October 2002 Hydrothermal Synthesis of Nanocrystalline Cerium(/V)Oxide Powders 2463 Table L. Chemical Compositions of the Used Precursor and the Hydrothermal Powder eaction temp pH value of Component content (wti and time used medium CeO, H O CO, S 40 Precursor 8312.71.53 N 1200°C,8h 12 88.66.81.42 95.34.40.20.4 in Table I It can be found that the lower the ph of the reaction medium, the lower the moisture content of the sample, which means that the dewatering of the precursor in the acid medium we more complete than that in the basic medium. The XRD patterns of the samples are shown in Fig. I. The used precursor and the 10 hydrothermal powders displayed all of the major diffraction peaks of CeO, with the fluorite structu The relationship between reaction temperature and the grain 0 size of the powders synthesized in the hydrothermal medium with different acidities is shown in Fig. 2. Figure 3 shows the relation- 100 200 hip between reaction time and the grain size of the powders synthesized at200°C TEMPERATURE (C) (2) Morphology Fig. 2. Relationship between the particle size of the powders and the reaction temperature. The reaction time was fixed at 18 h.(.) Acidic TEM photographs of the used precursor and the powders medium neutral medium:(A) basic medium. synthesized at different hydrothermal reaction temperature for 18 h are shown in Fig. 4. Some TEM photographs of the powders synthesized at 200C for different reaction times are acidities are displayed in Fig. 6. It is well known that the broad absorption band located in the area from 3200 to 3600 cm - In this paper, it is obvious that hydrothermal treatment of approximately corresponds to the O-H stretching vibration,and Ce(OH), is an Ostwald ripening process and the acidity has he one located in the area from 400 to 750 cm to the Ceo considerably influenced the hydrothermal treatment process. stretching vibration. The absorption peaks at 1600. 1500, or 1340 cm, and 1060 cm correspond to the H,O bending vibration. (3) IR Spectrograms the CO; stretching vibration, and the Ce-OH or SO4 stretching c IR spectrograms of the used precursor and hydrothermal pow vibration, respectively. From Fig. 6. it can be concluded that the synthesized at 200 C for 18 h in the medium with various crystallization of the powders synthesized in an acidic hydrother- mal medium was more complete than others, which agrees with the (111) processes are associated with SO? and Co? othermal t given in analysis for the chemical compositions of the samples Table L. Figure 6 and Table I show that the hydr treatmet (D) 220) (311) (4) TG-DTA Spectrograms The TG-DTA spectrograms of the used precursor and the hydrothermal powders are shown in Fig. 7. The ascent of the TG curve denotes an increase of sample weight, and it is expressed by positive Am: and the descent of the TG curve denotes a decrease (C) 、∧八 且 0 100 20(deg TIME (h) Fig. 1. X-ray diffraction patterns of the used precursor(A)and the Fig. 3. Relationship between the particle size of the powders and the hydrothermal powders synthesized at 200 C for 18 h in the reaction reaction time. The reaction temperature was fixed at 200C(+)Acidic medium with a pH of 12(B), 7(C), and 2(D) medium;() neutral medium: (X) basic mediumOctober 2{)O2 Hvdrothermal Synthesis of Nanocry.<italline Cerium(lV) O.xide Powders 2463 Tiihtc I. Chemical Compositions of the I sed Precursor and the Hydrothermal Powders u....,.,i .,„ „» f .!,» Componcni L-onreni |wi*} Precursor Powder 1 Powder 2 Powder 3 unJ liiTiL' 200"C. 18 h usL'd mciiiuin 12 7 2 CeO- 83 88.6 90.1 95.3 H.O 12.7 6.8 5.8 4.4 CO, 1.5 1.4 1.1 0.2 so.. 3.2 2.9 2.7 0.4 in Table I. It can be found that the lower the pH of the reaction niedium. the lower the moisture content of the sample, which means ihiit the dewalering of ihe precursor in the acid medium was more complete than thai in the basic medium. The XRD paitems of the samples are shown in Fig. I, The used precursor and the hydrothermal powders displayed all of the major diffraction peaks of CeOi with the fluorite structure. The relationship between reaction temperature and the grain size of Ihe powders synthesized in the hydrothermal medium with different acidities is shown in Fig. 2. Figure 3 shows the relation￾ship between reaction time and Ihe grain size of the powders synthesized at 200"C. (2) Morphology TEM photographs of the used precursor and the powders synthesized at different hydrothermal reaction temperature for 18 h are shown in Fig. 4. Some TEM photographs of the powders synthesized at 200°C for different reaction times are shown in Fig. 5. In this paper, it is obvious that hydrothermal treatment of Ce{OH)4 is an Ostwald ripening process and the acidity has considerably influenced the bydrothermal treatment process. ii) IR Spectrograms IR spectrograms of the used precursor and hydrothermal pow￾ders synthesized at 2OO''C for 18 h in the medium with various (111) 5 N So 50 40 30 20 10 0 100 200 300 TEMPERATURE (°C) Fig. 2. Relationship between the particle size of the powders and llic reaction lemperature. The reaction lime was fixed al 18 h. (•) Acidic medium: (•) neulral nieiiium; (A) basic niedium. acidities are displayed in Fig. 6. It is well known ihat ilie broad absorption hand located in the area from 3200 to 3f)(H) cm ' approximately corresponds to the 0-H stretching vibration, and the one located in the area from 400 to 750 cm ' to the CeO, stretching vibrution. The absorption peaks at 1600. 1500. or 1340 cm the vibration, respectively. From Fig. 6. it can be concluded tbat the crystallization of the powders synthesized in an acidic hydroiher￾mal medium was more complete than others, which agrees wilh the analysis for the chemical compositions of the satnples given in Table I. Figure 6 and Table I show that tbe hydrothermat treatment processes are associated witb SOj " and CO^ . ~', and 1060 cm ' correspond to the H,O bending vihralion, COt" stretching vibration, and ihe Ce-OH or SOj stretching (4) TG-DTA Spectrograms The TG-DTA spectrograms of the used precursor and the bydiotbermal powders are shown in Fig. 7. The ascent of the TG curve denotes an increase of sample weight, and it is expressed by positive Am: and the descent of the TG curve denotes a decrease 20 50 100 TIME (h) 150 Fig, 1. X-ray diffraction patterns of the used precursor (A) and the hydrothermal powders synthesized at 200°C for 18 h in the reaction medium wUh a pH of 12 (B). 7 (C), and 2 (D). Fig. 3. Relationship between the particle si7e oi ihe powders and the reaction time. The reaction temperature was fixed al 20O''C. (•) Acidic medium; (•) neuiral medium: (X) basic medium