Magnetic response of photonic crystals based on nucleating agents of binuclear complexes

The solvothermal reduction method, utilizing a single FeCl3 as the iron source, is considered to be one of the effective ways to synthesize Fe3O4 magnetically responsive photonic crystals (MRPCs). However, some studies have revealed that Fe3O4 synthesized via this method cannot form a photonic crystal structure when subjected to a magnetic field, and the underlying reason for this inconsistency remains unreported. In our investigation, we have discovered that the presence of trace amounts of Fe2+ in the FeCl3 reagent is essential for successfully producing Fe3O4 MRPCs through this method. Moreover, the size, saturated magnetization value, and photonic tuning range of MRPCs can be controlled by adjusting the Fe2+ content. By analyzing the structure of the precursor formed during the reaction, we identified the formation of a binuclear iron structure in the precursor. This binuclear iron structure plays a pivotal role in the formation of Fe3O4 MRPCs. Based on these findings, we propose a new perspective that the binuclear iron complex serves as a nucleating agent for the formation of Fe3O4 MRPCs. The proposed view of nucleation could have considerable effect on the preparation of mixed-valence metal oxides through the solvothermal reduction route, and the results provide a facile method to modulate MRPCs, which is important for future applications in sensors, color displays, and gratings. In the synthesis of Fe3O4 MRPCs via the solvothermal reduction route, The formation of a binuclear iron complex involving both Fe2+ and Fe3+ is key to imparting the product with monodispersity, superparamagnetism and magnetochromatic properties.