Deciphering the role of water and a zinc-doping process in a polyol-based approach for obtaining Zn/Co/Al-based spinels: toward “green” mesoporous inorganic pigments†
Two new families of zinc/cobalt/aluminum-based pigments, with a unique composition, were obtained through the polyol method. The hydrolysis process of a mixture of Co(CH3COO)2, Zn(acac)2 and Al(acac)3 (acac− = acetylacetonate ion) in 1,4-butanediol afforded dark blue gels (wPZnxCo1−xAl), in the presence of a supplementary amount of water, and light green powders (PZnxCo1−xAl), respectively, for the water-free procedure (x = 0, 0.2, 0.4). The calcination of the precursors yielded dark green (wZnxCo1−xAl) and blue (ZnxCo1−xAl) products. XRD measurements and Rietveld refinement indicate the co-existence of three spinel phases, in different proportions: ZnxCo1−xAl2O4, Co3O4 and the defect spinel, γ-Al2.67O4. The Raman scattering and XPS spectra are in agreement with the compositions of the samples. The morphology of wZnxCo1−xAl consists of large and irregular spherical particle aggregates (ca. 5–100 mm). Smaller agglomerates (ca. 1–5 mm) with a unique silkworm cocoon-like hierarchical morphology composed of cobalt aluminate cores covered with flake-like alumina shells are formed for ZnxCo1−xAl. TEM and HR-TEM analyses revealed the formation of crystalline, polyhedral particles of 7–43 nm sizes for wZnxCo1−xAl, while for ZnxCo1−xAl, a duplex-type morphology, with small (7–13 nm) and larger (30–40 nm) particles, was found. BET assessment showed that both series of oxides are mesoporous materials, with different pore structures, with the water-free samples exhibiting the largest surface areas due, most likely, to the high percent of aluminum oxide. A chemical mechanism is proposed to highlight the role of the water amount and the nature of the starting compounds in the hydrolysis reaction products and, further, in the morpho-structural features and composition of the resulting spinel oxides. The CIE L*a*b* and C* colorimetric parameters indicate that the pigments are bright, with a moderate degree of luminosity, presenting an outstanding high blueness.