# Preparation Techniques of TiO\({}_{2}\) Nanofluids and Challenges: A Review
## 2 Preparation Methods
### Hydrothermal Synthesis  
The hydrothermal synthesis method has been utilized to synthesize single crystals from an aqueous solution in an apparatus consisting of a thin-walled steel pressure vessel (called an autoclave) at higher temperature and pressure. [PERSON] et al. [51] produced anatase type TiO\({}_{2}\) nanowires (30-45 nm) from TiO\({}_{2}\) nanoparticles using hydrolysis synthesis, 1-gram anatase type TiO\({}_{2}\) was placed inside a Teflon-lined autoclave filled with 10 M aqueous solution of NaOH. For heating and cooling purposes, the autoclave was sealed into a stainless-steel tank and heated at 200 \({}^{\circ}\)C for 2 h. The heated mixture was cooled down gradually to room temperature by placing it in the atmospheric air. After that, the prepared solution was washed out with distilled deionized water, HCl aqueous solution and absolute ethanol several times and dried for 6 h at 70 \({}^{\circ}\)C.  
[PERSON] and [PERSON] [52] used the hydrothermal technique for the preparation of nanotubes by using 150 mg TiO\({}_{2}\) of anatase type powder as a starting material. The mixture of TiO\({}_{2}\) powder and an aqueous solution of NaOH was placed into a Teflon-lined stainless autoclave for 72 h at 110 \({}^{\circ}\)C. The precipitated powder was then washed out with distilled water and an aqueous solution of HCl after cooling it at room temperature. Finally, the powder was dried at 60 \({}^{\circ}\)C for 2 h.  
[PERSON] and [PERSON] [53] observed while preparing the anatase type TiO\({}_{2}\) nanorods from nanotubes, that the crystal size of the particles increased by increasing the pH value of suspension. The key steps involved during the transformation of nanotubes to nanorods were the oriented attachment of crystallites and the shrinkage of tube walls. Experimental procedures involved the mixing of commercially available powder (70% anatase, 30% rutile) into the solution of 10N NaOH and placing it inside the Teflon-lined autoclave at 130 \({}^{\circ}\)C for 20 h. The sample was then washed out with the solution of HNO\({}_{3}\) by varying the amount to give different pH values. Hydrothermal treatment was carried out in an autoclave for 48 h at 175 \({}^{\circ}\)C, the developed slurries were then dried at 100 \({}^{\circ}\)C for 3 h after the process of filtration to get the final product. [PERSON] et al. [54] formulated anatase and rutile type phase pure TiO\({}_{2}\) nanocrystallites by autoclaving the amorphous TiO\({}_{2}\) in the presence of HF and HCl, and citric and nitric acids, respectively. The anatase type TiO\({}_{2}\) prepared by using HF with nitric acid as a cooperative catalyst has an irregular surface, while the particles formulated by using HCl as a cooperative catalyst gave a regular crystalline surface. Pure phase rutile TiO\({}_{2}\) was also obtained by autoclaving in the presence of cooperative catalysts citric and nitric acids. [PERSON] et al. [55] studied the photocatalytic activity and found that the mesoporous anatase TiO\({}_{2}\) nanopowder (prepared by the hydrothermal method) showed much better results as compared to nanorods, mesoporous, nanofibers TiO\({}_{2}\) (prepared by sol-gel method) and commercial (P-25, JRC-03, and JRC-01) TiO\({}_{2}\). Distilled water of 40 mL was added to a mixture of (titanium (IV) butoxide (Aldrich, St. Louis, MO, USA) and acetylacetone) and stirred for 5 min. The obtained solution was cooled down at room temperature after heating it in the Teflon-lined stainless-steel autoclave for 1 h at 130 \({}^{\circ}\)C. At the final stage, the obtained solution was washed out with water and 2-propanol. [PERSON] et al. [56] made a comparison of conventional and microwave hydrothermal synthesis for the preparation of TiO\({}_{2}\) nanopowder and found that the powder synthesized via microwave hydrothermal synthesis was more crystalline and took less time compared to the conventional one.