Photocatalytic activity of ZrO2 nanoparticles prepared by electrical arc discharge method in water

Article abstract of DOI:10.1016/j.poly.2010.01.003

ZrO₂ nanoparticles were synthesized through arc discharge of zirconium electrodes in deionized (DI) water. X-ray diffraction (XRD) analysis of the as prepared nanoparticles indicates formation a mixture of nanocrystalline ZrO₂ monoclinic and tetragonal phase structures. Transmission electron microscopy (TEM) images illustrate spherical ZrO₂ nanoparticles with 7-30 nm diameter range, which were formed during the discharge process with 10 A arc current. The average particle size was found to increase with the increasing arc current. X-ray photoelectron spectroscopy (XPS) analysis confirms formation of ZrO₂ at the surface of the nanoparticles. Surface area of the sample prepared at 10 A arc current, measured by BET analysis, was 44 m²/g. Photodegradation of Rhodamine B (Rh. B) shows that the prepared samples at lower currents have a higher photocatalytic activity due to larger surface area and smaller particle size.

Full text of DOI:10.1016/j.poly.2010.01.003

Polyhedron 29 (2010) 1370–1374
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Photocatalytic activity of ZrO2 nanoparticles prepared by electrical arc
discharge method in water
*
Ali Akbar Ashkarran , Seyedeh Arezoo Ahmadi Afshar, Seyed Mahyad Aghigh, Mona kavianipour
Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, P.O. Box 14665-678, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
ZrO2 nanoparticles were synthesized through arc discharge of zirconium electrodes in deionized (DI)
water. X-ray diffraction (XRD) analysis of the as prepared nanoparticles indicates formation a mixture
of nanocrystalline ZrO2 monoclinic and tetragonal phase structures. Transmission electron microscopy
(TEM) images illustrate spherical ZrO2 nanoparticles with 7–30 nm diameter range, which were formed
during the discharge process with 10 A arc current. The average particle size was found to increase with
the increasing arc current. X-ray photoelectron spectroscopy (XPS) analysis confirms formation of ZrO2 at
the surface of the nanoparticles. Surface area of the sample prepared at 10 A arc current, measured by BET
analysis, was 44 m2/g. Photodegradation of Rhodamine B (Rh. B) shows that the prepared samples at
lower currents have a higher photocatalytic activity due to larger surface area and smaller particle size.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 25 November 2009
Accepted 4 January 2010
Available online 11 January 2010
Keywords:
ZrO2 nanoparticles
Arc discharge
Characterization methods
Photocatalytic activity
1. Introduction
The early works by arc discharge method in liquids were based
on production of carbonaceous nanostructures such as MWCNTs
ZrO2 nanoparticles have attracted much interest due to their
specific optical and electrical properties and potential applications
in transparent and optical devices, sensors, fuel cells, advanced
ceramics and photocatalysts [1–4]. Specifically, much attention
has been drawn towards their photocatalytic properties because
of its application in environmental purification and decomposition
of toxic and organic compounds. A key requirement for improving
the photocatalytic activity is to increase the specific surface area
and enhance the crystallinity [5]. These requirements are met by
crystalline nanostructured materials. Several methods including
hydrothermal [6], sol–gel [7], chemical vapor deposition (CVD)
[8] and sputtering [9] have been used to prepare ZrO2 nanoparti-
cles. For efficient photocatalytic activity, nanomaterials need to
be crystalline, that is, should be grown at high temperatures or
at very slow rates. In general, synthesis of ZrO2 usually results in
amorphous structures but, electrical arc discharge in water has
the advantage in this regard as it produces self-crystallized nano-
particles due to high temperature caused by joule heating. More-
over, compared with other techniques, electrical arc discharge in
water is an attractive method because of simplicity of experimen-
tal set up, lack of need for complicated equipments, low impurity,
less production steps leading to a high-throughput and cost-effec-
tive procedure to generate a high yield of nanoparticles. Also the
simplicity of this method allows scaling up for mass production.
[10–13], SWCNTs, SW-CNHs [11] and nano-onions [13]. In past
few years, few papers have been published regarding synthesis of
metal and metal oxide nanostructures such as MoS2 [14], CuO
and Cu2O [15], Ag [16,17], Au [18,19], WO3 [20] and ZnO [21] by
arc discharge in liquids.
In the present study, we report a simple, inexpensive and one-
step synthesis route of ZrO2 nanoparticles by arc discharge method
in DI water. We have studied the effect of arc current on size dis-
tribution and the photocatalytic activity of the produced nanopar-
ticles. The main advantage of the present method is the direct
formation of ZrO2 nanoparticles from discharge of zirconium elec-
trodes within water. To the author’s knowledge, to date there has
been no published report on synthesis of ZrO2 nanoparticles based
on arc discharge using zirconium electrodes in DI water.
2. Experimental
2.1. Synthesis of ZrO2 nanoparticles
The experimental set up consists of two main parts: a high cur-
rent DC power supply and a reactor including anode, cathode and a
micrometer which moves the anode towards the cathode. In this
work, a 10–20 A current was applied between two zirconium
metallic electrodes. The voltage was dropped to about 2–3.5 V dur-
ing the arc formation while the current was fixed to a desired
amount. Both the anode and cathode were wire shaped, 1 mm in
diameter and had 99.2% purity (from Goodfellow). Initially, we
bring the two electrodes into touch leading to a small contact cross
* Corresponding author. Tel.: +98 21 44869632; fax: +98 21 44869640.
E-mail address: ashkarran@srbiau.ac.ir (A.A. Ashkarran).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.01.003

A.A. Ashkarran et al. / Polyhedron 29 (2010) 1370–1374
1371
section and thus to a high current density. Then we separate them
from each other and as a result zirconium is ablated from the an-
ode and then condensed in water and forms different
nanoparticles.
2.2. Characterization
Analysis of the crystalline structures was performed by XRD dif-
fractometer (X’pert Philips) with wavelength of Cu Ka radiation in
2h range from 10° to 80° by 0.005° sꢀ1 steps. UV–Vis spectroscopy
of the samples was taken out by a Lambda 950 spectrophotometer
(Perkin–Elmer) from 200 nm to 1100 nm wavelengths. TEM analy-
sis was performed by a LEO 912 AB instrument at 200 keV acceler-
ating energy by deposition of ZrO2 nanoparticles onto the copper
grid at room temperature. XPS analysis was taken out by a dual
Mg–Al anode X-ray source. A concentric hemispherical analyzer
(from Specs company model EA10 plus) was employed to analyze
the emitted electrons from the surface. The energy axis was cali-
brated by adjusting the carbon peaks at 285 eV. Surface area of
the nanoparticles was determined by Belsorp adsorption–desorp-
tion (BEL Japan Inc.) instrument. Photocatalytic activity of the
nanoparticles was measured by photodegradation of Rh. B with
the initial concentration and volume of 10ꢀ5 M and 30 mL respec-
tively at the presence of 30 mL ZrO2 solution. First the mixed Rh. B
and ZrO2 solution was stirred in dark for 30 min to equilibrate the
Fig. 1. XRD pattern of ZrO2 nanoparticles was made at 10 A arc current (a) as
prepared and (b) annealed at 500 °C.
Fig. 2. XPS analysis for ZrO2 nanoparticles formed at 10 A arc current, (a) wide range of energy survey, (b) energy range related to Zr peaks and (c) energy range related to O 1s
peak.

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Fig. 3. TEM images of ZrO2 nanoparticles prepared at (a) 10 and (b) 20 A electrical arc currents.
absorption–desorption between dye molecules and ZrO2 photocat-
alysts. Then it was irradiated at room temperature by a 125 W low-
pressure mercury lamp with 254 nm strongest peak wavelength.
The degradation rate was measured by UV–Vis spectrophotometer
at the maximum absorption wavelength of Rh. B. DI water was
supplied from a Millipore water purification system (Direct-Q-3)
treatment was 17.5 nm which was approximately constant com-
pared with before annealing. These results correspond to other
works such as Zhang et. al. [22] and Liang et. al. [23].
with 18.2 MX cm (0.055 ls) resistance.
3. Results and discussion
3.1. XRD analysis of the products
XRD result for as prepared samples at 10 A arc current demon-
strates formation of crystalline ZrO2 with a mixture of monoclinic
and tetragonal phase which is depicted in Fig. 1. The obtained spec-
trum has the ZrO2 monoclinic phase peaks at 2h = 24.5, 28.2, 31.5,
35.3, 50.2, 50.7, 60.1, 62.8, 72.9, 73.5 and 74.6° and tetragonal
phase peaks at 2h = 30.2, 34.6 and 59.3° which is in agreement with
the 37–1484 and 42–1164 standard card from the JCPDS. The sin-
gle crystalline size determined by Debye–Scherrer formula was
16.5 nm. In this method the as prepared particles are usually crys-
tallized due to high temperatures caused by joule heating. Anneal-
ing the extracted powder at 500 °C in air for 1 h did not result in
any remarkable changes in the intensity or position of the peaks
as is illustrated in Fig. 1b. The single crystalline size after heat
Fig. 4. BET adsorption–desorption analysis of as prepared ZrO2 nanoparticles at
10 A arc current.

A.A. Ashkarran et al. / Polyhedron 29 (2010) 1370–1374
1373
3.2. XPS analysis
that the particles are nanosized and spherical. This means that
the aspect ratio is close to 1. The histogram of the size distribution
is shown in Fig. 3b. It imparts that the size of the particles is be-
tween 7 and 30 nm and the average diameter is 21 nm. TEM image
and the related size distribution of the ZrO2 nanoparticles synthe-
sized at 20 A arc current is shown in Fig. 3c and d. It reveals that
the size of the particles is between 30 and 52 nm and the average
diameter is 42 nm. We can conclude that the mean particle size in-
creases by increasing the arc current. The reason for this behavior
has been discussed in detail in our previous work [19].
To be sure about the composition of the product we performed
XPS analysis. Fig. 2(a–c) shows the XPS data for the as-synthesized
10 A sample. From XPS spectrum the binding energies of O 1s, Zr
4p, Zr 3d, Zr 3p3/2, Zr 3p½ and Zr 3s for the as prepared ZrO2 nano-
particles are 530.7, 28.5, 181.5, 333, 346.5 and 433.5 eV, respec-
tively. In addition a weak C 1s peak at 285 eV is observed due to
ambient air contamination. To determine the chemical state we
used Auger parameter of Zr (using Zr 3p3/2 peak and Auger peak
at 459 eV) which is equal to 2011.2 eV. This value confirms forma-
tion of ZrO2 at the surface of samples. Also O 1s peak (Fig. 2c) has
two parts: main part at 530.7 which reflects ZrO2 state and another
part at 532 eV which is due to C–O bound. The ratio of oxygen to
zirconium peaks determines O to Zr ratio about 2.2. The excess ra-
tio from 2 is due to adsorbed oxygen at the surface of the sample.
3.4. BET analysis
Photocatalytic activity of ZrO2 nanoparticles depends on their
surface area. In order to evaluate its activity we first determined
the active surface area of the nanoparticles. BET analysis gives
the surface area of the nanoparticles about 44 m2/g which is shown
in Fig. 4. The adsorption–desorption isotherm shows a hysteresis
loop that is very similar to type V isotherm [24]. The relationship
between the specific surface area and the particle size in BET mod-
el is
3.3. TEM studies
The shape and size distribution of nanoparticles were character-
ized by TEM. Fig. 3 illustrates typical morphologies of ZrO2 nano-
particles obtained at 10 and 20 A arc currents. It was observed
Fig. 5. Evaluating of photocatalytic activity of ZrO2 nanoparticles by photodegradation of Rh. B. (a) Rh. B without ZrO2 nanoparticles, (b) 10 A, (c) 15 A and (d) 20 A arc current.

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A.A. Ashkarran et al. / Polyhedron 29 (2010) 1370–1374
of K. The calculated reaction rate constant for the samples prepared
at 10, 15 and 20 A arc currents were 16 ꢁ 10ꢀ3, 14 ꢁ 10ꢀ3 and
5 ꢁ 10ꢀ3 minꢀ1, respectively. Furthermore, compared with com-
mercial TiO2 photocatalyst (Degussa P25) in an equal situation,
our 10 A sample shows near two times more photocatalytic activity.
4. Conclusion
We have prepared ZrO2 nanoparticles in a one-step synthesis
process by a high current electrical arc discharge of Zr electrodes
in DI water. XRD results indicated formation of a mixture of nano-
crystalline ZrO2 monoclinic and tetragonal phase and 26.5 nm sin-
gle crystalline domain size. Surface chemical composition of the
nanoparticles determined by XPS shows formation of ZrO2. TEM
images revealed spherical nanoparticles with 21 and 42 nm mean
particle size at 10 and 20 A arc currents, respectively. The particle
size was found to increase with an increase in the arc current. Sur-
face area of the sample prepared at 10 A arc current, measured by
BET analysis, was 44 m2/g. Further, the photocatalytic activity of
ZrO2 nanoparticles demonstrated that by increasing UV illumina-
tion time, the maximum absorption peak and concentration of
Rh. B decreases in the presence of ZrO2 nanoparticles. In fact, de-
crease of concentration of the sample prepared at 10 A was found
to be more pronounced than the sample prepared at 15 and 20 A
arc current which was due to increased surface to volume ratio.
Fig. 6. Changes of rhodamine
samples prepared at 10, 15 and 20 A arc currents and the blank sample.
B concentration under UV illumination for the
6
S ¼
qd
which
q is the solid density, S is the specific surface area and d is the
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