4th International Science, Social Science, Engineering and Energy Conference
11th-14th December, 2012, Golden Beach Cha-Am Hotel, Petchburi, Thailand
I-SEEC 2012
www.iseec2012.com
Enhanced Photocatalytic Activities of Ag/ZnO Powders
Modified by Diblock Copolymer
P. Amornpitoksuka,d,e1, S. Suwanboonb,d,e2, N. Muensitc,d,e3
b
a
Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla, 90112, Thailand
Department of Materials Science, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla, 90112, Thailand
c
Department of Physics, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla, 90112, Thailand
d
Center of Excellence in Nanotechnology for Energy (CENE), Prince of Songkla University,
Hat-Yai, Songkhla, 90112, Thailand
e1
ampongsa@yahoo.com, e2ssuwanboon@yahoo.com, e3nantakan.m@psu.ac.th
Abstract
Ag/ZnO powders were synthesized through a precipitation method, using poly(ethylene oxide)-block-poly(polylene
oxide) as stabilizer. X-ray diffraction patterns of all prepared powders showed a mixed phase of Ag and ZnO. The
existence of metallic Ag on the surface of all prepared ZnO powders was confirmed by X-ray photoelectron
spectroscopy. Their photocatalytic activities were investigated through the degradation of a methylene blue solution
under blacklight illumination. The Ag/ZnO prepared from the Zn 2+ solution containing 7 mol% of Ag+ had the
highest photocatalytic activity and this also showed better photocatalytic activity than a commercial ZnO powder.
Keywords: ZnO, Photocatalyst, Diblock copolymer.
Introduction
Zinc oxide (ZnO) is an n-type II-VI semiconductor with a wide direct energy band gap of 3.37 eV
and a large free-exciton binding energy. According to its properties, ZnO has attracted attention for use in
electrical and optical applications such as light emitting diodes, piezoelectric transducers, etc [1-3].
Nowadays, it can also be used as a photocatalyst for the treatment of pollutants in water because ZnO has
a high photocatalytic activity, is a low cost material and also is environmental friendly [4].
In recent years, the control of the morphology and size of ZnO particles has been extensively
investigated because these are key factor for improving its photocatalytic activity. At present, the
investigation of a stabilizer or capping agent by a group of block copolymers has aroused interest. A
diblock copolymer is one of the stabilizers with an effective potential to control both the morphology and
the size of ZnO powders and results in a product with improved photocatalytic properties [5]. Not only
the shape and size of the ZnO particles changed but also some dopants can effectively improve the
photocatalytic activity of ZnO powder. In previous publications [6-7], scientists have found that silver
(Ag) is the most popular metal dopant for coupling with ZnO powder to enhance its photocatalytic
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properties because silver can reduce or inhibit a charge-recombination between the photogenreated
electrons in a conduction band and the photogenerated holes in the valence band.
In this work, we have investigated the effect of Ag loading on the morphology and photocatalytic
properties of Ag/ZnO powders, prepared by a precipitation method, using poly(ethylene oxide)-bpoly(propylene oxide) or (PEO) 19-b-(PPO)3 as a stabilizer.
Experimental
Ag/ZnO powders were synthesized through hydrolysis of zinc acetate dihydrate
(Zn(C2H3O)2.2H2O, Aldrich) in the presence of PEO-b-PPO (MW1000, Huntsman) in an alkaline
solution. In a typical procedure, 0.015 mole of Zn(C 2H3O2)2.2H2O was dissolved in 150 mL of distilled
water containing various amounts of AgNO3 (1-12 mol% Ag). Then, 0.015 mol of PEO-b-PPO was added
into the above solution to obtain a suspension and the mole ratio of Zn 2+ to PEO-b-PPO was kept at 1:1
throughout the experiment. After that, 150 mL of 1 M NaOH was added dropwise to the above
suspension. The precipitates were stirred at 70 OC for 1 h. After cooling to room temperature, these
precipitates were filtered, rinsed with distilled water several times, then collected and dried at 100 OC for
1 h before further studies.
The X-ray powder diffraction (XRD) patterns of these Ag/ZnO powders were recorded on a XPart (Phlilip) diffractometer using Cu K radiation (0.15406nm) at a scanning rate of 5 o/min for 2
ranging from 30o to 70o. The morphology of all samples was investigated by scanning electron
microscopy (Quanta 400, FEI). The oxidation state of Ag on the surface of the Ag/ZnO powder was
studied by the X-ray photoelectron spectroscopy technique. The photocatalytic activity of Ag/ZnO
powders was investigated through the discoloration of a methylene blue (MB) dye solutions under
illumination from three blacklight fluorescent tubes (15W, Sylvania) The remaining dye concentration
after the required irradiation times was measured by a UV-Vis spectrophotometer (UV 2450, Perkin
Elmer).
Results and discussion
Figure 1 shows the x-ray diffraction patterns (XRD) of selected ZnO powders at various Ag
loadings. The ZnO powders containing Ag loadings between 1 and 9 mol%, clearly showed two different
characteristic diffraction patterns of the ZnO crystals in a wurtzite structure (JCPDS card number 361451) and a cubic structure of metallic Ag (JCPDS card number 87-0718). The metallic Ag content in the
ZnO powder increased with the Ag loading as indicated by an increase of the peak intensity of the
metallic Ag. The coexistence of the ZnO and metallic Ag detected in the XRD patterns probably
confirmed that all samples had the heterostructure of Ag/ZnO as usually reported in the literature [6-7].
On the other hand, when the Ag loading exceeded 9 mol%, the XRD pattern showed an additional XRD
peak or third phase that closely matched the cubic Ag2O (JCPDS card number 41-1104).
Figure 2 shows the scanning electron microscopy (SEM) images of selected Ag/ZnO powders. In
the absence of AgNO3, the ZnO shows an urchin-like shape because of its anisotropic growth along the
(0002) plane in all directions. The velocity of the growth of the ZnO crystals, for a solution method, in a
different direction is reported to be: V(0001) > V(-101-1) > V(-1010) > V(-1011) > V(000-1) [8]. With an
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increase of the Ag+ ions in the starting solution, the branching rod’s dimension decreased and tended to
form a pallet-like structure.
Fig. 1. XRD patterns of ZnO with (a) 0, (b) 1, (c) 7 and (d) 15 mol% of Ag loading. The (111) of metallic Ag and (111) of Ag 2O
phases are marked in “” and “@”, respectively.
Fig. 2. SEM images of ZnO with (a) 0, (b) 3, (c) 9 and (d) 15 mol% of Ag loading.
The oxidation state of the Ag in the ZnO powders was characterized by the XPS technique. Figure 3
(a) shows the XPS spectrum of a ZnO with 7 mol% of Ag loading and all the indexed peaks can be
identified to C, O, Ag and Zn. The presence of C in the XPS spectrum could come from the oil pump in
the vacuum system of the XPS equipment [9]. Figure 3 (b) shows the Ag 3d core-level XPS spectrum of
ZnO with 7 mol% of Ag loading. The binding energies of Ag 3d 5/2 and 3d3/2 were clearly observed at
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367.68 and 373.68 eV, respectively. The obtained binding energy of the Ag 3d5/2 was found to shift to
lower values compared to the referenced binding energy of Ag 3d 5/2 for metallic Ag (368.2 eV) and the
evidence for this comes from the interaction between the metallic Ag and ZnO as usually mentioned in
the literature [6-7,10-11]. Furthermore, as the splitting of the 3d doublet is about 6 eV, this confirms the
nature of the metallic Ag [12].
Fig. 3. The full spectrum of XPS (a) and the 3d doublet of the XPS spectrum of ZnO with 7 mol% of Ag loading.
Fig. 4. The comparison of the degradation efficiency for ZnO powders with various Ag loadings after being irradiated for 1 h and
comparison with a commercial ZnO (com.ZnO) powder.
The photocatalytic activity was investigated by the degradation of a methylene blue (MB) dye
solution and the degradation efficiency was calculated by the following equation;
Degradation (%) = [(Ao – At)/Ao] x 100
where Ao is the absorbance of the MB dye solution before irradiation and At is the absorbance of the MB
dye solution after being irradiated for the required time.
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Figure 4 shows the variation of the photocatalytic efficiency of the ZnO powders with various Ag
loading after being irradiated for 1 h. To ensure that the degradation efficiency did not derive from either
adsorption or self-degradation by light, the powder samples were stirred in the MB solutions in the dark
and only the MB solution without any photocatalyst was irradiated under a blacklight for 3 h. The results
(data are not shown) show that these effects can slightly reduce the absorbance of the MB solution but
these were negligible compared to the results from the photocatalytic degradation. As seen in Figure 4,
the ZnO powder with 7 mol% of Ag loading showed the best performance for MB degradation. The
photocatalytic efficiency of the Ag/ZnO increased with the Ag loading as the Ag content was increased
from 0 to 7 mol%. In the heterostructure of the Ag/ZnO, the Fermi level of Ag was lower than the
conduction band of ZnO so that the photogenerated electrons in the conduction band of the ZnO could be
transferred to this level and react with oxygen to produce a superoxide anion radical ( O2-). Moreover, this
level can act as a sink of electrons to reduce the charge-recombination reaction. However, when the Ag
loading exceeds 7 mol%, the photocatalytic efficiency of Ag/ZnO powders decreased with an increase of
the Ag content. This could arise from a decrease of the ZnO surface from more Ag or Ag 2O coverage. In
addition, the ZnO powder with a 7 mol% of Ag loading also showed more photocatalytic activity than the
commercial ZnO grade (Figure 4).
Conclusion
Ag/ZnO powders can be synthesized by the precipitation method at low temperatures without any
calcination process. The Ag on the surface of ZnO is in a metallic form and helps to enhance the
photocatalytic activity of the ZnO powders. The ZnO with 7 mol% of Ag loading exhibited the best
activity and, moreover, showed a better photocatalytic performance than a commercial ZnO.
Acknowledgments
This research is supported by The Thailand Research Fund (TRF), Office of the Higher Education
Commission and Prince of Songkla University under contact number MRG5480071. The authors would
like to thank Dr.Brian Hodgson for assistance with the English.
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