- Email: [email protected]
Hydrothermal synthesis of nanostructures Bi12TiO20 and their photocatalytic activity on acid orange 7 under visible light
Chemosphere 78 (2010) 1350–1355
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Hydrothermal synthesis of nanostructures Bi12TiO20 and their photocatalytic activity on acid orange 7 under visible light Xiangqi Zhu a, Jinlong Zhang a,b,*, Feng Chen a a b
Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China School of Chemistry and Material Science, Guizhou Normal University, Guiyang 550001, PR China
a r t i c l e
i n f o
Article history: Received 28 June 2009 Received in revised form 4 January 2010 Accepted 5 January 2010 Available online 8 February 2010 Keywords: Bi12TiO20 Nanostructures Acid orange 7 Photocatalytic degradation Visible light irradiation BET surface area
a b s t r a c t Bismuth titanate (Bi12TiO20) nanostructures with different morphologies were synthesized hydrothermally using Bi(NO3)3 and Ti(SO4)2 in the presence of polyethylene glycol (PEG). X-ray diffraction (XRD) proved that the samples were in pure cubic phase. UV–visible diffuse reflection spectra showed the band gap of Bi12TiO20 is about 2.7 eV. Brunauer–Emmett–Teller (BET) analysis proved that the Bi12TiO20 samples have higher surface areas than samples prepared by methods reported previously. Photocatalytic degradation of acid orange 7 (AO7) under visible light illumination was used to evaluate the photocatalytic ability of samples. The photocatalytic results showed Bi12TiO20 could degrade AO7 very efficiently and had higher photocatalytic activity than traditional N-doped TiO2. In addition, we have also discussed factors that have major effect on reaction efficiency. BET surface area played the most important role in the photocatalytic degradation of AO7. The crystallinity of the samples is another important factor which can also influence photocatalysis results. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Dyes are important organic pollutants, and their release as wastewater in the ecosystem is a dramatic source of esthetic pollution, eutrophication, and perturbations in aquatic life (Xu and Langfor, 2001; Bao et al., 2004; Horikoshi et al., 2004; Fu et al., 2005; Dillert et al., 2007). Since the discovery of the photocatalytic splitting of water on the TiO2 electrodes by Fujishima and Honda (1972), the application of semiconductor photocatalysts on degradation of pollutants has received great attention (Linsebigler et al., 1995; Zhao et al., 1998; Fujishima et al., 2000; Ho et al., 2004; Dai et al., 2007; Huang et al., 2008; Echavia et al., 2009). Among all photocatalysts, TiO2 attracts the most attention because of its chemical stability, nontoxicity, and high photocatalytic reactivity (Frank and Bard, 1977; Herrmann et al., 1988; Muggli et al., 1998; Phanikrishna Sharma et al., 2008). However, the band gap of the TiO2 is 3.2 eV. It absorbs only the ultraviolet light (k < 400 nm) which only accounts for about 4% of the sunlight (Asahi et al., 2001; Yu et al., 2003; Tang et al., 2004; Shang et al., 2008). In order to improve the efficiency of the sunlight utilization, the development of photocatalysts with high activity under a wide
* Corresponding author. Address: Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China. Tel./fax: +86 21 64252062. E-mail address: [email protected] (J. Zhang). 0045-6535/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2010.01.002
range of visible light irradiation is highly desirable (Hou et al., 2008; Ménesi et al., 2008; Gomathi Devi et al., 2009). In recent years, ternary bismuth oxide semiconductors, such as Bi2WO6 (Kudo and Hijii, 1999; Fu et al., 2005; Zhang et al., 2007), BiVO4 (Tokunaga et al., 2001; Kohtani et al., 2005), NaBiO3 (Kako et al., 2007), Bi12TiO20 (Yao et al., 2003; Xie et al., 2006) have been widely studied as a class of promising photocatalyst which can respond under visible light. Among all ternary bismuth oxide, Bi12TiO20 crystals show high photocatalytic ability in degrading the organic pollutants exposed to UV light irradiation (Yao et al., 2003). However, Bi12TiO20 samples were usually obtained by mechanical method (Stojanovic et al., 2006) and chemical solution decomposition (CSD) method (Yao et al., 2003). The mechanical method is apt to destroy the microstructure of the crystal while the CSD method always involves a very complex procedure. Furthermore, the sample obtained from the above two methods bears limited surface areas and there have been few studies (Yao et al., 2003) about the visible light-induced photocatalytic activity of Bi12TiO20. Here, we report a polyethylene glycol (PEG)-assisted hydrothermal method to synthesize Bi12TiO20 nanostructures with relative high surface areas. Different kinds of PEGs can influence the morphology and the Brunauer–Emmett–Teller (BET) surface area of the samples. The photodegradation of AO7 was employed to evaluate the photocatalytic activities of Bi12TiO20 under visible light illumination. The Bi12TiO20 showed high photoactivity towards degradation of AO7 and has been proven to be a promising alterna-
X. Zhu et al. / Chemosphere 78 (2010) 1350–1355
1351
tive for dye-containing wastewater treatment under visible light irradiation.
(TOC) of the solution was measured by a Liqui TOC analyzer (Elementar).
2. Experiment
3. Results and discussion
2.1. Materials
3.1. XRD
All major reagents were of analytical purity and were supplied by China National Medicines Corporation while AO7 was purchased from Acros and used without further purification. The samples prepared for comparison are (i) N–TiO2 synthesized according to Yu et al. (2006) using Ti(SO4)2 as starting materials and (ii) P-25 titania (TiO2; ca. 80% anatase, 20% rutile; BET area, ca. 50 m2 g1) purchased from the Degussa. Double distilled water was used throughout this study.
The XRD spectra of the samples prepared under different PEGs and hydrothermal time are shown in Figs. 1 and 2. Fig. 1 indicates the XRD diffraction of the samples using different kinds of PEGs. All diffraction peaks can be indexed to cubic Bi12TiO20 according to the JCPDS Card No. 34-0097 and all peaks of three spectra are very sharp indicating high crystallinities of samples. However, the intensities of the peaks from three curves are different which prove that there are some differences among the samples prepared with different PEGs. PEG2000 and PEG600 have stronger peak intensity than PEG10000. The relationship between the crystallinity of Bi12TiO20 and different PEGs will be discussed later. On the other hand, the similar peaks’ positions indicate that different PEG chain length has almost the same effect on the phase of Bi12TiO20 sample. To reveal the effect of hydrothermal treatment time on the Bi12TiO20, time-dependent experiments were carefully conducted. Fig. 2 shows the XRD spectra of samples synthesized with different hydrothermal time. In the preparation of pure sillenite Bi12TiO20, main peaks of cubic Bi12TiO20 have been found when hydrothermal time was just 8 h. But a metastable phase (such as peaks at 32.0° and 23.9°) also appeared which means 8 h hydrothermal time is
2.2. Preparation of Bi12TiO20 photocatalysts The Bi12TiO20 synthesis was as follows: 5 mM of Bi(NO3)35H2O and 0.417 mM of Ti(SO4)2 (Bi:Ti = 12:1) were dissolved in 10 mL 20% (volume ratio) HNO3 under vigorous stirring for 1 h. The 1.8 g different kinds of PEGs (PEG600, PEG2000, PEG10000) were dissolved into 10 mL of double distilled water and added dropwise into the reaction solution. One molar NaOH aqueous solution was also added slowly to adjust the pH to 12 and the transparent solution changed to a white suspension. This suspension was stirred for 30 min and then transferred into a 100 mL stainless steel autoclave up to about 80% of its capacity. The autoclave was kept at 180 °C for 8 h, 24 h and 40 h, respectively, and cooled to room temperature after the reaction time. The precipitates were washed with deionized water and ethanol five times and then dried at 100 °C for 2 h to obtain the final products. 2.3. Characterization of Bi12TiO20 photocatalysts The powder X-ray diffraction (XRD) analysis of the prepared catalyst was carried out at room temperature with a Rigaku D/ max 2550 VB/PC apparatus using Cu Ka radiation (k = 1.5406 Å) and a graphite monochromator, operated at 40 kV and 30 mA. The BET of the sample was determined through nitrogen adsorption at 77 K (Micromeritics ASAP 2010). The sample was degassed at 473 K before the measurement. The UV–vis absorbance spectra were recorded for the dry-pressed disk sample using a Scan UV– vis spectrophotometer (Varian, Cary 500) equipped with an integrating sphere assembly. Transmission electron microscopy (TEM) analysis of the samples was done using a JSM-2100F (JEOL) instrument and the electron beam accelerating voltage was 200 kV.
Fig. 1. XRD patterns of Bi12TiO20 samples prepared with different kinds of PEG (a) PEG600, (b) PEG2000 and (c) PEG10000; hydrothermal temperature: 180 °C; hydrothermal time: 24 h; pH = 12.
2.4. Photocatalytic activities test The photocatalytic degradation of AO7 with visible light was used to evaluate the photocatalytic activities of the samples. A 1000 W halogen lamp was used as the light source with a 420 nm cutoff filter to provide visible light irradiation. In each experiment, 0.1 g of photocatalyst was added to 100 mL of AO7 solution (20 mg L1). Before illumination, the suspensions were magnetically stirred in the dark for 30 min to ensure the adsorption–desorption equilibrium between the photocatalyst and AO7. Then the solution was exposed to visible light irradiation under magnetic stirring. At given time intervals, 4 mL of suspension was sampled and centrifuged to remove the photocatalyst particles. Then, the catalyst-free dye solution were analyzed by a Cary 100 UV–vis spectrometer to record intensity of the maximum band at 484 nm in the UV–vis absorption spectra. Total organic carbon
Fig. 2. XRD patterns of Bi12TiO20 samples prepared with different hydrothermal time (a) 40 h, (b) 24 h, (c) 8 h; hydrothermal temperature: 180 °C; PEG: PEG10000; pH = 12.
1352
X. Zhu et al. / Chemosphere 78 (2010) 1350–1355
not enough for the formation of pure cubic Bi12TiO20. Gradual transformation from metastable phase into sillenite phase was observed with the hydrothermal time changing from 8 h to 40 h. Pure cubic Bi12TiO20 was finally obtained with 24 h hydrothermal treatment. After the hydrothermal conversion proceeded for 40 h, the peak intensity got stronger which meant the crystallinity of the sample was developing with the prolongation of hydrothermal time. The result of FT-IR (data not shown) showed that with the extension of the hydrothermal time, the peaks got sharper and sharper which indicated that the extension of hydrothermal time was beneficial for the formation and strengthening of Bi–O and this result agreed well with the XRD results discussed above.
3.2. UV–vis diffuse reflection spectrum Fig. 3 shows the UV–vis diffuse reflection spectrum of the prepared Bi12TiO20 (PEG: PEG600, hydrothermal temperature: 180 °C; hydrothermal time: 24 h; pH = 12). As a comparison, the spectra of P-25 titania and N–TiO2 were also measured. The absorption onset wavelength of Bi12TiO20 was around 500 nm which was shifted 100 nm to visible region compared to P-25. The color of synthesized Bi12TiO20 was yellow, which was in accordance with its absorption spectrum. The absorption spectrum of Bi12TiO20 had steep shape which indicated the absorption relevant to the band gap was due to the intrinsic transition of the nanomaterials rather than the transition from impurity levels (Kudo et al., 2002). It is well-known that for a crystalline semiconductor (Butler, 1977; Fu et al., 2005), the optical absorption near the band edge follows the formula: n
ahm ¼ Aðhm EgÞ2
ð1Þ
where a, h, m, Eg, and A are the absorption coefficient, Planck constant, the light frequency, the band gap, and a constant, respectively. Among these, n describes the characteristics of the transition in a semiconductor. For Bi12TiO20, the value of n is 1. The plot of (ahm)2 vs. hm based on the direct transition is shown in the inset of Fig. 3. Therefore, the energy of the band gap could be estimated to be about 2.70 eV for Bi12TiO20 in our case. This indicates that the nanosized Bi12TiO20 has a suitable band gap for photocatalytic decomposition of organic contaminants under visible light irradiation.
Fig. 3. UV–vis diffuse reflectance spectra of as-prepared Bi12TiO20 powders (PEG: PEG600, hydrothermal temperature: 180 °C; hydrothermal time: 24 h; pH = 12), N– TiO2 and P-25. The inset is the (ahm)2 vs. hm curve.
3.3. Degradation of AO7 using Bi12TiO20 photocatalysts Photodegradation experiments of AO7 were carried out under visible light illumination at wavelengths longer than 420 nm in order to test the photocatalytic performance of the Bi12TiO20 sample. For comparison, the photodegradation of AO7 by N-doped TiO2 was also carried out. The results displayed in Fig. 4a shows that AO7 solution was stable under visible light irradiation in the absence of any catalyst. When Bi12TiO20 was added to the AO7 solution, the total degradation rate was over 90% within 6 h irradiation, much higher than the N-doped TiO2 which only reached 15% of the total decomposition. As shown in Fig. 4b, photocatalytic activity of Bi12TiO20 synthesized under different synthesizing conditions is studied. Samples synthesized using PEG10000 showed higher activity than the samples synthesized using PEG600 and PEG2000. Meanwhile when the hydrothermal time was 24 h, Bi12TiO20 showed the fastest degradation speed/rate compared with other hydrothermal times. TOC removal of the solution is shown in Fig. 4c while the corresponding UV/vis spectral changes of these solutions are displayed in the inset of Fig. 4c. The rate of TOC reduction was slower than that of the degradation of the dye; 80% of TOC still remained in the suspension after 6 h of irradiation when AO7 solution was bleached. The inset of Fig. 4c shows that the peaks at 310 and 228 nm corresponding to naphthalene ring and benzene ring in the dye molecule were also reduced (Stylidi et al., 2004). A lot of small organic intermediates which were not completely mineralized were generated in the process of photocatalysis. So the remaining organic carbon is probably due to these refractory organic compounds. The photocatalytic decolorization of AO7 follows the pseudofirst-order reaction kinetics (Yu et al., 2000; Hammami et al., 2008; Ji et al., 2009) and its kinetics might be expressed as follows:
ln
C0 ¼ kt C
ð2Þ
where k is the apparent reaction rate constant, C0 is the initial absorbance of aqueous AO7, t is the reaction time, and C is the absorbance of aqueous AO7 at the reaction time of t. According to Eq. (2), the kinetic of AO7 decolorization under different photocatalysts is presented in Fig. 4d by plotting the logarithm of the normalized dye concentration against irradiation time. The calculated apparent reaction rate constants of each catalyst are listed in Table 1. In order to study the possible relationship between the photocatalytic activity and the property of different samples, TEM images (Fig. 5) and BET surface area data (Table 1) of different samples are also displayed. Table 1 shows the BET surface area, apparent reaction rate constants of Bi12TiO20 under different preparation conditions as well as the standard deviations. Samples synthesized in the presence of PEG10000 (sample 1 and sample 2) had relatively high BET surface areas. Sample 1 reached over 15 m2 g1, which was much higher than the samples prepared by other methods, such as coprecipitation processing (4.89 m2 g1) (Xu et al., 2007). But when PEG2000 and PEG600 were used, the BET surface area decreased. In order to study the effects of PEGs on the surface area, TEM was used to observe the morphologies and structure details of these bismuth titanate products. Fig. 5a gives the TEM image of a typical example of Bi12TiO20 nanoparticles which were prepared using PEG10000. It showed that products synthesized were composed of nano-sized particles, which were about 10 nm in size, and each particle was nearly spherical in shape and had higher density. As shown in Fig. 5b, Bi12TiO20 nanobelts with width of 50–80 nm and lengths up to several micrometres and Bi12TiO20 nanoparticles (pointed out by the arrow) were all synthesized in
1353
X. Zhu et al. / Chemosphere 78 (2010) 1350–1355
Fig. 4. (a) Temporal course of the photodegradation of AO7 in different catalyst aqueous dispersions; (b) temporal course of the photodegradation of AO7 by Bi12TiO20 prepared under different conditions. (c) TOC of AO7 solutions degraded by Bi12TiO20 which was prepared under 180 °C for 24 h in the presence of PEG10000. The inset is the corresponding UV/vis spectral changes. (d) The corresponding kinetic analysis assuming the pseudo-first-order reaction for AO7 dye. Experimental condition: [AO7] = 20 mg L1, [catalyst] = 1 g L1, T = 298 K, initial pH = 6.2.
Table 1 BET surface area of the prepared Bi12TiO20 under different synthetic conditions, their photocatalytic reaction rate constants and the standard deviations. Name Sample Sample Sample Sample Sample Sample
1 2 3 4 5 6
Synthesizing conditions
BET surface area (m2 g1)
Photocatalytic reaction rate constants, k (h1)
Standard deviation SD (%)
PEG10000, 8 h PEG10000, 24 h PEG10000, 40 h PEG600, 24 h PEG2000, 24 h N–TiO2
17.98 13.50 5.92 6.60 6.00 –
0.315 0.327 0.134 0.140 0.125 0.029
5.7 1.8 3.3 4.0 1.3 0.9
the process of using PEG2000. Although some small nanoparticles could also be observed in Fig. 5b, the formation of big-size nanobelts could decrease the BET surface area of sample dramatically. The different morphologies mainly depend on the different chain length of PEGs. PEG with uniform and ordered chain structure can easily adsorb on the surface of metal oxide colloid which has been investigated in previous reports (Liu et al., 2000; Dobryszycki and Biallozor, 2001). This type of polymer greatly decreases the activity of colloid surface when adsorbed on them (Li et al., 2003). From the view of kinetics of colloid growth, if the colloid adsorbs the polymer on some parts of its surface, the growth rate of the colloids in certain direction will be confined. In the present work, PEG2000 which has relative short-chain length, uniform and ordered chain can modify the growth kinetics of the growing colloids and finally leads into anisotropic growth of the crystals.
There were a few spherical nanoparticles in the prepared sample because the concentration of PEG2000 was high enough to confine all aspects of the colloids. In the previous research using longlength PEG (Yu et al., 2001), lots of spherical particles were also obtained, which is due to the fact that a long-chain PEG can confine more than one direction of the growing colloid and finally form spherical colloids. On the other hand, long-chain polymers are easily intertwisted which leads to detrimental effects in the formation of 1D nanostructures, while short-chain polymers greatly decrease the possibilities of intertwisting under these conditions. As a result, sample 2 prepared using PEG10000 was spherical nanoparticles and it had larger BET surface areas than sample 5. On the other hand, the different morphologies of Bi12TiO20 resulting from different PEGs might be the explanation of the differences among the crystallinities of Bi12TiO20 prepared using various PEGs. The big-
1354
X. Zhu et al. / Chemosphere 78 (2010) 1350–1355
Fig. 5. (a) TEM images of Bi12TiO20 nanoparticles prepared using PEG10000; (b) TEM images of Bi12TiO20 nanobelts and nanoparticles prepared using PEG2000.
size-nanobelt-Bi12TiO20 had the better crystallinity than the smallsize-nanocrystal-Bi12TiO20. Higher BET surface area provides more active sites for photocatalytic reaction, while a perfect crystallinity is helpful for inhibiting the recombination rate of photo-generated electrons and holes, improving the photocatalytic activity (Sclafani et al., 1990). But crystallinity and BET surface area always have the competing relationship (Li et al., 2005; Todorova et al., 2008). In this study, sample 1 and sample 2 which have relatively high BET surface area have much higher photocatalytic activity than other samples. This suggests that the rates of photocatalytic activity primarily depend on the BET surface area of the sample. When the samples have different crystallinities and phase compositions, such as sample 1 and sample 2, the result have been affected by both factors. As shown in Fig. 2, sample 2 prepared with 24 h hydrothermal time has pure cubic phase and better crystallinity than sample 1 which is prepared with only 8 h hydrothermal time. The sample 2 showed the relative higher photocatalytic activity as compared to sample 1 (Fig. 4b and d). This result proved that crystallinity can also affect the photocatalyst results. Therefore, considering the two factors (BET surface area and crystallinity), sample 2 having the relative high surface area and the perfect crystallinity performs the best.
peaks in the XRD pattern were almost the same to those of Bi12TiO20 before irradiation. As shown in this result, Bi12TiO20 is considered to be relatively stable to visible light under the present experimental conditions. This result offers a possibility for application of Bi12TiO20 photocatalyst in the wastewater treatment. 4. Conclusions Bi12TiO20 nanostructures with relatively large BET surface area were successfully synthesized by hydrothermal method in the presence of PEG. PEG with different chain length had similar effect on the crystallinity of samples, but could induce differences to the morphology and BET surface area. The optical band gap was estimated to be about 2.7 eV which proved Bi12TiO20 can respond to the visible light. The results of photocatalytic degradation of AO7 showed that BET surface area had more important influence to the activity of degrading AO7 than crystallinity. The Bi12TiO20 photocatalyst prepared with the PEG10000 and the hydrothermal time of 24 h showed the highest photocatalytic activity for photocatalytic degradation of AO7. Acknowledgements
3.4. The stability of Bi12TiO20 as the photocatalyst Fig. 6 indicates the XRD patterns of the sample after 6 h of visible light irradiation. Both the position and the intensity of the
This work was supported by Science and Technology Commission of Shanghai Municipality (07JC14015); National Nature Science Foundation of China (20773039, 20977030); National Basic Research Program of China (973 Program, 2007CB613301, 2010CB732306); the Ministry of Science and Technology of China (2006AA06Z379, 2006DFA52710) and ‘‘The Zhuoyue Project” of East China University of Science and Technology. References
Fig. 6. XRD patterns of Bi12TiO20 before and after visible light irradiation.
Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K., Taga, Y., 2001. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 293, 269–271. Bao, N., Feng, X., Yang, Z., Shen, L., Lu, X., 2004. Highly efficient liquid-phase photooxidation of an azo dye methyl orange over novel nanostructured porous titanate-based fiber of self-supported radially aligned H2Ti8O171.5H2O nanorods. Environ. Sci. Technol. 38, 2729–2736. Butler, M.A., 1977. Photoelectrolysis and physical properties of the semiconducting electrode WO3. J. Appl. Phys. 48, 1914–1920. Dai, K., Chen, H., Peng, T.Y., Ke, D.N., Yi, H.B., 2007. Photocatalytic degradation of methyl orange in aqueous suspension of mesoporous titania nanoparticles. Chemosphere 69, 1361–1367. Dillert, R., Bahnemann, D., Hidaka, H., 2007. Light-induced degradation of perfluorocarboxylic acids in the presence of titanium dioxide. Chemosphere 67, 785–792. Dobryszycki, J., Biallozor, S., 2001. On some organic inhibitors of zinc corrosion in alkaline media. Corros. Sci. 43, 1309–1319.
X. Zhu et al. / Chemosphere 78 (2010) 1350–1355 Echavia, G.R.M., Matzusawa, F., Negishi, N., 2009. Photocatalytic degradation of organophosphate and phosphonoglycine pesticides using TiO2 immobilized on silica gel. Chemosphere 76, 595–600. Frank, S.N., Bard, A.J., 1977. Heterogeneous photocatalytic oxidation of cyanide and sulfite in aqueous solutions at semiconductor powders. J. Phys. Chem. 81, 1484– 1488. Fu, H.B., Pan, C.S., Yao, W.Q., Zhu, Y.F., 2005. Visible-light-induced degradation of Rhodamine B by nanosized Bi2WO6. J. Phys. Chem. B 109, 22432–22439. Fujishima, A., Honda, K., 1972. Electrochemical photolysis of water at a semiconductor electrode. Nature 238, 37–38. Fujishima, A., Rao, T.N., Tryk, D.A., 2000. Titanium dioxide photocatalysis. J. Photochem. Photobiol. C 1, 1–21. Gomathi Devi, L., Narasimha Murthy, B., Girish Kumar, S., 2009. Heterogeneous photo catalytic degradation of anionic and cationic dyes over TiO2 and TiO2 doped with Mo6+ ions under solar light: correlation of dye structure and its adsorptive tendency on the degradation rate. Chemosphere 76, 1163–1166. Hammami, S., Bellakhal, N., Oturan, N., Oturan, M.A., Dachraoui, M., 2008. Degradation of acid orange 7 by electrochemically generated OH radicals in acidic aqueous medium using a boron-doped diamond or platinum anode: a mechanistic study. Chemosphere 73, 678–684. Herrmann, J.M., Disdier, J., Pichat, P., 1988. Photocatalytic deposition of silver on powder titania: consequences for the recovery of silver. J. Catal. 113, 72–81. Horikoshi, S., Hojo, F., Hikaka, H., Serpone, N., 2004. Environmental remediation by an integrated microwave/UV illumination technique. 8. Fate of carboxylic acids, aldehydes, alkoxycarbonyl and phenolic substrates in a microwave radiation field in the presence of TiO2 particles under UV irradiation. Environ. Sci. Technol. 38, 2198–2208. Hou, Y.D., Wang, X.C., Wu, L., Chen, X.F., Ding, Z.X., Wang, X.X., Fu, X.Z., 2008. NDoped SiO2/TiO2 mesoporous nanoparticles with enhanced photocatalytic activity under visible-light irradiation. Chemosphere 72, 414–421. Ho, W., Yu, J.C., Lin, J., Li, P., 2004. Preparation and photocatalytic behavior of MoS2 and WS2 nanocluster sensitized TiO2. Langmuir 20, 5865–5869. Huang, H.H., Tseng, D.H., Juang, L.C., 2008. Titanium dioxide mediated photocatalytic degradation of monochlorobenzene in aqueous phase. Chemosphere 71, 398–405. Ji, P.F., Zhang, J.L., Chen, F., Anpo, M., 2009. Study of adsorption and degradation of acid orange 7 on the surface of CeO2 under visible light irradiation. Appl. Catal. B 2009 (85), 148–154. Kako, T., Zou, Z.G., Katagiri, M., Ye, J.H., 2007. Decomposition of organic compounds over NaBiO3 under visible light irradiation. Chem. Mater. 19, 198–202. Kohtani, S., Hiro, J., Yamamoto, N., Kudo, A., Tokumura, K., Nakagaki, R., 2005. Adsorptive and photocatalytic properties of Ag-loaded BiVO4 on the degradation of 4-n-alkylphenols under visible light irradiation. Catal. Commun. 6, 185–189. Kudo, A., Hijii, S., 1999. H2 or O2 evolution from aqueous solutions on layered oxide photocatalysts consisting of Bi3+ with 6s2 configuration and d0 transition metal ions. Chem. Lett., 1103–1104. Kudo, A., Tsuji, I., Kato, H., 2002. AgInZn7S9 solid solution photocatalyst for H2 evolution from aqueous solutions under visible light irradiation. Chem. Commun. 17, 1958–1959. Li, D., Haned, H., Hishita, S., Ohashi, N., Labhsetwar, K.N., 2005. Fluorine-doped TiO2 powders prepared by spray pyrolysis and their improved photocatalytic activity for decomposition of gas-phase acetaldehyde. J. Fluorine Chem. 126, 69–77. Linsebigler, A.L., Lu, G., Yates, J.T., 1995. Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results. Chem. Rev. 95, 735–758. Liu, X.H., Yang, J., Wang, L., Yang, X.J., Lu, L.D., Wang, X., 2000. An improvement on sol–gel method for preparing ultrafine and crystallized titania powder. Mater. Sci. Eng. A 289, 241–245. Li, Z.Q., Xiong, Y.J., Xie, Y., 2003. Selected-control synthesis of ZnO nanowires and nanorods via a PEG-assisted route. Inorg. Chem. 42, 8105–8109.
1355
Ménesi, J., Körösi, L., Bazsó, E., Zöllmer, V., Richardt, A., Dékány, I., 2008. Photocatalytic oxidation of organic pollutants on titania–clay composites. Chemosphere 70, 538–542. Muggli, D.S., McCue, J.T., Falconer, J.L., 1998. Mechanism of the photocatalytic oxidation of ethanol on TiO2. J. Catal. 173, 470–483. Phanikrishna Sharma, M.V., Durga Kumari, V., Subrahmanyam, M., 2008. TiO2 supported over SBA-15: an efficient photocatalyst for the pesticide degradation using solar light. Chemosphere 73, 1562–1569. Sclafani, A., Palmisano, L., Schiavello, M., 1990. Influence of the preparation methods of titanium dioxide on the photocatalytic degradation of phenol in aqueous dispersion. J. Phys. Chem. 94, 829–832. Shang, M., Wang, W.Z., Sun, S.M., Zhou, L., Zhang, L., 2008. Bi2WO6 nanocrystals with high photocatalytic activities under visible light. J. Phys. Chem. C 112, 10407– 10411. Stojanovic, B.D., Paiva-Santos, C.O., Jovalekic, C., Simoes, A.Z., Filho, F.M., Lazarevic, Z., Varela, J.A., 2006. Mechanically activating formation of layered structured bismuth titanate. Mater. Chem. Phys. 96, 471–476. Stylidi, M., Kondarides, D.I., Verykios, X.E., 2004. Visible light-induced photocatalytic degradation of acid orange 7 in aqueous TiO2 suspensions. Appl. Catal. B: Environ. 47, 189–201. Tang, J., Zou, Z., Ye, J., 2004. Efficient photocatalytic decomposition of organic contaminants over CaBi2O4 under visible light irradiation. Angew. Chem., Int. Ed. 43, 4463–4466. Todorova, N., Giannakopoulou, T., Vaimakis, T., Trapalis, C., 2008. Structure tailoring of fluorine-doped TiO2 nanostructured powders. Mater. Sci. Eng. B – Adv. 152, 50–54. Tokunaga, S., Kato, H., Kudo, A., 2001. Selective preparation of monoclinic and tetragonal BiVO4 with scheelite structure and their photocatalytic properties. Chem. Mater. 13, 4624–4628. Xie, L.J., Ma, J.F., Tian, H., Zhou, J., Zhao, Z.Q., Wu, P.W., Hu, Y.M., Wang, Y.G., Tao, J.T., Zhu, X.Y., 2006. Isopropanol-assisted hydrothermal synthesis of bismuth titanate nanophotocatalysts. Mater. Lett. 60, 284–286. Xu, S.H., Shangguan, W.F., Yuan, J., Shi, J.W., Chen, M.X., 2007. Photocatalytic properties of bismuth titanate Bi12TiO20 repaired by co-precipitation processing. Mater. Sci. Eng. 137, 108–111. Xu, Y.M., Langfor, C.H., 2001. UV or visible-light induced degradation of X3B on TiO2 nanoparticles: the influence of adsorption. Langmuir 17, 897–902. Yao, W.F., Wang, H., Xu, X.H., Cheng, X.F., Huang, J., Shang, S.X., Yang, X.N., Wang, M., 2003. Photocatalytic property of bismuth titanate Bi12TiO20 crystals. Appl. Catal. A – Gen. 243, 185–190. Yu, J.C., Zhang, L.Z., Zheng, Z., Zhao, J., 2003. Synthesis and characterization of phosphated mesoporous titanium dioxide with high photocatalytic activity. Chem. Mater. 15, 2280–2286. Yu, J.G., Zhao, X.J., Zhao, Q.N., 2000. Effect of surface structure on photocatalytic activity of TiO2 thin films prepared by sol–gel method. Thin Solid Films 379, 7– 14. Yu, J.G., Zhao, X.J., Zhao, Q.N., Wang, G., 2001. Preparation and characterization of super-hydrophilic porous TiO2 coating films. Mater. Chem. Phys. 68, 253–259. Yu, J.G., Zhou, M.H., Cheng, B., Zhao, X.J., 2006. Preparation, characterization and photocatalytic activity of in situ N, S-codoped TiO2 powders. J. Mol. Catal. A: Chem. 246, 176–184. Zhang, L.S., Wang, W.Z., Zhou, L., Xu, H.L., 2007. Bi2WO6 nano- and microstructures: shape control and associated visible-light-driven photocatalytic activities. Small 3, 1618–1625. Zhao, J.C., Wu, T.X., Wu, K.Q., Oikawa, K., Hidaka, H., Serpone, N., 1998. Photoassisted degradation of dye pollutants (III) degradation of the cationic dye rhodamine B in aqueous anionic surfactant/TiO2 dispersions under visible light irradiation: evidence for the need of substrate adsorption on TiO2 particles. Environ. Sci. Technol. 32, 2394–2400.
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Hydrothermal synthesis of nanostructures Bi12TiO20 and their photocatalytic activity on acid orange 7 under visible light Xiangqi Zhu a, Jinlong Zhang a,b,*, Feng Chen a a b
Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China School of Chemistry and Material Science, Guizhou Normal University, Guiyang 550001, PR China
a r t i c l e
i n f o
Article history: Received 28 June 2009 Received in revised form 4 January 2010 Accepted 5 January 2010 Available online 8 February 2010 Keywords: Bi12TiO20 Nanostructures Acid orange 7 Photocatalytic degradation Visible light irradiation BET surface area
a b s t r a c t Bismuth titanate (Bi12TiO20) nanostructures with different morphologies were synthesized hydrothermally using Bi(NO3)3 and Ti(SO4)2 in the presence of polyethylene glycol (PEG). X-ray diffraction (XRD) proved that the samples were in pure cubic phase. UV–visible diffuse reflection spectra showed the band gap of Bi12TiO20 is about 2.7 eV. Brunauer–Emmett–Teller (BET) analysis proved that the Bi12TiO20 samples have higher surface areas than samples prepared by methods reported previously. Photocatalytic degradation of acid orange 7 (AO7) under visible light illumination was used to evaluate the photocatalytic ability of samples. The photocatalytic results showed Bi12TiO20 could degrade AO7 very efficiently and had higher photocatalytic activity than traditional N-doped TiO2. In addition, we have also discussed factors that have major effect on reaction efficiency. BET surface area played the most important role in the photocatalytic degradation of AO7. The crystallinity of the samples is another important factor which can also influence photocatalysis results. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Dyes are important organic pollutants, and their release as wastewater in the ecosystem is a dramatic source of esthetic pollution, eutrophication, and perturbations in aquatic life (Xu and Langfor, 2001; Bao et al., 2004; Horikoshi et al., 2004; Fu et al., 2005; Dillert et al., 2007). Since the discovery of the photocatalytic splitting of water on the TiO2 electrodes by Fujishima and Honda (1972), the application of semiconductor photocatalysts on degradation of pollutants has received great attention (Linsebigler et al., 1995; Zhao et al., 1998; Fujishima et al., 2000; Ho et al., 2004; Dai et al., 2007; Huang et al., 2008; Echavia et al., 2009). Among all photocatalysts, TiO2 attracts the most attention because of its chemical stability, nontoxicity, and high photocatalytic reactivity (Frank and Bard, 1977; Herrmann et al., 1988; Muggli et al., 1998; Phanikrishna Sharma et al., 2008). However, the band gap of the TiO2 is 3.2 eV. It absorbs only the ultraviolet light (k < 400 nm) which only accounts for about 4% of the sunlight (Asahi et al., 2001; Yu et al., 2003; Tang et al., 2004; Shang et al., 2008). In order to improve the efficiency of the sunlight utilization, the development of photocatalysts with high activity under a wide
* Corresponding author. Address: Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China. Tel./fax: +86 21 64252062. E-mail address: [email protected] (J. Zhang). 0045-6535/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2010.01.002
range of visible light irradiation is highly desirable (Hou et al., 2008; Ménesi et al., 2008; Gomathi Devi et al., 2009). In recent years, ternary bismuth oxide semiconductors, such as Bi2WO6 (Kudo and Hijii, 1999; Fu et al., 2005; Zhang et al., 2007), BiVO4 (Tokunaga et al., 2001; Kohtani et al., 2005), NaBiO3 (Kako et al., 2007), Bi12TiO20 (Yao et al., 2003; Xie et al., 2006) have been widely studied as a class of promising photocatalyst which can respond under visible light. Among all ternary bismuth oxide, Bi12TiO20 crystals show high photocatalytic ability in degrading the organic pollutants exposed to UV light irradiation (Yao et al., 2003). However, Bi12TiO20 samples were usually obtained by mechanical method (Stojanovic et al., 2006) and chemical solution decomposition (CSD) method (Yao et al., 2003). The mechanical method is apt to destroy the microstructure of the crystal while the CSD method always involves a very complex procedure. Furthermore, the sample obtained from the above two methods bears limited surface areas and there have been few studies (Yao et al., 2003) about the visible light-induced photocatalytic activity of Bi12TiO20. Here, we report a polyethylene glycol (PEG)-assisted hydrothermal method to synthesize Bi12TiO20 nanostructures with relative high surface areas. Different kinds of PEGs can influence the morphology and the Brunauer–Emmett–Teller (BET) surface area of the samples. The photodegradation of AO7 was employed to evaluate the photocatalytic activities of Bi12TiO20 under visible light illumination. The Bi12TiO20 showed high photoactivity towards degradation of AO7 and has been proven to be a promising alterna-
X. Zhu et al. / Chemosphere 78 (2010) 1350–1355
1351
tive for dye-containing wastewater treatment under visible light irradiation.
(TOC) of the solution was measured by a Liqui TOC analyzer (Elementar).
2. Experiment
3. Results and discussion
2.1. Materials
3.1. XRD
All major reagents were of analytical purity and were supplied by China National Medicines Corporation while AO7 was purchased from Acros and used without further purification. The samples prepared for comparison are (i) N–TiO2 synthesized according to Yu et al. (2006) using Ti(SO4)2 as starting materials and (ii) P-25 titania (TiO2; ca. 80% anatase, 20% rutile; BET area, ca. 50 m2 g1) purchased from the Degussa. Double distilled water was used throughout this study.
The XRD spectra of the samples prepared under different PEGs and hydrothermal time are shown in Figs. 1 and 2. Fig. 1 indicates the XRD diffraction of the samples using different kinds of PEGs. All diffraction peaks can be indexed to cubic Bi12TiO20 according to the JCPDS Card No. 34-0097 and all peaks of three spectra are very sharp indicating high crystallinities of samples. However, the intensities of the peaks from three curves are different which prove that there are some differences among the samples prepared with different PEGs. PEG2000 and PEG600 have stronger peak intensity than PEG10000. The relationship between the crystallinity of Bi12TiO20 and different PEGs will be discussed later. On the other hand, the similar peaks’ positions indicate that different PEG chain length has almost the same effect on the phase of Bi12TiO20 sample. To reveal the effect of hydrothermal treatment time on the Bi12TiO20, time-dependent experiments were carefully conducted. Fig. 2 shows the XRD spectra of samples synthesized with different hydrothermal time. In the preparation of pure sillenite Bi12TiO20, main peaks of cubic Bi12TiO20 have been found when hydrothermal time was just 8 h. But a metastable phase (such as peaks at 32.0° and 23.9°) also appeared which means 8 h hydrothermal time is
2.2. Preparation of Bi12TiO20 photocatalysts The Bi12TiO20 synthesis was as follows: 5 mM of Bi(NO3)35H2O and 0.417 mM of Ti(SO4)2 (Bi:Ti = 12:1) were dissolved in 10 mL 20% (volume ratio) HNO3 under vigorous stirring for 1 h. The 1.8 g different kinds of PEGs (PEG600, PEG2000, PEG10000) were dissolved into 10 mL of double distilled water and added dropwise into the reaction solution. One molar NaOH aqueous solution was also added slowly to adjust the pH to 12 and the transparent solution changed to a white suspension. This suspension was stirred for 30 min and then transferred into a 100 mL stainless steel autoclave up to about 80% of its capacity. The autoclave was kept at 180 °C for 8 h, 24 h and 40 h, respectively, and cooled to room temperature after the reaction time. The precipitates were washed with deionized water and ethanol five times and then dried at 100 °C for 2 h to obtain the final products. 2.3. Characterization of Bi12TiO20 photocatalysts The powder X-ray diffraction (XRD) analysis of the prepared catalyst was carried out at room temperature with a Rigaku D/ max 2550 VB/PC apparatus using Cu Ka radiation (k = 1.5406 Å) and a graphite monochromator, operated at 40 kV and 30 mA. The BET of the sample was determined through nitrogen adsorption at 77 K (Micromeritics ASAP 2010). The sample was degassed at 473 K before the measurement. The UV–vis absorbance spectra were recorded for the dry-pressed disk sample using a Scan UV– vis spectrophotometer (Varian, Cary 500) equipped with an integrating sphere assembly. Transmission electron microscopy (TEM) analysis of the samples was done using a JSM-2100F (JEOL) instrument and the electron beam accelerating voltage was 200 kV.
Fig. 1. XRD patterns of Bi12TiO20 samples prepared with different kinds of PEG (a) PEG600, (b) PEG2000 and (c) PEG10000; hydrothermal temperature: 180 °C; hydrothermal time: 24 h; pH = 12.
2.4. Photocatalytic activities test The photocatalytic degradation of AO7 with visible light was used to evaluate the photocatalytic activities of the samples. A 1000 W halogen lamp was used as the light source with a 420 nm cutoff filter to provide visible light irradiation. In each experiment, 0.1 g of photocatalyst was added to 100 mL of AO7 solution (20 mg L1). Before illumination, the suspensions were magnetically stirred in the dark for 30 min to ensure the adsorption–desorption equilibrium between the photocatalyst and AO7. Then the solution was exposed to visible light irradiation under magnetic stirring. At given time intervals, 4 mL of suspension was sampled and centrifuged to remove the photocatalyst particles. Then, the catalyst-free dye solution were analyzed by a Cary 100 UV–vis spectrometer to record intensity of the maximum band at 484 nm in the UV–vis absorption spectra. Total organic carbon
Fig. 2. XRD patterns of Bi12TiO20 samples prepared with different hydrothermal time (a) 40 h, (b) 24 h, (c) 8 h; hydrothermal temperature: 180 °C; PEG: PEG10000; pH = 12.
1352
X. Zhu et al. / Chemosphere 78 (2010) 1350–1355
not enough for the formation of pure cubic Bi12TiO20. Gradual transformation from metastable phase into sillenite phase was observed with the hydrothermal time changing from 8 h to 40 h. Pure cubic Bi12TiO20 was finally obtained with 24 h hydrothermal treatment. After the hydrothermal conversion proceeded for 40 h, the peak intensity got stronger which meant the crystallinity of the sample was developing with the prolongation of hydrothermal time. The result of FT-IR (data not shown) showed that with the extension of the hydrothermal time, the peaks got sharper and sharper which indicated that the extension of hydrothermal time was beneficial for the formation and strengthening of Bi–O and this result agreed well with the XRD results discussed above.
3.2. UV–vis diffuse reflection spectrum Fig. 3 shows the UV–vis diffuse reflection spectrum of the prepared Bi12TiO20 (PEG: PEG600, hydrothermal temperature: 180 °C; hydrothermal time: 24 h; pH = 12). As a comparison, the spectra of P-25 titania and N–TiO2 were also measured. The absorption onset wavelength of Bi12TiO20 was around 500 nm which was shifted 100 nm to visible region compared to P-25. The color of synthesized Bi12TiO20 was yellow, which was in accordance with its absorption spectrum. The absorption spectrum of Bi12TiO20 had steep shape which indicated the absorption relevant to the band gap was due to the intrinsic transition of the nanomaterials rather than the transition from impurity levels (Kudo et al., 2002). It is well-known that for a crystalline semiconductor (Butler, 1977; Fu et al., 2005), the optical absorption near the band edge follows the formula: n
ahm ¼ Aðhm EgÞ2
ð1Þ
where a, h, m, Eg, and A are the absorption coefficient, Planck constant, the light frequency, the band gap, and a constant, respectively. Among these, n describes the characteristics of the transition in a semiconductor. For Bi12TiO20, the value of n is 1. The plot of (ahm)2 vs. hm based on the direct transition is shown in the inset of Fig. 3. Therefore, the energy of the band gap could be estimated to be about 2.70 eV for Bi12TiO20 in our case. This indicates that the nanosized Bi12TiO20 has a suitable band gap for photocatalytic decomposition of organic contaminants under visible light irradiation.
Fig. 3. UV–vis diffuse reflectance spectra of as-prepared Bi12TiO20 powders (PEG: PEG600, hydrothermal temperature: 180 °C; hydrothermal time: 24 h; pH = 12), N– TiO2 and P-25. The inset is the (ahm)2 vs. hm curve.
3.3. Degradation of AO7 using Bi12TiO20 photocatalysts Photodegradation experiments of AO7 were carried out under visible light illumination at wavelengths longer than 420 nm in order to test the photocatalytic performance of the Bi12TiO20 sample. For comparison, the photodegradation of AO7 by N-doped TiO2 was also carried out. The results displayed in Fig. 4a shows that AO7 solution was stable under visible light irradiation in the absence of any catalyst. When Bi12TiO20 was added to the AO7 solution, the total degradation rate was over 90% within 6 h irradiation, much higher than the N-doped TiO2 which only reached 15% of the total decomposition. As shown in Fig. 4b, photocatalytic activity of Bi12TiO20 synthesized under different synthesizing conditions is studied. Samples synthesized using PEG10000 showed higher activity than the samples synthesized using PEG600 and PEG2000. Meanwhile when the hydrothermal time was 24 h, Bi12TiO20 showed the fastest degradation speed/rate compared with other hydrothermal times. TOC removal of the solution is shown in Fig. 4c while the corresponding UV/vis spectral changes of these solutions are displayed in the inset of Fig. 4c. The rate of TOC reduction was slower than that of the degradation of the dye; 80% of TOC still remained in the suspension after 6 h of irradiation when AO7 solution was bleached. The inset of Fig. 4c shows that the peaks at 310 and 228 nm corresponding to naphthalene ring and benzene ring in the dye molecule were also reduced (Stylidi et al., 2004). A lot of small organic intermediates which were not completely mineralized were generated in the process of photocatalysis. So the remaining organic carbon is probably due to these refractory organic compounds. The photocatalytic decolorization of AO7 follows the pseudofirst-order reaction kinetics (Yu et al., 2000; Hammami et al., 2008; Ji et al., 2009) and its kinetics might be expressed as follows:
ln
C0 ¼ kt C
ð2Þ
where k is the apparent reaction rate constant, C0 is the initial absorbance of aqueous AO7, t is the reaction time, and C is the absorbance of aqueous AO7 at the reaction time of t. According to Eq. (2), the kinetic of AO7 decolorization under different photocatalysts is presented in Fig. 4d by plotting the logarithm of the normalized dye concentration against irradiation time. The calculated apparent reaction rate constants of each catalyst are listed in Table 1. In order to study the possible relationship between the photocatalytic activity and the property of different samples, TEM images (Fig. 5) and BET surface area data (Table 1) of different samples are also displayed. Table 1 shows the BET surface area, apparent reaction rate constants of Bi12TiO20 under different preparation conditions as well as the standard deviations. Samples synthesized in the presence of PEG10000 (sample 1 and sample 2) had relatively high BET surface areas. Sample 1 reached over 15 m2 g1, which was much higher than the samples prepared by other methods, such as coprecipitation processing (4.89 m2 g1) (Xu et al., 2007). But when PEG2000 and PEG600 were used, the BET surface area decreased. In order to study the effects of PEGs on the surface area, TEM was used to observe the morphologies and structure details of these bismuth titanate products. Fig. 5a gives the TEM image of a typical example of Bi12TiO20 nanoparticles which were prepared using PEG10000. It showed that products synthesized were composed of nano-sized particles, which were about 10 nm in size, and each particle was nearly spherical in shape and had higher density. As shown in Fig. 5b, Bi12TiO20 nanobelts with width of 50–80 nm and lengths up to several micrometres and Bi12TiO20 nanoparticles (pointed out by the arrow) were all synthesized in
1353
X. Zhu et al. / Chemosphere 78 (2010) 1350–1355
Fig. 4. (a) Temporal course of the photodegradation of AO7 in different catalyst aqueous dispersions; (b) temporal course of the photodegradation of AO7 by Bi12TiO20 prepared under different conditions. (c) TOC of AO7 solutions degraded by Bi12TiO20 which was prepared under 180 °C for 24 h in the presence of PEG10000. The inset is the corresponding UV/vis spectral changes. (d) The corresponding kinetic analysis assuming the pseudo-first-order reaction for AO7 dye. Experimental condition: [AO7] = 20 mg L1, [catalyst] = 1 g L1, T = 298 K, initial pH = 6.2.
Table 1 BET surface area of the prepared Bi12TiO20 under different synthetic conditions, their photocatalytic reaction rate constants and the standard deviations. Name Sample Sample Sample Sample Sample Sample
1 2 3 4 5 6
Synthesizing conditions
BET surface area (m2 g1)
Photocatalytic reaction rate constants, k (h1)
Standard deviation SD (%)
PEG10000, 8 h PEG10000, 24 h PEG10000, 40 h PEG600, 24 h PEG2000, 24 h N–TiO2
17.98 13.50 5.92 6.60 6.00 –
0.315 0.327 0.134 0.140 0.125 0.029
5.7 1.8 3.3 4.0 1.3 0.9
the process of using PEG2000. Although some small nanoparticles could also be observed in Fig. 5b, the formation of big-size nanobelts could decrease the BET surface area of sample dramatically. The different morphologies mainly depend on the different chain length of PEGs. PEG with uniform and ordered chain structure can easily adsorb on the surface of metal oxide colloid which has been investigated in previous reports (Liu et al., 2000; Dobryszycki and Biallozor, 2001). This type of polymer greatly decreases the activity of colloid surface when adsorbed on them (Li et al., 2003). From the view of kinetics of colloid growth, if the colloid adsorbs the polymer on some parts of its surface, the growth rate of the colloids in certain direction will be confined. In the present work, PEG2000 which has relative short-chain length, uniform and ordered chain can modify the growth kinetics of the growing colloids and finally leads into anisotropic growth of the crystals.
There were a few spherical nanoparticles in the prepared sample because the concentration of PEG2000 was high enough to confine all aspects of the colloids. In the previous research using longlength PEG (Yu et al., 2001), lots of spherical particles were also obtained, which is due to the fact that a long-chain PEG can confine more than one direction of the growing colloid and finally form spherical colloids. On the other hand, long-chain polymers are easily intertwisted which leads to detrimental effects in the formation of 1D nanostructures, while short-chain polymers greatly decrease the possibilities of intertwisting under these conditions. As a result, sample 2 prepared using PEG10000 was spherical nanoparticles and it had larger BET surface areas than sample 5. On the other hand, the different morphologies of Bi12TiO20 resulting from different PEGs might be the explanation of the differences among the crystallinities of Bi12TiO20 prepared using various PEGs. The big-
1354
X. Zhu et al. / Chemosphere 78 (2010) 1350–1355
Fig. 5. (a) TEM images of Bi12TiO20 nanoparticles prepared using PEG10000; (b) TEM images of Bi12TiO20 nanobelts and nanoparticles prepared using PEG2000.
size-nanobelt-Bi12TiO20 had the better crystallinity than the smallsize-nanocrystal-Bi12TiO20. Higher BET surface area provides more active sites for photocatalytic reaction, while a perfect crystallinity is helpful for inhibiting the recombination rate of photo-generated electrons and holes, improving the photocatalytic activity (Sclafani et al., 1990). But crystallinity and BET surface area always have the competing relationship (Li et al., 2005; Todorova et al., 2008). In this study, sample 1 and sample 2 which have relatively high BET surface area have much higher photocatalytic activity than other samples. This suggests that the rates of photocatalytic activity primarily depend on the BET surface area of the sample. When the samples have different crystallinities and phase compositions, such as sample 1 and sample 2, the result have been affected by both factors. As shown in Fig. 2, sample 2 prepared with 24 h hydrothermal time has pure cubic phase and better crystallinity than sample 1 which is prepared with only 8 h hydrothermal time. The sample 2 showed the relative higher photocatalytic activity as compared to sample 1 (Fig. 4b and d). This result proved that crystallinity can also affect the photocatalyst results. Therefore, considering the two factors (BET surface area and crystallinity), sample 2 having the relative high surface area and the perfect crystallinity performs the best.
peaks in the XRD pattern were almost the same to those of Bi12TiO20 before irradiation. As shown in this result, Bi12TiO20 is considered to be relatively stable to visible light under the present experimental conditions. This result offers a possibility for application of Bi12TiO20 photocatalyst in the wastewater treatment. 4. Conclusions Bi12TiO20 nanostructures with relatively large BET surface area were successfully synthesized by hydrothermal method in the presence of PEG. PEG with different chain length had similar effect on the crystallinity of samples, but could induce differences to the morphology and BET surface area. The optical band gap was estimated to be about 2.7 eV which proved Bi12TiO20 can respond to the visible light. The results of photocatalytic degradation of AO7 showed that BET surface area had more important influence to the activity of degrading AO7 than crystallinity. The Bi12TiO20 photocatalyst prepared with the PEG10000 and the hydrothermal time of 24 h showed the highest photocatalytic activity for photocatalytic degradation of AO7. Acknowledgements
3.4. The stability of Bi12TiO20 as the photocatalyst Fig. 6 indicates the XRD patterns of the sample after 6 h of visible light irradiation. Both the position and the intensity of the
This work was supported by Science and Technology Commission of Shanghai Municipality (07JC14015); National Nature Science Foundation of China (20773039, 20977030); National Basic Research Program of China (973 Program, 2007CB613301, 2010CB732306); the Ministry of Science and Technology of China (2006AA06Z379, 2006DFA52710) and ‘‘The Zhuoyue Project” of East China University of Science and Technology. References
Fig. 6. XRD patterns of Bi12TiO20 before and after visible light irradiation.
Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K., Taga, Y., 2001. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 293, 269–271. Bao, N., Feng, X., Yang, Z., Shen, L., Lu, X., 2004. Highly efficient liquid-phase photooxidation of an azo dye methyl orange over novel nanostructured porous titanate-based fiber of self-supported radially aligned H2Ti8O171.5H2O nanorods. Environ. Sci. Technol. 38, 2729–2736. Butler, M.A., 1977. Photoelectrolysis and physical properties of the semiconducting electrode WO3. J. Appl. Phys. 48, 1914–1920. Dai, K., Chen, H., Peng, T.Y., Ke, D.N., Yi, H.B., 2007. Photocatalytic degradation of methyl orange in aqueous suspension of mesoporous titania nanoparticles. Chemosphere 69, 1361–1367. Dillert, R., Bahnemann, D., Hidaka, H., 2007. Light-induced degradation of perfluorocarboxylic acids in the presence of titanium dioxide. Chemosphere 67, 785–792. Dobryszycki, J., Biallozor, S., 2001. On some organic inhibitors of zinc corrosion in alkaline media. Corros. Sci. 43, 1309–1319.
X. Zhu et al. / Chemosphere 78 (2010) 1350–1355 Echavia, G.R.M., Matzusawa, F., Negishi, N., 2009. Photocatalytic degradation of organophosphate and phosphonoglycine pesticides using TiO2 immobilized on silica gel. Chemosphere 76, 595–600. Frank, S.N., Bard, A.J., 1977. Heterogeneous photocatalytic oxidation of cyanide and sulfite in aqueous solutions at semiconductor powders. J. Phys. Chem. 81, 1484– 1488. Fu, H.B., Pan, C.S., Yao, W.Q., Zhu, Y.F., 2005. Visible-light-induced degradation of Rhodamine B by nanosized Bi2WO6. J. Phys. Chem. B 109, 22432–22439. Fujishima, A., Honda, K., 1972. Electrochemical photolysis of water at a semiconductor electrode. Nature 238, 37–38. Fujishima, A., Rao, T.N., Tryk, D.A., 2000. Titanium dioxide photocatalysis. J. Photochem. Photobiol. C 1, 1–21. Gomathi Devi, L., Narasimha Murthy, B., Girish Kumar, S., 2009. Heterogeneous photo catalytic degradation of anionic and cationic dyes over TiO2 and TiO2 doped with Mo6+ ions under solar light: correlation of dye structure and its adsorptive tendency on the degradation rate. Chemosphere 76, 1163–1166. Hammami, S., Bellakhal, N., Oturan, N., Oturan, M.A., Dachraoui, M., 2008. Degradation of acid orange 7 by electrochemically generated OH radicals in acidic aqueous medium using a boron-doped diamond or platinum anode: a mechanistic study. Chemosphere 73, 678–684. Herrmann, J.M., Disdier, J., Pichat, P., 1988. Photocatalytic deposition of silver on powder titania: consequences for the recovery of silver. J. Catal. 113, 72–81. Horikoshi, S., Hojo, F., Hikaka, H., Serpone, N., 2004. Environmental remediation by an integrated microwave/UV illumination technique. 8. Fate of carboxylic acids, aldehydes, alkoxycarbonyl and phenolic substrates in a microwave radiation field in the presence of TiO2 particles under UV irradiation. Environ. Sci. Technol. 38, 2198–2208. Hou, Y.D., Wang, X.C., Wu, L., Chen, X.F., Ding, Z.X., Wang, X.X., Fu, X.Z., 2008. NDoped SiO2/TiO2 mesoporous nanoparticles with enhanced photocatalytic activity under visible-light irradiation. Chemosphere 72, 414–421. Ho, W., Yu, J.C., Lin, J., Li, P., 2004. Preparation and photocatalytic behavior of MoS2 and WS2 nanocluster sensitized TiO2. Langmuir 20, 5865–5869. Huang, H.H., Tseng, D.H., Juang, L.C., 2008. Titanium dioxide mediated photocatalytic degradation of monochlorobenzene in aqueous phase. Chemosphere 71, 398–405. Ji, P.F., Zhang, J.L., Chen, F., Anpo, M., 2009. Study of adsorption and degradation of acid orange 7 on the surface of CeO2 under visible light irradiation. Appl. Catal. B 2009 (85), 148–154. Kako, T., Zou, Z.G., Katagiri, M., Ye, J.H., 2007. Decomposition of organic compounds over NaBiO3 under visible light irradiation. Chem. Mater. 19, 198–202. Kohtani, S., Hiro, J., Yamamoto, N., Kudo, A., Tokumura, K., Nakagaki, R., 2005. Adsorptive and photocatalytic properties of Ag-loaded BiVO4 on the degradation of 4-n-alkylphenols under visible light irradiation. Catal. Commun. 6, 185–189. Kudo, A., Hijii, S., 1999. H2 or O2 evolution from aqueous solutions on layered oxide photocatalysts consisting of Bi3+ with 6s2 configuration and d0 transition metal ions. Chem. Lett., 1103–1104. Kudo, A., Tsuji, I., Kato, H., 2002. AgInZn7S9 solid solution photocatalyst for H2 evolution from aqueous solutions under visible light irradiation. Chem. Commun. 17, 1958–1959. Li, D., Haned, H., Hishita, S., Ohashi, N., Labhsetwar, K.N., 2005. Fluorine-doped TiO2 powders prepared by spray pyrolysis and their improved photocatalytic activity for decomposition of gas-phase acetaldehyde. J. Fluorine Chem. 126, 69–77. Linsebigler, A.L., Lu, G., Yates, J.T., 1995. Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results. Chem. Rev. 95, 735–758. Liu, X.H., Yang, J., Wang, L., Yang, X.J., Lu, L.D., Wang, X., 2000. An improvement on sol–gel method for preparing ultrafine and crystallized titania powder. Mater. Sci. Eng. A 289, 241–245. Li, Z.Q., Xiong, Y.J., Xie, Y., 2003. Selected-control synthesis of ZnO nanowires and nanorods via a PEG-assisted route. Inorg. Chem. 42, 8105–8109.
1355
Ménesi, J., Körösi, L., Bazsó, E., Zöllmer, V., Richardt, A., Dékány, I., 2008. Photocatalytic oxidation of organic pollutants on titania–clay composites. Chemosphere 70, 538–542. Muggli, D.S., McCue, J.T., Falconer, J.L., 1998. Mechanism of the photocatalytic oxidation of ethanol on TiO2. J. Catal. 173, 470–483. Phanikrishna Sharma, M.V., Durga Kumari, V., Subrahmanyam, M., 2008. TiO2 supported over SBA-15: an efficient photocatalyst for the pesticide degradation using solar light. Chemosphere 73, 1562–1569. Sclafani, A., Palmisano, L., Schiavello, M., 1990. Influence of the preparation methods of titanium dioxide on the photocatalytic degradation of phenol in aqueous dispersion. J. Phys. Chem. 94, 829–832. Shang, M., Wang, W.Z., Sun, S.M., Zhou, L., Zhang, L., 2008. Bi2WO6 nanocrystals with high photocatalytic activities under visible light. J. Phys. Chem. C 112, 10407– 10411. Stojanovic, B.D., Paiva-Santos, C.O., Jovalekic, C., Simoes, A.Z., Filho, F.M., Lazarevic, Z., Varela, J.A., 2006. Mechanically activating formation of layered structured bismuth titanate. Mater. Chem. Phys. 96, 471–476. Stylidi, M., Kondarides, D.I., Verykios, X.E., 2004. Visible light-induced photocatalytic degradation of acid orange 7 in aqueous TiO2 suspensions. Appl. Catal. B: Environ. 47, 189–201. Tang, J., Zou, Z., Ye, J., 2004. Efficient photocatalytic decomposition of organic contaminants over CaBi2O4 under visible light irradiation. Angew. Chem., Int. Ed. 43, 4463–4466. Todorova, N., Giannakopoulou, T., Vaimakis, T., Trapalis, C., 2008. Structure tailoring of fluorine-doped TiO2 nanostructured powders. Mater. Sci. Eng. B – Adv. 152, 50–54. Tokunaga, S., Kato, H., Kudo, A., 2001. Selective preparation of monoclinic and tetragonal BiVO4 with scheelite structure and their photocatalytic properties. Chem. Mater. 13, 4624–4628. Xie, L.J., Ma, J.F., Tian, H., Zhou, J., Zhao, Z.Q., Wu, P.W., Hu, Y.M., Wang, Y.G., Tao, J.T., Zhu, X.Y., 2006. Isopropanol-assisted hydrothermal synthesis of bismuth titanate nanophotocatalysts. Mater. Lett. 60, 284–286. Xu, S.H., Shangguan, W.F., Yuan, J., Shi, J.W., Chen, M.X., 2007. Photocatalytic properties of bismuth titanate Bi12TiO20 repaired by co-precipitation processing. Mater. Sci. Eng. 137, 108–111. Xu, Y.M., Langfor, C.H., 2001. UV or visible-light induced degradation of X3B on TiO2 nanoparticles: the influence of adsorption. Langmuir 17, 897–902. Yao, W.F., Wang, H., Xu, X.H., Cheng, X.F., Huang, J., Shang, S.X., Yang, X.N., Wang, M., 2003. Photocatalytic property of bismuth titanate Bi12TiO20 crystals. Appl. Catal. A – Gen. 243, 185–190. Yu, J.C., Zhang, L.Z., Zheng, Z., Zhao, J., 2003. Synthesis and characterization of phosphated mesoporous titanium dioxide with high photocatalytic activity. Chem. Mater. 15, 2280–2286. Yu, J.G., Zhao, X.J., Zhao, Q.N., 2000. Effect of surface structure on photocatalytic activity of TiO2 thin films prepared by sol–gel method. Thin Solid Films 379, 7– 14. Yu, J.G., Zhao, X.J., Zhao, Q.N., Wang, G., 2001. Preparation and characterization of super-hydrophilic porous TiO2 coating films. Mater. Chem. Phys. 68, 253–259. Yu, J.G., Zhou, M.H., Cheng, B., Zhao, X.J., 2006. Preparation, characterization and photocatalytic activity of in situ N, S-codoped TiO2 powders. J. Mol. Catal. A: Chem. 246, 176–184. Zhang, L.S., Wang, W.Z., Zhou, L., Xu, H.L., 2007. Bi2WO6 nano- and microstructures: shape control and associated visible-light-driven photocatalytic activities. Small 3, 1618–1625. Zhao, J.C., Wu, T.X., Wu, K.Q., Oikawa, K., Hidaka, H., Serpone, N., 1998. Photoassisted degradation of dye pollutants (III) degradation of the cationic dye rhodamine B in aqueous anionic surfactant/TiO2 dispersions under visible light irradiation: evidence for the need of substrate adsorption on TiO2 particles. Environ. Sci. Technol. 32, 2394–2400.