Structural evolution in crystalline MoO3 nanoparticles with tunable size

In this study MoO3 nanoparticles were prepared in porous Vycor glass by impregnation–decomposition cycles (IDC) with molybdenum(VI) 2-ethylhexanoate. X-ray diffraction data show that the nanoparticles are crystalline and are in the orthorhombic α-MoO3 phase. Raman spectroscopy data also indicate the formation of this phase. The profiles in the Raman spectra changed with the number of IDC, indicating a structural evolution of the MoO3 nanoparticles. The IDC methodology promoted a linear mass increase and allowed tuning the nanoparticle size. Analysis of HRTEM images revealed that for 3, 5 and 7 IDC, the MoO3 nanoparticle average diameters are 3.2, 3.6 and 4.2 nm. Diffuse reflectance spectroscopy indicates a consistent red shift in the band gap from 3.35 to 3.29 eV as the size increases from 3.2 to 4.2 nm. This observed red shift in the band gap of the MoO3 nanoparticles is presumably due to quantum confinement effects.