Super-hydrophobic surfaces of SiO2-coated SiC nanowires Fabrication

Short Communication

Super-hydrophobic surfaces of SiO 2-coated SiC nanowires:Fabrication,mechanism and ultraviolet-durable

super-hydrophobicity

Jian Zhao a ,Zhenjiang Li a ,?,Meng Zhang a ,Alan Meng b ,?

a

Key Laboratory of Polymer Material Advanced Manufacturings Technology of Shandong Provincial,Qingdao University of Science and Technology,Qingdao 266061,Shandong,PR China

b

Qingdao 266042,Shandong,a r t i c l e i n f o Article history:

Received 4November 2014Accepted 17December 2014

Available online 26December 2014Keywords:

SiO 2-coated SiC nanowires Super-hydrophobic treatment UV-durable super-hydrophobicity

a b s t r a c t

The interest in highly water-repellent surfaces of SiO 2-coated SiC nanowires has grown in recent years due to the desire for self-cleaning and anticorrosive surfaces.It is imperative that a simple chemical treat-ment with ?uoroalkylsilane (FAS,CF 3(CF 2)7CH 2CH 2Si(OC 2H 5)3)in ethanol solution at room temperature resulted in super-hydrophobic surfaces of SiO 2-coated SiC nanowires.The static water contact angle of SiO 2-coated SiC nanowires surfaces was changed from 0°to 153°and the morphology,microstructure and crystal phase of the products were almost no transformation before and after super-hydrophobic treatment.Moreover,a mechanism was expounded reasonably,which could elucidate the reasons for their super-hydrophobic behavior.It is important that the super-hydrophobic surfaces of SiO 2-coated SiC nanowires possessed ultraviolet-durable (UV-durable)super-hydrophobicity.

ó2014Elsevier Inc.All rights reserved.

1.Introduction

In nature,the super-hydrophobic lotus leaves,water-walking insects and spiders are highly water-repellent [1–3].When water droplets land on their surfaces,they can move freely and roll off with dust particles and surface contaminants.The phenomenon has inspired considerable research on realizing super-hydrophobic behavior and self-cleaning property [4–7].During daily life,super-hydrophobic surfaces with highly water-repellent properties can be used for anti-icing surfaces [8],?uid drag reduction [9]and drop-let manipulation [10].It has been theoretically and experimentally understood that super-hydrophobic surfaces can be obtained by the cooperative effect of low surface energy and topographically rough-ened surface structures.In accordance with the conception,researchers have prepared arti?cial super-hydrophobic surfaces through creating hierarchical micro/nanostructures on specimen

surfaces [11]and chemically modifying their surfaces to reduce sur-

https://www.360docs.net/doc/fd18178155.html,/10.1016/j.jcis.2014.12.057

0021-9797/ó2014Elsevier Inc.All rights reserved.

?Corresponding authors.

E-mail addresses:zjli126@https://www.360docs.net/doc/fd18178155.html, (Z.Li),mengalan@https://www.360docs.net/doc/fd18178155.html, (A.Meng).

face energy[12].Meanwhile,various methods have been reported for creating super-hydrophobic surfaces such as sol–gel processing [13],plasma processing[14]and spray pyrolysis[15].However,all these techniques require either speci?c equipments or dif?cult pro-cess controls.Moreover,the surface morphology and microstruc-ture of the products may be affected greatly during the preparation process.Fortunately,chemical coating approach is completely different from the above-mentioned ones,which exhib-its simple,effective and facile advantages to the fabrication of arti-?cial super-hydrophobic surfaces of nanomaterials[12].

It is well known that SiO2-coated SiC nanowires possess supe-rior mechanical property[16,17],which can hold great promise for application as polymer reinforcements.Unfortunately,SiO2-coated SiC nanowires present super-hydrophilic state themselves, resulting in poor compatibility with the polymer and uneven dis-persion in the matrix.Therefore,super-hydrophobic treatment of SiO2-coated SiC nanowires is of great signi?cance for applying the products to polymer(such as rubber and plastic).In addition, as far as any practical application is concerned,we must consider seriously UV-durability of the super-hydrophobic surfaces of SiO2-coated SiC nanowires,which is highly signi?cant for meeting the needs of people in some special industrial?elds such as high UV irradiation and outdoor applications.To the best of our knowl-edge,the relevant research has rarely been reported so far.There-fore,it is necessary to carry out super-hydrophobic treatment and investigate UV-durability for expanding the scope of their applications.2.Experimental section

2.1.Fabrication of super-hydrophobic surfaces of SiO2-coated SiC Nanowires on the graphite substrate

Firstly,the ethanol solution of FAS with a volume fraction of1% was prepared and the pH value of the solution was adjusted to4.0 using glacial acetic acid.Furthermore,modi?er FAS was deposited onto the nanostructured surfaces by immersing the substrates with SiO2-coated SiC nanowires in the above-prepared solution for24h at room temperature.Finally,the products after FAS coat-ing were subsequently washed with ethanol to remove excess reactants and then they were dried in oven for12h at60°C.

2.2.Characterization

The morphology and chemical elements were characterized by using a JEOL JSM-6?eld emission scanning electron microscopy (FESEM)equipped with an energy dispersive X-ray spectroscopy (EDS)equipment.Further detailed structural information was obtained by using a Hitachi(Tokyo,Japan)H-8100transmission electron microscope(TEM),and the corresponding selected area electron diffraction(SAED).The X-ray powder diffraction(XRD) pattern of the products was recorded by using a Advance D8X-ray diffractometer.The water contact angle(CA)was measured with5l L droplets of deionized water using a contact angle mea-

droplet shape on(a)the surfaces of untreated SiO2-coated SiC nanowires,(b)the super-hydrophobic surfaces of SiO2-coated SiC nanowires.

water droplets on the super-hydrophobic surfaces.(d)XRD pattern,(e)TEM image and corresponding SAED pattern(inset),

spectrum(inset)of SiO2-coated SiC nanowires after super-hydrophobic treatment.

34J.Zhao et al./Journal of Colloid and Interface Science444(2015)33–37

dropped onto the surfaces,it spreads very quickly,suggesting that they have a good af?nity with water.Such a phenomenon is ascribed to the presence of the large amounts of hydroxyl groups on the surfaces of SiO 2-coated SiC nanowires,resulting in super-hydrophilicity.Fig.1b illustrates the water droplet shape on the super-hydrophobic surfaces of SiO 2-coated SiC nanowires.The sta-tic water contact angle of the surfaces is measured to be around 153°,both advancing (154.5°)and receding (152°)water contact angle were almost the same,clearly revealing that the surfaces transform from super-hydrophilicity to super-hydrophobicity.Fig.1c shows a digital photograph of water droplets on the super-hydrophobic surfaces (same sample as in Fig.1b).It can be seen from Fig.1c that the water droplets with the uniform shape deposited on the super-hydrophobic surfaces are nearly spherical.We can believe that it gives a direct demonstration of super-hydro-phobicity across the whole surfaces.Fig.1d shows the typical XRD pattern of the SiO 2-coated SiC nanowires after super-hydrophobic treatment.The broad peak at about 20°corresponds to the amor-phous SiO 2coatings.Besides the diffraction peaks of graphite sub-strate,the major diffraction peaks are assigned to the (111),(200),(220),and (311)re?ections of cubic b -SiC.These values are con-sistent with the known values for b -SiC (JCPDS Card No.29-1129).These results suggest that the crystal phase of the SiO 2-coated SiC nanowires after super-hydrophobic treatment are in agreement with the untreated products (see Fig.S1in Supporting Information ).TEM image and corresponding SAED pattern of the SiO 2-coated SiC nanowires in Fig.1e depict that the nanowires are consist of SiC core and SiO 2shell.Typically,the thickness of the shell is around 15nm,and the diameter of the core is in the range of 20–30nm.Fig.1f depicts the overall surfaces morphology of SiO 2-coated SiC nanowires after super-hydrophobic treatment,which indicates that large quantities of wire-like nanostructures randomly distribute on the substrate.The nanowires with uniform diameters are dense.Their corresponding EDS spectrum,shown in the inset of Fig.1f,illustrates that the wire-like structures surfaces are composed of C,Si,O,and F.And the elemental F peak can be clearly observed in this spectrum,which demonstrates that the elemental F exist on the surfaces of SiO 2-coated SiC

nanowires.

2-coated SiC nanowires surfaces,the black line and the red line represents untreated sample and FAS-treated spectrum (a),O 1s region (b),Si 2p region (c),C 1s region (d),F 1s region (e).

Compared with the untreated products(see Fig.S2in Supporting Information),the morphology of super-hydrophobic SiO2-coated SiC nanowires surfaces does not exhibits obvious change.Conse-quently,according to above analysis results,the super-hydropho-bic surfaces of SiO2-coated SiC nanowires are obtained successfully.

The chemical bonding states of the SiO2-coated SiC nanowires surfaces before and after super-hydrophobic treatment were inves-tigated by XPS analysis as shown in Fig.2.

In Fig.2a,it can be observed that a typical survey spectrum of the untreated sample surfaces(black line)indicates the peaks at 532.6eV and103.2eV are con?rmed to come from the

Si2p in SiO2shell respectively.The information recorded

the super-hydrophobic sample surfaces(red line)shows

the peaks of O1s and Si2p but also C1s and F1s peaks

from FAS molecules.The peaks of O1s and Si2p with

tion spectra can be more clearly observed in Fig.2b and

tively.It can be seen from the above two?gures that intensities of both O1s and Si2p peaks from the super-hydropho-bic sample surfaces are lower than that of the untreated

The reason for this phenomenon is that FAS molecules

to the SiO2coatings surfaces.Furthermore,Fig.2d shows

1s peak at284.8eV,291.4eV,293.3eV assigned to hydrophobic sample surfaces can be unambiguously

CH2,CF2and CF3functional groups,respectively.In

strong peak at688.6eV corresponds well to F1s after super-hydro-phobic treatment,which ascribes to the CF2and CF

groups in the framework of the FAS molecules as

Fig.2e.Notably,there are no peaks observed for C1s

responding to the untreated sample.So XPS characterization stantiates that FAS molecules were successfully bonded surfaces of SiO2-coated SiC nanowires.

As described previously,Wong and his co-workers

and OTS as modi?ers to treat ZnO nanorod/epoxy composite?lms for obtaining the super-hydrophobic surface.And the reaction model of hydrolysis and condensation of the modi?ers was inves-tigated in detail,importantly,which provided precise guideline for functionalizing surfaces of nanomaterials with other modi?ers [18].Herein,on the basis of the concept,a reasonable mechanism of FAS-treated SiO2-coated SiC nanowires surfaces presenting super-hydrophobicity was elaborated as displayed in Fig.3.

The FAS molecules can be hydrolyzed into reactive silanol groups(Si–OH)as shown in Fig.3a.The hydrolysis equation can be placed as following:

M—SieOC2H5TtH2O!M—SiOHtC2H5OHe1T

schematic diagram of super-hydrophobic treatment of SiO2-coated SiC nanowires surfaces.The process of the hydrolysis and polymerization of FAS treatment effect of SiO2-coated SiC nanowires surfaces functionalized with FAS(b).

Fig.4.Changes in the static contact angles of the super-hydrophobic of the surfaces

SiO2-coated SiC nanowires exposed to UV irradiation for different time.The error

bars for each plot represent the maximum and minimum water contact angles.

where M represents the hydrophobic group of the FAS.A large num-ber of hydroxyl groups are inherently present on the untreated SiO2-coated SiC nanowires surfaces as shown in the FTIR spectros-copy recorded from them(see Fig.S3in Supporting Information), imparting a super-hydrophilic state of the nanowires surfaces with very low water contact angle.Moreover,a large amount of silanol groups can condense with the hydroxyl groups on SiO2-coated SiC nanowires surfaces to form stable chemical bonds since silanol groups show much higher chemical activities as displayed in Fig.3a.

The condensation reaction equation can be described as following:

M—SiOHtOH—R!M—Si—O—RtH2Oe2THerein,R stands for the SiO2-coated SiC nanowires surfaces.After condensation reaction,the FAS molecules are bonded to the SiO2-coated SiC nanowires surfaces,and the hydrophobic groups of the FAS molecules are exposed to the outside,which effectively prevent water droplets from contacting of the surfaces.

Fig.3b illustrates that the surfaces of SiO2-coated SiC nanowires can be completely covered by a large amount of FAS molecules, forming self-assembled monolayers.The surface energy of mono-layers can be decreased dramatically because the large electroneg-ativity of?uorine atom combines with the high bond energy of C–F.So,the SiO2-coated SiC nanowires surfaces can exhibit a large static water contact angle up to153°and a hysteresis as low as 2.5°,which belongs to the type of super-hydrophobic surfaces.

It is very vital to investigate the UV-durability of super-hydro-phobic surfaces of SiO2-coated SiC nanowires so as to apply them to special industrial?elds.The UV-durability of the surfaces is esti-mated by measuring the water contact angles of them,which are directly illuminated using the100W mercury lamp for different time.All of the plots show the averaged values of water contact angles measured at three different points on the same sample as dis-played in Fig.4.The error bars for each plot represent maximum and minimum water contact angles.As can be seen from Fig.4,with increasing UV exposure time,the values of water contact angles remain almost unchanged.Notably,after2h UV light irradiation, the super-hydrophobic surfaces can show UV-durable super-hydro-phobicity(water contact angle 151.5±0.5°),which can be attrib-uted to the following factors:The con?nement effect of SiO2shell on the photoactivity of SiC nanowires,since the valence and conduc-tion bands of SiO2(broad forbidden gap of 8.9eV)lie far lower and higher in energy than the corresponding bands of SiC,respec-tively[17].Under UV irradiation,the photogenerated holes,gener-ated in SiC,cannot get across the potential barrier existing between the SiC core and dense SiO2shell.As a result,they cannot react with the FAS monolayer modi?ed on the surfaces of SiO2shell. Moreover,owing to the fact that C–F bonds(the large electronega-tivity of?uorine atom as well as the high bond energy of C–F)are stronger than C–H bonds,so a large number of linear?uorocarbon chains bonded to the SiO2shell surfaces cannot be decomposed under UV irradiation[19].Notably,the intensity( 9mW/cm2)of UV irradiation was several times stronger than that in sunlight,indi-cating that the super-hydrophobic SiO2-coated SiC nanowires may have potential applications in our daily life such as high UV irradiation and outdoor applications.This supports the fact that the chemical coating method is an effective strategy for obtaining of super-hydrophobic surfaces of SiO2-coated SiC nanowires.4.Conclusions

The super-hydrophobic surfaces of SiO2-coated SiC nanowires were obtained by a simple,effective and mild chemical coating approach.Notably,there was almost no change in the morphology, microstructure and crystal phase of SiO2-coated SiC nanowires before and after super-hydrophobic treatment.Moreover,the rea-son for the SiO2-coated SiC nanowire surfaces exhibiting super-hydrophobic behavior was that FAS molecules were successfully bonded to the product surfaces,on which the hydrophobic groups of the FAS molecules point the outside.In addition,the analysis results reveal that the super-hydrophobic surfaces of SiO2-coated SiC nanowires can exhibit UV-durability.Consequently,it is entirely reasonable for us to believe that the technique can achieve large-scale production of the super-hydrophobic SiO2-coated SiC nanowires in new industrial applications.

Acknowledgments

The work reported here was supported by the National Natural Science Foundation of China under Grant Nos.51272117, 51172115,the Natural Science Foundation of Shandong Province under Grant Nos.ZR2011EMZ001,ZR2011EMQ011, ZR2013EMQ006,the Research Award Fund for Outstanding Young Scientists of Shandong Province Grant No.BS2013CL040,the Spe-cialized Research Fund for the Doctoral Program of Higher Education of China under Grant No.20123719110003and the Tackling Key Program of Science and Technology in Shandong Province under Grant No.2012GGX10218,the Application Foundation Research Program of Qingdao under Grant No.13-1-4-117-jch.We express our grateful thanks to them for their?nancial support.

Appendix A.Supplementary material

Supplementary data associated with this article can be found,in the online version,at https://www.360docs.net/doc/fd18178155.html,/10.1016/j.jcis.2014.12.057. References

[1]W.Barthlott,C.Neinhuis,Planta202(1997)1–8.

[2]D.L.Hu,J.W.M.Bush,Nature437(2005)733–736.

[3]M.Dickinson,Nature424(2003)621–622.

[4]L.Zhu,P.Shi,J.Xue,Y.Y.Wang,Q.M.Chen,J.F.Ding,Q.J.Wang,ACS Appl.Mater.

Interfaces6(2014)8073–8079.

[5]L.Y.Shen,B.L.Wang,J.L.Wang,J.H.Fu,C.Picart,J.Ji,ACS Appl.Mater.Interfaces

4(2012)4476–4483.

[6]J.Y.Lee,S.Pechook,D.J.Jeon,B.Pokroy,J.S.Yeo,ACS Appl.Mater.Interfaces6

(2014)4927–4934.

[7]T.H.Li,Q.G.Li,J.Yan,F.Li,Dalton Trans.43(2014)5801–5805.

[8]J.B.Boreyko,C.P.Collier,ACS Nano7(2013)1618–1627.

[9]Y.C.Jung,B.Bhushan,ACS Nano3(2009)4155–4163.

[10]H.Mertaniemi,V.Jokinen,L.Sainiemi,S.Franssila,A.Marmur,O.Ikkala,R.H.A.

Ras,Adv.Mater.23(2011)2911–2914.

[11]H.C.Barshilia,N.Selvakumar,N.Pillai,L.M.Devi,K.S.Rajam,Appl.Surf.Sci.257

(2011)4410–4417.

[12]J.Seo,S.Lee,J.Lee,T.Lee,ACS Appl.Mater.Interfaces3(2011)4722–4729.

[13]R.Taurino,E.Fabbri,M.Messori,F.Pilati,D.Pospiech,A.J.Synytska,J.Colloid

Interface Sci.325(2008)149–156.

[14]B.Balu,V.Breedveld,D.W.Hess,Langmuir24(2008)4785–4790.

[15]N.L.Tarwal,P.S.Patil,Appl.Surf.Sci.256(2010)7451–7456.

[16]Z.L.Wang,Z.R.Dai,R.P.Gao,Z.G.Bai,J.L.Gole,Appl.Phys.Lett.77(2000)3349–

3351.

[17]Z.J.Li,J.Zhao,M.Zhang,J.Y.Xia,A.L.Meng,Nano Res.7(2014)462–472.

[18]Y.Liu,Z.Y.Lin,W.Lin,K.S.Moon,C.P.Wong,ACS Appl.Mater.Interfaces4

(2012)3959–3964.

[19]Y.H.Xiu,D.W.Hess,C.P.Wong,J.Colloid Interface Sci.326(2008)465–470.

J.Zhao et al./Journal of Colloid and Interface Science444(2015)33–3737