# NiO-Based Electronic Flexible Devices
## 2 Synthesis and Properties of NiO Nanoparticles and Thin Films  
The NiO synthetic procedures largely condition its properties and make it suitable for different applications. Solvothermal (including sol-gel) and hydrothermal syntheses are among the most largely used methods, since they usually provide a facile way to tune size and shape of the NiO particles. Several parameters can be varied in both cases, such as type of Ni salt, type of base, temperature, solvent, and possible employment of a template. The employment of an autoclave also influences the course of the reaction due to the vapor pressure in a sealed vessel. Finally, the calcination phase, which typically follows, also plays a role, since it can shape the material depending on the temperature ramp and final temperature. In addition, the phytogenic synthesis has become an interesting variant of the sol-gel method, where leaf extracts of plants such as _G. Sylvestre_[78] or _Ageratum conyzoides_ L. [79] are used to form a gel which is to be removed during the calcination phase. NiO thin films can be achieved through several techniques: RF sputtering, electron beam evaporation, DC magnetron sputtering, anodic and cathodic electrodeposition, chemical vapor deposition, and nanoparticles electrospray [80]. A sketch of common synthetic pathways of NiO is reported in Figure 1, along with two examples of NiO thin film synthesis. Finally, the dealloying technique has been successfully employed for achieving all-solid flexible materials, starting from various metals and metal oxides as an implementation of the achievement of flexible porous gold by the removal of silver from a gold-silver alloy with nitric acid [81]. The type of conditions employed in the synthesis reflect onto the NiO properties. There are several examples of tuning the NiO features via the employment, for instance, of different types of bases which may influence the precursors structural parameters, such as the planar distances. The NiO morphology and the ensuing capacitance can be influenced by the calcination procedure [52; 64; 82]. The size of NiO nanoparticles is directly correlated to the band gap and can be estimated through the Brus relationship for spherical shapes [83]. Similarly, size and shape of the nanoparticles impact the NiO capacitance which is among the mostly exploited properties. Other relevant properties useful for building devices are the resistance variation with the temperature (Seebeck effect), the refractive index variation as a function of light wavelength, the hole mobility, and the resistance variation upon voltage application, which can be used for temperature devices, electrochromic devices, solar cells, and ReRAM, respectively. The employment of NiO, either as nanoparticles or as thin layers in the actual construction of the various devices requires the embedding of NiO into matrices or the deposition on supports and ensuing incorporation in the device, with associated issues of integration feasibility and configuration design. The interaction NiO-matrix or NiO-support may influence the NiO properties through synergistic effects. Furthermore, the flexibility requirement may introduce the deterioration upon deformation, thus affecting the overall properties of the devices. In Table 1, a few examples of capacitance dependency of NiO from morphologies as well as matrix and coating interactions are reported. Figure 1: Sketch of the most common preparation methods of NiO nanoparticles and thin films: (**a**) photosynthesis, (**b**) precipitation; (**c**) hydrothermal synthesis, (**d**) electrospray of nanoparticles to achieve thin film, (**e**) an example of RF sputtering technique for thin film synthesis. The blue and orange balls represent Ar and O\({}_{2}\) gases. The employment of NiO on a large scale needs to be offset against major drawbacks such as allergenicity and toxicity. In contact with the skin, NiO can cause allergic reactions [94] which is a problem for wearable devices exposing NiO on the external layer.