Microstructural tuning and band gap engineering of calcium hydroxides: a novel approach by pH variation

Here we report a simple, inexpensive, energy benign, yet novel pH-driven chemical precipitation technique to achieve microstructural and band gap engineering of calcium hydroxide nanoparticles (CHNPs). The chemical precipitation route involved the use of 0.4-1.6 M Ca(NO₃ )₂ .4H₂ O solutions as the precursor and 1 M NaOH solution as the precipitator. The simple variation in precursor molarity induces a pH change from about 12.4 to 11.3 in the reactant solution. The CHNPs characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), and ultraviolet-visible (UV-Vis) spectroscopy techniques confirm a jump of nanocrystallite size from ~50-70 nm with a concomitant reduction of direct optical band gap energy from ~5.38-5.26 eV. The possible mechanisms that could be operative behind obtaining microstructurally tuned (MT)-CHNPS and band gap engineering (BGE) are discussed from both theoretical and physical process perspectives. Furthermore, the implications of these novel results for possible futuristic applications are briefly hinted upon.