## 2 Synthesis of Yttrium
### Hydrothermal Process  
According to wet chemistry methods, the hydrolysis of yttrium salt solutions can be scaled up. The lower solubility of Y(OH)\({}_{3}\) (Ks \(\sim\)(10-22.1)) is highly challenging during the synthesis of Y\({}_{2}\)O\({}_{3}\) nanoparticles in hydrothermal processes with uniform nuclei sizes and growth [42]. Y\({}_{2}\)O\({}_{3}\) nanocubes were synthesized using Hexamethylenetetramine (HMTA) and YCl\({}_{3}\) solutions (required amount dissolved in 60 mL of deionized water). The mixture was stirred for 1 min, and then transferred to a 100 mL Teflon-lined autoclave for heating at 180 \({}^{\circ}\)C for 24 h. After 24 h, the obtained products were cooled to room temperature (RT), washed, and dried at 50 \({}^{\circ}\)C for 24 h (Table 2).  
Y\({}_{2}\)O\({}_{3}\) nanoparticles in the shape of flowers can be synthesized using HMTA, YCl\({}_{3}\), and phenol solutions (all dissolved in 60 mL of deionized water). The same procedure was followed for the synthesis of Y\({}_{2}\)O\({}_{3}\) flowers as Y\({}_{2}\)O\({}_{3}\) cubes, as shown in Figure 1, to form the composite Y\({}_{2}\)O\({}_{3}\) plate/phenolic resin structures. The phenolic resin is formed during the reaction between phenol and formaldehyde, which is the product of the reaction between HMTA and deionized water. Subsequently, the composite Y\({}_{2}\)O\({}_{3}\)/phenolic resin  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \# & **Precursors** & **Medium** & **Temperature** & **Obtained** & **Ref.** \\ \hline
1 & Yttrium salts, ammonia, water & Chloride, nitrate & (200–270) \({}^{\circ}\)C & Yttrium grains & [43] \\ \hline
2 & Y(NO\({}_{3}\))\({}_{3}\) 6H\({}_{2}\)O (3.83 g), and & & & & \\  & urea (0.5 g) & & & & \\ \hline
3 & Y(NO\({}_{3}\))\({}_{3}\) 6H\({}_{2}\)O, & & & & \\  & Glycine, water, & & & & \\ \hline
4 & Y\({}_{2}\)O\({}_{3}\), nitric acid & ammonia & (08–220) \({}^{\circ}\)C for 24 h & Hexagonal Y(OH)\({}_{3}\), & [46] \\ \hline
5 & Pure Y\({}_{2}\)O\({}_{3}\) was dissolved in & & & & \\  & an HNO\({}_{3}\) solution & NaOH solution & 110 \({}^{\circ}\)C for 72 h & Y\({}_{2}\)O\({}_{3}\) nanotubes (NTs) & [47] \\ \hline
6 & Pure Y\({}_{2}\)O\({}_{3}\), & NaOH solution & 150 \({}^{\circ}\)C for 12 h & Hexagonal Y(OH)\({}_{3}\) with & \\  & & & & & \\  & & & & & \\ \hline
7 & Y(NO\({}_{3}\))\({}_{3}\)-8H\({}_{2}\)O, 0.5 g & & & & \\  & PEG-6000 and 20 mL water & NaOH & 60 \({}^{\circ}\)C for 4 h & Y(OH)\({}_{3}\) and Y\({}_{2}\)O\({}_{3}\) nanotubes & [48] \\ \hline
8 & hydrate yttrium nitrate and & & & & \\  & methyl salicate & ethyl acetate & 80 \({}^{\circ}\)C for 120 min & Porous cubic yttrium oxides & [49] \\ \hline \hline \end{tabular}
\end{table}
Table 2: Synthesis of yttrium oxide by hydrothermal method.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \# & **Method** & **Application** & **Precursor** & **Ref.** \\ \hline
8 & Biosynthesis from _Acalypha_ & Increasing the rate of antibacterial & Yttrium nitrate hexahydrate + \(A\). & [17] \\  & _indica_ leaf extract size (23 \(\upmu\)m & behavior & _indica_ leaf & [17] \\  & to 66 nm) & Yttrium oxalate hydrate & Narrow particle size distribution, used for industrial processing & [38] \\ \hline
9 & Yttrium oxide in the & & & \\  & microwave field & Yttrium oxalate hydrate & Narrow particle size distribution, used for industrial processing & [38] \\ \hline
10 & Green method synthesis & Efficient anticancer activity against & Plant-based & [39] \\ \hline
11 & Sono-chemical irradiation method & Antimicrobial, anti-biofilm production & Yttrium III acetate tetrahydrate & [40] \\ \hline
12 & Chemical precipitation method & Transparent ceramic & Carbonate yttrium + nitric acid & [41] \\ \hline \hline \end{tabular}
\end{table}
Table 1: _Cont._structures were further annealed at 600 \({}^{\circ}\)C for 8 h to remove the phenolic-resin template, which resulted in the formation of Y\({}_{2}\)O\({}_{3}\) flowers [50].