EXECUTION STAGE 1/2022

 Development of metal oxide nanoparticles, fluorescent organic precursors and of the corresponding polyurethanes

The activities proposed for the first stage of the project were:

1.1. Synthesis and functionalization of metal oxide nanoparticles (TiO₂, ZnO, MnO₂) 

1.2. Preparation of fluorescent monomers (diols of coumarin, dansyl or pyrene) to be used in polyurethane synthesis 

1.3. Design and synthesis of new polyurethane elastomers bearing fluorescent sequences 

All the proposed activities were addressed and completed, the main results being summarized as follows:

In accordance with the proposed objectives, the first approached activity consisted in the synthesis of metal oxide nanoparticles, the studies carried out at this stage aiming the design and preparation of new zinc oxide nanoparticles. Although ZnO nanoparticles have a wide band gap energy (Eg = 3.37 eV), their major advantage compared to other photocatalysts is that they absorb a larger fraction of the UV spectrum. However, the wide band gap of the ZnO semiconductor results in low photocatalytic efficiency in the UV region due to the rapid recombination of photogenerated electron-hole pairs, which is why preventing the recombination of electron-hole pairs by reducing the band gap of ZnO nanoparticles (ZnO NPs) improves the photocatalytic performance. Therefore, a series of zinc oxide nanoparticles was synthesized, using various preparative conditions, their characterization being carried out by specific methods (FTIR spectroscopy, X-ray diffraction, scanning electron microscopy, EDX spectroscopy). In addition, the estimation of the band gap energy values shows that these varied for the synthesized ZnO NPs between 3.14 and 3.21 eV, and after functionalization with a silane-type derivative, the Eg value decreases to 2.97 eV.

The organic functionalization of ZnO nanoparticles with silane-type units and iodide ions was proposed as a convenient method of linking the inorganic units to the polyurethane chain through quaternization reactions, aiming a better dispersion of the nanoparticles in the organic matrix and preventing their migration during the photocatalytic activity testing.

In order to introduce fluorescent sequences into the polyurethanes chains, the synthesis and characterization (¹H NMR, ¹³C NMR, FTIR, UV-Vis and fluorescence spectroscopy) of two types of diols bearing fluorescent units derived from coumarin was carried out, and the monomer with the best optical characteristics has been used in various ratios in the preparation of fluorescent polyurethane elastomers. The synthesized polyurethanes were also spectrally characterized to confirm the structure and to evaluate the optic characteristics, subsequent studies being directed towards the preparation of fluorescent polyurethane/ZnO nanoparticle hybrid nanocomposites and the testing of their photocatalytic activity under visible light irradiation.

EXECUTION STAGE 2/2023

 Development of metal oxide nanoparticles, fluorescent organic precursors and of the corresponding polyurethanes

The activities proposed for this stage of the project were:

2.1. Synthesis and functionalization of metal oxide nanoparticles (TiO₂, ZnO, MnO₂) (objective 1.1.)

2.2. Preparation of fluorescent monomers (diols of coumarin, dansyl or pyrene) to be used in polyurethane synthesis (objective 1.2.)

2.3. Design and synthesis of new polyurethane elastomers bearing fluorescent sequences (objective 2.)

2.4. Development of hybrid materials through covalent attachment of functionalized metal oxide nanoparticles to polyurethane chains (objective 3.)

2.5. Thoroughly characterization of the fabricated nanocomposites. Evaluation of the photocatalytic abilities of the proposed materials (objective 4.).

All the proposed activities were addressed and completed, the main results being summarized as follows:

In this stage, the series of metal oxide nanoparticle was diversified with the design and preparation of new titanium and manganese dioxide nanoparticles, special attention being paid to the shape and size of the nanoparticles as well as to the band gap energy variation. 9 types of TiO₂ nanopowders and 3 types of manganese oxide nanoparticles were prepared, which differ from each other by their crystalline structure, lattice parameters, size and morphology of the final particles, etc., aspects that were evaluated with the help of specific characterization techniques. Further, some selected TiO₂, MnO₂ and ZnO (synthesized in stage I) nanoparticles were functionalized with silane units and chloride ions (using 3-chloropropyl trimethoxysilane derivative) through a sol-gel reaction in order to immobilize them by quaternization in the polyurethane chain. The band gap energy values (Eg) for inorganic NPs varied between 3.00 and 3.16 eV (3.16 eV for ZnO, 3.15 eV for TiO₂ and 3.00 eV for MnO₂) and after the functionalization with a silane-type derivative, Eg decreased to 2.95 eV for ZnO-Si, 2.83 eV for TiO₂-Si and 2.54 eV for MnO₂-Si.

Also, during this stage, fluorescent diols with dansyl or pyrene units were synthesized, the proposed structures being characterized by spectral methods (¹H NMR, ¹³C NMR, FTIR, UV-vis, PL). Then, a series of polyurethane elastomers with dansyl-type fluorescent units was obtained, the ratio of dansyl diol used in the synthesis of polyurethanes being of 0.5 mol % and 1.0 mol %, respectively.

To generate hybrid materials, the ZnO NPs synthesized in the previous stage, functionalized with 3-iodopropyl trimethoxysilane, were attached by quaternization to the tertiary nitrogen atoms in the structure of polyurethanes with coumarin units (also reported in stage I). The prepared samples include various molar percents of coumarin chromophore (1 mol %, 5 mol % and 10 mol %, respectively) and two different types of silanized ZnO NPs (nanosheets or nanorods).

Investigation of the optical properties for the polyurethane/ZnO NPs hybrid films with or without coumarin-type fluorophore was carried out by UV-Vis and fluorescence spectroscopy, the band gap energy (Eg) of the hybrid films being calculated from their reflectance spectra using the Kubelka-Munk method. Based on the Eg values, all the synthesized samples (A_0 – 10 and B_0 – 10) were tested as photocatalysts under visible light irradiation, using malachite green (MG) as a model pollutant.

Malachite green photodecomposition takes place in the presence of all hybrid materials containing ZnO NPs, the time required for the complete removal of MG varies between 18 and 100 min, depending on the sample used. The absence of ZnO NPs from PU films does not activate MG photodegradation, and the adsorption effect of polyurethane films can be ignored. Also, the reusability and stability of the hybrid catalyst films were tested, the removal efficiency of MG as a single pollutant in aqueous solution varied between 97.3 % and 100 %, and for MG as a component of the mixture with Congo Red (MG-CR) varied between 95.5 % and 100 %, after 7 successive catalysis cycles.