MECHANICALLY ACTIVATED MOO3 .2. CHARACTERIZATION OF DEFECT STRUCTURES

The influence of mechanical activation in a planetary mill upon the na ture and concentration of defects in MoO3 powders was investigated by means of diffuse reflectance spectroscopy in the UV/vis regime (DR-UV- vis) and by electron spin resonance (ESR). Defects located at the crys tallite surfaces were characterized by infrared spectroscopy of adsorb ed CO as a probe molecule. For this purpose, MoO3 was ball-milled in a planetary mill during 600 min. In the DR-UV-vis spectra, the valence- to-conduction band transition exhibits a considerable blue shift with decreasing particle size. Furthermore, excitonic absorptions observed in these spectra are also drastically affected by the smaller particle size and probably by the altered crystallite surfaces. An increasing intensity of the polaron bands was observed. In addition, a linear dep endence was obtained between the position of the band attributed to po laron conductance and the logarithm of the carrier concentration per M o atom. Both the increasing intensity and the shift of the polaron ban d revealed that a substoichiometric MoO3-x was formed upon mechanical treatment. ESR spectroscopy showed that MoO3 milled for 600 min, and u nmilled MoO3 although in much smaller concentration, contained Mo5+ ce nters. The main part of these Mo5+ ions had C-2v or C-4v symmetry. Bot h samples also contained Mo5+ centers interacting with protons in clos e vicinity. Adsorption of O-2 did not lead to paramagnetic broadening; hence these Mo5+ centers are located within the bulk MoO3. In additio n, a signal in the ESR spectra of both samples is assignable to free e lectrons at the crystallite surfaces as revealed by paramagnetic broad ening upon O-2 adsorption. One Mo5+ defect species, however, was only detected in milled MoO3 and attributed to the precursor structure of s hear defects, thus corroborating the reported XRD and HRTEM results. T he high surface sensitivity of the IR technique using adsorbed probe m olecules revealed the formation of coordinatively unsaturated (cus) Mo 4+ surface states in MoO3 samples which were mechanically activated.