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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2023-08, Vol.52 (3), p.1386-141
• Main Authors: Alexandru, Maria-Gabriela, Ianculescu, Adelina-Carmen, Carp, Oana, Culita, Daniela C, Preda, Silviu, Ene, Cristian D, Vasile, Bogdan Stefan, Surdu, Vasile-Adrian, Nicoara, Adrian-Ionut, Neatu, Florentina, Pintilie, Ioana, Visinescu, Diana
• Format: Article
• Language: English
• Subjects: Aluminum; Aluminum oxide; Butanediol; Cobalt aluminate; Cobalt oxides; Composition; Crystal defects; Flakes (defects); Gels; Hydrolysis; Luminosity; Moisture content; Morphology; Pigments; Raman spectra; Reaction products; Silkworms; Spinel; X ray photoelectron spectroscopy; Zinc
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d3dt00972f

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(CH 3 COO) 2 , Zn(acac) 2 and Al(acac) 3 (acac − = acetylacetonate ion) in 1,4-butanediol afforded dark blue gels ( wP Zn x Co 1− x Al ), in the presence of a supplementary amount of water, and light green powders ( P Zn x Co 1− x Al ), respectively, for the water-free procedure ( x = 0, 0.2, 0.4). The calcination of the precursors yielded dark green ( w Zn x Co 1− x Al ) and blue ( Zn x Co 1− x Al ) products. XRD measurements and Rietveld refinement indicate the co-existence of three spinel phases, in different proportions: Zn x Co 1− x Al 2 O 4 , Co 3 O 4 and the defect spinel, γ-Al 2.67 O 4 . The Raman scattering and XPS spectra are in agreement with the compositions of the samples. The morphology of w Zn x Co 1− x Al 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 Zn x Co 1− x Al . TEM and HR-TEM analyses revealed the formation of crystalline, polyhedral particles of 7-43 nm sizes for w Zn x Co 1− x Al , while for Zn x Co 1− x Al , 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. Two series of Zn/Co/Al spinel type pigments were obtained through polyol-assisted synthesis, the influence of the water amount and Zn-doping process on their composition, morphology, and chromatic performances being investigated here.