Effect of pH on Structural and Optical Properties of MagnesiumChromatesNanocrystalsSynthesized by Combustion Method

Abstract

In this work magnesium chromate (MgCr2O4)samples were produced by combustionmethod. Sample1 was prepared without adding ammoniahydroxide (pH = 0.96) during synthesis steps whilst sample 2 was prepared with ammonia hydroxide (pH = 8.62).Sample 1 was obtained in 2:10 h whilstsample 2 was produced in 45 min. Effect of pH upon structural properties wereinvestigated by X-rays diffraction (XRD) technique. Theresults confirm the formation of the cubic spinel structurewith average crystallite sizes of 13.0 nm and 8.0 nm for sample 1 and sample 2,respectively. The lattice parameters were calculated usingBragg’s law and found to be 8.30Å and8.33Å for sample 1 and sample 2, respectively.The X-rays densitiesvalues were 2.227× 106g/cm3 and 2.206× 106 g/cm3 for the sample 1 and sample 2, respectively.The obtainedmicrostrain values were0.007148 and 0.015206 for sample 1 and sample 2,respectively. The opticalproperties were studied using Ultra violate- Visible (Uv-Vis) absorption technique. The resultsshow that MgCr2O4 samples have a direct band gap of3.007 eV and 3.057 eV for the sample 1 and sample 2, respectively.It observed that the band gapof sample 2 was slightlygreater compared to sample 1. This associated with crystallite sizes of the samples. It can be concluded that control of pH can significantly reduce the duration ofcombustion synthesis method.