Super-Hydrophobic Multi-Walled Carbon Nanotube Coatings for Stainless Steel

Francesco De Nicolaa,b,∗, Paola Castruccia,b, Manuela Scarsellia,b, Francesca Nannic, Ilaria Cacciottid,Maurizio De Crescenzia,b,e

aDipartimento di Fisica, Universita di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, ItalybIstituto Nazionale di Fisica Nucleare, Universita di Roma Tor Vergata (INFN-Roma Tor Vergata), Via della Ricerca Scientifica 1, 00133

Roma, ItalycDipartimento di Ingegneria dell’Impresa, Universita di Roma Tor Vergata (INSTM-UdR Roma Tor Vergata), Via del Politecnico 1, 00133

Roma, ItalydUniversita di Roma Niccolo Cusano (INSTM-UdR), Via Don Carlo Gnocchi 3, 00166 Roma, Italy

eIstituto di Struttura della Materia, Consiglio Nazionale delle Ricerche (ISM-CNR), Via del Fosso del Cavaliere 100, 00100 Roma, Italy

Abstract

We have taken advantage of the native surface roughness and the iron content of AISI 316 stainless steel to direct growmulti-walled carbon nanotube (MWCNT) random networks by chemical vapor deposition (CVD) at low-temperature(< 1000C), without the addition of any external catalysts or time-consuming pre-treatments. In this way, super-hydrophobic MWCNT films on stainless steel sheets were obtained, exhibiting high contact angle values (154) and highadhesion force (high contact angle hysteresis). Furthermore, the investigation of MWCNT films at scanning electronmicroscopy (SEM) reveals a two-fold hierarchical morphology of the MWCNT random networks made of hydrophiliccarbonaceous nanostructures on the tip of hydrophobic MWCNTs. Owing to the Salvinia effect, the hydrophobic andhydrophilic composite surface of the MWCNT films supplies a stationary super-hydrophobic coating for conductivestainless steel. This biomimetical inspired surface not only may prevent corrosion and fouling but also could providelow-friction and drag-reduction.

1. Introduction

Super-hydrophobic surfaces (i.e., water contact anglegreater than 150) have attracted recently much atten-tion in fundamental research [1–5] and potential indus-trial applications, such as waterproof surfaces [6], anti-sticking [7], anti-contamination [8], self-cleaning [5], anti-fouling [9], anti-fogging [10], low-friction coatings [11], ad-sorption [12], lubrication [13], dispersion [14], and self-assembly [4]. In general, artificial super-hydrophobic sur-faces can be realized governing both the chemical compo-sitions and morphological structure of the solid surfaces.In particular, surface roughness [15, 16] (micro- and nano-morphology) may also be enhanced especially by hierarchi-cal [2, 8, 11, 16–20] and fractal structures [3, 19], possiblyallowing air pocket formation to further repel water pene-tration [21]. Nevertheless, realizing a permanent super-hydrophobic surface remains quite a challenge. Lately,chemical [22], mechanical [11], thermal stability [23], andtime durability [24] have been addressed.

However, the best and most efficient surfaces known sofar evolved in 460 million years in plants and animals oweto adaptation to different environments and now they serve

∗Corresponding authorEmail address: fdenicola@roma2.infn.it (Francesco De

Nicola)URL: 0039 0672594532 (Francesco De Nicola)

as models for the development of artificial biologically in-spired or biomimetic materials [16, 25, 26]. Recent studiesdemonstrate that super-hydrophobicity of many naturalsurfaces [2, 16, 17, 20] principally results from the pres-ence of at least two-fold morphology at both micro- andnano-scales and the low energy materials on the surfaces.For instance, the hierarchical architecture of the Salvinialeaf surface is dominated by complex elastic papillae mil-limetric in size coated with self-assembly nano-scaled epi-dermal wax crystals ranging in sizes from 0.2 to 100 µm[16, 20]. The terminal cells of each super-hydrophobicpapilla lack the wax crystals and form evenly distributedhydrophilic cells that cover only 2% of the surface [20].These hydrophilic cells stabilize the air layer by pinningthe liquid-vapor interface to the tips of the papillae. Thisprevents the loss of air caused by formation and detach-ment of gas bubbles due to instabilities, such as pressurefluctuations, especially in a turbulent water flow environ-ment. The unique combination of hydrophilic cells onsuper-hydrophobic papillae provide a promising conceptfor the development of a coating with a long-term super-hydrophobic behaviour. In general, the adhesion with thewater is so strong that the elastic papillae bend and swingback when the tips snap off the droplets [20].

The so-called Salvinia or petal effect [16, 17, 20] istherefore referred to super-hydrophobic adhesive surfaceswith hydrophilic and hydrophobic hierarchical morphologyproviding sufficient roughness for exhibiting both a large

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contact angle and a high contact angle hysteresis, con-versely to the lotus effect [2, 17] (high contact angle valueand low contact angle hysteresis). Consequently, a waterdroplet on such a surface is nearly spherical in shape andcannot roll off even when the leaf is turned upside down.However, larger drops can roll off the surface at the slight-est tilting or vibration [27].

In general, it is very difficult to fabricate an applicableengineering super-hydrophobic surface on stainless steel,because the textured films easily fall off from the stainlesssteel substrate. Lately, some achievements on the realiza-tion and characterization of stable super-hydrophobic sur-faces on stainless steel have been made [28–31] and partic-ularly using carbon nanotube coatings [32, 33]. Further-more, stainless steel potential applications include elec-trodes for super-capacitors [34], fuel cells [35], capacitivedeionization [36] and capacitive mixing for extracting en-ergy from salinity difference of water resources [37], fieldemission probes [38], sensors [39], catalyst support for wastew-ater treatment [40] and tribological applications [41]. There-fore, stainless steel may be considered as a valid candidatefor direct growth [42] of carbon nanotubes by CVD, alsobecause of its high content of iron as the catalyst element.In particular, direct growth is widely used due to severaladvantages, such as capability to produce dense and uni-form deposits, reproducibility, strong adhesion, adjustabledeposition rates, ability to control crystal structure, sur-face morphology and orientation of the CVD products,reasonable cost and wide scope in selection of chemicalprecursors.

Recently, we have shown [43–46] that the direct growthof high quality MWCNTs on stainless steel in the absenceof any external catalysts is possible. Moreover, acid treat-ments and oxidation-reduction stages on this type of sur-face are not necessary because of the native nano-scaleroughness of the substrate and the iron-rich substrate sur-face both act as an efficient catalyst or template in the syn-thesis of MWCNTs. Particularly, at our working temper-ature mostly iron nanoparticles are involved in the growthmechanism. Furthermore, after the first growth, the stain-less steel substrate may be used again, just carefully re-moving the synthesized carbon nanotubes in an ultrasonicbath. We remark that ultrasonication is generally neededto detach the MWCNT film from the steel substrate, dueto its strong adhesion [42, 43].

Here, we illustrate a simplified recipe to synthesizeMWCNTs on a sheet of AISI 316 stainless steel by CVD,without any external catalysts. Moreover, we will investi-gate the MWCNT hierarchical morphology from the SEMmicrographs of the films and their super-hydrophobic prop-erties will be characterized. In particular, we will showthat owing to their particular hierarchical architecture, thesuper-hydrophobic MWCNT coatings for stainless steel ex-hibit long-term high contact values and also high adhesiveforce with water (high contact angle hysteresis). There-fore, the super-hydrophobic state achieved is stationary.

2. Experimental

A 30 × 40 mm2 piece of AISI 316 stainless steel sheet(Fe 70%, Cr 18%, Ni 10%, and Mo 2%, Goodfellow Cam-bridge, Ltd.) was carefully sonicated in deionized waterand degreased in isopropyl alcohol for 10 min. Then, thesteel substrate was placed on a molybdenum sample holderacting also as resistive heater and inserted into an ultrahigh vacuum chamber and the pressure was brought upto 10−2 Torr by a rotary pump. At this stage, argon gas(500 sccm) was inserted at 12 Torr and then the heater wasincreased at the working temperature ≈ 730 C. The sam-ple temperature was controlled with an optical pyrometer,so when the substrate reached the working temperature,acetylene (C2H2) was introduced (200 sccm) in the cham-ber and MWCNTs grew in dynamic condition, since therotary pump was kept in operation during the process. Af-ter 10 min of growth, Ar gas (500 sccm) was inserted inchamber for 5 min.

3. Results and discussion

3.1. Microscopic characterization of MWCNT films

In Figure 1a,d SEM (Zeiss Leo Supra 35) images ofour synthesized nanostructures are reported, showing thata high density of randomly oriented MWCNTs uniformlygrew on the stainless steel sheet with an average film thick-ness of 1.6 ± 0.8 µm. Moreover, MWCNTs come with awide distribution of tube diameters, with average value58.19 ± 18.35 nm. Also, we have shown in our past works[43–46] transmission electron microscopy (TEM) imagesconfirming the multi-walled nature of the as-grown car-bon nanotubes. Furthermore, in Figure 1b,d it may beobserved that MWCNTs are mostly capped and often theypresent carbonaceous nanostructures (amorphous and/orgraphitic carbon) around the tips (Figure 1d) and close tothe stainless steel surface (Figure 1c), with a characteris-tic dimension of hundred nanometers, as also reported byother authors [42].

3.2. Wetting properties of MWCNT films

Moreover, we characterized the wettability of MWCNTfilms acquiring images of sessile water drops cast on thecarbon nanotube films by a custom setup with a CCDcamera. Static advanced contact angles were measured in-creasing the volume of the drop by step of 1 µL. A plugin[47] for the open-source software ImageJ was exploited toestimate the contact angle values by using cubic B-Splineinterpolation of the drop contour to reach subpixel resolu-tion, with an accuracy of 0.02. The deionized water (18.2MΩ cm) drop volume used to achieve the contact anglesof samples was 10 µL. Moreover, every contact angle wasmeasured 15 s after drop casting to ensure that the dropletreached its equilibrium position. In Figure 2a the imageof a water droplet cast on the MWCNT film is shown.The experimental contact angle value is Θ = 154 ± 4

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Figure 1: Scanning electron microscopy images of MWCNTs direct grown by CVD on stainless steel without any external catalysts or pre-treatments, at different magnifications, 30,000× (a,c) and 100,000× (b,d). Carbonaceous nanostructures may be seen close to the stainlesssteel surface and around MWCNT tips (red circles).

with no observable roll-off angle, even if the substrate isturned upside down. Therefore, we infer that the contactangle hysteresis is so high to pin the water droplet on theMWCNT surface. The adhesive force in unit of length ofa surface in contact with water is given by [13]

Fadh = γLV (1 + cos Θ) , (1)

where γLV = 72.5 mN/m and for our MWCNT samplesFadh = (7.33 ± 0.02) mN/m. Therefore, for a water dropwith diameter 1 mm (Figure 2a), the adhesive force of theMWCNT film in contact with the drop is F = (7.33±0.02)µN. The obtained result is about 25% lesser than the ad-hesive force of a single gecko foot-hair (i.e., seta) [48], but10 times higher than that of Salvinia leaf [49]. Interest-ingly, the contact angle value achieved is among the high-est reported in literature for not chemically treated [50–54], functionalized [55–58], or suitably textured [32, 33]randomly distributed MWCNT films.

Furthermore, Figure 2b reports the variations of con-tact angle and droplet radius as functions of the elapsedtime from drop cast on the MWCNT films. In such suctionexperiment we show that although samples are porous, thecontact angle trend is constant to demonstrate the stabil-ity in time of the super-hydrophobic state of MWCNT

coatings. On the other hand, the droplet radius linearlydecreases of ≈ 15% within 10 min owing to the liquidevaporation and not to the suction process, otherwise thecontact angle would also linearly decrease [14]. Our re-sults are particularly remarkable, since the water contactangle of MWCNT films has been reported [24] to decreaselinearly with time, from an initial value of Θ ≈ 146 toΘ ≈ 0 within 15 min.

3.3. Salvinia effect in MWCNT films

In addition, the obtained high contact angle value maybe attributed to hydrophilic (≈ 86) carbonaceous nanos-tructures around the tips of the hydrophobic randomlyarranged MWCNTs (≈ 92-138 [53, 54, 59]), constitut-ing a two-fold hierarchical morphology able to stabilizethe super-hydrophobicity of the film by the Salvinia effect(Figure 3). As it occurs in Salvinia, water droplets arepinned by the attractive interaction due to hydrophiliccarbonaceous nanostructures, while they exhibit super-hydrophobic contact angles with a large amount of airpockets, owing to the repulsive interaction of hydrophobicMWCNTs. In this way, the film results in a air-retainingsuper-hydrophobic surface. The super-hydrophobic effectdue to the presence of carbonaceous nanostructures on

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Figure 2: (a) The water contact angle value measured on the MWCNT films deposited on stainless steel is Θ = 154 ± 4. (b) Variationsof contact angle and drop radius as functions of the elapsed time from drop cast on the porous MWCNT films. The constant trend of thecontact angle value proves the stability in time of the super-hydrophobic state, while the linear decrease of droplet radius is due to the liquidevaporation and not to the suction process.

the MWCNT tips has been also reported by Han et al[60]. In their work, vertically aligned MWCNT were pro-cessed by plasma immersion ion implantation in order tocap them by hydrophilic amorphous carbon nanoparticles.The authors report a measured water contact angle valueof ≈ 180 and zero roll-off angle. However, the difference incontact angle values between our and Han’s results couldbe attributed to the lower density of carbonaceous nanos-tructures and to the random distribution of the MWCNTsin our samples. Evidently, they observed the lotus effectrealizing in that way a waterproof surface, while we recog-nized the Salvinia effect, thus fabricating an air-retainingsuper-hydrophobic surface.

In order to analyze in more detail the super-hydrophobicityof our samples, we used the Cassie-Baxter equation [21] inthe hydrophobic regime

cos θ∗ = (1 − φ) cos θ − φ, 1 = φ+ φs, (2)

with φs the surface solid fraction, φ the surface air fraction,cos θ∗ the apparent contact angle, and cos θ the Young’scontact angle of the surface defined as

cos θ =γSV − γSL

γLV. (3)

The surface tensions of the solid-vapor, the solid-liquid,and the liquid-vapor interfaces are denoted by γSV , γSL,and γLV , respectively. Moreover, if we consider the experi-mental contact angle θ = 97±8 measured for highly pureMWCNT (Nanocyl, NC7000, assay > 90%, diameter: 5-50nm) random network films realized as in Ref. [61], as theYoung’s contact angle of the hierarchical MWCNT com-posite surface with apparent contact angle θ∗ ∼= 154, weeasily obtain an air fraction φ ∼= 0.88. This result suggeststhe formation of a large amount of air pockets. However,we have recently reported [59] that highly hydrophobicMWCNT random network films with a hierarchical surfacemorphology, owing to their Young’s contact angle close to

90 are in a metastable Wenzel-Cassie-Baxter state, whichis stationary. Indeed, Figure 2b suggests that air pock-ets are very stable in time. It is worth noting that themetastability of the MWCNT film is coherent with theSalvinia effect, in which although the liquid droplets arepinned on Salvinia leaves, they can roll off from the surfaceby a slight vibration.

Figure 3: Scheme of the Salvinia effect in our MWCNT random net-work films. Water droplets are pinned by the attractive interactiondue to hydrophilic carbonaceous nanostructures, while they exhibitsuper-hydrophobic contact angles owing to the repulsive interactionof hydrophobic MWCNTs.

4. Conclusions

In summary, we have realized super-hydrophobic MWCNTfilms on AISI 316 stainless steel by CVD without the ad-dition of any external catalysts or pre-treatments, at low-temperature. Furthermore, the investigation at SEM re-veals that the MWCNT coatings are carbon nanotuberandom networks with a two-fold hierarchical morphologyowing to the presence of hydrophilic carbonaceous nanos-tructures on the top of the hydrophobic MWCNTs. Thesurface hierarchical architecture of the MWCNT films pro-vides a stationary super-hydrophobic state for the coatingsbecause of the Salvinia effect. Such MWCNT films maybe used for super-hydrophobic stainless steel realizations,

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such as drag reduction [11, 20], anti-corrosion [30], anti-fouling [9], and anti-contamination [16].

Acknowledgements

We thank R. De Angelis, F. De Matteis, and P. Prosposito(Universita di Roma Tor Vergata, Roma, Italy) for theircourtesy of contact angle instrumentation. This projectwas financial supported by the European Office of AerospaceResearch and Development (EOARD) through the Air ForceOffice of Scientific Research Material Command, USAF,under Grant No. FA9550-14-1-0047.

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