Influence of pH on structural, morphological, optical, photocatalytic, and antibacterial properties of yttrium oxide nanoparticles via co-precipitation method

Yttrium oxide nanoparticles with multiform morphologies have been synthesized by the coprecipitation method. The structure, morphology, functional groups, optical and photoluminescence properties were examined through X-ray diffraction (XRD), Scanning electron microscope (SEM), Fourier transform infrared spectrometer (FTIR), UV-Visible (UV-Vis), Photoluminescence spectra (PL). The XRD patterns obtained for the samples synthesized at various pH values confirmed the cubic structure of Y 2 O 3 . The patterns obtained on the samples at pH values of 8 and 9 appeared as have sharp peaks suggested, that the samples were well crystallized. From UV-vis spectra, it revealed that the bandgap energy exhibits a blue shift in the absorption edge for the samples with the increase of pH due to their changing morphologies and surface structures. In the PL spectra, the obtained Y 2 O 3 samples demonstrate an intense and bright UV and blue emission under the excitation wavelength range of 250 nm. The photocatalytic degradation of the Y 2 O 3 nanostructure was studied against the Methylene blue (MB) dye under sunlight irradiation. The results showed good recital under solar light irradiation. Further, the antimicrobial activities of Y 2 O 3 nanostructure against foodborne pathogens ( Staphylococcus aureus and Salmonella typhi ) were examined by using the disc diffusion method. Moreover, the Y 2 O 3 nanostructure was found to be biocompatible.


Introduction
Currently, tremendous efforts have been focused on the studies of modern nanomaterials and their impending applications. With increasing industrial demands, it exhibits a smaller size, well crystalline nature, and better nanostructures adopted for electronic devices and wastewater remediation applications. Rare earth metal oxide nanoparticles are extensively studied in the field of nanomaterials due to their peculiar nanostructures, optical, electronic, and catalytic properties [1]. All these features are favorable to several technological applications like as optical communication, optical display device, photocatalytic, medical diagnosis, and UV shielding [1][2][3][4][5][6][7]. In addition, Fluorescent based nanomaterials versatile role in the area of UV and visible light emission devices [8,9]. The compounds based on rare earth metals have been widely utilized as high recital luminescent materials, because of their properties within the nanoscale range might be related to their morphology [10,11]. Among all rare-earth oxides, yttrium oxide (Y2O3) nanoparticles have also been significantly reviewed as a host material that used upconversion efficiency because of their extensive range of optical transparency large bandgap (5.8 eV) and other optical and biological applications [12,13].
The morphology and structure of materials chiefly depend upon the fabrication circumstances and limitations [14]. So far, numerous techniques employed for the fabrication of Y2O3 nanostructures, such as sol-gel [15], hydrothermal [16], and combustion synthesis [17]. Amid these, the co-precipitation route extends renowned benefits counting sample purity, low cost, and eases to production. In this method, the preparation of particles is below hundred nanometers, easily control the particle size by changing the hydrolysis time and regulate the shape of the particles by changing the pH value of the aqueous solution [18]. This size control and shape are desirable to generate well optoelectronic material. Hence, in this work synthesize Y2O3 nanoparticles with well-ordered particle size and shape by the trouble-free alteration of the pH value of the solution. The manipulate of pH on the purity, crystallinity, morphology, optical, and emission properties has been investigated through XRD, FTIR, SEM, UV-Vis, photoluminescence, photocatalytic and biological studies.

Synthesis of yttrium oxide nanoparticles
Yttrium oxide nanoparticles were synthesized by the co-precipitation route. The initial chemicals consumed in this process were of analytical grade without any supplementary cleansing. The yttrium nitrate hexahydrate (Y(NO)3.6H2O) was used as a forerunner material and ammonium hydroxide employed as a precipitating agent (varying pH values). Then, the precise amount of yttrium nitrate was suspended in 50 mL of deionized water (DD) and stirred well through a magnetic stirrer. The pH (7, 8, and 9) readings of the solution were separately regulated by ammonium hydroxide; the above were combined through steady stirring for 2 h at 353 K.
Finally, the white precipitate was obtained. The accomplished precipitate was filtered off and cleaned a several times with DD and ethanol. The resultant white gel was dehydrated at 353 K for approximately 3 h in a hot air oven and next calcinated at 773 K for 2 h.

Characterization techniques
The phase of the fabricated samples were examined by XRD operating the PANalytical model X'PERT-PRO spectrometer Kα radiations (λ = 1.5418 Ǻ). FTIR spectra were recorded using the Shimadzu model 8400S spectrometer (4000 cm -1 to 400 cm -1 ). For this analysis, a small amount of Y2O3 samples was blended with KBr and then pressed into pellets for the measurement. The changes in surface morphology of the products were observed by (SEM, S-4200, Hitachi) functioned at 15 kV. The UV-vis absorption spectra were obtained on the Agilent 8453 diode array UV-Vis spectrophotometer. Photoluminescence was examined on a Cary eclipse spectrophotometer with a UV light as the excitation light source.
The photocatalytic action of the Y2O3 (pH=9) was appraised by handling a strategy interpreted in [35] although the antibacterial action of the Y2O3 (pH=9) was explored by agar well diffusion method portrayed elsewhere [35].

Structural analysis
The XRD peaks of yttrium oxide nanoparticles with various pH values that were different from 7 to 9 are revealed in Fig which designates that are highly phase pure compounds. At the optimum value of pH, supersaturation becomes the highest value and the radius of crystallites will be formed as a lowest, which is due to the circumstance that larger concentration of solute (supersaturation) guides to the initiation of a huge numeral of nucleation sites and more nuclei means smaller sized nuclei. For pH values 8 and 9 of the samples, more sharp peaks obtained which suggested that the synthesized samples were highly crystallized. Though, the Y2O3 nanoparticles achieved from the solution of pH=7 revealed reasonably less crystallinity and crystallite size. Fig. 1 exhibits a slightly broad peak for pH = 7 and 8 which results from diminished crystallite size contrasted to the higher value of pH = 9 illustrated in Table 1.
The average crystallite size (Debye-Scherrer) of yttrium oxide nanoparticles was attained by this equation 1 [19] (1) Where 'β' signifies the full width at half maximum (FWHM) of XRD peaks, 'θ' is the Bragg's angle of diffraction lines, 'λ' is an incident wavelength of X-rays. The crystallite size is measured from the XRD and it is found to be 13 -16 nm which was displayed in Table 1. For cubic structure, the Lattice constant (a) is obtained from dhkl as given below (2) Where 'd' is the Inter planar spacing, (h k l) is the Miller indices and 'a' is Lattice constant. The calculated lattice value is 10.6020 and it is in fine conformity with the customary JCPDS card no-89-5591.

Fig.1. XRD patterns of Y2O3 nanoparticles prepared with different pH values
The surface states will take part in a key function in the nanoparticles, because of their huge surface to volume ratio with a lessening in crystallite size [20]. The specific surface area is calculated by using the formula, Where 'S' is the specific surface area, 'Dp ' is the particle size and 'ρ' is the density of Y2O3 (5.01 g/cm -3 ). The specific surface area of Y2O3 nanoparticles depends on the association amid crystallite size and shape. The dislocation density strongly influences many material properties including strength, size, and defects. The evaluated dislocation density for Y2O3 nanoparticles has shown in Table 1. Table 1 Variation of lattice constants, crystallite size, and energy bandgap of Y2O3 nanoparticles synthesized at different pH values.

W-H analysis
Williamson-Hall method (W-H) can be applied to compute the microstrain of the fabricated Y2O3 nanoparticles. The strain was computed from the horizontal line intercept at the Y-axis, it has a non-zero slope (Fig. 2). From the W-H plot, it is noticed that the strain values are very little and therefore their effect on the broadening of peaks is insignificant [21,22]. However, there is a minor deviation in the particle size for different pH synthesized samples, due to the presence of strain is account obtained.

Morphological Analysis
SEM micrographs of the fabricated Y2O3 nanoparticles with a different pH value are exhibited in Fig. 3 (a-c). The sample synthesized by the solution of pH=7 appeared agglomerated with a spherical nanostructure (Fig. 3a). For pH=8, the Y2O3 sample exhibited cube like morphology but it shows a much thinner surface (Fig. 3b). Alternatively, From the SEM image of Y2O3 nanoparticles found from the solution of pH=9, the surface morphology different from other samples was detected. The rod-like morphology was observed for the sample synthesized from pH=9 (Fig. 3c). Fig. 3c shows rods like morphology and their average length: 5.75 µm, diameter: 120-450 nm. This designates that pH value is a substantial parameter in adjusting the morphology of Y2O3 sample. It is evident from fig. 3 that the increase in pH affected the Yttrium oxide particles owing to the attendance of OHions.

FTIR studies
The FTIR spectra for the Y2O3 samples fabricated with a diverse pH value are depicted in Fig. 4. The peak monitored at 3451 cm -1 is because of the stretching vibration of the water molecule [23]. The vibrational peak found at 1633 cm -1 was ascribed to the OH vibration. The peak observed at 589 cm -1 suggests the stretching vibration of Y-O, which reveals the presence of yttrium oxide in the crystalline phase. It was obvious that the Y-O absorption bands become broader as the particle size diminishes. There were no additional peaks observed in the spectra, indicating its high phase purity of the sample. Although, the slight change in the wavenumber of the samples fabricated at variant pH values (see Fig.4). This is well matched with the result of the XRD studies.

UV-DRS spectra
The UV-Vis absorbance spectra of Y2O3 nanoparticles fabricated with a diverse pH values are displayed in Fig. 5. They exhibit strong absorbance band at around 270 nm; it could be connected to the photoexcitation of electrons from the valance band to conduction band [24].

Fig. 5. (a) UV-Vis absorption spectra of Y2O3 sample and (b-d) bandgap determination of the Y2O3 nanoparticles prepared at different pH 7, 8 and 9
The optical bandgap (Eg) of Y2O3 samples was estimated by using Tauc's plot relation.
The absorption coefficient is given by equation (4): (4) Where 'hν' is photon energy, 'α'is absorption coefficient, 'Eg' is bandgap energy, and 'A' is the constant related to the material. The exponent n= 1/2, 2, 3/2, and 3 corresponds to the allowed direct and indirect, forbidden direct and forbidden indirect transitions, individually [25,26]. The bandgap was analyzed from a plot of (αhν) 2 vs hν. The bandgap was found to be 3.42, 4.15, 4.47 eV for the Y2O3 nanoparticles prepared at pH= 7, 8, and 9 respectively. It has been observed that the bandgap energy reveals a blue move in the absorption edge for the samples with enhancing of pH due to their changing morphologies and surface microstructures [27].

Photocatalytic activity
The photocatalytic absorbance spectrum of Y2O3 sample (pH=9) beneath sunlight presented in is due to the partition of charge carriers, the reduction of the bandgap, and the rod-like (1-D) structure [33,34].

Antibacterial activity
The antibacterial studies of Y2O3 (pH=9) were assessed adjacent to Staphylococcus aureus and Salmonella typhi bacterial strains and were checked at diverse concentrations (50 and 100 μg/mL). Y2O3 nanorods demonstrated good antibacterial action against Staphylococcus aureus (20 mm) at 100 μg/mL. Table 2 recapitulates the zone of inhibition (ZOI) values acquired adjacent to both tested microbes.
The antibacterial mechanism of the fabricated Y2O3 can be elucidated by [35][36][37] I. Production of reactive oxygen species (ROS) II.

Release of heavy metal ions
The production of ROS on the surface of prepared nanomaterials has accounted by Karthik et al. [38].

Conclusion
The yttrium oxide nanomaterials were effectively fabricated by the co-precipitation route and their structural, surface morphology, optical, photocatalytic, and antibacterial properties were (pH=9) sample proves to be an appropriate material for wastewater remediation and biomedical applications.