Tuning of superhydrophobic to hydrophilic surface: A facile one step electrochemical approach

Journal of Alloys and Compounds xxx (2016) 1e4

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Tuning of superhydrophobic to hydrophilic surface: A facile one step electrochemical approach S. Ashoka a, b, *, N. Saleema c, D.K. Sarkar b, ** a

Department of Chemistry, Dayananda Sagar University, Bengaluru, 560068, India Centre Universitaire de Recherche sur l'Aluminium (CURAL), University of Quebec at Chicoutimi, Saguenay, Quebec, G7H2B1, Canada c Aluminum Technology Centre, National Research Council of Canada, Saguenay, Quebec, G7H 8C3, Canada b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 June 2016 Received in revised form 21 September 2016 Accepted 29 October 2016 Available online xxx

Tuning surface wettability is of great interest for both scientific research and practical applications. We report a facile electrochemical route for tuning the superhydrophobic surface of stearic acid (SA) modified copper with over-oxidized polypyrrole (OPPy). A systematic study was performed on the relationship between pyrrole concentrations and wetting states of SA modified copper surface. The wettability of the SA modified copper surface was monotonically converted from superhydrophobicity to hydrophilicity by increasing the pyrrole concentration during the process of electrochemical deposition. © 2016 Elsevier B.V. All rights reserved.

Keywords: Electrodeposition Over-oxidized polypyrrole Superhydrophobic Contact angle hysteresis

1. Introduction The studies on wetting behaviors of a solid surface have become an area of increasing investigation in recent years. Significant attention has been paid on controlling the surface wettability, owing to their wide range of biological, chemical and electronic applications [1]. In general, solid surfaces with water contact angle greater than 150
and close to 0
are described as superhydrophobic and superhydrophilic, respectively. The wettability of a solid surface is directed by both the chemical composition and the surface topography. Generally, superhydrophobic surfaces can be achieved by controlling the roughness and topography of hydrophobic surfaces [2]. Whereas, superhydrophilic surfaces can be obtained by utilizing capillary effect where increasing the hydrophilic surface roughness will usually increase the apparent hydrophilicity of the surface [3]. Such distinguished switchable surfaces may have wide range of applications. For example, superhydrophobic surfaces are extensively used in drug release, wind

* Corresponding author. Department of Chemistry, Dayananda Sagar University, Bengaluru, 560068, India. ** Corresponding author.Centre Universitaire de Recherche Sur 1’ Aluminium (CURAL), University of Quebec at Chicoutimi, Quebec, Canada. E-mail addresses: [email protected] (S. Ashoka), [email protected] (D.K. Sarkar).

shields, automobiles, self-cleaning, photovoltaics, corrosion resistance, green houses, heat transfer, etc. [4e6]. Contrary, superhydrophilic surfaces are extensively used in creating anti-fogging surfaces, bio-fouling and its prevention, medical fields [7], etc. Hence by tuning the surface roughness and surface energy, one can achieve the desired surfaces including hydrophobic, superhydrophobic, hydrophilic as well as superhydrophilic. Recently, wettabbility of hydrophobic and superhydrophobic surfaces of many metal oxides have been studied under UV and visible light illumination [8e12]. In our previous publication, a stable and anti-corrosive superhydrophobic stearic acid modified copper surface has been reported [13]. In the present work, we have investigated the wetting behaviour of stearic acid modified copper surface by the addition of non conducting polymer, over-oxidized polypyrrole (OPPy). The OPPy addition to the superhydrophobic SA modified copper surface has been carried out in a single step, via an electrochemical deposition. That is, electro-deposition of an Oppy together with SA to an anodic copper surface. We believe that the fabricated electrically inactive OPPy coating with special wetting behaviour emerging as a novel material for biosensors construction [14]. 2. Experimental Before electrochemical deposition, the copper substrates were

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Please cite this article in press as: S. Ashoka, et al., Tuning of superhydrophobic to hydrophilic surface: A facile one step electrochemical approach, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.294

S. Ashoka et al. / Journal of Alloys and Compounds xxx (2016) 1e4

3. Results and discussion The SA deposited copper substrate exhibits green (Fig. S1(b), ESI) colour due to the formation of copper stearate from a reaction between SA and copper upon application of DC voltage [13]. On the other hand, OPPy plus SA deposited copper substrate shows black colour (Fig. S1(c), ESI) due the formation of OPPy [15]. The copper substrate electrodeposited only in ethanolic pyrrole solution (without SA) shows reddish colour similar to that of as-received copper substrate (Fig. S1(a), ESI). This observation confirms that the SA helps pyrrole to deposit on an anodic copper substrate. That is, incorporating a SA in the course of the electrochemical deposition increases the adhesion of PPy to the copper substrate. The interactions between PPy and SA on copper surface create hydrophobic environment, thereby enhancing the formation of more nucleation sites for PPy growth. The formation of more nucleation sites on the copper surface is due to the electrostatic interaction of unshared electron pairs of PPy and alkyl chain of the SA. That is, the p-H and N-H interactions between the PPy backbone bearing p system and the H atoms of the eCH3 terminated SA [16]. Then the deposited PPy undergoes an over oxidation at b position of pyrrole rings (Fig. S2, ESI). Shustak et al. reported a similar result for PPy deposition on stainless steel substrate in two step electrochemical method [17]. Where, the author's first monolayer of decanoic acid was assembled on stainless steel surface and then PPy has been deposited in the second step. Fig. 1 shows the IRRAS spectrum of SA and SA plus OPPy deposited copper surface. The two infrared absorption peaks at 2852 and 2923 cm1 originates from the symmetric and asymmetric C-H stretching modes of -CH2 groups of copper stearate, respectively, and the other peak at 2955 cm1 is ascribed to the asymmetric in-plane CeH stretching mode of the eCH3 group [13]. The symmetric and asymmetric vibration modes of eCOO (copper stearate) appeared, respectively at 1470 and 1583 cm1. However, these two symmetric and asymmetric vibration modes of eCOO are quite difficult to differentiate in the present case as the pyrrole ring of PPy chain is also showed a symmetric and antisymmetric ring stretching vibrations at the same wavenumbers [18]. The Infrared absorption peak at 1705 cm1 is assigned to the carbonyl -C]O groups fixed at b position of pyrrole rings and which result from an

CH2

COO

sequentially cleaned with detergent, deionized water and ethanol for 15 min, respectively. The cleaned substrates were electrochemically modified by immersing copper electrode in an ethanolic solution of SA and pyrrole with the application of potential, 20 V DC for 2 h. Using two electrodes configuration, the anodic and cathodic copper substrates were separated by a distance of 1.5 cm. The deposited anodic substrates were removed from the solution, rinsed with deionized water followed by ethanol, and then dried at 70
C. The Nicolet 6700 Infrared reflection absorption spectrometer (IRRAS) was employed to confirm the formation of OPPy on SA modified copper surface. The morphology was performed using JEOL JSM-6480 LV scanning electron microscopy (SEM). The surface roughnesses of the films were measured using an optical profilometer (MicroXAM-100 HR 3D surface profilometer). The wetting characteristics of the films were carried out by measuring both static and dynamic contact angles using a First Ten Angstrom contact angle goniometer at five positions on each substrate using 10 mL deionized water drop. The dynamic contact angle was measured by holding the water drop with a stationary needle in contact with the sample surface and moving the goniometer stage in one direction.

Absorbance (a.u)

2

CH3

SA on Cu

a

SA+Py on Cu

b

C=O 1500

1800

2100

2400

2700

3000

-1 Wavenumber (cm ) Fig. 1. IRRAS spectrum of (a) stearic acid and (b) stearic acid plus OPPy coated copper surface.

over-oxidation [19]. The oxidation of pyrrole increases the hydrophilic nature of deposited film and thereby produces super hydrophilic surface on the surface of the stearic modified copper substrate [20]. SEM has been utilized to gain the information about the morphological changes occur on as-received copper surfaces after electrodeposition. SEM images of as-received copper substrate shows an even surface ((Fig. 2(a)). Evident from Fig. 2(b) that the SA deposited copper surface is composed of randomly distributed fewer micro-particles of copper stearate [21] and each microparticle is composed of thin leaf like nanostructures (inset of Fig. 2(b)). These random distributions of micro-particles together with leaf like structures leading to a porous surface promoting entrapment of air beneath a water drop placed on its surface. The SA plus OPPy ([Py/SA] ¼ 0.0048) deposited copper surfaces are also shows the similar morphology as that of SA deposited copper surface (Fig. 2(c)). However, the density of the micro-particles increases tremendously. Each micro-particle is composed of spherical OPPy particles together with thin leaf like nanostructures. Interestingly, spherical OPPy particles appear at the pores created by leaf like structures (Fig. S3(a), ESI). Further increase of [Py/SA] to 0.01921, the density and diameter of the micro-particles decreases (Fig. 2(d)). Additionally, the content of OPPy particles increases in the micro-particles of copper stearate and thereby fills the pores (Fig. S3(b), ESI). The surface is completely covered by OPPy particles with further addition of pyrrole (Py/SA] ¼ 0.0762), as shown in Fig. 2(d) and this makes the surface superhyfrophilic. The wetting behaviour of a surface is dependent on both surface chemistry and surface topography. Fig. 3(a) depicts the surface roughness variation of OPPy coated copper as a function of [Py/SA] molar ratio. The surface roughness 0.43 mm on as-received copper surface was found to increase to 3.86 ± 0.5 mm for SA deposited copper surface. The increase in the roughness value is due to the formation of copper stearate nano-microstructure on an anodic copper surface. Addition of pyrrole into the ethanolic solution of SA leads to further increase in roughness. The surface roughness of asreceived copper surface was found to increases to 6.0 ± 1.3 mm and 7.7 ± 1.4 mm for 0.0024 and 0.0048 [Py/SA] molar ratio. The increase in roughness is due the increase of density and size of microparticles as observed from the SEM image of Fig. 2(c). However, further increase in [Py/SA] molar ratio to 0.0096 and 0.01921 results in the decrease of surface roughness value to 4.1 ± 0.38 mm. The decrease in roughness value is due to the formation of thick layer of OPPy between the pores. The decrease of surface roughness is well

Please cite this article in press as: S. Ashoka, et al., Tuning of superhydrophobic to hydrophilic surface: A facile one step electrochemical approach, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.294

S. Ashoka et al. / Journal of Alloys and Compounds xxx (2016) 1e4

3

Fig. 2. SEM images of (a) as-received copper surface, and OPPy surfaces as a function of [Py/SA]: (b) 0, (c) 0.0048, (d) 0.01921 and (e) 0.0768.

supported by the work of Abdelsalam et al. [22]. The author studied the wetting properties of honeycomb structured gold surfaces formed by electrodeposition of gold through a sub-micrometer template. The film thickness increases up to the radius of the cavities, the apparent water contact angle increases but further increases in thickness leads to decrease of water contact angle. The observed decrease in water contact angle with increase of thickness is due to the decrease of surface roughness. The formation of thick layer of OPPy is further confirmed by 3D image of optical profilometry, revealing apparently flat surface (inset of Fig. 3(a)). The surface wettability has been evaluated by measuring the water contact angle and CAH. Fig. 3(b and c) shows the variation of contact angle and CAH as a function of pyrrole concentration. The contact angle of 159 ± 2
and CAH 22.3 ± 3
is observed for the copper surface prepared without pyrrole. The high CAH is due to the lower roughness attributed by fewer and less density microparticles [23,24]. The contact angle slightly increases to 161 ± 3
and CAH decreases to 8 ± 1
for [Py/SA] molar ratio 0.0024. The increase in contact angle and decrease of CAH is due the higher surface roughness, which results from the higher density and lesser distance between the microparticles [23,24]. The contact angle remains same as 161 ± 2
, when the [Py/SA] molar ratio increases to 0.0048. However, the CAH increases to 12 ± 1
. The contact angle

drops to 152 ± 7
and 151 ± 6
with CAH of 15 ± 2, and 12 ± 1, respectively for [Py/SA] molar ratio to 0.009 and 0.0192. Similar observations were made by Xu et al., for superhydrophobic silica nanoparticles [25]. Where, the authors observed a slight decrease of the apparent water contact angle with increasing the nanoparticle concentration, although the nanoparticle films remained superhydrophobic. The copper surfaces prepared using higher concentration of pyrrole showed relatively flat and thick surface due to the pores filled by OPPy particles [25]. Therefore, the underlying substrate was exposed and water was able to impregnate between the microspots and increase solidliquid contact, leading to larger water CAH. Further increase in pyrrole concentration leads to superhydrophilic surface as OPPY itself hydrophilic (Figure not shown). 4. Conclusion The paper presents a tuning of surface wettability using non conducting polymer, OPPy. We think a similar concept could be applied to make other conducting polymers. The possibility to obtain superoleophobic and hydrophilic properties using non conducting polymers also opens the door for other specific applications.

Please cite this article in press as: S. Ashoka, et al., Tuning of superhydrophobic to hydrophilic surface: A facile one step electrochemical approach, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.294

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S. Ashoka et al. / Journal of Alloys and Compounds xxx (2016) 1e4 10

175

Roughness (μm)

Contact angle (degree)

a

8 6 4 2 0

165 160 155 150 145 140

0.000

0.005

0.010

0.015

[Py/SA] ratio

0.020

b

170

0.000

0.010

0.015

0.020

[Py/SA] ratio

28

c

24

CA Hys (degree)

0.005

20 16 12 8 4 0

0.000

0.005

0.010

0.015

0.020

[Py/SA] ratio Fig. 3. Variation of (a) surface roughness, (b) contact angle, and (c) CAH as a function of [Py/SA] molar ratio. The inset images of (b and c) show the water drop shape and surface topography, respectively.

Acknowledgment We sincerely thank the FQRNT for awarding Quebec-India postdoctoral fellowship to one of the authors S. Ashoka to pursue this research work. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.10.294. References [1] [2] [3] [4]

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Please cite this article in press as: S. Ashoka, et al., Tuning of superhydrophobic to hydrophilic surface: A facile one step electrochemical approach, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.294