Synthesis, Structural and Photophysical Properties of Gd2O3:Eu3+ Nanostructures Prepared by a Microwave Sintering Process


In this paper, we report the obtention of gadolinium oxide doped with europium (Gd2O3:Eu+3) by thermal decomposition of the Gd(OH)3:Eu3+ precursor prepared by the microwave assisted hydrothermal method. These systems were analyzed by thermalgravimetric analyses (TGA/DTA), X-ray diffraction (XRD), structural Rietveld refinement method, fourrier transmission infrared absorbance spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM) and photoluminescence (PL) measurement. XRD patterns, Rietveld refinement analysis and FT-IR confirmed that the Gd(OH)3:Eu3+ precursor crystallize in a hexagonal structure and space group P6/m, while the Gd2O3:Eu3+ powders annealed in range of 500°C and 700°C crystallized in a cubic structure with space group Ia-3. FE-SEM images showed that Gd(OH)3:Eu3+ precursor and Gd2O3:Eu3+ are composed by aggregated and polydispersed particles structured as nanorods-like morphology. The excitation spectra consisted of an intense broad band with a maximum at 263 nm and the Eu3+ ions can be excitated via matrix. The emission spectra presented the characteristics transitions of the Eu3+ ion, whose main emission, , is observed at 612 nm. The photophysical properties indicated that the microwave sintering treatment favored the Eu3+ ions connected to the O-Gd linkages in the Gd2O3 matrix. Also, the emission in the Gd2O3:Eu3+ comes from the energy transfered from the Gd-O linkages to the clusters in the crystalline structure.

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Moura, A. , Oliveira, L. , Nogueira, I. , Pereira, P. , Li, M. , Longo, E. , Varela, J. and Rosa, I. (2014) Synthesis, Structural and Photophysical Properties of Gd2O3:Eu3+ Nanostructures Prepared by a Microwave Sintering Process. Advances in Chemical Engineering and Science, 4, 374-388. doi: 10.4236/aces.2014.43041.

1. Introduction

One-dimensional nanomaterials, such as nanotubes, nanowires, nanobelts or nanoribbons have attracted much interest in the past decade due to their physical properties and potential applications in nanotechnology fields [1] -[8] . Moreover, these materials can be applied as displays, catalysts, biological sensing, and other optoelectronic devices [9] -[11] .

The demand for efficiency and high resolution waveguides, lamps and other optical devices has also stimulated the discovery of new luminescent materials with superior properties. Thus, there has been a tremendous interest in the subject of materials science for the development of new luminescent materials. The improved performance of display requires high-quality phosphors for sufficient brightness and long-term stability. To enhance the luminescent characteristics of phosphors, extensive research has been carried out on rare-earth activated oxide phosphors due to their superiority in color purity, chemical and thermal stabilities [12] -[14] . In this context, lanthanide hydroxides and oxides have actively been investigated for its application in multilayered capacitors, luminescent lamps and displays, solid-laser devices, optoelectronic data storages, waveguides, and heterogeneous catalysts. Their composition, structure and particle size depend on the synthesis method. Moreover, the chemical homogeneity and morphology of the synthesized products determine the effectiveness of their properties [11] [15] [16] . When they are applied for a fluorescent labeling, for instance, there are several advantages such as sharp emission spectra, long lifetimes, and high resistance against photobleaching in comparison with conventional organic fluorophores and quantum dots [17] -[19] .

In particular, the gadolinium oxide doped with Eu3+ (Gd2O3:Eu3+) exhibits a strong paramagnetic behavior (S 1/4 72) as well as strong UV and cathode-rays have also been observed in the lanthanide (Sm3+, Er3+) doped Gd2O3 excited luminescence, which are useful in biological fluorescent label, contrast agent, and display applications [20] -[22] . In addition, Gd2O3:Eu3+ is a very efficient X-ray and thermo-luminescent phosphor [23] .

Europium ion in a trivalent state is one of the most studied rare earth element because of the simplicity of its emission spectra and due to the wide application as red phosphor in color TV screens. Eu3+ f-f transitions are sensitive to its local environment. The monitoring of different concentrations of the Eu3+ content into a ceramic material is very interesting in understanding the nature of the lattice modifiers as well as the degree of order-disorder into its crystalline structure. The most intense f-f transition is the transition at 616 nm. When this ion is presented in a non-centrosymmetric site, it can be used as an activator ion with red emission which has been used in the most commercial red phosphor. Moreover, the intensity of Eu3+ excitations at around 394 and 465 nm is improved in these materials as compared with most other Eu3+ doped phosphors [24] [25] . Because of it, this ion is able to be applied as biological sensors, phosphors, electroluminescent devices, optical amplifiers or lasers when it is used as a dopant in a variety of ceramic materials [26] -[28] .

A variety of preparation methods have been developed to reduce the reaction temperature and achieve a small particle size of high quality Gd2O3:Eu3+ phosphors [11] [29] -[32] .

Microwave heat processing has been successfully applied for the preparation of micro or nanosized inorganic materials [33] -[38] . The microwave-assisted heating is a greener approach to synthesize materials in a shorter time (from several minutes to a few hours) and with lower power consumption (hundreds of Watts) compared to the conventional heating at the same temperatures [39] -[43] . This is a consequence of directly and uniformly heating of the components, and exchange in the reaction selectivity, which can increase the reactional rates (microwave catalysis). Consequently, microwave synthesis is becoming quite common in several material sciences areas, nanotechnology, inorganic, organic, biochemical, or pharmaceutical laboratories [44] -[50] .

In the present work, we investigated the photo-physical properties of Gd2O3: Eu3+ phosphors obtained by the thermal decomposition in range of 500˚C and 700˚C of the Gd(OH)3:Eu3+ precursor prepared by the microwave assisted hydrothermal method. These materials were structured and microstructurally analyzed by means of X-ray diffraction (XRD), Rietveld refinement method, fourrier transmission infrared absorbance spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM). The photo-physical properties were investigated through the excitation and emission spectra of the Eu3+ ion as well as lifetime measurements.

2. Experimental Procedure

2.1. Synthesis of the Precursors

The synthesis of the precursors was performed using the following procedure: In a typical synthesis, 1.8 g of Gd2O3 and 0.018 g of Eu2O3 were dissolved in 3.0 mL of the HNO3 solution. After the formation of a clear solution, this solution was kept under constant heating until complete evaporation of the acid. Then 80 mL of distilled water were added to the solution and stirred for 30 min at room temperature. After that, an aqueous KOH (2.0 M) solution was added until the pH of solution was adjusted to be in the range of 12 giving rise to a colloidal precipitates. After stirring for about 30 min, the resultant solution was transferred to a Teflon lined stainless autoclave. This autoclave was then sealed and placed into a microwave system (MH) using 2.45 GHz microwave radiation with maximum power of 800 W. The MH conditions were kept at 140˚C for 1 minute. The white powders obtained (Gd(OH)3:Eu3+) were collected, washed with water and ethanol, and then dried at 60˚C for 8 h under atmospheric air in a conventional furnace.

2.2. Synthesis of Gd2O3:Eu3+ Powders

The Gd2O3:Eu3+ powders were obtained from thermal decomposition of the Gd(OH)3:Eu3+ precursors. These precursor powders were placed in ceramic crucibles and heated in a microwave sintering furnace at 500˚C, 550˚C, 600˚C, 650˚C and 700˚C for 5 min under an ambient atmosphere using a heating rate of 5˚C/min producing white powders denoted as Gd2O3:Eu3+.

2.3. Characterization

The Gd(OH)3:Eu3+ and Gd2O3:Eu3+ powders were structurally characterized by X-ray diffraction (XRD) in normal routine and Rietveld routine using a Rigaku-DMax/2500PC (Japan) with Cu-Kα radiation (λ = 1.5406 Å) and in the 2θ range from 10˚ to 130˚ with a scanning rate of 0.02˚/min. Fourier Transmission Infrared absorbance spectroscopy (FT-IR) analysis were taken in a FT-IR Bruker model EQUINOX spectrophotometer in range of 500 and 4000 cm−1. Crystals morphologies were verified using a Scanning Electron Microscope (Jeol JSM-6460LV microscope). Photoluminescence (PL) was measured with a Thermal Jarrel-Ash Monospec 27 monochromator and a Hamamatsu R446 photomultiplier. The 350.7 nm exciting wavelength of a krypton ion laser (Coherent Innova) was used, with the nominal output power of the laser power kept at 200 mW. All the measurements were taken at room temperature. The excitation and emission spectra of the Gd2O3:Eu3+ powders were measured in a Jobin Yvon-Fluorolog 3 spectrofluorometer at room temperature using a 450 W xenon lamp as excitation energy source. Lifetime data of the Eu3+ (lexc = 394 nm, lem = 612 nm) transition in the Gd2O3:Eu3+ samples were evaluated from the decay curves using the emission wavelength set at 612 nm and excitation wavelength set at 393 nm.

3. Conclusion

In summary, the obtained results showed that the Gd(OH)3:Eu3+ (precursor) was synthesized by the microwave assisted hydrothermal method in a short period of time (30 minutes). After heated treated from 500˚C to 700˚C,

Figure 10. Emission spectra of Gd2O3:Eu3+ samples calcined at 500˚C, 550˚C, 600˚C, 650˚C and 700˚C, lex = 263 nm.

Figure 11. Decay curves and lifetime of the 5D07F2 transition characteristic of the Eu3+ of the Gd2O3:Eu3+ nanorods heat treated at 500˚C, 550˚C, 600˚C, 650˚C and 700˚C (lex = 612 nm and lem = 612 nm).

the XRD patterns and Rietveld refinement and FT-IR analyses indicated the formation of Gd2O3:Eu3+ powders which crystallizes in a cubic structure of crystalline Gd2O3 and space group Ia-3. No secondary phases related to the Eu3+ ions were detected indicating that these ions were incorporated to the hydroxide and oxide matrixes in the analyzed powders. FE-SEM images indicated that the Gd(OH)3:Eu3+ precursor and Gd2O3:Eu3+ powders are composed by several aggregated particles with nanorods-like morphology, which sizes are in the range of 8 and 20 nm. Eu3+ emission and excitation spectra pointed out that the emission in the Gd2O3:Eu3+ powders comes from the energy transfer from the Gd-O and clusters in the crystalline structure. Moreover, these are in accordance to the lifetime values, which presented an increase as the temperature increases. This method is very simple and effective, and can be extended to synthesize some other rare earth and metal oxide nanorods.


The authors acknowledge the financial support of the Brazilian research financing institutions: CNPq (INCTMN), CAPES and FAPESP (CEPID). A special thanks for Maria Fernanda Cgnin de Abreu.


*Corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.


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