Highly Efficient SnIn4S8@ZnO Z-Scheme Heterojunction Photocatalyst for Methylene Blue Photodegradation

1. Introduction

With the rapid development of human society, the problem of pollution is becoming more and more serious, and people have adopted many methods to deal with the problem of pollution [1,2,3]. Photocatalytic technology, as a new low-consumption, renewable and non-secondary pollution solution to pollution problems, has received extensive attention from researchers [4,5,6,7,8]. Through the continuous efforts of researchers, the photocatalytic materials have been greatly expanded, including oxides and sulfides such as ZnO、CdS、NiCo₂O₄ and CuInS₂ [9,10,11,12], perovskite such as CsPbCl₃ [13], graphite-like phase g-C₃N₄ [14] and mono-elemental red phosphorus [15], and so on. Among numerous photocatalytic materials, ZnO is a promising photocatalyst because it possesses many excellent characteristics such as high electron mobility, low cost, nontoxicity and easy preparation [16,17]. However, it is very easy for photoinduced electrons and holes of ZnO to recombine, which brings the low quantum yield, thus limiting its photocatalytic activity. For purpose of inhibiting the recombination between the photoinduced electrons and holes in ZnO, the following methods have been investigated by researchers: (i) Doping metal or non-metallic ions into ZnO can change its electron-hole concentration, thereby inhibiting the recombination between the photoinduced electrons and holes [18,19]. However, it would be usually difficult for doping elements to be doped into the lattice of ZnO and improper doping will actually inhibit its activity. (ii) Deposition of noble metals, such as Pt, Ag and Au, on the surface of ZnO is a typical surface modification strategy for the improvement of its optical quantum efficiency [20,21]. However, the high cost of these precious metals is not conducive to the promotion and practical application of this technology. (iii) The technique of coupling ZnO with other semiconductors which possess proper energy bands will produce heterogeneous structures. The photoinduced charge carriers will be separated through directional transfer in the presence of potential difference or internal electric field generated by the heterojunction, which will result in the significant improvement of the photocatalytic efficiency of the ZnO [22,23]. Therefore, the construction of heterojunction is a very promising technology for facilitating the progress of ZnO photocatalytic research [24,25,26].

Snln₄S₈, a typical polymetallic sulphide, has shown great latent capacity in the fields of the photocatalysis due to its unique structure. The SnIn₄S₈ material has a primary structure of nanoparticles and a secondary structure of micron-sized monodisperse sphere constructed from primary structure. This novel structure has two huge advantages for the application in the field of photocatalysis: (i) the primary structure provides a large specific surface area to adsorb more dye molecules; (ii) the secondary and porous structure can act as a light scattering center to improve the utilization of light. So far, Snln₄S₈ has been successfully prepared and applied to photocatalytic degradation of organic pollutants [27]. Most notably，benefitting from appropriate structure of Snln₄S₈ energy band, the photocatalytic activities of some materials can be enhanced by building a heterojunction with Snln₄S₈ [28,29,30]. However, the investigation on the application of the Snln₄S₈/ZnO heterojunction in the field about photocatalytic degradation of effluent containing organic contaminant has not been reported yet.

As a common organic dye, methylene blue (MB) is one of the main components of wastewater from the printing and dyeing industry. The wastewater containing MB will cause serious ecological pollution and destruction. Furthermore, MB can result in adverse effects on human health, such as irreversible damage of the eyes, increased heart rate, vomiting, shock, unhealthy pallor, jaundice, and tissue necrosis. Therefore, how to degrade MB in wastewater has been the direction explored by many researchers.

In this work, for purpose of enhancing the photo quantum efficiency of ZnO, SnIn₄S₈@ZnO Z-scheme heterojunction was synthesized by a convenient two-step hydrothermal approach. Furthermore, the photocatalytic activity of SnIn₄S₈@ZnO was assessed through the degradation of MB under the irradiation of ultraviolet (UV) light.

2. Materials and Methods

2.1. Synthesis of ZnO

ZnO in the form of nanosheets was prepared by a simple hydrothermal process. ZnCl₂ aqueous solution (50 mL, 1 mol/L) and KOH aqueous solution (20 mL, 7.5 mol/L) were prepared, respectively. The above two solutions were mixed, agitated for 20 min, then poured into a stainless steel autoclave with Teflon-lining whose volume was 100 mL. Afterwards, the sealed autoclave was heated in the electric oven at 180°C for 10 h, and then naturally cooled to ambient temperature. Finally, the product was separated through centrifugation, washed several times using deionized water and anhydrous alcohol, dried at 60°C for 10 h, and then ground for the formation of ZnO powders.

2.2. Synthesis of SnIn₄S₈@ZnO heterojunction

The typical procedure for SnIn₄S₈@ZnO heterojunction synthesis was as follows. Firstly, 70 mL of anhydrous ethanol solution with a certain quantity of SnCl₄·5H₂O was prepared. Secondly, a certain quantity of InCl₃·4H₂O was dissolved in the solution which was stirred for 10 min. Thirdly, 2.0345g of the as-prepared ZnO was mixed into the solution which was agitated for 20 min. Fourthly, a certain quantity of CH₄N₂S was mixed into the solution which was agitated for 20 min. Fifthly, the solution was poured into a stainless steel autoclave with Teflon-lining whose volume was 100 mL, the sealed autoclave was heated in an electric oven at 160°C for 10 h and then naturally cooled to ambient temperature. Finally, the products were washed using deionized water, dehydrated under vacuum at 70°C for 10 h, and then ground for further use. The obtained samples were recorded as ZSX (X = 200, 400, 600, 800, and 1000), where X represented the molar ratio of ZnO to SnIn₄S₈. In addition, SnIn₄S₈ was prepared through the same approach mentioned above except addition of as-synthesized ZnO. The flow chart of the synthesis of the SnIn₄S₈, ZnO and SnIn₄S₈@ZnO samples is exhibited in Figure 1.

2.3. Characterizations

The phase compositions of the as-synthesized samples were analyzed via an X-ray diffractometer (XRD, Rigaku Smartlab3kw, Japan). The morphologies of the samples were described using a field emission scanning electron microscope (FESEM, Zeiss Sigma 300, Germany) and a transmission electron microscope (TEM, FEI Titan G2 60-300, US). The distribution of elements was analyzed through energy dispersive X-ray (EDX) detector (Oxford X-Max, UK) which was merged into the FESEM. A UV-visible spectrophotometer (Shimadzu UV-2550, Japan) was used to characterize the absorption spectra of specimens. A fluorescence spectrophotometer (Lengguang Tech. F97XP, China) was used to test the photoluminescence (PL) spectra of specimens.

2.4. Photocatalytic degradation of MB

The photocatalytic efficiencies of specimens were evaluated through choosing MB as model pollutant. 15 mg of catalyst powders were added into MB solution (100 mL, 10 mg/L). The mixture was treated for 10 min through ultrasonic vibration and then agitated for 30 min in the dark to reach adsorption/desorption equilibrium. After that, the constantly stirred mixture was illuminated by an ultraviolet lamp whose power and irradiation peak wavelength was 36 W and 365 nm, respectively. 5 ml of the mixture was extracted at the interval of 5 min, and then the photocatalyst was removed through centrifugation. Finally, the UV-Vis spectrophotometer was used to determine the absorbance of solution at 664 nm.

3. Results and Discussion

3.1. XRD analysis

The XRD spectra of ZnO, SnIn₄S₈ and ZSX with a scanning speed of 10°/min are displayed in Figure 2a. Among them, the characteristic peaks of ZnO can correspond to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) crystalline facets of the hexagonal wurtzite phase of ZnO (JCPDS card No. 36-1451) [31]. The characteristic peaks of SnIn₄S₈ at 2θ = 18.7°, 28.6°, 33.2°, 48.3° and 50.1°, which are consistent with the (202), (222), (400), (440) and (531) crystal faces of cubic phase of SnIn₄S₈ (JCPDS card No. 42-1305), separately, can be observed [32]. Furthermore, no characteristic peaks of any foreign substance can be found in the XRD spectra, suggesting an excellent purity of the as-prepared SnIn₄S₈. In addition, the characteristic peaks of ZSX samples can be ascribed to the diffraction planes of ZnO (JCPDS card No. 36-1451). However, no diffraction peaks corresponding to SnIn₄S₈ can be found in the XRD patterns of ZSX samples, which should be attributed to the low level of SnIn₄S₈ in ZSX specimens and relatively fast scanning rate of XRD. In order to verify the existence of SnIn₄S₈ in ZSX samples, the as-prepared ZS1000 was test through XRD with a scanning rate of 4°/min and the pattern (Figure 2b) clearly displays that the diffraction peaks at 18.7° and 28.6° can be ascribed to (202) and (222) plane of SnIn₄S₈ (JCPDS card No. 42-1305).

3.2. Morphological analysis

The FSEM photographs of ZnO, SnIn₄S₈ and ZSX samples are presented in Figure 3. It can be seen that the ZnO showed flaky morphology with smooth surface (Figure 3a). The as-prepared SnIn₄S₈ showed a morphology of 3D hierarchical reticular microspheres with diameters of 2-4 μm (Figure 3b,c). From the magnified FSEM picture, it can be observed that the microspheres were composed of many two-dimensional nanoflakes with a thickness of about 20 nm. Furthermore, the microspheres possessed a porous appearance because of the interlacing of nanoflakes. The SEM images of ZSX samples show that many SnIn₄S₈ nanoparticles were unevenly distributed on the surface of the ZnO Substrate (Figure 3d-h). Furthermore, the number of nanoparticles gradually decreased with the increase of the molar ratio of ZnO to SnIn₄S₈. Moreover, EDX elemental mapping analysis of ZS200 demonstrated that the elements Zn, O, Sn, In and S existed in ZSX samples (Figure 4), which can further suggest that the large flake-like substrates were ZnO and the small nanoparticles were SnIn₄S₈.

The TEM photo of ZS800 obviously indicates that the SnIn₄S₈ nanoparticles densely covered the surface of the nanoflake-like ZnO to obtain the SnIn₄S₈@ZnO heterojunction (Figure 5a). Two kinds of lattice fringes are clearly displayed in HRTEM image of ZS800 , as shown in Figure 5b, the observed interplanar spacing of 0.247nm corresponded to ZnO (101) plane (JCPDS card No. 36-1451), and the characteristic interplanar spacing of 0.268nm belonged to SnIn₄S₈ (400) plane (JCPDS card No. 42-1305) [31,33]. Furthermore, it can be observed that the contact interface between ZnO and SnIn₄S₈ is very tight. The results mentioned above demonstrate that the SnIn₄S₈@ZnO heterojunction has been successfully constructed. The tight contact interface between SnIn₄S₈ and ZnO should be beneficial for the migration of photogenerated charge carriers and the stability of structure. As a consequence, the SnIn₄S₈@ZnO heterojunction would exhibit the enhanced photocatalytic activity.

3.3. UV-Vis absorption spectra analysis

The light-harvesting ability can be regarded as a crucial factor for the photocatalytic activity of semiconductors. It can be found from the UV-vis absorption spectra (Figure 6a) that all the samples exhibited strong optical absorption in the region of UV. Moreover, the ZSX samples exhibited absorption cutoff wavelengths with slight redshift and the improvements of absorption capability in the Vis range compared to pure ZnO. The band gap (E_g) of the crystalline semiconductors can be obtained using the Kubelka:

$$\mathsf{\alpha }\mathrm{hv}=\mathrm{A}{\left(\mathrm{hv}-{\mathrm{E}}_{\mathrm{g}}\right)}^{\mathrm{n}/2}$$

(1)

Where α is the absorbance, hv is the photon energy, and A is a constant [34]. The selection of n value (1 for direct transition, while 4 for indirect transition) is based on the transition property in the semiconductor [35]. The band gap energy of as-synthesized specimens was determined through selecting n as 1 because both SnIn₄S₈ and ZnO are direct transition semiconductors. Figure 6b presents the (αhν)² curves with hv of ZnO and ZS800, the band gaps can be calculated through lengthening the linear portion to the x-axis, which indicates that the band gaps of ZnO and ZS800 were about 3.22eV and 3.19eV, respectively. The narrowing of bandgap energy suggested that light-harvesting capability of ZnO was improved by the hybridization with SnIn₄S₈.

3.4. PL spectra analysis

The recombination rate of photoinduced charge carriers can be reflected through the peak intensity of PL spectrum. Normally, a weaker peak intensity reflects a higher separation efficiency of the photogenerated charge carriers, which results in a better photocatalytic performance [36,37]. The PL spectra of the as-synthesized ZnO and ZSX specimens are displayed in Figure 7, which reveals that all specimens had similar PL spectra with two distinct luminescence peaks at approximate 400 and 460 nm. The peak at about 400 nm was the emission near the edge of the band, which originated from the recombination of the free-excitons through the collision between the excitons, and the peak at about 460nm may be the result from the inherent defects of ZnO [38,39]. Furthermore, compared with ZnO, the intensity of the fluorescence spectra of ZSX samples were significantly reduced. It indicated that SnIn₄S₈@ZnO heterojunction had a lower recombination rate of photogenerated charge carriers, which would be beneficial for the improvement of photocatalytic performance of catalyst [40].

3.5. Photocatalytic activities of the specimens

The MB photodegradation curves over ZnO and ZSX samples are shown in Figure 8a, and the ultimate degradation rates of MB after 20 min are exhibited in Figure 8b. The changes of MB concentrations were very small when MB was adsorbed for 30 min using all samples in the dark, which indicated that the adsorption effects of the ZnO and ZSX samples on MB were negligible. After 20 min of UV illumination, the degradation rates of MB degraded by ZnO, ZS200, ZS400, ZS600, ZS800 and ZS1000 were 66%, 88%, 88%, 86%, 91% and 87%, respectively. Compared with the ZnO, the ZSX samples exhibited obviously improved photocatalytic activities. Furthermore, ZS800 had the highest photocatalytic efficiency for MB degradation.

Generally speaking, the photodegradation of MB complies with the pseudo-first-order kinetic equation:

$$\ln \left(\frac{{\mathrm{C}}_0}{{\mathrm{C}}_{\mathrm{t}}}\right)=\mathrm{kt}$$

(2)

Where k, t, C₀, and C_t are the apparent rate constant, the photocatalytic reaction time, the MB concentrations at the moment of t = 0 and a certain time, respectively [41]. The kinetic curves of MB photodegradation in presence of ZnO and ZSX samples were fitted by plotting ln(C₀/C_t) versus time (Figure 8c), and the good inear relationship can be observed. The gradient of kinetic curve denotes k value which corresponds to the degradation efficiency. The k values of MB degradation over ZnO, ZS200, ZS400, ZS600, ZS800 and ZS1000 were 0.054, 0.108, 0.109, 0.101, 0.121 and 0.102 min^-1, separately (Figure 8d). It can be found that k value of ZSX samples for MB degradation were obviously higher than that of ZnO. Furthermore, ZS800 had the highest k value which was approximately 2.2 times over that of ZnO. These results indicate that the photocatalytic activity of the ZnO can be significantly boosted through hybridization with SnIn₄S₈ for the formation of SnIn₄S₈@ZnO heterojunction which is an efficient photocatalyst.

3.6. Photocatalytic mechanism analysis

According to the results in this work and several previous investigations, a Z-scheme mechanism can be proposed for efficient degradation of MB by the SnIn₄S₈@ZnO heterojunction, as shown in Figure 9. The previous investigations indicate that the conduction band (CB) position of SnIn₄S₈ is higher than that of ZnO, while the valence band (VB) position of ZnO is lower than that of SnIn₄S₈ [42,43,44]. Under the irradiation of UV light, the photoinduced electrons can transfer from the VB of the ZnO and SnIn₄S₈ to the CB of them, respectively, keeping the photogenerated holes in the VB of them. Then the photoinduced electrons in the CB of the ZnO can migrate to the VB of the SnIn₄S₈ owing to the appropriate potential difference. Eventually, a Z-shaped migration path of photogenerated electrons is formed in the SnIn₄S₈@ZnO heterojunction. This mechanism results that the photoinduced electrons collect in CB of SnIn₄S₈ while the photoinduced holes gather in VB of ZnO. These photoexcited electrons and holes are efficiently separated in the opposite side of SnIn₄S₈@ZnO heterojunction. Therefore, the life span of photo-induced carriers can be greatly improved, leading to remarkable improvement of photocatalytic activity. The photoinduced holes in VB of ZnO are vital oxidants, they can quickly oxidize hydroxyl anions (OH^-) to hydroxyl radicals (·OH). The photogenerated electrons in CB of SnIn₄S₈ are robust reductants, they can rapidly reduce dissolved oxygen (O₂) into superoxide radicals (·O₂^-). Finally, the MB molecules can be degraded into the inorganic small molecules by these highly active ·OH and ·O₂^- species. Overall, the main reactions of this mechanism can be expressed as below.

(1)

$\mathrm{ZnO}/{\mathrm{SnIn}}_4{\mathrm{S}}_8+\mathrm{hv}\to \mathrm{ZnO}\left({\mathrm{h}}^{+}\left(\mathrm{VB}\right)+{\mathrm{e}}^{-}\left(\mathrm{CB}\right)\right)/{\mathrm{SnIn}}_4{\mathrm{S}}_8\left({\mathrm{h}}^{+}\left(\mathrm{VB}\right)+{\mathrm{e}}^{-}\left(\mathrm{CB}\right)\right)$

(2)

$\mathrm{ZnO}\left({\mathrm{h}}^{+}\left(\mathrm{VB}\right)+{\mathrm{e}}^{-}\left(\mathrm{CB}\right)\right)/{\mathrm{SnIn}}_4{\mathrm{S}}_8\left({\mathrm{h}}^{+}\left(\mathrm{VB}\right)+{\mathrm{e}}^{-}\left(\mathrm{CB}\right)\right)\to \mathrm{ZnO}\left({\mathrm{h}}^{+}\left(\mathrm{VB}\right)\right)+{\mathrm{SnIn}}_4{\mathrm{S}}_8\left({\mathrm{e}}^{-}\left(\mathrm{CB}\right)\right)$

(3)

$\mathrm{ZnO}\left({\mathrm{h}}^{+}\left(\mathrm{VB}\right)\right)+{\mathrm{OH}}^{-} \to ·\mathrm{OH}$

(4)

${\mathrm{SnIn}}_4{\mathrm{S}}_8\left({\mathrm{e}}^{-}\left(\mathrm{CB}\right)\right)+{\mathrm{O}}_2 \to ·{\mathrm{O}}_2^{-}$

(5)

$·\mathrm{OH},·{\mathrm{O}}_2^{-}+\mathrm{MB} \to \mathrm{inorganic}\mathrm{small}\mathrm{molecules}$

4. Conclusions

In this work, SnIn₄S₈@ZnO Z-scheme heterojunction photocatalyst was successfully acquired through a convenient two-step hydrothermal approach. The as-synthesized SnIn₄S₈@ZnO exhibited significantly enhanced photocatalysis activity under UV light irradiation. The degradation ability of the SnIn₄S₈@ZnO structure towards MB was significantly stronger than that of pure ZnO. The remarkable performance for MB photodegradation can be ascribed to the efficient spatial separation of photoinduced electrons and holes through a Z-scheme heterojunction with intimate contact interface. To conclude, the results in this paper will bring a novel insight into constructing excellent ZnO-based photocatalytic systems for wastewater purification.