Facile fabrication of Mn2+-doped ZnO photocatalysts by electrospinning

In this study, a high-efficiency photocatalyst was synthesized by Mn2+-doped ZnO nanofibres (NFs) fabricated by facile electrospinning and a following annealing process, in which Mn2+ successes incorporate to ZnO NFs lattice without changing any morphology and crystalline structure of ZnO. The photodegradation properties of ZnO loading with different concentrations of Mn2+ (5, 10, 15 and 50 at%) were investigated. The 50% MnO–ZnO composite owns excellent active photocatalytic performance (quantum efficiency up to 7.57%) compared to pure ZnO (0.16%) under visible light and can be considered as an efficient visible light photocatalyst material. We systematically analysed its catalytic mechanism and found that the enhancement belongs to the Mn doping effect and the phase junction between MnO and ZnO. The dominant mechanism of Mn doping leads to the presence of impurity levels in the band gap of ZnO, narrowing the optical band gap of ZnO. In addition, doped Mn2+ ions can be used as electron traps that inhibit the recombination process and promote electron–hole pair separation. In summary, this paper provides a convenient method for fabricating highly efficient visible light photocatalysts using controlled annealing.

SEM, TEM and EDS mapping. It was found that the amount of Mn in ZnO could affect the photocatalytic activity of the final product. ZnO with 50% of Mn exhibited the strongest photocatalytic performance for degradation of RhB. Although some findings are interesting, the present MS cannot be accepted owing to the below reasons: 1. The authors claimed that N-doped ZnO was prepared, but no evidence proved the presence of N. In addition, the authors should experimentally prove the influence of N on the photocatalytic performance of the target catalyst. 2. The distance between Xe lamp and RhB solution should be mentioned. 3. As shown in Fig.1a, the authors claimed that as-spun Mn2+-doped ZnO nanofibers with a diameter of ~600 nm were prepared. However, according to experimental description the smooth nanofibers should be the precursor to obtain the final product before annealing. Here, PVP should exist in these smooth nanofibers. 4. In EDS analyses, no N element was detected. The related description in P5 is error. 5. In P6, Line 17-20: The sentence of "There is no MnO...ZnO composite." is incomplete. 6. "50% Mn-doped ZnO" was non-scientific expression. In general, the dopant is a small amount in a host. Also, ZnO and MnO were detected in the final catalyst, respectively, according to HRTEM and XRD characterization. 7. In P6, Line 27-30: The authors claimed that Mn readily donated its electrons to O, ... In this work, Mn2+ ions were used in fact. Were Mn ions with higher valences formed (Mn4+ ions were mentioned.)? I cannot understand the authors' meanings. 8. The authors should note the superscript and subscript.

Review form: Reviewer 3
Is the manuscript scientifically sound in its present form? No

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation? Reject
Comments to the Author(s) 1.
Some sentences should be rewritten, as marked in the following paragraph: In this study, we report a high efficiency photocatalyst synthesized by Mn2+ doped ZnO nanofibers (NFs) fabricated by facile electrospinning and a following annealing process, in which Mn2+ successes incorporate to ZnO NFs lattice without changing any morphology and crystalline structure of ZnO. The photodegradation properties were studied for ZnO doping with different concentrations of Mn2+ (5, 10, 15 and 50 at.%). The 50 % Mn2+-doped ZnO NFs owns excellent active photocatalytic performance (quantum efficiency up to 7.57 %) compared to pure ZnO (0.16 %) under visible light and can be considered as an efficient visible-light photocatalyst material. We systematically analyzed the catalytic mechanism and showed that the enhancement belongs to the Mn doping effect and the phase junction between MnO and ZnO. The dominant mechanism of Mn doping leads to the presence of impurity levels in the band gap of ZnO, narrowing the optical band gap of ZnO. In addition, doped Mn2+ ions can be used as electron traps that inhibit the recombination process and promote electron-hole pair separation. In summary, this paper provides a convenient method for fabricating highly efficient visible light photocatalysts using controlled annealing. Due to the low energy consumption, low cost, low toxicity and none secondary pollution, photocatalytic degradation has been considered as a promising strategy to address the current environmental issue1-4. Photocatalysts which could effectively reduce the barrier in photocatalytic reaction is one of the key factors getting much attention5,6. TiO2 has been considered as one of photocatalyst candidate, but its low efficiency in the visible light range limit its application7. Although the visible light response can be achieved by doping, the efficiency is still low. Therefore, the development and design of high-efficiency new photocatalysis has become a strategy to solve the low efficiency of visible light catalysis. There are too many sentences and they could not be listed one by one. 2. What's the meaning of PVV? 3. Is there any defined evidence to show Mn cation was doped into the lattice of ZnO crystal? And XPS results of the samples should be provided. If 50% Mn2+-doped ZnO was fabricated successfully, the sample should be called as MnO2 and ZnO composite, instead of Mn2+-doped ZnO. 4. What's the meaning of "The 50 % Mn2+-doped ZnO NFs "? Does that mean 50% of Mn in the precursor was doped into the lattice of ZnO crystal or 50% of Zn was replaced by Mn? 4. The mechanism of the paper proposed, there is also no enough evidence to support it, especially for the phase junction between MnO and Mn2+ doped ZnO, no microstructure of the sample to support this structure. On the other hand, if there is a junction between the two phases, MnO and Mn2+ doped ZnO, it is clear that no all Mn2+ cations were doped into the lattice of ZnO crystal. In this case, how to explain the mechanism that the authors proposed? The editor assigned to your manuscript has now received comments from reviewers. We would like you to revise your paper in accordance with the referee and Subject Editor suggestions which can be found below (not including confidential reports to the Editor). Please note this decision does not guarantee eventual acceptance.
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4.
In the Figure 5b, the bandgap of the whole 50% Mn2+-doped ZnO composite is calculated as 2.25 eV. However in the Figure 6, the energy band diagram of 50% Mn2+-doped ZnO nanofiber is divided into Mn2+-doped ZnO (2.2 eV) and MnO (3.6 eV). Is it reasonable to use 2.2 eV for part of 50% Mn2+-doped ZnO nanofiber in energy band diagram? please explain this contradiction. 5. In the page 11, the authors said "doped Mn2+ ions can be used as electron traps, which can suppress the recombination process and promote the electron-hole pair separation" . The author may need offer the experimental data about charge carrier recombination of samples to prove it. 6. In the page 12, the authors said "Mn2+-doped ZnO nanofibers show much higher photocatalytic activity than Mn2+-doped ZnO and pure ZnO nanofibers under the visible light". This sentence should be revised. Comments to the Author(s) In this MS, Wang et al prepared Mn-doped ZnO photocatalyst through the electrospinning technology with subsequent annealing. The as-obtained products were characterized by XRD, SEM, TEM and EDS mapping. It was found that the amount of Mn in ZnO could affect the photocatalytic activity of the final product. ZnO with 50% of Mn exhibited the strongest photocatalytic performance for degradation of RhB. Although some findings are interesting, the present MS cannot be accepted owing to the below reasons: 1. The authors claimed that N-doped ZnO was prepared, but no evidence proved the presence of N. In addition, the authors should experimentally prove the influence of N on the photocatalytic performance of the target catalyst. 2. The distance between Xe lamp and RhB solution should be mentioned. 3. As shown in Fig.1a, the authors claimed that as-spun Mn2+-doped ZnO nanofibers with a diameter of ~600 nm were prepared. However, according to experimental description the smooth nanofibers should be the precursor to obtain the final product before annealing. Here, PVP should exist in these smooth nanofibers. 4. In EDS analyses, no N element was detected. The related description in P5 is error. 5. In P6, Line 17-20: The sentence of "There is no MnO...ZnO composite." is incomplete. 6. "50% Mn-doped ZnO" was non-scientific expression. In general, the dopant is a small amount in a host. Also, ZnO and MnO were detected in the final catalyst, respectively, according to HRTEM and XRD characterization. 7. In P6, Line 27-30: The authors claimed that Mn readily donated its electrons to O, ... In this work, Mn2+ ions were used in fact. Were Mn ions with higher valences formed (Mn4+ ions were mentioned.)? I cannot understand the authors' meanings. 8. The authors should note the superscript and subscript.

Reviewer: 3
Comments to the Author(s) 1. Some sentences should be rewritten, as marked in the following paragraph: In this study, we report a high efficiency photocatalyst synthesized by Mn2+ doped ZnO nanofibers (NFs) fabricated by facile electrospinning and a following annealing process, in which Mn2+ successes incorporate to ZnO NFs lattice without changing any morphology and crystalline structure of ZnO. The photodegradation properties were studied for ZnO doping with different concentrations of Mn2+ (5, 10, 15 and 50 at.%). The 50 % Mn2+-doped ZnO NFs owns excellent active photocatalytic performance (quantum efficiency up to 7.57 %) compared to pure ZnO (0.16 %) under visible light and can be considered as an efficient visible-light photocatalyst material. We systematically analyzed the catalytic mechanism and showed that the enhancement belongs to the Mn doping effect and the phase junction between MnO and ZnO. The dominant mechanism of Mn doping leads to the presence of impurity levels in the band gap of ZnO, narrowing the optical band gap of ZnO. In addition, doped Mn2+ ions can be used as electron traps that inhibit the recombination process and promote electron-hole pair separation. In summary, this paper provides a convenient method for fabricating highly efficient visible light photocatalysts using controlled annealing. Due to the low energy consumption, low cost, low toxicity and none secondary pollution, photocatalytic degradation has been considered as a promising strategy to address the current environmental issue1-4. Photocatalysts which could effectively reduce the barrier in photocatalytic reaction is one of the key factors getting much attention5,6. TiO2 has been considered as one of photocatalyst candidate, but its low efficiency in the visible light range limit its application7. Although the visible light response can be achieved by doping, the efficiency is still low. Therefore, the development and design of high-efficiency new photocatalysis has become a strategy to solve the low efficiency of visible light catalysis. There are too many sentences and they could not be listed one by one. 2. What's the meaning of PVV? 3. Is there any defined evidence to show Mn cation was doped into the lattice of ZnO crystal? And XPS results of the samples should be provided. If 50% Mn2+-doped ZnO was fabricated successfully, the sample should be called as MnO2 and ZnO composite, instead of Mn2+-doped ZnO. 4. What's the meaning of "The 50 % Mn2+-doped ZnO NFs "? Does that mean 50% of Mn in the precursor was doped into the lattice of ZnO crystal or 50% of Zn was replaced by Mn? 4. The mechanism of the paper proposed, there is also no enough evidence to support it, especially for the phase junction between MnO and Mn2+ doped ZnO, no microstructure of the sample to support this structure. On the other hand, if there is a junction between the two phases, MnO and Mn2+ doped ZnO, it is clear that no all Mn2+ cations were doped into the lattice of ZnO crystal. In this case, how to explain the mechanism that the authors proposed?
Author's Response to Decision Letter for (RSOS-191050.R0) See Appendix A.

Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

Recommendation?
Accept as is

Reviewers' Comments to Author
We would like to thank the editor's effort and referees' very constructive suggestions. Here, we have revised the manuscript according to the comments by point-by-point.

Reviewer: 1
I'd like to suggest authors to make some revision. Detail comments are listed below.
Comment 1: In the page 8, the authors said "there is no MnO phase in the 0, 5, 10, 15% Mn2+ doping samples but a clear MnO phase peak in the Mn2+-doped ZnO composite", the "in the Mn2+-doped ZnO composite" should be revised as "in the 50% Mn2+-doped ZnO composite".

Response 1:
We thank for referee's very carefully reviewing. We are so sorry for our carelessness. In this revision, we have revised the text to "There is no MnO phase in the 0, 5, 10, 15% Mn 2+ doping samples, but there was a clear MnO phase peak in the MnO-ZnO composite." on Page 6, Line 7.

Response 2:
Thank for referee's kind comment. In this revision, we have revised the legend of Figure 4c and added the legend of Figure 4d.
Comment 3 In the Figure 5, it will be clearer to use the same name for the green line.

Response 3:
Many thanks for referee's good suggestion. In this revision, we have modified the title of the green line in Figure 5. Figure 5b, the bandgap of the whole 50% Mn2+-doped ZnO composite is calculated as 2.25 eV. However in the Figure 6, the energy band diagram of 50% Mn2+-doped ZnO nanofiber is divided into Mn2+-doped ZnO (2.2 eV) and

Comment 4 In the
MnO (3.6 eV). Is it reasonable to use 2.2 eV for part of 50% Mn2+-doped ZnO nanofiber in energy band diagram? please explain this contradiction.

Response 4:
Thank you for the comment. The whole 50% Mn 2+ -doped ZnO composite is calculated as 2.25 eV, thus, the part of 50% Mn 2+ -doped ZnO nanofiber is not 2.25 eV.

Appendix A
We are sorry for the inaccuracy and we revised the mechanism of the electron transfer part to ZnO and MnO junction. We have revised the band gap of ZnO nanofiber to 3.25 eV in Figure 6.
Comment 5 In the page 11, the authors said "doped Mn2+ ions can be used as electron traps, which can suppress the recombination process and promote the electron-hole pair separation" . The author may need offer the experimental data about charge carrier recombination of samples to prove it.

Response 5:
We thank for referee's good suggestion. We fully agree with referee that additional experiment data about charge carrier recombination will make sense. However, currently we are unable to perform this experiment because it is still a challenge to accurately display the charge carrier recombination on power-like sample. Instead of it, in this revision, we have provide a few theoretical literatures (Ref. 1 and 32) where the suppressing the recombination by the doped ions has been demonstrated. The text has been revised on Page 9, Last 4 Line.

Comment 6
In the page 12, the authors said "Mn2+-doped ZnO nanofibers show much higher photocatalytic activity than Mn2+-doped ZnO and pure ZnO nanofibers under the visible light". This sentence should be revised.