Hydrothermal synthesis has many unique advantages in the preparation of nanopowders. But the reaction mechanisms and kinetics have not been clearly understood because the reaction was generally occurred in a sealed vessel. In-situ techniques can be used to collect information about the precursors, phase evolution, the fate of the transient intermediates and better understand the nucleation, crystallization and growth by monitoring the whole reaction process in real time. Thus the controlled synthesis of materials can be achieved. This review summarizes the in-situ techniques, in-situ reactors and their applications in understanding the hydrothermal formation mechanisms of nanopowders. These in-situ techniques include synchrotron energy dispersive X-ray diffraction (EDXRD), angular dispersive X-ray diffraction (ADXRD), X-ray absorption fine structure (XAFS), small-angle X-ray scattering (SAXS), neutron powder diffraction (NPD), small-angle neutron scattering (SANS) and several selected spectroscopic methods (UV-Vis, Raman, FTIR).
