Effect of Fe₂O₃@SiO₂ Nanoparticle Doping Ratio on the Colour of Synthetic Precious Fire Opal

Precious opal displays iridescent play-of-colour which is due to the diffraction effects of visible light by the three-dimensional periodic ordering of uniformly sized silica nanoparticles^[1]. These properties make precious opal a useful prototype for 3D photonic crystals. More and more researchers attempted to synthesize precious opal not just for gemmological application but for potential applications in photonics, optoelectronics, and sensors^[2].

Among the wide variety of opal types, precious fire opal has captured people due to its simultaneous display of play-of-colour and red-orange-yellow body colour. Some previous studies believe that the dispersion of Fe₂O₃ nanoparticles in precious fire opal is one of the main causes of its body colour. The body colour would become darker when the concentration of Fe₂O₃ nanoparticles increases^[3]. In addition, the presence of Fe₂O₃ nanoparticles is equivalent to introducing defects in the opal structure. The increasing concentration of Fe₂O₃ nanoparticles will change the periodic dielectric structure of precious fire opal and ultimately affect the play-of-colour. To study these phenomena quantitatively, the best strategy is to synthesize a series of fire opal with different Fe₂O₃ nanoparticle doping ratios.

Fe₂O₃ nanoparticle is one kind of wildly used red inorganic pigments with strong tinting strength^[4]. However, only a few studies have been reported on synthetic opal with doped Fe₂O₃ nanoparticles, maybe due to the unstable properties of bare Fe₂O₃ nanoparticles. Bare Fe₂O₃ nanoparticles are easy to agglomerate in its dispersed system. At high temperatures, bare Fe₂O₃ nanoparticles can be reduced to Fe₃O₄ and cause the colour to turn dark red. In addition, environmental pollution or light exposure can lead to colour fading^[5]. These problems can be solved by encapsulating Fe₂O₃ nanoparticle in the SiO₂ shell. The SiO₂ shell can prevent the aggregation of dispersed particles, adjust the distance between assembled particles, enhance the chemical stability of Fe₂O₃ core particles, and keep the colour stability of synthetic precious fire opal^[6].

The present work aims to deepen the understanding of the colour genesis of precious fire opal and to improve synthetic techniques. We used Fe₂O₃ core with SiO₂ shell nanoparticles (Fe₂O₃@SiO₂) as red pigments and synthesized a series of fire opal films. First, the Fe₂O₃@SiO₂ nanoparticles were prepared by a sol-gel process. Then, Fe₂O₃@SiO₂ nanoparticles with different volume fractions were mixed in a well-dispersed, similar-sized SiO₂ nanoparticle suspension. Finally, the fire opal films were synthesized by the vertical deposition self-assembly method. The nanostructure and colour evolution of synthetic fire opal films with different Fe₂O₃@SiO₂ nanoparticle doping ratios were studied and compared to the results of previous reports. It showed that the saturation and brightness of the body colour were enhanced compared to bare Fe₂O₃ particle-doped opal. This is mainly due to the more even distribution of Fe₂O₃@SiO₂ nanoparticles and the doping of Fe₂O₃@SiO₂ nanoparticles, which leads to fewer structural defects in synthetic fire opal. In terms of structural colour, no effect of Fe₂O₃@SiO₂ doping on the hue of structural colour has been found. However, the doping ratio has an effect on the intensity of structural colour. When the thickness was the same, the structural color intensity decreased first and then increased with the increase of doping ratio, finally exceeding the structural colour intensity of opal films assembled with pure SiO₂ nanoparticles. The present experimental work shows that Fe₂O₃@SiO₂ nanoparticles can effectively improve the colour properties of synthetic fire opal as doped pigments. In the next stage of this study, the physical mechanism of Fe₂O₃@SiO₂ affecting the structural colour intensity of fire opal will be investigated by numerical simulation to provide theoretical support for the application of Fe₂O₃@SiO₂ nanoparticles in synthetic fire opal.