Introduction

Recently, phosphor compounds have been a subject of interest and controversy, and among them Y2O3 has an exceptional status. Yttrium oxide with a space group Ia3 (No. 206) and cubic crystal structure, density of about 5.01 g/cm3, 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 (ΔR = 0). Among the lanthanide ions, Eu3+ with 5D0–7F2 transition has been used extensively as a source of red light at an approximate wavelength of 618 nm [5].

The studies about Y2O3 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. H3BO3 with the melting point of 171 °C is one of the most prominent flux materials, employed for synthesizing Y2O3/Gd2O3 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 (Y0.96Eu0.04)2O3 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 (Y0.96Eu0.04)2O3 phosphor, analytical grades of yttrium acetate Y(CH3COO)3.xH2O, europium oxide (Eu2O3) 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 H3BO3, 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.

Table 1 Name of different compounds due to magnitude of added H3BO3
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Characterization

The crystal structures of synthesized Y2O3/YBO3:Eu3+ phosphors were examined by X-ray diffraction (XRD, Rigaku, Japan) with CuKα radiation (λ = 1.54 A). To study the microstructure and size of these phosphors, field emission scanning electron microscope (FESEM, Hitachi SU70, Japan) and also Brunauer–Emmett–Teller (BET, Sensiran, Toseye Hesgarsazan Asia, Iran) were employed. In addition, the optical characteristics of synthesized phosphors were investigated through use of photoluminescence (PL, Horiba Jobin–Yvon Fluorolog-3, Japan) analyzer.

Results and discussion

XRD analysis

Figure 1a shows the XRD spectra of Y2O3/YBO3:Eu3+ 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 = 0.9λ/βcosθ. In this equation, D is the average grain size, λ is the wavelength of X-ray (0.154 nm), and β and θ are the full-width at half-maximum 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 Y2O3, 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).

Fig. 1
figure 1

XRD spectra of Y2O3/YBO3:Eu3+ phosphors: a A-H and b A-D luminescent compounds

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Table 2 Results obtained from the XRD spectra of A-H composites
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As a matter of fact, H3BO3 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 H3BO3 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 H3BO3 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 H3BO3 suppress the growth of Y2O3 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 H3BO3, 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 YBO3. The main diffractions are related to (002), (010), (102), (110), (104), (112), (200), (202), (114) planes, where (010) has the highest intensity. In other words, it was found that through consuming large amounts of H3BO3, the formation of yttrium borate (YBO3), with hexagonal structure space group P63/mmc and lattice parameters of a = 3.778 and c = 8.814 Å, may be feasible.

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 H3BO3 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 H3BO3 crystal structure. Conversely, further addition of H3BO3 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 H3BO3, the (010) inter-planar distance decreases from 3.275 to 3.266 Å.

FESEM observations

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 H3BO3 results in the increase in particle size. In other words, the particle size of solid-state synthesized Y2O3:Eu3+ 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 H3BO3 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.

Fig. 2
figure 2

FESEM images of A-H luminescent compounds

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It is very interesting that when the added flux material is equal to the stoichiometric values, required for the formation of YBO3, 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 µm, respectively. Henceforth, whenever the amount of boric acid exceeds the stoichiometric quantity, the particle size decreases gradually. This issue is because H3BO3 material plays the role of obstacles, preventing grain growth.

PL analysis

Figure 3 shows the photoluminescence excitation and emission spectra of A-H compounds. The photoluminescence excitation of Y2O3:Eu3+ 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 O2− to Eu3+ ions with the levels of 2p and 4f, respectively [12]. Figure 3b shows the photoluminescence emission properties of Y2O3:4%Eu/H3BO3 phosphors at the ambient temperature. The peaks are attributed to the trivalent Eu3+ ions with the forbidden transition of f–f, known as 5D0–7FJ (J = 0, 1, 2). The weaker emissions which appear at about 587.8, 593.4 and 599.6 nm correspond to the 5D0–7F1 transition. The 5D0–7F1 magnetic dipole transition is known as the parity allowed transition (∆J = 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 5D0–7F2 transition. The 5D0–7F2 electric dipole transition (∆J = 2) is highly affected by the local environment around Eu3+ 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 5D0–7F2 peak has occurred at 612 nm [19]. Furthermore, it is seen that the addition of small amounts of boric acid to the rare earth-doped Y2O3 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].

Fig. 3
figure 3

Photoluminescence: a excitation of A, b emission of A-H, c emission of A-D and d emission of E–H compounds

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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 5D0–7F1 of Eu3+ ions [18]. YBO3 host material has a hexagonal crystal structure in which Y3+ ions are surrounded by BO3 groups. By doping of Eu element in YBO3 oxide material, Eu ions are substituted into trivalent Y ions, providing a symmetry center and also a strong 5D0–7F1 transition. In addition, two relatively weaker bands at the approximate wavelengths of 612 and 631 nm are related to the 5D0-7F2 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 Eu3+ activator. Meanwhile, the color purity of Eu3+ emission is characterized by (5D0–7F2)/(5D0–7F1) ratio [22]. For YBO3, the Ci and C1 sites are the well-known locations for the trivalent Eu ions. For 5D0–7F1 transition, the photoluminescence emission appears from both Ci and C1 sites, while for 5D0–7F2, only C1 sites are considered since the Ci 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 YBO3:Eu phosphors on chromaticity characteristics [24].

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 H3BO3 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 H3BO3, providing more perfect crystal structure and stronger luminescence properties. Further increase in H3BO3 (G, H phosphors), providing some impurities in the compounds, suppresses the intensity of PL.

As an overall conclusion, the emission wavelength of Y2O3:Eu3+/YBO3:Eu3+-based phosphors is considered. The wavelength spectrum of compounds B and F is shown in Fig. 4.

Fig. 4
figure 4

Wavelength spectrum of synthesized compounds

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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 Y2O3:Eu3+-based compounds.

Conclusions

Nanoparticles of (Y0.96Eu0.04)2O3 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 Y2O3:Eu3+ phosphors, dramatically. Moreover, the PL emission wavelengths of obtained productions are influenced by the 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.