Journal of Colloid and Interface Science 364 (2011) 219–229

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Journal of Colloid and Interface Science

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Superhydrophobic silicon surfaces with micro–nano hierarchical structuresvia deep reactive ion etching and galvanic etching

Yang He, Chengyu Jiang ⇑, Hengxu Yin, Jun Chen, Weizheng Yuan ⇑Micro and Nano Electromechanical Systems Laboratory, Northwestern Polytechnical University, Xi’an 710072, PR China

a r t i c l e i n f o

Article history:Received 18 April 2011Accepted 9 July 2011Available online 5 August 2011

Keywords:SuperhydrophobicHierarchical structureDeep reactive ion etchingGalvanic etchingTwo-scale model

0021-9797/$ - see front matter � 2011 Published bydoi:10.1016/j.jcis.2011.07.030

⇑ Corresponding authors.E-mail addresses: jiangcy@nwpu.edu.cn (C. Jia

(W. Yuan).

a b s t r a c t

An effective fabrication method combining deep reactive ion etching and galvanic etching for siliconmicro–nano hierarchical structures is presented in this paper. The method can partially control the mor-phology of the nanostructures and enables us to investigate the effects of geometry changes on the prop-erties of the surfaces. The forming mechanism of silicon nanostructures based on silver nanoparticlegalvanic etching was illustrated and the effects of process parameters on the surface morphology werethoroughly discussed. It is found that process parameters have more impact on the height of silicon nano-structure than its diameter. Contact angle measurement and tilting/dropping test results show that as-prepared silicon surfaces with hierarchical structures were superhydrophobic. What’s more, two-scalemodel composed of micropillar arrays and nanopillar arrays was proposed to study the wettability ofthe surface with hierarchical structures. Wettability analysis results indicate that the superhydrophobicsurface may demonstrate a hybrid state at which water sits on nanoscale pillars and immerses intomicroscale grooves partially.

� 2011 Published by Elsevier Inc.

1. Introduction

In recent years, superhydrophobic surfaces have attracted aconsiderable amount of attention. Superhydrophobic surfaces arecharacterized by high contact angles (usually >150�) and low con-tact angle hysteresis, which causes water to roll off the surfaces,thereby leading to water repellency and self-cleaning characteris-tics [1,2]. Superhydrophobicity occurs naturally in some plantleaves (e.g., lotus and rice) and insect surface (e.g., butterfly, waterstrider and spider). It is suggested that the micro–nano hierarchicalmorphology on these natural surfaces composed of two roughnesspatterns at different length scale is the key factor leading to super-hydrophobicity [3–8].

Superhydrophobicity is critical in antistiction, friction reduc-tion, and anticorrosion applications. In addition, superhydrophobicsurfaces offer much promise for the formation of high-performancemicro–nano structured surfaces with multifunctionality that canbe used in optical, photoelectric, microelectronic, catalytic, andbiomedical applications [9–11].

From a practical viewpoint, superhydrophobic surfaces on sili-con are particularly attractive since they can be integrated withother electronic components to protect them from the deleteriouseffects of water and moisture in the environment. Therefore, the

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ng), yuanwz@nwpu.edu.cn

fabrication of superhydrophobic silicon surface with micro–nanohierarchical structure appears to be a prerequisite to the develop-ment of biomimetic functional surfaces and would be potential inmicro and nano electromechanical systems (MEMS/NEMS).

Considerable efforts have been expended to create hierarchi-cally structured surfaces that mimick the natural surface struc-tures. A number of techniques to fabricate hierarchical structureson different materials like polymers and metals have been reportedin recent years, such as combining colloidal self-assembly and Ausputter deposition [12], a two-step temperature directed capillarymolding technique [13], modification by long-chain fatty acids[14], replicating prepatterned structures and plasma treatment[15], utilizing the reaction between the hydrolyzed solution andmetallic surfaces [16], combined replica molding and layer-by-layer assembly [17], UV-assisted capillary force lithography [18],replication of micropattern and self-assembly of hydrophobic al-kanes [19], a two-step solution approach [20], using mixtures ofmetal oxide nanoparticles with diameters on two different lengthscales [21], electrodeposition [22], lithography and plasma etching[23], lotus and rice leaf template and PDMS replica [24], in situhydrothermal crystallization method [25], self-assembling underhydrothermal conditions [26], and so on.

However, only a few techniques have been proposed for fabri-cating silicon micro–nano hierarchical structures. Choi et al. [27]proposed a method for fabricating hierarchical silicon structuresvia the combination of scanning probe lithography and wetchemical etching. It is a maskless process and can realize three

(a) exposure

(b) development

Silicon substrate

(c) deep reactive ion etching

(d) silver nanoparticle forming

220 Y. He et al. / Journal of Colloid and Interface Science 364 (2011) 219–229

dimensional hierarchical structures. However, this method is notefficient for batch fabrication of large area hierarchical structuressince it need much time and cost high. Wang et al. [28] proposeda method that incorporates electrochemical surface modificationand wet etching to fabricate a superhydrophobic silicon surface.Xiu et al. [29] generated silicon pyramid surfaces by KOH aniso-tropic etching and constructed hierarchical structures from Si pyr-amids where nanostructures were added by Au-assisted electrolessHF/H2O2 etching. These methods cost low, however, the morphol-ogy of silicon micro–nano hierarchical structures is not controlla-ble. Kwon et al. [30] combined the two readily accessible etchingtechniques, a standard deep silicon etching, and a gas phase isotro-pic etching (XeF2) for the uniform formation of hierarchical struc-tures on a silicon surface. The fabricated surface shows durablesuper water repellency. Sun et al. [31] presented a method basedon an improved Bosch deep reactive ion etching (DRIE) processand its black silicon effect. They created micro–nano hierarchicalsilicon structures over an entire 4 in. wafer. These methods canachieve fabrication of hierarchical structures, but they can hardlycontrol the morphology of the nanostructures.

Here, we reported an effective method combining deep reactiveion etching and galvanic etching to fabricate silicon surfaces withmicro–nano hierarchical structures. Experimental procedures areshown in Fig. 1. Micropatterns were transferred from a photolitho-graphic mask to the photoresist on the silicon surface by exposure(a) and development (b). Then silicon micropillars were formed viadeep reactive ion etching (c). After that, silver nanoparticles weredeposited on the micropillars by immersing into mixed solution(d) and nanostructures were formed on the micropillars (e) by sil-ver nanoparticle assisted galvanic etching [32,33]. Finally the sili-con surfaces with micro–nano hierarchical structures wereobtained after cleaning (f). The proposed method has gained themerits of both deep reactive etching and galvanic etching. Themicroscale patterns can be predefined and fabricated by lithogra-phy and deep reactive etching, and morphology of the nanostruc-tures can be partially controlled by galvanic etching with lowcost, thus enables us to investigate the effects of geometry changeson the properties of the surfaces. In addition, the forming mecha-nism of silicon nanostructures was illustrated, and the effects ofdifferent process parameters on the surface morphology werethoroughly discussed. Contact angles were measured and tilting/dropping test were implemented to investigate the wettability ofthe surface. What’s more, two-scale model composed of micropil-lar arrays and nanopillar arrays was proposed. Wettability analysiswas taken by comparing theoretical calculations with experimen-tal measurements.

(e) silver nanoparticle assited galvanic etching

(f) micro and nano pillar forming

Fig. 1. Schematic of experimental procedures of deep reactive ion etching andgalvanic etching.

2. Experiments

2.1. Pattern transfer by lithography

(100)-oriented p-type silicon wafers were used in the experi-ments. A mask was designed with arrayed square posts of a certainwidth (20 lm) and different spaces (10, 20, 30, 40, 50, 60 and80 lm) in order to study the effects of geometric parameters onthe surface morphology. The wafer was cleaned with acetone(5 min), ethanol (5 min), deionized water (2–3 times), H2SO4/H2O2 (3:1 H2SO4 (97%)/H2O2 (30%), 10 min) sequentially. Negativephotoresist (EPG533) was spin-coated on the wafer afterwards(50 rpm, 50r/s, 15 s; 500 rpm, 250r/s, 10 s; 3000 rpm, 1500r/s,40 s). Then the wafer was exposed to UV light (MA6, Karl SUSScompany Ltd., 80 s) and was developed in TMAH resolution. Thepattern was transferred from a mask to the photoresist afterlithography.

(a) (b)

(c)

(d)

(e)

Fig. 2. SEM images of silicon surface with micro–nano hierarchical structures. (a) Periodically arranged micropillar arrays, (b) a micropillar with nanostructures, (c)nanostructures formed on the top, (d) nanostructures formed on the sidewall, (e) nanostructures formed on the bottom.

Si

Ag+

e-

SiO2

Si e-

Ag+6

2- Ag+

Ag

SiO2 e-

Si

Ag+

HF SiF

HF SiF62- Ag+

Ag Ag Ag

(a) (b) (c)

(d) (e)

Si SiO2

e-

Ag

HF SiF62-

Fe3+ Fe2+

SiO2 e- Si

Ag Ag Ag

HF SiF62-

Fe3+ Fe2+

Fig. 3. A model illustrating the mechanism of silver nanoparticle assisted galvanic etching. (a) Nucleation of silver nanoparticles on the silicon surface in HF/AgNO3 solution,(b) growth of silver nanoparticles while SiO2 forms simultaneously underneath the silver nanoparticles, (c) reduction of Fe3+ ions, etching of silicon, and sinking of silvernanoparticles in HF/Fe(NO3)3 solution, (d) forming silver nanoparticle arrays on the silicon surface, (e) forming silicon nanopore arrays (inversely silicon nanopillar arrays) inthe bulk silicon.

Y. He et al. / Journal of Colloid and Interface Science 364 (2011) 219–229 221

Fig. 4. SEM images of silver nanoparticles and dendrites deposited on the silicon surfaces in HF–AgNO3 solution with different AgNO3 concentration (a) 0.01 mol/L, (b)0.02 mol/L and (c) 0.03 mol/L.

Fig. 5. (a) Enlarged SEM images of silver nanoparticle formed on the silicon surface in HF–AgNO3 solution with AgNO3 concentration 0.01 mol/L and (b) corresponding etchedsilicon pores after 1 min etching.

Fig. 6. SEM cross-section images of silicon nanostructures etched in HF–Fe(NO3)3 solution with different HF concentration (a) 0.9 mol/L, (b) 2.7 mol/L, (c) 4.6 mol/L, (d)6.4 mol/L, (e) 8.2 mol/L and (f) 10 mol/L.

222 Y. He et al. / Journal of Colloid and Interface Science 364 (2011) 219–229

2.2. Forming silicon micropillar arrays by deep reactive ion etching

Then deep reactive ion etching (ICP ASE, STS company Ltd.) wasused to form the micropillar arrays. Key process parameters forbetter micropillar morphology were determined based on lots of

experiments (etching process: pressure 35 mT, SF6 180 sccm/min,coil power 600 W, platen power 12 W, process time 14 s; passiv-ation process: pressure 35 mT, C4F8 85 sccm/min, coil power600 W, platen power 0 W, process time 7 s). The etching rate isnearly 3 lm/min with these process parameters. The height of

Fig. 7. SEM cross-section images of silicon nanostructures etched in HF–Fe(NO3)3 solution with different Fe(NO3)3 concentration (a) 0.07 mol/L, (b) 0.09 mol/L, (c) 0.11 mol/L,(d) 0.13 mol/L, (e) 0.15 mol/L and (f) 0.17 mol/L.

Fig. 8. The effect of Fe(NO3)3 concentration on the height of silicon nanostructure.

Y. He et al. / Journal of Colloid and Interface Science 364 (2011) 219–229 223

the micropillar was controlled by adjusting the total process time.For example, total process time was adjusted to 6.7 min to get themicropillar with the height of 20 lm. O2 was used to remove thephotoresist. At last, the wafer was diced into many small sampleswith micropillar arrays.

2.3. Forming silicon nanostructures by galvanic etching

The sample with micropillars were cleaned with acetone(5 min), ethanol (5 min), deionized water (2–3 times). Then theywere thoroughly rinsed into mixed solution of HF (4.6 mol/L)/AgNO3 (0.01 mol/L) for 1 min. They were further immersed intomixed solution of HF (4.6 mol/L)/Fe(NO3)3 (0.05 mol/L) for 30 minat 30 �C. Finally, they were rinsed copiously in deionized waterand dried by N2 flow at room temperature. After the above pro-cesses, the samples with micro–nano hierarchical structures wereobtained.

2.4. Surface modification

Silicon micro–nano hierarchical structures without surfacemodification are not stable for superhydrophobicity since theymay be oxidized, thus surface modification is necessary in realapplications. Here, hydrophobic monolayer was covered on thesurface by immersing into (Heptadecafluoro-1,1,2,2-tetradecyl)tri-methoxysilane solution using the dip coater(DC, KSV company Ltd.).

2.5. Surface characterization

The morphology of the silicon surface was observed with scan-ning electron microscope (JEOL, JSM-6309A) at 20 kV. The contactangles (CA) of the as-prepared silicon surfaces with micro–nanohierarchical structures were measured based on the sessile dropletmethod using an optical contact angle measurement device(OCA15EC, DataPhysics Company Ltd.). The CA was determinedby fitting a Young–Laplace curve around the water drop. Deionizedwater droplets with volumes of 2 lL were employed for the CAmeasurements. The contact angle hysteresis (CAH) was obtainedby subtracting receding angle (hR) from advancing angle (hA).Deionized water droplets with volumes of 4 lL and increment of1 lL were employed for advancing and receding angle measure-ments. The mean value was calculated from at least five individualmeasurements. The experiments were performed at 20 �C and 30%relative humidity.

3. Results and discussion

3.1. Surface morphology

Silicon surfaces with micro–nano hierarchical structures werefabricated using the proposed method. Fig. 2 shows the SEMimages of as-prepared micro–nano hierarchical structures. Fig. 2ashows the periodically arranged micropillar arrays with nanostruc-tures. Fig. 2b shows a micropillar with grass-like nanostructures onits top, sidewall and bottom. Fig. 2c–e shows the enlarged image ofthe nanopores and nanopillars formed on the top, sidewall and

Fig. 9. SEM cross-section images of silicon nanostructures etched in HF–Fe(NO3)3 solution with different etching time (a) 30 min, (b) 60 min, (c) 90 min and (d) 120 min.

224 Y. He et al. / Journal of Colloid and Interface Science 364 (2011) 219–229

bottom of the micropillar, respectively. It is noted that the edge ofthe micropillar was eliminated since nanopillars on top and side-wall overlapped. It means that three dimension nanopillars canbe fabricated by this process. The height, width and space of micro-pillars are 20 lm, 20 lm, and 10 lm, respectively, and those ofnanopillars are approximately 2 lm, 200 nm, and 200 nm, respec-tively. The results demonstrate that micro–nano hierarchical struc-tures have been successfully achieved by deep reactive ion etchingand galvanic etching.

Fig. 10. The effect of etching time on the height of silicon nanostructure.

3.2. Forming mechanism of silicon nanostructures

Silicon nanostructures were formed by silver nanoparticle as-sisted galvanic etching process, during which the cathodic (sil-ver-deposition and sinking) and anodic reactions (siliconoxidation and dissolution) occurred simultaneously.

Generally, the metal nanoclusters, such as Pt and Cu, have astrong tendency to coalesce and form a continuous grain film inthe reaction process. However, silver nanoclusters deposited onthe silicon surfaces form silver particles or dendrites rather thana continuous grain film in the HF–AgNO3 system. The silver parti-cles or dendrites can consume spare deposited silver atoms to re-strain the coalescence of silver nanoclusters effectively, so thesilicon surfaces can be selectively etched to form nanopillars.

The mechanism of silver nanoparticle assisted galvanic etching[34–36] is detailed in Fig. 3. Ag+ ions adjacent to the silicon surfacecapture electrons from the valence band of Si, and are deposited inthe form of metallic Ag nuclei on nanoscale; generally, electron ex-change between Ag+ ions and Si is more likely to take place at de-fects (Fig. 3a). The Ag nuclei grow into larger particles as more Agions are deposited. Simultaneously, because the Si underneath theAg particles releases as many electrons as are required by Ag ions

to be reduced, excess local oxidation occurs, and SiO2 is producedunderneath these Ag nanoparticles. Shallow pits would immedi-ately form underneath the Ag nanoparticles, due to SiO2 is etchedaway as SiF6 by the HF solution; then the Ag particles enter theforming pits (Fig. 3b). In the solution of HF/Fe(NO3)3, Fe3+ ions havethe strong tendency to preferentially obtain electrons from silvernanoparticles and be reduced to Fe2+ ions, while the silicon under-neath the silver nanoparticles is oxidized to SiO2. The Ag+/Ag sys-tem has a more positive redox potential than that of the Fe3+/Fe2+ couple, so silver nanoparticles are stable and can continue to

Fig. 11. SEM cross-section images of silicon nanostructures etched in HF–Fe(NO3)3 solution with different heating temperature (a) 20 �C, (b) 35 �C, (c) 50 �C and (d) 60 �C.

Y. He et al. / Journal of Colloid and Interface Science 364 (2011) 219–229 225

enhance the reduction of Fe3+ ions and the oxidation of silicon inthe HF/Fe(NO3)3 solution. Thus initial shallow pits in silicon couldbe developed into deeper pores owing to the continuous etchingaway of SiO2 and further sinking of silver nanoparticles upon pro-longed immersion in the HF/Fe(NO3)3 solution (Fig. 3c).

Based on above electroless-deposition and catalyzed galvanicetching, silver nanoparticle arrays were formed on the silicon sur-face (Fig. 3d) and silicon nanopore arrays (inversely silicon nano-pillar arrays) were formed finally (Fig. 3e).

3.3. Effects of process parameters on surface morphology

The effects of process parameters on surface morphology werethoroughly investigated. Particularly, galvanic etching was mainlyconsidered since deep reactive etching is widely used in MEMSprocess and is quite well understood. The effect of each parameterin galvanic etching process was investigated individually whileother parameters been fixed (fixed parameters can be found inSection 2.3).

Fig. 4 shows the SEM images of silver nanoparticles and nano-dendrites formed on the silicon surfaces in HF–AgNO3 solutionwith different AgNO3 concentration ranged from 0.01 mol/L to0.03 mol/L. It is clear that silver nanoparticles grow up to dendriteswhile AgNO3 concentration increased. The growth mechanism ofthese silver dendrites [37,38] can be illustrated by diffusion-lim-ited aggregation (DLA) model [39] which involves cluster forma-tion by the adhesion of a particle with a random path to aselected seed on contact and allows the particle to diffuse and stickto the growing structure.

Fig. 5 shows the enlarged SEM images of silver nanoparticles inHF–AgNO3 solution with AgNO3 concentration 0.01 mol/L and cor-responding etched silicon pores after 1 min etching. It can be seenthat diameters of the etched silicon pores (100 ± 50 nm) are similarto those of silver nanoparticles (100 ± 50 nm), proving that diame-

ters of silicon nanostructures are determined by those of silvernanoparticles.

Fig. 6 shows SEM cross-section images of silicon nanostructuresetched in HF–Fe(NO3)3 solution with different HF concentration. Itis seen that the silicon structures will be disordered if the concen-tration of HF is too small (such as 0.9 mol/L) or too large (such as10 mol/L). Thus it indicates that HF concentration should be care-fully chosen for regular nanostructures. Here, HF concentration4.6 mol/L was taken as an optimal parameter in trade off betweenheight and uniformity of the nanostructures.

Fig. 7 shows SEM cross-section images of silicon nanostructuresetched in HF–Fe(NO3)3 solution with different Fe(NO3)3 concentra-tion and Fig. 8 plotted the effect of Fe(NO3)3 concentration on theheight of silicon nanostructure. Increasing Fe(NO3)3 concentrationwill obviously increase the height of silicon nanostructure withapproximate linearity, as shown in Fig. 8.

Fig. 9 shows SEM cross-section images of silicon nanostructuresetched in HF–Fe(NO3)3 solution with different etching time andFig. 10 plotted the effect of etching time on the height of siliconnanostructure. Increasing etching time from 30 min to 90 min willlinearly increase the height of silicon nanostructure (the height ofsilicon nanostructure increased nearly 2 lm when the heating timeincreased 30 min), as seen from Fig. 10. However, too long etchingtime (such as 120 min) will result in disordered nanostructures be-cause of over-etching.

Fig. 11 shows SEM cross-section images of silicon nanostruc-tures etched in HF–Fe(NO3)3 solution with different heating tem-perature(note that location of the nanostructures in images isinverse to previous images) and Fig. 12 plotted the effect of heatingtemperature on the height of silicon nanostructure. Increasingheating temperature from 20 �C to 50 �C will linearly increase theheight of silicon nanostructure (the height of silicon nanostructureincreased nearly 2 lm when the heating temperature increased15 �C), as seen from Fig. 12. Nevertheless, the experiments showthat temperature higher than 50 �C may lead to structure disorder.

Fig. 12. The effect of heating temperature on the height of silicon nanostructure.

Table 2Contact angle and contact angle hysteresis measurement results.

hm (lm) am (lm) bm (lm) CA (�) CAH (�)

20 20 10 149.2 3.620 20 20 149.7 3.420 20 30 148.8 3.420 20 40 149.5 2.720 20 50 149.5 2.720 20 60 152.3 320 20 80 151.8 2.2

226 Y. He et al. / Journal of Colloid and Interface Science 364 (2011) 219–229

Based on above discussions and series of experiments, an opti-mized range of process parameters for uniform silicon nanostruc-tures with adjustable length can be obtained, as seen in Table 1.The concentration of AgNO3 should be less than 0.01 mol/L to pre-vent silver dendrites growing. The concentration of HF should benear 4.6 mol/L to gain better uniformity of silicon nanostructures.The concentration of Fe(NO3)3 ranging from 0.05 mol/L to0.12 mol/L, the etching time ranging from 30 min to 60 min, andthe heating temperature ranging from 30 �C to 50 �C will lead touniform silicon nanostructures with different length ranging fromaround 2 lm to 5 lm. For example, an group of stable optimizedprocess parameters for silicon nanostructures with height 2 lmand width 200 nm are shown in Section 2.3 (HF 4.6 mol/L, AgNO3

0.01 mol/L, Fe(NO3)3 0.05 mol/L, etching time 30 min, heating tem-perature 30 �C). Silicon nanostructures may also be generated byusing the process parameters out of this optimized range, however,the uniformity of the structures is not as good as those by usingprocess parameters inside the optimized range.

According to previous experiments, it is also found that processparameters have more impact on the height of silicon nanostruc-ture than its diameter, which is due to galvanic etching mecha-nism. As mentioned before, sinking of silver nanoparticles andetching away of SiO2 lead to silicon deeper pores, thus the diame-ters of silicon pores are determined by those of silver nanoparti-

Table 1Optimized range of process parameters for uniform silicon nanostructures.

AgNO3

concentration(mol/L)

HFconcentration(mol/L)

Fe(NO3)3

concentration(mol/L)

Etchingtime(min)

Heatingtemperature(�C)

<0.01 5–6 0.05–0.12 30–60 30–50

Fig. 13. Static contact angle measurement result.

cles. In addition, orthogonal experiment results proved that thediameter of silver nanoparticle is determined by AgNO3 concentra-tion. Therefore the diameter of nanostructures varies less com-pared with its height if the AgNO3 concentration is fixed inexperiments.

Above all, process parameters should be carefully chosen for or-dered silicon nanostructures. Particularly, increasing etching timeor heating temperature or concentration of Fe(NO3)3 will linearly(or approximately linearly) increase the height of silicon nano-structure, thus these process parameters are preferable for control-ling the morphology of silicon nanostructures.

3.4. Surface characterization results

Contact angle (CA) and contact angle hysteresis (CAH) of thesamples with various geometric parameters were measured, andthe results are listed in Table 2, where am, hm, bm denote width,height and space of micropillars, respectively. It is clearly seen thatCA of the hierarchical structures with different geometry are allclose to 150�, and CAH of them are basically less than 4�. Fig. 13shows CA measurements of a sample (hm = 20 lm, am = 20 lm,bm = 10 lm). Fig. 14 plotted CAH of all as-prepared samples. It isobvious that the as-prepared silicon surfaces demonstrate highcontact angles and low contact angle hysteresis.

To further investigate the wettability of the surface, tilting testwas implemented and recorded by optical contact angle measure-ment device. Fig. 15 shows two snapped picture of tilting test (Vi-deo can be seen from Supplement material). It is seen that waterdrop easily rolled down when the substrate was titled. Otherwise,dropping test was also implemented and recorded by optical con-tact angle measurement device. As Fig. 16 shows, a water droplet(10 lL) was dropped on the surface with micro–nano hierarchicalstructure (hm = 20 lm, am = 20 lm, bm = 10 lm). The shot imagewas taken at a rate of 560 frames per second. When the droplet

Fig. 14. CAH measurement result.

Fig. 15. Tilting test result.

(a) (b)Fig. 17. Classic wetting state models.

Y. He et al. / Journal of Colloid and Interface Science 364 (2011) 219–229 227

landed on the surface with micro–nano hierarchical structure, itbounced back from the surface. The droplet kept bouncing forabout 0.1 ms, until the initial potential energy was consumedthrough the internal flow and the surface friction. Tilting and drop-ping test demonstrated the effectiveness of the micro–nano hierar-chical structure in realizing the water repellent surface.

According to above surface characterization results, it is con-cluded that the as-prepared silicon surfaces with hierarchicalstructures were superhydrophobic. In addition, the characteriza-tion results of the silicon surfaces with hierarchical structures werecompared with those of flat silicon surfaces to verify the role of thehierarchical structure. The experimental results showed that theaverage contact angle on flat silicon surfaces is 59.9�, whilst thewater droplet did not bounce back from the flat silicon surfaces.However, the contact angle increased to near 150� and water drop-let bounced back from the surfaces when hierarchical structureswere generated. In other word, the hydrophobicity of the siliconsurfaces with hierarchical structures are enhanced significantlycompared with flat silicon surfaces. It is indicated that the hierar-chical structure plays the key role in surface superhydrophobicity.

3.5. Wettability analysis

In 1936, Wenzel [40] proposed a model to describe the relation-ship between surface roughness and water wetting behavior. Thecontact angle hW on the rough surface is given by the followingequation:

cos hW ¼ r cos he ð1Þ

where he stands for the equilibrium contact angle on a flat surfaceand r represents the surface roughness that defined as the ratio ofthe actual area of liquid–solid contact to the projected area. In thiscase, liquid fills up the rough surface to form a completely wettedcontact with the surface, as shown in Fig. 17a.

On the other hand, a liquid droplet sitting on a composite sur-face composed of solid and air is known as the Cassie state model[41]. The contact angle in the Cassie state can be formulated by thefollowing equation:

Fig. 16. Sequential shot of the droplet bouncin

cos hC ¼ rf ð1þ cos heÞ � 1 ð2Þ

where hC stands for the contact angle of a liquid droplet at the Cas-sie state and the solid fraction f represents the fraction of solid–li-quid contact area. In this case liquid sits on the top of the roughsurface, as shown in Fig. 17b.

Nevertheless, these models have their limits and cannot fullydescribe the wetting phenomena on micro–nano hierarchicalstructures. Therefore, two-scale model were proposed to deal withwettability of the surface with hierarchical structures [42].

Here, micro–nano hierarchical structure was simplified as two-scale model composed of micropillar arrays and nanopillar arrays,as diagrammed in Fig. 18, where an, hn, bn denote width, height andspace of nanopillars, respectively, and am, hm, bm denote width,height and space of micropillars, respectively, whereas subscriptn, m denote nanoscale and microscale individually.

It is suggested that there are four wetting states which are dif-ferent combination of Wenzel state and Cassie state for the surfacewith micro–nano hierarchical structures [43] as Fig. 19 shows,where w, c, n and m denote Wenzel state, Cassie state, nanoscaleand microscale, respectively. wn-cm means the surface demon-strate Wenzel state on nanoscale and Cassie state on microscale.The rest can be deduced by analogy.

Consider wn-cm state (Fig. 19a), when three phase contact linehas a small displacement dx, the change of free energy of micro–nano hierarchical structure is as follows:

dE ¼ fmðcSL � cSV Þrndxþ ð1� fmÞcLV dxþ cLV dx cos hA ð3Þ

where fm is solid–liquid contact area fraction on microscale, rn isroughness factor on nanoscale, hA is the apparent contact angle ofmicro–nano hierarchical structure, cSV, cSL, cLV is solid–vapor, so-lid–liquid and liquid–vapor surface tension, respectively.

g on the superhydrophobic silicon surface.

ma

ma

mb

na nb

na nh

mh

Fig. 18. Two-scale model for micro–nano hierarchical structure.

Fig. 19. Wetting state of micro–nano hierarchical structure (a) wn-cm, (b) cn-cm,(c) cn-wm and (d) wn-wm.

228 Y. He et al. / Journal of Colloid and Interface Science 364 (2011) 219–229

Make the change of free energy smallest:

dE ¼ 0 ð4Þ

And take Young formula into account:

cos he ¼cSL � cSV

cLVð5Þ

Then apparent contact angle of micro–nano hierarchical struc-ture at wn-cm state is given as follows:

cos hA ¼ �1þ fmð1þ rn cos heÞ ð6Þ

Similarly, apparent contact angles of micro–nano hierarchicalstructure at cn-cm, cn-wm, wn-wm states are given as follows:

cos hA ¼ �1þ fmfnð1þ cos heÞ ð7Þ

cos hA ¼ rmð�1þ fSnð1þ cos heÞÞ ð8Þ

cos hA ¼ rmrn cos he ð9Þ

Here, rm is roughness factor on microscale:

rm ¼ 1þ 4amhm

ðam þ bmÞ2ð10Þ

fm is solid–liquid contact area fraction on microscale:

fm ¼a2

m

ðam þ bmÞ2ð11Þ

rn is roughness factor on nanoscale:

rn ¼ 1þ 4anhn

ðan þ bnÞ2ð12Þ

fn is solid–liquid contact area fraction on nanoscale:

fn ¼a2

n

ðan þ b2nÞ

ð13Þ

Substitute the geometric parameters of the structure listed in Sec-tion 3.1 into above formulas and the theoretical calculation resultswere obtained. In theoretical calculation, the equilibrium contactangle is set as 59.9� according to our experimental measurement re-sult on flat silicon surface. Comparison of theoretical calculationand measurements is shown in Table 3. Plotted comparison resultsare shown in Fig. 20.

It is seen from Table.3 and Fig. 20 that measurement results lo-cated between theoretical calculation curve of cn-cm state and cn-wm state. It indicates that micro–nano hierarchical structures notbehave at any kind of four theoretical wetting states as shown inFig. 19. Instead, it may demonstrate a hybrid state at which watersits on nanoscale pillars and immerses into microscale groovespartially, as Fig. 21 shows. Hybrid state may be the reason forthe deviation between measurement and calculation results.

Table 3Theoretical calculations and measurements of contact angles of micro–nano hierarchical structure (he = 59.9�).

hm (lm) am (lm) bm (lm) bmam

wn-cm (�) cn-cm (�) cn-wm (�) wn-wm (�) Measurement (�)

20 20 10 0.5 – 146.4 – – 149.220 20 20 1 51.0 154.9 – – 149.720 20 30 1.5 87.5 160.0 – – 148.820 20 40 2 106.0 163.3 154.4 – 149.520 20 50 2.5 117.9 165.7 145.9 – 149.520 20 60 3 126.3 167.5 141.3 – 152.320 20 80 4 137.6 170.0 136.4 – 151.8

Where, – denote that calculation result is out of the contact angle range (�1 < cos h < 1), thus it does not exist.

Fig. 20. Comparison of theoretical calculations and measurements of contact anglesof micro–nano hierarchical structure (hm = 20 lm, am = 20 lm).

Fig. 21. Hybrid state of micro–nano hierarchical structure.

Y. He et al. / Journal of Colloid and Interface Science 364 (2011) 219–229 229

4. Conclusion

An effective fabrication method combining deep reactive ionetching and galvanic etching for silicon micro–nano hierarchicalstructures is presented in the paper. The method can partially con-trol the morphology of the nanostructures and enables us to inves-tigate the effects of geometry changes on the properties of thesurfaces.

The forming mechanism of silicon nanostructures based on sil-ver nanoparticle galvanic etching was illustrated. The effects ofprocess parameters on the surface morphology were thoroughlydiscussed. It is found that process parameters have more impacton the height of silicon nanostructure than its diameter. Particu-larly, increasing etching time or heating temperature or Fe(NO3)3

concentration will linearly (or approximately linearly) increasethe height of silicon nanostructure, thus these process parametersare preferable for controlling the morphology of siliconnanostructures.

Using the deep reactive ion etching and galvanic etching meth-od, silicon micro–nano hierarchical structures with different geo-metric parameters were successfully fabricated. Contact anglemeasurement and tilting/dropping test results show that as-pre-pared silicon surfaces with hierarchical structures weresuperhydrophobic.

Two-scale model composed of micropillar arrays and nanopillararrays was established to study the wettability of the hierarchicalstructures. Theoretical calculations of contact angles were com-pared with experimental measurements. The results show thatthe superhydrophobic surface may demonstrate a hybrid state atwhich water sits on nanoscale pillars and immerses into micro-scale grooves partially.

Acknowledgments

This work is supported by National Natural Science Foundationof China (Grant No. 51005187), Natural Science Basic Research Planin Shaanxi Province of China (Grant No. 2010JM7012), NPU Foun-dation for Fundamental Research (Grant No. JC200826), Ao-XiangStar Program of NPU. The authors thank the helpful discussion withDr. Jie Kong (School of Science, Northwestern PolytechnicalUniversity).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jcis.2011.07.030.

References

[1] P. Roach, N.J. Shirtcliffe, M.I. Newton, Soft Matter 4 (2008) 224–240.[2] N.J. Shirtcliffe, G. McHale, S. Atherton, M.I. Newton, Adv. Colloid Interface Sci.

161 (2010) 124–138.[3] L. Feng, S.H. Li, Y.S. Li, H.J. Li, L.J. Zhang, J. Zhai, Y. Song, B.Q. Lin, L. Jiang, D.B.

Zhu, Adv. Mater. 14 (2002) 1857–1860.[4] X.F. Gao, L. Jiang, Nature 432 (2004) 36.[5] Y.M. Zheng, X.F. Gao, L. Jiang, Soft Matter 3 (2007) 178–182.[6] A. Marmur, Langmuir 20 (2004) 3517–3519.[7] W.J.P. Barnes, Science 318 (2007) 203–204.[8] Y.M. Zheng, H. Bai, Z.B. Huang, X. Tian, F.Q. Nie, Y. Zhao, J. Zhai, L. Jiang, Nature

463 (2010) 640–643.[9] A. Lafuma, D. Quere, Nat. Mater. 2 (2003) 457–460.

[10] R. Blossey, Nat. Mater. 2 (2003) 301–306.[11] Z.G. Guo, F. Zhou, J.C. Hao, W.M. Liu, J. Am. Chem. Soc. 127 (2005) 15670–

15671.[12] Y.H. Xiu, L.B. Zhu, D.W. Hess, C.P. Wong, Langmuir 22 (2006) 9676–9681.[13] H.E. Jeong, S.H. Lee, J.K. Kim, K.Y. Suh, Langmuir 22 (2006) 1640–1645.[14] S.T. Wang, Y.L. Song, L. Jiang, Nanotechnology 18 (2007) 015103.[15] B. Cortese, S. D’Amone, M. Manca, I. Viola, R. Cingolani, G. Gigli, Langmuir 24

(2008) 2712–2718.[16] W.G. Xu, H.Q. Liu, S.X. Lu, J.M. Xi, Y.B. Wang, Langmuir 24 (2008) 10895–

10900.[17] Y. Zhao, M. Li, Q.H. Lu, Z.Y. Shi, Langmuir 24 (2008) 12651–12657.[18] H.E. Jeong, M.K. Kwak, C.I. Park, K.Y. Suh, J. Colloid Interface Sci. 339 (2009)

202–207.[19] B. Bhushan, K. Koch, Y.C. Jung, Ultramicroscopy 109 (2009) 1029–1034.[20] J. Wang, J. Chen, S.H. Cao, S.H. Xia, Y.J. Zhu, G.J. Xu, Mater. Chem. Phys. 117

(2009) 183–186.[21] B. Hipp, I. Kunert, M. D€urr, Langmuir 26 (2010) 6557–6560.[22] T. Hang, A.M. Hu, H.Q. Ling, M. Li, D.L. Mao, Appl. Surf. Sci. 256 (2010) 2400–

2404.[23] J. Marquez-Velasco, M.E. Vlachopoulou, A. Tserepi, A. Tserepi, Microelectron.

Eng. 87 (2010) 782–785.[24] W.J. Zhao, L.P. Wang, Q.J. Xue, J. Phys. Chem. C 114 (2010) 11509–11514.[25] J. Wang, D.D. Li, Q. Liu, X. Yin, Y. Zhang, X.Y. Jing, M.L. Zhang, Electrochim. Acta

55 (2010) 6897–6906.[26] Z. Chen, M.H. Cao, Mater. Res. Bull. 46 (2011) 555–562.[27] I.H. Choi, Y.H. Kim, J.H. Yi, Ultramicroscopy 108 (2008) 1205–1209.[28] M.F. Wang, N. Raghunathan, B. Ziaie, Langmuir 23 (2007) 2300–2303.[29] Y.H. Xiu, L.B. Zhu, D.W. Hess, Nano Lett. 7 (2007) 3388–3393.[30] Y.J. Kwon, N. Patankar, J.K. Choi, J.H. Lee, Langmuir 25 (2009) 6129–6136.[31] G.Y. Sun, T.L. Gao, X. Zhao, H.X. Zhang, Micromech. Microeng. 20 (2010)

075028.[32] X.C. Li, B.K. Tay, P. Miele, A. Brioude, D. Cornu, Appl. Surf. Sci. 255 (2009) 7147–

7152.[33] W.F. Kuan, L.J. Chen, Nanotechnology 20 (2009) 035605.[34] K.Q. Peng, Y. Wu, H. Fang, X.Y. Zhong, Y. Xu, J. Zhu, Angew. Chem. Int. Ed. 44

(2005) 2737–2742.[35] K.Q. Peng, J.J. Hu, Y.J. Yan, Y. Wu, H. Fang, Y. Xu, S.T. Lee, J. Zhu, Adv. Funct.

Mater. 16 (2006) 387–394.[36] H. Fang, Y. Wu, J.H. Zhao, J. Zhu, Nanotechnology 17 (2006) 3768–3774.[37] T. Qiu, X.L. Wu, Y.F. Mei, P.K. Chu, G.G. Siu, Appl. Phys. A 81 (2005) 669–671.[38] W.C. Ye, C.M. Shen, J.F. Tian, C.M. Wang, L.H. Bao, H.J. Gao, Electrochem.

Commun. 10 (2008) 625–629.[39] T.A. Witten, L.M. Sander, Phys. Rev. B 27 (1983) 5686–5697.[40] R.N. Wenzel, Ind. Eng. Chem. 28 (1936) 988.[41] A.B.D. Cassie, S. Baxter, Trans. Faraday Soc. 40 (1944) 0546.[42] N.A. Patankar, Langmuir 20 (2004) 8209–8213.[43] T.G. Cha, J.W. Yi, M.W. Moon, K.R. Lee, H.Y. Kim, Langmuir 26 (2010) 8319–

8326.