Synthesis and Structural Properties of Eu3+:TiO2 Nanoparticles

INTRODUCTION

Recently, several types of published research have been emphasized the preparation and usage of Titanium dioxide (TiO2) nanoparticles in many applications such as hydrogen (H2) generation, solar cells, photocatalyst, and gas sensors [1]. This is due to its low cost, high stability against photo-induced corrosion and chemicals [2]. It is non-toxic, has strong oxidizing power, a wide energy bandgap of 3.2 eV and maximum light scattering with virtually no absorption [3, 4]. The three different crystalline forms of TiO2 are rutile (tetragonal), anatase (tetragonal), and brookite (orthorhombic) [5]. The formation of each crystalline phase depends on the nature of starting material, sol composition, deposition method and calcination temperatures [6, 7]. TiO2 bulk can be transformed from an amorphous phase into crystalline by calcination [8]. Several methods have been employed to synthesis TiO2, such as microwave [9-11], hydrothermal [12] polyol mediate solvothermal [13] and ultrasonic methods [14]. Among these methods, the sol-gel is widely used due to its simplicity, low cost, high flexibility, high yield and homogeneous product [15]. Sol-gel procedures consist of four steps: the main processes are hydrolysis condensation, growth and gel formation. In the initial steps, catalysts such as acid or base are used to speed up the reaction process [16]. Rare earth (RE) ions doped metal oxides (MO) had received great attention because they are applicable in several fields such as optical detectors and fibers, solid-state lasers, light-emitting diodes, and screens. Owing to the important properties of REs such they include several excited energy levels, promising optical properties such as long life-time and have absorption and emission spectra with sharp peaks shifted from ultraviolet to the infrared region [17].

Usually, TiO2 nanoparticles are used as an electrode and an active layer in solar cells due to their transparency and conductivity. The photoactivity of TiO2 depends on several factors, including crystal modification, particle size, preparative route, surface area, porosity, thermal history, foreign ions and so on. Doping TiO2 with RE metals increases the photoactivity. Europium (Eu3+) ions represent the most promising and applicable ions compared to other RE. This is due to the fact that the Eu3+ ions have narrow fluorescence emission with long lifetimes [18].

Recently, doping TiO2 nanoparticles with RE ions especially Eu3+ have been used in several prospective applications such as biomedical, sensors, photocatalytic, electrochemical, solar cells and so on [19]. Therefore, TiO2 doped Eu3+ attracted a few researcher’s considerations. Whereas, the preparation methods were complicated and required high temperature [20].

Our proposed preparation method solves this problem by developing a new method for preparation and doping TiO2 with Eu3+ with various doping levels without the requirement of complicated instruments or high temperatures. To the best of our knowledge, this is the first report which addresses the doping of TiO2 with Eu3+ at room temperature. The novelty of this work is to use a simple, green, inexpensive preparation method at room temperature which is considered an energy-saving method. The structural and morphological properties were investigated for the synthesized samples via XRD and SEM.

MATERIALS AND METHODS

The TiO2 solution was synthesized using Titanium (IV) isopropoxide (TTIP) purity (97%), as a precursor material supplied by Aldrich Company. Ethanol (EtOH) purity (99.9%), as a solvent was purchased from GCC. Hydrochloric Acid (HCl) of purity (37 %) was provided by BDH as a catalyst. The composition of the materials consisted of HCl:TTIP:H2O: EtOH with a molar ratio of 0.1:1:1:10. HCl, H2O and EtOH were added to the TTIP under continuous stirring. By using Europium (III) nitrate hydrate purity (99.9%) from Aldrich Company the doping with Europium Eu3+ is investigated. The doping rate of Eu3+ into TiO2 were about 0.0, 0.7, 1.1, 1.9 and 2.3% molar ratio. The prepared solution was stirred vigorously for about 1-2 hours until a clear transparent solution was achieved. Following to hydrolysis and condensation process, the prepared solutions were aged for 24 hour. These preparation steps were totally done at room temperature to obtain a colorless powder. The samples were annealed in the furnace at 500°C for 2 hours. Finally, a yellowish powder was collected due to the doping of Eu inside TiO2.

XRD patterns were obtained using (X’PERT PRO, PHILIPS) by CuKα radiation at 40 kV and 30 mA. The crystallite size of TiO2 and TiO2 doped Eu3+ powder were estimated by applying the Scherrer equation [21].

Ds = K λ / β Cos θ (1)

Where K is a constant (shape factor) and the value 0.94 was used for the calculation, λ is the X-ray wavelength which is 0.15405 nm, β is the full-width at half maximum of the diffraction line and θ is the half diffraction angle of the centroid of the peak in degree. To calculate the lattice parameters (a and c) and the unit cell volume (V) of the samples, the following relations (belongs to the tetragonal crystal system), were applied respectively [22, 23].

=+ (2)

Where, d is the inter-planar spacing and (h,k,l ) are Miller indices.

(3)

The surface morphology of the prepared samples was monitored by FESEM (MIRA3 TESCAN).

RESULTS AND DISCUSSION

X-Ray Diffraction analysis

Structural properties were investigated using the X-Ray diffractometer. The XRD patterns of the un-doped and Eu doped TiO2 samples prepared by the sol-gel method after annealing at 500 ̊C are presented in Fig. 1. It is clear that all samples have anatase phase TiO2, which corresponds to the standard diffraction data (ICSD 01-073-17٦٤). This is due to the fact that the TiO2 grows in three crystal structures: “anatase” and “brookite” at low temperature while “rutile” crystallizes above 600 ̊C [8]. The presence of a peak corresponding to Eu3+ ions is an indicator of TiO2 doping with Eu3+ ions. The absence of peaks corresponds to other materials, is evidence of the high purity prepared structure. It can be observed from Fig. 1 that the diffraction peaks of the samples were slightly shifted to a lower angle due to the presence of Eu3+ ions into TiO2. These results are matched well with previous work [24].

The peak intensity decreased dramatically due to the addition of Eu3+. The crystal size of Eu3+: TiO2 = 0.0, 0.7, 1.1, 1.9 and 2.3% were estimated as 20.7, 18.4, 13.79, 20.70 and 33.13 nm, respectively. It can be seen that the crystal size decreases with increasing the doping ratio of Eu3+ as shown in Fig. 2. The crystal size and full width at half maximum β at Miller indices (101) for the samples used in this study are explained in (Table 1). The reduction of anatase particle crystallite coherent sizes after doping with RE ions has been already reported in the literature [25,26] and can be attributed to the presence of RE-O-Ti bonds that inhibit the growth of crystals. The presence of ions in octahedral coordination with a larger ionic radius than the original (Eu3+ = 0.95Å > Ti4+ =0.68 Å) [25], induces the expansion of the unit cell volume and causes lattice disorder.

The selected miller indices (004) and (200) exhibited the following lattice parameters: a=3.7760 A° and c= 9.4860 A°, and α=β=γ=90°. It can be seen from (Table 1) that all the samples have good agreement with the TiO2 stander data. As its listed in Table 1 the crystal size varied from 20 to 33 nm which is in agreement with previous work [27].

Several studies reported that by doping TiO2 nanoparticles with Eu, the crystal size decreased which is consistent with our study [3]. The effect of doping Eu3+ ions concentration on TiO2 nanoparticles was also studied. It was found that the crystal size reduced with increasing Eu content [18, 26].

Moreover, we did not observe any peaks of the europium complex and no shifting of the anatase peaks was observed after grafting Eu3+ steroids. That is revealing the Eu ions do not disperse in the amorphous Titania region instead of occupying Ti Locations within anatase nanocrystals.

FESEM results

Field Emission Scanning Electron Microscopy (FESEM) was used to probe the changes in the morphology and particle size of the TiO2 powder. Fig. ٣ (a, b, c, d and e) shows the FESEM images of TiO2 powder doped with Eu = (0.0, 0.7, 1.1, 1.9 and 2.3 %), respectively. It was found that the average particle size of TiO2 before Eu doping is (30 ± 5 nm). It can be observed that the average particle size increases by increasing the doping Eu content. Moreover, the amplified images of the samples illustrate the growth and agglomerations of the nanoparticles due to doping with Eu. It was reported that the incorporation of europium into titanium dioxide by the sol-gel technique strongly affects the structural properties of TiO2, where the particles are joined together to form irregular shapes with size in the range from 3 to 136 µm [26].

Optical characterization of TiO2:Eu3+

From the observation of the transmittance spectrum of titanium doped with Eu3+ Fig. 4, there is a behavior similar to that of pure titanium, and there is a clear concavity at the wavelength 394, as well as 464 nm.

When calculating the energy gap for pure titanium, the gap value was (3.48 eV), as shown in Fig. 5 which is the highest possible, and the reason is due to the discrepancy in the edge of the optical range due to localized deformation states of the EU+3, and this is consistent with the researcher’s findings [27].

After that, the energy gap was calculated as shown in Fig. 6, and we note that there is a decrease in the value of the energy gap at some concentrations of Eu3+, especially when concentration (1.9). Where the lowest value was (2.18 eV), and the highest value was 2.518 eV at concentration of (1.1). By comparing these results with previous work where the pure TiO2 is around 3.1 that reduced to 2.7 eV by increasing doping level with Eu [28].

As mentioned before the preparation method plays an important rule to control the properties of the prepared materials. As example in this manuscript the prepared materials exhibited promising properties which is divers from those obtained in previous work due to the preparation method which is totally different from that which have been used in the mentioned work [18].

CONCLUSION

High purity Eu3+ doped TiO2 nanoparticles with doping rates of 0.0, 0.7, 1.1, 1.9 and 2.3% were successfully prepared by the sol-gel technique. The results showed that significant effects on the structural properties were achieved by doping Eu3+ into TiO2. XRD patterns revealed that the prepared samples had good crystallinity and shown that the prepared Eu3+ doped TiO2 samples have a polycrystalline structure in the anatase phase. As well it proved the decrement in crystallite size by increasing the Eu3+content. Consequently, the average particle size increase when the doping ratio increased as revealed by FESEM.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interests regarding the publication of this manuscript.