Red , White and Blue Light Emission from Europium Doped Al 2 O 3 Confined into a Silica Matrix

The main drawbacks of fluorescent activated lanthanide luminescent materials in practical optoelectronic applications are the high cost of raw material and the limitation of a specific color depending on the lanthanide ion. In this work, the synthesis of red, white and blue light emitting composite powders of Al2O3:Eu@SiO2 were successfully synthesized by the hydrolysis and condensation reactions of TEOS as the raw precursor of silica matrix on the Al2O3:Eu surface. With the aim to change the CIE coordinates different weight ratios of Al2O3:Eu/SiO2 powders from 13Al/87Si to 5Al/95Si were prepared. Besides, various excitation wavelength from 280 to 340 nm was used in order to change the ratio intensity; Ired/Iblue, between the red emission coming from the D0→F2 transition of Eu ions and the blue band of silica matrix. Chemical evolution of the hydrolysis and condensation reaction of TEOS were conducted by FTIR spectroscopy. The results indicate that from 500 ̊C a complete TEOS to SiO2 transformation is carried out.


Introduction
It is well known that rare-earth elements such as erbium, terbium, europium, cerium, and ytterbium are the most important activator centers of luminescence [1] [2], and this has given great impetus to the study of a variety of luminescent materials using a host crystal lattice: oxides, sulfides, selenides, and titanates doped with rare-earth ions for a broad application, such as in lasers, ceramics, sensors, lighting, displaying, and heat-resistant materials [3] [4] [5] [6] [7].The most important reason for using rare-earth ions as activator centers in matrices is undoubtedly their narrow band of emission and absorption, which is due to 4f→4f electronic transitions [8].Europium is the most studied lanthanide ion because of its ability to be incorporated into different host.The Eu 3+ ions show a red emission centered at around 613 nm coming from the 5 D 0 → 7 F 2 transitions giving one of the three fundamental colors (red, blue, and green) [9].Concerning host materials, special attention is put on metal oxides due to their high physical and chemical stability and easy fabrication.The Al 2 O 3 crystal is a material with a significant technological importance because of the large optical transparency from ultraviolet to near-infrared, and because of its excellent mechanical properties and good chemical stability [10] [11] [12].
However, despite their excellent optical properties of lanthanide ions, the main drawback of these materials in practical optoelectronic applications is the high cost of salts lanthanides.In this regard, the amorphous silica (SiO 2 ) is a cheap material which can be obtained easily by soft chemical methods, i.e. sol-gel [13] [14].Many recent reports indicate that silica can be used to confine luminescent material such as lanthanide doped oxide materials in order to protect the luminescent component from the environment or as functional silica nanoparticles for biological application or even increase the emission intensity [15] [16] [17] [18].Moreover, for obtaining white light emission is common to mix the three fundamental colors (blue, green and red), which results in using different lanthanide ion making the final cost increased too.Interestingly, the cheaper amorphous silica matrix when excited with UV light shows a broad blue emission centered at 440 nm [19].Thus, combining the optical properties of Al 2 O 3 :Eu 3+ (red emission) and silica (blue emission) it is expected to have a different color emission from their raw emission.
In this work, Al 2 O 3 :Eu 3+ nanoparticles were successfully embedded within a silica matrix in order to reduce the high cost of raw precursor and obtain a near white color by exciting the composite powders at different wavelengths from 270 to 340 nm with the aim to change the ratio emission intensity between the red emission coming from the 5 D 0 → 7 F 2 transition of Eu 3+ ions and the wide blue band of silica matrix, I red /I blue .By changing this ratio, red, magenta, near white and blue light emission is obtained when exciting at 270, 290, 310 and 340 nm.

Synthesis of SiO2 Sol
The precursors used to synthesize the silica matrix by the sol-gel method were tetraethyl orthosilicate ((Si(OC 2 H 5 ) 4 , 98%, TEOS), ethanol (CH 3 CH 2 OH, 99.9% Fermont), hydrochloric acid (HCl 37.1 %, Fermont), and distilled water.For the synthesis, 22.4 mmol of TEOS was dissolved in ethanol; a solution of ethanol/TEOS with a molar ratio of 4:1 was prepared according to Klein's diagram Open Journal of Applied Sciences kept stirring for 1 h.Finally, this last solution was added to the ethanol/TEOS solution and kept stirring for 21 h.

Synthesis of Al2O3:Eu 3+ NPs@SiO2 Composites
The Al 2 O 3 :Eu 3+ powders were weighed and added to the silica solution previously synthesized in such a way that the following weight ratios for Al 2 O 3 :Eu 3+ /SiO 2 were obtained: 13/87, 11/89, 9/91, 7/93 and 5/95.Subsequently, the mixture was placed under magnetically agitation at 60˚C until a partial hydrolysis and condensation reaction of the SiO 2 sol to SiO 2 powders with a white appearance.
With the aim to determinate the complete reaction transformation of SiO 2 and eliminate organic matter which is present in the xolegel silica powders, various annealing treatments at 200˚C, 300˚C, 500˚C and 700˚C for 1 h were conducted.

Characterization
In order to evaluate the chemical evolution, Fourier Transform Infrared spectroscopy (FTIR) measurements were performed on a Perkin Elmer 2000 spectrometer, using the KBr pellet method.For all measurements, the spectra were recorded from 4000 to 400 cm −1 .The emission and excitation spectra of the samples were recorded on a F-7000 fluorescence spectrophotometer, using a 450 W xenon lamp.The CIE coordinates for each sample were computed from the respective light emission spectra using a MATLAB code The final uncertainty in the emission and excitation spectra was 16.8 a.u. of intensity (y axis) and 0.18 nm of wavelength (on the x axis),with a confidence level of 95%.

Chemical and Morphological Analyses
Figure 1 shows the FTIR spectra of the Al 2 O 3 :Eu 3+ @SiO 2 composite powders obtained by the sol-gel method and annealed from 60˚C to 700˚C for 1 h.Despite the different silica concentration of each of the synthesized sample, the same bond vibrations related to TEOS and their decomposition to silica powders can be found.The bands in the 3000 -3600 cm −1 range correspond to the stretching vibrations of the hydroxyl groups (υOH), due to the presence of water in the sample, and the band around 1634 cm −1 corresponds to the OH deformation vibrations (δOH) [20], the intensity of these bands is decreased as the temperature increases, which is related to the complete removal of chemically absorbed water.The absorption bands centered at 960 cm −1 are associated with silanol groups (SiOH), and the band centered at 585 cm −1 can be assigned to ethoxy groups (Si(OC 2 H 5 )); both are products of the hydrolysis of TEOS [13], at the same time, a condensation reaction between SiOH and the ethoxy groups (SiOC 2 H 5 ) is carried out, and siloxane groups (Si-O-Si) is formed, which is reflected at 1080 cm −1 in the symmetry stretching of SiO 4 tetrahedra, at 798 and Open Journal of Applied Sciences (SiOH) and Si(OC 2 H 5 ) at 960 and 585 cm −1 disappear in the spectra at 500 and 700˚C.This phenomenon can be attributed to the complete hydrolysis of ethoxy groups (Si(OC 2 H 5 )) to silanol groups and from silanol to siloxane groups.

Photoluminescence Characterization of Al2O3:Eu 3+ @SiO2 Composites
The luminescent properties of Al 2 O 3 :Eu 3+ @SiO 2 composite powders were evaluated.At first, the light emission properties of the composites were analyzed as a function of the weight quantity as depicted in Figure 2. The samples were excited at a wavelength of 280 nm.The main emission peak at 613 nm can be ascribed to the transition 5 D 0 → 7 F 2 , and the emissions at 577, 592 and 650 nm correspond to the transitions 5 D 0 → 7 F 0 , 5 D 0 → 7 F 1 and 5 D 0 → 7 F 3 respectively [22].From the spectra, it is observed that the bare Al 2 O 3 :Eu 3+ sample show the maximal emission intensity.When silica is incorporated to the alumina powders to form the composite powders, the emission decreases in half for all the samples: Al/Si equal to 13/87, 11/89, 9/91, 7/93 and 5/95.Thus, even with a higher concentration of 95 w% the emission intensity is kept.Besides, with the addition of silica, a broad band centered at 440 nm appears but the intensity emission is poor because the samples were excited at 280 nm.Besides, with the addition of silica, a broad band centered at 440nm appears but the intensity emission is poor because the adequate excitation for Al 2 O 3 is at 350nm.It is an excellent approach to reduce the cost and obtain a bigger quantity of luminescent material.
On the other hand, by varying the excitation wavelength from 270 to 340 nm, the intensity emission of the composite powders of the sample 5/95 is highly affected (Figure 3).For instance, when the sample is exited at 270 nm the intensity at 610 nm is the more intense, but as the wavelength increases to 290 to 310 and  340 nm, this transition is highly reduced.Nevertheless, as wavelength increases, the silica shows a broader and a higher emission intensity.Thus, it is possible that controlling the intensity ratio between the blue from silica and red from Al 2 O 3 :Eu 3+ , the emission color changes from red to magenta to white to blue as wavelength increases from 280 to 340 nm.
Figure 3 shows the emission intensity of sample 5:95 exited at 280, 310 and 340 nm.When the composite powders are excited at 270 nm, the europium transition 5 D 0 → 7 F J (where J = 0, 1, 2, 3) are more intense and a low intense broadband covering the blue region is observed.However, when the sample is excited at 310 nm the blue emission centered at 440 nm is increased but the red emission is reduced.The same tendency is shown when the sample is excited at Open Journal of Applied Sciences  Figure 4 shows the CIE coordinates of sample 5:95 exited at 270, 310 and 340 nm, in addition, the pure Al 2 O 3 :Eu 3+ CIE coordinates are put as a comparison.
For pure Al 2 O 3 :Eu 3+ powders, a red emission is obtained because of the main 5 D 0 → 7 F 2 transitions from Eu 3+ .By exciting the composite powders at 270 nm, the intensity ratio, I blue /I red = 0.4 and the emission is a white near color with CIE coordinates about x = 0.34, y = 0.27.The europium transition is more intense than the broad band centered at 440 nm covering the blue region.However, when the sample is excited at 310 and 340 nm the blue emission centered at 440 nm is highly increased, giving I blue /I red = 5.7 with an excitation of 340 nm.
The Table 1 shows the optical properties of the composite powders.As can be seen, different color emission can be obtained as a function of wavelength.For the bare Al 2 O 3 :Eu 3+ sample, the transition 5 D 0 → 7 F 2 gives a strong band at 613 nm (red emission) while increasing the wavelength excitation to 310 and 340 the emission intensity decreases.

Conclusion
Europium doped alumina powders embedded in a silica matrix with different weight ratios from 13Al/87Si to 5Al/95Si were successfully obtained by the sol-gel Open Journal of Applied Sciences method.The emission intensity for all samples synthesized was kept similar, proving it is possible to use very high concentration of inert SiO 2 matrix without loss the optical properties of Al 2 O 3 :Eu 3+ .The composite powders exhibited luminescent properties in the red, white and blue region of the visible spectra.In the case of near white emission, the computed CIE coordinates were (x = 0.38, y = 0.27) which was a function of the intensity ratio, I blue /I red = 0.4.Finally, by FTIR spectroscopy, it is determined that the complete transformation of TEOS molecules to amorphous SiO 2 is reached at 500˚C.

Table 1 .
Optical properties of the composites excited at varios wavelengths.