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Applied Surface Science 255 (2008) 3371–3374

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Superhydrophobic or superhydrophilic surfaces regulated by micro-nanostructured ZnO powders

Xingfu Zhou a,*, Xuefeng Guo b, Weiping Ding b,**, Yi Chen b

a College of Chemistry and Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR Chinb Lab of Mesoscopic Chemistry and School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China

A R T I C L E I N F O

Article history:

Received 30 July 2008

Received in revised form 21 September 2008

Accepted 21 September 2008

Available online 14 October 2008

Keywords:

ZnO powder

Micro-nanostructure

Superhydrophobic surface

Superhydrophilic surface

A B S T R A C T

The present study demonstrates a simple technique to decorate a surface for superhydrophobic o

superhydrophilic properties by randomly coating an inorganic oxides powder. The superior propertie

are stable in air for more than half a year. The particulates of the powder are made from aligned singl

crystalline ZnO nanorods, which aggregate to microspheres with ð0 0 0 1̄Þ ends pointed outside of th

spheres. The density of surface hydroxyls of ZnO nanorods and the micro-nanostructures of th

particulates are responsible for their superior properties.

� 2008 Elsevier B.V. All rights reserved

Contents lists available at ScienceDirect

Applied Surface Science

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1. Introduction

In recent years, the study of superhydrophilic and/or super-hydrophobic techniques has been a focus of researchers [1–3]. Ingeneral, superhydrophobic surfaces with a water contact angle(CA) greater than 1508 can be obtained by controlling thetopography of hydrophobic surfaces, while superhydrophilicsurfaces with a CA less than 58 can be realized through a three-dimensional or two-dimensional capillary effect on hydrophilicsurfaces [4,5]. In the Wenzel case, the liquid completely fills thegrooves of the rough surface where they contact, the surfaceroughness dramatically enhances the CA on the hydrophobicsurface but decreases the CA on the hydrophilic surface owing tothe capillary effect [6]. In the Cassie theory, vapor pockets areassumed to be trapped underneath the liquid: the larger thefractional interfacial area of the trapped air, the more hydrophobicthe surface [7]. To date, superhydrophilic or superhydrophobicsurfaces directly regulated by powder have been scarcely reportedsince powders cannot give well-defined surfaces with such uniqueproperty. The fundamental of these phenomena proposes theessential of the combination of hierarchical micro-nanostructuresfor superhydrophilicity or superhydrophobicity. Based on thisideal, how to obtain the superhydrophilic or superhydrophobic

* Corresponding author. Tel.: +86 25 83587773.

** Corresponding author.

E-mail address: zhouxf@njut.edu.cn (X. Zhou).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.09.080

surfaces directly regulated by inorganic oxides powder? Themicro/nanostructure of the constituent units of powder should bedexterously designed. Firstly, the geometric structure of consti-tuent building blocks of the powder should be isotropic spherewith high symmetry, which provided the same structure whenstacking onto the surfaces. Secondly, the constituent sphericalpowders should have micro/nanostructures which enhance theirwettability.

ZnO, an n-type II–VI semiconductor with a direct wide band gap(3.37 eV) and a large exciton binding energy (60 meV) at roomtemperature, possessing unique electrical and optical properties, isa widely used material [8–10]. There have been some studiesreporting the superior wettability of a film made from aligned ZnOnanorods [11,12], and controllable wettability of an aligned ZnOnanorod film is also reported [13–15], of which superhydropho-bicity or superhydrophilicity can be reversibly switched byalternation of UV irradiation and dark storage or oxygen plasmatreatment. The arrayed ZnO nanowires treated with stearic acidalso has been used to fabricate superhydrophobic or super-hydrophilic surfaces [16,17]. The phenomena are mainly attrib-uted to the low surface energy and the air holding rough surface. Itshould be pointed out, however, that such property of the films isrelated to their orientation growth on the substrate and difficultlyapplied to large areas. Decorating a large area of surfaces in variedshapes is much important for the future industrial applications.Taking ZnO as an example, here we demonstrate a dexterousdesign using a powder of spherical assembly of one-dimensionalZnO nanorods, of which particulates possess the same external

Fig. 1. SEM images of the microspherical assemblies of ZnO nanorods at various times of hydrothermal reaction at 453 K: (a) NZ-7; (b) NZ-20; (c) NZ-100, and the inset shows

a closer observation. (d) Typical overview of the ZnO powder obtained with reaction time of 20 h; inset shows their typical XRD spectra.

X. Zhou et al. / Applied Surface Science 255 (2008) 3371–33743372

surface at all directions. It endows superhydrophilicity or super-hydrophobicity and ultraviolet-irradiation resistance to a surfaceonto which the powder is randomly coated.

2. Experimental

The special micro-nanostructured ZnO powder was prepared bya hydrothermal approach suitable for the large-scale of production,similar to the route we have reported previously [18,19]. Fortypical experiments, 2 g of poly ethylene glycol (PEG 200) wasdissolved in a mixed solvent of 50 mL anhydrous ethanol and10 mL deionized water with stirring, and 0.003 mole ofZn(NO3)2�6H2O and 0.06 mole of NaOH were dissolved in 5 mLof de-ionized distilled water. Then the mixture of the above twosolutions was provided a 10-min supersonic (20 kHz) pretreat-ment in a pulverizer with a power of 200–600 W and then washydrothermally treated at 453 K in a Teflon-lined autoclave forvaried periods of time. After the reactions, white crystallineproducts were harvested by centrifugation and thoroughly washedwith absolute alcohol and hot water. The superhydrophilic ZnOpowder with particulates of micro-spherically assembled ZnOnanorods was obtained by the hydrothermal treatment at 453 K for7 h, while prolonged time (100 h) of hydrothermal reactionproduced superhydrophobic ZnO powder with similar structuredparticulates. Powder XRD measurements were performed on aPhilips X’Pert MPD Pro X-ray diffractometer with graphitemonochromatized high-intensity Cu KR radiation at 50 kV.Scanning electron microscopy (SEM) images were taken on aJSM-5900 instrument and High-resolution Scanning electronmicroscopy (SEM) pictures were recorded on a LEO-1530 FESEMinstrument. The wettability was evaluated by the water contactangle measurement of the as-prepared powder spread andplanished on the glass three times.

3. Results and discussion

Fig. 1a shows the morphology of the microspherical assembly ofZnO nanorods which were obtained by hydrothermal treatment at453 K for 7 h; the sample was named as NZ-7. The ends of thenanorods outwards pointed are not so smooth at the relative shorttime of reaction. Fig. 1b shows that ZnO nanorods transform tonanocones with diametric steps along c-axis when the hydro-thermal treatment was prolonged to 20 h, which was denoted asNZ-20. By the ideal model of growth previously reported for ZnO[20], it can be concluded that all the polar O-terminated ð0 0 0 1̄Þsurfaces of the nanorods are arranged at the external surface of themicrospherical assembly. As shown in Fig. 1c, the nanoconeseventually changed into little short but thicker nanorods withsmooth surfaces when the hydrothermal reaction prolonged to100 h at the same temperature. The sample was named as NZ-100,of which the flat end of the nanorods outwards pointed can beattributed to ð0 0 0 1̄Þ plane by their shapes. Fig. 1d is the typicalSEM overview of the micro-nanostructured ZnO powder obtainedby the hydrothermal treatment at 453 K for 20 h, it is clearly shownthat the obtained powder is uniformly constituted with micro-sphere assembly of ZnO nanoneedles. XRD measurements (inset ofFig. 1d) show that ZnO is crystallized in the hexagonal wurtzitestructure.

Fig. 2 shows the optical images of a droplet of water on the ZnOmodified surfaces of glass; the structures of the powders are alsoschematically shown in the figure. A water contact angle of 1.28 onthe glass decorated with NZ-7 was obtained (Fig. 2a), while a waterCA of about 163.48 was observed on the NZ-100 coated glass(Fig. 2c). The result indicates that the wettability of the surfaceschange from superhydrophilicity to superhydrophobicity,although the morphologies of the ZnO powders appear to besimilar. The glass coated with NZ-20 shows a CA of about 188

Fig. 2. Photographs of the shape of a water droplet on the glass decorated with NZ-7 (a), NZ-20 (b) and NZ-100 (c).

Fig. 3. XPS profiles at O region of the as-prepared samples of (a) NZ-7, (b) NZ-20 and

(c) NZ-100.

X. Zhou et al. / Applied Surface Science 255 (2008) 3371–3374 3373

(Fig. 2b), in between those of the NZ-7 and the NZ-100. Thesuperhydrophilic or superhydrophobic properties of the glasssurfaces were kept unchanged in air for as long as half a year atroom temperature.

The surface free energy and the surface roughness are two mainfactors governing the surface wettability. It is well known fromnature that the optimum surface morphologies to achieve highhydrophobicity are micro-nanobinary structures with low surfacefree energy. Study shows that only when the surface free energiesof the various crystallographic planes differ significantly couldanisotropic nanorod growth be realized [21], and a fast growingplane generally tends to disappear leaving behind slower growingplanes with lower surface energy with increasing the reactiontime. For the anisotropic ZnO nanorods grown in basic solution, itsgrowth rates in different directions were reported to beRð0 0 0 1Þ>Rð0 1 1̄ 1̄Þ>Rð0 1 1̄ 0Þ>Rð0 1 1̄ 1Þ>Rð0 0 0 1̄Þ, i.e., theplane ð0 0 0 1̄Þ possesses the lowest surface energy. The slowergrowing ð0 0 0 1̄Þ planes with lower surface energy increase withthe increase of the reaction time. Eventually, the ð0 0 0 1̄Þ planeswith the lowest surface energy of the nanorods locate at theexternal surface of the current spherical assembly at all directions.

The surface hydroxyls also play an important role for theanisotropic growth and the wettability of ZnO nanorods. Thecurrent as-prepared samples were measured by XPS to analyzethe changes in surface hydroxyls with hydrothermal reaction time.The O1S binding energy spectra are shown in Fig. 3. All the O1S

profiles for superhydrophobic and superhydrophilic samples, i.e.,NZ-7, NZ-7 and NZ-100, are asymmetric, indicating that at leasttwo oxygen species are present on the surface. By deconvolution,the asymmetric peak can be divided into two peaks respectively,one at 530.4 eV attributed to lattice oxygen and the other at about532 eV caused by surface hydroxyls [22]. It is clear that the amountof surface hydroxyls decreased with increasing time of hydro-thermal reaction, which is accompanied by the changes inwettability of the samples. Under certain surface roughness, themore the surface hydroxyls, the more hydrophilic the surfaces.

In general, surface roughness is the key factor affecting thesurface wettability. The wettability of a surface can be enhanced byincreasing the surface roughness. Han et al. pointed out that themicronanobinary structure has the greatest ability to increasethe surface wettability and the nanostructure is better than themicrostructure at improving the surface superhydrophobicity[23,17]. In our experiment, the individual ZnO isotropic micro-sphere with high symmetry has been assembled from manynanorods; the nanoscale grooves between the adjacent ZnOnanorods are clearly shown by the SEM picture of an individualmicrosphere in Fig. 1. Furthermore, when stacking the micro-nanostructured ZnO powder onto the surfaces, the adjacent ZnOmicrospheres will give a microscale grooves. Therefore, the surface

Fig. 4. Optical images of the glass surfaces randomly coated with the ZnO powders: (a) NZ-7 and (b) NZ-100. Inset shows typical SEM image.

X. Zhou et al. / Applied Surface Science 255 (2008) 3371–33743374

wettability regulated by the ZnO powder has been greatlyenhanced by their micro-nanostructures. If the surface wettabilityof the substrates can be tuned between hydrophobic andhydrophilic, their wettability can be further amplified by theintroducing micro-nanoscale surface roughness. From our XPSresults, it is clear that the amount of surface hydroxyls decreasedwith increasing time of hydrothermal reaction, i.e., the surfacewettability of the substrates can be tuned between hydrophilicityand hydrophobicity with the increasing time of hydrothermalreaction. So their surface wettability can be further amplifiedbetween superhydrophilicity and superhydrophobicity by themicro-nanostructured ZnO powder.

Fig. 4 shows the optical and SEM micrographs of the surfaceswith randomly coating of the ZnO powders. The stacking of as-prepared micro particulates assembled by ZnO nanorods endowscertain roughness to the surface of glass in micrometer andnanometer scales and the air can be held among the spheres andnanorods, which intensifies the hydrophobic or hydrophilicproperty of the surface by increasing the proportion of air/waterinterfaces. Both the surface hydroxyls and the hierarchical micro-nanostructures of the surface contribute to the superhydrophilicityor superhydrophobicity. The current results are important todevelop potential techniques for surface processing. For example,the process of coating a surface in any shape with polymeradhesives and then (onto which) spraying the inorganic powder asa compact layer must functionalize the surface with super-hydrophobic or superhydrophilic properties together with ultra-violet irradiation resistance.

4. Conclusions

In summary, the superhydrophilic or superhydrophobic pow-ders consisting of ZnO nanorods assembled microspheres arefabricated with a facile method. The powders are easy to be coatedonto surfaces in any shape and endow the surfaces with super-hydrophilic or superhydrophobic properties, decided by the type ofZnO powders. The surface property of ZnO nanorods and the

designed micro-nanostructures of ZnO nanorods assembledmicrospheres accounts for the superior wettability. The findingis significant for future industrial applications.

Acknowledgements

This work was supported by the Ministry of Science andTechnology of China (contract no. 2003CB615804) and the NSF ofChina (grant nos. 20403008 and 20673054).

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