Abstract
LaMnO3 (LMO) nanopowder was synthesized by the microwave combustion method using glycine and nitrate salts of La and Mn as precursors. The as-prepared LMO powder was pressed at high pressure and annealed at 1000°C for 8 hours to make a target for thin film deposition. The structural and elemental analysis was obtained by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). Thin films of LMO were fabricated using pulsed electron deposition (PED) at room temperature. The effects of discharge voltage and oxygen/argon flux ratio on the produced thin films were studied. The study shows that stoichiometry and structure of the target was preserved well in the thin films prepared with a discharge voltage from 14 to 15 kV, while the oxygen/nitrogen flux ratio did not show a clear effect on the quality of thin films.
1. Introduction
Recently, semiconductor magnetic nanomaterials have received much awareness than that of their same bulk materials, due to their size and surface effects, which exhibit unique properties such as photoluminescence, magneto-optical, electrochemical, and photocatalytic activity [1–13]. These unique properties of nanomaterials in turn help to solve a lot of urgent problems related to exhaustion of fossil fuels and environment pollution. In this aspect, production and application of clean fuels based on nanomaterials become more and more critical for the development of human society [14–17]. In particular, solid oxide fuel cells (SOFCs) have been extensively studied and attracted much attention as a promising way to generate electricity at high efficiency and low cost [18–20]. For such applications, the cathode material must satisfy several criteria such as good electrical conductivity, high porosity to allow gas diffusion, thermal expansion coefficient matching well with that of the solid electrolyte, and chemical stability at a high temperature [21–23]. Most of the requirements listed above are satisfied by LaMnO3 perovskites, making it one of the most suitable materials for cathode in SOFCs [24].
Among many effective methods developed to produce nanopowders, microwave-assisted synthesis was first developed in 1985 and has quickly become a major area of study. Since microwave radiations were discovered, these have been widely utilized for synthetic chemistry in order to activate chemical reactions more rapidly and homogeneously [25–29]. Moreover, a self-combustion reaction assisted with microwave is extremely suitable for preparation of perovskite nanomaterials due to notable advantages such as simple and convenient experimental set-up, extremely time, and energy saving, as well as offering homogeneous product [30–32]. These advantages result from a thorough blending among the constituents in an aqueous media under microwave irradiation. Furthermore, an exothermic redox reaction between the fuel and an oxidizer provides a great amount of heat which is necessary for the formation of the perovskite phase [33, 34].
In general, thin films can be manufactured by a variety of methods such as sputtering, thermal evaporation, and molecular-beam epitaxy [35–37]. Pulsed electron deposition (PED) is also a physical thin film deposition technique offering many advantages such as high film homogeneity and low cost. However, this method seems to be not explored for preparation of perovskite thin films even though it is convenient to transfer the target stoichiometry to the deposited films by PED [38].
In this paper, LaMnO3 (LMO) nanopowder was prepared by microwave combustion where glycine was used as a fuel in a combustion reaction with nitrate salts of lanthanum and manganese. The as-prepared LMO powder was pressed and annealed at 1000°C for 8 hours to make a target for thin film deposition. The fabrication of LMO thin films by PED was investigated in details.
2. Experiment
LaMnO3 nanopowder was prepared by the microwave combustion method. Analytical grade La2O3 (99.99%), Mn(NO3)2·4H2O (99%), and glycine NH2CH2COOH (99%) were used as the starting materials. Stoichiometric amount of La2O3 was dissolved in HNO3 to obtain La(NO3)3 solution. Then, La(NO3)3 and Mn(NO3)2 solutions in water at molar ratio of 1 : 1 were mixed well before dropwise addition of glycine.
We first prepared a set of samples with different molar ratios of glycine to the metal ion (F = 2.5, 3, 3.5, 4, and 4.5) to study the effect of fuel on the nanoproduct. After heating on a hot plate at 150°C, the light pink solution evolved into a colorless transparent one and then viscous brown gel. Gel was stored in a glass container covered with an open lid and then quickly transferred to a microwave oven. After a few seconds, the viscous gel bubbled up and autoignited to produce brown fine powder.
The LMO powder was milled, pressed, and annealed at 1000°C for 8 hours to make a target for fabrication of the thin film by pulsed electron deposition system PEBS-20 from Neocera, Inc. All the LaMnO3 films were deposited on silicon substrates with the repetition rate of pulses maintained at 5 Hz, pulse width of 100 ns, and 20 000 pulses.
Two sets of thin film samples were prepared to study the effect of discharge voltage (LMO-1) and O2/N2 flux ratio (LMO-2). The deposition of the LMO-1 films was carried out at room temperature and at five discharge voltages: 11, 12, 13, 14, and 15 kV. The N2 gas and O2 gas were introduced at a pressure of 9 × 10−3 Torr for enhancing the electron beam and stabilizing the beam propagation to the target with a gas flux of 10 sccm and 15 sccm for oxygen and nitrogen, respectively. LMO-2 films were fabricated at 15 kV with various flux ratios of oxygen and nitrogen: 0 : 25; 5 : 20; 10 : 15; 20 : 5; and 25 : 0 sccm. During the deposition process, the pressure was maintained by balancing between the rate of the turbopump and the flow rate of oxygen and nitrogen gas introduced into the chamber. The as-deposited thin films were annealed at 400, 600, and 800°C for 2 h.
The crystallinity of the thin films was characterized by the X-ray diffraction system (Siemens D5005, Bruker, Germany) with Cu Kα1 (λ = 0.154056 nm) radiation. The morphology of the product was investigated by a scanning electron microscope (Nova Nano SEM 450). The composition of the samples was verified by energy dispersive X-ray (EDX) spectrometry (Oxford Isis 300) integrated into the JEOL-JSM 5410 scanning electron microscope.
3. Results and Discussion
Figure 1 shows XRD patterns of LaMnO3 nanoparticles prepared by the microwave irradiation method when using glycine as fuel in the combustion reaction with the glycine-nitrate molar ratio (G/N) (F = 2.5, 3, 3.5, 4, and 4.5).
The sample prepared with F = 2.5 is amorphous because the released energy is not enough for formation of the perovskite phase. With F = 3, 3.5, 4, and 4.5, the LaMnO3 perovskite phase has formed. Sharp and intense XRD peaks of the samples with F = 3 suggested that crystal quality of those samples is better. The combustion reaction with a higher value of F liberated more heat. However, gas generated during the reaction can bring out heat from the reaction chamber and reduced the heat used for synthesis of the perovskite material [39]. Heat generated by the reaction might be reduced too much and resulted in an amorphous state of samples with F greater than 4. XRD patterns indicate that LaMnO3 nanoparticles, prepared with F = 3; 3.5; and 4, crystallized in a hexagonal structure. Lattice parameters and crystalline size of the samples prepared with different ratios of F are shown in Table 1.
XRD analysis of the LaMnO3 nanoparticles prepared by combustion assisted with microwave irradiation also shows the presence of both LaMnO3 and La2CO5. La2CO5 is an unwanted phase due to the unavoidable reaction of metal ions with organic precursors.
A pure perovskite is necessary for making cathode of good performance fuel cell. Therefore, such secondary phases need to be removed. As an unsolvable material, La2CO5 cannot be washed away using water, so the nanopowder was annealed at 1000°C to convert the remaining La2CO5 into LaMnO3. Figure 2 shows the XRD patterns of LaMnO3 samples with a molar ratio of glycine to metal ion of 3 and 3.5 before and after annealing at 1000°C in 8 h. After annealing, no peak related to La2CO5 can be seen in the XRD pattern. The result implies the purity of the final nanopowder. Sharper and stronger diffraction peaks show that crystalline sizes of the LaMnO3 powder increase clearly after annealing, as expected due to the crystal growth during the annealing process at a high temperature. It should be also noted that, after annealing, a clear peak shift to a higher angle was observed. The reason for lattice expansion in the sample should be taken into account. One possible explanation for lattice expansion in the sample before annealing is small particles size. In other words, in nanomaterials of poor structural order, defect might be introduced at a high concentration, which results in an internal stress in the lattice due to the presence of large La3+ at interstitial sites in the lattice. The internal stress is responsible for the lattice expansion, as observed for as-prepared nanopowder samples. The corresponding decrease of the lattice parameter is shown in Table 2.
However, the results show that annealing the samples at 1000°C does not result in transition of the phase structure. Such peak shift to a higher angle, combining with the disappearance of some peaks in the XRD pattern, can be an indication of a phase transition. The change in the lattice parameter suggests that annealing phase transition from hexagonal to other structures can occur at a higher temperature or in longer time. However, this is not being mentioned in this paper but would be reported in another study.
The energy dispersive X-ray spectra (Figure 3) of the LaMnO3 nanopowder with F = 3 before and after annealing at 1000°C in 8 hours only show the peaks of La, O, and Mn. This result indicates that samples are clean and pure. LaMnO3 nanopowder prepared with a molar ratio of glycine/nitrate (F = 3) was used to make a target because the results showed that better crystallinity was achieved for this sample. The as-prepared LMO powder was remilled, pressed at high pressure, and annealed at 1000°C for 8 hours to make a target. The structure and phase purity of the LMO target was examined by XRD measurements, as shown in Figure 4. The XRD patterns reveal that the structure of the LMO target is still hexagonal. The LMO target showed peaks corresponding to reflection from (102), (110), (104), (202), (204), (212), (214), and (220) planes, where (110) and (104) peaks have the strongest intensity clearly demonstrating the preferred crystal growth during the annealing process.
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Even though the LMO target can be fabricated by the normal solid-state reaction method from oxides precursors, using nanopowder of LMO to make the target help in reducing the treating temperature to 1000°C, the target prepared by the solid-state reaction normally required a much higher annealing temperature (1300–1400°C) and longer time (12–24 h).
Figure 5 shows XRD patterns of the LaMnO3 thin films deposited at 15 kV before and after annealing at different temperatures. It can be seen that the as-deposited films and films annealed at 400°C and 600°C were amorphous because no diffraction peak is observed. As annealing temperature is raised up to 800°C, the thin film is crystallined in the hexagonal structure.
Figure 6 shows SEM images of LMO thin films deposited at 15 kV before and after annealing at different temperatures.
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SEM images of thin films deposited at 15 kV show that the particle size distribution is quite uniform and postannealing does not change the morphology of the produced thin films. The film is crack free both before and after being annealed. SEM images also reveal that the films have uniform particulates which are uniformly distributed on the surface.
XRD analysis (Figure 7) shows that all thin films are amorphous for all discharge voltage. Postannealing at a temperature higher than 800°C is required to obtain crystalline films. However, transition from the amorphous to hexagonal structure occurs only in films deposited at 14 and 15 kV. Other films prepared at voltage lower than 14 kV remains in the amorphous state despite postannealing.
The diffraction peaks were higher at 15 kV which is the maximum voltage available for the used PED system. The above results can be understood that discharge voltage is a critical factor determining the growth of thin films by PED. Voltage higher than 14 kV is required to preserve the stoichiometry composition of the target to the thin films. However, the energy transferred by the electron beam is not enough for the atom to arrange on the substrate in a crystalline structure. Hence, postannealing is necessary to activate the crystallization of the thin films. Lattice parameters of the films prepared at 14 and 15 kV after annealing are shown in Table 3.
In vacuum deposition techniques, gas ambient may also have a great influence on the structure and morphology of the thin films [40]. We also studied the effect of the N2/O2 flux ratio on the LMO thin film prepared by pulse electron deposition. Figure 8 shows XRD patterns of the LaMnO3 thin films deposited at 15 kV and 5 Hz with different ratios of N2/O2 flux (25 : 0, 20 : 5, 10 : 15, and 0 : 25 sccm).
It is clear that the films after postannealing are well crystallined at all different ratios of N2/O2 flux. The highest intensity was achieved for film grown at N2/O2 flux (10 : 15 sccm), indicating that partial pressure of oxygen also has a slight effect on the structure of the thin films.
Figure 9 shows SEM images of the LMO-2 films, which were prepared at a different oxygen partial pressure. While the sample prepared in pure nitrogen ambient possesses fine particulates, the samples prepared in oxygen show more particulates. This could be due to the fragmentation of the oxide particulates in the plasma region at higher oxygen partial pressures [41].
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4. Conclusion
LaMnO3 nanopowder was successfully synthesized using combustion assisted with microwave irradiation approach. Using nanopowder helps to lower the temperature of heat treatment of the target. Detailed investigation of LaMnO3 thin films preparation shows that discharge voltage is a critical parameter for film deposition by PED. The obtained films were amorphous but could be converted into the crystalline phase after annealing at relatively a low temperature of 800°C. Suitable ratio of oxygen/nitrogen flux also contributes to better crystal quality of the thin films. The results demonstrate the potential of using PED as a tool for fabrication of perovskite thin films of high quality for various applications in electronic fields.
Data Availability
All data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The authors would like to thank Faculty of Physics, VNU University of Science, for allowing them to use the equipment. This research was funded by the Vietnam National University (VNU), Hanoi, under project number QG.17.11.