## 3 Results and Discussion
### Copper Oxide  
For the synthesis of the CuO, the coprecipitation method was used. The obtained gel was subjected to different treatments during the crystallization stage: microwave (MW), ultrasound (US), combined mode (MW-US), and also a conduction heating (CH). After the treatment, a change of color took place in the gel from blue to brown. This change is attributed to the transformation suffered by the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Cellulose** & **Hemicellulose** & **Lignin** & **Ash** & **Extractives** & **Reference** \\ \hline
**32.5\%** & **30.7\%** & **26.5\%** & **2.4\%** & **7.9\%** & **Sugar cane bagasse** \\
35.28\% & 33.28\% & 25.20\% & 4.1\% & 2.14\% & [23] \\
43.1\% & 31.1\% & 11.4\% & 5.5\% & 8.5\% & [26] \\
49.8\% & 28.4\% & 14.9\% & — & [27] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Composition of cane bagasse. Figure 2: Pyrosis of sugarcane bagasse system: (**a**) nitrogen generator; (**b**) K type thermocouple; (**c**) Dean-Stark type trap; (**d**) \(\alpha\) alumina base; (**e**) multimodal microwave; and (**f**) template glass reactor. precursors to copper oxo-hydroxide (Cu\({}_{x}\)O\({}_{y}\)H\({}_{2}\)). These hydrates contain Cu-OH and Cu-O groups that are transformed into CuO [28]. Thermogravimetric analysis was made to the resulting materials (Figure 3) to know the calcination temperature at which the Cu\({}_{x}\)O\({}_{y}\)H\({}_{z}\) are completely transformed into CuO. x, y, and z subscripts were calculated from the information provided from the thermographs: Cu\({}_{0.31}\)O\({}_{0.44}\)H\({}_{0.26}\) for microwaves, and Cu\({}_{0.13}\)O\({}_{0.38}\)H\({}_{0.49}\) for the other three methods. The thermographs show that all materials have a weight loss in the temperature interval between 170 and 260 \({}^{\circ}\)C and this is due to the release of water generated when transforming the Cu\({}_{x}\)O\({}_{y}\)H\({}_{z}\) into CuO. The weight loss in the combined, ultrasound only, and conventional treatments was close to 30%, while the microwave heat treatment had a lower weight loss of approximately 10%. This is attributed to the fact that microwave thermal treatment favors the generation of CuO in greater measure when compared to the other treatments studied. Therefore, an adequate temperature to ensure the complete transformation into CuO is higher than 300 \({}^{\circ}\)C and for this reason, in all cases the samples were subjected to 350 \({}^{\circ}\)C for 8 h.  
Figure 4 shows the X-ray diffraction patterns (XRD) of materials synthesized with different treatments and Table 2 shows lattice parameters and crystal sizes for the same materials. We can observe the characteristic reflections of the monoclinic CuO in all the synthesized materials and no crystalline impurities were detected. The Inorganic Crystal Structure Database (ICSD) 00-048-1548 was used as a reference. The CuO MW presents a greater dispersion in the crystal sizes for each crystallographic direction. Lower order for the crystallographic direction (111) in comparison with the other treatments. This effect can be attributed to the vibrations generated by microwaves; it has been reported that microwave-assisted synthesis generates changes in the crystal structure (phase and degree [29; 30]. According to the results shown in Table 2, it is worth mentioning that there are no significant changes in the lattice parameters for the synthesized materials. On the other hand, CuO CH has the largest crystal size while the material with microwave treatment has the smallest (3.4 times smaller than CuO CH). Smaller crystal size is due to overheating generated by microwaves. The microwaves distribute the thermal energy in the system more homogeneously compared to the conventional method where there are greater differences in temperature. Therefore, more homogeneous heating promotes a greater formation of seeds (nucleation stage) which have a lower growth due to the quick decrease in the concentration of the reactant in the surrounding space [31].