Enhancing photoluminescence dynamics, visible light driven photocatalytic dye degradation and distinctive electrical properties of cerium doped pyrimidine based covalent organic frameworks: Sustainable synthesis and structural characterization

Abstract

Covalent organic frameworks (COFs) are an important class of compounds having exceptional long-range ordered structures with porosity and crystallinity. In the present study, 6-chloropyrimidine 2, 4-diamine and terephthalaldehyde were made to react in ethanol solvent at reflux conditions using the solvothermal method resulting in the formation of pyrimidine based covalent organic frameworks (PyCOFs). The synthesized PyCOFs were then functionalized with cerium (Ce) and obtained as Ce@PyCOFs. The crystal structure, microstructure, purity, and optical properties of the synthesized PyCOFs and Ce@PyCOFs were confirmed by X-ray diffraction (XRD), field emission scanning microscope (FESEM), Fourier transform infrared spectroscopy (FT-IR), and UV–visible analytical techniques. Optical studies of PyCOFs and Ce@PyCOFs revealed redshift in maximum absorbance bands with decreased the optical band gap (E_g). The mean Zeta potential of PyCOFs, and Ce@PyCOFs varies between 15.60 mV and 37.84 mV. Room temperature photoluminescence (RTPL) studies of PyCOFs and Ce@PyCOFs unveiled broad yellow-green emission peaks centered at 567.9 nm, 571.3 nm, 380 nm, 559 nm and few weak sharp emission bands at various excitation wavelengths. Further, visible light photocatalytic dye degradation studies of PyCOFs and Ce@PyCOFs against methylene blue (MB) and methyl red (MR) dyes showed degradation efficiency of 73.50 %, 78.99 % and 89.18 %, 87.34 % in 140 min and 180 min of irradiation. Further, the effect of pH on photocatalysis showed the basic medium (pH = 11) necessary for the degradation of MB dye while the acidic medium (pH = 4) was necessary for the degradation of MR dyes. The effective dosage of the catalysts was found to be 0.04–0.06 g/L. The Ce@PyCOFs showed better photocatalytic degradation efficiency than PyCOFs. Further, PyCOFs and Ce@PyCOFs showed non-linear electrical susceptibility with the involvement of rotational diffusion anisotropy (ξ). The PyCOFs and Ce@PyCOFs showed dielectric loss (tan δ) at higher applied frequency. The real (Z′) and imaginary parts (Z″) of the impedance decrease monotonically with an increase in applied frequency and become linear. Cole-Cole studies for PyCOFs showed Nyquist behavior with non-linear electrical susceptibility and change in the phase angle (θ). The resistance (R) decreases with an increase in applied frequency. However, Ce@PyCOFs, showed linear variation in complex impedance spectra indicating no change in the phase angle (θ) and the voltage (V)/current (I) varies linearly with applied frequency. The synthesized Ce@PyCOFs can be utilized for photonics, optoelectronics and photocatalytic device constructions and related applications.