Photocatalytic degradation of methyl orange using ZnO nanopowders synthesized via thermal decomposition of oxalate precursor method

Article abstract of DOI:10.1016/j.physb.2013.02.005

ZnO nanoparticles were synthesized by thermal decomposition of oxalate precursor method. The nanopowders were characterized using powder X-ray diffraction technique and scanning electron microscopy and sample was found to have hexagonal wurtzite structure of ZnO. Average crystallite size of ZnO was found to be ～27 nm. The photocatalytic activity of ZnO was evaluated by using methyl orange (MO) as probe molecule. It was inferred from control experiments that presence of both ZnO and UV light is necessary for photodegradation. From photocatalytic experiment it was observed that MO is completely degraded in 80 min when the amount of catalyst is 1 g/L and initial concentration of MO is 0.03 mM. The effect of dye concentration, catalyst loading and solution pH on photodegradation rate was also investigated. It was found that optimal conditions for photodegradation of MO are 1 g/L catalyst at a solution pH 9. Photodegradation of MO was found to follow pseudo-first order kinetics. Langmuir-Hinshelwood model was also used to describe the photodegradation process.

Full text of DOI:10.1016/j.physb.2013.02.005

Author’s Accepted Manuscript
Photocatalytic degradation of Methyl Orange using
ZnO nanopowders synthesized via thermal decom-
position of oxalate precursor method
Japinder Kaur, S. Bansal, Sonal Singhal
www.elsevier.com/locate/physb
PII:
S0921-4526(13)00081-1
DOI:
Reference:
http://dx.doi.org/10.1016/j.physb.2013.02.005
PHYSB307556
To appear in:
Physica B
Received date: 10 December 2012
Revised date: 29 January 2013
Accepted date: 3 February 2013
Cite this article as: Japinder Kaur, S. Bansal and Sonal Singhal, Photocatalytic
degradation of Methyl Orange using ZnO nanopowders synthesized via thermal
decomposition of oxalate precursor method, Physica B, http://dx.doi.org/10.1016/
j.physb.2013.02.005
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TITLE: Photocatalytic degradation of Methyl Orange using ZnO nanopowders
synthesized via thermal decomposition of oxalate precursor method
AUTHORS: Japinder Kaur, S. Bansal and Sonal Singhal
CORRESPONDING AUTHOR: Sonal Singhal
Assistant Professor,
Department of Chemistry,
Panjab University,
Chandigarh, India - 160 014
Ph. No. +91-172-2534421(o)
+91-09872118810(m)
Fax No. +91-172-2545074
E-mail: sonal1174@gmail.com
DECLARATION:
The article is original, has been written by the stated authors who are all aware of
its content and approve its submission, has not been published previously, it is not
under consideration for publication elsewhere, no conflict of interest exists, or if
such conflict exists, the exact nature of the conflict must be declared and if
accepted, the article will not be published elsewhere in the same form, in any
language, without the written consent of the publisher.
Manuscript contains:
Manuscript
Tables
11 pages
3
Figures
11
0

Photocatalytic degradation of Methyl Orange using ZnO nanopowders synthesized via
thermal decomposition of oxalate precursor method
Japinder Kaur1, S. Bansal2 and Sonal Singhal1*
1Department of Chemistry, Panjab University, Chandigarh-160 014
2Dept. Sci. Tech., New Delhi
*sonal1174@gmail.com
Abstract
ZnO nanoparticles were synthesized by thermal decomposition of oxalate precursor
method. The nanopowders were characterized using Powder X-Ray Diffraction Technique and
Scanning Electron Microscopy and sample was found to have hexagonal wurtzite structure of
ZnO. Average crystallite size of ZnO was found to be ~ 27 nm. The photocatalytic activity of
ZnO was evaluated by using Methyl Orange (MO) as probe molecule. It was inferred from
control experiments that presence of both ZnO and UV light is necessary for photodegradation.
From photocatalytic experiment it was observed that MO is completely degraded in 80 min when
the amount of catalyst is 1g/L and initial concentration of MO is 0.03 mM. The effect of dye
concentration, catalyst loading and solution pH on photodegradation rate was also investigated. It
was found that optimal conditions for photodegradation of MO are 1g/L catalyst at a solution pH
9. Photodegradation of MO was found to follow pseudo-first order kinetics. Langmuir-
Hinshelwood Model was also used to describe the photodegradation process.
Key Words: Photocatalytic degradation, Zinc oxide, Methyl Orange, Oxalate Precursor method
1

1. Introduction
Methyl Orange (MO) is an important example of azo dye and is widely used as a coloring
agent in textile, food, leather and pharmaceutical industries. MO can also be used as coloring
agent for the detection of hydrogen gas and for the determination of Itopride hydrochloride [1,
2]. But discharge of MO from dye industries into water bodies is causing many health hazards.
MO is toxic, mutagenic and carcinogenic [3-5]. MO is not readily biodegradable and thus special
methods are required for its degradation. Some of the methods are Physical Methods
(coagulation, reverse osmosis, membrane filtration), chemical methods (reduction, oxidation, ion
exchange, complexometric methods) and biological methods (aerobic, anaerobic) [6]. Recently,
Advanced Oxidation Processes (AOP) is gaining attention for the removal of organic pollutants
from water [7-10]. These processes involve the treatment of wastewater through the generation
of hydroxyl radicals [11]. These radicals are strong oxidizing agents and thus oxidize pollutant to
CO2 and H2O. This is an extremely fast process and no solid waste is generated. Generally, these
are carried out in the presence of ozone (O3), hydrogen peroxide (H2O2) and/or UV light. In
these, semiconductor metal oxides, such as ZnO act as catalyst for the degradation of pollutant.
ZnO is an n-type semiconductor with a wide band gap of 3.37 eV and a large exciton
binding energy of 60 meV [12]. Due to these properties, it has found applications in many fields,
such as in solar cells, gas sensors, photoelectrochemical cells, antibacterial activity and
photocatalysis [13-18]. ZnO can be synthesized with wide range of morphologies like nanorods,
nanobelts, nanowires, nanotubes, and whiskers using different methods such as ball-milling, sol-
gel process, hydrothermal, spray pyrolysis, chemical vapor deposition, etc [19-24].
A large number of reports are there on the photocatalytic degradation of Methyl Orange
using ZnO as a catalyst in the presence of UV light [25-33]. Zhu et al. [29] synthesized ZnO rod-
2

assembled microspheres using microwave-assisted hydrothermal method and observed that
complete degradation of MO takes place in 9 h. ZnO powders with various size scale were
synthesized by Wang et al. [30] via thermal evaporation method and chemical deposition
method. It was observed that 50 nm ZnO powder degraded 80% MO in 2 h. Hong et al. [31]
prepared Fe3O4/ZnO core/shell magnetic nanoparticles by co-precipitation method and noticed
83% degradation of MO after 6 h. Cu-doped ZnO nanoparticles were prepared by Fu et al. [32]
and 88% degradation of Methyl Orange was observed in 4h. ZnO nanorods on indium-tin oxide
substrate were synthesized by Zheng et al. [33] via thermal evaporation method and 71%
degradation of MO was observed after 2.5 h.
Although ZnO nanoparticles have been investigated for photodegradation of MO, but the
time period required for the degradation of MO is very high. The present work is an attempt to
reduce the time required for the degradation of MO. This work deals with the synthesis of ZnO
nanopowders by thermal decomposition of oxalate precursor, which is an economical, facile and
low temperature solution route. Then photocatalytic activity of synthesized ZnO is evaluated by
studying the photodegradation of MO. Also a systematic investigation has been carried out to
find the optimum conditions for MO degradation by varying the dye concentration, catalyst
loading and solution pH.
2. Experimental
2.1 Synthesis
Nanocrystalline powders of ZnO were synthesized by thermal decomposition of oxalate
precursor. This method is an economical and eco-friendly method without the involvement of
solvents. Oxalate precursor was prepared by mixing aqueous solutions of zinc chloride and
oxalic acid. The mixture was then stirred for 2 h. This resulted in the formation of zinc oxalate
3

precipitates which were filtered and washed thoroughly with distilled water and then dried. The
zinc oxalate was used as precursor for the synthesis of ZnO. Fig. 1 shows the flow chart
depicting the synthesis of ZnO nanopowders.
2.2 Physical Characterization
To determine the temperature at which zinc oxalate decomposes to zinc oxide, the oxalate
precursor was subjected to Thermogravimetric Analysis upto 1000 °C at the heating rate of 10
°C/min using TA, SDT Q600. X-Ray Diffraction (XRD) patterns were recorded using
PANalytical’s X’Pert PRO spectrophotometer equipped with Cu-Kα radiation (λ= 1.5404 Ǻ) in
2θ range from 10° to 100°. The crystallite size was calculated by employing Debye-Scherrer
equation. The Scanning electron micrographs were taken on JEOL, JSM-6610LV. The UV-Vis
spectra were recorded on JASCO, V-530 spectrophotometer.
2.3 Photocatalytic Measurements
The photocatalytic activity of zinc oxide powder was evaluated by measuring
photocatalytic degradation of MO in water under the illumination of UV light. The molecular
structure of MO is shown in Fig. 2. The experiment was carried out by dispersing 100 mg of
photocatalyst in 100 mL MO aqueous solution having concentration of 0.03 mM, after which, it
was magnetically stirred in dark for 30 min to ensure the establishment of adsorption/desorption
equilibrium between MO molecules and the surface of the photocatalyst. Subsequently, the
suspension was irradiated with UV light. 4 mL samples were withdrawn from the above
suspension at different time intervals and it was filtered through a syringe filter to completely
remove the catalyst particles. The obtained samples were then analyzed using UV-Vis
spectroscopy by recording variations in absorbance at wavelength of 463 nm. The percentage of
degradation was calculated using the following equation (eq. 1) [34]:
4

Co −Ct
Degradation(%) =
×100
(1)
Co
where Co is the initial dye concentration and Ct is the dye concentration at certain reaction time t
(min). Unless otherwise specified, all the experiments were carried out at natural pH of MO
(8.06). Effect of variation of initial dye concentration (0.01-0.09 mM), catalyst loading (0.25-
2.00 g/L) and pH of the solution (pH 5-11) on rate of photocatalytic degradation was also
studied. The pH of solution was adjusted to desired level by adding NaOH and HNO3 before the
addition of photocatalyst. For comparison, blank experiment was carried out in the absence of
UV light and in presence of the photocatalyst. Another experiment was performed in the
presence of UV light and in the absence of photocatalyst.
3. Results and Discussion
3.1 Thermogravimetric Studies
The TGA curve for the precursor is presented in Fig. 3 and it exhibits two discernible
stages of weight loss. The first weight loss of 20% between 90 and 100 °C corresponds to the
elimination of two molecules of water from zinc oxalate dihydrate (theoretical value of 19%) to
form anhydrous zinc oxalate. The second weight loss of 40% between 370 and 400 °C is due to
decomposition of anhydrous zinc oxalate to zinc oxide with the elimination of CO2 and CO
(theoretical value of 38%) followed by formation of clear plateau between 400 and 1000 °C.
Since there was no appreciable weight loss above 400 °C, the oxalate precursor was annealed at
450 °C to obtain ZnO powders. The decomposition of zinc oxalate dihydrate can be expressed as
the following reaction:
ZnC2O4.2H2O Æ ZnC2O4 + 2H2O
ZnC2O4 Æ ZnO + CO + CO2
(2)
(3)
5

3.2 Powder X-Ray Diffraction Studies
Fig. 4 shows XRD pattern of annealed sample. All the diffraction peaks can be indexed to
the hexagonal wurtzite structure of zinc oxide with lattice constants a = 3.2459 Å and c = 5.215
Å. The sharp and intense peaks indicate that sample is highly crystalline. The average crystallite
size was found to be ~ 27 nm by applying Debye Scherrer equation (eq. 4) [35]:
0.9λ
d =
βcosθ
(4)
where d is the average crystallite size, λ = 1.541 Å (wavelength of radiation used), θ is the Bragg
angle and β is the angular width of the diffraction peak at the half maximum (FWHM) on the 2θ
scale.
3.3 Scanning Electron Microscopy
The surface morphology of synthesized ZnO powders was investigated using SEM (Fig.
5). It can be seen that ZnO powder is nanocrystalline and exist in the form of bundles. The
bundles are composed of aggregates of nanoparticles. This aggregation of nanoparticles is due to
large surface area to volume ratio and high surface energy.
3.4 Photocatalytic Activity
3.4.1 UV-Vis spectra study
The photocatalytic activity of ZnO powder was examined by degradation of MO. Fig. 6
shows the UV-Vis spectra of aq. solution of MO (0.03 mM) in presence of ZnO (1 g/L) at
different irradiation times (inset shows the photograph of MO solution). It can be seen from the
graph that the maximum absorbance at 463 nm decreased gradually and finally disappeared after
80 min indicating the complete degradation of dye. However when the same experiment was
6

carried out in the absence of ZnO nanoparticles, only 9% degradation was observed even after
irradiation for 5 hr. In the presence of ZnO, but in the absence of UV light, 21% degradation was
observed after 5 hr, which could be due to the adsorption of dye on the surface of ZnO. This
observation indicated that presence of both catalyst and UV light are required for an effective
degradation of dye.
3.4.2 Mechanism
When ZnO is irradiated by light with energy greater than or equal to its band gap, an
electron in the valence band can be excited to the conduction band, with the simultaneous
generation of hole in the valence band (eq. 5). These photogenerated electron-hole pair can either
recombine (eq. 6) or interact separately with other molecules. The holes in the valence band can
react with water on the surface of ZnO or hydroxide ions to form highly reactive hydroxyl
radicals (˙OH) (eq. 7 and 8) while electrons are accepted by adsorbed oxygen to form superoxide
radical anion (O2˙¯) (eq. 9). This radical may form organic peroxides or H2O2 in the presence of
organic scavenger (eq. 10 and 11). The hydroxyl radical is a powerful oxidizing agent and
attacks the dye molecule to give the oxidized product (eq. 12). The reactions are summarized as
below [36]:
+
ZnO + hν Æ h + e¯
(5)
(6)
(7)
+
h + e¯ Æ heat
+
+
H2O + h Æ H + ˙OH
+
h + OH¯ Æ ˙OH
(8)
(9)
e¯ + O2 Æ O2˙¯
7

+
O2˙¯ + HO2˙ + H Æ H2O2 + O2
(10)
O2˙¯ + organic species Æ organic species OO˙
˙OH + organic species Æ oxidized product
3.4.3 Effect of initial dye concentration
(11)
(12)
The effect of initial MO concentration on the photocatalytic degradation was studied by
varying the concentration of MO from 0.01 to 0.09 mM, at a constant catalyst loading of 1 g/L
for 45 min. It was found that degradation percentage is strongly influenced by the initial dye
concentration and decreased from 98.1% to 50% with increase in concentration of dye from 0.01
to 0.09 mM (Fig. 7). The rate of degradation is related to the available catalyst surface for the
generation of electron-hole pair, which in turn generates hydroxyl radicals. In this case, the
amount of catalyst is kept constant and thus number of hydroxyl radicals generated remains
same, while dye concentration is increased. Thus the number of hydroxyl radical which attack
dye molecule decreases and thus rate of degradation decreases.
The kinetics of photocatalytic degradation of MO can be depicted by first-order equation
(eq. 13) [37]:
⎛
⎜
⎝
⎞
⎟
⎠
Co
Ct
In
= Kapp t
(13)
where Kapp is the pseudo-first order rate constant (min-1), Co is initial concentration, Ct is the
concentration of MO at time t (min). Fig. 8 shows the plot of ln(Co/Ct) vs. time at different initial
concentration of dye at fixed catalyst loading of 1 g/L. Linear plots were observed (with R2
values higher than 0.9) which attested that photodegradation of MO obeys pseudo-first order
kinetics. The calculated pseudo-first order rate constant and corresponding R2 values are
presented in Table 1. It can be seen that with increase in dye concentration from 0.01 mM to 0.09
8

mM, degradation rate constant decreases from 0.0877 to 0.017 min-1. Many researchers have
used Langmuir-Hinshelwood model to scrutinize the heterogeneous photocatalytic reaction of
organic compounds in aqueous solutions. The experimental data was restructured in terms of the
Langmuir-Hinshelwood model to accommodate reactions taking place at solid-liquid interface
(eq. 14 and 15) [37].
dC
K1K2C
ro = −
=
= KappC
dt (1+ K2Co )
(14)
1
1
Co
=
+
Kapp K1K2 K1
(15)
where ro is the rate of disappearance of dye, K1 is the surface reaction rate constant (mol L-1 min-
1), K2 is the Langmuir-Hinshelwood adsorption equilibrium constant (L mol-1). A plot of 1/Kapp
vs. Co for the photocatalytic degradation of MO is shown in Fig. 9. A linear relationship between
1/Kapp and Co was observed (R2 = 0.9811), which established the validity of Langmuir-
Hinshelwood model for photocatalytic degradation of MO by ZnO photocatalyst. The values of
K1 and K2 were found to be 1.637 × 10-6 mol L-1 min-1 and 3.092 × 105 L mol-1 respectively.
3.4.4 Effect of Catalyst Loading
To study the effect of catalyst loading on the photodegradation of MO, experiments were
carried out by varying the amount of ZnO from 0.25–2.00 g/L at a fixed dye concentration of
0.03 mM for 40 min. It was observed that percentage of MO degraded after 40 min increased
from 58.9 to 95.3% with increase in catalyst loading from 0.25–2.00 g/L (Table 2). Fig. 10
shows the plot of ln(Co/Ct) vs. time for different amount of ZnO. The change of rate constant
with change in catalyst loading is shown in Fig. 11. It can be seen that rate constant increases
with increase in catalyst loading. However two regions of increase are there. First region from
0.25–1.00 g/L where slope of curve is very steep, and second region from 1.25–2.00 g/L where
9

increase in rate constant is relatively less. The increase in rate constant with increase in amount
of ZnO is due to an increase in number of active sites on photocatalyst surface. This result in an
increase in generation of hydroxyl radicals and thus leads to an increase in the number of dye
molecules degraded. However, increase in catalyst dosage also increases the turbidity of
solution, and results in decrease in UV light penetration due to increased light scattering. This
factor comes into existence at higher catalyst dosage (above 1 g/L) and thus results in relatively
smaller increase in rate constant with increase in catalyst dosage. Keeping these observations in
mind, other experiments were carried out with 1.00 g/L ZnO so as to avoid the excessive usage
of catalyst.
3.4.5 Effect of pH
To study the role of pH on photodegradation of MO, experiment was carried out in the
pH range of 5 - 11 at 0.03 mM MO concentration and 1 g/L catalyst loading for 40 min. Fig. 12
shows the effect of pH on the percentage of dye degraded after 40 min. Fig. 13 shows the plot of
ln (Co/Ct) vs. time for different values of pH. Calculated rate constants are presented in Table 3.
It can be seen that degradation rate increases with increase in pH, exhibiting maximum at pH 9.
Further increase in pH resulted in decrease in degradation rate. So, maximum degradation rate
was observed in alkaline region. Alkaline medium facilitate the formation of OH¯ ions which are
responsible for the generation of OH˙ radicals, which leads to an increase in rate of
photodegradation [38]. The decrease in degradation rate at pH 11 may be due to dissolution of
ZnO. So the optimum pH for photodegradation of MO is 9.
10

4. Conclusions
ZnO nanoparticles, synthesized by thermal decomposition of oxalate precursor, were
found to be efficient catalyst for the photodegradation of MO dye under UV irradiation. Control
experiments revealed that presence of both ZnO and UV light is necessary for the degradation of
dye. The reaction was found to follow pseudo-first order kinetics and Langmuir-Hinshelwood
Model described it well. The surface reaction rate constant and Langmuir-Hinshelwood
adsorption equilibrium constant were found to be 1.637 × 10-6 mol L-1 min-1 and 3.092 × 105 L
mol-1 respectively. It was observed that degradation of MO is dependent upon operational
parameters, such as, concentration of dye, catalyst loading and pH. The optimal conditions for
photodegradation of MO were found to be 1g/L catalyst at a solution pH 9.
Acknowledgements
The authors would like to acknowledge the Council of Scientific and Industrial Research
(CSIR), New Delhi, for the financial support.
11

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[38] S. Sakthivel, B. Neppolian, M. V. Shankar, B. Arabindoo, M. Palanichamy, V.
Murugesan, Solar photocatalytic degradation of azo dye: comparison of photocatalytic
efficiency of ZnO and TiO2, Sol. Energy Mater. Sol. Cells, 77 (2003) 65-82.
16

Table 1 Effect of initial dye concentration on photocatalytic degradation of MO.
Initial dye
Kapp
concentration
(mM)
0.01
Degradation (%)
R2
(min-1)
98.09
90.67
77.86
60.19
50.01
0.0876
0.057
0.9975
0.9976
0.9967
0.9423
0.9116
0.03
0.05
0.0334
0.0223
0.017
0.07
0.09
17

Table 2 Effect of catalyst loading on photocatalytic degradation of MO.
[39]
Catalyst Loading
Kapp
Degradation (%)
R2
(g/L)
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
(min-1)
0.022
58.92
74.41
86.24
90.10
93.26
93.64
94.80
95.29
0.9953
0.9896
0.9847
0.9976
0.992
0.0358
0.0512
0.057
0.065
0.0678
0.0707
0.0726
0.9911
0.9922
0.9911
18

Table 3 Effect of solution pH on photocatalytic degradation of MO.
pH
Kapp
Degradation (%)
R2
(min-1)
0.0411
0.0449
0.057
5
79.72
83.06
90.10
95.72
79.54
0.9885
0.9903
0.9976
0.995
7
Natural
9
0.0761
0.0382
11
0.9931
19

Zinc Chloride
solution
Oxalic Acid
Solution
Mixed the solutions
Filter and wash the
precipitates
Dried in oven
Annealed
Fig. 1. Flowchart depicting the synthesis of ZnO powders
20

O
N
N N
S O Na
O
Fig. 2. Molecular Structure of Methyl Orange
21

Temperature (°C)
Fig. 3. TGA Curve of oxalate precursor
22

3000
2000
1000
0
10
20
30
40
50
60
70
80
90
Angle (2θ)
Fig. 4. X-Ray Diffraction pattern of ZnO
23

Fig. 5. SEM image of ZnO nanopowders.
24

0 min.
10 min.
20 min.
30 min.
80 min.
Wavelength (nm)
Fig.6. UV-Vis spectra of photodegradation of MO with ZnO as a
function of time (inset shows the photographs of MO solution).
25

Initial dye concentration (mM)
Fig. 7. Comparison of percent degradation at different initial
dye concentration.
26

Time (min)
Fig. 8. Effect of initial dye concentration on the photocatalytic degradation
of MO using ZnO.
27

Initial dye concentration (mM)
Fig. 9. Plot of 1/Kapp and initial concentration of MO
28