As nanotechnology becomes increasingly important and ubiquitous, new and scalable synthetic approaches are needed to meet the growing demand for industrially viable routes to nanomaterial production. Continuous-flow hydrothermal synthesis or supercritical water hydrothermal synthesis (scWHS) is emerging as a versatile solution to this problem. The process was initially developed to take advantage of the tunable chemical and physical properties of superheated water to produce metal oxide nanoparticles by rapid nucleation and precipitation. The development of new mixing regimes and reactor designs has been facilitated by the modelling of reactor systems. These new reactor designs further exploit the properties of supercritical water to promote faster and more uniform mixing of reagent streams. The synthetic approach has been expanded beyond the metal oxide systems for which it was conceived, and now encompasses metal sulfides, metal phosphates, metal nanoparticles and metal-organic frameworks. In many of these cases, some degree of size and shape control can be achieved through careful consideration of both chemistry and reactor design. This review briefly considers the development of scWHS reactor technology, before highlighting some of our recent work in expanding the scope of this synthetic method to include a wide range of materials.

Molybdenum disulphide (MoS2) has been widely used as a catalyst and high temperature lubricant. It has been heavily researched recently as a graphene analogue and member of the so-called inorganic fullerenes. Here we report the first continuous flow hydrothermal synthesis of MoS2. With fast reaction times and flexibility the continuous flow hydrothermal system allowed MoS2 to be produced in a stepwise fashion, offering an insight into the mechanism involved. It has been found that the synthesis of MoS2 proceeded via the sulphidation of molybdate anions to thiomolybdate species, which are transformed to amorphous MoS3 by acidification in flow, before further hydrothermal treatment decomposes this amorphous precursor to tangled MoS2 nanosheets.

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