Optical and magnetic properties of cobalt(2+) ions in dried and heated silica gels prepared by the sol-gel process

Cobalt–silicon mixed oxide nanocomposites by modified sol–gel method

“…3). At this stage of heat treatment, the tetrahedral complex is likely to be Co(OH) 4 or CoO 4 where ligands come from silica matrix [32]. On the contrary, the spectra of the 20Co-400 and 30Co-400 mirror the partial oxidation of Co 2+ to Co 3+ with the simultaneous Co 3 O 4 formation.…”

“…Therefore, the actual molar ratio required to obtain the siloxane network is less then 1:2 and the use of higher water amounts mainly affects the rate of the above reactions [28,30]. The addition of Co(NO 3 ) 2 Á 6H 2 O to the partially hydrolysed TEOS solution gives rise to a pinkish coloured solution that is indicative of the presence of the [Co(H 2 O) 6 ] 2+ aquo-ion [31,32]. It is known that, in aqueous environment, this complex ion can undergo the acid-base equilibria forming Co oxo-hydroxo complexes that, in the presence of growing clusters containing hydroxo functionalities, Si(OR) x (OH) y , could give rise to condensation reactions forming Co-O-Si bridges [30,31].…”

“…The d-d transitions of Co 2+ ions and the O-Co 2+ charge transfer transitions are expected to occur in the visible and ultraviolet region, respectively [36]. In the visible region, the 10Co dried gel shows three bands at 525, 575 and 650 nm that are related to the 4 [24,32,37]. The relative intensity of the triplet components does not correspond to the one of a typical complex ion in aqueous solution since the intensity of the component at 525 nm appears enhanced, while that of the component at 650 nm results to be reduced.…”

Influence of sol–gel conditioning on the cobalt phase and the hydrothermal stability of cobalt oxide silica membranes

“…On the other hand, Co 2 þ Si was blue and major peaks at 524, 584 and 650 nm attributed to the 4 A 2 (F)-4 T 1 (P) transition of Co 2 þ in a tetrahedral environment (T d ) were observed. The tetrahedral environment is assumed to be [CoO 4 ] and/or [Co(OH) 4 ] where the ligands originate from the silanol and/or siloxane groups of silica matrix [37][38].…”

Fe3+ doped SiO2 nanostructured gel-glasses: Structural, optical and magnetic properties

“…Some of the studies on nanostructured gel-glasses are either concerned with properties related to isolated transition metal ions [1,2], the influence of doping on the gel-glass structure [3] or with the incorporation of nanoscale metal or metal oxide particles [4]. Incorporation of magnetic metal ions in the silica gel-glass matrices to obtain magneto-optical nanostructured glasses is an interesting field of study.…”