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Embedment of ZnO nanoparticles in the natural photonic crystals within peacock feathers

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2008 Nanotechnology 19 365602

(http://iopscience.iop.org/0957-4484/19/36/365602)

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Nanotechnology 19 (2008) 365602 (6pp) doi:10.1088/0957-4484/19/36/365602

Embedment of ZnO nanoparticles in thenatural photonic crystals within peacockfeathersJie Han1, Huilan Su1,3, Chunfu Zhang2, Qun Dong1, Wang Zhang1

and Di Zhang1,3

1 State Key Lab of Metal Matrix Composites, Shanghai Jiaotong University,Shanghai 200240, People’s Republic of China2 Med-X Research Institute, Shanghai Jiaotong University, Shanghai 200240,People’s Republic of China

E-mail: hlsu@sjtu.edu.cn and zhangdi@sjtu.edu.cn

Received 9 May 2008, in final form 12 June 2008Published 28 July 2008Online at stacks.iop.org/Nano/19/365602

AbstractInspired by the embedment of emission species in synthetic photonic crystals to display noveloptical properties, the natural photonic crystals within peacock feathers are chosen as the matrixto embed ZnO nanoparticles through an in situ approach. Peacock feathers function as thesupporting substrate and provide reactive sites for the in situ synthesis of hexagonal ZnOnanoparticles. Herein, ZnO nanoparticles exhibit photoluminescence in the visible range andare supposed to be tailored by the peacock feather, having potential applications inoptoelectronics and optical communications.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

The introduction of light-emitting materials into photoniccrystals has attracted a great deal of research interest,since photonic crystals are proposed to modify spontaneousemission [1]. Several synthetic photonic crystals, such asopals made up of polystyrene [2–5], silica [6], PMMA [7]and PMMA/PMAA [8] have already been chosen as thestructural matrices to incorporate lasing dyes [9] or quantumdots inside, and finally achieve hybrid materials and inversestructures. Moreover, the relationship between the embeddedemission species and the photonic crystal structures has beeninvestigated. Lin et al reported the CdS nanocrystal-in-PMMA/PMAA photonic crystal system and observed theinteraction of the photonic stop band and light emission [8]. In2004, Lodahl et al investigated a QD-CdSe-in-titania inverseopal system and reported the frequency-dependent emissionrate of quantum dots under the influence of modified vacuumfluctuations in photonic crystals [10]. These structures arequite promising to perform directional and tunable emission,which is essential for diverse applications in optoelectronicsand optical communications [10].

3 Authors to whom any correspondence should be addressed.

Synthetic photonic crystals with limited patterns areusually fabricated via highly expensive equipment andsophisticated processes, which limits their applications. Incontrast, natural photonic crystals contain various patterns [11]and abundant reactive sites according to their chemicalcomponents, which make them promising candidates to createnovel optical devices. Peacock feathers, whose iridescentcolors are derived from the 2D photonic crystal structure insidethe cortex, are one such example. As revealed by Yoshioka’sgroup and Zi’s group, the 2D photonic crystal structure beneaththe surface keratin layer is made up of melanin rods connectedby keratin. By varying the lattice constant and the numberof periods in the photonic crystal structure, peacock feathersprovide several 2D photonic crystal structures with differentcolors. The control of light in such structures has beenobserved by the angular dependence reflection spectra in thevisible range [12, 13].

ZnO is a highly interesting optoelectronic material with awide electronic bandgap ∼3.37 eV at room temperature, whichmakes it an efficient emitter in the near-UV spectrum. Also,ZnO has defect emission in the visible range [14]. Both thenear-UV emission and the visible emission are supposed to betunable by embedding the nanoparticles into different photonic

0957-4484/08/365602+06$30.00 © 2008 IOP Publishing Ltd Printed in the UK1

Nanotechnology 19 (2008) 365602 J Han et al

Figure 1. FESEM and HRTEM (inset in (b)) images of an original peacock feather: (a) shows the surface of the original feather, while theinset shows the corresponding transverse cross section; (b) shows the longitudinal cross section of the original feather, while the inset displaysthe HRTEM image of a single keratin-coated melanin rod.

structures. ZnO-infiltrated opals and inverted opals have beenachieved by various methods such as the sol–gel process [2],focused-ion-beam etching [15], atomic layer deposition [3],electrochemical infiltration [4, 5], etc. Moreover, the photoniccrystal structures have been demonstrated to have influence onthe room-temperature UV lasing in a 3D ZnO inverse opalobtained by Chang’s group [16] and Teh’s group [4]. Sincea peacock feather has the ability to control light in the visiblerange, the visible defect emission from the ZnO nanoparticlesmight be influenced once ZnO nanoparticles are embedded ina peacock feather. Thus, the infiltration of ZnO nanoparticlesinto peacock feathers might achieve novel optical properties.

In this paper, peacock feathers are chosen as the matrix toembed ZnO nanoparticles through an in situ approach. Boththe surface keratin layer and the keratin component connectingmelanin rods in the feather cortex could provide reactive sitesfor the formation of ZnO nanoparticles. In the resulting nano-ZnO/peacock feather, the feather not only functions as thesupport for ZnO nanoparticles, but also should serve as thelight controller according to its 2D photonic crystal structure,which is still under investigation.

2. Experimental details

The peacock has several kinds of feathers with differentcolors [17]. Yoshioka’s group and Zi’s group [12, 13]have observed the 2D photonic crystal structure composedof keratin-coated melanin rods beneath the surface keratinlayer of peacock feathers from the eye region and the bodycovering. In this investigation, we chose the abundant redfeather loosely arranged along the white tail stem under theattractive eye region. Figure 1(a) displays the FESEM imageof the surface of the red peacock feather and its inset shows itstransverse cross section, revealing the ordered array of circlesbeneath the feather surface. These circles in figure 1(a) arefurther investigated under FESEM observation (figure 1(b))and HRTEM observation (inset in figure 1(b)), and present therod feature. Therefore, the red feather contains a 2D photoniccrystal structure (lattice constant: 137 nm), which is similar tothe observation by Yoshioka’s group and Zi’s group [12, 13].

For the embedment of ZnO nanoparticles in peacockfeathers, zinc acetate dihydrate (0.274 g, 1.25 mmol) wasdissolved in ethanol (125 ml), followed by the addition of thefeather into the Zn-precursor solution. The system was kept atabout 70 ◦C under vigorous stirring. Then, the OH− precursorwas prepared by dissolving 0.078 g NaOH in 65 ml ethanol,and was then added dropwise to the system. Until evaporationto 50 ml, the system was subsequently placed in an autoclavekept in 70 ◦C for 0–60 h. Finally, the treated feather was takenout of the final solution in the autoclave, rinsed with ethanoland dried to obtain the product sample.

FESEM images were obtained on a FEI Sirion 200field emission gun scanning electron microscope withsamples presputtered with Au on their surface to preventcharging. XRD measurements were taken on a Bruker-AXSinstrument. HRTEM measurements were taken on a JEM-2100F instrument under an acceleration voltage of 200 kV.FTIR measurements were recorded on a Perkin-Elmer Paragon1000 with samples crushed to powder and compressed intoa KBr pellet. PL (photoluminescence) spectra were takenon a Perkin-Elmer LS 55. UV–vis spectra were taken on aThermo-Electron Evolution 300. Samples for HRTEM, PL andUV measurements were prepared by dispersing the product inethanol via ultrasonic agitation.

3. Results and discussion

The XRD results of the ZnO-embedded feather and the originalfeather are shown in figure 2. The ZnO-embedded featherpresents a peak around 35◦, corresponding to the relativelysmall amount of embedded ZnO nanoparticles compared tothe original feather. The products formed outside the peacockfeather under the same conditions were also investigated.After being treated in an autoclave (0 and 40 h), theimpregnated solution was continuously evaporated at 70 ◦Cto obtain powder samples (products outside the feather) forXRD measurement. The main reflection peaks of the powdersamples can be indexed as hexagonal ZnO (JCPDS card no. 36-1451) and the XRD line broadening demonstrates the existenceof nanocrystalline ZnO.

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Nanotechnology 19 (2008) 365602 J Han et al

Figure 2. XRD patterns of the original peacock feather, theZnO-embedded feather and the products outside the feather(treated in autoclave for 0 and 40 h).

HRTEM measurements provide further information of theembedded ZnO nanoparticles. During the ultrasonic agitationfor TEM observation, the feather was broken into fragments

and some of the embedded ZnO nanoparticles were shakenoff from the substrate feather. Compared with the HRTEMimage of the original feather (inset of figure 1(b)), figure 3(a)displays the ZnO nanoparticles remained on the connectingkeratin that coat the melanin rod, which suggests the successfulembedment of ZnO nanoparticles in the photonic crystalsby the binding effect between the feather keratin and ZnOnanoparticles. The top right corner in figure 3(a) presentssome dispersed ZnO nanoparticles shaken off from the treatedfeather. In addition, higher-magnification images of the shakenZnO nanoparticles are displayed in figures 3(b) and (d),corresponding to the samples treated in an autoclave for 0and 40 h, respectively. The SAED patterns for both samples(inset in figure 3(a) and (c)) can be indexed as hexagonal ZnO(JCPDS card no. 36-1451) and the relevant planes are markedin figure 3(c). It is clear that the ZnO nanoparticles formed insitu in the peacock feather have the same crystalline featuresas those formed outside the feather that are characterized byXRD measurements as shown in figure 2. In addition, thenanoparticles exhibit round spheres and the treatment in theautoclave leads to homogeneous growth of ZnO nanoparticleswith diameters from 8.5 nm (0 h) to 13.5 nm (40 h) as indicatedby the dashed circles in figures 3(b) and (d). Therefore, bycontrolling the treatment time in the autoclave, spherical ZnO

Figure 3. HRTEM images and corresponding SAED patterns of as-synthesized nano-ZnO/peacock feather dispersed in ethanol via ultrasonicagitation, corresponding to treatment time in an autoclave for 0 h ((a) and (b)) and 40 h ((c) and (d)). (b) represents the nanoparticles at the topright corner in (a), which were shaken off from the feather.

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Figure 4. FESEM images of as-synthesized nano-ZnO/peacock feather (a) and the sample prepared by the control experiment (b), implyingthat the peacock feather plays important roles during the embedment process. Arrows in (b) indicate the undesired aggregations.

nanocrystallites with hexagonal structure and different sizeswere synthesized in situ in a peacock feather.

FESEM images reveal the distribution of ZnO nanoparti-cles on the feather’s surface. The original feather has a smoothsurface with only a small amount of microspores (figure 1(a)).However, by the embedment procedure, the feather surface iscovered by ZnO nanoparticles (figure 4(a)). Thus, both thesurface keratin and the keratin connecting melanin rods (as re-vealed in figure 3) are involved in the in situ formation of ZnOnanoparticles. A control experiment was carried out to investi-gate the embedment synthesis. In this case, the feather was notintroduced into the reaction system until it was transferred intothe autoclave. Thus ZnO nanoparticles should come into be-ing without the existence of the feather in the reaction system,considering that all other procedures are the same as describedin section 2. The corresponding FESEM image of ZnO/featherobtained in the control experiment (figure 4(b)) displays theinhomogeneous distribution and undesired aggregation of ZnOparticles on the feather surface. Therefore, the comparison offigures 4(a) and (b) implies that the peacock feather plays im-portant roles during the successful embedment process, whichis further confirmed by the FTIR measurement.

As shown in figure 5, the FTIR spectrum of the originalfeather presents characteristic bands of protein, relating to thekeratin component. The band at 1640 cm−1 corresponds toamide I (C=O stretching), the band at 1540 cm−1 correspondsto amide II (secondary NH bending) and the band at 1242 cm−1

corresponds to amide III (C–N stretching) [18]. In addition,the C–S stretching (616 cm−1), C–C stretching (1186 cm−1),CH3 symmetric bending (1384 cm−1) and CH2 scissoring(1454 cm−1) are also observed [19, 20]. By embedding ZnOnanoparticles in a peacock feather, the 1728 cm−1 band (C=Ostretching from COOH of aspartic and glutamic acid residues)disappeared [21], along with the appearance of the 1402 cm−1

band (COO− symmetric stretching) [20], which indicates thebinding effect between ZnO nanoparticles and the carboxylgroups of keratin in the feather. Moreover, the C–C stretchingband (1186 cm−1) reduces and moves to lower frequency,which might be due to the addition of Zn2+ ions during theprocess [22]. A mechanism is proposed in scheme 1 basedon the above analyses. In the Zn precursor, the peacockfeather binds Zn2+ ions via carboxyl groups of aspartic and

2000 1800 1600 1400 1200 1000 800 600

Figure 5. FTIR spectra of the original peacock feather andas-synthesized nano-ZnO/peacock feather (treated in an autoclave for0 and 40 h, respectively).

glutamic acid residues in keratin. Both the outside keratin layerand the inside keratin coat that connect the melanin rods areinvolved. Then OH− is introduced into the reaction systemand ZnO nanoparticles are in situ formed and anchored ontothe binding sites in the peacock feather. Further treatment inthe autoclave causes the growth of ZnO nanoparticles. Finally,nano-ZnO/peacock feather hybrids are obtained.

As mentioned before, the infiltration of ZnO nanoparticlesinto peacock feathers might achieve novel optical properties.Herein, PL and UV–vis measurements are taken to primarilyinvestigate the emission of the as-prepared ZnO nanoparticlesdispersed in ethanol. Figure 6 displays the optical properties ofthe original feather and nano-ZnO/peacock feather dispersedin ethanol via ultrasonic agitation. The original featherhas emission peaks between 400 and 450 nm. However,the nano-ZnO/peacock feather has a broad emission between500 and 650 nm. Usually ZnO nanoparticles exhibit twocharacteristic emission peaks, the UV emission related to theband edge emission and the green luminescence related todefects [14]. Herein, the broad emission in figure 6(a) shouldarise from the defect emission of ZnO nanoparticles and theabsence of UV excitonic emission of ZnO at 375 nm could

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Nanotechnology 19 (2008) 365602 J Han et al

surface keratin

the array of keratin coated melanin rods

Zn2+ OH-

original feather

nano-ZnO

nano-ZnO/ feather

Scheme 1. Illustration of the embedment of ZnO nanoparticles in a peacock feather. The peacock feather binds Zn2+ ions via carboxyl groupsof aspartic and glutamic acid residues in keratin →in situ ZnO nucleation on the binding sites in a peacock feather → the formation of ZnOnanoparticles → nano-ZnO/peacock feather hybrids are obtained.

400 500 600300 400

300 400 500

Figure 6. Photoluminescence emission (a), UV–vis absorption (b) and excitation (c) spectra of the original peacock feather and theas-synthesized nano-ZnO/peacock feather, which was dispersed in ethanol via ultrasonic agitation (excitation wavelength in (a): 360 nm,emission wavelength in (c): 550 nm).

Figure 7. Photoluminescence emission spectrum of the free ZnOobtained from the same preparation batch outside the feather(excitation wavelength: 360 nm). Inset shows the correspondingexcitation spectra (emission wavelength: 420 and 550 nm).

be attributed to the presence of a high concentration of non-radiative recombination centers. In addition, the excitationspectrum of the nano-ZnO/peacock feather recorded for

550 nm emission (figure 6(c)) presents an obvious absorptionbelow 365 nm, which is coherent with the UV–vis absorptionspectrum (figure 6(b)) and consistent with the feature of defectemission. Figure 7 provides the emission spectrum (excitationwavelength: 360 nm) of the corresponding free ZnO obtainedoutside the peacock feather in the same preparation batch forcomparison. Free ZnO displays two emission bands, the violetemission around 420 nm and the green luminescence centeredat 550 nm. The additional violet emission is probably dueto radiative defects related to the interface traps existing atthe grain boundaries [23], which possibly relate to the specialundesired aggregation of ZnO as shown in figure 4(b). Thecomparison suggests that the ZnO prepared with and withoutthe existence of the feather have quite different optical features,which might due to the influence of the feather during thesynthesis or the interaction between the ZnO and the keratincomponents in the final product. Moreover, considering thevisible emission of embedded ZnO nanoparticles and the 2Dphotonic structure provided by the colorful peacock feather, itis possible to control the spontaneous emission by the peacockfeather, which is still under investigation.

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4. Conclusion

In conclusion, hexagonal ZnO nanocrystallites with diametersfrom 8.5 to 13.5 nm have been embedded in the naturalphotonic crystal structure within a peacock feather throughan in situ procedure. During the synthesis, both the outsidekeratin layer and the inside keratin coat that connect themelanin rods of the peacock feather provide functional sitesfor the in situ formation of ZnO nanoparticles. The embeddedZnO nanoparticles perform a broad visible emission thatcorresponds to defects, which are supposed to be controllableby the peacock feather biosubstrate. As-prepared nano-ZnO/peacock feather hybrids would have valuable applicationsin optoelectronics and optical communications.

Acknowledgments

Financial support from the National Science Foundationsof China and the Major Fundamental Research Project ofShanghai Science and Technology Committee (grant no.07DJ14001) is gratefully acknowledged. The authors alsothank SJTU Instrument Analysis Center for the measurements.

References

[1] Yablonovitch E 1987 Phys. Rev. Lett. 58 2059[2] Nair R V and Vijaya R 2007 J. Phys. D: Appl. Phys. 40 990[3] Scharrer M, Wu X, Yamilov A, Cao H and Chang R P H 2005

Appl. Phys. Lett. 86 151113[4] Teh L K, Wong C C, Yang H Y, Lau S P and Yu S F 2007

Appl. Phys. Lett. 91 161116

[5] Sumida T, Wada Y, Kitamura T and Yanagida S 2001Chem. Lett. 1 38

[6] King J S, Neff C W, Summers C J, Park W, Blomquist S,Forsythe E and Morton D 2003 Appl. Phys. Lett. 83 2566

[7] Paquet C, Yoshino F, Levina L, Gourevich I, Sargent E H andKumacheva E 2006 Adv. Funct. Mater. 16 1892

[8] Lin Y, Zhang J, Sargent E H and Kumacheva E 2002Appl. Phys. Lett. 81 3134

[9] Shkunov M N, Vardeny Z V, DeLong M C, Polson R C,Zakhidov A A and Baughman R H 2002 Adv. Funct. Mater.12 21

[10] Lodahl P, van Driel A F, Nikolaev I S, Irman A, Overgaag K,Vanmaekelbergh D L and Vos W L 2004 Nature 430 654

[11] Vukusic P and Sambles J R 2003 Nature 424 852[12] Yoshioka S and Kinoshita S 2002 Forma 17 169[13] Zi J, Yu X, Li Y, Hu X, Xu C, Wang X, Liu X and Fu R 2003

Proc. Natl Acad. Sci. 100 12576[14] Kar S, Dev A and Chaudhuri S 2006 J. Phys. Chem. B

110 17848[15] Wu X, Yamilov A, Liu X, Li S, Dravid V P, Chang R P H and

Cao H 2004 Appl. Phys. Lett. 85 3657[16] Scharrer M, Yamilov A, Wu X, Cao H and Chang R P H 2006

Appl. Phys. Lett. 88 201103[17] Burgess S C, King A and Hyde R 2006 Opt. Laser Technol.

38 329[18] Iridag Y and Kazanci M 2006 J. Appl. Polym. Sci. 100 4260[19] Edwards H G M, Hunt D E and Sibley M G 1998

Spectrochim. Acta A 54 745[20] Taddei P, Monti P, Freddi G, Arai T and Tsukada M 2003

J. Mol. Struct. 650 105[21] Church J S, Corino G L and Woodhead A L 1997

Biopolymers 42 7[22] Barone J R, Dangaran K L and Schmidt W F 2007

J. Appl. Polym. Sci. 106 1518[23] Jin B J, Im S and Lee S Y 2000 Thin Solid Films 366 107

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