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Controlled synthesis of praseodymium oxide nanoparticles obtained by combustion route: Effect of calcination temperature and fuel to oxidizer ratio
Applied Surface Science 471 (2019) 246–255
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Full Length Article
Controlled synthesis of praseodymium oxide nanoparticles obtained by combustion route: Effect of calcination temperature and fuel to oxidizer ratio
T
Bahaa M. Abu-Zied Chemistry Department, Faculty of Science , King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
ARTICLE INFO Keywords: Pr6O11 nanoparticles Praseodymium oxide Rare earth oxides Combustion synthesis Nanomaterials
ABSTRACT
This investigation focuses on the employment of the solution combustion route for synthesis of praseodymium oxide nanoparticles. The fabrication was be carried out by using the solution combustion of a fuel (urea) and oxidizer (praseodymium nitrate) mixture. The influence of fuel/oxidizer ratio and the temperature of calcination on the phases formed, morphological changes and the crystallite size of Pr6O11 have been checked. Various characterization tools have been used for the identification of the thermal behaviour of the precursors, structure and morphology of the prepared Pr6O11 nanoparticles, which include TGA-DTA, XRD, FT-IR, XPS, SEM, and TEM. With respect to the effect of the fuel content, it was found that using F/O ratio ≥4.0 at 500 °C praseodymium carbonate starts to form at the expense of the oxide phase. These phase changes were, also, companied by a noticeable morphological modifications. On the other hand, it was shown that Pr6O11 can be obtained as a single phase upon calcining at temperatures ≥500 °C. The electrical conductivity of Pr6O11 nanoparticles synthesized at 400–700 °C has been investigated over a measuring temperature range of 200–400 °C. The obtained results have led to a better understanding of the factors influencing the electrical conductivity behaviour of Pr6O11 nanoparticles, viz. the availability of Pr3+/Pr4+ pairs and the crystallite size.
1. Introduction Among the various rare earth metal oxides, praseodymium oxide (PrOx) possesses many phases with various oxygen contents, with × ranging from 1.5 to 2 [1–5]. In this sense, different formulations have been reported comprising × = 1.833, 1.818, 1.810, 1.800, 1.780, 1.777, 1.714, 1.667, and 1.600. PrO2 adopts the calcium fluoride structure whereas Pr2O3 structurally related to the hexagonal lanthanum oxide structure [3,4]. Oxides with O/Pr > 1.5 crystallize in the fluorite structure [4]. Pr6O11 was reported to be the most stable composition at ambient temperature and pressure [4]. Regarding the electrical conductivity of praseodymium oxides, it was also shown that PrOx phases with × < 1:73 are a p-type semiconductors; at higher value of ×, it shows the n-type semi-conductivity behaviour [5]. The electron hopping, occurring between the lattice mixed-metal valence states, was proposed to be responsible for the high electrical conductivity of Pr6O11 [2,6,7]. Recently, praseodymium oxides have been investigated as a subject of various emerging applications or processes. Pr6O11 was used as hydrocarbon sensor [8]. When Pr6O11 is added to ZnO, CuO, WO3 and
SnO2⋅Co2O3⋅Ta2O5 the product is used as varistors, which is applied in electronic devices for the protection of these devices from the voltage surge [9–12]. Praseodymium oxide can form with ceria solid solution which can be used as oxygen-storage component, which is used in solid oxide fuel cell (SOFC) as a cathode material [13–15]. Doping of the CeO2 lattice with the Pr6O11 chromophore yields ceramic glazes with colours that ranged from red-brown to magenta-orange depending on the chromophore content, synthesis protocol and temperatures of pretreatment [16–19]. Praseodymium oxides are largely employed as catalysts or in catalyst formulations (as promoters or stabilizers) in various processes in the area of both industry and environment. For example, Pr6O11 oxide was a catalyst for the dehydration and dehydrogenation of 1,3-butanediol and 2-butanol [20]. It was concluded that the praseodymium oxide addition to ceria increases its activity during CO oxidation and methane combustion [21,22]. Bulk Pr6O11 is usually prepared by several methods such as the thermolysis of praseodymium salts, such as the nitrate and the acetate salts, at high-temperatures [5,6]. Over the past few years, novel protocols in the synthesis praseodymium oxide nanoparticles have been
E-mail address: [email protected]. https://doi.org/10.1016/j.apsusc.2018.12.007 Received 17 October 2018; Received in revised form 21 November 2018; Accepted 2 December 2018 Available online 03 December 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 2. XRD patterns recorded for the Pr-samples with the F/O ratios of 0.5 (a), 1.0 (b), 2.0 (c), 4.0 (d), 8.0 (e) and 16.0 (f) calcined at 500 °C.
Fig. 1. TGA-DTA curves obtained for urea/praseodymium nitrate mixture (a), bare praseodymium nitrate (b) and bare urea (c).
developed. For instance, Pr6O11 nanofibers were successfully prepared by the electrospinning of an aqueous mixture of polyvinyl acetate and praseodymium nitrate [23]. Pr6O11 nanorods were obtained by the calcination, at 400–600 °C, of Pr(OH)3 nanorods precursor prepared via precipitation method [24,25]. Kang et al. [26] prepared Pr6O11 nanorods hydrothermal method employing the hydrothermal route and subsequent calcination at temperatures ≥550 °C. Over the past two decades, combustion synthesis (CS) has gained increased attention as facile technique for the preparation of nanomaterials. It comprises a self-sustained redox reaction with high exothermicity between a fuel (organic substance like hydrazides, glycine, urea, sucrose etc.) and an oxidizer (usually metal nitrate). Depending upon the nature of reactants and the reaction exothermicity CS is classified to (i) solid state combustion (SSC) where all the starting materials as well as the products are in the solid state, and (ii) solution combustion synthesis (SCS) which involves self-sustained reaction by rapidly heating a homogeneous solution containing a mixture of the fuel and the oxidizer [27]. Focusing our attention to the SCS, it was successfully employed in the preparation of a variety of nanomaterials such as CdCr2O4 [28], MgO [29], CdO [30] and Co3O4 [31,32]. The role of the combustion fuel can be indicated as follows: (i) it can complexes with the metal cations leading to a homogeneity increasing of the mixture,
Fig. 3. FT-IR spectra obtained for the Pr-samples with the F/O ratios of 0.5 (a), 1.0 (b), 2.0 (c), 4.0 (d), 8.0 (e) and 16.0 (f) calcined at 500 °C. 247
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Fig. 4. SEM images for the Pr-0.5-500 (a), Pr-4-500 (b) and Pr-16-500 (c) samples.
and (ii) it is considered as carbon and hydrogen source, which are essential in the combustion process and the evolution of a plenty of gases. These gases are hypergolic, i.e. they can easily ignite upon acquiring the critical density, even at ambient pressure, at their ignition temperature. As far as we are aware, there is a lack of papers reporting the synthesis of nanocrystalline Pr6O11 via the combustion route. Thus, the main target of the present investigation is to synthesize praseodymium oxide nanoparticles using the combustion route. Moreover, we aimed to address the influence of: (i) the fuel/ oxidized molar ratio of, and (ii) the temperature of calcination on the formation, structural and textural features of the praseodymium oxide. Different techniques have been employed for characterization of the obtained nanoparticles: TGA-DTA, XRD, FT-IR, XPS, SEM and TEM. Moreover, the electrical resistivity of the Pr-samples pre-treated at 400–700 °C range has been investigated.
over a hotplate till a viscous gel was formed. Finally, the obtained gels have been calcined in air atmosphere at 500 °C for 1 h. In the second series, effect of increasing the temperature of calcination on the formation, morphology and crystallite size of Pr6O11 nanoparticles has been studied. For the preparation of this series, the required amounts of urea and praseodymium nitrate for obtaining a F/O ratio of 0.5 have been dissolved in distilled water. The hotplate dried mixtures have been calcined in air atmosphere at 400–700 °C range for 1 h. The various samples obtained will be denoted in the text using notations Pr-x-y, where × refers to the F/O and y the calcination temperature. 2.2. Techniques TGA and DTA analyses have been used to check the thermal stability of the reaction precursors, viz. urea and praseodymium nitrate, and their mixture. The measurements have been performed on a Shimadzu DTG-60 thermal analyser. The measurements conditions were: 10 mg sample weight, 10 °C/min heating rate, and air flow of 40 ml. min−1. The XRD patterns of the various Pr-containing solids were obtained using a Philips (type PW 103/00) diffractometer with Cu Kα (λ = 1.5405 Å) radiation operated at 35 kV, 20 mA and a scanning rate of 0.06 min−1 over the 2θ range of 4–80°. FT-IR spectra were collected using the Attenuated total reflection (ATR) attachment of Nicolet iS50 FT-IR spectrometer. The microscopic investigations of some selected samples have been conducted on: (i) JEOL JSM-5400 LV scanning electron microscope (SEM) and (ii) JEOL (model JEMTH-100 II) transmission electron microscope (TEM). The nature of surface elements and their chemical binding has been investigated using SPECS
2. Experimental 2.1. Materials and samples preparation The combustion fuel (urea) and the praseodymium nitrate (Pr (NO3)3·6H2O), both obtained from BDH, were of > 99% purity. The preparation of Pr6O11 nanoparticles were briefly described below. Two series of samples have been synthesized. The first one addresses the effect of changing the fuel (F) to the oxidizer (nitrate anion, O) molar ratio on the phase composition of the obtained solids has been investigated. For the synthesis of this series, the required amounts of urea and praseodymium nitrate for the F/O molar ratios of 0.5, 1, 2, 4, 8 and 16 were dissolved in distilled water. The obtained solutions were dried 248
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to a certain thermal events, the thermal behaviour of the parent mixture components has been investigated using the same thermal analyser under the same conditions. Fig. 1(b) depicts the obtained TGA-DTA thermograms of bare praseodymium nitrate. This thermal behaviour can be interpreted using the previous results of Hussein et al. [5]. The consecutive early WL (24.85%), which extends from 40 to 300 °C, is very close to that ascribed to the elimination of the hydration water molecules (24.82%). Another consecutive WL steps can be observed at the 305–550 °C temperature range that ends with the formation of a stable residue (39.17%). This value is similar to that (39.13%) attributed to the Pr6O11 formation via the formation of various forms of nitrogen oxides (NO, NO2 and N2O5) [5]. Meanwhile, the DTA curve indicates the endothermic behavior of these steps. The obtained TGA-DTA patterns of bare urea are depicted in Fig. 1(c). Schaber et al. [33] published an interesting study on the thermal behaviour of urea using various techniques under open reaction vessel and in nitrogen atmosphere. They pointed out that the reaction pathway is complex that yields a diverse compounds. With the aid of that study and others [28,34], the weight invariant endothermic peak at 138 °C could be ascribed to the urea melting process. The WL steps locates at the 143–450 °C temperature range (Fig. 1(c)) could be related to urea decomposition with the formation of cyanuric acid, ammonia, biuret, mmelide, ammeline and melamine. The DTA curve in Fig. 1(c) clearly indicates the endothermic nature of these steps. Taking into account that the measurements were conducted in air flow (Fig. 1(c)), the exothermic peak occured at 460 °C could be assigned to the oxidation of carbon residue formed at the end of urea decomposition. Bearing in mind these findings, the DTA behaviour at the 160–335 °C range (Fig. 1(a)) could be ascribed to the super-position of two opposing effects: (i) dehydration of praseodymium nitrate as well as urea decomposition (endothermic), and (ii) combustion of the organic compounds vapours formed at this temperature range (exothermic). From the absence of any endothermic effect for bare praseodymium nitrate and urea, it is plausible to suggest that the presence of praseodymium nitrate has a catalytic effect on the combustion of the organic compounds formed at this temperature range. Finally, the low-intensity exothermic peak at 559 °C could be attributed the Pr3+ → Pr4+ oxidation process, which leads to the formation of Pr6O11 phase. 3.2. Influence of the F/O ratio change Non-isothermal TGA-DTA results (Fig. 1) revealed that the heat treatment at 500 °C of the urea/praseodymium nitrate mixture is sufficient to decompose its components completely. Therefore, in this section, the influence of the F/O ratio change on the structure and surface characteristics of the praseodymium oxide has been investigated using 500 °C calcination temperature for 1 h. Fig. 2 depicts the obtained XRD diffraction patterns for the samples with F/O ratios in the 0.5–16 range. As can be shown, the diffractogram of sample with F/O = 0.5 exhibits reflections at 2θ = 28.45°, 32.89°, 47.09°, 55.84°, 58.54°, 68.67°, 75.86° and 78.26°, which can be assigned to the Pr6O11 phase (JCPDS card No. 42-1121) with cubic fluorite structure, lattice constant of ao = 0.54678 nm and the space group Fm3m. No other peaks attributable to the parent mixture, other Pr-oxides or impurities were detected. In this context, the exothermic peak at 559 °C (Fig. 1(a)) was correlated with the formation of Pr6O11 phase. From the detection of that phase for the 500 °C product, it is reasonable to suggest that the isothermal heating of the parent mixture at 500 °C for 1 h has shifted that effect towards lower temperatures. For of the prepared samples employing the F/O ratios of 1.0 and 2.0, the relevant XRD patterns (Fig. 2(b, c)) indicate that these solids exhibit the same reflections attributable to the cubic fluorite phase of Pr6O11. Upon changing the F/O ratio to 4.0, the obtained XRD pattern (Fig. 2(d)) reveals the broadening of the Pr6O11 peaks and the appearance of a new shoulder peak at 2θ = 30.03°. Upon further rise in the F/O vale up to 16.0, the intensity of the peaks due to Pr6O11 phase continue to decrease and additional XRD peaks appeared at 2θ = 13.57°,
Fig. 5. TEM images for the Pr-0.5-500 (a), Pr-4-500 (b) and Pr-16-500 (c) samples.
GmbH spectrometer with a monochromatic Al Kα (hν = 1486.6 eV) Xray source. The C1s peak (284.6 eV) was used as a reference position in analysing the obtained results. A Pyrex glass cell, operated at the 200–400 °C temperature range, was using for the electrical resistivity measurements of the Pr6O11 samples calcined at 400–700 °C. The resistance of the tested samples was measured with the aid of Keithley 610C Solid state electrometer. 3. Results and discussion 3.1. Non-isothermal behaviour Fig. 1(a) depicts the TGA-DTA patterns obtained upon non-isothermal heat treatment, from ambient to 700 °C, of the dried urea/ praseodymium nitrate mixture, with F/O ratio = 0.5. The TGA pattern clearly reveals a consecutive weight loss (WL) events that start from 40 to 500 °C. The relevant DTA thermogram shows a multi endo-exothermic peaks. In order to understand and to relate these thermal steps 249
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Scheme 1. The preparation of Pr-nanostructures sing various F/O ratios.
23.27°, 31.50°, 41.85° and 45.33°. These newly emerged peaks could be assigned to the monoclinic phase of Pr2O2CO3 (JCPDS card No. 250696). From the inspection of the data presented in Fig. 2, it appears that increasing the F/O value ≥4.0 leads to the coexistence of both Pr6O11 and Pr2O2CO3 phases. Moreover, a crystallinity loss of these phases is evident. Concurrently, the coexistence of these phases was observed during the thermal degradation of praseodymium acetate [6]. Based on the X-ray line-width broadening, the crystallite sizes of the obtained Pr6O11 were calculated, using four main peaks in Fig. 2, employing the Scherrer equation. The values obtained were 13, 10, 18, 6, 5, and 5 nm for the samples with F/O ratio of 0.5, 1, 2, 4, 8, and 16, respectively. FT-IR spectra for the solid products formed at 500 °C using a F/O values within the 0.5–16.0 range are shown in Fig. 3. The spectra for the samples synthesized with a F/O ratios of 0.5–2.0 displayed two absorptions 666–671 and 444–516 cm−1, which can be ascribed the PreO stretching vibration [5,35,36]. Another absorption appear at approximately 851, 1067, 1100, 1198, 1390 and 1472 cm−1, which could be related to the carbonate anion modes of vibration [6,36–38]. A strong distinctive absorption appear at 3605 cm−1. This peak, which is associated with the broad one due to the OeH bond of water molecules (3000–3700 cm−1), could be related to the PreOH vibration [7,39]. In this context, the detection of carbonate vibrations is not surprising and agrees with the previous reports about the ability of the various rare earth oxides to form surface carbonate structures [40,41]. For the samples with F/O ratio ≥4.0, the obtained spectra reveal the following modifications: (i) the disappearance of the PreOH peak, (ii) an increase
in the carbonate anions peaks-intensity, and (iii) a shift of the absorptions due to PreO stretching vibration 513–521 and 432–440 cm−1. These findings agree with the detection of Pr2O2CO3 phase upon the F/ O ratio increase (Fig. 2). The surface morphology of the samples having F/O ratios of 0.5, 4 and 16 was examined using SEM and the obtained data are shown in Fig. 4. This Figure clearly elucidate the grain structure modifications upon the F/O ratio increase. Inspection of Fig. 4(a) reveals that the sample with F/O = 0.5 exhibits a layered-flake structure that is densely packed. The large amount of gases evolved during the precursor decomposition could be responsible for this porosity development. Increasing the F/O ratio to 4, i.e. for Pr-4-500 sample, the obtained SEM image (Fig. 4(b)) reveals the etching of the flakes edges. In addition, an obvious densification of the material occurred as shown by the decrease of the space among the particles. More obvious change can be observed as a result of increasing the F/O ratio to 16 (Fig. 4(c)). This sample shows particles with a rod-like morphology and the average length of these rods is 8–19 μm. These observed morphological changes could be considered as a direct response to the obtained phase changes upon the 0.5 to 16 rise in the F/O ratio. The morphology of the samples presented in Fig. 4 was further characterized by TEM analysis. It can be seen from Fig. 5(a) that the layered-flake structure of the Pr-0.5-500 sample is composed of quasi-spherical nanoparticles having 11–27 nm diameters. Similar morphology can be seen for the Pr-4-500 sample, Fig. 5(b), but with slightly smaller particles size (6–18 nm). TEM image depicted in Fig. 5(c) indicates that the rods of the Pr-16-500 sample are 250
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Fig. 6. XRD patterns recorded for Pr-nitrate (a) and the various Pr-samples with the F/O = 0.5 and calcined at 300 °C (b), 400 °C (c), 500 °C (d), 600 °C (e) and 700 °C (f). Fig. 7. FT-IR spectra obtained for Pr-nitrate (a) and the various Pr-samples with the F/O = 0.5 and calcined at 300 °C (b), 400 °C (c), 500 °C (d), 600 °C (e) and 700 °C (f).
composed of particles with dimensions in the range 7–25 nm. Schematic diagram that illustrates the preparation of Pr-nanostructures sing various F/O ratios is depicted in Scheme 1. 3.3. Influence of the calcination temperature rise
characterizing the Pr6O11 phase, and (ii) an intensity decrease of the bands due to the nitrate anion and water molecules. This suggests the co-existence of Pr6O11 and Pr(NO3)3·6H2O as a constituents of the Pr0.5-400 sample. The XRD patterns of the 500–700 °C calcined samples clearly indicate the presence of only one phase, viz. Pr6O11 (JCPDS card No. 42-1121). Meanwhile, the relevant FT-IR spectra illustrated the disappearance of all nitrate anions absorptions and the development of new ones due to carbonate anions and PreOH vibrations. It is evident that the intensity of the carbonate anions vibrations is lowered upon the calcination temperature rise. This finding clearly reveals the ability of the ability of the formed Pr6O11 at temperatures such as 700 °C to form surface carbonate. The crystallite size of the 500–700 calcined products was calculated using the Sherrer equation. The obtained values were 13, 16 and 20 nm for the Pr6O11 obtained at 500, 600 and 700 °C, respectively. Inspection of the SEM micrographs of the Pr-0.5-700 sample (Fig. 8(a)) reveals the uniform distribution of homogeneous microstructures. Similar to the Pr-0.5-500 sample morphology, these microstructures possess the layered-flake morphology. However, it is obvious that the Pr-0.5-700 sample has lower agglomerate size. TEM image of the Pr-0.5-700 sample (Fig. 8(b)) clearly shows that these flakes are composed of quasi-like spheres with diameters of these particles in the range of 14–62 nm, which are larger than those of Pr-0.5-500 sample. In this context, it was demonstrated that the very small particle size are responsible for the pronounced agglomeration [43]. This suggestion is supported by the observed development of the particles size, together with the reduction of the agglomerates size, accompanying the rise of pre-treatment temperature from 500 to 700 °C. Similar argument was used by Matović et al. [43] for the interpretation of the effect of the pre-
In the previous section, it was concluded that the F/O ratio increase leads to the Pr2O2CO3 phase formation at the expense of Pr6O11. Therefore, in order to study the influence of the temperature of calcination on the formation and the physicochemical features of bare Pr6O11 the precursor with the lowest F/O ratio was selected. Figs. 6 and 7 show the XRD diffractograms and FT-IR spectra of the 300–700 °C calcined samples, respectively. For comparison the data of bare hydrated praseodymium nitrate was also provided. In comparison with the bare praseodymium nitrate, the XRD pattern of Pr-0.5-300 shows an intensity decrease and disappearance of some reflections due to that precursor. Moreover, new reflections appeared at 2θ = 21.54°, 44.72° and 48.26°. The FT-IR spectrum of this sample shows absorptions at 1450, 1324, 1040, 815, and 838 cm−1 attributable to Pr(NO3)3 [5,42]. A broad absorption can be observes at 700–400 cm−1, which could be ascribed to PreO bond [5,35,36]. The two bands at 3420 and 1636 cm−1 could be, respectively, attributed to the ν(OH) and δ(HOH) vibration modes of water molecules [37]. In addition, the FT-IR spectrum of the Pr-0.5-300 sample revealed the absence of any peak due to urea. This picture suggests that the Pr-0.5-300 sample is composed of praseodymium nitrate with a lower hydration order; probably in the form of mon-hydrated salt as reported by Hussein et al. [5]. Increasing the heating temperature to 400 °C leads to further intensity decrease and the disappearance of other peaks due to hydrated praseodymium nitrate together (Fig. 6). Moreover, development of new reflections attributable to Pr6O11 (JCPDS card No. 42-1121) can be observed. The relevant FR-IR spectrum of Pr-0.5-400 sample (Fig. 7) reveals the following modifications: (i) emergence new peaks at 618 and 523 cm−1 251
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Fig. 8. SEM (a) and TEM (b) images of Pr-0.5-700 sample.
treatment temperature on the morphological modifications of Pr6O11 synthesized by the room temperature self-propagating method. In order to gain more structural information about the upper layers of the prepared Pr6O11, the Pr-0.5-700 sample was subjected to XPS analysis and the obtained results are presented in Fig. 9. The survey spectrum (not shown) showed the existence of C, O and Pr elements on the surface of Pr-0.5-700 sample. The Pr 3d spectrum (Fig. 9(a)) shows two asymmetric peaks being maximized at a binding energy (BE) values of 954.75 and 934.42 eV. These peaks that could be indexed to the 3d3/ 2 and 3d5/2 core-levels of Pr, respectively [44–46]. The spin-orbitsplitting energy, 20.33 eV, agrees well with the literature data [44–46]. The 3d5/2 peak can be fitted into two contributions at 934.63 and 930.22 eV, which could be assigned to the Pr4+ and Pr3+ oxidations states, respectively [44–47]. The O 1s spectrum (Fig. 9(b)) shows one asymmetric peak, which can be fitted to four symmetric ones. We could assign the weak-intensity contributions at BE of 525.47 and 528.21 eV to the unstable oxygen moiety [40] and the PreO bond [45], respectively. The peak at BE of 529.79 eV is usually assigned to adsorbed oxygen and/or oxygen in carbonate and/or hydroxyl species [40,45,47]. The peak appeared at 532.59 eV could be related to adsorbed water molecules [40]. The peak-fitted data of the C 1s core-level (Fig. 9(c)) shows the presence of five peaks corresponding to five carbon species. The main contributions at BE of 284.60 eV and 286.15 eV cold be ascribed to the CeH and CeC bonding, respectively [48–51]. The peak at BE of 288.37 eV can be attributed to the C]O bond [48–51]. The feature at BE of 290.47 eV corresponds to the oxygen bonds in CO32− [48–51] that are adsorbed onto PrOx surface [52]. In this context, the carbonate species detection, via XPS analysis, agrees well with detection of surface carbonates on this series of samples via FT-IR spectroscopy. Regarding the weak feature at 282.59 eV (Fig. 9(c)), literature survey revealed the lack of information about such peak for PreOeC system(s). Within the same BE range, Chen et al. [48]
assigned a peak at 282.4 eV to the carbon–boron bond. Shutthanandan et al. [53] reported a peak at 283.0–283.5 eV attributable to high concentration of surface LieC formed during the reduction of ethylene carbonate. Dev [54] reported an XPS peak at ∼282 eV for the TieC bonding. Based on these literature data, it is reasonable to ascribe the feature at 282.59 eV (Fig. 9(c)) to the PreC bonding. Fig. 10(a) shows the DC electrical conductivity dependence on the calcination temperature for the praseodymium samples synthesized employing a F/O ratio of 0.5 at a measuring temperature range 200–400 °C in static air atmosphere. It is evident that the conductivity rise with the measuring temperature for the investigated samples. The lowest and highest conductivity values were exhibited by the samples pre-treated at 400 and 500 °C, respectively. Further temperature increase above 500 °C leads to a gradual conductivity decrease. Within the both the pre-treatment and the measuring temperature ranges, Pr6O11 is the stable phase in air atmosphere [55]. Bearing in mind the multiple praseodymium oxidation states (+3 and +4) in Pr6O11 phase, the electron hopping between the lattice Pr3+/Pr4+ pairs is responsible for the reported its high electrical conductivity [7]. In this respect, Kharton et al. [56] reported that the incorporation of Pr into Ce0.8Gd0.2O2−δ has led to a 2.5–4 times increase in its electrical conductivity and a decrease in the activation energy of conductance from 145 to 125 kJ/mol. Wang et al. [35] reported an electrical conductivity improvement of Pr6O11/polypyrrole composite as a result of increasing the Pr6O11 concentration from 5 to 10 wt%. The existence of the mixed valence Pr3+/Pr4+ pairs was responsible for the electrical conductivity improvement of other Pr-containing systems [37,57]. Accordingly, the observed increase in the conductivity during the pre-treatment temperature rise from 400 to 500 °C could be ascribed to the completion of the formation of Pr6O11 phase, i.e. formation of more Pr3+/Pr4+ pairs. For Pr6O11 prepared via the calcination of its nitrate, hydroxide and hydroxy-carbonate precursors, an inverse proportionality was reported 252
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Fig. 10. Plots of ln σ versus (a) the calcinations temperature and (b) the reciprocal of the absolute temperature for the Pr-samples.
Fig. 9. Pr 3d (a), O 1s (b) and C 1s (c) XPS spectra of Pr-0.5-400 sample.
4. Conclusions
between the crystallites dimensions and the electrical conductivity; upon decreasing the grains dimensions, the resistance of grain boundaries decreased too [7,55]. Hence, the same argument was used in the interpretation of the observed gradual conductivity decrease of Pr6O11 upon increasing the temperature of calcination above 500 °C (Fig. 10(a)). Fig. 10(b) depicts the Arrhenius plot of lnσ versus 1/T for the samples calcined at 400–700 °C. A good linearity can be observed (correlation coefficient > 0.99) with no inflection point. The activation energy, Ea, values were computed from the slope of these lines and the obtained values were 46.1, 40.9, 39.3 and 39.8 kJ/mol for the samples heated at 400, 500, 600 and 700 °C, respectively. This finding indicates that calcining the precursor at the 500–700 °C temperature range of has a little effect on the activation energy values. Moreover, the higher Ea value of the Pr-0.5-400 sample cold be related to the presence of Pr(NO3)3 and Pr6O11 mixture, i.e. less concentration of Pr3+/Pr4+ pairs. The obtained Ea values of Pr6O11 in this study are lower than that reported for Pr6O11 nano-rods prepared via the precipitation method (84.9 kJ/mol) [58] or that prepared by the oxidation of bulk Pr-metal (84.9–168.9 kJ/mol) [59].
In conclusion, cubic phase Pr6O11 nanoparticles was successfully prepared via the fine tuning of both the F/O ratio and the calcination temperature. Using a F/O ratio ≥4.0 leads to the formation of praseodymium carbonate as un-wanted by-product. On the other hand, calcining the parent Pr-precursor at 500 °C was necessary for its complete decomposition and obtaining Pr6O11 as a single phase. Moreover, the morphology of the obtained Pr-containing solids was more sensitive to the change in the F/O ratio. In comparison with Pr6O11 prepared by other routes, the higher electrical conductivity with low activation energy of the prepared Pr6O11 nanoparticles was attributed to the structure-related effect, which leads to the formation of Pr3+/Pr4+ ion pairs. Moreover, an inverse proportionality of the electrical conductivity of Pr6O11 with its crystallites size was obtained, which was correlated with the resistance decrease of the grain boundaries. The improved electrochemical properties of the prepared nanocrystalline Pr6O11 nominate it as a promising material in ceramics, high space charge carriers and electrical and sensing devices.
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Acknowledgement This work was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant No. (130-108-D1434). The author, therefore, acknowledge with thanks DSR technical and financial support.
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Full Length Article
Controlled synthesis of praseodymium oxide nanoparticles obtained by combustion route: Effect of calcination temperature and fuel to oxidizer ratio
T
Bahaa M. Abu-Zied Chemistry Department, Faculty of Science , King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
ARTICLE INFO Keywords: Pr6O11 nanoparticles Praseodymium oxide Rare earth oxides Combustion synthesis Nanomaterials
ABSTRACT
This investigation focuses on the employment of the solution combustion route for synthesis of praseodymium oxide nanoparticles. The fabrication was be carried out by using the solution combustion of a fuel (urea) and oxidizer (praseodymium nitrate) mixture. The influence of fuel/oxidizer ratio and the temperature of calcination on the phases formed, morphological changes and the crystallite size of Pr6O11 have been checked. Various characterization tools have been used for the identification of the thermal behaviour of the precursors, structure and morphology of the prepared Pr6O11 nanoparticles, which include TGA-DTA, XRD, FT-IR, XPS, SEM, and TEM. With respect to the effect of the fuel content, it was found that using F/O ratio ≥4.0 at 500 °C praseodymium carbonate starts to form at the expense of the oxide phase. These phase changes were, also, companied by a noticeable morphological modifications. On the other hand, it was shown that Pr6O11 can be obtained as a single phase upon calcining at temperatures ≥500 °C. The electrical conductivity of Pr6O11 nanoparticles synthesized at 400–700 °C has been investigated over a measuring temperature range of 200–400 °C. The obtained results have led to a better understanding of the factors influencing the electrical conductivity behaviour of Pr6O11 nanoparticles, viz. the availability of Pr3+/Pr4+ pairs and the crystallite size.
1. Introduction Among the various rare earth metal oxides, praseodymium oxide (PrOx) possesses many phases with various oxygen contents, with × ranging from 1.5 to 2 [1–5]. In this sense, different formulations have been reported comprising × = 1.833, 1.818, 1.810, 1.800, 1.780, 1.777, 1.714, 1.667, and 1.600. PrO2 adopts the calcium fluoride structure whereas Pr2O3 structurally related to the hexagonal lanthanum oxide structure [3,4]. Oxides with O/Pr > 1.5 crystallize in the fluorite structure [4]. Pr6O11 was reported to be the most stable composition at ambient temperature and pressure [4]. Regarding the electrical conductivity of praseodymium oxides, it was also shown that PrOx phases with × < 1:73 are a p-type semiconductors; at higher value of ×, it shows the n-type semi-conductivity behaviour [5]. The electron hopping, occurring between the lattice mixed-metal valence states, was proposed to be responsible for the high electrical conductivity of Pr6O11 [2,6,7]. Recently, praseodymium oxides have been investigated as a subject of various emerging applications or processes. Pr6O11 was used as hydrocarbon sensor [8]. When Pr6O11 is added to ZnO, CuO, WO3 and
SnO2⋅Co2O3⋅Ta2O5 the product is used as varistors, which is applied in electronic devices for the protection of these devices from the voltage surge [9–12]. Praseodymium oxide can form with ceria solid solution which can be used as oxygen-storage component, which is used in solid oxide fuel cell (SOFC) as a cathode material [13–15]. Doping of the CeO2 lattice with the Pr6O11 chromophore yields ceramic glazes with colours that ranged from red-brown to magenta-orange depending on the chromophore content, synthesis protocol and temperatures of pretreatment [16–19]. Praseodymium oxides are largely employed as catalysts or in catalyst formulations (as promoters or stabilizers) in various processes in the area of both industry and environment. For example, Pr6O11 oxide was a catalyst for the dehydration and dehydrogenation of 1,3-butanediol and 2-butanol [20]. It was concluded that the praseodymium oxide addition to ceria increases its activity during CO oxidation and methane combustion [21,22]. Bulk Pr6O11 is usually prepared by several methods such as the thermolysis of praseodymium salts, such as the nitrate and the acetate salts, at high-temperatures [5,6]. Over the past few years, novel protocols in the synthesis praseodymium oxide nanoparticles have been
E-mail address: [email protected]. https://doi.org/10.1016/j.apsusc.2018.12.007 Received 17 October 2018; Received in revised form 21 November 2018; Accepted 2 December 2018 Available online 03 December 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 2. XRD patterns recorded for the Pr-samples with the F/O ratios of 0.5 (a), 1.0 (b), 2.0 (c), 4.0 (d), 8.0 (e) and 16.0 (f) calcined at 500 °C.
Fig. 1. TGA-DTA curves obtained for urea/praseodymium nitrate mixture (a), bare praseodymium nitrate (b) and bare urea (c).
developed. For instance, Pr6O11 nanofibers were successfully prepared by the electrospinning of an aqueous mixture of polyvinyl acetate and praseodymium nitrate [23]. Pr6O11 nanorods were obtained by the calcination, at 400–600 °C, of Pr(OH)3 nanorods precursor prepared via precipitation method [24,25]. Kang et al. [26] prepared Pr6O11 nanorods hydrothermal method employing the hydrothermal route and subsequent calcination at temperatures ≥550 °C. Over the past two decades, combustion synthesis (CS) has gained increased attention as facile technique for the preparation of nanomaterials. It comprises a self-sustained redox reaction with high exothermicity between a fuel (organic substance like hydrazides, glycine, urea, sucrose etc.) and an oxidizer (usually metal nitrate). Depending upon the nature of reactants and the reaction exothermicity CS is classified to (i) solid state combustion (SSC) where all the starting materials as well as the products are in the solid state, and (ii) solution combustion synthesis (SCS) which involves self-sustained reaction by rapidly heating a homogeneous solution containing a mixture of the fuel and the oxidizer [27]. Focusing our attention to the SCS, it was successfully employed in the preparation of a variety of nanomaterials such as CdCr2O4 [28], MgO [29], CdO [30] and Co3O4 [31,32]. The role of the combustion fuel can be indicated as follows: (i) it can complexes with the metal cations leading to a homogeneity increasing of the mixture,
Fig. 3. FT-IR spectra obtained for the Pr-samples with the F/O ratios of 0.5 (a), 1.0 (b), 2.0 (c), 4.0 (d), 8.0 (e) and 16.0 (f) calcined at 500 °C. 247
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Fig. 4. SEM images for the Pr-0.5-500 (a), Pr-4-500 (b) and Pr-16-500 (c) samples.
and (ii) it is considered as carbon and hydrogen source, which are essential in the combustion process and the evolution of a plenty of gases. These gases are hypergolic, i.e. they can easily ignite upon acquiring the critical density, even at ambient pressure, at their ignition temperature. As far as we are aware, there is a lack of papers reporting the synthesis of nanocrystalline Pr6O11 via the combustion route. Thus, the main target of the present investigation is to synthesize praseodymium oxide nanoparticles using the combustion route. Moreover, we aimed to address the influence of: (i) the fuel/ oxidized molar ratio of, and (ii) the temperature of calcination on the formation, structural and textural features of the praseodymium oxide. Different techniques have been employed for characterization of the obtained nanoparticles: TGA-DTA, XRD, FT-IR, XPS, SEM and TEM. Moreover, the electrical resistivity of the Pr-samples pre-treated at 400–700 °C range has been investigated.
over a hotplate till a viscous gel was formed. Finally, the obtained gels have been calcined in air atmosphere at 500 °C for 1 h. In the second series, effect of increasing the temperature of calcination on the formation, morphology and crystallite size of Pr6O11 nanoparticles has been studied. For the preparation of this series, the required amounts of urea and praseodymium nitrate for obtaining a F/O ratio of 0.5 have been dissolved in distilled water. The hotplate dried mixtures have been calcined in air atmosphere at 400–700 °C range for 1 h. The various samples obtained will be denoted in the text using notations Pr-x-y, where × refers to the F/O and y the calcination temperature. 2.2. Techniques TGA and DTA analyses have been used to check the thermal stability of the reaction precursors, viz. urea and praseodymium nitrate, and their mixture. The measurements have been performed on a Shimadzu DTG-60 thermal analyser. The measurements conditions were: 10 mg sample weight, 10 °C/min heating rate, and air flow of 40 ml. min−1. The XRD patterns of the various Pr-containing solids were obtained using a Philips (type PW 103/00) diffractometer with Cu Kα (λ = 1.5405 Å) radiation operated at 35 kV, 20 mA and a scanning rate of 0.06 min−1 over the 2θ range of 4–80°. FT-IR spectra were collected using the Attenuated total reflection (ATR) attachment of Nicolet iS50 FT-IR spectrometer. The microscopic investigations of some selected samples have been conducted on: (i) JEOL JSM-5400 LV scanning electron microscope (SEM) and (ii) JEOL (model JEMTH-100 II) transmission electron microscope (TEM). The nature of surface elements and their chemical binding has been investigated using SPECS
2. Experimental 2.1. Materials and samples preparation The combustion fuel (urea) and the praseodymium nitrate (Pr (NO3)3·6H2O), both obtained from BDH, were of > 99% purity. The preparation of Pr6O11 nanoparticles were briefly described below. Two series of samples have been synthesized. The first one addresses the effect of changing the fuel (F) to the oxidizer (nitrate anion, O) molar ratio on the phase composition of the obtained solids has been investigated. For the synthesis of this series, the required amounts of urea and praseodymium nitrate for the F/O molar ratios of 0.5, 1, 2, 4, 8 and 16 were dissolved in distilled water. The obtained solutions were dried 248
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to a certain thermal events, the thermal behaviour of the parent mixture components has been investigated using the same thermal analyser under the same conditions. Fig. 1(b) depicts the obtained TGA-DTA thermograms of bare praseodymium nitrate. This thermal behaviour can be interpreted using the previous results of Hussein et al. [5]. The consecutive early WL (24.85%), which extends from 40 to 300 °C, is very close to that ascribed to the elimination of the hydration water molecules (24.82%). Another consecutive WL steps can be observed at the 305–550 °C temperature range that ends with the formation of a stable residue (39.17%). This value is similar to that (39.13%) attributed to the Pr6O11 formation via the formation of various forms of nitrogen oxides (NO, NO2 and N2O5) [5]. Meanwhile, the DTA curve indicates the endothermic behavior of these steps. The obtained TGA-DTA patterns of bare urea are depicted in Fig. 1(c). Schaber et al. [33] published an interesting study on the thermal behaviour of urea using various techniques under open reaction vessel and in nitrogen atmosphere. They pointed out that the reaction pathway is complex that yields a diverse compounds. With the aid of that study and others [28,34], the weight invariant endothermic peak at 138 °C could be ascribed to the urea melting process. The WL steps locates at the 143–450 °C temperature range (Fig. 1(c)) could be related to urea decomposition with the formation of cyanuric acid, ammonia, biuret, mmelide, ammeline and melamine. The DTA curve in Fig. 1(c) clearly indicates the endothermic nature of these steps. Taking into account that the measurements were conducted in air flow (Fig. 1(c)), the exothermic peak occured at 460 °C could be assigned to the oxidation of carbon residue formed at the end of urea decomposition. Bearing in mind these findings, the DTA behaviour at the 160–335 °C range (Fig. 1(a)) could be ascribed to the super-position of two opposing effects: (i) dehydration of praseodymium nitrate as well as urea decomposition (endothermic), and (ii) combustion of the organic compounds vapours formed at this temperature range (exothermic). From the absence of any endothermic effect for bare praseodymium nitrate and urea, it is plausible to suggest that the presence of praseodymium nitrate has a catalytic effect on the combustion of the organic compounds formed at this temperature range. Finally, the low-intensity exothermic peak at 559 °C could be attributed the Pr3+ → Pr4+ oxidation process, which leads to the formation of Pr6O11 phase. 3.2. Influence of the F/O ratio change Non-isothermal TGA-DTA results (Fig. 1) revealed that the heat treatment at 500 °C of the urea/praseodymium nitrate mixture is sufficient to decompose its components completely. Therefore, in this section, the influence of the F/O ratio change on the structure and surface characteristics of the praseodymium oxide has been investigated using 500 °C calcination temperature for 1 h. Fig. 2 depicts the obtained XRD diffraction patterns for the samples with F/O ratios in the 0.5–16 range. As can be shown, the diffractogram of sample with F/O = 0.5 exhibits reflections at 2θ = 28.45°, 32.89°, 47.09°, 55.84°, 58.54°, 68.67°, 75.86° and 78.26°, which can be assigned to the Pr6O11 phase (JCPDS card No. 42-1121) with cubic fluorite structure, lattice constant of ao = 0.54678 nm and the space group Fm3m. No other peaks attributable to the parent mixture, other Pr-oxides or impurities were detected. In this context, the exothermic peak at 559 °C (Fig. 1(a)) was correlated with the formation of Pr6O11 phase. From the detection of that phase for the 500 °C product, it is reasonable to suggest that the isothermal heating of the parent mixture at 500 °C for 1 h has shifted that effect towards lower temperatures. For of the prepared samples employing the F/O ratios of 1.0 and 2.0, the relevant XRD patterns (Fig. 2(b, c)) indicate that these solids exhibit the same reflections attributable to the cubic fluorite phase of Pr6O11. Upon changing the F/O ratio to 4.0, the obtained XRD pattern (Fig. 2(d)) reveals the broadening of the Pr6O11 peaks and the appearance of a new shoulder peak at 2θ = 30.03°. Upon further rise in the F/O vale up to 16.0, the intensity of the peaks due to Pr6O11 phase continue to decrease and additional XRD peaks appeared at 2θ = 13.57°,
Fig. 5. TEM images for the Pr-0.5-500 (a), Pr-4-500 (b) and Pr-16-500 (c) samples.
GmbH spectrometer with a monochromatic Al Kα (hν = 1486.6 eV) Xray source. The C1s peak (284.6 eV) was used as a reference position in analysing the obtained results. A Pyrex glass cell, operated at the 200–400 °C temperature range, was using for the electrical resistivity measurements of the Pr6O11 samples calcined at 400–700 °C. The resistance of the tested samples was measured with the aid of Keithley 610C Solid state electrometer. 3. Results and discussion 3.1. Non-isothermal behaviour Fig. 1(a) depicts the TGA-DTA patterns obtained upon non-isothermal heat treatment, from ambient to 700 °C, of the dried urea/ praseodymium nitrate mixture, with F/O ratio = 0.5. The TGA pattern clearly reveals a consecutive weight loss (WL) events that start from 40 to 500 °C. The relevant DTA thermogram shows a multi endo-exothermic peaks. In order to understand and to relate these thermal steps 249
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Scheme 1. The preparation of Pr-nanostructures sing various F/O ratios.
23.27°, 31.50°, 41.85° and 45.33°. These newly emerged peaks could be assigned to the monoclinic phase of Pr2O2CO3 (JCPDS card No. 250696). From the inspection of the data presented in Fig. 2, it appears that increasing the F/O value ≥4.0 leads to the coexistence of both Pr6O11 and Pr2O2CO3 phases. Moreover, a crystallinity loss of these phases is evident. Concurrently, the coexistence of these phases was observed during the thermal degradation of praseodymium acetate [6]. Based on the X-ray line-width broadening, the crystallite sizes of the obtained Pr6O11 were calculated, using four main peaks in Fig. 2, employing the Scherrer equation. The values obtained were 13, 10, 18, 6, 5, and 5 nm for the samples with F/O ratio of 0.5, 1, 2, 4, 8, and 16, respectively. FT-IR spectra for the solid products formed at 500 °C using a F/O values within the 0.5–16.0 range are shown in Fig. 3. The spectra for the samples synthesized with a F/O ratios of 0.5–2.0 displayed two absorptions 666–671 and 444–516 cm−1, which can be ascribed the PreO stretching vibration [5,35,36]. Another absorption appear at approximately 851, 1067, 1100, 1198, 1390 and 1472 cm−1, which could be related to the carbonate anion modes of vibration [6,36–38]. A strong distinctive absorption appear at 3605 cm−1. This peak, which is associated with the broad one due to the OeH bond of water molecules (3000–3700 cm−1), could be related to the PreOH vibration [7,39]. In this context, the detection of carbonate vibrations is not surprising and agrees with the previous reports about the ability of the various rare earth oxides to form surface carbonate structures [40,41]. For the samples with F/O ratio ≥4.0, the obtained spectra reveal the following modifications: (i) the disappearance of the PreOH peak, (ii) an increase
in the carbonate anions peaks-intensity, and (iii) a shift of the absorptions due to PreO stretching vibration 513–521 and 432–440 cm−1. These findings agree with the detection of Pr2O2CO3 phase upon the F/ O ratio increase (Fig. 2). The surface morphology of the samples having F/O ratios of 0.5, 4 and 16 was examined using SEM and the obtained data are shown in Fig. 4. This Figure clearly elucidate the grain structure modifications upon the F/O ratio increase. Inspection of Fig. 4(a) reveals that the sample with F/O = 0.5 exhibits a layered-flake structure that is densely packed. The large amount of gases evolved during the precursor decomposition could be responsible for this porosity development. Increasing the F/O ratio to 4, i.e. for Pr-4-500 sample, the obtained SEM image (Fig. 4(b)) reveals the etching of the flakes edges. In addition, an obvious densification of the material occurred as shown by the decrease of the space among the particles. More obvious change can be observed as a result of increasing the F/O ratio to 16 (Fig. 4(c)). This sample shows particles with a rod-like morphology and the average length of these rods is 8–19 μm. These observed morphological changes could be considered as a direct response to the obtained phase changes upon the 0.5 to 16 rise in the F/O ratio. The morphology of the samples presented in Fig. 4 was further characterized by TEM analysis. It can be seen from Fig. 5(a) that the layered-flake structure of the Pr-0.5-500 sample is composed of quasi-spherical nanoparticles having 11–27 nm diameters. Similar morphology can be seen for the Pr-4-500 sample, Fig. 5(b), but with slightly smaller particles size (6–18 nm). TEM image depicted in Fig. 5(c) indicates that the rods of the Pr-16-500 sample are 250
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Fig. 6. XRD patterns recorded for Pr-nitrate (a) and the various Pr-samples with the F/O = 0.5 and calcined at 300 °C (b), 400 °C (c), 500 °C (d), 600 °C (e) and 700 °C (f). Fig. 7. FT-IR spectra obtained for Pr-nitrate (a) and the various Pr-samples with the F/O = 0.5 and calcined at 300 °C (b), 400 °C (c), 500 °C (d), 600 °C (e) and 700 °C (f).
composed of particles with dimensions in the range 7–25 nm. Schematic diagram that illustrates the preparation of Pr-nanostructures sing various F/O ratios is depicted in Scheme 1. 3.3. Influence of the calcination temperature rise
characterizing the Pr6O11 phase, and (ii) an intensity decrease of the bands due to the nitrate anion and water molecules. This suggests the co-existence of Pr6O11 and Pr(NO3)3·6H2O as a constituents of the Pr0.5-400 sample. The XRD patterns of the 500–700 °C calcined samples clearly indicate the presence of only one phase, viz. Pr6O11 (JCPDS card No. 42-1121). Meanwhile, the relevant FT-IR spectra illustrated the disappearance of all nitrate anions absorptions and the development of new ones due to carbonate anions and PreOH vibrations. It is evident that the intensity of the carbonate anions vibrations is lowered upon the calcination temperature rise. This finding clearly reveals the ability of the ability of the formed Pr6O11 at temperatures such as 700 °C to form surface carbonate. The crystallite size of the 500–700 calcined products was calculated using the Sherrer equation. The obtained values were 13, 16 and 20 nm for the Pr6O11 obtained at 500, 600 and 700 °C, respectively. Inspection of the SEM micrographs of the Pr-0.5-700 sample (Fig. 8(a)) reveals the uniform distribution of homogeneous microstructures. Similar to the Pr-0.5-500 sample morphology, these microstructures possess the layered-flake morphology. However, it is obvious that the Pr-0.5-700 sample has lower agglomerate size. TEM image of the Pr-0.5-700 sample (Fig. 8(b)) clearly shows that these flakes are composed of quasi-like spheres with diameters of these particles in the range of 14–62 nm, which are larger than those of Pr-0.5-500 sample. In this context, it was demonstrated that the very small particle size are responsible for the pronounced agglomeration [43]. This suggestion is supported by the observed development of the particles size, together with the reduction of the agglomerates size, accompanying the rise of pre-treatment temperature from 500 to 700 °C. Similar argument was used by Matović et al. [43] for the interpretation of the effect of the pre-
In the previous section, it was concluded that the F/O ratio increase leads to the Pr2O2CO3 phase formation at the expense of Pr6O11. Therefore, in order to study the influence of the temperature of calcination on the formation and the physicochemical features of bare Pr6O11 the precursor with the lowest F/O ratio was selected. Figs. 6 and 7 show the XRD diffractograms and FT-IR spectra of the 300–700 °C calcined samples, respectively. For comparison the data of bare hydrated praseodymium nitrate was also provided. In comparison with the bare praseodymium nitrate, the XRD pattern of Pr-0.5-300 shows an intensity decrease and disappearance of some reflections due to that precursor. Moreover, new reflections appeared at 2θ = 21.54°, 44.72° and 48.26°. The FT-IR spectrum of this sample shows absorptions at 1450, 1324, 1040, 815, and 838 cm−1 attributable to Pr(NO3)3 [5,42]. A broad absorption can be observes at 700–400 cm−1, which could be ascribed to PreO bond [5,35,36]. The two bands at 3420 and 1636 cm−1 could be, respectively, attributed to the ν(OH) and δ(HOH) vibration modes of water molecules [37]. In addition, the FT-IR spectrum of the Pr-0.5-300 sample revealed the absence of any peak due to urea. This picture suggests that the Pr-0.5-300 sample is composed of praseodymium nitrate with a lower hydration order; probably in the form of mon-hydrated salt as reported by Hussein et al. [5]. Increasing the heating temperature to 400 °C leads to further intensity decrease and the disappearance of other peaks due to hydrated praseodymium nitrate together (Fig. 6). Moreover, development of new reflections attributable to Pr6O11 (JCPDS card No. 42-1121) can be observed. The relevant FR-IR spectrum of Pr-0.5-400 sample (Fig. 7) reveals the following modifications: (i) emergence new peaks at 618 and 523 cm−1 251
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Fig. 8. SEM (a) and TEM (b) images of Pr-0.5-700 sample.
treatment temperature on the morphological modifications of Pr6O11 synthesized by the room temperature self-propagating method. In order to gain more structural information about the upper layers of the prepared Pr6O11, the Pr-0.5-700 sample was subjected to XPS analysis and the obtained results are presented in Fig. 9. The survey spectrum (not shown) showed the existence of C, O and Pr elements on the surface of Pr-0.5-700 sample. The Pr 3d spectrum (Fig. 9(a)) shows two asymmetric peaks being maximized at a binding energy (BE) values of 954.75 and 934.42 eV. These peaks that could be indexed to the 3d3/ 2 and 3d5/2 core-levels of Pr, respectively [44–46]. The spin-orbitsplitting energy, 20.33 eV, agrees well with the literature data [44–46]. The 3d5/2 peak can be fitted into two contributions at 934.63 and 930.22 eV, which could be assigned to the Pr4+ and Pr3+ oxidations states, respectively [44–47]. The O 1s spectrum (Fig. 9(b)) shows one asymmetric peak, which can be fitted to four symmetric ones. We could assign the weak-intensity contributions at BE of 525.47 and 528.21 eV to the unstable oxygen moiety [40] and the PreO bond [45], respectively. The peak at BE of 529.79 eV is usually assigned to adsorbed oxygen and/or oxygen in carbonate and/or hydroxyl species [40,45,47]. The peak appeared at 532.59 eV could be related to adsorbed water molecules [40]. The peak-fitted data of the C 1s core-level (Fig. 9(c)) shows the presence of five peaks corresponding to five carbon species. The main contributions at BE of 284.60 eV and 286.15 eV cold be ascribed to the CeH and CeC bonding, respectively [48–51]. The peak at BE of 288.37 eV can be attributed to the C]O bond [48–51]. The feature at BE of 290.47 eV corresponds to the oxygen bonds in CO32− [48–51] that are adsorbed onto PrOx surface [52]. In this context, the carbonate species detection, via XPS analysis, agrees well with detection of surface carbonates on this series of samples via FT-IR spectroscopy. Regarding the weak feature at 282.59 eV (Fig. 9(c)), literature survey revealed the lack of information about such peak for PreOeC system(s). Within the same BE range, Chen et al. [48]
assigned a peak at 282.4 eV to the carbon–boron bond. Shutthanandan et al. [53] reported a peak at 283.0–283.5 eV attributable to high concentration of surface LieC formed during the reduction of ethylene carbonate. Dev [54] reported an XPS peak at ∼282 eV for the TieC bonding. Based on these literature data, it is reasonable to ascribe the feature at 282.59 eV (Fig. 9(c)) to the PreC bonding. Fig. 10(a) shows the DC electrical conductivity dependence on the calcination temperature for the praseodymium samples synthesized employing a F/O ratio of 0.5 at a measuring temperature range 200–400 °C in static air atmosphere. It is evident that the conductivity rise with the measuring temperature for the investigated samples. The lowest and highest conductivity values were exhibited by the samples pre-treated at 400 and 500 °C, respectively. Further temperature increase above 500 °C leads to a gradual conductivity decrease. Within the both the pre-treatment and the measuring temperature ranges, Pr6O11 is the stable phase in air atmosphere [55]. Bearing in mind the multiple praseodymium oxidation states (+3 and +4) in Pr6O11 phase, the electron hopping between the lattice Pr3+/Pr4+ pairs is responsible for the reported its high electrical conductivity [7]. In this respect, Kharton et al. [56] reported that the incorporation of Pr into Ce0.8Gd0.2O2−δ has led to a 2.5–4 times increase in its electrical conductivity and a decrease in the activation energy of conductance from 145 to 125 kJ/mol. Wang et al. [35] reported an electrical conductivity improvement of Pr6O11/polypyrrole composite as a result of increasing the Pr6O11 concentration from 5 to 10 wt%. The existence of the mixed valence Pr3+/Pr4+ pairs was responsible for the electrical conductivity improvement of other Pr-containing systems [37,57]. Accordingly, the observed increase in the conductivity during the pre-treatment temperature rise from 400 to 500 °C could be ascribed to the completion of the formation of Pr6O11 phase, i.e. formation of more Pr3+/Pr4+ pairs. For Pr6O11 prepared via the calcination of its nitrate, hydroxide and hydroxy-carbonate precursors, an inverse proportionality was reported 252
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Fig. 10. Plots of ln σ versus (a) the calcinations temperature and (b) the reciprocal of the absolute temperature for the Pr-samples.
Fig. 9. Pr 3d (a), O 1s (b) and C 1s (c) XPS spectra of Pr-0.5-400 sample.
4. Conclusions
between the crystallites dimensions and the electrical conductivity; upon decreasing the grains dimensions, the resistance of grain boundaries decreased too [7,55]. Hence, the same argument was used in the interpretation of the observed gradual conductivity decrease of Pr6O11 upon increasing the temperature of calcination above 500 °C (Fig. 10(a)). Fig. 10(b) depicts the Arrhenius plot of lnσ versus 1/T for the samples calcined at 400–700 °C. A good linearity can be observed (correlation coefficient > 0.99) with no inflection point. The activation energy, Ea, values were computed from the slope of these lines and the obtained values were 46.1, 40.9, 39.3 and 39.8 kJ/mol for the samples heated at 400, 500, 600 and 700 °C, respectively. This finding indicates that calcining the precursor at the 500–700 °C temperature range of has a little effect on the activation energy values. Moreover, the higher Ea value of the Pr-0.5-400 sample cold be related to the presence of Pr(NO3)3 and Pr6O11 mixture, i.e. less concentration of Pr3+/Pr4+ pairs. The obtained Ea values of Pr6O11 in this study are lower than that reported for Pr6O11 nano-rods prepared via the precipitation method (84.9 kJ/mol) [58] or that prepared by the oxidation of bulk Pr-metal (84.9–168.9 kJ/mol) [59].
In conclusion, cubic phase Pr6O11 nanoparticles was successfully prepared via the fine tuning of both the F/O ratio and the calcination temperature. Using a F/O ratio ≥4.0 leads to the formation of praseodymium carbonate as un-wanted by-product. On the other hand, calcining the parent Pr-precursor at 500 °C was necessary for its complete decomposition and obtaining Pr6O11 as a single phase. Moreover, the morphology of the obtained Pr-containing solids was more sensitive to the change in the F/O ratio. In comparison with Pr6O11 prepared by other routes, the higher electrical conductivity with low activation energy of the prepared Pr6O11 nanoparticles was attributed to the structure-related effect, which leads to the formation of Pr3+/Pr4+ ion pairs. Moreover, an inverse proportionality of the electrical conductivity of Pr6O11 with its crystallites size was obtained, which was correlated with the resistance decrease of the grain boundaries. The improved electrochemical properties of the prepared nanocrystalline Pr6O11 nominate it as a promising material in ceramics, high space charge carriers and electrical and sensing devices.
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Acknowledgement This work was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant No. (130-108-D1434). The author, therefore, acknowledge with thanks DSR technical and financial support.
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