With the increase in demand of textiles products, the polluted groundwater problem in the world is increasing proportionally and leading to environmental and health problems. Hence in this present work, we report on the dye removal efficacy of nano Ca(OH) 2 by changing the molarity of the reactants. In the process, the chemical precipitation technique is used to synthesize the phase pure nano Ca(OH) 2 powders using equimolar concentrations (e. g., 0.4 M, 0.6 M, 0.8 M and 1 M) of [Ca(NO 3 ) 2 ⋅ 4H 2 O] and NaOH solutions. The prepared material was characterized using XRD, FTIR, and UV-Vis spectroscopy. The results from the characterization study show, that the dye removal behaviour is complex and is sensitive to a simultaneous combination of many factors e. g., the molarities of the reactants, the resultant nanocrystallite size, band gap energy, the concentration of the dye and, the temperature of adsorption. Therefore, these results are discussed in terms of relative variations in the microstructure, lattice strain, band gap energy, defect structure, and the amount of (OH À ) ions. Further, the probable mechanism of dye removal behaviour is suggested. Finally, the consequences of these results in terms of microstructurally tuned nano Ca(OH) 2 materials development for prospective futuristic applications are highlighted.

Nickel hydroxide nanoparticles (NHNPs) are extremely important semiconducting materials for applications in energy storage and energy harvesting devices. This study uses a novel variation in molarity of the sodium hydroxide (NaOH) precipitator solution to enhance the direct optical band gap in the NHNPs chemically synthesized by using nickel nitrate hexahydrate (Ni(NO 3 ) 2 Á6H 2 O) as the precursor. The simple, energy benign chemical precipitation route involved the usage of 1 M (Ni (NO 3 ) 2 Á6H 2 O) solutions as the precursor and 0.4 M, 0.6 M, and 0.8 M NaOH solutions as the precipitator solutions. The simple variation in precipitator molarity induces an increase in pH from about 6.9 to 7.5 of the reactant solution. As the molarity of the precursor solution does not change, the change in pH of the reactant solution is equivalent to the change in the pH of the precipitator solution. The NHNPs characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), dynamic light scattering (DLS), Fourier-transform infrared (FTIR) and ultraviolet-visible (UV-vis) techniques confirm a reduction of the nanocrystallite size from about 6.8 to 4.5 nm with a concomitant enhancement in the direct optical band gap energy from about 2.64 to 2.74 eV. The possible mechanisms that could be operative behind obtaining microstructurally tuned (MT)-NHNPs and band gap engineering (BGE) of the MT-NHNPs are discussed from both theoretical and physical process perspectives. Further, the implications of these novel results for possible future applications are briefly touched upon. The reported results might be useful to assess the material as an active electrode to improve the performance of batteries.