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Structural, optical, and magnetic properties of Ca2+ doped La2CuO4 perovskite nanoparticles
Vacuum 167 (2019) 407–415
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Structural, optical, and magnetic properties of Ca2+ doped La2CuO4 perovskite nanoparticles
T
M. Sukumara, L. John Kennedya,∗, J. Judith Vijayab, B. Al-Najarc, M. Bououdinac, Gopinath Mudhanaa a
Materials and Physics Division, School of Advanced Sciences, Vellore Institute of Technology (VIT), Chennai, 600 127, Tamil Nadu, India Catalysis and Nanomaterials Research Laboratory, Department of Chemistry, Loyola College (Autonomous), Chennai, 600 034, Tamil Nadu, India c Department of Physics, College of Science, University of Bahrain, P.O. Box 32038, Bahrain b
ARTICLE INFO
ABSTRACT
Keywords: Microwave synthesis La2CuO4 nanoparticles Phase transformation Optical properties Magnetic properties
Ca2+ substituted La2CuO4 (LCC) perovskite nanostructures have been prepared by microwave combustion. X-ray diffraction analysis indicates that the undoped La2CuO4 (LCC0) crystallizes within a single perovskite-type phase with an orthorhombic crystal structure. For smaller Ca2+ doping of 0.1 (LCC1), a minor impurity was observed, while for concentrations 0.2 (LCC2) and 0.3 (LCC3), orthorhombic-to-tetragonal phase transformation occurs. In contrast, the tetragonal phase disappears at 0.4 and 0.5 (LCC4 and LCC5) as confirmed by XRD and Rietveld refinements. The orthorhombic and tetragonal perovskite phases have an average crystallite size in range 34.2–54.3 nm and 39.8–39.4 nm, respectively. Fourier transform infrared spectroscopy study established the characteristic absorption bands of La2CuO4 perovskite orthorhombic structure; the correlated bands are observed at 683 cm−1 for La-O and 516 cm−1 for Cu-O stretching modes. The optical band gap as determined by diffuse reflectance spectroscopy (DRS) increases then decreases with increasing Ca2+ content due to the quantum confinement effect. The surface morphology observations using scanning electron microscopy display nano-sized pores and pore walls are fused grain boundaries. Magnetization-field curves exhibit a ferro-/paramagnetic behavior for undoped La2CuO4 while Ca-doped system has only a paramagnetic behavior due to the exchange of A and B sites within La2CuO4 host lattice.
1. Introduction Magnetic nanomaterials, perovskites and perovskite doped with alkaline earth metal ions (Ca, Sr and Ba) or spinels, have been extensively studied because of the numerous promising and potential applications associated with their fascinating structural, electrical and magnetic properties [1–3]. Moreover, perovskite-based nanomaterials possess additional characteristics such as high temperature stability, excellent catalytic oxidation activity and oxidative reforming of diesel accordingly [4,5]. Currently, the perovskites oxides have been investigated for environment and energy related applications such as automobile exhaust purification, fuel cells, N2O decomposition, water splitting [5], redox and electrode materials [6], gas sensor [7], superconductor [8], catalysis [9] and etc. A2BO4 perovskite oxides are composed of a lanthanide element (trivalent cation) at position A-site and a transition metal (divalent cation) at position B-site, due to electroneutrality of the oxides (A23+B2+O42−). These perovskite oxides are used more frequently in
∗
heterogeneous catalysis, since transition metals show excellent catalytic activity for a variety of reactions due to their electronic structure [5,10]. La2CuO4 perovskite-like phase with an orthorhombic structure (Bmab space group) has a lattice consisting of CuO2 planes perpendicular to the c-axis [6], and are composed of alternated (LaO)2 and CuO2 rock-salt layers. The A-site cation (La3+) has nine-fold coordination and B-site cation (Cu2+) has octahedral coordination [6,11]. Naturally, the structure of perovskite type oxides (La2CuO4) could be changed (from orthorhombic to tetragonal system) depending on various parameters such as temperature, nature of the doping element of its concentration, and under pressure [4,12]. Among the observed phases, high temperature tetragonal (HTT), low temperature orthorhombic (LTO) and tetragonal (LTT) with P42/ncm and Bmab, I4/mmm symmetry, respectively are observed [12]. Lanthanum cuprate is a p-type anti-ferromagnetic semiconductor and becomes a supertconductor by replacing Lanthanum ions with alkaline rare earth cations like Sr, Ba and Ca or by intercalating excess of
Corresponding author. E-mail addresses: [email protected], [email protected] (L.J. Kennedy).
https://doi.org/10.1016/j.vacuum.2019.06.036 Received 21 December 2018; Received in revised form 26 June 2019; Accepted 27 June 2019 Available online 28 June 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 2. The peaks shift positions of diffraction planes as a function of Ca2+ concentration.
Fig. 1. X-ray diffraction patterns of LCC samples.
specified reaction time, heating stops immediately, and the reaction products are essentially being quenched leading to new properties in comparison to conventional methods [16]. Midouni et al. have prepared Ca2+ doped La2CuO4 perovskite employing conventional high temperature ceramic method. The samples were prepared at 900 °C for 12 h, then pressed into pellets and fired at 1000 °C for 24 h. The results confirmed that Ca2+ ion substituted La3+ ion site (x = 0.15) followed by phase transition from orthorhombic to tetragonal structure [17]. Maluf et al. also reported on the synthesis of Ca2+ doped La2CuO4 perovskite by co-precipitation method; the samples were pre-calcined at 350 °C for 2 h and then calcined at 700 °C for 4 h. A phase transition occurred from orthorhombic to tetragonal structure when La3+ is replaced by Ca2+ [18]. The present study focuses on the synthesis of Ca2+ doped La2CuO4 perovskite under microwave influence to undergo phase transition by the appearance of biphasic perovskite system (orthorhombic and cubic or orthorhombic and tetragonal). The correlation between structural, functional, optical, morphologcal, elemental and magnetic properties are investigated.
oxygen into the interstitial sites. From the literature, several methods have been adopted for the preparation of La2CuO4 perovskite nanomaterials, including solid-state [13], co-precipitation [7], sol-gel [6], combustion [9], hydrothermal [14,15] and so on. Velasquez et al. prepared La2CuO4 perovskite nanoparticles employing self-combustion using glycine as fuel followed by calcination at 700 °C for 6 h [9]. However, the synthesis of La2CuO4 using microwave assisted combustion method is scarce in the literature. Microwave synthesize of materials is of scientific importance as this approach is energy efficient and more rapid than the conventional heating methods. It also favors structural and physicochemical modifications in the final products. During the microwave heating process, microwave radiation interacts directly with the reaction components, where one of the reactants shall be capable of converting microwave energy into heat energy. As a result, uniform heating takes place within the sample and generates extremely high temperature during combustion, hence external heat supply is not required than conventional methods. Similarly, when microwave power is terminated after the Table 1 Rietveld refined XRD parameters of LCC samples. Titles a
Structure JCPDS Card no: Space group Crystallite size D (nm)b Microstrain Lattice parameter (Å) Cell volume V (Å3) R-factors Rwp (%) Rp(%) Re (%) S χ2 a b
LCC0
LCC1
Ortho 88–0940 Bmab 39.6 0.045 a = 5.3589 b = 5.3959 c = 13.1525 380.31
Ortho 80–1483 Fmmm 45.1 0.155 a = 5.3477 b = 5.3800 c = 13.1580 378.56
13.86 10.36 11.50 1.20 1.45
15.68 11.48 12.74 1.22 1.50
LCC2 Cubic 77–7215 Im-3m 2.2 1.680 a = 5.1 132.65
LCC3
Ortho 80–1483 Fmmm 38.1 0.061 a = 5.3670 b = 5.3480 c = 13.1590 377.69 15.06 11.11 12.73 1.18 1.39
Tetra 80–1074 I4/mmm 39.8 0.186 a = 5.3452 c = 13.1790 376.53
Ortho 80–1483 Fmmm 54.3 0.193 a = 5.3760 b = 5.3500 c = 13.1540 378.33 14.02 10.28 12.75 1.09 1.20
Ortho – Orthorhombic, Tetra – Tetragonal. Average crystallite size D (nm) by Scherrer's formula. 408
Tetra 80–1074 I4/mmm 39.4 0.207 a = 5.3443 c = 13.1730 376.24
LCC4
LCC5
Ortho 44–0151 Fmmm 42.0 0.235 a = 5.3472 b = 5.3678 c = 13.1700 378.01
Ortho 44–0151 Fmmm 34.2 0.210 a = 5.3420 b = 5.3600 c = 13.1800 377.38
13.86 10.59 12.44 1.11 1.23
12.73 09.57 12.17 1.04 1.09
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Fig. 3. Rietveld refined XRD pattern for LCC samples.
2. Experimental and methods
Table 2 Bulk density, X-ray density, porosity (%) and bandgap of LCC samples. Sample code
LCC0 LCC1 LCC2 LCC3 LCC4 LCC5
Molecular weight, M (g)
Bulk density, db (g/cm3)
X-ray density, dx (g/cm3)
Porosity, P (%)
Band gap, Eg (eV)
405.36 395.47 385.58 375.70 365.82 355.94
3.254 3.125 3.105 2.953 2.881 2.629
7.078 6.929 6.779 6.595 6.427 6.263
54.02 54.89 54.19 55.22 55.17 58.02
1.88 2.05 2.07 2.10 2.13 2.04
2.1. Synthesis of LCC perovskite nanoparticles All chemicals including lanthanum nitrate (La(NO3)3·6H2O), calcium nitrate (Ca(NO3)2·4H2O), copper nitrate (Cu(NO3)2·3H2O) and urea (NH2CONH2) were analytical grade (99.9%), purchased from SD fine-chemicals, India. All these chemicals were used as received without further purification. Double distilled water was used during sample preparation. Appropriate amounts of La(NO3)3·6H2O, Ca(NO3)2·4H2O and Cu (NO3)2·3H2O (precursors-oxidizers) and NH2CONH2 (fuel) were used for the synthesis of perovskite nanoparticles by microwave combustion method [1,16,17]. An homogeneous solution was obtain by dissolving 409
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LCC4 and LCC5, respectively. The La2-xCaxCuO4 (0 ≤ x ≤ 0.5) compositions are generally termed as LCC systems. 2.2. Characterization The structure was characterized by X-ray diffraction (XRD) with rietveld refinement using Rigaku Model Smartlab 3 kW X-ray diffractometer with CuKα radiation (λ = 1.5406 Å). Experimental conditions are: Current 40 mA, Voltage 40 kV giving a power of 1.6 kW, initial angle: 20°, final angle 80°, step angle 0.04° and counting time 1 s. Rietveld analysis, were performed to determine qualitative (phase identification using a database by ICCD which is incorporated with PDXL program) and quantitative (determination of phase composition, crystallite size and microstrain of each phase, lattice parameters of each phases, etc) analysis. The microstrain were also calculated from the peak shape directly from PDXL program. The functional groups were determined by Thermo scientific NICOLET iS10 OMNI FTIR spectrophotometer. The operational parameters of OMNI FTIR spectrophotometer include, photon source – infrared (IR), power – 24 VDC, data scan/acquisition time – 1 scan per every 4 s at 0.5 cm−1, dwell time - 5 min. The optical properties were determined at room temperature by Thermo scientific Evolution 300 UV–visible spectrophotometer. The morphology and elemental analysis were performed using HITACHI S4800 High Resolution-Scanning Electron Microscopy equipped with HORIBA EMAX energy dispersive X-ray spectroscopy. The operating parameters in SEM analysis include, electron source cold field emission gun; resolution 1 nm at accelerating voltage 15 kV and working distance 4 mm; imaging mode - back scattered electron imaging. and for elemental analyzes using EDX include, detection area – 30 mm2; window – ATW2; resolution at 5.9 keV–133 eV; and SiLi detector. PMC MicroMag 3900 model vibrating sample magnetometer equipped with a 1 Tesla magnet was used to study magnetic properties. The operating parameters are as follows, the powdered sample is introduced in a Al foil, then pressed so the powder does not move during measurements. The powder mass is weighted with 4-digit micro balance. Then the sample is fixed using silica (non-magnetic) grease to the rod (non-magnetic) which in turn is fixed in the VSM apparatus. The measurement is launched under the following experimental conditions: field step 50 Oe, counting time 1 s, the field decreases from 10.000 Oe to Zero goes to Zero, then increases in the negative field up to −10.000 Oe, return to Zero then increases to maximum value again of 10.000 Oe. This is what is called full hysteresis magnetization-field loop.
Fig. 4. FT-IR spectra of LCC samples.
the constituents in the desired mole ratio in de-ionized water followed by stirring at room temperature for 45 min 60 mL of homogeneous liquid solution was poured into a cylindrically shaped pure silica crucible (density 1.95 g/cm3, volume 100 mL) and placed at the centre of microwave oven for microwave exposure (2.45 GHz multimode cavity, 900 W) for a short period of 15 min. Initially, the solution boiled and underwent dehydration followed by decomposition with the evolution of gases. The fuel/oxidizer ratio (F/O) was equal to 1 as per the concept used in propellant chemistry [1,21] and employs the elemental stoichiometric coefficient (Φe). This parameter uses the total oxidizing and reducing valences of the components, which serve as numerical coefficients for the stoichiometric balance. When Φe = 1, the mixture is stoichiometric and is known to release maximum energy in condensed fuel/oxidizer systems. The mixture is fuel-lean (deficient) when Φe > 1 and rich when Φe < 1. The oxidizer-to-fuel molar ratio required for a stoichiometric mixture is determined by summing the total oxidizing and reducing valences in the oxidizer (lanthanum (calcium) nitrate, copper nitrate) and dividing by the sum of the total oxidizing and reducing valencies in the fuel (urea) as expressed in the following equation:
Oxidizer / Fuel =
3. Results and discussion 3.1. X-ray diffraction analysis Fig. 1 displays the evolution of X-ray diffraction (XRD) patterns as function of Ca2+ doping content. The undoped La2CuO4 (LCC0) sample exhibits characteristic peaks of single phase perovskite oxide having an orthorhombic structure with space group Bamb, in agreement with JCPDS card No: 88-0940). With Ca2+ doping, LCC1 reveals the presence of the orthorhombic phase with the appearance of new peak at 2θ ≈ 28.09° (110), attributed to the minor impurity of Ca with cubic phase and space group Im-3m in agreement with JCPDS card No: 100348. However, with increasing Ca2+ concentration, LCC2 and LCC3 samples, in addition to the main orthorhombic phase, a new peak appears at 2θ ≈ 39.9°, indicating the occurrence of structural phase transformation from orthorhombic to tetragonal (space group I4/mmm, JCPDS No: 80-1074), these similar results are reported by Maluf et al. [18] and Midouni et al. [19]. In contrast, the tetragonal phase disappears at higher Ca2+ doping concentration (LCC4 and LCC5) as confirmed by Rietveld results (Table 1). On the other hand, Fig. 2 reveals a shift of the diffraction peaks toward higher 2θ angles with increasing Ca2+ concentration, which can be explained by the reduction
all oxidizing and reducing elements in oxidizer ( 1) oxidizing and reducing elements in fuel (1)
This ratio is required for the complete combustion of all materials. When the solution reacted at the point of spontaneous combustion, ignition took place resulting in a rapid flame, which yielded the solid fluffy block powder. Finally, the powder was subjected to calcination at 500 °C for 2 h, then washed with distilled water repeatly. The product was then dried in a hot air vacuum oven at 150 °C for 1 h to avoid water molecules contamination and further to obtain the final La2CuO4 perovskite nanoparticles. The obtained powders with x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5 Ca2+ fractions were labeled as LCC0, LCC1, LCC2, LCC3, 410
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Fig. 5. Plot of (F(R)hν)2 versus hν for LCC samples.
of lattice parameters (a, b, V) due to the smaller ionic radius of Ca2+ (0.134 nm) compared to that of La3+ (0.136 nm) [19], which indicates the successful Ca2+ doping in La2CuO4 structure. The average crystallite size (D) of LCC perovskite oxides has been estimated using the Debye-Scherrer formula [20]:
D=
0. 89 cos
54.3 nm. On the other hand, the crystallite size of the secondary cubic phase is found to be very small at the quantum size level; i.e. 2.2 nm (LCC1), but with higher Ca2+ content (LCC2 and LCC3 samples where phase transformation occurred, the mean crystallite size seems not affected; i.e. 39.8 and 39.4 nm, respectively (Table 1). The lattice parameters of the LCC system has been calculated from the XRD patterns using eqns. (3)–(5):
(2)
where, λ - the X-ray wavelength, θ - the Bragg diffraction angle and β the full width at half maximum (FWHM). The average crystallite size of the orthorhombic phase for LCC0 samples is found to be 39.6 nm, meanwhile with Ca2+ doping (LCC1 to LCC5), it ranges from 34.2 to 411
1 h2 k2 l2 = 2 + 2 + 2 2 d a b c
(3)
1 h2 + k 2 + l 2 = d2 a2
(4)
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(Z = 4) [17], M - the molecular weight, V - the unit cell volume and N the Avogadro's number. It can be noticed that with increasing Ca2+ content, the value of dx gradually decreases from 7.078 to 6.263 g/cm3 (Table 2), due to the atomic mass of Ca2+ being much lower compared to that of La3+; 40.078 and 138.906 g/mole, respectively. The percentage of porosity of LCC perovskite samples has been also calculated by eqn. (8) [16]:
P =
1
X
Bulk density ray density
× 100
(8)
Table 2 reveals that the P value steadily decreases with the increase in Ca2+ content, in good agreement with the dx. The values of bulk density are found less than that of X-ray density, which is due to the occurrence of unavoidable pores during the period of the microwave combustion process. It can be seen that with the increase in Ca2+ content, the percentage of porosity ranges from 54.02 to 58.02 (%), which can be attributed to the phase transformation associated with the doping concentration. 3.2. FT-IR analysis Fig. 4 shows Fourier transform infrared (FTIR) spectra of LCC perovskite samples recorded in the range between 4000 and 400 cm−1 at room temperature. The spectra display characteristic broad bands at 3745 and 2921 cm−1 ascribed to O-H stretching vibration of adsorbed water molecules. The small band appearing at 2353 cm−1 can be associated with O-O bond while the bands at 1752 and 1607 cm−1 can be attributed to the of H-O-H bond vibration [22]. Additionally, the characteristic absorption bands at 683 and 516 cm−1 are related to the metal oxide (lanthanum and copper) stretching modes of orthorhombic La2CuO4 perovskite structure [23]. These bands are steady lowering by increasing Ca2+ concentration.
Fig. 6. HR-SEM image of LCC samples.
1 h2 + k 2 l2 = + 2 d2 a2 c
(5)
3.3. Optical properties
where, d the inter-atomic spacing a, b and c are the lattice constants. The lattice constants (a, b and c) and unit cell volume (V) are displayed in Table 1. The observed changes in the lattice parameters are due to the dissolution of calcium (Ca2+) with in the La2CuO4 host lattice by occupying La3+ sites, since both ions have slight difference in their corresponding ionic radii (La3+ - 0.136 nm and Ca2+ - 0.134 nm, respectively), alongside with the effect of doping concentration in a solid solution which is dependent on the miscibility limit of the resultant phase formation [1]. Fig. 3 displays the Rietveld refinements of LCC system carried out based on three phases such as orthorhombic, tetragonal and cubic as performed by using PDXL software. It is clear that the experimental data are well fitted with the theoretical model, as the calculated quality fit parameters for instance the values of χ2 given in Table 1 are closer to 1. The goodness of fit parameter (S) is calculated from S = Rwp/Re where Rwp and Re are the weighted profile and expected weighted profile reliability factors, respectively. The value of ‘S’ around unity indicates the excellent goodness of fit and confirms that the refined parameters are determined more precisely [16]. The bulk density (bd) of LCC perovskite has been calculated according to eqn. (6) [21].
bd =
M r 2h
Fig. 5 displays the optical properties of LCC systems have been studied by UV–visible diffuse reflectance spectroscopy. The modified Tauc relation eqn. (9) used to calculate optical band gap energy [24].
F (R ) h = A ( h
ZM NV
(9)
where, n = 1/2 and 2 represents direct and indirect transition, thereby giving direct and indirect band gaps respectively. The optical band gap has been determined by using the Kubelka-Munk (K-M) function. The KM function is generally applied to convert the diffused reflectance into equivalent absorption it is mostly used for analyzing the powder sample.
= F (R ) =
(1
R )2 2R
(10)
where, F(R) is the K–M function, α isthe absorption coefficient and R is the reflectance [25]. A graph is plotted between (F(R)hν)2 and hν, the intercept value is the band gap energy as shown in Fig. 5. The extrapolation of the linear region of the plots to (F(R)hν)2 = 0 gives the direct band gap value. The band gap value of pure La2CuO4 is found to be 1.88 eV, which is significantly higher than some values reported in the literature; i.e., 1.30 and 1.24 eV [6,26], and this may be caused by quantum confinement effect. With increasing Ca2+ content (LCC1 to LCC4), the band gap value increases from 2.05 eV to 2.13 eV. This is due to an increase in bond-mismatch, which represents the variation of the lattice parameters a, b and c of the unit cell. The substitution of La3+ (large radius) by Ca2+ (small radius) within the La2CuO4 host lattice will enhance the bond-mismatch by contracting the (LaO)2 layers [6]. While for higher Ca2+ content (LCC5), the band gap decreases up to 2.04 eV giving rise to a red shift, which may be attributed to the quantum confinement effect occurring at nano-scale regime [27].
(6)
where, M is the mass of sample pellet, r is the radius of pellet, and h is the height of the pellet. The X-ray density (dx) of LCC perovskite samples has been calculated according to eqn. (7) [20]:
dx =
Eg )n
(7)
where, Z - the number of molecules per unit cell of perovskite lattice 412
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Fig. 7. EDX spectrum of LCC samples.
3.4. SEM and EDX analysis Fig. 6 shows the surface morphology of LCC perovskite samples as prepared by microwave combustion, a technique that involves escaping of water molecules and volatile gases (nitrogen, carbon dioxide and oxygen) which results in the formation of pores. It can be seen that LCC0, LCC1 are formed of nano-sized crystallized grains with isolated particles. For LCC2, LCC3 and LCC4, it can be observed the appearance of particles around the pore and pore walls are of fused grains while LCC5 reveals particles with fused grains boundaries. Fig. 7 displays the EDX spectra for elemental composition, revealing the presence of lanthanum, calcium, copper and oxygen only for all LCC perovskite system. The inset table in Fig. 7 shows the relevant fraction of the corresponding elemental composition. 3.5. Magnetic properties Fig. 8 illustrates the magnetic hysteresis curves of LCC perovskite samples. Pure LCC0 sample reveals a ferro-/para-magnetic behavior (presence of both paramagnetic and ferromagnetic phases) with a saturation magnetization (Ms) of 2.547 memu/g (obtained after the
Fig. 8. Magnetic hysteresis curves of LCC samples.
413
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performing some tests, such as hyperthermia (generation of heat under AC magnetic field) as well as possible application as magneto-caloric effect, photo-electrochemical decomposing of water, catalysis and etc.
Table 3 Magnetic parameters of LCC samples. Sample code
LCC0 LCC1 LCC2 LCC3 LCC4 LCC5
Magnetic behavior
Coercivity, Hc (Oe)
Remanence, Mr (μemu/g)
Saturation Magnetization, Ms (memu/g)
Ferro/para Para Para Para Para Para
542.3/519.9 48.46 101.97 93.98 15.49 49.66
910.6 125.58 55.72 234.39 28.58 198.96
2.547/3.528 6.97 4.14 5.94 6.12 7.70
Acknowledgements All the authors sincerely thank VIT University Chennai Campus, Chennai, Tamil Nadu, India, for providing financial assistance through research associateship to the first author. References [1] M. Sukumar, L.J. Kennedy, J.J. Vijaya, B. Al-Najar, M. Bououdina, Co2+ substituted La2CuO4/LaCoO3 perovskite nanocomposites: synthesis, properties and heterogeneous catalytic performance, New J. Chem. 42 (2018) 18128–18142. [2] P. Karpagavinayagam, C. Vedhi, Green synthesis of iron oxide nanoparticles using Avicennia marina flower extract, Vacuum 160 (2019) 286–292. [3] J. Wang, D. Zhou, Y. Li, P. Wu, Experimental and first-principle studies of ferromagnetism in Na- doped SnO2 nanoparticles, Vacuum 141 (2017) 62–67. [4] Z. Zhao, L. Wang, J. Ma, Y. Feng, X. Cao, W. Zhan, Y. Guo, Y. Guo, G. Lu, Deoxygenation of coal bed methane on LaCoO3 perovskite catalyst: the structure evolution and catalytic performance, RSC Adv. 7 (2017) 15211–15221. [5] N. Labhasetwar, G. Saravanan, S. Kumar Megarajan, N. Manwar, R. Khobragade, P. Doggali, F. Grasset, Perovskite-type catalytic materials for environmental applications, Sci. Technol. Adv. Mater. 16 (2015) 036002. [6] Y. Li, J. Huang, L. Cao, J. Wu, J. Fei, Optical properties of La2CuO4 and La2−xCaxCuO4 crystallites in UV–vis–NIR region synthesized by sol–gel process, Mater. Char. 64 (2012) 36–42. [7] X. Zhou, Q. Cao, Y. Hu, J. Gao, Y. Xu, Sensing behavior and mechanism of La2CuO4SnO2 gas sensors, Sensor. Actuator. B Chem. 77 (2001) 443–446. [8] Q.Y. Tu, X.L. Chen, B.K. Ma, Z.X. Zhao, J.Q. Li, J.K. Liang, Structural and superconducting properties of La2CuO4+δ chemically oxidized by NaClO, Phys. C Supercond. Its Appl. 370 (2002) 94–100. [9] M. Velasquez, A. Santamaria, C. Batiot-Dupeyrat, Selective conversion of glycerol to hydroxyacetone in gas phase over La2CuO4 catalyst, Appl. Catal. B Environ. 160–161 (2014) 606–613. [10] T. Uchiyama, R. Karita, M. Nishibori, H. Einaga, Y. Teraoka, Preparation and characterization of Pd loaded Sr-deficient K2NiF4-type (La,Sr)2MnO4 catalysts for NO–CO reaction, Catal. Today 251 (2015) 7–13. [11] J. Zhu, Z. Zhao, D. Xiao, J. Li, X. Yang, Y. Wu, Characterization and catalytic activity in NO decomposition of La2-xSrxCuO4 (0 ≤ x ≤1) compounds with T* phase structure, Mater. Chem. Phys. 94 (2005) 257–260. [12] A. Tavana, M. Akhavan, C. Draxl, First-principles study on lattice instabilities and structural phase transitions in Ba doped La2CuO4, Phys. C Supercond. Its Appl. 517 (2015) 20–25. [13] X. Chen, K. Tang, S. Zeng, Q. Hao, D. Wang, Z. Gao, Y. Wang, Fluorination of La2xSrxCuO4 (x = 0, 0.15, 0.3) and study on the crystal structures, magnetic properties of their fluorinated products, J. Alloy. Comp. 626 (2015) 239–244. [14] L. Gao, X. Wang, H. Tong Chua, S. Kawi, Growth of La2CuO4 nanofibers under a mild condition by using single walled carbon nanotubes as templates, J. Solid State Chem. 179 (2006) 2036–2040. [15] D. Chen, C. Lin, A. Maljuk, F. Zhou, Growth and Characterization of Bulk Superconductor Material, Springer, 2016, https://doi.org/10.1007/978-3-31931548-5 243. [16] M. Sukumar, L. John Kennedy, J. Judith Vijaya, B. Al-Najar, M. Bououdina, Structural, magnetic and catalytic properties of La2-xBaxCuO4 (0 ≤ x ≤ 0.5) perovskite nanoparticles, Ceram. Int. 44 (2018) 18113–18122. [17] M. Sukumar, L.J. Kennedy, J.J. Vijaya, B. Al-Najar, M. Bououdina, Facile microwave assisted combustion synthesis, structural, optical and magnetic properties of La2−xSrxCuO4 (0 ≤ x ≤ 0.5) perovskite nanostructures, J. Magn. Magn. Mater. 465 (2018) 48–57. [18] S.S. Maluf, P.A.P. Nascente, C.R.M. Afonso, E.M. Assaf, Study of La2-xCaxCuO4 perovskites for the low temperature water gas shift reaction, Appl. Catal. Gen. 413–414 (2012) 85–93. [19] A. Midouni, M.I. Houchati, N. Chniba-boudjada, A.H. Hamzaoui, From n = 1 to n = 2 of the Ruddlesden−Popper Phases via Ca-doping and induced effects on electrical and optical properties of La2-xCaxCuO4-δ, J. Phys. Chem. Solids 110 (2017) 76–86. [20] G.T. Anand, L.J. Kennedy, J.J. Vijaya, K. Kaviyarasan, M. Sukumar, Structural, optical and magnetic characterization of Zn1-xNixAl2O4 (0 ≤ x ≤ 5) spinel nanostructures synthesized by microwave combustion technique, Ceram. Int. 41 (2014) 603–615. [21] D. Ravinder, T.A. Manga, Elastic behaviour of Cu–Cd ferrites, J. Alloy. Comp 299 (2000) 5–8. [22] V.B. Kumar, D. Mohanta, Formation of nanoscale tungsten oxide structures and colouration characteristics, Bull. Mater. Sci. 34 (2011) 435–442. [23] D.V. Dharmadhikari, S.K. Nikam, A.A. Athawale, Template free hydrothermal synthesis and gas sensing application of lanthanum cuprate (La2CuO4): effect of precursors on phase formation and morphology, J. Alloy. Comp. 590 (2014) 486–493. [24] M. Sukumar, L.J. Kennedy, Catalytic conversion of methanol to formaldehyde over La2CuO4 nanoparticles, J. Nanosci. Nanotechnol. 19 (2019) 826–832.
removal of the paramagnetic component). The LCC perovskite samples show only paramagnetic behavior, the corresponding magnetic parameters are reported in Table 3. Upon the application of higher magnetic field (either -ve or + ve field), the magnetization of samples tends to saturate and ferro and paramagnetic behavior become dominant. The magnetic parameters such as Ms, Mr and Hc (magnetization saturation, remanence, coercivity) are found to change dramatically, ranging from 4.14 to 7.70 memu/g, 28.58–234.39 μemu/g and 15.49 to 101.97 Oe by increasing Ca2+ concentration. The observed change in the magnetic behavior can be related to several factors, but mostly to the exchange of A and B site cations within La2CuO4 host lattice upon substitution of Ca2+ ions; as alongside with La2CuO4 orthorhombic to tetragonal phase transition which corroborate the ferro-/para-magnetic transition. Indeed, the magnetization value has been found to change suddenly at certain doping Ca2+ level, which can be due to the transition between single domain and multi-domain at the critical size [28,29]. The drastic change occurs even at the smallest Ca concentration (x = 0.1), where there is a transition from para/ferro to fully paramagnetic behavior. Since both ionic radii are almost closer La3+ (0.136 nm) and Ca2+ (0.134 nm) [30], upon the replacement of La3+ by Ca2+, a lattice distortion resulting in the creation of point defects (vacancies or/and interstitials). For instance, the difference in valence, 2+ state for Ca and 3+ state for La, will induce unbalanced total charge of La2CuO4 phase (+ve charge in excess) following the substitution of La3+ by Ca2+, and as a result oxygen vacancies will be created. However, a more complex combination of several intrinsic/extrinsic factors may contribute simultaneously, mainly the creation of vacancies, short/long exchange interactions between the constituent ions (La3+, Ca2+, Cu2+ and O2−), as well as the occupancy of 3p alkali metal Ca2+ with a paramagnetic contribution into rare earth La3+ sites having a weak paramagnetic moment. 4. Conclusion The influence of Ca2+ doping concentration on the evolution of structure, optical and magnetic properties of La2CuO4 has been investigated. It is found that La2CuO4 crystallizes into single orthorhombic perovskite structure, where as lower Ca (0.1–0.3) doping favors orthorhombic to tetragonal phase transformation then surprisingly followed by the disappearance of the tetragonal phase at higher doping concentration (0.4 and 0.5) as confirmed by Rietveld refinements. The appearance of absorption bands at 683 and 516 cm−1 confirms the perovskite structure. The optical band gap calculated using K-M function increases from 1.88 to 2.13 eV and then decreased 2.04 eV with increasing Ca2+ content. Morphological observations reveal the formation of nano-sized crystallized grains with pores and pore walls are of fused grains boundaries. Ferro-/para-, ferro- or para-magnetic behaviors have been identified from magnetization-field hysteresis curves. The studied system indicates how Ca2+ doping possesses an influential role on perovskite La2CuO4 in terms of the change in crystal structure and phase composition alongside with morphology and size of particles, which will have direct influence on properties (optical and magnetic). This will affect the performance of the obtained nanopowders when 414
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M. Sukumar, et al. [25] R. Bhardwaj, J. Pal, K. Hwa, N. Goyal, S. Gautam, Electronic and magnetic structure investigation of vanadium doped ZnO nanostructure, Vacuum 158 (2018) 257–262. [26] M. Enhessari, M. Shaterian, M.J. Esfahani, M.N. Motaharian, Synthesis, characterization and optical band gap of La2CuO4 nanoparticles, Mater. Sci. Semicond. Process. 16 (2013) 1517–1520. [27] G. Fan, J. Tong, F. Li, Visible-light-induced photocatalyst based on cobalt-doped zinc ferrite nanocrystals, Ind. Eng. Chem. Res. 51 (2012) 13639–13647. [28] B.P.S. Upadhyay, K. Parekh, Influence of crystallite size on the magnetic properties
of Fe3O4 nanoparticles, J. Alloy. Comp. 678 (2016) 478–485. [29] M. Sukumar, L.J. Kennedy, J.J. Vijaya, B. Al-Najar, M. Bououdina, Facile synthesis of Fe3+ doped La2CuO4/LaFeO3 perovskite nanocomposites : structural , optical , magnetic and catalytic properties, Mater. Sci. Semicond. Process. 100 (2019) 225–235. [30] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. A 32 (1976) 751–767.
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Structural, optical, and magnetic properties of Ca2+ doped La2CuO4 perovskite nanoparticles
T
M. Sukumara, L. John Kennedya,∗, J. Judith Vijayab, B. Al-Najarc, M. Bououdinac, Gopinath Mudhanaa a
Materials and Physics Division, School of Advanced Sciences, Vellore Institute of Technology (VIT), Chennai, 600 127, Tamil Nadu, India Catalysis and Nanomaterials Research Laboratory, Department of Chemistry, Loyola College (Autonomous), Chennai, 600 034, Tamil Nadu, India c Department of Physics, College of Science, University of Bahrain, P.O. Box 32038, Bahrain b
ARTICLE INFO
ABSTRACT
Keywords: Microwave synthesis La2CuO4 nanoparticles Phase transformation Optical properties Magnetic properties
Ca2+ substituted La2CuO4 (LCC) perovskite nanostructures have been prepared by microwave combustion. X-ray diffraction analysis indicates that the undoped La2CuO4 (LCC0) crystallizes within a single perovskite-type phase with an orthorhombic crystal structure. For smaller Ca2+ doping of 0.1 (LCC1), a minor impurity was observed, while for concentrations 0.2 (LCC2) and 0.3 (LCC3), orthorhombic-to-tetragonal phase transformation occurs. In contrast, the tetragonal phase disappears at 0.4 and 0.5 (LCC4 and LCC5) as confirmed by XRD and Rietveld refinements. The orthorhombic and tetragonal perovskite phases have an average crystallite size in range 34.2–54.3 nm and 39.8–39.4 nm, respectively. Fourier transform infrared spectroscopy study established the characteristic absorption bands of La2CuO4 perovskite orthorhombic structure; the correlated bands are observed at 683 cm−1 for La-O and 516 cm−1 for Cu-O stretching modes. The optical band gap as determined by diffuse reflectance spectroscopy (DRS) increases then decreases with increasing Ca2+ content due to the quantum confinement effect. The surface morphology observations using scanning electron microscopy display nano-sized pores and pore walls are fused grain boundaries. Magnetization-field curves exhibit a ferro-/paramagnetic behavior for undoped La2CuO4 while Ca-doped system has only a paramagnetic behavior due to the exchange of A and B sites within La2CuO4 host lattice.
1. Introduction Magnetic nanomaterials, perovskites and perovskite doped with alkaline earth metal ions (Ca, Sr and Ba) or spinels, have been extensively studied because of the numerous promising and potential applications associated with their fascinating structural, electrical and magnetic properties [1–3]. Moreover, perovskite-based nanomaterials possess additional characteristics such as high temperature stability, excellent catalytic oxidation activity and oxidative reforming of diesel accordingly [4,5]. Currently, the perovskites oxides have been investigated for environment and energy related applications such as automobile exhaust purification, fuel cells, N2O decomposition, water splitting [5], redox and electrode materials [6], gas sensor [7], superconductor [8], catalysis [9] and etc. A2BO4 perovskite oxides are composed of a lanthanide element (trivalent cation) at position A-site and a transition metal (divalent cation) at position B-site, due to electroneutrality of the oxides (A23+B2+O42−). These perovskite oxides are used more frequently in
∗
heterogeneous catalysis, since transition metals show excellent catalytic activity for a variety of reactions due to their electronic structure [5,10]. La2CuO4 perovskite-like phase with an orthorhombic structure (Bmab space group) has a lattice consisting of CuO2 planes perpendicular to the c-axis [6], and are composed of alternated (LaO)2 and CuO2 rock-salt layers. The A-site cation (La3+) has nine-fold coordination and B-site cation (Cu2+) has octahedral coordination [6,11]. Naturally, the structure of perovskite type oxides (La2CuO4) could be changed (from orthorhombic to tetragonal system) depending on various parameters such as temperature, nature of the doping element of its concentration, and under pressure [4,12]. Among the observed phases, high temperature tetragonal (HTT), low temperature orthorhombic (LTO) and tetragonal (LTT) with P42/ncm and Bmab, I4/mmm symmetry, respectively are observed [12]. Lanthanum cuprate is a p-type anti-ferromagnetic semiconductor and becomes a supertconductor by replacing Lanthanum ions with alkaline rare earth cations like Sr, Ba and Ca or by intercalating excess of
Corresponding author. E-mail addresses: [email protected], [email protected] (L.J. Kennedy).
https://doi.org/10.1016/j.vacuum.2019.06.036 Received 21 December 2018; Received in revised form 26 June 2019; Accepted 27 June 2019 Available online 28 June 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 2. The peaks shift positions of diffraction planes as a function of Ca2+ concentration.
Fig. 1. X-ray diffraction patterns of LCC samples.
specified reaction time, heating stops immediately, and the reaction products are essentially being quenched leading to new properties in comparison to conventional methods [16]. Midouni et al. have prepared Ca2+ doped La2CuO4 perovskite employing conventional high temperature ceramic method. The samples were prepared at 900 °C for 12 h, then pressed into pellets and fired at 1000 °C for 24 h. The results confirmed that Ca2+ ion substituted La3+ ion site (x = 0.15) followed by phase transition from orthorhombic to tetragonal structure [17]. Maluf et al. also reported on the synthesis of Ca2+ doped La2CuO4 perovskite by co-precipitation method; the samples were pre-calcined at 350 °C for 2 h and then calcined at 700 °C for 4 h. A phase transition occurred from orthorhombic to tetragonal structure when La3+ is replaced by Ca2+ [18]. The present study focuses on the synthesis of Ca2+ doped La2CuO4 perovskite under microwave influence to undergo phase transition by the appearance of biphasic perovskite system (orthorhombic and cubic or orthorhombic and tetragonal). The correlation between structural, functional, optical, morphologcal, elemental and magnetic properties are investigated.
oxygen into the interstitial sites. From the literature, several methods have been adopted for the preparation of La2CuO4 perovskite nanomaterials, including solid-state [13], co-precipitation [7], sol-gel [6], combustion [9], hydrothermal [14,15] and so on. Velasquez et al. prepared La2CuO4 perovskite nanoparticles employing self-combustion using glycine as fuel followed by calcination at 700 °C for 6 h [9]. However, the synthesis of La2CuO4 using microwave assisted combustion method is scarce in the literature. Microwave synthesize of materials is of scientific importance as this approach is energy efficient and more rapid than the conventional heating methods. It also favors structural and physicochemical modifications in the final products. During the microwave heating process, microwave radiation interacts directly with the reaction components, where one of the reactants shall be capable of converting microwave energy into heat energy. As a result, uniform heating takes place within the sample and generates extremely high temperature during combustion, hence external heat supply is not required than conventional methods. Similarly, when microwave power is terminated after the Table 1 Rietveld refined XRD parameters of LCC samples. Titles a
Structure JCPDS Card no: Space group Crystallite size D (nm)b Microstrain Lattice parameter (Å) Cell volume V (Å3) R-factors Rwp (%) Rp(%) Re (%) S χ2 a b
LCC0
LCC1
Ortho 88–0940 Bmab 39.6 0.045 a = 5.3589 b = 5.3959 c = 13.1525 380.31
Ortho 80–1483 Fmmm 45.1 0.155 a = 5.3477 b = 5.3800 c = 13.1580 378.56
13.86 10.36 11.50 1.20 1.45
15.68 11.48 12.74 1.22 1.50
LCC2 Cubic 77–7215 Im-3m 2.2 1.680 a = 5.1 132.65
LCC3
Ortho 80–1483 Fmmm 38.1 0.061 a = 5.3670 b = 5.3480 c = 13.1590 377.69 15.06 11.11 12.73 1.18 1.39
Tetra 80–1074 I4/mmm 39.8 0.186 a = 5.3452 c = 13.1790 376.53
Ortho 80–1483 Fmmm 54.3 0.193 a = 5.3760 b = 5.3500 c = 13.1540 378.33 14.02 10.28 12.75 1.09 1.20
Ortho – Orthorhombic, Tetra – Tetragonal. Average crystallite size D (nm) by Scherrer's formula. 408
Tetra 80–1074 I4/mmm 39.4 0.207 a = 5.3443 c = 13.1730 376.24
LCC4
LCC5
Ortho 44–0151 Fmmm 42.0 0.235 a = 5.3472 b = 5.3678 c = 13.1700 378.01
Ortho 44–0151 Fmmm 34.2 0.210 a = 5.3420 b = 5.3600 c = 13.1800 377.38
13.86 10.59 12.44 1.11 1.23
12.73 09.57 12.17 1.04 1.09
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Fig. 3. Rietveld refined XRD pattern for LCC samples.
2. Experimental and methods
Table 2 Bulk density, X-ray density, porosity (%) and bandgap of LCC samples. Sample code
LCC0 LCC1 LCC2 LCC3 LCC4 LCC5
Molecular weight, M (g)
Bulk density, db (g/cm3)
X-ray density, dx (g/cm3)
Porosity, P (%)
Band gap, Eg (eV)
405.36 395.47 385.58 375.70 365.82 355.94
3.254 3.125 3.105 2.953 2.881 2.629
7.078 6.929 6.779 6.595 6.427 6.263
54.02 54.89 54.19 55.22 55.17 58.02
1.88 2.05 2.07 2.10 2.13 2.04
2.1. Synthesis of LCC perovskite nanoparticles All chemicals including lanthanum nitrate (La(NO3)3·6H2O), calcium nitrate (Ca(NO3)2·4H2O), copper nitrate (Cu(NO3)2·3H2O) and urea (NH2CONH2) were analytical grade (99.9%), purchased from SD fine-chemicals, India. All these chemicals were used as received without further purification. Double distilled water was used during sample preparation. Appropriate amounts of La(NO3)3·6H2O, Ca(NO3)2·4H2O and Cu (NO3)2·3H2O (precursors-oxidizers) and NH2CONH2 (fuel) were used for the synthesis of perovskite nanoparticles by microwave combustion method [1,16,17]. An homogeneous solution was obtain by dissolving 409
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LCC4 and LCC5, respectively. The La2-xCaxCuO4 (0 ≤ x ≤ 0.5) compositions are generally termed as LCC systems. 2.2. Characterization The structure was characterized by X-ray diffraction (XRD) with rietveld refinement using Rigaku Model Smartlab 3 kW X-ray diffractometer with CuKα radiation (λ = 1.5406 Å). Experimental conditions are: Current 40 mA, Voltage 40 kV giving a power of 1.6 kW, initial angle: 20°, final angle 80°, step angle 0.04° and counting time 1 s. Rietveld analysis, were performed to determine qualitative (phase identification using a database by ICCD which is incorporated with PDXL program) and quantitative (determination of phase composition, crystallite size and microstrain of each phase, lattice parameters of each phases, etc) analysis. The microstrain were also calculated from the peak shape directly from PDXL program. The functional groups were determined by Thermo scientific NICOLET iS10 OMNI FTIR spectrophotometer. The operational parameters of OMNI FTIR spectrophotometer include, photon source – infrared (IR), power – 24 VDC, data scan/acquisition time – 1 scan per every 4 s at 0.5 cm−1, dwell time - 5 min. The optical properties were determined at room temperature by Thermo scientific Evolution 300 UV–visible spectrophotometer. The morphology and elemental analysis were performed using HITACHI S4800 High Resolution-Scanning Electron Microscopy equipped with HORIBA EMAX energy dispersive X-ray spectroscopy. The operating parameters in SEM analysis include, electron source cold field emission gun; resolution 1 nm at accelerating voltage 15 kV and working distance 4 mm; imaging mode - back scattered electron imaging. and for elemental analyzes using EDX include, detection area – 30 mm2; window – ATW2; resolution at 5.9 keV–133 eV; and SiLi detector. PMC MicroMag 3900 model vibrating sample magnetometer equipped with a 1 Tesla magnet was used to study magnetic properties. The operating parameters are as follows, the powdered sample is introduced in a Al foil, then pressed so the powder does not move during measurements. The powder mass is weighted with 4-digit micro balance. Then the sample is fixed using silica (non-magnetic) grease to the rod (non-magnetic) which in turn is fixed in the VSM apparatus. The measurement is launched under the following experimental conditions: field step 50 Oe, counting time 1 s, the field decreases from 10.000 Oe to Zero goes to Zero, then increases in the negative field up to −10.000 Oe, return to Zero then increases to maximum value again of 10.000 Oe. This is what is called full hysteresis magnetization-field loop.
Fig. 4. FT-IR spectra of LCC samples.
the constituents in the desired mole ratio in de-ionized water followed by stirring at room temperature for 45 min 60 mL of homogeneous liquid solution was poured into a cylindrically shaped pure silica crucible (density 1.95 g/cm3, volume 100 mL) and placed at the centre of microwave oven for microwave exposure (2.45 GHz multimode cavity, 900 W) for a short period of 15 min. Initially, the solution boiled and underwent dehydration followed by decomposition with the evolution of gases. The fuel/oxidizer ratio (F/O) was equal to 1 as per the concept used in propellant chemistry [1,21] and employs the elemental stoichiometric coefficient (Φe). This parameter uses the total oxidizing and reducing valences of the components, which serve as numerical coefficients for the stoichiometric balance. When Φe = 1, the mixture is stoichiometric and is known to release maximum energy in condensed fuel/oxidizer systems. The mixture is fuel-lean (deficient) when Φe > 1 and rich when Φe < 1. The oxidizer-to-fuel molar ratio required for a stoichiometric mixture is determined by summing the total oxidizing and reducing valences in the oxidizer (lanthanum (calcium) nitrate, copper nitrate) and dividing by the sum of the total oxidizing and reducing valencies in the fuel (urea) as expressed in the following equation:
Oxidizer / Fuel =
3. Results and discussion 3.1. X-ray diffraction analysis Fig. 1 displays the evolution of X-ray diffraction (XRD) patterns as function of Ca2+ doping content. The undoped La2CuO4 (LCC0) sample exhibits characteristic peaks of single phase perovskite oxide having an orthorhombic structure with space group Bamb, in agreement with JCPDS card No: 88-0940). With Ca2+ doping, LCC1 reveals the presence of the orthorhombic phase with the appearance of new peak at 2θ ≈ 28.09° (110), attributed to the minor impurity of Ca with cubic phase and space group Im-3m in agreement with JCPDS card No: 100348. However, with increasing Ca2+ concentration, LCC2 and LCC3 samples, in addition to the main orthorhombic phase, a new peak appears at 2θ ≈ 39.9°, indicating the occurrence of structural phase transformation from orthorhombic to tetragonal (space group I4/mmm, JCPDS No: 80-1074), these similar results are reported by Maluf et al. [18] and Midouni et al. [19]. In contrast, the tetragonal phase disappears at higher Ca2+ doping concentration (LCC4 and LCC5) as confirmed by Rietveld results (Table 1). On the other hand, Fig. 2 reveals a shift of the diffraction peaks toward higher 2θ angles with increasing Ca2+ concentration, which can be explained by the reduction
all oxidizing and reducing elements in oxidizer ( 1) oxidizing and reducing elements in fuel (1)
This ratio is required for the complete combustion of all materials. When the solution reacted at the point of spontaneous combustion, ignition took place resulting in a rapid flame, which yielded the solid fluffy block powder. Finally, the powder was subjected to calcination at 500 °C for 2 h, then washed with distilled water repeatly. The product was then dried in a hot air vacuum oven at 150 °C for 1 h to avoid water molecules contamination and further to obtain the final La2CuO4 perovskite nanoparticles. The obtained powders with x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5 Ca2+ fractions were labeled as LCC0, LCC1, LCC2, LCC3, 410
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Fig. 5. Plot of (F(R)hν)2 versus hν for LCC samples.
of lattice parameters (a, b, V) due to the smaller ionic radius of Ca2+ (0.134 nm) compared to that of La3+ (0.136 nm) [19], which indicates the successful Ca2+ doping in La2CuO4 structure. The average crystallite size (D) of LCC perovskite oxides has been estimated using the Debye-Scherrer formula [20]:
D=
0. 89 cos
54.3 nm. On the other hand, the crystallite size of the secondary cubic phase is found to be very small at the quantum size level; i.e. 2.2 nm (LCC1), but with higher Ca2+ content (LCC2 and LCC3 samples where phase transformation occurred, the mean crystallite size seems not affected; i.e. 39.8 and 39.4 nm, respectively (Table 1). The lattice parameters of the LCC system has been calculated from the XRD patterns using eqns. (3)–(5):
(2)
where, λ - the X-ray wavelength, θ - the Bragg diffraction angle and β the full width at half maximum (FWHM). The average crystallite size of the orthorhombic phase for LCC0 samples is found to be 39.6 nm, meanwhile with Ca2+ doping (LCC1 to LCC5), it ranges from 34.2 to 411
1 h2 k2 l2 = 2 + 2 + 2 2 d a b c
(3)
1 h2 + k 2 + l 2 = d2 a2
(4)
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(Z = 4) [17], M - the molecular weight, V - the unit cell volume and N the Avogadro's number. It can be noticed that with increasing Ca2+ content, the value of dx gradually decreases from 7.078 to 6.263 g/cm3 (Table 2), due to the atomic mass of Ca2+ being much lower compared to that of La3+; 40.078 and 138.906 g/mole, respectively. The percentage of porosity of LCC perovskite samples has been also calculated by eqn. (8) [16]:
P =
1
X
Bulk density ray density
× 100
(8)
Table 2 reveals that the P value steadily decreases with the increase in Ca2+ content, in good agreement with the dx. The values of bulk density are found less than that of X-ray density, which is due to the occurrence of unavoidable pores during the period of the microwave combustion process. It can be seen that with the increase in Ca2+ content, the percentage of porosity ranges from 54.02 to 58.02 (%), which can be attributed to the phase transformation associated with the doping concentration. 3.2. FT-IR analysis Fig. 4 shows Fourier transform infrared (FTIR) spectra of LCC perovskite samples recorded in the range between 4000 and 400 cm−1 at room temperature. The spectra display characteristic broad bands at 3745 and 2921 cm−1 ascribed to O-H stretching vibration of adsorbed water molecules. The small band appearing at 2353 cm−1 can be associated with O-O bond while the bands at 1752 and 1607 cm−1 can be attributed to the of H-O-H bond vibration [22]. Additionally, the characteristic absorption bands at 683 and 516 cm−1 are related to the metal oxide (lanthanum and copper) stretching modes of orthorhombic La2CuO4 perovskite structure [23]. These bands are steady lowering by increasing Ca2+ concentration.
Fig. 6. HR-SEM image of LCC samples.
1 h2 + k 2 l2 = + 2 d2 a2 c
(5)
3.3. Optical properties
where, d the inter-atomic spacing a, b and c are the lattice constants. The lattice constants (a, b and c) and unit cell volume (V) are displayed in Table 1. The observed changes in the lattice parameters are due to the dissolution of calcium (Ca2+) with in the La2CuO4 host lattice by occupying La3+ sites, since both ions have slight difference in their corresponding ionic radii (La3+ - 0.136 nm and Ca2+ - 0.134 nm, respectively), alongside with the effect of doping concentration in a solid solution which is dependent on the miscibility limit of the resultant phase formation [1]. Fig. 3 displays the Rietveld refinements of LCC system carried out based on three phases such as orthorhombic, tetragonal and cubic as performed by using PDXL software. It is clear that the experimental data are well fitted with the theoretical model, as the calculated quality fit parameters for instance the values of χ2 given in Table 1 are closer to 1. The goodness of fit parameter (S) is calculated from S = Rwp/Re where Rwp and Re are the weighted profile and expected weighted profile reliability factors, respectively. The value of ‘S’ around unity indicates the excellent goodness of fit and confirms that the refined parameters are determined more precisely [16]. The bulk density (bd) of LCC perovskite has been calculated according to eqn. (6) [21].
bd =
M r 2h
Fig. 5 displays the optical properties of LCC systems have been studied by UV–visible diffuse reflectance spectroscopy. The modified Tauc relation eqn. (9) used to calculate optical band gap energy [24].
F (R ) h = A ( h
ZM NV
(9)
where, n = 1/2 and 2 represents direct and indirect transition, thereby giving direct and indirect band gaps respectively. The optical band gap has been determined by using the Kubelka-Munk (K-M) function. The KM function is generally applied to convert the diffused reflectance into equivalent absorption it is mostly used for analyzing the powder sample.
= F (R ) =
(1
R )2 2R
(10)
where, F(R) is the K–M function, α isthe absorption coefficient and R is the reflectance [25]. A graph is plotted between (F(R)hν)2 and hν, the intercept value is the band gap energy as shown in Fig. 5. The extrapolation of the linear region of the plots to (F(R)hν)2 = 0 gives the direct band gap value. The band gap value of pure La2CuO4 is found to be 1.88 eV, which is significantly higher than some values reported in the literature; i.e., 1.30 and 1.24 eV [6,26], and this may be caused by quantum confinement effect. With increasing Ca2+ content (LCC1 to LCC4), the band gap value increases from 2.05 eV to 2.13 eV. This is due to an increase in bond-mismatch, which represents the variation of the lattice parameters a, b and c of the unit cell. The substitution of La3+ (large radius) by Ca2+ (small radius) within the La2CuO4 host lattice will enhance the bond-mismatch by contracting the (LaO)2 layers [6]. While for higher Ca2+ content (LCC5), the band gap decreases up to 2.04 eV giving rise to a red shift, which may be attributed to the quantum confinement effect occurring at nano-scale regime [27].
(6)
where, M is the mass of sample pellet, r is the radius of pellet, and h is the height of the pellet. The X-ray density (dx) of LCC perovskite samples has been calculated according to eqn. (7) [20]:
dx =
Eg )n
(7)
where, Z - the number of molecules per unit cell of perovskite lattice 412
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Fig. 7. EDX spectrum of LCC samples.
3.4. SEM and EDX analysis Fig. 6 shows the surface morphology of LCC perovskite samples as prepared by microwave combustion, a technique that involves escaping of water molecules and volatile gases (nitrogen, carbon dioxide and oxygen) which results in the formation of pores. It can be seen that LCC0, LCC1 are formed of nano-sized crystallized grains with isolated particles. For LCC2, LCC3 and LCC4, it can be observed the appearance of particles around the pore and pore walls are of fused grains while LCC5 reveals particles with fused grains boundaries. Fig. 7 displays the EDX spectra for elemental composition, revealing the presence of lanthanum, calcium, copper and oxygen only for all LCC perovskite system. The inset table in Fig. 7 shows the relevant fraction of the corresponding elemental composition. 3.5. Magnetic properties Fig. 8 illustrates the magnetic hysteresis curves of LCC perovskite samples. Pure LCC0 sample reveals a ferro-/para-magnetic behavior (presence of both paramagnetic and ferromagnetic phases) with a saturation magnetization (Ms) of 2.547 memu/g (obtained after the
Fig. 8. Magnetic hysteresis curves of LCC samples.
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performing some tests, such as hyperthermia (generation of heat under AC magnetic field) as well as possible application as magneto-caloric effect, photo-electrochemical decomposing of water, catalysis and etc.
Table 3 Magnetic parameters of LCC samples. Sample code
LCC0 LCC1 LCC2 LCC3 LCC4 LCC5
Magnetic behavior
Coercivity, Hc (Oe)
Remanence, Mr (μemu/g)
Saturation Magnetization, Ms (memu/g)
Ferro/para Para Para Para Para Para
542.3/519.9 48.46 101.97 93.98 15.49 49.66
910.6 125.58 55.72 234.39 28.58 198.96
2.547/3.528 6.97 4.14 5.94 6.12 7.70
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removal of the paramagnetic component). The LCC perovskite samples show only paramagnetic behavior, the corresponding magnetic parameters are reported in Table 3. Upon the application of higher magnetic field (either -ve or + ve field), the magnetization of samples tends to saturate and ferro and paramagnetic behavior become dominant. The magnetic parameters such as Ms, Mr and Hc (magnetization saturation, remanence, coercivity) are found to change dramatically, ranging from 4.14 to 7.70 memu/g, 28.58–234.39 μemu/g and 15.49 to 101.97 Oe by increasing Ca2+ concentration. The observed change in the magnetic behavior can be related to several factors, but mostly to the exchange of A and B site cations within La2CuO4 host lattice upon substitution of Ca2+ ions; as alongside with La2CuO4 orthorhombic to tetragonal phase transition which corroborate the ferro-/para-magnetic transition. Indeed, the magnetization value has been found to change suddenly at certain doping Ca2+ level, which can be due to the transition between single domain and multi-domain at the critical size [28,29]. The drastic change occurs even at the smallest Ca concentration (x = 0.1), where there is a transition from para/ferro to fully paramagnetic behavior. Since both ionic radii are almost closer La3+ (0.136 nm) and Ca2+ (0.134 nm) [30], upon the replacement of La3+ by Ca2+, a lattice distortion resulting in the creation of point defects (vacancies or/and interstitials). For instance, the difference in valence, 2+ state for Ca and 3+ state for La, will induce unbalanced total charge of La2CuO4 phase (+ve charge in excess) following the substitution of La3+ by Ca2+, and as a result oxygen vacancies will be created. However, a more complex combination of several intrinsic/extrinsic factors may contribute simultaneously, mainly the creation of vacancies, short/long exchange interactions between the constituent ions (La3+, Ca2+, Cu2+ and O2−), as well as the occupancy of 3p alkali metal Ca2+ with a paramagnetic contribution into rare earth La3+ sites having a weak paramagnetic moment. 4. Conclusion The influence of Ca2+ doping concentration on the evolution of structure, optical and magnetic properties of La2CuO4 has been investigated. It is found that La2CuO4 crystallizes into single orthorhombic perovskite structure, where as lower Ca (0.1–0.3) doping favors orthorhombic to tetragonal phase transformation then surprisingly followed by the disappearance of the tetragonal phase at higher doping concentration (0.4 and 0.5) as confirmed by Rietveld refinements. The appearance of absorption bands at 683 and 516 cm−1 confirms the perovskite structure. The optical band gap calculated using K-M function increases from 1.88 to 2.13 eV and then decreased 2.04 eV with increasing Ca2+ content. Morphological observations reveal the formation of nano-sized crystallized grains with pores and pore walls are of fused grains boundaries. Ferro-/para-, ferro- or para-magnetic behaviors have been identified from magnetization-field hysteresis curves. The studied system indicates how Ca2+ doping possesses an influential role on perovskite La2CuO4 in terms of the change in crystal structure and phase composition alongside with morphology and size of particles, which will have direct influence on properties (optical and magnetic). This will affect the performance of the obtained nanopowders when 414
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