Ceria or ceriuim oxide is an excellent semiconducting material with a wide
band gap which is familiar for its catalytic properties (Trovarelli
et al., 1997). Cerium, one of the elements in lanthanide series exhibit
both oxidation states +3 and +4 and has the capacity to switch over these oxidation
states very easily. Because of this excellent property cerium oxide is used
in various applications such as catalytic converters, solid oxide fuel cells
(Laberty-Robert et al., 1997) and oxygen buffers.
Nowadays nanocrystalline cerium oxide particles are studied due to its various
applications. These cerium nanoparticles are used as UV absorbents and filters
(Hu et al., 2006). Nanoceria is now extensively
used in sensing applications especially in gas sensors (Jalilpour
and Fathalilou, 2012) and electrochemical sensors. In electrochemical biosensors
the cerium oxide nanoparticles are used as nanointerface to improve the basic
characteristics of the sensors such as response time, sensitivity, selectivity,
linear response. In these sensors the cerium oxide nanoparticles are used as
the interface between the electrode and the biological element. The presence
of these nanoceria as interface in these sensors increases the electron transfer
rate between the electrode and the recognition element. Hence, the basic characteristics
of the sensors improved.
Cerium oxide nanoparticles have wide range of biological and biomedical applications. The Reactive Oxygen Species (ROS) is responsible for number of diseases (e.g., vision loss). The specific redox properties of the cerium oxide nanoparticles make it interact with a large number of ROS and minimize their harmful effects. The introduction of the cerium oxide nanoparticles into the in-vitro cell culture increases the life span of the cells and decreases the cell damage caused by H2O2 (ROS).
A recent study revealed the effective role of cerium oxide nanoparticles in spinal cord damage and some diseases related to the Central Nervous System (CNS) due to oxidative stress. The presence of the cerium nanoparticles increased the life span of the neurons in the spinal cord and in the central nervous system. Because of the wide range of applications of the cerium oxide nanoparticles it was synthesized by hydroxide mediated approach and characterized.
MATERIALS AND METHODS
Cerium oxide nanoparticles are prepared by several methods such as hydrothermal,
sol-gel, flame spray pyrolysis, solvothermal methods etc.
|| Preparation of cerium oxide nanoparticles
These methods depend on high temperature, pressure and the use of capping agents.
We have developed a simple and novel method for the synthesis of cerium oxide
nanoparticles. The precursor for the production of the cerium oxide nanoparticles
by hydroxide mediated method was cerium nitrate and sodium hydroxide. 0.1 M
cerium nitrate solution and the 0.3 M NaOH solution was prepared using double
distilled water. NaOH solution was added dropwise to the precursor solution
for about 2-3 h at room temperature under constant stirring. A pinkish white
precipitate was obtained. The precipitate was centrifuged at 8000 rpm for 15
min and the pellet was collected by discarding the supernatant. The pellet was
washed several times with distilled water and once with ethanol. The pellet
was then dried at 80°C for 1 h in hot air oven and then annealed at 270°C
RESULTS AND DISCUSSION
Structural and morphological studies: The structural characterization, phase identification and the grain size of the cerium oxide nanoparticles were studied by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM).
Flow chart for the synthesis of cerium oxide nanoparticles
X-ray diffraction pattern of cerium oxide nanoparticles: The XRD pattern
of CeO2 nanoparticles is shown in Fig. 2. The XRD
pattern was scanned from 10-80 degrees with the scan rate 2θ min-1.
The XRD profile confirmed the polycrystalline nature of the cerium oxide nanoparticles.
The high intensity peaks were observed at 28.53, 33.09, 47.5, 56.26, respective
to the 111, 200, 220, 311 crystal planes. The crystal planes were in well accordance
with JCPDS No: 34-0394 of CeO2 crystal. The diffraction peaks in
these XRD spectra indicates the pure cubic fluorite structure.
|| XRD pattern of the CeO2 nanoparticles
|| SEM of cerium oxide nanoparticles
Crystallite size was obtained by using the Debyes Scherrer equation:
where, K is the shape factor, λ is the X-ray wavelength, β is the line broadening at half the maximum intensity (FWHM) in radians and θ is the Bragg angle. The crystallite size was found to be in the range from 9-16 nm.
SEM of cerium oxide nanoparticles: SEM of cerium oxide nanoparticles is shown in Fig. 3. Surface and morphological characterization of cerium oxide nanoparticles were carried out using scanning electron microscopy. Nanosized spherical shaped CeO2 particles obtained was confirmed. The mean size of the particles varies from 18- 30.4 nm.
Fourier transform infra-red spectroscopy of cerium oxide nanoparticles:
The FTIR spectrum of cerium oxide nanoparticles is shown in Fig.
4. The spectrum was recorded in the wave number range of 400-4000 cm-1.
|| FTIR spectrum of cerium oxide nanoparticles
||UV-visible absorbance spectra of cerium oxide nanoparticles
The bands at 1622.71 and 3375.80 cm-1 represents the water and the
hydroxyl stretches, respectively. The intensive band at 1384.23 cm-1
represents N-O stretch due to the presence of nitrate. The absorption band at
wavenumber 550.84 cm-1 represents the Ce-O stretch (Ansari
et al., 2009). We confirmed the formation of nanoparticles using
Optical studies: A UV-visible absorbance spectrum of cerium oxide nanoparticles is shown in Fig. 5. The absorbance spectra was recorded for the nanoparticles dispersed in water. Here, water is used as a blank. A strong absorption below 400 nm was observed. A very well defined absorbance peak was observed around 325 nm for the synthesized cerium oxide nanoparticles.
The cerium oxide nanoparticles were synthesized by hydroxide mediated method.
XRD pattern of CeO2 particles shows that the particles were in polycrystalline
in nature. The SE micrograph shows the particles are in spherical shape with
size range of 18-30.4 nm. The FTIR spectrum confirmed the formation of CeO2
particles. The optical studies of CeO2 particles were studied
by UV-visible absorbance spectrophotometry. Thus the cerium oxide nanoparticles
were synthesized and characterized.
The authors are grateful to the Department of Science and Technology for the financial support. We also thank SASTRA University, Thanjavur, for the infrastructural support.