Structure and photoluminescence properties of Ag-coated ZnO nano-needles

Article abstract of DOI:10.1016/j.jallcom.2011.01.118

A large number of zinc oxide (ZnO) nano-needles were synthesized by thermal oxidation of pure zinc. The surfaces of ZnO nano-needles were coated with a layer of Ag by pulse electro-deposition technique. The uncoated and coated ZnO nano-needles were characterized by using the X-ray diffraction and the scanning electron microscope (SEM). The results showed that the uncoated samples were close-packed hexagonal structure, which showed needle-like morphology. Their average diameter is about 40 nm, lengths up to 5 μm. At the same time we observed that the prepared ZnO nano-needles have been coated with Ag successfully. The photoluminescence spectrums of ZnO nano-needles with Ag-coated and uncoated were analyzed, finding that the uncoated ZnO nano-needles have two fluorescence peaks at 388 nm and 470.8 nm, respectively, the relative intensity of 143.4 and 93.61; and the Ag-coated ZnO nano-needles showed a pair of strong peaks at 387.4 nm and 405.2 nm, the relative intensity of 1366 and 1305, respectively, indicating that the Ag-coated ZnO nano-needles can increase the absorption of UV light.

Full text of DOI:10.1016/j.jallcom.2011.01.118

Journal of Alloys and Compounds 509 (2011) 5765–5768
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Structure and photoluminescence properties of Ag-coated ZnO nano-needles
Xiaozhu Lia,b,∗, Yongqian Wangc
a Department of Physics, Shaoguan University, Shaoguan, Guangdong 512005, China
b Department of Physics and Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University, Wuhan, Hubei 430072, China
c Engineering Research Center of Nano-Geomaterials of Ministry of Education (China University of Geosciences), Wuhan, Hubei 430074, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A large number of zinc oxide (ZnO) nano-needles were synthesized by thermal oxidation of pure zinc.
The surfaces of ZnO nano-needles were coated with a layer of Ag by pulse electro-deposition technique.
The uncoated and coated ZnO nano-needles were characterized by using the X-ray diffraction and the
scanning electron microscope (SEM). The results showed that the uncoated samples were close-packed
hexagonal structure, which showed needle-like morphology. Their average diameter is about 40 nm,
lengths up to 5 ␮m. At the same time we observed that the prepared ZnO nano-needles have been
coated with Ag successfully. The photoluminescence spectrums of ZnO nano-needles with Ag-coated
and uncoated were analyzed, finding that the uncoated ZnO nano-needles have two fluorescence peaks
at 388 nm and 470.8 nm, respectively, the relative intensity of 143.4 and 93.61; and the Ag-coated ZnO
nano-needles showed a pair of strong peaks at 387.4 nm and 405.2 nm, the relative intensity of 1366 and
1305, respectively, indicating that the Ag-coated ZnO nano-needles can increase the absorption of UV
light.
Received 15 November 2010
Received in revised form 10 January 2011
Accepted 19 January 2011
Available online 28 January 2011
Keywords:
Ag-coating
ZnO nano-needles
Pulse electro-deposition
Photoluminescence
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
doped ZnO thin films were enhanced; Sun et al. [3] have found the
light absorption ability of Ag-doped ZnO thin films were enhanced,
Zinc oxide (ZnO) is an excellent wide band gap semiconduc-
tor functional material because of its wide band gap (3.37 eV) at
room temperature, and large exciton binding energy (60 meV). ZnO
with a strong free exciton transition luminescence characteristic in
the UV band has been widely used in the fields of antibacterial,
antistatic, absorption of ultraviolet light and electrical conductiv-
ity, etc. Furthermore, one-dimensional ZnO nano-materials have
practical value in the UV detection due to their faster response
to ultraviolet light, good thermal stability, structure easy to con-
trol and low-cost. Since Yang et al. [1] synthesized ultraviolet
diode based on single ZnO nanowire, one-dimensional ZnO nano-
materials have attracted widely attention in the UV optoelectronic
applications. However, the photosensitivity of one-dimensional
ZnO nano-materials is always limited due to its surface states,
the photon capture efficiency, defecting distribution of photo-
electric sensitivity factors, so that it can not fully reflect the
nano-effect, and greatly limited the application of one-dimensional
ZnO nano-materials. Therefore, it is necessary to optimize zinc
oxide nanostructures, such as doping, coating and surface modi-
fication. Zhang et al. [2] have reported the ultraviolet of silver (Ag)
and the UV absorption edge of the films have a obvious red shift. Fan
et al. [4] have reported the gas sensing properties of the Ag-doped
ZnO nanowires were greatly improved as reducing the diameter of
the nanowires and their surface area increase; Li et al. [5] have
found that the UV emission properties of ZnO nanowires were
enhanced which were coated by ZnS, due to the surface oxygen
vacancies were filled by the S element; it is showed that the surface
modified or doped nano-ZnO have become the research hotspot
in recent years. In this paper, the process were introduced that
ZnO nano-needles were prepared by thermal oxidation and the Ag-
coated ZnO nano-needles by pulsed electro-deposition means, and
micro-structure and optical performance of these Ag-coated ZnO
nano-needles have been investigated.
2. Experimental
2.1. Preparation of ZnO nano-needles
Large areas of pure zinc (purity of 99.9%, mass fraction) was slit into the small
pieces of 1 cm × 1 cm, After the mechanical grinding and polishing of the substrate
surface; mirror-like small zinc foil was washed by distilled water, absolute alcohol,
respectively, and it was further cleaned using ultrasonic cleaner for 10 min to remove
impurities, dried. The cleaned zinc foil was then placed into a alumina boat that
was put in a box-type furnace. The heating was done at 450 ◦C for 10 h, and then
naturally cooled to room temperature. An off-white substance can be obtained on
zinc foil, placing it under scanning electron microscope. A layer of hairy needle-like
nano-material can be seen on the zinc surface.
∗
Corresponding author at: Department of Physics, Shaoguan University,
Shaoguan Guangdong 512005, China.
E-mail address: Lixiaozhu1019@21cn.com (X. Li).
0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.01.118

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X. Li, Y. Wang / Journal of Alloys and Compounds 509 (2011) 5765–5768
Fig. 1. X-ray diffraction patterns of uncoated and Ag-coated ZnO nanoneedles: (a) uncoated; (b) Ag-coated.
Fig. 2. The TEM and SEAD diffraction pattern of ZnO nano-needles.
Fig. 3. SEM morphologies of ZnO nano-needles before and after Ag- coated: (a) (b) the uncoated; (c) Ag-coated, the closeup view is on the upper right corner.

X. Li, Y. Wang / Journal of Alloys and Compounds 509 (2011) 5765–5768
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Fig. 4. Comparison of photoluminescence spectra of uncoated Ag and Ag-coated ZnO nano-needles at room temperature.
2.2. Preparation of Ag-coated ZnO nano-needles
weakening. It indicates that the ZnO nano-needles have been suc-
cessfully achieved Ag coating by using pulsed electro-deposition
technique.
The surface of ZnO nano-needles was coated with Ag by using pulsed electro-
deposition technique. Process as follows: silver iodide (AgI) solution was used as
electro-deposition solution. The addition of potassium iodide (KI) was to dissolve
insoluble silver iodide. Electro-deposition liquid ingredients are shown in Table 1.
During pulsed electro-deposition, the anode was a standard graphite electrode
(mass fraction is 99.9%) sized 50 cm × 50 cm, and the cathode was a zinc sheet sub-
strate which was covered with nanoneedle-shaped material. Both electrode and
substrate were placed in a standard Hull cell, and the electrolyte was mixed using
a magnetic stirrer with a digital thermostat for preventing the precipitation. The
power supply was a numerical control double-pulse (square-wave pulse) plating
electric source (GKDM 30-15, Xin Du, China), and the power supply adopted rectan-
gular wave output current. The plating parameters includes output pulse frequency
(f = 100 Hz) and duty cycle (r = 10%). The plating was carried out at 60 ◦C with a plat-
ing time 5 min. The pH value of the electrolyte was measured by a PHS-25 type pH
meter, and adjusted to pH 2.0 by adding sulfuric acid (H2SO4).
3.2. TEM analysis
Fig. 2(a) and (b) shows the TEM and SEAD diffraction patterns
of ZnO nano-needles. According to the TEM photographs of a single
nano-needle, we found that the surface of nano-needle is smoothed
from bottom to top, gradually reduced in diameter, showing acicu-
lar form. Draw according to SEAD diffraction pattern, the distance
between planes is 0.28 nm, corresponding to (0 0 0 2) plane of the
hexagonal wurtzite ZnO. It shows that the ZnO nano-needles grow
along [0 0 0 1] direction. It is illustrate that pure ZnO nano-needles
can be obtained by the method of thermal oxidation. The result is
consistent with that of XRD.
2.3. Characterization and testing
The phase of samples were analyzed by power X-ray diffraction (XRD) on
German Bruker AXS/D8 advanced X-ray diffractometer with Cu Ka radiation
(ꢀ = 1.5418A). The morphologies and the lengths of the samples were observed by
3.3. SEM analysis
˚
using a scanning electron microscope (SIRION SEM, FEI the Netherlands) and a trans-
mission electron microscope (JEOL JEM 2010 TEM, Japan). The photoluminescence
spectrums of ZnO nano-needles with Ag-coated and uncoated were measured by
using fluorescence spectrophotometer at room temperature (HITACHI F-4600, using
Xe lamp as excitation source).
Fig. 3 shows the SEM photographs of the uncoated and Ag-
coated ZnO nano-needles. It can be seen from Fig. 3(a) and (b), when
no Ag-coated, the surface of ZnO nano-needles is smoothed, show-
ing acicular form. The length is estimated about 2–5 ␮m according
to Digitalmicrograph Test Software. Fig. 3(c) shows the SEM pho-
tographs of ZnO/Ag nano-needles. It can be clearly seen that the
closeup view on the upper right corner, the ZnO nano-needles
have been buried by Ag. It is obviously that the surfaces of ZnO
nano-needles have been successfully achieved the Ag coating.
3. Results and discussion
3.1. XRD analysis
Fig. 1 shows the XRD pattern of Ag-coated and uncoated ZnO
nano-needles. Fig. 1(a) shows the XRD pattern of ZnO nano-needles
synthesized by thermal oxidation method. All the peaks can be
indexed to ZnO. It is fully consistent with the standard spec-
trum of ZnO (JCPDS 79-0205), no other peaks of impurities can
be detected. Fig. 1(b) shows the XRD patterns of the Ag-coated
ZnO(ZnO/Ag) nano-needles. A strong diffraction peak correspond-
ing to Ag(III) plane can be seen in the 2ꢁ = 38.088◦ from the figure,
and the corresponding diffraction peaks of Ag have been emerged in
44.2720◦,64.3960◦ and 77.3370◦. While the corresponding diffrac-
tion peaks of ZnO also still exist, but their intensities are significant
3.4. The test and analysis of the photoluminescence (PL) spectra
In order to understand the optical properties of these products,
we have carried out their photoluminescence spectra test. Fig. 4
shows the photoluminescence of uncoated Ag and Ag-coated ZnO
nano-needles under excitation at 325 nm at the room temperature.
It exhibits two fluorescence peaks at 388 nm and 470.8 nm, the
relative intensity of 143.4 and 93.61, respectively. Ag-coated ZnO
nano-needles showed strong double peaks, which were located at
387.4 nm violet luminescence peak and 405.2 nm blue–violet peak,
the relative intensity of 1366.0 and 1305.0, respectively. Compared
with the non-coated Ag ZnO nano-needles, the UV light inten-
sity of Ag-coated ZnO nano-needles was more intense than that
of uncoated ZnO nano-needles, just enhances nearly ten times. The
blue-shift of the near band edge emission peak of the ZnO nanonee-
dles was observed comparing to that of no-coated Ag. It can be
concluded that the Ag-coated affected the photoluminescence of
ZnO nano-needles. Its spectrum consists of a wide and strong deep-
level blue-green emission belt at 450–500 nm, a weak and narrow
UV emission bands at 375 nm. Where the UV light was stimulated
by radiant combination from the exciton; the blue–green belt is
Table 1
Bath compositions and pulse plating conditions.
Composition or parameter
Data
Silver iodide (AgI)
Potassium iodide (KI)
pH value
0.05 mol/L
0.05 mol/L
2.0
Positive pulse work time
Negative pulse work time
Positive pulse peak value current
Negative pulse peak value current
8T(T = ton + toff)
2T(T = ton + toff)
0.5 A
0.3 A

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X. Li, Y. Wang / Journal of Alloys and Compounds 509 (2011) 5765–5768
attributed to the oxygen vacancy and interstitial zinc [6]. Through
the PL spectrum test, we found that the relative intensity of blue
emission peak at 470.8 nm is 93.61 for pure ZnO nano-needles.
The association point defects in ZnO thin films and the energy
levels of co-introduction defects can be calculated according to
full-potential linear muffin-tin orbita1, that is FP-LMTO method [7],
and 470 nm peak derived from the transitions between the Zn gap
and the Zn vacancy. It could be deduced that there is Zn intersti-
tial donor-type defects in the eigen ZnO nano-needles. The light
emission peaks of Ag-coated ZnO nano-needles are at 387.4 nm
and 405.2 nm. Fig. 4 shows that the near-band-edge emission peak
at about 375 nm greatly strengthened, and the deep-level green
emission peak at 450–500 nm disappeared. So the quality of Ag-
coated crystal is improved. UV light enhancement is due to metallic
Ag was electrodeposited on the surface of ZnO nano-needles. On
the one hand the exciton was formed at the interface between
Ag nano-crystalline and ZnO nano-needles, and the performance
of ultraviolet light emission was enhanced; The other hand, Ag
nano-crystalline coating in the surface of the ZnO nano-needles
interacts with the ever-increasing ultraviolet light by ZnO nano-
needles issued. When the interact is to achieve a certain degree,
plasma resonance is generated in the surface of Ag nano-crystals,
making the local field intensity surrounding Ag nano-crystals dra-
matically increased, which led to enhancement of ZnO ultraviolet
light.
were successfully prepared by using the electro-deposition tech-
nique in AgI bath. Comparing the photoluminescence spectrums
of uncoated and Ag-coated ZnO nano-needles, the results showed
that the UV light emission peak intensity of Ag-coated ZnO nano-
needles significantly increased, increased nearly 10 times. The
results indicated that Ag-coated ZnO nano-needles improved the
luminous efficiency of ZnO nano-needles; the near band-edge
emission peak of Ag-coated ZnO nano-needles greatly strength-
ened, and the deep level green emission peak disappeared, these
indicated that the ZnO nano-needles crystal quality improved after
Ag-coated.
Acknowledgement
This study is supported by the National Nature Science Founda-
tion, no. 10747146.
References
[1] Y.H. Yang, C.X. Wang, B. Wang, N.S. Xu, G.W. Yang, Chemical Physics Letters 403
(2005) 248–251.
[2] Yang Zhang, Ziyu Zhang, Bixia Lin, Zhuxi Fu, Jin Xu, et al., Journal of Physical
Chemistry B 109 (2005) 19200–19203.
[3] Sun Wang-dian, Wang Ying-lian, Chinese Journal of Vacuum Science and Tech-
nology 27 (2007) 309–311.
[4] Fan Xin-hui, Lin He, Ma Xue hong, Yan Wen, et al., Nanoscience and Nanotech-
nology 6 (2009) 61–65.
[5] J. Li, D. Zhao, Journal of Physical Chemistry B 110 (2006) 14685–14687.
[6] Jin Yun-jiang, Yu Qing-xuan, Liu Shu-jian, Liao Yuan., et al., Chinese Journal of
Quantum Electronics 25 (2008) 43–48.
4. Conclusions
[7] Tan Tian-ya, Chen Jun-jie, Shan Jing, Jiang Xue, Wu Wei, Guo Yong-xin, et al.,
Journal of Liaoning university (Natural Sciences Edition) 36 (2009) 6–8.
ZnO nano-needles were prepared at 450 ◦C by the thermal
oxidation of pure Zn film. Further Ag-coated ZnO nano-needles