**2.5 3D printing method**

In the last few years, several works have been developed to fabricate 3D porous materials; principally 3D porous TiO2 based materials.

Arango et al. [47] prepared a porous TiO2 by 3D printing. They suggested that a large surface area could be realized for the TiO2 via 3D printing technology.

Liu et al. [48] used 3D printing to prepare the porous Pb/TiO2 composites applied to remove the organic contamination in the wastewater. The obtained materials exhibited high catalytic activity, good stability, and reusability against the treatment of high concentration 4-NP wastewater. The optical images of the Pb/TiO2 scaffolds with 4, 8, 12, and 16 layers are presented in **Figure 8**.

Additionally, Aleni et al. [49] used 3D printing to fabricate a 3D dense and porous TiO2 structure. The final products exhibited similar mechanical properties to those of porous ceramics prepared via conventional methods.

Xu et al. [50] developed 3D printing to assembly TiO2 powders into hierarchical porous structures at macro and microscale. The schematic illustration of 3D printing process is presented in **Figure 9**. The obtained results showed that the TiO2 structures with abundant light absorption sites and high surface area could enhance the conversion efficiency of N2 and NH3.

Furthermore, Wang et al. [51] synthesized TiO2 NPs containing macrostructures by 3D printing for Arsenic (III) removal in water. They showed that 3D printing could fabricate and design macrostructures with special functions.

**Figure 8.**

*Optical images of the Pb/TiO2 scaffolds with (a): 4, (b): 8, (c): 12, and (d): 16 layers.*

**Figure 9.**

*Schematic illustration of 3D printing of a hierarchical porous TiO2 [50].*
