Novel β-In_2.77S₄ nanosheet-assembled hierarchical microspheres: synthesis and high performance for photocatalytic reduction of Cr(VI)

Abstract

A novel nanosheet-assembled hierarchical sulfur-deficient β-In_2.77S₄ microsphere photocatalyst was prepared via a facile hydrothermal method. The phase evolution and possible formation mechanism of the nanosheet-assembled hierarchical β-In_2.77S₄ microspheres have been explored by virtue of temperature- and time-dependent experiments. The results demonstrated that the β-In_2.77S₄ microspheres were composed of many thin nanosheets, and underwent the Oswald ripening and self-assembly processes. This unique hierarchical structure showed faster charge separation efficiency of photogenerated electron–hole pairs. It was found that the product synthesized under suitable hydrothermal conditions showed a large specific surface area (157.86 m² g⁻¹) and high visible light photocatalytic activity in the reduction of aqueous Cr(VI). The highest activity is obtained for β-In_2.77S₄ (140 °C, 24 h), over which more than 99% of Cr(VI) was reduced after 50 min. The results suggested that nanosheet-assembled hierarchical β-In_2.77S₄ microspheres have potential application in the efficient utilization of solar energy for the treatment of Cr(VI)-containing wastewater.

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1. Introduction

Hexavalent chromium (Cr(VI)) is an awkward pollutant in the effluents from chromate-related industries, such as ceramics, mining, cosmetics, wood preservation, chromate production, leather tanning and pigments. Cr(VI) is only weakly adsorbed onto inorganic substrates. It has high toxicity, high solubility and mobility in water, and can do great harm to the environment and human health.^1–4 Therefore, it is necessary to explore effective, economical, and environmentally friendly methods for the treatment of wastewater contaminated with Cr(VI).^5–8 Conventional methods for the treatment of Cr(VI) pollution including chemical reduction, ion-exchange, sorption, and bacterial reduction require either high energy or large quantities of chemicals, and are not used widely.^9–12 In contrast, semiconductor-based photocatalytic reduction of Cr(VI) has exceptional advantages, such as (i) low cost, (ii) direct use of clean and safe solar energy, (iii) no use and no release of other undesirable chemicals, and (iv) reusability.^13–15 Therefore, semiconductor-based photocatalytic reduction is widely recognized as a promising way for Cr(VI) wastewater treatment.

As a typical metal sulfide, indium sulfide has been widely explored for many different kinds of applications, such as photocatalysis,^16,17 cancer diagnoses,^18,19 color televisions,²⁰ lithium ion batteries,²¹ solar cells^22,23 and so on. Generally, indium sulfide has two kinds of composition forms, InS and In₂S₃, with band gaps of 2.44 eV and 2.0–2.2 eV, respectively.²⁴ Between these two compounds, most of the research works have been focused on In₂S₃. In₂S₃ with various morphologies (nanowires,²⁵ nanorods,²⁶ nanosheets²⁷ and hollow nanosphere,²⁸ etc.) and its composites have been synthesized and applied to produce hydrogen and photocatalytic degradation dyes. For example, Qiu et al.²⁹ has synthesized In₂S₃ by biomolecule-assisted hydrothermal method and the In₂S₃ shown enhanced photocatalytic performances for degradation of methyl orange. Fu et al.³⁰ has prepared tetragonal and cubic β-In₂S₃ by hydrothermal method and the as-prepared β-In₂S₃ photocatalysts exhibited excellent photocatalytic activity for H₂ production. Chen et al.³¹ has synthesized Bi₂S₃/In₂S₃ composite with efficient photocatalytic degradation of 2,4-dichlorophenol under visible light irradiation. In our group, Xing et al.³² prepared In₂S₃/g-C₃N₄ heterojunctions with high photocatalytic activity in the degradation of RhB. However, the study on the photocatalytic reduction of Cr(VI) was still limited, and therefore, exploration of indium sulfide in photocatalytic remove of Cr(VI) is highly desirable.

Herein, we reported the synthesis of novel nanosheet-assembled hierarchical sulfur deficient β-In_2.77S₄ microsphere photocatalyst via a facile hydrothermal method. The photocatalytic activity of the β-In_2.77S₄ samples were evaluated by the reduction of Cr(VI) under visible light irradiation. The as-prepared β-In_2.77S₄ hierarchical microsphere exhibited high photocatalytic activity for Cr(VI) reduction. In addition, the possible mechanism of the photocatalytic reduction reaction was proposed by introducing scavengers into the photocatalytic reaction system.

2. Experimental section

2.1. Materials

Indium nitrate (In(NO₃)₃·4.5H₂O), L-cysteine (C₃H₇NO₂S), and potassium persulfate (K₂S₂O₈). Absolute ethanol was purchased from Sinopharm Chemical Reagent Co., Ltd, China and was used as received without any further purification. Deionized water was used in the experiments.

2.2. Preparation of the β-In_2.77S₄

The β-In_2.77S₄ hierarchical spheres were prepared by a facile hydrothermal process. In a typical procedure, In(NO₃)₃·4.5H₂O (0.3820 g) was dissolved in deionized water (80 mL) under magnetic stirring, then L-cysteine (0.4838 g) was added into the above solution and stirring for 1 h. The mixed solution was transferred into a 100 mL Teflon-lined stainless steel autoclave, sealed and maintained at 140 °C for 24 h. Meanwhile, the samples were also synthesized at other two different temperatures (120 °C and 160 °C). The samples synthesized at different temperature were denoted as β-In_2.77S₄-120, β-In_2.77S₄-140, β-In_2.77S₄-160, respectively. The as-fabricated products were cooled down to room temperature and separated by centrifugation, washed with deionized water and absolute ethanol three times and dried under air at 60 °C for 12 h before further characterizations.

2.3. Characterization

The phase purity and crystal structure of the obtained samples were examined by X-ray diffraction (XRD, Bruker D8 Advance X-ray diffractometer) with Cu-Kα radiation (λ = 1.5406 Å). Scanning electron micrograph (SEM) images were obtained using a field emission SEM (FESEM) instrument (Hitachi S-4800 II, Japan). Transmission electron microscopy (TEM) was done using a JEOL-JEM-2010 (JEOL, Japan) operating at 200 kV. Surface analysis of the sample was examined by X-ray photoelectron spectroscopy (XPS) using a ESCA PHI500 spectrometer. The surface areas of the samples were measured by a TriStar II 3020-BET/BJH Surface Area. UV-vis diffuse reflectance spectra (DRS) were performed on a Shimadzu UV-2401 spectrophotometer equipped with spherical diffuse reflectance accessory. The Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution of the samples were characterized by nitrogen adsorption with a TristarII3020 Instrument. The photoluminescence (PL) spectra of the photocatalyst were obtained by a Varian Cary Eclipse spectrometer with an excitation wavelength of 426 nm.

2.4. Photocatalytic reduction of Cr(VI)

The photocatalytic activity of the as-prepared β-In_2.77S₄ for the degradation of Cr(VI) aqueous solution (50 mg L⁻¹, which was K₂Cr₂O₇ solution) in a quartz vial. In a typical measurement, the mixed suspension was exposed to visible light irradiation produced by a 300 W xenon lamp with a 420 nm cut-off filter (λ > 400 nm). All experiments were conducted at room temperature in air. 50 mg of the photocatalyst was added into 50 mL of 50 mg L⁻¹ Cr(VI) aqueous solution, then the above suspension was stirred in dark for 30 min to ensure the establishment of adsorption–desorption equilibrium between the sample and the reactant before being exposed to visible light illumination. 5 mL of sample solution was collected at a certain time and centrifuged (8000 rpm, 10 min) to remove the photocatalyst. Then the catalyst-free solution was detected using a UV-vis spectrophotometer. And the recyclability of the β-In_2.77S₄ photocatalyst in this case was also studied after it was washed with ethanol and deionized water for four times. The recycled photocatalytic activity experiment on the used catalyst was done as follows. Typically, after the reaction of the first run under visible light irradiation the photocatalyst was separated by centrifugation. The recycled catalyst was washed with deionized water and ethanol carefully. The fresh liquid Cr(VI) aqueous solution, was mixed with this used catalyst and was subjected to the second run photocatalytic activity testing. By analogy, the third and fourth run tests were also performed. In addition, controlled photoactivity experiments using K₂S₂O₈ as a scavenger for electrons were carried out similarly to the above photocatalytic reduction of Cr(VI) except that the radical scavengers were added to the reaction system.

2.5. Photoelectrochemical measurements

To investigate the photoelectrochemical properties of as-prepared samples, the modified electrodes were prepared as follows: 4 mg of the as-prepared photocatalyst was suspended in 1 mL ethanol and 20 μL 5 wt% Nafion solution to produce a slurry. Then, 30 μL of the resulting colloidal dispersion then dropped onto a piece of FTO slice with a fixed area of 1 cm². All the photoelectrochemical measurements were measured on an electrochemical analyzer in a standard three-electrode system using the prepared samples as the working electrode, a Pt wire as the counter electrode, and Ag/AgCl as a reference electrode. A 500 W Xe are lamp served as a light source. The photocurrent and electrochemical impedance spectroscopy (EIS) was performed in 0.2 M Na₂SO₄ aqueous solution.

3. Results and discussion

XRD was used to identify the phase and composition of the as-prepared samples. Fig. 1a shows the XRD patterns of the β-In_2.77S₄ hierarchical microspheres synthesized at different hydrothermal reaction temperatures. For the β-In_2.77S₄-140 and β-In_2.77S₄-160 samples, the distinct diffraction peaks at 2θ = 27.5°, 33.3°, 43.8°, 47.9°, 56.8° and 59.6° correspond to the (311) (400) (511) (440) (622) and (444) planes of the cubic structure of β-In_2.77S₄ (JCPDS card no. 88-2495), respectively. There are no other impurity peaks observed, which indicates that the pure phase of β-In_2.77S₄ was obtained using the current hydrothermal method. In contrast, the diffraction peak intensities of the β-In_2.77S₄-120 were relatively weak because of the imperfect crystallization.

Fig. 1 (a) XRD pattern of the β-In_2.77S₄ samples prepared at different reaction temperatures, (b) survey XPS spectra of β-In_2.77S₄-140 with high resolution spectra for (c) In 3d and (d) S 2p, respectively.

X-ray photoelectron spectroscopy (XPS) was employed to further study the surface elemental of β-In_2.77S₄-140 sample. The typical full survey scans spectrum indicates that the presence of S, In, C and O. C 1s peaks of the β-In_2.77S₄ are mainly ascribed to the hydrocarbon from the XPS instrument itself (Fig. 1b). The oxygen peak at a binding energy of 532.6 eV is due to the presence of H₂O absorbed on the sample surface. The binding energies obtained in XPS analysis were corrected for specimen charging by referencing C 1s to 284.6 eV. As shown in Fig. 1c, two strong peaks located at approximately 444.6 and 452.6 eV were assigned to In 3d_5/2 and In 3d_3/2, respectively.³³ For the spectrum of S 2p shown in Fig. 1d, the peaks at 161.6 and 162.2 eV can be attributed to S 2p_3/2 and S 2p_1/2, respectively.³⁰ The above XPS results confirm that the as-obtained product is only composed of In and S element.

The morphology and microstructure of β-In_2.77S₄-120, 140 and 160 samples were firstly investigated by SEM. As shown in Fig. 2a, the β-In_2.77S₄-120 microsphere with sizes of about 2 μm was formed. The magnified image of the sample (Fig. 2b) indicates that the surface of sphere was partially smooth. As the reaction temperature increases to 140 °C (Fig. 2c), the SEM image clearly demonstrates that the β-In_2.77S₄ microspheres had a unique marigold-like spherical superstructure. It was observed that the microsphere was composed of numerous nanosheets (Fig. 2d). When the reaction temperature increases to 160 °C (Fig. 2e and f), the flower-like microsphere can be found as prepared at 140 °C, the surface of β-In_2.77S₄-160 microsphere was also composed of a large number of nanosheets. The comparative experiments indicate that the appropriate temperature is important for the preparation of nanosheet-assembled hierarchical β-In_2.77S₄ microspheres.

Fig. 2 The low-magnification SEM and the corresponding high magnification images of β-In_2.77S₄ prepared at different reaction temperature for 24 h: (a, b) 120 °C, (c, d) 140 °C and (e, f) 160 °C.

The microstructure and morphology of β-In_2.77S₄-140 microspheres was further investigated by TEM. From the low-magnification TEM image (Fig. 3a and b) it can be found that the as-obtained hierarchical microsphere surface was composed of nanosheets, which is consistent with the SEM results. HRTEM image of the β-In_2.77S₄ sample was presented in Fig. 3c. The clear lattice fringe with d = 0.267 nm matches the (400) planes of cubic phase β-In_2.77S₄. The above results manifest that the uniform nanosheet-assembled β-In_2.77S₄ microspheres have been successfully prepared by the one-pot hydrothermal route developed here.

Fig. 3 (a, b) TEM and (c) HRTEM images of β-In_2.77S₄-140.

In order to understand the evolution process of the nanosheet-assembled β-In_2.77S₄ microspheres, time-dependent experiments were carried out, during which the samples were collected at different times. Their structural information and phase composition were subjected to the XRD investigation. Fig. 4 shows the XRD patterns of the β-In_2.77S₄ samples collected at 2 h, 8 h, 12 h, 16 h, 24 h, respectively. When the reaction time was 2 h, main diffraction peaks ascribed to the β-In_2.77S₄ almost do not appear (Fig. 4a). It is obvious to find that the main diffraction peaks gradually strengthen with time prolonging from 8 to 16 h (Fig. 4b–d). With further increases the reaction time to 24 h, as illustrated in Fig. 4e, all the characteristic XRD peaks for β-In_2.77S₄ can be easily detected.

Fig. 4 XRD patterns of the β-In_2.77S₄-140 obtained at (a) 2 h, (b) 8 h, (c) 12 h, (d) 16 h and (e) 24 h.

Fig. 5 shows the SEM images of the products obtained after 2, 8, 12 and 16 h, respectively, which reveals that the morphologies of these intermediates. Several obvious evolution stages can be observed. In the first stage (2 h), the microsphere was not fully formed (Fig. 5a and b). In the second stage (8 h), the microsphere was formed with small size (Fig. 5c). The corresponding high magnification image of the sample (Fig. 5d) indicated that the surface of microsphere was relatively smooth. As the reaction proceeds (12 h), the microsphere size was increased and the magnified image showed that the flakes began to form on the surface of microsphere (Fig. 5e and f). Notably, with the extension of the reaction time (16 h), the β-In_2.77S₄ microsphere grows larger (Fig. 5g). The magnification SEM indicated that the thin nanosheets were not fully formed (Fig. 5h). Upon prolonging reaction time to 24 h (Fig. 2c), the product was composed of flakes-built nanostructures, which reveals that extending the reaction time to 24 h is favorable for the formation of uniform nanosheet-assembled hierarchical microsphere nanostructure.

Fig. 5 SEM and the corresponding high magnification images of the β-In_2.77S₄-140 obtained at (a, b) 2 h, (c, d) 8 h, (e, f) 12 h, (g, h) 16 h.

Based on the above experimental results, we proposed a possible formation mechanism for the β-In_2.77S₄ microspheres, as shown in Fig. 6. First, β-In_2.77S₄ nuclei are formed through the reaction among In(NO₃)₃·4.5H₂O and L-cysteine (C₃H₇NO₂S). Then, these newly produced nuclei self-assemble in situ, and driven by the anisotropy growth tendency of hexagonal crystal structure. The β-In_2.77S₄ rudiment underwent the process of Oswald ripening, and further generated nanosheet-assembled microspheres as a result of an intrinsic lamellar structure of the cubic β-In_2.77S₄ phase under certain hydrothermal conditions.

Fig. 6 Schematic illustration of the possible formation mechanism for the β-In_2.77S₄ microspheres.

Fig. 7a presents the UV-vis diffuse reflectance spectra (DRS) of β-In_2.77S₄ samples prepared at different reaction temperatures. On the basis of the DRS absorbance, the samples absorption edge is about from 620 to 640 nm. The absorption edges of the samples shift to longer wavelengths as the temperature increases, which can be clearly reflected by the color change of the as-prepared sample powder (see the digital pictures in Fig. 7a, inset). The energy level and band gap of the semiconductors play a crucial role in determining their physical properties. The band gap energy of a semiconductor can be calculated by the following equation:

αhν = A(hν − E_g)^n/2 (1)

where α, ν, E_g, and A are the absorption coefficient, light frequency, band gap, and a constant, respectively. n is determined from the type of optical transition of a semiconductor (n = 1 for direct transition and n = 4 for indirect transition). By fitting the experimental data using the above equation, the value of n for β-In_2.77S₄ was estimated to be 1, which indicates direct transitions in the β-In_2.77S₄.³² Therefore, the band gap energy (E_g value) of the resulting samples can be estimated from a plot of (αhν)² versus photon energy (hν). The intercept of the tangent to the X axis would give a good approximation of the band gap energy of the samples. As shown in Fig. 7b, the estimated band gaps (E_g) of the β-In_2.77S₄ are in the range of 2.07 to 2.17 eV, and the slight shift in the absorption edge may be due to the variation of particle size and morphology.

Fig. 7 (a) UV-vis diffuse reflectance spectra of the samples collected at different reaction temperature; the inset shows the photographs of the samples. (b) Plot of (αhν)² versus energy (hν) for the band gap energy of the samples collected at different reaction temperature.

The photocatalytic activity of the as-prepared β-In_2.77S₄ nanophotocatalysts were investigated by visible light reduction of Cr(VI) in aqueous solution at room temperature (Fig. 8). The time-dependent absorption spectra of Cr(VI) solutions under visible light illumination in the presence of β-In_2.77S₄, which prepared at different reaction temperature are first shown in Fig. 8a–c. The β-In_2.77S₄-120 and β-In_2.77S₄-160 samples could reduce Cr(VI) reaching 78.1% and 82.6% after 50 min under visible light irradiation, while approximately 99% reduction of Cr(VI) is observed for the β-In_2.77S₄-140 sample (Fig. 8a–c). Fig. 8d shows the photocatalytic reduction of Cr(VI) (pH = 4) as a function of irradiation time over different photocatalysts. Prior to illumination, the suspension was magnetically stirred for 30 min in the dark to achieve the adsorption equilibrium of the Cr(VI) on the photocatalyst powders. The photocatalytic reduction of Cr(VI) in the absence of photocatalyst was almost negligible under the same condition. The best visible light photocatalytic activity was achieved at β-In_2.77S₄-140. Compared with other reported photocatalysts for the reduction of Cr(VI), our β-In_2.77S₄-140 microspheres shown high photocatalytic performance. Zhang et al.⁹ has synthesized SnS₂ by hydrothermal method and more than 85% of Cr(VI) was reduced after 50 min. Ku et al.³⁴ has prepared TiO₂ shown photocatalytic reduction 68% of Cr(VI) (pH = 2 ± 0.1) after 300 min. However, β-In_2.77S₄ shown enhanced photocatalytic performances for reduction of Cr(VI), Which more than 99% of Cr(VI) was reduced after 50 min.

Fig. 8 UV-vis absorption spectral changes of Cr(VI) aqueous solution in the presence of (a) β-In_2.77S₄-120, (b) β-In_2.77S₄-160, (c) β-In_2.77S₄-140; (d) photocatalytic reduction of Cr(VI) (pH = 4) over different samples under visible light irradiation.

To further understand the reaction kinetics of Cr(VI) reduction, the apparent pseudo-first-order model expressed by eqn (2) was applied in the experiments:

ln(C₀/C) = k_appt (2)

where C is the concentration of the Cr(VI) at time t, C₀ is the initial concentration of the Cr(VI) solution, and the slope k is the apparent reaction rate constant. In Fig. 9, the results imply that the highest k_app for β-In_2.77S₄-140 was gained. It can be found that the k_app of β-In_2.77S₄-140 (0.0675 min⁻¹) is about 2.6 times that of β-In_2.77S₄-120 (0.0256 min⁻¹) and about 2.1 times that of β-In_2.77S₄-160 (0.0321 min⁻¹).

Fig. 9 ln(C₀/C) versus time curves of Cr(VI) reduction over β-In_2.77S₄-120, 140 and 160 samples. The inset is the comparison of reaction rate constant, k, obtained from linear fitting.

The different photocatalytic activity of the samples can be influenced by many factors, such as the crystallinity, band gap, surface area, lifetime of the photocarriers and grain boundary effect, etc.³⁵ It is well-known that photocatalysts with higher specific surface areas and bigger pore volumes are beneficial for the enhancement of photocatalytic performance due to there being more surface active sites for the adsorption of the reactant molecules, ease of transportation of the reactant molecules and products through the interconnected porous networks, and enhanced harvesting of light.³⁶ Nitrogen (N₂) adsorption–desorption isotherms and BJH pore size distribution curves (inset), as presented in Fig. 10, reveal the BET surface area and porosity of β-In_2.77S₄ materials. It is observed that the BET specific surface area of β-In_2.77S₄-140 (157.86 m² g⁻¹) is the highest among those samples. The surface area for the β-In_2.77S₄-120 and β-In_2.77S₄-160 is 62.13 and 132.68 m² g⁻¹, respectively. Meanwhile, the inset reflects that the samples are mesoporous structure (about 2–5 nm), β-In_2.77S₄-140 has bigger pore volumes. It is easy to find that the β-In_2.77S₄-140 exhibits stronger reduction ability toward Cr(VI) with a higher surface area, which suggests that photocatalysts with higher specific surface areas and bigger pore volumes are beneficial for the enhance photocatalytic activity toward Cr(VI) reduction.

Fig. 10 Nitrogen sorption isotherms of the β-In_2.77S₄-120, 140 and 160; the inset is the corresponding pore diameter distribution.

As well known that photoluminescence (PL) spectrum is widely used to investigate the migration, transfer and recombination of electron–hole pairs.³⁷ In order to investigate the photocatalytic mechanism, the photoluminescence (PL) spectrum and transient photocurrent analysis were employed. Fig. 11a shows that at an excitation wavelength of 426 nm. The main emission peak centers at about 430 nm for the β-In_2.77S₄, which can be ascribed to the band gap recombination of electron–hole pairs. The β-In_2.77S₄-140 displayed the lowest PL intensity, which means that β-In_2.77S₄-140 has a higher separation efficiency of photogenerated electron–hole pairs.

Fig. 11 (a) Photoluminescence (PL) spectra of the β-In_2.77S₄-120, 140 and 160; (b) transient photocurrent responses of the β-In_2.77S₄-120, 140 and 160.

To further improved carrier separation efficiency of the β-In_2.77S₄ samples, the photocurrent–time measurement was performed. Fig. 11b shows a comparison of the photocurrent–time (I–t) curves for these samples with typical on–off cycles of intermittent visible light irradiation. The photocurrent intensity remained at a relatively high constant value when the light was on, and rapidly decreased to zero as soon as the light was turned off. Notably, the photocurrent value of β-In_2.77S₄-140 is several times higher than β-In_2.77S₄-120 and β-In_2.77S₄-160. This observation indicated that the as-prepared β-In_2.77S₄-140 material allows for more efficient separation of photogenerated electron–hole pairs and the photoinduced carrier recombination will be reduced. As a result, the improved transfer efficiency of charge carriers could lead to the enhanced photocatalytic activity of β-In_2.77S₄-140 microsphere.

To elucidate the charge separation process of the β-In_2.77S₄ microsphere, the band positions of β-In_2.77S₄ could be calculated using following equations:

E_CB = χ − E_e − 0.5E_g (3)

where E_VB and E_CB are the valence and conduction band edge potentials, respectively, χ is the absolute electronegativity of the semiconductor, E_e is defined as the energy of free electrons on the hydrogen scale and E_VB can be determined by E_VB = E_g + E_CB. Based on the band gap positions, the CB and VB edge potentials of β-In_2.77S₄ are determined to be −0.86 and 1.26 eV, respectively.

Taking K₂S₂O₈ as scavenger for photogenerated electrons,^10,38 the control experiments for the photoreduction of Cr(VI) under visible light illumination have been performed. As shown in Fig. 12, the adding of K₂S₂O₈ scavenger for electrons inhibit the photocatalytic reduction of Cr(VI). The results evidently demonstrate that the photogenerated electron in the hierarchical β-In_2.77S₄ microspheres indeed plays a motivational role to the photocatalytic reduction of Cr(VI). On the basis of the related literature work and the experimental results described above,^39,40 the corresponding photocatalytic reduction of Cr(VI) reactions may be expressed by the following equations:

β-In_2.77S₄ + hv → β-In_2.77S₄ (h⁺ + e⁻) (4)

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (5)

2H₂O + 4h⁺ → O₂ + 4H⁺ (6)

Fig. 12 Controlled experiments for photoreduction of Cr(VI) over the samples of β-In_2.77S₄ using K₂S₂O₈ as scavenger for photogenerated electrons under visible light irradiation.

Based on the above experiments, a tentative photocatalytic reaction mechanism for reduction of Cr(VI) over the β-In_2.77S₄ can be schematically proposed in Scheme 1. Upon the visible light irradiation, the electrons in the valence band (VB) of β-In_2.77S₄ are photoexcited to the conduction band (CB), leaving holes in the valence band. Due to the faster photogenerated electrons transfer to the surface of β-In_2.77S₄-140, which hampers the recombination of electrons and holes efficiently. As a result, the improved lifetime and transfer of photogenerated electrons significantly contribute to the remarkably enhanced photoactivity of β-In_2.77S₄-140 toward reduction of Cr(VI) to Cr(III).

Scheme 1 Possible schematic mechanism for photocatalytic reduction of Cr(VI) over β-In_2.77S₄ samples under visible light irradiation.

The stability and recyclability of the photocatalysts are significant factors in their practical application. As shown in Fig. 13a, the recycling experiment for Cr(VI) reduction was conducted with β-In_2.77S₄-140 photocatalyst. The photocatalyst exhibits a slight loss of activity after four cycles, suggesting that the β-In_2.77S₄-140 material is a stable photocatalyst for the reduction of Cr(VI). The XRD pattern of the β-In_2.77S₄-140 sample was also investigated after four recycling runs for reduction of Cr(VI) (shown in Fig. 13b). It was found that the catalyst exhibits no obvious change compared with the catalyst before the photocatalytic reaction. As a result, it can be concluded that the β-In_2.77S₄-140 photocatalyst is stable and can be easily recycled for their practical application.

Fig. 13 (a) Recycling photocatalytic reduction of Cr(VI) over the sample of β-In_2.77S₄-140 under visible light irradiation; (b) XRD patterns of β-In_2.77S₄-140 before reduction and after reduction for the four cycles.

Fig. 14 shows the XPS spectra of β-In_2.77S₄ recovered before and after use in the photocatalytic reduction of Cr(VI). The survey XPS spectrum of β-In_2.77S₄ revealed the presence of C, O, S, In and Cr elements. The In 3d XPS spectrum displayed two peaks at 444.6 and 452.6 eV (Fig. 14b), the S 2p XPS spectrum showed two peaks at 161.6 and 162.2 eV (Fig. 14c), respectively. The binding energies of In 3d and S 2p of β-In_2.77S₄ (Fig. 14b and c) were almost the same as those of fresh β-In_2.77S₄ (Fig. 1), suggesting the chemical states of the In and S components of β-In_2.77S₄ remained unchanged after the four cycle photocatalytic test. While the Cr 2p peaks of β-In_2.77S₄ were observed at the binding energies of 577.1 and 588.1 eV, respectively (Fig. 14d), which can be assigned to Cr(III).^41,42 The result suggested that Cr(VI) was reduced during photoreaction and the resultant Cr(III) adsorbed on the surface of catalyst.

Fig. 14 XPS spectra of the used β-In_2.77S₄ before and after washing with ethanol and deionized water: (a) survey spectrum, (b) In 3d, (c) S 2p and (d) Cr 2p.

4. Conclusions

In summary, the β-In_2.77S₄ nanosheet-assembled microspheres materials have been synthesized via a facile hydrothermal process. It was demonstrated that this hierarchical nanosheet microspheres structure with high specific surface areas could effectively improve the photoreduction efficiency of Cr(VI) by promoting the charge separation and inhibiting the recombination of electron–hole pairs. In addition, we have also used the radical scavengers technique to study the role of photoactive species involved in the photocatalytic reduction of Cr(VI). This work could promote further interest in adopting such strategy to synthesize hierarchical nanosheet microsphere materials with controlled architectural morphology and enhanced photocatalytic performance toward photocatalytic applications.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the financial supports of National Nature Science Foundation of China (No. 21406091 and 21576121), Natural Science Foundation of Jiangsu Province (BK20140530), College Natural Science Research Program of Jiangsu Province (13KJB610003), and College Students Practice Innovation Training Program of Jiangsu Province (201510299023Z).

References

1. Y. C. Zhang, M. Yang, G. Zhang and D. D. Dionysiou, Appl. Catal., B, 2013, 142, 249–258.
2. M. I. Litter, Adv. Chem. Eng., 2009, 36, 37–67.
3. Q. Cheng, C. Wang, K. Doudrick and C. K. Chan, Appl. Catal., B, 2015, 176, 740–748.
4. P. Mohapatra, S. K. Samantaray and K. Parida, J. Photochem. Photobiol., A, 2005, 170, 189–194.
5. Y. Zhao, Y. Zhang, J. Li and Y. Chen, Sep. Purif. Technol., 2014, 129, 90–95.
6. J. M. Meichtry, C. Colbeau-Justin, G. Custo and M. I. Litter, Catal. Today, 2014, 224, 236–243.
7. J. M. Meichtry, C. Colbeau-Justin, G. Custo and M. I. Litter, Appl. Catal., B, 2014, 144, 189–195.
8. Y. C. Zhang, L. Yao, G. Zhang, D. D. Dionysiou, J. Li and X. Du, Appl. Catal., B, 2014, 144, 730–738.
9. Y. C. Zhang, J. Li, M. Zhang and D. D. Dionysiou, Environ. Sci. Technol., 2011, 45, 9324–9331.
10. B. Weng, S. Liu, N. Zhang, Z. R Tang and Y. J. Xu, J. Catal., 2014, 309, 146–155.
11. X. Liu, L. Pan, T. Lv, G. Zhu, Z. Sun and C. Sun, Chem. Commun., 2011, 47, 11984–11986.
12. B. Weng, X. Zhang, N. Zhang, Z. R. Tang and Y. J. Xu, Langmuir, 2015, 31, 4314–4322.
13. Y. C. Zhang, J. Li and H. Y. Xu, Appl. Catal., B, 2012, 123, 18–26.
14. Y. C. Zhang, Q. Zhang, Q. C. Shi, Z. Y. Cai and Z. J. Yang, Sep. Purif. Technol., 2015, 142, 251–257.
15. H. T. Wei, Q. Zhang, Y. C. Zhang, Z. J. Yang, A. Zhu and D. D. Dionysiou, Appl. Catal., A, 2015 DOI:10.1016/j.apcata.2015.11.005.
16. W. Du, J. Zhu, S. Li and X. Qian, Cryst. Growth Des., 2008, 8, 2130–2136.
17. Y. He, D. Li, G. Xiao, W. Chen, Y. Chen, M. Sun, H. Huang and X. Fu, J. Phys. Chem. C, 2009, 113, 5254–5262.
18. Y. H. Kim, J. H. Lee, D. W. Shin, S. M. Park, J. S. Moon, J. G. Namc and J. B. Yoo, Chem. Commun., 2010, 46, 2292–2294.
19. D. K. Nagesha, X. Liang, A. A. Mamedov, G. Gainer, M. A. Eastman, M. Giersig, J. J. Song, T. Ni and N. A. Kotov, J. Phys. Chem. B, 2001, 105, 7490–7498.
20. W. Chen, J. O. Bovin, A. G. Joly, S. Wang, F. Su and G. Li, J. Phys. Chem. B, 2004, 108, 11927–11934.
21. G. Li and H. Liu, J. Mater. Chem., 2011, 21, 18398–18402.
22. M. R. Menon, M. V. Maheshkumar, K. Sreekumar, C. Kartha and K. P. Vijayakumar, Sol. Energy Mater. Sol. Cells, 2010, 94, 2212–2217.
23. J. Rousset, F. Donsanti, P. Genevee, G. Renou and D. Lincot, Sol. Energy Mater. Sol. Cells, 2011, 95, 1544–1549.
24. A. Timoumi, H. Bouzouita, M. Kanzari and B. Rezig, Thin Solid Films, 2005, 480, 124–128.
25. A. Datta, G. Sinha, S. K. Panda and A. Patra, Cryst. Growth Des., 2008, 9, 427–431.
26. M. E. Norako, M. A. Franzman and R. L. Brutchey, Chem. Mater., 2009, 21, 4299–4304.
27. W. Zhang, D. K. Ma, Z. Huang, Q. Tang, Q. Xie and Y. T. Qian, J. Nanosci. Nanotechnol., 2005, 5, 776–780.
28. Y. Liu, M. Zhang, Y. Gaoa, R. Zhang and Y. Qian, Mater. Chem. Phys., 2007, 101, 362–366.
29. W. Qiu, M. Xu, X. Yang, F. Chen, Y. Nan, J. Zhang and H. Chen, J. Mater. Chem., 2011, 21, 13327–13333.
30. X. Fu, X. Wang, Z. Chen, Z. Zhang, Z. Li, D. Y. Leung and X. Fu, Appl. Catal., B, 2010, 95, 393–399.
31. Y. Chen, G. Tian, Q. Guo, R. Li, T. Han and H. Fu, CrystEngComm, 2015, 17, 8720–8727.
32. C. S. Xing, Z. D Wu, D. L. Jiang and M. Chen, J. Colloid Interface Sci., 2014, 433, 9–15.
33. L. Y. Chen, Z. D. Zhang and W. Z. Wang, J. Phys. Chem. C, 2008, 112, 4117–4123.
34. Y. Ku and I. L. Jung, Water Res., 2001, 35, 135–142.
35. J. Yin, Z. Zou and J. Ye, J. Phys. Chem. B, 2004, 108, 8888–8893.
36. D. Chandra, K. Saito, T. Yui and M. Yagi, Angew. Chem., Int. Ed., 2013, 52, 12606–12609.
37. K. Lv, J. Hu, X. Li, M. Li and J. Mol, J. Mol. Catal. A: Chem., 2012, 356, 78–84.
38. Y. Zhang, Z. Chen, S. Liu and Y. J. Xu, Appl. Catal., B, 2013, 140–141, 598–607.
39. J. Yu, S. Zhuang, X. Xu, W. Zhu, B. Feng and J. Hu, J. Mater. Chem. A, 2015, 3, 1199–1207.
40. Q. Sun, H. Li, S. L. Zheng and Z. M. Sun, Appl. Surf. Sci., 2014, 311, 369–376.
41. X. Hu, H. Ji, F. Chang and Y. Luo, Catal. Today, 2014, 224, 34–40.
42. J. Wu, J. Wang, Y. Du, H. Li and Y. Yang, Appl. Catal., B, 2015, 174, 435–444.

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