1 Introduction

Dye wastewater pollution has attracted increasing attention due to the rapid development of printing and dyeing industry. At present, the main methods for treatment of dye wastewater are adsorption, chemical oxidation, ultrasonic degradation, biological degradation, and photocatalytic oxidation. Photocatalytic oxidation [1] has the unique advantages of low energy consumption and high degradation efficiency; this method has gained increasing attention in the academia and production enterprises. Research and development of semiconductor catalysts, such as TiO2 [2,3,4,5], is the core of photocatalytic oxidation for dye wastewater treatment. SnO2 is a wide band gap semiconductor (Eg = 3.6 eV) and has excellent photoelectric performance and chemical stability; as such, SnO2 is a common photocatalytic material [6, 7]. However, some disadvantages of SnO2 that seriously limit its application in the field of photocatalysis include fast recombination of carrier and hole pairs, low light conversion efficiency, and low visible-light response. Different semiconductors with band structure have been matched to build heterostructures with improved photocatalytic efficiency [8, 9]. At present, SnO2 is combined with TiO2 [10], ZnO [11], Fe2O3 [12], C3N4 [13], graphene [14], and other materials to obtain significantly improved visible photocatalytic performance.

SiC, as a traditional non-metallic semiconductor material, is rich in carbon and silicon and found in the Earth’s crust. SiC has high carrier transport rate and chemical stability and is thus a potential photocatalyst [15, 16]. SiC and SnO2 have matching conduction band and valence band potentials. SiC/SnO2 composites can be used in photocatalytic water splitting [17, 18] and electrochemical oxidation of wastewater [19]. However, no report is available about heterojunction materials constructed by SnO2 and SiC for photocatalytic degradation.

In our previous study [20], Sn6O4(OH)4 powders were prepared by mechanical alloying with SnCl2·2H2O and NH4HCO3 as raw materials; SnO2 nanoparticles were obtained by aging Sn6O4(OH)4 powders at a certain time in air. However, the as-synthesized SnO2 material did not have visible-light catalytic degradation performance. In the present study, we prepared SnO2/SiC composites through mechanical alloying and subsequent aging. The visible-light photocatalytic degradation of methyl orange (MO) was also studied.

2 Experimental Procedure

Reagent-grade SnCl2·2H2O, 2-methylimidazole, and NH4HCO3 were obtained from Tianjin Beichen Fangzheng Reagent Factory. SiC powders (average size of 40 nm, 99.9% purity) were purchased from Beijing Deke Island Gold Technology Co., Ltd. The raw material formula is listed in Table 1. These raw material powders were weighted and mechanically ground on a small grinder for 25 min. The ground powders were allowed to stand in air for 30 min before immersing in deionized water for 10 days. Finally, the powders were washed several times and dried. The obtained powders were named SnO2, 1% SiC/SnO2, 3% SiC/SnO2, 5% SiC/SnO2, 7% SiC/SnO2, 9% SiC/SnO2, and 11% SiC/SnO2. The detailed test equipment and conditions were reported in previous research [16]. In the present work, the concentrations of the photocatalyst and MO are 1 g/L and 15 mg/L, respectively.

3 Results and Discussion

Figure 1 shows that the XRD pattern of different photocatalytic materials. The synthesized products were mainly composed of SnO2 and SiC. MA-synthesized Sn6O4(OH)4 precursor was basically transformed to SnO2 by aging. The reaction equation for the formation of SnO2 was as follows.

$$6{\text{SnCl}}_{{{2}}} \cdot 2{\text{H}}_{{{2}}} {\text{O }} + {\text{ 12NH}}_{{\text{4}}} {\text{HCO}}_{{{3}}} \to {\text{Sn}}_{{{6}}} {\text{O}}_{{{4}}} \left( {{\text{OH}}} \right)_{{t{4}}} + {\text{ 12NH}}_{{{4}}} {\text{Cl}} + {\text{ 16H}}_{{{2}}} {\text{O}} + {\text{12CO}}_{{{2}}}$$
(1)
$${\text{Sn}}_{{{6}}} {\text{O}}_{{{4}}} \left( {{\text{OH}}} \right)_{{{4}}} + {\text{3O}}_{{{2}}} \to {\text{ 6SnO}}_{{{2}}} + {\text{ 2H}}_{{{2}}} {\text{O}}$$
(2)

Compared with other fabrication technologies [10,11,12,13,14], this aging technology had the advantages of mild conditions, cost-effectiveness, and simplicity.

When the SiC content was low (≦ 2%), the SiC peaks were difficult to find. The SiC peaks began to appear when the SiC content was 4%. With increasing SiC content, the intensity of the diffraction peaks of SiC increased. Weak Sn6O4(OH)4 peak began to appear when the SiC content was increased to 7%. The intensity of the Sn6O4(OH)4 peak obviously increased when the SiC content was increased to 11%. Hence, adding excess SiC may inhibit the transformation from Sn6O4(OH)4 into SnO2 and could degrade the photocatalytic properties of the composites.

The diffraction peaks of SnO2 widened, indicating the fine grains of SnO2. Average grain size was estimated from the highest intensity peaks of SnO2 (2θ = 26.56°) by using the Scherrer’s equation. The average grain sizes were 6–7 nm for SnO2 and SnO2/SiC composites. Hence, we obtained very fine SnO2 nanocrystallines by using the proposed method.

Figure 2 shows the SEM images of (a) SnO2 and (b) 7%SnO2/SiC composite and present similar serious aggregates. These aggregates were composed of nanoparticles with size of several to dozens of nanometer.

Figure 3a shows that the as-synthesized SnO2 mainly consisted of severely agglomerated nanocrystallines with size of less than 10 nm; this finding is consistent with the XRD results. Nanosheets with thickness of about 3 nm and length of about 30 nm were also detected. The TEM image (Fig. 3b) of 7%SnO2/SiC composite was identical to that of the SnO2 sample. Figure 3c shows the HRTEM image of 7%SnO2/SiC composite. The tight interface of SiC and SnO2 was observed, indicating the formation of a good heterostructure. This SnO2/SiC heterostructure may enhance the separation of photoelectron pairs, thereby enhancing the photocatalytic performance.

The compositional information and elemental chemical states of 7% SiC/SnO2 were probed with X-ray photoelectron spectroscopy (XPS). Spectrum decomposition was performed using the casaXPS program with Gaussian functions after the subtraction of a Shirley background.

Figure 4a shows the survey spectra, which confirmed the existence of Si, C, Sn, and O. As shown in Fig. 4b, the peaks centered at 486.98 and 495.38 eV are assigned to the Sn3d5/2 and Sn3d3/2 of Sn4+ in SnO2, respectively. The O peaks (Fig. 4c) at 530.74, 531.81, and 534.29 eV are assigned to O–Sn–O, surface hydroxyl oxygen (–OH), and adsorption oxygen groups, respectively. Figure 4d and e show the high-resolution spectra of Si 2p and C 1s, respectively. The peaks centered at 101.8 and 284.6 eV are attributed to Si 2p and C 1s of SiC, respectively. The C=O bond mainly rose from oxygen absorbed on the surfaces of the SiC nanostructures.

The photoluminescence spectrum of the sample was recorded to investigate the separation effect of photogenerated electrons and holes in the composites (Fig. 5). The intensity of all emission peaks of SnO2/SiC composites was lower than that of SnO2, suggesting the low recombination probability of electron holes in the composites. The peak intensity of 7% SnO2/SiC composite was the lowest, indicating that the electron–hole recombination probability of the catalyst was the lowest. This composite was considered the most conducive to the separation of electrons and holes in the photocatalytic system. Hence, the introduction of SiC effectively improved the luminescent properties of SnO2 and rendered them conducive to photocatalytic reaction.

The typical UV–Vis diffuse reflection spectra of SiC, SnO2, and composites are shown in Fig. 6a. SnO2 showed weak absorption capacity, indicating that it possessed a wide band gap, whereas pure SiC had an obvious absorption band from 200 to 800 nm. SiC/SnO2 composites had obviously stronger absorption capacity than SnO2 in the visible region. These results indicated that the SiC/SnO2 composites exhibited efficient visible-light utilization, which can result in good photocatalytic performance.

Figure 6b shows that the band-gap energy (Eg) values of SiC/SnO2 composites were about 2.65 eV. Single-phase SnO2 and SiC had a high Eg value of 2.78 and 2.99 eV, respectively. Thus, their combination significantly reduced their band gap, thereby improving the utilization efficiency of visible light.

Figure 7 compares the photocatalytic degradation of MO by SnO2 and SnO2/SiC composites under visible light. SnO2 exhibited weak ability to photocatalytic degradation of MO. Moreover, SnO2/SiC composites had good degradation ability. The photocatalytic degradation abilities of 1% SiC/SnO2, 3% SiC/SnO2, 5% SiC/SnO2, and 7% SiC/SnO2 composites were better, and they degraded 99% MO within 45 min. The enhancement can be explained by the synergetic effect on the efficient electron–hole separation at the SnO2/SiC photocatalyst interface. Figure 1 shows that the 9%SiC/SnO2, and 11% SiC/SnO2 composites contained some Sn6O4(OH)4 impurity. Obviously, adding excess SiC inhibited the transformation of Sn6O4(OH)4 to SnO2, which decreased the content of SnO2 photocatalysts in the composites.

Table 2 shows the visible photocatalysis results of the fabricated SnO2/SiC composite and SnO2-based catalysts in other studies. The SnO2/SiC composite showed higher photocatalytic degradation rate than the other SnO2-based photocatalysts. Hence, SnO2/SiC photocatalyst could be a new highly efficient catalyst.

To evaluate the photocatatyic cyclic performance of the 7% SnO2/SiC composite, four cycling experiments were performed for MO degradation. Figure 8a shows that after several cycles, no obvious change occurred in the photocatalytic-degradation activity. Additionally, the XRD (Fig. 8b) pattern and XPS (Fig. 8c–g) data remained stable. These findings indicated the reusability and high stability of the 7% SnO2/SiC composite.

The proposed mechanism of photocatalytic degradation over SnO2/SiC nanosheets is shown in Fig. 9. The locations of the conduction and valence bands of SiC and SnO2 were confirmed by the Mott–Schottky plot. The valence-band maximum values of SiC and SnO2 were estimated at 1.6 and 3.78 eV, and the corresponding conduction-band minimum values of SiC and SnO2 were around − 1.4 and 0 eV, respectively. The conduction band of SiC was more negative than that of SnO2, resulting in the transfer of electrons generated in the conduction band from SiC to SnO2 nanoparticles. The valence band of SnO2 was more positive than that of Si; as such, the hole transferred from the valence band of SnO2 to the valence band of SiC. This migration of electrons and holes between SnO2 and SiC effectively reduced the band gap of SnO2. This phenomenon further improved the mobility of electrons, inhibited the recombination of electrons and holes, and prolonged the lifetime of electrons and holes, which are conducive to photocatalytic degradation. The holes in the valence band oxidized MO into CO2 and H2O and interacted with OH− to form ·OH radicals, which also oxidized MO into CO2 and H2O. In addition, the electrons on the conduction band reduced the adsorbed O2 into O2− and oxidized MO into CO2 and H2O.

4 Conclusions

In this work, SiC/SnO2 composites were synthesized by mechanical alloying combined with aging. The TEM, XPS, UV–Vis DRS, and PL studies showed the formation of SiC/SnO2 heterojunction, which inhibited the recombination of photogenerated electron–hole pairs. Hence, the fabricated SiC/SnO2 composites exhibited excellent visible-light photodegradation of MO. In particular, 7%SiC/SnO2 composite possessed the highest photodegradation rate, that is, it degraded 99% MO within 45 min.

Table 1 Raw material formula
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Fig. 1
figure 1

The XRD patterns of SiC/SnO2 composites

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Fig. 2
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FE-SEM images of a, b SnO2 and c, d 7%SiC/SnO2 composite

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Fig. 3
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The typical a TEM and b HRTEM images of 7%SiC/SnO2 composite

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Fig. 4
figure 4

The XPS spectra of a survey, b Sn 3d, c O 1s, d Si 2p and e C 1s for 7%SiC/SnO2 composite

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Fig. 5
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Photoluminescence spectra of SiC/SnO2 composites

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Fig. 6
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a UV–Vis DRS spectra and b band gap energy (Eg) values of SiC/SnO2 composites

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Fig. 7
figure 7

The photocatalytic activity of the SiC/SnO2 composites to MO dyes

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Table 2 Photocatalytic activities of different SnO2 matrix photocatalysts under visible light
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Fig. 8
figure 8

The four recycled testing of 7% SnO2/SiC composite for MO degradation (a), and the XRD pattern (b), XPS survey spectrum (c), high-resolution spectra of Sn 3d (d), O 1s (e), Si 2p (f) and C 1s (g) of the composite after cycles for MO degradation

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Fig. 9
figure 9

The photocatalytic routes of the SiC/SnO2 heterostructure

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