Investigating the physical and electrical properties of La2O3 via annealing of La(OH)3

A simple technique was utilized to fabricate pure hexagonal La2O3 nanorods by utilizing lanthanum(III) nitrate hexahydrate (La(NO3)3·6H2O) and ammonia (NH4OH). The La2O3 nanoparticles were analyzed using XRD, TGA, Raman, SEM, FTIR, TEM, PL spectroscopy, and Mott–Schottky techniques. The XRD analysis confirmed the production of La(OH)3 nanorods under appropriate conditions, which were then successfully converted into La2O2CO3 and finally into La2O3 nanorods through annealing. The TGA analysis showed that the total weight loss was due to water evaporation and the dissolution of minimal moisture present in the environment. The FTIR analysis confirmed the presence of functional groups. The SEM analysis revealed changes in morphology. The TEM analysis to determine the particle size. The PL findings showed three emission peaks at 390, 520, and 698 nm due to interband transitions and defects in the samples. The Mott–Schottky analysis demonstrated that the flatband potential and acceptor density varied with annealing temperature, ranging from 1 to 1.2 V and 2 × 1018 to 1.4 × 1019 cm−3, respectively. Annealing at 1000 °C resulted in the lowest resistance to charge transfer (Rct).

were synthesized at 60 °C.Then, the nanorods were calcinated to 400 °C for two hours in a furnace.After calcining the La(OH) 3 nanorods for two hours at 800 °C, pure La 2 O 3 nanorods could be formed effectively 24 .Jun-Gill Kang et al.The hexagonal phase La(OH) 3 nanowires and La 2 O 3 microboards were effectively manufactured using a hydrothermal technique at 900 °C.On the other hand, hexagonal La 2 O 3 is unstable in surrounding conditions.Due to the hygroscopic nature of La 2 O 3 , it progressively re-transforms into a hexagonal phase La(OH)3 via rehydroxylation 25 .Xing Wang et al. fabricated La 2 O 3 films at different annealing temperatures of 400, 600, and 800 °C, for 60 s through atomic layer deposition.It was found that after 400 °C annealing temperatures, amorphous disordered structures of the film can be obtained.After being annealed at 600 and 800 °C, only weak crystalline planes such as hexagonal (101) appear, indicating the possibility of converting the 10 nm La 2 O 3 film into complete crystallized.After being crystallized, the refractive index increases dramatically while the bandgap is slightly decreased 26 .The annealing temperature can improve the lattice structure and NPs characteristics in our work.In biological applications, the temperature-dependent crystal size is significant.In this work, La 2 O 3 nanoparticles were synthesized via hydrothermal method at different annealing temperatures ranging from 500 and 1000 °C.The synthesized nanoparticles are intended to be used for biological applications.To investigate the effect of annealing temperature on the microstructures, morphology, optical and photoelectrochemical characteristics of La 2 O 3 nanoparticles, various characterization techniques were employed.

Experimental methods
La 2 O 3 nanoparticles were synthesized using a hydrothermal technique.The chemical solutions used in the process included lanthanum (III) nitrate hexahydrate (La(NO 3 ) 3 •6H 2 O; 99%; BDH) and ammonia (NH 4 OH; 32%; PIOCHEM, EGYPT).To begin, 0.1 M lanthanum nitrate was dissolved in 100 mL of distilled water (Milli-Q, 18MΩ cm) at 60 °C.The mixture was vigorously stirred until it became clear.Then, liquid ammonia was added slowly, drop by drop, to adjust its pH to 10.The solution was then left undisturbed to allow the precipitation to occur.The precipitate obtained was gathered and rinsed with water and ethanol to ensure no unreacted substances remained.The washed powder was then dried in an 80 °C hot air oven for an entire night.Afterward, the dry powder samples were collected and crushed using a mortar before analysis.The experiment was conducted at different annealing temperatures ranging from 500 to 1000 °C.The following analysis techniques were used: X-ray diffraction (Shimadzu), Thermo Gravimetric analysis (TGA-Setaram Themys one +) to determine the total weight loss, Raman spectra using a WITec alpha300 R system, and Fourier transform infrared FTIR with single beam (Nicolet.iS10-U.S.A.) to identify the functional groups present in the synthesized samples.Scanning electron microscopy (SEM) (JSM-651OLV) was employed to analyze the morphological properties.Transmission electron microscopy (TEM) images were taken by (JEOL 2010-200 kV).The optical characteristics of La 2 O 3 nanoparticles were examined by PL with a Kimmon He-Cd laser of 325 nm (3.82 eV).The CHI660E electrochemical workstation with the help of 0.5 M Na 2 SO 4 was utilized to make Mott-Schottky and EIS.
where D corresponded the crystal size, β is the FWHM indicated in radians, λ is X-ray wavelength and θ is the diffraction angle 33 .
It was observed that the crystallite size increased from 12 to 33.26 nm as the annealing temperature was raised.This could be attributed to two possible reasons; Firstly, higher temperatures may have facilitated ion diffusion in the samples.Secondly, the reduction of defects in the samples could have lowered the barriers between the particles, leading to the production of larger particles [34][35][36][37][38] .In order to determine microstrain (ε), the following equation is utilized 39 : The density of dislocation (δ) is represented by the following Eq. 40: The values for microstructural properties are summarized in Table 1. Figure 2 displays that annealing at a higher temperature leads to fewer surface defects compared to non-annealed samples 41 .

TGA analysis
To evaluate the La 2 O 3 NPs' thermal stability, a Thermo Gravimetric analysis was performed in a nitrogen environment between room temperature and 1000 °C at a heating rate of 10 °C /min 42 .The TGA curve for the La 2 O 3 NPs produced under nitrogen environment up to 1000 °C is shown in Fig. 3.
At a temperature of 100 °C, weight loss occurred in the low-temperature region due to the reduction of water molecules 43 .There was also a sharp weight loss observed at around 400 °C, indicating an endothermic process.The weight loss observed is related to the loss of carbon dioxide, and it amounts to approximately 6.77% 2,44 .At the high-temperature area, the breakdown of lanthanum carbonate and weight loss is approximately 1.5%.Finally, at (1) D = 0.9 βCOSθ  temperatures above 700 °C, lanthanum oxy-carbonate dissolves and produces pure La 2 O 3 nanoparticles 45 .It has been observed that the total weight loss of La 2 O 3 NPs is caused by the evaporation of water and the dissolution of minimal moisture present in the particles.From the TGA curve, it has been estimated that only 12.77% of the total weight loss occurs.These findings clearly demonstrate that ammonia, which is used as a reducing agent in the fabrication process of La 2 O 3 NPs, is highly stable.The ammonia is entirely removed by the washing process of water and ethanol, and annealing at 500 °C is sufficient to remove any unprocessed ammonia and additional impurities 2 .The decomposition pathway of La (OH) 3 is proposed as follows 24 :

Raman analysis
Raman spectra are used to confirm the results obtained from X-ray powder diffraction 46 .A laser Raman analysis was conducted to identify the phases, which ranged from 100 to 1300 cm −1 , as shown in Fig. 4. At 60 °C, three vibrational modes were observed, which are related to only La(OH) 3 phase 47 .These vibrational modes are A 1g , E 2g , and E 1g at 280, 340, and 450 cm −1 , respectively.A 1g is a non-degenerate total-symmetric vibration, that means it is symmetric under all symmetry operations (rotation, mirror plane, inversion center).E 1g , E 2g is ( 4)  www.nature.com/scientificreports/ a double-degenerate vibration that is symmetric under inversion 48 .By annealing at 500 °C and 600 °C, it was observed that four vibrational modes appeared around 340, 405, 450 and 1057 cm −1 , which contained a mix of La (OH) 3 , La 2 O 2 CO 3 , and La 2 O 3 phases 48 .With an increase in annealing temperature up to 1000 °C, vibrational modes were observed, which were related to only La 2 O 3 phase 49 .It was found that the sharpness of Raman peaks increased with an increase in annealing temperature, which might be related to the increase in particle size 50 .These results are consistent with the XRD measurements.

Fourier transform infrared spectroscopy analysis
Figure 5 shows the FTIR spectrum of La 2 O 3 NPs samples at different annealing temperatures, ranging from 4000 to 400 cm −1 .The aim was to identify the functional groups present in the prepared samples 51 .The wide peak observed at 3609 cm −1 indicates the presence of O-H stretching vibration due to absorbed moisture on the nanoparticle surfaces 3,52 .The bands at 1620 cm −1 and 1366 cm −1 are caused by the O-H vibration in absorbed water on the sample surface 24,53 .The intensity of the peaks at 1620 cm −1 and 1366 cm −1 decreased with increasing annealing temperature.Two small peaks at 852 cm −1 , observed at 500 °C and 600 °C, were related to C-O bending vibrations 2,16 .The broad absorption band measured at 660 cm −1 is thought to occur due to the La-O stretching vibration 24,54 .These bands demonstrated the existence of the La 2 O 3 phase in the synthesized nanoparticles.

Surface morphology evaluation
The SEM technique was used to analyze the morphology of the La 2 O 3 that was synthesized.Figure 6 The samples exhibit rod-shaped nanostructures with nanoparticle aggregation, which become more spherical at high temperatures, hindering size identification 24,55 .This aggregation could be influenced by annealing temperature that increases the mobility of atoms 52 .As the annealing temperature is increased, the nanoparticles' grain size also increases.The merging process is enhanced by the oxygen or lanthanum defects at the grain boundaries through the annealing of additional grains at high temperatures.It is proposed that grains with lower surface energy will expand more at higher temperatures.Moreover, higher temperatures provide more energy for the atoms to move and occupy their correct positions in the crystal lattice 35 .When the annealing temperature rises, this aggregation increases and produces large particles, as confirmed by XRD results.

TEM pattern
Figure 7a,b, display TEM images of two samples, La 2 O 2 CO 3 and La 2 O 3 , that were fabricated and annealed at 500 °C and 1000 °C, respectively.The sample annealed at 500 °C showed aggregation and had nanorods with an average size of 11 ± 2 nm.As the annealing temperature increased to 1000 °C, the aggregation of particles increased which become more spherical at high temperatures with an average particle size of 31 ± 2 nm 56 .These findings agree with XRD results 57 .

PL analysis
The PL emission is caused by the recombination of free carriers.This makes PL spectra useful for analyzing the efficiency of trapped charges 58 .In Fig. 8, you can see the PL spectrum of all samples taken at room temperature with an excitation wavelength of 320 nm.The measurements were taken between 300 and 800 nm.The PL spectrum has three peaks: the first is around 390 nm, the second is located at 520 nm, and the third peak is found at 698 nm.The first peak is correlated with inter-band transitions in La 2 O 3 .The second peak is the result of cation vacancy defects in La 2 O 3 59,60 .The third peak results from radiative relaxational transitions phenomena of La 3+ ions 61 .It was observed that the peaks' intensity increase with annealing temperature, showing that lanthanum vacancies may be enhanced by increasing of annealing temperature 61 .

Electrochemical impedance spectroscopy (EIS)
The behavior of charge transfer at the interface between semiconductor and electrolyte was studied by examining the electrical responses of La 2 O 3 nanoparticles using the electrochemical impedance spectra technique over a wide frequency range of 10 4 HZ 62,63 .Figure 9 shows the Nyquist diagram of La 2 O 3 nanoparticles, which were fabricated at various annealing temperatures ranging from 500 to 1000 °C.The Nyquist curves (Z" vs. Z') were fitted using ZSimpwing software )https:// www.amete ksi.com/ produ cts/ softw are/ zsimp win (and an electrical circuit model with the corresponding components of solution-resistance (Rs), charge-transfer resistance (RRCT), and double-layer capacitance (C dl ) 64 .The plots are composed of semicircles, and their diameters decrease as the annealing temperature increases 41 .This suggests that there is a higher rate of charge transfer and a lower rate of charge recombination 65,66 .The reason for this is that when the annealing temperature increases, the electrons present in the valence band acquire enough energy to move into the conduction band.Because the number of electrons in the conduction band rose, conductivity is improved and resistivity declines 67 .

Mott-Schottky measurements
Mott-Schottky technique was used to determine the type of conductivity and compute the density of donors or acceptors in the fabricated samples 68 .The Mott-Schottky plot (1/C 2 vs. V) is obtained from the electrochemical impedance data using the following relation 69 : where f is 10 kHz and Xc represents the imaginary part (Z").Figure 10 shows Mott-Schottky plots for the prepared La 2 O 3 nanoparticles at various annealing temperatures.According to the Mott-Schottky formula, the acceptor density (N A ), the semiconductor capacitance (C), and the built-in voltage (V fb ) are related by the following formula 68 : where ε, ε 0, e, k B , A, T represents dielectric constant (ε for La 2 O 3 = 27), vacuum permittivity, charge of electron, Boltzmann constant, active surface area of the photoelectrode and temperature, respectively 70 .V fb could be obtained experimentally by interception of the linear part of the Mott-Schottky plot on the x-axis and it was observed to increase from 1 to 1.21 V with the raise in annealing temperature.The more the built-in voltage, the more the charge separation at the interface between the semiconductor and electrolyte, the acceptors density was estimated by the following formula 71 .
where S represents the slope of the Mott-Schottky graph 70 .M-S plots exhibited a negative slope, showing that all samples were p-type semiconductors.The acceptors density improved from 1.3 × 10 17 to 3.2 × 10 17 cm -3 with the raise in annealing temperature, signifying a higher density of the lanthanum vacancies in La 2 O 3 after it's annealed at 1000 °C.Table 2 shows the calculated acceptors density and flat band potentials for La 2 O 3 nanoparticles at different annealing temperatures.

Conclusions
In this study, La 2 O 3 nanoparticles were synthesized via hydrothermal method and characterized for microstructure, morphology, thermal, optical, and electrical properties.The XRD findings revealed that La(OH) 3 nanorods were produced at 60 °C under appropriate conditions and could be converted into La 2 O 2 CO 3 by annealing at temperatures of 500 °C and 600 °C for 3 h.Finally, the annealing temperature was raised until 1000 °C to obtain La 2 O 3 .According to TGA analysis, the weight loss observed in the manufactured La 2 O 3 NPs was due to water evaporation and the dissolution of a minimal amount of moisture present in the environment.FTIR analysis confirmed the presence of functional groups related to La(OH) 3 , La 2 O 2 CO 3 , and La 2 O 3 .SEM results showed that the nanoparticles' aggregation increased with higher annealing temperatures.that the nanostructures were rodshaped and became more spherical at high temperatures as the particle size increased.PL analysis revealed three peaks: the first peak was associated with inter-band transitions in La 2 O 3 , the second peak resulted from cation vacancy defects in La 2 O 3 , and the third peak resulted from radiative relaxational transitions phenomena of La 3+ ions.The Mott-Schottky test results show that the flatband potential and acceptor density change depending on the annealing temperature.Specifically, the flatband potential ranges from 1 to 1.2 V, while the acceptor density ranges from 2 × 10 18 to 1.4 × 10 19 cm −3 .Moreover, the annealing temperature of 1000 °C resulted in the lowest resistance to charge transfer (Rct).

Figure 2 .
Figure 2. Variation of strain and crystallite size for La(OH) 3 , La 2 O 2 CO 3 and La 2 O 3 nanoparticles.

Figure 3 .
Figure 3. TGA curve of the dried powder at 60 °C.

Table 1 .
Structural parameter of Lanthanum Oxide at different annealing temperatures.

Table 2 .
Effect of annealing on acceptors density and flat band potentials for La 2 O 3 nanoparticles.