A Two-Dimensional Guidance Strategy to Fabricate Perovskite Gadolinium Aluminate Ceramic Film

Abstract

Gadolinium aluminate is an effective host for doping with various ions, and it can emit various colors. However, it is not easy to prepare transparent ceramics of gadolinium aluminate using traditional methods, although transparent ceramics are very suitable for solid lighting. In this work, a two-dimensional guidance strategy has been successfully carried out for perovskite-structured aluminate ceramic film. Through the two-dimensional interfacial reaction, GdAlO₃:Eu³⁺ (GAP:Eu³⁺) transparent ceramic films were successfully fabricated using nanosheets exfoliated from layered gadolinium hydroxide, a rare earth source. The final films were tested by characterization techniques, including XRD, SEM, TEM, FT-IR, PLE/PL spectroscopy, temperature-dependent PL spectroscopy, and luminescence decay analysis. The perovskite film of transparent ceramics can be obtained by calcining LRH nanosheets on the substrate of amorphous alumina at 1550 °C in air with a reaction time of 2 h. During the interface reaction, temperature-dependent element diffusion takes the dominant role, and increased reactants take in the reaction with increasing calcination temperature. The grain for ceramic film is only 2–5 μm, which is much smaller than that for bulk ceramic. This is mainly due to the lower temperature and the interface diffusion. Ceramic film has a high transmittance larger than 90% at the visible range. Upon UV excitation at 254 nm, the film exhibits intense emission at the red wavelength range. The outcomes described in this work may have wide implications for transparent ceramics and layered rare-earth hydroxides.

1. Introduction

Recently, light-emitting diodes (LEDs) have attracted increased attention because of their high luminous efficiency, low energy costs and environmentally friendly nature [1,2]. Traditionally, LEDs were fabricated by combining an LED chip (such as InGaN) and phosphor powders. Taking the white LED as an example, the blue LED chip and the yellow garnet phosphor of Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) are the two most important components. However, the glue used for the phosphor powders in LED chips is sensitive to high temperatures and varied humidity, so the mixture of glue and phosphor will age and peel off as time goes by [3,4,5]. Now, transparent ceramic is widely accepted as the preferred luminophore for LEDs because of its stability and high transparency [6]. Because transparent ceramics have high transmittance and can easily be prepared into bulk ceramics with various shapes, they are suitable for application in high-power and high-density devices, except for light-emitting devices [6]. Transparent ceramics can be used in optical components, scintillators, transparent armor, solid-state lasers, and solid-state lighting. Except for the transparent ceramics of garnet rare earth aluminates, such as Y₃Al₅O₁₂ and Lu₃Al₅O₁₂, perovskite rare-earth aluminate is another system for transparent ceramics [7,8,9]. The investigations of perovskite rare-earth aluminate mainly focus on the three systems of GdAlO₃, LaAlO₃ and YAlO₃, because in the 4f sublayer of Gd³⁺, La³⁺, and Y³⁺, the electrons are half, fully, or not filled [7,8]. Therefore, they are suitable hosts that can be doped with various activators and emit various emissions.

By comparison, there are a lot of investigations into YAlO₃ and LaAlO₃, while the studies on GdAlO₃ are rather limited. GdAlO₃ (GAP) has a tetrahedron unit cell structure with a = 5.305 Å, b = 7.448 Å, c = 5.254 Å, and the unit cell belongs to the orthogonal perovskite structure of ABO₃. However, GdAlO₃ is not a perfect perovskite; it is a distorted perovskite [9]. In the ideal perovskite, Gd³⁺ has various coordinations with oxygen atoms, changing from 12 to 8, while Al³⁺ only has one kind of 6 coordination with oxygen atoms. The structure has the space group of Pbnm, deviating from the cubic space group Pm3m, which is due to distortion from the octahedron of [BO₆]. The octahedral cluster of [AlO₆] is arranged along the c axis, and the polyhedron of [GdO₈] connects with the octahedron of [AlO₆] in a collinear or coplanar manner [9]. Because of its special crystal structure, it is possible to optimize or realize the varied luminescence through a modification of crystal structure and composition. Up to now, the doping ions employed for GdAlO₃ hosts are mainly Eu³⁺ (red) and Ce³⁺ (yellow), and a small number of other ions, including Yb³⁺ (yellow), Tb³⁺ (blue and green), Pr³⁺ (blue and red), Er³⁺ (green), Er³⁺/Yb³⁺/Tm³⁺ (white), and Cr³⁺ and Mn⁴⁺ (near infrared) [10,11,12,13,14,15,16,17,18]. In addition, previous studies have focused on the optical properties of GdAlO₃ nanocrystals and phosphor powders, and the reports on GdAlO₃ transparent ceramics and ceramic films are rather limited.

More recently, we proposed a novel strategy to fabricate transparent ceramic film of garnet-structured Y₃Al₅O₁₂ (YAG) through an interface reaction using the exfoliated nanosheets of layered rare-earth hydroxides (LRHs) as the rare-earth source [6,19]. The outcomes pave a two-dimensional guidance strategy for transparent ceramic film, and they may have a demonstrative effect on other systems, including the perovskite system. LRHs can be exfoliated into nanosheets with single- or several-layer thickness, which are the building units for the fabrication of films with multifunctions owing to 2D morphologies [20]. It is not easy for larger rare earths to form LRHs, and Gd might be the boundary, so most layered rare-earth hydroxides are formed for the rare earths with smaller ionic radii, such as Eu, Tb, Sm, Ho, and Y [21]. However, most rare-earth aluminates are garnet-structured with the composition of RE₃Al₅O₁₂ for the smaller rare earths, while they are perovskite-structured for the larger rare earths with the composition of REAlO₃ [22]. Therefore, layered rare-earth hydroxides with larger rare-earth ionic radii are the possible candidates to fabricate perovskite-structured rare-earth aluminates films. On this basis, layered gadolinium hydroxides are the best choice to fabricate perovskite-structured aluminates films, because gadolinium is almost the largest element for layered rare-earth hydroxides in the processing window.

In this work, a two-dimensional guidance strategy has been successfully carried out for perovskite-structured aluminate ceramic film. Through the interfacial reaction, GdAlO₃:Eu³⁺ (GAP:Eu³⁺) films of transparent ceramic were fabricated using the exfoliated layered gadolinium hydroxide nanosheets as the rare-earth source. The final films were tested by characterization techniques, including XRD, SEM, TEM, FT-IR, PLE/PL spectroscopy, temperature-dependent PL spectroscopy, and luminescence decay analysis. The transparent film exhibits intense emission at the red wavelength range. In the following section, the synthesis and properties of the ceramic films are investigated in detail.

2. Experimental Section

2.1. Materials and Synthesis

The raw materials are rare-earth oxides of Gd₂O₃ and Eu₂O₃ with 99.99% purity (Huizhou Ruier Rare-Chem. Hi-Tech. Co., Ltd., Huizhou, China), and are the rare-earth source. Ammonia hydroxide (NH₄OH, 25 wt %), nitric acid (HNO₃, 67 wt %), and anhydrous ethanol (C₂H₅OH, 99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) The aqueous solutions of rare-earth nitrates were made by dissolving the powder of rare-earth oxides in hot nitric acid.

Synthesis of LRH crystals and exfoliation of LRH nanosheets. First, 2 mmol rare-earth nitrate solution of Gd(NO₃)₃ and Eu(NO₃)₂ (Gd:Eu molar ratio of 95:5) was prepared by stirring the mixed solution at room temperature. After adding a proper NH₄OH, the colloidal suspension was adjusted with a pH value around 7. After that, the white mixed suspension was moved to a 100 mL Teflon-lined autoclave. After the hydrothermal reaction, the products were collected through centrifugation. The LRH crystals were exfoliated into nanosheets by hydrothermal anion exchange followed by mechanical agitation in formamide, according to our previous work [20]. The dodecyl sulfate (DS⁻, C₁₂H₂₅OSO₃⁻)-intercalated LRH was obtained like the first preparation [23]. The products after ion exchange are called LRH-DS in the later text.

Film fabrication of GAP transparent ceramic. Exfoliated nanosheets were spin-coated on a substrate of amorphous alumina and then calcined at selected temperatures with the reaction time of 2 h. Through an interfacial reaction, films of GdAlO₃ (GAP) transparent ceramic were grown on the substrate.

2.2. Characterization

Phase identification was performed by X-ray diffractometry (XRD, Model SmartLab, Rigaku, Tokyo, Japan) under 40 kV/40 mA, using nickel-filtered Cu-Kα radiation (λ = 0.15406 nm). The scanning speed was 6.0°/2θ per minute and the scanning range was 5°–60°. The FT-IR spectra were performed by Fourier transform infrared spectroscopy (FT-IR, Model Nicolet iS5, Thermo Fisher Scientific, Madison, WI, USA) using the standard KBr method. The morphology and microstructure of the products were analyzed by field emission scanning electron microscopy (FE-SEM, Model JSM-7001F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM, Model JEM-2000FX, JEOL, Tokyo, Japan). A fluorescence spectrophotometer (Model FP-8600, JASCO, Tokyo, Japan) was used for luminescence analysis. It was equipped with a 150 W Xe-lamp and an integrating sphere (Model ISF-513, JASCO, Tokyo, Japan).

3. Results and Discussion

3.1. Synthesis of LRH Crystals and Exfoliation of Nanosheets

The XRD patterns of LRH are shown in Figure 1a. The synthesized product is layered rare-earth hydroxide, since the diffraction peaks match well to the reported diffraction data. The diffraction peaks of (002) and (004) confirm the unique layered structure, while the appearance of (220) confirms that the compounds have a well-developed layered structure [20,21]. It is interesting to find that the (00l) reflections shift strongly to the lower-angle side, whereas the (hk0) reflections remain at the same position after the anion exchange with DS⁻. This phenomenon can be well understood by the layer distance increase in LRH-DS, which results in a significant change in the crystal plane perpendicular to the host layer without affecting other planes [24,25]. There are no other impurities, implying that the products have a single phase of LRH. Therefore, the incorporation of Eu³⁺ did not affect the XRD diffraction peaks of LRH (Figure 1a). Through the anion exchange with DS⁻, the LRH-DS still remains the layered characteristic, but the layer distance becomes significantly larger than that for the original LRH (Figure 1a). Close observation of the small-angle XRD in Figure 1b shows that the 2θ value of LRH-DS shifts to the side of the smaller angle, so the interlayer distance increases from ~0.84 nm for the original LRH to ~2.61 nm for the ion-exchanged LRH-DS. Figure 1c shows FT-IR spectra for the original LRH and ion-exchanged LRH-DS, which further confirms the completed reaction of ion exchange. The absorption peak at a wavenumber larger than 3500 cm⁻¹ indicates the existence of o hydroxyl (OH⁻), and the absorption peaks at a wavenumber range larger than 3000 cm⁻¹ but smaller than 3500 cm⁻¹ indicate the existence of H₂O. At the same time, the shoulder peak at ~1634 cm⁻¹ also indicates the existence of H₂O. The above results confirm that there is molecular water in the compounds. For the LRH sample, there is a sharp absorption peak at ~1389 cm⁻¹, which confirms the existence of free NO₃⁻ [26,27,28]. However, the vibration of free NO₃⁻ disappears for ion-exchanged LRH-DS. Instead, new absorption peaks at ~1050 cm⁻¹, ~1173 cm⁻¹, ~1466 cm⁻¹, ~2845 cm⁻¹, ~2915 cm⁻¹, and ~2962 cm⁻¹ are found through anion-exchange processing, which are assigned to the vibrations of OSO₃⁻, -CH₂, and -CH₃, respectively, indicating that C₁₂H₂₅SO₄⁻ (DS⁻) has replaced the NO₃⁻ of LRH via anion exchange [26,27]. The FE-SEM images of LRH are shown in Figure 2a. The products are platelets with a lateral size of 1–2 μm, and the surface is smooth. However, they are irregular platelets rather than hexagon platelets, which are usually observed for LRHs [20,21]. After anion exchange, the thickness of LRH crystals significantly increases, and the interlayers are found to be obviously loose for the LEH-DS sample (Figure 2b). Dispersing the LRH-DS in formamide yielded the transparent suspension containing exfoliated nanosheets (Figure 2c,d). Under laser beam irradiation, the suspension exhibited a clear observable Tyndall effect (Figure 2e). The TEM observation finds that the exfoliated nanosheets are rather thin, but the surface is not smooth, with flocculent molecules on the nanosheets, which may be the residual DS⁻. The nanosheets exfoliated from LRH are the important reaction source of rare earth for the fabrication of ceramic films.

3.2. Preparation and Characterization of GAP Ceramic Film

Figure 3 shows the schematic illustration for fabricating ceramic film of GAP and the formation mechanism. The exfoliated LRH nanosheets were directly prepared on the substrate of Al₂O₃ and calcined at selected temperatures for 120 min. The XRD patterns of prepared films at different temperatures are shown in Figure 4. After calcination at 800 °C, only cubic-structured Gd₂O₃ is found, which is converted from the exfoliated LRH nanosheets on Al₂O₃ substrate (Figure 4). Elevating the temperature from 800 °C to 1000 °C yields a small trace of Gd₄Al₂O₉ (GAM) along with the main Gd₂O₃, indicating that Al₂O₃ begins to react with Gd₂O₃ at the interface. When the temperature reaches 1300 °C, the main phase for the ceramic film is GdAlO₃, along with a small amount of Gd₃Al₅O₁₂ (GAG), Gd₄Al₂O₉ and crystalline Al₂O₃. Further increasing the temperature up to 1550 °C finally yields GdAlO₃ (GAP). The above results indicate that GAP is formed at a temperature higher than 1300 °C. Because the LRH nanosheets react with the amorphous alumina substrate at the interface, the temperature-dependent element diffusion takes the dominant role, and more reactants participate in the reaction with increasing calcination temperature. At the low temperature of 800 °C, the LRH transforms into Gd₂O₃ and Al₂O₃ does not react with Gd₂O₃, because element diffusion does not happen. However, at a temperature of 1000 °C, some element diffusion takes place from Gd₂O₃ and Al₂O₃, which leads to the appearance of a small amount of Gd₄Al₂O₉ along with the main-phase Gd₂O₃. The element diffusion becomes stronger at the higher temperature of 1300 °C, and the reaction between Gd₂O₃, Al₂O₃, and Gd₄Al₂O₉ is more thorough, which finally contributes to the main GdAlO₃ and a small amount of Gd₃Al₅O₁₂ and Gd₄Al₂O₉. Because the thicknesses of the films are not uniform, the amount of Gd₂O₃ that participates in the reaction is not the same, and the resultant phases are not the same at different areas of the ceramic film. However, the almost uniform film that makes the dominant phase is GdAlO₃. At a temperature of 1300 °C, the crystallization of the amorphous alumina takes place, so some crystalline Al₂O₃ appears in the films. However, the Al₂O₃ for the interface reaction is completely in excess (Figure S1), so all the Gd₂O₃ reacts with Al₂O₃ to form GdAlO₃ at a temperature of 1550 °C. All crystalline Al₂O₃ at the interface participates in the reaction, so there is no crystalline Al₂O₃ in the film or a small trace of crystalline Al₂O₃ under the film that is undetectable.

The SEM morphology of the films calcined at 1300 and 1550 °C is shown in Figure 5a,b. Traditionally, the obtained grain size of bulk ceramic is around 10–30 μm, because the ceramic is prepared by the traditional method at a high temperature range from 1700 °C to 1800 °C with the help of vacuum sintering [29,30,31]. Here, the sizes of most grains for the ceramic film at 1300 °C are 0.3–1 μm (Figure 5a). The grain size is up to 2–5 μm with the temperature elevated up to 1550 °C (Figure 5b). However, the grains are much smaller than those in bulk ceramic. First, the reaction temperature for ceramic film (1550 °C in air) is lower than that for bulk ceramic. Second, the reaction takes place on the interface, so the element diffusion is interface diffusion, which is rather slower than the volume diffusion for bulk ceramic. Therefore, the growth speed of the grains is much smaller than that for bulk ceramic.

3.3. Optical Properties of GAP:Eu³⁺ Transparent Ceramic Film

Transmittance of the prepared GAP:Eu³⁺ ceramic film was analyzed in Figure 5c. A broad and strong band is found in the transmittance curve at the range of 250–400 nm, which is arising from the charge-transfer absorption of O²⁻-Eu³⁺ [32]. The shoulder near the intense band is assigned to the ⁸S_7/2-⁶I_J transition of Gd³⁺ [33]. However, the transmittance curve is gentle and the value keeps above 90%, indicating that the transmittance of ceramic film is higher than 90%. Because the bare amorphous alumina substrate has a transmittance of ~99%, the ceramic film with the transmittance higher than 90% is highly transparent. Putting ceramic film (coated on the amorphous alumina) on the words shows that the words can also be observed clearly (Figure 5d), which confirms that GAP:Eu³⁺ ceramic film has high transmittance.

Figure 6 shows the photoluminescence excitation emission spectra of GAP:Eu³⁺ transparent ceramic film. Monitoring the wavelength at 618 nm, a broad and strong band is observed in PLE spectra at the range of 200–300 nm, whose maximum is located at 254 nm. The electron transition of O²⁻-Eu³⁺ mainly contributes to the intense band (Figure 6a), which is usually called the charge-transfer band (CTB) [32]. However, the very weak peaks at the wavelength range of 300–500 nm are assigned to the intra-4f electronic transitions of the Eu³⁺ ion, but they are almost invisible because of the rather strong CTB. Under the UV light excitation at 254 nm, the GAP:Eu³⁺ transparent ceramic film outputs a red emission with multiple sharp peaks. The emission peaks at the range of 500–800 nm are assigned to the typical transition of Eu³⁺ from the excited energy level of ⁵D₀ to the ground energy level of ⁷F_J (J = 0, 1, 2, 3, 4) [32]. Because the relative intensity of different J-level transitions is closely related to the symmetrical environment of Eu³⁺ ion, the coordination environment of Eu³⁺ in the host can be evaluated by the intensity of Eu³⁺ transitions. On the basis of the Judd–Ofelt theory, when the Eu³⁺ ion occupies a site with an inversion center, the magnetic dipole transition of ⁵D₀-⁷F₁ is allowed, but the electric dipole transition of ⁵D₀-⁷F₂ is forbidden [34,35]. In Figure 6b, the intensity of ⁵D₀-⁷F₁ transition at 595 nm is obviously weaker than that of ⁵D₀-⁷F₂ transition at 618 nm, so most Eu³⁺ ions occupy an asymmetry site. The symmetry of these sites is lower than that of the perfect crystal (D_2d), which leads to the appearance of ⁵D₀-⁷F₂ transition at 618 nm and thus contributes to its emission intensity. Notably, the width of the peak around 617 nm indicates the overlap of bands corresponding to ⁵D₀-⁷F₂ transitions (Figure 6b). Therefore, the outputted signal is red instead of orange. The CIE coordinate diagram of transparent ceramic film indicates that the color coordinate is (0.644, 0.355), located in the region of red color (Figure 6c). The CIE coordinate for the red emission of Eu³⁺ ions is close to the value in Ref. [13]. It not only depends upon the asymmetric ratio, but also on the higher energy emission levels. The appearance of the transparent ceramic film under UV light directly confirms the emission color is red. The transparent ceramic film also exhibits a stable thermality since its emission intensity only loses about 17% by increasing the environment temperature from 25 to 150 °C (Figure S2).

The lifetime of GAP:Eu³⁺ ceramic film is analyzed in Figure 6d through the fluorescence decay curve. The tested data are matched well with a single exponential, according to the following formula: $I=A\exp \left(-t/\tau \right)+B$, where τ is fluorescence lifetime (ms), t is decay time (ms), I is fluorescence intensity, and A and B are constants [19,36,37]. Through the calculation from the formula, the fluorescence lifetime of the film is determined to be ~1.570 ms. For most Eu³⁺-doped oxides, the lifetimes in the literature are in the range of 1.4–2.0 ms, which are in the domain of the lifetimes for transparent ceramic film [38,39,40].

4. Conclusions

In this work, the two-dimensional guidance strategy has been successfully carried out for the perovskite-structured aluminate ceramic film. Through the interfacial reaction, GdAlO₃:Eu³⁺ (GAP:Eu³⁺) films of transparent ceramic were fabricated using the exfoliated layered gadolinium hydroxide nanosheets as the rare-earth source. The final films were tested by the characterization techniques, including XRD, SEM, TEM, FT-IR, PLE/PL spectroscopy, temperature-dependent PL spectroscopy, and luminescence decay analysis. The perovskite film of transparent ceramic can be obtained by calcining the LRH nanosheets on the substrate of amorphous alumina at 1550 °C in air for 120 min. Because the reaction takes place at the interface, the temperature-dependent element diffusion takes the dominant role and more reactants participate in the reaction with increasing calcination temperature. The grains for ceramic film increase up to 2–5 μm by elevating the temperature to 1550 °C, but the grains are much smaller than that for bulk ceramic. The lower temperature and interface diffusion contribute to the smaller grains. The ceramic film exhibits a high transmittance above 90% at the visible wavelength range. Upon UV excitation at 254 nm, the ceramic film emits intense red light with a lifetime of ~1.570 ms.