## 3 Results and Discussion
### Spectrum Characterizations of CQDs/TiO\({}_{2}\) Particles  
The synthesized CQDs/TiO\({}_{2}\) particles were characterized with spectrum techniques to understand their structural and optical properties. Figure 2: **(a–b)** TEM images of CQDs/TiO\({}_{2}\) particles; **(c–e)** Distribution images of CQDs/TiO\({}_{2}\) particles Ti, O and C; **(f)** EDS analysis results of CQDs/TiO\({}_{2}\) particles. Figure 3a shows the XRD spectra of the synthesized CQD/TiO\({}_{2}\) particles under different reaction periods from 10 to 12, and 14 h. All of the peaks in the XRD spectra could be seen on the standard card of anatase [31], which confirms that the prepared CQDs/TiO\({}_{2}\) is an anatase type. Anatase is more favorable for the photocatalytic reaction due to more defects in the crystal lattice to improve the separate of electrons and holes. As the reaction time increases, the intensity of each crystal plane diffraction peak gradually increases and the peak width at half height becomes narrower. Moreover, the high-energy crystal plane of TiO\({}_{2}\) can promote the catalytic effect of TiO\({}_{2}\)[32]. With more exposure of the high energy crystal plane, better photocatalytic effect can be obtained. The percentages of the high-energy lattice plane with different reaction times were calculated and shown in Table 1, the equation is displayed in Supplementary Information. It can be found that the crystallinity of TiO\({}_{2}\) is poor and the exposure fraction of high energy crystal plane is low when the reaction time is too short (for instance 10 h). However, a too long reaction time (for instance 14 h) will lead to the breakage of particles and agglomeration of CQDs deposition, thus losing the characteristics of CQDs. Figure 3b gives the FTIR spectra of the obtained CQDs/TiO\({}_{2}\) particles, which reveal the composition and functional group information of the tested materials. The peak at 565 cm\({}^{-1}\) generated by the role of Ti-O in TiO\({}_{2}\), which confirms the existence of TiO\({}_{2}\) in the CQDs/TiO\({}_{2}\) composites [31, 33]. The corresponding peak of 1400 cm\({}^{-1}\) is assigned to the C-O bond, and the vibration peak at 1625 cm\({}^{-1}\) is ascribed to the unsaturated C = C bond. The peak at 2925 cm\({}^{-1}\) is related to the absorption peak of C-H in aldehyde group, indicating some oxygen-containing functional groups were carried on CQDs/TiO\({}_{2}\). Moreover, the absorption peaks of CQDs/TiO\({}_{2}\) with 12 h reaction show highest intensity strongest among the reaction time of these different lengths. Figure 4a shows the AFM image of the supernatant solution of CQDs/TiO\({}_{2}\), in which some of the CQDs are not deposited on the surface of TiO\({}_{2}\) and can therefore be measured with AFM. The size of the CQDs was about 4 nm. According to the section analysis in the graph (Figure 4b), the synthesized CQDs are roughly homogeneous in sphere-shape. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Samples** & **Average Thickness/nm** & **Average Length /nm** &
\begin{tabular}{c} **Percentage of High-Energy** \\ **Lattice Plane** \\ \end{tabular} \\ \hline
10 h CQDs/TiO\({}_{2}\) & 5.3 & 6.1 & 22\% \\
12 h CQDs/TiO\({}_{2}\) & 6.5 & 9.5 & 30\% \\
14 h CQDs/TiO\({}_{2}\) & 5.8 & 9.1 & 32\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Different sizes of carbon quantum dots (CQDs)/TiO\({}_{2}\) synthesized by hydrothermal method. Figure 3: (**a**) X-ray diffraction (XRD) patterns and (**b**) Fourier transform infrared spectroscopy (FTIR) spectra of CQDs/TiO\({}_{2}\) particles that synthesized with different hydrothermal reaction periods of 10, 12, and 14 h.  
To further prove the formation of CQDs/TiO\({}_{2}\) composites, UV-vis (Figure 4c) and fluorescence (Figure 4d) spectroscopy were used to characterize the supernatant of CQDs/TiO\({}_{2}\). There is a peak at 320 nm for the UV-vis spectrum of CQDs/TiO\({}_{2}\), which is absent for the spectrum of CQDs.