Enhanced photocatalytic properties of ZnFe₂O₄-doped ZnIn₂S₄ heterostructure under visible light irradiation

Abstract

ZnFe₂O₄-doped ZnIn₂S₄ heterostructures were fabricated with a series of different proportions of ZnFe₂O₄ using a two-step solvothermal method. The as-synthesized ZnFe₂O₄–ZnIn₂S₄ heterostructure exhibited enhanced photocatalytic performance in the degradation of organic pollutants compared to bare ZnIn₂S₄ and ZnFe₂O₄ under visible light irradiation, and the 2.5 wt% ZnFe₂O₄–ZnIn₂S₄ showed the highest activity. The enhanced mechanism of photocatalytic activity can be mainly attributed to the stable heterojunction interface between ZnFe₂O₄ and ZnIn₂S₄, which can efficiently improve the separation of photogenerated carriers. Meanwhile, the increased surface-active sites and extended light absorption of ZnIn₂S₄ after the decoration of ZnFe₂O₄ nanoparticles may also play a certain role in enhancing photocatalytic activity.

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1. Introduction

With the development of different industries, our living environment has become more colorful, but at the same time, a large number of organic contaminants have been discharged into the water. These organic pollutants are generally very complex, heterogeneous, toxic and harmful to human beings.¹ Therefore, considerable attention has been drawn to developing effective technology for the removal of organic pollutants from aqueous solutions. The main techniques used for the removal of harmful organic dyes from aqueous solutions, such as adsorption by the use of activated carbon, biological treatment, and filtration, do not eliminate these contaminants completely.² Therefore, a novel approach for water purification needs to be developed. Heterogeneous photocatalysis, which can be classified as an advanced oxidation method, is considered a promising technology for the elimination of most organic contaminants.^1,3 Compared with the conventional wide-bandgap semiconductors, for example TiO₂ and ZnO, which can only absorb UV light and thus has greatly restricted practical application,^1,4–6 metal sulfides have been intensively studied in photocatalysis because of their suitable bandgap and catalytic functions.^7–10 Among these metal sulfides, ternary metal sulfide ZnIn₂S₄, an eco-friendly and chemically stable, visible light-driven photocatalyst, shows high activity in the degradation of contaminants.^11–18 However, the photocatalytic activity of ZnIn₂S₄ does not satisfy the requirements for effective removal of various organic pollutants. For the visible light-driven photocatalyst ZnIn₂S₄, one of the fundamental issues limiting its further applicability is that the photogenerated electron–hole pairs in the excited states are unstable and can easily recombine at or near its surface, resulting in relatively low photocatalytic efficiency.^19,20 Clearly, efficiently improving the separation and transfer of the photogenerated charge carriers and prolonging their lifetime are keys to obtaining high photocatalytic activities. Recent efforts have demonstrated that the charge separation and photocatalytic activity could be improved by selectively doping ZnIn₂S₄ nanomaterials using other semiconductors with tunable electronic structures to form unique heterostructures.^20–26

Spinel ZnFe₂O₄, an emerging inorganic semiconductor material, has great application potential in photocatalytic degradation due to its visible light response, low price, good chemical stability and non-toxicity.^27–29 From the viewpoint of band structure, the bandgap of ZnFe₂O₄ (∼1.92 eV) is smaller than that of ZnIn₂S₄ (∼2.4 eV).^26,29 The position of the conduction band (CB) of ZnFe₂O₄ (−1.54 eV vs. NHE) is more negative than that of ZnIn₂S₄ (−0.68 eV vs. NHE), and it thus provides the possibility for directional transfer of the photogenerated electrons from ZnFe₂O₄ to ZnIn₂S₄.^30,31 Moreover, due to the valence band (VB) of ZnIn₂S₄ (+1.56 eV vs. NHE) being more positive than that of ZnFe₂O₄ (+0.38 eV vs. NHE), the holes on VB of ZnIn₂S₄ will migrate to that of ZnFe₂O₄, improving the separation of charge carriers in composites. Therefore, it can be supposed that ZnFe₂O₄-doped ZnIn₂S₄ may result in enhanced photocatalytic efficiency in comparison to individual ZnFe₂O₄ or ZnIn₂S₄. However, to the best of our knowledge, no work related to the ZnFe₂O₄-doped ZnIn₂S₄ photocatalyst has been reported.

In this work, we successfully synthesized a series of ZnFe₂O₄-doped ZnIn₂S₄ composites (ZFO–ZIS) by a facile, low-temperature solvothermal method. It can be seen that the pure ZnIn₂S₄ microspheres consist of many nanosheets. Furthermore, the doping of the ZnFe₂O₄ nanoparticles can affect the morphology of ZnIn₂S₄, resulting in destruction and gradual disappearance of these microspheres with increasing doping of ZnFe₂O_4. The photocatalytic properties of ZFO–ZIS composites were studied by the degradation of organic pollutants in water. Compared with bare ZnIn₂S₄, the ZFO–ZIS composites showed enhanced photocatalytic performance under visible light irradiation, and the content of ZnFe₂O₄ in composites obviously affected the photocatalytic performance. Finally, the mechanism of the enhanced photocatalytic performance for the ZFO–ZIS composites is also explained.

2. Experiment

2.1 Synthesis of the ZFO–ZIS composites

All of the reagents were of analytical grade and used without further purification. ZFO–ZIS composites with different mass percentages of ZnFe₂O₄ were prepared via a two-step hydrothermal method. Firstly, ZnFe₂O₄ nanoparticles were synthesized by the following method. 0.2975 g of Zn(NO₃)₂·6H₂O and 0.8080 g of Fe(NO₃)₃·9H₂O were dissolved in 20 mL of absolute ethanol. 2 mL of 6 mol L⁻¹ NaOH solution was added into the above solution under magnetic stirring. After 30 min, the mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave, heated to 180 °C and maintained for 12 h. The obtained precipitate was washed using adequate deionized water and ethanol after naturally cooling to room temperature. The prepared ZnFe₂O₄ nanoparticles were collected by centrifugation, and then dried at 80 °C for 24 h. Secondly, the prepared ZnFe₂O₄ nanoparticles were added into a solution with 20 mL deionized water and 5 mL ethylene glycol to obtain a uniform suspension under ultrasonic vibration (100 W) for 30 min. Then, 0.14875 g of Zn(NO₃)₂·6H₂O, 0.382 g of In(NO₃)₃·4.5H₂O and 0.3029 g of L-cysteine were added into the above solution under ultrasonic vibration for another 30 min. The above mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave, heated to 150 °C and maintained for 12 h. After naturally cooling to room temperature, the as-synthesized precipitates were rinsed with deionized water and ethanol, and then dried at 70 °C for 12 h. For comparison, blank ZnIn₂S₄ was prepared using the same experimental conditions without ZnFe₂O₄ nanoparticles.

2.2 Characterization

The crystal phases of the powder samples were examined by X-ray diffraction (XRD, Bruker D8-25A Advance diffractometer) with monochromatized Cu Kα radiation (λ = 1.542 Å). Scanning electron microscope (SEM, JSM-6700F) and transmission electron microscope (TEM, JEOL 2100) were used to observe the morphology and structure of the samples. Nitrogen adsorption/desorption isotherms were determined using TriStar II 3020 surface area and pore size analyzer (Micromeritics). The fluorescence spectra of the samples were measured using the Hitachi F-7000 fluorescence spectrophotometer under the excitation wavelength of 320 nm.

2.3 Photocatalytic activity measurement

The photocatalytic performance of the as-prepared samples was measured by the photocatalytic degradation of 4-nitrophenol (4-NP) under visible light irradiation. 50 mg of as-prepared catalyst was added to 100 mL of 10 mg L⁻¹ 4-NP solution in a Pyrex photocatalytic reactor with a circulating water system, which could keep the reaction temperature at 25 ± 1 °C. Before irradiation, the mixed suspensions were magnetically stirred in the dark for 30 min to ensure that the catalyst and solution reach the adsorption–desorption equilibrium. Then, the mixed suspensions were exposed to visible light irradiation by a 300 W Xe lamp (Perfectlight, Beijing, China) with cut-off filter (λ > 420 nm). Analytical samples were taken from the mixture solution at given time intervals and immediately filtered to remove the catalyst. The concentrations of 4-NP in filtrate were measured using a Hitachi U-3900H UV-vis spectrophotometer. The degradation rate of 4-nitrophenol was calculated as follows:

(A₀ − A_t)/A₀ × 100% (1)

where A₀ is the initial concentration of the 4-NP after adsorption–desorption equilibrium and A_t is the concentration of the 4-NP after illumination at the given time. In addition, the self-degradation rate of 4-NP was also tested under similar conditions without photocatalysts.

2.4 Hydroxyl radical analysis

The generation of ˙OH at different times was analyzed by photoluminescence technique using terephthalic acid as probe.^9,32–36 50 mg of catalyst was dispersed in a 100 mL solution containing 5 mM terephthalic acid and 10 mM sodium hydroxide. After magnetic stirring for 30 min under dark condition, this suspension was exposed to visible light irradiation (λ > 420 nm). During irradiation, 3 mL suspensions were taken at defined time intervals, and the catalyst was immediately removed by filtration. The fluorescence spectra of the filtrates were measured using the Hitachi F-7000 fluorescence spectrophotometer, and the strongest emitted photoluminescence was recorded at 434 nm under the inherent excitation wavelength of 315 nm.

3. Results and discussion

3.1 Morphology and structure

Fig. 1 shows the XRD diffraction patterns of the as-synthesized bare ZnIn₂S₄, ZnFe₂O₄ and ZFO–ZIS composites with different ZnFe₂O₄ doping. For the pure ZnIn₂S₄, all the characteristic peaks could be indexed to a hexagonal phase of ZnIn₂S₄ (JCPDS no. 72-0773). No impurity peaks attributable to binary sulfides, oxides, or organic compounds related to the reactants can be detected, indicating the good phase purity of the as-prepared ZnIn₂S₄. The broad diffraction peaks of ZnIn₂S₄ may be attributed to the slow decomposition of L-cysteine to release S²⁻ ions into the solution, which can regulate the nucleation rate of ZnIn₂S₄ particles, resulting in a much smaller crystallite size. The XRD patterns of ZFO–ZIS with mass percentages of ZnFe₂O₄ doping from 1 to 10 wt% mainly revealed diffraction peaks of ZnIn₂S₄, which could be attributed to the low content or poor crystallinity of ZnFe₂O₄ nanoparticles. When the mass percentage of ZnFe₂O₄ in composites was increased to 30 and 50 wt%, respectively, we can see several characteristic peaks assigned to ZnFe₂O₄ (JCPDS no. 77-0011).

Fig. 1 XRD patterns of bare ZnIn₂S₄, ZnFe₂O₄, and ZFO–ZIS composites.

Fig. 2 shows the typical SEM images of the as-synthesized bare ZnIn₂S₄, ZnFe₂O₄ and ZFO–ZIS composites. For pure ZnIn₂S₄ sample (Fig. 2a), some monodisperse microspheres with size of around 3 μm can be observed, and other small spheres are adhered to each other. The inset of Fig. 2a shows the local magnified image of the surface of ZnIn₂S₄ microspheres, and we can see that these microspheres are composed of a large number of nanosheets with the average thickness of 10 nm. The pure ZnFe₂O₄ (Fig. 2b) shows evenly dispersed nanoparticles. When the mass percentage of ZnFe₂O₄ in composites was increased from 1 to 10 wt% (Fig. 2c–f), the sphere-like structure assembled by nanosheets can still be observed, but the space between these nanosheets are gradually increased with the increasing content of ZnFe₂O₄ in ZFO–ZIS composites. This means that the sphere-like structure of ZFO–ZIS composites is much looser than that of bare ZnIn₂S₄. Due to the special structure of these ZnIn₂S₄ microspheres and the size of ZnFe₂O₄, it is difficult to find these ZnFe₂O₄ nanoparticles on the ZnIn₂S₄ microspheres from Fig. 2c–f. When the loading of ZnFe₂O₄ was increased to 30 wt% (Fig. 2g) or 50 wt% (Fig. 2h), the sphere-like structure of composites was destroyed completely, and a lot of ZnFe₂O₄ nanoparticles can be observed on ZnIn₂S₄ nanosheets.

Fig. 2 Typical images of (a) ZnIn₂S₄, (b) ZnFe₂O₄, and ZFO–ZIS composites with different mass percentages of doping ZnFe₂O₄: (c) 1 wt%, (d) 2.5 wt%, (e) 5 wt%, (f) 10 wt%, (g) 30 wt% and (h) 50 wt%.

To further explore the structure of the microspheres, TEM images of ZFO–ZIS composites with 2.5 wt% loading of ZnFe₂O₄ are presented in Fig. 3. From Fig. 3a, sphere-like structures can be clearly observed, and these microspheres adhere to each other, which is consistent with the SEM result in Fig. 2d. From Fig. 3b, we can obviously see that these microspheres consist of nanosheets, and some shed nanosheet networks are attached to the surface of these microspheres. The EDS result in Fig. S1† exhibits the chemical elements Zn, In, S, Fe and O on the surface of the composites, which suggests that the ZnFe₂O₄ nanoparticles attached onto the surface of ZnIn₂S₄ nanosheets successfully. From Fig. 3c and the HRTEM image in Fig. 3d, it can be clearly seen that ZnFe₂O₄ nanoparticles are supported on the ZnIn₂S₄ nanosheet surface. In addition, the clear lattice fringes shown in Fig. 3d with spacing values of 3.30 Å and 2.54 Å can be attributed to the (101) crystalline planes of hexagonal ZnIn₂S₄ (JCPDS no. 72-0773) and the (311) crystalline planes of cubic ZnFe₂O₄ (JCPDS no. 77-0011), respectively. The interface (blue dotted line) between ZnIn₂S₄ nanosheets and ZnFe₂O₄ nanoparticles also can be clearly observed in Fig. 3d.

Fig. 3 Typical TEM and HRTEM images of 2.5 wt% ZFO–ZIS composites.

The nitrogen adsorption–desorption isotherms of bare ZnIn₂S₄, ZFO–ZIS composites and pure ZnFe₂O₄ are shown in Fig. 4a and S2.† All the isotherms exhibit similar type IV (IUPAC classification) isotherms with a typical H₃ hysteresis loop,³⁷ indicating that mesoporous structure and slit-like pores exist in the composites. On the basis of the BET equation, the specific surface areas are 77.56, 138.93, 170.8, 167.8, 104.1, 75.89, 65.83, and 132.1 m² g⁻¹ for the bare ZnIn₂S₄, pure ZnFe₂O₄,1 wt% ZFO–ZIS, 2.5 wt% ZFO–ZIS, 5 wt% ZFO–ZIS, 10 wt% ZFO–ZIS, 30 wt% ZFO–ZIS and 50 wt% ZFO–ZIS, respectively. It is obvious that the specific surface areas were increased after the ZnFe₂O₄ was doped in ZnIn₂S₄ at the low mass ratios from 1 to 5 wt%, which may contribute to the loose, sphere-like structure and reduced crystallinity of ZnIn₂S₄. When the content of ZnFe₂O₄ is up to 10 wt% and more, the sphere-like structure was destroyed gradually with increasing ZnFe₂O₄ nanoparticles in composites, resulting in the decrease of the specific surface areas of composites by degrees. However, the specific surface area of 50 wt% ZFO–ZIS was up to 132.1 m² g⁻¹ because of the main role of ZnFe₂O₄ nanoparticles in composites.

Fig. 4 (a) N₂ adsorption–desorption isotherms and (b) the pore size distribution curves of the pure ZnIn₂S₄ and ZFO–ZIS composites.

As we know, a large surface area is considered a basic requirement for a highly effective photocatalyst. The large surface area and multiple scattering effects of the mesoporous composites with the special porous structure can enhance the light-harvesting performance of the photocatalyst.^38–41 Meanwhile, the large surface area of the photocatalyst can provide more surface-active sites for the reactant molecules, allowing the reactant and catalyst to contact more fully, which can improve the photocatalytic performance. This indicates that the 1 and 2.5 wt% ZFO–ZIS samples with the larger surface areas may possess higher photocatalytic performance in the degradation of organic contaminants.

In addition, the pore size (Fig. 4b) distributions of the samples calculated from the BJH model show similar structure for the bare ZnIn₂S₄ and ZFO–ZIS composites. The details of nitrogen adsorption/desorption measurements for bare ZnIn₂S₄ and the ZFO–ZIS composites are listed in Table S1.†

3.2 Photocatalytic properties

The photodegradation of 4-NP under visible light irradiation (λ > 420 nm) of a 300 W xenon lamp was carried out to evaluate the photocatalytic activity of the as-prepared samples. The variation of 4-NP concentration during the photocatalytic process is shown in Fig. 5a, which clearly shows that the adsorption effect of 4-NP on the surface of the samples is almost negligible in dark condition, and 4-NP was stable under irradiation without photocatalyst. In addition, only 4.4 wt% 4-NP was degraded under visible light irradiation using P25 as photocatalyst. With pure ZnIn₂S₄ and pure ZnFe₂O₄ as photocatalyst, no more than 12.6% and 28.2% of the 4-NP was degraded in 180 min. However, about 50–90% of the 4-NP was photodegraded by the ZFO–ZIS composites during the same period. To further understand the photocatalytic activity of all as-prepared samples, the kinetic model is discussed. The degradation of 4-NP followed pseudo-first-order kinetics, as shown in Fig. 5b. Among all the photocatalysts, the degradation rate constant (K) of bare ZnIn₂S₄ was the smallest of pseudo-first-order plots at 0.076 h⁻¹. After ZnIn₂S₄ was combined with ZnFe₂O₄ nanoparticles, enhanced photocatalytic performance was achieved, and the corresponding rate constants for 4-NP degradation with 1 wt% ZFO–ZIS, 2.5 wt% ZFO–ZIS, 5 wt% ZFO–ZIS, 10 wt% ZFO–ZIS, 30 wt% ZFO–ZIS and 50 wt% ZFO–ZIS were 0.95, 1.25, 1.03, 0.85, 0.47, and 0.13 h⁻¹, respectively. This indicates that 2.5 wt% ZFO–ZIS possesses the highest photocatalytic performance among the concentration values tried in the test. In addition, to explore the importance of the interface between ZnIn₂S₄ and ZnFe₂O₄, the degradation 4-NP with 2.5 wt% ZFO/ZIS by physical mixing was also measured, and the corresponding rate constant for 4-NP was 0.86 h⁻¹. The 4-NP degradation rate of 2.5 wt% ZFO–ZIS was up to 16.4-fold, 13.9-fold and 1.43-fold higher than that of the pure ZnIn₂S₄, pure ZnFe₂O₄ and 2.5 wt% ZFO/ZIS. This indicates that incorporation of ZnFe₂O₄ and ZnIn₂S₄ with stable interface is very important to enhance the photocatalytic performance.

Fig. 5 (a) Photodegradation of 4-NP under visible light and (b) the corresponding pseudo-first-order kinetics.

The improved photocatalytic performance of ZFO–ZIS can be explained by the PL spectra shown in Fig. 6. PL spectra can be used primarily to determine the effectiveness of trapping, migration and transfer of charge carriers, as well as to understand the fate of the photo-induced electrons and holes pairs in semiconductors.^42–44 PL is the result of the recombination of excited electrons and holes, and the lower PL intensity indicates a lower recombination rate.^45,46 The PL intensity of ZFO–ZIS was obviously weaker than that of pure ZnIn₂S₄, suggesting that the recombination rate of charges and holes for the ZFO–ZIS system was lower than those of pure ZnIn₂S₄. It revealed that the ZFO–ZIS system can effectively suppress the photogenerated charge/hole recombination. The improved separation of photogenerated carriers should be attributed to the suitable band potentials of ZnFe₂O₄ nanoparticles and ZnIn₂S₄ nanosheets. The possible mechanism of separation and transportation of electron–hole pairs at the interface between ZnFe₂O₄ and ZnIn₂S₄ in composites under visible light is proposed and illustrated in Scheme 1. Under visible light illumination, both ZnFe₂O₄ and ZnIn₂S₄ can be excited and produce photogenerated electron–hole pairs. Since the CB position of ZnFe₂O₄ (−1.54 eV vs. NHE) is more negative than that of ZnIn₂S₄ (−0.68 eV vs. NHE), the photogenerated electrons on the CB of ZnFe₂O₄ can easily transfer to the surface of ZnIn₂S₄. Moreover, the VB of ZnIn₂S₄ (+1.56 eV vs. NHE) is more positive than that of ZnFe₂O₄ (+0.38 eV vs. NHE); the photogenerated holes on the VB of ZnIn₂S₄ will migrate to the VB of ZnFe₂O₄. Therefore, the photogenerated electrons and holes can move in opposite directions, which can effectively reduce the recombination probability and improve the separation of charge carriers, resulting in remarkable enhanced photocatalytic activities of these ZnFe₂O₄–ZnIn₂S₄ composites in comparison to individual ZnIn₂S₄. Furthermore, compared with the transient interface derived from the independent ZnFe₂O₄ nanoparticles and ZnIn₂S₄ microspheres by physical mixing in the 2.5 wt% ZFO/ZIS mixture, the interface in the 2.5 wt% ZFO–ZIS was more stable due to their tight heterojunction structure (Fig. 3d). Therefore, the 2.5 wt% ZFO–ZIS exhibited the highest catalytic degradation ability in all samples.

Fig. 6 PL spectra of the bare ZnIn₂S₄, ZnFe₂O₄, ZFO–ZIS and ZFO/ZIS.

Scheme 1 The possible mechanism of separation and transportation of electron–hole pairs in the interface between ZnFe₂O₄ and ZnIn₂S₄.

Apart from these advantages of high specific surface area and the tight heterojunction structure for ZFO–ZIS composites, UV-vis diffuse reflectance spectra of the ZnIn₂S₄ and ZFO–ZIS also confirm that extended light absorption of ZnIn₂S₄ after the decoration of ZnFe₂O₄ nanoparticles may also play a certain role in enhancing photocatalytic activity. Fig. S3† shows the UV-vis diffuse reflectance spectra of the ZnIn₂S₄ and ZFO–ZIS; we can clearly observe that all of the samples show a strong characteristic absorption in the UV region. The pure ZnIn₂S₄ shows the narrowest absorption range (below 450 nm), and ZnFe₂O₄ exhibits the widest absorption range, almost including the whole of the visible light region. After ZnFe₂O₄ was doped in the ZnIn₂S₄, the absorption band of ZnIn₂S₄ in the visible light region was obviously enhanced. This indicates that more visible light can be absorbed, and it may provide more energy to excite more electron–hole charge carriers, thus improving the photocatalytic performance of the material.

Hydroxyl radicals (˙OH) are usually suggested as the primary active species in photocatalytic oxidation processes. The formation of ˙OH radicals on the surface of samples was detected by the PL method using terephthalic acid as a probe molecule. Fig. 7a shows the PL spectra of the 2.5 wt% ZFO–ZIS in a solution of 5 mM terephthalic acid and 10 mM NaOH under visible light irradiation. The PL emission intensity was very weak and almost negligible under dark conditions, and then the intensity of PL increased gradually with the increasing irradiation time, which indicates that the ˙OH radicals were generated by 2.5 wt% ZFO–ZIS under visible light irradiation and that the mass of ˙OH in the solution can be increased with the increasing irradiation time. Fig. 7b displays the changes of the fluorescence intensity between the different samples under visible light irradiation. It is observed that 2.5 wt% ZFO–ZIS shows the strongest PL intensity, and the order of PL intensity over all photocatalysts is well in agreement with the results of the photocatalytic degradation of 4-NP in Fig. 5, which indicates that the 2.5 wt% ZFO–ZIS could generate more ˙OH on its surface under the same conditions and provide more active substances for the degradation of 4-NP than other photocatalysts.

Fig. 7 (a) Fluorescence spectra of 2-hydroxy-terephthalic acid (TAOH) formed by the reaction of terephthalic acid (TA) with ˙OH radicals generated from 2.5 wt% ZFO–ZIS under visible light irradiation; (b) the changes of the fluorescence intensity between the different samples under visible light irradiation.

To further understand the degradation process of 4-NP, the photodegradation solution over 2.5 wt% ZFO–ZIS was analyzed by UV-vis spectrophotometer and high-performance liquid chromatography (HPLC). Fig. 8a shows the absorption spectra of the degraded samples of 4-NP at different times; we can clearly see that the strongest absorption of 4-NP was located at 315 nm, and with the increase of illumination time, the absorption peak was reduced obviously, and no other absorption peaks were observed in the range of 200–800 nm. This indicates that no other impurities appeared, but only 4-NP decreased, which suggests that the degradation mechanism of 4-NP may be the mineralization of 4-NP to form carbon dioxide and water directly. This speculation can be confirmed by the results of HPLC displayed in Fig. 8b. Before irradiation, we can observe that there was a larger peak corresponding to 4-NP at the retention time of 4.5 min, which proved that there is good response for 4-NP in present chromatographic conditions, and the concentration of 4-NP in the samples could be detected accurately. When the irradiation time was increased to 60 min, the intensity of the peak at 4.5 min was decreased sharply, and the peak almost disappeared after 120 min. As we know, the peak area reflects the concentration of 4-NP; the decreasing peak area demonstrates that the concentration of 4-NP decreased sharply with increasing irradiation time until the limit of detection.

Fig. 8 (a) Absorption spectra and (b) HPLC chromatogram of the 4-NP solution after photodegradation under visible light.

The stability of photocatalysts is a key factor for practical application. The cycle degradation of 4-NP by 2.5 wt% ZFO–ZIS under visible light irradiation was performed and shown in Fig. 9. 90 wt% of 4-NP at the 1st cycle and 78 wt% of 4-NP at the 3rd cycle was photodegraded, respectively. This indicates that the 2.5 wt% ZFO–ZIS is stable under visible light irradiation and can be promoted for practical applications.

Fig. 9 Cycle degradation of 4-NP by 2.5 wt% ZFO–ZIS under visible light irradiation.

In addition, the as-prepared ZFO–ZIS photocatalyst can also be used for the decolorization of wastewater containing organic dye. Fig. S4† shows the treatment results of mixing wastewater containing 20 mg L⁻¹ methyl orange and 20 mg L⁻¹ Congo red using the 2.5 wt% ZFO–ZIS. The degradation mechanism of methyl orange and Congo red are the same; both methyl orange and Congo red in water decrease with the increase of irradiation time. When the time was up to 240 min, the decolorization rates of methyl orange and Congo red were 98% and 97.9%, respectively. The inset of Fig. S4† displays the absorption spectra of the aqueous solution containing methyl orange and Congo red; the absorption peaks at 464 nm and 494 nm decreased gradually with increasing irradiation time and almost entirely disappeared after 4 h irradiation. In addition, no other new absorption peaks appeared in the range from 250 nm to 700 nm, indicating the complete catalytic decolorization of the methyl orange and Congo red in aqueous solution. The photograph inset of Fig. S4† shows the color change during irradiation; we can clearly observe that as time goes on, the color changes from orange red to orange, and then to colorless.

4. Conclusion

In summary, ZnFe₂O₄-doped ZnIn₂S₄ heterostructures with a series of different proportions of ZnFe₂O₄ were successfully synthesized by a two-step solvothermal method. The as-prepared ZnFe₂O₄–ZnIn₂S₄ presented flower-like microspheres when the mass percentage of ZnFe₂O₄ was up to 10%. When the loading of ZnFe₂O₄ was increased to 30 wt% or more, the structure of spheres disappeared. As used for the degradation of 4-NP in aqueous solution and the decolorization of dye wastewater, the ZnFe₂O₄–ZnIn₂S₄ exhibited enhanced photocatalytic performance compared to bare ZnIn₂S₄ under visible light irradiation, and the 2.5 wt% ZnFe₂O₄–ZnIn₂S₄ showed the highest activity. The main factor for enhanced photocatalytic activity can be attributed the improved separation of photogenerated carriers by the stable heterojunction interface between ZnFe₂O₄ and ZnIn₂S₄. In addition, the increased surface-active sites and extended light absorption of ZnIn₂S₄ by the doping of ZnFe₂O₄ nanoparticles also play a certain role for the enhancement of photocatalytic activity.

Acknowledgements

We gratefully appreciate the support by NFSC (Grant No. 51402146, 51238002 and 51368045), National Science Funds for Excellent Young Scholars (Grant No. 51422807), the National Natural Science Fund (Grant No. 20142BAB213018) of Jiangxi province, the Science and Technology Planning Project (Grant No. 20151BBG70019) of Jiangxi province, and the Graduate Student Innovation Special Fund (Grant No. YC2015016) of Nanchang Hangkong University. In addition, Dr Chen appreciates the State Scholarship Fund from China Scholarship Council (Grant No. 201508360071), and Prof. Guo appreciates the support of the GanPo 555 Talent Project funded by Jiangxi Province gratefully.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14847f

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