Defect-enhanced photocatalytic activity of ZnO micro/nanostructures

Article abstract of DOI:10.1002/cjoc.201300477

ZnO micro/nanocrystals with controlled oxygen defects are successfully prepared through annealing precursors obtained via a simple solvothermal method. The size and surface defects of the as-synthesized ZnO micro/nanocrystals increase with the solvent volume ratio of water/ethanol increasing; the BET surface areas of the corresponding samples decrease during these processes; photoluminescence reveals that the type and concentration of surface defects (oxygen vacancy and interstitial oxygen) are quite different for the samples prepared via different solvent volume ratio of water/ethanol. In addition, it is found that the photocatalytic activity of the synthesized ZnO nanocrystals is mainly dependent on the concentration of surface defects. The sample with more surface defects exhibits higher photocatalytic activity toward the degradation of methylene blue (MB). The possible photocatalytic mechanism is discussed in detail. Copyright

Full text of DOI:10.1002/cjoc.201300477

FULL PAPER
DOI: 10.1002/cjoc.201300477
Defect-enhanced Photocatalytic Activity of
ZnO Micro/nanostructures
Yu Wang, Mei Yan, Qinguo Chen, and Yin Peng*
The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Key Laboratory of Molecular
Based Materials, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China
ZnO micro/nanocrystals with controlled oxygen defects are successfully prepared through annealing precursors
obtained via a simple solvothermal method. The size and surface defects of the as-synthesized ZnO mi-
cro/nanocrystals increase with the solvent volume ratio of water/ethanol increasing; the BET surface areas of the
corresponding samples decrease during these processes; photoluminescence reveals that the type and concentration
of surface defects (oxygen vacancy and interstitial oxygen) are quite different for the samples prepared via different
solvent volume ratio of water/ethanol. In addition, it is found that the photocatalytic activity of the synthesized ZnO
nanocrystals is mainly dependent on the concentration of surface defects. The sample with more surface defects ex-
hibits higher photocatalytic activity toward the degradation of methylene blue (MB). The possible photocatalytic
mechanism is discussed in detail.
Keywords ZnO micro/nanocrystals, surface defects, photocatalytic activity
Introduction
oxygen vacancy plays an important role in defect-me-
diated energy transfer between ZnO nanocrystals and a
conjugated dye.[18] Zheng’s investigation exhibited that
the photocatalytic activity of ZnO nanocrystals is
mainly dependent on the type and concentration of the
oxygen defects.[19] Further mechanistic researches
clearly demonstrated that decreasing the relative con-
centration ratio of bulk defects to surface defects in
TiO2 nanocrstals could significantly improve the separa-
tion efficiency of photogenerated electrons and holes.[21]
However, how to obtain different types of surface de-
fects in ZnO nanocrystals is still a great challenge.
Semiconductor-assisted photocatalysis has attracted
considerable attention as a promising tool for imple-
menting the purification of wastewater and hydrogen
energy production.[1-3] A major limitation of achieving
high photocatalytic efficiency is the quick recombina-
tion of charge carries. Recombination, which has faster
kinetics than surface redox reactions, is a major draw-
back as it reduces the quantum efficiency of photocata-
lysis. Therefore, it is an important way to promote the
photocatalysis efficiency by decreasing the recombina-
tion rate. Some studies have highlighted that ZnO ex-
hibits better photocatalytic efficiency than TiO2 for re-
moving organic compounds in water matrices and pho-
toelectric conversion.[4,5] Unfortunately, ZnO holds the
critical drawback of photocorrosion effect during light
irradiation and causes the decrease in photocatalytic
activity and stability.
Many works have been devoted to reduce the re-
combination of charge carriers by coupling the ZnO
photocatalyst with other materials, such as noble met-
als,[6-14] semiconductors,[15] and carbon materials.[16,17]
Recently, some researchers find that generating the sur-
face defects (such as oxygen vacancy and interstitial
oxygen, etc.) in the semiconductors could also improve
the quantum efficiency of photocatylysis.[17-21] It is
well-known that the existence of defects in a semicon-
ductor would lead to a corresponding defect energy
level in the band gap. Beane et al. demonstrated that the
Zheng et al.[19] reported the controlled-synthesis of
oxygen vacancy defects in ZnO by an ethanol-mediated
solvothermal method, and then annealing as-synthesized
ZnO sample at the different temperature to control in-
terstitial oxygen defects. Wang et al.[20] obtained ZnO
nanoparticles with the oxygen vacancy defects in
Nafion using ethanol solution of sodium hydroxides and
aqueous solution of zinc nitrate. In this study, the ZnO
micro/nanocrystals with different defect types of surface
defects are prepared by annealing precursors obtained
via a simple solvothermal method. ZnO precursors are
synthesized using zinc acetate dihydrate (ZnC4H6O4•
2H2O) and oxalic acid dihydrate (C2H2O4•2H2O) as raw
materials in mixed solvent of water and ethanol with
different volume ratio. The photoluminescence emission
spectra (PL) and photocatalytic properties of the
as-prepared samples are investigated in detail. The re-
*
E-mail: kimipeng@mail.ahnu.edu.cn; Tel.: 0086-553-33937135; Fax: 0086-553-3869303
Received June 19, 2013; accepted August 28, 2013; published online October 17, 2013.
Chin. J. Chem. 2013, 31, 1557—1563
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light source. Before irradiation, the system was stirred
in the dark for 30 min to ensure a sorption-desorption
equilibrium. Then, the system was irradiated for various
durations. The concentration changes of MB were re-
corded by the UV-Vis absorption spectra of solutions.
sults show that the type and concentration of surface
defects in the synthesized ZnO micro/nanocrystals can
be remarkably changed with the changing of the ratio of
water to ethanol in the mixed solvent. The photocata-
lytic properties of ZnO micro/nanocrystals are depend-
ent on concentration of surface defects.
Scheme 1 The band structure and charge-transfer process of
ZnO micro/nanostructure with oxygen defects
Experimental
Preparation
All chemicals and reagents were analytical grade,
purchased from Shanghai Chemical Reagent Co. Ltd.
and used without further purification. To obtain the
rod-like ZnC2O4 precursor, 5 mmol of Zn(Ac)2•2H2O
was dissolved in 20 mL of water to form a clear solution.
Then, an equal volume of an aqueous solution contain-
ing 5 mmol of oxalic acid (C2H2O4•2H2O) was drop-
wise added into the above solution under magnetic stir-
ring. After that, the solution was transferred into a Tef-
lon-lined stainless steel autoclave, heated at 180 ℃ for
10 h, and then cooled to room temperature naturally.
The precipitate was collected and washed with deion-
ized water and anhydrous ethanol several times each,
and dried in air at 60 ℃ for 4 h. As a contrast, ZnC2O4
precursors were also synthesized in ethanol and mixed
solvent of water and ethanol (volume ratio＝1∶1) with
the same procedure. Finally, the as-obtained precursors
were calcined in air at 500 ℃ for 2 h to produce po-
rous ZnO microrods. The porous ZnO microrods ob-
tained from ZnC2O4 precursors that were synthesized in
water，water and ethanol, and ethanol were labeled as
ZnO-1, ZnO-2, and ZnO-3, respectively.
Results and Discussion
The morphologies of the obtained samples were
examined with scanning electron microscopy (SEM)
measurements. As shown in Figure 1a, the as-prepared
ZnC2O4 precursor obtained in water displays short
rod-like shapes with 3－5 μm in diameter and 4－7 μm
in length, and their surfaces are smooth. After being
calcined in air at 500 ℃ for 2 h, the precursor turns
into ZnO-1 microrods, which was shown in Figure 1b,
1c. It is obvious that the ZnO-1 microrods still maintain
the rod-like morphology of the precursor. Furthermore,
the ZnO-1 microrods show porous structure, and are
comprised of abundant ZnO nanoparticles with the
mean size of 50－100 nm (Figure 1c).
XRD technique was applied to investigate the crys-
tallographic structure of the as-obtained products, as
shown in Figure 1d. The XRD pattern of the precursor
can be indexed as the monoclinic phase of zinc oxalate
(JCPDS card files No. 25－1029), and the XRD patterns
of the ZnO-1, ZnO-2 and ZnO-3 samples obtained
through calcining the precursors can be indexed as the
hexagonal ZnO (wurtzite structure: space group P63 mc,
JCPDF Card No. 36－1451). No peaks from precursor
are detected in the ZnO XRD patterns, indicating that
the precursors have been completely converted after
calcination.
Characterization
Powder X-ray diffraction (XRD) of the products was
recorded on a Japan Shimadzu X-ray diffractometer
(XRD-6000) equipped with graphite monochromated
Cu Kα radiation (λ＝0.15406 nm). Scanning electron
microscopy (SEM) images were recorded on a Hitachi
S-4800 field emission scanning electron microscope.
Photoluminescence emission spectra (PL) were re-
corded on an FLS920 spectrophotometer equipped with
a 150-W xenon lamp as the excitation source at room
temperature. UV-Vis diffuse-reflectance spectra (DRS)
were recorded with a UV-2450 spectrophotometer in the
wavelength range of 200－800 nm at room temperature.
BET surface area was recorded on a Tristar II 3020 M
Automatic Surface Area. The infrared (IR) absorption
spectra of the samples were recorded over the frequency
range from 450 to 4000 cm−1 using a IRPrestige-21
FTIR spectrophotometer.
The solvents have important effect on the morphol-
ogy and size of the ZnC2O4 precursors. Typical SEM
images of the samples obtained in different solvents are
shown in Figure 2. Comparing with the ZnC2O4 precur-
sor synthesized in water, the morphologies and sizes of
the precursor obtained in ethanol and water/ethanol are
obviously changed. The ZnC2O4 precursor obtained in
water/ethanol displays microrods with 0.5－3 μm in
diameter and 1－6 μm in length (Figure 2a), and the
diameter of the microrods is thinner than that of ZnC2O4
Photocatalytic activity measurements
To investigate the photocatalytic activity of the as-
prepared ZnO micro/nanostructures, 40 mg of photo-
catalyst was added into a 100 mL methylene blue (MB)
solution (10 mg/L). A 300 W high-pressure Hg lamp
with main emission wave of 365 nm was used as the
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Defect-enhanced Photocatalytic Activity of ZnO Micro/nanostructures
synthesized in water. However, when only ethanol is
used as solvent, the obtained ZnC2O4 precursor is com-
posed of nanorods with 70－100 nm in diameter and
0.2－1 μm in length (Figure 2c). After being calcined in
air at 500 ℃ for 2 h, it can be clearly seen that the ob-
tained ZnO samples still maintain the original precursor
shapes. The ZnO-2 sample by calcining ZnC2O4 pre-
cursor obtained in water/ethanol is comprised of nano-
particles with mean size of 50－100 nm (Figure 2b),
and the ZnO-3 sample by calcining ZnC2O4 precursor
obtained in ethanol is made of nanoparticles with size of
20－50 nm (Figure 2d).
d
ZnO-3
ZnO-1
ZnO-2
ZnC O
2
4
10
20
30
40
50
60
70
2 θ/(o)
Figure 2 SEM images of ZnC2O4 precursor (a, c) synthesized in
water and ethanol (1∶1) (a) and (c) ethanol, and their respective
calcining product ZnO-2 (b) and ZnO-3 (d).
Figure 1 Typical SEM images (a－c) and XRD pattern (d) of
the obtained ZnC2O4 precursor and ZnO, (a) synthesized in water
and its calcining product ZnO-1 micro/nanostructures (b, c), (b)
low magnification SEM image and (c) high magnification image.
The UV-Vis diffuse-reflectance spectra of ZnO sam-
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ples are shown in Figure 3a. It is interesting to find that,
with the solvents changing from water to ethanol, the
absorption strength of visible light increases (ZnO-1→
ZnO-2→ZnO-3), but the band-edge absorption has no
obvious change (3.2 eV).
cancies and photoexcited holes,[25] transition between
electrons close to the conductive band and deeply
trapped holes at VO＋＋
,
surface defects,[27] etc. The
[26]
yellow-orange emissions have been assigned to intersti-
tial oxygen and dislocation, as well as Li dopants.[28]
Recently, Zeng et al.[29] reported that the green emission
is usually only efficient under the above-Eg excitation,
which suggests it is the transition from the conduction
band to deep levels. Effective transitions from the inter-
stitial zinc (Zni) energy level to the valance band and
other deep levels will induce violet, blue and yellow
emissions according to the energy differences.
The IR spectra of ZnO samples are shown in Figure
4. It can be seen that the absorption band is redshifted
from 504 to 484 cm−1 (ZnO-3→ZnO-2→ZnO-1) (Fig-
ure 4b). This phenomenon should be ascribed to the
variation of the oxygen defect type and density.[19] The
increase of oxygen defects decreases the strength of the
Zn－O bond, resulting in a shift of the IR absorption
peak toward a low wavenumber (red shift). Moreover,
with variation in the oxygen defects, the intensity of the
IR absorption peak (I∝Ev) would change accordingly.
According to the IR peak intensity of ZnO, it is con-
cluded that the density of oxygen defects for ZnO-1
samples should be the greatest and that for ZnO-3 is
weakest among three ZnO samples, which is consistent
with the result of PL characterization.
The room temperature visible PL spectrum (Figure
3b) of the as-prepared samples show sharp emission
peak at 383－386 nm resulting from free-exciton anni-
hilation, and a broad band at 450－800 nm which is
related to deep defect levels located in the gap.[23] From
Figure 3b, we can see that the intensity of the peaks at
450－800 nm is obviously different, the emission peak
of the ZnO-1 sample is the strongest and ZnO-3 is the
weakest among the three samples. On the other hand,
their main emission peaks shift from 550 nm (green) to
572 nm (yellow) and 605 nm (orange) with the solvent
changing from water to ethanol (ZnO-1→ZnO-2→
ZnO-3). The orange (a main peak at 605 nm) emission
is the same as that reported by Zheng et al.,[19] Greene
et al.,[22] Cross et al.[23] and Xu et al.[24]
a
100
90
80
70
60
50
40
30
7x104
ZnO-3
ZnO-2
ZnO-1
b
6x104
ZnO-1
ZnO-2
5x104
ZnO-3
4x104
3x104
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber/cm-1
2x104
1x104
0
b
ZnO-3
ZnO-2
ZnO-1
80
70
60
50
40
30
400
500
600
700
800
Wavelength/nm
Figure 3 UV-Vis diffuse-reflectance spectra (a) and room tem-
perature PL spectra (b) of ZnO.
The origin of these visible emissions (e.g., green,
yellow, orange, and red emissions) is still highly con-
troversial. For example, a number of different hypothe-
ses have been proposed to explain the green emission,
such as transition between singly ionized oxygen va-
580
560
540
520
500
480
460
Wavenumber/cm-1
Figure 4 The IR spectra of three ZnO samples.
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Defect-enhanced Photocatalytic Activity of ZnO Micro/nanostructures
According to above discussions, in our three samples,
the PL intensity of ZnO-1 is the strongest and that of
ZnO-3 is the weakest, which implies that the defects of
ZnO-1 are the most abundant and the defects of ZnO-3
are the fewest. According to Zheng’s investigation, the
defect is benefit for enhancing the photocatalytic activ-
ity. Morever, the green defect (550 nm) is more effective
than the yellow defects (605 nm).
To investigate the photocatalytic activity of the
as-prepared porous ZnO microrods, we select methylene
blue (MB) as the model pollutants. Total concentration
of dye aqueous solution was simply determined from
the maximum absorption (664 nm) of UV-Vis absorp-
tion spectra. C/C0 was used to describe the degradation
which stands for the concentration ratio after and before
a certain period of reaction time. As shown in the Figure
5, ZnO-1 micro/nanostructure displays superior photo-
catalytic activity and 100% of MB dye molecules can be
degraded within 15 min. However, within the same irra-
diation durations, 96% and 88% of MB were respec-
tively degraded when ZnO-2 and ZnO-3 micro/nano-
structures were used as photocatalysts. Figure 5(b－d)
clearly exhibit the UV-vis absorption spectra of MB
dyes irradiated by 365 nm UV light for various dura-
tions in the presence of ZnO-1, ZnO-2 and ZnO-3.
Since the adsorption of photocatalysts to dyes can be
ignored, the above experiments show that the present
ZnO micro/nanostructures can promote the photodegra-
dation of MB dye molecules in short irradiation time,
and the ZnO-1 sample has the highest and ZnO-3 has
the weakest photocatalytic activity.
To quantitatively understand the reaction kinetics of
MB degradation in our experiments, we applied the
Langmuir-Hinshelwood model, which is well-estab-
lished for photocatalytic experiments when the concen-
tration of the organic pollutant is in the millimolar
range,[30] as expressed by
ln(C/C )
kt
−
＝
0
where C0 and C are the concentrations of pollutant in
solution at time t0 and t, respectively, and k is the ap-
parent forst-order rate constant. The apparent rate con-
stant k is calculated to be 0.348, 0.242 and 0.163 min−1
for ZnO-1, ZnO-2 and ZnO-3, respectively (Figure 6).
That is, the photocatalytic activity of the ZnO-1 is about
2 times higher than that of ZnO-3, and 1.5 times hight
than that of ZnO-2.
What causes the great differences in photocatalytic
activity of ZnO samples? As we know the sizes, mor-
phologies and surface areas have important effect on the
1.0
a
2.5
origin
ZnO-1
ZnO-2
ZnO-3
adsorption
10 min
15 min
20 min
25 min
b
0.8
0.6
0.4
0.2
0.0
2.0
1.5
1.0
0.5
0.0
0
5
10
15
20
25
30
200
300
400
500
600
700
800
t/min
Wavelength/nm
origin
adsorption
10 min
15 min
20 min
2.5
2.0
1.5
1.0
0.5
0.0
2.5
origin
adsorption
10 min
15 min
20 min
d
c
2.0
1.5
1.0
0.5
0.0
25 min
25 min
200
300
400
500
600
700
800
200
300
400
500
600
700
800
Wavelength/nm
Wavelength/nm
Figure 5 The photodegration curves (a) and the UV-vis absorption spectra of MB dye irradiated by UV light (λ＝365 nm) for different
durations in the presence of (b) ZnO-1, (c) ZnO-2 and (d) ZnO-3.
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However, the ZnO-3 shows poor photostability, after the
second cycle, the photocatlytic efficiency reduced by
6.4%. From their SEM images, we also find that the
reused ZnO-1 (nine times) still retained the original
structure with a few attached particles (Figure 7b),
which shows that they have high photostability in the
photocatalytic process. However, the morphology of the
7
6
5
4
3
2
1
0
ZnO-1
ZnO-2
ZnO-3
0.20
a
ZnO-1
ZnO-2
ZnO-3
0.15
0
5
10
15
20
25
t/min
0.10
Figure 6 Photocatalytic degradation reaction kinetics of MB
over ZnO-1, ZnO-2 and ZnO-3.
0.05
0.00
photocatalytic activity, and the small size and large BET
surface area will improve photocatalytic activity of the
semiconductors. In our as-prepared ZnO samples,
ZnO-1 with the biggest size has the smallest BET sur-
face areas (9.8126 m²/g), ZnO-2 with the middle size
has the middle BET suface areas (11.9435 m²/g), and
ZnO-3 with the smallest size has the largest BET sur-
face areas (17.4878 m²/g). Therefore, the ZnO-3 should
have the highest photocatalytic activity among the three
samples. However, contrary result is obtained. So we
conclude that their difference in photocatalytic activity
can mainly be attributed to the different types and con-
centrations of defect among these three samples.
1
2
3
4
5
6
7
8
times
It is generally accepted that, when a photon with en-
ergy of hν matches or exceeds the bandgap energy (ΔEg)
of the semiconductor, the electron and hole pairs can be
generated. If a suitable scavenger or surface defect state
is available to trap the electrons or holes, recombination
is prevented and subsequent redox reactions may occur.
As shown in Scheme 1, two impurity levels, which can
enhance the electron and hole pairs separation rate, are
generated according to Zheng’s research.[19] The Vo''
defects work as electron acceptors and can trap the
photogenerated electrons temporarily to reduce the sur-
face recombination of electrons and holes, whereas deep
defects Oi'' serve as photogenerated holes’ shallow trap-
pers to restrain the recombination of photogenerated
electrons and holes. As the redox reactions might occur
on the surface of these defects, it can be considered to
be the active sites of the ZnO photocatalyst. On the ba-
sis of the above discussion, higher concentration of de-
fects results in higher photocatalytic activity (ZnO-1＞
ZnO-2＞ZnO-3).
To investigate the stability of photocatalytic per-
formance in UV region, the ZnO samples were used to
degrade MB dye in eight repeated cycles, and the results
were shown in Figure 7. It was noteworthy that the
photocatalytic performance of ZnO-1 and ZnO-2 exhib-
ited effective photostability under UV irradiation (Fig-
ure 7a), where the photocatalytic efficiency reduced
only 0.5% and 0.2% after eight cycles, respectively.
Figure 7 (a) eight photocatalytic degradation cycles of MB
using ZnO-1 and ZnO-2 under UV, the FESEM images of ZnO-1
(b) and ZnO-2 sample (c) which had been reused after eight cy-
cles , and (d) which had been reused after two cycle.
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Defect-enhanced Photocatalytic Activity of ZnO Micro/nanostructures
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(Figure 7c), and ZnO-3 (two times) changes from po-
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It has been reported that the photocorrosion mainly
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Conclusions
ZnO micro/nanostructures made of nanopartiles with
controlled size, surface defect type and concentration
are successfully prepared through simple thermal treat-
ment methods. With the solvent changing from water to
ethanol, the size, surface defect type and concentration
of as-prepared ZnO samples are obviously different.
The luminescence and photocatalytic activity of these
ZnO samples are mainly dependent on the type and
concentration of oxygen defects formed in the samples.
The sample with higher surface defects concentration
exhibits better photocatalytic activity toward MB deg-
radation under UV light irradiation. The surface defects
are proposed to be the active sites of ZnO photocatalyst
in our system.
Acknowledgments
This work was supported by the National Natural
Science Foundation of China (No. 21101006)
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Chin. J. Chem. 2013, 31, 1557—1563
© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
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