A visible light activity of TiO2 based photocatalysts

JOURNAL OF SCIENCE OF HNUE 2011, Vol. 56, N◦. 1, pp. 11-20 A VISIBLE LIGHT ACTIVITY OF TiO2 BASED PHOTOCATALYSTS Nguyen Minh Thuy(∗), Le Thi Hong Hai Duong Quoc Van and Bui Thi Hau Hanoi National University of Education (∗)E-mail: thuynm@hnue.edu.vn Abstract. Vanadium doped TiO2 nanoparticles were synthesized by sol- gel and hydrothermal methods. The samples were characterized by X-ray diffraction, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and UV-vis diffuse reflectance spectroscopy. The Vanadium doped TiO2 nanoparticles had identical anatase phase with aver- age crystal size of 15-20nm and exhibited the long tailed absorption in the visible light region above 380nm. The photocatalystic activity under the ir- radiation of visible light was evaluated by the degradation of phenol aqueous solution . The samples synthesized by hydrothermal method show better photocatalystic activity: Under the condition of 360 min of Vis-irradation the photolacatalyst TiO2:0.5%V makes the phenol concentration decreased to 0.3%. Keywords: semiconductors, nanocrystals, photocatalysts, absorp- tion, irradiation. 1. Introduction Titanium dioxide (TiO2) photocatalysts have been investigated with consid- erable attention because they can be applicable for the decomposition of undesired compounds in air as well as waste water, solar energy conversion and production of clean energy resources through the water splitting reaction. Especially, applications for environmental issues such as purification of waster water using natural solar light are vital interest to practical utilizations and challenging topics. However, the ap- plication of TiO2 as a photocatalyst for visible light-induced chemical reactions was hampered by its large band-gap energy (3.2 eV for anatase TiO2), which requires ultraviolet (UV) light to activate and leads to the lower energy efficiency. Widening the absorption edge of TiO2 from UV to visible spectral range could provide the groundwork to develop TiO2 catalysts with visible light activity. Many studies had been attributed to the doping of transition metals into TiO2 to develop vis-photocatalysts, such as V, Cr, Mo, Fe... [1-5]. Different methods had 11 Nguyen Minh Thuy, Le Thi Hong Hai, Duong Quoc Van and Bui Thi Hau been chosen to prepare V-doped TiO2 catalysts, such as sol-gel method, metal ion- implantation method, co-precipitation method, hydrothermal method and so on[2- 4]. It is promising to synthesize V-doped TiO2 powders economically and practically by the hydrothermal method for its low-cost, effectiveness and easy execution. In this work, V-doped TiO2 nanopoders were successfully synthesized by two synthesized methods: sol-gel citrate and hydrothermal. The samples prepared by hydrothermal method exhibited highest photocatalystic activity. The visible light phototcatalystic activity of V-doped TiO2 prepared by hydrothermal method sug- gested an application for an environmental treatment of waste water. 2. Content 2.1. Experimental The used chemicals were TiCl4, acid Citric (CA), NH4NO3, NH3 10 %, V2O5/HCl 0.03 M. All chemicals were of analytical reagent grad. Pure titanium catalyst was prepared using the hydrothermal method with the following procedure: Firstly 3.0 ml TiCl4 was added drop by drop to 100 ml distilled water vigoroustirring, which was continuously stirred for 20 minutes. Afterwards, ammonia aqueous solution NH3 (25%) was added to adjust the pH value to 7-8, and then stirred for 30 min- utes. Then the distilled water was added and the mixture was transferred into an autoclave and kept at 2000C for 5h. After centrifugation, washing and drying, the samples were calcined at 6500C for 1h (see Figure 1a). A series of V-doped TiO2 hydrosol were prepared by changing the V/Ti ratio. The Vanadium doping content was calculated by following equation: %V = nV /(nV + nTi) where nV and nTi were the mole of Vanadium and Titanium respectively. Pure TiO2 was prepared using the sol-gel citrate method with the following procedure. The starting solutions were prepared by addition of aqueous solution of citric acid (CA) and NH4NO3 with the molar ratio of 1:9. Then TiCl4 was added drop by drop to the solution vigoroustirring, which was continuously stirred for 30 min. The molar ratio CA: TiCl4 was 1.2:1. Afterwards, ammonia aqueous solution NH3 was added to adjust the pH value to 7-8, then the mixture was strong stirred at 800C to completely get a sol. The sol was burned and then calcined at 6500C for 3h. A series of V-doped TiO2 hydrosol were prepared by changing the V/Ti ratio. The V-doped samples prepared by the sol-gel citrate method were designated as S0; S1; S3; S5; S7 and S9 with the yields of the V doping contents of 0.0; 0.1; 0.3; 0.5; 0.7 and 0.9%, respectively. The samples prepared by the hydrothermal method were designated as T0; T1; T3; T5; T7 and T9 with the yields of the V doping contents of 0.0; 0.1; 0.3; 0.5; 0.7 and 0.9%, respectively. 12 A visible light activity of TiO2 based photocatalysts Figure 1. Preparation processes of Vanadium doped TiO2 by sol-gel citrate (a) and hydrothermal (b) methods The structure and crystalline characters were analyzed by X- ray diffraction (XRD) SIEMENS D5005 and by Raman scattering measurements with LABRAM micro-Raman spectroscopy. The morphology was investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). Absorption measurements were obtained with JASCO V-670 spectrometer. X-ray photoelectron spectroscopy (XPS) mea- surements were performed in a commercial Microlab 350 XPS system equipped with an Al Kα source, in the UHV chamber (∼ 10−9Torr) with 600-take off angle. The irradiation was performed with a 100 W-wolfram lamp being used for visible light sources. The photocatalytic activity of samples was evaluated by degra- dation of phenol (C6H5OH) diluted in water. 80 ml phenol solution (5.10 −5 mol/l) was stirred under dark condition. Then 200 mg TiO2 -based photocatalysts was added to the phenol solution and the mixture was stirred for 60 min under dark condition. And then, visible light irradiation was carried out using 100 W-wolfram lamp. The progress of the reactions was monitored by High performance liquid Chromatography - HPLC for 360 min. 2.2. Results and discussion 2.2.1. Sample characterization Figure 2 presented the XRD patterns of V-doping TiO2 powder prepared by hydrothermal methods. For all doping samples (up to 0.9%) the XRD patterns show 13 Nguyen Minh Thuy, Le Thi Hong Hai, Duong Quoc Van and Bui Thi Hau Figure 2. The XRD patterns of TiO2 (a in left); TiO2:0.05V (b in left) and TiO2:0.09V (in right) nanoparticles prepared by hydrothermal method Figure 3. The XRD patterns of TiO2 and TiO2:V nanoparticles prepared by sol-gel method and the 101-peaks by different preparation methods only anatase TiO2 peaks. The average crystallite size calculated using the Scherers formula gave a value of about 15 nm. The XRD patterns of V-doped TiO2 powder prepared by sol- gel method are presented in Figure 3a, b, which showed that the nanoparticles present the anatase phase. The average crystallite size is about 20 nm. The inserted picture in Figure 3 shows the XRD-(101) peaks of the samples TiO2:0.5%V prepared by different methods. One can see that the (101) peak of sample prepared by hydrothermal method is clearly broadened than that by sol-gel method. This indicated a smaller grain size in the samples prepared by the hydrothermal method. Figure 4 shows the SEM image of TiO2:0.5%V powder (a) and their TEM image (b). The sample is composed of cubic grains. The average size of particles is about 20 nm. 14 A visible light activity of TiO2 based photocatalysts Figure 4. SEM and TEM images of TiO2:0.5%V nanoparticles prepared by hydrothermal method Figure 5 i) and ii) present the Raman patterns of undoped TiO2 and V-doped TiO2. The observed peaks showed that the nanoparticles were complete anatase phase. Figure 5 iii) and iiii) below show more detailed Raman peaks of the doped samples. Figure 5. The Raman spectra of TiO2 and TiO2:V prepared by hydrothermal method 15 Nguyen Minh Thuy, Le Thi Hong Hai, Duong Quoc Van and Bui Thi Hau Compared with the undoped sample, the Eg-vibrations at 146 and 197 cm−1 presented a lightly blue-shift after V doping. The other vibrations presented no changed with the doping effect up to 0.9%V. All peaks became wide and lightly unsymmetrical after V doping. As well-known for the nanomaterials the blue-shift of Raman peaks can be attributed to the quantum size effect that comes from the small particle size, which implied that the smaller the grain size, the more noticeable the blue-shift. In this study the observed blue shift can not be ascribed to the decrease of grain sizes because the grain size increased with the doped V concentration, we supposed the blue shift can be ascribed to the TiO2 lattice structure distortion due to V doping. According to [3], the blue shift can be partly attributed to the oxygen defects. The lattice structure distortion due to V doping can be remarked also in the absorption spectra. Figure 6. The Raman spectra of TiO2 and TiO2:V prepared by hydrothermal method Figure 7. The Ti 2p and O 1s XPS spectra of undoped and vanadium doped samples 16 A visible light activity of TiO2 based photocatalysts The optical absorption spectra of TiO2 and V-doped TiO2 samples prepared by sol-gel (left) and hydrothermal (right) methods are presented in Figure 6. Pure TiO2 (curve a) exhibited the absorption edge at around 380 nm, which corresponded to the band gap of anatase type of TiO2. The V-doped TiO2 exhibited the long tailed absorption in the visible light above 380 nm. Compared with the spectrum of undoped TiO2 (curve a), the red (curve b) shift was observed in doped samples. The tailing of the absorption band of doped samples can be assigned to the charge- transfer transition from the d orbital of V4+ to the conduction band of TiO2 [4,5]. The weak absorption band in 650-700 nm can due to the d-d transitions of V3d electrons. Reference [6] shown that besides the d-d transition, the gap states intro- duced by V doping was another important reason that resulted in the visible light absorption. The Reference [7,8] obtained that the weak band tail of undoped TiO2 was from the momentary localization of excitons due to the phonon interaction, and the strong band tail of the V-doped TiO2 nanoparticles can mainly contributed to the impurities and lattice disorder, which also indicated the formation of gap states, in accordance with the result of calculation performed by [9]. In present work we suppose that the strong band tail in the visible light of V-doping TiO2 nanopow- ders is related to the d-d transition of V4+ in the host TiO2 and also to the lattice disorder. The evidence of the V4+ in host TiO2 and lattice disorders can be also observed in XPS spectra (see Figure 7). The oxygen defects and lattice disorder can be seen by the XPS spectra. Figure 7a shows the Ti 2p core-level spectra of undoped and vanadium doped samples. As well known, the Ti 2p peak split into Ti 2p3/2 (∼459 eV) and Ti 2p1/2 (∼ 464.5 eV) due to self-orbital coupling effect. For the undoped sample, these peaks were symmetrical, that indicates that Ti+4 was mainly presented in the undoped sample. The Ti 2p peak became wide and unsymmetrical after V doping, which may be related to more oxygen defects after V doping (it can lead to Ti2O3 formation). Figure 7b illustrates the O 1s core level XPS spectra of undoped and vanadium doped samples. The O 1s peak (from 530 to 532 eV) is often believed to be composed of several different oxygen species, such as Ti-O bonds in TiO2 or Ti2O3, hydroxyl groups, C-O bonds and adsorbed H2O. In the spectra this peak is broaden, which can be analyzed to three components: (i) the man peak (of undoped and doped samples) at ∼531 eV could be ascribed to lattice oxygen in TiO2 (Ti-O-Ti); (ii) the other peak at 532.3 eV could be associated to surface hydroxyl (Ti-OH) groups [10]; and (iii) for the doping samples there could be other peak at ∼533 eV of V-O groups [7]. The latest is increased with the doping effect. The V doping lead to the O 1s peak shifting to higher binding energy, indicative of the O valence increase. 2.2.2. Photocatalytic activity measurement The photocatalystic degradation of phenol has been chosen as a model reac- tion to evaluate the photocatalystic activities of the obtained TiO2 based catalysts. 17 Nguyen Minh Thuy, Le Thi Hong Hai, Duong Quoc Van and Bui Thi Hau Phenol was selected as a model pollutant for the photocatalytic oxidation experi- ments because it was a common toxicant in waste water. All phenol diluted water samples were prepared as following: 80 ml (C6H5OH) solution (5.10 −5 mol/l) was stirred under dark condition. Then 200 mg TiO2 -based photocatalysts was added to the phenol solution and the mixture was stirred for 60 min under dark condition. And then, visible light irradiation was carried out using 100 W wolfram lamp with emission spectral range from 400 to 800 nm. Figure 8 presented the chromatographs of phenol degradation of phenol pol- lution water using TiO2:0.5%V as photocatalysts, which were recorded by HPLC at 0 and 360 min after a visible irradiation. Figure 8. Chromatographs of phenol degradation using TiO2:0,5%V under vis-irradiation at 0 (over) and 360 min(below)) Figure 9. Photocatalytic phenol degradation curves of the 0,5% doped TiO2 prepared by different method By calculating and comparing the squares of phenol peaks (at trent.=5.6 min.) in the chromatographs we obtained the phenol degradation in water samples in photocatalyctic process. Using result of our previous study, the 0.5%V doped TiO2 sample was chosen for the photocatalystic investigation. Figure 9 described the photocatalystic activity of the 0.5%V- doped TiO2 samples prepared by different methods. One can see that the sample prepared by hydrothermal method (c curve) exhibited highest photocatalystic activity. After 360 min irradiation, the current normalized phenol concentration decreased to 0.43%. This result can be related to the smaller grain size of the samples prepared by hydrothermal method, which is agreed with the XRD result above. Toward practical use of these materials we have measured its photocatalystic degradation of phenol in waste water of Vanphuc textile village in Hanoi. We choose 18 A visible light activity of TiO2 based photocatalysts the 0.5%V doped TiO2 samples prepared by sol gel (S5) and hydrothermal (T5) methods. The current phenol concentrations in waste water sample after visible irradiation are presented in Table 1. Table 1. The visible light photocatalystic degradation of phenol in waste water by using TiO2:0.5%V powders Irradiation time (min) 0 60 120 180 240 Phenol content (mg/l) case of S5 sample 0.9106 0.7318 0.5714 0.2739 0.0036 Phenol content (mg/l) case of T5 sample 0.7712 0.6992 0.6271 0.3135 0.0030 The current phenol content in waste water decreased to 0.003 mg/l after 240 min. of visible irradation.This value is less than the allowable phenol value on industrial waste water by discharge standards (TCVN5945-2005: < 0.1mg/l). The visible light phototcatalystic activity of V doped TiO2 suggested an application for an environmental treatment of waste water. 3. Conclusion The pure and vanadium doping TiO2 nanopowders were prepared by sol-gel citrate and hydrothermal methods. The dopant V-concentration is in a range from 0.1% to 0.9%. All obtained samples are single anatase crystal phase. The average crystal size is about 15-20 nm, the grain are cubic form and have the size dis- tributed of 15-25 nm. 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