A Study and Comparison of the Preparation of Gadolinium Aluminate Nanoparticles Using γ-Irradiated and Unirradiated Precursors

-e effects of c-irradiation and the application of different precursors on the formation of gadolinium aluminate (GdAlO3) nanoparticles (NPs) have been studied in detail. GdAlO3 NPs were prepared by using different gadolinium-based precursors including gadolinium acetate (Gd(CH3COO)3·4H2O) and gadolinium nitrate (Gd(NO3)3·6H2O), while Al2O3 and Al(NO3)3·9H2O were used as the source of Al. -e preparation of GdAlO3 was carried out by two different methods, solid-state reaction and sol-gel process. To study the effect of c-irradiation, both irradiated and unirradiated Gd(CH3COO)3·4H2O have been tested for the preparation of gadolinium aluminate (GdAlO3). Notably, Gd(CH3COO)3·4H2O did not produce GdAlO3 in both solid-state and sol-gel processes even after optimizing various parameters, including the application of c-irradiation. However, single-phase nanocrystalline GdAlO3 NPs were successfully obtained from the reaction of gadolinium nitrate Gd(NO3)3·6H2O and Al(NO3)3·9H2O by a sol-gel process. -e formation of NPs has been confirmed by X-ray diffraction analysis (XRD) and Fourier-transform infrared (FT-IR) spectroscopy. -e results indicate towards the formation of an orthorhombic perovskite structure of GdAO3 in the Pbnm space group. Transmission electron microscopy (TEM) has been employed for the particle-size analysis, which revealed the formation of spherical-shaped nanoparticles with the size range of 50–70 nm. Surface morphology of the sintered pellet was obtained from high-resolution scanning electron microscopy (HR-SEM). Besides, the effect of irradiation with c-rays on the quality of resultant NPs has also been studied.


Introduction
Among various rare-earth materials, perovskite-type rareearth oxides and aluminates have gained immense interest as a promising class of materials [1].Due to their high chemical stability, these types of materials have been commonly used in various device-based applications including magnetic, optical, and electronic devices [2,3].Particularly, the perovskite aluminates, such as gadolinium aluminum perovskites, are a kind of ceramics based on the Ln 2 O 3 -Al 2 O 3 system (Ln-lanthanide element) offering advantages of longer lifetimes and higher, polarized cross sections with respect to most other oxide matrices.ese materials can be potentially applied for various optical, electronic, and structural applications [4].
Among these aluminates, gadolinium aluminate (GdAlO 3 ) belongs to an important class of rare-earth aluminates from the perovskite family of compounds with the ABO 3 structure [5].
is material at the nanoscale level possesses superior powder properties including better phase homogeneity, good sinter ability, and enhanced physicochemical properties [6].e synthesis of GdAlO 3 is often challenging and usually carried out by solid-state reaction of Gd 2 O 3 and Al 2 O 3 at elevated temperatures [7].However, this technique usually requires extensive mixing of materials or vigorous grinding and high temperatures, which may negatively impact the microstructure of the resultant material [8].erefore, GdAlO 3 has also been prepared using different wet-chemical methods such as, polymerized complex route, combustion synthesis, and sol-gel process [9].Using these methods, the powder characteristics of the resultant material such as its sinter ability and phase purity can be potentially improved [10].
Notably, during the preparation of aluminates, the method, conditions of preparation, and precursors of the reaction play an important role in determining the morphology and physicochemical properties of the resultant material.ese properties can also be affected by doping with certain foreign cations and subjecting to ionizing radiation.Particularly, irradiation of precursors with c-rays may potentially modify one or more properties of the material in a significant way by creating point defects or by increasing the number of nucleation sites for the formation of product on the surface of reacting materials [11].Besides, choosing the right precursor during aluminate synthesis is a key factor in forming aluminate NPs with desirable properties.
In this study, approbation of solid-state reaction and solgel process for the synthesis of GdAlO 3 NPs was performed in order to compare the efficiency of both methods.e solid-state reaction was performed using gadolinium acetate as the precursor, while in the case of sol-gel technique, apart from gadolinium acetate, gadolinium nitrate has also been used.Moreover, in order to study the effect of c-irradiation on the properties of resultant NPs, all the precursors used in this study have been irradiated using 100 and 300 kGy c-irradiation.e properties of NPs prepared from irradiated precursors are also compared with GdAlO 3 NPs obtained from unirradiated precursors.ereafter, a small amount of gelatin was gradually added under continuous stirring, until a hard transparent solid resin formed.is resultant resin was kept in a furnace at 200 °C for 40 min.Finally, the dark gel powder was subjected to further calcination at 1000 °C for 10 h.

Materials and Methods
(2) Using c-Irradiated Gadolinium Acetate.Prior to use, the mixture of Gd(CH 3 COO) 3 with Al 2 O 3 in appropriate ratio is irradiated with 100 or 300 kGy c-rays.For irradiation, samples were encapsulated under vacuum in glass vials and were exposed to successively increase doses of radiation at a constant intensity using Co-60 c-ray cell 220 (Nordion International Inc., Ontario, Canada) at a dose rate of 104 Gy h −1 .e aforementioned mixture (1.3-1.5 g) was dispersed in 10 ml of distilled water.To this mixture, 2 drops of benzyl alcohol and 2 drops of oleic acid were added under stirring.ereafter, a small amount of gelatin was added gradually with continuous stirring till a hard transparent gel formed.
e gel was left in the furnace at 100 °C for 2 h.Finally, the gel powder was further calcined at various temperatures, including at 800 °C for 24 h, 850 °C for 24 h, and finally at 1000 °C for 3 h in order to find the best conditions to synthesize GdAlO 3 nanocrystals by this method.
(3) Using c-Irradiated Gadolinium Nitrate as Precursor.For this purpose, the gel was prepared using stoichiometric amounts of 100 and 300 kGy c-irradiated Gd(NO 3 ) 3 •6H 2 O and 100 kGy Al(NO 3 ) 3 •9H 2 O. First, ∼2.25 g (100 or 300) kGy Gd(NO 3 ) 3 •6H 2 O was dispersed in 50 ml of 0.2 M acetic acid at 65 °C.Separately, ∼1.87 g 100 kGy Al(NO 3 ) 3 •9H 2 O was dispersed in 50 ml of distilled water at 65 °C.Subsequently, this solution is added to the Gd 3+ solution.e resulting mixture was stirred for 1 hour at 65 °C.ereafter, 2 ml of 1,2-ethanediol was added to the reaction solution as a complexing agent.e solution was concentrated by slow evaporation at the same temperature under stirring for one day, and the Gd-Al-O acetate-nitrate-glycolate sol turned into a white transparent gel.e gel was left to dry for few days.It was then oven-dried at 200 °C for 1 h.e resultant gel powder was preheated for 3 h at 800 °C in air.Finally, the powder was additionally sintered in air for 10 h at 1000 °C.

Characterization.
XRD diffractograms were collected on a Altima IV (Rigaku, Tokyo, Japan) X-ray powder diffractometer using Cu-Kα radiation (λ � 1.5418 Å).Transmission electron microscopy (TEM) was performed on a JEOL (Peabody, MA, USA) JEM 1101 microscope.e samples for TEM were prepared by placing a drop of the primary sample on a holy carbon copper grid and then dried 2 Advances in Materials Science and Engineering for 6 h at 80 °C in an oven.e SEM images were recorded on a scanning electron microscope (NNL200; FEI).A Perki-nElmer spectrum BX FT-IR system was used to measure FT-IR spectra.

Results and Discussion
3.1.Solid-State Reaction.e solid-state reaction is one of the most common methods applied for the preparation of rare-earth polycrystalline material including GdAlO 3 .ese materials are prepared by using solid gadolinium acetate or gadolinium nitrate as precursors.ese solid starting materials do not react at mild temperatures and often require very high temperatures (∼1500 °C).During these reactions, repeated grinding and heating are also applied to accelerate the diffusion of atoms between raw materials.During this study, both irradiated and unirradiated gadolinium acetate and Al 2 O 3 were calcined at different temperatures in the range of 600 °C to 1000 °C.To stabilize the resultant NPs, two types of surfactants were used including oleic acid and SDS.
Initially, to confirm the formation of GdAlO 3 NPs from various solid-state reactions of both irradiated and unirradiated gadolinium acetate precursor, FT-IR spectra of all the products were recorded (data are not provided here).Typically, perovskite-type rare-earth aluminates with ABO 3 structures consist of some characteristic bands in the lowfrequency region of 400-1000 cm −1 [12].However, all the recorded spectra did not show these characteristic bands, which are expected to appear at 670 and 460 cm −1 in the case of GdAlO 3 NPs [8].Even after optimizing various reaction parameters, including varying the concentration of the reactants (0.1Gd/0.9Al, 0.3Gd/0.7Al,and 0.5Gd/0.5Al),calcination temperature, and type of surfactants, the reactions did not succeed; this was clearly indicated by the IR spectra.
is was further confirmed by XRD analysis, as in all the products obtained from different reactions, the XRD pattern indicated the presence of physical mixture of Gd 2 O 3 as the major phase and Al 2 O 3 besides the presence of some unknown phases (Figure 1).Indeed, after irradiation of gadolinium acetate precursor with c-rays, no significant effect of c-irradiation was observed where all these patterns reveal that the major phase is Gd 2 O 3 besides Al 2 O 3 and unknown minor phases.

Sol-Gel
Process.Among various wet-chemical methods, including polymerized complex route, combustion synthesis, and homogeneous precipitation methods, the sol-gel process is widely applied as it facilitates proper chemical interactions among reacting species of the initial mixture which leads to the evolution of a homogeneous solid-state structure at the atomic level [13].In this study, GdAlO 3 NPs are prepared using both irradiated and unirradiated gadolinium acetate and Al 2 O 3 or Al(NO 3 ) 2 •9H 2 O precursors.Initially, IR analysis was performed to evaluate the formation of GdAlO 3 NPs.e results reveal that using Al 2 O 3 as the source of Al 3+ with both irradiated and unirradiated gadolinium acetate precursor did not yield GdAlO 3 NPs.
is is clearly indicated by the absence of characteristic peaks in the range of 400 to 800 cm −1 for the perovskite-type GdAlO 3 NPs, whereas, the application of Al(NO 3 ) 2 •9H 2 O with both irradiated and unirradiated gadolinium acetate precursors yielded GdAlO 3 NPs. is is confirmed by the IR spectra (spectra are not provided here) of resultant NPs which displayed characteristic M-O vibration peaks in the range of 850-400 cm −1 .IR analysis also revealed that the irradiation does not have significant effect on the formation of resultant NPs as the IR spectra of GdAlO 3 NPs prepared using both irradiated and unirradiated gadolinium acetate precursors are qualitatively identical.e FT-IR results were found to be consistent with crystallization process observed by the XRD measurements.
e absence of the characteristic diffraction peaks of GdAlO 3 in the XRD pattern shown in Figure 2(a) clearly demonstrates that the reaction between unirradiated gadolinium acetate and Al 2 O 3 did not result in the formation of GdAlO 3 NPs, whereas, the product obtained from the reaction of unirradiated gadolinium acetate and Al(NO 3 ) 3 •9H 2 O produced GdAlO 3 NPs which is confirmed by the X-ray diffraction patterns as shown in the Figure 2(b).e XRD was measured after the product was sintered for 10 h at 1000 °C.
e XRD result revealed that, after sintering, fully crystalline single-phase GdAlO 3 ceramic was formed.e diffraction pattern indicates the presence of perovskite crystal structure of GdAlO 3 (JCPD card # 01-073-9322) [12].However, along with perovskite gadolinium aluminates, the formation of the impurity phases is evident.e X-ray line broadening method was used to calculate the crystallite size of the prepared GdAlO 3 nanopowder using the Scherrer formula.
e synthesized GdAlO 3 powder was found to be nanocrystalline in nature consisting of a crystallite size of 22 nm.
To study the effect of irradiation on the precursors as well as on the quality of resultant GdAlO 3 NPs, the reactant Gd(CH 3 COO) 3 was irradiated with 100 and 300 kGy c-rays, while Al(NO 3 ) 3 •9H 2 O was irradiated with only 100 kGy c-rays.Notably, the reaction of both irradiated (100 kGy) and unirradiated Gd(CH 3 COO) 3 with Al 2 O 3 has produced similar results, i.e., did not yield GdAlO 3 NPs.Even the increased intensity (300 kGy) of the irradiation did not have any effect on the formation of the product.is was further confirmed by measuring the XRD pattern of the products obtained from both 100 and 300 kGy irradiated precursors, which did not exhibit any difference in their diffraction pattern (data not shown here).
erefore, to further study the effect of irradiation, another precursor Gd(NO 3 ) 3 •6H 2 O was used, which was also irradiated with both 100 and 300 kGy c-rays.In this case, 100 kGy irradiated Al(NO 3 ) 3 •9H 2 O was used as a source of Al ions.ese results were found to be consistent with the crystallization process observed by the XRD measurements (Figure 3).According to this analysis, both gels produced fully crystalline phase GdAlO 3 ceramic, where all diffraction patterns in these XRD could be attributed to the perovskite crystal structure of GdAlO 3 .All reflections in the diffraction pattern as shown in Figure 2 are in good agreement with ICDD 46-0395, which confirms the formation of GdAlO 3 phase with orthorhombic perovskite structure [9].e average size of the crystallites calculated from the XRD using the Scherrer formula was Advances in Materials Science and Engineering  .SEM images depict that individual particles seemed to exhibit the shape of microsized volumetric plate-like crystals which were partially fused to form soft agglomerates.Besides, the plate-like grains coexist with spherically shaped particles.ese images also reveal that increasing the c-ray doses from 100 to 300 kGy leads to the production of smaller size particles.
is effect is clearly visible in the TEM images of the product obtained from both 100 and 300 kGy precursors.
e TEM images reveal that the spherical nanoparticles with the size between 50 and 70 nm were obtained in case of 300 kGy Gd(NO 3 ) 3 •6H 2 O with 100 kGy Al(NO 3 ) 3 •9H 2 O as shown in Figures 4(c) and 4(d), while the 100 kGy irradiated sample produced slightly large nanoparticles with size between 70 and 100 nm.e difference between the particle sizes obtained from TEM results and XRD results is due to the serious agglomeration of crystallites.e particle size seemed to be higher than that corresponding to crystallite average sizes, which means that either the particles of the powder are polycrystalline, or actually, the so-called "particles" are not the ultimate structural units, but rather they are agglomerates consisting of 2-3 single crystal particles very hard to be dispersed.erefore, the irradiation by c-ray can be considered as a new factor to prepare nanocrystalline GdAlO 3 NPs where the particles size of product can be optimized.

Conclusions
In this work, single-phase, perovskite-type GdAlO 3 NPs were prepared using c-rays-irradiated Gd(NO 3 ) 3 •6H 2 O and Al(NO 3 ) 3 •9H 2 O precursors in a sol-gel process.e surface morphology of the GdAlO 3 phase reveals spherical particles of smaller size coexisting with agglomerates.
e TEM image shows the particles sizes of about 30-50 nm.During this study, it was revealed that both the precursor and irradiation with c-rays have significant effect on the formation and quality of the resultant NPs.Since, the reaction with irradiated Gd(NO 3 ) 3 •6H 2 O with Al 2 O 3 did not produce GdAlO 3, whereas irradiated Gd(NO 3 ) 3 •6H 2 O and Al(NO 3 ) 3 •9H 2 O precursors have successfully produced GdAlO 3 NPs.Moreover, with increasing the intensity of the irradiation, the size of the NPs also decreased.

Data Availability
e spectroscopic and microscopic data used to support the findings of this study are included in the article.