## 4 Copper Nanoparticles Synthesis
### Chemical Methods  
Chemical methods are the most widely used to obtain copper nanoparticles. One major drawback is the use of toxic materials during the synthesis phase. Considering that nanoparticles are being used more frequently and have increasing human contact, it is essential to develop environmentally friendly processes [41].  
Various chemical methods are used to obtain nanoparticles, such as sonochemical reduction [42], hydrothermal synthesis [43], electrochemical [44] and chemical reduction [45]. The latter is the most commonly used one. It involves using hydrazine, ascorbic acid or sodium borohydride as a reducing agent. The chemical reduction method is often used to obtain CuNPs because it is simple, has high yield efficiency and requires limited equipment.  
[PERSON] et al. [46] used reducing agents (NaH\({}_{2}\)PO\({}_{2}\), N\({}_{2}\)H\({}_{4}\) and NaBH\({}_{4}\)) and polymeric capping agents and obtained CuNPs with sizes between 3 and 9 nm. The authors discovered that the lack of water in nanoparticle synthesis had a small beneficial effect on the stability of obtained nanoparticles.  
In another recent study, [PERSON] et al. [47] used anhydrous copper (II) chloride (135 mg, 1.0 mmol) in a suspension of lithium powder (14 mg, 2.0 mmol) under an argon atmosphere to obtain CuNPs.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Used Substances** & **Size** & **Shape** & **UV-VIS** & **References** \\ \hline Copper(II) sulphate pentahydrate & 28 nm & Spherical & 579 nm & [30] \\ Citrus lemon fruits & & & \\ \hline Extract of _O. sanctum_ leaves & 122.7 nm & Spherical & 586 nm & [31] \\ Copper(II) sulphate pentahydrate & & & \\ \hline Extract of _Rluscoraria_ L. fruits extract & 70 nm & Spherical & 568 nm & [32] \\ Copper(II) sulphate pentahydrate & & & \\ \hline Extract of the flower _Milletta pinnata_ & 23 \(\pm\) 1.1 nm & Spherical & 378 nm & [29] \\ Copper acetate solution & & & \\ \hline Leaf extract of _Ageratum huostonianum_ Mill. & 80 nm & Cubic, hexagonal, rectangular & 326 nm & [21] \\ Copper(II) chloride & 15–30 nm & Spherical & 580 nm & [33] \\ \hline  _Ginko biloba_ L. leaf & 15–20 nm & Spherical & 560–580 nm & [34] \\ Copper(II) chloride & & & \\ \hline _Magrolia kobus_ leaf & 45–110 nm & Spherical & 560 nm & [35] \\ Copper sulphate pentahydrate & & & \\ \hline _Syzgium aromaticumm_ bud & 12 nm & Spherical & 580 nm & [36] \\ Copper(I) acetate & & & \\ \hline _Azadirachta indica_ leaf & 48 nm & Cubic & 560 nm & [37] \\ Copper(II) chloride & & & \\ \hline Ripened _Duranta arecta_ fruit & 76 nm & Spherical & 588 nm & [38] \\ Copper(II) sulphate pentahydrate & & & \\ \hline Tilia extract & 27.6 nm & Spherical & 563 nm & [39] \\ Copper(II) sulphate pentahydrate & & & \\ \hline _Punica granatum_ peel & 15–20 nm & Spherical & 585 nm & [40] \\ Copper(II) sulphate pentahydrate & & & \\ \hline Copper(II) sulphate pentahydrate Cetyl trimethil & & & \\ ammonium bromide & 78 nm & Spherical & 576 nm & [20] \\ Extract of seedless dates & & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Green synthesis methods of copper nanoparticles.  
By varying the concentration of copper salts, the reducing agent, the pH of the solution and synthesis temperature, we obtain nanoparticles with different sizes, shapes and activity [25].  
Table 4 presents various substances used to obtain copper nanoparticles. The obtained copper nanoparticles are characterized using UV-VIS and their size and shape are recorded.