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