Solar Energy Materials & Solar Cells 107 (2012) 219–224

Contents lists available at SciVerse ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

Nanometric multi-scale rough, transparent and anti-reflective ZnOsuperhydrophobic coatings on high temperature solar absorber surfaces

Harish C. Barshilia n, Siju John, Vishal Mahajan

Surface Engineering Division, CSIR-National Aerospace Laboratories, Post Bag No. 1779, Bangalore 560017, India

a r t i c l e i n f o

Article history:

Received 14 October 2011

Received in revised form

15 February 2012

Accepted 19 June 2012Available online 20 July 2012

Keywords:

High temperature solar selective coating

Superhydrophobicity

Sputtering

Broadband anti-reflection

48/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.solmat.2012.06.031

esponding author. Tel.: þ91 80 2508 6494; fa

ail address: harish@nal.res.in (H.C. Barshilia).

a b s t r a c t

We report the fabrication of a nanometric multi-scale rough, transparent and anti-reflective zinc oxide

(ZnO) superhydrophobic coating on TiAlN/TiAlON/Si3N4 spectrally selective solar absorber surface,

which has been developed previously for solar thermal power generation applications. The optimized

ZnO superhydrophobic coating on the absorber surface demonstrates extraordinary water repellency

(with contact angle 41551), improved absorptance (40.96) and excellent broadband anti-reflection in

the visible range of the solar spectrum. The multi-functional ZnO coating was stable up to 450 1C (in air

and vacuum), indicating its reliability for high temperature photothermal conversion applications.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

The photothermal conversion efficiency of solar thermal col-lectors greatly depends on their spectral selectivity [1]. Highspectral selectivity is reportedly achieved by designing absorbercoatings on collectors having high absorptance (a) in the wave-length (l) range 0.3–2.5 mm and low emittance (e) at higheroperating temperatures (Top) in the IR region (2.5–100 mm) [2].The ideal characteristics of a photothermal converter can beapproximated by an absorber–reflector tandem, in which thereflector is coated with a layer which is highly absorbing inthe visible region and is transparent in the IR region to allowthe reflector to transmit through in this region and to determinethe thermal emittance. Other properties of absorber coatingslike thermal stability, abrasion resistance, UV stability and self-cleaning need to be considered in order to retain their persistenceperformance for long service life. Furthermore, the spectrallyselective coatings reflect significant amount of solar radiation,thus, degrading the conversion efficiency [1]. Even though singlelayer anti-reflection coatings (ARC) are used in the spectrallyselective surfaces, these are far from being the most efficientsolutions since the ARCs allow a reduction in the reflectance (R)only in a narrow wavelength of the solar spectrum and asubstantial amount of solar radiation is reflected from theabsorber surfaces [3]. Texturing of the absorber surfaces atsub-micron level has been reported previously for improved

ll rights reserved.

x: þ91 80 2521 0113.

aborptivity and very low front surface reflectance [4,5]. Still,there is a need to develop solar absorber coatings that preferen-tially exhibit multifunctionalities such as: high spectral selectiv-ity, self-cleaning, broadband anti-reflection phenomenon, etc.Such coatings have been fabricated recently for photovoltaicdevices [6–10], however, there are no reports on functionalcoatings with self-cleaning and anti-reflective properties for hightemperature photothermal conversion applications.

It has been reported extensively that for a carefully designedrough surface (especially with low surface free energy (SFE)), theair bridging effect between consecutive mounts reduces intimatecontact between liquid and solid, leading to high water contactangle (CA). Such nanostructured surfaces with static contactangle 41501 and sliding angle o101 bestow superhydrophobi-city [11,12]. Recently, there has been a great interest in thedevelopment of superhydrophobic surfaces, which are expectedto have a wide range of technological applications in self-clean-ing, anti-icing, anti-biofouling, anti-fogging, micro-fluidics, etc.[13,14]. Because of their potential applications, a large number ofsuperhydrophobic coatings based on organic, inorganic andorganic-inorganic hybrid systems have been developed usingdifferent methods in the last decade [12]. It has been shown thatsuperhydrophobicity of a given surface is attributed to a combi-nation of dual scale roughness (micro- and nanometric) and lowsurface free energy. Recent reports also suggest that even thoughmulti-scale roughness is necessary to achieve superhydrophobi-city, the upper scale of the roughness may not necessarily be inthe micrometric scale [15,16].

Incorporation of other important multifunctional characteris-tics in the superhydrophobic surfaces, viz., optical transparency,

Si3N4TiAlONTiAlN

Substrate

ZnO

Fig. 1. Schematic of rough and transparent ZnO superhydrophobic coating on

TiAlN/TiAlON/Si3N4 tandem absorber deposited on SS substrate. The TiAlN, TiAlON

and Si3N4 layer thicknesses were: approximately 64, 24, 34 nm, respectively.

H.C. Barshilia et al. / Solar Energy Materials & Solar Cells 107 (2012) 219–224220

electrical conductivity, reversibility, thermal stability and abra-sion resistance is expected to open new frontiers in designingnovel materials with exotic properties [17–19]. Remarkablyenough, it is extremely difficult to achieve superhydrophobiccoatings with multifunctional properties. For example, higherroughness is necessary to achieve superhydrophobicity, on thecontrary, the transmittance (T) of a coating in the visible rangedecreases with increasing roughness, especially if the roughnessexceeds the wavelength of light (400–750 nm) [3,18]. Thus,fabrication of a superhydrophobic coating with anti-reflectivebehavior suitable for an underlying spectrally selective coatingrequires precise control of microstructure, thickness and opticalproperties.

Several transparent superhydrophobic coatings based on silicaand zinc oxide (ZnO) have been developed in recent years fromboth the physical and chemical routes on glass substrates[6,17,20–22]. ZnO is an interesting material as it exhibits a widerange of surface morphologies such as nanorods, nanonails,nanoneedles and nanowires. In our previous research we haveshown that thermally annealed, sputtered Zn coatings (i.e., Zn–ZnO) exhibit superhydrophobicity [23]. When Zn–ZnO coatingsare annealed at higher temperature in vacuum for longer dura-tions, Zn evaporates and forms highly transparent ZnO coatings[24]. Because of its large band gap (�3.3 eV), ZnO shows very hightransmittance in the visible region and high absorption in the UVregion. Of course, the optical properties of nanostructured ZnOvary significantly with the defect density and the microstructure[25]. Even though, superhydropohobic ZnO coatings have beenstudied widely no attempts have been made to use ZnO for ant-reflection applications, especially for solar thermal conversion. Inthis paper, we report the fabrication of a nanometric multi-scalerough, transparent and anti-reflective zinc oxide superhydropho-bic coating on a sputter deposited high temperature solar selectivecoating (SHC–SSC coating hereafter, with a schematic shown inFig. 1). The solar selective coating (TiAlN/TiAlON/Si3N4), which isstable at Top4450 1C (in vacuum), has been developed previouslyby our group using a tandem absorber concept for solar thermalpower generation applications [26,27]. The optimized SHC-SSCcoating demonstrates extraordinary water repellency, improvedabsorptance and excellent broadband anti-reflection in the visiblerange of the solar spectrum.

2. Experimental details

The tandem absorber deposited on stainless steel (SS) sub-strate (3.5�3.5 cm2) was used as the spectrally selective coatingwith a¼0.940 and e¼0.17. More details about the preparationand characterization of the absorber coating can be found in Refs.[26–28]. Thermally oxidized ZnO coatings with varying thick-nesses (300–1800 nm) were deposited on the tandem absorber bysputtering a Zn target in Ar plasma followed by 1 h oxidation at

350 1C in O2 environment. ZnO coatings prepared under theseconditions were further annealed for 2 h at 450 1C in vacuum toimprove the optical transparency. Because the spectrally selectivecoating was unstable for temperatures greater than 500 1C, theannealing temperature was restricted to 450 1C in the presentstudy. Further details about ZnO coating preparation and char-acterization can be found in Refs. [23,24].

The X-ray diffraction (XRD) patterns of the coatings wererecorded in a Rigaku D/max 2200 Ultima X-ray powder diffract-ometer with thin film attachment (a¼31). The X-ray source was aCuKa radiation (l¼0.15418 nm), which was operated at 40 kVand 30 mA. The bonding structure of the coatings was character-ized by X-ray photoelectron spectroscopy (XPS) using an ESCA3000 (V. G. Microtech) system with a monochromatic AlKa X-raybeam (energy ¼1486.5 eV and power¼150 W). Three-dimen-sional surface imaging and the roughness of the coatings weremeasured using atomic force microscopy (AFM), Surface ImagingSystems. The microstructure of the coatings was studied usingfield-emission scanning electron microscopy (FESEM), Supra 40VP, Carl Zeiss. The static contact angle was measured according tothe sessile drop method using a contact angle analyzer (Phoenix300 goniometer) with deionized water. The system mainly con-sists of a CCD video camera with a resolution of 768�576 pixels.The drop image was stored by the video camera and an imageanalysis system calculated both the left and right angles from theshape of the drop with an accuracy of 711. The droplet size of thefluid was about 5 ml, therefore, the gravitational effects can beneglected. The contact angle of the samples was measured atthree places and the values reported herein are the averages ofthree measurements. The dynamic contact angle measurements(advancing (yA) and receding (yR) contact angles) were carried outusing a Rame-Hart contact angle goniometer (model 100-00)equipped with a CCD camera. The difference between the twoangles (yA�yR) is called contact angle hysteresis. A water droplet(about 10 mg) was slowly and carefully attached to the substrateby a micro-syringe. The sliding angle of the water droplet wasobserved by first placing the water droplet on horizontal surfaceand then slowly tilting the film surface until the droplet startsmoving.

The optical properties (a and e) of the SHC–SSC coatings weremeasured using solar spectrum reflectometer (Model SSR) andemissometer (Model AE) of M/s. Devices and Services. For the solarspectrum reflectometer, the source of the illumination was atungsten–halogen lamp. The radiation reflected by the samplewas measured at an angle of 201 from the normal, with fourfiltered detectors (UV, blue, red and infrared). By summing thefour outputs in the appropriated proportions, a solar spectrummeasurement was achieved. Air mass 2 was used to calibrate thesolar reflectometer. The emissometer was heated to 82 1C, so thatthe sample to be measured need not be heated. This emissometeris a special purpose instrument to measure the emittance ofabsorber coatings used for flat plate solar thermal collector,wherein the maximum working temperature of the collector isof the order of 80–85 1C. At 82 1C, the spectral range of the thermalradiation emitted from the surface is in the range 3–30 mm. Thedetector in the emissometer consists of a differential thermopilewith low and high emittance areas, which ensures near constantresponse to the emitted radiation in this wavelength range. Boththe instruments were calibrated using standard samples. Theaccuracies of the measured a values are72% with a drift of71%þ0.003/hr and the emissometer has a repeatability of70.01 units. The absorptance and the emittance values weremeasured at four different positions and the values reportedherein are the average of four measurements. The optical trans-mittance and reflectance of the ZnO and SHC–SSC coatings weremeasured using a Systronic make UV–vis 119 spectrophotometer.

H.C. Barshilia et al. / Solar Energy Materials & Solar Cells 107 (2012) 219–224 221

3. Results and discussion

The XRD data of approximately 1.3 mm thick optimized ZnOcoating deposited on glass substrate shown in Fig. 2 in the 2yrange 25–601. Peaks centered at 2y¼31.8, 34.3, 36.3, 47.6 and56.51 correspond to (100), (002), (101), (102) and (110) planes,respectively of hexagonal-wurtzite ZnO [29]. The formation ofZnO was further confirmed by X-ray photoelectron spectroscopy.The core level XPS spectra of the optimized ZnO coating areshown in Fig. 3 (a) and (b). Zn 2p3/2 spectrum of the coatingconsisted of a sharp peak centered at 1021.7 eV, which corre-sponds to stoichiometric ZnO [30]. The O 1 s spectrum indicatedthat the peak centered at 531.5 eV corresponds to O2� ions thatare in oxygen-deficient regions within the matrix of ZnO [30].

The multifunctionality of the SHC–SSC coating was optimizedby varying the ZnO coating thickness (tZnO) to achieve highabsorptance, and contact angle and low emittance. The measure-ments indicated that a and e increased from 0.944 to 0.972 andfrom 0.17 to 0.18, respectively when the coating thicknessincreased to 1300 nm (Fig. 4(a)). Further increase in the coatingthickness did not affect a but a significant increase in e wasobserved. For example, the emittance increased to 0.21 for a SHC–SSC coating with tZnO¼1800 nm and it increased drasticallythereafter (results not shown). The contact angle observed onuncoated tandem absorber was 1001, which scaled up almostlinearly with tZnO and at tZnO¼1588 nm, CA as high as 1571 wasobtained but with high emittance (Fig. 4(a) and (b)). FortZnO41600 nm a decrease in CA was observed because of thechange in surface roughness as will be discussed later. The resultsrevealed that the SHC–SSC coating with tZnO¼1300 nm demon-strated high a (0.972), low e (0.18) and CA41501. The waterdroplets on these surfaces rolled off very easily.

25 30 35 40 45 50 55 60

(102)

(002)(110)

(100)

Inte

nsity

(arb

. uni

ts)

2 theta (deg.)

(101)

Fig. 2. XRD pattern of 1300 nm thick optimized ZnO coating on silicon substrate.

1021.7 eV

1015 1022 1029

Inte

nsity

(arb

. uni

ts)

Binding Energy (eV)

Fig. 3. Deconvoluted XPS spectra of optimized ZnO coating:

It is known that for a given material, it is possible to lower theSFE by suitably modifying the surface morphology, and a precisecontrol of roughness is necessary to achieve high transparency[3,12,18,19]. The microstructure and roughness of ZnO coatingswere judiciously controlled by varying ZnO coating thickness,annealing temperature and duration. The FESEM image of the ZnOcoating with low tZnO (433 nm) displayed a porous microstructurewith solid component of ZnO nanoclusters as whitish regions andair-pockets as darker regions (Fig. 5(a)). The roughness profile ofthis sample (Fig. 5(b)), measured using AFM, exhibited an averageroughness (Ra) of 36 nm. This surface morphology resulted in acontact angle of 1251. For ZnO coating with tZnO¼1300 nm, themicrostructure manifested more of whitish regions (Fig. 5(c)).This sample demonstrated Ra¼82 nm (Fig. 5(d)) and a contactangle of 1541. It must be emphasized that the roughness is one ofthe factors determining wettability of a given surface. Thestructure (i.e., shape) of the roughness affects the wettabilitysignificantly rather than its magnitude. This is due the fact thatthe structure of the roughness defines the shape and continuity ofthe three phase contact line which critically determines wett-ability of a given surface [31,32]. According to cross-sectionalFESEM studies, the porous microstructure existed even along thecoating thickness (Fig. 5(e)). It is believed that presence of thesolid components and the air-pockets forms the first scale ofroughness on higher scale. Detailed high-resolution FESEM inves-tigations led to conclude that the ZnO nanoclusters were formedas a result of fusion of individual nanoclusters, which generated atextured surface (marked with arrows in inset of Fig. 5(f)), givingrise to the formation of localized roughness on nanometric scale.This nanometric scale roughness attributes to the lower scale ofhierarchal multi-scale roughness. A graphical representation of awater droplet on the ZnO coating is displayed in Fig. 5(e). It can beseen that the water droplet balances on the solid surface bridgingthe air-pockets giving rise to a high CA. The water droplet on suchporous microstructure follows the well known Casie–Baxtermodel [33]. The dynamic contact angle measurements on SHC–SSC coating with tZnO¼1300 nm showed apparent advancing andreceding contact angles of 163 and 1591, respectively with asliding angle of 41. This confirmed that the water droplet on suchsurface was indeed in the Casie–Baxter state. For tZnO41600 nm,the roughness increased and the CA decreased considerably. Weconclude from these observations that in order to achieveextreme water repellency in ZnO coatings, a microstructure withan optimum combination of solid regions (i.e., textured nanoclus-ters) and air-pockets, resulting in hierarchal nanometric multi-scale roughness, is necessary.

To investigate the self-cleaning and anti-reflective propertiesof the ZnO coating, the tandem absorber and glass substrateswere coated with 1300 nm thick ZnO. Fig. 6(a) shows the photo-graph of water droplets on the optimized SHC–SSC sample,

531.5 eV

525 532 539

Inte

nsity

(arb

. uni

ts)

Binding Energy (eV)

(a) core level Zn 2p3/2 spectrum and (b) O 1 s spectrum.

0 300 600 900 1200 1500 1800

125

130

135

140

145

150

155

160

O → 154º

Coating thickness (nm)

Em

ittan

ceC

onta

ct A

ngle

(deg

.)

0.00.10.20.30.40.50.60.70.80.91.0

0 300 600 900 1200 1500 18000.00.10.20.30.40.50.60.70.80.91.0

Absorptance (α)Emittance (ε)

Abs

orpt

ance

Coating thickness (nm)

Ref.

Ref.

O

O

O→ 0.9720.18 Ref. → 0.944 (α)

0.17 (ε)

O

Fig. 4. (a) Variations of absorptance and emittance with successive increase in ZnO coating thickness on tandem absorber surface. (b) Respective variation in contact angle

as a function of thickness. Ref., with dashed line, shows a and e values of uncoated tandem surface, whereas, O represents the optimum coating thickness with maximum

possible increase in the absorptance corresponding to lowest emittance.

50

10000 20000 300000Length (nm)

100

0Ra= 36 nm

Hei

ght (

nm)

1 μm

154º

4 μm

4 μm

125º

790

40000

nm

nm

40000

2000020000

0

0 Ra= 82 nm

1 μm

Water droplet

Air pocketSolidregion

Fig. 5. FESEM micrographs of ZnO nanoclustered film (a) with 433 nm thickness, (b) corresponding AFM 2-D roughness profile. (c) the FESEM image of 1300 nm optimized

thickness with its (d) 3-D morphology, (e) cross-sectional view from a fractured coating deposited on Si substrate and (f) high-resolution image. The inset in (f) shows ZnO

nanoclusters with nanometric roughness added to porous microstructure responsible for superhydrophobicity. The thickness of ZnO nanostructured coating was

controlled by sputtering time.

H.C. Barshilia et al. / Solar Energy Materials & Solar Cells 107 (2012) 219–224222

Transmittance (T)Reflectance (R)

400 500 600 700 800Wavelength (nm)

T an

d R

(%)

010

20304050

60

7080

90100

Fig. 6. Photographs of colored water droplets on the solar selective coating deposited on SS substrates: (a) with and (b) without optimal superhydrophobic ZnO coating.

(c) The measured transmittance and reflectance spectra of ZnO film on glass substrate. (d) ZnO superhydrophobic coated glass substrate with spherical water droplets.

400 500 600 700 8000

5

10

15

20

With ZnO

Ref

lect

ance

(%)

Wavelength (nm)

Without ZnO

Fig. 7. Reflectance data of tandem absorber deposited on SS substrate with and

without ZnO coating.

H.C. Barshilia et al. / Solar Energy Materials & Solar Cells 107 (2012) 219–224 223

displaying almost spherical drops with CA¼1541, whereas, thewater droplet on the uncoated tandem absorber was non-sphe-rical (Fig. (6)b). The SHC–SSC coating exhibited excellent self-cleaning behavior. The measured reflectance and transmittancedata of the optimized ZnO coating on corning glass substrateshowed T470% for l4600 nm and low R (o5% for lr700 nm)(Fig. 6(c)). The bare glass substrate had a transmittance of �90%in the spectral range 400–850 nm. The multifunctional propertiesof ZnO coating deposited on a glass substrate are further eluci-dated in Fig. 6(d), displaying good readability of the lettersunderneath the ZnO coated glass substrate and spherical shapeof the water droplets. The reflectance data of the tandem absorberwith and without ZnO coating are shown in Fig. 7. The coatingroughness, which becomes a source of scattering of light and

untreated Zn impurities (in any, even on ppm level) as a result ofthermal oxidation, reduces the transparency of ZnO coated glasssubstrate [18]. It is interesting to note that the reflectance of ZnOcoated tandem absorber decreases considerably compared touncoated sample. As discussed earlier, a single layer ARC displayslow reflectance only in a narrow wavelength range [3]. Most ofthe photothermal applications, however, require excellent broad-band anti-reflection properties, which is achieved by depositingan additional ARC layer with lower refractive index (n) on top ofthe single layer ARC. Unlike, single layer ARC, wherein, thethickness is few tens of nm, for a double-layer ARC it extends tofew hundreds of nm [34]. This coating design with optimal n andcoating thickness reduces broadband reflectance. In the presentwork, for the tandem absorber, n decreases gradually fromsubstrate to the surface of the coating (i.e., nSubstrate4nTiAlN4nTiAlON4nSi3N4) [26]. The n of ZnO varies from 2.1 to 1.9 in thewavelength range 4 mm4lZ0.5 mm and the n values of ZnO(e.g., n0.5 mm¼2.05) are reported to be lower than Si3N4 (e.g.,n0.5 mm¼2.20) layer [26,35]. Interestingly, the porous microstruc-ture of ZnO is advantageous, which gradually increased thematerial density from air to the coating and extends the anti-reflective behavior in both visible and near-infrared region (NIR)of the solar spectrum as it allows a continuous transition from alow (i.e., air) to a high refractive index (i.e., ZnO) [3]. Hence, it canbe inferred that this gradual change in the refractive indexdecreased the front surface reflectance [R¼(n�1)2/(nþ1)2] ofthe SHC–SSC coating (Fig. 7) and consequently increased theabsorptance. The reflectance data of SHC–SSC sample is consistentwith the absorptance data, which shows an increase in a with tZnO

(Fig. 4(a)). A direct application of the SHC–SSC coating of thepresent work can be envisaged for linear Fresnel technology,wherein the receiver tube coated with an absorber is exposed tohigh temperature in the air. The performance of the linear Fresnel

H.C. Barshilia et al. / Solar Energy Materials & Solar Cells 107 (2012) 219–224224

tube is expected to improve considerably if a suitable transparentand superhydrophobic coating is applied on its surface. Addition-ally, the applications of transparent superhydrophobic ZnO coat-ing of the present work can be extended for other solar/opticaldevices such as: solar cells, dichroic mirrors, optical lenses,photodetectors, night-vision devices, NIR sensors, etc.

4. Conclusions

Nanometric multi-scale rough ZnO coating was fabricatedwith its full compatibility on TiAlN/TiAlON/Si3N4 high tempera-ture solar selective coating. In the development towards real lifeapplications of superhydrophobic concept, this successful effort toattain extreme water repellency on high temperature solarabsorber coating is first of its kind. The ZnO thin film responsiblefor such fascinating behavior was stable up to 450 1C (in vacuumand air), indicating its reliability for high temperature photo-thermal conversion applications. Beyond self-cleaning, ZnO coat-ing is expected to impart the absorber surface a long lasting lifeand resistance against degradation of its optical properties mainlydue to water staining and dry contaminants.

Acknowledgments

This research was partially supported by the Council ofScientific and Industrial Research (CSIR), New Delhi (Grant No.:FAC-00-01-11). The authors thank the Director, National Aero-space Laboratories (CSIR-NAL), Bangalore for giving permission topublish these results.

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