Recently, many researches have been conducted on improving the quality and use of materials for meeting the new requirement. Oxide Ceramics are extraordinary materials (especially non-Centrosymmetric ones) that are exploited in many scientific and industrial fields such as signal processing (electronics, telecommunications, etc.) where electrochemical impedances and electrical resonance phenomena commonly used in many technologies [1-4], and in sensors and actuators field [5-10] (by capturing or transforming physical quantities) where piezoelectric materials have been increasingly used. In addition, nano-crystalline ceramic-oxides have been also studied for their quantum size effects [11-15].
Our particular interest to study the ABO3 ceramic materials is raised in the contentious literature [16-20]. For example, Sr-doped LaYO3 has been introduced as potential anode materials for solid oxide fuel cells [21,22]. These materials have catalytic properties, especially the perovskite-type oxides [23-28], which are also known to show an excellent proton conductivity at intermediate temperatures [29,30].
Both GdYO3 and YbYO3 have high thermal and chemical stability and high luminescence efficiency . As a very good luminescent material, the mixed oxide GdYO3 ceramic doped with Eu3+ ions has been used to provide red light emissions in modern optoelectronic devices [32,33]. The YbYO3 doped ceramics are considered among the most promising host transparent materials for laser applications. Yb3+: (LaxY1–x)2O3 nano-particles could be, in particular, a good gain medium for ytterbium high power pulse lasers as reported in [34,35].
The majority of scientific publications focused on the synthesis and characterization of oxides nano-crystals, their functional properties and possible applications in different fields. In this way, several chemical approaches have also been applied to obtain nano-crystals and control particle sizes and morphologies [36-41] Hydrothermal synthesis, for example, is often used due to its simplicity, allowing the control of particle shape and particle size distributions by making easy changes in the experimental conditions [42,43]. However, in wider experimental conditions, the co-precipitation method followed by a thermal treatment has also been successfully used for the fabrication and modification of nano-sized oxides [44,45].
In the present paper, therefore, two kinds of pure nano-powders Gd0.745Y1.255O3 and Yb1.4Y0.6O3 of mixed rare-earth-yttrium sesqui-oxides (diluted magnetics [46,47]), were successfully synthesized using the co-precipitation method. These mixed oxides nano-powders have been synthesized by capping its hydroxides to form precipitates and maintain the pH value. Addedly, the influence of the thermal treatment on particle sizes, crystallinity, morphological behaviour and optical properties was also investigated. This method offered advantages of simplicity and efficiency.
In general, any mixed oxide of yttrium with other rare earth element corresponding to formula Y2-xRExO3 , the rare earth element is used as a dopant in combination with yttrium oxide Y2O3 which has been considered as one of the most promising compounds for many applications such as optical amplifiers [48,49], cathode ray tubes (CRT), plasma display panels (PDP), high-temperature protective coatings, and it is also used in the manufacture of colored fluorescent lamps (yttrium oxide is a red emitting material under UV) [50-52]. The ability to give its particular physical properties (like luminescent properties) to rare earth elements and its high crystallographic stability, the yttrium oxide has been used as a host material.
Furthermore, the mixed rare earth-yttrium oxides Y2-xRExO3 are an important group of diluted magnetic semiconductors [46,47], especially when prepared in a nano-structured form. Particularly, the ytterbium–yttrium mixed oxides Y2-xYbxO3 are potential emitter materials for thermo-photovoltaic energy converters  and also promising ceramic lasers [54-56] due to the effect of Yb3+ ions (Yb3+ is a very attractive rare earth ion in the lanthanide series with unfilled f shells). On the other hand, the gadolinium-yttrium mixed oxides Y2-xGdxO3 (x = 0.10, 0.18, 0.41, 0.74 and 1.26) are also a special class of semi-magnetics semiconductors. The magnetic properties of all of them were studied in .
MATERIALS AND METHODS
Following a typical synthesis procedure for the preparation of La1-xCaxAlO3 (0≤ x≤ 0.6) nano-powders presented by Malika Diafi , a co-precipitation method was adopted to prepare Gd0.745Y1.255O3 and Yb1.4Y0.6O3 nano-powders following the same steps. First, all chemical reagents were used without any further purification. 0.014 mol of Gd(NO3)3.6H2O and 0.016 mol of Y(NO3)3.6H2O were dissolved separately in 25 ml of distilled water to form transparent solutions after that they were mixed. Under magnetic stirring, NaOH solution (12N) was slowly added drop by drop to the mixture to adjust pH values and obtain a precipitate. The precipitate was filtered, washed with distilled water several times and dried at 90°C for 24h. Finally, the product obtained was calcined at different temperatures 600 °C, 800 °C and 1000°C in a tube oven for 4 h to form nano-powders.
Following the same steps and using Yb(NO3)3.5H2O precursor (0.014 mol), Yb1.4Y0.6O3 nano-powder was also synthesized.
Powder X-ray diffraction analysis (XRD) of Gd0.745Y1.255O3 and Yb1.4Y0.6O3 was carried out at room temperature on a Philips PW 3830 diffractometer employing Co Kα (λ = 1.7889 Å) radiation with 0.016 steps. Peak positions and full width at half maximum (FWHM) were determined in the range (2θ) 15–75° to the recorded data, using the peak fitting module of the Origin program.
In addition, SEM images of the products were obtained using a scanning electron microscope (Tescan VEGA3 model), and the mean particle size was determined by Scherrer’s formula :
D = Kλ/βcosθ (1)
Where θ is the Bragg’s angle of X-ray diffraction, λ is the wavelength of X-ray (1.7889 Å). K is a shape factor taken as 0.9, and β is full width at half maximum (FWHM).
The different nano-powders of our compounds calcined at 600, 800, and 1000°C were dispersed in distilled water with concentration (0.2g/l) to form suspension solutions, which were then utilized for the measurement of optical properties of absorption by an ultraviolet-visible spectrophotometer with optima SP-3000nano using quartz cells. All UV-Vis spectra were registered in the wavelength range from 200 nm to 500 nm at room temperature.
RESULTS AND DISCUSSION
XRD patterns of Gd0.745Y1.255O3 and Yb1.4Y0.6O3 nano-powders synthesized via Co-precipitation method and calcined at different temperatures (600 °C, 800 °C and 1000°C) are shown in Fig. 1 and Fig. 2, respectively.
Without calcination, XRD patterns of Gd0.745Y1.255O3 and Yb1.4Y0.6O3 show amorphous phases. Contrarily, both Gd0.745Y1.255O3 and Yb1.4Y0.6O3 powders which were calcined at 600, 800 and 1000 °C for 4 h can be indexed as pure-phases that are in good agreement with the data of ICSD Card No.73659  and ICSD Card No. 84136  belonging to the space group I a -3 (parameter a = 10.703Å and a = 10.4807Å), respectively.
In these experiments, the mixed oxides Gd0.745Y1.255O3 and Yb1.4Y0.6O3 were synthesized coincidentally instead of GdYO3 and YbYO3. It’s worth noting that with the co-precipitation method Gd0.745Y1.255O3 and Yb1.4Y0.6O3 nano-powders could be synthesized using NaOH solution as a precipitate agent to form the hydroxide precipitations (amorphous phases):
Gd(NO3)3 + Y(NO3)3 + 6OH- → Gd(OH)3 + Y(OH)3 + 6NO3- (this work) (2)
Yb(NO3)3 + Y(NO3)3 + 6OH- → Yb(OH)3 + Y(OH)3 + 6NO3- (this work) (3)
In this work, NaOH solution was added drop by drop to the mixture to adjust pH values to pH ≤ 9.5 to avoid the formation of [Gd(OH)6]3–, [Yb(OH)6]3– and [Y(OH)6]3– ions before the precipitation of Gd(OH)3, Yb(OH)3 and Y(OH)3.
Zeheng Yang et al. , discussed the controlling molar ratio of NaOH in the synthesis of CuO nanoribbons. They proved experimentally that the molar ratio of NaOH to Cu(NO3)2 is an important parameter and that at high concentration of NaOH aqueous solutions, the hydroxide ions are first formed.
Moreover, Zhiwu Chen et al. in  reported that in the synthesis of bismuth ferrite powders, if ethanol is added to water, the surface tension of the solvent decreases and the hydroxide precipitations form more quickly and thus have better dispersibility and are more easily dehydrated to form the powders.
In the thermal treatment (calcination) at various temperatures, Fig. 1 shows characteristic peaks of pure Gd0.745Y1.255O3 single crystallographic structure. It can be seen that the XRD patterns of nano-powders at 600°C, 800°C and 1000°C were almost the same, which suggests that the crystallization of Gd0.745Y1.255O3 structure could be at temperature under 600°C. In the case of Yb1.4Y0.6O3 (Fig. 2), XRD patterns of nano-powders at 600°C, 800°C and 1000°C are also identical. Specially, two peaks in 2θ = 44.99° and 52.42° for Yb1.4Y0.6O3 structure are clearly observed in the XRD pattern without calcination. Therefore, it can be confirmed that this is the first crystallization of the Yb1.4Y0.6O3 phase.
It is evident that the width of the diffraction peaks of all precedent nano-powders indicates that the crystallite sizes are very small. As shown in Table 1, the average crystallite sizes calculated using the Debye-Scherrer formula for the Gd0.745Y1.255O3 and Yb1.4Y0.6O3 prepared samples increased as thermal treatment temperatures and crystallinity increased. This observation is in good agreement with literature [61-64]. In , Juliana B. Silva et al. stated that “the increase in peak intensities with temperature is due to the increase in crystallinity and particle size during the calcination process”.
A similar behavior is in fact found for Gd0.745Y1.255O3 and Yb1.4Y0.6O3. From Fig. 3, in the case of Gd0.745Y1.255O3 and around 2θ = 33.69°, the peak intensities increased with the temperatures of thermal treatment and crystallinity, which was also in accordance with the obtained values for crystallite size (increased from 38 nm to 78 nm).
There are many factors that influence the quality of the Gd0.745Y1.255O3 and Yb1.4Y0.6O3 ceramic nano-powders, such as powder agglomeration level, the temperature and the calcination time and especially the final particle size. According to the SEM images of prepared samples (Gd0.745Y1.255O3 and Yb1.4Y0.6O3) with thermal treatment, which are shown comparatively in Fig. 4, the nano-crystallites were not clearly visible in these images due to the SEMs resolution limit as well as the agglomeration of nano-powders.
However, a higher temperature motivated the grain growth as shown in image (D) (see Fig. 4). N. M. Al-Hada et al.  suggested that “as the temperature increases, several neighboring particles cling to each other, enlarging the particle size by melting their surfaces at higher temperature”. This proposal has been previously discussed in literature [65,66].
UV–Vis absorption spectra of calcined Gd0.745Y1.255O3 and Yb1.4Y0.6O3 nano-powders are shown in Fig. 5 (a) and 6 (a), respectively. For all samples, as is observed clearly in UV spectra, strong absorption bands were detected at low wavelengths which corresponding to the band-to-band transition of the different phases. The obtained values of exciton energy Eex, shown in Table 2, were calculated using the equation:
Eex = hc/λabs (4)
Where, h is Planck’s constant, c the speed of light, and λabs is the wavelength of the absorption (nm).
On the other hand, it should be also noted that the absorbance decreases with the increasing wavelength, as reported in , this indicates the presence of an optical band-gap. The optical band-gap of Gd0.745Y1.255O3 and Yb1.4Y0.6O3 nano-powders were obtained using the Tauc plot method  as presented in Fig. 5 (b-d) and Fig 6 (b-d), respectively. The Tauc plot method based on the graph of (αhν)2 as a function of energy hν (eV), where α is the absorption coefficient and ν is the frequency.
For the Yb1.4Y0.6O3, the values of the band-gap energy for the calcined nano-powders decrease with the temperature of calcination (from 4.22 to 3.95 eV) and consequently, with the increase in the particle size. These results are in good agreement with those reported for TiO2 nanoparticles by S. Sharma et al. in , who added that: “Smaller crystallite size will have a larger band-gap and larger crystallite size will have a smaller band-gap”.
In contrast, for Gd0.745Y1.255O3, it can be seen that isn’t the same behavior. The value of Eg was calculated to be ~3.61, ~ 3.82, and ~3.80 eV for the samples calcined at 600, 800, and 1000°C, respectively. Actually, the larger band-gap was observed for the nano-powder calcined at 800°C. This comportment may be attributed to the decreasing content of gadolinium in the mixed oxide phases, comparison to the Ytterbium content.
From these results and as a comparison, it should be noted that the Eg values of different nano-powders for Yb1.4Y0.6O3 (with smaller particle sizes) are larger than those for Gd0.745Y1.255O3. In the case of yttrium oxide nanoparticles, and based on photoluminescence and UV–vis results as is presented in , the influence of the particle size on the optical properties was also confirmed.
In this paper, pure Gd0.745Y1.255O3 and Yb1.4Y0.6O3 nano-powders of mixed rare-earth-yttrium sesquioxides (diluted magnetics) were successfully synthesized using the co-precipitation method with thermal treatment (calcination). The nano-powders were characterized by X-ray powder diffraction; their morphologies were analyzed by scanning electron microscope, their average sizes were calculated using the Scherrer formula, and their optical properties were studied using UV-Vis measurements and a Tauc plot calculation. We confirmed the good crystallinity of the Gd0.745Y1.255O3 and Yb1.4Y0.6O3 pure phases at different temperatures of thermal treatment (600°C, 800°C and 1000°C) and nano-metric particle sizes.
Under various pH values, the experiments were carried out to reveal the role of pH on the “co-precipitation” synthesis of Gd0.745Y1.255O3 and Yb1.4Y0.6O3 nano-powders. The pH values were adjusted by adding NaOH which, as a complexing agent, has a key role in the precipitation process.
The X-ray diffraction and the particle-size distribution demonstrated that, at different calcination temperatures 600°C, 800°C and 1000°C, particle size and cristallinity increased slightly as the annealing temperature increased. They were found to increase from ~ 42 nm at 600 °C to ~ 100 nm at 1000 °C for Gd0.745Y1.255O3 and from ~ 13 nm at 600°C to ~ 50 nm at 1000°C for Yb1.4Y0.6O3 which proved that higher temperatures motivated the grain growth. SEM analyses have also confirmed the growth and agglomeration of Gd0.745Y1.255O3 and Yb1.4Y0.6O3 nano-powders.
On the other hand, the band-gap of Yb1.4Y0.6O3 nano-powders were found to be decreasing from 4.22 to 3.95 eV when the particle sizes increasing, but for Gd0.745Y1.255O3 were found to be ~3.61, ~ 3.82, and ~3.80 eV for the samples calcined at 600, 800, and 1000°C, respectively. This confirmed the influence of the particle size on the optical properties.
From these results, it can be concluded that the formation of Gd0.745Y1.255O3 and Yb1.4Y0.6O3 nano-powders using a typical co-precipitation method has advantages of simplicity and efficiency, and also proves that thermal calcination is very effective for increasing cristallinity but with an increase in particle sizes which influence on the optical properties.
The authors thank Pr. A. Guibadj for his helpful suggestions and support during the experiments.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding this article.