Yttrium Oxide Nanoparticles Prepared by Heat Treatment of Cathodically Grown Yttrium Hydroxide

An easy two-step synthetic route is reported for the manufacture of yttrium oxide nanoparticles utilizing aqueous yttrium nitrate solution. In the first step, yttrium hydroxide precursor was grown on stainless steel electrode using a simple cathodic electrodeposition at room temperature. The subsequent second step includes the thermal decomposition of yttrium hydroxide powder at different temperatures for two hours. The synthesized products were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetery (DSC), FT-IR and Raman spectroscopy. Results showed that the as-deposited Y(OH)3 is composed of nanoparticles with grain size of approximately 40–50 nm. Cubic-structured Y2O3 phase with a porous morphology was finally formed when temperature was raised to 600◦C. Results suggested that the final oxide nanoparticles are crystalline and consist only of yttrium oxide phase forming agglomerates of many primary particles with average diameter around 30 nm.


Introduction
Nanosized metal oxides, in a variety of morphologies (such as particles, spheres, rods, and sheets), have attracted a great deal of attention.This is due to their novel physicochemical properties, associated with their nanometer-scale size and excellent technological applications.Yttrium oxide, as an important member among rare-earth compounds, has been actively studied in the recent years.It is one of the most promising elements for the fabrication of optoelectronics devices and chemical catalysis.Y 2 O 3 can be used as highefficient additives and functional composite materials like yttria stabilized zirconia films [1][2][3][4].This material could be synthesized via several methods, including cathodic electrodeposition [5][6][7][8][9], gas-phase condensation [10], precipitation [11], sol-gel [12], pyrolysis [13], solvothermal [14], and hydrothermal synthesis [15][16][17].Among these methods, the easy and clean cathodic electrodeposition method has been proven to be a versatile and controllable technique for the preparation of nanostructured materials.
Preparation of yttrium oxide by cathodic electrodeposition is simple and inexpensive [9].In this method yttrium oxide is synthesized by two steps.Firstly, yttrium hydroxide species were grown cathodically on the steel substrate from a bath containing yttrium nitrate solution.The subsequent second step includes the thermal decomposition of yttrium hydroxide to yttrium oxide.In the first step, formation of yttrium hydroxide intermediates is inevitable.These are mainly composed of different yttrium oxide nitrate hydroxides (such as Y 4 O(OH) 9 (NO 3 ), Y 2 (OH) 5.14 (NO 3 ) 0.86 •H 2 O, and hexagonal Y(OH) 3 ) [16].It seems that electrochemical precipitation process of yttrium hydroxide in nitrate salt solution is greatly influenced by nitrate ions.It could be expected that changes of the structural/morphological properties of the prepared yttrium hydroxide precursor may give rise to improved properties and evaluate the important structural and morphological characteristics and enlarge the application field of yttrium oxide.These properties can be tuned readily via electrodeposition parameters.In this relation, however, relatively few reports are available in the published literature.As far as the authors know, nanoparticles formed in this way have not been recorded earlier in the literature.Thus, the objective of the present study was to investigate the feasibility of using electrochemical parameters for the preparation and modifications of yttrium hydroxide precursor and synthesis of Y 2 O 3 nanoparticles.The latter is obtained by heat treatment of the hydroxide precursor, which is cathodically grown on steel substrate in an aqueous yttrium nitrate solution at ambient temperature.

Experimental Procedure
Electrodeposition experiments were performed using 0.01 M Y(NO 3 ) 3 •6H 2 O (Merck) bath at room temperature.The electrochemical cell includes a cathodic steel substrate centered between two parallel graphite counter electrodes.Prior to each deposition, steel substrates have given an galvanostatically electropolishing treatment at a current density of 20 A (0.2 A cm −2 ) for 5 min in a bath (70 • C) containing 50 vol.%phosphoric acid, 25 vol.%sulfuric acid, and balanced deionized water.All electrodeposition experiments were performed galvanostatically at a cathode current density of 1 mA cm −2 at room temperature.After drying, the as-deposited samples were scraped from the steel electrodes for further characterization and thermal analysis.Crystal structures of the samples were determined by X-ray diffraction (XRD) with a diffractometer (Phillips, PW-1800) using monochromatized Cu Kα radiation at a scanning rate of 0.5 • min −1 .Thermogravimetric analysis (TGA) and thermal behavior of the as-deposited sample were investigated by means of differential scanning calorimeter (DSC, STA-1500).Carbon, nitrogen, and hydrogen contents were determined by CHN analysis.
The samples were heated in air between room temperature and 600 • C at a heating rate of 10 • C min −1 .The morphology of the samples was examined using a scanning electron microscopy (Philips 515).Raman and FT-IR spectra were obtained by means of a LAB RAM 800 and a Perkin-Elmer spectrometer (RX FT-IR), respectively.

Results and Discussion
Controlling of the reaction rate and the speed of the growth process through experimental parameters including current density, bath temperature, and reaction medium are key factors in designing new products with desired crystallinity and morphologies [24][25][26][27].Preparative parameters such as nature of precipitating agent, nature of rare-earth supplier, and the pH of the precipitation medium play major roles in physic-chemical characteristics of the precursor [28].It could be noted that an important aspect in the cathodic electrodeposition process of Y(OH) 3 is the reduction of an oxygen precursor at the electrode/solution interface [29].This would control the growth rate and significantly affects the structural and morphological characteristics of the obtained products.Three kinds of oxygen precursor have been reported up to now, that is, nitrate ions, dissolved molecular oxygen, and hydrogen peroxide.Among them, nitrate ions-based oxygen precursor has received considerable interests [30].The general scheme of electrodeposition of Y(OH) 3 from aqueous yttrium nitrate solution could be supposed as follows [31,32]: ( Simultaneously, oxidation of water occurs at the anode: Cathodic electroreduction of nitrates ions on the electrode surface liberates hydroxide ions known as electrogeneration of base.Then, yttrium ions precipitate with the hydroxyl anions, resulting in the formation of metal hydroxide and yttrium hydroxy-nitrate compounds [33].Reduction of nitrate ions at the electrode/solution interface controls the growth rate affecting the crystallinity and morphology of the obtained products significantly.Probably, different polarity and hydrogen-bonding ability of the solvent may modify the formation and crystal growth kinetics of the formed nanoparticles.On the other side, the morphological and structural properties of the oxide material could be considered in terms of growth kinetics prevailing during phase conversion of the precursor in the course of annealing.This is due to the fact that some important physicochemical phenomena like structural transformation, volume expansion, release of chemisorbed and physisorbed water are accompanied with oxidative heat treatment process [34].The appearance and speed of these phenomena could be described as important factors affecting the properties of oxide nanoparticles.

Structural and Morphological
Characterization.X-ray diffraction patterns have shown that the as-deposited sample was poorly crystallized (Figure 1(a)).This could be explained by crystal growth kinetics under lower bath temperature and the accompanied intercalation of nitrate ions.The existence of nitrate ions in the as-deposited yttrium hydroxide is confirmed by EDAX analysis (Figure 2(e)).XRD patterns of the sample annealed at 600 • C can be readily indexed to a pure cubic phase of Y 2 O 3 (Figure 1(e)) [35].As could be seen in Figure 1, during thermal procedure, Y(OH) 3 transformed to various crystallographic patterns at 200, 360, 450, and 600 • C (Figures 1(b)-1(d)).These compounds mainly consist of different yttrium oxide nitrate hydroxides such as Y 4 O(OH) 9 (NO 3 ), Y 2 (OH) 5.14 (NO 3 ) 0.86 •H 2 O, and hexagonal Y(OH) 3 [24].Formation and coexistence of such species are strongly depending on experimental conditions such as annealing temperature and pH of the electrolyte solution.It is reported that the content of hydroxyl group in these compounds may influence the formation temperature of Y 2 O 3 .This temperature is around 530 • C under our experimental condition as confirmed by DSC analysis (Figure 3).
Figure 2 shows SEM images of yttrium hydroxide and that of the annealed sample.EDAX diagram (Figure 2(e)) shows a small amount (approximately 2.5%) of nitrogen content resulting from accompanied intercalation of nitrate ions in the as-deposited precursor sample.The latter consists of agglomerated particles with diameters around 20-30 nm (Figures 2(a particulates.In comparison to Y(OH) 3 precursor, it could be clearly seen from Figure 2(c) that the particle size of yttrium oxide is drastically reduced and a more smooth surface was obtained (Figure 2(c)).However, an enlarged SEM image revealed that the oxide film still consists of agglomerated smaller nanoparticles with an average diameters of about 30-40 nm (Figure 2(d)).This could be related to the phenomena like dehydration and release of N-O species which occurs with phase transformations during calcination.These phenomena may cause particles agglomeration.[9,24].The endothermic peak at around 450 • C confirms dehydration of structural water from YOOH phase [9,36].Decomposition profile of yttrium hydroxide-nitrate compounds is fairly complex with overlapping steps.It is worth noting that NO 3 − ions and traces of carbonate ions are also present in our samples due to an incomplete precursor wash or by atmospheric CO 2 absorption on the gels surface during the precipitation stage [35].According to [27], ion chromatography analysis indicated that the NO 3 − content in intermediate products at different calcination steps kept increasing when calcination temperature was lower than 450 • C, while it decreased dramatically during 450-530

Thermal and Spectral Analysis
] takes place at temperatures above 500 • C [33].Because of presence of these compounds in our hydroxide samples (as justified by EDAX), this result could be consistent with our experimental data showing decomposition temperature of these compounds  As could be seen from TG diagram, approximately 12% of the total weight loss corresponds to physically adsorbed water.It can be concluded that the weight loss of 22.5% below 450 • C corresponds to the evaporation of physically and chemically adsorbed water and OH − departure, while the weight loss at temperatures higher than 450 • C (in order of 13%) is associated with the release of N-O species.
CHN analysis of as-deposited hydroxide and its annealed sample at 600 • C was carried out from 25 to 600 • C. The amount of nitrogen in the hydroxide sample did not exceed 2.72% as shown in CHN analysis result in Table 1.The nitrogen content of the oxide sample was found to be lower than the detection limit of the instrument (i.e., ≤0.03%).The intercalation of nitrate ions in electrochemical step is consistent with results of XRD and TG analysis of the hydroxide sample.Results of CHN and EDAX analysis of the latter show the existence of approximately equal amount of nitrogen (Figure 2(e)).Obviously, structural changes of the oxide material, as visible in XRD, could be attributed to the presence and effects of nitrogen during heat treatment process.
FT-IR spectra of the as-prepared Y(OH) 3 and that of the annealed one at 600 • C are shown in Figure 4. IR spectra of our compounds show peaks between 3100 and 3800 cm −1 , which may be due to the O-H stretching of water molecule [37].The peaks of OH stretching vibration appears to be around 3440 cm −1 and vibrations peaks due to intercalated/adsorbed nitrate ions at about 1385 cm −1 [38].Another peak observed between 1450 and 1670 cm −1 corresponds to the O-H bending of water molecules.The peaks at about 1527 cm −1 and 1407 cm −1 can be attributed to the split antisymmetrical stretching vibration of carbonate and nitrate group, respectively [39].After the precursor was calcined at 600 • C for 2 h, new bands at 543, 455, and 428 cm −1 appeared (inside graph of Figure 4) [40].
The frequencies below 600 cm −1 could be attributed to Y-O stretching mode of Y 2 O 3 structure.Figure 5 displays the Stokes Raman spectrum (λ ex = 532 nm) of Y 2 O 3 nanoparticles sintered at 600 • C. It shows an intense peak at 378 cm −1 and a weak peak around 410 cm −1 .These results are also in agreement with recent reports on Raman spectrum of Y 2 O 3 [41,42].

Conclusion
A two-step synthetic route was employed for the synthesis of yttrium oxide nanoparticles under mild conditions.Firstly, Y(OH) 3 precursor was cathodically grown on steel substrate in nitrate medium at RT.This is followed by heat treatment of the precursor in air for two hours.The properties of the yttrium hydroxide are strongly influenced by the electrochemical parameters and experimental conditions, which could be tuned to get the desired oxide product after calcination process.During this process, dehydration and structural changes occur, and finally crystalline Y 2 O 3 could be obtained at 600 • C. The resulting oxide material exhibits nanoparticles with an average particle size of 30 nm. Results of this work showed that adequate adjusting of the electrochemical parameters and bath conditions induce important physicochemical changes of the hydroxide precursor, which translate into important physicochemical changes of the Y 2 O 3 nanoparticles with desired properties.

Figure 3
presents results of thermal DSC and related TG analysis of the as-deposited Y(OH) 3 powder.Three endothermic peaks are observed at 95, 350, and 450 • C. The broad endothermic peak around 95 • C is related to dehydration of free and physically absorbed molecular water.The next endothermic peak at 350 • C is supposed to be related with the first dehydration of chemisorbed and combined water from Y(OH) 3 and α-Y(OH) 3

Figure 4 :
Figure 4: FT-IR spectra of the as-prepared Y(OH) 3 and annealed at 600 • C.

Figure 5 :
Figure 5: Raman spectra of Y 2 O 3 annealed in air at 600 • C.

Table 1 :
CHN analysis results of the as-deposited sample and annealed at 600 • C.