New generation of continuous hydrothermal flow synthesis materials for environmental applications

The demands for new materials such as carbonaceous nanomaterials and their nanocomposites that are produced in a more efficient, economical, and environmentally friendly manner, as well as the need to integrate them into applications that address global issues, is a current challenge. The focus of this doctoral research project was to develop faster, cleaner synthetic methods to produce materials with superior physical and chemical properties utilising sustainable and/or renewable precursors, ultimately delivering solutions to challenges in environmental applications and beyond. This was accomplished using a continuous hydrothermal flow synthesis approach.
Continuous hydrothermal flow synthesis is an unconventional method that uses supercritical water as a reaction environment, and it was designed for the very fast, continuous flow production of high quality and high quantity nanomaterials, with real-time control over the process parameter (temperature, pressure, precursors concentration, pH, and flow rates). For the first time, by developing and applying CHFS methodologies, carbon quantum dots (CQDs) and nitrogen-doped carbon quantum dots (NCQDs) were synthesized. The CHFS CQDs and NCQDs were produced from biomass-related precursors: glucose and citric acid; the synthetic process can be classified not only as green but sustainable too. The reduction of graphene oxide with a non-corrosive and reusable reducing agent (formic acid) was achieved. The reduced graphene oxide with different oxygen content (17.37 at%, 16.82 at% and 13.3 at%) was synthesised. A new CHFS method to produce nano TiO2 (anatase) has been developed to pioneer the one-pot synthesis of carbonaceous nanocomposites of TiO2 with NCQDs and reduced graphene oxide (rGO).
All the materials produced in this study have been characterised and tested in environmental-related applications: a) toxic ions sensing (CQDs, NCQDs) with promising limits of detection for hexavalent chromium (LODCQDs= 3.62 ppm and LODNCQDs=0.365 ppm),
b) graphene-related membrane-based water treatment with good performances, and photocatalysis where the TiO2 and its carbonaceous nanocomposites were tested for photodegradation of methylene blue with excellent conversions and rates constants (the best photocatalyst, TiO2-NCQDs-rGO(1), showed a conversion of 93.45% and a rate constant of 25.24 x10-5 s-1). This research project designed and engineered new promising carbonaceous materials, expanding the CHFS portfolio to new frontiers. The as-prepared materials exhibited superior physical, chemical, and morphological characteristics demonstrating their potential for environmental applications and beyond.