1
Poly-acrylic acid embedded with TiO2 photocatalyst is successfully fabricated.
2
The cross-linking between poly-acrylic acid and TiO2 is formed under UV light irradiation.
3
The fabricated hybrid hydrogel is able to perform self-healing autonomously.
4
The self-healing mechanism in PT hydrogel is triggered by hydrogen bonds.
5
PT hydrogel is applied for reduction of 4-nitrophenol to 4-aminophenol.
6
Highlights (for review)
Journal: Materials Chemistry Physics
Manuscript ID: MATCHEMPHYS-D-23-01497
Title: "Facile Synthesis of Self-Healing Poly-Acrylic Acid/TiO2 Hybrid Hydrogel for Photocatalytic Hydrogenation of 4-Nitrophenol to 4-Aminophenol"
Author(s): Pasaribu, Subur P.; Masmur, Indra; Hestina; and Panggabean, Aman Sentosa
Dear Prof. Alessandro Martucci, Ph.D,
We would like to thank the editor for very careful review and for the constructive suggestions on our manuscript. We also appreciate the time and efforts by the editor and reviewers in reviewing this manuscript. We have adopted the suggestions and the appropriate changes have been introduced to the revised manuscript. Therefore, hopefully you find our responses satisfactory and that the revised manuscript is now completely acceptable for publication in Materials Chemistry Physics.
We look forward to receive further communications.
Sincerely yours,
Dr. Subur P. Pasaribu Associate Professor Department of Chemistry Mulawarman University
Email: [email protected]
Response to Reviewers
We would like to thank the reviewers for careful and thorough reading of this manuscript and for the thoughtful comments and suggestions. Comments and Responses to the comments are given below.
Reviewer(s)' Comments to Author:
Reviewer: #1
Recommendation: major revision.
Comments:
1. Is self-healing related to swelling of hydrogel? Please explain.
Response: Thank you for the suggestion. As can be seen from Fig. 4a, the degree of swelling is related with the self-healing efficiency of hydrogels (Fig. 5b) in which the most optimum self-healing efficiency occurred at lower degree of swelling (Fig. 4a), indicating better self- healing activity of hydrogels at higher cross-linking network.
2. What is the maximum swelling obtained? Swelling should be optimized by varying different reaction parameter like pH, solvent, acrylic acid concentration, time, temperature etc.
Response: Thank you for the suggestion. In the revised manuscript, the swelling study of PT- hydrogels was carried out at varied pH, solvent, acrylic acid concentration, and temperature.
Furthermore, the swelling characteristic of PT-4 hydrogel was also investigated at varied pH, solvent, AA concentration, and temperature. For swelling at different pH, the SD of PT-4 hydrogel increased from 63% at pH 1 to the maximum of 179% at pH 7 (Fig. 4b). Subsequently, with increasing pH, the SD decreased gradually from 179% to 103% at pH 1326. The swelling experiment in different solvents was performed in polar and nonpolar solvents (e.g. acetone, methanol, isopropanol, ethanol, and DMSO). Among the alcohols, the SD in methanol indicates the lowest SD value (Fig. 4c). This is because methanol is more polar than ethanol and isopropanol which could generate hydrogen bonding between the solvent and hydrogel, therefore limit the expansion of hydrogel networks. On the other hand, the highest SD value is achieved by swelling in acetone which due to the nature of polar aprotic solvent that could not form hydrogen bonds27. On the other hand, it is noticed that SD value of PT-4 hydrogel was found to be relatively low (~120%) which because of the disintegration of PT-4 sample. Fig.
4d shows the swelling behavior of PT hydrogels as a function of AA concentration and a fixed water amount of 4 mL. The SD decreases gradually at higher AA concentration where a plateau is achieved at AA concentration of 2 mL. Moreover, the SD with temperature variation during
swelling experiment was shown in Fig. 4e. The result clearly indicates that the SD value of PT- 4 hydrogel increased with increasing swelling temperature.
Fig. 4 (a) Degree of swelling and gel fraction of PT hydrogels. The swelling characteristic of PT hydrogels at varied (b) pH, (c) solvent, (d) AA concentration, and (e) temperature.
3. Have author used initiator or crosslinker to synthesize hydrogel?
Response: Thank you for the suggestion. In this study, there is no utilization of initiator to synthesize the hydrogels. Instead, the hydrogels are formed through the hydrogen bonding (physical crosslinking) between the carboxylic groups from polyacrylic acid and TiO2.
4. How authors justify hydrogen bonding between TiO2 and carboxylic functional groups in poly-acrylic acid. Please give evidences.
Response: Thank you for the comment. The hydrogen bonding between TiO2 and carboxylic functional groups in PAA was evidenced by FTIR analysis as shown in Fig. 3b. The cross- linking interaction between TiO2 and carboxylic functional groups through hydrogen bonds is evidenced by the shifting peak from 1734 cm-1 in AA, then shift to lower wavelength of 1715 cm-1 (Fig. 3b).
5. Who is more suitable for photocatalytic rigid or spongy hydrogel? Is spongy hydrogel stable?
Response: Thank you for the suggestion. The authors have performed the photocatalytic hydrogenation of 4-NP to compare the photocatalytic activity between rigid (PT-2) and spongy hydrogels (PT-4). As can be seen in figure below, spongy hydrogel (PT-4) performed better
photocatalytic hydrogenation activity that rigid hydrogel (PT-2), in which the intensity of 4- NP decreases from 1 to 0.27 using PT-4 within two hours, while it is only 0.4 for PT-2. The spongy hydrogel was stable without any deformation for PT-4 before and after photocatalytic reaction.
6. Characterization part is poor. TGA and BET study should be carried out to justify the results.
Response: Thank you for the suggestion. Fig. 3c presents the TGA profiles of PT hydrogels. The sharp weight loss was observed at temperature of ~230 C indicating the anhydride the formation of PAA. The weight loss at temperature ~395 C was accounted for the degradation of anhydride PAA. The contained TiO2 did not affect the thermal stability of the hydrogels. The residual weight of all hydrogels are in the range from 3.6-15.1 wt%. In addition, BET analysis was carried out to determine the specific surface area, average pore diameter and volume. BET analysis clearly shows that the surface area, pore volume and diameter of PT hydrogels gradually increase with increasing water amount during synthesis as shown in Table 1.
Fig. 3 (a) Diffractogram of PT hydrogels with varied water content. (b) FTIR spectra of acrylic acid and PT hydrogels with different water content. (c) Thermogravimetric analysis (TGA) profiles of PT hydrogels.
0.0 0.2 0.4 0.6 0.8 1.0
Intensity of 4-NP (a.u.)
Time (min)
PT-4 hydrogel (Spongy) PT-2 hydrogel (Rigid)
120 100 80 60 40 20 0 -10 -20 -30
Table 1. BET surface area and pore size analysis for PT hydrogels.
Samples BET surface area (m2 g-1)
Pore volume (cm3 g-1)
Pore diameter (nm)
PT-1 23 0.11 4.2
PT-2 37 0.17 4.9
PT-3 51 0.29 5.7
PT-4 83 0.42 6.4
PT-5 90 0.61 7.1
7. XRD and SEM study of polyacrylic acid hydrogel without TiO2 should be given to make the effective explanation.
Response: Thank you for the suggestion. In this study, the precursors are acrylic acid and TiO2. Polyacrylic acid hydrogel can only be formed with the existence of TiO2 that can polymerize the acrylic acid by radical species generated during the UV-light irradiation. Therefore, XRD and SEM analysis of polyacrylic acid hydrogel without TiO2 cannot be characterized.
8. There some recent articles, which must be added to literature review:
https://doi.org/10.1016/j.chemosphere.2023.138015, https://doi.org/10.1016/j.apmt.2022.101676,
https://doi.org/10.1007/s11356-022-24940-3, https://doi.org/10.1007/s13204-022-02743-9, https://doi.org/10.1016/j.materresbull.2023.112238
Response: Thank you for the suggestion. All the abovementioned literature reviews have been cited in the revised manuscript.
9. What are new insights in your photocatalytic hydrogenation using hydrogel?
Response: Thank you for the suggestion. Hydrogels have the advantages to absorb the chemical contaminant in their swellable three-dimensional (3D) networks. By combining TiO2
photocatalyst and poly-acrylic acid hydrogel, the authors found that the photocatalytic activity of PT hydrogels was enhanced through the initial saturated absorption of chemical contaminant and followed by photoreaction to degrade the contaminant.
10. Schematic photocatalytic hydrogenation mechanism should be provided.
Response: Thank you for the suggestion. The schematic photocatalytic hydrogenation mechanism of 4-NP to 4-AP has been added as Fig. 8 in the revised manuscript. The
photocatalytic hydrogenation mechanism of 4-NP to 4-AP by PT-4 hydrogel is initially started by saturated absorption of 4-NP in the three-dimensional networks of PT-4 hydrogel, then followed by photoreaction by TiO2 to reduce 4-NP to 4-AP as can be seen in Fig. 8.
Fig. 8 Schematic photocatalytic hydrogenation of 4-NP by PT-4 hydrogel.
11. Is synthesized hydrogel reusable for photocatalytic hydrogenation?
Response: Thank you for the suggestion. The stability of PT-4 hydrogel was determined by running photocatalytic 4-NP hydrogenation for five-consecutive cycles. The results show that PT-4 hydrogel is robust and reusable with the stability remains higher than 90% after five- consecutive cycles of 4-NP photocatalytic hydrogenation as shown in figure below.
0 20 40 60 80 100
5th 4th
3rd 2nd
1st
Reusability (%)
Cycles
Journal: Materials Chemistry Physics
Manuscript ID: MATCHEMPHYS-D-23-01497
Title: "Facile Synthesis of Self-Healing Poly-Acrylic Acid/TiO2 Hybrid Hydrogel for Photocatalytic Hydrogenation of 4-Nitrophenol to 4-Aminophenol"
Author(s): Pasaribu, Subur P.; Masmur, Indra; Hestina; and Panggabean, Aman Sentosa
Dear Prof. Alessandro Martucci, Ph.D,
We would like to thank the editor for very careful review and for the constructive suggestions on our manuscript. We also appreciate the time and efforts by the editor and reviewers in reviewing this manuscript. We have adopted the suggestions and the appropriate changes have been introduced to the revised manuscript. Therefore, hopefully you find our responses satisfactory and that the revised manuscript is now completely acceptable for publication in Materials Chemistry Physics.
We look forward to receive further communications.
Sincerely yours,
Dr. Subur P. Pasaribu Associate Professor Department of Chemistry Mulawarman University
Email: [email protected]
Response to Reviewers
We would like to thank the reviewers for careful and thorough reading of this manuscript and for the thoughtful comments and suggestions. Comments and Responses to the comments are given below.
Reviewer(s)' Comments to Author:
Reviewer: #3
Recommendation: major revision.
Comments:
1. English language need to be improved
Response: Thank you for the suggestion. The appropriate English editing has been introduced to the revised manuscript.
2. Please remove (TiO2 anatase, 95%) is not correct
Response: Thank you for the suggestion. In the revised manuscript, the authors have removed the above-mentioned wording of (TiO2 anatase, 95%).
3. In synthesis part the author adds 2ml of acrylic acid with five water contents so he has Five sample so why he used 12-well plates
Response: Thank you for the comment. In our laboratory, we only have the 12-well plates and 36-well plates. However, we did not use the 36-well plates due to the small volume which cannot fit for the total solution, therefore we used the 12-well plates.
4. The scale bar of SEM image is loss
Response: Thank you for the comment. The scale bar for SEM images is 10 m and has been added in the revised manuscript.
Fig. 2 Surface morphology of (a,b) PT-1, (c,d) PT-2, (e,f) PT-3, (g,h) PT-4, and (i,j) PT-5 hydrogels. Photographs of (k) side-view and (l) top-view of the synthesized PT hydrogels. The scale bar in SEM images is 10 m.
5. Why author choose PT-4 sample in the application section
Response: Thank you for the comment. The author chose PT-4 hydrogel to use for the application is based on the mechanical properties, self-healing efficiency, and gel fraction.
These characterizations demonstrated that the plateau of data was achieved at PT-4 hydrogel.
6. Please study the Photocatalytic hydrogenation of 4-nitrophenol to 4-aminophenol for all sample and without sample (free)
Response: Thank you for the suggestion. The study regarding photocatalytic hydrogenation of 4-NP to 4-AP has been carried out. The results showed that the photocatalytic activity of PT hydrogels enhanced with increased water amount during synthesis, while there is no significant alteration between PT-4 and PT-5 hydrogels as shown in figure below.
7. The following references should be used to improve the content of manuscript (Ultrasonics Sonochemistry 37 (2017): 529-535.)
(ACS omega, 5(34), pp.21476-21487.)
(Arabian Journal of Chemistry. 2020 Feb 1;13(2):3762-74.) (Polymers for Advanced Technologies, 32(2), pp.514-524.)
(Macromolecular Chemistry and Physics. 2020 Oct;221(19):2000218) (Journal of environmental chemical engineering, 5(3), pp.2325-2336.) (Composites Communications 3 (2017): 18-22.)
Response: Thank you for the suggestion. All the abovementioned references have been cited to improve the content of revised manuscript.
0.0 0.2 0.4 0.6 0.8 1.0
Intensity (a.u.)
Time (min)
sample-free PT-1 PT-2 PT-3 PT-4 PT-5
120 100 80 60 40 20 0 -10 -20 -30