Rapid one-pot hydrothermal reaction for preparing BaCu2Si2O7 fine particles with controlled blue colour tonality

Abstract

A new chemical approach to control the preparation of BaCu2Si2O7 fine particles employing alkaline NaOH solutions coupled with a variation of the nominal mixed Cu2+ content was investigated aiming to crystallise BaCu2Si2O7 particles successfully. The concentration of the NaOH solutions varied between 0.25 and 3.0 M, and the mixed Cu2+ content varied between 1.6 and 2.0 mol. The synthesis of the BaCu2Si2O7 particles occurred at 180 °C for an interval as short as 24 h, even in a 0.25 M NaOH solution with a Cu2+ content of 2.0 mol. The crystallisation of BaCu2Si2O7 particles, free of by-products, was carried out via a one-pot process involving a single-step reaction. The crystallisation of fine octahedral-shaped particles with a size varying from 75 to 200 nm occurred in a 0.25 M NaOH solution at 200 °C for 6 h. These particles exhibited a 3D self-assembling process forming semi-spherical shaped agglomerates (2.25 μm average size) with a pale blue hue. On the contrary, the optical reflectance of the BaCu2Si2O7 particles was tuned in the UV–vis, and blue tonality occurred when the mixed Cu2+ content varied between 1.7 and 1.9 mol. The brighter blue colour tonalities with high UV–vis reflectance is favoured by the formation of fine block-like shaped particles with sizes within the range of 60–125 nm; these particles formed oval-shaped 3D hierarchical agglomerates with an average size of 0.6 μm. All the powders prepared by the present method exhibited CIE-L*a*b* values that correspond to the Han blue colour spectral space, but the differences in the colour values are strongly affected by the particle morphological variations.