The crystal structures are identified by means of XRD analysis. Figure 1 shows XRD patterns of the as-prepared samples. It can be seen that the dispersing diffraction peak at about 25° of the 2θ appeared for the carbon spheres, which could be related to the formation of the amorphous carbon, as shown in Fig. 1c. After calcination at 550 °C for 3 h, carbon sphere was oxidized into gas to flow out. The pattern of the ZnO-SnO2 hollow spheres (Fig. 1b) exhibits two sets of diffraction peaks. One set of diffraction peaks is indexed to the hexagonal wurtzite ZnO phases (space group - P63/mc, JCPDS card no. 36-1451). The other set of peaks is connected to the tetragonal rutile SnO2 phases (space group - P42/mnm, JCPDS card no. 41-1445). No evidence of impurities is detected, indicating the as-prepared sample has a heterostructure; compared to the pattern of ZnO hollow spheres (Fig. 1a), its diffraction peaks are obviously broadened, indicating that the crystal size becomes smaller.
In order to get more information about the templates and hollow structures, the obtained samples are further investigated by SEM and TEM, as shown in Fig. 2. Figure 2a, b shows SEM and TEM images of carbon spheres prepared via a typical hydrothermal treatment. Apparently, all of the carbon spheres exhibit very similar spherical morphology with uniform morphology. It can be seen that the average diameters of carbon spheres are about 400 nm. Figure 2c, d shows numerous ZnO-SnO2 hollow spheres with a diameter of about 150 nm. It is obviously found that the diameter is smaller than its templates. The shrinkage could be attributed to the decarbonization and dehydration of carbon spheres during the process of calcination, and carbon sphere is oxidized into gas to flow out. Compared to the ZnO hollow spheres (Fig. 2e, f), ZnO-SnO2 hollow spheres have a smaller crystal size about 10 nm, indicating that ZnO-SnO2 heterostructure prepared through adding extra SnCl4·5H2O could inhibit the grain growth. It is easy to understand that the nucleation and crystal growth of ZnO (or SnO2) facilitates heterogeneous nucleation of SnO2 (or ZnO). Subsequently, crystal growth of SnO2 (or ZnO) suppresses growth of ZnO (or SnO2), which is caused by growth competition. As is suggested by TEM and XRD results, the ZnO-SnO2 heterostructures have smaller particle and crystal size than ZnO hollow spheres.
The UV–vis absorption spectra of ZnO and ZnO-SnO2 hollow spheres are shown in Fig. 3. It can be seen that the wavelength of the absorption edge for ZnO hollow spheres is located at 375 nm, which can be attributed to the intrinsic absorption band derived from the band gap transition. The obtained ZnO-SnO2 hollow spheres showed much stronger absorbance intensities and larger absorbance region than ZnO hollow spheres. This result indicates that the formation of heterostructure is beneficial for photocatalytic performance in the UV and visible light regions [21].
The formation of ZnO and ZnO-SnO2 hollow spheres involves using carbon spheres as template, and the schematic illustration of the formation is demonstrated in Fig. 4.
During the hydrothermal process, the growth of carbon spheres follows the LaMer model [22]. As shown in Fig. 5, the as-prepared carbon spheres have a large number of functional groups in the surface such as –OH, C=O, which is conducive to the adsorption of metal ions to form core-shell composite spheres [23, 24]. Then calcined in air at 550 °C, the hollow spheres can be obtained by the oxidation of carbon spheres into CO2 simultaneously and precursor with metal ions translates into metallic oxide. With further analysis of FT-IR spectra, it can be seen that the ZnO bond is assigned to the stretching frequency at 435 cm−1 for pure ZnO which is shifted to higher frequency as 476 cm−1 for ZnO-SnO2 hollow spheres. In addition, the weak band at 633 cm−1 for ZnO-SnO2 hollow spheres is assigned to O–Sn–O bond, which is consisted with XRD results.
Photocatalytic activity of the ZnO hollow spheres and ZnO-SnO2 hollow spheres was examined by using RhB degradation. The degradation efficiency can be expressed as follows:
$$ \eta =\frac{C_0-{C}_t}{C_0}\kern0.5em \times \kern0.5em 100\%=\frac{A_0-{A}_t}{A_0}\kern0.5em \times \kern0.5em 100\% $$
(1)
where C
0 is the initial concentration of RhB and C
t
represents the concentration after t min reaction. UV–vis spectra of RhB in contact with different catalyst samples after 30-min dark adsorption/desorption are shown in Fig. 6a, b. Figure 6c shows that the degradation efficiency of the RhB (20 mg/L) in aqueous catalyst dispersion with ZnO-SnO2 hollow spheres could reached 99.85% within 40 min while the ZnO hollow spheres need more time. The high degradation efficiency of the RhB could be related to the heterostructure and the reduced crystal size.
In order to further investigate the photocatalytic activities of these catalysts, pseudo-first-order kinetics are used to analyze the photocatalytic degradation kinetics:
$$ \ln \frac{C_0}{C_t}=kt $$
(2)
where k is the kinetic rate constant (min−1) and t is the irradiation time (min). The relationship curves of ln(C
0/C
t
) versus irradiation time are plotted in Fig. 6d. The kinetic rate constant of ZnO hollow spheres and ZnO-SnO2 hollow spheres are 0.092 and 0.120 min−1, respectively. Meanwhile, the ZnO-SnO2 hollow spheres used in the photocatalytic activity measurements are centrifuged, dried at 80 °C for 3 h before it is reused as such in the succeeding photocatalytic experiment. It can be seen that ZnO-SnO2 hollow spheres exhibit excellent reusability in photocatalytic degradation of RhB, and the degradation efficiency could reach 90.34% after used two times (Fig. 6c). These results indicate that the ZnO-SnO2 hollow spheres have higher photocatalytic activities than ZnO hollow spheres. Due to the presence of the ZnO-SnO2 heterostructure, the probability of the recombination of electron-hole pairs is significantly reduced and the photo-responding range is extended [12–15]. In addition, the unique hollow nanostructure with mesoporous spheres provides efficient molecular transport pathways to their interior surface, increases the catalyst surface area, and provides more reaction site for photodegrading RhB.