It is well known that rare-earth elements such as erbium, terbium, europium, cerium, and ytterbium are the most important activator centers of luminescence   , 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      . 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  . Europium is the most studied lanthanide ion because of its ability to be incorporated into different host. The Eu3+ ions show a red emission centered at around 613 nm coming from the 5D0→7F2 transitions giving one of the three fundamental colors (red, blue, and green)  . Concerning host materials, special attention is put on metal oxides due to their high physical and chemical stability and easy fabrication. The Al2O3 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    .
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 (SiO2) is a cheap material which can be obtained easily by soft chemical methods, i.e. sol-gel   . 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     . 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  . Thus, combining the optical properties of Al2O3:Eu3+ (red emission) and silica (blue emission) it is expected to have a different color emission from their raw emission.
In this work, Al2O3:Eu3+ 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 5D0→7F2 transition of Eu3+ ions and the wide blue band of silica matrix, Ired/Iblue. By changing this ratio, red, magenta, near white and blue light emission is obtained when exciting at 270, 290, 310 and 340 nm.
2.1. Synthesis of SiO2 Sol
The precursors used to synthesize the silica matrix by the sol-gel method were tetraethyl orthosilicate ((Si(OC2H5)4, 98%, TEOS), ethanol (CH3CH2OH, 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  . Later, a solution of distilled water and hydrochloric acid was prepared and kept stirring for 1 h. Finally, this last solution was added to the ethanol/TEOS solution and kept stirring for 21 h.
2.2. Synthesis of Al2O3:Eu3+ NPs@SiO2 Composites
The Al2O3:Eu3+ powders were weighed and added to the silica solution previously synthesized in such a way that the following weight ratios for Al2O3:Eu3+/SiO2 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 SiO2 sol to SiO2 powders with a white appearance. With the aim to determinate the complete reaction transformation of SiO2 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.
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%.
3. Results and Discussion
3.1. Chemical and Morphological Analyses
Figure 1 shows the FTIR spectra of the Al2O3:Eu3+@SiO2 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)  , 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(OC2H5)); both are products of the hydrolysis of TEOS  , at the same time, a condensation reaction between SiOH and the ethoxy groups (SiOC2H5) is carried out, and siloxane groups (Si-O-Si) is formed, which is reflected at 1080 cm−1 in the symmetry stretching of SiO4 tetrahedra, at 798 and 450 cm−1  . As expected, as temperature increases, the bands related to
Figure 1. FTIR spectra of sample 5Al2O3:Eu3+@95% SiO2 annealed at a temperature range of 60˚C to 700˚C.
(SiOH) and Si(OC2H5) 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(OC2H5)) to silanol groups and from silanol to siloxane groups.
3.2. Photoluminescence Characterization of Al2O3:Eu3+@SiO2 Composites
The luminescent properties of Al2O3:Eu3+@SiO2 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 5D0→7F2, and the emissions at 577, 592 and 650 nm correspond to the transitions 5D0→7F0, 5D0→7F1 and 5D0→7F3 respectively  . From the spectra, it is observed that the bare Al2O3:Eu3+ 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 Al2O3 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
Figure 2. Emission spectra of Al2O3:Eu3+@SiO2 as function of SiO2 concentration.
Figure 3. Emission spectra of Al2O3:Eu3+@SiO2 powder exited at different wavelengths.
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 Al2O3:Eu3+, 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 5D0→7FJ (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 340 nm.
Figure 4. CIE diagram of Al2O3:Eu3+@SiO2 by varying wavelength excitation.
Table 1. Optical properties of the composites excited at varios wavelengths.
Figure 4 shows the CIE coordinates of sample 5:95 exited at 270, 310 and 340 nm, in addition, the pure Al2O3:Eu3+ CIE coordinates are put as a comparison. For pure Al2O3:Eu3+ powders, a red emission is obtained because of the main 5D0→7F2 transitions from Eu3+. By exciting the composite powders at 270 nm, the intensity ratio, Iblue/Ired = 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 Iblue/Ired = 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 Al2O3:Eu3+ sample, the transition 5D0→7F2 gives a strong band at 613 nm (red emission) while increasing the wavelength excitation to 310 and 340 the emission intensity decreases.
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 method. The emission intensity for all samples synthesized was kept similar, proving it is possible to use very high concentration of inert SiO2 matrix without loss the optical properties of Al2O3:Eu3+. 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, Iblue/Ired = 0.4. Finally, by FTIR spectroscopy, it is determined that the complete transformation of TEOS molecules to amorphous SiO2 is reached at 500˚C.
This work was supported by CONACyT project 254280. Thanks M. Pacheco Ramos and Dr. Veronica Melo for their help in structural characterization PL and FTIR techniques.
Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.