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Functionalization of Ceramic Metal Oxide Powders and Ceramic Membranes by Perfluroalkylsilanes and Alkylsilanes Possessing Different Reactive Groups –Physicochemical and Tribological Properties Joanna Kujawa, and Wojciech Kujawski ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11975 • Publication Date (Web): 26 Feb 2016 Downloaded from http://pubs.acs.org on February 28, 2016

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ACS Applied Materials & Interfaces

Functionalization of Ceramic Metal Oxide Powders and Ceramic Membranes by Perfluroalkylsilanes and Alkylsilanes Possessing Different Reactive Groups – Physicochemical and Tribological Properties Joanna Kujawa*, Wojciech Kujawski Nicolaus Copernicus University in Toruń, Faculty of Chemistry, 7 Gagarina St., 87-100 Toruń, Poland, Tel: +48 56 611 43 15, Fax: +48 56 611 45 26. KEYWORDS: ceramic membranes, TiO2, ZrO2, perfluroalkylsilanes, alkylsilanes

ABSTRACT:

The functionalization of ceramic materials, metal oxide powders (TiO2 and ZrO2) and ceramic membranes (5kD TiO2 and 300kD TiO2) was performed and thoroughly discussed. The objective of the functionalization was to change the natively hydrophilic character to the hydrophobic. The hydrophilic character of the ceramics generates limitations in wider application of such materials.

Material

functionalization

was

performed

using

perfluoroalkylsilanes

and

trifunctional(octyl)silanes possessing three different reactive functional groups: -Cl, -OMe, and OEt. The characterization of functionalized metal oxide powders and ceramic membranes was

ACS Paragon Plus Environment

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assessed by a combination of various analytical methods and techniques: NMR, TGA, HR-TEM, FT-IR, SEM-EDX, AFM, and contact goniometry. The impact of molecular structure of grafting agents (type of reactive group), time of functionalization process (5 – 15 min), and type of membrane morphology on the material, physicochemical, and tribological properties was studied. Effectiveness of hydrophobization was confirmed by HR-TEM technique. The thickness of the attached hydrophobic nanolayer on the surface of ceramics was around 2.2 nm. It was found that the stable hydrophobic surfaces were obtained by functionalization with both fluorinated and non-fluorinated modifiers. The materials modified with perfluoroalkylsilanes (FC6OEt3) and trichloro(octyl)silanes (C6Cl3) during 15-minutes hydrophobization possess comparable properties: contact angle (CA) equal to 130o and 133o; roughness RMS 10.2nm and 12nm; adhesive force 4.1nN and 5.7nN; Young modulus 135GPa and 130GPa, respectively. The relation between hydrophobicity level and ceramic membrane roughness was discussed applying the Kao diagram concept.

29

Si NMR results show that type of modifiers has an important

influence on grafting efficiency and on the mode of the grafting molecules attachment. In case of grafting with n-octyltrichlorosilane (C6OCl3) and n-octyltrimethoxysilane (C6OMe3) an increase of lateral polymerization across the octylsilane layer was observed.

1. INTRODUCTION The fabrication, design and characterization of materials with controlled properties (e.g. hydrophobicity, hydrophilicity, wettability or micro-, and nano-architecture) are important in material science

1,2

. Specifically, the control of wetting properties is significant from practical

point of view, e.g. in creation of anti-fogging, self-cleaning, anti-icing, and antibacterial


2

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1, 3

materials

. Highly hydrophobic coatings are applied also for the creation of an ultra-dry

surfaces 4-6. Surface wettability is directly correlated with the surface character. The level of water repellence can be governed by chemical and/or the mechanical/geometrical modification. The studied

materials

can

be

basically

classified

as

hydrophilic/superhydrophilic

or

hydrophobic/superhydrophobic depending on the physicochemical properties (e.g. high or low surface energy) 1, 3, 7. Hydrophobic and superhydrophobic surfaces exist also in nature. One example of natural hydrophobic self-cleaning surface is the lotus leafs which are characterized by high water repellence on the surface

1, 8

. The hydrophobic layers can be also utilized to protect grids and

circuits 1, 9-14. Such hydrophobic coatings are frequently applied in maritime industry. The ship's hulls covered by hydrophobic surfaces are more resistant to corrosion and growth of marine organisms

11, 13

. Moreover, coated surfaces of ship’s hulls are characterized by a lower friction

drag, thus decreasing fuel consumption. Despite the fact that hydrophobic surfaces possess many important applications, safety of the environment, especially the water environment, should be taken into account. This is a big issue due to the utilization of fluorinated compounds for the surface functionalization. International Maritime Organization published a number of protocols and procedures regarding water protection against potentially dangerous additives (e.g. fluorinated chemicals, such as perfluorooctane sulfonic acid and perfluorooctanoic acid), as the persistent and global contaminants 12, 14, 15. Highly hydrophobic surfaces can be also created by micro or nano-texturing, however such structures can be easily damaged by cleaning or abrasion. For these reasons, it is necessary to create mechanically and chemically stable surfaces by modification with environmentally


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friendly compounds, possibly without fluorine content. Such surfaces can be subsequently utilized in advanced molecular separation, catalysis, photonics, sensing, and drug delivery. The interesting example is the modified Nanoporous Anodic Alumina (NAA). Modifying the wettability of NAA surface by grafting with silanes e.g. alkyl-trichloro-silanes or perfluoroalkylsilanes it is possible to achieved materials for transporting and separation (solvent-extraction and langmuir-adsorption-based transport) 16, 17. Anodic aluminium oxide membranes coated by silica possessing controlled pore size and specific surface properties are applicable for, cell culture, tissue engineering and biosensing

17, 18

. In the creation of organic polymer/inorganic hybrid

nanocomposites, silica is widely applied as a ﬁller or reinforcement agent in polymeric matrix. However, prior to its utilization, the chemical modification of the silica particles is required. For the tuning its properties and improve the stability, solubility as well as dispersion in various solvents, organosilanes can be used 19. In general, the surface hydrophobicity depends on two factors: chemical and geometrical 21

4, 20,

. The fabrication of hydrophobic surfaces typically involves surface grafting resulting in

surface roughness (geometrical factor) at nanoscale (i.e., nanoparticles, photolithography, mesoporous polymers, surface etching), and/or chemical modification (chemical factor) leading to the surface energy diminution

1, 13, 22

. Shibuichi et al.

4, 20

created various well-characterized

fractal surfaces from alkylketene dimer (AKD) displaying a water contact angle of up to 174°. These random and highly rough fractal surfaces were compared with the flat ones possessing much lower values of contact angle - smaller than 109o. Both surfaces, flat and rough, were prepared from the material presenting alike chemical structure. The relation between surface roughness (geometrical factor) and the chemical factor is usually presented graphically

4, 20

as

follows: the value of the measured contact angle on a rough surface (θr) is plotted against the


4

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Young contact angle achieved on a flat smooth surface (θs). The results are collected from different liquids possessing various surface tensions. Subsequently, angles on the rough and flat surfaces are expressed by their respective cosine values. The resulting graph is known as a “Kao diagram”

4, 7, 20, 23

(Figure 1). Using a concept of Kao diagram, it is possible to discuss

physicochemical properties of modified materials in terms of surface roughness and wetting abilities (Figure 1). In the first quadrant of the coordinate system, hydrophilic and highly hydrophilic surfaces are located, independently from the roughness. Due to the hydrophilic surface character, the soaking of polar solvents into the material can be observed (Figure 1). Subsequently, with an increase of hydrophobicity level on the flat surface e.g. by chemical modification, it is possible to reach value of cosθs on the abscissa never smaller than -0.3, what corresponds to the maximum existing chemical hydrophobicity of ca. 120o 24. These surfaces are classified to the Wenzel region, in which surfaces characterized by homogeneous morphology and hydrophobic character are located

25

. As soon as the nano- or microstructure domains are

introduced onto the surface, the apparent contact angle changes significantly and reaches values higher than 150o (third quadrant of the coordinate system). This difference in the surface wettability is associated with presence of the roughness and can be classified as the CassieBaxter region

26

. In the Cassie-Baxter zone highly and super hydrophobic surfaces possessing

well-developed surface roughness are represented (Figure 1).


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Figure 1. Kao diagram – correlation between hydrophilicity and hydrophobicity of the flat and rough surfaces. Ceramic materials (e.g. titania, zirconia, alumina, and silica) are characterized by high chemical, thermal and mechanical stability and therefore they are ideal materials for many applications in the chemical, biotechnological and pharmaceutical industries as well as in water and wastewater processing

9, 10, 27-29

. However, the ceramic materials are hydrophilic by nature.

This character of the surface, related to the occurrence of hydroxyl group, can limit the possible range of applications of the ceramic materials, where the hydrophobic character of the surface is needed (e.g. vacuum membrane distillation)

27-30

. In order to extend ceramic membranes

application, the hydrophobization process of the ceramic materials must be performed

31-34

.

Perfluoroalkylsilanes are very common agents applied for modification of ceramics 27, 31, 32, 35-39. However, this type of compounds possesses fluorine atoms and this can create problems in the case of utilization of such prepared hydrophobic surfaces, e.g. for water treatment 9, 40, 41. The purpose of the research was to highlight the possibility of utilization of alkylsilanes instead of perfluoroalkylsilanes.


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The main aim of the research was to investigate the functionalization of metal oxide powders and ceramic membranes leading to the creation of the stable hydrophobic layer on these materials. The efficiency of the hydrophobization process by various trifunctional alkylsilanes was evaluated. Additionally, important aspect of the work was related to the comprehensive material characterization of the obtained hydrophobic surfaces and correlation between types of grafting molecules (type of reactive groups and the presence or lack of the fluorine atoms) and physicochemical and tribological properties of the created surfaces. 2. EXPERIMENTAL SECTION The commercially available metal oxide powders: TiO2 and ZrO2 were supplied by SigmaAldrich (USA). The following grafting compounds were purchased from Linegal Chemicals (Poland):

n-octyltrichlorosilane, C6H13C2H4Si(Cl)3 – denoted as C6Cl3,

n-octyltrimethoxysilane, C6H13C2H4Si(OCH3)3 – denoted as C6OMe3,

n-octyltriethoxysilane, C6H13C2H4Si(OC2H5)3 – denoted as C6OEt3,

1H,1H,2H,2H-perfluorooctyltriethoxysilane, C6F13C2H4Si(OC2H5)3 – denoted as FC6OEt3.

The structure of the grafting agents are presented in Figure S1 – Supplementary Information. Planar (5kD and 300kD) and tubular (300kD) titania ceramic membranes (TAMI Industries, France) were used in the presented research. Commercial membranes originally possess hydrophilic character. The following solvents: glycerine, dimethyl sulfoxide, pyridine, ethyl iodide, butyl acetate, butanol, methyl tert-butyl ether, n-hexane, perfluorohexane, acetone, ethanol, and chloroform (stabilized by ethanol) were purchased from Avantor Performance Materials Poland S.A (Poland).


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Grafting process. Hydrophobization process of ceramic materials (metal oxide powders and membranes) was preceded by washing samples in acetone, ethanol and water (10 min in each solvent followed by drying in an oven at 110oC for 12h). Subsequently, ceramics were immersed in the grafting solution for a short period of time equal to 5 min and the procedure was repeated three times, what resulted in the total modification time equal to 15 min. The details of the hydrophobization process of ceramics are described elsewhere

22, 34, 42

. Eventually, the

hydrophobized samples were characterized by the below-mentioned analytical methods. Ceramic materials characterization: Thermogravimetric analysis (TGA). The TGA technique was used to determine the effectiveness of the metal oxide powders functionalization process. Pristine and modified powders were analysed on the Simultaneous TGA-DTA SDT 2960 Thermal Analysis apparatus. In addition, TGA was applied to establish the amount of hydroxyl (OH) groups utilized during grafting process. The details regarding the calculation of the amount of OH groups on the metal oxide powders have been presented in our previous work 33. Nuclear magnetic resonance (NMR). In case of solid samples (metal oxide powders – pristine and modified)

29

Si NMR CP/MAS (Nuclear Magnetic Resonance of solid-state cross

polarization/magic angle spinning) analyses were done. Bruker Avance 700 MHz equipment was used to collect NMR spectra. High resolution transmission electron microscopy (HR-TEM). HR-TEM analysis was done for unmodified and modified metal oxide powders in order to prove the presence of hydrophobic layer. Samples were dispersed in water and subsequently were placed on the copper grid. Tecnai G2 F20 X-Twin, FEI Europe apparatus was utilized, applying an accelerating voltage of 200kV.


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FT-MIR spectroscopy. The FT-MIR technique was used to confirm the hydrophobization process. Spectra for native (hydrophilic) and hydrophobized metal oxides powders were performed on Perkin-Elmer Spectrum 200 apparatus in the mid infrared range (4000–400 cm−1). Analysed samples were prepared by the deposition of a thin film on KBr plate. Contact angle (CA) and surface free energy (SFE) Static and dynamic contact angles (CA) measurements were performed at room temperature using goniometer PG-X (FibroSystem AB) and deionized water (18 MΩ.cm) as a liquid. CA were established on the pristine and hydrophobized membranes, based on sessile drop (static CA) and tilting plate (dynamic CA) methods and were described in detail elsewhere 43, 44. The apparent CA values were calculated by Image J software (Image J, NIH – freeware version), with an accuracy of ± 2o. Additionally, goniometer was used for the assessment of surface free energy (SFE) for all investigated planar ceramic membranes. SFE was calculated according to the Owens-Wendt method 6. The results are presented as an average value obtained from 20-30 measurements (mean accuracy ± 4%). Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX). SEM-EDX (Quantax 200 with XFlash 4010 detector - Bruker AXS) was used for the characterization of membrane morphology. The analysis was performed in order to validate whether molecules of grafting agents were anchored uniquely on the surface or also inside the membrane structure. Prior to the SEM-EDX analyses, the hydrophobized samples of planar ceramic membranes were broken into small pieces. Atomic Force Microscopy (AFM). Planar membranes (topography and phase analysis) were tested by AFM technique (apparatus - NanoScope MultiMode SPM System and NanoScope IIIa i Quadrex controller, Veeco, Digital Instrument, UK). According to the AFM measurements, the following surface geometry parameter – RMS (Root mean square average of height deviations


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taken from the mean data plane) was determined applying a tip scanning. This parameter was determined for all samples using the mathematical algorithm, built-in in the NanoScope Analysis software (version 1.4, Built R3Sr5.96909, Bruker Corporation). The root mean square roughness (RMS) and average roughness are the most used geometry - amplitude parameters. Particularly, the RMS one is used to study temporal changes during the creation of new surfaces as well as spatial differences when studying the surface feature using different scales. This parameter is more sensitive to large deviations with respect to the mean line. Scan size was equal to 5µm x 5µm. The AFM analysis was applied in order to characterize surfaces and to investigate an impact of grafting process on the physicochemical properties of membranes (hardness – H, Young modulus - E and adhesive forces - Fadh)

46

. Nanoindentation tests were performed to

assess the stability of hydrophobized samples. The measurements were done in a contact mode configuration. Three-sided pyramid diamond cantilever (cantilever spring constant 859 N/m) with a 60o apex angle was used. Physicochemical properties of the ceramic surfaces (H, E and Fadh) were established from the load–displacement curves recorded in the real time. All samples were analysed five times and an average value was calculated (accuracy ± 4%). In case of adhesion force determination, the measurements were repeated twenty times to achieve representative results. Fadh evaluation was performed at contact-mode silicon nitride (Si3N4) probes (NP-1) - spring constant provided by the manufacturer (Veeco) 0.58 Nm-1. The tip velocity was 7.88 µm s-1, and the ultimate loading force was 50–70 nN. Ambient temperature conditions were kept during all experiments. 3. RESULTS AND DISCUSSION 3.1. Characterization of pristine materials. The utilized pristine TiO2 and ZrO2 metal oxide powders were characterized by specific surface area (SSA) of 83 m2 g-1 and 15 m2 g-1,


10

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respectively. Specific surface area was determined based on the adsorption/desorption isotherms and BET (Brunauer–Emmett–Teller) model

33, 34

. In our previous work

34

, crystal forms and

phase composition of metal oxides were determined. TiO2 was composed mainly of anatase form (77%) and rutile form (23%), whereas ZrO2 was mainly monoclinic ZrO2 (96.4%) and tetragonal (3.6%) 33, 34. The amount of OH groups available for grafting on the pristine metal oxide powders were as following: 2.06 ± 0.11 and 0.29 ± 0.02 mmol g-1 for TiO2 and ZrO2, respectively. The differences in the amount of OH groups are related to the textural properties of metal oxide powder, particularly specific surface area. TiO2 samples are characterized by SSA = 83 m2 g-1 and there were much more available OH groups than in the case of ZrO2 samples (SSA = 15 m2 g-1). In order to characterize the pristine ceramic membranes, the following physicochemical and tribological parameters were determined: CA, SFE, RMS, H, E and Fadh. The received values of aforementioned parameters are gathered in Table 1. Contact angle as well as SFE were independent of the membrane morphology (pore size) and were equal to 40o and 141 nN m-1. However, pore size has an impact on the others parameters. RMS, Fadh and H were slightly smaller for microporous 5-kD samples. An opposite situation was observed for Young modulus which was a slightly bigger for 5kD membranes than for 300kD ones. Table 1. Characterization of virgin ceramic membrane samples. Parameter

TiO2 5kD

TiO2 300kD

Contact angle for water, CA (o)

40 ± 2

Surface free energy, SFE (mN m-1)

141 ± 6

Root mean square, RMS (nm)

58.0 ± 1.8

67.0 ± 2.6

Adhesion force, Fadh (nm)

27.7 ± 0.8

31.1 ± 0.9


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Nano-hardness, H (GPa)

5.1 ± 0.1

4.6 ± 0.1

Young modulus, E (GPa)

120.0 ± 2.4

113.1 ± 2.3

3.2. Characterization of functionalized metal oxide powders with alkylsilanes and perfluoroalkylsilanes The FT-MIR, HR-TEM, TGA and NMR techniques were used to confirm the effectiveness of hydrophobization process. The FT-MIR spectra presented in Figure 2 and Figure S2 – Supplementary Information were recorded before and after the functionalization by alkylsilanes and perfluoroalkylsilanes. Based on the obtained results, it can be stated that both TiO2 and ZrO2 metal oxide powders were efficiently modified using grafting compounds. As a consequence of anchoring of alkylsilanes or perfluoroalkylsilanes molecules, a change of the surface character from hydrophilic to hydrophobic one was also noticed 33, 34.

Figure 2. FT-MIR spectra of native and modified TiO2 powder. Grafting conditions: QTiO2 = 0.75 mmol g−1, Cmod = 0.05 M, Tmod = 21oC, tmod = 15min. (Spectra for ZrO2 – Figure S2).


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The presence of the Si-O-X (X = Ti or Zr) bond at frequency of around 600 cm−1 is correlated to a chemical reaction between the reactive groups of modifiers (e.g. ethoxy, methoxy or chlorine atoms) and the available OH groups on the surface of the powder materials

32, 33

. The

presence of specific vibration bands of alkylsilanes in the region 1300–900 cm−1 is a consequence of the grafting process (Figure 2)

32, 33

. The bands of the perfluorinated chains

related to modification by FC6OEt3 molecules are observed at the frequency range of 1242– 1018 cm−1, symmetric (1181 cm-1) and asymmetric (1230 cm-1) stretching frequency of CF2 functional group of FC6OEt3. The vibration of the Si-O bond is detected at frequency of ca. 1119 cm−1. The obtained results for grafting by fluorinated alkylsilanes molecules are consistent with the literature data 32-34. In the case of functionalization with the non-fluorinated compounds (C6Cl3, C6OMe3 and C6OEt3), the peaks at frequency of 2850 cm-1 and 2920 cm-1 resulting from the asymmetric and symmetric CH2 stretching vibrations of alkyl chain can be observed. Moreover, bands at 1462 cm-1 (asymmetric vibration bands of CH3) and at 1446 cm-1 (CH2 inplane deformation) are detected in FT-IR spectra for powders modified by C6OMe3 and C6OEt3. Bands at 690 cm-1 are regarded as an evidence of C-H out-of-plane bending vibrations. Bands characterized by different intensity and correlated to C-C-C stretching symmetric vibrations were found at 1122 cm-1 and 1205 cm-1. According to the achieved data, it can be concluded that hydrophobization process was efficient. This remark can be proved by the very weak frequency of OH stretching (3400 cm-1, 3363 cm-1), Ti-OH stretching and Ti-O (455 cm-1) bands. Furthermore, in the case of modification by C6OMe3 and C6OEt3 the peaks located in the region of 1060 cm-1 and 1070 cm-1 are attributed to polymerized siloxane network Si-O-Si. This observation was related to the lateral polymerization of silanol groups in alkylsilanes and creation of 2D siloxane network. This kind of polymerization was limited in functionalization


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with perfluoroalkylsilanes (FC6OEt3). The same findings were highlighted by Prakash et al. for the modification process of titanium dioxide nanoparticles by 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane and octyltrichlorosilane 47. Based on the FT-MIR analysis it was possible to prove that titania as well as zirconia oxide powders were efficiently functionalized. Additionally, the confirmation of functionalization was assessed by HR-TEM analysis (Figure 3).

Figure 3. HR-TEM images of ZrO2 non-modified (A) and modified (B) ones. A2, B2 and B3 selected area of election diffraction patterns. Based on the HR-TEM the nature of single crystalline and high crystallinity of the particle was clearly indicated. It is possible to see the difference between pristine (Figure 3A) and functionalized powders (Figure 3B). The changes are related to the creation of hydrophobic nanolayer with the thickness around 2.2 nm. In the HR-TEM images of pristine sample a sharp edge of crystallites can be seen, however after modification the shape became an indistinct due to


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presence of amorphous fluorine-carbon chains attached to the ZrO2. These differences can be also detected on the selected area of election diffraction (SAED) patterns. In the case of native samples, only diffraction spots of ZrO2 can be seen (Figure 3.A2). The calculated distance value betwixt parallel fringes (Figure 3.A1) was equal to 0.318nm. Thus, it can be referred to the interplanar spacing of monoclinic zirconia plane (111) 48. Additionally, the diffraction spots can be indexed to a number of planes related to the monoclinic zirconia (Figure 3.A2). However, in case of functionalized powder, two types of SEED patterns were observed: possessing only diffraction spots (Figure 3.B2) and circles from amorphous phase of hydrophobic brush occurring near the edge of crystallites (Figure 3.B3). The presence of circles can be explained by molecules of modifiers attached to the ceramic surface. The further detailed studies of grafting efficiency were done applying TGA and NMR analysis. It should be remembered that hydrophobization occurs between available hydroxyl groups on the ceramic surface and reactive groups of modifiers

33, 34

. Modification process showed high

effectiveness, independently on the type of grafting agents, resulting in the utilization of OH groups for covalent bonds creation higher than 60% (Figure 4A). This observation was also confirmed by FT-MIR results (by a detection of very weak frequency of OH, Ti-OH and Ti-O bands) 32-34. The highest percentage of applied OH was noticed for the modification with C6Cl3 (Figure 4A). In case of utilization of non-fluorinated grafting molecules, the amount of used hydroxyl groups changes in the following order C6Cl3 > C6OMe3 > C6OEt3. This tendency can be correlated with the bond energy dissociation which for Si-Cl, Si-OMe and Si-OEt is equal to 489.5 kJ mol-1, 514.6 kJ mol-1 and 510.5 kJ mol-1, respectively 49. Taking into consideration type of ceramic surface, it can be stated that titania oxide powders are modified with higher efficiency than zirconia powders. This is related to the textural properties of metal oxide powders, e.g.


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samples with higher specific surface area are modified much better

Page 16 of 37

32, 34

. In the case of

functionalization with perfluoroalkylsilanes molecules, low utilization of OH groups is observed (Figure 4A). The obtained values of 64% for TiO2 and 60% for ZrO2 are in a very good accordance with our previous findings

33

. Higher amount of OH groups remaining after

modification process with FC6OEt3 is related to a bigger size of perfluorinated chains, comparing to modification with other compounds (C6Cl3, C6OMe3 and C6OEt3). The lattice spacing is equal to 5.9 Å and 4.79 Å for fluorinated chains and non-fluorinated chains, respectively 50, 51.

Figure 4. A - Amount of utilized OH groups in percentage during grafting process. B – surface concentration of grafting molecule. Grafting conditions: QTiO2, ZrO2 = 0.75 mmol g−1, Cmod = 0.05 M, Tmod = 21oC, tmod = 15min. NMR results delivered molecular insight into the hydrophobization mode of the alkylsilanes and perfluoroalkylsilanes on the ceramic surface as well as provided significant information about the changes occurring at the TiO2 and ZrO2 surface during grafting. In

29

Si NMR spectra

of TiO2 and ZrO2 modified by alkylsilanes and perfluoroalkylsilanes two sets of signals are


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observed: between −50 and −70 ppm (Tn signals) and between −87 and −115 ppm (Qn signals). According to the literature 33, 52, 53, the signals can be assigned to the appropriate Tn structures T1 (OSi(OEt)2R), T2 (O2Si(OEt)R), T3 (O3SiR) at chemical shifts -42 do -51 ppm, -53 do -59 ppm, and -62 do -69 ppm, respectively. The signals for the following Qn structures are located at the chemical shifts -92 do -100 ppm (Q2 – O2Si(OH)2), -102 do -108 ppm (Q3– O3SiOH), and -109 do – 115 ppm (Q4– O4Si), accordingly

33, 52, 53

. Table 2 gathered picks intensity of all detected signals. The data are

presented as one group of signals (the sum of Tn and Qn intensity is equal to 100%). Presentation of the Tn and Qn structures together can highlight the contribution of Qn structures, which in many cases is quite small (Table 2). Table 2. Percentage contribution of picks intensity in the

29

Si NMR spectra of modified metal

oxide powders. Grafting conditions: QTiO2, ZrO2 = 0.75 mmol g−1, Cmod = 0.05 M, Tmod = 21oC, tmod = 15min. Titania

Zirconia

C6Cl3 C6OMe3 C6OEt3 FC6OEt3

C6Cl3

C6OMe3

C6OEt3

FC6OEt3

T1*

10

15

24

20

53

26

30

11

T2*

20

38

42

49

24

35

41

51

T3*

9

32

30

29

24

14

29

34

Q2*

40

10

1

2

0

17

0

0

Q3*

13

3

0

0

0

5

0

3

Q4*

7

3

0

0

0

3

0

2

Comparing data presented in Figure 4 and in Table 2 for perfluorinated samples, the interesting tendency can be seen. Higher surface concentrations of FC6OEt3 molecules was obtained for samples characterized by lower level of OH group utilization in the grafting process. This is


17


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associated to mode of molecules attachment and it can be explained using the NMR results (Table 2). FC6OEt3 are attached mainly by T2 and T3 structures to the ZrO2 and TiO2 surfaces. The occurrence of T3 structure is characteristic for surface functionalization with molecules possessing shorter fluoro-carbon chains i.e. 6 carbon atoms. According to the previous results, it can be stated that with an extension of the fluorocarbon chains length, a contribution of T3 structure decreases 33, 34. Moreover, the low contribution of Qn structure is noticed ( ≤ 3%) during grafting by FC6OEt3, what is related to a very limited lateral polymerization of silanol groups 47. Alkylsilanes agents are anchored mainly by T2 and T3 structure to the ceramic surfaces (Table 2). Moreover, the modification with alkylsilanes results in a lower surface concentration of grafting agents (i.e. C6OMe3 and C6OEt3) (Figure 4B) and higher utilization of OH groups, comparing to samples modified by C6Cl3. This statement is associated with a smaller lattice spacing of alkylsilane chains 50, 51. However, the presence of Qn structures is more visible, especially in the case of functionalization with C6OMe3 (ZrO2 and TiO2) and C6Cl3 (TiO2), what corresponds to a partial polymerization of silanol groups of grafting agents and creation of 2D siloxane network. Much higher surface concentration of C6Cl3 molecules on the ZrO2 surface than on TiO2 can be explained by the mode of grafting molecules anchoring by T1, T2 and T3 structure as well as the lack of Qn ones. Summarising, the type of grafting agents (presence of fluorine atoms, type of reactive group), type of ceramic surface (textural properties) as well as mode of modification molecules attachment have an important influence on the efficiency of functionalization process by various alkylsilanes. 3.3. Characterization of functionalized ceramic membranes with alkylsilanes and perfluoroalkylsilanes. Prior to the determination of hydrophobicity level of functionalized ceramic membranes, the SEM-EDX analysis of the membranes cross sections was done (Figure


18

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S3 – Supplementary Information). SEM-EDX technique was used to determine the location of grafting molecules attachment. In case of microporous membranes, grafting molecules are located mainly on the membrane surface. This fact is associated with the size of perfluoroalkylsilanes molecules (1.5 – 2.2nm), which is similar to the membrane pore size diameter (2-4nm). On the other hand, in the case of macroporous membranes with much bigger pores (around 200nm), it can be observed that hydrophobic molecules are anchored on the membrane surface as well as inside the porous structure of the membranes. The chosen results from SEM-EDX analysis are presented in Figure S3 of Supplementary Information. All planar ceramic membranes were efficiently modified and the surface character expressed by CA changed from hydrophilic (CA 90o) - Figure 5. As a result of functionalization by grafting with various alkylsilanes and perfluoroalkylsilanes, the changes in physicochemical and tribological properties could be also observed.

Figure 5. Contact angle values for 5kD and 300kD membranes. Grafting conditions: Cmod = 0.05 M, Tmod = 21oC, tmod = 5, 10 and 15min. The measurement of contact angle is one of the most frequently applied methods to evaluate the hydrophobic/hydrophilic quality of functionalized surfaces. It is evident from data presented


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Page 20 of 37

in Figure 5 that the apparent contact angle values for all membranes were higher than 90o. This observation is in a good accordance with the results presented by other research groups 12, 22, 52. Based on the obtained data (Figure 5), it can be also stated that following parameters: membrane morphology, grafting duration and type of the modification molecules have strong impact on the contact angle values. Interestingly, higher values of CA are observed for 300kD membranes comparing with 5kD membranes, independently of utilized modifiers (Figure 5). It should be recalled that surface hydrophobicity depends on two factors, chemical and geometrical (surface structure - roughness) ones 4, 20, 21. Considering the statements from literature 4, 7, 20, 21, 23, 54

, it is proved that after chemical modification under the same experimental conditions, higher

CA value will be observed for rougher surface (Figure 1). The second parameter influencing the resulting hydrophobicity is the modification time. An extension of grafting duration contributes to higher CA values (Figure 5). It should be highlighted that in this work hydrophobic surfaces were achieved by a very fast grafting processes, the total modification time varied between 5 and 15 min. It was found that this short time was sufficient to turn the properties of hydrophilic ceramic membranes into hydrophobic ones. Time extension from 5 to 15 min contributed to around 10% increase of CA values (e.g. from 112 o ± 2o to 125o ± 2o for 300kD grafted by C6OMe3). The type of grafting molecules has also a visible effect on the hydrophobicity, expressed by CA. The highest CA values are obtained for surface modified by perfluoroalkylsilanes (130o ± 2o for 15min modification of 300kD by FC6OEt3) than for membranes grafted by alkylsilanes (115o ± 2o for 15min modification of 300kD by C6OEt3). This fact is correlated to the larger diameter of perfluorinated chains that creates a higher energy penalty for hydration

50, 52

. The achieved values are analogous to data

shown in scientific literature for similar grafting compounds

52, 55

. Independently on membrane


20

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morphology, the lowest values were noticed for membranes functionalized with C6OMe3 and C6Cl3 molecules. This observation can be associated with the mode of molecules anchoring to the ceramic surface. Taking into account the NMR results, it can be seen that these compounds tend to partially polymerize and to create 2D siloxane network 4, 20, 21, 54. The changes in wetting behaviour of the hydrophobized surface become more visible if surface free energy (SFE) is taken into consideration (Figure 6).

Figure 6. Surface free energy values and their components for 5kD and 300kD membranes. Grafting conditions: Cmod = 0.05 M, Tmod = 21oC, tmod = 5, 10 and 15min. Various approaches can be implemented to evaluate the SFE of ceramics. The most common methodology links SFE and contact angle measurements of different solvents

6, 52, 56

. In this


21


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Page 22 of 37

research Owens-Wendt method was chosen for determination of SFE. According to OwensWendt method, SFE consists of two components – polar and dispersive one

6, 22

. The dispersive

part of the SFE of a solid or the surface tension of a liquid is associated with dispersive interactions. These types of interactions are based on temporary variations in the electron density which is not permanently localized in the molecule. This produces momentary dipoles that can also induce temporary dipoles in adjacent molecules. The dispersive interactions are very weak and are known as London force

6, 22

. The polar interactions occur in molecules with a dipole

moment. This type of interaction is related to the polar, hydrogen, induction (Debye) and acidbase forces 6, 22,52. These are molecules with a permanent inequality of the electron density due to different electronegativities of the bonding partners while at the same time the molecule is asymmetrical (e.g. water). Molecules with a dipole moment can form polar interactions with one another. The lower contribution of polar components is observed in the case of surfaces possessing more hydrophobic character

56

. It was found that for more hydrophobic membranes

(i.e. showing higher CA), smaller contribution of polar component is noticed (Figure 6). Functionalization has a strong impact on the SFE value (Figure 6). It is clearly seen that after hydrophobization process, independently on the conditions, the SFE value is reduced. The values of SFE depend on the grafting conditions and change from 141 ± 6.1 mN m-1 for pristine surfaces (both 5kD and 300kD) to the values included in the range from 30.5 ± 1.3 mN m-1 (300kD membrane grafted 15 min by C6Cl3) to 73.3 ± 3.1 mN m-1 (5kD membrane grafted 5 min by C6OEt3) - Figure 6. Considering the changes in SFE components separately, it can be seen that in case of samples grafted with C6Cl3 and C6OEt3 there is no influence of the grafting conditions on the value of polar components. This observation is directly correlated with values of CA (Figure 5). The interesting impact on the value of polar component of SFE has a presence


22

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of fluorine atoms. In case of fluorine-free surfaces there is no effect of grafting time on the polar part of SFE. However, for fluorinated sample with increasing duration of grafting the diminution of polar part was observed (Figure 6). Dispersive component of 5kD membranes was in the range of 79% to 88% of overall SFE and in the range of 74% to 94% for 300kD ones.

Figure 7. Surface roughness – RMS. Grafting conditions: Cmod = 0.05 M, Tmod = 21oC, tmod = 5, 10 and 15min. The achieved SFE values were inversely proportional to the contact angle values, the highest SFE values were found for the least hydrophobic membranes i.e. grafted by C6OEt3 as well as for all membranes hydrophobized during 5 min (Figure 6). An interesting remark related to the observed higher hydrophobicity is that the free energy of hydration per unit hydrophobic surface is comparable for fluorocarbons and hydrocarbons

50, 57

. In addition, C-F bond is characterized

by a higher dipole moment than the C-H one. For that reason, a stronger binding with dipolar water could be expected. Moreover, the polarizability of fluorine in the C-F bond is moderately low, because of the localization in the periodic table. However, free energy of hydration is similar and not smaller than for C-H bond 58. Because of that, the dispersion interactions of C-F


23


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Page 24 of 37

with water will be probably more attractive than in case of C-H. Thus, a surface functionalized by fluorinated modifier could have been even more hydrophilic comparing with the sample modified by non-fluorinated compounds

49, 51

. However, according to smaller lattice spacing of

alkylsilane chains, non-fluorinated surfaces will be more hydrophilic 49, 51. In Figure 6 it can be seen that not only duration of modification or type of modification agent have an influence on the SFE but also a size of membrane pores (5kD or 300kD). The extension of grafting process duration and bigger membrane pore diameters contribute to the lowering of overall surface free energy. The samples grafted by fluorinated compounds (2.9 ± 0.1 mN m-1 for 300kD membrane grafted 15 min by FC6OEt3) show much more reduced value of polar component than nonfluorinated ones (7.2 ± 0.3 mN m-1 for 300kD membrane grafted 15 min by C6Cl3). On the other hand, the total SFE value for non-fluorinated sample was lower (30.5 ± 1.3 mN m-1 for FC6OEt3 and 33.8 ± 1.4 mN m-1 for C6Cl3). This fact results from the presence of fluorine atoms in FC6OEt3 molecules. Furthermore, such small value of polar component for surface modified by FC6OEt3 is associated with low amount of polar groups 50, 52. Additionally, the physical meaning of a lower polar component can be linked to the creation of monolayer on the ceramics. This finding is additionally demonstrated by AFM results showing reduced roughness for fluorinated ceramic surfaces (Figure 7)

52

. In case of surfaces modified by non-fluorinated molecules, the

observed higher values of polar component suggest either the presence of disordered and multilayer structure or an incomplete coating of the surface by grafting molecules. Dispersive part was also lower comparing with membranes possessing less hydrophobic character. In the case of dispersive components an interesting relation is observed for samples modified by


24

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compounds possessing different reactive groups (e.g. -Cl, -OMe, -OEt). There were almost no changes in dispersive components with increasing grafting time (Figure 6). Describing the physicochemical properties of the functionalized surfaces, it is particularly important to take into consideration roughness of the examine samples. The roughness determined by AFM analysis was shown in Figure 7 as a root square mean (RMS) of the surface 59, 60

. Generally, RMS values depend on the grafting time, membrane morphology, and the type

of grafting agents. Pristine samples were characterised by RMS values 58 ± 1.8 nm and 67 ± 2.6 nm for 5kD and 300kD membranes, respectively. As a consequence of functionalization, the surface smoothing is observed (Figure 7). This relationship is in a good accordance with literature data

61, 62

. The accomplished values of RMS for hydrophobized samples were in the

range of 5.6 ± 0.2 – 48 ± 1.5 nm in case of 5kD membranes and in the range of 10.2 ± 0.4 – 62 ± 2.1 nm in the case of 300kD membranes, respectively. The highest reduction of RMS value was noticed after extension of the grafting duration. Extension of modification time from 5 min to 10 min contributed to 23-42% diminution of RMS. Whereas, the reduction of RMS by 70-84% was noticed as a result of grafting length from 5 min to 15 min. The smoothest surfaces were obtained on the ceramics grafted with perfluoroalkylsilanes (5.6 ± 0.2 nm for 5kD membrane grafted 15 min by FC6OEt3). This fact is related to the high hydrophobicity of perfluorinated ceramic surfaces as well as to the differences at the molecular level e.g. higher lattice spacing of fluorinated chains in FC6OEt3 modifier

50,

51

. Taking into consideration membranes

functionalized by alkylsilanes molecules (C6Cl3, C6OMe3 and C6OEt3), RMS results for C6Cl3 and C6OMe3 were comparable. While, the highest RMS were observed for 5kD and 300kD ceramic membranes grafted by C6OEt3. Smaller RMS values for C6Cl3 and C6OMe3 can be associated with the possibility of a silanol 2D network creation (Table 2, Figs. 4, 6). In addition,


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the observed changes are associated with mode of grafting of molecules attachment 33 what is the most important factor in the functionalization process. Furthermore, these surfaces modified by C6OMe3 and C6OCl3 possessed higher contact angle values than membranes grafted with C6OEt3 molecules (Figure. 5). Membrane morphology has a pronounced effect on the measured RMS values and wettability properties. 300kD membranes possessing bigger pore size are characterized by higher values of apparent CA (Figure 5) and subsequently higher roughness (Figure 7). These observations can be further discussed based on the Wenzel or Cassie-Baxter models

25, 26

Wenzel model describes the homogeneous wetting regime, while Cassie-Baxter

model suggests the heterogeneous surfaces with air pockets existing between so-called “pillars” 25, 26

. Based on this information, rougher and more heterogenic 300kD membranes characterized

by higher CA (123 – 130o for modification with FC6OEt3) can be described by Cassie-Baxter model. On the other hand, more homogenous and less hydrophobic 5kD membranes should be described using Wenzel model.

Figure 8. Kao diagram: A–functionalized 5kD TiO2 membrane; B - functionalized 300kD TiO2 membrane.


26

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In order to present comprehensive characteristics of functionalized surfaces, both physicochemistry and wetting behaviour of the obtained hydrophobic layer are taken into account. In that case, the Kao diagrams

4, 20, 21, 54, 63

correlating surface hydrophobicity and its

roughness, were created for 5kD and 300kD membranes (Figure 8). Based on the presented results it is seen that with increase of the hydrophobicity, the value of cosθr decreases. This observation corresponds to increase of the CA to 112o and 131o in the case of 5kD and 300kD membranes, respectively (Figure 8A,B). Moreover, the transition from the soaking region (pristine – hydrophilic ceramic membranes) to the Wenzel’s region is observed. According to the presented data, it is seen that physicochemical properties of the surface obtained for 300kD membranes substantially varies from those for the 5kD membranes, and it can be directly related to the sample roughness (Figure 8A). On the rougher 5-kD surface (Figure 8A) it can be noticed that cosθr is around -0.3 or higher, what corresponds to the maximum value of CA equal to ca. 110o - 112o. These results were achieved for the water as a testing liquid. The similar observation was highlighted by Quéré et al.

63

, who proved that the value of CA on a

perfectly flat solid surface would be never higher than ~ 120◦, what corresponds to the maximum existing chemical hydrophobicity

63

. However, for the rougher surface a different behaviour is

noticed (Figure 8B). The experimental points are located much closer to the Cassie-Baxter region, than in the case of 5kD membranes (samples grafted by FC6OEt3 and C6Cl3) - Figure 8. It can be pointed out that on the base of the Kao diagram, it is possible to assess the quality of functionalized surfaces. 3.4. Nanotribological study of functionalized ceramics. AFM technique was also used to discuss the tribological properties of hydrophobized surfaces. Based on the obtained results for pristine and modified samples, it can be highlighted that titania surfaces modified by


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perfluoroalkylsilanes show higher CA, lower value of SFE (Figure 6) and lower RMS-roughness (Figure 7) comparing with non-modified samples. The surfaces with lower SFE and higher CA values show smaller values of the adhesive forces (Figure. 9A). The diminution of Fadh values is mainly related to the fact that the friction forces are controlled by the surface interactions under low contact pressures

38, 64

. In that case, the adhesion plays the most important role. Fadh results

from electrostatic, chemical covalent bonding, van der Waals forces, and capillary forces between the two surfaces in contact

38, 64

. The measured values of adhesive forces for the

functionalized samples were in the range of 4.1 ± 0.1 nN to 14.0 ± 0.4 nN (Figure 9A). On the other hand, values of pristine TiO2 ceramic samples were equal to 27.7± 0.8 nN and 31.0 ± 0.9 nN for 5kD and 300kD, respectively. These values are in a good accordance with the literature data TiO2

36, 38, 64

and

. For instance Cichomski et al. Fadh

=

10

nN

and

36, 38

22

obtained adhesive force Fadh = 25 nN for virgin

nN

for

TIO2

modified

by

1H,1H,2H,2H-

perfluorodecyltrichlorosilane and (3,3,3-trifluoropropyl)trichlorosilane molecules, respectively. Taking into consideration the surfaces modified by alkylsilanes, it can be also seen that for samples characterized by higher CA and lower SFE, the adhesion force is also lower (Figure 9A). However, non-fluorinated surfaces possess higher Fadh that fluorinated ones (e.g. 5.6 ± 0.2 nN for 300kD modified during 10 min by FC6OEt3 and 10.7 ± 0.3 nN for 300kD modified during 10 min by C6OEt3). Generally, for all investigated samples it was observed that the most visible effect on the Fadh value comes from the change of the grafting duration. The extension of grafting time from 5 min to 10 min contributed to ca. 20% reduction of adhesive force, whereas the extension from 5 min to 15 min resulted in 50% diminution of Fadh (Figure 9A).


28

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Figure 9. Nanotribological characterization of 5kD and 300kD membranes: A – Adhesive force; B – Nanohardness, C – Young modulus. Grafting conditions: Cmod = 0.05 M, Tmod = 21oC, tmod = 5, 10 and 15min. The nanotribological characterization consisted of nanohardness (H) and Young modulus (E) determination (Figure 9B,C and Figure S4 –S6 – Supporting Information) 65-70. Nanohardness of functionalized ceramics varied in the range of 6.0 ± 0.1 GPa (300kD modified 5 min by C6OEt3) and 8.6 ± 0.2 GPa (5kD modified 15 min by FC6OEt3). The measured values of Young modulus


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changed from 123 ± 2.5 GPa (300kD modified 5 min by C6OEt3) to 150 ± 3 GPa (5kD modified 15 min by FC6OEt3). Both values are in a good accordance with the literature data

67-70

.

Depending on the characteristics of the presented titania surface, chemical nature as well as on the type of applied modifiers a large variety of Young modulus (4 - 13 GPa) and harnesses (116 – 209 GPa) have been reported 67-70. However, it was found that in this research type of modifier has a little impact on the E and H parameters (Figure 9B,C). This is mainly caused by the small thickness of the hydrophobic layer on the ceramic surfaces, regardless the applied modifier. All utilized grafting molecules possess the same amount of carbon atoms in the alkylsilanes chain. However, the marginally higher values of hardness and Young modulus are observed for samples grafted with fluorinated compounds (Figure 9B,C). These differences can be correlated with the higher stiffness of the fluorocarbon chains and fact that backbone structure rotation of fluorinated chains is much lower than non-fluorinated ones because of a bigger size of fluorine atom comparing to hydrogen one

66-71

. Furthermore,

hydrophobic layer obtained from perfluorinated chains is more rigid than the layer created by alkylsilanes molecules. The carbon-carbon bonds in an alkyl chain can freely rotate contrary to the fluorinated chain 64. Bhushan et al.

65, 66

emphasized that the self-assembled monolayer with

stiffer backbone structure needs more energy for the elastic deformation. This statement explains the higher value of friction and hardness of the fluorinated surface 65, 66. 4. CONCLUSIONS The variation of surface, microscopic, spectroscopic and goniometric techniques - NMR, HRTEM, TGA, FT-IR, SEM-EDX, AFM was applied for the extended characterization of the structure of functionalized ceramic materials. Although the dissimilar nature of the used ceramic substrates (metal oxide powders and ceramic membranes), the various analytical methods


30

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provide a coherent picture of the perfluoroalkylsilane and trifunctional(octyl)silanes structure as a function of experimental grafting conditions. Based on the SEM-EDX analysis, it was shown that grafting molecules are attached on the surface as well as inside the porous structure of the ceramic membranes. However, in case of 5kD membranes, molecules bonding inside the pores is limited by the size of modifiers (1.5 – 2.2nm) comparable with membrane pores dimension (2-4nm). In the case of metal oxide powders, it was shown, that type of grafting agents (presence of fluorine atoms, type of reactive group), type of ceramic surface (textural properties) and mode of modification molecules attachment have significant impact on the functionalization efficiency. In the case of planar membranes, the functionalization process has an important influence on the nanotribilogical properties of examined samples. As a consequence of hydrophobization process, harder and more mechanically-resistant surfaces are obtained. Moreover, modified samples were characterized by lower values of adhesive forces what is extremely important from an application point of view and creation of self-cleaning surfaces, microfluidics, microarrays and lab on chip microsystems. Presented research is also valuable from the point of view of designing or controlling the chemical nature of the surface (Young modulus, adhesion, hardness, coefficient of friction or wear) and their nano- or micro-architecture, by anchoring alkylsilane or perfluoroalkylsilanes derivatives to metal oxide surfaces. The reduction of surface free energy and surface roughness (RMS) as well as an increase of CA values were observed with an extension of grafting time from 5 min to 15 min. Independently of the hydrophobization conditions (time and/or type of modifiers), higher values of water contact angle were achieved on the 300kD membranes. This pronounced impact of the membrane morphology on the hydrophobicity level was gathered in Kao diagrams.


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The most important finding was the fact that by grafting with non-fluorinated modifiers it is possible to create surfaces possessing material and tribological properties alike to samples modified by fluorinated compounds. This outcome provides significant development as well as insight into the potential applicability of environmental friendly hydrophobic and superhydrophobic surfaces. The successfully created fluorine-free surfaces can be possibly utilized in e.g. analytical application to non-solvent sample concentration or as an element of chemical sensor for hydrocarbon detection in water. The potential applications of fluorine-free surfaces is due to the elimination of harmful fluorine atoms.

ASSOCIATED CONTENT Supporting Information. The molecular structures of grafting compounds. FT-MIR spectra of native and modified ZrO2 powder. SEM-EDX results - presence of fluorine and silicon for 5kDTiO2 and 300kD-TiO2 samples. Nanoindentation analysis of TiO2-5kD modified by C6OEt3, TiO2-300kD modified by C6OEt3 and TiO2-300kD modified by FC6OEt3. All in PDF. AUTHOR INFORMATION Corresponding Author * Corresponding author: (J.Kujawa) Nicolaus Copernicus University in Toruń, Faculty of Chemistry, 7 Gagarina St., 87-100 Toruń, Poland, Tel: +48 56 611 43 15, Fax: +48 56 611 45 26, E-mail address: [email protected]. Funding Sources


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The research was supported by 2012/07/N/ST4/00378 (Preludium 4) grant from the National Science Centre Po-land. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT This research was supported by 2012/07/N/ST4/00378 (Preludium 4) grant from the National Science Centre Poland. Special thanks are due to Ms. Karolina Jarzynka for her kind assistance with the text editing. ABBREVIATIONS θs, contact angle on flat surface; θr, contact angle on rough surface; AKD, alkyl ketene dimer; E, Young modulus; Fadh, adhesive force; FC6OEt3, 1H,1H,2H,2H-perfluorooctyltriethoxysilane; H, nanohardness; C6Cl3, n-octyltrichlorosilane; C6OMe3, n-octyltrimethoxysilane; C6OEt3, noctyltriethoxysilane; CA, contact angle; OH, hydroxyl group; NAA, Nanoporous Anodic Alumina; RMS, root square mean; SFE, surface free energy; SSA, specific surface area; TiO2, titania; ZrO2, zirconia. REFERENCES (1) Gogolides, E.; Ellinas, K.; Tserepi, A., Hierarchical Micro and Nano Structured, Hydrophilic, Superhydrophobic and Superoleophobic Surfaces Incorporated in Microfluidics, Microarrays and Lab on Chip Microsystems. Microelectron. Eng. 2015, 132, 135-155. (2) Wang, J.; Duan, G.; Liu, G.; Li, Y.; Xu, L.; Cai, W., Fabrication of Gold and Silver Hierarchically Micro/Nanostructured Arrays by Localized Electrocrystallization for Application as Sers Substrates. J. Mater. Chem. C 2015, 3, 5709-5714. (3) Ahmad, N. A.; Leo, C. P.; Ahmad, A. L., Superhydrophobic Alumina Membrane by Steam Impingement: Minimum Resistance in Microfiltration. Sep. Purif. Technol. 2013, 107, 187-194. (4) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K., Super-Water-Repellent Fractal Surfaces. Langmuir 1996, 12, 2125-2127. (5) Wang, H.; He, G.; Tian, Q., Effects of Nano-Fluorocarbon Coating on Icing. Appl. Surf. Sci. 2012, 258, 7219-7224.


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