  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.
