## 3 Results and Discussion
### Photocatalytic Characterization  
The synthesized g-C\({}_{3}\)N\({}_{4}\) was characterized via X-ray diffraction to confirm the structure of the obtained material (Figure 2a). One can see in the diffraction pattern of g-C\({}_{3}\)N\({}_{4}\) two main peaks at \(\approx\)12.8\({}^{\circ}\) and \(\approx\)27.7\({}^{\circ}\), corresponding to intraplanar and interplanar distances of g-C\({}_{3}\)N\({}_{4}\), respectively. Based on the presence of additional peaks at \(\approx\)17.7\({}^{\circ}\) and \(\approx\)21.7\({}^{\circ}\), the structure was indexed in the orthorhombic \(P\)21212 cell. Interplanar distances d\({}_{002}\) = 3.23 A and d\({}_{210}\) = 6.93 A are in accordance with the literature data [49; 50]. The 5% Pt/g-C\({}_{3}\)N\({}_{4}\) and 5% PtO/g-C\({}_{3}\)N\({}_{4}\) photocatalysts were also characterized via XRD (Figure S1). The formation of platinum metal particles was confirmed for the photocatalyst prepared via  
Figure 1: (**a**) Scheme of reactor for photocatalytic study; (**b**) spectrum of LED with the wavelength at maximum intensity. the chemical reduction method. No peaks related to platinum or its oxide are observed in the XRD of the photocatalyst 5% PtO/g-C\({}_{3}\)N\({}_{4}\), confirming the partial reduction of platinum ions. It is known that platinum oxide is generally poorly crystallized and hardly seen via XRD [51,52]. The diffuse reflectance spectra are presented in Figure 2b. One can see that the platinum deposition leads to an increase in light absorption beyond the 450 nm region. The increase in light absorbance is due to the surface plasmon resonance of Pt nanoparticles. The shift in the absorption edge leads to a broadening of the action spectrum of the photocatalyst to the visible light. The obtained band gap value of g-C\({}_{3}\)N\({}_{4}\) (2.7 eV) agrees well with the literature data (Figure S2). To confirm the formation of metallic platinum or PtO nanoparticles, we applied a combination of surface- and volume-sensitive methods--XPS and XANES spectroscopy. The Pt 4\(f\) core-level spectra of 0.5% Pt/g-C\({}_{3}\)N\({}_{4}\), and 0.5% PtO/g-C\({}_{3}\)N\({}_{4}\) are presented at Figure 3a. One can see that the Pt4f core-level spectrum of 0.5% Pt/g-C\({}_{3}\)N\({}_{4}\) could be well fitted by one Pt 4_f_/72-Pt 4_f_/5_/2 doublet with a Pt 4_f_/72-binding energy of 70.7 eV. In the case of 0.5% PtO/g-C\({}_{3}\)N\({}_{4}\), the Pt4f core-level spectrum of 0.5% Pt/g-C\({}_{3}\)N\({}_{4}\) is fitted by one Pt 4_f_/72-Pt 4_f_/5_/2 doublet with a Pt 4_f_/72-binding energy of 72.9 eV. According to the literature data, the peak at 70.7 eV could be attributed to the platinum in the metallic state, and the peak at the higher binding energy (72.9 eV) to the platinum in the oxidized state, Pt\({}^{2+}\) in PtO [40,53,54,55]. The surface atomic ratios [Pt]/[C\({}_{3}\)N\({}_{4}\)] for 0.5% Pt/g-C\({}_{3}\)N\({}_{4}\) and 0.5% PtO/g-C\({}_{3}\)N\({}_{4}\) are equal to 0.002 and 0.005 (Table 1). The observed difference in the surface ratios could be due to the formation of larger metallic platinum nanoparticles more than the deposition of PtO nanoparticles, which will be further confirmed via TEM. As mentioned above, an X-ray absorption spectroscopy allowed us to clarify the volume chemical state of the platinum nanoparticles deposited on g-C\({}_{3}\)N\({}_{4}\). The XANES spectra of the Pt \(L_{3}\)-edges of the 0.5%Pt/g-C\({}_{3}\)N\({}_{4}\) and 0.5% PtO/g-C\({}_{3}\)N\({}_{4}\) are presented in Figure 3b. It can be seen that the XANES spectra of the Pt \(L_{3}\)-edge of 0.5%Pt/g-C\({}_{3}\)N\({}_{4}\) differs from that of 0.5% PtO/g-C\({}_{3}\)N\({}_{4}\), but has features (peaks A-D) similar to those of \begin{table}
\begin{tabular}{c c c c} \hline
**Photocatalyst** & **[Pt]/[C\({}_{3}\)N\({}_{4}\)]** & **Pt4f Binding Energy, eV** & **Pt\({}^{0}\), \%** \\ \hline
0.5\% Pt/g-C\({}_{3}\)N\({}_{4}\) & 0.002 & 70.7 & 100 \\
0.5\% PO/g-C\({}_{3}\)N\({}_{4}\) & 0.005 & 72.9 & 0 \\ \hline \end{tabular}
\end{table}