AMINO-(GD)NITRATE COMBUSTION PROCESS: THE INFLUENCE OF AN AMINO ACID ON THE FINAL PRODUCT

Combustion methods are a promising strategy for the lanthanide oxide nanomaterials preparation. They are environmentally friendly, they have feasible experimental setup and can be easily upscaled. As a part of our broader research dealing with amino acid-nitrate processes, several samples of the Gd2O3, synthesized with four amino acids (alanine, glycine, proline, and serine) were prepared to observe an influence of amino acids on the final product’s nature and morphology. All of the four samples were prepared via thermal decomposition of a transient complex formed in situ from Gd(NO3)3·6H2O and selected amino acid. The resulting products were characterized by the X-ray powder diffraction analysis, which demonstrated an influence of amino acids on the crystal structure of the final Gd2O3 nanocrystallites. The elemental composition and morphology of the samples of Gd2O3 were examined by the scanning electron microscopy with energy dispersive X-ray spectroscopy. We also measured particle size and particle size distribution as all final products (Gd2O3) had tendency to form agglomerates. Fourier-transform infrared spectroscopy and X-ray powder diffraction analysis revealed that glycine and proline, used as an organic fuel, yields the cubic structure of Gd2O3 nanocrystallites.


INTRODUCTION
Lanthanide oxides have gained a lot of attention due to their diverse use for applications such as in the photonics, catalysis, nuclear industry, and electronics [1]. Among the important lanthanides oxides, Gd2O3 gains a significant importance due to its good crystallographic stability up to 2325 °C, high mechanical strength, and excellent thermal conductivity together with relatively high dielectric constant, wide band gap [2], and versatile and tunable magnetic properties [3]. Nanomaterials based on Gd2O3 have been used for example in the solid-state lasers [4], in the solid oxide fuel cells as an electrolyte [5], in the temperature sensors [6] or as the contrast agents in the magnetic resonance imaging [7].
The promising perspective method for preparation of Gd2O3 could be amino acid-nitrate combustion process [8,9] which appear to be effective, low-cost, environmentally friendly, experimentally undemanding and perfectly scalable [10]. Amino acid-nitrate process use amino acid as a fuel and nitrate (or nitrate hydrate) as an oxidant in the presence of source of the inner transition metal [11,12]. Different amino acids have already been used in the combustion processes for preparation of nanomaterials based on lanthanides, such as for https://doi.org/10.37904/nanocon.2019.8590 example alanine [4,6,13,14], glycine [5,13,14] or serine [13,14]. It has been found that the type of source of the inner transition metal and amino acids has an influence on the shape and grain size of the nanocomposite [13,14]. It was further observed that different types of organic fuels (urea and alanine or their mixture) have an influence on the temperature of combustion, crystallographic structure, morphology and resulting particle sizes [6]. It has also been found that the particle size increased with increasing calcination temperature in the combustion synthesis (from 25 nm at 800 ºC to 200 nm at 1100 ºC) [4].
Consequently we chose to prepare four various samples of Gd2O3 nanocrystallites using amino acid-(Gd)nitrate combustion process for observation an influence of an amino acids on the nature and morphology of the final product. Four different amino acids were selected for preparation. Specifically, glycine and alanine as a representative of amino acids with a side aliphatic chain, serine as a representative of amino acids with a hydroxyl group on the side chain, and proline as a representative of an imino acids.
Samples of Gd2O3 were prepared according to the protocol used as in our previous study [15] by mixing equivalent portions of 0.5 mol•dm -3 solutions of Gd(NO3)3•6H2O and particular amino acid (alanine -Ala, glycine -Gly, proline -Pro, serine -Ser) and dried at 120 °C until viscous gel was formed. The gel was then calcined (High Temperature Equipment LAC/LMH) in the temperature 600 °C for 1 hour. Prepared samples, marked Gd_Ala, Gd_Gly, Gd_Pro, and Gd_Ser, were subsequently analysed by X-ray powder diffraction analysis (XRPD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS), and particle size and particle size distribution technique (PS/PSD).

Characterization of the samples
XRPD was performed using the X-ray diffractometer Ultima IV (Rigaku, Japan), operated at 40 kV and 40 mA with CuKα radiation of wavelength λ = 0.154060 nm (reflection mode, Bragg-Brentano arrangement with scintillation detector). The XRPD patterns were recorded in the 10 -70° (2θ) range with a scanning rate of 2.4 °/min. The samples were placed in a ground glass depression in the sample holder and flattened with a glass slide. Phase analysis was evaluated by database PDF-2 Release 2011. Graphics processing XRPD patterns was made using OriginPro8. The crystallite size of the most intense reflection of Gd_Ala, Gd_Gly, Gd_Pro, and Gd_ Ser were determined using the Scherrer formula [16].
Single reflection ATR technique with diamond crystal was used for measurements of mid-FTIR spectra (Nicolet iS10, Thermo Scientific, USA). Spectra were recorded in the range of 400 -4000 cm -1 , however, only region of interest (400 -1800 cm -1 ) is shown in FTIR spectra. All obtained data were processed using the OMNIC software.
For the SEM/EDS analysis, samples were attached to the carbon conductive tape and sputtered with two layers of gold (POLARON SC 7640 sputter coater). Scanning electron microscope Quanta 450 FEG (FEI) was used for revealing of the morphology and EDS analysis was performed with analyser APOLLO X (EDAX) for qualitative analysis of the presented elements. Images were taken by using SE detector (secondary electron detector) at 15 kV. The resulting images were edited by ImageJ program.
Particle size (PS) and particle size distribution (PSD) analyses were performed by dynamic light scattering (DLS) method on the analyzer Malvern Zen 3600 Zetasizer. The same instrument was used to measure their zeta potential. To perform these analyses, the powder samples (0.01 g) were diluted in 50 ml of demineralized water and disintegrated in in self-constructed ultrasonic reactor with the acoustic power of 0.5 kW/l for 3 minutes [17]. The samples were then immediately injected into a cell for zeta potential and particle size distribution measurement. The refractive indexes of lanthanide oxides (1.950) and distilled water (1.333) were determined in our previous research [15].

RESULT AND DISCUSSION
The X-ray diffraction patterns of prepared gadolinium oxides are shown in  FTIR spectra are presented in the Figure 2. All spectra show bands typical for the Gd2O3 [18]. By comparing the spectra, it is clear, that Gd_Ala and Gd_Ser have similar features and as well Gd_Gly with Gd_Pro. These results are in the agreement with the previous XRPD results. It could be also mentioned that main band corresponding Gd-O (538 cm -1 , arrow in the Figure 2) bond is more intensive and narrow for Gd_Gly and Gd_Pro samples, which is probably caused by the cubic crystal structure. From the SEM images (Figure 3) we can see that Gd2O3 forms an aggregated meso-and macro-porous network structure like foam. We can see in the taken SEM images that the type of the amino acid did not affected resulting morphology, so it has to be influenced by the reaction mechanisms. However, detailed description and explanation of this phenomenon is under investigation. EDS then confirmed the expected presence of only gadolinium and oxygen in the prepared samples. These results are in correlation with the results obtained by XRPD and FTIR. The particle size measurement in the Figure 4 shown bimodal distribution with corresponding mean sizes dm1 and dm2 summarized in the inserted table. The bimodal character may have origin in the intensive reaggregation of smaller particles and in the incomplete disintegration of all aggregates to the same size. The zeta potential values ( Table 1) show low values for all samples, which indicates unstable dispersion and the particle re-aggregation is highly expected. The extraordinary low zeta potential for the Gd_Ser sample then caused more rapid aggregation during measurement which can be seen as the shift in particle size distribution in comparison to the other samples.

CONCLUSION
We prepared four samples of nanocrystalline Gd2O3 (Gd_Ala, Gd_Gly, Gd_Pro, and Gd_Ser) using combustion process to observe an influence of selected amino acids on the final product's nature and morphology. Obtained samples were characterized by XRPD, FTIR, SEM/EDS, and PS/PSD techniques. We proved that amino acid-nitrate combustion process allows synthetize pure and homogeneous Gd2O3 structures. Electron microscopy revealed net-like structure created by aggregation of nanoparticles. The glycine and proline produced cubic Gd2O3 with comparable particles size, but alanine and serine produced monoclinic Gd2O3 with much different particles size. In addition, the same type of amino acid does not produce Gd2O3 of the same crystalline structure as would be expected. Finally, we discovered that type of the amino acid does not affect the morphology of the resulting product.