well-aligned zno nanorod arrays derived from 2d photonic crystals within peacock feathers

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Well-aligned ZnO nanorod arrays derived from 2D photonic crystals within peacock feathers Jing Cao, a Huilan Su,* a Jianjun Chen, a Jie Han, a Won-Jin Moon b and Di Zhang* a Received 17th January 2012, Accepted 15th May 2012 DOI: 10.1039/c2ce25085c Well-aligned ZnO nanorod arrays are successfully synthesized via a facile immersion and multi-step calcination process using the natural 2D photonic crystals found within peacock feathers as templates. The as-prepared ZnO nanorod arrays possess an array of melanin rods from the peacock feathers and exhibit tunable as well as enhanced photoluminescence in the visible range. The synthesis mechanism is investigated to reveal that the keratin component in peacock feathers can be activated by an EDTA–DMF suspension and provide more COO 2 groups, which could bind Zn 2+ when immersed in the precursor solution. In the succeeding multi-step calcination process, keratin first breaks down at low temperature, so that melanin rods can act as the substrate template for the formation of the ZnO nanorod arrays. Introduction Zinc oxide (ZnO) has a wide band gap (3.37 eV), large exciton binding energy (60 meV) and high electromechanical coupling constant, 1,2 etc. and has been widely applied in areas such as gas sensing, 3 light emitting diodes, 4,5 electron emission devices 6 and solar cells, 7 etc. Up to now, ZnO micro/nanostructures, especially patterned ZnO nanocrystals, have been investigated with great interest because of their ability to manipulate light, 8 to give reliable and enhanced field emission characteristics, enhanced light extraction efficiency 4,9 and they are important for fabricating nanooptoelectronic devices and lasers which can operate at room temperature. Photonic crystal structures have attracted particular attention 10,11 due to their prominent abilities to control the emission and propagation of light waves 12 as well as their potential applications in optoelectronics and information proces- sing. 13 In this sense, ZnO arrays with photonic crystal structures would be highly promising in terms of photoelectric materials. Techniques such as e-beam lithography, nanoimprinting and nanotemplate-assisted growth, etc. are usually employed to fabricate artificial photonic crystals. However, the expensive and time-consuming procedures and limited pattern have nar- rowed their applications. In contrast, natural photonic crystals contain various patterns 14,15 and abundant reactive sites according to their chemical components, which make them a promising alternative towards creating novel optical devices. Peacock feathers, whose iridescences (mainly blue, green, yellow and brown) are caused by 2D photonic crystal structures, 16 are a case in point. The 2D photonic crystals consist of melanin rods which are connected by keratin. In the 2D photonic crystals, melanin rods are periodically arranged, forming different lattice constants which tune the reflective light via multiple internal light reflections, refractions, and interference events. Lattice constants of 140, 150, 165 and 180 nm produce structural colours of blue, green, yellow and brown in the barbules, respectively. 17,18 Therefore it is an interesting prospect to replace the photonic crystal structures consisting of melanin rods with semiconductor nanoparticles. In our previous work, semiconductor nanoparticles have been embedded into peacock feathers 19,20 and exhibit tunable optical properties in terms of type, size and the amount of nanoparticles. In a progressive step, we herein obtain pure ZnO materials with well-aligned structures which are derived from peacock feathers. In detail, peacock feathers are pre-treated by an EDTA–DMF suspension, which is prepared by dissolving EDTA (ethylene- diaminetetraacetic acid) in DMF (N,N-dimethylformamide), to increase the amount of COO 2 groups 19 on the keratin. Then, in a subsequent low-temperature immersion process, the increased amounts of COO 2 would bind more Zn 2+ precursors onto the keratin component. 20 Afterwards, a multi-step calcination process is introduced to remove the organic template composed of keratin and melanin. 21 In a low temperature range, the keratin component begins to break down while the melanin component could still be stable. At elevated temperatures, the melanin component decomposes leaving ZnO to develop into well- aligned nanorods. The as-obtained well-aligned ZnO nanorod arrays are more desirable for tuning photoluminescence, because the pure ZnO materials would not be confined by the organic ingredients in the composite materials, such as optical losses and an inability to withstand harsh conditions (e.g. high tempera- ture). FTIR spectra and TGA curves are used to further a State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai, China. E-mail: [email protected], [email protected]; Fax: 86-21-34202749; Tel: 86-21-34202584 b Korea Basic Science Institute, Gwangju Center, Yongbong-dong, Buk-gu, Gwangju, 500-757, Korea CrystEngComm Dynamic Article Links Cite this: CrystEngComm, 2012, 14, 5262–5266 www.rsc.org/crystengcomm PAPER 5262 | CrystEngComm, 2012, 14, 5262–5266 This journal is ß The Royal Society of Chemistry 2012 Published on 15 May 2012. Downloaded by Northeastern University on 26/10/2014 16:25:30. View Article Online / Journal Homepage / Table of Contents for this issue

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Well-aligned ZnO nanorod arrays derived from 2D photonic crystals withinpeacock feathers

Jing Cao,a Huilan Su,*a Jianjun Chen,a Jie Han,a Won-Jin Moonb and Di Zhang*a

Received 17th January 2012, Accepted 15th May 2012

DOI: 10.1039/c2ce25085c

Well-aligned ZnO nanorod arrays are successfully synthesized via a facile immersion and multi-step

calcination process using the natural 2D photonic crystals found within peacock feathers as

templates. The as-prepared ZnO nanorod arrays possess an array of melanin rods from the peacock

feathers and exhibit tunable as well as enhanced photoluminescence in the visible range. The synthesis

mechanism is investigated to reveal that the keratin component in peacock feathers can be activated

by an EDTA–DMF suspension and provide more COO2 groups, which could bind Zn2+ when

immersed in the precursor solution. In the succeeding multi-step calcination process, keratin first

breaks down at low temperature, so that melanin rods can act as the substrate template for the

formation of the ZnO nanorod arrays.

Introduction

Zinc oxide (ZnO) has a wide band gap (3.37 eV), large exciton

binding energy (60 meV) and high electromechanical coupling

constant,1,2 etc. and has been widely applied in areas such as gas

sensing,3 light emitting diodes,4,5 electron emission devices6 and

solar cells,7 etc. Up to now, ZnO micro/nanostructures, especially

patterned ZnO nanocrystals, have been investigated with great

interest because of their ability to manipulate light,8 to give

reliable and enhanced field emission characteristics, enhanced

light extraction efficiency4,9 and they are important for fabricating

nanooptoelectronic devices and lasers which can operate at room

temperature. Photonic crystal structures have attracted particular

attention10,11 due to their prominent abilities to control the

emission and propagation of light waves12 as well as their

potential applications in optoelectronics and information proces-

sing.13 In this sense, ZnO arrays with photonic crystal structures

would be highly promising in terms of photoelectric materials.

Techniques such as e-beam lithography, nanoimprinting and

nanotemplate-assisted growth, etc. are usually employed to

fabricate artificial photonic crystals. However, the expensive

and time-consuming procedures and limited pattern have nar-

rowed their applications. In contrast, natural photonic crystals

contain various patterns14,15 and abundant reactive sites

according to their chemical components, which make them a

promising alternative towards creating novel optical devices.

Peacock feathers, whose iridescences (mainly blue, green, yellow

and brown) are caused by 2D photonic crystal structures,16 are a

case in point. The 2D photonic crystals consist of melanin rods

which are connected by keratin. In the 2D photonic crystals,

melanin rods are periodically arranged, forming different lattice

constants which tune the reflective light via multiple internal

light reflections, refractions, and interference events. Lattice

constants of 140, 150, 165 and 180 nm produce structural

colours of blue, green, yellow and brown in the barbules,

respectively.17,18 Therefore it is an interesting prospect to replace

the photonic crystal structures consisting of melanin rods with

semiconductor nanoparticles.

In our previous work, semiconductor nanoparticles have been

embedded into peacock feathers19,20 and exhibit tunable optical

properties in terms of type, size and the amount of nanoparticles.

In a progressive step, we herein obtain pure ZnO materials with

well-aligned structures which are derived from peacock feathers.

In detail, peacock feathers are pre-treated by an EDTA–DMF

suspension, which is prepared by dissolving EDTA (ethylene-

diaminetetraacetic acid) in DMF (N,N-dimethylformamide), to

increase the amount of COO2 groups19 on the keratin. Then, in

a subsequent low-temperature immersion process, the increased

amounts of COO2 would bind more Zn2+ precursors onto the

keratin component.20 Afterwards, a multi-step calcination

process is introduced to remove the organic template composed

of keratin and melanin.21 In a low temperature range, the keratin

component begins to break down while the melanin component

could still be stable. At elevated temperatures, the melanin

component decomposes leaving ZnO to develop into well-

aligned nanorods. The as-obtained well-aligned ZnO nanorod

arrays are more desirable for tuning photoluminescence, because

the pure ZnO materials would not be confined by the organic

ingredients in the composite materials, such as optical losses and

an inability to withstand harsh conditions (e.g. high tempera-

ture). FTIR spectra and TGA curves are used to further

aState Key Lab of Metal Matrix Composites, Shanghai Jiao TongUniversity, Shanghai, China. E-mail: [email protected],[email protected]; Fax: 86-21-34202749; Tel: 86-21-34202584bKorea Basic Science Institute, Gwangju Center, Yongbong-dong, Buk-gu,Gwangju, 500-757, Korea

CrystEngComm Dynamic Article Links

Cite this: CrystEngComm, 2012, 14, 5262–5266

www.rsc.org/crystengcomm PAPER

5262 | CrystEngComm, 2012, 14, 5262–5266 This journal is � The Royal Society of Chemistry 2012

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investigate the mechanism of the well-aligned ZnO nanorod

arrays.

Experimental details

The peacock feathers chosen in this work were Pavo cristatus

feathers purchased from the Shanghai Huayang Peacock

Cultivation Co., Ltd. The field emission scanning electron

microscopy (FESEM) images of the barbules clearly reveal the

periodically-aligned melanin rod arrays beneath the surface

keratin. As the images show, peacock feathers contain a 2D

photonic crystal structure16–18 (Fig. 1) and the melanin rods are

about 700–800 nm in length and 100–150 nm in diameter.

The peacock feathers were first immersed into an EDTA–

DMF (approximately 1 : 10 v/v) suspension at 110 uC for 6 h to

become activated,19 henceforth referred to as E/D feathers. The

activated feathers were then immersed into the Zn2+ precursor

solution (containing 0.33 g of ZnAc2?2H2O (zinc acetate

dihydrate) and 36 mL DMF) for 20 min, taken out and rinsed

thoroughly with DMF and distilled water and henceforth

defined as the immersed feathers. Finally, the immersed feathers

were calcined at 570 uC for 1–2 h to achieve the final product. In

the meantime, a control sample was formed under the same

conditions but in the absence of peacock feathers.

The morphology and structure of the products were examined/

characterized using a FEI QUANTA 250 field emission gun

scanning electron microscope (FESEM) with samples pre-

sputtered with Au on the surface to prevent charging. The

crystal structure of the products was analyzed by the X-ray

diffraction (XRD) patterns obtained on a Bruker-AXS instru-

ment with a Cu–Ka line of 0.1541 nm. HRTEM measurements

were taken on a JEM-2100F instrument under an acceleration

voltage of 200 kV. The photoluminescence (PL) spectra were

taken on a Perkin-Elmer LS 55. The samples for both the

HRTEM and PL measurements were prepared by dispersing the

products in ethanol via ultrasonic agitation. FTIR measurements

were recorded on a Perkin-Elmer Paragon 1000 with the samples

crushed to a powder and compressed into a KBr pellet. The

TGA data were recorded using Thermal Analyst equipment

TGA 2050 (TA Instrument Inc.) under an atmosphere of air and

a heating rate of 5 uC min21.

Results and discussion

Fig. 2 shows the XRD results and HRTEM images of the final

product and the control sample. All the peaks in the XRD

pattern (Fig. 2a) can be indexed to the hexagonal phase of ZnO

(JCPDS 36-1451), confirming both the final product and the

control sample to be wurtzite ZnO. The HRTEM image (Fig. 2b)

shows that these final products are ZnO nanorods with a

diameter of approximately 50 nm and length of about 300–

400 nm. The selected area electron diffraction (SAED) pattern

(the inset in Fig. 2b) can be indexed as hexagonal ZnO (JCPDS

card no. 36-1451). Fig. 2c reveals well-resolved lattice fringes,

indicating the high crystallinity of the as-synthesized rod. The

lattice spacing of 0.248 nm corresponds to the d-spacing between

the (1011) planes of ZnO. In all, ZnO nanorods with a hexagonal

structure were synthesized under the direction of peacock

feathers as templates through a low-temperature immersion

process.

The morphologies and structures of the melanin rods in the

original peacock feathers and the final ZnO nanorod arrays can

be seen in the FESEM images. As revealed in Fig. 3a, the

melanin rods in the original peacock feathers (100 nm in

diameter and 700 nm in length) are well-arrayed to form a 2D

photonic crystal structure, with a lattice constant of 137 nm.20

After the immersion and calcination procedure, ZnO nanorods

are developed: they are, on average, approximately 300–400 nm

long and 50 nm in diameter (Fig. 3b), with an aspect ratio of 5–

10, confirming the results revealed by the HRTEM images. As

revealed in Fig. 3b, the ZnO nanorods have replaced the melanin

rods and are arranged parallel to the surface, which is the same

arrangement as the melanin rods, indicating the successful

replication of the original well-arrayed structure.

So far, well-aligned ZnO nanorod arrays have been fabricated

in the same arrangement as the melanin rods in the original

feathers and they may achieve novel optical properties, as

mentioned previously. The room temperature photolumines-

cence (PL) spectra were measured to primarily investigate the

emission of the as-prepared ZnO nanorods. The samples were

excited at a wavelength of 360 nm and the broad bands in the

region were fitted with a Gaussian function, as presented in

Fig. 4. The original feather has broad emission peaks ranging

from 450 nm to 600 nm (Fig. 4a), due to molecular luminescence.

The immersed feather has the same curve as the original feathers:

ZnO nanocrystallites are undeveloped and thus molecular

luminescence dominants. The well-aligned ZnO nanorod arrays

and the control sample both exhibit three emission peaks in the

visible green emission region, as revealed in Fig. 4b and Fig. 4c.

The commonly observed visible emission in ZnO can be

attributed to structural defects, surface defects and impurities.22

Djurisi et al. have reported such defect emissions in ZnO as

green, yellow, and orange-red.23 Due to the Zn-rich conditions in

our aqueous growth method, the green emission of the well-

aligned ZnO nanorod arrays and the control sample here is

mainly caused by oxygen vacancies.22,24,25 However, the obvious

differences between the two PL spectra cannot be neglected: the

emission peaks of the well-aligned ZnO nanorods array have

shifted by about 50–90 nm to the longer wavelength region in

comparison with the control sample. In addition, the relative

intensity of the well-aligned ZnO nanorod arrays is much

Fig. 1 FESEM images of the peacock feathers: (a) shows the cross

section of the barbule and (b) shows the longitudinal section of the

barbule. The insets are illustrations of the 2D photonic crystal structure.

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stronger than that of the immersed feathers. As is widely

accepted, photonic crystals could control the emission and

propagation of light waves12 and enhance the light emitting

properties of the materials.4,9 Thus it is reasonable that the

intensified and red-shifted photoluminescence spectra, compared

to that of the control sample, might be the result of the photonic

crystal structure. Therefore, well-aligned ZnO nanorod arrays

with a tunable emission wavelength and enhanced intensity have

been obtained under the guidance of such patterned structures,

which makes sense in the fabrication of optically functional

materials.

The formation mechanism of the well-aligned ZnO nanorod

arrays was investigated by FTIR and TGA measurements. As

revealed in Fig. 5a, the original feather shows characteristic

bands from proteins.20 Compared to the original feather, and the

curve from the E/D feather, the absent 1728 cm21 band (CLO

stretching from COOH) and the movement of the 1384 cm21

band to a longer wavelength verify the successful introduction of

COO2 sites onto the keratin by an EDTA–DMF suspension.19

That the band at approximately 1300 cm21 (OH deformation)

has red-shifted by about 30 cm21, and the absent band at

1100 cm21 (C–O stretching from hydroxyl groups)20 both indicate

the bonding effect between the Zn2+ precursor and keratin.

Taking the above analysis into consideration, it is reasonable to

conclude that Zn2+ interacts with the feather keratin via the

COO2 group in the E/D feathers and the COO2 group should be

considered as the reactive site during the immersion process.

Moreover, the C–C stretching band (1186 cm21) is reduced and

moves to a lower wavelength, which might be due to the addition

of Zn2+ ions during the process. In addition, melanin can also

adsorb Zn2+.26 Therefore, both the keratin component and the

melanin rods are involved in the reaction.

In the calcination process, ZnAc2 would undergo hydrolysis

to form metal complexes at 70 uC, as eqn (1) describes. Since the

Zn2+ precursor infiltrates and binds with the keratin component

and melanin rods, the metal complexes would be formed

in situ. At higher temperatures, these complexes will dehydrate

and remove acetic acid to form pure ZnO, as indicated in eqn

(2).27

Zn OH{ð Þx CH3COO{ð Þ2{x �?D

ZnOz

x{1ð ÞH2Oz 2{xð ÞCH3COOH(1)

Fig. 2 (a) XRD patterns of the final product and control sample, (b–c) HRTEM images and the inset in (b) correspond to the SAED pattern of the

final product.

Fig. 3 FESEM images of (a) the well-aligned melanin rods in the

original peacock feathers and (b) the well-aligned ZnO nanorods in the

final products.

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Zn CH3COOð Þ2zxH2O �?D

Zn OH{ð Þx CH3COO{ð Þ2{xzxCH3COOH(2)

The TGA curve of the original peacock feather further

reveals the degradation of the organic components from the

peacock feathers. As revealed in Fig. 5b, the peacock feather

begins to break down at 243 uC and is completely broken

down at 345 uC, which corresponds to the pyrolysis of

keratin.28–30 Soon after, the weight-losing rate slows but then

speeds up again after 470 uC until 570 uC, which could be

attributed to the pyrolysis of the melanin component. It has

been reported that melanin has a well-developed graphitic-like

structure and is capable of withstanding higher temperatures

than keratin. It begins to lose water at 280 uC and quickly

decomposes between 500 uC and 600 uC.26,31 Therefore, the

second rapid degradation could be attributed to the behaviour

of the melanin rods. In this case, keratin could be decomposed

earlier than melanin and the decomposed keratin, including

the Zn2+ complexes formed in situ, would wrap around the

stable melanin rods. To put it another way, the 2D photonic

crystal structure could remain over a range of low tempera-

tures when the keratin begins to decompose but the melanin

rods remain to maintain structure. The thermal insulation at

570 uC for 1–2 h and the subsequent annealing could guarantee

the complete decomposition of the organic component to give

pure semiconductor materials. Based on the above analysis, a

formation mechanism of the well-aligned ZnO nanorod arrays

under the direction of peacock feathers is put forward in

Scheme 1.

Fig. 4 (a) Photoluminescence emission: (excitation wavelength: 360 nm); (b) is the fitting results of the Gaussian function from the well-aligned ZnO

nanorod arrays and (c) is the fitting results of the Gaussian function from the control sample.

Fig. 5 (a) FTIR spectra of the original feather, E/D feather and

immersed feather; and (b) the TGA curve of the original peacock feather.

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Conclusions

Well-aligned ZnO nanorod arrays have been successfully

synthesized using peacock feathers activated by an EDTA–

DMF suspension as a template. A facile immersion along with a

multi-step calcination process has guaranteed the successful

replication of ZnO nanorods from the melanin rods arrange-

ment. By immersion, the Zn2+ precursors are adsorbed and

bound with COO2 in the keratin component. In the following

calcination process, the keratin component begins to break down

at 243 uC, at which temperature the melanin rods are still stable

to be able to keep the 2D photonic crystal structure. At 470 uC,

the melanin component breaks down leaving the ZnO nanopar-

ticles to develop into nanorods. The well-aligned ZnO nanorods

are, on average, 300–400 nm long and 50 nm in diameter. The

intensified and red-shifted PL spectrum of the well-aligned ZnO

nanorods array verified the regulation of the photonic crystal

structures. The as-synthesized well-aligned ZnO nanorod arrays

would be promising in optoelectronics and optical communica-

tion. Achieving such a small feature is very expensive when using

lithography-based techniques and this technique has created a

new pathway for the much more economical, large-scale

fabrication of 2D photonic crystal (PC) devices.

Acknowledgements

Financial support from the NSFC (No. 51102165), the 973

National Project (No. 2011CB922200), Shanghai Science and

Technology Committee (Nos. 10JC1407600) and KBSI grant

(T31903) to W. J. Moon is gratefully acknowledged.

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Scheme 1 Mechanism of the well-aligned ZnO nanorod arrays.

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