Synthesis and characterization of Gd2Zr2O7 defect-fluorite oxide nanoparticles via a homogeneous precipitation-solvothermal method

College of Physical Science and Technology R. China. E-mail: qijianqi@scu.edu.cn; lutie Key Laboratory of High Energy Density Ph University, Chengdu 610064, Sichuan, P. R. Key Laboratory of Radiation Physics and Sichuan University, Chengdu 610064, Sichu Department of Chemistry, Washington State USA The Alexandra Navrotsky Institute for Exp State University, Pullman, Washington 9916 Cite this: RSC Adv., 2017, 7, 54980


Introduction
Gd 2 Zr 2 O 7 is one of the most important rare-earth-stabilized zirconia (RESZ) materials due to its potential application as a ceramic waste matrix for immobilizing actinides and ssion products. 1,2 This crystalline system has a general formula A 2 B 2 O 7 , with one eight-coordinated A site (16c) and one sixcoordinated B site (16d) that are capable of incorporating a large amount of radionuclides (e.g. Pu, U, Np, Hf, Th, Am and Cm). 3 Previous studies show that Gd 2 Zr 2 O 7 has a high thermal stability, high chemical resistance and durability, 4-7 which suggests that it can serve as a stable, robust and long-lived waste host. In addition, due to the good mechanical properties (strength and hardness), thermodynamic stability and leaching durability, Gd 2 Zr 2 O 7 also nds versatile applications in many elds, such as thermal barrier coatings, solid electrolytes in solid oxide fuel cells, transparent ceramics, photoactive materials, etc. [8][9][10][11] Controls of sizes and morphologies of grains can greatly affect the physical and chemicals properties of material in related to its applications. Due to the signicant advantages in small grain sizes, nanostructured materials have been reported to display enhanced or discrepant properties comparing to coarse-grained counterparts. [12][13][14] Previous studies on RESZ show that compared to the conventionally synthesized microcrystalline structures, nanosized ceramics show enhanced radiation resistance, decreased thermal conductivity and increased conductivity. [15][16][17][18][19] So far, there are reports on using wet chemistry methods, the sol-gel processing, 5 the gel combustion processing, 20 the co-precipitation method, 21 the hydrothermal method 22 and others, 6 to prepare Gd 2 Zr 2 O 7 nanoparticles with different chemical homogeneity and phase assemblage. However, those former methods cannot produce nanoparticles with wellcontrolled grain sizes due to the agglomeration as a result of pH variation.
Thus, in this paper, we introduced a facile homogeneoussolvothermal precipitation method to synthesize wellcrystallized and dispersed Gd 2 Zr 2 O 7 nanoparticles with grain sizes of 20-30 nm. We demonstrated that this method has a better control of the grain size distribution through pH variation by adjusting the molar ratio of urea. In addition, it provides a more efficient way to synthesis well-dispersed nanoparticles than other wet chemistry routes.

Experimental section
In the synthesis experiment, Gd(NO 3 ) 3 $6H 2 O (>99.99%, Ruike RE, China), ZrOCl 2 $8H 2 O (>99.99%, Aladdin) were mixed with a stoichiometric ratio of 1 : 1 and dissolved in deionized water. Ethanol and different amounts of urea were added to the mixed solution while it was constantly stirring. Aer a fully mixing, the solution was heated at 200 C for 24 h in a Teon beaker and a precipitate was yielded. The precipitate was then ltered and washed with deionized water and ethanol in turns for several times. The reactants diluted by ethanol were dried at 60 C. Finally, the dry precursors were calcined at 800 C for 2 h. The synthesis scheme is shown in Fig. 1. We compared our synthetic samples by the homogeneous precipitation-solvothermal method with those obtained by the co-precipitation method. In the co-precipitation method, the dilute ammonium hydroxide was used as the precipitant, which was mixed with the solution containing Gd 3+ and Zr 4+ . Aer stirring the solution for 60 min and aging for 24 h, the formed gel-like precipitate was obtained from the solution through the centrifugal separation. Then the precipitate was washed with deionized water and ethanol for several times and dried at 60 C. Gd 2 Zr 2 O 7 powders were nally synthesized aer a calcination at 800 C for 2 h.
The synthetic products were characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM), transmission electron microscopy (TEM), Infrared spectroscopy (IR), and Brunauer-Emmett-Teller (BET) surface area measurements. The quantitative phase analysis was derived from the renement of the XRD patterns using MAUD Rietveld program.

Results and discussion
The XRD patterns (Fig. 2) of Gd 2 Zr 2 O 7 synthesized by the homogeneous-solvothermal precipitation method suggest a good crystallinity. Particularly, when the molar ratio of urea : Gd 3+ : Zr 4+ is 30 : 1 : 1, the phase is completely crystallized in the defect-uorite structure, evidenced by the strong and sharp peaks resulting from the (111), (200), (220), (311) and (222) reections. By using the Scherrer equation implemented by JADE 6, we performed a pattern prole tting that determined the average crystallite size to be 7.0 AE 0.3 nm which is similar to that reported by Popov et al. ($10 nm). 23,24 Because of our powders were well crystallized in small sizes, we can control the particle size by applying different calcination temperatures to the preliminary particles. Furthermore, the XRD patterns suggest that the molar ratio of urea : Gd 3+ : Zr 4+ has an inuence on the formation of the nal products ( Fig. 2). At a lower concentrated level of urea, we observed a weak peak attributed to the (440) reection of Gd 2 O 3 , which was not shown in the product when the urea molar ratio is 30. Though at a very high urea concentration (molar ratio of 60), the XRD pattern shows no formation of Gd 2 Zr 2 O 7 but Gd 2 O 3 and ZrO 2 instead (Fig. 3).
The relationship between the urea concentration and the nal product can be explained by the pH variation. When the temperature of mixed solution increases, the hydrolysis reaction takes place and provides hydroxide ions described in eqn (1) and (2), 3 (1) The hydrolysis reaction in the solution can constantly provide hydroxide ions with which Gd 3+ and Zr 4+ are combined, so that the mixing of Gd and Zr happens in a molecular level which is the basis for synthesizing homogeneous Gd 2 Zr 2 O 7 nanocrystals. Thus, the concentration of hydroxide ions is critical as it can determine the phases formed in the nal products. If the concentration of urea is low, the concentration of hydroxide ions is also low and not enough for OH À to be   reacted with Gd 3+ and Zr 4+ . In addition, though theoretically urea could provide enough hydroxide ions for reactions with Gd 3+ and Zr 4+ , the hydrolysis reactions described in eqn (1) and (2) are in dynamic equilibrium in a hermetic reactor, which could result in an insufficient concentration of hydroxide ions. For instance, no precipitate is formed when the molar ratio of urea : Gd 3+ : Zr 4+ is lower than 2 : 1 : 1. As the concentration of urea increased, the content of hydroxide ions is also increased as seen from eqn (1) and (2). Aer the molar ratio of urea reaches 3.5, the solution contains enough hydroxide ions to be combined with Gd 3+ and Zr 4+ (Fig. 2) that yielded phases consist of a defect-uorite phase Gd 2 Zr 2 O 7 as the main phase with a tiny amount of Gd 2 O 3 as a secondary phase. We found out that the amount of the impurity (Gd 2 O 3 ) decreases as the mole ratio of urea increases up to certain values; and at a proper mole rate (30 : 1 : 1), we produced pure Gd 2 Zr 2 O 7 in defect-uorite structure (Fig. 2). However, on the other hand, if the concentration of urea is too high, such as 60 : 1 : 1, the hydroxide ions would segregate Gd 3+ and Zr 4+ , and react with them separately to form Gd(OH) 3 and Zr(OH) 4 instead of (Gd, Zr) hydroxide that eventually leads to products as Gd 2 O 3 and ZrO 2 (Fig. 2).
The hydrolysis reactions described by eqn (1) and (2) are known as homogeneous precipitations that produce a homogeneous precipitation of (Gd, Zr) hydroxide as described in eqn (3). 22 Then the next step occurred under heating is the solvothermal process, during which the added ethanol can effectively break up the aggregates in the (Gd, Zr)-oxides at 200 C, and the alkoxyl group can substitute hydroxyl group as described in eqn (4), In this homogeneous precipitation-solvothermal method, the solvothermal process is the key step for synthesizing pure defect-uorite phase Gd 2 Zr 2 O 7 nanocrystals. As a comparison, we performed a co-precipitation synthesis without the solvothermal process. The mixed solution resulted from the homogeneous precipitation was heated in a thermostatted oil bath that maintained at 120 C for 2 h with a constant stirring. Then the suspension was cooled and the white precipitate was washed multiple times with deionized water and ethanol. Then the precursors were dried in vacuum at 60 C and then calcined at 800 C for 2 h. The XRD patterns of the products are given in Fig. 4. It's clear that without a solvothermal process, the synthesized powder is pure ZrO 2 . Numbers of studies show both A 2 B 2 O 7 oxides and rare earth oxides can be synthesized by homogeneous precipitation method using urea as precipitant. 25,26 Nevertheless, due to the low-temperature calcination, using only homogeneous precipitation method could lead to A 2 B 2 O 7 oxides mixed with BO 2 crystalline phases and perhaps poor crystallized AO 2 or A-containing polymers, partly due to the closed formation enthalpies of these phases at 800 C. Similar phenomena were observed in other complex oxide systems, such as garnet. 27,28 During the solvothermal process, Gd 2 Zr 2 O 7 was formed because of more OH À was produced by the hydrolysis of urea with the aids of high temperature and high pressure. Compared with Gd 3+ , Zr 4+ can be precipitated at lower pH value. With the absence of solvothermal process, only Zr 4+ , rather than Gd 3+ , was precipitated because of the relatively low pH value provided by the insufficient hydrolysis of urea, giving rise to the formation of the precursor of ZrO 2 . Thus, our homogeneous precipitation-solvothermal method was proved to be a new feasible route for synthesizing pure phase Gd 2 Zr 2 O 7 as a base for various functional nanomaterials.  IR absorption spectra are used to determine the site preferences of Gd and Zr in Gd 2 Zr 2 O 7 as it is sensitivity to the change of the local structure resulting from distortion of polyhedra and element substitutions. The IR spectra of samples prepared by the homogeneous precipitation-solvothermal method with different concentrations of urea are presented in Fig. 5. According to former studies, the infrared spectra of A 2 B 2 O 7 compounds contain seven IR-active optic modes. 29,30 Fig. 5(a) shows the IR absorption spectra of powders with different concentrations of urea in the wave range of 400 cm À1 to 4000 cm À1 . There are several characteristic absorption bands at about 436 cm À1 , 540 cm À1 , 1380-1630 cm À1 , and 3500 cm À1 . It has been known that the appearances of the band centered at approximately the 1380-1630 cm À1 and 3500 cm À1 bands are evidence of the existence of water molecules attached to the powders. In Fig. 5(b), the spectrum of the sample with the mole ratio of urea as 60 shows peaks in the range of 400 cm À1 to 600 cm À1 , from which the peak at 540 cm À1 is attributed from Zr-O stretching modes, and that at 436 cm À1 is caused by the O-Gd-O bending modes. These are another evidence besides XRD that show the presence of Gd 2 O 3 and ZrO 2 when the urea is high concentrated during the homogeneous precipitationsolvothermal synthesis.
In order to understand the micro-morphologies and microstructures of synthetic products made by the homogeneous precipitation-solvothermal method as compared to coprecipitation method, we performed SEM on Gd 2 Zr 2 O 7 powders synthesized by these two methods (Fig. 6). Fig. 6(b) shows large particles of pure defect-uorite nanocrystalline phase Gd 2 Zr 2 O 7 powders prepared by the homogeneous precipitation-solvothermal method with urea mole ratio as 30. The morphology suggests well-dispersed and homogeneous features. While in Fig. 6(a), particles prepared by the coprecipitation method show elevated agglomerations that grains are interconnected with each other to form larger microstructures that have irregular morphologies and porous networks. Compared to co-precipitation method, our new method produces Gd 2 Zr 2 O 7 powders with a better dispersity and a low level of aggregation. Furthermore, the particle sizes of samples synthesized by our method estimated by Nano Measurer 1.2 are in a relatively-narrow particle-size distribution within the range of 20-30 nm with a small mean particle size as 28.7 AE 0.8 nm (shown in Fig. 7).
TEM micrographs of our synthetic samples suggest the nature of nanocrystalline as evidenced in Fig. 8. It is concluded by the morphological image ( Fig. 8(a)) that Gd 2 Zr 2 O 7 powders are well-dispersed nanoparticles without tight aggregations. The crystal structure is conrmed by selected area diffraction (SAED) to be defect-uorite shown in Fig. 8(b). The crystallite size determined by HRTEM (Fig. 8(c)) is $7 nm in agreement   with the one by Scherrer formula; and the clear and regular crystal lattice distance indicates that Gd 2 Zr 2 O 7 phases are highly crystalline. Well-dispersed Gd 2 Zr 2 O 7 nanoparticles shown in the Fig. 8(d) HRTEM image have sphere-like or ellipsoid shapes. As mentioned above, this homogeneous precipitation-solvothermal method can be used to synthesize Gd 2 Zr 2 O 7 in smaller particle sizes and with a good dispersity at a relatively low temperature about 200 C.
The BET surface area and total pore volume of the calcined powder were studied by the gas absorption analysis. The N 2 adsorption/desorption isotherms of the powders which produced by the mole ratio of urea, Gd 3+ and Zr 4+ is 30 : 1 : 1 are shown in Fig. 9. The Gd 2 Zr 2 O 7 powders prepared via homogeneous precipitation-solvothermal processing displays a type H3 hysteresis loop (according to the IUPAC classication scheme), indicating smaller particle sizes and a good dispersity. The specic surface area of the pure defect-uorite phase Gd 2 Zr 2 O 7 powder is 76.9 m 2 g À1 , and the total pore volume is about 0.63 m 2 g À1 , and with an average pore radius of 26.04 nm, which further proves our conclusion compared with the SEM result. Thus, we can conclude that via the homogeneous precipitationsolvothermal method, Gd 2 Zr 2 O 7 nanoparticles are well dispersed and the particle sizes are small.

Conclusions
In summary, well-crystalized and well-dispersed defect-uorite phase Gd 2 Zr 2 O 7 nanocrystalline powders have been successfully synthesized by the homogeneous precipitationsolvothermal method. The reactions of precipitation and crystallization both occur at a relatively low temperature so that the high-temperature calcination can be eliminated. We discovered that under the appropriate mole ratio of urea : Gd 3+ : Zr 4+ as 30 : 1 : 1, the formed Gd 2 Zr 2 O 7 nanocrystalline powder is most sphere-like (with some oval shape) and has a narrow particle distribution with an average diameter of 20-30 nm. This research provides a new facile and efficient route to prepare Gd 2 Zr 2 O 7 nanocrystals at a lower temperature with shortened reaction time, which can also be extended to the synthesis of Gd 2 Zr 2 O 7 -based functional nanomaterials and other uoriteoxide nanocrystals (like La 2 Zr 2 O 7 ).

Conflicts of interest
There are no conicts to declare.