Synthesis, Characterization, and Applications of Y2O3 Nanoparticles Synthesized via Thermal Decomposition of the [Y(Cup)2(Gly)∙2H2O] Complex

Introduction

The synthesis of nanoparticles has emerged as a fascinating field of research with numerous applications in various scientific and technological domains. Yttrium oxide nanoparticles have gained considerable interest among various nanoparticles, thanks to their distinctive properties and promising utilization in diverse domains, including electronics, catalysis, biomedical engineering, and energy storage.¹ Yttrium oxide (Y₂O₃) is a binary compound composed of yttrium, a rare earth metal, and oxygen. The synthesis of yttrium oxide nanoparticles involves the controlled fabrication of these particles at the nanoscale, typically ranging from 1 to 100 nanometers in size.²

Yttrium oxide nanoparticles exhibit a spectrum of favorable attributes, encompassing high thermal stability, superb chemical resistance, and low electrical conductivity. These attributes make them highly suitable for diverse applications, including solid-state electrolytes in fuel cells, luminescent materials in displays and lighting devices, and as additives for enhancing the mechanical and thermal properties of materials. Furthermore, their unique optical, electronic, and magnetic properties offer potential in areas such as photonics, optoelectronics, and magnetic data storage.³

Various methods have been developed to synthesize yttrium oxide nanoparticles, each with its own advantages and limitations. Common techniques include sol-gel synthesis, thermal decomposition, precipitation, hydrothermal synthesis, and aerosol processes. These methods enable the controlled formation of yttrium oxide nanoparticles with tailored sizes, shapes, and surface properties, allowing researchers to optimize their properties for specific applications.⁴

In this article, we will explore the synthesis of yttrium oxide nanoparticles in detail, discussing the principles behind different synthesis methods and highlighting their respective advantages and challenges. We will delve into the key parameters that influence the size, shape, and properties of the nanoparticles, such as precursor materials, reaction conditions, and post-synthesis treatments. Additionally, we will explore the characterization techniques employed to analyze the structure, morphology, and composition of yttrium oxide nanoparticles.

By understanding the synthesis techniques and properties of yttrium oxide nanoparticles, researchers can further advance their applications across various fields. The ability to engineer nanoparticles with precise control over their characteristics opens up new avenues for innovation and exploration in nanotechnology, leading to advancements that can revolutionize industries and contribute to scientific progress.

Experimental Section

Materials

The chemicals employed in the synthesis of mixed ligand complexes were purchased from E. Merck Pharma. Yttrium chloride hexahydrate and the amino acid L-glycine were used without purification for the synthesis of metal complexes. The primary ligand, Cupron, was also utilized in the complex synthesis. Solvents such as DMF, DMSO, ethanol, and acetone were used in the research work after distillation and purification following standard protocols for complex synthesis. Analytical balance and Borosil glasswares were used, and standard calibration protocols were followed for accurate measurements.

Physical Measurements

In the course of this study, several analytical techniques and instruments were employed. Elemental analysis, encompassing the determination of carbon (C), hydrogen (H), and nitrogen (N) content, was executed at IIT Mumbai using a Thermo Finnigan Elemental Analyser with model number FLASH EA 1113 series. Yttrium content estimation was accomplished through complexometric analysis, following established protocols. Fourier Transform Infrared (FT-IR) spectra were meticulously recorded in the spectral range of 4000-400 cm^-1, utilizing a Perkin Elmer Spectrum FT-IR instrument with model number 1500. UV-Vis spectra were captured using a Perkin Elmer Lambda-950 UV-VIS spectrometer, with dimethyl sulfoxide (DMSO) as the solvent, spanning the wavelength range from 200 to 800 nm. Thermal analysis was carried out under controlled nitrogen atmosphere, employing a Perkin Elmer Diamond TG-DTA instrument. Furthermore, to characterize the Yttrium oxide nanoparticles synthesized in this study, scanning electron microscopy (SEM) was conducted with a Hitachi S-4800 instrument, and X-ray diffraction (XRD) analysis was performed using a Bruker D 8 Advance X-ray diffractometer.

Synthesis of Mixed Ligand Yttrium (III) Complexes

To prepare the complex, we initiated the process by blending an aqueous solution of yttrium chloride hexahydrate (303 mg, 1 mmol) with 10 ml of an alcoholic solution of (2Z)-2-(N-hydroxyimino)-1,2-diphenylethan-1-ol (454 mg, 2 mmol) while ensuring continuous stirring. This mixture was subsequently subjected to heating in a water bath for 10-15 minutes. Following this step, we added an aqueous solution of amino acids (1 mmol) drop by drop into the reaction mixture, maintaining continuous stirring. The entire mixture was once again heated, this time in a boiling water bath, for an additional 10 minutes.  After the heating process, we carefully transferred the reaction mixture to an asbestos sheet, and in a precise manner, introduced dilute ammonia solution drop by drop into the beaker. The formation of the complex became evident as the pH gradually increased upon the addition of ammonia solution. The resulting solution, which contained the complex precipitate, was then efficiently filtered through a Buchner funnel, and the solid complex underwent thorough washing with ethyl alcohol and water before being subjected to drying in an oven.⁵

Synthesis of nanoparticles by thermal decomposition of Y(III) complex

Approximately 0.5g of a newly synthesized Y(III) complex underwent a thermal decomposition process within an oven, maintained at a temperature of 400°C for a duration of 30 minutes. This thermal treatment yielded a powder with an off-white hue, indicating the successful production of Yttrium oxide (Y₂O₃) nanoparticles. Subsequently, the resultant powder was allowed to naturally cool to room temperature, followed by meticulous washing with ethanol multiple times to eliminate any lingering impurities. Ultimately, the prepared Y₂O₃ nanoparticles were subjected to an oven-drying process, resulting in the attainment of a dry and stable form. This synthesis method effectively facilitated the conversion of the Y(III) complex into Yttrium oxide nanoparticles, thereby paving the way for further characterization and potential applications.⁶

Biological Activity

Agar Cup Method

In this method, a 1000 ml solution of Mueller Hinton Agar (MHA) is prepared using distilled water and sterilized at 121°C for 20 minutes. After sterilization, it is allowed to cool down to 40°C. All glassware used in the experiment is sterilized as well. An 18-hour-old bacterial culture with a density of 10⁶ cells/cm³ is incubated in the Mueller Hinton Agar, and 20 cm³ of the agar is poured into sterile plates, which are then cooled and allowed to solidify. Each plate is punctured with three wells of 8 mm diameter. Tetracycline (0.1 cm³) is used as a control. The wells are filled with 0.1 cm³ solutions of the prepared complexes in DMSO, labeled with the sample names. The sterile plates are then incubated for 24 hours at 37°C, and the zones of inhibition are measured to determine the antibacterial activity of the complexes.^7-9

Tube Dilution Method

To find the Minimum Inhibitory Concentration (MIC) of the complexes, the tube dilution method is employed. Stock solutions of all complexes in DMSO are prepared at a concentration of 1 mg/cm³, and further dilutions are made. The bacterial culture stock is also diluted with sterile broth to achieve a concentration of 10⁶ cells/cm³. Four sets of sterile tubes are taken for each bacterial strain and labeled with various concentrations of the prepared complexes. Each sterile tube is filled with 5 cm³ of the diluted solution of the yttrium complexes, and the diluted bacterial culture is added to each tube. The tubes are then sealed and incubated for 24 hours at 37°C. Tetracycline is used as the standard. Positive controls are maintained by incubating 5 cm³ sterile Mueller Hinton Broth with bacterial culture. Additionally, 5 cm³ DMSO of 10^-3 M dilution in three tubes are kept for each bacterial strain to observe bacterial growth. The tubes are examined for turbidity, and the minimum inhibitory concentration is determined by the absence of visible change in turbidity.^10-13

Table 1: Antibacterial Activity by Agar Cup Method.

Sr. No.	Complex	Antibacterial Activity (mm)
		S.                aureus	C.    diphtheriae	S.                        typhi	P. aeruginosa
2.	[Y(Cup)2(Ala)∙2H2O]	16	13	8	9
2	Y2O3	20	18	12	14
—	Tetracycline	30	23	25	16

Table 2: Data of Antibacterial Activity by Tube Dilution Method.

Sr. No.	Complex	MIC (µg/cm3)
		S.             aureus	C.              diphtheriae	S.                       typhi	P. aeruginosa
2.	[Y(Cup)2(Ala)∙2H2O]	70	75	100	117
4.	Y2O3	80	82	118	100

 

Table 3: MIC of Metal Salts, Ligand and Tetracycline by Tube Dilution Method.

Salt / Ligand /    Tetracycline	MIC                                                                                                        (µg/cm3)
	S. aureus	C. diphtheriae	S. typhi	P. aeruginosa
Y.Cl3.6H2­O	100	140	1450	180
Cupron	100	200	150	140
Tetracycline	1.5	2.0	1.5	7.0

 

The antibacterial activity data presented in Tables 2 and 3 reveal insights into the inhibitory potential of the tested compounds against various bacterial strains. Table 2 indicates that both the [Y(Cup)₂(Ala)∙2H₂O] complex and Y₂O₃ nanoparticles exhibit moderate antibacterial activity, with smaller inhibition zones compared to the standard tetracycline. Table 3 demonstrates the Minimum Inhibitory Concentration (MIC) values, with lower values indicating stronger inhibition. Here, [Y(Cup)2(Ala)∙2H₂O] complex showcases better inhibitory potential compared to Y₂O₃ nanoparticles. Tetracycline exhibits the lowest MIC values, displaying potent antibacterial action. In summary, the data underscores the varying antibacterial efficacy of the tested compounds. While tetracycline stands out with strong inhibition, the tested complexes and Y₂O₃ nanoparticles display moderate antibacterial activity.

Results and Discussion

This Chapter study mainly focused on the preparation of mixed ligand Yttrium complexes, study of their physicochemical properties and spectral interpretation of Yttrium(III) complexes.

Scheme 1 Click here to View Scheme

Where,

HCup:  Cupron
HL:  Amino acid

Where HCup represents (2Z)-2-(N-hydroxyimino)-1,2-diphenylethan-1-ol, and HL corresponds to alanine. The resulting complex exhibits a light white color, possesses non-hygroscopic properties, and stands as a thermally stable solid, signifying the presence of strong metal-ligand bonds. This complex demonstrates partial solubility in various solvents, including methanol, ethanol, chloroform, DMF, and DMSO. The elemental analysis data for the metal complex aligns with a general formulation of 1:2:1, indicative of a complex of the form [Y(Cup)2(L)∙2H2O]. Furthermore, when the complex is dissolved in DMF at a concentration of 10^-3 M, its molar conductance values are consistently low (<1), suggesting its non-electrolytic nature.^14,15

 UV Spectra

In methanol, the electronic spectra of the metal complex, recorded in the UV region, reveal the presence of intra-ligand and charge transfer transitions. Specifically, three distinct transitions are discernible: one at 203-263 nm (n→π*), another at 337 nm (π→π*), and a third in the range of 384-386 nm, signifying a charge transfer from ligands to the metal center. In chloroform, the spectra of the metal complexes in the visible region display a notable charge transfer transition occurring at 400-420 nm. Furthermore, additional transitions manifest at 560-600 nm and 820-840 nm, characteristic of d-d transitions commonly observed in transition metal complexes. It is noteworthy that the UV-vis spectrum of Y₂O₃ nanoparticles differs significantly from that of the initial complex. This divergence confirms that the 288 nm band originates from the Y₂O₃ nanoparticles and not from the Y(III) complex. Importantly, the UV-vis spectrum of commercially available bulk Y₂O₃ powder does not display any discernible absorption bands.^16,17

Figure 1: UV spectra of Y2O3 nanoparticles. Click here to View Figure

FTIR Spectra

The IR spectrum displays distinctive absorption bands between 3400-3600, 2940, 3238, 1267-1212, 600-590, and 510 cm−1. These correspond to coordinated-H₂O, -NH₂, CO, N–O, and M–N stretching vibrations, respectively. After thermal decomposition at 400 °C, the specific bands related to the HCup and HL compounds vanish. Instead, a robust 440 cm−1 band arises, attributed to Y–O stretching in Y₂O₃ NPs.¹⁸

SEM

Figure 2 displays SEM micrographs illustrating the Y(III) complex and its resulting decomposition product at 400°C. A discernible transformation is evident in these images. The initial complex powder, as depicted in Figure 2a, consists of substantial block crystals varying in size. In stark contrast, the SEM image of the product (Figure 2b) vividly showcases particles with distinct shape and size characteristics compared to the precursor complex. The product’s SEM image reveals the presence of exceedingly fine semi-spherical particles, loosely aggregated in nature. Notably, the product’s morphology stands in stark contrast to that of the complex, indicating a significant shift in structure during decomposition. The absence of characteristic morphological traits from the complex further underscores the completion of decomposition, resulting in the formation of these finely textured spherical particles. This visual evidence reinforces the profound alteration from large block crystals to delicately structured, loosely aggregated particles.^19,20

Figure 2: (a). SEM image Y(III) complex and (b). SEM image of Y2O3 nanoparticles Click here to View Figure

TEM

Illustrated in Figure 3 are the TEM images depicting both the complex and its resultant decomposition product at 400°C. Initial observations reveal that the TEM micrograph of the initial complex powder (Figure 3a) is dominated by large block crystals of varying dimensions. Conversely, the thermal decomposition process yields uniformly shaped Y₂O₃ nanoparticles (Figure 3b), exhibiting spherical morphology with mild agglomeration. Upon decomposition, the particle size distribution demonstrates a remarkable uniformity, spanning from 8 to 12 nm, and culminating in an average particle diameter of approximately 10 nm. Notably, this average particle size, as determined through TEM analysis, closely mirrors the previously computed value attained through alternative methods. This concurrence underscores the reliability and consistency of the particle size estimation technique employed. The TEM images distinctly capture the metamorphosis from bulky crystalline structures to finely defined nanoparticles, providing visual confirmation of the successful synthesis and ensuing decomposition process.^21,22

Figure 3: (a). TEM image Y(III) complex and (b). TEM image of Y2O3 nanoparticles. Click here to View Figure

XRD

Y₂O₃ powders were synthesized utilizing a modified thermal decomposition process conducted at 400°C for a duration of 30 minutes. The crystal structure and morphology of the resulting Y₂O₃ nanoparticles were comprehensively characterized employing X-ray diffraction (XRD), thermogravimetric analysis with differential scanning calorimetry (TG-DSC), and field-emission scanning electron microscopy (FE-SEM). The XRD pattern unveiled four distinctive, strong, and broad peaks occurring at 2θ values approximately equal to 29.2, 33.8, 48.5, and 57.60 degrees. These peaks corresponded to the (222), (400), (440), and (622) reflections of cubic Y₂O₃, respectively, as per the JCPDS card number 89-5591. Additionally, the determination of the average Y₂O₃ nanoparticle size was accomplished employing the Debye-Scherrer equation, based on the relative intensity peak (222), resulting in an estimated size of approximately 10 nm. The increased sharpness of the XRD peaks signifies the crystalline nature of the particles. The calculation formula employed for this determination is as follows:

D = (0.94λ) / (βcosθ)

In the above formula , λ denotes the X-ray wavelength (λ = 1.542 Å) corresponding to Cu-Kα radiation, β signifies the full width at half maximum (FWHM) of the diffraction line, and θ represents the diffraction angle. The utilization of this analytical approach offers valuable insights into the structural and size attributes of the Y₂O₃ nanoparticles. Such insights prove instrumental in comprehending the potential applications of these nanoparticles across various fields. ^24,25

Figure 4: XRD Click here to View Figure

Conclusion

In conclusion, the comprehensive analysis of Y₂O₃ nanoparticles and the Y(III) complex has yielded valuable insights into their structural, electronic, and morphological properties. Electronic spectra in methanol unveiled distinct transitions: 203-263 nm (n→π*), 337 nm (π→π*), and 384-386 nm (charge transfer). In chloroform, a charge transfer transition at 400-420 nm, along with d-d transitions at 560-600 nm and 820-840 nm, indicated the complex’s solvent-dependent behaviour. The UV-vis spectrum of Y₂O₃ nanoparticles distinctly differed from the initial complex, confirming the source of the 288 nm band. FTIR analysis revealed shifts in absorption bands after decomposition: coordinated-H₂O, -NH₂, CO, N–O, and M–N stretching vibrations. SEM images depicted the transformation from large block crystals in the complex to finely shaped, semi-spherical particles in the product. TEM analysis further supported this transformation, with particle sizes averaging around 10 nm post-decomposition. XRD patterns validated the crystalline nature of the nanoparticles, with strong peaks at 2θ ≈ 29.2, 33.8, 48.5, and 57.60 corresponding to (222), (400), (440), and (622) reflections of cubic Y₂O₃, respectively. This structural information aligns with the uniformity observed in the TEM images. Collectively, these results provide a comprehensive understanding of the synthesis, decomposition, and properties of Y₂O₃ nanoparticles, offering numerical evidence to guide their potential applications in various fields.

The conducted antibacterial activity assays highlight the moderate inhibitory potential of the [Y(Cup)₂(Ala)∙2H₂O] complex and Y₂O₃ nanoparticles against bacterial strains. While these substances demonstrate promising activity, their efficacy is notably surpassed by the standard antibiotic tetracycline.

Acknowledgement

The author Dr. Jitendra M. Pawara extends gratitude to Changu Kana Thakur Arts, Commerce & Science College (Autonomous), New Panvel, Navi Mumbai. The college provided instrumental and laboratory resources, along with research funding through the ‘RUSA grant Scheme of Seed Money for Research to Teacher Rashtriya Uchchatar Shiksha Abhiyan, enabling the completion of this research work.

Conflict of Interest

The authors declare that there are no conflicts of interest related to this research work.

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