Open Journal of Synthesis Theory and Applications, 2012, 1, 18-22 Published Online July 2012 (
Preparation and Characterization of Silver Doped ZnO
Nguyen Van Nghia*, Tran Nam Trung, Nguyen Ngoc Khoa Truong, Doan Minh Thuy
Department of Physics, Quy Nhon University, Quy Nhon, Vietnam
Email: *
Received April 25, 2012; revised June 7, 2012; accepted July 3, 2012
ZnO was prepared by hydrothermal method. The result of scanning electron microscopy showed that the materials had
nano rod structures. Ag-doped ZnO was prepared by UV-photoreduction. Crystalline phases and optical absorption of
the prepared Ag-doped ZnO samples were determined by X-ray diffraction, Raman spectrum, UV-visible, and UV-
photoreduction spectrophotometer. X-ray analyses revealed that Ag was doped ZnO crystallizes in hexagonal wurtzite
structure. The incorporation of Ag+ in the site of Zn2+ provoked an increase in the size of nanocrystals as compared to
pure ZnO. The photocatalytic and photoluminescence properties of materials were considered.
Keywords: Nanostructures; Photocatalysis; Hydrothermal; ZnO; Silver Doping
1. Introduction
Nanostructured ZnO materials have received consider-
able interest from scientists due to their remarkable per-
formance in electronics, optics and photonics. ZnO is a
wide band gap (3.37 eV at room temperature) compound
semiconductor that is appropriate for short wavelength
optoelectronics applications. The large exciton binding en-
ergy (60 meV) in ZnO crystal allows efcient excitonic
emission at room temperature. Therefore, ZnO nanos-
tructures have had a wide range of high technology ap-
plications like surface acoustic wave lters, photonic
crystals, gas sensors, photocatalysis [1-3]. Because of
having a wide bandgap, ZnO can only be activated by
ultraviolet light of wavelength below 385 nm. The ultra-
violet light reaching the earth’s surface is less than 5% of
the solar energy, which is too low to attain significant
photodegradation in commercial application. Some in-
teresting approaches have been adopted to extend the
photoresponse of ZnO toward the visible spectral region,
such as implanting transitional metal ions [4,5].
Metal silver is also a significant visible light photosen-
sitizer, which is stable and nontoxic. Ag is also relatively
cheap; thus Ag modification is of great significance for
industrial practice. The improvement in efficiency of
photocatalytic reactions under visible light is explained
as the result of a vectorial transfer of photogenerated elec-
trons and holes from metal to semiconductor. Moreover,
ZnO modified by Ag can improve the distribution of
surface charges, accept a conduction band generated by
solar light irradiation during photoreaction, prevent the
recombination of the photogenerated electron-hole. Many
researchers reported that ZnO thin film with Ag doping,
which enhances ultraviolet emission and improves elec-
trical and optical properties, was prepared by wet chemi-
cal [6], DC magnetron sputtering [7] and pulsed laser
deposition [8].
In this work, silver nanoparticles were deposited on
the surface of ZnO nanorods (prepared by hydrothermal
method) by using a photochemical reduction under UV
irradiation. We also have compared the photocatalytic
properties of Ag-doped ZnO and ZnO nanorods (ZnO-
2. Experimental
2.1. ZnO -NRs Preparation
4,6 g of zinc acetate (Zn(OAc)2 powders were dissolved
in 40 ml a solvent etanol under stirring for 1 h (called M1
solution). The M2 solution was prepared by adding 2,5 g
oxalic acid (H2C2O4) into 40 ml etanol under stirring. M2
was slowly poured in the M1 solution under ultrasonic
wave (35 kHz, 100 W) for 30 minutes. The sonicated
solution was then moved to a teflon vessel and put in a
stainless steel autoclave for carrying out the hydrother-
mal treatment at 140˚C for 20 hours. The stainless steel
was then opened at room temperature and the precipitates
were separated and washed repeatedly by deionized wa-
ter until the pH value of the washing solution became
lower than 7. The final as-prepared product was dried
under vacuum at temperature of 80˚C for 12 hours. Then
*Corresponding author.
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N. Van NGHIA ET AL. 19
the product was calcined at temperature of 450˚C for 1
hour in air.
2.2. Ag-Doped ZnO Preparation
0,5 g of ZnO-NRs were dispersed into deionized water
under stirring. The suspension was sonicated for 30 min
by ultrasonic wave. After ultrasonically treated the sus-
pension was further magnetically stirred for 30 min un-
der UV irradiation. Then 0.007 g AgNO3 were added into
the suspension (mAg:mZn = 1%) and followed by UV il-
lumination for 4 h under stirring. The black powder was
centrifuged, rinsed with deionized water repeatedly to
purify the product, and nally dried at 70˚C under air for
5 h. The product was denoted Zn-Ag1.
Using the same method, we prepared products Zn-Ag2,
Zn-Ag3, Zn-Ag4, Zn-Ag5. The more silver ion was doped,
the darker in colour of the product was.
2.3. Characterization and Measurement
The morphology dimensions and microstructure meas-
urements of the samples were performed using a Hitachi
S4800 scanning electron microscope (SEM). The crystal-
line phases of the obtained samples were characterized
by using a Siemens D-5005 X-ray powder diffractometer
(XRD) with a monochromatized Cu-Kα irradiation (λ =
1.54056 Å). The Raman spectra were recorded on a
Nicolet spectrometer equipped with an optical micro-
scope at room temperature. UV-Vis spectra were meas-
ured by a Scan UV-Vis spectrophotometer (Varian, Cary
The photocatalytic activity was evaluated by measur-
ing the decomposition of the aqueous solution of me-
thylene blue (with a concentration of 10 mg/L) under
sunlight irradiation for 30 min of pure ZnO-NRs and
Ag-doped ZnO. The reactor was equipped with water
circulation in the outer jacket in order to maintain a
constant temperature. Prior to irradiation, the suspensions
were magnetically stirred in dark for 1 h to ensure an
establishment of adsorption/desorption equilibrium. Then
the solution was filtered to remove “particles”. The re-
sulted solution was analyzed by recording variations in
the absorption in UV-visible spectra of methylene orange
using a Shimadzu 1601-PC Ultraviolet-visible spectro-
3. Results and Discussion
The XRD patterns of ZnO-NRs and Zn-Ag5 are exhibited
in Figure 1.
As can be seen from Figure 1, all the samples were
well crystalline, and of hexagonal wurtzite phase (JCPDS
le No. 36-1451). However, the Ag-doped samples re-
vealed some additional diffraction peaks marked with
“*” associated with the face-centered-cubic phase of me-
tallic Ag (JCPDS le No. 04-0783). The appearance of
Ag peaks in the diffraction patterns indicates clearly the
formation of crystalline silver clusters in the nanoparti-
cles. This can be demonstrated by SEM image of Zn-Ag5.
Figure 2(a) shows the image of ZnO nanorods cal-
cined at 450˚C, a large number of open-ended ZnO-NRs
with uniform diameters around 300 nm and lengths about
several micrometer can be clearly seen in the picture. No
obvious damage is found and the materials remain in
good shape. Figure 2(b) shows that Ag-sensitized ZnO
prepared by photoreduction does not cause any change in
the morphology compared with pure ZnO. But some
newly formed small Ag particles with diameter a few
nanometer on the surface of ZnO can be observed clearly.
The EDS spectrum (Figure 3) showed that the Zn-Ag5
material include elements such as Zn, O and Ag. The
quality of Ag content deduced from the EDS spectrum
about 3% indicate that Ag particles are losed in the reac-
tion process.
The room temperature UV-Vis spectrum of undoped
ZnO and Zn-Ag5 are presented in Figure 3. As can be
seen from curve A in Figure 4, ZnO-NRs have a broad
intense absorption below wavelength 400 nm. It is the
characteristic absorption of ZnO corresponding to the
charge transfer process from the valence band to conduc-
tion band in ZnO. In the UV-Vis spectra of the Zn-Ag5
(curve B), it can be seen that there is a absorption peak
Figure 1. XRD patterns of ZnO and Zn-Ag5.
Figure 2. SEM images of ZnO nanorods (a) and Zn-Ag5 (b).
Copyright © 2012 SciRes. OJSTA
Figure 3. EDS spectrum of Zn-Ag5.
Figure 4. UV-Vis spectrums of ZnO and Zn-Ag5.
around 460 nm in the visible range. This is the character-
istic of surface plasmon absorption corresponding to Ag0
particles [9]. So the modication decrease in bandgap
energy of ZnO.
Figure 5 presents the room temperature Raman spec-
trum of ZnO-NRs and Zn-Ag5. It can be seen that the
spectrum of ZnO (curve A) consists of five peaks located
at about 100, 140, 380, 437, and 580 cm–1 which corre-
spond to the fundamental phonon modes of hexagonal
ZnO, respectively. The curve B which is the spectrum of
Zn-Ag5 has four peaks located at about 100, 140, 241,
and 580 cm–1. There appeared a broad Raman peak at
about 241 cm–1 exclusively for the Ag-doped samples.
The intensity of this peak decreased drastically on doping
the silver in the samples. We can recall that the incorpo-
ration of Ag in our ZnO nanoparticles reduces their crys-
tallinity [8].
Figure 6 shows room-temperature PL spectrums of
ZnO and Zn-Ag5 at the excitation wavelength of 325 nm.
We can see that a blue-green emissions at about 520 nm
200 400 600
200 400 600
Intensity (A.u)
B: Zn-Ag5
A: ZnO-NRs
Wavenumber (cm-1)
Figure 5. Raman spectrums of ZnO and Zn-Ag5.
were observed in them. The emission centered at 520 nm
of Zn-Ag5, which can be tted to two peaks centered at
516 nm and 630 nm, respectively, as shown in the inset
of Figure 5. The peak 516 might be attributed to the in-
trinsic defects (O and Zn vacancies or interstitials and
their complexes) in ZnO. The other hand, the Ag-doped
ZnO, which prepared, exhibits a new and unusual PL
phenomenon at 615 nm; there is the possibility of the
formation of a new surface state. The dopant Ag has a
great effect on the separation and recombination process
of photo-induced charge carriers of ZnO, which can fur-
ther effect on PL performance. It indicates that the pho-
toluminescence mechanism of Ag/ZnO is very complex
and further research is needed.
Figure 7 shows the effect of the Ag content on the
photocatalytic activity of Ag-doped ZnO. The photo-
catalytic degradation ratio of methyl blue (MB) increases
rather rapidly initially with the increase of the Ag content
and reaches a plateau at the Ag content of 4%. It is worth
that the amount of doped silver ion is very important to
photoactivity. But an increase in dopant ion can make
increasable rate recombination of electron-hole pairs,
because silver ions play a central role of recombination,
that can make decreasing the photocatalytic activity of
material. The photocatalytic mechanism of Ag/ZnO is
also complex and it’s studied deeply.
We considered the UV-photoreduction mechanisms
from the view point of photolysis at ZnO catalyst. Oxida-
tion and reduction occur at the same time in Ag ion
photoreduction. In the reduction, the conduction band
electrons generated in the ZnO (e(CB)) by UV irradition
can reduce adsorbed Ag+ ions, giving rise to Ag atoms
(Ag0). The reduced Ag is deposited on the ZnO surface.
Photochemical reactions induced by ZnO-light are sum-
maized as: r
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400 500 600 700800 900400 500 600 700800 900
Intensity (A.u)
Wavelength (nm)
500600 700 800 9001000
Fit Curve
Intensity (au)
Wavelength (nm)
Figure 6. Room-temperature PL spectrum of Zn-Ag5. In the inset, the curves “…”show Gaussian curve fitting.
The degradation of MB (%)
The ratio Ag-Doped (%)
Figure 7. The degradation of MB on the Ag-doped ZnO.
ZnO + h
e(CB) + h+(VB) (1)
Ag+ + e(CB) Ag0 (2)
2H2O + h+(VB) O2 + 4H+ (3)
4. Conclusion
Nanorod ZnO was successfully synthesized by a simple
hydrothermal method. Ag-doped ZnO composites were
prepared by method of UV-photoreduction with as-syn-
thesized ZnO-NRs. Photocatalytic reactions show that
doping Ag into ZnO hole remarkably improves the pho-
tocatalytic activity of ZnO under simulated solar light.
The PL spectrum result shows that Ag-sensitized ZnO
composite not only has the emission intensities at about
318 nm and 520 nm, but also exhibits a new and unusual
PL phenomenon at 615 nm, indicating that the dopant Ag
has great effects on separation and recombination proc-
esses of photo-induced charge carriers of ZnO.
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