Enhanced Photocatalytic Activity of ZnO–CdS Composite Nanostructures towards the Degradation of Rhodamine B under Solar Light
Round 1
Reviewer 1 Report
In their paper, the authors have synthesized ZnO-CdS composite nanostructures and used it for the photocatalytic degradation of RhB aqueous solution.
Some comments are given below:
1-Several papers have investigated the photocatalytic activity of ZnO-CdS composites, authors are invited to briefly discuss the novelty of their work.
2-In Materials and methods, please add the solar light source? was it a lamp or solar light simulator?
3-A standard XRD pattern of CdS and ZnO should be added to fig.1
4-Fig 5b, show the y-axis starting from zero. Band gaps are determined when the y-axis starts from zero
5-The values of ECB and EVB of both CdS and ZnO were calculated or taken from the bibliography? if so, please add references
6- What is the TOC/COD removal of the system
7- Experimental works should be done in replicates. If so, error bars should be added to figures
8- The application part lacks discussion, authors must correlate between the MET, optical properties… of the catalysts and the photocatalytic properties. There is a minimum discussion about the results and their relevance and importance to the study.
9- Also, the performance of the obtained catalysts should be compared with the reported literature to establish the superiority of the work.
10-The given mechanism is general, EPR or scavenging experiments are recommended to be carried out or at least RhB degradation pathways.
Author Response
catalysts
Ms. Ref. No.: catalysts-1458262 
Title: Enhanced photocatalytic activity of ZnO-CdS composite nanostructures towards the degradation of Rhodamine B under sun light
Response to Reviewer-1 Comments
We appreciate the efforts of the reviewers for their detailed and insightful comments, which have helped us to improve the quality of our manuscript. A point-by-point response to the reviewer-1 comments is appended below for your convenience.
Comment 1: Several papers have investigated the photocatalytic activity of ZnO-CdS composites, authors are invited to briefly discuss the novelty of their work.
Response: We acknowledge the reviewer’s opinion. Semiconducting nanostructures, especially zero-dimensional (0D) and two-dimensional (2D) structures have superior photocatalytic activity as a result of improved electron-hole separation and a low recombination rate. Because of the unusual structural, physical, and chemical properties of 0D/2D materials, the fabrication of a 0D nanoparticles (NPs) and 2D flake-like structures have aroused wide interest as promising photocatalysts with several merits: (i) The bandgap and the light absorption of 0D/2D semiconductors can be adjusted. (ii) The recombination of electrons and holes in the bulk can be reduced due to the 0D/2D dimensional formation of heterostructures. (iii) Most of the active sites can be exposed on the surface and take part in the photocatalytic reaction. Moreover, various semiconducting nanomaterials such as metal oxides, metal sulfides, noble metal nanoparticles, etc., have been utilized to fabricate solar-light harvesting photocatalysts that are capable of degrading the toxic wastewater contaminants. However, the development of suitable bandgap materials with sufficient valence band and conduction band potentials is crucial to efficient photodegradation under the solar-light irradiation. Thus, it appears that functionalization of structurally combined binary nanocomposite is a successful methodology for achieving the photocatalytic degradation of organic pollutants. Furthermore, we wish to employ an efficient, and cost-effective method to synthesize ZnO-CdS nanocomposites in the present study. Thank you for your valuable comment.
Comment 2: In Materials and methods, please add the solar light source? was it a lamp or solar light simulator?
Response: We acknowledge the reviewer’s opinion. The photocatalytic performance of the prepared catalysts was evaluated by RhB photocatalytic degradation experiments using a 100-W solar simulator. As suggested, the specifications of the light source are added in the experimental section. Thank you for your valuable suggestion.
Comment 3: A standard XRD pattern of CdS and ZnO should be added to fig.1.
Response: We acknowledge the reviewer’s opinion. As suggested, the standard XRD pattern of ZnO and CdS were provided in Fig. 1 of the revised manuscript. Thank you for your valuable suggestion.
Comment 4: Fig 5b, show the y-axis starting from zero. Band gaps are determined when the y-axis starts from zero.
Response: We acknowledge the reviewer’s opinion. As suggested, Fig. 5(b) was clearly presented in the revised manuscript. Thank you for your valuable suggestion.
Comment 5: The values of ECB and EVB of both CdS and ZnO were calculated or taken from the bibliography? if so, please add references
Response: We acknowledge the reviewer’s opinion. The values of ECB and EVB for both CdS and ZnO were calculated in the following manner. Initially, the bandgap (Eg) values of ZnO and CdS were estimated from the Tauc plots (Fig. 5(b)). Further, in order to estimate the valance band (VB) and conduction band (CB) potentials of ZnO and CdS, we have chosen the individual atomic electron affinity (Eea) and first ionization energy (Eion) values of Zn, O, Cd, and S components. After, we have measure the susceptibility (χ) of individual components using Eea and Eion. i.e., χ = ½(Eea + Eion). Subsequently, we have estimate the susceptibility (χ) of ZnO and CdS. Based on these values, we have measure the VB and CB potentials of ZnO and CdS. Thank you for your valuable comment.
Comment 6: What is the TOC/COD removal of the system.
Response: We acknowledge the reviewer’s opinion. We regret that, we don’t have the facility of the Total Organic Carbon (TOC) analyzer, which could definitely give us additional information about the mechanism. We respect the reviewer’s suggestion and will try to include it in the future communications. Instead of the TOC analysis, the results observed from the PL spectra (intensity decreased for heterostructured nanocomposite), the degradation activity (98.16% of RhB degradation within 80 min), and the radical trapping results strongly supports the photocatalytic activity. We hope the reviewer understand the experimental deficiencies at the stage of the present experiments. Thank you for your valuable suggestion for the improvement of the manuscript.
Comment 7: Experimental works should be done in replicates. If so, error bars should be added to figures.
Response: We acknowledge the reviewer’s opinion. As suggested, the error bars were added for the obtained results in the revised manuscript. Thank you for your valuable suggestion.
Comment 8: The application part lacks discussion, authors must correlate between the MET, optical properties… of the catalysts and the photocatalytic properties. There is a minimum discussion about the results and their relevance and importance to the study.
Response: We acknowledge the reviewer’s opinion. As suggested, more discussion about obtained results was included in the revised manuscript and correlated the obtained results with photocatalytic properties. Thank you for your valuable suggestion.
Comment 9: Also, the performance of the obtained catalysts should be compared with the reported literature to establish the superiority of the work.
Response: We acknowledge the reviewer’s opinion. As suggested, the catalyst performance was compared with the reported literature (Table 2) to establish the superiority of the work. Thank you for your valuable suggestion.
Comment 10: The given mechanism is general, EPR or scavenging experiments are recommended to be carried out or at least RhB degradation pathways.
Response: We acknowledge the reviewer’s opinion. We have conducted the radical-scavenging experiments to confirm the proposed mechanism. In order to determine the reactive radical species, radical-scavenging experiments were performed by adding scavengers. Benzoquinone (BQ), isopropyl alcohol (IPA) and, triethanolamine (TEOA) were chosen as superoxide radical (•O2−), hydroxyl radical (•OH) and hole (h+) scavenger, respectively. The trapping experiments were conducted as similar to the photocatalytic activity test. Based on the radical-scavenging experiments, we have proposed an upgraded tentative type-II photocatalytic reaction mechanism for the ZnO-CdS nanocomposite. However, we regret that we were not able to investigate the photocatalytic process by EPR measurements, which could definitely give us additional information about the mechanism. Unfortunately, we don’t have the facility of EPR instruments. We hope the reviewer understand the experimental deficiencies at the stage of the present experiments. We deeply appreciate the comment raised by the reviewer. Thank you very much.
The authors are very thankful to the Reviewer for their valuable suggestions for the improvement of the manuscript.
All the modifications are shown in yellow color in the revised manuscript.
With regards
Ravindranadh Koutavarapu, Ph.D
Author Response File: 
Author Response.pdf
Reviewer 2 Report
1. Already so many people have been working in Rhodamine B degradation for almost a decade. Why do you want to decompose the dye? How is your work superior to others? Is there any purpose of your work to favor the earth from a practical point of view?
2. I wish to have the connection between semiconductor photon energy (Fig. 5) and the degradation profile of your experiment in Fig 7. Because its visible CdS has more semiconducting properties than the rest in Fig. 5b.
3. I would appreciate your Tauc plot. But, I need to see your units of Y-coordinates as Y-axis description, i.e, (eV cm-1)2
4. In fig. 2, can I know what you are trying to say? Because, for me, I can't see any difference between the 3 pictures in the figure. Also, I wish to see the original scale bar.
5. Need clarity in the mechanism.
6. In Fig. 7a and Fg. 5a, I wish to see the y-axis coordinates.
7. In your whole reaction, the degradation of Rhodamine B dye by your catalysts (different catalysts), the relationship of concentration with optical density is based on which law. If it's based on BEER'-LAMBERT'S LAW, in fig. 7a there is a possibility of dimer formation in the dye due to the shoulder peak of neat Rhodamine B even below OD 1. You must make some proper explanation in this review.
The rest of the work is ok. All the best.
Author Response
catalysts
Ms. Ref. No.: catalysts-1458262 
Title: Enhanced photocatalytic activity of ZnO-CdS composite nanostructures towards the degradation of Rhodamine B under sun light
Response to Reviewer-2 Comments
We appreciate the efforts of the reviewers for their detailed and insightful comments, which have helped us to improve the quality of our manuscript. A point-by-point response to the reviewer-2 comments is appended below for your convenience.
Comment 1: Already so many people have been working in Rhodamine B degradation for almost a decade. Why do you want to decompose the dye? How is your work superior to others? Is there any purpose of your work to favor the earth from a practical point of view?
Response: We acknowledge the reviewer’s opinion. As mentioned, so many people have been working in RhB dye degradation. RhB is a carcinogenic agent which damages the human health. To reduce the impact of RhB on the environment, it is necessary to degrade the harmful dye. In view of this, we synthesized the ZnO-CdS NCs by simple and cost-effective method. Furthermore, semiconducting nanostructures, especially zero-dimensional (0D) and two-dimensional (2D) structures have superior photocatalytic activity as a result of improved electron-hole separation and a low recombination rate. Because of the unusual structural, physical, and chemical properties of 0D/2D materials, the fabrication of a 0D nanoparticles (NPs) and 2D flake-like structures have aroused wide interest as promising photocatalysts with several merits: (i) The bandgap and the light absorption of 0D/2D semiconductors can be adjusted. (ii) The recombination of electrons and holes in the bulk can be reduced due to the 0D/2D dimensional formation of heterostructures. (iii) Most of the active sites can be exposed on the surface and take part in the photocatalytic reaction. Thus, it appears that functionalization of structurally combined binary nanocomposite is a successful methodology for achieving the photocatalytic degradation of organic pollutants. Thank you for your valuable comment.
Comment 2: I wish to have the connection between semiconductor photon energy (Fig. 5) and the degradation profile of your experiment in Fig 7. Because its visible CdS has more semiconducting properties than the rest in Fig. 5b.
Response: We acknowledge the reviewer’s opinion. From XRD analysis, a slight shift in the XRD peak positions was observed in the ZnO-CdS nanocomposite compared with their pure samples, which is the evidence for the formation of heterostructure. From optical absorption studies, the light harvesting capability of the ZnO-CdS nanocomposite was shifted into the visible region compared to pure ZnO with a bandgap of 2.9796 eV. This demonstrates that the development of heterostructure can significantly hinder the reunion of charge carriers and subsequently shift the light response ability towards the visible region. Further, from PL studies, it is very clear that the ZnO-CdS nanocomposite showed low PL intensities compared to the pure samples, which is due to the electron-trapping defects. The weak PL intensity signifies the suppression of electron-hole recombination rate, thus the ZnO-CdS nanocomposite exhibit an extended lifetime of the photogenerated carriers. Thank you for your valuable comment.
Comment 3: I would appreciate your Tauc plot. But I need to see your units of Y-coordinates as Y-axis description, i.e, (eV cm-1)2
Response: We acknowledge the reviewer’s opinion. As suggested, the y-coordinates of Fig. 5(b) are added in the revised manuscript. Thank you for your valuable suggestion.
Comment 4: In Fig. 2, can I know what you are trying to say? Because, for me, I can't see any difference between the 3 pictures in the figure. Also, I wish to see the original scale bar.
Response: We acknowledge the reviewer’s opinion. As suggested, the original scale bars are provided in Fig. 2. SEM and TEM were utilized to examine the morphology of the synthesized samples. SEM micrographs of pure ZnO, CdS, and ZnO-CdS NCs are shown in Figure 2. The ZnO nanoflakes (NFs) with low agglomerates are formed, as illustrated in Figure 2(a). The evenly dispersed spherical shaped CdS nanoparticles (NPs) are seen in Figure 2(b). CdS NPs are detected on the surface of ZnO NFs in ZnO-CdS NCs, implying the development of CdS NPs adorned ZnO NFs heterostructures as seen in Figure 2(c). Thank you for your valuable suggestion.
Comment 5: Need clarity in the mechanism.
Response: We acknowledge the reviewer’s opinion. As suggested, the detailed mechanism was provided in the revised manuscript. Thank you for your valuable suggestion.
Comment 6: In Fig. 7a and Fig. 5a, I wish to see the y-axis coordinates.
Response: We acknowledge the reviewer’s opinion. As suggested, the y-coordinates of Fig. 5(a) and Fig. 7(a) are provided in the revised manuscript. Thank you for your valuable suggestion.
Comment 7: In your whole reaction, the degradation of Rhodamine B dye by your catalysts (different catalysts), the relationship of concentration with optical density is based on which law. If it’s based on BEER-LAMBERT’S LAW, in fig. 7a there is a possibility of dimer formation in the dye due to the shoulder peak of neat Rhodamine B even below OD 1. You must make some proper explanation in this review.
Response: We acknowledge the reviewer’s opinion. The relationship of concentration with absorption is based on the Beer-Lambert law. The Beer-Lambert law states that there is a linear relationship between the concentration and the absorbance of the solution, which enables the concentration of a solution to be calculated by measuring its absorbance. According to the obtained results, as the solar light illuminated over the solution, the concentration of RhB gradually decreased w.r.t time and accordingly the absorption spectra also decreased. As suggested, the degradation pathway of RhB dye was provided in the revised manuscript. Thank you for your valuable comment.
The authors are very thankful to the Reviewer for their valuable suggestions for the improvement of the manuscript.
All the modifications are shown in yellow color in the revised manuscript.
With regards
Ravindranadh Koutavarapu, Ph.D
Author Response File: Author Response.pdf
Reviewer 3 Report
This manuscript may be considered for publication in a reputed journal like Catalysts. This manuscript is prepared according to the authors instructions of the journal. Enough care was taken while preparing this manuscript for publication. Authors report preparation and characterisation of a composite nanostructures. The photocatalytic activity of the Zn/Cd composite nanostructures was improved by the reduction in recombination rate of photoinduced e– 260 /h+ pairs. The work has impact and I find the manuscript appropriate for publication.
One small note: In my opinion, the introduction section contains too much unnecessary information and can be shortened.
Author Response
catalysts
Ms. Ref. No.: catalysts-1458262 
Title: Enhanced photocatalytic activity of ZnO-CdS composite nanostructures towards the degradation of Rhodamine B under sun light
Response to Reviewer-3 Comments
We appreciate the efforts of the reviewers for their detailed and insightful comments, which have helped us to improve the quality of our manuscript. A point-by-point response to the reviewer-3 comments is appended below for your convenience.
This manuscript may be considered for publication in a reputed journal like Catalysts. This manuscript is prepared according to the authors instructions of the journal. Enough care was taken while preparing this manuscript for publication. Authors report preparation and characterization of a composite nanostructures. The photocatalytic activity of the Zn/Cd composite nanostructures was improved by the reduction in recombination rate of photoinduced e–/h+ pairs. The work has impact and I find the manuscript appropriate for publication.
Comment 1: In my opinion, the introduction section contains too much unnecessary information and can be shortened.
Response: We acknowledge the reviewer’s opinion. As suggested, the introduction section was improved in the revised manuscript. Thank you for your valuable suggestion.
The authors are very thankful to the Reviewer for their valuable suggestions for the improvement of the manuscript.
All the modifications are shown in yellow color in the revised manuscript.
With regards
Ravindranadh Koutavarapu, Ph.D
Author Response File: Author Response.pdf
Reviewer 4 Report
File is attached
Comments for author File: Comments.pdf
Author Response
catalysts
Ms. Ref. No.: catalysts-1458262 
Title: Enhanced photocatalytic activity of ZnO-CdS composite nanostructures towards the degradation of Rhodamine B under sun light
Response to Reviewer-4 Comments
We appreciate the efforts of the reviewers for their detailed and insightful comments, which have helped us to improve the quality of our manuscript. A point-by-point response to the reviewer-4 comments is appended below for your convenience.
Comment 1: There are a few grammatical errors and typos, and therefore, a careful checkup is needed.
Response: We acknowledge the reviewer’s opinion. As suggested, the manuscript was thoroughly revised to avoid the grammatical and typographic errors. Thank you for your valuable suggestion.
Comment 2: In a few places it is stated as ZnO. However, the most appropriate term is ZnO NFs. Also, stated in many places as Zn-Cd instead of ZnO-CdS NCs.
Response: We acknowledge the reviewer’s opinion. As suggested, the terms ZnO, CdS, and Zn-Cd were replaced with ZnO NFs, CdS NPs and ZnO-CdS NCs, respectively. Thank you for your valuable suggestion.
Comment 3: The novelty of the research is not specified by the authors, other than its low cost and single step. Further, the novelty cannot be justified as they have used similar approaches to synthesize CdS NPs and ZnO-CdS NCs as reported by the other authors. Besides, if the procedure used for ZnO NFs is different even, it cannot be justified by the cost as there are other methodologies which are cheaper than the methodology mentioned by the authors.
Response: We acknowledge the reviewer’s opinion. Semiconducting nanostructures, especially zero-dimensional (0D) and two-dimensional (2D) structures have superior photocatalytic activity as a result of improved electron-hole separation and a low recombination rate. Because of the unusual structural, physical, and chemical properties of 0D/2D materials, the fabrication of a 0D nanoparticles (NPs) and 2D flake-like structures have aroused wide interest as promising photocatalysts with several merits: (i) The bandgap and the light absorption of 0D/2D semiconductors can be adjusted. (ii) The recombination of electrons and holes in the bulk can be reduced due to the 0D/2D dimensional formation of heterostructures. (iii) Most of the active sites can be exposed on the surface and take part in the photocatalytic reaction. Moreover, various semiconducting nanomaterials such as metal oxides, metal sulfides, noble metal nanoparticles, etc., have been utilized to fabricate solar-light harvesting photocatalysts that are capable of degrading the toxic wastewater contaminants. However, the development of suitable bandgap materials with sufficient valence band and conduction band potentials is crucial to efficient photodegradation under the solar-light irradiation. Thus, it appears that functionalization of structurally combined binary nanocomposite is a successful methodology for achieving the photocatalytic degradation of organic pollutants. Furthermore, we wish to employ an efficient, and cost-effective method to synthesize ZnO-CdS nanocomposites in the present study. Thank you for your valuable comment.
Comment 4: There are several publications on ZnO-CdS NCs using the same approach or different approaches, however, the authors have not specified how the methodology or the results obtained are different from the published work.
Response: We acknowledge the reviewer’s opinion. As suggested, the superiority of the present work was specified by comparing with other reports in the revised manuscript. Thank you for your valuable suggestion.
Comment 5: It could be better to specify the other applications of the ZnO NP, CdS NPs and ZnO-CdS NCs.
Response: We acknowledge the reviewer’s opinion. As suggested, the other applications of ZnO, CdS, and ZnO-CdS NCs were provided in the revised manuscript. Thank you for your valuable suggestion.
The authors are very thankful to the Reviewer for their valuable suggestions for the improvement of the manuscript.
All the modifications are shown in yellow color in the revised manuscript.
With regards
Ravindranadh Koutavarapu, Ph.D
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors have properly answered the asked comments and suggestions
The revised MS can be accepted for publication
Author Response
catalysts
Ms. Ref. No.: catalysts-1458262 
Title: Enhanced photocatalytic activity of ZnO-CdS composite nanostructures towards the degradation of Rhodamine B under solar light
Response to Reviewer-1 Comments
We appreciate the efforts of the reviewers for their detailed and insightful comments, which have helped us to improve the quality of our manuscript.
Comment 1: The authors have properly answered the asked comments and suggestions. The revised MS can be accepted for publication.
Response: We acknowledge the reviewer’s opinion. Thank you for your appreciation and support.
The authors are very thankful to the Reviewer for their valuable suggestions for the improvement of the manuscript.
With regards
Ravindranadh Koutavarapu, Ph.D
Author Response File: Author Response.pdf
Reviewer 2 Report
Comment 1: need proper explanation with respect to the comment
Comment 2: more clarity expected since CdS is already good, why you’re preparing this catalyst?
Comment 3: not corrected in the flow of the paper
Comment 4: for my eyes all are same pictures look like different position.
Comment 7: Since, you are stating that you’re considering Beer-Lambert law, you must consider limitations in this law. Your OD value over 1 (more or less 1.05) in the figure. 7.a. also dimer peak appeared, you’re completely out of box. In figure 7.b, your first data start from 1. How it’s possible?
Comments for author File: Comments.pdf
Author Response
catalysts
Ms. Ref. No.: catalysts-1458262 
Title: Enhanced photocatalytic activity of ZnO-CdS composite nanostructures towards the degradation of Rhodamine B under solar light
Response to Reviewer-2 Comments
We appreciate the efforts of the reviewers for their detailed and insightful comments, which have helped us to improve the quality of our manuscript. A point-by-point response to the reviewer-2 comments is appended below for your convenience.
Comment 1: Need proper explanation with respect to the comment
Response: We acknowledge the reviewer’s opinion. Organic dyes are widely utilized in paper, textiles, plastics, rubber, and coatings as major industrial components. The removal of these toxic dyes before they are discharged into the natural system is a critical concern from an environmental standpoint. These dye compounds, which have underlying carcinogenicity as well as virulent and refractory biodegradability, have sparked widespread toxicological concern due to the significant hazards they pose to human health. Among these, Rhodamine B (RhB) dye is extremely toxic to the human body, causing skin and visceral redness, minor blood vessel congestion between the brain, cardiac fiber rupture, blurred and vanished horizontal lines, and other symptoms. When patients consume RhB, they may get pulmonary edema. The alveolar cavity, renal interstitial vascular congestion, and renal tubule lumen all have a lot of uniform red stain. Ingestion over an extended period of time will result in death. According to the World Health Organization’s International Agency for Research on Cancer (IARC), the ingestion and contact of RhB dye can cause acute and chronic toxic injuries. China’s Ministry of Health has also banned it in food. The European Food Safety Authority (EFSA) also confirmed the carcinogenicity of RhB. In order to protect people’s health and safety, it is very important to establish a rapid degradation method for these dyes and heavy metal ions. As we mentioned in Table 2, the synthesized ZnO-CdS nanocomposite performed superior photocatalytic activity towards the degradation of RhB dye than other reports. Thank you for your valuable suggestion.
Comment 2: More clarity expected since CdS is already good, why you’re preparing this catalyst?
Response: We acknowledge the reviewer’s opinion. We have selected two different bandgap semiconductors and proposed to synthesize a coupled ZnO-CdS nanocomposite for photocatalysis to reduce the impact of organic pollutants on the environment. The recent reports revealed that the nanocomposites with diverse morphology exhibit superior photocatalytic performance than pristine ZnO NPs and CdS NPs. The CdS forms a type-II heterostructure with ZnO due to its well-matched crystal structure (hexagonal-hexagonal) and intrinsic properties between these semiconductors. Which facilitates a very fast inter band charge transfer from CdS to ZnO. Moreover, the life time of photogenerated charge carriers in ZnO–CdS nanocomposite was observed to be higher than that of pure ZnO nanoparticles (NPs) and CdS NPs. The construction of heterostructure by coupling CdS with ZnO can broaden the visible light response range, thereby improving the photocatalytic efficiency. In view of this, we synthesized ZnO-CdS nanocomposite material to enhance its photocatalytic performance under solar light. The formation of heterostructure between ZnO and CdS NPs separated the photo-generated electron-hole pairs and reduced the recombination rate. The holes generating hydroxyl radicals (•OH) and electrons generates superoxide radicals, which subsequently gives •OH radicals. These hydroxyl radicals are responsible for the degradation of pollutants from water bodies. Thank you for your valuable suggestion.
Comment 3: Not corrected in the flow of the paper
Response: We acknowledge the reviewer’s opinion. As suggested, the description of the y-coordinates of Fig. 5(b) are corrected in the revised manuscript. Thank you for your valuable suggestion.
Comment 4: For my eyes all are same pictures look like different position.
Response: We acknowledge the reviewer’s opinion. As suggested, we have provided the original images in Figure 2. SEM micrographs of pure ZnO, CdS, and ZnO-CdS NCs are shown in Figure 2. The ZnO NPs with low agglomerates are formed, as illustrated in Figure 2(a). The evenly dispersed spherical shaped CdS NPs are seen in Figure 2(b). CdS NPs are detected on the surface of ZnO NPs in ZnO-CdS NCs, implying the development of CdS NPs adorned ZnO NPs heterostructures as seen in Figure 2(c). We have tried our best to describe the SEM images, even TEM results also supports the SEM results. Thank you for your valuable suggestion.
Comment 7: Since, you are stating that you’re considering Beer-Lambert law, you must consider limitations in this law. Your OD value over 1 (more or less 1.05) in the figure. 7(a) also dimer peak appeared, you’re completely out of box. In figure 7(b), your first data start from 1. How it’s possible?
Response: We acknowledge the reviewer’s opinion. The Beer-Lambert law states that there is a linear relationship between the concentration and the absorbance of the solution, which enables the concentration of a solution to be calculated by measuring its absorbance. According to the obtained results, as the solar light illuminated over the solution, the concentration of RhB gradually decreased w.r.t time and accordingly the absorption spectra also decreased. Furthermore, there is no shifting of absorption peak at around 554 nm, which is under the limitations. Moreover, the absorption of RhB from Figure 7(a) is around 1 only. Nevertheless, in the present study, we mainly focused on the construction of facile, and low-cost ZnO-CdS NCs for the environmental remediations over the solar light illumination. Thank you for your valuable comment.
The authors are very thankful to the Reviewer for their valuable suggestions for the improvement of the manuscript.
All the modifications are shown in yellow color in the revised manuscript.
With regards
Ravindranadh Koutavarapu, Ph.D
Author Response File: Author Response.pdf
Reviewer 4 Report
Attached
Comments for author File: Comments.pdf
Author Response
catalysts
Ms. Ref. No.: catalysts-1458262 
Title: Enhanced photocatalytic activity of ZnO-CdS composite nanostructures towards the degradation of Rhodamine B under solar light
Response to Reviewer-4 Comments
We appreciate the efforts of the reviewers for their detailed and insightful comments, which have helped us to improve the quality of our manuscript. A point-by-point response to the reviewer-4 comments is appended below for your convenience.
Comment 1: It still appears as ZnO, not ZnO NPs, and the places where the changes made by the authors appear as ZnO NFs, which is entirely wrong.
Response: We acknowledge the reviewer’s opinion. As suggested, the term ZnO was replaced with ZnO NPs. Thank you for your valuable suggestion.
Comment 2: Also, still stated in many places as Zn-Cd instead of ZnO-CdS NCs.
Response: We acknowledge the reviewer’s opinion. As suggested, the term Zn-Cd was replaced with ZnO-CdS NCs. Thank you for your valuable suggestion.
Comment 3: The novelty of the research is not justified by the authors
Response: We acknowledge the reviewer’s opinion. We have selected two different bandgap semiconductors and proposed to synthesize a coupled ZnO-CdS nanocomposite for photocatalysis to reduce the impact of organic pollutants on the environment. The recent reports revealed that the nanocomposites with diverse morphology exhibit superior photocatalytic performance than pristine ZnO NPs and CdS NPs. The CdS forms a type-II heterostructure with ZnO due to its well-matched crystal structure (hexagonal-hexagonal) and intrinsic properties between these semiconductors. Which facilitates a very fast inter band charge transfer from CdS to ZnO. Moreover, the life time of photogenerated charge carriers in ZnO–CdS nanocomposite was observed to be higher than that of pure ZnO nanoparticles (NPs) and CdS NPs. The construction of heterostructure by coupling CdS with ZnO can broaden the visible light response range, thereby improving the photocatalytic efficiency. In view of this, we synthesized ZnO-CdS nanocomposite material to enhance its photocatalytic performance under solar light. The formation of heterostructure between ZnO and CdS NPs separated the photo-generated electron-hole pairs and reduced the recombination rate. The holes generating hydroxyl radicals (•OH) and electrons generates superoxide radicals, which subsequently gives •OH radicals. These hydroxyl radicals are responsible for the degradation of pollutants from water bodies. Thank you for your valuable suggestion.
Comment 4: It could be better to specify the other applications of the ZnO NPs, CdS NPs and ZnO-CdS NCs. e.g. ZnO NPs – fluorescence quenching, fluorescence resonance energy transfer, photoluminescence, antioxidant, antibacterial, etc.
Response: We acknowledge the reviewer’s opinion. As suggested, the other applications of ZnO NPs, CdS NPs, and ZnO-CdS NCs were provided in the revised manuscript. Thank you for your valuable suggestion.
ZnO NPs: ZnO has a large application potential in photocatalysis, solar cell, optoelectronics, sensors and light emitting devices, which has a wide direct bandgap of ≈3.37 eV and a large exciton binding energy of 60 meV at room temperature.
CdS NPs: Chalcogenide semiconductors such as CdS is a well-characterized II-VI group inorganic semiconductor with a direct bandgap of ≈2.42 eV and it has wide application potential in optoelectronics, such as nonlinear optics, visible-light emitting diodes and lasers.
ZnO-CdS NCs: ZnO–CdS nanocomposites have attracted a great deal of attention in recent years because of their electronic and optical properties that makes them suitable for use as light emitting devices, optoelectronics, photoconductive devices, photocatalysts, photovoltaic solar cells and fluorescence probes for biomedical applications.
The authors are very thankful to the Reviewer for their valuable suggestions for the improvement of the manuscript.
All the modifications are shown in yellow color in the revised manuscript.
With regards
Ravindranadh Koutavarapu, Ph.D
Author Response File: Author Response.pdf
