Synthesis, Characterization, and Anti-Algal Activity of Molybdenum-Doped Metal Oxides

Round 1

Reviewer 1 Report

The manuscript reports of anti-algae properties of doped Mo oxides. The application is new and on this viewpoint the degree of novelty is good. However a deep revisions is needed, since the XRD part needs some major adjustments. Looking to figure 3, the MoZO samples is more a mixture of Mo and Zn oxides than a Mo-doped Zn oxide. Moreover, the MO3 legend is to be corrected on MoO3. Table 2: the number of digit of crystal size is too large: the first and second digit are non sense. For lattice edge the third digit at least must be added since a 0.00 error is totally non sense. A proof that a Mo-doped oxide and not a mixture of oxides is obtained should be given: for instance some point edx indicating the presence of both Mo and Zn element in the same crystallite?  Line 99: “MoCl5” should be “(MoCl5)2” at first. This is a rather volatile solid. Again a mixture of (MoCl5)2 and ZnO is different from a Mo-doped  ZnO. If “(MoCl5)2 is present as in the discussion, why it is not evident in the XRD data? At line 152 “Na2MoO4” appears without correspondence in the XRPD data.  This section must be rewritten giving a more detailed formation of a Mo-doped Zn oxide. Conversely the manuscript can not be suitable for publication. Moreover some minor notes are listed here below.

Minor notes:

• Line 19. The “MoZO” acronym is not clear neither unique: Zn or Zn?
• Line 44 use “significant semiconductor materials” instead of “a significant semiconductor material”
• Line 91 and Figure 1: is really “a.u.” on Y axis? Or simply number of particles? Moreover the Gaussian interpolation of the histogram is rather bad fitting: what about using another interpolation? Asymmetric Gaussian-like function? “with a frequency value of 360 a.u.”: not clear the meaning. Is this the total number of counted particles? To explain better.
• Line 101. The second digit of atomic% is non sense. Please remove it. Also evaluate if keeping the first decimal digit: what’s the error associated to the atomic%? The same for Table 1.
• Figure 2, EDS data: what about the not assigned 1.9 keV peak? The quality of this picture should be improved.
• Line 162 “New peaks were appeared” is not English
• Line 173 “data not shown” is not acceptable. Please put the data in a supplementary file
• Line 212: why MZO in the equation? The quality of the equation must be improved
• Line 290 “after treated” is not English
• Line 292 “Therefore, agglomeration can be considered an anti-algal mechanism of MoZO.” To be explained better

 

Author Response

We wish to thank you for your constructive comments in this round of review. Your comments provided valuable insights to refine contents and analysis of the manuscript. In this document, we try to address the issues raised as best as possible. All the changes made in the manuscript and supplementary files are highlighted with yellow color.               

                                                               

                                                               

1. Looking to figure 3, the MoZO samples is more a mixture of Mo and Zn oxides than a Mo-doped Zn oxide.

XRD of MoZnO showed the presence of MoO₃ along with a shift in the position of ZnO peaks, which indicated that some of the Zn was replaced by Mo in ZnO crystal lattice [1,2]. The intensity along all planes was decreased in MoZnO. This indicated that incorporated Mo into the ZnO lattice inhibited grain growth in all directions [3,4]. Similarly, the broadening of the diffraction peak along (201), (004), and (202) planes indicated the strain was developed after the incorporation of Mo in ZnO lattice [2–6]. EDX showed the presence of both Mo and Zn elements in the same crystallite. A new peak appeared in the FT-IR spectrum of MoZnO, which indicated the Mo-O-Zn interaction formed in MoZnO. All these results indicated that though XRD of MoZnO showed the existence of MoO₃ on the surface of MoZnO, some of Mo get incorporated in ZnO crystal.

2. Moreover, the MO3 legend is to be corrected on MoO3.

In figure 3, MO₃ legend is corrected on MoO₃.

3. Table 2: the number of digit of crystal size is too large: the first and second digit are non sense.

In Table 2 the digits of crystal size are corrected. 45.46 ± 3.56 and 4.48 ± 0.61 are rewritten as 45 ± 3.5 and 4.4 ± 0.61.

4. For lattice edge the third digit at least must be added since a 0.00 error is totally non sense.

In Table 2 the third digit is added to 0.00 error. 3.25 ± 0.00 and 3.25 ± 0.00 are rewritten as 3.25 ± 0.002 and 3.25 ± 0.002.

5. A proof that a Mo-doped oxide and not a mixture of oxides is obtained should be given: for instance some point edx indicating the presence of both Mo and Zn element in the same crystallite?  

XRD of MoZnO showed the presence of MoO₃ along with a shift in the position of ZnO peaks, which indicated that some of the Zn was replaced by Mo in ZnO crystal lattice [1,2]. The intensity along all planes was decreased in MoZnO. This indicated that incorporated Mo into the ZnO lattice inhibited grain growth in all directions [3,4]. Similarly, the broadening of the diffraction peak along (201), (004), and (202) planes indicated the strain was developed after the incorporation of Mo in ZnO lattice [2–6]. EDX showed the presence of both Mo and Zn elements in the same crystallite. A new peak appeared in the FT-IR spectrum of MoZnO, which indicated the Mo-O-Zn interaction formed in MoZnO. All these results indicated that though XRD of MoZnO showed the existence of MoO₃ on the surface of MoZnO, some of Mo get incorporated in ZnO crystal. Further, MoZnO showed very high anti-algal activity under visible light, whereas ZnO+Na2MoO4∙2H2O and ZnO+MoCl₅ were inactive. Jaikumar et al. found that nano MoO3 increased the growth of Anabaena spp. in a concentration-dependent manner [7]. The high anti-algal activity of MoZnO was indirect evidence of the successful doping of Mo in ZnO. Based on these results, we think that no additional evidence is required to prove the doping of Mo in ZnO.

6. Line 99: “MoCl5” should be “(MoCl5)2” at first. This is a rather volatile solid. Again a mixture of (MoCl5)2 and ZnO is different from a Mo-doped  ZnO.

“MoCl₅” is rewritten as “(MoCl₅)₂” (Line: 17, 84, 99, 251, 299, 305, 372, 373, 452, 459, and 460). “(MoCl₅)₂” is volatile and highly reactive solid. Due to its high reactivity, it will react more easily with unreactive metal oxides to form Mo-doped metal oxides [8]. The mixture of (MoCl₅)₂ and ZnO did not show anti-algal activity, whereas MoZnO showed very high anti-algal activity. This indicated that MoCl₅ impurity present in the MoZnO did not contribute to the antialgal activity of MoZnO.

7. If “(MoCl₅)₂” is present as in the discussion, why it is not evident in the XRD data?

The XRD peak for “(MoCl₅)₂” was appeared at ~ 17° in figure 3. Kim et al. showed a similar peak of (MoCl₅)₂ at 2θ = 17 ∼ 19° [8]. (Line:121)

8. At line 152 “Na2MoO4” appears without correspondence in the XRPD data.

At line 152, “Na₂MoO₄” was used to compare with the FT-IR data of MoZnO. Na2Na₂MoO₄ showed the band of Mo-O bond at 833 cm^-1.

9. Line 19. The “MoZO” acronym is not clear neither unique: Zn or Zn?

The “MoZO” acronym is rewritten as MoZnO. (Line: 19).

10. Line 44 use “significant semiconductor materials” instead of “a significant semiconductor material”

Article “a” is removed from “a significant semiconductor material”. (Line 44).

11. Line 91 and Figure 1: is really “a.u.” on Y axis? Or simply number of particles? Moreover the Gaussian interpolation of the histogram is rather bad fitting: what about using another interpolation? Asymmetric Gaussian-like function? “with a frequency value of 360 a.u.”: not clear the meaning. Is this the total number of counted particles? To explain better.

In figure 1. The Y-axis title is changed to the number of particles. We corrected the Gaussian interpolation fitting of the histogram. No other interpolation is used to improve the fitting. For ZnO random particles are used to draw a histogram, but for MoZnO total number of surface-particles is used to draw a histogram.

12. Line 101. The second digit of atomic% is non sense. Please remove it. Also evaluate if keeping the first decimal digit: what’s the error associated to the atomic%? The same for Table 1.

The second digit of atomic% is removed (Line: 101 and Table 1). We are sorry, but we do not think the calculation of error is important here.

13. Figure 2, EDS data: what about the not assigned 1.9 keV peak? The quality of this picture should be improved.

1.9 keV peak may be assigned to silicon. Same peak was appeared in the EDS analysis of ZnO. Good quality EDS image is provided in the manuscript.

 

14. Line 162 “New peaks were appeared” is not English

The last sentence of FT-IR result is corrected as “The FT-IR spectrum of MoZrO, MoWO, and MoSrTiO shows a new band at 983 cm^-1, 953 cm^-1, and 954 cm^-1 (indicated with an arrow), which may correspond to Zr−O−Mo [39], W−O−Mo [40], and SrTi−O−Mo linkages, respectively”. (Line 162)

15. Line 173 “data not shown” is not acceptable. Please put the data in a supplementary file.

Plots of preliminary anti-algal activity and rate of reaction of molybdenum-doped metal oxides are provided in a supplementary file. Only MoZnO showed anti-algal activity. We refer to and recognize the key paper of Hoque in the discussion. We plot the graph between the rate of reaction versus the concentration of molybdenum doped metal oxides. Based on the concept of Hoque paper, MoZnO is considered as ideal photocatalyst. (line: 260-268)

16. Line 212: why MZO in the equation? The quality of the equation must be improved.

We rewrite the MZO as MoZnO and improved the quality of the equation.

17. Line 290 “after treated” is not English

We corrected the sentence “SEM image showed the release of organic matter by M. aeruginosa incubated with MoZnO”. (Line 290)

18. Line 292 “Therefore, agglomeration can be considered an anti-algal mechanism of MoZO.” To be explained better.

We explained why agglomeration is considered an anti-algal mechanism of MoZO? MoZnO generated ·OH radical in the BG-11 medium, which oxidized the lipid membrane of algal cells, resulting in cell damage. The discharge of organic matter from damage cells in BGM diminishes the stability of the algal suspension, facilitating agglomeration. Therefore, agglomeration can be considered an anti-algal mechanism of MoZnO (Line: 291-293 and 450-452)

 

 

References:

[1]          S.H. Nam, S.J. Cho, J.H. Boo, Physical properties of metal-doped zinc oxide films for surface acoustic wave application, Nanoscale Res. Lett. 7 (2012) 1–5. https://doi.org/10.1186/1556-276X-7-25.

[2]          K. Joshi, M. Rawat, S.K. Gautam, R.G. Singh, R.C. Ramola, F. Singh, Band gap widening and narrowing in Cu-doped ZnO thin films, J. Alloys Compd. 680 (2016) 252–258. https://doi.org/10.1016/j.jallcom.2016.04.093.

[3]          R. Swapna, M.C. Santhosh Kumar, Growth and characterization of molybdenum doped ZnO thin films by spray pyrolysis, J. Phys. Chem. Solids. 74 (2013) 418–425. https://doi.org/10.1016/j.jpcs.2012.11.003.

[4]          A. Khorsand Zak, W.H. Abd. Majid, M.E. Abrishami, R. Yousefi, X-ray analysis of ZnO nanoparticles by Williamson-Hall and size-strain plot methods, Solid State Sci. 13 (2011) 251–256. https://doi.org/10.1016/j.solidstatesciences.2010.11.024.

[5]          R. Yousefi, A.K. Zak, F. Jamali-Sheini, Growth, X-ray peak broadening studies, and optical properties of Mg-doped ZnO nanoparticles, Mater. Sci. Semicond. Process. 16 (2013) 771–777. https://doi.org/10.1016/j.mssp.2012.12.025.

[6]          S. Muthukumaran, R. Gopalakrishnan, Structural, FTIR and photoluminescence studies of Cu doped ZnO nanopowders by co-precipitation method, Opt. Mater. (Amst). 34 (2012) 1946–1953. https://doi.org/10.1016/j.optmat.2012.06.004.

[7]          S.S. Jaikumar, R. Yuvakkumar, R.S. Prabha, G. Karunakaran, V. Rajendran, S.I. Hong, Facile and novel synthetic method to prepare nano molybdenum and its catalytic activity, IET Nanobiotechnology. 9 (2015) 201–208. https://doi.org/10.1049/iet-nbt.2014.0015.

[8]          D. Kim, G. Kim, H. Bae, E. Kim, B. Moon, D. Cheon, An External Energy Independent WO3/MoCl5 Nano-Sized Catalyst for the Superior Degradation of Crystal Violet and Rhodamine B Dye, Catalysts. 9 (2019) 642.

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

General Comments

 

The interesting study deals with the synthesis of Mo-doped ZnO photocatalyst, which is tested for the inhibition of Microcystis aeruginosa growing under visible light. Leaving the anti-algal assay (section 2.3) for actual evaluation by experts on that subject, specific issues below should be addressed in the revised manuscript (not just in the response letter) for improving its outcome.

 

Specific Comments

 

• Because only the material with Zinc is of interest to the manuscript, the other metal oxides should be moved to the supplement.

 

• How was the 65% yield of MoZO determined?

 

• Looking carefully at Figure 1B with a reference mark for 100 um, it appears impossible that the structures are represented by the histogram in Figure 1D for MoZO with diameters < 25 uM. The interpretation in lines 91-95 does not seem to make sense based on the large size of materials in the image.

 

• Why is isopropanol added for the control in the essay of HO radical? This is unclear as isopropanol can also scavenge HO.

 

• The very week shoulder at 1038/cm in Figure 4A (and lines 155-158) appears to suggest a new stretch is present but the evidence is not strong enough to fully support the Zn-O-Mo linkage is he cause. Can additional experimental support be provided (e.g., with other method/instrument)?

 

• The FT-IR spectra in Figure 4 need to indicate in the y-axes the correct information, units and provide a step size or reference numbers for the scale.

 

• When discussing the catalytic activity of the photocatalysts in lines 261-265, the revised manuscript should refer to and recognize the key paper of Hoque, which has summarized the standard methods for reporting photocatalytic activity: Photocatalytic Activity: Experimental Features to Report in Heterogeneous Photocatalysis. Materials 2018, 11(10), 1990.

Author Response

We wish to thank you for your constructive comments in this round of review. Your comments provided valuable insights to refine contents and analysis of the manuscript. In this document, we try to address the issues raised as best as possible.         All the changes made in the manuscript and supplementary files are highlighted with yellow color.

 

1. Because only the material with Zinc is of interest to the manuscript, the other metal oxides should be moved to the supplement.

We moved the other metal oxides to the supplement. (Figure S1)

2. How was the 65% yield of MoZO determined?

Here instead of using a number of moles of reactants and products, we use the weight of reactants and products for the calculation of %yield. (line 87)

3. Looking carefully at Figure 1B with a reference mark for 100 um, it appears impossible that the structures are represented by the histogram in Figure 1D for MoZO with diameters < 25 uM. The interpretation in lines 91-95 does not seem to make sense based on the large size of materials in the image.

We agree with the above comments and corrected the interpretation in lines 91-95. The majority of the ZnO particles had a size of up to 300-500 nm (Figure 1C). The average particle size of ZnO was 392 nm, and the particle size decreased upon doping ZnO with Mo (Figure 1D). Most particles had a size of 27-45 nm. The average particle size of MoZnO was 40 nm. It was found that particle shape and size were strongly altered by ball milling and doping. The average particle size decreased 9.8-fold with ball milling and Mo doping. (line: 91-95)

Additionally, in figure 1. the Y-axis title changed to the number of particles and corrected the Gaussian interpolation fitting of the histogram.

 

4. Why is isopropanol added for the control in the essay of HO radical? This is unclear as isopropanol can also scavenge HO.

Right, isopropanol is HO scavenger. MoZnO produces ·OH radical in the BG-11 medium, which can oxidize non-fluorescent terephthalic acid to fluorescent hydroxy terephthalic acid. In the presence of isopropanol (HO scavenger), total ·OH radical and the number of fluorescent hydroxy terephthalic acid will decrease. Therefore, here isopropanol is used as a control, which gives additional conformation of ·OH radical production by MoZnO under visible light.

 

5. The very week shoulder at 1038/cm in Figure 4A (and lines 155-158) appears to suggest a new stretch is present but the evidence is not strong enough to fully support the Zn-O-Mo linkage is he cause. Can additional experimental support be provided (e.g., with other method/instrument)?

XRD of MoZnO showed the presence of MoO₃ along with a shift in the position of ZnO peaks, which indicated that some of the Zn was replaced by Mo in ZnO crystal lattice [1,2]. The intensity along all planes was decreased in MoZnO. This indicated that incorporated Mo into the ZnO lattice inhibited grain growth in all directions [3,4]. Similarly, the broadening of the diffraction peak along (201), (004), and (202) planes indicated the strain was developed after the incorporation of Mo in ZnO lattice [2–6]. EDX showed the presence of both Mo and Zn elements in the same crystallite. A new peak appeared in the FT-IR spectrum of MoZnO, which indicated the Mo-O-Zn interaction formed in MoZnO. All these results indicated that though XRD of MoZnO showed the existence of MoO₃ on the surface of MoZnO, some of Mo get incorporated in ZnO crystal. Further, MoZnO showed very high anti-algal activity under visible light, whereas ZnO+Na2MoO4∙2H2O and ZnO+MoCl₅ were inactive. Jaikumar et al. found that nano MoO3 increased the growth of Anabaena spp. in a concentration-dependent manner [7]. The high anti-algal activity of MoZnO was indirect evidence of the successful doping of Mo in ZnO. Based on these results, we think that no additional evidence is required to prove the doping of Mo in ZnO.

 

6. The FT-IR spectra in Figure 4 need to indicate in the y-axes the correct information, units and provide a step size or reference numbers for the scale.

In Figure 4 X and Y axis labels are corrected.

7. When discussing the catalytic activity of the photocatalysts in lines 261-265, the revised manuscript should refer to and recognize the key paper of Hoque, which has summarized the standard methods for reporting photocatalytic activity: Photocatalytic Activity: Experimental Features to Report in Heterogeneous Photocatalysis. Materials 2018, 11(10), 1990.

We refer to and recognize the key paper of Hoque in the discussion. We plot the graph between the rate of reaction versus the concentration of molybdenum doped metal oxides. Based on the concept of Hoque paper, MoZnO is considered as ideal photocatalyst. (line: 260-268 and 450-452)

8. Additionally, the XRD peak for “(MoCl₅)₂” was showed in figure 3. Kim et al. showed a similar peak of (MoCl₅)₂ at 2θ = 17 ∼ 19° [8]. (Line:121)

 

References:

[1]          S.H. Nam, S.J. Cho, J.H. Boo, Physical properties of metal-doped zinc oxide films for surface acoustic wave application, Nanoscale Res. Lett. 7 (2012) 1–5. https://doi.org/10.1186/1556-276X-7-25.

[2]          K. Joshi, M. Rawat, S.K. Gautam, R.G. Singh, R.C. Ramola, F. Singh, Band gap widening and narrowing in Cu-doped ZnO thin films, J. Alloys Compd. 680 (2016) 252–258. https://doi.org/10.1016/j.jallcom.2016.04.093.

[3]          R. Swapna, M.C. Santhosh Kumar, Growth and characterization of molybdenum doped ZnO thin films by spray pyrolysis, J. Phys. Chem. Solids. 74 (2013) 418–425. https://doi.org/10.1016/j.jpcs.2012.11.003.

[4]          A. Khorsand Zak, W.H. Abd. Majid, M.E. Abrishami, R. Yousefi, X-ray analysis of ZnO nanoparticles by Williamson-Hall and size-strain plot methods, Solid State Sci. 13 (2011) 251–256. https://doi.org/10.1016/j.solidstatesciences.2010.11.024.

[5]          R. Yousefi, A.K. Zak, F. Jamali-Sheini, Growth, X-ray peak broadening studies, and optical properties of Mg-doped ZnO nanoparticles, Mater. Sci. Semicond. Process. 16 (2013) 771–777. https://doi.org/10.1016/j.mssp.2012.12.025.

[6]          S. Muthukumaran, R. Gopalakrishnan, Structural, FTIR and photoluminescence studies of Cu doped ZnO nanopowders by co-precipitation method, Opt. Mater. (Amst). 34 (2012) 1946–1953. https://doi.org/10.1016/j.optmat.2012.06.004.

[7]          S.S. Jaikumar, R. Yuvakkumar, R.S. Prabha, G. Karunakaran, V. Rajendran, S.I. Hong, Facile and novel synthetic method to prepare nano molybdenum and its catalytic activity, IET Nanobiotechnology. 9 (2015) 201–208. https://doi.org/10.1049/iet-nbt.2014.0015.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Suggested changes were made and the manuscript can be accepted for publication.

Only two small notes:

• The sentence at line 121 “The XRD peak for (MoCl5)2 was appeared at ~17” must be rewritten
• The sentence written in the answer “EDX showed the presence of both Mo and Zn elements in the same crystallite” should be pointed out in the manuscript

Author Response

We wish to thank you for your constructive comments in second round of review. Your comments provided valuable insights to refine contents and analysis of the manuscript. In this document, we try to address the issues raised as best as possible. All the changes made in the manuscript are highlighted with yellow color.

                                                               

1. The sentence at line 121 “The XRD peak for (MoCl5)2 was appeared at ~17” must be rewritten.

The sentence at line 121 is rewritten as “The XRD peak at ~17° indicated the presence of (MoCl₅)₂ on the surface of MoZnO.”

2. The sentence written in the answer “EDX showed the presence of both Mo and Zn elements in the same crystallite” should be pointed out in the manuscript.

The sentence “EDX showed the presence of both Mo and Zn elements in the same crystallite” is pointed out in the manuscript. (Line 99 and 458).

Author Response File: Author Response.pdf

Reviewer 2 Report

The revised manuscript is now recommended for publication in Catalysts as is.

Author Response

We wish to thank you for your constructive comments in second round of review.