Materials Chemistry and Physics
Facile Synthesis of Self-Healing Poly-Acrylic Acid/TiO2 Hybrid Hydrogel for Photocatalytic Hydrogenation of 4-Nitrophenol to 4-Aminophenol
--Manuscript Draft--
Manuscript Number: MATCHEMPHYS-D-23-01497R1
Article Type: Full Length Article
Keywords: Self-healing hydrogel; poly-acrylic acid; TiO2; 4-nitrophenol; 4-aminophenol
Corresponding Author: Subur Pasaribu
Samarinda, INDONESIA
First Author: Subur Pasaribu
Order of Authors: Subur Pasaribu
Indra Masmur Hestina Hestina
Aman Sentosa Panggabean
Abstract: Self-healing poly-acrylic acid/TiO2 (PT) hybrid hydrogel with different water amount have been simply fabricated by UV light irradiation. The cross-linking (matrix) formation of PT hydrogels was formed due to UV light absorption by TiO2 generating the radical species to polymerize acrylic acid monomer to poly-acrylic acid. XRD analysis successfully revealed the presence of TiO2 in the hydrogel. Meanwhile, the
polymerization of acrylic acid to poly-acrylic acid was revealed by FTIR analysis. It is obvious that the physical appearance of as-prepared PT hydrogels with different water amount was different, where a rigid hydrogel was obtained with less water amount and a spongy hydrogel was formed with more water amount. The highest self-healing efficiency achieved ~85% exhibited by PT-4 hydrogel in which the self-healing ability is due to hydrogen bond occurred between TiO2 and carboxylic functional groups in poly- acrylic acid. For the application, the healed PT-4 hydrogel demonstrated the capability to convert toxic 4-nitrophenol to 4-aminophenol through photocatalytic hydrogenation reaction within 2 h UV light irradiation.
Suggested Reviewers: Hardy Shuwanto
[email protected] Jenni Lie
[email protected] Hairus Abdullah [email protected] Suryadi Ismadji
[email protected] Osman Zelekew
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Declaration of interests
☒The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Declaration of interests: none Declaration of Interest Statement
Facile Synthesis of Self-Healing Poly-Acrylic Acid/TiO
2Hybrid Hydrogel for Photocatalytic Hydrogenation of 4-Nitrophenol to 4-Aminophenol
Subur P. Pasaribua*, Indra Masmurb, Hestinac, and Aman Sentosa Panggabeana
aDepartment of Chemistry, Faculty of Mathematics and Natural Sciences, Mulawarman University, Samarinda-75123, Indonesia
bDepartment of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Sumatera Utara, Medan-20155, Indonesia
cDepartment of Chemistry, Universitas Sari Mutiara Indonesia, Medan-20123, Indonesia
*Corresponding author: [email protected]
Title Page
Mar 20th, 2023 Editor-in-Chief
Materials Chemistry and Physics
Manuscript Title: Facile Synthesis of Self-Healing Poly-Acrylic Acid/TiO2 Hybrid Hydrogel for Photocatalytic Hydrogenation of 4-Nitrophenol to 4-Aminophenol
Authors: Subur P. Pasaribu, Indra Masmur, Hestina, and Aman Sentosa Panggabean Dear Editor-in-Chief,
Enclosed please find a copy of the manuscript entitled “Facile Synthesis of Self-Healing Poly-Acrylic Acid/TiO2 Hybrid Hydrogel for Photocatalytic Hydrogenation of 4-Nitrophenol to 4-Aminophenol”. In this study, we demonstrated a facile synthesis of poly-acrylic acid hydrogel embedded with TiO2 prepared under UV light irradiation. The fabricated hybrid hydrogel exhibits self-healing ability and was applied for reduction of 4-nitrophenol to 4-aminophenol. The characterization was carried out by SEM, FTIR, XRD, EDX, physicochemical and mechanical properties and UV spectroscopy analyses. To the best of our knowledge, this is the first study to prepare hybrid hydrogel under a simple and facile UV light irradiation.
This manuscript was suggested to transfer from Journal of Catalysis to Materials Chemistry and Physics. This article is original work by author and co-authors and is not under consideration for publication or published previously in another journal at the time of submission.
The authors declare that there are no known competing financial interests or personal relationships that could have appeared to influence the work. If accepted at a later time, this manuscript will not be published elsewhere in the same form, in any language, without the written content of the publisher.
We hope that this original full paper fits the journal scope and its standard quality to be considered for publication in Materials Chemistry and Physics. I am looking forward to hearing from you.
Sincerely yours,
Dr. Subur P. Pasaribu Associate Professor Department of Chemistry Mulawarman University
Cover Letter
Facile Synthesis of Self-Healing Poly-Acrylic Acid/TiO
2Hybrid Hydrogel for Photocatalytic Hydrogenation of 4-Nitrophenol to 4-Aminophenol
Subur P. Pasaribua*, Indra Masmurb, Hestinac, and Aman Sentosa Panggabeana
aDepartment of Chemistry, Faculty of Mathematics and Natural Sciences, Mulawarman University, Samarinda-75123, Indonesia
bDepartment of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Sumatera Utara, Medan-20155, Indonesia
cDepartment of Chemistry, Universitas Sari Mutiara Indonesia, Medan-20123, Indonesia
*Corresponding author: [email protected]
Revised Manuscript (clean copy) Click here to view linked References
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Graphical Abstract
3 Abstract
Self-healing poly-acrylic acid/TiO2 (PT) hybrid hydrogel with different water amount have been simply fabricated by UV light irradiation. The cross-linking (matrix) formation of PT hydrogels was formed due to UV light absorption by TiO2 generating the radical species to polymerize acrylic acid monomer to poly-acrylic acid. XRD analysis successfully revealed the presence of TiO2 in the hydrogel. Meanwhile, the polymerization of acrylic acid to poly-acrylic acid was revealed by FTIR analysis. It is obvious that the physical appearance of as-prepared PT hydrogels with different water amount was different, where a rigid hydrogel was obtained with less water amount and a spongy hydrogel was formed with more water amount. The highest self-healing efficiency achieved ~85% exhibited by PT-4 hydrogel in which the self- healing ability is due to hydrogen bond occurred between TiO2 and carboxylic functional groups in poly-acrylic acid. For the application, the healed PT-4 hydrogel demonstrated the capability to convert toxic 4-nitrophenol to 4-aminophenol through photocatalytic hydrogenation reaction within 2 h UV light irradiation.
Keywords: Self-healing hydrogel; poly-acrylic acid; TiO2; 4-nitrophenol; 4-aminophenol
4 1. Introduction
Self-healing hydrogels are three-dimensional cross-linked polymeric networks with high water content and able to heal the damage parts autonomously which inspired by self- repairing system of living creatures. Self-healing hydrogels could perform the autonomous self-repairing based on either dynamic covalent bonds (e.g. Diels-Alder reaction1, coordination bond2, Schiff base3, boronate ester bond4, and disulfide bond5) or noncovalent interactions (e.g.
ionic interaction6-8, hydrogen bonds9, hydrophobic interaction10, and host-guest interaction11).
Self-healing hydrogels had been widely utilized particularly in biomedical related applications such as diabetic wound12, cartilage tissue engineering13, drug delivery14, and biosensors15.
Environmental contamination due to chemical pollutants remain as serious global problems which had gained considerable attention. In majority, the chemical pollutants are nitroaromatic organic compounds which are acutely toxic, low solubility in water and highly stable. For instance, 4-nitrophenol (4-NP) has been widely used as precursors for dyestuff, explosives, drugs, etc.)16. The exposure to 4-NP lead to serious health issues such as skin irritation, organ disfunction, etc. If marine creatures are exposed to this toxic chemical, it will bring the domino effect to human as well via the food chain. Therefore, several techniques were developed to eliminate such toxic chemicals from water bodies17. The well-known techniques are photocatalysis, electrocatalysis, adsorption, and Fenton reaction18-23. Adsorption method is less preferable because this method only creates another secondary waste which in the end, the generated waste would be dumped into the landfill. Meanwhile, electrocatalysis method is limited due to its low efficiency and poor durability that would be critical issue for scale up purposes. Therefore, photocatalysis is more preferable since it requires only sunlight to proceed the catalytic reaction on the catalysts24-26. Many photocatalysis works have been developed for reduction of 4-nitrophenol to 4-aminophenol27-29, however, most of the works only focus on the photocatalytic performances without considering the recyclability and
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reusability30. For instance, Shuwanto et. al. (2021) reported the fabrication of nanosized powder Ni-Mg codoped Zn(O,S) photocatalyst for environmental remediation purposes and hydrogen production under solar light irradiation31. Eventhough, the authors reported the superior photocatalytic performances, the after-used photocatalysts collection would be burdensome responsibilities in real-life practical application.
With this regard in mind, we are developing photocatalyst embedded in hydrogel with self-healing ability that can extend its lifetime for environmental remediation. In this study, we demonstrated a simple and facile preparation of poly-acrylic acid hydrogel embedded with TiO2 photocatalyst (PT hydrogel) synthesized by UV light irradiation without any cross-linking agents. The as-prepared hybrid catalyst was applied to reduce 4-NP to 4-AP and the characterizations and catalytic experiments are thoroughly elaborated in this work.
2. Experimental Methods 2.1.Materials
Acrylic acid (99.1%, Sigma Aldrich) and titanium dioxide (Showa chemical) were analytical grade and used without further purification. DI water was obtained using Millipore filtration system with the resistance of 18.2 M.
2.2. Preparation of polyacrylic acid-TiO2 hybrid hydrogel
In a typical synthesis, 2 mL of acrylic acid was placed into a 12-well plate followed by adding a certain amount of DI water (1, 2, 3, 4, and 5 mL). Subsequently, 2 mg of TiO2 was added into the diluted acrylic acid solution and dispersed using an ultrasonicator (Branson M5800H) for 30 min. The dispersion was then irradiated by UV light for 45 min (360 nm, 20 W) with a distance of 5 cm between UV lamp and the dispersion. In this study, DI water amount was varied to study the cross-linking effect on the resulted polyacrylic acid-TiO2 hybrid
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hydrogels. The synthesis illustration was schemed in Fig. 1 and the synthesized hydrogels were named as PT-x, where x represents the amount of water in mL.
Fig. 1 Schematic illustration for PAA-TiO2 hydrogels prepared under UV light irradiation.
2.3. Characterizations of hydrogels
In this study, the surface morphology was observed using scanning electron microscopy (FE-SEM JEOL, JSM-7900F) at an operating voltage of 20 kV. The crystallinity of samples was determined by X-ray diffractometer (XRD, Bruker D2 phaser) using Cu K radiation (=
0.1514 nm). The functional groups in hydrogel samples was analyzed through fourier transform infrared (FTIR) spectroscopy using KBr pellet method. Moreover, the mechanical properties of hydrogels in terms of tensile stress () and strain was examined using Testometric (M500- 25AT) with a 100 N load cell and a crosshead speed of 50 mm/min, where the hydrogels were initially cut into a dog bone shape prior to the mechanical testing. The swelling degree of hydrogels was determined by allowing the hydrogels to swell in DI water at room temperature (35 2 C) until saturated swollen was achieved. Afterwards, the excess water on hydrogel surfaces was gently blotted and weighed as Ws, while, initial weight of hydrogels was Wi. The equilibrium swelling degree was calculated using Eq. 1. Accordingly, gel fraction of hydrogels was investigated to determine the cross-linking degree of hydrogels (insoluble parts). The
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saturated swollen hydrogels were subsequently dried and weighed as Wi’ and the gel fraction was further calculated using Eq. 2. Furthermore, the sell-healing efficiency of hydrogels was determined by rupturing the hydrogels off into two symmetrical parts, then, put the damaged parts in contact at room temperature (35 2 C) for 12 h for healing process. The healed samples was subjected to tensile test and the results were recorded as ’. Therefore, the self- healing efficiency could be calculated using Eq. 3.
SD= WsW- Wi
i 100% (1)
GF= WWi'
i 100% (2)
SHE= ′ 100% (3)
3. Results and Discussions
3.1.Surface morphology of PT hydrogels
In this study, the synthesis of PT hydrogel was occurred through several steps, first, the dispersion containing TiO2 and acrylic acid monomer was irradiated with UV light. Herein, radical species are generated as a result of TiO2 absorbing UV light. Accordingly, the radical species induce polymerization on acrylic acid to poly-acrylic acid. Second, the contained carboxylic functional groups in poly-acrylic acid react with TiO2 through the formed hydrogen bonds to yield cross-linking formation of hydrogel32.
The surface morphology of the as-synthesized PT hydrogels observed using SEM was shown in Fig. 2a-j. The hydrogel samples was firstly freeze-dried prior to SEM analysis. The surfaces of PT-1 and PT-2 hydrogels were rough and contain porosity (Fig. 2a-d). It is because the preparation of PT 1 and PT-2 hydrogels only applied a less amount of water (1 – 2 mL).
Meanwhile, there are no significant differences among PT-3, PT-4, and PT-5 hydrogels, where the hydrogels exhibit relatively smooth surfaces (Fig. 2e-j). Fig. 2k and 2l display the
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photographs of as-prepared PT hydrogels, where PT-1 and PT-2 hydrogels possessed relatively firm and transparent gel, meanwhile, PT-3, PT-4, and PT-5 hydrogels were found to be white gel with more springy (tofu-like) structure. On the other hand, there are no TiO2 particles on hydrogel surfaces indicating that TiO2 particles were probably embedded inside the PT hydrogels.
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.
3.2. Phase identification of PT hydrogels
To identify the phase structural and the TiO2 existence in PT hydrogels, XRD analysis was carried out by scanning from 10 – 70. Fig. 3a showed the diffractograms of PT hydrogels with varied water content. All PT hydrogels exhibit similar XRD patterns indicating the amorphous phase of PAA hydrogels with several peaks of TiO2 anatase. Those several peaks of TiO2
anatase are located at 2 of 25.3 (101), 37.1 (103), 38 (004), 38.8 (112), 48.1 (200), 54.2
(105), 55 (211), 62 (213), and 68.8 (116) matched with TiO2 anatase standard with JCPDS no. 21-1272.
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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.
In addition, FTIR analysis was conducted to identify the functional groups in PT hydrogel. As can be seen in Fig. 3b, the functional groups of -OH, C=O, and C=C in acrylic acid was appeared at 3180, 1734 and 1615 cm-1, respectively6, 33-35. Such narrow C=C peak at 1615 cm-1 becomes broaden after polymerized to form PT hydrogels. The intensity of broaden peak of C=C group gradually decreased with increasing water amount during synthesis. This result proves that higher polymerization degree took place with higher water amount during the synthesis. Furthermore, 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)8, 36.
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.
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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
3.3 Swelling degree and gel fraction of PT hydrogels
Swelling degree experiment demonstrates the capability of hydrogels absorbing water in the cross-linking networks, which in this study was determined by gravimetric method.
Meanhwile, the gel fraction represents the cross-linking degree of hydrogels. As can be seen in Fig. 4, the swelling degree of PT hydrogels decreased gradually with increasing water content. PT hydrogels could absorb water in their cross-linked networks mainly due to the hydrophilical carboxylic organic functional groups (-COO)6, 37. On the other hand, the cross linking degree of PT hydrogels increased with increasing water amount during synthesis. This phenomenon explains that PT hydrogels with higher gel fraction contain denser cross-linking networks and limit the water uptake, therefore resulted in lower swelling ability.
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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.
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 1338. 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 bonds39. 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.
3.4 Mechanical properties and self-healing efficiency of PT hydrogels
The mechanical properties was determined by tensile test method which performed on pristine and after-healed PT hydrogels. Fig. 5a shows the stress-strain curves of pristine and after-healed PT hydrogels. The tensile strain of pristine PT-1 hydrogel showed a relatively brittle and fractured properties as its elongation only 31% at stress of 3.2 MPa. Moreover, the
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strain of PT hydrogels increased dramatically with increasing water content, where it is contratrily with the tensile stress. Fig. 5b displays the tensile stress of initial and after-healed PT hydrogels with their self-healing efficiency. It is obvious that the self-healing efficiency of PT hydrogels increased from ~21 to ~85 % with increasing water content revealing that water plays vital role for healing process along the ruptured-off area of PT hydrogels. This results also revealed that 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.
Fig. 5 (a) Typical tensile stress-strain curve of pristine and after-healed PT hydrogel samples.
(b) Self-healing efficiency of PT hydrogels based on initial and after-healed tensile stress properties.
3.5 Self-healing mechanism of PT hydrogels
Fig. 6a-d show the healing process of PT-4 hydrogel at varied healing time observed using SEM analysis. As can be seen, there are no significant differences for initial, 3 h, and 6 h healing time of PT-4 hydrogel (Fig. 6a-c), which there is only a flimsy line of ruptured-off at hydrogel interfaces. Afterwards, the separated two-halves of hydrogel was attached to each other after 12 h healing time (Fig. 6d). The optical microscope images clearly indicates the self-healing ability of PT-4 hydrogel as shown in Fig. 6e-g. The healed PT-4 hydrogel was
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subsequently subjected to stretching force to demonstrate the successful recovery as can be seen in Fig. 6h. Interestingly, the self-healing process in PT-4 hydrogel occurred due to hydrogen bonds between TiO2 and carboxylic groups as illustrated in Fig. 6i40.
Fig. 6 SEM images of cut-off PT-4 hydrogels at different healing time (a) initial, (b) 3 h, (c) 6 h, and (d) 12 h. Optical microscopy images of (e) pristine, (f) cut-off, and (g) healed PT-4 hydrogel. (h) Photograph of self-healing PT-4 hydrogel (coloured by adding orange dye) at stretching condition. (i) Illustration for self-healing mechanism of hydrogen bond in PT-4 hydrogel.
3.6 Photocatalytic hydrogenation of 4-nitrophenol to 4-aminophenol
For the application, the as-prepared PT-4 hydrogel was applied for photoreduction of 4-nitrophenol to 4-aminophenol in water containing 20% EtOH as hole scavenger at UV light (36 W) irradiated condition. The weight of PT-4 hydrogel was 0.1 g in a 100 mL solution of 30 ppm of 4-nitrophenol. As can be seen in Fig. 7a, the peak of 4-nitrophenol was arose at 318 nm determined by UV-Vis spectrophotometer. Since PT-4 hydrogel possesses the ability to swell, the hydrogel was allowed to absorb until reach saturation for 30 min. After irradiated for 120 min, such peak shifted from 318 nm to lower wavelength of 305 nm indicating the
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conversion of 4-nitrophenol to 4-aminophenol (Fig. 7a)41. In addition, we also compare the photocatalytic performance of as-prepared and after-healed PT-4 hydrogels. The photocatalytic performance between as-prepared and after-healed samples do not have significant alteration as shown in Fig. 7b. Moreover, the reusability of PT-4 hydrogel was examined by running five- consecutive cycles for 4-NP photocatalytic hydrogenation to determine the robustness of sample. 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 Fig. 7c. To clarify the photohydrogenation of 4-nitrophenol to 4-aminophenol, GC-MS analysis was carried out on initial and after irradiated solution. Fig. 7d and e confirm the existence of 4-nitrophenol and 4-aminophenol in initial and after irradiated solution, respectively. In a big picture, this result proves that after being damaged and recovery through the healing process, PT-4 hydrogel still could proceed the photocatalysis reaction and also emphasize the robustness of PT-4 hydrogel. 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.
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Fig. 7 (a) UV-vis spectrum of 4-nitrophenol during hydrogenation reaction by PT-4 hydrogel under UV light irradiation. (b) Intensity comparison of pristine and healed PT-4 hydrogels for photocatalytic hydrogenation of 4-nitrophenol. (c) The reusability performance of PT-4 hydrogel for photocatalytic hydrogenation of 4-NP for five-consecutive cycles. The chromatogram results of (d) initial 4-nitrophenol and (e) after hydrogenation solutions.
Fig. 8 Schematic photocatalytic hydrogenation of 4-NP to 4-AP by PT-4 hydrogel.
Conclusion
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In conclusion, a facile approach to fabricate poly-acrylic acid/TiO2 hybrid hydrogel is conducted utilizing UV light irradiation at varied water content. The resulted PT hydrogels are completely unsimilar when different amount of water is used. A rigid and firm hydrogel is formed with less amount of water, meanwhile, a springy hydrogel will be obtained with increasing water content. The presence of TiO2 and poly-acrylic acid are confirmed by XRD and FTIR analyses, respectively. The water absorption capability of PT hydrogels increased gradually with increasing water content, vice versa with the gel content in PT hydrogels. The highest self-healing efficiency was demonstrated by PT-4 hydrogel which reached ~85%. In this work, the self-healing process is mainly ascribed to the hydrogen bonds between TiO2 and carboxylic functional groups from polyacrylic acid. At last, the self-healing PT-4 hybrid hydrogel can be applied for reduction of 4-nitrophenol to 4-aminophenol through photocatalytic hydrogenation reaction.
CRediT authorship contribution statement
Subur P. Pasaribu : Methodology, Conceptualization, Data curation, Formal analysis, Writing - original draft.
Indra Masmur : Investigation, Validation, Writing - review & editing.
Hestina : Formal analysis, Writing - review & editing.
Aman Sentosa Panggabean : Validation, Writing - review & editing.
Declaration of competing interest
The authors declare that there are no conflicts of interest that could have appeared in this reported work.
Acknowledgements
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The authors would like to thanks Mulawarman University and Universitas Sumatera Utara for the facilities and instrumentation to conduct the research.
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Facile Synthesis of Self-Healing Poly-Acrylic Acid/TiO2 Hybrid Hydrogel for Photocatalytic Hydrogenation of 4-Nitrophenol to 4-Aminophenol
Subur P. Pasaribua*, Indra Masmurb, Hestinac, and Aman Sentosa Panggabeana
aDepartment of Chemistry, Faculty of Mathematics and Natural Sciences, Mulawarman University, Samarinda-75123, Indonesia
bDepartment of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Sumatera Utara, Medan-20155, Indonesia
cDepartment of Chemistry, Universitas Sari Mutiara Indonesia, Medan-20123, Indonesia
*Corresponding author: [email protected]
Revised Manuscript with tracked changes Click here to view linked References
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Graphical Abstract 1
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3 Abstract
Self-healing poly-acrylic acid/TiO2 (PT) hybrid hydrogel with different water amount have been simply fabricated by UV light irradiation. The cross-linking (matrix) formation of PT hydrogels was formed due to UV light absorption by TiO2 generating the radical species to polymerize acrylic acid monomer to poly-acrylic acid. XRD analysis successfully revealed the presence of TiO2 in the hydrogel. Meanwhile, the polymerization of acrylic acid to poly-acrylic acid was revealed by FTIR analysis. It is obvious that the physical appearance of as-prepared PT hydrogels with different water amount was different, where a rigid hydrogel was obtained with less water amount and a spongy hydrogel was formed with more water amount. The highest self-healing efficiency achieved ~85% exhibited by PT-4 hydrogel in which the self- healing ability is due to hydrogen bond occurred between TiO2 and carboxylic functional groups in poly-acrylic acid. For the application, the healed PT-4 hydrogel demonstrated the capability to convert toxic 4-nitrophenol to 4-aminophenol through photocatalytic hydrogenation reaction within 2 h UV light irradiation.
Keywords: Self-healing hydrogel; poly-acrylic acid; TiO2; 4-nitrophenol; 4-aminophenol 1
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4 1. Introduction
Self-healing hydrogels are three-dimensional cross-linked polymeric networks with high water content and able to heal the damage parts autonomously which inspired by self- repairing system of living creatures. Self-healing hydrogels could perform the autonomous self-repairing based on either dynamic covalent bonds (e.g. Diels-Alder reaction1, coordination bond2, Schiff base3, boronate ester bond4, and disulfide bond5) or noncovalent interactions (e.g.
ionic interaction6-8, hydrogen bonds9, hydrophobic interaction10, and host-guest interaction11).
Self-healing hydrogels had been widely utilized particularly in biomedical related applications such as diabetic wound12, cartilage tissue engineering13, drug delivery14, and biosensors15.
Environmental contamination due to chemical pollutants remain as serious global problems which had gained considerable attention. In majority, the chemical pollutants are nitroaromatic organic compounds which are acutely toxic, low solubility in water and highly stable. For instance, 4-nitrophenol (4-NP) has been widely used as precursors for dyestuff, explosives, drugs, etc.)16. The exposure to 4-NP lead to serious health issues such as skin irritation, organ disfunction, etc. If marine creatures are exposed to this toxic chemical, it will bring the domino effect to human as well via the food chain. Therefore, several techniques were developed to eliminate such toxic chemicals from water bodies17. The well-known techniques are photocatalysis, electrocatalysis, adsorption, and Fenton reaction18-23. Adsorption method is less preferable because this method only creates another secondary waste which in the end, the generated waste would be dumped into the landfill. Meanwhile, electrocatalysis method is limited due to its low efficiency and poor durability that would be critical issue for scale up purposes. Therefore, photocatalysis is more preferable since it requires only sunlight to proceed the catalytic reaction on the catalysts24-26. Many photocatalysis works have been developed for reduction of 4-nitrophenol to 4-aminophenol27-29, however, most of the works only focus on the photocatalytic performances without considering the recyclability and 1
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reusability30. For instance, Shuwanto et. al. (2021) reported the fabrication of nanosized powder Ni-Mg codoped Zn(O,S) photocatalyst for environmental remediation purposes and hydrogen production under solar light irradiation31. Eventhough, the authors reported the superior photocatalytic performances, the after-used photocatalysts collection would be burdensome responsibilities in real-life practical application.
With this regard in mind, we are developing photocatalyst embedded in hydrogel with self-healing ability that can extend its lifetime for environmental remediation. In this study, we demonstrated a simple and facile preparation of poly-acrylic acid hydrogel embedded with TiO2 photocatalyst (PT hydrogel) synthesized by UV light irradiation without any cross-linking agents. The as-prepared hybrid catalyst was applied to reduce 4-NP to 4-AP and the characterizations and catalytic experiments are thoroughly elaborated in this work.
2. Experimental Methods 2.1.Materials
Acrylic acid (99.1%, Sigma Aldrich) and titanium dioxide (TiO2 anatase, 95%, Showa chemical) were analytical grade and used without further purification. DI water was obtained using Millipore filtration system with the resistance of 18.2 M.
2.2. Preparation of polyacrylic acid-TiO2 hybrid hydrogel
In a typical synthesis, 2 mL of acrylic acid was placed into a 12-well plate followed by adding a certain amount of DI water (1, 2, 3, 4, and 5 mL). Subsequently, 2 mg of TiO2 was added into the diluted acrylic acid solution and dispersed using an ultrasonicator (Branson M5800H) for 30 min. The dispersion was then irradiated by UV light for 45 min (360 nm, 20 W) with a distance of 5 cm between UV lamp and the dispersion. In this study, DI water amount was varied to study the cross-linking effect on the resulted polyacrylic acid-TiO2 hybrid 1
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hydrogels. The synthesis illustration was schemed in Fig. 1 and the synthesized hydrogels were named as PT-x, where x represents the amount of water in mL.
Fig. 1 Schematic illustration for PAA-TiO2 hydrogels prepared under UV light irradiation.
2.3. Characterizations of hydrogels
In this study, the surface morphology was observed using scanning electron microscopy (FE-SEM JEOL, JSM-7900F) at an operating voltage of 20 kV. The crystallinity of samples was determined by X-ray diffractometer (XRD, Bruker D2 phaser) using Cu K radiation (=
0.1514 nm). The functional groups in hydrogel samples was analyzed through fourier transform infrared (FTIR) spectroscopy using KBr pellet method. Moreover, the mechanical properties of hydrogels in terms of tensile stress () and strain was examined using Testometric (M500- 25AT) with a 100 N load cell and a crosshead speed of 50 mm/min, where the hydrogels were initially cut into a dog bone shape prior to the mechanical testing. The swelling degree of hydrogels was determined by allowing the hydrogels to swell in DI water at room temperature (35 2 C) until saturated swollen was achieved. Afterwards, the excess water on hydrogel surfaces was gently blotted and weighed as Ws, while, initial weight of hydrogels was Wi. The equilibrium swelling degree was calculated using Eq. 1. Accordingly, gel fraction of hydrogels was investigated to determine the cross-linking degree of hydrogels (insoluble parts). The 1
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saturated swollen hydrogels were subsequently dried and weighed as Wi’ and the gel fraction was further calculated using Eq. 2. Furthermore, the sell-healing efficiency of hydrogels was determined by rupturing the hydrogels off into two symmetrical parts, then, put the damaged parts in contact at room temperature (35 2 C) for 12 h for healing process. The healed samples was subjected to tensile test and the results were recorded as ’. Therefore, the self- healing efficiency could be calculated using Eq. 3.
SD= WsW- Wi
i 100% (1)
GF= WWi'
i 100% (2)
SHE= ′ 100% (3)
3. Results and Discussions
3.1.Surface morphology of PT hydrogels
In this study, the synthesis of PT hydrogel was occurred through several steps, first, the dispersion containing TiO2 and acrylic acid monomer was irradiated with UV light. Herein, radical species are generated as a result of TiO2 absorbing UV light. Accordingly, the radical species induce polymerization on acrylic acid to poly-acrylic acid. Second, the contained carboxylic functional groups in poly-acrylic acid react with TiO2 through the formed hydrogen bonds to yield cross-linking formation of hydrogel32.
The surface morphology of the as-synthesized PT hydrogels observed using SEM was shown in Fig. 2a-j. The hydrogel samples was firstly freeze-dried prior to SEM analysis. The surfaces of PT-1 and PT-2 hydrogels were rough and contain porosity (Fig. 2a-d). It is because the preparation of PT 1 and PT-2 hydrogels only applied a less amount of water (1 – 2 mL).
Meanwhile, there are no significant differences among PT-3, PT-4, and PT-5 hydrogels, where the hydrogels exhibit relatively smooth surfaces (Fig. 2e-j). Fig. 2k and 2l display the 1
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photographs of as-prepared PT hydrogels, where PT-1 and PT-2 hydrogels possessed relatively firm and transparent gel, meanwhile, PT-3, PT-4, and PT-5 hydrogels were found to be white gel with more springy (tofu-like) structure. On the other hand, there are no TiO2 particles on hydrogel surfaces indicating that TiO2 particles were probably embedded inside the PT hydrogels.
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.
3.2. Phase identification of PT hydrogels
To identify the phase structural and the TiO2 existence in PT hydrogels, XRD analysis was carried out by scanning from 10 – 70. Fig. 3a showed the diffractograms of PT hydrogels with varied water content. All PT hydrogels exhibit similar XRD patterns indicating the amorphous phase of PAA hydrogels with several peaks of TiO2 anatase. Those several peaks of TiO2
anatase are located at 2 of 25.3 (101), 37.1 (103), 38 (004), 38.8 (112), 48.1 (200), 54.2
(105), 55 (211), 62 (213), and 68.8 (116) matched with TiO2 anatase standard with JCPDS no. 21-1272.
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