## 3 Results and Discussion  
The achievement of the composite CQDs@NiO was evaluated in several ways. The synthesis of CQDs via CA pyrolysis yielded the typical UV-Vis spectrum with a maximum absorption at 352 nm [42], as reported in Figure 1A. The NiO sample and the composites were analyzed by X-ray diffraction, as shown in Figure 1B. NiO presented diffraction peaks at 20 = 37.4\({}^{\circ}\), 43.4\({}^{\circ}\), 63.0\({}^{\circ}\), 75.5\({}^{\circ}\), and 79.6\({}^{\circ}\) which were assigned to the (111), (200), (220), (222), and (311) reflections, respectively, pointing to a pure phase [53]. The average particle size was estimated by analysis of the peak's full width at half maximum (FWHM) using the Scherrer formula D = K \(\surd\beta\)cos(\(\theta\)), where K is a constant (ca. 0.9), \(\lambda\) is the X-ray wavelength used to collect the XRD patterns (i.e., 1.5418 A), \(\theta\) is the Bragg angle, and \(\beta\) is the pure diffraction broadening of a peak at half-height due to the crystallite dimensions, yielding an appraised value of 35 nm. From the morphological point of view, NiO synthesized via urea and nickel nitrate and calcined at 500 \({}^{\circ}\)C has a peculiar texture of polygonal-shaped nanoparticles, with an average size of 40 nm, which tend to merge to yield a sort of bidimensional extended structure (Figure 2(a1)). The texture was retained by adding 1% CQDs, although the porosity appeared affected to some extent. The 2%CQDs, and 3%CQDs samples still retained the polygonal-shaped structure, but the deposition of the CQDs definitively helped the merging of the nanoparticles, resulting in an extended structure (Figure 2(b1-d1)). This reflects the distribution of the carbon on the NiO surface as mapped by EDS and shown in Figure 2(b2-d2), which was quite homogeneous for the composite 1% CQDs and had scattered islands in the 2%CQDs and 3%CQDs composites. The typical TEM image of the synthesized NiO is shown in Figure 2(c1). The corresponding HRTEM image is presented in Figure 2(b1), where an interplanar spacing of 0.208 nm was measured from the fringe pattern, corresponding to the (200) plane. From Figure 2(b3-d3), it can be found that the CQDs were homogeneously distributed onto the NiO surface in the 1% CQDs sample (round dots embedded in the NiO structure), and that their density increased in the 2% CQDs sample and tended to merge in the 3% CQDs sample. Surface area, average pore size, and total pore volume measurements were in line with the structure and morphology analysis (see Table 1). The surface and pore parameters of NiO were slightly modified by the presence of 1% CQDs, resulting in a small reduction of the average pore size and increase of the surface area. A higher load of QDs resulted in a partial pore occlusion and reduced  
Figure 1: (**A**) Ultraviolet-visible light (UV–Vis) spectrum of the carbon quantum dots (CQDs); (**B**) X-ray diffraction (XRD) of NiO and composite samples: (a) pure NiO, (b) 1%CQDs@NiO, (c) 2%CQDs@NiO, and (d) 2%CQDs@NiO. The inset is a magnification of the 20 region of 25–35\({}^{\circ}\). surface area as expected for a more consistent deposition of the QDs into the pores and an aggregation of the quantum dots. A higher load resulted in a more pronounced effect. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{2}{c}{**Surface Area**} & \multirow{2}{*}{**Average Pore Size (nm)**} & \multicolumn{2}{c}{**Total Pore Volume**} \\  & **(m\({}^{2}\)**-g\({}^{-1}\))** & & **(cm\({}^{3}\)**-g\({}^{-1}\))** \\ \hline NiO & 186.7 & 18.6 & 0.53 \\
1\%CQDs@NiO & 196.3 & 18.3 & 0.56 \\
2\%CQDs@NiO & 156.2 & 15.4 & 0.43 \\
3\%CQDs@NiO & 146.1 & 12.5 & 0.32 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Surface area, average pore size, and total pore volume of the synthesized samples. Figure 2: SEM images of (**a1**) NiO, (**b1**) 1% CQDs@NiO, (**c1**) 2% CQDs@NiO, and (**d1**) 3% CQDs@NiO. Energy-dispersive spectroscopy (EDS) carbon distribution maps of (**b2**) 1% CQDs@NiO, (**c2**) 2% CQDs@NiO, and (**d2**) 3% CQDs@NiO.