Fabrication, characterization, and electrical conductivity properties of Pr6O11 nanoparticles

JOURNAL OF RARE EARTHS, Vol. 31, No. 7, July 2013, P. 701

Fabrication, characterization, and electrical conductivity properties of Pr6O11 nanoparticles Bahaa M. Abu-Zied1,2,3,*, Youssef A. Mohamed2, Abdullah M. Asiri1,3 (1. Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia; 2. Chemistry Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt; 3, Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia) Received 27 February 2013; revised 28 May 2013

Abstract: Pr6O11 nanoparticles were obtained by subsequent thermal decomposition of the as-prepared precipitate formed under ambient temperature and pressure using NaOH as precipitant. The calcination process was affected, for 1 h in static air atmosphere, at 400–700 °C temperature range. The different samples were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), thermogravimetric analysis (TGA), in situ electrical conductivity, and N2 adsorption/desorption. The obtained results demonstrated that nano-crystalline Pr6O11, with crystallites size of 6–12 nm, started to form at 500 °C. Such value increased to 20–33 nm for the sample calcined at 700 °C. The as-synthesized Pr6O11 nanoparticles presented high electrical conductivity due to electron hopping between Pr(III)-Pr(IV) pairs. Keywords: nanocrystalline praseodymium oxide; Pr6O11; nanostructured materials; electrical conductivity; rare earths

Studies of praseodymium oxides have received a renewed attention in the recent years because of their electronic, optical properties and chemical properties originating from the 4f shell of electrons. The PrOx system comprises a homologous series having the general formula PrnO2n−2 with n=4, 5, 6, 7, 9, 10, 11, 12, ∞[1,2]. The Pr3+ oxide phase, i.e. Pr2O3, has hexagonal structure. The fluorite structure is characteristic also for stoichiometric supreme Pr4+ oxide (α-PrO2). The phases intermediate between Pr3+ and Pr4+ oxides (β-Pr6O11, δ-Pr11O20, ε-Pr5O9, ξ -Pr9O16, ι-Pr7O12, Pr6O10 and Pr5O8) are classified into the oxygen-deficient modifications of the fluorite structure. Among these oxides Pr6O11 (or PrO1.833) is the most stable form at atmospheric pressure and room temperature[1,2]. Praseodymium oxides have been largely used in the last decade in catalyst formulations for a variety of processes in the areas of both chemicals and environment. In this context, among 14 rare earth oxides, La2O3, CeO2, Pr6O11, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb4O7, Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, and Lu2O3, praseodymium oxide showed the highest yield (80 %) at 350 °C for acetic acid ketonization[3]. Pr6O11 catalyzes decomposition of 1,3-butanediol into 2-propanol, propanone, methanol, and ethanol[4]. As promoter, it was shown that Pr6O11 enhances the individual photocatalysis of the oxidation of oxalic acid over TiO2, ZnO, CuO, Bi2O3, and Nb2O5 catalysts[5]. It is demonstrated that doping with Pr6O11 led to an enhancement in catalytic performance of Cu/H-Sep

in the SCR of NO with C3H6 [6]. Recently, it was reported that the presence of Pr can promote the production of oxygen vacancies and improve oxygen mobility, which result in enhancing the oxygen-storage capacity of the flower-like ceria and its catalytic performance for the methane combustion[7]. More recently, Ming et al.[8] have reported that the addition of small amounts of Pr6O11 to cobalt-silica gel catalysts greatly enhances their activity and selectivity in the Fischer-Tropsch synthesis. The cobalt species deposited over the praseodymium-modified silica gel surface tends to be dispersed much more homogenously and to uptake more hydrogen than un-promoted cobalt catalyst, thus increasing the catalytic activity[8]. Pr was adopted as a promoter for SnO2/Ti electrode to improve the electrocatalytic performance for pharmaceutical wastewater treatment[9]. There are also several emerging applications or processes for which praseodymium oxides are currently being actively investigated. For example, due to its high electrical conductivity, arising from electron hopping between the mixed metal ion valence-states of the lattice[10], Pr6O11 are used as hydrocarbon sensor[11]. Zn-doped Pr6O11 [12,13] is used as varistors, electronic ceramic devices that are widely used to protect electrical and electronic equipment against transient over-voltages. Solid solution Ce1–xPrxO2, prepared by high-temperature calcination of the basic starting oxides CeO2 and Pr6O11, give interesting pink-orange hues in ceramic glazes which is used as inorganic pigment[14]. The same material is used

* Corresponding author: Bahaa M. Abu-Zied (E-mail: [email protected]; Tel.: +966-0563737563) DOI: 10.1016/S1002-0721(12)60345-7

702

as a composite active layer of a solid oxide fuel cell (SOFC) cathode[15]. Nanostructured materials have attracted great interest in recent years due to their particular physical and chemical properties. The origin of their unusual properties is due to two facts: (i) the particle size is similar which leads to special electrical properties in nanostructured ionic materials and nano-semiconductors, and (ii) surface effects dominate the thermodynamics and energetic of the particles that leads to nanocrystals adopting different morphologies and to the variation in the catalytic activity[16]. Over the past few years it has been shown that the successful synthesis of Pr6O11 nanoparticles can be achieved by a number of processes. Irregular shape Pr6O11 was obtained via the thermal decomposition of praseodymium nitrate[10]. Well-defined and uniform Pr6O11 nanofibers were synthesized by electrospinning of an aqueous sol-gel consisting of praseodymium nitrate hexa-hydrate and polyvinyl acetate[17]. Wang et al.[18] reported on the synthesis of single crystalline Pr6O11 nanotubes by a molten salt method at 840 °C using bulk Pr6O11 as the Pr source and NaCl or KCl as the salt agents. Different morphologies were reported for Pr6O11 being prepared via the precipitation route. Nanocrystalline Pr6O11 spheres were prepared by precipitation of praseodymium nitrate solution using hexamethylenetetramine (HMTA) followed by heat treatment at 500 °C for 1 h[10]. Pr6O11 nanorods were synthesized using NH4OH[19] and KOH[20] as precipitants followed by calcination at 500– 650 °C temperature range. The objective of the present work was to obtain nanocrystalline Pr6O11 via the precipitation method using NaOH as precipitant. The work was focused on the effect of temperature of calcination in obtaining the nanoparticles. The solid products formed at 60–700 °C were characterized using XRD, FTIR, SEM, TEM and N2 adsorption techniques. Thermal events accompanying the thermal decomposition of the starting material were examined by TGA and in situ electrical conductivity measurements. After discussing the synthesis and the characterization of the resulting Pr6O11 nanoparticles, the electrical conductivity properties of the samples calcined at the 400–700 °C temperature range were discussed in terms of the structural modifications accompanying the heat treatment of these materials.

1 Experimental

JOURNAL OF RARE EARTHS, Vol. 31, No. 7, July 2013

temperatures till 700 °C. First, 2 g of Pr6O11 was dissolved in dilute HNO3 resulting in the formation of a pale green stock solution of Pr(NO3)3. In a typical synthesis, a solution of NaOH (2 mol/L) was added drop-wise to the obtained Pr(NO3)3 solution with vigorous stirring until the pH of 9.1 was reached. The as-obtained green colloidal precipitate was stirred for 30 min and aged for 16 h before it was filtered. The precipitate was filtered using centrifuge, and then washed with distilled water several times till a pH value of about 7 was reached, then washed with alcohol for one time. The white product was dried at 60 °C overnight. The calcination was carried out for 1 h in static atmosphere in the 400–700 °C temperature range. For simplicity, the samples will be referred to by abbreviations Pr-x, where x indicates the calcination temperature. 1.2 Characterization The TGA-DTG curves were recorded with a Shimadzu DT-60 instrument apparatus using a heating rate of 10 °C/min in air atmosphere. Powder X-ray diffraction (XRD) patterns were recorded using a Philips diffractometer (type PW 103/00) with Cu Kα radiation (λ= 0.15405 nm) at 35 kV and 20 mA with a scanning rate in 2θ of 0.06 min–1. FTIR spectrum of Pr-60 sample was performed by the KBr disc technique in the wavelength range of 4000–400 cm–1, using a Thermo-Nicolet-6700 FTIR spectrophotometer. The average particle sizes of the calcined Pr6O11 samples were estimated from X-ray line width broadening using the Scherrer equation[21]. The morphologies of the Pr-500 and Pr-700 samples were analyzed by a field-emission scanning electron microscope (FE-SEM) on a JEOL model JSM-7600F microscope. Nitrogen adsorption isotherms for Pr-500, Pr-600, and Pr-700 samples (measured at −196 °C) were constructed using Quantachrome (Nova 3200 series) multi-gas adsorption apparatus. Surface areas were calculated by BET analysis of the corresponding adsorption isotherms whereas the external surface areas were calculated using the Va-t plots of de Bore[22]. Transmission electron images were taken using a JEOL transmission microscope (model JEMTH-100 II). Electrical conductivity measurements were carried out using a Pyrex glass conductivity cell. The resistance measurements were carried out using a Keithley 610C solid-state electrometer. In each run 500 mg of the catalyst were placed between two electrodes (1.0 cm in diameter) and pressed by the upper electrode in order to ensure a good contact among the particles.

1.1 Nanocrystalline Pr6O11 preparation Analytical grade chemicals were used. Pr6O11 was prepared via the thermal decomposition of praseodymium acetate. The Pr6O11 samples were prepared by the precipitation method followed by calcination at elevated

2 Results and discussion 2.1 XRD and FTIR analyses X-ray diffraction patterns obtained for the as-formed

Bahaa M. Abu-Zied et al., Fabrication, characterization, and electrical conductivity properties of Pr6O11 nanoparticles 703

sample of praseodymium dried at 60 °C and its calcination products formed in air at the 400–700 °C temperature range are shown in Fig. 1. XRD pattern for Pr-60 sample manifests the presence of some reflections at 2θ= 15.97°, 20.67°, 24.08°, 26.67°, 28.14°, 30.40°, 33.97°, 35.92°, 38.67°, 43.86°, 45.32°, 46.78°, and 49.54°. Checking the available Joint Committee on Powder Diffraction Standards (JCPDS) data bank showed that such reflections belong to praseodymium carbonate hydroxide, Pr(CO3)(OH)2·H2O, (JCPDS 46-0370). In this context, the detection of the hydroxy-carbonate phase for the Pr-60 sample indicates the high affinity of the praseodymium hydroxide, formed at the instant of the precipitation process, to the atmospheric CO2, thus transforming to the hydroxy-carbonate phase during the aging of the precipitate. The FTIR spectrum of the Pr-60 sample is shown in Fig. 2. The spectrum manifests the presence of absorptions at 685, 836, 1050, 1435, 1520, broad one at 3450 cm–1. The bands located at 685, 836, 1050, 1435, and 1520 cm–1 can be assigned to the carbonate structure[23–27], whereas the band at 3450 and 1651 cm–1 are assigned to the structural OH in the product[24,25–27]. Such finding also suggests that Pr-60 samples are composed of praseodymium hydroxy-carbonate phase. Heating the Pr-parent at 400 °C, shown in Fig. 1(2), results in the disappearance of the reflections due to the hydroxy-carbonate phase together with the emergence of new ones that can be indexed to the pure monoclinic phase Pr2O2CO3 (JCPDS 25-0696)[21]. Raising the pretreatment temperature to 500 °C, shown in Fig. 1(3), results in the development of new reflections at 2θ=28.14°, 32.83°, 47.27°, and 56.03°. These reflections are assigned

Fig. 2 IR spectra of the Pr-60 sample

to the cubic PrO1.833 (JCPDS 06-0329) phase. Meanwhile, the reflections due to Pr2O2CO3 phase showed marked intensity decrease. XRD patterns for the samples calcined at the 600 and 700 °C revealed the presence of only one phase, PrO1.833 (JCPDS 42-1121). In this context, it was reported that heavy rare earth oxides (REO) such as Lu2O3, Yb2O3, Tm2O3, Er2O3, Y2O3, Ho2O3, and Dy2O3, transformed from monoclinic to cubic structure with calcination temperature increasing up to 1100 °C, while light REOs, such as Sc2O3, CeO2, La2O3, Pr6O11, Nd2O3, Sm2O3, Eu2O3, Gd2O3, and Tb4O7, showed no changes in their crystal structure during calcination[28]. Concurrently, using TPR and in situ XRD analyses, it was demonstrated that Pr6O11 transformed to Pr2O3 at around 800 °C[29]. The experimental diffraction peaks of Pr-700 and that of standard JCPDS card (JCPDS 42-1121) data are listed in Table 1. From the table, it is evident that the experimental values and the literature values are similar, which confirms that the decomposition product is Pr6O11. Fig. 1 illustrates that on increasing the calcination temperature the width of diffraction peaks decreases slightly and the intensity of the diffraction peaks increases. By the width Table 1 Experimental and JCPDS data diffraction peak positions, interplaner distance (d) values of the cubic Pr6O11 phase being obtained by the calcination of Pr-parent at 700 ºC Experimental

Literature

peak position

peak position

interplanar

interplanar

2θ/(º)

2θ/(º)

distance d/nm

distance d/nm

111

28.274

28.250

3.15384

3.15650

200

32.769

32.740

2.73068

2.73310

220

46.975

46.995

1.93272

1.93200

311

55.726

55.707

1.64820

1.64870

222

58.423

58.426

1.57834

1.57830

400

68.612

68.590

1.36672

1.36710

331

75.744

75.734

1.25476

1.25490

420

78.082

78.085

1.22294

1.22290

(h k l)

Fig. 1 XRD powder diffractograms obtained for the precipitate dried at 60 °C (1) and its calcination products at 400 °C (2), 500 °C (3), 600 °C (4), and 700 °C (5)

Experimental

Literature

704

of the XRD diffraction peak (111) and Scherrer equation we obtained crystallites size values of 8, 19, and 28 nm for PrO1.833 formed at 500, 600, and 700 °C, respectively. 2.2 Thermal analysis and in situ electrical conductivity measurement Fig. 3 depicts the TGA and DTG curves obtained upon heating the parent sample, dried at 60 °C, in air atmosphere till 700 °C. Inspection of the TGA curve reveals the presence of several mass loss (ML) steps. The first two steps, which are maximized at 38 and 78 °C (DTG curve), bring a total ML of around 6.6% which is higher than that attributed to the removal of one H2O molecule (4%) from the parent salt. The higher observed ML could plausibly be related to the presence of adsorbed water. The second step is not a simple one, but it is a composite step with two maxima at 292 and 369 °C (DTG curve) which corresponds to a total weight loss of 12.61%. This step indicates a complex series of overlapping dehydration and de-carbonation processes leading eventually to the formation of Pr2O2CO3 (ML=13.36%). Such phase was confirmed by using XRD analysis for the 400 °C calcination product. In this context, D’assunção et al.[30] have reported the thermal decomposition of hydrated basic carbonated of lanthanides. They have attributed the observed mass losses for La, Pr and Gd compounds between 230 and 463 °C to the simultaneous loss of water of crystallization “hydroxyl” water and one molecule of CO2 yielding Ln2O2CO3 as intermediates. In agreement it was shown that PrOHCO3 decomposed to Pr2O2CO3 intermediate at 440–480 °C temperature range[26,27]. The oxycarbonates of Pr, Nd, Sm, and Gd decompose to the corresponding oxides, as final products, throughout the formation of the intermediates Pr 2 O 2.6 (CO 3 ) 0.4 , Nd2O2.5(CO3)0.5, Sm2O2.5(CO3)0.5, and Gd2O2.4(CO3)0.6 [30]. A third ML step, being maximized at 438 °C (Fig. 3), brings a residue of 76.2% which is very close to that,

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76%, anticipated to the formation of Pr2O2.5(CO3)0.5 intermediate. The final ML step is in Fig. 3, which covers a wide temperature range from 450–650 °C can be attributed to the decomposition of Pr2O2.5(CO3)0.5 intermediated yielding Pr6O11 as a final product. In-situ electrical conductivity measurement has become one of the important methods to study the physical properties of solids, which can provide a series of valuable data about the variation of electrical properties associated with the thermal treatment of solids[21,24,31] or under catalytic conditions[32,33]. Fig. 4 depicts the plot of lg(σ) vs. temperature during decomposition of praseodymium parent salt in air atmosphere. Two points could be raised from the inspection of Fig. 4: (i) the electrical conductivity increases continuously during the temperature increase from ambient till 500 °C (the temperature limit of our conductivity cell), and (ii) we can distinguish three steps in the conductivity-temperature curve. Such steps are maximized at 97, 273, and 431 °C. Combining these data with the information abstracted from XRD and TGA analyses gives a much more detailed view of the parent salt decomposition processes. In this way, the observed first conductivity increase, which ends at around 150 °C, can be correlated with the evolution of water molecules giving Pr2(OH)2CO3. The observed conductivity increase in the second and the third steps can be ascribed to the formation of Pr2O2CO3 and Pr6O11 phases, respectively. This suggestion finds support from: (i) the fact that Pr2O2CO3 is the only phase detected for the sample calcined at 400 °C whereas a mixture of Pr6O11 and Pr2O2CO3 are the components of the 500 °C calcined sample as indicated by X-ray diffraction analysis, and (ii) recently, we have reported an electrical conductivity increase, during Pr(CH3COO)3·H2O decomposition, as a result of PrO(CH3COO)→Pr2O2CO3 and Pr2O2CO3→ Pr6O11 transformations at the same temperature range[21]. 2.3 Texture analysis and grains morphology Adsorption-desorption isotherms of nitrogen, meas-

Fig. 3 TGA (1) and DTA (2) thermograms obtained by heating the praseodymium precipitate dried at 60 °C in air atmosphere till 700 °C

Fig. 4 Plot of lg σ vs. temperature for praseodymium basic carbonate during decomposition in air atmosphere

Bahaa M. Abu-Zied et al., Fabrication, characterization, and electrical conductivity properties of Pr6O11 nanoparticles 705

ured at –196 ºC, over Pr-500, Pr-600, and Pr-700 samples are depicted in Fig. 5(a). On analyzing these isotherms it is possible to drive the values of specific area (SBET), external surface area (St), the total pore volume (Vp), and the average pore radius of each sample. The obtained values are listed in Table 2. The obtained isotherm and the hysteresis loop for Pr-500 sample belongs nearly to type II and type H3, respectively, according to the IUPAC classification[22]. Moreover, the closure point of the hysteresis loop is approximately at P/Po=0.1, which indicates either a strong affinity of adsorbate towards the surface or the existence of ultra-micropores[24]. Calcining the Pr-parent at 600 and 700 °C, i.e. for Pr-600 and Pr-700 samples, results in the noticeable decrease in the type II character of the obtained isotherms. The SBET value of Pr-500 is 37.66 m2/g, as shown in Table 2, such value is higher than that obtained from the thermal de-

composition of praseodymium acetate at 500 °C [21]. Raising the calcination temperature to 600 and 700 °C produces a 28.23% and 47.45% loss in the SBET values, respectively. Meanwhile, the obtained St values follow approximately similar trend. Such behavior is a direct response to the sintering and densification effects which predominated with temperature rise. The activation energy of sintering (Es) was calculated from the plot of ln(1/SBET) vs. 1/T using Arrhenius equation as shown in Fig. 5(a) (inset)[34]. The calculated Es was 20.05 kJ/mol. The relevant Va-t plots are shown in Fig. 5(b). All samples show a negative deviation (downward deviation) which indicates the presence of micropores for all samples. Fig. 6 shows the surface morphology of the samples calcined at 500 and 700 °C as shown by the SEM. The figures indicate that Pr-500, as shown in Fig. 6(a), is composed of network of well dispersed fine particles

Fig. 5 N2 adsorption (solid symbols)/desorption (open symbols) isotherms together with Arrhenius plot of activation energy of sintering (inset) (a), and t-curves of Pr-500, Pr-600, and Pr-700 samples (b) Table 2 texture data for the samples calcined at the 500–700 °C temperature range Calcination

SBET/

External surface

Micropore surface

Micropore volume/

Total pore

Average pore

temperature/°C

(m2/g)

area/(m2/g)

area/(m2/g)

(mL/g)

volume/(mL/g)

radius/nm

500

37.66

35.8

1.86

0.000

0.03

1.728

600

27.03

20.9

6.13

0.003

0.02

1.53

700

19.79

16.78

3.00

0.001

0.01

0.988

Fig. 6 Scanning electron micrographs obtained for Pr-500 (a) and Pr-700 (b) samples

706

having a fibre like structure. There is a significant amount of agglomeration of particles observed in the nanographs. Moreover, irregular holes distributed among the various particles without a characteristic size or shape. Inspection of Fig. 6(b) reveals that Pr-700 has spheres like morphology having uniform distribution and considerable particle agglomeration. Moreover, it exhibits morphology with considerable porosity and the formation of agglomerated nanometric particles (15–20 nm). Such porous structure results from the gases evolved during the calcination process. The morphology difference between Pr-500 and Pr-700 samples is a direct response to the structural modifications accompanying the thermal treatment of the parent salt, since XRD analysis revealed the coexistence of Pr2O2CO3 and PrO1.833 for the Pr-500 sample whereas pure PrO1.833 was the only constituent of the 700 °C calcined sample. Due to the obtained low particle size, a transmission electron microscopy (TEM) study was carried out. The Pr-500 image in Fig. 7(a), confirms the existence of very small dense clusters of nanoparticles having irregular shape. Moreover, the particles are nearly uniform and the diameters are in the range of 6–12 nm. Such value is in close agreement with the average crystallite size (8 nm) as calculated from XRD line broadening. Fig. 7(b) reveals that Pr-700 sample has nearly spherical morphol-

Fig. 7 Transmission electron pictures for Pr-500 (a) and Pr-700 (b) samples

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ogy of the particles and this observation is also in agreement with the SEM image of this sample. A significant amount of agglomeration is observed in the particles. The estimated diameters of the spherical nanoparticles are about 20–33 nm. This is in close agreement with the average crystallite size (28 nm) as calculated from XRD line broadening. It is obvious that the calcination temperature has a profound effect on the particles size. During the sintering process, the thermal energy drives the grain boundary growing over pores, decreasing the pore volume and densifying the material. When the driving force of the grain boundary is homogeneous, a uniform grain is formed; in contrast, abnormal grain growth occurs if the driving force is inhomogeneous[35]. In this context, higher calcination temperature facilitates large grains growing (Fig. 7) but it leaves some porosity in the obtained Pr6O11 (Fig. 6). 2.4 Electrical conductivity measurements Fig. 8 shows the variation of ln σ values, as obtained in air atmosphere at the 150–500 °C temperature range, with the calcination temperature of the different calcined Pr-samples. Considering the conductivity values at 150 °C, it appears that the calcination product at 400 °C is characterized by a specific conductivity value of (1.22×10–7 Ω–1cm–1). This value abruptly increases upon raising the calcination temperature up to 500 °C. Then, an electrical conductivity increase can be observed upon further increase of the calcination temperature up to 600 °C which displays a specific conductivity of 7.41×10–5 Ω–1cm–1. Further increase in the calcination temperature to 700 °C is accompanied by a mild electrical conductivity decrease (3.39×10–5 Ω–1cm–1). In addition, the trend of conductivity variation with the calcination temperatures is the same all over the whole range of temperature

Fig. 8 Dependence of lnσ on the calcinations temperature of Pr-x samples

Bahaa M. Abu-Zied et al., Fabrication, characterization, and electrical conductivity properties of Pr6O11 nanoparticles 707

measurements (150–500 °C). The trend of conductivity change could be correlated with the structural changes accompanying the heat pretreatment of the parent salt in the 400–700 °C range. XRD analysis demonstrated that the 400 °C heated sample composed of Pr2O2CO3 phase whereas PrO1.833 starts to form at 500 °C and becomes the only phases in the 600–700 °C range. Thus, the lower conductivity values for the 400 °C calcined sample can be related to the presence of Pr3+ containing compound. The higher conductivity values as well as the continuous increase in the conductivity till 600 °C can be correlated with the presence of Pr3+-Pr4+ redox couple in the form of Pr6O11. In this context, de Larramendi et al.[16] have reported an electrical conductivity increase for nanocrystalline Ce0.8Pr0.2O2 at 600 °C. They concluded that the existence of a mixed valence Pr3+/Pr4+ ions can generate an electronic conduction which is more important at higher concentrations of Pr. Shrestha et al.[10] have reported that Pr6O11 shows exceptionally high electrical conductivity due to exceptionally high electronic mobility resulting from electron hopping between the mixed metal ion valence states of the lattice. Concurrently, Wang et al.[2] have reported an electrical conductivity increase as a result of increasing the Pr6O11 content in the praseodymium oxide/polypyrrole composite to 5 wt.%–10 wt.%. In agreement, the results of Liu et al.[36] indicated that Pr6O11 can be a good additive to improve the properties of Bi4Ti3O12. Regarding the mild conductivity decrease associated with the calcination temperature rise from 600 to 700 °C (Fig. 8), our XRD analysis indicated that such temperature rise is accompanied by Pr6O11 crystallite size increases from 19 to 28 nm. Accordingly, it is reasonable to relate the obtained mild conductivity decrease to the obtained crystallite size increase. In agreement, Shrestha et al.[10] have prepared Pr6O11 with different crystallites sizes employing different preparation routes. They concluded that the electrical conductivity is inversely proportional to the oxide particle size, which is attributed to the decreasing resistance of grain boundaries as the grain size decreases.

3 Conclusions Our primary results can be summarized as follows. Treating Pr(NO3)3 with NaOH led to the formation of Pr(CO3)(OH)2·H2O precipitate. Pr6O11 nanoparticles were obtained by calcining such precipitate at 500–700 °C range. It was found that the calcination temperature was the crucial factor that controlled not only the formation of Pr6O11 nanoparticles but also the size and morphology of such solid. The electrical conductivity of the prepared Pr6O11 was controlled by Pr3+-Pr4+ redox couple as well as its crystallite size. Thus, the present route for preparing Pr6O11 nanoparticles could be applied for the prepa-

ration of other nano-crystalline rare earth oxides due to: (i) the low temperature employed in the process; (ii) no inert gases, templates, or other special chemicals are required; and (iii) the rapid reaction time. Acknowledgements: The authors would like to thank Prof. S. Weber (Pittsburg University, USA) for his generous donation of the terbium acetate sample used in the preparation of Pr6O11 starting material.

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