Figure 2a,b show the SEM results of the ZnSnO
3 samples prepared after calcination at 450 °C for 2 h in an air atmosphere. From
Figure 2a, the ZnSnO
3 samples after calcination at 450 °C for 2 h in an air atmosphere are coarse and uniformly fibrous, with no breakage of the continuous fibers. The average diameter of the ZnSnO
3 sample is about 300–400 nm, as shown in
Figure 2b.
Figure 2c,d show the SEM results of ZnSnO
3/ZnO samples prepared after calcination at 450 °C for 2 h in an air atmosphere. As can be seen in
Figure 2c, the ZnSnO
3/ZnO samples exhibit uniformly coarse and fine fibers with no fracture in the fiber continuity. The average diameter of the ZnSnO
3/ZnO nanofibers is even smaller, around 200 nm, as shown in
Figure 2d. The TEM image in
Figure 2d indicates that the ZnSnO
3/ZnO nanofibers have a hollow structure. According to the results of Li et al.
[21], the morphology of nanofibers or nanotubes is mainly dependent on the calcination temperature. During calcination, PVP is gradually removed as the temperature increases. The decomposition of the drug and the evaporation of the solvent will generate a variety of gases. When the calcination temperature reaches 450 °C, the rate of gas production from the decomposition of pharmaceuticals is too fast, and the pressure difference between the interior and exterior of the nanofibers is more considerable, so the hollow structure is formed
[21]. The above experiments show that the diameter of the ZnSnO
3/ZnO nanofibers obtained by compounding ZnSnO
3 with ZnO is significantly reduced compared to the ZnSnO
3 nanofiber samples. The BET analysis shows that the specific surface area of the ZnSnO
3/ZnO nanofibers (31.24 m
2·g
−1) was substantially larger than that of pristine ZnSnO
3 nanofibers (20.15 m
2·g
−1), as shown in
Figure 2e,f.
XPS is carried out to study the elemental composition and valence states in ZnSnO
3/ZnO, and the results are shown in
Figure 3.
Figure 3a shows the full XPS spectrum of the ZnSnO
3/ZnO nanofibers; the researchers can observe that the obtained ZnSnO
3/ZnO nanofibers contain the elements Zn, Sn, C, and O. The peak at 284.8 eV corresponds to the spin-orbit peak of C 1s. The broad XPS spectrums of the Zn 2p range, Sn 3d, and O 1s range are shown in
Figure 3b–d. In the case of the Zn 2p spectrum, the Zn 2p peak can be distributed into two signals, as shown in
Figure 3b. The Zn-2p
3/2 and Zn-2p
1/2 signals are focused at 1021.1 eV and 1044.1 eV, respectively, which indicates that the chemical valence of Zn in the system is +2
[22]. As shown in
Figure 3c, the two peaks at 486.8 and 495.1 eV correspond to the Sn 3d
5/2 and Sn 3d
3/2 spin-orbit peaks, respectively. The bimodal spin-orbit shows a split value of approximately 8.3 eV, indicating the presence of Sn
4+ cations
[23]. The O 1s spectrum of the ZnSnO
3 and ZnSnO
3/ZnO is illustrated in
Figure 3d,e, which is deconvolved into three characteristic peaks by Gaussian fitting
[16][24]. The three fitting peaks are attributed to three essential oxygen species
[25], which are denoted as O
L, O
V, and O
C, respectively, corresponding to O
2− species in the lattice oxygen species, vacancy oxygen species (VO
S), and chemically adsorbed or dissociated oxygen species, respectively
[26]. The three characteristic peaks of O 1s in the ZnSnO
3 nanofibers are located at 530.06 eV, 531.65 eV, and 533.13 eV. In contrast, the distinct peaks of O 1s in the ZnSnO
3/ZnO nanofibers are located at 529.99 eV, 531.33 eV, and 532.27 eV, which can be ascribed to the three significant oxygen species, represented as O
L, O
V, and O
C, respectively
[17][27]. It must be pointed out that the gas-sensing properties may be highly dependent on the type of VO (vacancy oxygen) present on the semiconductor surface
[28]. The proportion of oxygen species in the obtained ZnSnO
3 and ZnSnO
3/ZnO nanofibers are shown in
Figure 3f and
Table 1. The results of
Figure 3f and
Table 1 clearly indicate that compared with the percentage of Ov of ZnSnO
3 nanofibers, that of ZnSnO
3/ZnO nanofibers is substantially larger.