Synergistic adsorption and kinetic studies of heterostructured g-C₃N₄/TiO₂ nano-photocatalyst under visible light for enhanced CO₂ reduction

Graphitic carbon nitride (g-C₃N₄) and titanium dioxide (TiO₂) were synthesized using sol-gel and ultrasonic impregnation technique followed by calcination for photocatalytic CO₂ reduction. The nano-photocatalysts were analyzed for their morphological, structural, and optical characteristics. Scanning electron microscopy (SEM) revealed the presence of spherical and layered sheet-like nanoparticles, as well as the occurrence of minor aggregations. The ultraviolet-visible spectroscopy (UV-vis) revealed that g-C₃N₄ has good photocatalytic properties with a medium band gap (2.7 eV), and TiO₂ has high charge transfer potentials, robust oxidation properties, and high band gap (3.20 eV). However, the larger band gap makes it unresponsive in the visible light spectrum. In order to circumvent this constraint, a hybrid heterostructured g-C₃N₄/TiO₂ catalyst with different compositions, viz., 1:1, 1:2, and 2:1, were fabricated using the ultrasonic impregnation technique followed by calcination process. The optical band gap of g-C₃N₄/TiO₂ nanocomposite shows a red shift towards 2.85 eV from 3.20 eV for bare TiO₂, inferring enhanced absorption in the visible light region. Further, the photocatalytic experiments were performed using visible light sources for all the catalysts. The g-C₃N₄/TiO₂ (2:1) reported higher photocatalytic activity due to its reduced crystallite size of 12.94 nm which were investigated using X-ray diffraction analysis (XRD) and lower band gap of 2.85 eV. The study infers that hybrid photocatalyst enhances the visible light absorption, electron-hole (e - /h +) pair separation rate, and photocatalytic reduction of CO₂. In addition, two adsorption models Langmuir and Freundlich were used and adsorption kinetic data were fitted to pseudo-first-order reaction for all the five catalysts. The adsorption isotherm of CO₂ by g-C₃N₄/TiO₂ (2:1) well fitted by the Freundlich adsorption equation. On the basis of adsorption magnitude (n) values (1.74), it was found that the interaction between CO₂ molecules and g-C₃N₄/TiO₂ occurs according to the chemisorption mechanism. The kinetic study infers that the highest value of apparent rate constant (k_app) was exhibited by g-C₃N₄/TiO₂ (2:1), which indicates that the products predominate at equilibrium.