C. LIU et al.: PHOTOCATALYTIC ACTIVITY OF Ce-DOPED ZnO NANOPARTICLES
349–353
PHOTOCATALYTIC ACTIVITY OF Ce-DOPED ZnO
NANOPARTICLES
FOTOKATALITI^NA AKTIVNOST S Ce DOPIRANIH ZnO
NANODELCEV
Chengyou Liu
1
, Jinghua Xu
2
,SuCui
2
, Hai Yu
2
1
Institute of Chemistry and Materials, Hainan Institute of Science and Technology, Hannan 570100, China
2
Department of Physics, Tonghua Normal University, Tonghua, Jilin, China
13943599862@163.com
Prejem rokopisa – received: 2017-11-03; sprejem za objavo – accepted for publication: 2017-11-23
doi:10.17222/mit.2017.188
Semiconductor materials are often used as photocatalysts. In order to increase the photocatalytic ability, rare earths are usually
added to change the properties of materials. In this paper, we report that Ce:ZnO nanoparticles were synthesized and their
photocatalytic properties researched. Wastewater containing methyl blue was used as the object of investigation. Ce:ZnO
samples were synthesized using a simple chemical method. Four samples were obtained: three containing Ce concentrations of
(1, 2 and 3) % (w/%) and one of pure ZnO. Optical properties of the samples were investigated with UV-Vis. X-ray diffraction
(XRD) and field-emission scanning electronic microscopy (FESEM) were employed to examine the chemical composition and
microstructures. It was found that the size of ZnO crystallites is suppressed after Ce-doping. Further, the samples were used as
efficient photocatalysts for a photocatalytic degradation of methyl blue. It was observed that Ce-doped ZnO nanoparticles
exhibited excellent photocatalytic properties. The degradation ability increased with the increasing Ce concentration. For the
sample of3%ofmass fractions of Ce-doped ZnO, a complete photodegradation was observed after 60 min.
Keywords: Ce-doped ZnO, nanoparticles, photocatalyst, methyl blue, photodegradation
Polprevodniki se pogosto uporabljajo kot fotokatalizatorji. Pove~anje kataliti~ne sposobnosti dose`emo tako, da materialom
dodamo elemente redkih zemelj in s tem spremenimo njihove lastnosti. V tem ~lanku avtorji poro~ajo o sintezi nanodelcev ZnO
z dodatkom cerija (Ce) in raziskavi njihovih fotokataliti~nih lastnosti. Kot predmet raziskovanja so izbrali odpadno vodo, ki je
vsebovala metilensko modrilo. Vzorce Ce:ZnO so sintetizirali z enostavno kemijsko metodo. Izdelali so {tiri vzorce; ~isti ZnO,
in tri vzorce, ki so vsebovali (1, 2, in 3) % masnih dele`ev Ce. Opti~ne lastnosti vzorcev so dolo~ili s spektroskopijo, ki temelji
na absorpciji ultravijoli~ne in vidne svetlobe (UV-Vis). Za dolo~itev kemijske sestave in mikrostrukture pa so uporabili
rentgensko difrakcijo (XRD) in vrsti~no elektronsko mikroskopijo na emisijo polja (FESEM). Avtorji ~lanka so ugotovili, da je
rast ZnO kristalitov zavrta po dopiranju s Ce. Nadalje so vzorce u~inkovito uporabili kot fotokatalizatorje za fotokataliti~no
degradacijo metilenskega modrila. Ugotovili so da imajo s cerijem dopirani nanodelci ZnO odli~ne fotokataliti~ne lastnosti.
Stopnja degradacije se je pove~evala s pove~evanjem koncentracije Ce. Pri vzorcu nanodelcev ZnO s 3 mas. % dodatkom Ce je
pri{lo do popolne degradacije po 60 minutah.
Klju~ne besede: s Ce dopiran ZnO, nanodelci, fotokatalizator, metilensko modrilo, fotodegradacija
1 INTRODUCTION
Environmental pollution has become a world prob-
lem. At the international level, the governments and
people pay more attention to our environment. With the
development of science and technology, it became more
important to protect our environment. Among environ-
mental challenges, the most serious problems are the
water and air pollution. With the development of modern
agriculture and industry, more and more wastewater is
poured into rivers, lakes and fields. The rivers are
becoming so dirty that no living beings can live in it. The
water is giving off a terrible smell, which is a threat to
people’s safety. Due to containing a large number of
organic and inorganic compounds, wastewater has
caused serious pollution in the world, especially in the
developing countries. The pollution can harm crops,
cause various diseases, such as digestive-system disease,
respiratory disease, cardio-cerebrovascular disease,
heavy-metal overdose, calculus, etc. Finding ways to
protect our water from pollution has been one of the
most difficult problems in the world. We must accept
that, due to society development, we daily produce
wastewater, polluting the waters. So, it is very important
to control water pollution and purify wastewater.
Up to now, nano-semiconductor oxides such as TiO2
and ZnO have been given a lot of attention in the field of
photocatalysis.
1–6
Their photocatalytic ability to degrade
organic pollutants in water and air were widely studied.
Both of them have many advantages such as a strong
oxidation power, chemical inertness and stability against
chemical corrosion and photocorrosion.
7–12
For the past few years, ZnO has been a hot academic
topic. Due to its excellent optical and electrical pro-
perties, large band gap (3.37 eV) and exciton energy
(60 MeV), ZnO is expected to be important in applica-
tions such as sensing, light-emitting diodes, photo-
catalysts, solar cells and transparent conducting
layers.
13–17
As we know, the photocatalytic efficiency is
relative to the surface chemical reaction and electron-
Materiali in tehnologije / Materials and technology 52 (2018) 3, 349–353 349
UDK 544.526.5:620.3:67.017 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(3)349(2018)

hole recombination. If the course of electron-hole recom-
bination is too fast, the surface chemical reaction will not
occur. The photocatalytic efficiency will be low. So,
finding a way to restrain electron-hole recombination is
the key to increasing the photocatalytic efficiency.
In order to improve optical, electronic and magnetic
properties, multiple elements are doped into ZnO to form
a ternary or a multicomponent. After doping, the band
gap, electron mobility and photocurrent response are
greatly changed. To achieve these changes, the rare earth
Ce doped into ZnO to get a Ce
x
Zn
1–x
O ternary is widely
researched and used. When Ce-doped ZnO is used in a
photocatalytic study, increasing the photocatalytic
efficiency is a problem that must be solved urgently.
18,19
In this paper, we report on our research work in-
cluding the synthesis of pure and Ce-doped ZnO nano-
materials with different concentrations of Ce, and on a
photocatalytic experiment. The photocatalytic-efficiency
dependence on the Ce-concentration was studied.
2 EXPERIMENT
Ce-doped ZnO samples were synthesized using a
simple chemical-precipitation method.
20
In brief, zinc
nitrate hexahydrate (Zn(NO
3
)
2
·6H
2
O) and ammonium
bicarbonate (NH
4
HCO
3
) were chosen as the starting ma-
terial and cerium nitrate hexahydrate (Ce(NO
3
)
3
·6H
2
O)
was used as the dopant source. Deionized water was used
as the solvent. All the analytical-grade reagents were
purchased from Sinopharm Chemical Regent Co., Ltd.,
without further purification. The synthesis method was
as follows: first, Zn(NO
3
)
2
·6H
2
O and Ce(NO
3
)
3
·6H
2
O
were dissolved in deionized water with a molar ratio of
Ce/(Ce+Zn) being x:1 (x = 0.00, 0.01, 0.02 and 0.03).
Second, the amounts of the NH
4
HCO
3
aqueous solution
were transferred into the ZnO/Ce aqueous solution
during constant stirring. Then, white precipitates were
formed in the above mixture solution and washed with
deionized water and alcohol for several times. After
having been dried in an oven at 60 °C for several hours,
the white precipitates were finally annealed under air
atmosphere for3hat450°C.
An X-ray diffractometer (XRD, D/Max-2400) was
used to analyze the crystal structures of synthesized
nanoparticles. The morphology was investigated with a
field scanning electron microscope (FSEM, S-4800).
Optical properties of the samples were studied by
measuring the transmittance in a range of 300–800 nm.
The photocatalytic activity of the light-degraded methyl
blue was investigated.
Experimental details of the photocatalysis are as
follows: 100 mL of wastewater containing methyl blue
was poured into a 200-mL breaker. The concentration of
methyl blue was 40 mg/L. 10 mg of Ce-doped ZnO
nanoparticles with different Ce molar ratios (Ce/(Ce+Zn)
was x:1 (x=0.00, 0.01, 0.02 and 0.03)) were put into the
wastewater solution. All the samples were dispersed
under magnetic stirring for 10 min. A 15-watt UV lamp
was used as the excitation light source, fixed 15 cm
above the treated solution. The decay of methyl blue was
monitored with UV-Vis absorbance measurements, made
at intervals of 20 min. All of the experiments were
carried out against a dark background and at room tem-
perature.
The photocatalytic efficiency can be characterized
with photodegradation percentage defined as #, where A
0
and A are the initial concentration and the concentration
obtained over a period of time, respectively. According
to the Beer Law, the concentration can be expressed with
absorbance.
3 RESULTS AND DISCUSSION
The absorption spectra of a sample were measured at
room temperature in the range of 300–800 nm. The
result is shown in Figure 1. The sample exhibited high
transmittance in the visible regions. With the increasing
Ce concentration, the absorption edge of the sample
moved slightly to the lower-energy direction. However,
as the Ce concentration changed only slightly, the
valence band of the sample could not be changed too
much. For our samples, the experimentally measured
valence-band gap is about 3.36 eV. The result is similar
to the report of Varughese et al.
21
The XRD patters of the samples are shown in Fig-
ure 2. It was found that all the diffraction peaks of ZnO
C. LIU et al.: PHOTOCATALYTIC ACTIVITY OF Ce-DOPED ZnO NANOPARTICLES
350 Materiali in tehnologije / Materials and technology 52 (2018) 3, 349–353
Figure 1: Transmittance spectra of the Ce:ZnO nanoparticles
Figure 2: XRD patterns of the Ce:Zno nanoparticles with Ce concen-
trations of (0, 1, 2 and 3) %

and Ce:ZnO can be indexed as a hexagonal wurtzite
structure, without the characteristic peaks for the other
impurities.
The intensity of diffraction peaks decreases with the
increasing concentration of Ce. The half-width of a peak
becomes wider when the concentration of Ce increases.
Besides, the positions of the corresponding diffraction
peaks of Ce-doped ZnO shift slightly toward the lower
angle.
This phenomenon can be explained as follows: due to
the fact that Ce
4+
(0.092 nm) and Ce
3+
(0.103 nm) are a
bit larger than Zn
2+
(0.074 nm), this shift of diffraction
peaks indicates that the Ce ions were incorporated into
the ZnO lattice, substituting the Zn ion sites. The infor-
mation about the lattice constants of pure and Ce-doped
ZnO obtained from the XRD data can be calculated
using the Scherrer formula, D = 0.89 /( ·cos  ), where
 = 0.154056 nm at the Cu K
 1
radiation,  is the half-
width of the diffraction peak and  is the Bragg angle.
In our case, the crystallite size is estimated to be
about 35–50 nm. It is found that the crystallite size
decreases with the increasing Ce concentration. This can
be explained as follows: In the course of the crystallite
growth, the particle size is dependent on the particle
boundary motion. When increasing the Ce concentration,
the lattice agglomerates and the distortion caused by the
addition of Ce decreases the particle mobility and, there-
fore, inhibits the growth of the ZnO crystallites.
The morphology of pure and Ce-doped ZnO was
observed with FSEM. The images of pure and Ce-doped
ZnO are shown in Figure 3. The usual nanoparticles
with a diameter of about 35–50 nm (Figure 3) were ob-
served, which accords with the results calculated from
the XRD data.
It was found that the size of nanoparticles decreases
with the increasing Ce concentration.
The absorbance changes of methyl blue with the UV
irradiation time are shown in Figure 4.
The absorption band at a wavelength of about 650 nm
decreases with the increasing irradiation time for all the
Ce-doped ZnO photocatalysts. After about 80 min of UV
irradiation, a complete photodegradation is observed for
the Ce (3 %) doped ZnO sample. After 60 min of UV
irradiation, the percentage of the photodegradation of
methyl blue was measured for the four samples. The
result is shown in Figure 5. It is very clear that the
photocatalytic effect increases with the increasing Ce
concentration.
Using the 650-nm absorption intensity related to the
concentration of methylene blue, variations A
0
and A
versus the irradiation time are shown in Figure 6.
C. LIU et al.: PHOTOCATALYTIC ACTIVITY OF Ce-DOPED ZnO NANOPARTICLES
Materiali in tehnologije / Materials and technology 52 (2018) 3, 349–353 351
Figure 5: Percentage of photodegradation vs Ce-doping after 60 min
of UV irradiation
Figure 3: FSEM images of Ce-doped ZnO, Ce concentrations of:
a)0%,b)1%,c)2%andd)3%
Figure 4: Absorbance changes of methyl blue with UV irradiation
time for: a) pure ZnO, b)1%C e ,c )2%C ea n dd )3%Ce:ZnO
nanoparticles as the photocatalysts
Figure 6: A0/A as a function of irradiation time for methyl blue
photodegradation using Ce:ZnO as the photocatalyst

The variation in the percentage of photodegradation
of methyl blue with irradiation time was investigated and
shown in Figure 7.
The experimental results show that the absorbance of
methyl-blue solution decreases with the increasing UV
irradiation time. It takes about 120 min for ZnO to
become completely photodegraded. However, for the
Ce-doped ZnO samples, the time of complete photo-
degradation reduces to 98 min (1 % Ce), 85 min (2 %
Ce) and 75 min (3 % Ce).
The experiments prove that Ce-doped ZnO improves
the photocatalytic ability. As we know, the mechanism of
photocatalysis is based on the migration of electrons and
holes to the surfaces of catalytic nanoparticles upon an
UV irradiation and their subsequent participation in a
redox reaction with the adsorbed methylene blue in the
solution. The reason for enhanced photocatalysis can be
explained as follows: Firstly, when the Ce
4+
ions are
doped into the ZnO lattice, the Zn site is replaced by
Ce
4+
to form electron-donor defects. In order to maintain
electronic neutrality, Ce ions must release electrons into
the conduction band, which increases the concentration
of free electrons. Hence, the increased transport of
electrons results in a higher response. Secondly, Ce
4+
can
also trap the conduction-band electrons to eliminate or
reduce the probability of electron-hole recombination.
Thirdly, the oxygen vacancy after doping Ce on photo-
lytic surfaces can serve as a trap for the electrons from
the conduction band. Fourthly, because Ce
4+
ion radius is
larger than that of Zn
+
, the average crystalline of
Ce-doped ZnO is smaller than that of pure ZnO. This
makes the specific surface increase and provide for more
active centers for oxygen molecules adsorbed on the
surface, which increases the electron transport. All of
these can efficiently enhance the transfer of photo-
generated electrons to participate in the redox reaction,
forming OH free radicals causing methylene-blue
degradation.
4 CONCLUSION
Pure ZnO and various Ce:ZnO samples were pre-
pared with a simple chemical-precipitation method. The
results of XRD and FESEM clearly revealed the
microstructures of the samples with a size of about
35–80 nm. Using methylene blue as the reactant,
photocatalytic properties of ZnO and Ce-doped ZnO
were investigated. The experimental results show that
Ce-doped ZnO is more photocatalytically active than
pure ZnO. The ability of photodegradation increases
with the increasing Ce concentration. For pure ZnO and
3 % Ce-doped ZnO, a complete degradation was ob-
served in 120 min and 75 min, respectively. Furthermore,
this work implies that suitable Ce-doping can improve
the photocatalytic ability of a ZnO NP-based photo-
catalyst. Moreover, more research will be carried out
within our forthcoming work.
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