Controllable synthesis of (NH₄)Fe₂(PO₄)₂(OH)·2H₂O using two-step route: Ultrasonic-intensified impinging stream pre-treatment followed by hydrothermal treatment

(NH₄)Fe₂(PO₄)₂(OH)·2H₂O samples with different morphology are successfully synthesized via two-step synthesis route – ultrasonic-intensified impinging stream pre-treatment followed by hydrothermal treatment (UIHT) method. The effects of the adoption of ultrasonic-intensified impinging stream pre-treatment, reagent concentration (C), pH value of solution and hydrothermal reaction time (T) on the physical and chemical properties of the synthesised (NH₄)Fe₂(PO₄)₂(OH)·2H₂O composites and FePO₄ particles were systematically investigated. Nano-seeds were firstly synthesized using the ultrasonic-intensified T-mixer and these nano-seeds were then transferred into a hydrothermal reactor, heated at 170 °C for 4 h. The obtained samples were characterized by utilising XRD, BET, TG-DTA, SEM, TEM, Mastersizer 3000 and FTIR, respectively. The experimental results have indicated that the particle size and morphology of the obtained samples are remarkably affected by the use of ultrasonic-intensified impinging stream pre-treatment, hydrothermal reaction time, reagent concentration, and pH value of solution. When such (NH₄)Fe₂(PO₄)₂(OH)·2H₂O precursor samples were transformed to FePO₄ products after sintering at 650 °C for 10 h, the SEM images have clearly shown that both the precursor and the final product still retain their monodispersed spherical microstructures with similar particle size of about 3 μm when the samples are synthesised at the optimised condition.