**3.2 Nanorods**

Nanorods (**Figure 3**) are one dimensional nanoscale objects. They have a width in the range of 1–100 nm. In the work of J. Ben Naceur et al. [5], the SEM images

**Figure 2.** *SEM image of TiO2 nanowires. (a) TEM image of ITO-TiO2 core-shell nanostructure (b).*

reveal that the entire surfaces of the FTO substrate is uniformly coated by TiO2 nanorods with an average length and a diameter equal to 1 μm and 60 nm, respectively. Titanium dioxide nanorods arrays (NRAs) photoanodes have been grown by the hydrothermal method on FTO coated glass substrates for different hydrothermal reaction time (5, 10, 15 h). Structural and morphological properties of TiO2 films confirms the formation of rutile phase with nanorods morphology. The wettability and photoelectrochemical performances of films were investigated. The wettability tests of the sample elaborated at 10 h revealed that this sample is more hydrophilic among all prepared samples for that, it has the best physical properties with a higher photocurrent density equal to 0.22 mA.cm−1 at 0.5 V vs. Ag/AgCl.

### **3.3 Nanotubes**

Nanotubes are typically long and thin cylindrical protrusions with sub-micron diameter and lengths in the order of several 100 μm. The SEM images (**Figure 4**), in the work of T-H. Meen et al. [6] show the formation of the TiO2 nanotubes. To prepared TiO2 nanotube arrays the electrochemical anodization was used and was tested as photoelectrode of dye-sensitized solar cells. In the SEM analysis, the lengths of TiO2 nanotube arrays prepared by electrochemical anodization was approximately 10 to 30 μm. After titanium tetrachloride (TiCl4) treatment, the walls of TiO2 nanotubes were coated with TiO2 nanoparticles. XRD patterns showed that the oxygen-annealed TiO2 nanotubes have a better anatase phase. The conversion efficiency with different lengths of TiO2 nanotube photoelectrodes is 3.21%, 4.35%, and 4.34% with 10, 20, and 30 μm, respectively. The electrochemical impedance spectroscopy analysis, show that the value of Rk (charge transfer resistance related to recombination of electrons) decreases from 26.1 to 17.4 Ω when TiO2 nanotubes were treated with TiCl4. The treated TiO2 nanotubes with TiCl4 show that the surface area of nanotubes increase, resulting the increase of dye adsorption and the increase of the conversion efficiency of DSSCs.

#### **Figure 4.**

*SEM images of TiO2 nanotubes (a) top view and (b) side view before TiCl4 treatment, (c) top view and (d) side view after TiCl4 treatment [6].*

#### **3.4 Nanosheets**

Nanosheet is a two-dimensional nanostructure with thickness in a scale ranging from 1 to 100 nm. As described in work of F. Li et al. [7] the scanning electron microscopy (SEM) image (**Figure 5**) show a single layered 2D morphology of TiO2 nanosheets. TiO2 nanosheets are a good carrier of photocatalytic materials and have become attractive materials in the new century because of their high active surface exposure characteristics and special morphology. The preparing TiO2 nanosheets, was made via hydrothermal calcination method. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and UV–visible diffuse reflection absorption spectra (DRS) were used to characterize the structure and morphology of the TiO2 nanosheets. The suitable calcination temperature was 400°C to obtain the TiO2 nanosheets, with a good hydrogen production rate of 270 μmol/h. The sheet structure of the material was beneficial for improving the photocatalytic water splitting

**Figure 5.** *SEM images of the single-layered 2D mesoporous TiO2 nanosheets [7].*

hydrogen production performance. The research in photocatalytic water splitting of TiO2 thin films to produce hydrogen are currently a promise topic.
