Synthesis and luminescence properties of hybrid organic–inorganic transparent titania thin film activated by in-situ formed lanthanide complexes

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Abstract

Stable transparent titania thin films were fabricated at room temperature by combining thenoyltrifluoroacetone (TTFA)-modified titanium precursors with amphiphilic triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO, P123) copolymers. The obtained transparent titania thin films were systematically investigated by IR spectroscopy, PL emission and excitation spectroscopy and transmission electron microscopy. IR spectroscopy indicates that TTFA coordinates the titanium center during the process of hydrolysis and condensation. Luminescence spectroscopy confirms the in-situ formation of lanthanide complexes in the transparent titania thin film. TEM image shows that the in-situ formed lanthanide complexes were homogeneously distributed throughout the whole thin film. The quantum yield and the number of water coordinated to lanthanide metal center have been theoretically determined based on the luminescence data.

Graphical abstract

Novel stable luminescent organic–inorganic hybrid titania thin film with high transparency activated by in-situ formed lanthanide complexes have been obtained at room temperature via a simple one-pot synthesis approach by using TTFA-modified titanium precursor with amphiphilic triblock copolymer P123. The obtained hybrid thin film displays bright red (or green), near-monochromatic luminescence due to the in-situ formed lanthanide complex.

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Introduction

Luminescent lanthanide organic complexes are of both fundamental and technical interest due to their characteristic luminescence properties, e.g., extremely sharp emission bands, long lifetime, and potential high internal quantum efficiency. This has drawn much attention because of their versatile applications, such as planar waveguide amplifiers, plastic lasers, and light-emitting diodes [1]. For technological applications, these complexes have to be incorporated into a stable solid matrix [2], [3], [4], [5], [6], [7]. Lanthanide complexes are generally incorporated into different substrates or matrixes, including sol–gel materials, supramolecules, and porous materials. An enhanced thermal stability, mechanical stability, and luminescent properties compared with the corresponding pure lanthanide complexes have been observed in these systems. Among them, mesostructured materials fabricated from polymerizable silica species and organic structure-directing agents are much more attractive because they have led to a number of advanced optical applications thanks to the ability to integrate specific functional molecules into distinct regions of the mesostructures [5], [6], [8], [9], for instance, when a laser dye is incorporated selectively into the organic domains of the mesostructure, silica-based hybrids perform a number of advanced optical applications, including optical switching and sensing, and low threshold waveguide microlasing [8], [9]. However, optical devices fabricated from silica have a limited refractive index (n=1.43) and must be supported by ultralow refractive index materials when efficient waveguiding is important [9]. This obstacle could be overcome by using a higher refractive index inorganic component such as mesostrucutred titania, which has been demonstrated recently by the fabrication of dye-doped hybrid waveguides with trifluoroacetate (TFA)-modified titania frameworks [10], [11], [12], [13]. The obtained transparent thin films have a high effective index of refraction (n=1.6–1.7). The stability and optical quality of the hybrid thin film obtained by this new approach can be ascribed to the presence of chelating TFA ligands within the titania network suppressing continued condensation of titainia network [12], [13]. Carboxylic acid and β-diketones are used to moderate the reactivity of titanium precursor, thus transparent titanium gel can be obtained [12], [13], [14]. Nevertheless, to the best of our knowledge, stable transparent titania thin films combined with the interesting luminescent properties of lanthanide compounds have not been reported yet due to the high activity of the titanium precursor.

Herein we report a controlled synthesis procedure for organic–inorganic transparent titaina thin film exhibiting sharp emission lines due to the in-situ formed highly luminescent 1,10-phenanthroline (phen) Eu3+ or Tb3+ complex by employing a TTFA stabilized titania precursor. The obtained organic–inorganic composites are composed of hydrophobic regions and the rigid inorganic framework due to the presence of Pluronic P123, enabling them act as ideal host materials for organic guest molecules because the hydrophobic regions are compatible with organic guest species and the rigid inorganic framework can protect the included molecules.

Section snippets

Materials

Eu2O3, Tb4O7, Ti(OiPr)4, P123, TTFA, 1,10-phenanthroline purchased from Aldrich. They were used without further purification. EuCl3·6H2O and TbCl3·7H2O were obtained by dissolving Eu2O3, Tb4O7 in HCl.

Synthesis of hybrid organic–inorganic titania thin film

Appropriate amount of EuCl3·6H2O (or TbCl3·7H2O) and 1,10-phenanthroline (molar ratio Eu3+:phen of 1:3) were added to 15 mL of EtOH containing 1 g of P123. Separately, 5.5 mL of Ti(OiPr)4 was reacted under stirring with 4.13 g of TTFA and 0.7 mL of concentrated HCl. After 10 m, the two solutions were

Results and discussion

The obtained titania thin films are stable under ambient conditions for several months without heat treatment for stabilization via conventional inorganic-acid-based route. This can be ascribed to the chelating of TTFA to titanium alkoxide, which can prevent extended three-dimensional cross-linking of the molecular titania units and enable the room temperature formation of stable glassy composite. The reaction of TTFA with Ti(OiPr)4 can be confirmed by the FTIR spectra as shown in Fig. 1. Fig. 1

Conclusions

Concluding, novel stable luminescent organic–inorganic hybrid titania thin film with high transparency activated by in-situ formed lanthanide complexes have been obtained at room temperature via a simple one-pot synthesis approach by using TTFA-modified titanium precursor with amphiphilic triblock copolymer P123. The obtained hybrid thin film displays bright red (or green), near-monochromatic luminescence due to the in-situ formed lanthanide complex. We can ascribe our result to the presence of

Acknowledgment

This work is financially supported by Hebei University of Technology.

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