Advances in Chemical Engineering and Science, 2011, 1, 9-14
doi:10.4236/aces.2011.11002 Published Online January 2011 (http://www.SciRP.org/journal/aces)
Investigation of Photocatalytic Degradation of Methyl
Orange by Using Nano-Sized ZnO Catalysts
Changchun Chen1*, Jiangfeng Liu1, Ping Liu2, Benhai Yu1
1College of Physics and Electronics Engineering, XinYang Normal University, XinYang, China
2College of Materials Science and Engineering, Nanjing University of Techno logy, Nanjing, China
Received December 30, 2010; revised January 19, 2011; accepted January 22, 2011
Abstract
Nano-sized ZnO catalysts were prepared by a direct precipitation method under the optimal conditions (cal-
cination of precursors at 550˚C for 120 min). The as-synthesized ZnO catalysts were characterized by X-ray
diffraction (XRD), scanning electron microscope (SEM) and UV-Vis spectroscopy. The photocatalytic prop-
erties of ZnO nanoparticles were investigated via methyl orange (MO) as a model organic compound under
UV light irradiation. The influence of operating parameters on MO degradation including the amount of ZnO
catalysts, pH value of solutions, and the photodegradation temperature was thoroughly examined. In addition,
the kinetic process of photocatalytic degradation of MO using nano-sized ZnO catalyst was also examined,
and the degradation of MO follow the first order kinetics.
Keywords: ZnO Nanoparticles, Photocatalytic Degradtion, Methyl Orange
1. Introduction
Semiconductor photocatalysts such as TiO2 and ZnO
nano-particles have attracted much attention in recent
years due to their various applications to the photocata-
lytic degradation of organic pollutants in water and air
and dye sensitized photovoltaic solar cell [1-3]. Among
these semiconductor photocatalysts, TiO2 is the most
commonly used owing to its stable, harmless and inex-
pensive properties. However, two typical defects includ-
ing only exciting by high energy UV irradiation and a
low quantum yield rate resulted from a low rate of elec-
tron transfer to oxygen and a high rate of recombination
between excited electron/hole pairs, limit the photo-
oxidation rate of TiO2 nanoparticles. In order to improve
the photocatalytic efficiency of TiO2 nanoparticles, most
studies have been focused on the modification of TiO2
doped by metal ions, especially transition metal ions,
which make it possible for TiO2 to absorb visible light by
increasing the charge separation [4,5]. In addition, com-
bination of different kinds of semiconductor photocata-
lysts also is a promising way to improve the photocata-
lytic efficiency [6]. Recently, ZnO nanoparticles appear
to be a suitable alternative to TiO2 nanoparticles used for
the photodegradation of pesticide carbetamide [7], herbi-
cide triclopyr [8], pulp milling bleaching wasterwater [9],
2-phenylphenol [10], phenol [11], reactive blue 19 [12],
and acid red 14 [13]. The substitution of TiO2 by ZnO
used for photo-degradation is ascribed to the
photo-degradation mechanism of ZnO being similar to
that of TiO2 [3,14]. K. Gouvea et al. has confirmed that
ZnO exhibits a better efficiency than TiO2 in photocata-
lytic degradation of some reactive dyes in aqueous solu-
tion [15]. As we known, ZnO nanoparticles can be syn-
thesized by various approaches including sol-gel proc-
essing, homogeneous precipitation, mechanical milling,
organometallic synthesis, microwave method, spray py-
rolysis, thermal evaporation and mechanochemical syn-
thesis. However, ZnO nanoparticles fabricated by the
abovementioned methods are prone to aggregate due to
the large surface area and high surface energy. In order to
improve the dispersion, it is necessary to modify the sur-
face of ZnO nanoparticles. Some researches have re-
vealed several physical and chemical methods for modi-
fying the surface of ZnO nanoparticles. The chemical
surface modification, which can be classified as surface
grafting and esterification, is the most promising method
because of the strong covalent bond between the surface
modified particles and polymer chains. In previous re-
searches, the ZnO nanoparticles were ever modified by
SiO2 [16], PMMA [17] and PSt [18], and the influence of
particles on the mechanical properties of polymer matrix

C. C. CHEN ET AL. 10
was studied.
In the present article, the nano-sized ZnO catalysts
were prepared by a direct precipitation method under the
optimal conditions (calcination of precursors at 550˚C
for 120 min). The surfaces of ZnO nanoparticles fabri-
cated by a direct precipitation method are not modified
by SiO2, PMMA and PSt. The effect of various experi-
ment parameters such as the amount of ZnO catalyst, pH
of solutions, the photodegradation temperature, and the
initial concentration of MO on the degradation of the
MO has been thoro ughly examined with an aim to qua n-
titatively probin g the regulation of photocatalytic activity
of ZnO nano-sized particles fabricated by a direct pre-
cipitation method.
2. Experimental
2.1. Preparation and Characterization
Nano-sized ZnO particles in this study were prepared by
a direct precipitation method. Zn(NO3)2, (NH4)2CO3,
ethanol and de-ionized water were used in the experi-
ments. All the reagents used in this study were the ana-
lytical grade. The synthetic procedures of nano-sized
ZnO particles were also thoroughly introduced elsewh ere
[19]. As is reported in our recent study [20], the nano-
sized ZnO particles fabricated by a direct precipitation
method via the calcination of precursors at 550˚C for
120 min have the optimal photocatalytic activity. As a
result, the nano-sized ZnO particles synthesized by the
calcination of precursor at 550˚C for 120 min were used
as catalysts in this study. The specific surface area of
nano-sized ZnO particles synthesized by the calcination
of precursor at 550˚C for 120 min was determined by
nitrogen absorption Brunauer-Emett-Teller (BET) method.
The BET measurements were performed on a Micromer-
itics ASAP 200 instrument. The 26.58 m2/g of BET spe-
cific surface area was obtained. The structural properties
of these nano-sized ZnO particles were investigated by
the -2 method of X-ray diffraction (XRD) with a Cu
K1 ( = 0.154 nm) radiation at 40 kV and 30mA using a
multipurpose XRD system (PANalytical). The morphol-
ogy and particle size of these nano-sized ZnO particles
were also analyzed by a scanning electron microscope
(SEM, JXA840). SEM photographs for the nano-sized
ZnO particles were recorded (LEO 435) at 30 kV from
samples covered with a thin gold film.
2.2. Photocatalytic Degradation
The MO solutions in concentrations varied from 5 to 50
mg/L (5, 10, 20, 30, 40 and 50) were prepared through
dissolving MO powders in ultra pure water, respectively.
The concentration of MO solution was determined by
measuring the value at approximately 464 nm using a
UV-Vis spectrophotometer 756PC (China). The reaction
suspensions were prepared by adding nano-sized ZnO
particles into the abovementioned MO solutions. The
suspensions were ultrasonically sonicated for 20 min and
magnetically stirred in dark for 45 min to ensure an ad-
sorption/desorption equilibrium. The reaction suspensions
containing MO and nano-sized ZnO photocatalyst were
irradiated by a 300 W high-pressure mercury lamp with
continuous stirring. In addition, the pH of MO solution
adjusted by adding NaOH or HCl solutions was measured
using Elico LI120. Absorbance measurements were also
recorded in the range of 200-600 nm, using a UV-vis
spectrophotometer. The photocatalytic degradation effi-
ciency of the MO solutions was calculated with the fol-
lowing formula: 0
0
100
AA %
A


 , where A0 is the
absorbance of MO dye solution before the illumination,
A is the absorbance of MO solutions in suspension after
time t.
3. Results and Discussion
The XRD pattern of ZnO nanoparticles synthesized via
the calcination of precursor at 550˚C for 120 min is
showed in Figure 1. It could be seen that the diffraction
peaks were more inten siv e and narr ower imp lyin g a good
crystalline nature of the as-synthesized ZnO product, and
all of the peaks can be well indexed to hexagonal phase
ZnO reported in JCPDS card (NO.36-1451, a = 0.3249 nm,
c = 0.5206 nm). Diffraction peaks rel ated to the im purities
were not observed in the XRD pattern, confirming the
high purity of the synthesized product. The average
crystalline size (L) of the nano-sized ZnO particles can
be calculated from the Debye-Scherrer formula [21]:
20 30 40 50 60 70
0
1000
2000
3000
4000
5000
6000
7000
(201)
(112)
(200)
Intensity/(a.u.)
2/(o)
(100)
(002)
(101)
(102)
(110) (103)
Figure 1. XRD patterns of nano-sized ZnO particles
synthesized by a direct precipitation method with the
calcination of precursor at 550˚C for 120 minutes.

C. C. CHEN ET AL. 11
089.
Lcos



, where L is the crystalline size (in nm),  is
the wavelength (in nm),  is the full width at half maxi-
mum intensity (FWHM--in radian), and  is the Bragg
diffraction angle (). The average crystalline size of ZnO
product synthesized was figured out from the De-
bye-Scherrer formula to be about 17.3 nm. Typical SEM
image of the ZnO nano-particles calcinated at 550˚C for
120 min is shown in Figure 2. The ZnO nanoparticles are
reasonably uniform in size, and the morphology of the
ZnO nano-particles takes on pseudo-spherical shape. The
average size of the ZnO nanoparticles is approximately
20 nm, which is consistent with the crystallite size esti-
mated from t he XR D anal y si s shown in Figure 1.
The photocatalytic activity of ZnO nanoparticles was
assessed according to the photo-degradation kinetics of
MO solutions. The dependence of the amount of ZnO
nanoparticles on the photodegradation efficiency of 1l
MO solutions with an initial concentration of 10 mg/l at
the neutral pH value was shown in Figure 3. Experi-
ments show that the photo-degradation efficiency of MO
solution increases with the increase of amount of ZnO
photocatalysts, after which it reaches the highest value of
catalyst amount (2.5 g/l), and then decrease. This obser-
vation can be explained in terms of availability of active
sites on the surface of catalyst and the penetration
of UV light into the suspension [22]. The total active
surface area of ZnO nano-particles increases with the
increase of catalyst dosage. On the other hand, with an
increase in the turbidity of th e MO suspension, the pene-
tration of UV light will decrease as a result of the in-
creased scattering effect.
The effect of pH in the range of pH 3.0 – 11.0 on the
photo-catalytic degradation rate of MO was investigated.
The photodegradation efficiency of 1l MO solution with
an initial concentration of 10 mg/l (ZnO concentration =
2.5 g/l) as a function of pH of MO solutions were shown
in Figure 4. The strong effect of pH on the photodegra-
dation efficiency of MO solution was observed. The
highest removal rate of MO was obtained at a pH of 7.0.
Figure 2. The SEM image of nano-sized ZnO particles
synthesized by a direct precipitation method with the
calcination of precursor at 550˚C for 120 minutes.
0 20406080100120
-60
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
100
012345
40
50
60
70
80
90
100
photodegradation efficiency (%)
ZnO co ncentration (g/L)
Photodegradation efficiency (%)
Irradiation tim e (min)
0.4g /L
1g/L
2g/L
2.5g /L
3g/L
5g/L
Figure 3. The effect of ZnO amount on the photodegradation efficiency of MO solutions.

C. C. CHEN ET AL. 12
0 20406080100120
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
100
24681012
65
70
75
80
85
90
95
100
Photod e gr adation efficien c y(%)
pH of M O solution
Photodegradation efficiency (%)
Irradiation time (min)
3
5
6
7
9
11
Figure 4. Effect of pH on the photodegradation efficiency of methyl orange at different irradiation times.
However, when TiO2 nanoparticles were utilized to
catalyze the photo-degradation of MO, the higher re-
moval rate of MO was obtained at lower pH values,
which is reported in literature [23]. The effect of pH on
the photodegradation of MO using ZnO catalysts can be
explained as follows. As is pointed out by E. Topoglidis
et al. [24], the point of zero charge (PZC) of nano-sized
ZnO particle is about a pH of 9.30. Above the pH value,
the surfaces of nano-sized ZnO particles are negatively
charge. Below the pH value, the surfaces of nano-sized
ZnO particles are positively charged. Methyl orange
molecules have negative charges in a wide pH value
range. Therefore, when the MO solution pH value is be-
low the PZC, the MO anions should be readily adsorbed
on the surfaces of nano-sized ZnO particles.
As is described by H. Tian et al. [1], the photo-
catalytic degradation efficiency of MO solutions with
nano-sized TiO2 catalysts changes with the temperature
variation of the MO solutions. Hence, the effect of MO
solution temperature on the degradation efficiency of
MO catalyzed by ZnO nano-particles was also discussed
in the range from 20 to 70˚C at 10˚C intervals in this
study. It can be seen in Figure 5 that at the first stage,
the photo-degradation ratios of MO ascend with the in-
crease of solution temperature from 20˚C to 50˚C, and
begin to decrease at a temperature beyond 50˚C. How-
ever, in many cases, the higher the temperature is, the
quicker the chemical reaction rate does. The experimen-
tal results shown in Figure 5 can be explained as follows.
At an elevated temperature, the adsorbability of nano-sized
ZnO particles to MO becomes low. The lower adsorbability
20 30 40 50 60 70
55
60
65
70
75
80
85
90
95
Photodegradation efficiency (%)
photocatalytic degradation tmep eratu re ( 0C)
Figure 5. Influence of solution temperature on photode-
gradation efficiency of methyl orange( MO initial concen-
tration of 10 mg/l, ZnO amount of 2.5 g/l, solution acidity of
pH 7.0 and total volume of 1l.
of MO will weaken the direct hole oxidation on the sur-
face of nano-sized ZnO catalysts.
The photocatalytic decomposition of MO organic pol-
lutants on the surface of ZnO nano-particles also follow a
pseudo first-order kinetic law, and can be expressed as
0
C
ln kt
C




 , where C and C0 are the reactant con-
centration at time t = t and t = 0, respectively, k and t are
the pseudo-first-order rate constant (reaction rate con-
stant) and time, respectively [25]. The relationships be-
tween –ln(C/C0) and irradiation time (Reaction time) are
Photocatalytic degradation temperature (oC)

C. C. CHEN ET AL. 13
shown in Figure 6. It is obviou s that there exists a linear
relationship between –ln(C/C0) and irradiation time. The
pseudo-first-order rate constant k and linear regression
coefficient (R) for MO solutions with different initial
MO concentrations are summarized in Table 1, respec-
tively.
According to the Langmuir-Hinshelwood model, the
fact that the decrease of reaction rate constant with the
increase of the initial concentration of MO solutions ob-
tained from Table 1 could be explained as follows. The
organic MO is firstly adsorbed on the surface of
nano-sized ZnO particles, and then the photocatalytic
decomposition takes place under UV irradiation. With
the increase of the initial MO concentrations, the MO
molecules congregate on the surface of nano-sized ZnO
catalysts. However, quenching between these excited
MO molecules irradiated by UV will take place [26]. The
quenching probability could also increase with the in-
crease of the initial MO concentrations. Consequently,
the photocatalytic efficiecncy of MO solutions is de-
creased with the increase of the initial MO concentra-
tions.
4. Conclusions
The nano-sized ZnO catalysts were prepared via the
0 20406080100120
0
1
2
3
4
5
6
7
85mg/L
10mg/L
30mg/L
40mg/L
50mg/L
ln(C0/Ct)
Irradiation time (min)
Figure 6. Kinetics of the methyl orange degradation cata-
lyzed by ZnO nanoparticles.
Table 1. Reaction rate constant of MO photocatalytic de-
composition with different initial concentration.
Experiments ZnO concentration
(g/L)
Initial
concentration
of MO
solutions
(mg/L)
k
(constant )R2
1 2.5 5 0.063812 0.9999
2 2.5 10 0.024304 0.9999
3 2.5 30 0.016148 0.9975
4 2.5 40 0.013099 0.9997
5 2.5 50 0.007942 0.9982
calcination of precursors at 550˚C for 120 min. A series
of experiments were carried out to study the effects of
the amount of photocat al y st , a pH value of M O soluti o ns,
and reaction temperature on the photocatalytic degrada-
tion efficiency of MO solutions. The photodegradation
efficiency increases with the increase in the amount of
ZnO photocatalyst. Once the amount of ZnO photocata-
lyst is beyond the highest value of catalyst amount (2.5 g.
l-1), the photo-degradation efficiency of MO solution
begin to decrease. The photocatalyst capacity of ZnO
towards the methyl or ange solutions strongly depend s on
the pH of MO solutions, and the MO solu tions catalyzed
by ZnO nano-particles has good photodegradation effi-
ciency at a central pH value. The photodegradation ratios
of MO solutions ascend with the increase of the MO so-
lution temperature from 20 to 50˚C, and they begin to
decrease at temperature beyond 50˚C.
5. Acknowledgements
This research was partially supported by the Planned
Science and Technology Project of Henan Province,
China, under Grant No. 082300410050.
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