Morphological and Electrochemical Properties of Crystalline Praseodymium Oxide Nanorods
Highly crystalline Pr6O11 nanorods were prepared by a simple precipitation method of triethylamine complex at 500°C. Synthesized Pr6O11 nanorods were uniformly grown with the diameter of 12–15 nm and the length of 100–150 nm without any impurities of unstable PrO2 phase. The Pr6O11 nanorod electrodes attained a high electrical conductivity of 0.954 Scm−1 with low activation energy of 0.594 eV at 850°C. The electrochemical impedance study showed that the resistance of electrode was significantly decreased at high temperature, which resulted from its high conductivity and low activation energy. The reduced impedance and high electrical conductivity of Pr6O11 nanorod electrodes are attributed to the reduction of grain boundaries and high space charge width.
KeywordsPraseodymium oxide Nanorods Impedance spectroscopy Electrochemical property
The crystalline praseodymium oxide (PrOx) is a promising material for many potential applications in nanodevices and microelectronics devices due to its high-K dielectric with an effective dielectric constant of around 30 and very low leakage currents . Praseodymium oxides have been used as high electrical conductive materials , a semiconducting oxide for dielectric materials , sensing materials for detection of ethanol vapor , organic light-emitting diode , oxygen-storage components of three-way automotive catalysts  and non-volatile ferroelectric random access memory (Nv-FRAM) devices . Among PrOx such as PrO2, Pr2O3 and Pr6O11, Pr6O11 shows exceptionally high electrical conductivity due to electron hopping between the mixed metal ion valence states of the lattice .
It was reported that Pr6O11 nanotubes and nanorods were synthesized by a molten salt method  and a hydrothermal method , respectively. The morphological properties of Pr6O11 nanomaterials were reported in literature [2, 11]. However, there are few reports on electrochemical properties such as conductivity and impedance of the praseodymium oxide nanomaterials, even though these materials are having advanced morphological and crystalline properties. In this paper, we report the electrochemical properties as well as morphological properties of the synthesized polycrystalline Pr6O11 nanorods by a simple precipitation method.
Synthesis of Pr6O11 Nanorods
In a typical synthesis, 2 g of Pr(NO3)3·6H2O powder (purity 99.5%, Sigma–Aldrich chemicals) was dissolved in a mixture of isopropanol and cyclohexane with the ratio of 1:4 and stirred until the solution become transparent. Triethylamine was added in the reaction mixture as complexing agent. The white precipitate was formed after a few minutes. The obtained precipitate was filtered and washed with distilled and deionized water and dried at 80°C in air. The crude synthesized powder was calcined at 500°C for 2 h in an ambient condition to produce Pr6O11 nanorods.
Electrochemical impedance spectroscopy (EIS) was taken with the thin film electrode of Pr6O11 coated on fluorine-doped tin oxide glass (FTO, Hartford Glass Co., 8 Ω/sq, 80% transmittance). For the preparation of Pr6O11 nanorods films, Pr6O11 nanorods slurry was prepared using aqueous polyethylene glycol (Fluka, MW 20,000) solution under vigorous grinding. Thus, prepared uniform slurry was coated on FTO glass with a thickness of about 10 μm and active area of ~0.5 × 0.5 cm2 by a doctor blade technique. The film was calcined at 400°C for 1 h. To fabricate an electrochemical cell, a Pt counter electrode glass was placed over the Pr6O11 electrode and the edges of the cell were sealed with 60-μm-thick sealing sheet (SX 1170-60, Solaronix) by pressing the two electrodes together on a double hot-plate at 70°C. Finally, an electrolyte of LiI (0.5 M) and I2 (0.05 M) in acetonitrile was introduced into the cell through one of two small holes drilled in the counter electrode. EIS measurement of fabricated Pr6O11 electrochemical cells was performed using an AC impedance analyzer (VersaSTAT 4) in the frequency range from 10 to 1 MHz with signal amplitude of 10 mV. Electrical conductivity of Pr6O11 nanorods material was measured in air by four-probe DC method in the temperature range of 20–850°C. Pr6O11 nanorod powder was pressed into cylindrical pallet and then calcined at 1,000°C for 5 h.
Results and Discussion
Where σo = pre-exponential factor, Ea = activation energy for hopping conduction, k = Boltzmann’s constant, T = absolute temperature. The maximum conductivity of as synthesized Pr6O11 nanorods is 0.954 Scm−1 at 850°C, which is much higher than those of reported bulk materials Pr0.97Sr0.07Ga0.85Mg0.15O3 (0.1 Scm−1 at 850°C) . The activation energy (Ea) of electrical conductivity of Pr6O11 nanorods is calculated to be 0.504 eV from the Arrhenius plot as shown in the inset of Fig. 5. Interestingly, Pr6O11 nanorod electrode shows the lower Ea (0.504 eV) when compared to the Pr0.97Sr0.07Ga0.85Mg0.15O3 bulk materials (Ea = 0.88 eV) . It is suggested that synthesized Pr6O11 nanorods constituted the high electronic carriers on the surface of grains upon the removal of chemisorbed oxygen with the increase of temperature, resulting in the high electrical conductivity with low enthalpy . Moreover, the low activation energy of metal oxide supplied the favorable route for charge carrier conduction at high temperature, which leads to the high electronic conductivity . This enhanced electrical conductivity with low activation energy delivers the improved electrochemical properties and high space charge carriers.
Highly crystalline Pr6O11 nanorods were successfully synthesized by a simple precipitation method. The morphological and crystalline analysis showed that as synthesized Pr6O11 nanorods possess a polycrystalline cubic phase grown in (111) direction with the diameter of 10–15 nm and the length of 100–150 nm. The synthesized Pr6O11 nanorod electrode exhibited the high electrical conductivity with low activation energy and the reduced impedance at high temperature. It is attributed to the reduction of grain boundaries and enlargement of grain size, resulting in the enhanced ionic interaction at electrolyte–electrode interface and low Rct. The synthesized Pr6O11 nanorod electrodes would be promising materials for various electrical and sensing devices.
This research is supported by the National Research Foundation (NRF) of Korea through the grant No. R01-2007-000-20810-0 and the Brain Korea 21(BK21) program of the “Center for Future Energy Materials and Devices” in Chonbuk National University. We also wish to thank Korea Basic Science Institute (KBSI) Jeonju branch, and Mr. Jong-Gyun Kang, Centre for University Research Facility (CURF) for performing SEM and TEM images, respectively.
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