稀有金属(英文版) 2018,37(03),181-190

Characterization of B site codoped LaFeO₃ nanoparticles prepared via co-precipitation route

Seyedeh Behnaz Varandili Alireza Babaei Abolghasem Ataie

School of Metallurgy and Materials Engineering,College of Engineering,University of Tehran

收稿日期：14 October 2015

基金：financially supported by University of Tehran(No.810729920/6/02);Iran Nanotechnology Initiative Council;

Characterization of B site codoped LaFeO₃ nanoparticles prepared via co-precipitation route

Seyedeh Behnaz Varandili Alireza Babaei Abolghasem Ataie

School of Metallurgy and Materials Engineering,College of Engineering,University of Tehran

Abstract：

LaFe_1-x-yCo_xPd_yO₃ [(x, y) =(0, 0),(0.40, 0),(0.38, 0.05)] nanoparticles were synthesized via a co-precipitation route using ammonium hydroxide, sodium hydroxide and ammonium carbonate as the precipitant and calcination at different temperatures to study the compositional driven structural changes in lanthanum ferrites.Analysis of X-ray diffraction(XRD) patterns confirms the formation of single-phase perovskite structure and existence of orthorhombic Pnma symmetry for calcined powders. Field emission scanning electron microscope(FESEM) observations show that Pd-doped powders yield finer particles along with narrower particle size distribution compared with LaFeO₃ and LaFe_0.6Co_0.4O₃. Moreover,using ammonia as the precipitant leads to a smaller mean particle size of powders compared to NaOH, as well as significant difference in morphology of the particles.Raman analysis reveals that both Co and Pd atoms substitute Fe site in perovskite structure with shifting of phonon modes. Comparing Raman spectra demonstrates the presence of more oxygen vacancies in Pd-doped perovskites. It can be concluded from the results that Pd is successfully incorporated into the perovskite structure by co-precipitation method.

Keyword：

Perovskites; Pd doping; Nanoparticles; Co-precipitation; Raman spectroscopy;

Author： Alireza Babaei e-mail:alireza.babaei@ut.ac.ir;

Received： 14 October 2015

1 Introduction

Perovskite compounds have been extensively investigated because of their large variety of unique properties.Typical structure of a perovskite consists of a large-sized 12coordinated cations at the A site and another small-sized 6coordinated cations at the B site.Introduction of certain doping agents to the crystal structure of perovskites results in significant persity in their catalytic,electrical and ionic conducting properties,which makes them an ideal candidate for catalytic conversion of pollutant emissions [ 1] ,as well as solid oxide cell (SOC) components [ 2, 3] .

Catalytic activity is usually assessed by the nature of B site cation.In order to attain a greater catalytic performance,particular properties such as specific surface area,micro structure,degree of crystallinity and the crystal morphology should be also considered.Thus a synthesis method which produces homogeneous micro structure of nano powders without impurities seems to be necessary.Among many available powder fabrication routes,solution-based techniques are especially demanded,because of their facility and low reaction temperature compared to solid-state reactions.Various solution synthetic methods have been reported for the synthesis of doped lanthanum ferrite nanoparticles,such as citrate complexation [ 2] ,solution combustion [ 4] and co-precipitation [ 5, 6, 7] .For example,Zhou et al. [ 5] prepared two kinds of catalysts,one palladium-doped perovskite (LaFe_0.77Co_0.17Pd_0.06O₃)and another perovskite supported palladium catalyst (Pd/LaFe_0.8Co_0.2O₃),The former was synthesized by a conventional co-precipitation method and the latter by impregnating a LaFe_0.8Co_0.2O₃ carrier.Then,the catalytic activities of two catalysts were evaluated under a condition approaching the real working cases of three way catalysts.They reported that Pd species in Pd/LaFe_0.8Co_0.2O₃catalyst can be reduced more easily compared to the other one.They attributed the higher activity of the Pd/LaFe_0.8-Co_0.2O₃ catalyst to the ease of reduction in palladium.Following the work of Tanaka et al. [ 7] ,an innovative LaFe_0.95-xCo_xPd_0.05O₃ perovskite catalyst,named the‘intelligent catalyst,was prepared by alkoxide method,and a thermal treatment was carried out to simulate the redox fluctuations of an automotive exhaust gas.They reported that these perovskites have a new self-regenerative function of the precious metal in the inherent fluctuations of automotive exhaust gas.

Not every conventional characterization technique is suitable for monitoring of doping and nanoscale related properties.But Raman spectroscopy has already evidenced to be an unrivaled tool for probing the atomic substitutions and resulting features such as charge transfer and structural distortion in multi oxide systems [ 8, 9, 10] .Raman effects in perovskites are originated from the vibrations of iono-covalent MO₆ octahedron entity due to the interaction with incoming photons [ 8] .Raman spectroscopy analysis has been carried out for the doped systems such as LaFe_1-xCr_xO₃ and LaMn_1-xCo_xO₃ [ 11, 12] .Very few investigations of Raman spectra for LaFeO₃ and LaFe_1-xCo_xO₃ materials [ 9] and almost no Raman results for LaFe_1-x-yCo_xPd_yO₃ were reported.

In this work,synthesis of LaFeO₃ nanoparticles was carried out via a co-precipitation method by using various alkali agents calcined at different temperatures.Then,the effects of Co and Pd substitution for Fe on the phase composition,morphology and Raman spectra of the samples were investigated.This work aims to optimize the preparation route for achieving a single-phase perovskite with proper morphological and structural characteristics of powders for further catalytic applications.

2 Experimental

2.1 Powder preparation

LaFeO₃ (LF),LaFe_0.6Co_0.4O₃ (LFC) and LaFe_0.57Co_0.38Pd_0.05O₃ (LFCP) were prepared via a co-precipitation method.In the synthesis process,an aqueous solution of stoichiometric amounts of La(NO₃)3·6H₂O,Fe(NO₃)3·9H₂O,Co(NO₃)2·6H₂O and Pd(NO₃)2·2H₂O (all from Merck),were prepared.Aqueous solutions of three precipitant agents including ammonium hydroxide (NH₄OH),sodium hydroxide (NaOH) and ammonium carbonate (C₂H₁₁N₃O₅)were added to the solution of metal nitrates in separate synthesis experiments,while magnetic stirring at 25℃,until the pH of solutions was adjusted to 10.The resultant precipitates were filtered off,washed with distilled water and dried at 90℃for 12 h.The dried precipitates were calcined in air at 800,900 and 1000℃for 1 h.In order to make a coarse-grained bulk sample for Raman analysis,1 g of LaFeO₃ powder was cold pressed by uniaxial pressing at400 MPa.Then,the green pellet was sintered at 1300℃

for 2 h.Table 1 summarizes the process conditions for powder preparation and sample codes.

2.2 Powder characterization

The phase constitutions of synthesized powders were determined by an X-ray powder diffractometer (XRD,Philips PW-1730 diffractometer) using Cu Kαradiation.The mean crystallite sizes were estimated,after amendment of instrumental contribution to the line broadening,from the Scherrer’s formula.Field emission scanning electron microscopy (FESEM) and energy-dispersive spectroscopy (EDS) were done by a TESCAN-MIRA3instrument to investigate the particles size,morphology and elemental composition of the powders.Microstructural image processing (MIP4student) software was employed to determine the particle size of synthesized powders.Raman spectra were recorded using a Bruker Senterra dispersive Raman spectrometer with a 785-nm laser beam as the light source and excitation power of 10 mW.

3 Results and discussion

3.1 XRD analysis

Figure 1 shows XRD patterns of LFC co-precipitated using NaOH,C₂H₁₁N₃O₅ and NH₄OH and calcined at 800,900and 1000℃.Analysis of XRD patterns reveals that the crystal framework of samples co-precipitated using NH₄OH and NaOH (except LFC-N-8) exhibits characteristic diffractions of perovskite single phase (JCPDS PDF No.44-0361) indexed to the orthorhombic Pnma space group.However,ammonium carbonate does not seem to be a proper precipitant for achieving a single-phase LCF perovskite.The powders precipitated by NaOH possess a single-phase structure at 900 and 1000℃,while they have Fe₃O₄ impurity phase (JCPDS PDF No.89-0950) at800℃.It seems that when the calcination temperature is adequately increased,all the intermediate complexes could contribute to the perovskite formation reaction.

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Table 1 Synthesized conditions and sample codes of synthesized powders

In the case of powders precipitated using NH₄OH,by lowering the calcination temperature from 900 to 800℃,the peak intensity in XRD pattern especially around2θ=23°and consequently the crystallinity of the phase are reduced significantly.As the lower calcination temperature is preferred for inhibiting particles from coarsening and achieving higher catalytic activity,900℃was selected as the optimum calcination temperature for further experiments.

Figure 2 compares XRD patterns of LF,LFC and LFCP powders co-precipitated separately using NaOH and NH₄OH and calcined at 900℃.Phase analysis of all patterns shows purely orthorhombic perovskite crystallinity(LaFeO₃ JCPDS PDF No.37-1493).Corresponding to each hkl index (Fig.2),the peak positions of XRD patterns for both LFC-N and LFC-AH groups tend to shift toward larger angles compared to those for LF-N and LF-AH,which is due to a decrement in the interplanar distance of crystals.The lattice parameters were calculated from XRD patterns based on d=(h²/a²+k²/b²+l²/C²).First,the diffraction patterns were indexed according to JCPDS cards.Then,three main high intensity reflections were preferably used to determine the d-values using Braggs law.The lattice parameters (a,b and c) were calculated and are listed in Table 2 along with the mean crystallite size of the samples.It is clear from Table 2 that for both kind of precipitants,the unit cell volume of LFC solid solution is smaller than that of LF.The crystallite size of LF-N-9 is67 nm,whereas it is equal to 42 nm in LFC-N-9.Also,the mean crystallite sizes are 43 and 31 nm for LF-AH-9 and LFC-AH-9,respectively.According to Shannon [ 13] ,the radius of Co³⁺(0.061 nm) is a little smaller than that of Fe³⁺(0.064 nm).So,the crystal shrinks without destroying its structure after partial replacement of the B site of the perovskite by Co³⁺,leading to a reduction in crystallite size.The resultant unit cell volume values in Table 2 are in agreement with the trend reported in Ref. [ 14] but opposite to the results in Ref. [ 15] .This contradiction might be due to the reduction in some of Co³⁺to Co²⁺during LFC processing.The larger radius of Co²⁺(0.0745 nm) [ 13] than that of Fe³⁺could increase the unit cell volume of LaFe_1-xCo_xO₃ oxides.

Fig.1 XRD patterns of LFC co-precipitated using NaOH,C₂H₁₁N₃O₅ and NH₄OH calcined at 800,900 and 1000℃

XRD patterns of LaFe_0.57Co_0.38Pd_0.05O₃ in Fig.2 displays a small shift to lower 2θvalues with respect to LaFe_0.6Co_0.4O₃.Existence of this small shift indicates that Pd is incorporated into the perovskite structure and the lattice parameters tend to be changed.Partial substitution of B site with larger radius Pd³⁺introduces an increase in unit cell volume compared to LaFe_0.6Co_0.4O₃.Furthermore,a few Pd ions may enter into the lattice with their 2+valence and create more oxygen vacancies [ 16] .Existence of oxygen vacancies is probably part of the reason why the cell volume values of LFCP samples are smaller than those of LF ones.No line corresponding to palladium oxide or metal can be observed in XRD patterns of LFCP samples.However,there is a broadening and drop off in the peaks intensity after incorporation of Pd in perovskite lattice,which can be explained as follows.The growth process of the precipitated particles in the solution is diffusion-limited.In a multicomponent system,the mass fluxes of solute species arise from the spatial concentration gradients of them.The diffusion coefficient of the solute species is dependent on their mobility and the mobility is the inverse of the Stokes-Einstein friction factor,which is given as6πμa for a spherical particle of radius (a),whereμis the fluid (continuous phase) viscosity [ 17] .Larger radius of Pd²⁺causes sluggish diffusivity and thus reduces the growth rate of initial nucleus.Therefore,smaller crystallites would be fabricated due to Pd doping,and according to Scherrer’s formula,these smaller crystallites lead to a larger full width at half maximum (FWHM) of peaks (peak broadening).

Fig.2 XRD patterns of LF,LFC and LFCP,co-precipitated using NH₄OH (up) and NaOH (down) calcined at 900℃

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Table 2 Lattice parameters and crystallite sizes of synthesized LF,LFC and LFCP powders

Column chart in Fig.3a shows the mean crystallite size of powders synthesized by various precipitation agents as a function of calcination temperature.In the co-precipitation reactions,the particle size and morphology of the products are controlled by the nucleation and growth kinetic [ 18] .When precipitation begins,numerous amorphous particles or small crystallites which are the hydroxides of each metal nitrates (M(OH)_n) would be initially formed under conditions of high super saturation (nucleation);however,they tend to quickly aggregate together to form more thermodynamically stable particles (growth).When the hydroxides were calcined,they first give MO(OH)_x [ 19] ;then,the mixture of the constituent metal hydroxides would react to form a single-phase mixed metal oxide (perovskite) at higher temperatures.

It is obvious from Fig.3a that NH₄OH produces smaller crystallites at identical calcination temperature compared to NaOH,which might be originated from the nature of these precipitants.Since the pH of both solutions containing different precipitants is equally adjusted at 10,the ionic strength of these solutions is equal.Thus,another factor might be the reason for smaller crystallites of samples synthesized by NH₄OH.Henrist et al. [ 20] stated that since the sodium ion is smaller than the ammonium ion,cations in the NaOH-salts nitrates solution migrate faster toward the growing colloids due to the more diffusivity of species.This encourages the growth kinetic and causes an increment in mean crystallite size.Column chart in Fig.3b lists the mean crystallite size of powders co-precipitated by two kinds of precipitation agents as a function of composition.It is seen that the mean crystallite size of powders precipitated by NaOH is much larger than that of powders synthesized by NH₄OH.However,in the case of doped samples,the mean crystallite size is controlled by the type and amount of doping agents and the mean crystallite size difference between two kinds of precipitants is negligible.

3.2 FESEM micrographs

Figure 4 presents the FESEM images of LF,LFC and LFCP synthesized using NaOH and NH₄OH as the precipitant and calcined at 900℃.The SEM image of LF-N-9(Fig.4a) shows a uniform particle size distribution and a fine particle size of around 200 nm.The SEM image of LFC-N-9 (Fig.4c) displays presence of clustered particles,forming a non-uniform distribution of particle size with the average particles diameter of about 450 nm.It seems that in the precipitation stage,Sample LF-N-9 should have smaller nucleation barriers than Sample LFC-N-9.Higher values of precipitant concentration,used for the synthesis of LF-N-9,provide a greater super saturation which leads to higher nucleation rate and results in smaller size of LF-N-9 particles.

FESEM image of LFC sample,co-precipitated using sodium hydroxide in Fig.4c,reveals the formation of agglomerates of relatively large particles of perovskite phase while using ammonium hydroxide as the precipitant yields formation of faceted prism-like particles (inset in Fig.4d).LFC-AH-9 powder has a narrower particle size distribution with the average dimension of about 85 nm in diameter and 150 nm in height.The higher degree of aggregation in LFC-N-9 could be a subsequence of the favored nucleation process at higher OH^-concentration with no separation of particles.It has been demonstrated that the addition of a stronger base into a metal salt solution made it difficult to generate monodispersed metal hydrous oxides [ 20] .Figure 4d shows that the partially isolated particles are formed in LFC-AH-9.Since ammonia is a fugitive alkaline substance with a high steam pressure,it should be brought out through intermediate steps in synthesis process.Higher volume of escaping gases for the sample synthesized using ammonia can easily break the agglomerations and create a high density of free surfaces.These free surfaces can prevent the particle growth and result in smaller particle size as expressed by Guo et al. [ 21] .Moreover,it can be seen in Fig.4d that the surface curvature of particles in Sample LFC-AH-9 turn into smooth facets despite the cessation of particle growth.According to Ref. [ 22] ,owing to calcination at sufficiently high temperatures,the crystals tend to grow on facets along the preferred crystallographic orientations to form the equilibrium shape.This eventuates in existence of nanoparticles with planar surfaces.

Fig.3 Variations in mean crystallite size of powders co-precipitated by NaOH and NH₄OH:a calcination temperature of LFC samples and b composition (after calcination at 900℃)

Fig.4 FESEM images of powders co-precipitated using a,c,e NaOH and b,d,f NH₄OH;a,b undoped;c,d cobalt doped;e,f cobalt and palladium doped

Fig.5 Schematic illustration of precipitation of cation hydroxides by increasing pH,and subsequent formation of a core-shell structure

The FESEM images of Pd-doped samples are shown in Fig.4e,f.It can be clearly seen that the addition of Pd affects the morphology of LaFe_0.6Co_0.4O₃.MIP measurements indicate that LFCP powders have relatively uniform particles size with a much smaller size compared to LFC ones;however,agglomerations between the particles are also observed.Furthermore,the LFCP-AH-9 powder has a finer average particle size (about 40 nm) and a narrower particle size distribution compared to the LFCP-N-9 powder.The reason for the significant difference in the particle size distribution of LFC and LFCP samples is probably originated from the formation steps of the precursors in the primary stages of the synthesis.Formation of initial precipitates is a consequence of mixing of cation hydroxides.Tang et al. [ 23] explicated that the ionic product of the cations and OH^-should reach the solubility product factor(K_sp) to cause precipitation of a hydroxide group.The hydroxides of Fe³⁺and Co³⁺would be formed at pH≤3,and the hydroxide of La³⁺would precipitate at pH value around 6 according to their solubility product factor [ 24] .While the hydroxide of Pd²⁺would be appeared at pH>9 [ 25] .It means that La(OH)3,Fe(OH)3 and Co(OH)3 are convenient to realize the precipitation unanimously in the preliminary stage of the process,while the colloidal Pd(OH)2 will ultimately cover the mixture of hydroxide of other cations at the higher pH value.Figure 5 shows a schematic illustration of formation of different cation hydroxides while increasing pH,and consequently a coreshell structure was constructed.This wrapped core-shell structure may decrease self-aggregation or Ostwald ripening of colloidal particles during the reaction,thus preventing broadening of particle size distribution [ 26] .

Figure 6 shows EDS results of Samples LFCP-N-9 and LFCP-AH-9.The result confirms relatively homogeneous mixing of atoms in Samples LFCP-N-9 and LFCP-AH-9.The atomic ratio is almost well-matched with the stoichiometric ratio of perovskite while taking oxygen at balance.Also there is no existence of Na,but Au with a relatively high content due to Au coating on the surface of the samples.Observed Pd contents in both samples affirm that palladium is incorporated successfully into the perovskite structure.However,it is noticeable that the atomic ratio of Pd/Fe in EDS results is much higher than that of stoichiometric LFCP compound.According to X-ray data,photon energies of L-shell emission lines for Pd and M-shell emission line for Au (coated on sample) are very close together.Thus,higher amount of Pd reported by EDS could be attributed to the overlap of M peak from Au and L peaks from Pd.

3.3 Micro-Raman spectroscopy

Figure 7 illustrates the Raman spectra of LF,LFC and LFCP synthesized using NaOH and NH₄OH as the precipitant and calcined at 900℃C.Raman spectra of samples in this work show 11-13 peaks depending on the dopant type.Table 3 tabulates the position of Raman peaks for the LaFeO₃ powders,together with the symmetry assignment of the modes suggested by previous studies [ 27, 28] .

The modes with frequencies of lower than 200 cm^-1 are related to La vibrations [ 27] .Modes around 280 cm^-1 are related to oxygen octahedral tilt [ 29] ,and modes located at385 and 437 cm^-1 are assigned to the bending of Fig.7 Raman spectra of doped and undoped LF,synthesized using different alkaline agents and calcined at 900℃.Inset being comparison of Raman spectra at frequencies of below 600 cm^-1 for Pd-and Co-doped compoundsoctahedral [ 27] .Martin-Carron et al. [ 30] reported that the asymmetric stretching around 480 cm^-1 in RMnO₃ is correlated with the JahnTeller distortion.Similar mode assignments can also be applied to the results.The line at around 631 cm^-1 is somewhat eristic.Venugopalan et al. [ 31] diagnosed a mode at 654 cm^-1 in LuFeO₃ as two phonon scattering.Koshizuka and Ushioda [ 32] allocate a mode at 645 cm^-1 in ErFeO₃ to impurity scattering.Martín-Carrón et al. [ 30] assigned the mode around610 cm^-1 in LaMnO₃ to a symmetric stretching of the basal oxygen ions of the octahedral (B_1g).However,the assignment of resultant modes is rather vague due to the impossibility of using scattering configurations where the line of certain modes is separately pronounced.

Fig.6 EDS spectra of a LFCP-N-9 and b LFCP-AH-9

Comparison of the Raman spectra of the bulk and powder samples of LF-N-9 and LF-AH-9 in Fig.7demonstrates that a red shift and broadening of the line of Raman modes occur in powders compared with the bulk ones,which is attributed to the confinement of phonons.Pursuant to the report by Arora et al. [ 33] ,when the size of the particles (D) approaches the nanoscale,the wave function of optical phonons is no longer a plane wave.It allows the contribution of all phonons with finite values of q (asymmetry parameter),includingq=0,in the Raman scattering process.This results in a decrement of the phonon vibrational frequency and an increment of the wave vector.

Comparison of Raman spectra of LF samples with LFC ones in Fig.7 indicates that substitution of Fe³⁺by Co³⁺increases the frequencies of the modes and reduces the peak intensities.This blue shift may be caused by different factors such as the contraction of the unit cell,as seen in Table 2,which results in the increment of bond strength.The interaction of Fe⁴⁺/Co²⁺electron configuration might be another impressive issue.Andreasson et al. [ 11] predicted that the orbitons can be excited in LaFe_1-xCr_xO₃,in which the Fe ions have Jahn-Teller active d⁴ electronic configuration.The interplay between Fe⁴⁺/Cr²⁺electron configuration is associated with a contraction of the oxygen octahedron surrounding Fe⁴⁺and an expansion of the adjacent octahedron surrounding Cr²⁺.This results in existence of an A_g-like mode.It is conjectured that possible presence of Jahn-Teller Fe⁴⁺in LFC samples could shift the frequency of the vibrations of around 490 cm^-1 to the higher values.Since according to some report [ 15] ,Co³⁺in LaFe_1-xCo_xO₃ could be partially reduced to Co²⁺,a number of Fe ions may take a 4+oxidation state in order to retain electroneutrality.It is worth noting that replacing Fe ions by Co ions propels the lattice geometry to a more symmetric rhombohedral system,thus reflecting a decrement of Raman intensities due to the displacement of oxygen ions into the higher symmetric sites of the lattice.

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Table 3 Symmetry assignments for observed peaks in LF-N-9 and LF-AH-9 (Labeling of modes following Refs. [27,28] )

Fig.8 Raman shifts (ω) for

mode and unit cell volume versus tolerance factor for samples synthesized by a NaOH and b NH₄OH

It is clear from Fig.7 that both LFCP samples have a red shift and broadening of A_g and B₃g modes compared to LFC ones.This decrement of frequency may be due to the substitution of Fe and Co ions by heavier Pd ions which enhance the average mass of the B site cation.Since the frequencies of the Raman modes are dependent on force constant and ionic mass,in the approximation of a harmonic oscillator [ 34] ,ω=(k/μ)^1/2 (k,force constant;μ,reduced mass),the heavier atoms of the structure such as Pd (atomic mass of 1.767×10^-22 g) are expected to vibrate in the lower frequencies compared to Fe (atomic mass of 9.272×10^-23 g) and Co (atomic mass of9.785×10^-23 g) [ 35] .So the normal modes related to B-O bond shift toward the lower wavenumber region.Furthermore,it is obvious from Fig.7 that the peak intensities of LFCP samples are lower than those of LFC samples synthesized with both types of precipitant.Most of Raman peaks which are corresponding to the modes of A_g and B₃g symmetry involve mainly oxygen motions [ 29] .Thus,it is comprehensible that an exchange of Fe³⁺and Co³⁺for Pd²⁺creates more oxygen vacancies in the structure of perovskite and causes a decrease in the intensity of the first-order peaks as mentioned in Ref. [ 11] .Moreover,the splitting of modes in LFCP-AH-9 and LFCP-N-9 in comparison with LF ones can be attributed to the simultaneous existence of three types of octahedra,namely FeO₆,CoO₆and PdO₆ in the composition as similarly reported by Gnezdilov et al. [ 12] .

Fig.9 Raman shift (ω) as a function of particle size (D) for Ag(4) mode in LF samples.Inset being Raman shifts as a function of particle size (in nano scale) for Ag mode in LF,LFC and LFCP samples

A plot of Raman experimental frequency positions for the mode and cell volume versus the Goldschmidt’s tolerance factor of Raman samples is depicted in Fig.8.For doing a reasonable comparison,it is assumed that all Fe,Co and Pd ions enter into the structure with their 3+oxidation state ideally,just to study the compatibility level of Raman spectra and XRD results.It can be simply stated that any decrement of the radius of B site cations in an ABO₃ perovskite results in an increase in the tolerance factor, ,where r_A,r_B and r_O are the size of A site cations,B site cations and O [ 36] .Therefore,when large size Fe³⁺(0.0645 nm) is replaced by smaller Co³⁺(0.0545 nm) in LFC composition,an increment in the tolerance factor and a decrease in the cell volume are expected.On the other hand,doping a small amount of Pd³⁺(0.076 nm) with its larger ionic radius reduces the tolerance factor and increases the cell volume compared to LFC.These variations of lattice parameters result in variation of bond strength and thus displacement of the Raman peak positions.The same trend is observed by using both NaOH and NH₄OH precipitants.Lower values of Raman frequencies in the samples synthesized by NH₄OH are well matched with their larger unit cell volume compared to the samples prepared by NaOH.

Figure 9 shows the variation of the peak position for the A_g(4) mode versus the particle size for different compositions.The rate of changes in A_g(4) frequency as a function of the particle size is similar with the one generally seen in nanomaterials [ 37] .It is clear from Fig.9 that as the particle size of LF samples decreases,the wavenumber reduces slowly until the dimension of the particles reaches below 100 nm and a sharp drop occurs in diagram.Different interpretations are given for this downward trend.One can understand the red shifts of the Raman peaks in terms of phonon confinement [ 33] as mentioned earlier.Moreover,Yassin et al. [ 37] reported that when the bulk and nanoparticles samples are exposed to the same laser power,the increment of temperature in nanoparticles is more significant compared to that of the bulk samples.This causes a larger red shift,where Yassin et al. [ 37] correlated that phenomenon to the anharmonic phonon-phonon interactions and thermal expansion of the lattice.

4 Conclusion

Poly crystalline nanoparticles of Pd-and Co-doped LaFeO₃perovskite were successfully prepared by co-precipitation method.The influence of Co and Pd doping into the LF compound on its structural,morphological and Raman characteristics was studied.XRD analysis confirms that the formation of single perovskite phase without any impurity is achievable using NaOH or NH₄OH as the alkaline agent.No line corresponding to palladium oxide or metal can be observed in XRD patterns of LFCP.A remarkable decrease in the mean particle sizes of LFCP samples compared with LFC ones demonstrates successful doping of Pd into the perovskite lattice.A reduction in particle size is observed in the samples co-precipitated by NH₄OH compared with the powders co-precipitated by NaOH.Evaluation of Raman spectra demonstrates active phonon modes for all samples.The shifting of phonon modes is related to the variation of MO₆ octahedral configuration as a result of doping concentration,which affirms the incorporation of Pd into the B site of perovskite lattice.

Acknowledgments This work was financially supported by University of Tehran (No.810729920/6/02) and Iran Nanotechnology Initiative Council.

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