**2.3 Sol-gel method**

Sol-gel process is a powerful pathway for the synthesis of multi-component materials because of its mild synthesis conditions and low temperature. Thus, several researches have been reported on the fabrication of TiO2 NPs by sol-gel method. For example, Sabry et al. [32] synthesized TiO2 NPs by sol-gel process. The prepared material showed efficient photocatalytic activity of up to 68% after 180 min. Hsiung et al. [33] investigated the structure of photocatalytic active sites of TiO2 NPs prepared by sol-gel. They concluded that the material exhibited an excellent photocatalytic activity. Additionally, TiO2 NPs used as catalyst was prepared by sol-gel in acid at pH 3 [34]. The result showed that the material exhibited excellent reactivity for the photocatalytic reduction of nitric oxide.

Venkatachalam et al. [35] prepared alkaline earth metal (Mg2+ and Ba2+) doped TiO2 NPs by sol-gel method using titanium isopropoxide as precursor. **Figure 4** illustrates the SEM image of the metal-doped TiO2 NPs, which are spherical in shape. Furthermore, the final product exhibited higher photocatalytic activity for the bisphenol.

Saravanan and Duby [36] investigated the optical and morphological properties of TiO2 NPs synthesized via sol-gel method using titanium butoxide as a precursor. UV-Visible analyses revealed the absorbance peak in the UV region (about 380 nm) and FTIR spectrum confirmed the existence of anatase TiO2 in the range of 400– 1000 cm−1. The average particle size of the TiO2 NPs determined by dynamic light scattering (DLS) was found 131 nm. Govindaraj et al. [37] synthesized TiO2 NPs to be used as a photo-anode by the sol-gel method. UV-Visible spectrum revealed the light absorption in the UV region with optical bandgap of 3.2 eV (see **Figure 5**).

Sinha et al. [38] studied the structural, optical, and antibacterial performance of the Mg doped TiO2 NPs prepared by the sol-gel method. They reported that optical transmittance increases from 3 to 3.07 eV. In addition, the photoluminescence emission shows inner UV to blue resign from pure and doped TiO2 NPs. Furthermore, Mugundan et al. [39] synthesized barium doped TiO2 NPs by the sol-gel method. They concluded that the pure TiO2 NPs revealed higher second harmonic generation efficiency compared to barium doped TiO2 NPs. Nachit et al. [40] investigated the photocatalytic activity of TiO2 NPs prepared by sol-gel process at low temperature. The mean crystallite size of TiO2 NPs reached 30 nm at 500°C using an acid. In addition, the photocatalytic activity of TiO2 NPs revealed that the degradation of Rhodamine B under UV light have a removal efficiency of 95% during 60 min.

### **Figure 4.**

*SEM image of metal-doped TiO2 NPs prepared by sol-gel [35].*

### **Figure 5.**

*Schematic diagram of the sol-gel route used for preparing TiO2 NPs [37].*
