# Photocatalytic Activity of TiNbC-Modified TiO2 during Hydrogen Evolution and CO2 Reduction
## 3 Results and Discussion
### SEM and TEM Characterizations  
Figure 3 shows a SEM image of the 5% TiNbC/TiO\({}_{2}\) heterostructure. It can be seen that most of the particles correspond to the manufacturer's declared sizes of 1-5 \(\upmu\)m (Figure 3). EDS analysis confirms the elemental composition of the heterostructure for all heterostructure concentrations (Figure S1). The element distribution map on a typical 5% TiNbC/TiO\({}_{2}\) particle shows a uniform distribution of the elements Ti, Nb, O, and C (Figure S2). The ratio of the elements was measured by the EDX method. As can be seen from Table 1, the contents of 1, 5, and 10% MXene satisfied the reported compositions of heterostructures. The TiNbC/TiO\({}_{2}\) heterostructures compound ratios are shown in Table 1. All MXenes were completely covered by TiO\({}_{2}\) nanoparticles (Figure S3). Figure 4 shows the structure of the obtained titanium dioxide and the MXene 5% TiNbC heterostructures obtained by transmission electron microscopy. The image of a typical particle (Figure 4, left) clearly shows hexagonal nanoparticles covering a large particle with no clear facets. Comparison of the EDX maps of the distribution of individual elements (Figure 4, right) allows us to conclude that the nanoparticles with sharp edges are titanium dioxide in the presented image, and the larger particles without sharp edges are MXene TiNbC. This fully corresponds to the granulometric characteristics of the components declared by the MXene manufacturer (see Section 2.1) and obtained by XRD. The electron diffraction on the particle in question forms a system of concentric rings and individual point reflexes. The analysis of their radii (inset in Figure 4) shows that the diffraction is caused mainly by anatase, which also correlates with the powder X-ray diffraction data. It is also possible to specify the parameters of the unit cell [36]. Figure 5 shows the extinction spectrum of the heterostructures with different concentrations of TiNbC. Figure 2: Powder X-ray diffraction patterns of the original MXene and titanium dioxide used to produce the heterostructures, as well as X-ray diffraction patterns of the synthesized heterostructures. The values of the band gap width for TiNbC/TiO\({}_{2}\) heterostructures were determined from the absorption spectra (Figure 6). The obtained band gap values were E\({}_{\text{g}}\) = 3.43 eV for the 1% of TiNbC and 3.33 eV for the 5% and 10% of TiNbC. The obtained values were quite close to the bandgap width of pure titanium dioxide, which was due to the small content of MXene in the heterostructure. Study of the Photocatalytic Activity of TiNbC/TiO\({}_{2}\) in the Process of CO\({}_{2}\) Reduction and Hydrogen Evolution  
The activities of the synthesized composite samples were tested in CO\({}_{2}\) reductions and hydrogen formations from aqueous solutions of ethanol under LED irradiation. The results of the photocatalytic reductions of CO\({}_{2}\) and hydrogen productions are summarized in Tables 2 and 3, respectively. Note that only the TiNbC/TiO\({}_{2}\) heterostructures showed photocatalytic activities in both processes. Figure 7 shows the results of the photocatalytic tests of the investigated titanium dioxide and MXene heterostructures in the photocatalytic CO\({}_{2}\) reduction. One can see that the main reduction products were CH\({}_{4}\) and CO; hydrogen was evolved as a by-product in the water-splitting reaction. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Ti, \%** & **Nb, \%** & **C, \%** & **O, \%** \\ \hline
1\% TiNbC/TiO\({}_{2}\) & 36.1 \(\pm\) 0.4 & 1.0 \(\pm\) 0.2 & 1.2 \(\pm\) 0.2 & 62.9 \(\pm\) 0.9 \\
5\% TiNbC/TiO\({}_{2}\) & 39.4 \(\pm\) 0.5 & 3.1 \(\pm\) 0.3 & 4.1 \(\pm\) 0.3 & 57.6 \(\pm\) 0.9 \\
10\% TiNbC/TiO\({}_{2}\) & 39.0 \(\pm\) 0.3 & 4.5 \(\pm\) 0.1 & 8.2 \(\pm\) 0.4 & 57.5 \(\pm\) 0.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: TiNbC/TiO\({}_{2}\) heterostructures compound ratios. Figure 3: SEM image of a typical pattern of 5% TiNbC/TiO\({}_{2}\) heterostructures.