Micro Structure and Magnetic Behavior of La 0.7 Pb 0.3 Mn 1-x Co x O 3 Perovskite Nanomaterials

: A perovskite solar cell is a solar cell using a perovskite type organic metal halide semiconductor as a light absorbing material.The perovskite solar cell precursors La 0.7 Pb 0.3 Mn 1 -x Co x O 3 was successfully prepared by the Sol-gel method. The experimental conditions were calcined at 950℃ for 10 hours. TG-DSC shows that the chemical reaction led to a slow endothermic process, when the temperature was above 300℃, and the formation of the lattice. The X-ray diffraction results confirmed all samples are perovskite rhombohedron (R-3C) structure, and no impurity is generated, the average grain size of the sample are 38.0nm to101.4nm. The SEM results show that the particle boundary is clear and there is little agglomeration between particles, the particle size increases and the agglomeration becomes serious with the increase of Co doping content. The FT-IR images show that the absorption peak in the low-frequency wavenumber phase of the main band at around 439 cm -1 and 608 cm -1 . The ferromagnetism of samples La 0.7 Pb 0.3 Mn 1 -x Co x O 3 ( x= 0~1.0) gradually weakens with the increase of Co doping ratio, the specific saturation magnetization of the samples reduced from 47.69 emu/g to 0.62 emu/g, while the coercivity increased from 36.79 Oe to 96.68 Oe.


Introduction
Manganese-based perovskite materials have attracted extensive attention of researchers, due to the giant magnetoresistance effect, magnetocaloric effect, catalytic oxidation, electrical conductivity and other outstanding features [1][2] .Perovskite type solar cells are solar cells using perovskite type organic metal halide semiconductors as light absorbing materials, belonging to the third generation of solar cells, also known as new concept solar cells [3][4] .Perovskite solar cells use organic-inorganic hybrid metal halides with perovskite crystal structure as the light absorption layer.Since 2009, they have attracted much attention due to their simple preparation method, low production cost and excellent photoelectric performance.The photoelectric conversion efficiency has rapidly increased from 3.8% to 23.7%, becoming the fastest developing photovoltaic technology at present and the most attractive emerging photovoltaic technology in the world.The industry expects that the perovskite technology can greatly reduce the manufacturing cost of photovoltaic modules while further improving the photoelectric conversion efficiency.
In recent years, in order to further expand the application scope of manganese-based perovskite materials in practice, more and more researches have been developed towards multi-component [3][4] .Among many candidate materials, transition metal (Fe, Co, Cr, Ni) ions have similar ionic radii to B-site manganese ions, and the average ionic radius of B-site can be kept small by doping transition metal ions by breaking the valence ratio of Mn 3+ and Mn 4+ at the B site, the double exchange between Mn 3+ and Mn 4+ is weakened, and the octahedral structure of MnO6 is distorted, resulting in a series of new physical and chemical properties [5][6][7] .The study of B-site co-doping of perovskite composite oxide La0.7Pb0.3MnO3 is of great significance for understanding the internal mechanism of the material and developing new functional materials.
In recent years, many scientific researchers have made important research on the B site co-doping of perovskite composite oxide La0.7Pb0.3MnO3.In 2007, Zhao [4] et al. studied the structure, magnetic and magnetocaloric properties of perovskite manganate La1-xPbxMn1-y-zCuyO3 , found to generate a large magnetic entropy changed near the Curie temperature TC, employing double-exchange interactions as well as competing effects between lattice distortions induced by Cu doping, explained The maximum magnetic entropy change of the sample with Cu doping ratio of 0.01 is obtained.In 2010, Dhahri N [5][6][7] et al. prepared La0.67Pb0.33Mn1-xCoxO3(0≤x≤0.3)material by sol-gel method and studied its structural and magnetic changes, and found that all the samples are all rhombohedral structures.With the increase of Co doping ratio, the Curie temperature decreases and the magnetic entropy change increases.When x=0.3, the maximum magnetic entropy change is 3.22 J/ (K• kg).Tokarz W [8][9] et al. used first-principles density functional theory to study the band structures and photoemission states of La2/3Pb1/3MnO3 and La0.67Pb0.33Mn1-xFexO3.From the reports in the literature, it can be found that the doping of different elements and different preparation methods and process conditions have a great influence on the structure, morphology and magnetic properties of the synthesized samples.
Therefore, in order to further improve the morphology of the sample and expand the application range of the perovskite composite oxide La0.7Pb0.3MnO3,according to the synthesis process of the sample explored, the Co ion pair La0.7Pb0.3MnO3manganese with a similar magnetic moment to the Mn ion was selected.The B site of the oxide is co-doped.Under this synthesis method, the change law of the microstructure and magnetic properties of perovskite solar cell precursors La0.7Pb0.3Mn1-xCoxO3 is mainly studied, and the ionic radius effect produced by the doping of transition metal Co and perovskite is revealed.Intrinsic link between microstructure and magnetic properties of manganese oxide materials.Lay the theoretical and experimental foundation for the development and application of new functional materials.

Material Preparation
In this experiment, La0.7Pb0.3Mn1-xCoxO3doped with different proportions of Co was compared and studied.The specific steps are as follows: According to the molar ratio of La0.7Pb0.3Mn1-xCoxO3,weigh the total amount of substances as 0.01mol La (NO3)3, Pb (NO3)2, and 0.01mol of Mn (NO3)2(50%) solution, Co (NO3)2• 6H2O; According to the mol ratio of the complexing agent and the metal ion being 1:1, take by weighing 11 parts of citric acid monohydrate whose amount is 0.02mol; The weighed nitrates and complexes were dissolved and mixed with deionized water, respectively, to obtain a precursor solution.Label the 11 solutions as N1~N11; The precursor solution was stirred in a water bath at 50℃, the pH of the solution was adjusted to about 7 with ammonia water, a small amount of ethylene glycol was added dropwise as a dispersant, and the stirring and evaporation were continued until the solution was evaporated to dryness to obtain a wet gel; The wet gel was placed in a digital display drying box and dried at 100℃ for 24h to obtain a dry gel; Use alcohol self-propagating combustion to obtain black precursor powder; After fully grinding the precursor powder, put it into a muffle furnace, control the heating rate to be 2.5℃ per minute, and calcinate at 950℃ for 10 hours to obtain a black shiny sample powder.

Characterization Method
In this paper, the X-ray diffractometer (D/max-2500v/pc) of Rigaku Company in Japan was used to characterize and analyze the phase structure of the sample; the FT-IR spectrometer of Perkin Elmer Company was used to analyze the possible functional groups and internal chemical bonds in the sample; The EVO18 electron scanning microscope of ZEISS company was used to test the microscopic morphology of the sample; The VSM-100 vibrating sample magnetometer of Yingpu Magnetics was used to test the hysteresis loop of the sample at room temperature.

DTG-DSC Analysis
In order to study the thermal decomposition process and quality change of the xerogel precursor at high temperature, TG-DSC test was carried out on the xerogel precursor of La0.7Pb0.3Mn0.9Co0.1O3. Figure 1 shows TG-DSC test results of the La0.7Pb0.3Mn0.9Co0.1O3under N2 atmosphere.It can be seen from the TG curve that the weight loss ratio of the xerogel is 3.85% at 0~100℃, and there is a corresponding endothermic peak in the DSC curve, which is mainly caused by the loss of water in the precursor; at 100℃ The weight loss ratio at 100~200℃ is 7.97%, and there are two small absorption peaks in the DSC curve, which are mainly caused by the gradual decomposition of crystal water and organic matter in the precursor [9][10] ; There is a large weight loss of about 49.21% near 262℃, and there is an obvious endothermic peak and exothermic peak in the DSC curve, which is attributed to the strong oxidation reaction of citric acid or the violent decomposition of different metal nitrates.The hydrocarbon components in the precursor almost completely disappeared, resulting in significant weight loss [10] ; when the temperature was above 300℃, the chemical reaction led to a slow endothermic process, and the mass of the precursor remained basically unchanged, and finally The composite material is about 26.56% of the total mass of the precursor, corresponding to the formation of the lattice.

Structural Analysis
Figure 2 is the XRD pattern of the sample La0.7Pb0.3Mn1-xCoxO3(x=0~1.0), the crystal structure of all samples is perovskite rhombohedron (R-3C) structure, and no impurity is generated.It shows that the single phase of the sample is good and the synthesis quality is high.Its lattice parameters are in perfect agreement with the standard card (JCPDS: 70-3942) values.And as the doping ratio of Co increases, the main diffraction peak shifts to a high angle; the lattice parameters under different doping ratios can be obtained by JADE fitting as shown in Table 1.

2dsinθ=nλ
(1) According to the Bragg equation (1), λ is constant, if θ is larger then d is smaller, and the unit cell parameter become smaller.From the enlarged image of the main diffraction peak in Fig. 2, it can be seen that the position of the main diffraction peak increases with the increase of the Co doping ratio.The angle is shifted, and the corresponding lattice parameter decreases.Figure 3 shows the relationship between the unit cell parameterand and the Co doping amount x for the sample La0.7Pb0.3Mn1-xCoxO3.The figure shows that the lattice constant and the unit cell volume.The decreasing trend may be related to the lattice distortion caused by Co atoms replacing Mn atoms into the LaMnO3 lattice.Through the change of Δ2θ, it is found that with the increase of Co doping amount x, Δ2θ also increases, indicating that the doping of Co does indeed cause the change of the lattice.Furthermore, using the Scherrer formula: The average grain size of the sample can be calculated, where β is the full width at half maximum of the main diffraction peak, λ is the x-ray wavelength, and θ is the diffraction angle.The calculated average grain sizes of the crystals are 81.0 nm, 60.4 nm, 54.3 nm, 41.4nm, 47.6nm, 60.9nm, 38.0nm, 73.4nm, 101.4nm, the samples are all nanomaterials.
Figure 4 shows the SEM images of La0.7Pb0.3Mn1-xCoxO3(x=0, 0.1, 0.5, 0.9) nanoparticles.It can be seen from the figure 4 that when the doping ratio is 0 and 0.1, the particle boundary of La0.7Pb0.3Mn0.9Co0.1O3manganese oxide is clear, the morphology is neat, and the average particle size is relatively uniform, and there is little agglomeration between particles; with the increase of Co doping ratio, the particle size of the sample increases, whenx= 0.5, some particles agglomerate; when x= 0.9, the agglomeration is the most serious, and the particle size increases, this may be due to the mismatch between adjacent layers caused by the increase of the Co doping ratio, which is easy to distort the lattice.In addition, with the increase of the Co substitution amount x, the perovskite structure will generate oxygen vacancies, which may also lead to changes in the unit cell structure and increase agglomeration, the macroscopic appearance of particle shape becomes irregular, and the particle size is inconsistent.The FT-IR image of the sample La0.7Pb0.3Mn1-xCoxO3(x=0.1~0.9) is shown in Figure 5, and the diagram shows that all samples appear obvious infrared absorption peak at around 439 cm -1 , 608 cm -1 , 1630 cm -1 , 3432 cm -1 .The illustration shows the absorption peak in the low-frequency wavenumber phase of the main band at around 439 cm -1 and 608cm -1 , which can be attributed to the bending vibration of O-Mn-O caused by the Jahn-teller distortion and the stretching vibration of Mn-O in the MnO6 regular octahedron.The changes of Mn-O bond length and O-Mn-O bond angle are all related to the conditions of Mn-O stretching vibration and O-Mn-O bending vibration [11] .The vibrational absorption peak of the Co-doped sample is strengthened at around 608 cm -1 , which indicates that the MnO6 octahedral distortion is partly caused by the Co doping.The symmetric stretching vibration of the carboxyl radical is found around the wavenumber of 2350 cm -1 [3] , there is a broad peak at about 3431 cm -1 corresponding to the O-H stretching vibration of water molecules.And there is a strong stretch vibration at 1630 cm -1 , it is attributed to the bending vibrations of the surface water molecules H-O-H [12] .

Magnetic Performance Analysis
The hysteresis loops of the sample La0.7Pb0.3Mn1-xCoxO3nanomaterials at room temperature are shown in Figure 6.The hysteresis loops of all samples are closed curves, and the samples La0.7Pb0.3MnO3showed obvious ferromagnetic at room temperature , the magnetization basically reaches saturation around 0.2T, and it is a soft magnetic material with small coercivity.
-  Table 2 shows the specific magnetic parameters of the La0.7Pb0.3Mn1-xCoxO3polycrystalline material.It can be seen from the table 2 that the specific saturation magnetization of the samples are 47.69 emu/g, 7.46 emu/g, 2.65 emu/g, 1.90 emu/g, 1.64 emu/g, 1.46 emu/g, 1.60 emu/g, 1.24 emu/g, 0.72 emu/g, 0.72 emu/g, 0.62 emu/g, respectively.The ferromagnetism of the sample La0.7Pb0.3Mn1-xCoxO3varies with the increase of Co doping amount showed a decreasing trend.In order to more clearly reflect the changing law of magnetic parameters with the Co doping amount x, the relationship between the specific saturation magnetization, coercivity, remanent magnetization and doping amountxas shown in Figure 7.The diagram shows that the specific saturation magnetization and remanent magnetization decrease rapidly with the increase of Co doping ratio, while the coercivity increases with the increase of Co doping ratio; when the doping ratio is x=0.1, the magnetization strength is still not saturated at 0.8T, and it is weak ferromagnetic.When the doping ratio is further increased, the ferromagnetism of the nanoparticle material basically disappears, which may be due to the fact that Co doping weakens the double exchange between Mn 4+ and Mn 3+ ions, resulting in the decrease in the mobility of the eg electrons of Mn 3+ , which makes the ferromagnetism is weakened.In addition, when Co is doped on the Mn site, because the ionic radii of Co and Mn 3+ are close, the substitution of Co on the Mn site will not change the bond length and bond angle of the Mn-O bond, so the lattice effect can be completely eliminated [13][14] .

Conclusion
The perovskite solar cell precursors La0.7Pb0.3Mn1-xCoxO3was successfully prepared by the Sol-gel method.The XRD results show that the lattice constant and the unit cell volume both show a decreasing trend with the increase of Co doping ratio.The Mn-O stretching vibration and O-Mn-O bending vibration peaks of the sample La0.7Pb0.3Mn1-xCoxO3were found at the low-frequency wavenumber stage of 608 cm -1 and 439 cm -1 , and the vibration absorption peak around 608 cm -1 of the sample is strengthened after Co doping, indicating that the MnO6 octahedral distortion is partly caused by the doping of Co 3+ .The SEM results show that the particle boundary is clear, the morphology is neat, and the average particle size is relatively uniform, and there is little agglomeration between particles for the sample La0.7Pb0.3Mn0.9Co0.1O3.According to the particle size statistics, the average particle size of the sample La0.7Pb0.3Mn1-xCoxO3(x=0.1, 0.5, 0.9) is 202.59 nm, 246.11 nm, 264.62 nm respectively, indicating that the samples obtained in our experiment are many crystal material.The VSM results show that the ferromagnetism of samples La0.7Pb0.3Mn1-xCoxO3(x=0~1.0)gradually weakens with the increase of Co doping ratio.With the increase of Co content, the specific saturation magnetization and remanent magnetization decrease rapidly, while the coercivity increases; when the doping ratio is x=0.1, the magnetization is not saturated at 0.8T, which is weak ferromagnetism; when the doping ratio is further increased, the ferromagnetism of nanoparticle materials basically disappears.