Study on the Preparation and Optical Properties of Ce³⁺ Doped GdAlO₃ Nanoparticles by Co-Precipitation Method

Abstract

Nanoparticles of GdAlO₃:Ce were synthesized with sodium dodecylbenzene sulfonate (SDBS) as the dispersant and ammonia as the precipitant by co-precipitation reaction to prepare precursors under different conditions. The phase composition of the precursors and the particle morphology were characterized by thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), and scanning electron microscope (SEM). The excitation and emission spectra of the resultant samples were analyzed using a photoluminescence spectroscope (PL). The results showed that the as-prepared, well-dispersed, nano-sized GdAlO₃:Ce powder displayed spherical morphology at the initial concentration of metallic salt in liquor of 0.3 mol/L; the synthesized temperature was 0 °C, and it was calcined at 1300 °C for 2 h. The relative intensity of the photoluminescence peak had the maximum value when the Ce³⁺ dopant content was 0.9 mol% (mole fraction). The concentration quench occurred when the Ce³⁺ dopant content exceeded 0.9 mol%, and the peak of the excitation spectrum appeared at a wavelength of 381 nm.

1. Introduction

GdAlO₃ is a remarkable rare earth aluminate luminescent matrix material. It exhibits excellent physical, chemical, and optical stability [1,2,3]. It can improve the optical properties by doping a small number of rare earth ions, such as Eu³⁺ and Tb³⁺. Chen [4] employed the co-precipitation method to synthesize (Gd_0.97–xEu_xTb_0.03)AlO₃ (GdAP:Tb/Eu). Subsequently, the effects of different calcination temperatures and doping amounts on its luminescence properties were investigated. (Gd_0.97–xEu_xTb_0.03)AlO₃ (x = 0.005–0.07) phosphors were synthesized with the co-precipitation method using ammonium bicarbonate as the precipitant. At the optimal excitation wavelength of 275 nm, the phosphors with good dispersion exhibited strong red emission at 617 nm. The emission intensity varied with the amount of Eu³⁺, and the quenching concentration was about 5 at%. The phosphors prepared in this research can be widely used in solid-state displays and light-emitting devices. Teng [5] synthesized the precursor by carbonate precipitation and obtained a powder of (Gd_1–xTb_x)₃Al₅O₁₂ (GdAG:Tb³⁺) solid solution by calcination at 1500 °C. Low Tb³⁺ doping could stabilize the crystal structure of GdAG garnet and prevent its thermal decomposition. GdAG:Tb³⁺ phosphors exhibited a series of ⁵D₄ → ⁷F_J transitions of Tb³⁺ under UV excitation at 277 nm, with the strongest green emission around 544 nm. The quenching concentration of GdAG:Tb³⁺ was about 10 at%. Maintaining the optimal Tb³⁺ content at around 10 at%, the (Gd_0.9Tb_0.1), AG fluorescent powder exhibited high internal and external quantum efficiencies of 88.7% and 73.6%, respectively, under excitation at 277 nm. The developed (Gd_1–xTb_x) AG garnet phosphor has the potential to be used as a novel luminescent material. Ce³⁺ has small optical electronegativity, so it can realize the complete transition from orbit 5d to 4f, and the luminescence attenuation time is less than 100 ns. Therefore, it has attracted much attention as an activated ion of scintillator ceramics [6,7,8]. Shilpa [9] prepared GdAlO₃:Ce phosphors using the solution combustion method. Due to the ⁵D_3/2 → ⁷F_J transition (J = 7/2 and 5/2) of Ce³⁺, the photoluminescence emission spectrum shows a broad peak centered at 466 nm. Photometric properties confirmed that the prepared samples emit a cyan color with high color purity. Using its high sensitivity and high resolution, the fingerprint details can be clearly collected on hydrophilic and hydrophobic substrates, and it is expected to be used in forensic research and anti-counterfeiting applications.

The co-precipitation process is simple; its synthetic products have high purity and good morphology. It is widely used in the preparation of luminescent powder [10,11,12]; however, there are few reports on the preparation of GdAlO₃:Ce powder using the co-precipitation method. In this study, we successfully prepared GdAlO₃:Ce nanopowder using the co-precipitation method. In order to understand the properties of the powder and the influencing factors in the preparation process, we used TG-DTA, XRD, SEM, and other analysis methods. Through these analytical methods, we conducted a detailed investigation into the effects of different calcination temperatures and dispersant dosages on phase changes and morphology during the synthesis of GdAlO₃:Ce powder. At the same time, we also investigated the influence of the Ce³⁺ doping concentration on the excitation and emission spectra of the powder. On this basis, we further analyzed the nucleation mechanism of GdAlO₃:Ce, aiming to identify the optimal preparation process conditions. This research outcome will provide a solid experimental basis and theoretical foundation for the application of GdAlO₃:Ce nanopowder, and the research results are expected to promote its widespread use in fields such as luminescent materials and scintillators.

2. Materials and Methods

2.1. Raw Materials and Synthesis Process

As materials, we used Gd₂O₃ (99.9%), Al(NO₃)₃·9H₂O (99%), Ce(NO₃)₃ (99.9%), C₁₈H₂₉NaO₃S (90.0%), NH₃·9H₂O (28%), and HNO₃ (68%), which were all purchased from China Pharmaceutical Group Co., Ltd. (Beijing, China). The samples were tested using a TG-DTA analyzer (HCT-3), a powder X-ray diffractometer (Rigaku D/max−2500Pc, Rigaku Corporation, Tokyo, Japan), a HITACHI S-3500N model scanning electron microscope (Hitachi High-Tech Corporation, Tokyo, Japan), and a photoluminescence spectroscope (Hitachi F-4600, Hitachi High-Tech Corporation, Tokyo, Japan), etc.

The GdAlO₃:Ce powder was prepared using Gd₂O₃, Al(NO₃)₃·9H₂O, and Ce(NO₃)₃ as starting materials with the co-precipitation method. A 500 mL mixed solution was prepared, which had a cation concentration of 0.3 mol/L and contained Gd³⁺ and Al³⁺ in stoichiometric amounts with a Gd:Al molar ratio of 1:1. Ce(NO₃) was added to the solution, and its mole fraction was adjusted within the range from 0.3 mol% to 1.1 mol%. A dispersant, C₁₈H₂₉NaO₃S (1–3%), was added to the solution. Then, the solution was titrated using 0.5 mol/L NH₃·H₂O as a precipitant with a titration rate of 3 mL/min; the temperature of the reaction system was 0 °C, and the titration endpoint pH was adjusted to 10. The solution was stirred at a constant temperature for 5 h until white flocculent precipitation was formed. The obtained precursor precipitate was washed three times with deionized water and anhydrous ethanol successively and then dried in a vacuum drying oven. Finally, the dried precursor was ground and calcined at 1100–1400 °C for 2 h with a heating rate of 5 °C/min, and thus GdAlO₃:Ce was obtained.

2.2. Characterization

The crystallization process of the GdAlO₃:Ce precursor was analyzed using a TG-DTA analyzer (HCT-3, Beijing Hengjiu Experimental Equipment Co., Ltd., Beijing, China). The phase composition of the samples was examined using an X-ray diffractometer (Rigaku D/max−2500Pc) in continuous scanning mode, covering a 2θ range from 20° to 70°. The morphology and size of the GdAlO₃:Ce particles were characterized using a HITACHI S-3500N scanning electron microscope. The excitation and emission spectra of GdAlO₃:Ce samples were analyzed using a photoluminescence spectroscope (Hitachi F-4600) with a PMT voltage of 700 V and a scanning rate of 240 nm/min.

3. Results

3.1. TG-DTA Analysis of Precursors

As depicted in Figure 1, the TG-DTA curve of the precursor precipitate with 0.9 mol% Ce³⁺ doping and 2% SDBS dispersant addition illustrated its weight loss process, which was primarily divided into three stages. The initial weight loss stage occurred between 0 and 275 °C, with the sample losing 9.30% of its weight. A small endothermic peak appeared at 196.31 °C, which was attributed to the evaporation of residual and crystalline water in the precursor. The second weight loss stage took place from 275 to 550 °C, resulting in a 27.19% weight loss. Three endothermic peaks were observed at 311.27 °C, 325.28 °C, and 476.78 °C, corresponding to the thermal decomposition of Al(OH)₃ and Gd(OH)₃ in the precursor to release H₂O. The exothermic peak at 476.78 °C was due to the combustion of the dispersant SDBS in the precursor. The third weight loss stage occurred between 550 and 1000 °C, with a weight loss of only 2.90%. An exothermic peak at 682.61 °C indicated a crystal transformation of GdAlO₃ at this temperature, transitioning from an amorphous to a crystalline state. At this temperature, the precursor shifted from an amorphous disordered structure to a crystalline ordered structure and from a high-energy state to a low-energy state, resulting in an exothermic phenomenon. In summary, the reaction formula during the calcination process can be encapsulated as follows.

$$\begin{gathered} \mathrm{Gd}{\left(\mathrm{OH}\right)}_3·\mathrm{x}{\mathrm{H}}_2\mathrm{O}+\mathrm{Al}{\left(\mathrm{OH}\right)}_3·\mathrm{y}{\mathrm{H}}_2\mathrm{O}+\mathrm{Ce}{\left(\mathrm{OH}\right)}_3·\mathrm{z}{\mathrm{H}}_2\mathrm{O} \underrightarrow{ 0 °\mathrm{C}–275 °\mathrm{C}} \\ \mathrm{Gd}{\left(\mathrm{OH}\right)}_3+\mathrm{Al}{\left(\mathrm{OH}\right)}_3+\mathrm{Ce}{\left(\mathrm{OH}\right)}_3 \underrightarrow{ 275 °\mathrm{C}–550 °\mathrm{C}} \\ {\mathrm{Gd}}_2{\mathrm{O}}_3+{\mathrm{Al}}_2{\mathrm{O}}_3+{\mathrm{Ce}}_2{\mathrm{O}}_3 \underrightarrow{ 550 °\mathrm{C}–1000 °\mathrm{C}} \mathrm{Gd}\mathrm{Al}{\mathrm{O}}_3:\mathrm{Ce} \end{gathered}$$

3.2. Influence of Phase and Morphology of GdAlO₃:Ce by Calcination Temperature

Figure 2 presents the XRD pattern of precursor precipitates doped with 0.9 mol% Ce³⁺ and calcined at various temperatures for 2 h. It was evident that the XRD spectra of the samples calcined at temperatures ranging from 1100 to 1400 °C were largely identical, with the diffraction peaks aligning closely with those in the GdAlO₃ standard PDF#46-0395. This was due to the small doping amount of Ce³⁺, which was not enough to affect the crystal structure of matrix GdAlO₃. By comparing the curves of four samples, it could be seen that the full width at half maximum (FWHM) of the diffraction peak was directly proportional to the crystallinity. As illustrated in Figure 2 and

Table 1. FWHM and peak intensity of (112) plane diffraction peaks at different calcination temperatures.

Temperature/°C	FWHM	Peak Intensity
1100	0.21616	190
1200	0.20598	234
1300	0.19570	240
1400	0.20014	229

, when the temperature was at 1100 °C, the XRD peak intensity of the calcined sample was relatively low, and the FWHM was broad. However, as the temperature rose to 1200 °C, the peak intensity of the calcined sample intensified, and the FWHM narrowed. Comparatively, it could be seen that the XRD diffraction peaks of the sample obtained at a calcination temperature of 1300 °C exhibited the best FWHM and peak values, indicating its superior crystallinity. As the calcination temperature increased to 1400 °C, the impurity peak started to emerge in the XRD pattern of the resulting sample.

Figure 3 presents the SEM images of GdAlO₃:Ce powder subjected to various calcination temperatures. It was evident that when the calcination temperature was 1100 °C, only a small part of the grains in the sample were nearly spherical, and the grains were not completely grown. When the calcination temperature reached 1200 °C, the spherical morphology of grains could be seen obviously, but the grain boundaries were still difficult to distinguish. After calcination at 1300 °C, the sample clearly showed the grain morphology and grain boundary. At 1400 °C calcination, the grain size of the sample grew significantly, and the agglomeration phenomenon was severe. The appearance of large particle agglomeration on the SEM images was a manifestation of the macroscopic aggregation state of materials. During the material preparation process, multiple grains aggregated due to physical effects such as van der Waals forces, electrostatic attraction, and precipitation rates, forming these larger solid units.

Using the Scherer formula, the grain size, lattice parameters, and cell volume of GdAlO₃:Ce at various calcination temperatures were calculated, with the results presented in

Table 2. The grain size, lattice parameters, and unit cell volume of GdAlO₃:Ce derived from precursors calcined at various temperatures for 2 h.

Temperature/°C	Grain Size/nm	Lattice Parameters/Å			Unit Cell Volume/Å3
		a	b	c
1100	116.4	5.2534	5.3007	7.4498	207.4523
1200	138.8	5.2574	5.3015	7.4441	207.4827
1300	143.3	5.2496	5.3075	7.4563	207.7493
1400	489.2	5.2450	5.3062	7.4762	208.0702

In PDF#46-0395, the values are as follows: a = 5.2511 Å; b = 5.3017 Å; c = 7.4450 Å.

.

The grain sizes recorded in Table 2 were focused on a single crystal unit and accurately reflect the size of the smallest periodic structural unit in which the atoms inside the crystal are arranged in an ordered manner. As observed in Table 2, the grain size of GdAlO₃:Ce expanded from 116.4 nm to 489.2 nm with a rise in calcination temperature from 1100 °C to 1400 °C. The grain size of GdAlO₃:Ce powder gradually increased with the increase in temperature, suggesting that grain growth tended toward completion, and all grains remained in the nanometer range. When the temperature hit 1300 °C, the lattice parameters of the powder neared the standard values, leading to the conclusion that this temperature represents the optimal calcination condition.

3.3. The Impact of Dispersant Concentration on GdAlO₃:Ce

Figure 4 presents the XRD pattern of the precursor precipitate synthesized by incorporating varying amounts of dispersant with subsequent calcination at 1300 °C for 2 h. It was evident that when the dispersant content was 1%, the XRD spectrum of the sample exhibited high peak intensity and precise peak positions. The peak shapes were essentially regular, featuring moderate half-height widths, and there were additional impurity peaks present in the spectrum. However, there were some weak heteropeaks in the spectrum. Compared with the XRD spectra of the samples with 1% and 2% dispersant addition, the intensity of the characteristic peaks of the sample XRD spectra with 3% dispersant addition was slightly weak; there was a deviation in the positions of the peaks, with some miscellaneous peaks. It was obvious that the diffraction peak of impurity Ce_0.8Gd_0.2O_2−x appeared at the 2θ = 33.026° position and the diffraction peak of impurity Gd₃Al₅O₁₂ appeared at the 2θ = 52.749° position. Gd₃Al₅O₁₂ was a body-centered cubic structure in a cubic system, while GdAlO₃ had to face-centered cubic structure, indicating that the doping of Ce³⁺ and the high calcination temperature changed the crystal structure of GdAlO₃. Compared with the XRD spectra of the samples with 1% and 3% dispersant, the samples with 2% dispersant had the strongest characteristic peak and the narrowest FWHM, and all the diffraction peaks on the spectra completely corresponded to the standard card diffraction peaks.

Figure 5 shows the SEM morphology of GdAlO₃:Ce powder prepared with different dispersant contents. It was evident that when the dosage of the dispersant reached 1%, a few near-spherical grains appeared, but the grains were not dispersed, and the agglomeration was obvious. When the amount of dispersant was 2%, the grains of the sample were well-dispersed, the nearly spherical grains and grain boundaries could be clearly seen, and the composition was relatively uniform. The grain and the grain boundary of the sample with a dispersant content of 3% were not obvious. Some other substances that were quite different from the main phase grain could be observed, which was due to the impurities contained in the sample. Hence, the ideal quantity of dispersant was ascertained to be 2%.

3.4. Effect of Ce³⁺ Dopant Content on GdAlO₃:Ce Nanoparticles

Figure 6a presents the XRD pattern of samples doped with varying Ce³⁺ dopant concentrations following calcination at 1300 °C for 2 h. It appeared that the characteristic peak intensity of the 0.3 mol% Ce³⁺ dopant sample was high, and the FWHM was narrow. The sample with 0.5 mol% Ce³⁺ had the weakest diffraction peak, an irregular peak type, and multiple heteropeaks. Although the intensity of the characteristic peak of the doped 0.7 mol% Ce³⁺ sample was moderate and the spectrum showed no impurity peak, the FWHM was wide. Compared with other doping concentration samples, the diffraction peak intensity of doped 0.9 mol% Ce³⁺ samples was the highest; the FWHM was narrow, and there was no impurity peak. When the Ce³⁺ doping concentration increased to 1.1 mol%, the diffraction peak intensity of the sample was weak, and the peak shape was irregular and accompanied by heteropeaks. At the same time, comparing the diffraction peaks of samples with different Ce³⁺ dopant content, it can be seen in Figure 6b that the main peak in the diffraction spectrum tended to shift to the left side with an increase in doping concentration.

3.5. Effect of Ce³⁺ Dopant Content on Luminescence Characteristics

Figure 7 shows the excitation spectra of GdAlO₃:Ce samples prepared with different Ce³⁺ dopant contents. GdAlO₃:Ce had strong excitation bands at 381 nm and 437 nm, corresponding to the transition absorption from the Ce³⁺ 4f to 5d energy levels. Since Ce³⁺ had only one electron in 4f, spin orbit coupling decomposed 4f into two levels: ²F_5/2 and ²F_7/2. When Ce³⁺ entered the lattice position of Gd³⁺, 4f was less affected by the crystal field due to the shielding of the outside 5s²5p⁶ electrons, and still retains the LS coupling energy level characteristics of free ions [13,14,15]. Because the 5d energy level was affected by the crystal field and was under the condition of low symmetry, the 5d energy level was divided into multiple sub-levels [16,17,18].

Figure 8 shows the emission spectra of different Ce³⁺ doped GdAlO₃:Ce samples excited at a 381 nm wavelength. Due to the influence of its own energy transfer and cross-relaxation mechanism, the sample had a wide luminescence spectrum in the wavelength range of 460–570 nm. The emission spectrum had maximum peaks at the wavelengths of 491 nm and 553 nm, corresponding to the 5d → ²F_5/2 and 5d → ²F_7/2 luminescence transitions of doped Ce³⁺ [19,20,21]. Under the excitation of the 381 nm wavelength, the fluorescence intensity reached the peak when the doping amount of Ce³⁺ was 0.9 mol%. When the doping amounts of Ce³⁺ were 0.7 mol%, 0.5 mol%, and 0.3 mol%, the emission spectral intensity decreased in turn. When the doping amount of Ce³⁺ increased to 1.1 mol%, the fluorescence intensity of the emission peak decreased significantly. Since Ce³⁺ was the light-emitting center in GdAlO₃:Ce crystal, with an increased Ce³⁺ dopant content, the number of light-emitting centers also increased and the transition intensity improved, which increased the peak intensity of the emission spectrum [22,23,24]. When the concentration of Ce³⁺ reached a certain value, the probability of non-radiation transition increased, so the luminous efficiency decreased and concentration quenching occurred [25,26].

4. Conclusions

In this study, GdAlO₃:Ce was effectively produced using the co-precipitation technique. By characterizing and analyzing the resulting powder, the optimal synthesis process and the fluorescence characteristics of the GdAlO₃:Ce were identified.

• Nano GdAlO₃:Ce powder exhibiting excellent dispersion and an almost spherical shape could be synthesized via the co-precipitation technique. In this process, sodium dodecylbenzene sulfonate was utilized as a dispersant at a mass fraction of 2%, and ammonia served as the precipitant. The resulting powder was then calcined at 1300 °C for a duration of 2 h.
• Ce³⁺ doping caused GdAlO₃ lattice distortion, but it did not change its crystal structure. When the Ce³⁺ dopant content was 0.9 mol%, the emission spectral intensity of GdAlO₃:Ce was the strongest after excitation. Concentration quenching occurred when the Ce³⁺ dopant content exceeded this concentration.
• In the spectral analysis, GdAlO₃:Ce had two wide excitation spectra, and the highest peaks were located at wavelengths of 381 nm and 431 nm. The formation mechanism of GdAlO₃:Ce luminescence due to the transition of 5d → ²F_5/2 and 5d → ²F_7/2 luminescence energy bands after doping Ce³⁺ was excited.