Development and photocatalytic properties of potassium titanate nanostructure embedded resin composites

Kapusuz, Derya
Structural and functional properties of potassium titanate (PT; K2O.nTiO2, n=2, 4) nanostructures synthesized by sol-gel and hydrothermal methods were investigated with respect to titanium/potassium (Ti/K) ratio and their nanogrowth mechanisms were elucidated. Produced powders were characterized by XRD, electron microscopy (SEM, TEM) and gas adsorption (BET) techniques for phase, morphology and surface analyses, respectively. PT nanostructures were mixed with three UDMA-TEGDMA (UT) resin monomer mixtures in the order of increasing TEGDMA molar ratio of UT-2 (30%), UT-1 (50%), UT-3 (70%). Polymerization of the composites was accomplished by halogen light curing. Tensile, hardness, wear, water sorption/solubility and bioactivity tests were performed to determine the performance of the composites. The energy band gap (Eg) of PT powders, the photocatalytic activity of PT powders and PTUT composites were measured by UV-Vis Spectroscopy equipped with diffuse reflectance (DR) module. Methylene Blue (MB) dye degradation tests were performed to evaluate photocatalytic performance. It was demonstrated that both sol-gel and hydrothermal growth of PTs originate from an amorphous-like network. Potassium hexa-titanate (PHT, n=6) nanowires could be obtained by calcination of sol-gel derived amorphous network at temperatures as low as 600 °C. As the calcination temperature increased to 800 °C, potassium tetra-titanate (PTT, n=4) formation became significant. In addition, the size and aspect ratio of PTs increased as the calcination temperature and time increased (1-3 h). In sol-gel process, potassium hollandite (PH, KTi8O16) was an intermediate phase upon PHT crystallization. However, PHT and PTT nanowires grew directly from the hydrothermal solution under autogeneous pressure. Analysis indicated that amorphous network combined into sheets, crystallized into PTs and then split due to increased strain. The highest average size of sol-gel derived PT whiskers depending on Ti/K ratio were measured as 214 ± 113 nm in diameter and 4.9 ± 2.5 µm in length. The size of split PT nanowires were measured as 39 ± 14 nm in diameter and 1.14 ± 0.4 µm in length, after 24 h of hydrothermal treatment at 180 °C. The size of the nanostructures decreased as the hydrothermal duration increased to 48 h due to further splitting. Hydrothermal treatment provided PTs having Eg lower than widely used commercial TiO2 (P25, Degussa®) powder. Analysis of PT-UT composites revealed that mechanical and functional performances of the composites were mainly related to the PT embedment amount and dispersion efficiency. The lowest specific wear rate was achieved for the composite containing 5 wt % hydrothermally synthesized PT embedded UT-1 (50% UDMA – 50% TEGDMA, molar) mixture as 6.5 ± 0.6 *10-6 mm3/Nm by 5 wt % hydrothermally produced PT embedded UT-1 (50% U-50% T, molar) mixture. Similarly, tensile strength and elastic modulus of UT-1 mixture increased from 40.4 ± 1.7 MPa to 42.5 ± 0.2 MPa, and from 0.7 ± 0.04 GPa to 0.9 ± 0.1 GPa, respectively by embedding 5 wt % hydrothermally synthesized PT. The water sorption and solubility values were higher than 40 µg/mm3. The MB degradation test and analyses after simulated body fluid immersion showed that PTs preserved their photocatalytic performance and bioactivity in the composite form. MB degradation of composites in 24 h increased with increased PT embedment in the composite.