Synthesis of Yttrium Oxide Nanocrystal via Solvothermal Process

JOURNAL OF RARE EARTHS Vol.24,Spec. Issue, Dec. 2006, p.47

Synthesis of Yttrium Oxide Nanocrystal via Solvothermal Process Hong Zhanglian (&#&)’* , Zhang Shizhu (%-k*)’, Zhang Pengyue ( %M&)”2, Guo Huang ($F B)’, Fan Xianping ( %3k,T)’ ( 1 . Department of Materials Science and Engineering , Zhejiang University , Hangzhou 310027, China ; 2 . Magnetism Key Laboratory of Zhejiang Province , China Jiliang University, Hangzhou 310018, China )

Abstract : Y203 nanomaterials have been widely used in transparent ceramics and luminescent devices. Recently there are many studies focusing on controlling the size and morphology of Y2O3 in order to obtain better materials performance. In present study, yttrium oxyhydroxide precursor was synthesized via a facile solvothermal process through the dissolution-recrystallization mechanism of Yz03 raw powders in the ethylenediamine solvent, then nanosized yttrium oxide crystal was prepared from the precursor through post heat treatment process . The effects of solvothermal treatment temperature, holding time, solvent kinds and post heat treatment parameters on crystalline structure, grain shape and size of nanocrystal were investigated by XRD, TEM and TGA-DTA measurements. TEM images reveal that the morphology of product after post heat treatment at 460 “c for 12 h is rice-like nanocrystal. XRD shows that this product is pure cubic Y z 0 3 cphase. Present study reveals that high purity Y203 with rice-like morphology can be easily prepared with average size around 30 nm under suitable post heat treatment parameters. In addition, the effects of solvents such as water and ethanol etc . on the crystal structure and morphology were also investigated. It is suggested that dissolution-recrystallization process may be the main mechanism for the formation of nano-sized YOOH precursors under solvothermal reaction condition, and the ethylenediamine solvent is likely to play an important role in controlling the transformation process of yttria precursors to they203 nanocrystal .

Key words : yttrium oxyhydroxide : yttrium oxide : nanocrystal ; solvothermal process : rare earths Document code: A CLC number: TQ174.75 Article ID: 1002 - 0721 (2006) - 0047 - 04

In recent years, nanocrystals have attracted researchers interest because of their unusual behaviors‘’,~’ . Rare earth related n a n o c r y ~ t a l ,s ~especially ~~~~ the nanosized yttrium oxide ( Y203 ) with controllable size and morphology, which may have better sintering ability and unique luminescent property, have been widely studied because of their potential applications , in transparent ceramics and luminescent devices‘5361 such as FED and CRT, etc . Wet chemistry methods, including sol-gel route”] , combustion synthesis procedure[*’, micro emulsions method‘” , hydrothemal“O”’l and solvothermal[121methods etc. , were applied to prepare nanocrystals including the Y,03 nanocrystal which was studied in present study. However, wet chemistry methods usually adopt catalyst or template to obtain various precursors for yttrium oxide. In general, the precursors include yttrium hydroxide and yttrium carbonate, etc. Therefore, in order to obtain desired

products of yttrium oxide, calcinations or post heat treatment procedures are necessary and the heat treatment temperature is usually as high as 800 “c . Therefore, the whole procedure to produce nanosized YzOR crystal is very complicate and, usually is easily suffered from the contamination of undesired impurity species in the final products. The impurities may be harmful for the luminescent performance. Therefore, it is still important and necessary to develop a simple and efficient fabrication process, free of catalyst or template species, to obtain the final nanosized Y203 crystal in current research activities.

In this paper, a facile solvothermal process free of any catalyst or template has been developed for the preparation of nano-YOOH precursor. The nano-YOOH precursor, which has high purity and better transformation ability to the final nanosized Yz03 crystal after the post heat treatment process, was subsequently used as

Received date: 2006 - 06 - 25 ; revised date : 2006 - 09 - 08 Foundationitem: Project supported by SRF for ROCS, SEM (2003-14) and Science and Technology Department of Zhejiang Province (2003C11027)

*

Biography: Guo Huang (1981 - ), Male, Master candidate Corresponding author (E-mail : hong - zhanglian@zju. edu .cn )

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precursor to prepare nanosized Y203 crystal. In addition, suitable synthesizing conditions to synthesize the YOOH and Y203nanocrystal were suggested in present reaction system.

1 Experimental In a typical synthesis condition, 5 mmol Y,03 powder ( purity: 99. 999%) was dissolved in 3 ml concentrated nitric acid (analytical purity) to prepare Y ( NO3)?solution. Followed by the dropwise addition of the 4 mole L-’ KOH solution used as the precipitant, the pH value of the mixture solution was adjusted to be about 11. After aging completely, yttrium gels was filtrated and washed with distilled water for several times, and then dried at 110 “c in a vacuum oven. Then, the as-prepared precipitate was moved into a quartz tube and then placed into a 250 ml capacity stainless steel autoclave, which was subsequently filled with ethylenediamine ( analytical purity) up to 85 % of the total volume. The autoclave was heated, and maintained at 250 300 C ‘ for 3 to 12 h . Then the autoclave was cooled to room temperature. The as prepared precursor was collected and washed with the distilled water and ethanol, and then dried at 80 “c . During the post heat treatment process, the dried precursor was heated at 460 “c for 2 h in furnace, finally the obtained product was collected for further measurements. The crystal structure and phase of products were determined by X-ray diffraction ( X R D ) . The XRD patterns were recorded with a standard diffractometer (Rigaku MAX-RD, Japan) equipped with a graphite monochromator using Cu Ka radiation ( A = 0. 15405 nm) operating at 40 mA and 40 kV. A scanning rate of 4 (“)amin-’ was employed in the 26’ range from 10” to 80”. The size and morphology of the products were examined by transmission electron microscopy (TEM , JEOL 200CX), using an accelerating voltage of 200 kV . TGA-DTA data was recorded with Thermal Analysis Instrument ( SDT 4600, TA Instruments, New Castle, DE) with the heating rate of 10 “camin-’ under air atmosphere.

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and morphology of the as-hydrothermaled precursors. Fig. 1 shows the XRD patterns of the as-prepared yttria precipitate and the precursors prepared via solvothermal process under different reaction temperatures for 12 h . Before the solvothermal treatment, the corresponding XRD pattern (1) shows a bumped background, which reveals the presence of large amount of amorphous phase, and the diffraction peaks are consist with that of a monoclinic lattice of Y ( O H ) , (JCPDS

No. 21-1447). After solvothermal reaction above 250 “c for 12 h , all diffraction peaks of pattern (2) and (3) can be well indexed to the monoclinic phase of YOOH (JCPDS No.20-14131, and no other phase can be detected. However, when the solvothermal reaction was conducted at 300 “c for 12 h or long time, a weak peak in the Fig. 1 ( d ) appears. Based on the phase diagram of Y203-H,0 ~ y s t e m “ ~ ’such , situation may occur when the transformation from YOOH to Y 2 0 3 (JCPDS No. 44-0399) starts. It is difficult to determine the new phase formed at 300 “c just from this weak peak with diffraction angle of about 28 degree. In addition, the influence of reaction time from 3 to 12 h at 300 “c was investigated too. The XRD patterns were essentially identical which could be attributed to the YOOH phase. With increasing reaction time, the peaks became sharp and strong, implying that particle growth takes place and the crystal structure grows perfectly with increasing reaction time. The morphology and grain size of the precursor were investigated by TEM observation. As shown in Fig. 2, the YOOH phase formed during solvothermal reaction is nanocrystal with well shape. The morphol-

2 Result and Discussion 2.1

1

Solvothermal treatment of as-prepared yttria precipitate

Under the solvothermal reaction condition, the as-prepared yttria precipitate tends to dehydrate and re-crystallize . The reaction temperature, holding time and solvent kinds may influence the phase transformation process, the final phase composition, particle size

20

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XRD patterns of the as-prepared precipitate ( 1) and the precursors prepared via solvothermal process under temperatures of 250 “c ( 2 ) , 280 “c ( 3 ) and 300 “c: ( 4 ) for 12 h

Guo H et a1 .

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Synthesis of Ymium Oxide Nanocrystal via Solvothermal Process

esent solvothermal system free of catalyst and template.

2.2 Post heat treatmnt of yttria precmr The TG-DTA measurements were carried out to determine thermal behavior and decomposition reaction of the YOOH precursor. As shown in Fig. 3 , a large amount of weight loss in the TGA curve was found in the temperature range of 370 460 T , and the weight loss is about 10% which is in agreement with the theoretical weight loss for the YOOH Y203 transformation, plus the weight loss of dehydration of absorbed water. A corresponding endothermic peak in the DTA curve was observed around 440 “c . This temperature is lower than that required for decomposition of yttrium hydroxide and yttrium carbonate, which was reported . at 700 800 “c for classical micro powder[79111 Based on above TG-DTA measurements, the YOOH precursor obtained via solvothermal reaction at 300 “c for 3 h was calcined at 460 “c for 2 h to prepare Y203powders. XRD analysis (Fig. 4) shows that the nanosized YOOH phase has transformed completely to cubic Y20:g3 phase (JCPDS No.41-1105) without any other impurity phase after heat treatment at 460 “c

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+

Fig. 2

TEM images or the as-prepared precipitate ( a ) and the products prepared via solvothermal process at 300 “c for different times of 3 h ( b ) , 6 h ( c ) and 12 h (d)

ogy of YOOH phase, which has small grain size of about several dozens nanometers, is different from that of the as - prepared precipitate. Furthermore, the grain size becomes large with increasing reaction time. Thus, dissolution-recrystallization process“41 could be applied to explain the growth process of nanosized YOOH precursor. Firstly, the as-prepared precipitate was amorphous and contained poorly crystallized Y (OH), precipitate phase (see Fig. 1 ( 1 ) ) . When the solvothermal reaction goes on, Y (OH), was likely to dehydrate and dissolve into the solvent, then the Y OOH phase re-crystallized from supersaturated solution. With the increasing reaction time, the crystallite degree of YOOH was improved, and the corresponding grain with rice-like morphology and good dispersion grew to 30, 50 and 100 nm when the reaction time increases from 3 , 6 and 12 h , respectively. In general, the grain growth based on the dissolution-recrystallization mechanism is sensitive to the reaction temperature and pressure of reaction system in which the solvent plays an important role. In order to investigate the influence of various solvents on the formation of YOOH phase, ethanol and water were replaced for ethylenediamine while other conditions kept unchanged. The XRD results showed that the major product was amorphous which contained small fraction of poorly crystallized YOOH phase. Therefore, it was suggested that, in either water or ethanol solvents, only initial transformation from Y (OH), to YOOH occurred. Above results clearly indicated that ethylenediamine may promote the transformation from Y (OH), to YOOH at relative low reaction temperature in pr-

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400 600 800 TcmpcraturcK

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1000 1200

TG-DTA curve of products prepared via solvothermal reaction at 300 ‘K for 3 h

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XRD patterns of the product calcined at 460 “c for 2 h (1) and JCPDS card 41-1105 of cubic Y203(2)

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Fig.5 TEM image of nano-sized T for 2 h

Y203

product calcined at 460

for 2 h . And TEM observation of the as-prepared cubic YzOl phase (Fig. 5 ) shows that the corresponding particle maintains its precursor's rice-like morphology, and the average grain size is about 30 nm, slightly smaller than that of the precursor.

3

Conclusion

In summary, YOOH nanocrystal was synthesized via a facile solvothermal process, the effects of solvothermal temperature , holding time and solvent kinds on the crystallite structure, grain size and morphology were investigated. The ethylenediamine solvent was found to promote phase transformation from Y (OH), to YOOH at lower reaction temperatures in present solvothermal system. It was found that the YOOH precursors prepared by solvothermal process have high purity and better post heat-treatment ability than classical yttria precursors, the nano-sized Y203 crystalline could be easily obtained from the YOOH precursors after heat treatment at 460 T for 2 h. Present study offered a method to prepare Yz03 nanocrystal with merits that the reaction could be conducted under relative mild conditions and free of catalyst or template.

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