Raman Spectroscopy Reveals N2 Intercalation in WO3 Lattice: Impact on Configuration and Optical Properties

Raman spectra at high wavenumber (Figure 5) were investigated to clarify the configuration and existence of N'2' intercalation into the WO'3' lattice, which is responsible for the N content. No peak was observed for WO'3'-0, however, Raman spectra of N'2'-intercalated WO'3' samples exhibited signals at 2327 - 2342 cm⁻¹ , which can be ascribed to the N≡N vibration of N'2' in the WO'3' lattice on the basis of N≡N vibration of gaseous N'2' (2330 cm⁻¹ ) and the N'2'-intercalated WO'3' reported previously as well as relevant compounds [49 - 52]. Additionally, only one peak at 2330 cm⁻¹ was observed for WO'3'-0.62. It can be attribute to the lower amount of N'2' into the WO'3'-0.62 lattice, because such one peak was observed for N'2'-intercalted WO'3' prepared by dodecylamine according to ours earlier report [30]. However, it is amazing to note that two peaks at 2328 cm⁻¹ and 2342 cm⁻¹ began to be observed from n W:n N'2'H'4' of 1:1.2 as the content of N'2' increased. This significant difference confirms that the addition of N'2'H'4' affect not only the N content but also the configuration of N'2' into the WO'3' lattice. Further, The N'2' intercalation can be also proved by our previously reported [17, 30], the XPS spectrum in an W 4f region was deconvoluted by four bands at 37.0, 34.9, 32.9, 31.2 eV for N'2' intercalated WO'3'. The bands at 37.0 and 34.9 eV in higher energy for N'2' intercalated WO'3' (37.1 and 34.9 eV for NH'3'-WO'3') are assigned to 4f 5/2 and W 4f 7/2 of the WO'3' lattice similarly to WO'3'-0. The bands at 32.9 and 31.2 eV in lower energy for N'2' intercalated WO'3' can be assigned to the binding energies of 4f 5/2 and W 4f 7/2 interacted with N'2' intercalated. XPS data of WO'3'-N'2'H'4', in which a peak at 396.9 eV assigned to the intercalated nitrogen bound with the tungsten center. This is obviously different from the band at 396 eV of N-doped metal oxides (WO'3', TiO'2', etc.) [53, 54] and metal nitrides (TiN) [55]. Additionally, the nitrogen intercalation hardly causes the oxygen defects, which can be beneficial to improving the optical properties of WO'3'