3-D/3-D Z-Scheme Heterojunction Composite Formed by Marimo-like Bi2WO6 and Mammillaria-like ZnO for Expeditious Sunlight Photodegradation of Dimethyl Phthalate
, Jin-Chung Sin 1,2,3,4,*, Sze-Mun Lam 2,3,4,5, Honghu Zeng 2,3,4,*, Hua Lin 2,3,4, Haixiang Li 2,3,4, Liangliang Huang 2,3,4 and Abdul Rahman Mohamed 6
Round 1
Reviewer 1 Report
The paper demonstrates enhanced sunlight photocatalytic degradation of toxic DMP using an optimally proportioned Z-scheme heterojunction composites of (marimo)BW/(mammillaria)ZnO. The work is comprehensively presented, and the data is exhaustive. The paper can be considered for publication to Catalysts Journal after the authors have made several major revisions:
- The paper needs a major structural revision. The results and discussion section (Section 2) should not preclude the materials and methods section. This reviewer suggests that Section 2 should be Materials and Methods which should include the entire contents of the current Section 3 and the current Section 2.1 in the very end of the section. The new section 3 should be Results and Discussion which should include the current section 2.2 all the way to the current Section 2.4.
- It is very clear from the data/results that a dramatic decrease in performance results as the percentage of the composite goes over 20%. It was explained that the increase in composite percentage will result in recombination centers. Is there any other mechanism that is causing the dramatic decline in performance of the material such as material structure and change in overall morphology? This should be discussed in the paper.
After these issues have been addressed by the authors, the paper can be published in Catalysts Journal.
Author Response
Point 1: The paper needs a major structural revision. The results and discussion section (Section 2) should not preclude the materials and methods section. This reviewer suggests that Section 2 should be Materials and Methods which should include the entire contents of the current Section 3 and the current Section 2.1 in the very end of the section. The new section 3 should be Results and Discussion which should include the current section 2.2 all the way to the current Section 2.4
Response 1: The authors are very appreciative to the reviewer for this comment.
However, the authors prepared the manuscript based on “instructions for authors” stated in “Catalysts” under “research manuscript sections”. The following link showed the guideline for author in writing the manuscript: https://www.mdpi.com/journal/catalysts/instructions#preparation. Therefore, the authors would like to remain the existing orientation of each section in this revised manuscript.
Point 2: It is very clear from the data/results that a dramatic decrease in performance results as the percentage of the composite goes over 20%. It was explained that the increase in composite percentage will result in recombination centers. Is there any other mechanism that is causing the dramatic decline in performance of the material such as material structure and change in overall morphology? This should be discussed in the paper.
Response 2: The authors are very grateful to the reviewer for this comment.
The authors would also like to apologize for leaving out this information in our manuscript. The authors take this comment seriously. The FESEM image for 25-BWZ was added in our revised manuscript. From our observation, further increasing the Bi2WO6 to 25 wt%, the 25-BWZ did not show any significant changes on the overall morphology (Section 2.1, page 4). Subha et al. 2022 have reported that the loading percentage beyond the optimum amount exhibited declined in performance which was due to the charge carrier recombination centers (Subha et al. Chemosphere 306 (2022) 135631). In addition, Durai et al. 2022 reported the similar situation in their study where 20 wt% was identified as the optimum loading. Beyond the optimum loading, it resulted in deterioration of photocatalytic performance due to lower charge carrier separation efficiency (Durai et al. Chemosphere 306 (2022) 135659).
In this manuscript, the transient photocurrent response and EIS analyses have also been carried out to investigate the interfacial charge separation efficiency. The transient photocurrent response result of 25-BWZ exhibited a weaker photocurrent density when compared to 20-BWZ, which indicated a lower charge separation efficiency over 25-BWZ. From the EIS result, the Nyquist circle with larger arc indicating that the 25-BZW has a relatively higher charge transfer resistance compared to 20-BWZ (Section 2.4.2, page 8). Therefore, this result insinuated that appropriate content of Bi2WO6 was crucial for optimizing the photocatalytic performance of the composite.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
In this manuscript, authors have reported Z-scheme for heterojunction composite formed by marimo- 2 like Bi2WO6 and mammillaria-like ZnO for expeditious sunlight 3 photocatalytic degradation of dimethyl phthalate. The manuscript is well-written with acceptable novelty and scientific soundness. Just it is recommended to concise the title of the manuscript. otherwise, it is recommended for publication as it is.
Author Response
Point 1: In this manuscript, authors have reported Z-scheme for heterojunction composite formed by marimo-like Bi2WO6 and mammillaria-like ZnO for expeditious sunlight photocatalytic degradation of dimethyl phthalate. The manuscript is well-written with acceptable novelty and scientific soundness. Just it is recommended to concise the title of the manuscript. otherwise, it is recommended for publication as it is.
Response 1: The authors are very appreciative to the reviewer for this comment.
The title was revised to “3-D/3-D Z-scheme heterojunction composite formed by marimo-like Bi2WO6 and mammillaria-like ZnO for expeditious sunlight photodegradation of dimethyl phthalate”. (Page 1)
Author Response File: Author Response.pdf
Reviewer 3 Report
General comment:
The authors studied the photocatalytic performance of new 3-D/3-D Z-scheme heterojunction composite formed by marimo-like Bi2WO6 and mammillaria-like ZnO scheme heterojunction composite for degradation of dimethyl phthalate (DMP). This work is interesting and well described. I would like to recommend this manuscript to be published after the comments below addressed.
Comment 1:
There is no background line for XPS fitting in Fig 1(c-f).
Comment 2:
The authors should firstly mention “3. Materials and Methods” and then “2. Results and Discussion.”
Author Response
Point 1: There is no background line for XPS fitting in Fig 1(c-f).
Response 1: The authors are very appreciative to the reviewer for this comment.
The authors would also like to apologize for leaving out this information in our manuscript. The manuscript has been revised in line with the reviewer comments (Section 2.1, page 4).
Point 2: The authors should firstly mention “3. Materials and Methods” and then “2. Results and Discussion.”
Response 2: The authors are very grateful to the reviewer for this comment.
However, the authors prepared the manuscript based on “instructions for authors” stated in “Catalysts” under “research manuscript sections”. The following link showed the guideline for author in writing the manuscript: https://www.mdpi.com/journal/catalysts/instructions#preparation. Therefore, the authors would like to remain the existing orientation of each section in this revised manuscript.
Author Response File: Author Response.pdf
Reviewer 4 Report
The authors investigated the activity of 3D/3D Z-scheme heterojunction photocatalyst composed by marimo-like Bi2WO6 and mammillaria-like ZnO for degradation of dimethyl phthalate (DMP). They demonstrated that the optimal composite with 20 wt% of Bi2WO6/ZnO (20-BWZ) exhibited a photodegradation rate constant of 0.0259 min-1, which reached 2.3 and 5.9-folds greater than those of pure ZnO and Bi2WO6, respectively. I suppose that this manuscript could be published in Catalysts after the following revisions:
1. There are a few formatting and spelling mistakes, which should to be corrected.
2. The authors should calculate the mean crystallite size of photocatalysts from the full width at half-maximum of the corresponding diffraction peaks by using the Scherrer equation.
3. Did the authors study the adsorption capacity of the prepared composites? How the amount of Bi2WO6 in the heterojunction influence on the adsorption capacity? Is there a correlation between adsorption capacity and photocatalytic acitivity? This reviewer suppose that this information could be included in the manuscript.
4. What products were formed during the photooxidation of DMP?
5. The authors should include a table where a comparison of this study with other investigations will be presented. This could help to assess the promise of the developed photocatalyst.
6. There are questions about the material stability. The photocatalyst is not very stable. I am not sure that this photocatalyst could be considered for the practical application.
7. The authors should refer to more new literature and keep abreast of the latest research trends related to the use of Bi2WO6-based photocatalysts (10.3390/pr10040789; 10.1016/j.jiec.2022.02.051; 10.1007/s10854-022-08781-x) and Z-scheme materials (10.1002/jctb.7091; 10.1002/adma.201601694).
Author Response
Point 1: There are a few formatting and spelling mistakes, which should be corrected.
Response 1: The authors are very appreciative to the reviewer for this comment.
We have revised the whole manuscript carefully and tried to avoid any formatting and spelling mistakes.
Point 2: The authors should calculate the mean crystallite size of photocatalysts from the full width at half-maximum of the corresponding diffraction peaks by using the Scherrer equation.
Response 2: The authors are very grateful to the reviewer for this comment.
The authors would like to apologize for not putting this information in this manuscript. In this revised manuscript, the average crystallite size of Bi2WO6 and ZnO were 34.7 nm and 18.8 nm, respectively. In 20-BWZ composite, the size of Bi2WO6 and ZnO were measured to be 30.4 nm and 18.3 nm, respectively. Therefore, the results indicated that the crystallite size of Bi2WO6 and ZnO did not change significantly in the composite (Section 2.1, page 3).
Point 3: Did the authors study the adsorption capacity of the prepared composites? How the amount of Bi2WO6 in the heterojunction influence on the adsorption capacity? Is there a correlation between adsorption capacity and photocatalytic activity? This reviewer suppose that this information could be included in the manuscript.
Response 3: The authors are very thankful to the reviewer for this comment.
The authors have studied the dark adsorption of DMP over 20-BWZ in this revised manuscript. The dark adsorption result showed that the concentration of DMP varied negligibly with time, indicating the necessity of both catalyst and light irradiation (Section 2.2, page 5).
In another report, Ma et.al. 2018 combined Bi2WO6 with Ag3PO4 to form heterostructures. In their study, the specific surface area and pore volume have no obvious change among their samples, which played a minor role in the photocatalytic enhancement of their composite (Ma et al. RSC Adv., 8 (2018) 15853-15862).
The high photoactivity of BWZ composite was attributed to the successful establishment of Z-scheme heterojunction for efficient charge separation. These results were solidly confirmed by photocurrent responses and EIS analyses. The 20-BWZ exhibited the highest photocurrent intensity and the smallest arc radius, insinuating that the constructed composite can indeed escalate the charge carrier migration efficiency and hence enhanced the photocatalytic performance (Section 2.4.2, page 8).
Point 4: What products were formed during the photooxidation of DMP?
Response 4: The authors are very grateful to the reviewer for this comment.
The authors would like to apologize for missing out this information in the manuscript. In this revised manuscript, the authors have discussed the formation of intermediate products during the photodegradation of DMP (Section 2.4.4, page 9).
In order to further understand the pathway of DMP degradation, a series of experiments for intermediate products detection over 20-BWZ were on going.
Point 5: The authors should include a table where a comparison of this study with other investigations will be presented. This could help to assess the promise of the developed photocatalyst.
Response 5: The authors are very appreciative to the reviewer for this comment.
The manuscript has been revised in line with the reviewer comment (Section 2.4.4, page 10). In light of these findings, it can be suggested that the synthesized 20-BWZ photocatalyst in this work demonstrated good photoactivity.
Point 6: There are questions about the material stability. The photocatalyst is not very stable. I am not sure that this photocatalyst could be considered for the practical application.
Response 6: The authors are very thankful to the reviewer for this comment.
The authors have carried out 4 cycles of recycling test (Section 2.3, page 6). However, the decline of photocatalytic efficiency can be attributed to the inevitable loss of photocatalyst during the recovery of photocatalyst from the solution. In addition, similar reports were found where their photocatalytic activity was reduced during the recycling test. Anand et al. (2023) reported the photocatalytic performance of BiOCl/BiVO4 dropped from 82.8% to 66.9% after 4 cycles of photocatalytic degradation which caused by catalysts loss during recycling process. Chen et al. (2022) also reported the use of CoFe2O4/Bi2WO6/BiOBr in degrading Rhodamine B where its photocatalytic performance declined from 92.08 to 77.71% after 3 cycles due to loss of samples during the recovery process (Anand et al. Chemosphere, 310 (2023) 136847; Chen et al. J. Alloys Compd., 929 (2022) 167297).
In addition, the authors also carried out the phytotoxicity assessment in this manuscript. It is another important parameter in practical applications as to understand the possible environmental risks of photocatalytically treated DMP solution. Impressively, the vigna radiata seeds exposed to the photocatalytically treated samples showed much higher germination rate compared to the untreated DMP solution, which validated the detoxification of parent DMP pollutant via the photocatalysis treatment. The higher radicle growth from seed germination in photodegraded samples as well as in deionized water as control indicated the decrease of phytotoxicity. The untreated DMP solution displayed an explicit high level of phytotoxicity (82.8%) as compared with the control (0%) and sample after photocatalytic treatment (12.7%) (Section 2.3, page 6).
Hence, the present findings revealed that the prepared 20-BWZ composite was not only held good stability in degrading DMP but also efficiently detoxified the treated DMP solution.
Point 7: The authors should refer to more new literature and keep abreast of the latest research trends related to the use of Bi2WO6-based photocatalysts (10.3390/pr10040789; 10.1016/j.jiec.2022.02.051; 10.1007/s10854-022-08781-x) and Z-scheme materials (10.1002/jctb.7091; 10.1002/adma.201601694).
Response 7: The authors are very grateful to the reviewer for this comment
The manuscript has been revised in line with the reviewer comment (Reference [13], [15], [16], [22] and [47]).
Author Response File: Author Response.pdf
Round 2
Reviewer 4 Report
The authors have addressed all the comments I originally proposed. Thus, the manuscript should be accepted in the current form.
