Zinc–Acetate–Amine Complexes as Precursors to ZnO and the Effect of the Amine on Nanoparticle Morphology, Size, and Photocatalytic Activity

Round 1

Reviewer 1 Report (New Reviewer)

In the manuscript “Zinc-acetate-amine complexes as precursors to ZnO and the effect of the amine on nanoparticle morphology, size and photocatalytic activity”, the authors have investigated the photocatalytic activity and the effect on the size and morphology of ZnO nanoparticles, synthesized by different amine precursors. The manuscript is very interesting, but the analysis is far from complete. The manuscript follows the lack of knowledge and scientific evidence. I recommend accepting the manuscript after major revision based on the comments as follows:

1. The language of the manuscript needs to improve significantly. It is difficult to understand what the authors are trying to convey.

2. The introduction part needs more attention. The authors should discuss the state-of-art materials and fill the gap with appropriate discussion.

3. It is suggested to analyze the results in more detail with relevant discussion.

4. The authors are suggested to include a schematic representation of the synthetic procedure and define the mechanism of ZnO nanoparticles synthesis via different amines.

5. The pH of the solution plays a vital role in crystal growth and nucleation. The authors should discuss the effect of pH (NaOH) and how the slow and rapid addition of NaOH affects the particle size.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Comments for author File: Comments.pdf

Author Response

Reviewer#1:

In the manuscript “Zinc-acetate-amine complexes as precursors to ZnO and the effect of the amine on nanoparticle morphology, size and photocatalytic activity”, the authors have investigated the photocatalytic activity and the effect on the size and morphology of ZnO nanoparticles, synthesized by different amine precursors. The manuscript is very interesting, but the analysis is far from complete. The manuscript follows the lack of knowledge and scientific evidence. I recommend accepting the manuscript after major revision based on the comments as follows:

 

1. The language of the manuscript needs to improve significantly. It is difficult to understand what the authors are trying to convey.

 

The manuscript was reviewed and edited by all of the authors and appropriate changes were made.  Additionally, the manuscript was reviewed by the software “Grammerly” to catch any grammatical errors.

 

2. The introduction part needs more attention. The authors should discuss the state-of-art materials and fill the gap with appropriate discussion.

 

The Introduction has now been lengthened and now includes a longer discussion of ZnO and ZnO composites used for photocatalysis and waste water treatment.  Appropriate references are cited.

 

3. It is suggested to analyze the results in more detail with relevant discussion.

 

This statement is very vague.  If you would be more specific on which sections you think need a deeper discussion, I am happy to accommodate you.

 

4. The authors are suggested to include a schematic representation of the synthetic procedure and define the mechanism of ZnO nanoparticles synthesis via different amines.

 

A schematic of the synthetic procedure has been added to the manuscript as Figure 2.  The mechanism for the growth of ZnO is discussed in the Results and Discussion section and appropriate citations are given.

 

5. The pH of the solution plays a vital role in crystal growth and nucleation. The authors should discuss the effect of pH (NaOH) and how the slow and rapid addition of NaOH affects the particle size.

 

The discussion of the mechanism for the growth of ZnO now includes information on how the pH influences the growth. The material prepared with slow addition of NaOH are shown in Figure 1.  Material prepared using rapid addition of NaOH are shown in Figure 3. Slow addition of NaOH yields large particles and rapid addition of NaOH produces small particles.  The manuscript has been reworded in many places to make this more clear and understandable.

 

Reviewer 2 Report (Previous Reviewer 3)

The revised version of the manuscript may acceptable to the journal standard.

Author Response

Reviewer #2:

The revised version of the manuscript may acceptable to the journal standard.

 

Thank you for taking the time to review the manuscript.

Reviewer 3 Report (New Reviewer)

I have carefully read this paper entitled with “Zinc-acetate-amine complexes as precursors to ZnO and the effect of the amine on nanoparticle morphology, size and photocatalytic activity".

They responded to all comments from the reviewers. The manuscript is improved. Please, publish subject to the following revisions.

1-      Rewrite the novelty statement at the end of the introduction section.

2-      Reviewer suggested to a more comprehensive background of the photocatalytic activity of ZnO should be cited in the introduction for a wider readership. Below I listed, some papers:

- ZnO quantum dots-graphene composites: Formation mechanism and enhanced photocatalytic activity for degradation of methyl orange dye

 Volume 663, 5 April 2016, Pages 738-749

- Photocorrosion suppression and photoelectrochemical (PEC) enhancement of ZnO via hybridization with graphene nanosheets, Volume 502, 1 February 2020, 144189

 - Novel Visible Light Photocatalytic and Photoelectrochemical (PEC) Activity of Carbon-doped Zinc Oxide/Reduced Graphene Oxide: Supercritical Methanol Synthesis with Enhanced Photocorrosion Suppression, Volume 723, 5 November 2017, Pages 1001-1010

Comments for author File: Comments.docx

Author Response

Reviewer #3:

I have carefully read this paper entitled with “Zinc-acetate-amine complexes as precursors to ZnO and the effect of the amine on nanoparticle morphology, size and photocatalytic activity". 

They responded to all comments from the reviewers. The manuscript is improved. Please, publish subject to the following revisions.

 

1-      Rewrite the novelty statement at the end of the introduction section.

 

The novelty of the work is at the end of the third paragraph of the Introduction section which states, “For this work, to minimize solubility issues, the [Zn(acetate)₂(amine)₂] precursors were generated in solution, and the ZnO nanoparticles were prepared by alkali precipitation without the additional steps of isolation and redissolution. Maintaining the precursors in solution simplifies the synthesis of ZnO, decreases the synthesis time, and allows more amines to be used in the synthesis while still using fully characterized precursors.” I have tried to rephrase this to make it clearer.

 

2-      Reviewer suggested to a more comprehensive background of the photocatalytic activity of ZnO should be cited in the introduction for a wider readership. Below I listed, some papers:

 

- ZnO quantum dots-graphene composites: Formation mechanism and enhanced photocatalytic activity for degradation of methyl orange dye  Volume 663, 5 April 2016, Pages 738-749

 

- Photocorrosion suppression and photoelectrochemical (PEC) enhancement of ZnO via hybridization with graphene nanosheets, Volume 502, 1 February 2020, 144189

 

 - Novel Visible Light Photocatalytic and Photoelectrochemical (PEC) Activity of Carbon-doped Zinc Oxide/Reduced Graphene Oxide: Supercritical Methanol Synthesis with Enhanced Photocorrosion Suppression, Volume 723, 5 November 2017, Pages 1001-1010.

 

The Introduction has now been lengthened and now includes a longer discussion of ZnO and ZnO composites used for photocatalysis and waste water treatment.  The above papers are now include in the discussion, and other appropriate papers are also cited.

Round 2

Reviewer 1 Report (New Reviewer)

The authors have improved the manuscript as per the suggestions. But I feel that there is still room for discussion. The authors have mentioned the studies from the different groups but haven't discussed how their studies are more appropriate and better than the cited literature. Also, the introduction lacks the motivation behind the present work. 

Author Response

Once again the Introduction has been modified to try to address your concerns.  The novelty and motivation of this work is that we characterize the precursors that are used to prepare the ZnO.  Most who report ZnO photocatalysis research do not characterize their precursors.  As a result of our characterization, we can correlate precursor composition to physical and chemical attributes of the ZnO, such as ZnO particle size and shape, the kind and mount of organic ligands associated with the ZnO, and the material’s photocatalytic performance. I have tried to state this as clearly as possible in the Introduction.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

The authors investigated in this study the catalytic efficiency of ZnO nanoparticles synthetized by alkali precipitation using different amines. The study is certainly of interest to the Journal’s audience as the impact of the morphological properties of ZnO nanoparticles on the catalytic performance has been presented. The current version of the manuscript however presents some formatting issues that should be amended in order to clarify the data discussion and better highlight the outcomes. Furthermore, the analysis of the role and contributions of such ligands is unclear. While the methodology seems to be robust, the discussion should be revised to provide a more in-depth scientific evaluation of the outcomes and to evaluate the performance against a benchmark.

An additional major review is recommended on the basis of the following comments:

 

• The introduction is very short with limited review of the state of the art provided, apart from one study previously published by the authors. Why is it important to control the physiochemical properties of ZnO nanoparticles? How the particle size and morphology affect the desirable catalytic properties? There is little to no context in the present introduction. Finally, please amend lines 60-67 that likely come from some sort of writing guidelines.

 

• Is the novelty of this study limited to the amended synthesis protocol, as hinted in the introduction (lines 46-47)? Please clarify the novelty statement.

 

• The authors mentioned briefly that the average particle size was measured by considering 50-60 particles (line 112). Please include this type of information in the Materials and Methods, rather than in the main discussion.

 

• The evaluation of the particle size reduction attributed to a new set of synthesis conditions is unclear. As two variables were changed, such as the solvent mixture and the synthesis temperature, the contribution of each parameter to the final size reduction should be discussed further. A schematic could also be provided to better describe the synthesis process and highlight the differences within figure 1 and figure 2.

 

• The analysis of the particle size carried out by TEM/SEM, Scherrer equation and BET could be summarize in a table to discuss the outcomes of each characterization method. Why would ethylenediamine lead to a more pronounced difference across the datasets?

 

• The authors referred to a Table 1 (line 326), which is however missing from the discussion. Please amend.

 

• The authors attributed a pseudo first order kinetic mechanism to the dye decomposition, however the k is reported in M min-1 (0 order). Please check the units as there could be a typo.

 

• The authors mentioned that some ligands in tris and 2-thiazolamine-derived ZnO might be hindering the catalytic efficiency. Without a table or a graph summarizing the data it is very hard for a reader to follow the flow of the discussion and try to draw correlations within datasets. Nevertheless, from CHN and TGA measurements it appears that the variation across the samples could be minor, unless the authors would like to point at some significant changes that would underpin their conclusions. As the authors hint at some sort of surface hinderance due to the presence of ligands, it could be appropriate to conduct XPS surface analysis of the ZnO samples to assess the presence and chemical configuration of the impurities.

 

• A blank and a ZnO reference should be reported to assess the catalytic efficiency of the ZnO derived from Zn acetate + amine against a benchmark. How do these NPs compare with other studies on ZnO NPs available in the literature?

Reviewer 2 Report

The paper is very interesting, though it contains several inaccuracies, thus it should be accepted only after answering the following points:

 

1-In Figure 5 it is shown the Tauc plot, actually it is missing the fitting curve that intersects the X axis and shows the exact value of Energy band gap. Anyway, this value is practically the same for all the samples as indeed one would expect. Usually the doping with a metal or other kinds of chemical/physical functionalizations can alter the band gap, why the presence of a capping ligand should influence the band gap? Moreover, from CHN analysis and TGA, it emerges that low boiling point amines, are basically absent on the surface of the final samples, less volatile amines, as 2-thiazolamine, are stick on the surface of ZnO but their percentage is anyway very small.

 

2-There is no comparison between the catalytic activity of the prepared ZnO samples and other heterogeneous catalysts used in photodegradation of malachite green dye: do the newly prepared catalysts are more active? The stability test of the catalysts is missing: did the authors try to recycle the catalyst?

 

3-The conclusion of the discussion section says “ from this study it is evident that size, morphology and capping ligands on the nanoparticles have a profound impact on the materials catalytic activity” I would say that this is obvious and there is no need to say it. On the other hand, it is missing an explanation of the possible mechanism of action of the organic amine: why changing the amine, does it change the size so dramatically? Moreover, in Fig. 1 there are beautiful SEM images of the ZnO nanoparticles grown in presence of different amines at 95°C, and the amine here influences dramatically the shape, what could be the reason? That is a pity that the authors do not develop further this study. In fact, there are several studies on the influence of organic amines on ZnO growth, see for instance: Chem. Eur. J. 2007, 13, 632, so it is not surprising that changing amine, the authors observe a change in shape or size. In conclusion, what is the real novelty introduced by the present paper?

 

Minor corrections:

Please in the Introduction remove the sentences comprised between line 60 and line 67.

Line 273: please rephrase the sentence: “ we observe that although the use of different amines can be used…differing amount” and please substitute differing with different.

Line 308, please add 2 before (HO^.)₂ in order to balance equation 8.

In the following references 20, 25, 39, 41, 43, 45 and 49 please substitute the full name of the journal with the appropriate short name. Ex. Chemistry of Materials should be substituted with Chem. Mater.

Comments for author File: Comments.docx

Reviewer 3 Report

Comments: This paper reports the preparation of different ligands capping ZnO nanoparticles using different amines such as tris(hydroxymethyl)aminomethane (Tris), 2-thiazolamine, hydrazine, and ethylenediamine and has been used for the photocatalytic degradation of malachite green. The structure of the ZnO nanoparticles have been characterized and the analyses are reasonable. The authors should address the following comments for its acceptance.

1. Author should provide the HRTEM images of ZnO nanoparticles grown using different amines for the determination of the degree of crystallinity and capping effects.
2. Author should supply the FT-IR and or XPS analysis of different ligands capping ZnO nanoparticles for determination of the interaction between the ligands and ZnO. Besides, it could improve the manuscript quality and for more understanding of readers.
3. There are no experimental results available for the degradation of malachite green. Hence, the author needs to provide the experimental results for the degradation of malachite green in the presence of different ligands capping ZnO nanoparticles.
4. The recyclability of ligands capping ZnO nanoparticles on the degradation of malachite green should be investigated.
5. Relative literature on carbon-based ZnO nanoparticles have been widely used for the degradation of dyes and other analysis fields, and are suggested to be cited if they are useful to this manuscript, such as (Sci. Rep., 9, 19509, 2019; Photochem. Photobiol. A, 350, 75–85, 2018; J. Electroanal. Chem., 816, 195–204, 2018; J. Photochem. Photobiol. A, 337, 100–111, 2017; J. Photochem. Photobiol. B, 162, 500–510, 2016)
6. Some of the trivial errors in the manuscript. Hence, the manuscript should be carefully checked and necessary corrections should be done.