Applied Surface Science 257 (2011) 5083–5087
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Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
Synthesis and optical properties of flower-like ZnO nanorods by thermal
evaporation method
J.H. Zheng, Q. Jiang, J.S. Lian ∗
Key Lab of Automobile Materials, Ministry of Education, College of Materials Science and Engineering, Jilin University, Nanling Campus, Changchun, 130025, PR China
a r t i c l e
i n f o
Article history:
Received 13 November 2010
Received in revised form
26 December 2010
Accepted 5 January 2011
Available online 12 January 2011
Keywords:
ZnO nanostructures
Thermal carbon reduction
Optical properties
a b s t r a c t
Flower-like ZnO nanorods have been synthesized by heating a mixture of ZnO/graphite powders using
the thermal evaporation and vapor transport on Si (1 0 0) substrates without any catalyst. The structures, morphologies and optical properties of the products were characterized in detail by using X-ray
diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and Raman spectroscopy. The synthesized products consisted of large quantities of
flower-like ZnO nanostructures in the form of uniform nanorods. The flower-like ZnO nanorods had high
purity and well crystallized wurtzite structure, whose high crystalline quality was proved by Raman spectroscopy. The as-synthesized flower-like ZnO nanorods showed a strong ultraviolet emission at 386 nm
and a weak and broad yellow–green emission in visible spectrum in its room temperature photoluminescence (PL) spectrum. In addition, the growth mechanism of the flower-like ZnO nanorods was discussed
based on the reaction conditions.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
The hexagonal wurtzite ZnO with wide band gap energy
(3.37 eV), high exciton binding energy (60 meV) and high mechanical and thermal stabilities is an important semiconductor material
and herewith has been widely investigated for its catalytic,
electrical, optoelectronic and photochemical properties [1–5]. Lowdimensional ZnO nanostructures, such as nanotubes, nanowires,
nanorods and nanoflowers, have attracted wide attention for their
potential applications in the fabrication of devices like nanolaser,
dye sensitized oxide solar cells, photo catalyst, piezoelectric, transparent light power electronics and windows materials for displays.
[6–9]. Especially, increasing interests have been devoted to develop
ZnO nanorods by various methods, including the metal-organic
chemical vapour deposition [10], thermal CVD [11], thermal evaporation [12], electrochemical deposition [13,14], molecular beam
epitaxy [15], hydrothermal method [16] and pulsed laser deposition [17]. However, some of them have drawbacks like long reaction
time, toxic templates and exotic metal catalysts, and low purity or
poor crystallite quality of products, which may influence the quality and applications of ZnO nanorods. So there is still the need for
developing a method that can produce the ZnO nanorods with high
quality, high repeatability and low cost process.
∗ Corresponding author. Tel.: +86 431 85095876; fax: +86 431 85095876.
E-mail address: [email protected] (J.S. Lian).
0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2011.01.025
In this paper, hexagonal structure of flower-like ZnO nanorods
on Si substrate were fabricated by a simple thermal evaporation method and their structure, morphologyand optical properties
were investigated.
2. Experimental
The growth of flower-like ZnO nanorods on Si substrate was
based on the thermal evaporation process of high-purity ZnO powder (99.99%) and graphite powder (99.99%) (a weight ratio 1:1)
grounded fully into a mixture before being loaded into the alumina boat as a source. Si (1 0 0) wafer was used as substrate, which
was cleaned firstly in an ultrasonic bath with absolute alcohol
and acetone for 15 min at room temperature, respectively, and
then washed using de-ionized water. First, the source material
was kept in the alumina boat and the Si substrate was kept at
downstream from the source of the alumina boat. And then the
alumina boat with the source and Si substrate was placed into a
slender quartz tube avoiding the pollution. At last the small quartz
tube was placed at the center of the furnace. Fig. 1 shows the
schematic illustration of the apparatus. The quartz tube was heated
to the reaction temperature of 900 ◦ C and maintained for 30 min.
High-purity Ar gas (99.9%) flow was used as carrier gas inside the
tube during thermal evaporation. After the evaporation was finished, the furnace was naturally cooled to room temperature. A
layer of white products were found on the surface of the Si substrate.
J.H. Zheng et al. / Applied Surface Science 257 (2011) 5083–5087
20
30
40
50
60
70
80
(203)
(104)
(110)
(103)
(200)
(112)
(201)
(004)
(101)
(102)
The morphological, structural, and optical properties of
the ZnO nanorods were investigated. X-ray diffraction was
employed to determine the phase structure, performed on
a D/max-Rigaku XRD diffraction spectrometer with a Cu K␣
line of 1.5417 Å and a monochromator at 50 kV and 300 mA.
Scanning electron microscopy (SEM, JSM-5310) was used to
observe the surface morphology of samples. XPS spectra of
the ZnO nanorods were acquired with an ESCALABMk II (Vacuum Generators) spectrometer using unmonochromatized Al Ka
X-rays (240 W). Cycles of XPS measurements were done in
a high vacuum chamber with a base pressure of 10−8 Torr.
The room-temperature PL and Raman spectra were measured with a fluorescence spectrophotometer using a He–Cd
laser with a wavelength of 325 nm as the excitation light
source.
(100)
Fig. 1. The experimental apparatus for the synthesis of flower-like ZnO nanorods.
Intensity(a.u.)
(002)
5084
90
2theta(degree)
Fig. 2. shows typical XRD images of as-prepared samples grown on Si substrate.
3. Results and discussion
Fig. 2 shows a typical XRD pattern of the synthesized products.
All diffraction peaks can be indexed to hexagonal wurtzite ZnO
(JCPDS Card No. 36-1451) with lattice constants of a = 3.249 Å and
c = 5.206 Å and no diffraction peaks from any other impurities have
Fig. 3. (a) SEM image of flower-like ZnO nanorods used on Si substrate, (b) and (c) are two higher magnification SEM images of flower-like ZnO nanorods.
J.H. Zheng et al. / Applied Surface Science 257 (2011) 5083–5087
b
Intensity (a.u.)
Zn2p 3/2
284. 6 eV
C1s
1044. 8 eV
Intensity (a.u.)
1021. 7 eV
a
5085
Zn2p 1/2
1015 1020 1025 1030 1035 1040 1045 1050 1055 1060 1065
276
278
280
282
Binding Energy (eV)
284
286
288
290
292
296
524
526
528
530
532
534
536
538
540
O1s
0
C1s
531. 2 eV
529. 8 eV
Zn3d
Zn3p
Zn3s
Intensity (a.u.)
Zn2p 1/2
O1s
Zn2p 3/2
d
c
Intensity(a.u.)
294
Binding Energy (eV)
150
300
Binding Energy (eV)
450
600
750
900
1050
1200
Binding Energy (eV)
Fig. 4. XPS spectra of flower-like ZnO nanorods: (a) scanned from 0 to 1200 eV, (b) binding energy of C 1s spectrum, (c) binding energy of O 1s spectrum, (d) binding energy
of Zn 2p1/2 and Zn 2p3/2 spectrum.
been detected in the spectrum, indicating that the as-synthesized
products are pure wurtzite ZnO structure. Compared with the standard spectrum of bulk ZnO crystal, the peak of (0 0 2) plane is much
stronger than those of (1 0 0) and (1 0 1) planes. The result indicates
that most ZnO nanostructures may preferentially grow along the
(0 0 1) direction, which is in accordance with the SEM observation
shown below.
Typical SEM images for ZnO products grown on the Si substrate are shown in Fig. 3(a), and Fig. 3(b) and (c) are two higher
magnification SEM images of ZnO products. It can be seen that
the as-deposited products have large quantities of flower-like
ZnO nanostructures and these ZnO flowers consist of multipod
petals. The morphologies of the flower-like ZnO nanostructures
are approximately uniform. It is clear that the multipod petals
of the flower-like ZnO nanostructures were composed of uniform
nanorods and almost all the nanorods are radial structures having several branches. The diameter of these nanorods was about
50–80 nm and the length was about 1.5–2.0 ␮m. Furthermore, a
small quantity of the ZnO lamellar crystal was also observed at some
roots of the nanorods.
Vapor–solid (VS) and vapor–liquid–solid (VLS) mechanisms had
been proposed to explain the self-catalysis of Zn or ZnO [18]. Based
Intensity (a.u.)
Intensity (a.u.)
E2H
350
400
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 5. Room temperature PL spectrum of the flower-like ZnO nanorods.
E2(high)-E2(low)
A1(TO)
200
300
400
E1(LO)
500
600
700
800
Wavenumber (cm-1)
Fig. 6. Raman spectra of flower-like ZnO nanorods.
900
5086
J.H. Zheng et al. / Applied Surface Science 257 (2011) 5083–5087
on our synthetic process and the experimental results, flower-like
ZnO nanorods growth may not follow exactly the conventional
metal-catalytic VLS mechanism, but their growth mechanism can
be described as self-catalytic VLS growth model. ZnO has the higher
melting point (1975 ◦ C), so in our growth process, ZnO is reduced
first by graphite at 900 ◦ C to form Zn by the following reactions:
ZnO(s) + C → Zn(g) + CO2
(1)
C + CO2 → 2CO
(2)
ZnO(s) + CO → Zn(g) + CO2
(3)
The decomposed Zn vapor, CO and CO2 were quickly transported
to the downstream low temperature region of the quartz tube by Ar
stream, where they can condense to form liquid droplets on Si substrates as nuclei, Zn liquid droplets themselves function as metal
catalysts during the VLS growth process, then Zn would be oxidized
into ZnO again [20] following the reaction:
Zn + CO2 → ZnO + CO
(4)
During the course of Eq. (4), a quantity of heat will be released.
So Eq. (4) will occur more easily at relatively low temperature.
X-ray photoelectron spectroscopy (XPS) measurement was used
to investigate the surface chemical states of the sensing element. A
wide survey scan of XPS spectra is taken in the range 0–1200 eV as
shown in Fig. 4(a), in which all of the peaks can be ascribed to Zn, O,
and C elements as labeled [19]. No extra peak corresponding to any
impurities other than Zn and O is observed. Fig. 4(b) shows the XPS
spectra of C 1s locating at 284.6 eV. This binding energy is used as
calibration for other binding energies in the spectrum [21]. The O 1s
spectrum is shown in Fig. 4(c). In the O 1s region, symmetric broad
peak related to the low and higher binding energy components at
529.8 eV and 531.2 eV, respectively. The 529.8 eV peak is attributed
to the O2− ions on the wurtzite structure of the hexagonal Zn2+ ion
array, which are surrounded by zinc atoms with the full supplement
of nearest-neighbour O2− ions. Therefore, the 529.8 eV peak of the
O1s spectrum can be attributed to the Zn–O bonds. Meanwhile, the
higher binding energy component at 531.2 eV is assigned to the
oxygen deficient region or oxygen vacancies within the ZnO matrix
[22,23]. The Zn 2p spectrum (Fig. 4(d)) shows two peaks whose
binding energies are 1021.7 and 1044.8 eV and can be identified as
Zn 2p3/2 and Zn 2p1/2 lines with a better symmetry, respectively.
They are assigned to the lattice zinc in zinc oxide [24,25]. The peak
separation between these two peaks is 23.1 eV, which is well lying
within the standard reference value of ZnO [26,27]. The binding
energy and the binding energy difference values calculated from
the XPS study show that Zn atoms are in +2 oxidation state. Therefore, the XPS characteristic results imply the presence of oxygen
deficient region or oxygen vacancies within the flower-like ZnO
nanorods.
The room temperature photoluminescence (PL) of the flowerlike ZnO nanorods was examined with a He–Cd laser (325 nm) as
the excitation source and the result was shown in Fig. 5. In the PL
spectrum, a strong UV emission peak located at around 386 nm and
a weak and broader emission situated in the yellow–green part of
the visible spectrum have been observed. The UV emission peaked
around 386 nm is attributed to the near band-edge transition of
ZnO [28], namely, the recombination of free excitons through an
exciton–exciton collision process [29]. The strong UV emission in
the PL spectrum indicates that the flower-like ZnO nanorods have
good crystal quality.
For the origin of yellow–green emission, a number of hypotheses
have been proposed, it is generally accepted that the yellow–green
emission is attributed to the singly oxygen vacancy in the ZnO
nanomaterials and the emission results from the radiative recombination of a photo-generated hole with an electron occupying of
oxygen vacancy [30]. Meanwhile, surface states have also been
identified as a possible cause of the visible emission in ZnO nanomaterials. Zhang et al. [31] have reported that surface state may play a
more important role in the visible emission. The structure of flowerlike nanorods has small size and large surface area. It is possible
that the surface defects contribute to the yellow–green emission.
Hence, in our case, it might be reasonably inferred that both oxygen vacancy and surface state may respond to the yellow-green
emission of the flower-like ZnO nanorods.
Raman scattering spectrum is sensitive to crystallization, structural disorder and defects of materials and was therefore now used
to study the optical properties of flower-like ZnO nanorods. According to the group theory, hexagonal wurtzite ZnO belongs to the C64v
space group, the optical phonons at the point of the Brillouin zone
are A1 + 2B1 + E1 + 2E2 . The B1 modes are Raman silent modes. The A1
and E1 are Raman and infrared active, they are split into longitudinal
optical (LO) and transverse optical (TO) components [32], whereas
the E2 modes are nonpolar having two frequencies: E2 (high) associated with oxygen displacement and E2 (low) associated with Zn
sub-lattice and are Raman active only [31–33].
Fig. 6 shows the typical Raman scattering spectra of the flowerlike ZnO nanorods observed at room temperature. A sharp, strong
and dominant E2 (high) mode of ZnO located at 438 cm−1 is
observed in the spectrum, this is the intrinsic characteristic of the
Raman-active mode of wurtzite hexagonal ZnO [34]. The E2 (high)
mode has high intensity indicates the good crystallization of the
flower-like ZnO nanorods, which further testifies the results of XRD
and PL patterns. Normally, Raman peaks observed in between 570
and 590 cm−1 are considered to be associated with structural disorders, such as oxygen vacancy, Zn interstitial and their combination
[35]. So, in our Raman scattering spectra, the suppressed peak at
575 cm−1 is attributed to the E1 (LO) mode, which is probably
caused by the oxygen defects or oxygen deficient region. In addition, the two relatively weak peaks located at 331.9 and 376.6 cm−1
may correspond well to multiple phonon scattering processes (E2
(high)–E2 (low)) and A1T mode, respectively. The presence of an
intense E2 (high) mode and a suppressed E1 (LO) mode in the
Raman spectrum indicates that the as-synthesized flower-like ZnO
nanorods are highly crystalline with a hexagonal wurtzite phase.
4. Conclusion
Flower-like ZnO nanorods were successfully synthesized
through thermal evaporation method. The average diameter and
length of the nanorods are about 50–80 nm and 1.5–2.0 ␮m, respectively. It is demonstrated that the Zn liquid droplets function as
catalyst plays an important role in the self-catalytic VLS growth
process of flower-like ZnO nanorods at low temperature deposition region. XRD results demonstrate that the as-grown flower-like
ZnO nanorods have a wurtzite ZnO structure. The X-ray photoelectron spectroscopy measurements reveal the presence of oxygen
deficient region or oxygen vacancies within the flower-like ZnO
nanorods. PL and Raman measurements show that the sample has
good PL behaviors and high quality. Room temperature PL spectra of the flower-like ZnO nanorods show a strong UV emission
peak located at around 386 nm and a weak and broader emission situated in the yellow–green part of the visible spectrum. The
UV emission is assigned to the near-band-edge emission and the
yellow–green emission probably originates from surface defects
and oxygen vacancies.
Acknowledgements
This work was supported by National Nature Science Foundation (Grant No. 50871046), the Foundation of National Key Basic
Research and Development Program (No. 2010CB631001) and the
J.H. Zheng et al. / Applied Surface Science 257 (2011) 5083–5087
Program for Changjiang Scholars and Innovative Research Team in
University.
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