Novel Superhydrophobic Aerogel on the Base of Polytetrafluoroethylene

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Surfaces, Interfaces, and Applications

Novel Superhydrophobic Aerogel on the Base of Polytetrafluoroethylene Sergey A. Baskakov, Yuliya V. Baskakova, Evgene N. Kabachkov, Nadezhda N. Dremova, Alexandre Michtchenko, and Yury Shulga ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10455 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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ACS Applied Materials & Interfaces

Novel Superhydrophobic Aerogel on the Base of Polytetrafluoroethylene Sergey A. Baskakov 1, Yulia V. Baskakova 1, Eugene N. Kabachkov 1,2, Nadezhda N. Dremova 1, Alexandre Michtchenko *,3 and Yury M. Shulga 1,4 1

Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka 142432, Moscow Region, Russian Federation

2

Chernogolovka Scientific Center, Russian Academy of Sciences, Chernogolovka 142432, Moscow Region, Russian Federation 3

Instituto Politécnico Nacional, SEPI-ESIME-Zacatenco, Av. IPN S/N, Ed.5, 3-r piso, Ciudad de Mexico, C.P. 07738, Mexico

4

National University of Science and Technology MISIS, Leninsky pr. 4, Moscow 119049, Russian Federation

ABSTRACT: Polytetrafluoroethylene-based aerogel was synthesized for the first time. Graphene oxide was used as a binder. After reduction with hydrazine and annealing at 370°C, the aerogel with a density of 29 ± 2 mg/cm3 became superhydrophobic. The aerogel was characterized by IR spectroscopy, scanning electron microscopy, and X-ray photoelectron

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spectroscopy. The sorption capacity of the aerogel for seven solvents and its sorption recyclability for hexane were measured.

KEYWORDS: aerogel, polytetrafluoroethylene, graphene oxide, super-hydrophobicity, IR spectroscopy, X-ray photoelectron spectroscopy, sorption capacity *Corresponding author. E-mail address: [email protected] (A. Michtchenko)

1. Introduction Aerogels are known since 1931.1 The unique properties of aerogels are formed due to their composition, mainly consisting of air. For example, widely known silica aerogels have a density of from 0.003 to 0.1 g/cm3 and are characterized by a high specific surface area (500– 1200 m2/g), a low dielectric constant (1.1–2.0), low thermal conductivity (0.013–0.14 W/m K), and superhydrophobicity.1–6 At the same time, we did not find a single publication about aerogels that included polytetrafluoroethylene (PTFE), which is known for its hydrophobicity. The reason for this may be that PTFE has a very low adhesion and is not wetted by most organic solvents. Therefore, obtaining aerogel based on PTFE is an interesting and technically challenging task. However, in the literature, we found several papers on the preparation of aerogels, which included fluorine-containing compounds. Graphene/polyvinylidene fluoride (G/PVDF) composite aerogel was reported, which was obtained by solvothermal reduction of mixed graphene oxide dispersions and PVDF.7 The contact wetting angle (CWA) for this aerogel was 152 degrees. It was shown that this type of aerogel is a promising material for cleaning up oil spills and extracting organic solvents. The article8 describes the synthesis of aerogels


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based on reduced graphene oxide by self-polymerization of dopamine. Aerogels are reinforced with chitosan and treated with 1H, 1H, 2H, 2H-perfluorodecanethiol. They are treated with a fluorine-containing compound, which showed high super-hydrophobicity (for some of the samples, CWA reached 163 degrees). An analysis of the literature has shown that graphene oxide (GO) is currently one of the most popular materials for creating aerogels.9–13 We also know that both hydrophilic and hydrophobic pores can simultaneously exist in reduced graphene oxide aerogel.14 PTFE is often used as a binder when forming electrodes from carbon materials. This information has allowed us to assume that while preparing PTFE-based aerogel, we can use graphene oxide as a binder. In this paper, we report on the successful synthesis of aerogel based on polytetrafluoroethylene (70%) and graphene oxide. After reduction with hydrazine and annealing at 370 °C, the aerogel with a specific weight of approximately 30 mg/cm3 became hydrophobic (CWA for some samples reached 163.7 degrees). The results of the aerogel studies, which obtained by means of IR spectroscopy, scanning electron microscopy and X-ray photoelectron spectroscopy, are furnished below. The material obtained showed high sorption properties with respect to such solvents as isopropanol, acetone and hexane. 2. Experimental 2.1. Materials Water suspension F-4D (Kirovo-Chepetsk Chemical Company (URALCHEM Group) and ТU 6-05-1246-81) were chosen as a source of PTFE. This suspension contained PTFE (molecular mass 140000–500000), surface-active compounds (a mixture of ethoxylated alkylphenols) as a stabilizer and water in the proportion 6:1:3.



We used a modified Hammers method15 and obtained graphite oxide (GO). The details of our graphene oxide production are described in the articles.16–17 The size of the graphene oxide sheets that we used in this work range from 0.2 to 3 nm. 2.2. Synthesis of the PTFE/rGO aerogel When preparing the aerogel, 30 ml of a water GO suspension with a concentration of 11 mg/ml were placed in a glass beaker and sonicated for 5 minutes. Next, 0.81 ml of a PTFE suspension was introduced in drops into the GO suspension, without ceasing the ultrasonic treatment. After the complete introduction of the PTFE-containing suspension, the ultrasonic treatment was continued for 5 more minutes. The resulting mixture was frozen on a special platform of copper cooled by liquid nitrogen. The platform was cooled to a temperature below -50°C and temperature control was carried out using an electronic thermometer, CENTER 307, equipped with a thermocouple type TR-K01. After cooling, a mold for casting cylindrical hydrogels of Ø20 mm and a height of 15 mm was installed on the platform and filled with its GO/PTFE suspension. The frozen hydrogel was removed from the mold and dried in a MartinChrist Alfa 1-2 DLPlus freeze dryer. The resulting aerogels had a density of 35 ± 2 mg/cm3. The recovery of GO in aerogel was carried out in hydrazine vapor, while the color of the aerogel from grey–brown was changed to black. To remove the surfactants present in the PTFE suspension, the resulting aerogel was annealed in a tubular quartz furnace in a stream of argon. The process of annealing was carried out in 2 stages. First, the sample was heated to T = 120°C to remove residual hydrazine and water and kept at that temperature for 20 minutes. At the next stage, the temperature was raised to 370°C and retained for 30 minutes. The aerogel density decreased to 29 ± 2 mg/cm3 because of this treatment. 2.3. Dry samples of F-4D suspension


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For certification of the composition and condition of the components, which are present in the suspension F-4D used by us, a dry powder (DP) was prepared. For this, 2 ml of F-4D suspension was dried in an oven in the air at T = 65°C for 24 hours. Next, samples of DP (argon370) and DP (air370) were obtained. For this, the white powder DP was annealed in a tubular quartz furnace at T = 370°C for 30 minutes in an atmosphere of argon and air, respectively. Three tablets were also made: T, T (argon370) and T(air370). The initial tablets T with a diameter of 13 mm were prepared by pressing the powder of the joint venture using oil press at a pressure of 11.8 N/mm2. Samples T (argon370) and T (air370) were obtained by annealing tablets T in a tubular quartz furnace at T = 370°С for 30 minutes in an atmosphere of argon and air, respectively. 2.4. Characterization and measurement The IR spectra of the samples were obtained in the range of 400 to 4000 cm-1 using a Simadzu Fourier spectrometer with an ATR attachment. The contact water-wetting angle was measured on an OCA 20 instrument (Data Physics Instruments GmbH, Germany) at room temperature. Electron micrographs were obtained on a JEOL JSM-5910LV scanning electron microscope (electron energy 20 kV, chamber pressure 2 × 10-5 Pa). The XPS spectra were obtained using a Specs PHOIBOS 150 MCD electron spectrometer and an X-ray tube with an Mg anode (hν = 1253.6 eV). The vacuum in the spectrometer chamber did not exceed 4×10-8 Pa. 3. Results and discussion 3.1. Appearance Figure 1 shows photographs of the aerogel before and after all treatments. We can see that the samples obtained by us had a form close to that of a cylinder. The deviation from the cylindrical shape was that the sample diameter in the middle part was slightly smaller than



that of the ends. Moreover, in appearance, the flat ends seemed denser than the lateral surface. Further, for this reason, when describing spectral or other data, it is indicated from which part of the sample the material was taken for the study.

Figure 1. Photographs of the aerogel before (A) and after reduction with subsequent annealing (B). 3.2. IR spectra The most intense in the IR spectrum of the DP sample (Figure 2A, curve 1) are the bands due to stretching vibrations (ν) of C-F bonds (1201 and 1146 cm-1).18 The bands in the low-frequency region of the spectrum (638, 554, and 501 cm-1) are associated with the wagging, deformation, and rocking oscillations of CF2 groups. The presence of a stabilizer in the PTFE suspension appears primarily as a ν(C–H) in the range of 2955 to 2872 cm–1. The aerogel spectrum (Figure 2A, curve 2) differs from the spectrum of the DP by its inclined background, which we associate with the fact that the aerogel, in contrast to the DP, is a conducting material. In the aerogel spectrum, all PTFE bands are present, but bands due


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to the stretching vibrations of C–H bonds almost completely disappear. We associate the wide band with a maximum at 1513 cm–1 with the presence of reduced graphene oxide in the sample.

Figure 2. IR spectra of the samples: A - DP (1) and aerogel (2); B – T (argon370) (1) and T(air370) (2). A comparison of the spectra of T (argon370) and T (air370) tablets (Figure 2B) indicates that annealing in the air is more effective for removing hydrocarbon surfactant fragments from a sample (decreases bands in the range of 2955 to 2872 cm-1). However, 30 minutes of annealing in the air is not enough to remove the stabilizer completely from the tablet, since it remains black. It is to be noted that if the powder is annealed in the air, and not a tablet, the color of the powder during annealing changes in time from white to black and again to white. Moreover, this transformation completes only during long-term (more than 5 hours) annealing.



3.3. XPS Figure 3A shows a survey XPS spectrum of the interior of the aerogel. In addition to the main peaks due to fluorine and carbon, peaks from oxygen, nitrogen and silicon can also be seen on the spectrum. The chemical composition of the surface, calculated using the integral intensities of the analytical lines (marked in the figure), corresponds to the formula C1.00F0.52O0.03N0.007Si0.014. The presence of oxygen in the sample means that either the reduction procedure described in the experimental part does not allow the complete reduction of the graphene oxide or storing the sample in the air is accompanied by its partial oxidation. Nitrogen, as is well known in the literature, remains in graphene oxide samples after its reduction with hydrazine.19 As for silicon, its presence may be associated with an impurity in the original graphite. Information on the state of the atoms in the sample under study can be obtained from an analysis of the fine structure its XPS spectra. The spectrum of F1s is represented by one rather narrow peak (Еb = 689.4 eV, the full width at half maximum (FWHM) is equal to 1.8 eV). In principle, this peak can be used for spectrum calibration. However, the sample has good conductivity. Therefore, here and further, the positions of the peaks are provided without any calibration. The spectrum of С1s (Figure 3B, Table 1) is dominated by two peaks (peaks 2 and 6) at Eb 292.2 and 284.6 eV, which are due to the carbon atoms of PTFE and reduced graphene oxide (rGO), respectively. In the interval between these peaks, peaks 4 and 5 are distinguished in the spectrum. These peaks can be attributed to the oxygen-containing functional groups of the rGO. Peak 1 is naturally associated with the carbon atom of the CF3 groups.


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Figure 3. A - survey XPS spectrum of the aerogel; B – C1s XPS spectrum of the aerogel. Table 1. C1s spectrum and its decomposition Peak

Binding energy, eV

Relative intensity, %

1

293.4

3.63

2

292.2

29.23

3

287.7

0.74

4

286.7

2.34

5

285.8

9.49

6

284.6

54.57

As can be seen from Figure 3, the peaks related to carbon atoms in PTFE and rGO are energetically separated. From the data on the relative intensities of the peaks (Table 1), it is



possible to easily calculate the polymer content in the aerogel in the region of XPS analysis. It is for 68% mass, which is close to the value that was laid in the synthesis of aerogel (70% mass). 3.4. SEM The morphology features of the flat surface of the aerogel can be seen in its photograph obtained using scanning electron microscopy (Figure 4A). The characteristic dimensions of the protruding particles are in the range of 0.1 to 1.0 μm. In shape, some protruding particles resemble tubes with jagged edges. It is most likely that a polymer forms these tubes.

Figure 4. A - Image of superhydrophobic aerogel surface (cylinder end) obtained by SEM. Inset: A photograph of a drop of water on the aerogel surface; B - SEM image of the material from the inside of the aerogel.


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In the inner part of our aerogel, there are areas consisting of sheets of salad arranged in the same direction. We note here that the shape of these sheets differs from the well-known form of graphene oxide nanosheets.20–23 The dimensions of the sheets, which can be seen in the figure, reach 200 microns. Obviously, the sheets are formed with the participation of the polymer. Such structures for PTFE were not previously described. 3.5. CWA The contact wetting angle (CWA) for sample T is only 69 degrees, which is not surprising since PTFE particles in dry suspension are coated with a surfactant. For the samples T2 and T3, CWA are 111 and 112 degrees, respectively. In the case of aerogel, obtained in the form of a cylinder (Figure 1), only the ends are flat surfaces. Therefore, measurements were carried out only on these surfaces. CWA value for these surfaces (Figure 4A, inset) was in the range of 161.9 to 163.7 degrees [average contact angle {(∑10 𝑖=1 𝐶𝑊𝐴(𝑖))/10} is equal to 162.2 degrees]. Silica aerogels, which are generally fabricated through a supercritical drying process, have contact angles ranging from 100 to 150 degrees.24–27 Graphene aerogels, which are obtained via high temperature treatment (1050oC) of graphene oxides aerogels, are hydrophobic with WCA between 110 and 130 degrees.27 Graphene aerogels can become superhydrophobic (150–160 degrees) only following the application of a fluorinated surfactant. Superhydrophobic (WCA = 151.1–153.9 degrees) graphene aerogel was made by facile chemical reduction.28 Perhaps, the remnants of the chemical reducing agent partially affected its properties. The poly (vinylidene fluoride) (PVDF) aerogel is superhydrophobic with a water contact angle of 151 degrees.29 3.6. On the properties of the aerogel as a sorbent



It was interesting to check the sorption properties of the aerogel with respect to other solvents. The data we obtained are summarized in Table 2. Usually, the literature uses the concept of “absorption capacity” or the parameter Qw {Qw = (Wm-Wd)/Wd, where Wd is the dry aerogel weight and Wm is the maximum weight of the aerogel with solvent}.30 According to this parameter, the aerogel obtained by us is inferior to many aerogels described in the literature,30–34 since the polytetrafluoroethylene used by us has a large molecular weight (see above) and retains the supramolecular structure. However, if we turn to such a parameter as Qv {Qv = Va/V0, where V0 is the volume of the original aerogel, Va is the volume of solvent sorbed by the aerogel}, then our aerogel is not inferior to the aerogel record-holders, and sometimes even surpasses them.

Table 2. Aerogel sorption properties Sorbate

Qw* (%)

Qv* (%)

i-propanol

2361

95.9

acetone

2315

94.7

hexane

1897

92.5

tetrahydrofuran

2323

82.9

petroleum ether

1615

78.8

toluene

1552

56.6

1,2dichlorobenzene

254

6.2

*For designation, see the text. For example, an ultralight aerogel was obtained from reduced graphene oxide (its density was only 5.0 mg/cm3).30 The absorption capacity of this aerogel for hexane was 100–107 g/g. From the data, it is easy to calculate that a cubic centimeter of their aerogel contains 500–


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535 mg of hexane. Consequently, the fraction of the volume of this aerogel, which is filled with a solvent, is 0.769–0.823, which is less than this parameter for our aerogel (0.925). We believe that the parameter Qv introduced by us is not inferior to the parameter Qw in its information content. From the data of Table 2, the solvents we studied by the parameter Qv can be divided into 3 groups. The first group includes isopropanol, acetone and hexane, which almost completely fill the free volume of aerogel. The second group (petroleum ether and tetrahydrofuran) can fill 80% of the aerogel volume. The third group includes solvents which occupy less than 60% of the volume of aerogel. The stability of the aerogel to sorption-desorption cycles was tested on two samples, which differed in heat treatment conditions after reduction with hydrazine vapor. The PTFE-rGO(Ar) aerogel was heat treated in a tubular quartz furnace in an argon flow at 370°C for 1 hour. The sample PTFE-rGO(air) was annealed in the air under the same conditions. It is seen (Figure 5) that the sorption capacity of the sample PTFE-RGO(air) exceeds that of the sample PTFE-RGO(Ar) up to the 7th cycle. However, after the 7th cycle, its sorption capacity becomes almost the same. In general, we should note the high resistance of the aerogel obtained by us to cyclic loading with solvents.



3

Weight, g

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2 weigth of dry PTFE-rGO(Ar) weight of PTFE-rGO(Ar) after hexane sorbtion weigth of dry PTFE-rGO(Air) weight of PTFE-rGO(Air) after hexane sorbtion

1

0 0

2

4

6

8

10

Cycle number Figure 5. The sorption recyclability of the aerogel for hexane. Conclusions In summary, we first report on the synthesis of superhydrophobic aerogel based on polytetrafluoroethylene. An aqueous suspension of PTFE stabilized with ethoxylated alkylphenols was used as base component for the synthesis, and a graphene oxide was used as a binder. The aerogel reduced with hydrazine and annealed at 370°C become superhydrophobic (the CWA value was in the range of 161.9 to 163.7 degrees). Both the multi-scale structure and chemical groups with a low surface energy (CF2 and CF3) of the aerogel contributed to its super-hydrophobicity. The sorption capacity of the aerogel for the same solvents were also measured. It was established that isopropanol, acetone and hexane almost completely fill the free volume of aerogel. The high resistance of the aerogel to cyclic loading with solvents was also demonstrated.


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Conflicts of interest There are no conflicts to declare.

Acknowledgements This work has been performed in the frame of the state task of the Russian Federation (state registration number 0089-2019-0008), using the equipment of the Multi-User Analytical Center of IPCP RAS and the Centre of Collective Usage of NUST MISIS. This study was partially carried out with the use of resources of Competence Center of National Technology Initiative in IPCP RAS.

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(28) Xu, L.; Xiao, G.; Chen, C.; Li, R., Mai, Y.; Sun, G.; Yan, D. Superhydrophobic and superoleophilic graphene aerogel prepared by facile chemical reduction J. Mater. Chem. A 2015, 3 (14), 7498-7504. (29) Chen, X.; Liang, Y.N.; Tang, X.-Z.; Shen, W.; Hu, X. Additive-free Poly (vinylidene fluoride) Aerogel for Oil/water Separation and Rapid Oil Absorption Chem. Eng. J. 2017, 308 (1), 18-26. (30) Xu, L.; Xiao, G.; Chen, C.; Li, R.; Mai, Y.; Sun, G.; Yan, D. Superhydrophobic and Superoleophilic Graphene Aerogel Prepared by Facile Chemical Reduction. J. Mater. Chem. A Mater. Energy Sustain. 2015, 3 (14), 7498–7504. (31) Bi, H.; Xie, X.; Yin, K.; Zhou, Y.; Wan, S.; He, L.; Xu, F.; Banhart, F.; Sun, L.; Ruoff, R. S. Spongy Graphene as a Highly Efficient and Recyclable Sorbent for Oils and Organic Solvents. Adv. Funct. Mater. 2012, 22 (21), 4421–4425. (32) Wang, J.; Shi, Z.; Fan, J.; Ge, Y.; Yin, J.; Hu, G. Self-Assembly of Graphene into ThreeDimensional Structures Promoted by Natural Phenolic Acids. J. Mater. Chem. 2012, 22 (42), 22459–22466. (33) Wu, T.; Chen, M.; Zhang, L.; Xu, X.; Liu, Y.; Yan, J.; Wang, W.; Gao, J. ThreeDimensional Graphene-Based Aerogels Prepared by a Self-Assembly Process and Its Excellent Catalytic and Absorbing Performance. J. Mater. Chem. A Mater. Energy Sustain. 2013, 1 (26), 7612–7621.



(34) Huang, X.; Sun, B.; Su, D.; Zhao, D.; Wang, G. Soft-Template Synthesis of 3D Porous Graphene Foams with Tunable Architectures for Lithium--O 2 Batteries and Oil Adsorption Applications. J. Mater. Chem. A Mater. Energy Sustain. 2014, 2 (21), 7973–7979.


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Figure 1. Photographs of the aerogel before (A) and after reduction with subsequent annealing (B).



Figure 2. IR spectra of the samples: A - DP (1) and aerogel (2); B – T (argon370) (1) and T(air370) (2).


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Figure 3. A - survey XPS spectrum of the aerogel; B – C1s XPS spectrum of the aerogel.



Figure 4. A - Image of superhydrophobic aerogel surface (cylinder end) obtained by SEM. Inset: A photograph of a drop of water on the aerogel surface; B - SEM image of the material from the inside of the aerogel.


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3

Weight, g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 weigth of dry PTFE-rGO(Ar) weight of PTFE-rGO(Ar) after hexane sorbtion weigth of dry PTFE-rGO(Air) weight of PTFE-rGO(Air) after hexane sorbtion

1

0 0

2

4

6

Cycle number Figure 5. The sorption recyclability of the aerogel for hexane.


8

10

Novel Superhydrophobic Aerogel on the Base of Polytetrafluoroethylene

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