**4. Conclusions**

In this chapter, we summarized some advances in the synthesis and properties of titanium dioxide nanoparticles. TiO2 is basically found in three crystalline forms: brookite, anatase and rutile. Its important potential application including its use as a food additive, in cosmetics, as a pigment, semiconductor, as well as in catalysis and photocatalysis, for UV adsorption and hydrogen storage has contributed to its massive elaboration by different methods and processes. On the other hand, materials with a nanometric structure display structural, mechanical, physical, chemical, optical and electrical properties that are distinctly improved in comparison to the materials with a micrometric structure. However, each synthesis method allows favoring one or more of the above mentioned properties, allowing to promote the application of the obtained material in a specific field.

Several researches have been made on the preparation and characterization of TiO2 NPs for various applications. Different synthesis methods have been presented to prepare titanium dioxide nanoparticles. For instance polyol process, which combines simplicity and low cost, allows to obtain TiO2 NPs with different shapes and sizes depending on the starting reagents and operating conditions for photocatalytic activities applications. Hydrothermal is the most used method for nanomaterials synthesis and titanium dioxide can be successfully synthesized with different nanoscale shapes as sensors including dispersed TiO2 NPs on the graphene surface. Nevertheless, the sol-gel method remains a powerful alternative for the synthesis of multi-component materials at mild and low temperature conditions leading to efficient photocatalytic activity of TiO2. However, the Chemical vapor deposition process is suitable for the single step synthesis of nanocomposite coatings with enhanced properties. In this context, single layer graphene devices doped with TiO2 have been obtained by CVD. This doping has shown that TiO2 modifies the electronic properties as well as the structure of the CVD grown graphene. On the other hand, 3D porous TiO2 based materials with high catalytic activity and good stability can be obtained through 3D printing technology. Among the simplest and most costeffective processes for nanostructured materials synthesis, mechanical alloying is a very powerful technique for rapid elaboration of TiO2 NPs with excellent photocatalytic activity Nevertheless, compared to conventional methods, green method has been proven to be far more efficient; low cost, and eco-friendly route to the synthesis of TiO2 NPs.

The results obtained in this work enable a better understanding of the synthesis methods as well as the different related properties of titanium dioxide nanoparticles. However, the selection of the synthesis method is conditioned by the required properties of the titanium dioxide NPs and the cost of the final material to be obtained. This is all the more sought after for a value-added and large-scale TiO2 elaboration, which promotes the development of more innovative applications.

XRD, SEM, and TEM are the most used techniques for the nanostructured titanium dioxide characterization. The structural, morphological, and intrinsic properties of TiO2 NPs were also discussed and related to its performance in various applications. Titanium dioxide was a prime candidate material because of its low-cost, high-abundance, and ease of synthesis.
