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The 11th International Chemical Engineering Congress &
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Synthesis of Manganese Dioxide (MnO2) Nanoparticles using
Supercritical Water
M. Golmohammadi1*, A. Esmaeili2 1Department of Chemical
Engineering, Birjand University of Technology, Birjand, Iran
2College of North Atlantic-Qatar
[email protected]
Abstract Supercritical water, because of its unique properties,
can be used as a appropriate environment for conducting a wide
range of reactions including the synthesis of different
nanoparticles. The main objective of the present work was to
synthesize MnO2 nanoparticles via supercritical water technique as
well as to investigate their properties. Structural and
morphological properties of synthesized nanoparticles were
determined using X-ray diffraction and scanning electron microscopy
(SEM), respectively. The results demonstrated that the crystalline
structure of MnO2 was well developed in supercritical water.
Furthermore, it was found that the spherical fine particles with
mean crystalline size of 35±4 were synthesized in supercritical
medium. Keywords: Nanoparticles, Supercritical Water,
Characterization, Manganese Dioxide.
Introduction In the last decades, nanoparticles has attracted
much attention due to their extraordinary properties. Decreasing
the particle size to nanometers leads to the change in magnetic,
electrical, chemical behaviors as well as increasing the
surface-to-volume ratio. There are various methods to manufacture
the nanoparticles such as sol-gel [1], hydrothermal [2], chemical
vapor deposition [3], ultrasonic irradiation [4] and hydrothermal
synthesis in supercritical water [5]. Most of these methods consume
large amounts of chemicals that can be harmful to the environment.
In contrast, the production of nanoparticles in the supercritical
water environment requires no additional chemicals and the
nanoparticles are produced solely from the aqueous precursor
solution. Therefore, this method can be considered as one of the
green methods for the production of nanoparticles. Water above its
critical point (374 °C and 22.1 MPa) possesses unique properties
that distinguish it from water at room temperature. The dielectric
constant (ε), as the most important factor controlling the
solubility, drastically decreases around the critical point [6].
This property is about 78 at room temperature, making polar
minerals soluble in water. Increasing the temperature at a given
pressure reduces the dielectric constant of water to below 10. In
this case, mineral salts are no longer soluble in water and
consequently precipitate in the solution [7]. In the supercritical
water medium, metallic hydrated ions are first hydrolyzed and then
the metal oxide crystals are formed through a dehydration reaction.
Accordingly supercritical water is an appropriate medium for
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The 11th International Chemical Engineering Congress &
Exhibition (IChEC 2020) Fouman, Iran, 15-17 April, 2020
the formation of nanoparticles. This method has many advantages
such as short synthesis time, fine nanoparticle production with
appropriate distribution, control of size and morphology only with
temperature manipulating, and the use of minimum chemicals [8]. In
the last two decades, the supercritical water has been employed to
synthesize a wide range of metals, metal oxides, and mixed metal
oxides. Synthesis of numerous nanoparticles in supercritical water
have been extensively reviewed by Yoko et al. [9]. Manganese
dioxide has a wide range of applications, such as high-density
magnetic storage, catalysts, ion exchange, molecular adsorption,
electrochemicals, varistors and solar energy conversion. MnOx-based
catalysts have also been identified as active phases in several
catalytic oxidation processes and hydrogenation reactions [10].
Accordingly, the main objective of the current work is the
synthesis of MnO2, as an effective catalyst, by using supercritical
water method. Experimental The MnO2 nanoparticles were synthesized
in a batch-wise stainless steel (316L) reactor with a capacity of
100 ml. Firstly, 0.2 M solution of Manganese (II) nitrate
tetrahydrate (Merck AG. Fur synthesis) was prepared by dissolving
this salt in a given amount of double distilled water (see Figure
1). Then, 30 ml of obtained solution was poured into the reactor
and its cap was tightly closed. The reactor was then placed in a
furnace with temperature of 480 °C for 3 hours. After this time,
the reactor was rapidly quenched by cold water and its contents
were evacuated. The fabricated nanoparticles were washed using
double distilled water as well as centrifuged three times by using
a high speed centrifugation (10000 rpm, 10 min). The resulting
precipitate was then poured into a petri dish to dry overnight at
ambient temperature. The crystal structure and composition of the
synthesized nanoparticles were analyzed by X-ray diffractometry
(XRD, Philips PW 1800). Moreover, the morphology of nanoparticles
was studied by a KYKY SEM-EM3200 apparatus.
Figure 1. (a): The salt of Mn(NO3)2.4H2O, (b) 0.2 M aqueous
solution of the salt.
Results and discussion Figure 2 illustrates the XRD pattern of
MnO2 nanoparticles. The highly resolved diffraction peaks implying
that the crystalline structure of mixed oxide nanoparticles is well
developed through supercritical water synthesis. The diffraction
peaks, that appeared at 2θ= 28, 37, 41, 43, 46, 57, 59, 65, 67, and
72° appertain to tetragonal structure of MnO2 with miller indices
(110), (101), (200), (111), (210), (211), (220), (002), (310),
(301) , respectively and with lattice parameters a= 4.42 Å, b= 4.42
Å and c = 2.87 Å (JCPDS File No. 12-0716). In addition, the
crystallite size was calculated about 25 nm by applying
Debye-Scherer equation.
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The 11th International Chemical Engineering Congress &
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Figure 2. The XRD pattern of MnO2 nanoparticles synthesized via
supercritical water method.
Moreover, in order to study the morphology as well as particles
size distribution, the SEM micrograph of produced nanoparticles was
taken and presented in Figure 3. From this figure it obvious that
the nanoparticles are spherical in shape, and are in the range of
20-60 nm.
Figure 3. SEM micrograph of MnO2 nanoparticles synthesized via
supercritical water method.
Moreover, no significant nanoparticle agglomeration is observed
indicating that supercritical water has been successful in
manufacturing nanoparticle with uniform size distribution. To prove
this claim, the mean size of the nanoparticles was determined using
an image processing software and then the particle size
distribution histogram was plotted based the mean size of 300
nanoparticles (Figure 4). As can be seen, the nanoparticles have a
fairly normal distribution with an average particle size of 37±6
nm.
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The 11th International Chemical Engineering Congress &
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Figure 4. Particle size distribution histogram of MnO2
nanoparticles synthesized via supercritical water
method. Conclusions In this study, MnO2 nanoparticles were
synthesized in SCW medium and also were characterized by various
analyses. The results of the XRD and SEM indicated that the
obtained nanoparticles possessed satisfactory size and morphology
with narrow particle size distribution. Acknowledgements We
gratefully acknowledge the Birjand University of Technology for the
support of this work.
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