Fabrication of superhydrophilic surface on copper substrate by electrochemical deposition and sintering process☆

Qiaopeng Liu*,Yong Tang*,Wenjie Luo,Ting Fu,Wei Yuan

College of Mechanical and Automotive Engineering,South China University of Technology,Guangzhou 510640,China

Keywords:Superhydrophilic Superhydrophobic Copper surface Electrochemical deposition Sintering process

ABSTRACT Superhydrophilic surfaces were fabricated on copper substrates by an electrochemical deposition and sintering process.Superhydrophobic surfaces were prepared by constructing micro/nano-structure on copper substrates through an electrochemical deposition method.Conversion from superhydrophobic to superhydrophilic was obtained via a suitable sintering process.After reduction sintering,the contact angle of the superhydrophilic surfaces changed from 155°to 0°.The scanning electron microscope(SEM)images show that the morphology of superhydrophobic and superhydrophilic surfaces looks like corals and cells respectively.The chemical composition and crystal structure of these surfaces were examined using energy dispersive spectrometry(EDS)and X-ray diffraction(XRD).The results show that the main components on superhydrophobic surfaces are Cu,Cu2O and CuO,while the superhydrophilic surfaces are composed of Cu merely.The crystal structure is more inerratic and the grain size becomes bigger after the sintering.The interfacial strength of the superhydrophilic surfaces was investigated,showing that the interfacial strength between superhydrophilic layer and copper substrate is considerably high.

1.Introduction

In recent years,wettability of solid surfaces has aroused a great deal of concern.Especially,super-hydrophobic surfaces,with water contact angle(CA)greater than 150°and superhydrophilic surfaces(with water CA below 5°)have attracted great interest for their special functions and potential applications[1–3].Superhydrophobic surfaces have many practical applications,such as corrosion resistance,friction drag reduction,self-cleaning and anti-icing[4–7].Similarly,superhydrophilic surfaces can be applied in heat transfer enhancement,biological medicine,self-cleaning,friction drag reduction,and so on[8–13].Generally,chemical component and rough surface structure have great influence on the wettability of a solid surface.There are two basic methods for superhydrophilic surface fabrication:one is photo-induced superhydrophilic(PIH),for example TiO2,ZnO and WO3will show superhydrophilic after irradiating by UV or visible light[14,15].The other method is constructing rough structure on hydrophilic surfaces.In the last decade,numerous techniques for artificial superhydrophilic surface fabrication have been developed based on the above methods.Liu et al.[16]prepared superhydrophilic surface by constructing strawberry-like microstructure on the molybdenum substrate via a hydrothermal method.Behners et al.[17]fabricated superhydrophilic surfaces on polymer film by photo-induced microfolding.Superhydrophilic TiO2–SiO2composite surfaces were prepared through a sol–gel route by Houmard et al.[18].Additionally,there are plasma vapor deposition process[19],self-assembly approach[20]and electrochemical method[21],etc.However,a lot of artificial methods have certain limitations,such as complicated process,expensive equipment and low interfacial strength.Therefore,further efforts should be made to develop more simple and economical techniques for fabricating superhydrophilic surfaces with high interfacial strength.

Copper is a kind of important engineering material which has been widely used in lots of industrial applications.Because of the special functions,superhydrophobic and superhydrophilic copper surfaces have great potential applications.Superhydrophobic surfaces have been prepared on copper substrates by various processes,such as the solution-immersion method[22]and electrochemical approach[23].However,preparation of superhydrophilic copper surfaces with high interfacial strength has not been reported.In this work a novel route for fabricating superhydrophilic copper surfaces with high interfacial strength is provided.Micro/nano-structure was first built on copper substrates via an electrochemical deposition method.The next step is a sintering heat treatment for the prepared cooper surfaces.This flexible and controllable method does not require any chemical modification.Furthermore,the interfacial strength between superhydrophilic layer and copper substrate is sufficiently high for practical application.

2.Experimental

2.1.Materials and sample preparation

After polished by 1500#silicon carbide paper,the copper plates with a size of 20 mm×20 mm×2 mm(99.9%,Dongguan Tongzhuang Metal Materials Co.,China)were ultrasonically cleaned in acetone(99.5%,AR)and deionized waterfor3–5 min,respectively,then dried in airatambient temperature.The electrolyte with 1 mol·L-1H+and 0.02 mol·L-1Cu2+was prepared using HCl(36%–38%,AR)and CuCl2·2H2O(99.0%,AR).The prepared copper plates used as anode and cathode were immersed into the electrolyte at ambient temperature while a DC regulated power supply(QJ21005X,Ningbo QJE Electronic Co.,China)was utilized.The space between the cathode and the anode was 10 cm and both electrodes were placed vertically.The current densities varied from 10 to 50 mA·cm-2and the deposition time ranged from 5 to 20 min.After the electrochemical deposition,the cathode copper plate was cleaned with deionized water and dried at 100°C in a thermostatic oven for 30 min,then cooled in air to ambient temperature.The next step was a sintering process.After heating for 80 min in a vacuum sintering furnace(ZSJ-45/45/60,ACEM,China)at an initial temperature of 30 °C,the temperature reached 400 °C and then thermal insulation for 20 min.To further heat for 30 min,the temperature reached 500°C and then thermal insulation for 60 min.After that heating was stopped to allow the furnace temperature to drop to 200°C,then the samples were taken out of the furnace and placed in air to cool further.In this sintering process,there was always protective gas in the furnace until the samples were taken out.The protective gas was nitrogen when the furnace temperature was lower than 400°C and it was hydrogen when the furnace temperature was above 400°C.

2.2.Surface characterization

The morphology of the obtained surfaces was characterized by SEM(S-3700N,Hitachi,Japan)at 20 kV,and the chemical composition was analyzed using EDS(Quantax,Bruker,Germany)at 20 kV.The crystal structure of the prepared surfaces was determined by XRD(D8 ADVANCE,Bruker,Germany)with a Cu Kαradiation(λ =0.15418 nm)at 40 kV and 40 mA.Contact angle measurements were performed on a contactangle system(JC2000D,Power each,China)at ambient temperature.The accuracy is ±1°.Water droplets(5 μl)were delivered onto five different sample spots for each specimen and the average of five contact angles was used for analysis.

3.Results and Discussion

3.1.Morphological analysis

SEM images of the non-sintered and sintered surfaces with different current densities and deposition times are depicted in Figs.1 and 2,respectively.It is obvious that current density and deposition time have a great effect on the density of copper sediments,but little impact on the morphology of the obtained surfaces.Fig.1(e1),(e2)and(e3)shows the morphology of the non-sintered surface under different magnifications.The global morphology in Fig.1(e1)looks like clusters of stamens whose average size is around 20 μm.In a larger magnification[Fig.1(e3)],the coralloid structure with a size of around 200 nm is given to exhibit the morphology of the fine scale structure.Under different magnifications,one can see that a fine scale structure exists in the coarse scale structure,i.e.,the structure on the non-sintered surfaces is a specific micro/nano-composite structure,which can account for the superhydrophobicity of the prepared surfaces.Comparing Fig.2 with Fig.1,it is clear that the sintering process has made a great impact on the morphology of the prepared surfaces.After the sintering process,the micro/nano-structure has great changes,which looks like aggregated cells with an average diameter about 500 nm[Fig.2(c3)].In the sintering process,diffusion and coalescence take place among the micro-particles,which makes the particles get together closely and changes the morphology markedly.As a result,compared with the non-sintered surfaces,the wettability of the sintered surfaces increases greatly and a transition from superhydrophobicity to superhydrophilicity happens.

Fig.1.SEM images of the non-sintered surfaces with different current densities and deposition times:(a)10 mA·cm-2,20 min;(b)20 mA·cm-2,15 min;(c)30 mA·cm-2,15 min;(d)40 mA·cm-2,12 min;and(e1)–(e3)50 mA·cm-2,12 min.

Fig.2.SEM images of the sintered surfaces with different current densities and deposition times:(a)10 mA·cm-2,20 min;(b)20 mA·cm-2,15 min;(c1)–(c3)30 mA·cm-2,15 min;(d)40 mA·cm-2,12 min;and(e)50 mA·cm-2,12 min.

3.2.Conversion from superhydrophobic to superhydrophilic

Fig.3(a)illustrates the impact of deposition time and current density on the CA of the non-sintered surfaces.The overall trend is that when the current density is constant,the CA increases quite fast at first and then decreases slowly with the increasing of deposition time.The CA of the superhydrophobic surface is as high as 155±1°when the current density is 30 mA·cm-2and the deposition time is 10 min.In Fig.3(b),it is evident that with a constant current density the hydrophilicity of the sintered copper surface enhances while the deposition time increases.As is depicted in Fig.3(b)the CA of the superhydrophilic surfaces achieves a minimum value of 0°.

The conditions of water droplet on the different obtained surfaces are shown in Fig.4.The surfaces of the non-sintered samples reveal superhydrophobicity with contact angles of 155± 1°[Fig.4(a),(b)and(c)],while the sintered surfaces are very hydrophilic with contact angles of 5± 1°[Fig.4(d)],10± 1°[Fig.4(e)]and 7± 1°[Fig.4(f)],respectively.As is shown in Fig.5(a),the water droplet is incapable of adhering to the superhydrophobic surface which has an anti-wetting property.Fig.3(b)describes the superhydrophilicity of the sintered surface(with a CA of 0°)where the water droplet spreads out rapidly.

3.3.Composition and structure of the prepared surfaces

The chemical composition of the prepared surfaces is examined by EDS(Energy Dispersive Spectrometry).The samples are prepared at a current density of 30 mA·cm-2and a deposition time of 15 min.In Fig.6(a),the EDS spectrum reveals that the non-sintered surface is mainly composed of Cu and O elements.The mass ratio of Cu to O is 98.53/1.47 and atomic ratio is about17/1,i.e.,both Cu and cooperoxides exist in the superhydrophobic surface.Fig.6(b)shows that the sintered surface almost consists of Cu whose mass fraction is 99.55%.According to the EDS results,one can see that the reduction of O results from the sintering process,in which the copper oxides are reduced to Cu.

XRD(X-ray diffraction)patterns are used to analyze the phases and crystal structures of the sample surfaces[Fig.7(a)and(b)].Analyzed by JADE software,referring to JCPDS standard card the non-sintered surface contains three components:Cu,Cu2O and CuO,while the sintered surface consists only of Cu.The most intensive peaks are Cu diffraction peaks,at 43.306°,50.419°,and 74.054°[Fig.7(a)],and 43.365°,50.460°,and 74.077°[Fig.7(b)],which are assigned to(111),(200)and(220)diffraction lattice planes,respectively.Comparing Fig.7(a)with Fig.7(b),the pattern of non-sintered surface includes several other diffraction peaks at 36.437°,42.300°and 61.364°,which can be attributed to the Cu2O(111),Cu2O(200)and CuO(220)planes of copper oxide crystals.According to the peak search report,the FWHM(full width at half maximum)of the copper diffraction peaks is 0.207°,0.204°,and 0.217°[Fig.7(a)],and 0.086°,0.106°,and 0.089°[Fig.7(b)].Apparently,the Cu diffraction peaks of the non-sintered surface are wider and more dispersive than those of the sintered surface.Additionally,the Cu diffraction peaks in Fig.7(b)are more intensive than those in Fig.7(a),i.e.,the crystal structure is more inerratic and the grain size becomes bigger after the sintering process.Combined with the SEM results,the morphology,chemical composition and crystal structure have great changes after the sintering process,which leads to a conversion of the prepared surfaces from superhydrophobic to superhydrophilic.

Fig.3.CAof the obtained surfaces with water:(a)non-sintered and(b)sintering at500°C.

3.4.The wetting mechanism

Most previous studies reported that both low surface free energy and rough micro structure are indispensable to acquire a superhydrophobic surface.However,some researchers hold different opinions[24,25]and propose that some special fine-scale composite structures(double or multiple roughness structures)can make the surfaces of any material become non-wetting and a droplet may be suspended on a specific composite structure in the Cassie state[Fig.8(a)]even on the surface of a hydrophilic material.As a result,hydrophobic or even superhydrophobic surfaces can be prepared by using hydrophilic materials without chemical modification.The chemical components of the superhydrophobic surfaces prepared in this study are copper and copper oxide,which have high surface free energy.The water droplets do not spread out on these superhydrophobic surfaces because they are in the Cassie state.Cassie considered the contact between water droplet and rough surface with hydrophobic structure as a composite contact.The liquid does not fill the grooves on the rough surface and there is air trapped between grooves and liquid.That is to say,the total interface actually consists of liquid–solid interface and liquid–gas interface.Based on the Cassie theory[26]:

where θris the apparent CA and θeis the intrinsic CA of a liquid drop on the ideal smooth surface.Φsis the area fraction of the liquid–solid contact.If Φs＜ 1/(cosθe+1),θrwill be greater than 90°,even on the surface with high surface free energy,i.e.,hydrophobicity and even superhydrophobicity can be achieved by building composite structures of specific parameters on hydrophilic substrates.

There is an energy barrier between the Cassie and Wenzel states.An external interference may transform one state to the other by overcoming the energy barrier[27,28].The conversion from superhydrophobic surface to superhydrophilic surface is ascribed to the sintering process which transforms the Cassie state on the prepared surface into the Wenzel state[Fig.8(b)],in which the liquid–solid contact area is considered greater than the apparent geometric area,because the liquid fills up the grooves on the rough surface.According to the Wenzel theory[29]:

Fig.4.Images of water droplet after sitting on non-sintered surfaces:(a)20 mA·cm-2,12 min;(b)30 mA·cm-2,10 min;and(c)40 mA·cm-2,12 min;and on sintered surfaces:(d)10 mA·cm-2,15 min;(e)20 mA·cm-2,12 min;and(f)50 mA·cm-2,8 min.

Fig.5.Images of a pendent water drop around the instant of contact with(a)adhesion resistance of the superhydrophobic surface,and(b)superhydrophilic sintered surface.

Fig.6.EDS spectra of the prepared surfaces:(a)non-sintered and(b)sintered.

By definition,r is the roughness factor which is equal to the ratio of the actual area to the projected area on the horizontal plane.On copper surfaces which have high surface free energy,θeis less than 90°,therefore,θr＜ θe.In the Wenzel state,microstructure makes the hydrophilic surface more hydrophilic.When r is very close to 1/cosθethe rough surface exhibits superhydrophilic property.

Fig.7.XRD patterns of the obtained surfaces:(a)non-sintered and(b)sintered.

Fig.8.(a)Cassie mode of superhydrophobicity;(b)Wenzel mode of superhydrophilicity.

3.5.Interfacial strength of the superhydrophilic surfaces

The interfacial strength between superhydrophilic layer and copper substrate is investigated in the following simple test.The superhydrophilic surface was made into a wall of a micro-channel(height:0.5 mm,width:7 mm,length:50 mm)and the water flux in the channel versus the pressure drop along the channel was measured by a pressure transmitter.The results in Fig.9 indicate that the superhydrophilic structure remains intact(with a contact angle of 0°)when the pressure drop is less than 50 kPa,and the deflected curve at 50 kPa suggests some extent of partial destruction of the superhydrophilic surface structure.In contrast,the superhydrophobic structure of the same channel configuration(originally with CA of 150°)was partly damaged and the contact angle decreased to below 130°when the applied pressure drop was more than 5 kPa.The experimental result shows that superhydrophilic copper surfaces have higher interfacial strength,which indicates the potential for practical application.

Fig.9.Relationship between flux and inletpressure of the superhydrophilic microchannel.

4.Conclusions

The superhydrophobic surface(with a contact angle of 155°)prepared by electrochemical deposition on copper substrate can be converted into a superhydrophilic surface(with a CA of 0°)via a reduction sintering process.After the sintering process,the morphology,chemical composition and crystal structure of the prepared surface have changed significantly,which overcomes the energy barrier between the Cassie and Wenzel states.The superhydrophilic surfaces fabricated in this experiment show high interfacial strength,which indicates the prospect for practical application.