Solvo-/hydrothermal synthesis method

The solvo-/hydrothermal method takes place in a closed reaction vessel called an autoclave, where high pressures can be obtained at relatively low temperatures, with the possibility to use different solvents like water, ethanol, or polyols (Yang and Park 2019; Islam et al. 2022). Apart from ensuring elevated product purity and crystalline quality, hydrothermal techniques regulate the ultimate nanostructure dimensions, configuration, and crystal phase within a minimally polluted closed system setting (Cavalu et al. 2018).

Through this method, control of morphology and crystallinity can be obtained for the fine powders. The solvothermal method is the simplest way to produce ZnO NPs, at low pressure and a temperature equal to or higher than the boiling point of the solvent used in the reaction. Depending on the polarities of the solvent, different morphologies can be obtained from nanorods, sheets, and even nanocomposites in which ZnO of about 5 nm is deposited in sheets of graphene (Kunjara Na Ayudhya et al. 2006; Lu et al. 2008; Wu et al. 2011).

The pH of the solution is important in the morphology of ZnO NP nanostructures, especially by the chemical precipitation method. Particle size decreases with increasing pH by dissociating OH ions at high pH (Magesh et al. 2018; Mahajan et al. 2019; Alias et al. 2010). The crystallinity and uniformity of the particles are also obtained by chemical precipitation, in an aqueous or non-aqueous medium, in the presence of a reducing agent, followed by calcination (ChangChun Chen et al. 2008; Ching-Fang Liu et al. 2018). As in the case of the hydro-/solvothermal method, the polarity of the solvent is important, and reproducible nanostructures can be obtained by adding a non-polar (hexane) or weakly polar (acetone) solvent which favors chemical precipitation of high surface area ZnO nanoparticles and reproducible morphological structures, but with a tendency to aggregate, which is why the stabilizing agent is also important (Halaciuga et al. 2011; Dutta et al. 2012). Low pH values lead to the dissociation of Zn²⁺ ions, in hydrothermal synthesis, the pH being almost neutral to alkaline to favor the hydrolysis of the Zn precursor in the presence of hydroxyl ions (Ching-Fang Liu et al. 2018). In the case of hydrothermal synthesis, the pH variation from 7 to 13 has an effect on the crystal growth rate and surface energy, obtaining various morphologies such as nanorods with hexagonal ends, spheroidal disc and hexagonal, porous hexagonal nanorods, and porous hexagonal nanorods assembled into nanoflower structures (Kumaresan et al. 2017).

The hydrothermal method uses high pressure and temperature, in the presence of which heterogeneous reactions take place in the presence of solvents (Adeola et al. 2022; Medina-Ramírez et al. 2015). TiO₂ nanorods can be obtained (Muduli et al. 2011; Gao et al. 2015), CuO (Outokesh et al. 2011; Prathap et al. 2012), ZnO (Bin Liu and Zeng 2003; Gerbreders et al. 2020), MnO₂ (Subramanian et al. 2005; Chu et al. 2017), etc. It is also possible to obtain hybrid composites used in degradation processes, such as TiO₂ doped with boron and nitrogen for the degradation of bisphenol A (BPA) and TiO₂-Bi₂WO₆ composite was developed for Rhodamine blue degradation (Abdelraheem et al. 2019; Hou et al. 2014).

Innovative photocatalysts composed of PVDF/ZnO/CuS were developed using electrospinning, hydrothermal treatment, and ion exchange techniques with the purpose to address the issue of particle aggregation in an aqueous environment (Zang et al. 2022). These photocatalysts demonstrated excellent stability during recycling and reuse. ZnO nanorods were firmly attached to PVDF nanofibers, serving as a support structure. Additionally, CuS NPs were introduced as photosensitizers to enhance the visible light photocatalytic efficiency and compensate for the relatively low quantum efficiency of ZnO. The results demonstrated superior photocatalytic performance in the degradation of MB under both UV and visible light, with kinetic constants of 9.01 × 10⁻³ min⁻¹ for UV irradiation and 6.53 × 10⁻³ min⁻¹ for visible light. Before the hydrothermal treatment, the morphology of PVDF nanofibers appeared relatively smooth, with each nanofiber having a diameter of approximately 300 nm (Fig. 2a). However, the diameter distribution was somewhat uneven. After the hydrothermal process, a multitude of neatly arranged ZnO nanowhiskers enveloped the nanofibers (Fig. 2b), significantly increasing their specific surface area. Subsequently, in situ reduction techniques uniformly distributed CuS nanoparticles on the ZnO nanorods (Fig. 2c). Transmission electron microscopy (TEM) images revealed crystal spacings of 0.282 nm and 0.305 nm, corresponding to the (100) crystal plane of ZnO (wurtzite-type) and the (102) crystal plane of CuS (Fig. 2d, e). The interface between ZnO and CuS, marked with a red line, confirmed the successful construction of the p-n heterojunctions (Fig. 2f).

SEM images of a PVDF, b ZnO@PVDF, and c PVDF/ZnO/CuS nanocomposites. d–f TEM images and g EDX mapping of PVDF/ZnO/CuS nanocomposites (Zang et al. 2022) (open access)

As the process is considered environmentally advantageous, it is incorporated into eco-friendly approaches for synthesizing ZnO NPs. Nevertheless, this approach comes with certain drawbacks. For instance, it necessitates the use of a highly costly autoclave and imposes restrictions on research due to the inability to keep the reactor open. Other disadvantages are represented by the toxicity of some solvents that are used in this process; the reactions can take place in long periods of time (5–48 h) (Verma et al. 2021).