The Interface Interaction of C3N4/Bi2S3 Promoted the Separation of Excitons and the Extraction of Free Photogenerated Carriers in the Broadband Light Spectrum Range
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsReview report for the manuscript titled “Interface Interaction of C3N4/Bi2S3 Promoted the Separation of Excitons and the Extraction of Free Photogenerated Carriers in Broadband Light Spectrum Range”
The authors have demonstrated a broad band optical sensitivity in what they called “Bi2S3/C3N4 heterojunction” material. They have investigated the impact of electrode material, optical power and light wavelength on the optical responsivity of their material. The materials and process they utilized, the test structure they used is very simple and low cost which are important factors that can leverage transformation of the lab experiments into potential industrial applications. After a careful review of the authors’ work, I am providing comments below which I want to authors to carefully address.
1- Abbreviations appearing for the first time in the text are not defined (eg. C3N4, Bi2S3, CO2, g-C3N4, Bi2S3-xOx, In2O3, NiWO4, BiVO4, ZnO, BiFeO3 BiFeO3, etc.). The authors need to define all such abbreviations.
2- Since presented in the manuscript as three important competing processes, it would make sense for the authors to define the important optoelectrical processes: fluorescence, photoelectric effect and scattering
3- Although the authors have stated that their focus in the present work is on the interface property and processes, they still need to provide the thicknesses of the Bi2S3 and C3N4 layers as the bulk is still influential on the overall result given the photoexcitation/ carrier generation takes place in the bulk as well.
4- Based on the authors' interpretation of the XRD data, only crystallographic peaks of Bi2S3 are exhibited and not that of C3N4. Can the authors justify why C3N4's signature is not observed in the XRD data given the fact that the C3N4 layer also should have crystallinity behavior?
5- RAMAN and PL measurements should give a good insight into the phase formation of the involved 2D materials and their optical properties, respectively. Can the authors provide such data for their Bi2S3/C3N4 heterostructure? That would be useful as the SEM image provided in the paper does not provide a meaningful information.
6- The authors have presented "photocurrent Vs time" data for different light wave lengths but it took them several figures. This makes the data presentation incomprehensive and leaves their readers to compare the different figures to understand a trend. It would be more efficient if the authors conduct a spectral responsivity measurement and provide a data from where readers can easily observe the present device's sensitivity across wavelengths.
7- The authors have claimed at several spots in their "Results and Discussion" section that their heterojunction device shows "fast photocurrent responses". Can the authors back this claim by quantifying the response speed, i.e. by extracting a value from their transient measurements?
8- Can the authors provide a clear reason for the negative photoconductive response they observed with the graphite electrodes with 4B pencil drawings?
9- The authors claimed that their Bi2S3/C3N4 heterojunction has good stability of photophysical properties citing that the heterojunction material has been stored for 4 years. Do the authors have optical response data of the same material before 4 years? It would be interesting to see the how good the response was for a fresh material as that would also reflect on how far the photophysical properties of the material have degraded over the 4 years period.
10- The authors need to modify the band schematic given in their "Scheme-2" as it is not to scale and misleading giving the impression that the two materials have a comparable band gap while one has double the energy gap of the other's. Also can the authors explain what made them think that the Bi2S3/C3N4 heterojunction forms a staggered (Type-II) gap and not a straddling (Type-I)? This is very important to clarify as it determines how the charge carrier transport takes place across the junction and this in turn impacts the extraction of the photo-generated electron/hole pair contributing the measured photocurrent.
11- According to the authors' description of the material preparation, it seems that the C3N4 and Bi2S3 composites were ultra-sonicated altogether the final solution is printed/drop-casted on the substrate followed by annealing. The authors should explain how they can call this material a "Heterojunction"? A Heterojunction would refer to the formation of a clear interface or junction between two different materials or the same material of different doping which does not seem possible in this case looking at the method of preparation
12- The schematic of the device used for the photocurrent measurement, which now is put at the very end, should be presented in the results and discussions section right before the corresponding results
13- As seen in the schematic described in comment 12, both electrodes are contacting only one side of Bi2S3/C3N4 film. I believe the material investigated is not a heterojunction and is rather a fully intercalated homogeneous film composed of Bi2S3 and C3N4. Nevertheless, even if one follows the authors' claim and accepts the material as a heterojunction, the the test structure/ measurement setup used is not appropriate. The correct way of characterizing a heterojunction would be by contacting either sides of the heterojunction by separate electrodes that are at different potentials to enable monitoring of the charge carrier transfer from one layer to the other across the heterojunction. Therefore, the authors' way of characterization clearly shows that they are actually measuring a photoconductivity of a homogeneous material composed of Bi2S3 and C3N4 and not an optical sensitivity across a "heterojunction".
Comments on the Quality of English LanguageI would say the quality of English language is fine but it still needs improvement to qualify for a publication standard. I noticed few typos as well as short and fragmented sentences and phrases that need to be rephrased, combined or separated to enable a sound and well coherent story flow. Therefore, I strongly encourage the editors to enforce this demand on the authors and also thoroughly check for such flaws before publishing the paper if the revised version would be recommended by the reviewers including myself.
Author Response
Dear Sir,
Thank you for your work and comments. We have revised it according to the comments. The changes have been highlighted in the revision.
Sincerely,
Xingfa Ma
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsManuscript inorganics-3487080 titled “Interface Interaction of C3N4/Bi2S3 Promoted the Separation of Excitons and the Extraction of Free Photogenerated Carriers in Broadband Light Spectrum Range” devoted to the study of the photoelectric responses which were observed on the C3N4/Bi2S3 suspension obtained by the authors. The performed experiment includes varying the spectral characteristics of the light source, the power of the light source and the way of investigated sample contact with the equipment. The authors show that the obtained composite exhibits sufficient photocurrents for practical applications and can be used with simple organization (the graphite electrodes drawing by 4B pencil on a printing paper) of sample contact with the equipment. Despite the progress towards photogenerated current devices developing, the paper cannot be published in the presented view.
- The title of the paper poorly corresponds to its content: it lacks studies of interaction of C3N4 and Bi2S3 on the interface and ‘Separation of Excitons’ processes.
- The experimental techniques used by the authors in their study are described not well enough:
(a) The synthesis and characteristics of C3N4 should be shown in this article;
(b) Please indicate what kind of equipment was used as a light source, how the radiation power was regulated and controlled, what was the light flux directivity diagram and exposed sample area.
- In my opinion, the phrases like “XRD results of Bi2S3/C3N4 heterojunction” and “UV–Vis– NIR results of Bi2S3/C3N4 heterojunction” is not correct. Really, the authors say about Bi2S3+C3N4 nanocomposite where Bi2S3/C3N4 heterojunction is the interface border.
- Is there possible some theoretical discussion about
(a) Photocurrent changing during the sample exposition at light conditions (Figs 3-12);
(b) Absence of linear dependence between the value of adsorption coefficient (Fig 2) and photocurrent at the fixed light flux (Figs 3b, 4, 9, 10);
(c) Absence of linear dependences between the light flux and photocurrent value (Figs 5-7, 12)?
6) Please verify the data presented in Fig. 3-12:
(a) Note that 650 nm light sources at 50 mW in Fig. 3b the magnitude of the photocurrent is about 0.9 mA, while in Fig. 5 is about 4 mA. How can these differences be explained?
(b) Note the negative value of the photocurrent in Fig. 12 at 5 mW. Are there any examples in the literature of photocurrent inversion at varying radiation power?
(c) Correct the captions of Figs. 14, 15.
7) Please explain why, despite the absorption maximum nanocomposite in the region of 780 nm (Fig. 2), the data on photocurrents during photogeneration at this wavelength are almost not presented? The exceptions are Figs. 10, 14, and 15
8) Where is the information about relative arrangement of energy levels of Bi2S3 and C3N4 (scheme 2) taken from? The energy scale should be placed on the scheme (take, please, the energy of free charges in vacuum as zero). In addition, it should be indicated that the charge transfer across the heterojunction boundary leads to the similar energy of charges in different phases due to the potential jump appearance. And then the external conditions are imposed: photoinduced phenomena and external electric field.
Author Response
Dear Sir,
Thank you for your work and comments. We have revised it according to the comments. The changes have been highlighted in the revision.
Sincerely,
Xingfa Ma
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors have tried to address my comments and suggestions in their revised manuscript but what is missing in their response to each of my comments are sentences guiding at which lines in the revised manuscript the claimed changes can be found. Without such guides, blindly searching which comment is addressed where in the manuscript would unnecessarily create lots of work for a reviewer. Moreover, I would like request the authors to still address my comments to their responses (see points 1, 6, 9, 11 and 13 below)
1- The authors have now tried to define abbreviations at first appearance. However, the way they have done that is not correct. I would suggest that they start with the definition followed by the abbreviation inside brackets like this: Bismuth trioxide (Bi2O3) not Bi2O3 (bismuth trioxide).
2- definition/ explanation of fluorescence and scattering are is still missing. Only photoelectric effect is addressed in the revised manuscript. Nevertheless, its fine, I don't want to press on this.
3- Fine.
4- Ok
5- Ok
6- That is fine, I understand. However, I would suggest you to try merging some of the "Photocurrent vs time" data currently distributed across several figures but that can potentially be put on the same plot while using different colors. This way comparing the different results and understanding trends would be helpful for their readers.
7- Ok
8- Ok
9- What is described in the response letter is not amended in the revised manuscript. Can the authors include in their revised manuscript the photoelectric data measured on the sample before four years?
10- Ok
11- Please refer to my response in point-13 below.
12- Ok
13- I do not agree with the authors argument in their response to my comment-13. I wouldn't refer a fully intercalated homogeneous film obtained by ultrasonic mixing of Bi2S3 and C3N4 as a heterojunction. However, I think calling it "nanocomposite" is appropriate as that is what it exactly is. Therefore, I would ask the authors to replace the term "heterojunction" by "nanocomposite" throughout the manuscript
Comments on the Quality of English LanguageIt seems to me that the authors have tried their best but the English language in their manuscript still needs further improvement. The ideal solution for them would be to get support from an English language proficient colleagues or fellow researchers.
Author Response
Dear Sir, Thank you for your work and comments. We have revised it according to the comments. The changes have been highlighted in the revision. Sincerely, Xingfa MaAuthor Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsManuscript inorganics-3487080 is devoted to the study of the photoelectric responses which were observed on the C3N4/Bi2S3 suspension obtained by the authors. The performed experiment includes varying the spectral characteristics of the light source, the power of the light source and the way of investigated sample contact with the equipment. The authors show that the obtained composite exhibits sufficient photocurrents for practical applications and can be used with simple organization (the graphite electrodes drawing by 4B pencil on a printing paper) of sample contact with the equipment.
Author Response
Dear Sir,
Thank you for your work and comments. We have revised it according to the comments. The changes have been highlighted in the revision.
Sincerely,
Xingfa Ma
Author Response File: Author Response.pdf
Round 3
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors have tried to address my comments and suggestions in their revised versions. Nevertheless, there are still two open points the authors would benefit from addressing. Here below is my feedback/comment on the authors 2nd point-by-point response and please see open points 6, 9.
1- Accepted 2nd revision
2- Accepted 2nd revision
3- Accepted 1st revision
4- Accepted 1st revision
5- Accepted 1st revision
6- I understand the authors’ concern to merging the plots in the same graphs due differences in the magnitude of the base line currents. Nevertheless, to make it easier for readers to understand and follow the authors’ argument, I would still recommend the authors to, at least, group four “Photocurrent vs time" plots into one figure such that each plot becomes a sub-figure. For example, figures 5-8 in the current version can be grouped into new Figure-5 such that current figure 5 becomes, new figure-5a, current figure 6 becomes new figure 5-b, … etc. Similarly, figures 10-13 in the current version should be grouped into one figure, i.e. new Figure-7 (because current figure 9 will become figure-6) where current figure-10 becomes new figure-7a, current figure 11 becomes new figure-7b, etc. This will make the data presentation more comprehensive and appealing and more importantly, will make comparison of the results easier.
7- Accepted 1st revision
8- Accepted 1st revision
9- In response to my earlier comment, the authors have said figures 1-13 are before and figures 14-18 are after the four years’ storage of their nanocomposite material. This fact should be clearly highlighted in their revised manuscript (especially the new fact that figures 1-13 contain data achieved four years ago, which, however, is not mentioned at all in the current version of the manuscript). Also, it would make sense that the authors provide a short discussion on their assessment of the difference between the two data sets.
10- Accepted 1st revision
11- Please refer to my response in point-13 below.
12- Accepted 1st revision
13- Technically, this part can be marked as “Done”, as the authors have revised the manuscript by replacing the term “Heterojunction” by “Nanocomposite (aggregate)” as suggested. This comment on their 2nd response is just to show the authors that my earlier comment was not based on a misunderstanding of their work as they thought, but rather was based on the description they themselves have provided in their manuscript. Here below, I quote their description of how their Bi2S3/C3N4 material is prepared, as given in sub-section 3.3: “To 20 mL of C3N4 suspensions, 40 mL of H2O were added. Then, 0.5 g of bismuth(III) nitrate pentahydrate and an appropriate amount of sodium thiosulfate were added, and the mixtures were ultra-sonicated about 10 min.”.
The point here is that I still doubt that even the hydrothermal step would turn the ultra-sonicated mixture into a heterojunction layer (they have to provide experimental proofs otherwise). Therefore, using the term “nanocomposite (aggregate)” like it is amended in the revised manuscript is important and appropriate.
Comments for author File: Comments.pdf
It seems to me that the authors have tried their best but the English language in their manuscript still needs further improvement. The ideal solution for them would be to get support from an English language proficient colleagues or fellow researchers. Nevertheless, I would leave this part up to the Editors.
Author Response
Dear Sir,
Thank you for your comments. We have revised it according to your comments.
Sincerely,
Xingfa Ma
Author Response File: Author Response.pdf
