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Efficient photocatalytic degradation of organic dyes over titanium dioxide coating upconversion luminescence agent under visible and sunlight irradiation
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Applied Catalysis A: General 334 (2008) 227–233 www.elsevier.com/locate/apcata
Efficient photocatalytic degradation of organic dyes over titanium dioxide coating upconversion luminescence agent under visible and sunlight irradiation Jun Wang a,*, Ronghe Li a, Zhaohong Zhang b, Wei Sun a, Rui Xu a, Yingpeng Xie a, Zhiqiang Xing a, Xiangdong Zhang a a b
Department of Chemistry, Liaoning University, Shenyang 110036, PR China Department of Environment, Liaoning University, Shenyang 110036, PR China
Received 9 July 2007; received in revised form 3 October 2007; accepted 10 October 2007 Available online 16 October 2007
Abstract In order to use the sunlight efficiently, a new titanium dioxide (TiO2) photocatalyst with high catalytic activity under visible light irradiation was prepared with sol–gel technique. In this work, an upconversion luminescence agent, crystallized Er3+:Y3Al5O12, was synthesized and its characters were determined. It is found that this crystallized Er3+:Y3Al5O12 can emit three upconversion fluorescent peaks below 387 nm under the excitation of 488 nm visible light. Hence, this upconversion luminescence agent could transform visible light into ultraviolet light, which could satisfy the genuine requirement of TiO2 photocatalyst. Additionally, the upconverison mechanisms were also discussed. Meanwhile, the prepared TiO2 photocatalysts coating upconversion luminescence agent were characterized by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The photocatalytic activity of prepared TiO2 powder was tested through the degradation of congo red in aqueous solution as a model compound under visible and sunlight irradiation. To affirm the complete mineralization, the ion chromatography and total organic carbon (TOC) were used to observe the mineralized anions and organic residues. The experimental results proved that the prepared TiO2 photocatalyst coating crystallized Er3+:Y3Al5O12 behaved much higher photocatalytic activity under visible light and sunlight irradiation, and was able to decompose the congo red in aqueous solution efficiently. Therefore, this method may be envisaged as a novel technology for treating dyes wastewater using solar energy, especially for textile industries in developing countries. # 2007 Elsevier B.V. All rights reserved. Keywords: Upconversion luminescence agent; TiO2 photocatalyst; Sunlight; Visible light; Congo red
1. Introduction Titanium dioxide (TiO2) as photocatalyst has widely been used to photodegrade the organic pollutants for its relatively high catalytic reactivity, physical and chemical stability, avirulence and cheapness [1,2]. However, it is generally accepted that the TiO2 powder is a poor absorber of photons in the solar spectrum. To destroy the organic pollutants in wastewater effectively, the ultraviolet light (l < 387 nm) must be required to irradiate the TiO2 photocatalyst for its broad band-gap (Eg = 3.2–4.5 eV) [3]. For a long time, much attention has been drawn on extending the range of absorption wavelength of TiO2 photocatalyst, namely,
* Corresponding author. Tel.: +86 24 62202053; fax: +86 24 62202380. E-mail address: [email protected] (J. Wang). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.10.009
making the TiO2 powders as photocatalyst absorb the visible light at large extent. Many reformative methods were adopted such as doping of transition-metal ions [4,5], combination of narrow band-gap semiconductors [6,7] and aggradation of noble metals [8,9]. These methods do its work in extending the range of absorption wavelength of TiO2 photocatalyst, but they suffer from a thermal instability, an increase of carrier-recombination centers or the requirement of an expensive ion-implantation facility and complex-treated process [10]. Still some research groups have found the N-doping method. These N-doping TiO2 powders could be excited by visible light, so it may be a promising way of achieving visible light activity [11]. Although some recent concerning experiments showed that the absorption edge of treated TiO2 powders could shift toward wavelength of 550 nm [12], they also suffer from the increased recombination of electron and hole pairs [13]. In contrast with the traditional
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ideas extending the range of absorption wavelength, with the same purpose of using visible light efficiently, we considered whether there were some substances which could transform visible light into ultraviolet light to satisfy the genuine requirement of TiO2 photocatalyst. Fortunately, the upconversion luminescence materials containing rare earth metals are just what we want to introduce into TiO2 powder. In this study, the upconversion luminescence agent, crystallized Er3+ (1.0%):Y3Al5O12 powder, was prepared and then coated by TiO2 film through the sol–gel process. Lastly, we obtained the TiO2 photocatalyst coating crystallized Er3+:Y3Al5O12, which is active in the photocatalytic degradation of organic dyes under visible light irradiation. Here, the congo red usually contained in the dye industry wastewater was selected as a model contaminant, the photocatalytic activity of prepared TiO2 powder for the degradation of organic contaminants under visible light irradiation were investigated. Additionally, the 700 8C was selected as heat-treated temperature for various TiO2 powders. The molecular structure of congo red anion is shown below (Scheme 1). 2. Materials and methods 2.1. Preparation of Er3+:Y3Al5O12 upconversion luminescence agent The nanocrystalline Er3+:Y3Al5O12 samples were prepared by the coprecipitation technique described elsewhere [14,15]. The influence of NO3 anions adsorbed on the surface of TiO2 particles has been affirmed to be very weak in photocatalytic activities in the previous studies [16], therefore, Er(NO3)3 was used in this work for the doping of Er3+ ions. The stoichiometric amounts of Er2O3 (99.99%) and Y2O3 (99.99%) were dissolved in dilute HNO3 (AR, purity) solution and then were mixed with Al(NO3)3 solution under vigorous agitation. The molar ratio of ions was controlled by 0.03Er3+:2.97Y3+:5.00Al3+, and the doping of Er3+ in Y3Al5O12 was realized. Then 2.0 mL of ethylene glycol and 2.0 mL of acetic acid were added to the mixed solution. The final solution was further stirred at 70 8C, until the jelly was formed. The obtained jelly was dried at 110 8C and ground to obtain a white powder. The samples were heated at 700 and 900 8C for 2.0 h to obtain a crystallized form, respectively. 2.2. Preparation of TiO2 photocatalyst coating upconversion luminescence agent The TiO2 photocatalyst coating Er3+:Y3Al5O12 upconversion luminescence agent was prepared through the sol–gel process.
The two precursor solutions, here denoted precursor A and B, were prepared as follows. Precursor A: (C4H9O)4Ti (40 mL), C2H5OH (50 mL) and CH3COOH (3.0 mL) were mixed and stirred for 10 min at room temperature. Precursor B: H2O (7.0 mL) and C2H5OH (50 mL) were mixed and the crystallized or amorphous (unheated) Er3+:Y3Al5O12 powders (0.5 g) were added under vigorous agitation to make the Er3+:Y3Al5O12 powder dispersed adequately in the solution. Afterwards, the precursor B was added to the precursor A dropwise with vigorous stirring. The resultant mixture was further stirred for 20 min. The sol solution was placed in a culture dish for 1.0 day to finish the sol–gel transition and filtrated. Filter residue was rinsed with water repeatedly, and then dried at 100 8C for 24 h to get a dried gel. The dried gel was ground and a light-yellow powder was obtained. The powder was then heat treated at 700 8C for 2.0 h to obtain stable TiO2 photocatalyst coating (crystallized or amorphous) Er3+:Y3Al5O12. The ratio of Er3+:Y3Al5O12 to TiO2 in photocatalyst is about 2.0%. The pure TiO2 powder was also prepared adopting the same procedure without the addition of Er3+:Y3Al5O12 during sol–gel process. The surface areas were determined as 105, 112, 120 m2/g, respectively, for TiO2 photocatalysts coating crystallized and amorphous Er3+:Y3Al5O12 and pure TiO2 photocatalyst according to BET method. 2.3. Characterization of prepared TiO2 photocatalyst The X-ray diffraction (XRD) patterns were determined by powder X-ray diffractometer (RINT 2500, XRD-Rigaku Corporation, Japan) using Ni-filtered Cu Ka radiation in the range of 2u from 108 to 708 for confirming the crystal phases of samples. The crystallite sizes were calculated from the diffraction peak widths using the Scherrer equation. The transmission electron microscope (TEM) (JEM-3010, JEOL Company, Japan) was used for observing the characters of prepared TiO2 particles and estimating the particle size. 2.4. Measurement of photocatalytic activity of prepared TiO2 photocatalyst The experiments of the photocatalytic degradation of congo red (696.68, AR, Tianjin Yazhong Chemical Company, China) in aqueous solution were carried out under the conditions such as 20 mg/L congo red concentration, 1.0 g/L prepared TiO2 photocatalyst, 50 mL total volume, 144 W irradiation power and 20.0 0.2 8C solution temperature. The congo red suspension containing various TiO2 powders was irradiated by triphosphor lamps (FL40T8EXD/36, Toshiba Company, Japan). The triphosphor lamp has a correlated color temperature (CCT)
Scheme 1. Molecular structure of congo red anion.
J. Wang et al. / Applied Catalysis A: General 334 (2008) 227–233
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4000 K and the illumination light intensity of triphosphor lamp is 964347 lx. The wavelength of the three colours of the spectra mainly spreads between 440–700 nm, the associated light intensity is 360 cd and the flux is 1380 lm. The suspensions at specific intervals were sampled to monitor the changes of leftover congo red concentration. Sampled suspensions were centrifuged at 4000 rpm for 20 min to remove the TiO2 powder and then analyzed by UV–vis spectrophotometer (LAMBDA-17, Perkin-Elmer Company, USA). The ion chromatography (ICS90, Dionex Company, USA) was used to observe the mineralized anions produced during degradation process in the solution. The total organic carbon (TOC) was determined by a Shimadzu TOC analyzer (TOC 500, Shimadzu Corporation, Japan). 3. Results and discussion 3.1. The XRD of Er3+:Y3Al5O12 upconversion luminescence agent Fig. 1 shows the XRD patterns of the Er3+:Y3Al5O12 powder. It indicates that the Er3+:Y3Al5O12 powder heat treated at 700 8C (Fig. 1a) barely presents a crystallized form, and the main peak of the cubic structure of Y3Al5O12 is centered at 2u = 33.58, while the crystal phases of Y4Al12O9 and YAlO3 powders are also found. However, the Er3+:Y3Al5O12 powder heat treated at 900 8C (Fig. 1b) presents a well-defined onefold crystal phasic structure of Y3Al5O12, and no secondary impurity phase such as Y4Al12O9 and YAlO3. 3.2. The XRD of prepared TiO2 photocatalyst coating upconversion luminescence agent The XRD patterns of various prepared TiO2 powders heat treated at 700 8C were shown in Fig. 2. It can be seen that for three cases (a: prepared pure TiO2 powder; b: prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 (unheated); c: prepared TiO2 powder coating crystallized Er3+:Y3Al5O12
Fig. 2. XRD of various TiO2 powders after 700 8C heat treatment: (a) prepared pure TiO2 powder, (b) prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 (unheated) and (c) prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 (900 8C heat treated).
(900 8C heat treated) all consist of a mixture of anatase (about 10%) and rutile (about 90%) phase TiO2 powders. Additionally, as the experimental XRD data (not shown in this paper) proved, the simplex anatase phase TiO2 powder was produced at 500 8C heat treatment, and mixed phase TiO2 powder of anatase and rutile almost in the equal proportion fraction was obtained at 600 8C, while both are high adsorptive for some dyes even though in the dark, compared to the one heat treated at 700 8C. Thus, the 700 8C heat treatment was controlled intentionally. From the heights and widths of the X-ray diffraction peaks, the average crystallite size was calculated according to the Debye– Scherer equation. The size of crystallized Er3+:Y3Al5O12 powder is about 30–35 nm, while the size of prepared pure TiO2 powder is about 45–55 nm based on Fig. 2a. The average size of the prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 is close to 65–75 nm. Hence, the measured XRD pattern of the prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 exhibits narrower and higher peaks compared with that of prepared pure TiO2 powder. Additionally, in the XRD pattern of Fig. 2c, there is a new and slight peak centered at 2u = 33.58 observed in the TiO2 sample coating crystallized Er3+:Y3Al5O12, which naturally attributes to the addition (2.0%) of crystallized Er3+:Y3Al5O12. However, any new peak is not found in Fig. 2b, because the Er3+:Y3Al5O12 powder is mostly amorphous at 700 8C heat treatment. 3.3. The TEM of TiO2 prepared photocatalyst coating upconversion luminescence agent
Fig. 1. XRD of Er3+:Y3Al5O12 powders at different heat-treated temperatures: (a) 700 8C heat treatment and (b) 900 8C heat treatment.
Fig. 3 shows the TEM images of the prepared TiO2 powder coating crystallized Er3+:Y3Al5O12. It was found from Fig. 3a that the prepared TiO2 powder is composed of a large quantity of grayer spherical particles with approximatively uniform size
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Fig. 4. UV–vis spectra of congo red solution in the presence of prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 under different irradiation time: (a) 0.0 h irradiation (original solution), (b) 1.0 h irradiation, (c) 2.0 h irradiation, (d) 3.0 h irradiation, (e) 4.0 h irradiation and (f) 5.0 h irradiation.
Fig. 3. TEM of prepared TiO2 particles coating crystallized Er3+:Y3Al5O12 after 700 8C heat treatment (ULA: upconversion luminescence agent): (a) 20,000 times magnification and (b) 100,000 times magnification.
and shape. The average size of these particles estimated from the TEM image is about 50–70 nm, which is approximatively consistent with the calculated results based on Debye–Scherer equation. The black parts in central regions of many grayer spherical particles can be found clearly, which can be confirmed as the crystallized Er3+:Y3Al5O12 particles. In order to observe the conformation and composition of the prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 more obviously, in which one TiO2 particle in high-magnification TEM image was chosen for making the details illumination. As seen in Fig. 3b, the TiO2 folium becomes distinguishable and the nearly spherical particle of crystallized Er3+:Y3Al5O12 may be used as a core. The size of central Er3+:Y3Al5O12 particle is about 30– 50 nm, so that the thickness of TiO2 folium should be about 20– 30 nm. Hence, the visible light may sufficiently penetrate through such TiO2 folium and excite the inner crystallized Er3+:Y3Al5O12 particles, which must result in the emission of ultraviolet light. 3.4. The UV–vis spectra of congo red solution during photocatalytic degradation To demonstrate the potential applicability of the prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 as photocatalyst utilizing visible light, its photocatalytic activity was studied through the degradation of congo red in aqueous solution. Fig. 4 shows the changes of absorption spectra of congo red solutions
(20 mg/L intitial concentration and 50 mL total volume) in the presence of prepared TiO2 powder (50 mg/50 mL) coating crystallized Er3+:Y3Al5O12 at different irradiation time. The maximum absorption peak at 500 nm corresponding to the azo bond of congo red molecule diminished gradually as the irradiation time increased. It disappeared completely after about 5.0 h. The absorption peaks at 345 and 280 nm in ultraviolet regions, corresponding to the naphthyl ring of congo red molecule, also disappeared eventually, though with a slight increase during initial degradation process. This phenomenon may be explained by the formation of intermediate products during the photocatalytic degradation, which have much bigger absorbency than that of original congo red molecule. Anyhow, these results suggest that the congo red molecule was completely decomposed. The sequence of the color change of congo red solutions, shown in the inset of Fig. 4, also displays the degradation process of congo red. It is clearly seen that the cardinal red color of starting solution gradually disappears along with increasing irradiation time. 3.5. The ion chromatograms of congo red solution during photocatalytic degradation The ion chromatograms of congo red solution during photocatalytic degradation are shown in Fig. 5. They clearly proved that the sulphur and nitrogen heteroatoms in congo red molecule were converted into simple inorganic SO42, NO3 and NO2 anions. Correspondingly, the peak intensity of NO3 and NO2 anion seems to be much lower compared with the theoretical content of nitrogen atom in congo red molecule. The reasons are as follows. On one hand, during the photocatalytic degradation, at the beginning the azo bond of congo red molecule is decomposed and then partly reduced to the NH4+ cation which can not be seen in the ion chromatogram. On the other hand, as the degradation reaction proceeds, a series of
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Fig. 5. Ion chromatograms of the congo red solution during photocatalytic degradation in the presence of prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 under visible light irradiation: (a) 1.0 h visible light irradiation, (b) 3.0 h visible light irradiation and (c) 5.0 h visible light irradiation.
nitrogen oxides (NOx) and nitrogen gas (N2) are produced. Some volatile nitrogen oxides and nitrogen gas will escape from the solution, which also results in the low peak intensity of NO3 and NO2 anion [17]. In fact, that is helpful for the elimination of nitrogen containing organic pollutants. Additionally, according to the results of TOC test, more than 95% of congo red was eliminated after 5.0 h irradiation. Hence, it can be confirmed that all carbon atoms in congo red solution were converted into nontoxic CO2 and then escaped from the solution. 3.6. The influences of irradiation time on the degradation ratio under visible and sunlight A further comparative experiment was also carried out to investigate the catalytic activities of various prepared TiO2 photocatalysts. The congo red solutions were subjected to a series of experimental conditions: (a) without any TiO2 catalyst only under visible light irradiation; (b) with prepared pure TiO2 powder in the dark; (c) with prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 in the dark; (d) with prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 in the dark; (e) with prepared pure TiO2 powder under visible light irradiation; (f) with prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 under visible light irradiation; (g) with prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 under visible light irradiation. The corresponding results of reaction kinetics studies are given in Fig. 6. From Fig. 6 it can be seen that the rate constant (k) related to curve a is smallest for all cases, which indicates that the congo red molecules are unable to be decomposed easily only under visible light irradiation. In addition, the rate constants for curves b–d are also very small within 6.0 h. It reveals that three kinds of prepared TiO2 photocatalysts all behave a little adsorption to congo red molecules in aqueous solution. But
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Fig. 6. Reaction kinetics of photocatalytic degradation of congo red under different conditions: (a) without any TiO2 catalyst only under visible light irradiation, (b) with prepared pure TiO2 catalyst in the dark, (c) with prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 in the dark, (d) with prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 in the dark, (e) with prepared pure TiO2 powder under visible light irradiation, (f) with prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 under visible light irradiation and (g) with prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 under visible light irradiation.
these adsorption amounts did not influence the calculation of following photocatalytic degradation ratios. Nevertheless, for the photocatalytic degradation related to curves e–g, the rate constants illuminate that the congo red molecules can be degraded quickly in the presence of various prepared TiO2 photocatalysts. Of course, the TiO2 powder coating crystallized Er3+:Y3Al5O12 under visible light irradiation displays the highest catalytic activity in these three prepared TiO2 photocatalysts. Hence, although some dyes could be degraded over pure TiO2 based on the self-photosensitized process of dyes under visible light irradiation [18], the degradation efficiency is much lower than that used TiO2 catalyst coating crystallized and amorphous Er3+:Y3Al5O12. In the following, the photocatalytic activity of the prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 under sunlight irradiation was further tested compared with prepared pure TiO2 powder. As shown in Fig. 7, the outdoor experiments (Shenyang City, China, E1238240 and N418500 ) were carried out from a.m. 08:30 to p.m. 16:30 of 20 June 2006, where the corresponding temperature range changes from 22.3 to 25.6 8C, respectively. The congo red solution in conical flask c in the presence of prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 becomes colorless after 8.0 h sunlight irradiation, while it still keeps light red color in conical flask b in the presence of prepared pure TiO2 powder. Whereas the congo red solution in conical flask a in the absence of any TiO2 catalyst is hardly decolorized naturally under sunlight irradiation. Similar experiments were also carried out for some other organic dyes. Through the photocatalytic degradations of congo red, crystal violet and rodanmine B dyes under sunlight irradiation, it is experimentally proved that the prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 has promising applied perspective
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Fig. 7. Photocatalytic degradation of congo red in aqueous solution under sunlight irradiation: (a) without any TiO2 catalyst, (b) with prepared pure TiO2 powder and (c) with prepared TiO2 powder coating crystallized Er3+:Y3Al5O12.
Fig. 8. Possible principle of exciting TiO2 particle coating crystallized Er3+:Y3Al5O12 with visible light.
in using solar energy. A more detailed understanding on the photocatalytic activities of prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 is explained as below.
upconversion process may easily take place in Er3+:Y3Al5O12 crystals. As illustrated in Fig. 8, the crystallized Er3+:Y3Al5O12 as upconversion luminescence agent coated by the TiO2 folium, under continuous excitation of visible light, can emit the ultraviolet light, which can effectively be absorbed by TiO2 parts to generate the electron–hole pairs. In the same way as general photocatalytic degradation, these holes not only directly decompose the congo red adsorbed on the surface of TiO2 particles, but also oxidize water molecule to form OH radicals with high activity and indirectly degrade the congo red in aqueous solution.
3.7. Principle of photocatalytic degradation under visible light irradiation Recently, some research groups have reported the luminescent characteristics of ultraviolet upconversion in Er3+:Y3Al5O12 crystals, and the upconversion mechanisms are also discussed in detail [19]. When the Er3+:Y3Al5O12 as upconversion luminescence agent is pumped by a 488 nm Ar+ laser, the upconversion signals at 271, 317 and 381 nm are observed. Otherwise, the upconversion signals at 320 nm is also found by a 647 nm laser excitation [19,20]. The upconversion fluorescence spectrum and energy-level diagram showing related transitions are also given. According to the energylevel data given in reference [20], the luminescence transitions, 271 nm (2H9/2 ! 4I15/2), 317 nm (2P3/2 ! 4I15/2) and 381 nm (4G11/2 ! 4I15/2), are assigned, respectively. According to the excitation conditions and the specific pump wavelengths used, the population in an excited state with an energy exceeding the energy of the pump photon may be achieved either by excited state absorption (ESA) or by energy transfer upconversion (ETU) or by photon avalanche (PA). In general, the upconversion processes could be identified by measuring the decay profiles of upconversion emission from different multiplets under one- or multiple-pulsed excitation [21]. Under one-pulsed excitation, the Er3+ ions were excited from the ground state to the 4F9/2 state by a ground state absorption process, ETU then takes place through different energy-flow pathways. Under the two-pulsed excitation, the Er3+ ions in the ground state absorb two photons through ESA from the 4F9/2 state, which leads to population and upconversion luminescence from the 2P3/2 state. Under multiple-pulsed excitation, the photon avalanche may take place. The visible light section in solar light contains continuous pump wavelengths (between 400 and 700 nm) used as excitation source, thus the
4. Conclusion In summary, the prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 showed much higher photocatalytic activity, especially in terms of using visible light, compared with the prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 and prepared pure TiO2 powder. Therefore, to obtain the TiO2 catalyst with high photocatalytic activity, the Er3+:Y3Al5O12 powder must be pre-crystallized through high temperature heat treatment and then coated by TiO2 folium through sol–gel process. Consequently, the crystallized Er3+:Y3Al5O12 as upconversion luminescence agent coated by TiO2 folium could transform visible light into ultraviolet light to satisfy the genuine requirement of TiO2 photocatalyst. Thus, the efficiency of utilizing visible light was largely enhanced. It is promising to use the idea to develop new TiO2 catalyst with high activity for photocatalytic degradation of organic pollutants, and to develop new energy materials using solar energy, such as solar cells. Acknowledgements The authors greatly acknowledge The National Natural Science Foundation of China for financial support. The authors also thank our colleagues and other students participating in this work.
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References [1] S.F. Villanueva, S.S. Martı´nez, Sol. Energy Mater. Sol. C 91 (2007) 1492. [2] J.S. Chen, M.C. Liu, L. Zhang, J.D. Zhang, L.T. Jin, Water Res. 37 (2003) 3815. [3] M.R. Hoffman, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [4] U. Diebold, Surf. Sci. Rep. 48 (2003) 53. [5] W.C. Hung, S.H. Fu, J.J. Tseng, H. Chu, T.H. Ko, Chemosphere 66 (2007) 2142. [6] P.V. Kamat, Chem. Rev. 93 (1993) 267. [7] Y. Bessekhouad, N. Chaoui, M. Trzpit, N. Ghazzal, D. Robert, J.V. Weber, J. Photochem. Photobiol. A: Chem. 183 (2006) 218. [8] D.G. Shchukin, R.A. Caruso, Adv. Funct. Mater. 13 (2003) 789. [9] B. Ohtani, K. Iwai, S. Nishimoto, S. Sato, J. Phys. Chem. B 101 (1997) 3349. [10] J.K. Zhou, M. Taheuchi, A.K. Ray, M. Anpo, X.S. Zhao, J. Colloid Interface Sci. 311 (2007) 497.
233
[11] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [12] K. Kobayakawa, Y. Murakami, Y. Sato, J. Photochem. Photobiol. A: Chem. 170 (2005) 177. [13] V. Pore, M. Heikkila, M. Ritala, M. Leskela, S. Areva, J. Photochem. Photobiol. A: Chem. 177 (2006) 68. [14] S. Hinojosa, O. Barbosa-Garcia, M.A. Meneses-Nava, J.L. Maldonado, E. de la Roa-Cruz, G. Ramos-Ortiz, Opt. Mater. 27 (2005) 1839. [15] R.A. Rodriguez-Rojas, E. de la Rosa-Cruz, L.A. Diaz-Torres, P. Salas, R. Melendrez, M. Barhoza-Flores, M.A. Meneses-Nava, O. Barbosa-Garcia, Opt. Mater. 25 (2004) 285. [16] K. Bourikas, T. Hiemstra, W.H. van Riemsdijk, Langmuir 17 (2001) 749. [17] L. Hinda, P. Eric, H. Ammar, K. Mohamed, E. Elimame, G. Chantal, H. Jean-Marie, Appl. Catal. B: Env. 39 (2002) 75. [18] J.W. Tang, Z.G. Zou, J.H. Ye, Angew. Chem. 116 (2004) 4563. [19] H.L. Xu, Z.K. Jiang, Chem. Phys. 287 (2003) 155. [20] H. Xu, Z. Dai, Z. Jiang, Eur. Phys. J. D 17 (2001) 79. [21] H.L. Xu, S. Kroll, J. Lumin. 111 (2005) 191.
Applied Catalysis A: General 334 (2008) 227–233 www.elsevier.com/locate/apcata
Efficient photocatalytic degradation of organic dyes over titanium dioxide coating upconversion luminescence agent under visible and sunlight irradiation Jun Wang a,*, Ronghe Li a, Zhaohong Zhang b, Wei Sun a, Rui Xu a, Yingpeng Xie a, Zhiqiang Xing a, Xiangdong Zhang a a b
Department of Chemistry, Liaoning University, Shenyang 110036, PR China Department of Environment, Liaoning University, Shenyang 110036, PR China
Received 9 July 2007; received in revised form 3 October 2007; accepted 10 October 2007 Available online 16 October 2007
Abstract In order to use the sunlight efficiently, a new titanium dioxide (TiO2) photocatalyst with high catalytic activity under visible light irradiation was prepared with sol–gel technique. In this work, an upconversion luminescence agent, crystallized Er3+:Y3Al5O12, was synthesized and its characters were determined. It is found that this crystallized Er3+:Y3Al5O12 can emit three upconversion fluorescent peaks below 387 nm under the excitation of 488 nm visible light. Hence, this upconversion luminescence agent could transform visible light into ultraviolet light, which could satisfy the genuine requirement of TiO2 photocatalyst. Additionally, the upconverison mechanisms were also discussed. Meanwhile, the prepared TiO2 photocatalysts coating upconversion luminescence agent were characterized by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The photocatalytic activity of prepared TiO2 powder was tested through the degradation of congo red in aqueous solution as a model compound under visible and sunlight irradiation. To affirm the complete mineralization, the ion chromatography and total organic carbon (TOC) were used to observe the mineralized anions and organic residues. The experimental results proved that the prepared TiO2 photocatalyst coating crystallized Er3+:Y3Al5O12 behaved much higher photocatalytic activity under visible light and sunlight irradiation, and was able to decompose the congo red in aqueous solution efficiently. Therefore, this method may be envisaged as a novel technology for treating dyes wastewater using solar energy, especially for textile industries in developing countries. # 2007 Elsevier B.V. All rights reserved. Keywords: Upconversion luminescence agent; TiO2 photocatalyst; Sunlight; Visible light; Congo red
1. Introduction Titanium dioxide (TiO2) as photocatalyst has widely been used to photodegrade the organic pollutants for its relatively high catalytic reactivity, physical and chemical stability, avirulence and cheapness [1,2]. However, it is generally accepted that the TiO2 powder is a poor absorber of photons in the solar spectrum. To destroy the organic pollutants in wastewater effectively, the ultraviolet light (l < 387 nm) must be required to irradiate the TiO2 photocatalyst for its broad band-gap (Eg = 3.2–4.5 eV) [3]. For a long time, much attention has been drawn on extending the range of absorption wavelength of TiO2 photocatalyst, namely,
* Corresponding author. Tel.: +86 24 62202053; fax: +86 24 62202380. E-mail address: [email protected] (J. Wang). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.10.009
making the TiO2 powders as photocatalyst absorb the visible light at large extent. Many reformative methods were adopted such as doping of transition-metal ions [4,5], combination of narrow band-gap semiconductors [6,7] and aggradation of noble metals [8,9]. These methods do its work in extending the range of absorption wavelength of TiO2 photocatalyst, but they suffer from a thermal instability, an increase of carrier-recombination centers or the requirement of an expensive ion-implantation facility and complex-treated process [10]. Still some research groups have found the N-doping method. These N-doping TiO2 powders could be excited by visible light, so it may be a promising way of achieving visible light activity [11]. Although some recent concerning experiments showed that the absorption edge of treated TiO2 powders could shift toward wavelength of 550 nm [12], they also suffer from the increased recombination of electron and hole pairs [13]. In contrast with the traditional
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ideas extending the range of absorption wavelength, with the same purpose of using visible light efficiently, we considered whether there were some substances which could transform visible light into ultraviolet light to satisfy the genuine requirement of TiO2 photocatalyst. Fortunately, the upconversion luminescence materials containing rare earth metals are just what we want to introduce into TiO2 powder. In this study, the upconversion luminescence agent, crystallized Er3+ (1.0%):Y3Al5O12 powder, was prepared and then coated by TiO2 film through the sol–gel process. Lastly, we obtained the TiO2 photocatalyst coating crystallized Er3+:Y3Al5O12, which is active in the photocatalytic degradation of organic dyes under visible light irradiation. Here, the congo red usually contained in the dye industry wastewater was selected as a model contaminant, the photocatalytic activity of prepared TiO2 powder for the degradation of organic contaminants under visible light irradiation were investigated. Additionally, the 700 8C was selected as heat-treated temperature for various TiO2 powders. The molecular structure of congo red anion is shown below (Scheme 1). 2. Materials and methods 2.1. Preparation of Er3+:Y3Al5O12 upconversion luminescence agent The nanocrystalline Er3+:Y3Al5O12 samples were prepared by the coprecipitation technique described elsewhere [14,15]. The influence of NO3 anions adsorbed on the surface of TiO2 particles has been affirmed to be very weak in photocatalytic activities in the previous studies [16], therefore, Er(NO3)3 was used in this work for the doping of Er3+ ions. The stoichiometric amounts of Er2O3 (99.99%) and Y2O3 (99.99%) were dissolved in dilute HNO3 (AR, purity) solution and then were mixed with Al(NO3)3 solution under vigorous agitation. The molar ratio of ions was controlled by 0.03Er3+:2.97Y3+:5.00Al3+, and the doping of Er3+ in Y3Al5O12 was realized. Then 2.0 mL of ethylene glycol and 2.0 mL of acetic acid were added to the mixed solution. The final solution was further stirred at 70 8C, until the jelly was formed. The obtained jelly was dried at 110 8C and ground to obtain a white powder. The samples were heated at 700 and 900 8C for 2.0 h to obtain a crystallized form, respectively. 2.2. Preparation of TiO2 photocatalyst coating upconversion luminescence agent The TiO2 photocatalyst coating Er3+:Y3Al5O12 upconversion luminescence agent was prepared through the sol–gel process.
The two precursor solutions, here denoted precursor A and B, were prepared as follows. Precursor A: (C4H9O)4Ti (40 mL), C2H5OH (50 mL) and CH3COOH (3.0 mL) were mixed and stirred for 10 min at room temperature. Precursor B: H2O (7.0 mL) and C2H5OH (50 mL) were mixed and the crystallized or amorphous (unheated) Er3+:Y3Al5O12 powders (0.5 g) were added under vigorous agitation to make the Er3+:Y3Al5O12 powder dispersed adequately in the solution. Afterwards, the precursor B was added to the precursor A dropwise with vigorous stirring. The resultant mixture was further stirred for 20 min. The sol solution was placed in a culture dish for 1.0 day to finish the sol–gel transition and filtrated. Filter residue was rinsed with water repeatedly, and then dried at 100 8C for 24 h to get a dried gel. The dried gel was ground and a light-yellow powder was obtained. The powder was then heat treated at 700 8C for 2.0 h to obtain stable TiO2 photocatalyst coating (crystallized or amorphous) Er3+:Y3Al5O12. The ratio of Er3+:Y3Al5O12 to TiO2 in photocatalyst is about 2.0%. The pure TiO2 powder was also prepared adopting the same procedure without the addition of Er3+:Y3Al5O12 during sol–gel process. The surface areas were determined as 105, 112, 120 m2/g, respectively, for TiO2 photocatalysts coating crystallized and amorphous Er3+:Y3Al5O12 and pure TiO2 photocatalyst according to BET method. 2.3. Characterization of prepared TiO2 photocatalyst The X-ray diffraction (XRD) patterns were determined by powder X-ray diffractometer (RINT 2500, XRD-Rigaku Corporation, Japan) using Ni-filtered Cu Ka radiation in the range of 2u from 108 to 708 for confirming the crystal phases of samples. The crystallite sizes were calculated from the diffraction peak widths using the Scherrer equation. The transmission electron microscope (TEM) (JEM-3010, JEOL Company, Japan) was used for observing the characters of prepared TiO2 particles and estimating the particle size. 2.4. Measurement of photocatalytic activity of prepared TiO2 photocatalyst The experiments of the photocatalytic degradation of congo red (696.68, AR, Tianjin Yazhong Chemical Company, China) in aqueous solution were carried out under the conditions such as 20 mg/L congo red concentration, 1.0 g/L prepared TiO2 photocatalyst, 50 mL total volume, 144 W irradiation power and 20.0 0.2 8C solution temperature. The congo red suspension containing various TiO2 powders was irradiated by triphosphor lamps (FL40T8EXD/36, Toshiba Company, Japan). The triphosphor lamp has a correlated color temperature (CCT)
Scheme 1. Molecular structure of congo red anion.
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4000 K and the illumination light intensity of triphosphor lamp is 964347 lx. The wavelength of the three colours of the spectra mainly spreads between 440–700 nm, the associated light intensity is 360 cd and the flux is 1380 lm. The suspensions at specific intervals were sampled to monitor the changes of leftover congo red concentration. Sampled suspensions were centrifuged at 4000 rpm for 20 min to remove the TiO2 powder and then analyzed by UV–vis spectrophotometer (LAMBDA-17, Perkin-Elmer Company, USA). The ion chromatography (ICS90, Dionex Company, USA) was used to observe the mineralized anions produced during degradation process in the solution. The total organic carbon (TOC) was determined by a Shimadzu TOC analyzer (TOC 500, Shimadzu Corporation, Japan). 3. Results and discussion 3.1. The XRD of Er3+:Y3Al5O12 upconversion luminescence agent Fig. 1 shows the XRD patterns of the Er3+:Y3Al5O12 powder. It indicates that the Er3+:Y3Al5O12 powder heat treated at 700 8C (Fig. 1a) barely presents a crystallized form, and the main peak of the cubic structure of Y3Al5O12 is centered at 2u = 33.58, while the crystal phases of Y4Al12O9 and YAlO3 powders are also found. However, the Er3+:Y3Al5O12 powder heat treated at 900 8C (Fig. 1b) presents a well-defined onefold crystal phasic structure of Y3Al5O12, and no secondary impurity phase such as Y4Al12O9 and YAlO3. 3.2. The XRD of prepared TiO2 photocatalyst coating upconversion luminescence agent The XRD patterns of various prepared TiO2 powders heat treated at 700 8C were shown in Fig. 2. It can be seen that for three cases (a: prepared pure TiO2 powder; b: prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 (unheated); c: prepared TiO2 powder coating crystallized Er3+:Y3Al5O12
Fig. 2. XRD of various TiO2 powders after 700 8C heat treatment: (a) prepared pure TiO2 powder, (b) prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 (unheated) and (c) prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 (900 8C heat treated).
(900 8C heat treated) all consist of a mixture of anatase (about 10%) and rutile (about 90%) phase TiO2 powders. Additionally, as the experimental XRD data (not shown in this paper) proved, the simplex anatase phase TiO2 powder was produced at 500 8C heat treatment, and mixed phase TiO2 powder of anatase and rutile almost in the equal proportion fraction was obtained at 600 8C, while both are high adsorptive for some dyes even though in the dark, compared to the one heat treated at 700 8C. Thus, the 700 8C heat treatment was controlled intentionally. From the heights and widths of the X-ray diffraction peaks, the average crystallite size was calculated according to the Debye– Scherer equation. The size of crystallized Er3+:Y3Al5O12 powder is about 30–35 nm, while the size of prepared pure TiO2 powder is about 45–55 nm based on Fig. 2a. The average size of the prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 is close to 65–75 nm. Hence, the measured XRD pattern of the prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 exhibits narrower and higher peaks compared with that of prepared pure TiO2 powder. Additionally, in the XRD pattern of Fig. 2c, there is a new and slight peak centered at 2u = 33.58 observed in the TiO2 sample coating crystallized Er3+:Y3Al5O12, which naturally attributes to the addition (2.0%) of crystallized Er3+:Y3Al5O12. However, any new peak is not found in Fig. 2b, because the Er3+:Y3Al5O12 powder is mostly amorphous at 700 8C heat treatment. 3.3. The TEM of TiO2 prepared photocatalyst coating upconversion luminescence agent
Fig. 1. XRD of Er3+:Y3Al5O12 powders at different heat-treated temperatures: (a) 700 8C heat treatment and (b) 900 8C heat treatment.
Fig. 3 shows the TEM images of the prepared TiO2 powder coating crystallized Er3+:Y3Al5O12. It was found from Fig. 3a that the prepared TiO2 powder is composed of a large quantity of grayer spherical particles with approximatively uniform size
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Fig. 4. UV–vis spectra of congo red solution in the presence of prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 under different irradiation time: (a) 0.0 h irradiation (original solution), (b) 1.0 h irradiation, (c) 2.0 h irradiation, (d) 3.0 h irradiation, (e) 4.0 h irradiation and (f) 5.0 h irradiation.
Fig. 3. TEM of prepared TiO2 particles coating crystallized Er3+:Y3Al5O12 after 700 8C heat treatment (ULA: upconversion luminescence agent): (a) 20,000 times magnification and (b) 100,000 times magnification.
and shape. The average size of these particles estimated from the TEM image is about 50–70 nm, which is approximatively consistent with the calculated results based on Debye–Scherer equation. The black parts in central regions of many grayer spherical particles can be found clearly, which can be confirmed as the crystallized Er3+:Y3Al5O12 particles. In order to observe the conformation and composition of the prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 more obviously, in which one TiO2 particle in high-magnification TEM image was chosen for making the details illumination. As seen in Fig. 3b, the TiO2 folium becomes distinguishable and the nearly spherical particle of crystallized Er3+:Y3Al5O12 may be used as a core. The size of central Er3+:Y3Al5O12 particle is about 30– 50 nm, so that the thickness of TiO2 folium should be about 20– 30 nm. Hence, the visible light may sufficiently penetrate through such TiO2 folium and excite the inner crystallized Er3+:Y3Al5O12 particles, which must result in the emission of ultraviolet light. 3.4. The UV–vis spectra of congo red solution during photocatalytic degradation To demonstrate the potential applicability of the prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 as photocatalyst utilizing visible light, its photocatalytic activity was studied through the degradation of congo red in aqueous solution. Fig. 4 shows the changes of absorption spectra of congo red solutions
(20 mg/L intitial concentration and 50 mL total volume) in the presence of prepared TiO2 powder (50 mg/50 mL) coating crystallized Er3+:Y3Al5O12 at different irradiation time. The maximum absorption peak at 500 nm corresponding to the azo bond of congo red molecule diminished gradually as the irradiation time increased. It disappeared completely after about 5.0 h. The absorption peaks at 345 and 280 nm in ultraviolet regions, corresponding to the naphthyl ring of congo red molecule, also disappeared eventually, though with a slight increase during initial degradation process. This phenomenon may be explained by the formation of intermediate products during the photocatalytic degradation, which have much bigger absorbency than that of original congo red molecule. Anyhow, these results suggest that the congo red molecule was completely decomposed. The sequence of the color change of congo red solutions, shown in the inset of Fig. 4, also displays the degradation process of congo red. It is clearly seen that the cardinal red color of starting solution gradually disappears along with increasing irradiation time. 3.5. The ion chromatograms of congo red solution during photocatalytic degradation The ion chromatograms of congo red solution during photocatalytic degradation are shown in Fig. 5. They clearly proved that the sulphur and nitrogen heteroatoms in congo red molecule were converted into simple inorganic SO42, NO3 and NO2 anions. Correspondingly, the peak intensity of NO3 and NO2 anion seems to be much lower compared with the theoretical content of nitrogen atom in congo red molecule. The reasons are as follows. On one hand, during the photocatalytic degradation, at the beginning the azo bond of congo red molecule is decomposed and then partly reduced to the NH4+ cation which can not be seen in the ion chromatogram. On the other hand, as the degradation reaction proceeds, a series of
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Fig. 5. Ion chromatograms of the congo red solution during photocatalytic degradation in the presence of prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 under visible light irradiation: (a) 1.0 h visible light irradiation, (b) 3.0 h visible light irradiation and (c) 5.0 h visible light irradiation.
nitrogen oxides (NOx) and nitrogen gas (N2) are produced. Some volatile nitrogen oxides and nitrogen gas will escape from the solution, which also results in the low peak intensity of NO3 and NO2 anion [17]. In fact, that is helpful for the elimination of nitrogen containing organic pollutants. Additionally, according to the results of TOC test, more than 95% of congo red was eliminated after 5.0 h irradiation. Hence, it can be confirmed that all carbon atoms in congo red solution were converted into nontoxic CO2 and then escaped from the solution. 3.6. The influences of irradiation time on the degradation ratio under visible and sunlight A further comparative experiment was also carried out to investigate the catalytic activities of various prepared TiO2 photocatalysts. The congo red solutions were subjected to a series of experimental conditions: (a) without any TiO2 catalyst only under visible light irradiation; (b) with prepared pure TiO2 powder in the dark; (c) with prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 in the dark; (d) with prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 in the dark; (e) with prepared pure TiO2 powder under visible light irradiation; (f) with prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 under visible light irradiation; (g) with prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 under visible light irradiation. The corresponding results of reaction kinetics studies are given in Fig. 6. From Fig. 6 it can be seen that the rate constant (k) related to curve a is smallest for all cases, which indicates that the congo red molecules are unable to be decomposed easily only under visible light irradiation. In addition, the rate constants for curves b–d are also very small within 6.0 h. It reveals that three kinds of prepared TiO2 photocatalysts all behave a little adsorption to congo red molecules in aqueous solution. But
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Fig. 6. Reaction kinetics of photocatalytic degradation of congo red under different conditions: (a) without any TiO2 catalyst only under visible light irradiation, (b) with prepared pure TiO2 catalyst in the dark, (c) with prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 in the dark, (d) with prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 in the dark, (e) with prepared pure TiO2 powder under visible light irradiation, (f) with prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 under visible light irradiation and (g) with prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 under visible light irradiation.
these adsorption amounts did not influence the calculation of following photocatalytic degradation ratios. Nevertheless, for the photocatalytic degradation related to curves e–g, the rate constants illuminate that the congo red molecules can be degraded quickly in the presence of various prepared TiO2 photocatalysts. Of course, the TiO2 powder coating crystallized Er3+:Y3Al5O12 under visible light irradiation displays the highest catalytic activity in these three prepared TiO2 photocatalysts. Hence, although some dyes could be degraded over pure TiO2 based on the self-photosensitized process of dyes under visible light irradiation [18], the degradation efficiency is much lower than that used TiO2 catalyst coating crystallized and amorphous Er3+:Y3Al5O12. In the following, the photocatalytic activity of the prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 under sunlight irradiation was further tested compared with prepared pure TiO2 powder. As shown in Fig. 7, the outdoor experiments (Shenyang City, China, E1238240 and N418500 ) were carried out from a.m. 08:30 to p.m. 16:30 of 20 June 2006, where the corresponding temperature range changes from 22.3 to 25.6 8C, respectively. The congo red solution in conical flask c in the presence of prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 becomes colorless after 8.0 h sunlight irradiation, while it still keeps light red color in conical flask b in the presence of prepared pure TiO2 powder. Whereas the congo red solution in conical flask a in the absence of any TiO2 catalyst is hardly decolorized naturally under sunlight irradiation. Similar experiments were also carried out for some other organic dyes. Through the photocatalytic degradations of congo red, crystal violet and rodanmine B dyes under sunlight irradiation, it is experimentally proved that the prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 has promising applied perspective
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Fig. 7. Photocatalytic degradation of congo red in aqueous solution under sunlight irradiation: (a) without any TiO2 catalyst, (b) with prepared pure TiO2 powder and (c) with prepared TiO2 powder coating crystallized Er3+:Y3Al5O12.
Fig. 8. Possible principle of exciting TiO2 particle coating crystallized Er3+:Y3Al5O12 with visible light.
in using solar energy. A more detailed understanding on the photocatalytic activities of prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 is explained as below.
upconversion process may easily take place in Er3+:Y3Al5O12 crystals. As illustrated in Fig. 8, the crystallized Er3+:Y3Al5O12 as upconversion luminescence agent coated by the TiO2 folium, under continuous excitation of visible light, can emit the ultraviolet light, which can effectively be absorbed by TiO2 parts to generate the electron–hole pairs. In the same way as general photocatalytic degradation, these holes not only directly decompose the congo red adsorbed on the surface of TiO2 particles, but also oxidize water molecule to form OH radicals with high activity and indirectly degrade the congo red in aqueous solution.
3.7. Principle of photocatalytic degradation under visible light irradiation Recently, some research groups have reported the luminescent characteristics of ultraviolet upconversion in Er3+:Y3Al5O12 crystals, and the upconversion mechanisms are also discussed in detail [19]. When the Er3+:Y3Al5O12 as upconversion luminescence agent is pumped by a 488 nm Ar+ laser, the upconversion signals at 271, 317 and 381 nm are observed. Otherwise, the upconversion signals at 320 nm is also found by a 647 nm laser excitation [19,20]. The upconversion fluorescence spectrum and energy-level diagram showing related transitions are also given. According to the energylevel data given in reference [20], the luminescence transitions, 271 nm (2H9/2 ! 4I15/2), 317 nm (2P3/2 ! 4I15/2) and 381 nm (4G11/2 ! 4I15/2), are assigned, respectively. According to the excitation conditions and the specific pump wavelengths used, the population in an excited state with an energy exceeding the energy of the pump photon may be achieved either by excited state absorption (ESA) or by energy transfer upconversion (ETU) or by photon avalanche (PA). In general, the upconversion processes could be identified by measuring the decay profiles of upconversion emission from different multiplets under one- or multiple-pulsed excitation [21]. Under one-pulsed excitation, the Er3+ ions were excited from the ground state to the 4F9/2 state by a ground state absorption process, ETU then takes place through different energy-flow pathways. Under the two-pulsed excitation, the Er3+ ions in the ground state absorb two photons through ESA from the 4F9/2 state, which leads to population and upconversion luminescence from the 2P3/2 state. Under multiple-pulsed excitation, the photon avalanche may take place. The visible light section in solar light contains continuous pump wavelengths (between 400 and 700 nm) used as excitation source, thus the
4. Conclusion In summary, the prepared TiO2 powder coating crystallized Er3+:Y3Al5O12 showed much higher photocatalytic activity, especially in terms of using visible light, compared with the prepared TiO2 powder coating amorphous Er3+:Y3Al5O12 and prepared pure TiO2 powder. Therefore, to obtain the TiO2 catalyst with high photocatalytic activity, the Er3+:Y3Al5O12 powder must be pre-crystallized through high temperature heat treatment and then coated by TiO2 folium through sol–gel process. Consequently, the crystallized Er3+:Y3Al5O12 as upconversion luminescence agent coated by TiO2 folium could transform visible light into ultraviolet light to satisfy the genuine requirement of TiO2 photocatalyst. Thus, the efficiency of utilizing visible light was largely enhanced. It is promising to use the idea to develop new TiO2 catalyst with high activity for photocatalytic degradation of organic pollutants, and to develop new energy materials using solar energy, such as solar cells. Acknowledgements The authors greatly acknowledge The National Natural Science Foundation of China for financial support. The authors also thank our colleagues and other students participating in this work.
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References [1] S.F. Villanueva, S.S. Martı´nez, Sol. Energy Mater. Sol. C 91 (2007) 1492. [2] J.S. Chen, M.C. Liu, L. Zhang, J.D. Zhang, L.T. Jin, Water Res. 37 (2003) 3815. [3] M.R. Hoffman, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [4] U. Diebold, Surf. Sci. Rep. 48 (2003) 53. [5] W.C. Hung, S.H. Fu, J.J. Tseng, H. Chu, T.H. Ko, Chemosphere 66 (2007) 2142. [6] P.V. Kamat, Chem. Rev. 93 (1993) 267. [7] Y. Bessekhouad, N. Chaoui, M. Trzpit, N. Ghazzal, D. Robert, J.V. Weber, J. Photochem. Photobiol. A: Chem. 183 (2006) 218. [8] D.G. Shchukin, R.A. Caruso, Adv. Funct. Mater. 13 (2003) 789. [9] B. Ohtani, K. Iwai, S. Nishimoto, S. Sato, J. Phys. Chem. B 101 (1997) 3349. [10] J.K. Zhou, M. Taheuchi, A.K. Ray, M. Anpo, X.S. Zhao, J. Colloid Interface Sci. 311 (2007) 497.
233
[11] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [12] K. Kobayakawa, Y. Murakami, Y. Sato, J. Photochem. Photobiol. A: Chem. 170 (2005) 177. [13] V. Pore, M. Heikkila, M. Ritala, M. Leskela, S. Areva, J. Photochem. Photobiol. A: Chem. 177 (2006) 68. [14] S. Hinojosa, O. Barbosa-Garcia, M.A. Meneses-Nava, J.L. Maldonado, E. de la Roa-Cruz, G. Ramos-Ortiz, Opt. Mater. 27 (2005) 1839. [15] R.A. Rodriguez-Rojas, E. de la Rosa-Cruz, L.A. Diaz-Torres, P. Salas, R. Melendrez, M. Barhoza-Flores, M.A. Meneses-Nava, O. Barbosa-Garcia, Opt. Mater. 25 (2004) 285. [16] K. Bourikas, T. Hiemstra, W.H. van Riemsdijk, Langmuir 17 (2001) 749. [17] L. Hinda, P. Eric, H. Ammar, K. Mohamed, E. Elimame, G. Chantal, H. Jean-Marie, Appl. Catal. B: Env. 39 (2002) 75. [18] J.W. Tang, Z.G. Zou, J.H. Ye, Angew. Chem. 116 (2004) 4563. [19] H.L. Xu, Z.K. Jiang, Chem. Phys. 287 (2003) 155. [20] H. Xu, Z. Dai, Z. Jiang, Eur. Phys. J. D 17 (2001) 79. [21] H.L. Xu, S. Kroll, J. Lumin. 111 (2005) 191.