Sol-Gel Auto-Combustion Synthesis and Luminescence Properties of GdCaAl3O7:RE3+(RE=Eu, Tb, Eu@Tb) Phosphors For Near-Ultraviolet Light-Triggered Indoor Illumination


 The color-tunable GdCaAl3O7: RE3+ (RE=Eu, Tb, Eu@Tb) phosphors were synthesized via an ultrafast sol-gel auto-combustion synthesis method. The XRD (X-ray diffraction) patterns confirmed the forming of GdCaAl3O7 tetragonal phase, and the SEM (Scanning electron microscopy) images indicated that the obtained products presented honeycomb-like structure due to the gas releasing during the auto-combustion process. The EDS (Energy dispersion spectra) results suggest that Eu3+ and Tb3+ were well incorporated into the host matrices of GdCaAl3O7. The luminescent properties, concentration quenching mechanism, energy transfer mechanism, luminescent dynamics, thermal stability and product practicability of GdCaAl3O7: R3+ (RE=Eu, Tb, Eu@Tb) phosphors were investigated systematically. All the GdCaAl3O7: RE3+ (RE=Eu, Tb, Eu@Tb) products presented the characteristic emission of doped RE3+. Using GdCaAl3O7: 50% Tb3+@3.0% Eu3+ as yellow-emitting phosphors in combination with commercial BAM: Eu2+ blue-emitting phosphors and a NUV chip can successfully package a warm white LED device with high color rendering index (CRI, 86.3) and low correlated color temperature (CCT, 3348 K) that was very suitable for near-ultraviolet light-triggered indoor illumination.

Among them, GdCaAl3O7 can be employed as an efficient luminescent host because of their outstanding optical damage threshold and high ultraviolet (UV) absorption, which was considered as one of the best phosphor candidates in pc-WLEDs [15,16]. In addition, GdCaAl3O7 belongs to the aluminate-based melilite family with the compound formula of ABC3O7 (A= Ca, Sr, Ba; B=Y, La, Gd; C=Al, Ga). These melilite compounds form tetragonal crystals belonging to the space group P-421m, containing five-membered rings constituted from AlO4 5tetrahedral in which A 2+ ions and B 3+ ions are distributed randomly at the eight-coordinated sites with Cs symmetry, while Al 3+ ions occupy two non-equivalent tetragonal sites with both S4 and Cs symmetries [17][18][19][20][21].
According to the present literatures, the respective luminescent properties of Eu 3+  phosphors for white light emission by suitably controlling the doping ratios of Eu 3+ and Tb 3+ [5,24].The above results suggest that Eu 3+ and Tb 3+ activated phosphors are excellent candidates for solid-sate lighting.
In general, phosphors can be prepared by many synthesis methods including sol-gel, hydrothermal synthesis, microwave heating, solid-state reaction and solution combustion [16,19,20,32,33]. The main disadvantages of the traditional solid-state method include high reaction temperature, large particle sizes and inhomogeneous mixing [16,18,21,34]. The sol-gel auto-combustion synthesis is a suitable method in preparing multi-component oxides owing to the high reaction rate at a relatively low ignition temperature. Furthermore, the experimental process is easy to manipulate and the total experimental period is short. It is very beneficial to prepare the multi-component oxides with uniform size and high purity [13,19,20] (1 − x − y) 2 3 + 2 3 + 2 3 + 2 3 + 3 + 6 ( 3 ) 3 + 5 2 5 O 2 → 2 1− − 3 7 + ↑.
In a representative synthesis process, solution A was prepared by dissolving the stoichiometric amounts of Gd2O3, Eu2O3 and Tb2O3 in about 6 mL nitric acid and mixed with the solution B which was prepared by dissolving the stoichiometric amounts of ingredients (Al(NO3)3·9H2O, CaCO3) in distilled water. Meanwhile, the C2H5NO2 was added to above solution to obtain a homogeneous aqueous solution (the mole amount of glycine was equal to the total mole amount of metal ions).
Subsequently, the mixed aqueous solution was stirred continuously under the heating temperature of 70°C until the sticky gels were formed. The obtained gels were shifted into an open alumina crucible for the next the combustion experiments. The gels were burnt by a self-propagating combustion manner at a relatively low ignition temperature. Initially, the solutions were dehydrated rapidly along with the gas volatilization. Subsequently, the dehydrated gels were ignited following by lots of brown fumes (CO2, N2, NOx and H2O) [35]. Finally, a mass of porous products were obtained, while the as-prepared products presented brown owing to the residual organic matters. The as-prepared products were heated at different temperatures for 2 h in order to eliminate the unreacted matters and form single-phase Gd1-xEuxTbyCaAl3O7 phosphors. Fig. 1 shows the flowchart of fabricating GdCaAl3O7: Eu 3+ @Tb 3+ phosphors by the sol-gel auto-combustion method using glycine as combustion agent.

Phase structure, morphology and luminescence characterizations
The components of all the samples were clarified using an X-ray diffraction (XRD, Bruker D8 Advance) with Cu Ka radiation (λ= 1.54051 Å). The morphology and elemental composition were analyzed by utilizing a field-emission scanning electron microscope (FE-SEM; HITACHI SU3500) with an attached energy dispersive spectrometer (EDS). The luminescence spectra and decay curves were recorded by means of an Edinburgh FS5 fluorescence spectrometer using a pulsed xenon lamp as excitation source. The surrounding temperature of samples in the range of 183-423 K was adjusted by the temperature-controlled system (Linkam HFS600EPB2).

Fabrication of NUV chip-based white-LED devices
The NUV chip-based white-LED lamps were fabricated by combining the commercial BaMgAl10O17:Eu 2+ (BAM: Eu 2+ ) blue phosphors and the obtained GdCaAl3O7: 3.0% Eu 3+ @50% Tb 3+ phosphors with epoxy resin, and then coated on the surface of the commercial NUV (B365 nm) chip. After heating at 80 °C for 2 h, the NUV chip-based white-LED lamp was obtained. The photoelectric properties, such as electroluminescence (EL) emission spectra, color rendering index (CRI), correlated color temperature (CCT), color coordinates and luminous efficiency were recorded by using a multi-channel spectroradiometer system (MEASUREFINE; SPEC-3000A) attached with an integrating sphere.

Structural and morphological analysis
The phase structures of samples were determined by XRD method. Fig. 2a In this formula, 1 ( ) represents the radius of the replaced ions, and 2 ( ) stands for the radius of dopant. In the case of the coordinate number 6, the radii of Gd 3+ , Eu 3+ and Tb 3+ were 1.053, 1.066 and 1.04 Å, respectively. Accordingly, all the of doped GdCaAl3O7 were calculated as approximately 1.2 %, much lower than required 15%. It is well consistent with the formation rule of substitution-type solid solution proposed by Davolos [37], further indicating the feasibility of fabricating the RE 3+ -doped GdCaAl3O7. In order to know clearly the crystal configuration and surrounding environment of the GdCaAl3O7 host lattices, a unit cell of GdCaAl3O7 crystal structure was given. The corresponding cell parameters of GdCaAl3O7 were a = b = 7.801 Å, c = 5.132 Å and α = β = γ = 90°, as shown in Fig. 2c. In GdCaAl3O7, the frames of crystal structure are consisted of five-membered rings by linked tetrahedral AlO4 5− at each corner. As reported, Al 3+ ions were coordinated with four surrounding O 2-, and formed tetragonal structures. The Ca 2+ and Gd 3+ are randomly distributed at the centers of these rings which were in the octahedral coordination surrounded by six oxygen ions [15][16][17][18].  The lattice constants and cell volumes of prepared GdCaAl3O7 samples were investigated by Jade analysis software using the XRD data, as shown in Table 1. It can be known that the lattice constants (a b c) of GdCaAl3O7 increased slightly as the annealing temperatures increased, and became close with the standard GdCaAl3O7. The grain sizes can be calculated by Scherrer's equation [38]: Where represents the X-ray wavelength ( = 1.541 Å), stands for the diffraction angle, and corresponds to the half-height width of diffraction peak at 2 . The grain sizes of GdCaAl3O7 exhibited an obvious increasing from 35 nm to 50 nm in the temperature range of 800-1000 °C. Tb 3+ were well doped into the host lattices. An obvious Pt peak at around 2.3 keV was observed in the EDS spectra since the Pt was employed as a measurement electrode [39]. The SEM images of GdCaAl3O7: 50% Tb 3+ @3.0% Eu 3+ samples at different magnifications are shown in Fig. 3b-d. The obtained products have honeycomb-like structure with plenty of irregular pores because of the gas releasing during the process of sol-gel combustion reaction. The particle sizes were dispersed from 50-300 nm. Totally speaking, the product microstructure displayed the features of cross linking and compact bonding.

Luminescent properties of GdCaAl3O7: x% Eu 3+
To investigate the luminescent properties of the obtained products, the representative photoluminescence (PL) excitation and emission spectra of the GdCaAl3O7: 10% Eu 3+ are exhibited in Fig. 4a. It can be seen that the excitation spectra were consisted of several main absorption peaks at about 316 nm ( 7 F0→ 5 HJ), 362 nm ( 7 F0→ 5 D4), 382 nm ( 7 F0→ 5 L7), 393 nm ( Fig. 4c. It is necessary to study in detail the involved interaction about concentration quenching. Generally, the electric multipolar interaction and radiation reabsorption leaded to the NR energy transfer processes. Nevertheless, we cannot find any overlaps from the emission spectra, suggesting that the electric multipolar interaction would be more possible to generate the NR energy transfer processes and leaded to the concentration quenching rather than the radiation reabsorption [33]. The below Eq. (3) given by Dexter theory can be used to explain the mechanism of concentration quenching [10]: Here, and stand for the emission intensity and doping concentration, respectively. and are coefficients dependent on and . As we know, the electric multipolar owns three kinds of distinct interactions, and the value represents the basic reasons of concentration quenching, wherein = 6, 8, and 10 correspond to dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions, respectively. The inset in Fig. 4c displays the relation between the ( ) and ( / ), and the experimental data were well matched to a straight line with a slope of -2.09, which means that the value was estimated as about 6, indicating that the dipole-dipole interaction was the most possible to control the concentration quenching in GdCaAl3O7: x% Eu 3+ .
The luminescence results in GdCaAl3O7: x% Eu 3+ can be explained by the luminescence energy transfer mechanism. The schematic diagram of energy transfer mechanism of Eu 3+ is presented in  The temperature-dependent emission of the GdCaAl3O7: 40% Eu 3+ were measured in order to investigate the thermal stability of phosphors for practical applications. In Fig. 5a, it can be found that the intensity of all the peaks decreased with increasing the temperatures in the range of 183-423 K due to the well-known thermal quenching effects. Fig. 5b shows the normalized temperature-dependent emission intensity of the dominated transition ( 5 D0→ 7 F2). In general, the phosphor stability can be justified by comparing the emission intensity at 303 K with that at 423 K. 42 It can be seen that the emission intensity at 423 K retained 81.01% of that at 303 K, suggesting that the GdCaAl3O7: x% Eu 3+ have good thermal stability as a kind of red-emitting phosphors for solid-state lighting. In order to study the thermal quenching phenomenon, the activation energy (∆ ) was calculated by the following expression [43]: where 0 and correspond to the emission intensity at the initial temperature (183 K) and , respectively, is a constant related with the emission intensity, and is the Boltzmann constant (8.6173 × 10 −5 / ). In addition, the formula can be rewritten as below [26]: As displayed in Fig. 5c, it is obvious that all the experimental data were linear with a slope of −0.163, suggesting that the ∆ value for thermal quenching was about 0.163 eV in the GdCaAl3O7: 40% Eu 3+ . Furthermore, the relationship between the activation energy and NR transition per unit time ( ) can be expressed as below [44]: where is the frequency factor ( −1 ). In this formula, it is reasonable to believe that the possibility of NR transition would be small and good thermal stability could be obtained when the

Luminescent properties of GdCaAl3O7: y% Tb 3+
A series of Tb 3+ single-doped GdCaAl3O7 phosphors were fabricated and their emission spectra were tested. The excitation and emission spectra of the GdCaAl3O7: y% Tb 3+ are shown in Fig. 6a.
Under 543 nm emission, it can be found that the excitation spectra had a broadband ranging from 230 nm to 296 nm with a maxima band at about 273 nm corresponding to the 4f 8 -4f 7 5d 1 transition of  Fig. 6b shows the emission spectra for various Tb 3+ -doped concentrations. The 5 D4→ 7 F5 transition at around 543 nm was the dominate peak in the emission spectra, originating from magnetic dipole interaction.
The highest emission intensity was found at 50% Tb 3+ doping concentration. When the doping concentration was more than this percent, the emission intensity decreased owing to the influence of concentration quenching, so the optimum doping concentration of GdCaAl3O7: y% Tb 3+ was believed as 50%. Meanwhile, in the emission spectra, no any overlaps were noticed, suggesting that the electric multipolar interaction dominated the concentration quenching. Furthermore, according to Eq. (3), the value was calculated as 6.21 for the GdCaAl3O7: y% Tb 3+ , which is close to 6, demonstrating that the dipole-dipole interaction dominated the concentration quenching in GdCaAl3O7: y% Tb 3+ , as displayed in the inset of Fig. 6c.
In addition, the energy transfer among Tb 3+ in the GdCaAl3O7 host material was studied, as shown in Fig. 6d  The thermal stability of the GdCaAl3O7: 50% Tb 3+ was investigated based on the temperature-dependent emission spectra, as shown in Fig. 7a. Due to the thermal quenching effect, the emission intensity of the GdCaAl3O7: 50% Tb 3+ decreased with increasing the temperatures. The emission intensity still reached 81.36% at 423 K compared with that at 303 K, suggesting that the GdCaAl3O7: y% Tb 3+ have good thermal stability as green-emitting phosphors for solid-state lighting, as seen in Fig. 7b. In addition, in Fig. 7c the activation energy for this phosphor was calculated as 0.152 eV according to Eq. (5), which was slightly higher than other Tb 3+ -doped phosphor materials, such as Na5Lu9F32: Tb 3+ (0.14 eV) and Sr2Gd8Si6O26: Tb 3+ (0.13 eV) [47,48].

Luminescent properties of color-tunable
Here, and 0 are the emission intensities of the single-doped Tb 3+ and co-doped Tb 3+ @Eu 3+ , respectively. Therefore, it can be calculated that the ET efficiency reached 93.65% when x was 11, as shown in Fig. 8c. The energy transfer diagram for the GdCaAl3O7: 50% Tb 3+ @x% Eu 3+ in Fig. 8d illustrated the ET processes between Tb 3+ and Eu 3+ in GdCaAl3O7 host material. Under 376 nm irradiation, the energy would be transferred from the ground state of 7 F6 of Tb 3+ to the 5 G6 level, and then jumped to the 5 D4 level by non-radiative transition. In the meantime, the energy would be transferred from the higher 5 D4 (Tb 3+ ) level to the lower 5 D0 (Eu 3+ ) level, emitting the characteristic light of Eu 3+ . Thus, it is reasonable to believe that the energy transfer from the Tb 3+ to Eu 3+ was feasible and the ET efficiency was high. The CIE chromaticity coordinates of the GdCaAl3O7: 50% Tb 3+ @x% Eu 3+ were calculated, as shown in Fig. 8e. It is noticeable from the digital images that the As mentioned above, the concentration quenching phenomenon was produced in GdCaAl3O7: 50% Tb 3+ @x% Eu 3+ and was different from Eu 3+ single-doped GdCaAl3O7. The electric multipolar interaction was responsible for the concentration quenching during the ET processes. The relationship between exchange and multipolar interaction during the ET processes can be defined as follows [49]: In this formula, 0 and have the same meaning as Eq. (7), stands for the sum of Tb 3+ and Eu 3+ , and has the same meaning as of Eq. (3). The dependence of 0 / on C n/3 in GdCaAl3O7: 50% Tb 3+ @x% Eu 3+ was investigated in Fig. 9a-c. It is obvious that the best linear relationship was = 10, indicating that the ET processes was dominated by quadrupole-quadrupole interaction in GdCaAl3O7: 50% Tb 3+ @x% Eu 3+ . The thermal stability of PL emission intensity of the GdCaAl3O7: 50% Tb 3+ @x% Eu 3+ was recorded under 376 nm excitation, as shown in Fig. 10a

Decay times of GdCaAl3O7: 50% Tb 3+ @x% Eu 3+
The decay curves of the obtained GdCaAl3O7: 50% Tb 3+ @x% Eu 3+ (x= 0, 1, 3, 5, 7 and 11) were recorded by monitoring 376 nm excitation and 543/617 nm emission, as shown in Fig. 11a-b. The relation between luminescent intensity and lifetime can be defined as below [38]: where 1 is the constant and 1 is related to the lifetime, ( ) and 0 are the PL intensity at time t and initial time, respectively. It can be found that the experimental data can be well-fitted by a single exponential function. According to the Eq. (9), the corresponding lifetimes of GdCaAl3O7: 50%

Electroluminescence (EL) properties of the designed white-LED lamp.
As discussed above, the CIE chromaticity coordinate of GdCaAl3O7: 50% Tb 3+ @3.0% Eu 3+ sample was close to the warm white region. In this regard, by combining the NUV (~365 nm) chip with the GdCaAl3O7: 50% Tb 3+ @3.0% Eu 3+ and BAM: Eu 2+ commercial phosphors, a white-LED device was fabricated, and the obtained color rendering index (CRI) and correlated color temperature (CCT) values were compared, as shown in Fig. 12a and Fig. 12b

Conclusions
In summary, the GdCaAl3O7: RE 3+ (RE=Eu, Tb, Eu@Tb) phosphors with particle sizes of 50-300 nm were successfully prepared by a sol-gel auto-combustion synthesis method. The SEM images indicated that the obtained products present honeycomb-like structure with many irregular continuous pores due to the releasing of a large amount of gas during the auto-combustion process.

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