These synthesis procedures are very frequently employed not only for the synthesis for QDs, but also for different range of materials having various size ranges and configuration. Mostly, it involves single-step procedure in which organic precursors are treated in a closed autoclave to prepare the QDs under high pressure and temperature as per the requirement. Here selection of precursors plays a vital role because the source used may not only contain carbon but may also have doping elements which can impact the synthesized QDs structure. Citric acid, L-cysteine, melamine, hydrazine, Polyethylene glycol-400, (1, 3, 6)-trinitropyrene, hydrazine hydrate are often used as CQD precursors where as bromobenzoic acid, citric acid are the different precursors used in synthesis of GQDs. Other than these precursors CQDs are also been prepared via dehydration of glucose in sulfuric acid and nitric acid
[3]. Additionally, the usage of further sulphur and nitrogen doping are observed to make better application outcome of the CQDs or GQDS. For doping purposes, nitrogen (N) and sulphur (S) sources included in the preparation are such as thiourea, urea, hexamethylenetetramine, ethylenediamine and diethyl diethanolamine which are used to synthesize N or S-doped QDs
[3]. However, rather than focusing only on one step approach, researchers have adapted two-step synthesis approach to mainly obtain doped QDs with improved structure and applicability
[16]. For an example, GQDs are prepared with citric acid taking as precursor with basic condition hydrothermal processes, and then the as-synthesized QDs are blended with hydrazine and heated for several hours to prepared N-doped QDs. Not only CQDs or GQDs, hydrothermal technique is also been used to prepare CeO
2/Ce
2O
3 quantum dots anchored on reduced graphene oxide sheets of different weight fractions where graphene oxide was synthesized initially using natural graphite flakes via employing modified Hummer’s method
[17]. Cerium nitrate hexahydrate was the precursor used in this preparation. Similarly, single-step solvothermal technique was also employed to synthesize paper-like layered CeO
2 quantum dots doped Ni-Co hydroxide nanosheets which was used as electrode in asymmetric supercapacitor
[18]. Utilizing citric acid, thiourea and ceria like precursors along with hydrothermal method, N and S co-doped graphene quantum dots were grown on CeO
2 nanoparticles and this combination of ceria and graphene quantum dots were found to be providing more active surface area, hence enhanced electrochemical activity
[19].
Hydrothermal method is also used to prepare CuO quantum dots via using copper (II) acetate monohydrate precursor and additionally single layer graphene was added on the CuO quantum dot surface for better structural stability (unique core-shell structure) and improved performance
[20]. Likewise, CuS, SnO
2, NiCo
2O
4 quantum dots were also produced using copper (II) dithiooxamide, stannous chloride dehydrate, nickel acetate tetra-hydrate and cobalt acetate tetra-hydrate precursors respectively
[21][22][23]. In order to synthesize WS
2 and MoS
2 QDs, solvothermal technique is been broadly used
[24]. Precursors like sodium molybdate and cysteine are been used to form MoS
2 QDs where as sodium tungstate and L-glutathione were used to prepare WS
2 QDs
[25][26]. L-glutathione and cysteine is mainly used as the source for sulfide. However, studies have also mentioned the use of thiocarbamide, dibenzyldisulfide, thiourea as the sulfide source in synthesizing WS
2 and MoS
2 QDs
[24]. The key benefits of this method are less hazardous, easier to conduct and less expensive. Moreover, controlling and altering the properties as well as composition of the prepared QDs is possible using this method, which are favorable for application purposes.