Comparison of selected preparative techniques for nonporous TiO₂ photocatalysts.

Preparation methods	Comparison
(1) Hydrothermal	(1) Preparation of TiO2 powder using this method usually uses liquid solutions as solvents to harvest the precursors. This usually occurs under increased temperature, <250 °C,50 and high-pressure conditions
	(2) Crystalline products are often formed (Fig. 8), considering the nucleation and crystal development. These products have different compositions, structures, and morphologies. TiO2 powders are obtained after critical washing and drying
	(3) NaOH or ethanol and water frequently serve as the solvents. TiCl4 and Ti(SO4)2 are also commonly used as precursors
	(4) The hydrothermal procedure is best used to improve crystallization on both the laboratory and commercial scales. The crystallization process51 is influenced by features such as reaction time, reaction temperature, the medium, and type of precursor
(2) Water-in-oil microemulsion techniques	(1) In recent years, scientists in this field have shifted a lot of attention to monodisperse nanoparticle preparation using this technique49 due to the following factors:
	(a) The water-in-oil microemulsion is thermodynamically stable52
	(b) It is also the optically isotropic dispersion of surfactant stabilized microdroplets of water in an external oil phase.52 These extremely dispersed nanosized microdroplets are well-matched for particle synthesis. This is due to the ability to control the microenvironment, the site for chemical reactions
3. Coating methods	(1) Studies have revealed that active commercial TiO2 powder possesses less photocatalytic activity than the TiO2 film.53 Photocatalytic activity is often influenced by coating the TiO2.53 TiO2 doped with elements such as C and N for doped TiO2 synthesis is less expensive29
	(2) There are two main coating methods:
	(a) Directly sintering or dip-coating (sometimes called wash-coating) the catalyst powders54
	(b) Formation of the TiO2 film on the support. This mostly uses the following preparative techniques:
	(i) Metal–organic CVD (MOCVD)19
	(ii) Chemical vapor deposition (CVD)
	(iii) Sol–gel40,55
	(iv) Spray coating