F-127-Assisted Sol-Gel Synthesis of Gd₂O₃:Eu³⁺ Powders and Films

Abstract

In the current work, the influence of Pluronic F-127 (S = F-127) and temperature on the luminescent properties of Gd₂O₃:Eu³⁺ (Gd:S = 1:2) powders and films was studied. In order to synthesize the powders and films (by the dip-coating technique), Gd₂O₃: Eu³⁺ (5 mol%) ceramics were elaborated by the sol-gel route, using gadolinium and europium nitrates as precursors. The results obtained by means of X-ray diffraction, confirmed the presence of the cubic structure of Gd₂O₃ (in 800 ºC heat-treated powders and 700 ºC heat-treated films), and crystals with nanometer sizes of ~19 nm, and ~15 nm, corresponding to the spherical and laminar-like morphologies of densified powders and films, respectively. Crystallites from the cubic and monoclinic structure were present on Gd₂O₃: Eu³⁺-modified films up to 800 ºC. Chemical identification of the bonds present in the films was performed by Fourier transform infrared spectroscopy, which identified representative infrared absorption at 543 cm^-1, attributable to the Gd-O vibration. Photoluminescence studies showed that when the powders and films were heat-treated at 800 ºC, the intensity of their luminescence at the ⁵D₀→⁷F₂ Eu³⁺ transition (618 nm) was enhanced by the presence of F-127.

Keywords:
Gd₂O₃; Pluronic F-127; sol-gel; powders; films; europium

1. Introduction

The luminescence of rare earth (RE)-doped oxides (Ln₂O₃, Ln = RE) has been of great interest in recent decades, due to their applications in optoelectronic devices and to their sufficient brightness, high chemical stability, low phonon energy, and long-term stability ^1,2,3. Specifically, Gd₂O₃, considered an appropriate matrix for doping with europium due to its good luminescent characteristics and low phononic energy, presents a characteristic emission at 612 nm, corresponding to the transition ⁵D₀ - ⁷F₂ in the Eu used in panel display devices, such as PDPs ⁴, electroluminescent devices (ELDs) ⁵, fluorescent lamps ⁶, and so forth. Surfactant-modified Gd₂O₃:Eu³⁺ systems are promising alternatives for practical applications involving the development of nanodevices ⁷. Moreover, the luminescent properties of these systems depend on their morphology, size, and synthetic route ⁸. There are several methods for preparing Gd₂O₃: Eu⁺³, such as by the combustion ⁹, Pechini ¹⁰, sol-gel ^11,12, polyol ^13,14 and hydrothermal ¹⁵ methods, as well as others, giving rise to different morphologies and particle sizes, making it possible to modify the intensity of the luminescent emissions.

Some reports point to nanoflowers particles as promising candidates for applications in field emitters because of their thin open edges ¹⁶. In this work, we report on the synthesis of Gd₂O₃:Eu³⁺ powders and films modified with a surfactant (Pluronic F-127) by the sol-gel route, and on their influence on emission properties.

Their influence of Gd₂O₃:Eu³⁺ on structural and morphological characteristics was analyzed by infrared spectroscopy (FTIR), X-ray diffraction, and SEM, and the emission characteristics of the as-prepared powders and films were investigated using photoluminescence.

2. Experimental

2.1 Synthesis of Gd₂O₃:Eu³⁺ with F-127 surfactant

Gd₂O₃: Eu³⁺ powders and films synthesized in the presence of F-127 were prepared using gadolinium nitrate Gd(NO₃)₃ (99.9% Signa Aldrich), ethanol (C₂H₆O) (Fermont 99.9%), and europium (III) nitrate (Eu(NO₃)₃) (99.9% Alfa Aesar). The molar composition of the sol was Gd:Eu:C₂H₆O = 1:0.18:0.038. The gadolinium nitrate was dissolved in ethanol for 15 min. Thereafter, europium nitrate was added to the gadolinium sol under vigorous stirring at room temperature for 1 h in order to get the desired concentration (5 mol% Eu³⁺). The established Eu was used because concentration quenching may occur in heavily Eu³⁺-doped systems, arising from the distance-dependent, non-radiative, cross relaxation between neighboring Eu³⁺ ions ¹⁷. In order to prepare surfactant-modified Gd₂O₃:Eu³⁺ systems, F-127 (monomer atomic weight = 102 gmol^-1, Sigma Aldrich) chemical modifier precursor was added to the stable europium-doped gadolinium sol. Both compounds were dissolved (Gd:S=1:2) at room temperature in a dry box under nitrogen flux (humidity level < 3%). The europium-doped modified gadolinium solution was filtered through 0.2 µm and deposited on carefully cleaned pure silica substrates using the dip-coating technique ¹⁸ with a withdrawal speed of 80 mm min^-1, in order to obtain six layers. The layers were heat treated at 350 ºC for 30 min between each coating. The Gd₂O₃:Eu³⁺ precursor solutions were dried at 100 ºC for 24 h. The processed powders and films were finally annealed in air for 1 h at the required temperatures, ranging from 300 to 800 ºC. Crack-free and transparent layers were obtained and studied at room temperature. Both Gd₂O₃:Eu³⁺ powders and films without F-127 were prepared for the purpose of comparison.

2.2 Experimental techniques

Fourier transform medium infrared (FTIR) spectrograms were recorded using a Perkin-Elmer Spectrum 65, in a range of 4000-400 cm^-1, using the KBr pelleting technique. X-ray diffraction (XRD) patterns were recorded at room temperature on a PANalytical θ/θ Bragg Brentano X´Pert MPD PRO diffractometer, with a Cu Ka beam at 40 kV, 20 mA over a 2-θ range of 20º-80º (0.1º/s). The structural properties were also investigated by FTIR (Lambda 2000 Spectrum One, Perkin Elmer) spectroscopy. Scanning electron microscopy (SEM) was performed on the samples with a JEOL JSM-7800F microscope operated at 15 kV. The emission spectra were measured at room temperature with a Hitachi F-7000 spectrophotometer equipped with a 150-W xenon lamp as the excitation source.

3. Results and Discussion

3.1 Structural and morphological characterization of Gd ₂O₃:Eu³⁺/F-127 powders and films

The X-ray diffraction patterns of the Gd₂O₃:Eu³⁺ (5 mol%) powders and films heat-treated at temperatures ranging from 300 to 800 ºC and synthesized with F-127 are depicted in Fig. 1a and Fig. 1b, respectively. Fig. 1c shows the X-ray diffraction patterns of the Gd₂O₃:Eu³⁺ (5 mol %) powders and films heat-treated at 800 ºC and synthesized without F-127.

Four main characteristic peaks are clearly observable in the XRD patterns in Fig. 1a at a temperature of 800 ºC.

Figure 1
X-ray diffraction patterns of Gd₂O₃:Eu³⁺ (a) powders, (b) films synthesized with F127 and heat-treated at different temperatures, (c) powders and films synthesized without F127 and heat-treated at 800 ºC, (d) crystallite sizes for Gd₂O₃:Eu³⁺ powders synthesized with and without F-127 and for films in presence of F-127 at different temperatures.

The observed peaks at (2 2 2), (4 0 0), (4 3 1), (4 4 0), (6 1 1), and (6 2 2) correspond to the characteristic reflection lines of gadolinium oxide powders crystallized into the cubic phase (JCPDS 12-0797). As the temperature decreased, the diffraction intensity remained stable up to 500 ºC, when crystallization started. At lower temperatures, the presence of the band observed around 30º is associated with the presence of an amorphous phase. The sharp and strong peaks demonstrate the high crystallinity exhibited by the Gd₂O₃ samples.

The XRD patterns of the as-prepared films heat-treated at different temperatures (Fig. 1b) show the transformation up to 600 ºC, corresponding to all Bragg reflections, according to standard data on the cubic phase. No peaks, shifts, or other phases appear in the presence of F-127 surfactant up to 700 ºC and 800 ºC for the films and powders, respectively that indicates the high purity of the precursors and their complete conversion to Gd₂O₃:Eu³⁺ at 600 ºC in the cubic phase. Nevertheless, some of the relative peak intensities are very different; this may be due to the <100> preferred orientation exhibited by the films ¹⁹. However, a previous report on Gd₂O₃ without F-127 does not show this behavior ²⁰. Cho et al. demonstrated that preferential orientation occurs when the film grown by nucleation at the lowest strain energy ²¹. An interesting change occurs in Gd₂O₃:Eu³⁺-modified surfactant films-some peaks associated with the monoclinic phase (JCPDS 43-1015) occur at 800 ºC; this effect is probably due to the presence of nanoclusters ²², with some residual stress ²³. This behavior has not been shown for Eu-doped Gd₂O₃ modified surfactant films in previous reports.

In both cases, the presence of doping concentrations of Eu ions in the Gd₂O₃ structure (with a similar electric charge) indicates that Eu³⁺ was uniformly incorporated into the host lattice by substitution. This was due to the fact that the ionic radii for both lanthanide ions were almost the same (0.938 Å for Gd³⁺, 0.947Å for Eu³⁺) ²⁴, which did not affect the host structure of the Gd₂O₃. Finally, the C-type structure of Gd₂O₃ offers two nonequivalent sites for lanthanide doping ions: C₂ (non-centrosymmetric) and S₆ (centrosymmetric). In both crystalline structures, the coordination environment of the lanthanide doping ions occupying the two possible sites is C₂:S₆ = 3:1 ²⁵.

The crystallite sizes of both cubic systems (powders and films in the presence of F-127) were simulated, taking into account a Rietveld refinement. The size of the coherent domains was derived from a refinement of the full-width at half-maximum (FWHM), β, of the patterns fitted with pseudo-Voigt functions, according to the following relations ²⁶:

, ${\mathit{\beta }}^2=\raisebox{1ex}{$\mathit{IG}$}\!\left/ \!\raisebox{-1ex}{$\left({\cos }^2\mathit{\theta }\right)$}\right.$

where IG is a measure of the isotropic size effect

$\mathit{d}=\raisebox{1ex}{$\left(180\mathit{K}\mathit{\lambda }\right)$}\!\left/ \!\raisebox{-1ex}{$\mathit{\pi }\sqrt{\mathit{IG}}$}\right.$

where d = the size [Å], λ = wavelength [Å], and K, the Scherrer constant, is equal to 4/3.

Fig. 1d shows the evolution of the crystallite sizes of the Gd₂O₃:Eu³⁺ powders and films as a function of temperature, compared to the Gd₂O₃:Eu³⁺ particles synthesized without F-127. These values were found to increase as the temperature increased; at 800 ºC, the crystallite size for the reference powders corresponded to the largest one, at 33 nm. For samples synthesized in the presence of F-127 and heat-treated at 800 ºC, the crystal sizes observed for the powders and films were 19 nm and 15 nm at the highest temperature. It has been observed that the influence of F-127 on nanophosphors provokes an increment in particle dispersion, blocking the growth of the nanoparticles and modifying their shape and size ^27,28.

In accordance with previous reports, the non-modified and F-127-modified Eu³⁺-doped Gd₂O₃ systems exhibited two important features: (1) there is evidence of some reflection associated with the monoclinic phase in densified films (at 800 ºC) and (2) the preferred orientation was observed only in the F-127-modified Gd₂O₃ sol-gel films, but not in the non-modified films or powders (Fig. 1c). These effects were produced by the presence of an organic additive, causing a reduction of the strain energy involved in the nucleation growth ²⁹.

3.2 FTIR studies

Fig. 2 shows the normalized FTIR spectra of the 5 mol% Eu-doped Gd₂O₃ powders synthesized in the presence of F-127, annealed from 600 to 800 ºC. The broad bands situated around 3500 cm^-1 and 1650 cm^-1 arise from the absorption of O-H stretching (ν) and O-H deformation (δ) vibrations. The peaks in the wavelength range from 1510 to 1390 cm^-1 assigned to NO₃ group (from gadolinium precursor) and the symmetric and asymmetric C=O vibrations ³⁰. All of these intensity bands decreased as the annealing temperature increased and were practically eliminated at 700 ºC, due to evaporation of absorbed water and -NO₃ gases. Nevertheless, the heat-treatment at air atmosphere was not enough to eliminate NO₃ group, but a combined annealing in vacuum and air atmosphere is more efficient to remove remaining organic molecules ³¹. A sharp peak associated with this ligand at 1384 cm^-1 was observed even after 700 ºC heat treatment. The bands around 544 cm^-1 and 438 cm^-1 are due to the characteristic (Gd-O) stretching vibrations of cubic Gd₂O₃^32,33.

Figure 2
IR spectra of the Gd₂O₃:Eu³⁺ powders synthesized in the presence of F-127.

These characteristic bands were observed for the powders heat-treated up to 500 ºC, suggesting that the crystallization process was just starting at this temperature. The Gd₂O₃:Eu³⁺ powders present an amorphous phase at temperatures lower than 500 ºC, in agreement with the XRD results.

3.3 Morphological characteristics

In order to examine the morphology of the Gd₂O₃:Eu³⁺ ceramics and establish the influence of Pluronic F-127 on powders and films, the as-prepared samples were analyzed by means of the SEM technique.

Figs. 3a and d show SEM micrographs of the Gd₂O₃:Eu³⁺ powders and films without F-127, heat-treated at 800 ºC for 1 h. The Gd₂O₃:Eu³⁺ powders synthesized without F-127 (Fig. 3a) exhibit a flower shape 4 µm in size. In the case of the Gd₂O₃:Eu³⁺ films synthesized without F-157 (Fig. 3b), the SEM image reveals a homogeneously dispersed surface constituted by closely packed particles, radially distributed and less than 250 nm in size.

Fig. 3
SEM micrographs of (a) the Gd₂O₃:Eu³⁺ powders and (b) the films without F-127, and (c) the Gd₂O₃:Eu³⁺ powders and (d) the films in the presence of F-127.

Fig. 3c and d show zones of the Gd₂O₃:Eu³⁺ powders and films, respectively, synthesized in the presence of F-127. The morphology of the europium-doped powders reveals a mix of homogeneous spherical particles, well distributed, of approximately 1 and 4 µm in size. The morphological analysis of the Gd₂O₃:Eu³⁺ films with Gd:S = 1:2 revealed a porous surface with round particles promoted by the high F-127 concentration in the layers ³⁴. It is well known that the presence of this molecule (F-127) in the sol plays a crucial role in synthesizing surfactant-modified ceramic films. Surfactants like F-127 reduce the hydrolysis and condensation reaction rates, due to the capping effect of F-127 on the metal precursor ^35,36. This molecule is used to prepare rough and mesoporous surfaces for a number of applications ³⁷. The established surfactant concentration F-127/Gd = 2 seems to determine the collapse of the inorganic ceramic being synthesized, producing a greater number of interstices between particles/aggregates, seemingly a consequence of removing the surfactant molecules before the calcination step.

3.4 Luminescent properties

Figs. 4and 5 show the PL spectra of Gd₂O₃:Eu³⁺ synthesized in the presence of F-127 as a function of temperature (Figs. 4a and 5a), and comparisons between the as-prepared systems synthesized with and without F-127, heat-treated at 800 ºC (Figs. 4b and 5b).

Figure 4
PL spectra of (a) the Gd₂O₃:Eu³⁺ powders synthesized in the presence of F-127 heat-treated at different temperatures and (b) comparison of the Gd₂O₃:Eu³⁺ powders with and without F-127, heat-treated at 800 ºC.

Figure 5
PL spectra of (a) the Gd₂O₃:Eu³⁺ films synthesized in the presence of F-127, heat-treated at different temperatures and (b) comparison of the Gd₂O₃:Eu³⁺ films with and without F-127, heat-treated at 800 ºC.

The emissions arising from the powders (Fig. 4) and films (Fig. 5) are characterized by an intense emission peak situated at 612 nm. Five groups of emission lines are assigned to ⁵D₀→⁷F_J (J = 0, 1, 2, 3), being the most intense the ⁵D₀→⁷F₂ transition, an electric dipole-allowed transition that is hypersensitive to the environment. This strong emission occurs when Eu³⁺ ions occupy the sites without inversion centers inside the host lattice. Fig. 4a (the powders) and Fig. 5a (the films) show the emission spectra for Gd₂O₃:Eu³⁺ synthesized in the presence of F-127, heat-treated from 300 to 800 ºC. The luminescence intensity of the Eu³⁺-modified powders increases with an increase in temperature and improves crystallization. A poor emission intensity was observed for the Gd₂O₃:Eu³⁺ /F-127 powders at temperatures lower than 600 ºC that may be associated with the presence of an amorphous phase for systems annealed at up to 500 ºC.

The effects of temperature on the luminescence properties of both powders and films with and without F-127 were analyzed and compared at 800 ºC. Fig. 4b (the powders) and Fig. 5b (the films) reveal an intensity improved by the presence of the surfactant. These results can be explained by a better densification process and by less associated grain boundary absorption, reducing internal reflections of the emitted light due to rougher surfaces ³⁸. In the case of the Gd₂O₃:Eu³⁺ films annealed at different temperatures, differences can be observed regarding the ⁵D₀→⁷F₀ (581-nm) and ⁵D₀→⁷F₁ (splits at 588, 592 and 599 nm) transitions, because they represent the local environment of the Eu³⁺. For the F-127-modified and non-modified films heat-treated at 800 ºC, the sharp lines located at 581, 588, 592 and 599 nm (Fig. 5b) are associated with the ⁵D₀→⁷F₁ transition, this one is magnetic-dipole-allowed; its intensity shows small variations, with a crystal field strength close to that of the Eu³⁺ ions.

Nevertheless, for the Gd₂O₃:Eu³⁺ in the presence of F-127 heat-treated films at temperatures lower than 800 ºC, these bands are weaker and broader than those observed for modified films thermally treated at 800 ºC. This discrepancy between the emission peaks cannot be attributed to the presence of particles crystallized into the monoclinic phase (Fig. 1b); rather, the existence of mixed cubic and monoclinic phases could induce a spectroscopic adjustment, because the most intense peak, at ⁵D₀→⁷F₂, shows a red shift in the non-modified and F-127-modified films, centered at 611.8 nm and 612.4 nm, respectively. This effect indicates that the emissions obtained from the modified films came from the presence of monoclinic particles aside from cubic ones, with a maximum emission peak at a longer wavelength and blue-shifted for only the cubic nanoparticles ³⁹. A small fraction of the monoclinic nanocrystallites present in the modified Gd₂O₃:Eu³⁺ films failed to produce a luminescent signal, due to the monoclinic environment, because the peaks seen in the emission spectrum of the modified film are narrower and upshifted by the cubic structure. This result shows that the Pluronic F-127 affected the structure and crystalline size of the as-prepared modified films. F-127 is a hydrophilic, non-toxic copolymer with a micellar structure that improves its aqueous dispersity; it plays an effective role in controlling hydrolysis and condensation reactions. In the case of the Gd₂O₃:Eu³⁺-modified films, thermal treatment up to 800 ºC may result in a destructive effect on the final phases, allowing for the formation of monoclinic nanocrystallites that induce a residual stress ⁴⁰. Finally, other factors may explain the differences in the PL emission intensity, depending on the crystal size ^41,42. Some authors have reported variations in the emission intensity of doped nanocrystalline phosphors ⁴³ and optical properties, due to a reduction in the size of particles in the nano-range scale ^44,45. They observed that a decrease in the particle size of doped nanocrystalline systems increases the intensity of PL emissions. This is because the decrease in size causes the exciton wave functions to overlap with those of the doping ions, enhancing the energy transfer rate from the excitons to the doping ions. This, in turn, reduces the non-radiative decay rate and increases the PL emission intensity ⁴⁶. Small crystallites promote a non-radiative de-excitation process, due to the larger number of atoms on the nanoparticles’ surface, unlike larger particles, favoring a Eu³⁺ red emission ⁴⁷.

4. Conclusion

Gd₂O₃:Eu³⁺ powders and films modified and synthesized in the presence of F-127 were prepared by the sol-gel method. The synthesized ceramics exhibit both structural and improved luminescence compared to those of non-modified systems. Europium-active cubic powder and oriented films in the presence of F-127 are characterized by small crystallites of 19 and 15 nm, respectively, after 800 ºC thermal treatment. The results show that a portion of cubic Gd₂O₃ transforms into the monoclinic phase. Nevertheless, at a 254-nm excitation of the Gd₂O₃ host, the monoclinic portion was ineffective, but led mainly to the characteristic emission of Eu³⁺ associated with a cubic structure. Luminescence analyses showed an improved emission from modified Gd₂O₃:Eu³⁺ in both powders and crack-free films due to the F-127 surfactant and a superior densification process. These modified, europium-active and F-127-modified films show promise for phosphors on which display technologies are based.

5. Acknowledgments

The authors gratefully acknowledge the financial support of this work by the SIP-IPN projects 20196322 and 20196329 and by CNMN-IPN experimental support. Victor H. Colín Calderón acknowledges the Conacyt Ms S scholarship. The authors also would like to thank Henry Jankiewicz for the editing work that he did for this paper and M. García Murillo for her assistance.

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Publication Dates

• Publication in this collection
  08 Apr 2019
• Date of issue
  2019

History

• Received
  18 Sept 2018
• Reviewed
  08 Feb 2019
• Accepted
  20 Feb 2019