Characterization of B site codoped LaFeO3 nanoparticles prepared via co-precipitation route
来源期刊:Rare Metals2018年第3期
论文作者:Seyedeh Behnaz Varandili Alireza Babaei Abolghasem Ataie
文章页码:181 - 190
摘 要:LaFe1-x-yCoxPdyO3 [(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 LaFe0.6Co0.4O3. 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.
稀有金属(英文版) 2018,37(03),181-190
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;
Seyedeh Behnaz Varandili Alireza Babaei Abolghasem Ataie
School of Metallurgy and Materials Engineering,College of Engineering,University of Tehran
Abstract:
LaFe1-x-yCoxPdyO3 [(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 LaFe0.6Co0.4O3. 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
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
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
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),LaFe0.6Co0.4O3 (LFC) and LaFe0.57Co0.38Pd0.05O3 (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.
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
Fig.1 XRD patterns of LFC co-precipitated using NaOH,C2H11N3O5 and NH4OH calcined at 800,900 and 1000℃
XRD patterns of LaFe0.57Co0.38Pd0.05O3 in Fig.2 displays a small shift to lower 2θvalues with respect to LaFe0.6Co0.4O3.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 LaFe0.6Co0.4O3.Furthermore,a few Pd ions may enter into the lattice with their 2+valence and create more oxygen vacancies
Fig.2 XRD patterns of LF,LFC and LFCP,co-precipitated using NH4OH (up) and NaOH (down) calcined at 900℃
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
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.
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
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 LaFe0.6Co0.4O3.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.
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
The modes with frequencies of lower than 200 cm-1 are related to La vibrations
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.
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.
Table 3 Symmetry assignments for observed peaks in LF-N-9 and LF-AH-9 (Labeling of modes following Refs.
Fig.8 Raman shifts (ω) for
It is clear from Fig.7 that both LFCP samples have a red shift and broadening of Ag 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
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
Figure 9 shows the variation of the peak position for the Ag(4) mode versus the particle size for different compositions.The rate of changes in Ag(4) frequency as a function of the particle size is similar with the one generally seen in nanomaterials
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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