Solid-state properties and colloidal stability of thorium(IV)–silica nanoparticles

Thorium(IV) is able to form meta-stable colloids with silica in aqueous solution at pH ⩾ 7 which results in concentrations of colloid-borne Th(IV) of 10−3 M. The colloid structure can be regarded as amorphous because it shows no long-range order, indicated by the absence of distinct structural periodicity >4 Å. The internal structure of the colloid particles consists of [Th(O(H))n] polyhedra, n = 8–9, coordinated partly by [SiO4] polyhedra with Th–Si distances of 3.25 ± 0.02 Å, and partly by [Th(O(H))n] polyhedra with Th–Th distances of 3.98 ± 0.02 Å. The near-order coordination shows similarity with that of the orthosilicates thorite, α-ThSiO4, and huttonite, β-ThSiO4. XPS analysis of the oxygen bonds revealed the presence of O2−, OH− and H2O. The colloids can be classified as oxyhydroxo colloids [(Th,Si)On(OH)4−n·xH2O]4−2n−(4−n). Silica occurs in the colloid structure either in mononuclear or oligomeric units, depending on the Si/Th ratio and the silica precursor formed in the initial solution. Silica is enriched at the colloid surface if the concentration of the initial solutions is increased. The solution behavior of the particles was analyzed by in situ methods. With rising silica content, the particles gradually change from metal oxide type colloids to silica type colloids which can dramatically increase their colloidal stability.