Highly Transparent and UV-Resistant Superhydrophobic SiO2?Coated ZnO Nanorod Arrays
Yangqin Gao,?Issam Gereige,?Abdulrahman El Labban,?Dongkyu Cha,?Tayirjan T.Isimjan,*,?
and Pierre M.Beaujuge*,?
?Physical Sciences and Engineering Division and?Advanced Nanofabrication,Imaging&Characterization Laboratory,King Abdullah University of Science and Technology(KAUST),Thuwal23955-6900,Saudi Arabia
*Supporting Information
solar cell,nanocrystal arrays
INTRODUCTION
Superhydrophobic coatings can have a signi?cant impact on the development of glass and plastics with self-cleaning1?4and anti-fogging5,6properties,while also adding value to a wide range of tribological7?9and micro?uidics applications.10?13In super-hydrophobic coatings,hierarchical roughness introduces multi-scale voids and gaps,trapping air at the material-water interface and,in turn,inducing high contact angles(>150°)with water droplets,low contact angle hysteresis and sliding angles.
The design and development of self-cleaning coatings for use in solar panels is especially important given that nearly half of the overall power conversion e?ciency of solar panels can be lost due to dust accumulation every year.14In addition to the water-and dust-repellent property requirements,superhydro-phobic coatings for photovoltaics must be highly transmissive to both visible and near-IR light as well as being UV-resistant and durable.A few recent studies discuss transparent superhydrophobic coatings applied to photovoltaics.3,15,16 One promising approach utilizes solution-processed SiO2-coated polymer microbeads that can be made hollow by calcination of the polymer template(350°C).15The resulting microcapsules can be bridged in a chemical vapor deposition (CVD)step with SiO2precursors to yield a mechanically resistant and highly transparent coating that can be applied to organic photovoltaic(OPV)devices on glass.15Meanwhile, approaches by which highly transmissive and nanostructured coating materials can be produced in a sequence of low-temperature steps(<150°C)would make them applicable to a variety of?exible plastics,which is one of the key directions of research in the OPV community.17
Nanostructured oxides such as ZnO and TiO2possess high band gaps of~3.2eV18and,as a result,high visible transmissivities,and can be either solution-processed19?22or hydrothermally grown using established methods.23,24Notably, as light scatters across oxide arrays,parasitic re?ectivity is suppressed,such that nanostructured oxides can be used to improve the transparency of glass.25?28Here,we describe the preparation of a highly transmissive(avg.93-95%)and UV-resistant superhydrophobic coating based on SiO2-coated ZnO nanorod arrays grown on ultrathin seed layer(~5nm),and demonstrate that the presence of the superhydrophobic nanocomposite does not signi?cantly impact the?gures of merit of OPV devices.We also show that the sequential steps used for the preparation of the SiO2/ZnO nanocomposite can be reproduced on a thin transparent sheet(2×2in.2)of polyethylene terephthalate(PET),which retains its super-hydrophobicity upon repeated bending,suggesting that the superhydrophobic nanocomposite can be applied to?exible photovoltaics and displays.
■RESULTS AND DISCUSSION
As part of the ZnO nanorod hydrothermal growth process,a seed layer of ZnO must be deposited on pre-cleaned glass(Fig. 1a,step1)to lower the activation barrier for crystal nucleation.29Here,the seed layer was deposited by magnetron sputtering(see experimental section in the Supporting Information),and the dependence of seeded-glass trans-missivity on the thickness of seed layer was examined by UV?vis spectroscopy.Figure1b shows that the substrate transmittance gradually decreases as the thickness of the seed
Received:December2,2013
Accepted:February4,2014
Published:February4,2014
layer increases,indicating that the seed layer should be as thin as possible in order to minimize both the parasitic absorption and re ?ection.With the refractive index of ZnO on the order of 2.0,30the re ?ection of incident light at the air ?ZnO interface is expected to be greater than that at the air-glass interface.For example,in glass coated with a 50nm seed layer (Figure 1b,orange curve),10%(avg.)loss in transmittance is detected in the visible region (400?800nm),while the transmittance drops by ca .50%in the UV region,thus signi ?cantly reducing the number of photons transmitted to the active layer of any solar cell in this portion of the spectrum.In contrast,when the seed layer thickness approaches 5nm (Figure 1b,red curve),the transmissivity of the ZnO-coated glass is nearly equivalent to that of glass alone across the full spectrum (300?800nm),with a minimal transmittance loss near the band gap of ZnO.Meanwhile,the seed layer should be su ?ciently stable,that is,thick enough,to allow e ?ective and homogenous ZnO nanorod crystal growth by the hydrothermal approach.31Experimentally,we found that a thickness of a few nanometers is required.We thus set the seed layer thickness at 5nm in all following steps.To develop a superhydrophobic coating with the highest possible level of transparency,a trade-o ?between su ?cient surface roughness and excessive scattering caused by overly long ZnO nanorods must be found.Figure 1c shows the evolution of the substrate transmittance as a function of the time allowed for crystal growth upon the seed layer (Figure 1a,step 2),and indicates a continuous loss of optical transmittance in the UV region as longer nanorods are formed.Interestingly,beyond 10min of nanocrystal growth,the ZnO-coated glass substrate becomes more optically transmissive than bare glass in the visible region.This e ?ect can be attributed to the graded transition of refractive index across the nanostructured coating acting as an anti-re ?ecting material at the interface between glass and air.28,32Meanwhile,as nanocrystal growth extends to 1h,the nanorod length reaches ~500nm on average (see Figure S1in the Supporting Information),thus falling in the range of visible light wavelengths and resulting in signi ?cant parasitic scattering.33The higher degree of visible transparency is achieved after 25min of nanocrystal growth (Figure S1in the Supporting Information),as the nanorods reach a critical average length of ~240nm with their diameters on the order of 30nm (Figure 1c,inset,and FigureS4in the Supporting Information):transmissivity values in the 93-95%range,compared with 91?92%for uncoated glass.The X-ray di ?raction (XRD)analysis of the corresponding nanostructured glass (see Figure S5in the Supporting Information)shows a pronounced di ?raction peak at 2θ=34.4°corresponding to the (001)crystal plane of ZnO and only a weak di ?raction peak at 2θ=62.8°assignable to the (103)plane,indicating that most
of the ZnO nanorods grow vertically on ZnO-seeded glass;this result is also shown qualitatively by scanning electron microscopy (SEM)(Figure 1c).It is worth noting that the surface of as-prepared ZnO nanorod arrays is hydrophilic,34meaning that water droplets will rapidly wet ZnO-coated glass in spite of the signi ?cant
roughness of the nanostructured oxide.Hydrophilic substrates exhibit small water contact angles and large sliding angles and,as such,they can easily accumulate water and dust.To circumvent the combined e ?ect of polar hydroxyl groups at the ZnO surface and the capillary forces inducing the wettability,
the nanorods are coated with a monolayer of per ?uorodecyl-triethoxysilane (PTES).At the ZnO nanorod surface,the PTES monolayer reduces the surface energy of the oxide,35and is expected to impart the superhydrophobicity.Figure 1d describes the superhydrophobic behavior of the per ?uorinated ZnO nanorod arrays on glass as a function of the time allowed for crystal growth.While simple ZnO-seeded glass per ?uori-nated with PTES (t=0)shows low static contact angle values of 120°,static contact angles greater than 150°can be reached rapidly beyond 5min of nanocrystal growth.Meanwhile,the ZnO-nanostructured glass remains dominated by large sliding angle values greater than 20°during the ?rst 20min of nanocrystal growth,and it is only after about 25min that the nanorod arrays become truly superhydrophobic,with static contact angles greater than 150°(as high as 157°,Figure 1d inset)combined with markedly low sliding angles on the order of 10°and less (estimated from 2μL water droplets).The in ?uence of droplet size on sliding angle measurements is emphasized in Figure S2in the Supporting Information.These results con ?rm that the surface state can be changed from Wenzel to Cassie ?Baxter as the ZnO nanorods grow in length,that is,as the surface roughness increases as shown in Figure 1e.AFM images of a 5nm ZnO seed layer sputtered on glass and the ZnO nanorod array after 25min of growth are shown in Figure S3a and Figure S3b,respectively,in the Supporting Information.Here,we note that the aspect ratio of the AFM tip can be expected to limit the accuracy of the height pro ?les
and
Figure 1.(a)Schematic description of the preparation of super-hydrophobic SiO 2-coated ZnO nanorod arrays on glass (PTES =per ?uorodecyltriethoxysilane).(b)Transmittance spectra of ZnO seed layers of various
thicknesses
(c)Transmittance spectra of ZnO
nanorod arrays grown on 5nm ZnO seed layer with di ?erent growth times.Inset:SEM cross-section of ZnO nanorods obtained after 25min.(d)Evolution of the static (red square)and sliding (blue triangle)contact angles of PTES-treated ZnO nanorod arrays grown on 5nm ZnO seed layer for various growth times.Inset:
Photograph
of
a 2μL
water droplet placed on selected PTES-treated ZnO nanorod arrays.(e)Evolution of the surface roughness (RMS)(black square)and average nanorod length (by SEM)(red circle)in relation to the ZnO-nanorod growth time.
surface roughness(RMS)measurements performed on densely nanostructured surfaces such as the ones shown in Figure S3b in the Supporting Information,yet the relative comparison between RMS values(Figure1e)remains reasonable. Considering both the high degree of transparency retained by the~240nm-long ZnO nanorod arrays obtained after25min of hydrothermal growth and their excellent superhydrophobic properties reached upon per?uorination with PTES,we used these arrays in the following steps and discussions.
While ZnO nanorods are typically prone to UV-triggered photo-oxidation reactions,36,37this shortcoming and the resulting gain in hydrophilicity can be circumvented by protecting the nanocrystal surface with a thin layer of UV-resistant material acting as a physical barrier.23,24Here,we chose to protect the ZnO surface by a thin layer of SiO2(Figure 1a,step3)deposited by chemical vapor deposition(CVD) from tetraethoxysilane in the presence of ammonia(see experimental section in the Supporting Information).15The X-ray di?raction(XRD)pattern of the SiO2-coated ZnO nanorods with varying SiO2deposition times(t=0,3,6,9,12 h)is shown in Figure S5in the Supporting Information,where the gradual reduction in the(001)di?raction peak intensity can be attributed to the screening of the ZnO nanorods by SiO2. Figure2a shows that layering SiO2on the ZnO nanorods does not further reduce the overall transmissivity of the nano-structured glass substrate.Thus,even after6h of SiO2 deposition at an estimated growth rate of ca.1nm h?1,the transmittance of the coating remains on the order of45%(avg) in the UV region and on the order of93?95%across the visible region;the thickness of the SiO2layer deposited after3and6h was determined by transmission electron microscopy(TEM) and the imaged SiO2-coated ZnO nanorods are shown in Figure S6in the Supporting Information,along with the energy-dispersive X-ray(EDX)analysis.As expected,the re?ectance data shown in Figure S7in the Supporting Information indicate that SiO2-coated ZnO-nanostructured glass is less re?ective(4?5%)than bare glass(8?9%). However,Figure S7in the Supporting Information also indicates that the drop in the visible transmissivity of the SiO2-coated ZnO nanorods beyond6h of SiO2deposition (Figure2a)can be correlated with an appreciable gain in the re?ectance after the critical SiO2layer thickness is reached.This empirical observation is in agreement with the coarsening of
the ZnO nanorod diameter upon SiO2deposition(Figure2b) and with the corresponding reduction of RMS roughness as shown in Figure2c upon SiO2deposition.The SEM cross-section of the SiO2-coated ZnO nanorods shown in Figure S8 in the Supporting Information emphasizes the coarsening of the nanorod arrays resulting from9h of SiO2deposition.In such conditions,the nanorods are almost fully bridged by SiO2,while a rather continuous top layer of oxide starts forming,which is expected to increase the overall re?ectivity of the coating. Figure2c con?rms that,upon per?uorination of the SiO2layer with PTES(Figure1a,step4),the SiO2-coated ZnO composite becomes superhydrophobic.While the static contact angle of ca.157°remains fairly constant for SiO2layer thicknesses in the range1-6nm,the sliding angle reaches values as low as ca.7°(~2?3nm)and then increases gradually when the SiO2layer thicknesses surpass~6nm.With a sliding angle larger than40°after6h of SiO2deposition(~7?8nm),the super-hydrophobicity of the nanostructured glass is compromised. This observation is consistent with the signi?cant coarsening of the ZnO nanorod diameter upon SiO2deposition(Figure2b).
Importantly,Figure2d shows that thin SiO2layers of ca.1 nm(1h deposition)are su?cient to protect the ZnO nanorod surface and to avoid the dramatic increase in hydrophilicity caused by UV-triggered photo-oxidation reactions.In parallel, the robustness of the SiO2-coated ZnO nanorod arrays on glass was qualitatively assessed with a“scotch tape”experiment in which the adhesive tape was pressed at10kPa against the nanostructured surface for1min(see details in the Supporting Information).Figure2e shows that,upon removing the tape, the adhesive peels o?from the backing material and remains bound to the SiO2-coated ZnO nanorod arrays.The magni?cation of the nanostructured coating under the bound adhesive shown in Figure2f con?rms that the integrity of the nanorod arrays is retained.
To demonstrate the minimal impact of the presence of the superhydrophobic nanorod arrays on solar cell performance,we prepared bulk-heterojunction(BHJ)devices with the polymer donor PBDTTPD38?40and the fullerene acceptor PC71BM (see device fabrication in the Supporting Information).The con?guration of the OPV device including the SiO2
/ZnO Figure2.(a)Transmittance spectra of SiO2-coated ZnO nanorods (grown for25min on a5nm ZnO seed layer)for various SiO2 deposition times.Inset:TEM image of SiO2-coated ZnO obtained after3h of SiO2deposition.(b)SEM image(top-view)of the SiO2-coated ZnO obtained after6h of SiO2deposition.(c)Evolution of contact angles and surface roughness(RMS)in PTES-treated SiO2-coated ZnO nanorod arrays(grown for25min on5nm ZnO seed layer)for various SiO2deposition times:static contact angle(red square),sliding angle(blue triangle),surface roughness(yellow diamond).(d)Evolution of contact angles on PTES-treated SiO2-coated ZnO nanorod arrays with UV irradiation time(365nm,~2 mW cm?2)(e)SEM image of the SiO2/ZnO nanocomposite surface after application and subsequent removal of a scotch tape pressed at10 kPa for1min:the adhesive(dark region)peels o?and remains bound to the nanostructured coating.(f)Magni?ed SEM image showing that the integrity of the nanorod arrays is retained under the bound adhesive(darker regions).
nanocomposite is shown in Figure 3a;Figure 3b shows perfectly spherical droplets positioned on the front of the superhydrophobic device (static angle,157°;sliding angle,13°).The superhydrophobic cell (red curve,circles)and bare reference cell (blue curve,squares)under AM1.5G solar illumination (100mW cm ?2)(Figure 3c)exhibit equivalent J ?V characteristics with comparable power conversion e ?ciencies (PCEs)of 6.9and 6.8%,respectively,values comparable within the limits of experimental accuracy.In parallel,their external quantum e ?ciency (EQE)spectra (Figure 3d)show comparably broad and e ?cient EQE responses,with values >60%in the 370?630nm range,and peaking at ca .70%at 550nm,which con ?rms the minimal impact of the presence of the superhydrophobic nanorod arrays on solar cell performance.Ultimately,approaches to the preparation of highly trans-missive superhydrophobic coatings in a sequence of low-temperature (<150°C)steps may be applicable to a variety of ?exible plastics,which can serve as front cell and backing materials in the manufacture of ?exible displays and solar cells.Noting that ZnO nanorod arrays can be grown hydrothermally on various plastics such as polyethylene terephthalate (PET),41polydimethylsiloxane (PDMS),42and polyimide,43we consid-ered reproducing the sequential steps used for the preparation of our SiO 2-coated ZnO superhydrophobic coating on a thin transparent sheet of PET (2×2in.2).Under the same experimental protocol (see experimental section in the Supporting Information),the PET surface,initially hydro-phobic (Figure 3e),became superhydrophobic while retaining its high visible transmissivity (Figure 3f).On PET,the excellent static contact angle of 160°,comparable to that obtained earlier on glass (157°),was found to be relatively invariant upon repeated bending (Figure 3g;×350),a promising result in the context of future ?exible thin-?lm device applications.
■CONCLUSION
In summary,we have described the preparation of a highly transmissive (avg.93?95%)and UV-resistant superhydropho-bic coating based on SiO 2-coated ZnO nanorod arrays.On the one hand,we showed that the highest degree of coating transparency can be achieved upon carefully optimizing (i)the seed layer thickness and (ii)the size of the ZnO nanorods.On the other hand,we emphasize the critical dependence of the superhydrophobicity on (i)the ZnO nanorod length,(ii)the thickness of the SiO 2layer and (iii)the presence of a UV-protective layer (here SiO 2,only a few nm su ?ce)for the ZnO nanorods.The superhydrophobic SiO 2/ZnO nanocomposite has minimal impact on solar cell device performance under AM1.5G illumination,and the sequential steps used for its preparation are applicable to both glass and plastics,thus validating the suggestion that the superhydrophobic arrays can be utilized in ?exible displays and solar cells.
■ASSOCIATED CONTENT *Supporting Information
Experimental and characterization methods;enlarged trans-
mittance spectra for ZnO nanorod arrays with di ?erent growth times;in ?uence of droplet size on sliding angle;AFM and
cross-section SEM image of ZnO nanorods with 25min growth
time;XRD,TEM,EDX,and re ?ectance spectra of SiO 2-coated ZnO nanorods;top-view and cross-section SEM image of ZnO nanorods with SiO 2deposition time of 3and 9h.Video S1demonstrates retained superhydrophobicity after repeated bending of superhydropbic coated PET.This material is
available free of charge via the Internet at https://www.sodocs.net/doc/1813278819.html,.■AUTHOR INFORMATION Corresponding Author *E-mail:pierre.beaujuge@https://www.sodocs.net/doc/1813278819.html,.sa.Notes The authors declare no competing ?nancial interest.■ACKNOWLEDGMENTS The authors acknowledge the ?nancial support of the O ?ce of Competitive Research Funds (OCRF)at King Abdullah University of Science and Technology (KAUST)under the
“Competitive Research Grant ”(CRG)program No.FIC/2010/02.The authors thank the Advanced Imaging and
Characterization Laboratories at KAUST for technical support.Y.G.thanks Prof.Boon Ooi and Dr.TienKhee Ng for helpful discussions and useful scienti ?c insights.The authors thank Dr.Cle m ent Cabanetos for providing the PBDTTPD
polymer.
Figure 3.(a)
Schematic of a BHJ
polymer
solar
cell including the
SiO2/ZnO nanocomposite.(b)Water droplets positioned on the front of the superhydrophobic device remain perfectly spherical.(c)J ?V characteristic of a superhydrophobic (SH)cell (red circle)
superimposed on that of a bare reference (Ref.)cell (blue square);AM1.5G solar
illumination
(100mW cm ?2).(d)EQE spectra of the
SH cell (red circle)and the Ref.cell (blue square).(e)Water droplets positioned on a bare transparent sheet of PET (2×2in.2).(f)Droplets positioned on superhydrophobic PET
(static angle:160°).
(g)PET retains its superhydrophobicity upon repeated bending (×350)(extracted from Video S1in the Supporting Information).
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