Multisite luminescence of rare earth doped TiO2 anatase nanoparticles

https://doi.org/10.1016/j.matchemphys.2012.06.016Get rights and content

Abstract

Eu3+, Sm3+ and Tb3+ ions have been incorporated into anatase TiO2 nanocrystals via hydrolytic sol–gel method. Pure anatase phase was confirmed with XRD and TEM measurements. Band gap energies change slightly with rare earth incorporation, from 3.32 eV for undoped TiO2 to 3.15 eV, 3.25 eV and 3.29 eV for Tb3+, Sm3+ and Eu3+ doped TiO2. Photoluminescence of Eu3+ and Sm3+ originated from three different sites in TiO2 nanocrystals have been identified with the laser-excited site-selective spectroscopy measurements at 10 K. One site exhibits broad emission peaks, which are ascribed to the distorted lattice site near the surface. Other two sites, associated with the inner lattice, show significantly sharper fluorescence lines as a consequence of an ordered crystalline environment. The emission decays of Eu3+ and Sm3+ have similar values for inner-lattice sites and longer lifetimes for near-surface sites. The luminescence of Tb3+ doped TiO2 nanocrystals was immeasurably weak.

Highlights

► Sm3+, Eu3+ and Tb3+ are incorporated into anatase nanocrystals via sol–gel route. ► Sm3+ and Eu3+ luminescence originate from 3 different sites in TiO2 nanocrystals. ► Details on multisite structure for Sm3+ doped TiO2 are presented for the first time.

Introduction

Titanium (IV)-oxide occurs in nature in three mineral forms: anatase, brookite and rutile. All three phases are characterized with high refractive index (nanatase = 2.488, nrutile = 2.609, nbrookite = 2.583), low absorption and low dispersion in visible and near-infrared spectral regions, high chemical and thermal stabilities. This important metal-oxide semiconductor with relatively wide band gap (3.25 eV for anatase, 3.0 eV for rutile, 1.9 eV for brookite) [1] and low phonon energy (<700 cm−1) is an excellent host for various rare earth (RE) impurities providing their efficient emission in visible range [2], [3], [4], [5], [6]. These systems are of possible interest in white light emission diode (LED) industry [7], [8], [9], [10] and as photocatalysts [11], [12]. At the same time, being non-toxic and biocompatible, rare-earth doped anatase has strong potential to replace standard types of fluorophores (quantum dots, organic dyes, etc.), traditionally used as fluorescent markers in medicine and biological applications [13].

In particular anatase phase is considered very promising and has been widely investigated for various applications in lithium-ion batteries, filters, waveguides, anti-reflective and highly reflective coatings [14], [15], [16], [17], [18], [19], but it still remains a challenge to keep this phase stable from easy transformation to rutile. Setiawati and Kawano [20] studied the stabilization of anatase phase with Eu3+ and Sm3+ ions, added in different concentration, ranging from 0.1 to 1 mol%. They claimed that the significant suppression of TiO2 nanoparticle growth and stabilization of anatase phase were achieved by RE doping, getting better results for higher dopant concentrations.

Following these findings, we decided to produce anatase nanoparticles via sol–gel method and to use as dopants Eu3+, Sm3+ and Tb3+ ions, adding them in concentration of 3 at.%. We documented the successful synthesis of stable and pure anatase phase through several experiments: basic characteristics of synthesized materials from thermal analysis (TG/DTA), X-ray diffraction (XRD), Fourier transmission infrared (FTIR), scanning and transmission electron microscopy (SEM and TEM), nitrogen sorption measurements, UV–vis and photoluminescence (PL) spectroscopy, and discussed obtained results. Using site-selective technique at low temperature (10 K) we were able to prove the incorporation of RE3+ ions into the TiO2 lattice. The existence of three nonequivalent sites of Eu3+ and Sm3+ in anatase matrix has been reported and discussed.

Section snippets

Synthesis

To produce anatase TiO2 in the form of nanopowder the hydrolytic sol–gel route has been adopted, starting from rare-earth nitrates and titanium (IV)-isopropoxide. The sol–gel technology offers several processing advantages as the starting materials are mixed at the nanoscale level. In this way a complete and controlled mixing of components is ensured at the preliminary stage, the reaction rate is increased and the processing temperature lowered.

For synthesis of undoped and 3 at.% Eu3+, Sm3+ and

Results and discussion

The TiO2 gels prepared with hydrolytic sol–gel method were of amorphous nature and under appropriate sintering they transformed to crystalline TiO2. In order to determine proper sintering temperature needed for the transformation to anatase phase, we performed a thermal analysis of the synthesized gels. Results of TG/DTA analysis clarified the existence of three temperature regions (see Fig. 1): i) the DTA endothermic peak between room temperature and 300 °C, which can be attributed to the

Conclusions

Thermal and infrared analysis revealed the typical behavior of materials produced by sol–gel, with removal of residual water and solvents at lower temperatures and organic compounds at higher ones, as well as the formation of crystalline anatase phase. Purity of the anatase phase in all studied samples has been confirmed through XRD measurements, and at the local level with TEM observations. The small decrease in band gap values is noted for rare earth doped nanocrystals and could be ascribed

Acknowledgments

Authors acknowledge the financial support of the Ministry of Education and Science of the Republic of Serbia (45020).

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