HEOPrAHEHECKEE MATEPHAXbl, 2010, moM 46, № 4, c. 458-464
UDC 541.14
PREPARATION OF Er3+:YAlO3/ZnO COATING COMPOUND BY SOL-GEL METHOD AND PHOTOCATALYTIC DEGRADATION OF ORGANIC DYES
UNDER SUN LIGHT IRRADIATION
© 2010 Jun Wang*, Jia Li*, Yingpeng Xie*, Liqun Zhang**, Guangxi Han*, Ying Li*, Rui Xu*, Xiangdong Zhang*
*Department of Chemistry, Liaoning University, Shenyang, China **Department of Pharmacy, Liaoning University, Shenyang, China e-mail: wangjun890@126.com Received 09.04.2009
In this work, the Er3+:YAlO3, as upconversion luminescence agent, was prepared and coated by ZnO film through sol-gel technique and the Er3+:YAlO3/ZnO coating compound, a novel photocatalyst, with high activity under sun light irradiation was subsequently prepared. The Er3+:YAlO3 and Er3+:YAlO3/ZnO were characterized by XRD. The activity of Er3+:YAlO3/ZnO was tested by photocatalytic degradation of acid red B in aqueous solution. The experimental results proved that the Er3+:YAlO3/ZnO was able to decompose the acid red B efficiently, and it is promising to use the idea to develop new photocatalyst with high activity for degradation under sun light irradiation.
INTRODUCTION
In the field of decontamination, various semiconductor materials still feature dominantly in most of the works reported, as they are the most efficient for most ofthe electronic and chemical processes involved in photocatalytic reactions [1—3]. However, all these applications are based on their remarkable activity to degrade almost any organic molecule under ultraviolet light irradiation [4]. To use sun light efficiently, many reformative methods were adopted such as doping of transition-metal ions [5], doping of non-metallic atoms [6], combination of narrow band-gap semiconductors [7] and aggradation of noble metals [8]. These methods result in a shift of light absorption into the visible range, but only in very few cases does it also enable generation of a current or a chemical reaction by visible light. We always think that the photogenerated electron-hole pairs excited by different wavelength light are different, as their oxidation-reduction potentials are different. We also insist that the oxidation potential of the holes excited by ultraviolet light must be higher than that excited by visible light, and only those holes with strong oxidation ability can more easily decompose the stable organic pollutants, or indirectly degrade them through fleetly oxidizing H2O molecules to generate the -OH radicals with high oxidative ability, without ready recombination.
With the same purpose ofusing sun light efficiently, we think of the method of coating upconversion luminescence agent, which can emit ultraviolet light under visible light excitation, with semiconductor film. When the semiconductor film is irradiated by sun light, the inner upconversion luminescence agent emits the ultraviolet light which can effectively excite the surrounding ZnO films. Based on this principle, in our previous work we
prepared nano-sized TiO2 photocatalyst coating Er3+:Y3Al5O12 upconversion luminescence agent [9], and the experimental results proved the validity during the degradation of some organic dyes. Our continual interest in enhancing the photocatalytic activity of TiO2 farther and using sun light effectively encourages us to report the present investigation. The Er3+:^AlO3 is known as another upconversion luminescence agent with quite efficient visible to ultraviolet upconversion system. The emission bands around 318.7 and 320.1 nm were observed by 486.5 and 542.4 nm (or 548.8 nm) pumping [10, 11], respectively. Also the 326—342 and 354—359 nm upconversion luminescence by 652.2 nm (or 657.8 nm) pumping were detected [12]. Thus the upconversion process may easily take place in Er3+:YAlO3 crystal.
In this study, the Er3+:YAlO3 was coated by ZnO film though sol-gel method. The Er3+:YAlO3/ZnO coating compound was obtained and its photocatalytic experimental parameters were also investigated through the degradation of some dyes. These dyes are the most important class of synthetic organic dyes used in the textile industry and are therefore common industrial pollutants. Here, the acid red B (Fig. 1) was selected as the primary model contaminant to detect the photocatalytic activity of the Er3+:YAlO3/ZnO under sun light irradiation.
MATERIALS AND METHODS
The Er3+:YAlO3 was prepared by a precipitation process following the steps described elsewhere [13]. The Er001Y099AlO3 of molar ratio was adopted as upconver-
OH
Fig. 1. Molecular structure of acid red B.
sion luminescence agent in this work. First, proper amounts of yttrium and erbium nitrate were dissolved in distilled water. In a separate flask, aluminum nitrate was dissolved in distilled water and stirred for 1.0 h. The rare earth ions were added to the aluminum solution. Both solutions were mixed together and solid citric acid was added (mol ratio citric acid: metal ion is 3 : 1). The final solution was further evaporated at 85°C on a water bath until a pale yellow transparent, viscous gel was formed. The obtained gel was dried at 130°C for 24 h and grinded to obtain a powder. A heat treatment of 1200°C for 2.0 h was applied to the sample. After cooling in air atmosphere, the Er3+:YAlO3 was obtained.
The ZnO powder coating Er3+:YAlO3 upconversion luminescence agent as photocatalyst was prepared through the sol-gel process [14]. Zinc acetate dihy-drate [Zn(CH3COO)2 • 2H2O] (A.R., Aldrich Company, USA) was used as the zinc precursor. Solid citric acid (A.R., Aldrich Company, USA) was used as a sol stabilizer. The molar ratio of Zn(CH3COO)2 : citric acid was controlled as 2 : 1 and the last quality ratio of ZnO : Er3+ : YAlO3 was controlled as (1 — x) : x (x = = 0.05, 0.1, 0.15, 0.20, 0.25). Firstly, citric acid was added into the prescribed amount of Zn(CH3COO)2 solution and the mixture was continuously stirred for 30 min. Secondly, Er3+:YAlO3 powder were added into the mixture solution, and the final solution was continuously stirred at 85°C for hydrolysis reaction of
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Fig. 2. The apparatus of sun light irradiation: 1 — without any catalyst (in the dark); 2 — ZnO (in the dark); 3 — Er+:YAlO3/ZnO (in the dark); 4 — without any catalyst + + sun light irradiation; 5 — ZnO + sun light irradiation; 6 - Er3+ :YAlO3/ZnO powder + sun light irradiation.
Zn(CH3COO)2. After aging for 48 h, Zn(CH3COO)2 was hydrolyzed to form Zn(OH)2 gel on the surface of Er:YAlO3 particles, and the precipitate was dried in air at 110°C for 24 h to obtain a dried gel which was grinded and heat-treated at 300, 500 and 700°C, respectively, for 30, 60 or 90 min under air atmosphere in a tubetype furnace with a heating rate 50°C/min. For comparison, the pure ZnO powder was also prepared adopting the same procedure without the addition of Er3+:YAlO3 during sol-gel process.
The XRD patterns were determined by powder XRD (RINT 2500, XRD-Rigaku Corporation, Japan) using Ni-filtered CuK"a radiation in the range of 29 from 10° to 70° for confirming the crystal phases of Er3+:YAlO3 and Er3+:YAlO3/ZnO.
The experiments of the photocatalytic degradation were carried out adopting a self-made irradiation apparatus (Fig. 2) under the conditions such as 10 mg/l acid red B concentration, 1000 mg/l Er:YAlO3/ZnO amount, 50 ml total volume and 19—25°C temperature. The reactor is made up of normal glass (Na2O • CaO • SiO2). The acid red B suspension containing Er3+:YAlO3/ZnO powders was irradiated by sun light (China, Shenyang area, E123°24'N41°50', 19-25°C temperature from March to April at midmorning of9:30—10:30 a.m.). The suspensions at specific intervals were sampled to monitor the changes of leftover acid red B concentration. Sampled suspensions were centrifuged at 4.000 rpm for 20 min to remove the catalyst powder and then analyzed by UV-vis spectrophotometer (LAMBDA-17, Perkin-Elmer Company, USA).
RESULTS AND DISCUSSION
The XRD of Er3+:YAlO3/ZnO coating compounds.
The XRD pattern for Er3+:YAlO3 heated at 1200 °C (Fig. 3b) is similar to the JCPDS file for YAP (№70-1677) (Fig. 3a), which shows no impurity phases, such as Y(Al12O9 and Y,Al5O12 in synthetic compound. Fig. 3d shows that the XRD pattern ofEr3+:YAlO3/ZnO with 15 % weight Er:YAlO3 retains the ZnO crystalline form (Fig. 3c). In addition, in the XRD pattern there is some new peaks belonging to Er3+:^AlO3, which implies that the Er3+:YAlO3 and ZnO have integrated well.
Influences of Er3+:YAlO3 content, heat-treated temperature and heat-treated time on photocatalytic activity of Er3+:YAlO3/TiO2 coating compounds. It is
found from Fig. 4a that the photocatalytic activity of Er3+:YAlO3/ZnO rises with the increase of Er3+:YAlO3 content except for 5.0%, and is higher than that of ZnO for the similar system. It may be attributed to the fact that the ZnO absorbs more ultraviolet light resulted from the upconversion luminescence process. Because of the presence of Al3+ it can be seen that the adsorption ratio of Er3+:YAlO3/ZnO is also enhanced
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Fig. 3. (a) JCPDS file of YAlO3 (YAP) (№ 70-1677); (b) XRD of Er3+:YAlO3; (c) XRD of ZnO; (d) XRD of Er3+:YAlO3/ZnO
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Fig. 4. Influence of Er3+ :YAlÜ3 content (a) heat-treated temperature (b) and heat-treated time (c) on the photocatalytic activity of E
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