**2.2 Hydrothermal method**

Hydrothermal method is one of the most used route for nanomaterials synthesis. BiOI nanoflowers/TiO2 nanotubes were developed for the detection of atrazine [24]. The sensing platform showed good analytical performance for detecting atrazine.

Alev et al. [25] prepared TiO2 nanorods, with diameter of 100 nm, by hydrothermal using titanium butoxide, hydrochloric acid and deonised water. They concluded that the sensor response was 200% for 1000 ppm H2. Additionally, TiO2 NPs with size of 20 nm were prepared by hydrothermal method [26]. **Figure 2** shows the TEM image of TiO2 NPs. The result obtained by UV-VIS analysis revealed that the decrease in size of TiO2 NPs is beneficial to the blue shift of their absorption peak.

Le et al. [27] synthesized TiO2/graphene by hydrothermal method using TiCl4 as a precursor. High performance was attained for the catalysts including well dispersed TiO2 NPs on the graphene surface with loadings ranging from 16.5% to 26%. Similarly, Yang et al. [28] prepared TiO2 NPs by hydrothermal. The results revealed that the peptization of the precipitate favored formation of the rutile phase and highly crystalline anatase. Europium (Er) doped TiO2 NPs were prepared by hydrothermal method for photonic application [29]. TEM analysis showed that the average particle size was about 50 nm. Indeed, the Er doping leads to a change in morphology of NPs from rodlike to triangular for Er ions increased from 1 to 3 mol%, respectively (as presented in **Figure 3**).

Ag doped TiO2 NPs, with crystallite size of 10/13 nm, were prepared via hydrothermal at temperature of 180°C for 120 min [30]. It was revealed that the maximum photodegradation of indigo blue attained 75% after irradiation time of 150 min. Dadkhah et al. [31] prepared anatase TiO2 NPs by hydrothermal. They achieved conversion efficiency higher than 2.61% with the influence of amine ligands as a shape controller.

**Figure 2.** *TEM image of TiO2 NPs [26].*

**Figure 3.**

*TEM images of Er doped TiO2 NPs: (a) 1 Mol %, (b) 2 Mol%, and (c) 3 Mol% Er2+ [29].*
