The chemical composition and surface valance state of the NTO/ Ti
3C
2 are conducted by XPS analysis. The XPS survey spectrum (
Fig. 3a) shows the co-existence of Al, C, Ti, O, F and Na elements in the hybrid material, in which the Na KLL (497 eV), F KLL (565/835 eV) and O KLL (976 eV) represent the Auger peaks for Na, F, K elements respectively [
29]. The weak Al and F peaks, are derived from the raw material, and the fluorine functional groups on the surface of MXene due to the HF etching progress, respectively. The high resolution Ti 2p XPS spectrum (
Fig. 3b) can be deconvolution into four pairs peaks, corresponding to Ti-C (455.3/461.1 eV), Ti
2+ (456.6/462.8 eV), Ti
3+ (458.3/464.1 eV) and Ti
4+ (458.8/465.0 eV), respectively [
26,
27]. The large proportion of Ti-C peaks and low chemical state of Ti peaks indicate that the Ti
3C
2 matrix are well preserved after the oxidation and alkalization processes, which allows the merits of Ti
3C
2 to be fully utilized to enhance the electrochemical performance for sodium storage. Furthermore, the visual increasing Ti
3+ and Ti
4+ contribution, compared to the pristine MXene [
25], reveals the successful formation of NTO in hybrid, along with the enhanced peak intense of elemental Na in the survey spectrum. The C 1s XPS spectrum of the hybrid (
Fig. 3c) shows three kinds of C orbitals in NTO/Ti
3C
2, namely C-Ti (281.7 eV), C—C (284.8 eV, 288.7 eV), and C—O (286.1 eV), respectively [
26]. The proportion of C—C peak increases obviously compared to pristine MXene [
25], which can be attributed to carbon layers because of the precipitation of Ti atoms from Ti
3C
2 crystal during hydrothermal process. Further Raman observation of the NTO/Ti
3C
2 (Fig. S1 in Supporting information) shows the two broad peaks located at 1376.20 and 1595.93 cm
-1, which are associated with the disorderly induced characteristic D-band and the sp2 hybridized graphitized G-band of carbon, respectively [
30]. The ratio of D- and G- band intensity (
ID/
IG), is ~4.78, suggesting the appearance of disordered (amorphous) carbon layers in hybrid due to the precipitation and oxidation of the outermost Ti atoms in the MXene structure. Finally, the high-resolution O 1s spectrum (
Fig. 3d) shows three components. Specifically, the main peak at 530.0 eV is attributed to the Ti—O bonds, and two broad peaks at the binding energy of 531.5 and 532.8 eV can be indexed well to the O—C and—OH groups, respectively [
27].