The present research demonstrates the synthesis and characterization of LaMnO3 perovskite powders using the sol-gel technique for dye-sensitive solar cell applications. With this respect, transparent solutions were prepared from La and Mn based precursors, distilled water and citric acid monohydrate. Ammonium hydroxide was incorporated into the La-Mn solution in order to neutralize/precipitate at 24oC for 1 hour in the air. The solution was allowed to evaporate on a hot plate device at 90 °C in the air. The obtained solutions were dried at 90 oC for 24 hours to form a xerogel structure, dried at 200 oC for 2 hours and consequently annealed at 500 and 850 oC for 2 hours in the air. Thermal, structural, microstructural, optical and magnetic properties of the powders were characterized through differential thermal analysis-thermogravimetry (DTA-TG), Fourier transform infrared (FTIR), X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), scanning electron microscopy (SEM), Malvern zeta sizer (PSD), UV-vis spectrometer and vibrating sample magnetometer (VSM). The obtained results indicate promise, especially the low band range, that LaMnO3 powders can be used in dye-sensitized solar cells and can positively affect performance and efficiency.
Research Article
Synthesis and Characterizations of LaMnO3 Perovskite Powders Using Sol-Gel Method
https://doi.org/10.21203/rs.3.rs-217058/v1
This work is licensed under a CC BY 4.0 License
Version 1
posted 12 Feb, 2021
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Lanthanum-based perovskite materials possess ostensible optical [1, 2], magnetic [3–5] and catalytic properties [6]. These materials have been extensively preferred in many areas such as catalysts [7–9], chemical sensors [10], solid oxide fuel cell electrodes [7], hydrogen storage [11], optoelectronic devices [12], solar cells [13], magnetic materials [14, 15] by virtue of stability, different oxidation states, flexible oxygen stoichiometry, power conversion efficiency and isomorphic substitution of metals in their structures [10, 16]. Foremost these applications, solar energy produced from perovskite-based solar cells is one of the most up-and-coming alternative energy sources on account of its relatively high energy conversion efficiency, low production cost, low production toxicity, ease of manufacture and flexibility [17].
Owing to their stability, suitable bandgap, a lower cost lanthanum-based perovskites such as lanthanum manganite (LaMnO3), lanthanum orthoferrite (LaFeO3), lanthanum nickel oxide (LaNiO3) and lanthanum cobaltite (LaCoO3) display power conversion efficiency and they became a great alternative for solar cell implementations [18–20]. Of the perovskite materials, LaMnO3 notably possesses photovoltaic, photocatalytic and magnetic applications [20–22] on account of high stability at high temperature for aggressive medium [23] and narrower bandgap [24] features.
As a magnetic material, LaMnO3 literally indicates gorgeous attributes in tandem with photovoltaic characteristics. It is an orthorhombic perovskite structure insulating antiferromagnetic material and its Neél temperature (TN) is approximately 140 K. The magnetic manners of LaMnO3 material are acquired from Mn3+ (3d4, ) captions super-exchange interactions inasmuch as the Mn3+ cations are located in an octahedral crystal field with six oxygen anions as MnO6 form. There is a strong correlation betwixt the eg and t2g orbitals’ electron spins and this triggers the Jahn–Teller distortion of MnO6 octahedra around Mn3+ ions. In addition to these, with the increase in the amount of oxygen, (LaMnO3 + x) structure can form more Mn4+ ions. It leads to ferromagnetic double-exchange interactions of Mn3+–Mn4+ pairs, and antiferromagnetic Mn4+–Mn4+ ones [25].
The efficiency of LaMnO3 material relies on the synthesis techniques [26] such as solid-state reaction [27], combustion synthesis [28], hydrothermal synthesis [29], sonochemical synthesis [30], co-precipitation method [31], EDTA glycine process [32], core-shell [33], nanowires [34] and sol-gel [16, 35]. In recent years, studies on LaMnO3 synthesis have been launched using different precursors and several synthesis techniques in order to be applied for alternative realms of use. To illustrate these, L. Phan et. al. [25] soundly scrutinized the magnetocaloric effect and critical behavior of Fe-doped LaMnO3 so as to engender the LaMn0.9Fe0.1O3 phase. In addition to these, T. Shou et. al. [9] reported the catalytic effect of Fe doped LaMnO3. Y. Song et. al. [36] synthesized the LaMnO3−δ including oxygen vacancy by making use of the method of molten salt. S. Priyatharshni et. al. [37] studied various morphologies of nanoscale LaMnO3 structures which were synthesized with a facile hydrothermal process with stable calcination temperature. Q. Sun et. al. [15] synthesized LaMnO3 thin films with post-annealing in the oxygen atmosphere to appraise the influence of the excess oxygen on the transport, structural and magnetic behaviors.
In our initial attempt [38], we reported synthesis and characterizations of sol-gel derived LaFeO3 perovskite powder in view of its superior properties. In the previous work, the obtained results indicate that the LaFeO3 powders possess the potential for photocatalytic applications notably in the continuously applicable and sustainable dye-sensitized solar cells as futuristic and innovative approaches. There is no universally enshrined LaMnO3 synthesis due to the lack of any specific studies on sustainable dye sensitive solar cells in the literature. Moreover, no such organized search has been systematically studied to shed light on the influence of the structural, optical, particle size and magnetic properties of LaMnO3 for two different synthesis temperatures. In this research, the fundamental goal is to logically comprehend the profound impacts of the annealing behaviors of LaMnO3 perovskite powder and to specify the effect of structural, optical and magnetic features depending on the annealing temperature.
2.1. Solution preparation
There exist four primary stages in preparing a solution, they are preparation and characterization of transparent solutions, neutralization/precipitation method, and xerogel production. Each of these steps is prominent for nanoscale particle synthesis, the nanoparticle size of the particles is a major criterion for the next stages. Each process conducted in the synthesis of powders is comprehensively denoted in the flow chart of Fig. 1.
For the synthesis of pure LaMnO3 powders, first, a transparent solution precisely should be prepared. As listed in Table 1, lanthanum (La) and manganese (Mn) based solution was prepared to carry out the syntheses of LaMnO3 by making use of nitrate-based precursors which produces another compound. The precursors used in the synthesis are adjusted according to stoichiometric ratios. Distilled water (H2O) and citric acid monohydrate (C6H8O7.H2O) were employed as the solvent and chelating agent (ligand) respectively. The role of this chelating agent in solution is a chemical compound whose structure allows two or more donor atoms (or sites) to bond to the same metal ion at the same time and forms one or more rings [39].
| Solution | Chemical products | Concentration |
|---|---|---|
| LMO | Lanthanum nitrate hexahydrate (La(NO3)3x6H2O) Manganese (II) nitrate tetrahydrate (Mn(NO3)2x4H2O) Citric acid monohydrate (HOC(COOH)(CH2COOH)2·H2O) Distilled water (H2O) Ammonium hydroxide (NH4OH) | 5.413 g (0.0125 mole) 3.235 g (0.0125 mole) 5.253 g (0.0250 mole) 50 ml 7 ml |
15 ml of distilled water was used as a solvent and 0.0125 mole of La (lanthanum nitrate hexahydrate - La(NO3)3x6H2O, 99.99%, Merck) and 0.0125 mole of Mn (manganese (II) nitrate tetrahydrate - Mn(NO3)2x4H2O, 99.99%, Sigma) sources were separately dissolved and vigorously mixed together. As well, 0,025 mol of citric acid monohydrate (HOC(COOH)(CH2COOH)2·H2O, 99.99%, Iso) was veraciously dissolved in distilled water (15 ml) and this mixture was subsequently put in the solution in order to effectuate stable complexes of metals, which prevents the precipitation of the salts of these metals and the subsequent homogenization of the system. The total water volume was accurately adjusted to 50 ml. The solution was then shuffled on a magnetic stirrer at room temperature for 1 hour in the air. Whole processes had no sooner completed than the solution was made clear, transparent and stable.
The turbidity and pH values of the transparent solution had to be measured for solution characterization. Prior to the precipitation process, the turbidity of the prepared solution was calculated via turbidimeter (VELP TB1) in the range of 0–1000 ntu (nephelometric turbidity unit). At this step, the La-Mn solution was the color pink. With a standard pH meter, the pH value of the solution was measured to guess its characteristics, it was found that this value of the solution was below 1, which strongly represents an acidic character.
In the case of the neutralization/precipitation process, ammonium hydroxide (NH4OH, ≥ 99.99%, Merc) was involved in the generation of hydroxide ions in this solution, boosting the pH to the point in which oxides were formed. NH4OH was attentively incorporated into the La-Mn solution in a controlled manner with dropper control and checked with pH meter and pH-indicator strips. After adding 7 ml of NH4OH, the pH value was measured as neutral (7) and the obtained solution was smartly stirred to notralize/precipitate at 24oC for 1 hour in air. The color of the La-Mn solution was milky white. The solution was allowed to evaporate for 24 hours on a hot plate device at 90°C in air after the stirring procedure. This was followed by the production of xerogel powder, by acquiring an atomic mixture in the final product before drying and high-temperature annealing.
2.2. Heat treatment
In view of the fact that, in heat treatment, drying is a mass transfer process that comprises the removal of water or other solvents from solid, semi-solid and liquid by evaporation, structure features can be obtained by controlling the diffusion rate and cooling rate within the microstructure by heat treatment. In the realm of this, heat treating is frequently used to alter the structural, microstructural, mechanical, optical, magnetic and solar cell efficiencies of the materials. For proper heat treatment, it requires precise control over temperature, rate of ascent to this temperature, holding time at this temperature and cooling rate. The main objective of process annealing is to produce a uniform microstructure and improve properties, as mentioned in literature [40–42]
The drying and annealing processes required for the growth of the LaMnO3 powder are posed in Fig. 2 (see for more details). Our material, which comes in the form of xerogel on the hot plate is divided into two and both are dried at 200 oC for 2 hours in the air. Subsequently, with a heating rate of 5 oC/minute, the samples are heated up to 500 and 850 oC and kept at these temperatures for 2 hours. Hereafter, they were left to cool down in the electric furnace. In accordance with the DTA-TG results presented in-dept in the ensuing sections, the heating regimes of the drying and annealing processes were sensibly simulated.
2.3. Characterizations
Thermal analyses of xerogel kept on a hot plate at 90 oC for 24 hours and ceramic powder heated at 500 oC for 2 hours were conducted by means of DTG-60H Model Differential Thermal Analysis-Thermogravimetric Analysis (DTA-TGA) machine in an air environment at 5°C/min heating rate. With these analyses, an optimum heat treatment procedure was gained for drying, heat treatment and annealing regimes as well as obtaining decomposition and phase formation.
Fourier Transform Infrared (FTIR) spectra of La-Mn based xerogel and powders were recorded to cast light on the bonds in their structures through an ATR attached FTIR spectrometer (Thermo Scientific Nicolet IS10).
X-ray Diffraction (XRD) patterns of the ceramic powders were employed to extensively clarify their crystal structures and temperature-dependent phase transformations at the temperature between 500 oC and 850 oC by way of a Thermo-Scientific, ARL-Kα diffractometer with working settings of 45 kV − 44 mA and CuKα irradiation (wavelength, λ = 0.15418 nm).
X-ray Photoelectron Spectroscopy (XPS, Thermo Scientific Al Kα) and monochromatic Al-Kα (1486.7eV) X-ray source with 400 nm diameter beam size was exploited to define the elemental composition and surface chemistry of the powder specimens synthesized at 500oC and 850oC. The curve fitting program supplied by the Product Company (ThermoAvantageV5.65 programs) was used to specify binding energies and deconvoluted spectra.
In order to observe the surface morphology of La-Mn based xerogel and ceramic powders, Zeiss Sigma 300 VP model Scanning Electron Microscope (SEM) equipment operating at 8–10 kV acceleration voltage was employed at 20 kV.
In order to measure the average particle sizes and particle size distributions of LaMnO3 powders, the Malvern Zeta Sizer ZS90 (PSD) was utilized. The powders must be dispersed in a liquid for particle size measurement and distilled water is used as the liquid at room temperature. To determine the particle size in the device, the type of the used liquid and the refractive indices (2.50) of LaMnO3 powders must be entered in its program [43, 44].
Optical measurements of the ceramic samples were identified with a UV Vis spectrophotometer (Thermo Scientific Evolution 600) with diffused reflectance apparatus in the wavelength range of 300–850 nm.
Magnetic features of the powders were found through a Dexing Magnet-Vibrating Sample Magnetometer (NanoMagnetic Instruments Inc. VSM). The saturated magnetization (Ms), remanence magnetization (Mr) and coercive force (Hc) values of LaMnO3 powders annealed at 500 oC and 850 oC were determined at 25 oC in air with this system.
3.1. Solution properties
In the sol-gel synthesis, two measurements are carried out in the solution step to determine whether the solution is prepared correctly and whether it can pose the desired properties. To be able to easily make sense of solution characteristics, one must have full knowledge of turbidity and acidic-basic manners. In this strict sense, these are measurements of turbidity and pH values determined bounding up with the precursors, solvent and chelating agents. The turbidity of the solutions is measured in NTU (Nephelometric Turbidity Units). This value, which is between 0 and 1000 NTU, approaches 0 NTU in case the precursors are entirely dissolved in the solution and a homogeneous solution is obtained and approaches 1000 NTU unless it is dissolved properly [39]. As might be expected, the turbidity value measured before the ammonium hydroxide addition is 0.79 NTU, which means that the precursors decently dissolve in the solvent and are appropriate for the production of the stoichiometric perovskite structure.
As we elaborately explained in our former study [38], the acidic and basic features of the prepared solution reasonably give paramount hints for hydrolysis and condensation processes. Taking into consideration of the pH values in the solution phase, it is well reckoned that strong acidic pH (2–3) values entail slow gelation, which is ideal for the formation of an expanded network, while basic pH (8–9) values trigger swift gelation, which is optimum for the formation of smaller aggregates. Keep in mind that the desired neutral pH value of the solution is readily adjusted by putting in ammonium hydroxide (NH4OH) so as to acquire the intended perovskite structure, as reported in Refs. [38, 45–47].
3.2. Thermal behavior
Due to the fact that thermal analysis is a sort of simulation of the actual conditions of the heat treatment regimes for xerogels and powders, it is greatly benefited from DTA-TGA as a thermal analysis technique. In the analysis, it aims to obtain the proper heating regime and annealing temperature for the whole process. The La-Mn based xerogel dried at room temperature and the powder heat-treated at 500 oC is analyzed at atmospheric conditions at a temperature range of 25–990°C at a heating rate of 5°C/min. The thermal analysis gives information about changes in material properties as a function of temperature. While DTA registers changes in the material where no mass loss occurs, e.g. crystal structure changes, melting, glass transition, etc, TGA only measures changes caused by mass loss.
DTA-TGA curves of the La-Mn-based xerogel sample dried at 90 oC are depicted in Fig. 3. Once the curves are examined, there are small endothermic peaks at nearly 100°C, which is interrelated to the removal of physically adsorbed H2O from the xerogel structure [48] coming from precursors, citric acid monohydrate and distilled water (see Table 1). It is also explicit that exothermic peaks range from 180°C to 280°C are existed and they are concerned to thermal combustion of organic groups [49] originating from citric acid monohydrate, and La-Mn xerogel’s large peak at 262°C can be defined as by the alteration of the chemical precursor type. The weeny peak at 400 oC reflects thermal decomposition of the salt group ((NO3)−1) stemming from lanthanum nitrate hexahydrate and manganese (II) nitrate tetrahydrate. As is to be expected, the small peaks in the temperature range of 430 oC and 550 oC are the same as the oxidation process and it is figured out that the oxide forms commence crystallizing at 430 oC in La and Mn based xerogel. It is an undeniable fact that oxide structures formed at these temperatures are like single oxides such as La2O3 and MnO2, which we do not want, or non-stoichiometric perovskite oxides. In order to eliminate these undesired oxide structures, temperatures above 500 oC were raised and pure perovskite phase peaks containing LaMnO3 were observed. Even if large changes in the curve cannot be observed due to the low mass of the DTA-TGA sample, a change in slope at approximately 600–720°C and slope curves at about 800–850°C were determined. From the TGA curves in Fig. 3, it can be obviously seen that the weight loss value of La-Mn xerogel is 17.16% and that a steep decline of this weight loss occurs as an exothermic reaction at temperatures between 180°C and 280°C. After 280°C, the weight loss pursued at a rate as low as about 1% until 800°C and is over.
DTA-TGA analysis of the LaMnO3 ceramic powder sample, which was subjected to air heat treatment at 500oC for 2 hours, was performed to reveal the topotactic and phase transformations after 500 oC more clearly (Fig. 4). As mentioned elsewhere [50, 51], after a gel dries, it is generally heated at much higher temperatures than drying, so that conversions to more stable phases can occur. Note that a crystalline transition phase is usually produced by heat treatments at a temperature between 100oC and 1000oC. To put it another way, while these phases are generally produced at high temperatures, new thermodynamically unstable phases are formed by topotactic transformation at intermediate temperatures. As these intermediate temperatures are headed, a series of topotactic transformations are observed and gradually more stable phases emerge until the most stable thermodynamic phase is fully formed. Intermediate stages are called transitional phases. Topotactic transformation sequences and formed intermediate phases hinge on the initial gel structure and accordingly the chemical recipe used in sol-gel processing. When the processing temperature is increased after 500 oC, the topotactic reactions begin in the temperature range of 540 oC and 680 oC. In detail, the first topotactic reactions engender at 513-516-520 oC and 540-560-580 oC, respectively, while the second topotactic reactions take place at 620–650 oC, 623–660 oC, 640–680 oC respectively. The most appealing aspect in the determination of heat treatment profile is that nucleation and growth of fully stoichiometric perovskite structures occur in the temperature ranges of 752–848 oC, 756–877 oC and 766–900 oC.
Pursuant to the DTA-TGA results, the heat treatment regime can be obtained to be implemented for La-Mn based ceramic specimens by using the exothermic and endothermic reactions. By taking into consideration these reactions in the temperature range from 24 oC to 900°C, evaporation/solvent removal at 100 oC, combustion/decomposition at 150–430 oC, oxidation/crystallization at 400–500 oC, topotactic reactions at 500–650 oC and phase formation at 650–850 oC can be easily determined from DTA-TGA results. DTA-TGA analysis simulates the heat treatment process in the electric furnace from 24 to 1000 oC. Based on these results, the heat treatment of the xerogel sample dried at 90 oC for 24 hours is divided into two processes. Both processes are heated in the electric furnace at a rate of 5 oC/min in air. The first process is drying one, which was performed at 200 oC for 2 hours and the second process is annealing one, which was carried out at 500 oC and 850 oC for 2 hours. The samples were then left to cool down in the furnace environment. Using the temperatures in the exothermic and endothermic reactions in the DTA-TG results, the optimum heating regimes of the drying and annealing processes were precisely determined.
3.3. FTIR analysis
To gain more insight into the chemical structures of La-Mn based materials in the temperature range of 25 oC and 900 oC, FTIR analysis was carried out in detail. FTIR spectra of LaMnO3 at heat treated at 90 oC, 500 oC and 850°C are illustrated in Fig. 5. In the form of xerogel dried at 90oC, antisymmetric stretching and strong antisymmetric in-plane bending of NO3− in nitrate groups are noticed approximately at 840–880 cm− 1 [52]. At 1360–1480 cm− 1 bands pertinent to the vibration mode of the carboxylic group (-COO), O–H stretching mode, =CH3– the structure and other phases in the co-precipitation sample exists, as explained in Ref. [53–55]. In the range of 3200 − 2800 cm− 1 symmetrical and asymmetrical expansions, O-H expansions, N-H expansions and vibrations are determined [56]. The hydroxyl (OH) group’s peak at 1626 cm− 1 [52] and a peak at 3214 cm− 1 appertains the N-H groups. It is apparent that after the annealing processes, these bands are eliminated from the structure. Note that after the annealing process, FTIR spectra of the samples at 500 and 850 oC denotes the existence of M-O bonds (M = La and Mn) in different sites. It is a remarkable result that, in the range of 400–1000 cm− 1, two bands are found for perovskite structures. The higher one at ~ 600 cm− 1 represents La-O and Mn-O stretching vibrations corresponding to octahedral LaO6 and MnO6 groups and the lower one at ~ 400 cm− 1 represents La-O-La-O and Mn-O-Mn deformation vibrations [57]. In Fig. 5, the clearest peak is at 604 cm− 1 for both powders and belongs to the vibration of the octagonal MnO6 in the structure ABO3 [5, 7, 8] and at 3609 cm− 1 there is another peak which belongs to La-O in La2O3 [58]. The La2O3 peak is obviously observed in the gel form but after the annealing process, it is disappearing in that lanthanum is made a bond with Mn in the structures.
3.4. Phase analysis
XRD is a powerful non-destructive technique so as to characterize structures, phases, crystal orientations, and structural parameters of crystalline materials. As given in Fig. 6, XRD analysis was used to be identified the phase and crystal structures of the LaMnO3 powders annealed at 500 oC and 850°C. In the XRD analysis, it is also obvious that the LaMnO3 phase was successfully obtained in LaMnO3 powder samples at both temperatures. Nonetheless, even though it can be emphasized that LaMnO3 synthesis is possible at both temperatures once the peaks in the XRD analysis are compared, the intensity of the peaks and the noise in the graph differ. In the production of pure perovskite, any crystalline impurities like La2O3 or MnO2 are not requested after annealing processes. Generally speaking, increasing the annealing temperature comes up as a result of the elimination of the unwanted phases with the growth of perovskite structure. This issue strongly backs up DTA-TGA and FTIR findings.
XRD results present that LaMnO3 is apparently formed in the orthorhombically distorted perovskite structure, consisting of oxygen octahedra with a central Mn atom. This is evident that the characteristic diffraction peaks of LaMnO3 powder with the orthorhombically distorted perovskite structure remarked good crystallinity (JCPDS Card No. 50–0298). In the LaMnO3 sample, the main characteristic reflexes are located at 2θ of 23.0º, 32.8º, 40.2º, 47.0º, 52.6º, 58.2º, 68.7º, and 77.9º being indexed to the (012), (110), (202), (024), (122), (214), (208) and (134) diffraction planes, respectively.
Making use of the XRD data, it is possible to estimate the crystalline size of the materials. For this, the diffraction peaks were employed with the Scherrer Equation to guest the crystallite sizes of LaMnO3 powders.
In this equation, the shape factor (K) is 0.94, λ = 1.5418 Å, β is the full width at half maximum (FWHM) (radians), and θ is Bragg’s angle (radians) [59]. The XRD data of the powders are calculated for crystallite sizes for all diffraction peaks, as shown in Table 2. As might be expected, according to estimations, the average crystallite sizes were determined to be 20.58 nm in 500 ºC, and 24.79 nm in 850 ºC for LaMnO3. Upon examining these data, it is perceived that when the annealing temperature rises from 500 ºC to 850 ºC, the grain growth takes place in ceramic powders, and in its parallel, this boosts the crystallite sizes.
| Peak position 2θ (°) | FWHM B size (°) | L (nm) | Ave. L (nm) | Peak position 2θ (°) | FWHM B size (°) | L (nm) | Ave. L (nm) | ||
|---|---|---|---|---|---|---|---|---|---|
| LMO 500 ºC | 22,92 | 0,65809 | 12,32 | 20,58 | LMO 850 ºC | 23,32 | 0,30173 | 26,89 | 24,79 |
| 32,58 | 0,33324 | 24,84 | 32,88 | 0,50852 | 16,29 | ||||
| 40,24 | 0,31331 | 27,01 | 40,48 | 0,425 | 19,93 | ||||
| 46,76 | 0,74123 | 11,68 | 47,12 | 0,35429 | 24,47 | ||||
| 57,92 | 0,53836 | 16,87 | 58,34 | 0,45001 | 20,22 | ||||
| 68 | 0,34288 | 27,95 | 68,34 | 0,22335 | 43,00 | ||||
| 77,44 | 0,43599 | 23,36 | 77,96 | 0,45001 | 22,71 |
3.5. Elemental analysis
Upon setting out with the aim to present all details of elemental analysis, XPS analysis is utilized in specifying the valence states of elements for LaMnO3 samples as a distinctive quality. Survey scans and high-resolution XPS spectra are illustrated in Fig. 7 and Table 3. It is a very dear outcome that valence states of manganese and lanthanum in LaMnO3 are reported to be La-3d5 and Mn-2p for powders at both synthesis temperatures. In Fig. 7, XPS results of both LaMnO3 powders synthesized at 500 and 850 ºC are presented. Whilst Fig. 7a exhibits the general survey scan data, Figs. 7b and 7c depict the La-3d and Mn-2p core-level XPS spectra respectively. The C1s peak in the XRD survey scan comes from the CO2 from the air and the carbon tape used during analysis, and it gives the C1s signal at 284.84 eV and O-1s peaks are determined at 530 eV in both [60, 61]. In Fig. 7b, namely La-3d XPS spectrum, there are four obvious peaks and these are 834 and 838 eV for La-3d5/2 and 851 and 855 eV for La-3d3/2 binding energy of and its satellite peak [61, 62]. La-3d peaks in the XPS spectrum indicate the presence of the La3+ oxidation state. In Fig. 7c, there are two signals in 642 and 654 eV and they represent Mn2p3/2 and Mn2p1/2. Mn-2p peaks in the XPS spectrum indicate the presence of the Mn2+ oxidation state.
| Name | Peak BE | FWHM (eV) | Area (P) CPS (eV) | Weight ( %) | Q | |
|---|---|---|---|---|---|---|
| LaMnO3 850o | La3d5 | 835.52 | 3.09 | 1236378 | 57.11 | 1 |
| O1s | 530.98 | 3.38 | 410936.52 | 18.82 | 1 | |
| Mn2p | 643.12 | 3.91 | 383637.74 | 15.45 | 1 | |
| C1s | 286.04 | 2.82 | 103696 | 8.63 | 1 | |
| LaMnO3 500o | La3d5 | 834.87 | 3.5 | 57107.6 | 44.92 | 1 |
| O1s | 530.15 | 3.46 | 28376.27 | 22.13 | 1 | |
| Mn2p | 642.34 | 3.55 | 26408.48 | 18.11 | 1 | |
| C1s | 285.09 | 2.22 | 10467.47 | 14.84 | 1 |
In a general overview of XPS, lanthanum peaks have the same amplitude and amplitude in both regions and the same amplitude and value in manganese peaks. When XPS analyses are compared with FTIR and XRD results, it is seen that there is a good agreement. To be quite frank, in these results, it can be expressed that the LaMnO3 ceramic perovskite structure was successfully synthesized at both temperatures.
3.6. Microstructure
SEM observations are concerned with the question of how heat treatment temperatures such as 90 oC, 500 oC and 850°C can be altered the profound impacts on microstructures of LaMnO3 powders in the sol-gel process. Due to this reason, SEM images were employed to examine the surface morphology of xerogel and ceramic perovskite powder materials and to approximately determine particle sizes. As is known, the surface morphology of LaMnO3 powder materials rests on sol-gel parameters such as drying and annealing conditions. SEM micrographs in Figs. 8, 9 and 10 respectively refer to LaMnO3 xerogel and ceramic powder materials derived from sol-gel solutions and subsequently heat treated at 90 oC, 500 oC and 850°C. The microstructures of La-Mn based xerogel dried at 90 oC are shown in Fig. 8a. The xerogel powder has an irregular shape. On the grounds of the charge of many physical specifications like rheological properties and water-holding, the xerogel powders can schematically be separated into fine-stranded and aggregated or particulate gels [63]. Once Fig. 8b is thoroughly examined, it is seen that the sizes of the particles are between ∼5 µm and 40 µm. In addition to these, it can be noted that there are pores in the structure and pore sizes are approximately 5 µm and 25 µm (see Fig. 8 for details). It is well-known that gel networks have water-holding properties and with the increase in the network structure, the water-holding properties increases [64, 65].
As to the surface morphologies of LaMnO3 ceramic powder annealed at 500 oC, as denoted in Fig. 9, it can be noticed that the powder has nearly round-shaped small-sized grains. When the average particle sizes of the powders are measured, they are in the range of 50-1500 nm, and these sizes are agreed by the Ref [13]. Notwithstanding these grains, there also is an agglomeration process during sol-gel synthesis and the agglomerated particle size of the fine powders is in the range of 20–100 µm [66]. Even the presence of agglomerations of very fine particles smaller than 100 nm can be intuited in the images, because of not having a surface protective layer. However, the particle shapes are not well defined and the powders contain large and small pores.
As far as LaMnO3 ceramic powder annealed at 850 oC is concerned, its surface morphologies can be clearly seen in Fig. 10. In a contextual sense with the powder sample annealed at 500 oC, the LaMnO3 ceramic powder annealed at 850 oC generally exhibits similar characteristic behaviors except for its particle size. It is clear in Fig. 10 that the powder has fine submicron and nano-scaled particles with nearly round shapes. Upon measuring the average particle sizes, it is found to be in the range of 50–250 nm, which is a good result in favor of dye-synthesized solar cell application. We have a good agreement about the sizes of the powders in Ref [16]. In the fundamental finding as to its particle size, such an essential distinction is annealing temperature in sol-gel processing. As indicated in these SEM images, the increase in the annealing temperature gradually reduces the porosity and agglomeration. It can be implied that though powders annealed at 850 oC are spherical, powders annealed at 500 oC possess a sphere-like structure. It is believed that the particle size and shape of LaMnO3 ceramic powder strongly influence the microstructure and power performance of dye-synthesized nanocomposite.
3.7. Particle size analysis
In order to compare and detail the particle sizes observed in the SEM images, the particle size analysis was evaluated by using the particle size distribution of the powders. These perovskite powders are planned to be used in dye-sensitive solar cell applications, and it is thought that the low particle size in these applications not only stabilizes the cell more but also increases the power conversion efficiency. The particle sizes and distributions of both LaMnO3 ceramic powders (annealed at 500°C and 850°C) prepared by dispersing in pure water were determined using Malvern zeta sizer (Fig. 11). It is especially indicated that these results are consistent with the SEM micrographs in Figs. 9–10.
The particle size values of the sol-gel derived LaMnO3 ceramic powders annealed at 500 oC were found to be approximately 571 nm (the range of 60-1400 nm) and that of at 850 oC were determined to be as nearly 305 nm (in the range of 58-1200 nm). In terms of these results, it can be apparently accentuated that particle sizes decrease as the annealing temperature increases. Depending on these outcomes, the particle size analysis confirms XRD and SEM results.
It is especially indicated in Fig. 11 that small and large particles are described with two obvious peaks. In this case, these particles are dispersed in a liquid (water) and randomly moving, with the help of the dynamic light scattering mechanism these are determined. In this mechanism, the movement speed of the particles is applied to determine the particle size, and this movement continues. Therefore, if we take two 'peaks' of the powder the amount of motion of the sample particles separated by a short time interval is determined and therefore their size can be calculated. The reason why the particles in the sample are calculated as large is that there is a minimal particle movement between the two 'peaks'; Likewise, the small size removal of particles in the sample may also result from a large amount of movement between the two 'peaks'. [67]. In the zeta sizer machine, light scattering is a very convenient method for identifying single particles and aggregates thanks to its sensitivity to larger particles.
3.7. Optical properties
Why do not we focus our attention on the optical properties of LaMnO3 powders? The LaMnO3 powders we synthesized are aimed to be used in dye-sensitive solar cell applications. Therefore their optical behaviors have a critical place in this study. With UV-Vis spectroscopy used in determining optical properties, reflectance and transmission graphs depending on wavelength were obtained, as indicated in Figs. 12 and 13. The UV-Vis absorption edge is directly pertaining to the bandgap energy of the material (Fig. 14), and this bandgap directly affects the power conversion efficiency in solar cell applications.
Transmittance values in 320 nm and 800 nm wavelength range of pure LaMnO3 powders annealed at 500 oC and 850 oC are demonstrated in Fig. 12 and reflectance values are portrayed in Fig. 13. Once the transmittance wavelength curve is examined, it is seen that the LaMnO3 transmittance higher than about 90% for both annealing temperatures. Upon examining the reflection wavelength curve, a reflection of less than 5% is observed for pure LaMnO3 between 350 nm and 500 nm, and this reflection rate plummets to 20% in the range of 320 nm to 350 nm and stabilizes up to 800 nm. When we assess these two curves in general, LaMnO3 is a pertinent candidate for dye-sensitive solar cell applications.
Owing to the increase in crystallite size associated with higher densification of powders, there has been an increase in its transmittance through wavelengths [68]. These changes pertain to the electronic transition between the valance and conduction bands which are relevant to samples’ optical bandgaps [69, 70]. Using the optical measurements of LaMnO3 in the Kubelka-Munk (K-M) function, the bandgap in both synthesis temperatures was calculated using the graphs given in Fig. 14 [71]. This Kubelka-Munk (K-M) function method is based on the transformation of the diffuse reflectance through the following relation;
In this formulation Kubelka-Munk function is “F(R)”, the molar absorption coefficient is \(\alpha\), the scattering factor is S and the reflectance of the material is R. UV–vis diffuse reflectance measurements are expoited to predict the bandgap (Eg) energy. From a plot of (F(R).hv)2 vs. hv, the bandgap energy can be determined by extrapolating the slope to F(R)→0 [72–75] and concisely summarized in Table 4 using the results of Fig. 14. According to Fig. 14, band gap values (Eg) were found as 1.27 eV for both temperatures. It is important to express here that as the band gaps of LaMnO3 obtained by different production techniques in different studies were around 2.50 eV [3, 37, 76], in our synthesis, the bandgap of both of our LaMnO3 powders was found to be 1.27eV. When these optical results are taken into consideration, it can be discerned that LaMnO3 powders prepared by the sol-gel method possess a very low bandgap value and this makes the LaMnO3 structure promising for solar cell applications.
| Materials | Temperature (oC) | Bandgap (Eg) |
|---|---|---|
| LaMnO3 | 500 | 1.27 eV |
| LaMnO3 | 850 | 1.27 eV |
3.8. Magnetic properties
A vibrating sample magnetometer (VSM) was used to investigate the magnetic properties of LaMnO3 materials annealed at 500 oC and 850 oC, magnetic hysteresis (M - H) cycles were acquired for both powders and the change of magnetic properties depending on the synthesis temperature is represented in Fig. 15 and Table 5. Sizes, sample shapes, crystallinities, magnetization directions, synthesis methods are the critical effects on magnetic properties e.g. magnetization, remanence and coercivity.
| Sample name | Temperature (oC) | Ms (emu/g) | Mr (emu/g) | Hc (Oe) |
|---|---|---|---|---|
| LaMnO3 | 500 | 0.96 | 0.1 | 82.96 |
| LaMnO3 | 850 | 1.69 | 0.1 | 82.95 |
Although lanthanum and manganese demonstrate paramagnetic properties [6, 77], magnetic hysteresis loops for two different annealing temperatures of LaMnO3 in Fig. 15 denote soft ferromagnetic properties. In Table 5, remanent magnetization (Mr) and coercivity (Hc) values of both powders were found to be approximately the same value, 0.1 emu/g and 82.95 Oe, respectively, despite them, saturation magnetization (Ms) value was obtained 0,96 emu/g for 500 oC and 1,69 emu/g for 850 oC. In spite of the fact that the remanent magnetization (Mr) and coercivity (Hc) values are independent of the synthesis temperature, also an increase in synthesis temperature increases the permanent magnetization values of LaMnO3. In other words, the increase in the annealing temperature in the synthesis causes the magnetic properties to increase likewise [78–81]. Considering that all the properties of the powders we obtain are the same except the synthesis temperature and also the particle sizes. It can be pointed out that the difference in the saturation magnetization (Ms) value is due to the difference in particle size.
The most striking feature is that the average particle sizes of LaMnO3 powders are higher at 500 oC than that of 850 oC, whereas the magnetic saturation (M ) values are higher at 850 oC than that of 500 oC. In general, for LaMnO3 magnetic saturation increases by virtue of the decrease in the particle size, the change in the density of the particle and the decrease in the possible difference in the magnetic effect on the particle surface and volume.
To sum up, LaMnO3 perovskite powders with orthorhombically distorted perovskite structure were successfully synthesized by using the sol-gel method at synthesis temperatures of 500 °C and 850 °C in the hope of being used in dye sensitive solar cell applications. We draw attention to the results of this research as follows:
- By DTA-TGA analysis, the heat treatment regime from 24oC to 900 °C was determined to implement for La and Mn based xerogel and ceramic samples using exothermic and endothermic phenomena such as evaporation/solvent removal at 100 oC, combustion/decomposition at 150-430 oC, oxidation/crystallization at 400-500 oC, topotactic reactions at 500-650 oC and phase formation at 650-850 oC.
- In FTIR analysis, bands at 604 cm-1 correspond to octagonal MnO6’s strong Mn-O bond vibration, bands at 3609 cm-1 correspond to La2O3’s La-O bond. In the La-Mn-based gel, a La2O3 peak is seen but in the wake of the heat treatment process, it is disappearing because lanthanum makes a bond with manganese in the LaMnO3
- In XRD analysis, LaMnO3 powder’s purer single-phase can be seen in the sample annealed at 850 oC compared to annealed at 500 o The crystallite sizes of both powders are estimated as 20.58 nm for 500 ºC and 24.79 nm for 850 ºC.
- According to survey scan XPS spectra and high-resolution XPS spectra, both LaMnO3 samples have La-3d5 and Mn-2p valance states.
- SEM images of the LaMnO3 powder heat-treated at 500 oC are small grains with irregular shapes, but the powders annealed at 850 oC have very fine submicron and nanoscale particles of more spherical shape. The particle sizes of the sample annealed at 800 oC were determined to be in the range of 50-250 nm.
- The average particle size of the powders was 571 nm for 500 ºC and 305 nm for 850 ºC. Particle sizes abate with increasing annealing temperature.
- The bandgap value of LaMnO3 powder was found to be 1.27 eV for both annealing temperatures. In this case, the bandgap of the LaMnO3 powders is independent of the processing temperature.
- VSM analysis revealed that LaMnO3 has soft ferromagnetic features, even though La and Mn are paramagnetic materials. Even though the remanent magnetization (Mr) and coercivity (Hc) values are independent of the synthesis temperature, also increase in annealing temperature increases the permanent magnetization values of LaMnO3. For LaMnO3 magnetic saturation increases on account of the decrease in the particle size, the amendment in the density of the particle and the decrease in the possible difference in the magnetic effect on the particle surface and volume.
- In general, these results point out that LaMnO3 perovskite powders have a potential for photocatalytic applications as futuristic and innovative approaches, notably in continuously applicable and sustainable dye-sensitized solar cells.
Acknowledgments
This research has been supported by Manisa Celal Bayar University Scientific Research Projects Coordination Unit. Project Number: 2018-049, Dokuz Eylul University Center of Fabrication and Application of Electronic Materials.
Conflict of Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
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