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.

Introduction

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 [1]. Praseodymium oxides have been used as high electrical conductive materials [2], a semiconducting oxide for dielectric materials [3], sensing materials for detection of ethanol vapor [4], organic light-emitting diode [5], oxygen-storage components of three-way automotive catalysts [6] and non-volatile ferroelectric random access memory (Nv-FRAM) devices [7]. 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 [8].

It was reported that Pr6O11 nanotubes and nanorods were synthesized by a molten salt method [9] and a hydrothermal method [10], 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.

Experimental

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.

Characterizations

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

Figure 1 shows the low and high magnification field emission scanning electron microscopy (FESEM, Hitachi 4700) images of synthesized Pr6O11 nanorods. The Pr6O11 nanorods were uniform and well grown with 12–15 nm diameter and 100–150 nm length. The energy-dispersive X-ray spectroscopy (EDX) analysis was performed to investigate the compositional analysis of synthesized Pr6O11 nanorods as shown in Fig. 2a. It is observed that the atomic percentage of each element is same as the stoichiometric ratio of Pr6O11 materials (Pr:O = 0.545:1). The variation in stoichiometric ratio is within the 2% accuracy limit of error. Therefore, the synthesized Pr6O11 nanorods contain only Pr and O elements without other impurity elements as confirmed by XRD result (Fig. 2b). All of the peaks of XRD pattern are perfectly indexed as pure face-centered cubic phase (space group: Fm3 m-225) with calculated lattice constant a = 5.4678 Å, which is consistent with the standard diffraction pattern of Pr6O11 (JCPDS 42-1121). The well-defined XRD peak indicates that Pr6O11 nanorods have good crystalline properties without any impurities.

Figure 1

a low and b high magnification FESEM images for Pr6O11 nanorods synthesized at 500°C

Figure 2

a EDX spectrum and b XRD pattern of Pr6O11 nanorods synthesized at 500°C

The morphology of synthesized Pr6O11 nanorods is further examined by transmission electron microscope (TEM, JEOL JEM-2010) and high-resolution TEM (HRTEM) as shown in Fig. 3a and 3c. The TEM images again affirm that the synthesized Pr6O11 are nanorods morphology with the average diameter of 12–15 nm and length of 100–150 nm. The size of synthesized Pr6O11 nanorods is smaller than that of the reported work [11]. The small size of nanorods may attribute to slow dehydration of Pr(OH)3 during the preparation. Figure 3b exhibits the selected area diffraction (SAED) patterns of a single Pr6O11 nanorod. The SAED pattern of Pr6O11 indexed as a cubic Pr6O11 single crystal, recorded from the [111] zone axis [11]. Figure 3c shows the corresponding HRTEM image with the interplanar distance d111 = 0.313 nm and is grown along (111) direction. The orientation of the planes and their calculated interplanar distances are consistent with the Fast Fourier Transform (FFT, Fig. 3d) of HRTEM and XRD analysis.

Figure 3

a TEM image of the Pr6O11 nanorods b SAED pattern, c HRTEM image of individual Pr6O11 nanorods and d Fast Fourier transform (FFT) of HRTEM

Two major X-ray photoelectron spectroscopy (XPS, AXIS–NOVA, Kratos Inc.) peaks of Pr (Pr 3d) and oxygen (O 1 s) are measured to explain the surface binding between Pr and O atoms as shown in Fig. 4. Pr 3d exhibits the two peaks at 933.2 eV and 953.6 eV (Fig. 4a), which correspond to Pr 3d3/2 and Pr 3d5/2 of Pr6O11 nanorods, respectively. It is demonstrated that the peaks at 930 eV and 945 eV in the Pr 3d XPS of Pr6O11 might ascribe to the existence of Pr3+ ions in the products [12]. However, Yan et al. reported that the peaks at 933 eV and 954 eV in Pr 3d attributed to Pr 3d3/2 and Pr 3d5/2 due to the existence of Pr3+ ions in Pr6O11[11]. Fig. 4b shows the resolved O 1s spectra at 528.1, 530.7 and 532 eV for Pr6O11 nanorods. The lower binding energy (528.1 eV) originated from the oxygen atoms in the lattice [13], and the higher binding energy (531 and 532 eV) can be ascribed to hydroxyl groups on or near the surface of the samples [14]. The relative intensity of lower binding energy (528.1 eV) in O 1s is weaker than those of higher binding energies (531 and 532 eV), indicating the presence of oxygen few deficiency on the surface of Pr6O11 nanorods due to interstitials oxygen chemisorbed on the surface during the synthesis of Pr6O11 nanorods [15]. Atomic ratio of O per Pr on the surface of the praseodymium oxide nanorods is calculated by ~1.86, which is almost consistent with the theoretical value of Pr6O11 (1.83). It is again confirmed that the synthesized praseodymium oxide Pr6O11 is stable when compared to unstable PrO2 phase, which is in good agreement with XRD results.

Figure 4

XPS spectra of a Pr 3d and b O 1s of synthesized Pr6O11

Figure 5 shows the conductivity of as grown Pr6O11 nanorods as a function of temperature and the Arrhenius plot of lnσT vs. 1/T is depicted in inset of Fig. 5, where σ is conductivity (Scm−1) and T is absolute temperature. The electrical conductivity linearly increases as increasing temperatures from 200° to 850°C. The temperature dependence of the electrical conductivity of Pr6O11 nanorods indicates a typical semiconductor material behavior. The electrical conductivity (σ) of material seems to be governed by small polaron hopping mechanism as following equation:

Figure 5

The conductivity of synthesized Pr6O11 nanorod as a function of temperature. Inset shows the Arrhenius plot of DC conductivity (lnσT) vs 1/T for as prepared Pr6O11 nanorods. The accuracy of obtained conductivities with the larger temperature variation (200–850°C) is to be accurate about ±1% error by measurement system

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) [16]. 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) [16]. 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 [17]. 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 [18]. This enhanced electrical conductivity with low activation energy delivers the improved electrochemical properties and high space charge carriers.

To elucidate the charge transport and space charge carrier mobility properties of Pr6O11 nanorod materials, EIS measurements were carried out by applying a 10 mV alternative current (AC) signal over the frequency range of 10–1 MHz. Fig. 6 shows the AC impedance curve of fabricated electrochemical cells by Pr6O11 nanorods in the temperature range of 25–80°C. By the EIS study, it can be demonstrated the charge-transfer across the electrode–electrolyte interface, the order of space charge width in the solid oxide matrix and impedance growth at the surface [19]. As shown in inset of Fig. 6, equivalent circuit can be proposed by consisting of the resistance of electrolyte solution (RS), the charge transfer resistance at the interface of electrolyte and Pr6O11 nanorods (Rct), the charge transfer resistance at the interface of Pr6O11 nanorods and FTO (RP/TCO), the capacitance of electron accumulation layer in Pr6O11 (Cacc) and space charge capacitance (CSC) [20]. The value of real impedance (Zre) at high and low frequencies represents the RP/TCO and Rct. The value of Rct at 25°C is estimated approximately ~7285.5 Ω in Pr6O11 nanorods electrodes, which is lower than that of reported value in the order of 104 Ω [2]. While the value of Rct at 80°C decreases to ~4816.2 Ω. The impedance results indicate that Pr6O11 nanorods exhibit the superior electrical conductivity and high magnitude of space charge width in the nanometric range. The superior electrical conductivity and enlargement of space charge width may attribute to the reduction of grain boundary of synthesized Pr6O11 nanorods [2]. It is known that a small Rct suggests the fast electron transfer in between the electrode to electrolyte interfaces, while a large Rct indicates slow electron transfer in between the electrode to electrolyte interfaces [20]. The impedance (Z) values were dramatically decreased as increasing the temperature in the range of 25°–80°C (Fig. 6). In general, AC impedance of bulk material cells related to their grain size, grain boundaries and electrolyte–electrode interface, which are responsible for high ionic conduction. The decreased resistance at high temperature is due to the reduction of grain boundaries and enlargement of grain sizes, which improves the ionic interaction at electrolyte–electrode interface, resulting in the low Rct. Therefore, the low Rct value of synthesized Pr6O11 nanorods is ascribed to the high charge transfer properties within the electrode to electrolyte interface and the reduction in grain boundaries.

Figure 6

a Impedance spectra (Zimvs. Zre) of synthesized Pr6O11 nanorod as a function of temperature in frequency range of 10–1 MHz and b the equivalent circuit of fabricated cells. The accuracy of impedance measurement is probably with the error limit of ±3% by AC impedance analyzer (VersaSTAT 4)

Conclusions

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.