Controlled synthesis of hierarchical α-nickel molybdate with enhanced solar-light-responsive photocatalytic activity: A comprehensive study on the kinetics and effect of operational factors

The morphology and structural defects of nanocrystalline photocatalysts (NCPHCs) play a pivotal role in their catalytic features. Herein, various morphologies of nickel molybdate (NiMoO ₄ ) NCPHCs were synthesized via a hydrothermal route in the presence of various shape controllers (SCs) followed by a calcination process. The SCs including β-cyclodextrin (β-CD), Tween 20, ethylenediaminetetraacetic acid, and sodium dodecyl sulfate led to the formation of the rod, urchin, sphere, and flake-like morphologies of NiMoO _4, respectively. X-ray diffraction, Raman, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy results corroborated a pure phase of monoclinic crystal structure of NiMoO ₄ by utilizing the SCs in the reaction medium, whereas the SC-free NiMoO ₄ demonstrated the mixed α- and β-phases. To investigate the effect of morphology on the photocatalytic activity of NiMoO ₄ NCPHCs, the photocatalytic decolorization of Everzol Yellow 4GL, a bio-resistant azo dye as a model pollutant, was conducted under simulated solar-light irradiation. β-CD-assisted NiMoO ₄ rod-like NCPHC resulted in a 2-fold higher photocatalytic activity in comparison to the other fabricated morphologies and also a 1.5-fold higher photocatalytic efficiency than that of a commercial nano-ZnO, that can be attributed to the greater availability of hydroxyl functional groups and defects in the crystalline structure. Moreover, the photo-decolorization rate was assessed by first-order, second-order, parabolic-diffusion, and modified-Freundlich kinetic models. Consequently, the result proved that photocatalytic performance is highly depends on the morphology and crystalline-phase of the NCPHCs.