Zeinhom M. El-Bahy

Enhancement of titania by doping rare earth for photodegradation of organic dye (Direct Blue)

Lanthanide ions (La(3+), Nd(3+), Sm(3+), Eu(3+), Gd(3+), and Yb(3+))/doped TiO2 nanoparticles were successfully synthesized by sol-gel method. Their photocatalytic activities were evaluated using Direct Blue dye (DB53) as a decomposition objective. The structural features of TiO2 and lanthanide ions/TiO2 were investigated by XRD, SEM, UV-diffuse reflectance, and nitrogen adsorption measurements. Our findings indicated that XRD data characteristic anatase phase reflections and also XRD analysis showed that lanthanides phase was not observed on Lanthanide ions/TiO2. The results indicated that Gd(3+)/TiO2 has the lowest bandgap and particle size and also the highest surface area and pore volume (V(p)) as well. Lanthanide ions can enhance the photocatalytic activity of TiO2 to some extent as compared with pure TiO2 and it was found that Gd(3+)/TiO2 is the most effective photocatalyst. The photocatalytic tests indicate that at the optimum conditions; illumination time 40 min, pH approximately 4, 0.3g/L photocatalyst loading and 100 ppm DB53; the dye removal efficiency was 100%. Details of the synthesis procedure and results of the characterization studies of the produced lanthanide ions/TiO2 are presented in this paper.

ENHANCEMENT OF THE PHOTOCATALYTIC PROPERTIES OF TITANIA NANOPARTICLES BY DOPING LANTHANIDE IONS

Lanthanide ions (La, Nd, Sm, Eu, Gd, and Yb)doped TiO2 nanoparticles were successfully synthesized by sol-gel method to enhance the photocatalytic activity of the developed materials. The structural features of TiO2 and lanthanide ions–doped TiO2 fired at 550oC were investigated by XRD, UV-diffuse reflectance, and nitrogen adsorption measurements. The effect of lanthanide ions-doped TiO2 on the photoactivity was evaluated by the degradation of direct blue dye (DB 53) as a probe reaction. Our findings indicated that XRD data verified the formation of typical characteristic anatase reflections without any separate dopant-related peaks in all the prepared lanthanide ion-doped TiO2 nanoparticles. The particle size of lanthanide ions-doped TiO2 nanoparticles was smaller than pure TiO2 indicating the improvement of its surface morphology. The results indicated that Gd-TiO2 has the lowest bandgap and particle size and the highest surface area and pore volume (Vp) as well. The photocatalytic behavior of lanthanide ions-doped TiO2 was tested for oxidation of DB 53 in the presence of UV light at λ = 365 nm. It was found that Gd-TiO2 is the most effective catalyst in the photocatalytic activity studies. That might be due to its special characteristics of particle size, surface texture and bandgap properties. Our results should provide a significant contribution to the understanding of the chemistry of lanthanide iondoped TiO2 systems. Details of the synthesis procedure and results of the characterization studies of the produced lanthanide ions-TiO2 are presented in this paper. Introduction During the recent decades, the photocatalytic application using semiconductors has been received much attention to solve the environmental problems [1-4]. TiO2 has turned out to be the semiconductor with the highest photocatalytic activity, being non-toxic, stable in aqueous solution and relatively inexpensive [5]. The photocatalytic property of TiO2 is due to its wide bandgap and long lifetime of photo generated holes and electrons. The high degree of recombination of the photo generated electrons and holes are a major limiting factor controlling its ZEINHOM M. EL-BAHY., et al., 64 photocatalytic efficiency and impeding the practical application of these techniques in the degradation of contaminants in water and air. Thus, a major challenge in heterogeneous photocatalysis is the need to increase the charge separation efficiency of the photocatalysts [6]. Although TiO2 is the most widely used photocatalyst, attention has been paid to metal ions-doped titania and testing their efficiency to replace pure TiO2 and enhance the photocatalytic conversions. In order to decrease the bandgap of parent titania photocatalyst (Eg = 3.2 ev), slow down the recombination rate of the e/h pairs and enhance interfacial charge-transfer efficiency, the properties of TiO2 have been modified by selective surface treatments such as surface chelation, surface derivatization, platinization, and by selective metal ions doping TiO2 [7]. Coupled semiconductor photocatalysts exhibited a very high photocatalytic activity for both gas and liquid phase reactions. Researchers had much interest in coupling two semiconductor particles with different bandgap widths such as TiO2-CdS, TiO2WO3, TiO2-SnO2 [8], TiO2 –MoO3 [9] TiO2-SiO2 [10] and TiO2–Fe2O3 [11,12]. Lanthanide ions are known for their ability to form complexes with various Lewis bases e.g. acids, amines, aldehydes, alcohols, thiols, .....etc) in the interaction of these functional groups with the ƒ-orbitals of the lanthanides. Particularly, REmodified TiO2 nanoparticles become of current importance for maximizing the efficiency of photocatalytic reactions, increase the stability of anatase phase and prevent the segregation of TiO2 [13-16]. Thus, incorporation of lanthanide ions into a TiO2 matrix could provide a means to concentrate on the organic pollutant at the semiconductor surface and consequently enhance the photoactivity of titania [1619]. It was reported in literature that the optimum level of RE-doping is 1-2% to hinder the crystal growth of titania during calcination [15]. Although doping of lanthanide ions into TiO2 attracted some attentions [20-25], such works are little so far. Investigating of the effect of the addition of lanthanide ions (1 wt%) doped TiO2 is a crucial task for collecting more information about lanthanide ions-TiO2 systems and its impact on the catalytic activity for the photodegradation of DB 53. The primary deriving force in this work is to prepare lanthanide ions-doped TiO2 by solgel method and study its impact on the structure, bandgap, surface texture of TiO2 nanoparticles. Moreover, the removal of one of the organic pollutants (namely Direct Blue 53 dye) was investigated as a pattern of organic pollutant to evaluate the relative photocatalytic activity of the prepared photocatalyst samples. ENHANCEMENT OF THE PHOTOCATALYTIC PROPERTIES ....... 65 Experimental Materials: Titanium isopropoxide, Ytterbium (III) nitrate tetrahydrate, Neodymium nitrate hexahydrate, Samarium nitrate hexahydrate, Europium acetate tetrahydrate, Gadolinium (III) oxide and Lanthanum nitrate hexahydrate were used as precursors in the sol-gel preparations. Distilled water was used and all other chemicals were analytical grade. Direct Blue 53 (DB53), (molecular formula = C34H24N6Na4O14S4, molecular weight = 960.81), Scheme 1, was used. The parent TiO2 and lanthanide ions-doped TiO2 nanoparticles were prepared by sol-gel technique. The sol corresponds to the overall molar ratio of Ti(C4H9O4) : C2H5OH : H2O : HNO3 = 1 : 20: 4: 0.001. Ti(C4H9O4) was first dissolved in ethanol medium to form titania sol; the lanthanide salts were dissolved into stoichiometric amount of water and nitric acid and then added drop wise into the titania sol through stirring for 30 minutes at room temperature. The prepared sol was left to stand for the formation of gel and dried at 120oC. Finally, the obtained gel was calcined at 550°C for 5 hrs to keep the lanthanide ions-TiO2 in the anatase phase that has high photoactive sites. The atomic ratio of Ti: lanthanide ions was kept as 99: 1 for all lanthanide ions -doped nanoparticles which still maintained a single anatase modification even at high temperature ≤ 900°C [14].