(NH4)2Mo4O13 and h–MoO3 nanocrystalline powders were synthesized by precipitation method at a varied pH range from 5.0 to 1.0. The crystal structure, morphology and optical property of samples were determined by X–ray diffractometer (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and UV–vis diffuse reflectance spectrophotometer (UV-vis DRS). At pH 2.0, 3.0 and 5.0, homogenous plate–like (NH4)2Mo4O13 was seen, whereas the heterogeneous hexagonal rod–shaped MoO3 was found at very low pH of 1.0 and 1.5. Band gap energy of the synthesized (NH4)2Mo4O13 and h–MoO3 were 3.38 and 3.18 eV, respectively. Photochromic properties of the products were illustrated by color difference before and after UV irradiation using CIE Lab color system. The synthesized h–MoO3 provided a strong photochromic performance, while the (NH4)2Mo4O13 showed non–photochromic properties. Intercalation of H+ in h-MoO3 were studied using electrochemical characterization by cyclic voltammetry (CV). The diffusion coefficient of the samples increases with decreasing pH of the solution.
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