Enhanced photocatalytic efficiency of TiO2 with doped Ni-immobilized on silica gel

JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2016-0044 Mathematical and Physical Sci., 2016, Vol. 61, No. 7, pp. 151-156 This paper is available online at ENHANCED PHOTOCATALYTIC EFFICIENCY OF TiO2 WITH DOPED Ni-IMMOBILIZED ON SILICA GEL Phung Thi Len1, Nguyen Manh Nghia1, Nguyen Cao Khang1, Duong Quoc Van1 and Nguyen Thi Hue2 1Faculty of Physics, Hanoi National University of Education 2Institute of Environmental Technology, Vietnam Academy of Science and Technology Abstract. Ni-doped TiO2, immobile on silica gel (Ti1−xNixO2/SiO2), was synthesized using the sol-gel method. The XRD patterns show that the TiO2 particles on SiO2 have an anatase crystalline structure with a particle size of around 7 nm in diameter. The results of absorption spectra indicate that the band gap of the anatase TiO2 powder was 3.3 eV, while the band gap of anatase TiO2:Ni was 3.1 eV. UV-vis spectrums proved that TiO2:Ni/SiO2 samples can absorb visible light. Photocatalytic testing results showed that Ti0.93Ni0.06O2/SiO2 samples decompose 90% of MB under visible irradiation. Keywords: TiO2/SiO2, doping Ni, photocatalysis. 1. Introduction Titanium dioxide (TiO2) is the most popular substance used for photocatalytic degradation of organic compounds. This semiconductor has the advantage of being efficient, biologically and chemically inert, inexpensive, resistant to photo corrosion and chemical corrosion, nontoxic, highly photoactive and recyclable [1-3]. However, the application of TiO2 powder in water treatment is limited due to separation of TiO2 from a liquid [4]. The most common technique used is immobilization of nano-TiO2 onto a support [5, 6]. Of the various supports, silica gel is the most promising support because of its excellent characteristics in light transmission and adsorption of pollutants [7]. However, TiO2 is limited due to its large band gap of about 3.2 eV for anatase which makes it active only under ultraviolet radiation. Many efforts have been made to utilize visible light for TiO2 material by doping with transitional metals like Cr, Mn, Fe, Co, Ni, Cu, Zn and Zr, or nonmetal element like S, N and F [1, 8], or by using dye sensitization [9]. Nickel is one of the most popular transition elements used to modify the TiO2 surface. The effects of Ni2+ on the photocatalytic properties of TiO2 have been investigated by several authors [10]. Ni2+ has easily Received July 19, 2016. Accepted September 4, 2016. Contact Nguyen Manh Nghia, e-mail address: nghianm@hnue.edu.vn 151 Phung Thi Len, Nguyen Manh Nghia, Nguyen Cao Khang, Duong Quoc Van and Nguyen Thi Hue gone into TiO2 lattice by substitution of Ti4+ and created an impurity energy level. These impurity energy levels lead to a visible light response for the TiO2 photocatalyst [4]. The aim of this work is to study the structure, surface morphology, chemical composition, absorption spectra and porosity of Ti1−xNixO2/SiO2 prepared via the sol-gel method. In addition, the MB degradation process is also discussed. 2. Content 2.1. Experiment Ti1−xNixO2 nanoparticles (x = 0.00; 0.01; 0.03; 0.06; 0.09) were synthesized via the sol-gel process. The sol solution was prepared using hydrolysis and condensation of Ti(O-iC3H7)4 alkoxide (TTIP). Ti1−xNixO2 was obtained using sol consisting of TTIP, Ni(NO3)2, 6H2O, aetylacetone and ethanol with a molar ratio of x:1:1-x:34. Silica gel with a diameter of about 1.7 - 4.0 mm provided by Fuji Silysia Chemical Ltd (Japan) was soaked in the sol solution for 60 minutes. A Ti1−xNixO2/SiO2 sample was synthesized by drying sol at 105 ◦C and annealing at 500 ◦C for 3 hours. The crystallization of Ti1−xNixO2/SiO2 was investigated using a X-ray diffractometer (D8 Advance Bruker) with CuKα radiation. Their surface morphology and element analysis were characterized using scanning electron microscopy (SEM JSM 6010LA) with X-ray microanalysis. A spectrophotometer (Jasco V-670) was used to investigate the absorption properties of the material. Porosity was determined using a nitrogen adsorption/desorption isotherm at 77 K and the static volume technique and the average pore width was calculated using the BJH desorption method with a 3Flex Surface Characterization Analyzer (Micromeritics). The photocatalytic performance of the material was evaluated by determining the degradation reactions of methylene blue (MB) under visible irradiation. The prepared 200 mL of MB was taken in a beaker and passed through the photocatalytic system. A 2 g sample of Ti1−xNixO2/SiO2 was placed in the glass tube which was 1 cm in diameter and 15 cm long. The flow rate was of 15 mL min −1. The concentration of MB was confirmed using UV-vis Shimadu 2450 analysis. 2.2. Result and discussion The XRD patterns recorded for Ti1−xNixO2/SiO2 with different doping contents (x = 0%, 1%, 3%, 6%, 9%) are given in Figure 1. For all samples, there were only typical diffraction peaks of anatase TiO2 (JCPDS Cards No. 21-1272) located at 25.3, 37.8, 48.2, 54.0 and 62.8 corresponding to planes , , , and , respectively. By using the Scherrer equation, the sample crystallite sizes were estimated from the broadening peak as shown in Table I. It was shown that the crystal diameter decreased from 6.8 nm to 5.8 nm as the Ni doping concentration increased. All samples had the same tetragonal unit cell with a = b ≈ 3.8 A˚ and c ≈ 9.5 A˚. We propose that the substitution of host atoms (Ti4+, 0.68 A˚) by guest atoms (Ni2+, 0.71 A˚) with similar radius does not drastically change lattice parameters. 152 Enhanced photocatalytic efficiency of TiO2 with doped Ni-immobilized on silica gel Figure 1. XRD patterns of Ti1−xNixO2/SiO2 samples (x = 0%, 1%, 3%, 6%, 9%) Figure 2. EDX spectra of Ti1−xNixO2/SiO2 samples (x = 0.01; x = 0.09) 153 Phung Thi Len, Nguyen Manh Nghia, Nguyen Cao Khang, Duong Quoc Van and Nguyen Thi Hue The results in Figure 2 show that both of the sample surfaces were quite uniform in the 20 mum range. It can be observed that Si, Ti and O were present on the surface of the samples. A Ni peak only appeared in the Ti0.91Ni0.09/SiO2 EDX spectra but it did not exist in Ti0.99Ni0.01O2/SiO2 spectra. This could be because the amount of Ni in the Ti0.99Ni0.01O2/SiO2 was too small to be detected when using the EDX method. Compared to the dense and the homogenous of Si, Ti, O and Ni, it is possible to have well dispersed Ti1−xNixO2 on silica gel surface. It is known that the absorption of light influences photocatalytic activity significantly. Figure 3 illustrates the light absorption properties of Ti1−xNixO2/SiO2. The optical band gap values were obtained by extrapolating the linear portion of the plots of (αhν)2 versus hν to α = 0. The undoped TiO2 sample shows an absorption edge at around 390 nm (3.3 eV). The Ti1−xNixO2/SiO2 samples (x = 0.00, 0.01, 0.06, 0.09) have absorption edges in the wavelength range 400 - 460 nm (visible region). The band gap energies of various photo catalysts are listed in Table I. In addition, the absorption spectra of the Ni-doped TiO2 samples had a red-shift that is larger than that of un-doped TiO2, indicating that Ni-doping could be a promising approach for increasing catalytic activity. Figure 3. Absorption spectra of Ti1−xNixO2/SiO2 samples (x = 0.0, 0.01, 0.03, 0.06, 0.09) Table 1. Properties of Ni - TiO2/SiO2 with different concentrations of Ni (d - Crystalline diameter, S - Surface area, V - Micropore volume, P - Pore Size) Abbreviation d (nm) a = b (A˚) c (A˚) Eg (eV) λ(nm) S (m2/g) V (cm3/g) P (A˚) TiO2/SiO2 6.8 3.79 9.49 3.3 390 163 0.669 163 Ti0.99Ni0.01O2/SiO2 6.6 3.79 9.50 3.27 396 156 0.657 167 Ti0.97Ni0.03O2/SiO2 5.8 3.79 9.43 3.24 411 188 0.778 165 Ti0.94Ni0.06O2/SiO2 5.6 3.79 9.47 3.19 430 156 0.657 168 Ti0.91Ni0.09O2/SiO2 5.8 3.79 9.50 3.10 445 156 0.648 166 154 Enhanced photocatalytic efficiency of TiO2 with doped Ni-immobilized on silica gel Table I indicates the variation in porosity of the silica gel before and after being modified with TiO2 and TiO2:Ni materials. It was found that the silica gel grains coated with both intrinsic and Ni-doped TiO2 decreased the BET surface area of this porous material. The BET surface area of TiO2/SiO2 was down from 163 m2/g to 156 m2/g after coating with TiO2:Ni. Meanwhile, the average pore width of the samples remained almost the same. We assessed the catalytic potential by carrying out a decomposition experiment of MB in solution. Figure 4 presents the degradation of MB at different time periods. The TiO2/SiO2 sample shows the highest adsorption of MB, and this is due to its large surface area. It can be seen that Ni-doped TiO2 shows greater activity for degradation of MB in an aqueous solution compared to pure TiO2. Because Ni2+ plays an important role in trapping electrons and in charge separation, photocatalytic activity is comparatively good. The Ti0.93Ni0.06O2/SiO2 sample showed the best performance in degradation MB. The performance of the Ti0.91Ni0.09O2/SiO2 sample was lower than that of Ti0.93Ni0.06O2/SiO2 perhaps due to the lighter resistance of the higher Ni content. Figure 4. Results of degrading MB of Ti1−xNixO2/SiO2 samples in dark conditions (a) and under visible irradiation (b) (x = 0.0, 0.01, 0.03, 0.06, 0.09) 3. Conclusion TiO2 doped Ni, immobile on silicagel nanoparticles, was successfully prepared using the sol-gel method. Crystal size decreases from 6.8 nm to 5.8 nm and the band gap energy falls from 3.3 eV to 3.1 eV as the doping concentration increases. Mapping EDX spectra shows that a Ni peak appears only with the 9% Ni-doped sample and there is no Ni peak in the 1% Ni-doped sample. Because doping Ni on TiO2 shifted the absorption edges of the UV-vis spectrum to the visible light region, the as-synthesized Ni-doped TiO2 nanoparticles show a better photo degradation rate of reactive MB under visible irradiation. The best performance in MB degradation was 90% using a Ti0.93Ni0.06O2/SiO2 sample. 155 Phung Thi Len, Nguyen Manh Nghia, Nguyen Cao Khang, Duong Quoc Van and Nguyen Thi Hue REFERENCES [1] Akpan, U.G. and H. Hameed, 2010. The advancements in sol–ge l method of doped-TiO2 photocatalysts. Applied Catalysis A: General, 375, pp.1-11. [2] U.I.Gay and A.Abdullah, 2008. 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