= ФОТОХИМИЯ И МАГНЕТОХИМИЯ
УДК 541.14
PREPARATION OF Er3+: YAlO3/Fe-DOPED ZnO COMPOSITE AND INVESTIGATION ON SOLAR LIGHT PHOTOCATALYTIC DEGRADATION OF ORGANIC DYES
© 2011 J. Q. Gao, X. Y. Luan, J. Wang, L. N. Yin, K. Li, Y. Li, B. X. Wang, P. L. Kang
Department of Chemistry, Liaoning University, Shenyang 110036, P. R. China E-mail: photocawj890@126.com Received November 09, 2010
Abstract — The Er3+:YAlO3/Fe-doped ZnO composite, a new photocatalyst which could effectively utilize visible light, was prepared. In succession, the Er3+:YAlO3/Fe-doped ZnO was characterized by XRD and SEM, respectively. Acid Red B dyes, was degraded under solar light irradiation to evaluate the photocatalytic activity of the Er3+:YAlO3/Fe-doped ZnO. In addition, the effects of Er3+:YAlO3 content, heat-treated temperature and time on the photocatalytic activity of Er3+:YAlO3/Fe-doped ZnO were reviewed. Otherwise, the effect of initial dye concentration, Er3+:YAlO3/Fe-doped ZnO amount and solar light irradiation time on the photocatalytic degradation of Acid Red B were also investigated. It was found that the photocatalytic activity of Er3+:YAlO3/Fe-doped ZnO is much higher than that Fe-doped ZnO and pure ZnO for the similar system. Perhaps, the use of the Er3+:YAlO3/Fe-doped ZnO may provide a new way to take advantage of ZnO in sewage treatment aspects using solar energy. Keywords: organic dyes, photocatalytic degradation.
INTRODUCTION
Zinc oxide (ZnO) has been a well-known photocatalytic material for the past few decades [1, 2]. When the ZnO is irradiated by ultraviolet light, the electron-hole pairs are generated and they will produce powerful active oxygen species, which can decompose most organic compounds. However, ZnO can only absorb a small portion of solar spectrum in the ultraviolet region [1] and has high recombination rate of photo-induced electron-hole pairs at or near its surface [3]. The efficient use of solar light becomes an appealing challenge for developing photocatalysis. One approach for achieving this objective is doping other materials, including metal ions and other semiconductors, into the surface of ZnO nanoparticles [4, 5]. Of which, the doped Fe3+ cation, as electron capturer, can restrain the recombination of photogenerated electron-hole pairs.
With the purpose of using solar energy more efficiently, some substances, which could transform visible light into ultraviolet light, could be used to combine with the Fe-doped ZnO composite. The visible-to-ultraviolet upconversion luminescence agent is a good choice. Due to the rich energy level structure, trivalent erbium ions hosted in fluoride and oxide crystal matrices can emit the light in the ultraviolet and visible wavelength range, depending on the excitation energy, concentration of dopant as well as the properties of the host crystal. In our previous work, the studies had shown that the use of 40CdF2 ■ 60BaF2 ■ 0.8Er2O3/TiO2 and
Er3+:YAlO3/ZnO composites significantly enhanced the photocatalytic activity of photocatalyst [6, 7].
In this article, the Er3+:YAlO3 was prepared as an efficient visible-to-ultraviolet upconversion luminescence agent, which was mixed with Fe-doped ZnO by an ultrasonic dispersion and liquid boil method. And its photocatalytic activity for the degradation of Acid Red B was investigated. The results showed a better photocatalytic performance than Fe-doped ZnO. The parameters for the photocatalytic degradation of Acid Red B dye under solar light have been studied adopting Er3+:YAlO3/Fe-doped ZnO. The mechanism about improvement for the activity of the Er3+:YAlO3/Fe-doped ZnO is proposed as well. Being different from the conventional methods, it was inferred that the Er3+:YAlO3 could supply the ample ultraviolet light for satisfying the requirement of Fe-doped ZnO. Furthermore, it is easy to discover the raw materials and prepare the composite. It is possible to provide a new way to utilize Er3+:YAlO3/Fe-doped ZnO under solar light irradiation and reduce the cost for wastewater treatment.
EXPERIMENTAL
Synthesis of Er3+:YAlO3. The nanocrystalline Er3+:YAlO3, as an upconversion luminescence agent, was prepared by nitrate-citric acid technique as described before [8-10]. Firstly, the proper yttrium and
2162
erbium oxides were mixed together in a beaker with distilled water. And then, the right amount of concentrated HNO3 was added drop by drop forming the nitrate solution. Secondly, the Al(NO3)3 solution were mixed into the above solution under vigorous agitation. Finally, solid citric acid was added as much as triple metal ion. The solution was further stirred to combine into the final solution and then evaporated at 85°C on a water-bath until a pale yellow transparent sol was obtained. The sol was dried at 130°C for 24 hours until became finely-ground powder following milling. The nano-sized Er0.01Y099AlO3 was obtained after heat treatment at 1200°C for 120 min.
Preparation of Fe-doped ZnO composite. The Fe-doped ZnO was prepared by the sol—gel process. Zn(CH3COO)2 • 2H2O, was used as the zinc precursor. Solid citric acid was used as a sol stabilizer and the molar ratio of Zn(CH3COO)2 : citric acid was controlled as 2 : 1. And the Fe(NO3)3 was also added for the metal doping. Firstly, citric acid was added into the prescribed amount of Zn(CH3COO)2 solution and the mixture was continuously stirred for 1.0 h. Secondly, Fe(NO3)3 was added into the mixture solution (Fe : Zn molar ratio 0.5%) and the pH value was controlled between 8—9 by ammonia. The mixture was continuously stirred at 85°C for complete hydrolysis reaction of Zn(CH3COO)2. After aging for 48 hours, Zn(CH3COO)2 was hydrolyzed to form Zn(OH)2 gel, and the precipitate was dried in air at 110°C for 24 h to obtain a dried gel. The dried gel was grinded and heat-treated at 400, 550 and 700°C, respectively, for 30, 50 and 70 min with a heating rate 50 K min-1 under air atmosphere in a tube-type furnace. Finally, a series of Fe-doped ZnO were obtained.
Preparation of Er3+:YAlO3/Fe-dopedZnO composite. The mass percentage of Er3+:YAlO3 to Fe-doped ZnO are from 0.00 to 5.00% at 1.00% intervals. And the mixture of Er3+:YAlO3 and Fe-doped ZnO were placed in 100 ml beaker diluted with 50 ml de-ionized water and well dispersed by ultrasonics apparatus (KQ-100, 80 kHz, 50 W, Kunshan ultrasonic apparatus Company, China) for 15 min. The suspended liquid was constantly heat treated for 30 min at boiling point. After filtrating and washing, the deposit was grinded and heat-treated at 400, 550 and 700°C, respectively, for 30, 50 or 70 min with a heating rate 50 K min-1 under air atmosphere in a tube-type furnace. Finally, a series of Er3+:YAlO3/Fe-doped ZnO were obtained. For comparison, the Fe-doped ZnO without Er3+:YAlO3 was also prepared by the same procedure.
Experiments of photocatalytic degradation. The experimental conditions such as 100 ml total volume, 10 mg/l Acid Red B initial concentration and 1.0 g/l catalyst loading were kept throughout this work if special requirement was not involved. The different catalysts were mixed into Acid Red B solutions well in conical flasks. The mixture solutions were placed in the dark
for 30 min with full agitation, and then a small quanity of suspension was taken from the different solutions to determine the adsorption ratio. The leftover solutions were irradiated by solar light and sampled at different time intervals to determine the degradation ratio. The sampled suspensions were centrifuged at 4,000 rpm for 20 min to remove the catalysts and then analyzed by UV-vis spectrophotometer. The adsorption ratio (ra) and degradation ration (rdeg) were determined from the change in intensity of ^max of the Acid Red B using the following equation:
ra, % = [(Co - Ca)/Co] X 100, ^deg, % = [(Co - Ct)/Co] X 100 , ^rel, % = [(Ca - Ct)/Ca]X 100 ,
where c0 is the initial concentration of Acid Red B solution, ca is the dye concentration after adsorption by catalysts and ct is the final concentration after solar light irradiation, rrel is relative degradation ratio.
RESULTS AND DISCUSSION
XRD and SEMof Er3+:YAlO3/Fe-dopedZnO composite
The prepared Er3+:YAlO3, Fe-doped ZnO and Er3+:YAlO3/Fe-doped ZnO were all investigated by powder X-ray diffractometer (RINT 2500, XRD-Rigaku Corporation, Japan) using Ni-filtered CuKa radiation in the range of 20 from 10° to 65° (or 70°). Fig. 1c shows a XRD pattern of 3.0 mol. % Er3+-doped YAlO3 heated to 1200°C for 120 min. Apparently, the X-ray pattern of Er3+:YAlO3 is similar to the JCPDS file data of YAlO3 (Fig. 1d). Figure 1a shows that the Er3+:YAlO3/Fe-doped ZnO with heat-treatment basically retatin the Fe-doped ZnO crystalline form (Fig. 1b). However, the diffraction peaks of Er3+:YAlO3/Fe-doped ZnO are slightly higher than those of Fe-doped ZnO. It is because that Er3+:YAlO3 added make the crystallinity of Fe-doped ZnO increase. In addition, the X-ray pattern of Er3+:YAlO3 centered at 20 = 34.5° in Er3+:YAlO3/Fe-doped ZnO is hardly found, which implies that the Er3+:YAlO3 particles have already distributed on the surface ofFe-doped ZnO particles uniformly.
The SEM image of the Er3+:YAlO3/Fe-doped ZnO in Fig. 1e reveals the nubbly form with a size of approximately 0.5-1.0 ^m. It can be seen clearly that there are some small grains encased on the surface of Fe-doped ZnO, which illuminates that the Er3+:YAlO3 were doped with the Fe-doped ZnO well. In contrast, the surface of the Fe-doped ZnO shown in Fig. 1f is composed of a great deal of slices with the same average size at the Er3+:YAlO3/Fe-doped ZnO.
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Fig. 1. The XRD (a) of Er3+:YAlO3/Fe-doped ZnO (with 3.0 wt % Er3+:YAlO3 under 550°C and 50 min heat-treatment); (b) Fe-doped ZnO (under 550°C and 50 min heat-treatment); (c) Er3+:YAlO3 (with 1.0 mol % Er3+ under 1200°C and 120 min heat-treatment) and (d) YAlO3 (JCPDS file), and SEM (e) of Er3+
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