Impact of cation redox chemistry on continuous hydrothermal synthesis of 2D-Ni(Co/Fe) hydroxides

TL;DR

This study demonstrates how cation redox chemistry influences the phase and nanostructure of Ni(Co/Fe) hydroxides synthesized via continuous hydrothermal flow synthesis, highlighting process control for scalable production.

Contribution

It reveals the role of redox chemistry and synthetic parameters in controlling phase transitions in Ni(Co/Fe) hydroxides during continuous hydrothermal synthesis.

Findings

Redox chemistry governs phase transition from brucite to LDH.

Oxidizing and complexing agents influence Ni/Co oxidation states.

NiFe-hydroxides form without oxidants due to Fe3+ stability.

Abstract

Continuous hydrothermal flow synthesis (CHFS) is a facile, upscalable and cost-efficient synthetic method enabling the nanostructuring of advanced functional materials in steady conditions, i.e. not in batch synthesis. In this paper, we use CHFS to crystallize NiCo- and NiFe-hydroxides in water solution with 2D nanofeatures. By tuning the synthetic parameters, we disclose the key role of the cation redox chemistry in the transition between two competitive phases: from 2D-nanoplatelets of brucite to layered double hydroxides (LDH). For controlling the precipitation of different Ni, Fe, Co-hydroxide phases, we propose the combined use of an oxidizing (H2O2) and a complexing (NH3) agent. At temperatures as low as 80 {\deg}C, the presence of H2O2 and a low concentration of NH3 favour the Ni2+/Co3+ over Ni2+/Co2+ oxidation states, shifting the product structure from brucite phase (temperatures > 80 {\deg}C) to LDH. Conversely, for the NiFe-hydroxides the transition from LDH (temperatures < 80 {\deg}C) to brucite phase (temperatures > 80 {\deg}C) is controlled by the reaction temperature only. Due to the high stability of Fe3+, the synthesis of NiFe products by CHFS does not require oxidizing and complexing agents, resulting in a robust process for large-scale production.