Synthesis of ZnO doped ceria nanoparticles via azeotropic distillation processing

SONG Xiao-lan(宋晓岚), QU Peng(曲  鹏), YANG Hua-ming(杨华明), QIU Guan-zhou(邱冠周)

School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China

Received 10 April 2006; accepted 25 April 2006

Abstract: The synthesis of nano-sized ZnO-doped CeO₂ of 20 nm in crystal size by a coprecipitation technique was investigated by different scanning calorimetries/thermalgravimetrics(DSC/TG), X-ray diffraction (XRD), transmission electron microscopy (TEM) and ultraviolet (UV) absorbance. Azeotropic distillation processing was performed to effectively eliminate the residual water inside the as-prepared precipitate. Doping of ZnO results in the formation of solid solution. The crystal size of the nanoparticles increases with the increase of the doped ZnO amount, the calcination temperature and time. Doped CeO₂ nanoparticles show excellent visible-light property and ultraviolet-absorption activity. Doping of ZnO doesn’t not weaken the UV-shielding property of ceria.

Key words: ceria nanoparticles; ZnO doping; inorganic compounds; nanostructures; oxides

1 Introduction

Because the ultraviolet (UV) light in solar energy deteriorates some organic materials and causes damage on human’s health, various organic and inorganic sun-care products have been developed. Fine zinc oxide (ZnO) powder has special characteristics to make it ideal for use as a broad-spectrum inorganic sunscreen in personal-care products. But ZnO has high refractive indices and make the skin look unnaturally white when ultraviolet light incorporates into the products. In addition, its high photocatalytic activity facilitates the generation of reactive oxygen species, which can degrade other ingredients[1]. Ceria, which possesses a lower refractive index, is relatively sensitive to visible light and appears natural on the skin without imparting an excessively pale white look. Nanosized ceria(CeO₂) has excellent ultraviolet radiation absorption properties, which makes it ideal as a broad-spectrum inorganic sunscreen in personal-care products [2, 3].

CeO₂ is a typical oxide with a fluorite structure. It is well known that CeO₂ property can be changed by doping other elements. In recent years, CeO₂-based materials have been widely studied as catalysts, ceramics capacitors, oxygen tolerance materials, structure and electronic cells, etc[4-10]. It is expected to dope zinc oxide into ceria to develop a safer inorganic sunscreen with superior UV absorption activity.

Recently, various methods have been developed to synthesize doped ceria powders[11, 12], including solid-state reaction, coprecipitation, sol-gel process, hydrothermal synthesis, micro-emulsion, etc. But through the above techniques, it is difficult and inconvenient to obtain the nanosized ceria powder.

It is known that agglomeration may be formed during the preparation by chemical techniques because there were strong chemical bonds between neighboring particles. But when the azeotropic distillation process is introduced, the excess water molecules in the colloid can be removed easily and the —OH group on the surface of the particle is replaced by —OC₄H₉ group. Consequently, the possibility for the particles to get close as well as the formation of chemical bonds was greatly eliminated. The technique has proved to be quite efficient to eliminate the residual water in the precipitate responsible for the formation of serious agglomeration. So the hydrous oxides can be completely dehydrated[13, 14]. The synthesis of nano-sized ZnO doped ceria via azeotropic distillation was studied in this paper. The main purpose of this work is to optimize the synthesis proceess in order to attain high UV absorption powders.

2 Experimental

2.1 Materials

AR-grade Zn(NO₃)₂ and Ce(NO₃)₃ were respectively dissolved into deionized water with proper concentration. Two solutions were then mixed with molar ratio of Zn to Ce 0-10%. Polyethylene glycol was added as dispersant. After heating the solution till 40 ℃ under the stirring rate of 800 r/min and adjusting the pH value to 8-9, AR-grade ammonium nitrate was quickly added and then thoroughly stirred the mixed solution for 10 min. The chemical reactions is as follows:

(2-2x)Ce(NO)₃+2xZn(NO₃)₂+(3-x)(NH₄)₂CO₃→Ce_2-2xZn_2x(CO₃)_3-x+(6-2x)NH₄NO₃                     (1)

After aging for 1 d, the colloid was filtered and washed with deionized water to completely remove the remaining ammonium nitrate and other impurities to produce a gel. Part of the as-prepared gel was dried in a vacuum oven at 60 ℃ for more than 10 h. Other gel was mixed with butanol under strong stirring to form suspended colloid. Then the azeotropic distillation process could be carried out. When raising the temperature of the colloid to the azeotropic point of the water-butanol system(93 ℃), the water in the colloid was removed in the form of azeotropic substance. After water was completely removed, the system temperature was increased up to the boiling point of butanol (118 ℃). This process was finished until most of the remaining butanol vaporized. The colloid was then dried in a vacuum oven at 60　℃ for more than 10 h. Then the as-dried powders were put into furnace. Thermal treatment of the powder at 300-1 000　℃ for 1-5 h resulted in the formation of the nanoparticles.

2.2. Characterization

Thermal decomposition of the precursor was monitored by different scanning calorimetries, DSC, and thermalgravimetric, TG (STA-499C, Netzsch). The crystalline phase was examined by X-ray diffraction (Shimadzu, XRD6000). The size distribution and shape of the particle were observed using transmission electron spectroscope (Hitachi, H-8000). The chemical constitution analysis was measured by atomic absorption spectroscope (WYZ402). The UV properties were evaluated by ultroviolet-visible spectrophotometer (Lengguang, 765MC).

3 Results and discussion

3.1 Analysis of DSC/TG and XRD of precursors

XRD patterns of the undoped CeO₂ synthesized by azeotropic distillation method and normal method are shown in Figs.1 and 2. The main phase of the undoped precursor by drying directly is Ce₂(CO₃)₃·H₂O, while the mixed phases of CeO₂ and Ce(CO₃)₂·H₂O constitute the phases of the undoped precursor by azeotropic distillation processing. In Fig.2, Ce₂(CO₃)₃·H₂O disappears because of high temperature of the later step of the azeotropic distillation. In this step, Ce₂(CO₃)₃·H₂O is oxidized into Ce(CO₃)₂·H₂O, some of which is decomposed into CeO₂.

Fig.1 XRD pattern of undoped precursor without azeotropic distillation processing

Fig.2 XRD pattern of undoped precursor by azeotropic distillation processing

DSC/TG curves of the undoped CeO₂ precursor by drying directly and azeotropic distillation processing are shown in Figs.3 and 4. It can be seen from Fig.3 that obvious endothermal peak appears at 135.6 ℃ because of the dehydration of physical planar water of the precursor, and 21.58% mass loss can be observed. The exothermal peak at 278.6 ℃ indicates that Ce₂ (CO₃)₃·H₂O is oxidized and decomposes into CeO₂, with a mass loss of 20.82%.

In Fig.4 no obvious endothermal peak and little mass loss are observed before 200 ℃. This can be attributed to the removal of the adsorbing water after butanol azeotropic distillation. The exothermal peak around 235 ℃ shows that the precursor is oxidized and decomposed, and about 10.8% mass loss was observed in the TG curve correspondingly. The exothermal peak around 341 ℃ indicates that the organics (butoxy) is carbonized and burned, and about 5.8% mass loss is observed in the TG curve correspondingly.

Fig.3 DSC/TG curves of the undoped precursorwithout azeotropic distillation processing

Fig.4 DSC/TG curves of undoped precursor by azeotropic distillation processing

3.2      Influence of azeotropic distillation on crystal size of CeO₂

Fig.5 shows the XRD pattern of the undoped CeO₂ synthesized by normal method and azeotropic distillation method. The single fluorite-phase is observed in both curves, and the peak intensities of the undoped CeO₂ synthesized by normal method are stronger. The crystal sizes are 6.12 nm (a) and 5.83 nm (b), respectively. So it is indicated that azeotropic distillation can reduce the crystal size of CeO₂.

It can be observed that the sizes of the CeO₂ particles are about 10-20 nm from Figs.6 and 7. Seriously agglomeration is observed from Fig.6(a) but good dispersion is shown in Fig.6(b). The azeotropic distillation evidently restrains agglomerating.

Fig.5 XRD pattern of undoped CeO₂ nanoparticles calcined at 300 ℃ for 1 h (a) synthesized without azeotropic distillation processing and (b) synthesized by azeotropic distillation processing

The small crystallite size is ascribed to the azeotropic distillation process[13,14]. The purpose of this method is to remove the water contained in the wet colloid in the form of azeotropic substance, so it can prevent the formation of hard agglomerates during the upcoming processes of drying and calcinations. Usually, in wet colloid, the excess water molecules interact with the free hydroxyls on the surface of the colloid particles through hydrogen bonds. When particles get close, this kind of water molecules will draw neighboring particles together. These bridging water molecules can be removed when the colloid begins to dry and the hydrogen bonds between hydroxyls on the surface of two neighboring particles will draw them closer. Further drying process will cause the formation of strong chemical bonds between neighboring particles, and then the hard agglomerates appears:

Ce—OH+HO—Ce→Ce—O—Ce+H₂O           (2)

However, after the azeotropic distillation process, the excess water molecules in the colloid were removed and the hydroxyl groups on the surface of the particles were replaced by-OC₄H₉ group. Then the possibility for the formation of chemical bonds was greatly eliminated. This can be explained by the following facts. Firstly, the excess butanol molecules cannot draw particles together through the formation of hydrogen bonds. Secondly, during the following solvent removal process, no hydrogen bond can be formed between neighboring particles because the hydroxyl groups on the particle surface are replaced by butoxy ones. Thirdly, the butoxy group has steric hindrance that can prevent the approach of particles[8]. As a result, the azeotropic distillation dramatically reduces the possibility of the formation of chemical bonds and prevents the formation of hard agglomerates.

Fig.6 TEM micrographs of undoped CeO₂ nanopartcles calcined at 300 ℃ for 1 h: (a) Synthesized without azeotropic distillation processi; (b) Synthesized by azeotropic distillation processing

Fig.7 TEM micrographs of n(ZnO)/n(CeO₂) nanopartcles calcined at 300 ℃ for 1 h: (a) Synthesized without azeotropic distillation processing; (b) Synthesized by azeotropic distillation processing

3.3 Influence of ZnO doping on crystal size of CeO₂

XRD patterns of CeO₂ doped with different amounts of ZnO are shown in Fig.8. The peaks of different doping amounts are similar and no other substance emerges as the doping amount increases. No residuary material such as ZnO is observed. The chemical constitution was measured by atomic absorption spectroscopy and the results show that the mass fraction of Zn is 0.66% in the powder of n(ZnO/n(CeO₂) of 10%, and 2.82% in the powder of n(ZnO)/n(CeO₂) of 10%. The theoretical contents are 0.63% and 3.04% correspondingly. It can be concluded that solid solution is formed when the doping amount changes from 2% to 10%.

Fig.8 XRD patterns of CeO₂ nanoparticles (doped with different molar fractions of ZnO) calcined at 700 ℃ for 1 h

Table 1 shows the crystal sizes of CeO₂ calculated by SCHERRER formula. The crystal size of CeO₂ increases as the doping amount increases. It indicates that crystal growth rate is changed by doping and the crystal size is growing. It can be explained by the forming of the solid solution, more vacancy comes out and the particle diffusing is promoted.

Table 1 Variation of crystal size of CeO₂(calcined at 700 ℃ for 1 h) with doping amount

3.4    Influence of calcined temperature and time on crystal size of CeO₂

XRD patterns of n(ZnO)/n(CeO₂) of 10% are shown in Fig.9. Similar diffraction peaks can be observed. The peak intensities become stronger but the half-widths become narrower as the temperature increases. So the crystal size increases and the particle grows more completely. Table 2 shows the variation of the crystal size of CeO₂ with doping amount. The particle is at the early stage of structure forming in the lower temperature. At this time the particle has little pervasion ability and the grain is difficult to grow. As the temperature increases, the diffusing of the particle becomes more active. And because of the grain boundary moving at high temperature, the crystal size becomes bigger at higher temperature.

Fig.9 XRD patterns of n(ZnO)/n(CeO₂) of 10% nanoparticles calcined at different temperatures

Table 2 Variation of crystal size of n(ZnO)/n(CeO₂) of 10%(calcined for 1 h) with calcination temperature

Fig.10 and Table 3 show the XRD patterns of n(Zn)/n(CeO₂) of 4% calcined at different time and the variation of the crystal size of n(ZnO)/n(CeO₂) of 10% with calcination temperature. The diffraction peak intensities become stronger as the temperature increases. The longer the calcination time, the more stable the crystal lattice and the bigger the grain size.

3.5      Influence of ZnO doping on UV-shielding properties

Fig.11 shows the optical transmittance of 6%-10% (molar fraction) ZnO-doped ceria as well as undoped samples, over 250 to 600 nm wavelength range. The transmittance spectra of all samples are almost identical, indicating high transparency in a visible light region above 400 nm and high UV-shielding property below 400 nm. These data suggest that doping with zinc oxide doesn’t result in the loss of UV-shielding ability and transparency in the visible light region. The results are in accord with those of some former reports[3,5].

Fig.10 XRD patterns of n(ZnO)/n(CeO₂) of 4% nanoparticles calcined at 1 000 ℃ for different times

Table 3 Variation of crystal size of n(ZnO)/n(CeO₂) of 4%(calcined at 1 000 ℃) with calcination time

Fig.11 Transmittance spectra of ethanol solution of undoped and ZnO-doped CeO₂: 1—Undoped CeO₂；2—6%(molar fraction) ZnO-doped CeO₂, 700 ℃, 4 h；3—10% ZnO-doped CeO₂, 700 ℃, 4 h

4 Conclusions

ZnO-doped ceria nanoparticles with crystal size of 10-20 nm were successfully prepared by the coprecipation technique. The use of azeotropic distillation decreases the size of the as-prepared powders. Solid solution is formed when 2%-10% ZnO was doped. Doping with ZnO can increase the crystal size of the product. The crystal size of the as-synthesized powder increases with the increase of the amount of adulterant, calcination temperature and time. Nano-sized CeO₂ shows transparency in visible-light region and has excellent UV-absorption activity. Doping of ZnO doesn’t lead to the reduction of UV-shielding ability of ceria.

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(Edited by CHEN Can-hua)

Foundation item: Project (2005DFBA028) supported by the International Cooperation of Science and Technology Ministry of PRC; Project (59925412) supported by the National Natural Science Foundation of China

Corresponding author: SONG Xiao-lan; Tel: +86-731-8877203, 13808422020, E-mail: xlsong@hnu.cn