Synthesis of monodisperse erbium aluminum garnet (EAG) nanoparticles via a microwave method

JOURNAL OF RARE EARTHS, Vol. 31, No. 5, May 2013, P. 490

Synthesis of monodisperse erbium aluminum garnet (EAG) nanoparticles via a microwave method HU Song (㚵ᵒ), LU Chunhua (䰚᯹ढ)*, WANG Wei (⥟ि), DING Mingye (ϕᯢ⚼), NI Yaru (‫׾‬Ѯ㤍), XU Zhongzi (䆌ӆṧ)* (State Key Laboratory of Material-Orient Chemical Engineering, College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, China) Received 23 August 2012; revised 10 April 2013

Abstract: Uniform Er3Al5O12 spheres are of great value for fabricating optical ceramics. The highly monodisperse and size-controllable erbium aluminum garnet (EAG) precursors for transparent ceramics were successfully synthesized through a new microwave process. The precursors constituted of ultrafine particles joining together by a hydroxyls formed compact network structure in the absence of SO42however, the morphologies of the precursors exhibited spheres with trace amount of SO42. With manipulated programming of microwave irradiation parameters, narrow distributed particles of 40–50 nm were finally obtained by a separation of nucleation and nanocrystal growth. The mechanism behind the influence of microwave irradiation parameters on the growth of EAG precursors was preliminarily analysed. Easily dispersible and pure phase EAG were obtained at 950 °C. The as-prepared EAG powders were used to fabricate transparent ceramics and transparent polycrystalline EAG ceramics were obtained under hydrogen furnace at 1750 °C for 8 h. Keywords: erbium aluminum garnet; microwave-assisted method; nano-scale; monodisperse; transparent ceramic; rare earths

Nanotechnologies nowadays are widely adopted to fabricate ceramics. The formation of monodisperse nanocrystals with size control and shape manipulation has been intensively pursued. This topic is of great importance for elucidating special size-shape dependent physiochemical properties[1]. Rare earth transparent ceramics have been investigated since the successful yield of translucent alumina. They actually have diverse applications in devices, including lamp-tubes, solid-state laser, scintillators, and some advanced military use[2–6]. Conventional processes to fabricate REAG (RE=Y, Dy–Er), such as a sol-gel method, co-precipitate method, and alcohol-water hydrolysis method, rely on thermal conduction of black body radiation to drive chemical reaction. Energy transfers from the heating source to the solvent, and finally to the reactant molecules. This kind of heating model inevitably causes thermal gradients throughout the bulk solution and results in nonuniform reaction conditions[7], which will consequently lead to undesired poor nucleation and broadened size distribution. In contrast, microwave dielectric heating pattern has fast reaction kinetics, and is able to avoid the disadvantages of inhomogeneous, which has been reported in the fabrication of diverse nanomaterials[8–10]. More importantly, it is a great superiority that the microwave irradiation parameters can be

programmed and optimization of experimental conditions can be easily realized according to the design of synthesizing process. Although many reports[3,11–13] have discussed the preparation of nanoscale rare earth doped YAG powders, few attentions have focused on EAG. Er3Al5O12 single crystals have been proved to fabricate laser devices[14]. As is generally recognized that subjecting to the segregation coefficient, lasing performance of single crystals is restricted. From the spectroscopic point of view, polycrystalline transparent ceramics can substitute single crystal as lasing crystal with extending capabilities. The increased compositional versatility of transparent ceramics enables tailoring improved laser materials. However, optical properties of transparent ceramics strongly depend on their microstructure, which is a direct consequence of sintering. Considering this, monodisperse nanoscale powders are currently considered promising for transparent ceramics for their narrow size distribution and fine shapes. There has not been any report of nanoscale EAG prepared by microwave-assisted heating previously. Herein, we have firstly explored it for preparing EAG precursors. A combination of the traditional homogeneous co-precipitation and microwave irradiation were realized simultaneously in our present work, contributing to a

Foundation item: Project supported by Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) * Corresponding author: LU Chunhua, XU Zhongzi (E-mail: [email protected], [email protected]; Tel.: +86-25-83587252, +86-25-83587220) DOI: 10.1016/S1002-0721(12)60308-1

HU Song et al., Synthesis of monodisperse erbium aluminum garnet (EAG) nanoparticles via a microwave method

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lower pure-phase formation temperature of 950 °C. In order to modify the morphology of the precursor, a small amount of ammonium sulfate was added to obtain fine particle morphologies. The dispersibility and morphology of the precursor were improved, which would benefit the formation of pure EAG phase.

K Br pellet method with Fourier transform infrared spectroscopy (FTIR; model Nexus 670FTIR, NICOLET Co., America). Optical properties of the prepared samples were studied via UV-Vis-NIR spectrophotometer over the wavelength region ranging from 400 to 2000 nm (Model UV3101, Shimadzu Co., Japan).

1 Experimental

2

1.1 Synthesis of EAG particle via microwave irradiation

A technique called urea-based homogeneous precipitation (UBHP), has been widely adopted to fabricate the yttrium aluminum garnet powders[15,16]. In alkaline or neutral mediums, reactions usually proceed as is shown below. The hydrolysis of urea takes place initially. CO(NH2)2NH4++OCN– (1) And then, cyanate ions react rapidly according to the following equation. OCN–+OH–+H2ONH3+CO32– (2) Cations in the solution integrate with the hydroxyl and CO32 to form a complex compound called subcarbonates. While it is difficult to obtain monodisperse particles through a UBHP method, for the reason that a high [U]/[M] ratio is essential in the synthesis of perfect nanoparticles via traditional approaches[16]. Whilst with the traditional heating model, no vast OH and CO32 generate instantaneously in the system because of the gentle hydrolysis of urea. Recent work of preparing yttrium aluminum garnet precursors showed that a low [U]/[M] ratio, together with traditional co-precipitation methods would lead to a slow rise in pH, which causes sequential precipitating of Er(OH)CO3 and Al(OH)CO3, due to their differences in the solubility[17]. By contrast, it is a novel way to prepare EAG precursors via a microwave method. In the solution, urea decomposes at above 80 °C with a elevated temperature. Under the microwave irradiation, temperature of the system rises rapidily. The in situ and sharp hydrolysis of urea releases a large amount of hydroxyl and carbonate species into the solution, which serve as precipitating ligands. The continuous release of OH and CO32 avoids localized distribution of the reactants. A uniform temperature distribution, and a “inside to surface” heating mechanism, make it possible to exercise control over nucleation and growth of the particles. The erbium and aluminum ions nucleate simultaneously with the large amounts of hydroxyl and carbonate anions, according to the following equation. Er3++Al3++OH+CO32+H2O Er3Al5(OH))24–2x·(CO3)x·nH2O (3) The chemical formula of the final products obtained by a microwave-assisted method can be authenticated by the IR spectra of the precursor after drying. The similar explanation has been clarified by Li et al.[18] The morphologies of the resultant precursors are displayed in Fig. 1. Fig. 1(a) shows the ultrafine and spawn-

The yttrium source for this study was erbium nitrate hexahydrate (Er(NO3)3·6H2O) (>99.95% pure, Yancheng External Trade Company, China). Aluminum nitrate nanohydrate (Al(NO3)3·9H2O) (>99.0% pure, Shanghai, China) provided the aluminum source. Urea (CO(NH2)2, A.R.) was used as the precipitant. And ammonium sulfate ((NH4)2SO4, A. R., Shanghai Ling Feng Chemical Reagent Co., Ltd., China) was also used as a starting material. The erbium and aluminum aqueous stock solutions were prepared by mixing erbium nitrate hexahydrate and Aluminum nitrate nanohydrate of given concentrations. The stoichimetric molarity ratio of Er3+/Al3+ ions was kept 3:5. Proper content of urea was selected to add in the solution to find a optimal [U]/[M] molar ratio for the final product. The homogeneous synthesis reactions were carried out in a microwave-ultrasound reaction system (microwave frequency 2.45 GHz, Xian Ou instrument Co., Ltd., China) under different irradiation conditions for several minutes till visible turbidity appeared. Details of these microwave parameters will be described in the later section. After the microwave treatment, the precursors were obtained through high-speed centrifugation from the resultant solutions. After being washed with distilled water and ethanol each for 3 times, they were dried at 60 °C in the electric thermostatic drying oven for more than 24 h. The precursors were subsequently calcined in air for 2 h at various temperatures ranging from 950 to 1100 °C. 1.2 Characterization techniques Thermogravimetric analysis and differential thermal analysis (TG/DTA) traces of the dried precursor were performed from room temperature to 1250 °C with a heating rate of 10 °C/min under flowing air using a thermal analyzer (Diamond, PE Co., America). The phase compositions of both precursors and the calcined samples were characterized by powder X-ray diffraction (XRD) on a Thermal ARL XTRA diffractometer employing Cu K radiation (=0.15406 nm). Particle morphologies were observed by field-emission scanning electron microscopy (FE-SEM; model Carl Zeiss 1550, Carl Zeiss Electron Co., Germany). The bonds of the precursor and its calcined products were identified by the

Results and discussion

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Fig. 1 Field-emission scanning electron micrographs of erbium aluminum garnet precursors obtained via microwave irradiation in the absence of sulfate ions (a), and in the presence of sulfate ions (b)

like EAG precursors, which were produced in the absence of sulfate ions. Since it enclosed a large amount of water in the channels of the network structure joined by hydroxyls, the precursors appeared gelatinous and translucent once separated from the solution. Fig. 2 shows the schematic diagram of this network structure connected by OH. It can be seen from Fig. 1(b) that monodisperse spheres are obtainable by the microwave-assisted methodology with a micro-scale addition of (NH4)2SO4, which was served as dispersant. The morphology of the precursors presented uniform particles of about 40 nm. When separated from the solution, the sphere precursors were easily dispersed in distilled water, compared with the former one. Furthermore, after irradiating in the microwave oven for 30–80 min, the SO42-contained products were similar to suspensions, which would quickly settle in the solution once standing still for a while. This can be attribute to the sulfate ions being adsorbed on the surface of the precursors by virtue of electrostatic force[15]. Sulfate ions took the place of partial hydroxyls adsorbed on the precursor surface because of their stronger affinity to the positive precursors. Negative charges of SO42 neutralized the positive charges, making a decline in -potential. And the network structure mentioned above was destroyed. In addition, the sulfate ions acted as a structural director, which made a tuning on nanoparticle growth, and they inhibited agglomeration of the nuclei, thus the spheres of the precursors were successfully obtained. Similar behavior of sulfate ions concerning morphologies

Fig. 2 Schematic diagram of the network structure of the precursor in the absence of sulfate ions

and structure transformation have been studied in fabricating Y2O3, SnO2, TiO2, and mesoporous silica[19–22]. As is mentioned above, optical quality of transparent ceramics, to some extent, depends on the powder size. It is obligatory to control the particle size in wet-chemistry synthesis. In the present work, two main approaches have been cited. On one hand, reaction time was prolonged for particle growth. Fig. 3(a), and (b) shows the FE-SEM micrographs of precursors synthesized within different time under 800 W microwave irradiation. With microwave irradiation time increasing, the particle size slightly grew

Fig. 3 FE-SEM photographs (Left) of EAG precursors synthesized in different processes (the bar graphs (Right) corresponds to size distribution of each precursor) (a) Microwave power of 800 W, react for 40 min; (b) Microwave power of 800 W, react for 80 min; (c) 1200 W for 5 min and followed by a process of 200 W for 40 min

HU Song et al., Synthesis of monodisperse erbium aluminum garnet (EAG) nanoparticles via a microwave method

due to continuous attachment of stock solution or ultrafine nanoparticles in the solution onto the larger scaled particles already formed. The average size of the sphere particles were 39 and 43 nm, respectively. On the other hand, a separation of the nucleation and crystal growth stages was realized by changing microwave parameters under a certain resolution concentration. For the first time, we carried out a “5–40 minutes two-step reaction” model for the precursor synthesis. 1200 W of microwave irradiation power was selected in the first step. It is known that nucleation rate is largely dependent on the supersaturation concentration to the saturation concentration, Css/Cs. A sharp increase in Css/Cs occurs under intense microwave irradiation, thus contributes to a burst of nucleation. The 5 min comparatively fierce reaction condition was designed for nuclei formation, followed by a lower power of 200 W for 40 min, providing a facile environment for the growth of crystal particles. Vast scale nuclei were generated in the solution simultaneously, and then ultrafine precursor materials grew onto the surface of formed particles when solution concentration decreased below Css/Cs. As is shown in Fig. 3(c), the particles are well shaped and dispersive. From the histogram embed in the Fig. 3(c), the size distribution of the particles is narrow, the majority are among the size of 40–50 nm. TG/DTA was performed on the precursor in the absence of sulfate ions. The TG curve displayed in Fig. 4 shows a total mass loss of 34 wt.%. The relatively sharp decline of the TG curve from room temperature to 600 °C is corresponding to dehydration process and decomposition of NH4+ and partial NO3. It can be clearly seen from the DTA curve that an obvious exothermic peak appears at the temperature of 936 °C, which corresponds to a phase transition. XRD profiles of the erbium aluminum garnet precursors after calcination in air at various temperatures ranging from 950–1100 °C are shown in Fig. 5, the residence time at each temperature was kept 2 h. It can be seen in Fig. 5(a) that pure EAG phase was obtained at as low as

Fig. 4 Thermogravimetry/differential thermal analysis traces showing the decomposition process of the erbium aluminum garnet precursor (in the absence of sulfate)

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Fig. 5 XRD spectrograms of precursor synthesized in the absence of (NH4)2SO4 (a) and in the presence of 8 wt.% (NH4)2SO4 (b), under MW-power of 800 W, and products calcined at different temperatures

950 °C when no (NH4)2SO4 was added into the source materials, much lower than the fabrication of REAG (RE=Y, Dy, etc.) with a solid-state method to obtain pure crystalline phase at about 1600 °C. This tremendous difference in phase-formation temperature is partially due to the nano-scaled size and uniform morphology of the precursors. Furthermore, the relative intensity of the diffraction peak is enhanced with a rising temperature, due to the rise in integrity of crystallization at higher temperatures. Fig. 6 exhibits the FE-SEM micrographs of EAG powders by calcining the precursors consisting of sulfate ions (b) and free of sulfate ions (a) at 1100 °C. In Fig. 6(b), dumbbell shaped particles are distributed, whilst Fig. 6(a)

Fig. 6 FE-SEM photographs of polycrystalline EAG obtained by calcining for 2 h at 1100 °C in the absence of sulfate ions (a), and in the presence of sulfate ions (b), respectively

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shows a blurry and aggregated morphology of EAG particles. It is an excellent dispersant of sulfate ions in mediating the external morphologies of the crystalline powders. The existence of sulfate ions on the surface of particles at high temperatures can inhabit crystalline aggregation, and also contributes to the dispersion of particles[23]. However, since the sulfate ions have a high decomposition temperature of about 1000 °C, which would influence the powder sinterability. So in the present work, 8 wt.% ammonium sulfate in the stock solution was proved to be the optimal addition. Fig. 7 shows FT-IR spectra of the precursor and its thermal-treated products. The broad-band peaks at ~3450 cm–1 are corresponding to the coupled effects of water molecular. The absorption band at ~1640 cm–1 is the diagnostic of HOH banding mode of water molecular. The (ErO) and (ErOEr) vibration band is clearly observed below 1000 cm1, and the resonance absorption is caused by the interaction between lattice vibration of EAG crystal and the photon. The absorption band at ~1120 cm–1 is characteristic of SO42–, which appears on the precursor curve in Fig. 7. The comparative band intensities of the calcinated products at ~1120 cm–1 gradually decreases with the temperature rising, and finally disappear totally at above 1000 ºC, indicating the occurrence of a gradual dehydration desulfurization. Because of the existence of SO42–, the formation of pure EAG phase is postponed to higher than 1000 °C, which would be too high temperature to get powders with excellent sintering activity. As is shown in Fig. 5(b), in the presence of sulfate, there is still residual Er4Al2O9 and NH4Er(SO4)2 peaks after calcining at 950–1000 °C. Finally, the pure EAG phase is obtained at 1100 °C. The high-purity powders were used to fabricate polycrystalline transparent ceramics. Transparent EAG ceramics were obtained by sintering pellets at 1750 ºC for 8 h under hydrogen furnace. Fig. 8 shows the photograph of mirror-polished transparent ceramic with 16 mm in diameter and 2 mm in thickness. Letters can be clearly seen through the sample. The energy level of Er3+ ions are rich, and they are

Fig. 7 FT-IR spectra showing the decomposition process of the precursor in the presence of sulfate ions

JOURNAL OF RARE EARTHS, Vol. 31, No. 5, May 2013

Fig. 8 Photograph of EAG transparent ceramics

Fig. 9 UV-Vis-NIR diffuse reflectance spectra of the precursor and calcined precursor of EAG

corresponding to several emission wavenumber. In Fig. 9, the UV-Vis-NIR diffuse reflectance spectra of the EAG precursor and EAG crystal are displayed, from which it can be discovered that there exist light absorption near 526, 790 and 980 nm. This feature can be applied in the fabrication of up-conversion luminescent materials. It has been reported that Er3+-coped materials has a wide use in the fields of solid-state laser and full color display[24,25]. Typical Er3+ absorption at ~1530 nm has been observed in the prepared EAG-hosted polycrystalline particles. This characteristic can be applied in the laser ranger[26,27]. With the thermal treatment temperature rising, the main absorption peaks occur a hypochromatic shift. Furthermore, at ~1530 nm, a split of spectrum occurred at the temperature of 1100 °C. The sharp peaks are 1472 and 1532 nm, respectively. It can be clarified from two aspects. On one hand, it is the nano-effects that contribute to the spectral characteristics of the products. Before thermal treatment in the furnace, the precursor size is about 30 nm, while after calcining at 1100 °C for a residence time, the particles grow to larger than 100 nm. Near the wavelength of energy level transition of Er3+, electrons are distributed dispersedly, which lead to a dispersion of electric field, and proceed to lead to spectrum broadened, which can be seen from the curve of the precursor in Fig. 9. On the other hand, the certain crystalline field forms after heat-treatment at 1100 °C, so Er3+ are in an unified environment. In contrast, the electron field of the precursor and the product calcined at 1000 °C

HU Song et al., Synthesis of monodisperse erbium aluminum garnet (EAG) nanoparticles via a microwave method

is comparatively complex, so their spectra exhibit a phenomenon of broadening.

3 Conclusions Monodisperse erbium aluminum garnet (EAG) precursors for transparent ceramics fabricating were successfully prepared via a novel microwave-assisted method. An amount of 8 wt.% of ammonium sulfate was added in the stock solution to act as a dispersant, which had further served as a structure director to control the growth of the nanoparticles. Sphere precursor was generated, and the sulfate ions adsorbed on the surface of the precursors had commendably prevented agglomeration of the synthesized powders during drying. Furthermore, the addition of sulfate ions had influenced the phase transformation of the precursor during calcination. Powders of pure EAG phase were obtained as low at as 950 °C in the absence of SO42–, while it was retarded to 1100 °C with an optimal amount of sulfate ions. The as-prepared powders had been proved to fabricate transparent ceramics. The particle size was manipulated by adjusting the microwave parameters and the reaction process according to the LaMer theory, thus to realize the separation of nucleation and crystal growth. The prolong of microwave irradiation time from 40 to 80 min led to a slight growth of precursor. However, a phenomenon of aggregation came up if the reaction time was extended. A uniform particle size of 40–50 nm was obtained by a 5–40 minutes synthesis model, and the particles were of good dispersibility. Sharp light absorption appeared at 526, 790, 980 and 1530 nm, where the erbium aluminum garnet polycrystalline powders acted as the host material. Hypochromatic shift happened at the absorption peaks, and splitting of spectrum took place near 1530 nm.

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