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 . 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 . in a typical synthesis , 2 g of pr(no3)36h2o 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 . the obtained precipitate was filtered and washed with distilled and deionized water and dried at 80c in air . the crude synthesized powder was calcined at 500c 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 /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 m and active area of ~0.5  0.5 cm by a doctor blade technique . the film was calcined at 400c 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-m - thick sealing sheet ( sx 1170 - 60 , solaronix ) by pressing the two electrodes together on a double hot - plate at 70c . 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 20850c . pr6o11 nanorod powder was pressed into cylindrical pallet and then calcined at 1,000c for 5 h. in a typical synthesis , 2 g of pr(no3)36h2o 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 . the obtained precipitate was filtered and washed with distilled and deionized water and dried at 80c in air . the crude synthesized powder was calcined at 500c 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 /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 m and active area of ~0.5  0.5 cm by a doctor blade technique . the film was calcined at 400c 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-m - thick sealing sheet ( sx 1170 - 60 , solaronix ) by pressing the two electrodes together on a double hot - plate at 70c . 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 20850c . pr6o11 nanorod powder was pressed into cylindrical pallet and then calcined at 1,000c for 5 h. 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 the energy - dispersive x - ray spectroscopy ( edx ) analysis was performed to investigate the compositional analysis of synthesized pr6o11 nanorods as shown in fig . it is observed that the atomic percentage of each element is same as the stoichiometric ratio of pr6o11 materials ( pr : o = 0.545:1 ) . therefore , the synthesized pr6o11 nanorods contain only pr and o elements without other impurity elements as confirmed by xrd result ( fig . 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  , 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 . a low and b high magnification fesem images for pr6o11 nanorods synthesized at 500c a edx spectrum and b xrd pattern of pr6o11 nanorods synthesized at 500c 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 1215 nm and length of 100150 nm . 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   zone axis . 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 . 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 . 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 pr ions in the products . 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 pr ions in pr6o11 . 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 , and the higher binding energy ( 531 and 532 ev ) can be ascribed to hydroxyl groups on or near the surface of the samples . 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 . 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 . 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 lnt vs. 1/t is depicted in inset of fig . 5 , where  is conductivity ( scm ) and t is absolute temperature . the temperature dependence of the electrical conductivity of pr6o11 nanorods indicates a typical semiconductor material behavior . the electrical conductivity (  ) of material seems to be governed by small polaron hopping mechanism as following equation : the conductivity of synthesized pr6o11 nanorod as a function of temperature . inset shows the arrhenius plot of dc conductivity ( lnt ) vs 1/t for as prepared pr6o11 nanorods . the accuracy of obtained conductivities with the larger temperature variation ( 200850c ) is to be accurate about 1% error by measurement system where o = 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 at 850c , which is much higher than those of reported bulk materials pr0.97sr0.07ga0.85mg0.15o3 ( 0.1 scm at 850c ) . 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 . 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 . 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 101 mhz . 6 shows the ac impedance curve of fabricated electrochemical cells by pr6o11 nanorods in the temperature range of 2580c . by the eis study electrolyte interface , the order of space charge width in the solid oxide matrix and impedance growth at the surface . 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 ) . the value of real impedance ( zre ) at high and low frequencies represents the rp / tco and rct . the value of rct at 25c is estimated approximately ~7285.5  in pr6o11 nanorods electrodes , which is lower than that of reported value in the order of 10  . while the value of rct at 80c decreases to ~4816.2 . 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 . 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 . the impedance ( z ) values were dramatically decreased as increasing the temperature in the range of 2580c ( fig . 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 . a impedance spectra ( zimvs . zre ) of synthesized pr6o11 nanorod as a function of temperature in frequency range of 101 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 ) the morphological and crystalline analysis showed that as synthesized pr6o11 nanorods possess a polycrystalline cubic phase grown in ( 111 ) direction with the diameter of 1015 nm and the length of 100150 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 . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
highly crystalline pr6o11 nanorods were prepared by a simple precipitation method of triethylamine complex at 500c . synthesized pr6o11 nanorods 
 were uniformly grown with the diameter of 1215 nm and the length of 100150 nm without any impurities of unstable pro2 phase . 
 the pr6o11 nanorod electrodes attained a high electrical conductivity of 0.954 scm1 with low activation energy of 0.594 ev at 850c . 
 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 .