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 optoelec- tronic devices [9] -[11] .

The demand for efficiency and high resolution waveguides, lamps and other optical devices has also stimu- lated 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 en- hance the luminescent characteristics of phosphors, extensive research has been carried out on rare-earth acti- vated 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 ca- pacitors, luminescent lamps and displays, solid-laser devices, optoelectronic data storages, waveguides, and he- terogeneous 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 advan- tages 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 appli- cations [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 or- der-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 (mi- crowave 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.

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,

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.

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