and recording isotherms is the method, which is processed by the BET model.
Our answer: Changed to “Nitrogen sorption desorption recording and modeling by the Brunauer–Emmett–Teller (BET) model.”
Comment: Reactive oxidative species (ROS) were not identified at all, although this was promised in the abstract.
Our answer: ESR analysis conducted. The results are attached in section 2.7.
- Introduction
Comment: A better introduction should be written, clearly highlighting the lack of scientific knowledge in the area covered by the study.
Our answer: The intro was updated, and we added a paragraph regarding the water matrix and testing of a commercial dye.
- Results and discussion
Comment: line 58: “XRD diffractometer” – It is XR diffractometer of simply XRD.
Our answer: Changed to XR diffractometer.
Comment: Still in the line 58 the sharp peaks of the diffraction pattern was attributed to high purity of the product. It is important to note that the sharp diffraction pattern simply means that the products crystallinity is high, i.e. not amorphous and there are large crystallites. Anyway, the crystallite sizes should have been collected in a table.
Our answer: XRD analysis was performed for the composite and the components of origin.
Comment: The surface area of 15.43 m2/g obtained for graphitic C3N4 seems to be low in photocatalysis. Anyway, it is recommended to compare the surface areas of TiO2, graphitic C3N4 and the composite in a table.
Answer: Currently, our institute’s BET is out of order and in any way has a waiting list of more than 2.5 weeks due to the extended downtime. Due to the short time required to submit the revision, we were not able to conduct this analysis on time.
- SEM images
Comment: SEM images shown in Figure 2 tell nothing about nanosize scale. Hundreds of nanometers is its resolution. I have doubt that mesoporosity can be revealed from images as stated in line 78. I recommend removal of these pictures from the manuscript.
However, EDX analysis has conclusively shown that the sheet-shaped particles can be composed of C3N4, while the spherical particles can be composed of TiO2. Although SEM cannot account for chemical analysis, Figure 2 can be part of the paper if it is connected to EDX analysis.
Our answer: Thanks for this important remark. The section was updated according to the reviewer’s recommendation.
- UV-vis and DRS analysis
Comment: According to the generally accepted terminology, it is recommended to use the “absorption spectrum” instead of the “UV-vis spectrum.”
Our answer: The entire paragraph was updated.
Comment: A Figure demonstrating Z scheme charge transfer route with exact position of valance and conduction bands of the two participating solids would be recommended and helpful for general understanding.
Our answer: Z scheme was added.
Comment: On Figure S8, spectrum of parent Titania should be shown as well.
Our answer: DRS analysis was conducted for the composite and original components.
Comment: The interpretation given for Figure S8 is very confusing. It is not clear how the bandgap of the composite would be reduced and in comparison to what. Anyway, a simpler interpretation based on the Z scheme would not require changing the bandgap of the participating materials.
Our answer: DRS analysis was performed for the composite and original components, including interpretation. Z-schene was added.
- Photocatalytic degradation of AG-25
Comment: Line 147: There is no Figure 5.
Comment: Figure 4 was not referenced in the text.
Our answer: The figures’ numbering is fixed.
- Materials and methods
Comment: Line 177-183: The editorial instructions remained in the manuscript.
Our answer: The text is fixed.
- Figures
Comment: Legends should be moved into the captions. Experimental conditions are missing from the captions. The captions must be worded in such a way that they can be understood even without reading the text.
Our answer: The legends of Figures 7 and 8 are described in the captions and removed from the graphs.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
In the manuscript, the authors prepared a TiO2/g-C3N4 photocatalyst and comprehensively characterised the catalyst. At the same time, the performance of the composite photocatalyst was investigated by testing the removal of Acid-green-25 in brine. The experimental design has a certain degree of innovation, and the data is accurate and reliable, but the research on the photocatalytic mechanism still needs to be strengthened.
1. The author needs to carefully check the errors in the manuscript, such as '…59, 86% and 98%...' in line 122 and '…g-C3N4…' in line 125, etc.
2. To explain the superior photocatalytic degradation performance of composite materials, the author must provide some necessary characterization means, not just cite existing reports, for example, photoelectrochemical testing and steady-state fluorescence testing.
3. The author needs to supplement the active species capture experiment, detect the catalyst's valence band and conduction band position, and propose a reasonable photocatalytic degradation mechanism.
4. Cycling experiments must be discussed for photocatalytic degradation, and the author must make supplements.
5. Suggestion: More recent papers involving photocatalytic environmental remediation can be referenced in the manuscript. (Liu C, Mao S, Wang H, et al. Chemical Engineering Journal, 2022, 430: 132806; Liu C, Mao S, Shi M, et al. Journal of Hazardous Materials, 2021, 420: 126613; Chemical Engineering Journal, 2022, 449: 137757).
Author Response
- Comment: The author needs to carefully check the errors in the manuscript, such as ‘…59, 86% and 98%...’ in line 122 and ‘…g-C3N4…’ in line 125, etc.
Our answer: Issues fixed.
- Comment: To explain the superior photocatalytic degradation performance of composite materials, the author must provide some necessary characterization means, not just cite existing reports, for example, photoelectrochemical testing and steady-state fluorescence testing.
Our answer: Thanks. Photoluminescence was conducted for composite and original components. See paragraph 2.6. Unfortunately, photoelectrochemical testing is not available for us.
- Comment: The author needs to supplement the active species capture experiment, detect the catalyst’s valence band and conduction band position, and propose a reasonable photocatalytic degradation mechanism.
Our answer: XPS was conducted for composite and original components. ESR spectroscopy was performed as well for ROSs detection.
- Comment: Cycling experiments must be discussed for photocatalytic degradation, and the author must make supplements.
Our answer: This is an excellent idea. However, the time given for corrections was insufficient to perform more experiments.
- Comment: Suggestion: More recent papers involving photocatalytic environmental remediation can be referenced in the manuscript. (Liu C, Mao S, Wang H, et al. Chemical Engineering Journal, 2022, 430: 132806; Liu C, Mao S, Shi M, et al. Journal of Hazardous Materials, 2021, 420: 126613; Chemical Engineering Journal, 2022, 449: 137757).
Our answer: Thanks for addressing us to these articles. We learned from reading them, and they helped to strengthen our introduction.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors answerred tha questions in appropriate manner. The paper can be published.
Reviewer 2 Report
Authors addressed very well most of my comments. Paper could be published now.
