Preparation and fluorescence properties of MgAl2O4: Tb3+ nanorod-like phosphors

A series of MgAl2O4: x% Tb3+ (x = 1.0–6.0) phosphors was prepared by a hydrothermal-assisted light burning method. The crystal structure, morphology and fluorescence properties of the samples were investigated. The results indicated that when the the hydrothermal and light burning temperatures were determined to be 120 °C and 1100 °C, respectively, the crystallinity for Mg4Al2(OH)14 and NH4Al(OH)2CO3 biphasic precursors is the best, conducive to generating pure phase MgAl2O4 nanorod-like phosphors. The entry of Tb3+ into the MgAl2O4 lattice can cause lattice distortion, but the main structure of the spinel does not change. The main emission peaks for the series of phosphors are located in the green light region of 546 nm. The MgAl2O4: 4.0% Tb3+ phosphor has the highest fluorescence intensity and shows the best colour quality. Its CIE coordinates (x, y), CCT and colour purity are determined to be (0.3171, 0.5290), 5845 K and 53.50%, respectively. The series of MgAl2O4: Tb3+ phosphors exhibits typical cold green light emission and have good application prospects in solid-state display devices.


Introduction
Rare earth-doped aluminate phosphors have attracted extensive attention due to their excellent performance in chemical stability, luminous efficiency and long afterglow, and their applications have covered pigments, light storage and luminescence devices [1][2][3]. Facing many similar products in the field of luminescent materials, aluminate-based phosphors need to be comprehensively considered in terms of host materials, preparation processes, economic benefits and environmental protection. Compared with the same family of Sr and Ba aluminate materials, spinel-type MgAl 2 O 4 is an important aluminate with abundant raw material resources, low price and less pollution. In particular, MgAl 2 O 4 not only has excellent performance in melting point, strength and stability, but also has a low phonon energy (∼670 cm −1 ), and is expected to be an ideal host material for new phosphors.
The preparation methods for MgAl 2 O 4 phosphors can be divided into three main categories: solid-phase, combustion and liquid-phase methods. The solid-phase method has a simple operation, but there are some problems, such as high calcination temperature, high energy consumption and poor crystallinity for the product. The combustion method utilizes the instantaneous heat released by the rapid combustion of urea, amino acids and other fuels, so that the reaction can be completed in a short time, which has the effect of energy savings. However, it is easy to cause local uneven combustion and temperature gradients, resulting in uneven dispersion for the final product [4]. The liquid-phase method includes coprecipitation, sol-gel, hydrothermal and other synthesis technologies [5][6][7]. Among these technologies, the sol-gel method has unique advantages in the preparation of nanosized MgAl 2 O 4 phosphor, but there are also problems such as irregular agglomeration for the nanosized powders and harmful alkoxide raw materials endangering human health [8][9][10]. In this context, the hydrothermal method has achieved good results as a pollution-free green synthesis technology in the regulation of special morphology [7,11]. In addition, to meet the functional characteristics for phosphors, the preparation method may no longer be limited to a single technology but a combination of two or even more technologies. For example, the hydrothermal-assisted light burning method has been applied to the preparation of MgAl 2 O 4 phosphors, showing promising applications [12,13].
Tb 3+ has a large absorption cross section at wide pump wavelength and is easily excited. Many scholars have used Tb 3+ widely in various fluorescent materials to obtain ideal green luminescence [14]. As a promising fluorescent material, MgAl 2 O 4 : Tb 3+ is expected to be used in various light-emitting devices and full-colour displays [15]. In recent years, some research results have been achieved on MgAl 2 O 4 : Tb 3+ phosphors, but these results are still relatively few. Gugliotti [16] and Motloung et al [17] studied the luminescence behaviour for low amounts of Tb 3+ (2 mol%) doped MgAl 2 O 4 phosphors. The MgAl 2 O 4 : 2% Tb 3+ phosphor presented the highest luminescence intensity, and the colour could be tuned by changing the Tb 3+ content (2 mol%). For the first time, Valiev et al [18,19] presented structural and spectroscopic investigations of MgAl 2 O 4 ceramics doped with different concentrations of Tb 3+ ions (0.1, 1 and 5 wt%) fabricated by spark plasma sintering (SPS). However, there are still the few reports on the luminescence regularity of MgAl 2 O 4 doped with higher concentrations of Tb 3+ ions. The formation of nonagglomerated powders is important to obtain high luminescence efficiency, especially the fluorescence intensity of nanorod-like phosphors with good dispersibility, which is significantly higher than the fluorescence intensity of other morphologies such as spherelike, flower-like and tetrahedron morphologies [20,21]. However, there are still few reports on nanorod-like MgAl 2 O 4 : Tb 3+ phosphors. Relevant studies have shown that nanorod-like phosphors with good dispersibility have more application advantages in light-emitting diode (LED) packaging materials [22,23].
In view of these results, nanorod-like MgAl 2 O 4 : x% Tb 3+ (x = 1.0-6.0) phosphors were prepared by a hydrothermal-assisted light burning method. Under the excitation of 245 nm ultraviolet light, the fluorescence spectrum of phosphors was regulated by changing the doping amount of Tb 3+ , and the chromaticity performance was characterized and analysed by CIE colour coordinates.

Experimental
and CO(NH 2 ) 2 are analytical pure reagents (99.50%) and purchased from Tianjin Damao Chemical Reagent Co. Ltd, Tb(NO 3 ) 3 ·6H 2 O is a high purity reagent (99.99%) and purchased from Energy Chemical Co. Ltd The experimental water is pure water made by laboratory equipment, and absolute ethanol is analytical pure reagent.

Synthesis of MgAl 2 O 4 : Tb 3+ phosphors
were weighed in stoichiometric amounts of MgAl 2 O 4 : x % (n/n) Tb 3+ (x=1.0, 2.0, 3.0, 4.0, 5.0, 6.0) and mixed with a certain amount of pure water to maintain the Mg 2+ ion concentration in the nitrate solution at 0.05 mol l −1 . Urea precipitants were added to the above nitrate solutions according to the ratio of n + Mg : 2 n + Al : 3 n CO NH2 2 ( ) = 1 : 2 : 30. All mixing was carried out at room temperature and stirring was continued for at least 30 min. The mixture was poured into a stainless-steel autoclave, and approximately 70% filling was maintained. The autoclave was placed in a drying oven at a certain temperature for 12 h. The hydrothermal products were washed several times alternately with pure water and absolute ethanol, and then dried at 80°C for 6 h. The precursors were lightly burned in a muffle furnace at a certain temperature for 3 h to obtain the final phosphors.

Characterization
The crystal structure was characterized by x-ray diffraction (XRD) analysis (Empyrean X, Eindhoven, The Netherlands) equipped with Cu Kα radiation at the rate of 1°min −1 in a 2θ range of 10°to 80°. The sample morphology was investigated by a field-emission scanning electron microscope (Supra 55, Zeiss, Oberkochen, Germany) with a working voltage of 5.0 kV. The emission spectra were recorded by using an F-7000 fluorescent spectra photometer equipped with a 150 W Xe lamp (Hitachi Limited, Tokyo, Japan).

Structural characterization
The XRD patterns for precursors and light burning end-products at different hydrothermal temperatures are shown in figures 1(a) and (b), respectively. The hydrothermal products obtained at 100°C-120°C are biphasic precursors composed of Mg 4 Al 2 (OH) 14 (PDF No. 35-0964) and NH 4 Al(OH) 2 CO 3 (PDF No. 42-0250), from equations (1)-(2) [24]. As the hydrothermal temperature reaches 160°C, the precursor phase of Mg 4 Al 2 (OH) 14 disappears, and the AlOOH (PDF No. 21-1307) phase appears. Because in an alkaline high-temperature hydrothermal environment, Al 3+ will be transformed into relatively stable AlOOH instead of easily decomposed Mg 4 Al 2 (OH) 14 . In addition, when the hydrothermal temperature is 120°C, the light burning end-product has the best pure phase structure of MgAl 2 O 4 (PDF No. 21-1152) ( figure 1(b)). The amorphous Mg(Al)O solid solution and Al 2 O 3 resulting from the thermal decomposition of Mg 4 Al 2 (OH) 14 and NH 4 Al(OH) 2 CO 3 were suggested to facilitate the formation of MgAl 2 O 4 spinel [25]. Furthermore, the biphasic precursors at 120°C have stronger peaks with better crystallinity ( figure 1(a)). Therefore, the decomposition of the biphasic precursors with better crystallinity promotes the formation of amorphous products, which is important to obtain the final pure phase spinel phosphors.  figure 2. The diffraction peaks for the phosphors match the crystal planes of the standard MgAl 2 O 4 structure. With increasing calcination temperature, the intensity of the diffraction peak increases and the peak width narrows. The results show that the crystallinity of the samples increases with increasing of light burning temperature.
From the above results, the hydrothermal and light burning temperatures were determined to be 120°C and 1100°C, respectively. The XRD patterns for MgAl 2   lattice without affecting the spinel structure. Although the ionic radius for Tb 3+ (0.92 Å) is larger than the ionic radius of Mg 2+ (0.57 Å) and Al 3+ (0.54 Å), when rare earth ions replace the Mg 2+ sites in the spinel structure, the structures are more stable in energy and configuration [26]. Therefore, a larger Tb 3+ ion occupying the Mg 2+ site will lead to lattice expansion and increase the crystal plane spacing, resulting in a gradual shift of diffraction peaks to lower angles [27], as shown in figure 3(b). Furthermore, the cell parameters and volumes gradually increase with the Tb 3+ ion doping content (table 1), which further confirms that Tb 3+ ions effectively incorporate into the MgAl 2 O 4 host by occupying the Mg 2+ sites. However, the occupation of Mg 2+ sites by Tb 3+ ions is a nonequivalent substitutional behaviour. To maintain charge balance, some Mg 2+ and Al 3+ ions may be inverted to form an inverse spinel structure, and some oxygen vacancies (V 0 +1 , V 0 +2 ) may both compensate for positive charge defects [13]. Figure 4 shows the SEM images for MgAl 2 O 4 : 1.0% Tb 3+ phosphors prepared at different hydrothermal temperatures (100°C-160°C). The microscopic morphology for the phosphors changes with increasing of the hydrothermal temperature. When the hydrothermal temperature is 100°C, the phosphors exhibit an uneven rod-like morphology with some agglomerates ( figure 4(a)). When the hydrothermal temperature is 120°C, the phosphors exhibit a uniform nanorod-like morphology under the action of the biphasic precursors with good crystallinity ( figure 4(b)). As the hydrothermal temperature reaches 140°C, the rods increase in size and become micron-scale rods with good dispersion ( figure 4(c)), but the phase analysis at this temperature is not satisfactory due to the inclusion of a small amount of MgO and amorphous Al 2 O 3 impurities ( figure 1(b)). Continuing to raise the hydrothermal temperature to 160°C, the products are seen to be almost completely turned into flocculent aggregates ( figure 4(d)), possibly caused by the high temperature hydrothermal generation of the boehmite AlOOH precursor ( figure 1(a)). The bond energy of AlOOH is relatively strong, and AlOOH easily causes local gelatinous agglomeration, which is unfavourable for the formation of spinel [28]. SEM images of MgAl 2 O 4 : 1.0% Tb 3+ phosphors prepared at different light burning temperatures are shown in figures 5(a)-(c). The microscopic morphologies of the phosphors change with increasing calcination temperature. When the temperature is 900°C, the prepared phosphors exhibit irregular microscopic morphology due to poor crystallinity. When the temperature is 1000°C, nanorods appear, but there are still a small number of agglomerates. Continuing to raise the temperature to 1100°C, the phosphors are regular  nanorods with uniform appearance and excellent dispersion, and the diameter of the rod is approximately 200 nm ( figure 5(c)). With an increasing Tb 3+ ion doping amount, the main nanorod structure for the phosphor does not change significantly, but the surface for the rod is smooth at 1.0% Tb 3+ doping, while the surface  becomes rough at 5.0% ( figure 5(d)). According to the XRD experimental results ( figure 3(a)), the diffraction peak intensity of the 1.0% Tb 3+ -doped sample is significantly higher than the diffraction peak intensity of 5%. Generally, the decrease in the peak intensity as Tb 3+ mol% is increased can be explained by the destruction of the crystalline quality during the growth of MgAl 2 O 4 crystals [17]. The more rare earth ions enter the host lattice, the easier it is to cause lattice distortion, resulting in a decrease in crystallinity and a rough surface morphology.

Fluorescence properties
The excitation spectrum for the MgAl 2 O 4 : 1.0% Tb 3+ phosphor was measured at 545 nm ( figure 6(a)). The sample has a broad excitation band between 220-270 nm in the UV region with a peak at 245 nm. This broad band is attributed to the matrix absorption band and the 4f 8 →4f 7 5d 1 transition for Tb 3+ . Figure 6 The colour performance for MgAl 2 O 4 : x% Tb 3+ (x = 1.0-6.0) phosphor application is evaluated by the CIE chromaticity diagram and coordinates (x, y), as shown in figure 8. The emission range for the series of phosphors is mainly in the green spectral region. The correlated colour temperature (CCT) and colour purity values for phosphor were calculated in table 2 using the obtained CIE coordinates (x, y) data as follows (from  where n=(x-x c )/(y-y c ) was defined, (x, y) is the CIE coordinate for the prepared samples, (x c , y c ) is the coordinate of the centre point that can be identified as (0.3320, 0.1858), (x 0 , y 0 ) is the coordinate of the white  illuminant point (0.33, 0.33), and (x g , y g ) is the coordinate of the dominant green wavelength for the prepared samples.

Conclusions
Nanorod-like MgAl 2 O 4 : Tb 3+ phosphors were obtained by a hydrothermal-assisted light burning method. When the the hydrothermal and light burning temperatures were determined to be 120°C and 1100°C, respectively, the crystallinity for Mg 4 Al 2 (OH) 14 and NH 4 Al(OH) 2 CO 3 biphasic precursors is the best, which is conducive to generating pure phase MgAl 2 O 4 rod-like phosphors with a diameter of 200 nm. In the MgAl 2 O 4 : x % Tb 3+ (x = 1.0-6.0) phosphors, Tb 3+ can enter the MgAl 2 O 4 lattice without changing the spinel structure, but it will cause lattice distortion and reduce the surface quality of the rods. The main emission peaks for series of phosphors are located in the green light region of 546 nm. The MgAl 2 O 4 : 4.0% Tb 3+ phosphor has the highest fluorescence intensity and shows the best colour quality. Its CIE coordinates (x, y), CCT and colour purity are determined to be (0.3171, 0.5290), 5845 K and 53.50%, respectively. The series of phosphors exhibit a typical cold green light emission and has good application prospects in solid-state display devices.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).