Nanostructured metal oxides are key to next-generation optoelectronic and energy devices, yet performance hinges on synthesis-controlled particle size, phase purity, and defect chemistry. Here, we report an additive-free aqueous precipitation route using ammonium heptamolybdate tetrahydrate (AHM, \(({\text{NH}}_{4}{)}_{6}{\text{Mo}}_{7}{\text{O}}_{24}\cdot 4{\text{H}}_{2}\text{O}\) ) at controlled concentrations, followed by annealing at 500 °C and brief ball milling (60–120 min), to produce phase-pure, nanostructured α-MoO₃ powders. X-ray diffraction confirms single-phase orthorhombic α-MoO₃; Williamson–Hall analysis indicates milling-time-driven crystallite-size reductions from 52.56 to 15.17 nm in the most dilute series and from 19.73 to 15.26 nm in the more concentrated series. Electron microscopy reveals the formation of α-MoO₃ nanoparticles (~ 20–100 nm) as agglomerates after only 1 h of milling, together with a concomitant transformation from micrometer-long rod-like crystallites to rounded nanoparticles. Energy-dispersive X-ray spectroscopy detects only Mo and O, and high-resolution transmission electron microscopy confirms retention of the characteristic lamellar α-MoO₃ architecture despite milling-induced rounding. Fourier–transform infrared spectroscopy reveals the Mo = O stretching vibration, consistent with the layered orthorhombic phase. UV–Vis diffuse reflectance shows a milling-time-dependent decrease in the optical band-gap from 3.72 to 3.66 eV, 3.73 to 3.62 eV, and 3.72 to 3.69 eV for the first, second, and third concentration series, respectively. Overall, with appropriate parameterization, brief mechanical milling lowers the band-gap without any phase change or loss of the lamellar architecture, providing a robust, scalable route to tailor α-MoO₃ for various applications in optoelectronics.
