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Influence of Mn substitution on morphological, thermal and optical properties of nanocrystalline GdFeO3 orthoferrite
Nano-Structures & Nano-Objects 15 (2018) 17–27
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Nano-Structures & Nano-Objects journal homepage: www.elsevier.com/locate/nanoso
Influence of Mn substitution on morphological, thermal and optical properties of nanocrystalline GdFeO3 orthoferrite Shahid Husain *, Ali O.A. Keelani, Wasi Khan Department of Physics, Aligarh Muslim University, Aligarh 202 002, India
highlights
graphical abstract
• Nanocrystalline Mn doped in GdFeO3 • •
•
•
were synthesized by solid state reaction route. SEM/TEM micrographs reveal the increase in grain size on Mn doping. FTIR spectra show the characteristic Fe–O band around 559 cm−1 in undoped sample. An anomaly in the specific heat was observed near the Neel temperature of GdFeO3 . Mn doping decreases the bandgap of GdFeO3 significantly.
article
info
Article history: Received 4 October 2017 Received in revised form 22 January 2018 Accepted 5 March 2018 Keywords: Orthoferrites Solid state reaction route SEM/EDX TEM Differential thermal analysis (DTA) Urbach energy
a b s t r a c t The current research work focuses on the preparation of nanocrystalline GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3) samples via solid state reaction route to explore their microstructural, thermal and optical properties in detail. Scaning electron microscopy (SEM)/Transmission electron microscopy (TEM) results revealed coarse surface and highly agglomerated form of the particles in nanoscale region with increasing particle size on Mn substitution. Elemental composition and homogeneity of the ions were ensured from energy dispersive x-ray (EDX) and SEM-EDX analyses respectively. Fourier transform infrared (FTIR) spectra reflected characteristic peak of Fe–O band at ∼559 cm−1 for GdFeO3 sample and shifted to 579 cm−1 by the substitution of Mn ions, signify the octahedral FeO6 group of perovskite structure. The variation in heat flow and specific heat at constant pressure (Cp ) with the increase in temperature were monitored through differential thermal analysis (DTA) technique. An anomaly in the specific heat near the Nèel temperature (565 K) of GdFeO3 has been observed. This peak further shifts towards the lower temperature on Mn doping. Moreover, Mn substitution in GdFeO3 reduces the values of Cp . UV–visible absorption spectra exhibited two noticeable bands in the ultraviolet region and the band gap was found to decrease with the increase in Mn content as estimated by employing Tauc’s relation. However, Urbach energy calculated using absorption coefficient enhances. These results demonstrate that the physical properties of GaFeO3 system could be tuned by the appropriate substitution of Mn for real applications. © 2018 Elsevier B.V. All rights reserved.
1. Introduction
* Corresponding author. E-mail address: [email protected] (S. Husain). https://doi.org/10.1016/j.nanoso.2018.03.002 2352-507X/© 2018 Elsevier B.V. All rights reserved.
Ferrites are considered most promising materials owing to their diverse and interesting technological applications. They are eco-friendly and have potential applications in transformer cores, modulators, capacitors and optical storage, piezoelectric sensors,
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memory core, high quality filters, antennas, radio frequency components etc. [1–4]. In this group, RFeO3 (where R represents to a rare-earth ion) belongs to orthoferrite perovskite crystal structure. Among these, GdMnO3 is an orthorhombically distorted A-type antiferromagnetic insulator that exists in Pbnm or Pnma crystallographic space group. In this structure, Fe3+ ions are ordered antiferromagnetically in the ‘a’ axis and somewhat canted along the ‘c’ axis to give weak ferromagnetism in the temperature below 660 K. Due to antisymmetric exchange of slight canting of Fe3+ spins give the weak ferromagnetic nature of the compound [5,6]. Most of the orthoferrites are distorted due to FeO6 octahedra that modify these materials into monoclinic, orthorhombic, rhombohedral and triclinic crystal structures [7]. Sometimes rare earth ion is not responsible for the Fe octahedron distortion [8]. However, it is reported that the decrease of the rare-earth ion’s radius leads to distortion enhancement of the polyhedra. Because of this distortion, the rareearth ion is surrounded by the 12 oxygen (O) ions that are divided into two kinds i.e. eight first nearest oxygen ions and four second nearest O ions. This structural distortion in the system affect the magnetic ordering and spin-state transitions [9,10]. Therefore, the potential technological applications of these orthoferrites has raised much interest in relation to the properties, such as magnetic field sensors based on magnetoresistance, actuators utilize piezoelectricity and oxygen sensors on electronic ionic mixed conductivity with nonlinear response to oxygen pressure [11–13]. These type of materials also exhibit visible-light photocatalytic activity to overcome environmental pollution and the crisis of energy over the globe [14,15]. In addition, rare-earth perovskites are also found excellent alternate for the degradation of organic compounds with the help of visible light [15,16]. Moreover, GdFeO3 is a kind of typical rare-earth perovskite-type oxide with a stable crystallographic structure that has unique characteristics of electromagnetic and catalysis [17]. Hence, GdFeO3 has several application prospects in the fields of magnetism and photocatalysis. But the synthesis of this structure in single phase still is a challenging task via solid state reaction route for some of the structures [18] although it has many advantages including phase purity and minimum change of any other impurity elements. Zhang et al. [19] have explored GdFeO3 type distortion in orthorhombic DyMnO3 system by the variation in grain size. They have suggested that this type of modification is also possible by the variation in particle size without any external perturbations. In another work, Shah et al. studied the effect of Mg2+ doping on the dielectric and magnetic properties of polycrystalline GdFeO3 samples. They have observed the increase in dielectric constant and magnetoelectric coupling coefficient for 6% Mg doped sample. These results conclude that the antiferromagnetic domain walls play a significant role in controlling these properties [20]. However, it is difficult to control particle size and surface morphology that play vital and significant role in the physical and chemical properties of nanomaterials. But chemical methods of nanomaterials synthesis in various shapes are quite easy and well established nowadays [21]. The physical properties of these materials can be enhanced and tuned by the doping of transition metal ion at Fe site. Thermal parameters such as heat flow in and out of the system, specific heat etc. and influence of the doping on these parameters are also important for the various applications especially in sensors. In view of this, Mn doped GdFeO3 nanocrystalline sample were synthesized and detailed morphological, thermal and optical properties have been studied. To the best of our knowledge, no reports are available on the effect of Mn doping on the thermal and optical properties of GdFeO3 system. 2. Experimental details Nanocrystalline samples of GdFe1−x Mnx O3 (x = 0.0, 0.1, 0.2 and 0.3) (GFMO) have been synthesized through standard solid
Fig. S1. SEM-EDX elemental mapping images of GdFe1−x Mnx O3 (x = 0.0, 0.1, 0.2 & 0.3) samples.
state reaction route from high purity (AR grade) chemical reagents. The detailed description of the samples preparation is given elsewhere [22]. Scanning electron microscope (SEM) (JSM6510) and energy dispersive x-ray spectroscopy (EDX) respectively were employed to examine the surface morphology and presence of chemical composition in the samples. Transmission electron microscopy (TEM) (Jeol, JEM2010) images were captured for the investigation of the particle/grain size and internal structure. Fourier transform infrared (FTIR) spectroscopy measurements were carried out by the Bruker-Tensor-37 spectrometer for the wavenumber range of 370–4000 cm−1 . Differential thermal analysis (DTA) was performed using STA-8000 (Perkin Elmer) thermal analyzer in the temperature range of 30 to 1000 ◦ C at the heating rate of 20 ◦ C/min. The investigation of optical properties was performed in the wavelength range of 200 to 800 nm with a UV/Vis spectrophotometer. 3. Results and discussion 3.1. Morphological studies X-ray diffraction (XRD) data analysis through Rietveld refinement confirmed polycrystalline and single phase nature of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3) samples. All the samples crystallize in orthorhombic crystal symmetry without any other measurable impurity phase [22]. Fig. 1 displays SEM images of the synthesized samples that exhibit quite different surface morphology having nearly spherical shape of the grains with a large size variation in the range of 26 to 61 nm. Moreover, grain size increases for higher Mn contents and the grain boundaries are clearly distinguishable. Further, SEM images clearly show uniform distribution of the grains and systematic change in morphology with the increase in Mn doping. The chemical composition of the samples examined through EDX analysis are shown in Fig. S1. It gives the qualitative elemental composition in the samples by the appearance of Mn, Gd, Fe and O peaks. This indicates high purity of the samples and presence of no other impurity elements. Some gold peaks are appearing in the EDX spectra due to gold coating used to avoid charging of the specimen, which would otherwise occur due to the accumulation of electrons on the sample surface. It is also helpful to increase the secondary electrons that reduces noise to signal ratio. SEM-EDX mapping images further ensure uniform and appropriate elemental distribution throughout the samples (Fig. S2). It is clear that the distribution of elements Gd, Fe, Mn and O are homogeneous and symmetrical in all samples, and as we increase the Mn content in GdFeO3 the percentage of Fe reduces in the same ratio.
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Fig. 1. SEM micrographs of GdFe1−x Mnx O3 (x = 0.0, 0.1, 0.2 and 0.3) samples.
Fig. 2. TEM images of GdFe1−x Mnx O3 (x = 0.0, 0.1, 0.2 and 0.3).
The typical internal structure of the samples is examined by the transmission electron microscope (TEM) operated at 200 kV. TEM images of GdFe1−x Mnx O3 (x = 0.0, 0.1, 0.2 and 0.3) with grain sizes are displayed in Fig. 2. These micrographs exhibit that the grains have nearly spherical shapes in the range of 32 to 57 nm. The larger grain size is observed for higher Mn concentration that support SEM results. However, due to the smaller grain sizes, enough agglomeration is observed in all the samples. The large agglomeration in the particles may be due to the strong magnetic dipole–dipole interactions among them. Usually, the particle sizes are somewhat larger than that observed using SEM attributed to the light scattering of the organic molecules adsorbed on the particle surface. FTIR spectra of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3) perovskite were recorded to recognize the occurrence of various vibrational bands
in the samples and are displayed in Fig. 3. Generally, the infrared spectra provide the information of the functional groups, molecular geometry and inter or intra molecular interactions existing in a system. It is known that the frequency of the bands within 1000 cm−1 are associated to the bonds of inorganic elements. Therefore, the appeared prominent band at ∼435 cm−1 is related to the bending vibration of O−Fe−O in GdFeO3 [23]. However, Mn doping shifts this band to 453 cm−1 for x = 0.3. Other strong band at ∼559 cm−1 is also influenced by the Mn substitution and shifted from 559 cm−1 (x = 0.0) to 579 cm−1 for x = 0.3 concentration. This sharp absorption band at 559 cm−1 ascribed to the B–O (Fe–O) stretching vibration, being features of the octahedral BO6 groups in the perovskite (ABO3 ) compounds, which is usually noticed in the region of 500–700 cm−1 . The bands present at ∼3453 cm−1
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Fig. 3. FTIR spectra of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3) recorded at room temperature. Fig. S2. Energy dispersive x-ray (EDX) spectra of all samples.
and 1637 cm−1 are ascribe to the O–H stretching and H–O–H bending vibrations of structural moisture. On Mn doping these bands shifted to the lower values 3430 cm−1 and 1604 cm−1 for 30% Mn doping. These shifts are attributed to the smaller atomic mass of Mn as compared to Fe and hence these measurements again ensure purity of the samples. 3.2. Thermal analysis DTA profiles of GdFe1−x Mnx O3 (x = 0.0, 0.1, 0.2 and 0.3) samples as a function of temperature are recorded with a controlled
nitrogen atmosphere, in a temperature range of 30 to 1000 ◦ C and are shown in Fig. 4. This figure presents the heat flow in and out of the system due to endothermic and exothermic reactions. The undoped GdFeO3 sample exhibits endothermic peaks at temperatures 70.2, 292 and 789 ◦ C, whereas an exothermic peak is observed at about 380 ◦ C. But on Mn doping these endothermic peaks have started to shift towards lower temperatures and for x = 0.3 these peaks are present at 68.1, 242.3 and 710.4 ◦ C, however the exothermic peak presented in pristine sample at 380 ◦ C has been disappeared. In addition, some additional exothermic peaks
Fig. 4. Heat flow as a function of temperature for GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
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Fig. 5. Heat capacity at constant pressure (Cp ) as a function of temperature for GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
are appeared in Mn substituted samples at higher temperatures. These peaks can be ascribed to the structural transition on Mn substitution at higher temperatures as the exchange interaction of the Mn3+ –Mn4+ depends on the bond angle and the bond length. The reduction of Mn–O bond length and Mn–O–Mn bond angle will make Mn3+ –Mn4+ exchange interaction stronger, leading to the structural transition. We have also calculated the value of heat capacity at constant pressure (Cp ) and its variation with temperature are displayed in Fig. 5. The absolute value of Cp is observed maximum (226.42 mW/gm.degrees) for pristine sample and Mn doping reduces this value to 99.22 mW/gm.degrees for x = 0.3 concentration sample. The anomaly at 292 ◦ C or 565 K in pristine sample can be ascribed to the phase transition from antiferromagnetic to paramagnetic state as this temperature is near to Néel temperature of GdFeO3 . The Néel temperature of bulk GdFeO3 is reported as 670 K, that reduces to the lower value in nanoscale GdFeO3 . In these samples, Néel temperature further reduces on Mn doping. It is well matched to the reported results that also reported reduction in Néel temperature on the substitution at Fe site in GdFeO3 [20]. 3.3. Optical properties The study of optical properties is useful to know the optoelectronic nature of materials. Further, optical properties of orthoferrites are extremely influenced by the characteristics of the incident radiation falling on it. In this way, optical absorption studies give a simple route to explain the band structure and estimation of the optical band gap of materials. Reflectance spectra of Mn substituted GdFeO3 samples were observed in ultraviolet–visible (UV/Vis.) spectral range at room temperature. The reflectance (R) has been changed into absorbance (A) using the relation A =
Fig. 6. Absorption spectra of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3) as a function of wavelength.
− log(R). The change in absorbance as a function of wavelength is displayed in Fig. 6. This figure exhibit two pronounced peaks at 234 and 368 nm for undoped GdFeO3 sample and these peaks have been slightly red shifted on Mn doping. The absorption coefficient (α ) is calculated using α = 2.303 (A/t), here t stands for breadth of the cuvette in which sample was filled. Then the Tauc’s relation as given below is used to estimate the band gap (Eg ) of the samples [24,25].
α hν = A(hν − Eg )m
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Fig. 7. (α hv)2 versus h ν plots of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
Fig. 8. lnα vs h ν plots to determine the Urbach energy near the first absorption peak of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
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Table 1 Bandgap (Eg ), Urbach energy (Eu ) and steepness parameter (S(T )) for GdFe1−x Mnx O3 (x = 0, 0.10, 0.20 and 0.30) samples. Mn concentration
x x x x
= 0.0 = 0.1 = 0.2 = 0.3
Bandgap (Eg ) (eV) estimated by
Near first absorption peak
Tauc relation
K-M function
Eu (meV)
S(T) × 10−3
Eu (eV)
Near second absorption peak S(T) × 10−3
3.47 3.17 3.12 2.79
3.89 3.80 3.73 3.15
433.8 551.0 784.5 1380.0
59.598 46.918 32.955 18.73
1.1471 1.4489 2.3635 3.1526
22.54 17.84 10.94 8.20
Fig. 9. (A/λ)2 versus 1/λ plots for GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
Fig. 10. Variation in skin depth (δ ) with energy of photon plots of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
where symbols have their usual meaning. In this relation, exponent value m depends on the nature of band gap which is 12 for direct
bandgap and has the value 2 for indirect band gap [26]. We have plotted the graphs between (α hν )1/2 versus hν (indirect transition) and (α hν )2 versus h ν (direct transition). But the plots between (α hν )1/2 versus hν not give any useful result therefore our samples follow the direct transition. By extrapolation of linear part of the curves to the x-axis (photon energy), values of the energy gap can be found i.e. (α hν )2 → 0 (see Fig. 7). In this way, value of Eg is estimated as 3.47 eV for GdFeO3 , however doping of Mn decreases the band gap to 2.79 eV for x = 0.3 sample as tabulated in Table 1. In order to confirm band gap values, we have drawn the graphs between [F (R)hν ]2 and hν (Fig. S3), where F (R) is Kubelka Munk function. These values are nearly the same as we have estimated using (α hν )2 versus hν plots. In case of pristine sample, energy band gap is calculated as 3.89 eV and its value decreases to 3.15 eV for x = 0.3 concentration. The decrease in the band gap can be attributed to the introduction of some defects associated with in Mn substitution that create localized states in the band-gap region. Urbach energy (Eu ) is a valuable optical parameter to have an idea of the localized states in the band gap due to effect of possible defects or vacancies on doping. According to Urbach law absorption coefficient (α ) near band edges is an exponential character and energy corresponds to this is known as Urbach energy
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Fig. S3. (F (R)hν )2 versus h ν plots of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
Fig. 11. Refractive index (n) versus wavelength for GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
i.e. α = αo exp(hν/Eu ) [27]. Therefore, Urbach energy (Eu ) is estimated from the first and second peaks by plotting the graph between lnα and hν as shown in Fig. 8 for the first peak while for the second peak it is exhibited in Fig. S4. Using inverse of the slope of the graph, value of Eu has been calculated for all the samples. The presence of Urbach energy tail is an indicative of broadening of donor levels into impurity band that merge with the conduction band [28–30]. The higher value of Urbach energy related to the higher density of localized states. In the case of GdFe1−x Mnx O3 , Urbach energy increases with the incorporation of Mn ions and its value is calculated as 433.8 meV for pristine and 1.38 eV for x = 0.3 sample near the first edge and 1.15 eV to 3.15 eV near the second
Fig. 12. Optical conductivity (σ ) as function of incident photon wavelength for GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
edge. The enhancement in the Eu values proposes that the Mn substitution presents defects and more number of localized states in GdFeO3 . In other words, Urbach energy (Eu ) can also written as Eu = kBT /S(T ), where S(T ) is known as steepness parameter. The calculated values of Urbach energy and steepness parameter are given in Table 1 for all the samples. The maximum wavelength of incident radiation required to eject the electrons from a surface is defined as threshold wavelength, which is an important parameter for a material. The maximum wavelength or threshold wavelength (λs ) of the incident radiation has been estimated using UV/Vis.
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Fig. S4. lnα versus h ν plots to determine the Urbach energy near the second absorption peak of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
absorption data by employing following relation [31,32]: (A/λ) = G(1/λ − 1/λs ) 2
where A and G stand for the absorption and empirical constant respectively. For all the samples, we have plotted (A/λ)2 as a function of 1/λ to find out the threshold wavelength as shown in Fig. 9. It reveals an increase in threshold wavelength with Mn substitution and attains a value of 732.6 nm for 30% Mn doped as compared to pristine GdFeO3 (610.8 nm). It is well established that the absorption of the electromagnetic radiation by the material depends on various factors, like material’s nature and its thickness, extinction coefficient, photo-conductivity and dopant ions and their ratios with the material. When a high intense radiation falls on a material, then insignificant quantity of it reflected and most of the incident radiation is absorbed by the surface. In this regard, skin effect and the optical conductivity are two important parameters associated with absorption of photons. It is well established that the exponential decay occurs in electromagnetic energy inside a material. This decay resulted due to various sources such as crystal structure, refractive index and density and surface morphological structure of the materials. The depth at which intensity of the radiation falls to 1/e of its initial value at the surface is referred skin depth or penetration depth (δ ). It depends on two factors, i.e. material’s conductivity and the frequency of incident radiations. As we know that the resistivity of semiconductors is greatly depend on the energy band gap, therefore in term of the absorption coefficient (α ), it could be related with the skin depth using a relation δ = 1/α [33–35]. Fig. 10 displays the alteration in skin depth (δ ) as a function of incident energy (hν ) for GdFe1−x Mnx O3 (x = 0.0, 0.1, 0.2 and 0.3)
samples. These plots clearly exhibit significant reduction in skin depth with the increase in photon energy for all the samples. This may be due to reduction in the energy of the incident photon that influences number of neighbouring Fe3+ ions near the surface. The refractive index (n) determined by reflectance data shows the highest value near the region of strong absorption (∼ less than 400 nm). The variation in the refractive index with the wavelength of incident photon in the range of 400–800 nm is shown in Fig. 11. It is evident from the figure that the refractive index has higher values in the wavelength range of 200 to 400 nm for all the compositions. This behaviour can be elucidated on the basis of resonance effect in the incident electromagnetic radiation and the polarization of the electrons, that resulted in the coupling of electrons in the oscillating electric field. Thereby, there is no propagation of electromagnetic radiation through the samples. At longer wavelengths, the refractive index reduces till it reaches to the lowest value of about 1.1 at the maximum value of measured wavelength (800 nm). We have also estimated extinction coefficient (k) with the help of absorption coefficient (α ) by the equation, k = αλ/4π [36]. The variation in k with the incident photon energy is presented in Fig. S5. This figure reveals reduction in the extinction coefficient at the higher value of incident energy. At lower energies, the extinction coefficient is higher for Mn substituted samples but at higher values of energies, undoped and Mn doped samples show similar value of extinction coefficient. This variation in extinction coefficient is associated with the change in refractive index by Mn doping and may be due to a distortion in crystal structure. Optical conductivity (σ ) greatly depends on the optical energy gap and apart from that its value also depends upon many other parameters, like absorption coefficient, refractive index, incident
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Fig. S5. Extinction coefficient (k) versus photon energy plots for GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
photons’s frequency and the extinction coefficient. For the present system, we have estimated the optical conductivity by employing the relation, σ = α nc /4π k, here c stands for speed of light in empty space [37,38]. Fig. 12 illustrates the behaviour of optical conductivity as a function of incident photon wavelength for pure and Mn substituted GdFeO3 samples. It is clear from the graph that the optical conductivity depends extremely on the composition and wavelength of incident radiation. The value of σ decreases with the rise in wavelength and attained minimum values at 800 nm for all the compositions. This may be due to the high absorbent behaviour of the samples in that region and the excitation of electrons by the photon energy.
heat capacity at constant pressure (Cp ) near the Neel temperature that shifts towards the lower temperature on Mn doping leads to decease in Neel temperature. The various optical parameters were evaluated using UV/Visible absorption data. The energy bandgap of the samples, estimated using Tauc’s relation and KM function give almost the same values, and found to decrease with Mn doping. However, Urbach energy (Eu ) calculated near the absorption edges enhanced on Mn doping in GdFeO3 . The increase in Eu suggests that Mn substitution introduces defects and more number of localized states in GdFeO3 . The values of skin depth (δ ), refractive index and optical conductivity decreases with the increase in wavelength of incident radiation for all the samples.
4. Conclusions
Acknowledgement
Nanocrystalline GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3) orthoferrites have been synthesized successfully by solid state reaction route. Influence of Mn doping on the morphological, thermal and optical properties were investigated in detail. SEM micrographs revealed uniform distribution of grains of size ranging from 26–61 nm. TEM images indicate highly agglomerated nature of the grains. SEM and TEM analyses confirmed increase in grain size with the increase in Mn concentration. EDX analysis certify the presence of all elements and chemical purity of the synthesized samples without any impurity. Elemental mapping established homogeneous and symmetrical distributions of the elements in the respective samples. FTIR spectra exhibit a strong band at ∼435 cm−1 . This band is assigned to bending vibration of O–Fe–O. The Mn doping shifts this band from 435 cm−1 for pristine sample to 453 cm−1 for x = 0.3 concentration. DTA measurement exhibits an anomaly in the
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Influence of Mn substitution on morphological, thermal and optical properties of nanocrystalline GdFeO3 orthoferrite Shahid Husain *, Ali O.A. Keelani, Wasi Khan Department of Physics, Aligarh Muslim University, Aligarh 202 002, India
highlights
graphical abstract
• Nanocrystalline Mn doped in GdFeO3 • •
•
•
were synthesized by solid state reaction route. SEM/TEM micrographs reveal the increase in grain size on Mn doping. FTIR spectra show the characteristic Fe–O band around 559 cm−1 in undoped sample. An anomaly in the specific heat was observed near the Neel temperature of GdFeO3 . Mn doping decreases the bandgap of GdFeO3 significantly.
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Article history: Received 4 October 2017 Received in revised form 22 January 2018 Accepted 5 March 2018 Keywords: Orthoferrites Solid state reaction route SEM/EDX TEM Differential thermal analysis (DTA) Urbach energy
a b s t r a c t The current research work focuses on the preparation of nanocrystalline GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3) samples via solid state reaction route to explore their microstructural, thermal and optical properties in detail. Scaning electron microscopy (SEM)/Transmission electron microscopy (TEM) results revealed coarse surface and highly agglomerated form of the particles in nanoscale region with increasing particle size on Mn substitution. Elemental composition and homogeneity of the ions were ensured from energy dispersive x-ray (EDX) and SEM-EDX analyses respectively. Fourier transform infrared (FTIR) spectra reflected characteristic peak of Fe–O band at ∼559 cm−1 for GdFeO3 sample and shifted to 579 cm−1 by the substitution of Mn ions, signify the octahedral FeO6 group of perovskite structure. The variation in heat flow and specific heat at constant pressure (Cp ) with the increase in temperature were monitored through differential thermal analysis (DTA) technique. An anomaly in the specific heat near the Nèel temperature (565 K) of GdFeO3 has been observed. This peak further shifts towards the lower temperature on Mn doping. Moreover, Mn substitution in GdFeO3 reduces the values of Cp . UV–visible absorption spectra exhibited two noticeable bands in the ultraviolet region and the band gap was found to decrease with the increase in Mn content as estimated by employing Tauc’s relation. However, Urbach energy calculated using absorption coefficient enhances. These results demonstrate that the physical properties of GaFeO3 system could be tuned by the appropriate substitution of Mn for real applications. © 2018 Elsevier B.V. All rights reserved.
1. Introduction
* Corresponding author. E-mail address: [email protected] (S. Husain). https://doi.org/10.1016/j.nanoso.2018.03.002 2352-507X/© 2018 Elsevier B.V. All rights reserved.
Ferrites are considered most promising materials owing to their diverse and interesting technological applications. They are eco-friendly and have potential applications in transformer cores, modulators, capacitors and optical storage, piezoelectric sensors,
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memory core, high quality filters, antennas, radio frequency components etc. [1–4]. In this group, RFeO3 (where R represents to a rare-earth ion) belongs to orthoferrite perovskite crystal structure. Among these, GdMnO3 is an orthorhombically distorted A-type antiferromagnetic insulator that exists in Pbnm or Pnma crystallographic space group. In this structure, Fe3+ ions are ordered antiferromagnetically in the ‘a’ axis and somewhat canted along the ‘c’ axis to give weak ferromagnetism in the temperature below 660 K. Due to antisymmetric exchange of slight canting of Fe3+ spins give the weak ferromagnetic nature of the compound [5,6]. Most of the orthoferrites are distorted due to FeO6 octahedra that modify these materials into monoclinic, orthorhombic, rhombohedral and triclinic crystal structures [7]. Sometimes rare earth ion is not responsible for the Fe octahedron distortion [8]. However, it is reported that the decrease of the rare-earth ion’s radius leads to distortion enhancement of the polyhedra. Because of this distortion, the rareearth ion is surrounded by the 12 oxygen (O) ions that are divided into two kinds i.e. eight first nearest oxygen ions and four second nearest O ions. This structural distortion in the system affect the magnetic ordering and spin-state transitions [9,10]. Therefore, the potential technological applications of these orthoferrites has raised much interest in relation to the properties, such as magnetic field sensors based on magnetoresistance, actuators utilize piezoelectricity and oxygen sensors on electronic ionic mixed conductivity with nonlinear response to oxygen pressure [11–13]. These type of materials also exhibit visible-light photocatalytic activity to overcome environmental pollution and the crisis of energy over the globe [14,15]. In addition, rare-earth perovskites are also found excellent alternate for the degradation of organic compounds with the help of visible light [15,16]. Moreover, GdFeO3 is a kind of typical rare-earth perovskite-type oxide with a stable crystallographic structure that has unique characteristics of electromagnetic and catalysis [17]. Hence, GdFeO3 has several application prospects in the fields of magnetism and photocatalysis. But the synthesis of this structure in single phase still is a challenging task via solid state reaction route for some of the structures [18] although it has many advantages including phase purity and minimum change of any other impurity elements. Zhang et al. [19] have explored GdFeO3 type distortion in orthorhombic DyMnO3 system by the variation in grain size. They have suggested that this type of modification is also possible by the variation in particle size without any external perturbations. In another work, Shah et al. studied the effect of Mg2+ doping on the dielectric and magnetic properties of polycrystalline GdFeO3 samples. They have observed the increase in dielectric constant and magnetoelectric coupling coefficient for 6% Mg doped sample. These results conclude that the antiferromagnetic domain walls play a significant role in controlling these properties [20]. However, it is difficult to control particle size and surface morphology that play vital and significant role in the physical and chemical properties of nanomaterials. But chemical methods of nanomaterials synthesis in various shapes are quite easy and well established nowadays [21]. The physical properties of these materials can be enhanced and tuned by the doping of transition metal ion at Fe site. Thermal parameters such as heat flow in and out of the system, specific heat etc. and influence of the doping on these parameters are also important for the various applications especially in sensors. In view of this, Mn doped GdFeO3 nanocrystalline sample were synthesized and detailed morphological, thermal and optical properties have been studied. To the best of our knowledge, no reports are available on the effect of Mn doping on the thermal and optical properties of GdFeO3 system. 2. Experimental details Nanocrystalline samples of GdFe1−x Mnx O3 (x = 0.0, 0.1, 0.2 and 0.3) (GFMO) have been synthesized through standard solid
Fig. S1. SEM-EDX elemental mapping images of GdFe1−x Mnx O3 (x = 0.0, 0.1, 0.2 & 0.3) samples.
state reaction route from high purity (AR grade) chemical reagents. The detailed description of the samples preparation is given elsewhere [22]. Scanning electron microscope (SEM) (JSM6510) and energy dispersive x-ray spectroscopy (EDX) respectively were employed to examine the surface morphology and presence of chemical composition in the samples. Transmission electron microscopy (TEM) (Jeol, JEM2010) images were captured for the investigation of the particle/grain size and internal structure. Fourier transform infrared (FTIR) spectroscopy measurements were carried out by the Bruker-Tensor-37 spectrometer for the wavenumber range of 370–4000 cm−1 . Differential thermal analysis (DTA) was performed using STA-8000 (Perkin Elmer) thermal analyzer in the temperature range of 30 to 1000 ◦ C at the heating rate of 20 ◦ C/min. The investigation of optical properties was performed in the wavelength range of 200 to 800 nm with a UV/Vis spectrophotometer. 3. Results and discussion 3.1. Morphological studies X-ray diffraction (XRD) data analysis through Rietveld refinement confirmed polycrystalline and single phase nature of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3) samples. All the samples crystallize in orthorhombic crystal symmetry without any other measurable impurity phase [22]. Fig. 1 displays SEM images of the synthesized samples that exhibit quite different surface morphology having nearly spherical shape of the grains with a large size variation in the range of 26 to 61 nm. Moreover, grain size increases for higher Mn contents and the grain boundaries are clearly distinguishable. Further, SEM images clearly show uniform distribution of the grains and systematic change in morphology with the increase in Mn doping. The chemical composition of the samples examined through EDX analysis are shown in Fig. S1. It gives the qualitative elemental composition in the samples by the appearance of Mn, Gd, Fe and O peaks. This indicates high purity of the samples and presence of no other impurity elements. Some gold peaks are appearing in the EDX spectra due to gold coating used to avoid charging of the specimen, which would otherwise occur due to the accumulation of electrons on the sample surface. It is also helpful to increase the secondary electrons that reduces noise to signal ratio. SEM-EDX mapping images further ensure uniform and appropriate elemental distribution throughout the samples (Fig. S2). It is clear that the distribution of elements Gd, Fe, Mn and O are homogeneous and symmetrical in all samples, and as we increase the Mn content in GdFeO3 the percentage of Fe reduces in the same ratio.
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Fig. 1. SEM micrographs of GdFe1−x Mnx O3 (x = 0.0, 0.1, 0.2 and 0.3) samples.
Fig. 2. TEM images of GdFe1−x Mnx O3 (x = 0.0, 0.1, 0.2 and 0.3).
The typical internal structure of the samples is examined by the transmission electron microscope (TEM) operated at 200 kV. TEM images of GdFe1−x Mnx O3 (x = 0.0, 0.1, 0.2 and 0.3) with grain sizes are displayed in Fig. 2. These micrographs exhibit that the grains have nearly spherical shapes in the range of 32 to 57 nm. The larger grain size is observed for higher Mn concentration that support SEM results. However, due to the smaller grain sizes, enough agglomeration is observed in all the samples. The large agglomeration in the particles may be due to the strong magnetic dipole–dipole interactions among them. Usually, the particle sizes are somewhat larger than that observed using SEM attributed to the light scattering of the organic molecules adsorbed on the particle surface. FTIR spectra of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3) perovskite were recorded to recognize the occurrence of various vibrational bands
in the samples and are displayed in Fig. 3. Generally, the infrared spectra provide the information of the functional groups, molecular geometry and inter or intra molecular interactions existing in a system. It is known that the frequency of the bands within 1000 cm−1 are associated to the bonds of inorganic elements. Therefore, the appeared prominent band at ∼435 cm−1 is related to the bending vibration of O−Fe−O in GdFeO3 [23]. However, Mn doping shifts this band to 453 cm−1 for x = 0.3. Other strong band at ∼559 cm−1 is also influenced by the Mn substitution and shifted from 559 cm−1 (x = 0.0) to 579 cm−1 for x = 0.3 concentration. This sharp absorption band at 559 cm−1 ascribed to the B–O (Fe–O) stretching vibration, being features of the octahedral BO6 groups in the perovskite (ABO3 ) compounds, which is usually noticed in the region of 500–700 cm−1 . The bands present at ∼3453 cm−1
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Fig. 3. FTIR spectra of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3) recorded at room temperature. Fig. S2. Energy dispersive x-ray (EDX) spectra of all samples.
and 1637 cm−1 are ascribe to the O–H stretching and H–O–H bending vibrations of structural moisture. On Mn doping these bands shifted to the lower values 3430 cm−1 and 1604 cm−1 for 30% Mn doping. These shifts are attributed to the smaller atomic mass of Mn as compared to Fe and hence these measurements again ensure purity of the samples. 3.2. Thermal analysis DTA profiles of GdFe1−x Mnx O3 (x = 0.0, 0.1, 0.2 and 0.3) samples as a function of temperature are recorded with a controlled
nitrogen atmosphere, in a temperature range of 30 to 1000 ◦ C and are shown in Fig. 4. This figure presents the heat flow in and out of the system due to endothermic and exothermic reactions. The undoped GdFeO3 sample exhibits endothermic peaks at temperatures 70.2, 292 and 789 ◦ C, whereas an exothermic peak is observed at about 380 ◦ C. But on Mn doping these endothermic peaks have started to shift towards lower temperatures and for x = 0.3 these peaks are present at 68.1, 242.3 and 710.4 ◦ C, however the exothermic peak presented in pristine sample at 380 ◦ C has been disappeared. In addition, some additional exothermic peaks
Fig. 4. Heat flow as a function of temperature for GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
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Fig. 5. Heat capacity at constant pressure (Cp ) as a function of temperature for GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
are appeared in Mn substituted samples at higher temperatures. These peaks can be ascribed to the structural transition on Mn substitution at higher temperatures as the exchange interaction of the Mn3+ –Mn4+ depends on the bond angle and the bond length. The reduction of Mn–O bond length and Mn–O–Mn bond angle will make Mn3+ –Mn4+ exchange interaction stronger, leading to the structural transition. We have also calculated the value of heat capacity at constant pressure (Cp ) and its variation with temperature are displayed in Fig. 5. The absolute value of Cp is observed maximum (226.42 mW/gm.degrees) for pristine sample and Mn doping reduces this value to 99.22 mW/gm.degrees for x = 0.3 concentration sample. The anomaly at 292 ◦ C or 565 K in pristine sample can be ascribed to the phase transition from antiferromagnetic to paramagnetic state as this temperature is near to Néel temperature of GdFeO3 . The Néel temperature of bulk GdFeO3 is reported as 670 K, that reduces to the lower value in nanoscale GdFeO3 . In these samples, Néel temperature further reduces on Mn doping. It is well matched to the reported results that also reported reduction in Néel temperature on the substitution at Fe site in GdFeO3 [20]. 3.3. Optical properties The study of optical properties is useful to know the optoelectronic nature of materials. Further, optical properties of orthoferrites are extremely influenced by the characteristics of the incident radiation falling on it. In this way, optical absorption studies give a simple route to explain the band structure and estimation of the optical band gap of materials. Reflectance spectra of Mn substituted GdFeO3 samples were observed in ultraviolet–visible (UV/Vis.) spectral range at room temperature. The reflectance (R) has been changed into absorbance (A) using the relation A =
Fig. 6. Absorption spectra of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3) as a function of wavelength.
− log(R). The change in absorbance as a function of wavelength is displayed in Fig. 6. This figure exhibit two pronounced peaks at 234 and 368 nm for undoped GdFeO3 sample and these peaks have been slightly red shifted on Mn doping. The absorption coefficient (α ) is calculated using α = 2.303 (A/t), here t stands for breadth of the cuvette in which sample was filled. Then the Tauc’s relation as given below is used to estimate the band gap (Eg ) of the samples [24,25].
α hν = A(hν − Eg )m
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Fig. 7. (α hv)2 versus h ν plots of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
Fig. 8. lnα vs h ν plots to determine the Urbach energy near the first absorption peak of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
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Table 1 Bandgap (Eg ), Urbach energy (Eu ) and steepness parameter (S(T )) for GdFe1−x Mnx O3 (x = 0, 0.10, 0.20 and 0.30) samples. Mn concentration
x x x x
= 0.0 = 0.1 = 0.2 = 0.3
Bandgap (Eg ) (eV) estimated by
Near first absorption peak
Tauc relation
K-M function
Eu (meV)
S(T) × 10−3
Eu (eV)
Near second absorption peak S(T) × 10−3
3.47 3.17 3.12 2.79
3.89 3.80 3.73 3.15
433.8 551.0 784.5 1380.0
59.598 46.918 32.955 18.73
1.1471 1.4489 2.3635 3.1526
22.54 17.84 10.94 8.20
Fig. 9. (A/λ)2 versus 1/λ plots for GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
Fig. 10. Variation in skin depth (δ ) with energy of photon plots of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
where symbols have their usual meaning. In this relation, exponent value m depends on the nature of band gap which is 12 for direct
bandgap and has the value 2 for indirect band gap [26]. We have plotted the graphs between (α hν )1/2 versus hν (indirect transition) and (α hν )2 versus h ν (direct transition). But the plots between (α hν )1/2 versus hν not give any useful result therefore our samples follow the direct transition. By extrapolation of linear part of the curves to the x-axis (photon energy), values of the energy gap can be found i.e. (α hν )2 → 0 (see Fig. 7). In this way, value of Eg is estimated as 3.47 eV for GdFeO3 , however doping of Mn decreases the band gap to 2.79 eV for x = 0.3 sample as tabulated in Table 1. In order to confirm band gap values, we have drawn the graphs between [F (R)hν ]2 and hν (Fig. S3), where F (R) is Kubelka Munk function. These values are nearly the same as we have estimated using (α hν )2 versus hν plots. In case of pristine sample, energy band gap is calculated as 3.89 eV and its value decreases to 3.15 eV for x = 0.3 concentration. The decrease in the band gap can be attributed to the introduction of some defects associated with in Mn substitution that create localized states in the band-gap region. Urbach energy (Eu ) is a valuable optical parameter to have an idea of the localized states in the band gap due to effect of possible defects or vacancies on doping. According to Urbach law absorption coefficient (α ) near band edges is an exponential character and energy corresponds to this is known as Urbach energy
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Fig. S3. (F (R)hν )2 versus h ν plots of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
Fig. 11. Refractive index (n) versus wavelength for GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
i.e. α = αo exp(hν/Eu ) [27]. Therefore, Urbach energy (Eu ) is estimated from the first and second peaks by plotting the graph between lnα and hν as shown in Fig. 8 for the first peak while for the second peak it is exhibited in Fig. S4. Using inverse of the slope of the graph, value of Eu has been calculated for all the samples. The presence of Urbach energy tail is an indicative of broadening of donor levels into impurity band that merge with the conduction band [28–30]. The higher value of Urbach energy related to the higher density of localized states. In the case of GdFe1−x Mnx O3 , Urbach energy increases with the incorporation of Mn ions and its value is calculated as 433.8 meV for pristine and 1.38 eV for x = 0.3 sample near the first edge and 1.15 eV to 3.15 eV near the second
Fig. 12. Optical conductivity (σ ) as function of incident photon wavelength for GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
edge. The enhancement in the Eu values proposes that the Mn substitution presents defects and more number of localized states in GdFeO3 . In other words, Urbach energy (Eu ) can also written as Eu = kBT /S(T ), where S(T ) is known as steepness parameter. The calculated values of Urbach energy and steepness parameter are given in Table 1 for all the samples. The maximum wavelength of incident radiation required to eject the electrons from a surface is defined as threshold wavelength, which is an important parameter for a material. The maximum wavelength or threshold wavelength (λs ) of the incident radiation has been estimated using UV/Vis.
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Fig. S4. lnα versus h ν plots to determine the Urbach energy near the second absorption peak of GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
absorption data by employing following relation [31,32]: (A/λ) = G(1/λ − 1/λs ) 2
where A and G stand for the absorption and empirical constant respectively. For all the samples, we have plotted (A/λ)2 as a function of 1/λ to find out the threshold wavelength as shown in Fig. 9. It reveals an increase in threshold wavelength with Mn substitution and attains a value of 732.6 nm for 30% Mn doped as compared to pristine GdFeO3 (610.8 nm). It is well established that the absorption of the electromagnetic radiation by the material depends on various factors, like material’s nature and its thickness, extinction coefficient, photo-conductivity and dopant ions and their ratios with the material. When a high intense radiation falls on a material, then insignificant quantity of it reflected and most of the incident radiation is absorbed by the surface. In this regard, skin effect and the optical conductivity are two important parameters associated with absorption of photons. It is well established that the exponential decay occurs in electromagnetic energy inside a material. This decay resulted due to various sources such as crystal structure, refractive index and density and surface morphological structure of the materials. The depth at which intensity of the radiation falls to 1/e of its initial value at the surface is referred skin depth or penetration depth (δ ). It depends on two factors, i.e. material’s conductivity and the frequency of incident radiations. As we know that the resistivity of semiconductors is greatly depend on the energy band gap, therefore in term of the absorption coefficient (α ), it could be related with the skin depth using a relation δ = 1/α [33–35]. Fig. 10 displays the alteration in skin depth (δ ) as a function of incident energy (hν ) for GdFe1−x Mnx O3 (x = 0.0, 0.1, 0.2 and 0.3)
samples. These plots clearly exhibit significant reduction in skin depth with the increase in photon energy for all the samples. This may be due to reduction in the energy of the incident photon that influences number of neighbouring Fe3+ ions near the surface. The refractive index (n) determined by reflectance data shows the highest value near the region of strong absorption (∼ less than 400 nm). The variation in the refractive index with the wavelength of incident photon in the range of 400–800 nm is shown in Fig. 11. It is evident from the figure that the refractive index has higher values in the wavelength range of 200 to 400 nm for all the compositions. This behaviour can be elucidated on the basis of resonance effect in the incident electromagnetic radiation and the polarization of the electrons, that resulted in the coupling of electrons in the oscillating electric field. Thereby, there is no propagation of electromagnetic radiation through the samples. At longer wavelengths, the refractive index reduces till it reaches to the lowest value of about 1.1 at the maximum value of measured wavelength (800 nm). We have also estimated extinction coefficient (k) with the help of absorption coefficient (α ) by the equation, k = αλ/4π [36]. The variation in k with the incident photon energy is presented in Fig. S5. This figure reveals reduction in the extinction coefficient at the higher value of incident energy. At lower energies, the extinction coefficient is higher for Mn substituted samples but at higher values of energies, undoped and Mn doped samples show similar value of extinction coefficient. This variation in extinction coefficient is associated with the change in refractive index by Mn doping and may be due to a distortion in crystal structure. Optical conductivity (σ ) greatly depends on the optical energy gap and apart from that its value also depends upon many other parameters, like absorption coefficient, refractive index, incident
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Fig. S5. Extinction coefficient (k) versus photon energy plots for GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3).
photons’s frequency and the extinction coefficient. For the present system, we have estimated the optical conductivity by employing the relation, σ = α nc /4π k, here c stands for speed of light in empty space [37,38]. Fig. 12 illustrates the behaviour of optical conductivity as a function of incident photon wavelength for pure and Mn substituted GdFeO3 samples. It is clear from the graph that the optical conductivity depends extremely on the composition and wavelength of incident radiation. The value of σ decreases with the rise in wavelength and attained minimum values at 800 nm for all the compositions. This may be due to the high absorbent behaviour of the samples in that region and the excitation of electrons by the photon energy.
heat capacity at constant pressure (Cp ) near the Neel temperature that shifts towards the lower temperature on Mn doping leads to decease in Neel temperature. The various optical parameters were evaluated using UV/Visible absorption data. The energy bandgap of the samples, estimated using Tauc’s relation and KM function give almost the same values, and found to decrease with Mn doping. However, Urbach energy (Eu ) calculated near the absorption edges enhanced on Mn doping in GdFeO3 . The increase in Eu suggests that Mn substitution introduces defects and more number of localized states in GdFeO3 . The values of skin depth (δ ), refractive index and optical conductivity decreases with the increase in wavelength of incident radiation for all the samples.
4. Conclusions
Acknowledgement
Nanocrystalline GdFe1−x Mnx O3 (0 ≤ x ≤ 0.3) orthoferrites have been synthesized successfully by solid state reaction route. Influence of Mn doping on the morphological, thermal and optical properties were investigated in detail. SEM micrographs revealed uniform distribution of grains of size ranging from 26–61 nm. TEM images indicate highly agglomerated nature of the grains. SEM and TEM analyses confirmed increase in grain size with the increase in Mn concentration. EDX analysis certify the presence of all elements and chemical purity of the synthesized samples without any impurity. Elemental mapping established homogeneous and symmetrical distributions of the elements in the respective samples. FTIR spectra exhibit a strong band at ∼435 cm−1 . This band is assigned to bending vibration of O–Fe–O. The Mn doping shifts this band from 435 cm−1 for pristine sample to 453 cm−1 for x = 0.3 concentration. DTA measurement exhibits an anomaly in the
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