The X-ray Photoelectron Spectroscopy was used to determine the TON material surface composition. Figure 5a shows that comparison of the core N 1s lines in the Degussa P25 and the meso-TON materials. Degussa P25 represents the absence of N 1s peaks. However, the meso-TON materials have strong N 1s spectra that indicate a high level of N-doping. The nitrogen dopant concentration was nearly 0.33 atm% and 0.5 atm% for the SE-meso-TON and SG-meso-TON. It is possible to rule out the substitution N (Ns) as the B 1s spectra of the meso-TON materials do not display XPS signals around 396 eV. However, 397-402.5 eV for the meso-TON-materials indicates the appearance of the N 1s spectra. It may be explained by the difference of nitrogen species.
Figure 5b represents the major O 1s core level that appears around 530 eV. It is possible to observe the O 1s broadening at 531 eV that was previously explained by the bond between O-N-Ti-O and N-Ti-O. The O 1s spectra broadening may be assigned to the groups of the surface hydroxyl on titania. Figure 5c shows three samples that are of very similar C 1s XPS spectra. It ruled out the species of the doped carbon in the meso-TON materials.
Figure 5d represents the meso-TON Ti 2p spectra and P25 photocatalysts. The binding energy (BE) is around 464.1-464.6 eV and 458.4-458.9 eV. Compared to P25, the Ti 2p meso-TON material spectra is a bit shifted to higher BE. The Ti 2p XPS BE shift is associated with the Nspecy nature in TON materials. In case of the N-substituted TiO2, its Ti 2p XPS would shift to lower BE due to the increased Ti electron density. It is the result of the bond between O-N-Ti-O and N-Ti-O.
Figure 6 represents the XPS spectra for the N-doped TiO2 region with different ratios of N/Ti. It has been found that there is a broad peak from 397 to 403 eV that is typical for the nitrogen-doped titanium dioxide. The curve fitting resulted in the two peaks, 399.2 eV and 401.2 eV.
Peak 1 (399.2 eV) is related to the anionic N¯ in O-Ti-N linkages. Its binding energy is higher than that of TiN. It may be explained by the fact that the nitrogen TiO2 lattice doping reduces the nitrogen electron density. Therefore, changes that occur in the nitrogen environment can cause substantial differences in the 1 sXPS spectral region of the nitrogen. It is supported by the XPS spectra results for the Ti 2p region (see Figure 7a). The Ti2p3/2 core levels of the pure Tio2 and N-TiO2 are at 459.05 and 458.25 eV.
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Figure 7b shows additional N-TiO2 peak at nearly 532 eV that was previously related to the Ti-O-N bonds. The presence of Ti-O-N is typical for high binding energy. It is evident that the chemical states of the TiO2 doped nitrogen may be different and tend to coexist in the form of Ti-O-N and N-Ti-O.
Table 2 shows the calculation of the fitting peak nitrogen percentage and total nitrogen percentage in the TiO2. It indicates the difference in the theoretical N content and the calculated value from the XPS. In the preparation process, not all nitrogen sources can be incorporated into the titania photocatalyst. Moreover, the calculated XPS value is not the whole nitrogen value doped in the titania as XPS is a surface characterization measurement technique. Therefore, only a part of the nitrogen is doped into the titania lattice.
Figure 6a shows the XPS Ti 2p spectra. Two Ti 2p spectra peaks were observed at 458.8 and 464.4 eV. The binding energy of Ti 2p3/2 and 2p1/2 are nearly 458.4 and 464.0- eV. In addition, there was a positive shift compared to N-TiO2. It suggested the interaction between Au particles and N-TiO2. Figure 6b represents the O 1s spectra Xps of N-TiO2 and 4.9Au/N-TiO2. Moreover, the O1s binding energies of all samples had higher values than 530.0 eV.
Figure 6d represents the N 1s region XPS spectra of N-TiO2 and 4.0Au/N-TiO2 samples and their fitting curves. Both samples have a broad peak that extends from 394 to 404 eV. After the curve fitting, two peaks were received at 399.0-399/7 eV and 400.3-401.0 eV. As a rule, the peak over 399 eV is explained by the anionic N- in O-Ti-N linkages. Some scientists find that the N 1s peaks at 400 and 402 eV binding energy to the species of the molecularly adsorbed nitrogen. There is a suggestion that it is related to the N atom in the O-Ti-N environment. The comparison of N-TiO2 and N1s of 4.0Au/N-Tio2 has shown that samples of the latter are positively shifted. It indicates that some nitrogen specie charged may be transferred to Au species located on the TiO2 surface. According to some researches, the Au preadsorption on TiO2 surfaces substantially increased the reachable N amount through the NH3 direct reaction with Au/TiO2. In addition, the embedded N stabilization was the result if the electron transfers from the Au 6s levels towards the N 2p levels. It means that there may be a strong interaction between the particles of Au and N on the TiO2 surface. However, the results of the current testing are different as there was a different preparation of Au/N-TiO2 system. First, we prepared the N-doped TiO2 catalyst and then Au on the N-doped TiO2 surface. There may be a strong interaction between N-TiO2 and Au particles and it may cause the change of the Au/N-TiO2 electronic property.