F-127-Assisted Sol-Gel Synthesis of Gd2O3:Eu 3+ Powders and Films

In the current work, the influence of Pluronic F-127 (S = F-127) and temperature on the luminescent properties of Gd2O3:Eu 3+ (Gd:S = 1:2) powders and films was studied. In order to synthesize the powders and films (by the dip-coating technique), Gd2O3: Eu 3+ (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: Eu 3+-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 D0→ F2 Eu 3+ transition (618 nm) was enhanced by the presence of F-127.


Introduction
The luminescence of rare earth (RE)-doped oxides (Ln 2 O 3 , 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, Gd 2 O 3 , 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 5 D 0 -7 F 2 in the Eu used in panel display devices, such as PDPs 4 , electroluminescent devices (ELDs) 5 , fluorescent lamps 6 , and so forth.Surfactant-modified Gd 2 O 3 :Eu 3+ 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 Gd 2 O 3 : 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 Gd 2 O 3 :Eu 3+ 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 Gd 2 O 3 :Eu 3+ 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.

Synthesis of Gd 2 O 3 :Eu 3+ with F-127 surfactant
Gd 2 O 3 : Eu 3+ powders and films synthesized in the presence of F-127 were prepared using gadolinium nitrate Gd(NO 3 ) 3 (99.9%Signa Aldrich), ethanol (C 2 H 6 O) (Fermont 99.9%), and europium (III) nitrate (Eu(NO 3 ) 3 ) (99.9% Alfa Aesar).The molar composition of the sol was Gd:Eu:C 2 H 6 O = 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% Eu 3+ ).The established Eu was used because concentration quenching may occur in heavily Eu 3+ -doped systems, arising from the distance-dependent, non-radiative, cross relaxation between neighboring Eu 3+ ions 17 .In order to prepare surfactantmodified Gd 2 O 3 :Eu 3+ 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 Gd 2 O 3 :Eu 3+ 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 Gd 2 O 3 :Eu 3+ powders and films without F-127 were prepared for the purpose of comparison.

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.

Structural and morphological characterization of Gd 2 O 3 :Eu 3+ /F-127 powders and films
The X-ray diffraction patterns of the Gd 2 O 3 :Eu 3+ (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 Gd 2 O 3 :Eu 3+ (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.
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 Gd 2 O 3 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 Gd 2 O 3 :Eu 3+ 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 Gd 2 O 3 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 Gd 2 O 3 :Eu 3+ -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 Gd 2 O 3 modified surfactant films in previous reports.
In both cases, the presence of doping concentrations of Eu ions in the Gd 2 O 3 structure (with a similar electric charge) indicates that Eu 3+ 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 Gd 3+ , 0.947Å for Eu 3+ ) 24 , which did not affect the host structure of the Gd 2 O 3 .Finally, the C-type structure of Gd 2 O 3 offers two nonequivalent sites for lanthanide doping ions: C 2 (non-centrosymmetric) and S 6 (centrosymmetric).In both crystalline structures, the coordination environment of the lanthanide doping ions occupying the two possible sites is C 2 :S 6 = 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 : , where IG is a measure of the isotropic size effect 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 Gd 2 O 3 :Eu 3+ powders and films as a function of temperature, compared to the Gd 2 O 3 :Eu 3+ particles synthesized without F-127.These values were found to increase as the temperature cos IG 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 Eu 3+ -doped Gd 2 O 3 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 Gd 2 O 3 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 .

FTIR studies
Fig. 2 shows the normalized FTIR spectra of the 5 mol% Eu-doped Gd 2 O 3 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 NO 3 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 -NO 3 gases.Nevertheless, the heat-treatment at air atmosphere was not enough to eliminate NO 3 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 Gd 2 O 3 32,33 .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 Gd 2 O 3 :Eu 3+ powders present an amorphous phase at temperatures lower than 500 ºC, in agreement with the XRD results.

Morphological characteristics
In order to examine the morphology of the Gd 2 O 3 :Eu 3+ 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 Gd 2 O 3 :Eu 3+ powders and films without F-127, heat-treated at 800 ºC for 1 h.The Gd 2 O 3 :Eu 3+ powders synthesized without F-127 (Fig. 3a) exhibit a flower shape 4 µm in size.In the case of the Gd 2 O 3 :Eu 3+ 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. 3c and d show zones of the Gd 2 O 3 :Eu 3+ 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 Gd 2 O 3 :Eu 3+ 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.

Luminescent properties
Figs. 4and 5 show the PL spectra of Gd 2 O 3 :Eu 3+ 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).
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 5 D 0 → 7 F J (J = 0, 1, 2, 3), being the most intense the 5 D 0 → 7 F 2 transition, an electric dipole-allowed transition that is hypersensitive to the environment.This strong emission occurs when Eu 3+ 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 Gd 2 O 3 :Eu 3+ synthesized in the presence of F-127, heat-treated from 300 to 800 ºC.The luminescence intensity of the Eu 3+ -modified  powders increases with an increase in temperature and improves crystallization.A poor emission intensity was observed for the Gd 2 O 3 :Eu 3+ /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 Gd 2 O 3 :Eu 3+ films annealed at different temperatures, differences can be observed regarding the 5 D 0 → 7 F 0 (581-nm) and 5 D 0 → 7 F 1 (splits at 588, 592 and 599 nm) transitions, because they represent the local environment of the Eu 3+ .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 5 D 0 → 7 F 1 transition, this one is magnetic-dipole-allowed; its intensity shows small variations, with a crystal field strength close to that of the Eu 3+ ions.
Nevertheless, for the Gd 2 O 3 :Eu 3+ 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 5 D 0 → 7 F 2 , 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 blueshifted for only the cubic nanoparticles 39 .A small fraction of the monoclinic nanocrystallites present in the modified Gd 2 O 3 :Eu 3+ 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, nontoxic 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 Gd 2 O 3 :Eu 3+ -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 deexcitation process, due to the larger number of atoms on the nanoparticles' surface, unlike larger particles, favoring a Eu 3+ red emission 47 .

Conclusion
Gd 2 O 3 :Eu 3+ 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 Gd 2 O 3 transforms into the monoclinic phase.Nevertheless, at a 254-nm excitation of the Gd 2 O 3 host, the monoclinic portion was ineffective, but led mainly to the characteristic emission of Eu 3+ associated with a cubic structure.Luminescence analyses showed an improved emission from modified Gd 2 O 3 :Eu 3+ 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.

Figure 1 .
Figure 1.X-ray diffraction patterns of Gd 2 O 3 :Eu 3+ (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 Gd 2 O 3 :Eu 3+ powders synthesized with and without F-127 and for films in presence of F-127 at different temperatures.

Figure 2 .
Figure 2. IR spectra of the Gd 2 O 3 :Eu 3+ powders synthesized in the presence of F-127.

Fig. 3 .
Fig. 3. SEM micrographs of (a) the Gd 2 O 3 :Eu 3+ powders and (b) the films without F-127, and (c) the Gd 2 O 3 :Eu 3+ powders and (d) the films in the presence of F-127.

Figure 4 .
Figure 4. PL spectra of (a) the Gd 2 O 3 :Eu 3+ powders synthesized in the presence of F-127 heat-treated at different temperatures and (b) comparison of the Gd 2 O 3 :Eu 3+ powders with and without F-127, heat-treated at 800 ºC.

Figure 5 .
Figure 5. PL spectra of (a) the Gd 2 O 3 :Eu 3+ films synthesized in the presence of F-127, heat-treated at different temperatures and (b) comparison of the Gd 2 O 3 :Eu 3+ films with and without F-127, heat-treated at 800 ºC.