The Impact of In Situ Polymerization Conditions on the Structures and Properties of PANI/ZnO-Based Multiphase Composite Photocatalysts
, Marijana Kraljić Roković 1, Zlata Hrnjak-Murgić 1,* and Mark Žic 3,*Round 1
Reviewer 1 Report
The manuscript "The impact of in situ polymerization conditions on structure and properties of PANI/ZnO composite photocatalyst" by Gilja et al. relates the two-step in situ preparation (polymerization) of Polyaniline/ZnO (PANI/ZnO) composite photocatalysts, their characterization and application for the degradation of Acid Blue25 dye.
The work is scientifically sound. The manuscript is clearly written (aside for 2 points, see after) and presented: it deserves publication after minor revisions listed hereafter (= text modifications = suggestions for improvement):
1-line 58, the structure of the dye is missing. It is necessary to help the reader understand the possible interactions between components (see after).
2- The lines 72-84 must be written again: it is not clear how the in situ polymerization in two-steps proceeds. Equation 1 is related to ZnO dissolution in acidic media (competitive process).
Please explain better "we perfomed in situ polymerization by adding ZnO suspension directly into PANI polymerization suspension". What is exactly the polymerization in the second step?
3- Some typos lines 287-288, 297.
4- Lines 303-310 are not clearly written.
I guess "adsorption of dye on ZnO" is due to the presence of specific functions on the dye. But the molecular structure is missing and no particular interaction is mentioned here to help the reader understand.
5- Line 314, what about the smaller PANI particles sizes mentioned earlier to explain the lower photocatalytic activity?
6- Table 4 and lines 332-333: the difference between ZnO_1 and ZnO_2 does not seem very significant. What about experimental error?
Author Response
Reviewer #1
The manuscript "The impact of in situ polymerization conditions on structure and properties of PANI/ZnO composite photocatalyst" by Gilja et al. relates the two-step in situ preparation (polymerization) of Polyaniline/ZnO (PANI/ZnO) composite photocatalysts, their characterization, and application for the degradation of Acid Blue25 dye.
The work is scientifically sound. The manuscript is clearly written (aside for 2 points, see after) and presented: it deserves publication after minor revisions listed hereafter (= text modifications = suggestions for improvement):
Answer: Thank you for your valuable comments and suggestions. The changes in the text are marked red.
1-line 58, the structure of the dye is missing. It is necessary to help the reader understand the possible interactions between components (see after).
Answer: We have introduced an additional Scheme with a molecular dye’s structure.
2- The lines 72-84 must be written again: it is not clear how the in situ polymerization in two-steps proceeds. Equation 1 is related to ZnO dissolution in acidic media (competitive process).
Answer: We have modified these lines and explained in more detail the two-step polymerization.
Please explain better "we perfomed in situ polymerization by adding ZnO suspension directly into PANI polymerization suspension". What is exactly the polymerization in the second step?
Answer: The amount of aniline monomer and APS leftovers (from the first step), were sufficient for the in situ polymerization in the second step.
3- Some typos lines 287-288, 297.
Answer: The typos have been clarified.
4- Lines 303-310 are not clearly written.
Answer: We have rewritten the aforementioned lines.
I guess "adsorption of dye on ZnO" is due to the presence of specific functions on the dye. But the molecular structure is missing and no particular interaction is mentioned here to help the reader understand.
Answer: We have introduced an additional Scheme with molecular dye’s structure.
5- Line 314, what about the smaller PANI particles sizes mentioned earlier to explain the lower photocatalytic activity?
Answer: The small PANI/ZnO_3 particles (see new SEM image; X20.000) are composed of oligomers that do not contribute to PANI/ZnO synergistic effect. Therefore, these oligomer-based (vs. PANI-based) particles are not so “active” under solar irradiation.
6- Table 4 and lines 332-333: the difference between ZnO_1 and ZnO_2 does not seem very significant. What about experimental error?
Answer: We have calculated errors in kapp values and introduced them into Table. Now, the difference between kapp values can be observed.
Reviewer 2 Report
The topic of the stydy is interesting and within the scope of the journal.
There are, however, some major faults and substantial revision is required before publication.
My main concern is that the authors do not show that these combined systems have a better efficiency in comparison with the simple ZnO. It is true that the photocatalytic activity of the combined systems is higher but overall there is a higher due removal with ZnO only, even if it is due to the combined action adsorption/photodegradation.
The authors should therefore explain the advantages of these systems compared to the simple ZnO.
Considering more specifically the results:
TGA data are incomplete and have to be integrated/improved. It would be advisable to have the tests performed also air atmosphere and not just in N2, to see the decomposition of the various components in the presence of oxygen. The behaviour in the two atmospheres should be compared.
Moreover, the tests should be extended to higher temperatures: the curves from figure 3 in fact show that the wieight starts to decrease again for T > 500 oC; it is possible that there are other weight losses for these intervals.
Also, the authors should explain more clearly the reasons for the different behavious between the samples, i.e. why there is a significant difference between sample PANIZnO1 and almost no difference between PANIZnO2 and PANIZnO3.
A minor point: I think there is a typing error at line 231, it should be 270 and not 279 oC.
Regarding SEM, images with higher magnifications should be provided, to see the morphology of the lamellae and highlight possible differences between the three samples.
Author Response
Reviewer #2
The topic of the study is interesting and within the scope of the journal.
Answer: Thank you for your valuable comments and suggestions. The changes in text are marked red.
There are, however, some major faults and substantial revision is required before publication.
My main concern is that the authors do not show that these combined systems have a better efficiency in comparison with the simple ZnO. It is true that the photocatalytic activity of the combined systems is higher but overall there is a higher due removal with ZnO only, even if it is due to the combined action adsorption/photodegradation.
The authors should therefore explain the advantages of these systems compared to the simple ZnO.
Answer: Herein, photocatalytic tests were conducted by using low AB 25 concentration, which is not common in real systems. Under heavier dye’s load, ZnO active sites might be blocked, which is a problem. Furthermore, ZnO is a well-known photocatalyst but, its major drawbacks are also recombination process and low stability at low pH. Therefore, this paper tries to resolve these well-known problems.
Considering more specifically the results:
TGA data are incomplete and have to be integrated/improved. It would be advisable to have the tests performed also air atmosphere and not just in N2, to see the decomposition of the various components in the presence of oxygen. The behaviour in the two atmospheres should be compared.
Moreover, the tests should be extended to higher temperatures: the curves from figure 3 in fact show that the wieight starts to decrease again for T > 500 oC; it is possible that there are other weight losses for these intervals.
Answer: This kind of investigation on the thermal stability of PANI salt and base was previously thoroughly studied. However, the purpose of this work is to synthesize PANI/ZnO composites and to test them as photocatalyst in wastewater; and thus, we have restricted our temperature region (> 600 oC) of interest.
Also, the authors should explain more clearly the reasons for the different behavious between the samples, i.e. why there is a significant difference between sample PANIZnO1 and almost no difference between PANIZnO2 and PANIZnO3.
Answer: In situ synthesis of PANI/ZnO_1 lasted for 60 (step 1) + 30 (step 2) min, whereas synthesis of PANI/ZnO_2(3) lasted for 60 (step 1) + 60(120) (step 2) minutes. Therefore, the composition of polymer in PANI/ZnO_1 is more similar to the composition of polymer in solution A (pH 2). On the other hand, PANI/ZnO_2(3) were longer stirred at pH 5, which yielded a rather different (vs. PANI/ZnO_1) polymer composition and properties.
A minor point: I think there is a typing error at line 231, it should be 270 and not 279 oC.
Answer: This issue was resolved.
Regarding SEM, images with higher magnifications should be provided, to see the morphology of the lamellae and highlight possible differences between the three samples.
Answer: We have introduced one image higher magnification (X20.000) that shows the morphology of small composite particles composed of lamellas and ZnO.
Reviewer 3 Report
Comments:
“The impact of in situ polymerization conditions on structure and properties of PANI/ZnO composite photocatalyst”
The author has explained the detailed method of synthesis of PANI/ZnO composite as a photocatalyst for AB 25 dye under solar light. Authors systematically studied the physicochemical and photocatalytic properties of the synthesized samples. But there are some major issues that must be addressed in order to improve the quality of the manuscript.
- Authors claimed that PANI/ZnO composite, however in composites showed multi-phases. Clarify it.
- Give more standpoints in the support of advantages of PANI/ZnO composite.
- What is the conductivity of PANI? Compare its conductivity with the composite with measured value.
- What about the band gap of ZnO and its composites?
- The author should measure the particle size using TEM analysis. SEM is not the proper analysis method.
- The authors need to compare and provide the performance of as-prepared materials with the existing reported Photocatalysis materials also ZnO/PANI
- Authors should clearly describe, why the high concentration of PANI is not favourable in degradation of AB 25 dye.
- PANI/ZnO-2 electrode showed higher performance compared to other composition, but there is no reason, why?
- What about the stability of active material?
- Some of the typographical errors are needed to be rectified throughout manuscript.
Author Response
Reviewer #3
Comments:
“The impact of in situ polymerization conditions on structure and properties of PANI/ZnO composite photocatalyst”
The author has explained the detailed method of synthesis of PANI/ZnO composite as a photocatalyst for AB 25 dye under solar light. Authors systematically studied the physicochemical and photocatalytic properties of the synthesized samples. But there are some major issues that must be addressed in order to improve the quality of the manuscript.
Answer: Thank you for your valuable comments and suggestions. The changes in the text are marked red.
Authors claimed that PANI/ZnO composite, however in composites showed multi-phases. Clarify it.
Answer: In order to prepare PANI/ZnO suitable for photocatalysis, it is necessary to prepare PANI salt. However, the salt can be prepared only at low pH under which ZnO dissolves. According to literature, PANI/ZnO is usually synthesized by in situ aniline polymerization (under low pH) in the presence of ZnO. However, at these conditions ZnO completely dissolves and XRD shows only “traces” of ZnO. Herein, we point to the problem of ZnO dissolution by demonstrating the presence of additional phases but, at the same time, we propose an approach to avoid complete ZnO dissolution.
Give more standpoints in the support of advantages of PANI/ZnO composite.
Answer: We have clearly presented that PANI/ZnO composites have more superior photocatalytic properties vs the pure ZnO (see kapp values). We have also explained that the recombination process, which is a problem, can be bypassed by using PANI/ZnO composites.
What is the conductivity of PANI? Compare its conductivity with the composite with measured value.
Answer: The PANI conductivity is related to pH of polymerization media, type of counter-ions, the concentration of aniline, the ratio [aniline] vs. [APS], etc. As we cannot prepare pure PANI under the same conditions as we have prepared PANI/ZnO composites, we did not provide any PANI conductivities values. The conductivity of highly doped PANI (prepared at low pH) can be several order of magnitude higher than our PANI/ZnO composites but, in the aforementioned pH conditions, ZnO would completely dissolve.
What about the band gap of ZnO and its composites?
Answer: We have extracted ZnO bandgap values, and introduced them in Table.
The author should measure the particle size using TEM analysis. SEM is not the proper analysis method.
Answer: At the time being we cannot provide TEM images.
The authors need to compare and provide the performance of as-prepared materials with the existing reported Photocatalysis materials also ZnO/PANI
Answer: The purpose of this work was to obtain PANI/ZnO composites (with PANI/ZnO synergistic effect) without the complete dissolution of ZnO, which is a major undertaking. So, we were not focused on the aforementioned comparison process.
Authors should clearly describe, why the high concentration of PANI is not favourable in degradation of AB 25 dye.
Answer: The high PANI concentration can block solar irradiation and dyes’ diffusion.
PANI/ZnO-2 electrode showed higher performance compared to other composition, but there is no reason, why?
Answer: It was explained that only a composite of the optimal size in which PANI salt is in contact with both ZnO and the dye can demonstrate the full PANI/ZnO synergistic effect. According to this paper, PANI/ZnO_2 satisfies the aforementioned conditions.
What about the stability of active material?
Answer: According to TG analyses, the active materials are stable under conditions that are required for wastewater treatment.
Some of the typographical errors are needed to be rectified throughout the manuscript.
Answer: They are clarified.
Round 2
Reviewer 2 Report
The authors did not address properly the points I raised, they just gave some incomplete explanation, without improving the quality of the work.
In my opinion if these points are not dealt with, the paper should be rejected.
*********************************************************
the authors did not address my first two comments (the most important
ones), as I detail below.
My first comment was about the lower efficiency that these composite
materials have in comparison with pure ZnO. In reply, the authors state
that the tests are performed with a low AB concentration, not common in
real systems, and that at higher concentrations ZnO would have been
saturated. This answer to me proves even more that their experiments
were not performed in a proper way. If they wanted to show ZnO
saturation, they should have tested all materials at higher
concentrations, and see if in these conditions the composites performed
better that pure ZnO. The answer they gave did not make much sense.
My second point was about the TGA data; I suggested they should have
done at temperatures higher than 600 oC and in air atmosphere (not just
N2). The reply of the authors was that they wanted to check the
stability of the composites in wastewater and that, because of this,
higher temperatures were not necessary. I disagree with this statement;
if their objective was to test the stability in wastewaters they should
have considered other parameters, such as dissolution, change in surface
area, etc. TGA measurement are generally performed to understand the
behaviour of the materials; for composites, in particular, they are done
to assess possible interactions between different components, etc.
Moreover, if the authors were interested to the stability in
wastewaters, this would be one more reason to test the composites in air
and not just in N2, as wastewaters are a quite reactive environment,
surely more reactive than N2 atmosphere.
Because of this, I do not think the manuscript should be accepted.
*******************************************************
Author Response
Dear reviewer,
thank you for your comments that can be summarized in two major comments/points.
First of all, Fig. 7b clearly shows that composites (vs. pure ZnO) show higher photocatalytic efficiency. So, our goal was accomplished as we have prepared composites that have higher efficiency under solar irradiation than ZnO. Second, our photocatalytic experiments were performed with [AB 25] value, which we have continuously used in our studies [1,2]. Third, as PANI is essential for the PANI/ZnO synergistic effect we focused our TG study up to 600 oC in order to monitor PANI degradation. The experiments in N2 atmosphere enabled us to precisely monitor the degradation of PANI chains. And finally, the stability of PANI-based photocatalysts was one of the focuses in our previous work [2]. We clearly presented that radicals released form solar treatment do not degrade PANI, due to its high chemical stability. SO, we did not repeat the same experiments in this work.
In continuation of this text, you will find more detail answers.
Please see comments related to the first reviewer’s point.
ZnO is well-known for its high adsorption of organic dyes. However, the majority of removed dye (by ZnO) under solar treatment is not photodegraded but, adsorbed which is a problem. An intense dyes’ adsorption onto ZnO can be observed during 30 min of the experiment in the dark (see Fig. 7a). Although ZnO adsorbed 71% of dye in the dark, its photo-degradation ability is the lowest among the samples. This statement is confirmed in Fig. 7b, which presents samples’ photo-degradation capability under solar treatment. Fig. 7a and 7b clearly confirm that the ZnO adsorption ability is more expressed than its photo-degradation ability. The low ZnO photo-degradation ability under solar irradiation is assigned to intensive recombination of e- and h+, which is the main problem when dealing with ZnO. However, in PANI/ZnO composites, the ZnO recombination process was suppressed, which yielded a higher composites’ (vs. pure ZnO) photo-degradation ability under solar treatment. This is also confirmed by kapp values extracted from Fig. 7b. The aforementioned discussion is also summarized in the manuscript.
The applied AB concentration value in this work (30 mg / L) is usual for this kind of experiment. The same concentration value we have previously used (see [1,2]) when investigating ZnO (and TiO2)/PAN composites. According to Fig. 7, this concentration enabled us to monitor fine changes in photo-degradation property of investigated samples.
Please see comments for the second reviewer’s point.
The primer objective of our TG analyses was to acquire more information regarding the composition of PANI in PANI/ZnO composites (i.e. to understand that effect of two-step synthesis on PANI properties). As the two-step synthesis method was conducted in two different pH region, the composition of organic matter (i.e. oligomers and PANI) was different. In this work, the composition of organic matter has a major impact on the PANI/ZnO synergistic effect and on PANI/ZnO photo-degradation properties that are the main purpose of this work. Therefore, we have restricted our TG analyses in the temperature region up to 600oC where PANI is present. By using N2 atmosphere we were able to probe different PANI properties, like the stability of PANI chain (see, e.g. [3,4] ). This information obtained from N2 atmosphere is especially valuable as it showed that the PANI portion increases with synthesis time.
The stability of PANI in the composites during the solar irradiation treatment was one of the goals of our previous work [2]. To emphasize, we used TG data to show that PANI is not degraded by the radicals (see Scheme 1) produced by solar irradiation. As we have previously shown that PANI is stable under solar treatment (the same solar treatment was applied herein and in [2]), we decided not to repeat the same analyses.
I hope that you will find these answers suitable.
References
- Gilja, V.; Vrban, I.; Mandic, V.; Zic, M.; Hrnjak-Murgic, Z. Preparation of a PANI/ZnO Composite for Efficient Photocatalytic Degradation of Acid Blue. Polymers 2018, 10, doi:10.3390/polym10090940.
- Gilja, V.; Novaković, K.; Travas-Sejdic, J.; Hrnjak-Murgić, Z.; Roković, M.K.; Žic, M. Stability and synergistic effect of polyaniline/TiO<inf>2</inf>photocatalysts in degradation of Azo dye in wastewater. Nanomaterials 2017, 7, doi:10.3390/nano7120412.
- Brožová, L.; Holler, P.; Kovářová, J.; Stejskal, J.; Trchová, M. The stability of polyaniline in strongly alkaline or acidic aqueous media. Polymer Degradation and Stability 2008, 93, 592-600, doi:https://doi.org/10.1016/j.polymdegradstab.2008.01.012.
- Ding, L.L.; Wang, X.W.; Gregory, R.V. Thermal properties of chemically synthesized polyaniline (EB) powder. Synthetic Metals 1999, 104, 73-78, doi:10.1016/s0379-6779(99)00035-1.
Reviewer 3 Report
The authors have improved the revised manuscript and have incorporated the comments as suggested by reviewers. However the following issues must be addressed.
- Previous comment: Authors claimed that PANI/ZnO composite, however in composites showed multi-phases. Clarify it.
Authors response is nt acceptable. They have claimed that " we propose an approach to avoid complete ZnO dissolution". However its nt achieved because XRD shows multi-phases not single.
2. Stability of active materials in Photo-catalysis experiment?.
Author Response
Dear reviewer,
thank you for your comments that are summarized in two issues.
First of all, we have clarified the “multiphase” issue, and we have modified the Manuscript title as well as the text. Now, is clear that the samples are multiphase composites. Second, FE SEM image in Fig. 4d evidently shows that ZnO particles are present in the composite even after 120 min of the stirring. So the dissolution of ZnO was clearly avoided during in situ polymerization. And finally, the stability of active material during the synthesis was one of the focuses of our previous work [1], i.e. we demonstrated that PANI is not degraded by the radicals released during the solar treatment.
Please see the comments related to the first issue.
The PANI/ZnO samples in this work are multiphase composites composed of several phases. According to the provided data, it can be concluded that PANI is present in two phases (PANI salt and PANI base form). Furthermore, XRD study obviously presented three phases, i.e. ZnO and two additional phases. We have changed the title and we introduced changes in the text. Now, it is clear that the composites in this work are PANI/ZnO-based multiphase composites.
Also, according to FE-SEM images, ZnO was not completely dissolved. However, a small portion of ZnO was dissolved (at the beginning of the second step) when ZnO was added to acidic homopolymeriazion solution. This was a source of Zn2+ ions that formed an additional two phases. These issues were also more thoroughly explained in the Manuscript.
The answer to the second issue.
The issue of stability of photocatalyst material is now commented on in the manuscript. It was explained that this problem was addressed in our previous paper [1]. Specifically, in [1] we explained that formed radicals (Scheme 1) that degrade the dye did not affect the PANI in a composite. This fact was explained by the high stability of PANI, which is expected as this polymer is highly stable even in strong acids and strong bases.
I hope that you will find these answers satisfactory.
References:
- Gilja, V.; Novaković, K.; Travas-Sejdic, J.; Hrnjak-Murgić, Z.; Roković, M.K.; Žic, M. Stability and synergistic effect of polyaniline/TiO<inf>2</inf>photocatalysts in degradation of Azo dye in wastewater. Nanomaterials 2017, 7, doi:10.3390/nano7120412.
Round 3
Reviewer 2 Report
See below.
