Applied Surface Science 256 (2009) 61–66

Superhydrophobic surfaces fabricated by microstructuring of stainless steelusing a femtosecond laser

Bo Wu a,b, Ming Zhou a,*, Jian Li a, Xia Ye a, Gang Li a, Lan Cai a

a Center for Photon Manufacturing Science and Technology, Jiangsu University, Zhenjiang, Jiangsu 212013, Chinab School of Mechanical Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China

A R T I C L E I N F O

Article history:

Received 10 May 2009

Received in revised form 17 July 2009

Accepted 17 July 2009

Available online 25 July 2009

Keywords:

Femtosecond laser

Stainless steel

Double-scale structure

Superhydrophobic surface

A B S T R A C T

Fabrication of superhydrophobic surfaces induced by femtosecond laser is a research hotspot of

superhydrophobic surface studies nowadays. We present a simple and easily-controlled method for

fabricating stainless steel-based superhydrophobic surfaces. The method consists of microstructuring

stainless steel surfaces by irradiating samples with femtosecond laser pulses and silanizing the surfaces.

By low laser fluence, we fabricated typical laser-induced periodic surface structures (LIPSS) on the

submicron level. The apparent contact angle (CA) on the surface is 150.38. With laser fluence increasing,

we fabricated periodic ripples and periodic cone-shaped spikes on the micron scale, both covered with

LIPSS. The stainless steel-based surfaces with micro- and submicron double-scale structure have higher

apparent CAs. On the surface of double-scale structure, the maximal apparent CA is 166.38 and at the

same time, the sliding angle (SA) is 4.28.� 2009 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

1. Introduction

Wettability is an important property of solid surfaces. Generallyspeaking, wettability is distinguished through CA. Hydrophobicsurfaces yield CAs higher than 908, and hydrophilic surfaces yieldCAs lower than 908. Superhydrophobic surface exhibits a water CAlarger than 1508 and a SA smaller than 108 [1–3]. Such surfaces areexpected to be widely used in the fields of national defense,industrial and agricultural production and people’s daily life [4].For instances, in preventing glass pollution, keeping satelliteantenna and radar’s surfaces clean, prohibiting microbe adhering,reducing resistance on the pipe wall etc.

The wettability of a solid surface depends on both its chemicalnature [5] and topology [6–8]. Lowering the surface energy of asmooth and flat substrate cannot yield water CAs higher than 1208[9,10]. Microstructuring on surfaces is one of the effective ways offabricating superhydrophobic surfaces. Two models were pro-posed to describe the effects of surface roughness on hydro-phobicity [11,12]. In the Wenzel model, the liquid is assumed to bein contact with every part of the rough surface. Wenzel proposedthat the apparent CA, uw

r , on the rough surface is given by [11]

cosuwr ¼ r cosue (1)

* Corresponding author. Tel.: +86 511 88791458; fax: +86 511 88791288.

E-mail address: zm_laser@126.com (M. Zhou).

0169-4332/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2009.07.061

where r is the ratio between the actual and projected surface areasand ue is the intrinsic CA of liquid droplet on flat surface. Because r

is always greater than 1, with ue lower than 908, roughnessdecreases the CA while with ue larger than 908, roughness increasesthe CA.

The Cassie model assumes that the liquid does not completelywet the rough substrate. The contact of a liquid droplet on therough surface should be a composite contact between the dropletand the compound surface made of substrate material and air.Hence, the apparent CA, uC

r , on the rough surface is given by [12]

cosuCr ¼ f 1ðcosue þ 1Þ � 1 (2)

Where f1 represents the fraction of the solid in contact with theliquid and ue is the intrinsic CA of the liquid droplet on the flatsurface. From Eq. (2), it can be predicted that decreasing the valueof f1 should increase the value of uC

r , independent of the value of ue.So the larger fraction of air trapped within the interstices of therough surface, the smaller the value of f1 and the larger theapparent CA on rough surface.

In recent years, methods have been employed in the prepara-tions of metal-based superhydrophobic surfaces by microstructur-ing on the surfaces of metal material, such as electrochemicaldeposition [13–16], electroless replacement deposition [17],chemical etching [18–22], plasma assisted thermal vapor deposi-tion [23], natural oxidation [24], laser etching [25] etc. However,studies of stainless steel-based superhydrophobic surfaces not

B. Wu et al. / Applied Surface Science 256 (2009) 61–6662

enough, which doesn’t match well the wide application of stainlesssteel in architecture, industrial establishments and medicalinstruments. In the above fields, water and material surfacesinteract frequently, and this provides a bright application prospectfor superhydrophobic surface studies.

The lotus leaf like surface with double-scale structures is moreeasily to become superhydrophobic [26–28]. Kietzig et al. havecreated certain double-scale rough structures by femtosecondlaser irradiation on different metal alloys. After about 30 days,these surfaces change from superhydrophilic to superhydrophobic[26]. Zorba et al. and Barberoglou et al. have prepared stablesuperhydrophobic and highly water repellent surfaces by irradiat-ing silicon wafers with femtosecond laser pulses and subsequentlycoating them with chloroalkylsilane monolayers. These surfacespossessing hierarchical micro- and nanostructures exhibitingcontrolled double-scale roughness, qualitatively and quantita-tively mimic both the structure and the water repellentcharacteristics of the natural lotus leaf [27,28].

Here, we present a simple and easily-controlled method forfabricating stainless steel-based superhydrophobic surfaces by theway of microstructuring using a femtosecond laser and furthersilanization. Furthermore, we give a series of simplified geometricmodels of these rough microstructures. By these geometricmodels, apparent CAs of these microstructures were calculated.The calculated values are consistent with the experiment result.These values of apparent CAs can then be used to identify the truewetting state of these surfaces.

2. Experimental section

In our experiment, we used AISI 316L type austenitic stainlesssteel plates with diameter 15 mm and thickness 0.2 mm. Thesamples were mechanically polished, and were cleaned with a30 min ultrasonic bath in deionized water, followed by a 30 minultrasonic bath in acetone.

A commercial Regenerative/Multi-pass Ti: sapphire lasersystem that provides 800 nm central-wavelength pulses with aduration of 130 fs at a repetition rate of 1 kHz was used for theexperiment. Single pulse energy can be adjusted up to 1 mJ byusing an attenuator. The laser beam was focused to a diameter ofabout 180 mm perpendicularly onto the samples. The laserscanning route consisted of some parallel beelines and thedistance between adjoining beelines was 30 mm. The laserscanning velocity was 1 mm/s, and achieved by two-dimensionalgalvanometric mirrors. The processing chamber was pumpeddown with a mechanical pump and a molecular pump to get thecondition under vacuum (3.5 � 10�3 Pa). Having been treated byfemtosecond laser, the samples were examined by scanningelectron microscopy (SEM).

Before measuring the CA, the samples were silanized in avacuum oven using silane reagent (Trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane, 97%, 90 8C, 70 min, 0.1 torr) to lower theirsurface energy. Apparent CAs on surfaces were measured by videooptic CA instrument modelled OCAH200. The CAs and SAs weremeasured by using a video-frequency optics goniometer. Theselected water droplet volume was V = 1 mL. The apparent CA andSA of the water droplet on each sample surface were measured forfive times and the average value was adopted.

3. Results and discussion

Fig. 1 shows a sequence of SEM micrographs of AISI 316L typeaustenitic stainless steel-based surfaces at various laser fluences.We obtained some different microstructures on the samplesurfaces at different incident laser fluences. Typical LIPSS, whichare perpendicular to the direction of the polarization of the laser

beam, are formed at fluences between 0.08 and 0.2 J/cm2. Theripples are covered with nanoparticles. The periodicity of LIPSS isabout 0.5 mm. The width and the height of the ridge of the rippleare both about 0.35 mm. Another type of periodic micron ripples,which are along the direction of the polarization of the laser beam,begins to appear at laser fluence 0.24 J/cm2. The periodicity of theripples is about 3.5 mm. The periodicity and the height areincreasing with increasing of laser fluence. The periodic rippleswith micron periodicity are covered with LIPSS, whose periodicityand direction are similar to the ones fabricated at lower laserfluences. A few separated protrusions, whose width is a littlebigger than that of the micron ripples, begin to appear on theperiodic micron ripples at laser fluence 0.4 J/cm2. In the meantime,the periodic micron ripples change gradually from continued tobroken. With laser fluence increasing, both the height and thenumber of the protrusions increase, and the protrusions change toconical-shaped spikes. At higher laser fluence of 0.8 J/cm2, theconical-shaped spikes covered with LIPSS be sprinkle the entiresurface. The base of each spike has an asymmetric shape, with theshort axis of the base always parallel to the direction of the laserpolarization. The lengths of the long and the short axis of thesespikes’ base are about 5–8 mm and 3–6 mm respectively. Both theheight and the distance between spikes increase with laser fluenceincreasing.

LIPSS with a periodicity on the submicron level can beinduced from the interference between the incident laser lightand the scattered tangential wave originated from previous laserpulses [29]. Usually, LIPSS shows a regular ripple structure witha periodicity on the incident laser wavelength scale and isoriented perpendicularly to the polarization of the incidentlight [30]. In our work, the LIPSS periodicity is about 62.5% ofthe laser wavelength. When laser intensity is low, LIPSS areformed, showing that the submicron structure is induced by lowlaser fluence [30,31]. When laser intensity is high, the centralfluence of the spot is high. Under repetitious impulses, thematerial in the spot center melts and due to extreme heat, liquidmetal undergoes phase explosion, cools rapidly, freezes and as aresult, microscale roughness is formed [29]. As spot centermoves, spot fringe with low fluence begins to induce materialand thus, LIPSS are formed on the microroughness. Finally,micro- and submicron double-scale structure is formed as shownin Fig. 1. So double-scale structure is caused by nonuniformityof high-intensity spot’s fluence distribution and uniform motionof the spot.

Wettability of the stainless steel-based surfaces is distin-guished through apparent CA. Samples microstructured by laserand flat samples are silanized to lower the surface energy. Fig. 2shows images of water droplets on flat, LIPSS and double-scalestructure stainless steel-based surfaces. The CA on the flat surfaceis 113.0 � 0.98 as shown in Fig. 2(a). As shown in Fig. 2(b) and (c), theapparent CAs on LIPSS and double-scale structure surfaces are150.3 � 1.38 and 166.3 � 1.18, respectively.

Fig. 3 shows the relationship between apparent CAs and laserfluences giving rise to microstructures. The data points on thevertical axis at zero laser fluence correspond to the CAs on flatstainless steel-based surfaces. The apparent CAs upon micro-structuring surfaces with laser fluences of 0.08 J/cm2, 0.8 J/cm2 and2.4 J/cm2 are 150.3 � 1.38, 162.7 � 1.18 and 166.3 � 1.18 respec-tively. Compared with that on flat surfaces, apparent CA at 0.08 J/cm2

laser fluence increases by 37.38 and apparent CA increases by 12.48when laser fluence is in the range of 0.08–0.8 J/cm2. Between 0.8 and2.4 J/cm2, apparent CA increases by 3.68. Having been treated withfemtosecond laser, apparent CAs on microstructure surfaces arelarger than 1508. This meets the requirement of superhydrophobicityto CA. Especially; when the laser fluences are higher than 0.6 J/cm2,apparent CAs stably reach above 1618.

Fig. 1. Top (left), side (308) (middle) and profile view (right) SEM micrographs of AISI 316L type austenitic stainless steel-based surfaces at various laser fluences.

B. Wu et al. / Applied Surface Science 256 (2009) 61–66 63

Fig. 2. Photographs of water droplets on flat (a), LIPSS (b) and double-scale structure (c) AISI 316L type austenitic stainless steel-based surfaces after silanization.

Fig. 3. Apparent water CAs (*) on microstructured stainless steel-based surfaces as

a function of corresponding laser fluences.

B. Wu et al. / Applied Surface Science 256 (2009) 61–6664

To identify the true wetting state of the surfaces, we made somesimplified geometric models of the microstructure. Fig. 4 showsschematic illustration of LIPSS forming at fluences 0.08 J/cm2.Many nanoparticles are arrayed on the surface of LIPSS. Assumingthat nanoparticles are hemispherical and closely arrayed (Fig. 4(a))and the water droplet is in contact with every part of nanoparticles,

Fig. 4. Schematic illustration of LIPSS covered with nanoparticles formed at fluences 0.08

the sort water interface with the top of LIPSS.

Fig. 5. Schematic illustration of periodic micron ripples covered with LIPSS forming at flu

local interaction of the water interface with the top of micron ripples.

on the basis of simple geometry, the apparent CA (uWr ) of the flat

surface coved with nanoparticles, according to the Wenzelformula, can be given by:

cosuWr ¼ 1þ p

4

� �cosue (3)

where the intrinsic CA (ue) is substituted by the CA of flat surface(113.08), as a result, uW

r ¼ 134:2�. For LIPSS, cross-section is ahemicycle with d = 0.35 mm. and P = 0.50 mm (Fig. 4(b)). Generallythe Wenzel model is applied to describe low hydrophobic surfaces(whose apparent contact angle is slightly larger than 908), whilethe Cassie model is applied to describe highly hydrophobicsurfaces [25,32]. Since the measured apparent CA is large, thenthe wetting state accords with the Cassie model, the apparent CA ofLIPSS (uC

r ) can be given by:

cosuCr ¼

dsinue

P

� �ðcosue þ 1Þ � 1 (4)

Where the intrinsic CA (ue) is substituted by the apparent CA ofthe flat surface coved with nanoparticles (134.28), we can get uC

r ¼148:0� which is almost consistent with the experiment result(150.3 � 1.38).

J/cm2. (a) The alternating array of LIPSS and nanoparticles. (b) The local interaction of

ences 0.24–0.6 J/cm2. (a) The alternating array of micron ripples and LIPSS. (b) The

Table 1Geometric parameters and CAs of periodic micron ripples obtained at different laser

fluences.

Laser

fluence

(J/cm2)

Geometric parameter uWr (8) uC

r (8) ur (8)

b

(mm)

H

(mm)

P

(mm)

0.24 3.71 0.80 4.01 163.7 156.9 154.6�1.2

0.4 4.20 1.50 4.47 cosuWr < � 1 161.9 159.1� 0.8

0.6 4.39 2.03 4.70 cosuWr < � 1 164.2 161.2�1.0

Table 2Geometric parameters and CAs of micron conical-shaped spikes obtained at

different laser fluences.

Laser

fluence(J/cm2)

Geometric parameter uCr (8) uC

r of

0.49H (8)ur (8)

d

(mm)

H

(mm)

P

(mm)

0.8 6.35 4.84 7.26 175.7 163.7 162.7�1.1

1.2 9.82 8.80 10.70 176.2 162.9 163.8�0.9

1.6 8.06 10.56 11.36 178.0 166.8 164.6�1.3

2 13.76 15.07 16.21 177.1 164.2 165.5�1.2

2.4 15.63 18.02 18.82 177.3 164.5 166.3�1.1

B. Wu et al. / Applied Surface Science 256 (2009) 61–66 65

Fig. 5 shows schematic illustration of periodic micron ripplescovered with LIPSS forming at fluences 0.24–0.6 J/cm2. The profileof periodic micron ripples is parabola (y = ax2). Width is b, height isH, periodicity is P (Fig. 5), and parameters are listed in Table 1. Onthe basis of simple geometry, cosuW

r and uCr can be given by:

cosuWr ¼

2H

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffib2

16H2þ 1

sþ b2

8Hln

4H

b

1þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffib2

16H2þ 1

s !!

þ p� b

!= p

!cosue (5)

cosuCr ¼ � b2tanue

4HP

!ðcosue þ 1Þ � 1 (6)

Where the intrinsic CA (ue) is substituted by the apparent CA ofLIPSS (150.38), and calculation result is shown in Table 1. There is aconsiderable discrepancy between calculated uW

r of 0.24 J/cm2 andthe experiment result. Both calculated cosuW

r of 0.4 J/cm2 and thatof 0.8 J/cm2 are less than �1. This is because Eq. (1) is notappropriate to be applied in calculating CA of extremely roughsurface. On contrary, calculated uC

r accords with the experimentresult, with the largest discrepancy 3.08. The wetting state ofperiodic micron ripples can be described by the Cassie model.

Fig. 6 shows schematic illustration of micron conical-shapedspikes covered with LIPSS forming at fluences 0.8–2.4 J/cm2. The

Fig. 6. Schematic illustration of micron conical-shaped spikes covered with LIPSS formin

The local interaction of the water interface with the top of micron spikes. (c) The loca

profile of spike is parabola (y = ax2) [33]. Diameter is d, height is H,distance between spikes is P (Fig. 6), and parameters are listed inTable 1. Since spikes are rougher than the micron ripples of 0.6 J/cm2, Eq. (1) is not appropriate. On the basis of simplified geometry,the apparent CA (uC

r ), according to the Cassie model, can be givenby:

cosuCr ¼ �

pd4tan2ue

64H2P2ðcosue þ 1Þ � 1 (7)

Where the intrinsic CA (ue) is substituted by the apparent CA ofLIPSS (150.38), calculation results are shown in Table 2. CalculateduC

r by Eq. (7) are larger than the measured results. Probably becausethe fraction of spikes in contact with the water droplet is very little,the very high local pressure is produced, and then the waterbottom interface falls towards the bottom of the spikes. Assumingthat the distance between the water bottom interface and thebottom of the spikes is zH (Fig. 6(c)), the apparent CA (uC

r ) can begiven by:

cosuCr ¼

pð1� zÞd2

4P2ðcosue þ 1Þ � 1 (8)

Measured results, intrinsic CA (ue = 150.38) and the geometricparameters of corresponding laser fluences are substituted in

g at fluences 0.8–2.4 J/cm2. (a) The alternating array of micron ripples and LIPSS. (b)

l interaction of the water interface in the middle of micron spikes.

Table 3Measured SAs (a) and calculated f1 of microstructured stainless steel-based

surfaces of corresponding laser fluences.

Laser fluence(J/cm2) a (8) f1

0.08 No sliding 0.43

0.24 34.9�2.3 0.32

0.4 15.0�1.8 0.22

0.8 9.9�1.5 0.15

1.2 5.9�1.3 0.13

1.6 5.1� 0.9 0.12

2.4 4.2�1.0 0.09

B. Wu et al. / Applied Surface Science 256 (2009) 61–6666

Eq. (8), and values of z are calculated. The average value is 0.49.Replacing the value of z (0.49), d, P and ue (150.38) into Eq. (8), weobtain the values of uC

r , which are showed in Table 2. The calculatedvalues of uC

r are consistent with the experiment results with thelargest discrepancy 2.28. Thus, the bottom of the water dropletlocated at 0.49H is far away from the bottom of the spikes.

It can be seen from Table 3 that the SAs (a) on microstructuredstainless steel-based surfaces at a fluence larger than 0.8 J/cm2 aresmaller than 108. For these fluences, the microstructured stainlesssteel-based surfaces meet the criteria for a superhydrophobicsurface. Especially, the SA at a fluence of 2.4 J/cm2 is 4.2 � 1.08.Comparatively, the microstructured surface at a fluence of 0.24 J/cm2

exhibits a SA (34.9 � 2.38) larger than 108.The intrinsic CA of water droplet on the surface covered with

nanoparticles (ue = 134.28) and the apparent CAs ðuCr Þ of the rough

surfaces are introduced into Eq. (2), and then f1 value is estimated(Table 3). The corresponding f1 value of 0.08 J/cm2 is 0.43, whichmeans the fraction of the surface in contact with the water is 43%.This indicates that the contact of water droplet with the ridge ofripples forms the stable and continuous contacting line. Hence,even though the surface becomes erect, the droplet will not fall.The a value is decreasing along with the decreasing of f1.The f1

value of 2.4 J/cm2 is estimated to be 0.09. The fraction of thedouble-scale structure surface in contact with the water droplet is9%, which shows that the contact of water droplet with the ridge ofLIPSS on the top of spikes forms the unstable and isolatedcontacting spots. This leads to extremely large apparent CA andextremely small SA (4.2 � 1.08), which meets the requirement ofsuperhydrophobicity of the surface. On the other hand, air occupiesabout 91% of the contact area between the water droplet and thedouble-scale structure, which also leads to superhydrophobicity ofthe surface [34].

4. Conclusion

We fabricated stainless steel-based superhydrophobic surfacesby microstructuring using a femtosecond laser and the method ofsilanization. We also investigated the influence of laser fluence onapparent CA. By adjusting laser fluences, we fabricated variousmicrostructures. LIPSS whose apparent CA is 150.38 can be inducedby the laser at low fluence. With the increase of laser fluence,periodic ripples and cone-shaped spikes on the micron scalecovered with LIPSS were formed. These micro- and submicrondouble-scale structure surfaces yield apparent CAs higher thanthose on single scale structure surfaces, and the maximal value is

166.38, and meanwhile, the extremely small SA (4.28) is alsoobtained. By the simplified geometric models of the microstruc-ture, we found that the fraction of air between the water dropletand the double-scale surface is large, which is the important reasonof superhydrophobicity of the surface. This provides a simple andeasily-controlled method for fabricating stainless steel-basedsuperhydrophobic surfaces.

Acknowledgements

This work is supported by the National Natural ScienceFoundation of China (Key Program, Grant No. 50435030,50775104), the Program for New Century Excellent Talents ofMinistry of Education of Chinese and Excellent Young ScholarsFoundation of Jiangsu (BK 2006507) and the National HighTechnology Research and Development Program of China (‘‘863’’Program, Grant No. 2006AA04Z307).

References

[1] M. Miwa, A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Langmuir 16 (13)(2000) 5754–5760.

[2] L. Jiang, Y. Zhao, J. Zhai, Angew. Chem. Int. Ed. 43 (2004) 4338–4341.[3] L. Feng, Z.Y. Zhang, Z.H. Mai, Y.M. Ma, B.Q. Liu, L. Jiang, D.B. Zhu, Angew. Chem. Int.

Ed. 43 (2004) 2012–2014.[4] B.L. Feng, S.H. Li, Y.S. Li, H.J. Li, L.J. Zhang, J. Zhai, Y.L. Song, B.Q. Liu, L. Jiang, D.B.

Zhu, Adv. Mater. 14 (24) (2002) 1857–1860.[5] Y. Thmos, Philos. Trans. R. Soc. London 95 (1805) 65–87.[6] R.N. Wenzel, Ind. Eng. Chem. 28 (1936) 988–994.[7] L. Jiang, R. Wang, B. Yang, T.J. Li, D.A. Tryk, A. Fujishima, K. Hashimoto, D.B. Zhu,

Pure Appl. Chem. 72 (1-) (2000) 73–81.[8] J. Tordeux, C. Bico, D. Quere, Europhys. Lett. 55 (2) (2001) 214–220.[9] N.J. Shirtcliffe, G. McHale, M.I. Newton, C.C. Perry, Langmuir 21 (2005) 937–943.

[10] T. Nishino, M. Meguro, K. Nakamae, M. Matsushita, Y. Ueda, Langmuir 15 (1999)4321–4323.

[11] R.N. Wenzel, Phys. Colloid Chem. 53 (1949) 1466–1467.[12] A.B.D. Cassie, Discuss. Faraday Soc. 3 (1948) 11.[13] C.H. Wang, Y.Y. Song, J.W. Zhao, X.H. Xia, Surf. Sci. 600 (2006) 38–42.[14] L. Wang, S.J. Guo, X.G. Hu, S.J. Dong, Electrochem. Commun. 10 (2008) 95–99.[15] N. Zhao, F. Shi, Z.Q. Wang, X. Zhang, Langmuir 21 (2005) 4713–4716.[16] Q.M. Pan, Y.X. Cheng, Appl. Surf. Sci. 255 (2009) 3904–3907.[17] W. Song, J.J. Zhang, Y.F. Xie, Q. Cong, B. Zhao, J. Colloid Interface Sci. 329 (2009)

208–211.[18] Q.M. Pan, M. Wang, H.B. Wang, Appl. Surf. Sci. 254 (2008) 6002–6006.[19] W.C. Wu, M. Chen, S. Liang, X.L. Wang, J.M. Chen, F. Zhou, J. Colloid Interface Sci.

326 (2008) 478–482.[20] Z.G. Guo, F. Zhou, J.C. Hao, W.M. Liu, J. Colloid Interface Sci. 303 (2006) 298–305.[21] Q. Wang, B.W. Zhang, M.N. Qu, J.Y. Zhang, D.Y. He, Appl. Surf. Sci. 254 (2008)

2009–2012.[22] D.K. Sarkar, M. Farzaneh, R.W. Paynter, Mater. Lett. 62 (2008) 1226–1229.[23] G.X. Li, B. Wang, Y. Liu, T. Tan, X.M. Song, H. Yan, Appl. Surf. Sci. 255 (2008) 3112–

3116.[24] X.M. Hou, F. Zhou, B. Yu, W.M. Liu, Mater. Sci. Eng. A 452 (2007) 732–736.[25] B.J. Li, M. Zhou, R. Yuan, L. Cai, J. Mater. Res. 23 (9) (2008) 2491–2499.[26] A.M. Kietzig, S.G. Hatzikiriakos, Peter Englezos, Langmuir 25 (2009) 4821–4827.[27] V. Zorba, E. Stratakis, M. Barberoglou, E. Spanakis, P. Tzanetakis, S. Anastasiadis, C.

Fotakis, Adv. Mater. 20 (2008) 4049–4054.[28] M. Barberoglou, V. Zorba, E. Stratakis, E. Spanakis, P. Tzanetakis, S.H. Anastasiadis,

C. Fotakis, Appl. Surf. Sci. 255 (2009) 5425–5429.[29] W. Jia, Z.N. Peng, Z.J. Wang, X.C. Ni, C.Y. Wang, Appl. Surf. Sci. 253 (2006) 1299–

1303.[30] A.Y. Vorobyev, C.L. Guo, Appl. Surf. Sci. 253 (2007) 7272–7280.[31] X.D. Guo, R.X. Li, Y. Hang, Z.Z. Xu, B.K. Yu, H.L. Ma, B. Lu, X.W. Sun, Mater. Lett. 62

(2008) 1769–1771.[32] W. Li, A. Amirfazli, J. Colloid Surf. Sci. 292 (2005) 195–201.[33] A. Marmur, Langmuir 20 (2004) 3517–3519.[34] M.N. Qu, B.W. Zhang, S.Y. Song, L. Chen, J.Y. Zhang, X.P. Cao, Adv. Funct. Mater. 17

(4) (2007) 593–596.