Development of superamphiphobic alumina nanofiber mats using trimethoxysilane with a short perfluoroalkyl chain

Abstract

To avoid the generation of hazardous, long-chain perfluoroalkyl carboxylic acids (C _n F₂ _n ₊₁COOH, n ≥ 7), we develop relatively safer superamphiphobic alumina nanofiber mats. Our fabrication process focuses on two principles: lowering the surface energy using trimethoxy(1H, 1H, 2H, 2H-nonafluorohexyl)silane (C₄F₉CH₂CH₂Si(OCH₃)₃), which has short-chain perfluoroalkyls that are relatively safer than long-chain ones; and creating a high-roughness surface from electrospun alumina nanofibers with an average fiber diameter of 155 nm and inter-fiber spacing of 451 nm. Such mats exhibit super-repellency for water (contact angle $θ^{*}$ = 157°, contact angle hysteresis $Δ θ^{*} = 2^{\circ}$ , advancing angle $θ_{adv}^{*} =$ 158°, receding angle $θ_{rec}^{*} =$ 156°), and n-hexadecane ( $θ^{*}$ = 151°, $Δ θ^{*} =$ 9°, $θ_{adv}^{*} =$ 152°, $θ_{rec}^{*} =$ 143°). Furthermore, superamphiphobicity is maintained up to 350℃.

Keywords

superamphiphobicity alumina nanofibers safety perfluoroalkylsilane short fluoroalkyl chain

Superamphiphobic surfaces exhibit both water and oil repellency, with contact angles $θ^{*}$ greater than 150° and contact-angle hysteresis $Δ θ^{*}$ less than 10°. Such surfaces have wide commercial applications, including in products for self-cleaning, anti-corrosion, and filtration.^1–6 There is no naturally occurring superamphiphobic surface.⁷ It is therefore necessary to prepare such surfaces artificially based on two key principles: decreasing surface energy and increasing surface roughness.⁸

To reduce surface energy, almost all traditional superamphiphobic surfactants consist of long-chain perfluoroalkyls (R^F, C _n F₂ _n _+ 1–; n refers to the perfluorinated carbon number ≥ 7), due to their highly crystallized structure and hydro- and oleophobic nature.⁹ However, such R^F chains inevitably degrade into hazardous long-chain perfluoroalkyl carboxylic acids (PFCAs, C _n F₂ _n ₊₁COOH, n ≥ 7).¹⁰ One typical long-chain PFCA, perfluorooctanoic acid (PFOA, C₇F₁₅COOH), is seriously harmful to people and the environment, owing to its extreme persistence, bioaccumulation, and toxicity.^11–13 Thus, the phase-out of long-chain PFCAs and their precursors has been underway since the US Environmental Protection Agency (EPA) initiated the 2010/15 PFOA Stewardship Program.¹⁴ As a result, there is an urgent need to replace traditional noxious super-repellent surfactants with alternatives that are securer and simultaneously oil-repellent. Since non-fluorinated alternatives lack oil-repellency, fluorinated substitutions are essential for superamphiphobicity.¹⁵ Short-chain PFCAs have been found to be relatively safer than long-chain ones in hazard assessments.¹⁶ Hence, surfactants with a shorter chain R^F (C _n F₂ _n _+1–, n < 7) could be a reasonable substitute. However, short-chain R^F is non-crystallized, leading to limitations that make super-repellency difficult to realize.¹⁷

To overcome this difficulty, we utilized the other principle that boosts the repellency of superamphiphobic surfaces, that is, enhanced roughness. Electrospinning (ES) is an efficient way to achieve high roughness.^18–22 Pan et al.²³ created a superamphiphobic chemical shield by ES of fluorodecyl polyhedral oligomeric silsesquioxane. Ganesh et al.²⁴ created a super-repellent TiO₂ surface using ES with modifications involving (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane. Both studies, however, used the long-chain R^F that is likely to degrade into toxic PFCAs. Meanwhile, little attention has been paid to the formation of relatively safer superamphiphobic surfaces with short-chain perfluoroalkyl surfactants.

In the textile field, alumina nanofibers (ANs) are considered the most promising candidate for high-temperature air filtration and wastewater treatment filtration, due to the advantages of their alumina material and nanofiber structure.^25–29 Alumina material is both cheap and thermally and chemically resistant, as well as abundant in active hydroxyl groups.^30,31 The nanofiber structure can provide high capture efficiency and low pressure drop for high-efficiency particulate filtration, due to its inherent high porosity and large specific surface area.^32,33 Meanwhile, the development of superamphiphobic ANs has generated fruitful research linking the self-cleaning and filtration fields.^34–36 Hayase et al.³⁷ created a superamphiphobic AN surface for crude oil antifouling applications using supercritical drying to achieve a rough substrate, and then reducing the surface energy by chemical vapor deposition with (1H, 1H, 2H, 2H-heptadecafluorodecyl)-trimethoxysilane, which are toxicological long-chain perfluorinated compounds. Zhang et al.³⁸ fabricated amphiphobic alumina microfiber mats (water $θ^{*}$ = 134 $^{\circ}$ and oil $θ^{*}$ = 122 $^{\circ}$ ) for anti-corrosion applications, but the fabrication process used hazardous long-chain perfluoroalkylsilanes (FAS), that is, (heptadecafluoro-1, 1, 2, 2-tetradecyl)trimethoxysilane.

To the best of our knowledge, no study has yet investigated the formation of relatively safer superamphiphobic ANs. Here, we increased the relative safety of super-repellent AN fabrication in two ways: firstly, we used water instead of hazardous or flammable solutions as a solvent and, secondly, short-chain perfluorinated compounds (the perfluorinated carbon number is 4) were used to reduce the risk to humans and wildlife.

ANs can easily be prepared by ES.^39–41 We previously fabricated well-formed ANs by heating electrospun composite nanofibers containing poly(vinyl alcohol) (PVA)/boehmite nanoparticles [AlO(OH)ċH₂O].⁴² FAS have become popular surfactants, given that they are commercially available and have high reaction activity in silanization.⁴³ Thus, in this study we utilize electrospun AN mats as a high-roughness surface and short-chain FAS as a surfactant to develop relatively safer superamphiphobic alumina nanofibrous mats for high-temperature air filtration.

Experimental details

Materials

PVA (degree of polymerization: 1500) was purchased from Wako Pure Chemical Industries, Ltd, Japan. Boehmite nanoparticles (AlO(OH)·H₂O, dispersed particle size, 25 nm) were a kind gift from Sasol Chemicals, Japan KK, Japan. In all, seven testing liquids with different surface tension (

γ_{LV}

) were used in the experiments (Table 1). Distilled water was produced with an automatic water distillation apparatus (GS-200; Advantec Toyo Kaisha, Ltd, Japan). Diiodomethane (CH₂I₂), tetradecane, and dodecane were purchased from Nacalai Tesque, Inc., Japan. Olive oil and ethylene glycol ((CH₂OH)₂) were purchased from Wako Pure Chemical Industries, Ltd, Japan, while n-hexadecane was purchased from Tokyo Chemical Industry Co., Ltd, Japan.

Table 1.

Surface tension of testing liquids⁶

Liquids	Water	Diiodomethane	Ethylene glycol	Olive oil	n-Hexadecane	Tetradecane	Dodecane
$γ_{LV}$ (mN/m)	72.1	50.9	47.3	32	27.3	26.5	25.4

Three different surfactants were obtained from Tokyo Chemical Industry Co., Ltd, Japan: trimethoxy(3, 3, 3-trifluoropropyl)silane (FAS-1, CF₃CH₂CH₂Si(OCH₃)₃), trimethoxy(1H, 1H, 2H, 2H-nonafluorohexyl)silane (FAS-4, C₄F₉CH₂CH₂Si(OCH₃)₃), and trimethoxy(1H, 1H, 2H, 2H-heptadecafluorodecyl)silane (FAS-8, C₈F₁₇CH₂CH₂Si(OCH₃)₃). In FAS-n, FAS refers to perfluoroalkylsilane and n indicates the number of perfluorinated carbon atoms. FAS surfactants were stored under an argon atmosphere to prevent auto-polymerization before use. Table 2 compares surfactants and relative degradation product PFCAs, as well as serum elimination half-lives (t_1/2) of PFCAs in humans. Degradation from n:2 fluorotelomer material (C _n F₂ _n ₊₁CH₂CH₂-R) to PFCAs (C _n F₂ _n ₊₁COOH) occurs with an intermediate product, n:2 fluorotelomer alcohol (C _n F₂ _n ₊₁CH₂CH₂-OH).^15,44,45 As for PFCAs, long-chained perfluorononanoic acid (C₈F₁₇COOH) has a serum elimination half-life of 8.5 years. However, for short-chain perfluoropentanoic acid (C₄F₉COOH) and trifluoroacetic acid (CF₃COOH), the serum elimination half-life is only 28 days and 68 h, respectively. Therefore, short-chain PFCAs are less bioaccumulative in humans than long-chain homologues, that is, FAS-1 and FAS-4 are relatively safer than FAS-8.

Table 2.

Surfactants, modified surfaces of alumina nanofibers, degradation product perfluoroalkyl carboxylic acids (PFCAs) from surfaces, and serum elimination half-lives (t_1/2) of PFCAs in humans^46–48

Surfac-tants	Degradation product PFCAs	Serum elimination half-lives of PFCAs
FAS-1	CF₃COOH	t_1/2 < 68 h ± 35 h⁴⁶
FAS-4	C₄F₉COOH	t_1/2 < 28 days⁴⁷
FAS-8	C₈F₁₇COOH	t_1/2 > 8.5 years⁴⁸

FAS: perfluoroalkylsilanes.

Formation of PVA/boehmite precursor nanofibers and alumina nanofibers

Water-dispersible boehmite nanoparticles were added to 10 wt.% PVA aqueous solution at PVA/Alumina =70/30 wt.% with moderate agitation at room temperature for 30 min, resulting in a homogeneous PVA/boehmite precursor solution. During ES, the precursor solution was loaded in a plastic syringe with a 0.4-mm needle diameter, and electrospun at a feeding speed of 1 ml/h, 20-kV voltage, and 150-mm collection distance. Two kinds of collectors, a square copper plate (size 150 mm × 150 mm) and a rotating drum collector (diameter 160 mm, rotation speed 4000 rpm), were used to obtain isotropic and aligned PVA/boehmite precursor nanofibers, respectively. In order to create inorganic ANs, precursor nanofibers were heated in an electric furnace (Nitto Kagaku Co., Ltd, NPCTD3, Japan) at 500℃ for 5 h in air, with a heating rate of 10℃/min.

Formation of smooth alumina film

PVA/boehmite precursor solution was spread on a glass plate and dried at 40℃ for 12 h, followed by calcination at 500℃ for 5 h to eliminate organic components.

Surface modification of alumina nanofibrous mats

AN mats were dipped into 3 wt.% FAS of hexane solution for 10 h at room temperature, washed with methanol to remove residual FAS, and finally heated at 80℃ for 1 h, and 100℃ for 1 h. In order to prevent auto-polymerization of FAS, an argon atmosphere was used to protect the initial dipping step during surface modifications. FAS with labile methoxy group (CH₃O-) easily reacted with the hydroxyl groups (HO-) of ANs to form covalent linkages with concomitant loss of methanol, enabling the development of an amphiphobic AN surface. In this paper, AN/FAS-n refers to AN mats modified by FAS.

Instrumentation

Morphological images of nanofiber samples were captured by a scanning electron microscope (SEM; VE-9800, Keyence, Japan). Fiber diameter and relevant standard deviation were calculated using image-processing software (Image J). The infrared spectrum was carried out with Fourier transform infrared spectroscopy (FT-IR; Shimadzu, IRAffinity-1, Japan) in the mode of attenuated total reflectance (ATR), and collected from 4000 to 400 cm^–1. Inter-fiber spacing data were calculated by an advanced capillary flow porometer (CFP-1200AEXLTC; Porous Materials, Inc., USA). The thermogravimetry of AN/FAS-n was analyzed in air from room temperature to 600℃ with a thermal analysis device (DTG-60; Shimadzu, Japan).

Contact-angle measurements (static, advancing, receding) were obtained with video optical contact-angle equipment (VCA Optima-XE; AST Products, Inc., USA). The testing temperature and humidity were kept at 25℃ and 50%, respectively. Each contact-angle value was the average of five drops at different points, and droplet volume was set to $5 μ l$ . Advancing angles ( $θ_{adv}^{*}$ ) and receding angles ( $θ_{rec}^{*}$ ) were measured by video optical contact-angle equipment, together with the dynamic image-capture function. This measurement method was based on changing the volume of droplets with the same three-phase line.⁴⁹ A syringe pump yielded a water droplet on the test surface, and controlled the speed at which water was added and withdrawn through the needle to perform the advancing and receding movement, respectively. After forming a water droplet, water was slowly and continuously added to/withdrawn from the droplet at a velocity of 0.03 ml/min. When the volume was dynamically increased, the drop reached its maximum volume without raising the three-phase line (i.e., $θ_{adv}^{*}$ ). The drop volume was then reduced until the minimum volume was reached without lowering the three-phase line (viz., $θ_{rec}^{*}$ ). At the same time, $θ_{adv}^{*}$ and $θ_{rec}^{*}$ were recorded in corresponding frames by the dynamic image-capture function. The contact-angle hysteresis ( $Δ θ^{*}$ ) represents the difference between $θ_{adv}^{*}$ and $θ_{rec}^{*}$ .

Results and discussion

Surface morphology of alumina nanofiber mats before surface modification

Figure 1 shows the fiber morphology and diameter distributions of precursor PVA/boehmite nanofibers, and corresponding ANs calcinated for 5 h at 600℃. In order to compare their textures, two types of fiber orientation were prepared, isotropic and aligned. Differing fiber orientation is due to different collectors during ES. The plate collector accumulated isotropic fibers, while the rotating drum collector with a rotation speed of 4000 rpm gathered aligned fibers. All fibers were well formed with three-dimensional (3D) structures. Compared to precursor nanofibers, the diameter of AN fibers were shrunken due to the sintering effect. Furthermore, ANs exhibited rough and reentrant textures, leading to the necessary robust Cassie–Baxter state for the design of a superamphiphobic surface.⁵⁰ The average fiber diameter for isotropic and aligned AN mats were 155 and 230 nm, respectively. According to the Cassie–Baxter equation, to calculate the theoretical contact angle it is necessary to obtain not only the fiber diameter, but also the inter-fiber spacing.⁵¹ Therefore, the inter-fiber spacing of AN mats was measured with a capillary flow porometer. The measures were 451 nm for isotropic angles and 720 nm for aligned angles.

Figure 1.

Scanning electron microscope images of: (a) isotropic and (c) aligned electrospun precursor poly(vinyl alcohol)/boehmite nanofibers; (b), (d) corresponding alumina nanofibers after calcination for 5 h at 600℃; (e), (g) histograms of fiber diameter distribution for precursors; (f), (h) for alumina. AN: alumina nanofiber.

Superamphiphobicity of fluoroalkoxysilane modified alumina nanofiber mats

Composition changes after modification

Figure 2 presents the FT-IR spectrum of ANs and FAS-modified ANs. Bands between 1000 and 500 cm^–1 can be attributed to Al-O vibrations.⁵² After modification with FAS, new peaks at 1400–1000 cm^–1 result from fluoroalkyl groups. In particular, the band at 1209 cm^–1 indicates symmetric stretching of the -CF₂- moiety, while that at 1146 cm^–1 is due to the stretching vibrations of the -CF₃ group.⁵³ Therefore, FAS modifications are successful in grafting fluoroalkyl groups onto ANs.

Figure 2.

Fourier transform infrared spectra of pure alumina nanofiber (AN) and AN modified with different perfluoroalkylsilanes (FAS).

The effect of perfluoroalkyl chain length on amphiphobicity

Before surface modification, AN mats possess capillary structures and many superhydrophilic hydroxyl groups, leading them to absorb water and oil quickly, that is, water ( $γ_{LV} =$ 72.1 mN/m, $θ^{*}$ = 0°) and n-hexadecane ( $γ_{LV} =$ 27.3 mN/m, $θ^{*}$ = 0°). In order to decrease the surface energy of AN mats, we selected three kinds of FAS with different perfluoroalkyl chain lengths. In addition, seven kinds of testing liquid with various surface tensions were used to measure contact angles and contact-angle hysteresis.

As shown in Figure 3, for the same testing liquid, contact angles increased in the order FAS-1 < FAS-4 < FAS-8, because the surface tension of modifiers declined in the sequence FAS-1 (33.5 mN/m) > FAS-4 (23 mN/m) > FAS-8 (16.6 mN/m).⁵⁴ For the same surface, when the liquid surface tension ( $γ_{LV}$ ) decreases, contact angles decrease and contact-angle hysteresis increases. This is because the lower the $γ_{LV}$ of a liquid, the easier it can spread on a solid surface.

Figure 3.

Surface contact angles and testing liquids in alumina nanofiber (AN)/FAS-1 mats, AN/FAS-4 mats, and AN/FAS-8 mats (inset contains photographs of testing liquid droplets on AN/FAS-4 mats; from left to right: water, diiodomethane ethylene glycol, olive oil, n-hexadecane, tetradecane, and dodecane). FAS: perfluoroalkylsilanes.

As expected, AN/FAS-8 displayed excellent superamphiphobicity to water ( $γ_{LV} =$ 72.1 mN/m, $θ^{*}$ = 156°, $Δ θ^{*} =$ 2°, $θ_{adv}^{*} =$ 157°, $θ_{rec}^{*} =$ 155°) and tetradecane ( $γ_{LV} =$ 26.5 mN/m, $θ^{*}$ = 151°, $Δ θ^{*} =$ 5°, $θ_{adv}^{*} =$ 153°, $θ_{rec}^{*} =$ 148°); meanwhile, drops were able to roll over the surface. However, R^F groups (C₈F₁₇-) on AN/FAS-8 surfaces are prone to degrade into poisonous perfluorononanoate (C₈F₁₇COOH), which has a more than 8.5-year serum elimination half-life in humans. It is noteworthy that AN/FAS-4 displayed great super-repellence to water ( $γ_{LV} =$ 72.1 mN/m, $θ^{*}$ = 157°, $Δ θ^{*} =$ 2°, $θ_{adv}^{*} =$ 158°, $θ_{rec}^{*} =$ 156°) and n-hexadecane ( $γ_{LV} =$ 27.3 mN/m, $θ^{*}$ = 151°, $Δ θ^{*} =$ 9°, $θ_{adv}^{*} =$ 152°, $θ_{rec}^{*} =$ 143°), but AN/FAS-4 absorbs dodecane drops quickly rather than repelling them like AN/FAS-8. This is because R^F chains on AN/FAS-4 are less closely packed and orientated than those on AN/FAS-8.¹⁵ AN/FAS-1, on the other hand, cannot be superamphiphobic, as the R^F chains are too short and scattered to form dense packing.

The effect of surface morphology on amphiphobicity

Table 3 shows the effect of surface morphology on amphiphobicity for three different texture surfaces: isotropic and aligned AN mats, and alumina film. After modification with FAS-4, fibrous mats yielded better repellency than film, with the isotopic mats demonstrating higher anti-wetting characteristics than the aligned ones.

Table 3.

Surface-contact angles and contact-angle hysteresis of different liquids on different texture surfaces modified by FAS-4

	Water		Ethylene glycol		n-Hexadecane
	$θ^{*}$ (°)	$Δ θ^{*}$ (°)	$θ^{*}$ (°)	$Δ θ^{*}$ (°)	$θ^{*}$ (°)	$Δ θ^{*}$ (°)
Isotropic AN/FAS-4	157 ± 1.9	2	154 ± 2.2	5	151 ± 1.4	9
Aligned AN/FAS-4	152 ± 2.1	5	149 ± 2.1	11	122 ± 3.6	24
Alumina film/FAS-4	115 ± 1.7	21	102 ± 4.1	33	48 ± 4.1	52

AN: alumina nanofiber; FAS: perfluoroalkylsilanes.

The Cassie–Baxter state is the preferred explanation for amphiphobic surfaces, in which small air pockets are trapped under liquid droplets, resulting in a composite solid–liquid–air interface.⁵⁵ The formula is as follows

\cos θ^{*} = f_{SL} \cos θ + f_{LV} \cos π = f_{SL} \cos θ - f_{LV}

(1)

Here $, θ^{*}$ is the apparent contact angle, and θ is the contact angle on smooth film; $f_{LV}$ represents the area ratio of the air–liquid interface, $f_{SL}$ represents the area ratio of the solid–liquid interface, and $f_{SL}$ + $f_{LV}$ = 1.

As the water-contact angle of smooth alumina film θ is 115°, and according to Equation (1), the $f_{LV}$ of isotropic and aligned mats are 85% and 81%, respectively, isotropic mats comprise more air pockets beneath the water droplets than aligned mats. In contrast with fibrous mats, film possesses no air pockets (f_LV = 0%), resulting in inferior hydrophobicity (115°), which is in accord with reports that the maximum water-contact angle on a smooth surface is about 120°.⁵⁶ Therefore, the isotropic AN was selected for the preparation of a high-performance superamphiphobic surface.

In isotropic fibrous mats, fiber radius (R) and inter-fiber spacing (two-dimensional (2D)) are key factors in contact angles ( $θ^{*}$ ), requiring a rearrangement of the Cassie–Baxter equation into a more advanced form,^57,58 as shown below

\cos θ^{*} = - 1 + \frac{R}{(R + D)} [(π - θ) \cos θ + \sin θ]

(2)

For isotropic AN/FAS-4 mats, the calculated contact angle from Equation (2) is 156°, which is very close to the experimental contact angle of 157°, indicating that the obtained superamphiphobic AN mats conformed to the theoretical model.

Heat resistance of superamphiphobic alumina nanofiber mats

High-temperature air filtration is widely used in environments with industrial exhaust, coal combustion, car exhaust, biomass burning, and other forms of air pollution. Temperatures in such exhausts can range from 50℃ to 300℃, which means that filters must have high-temperature stability.⁵⁹ We thus measured the high-temperature stability of AN/FAS-4 superamphiphobicity after annealing in air from 100℃ to 500℃ for 1 h. Figure 4 shows that the contact angles of water and n-hexadecane stayed equal or higher than 150° in the temperature range of 25–350℃; moreover, the contact-angle hysteresis of water and n-hexadecane remained below 10°. Such results demonstrate that AN/FAS-4 maintained super-repellency to water and oil up to 350℃, indicating good thermal resistance. When the temperature reached 400℃, all repellency disappeared due to the decomposition of hydro- and oleophobic R^F, which is identical to the decomposition starting point of 308℃ in the thermal gravimetric graph.

Figure 4.

Surface contact angles on alumina nanofiber/FAS-4 after heat treatment for 1 h at increasing temperatures. FAS: perfluoroalkylsilanes.

Conclusions

Here, for the first time, we combined rough AN mats with surfactant trimethoxy(1H, 1H, 2H, 2H-nonafluorohexyl)silane (FAS-4) to realize a surface that is both relatively safer and superamphiphobic. Compared to the aligned nanofibrous texture, the isotropic nanofibrous texture was more suitable for producing a super-repellent surface. The AN mats displayed super-repellency to water ( $γ_{LV} =$ 72.1 mN/m, $θ^{*}$ = 157°, $Δ θ^{*} =$ 2°, $θ_{adv}^{*} =$ 158°, $θ_{rec}^{*} =$ 156°) and n-hexadecane ( $γ_{LV} =$ 27.3 mN/m, $θ^{*}$ = 151°, $Δ θ^{*} =$ 9°, $θ_{adv}^{*} =$ 152°, $θ_{rec}^{*} =$ 143°). Furthermore, such mats were thermally stable in terms of superamphiphobicity up to 350℃, which meets the demand of high-temperature air filtration.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

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