The effects of boric acid on crystal structure, nano-/microstructure and photoluminescence characteristics of rare earth-doped Y2O3/YBO3 composite compounds

In this investigation, (Y0.96Eu0.04)2O3 phosphor nanoparticles with an average size of 85 nm were synthesized through a facile solid-state method. It was found that small magnitudes of H3BO3 improve both growth and crystallinity of nanoparticles, significantly. Consuming specific amounts of boric acid not only results in dramatic changes in crystal structure, from cubic to hexagonal, but also influences size and morphology of particles. Noteworthy, further increase in the additive suppresses the crystal growth procedure. The luminescent properties of the synthesized phosphors were investigated by PL analyzer. It was found that the change in crystal structure leads to the huge differences in the wavelength and the photoluminescence characteristics. The main emission bands of Y2O3- and YBO3-based compounds are attributed to 5D0–7F2 electric dipole and 5D0–7F1 magnetic dipole transitions, respectively. In other words, it was concluded that sufficient quantities of H3BO3 cause a noticeable decrease in the main emission wavelength from 612 to 592 nm.

the synthesized phosphors were investigated by PL analyzer. It was found that the change in crystal structure leads to the huge differences in the wavelength and the photoluminescence characteristics. The main emission bands of Y 2 O 3 -and YBO 3 -based compounds are attributed to 5 D 0 -7 F 2 electric dipole and 5 D 0 -7 F 1 magnetic dipole transitions, respectively. In other words, it was concluded that sufficient quantities of H 3 BO 3 cause a noticeable decrease in the main emission wavelength from 612 to 592 nm.

Introduction
Recently, phosphor compounds have been a subject of interest and controversy, and among them Y 2 O 3 has an exceptional status. Yttrium oxide with a space group Ia3 (No. 206) and cubic crystal structure, density of about 5.01 g/cm 3 , refractive index more than 1.9, a wide band gap of 5.8 eV and melting point over 2400°C has been known as one of the most efficient phosphors in the area of optic materials. Yttria is widely used in many applications in various fields such as white LEDs [1], photovoltaic cells, up-conversion phosphors [2], lighting, sensors, and transparent ceramics [3], due to its excellent chemical stability. In addition, the literature surveys show that Yttria has been widely employed as host lattice for many lanthanide dopants like Eu and Tb ions [4]. The rare earth ions are characterized by semi-filled 4f shells. In these ions, 4f orbitals are located in inner layers shielded by filled 5 s and 5p orbitals. Correspondingly, referring to this shielding phenomenon, the effect of host lattice on optical transitions through 4f configuration is not very significant. Also, in configurational coordinate diagram (CCD) the levels of energy can be considered as parallel graph (DR = 0). Among the lanthanide ions, Eu 3? with 5 D 0 -7 F 2 transition has been used extensively as a source of red light at an approximate wavelength of 618 nm [5].
The studies about Y 2 O 3 reveal that different methods have been used to synthesize it, e.g., hydrothermal [6,7], sol-gel [8], spray pyrolysis [9], combustion [10,11], co-precipitation [12], micro-emulsion microwave [13] and electrospinning [14]. From the mentioned techniques, many researchers prefer to prepare phosphor materials through relatively simple solid-state method. But it should be noticed that this method has its own disadvantages, like improper crystallinity and calcinations at high temperatures. Therefore, to overcome these weak points many flux materials are used through solid-state process [15]. As a matter of fact, the melting point of flux compounds is less than the temperature of solid-state reaction, resulting in simplifying the reaction at lower temperatures. H 3 BO 3 with the melting point of 171°C is one of the most prominent flux materials, employed for synthesizing Y 2 O 3 /Gd 2 O 3 compounds [16].
Motivated by this background, the effect of different amounts of boric acid on microstructure, crystal structure and optical properties of synthesized phosphors was studied. Also, the influences of the magnitude of this additive on the properties of (Y 0.96 Eu 0.04 ) 2 O 3 phosphors were investigated. In this work, XRD, FESEM and PL analyses were employed to evaluate the crystal structure, size and morphology of synthesized powders and also their photoluminescence characteristics.

Experimental Preparation
To synthesize (Y 0.96 Eu 0.04 ) 2 O 3 phosphor, analytical grades of yttrium acetate Y(CH 3 COO) 3 .xH 2 O, europium oxide (Eu 2 O 3 ) were purchased from Aldrich Company with the highest purity (99.99%), used without any purification. In a typical procedure, 1 g of yttrium acetate together with 0.026 g of europium oxide was ground in an alumina crucible. Then, this mixture was calcined at 1100°C for 2 h in air, using a tube furnace. In this research, the considered productions nominated as A, B, C, D, E, F, G, H and I compounds were synthesized through using different amounts of H 3 BO 3 , added to the explained initial materials (see Table 1). The produced materials are hereafter. Accordingly, R shows the ratio of used boric acid to the required stoichiometric boric acid.
Results and discussion XRD analysis Figure 1a shows the XRD spectra of Y 2 O 3 /YBO 3 :Eu 3? phosphors (A-H compounds). The spectra of A-D compounds show that the main peaks of diffraction are attributed to (211), (222), (400), (440) and (622) crystallographic planes indicating that the synthesized phosphor has cubic structure (JCPDS No. 41-1105). For the sample A, the crystallite size of 45.6 nm was calculated employing the Scherrer formula, D = 0.9k/bcosh. In this equation, D is the average grain size, k is the wavelength of X-ray (0.154 nm), and b and h are the full-width at halfmaximum and diffraction angle of an observed peak, respectively. Accordingly, it is obvious that whenever R varies from 0 to 0.25, the structure of synthesized compound is very similar to that of Y 2 O 3 , with a cubic crystal structure. Among the mentioned crystallographic planes, the intensity of (222) peak has the highest value. In addition, up to a certain point, the addition of boric acid results in slight increase in (222) peak intensity. By contrast, further use of the additive, significantly decreases the diffraction intensity (see Table 2).
As a matter of fact, H 3 BO 3 acts as a flux material. With the use of small amounts of the additive, the crystallinity of synthesized phosphor is improved. This issue is related to the fact that small quantities of H 3 BO 3 with a relatively lower melting point act as flux material and enhance the growth of yttria crystals [17]. In other words, it was found that the addition of small amounts of boric acid has enlarged the distance of (222) plane from 3.035 to 3.059 Å (see Fig. 1b), implying the expansion of Yttria unit cell. But higher amounts of H 3 BO 3 not only decrease the peak intensity, but also get constrict the inter-planar distance from 3.059 to 3.053 and 3.052 Å . This observation reveals that relatively larger amounts of H 3 BO 3 suppress the growth of Y 2 O 3 material. It seems that, the flux material provides efficient obstacles among the produced oxide nanoparticles.
The study of XRD spectra of E-H compounds shows that the main diffraction peaks have occurred in different angles compared to those observed for A-D compounds. For instance, it is interestingly seen that through the addition of high magnitudes of H 3 BO 3 , a relatively weak peak has emerged at approximately 27.4°. The evaluation of XRD spectra of E-H phosphors confirms that the synthesized materials are well crystallized with a hexagonal crystal structure (JCPDS No. 16-0277), consistent with Similar to what was explained for crystallite size estimation of phosphor A, the crystallite size of 75.7 nm can be calculated for phosphor E. Noteworthy, since the melting point of boric acid, 171°C, is much lower than the temperature of solid-state procedure, remarkable amounts of this flux material are subject to evaporation via solid-state procedure. Hence, slight increase in H 3 BO 3 provides a situation to compensate the lost flux, improving the peak intensity of (010) (compare phosphors E and F). Also, the inter-planar distance of (010) plane enlarges from 3.250 to 3.268 Å , explaining the expansion of the H 3 BO 3 crystal structure. Conversely, further addition of H 3 BO 3 decreases the peak intensity, sensitively. This result means that the addition of relatively high amounts of boric acid decelerates the growth of the oxide materials. Clearly, the comparison of G, H phosphors shows that via the increase in H 3 BO 3 , the (010) inter-planar distance decreases from 3.275 to 3.266 Å . Figure 2 shows the FESEM images of A-H compounds. Figure 2a shows that the employment of solid-state procedure results in the formation of nanoparticles. It can be easily found that the addition of H 3 BO 3 results in the increase in particle size. In other words, the particle size of solid-state synthesized Y 2 O 3 :Eu 3? phosphor is approximately 85 nm, while for compounds B, C and D, the particle size increased to 100, 120 and 200 nm, respectively. So, it is obvious that the presence of H 3 BO 3 via solid-state procedure leads to severe grain growth. In addition, the surface area and the particle size of productions were characterized by BET equipment. The particle size of prepared compounds measured by the employed BET analyzer confirmed the observed particle sizes of FESEM images. It is very interesting that when the added flux material is equal to the stoichiometric values, required for the formation of YBO 3 , the particle size and morphology change dramatically. As a case in point, it is seen that the particle sizes of phosphors A and F are about 100 nm and 1 lm, respectively. Henceforth, whenever the amount of boric acid exceeds the stoichiometric quantity, the particle size decreases gradually. This issue is because H 3 BO 3 material plays the role of obstacles, preventing grain growth. Figure 3 shows the photoluminescence excitation and emission spectra of A-H compounds. The photoluminescence excitation of Y 2 O 3 :Eu 3? phosphor is shown in Fig. 3a. Accordingly, there is a wide band from 200 to 300 nm that is due to the charge transfer (CT) from O 2to Eu 3? ions with the levels of 2p and 4f, respectively [12]. Figure 3b shows the photoluminescence emission properties of Y 2 O 3 :4%Eu/H 3 BO 3 phosphors at the ambient temperature. The peaks are attributed to the trivalent Eu 3? ions with the forbidden transition of f-f, known as 5 D 0 -7 F J (J = 0, 1, 2). The weaker emissions which appear at about 587.8, 593.4 and 599.6 nm correspond to the 5 D 0 -7 F 1 transition. The 5 D 0 -7 F 1 magnetic dipole transition is known as the parity allowed transition (DJ = 1), and its intensity usually remains independent of the host crystal. But the prominent emission band at approximate wavelength of 612 and 631.2 nm is due to the 5 D 0 -7 F 2 transition. The 5 D 0 -7 F 2 electric dipole transition (DJ = 2) is highly affected by the local environment around Eu 3? ions. So the emission intensity depends on the symmetry of the crystal field around the europium ions [18]. Figure 3c, which belongs to the emission behavior of A-D compounds, shows that the prominent 5 D 0 -7 F 2 peak has occurred at 612 nm [19]. Furthermore, it is seen that the addition of small amounts of boric acid to the rare earthdoped Y 2 O 3 oxide (phosphor B) enhances the luminescence properties, efficiently. This issue, which is consistent well with the XRD spectra, reveals that use of boric acid has improved crystallinity of yttria host lattice. By contrast, it was found that higher quantities of the flux material result in the suppression of luminescence behavior that is very similar to the results reported by other researchers [20]. According to Fig. 3d, under the excitation wavelength of 240 nm, a strong band is seen at 592 nm which is related to the magnetic dipole transition of 5 D 0 -7 F 1 of Eu 3? ions [18]. YBO 3 host material has a hexagonal crystal structure in which Y 3? ions are surrounded by BO 3 groups. By doping of Eu element in YBO 3 oxide material, Eu ions are substituted into trivalent Y ions, providing a symmetry center and also a strong 5 D 0 -7 F 1 transition. In addition, two relatively weaker bands at the approximate wavelengths of 612 and 631 nm are related to the 5 D 0 -7 F 2 electric dipole transition. In other words, with the crystal change from cubic to hexagonal, the type of emission has been changed significantly. As per Judd-Ofelt theory [21], the intensity of transition between different levels depends upon the symmetry of the local environment of the Eu 3? activator. Meanwhile, the color purity of Eu 3? emission is characterized by ( 5 D 0 -7 F 2 )/( 5 D 0 -7 F 1 ) ratio [22]. For YBO 3 , the C i and C 1 sites are the well-known locations for the trivalent Eu ions. For 5 D 0 -7 F 1 transition, the photoluminescence emission appears from both C i and C 1 sites, while for 5 D 0 -7 F 2 , only C 1 sites are considered since the C i locations do not contribute to the emission characteristics [23]. As a matter of fact, the optical properties of phosphors depend on wide ranges of parameters. Zhenggui Wei et al. reported about the influence of particle size of YBO 3 :Eu phosphors on chromaticity characteristics [24].

PL analysis
It was found that the strongest and weakest intensities of PL spectra are related to F and H compounds, respectively. The relatively lower PL intensity of sample E compared to that of sample F may be attributed to the fact that H 3 BO 3 has a boiling point of 300°C and there is a huge loss of this material at the employed high-temperature solid-state procedure. So, it seems that the higher quantity of boric acid consumed in sample F compensates the loss of H 3 BO 3 , providing more perfect crystal structure and stronger luminescence properties. Further increase in H 3 BO 3 (G, H phosphors), providing some impurities in the compounds, suppresses the intensity of PL.
As an overall conclusion, the emission wavelength of Y 2 O 3 :Eu 3? /YBO 3 :Eu 3? -based phosphors is considered. The wavelength spectrum of compounds B and F is shown in Fig. 4.
It can be found that the use of boric acid results in change of emitted color from reddish (612 nm) to orangish (592 nm) for Y 2 O 3 :Eu 3? -based compounds.

Conclusions
Nanoparticles of (Y 0.96 Eu 0.04 ) 2 O 3 phosphor were synthesized through a facile solid-state method. It was found that using different amounts of boric acid not only changes the crystal structure of phosphors from cubic to hexagonal, but also affects the morphology and particle size of Y 2 O 3 :Eu 3? phosphors, dramatically. Moreover, the PL emission wavelengths of obtained productions are influenced by the Fig. 4 Wavelength spectrum of synthesized compounds addition of flux, effectively. Interestingly, it was found that the color of emission varies from reddish to orangish wavelengths by the use of boric acid.