Enhancement of the photocatalytic synchronous removal of Cr(VI) and RhB over RP-modified flower-like SnS₂

Abstract

Although photocatalysis is frequently employed to remove various pollutants in water, it still suffers from low efficiency due to the rapid recombination of photogenerated electrons and holes. In this study, a red phosphorus/tin disulfide (RP/SnS₂) composite photocatalyst is fabricated by loading nano-sized RP on flower-like SnS₂ films with a facile hydrothermal method. It is noteworthy that the 2D heterojunction formed between SnS₂ and RP provided channels for the rapid transfer of photon-generated carriers and their effective separation. Furthermore, the separated electrons can react with absorbed O₂ for the generation of superoxide radicals (˙O₂⁻), thereby impacting the photocatalytic degradation oxidation reaction. The application of photocatalytic synchronous removal of Cr(VI) and RhB over RP/SnS₂ was implemented first. Compared with pristine SnS₂, the photocatalytic degradation activity of Cr(VI) and RhB over the RP/SnS₂ composite was significantly enhanced and the kinetic rate constant reached 8.2, which is 10.8 times that of pristine SnS₂. Moreover, the hybrid photocatalysts exhibited prominent reusability and stability. Therefore, a photocatalytic degradation mechanism and pathway of carriers are proposed in the study. Furthermore, it is considered that the present study is a promising method in the treatment of wastewater by photocatalysis.

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1. Introduction

Currently, various semiconductors as photocatalysts, characterized by low cost, minimum efficiency, less stability and low toxicity are investigated extensively for water treatment.^1–5 Under sufficient light energy, electrons and holes are generated and transferred to undergo oxidation–reduction reactions on the surfaces of photocatalytic materials.⁶ Consequently, the energy band gap (E_g) and position of the conduction band (CB) and valence band (VB) critically impact this process. Given the diversity of contaminants in their surroundings and production, photocatalysts are specifically designed for multiple reactions.^7,8 Thus, the photocatalytic activity is dependent on the REDOX potential of semiconductors. Furthermore, rapid separation and migration capabilities result in photocatalytic performance with higher efficiency.^9–13 Accordingly, nano-sized semiconductors, especially the two-dimensional (2D) materials have been recognized as one of the most promising strategies to synthesize catalysts in a conducive environment exhibiting high efficiency.^14–17

Tin disulfide (SnS₂) is a metal sulfide photocatalyst with response to visible light and band gap of 2.0–2.25 eV.^18,19 It also exhibits good stability in various aqueous solutions, making it a promising photocatalyst for pollutant degradation.^20–26 The practical performance of SnS₂ materials is associated with crystallinity, morphology, size, etc. Intensive research has been performed to regulate the morphology of SnS₂, and various nanostructures were successfully built over the past few years.^27–29 In general, photocatalysts with large visible-light absorbance do not guarantee photocatalytic efficiency. The development of high performance SnS₂-based photocatalysts remains vital for the application of photocatalysis on large scale (Table S1†). Red phosphorus (RP) is an earth abundant element that exhibits high photocatalytic activity with narrow band gap in the range of 1.4–2.0 eV (ref. 30) and prevails over most of the traditional metal-based photocatalysts with its good visible light response.³¹ Fortunately, SnS₂ and RP have matched band potentials with both CB and VB potentials of RP being more negative than those of SnS₂, and thereby, thermodynamically allowing the photogenerated electron transfer from the CB of RP to the CB of SnS₂ under visible light irradiation. Moreover, the reverse transmission of holes occurs from the VB of SnS₂ to the VB of RP. Promoted by the photogenerated separation of electrons and holes in SnS₂, more efficient photocatalytic reaction can be achieved. It is noteworthy that the 2D/2D heterostructure between SnS₂ and RP shortens the transmission path, thereby accelerating carrier transportation and limiting the recombination process.

In this study, the synthesis of RP-modified flower-like SnS₂ for the synchronous removal of Cr(VI) and RhB is reported. The uniform dispersible 2D structure was built by a facile hydrothermal method, resulting in effective charge separation and significantly enhanced photocatalytic activity. Moreover, the quick e⁻ transfer significantly inhibited the electron–hole pair recombination and facilitated the release of photocatalytic active groups (˙O₂⁻). The photocatalytic activity of SnS₂ is significantly enhanced by the presence of RP. The K_app of the RP/SnS₂ composite for the removal of Cr(VI) and RhB is 8.2 and 10.8 times higher than the degradation kinetic constant of pure SnS₂, as well as 8.6 and 15.7 times higher than that of pure RP. Overall, the prominent photocatalytic efficiency and stability of the simultaneous removal of Cr(VI) and RhB with the RP/SnS₂ composite makes a significant step toward the application of photocatalysis in actual water treatment.

2. Materials and methods

2.1 The preparation of RP-modified flower-like SnS₂ hybrid

All chemicals were of analytical grade and employed without further purification. Moreover, the RP-modified flower-like SnS₂ was generated following the 3 steps below:

(1) The commercial RP was milled with water and sieved to remove the coarse part. Subsequently, the hydrothermal treatment was conducted at 200 °C for 12 h, the RP was washed, and centrifugal separation with deionized water was performed repeatedly to produce nanostructured RP for subsequent use.

(2) Then, 2 mmol SnCl₄·5H₂O and 5 mmol CH₃CSNH₂ (thioacetamide) were adequately dissolved in 60 mL of ethanol to generate a transparent solution. Different amounts of the nanostructured RP (i.e. 80 mg, 137 mg, 213 mg, 320 mg and 480 mg) were dispersed in the transparent solution, stirred for 30 min to mix well, then ultrasonic treatment was performed for 2 h.

(3) The suspension was transferred to a high-pressure reactor and heated at 180 °C for 12 h. After being cooled down to ambient temperature, the samples were harvested by centrifugation, washed with deionized water several times, and then dried at 60 °C with a drying oven. The obtained samples were respectively labeled as 20%-RP/SnS₂, 30%-RP/SnS₂, 40%-RP/SnS₂, 50%-RP/SnS₂ and 60%-RP/SnS₂. The specific preparation processes of the samples are illustrated in Scheme 1.

Scheme 1 Schematic illustration of the preparation of RP/SnS₂.

2.2 Characterization

The crystalline phases were characterized by a Shimadzu XRD-6000 powder diffractometer. Under a scanning electron microscope (SEM, Carl Zeiss SIGMA) and a transmission electron microscope (TEM, Tecnai G2 F20S-TWIN), the morphology and microstructure of photocatalysts were identified. X-ray photoelectron spectroscopy (XPS) was performed with a Kratos AXIS NOVA spectrometer. UV-vis diffuse reflectance spectra were collected on a Shimadzu UV-3600 UV/vis/NIR spectrophotometer. Photoluminescence (PL) spectra were obtained on a florescence spectrophotometer (Hitachi F-7000). With a three-electrode system (CHI-660E, Chenhua Instruments Co., Shanghai, China), the photoelectrochemical experiments were performed. A Pt wire and saturated calomel electrode (SCE) acted as counter electrode and reference electrode, respectively. The catalyst powder was deposited on a fluoride tin oxide (FTO) substrate to serve as the working electrode. A 0.5 M Na₂SO₄ aqueous solution acted as the electrolyte. A 300 W xenon lamp (MICROSOLAR300UV, Beijing Perfect light) equipped with a 420 nm cutoff filter (k > 420 nm) was employed as a visible light source.

2.3 Photocatalytic activity evaluation

The photocatalytic activity of the as-synthesized x-RP/SnS₂ (x = 20, 30, 40, 50 and 60) was examined by the simultaneous removal of Cr(VI) (K₂Cr₂O₇ as the source of Cr(VI)) and RhB. A Xe lamp (300 W) was used as the irradiation source. Approximately 35 mg of photocatalyst was dispersed in the 100 mL of solution containing Cr(VI) ions (40 ppm) and RhB (10 ppm). Prior to irradiation, the suspension was stirred in the dark for 30 min to reach the adsorption–desorption equilibrium, after which the process went on for 50 min under light irradiation. 3.5 mL of the solution was harvested and subsequently centrifuged at 10 000 rpm to separate the photocatalyst. The clear solution was used to determine the content of Cr(VI) and RhB by UV-vis spectroscopy, and the absorbance maximum (λ_max) of Cr(VI) and RhB was at 356 nm and 554 nm, respectively.

3. Results and discussion

3.1 Structural characterization and photocatalytic activity evaluation

Fig. 1 presents the typical X-ray diffraction (XRD) patterns of the as-fabricated samples. The pattern of RP features one distinct peak at 15.19°, which corresponds to the (102) plane of the amorphous RP. For the SnS₂ sample, all the peaks can be readily indexed to hexagonal SnS₂ (JCPDS 01-1010), in which peaks at 15.0°, 28.3°, 32.1°, 42.2°, 50.1° and 52.551° correspond to the (001), (100), (101), (102), (110) and (111) planes, respectively. No difference is identified in the diffraction peaks of SnS₂ before and after hydrothermal treatment with RP (Fig. 1(a)). However, the position of the sample peak at 15.0° shifts from the pure SnS₂ sample position to a lower degree, according to Fig. 1(b), which is probably caused by the formation of a heterojunction between RP and SnS₂ and the lattice deformation of SnS₂.³² Besides, the x-RP/SnS₂ structural transition can be attributed to the introduction of RP in the SnS₂ layered structure (Fig. 3).

Fig. 1 XRD patterns of (a) full spectra and (b) magnified spectra of samples RP, SnS₂, and x-RP/SnS₂.

To assess the photocatalytic activities, the as-prepared samples were applied for the simultaneous photocatalytic removal of Cr(VI) and RhB. Fig. 2 illustrates the removal rate–time relationship of the samples during 30 min of stirring in the dark and 50 min illumination, in which only 8.0% and 1.9% of Cr(VI) and RhB remained after 50 min over 50%-RP/SnS₂. However, pure SnS₂ and RP exhibited relatively weak capability toward the reduction of Cr(VI) and the decolorization of RhB; the residual amounts were nearly 50.2% and 18.5% for SnS₂, and 88% and 65.3% for RP, respectively. Fig. S1† presents the reduction of Cr(VI) (with UV absorption peak at 554 nm) and decolorization of RhB (with UV absorption peak at 354 nm) by 50%-RP/SnS₂, SnS₂ and RP. As expected, the modification of SnS₂ by RP improved the photocatalytic reduction of Cr(VI) and oxidation of RhB simultaneously. Besides, the reactions were identified to be in accordance with −ln(C/C_o) = K_appt (first-order kinetic equation), where K_app denotes the kinetic rate constant, C_o represents the original concentration of solution, t indicates the reaction time and C is the solution concentration at reaction time t. Fig. 2(c) suggests that the slope value of the fitting lines are 0.04494 min⁻¹ and 0.06692 min⁻¹, equated with the K_app of the 50%-RP/SnS₂ composite for the removal of Cr(VI) and RhB, exhibiting 8.2 and 10.8 times higher degradation kinetic constants than that of pure SnS₂ (as shown in Fig. S4(a)†) and 8.6 and 15.7 times higher than that of pure RP (as shown in Fig. S4(b)†). Moreover, by cycling the photocatalytic experiments, the performance stability and reusability of 50%-RP/SnS₂ photocatalysts were investigated. As revealed from the results of the recycling degradation experiments of Cr(VI) and RhB simultaneously in Fig. 2(d), there was a minor decrease after five cycles. Overall, 50%-RP/SnS₂ exhibited a relatively optimal and comprehensive performance. Thus, the analyses of the other samples are not presented in the following discussion.

Fig. 2 Comparison of the effect of photocatalytic simultaneous removal of (a) Cr(VI) and (b) RhB by RP, SnS₂, and x-RP/SnS₂, (c) the fitted reaction kinetic curves (−ln(C/C_o) = kt) and (d) recycling test runs for five times in the photocatalytic removal of Cr(VI) and RhB by 50%-RP/SnS₂.

The surface and interface characteristics of the composite significantly enhanced the photocatalytic efficiency. SEM and TEM images were examined for the interfacial state and morphological structure of RP-modified flower-like SnS₂. As revealed from Fig. 3(a) and (b), the prepared SnS₂ and RP exhibited nano-sized sheets with flower and fragment structures, respectively. The morphology of 50%-RP/SnS₂ presented in Fig. 3(c) indicated that the fragments were uniformly scattered on the planes. In this nanocomposite, RP nanosheets act as a support to amplify the inner space of the SnS₂ nanostructure, which may provide more transmission channels for carriers. The elemental mapping images of P, S and Sn in-plane and the EDS spectrum of 50%-RP/SnS₂ (Fig. S2†) also confirmed the uniform dispersibility of RP on SnS₂. TEM and HRTEM images (Fig. 3(d)–(f)) further showed the morphological structure between SnS₂ and RP, exhibiting effective contact and close interfaces, in which the lattice fringes of the sample with a d spacing of 0.312 nm can be assigned to the (100) lattice planes of SnS₂. Obviously, the RP modification strategy did not modify the flower-like structure of SnS₂. The construction of a 2D heterojunction between RP and SnS₂ may reduce the resistance and speed up the transmission of charge carriers, which was verified by the electrochemical characterizations, as shown in Fig. 7. Furthermore, N₂ sorption was measured to determine the specific surface area (S_BET) and corresponding pore structure (Fig. 4). Moreover, the S_BET of SnS₂, RP and 50%-RP/SnS₂ samples was determined as 35.38, 36.79 and 40.96 m² g⁻¹, respectively (Table 1). Minor variations were identified in the S_BET and pore volume of the 50%-RP/SnS₂ compared with SnS₂, suggesting that the composite does not generate agglomeration and has good dispersibility. It is noteworthy that the pathway between them might lead to an efficient charge transfer.

Fig. 3 SEM images of (a) SnS₂, (b) RP, (c) 50%-RP/SnS₂, (d and e) TEM and (f) HRTEM images of 50%-RP/SnS₂.

Fig. 4 (a) N₂ adsorption–desorption isotherms and (b) the pore size distribution of SnS₂, RP and 50%-RP/SnS₂ samples.

Table 1 S _BET, pore diameter and pore volume of SnS₂, RP and 50%-RP/SnS₂ samples

Samples	S BET (m^2 g^−1)	Pore diameter (nm)	Pore volume (cm^3 g^−1)
SnS2	35.38	3.05	0.20
RP	36.79	3.74	0.21
50%-RP/SnS2	40.96	3.76	0.23

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3.2 X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) was performed to delve into the chemical composition and analyze the chemical states of 50%-RP/SnS₂. Fig. 5(a) presents the full survey spectrum, suggesting that the elements the composites contained complied with the chemical composition of the photocatalyst. The high-resolution P 2p spectrum of RP illustrated in Fig. 5(b) displays two main peaks at 130.3 eV and 131.4 eV, assigned to P 2p_3/2 and 2p_1/2, respectively. The corresponding peak positions of RP/SnS₂ shift to 130.6 and 133.1 eV. The Sn 3d spectrum of SnS₂ (Fig. 5(c)) displays two main peaks at 487.1 and 495.5 eV, assigned to Sn 3d_5/2 and 3d_3/2, respectively.^33,34 The corresponding peak positions of RP/SnS₂ show minor shifts to 487.2 and 495.6 eV. Likewise, Fig. 5(d) suggests that the high-resolution S 2p spectrum for SnS₂ can be deconvoluted into two peaks at 162.1 and 163.3 eV, assigned to S 2p_3/2 and 2p_1/2,³⁵ expressing minor shifts to lower binder energies 161.9 and 163.1 eV for RP/SnS₂. As revealed from the mentioned results, the coupling between SnS₂ and RP altered the elemental binding energies for both of them, probably resulting from the electron transfer between semiconductors with different E_F levels.³⁶ This also indirectly implied that a nano-heterojunction was successfully established between SnS₂ and RP.

Fig. 5 (a) XPS survey spectra and corresponding high-resolution XPS spectra of (b) P 2p, (c) Sn 3d, (d) S 2p.

3.3 Photogenerated carrier transmission performance

As clearly identified from the above results, RP helps enhance the photocatalytic activity of SnS₂ by forming a nano-heterojunction. There into, the transmission mechanisms of carriers between semiconductors can be examined for an explanation. Photoluminescence (PL) spectra of SnS₂ and RP/SnS₂ under an excitation of 645 nm are presented to investigate the transfer behavior of photoinduced electrons and holes in photocatalysts. Fig. 6(a) suggests that the PL intensity of RP/SnS₂ was obviously lower than that of SnS₂, demonstrating that the SnS₂ nanosheets with moderate amounts of RP can suppress the electron–hole recombination. To delve into the reason for the enhanced photocatalytic activity, time-resolved photoluminescence (TR-PL) spectra were collected (Fig. 6(b)) with radiative lifetimes (τ₁ and τ₂) and their corresponding coefficients (A₁ and A₂) in the inset of Fig. 6(b). Based on the mentioned data, average fluorescence lifetimes (τ) can be obtained by the following formula,³⁷i.e., 0.69 ns and 0.73 ns for RP/SnS₂ and SnS₂, respectively. The decreased average fluorescence lifetime means that the transfer of electrons between SnS₂ and RP was accelerated, which is conducive to facilitating photocatalytic reactions.

Fig. 6 (a) PL and (b) time-resolved fluorescence decay spectra and calculated average lifetime (inset) of samples.

To further verify the role of RP in accelerating charge carrier separation, electrochemical measurements were performed, as shown in Fig. 7. It is known that the transient photocurrent response could manifest the separation and collection efficiency of hole–electron pairs occurring on the photocatalyst surface/interface.³⁸Fig. 7(a) suggests that the number of photoinduced charge carriers generated under the light exposure of RP/SnS₂ was significantly higher than those of SnS₂ and RP (nearly 2–3 times), in agreement with the photocatalytic performance results reported above. Moreover, the radii of these samples identified in the Nyquist plots (Fig. 7(b)) complied with the sequence of RP/SnS₂ < SnS₂ < RP, revealing that RP/SnS₂ possessed the lowest resistance and fastest charge transfer kinetics for charge carriers.

Fig. 7 (a) Transient photocurrent response and (b) EIS spectra of the samples.

3.4 Optical performance analysis

The optical properties are another vital factor in increasing the photoactivity and quantum yield. The enhanced light capture capacity of RP/SnS₂ was verified by UV-vis absorption spectroscopy. The tangent lines in the plots of the square root of the Kubelka–Munk functions against the photon energy were used to assess the band gaps (E_g) of SnS₂ and RP. As displayed in Fig. 8(a) and (b), the absorption edge of SnS₂ was around 634 nm with a band gap of nearly 2.21 eV. However, RP exhibits a considerably strong absorption in the visible region, the absorption edge of which is at approximately 689 nm with a band gap of nearly 1.97 eV. An obvious absorption edge red-shift could be identified in the RP/SnS₂ composites and the light absorption coefficient was integrally up-regulated in the 500–800 nm range. As revealed from the above results, the loading of RP nanosheets enhanced the optical properties of SnS₂, thereby partially facilitating the enhancement of photocatalytic activity, in agreement with the previous results.

Fig. 8 (a) UV-vis diffuse reflection spectra, (b) Kubelka–Munk plots converted from the UV-vis diffuse reflection spectra, valence band spectrum of (c) RP and (d) SnS₂.

Moreover, Fig. 8(c) and (d) shows the XPS valence spectrum of SnS₂ and RP, in which the obtained valence band (VB) positions were identified as 1.53 eV and 1.83 eV, respectively. Meanwhile, the E_g measured via the optical absorption spectra above (E_g (SnS₂) = 2.21 eV and E_g (RP) = 1.97 eV) indicates that the conduction band (CB) positions would respectively occur at about −0.44 eV and −0.38 eV for SnS₂ and RP. To identify the major active species accounting for organic pollutant degradation, disodium ethylenediaminetetraacetic acid (EDTA-Na₂), benzoquinone (BQ), and isopropyl alcohol (IPA) solutions were respectively added to trap holes (h⁺), hydroxyl radicals (˙OH), and superoxide radicals (˙O₂⁻). According to Fig. 9, IPA slightly impacted the RhB degradation, demonstrating that ˙OH are not the major reactive species in the photocatalytic oxidation process. However, upon the addition of EDTA-Na₂ and BQ, the efficiency decreased significantly, revealing that ˙O₂⁻ and h⁺ are the main reactive species in the photocatalytic oxidation. Moreover, as indicated by their relative positions and band structure, e⁻, h⁺ and ˙O₂⁻ are involved in the reactions theoretically.^39,40

Fig. 9 Comparison of the effect of different quenchers on the photocatalytic reactions.

The reactive oxygen species was further detected by EPR for verification, in which DMPO was applied in aqueous and methanolic dispersions as a trapping reagent for the ˙OH and ˙O₂⁻. As expected, the signal of the DMPO–˙OH was not found but the signal of the DMPO–˙O₂⁻ was obvious, as shown in Fig. 10. These findings along with scavenger trapping experiments demonstrated that ˙O₂⁻ and h⁺ were the predominant active species in this photocatalytic reaction system.

Fig. 10 EPR spectra of (a) DMPO–˙OH in aqueous dispersion and (b) DMPO–O₂˙ in methanolic dispersion under light irradiation.

As revealed from the analysis, the nano-heterojunction was built between SnS₂ and RP when the flower-like SnS₂ was modified by RP. The enhanced photocatalytic performance of RP/SnS₂ and photocatalytic synchronous removal of Cr(VI) and RhB was attributed to the critical role of nano RP. As described in Scheme 2, RP acts as the optical receiving active site to enhance the visible light response capability, thereby generating and transmitting more electrons to the CB of SnS₂. Additionally, RP nanosheets created a proper transmission channel of carriers to inhibit the recombination of photogenerated holes and electrons. Furthermore, photogenerated electrons and the absorbed O₂ might be trapped on the SnS₂ surface to generate an abundant superoxide radical (˙O₂⁻), thereby significantly impacting the photocatalytic degradation.

Scheme 2 Schematic illustration of photocatalytic removal mechanism of Cr(VI) and organic pollutants over RP/SnS₂ composite.

4. Conclusions

The proposed method for synchronous removal of Cr(VI) and RhB with RP/SnS₂ is promising based on the significantly increased degradation rate, expressing 8.2 and 10.8 times that of pristine SnS₂, as well as its high reusability. In the mentioned process, nano RP was introduced to act as an active site for both absorption enhancement and carrier separation with excellent dispersibility on the SnS₂ films. Moreover, the 2D heterojunction of the RP/SnS₂ photocatalyst created channels for the rapid transfer of photon-generated carriers; thus, photo-generated carriers can be effectively separated. Furthermore, the generation of abundant ˙O₂⁻ impacts the photocatalytic degradation oxidation reaction process. Overall, the application of RP/SnS₂ in the removal of coexisting Cr(VI)/RhB was demonstrated as a promising path toward treating wastewater.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The present study was supported by the National Natural Science Foundation of China (Nos 21476183, 21676213, 21807087 and 21673173), the China Postdoctoral Science Foundation (No. 2016M600809), the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2020JM422), Key Research and Development Program of Shaanxi (No. 2019KWZ-07), the Key Science and Technology Innovation Team of Shaanxi Province (No. 2017KCT-37), Technology Innovation Leading Program of Shaanxi (No. 2020TG-031) and the Xi'an City Science and Technology Project (No. 2019218214GXRC018CG019-GXYD18.4).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0na00489h

‡ Xue Bai and Yanyan Du contributed equally to the present study.

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