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SnO2 heterostructured nanosheet arrays grown on carbon cloth for efficient photocatalytic reduction of Cr(VI)
Journal of Colloid and Interface Science 514 (2018) 306–315
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Regular Article
SnS2/SnO2 heterostructured nanosheet arrays grown on carbon cloth for efficient photocatalytic reduction of Cr(VI) Guping Zhang, Dongyun Chen ⇑, Najun Li, Qingfeng Xu, Hua Li, Jinghui He, Jianmei Lu ⇑ Collaborative Innovation Center of Suzhou Nano Science and Technology, College of Chemistry Chemical Engineering and Materials Science Soochow University, 199 Ren’ai Road, Suzhou 215123, PR China
g r a p h i c a l a b s t r a c t SnS2/SnO2 composites on carbon cloth (CC) are fabricated via hydrothermal method and calcination, for efficient photocatalytic reduction of aqueous Cr (VI).
a r t i c l e
i n f o
Article history: Received 15 October 2017 Revised 12 December 2017 Accepted 17 December 2017 Available online 18 December 2017 Keywords: SnS2/SnO2composites Carbon cloth (CC) Visible light Hexavalent chromium (Cr(VI))
a b s t r a c t Nowadays, among the many heavy metal pollutants, hexavalent chromium (Cr(VI)) seriously threatens ecological systems and human health due to its high solubility, acute toxicity and potential carcinogenicity in wastewater. Meanwhile, semiconductor photocatalytic reduction is continuously gaining increasing significant research attention in the treatment of Cr(VI). Hence, we report an efficient preparation method for SnS2/SnO2 composites on carbon cloth (CC), for efficient photocatalytic reduction of aqueous Cr(VI). The morphology, composition, surface elements and optical properties of CC@SnS2/SnO2 composites were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV–vis diffuse reflectance spectroscopy. It was found that carbon cloth (CC) could be effectively used as a catalyst support in the obtained SnS2/SnO2 composites. In addition, the CC@SnS2 calcined 30 min exhibited the best efficiency for photocatalytic reduction of aqueous Cr(VI), which can be attributed to the formation of a heterostructure and the effective separation of photogenerated electrons (e) and holes (h+). It was also found that acidic conditions are more favorable for the photocatalytic reduction of aqueous Cr(VI) due to the presence of abundant H+. The photocatalytic mechanism of as-prepared composites is also discussed in detail. Ó 2017 Elsevier Inc. All rights reserved.
⇑ Corresponding authors. E-mail addresses: [email protected] (D. Chen), [email protected] (J. Lu). https://doi.org/10.1016/j.jcis.2017.12.045 0021-9797/Ó 2017 Elsevier Inc. All rights reserved.
G. Zhang et al. / Journal of Colloid and Interface Science 514 (2018) 306–315
1. Introduction
2. Experimental section
Rapid industrialization across the globe means that untreated industrial wastewater is often discharged directly into natural water sources, leading to increased heavy metal pollution [1–3]. Hexavalent chromium (Cr(VI)) is a common heavy metal contaminant in wastewater resulting from industrial processes such as electroplating, paint making and leather tanning [4–6]. Cr(VI) can seriously endanger both the ecological environment and human health owing to its high solubility and acute toxicity in water [7,8]. Many countries and organizations have strictly regulated the concentration of Cr(VI) in drinking water. For example, the World Health Organization (WHO) has set a limit for chromium in drinking water of 0.05 mg/L [9,10]. This has led to an intensive research focus on how to quickly and effectively treat chromium contaminated wastewater. Various conventional methods including membrane separation [11], ion exchange [12], chemical precipitation and adsorption [13–15], have been used to convert Cr(VI) to Cr(III). Cr(III) is comparatively less toxic than Cr(VI) and can be more easily precipitated and removed in the form of Cr(OH)3 [16–18]. However, commonly used methods require significant amounts of reducing agent and produce hazardous byproducts, which can also be harmful to the environment and human health [19]. Compared with the conventional methods, photocatalytic reduction of Cr(VI) to Cr(III) using semiconductor photocatalysts has numerous distinct advantages, such as high efficiency, low cost, fewer harmful byproducts, and direct use of natural solar energy [20–22]. As a result, semiconductor photocatalytic reduction is gaining significant research attention in the treatment of Cr(VI). Semiconducting metal sulfides, such as CdS, Ag2S and SnS2, are good absorbers of light in the visible range, making them promising photocatalysts [23–25]. SnS2 has been used in preference to CdS and Ag2S for Cr(VI) reduction under visible light irradiation, because of its lower toxicity, lower relative cost and wider spectral response [26]. It is now accepted that, in contrast with using a single semiconductor, using semiconductors composites can improve photocatalytic activity due to the effective separation of photogenerated electrons (e) and holes (h+) [27–31]. SnO2 is a stable, wide band gap, oxide semiconductor (Eg = 3.5–3.6 eV), and can form a heterostructure with SnS2 according to their matched band potentials, which brings about the sensitization of SnO2 and enhances the separation of photogenerated electrons and holes [32–34]. However, it can be difficult to control the structure and morphology of SnS2/SnO2 composites using standard synthesis methods. It is therefore necessary to develop an effective approach which allows for control over catalyst morphology and composition. Carbon cloth (CC) composites, consisting of reinforcing carbon fibers, are widely used in supercapacitors, Li-ion batteries, electrocatalysis and microwave absorption [35,36]. Using carbon cloth as a catalyst support can ensure uniform growth of materials with nanostructural features and generate hierarchical structures [37]. CC composites have the further advantage of providing a conducting pathway for rapid and efficient electron transport [38,39]. Carbon cloth is therefore showing a lot of promise as a catalyst support. In this study, SnS2/SnO2 composites on CC are fabricated for the photocatalytic reduction of aqueous Cr(VI). The preparation of the CC@SnS2/SnO2 composites is outlined in Scheme 1. The SnS2 nanosheets are produced from carbon cloth using a solvothermal method. CC@SnS2/SnO2 composites are then obtained by calcining CC@SnS2 at 400 °C in air. Moreover, the photocatalytic properties of the obtained CC@SnS2/SnO2 composites are determined for the reduction of aqueous Cr(VI) under visible light irradiation. Finally, the photocatalytic mechanism of the composite is proposed in light of the experimental results.
2.1. Materials and reagents
307
All chemicals and reagents were used directly without further purification. Absolute ethanol, isopropyl alcohol, acetone, and nitric acid (65%) were of analytical grade and purchased from Sinopharm Chemical Reagent Co. Ltd. (China). Tin(IV) chloride pentahydrate (SnCl45H2O) and thioacetamide (C2H5NS) were bought from Sigma-Aldrich. Carbon cloth (CC, W0S1002, 135 gm2) was purchased from Taiwan CeTech Co. Ltd. (China) with the thickness of 0.36 mm. The water used in all experiments was obtained from a Millipore system (18.2 MX cm). 2.2. Pretreatment of carbon cloth Prior to use, CC was first cleaned with acetone, ethanol, and deionized (DI) water by sonification continuously for 20 min each, respectively. Then, the cleaned CC was soaked in HNO3 with a concentration of 65% at room temperature for 24 h in order to enhance its surface hydrophilic property. At last, the treated CC was taken out, washed with DI water and dried at 60 °C overnight. 2.3. Preparation of CC@SnS2 The SnS2 nanosheets onto carbon cloth were synthesized by a solvothermal method according to a slightly modified method reported by Li et al. [37]. In a typical experiment, thioacetamide (4 mmol) and tin chloride pentahydrate (SnCl45H2O, 1.5 mmol) were added in a 50 mL Teflon-lined stainless steel autoclave including isopropanol (30 mL) and magnetic stirred for 30 min until clear. After that, a piece of carbon cloth (2 cm 2 cm) was immersed into the mixed solution, and heated at 180 °C for 24 h. After the autoclave cooling to room temperature naturally, the carbon cloth was collected and rinsed with DI water and ethanol repeatedly and finally dried in an oven at 60 °C overnight. 2.4. Preparation of CC@SnS2/SnO2 and CC@SnO2 The obtained CC@SnS2 was placed in a porcelain boat, and calcined at 400 °C in the air to prepare the CC@SnS2/SnO2 and CC@SnO2. The calcination time is ranging from 15 min to 90 min [40,41]. For ease of description, the CC@SnS2/SnO2 obtained under different conditions were named as ‘‘CC@SnS2/SnO2-1”, ‘‘CC@SnS2/ SnO2-2”, ‘‘CC@SnS2/SnO2-3”, and ‘‘CC@SnS2/SnO2-4”, as shown in Table 1. 2.5. Characterization The morphologies and structures of the prepared samples were examined by Scanning electron microscopy (SEM, Hitachi S-4700) and Transmission electron microscopy (TEM, Tecnai G200). The chemical element and the detailed microscopic structure of the samples were investigated by using X-ray energy dispersive spectroscopy (EDS) and high-resolution transmission electron microscopy (HRTEM, Tecnai G2 F20 S-TWIN). The crystal phase of the samples were analyzed by X-ray diffraction (XRD, X’ Pert-Pro MPD). The diffuse reflectance spectra were measured using a spectrophotometer (UV–vis, DRS Shimadzu UV-3600). X-ray photoelectron spectroscopy (XPS) data of the samples were obtained using an X-ray photoelectron spectrometer (ESCALAB MK II) with Al-Ka radiation.
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Scheme 1. Schematic illustration of the fabrication of CC@SnS2/SnO2 composites.
Table 1 Composition of catalysts prepared under different calcination conditions. Samples name
Temperature (°C)
Time (min)
Atomic (%) S K: O K
Molar ratio SnS2:SnO2
DAA (%)
CC@SnS2 CC@SnS2/SnO2-1 CC@SnS2/SnO2-2 CC@SnS2/SnO2-3 CC@SnS2/SnO2-4 CC@SnO2
– 400 400 400 400 400
– 15 30 45 60 90
– 5.3:1.3 5.4:4.6 2.4:6.6 0.9:12.7 –
– 4.08 1.17 0.36 0.07 –
15.59 20.37 22.24 24.71 27.14 30.65
DAA = dark adsorption amount for Cr(VI).
2.6. Photocatalytic activity measurements Photocatalytic activities of CC@SnS2, CC@SnS2/SnO2-(1–4), and CC@SnO2 were evaluated by the photocatalytic reduction of Cr (VI) under visible light irradiation using a 300 W Xe lamp with a 400 nm cutoff filter. ln a typical process, 120 mg of photocatalysts (2 cm 2 cm, 105 mg of CC) was added into 50 mL of Cr(VI) solution (10 mg/L) which were prepared by dissolving K2Cr2O7 into distilled water. At the same time, the pH of aqueous Cr(VI) was adjusted by 1 M H2SO4 and 1 M NaOH solution so as to research the influence of solution pH on catalytic efficiency. Before illumination, the suspension were magnetically stirred in the dark for 1 h at room temperature to make sure that the absorption-desorption equilibrium was achieved. During illumination, 3 mL suspension was sampled from the reactor at given time intervals, and the Cr(VI) content was measured colorimetrically at 540 nm using the standard diphenylcarbazide method [33,34]. 3. Results and discussion 3.1. Morphology and structure SEM and TEM were measured to establish the morphology and structure of the CC, CC@SnS2, CC@SnS2/SnO2-(1–4), and CC@SnO2. As shown in Fig. S1a and 1b (Supporting Information), the CC is made up of rod-like carbon fibers. The fibers have a smooth surface and an average diameter of 10 lm, providing a favorable environment for the support of the photocatalyst. After CC@SnS2 composite formation, the SnS2 nanosheets have smooth surfaces and are homogeneously grown on the carbon cloth in a close-packed arrangement, as illustrated in Fig. 1a and S1c. During calcination in air, the sulfide is gradually converted to oxide, as revealed by SEM-EDS (Fig. S4, Supporting Information). TEM and SEM images (Fig. S3a–f, magnified images in Fig. 1a–f) also show that in the initial 30 min of calcination the SnO2 nanoparticles grow on the surface of the SnS2 nanosheets [33,34,40]. When the calcination time
is greater than 30 min, SnO2 nanoparticles tend to grow in colonies with pores, and finally SnO2 nanosheets (Fig. S1d) composed of SnO2 nanoparticles are formed. However, the integral morphology of samples remains unchanged, as displayed in Figs. 1a–f and S2a– f. It is expected that the nanosheet structure is retained during calcination because the CC is able to effectively support the catalyst without changes to morphology, composition or mechanical properties [37]. Additionally, the calcination temperature in this case is relatively low. The HRTEM image of the SnS2/SnO2-2 nanosheets in Fig. 1 g shows a lattice spacing of 0.335 nm, which corresponds to the spacing of the (110) planes of SnO2 [33,34]. Lattice fringes with a spacing of 0.278 nm can also be discerned, which match up with the (101) planes of SnS2 [33]. The SEM and TEM images suggest that SnS2/SnO2 nanosheets are formed in a uniform, close-packed configuration on the CC. HRTEM indicates the formation of SnS2/ SnO2 heterostructures, which can promote electron transfer and separation, and accordingly increase the photocatalytic activity. A photograph, SEM image and elemental maps of CC@SnS2/ SnO2-2 are shown in Fig. 2. These images confirm that C, Sn, S and O were uniformly grown on the surface of the CC, and SnO2 nanoparticles were dispersed on the surface of the SnS2 nanosheets. Moreover, SEM-EDS analysis of CC@SnO2 (Fig. S4) shows elemental maps of C, Sn and O, and further confirms that SnO2 nanosheets consist of numerous SnO2 nanoparticles, supporting previous SEM and TEM findings. 3.2. Phase and composition The phase compositions of the as-prepared CC@SnS2, CC@SnS2/ SnO2-(1–4), and CC@SnO2 obtained over different calcination periods, were examined by XRD, as shown in Fig. 3. The diffraction peaks at 26.23° and 44.36° 2h came from the carbon cloth substrate [36]. For the SnS2 nanosheets present in CC@SnS2, all characteristic diffraction peaks matched with hexagonal phase SnS2 (JCPDS card No. 23-0677). For the CC@SnO2 sample, all of the peaks corresponded to rutile tetragonal phase SnO2 (JCPDS card No. 41-
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and 530.75 eV (Fig. 4f), respectively. These are consistent with the values for S2, and O2– in CC@SnS2 and CC@SnO2. The XPS study demonstrates the existence of SnS2 and SnO2 on carbon cloth. 3.3. Optical properties Fig. 5 shows the UV–vis diffuse reflectance spectra of SnS2, SnS2/ SnO2-(1–4), and SnO2, and their optical band gaps. As can be seen from Fig. 5a, SnS2 and SnS2/SnO2-(1–4) showed photoabsorption in the visible light region, however SnO2 demonstrated no absorption of visible light. According to a previous study, the band gaps (Eg) of the above samples can be determined based on the theory of optical absorption for direct band gap semiconductors (Eq. (1)):
ahm ¼ Bðhm EgÞ1=2
ð1Þ
where hm is the discrete photon energy and B is a constant related to the material. a can be calculated from the diffuse reflectance data using the Kubelka-Munk function [16,32–34]. Consequently, the band gaps (Eg) of SnS2, SnS2/SnO2-(1–4), and SnO2 nanosheets were estimated to be 2.22, 2.30, 2.25, 2.44, 2.56, 3.55 eV (Fig. 5b), respectively. It is therefore be expected that CC@SnS2/SnO2-2 will exhibit the best light response of the SnS2/SnO2-(1–4) materials, owing to it having the lowest energy band gap. 3.4. Photocatalytic activity It is well known that solution pH is an important factor in photocatalytic processes. Fig. 6 shows the influence of pH on the photocatalytic reduction of Cr(VI) by CC@SnS2/SnO2-2. It can be seen that the Cr(VI) reduction rate clearly decreased with increasing pH (Fig. 6a). The reduction rate of Cr(VI) over CC@SnS2/SnO2-2 is almost 100% at pH 2, after irradiation for 120 min. This decreased relatively by 23.81% at pH 5, and by 75.15% at pH 8. These findings are reflected in the UV–vis absorption spectra (Fig. 6b–d). The photocatalytic results can be explained as follows: first, under acidic conditions, Cr(VI) mainly exists in the form of Cr2O2 7 , for which reduction occurs as follows (Eq. (2)): Fig. 1. SEM images of the as-prepared (a) CC@SnS2, (b) CC@SnS2/SnO2-1, (c) CC@SnS2/SnO2-2, (d) CC@SnS2/SnO2-3, (e) CC@SnS2/SnO2-4 and (f) CC@SnO2. Insets show magnified images. (g) HRTEM images of SnS2/SnO2-2 nanosheets.
1445). It is clear that the increasing calcination time led to an increase in the peak intensity of the SnO2 phase at the expense of decreasing the peak intensity of SnS2, demonstrating that SnS2 on carbon cloth gradually becomes SnO2 when calcined. However, the characteristic peak intensity of SnO2 is weak, probably due to the peak intensity of both SnS2 and carbon cloth being relatively strong. It is worth noting that no unassigned diffraction peaks were present for any sample, which illustrates the high purity of the catalysts. The SnS2/SnO2 molar ratio in the CC@SnS2/SnO2 composites was determined by EDX (Fig. S5), and the results are shown in Table 1. X-ray photoelectron spectroscopy (XPS) was used to further determine the chemical states of CC@SnS2, CC@SnS2/SnO2-2, and CC@SnO2, as shown in Fig. 4. The XPS survey spectra in Fig. 4a and b shows that CC@SnS2/SnO2-2 contained C, Sn, S and O, without evidence of other peaks relating to impurities. The C 1 s peak (Fig. 4c) is attributed to CC. The high-resolution XPS spectra (Fig. 4d) show that the binding energy of Sn 3d3/2 (495.65 eV) and the binding energy of Sn 3d5/2 (487.28 eV) in CC@SnS2/SnO2-2 lie between those in CC@SnS2 (495.58 eV), (487.20 eV) and CC@SnO2 (495.79 eV), (487.34 eV) respectively. This could be due to the heterostructure effect between SnS2 and SnO2 [16,34,42]. In addition, it can be seen that the binding energies of S 2p3/2 and O 1 s in CC@SnS2/SnO2-2 are observed at 162.12 eV (Fig. 4e)
3þ þ Cr2 O2 þ 7H2 O 7 þ 14H þ 6e ¼ 2Cr
ð2Þ
The Nernst equation for this system can be expressed as (Eq. (3)) [43]: 14
u ¼ uh þ
þ 0:059 ½Cr2 O2þ 7 ½H lg 2 6 ½Cr3þ ½H O7
ð3Þ
2
According to Eq. (3), the value of u increases with increasing H+ concentration. The equilibrium (Eq. (2)) will shift to the right, meaning that Cr(VI) is more easily reduced in the more acidic environment. It also indicates that the reduction of Cr(VI) consumes H+. Second, the product of reduction, Cr3+, reacts with OH– to form Cr (OH)3, which deposits on the CC@SnS2/SnO2-2 surface. This partly blocks active sites and causes the photocatalytic activity to decrease. Therefore, the photocatalytic activities of CC@SnS2, CC@SnS2/SnO2-(1–4), and CC@SnO2 were evaluated in terms of the photocatalytic reduction of Cr(VI) solution at pH 2. Fig. 7 shows the photocatalytic reduction of Cr(VI) solution (10 mg/L, pH 2) in the presence of 120 mg of CC@SnS2, CC@SnS2/SnO2(1–4), and CC@SnO2, under visible light irradiation. C0 is the initial concentration of Cr(VI) and Ct is the concentration of Cr(VI) after visible light irradiation. Prior to visible light irradiation, the suspensions were magnetically stirred in the dark for 1 h to achieve an absorption–desorption equilibrium, and the dark adsorption amounts for Cr(VI) of samples are shown in Table 1. As can be seen from Fig. 7a–d, CC@SnO2 showed no visible-light-driven photocatalytic activity and CC@SnS2/SnO2-2 displayed the highest photocatalytic activity. The photocatalytic activities of CC@SnS2/SnO2-1
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Fig. 2. (a) Photograph of CC@SnS2/SnO2-2. (b) SEM image of CC@SnS2/SnO2-2 and (c–f) EDS mapping images of C (red), Sn (purple), S (green) and O (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. XRD patterns of CC@SnS2, CC@SnS2/SnO2-(1–4), and CC@SnO2.
and CC@SnS2/SnO2-2 were higher than that of CC@SnS2. However, CC@SnS2/SnO2-3 and CC@SnS2/SnO2-4 exhibited lower photocatalytic activity compared with CC@SnS2 because the extended calci-
nation times led to a larger proportion of the SnS2 nanosheets being converted into SnO2 nanosheets on CC, making them less able to absorb and utilize visible light. When irradiated for 60 min, the reduction rate of Cr(VI) solution for the CC@SnS2 and CC@SnS2/SnO2-(1–4) composites were 61.61%, 77.37%, 98.67%, 22.27% and 10.12%, respectively (Fig. 7b). The CC@SnS2/SnO2-2 composite exhibited the greatest efficiency for photocatalytic reduction of aqueous Cr(VI), which could be due to the formation of a heterostructure and the effective separation of photogenerated electrons (e) and holes (h+). In addition, it was found that the photocatalytic activity of CC@SnS2/SnO2-2 is slightly higher than that of SnS2/SnO2-2 under the same experimental condition (Fig. S6). Therefore, another potential contributing factor is that the CC provides a conducting support to transport electrons and to some extent prevents the combination of electrons and holes. The separation efficiency of the electron-hole pairs (e-h+) was evaluated by measuring the photocurrent density vs time curve. Fig. 8a shows the density – time curves for SnS2, SnS2/SnO2-2 and SnO2 under visible light irradiation. As shown in Fig. 8a, the photocurrent densities of SnO2 were approximately zero due to the lack of response to visible light. At the same time, the order of photocurrent intensity is SnS2/SnO2-2 > SnS2/SnO2-1 > SnS2 > SnS2/SnO2-3 > SnS2/SnO2-4, which agrees well with the observed experiment results of their photocatalytic activity mentioned
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Fig. 4. XPS spectra of CC@SnS2, CC@SnS2/SnO2-2, and CC@SnO2 (a) and (b) and high-resolution XPS spectra of C 1s (c), Sn 3d (d), S 2p (e), O 1s (f).
Fig. 5. (a) UV–vis diffuse reflectance spectra of SnS2, SnS2/SnO2-(1–4), and SnO2 nanosheets. (b) Plots of (F(R1)hm)2 versus (hm) for estimation of the optical band gaps of SnS2, SnS2/SnO2-(1–4), and SnO2.
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Fig. 6. (a) Photocatalytic reduction of 50 mL of 10 mg/L Cr(VI) solution at pH 2, 5, and 8 under visible light irradiation with 120 mg of CC@SnS2/SnO2-2. The UV–vis absorption spectral changes of Cr(VI) solution over 120 min of irradiation at (b) pH 2, (c) pH 5, and (d) pH 8.
Fig. 7. Photocatalytic activities of the samples for Cr(VI) solution (a), (b). UV–vis absorption spectra of Cr(VI) in presence of (c) CC@SnS2 and (d) CC@SnS2/SnO2-2.
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Fig. 8. (a) The transient photocurrent responses of the SnS2, SnS2/SnO2-(1–4) and SnO2. (b) Cyclic performance of CC@SnS2/SnO2-2 for the reduction of Cr(VI) solution under visible light irradiation.
above. In addition, SnS2/SnO2-2 showed an approximately twofold improvement in photocurrent response when compared with SnS2 under the same conditions. This supports the band gap data which suggested that the SnS2/SnO2-2 composite would show more efficient charge separation and migration. The stability of the catalyst is also an important parameter in practical applications, therefore recycle experiments for the reduction of Cr(VI) were conducted with CC@SnS2/SnO2-2. As presented in Fig. 8b, the reduction rate of Cr(VI) remained high, 91.16%, after three cycling runs. The XRD pattern (Fig. S7) of the recycled CC@SnS2/SnO2-2-AP was consistent with that of the initial sample with no obvious deviations in the locations of the peaks, indicating that CC@SnS2/SnO2-2 composite is a stable photocatalyst. Moreover, the CC@SnS2/SnO2-2 after three cycle photocatalytic use was further characterized by XPS. As can be seen in Fig. S8, the CC@SnS2/SnO2-2-AP contained
C, Sn, S and O components, as well as Cr contaminants. Meanwhile, the binding energy of Cr 2p3/2 over CC@SnS2/SnO2-2-AP was observed at 577.4 eV (Fig. S8b), which was consistent with Cr(III) in Cr(OH)3 [16]. Additionally, the solution after the cycle photocatalytic reduction of Cr (VI) over CC@SnS2/SnO2-2 was detected three times using ICP-MS with Cr(III) standard curve, in order to confirm the content of Cr ions in solution. The results show that the average contents of Cr ions are 0.150 mg/L, which indicate the Cr(III) was mainly located on the surface of photocatalyst rather than in solution. In conclusion, the above results demonstrate that CC@SnS2/ SnO2-2 has a good stability, and the appearance of Cr(III) further confirms the reduction of Cr(VI) under visible light irradiation. Scheme 2 shows a proposed mechanism for the photocatalytic reduction of Cr(VI) based on the experimental data reported, In general, upon irradiation the semiconductor photocatalyst absorbs
Scheme 2. Schematic illustration of the reaction mechanism for reduction of Cr(VI) over CC@SnS2/SnO2 composite under visible-light irradiation.
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light energy and the electrons (e) in the valence band (VB) are excited to the conduction band (CB), simultaneously producing the same number of holes (h+) in the VB. The e and h+ can either move to the surface of the semiconductor photocatalyst and take part in redox reactions with adsorbed species; or photogenerated e and h+ can recombine, decreasing photocatalytic activity, which is a significant limitation of semiconductor photocatalysts [1,2,16,19,20,33,34]. For CC@SnS2/SnO2 composites under visiblelight irradiation (Eq. (4)), the e in the VB of SnS2 can be excited to the CB and an equal amount of h+ are generated in the VB, but SnO2 shows no capacity to absorb visible light. However, e from SnS2 can transfer to the CB of SnO2 and h+ remain in the VB of SnS2 due to the VB and CB potentials of SnS2 both being more negative than those of SnO2. To some extent, the carbon cloth (CC) in this system transports e and prevents the combination of e and h+. Therefore, e and h+ can be effectively separated and their recombination is retarded, leading to greater photocatalytic activ3+ ity. Finally, Cr2O2 by the photogenerated elec7 is reduced to Cr trons and O2 is produced by oxidation of H2O, as shown in Eqs. (5) and (6).
CC@SnS2 =SnO2 þ hm ! e þ h
ð4Þ
3þ þ Cr2 O2 þ 7H2 O 7 þ 14H þ 6e ! 2Cr
ð5Þ
þ
þ
2H2 O þ 4h ! O2 þ 4Hþ
ð6Þ
4. Conclusions In summary, a series of CC@SnS2/SnO2 composites were successfully prepared using an efficient method. Carbon cloth (CC) not only favored the uniform and vertical growth of SnS2 nanosheets, but also effectively supported catalysts leading to unchanged morphology and mechanical properties during the calcination process. Of the composites prepared (CC@SnS2/SnO2-(1– 4)), CC@SnS2/SnO2-2 exhibited the highest photocatalytic activity compared with pure CC@SnS2. It is believed that the matched band potentials and good interfacial contact between the SnS2 and SnO2 on carbon cloth (CC) can facilitate charge transfer and suppress recombination of electrons and holes, leading to the enhanced photocatalytic reduction of Cr(VI) solution. Importantly, catalyst stability was maintained after three cycles. This study demonstrates that CC@SnS2/SnO2 composites are a promising alternative for the reduction of Cr(VI) in wastewater in the future. Furthermore, the current work could provide a general guiding line for designing catalysts on suitable conductive supports (for example, carbon cloth (CC), carbon fibers (CNF), nickel foam or stainless steel mesh), which would provide opportunities for both fundamental research and promising applications in environmental purification or energy conversion.
Acknowledgements We gratefully acknowledge the financial support provided by the National Key R&D Program of China (2017YFC0210901, 2017YFC0210906), National Natural Science Foundation of China (51573122, 21722607, 21776190), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (17KJA430014, 17KJA150009), the Science and Technology Program for Social Development of Jiangsu (BE2015637) and the project supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2017.12.045.
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
SnS2/SnO2 heterostructured nanosheet arrays grown on carbon cloth for efficient photocatalytic reduction of Cr(VI) Guping Zhang, Dongyun Chen ⇑, Najun Li, Qingfeng Xu, Hua Li, Jinghui He, Jianmei Lu ⇑ Collaborative Innovation Center of Suzhou Nano Science and Technology, College of Chemistry Chemical Engineering and Materials Science Soochow University, 199 Ren’ai Road, Suzhou 215123, PR China
g r a p h i c a l a b s t r a c t SnS2/SnO2 composites on carbon cloth (CC) are fabricated via hydrothermal method and calcination, for efficient photocatalytic reduction of aqueous Cr (VI).
a r t i c l e
i n f o
Article history: Received 15 October 2017 Revised 12 December 2017 Accepted 17 December 2017 Available online 18 December 2017 Keywords: SnS2/SnO2composites Carbon cloth (CC) Visible light Hexavalent chromium (Cr(VI))
a b s t r a c t Nowadays, among the many heavy metal pollutants, hexavalent chromium (Cr(VI)) seriously threatens ecological systems and human health due to its high solubility, acute toxicity and potential carcinogenicity in wastewater. Meanwhile, semiconductor photocatalytic reduction is continuously gaining increasing significant research attention in the treatment of Cr(VI). Hence, we report an efficient preparation method for SnS2/SnO2 composites on carbon cloth (CC), for efficient photocatalytic reduction of aqueous Cr(VI). The morphology, composition, surface elements and optical properties of CC@SnS2/SnO2 composites were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV–vis diffuse reflectance spectroscopy. It was found that carbon cloth (CC) could be effectively used as a catalyst support in the obtained SnS2/SnO2 composites. In addition, the CC@SnS2 calcined 30 min exhibited the best efficiency for photocatalytic reduction of aqueous Cr(VI), which can be attributed to the formation of a heterostructure and the effective separation of photogenerated electrons (e) and holes (h+). It was also found that acidic conditions are more favorable for the photocatalytic reduction of aqueous Cr(VI) due to the presence of abundant H+. The photocatalytic mechanism of as-prepared composites is also discussed in detail. Ó 2017 Elsevier Inc. All rights reserved.
⇑ Corresponding authors. E-mail addresses: [email protected] (D. Chen), [email protected] (J. Lu). https://doi.org/10.1016/j.jcis.2017.12.045 0021-9797/Ó 2017 Elsevier Inc. All rights reserved.
G. Zhang et al. / Journal of Colloid and Interface Science 514 (2018) 306–315
1. Introduction
2. Experimental section
Rapid industrialization across the globe means that untreated industrial wastewater is often discharged directly into natural water sources, leading to increased heavy metal pollution [1–3]. Hexavalent chromium (Cr(VI)) is a common heavy metal contaminant in wastewater resulting from industrial processes such as electroplating, paint making and leather tanning [4–6]. Cr(VI) can seriously endanger both the ecological environment and human health owing to its high solubility and acute toxicity in water [7,8]. Many countries and organizations have strictly regulated the concentration of Cr(VI) in drinking water. For example, the World Health Organization (WHO) has set a limit for chromium in drinking water of 0.05 mg/L [9,10]. This has led to an intensive research focus on how to quickly and effectively treat chromium contaminated wastewater. Various conventional methods including membrane separation [11], ion exchange [12], chemical precipitation and adsorption [13–15], have been used to convert Cr(VI) to Cr(III). Cr(III) is comparatively less toxic than Cr(VI) and can be more easily precipitated and removed in the form of Cr(OH)3 [16–18]. However, commonly used methods require significant amounts of reducing agent and produce hazardous byproducts, which can also be harmful to the environment and human health [19]. Compared with the conventional methods, photocatalytic reduction of Cr(VI) to Cr(III) using semiconductor photocatalysts has numerous distinct advantages, such as high efficiency, low cost, fewer harmful byproducts, and direct use of natural solar energy [20–22]. As a result, semiconductor photocatalytic reduction is gaining significant research attention in the treatment of Cr(VI). Semiconducting metal sulfides, such as CdS, Ag2S and SnS2, are good absorbers of light in the visible range, making them promising photocatalysts [23–25]. SnS2 has been used in preference to CdS and Ag2S for Cr(VI) reduction under visible light irradiation, because of its lower toxicity, lower relative cost and wider spectral response [26]. It is now accepted that, in contrast with using a single semiconductor, using semiconductors composites can improve photocatalytic activity due to the effective separation of photogenerated electrons (e) and holes (h+) [27–31]. SnO2 is a stable, wide band gap, oxide semiconductor (Eg = 3.5–3.6 eV), and can form a heterostructure with SnS2 according to their matched band potentials, which brings about the sensitization of SnO2 and enhances the separation of photogenerated electrons and holes [32–34]. However, it can be difficult to control the structure and morphology of SnS2/SnO2 composites using standard synthesis methods. It is therefore necessary to develop an effective approach which allows for control over catalyst morphology and composition. Carbon cloth (CC) composites, consisting of reinforcing carbon fibers, are widely used in supercapacitors, Li-ion batteries, electrocatalysis and microwave absorption [35,36]. Using carbon cloth as a catalyst support can ensure uniform growth of materials with nanostructural features and generate hierarchical structures [37]. CC composites have the further advantage of providing a conducting pathway for rapid and efficient electron transport [38,39]. Carbon cloth is therefore showing a lot of promise as a catalyst support. In this study, SnS2/SnO2 composites on CC are fabricated for the photocatalytic reduction of aqueous Cr(VI). The preparation of the CC@SnS2/SnO2 composites is outlined in Scheme 1. The SnS2 nanosheets are produced from carbon cloth using a solvothermal method. CC@SnS2/SnO2 composites are then obtained by calcining CC@SnS2 at 400 °C in air. Moreover, the photocatalytic properties of the obtained CC@SnS2/SnO2 composites are determined for the reduction of aqueous Cr(VI) under visible light irradiation. Finally, the photocatalytic mechanism of the composite is proposed in light of the experimental results.
2.1. Materials and reagents
307
All chemicals and reagents were used directly without further purification. Absolute ethanol, isopropyl alcohol, acetone, and nitric acid (65%) were of analytical grade and purchased from Sinopharm Chemical Reagent Co. Ltd. (China). Tin(IV) chloride pentahydrate (SnCl45H2O) and thioacetamide (C2H5NS) were bought from Sigma-Aldrich. Carbon cloth (CC, W0S1002, 135 gm2) was purchased from Taiwan CeTech Co. Ltd. (China) with the thickness of 0.36 mm. The water used in all experiments was obtained from a Millipore system (18.2 MX cm). 2.2. Pretreatment of carbon cloth Prior to use, CC was first cleaned with acetone, ethanol, and deionized (DI) water by sonification continuously for 20 min each, respectively. Then, the cleaned CC was soaked in HNO3 with a concentration of 65% at room temperature for 24 h in order to enhance its surface hydrophilic property. At last, the treated CC was taken out, washed with DI water and dried at 60 °C overnight. 2.3. Preparation of CC@SnS2 The SnS2 nanosheets onto carbon cloth were synthesized by a solvothermal method according to a slightly modified method reported by Li et al. [37]. In a typical experiment, thioacetamide (4 mmol) and tin chloride pentahydrate (SnCl45H2O, 1.5 mmol) were added in a 50 mL Teflon-lined stainless steel autoclave including isopropanol (30 mL) and magnetic stirred for 30 min until clear. After that, a piece of carbon cloth (2 cm 2 cm) was immersed into the mixed solution, and heated at 180 °C for 24 h. After the autoclave cooling to room temperature naturally, the carbon cloth was collected and rinsed with DI water and ethanol repeatedly and finally dried in an oven at 60 °C overnight. 2.4. Preparation of CC@SnS2/SnO2 and CC@SnO2 The obtained CC@SnS2 was placed in a porcelain boat, and calcined at 400 °C in the air to prepare the CC@SnS2/SnO2 and CC@SnO2. The calcination time is ranging from 15 min to 90 min [40,41]. For ease of description, the CC@SnS2/SnO2 obtained under different conditions were named as ‘‘CC@SnS2/SnO2-1”, ‘‘CC@SnS2/ SnO2-2”, ‘‘CC@SnS2/SnO2-3”, and ‘‘CC@SnS2/SnO2-4”, as shown in Table 1. 2.5. Characterization The morphologies and structures of the prepared samples were examined by Scanning electron microscopy (SEM, Hitachi S-4700) and Transmission electron microscopy (TEM, Tecnai G200). The chemical element and the detailed microscopic structure of the samples were investigated by using X-ray energy dispersive spectroscopy (EDS) and high-resolution transmission electron microscopy (HRTEM, Tecnai G2 F20 S-TWIN). The crystal phase of the samples were analyzed by X-ray diffraction (XRD, X’ Pert-Pro MPD). The diffuse reflectance spectra were measured using a spectrophotometer (UV–vis, DRS Shimadzu UV-3600). X-ray photoelectron spectroscopy (XPS) data of the samples were obtained using an X-ray photoelectron spectrometer (ESCALAB MK II) with Al-Ka radiation.
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Scheme 1. Schematic illustration of the fabrication of CC@SnS2/SnO2 composites.
Table 1 Composition of catalysts prepared under different calcination conditions. Samples name
Temperature (°C)
Time (min)
Atomic (%) S K: O K
Molar ratio SnS2:SnO2
DAA (%)
CC@SnS2 CC@SnS2/SnO2-1 CC@SnS2/SnO2-2 CC@SnS2/SnO2-3 CC@SnS2/SnO2-4 CC@SnO2
– 400 400 400 400 400
– 15 30 45 60 90
– 5.3:1.3 5.4:4.6 2.4:6.6 0.9:12.7 –
– 4.08 1.17 0.36 0.07 –
15.59 20.37 22.24 24.71 27.14 30.65
DAA = dark adsorption amount for Cr(VI).
2.6. Photocatalytic activity measurements Photocatalytic activities of CC@SnS2, CC@SnS2/SnO2-(1–4), and CC@SnO2 were evaluated by the photocatalytic reduction of Cr (VI) under visible light irradiation using a 300 W Xe lamp with a 400 nm cutoff filter. ln a typical process, 120 mg of photocatalysts (2 cm 2 cm, 105 mg of CC) was added into 50 mL of Cr(VI) solution (10 mg/L) which were prepared by dissolving K2Cr2O7 into distilled water. At the same time, the pH of aqueous Cr(VI) was adjusted by 1 M H2SO4 and 1 M NaOH solution so as to research the influence of solution pH on catalytic efficiency. Before illumination, the suspension were magnetically stirred in the dark for 1 h at room temperature to make sure that the absorption-desorption equilibrium was achieved. During illumination, 3 mL suspension was sampled from the reactor at given time intervals, and the Cr(VI) content was measured colorimetrically at 540 nm using the standard diphenylcarbazide method [33,34]. 3. Results and discussion 3.1. Morphology and structure SEM and TEM were measured to establish the morphology and structure of the CC, CC@SnS2, CC@SnS2/SnO2-(1–4), and CC@SnO2. As shown in Fig. S1a and 1b (Supporting Information), the CC is made up of rod-like carbon fibers. The fibers have a smooth surface and an average diameter of 10 lm, providing a favorable environment for the support of the photocatalyst. After CC@SnS2 composite formation, the SnS2 nanosheets have smooth surfaces and are homogeneously grown on the carbon cloth in a close-packed arrangement, as illustrated in Fig. 1a and S1c. During calcination in air, the sulfide is gradually converted to oxide, as revealed by SEM-EDS (Fig. S4, Supporting Information). TEM and SEM images (Fig. S3a–f, magnified images in Fig. 1a–f) also show that in the initial 30 min of calcination the SnO2 nanoparticles grow on the surface of the SnS2 nanosheets [33,34,40]. When the calcination time
is greater than 30 min, SnO2 nanoparticles tend to grow in colonies with pores, and finally SnO2 nanosheets (Fig. S1d) composed of SnO2 nanoparticles are formed. However, the integral morphology of samples remains unchanged, as displayed in Figs. 1a–f and S2a– f. It is expected that the nanosheet structure is retained during calcination because the CC is able to effectively support the catalyst without changes to morphology, composition or mechanical properties [37]. Additionally, the calcination temperature in this case is relatively low. The HRTEM image of the SnS2/SnO2-2 nanosheets in Fig. 1 g shows a lattice spacing of 0.335 nm, which corresponds to the spacing of the (110) planes of SnO2 [33,34]. Lattice fringes with a spacing of 0.278 nm can also be discerned, which match up with the (101) planes of SnS2 [33]. The SEM and TEM images suggest that SnS2/SnO2 nanosheets are formed in a uniform, close-packed configuration on the CC. HRTEM indicates the formation of SnS2/ SnO2 heterostructures, which can promote electron transfer and separation, and accordingly increase the photocatalytic activity. A photograph, SEM image and elemental maps of CC@SnS2/ SnO2-2 are shown in Fig. 2. These images confirm that C, Sn, S and O were uniformly grown on the surface of the CC, and SnO2 nanoparticles were dispersed on the surface of the SnS2 nanosheets. Moreover, SEM-EDS analysis of CC@SnO2 (Fig. S4) shows elemental maps of C, Sn and O, and further confirms that SnO2 nanosheets consist of numerous SnO2 nanoparticles, supporting previous SEM and TEM findings. 3.2. Phase and composition The phase compositions of the as-prepared CC@SnS2, CC@SnS2/ SnO2-(1–4), and CC@SnO2 obtained over different calcination periods, were examined by XRD, as shown in Fig. 3. The diffraction peaks at 26.23° and 44.36° 2h came from the carbon cloth substrate [36]. For the SnS2 nanosheets present in CC@SnS2, all characteristic diffraction peaks matched with hexagonal phase SnS2 (JCPDS card No. 23-0677). For the CC@SnO2 sample, all of the peaks corresponded to rutile tetragonal phase SnO2 (JCPDS card No. 41-
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and 530.75 eV (Fig. 4f), respectively. These are consistent with the values for S2, and O2– in CC@SnS2 and CC@SnO2. The XPS study demonstrates the existence of SnS2 and SnO2 on carbon cloth. 3.3. Optical properties Fig. 5 shows the UV–vis diffuse reflectance spectra of SnS2, SnS2/ SnO2-(1–4), and SnO2, and their optical band gaps. As can be seen from Fig. 5a, SnS2 and SnS2/SnO2-(1–4) showed photoabsorption in the visible light region, however SnO2 demonstrated no absorption of visible light. According to a previous study, the band gaps (Eg) of the above samples can be determined based on the theory of optical absorption for direct band gap semiconductors (Eq. (1)):
ahm ¼ Bðhm EgÞ1=2
ð1Þ
where hm is the discrete photon energy and B is a constant related to the material. a can be calculated from the diffuse reflectance data using the Kubelka-Munk function [16,32–34]. Consequently, the band gaps (Eg) of SnS2, SnS2/SnO2-(1–4), and SnO2 nanosheets were estimated to be 2.22, 2.30, 2.25, 2.44, 2.56, 3.55 eV (Fig. 5b), respectively. It is therefore be expected that CC@SnS2/SnO2-2 will exhibit the best light response of the SnS2/SnO2-(1–4) materials, owing to it having the lowest energy band gap. 3.4. Photocatalytic activity It is well known that solution pH is an important factor in photocatalytic processes. Fig. 6 shows the influence of pH on the photocatalytic reduction of Cr(VI) by CC@SnS2/SnO2-2. It can be seen that the Cr(VI) reduction rate clearly decreased with increasing pH (Fig. 6a). The reduction rate of Cr(VI) over CC@SnS2/SnO2-2 is almost 100% at pH 2, after irradiation for 120 min. This decreased relatively by 23.81% at pH 5, and by 75.15% at pH 8. These findings are reflected in the UV–vis absorption spectra (Fig. 6b–d). The photocatalytic results can be explained as follows: first, under acidic conditions, Cr(VI) mainly exists in the form of Cr2O2 7 , for which reduction occurs as follows (Eq. (2)): Fig. 1. SEM images of the as-prepared (a) CC@SnS2, (b) CC@SnS2/SnO2-1, (c) CC@SnS2/SnO2-2, (d) CC@SnS2/SnO2-3, (e) CC@SnS2/SnO2-4 and (f) CC@SnO2. Insets show magnified images. (g) HRTEM images of SnS2/SnO2-2 nanosheets.
1445). It is clear that the increasing calcination time led to an increase in the peak intensity of the SnO2 phase at the expense of decreasing the peak intensity of SnS2, demonstrating that SnS2 on carbon cloth gradually becomes SnO2 when calcined. However, the characteristic peak intensity of SnO2 is weak, probably due to the peak intensity of both SnS2 and carbon cloth being relatively strong. It is worth noting that no unassigned diffraction peaks were present for any sample, which illustrates the high purity of the catalysts. The SnS2/SnO2 molar ratio in the CC@SnS2/SnO2 composites was determined by EDX (Fig. S5), and the results are shown in Table 1. X-ray photoelectron spectroscopy (XPS) was used to further determine the chemical states of CC@SnS2, CC@SnS2/SnO2-2, and CC@SnO2, as shown in Fig. 4. The XPS survey spectra in Fig. 4a and b shows that CC@SnS2/SnO2-2 contained C, Sn, S and O, without evidence of other peaks relating to impurities. The C 1 s peak (Fig. 4c) is attributed to CC. The high-resolution XPS spectra (Fig. 4d) show that the binding energy of Sn 3d3/2 (495.65 eV) and the binding energy of Sn 3d5/2 (487.28 eV) in CC@SnS2/SnO2-2 lie between those in CC@SnS2 (495.58 eV), (487.20 eV) and CC@SnO2 (495.79 eV), (487.34 eV) respectively. This could be due to the heterostructure effect between SnS2 and SnO2 [16,34,42]. In addition, it can be seen that the binding energies of S 2p3/2 and O 1 s in CC@SnS2/SnO2-2 are observed at 162.12 eV (Fig. 4e)
3þ þ Cr2 O2 þ 7H2 O 7 þ 14H þ 6e ¼ 2Cr
ð2Þ
The Nernst equation for this system can be expressed as (Eq. (3)) [43]: 14
u ¼ uh þ
þ 0:059 ½Cr2 O2þ 7 ½H lg 2 6 ½Cr3þ ½H O7
ð3Þ
2
According to Eq. (3), the value of u increases with increasing H+ concentration. The equilibrium (Eq. (2)) will shift to the right, meaning that Cr(VI) is more easily reduced in the more acidic environment. It also indicates that the reduction of Cr(VI) consumes H+. Second, the product of reduction, Cr3+, reacts with OH– to form Cr (OH)3, which deposits on the CC@SnS2/SnO2-2 surface. This partly blocks active sites and causes the photocatalytic activity to decrease. Therefore, the photocatalytic activities of CC@SnS2, CC@SnS2/SnO2-(1–4), and CC@SnO2 were evaluated in terms of the photocatalytic reduction of Cr(VI) solution at pH 2. Fig. 7 shows the photocatalytic reduction of Cr(VI) solution (10 mg/L, pH 2) in the presence of 120 mg of CC@SnS2, CC@SnS2/SnO2(1–4), and CC@SnO2, under visible light irradiation. C0 is the initial concentration of Cr(VI) and Ct is the concentration of Cr(VI) after visible light irradiation. Prior to visible light irradiation, the suspensions were magnetically stirred in the dark for 1 h to achieve an absorption–desorption equilibrium, and the dark adsorption amounts for Cr(VI) of samples are shown in Table 1. As can be seen from Fig. 7a–d, CC@SnO2 showed no visible-light-driven photocatalytic activity and CC@SnS2/SnO2-2 displayed the highest photocatalytic activity. The photocatalytic activities of CC@SnS2/SnO2-1
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Fig. 2. (a) Photograph of CC@SnS2/SnO2-2. (b) SEM image of CC@SnS2/SnO2-2 and (c–f) EDS mapping images of C (red), Sn (purple), S (green) and O (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. XRD patterns of CC@SnS2, CC@SnS2/SnO2-(1–4), and CC@SnO2.
and CC@SnS2/SnO2-2 were higher than that of CC@SnS2. However, CC@SnS2/SnO2-3 and CC@SnS2/SnO2-4 exhibited lower photocatalytic activity compared with CC@SnS2 because the extended calci-
nation times led to a larger proportion of the SnS2 nanosheets being converted into SnO2 nanosheets on CC, making them less able to absorb and utilize visible light. When irradiated for 60 min, the reduction rate of Cr(VI) solution for the CC@SnS2 and CC@SnS2/SnO2-(1–4) composites were 61.61%, 77.37%, 98.67%, 22.27% and 10.12%, respectively (Fig. 7b). The CC@SnS2/SnO2-2 composite exhibited the greatest efficiency for photocatalytic reduction of aqueous Cr(VI), which could be due to the formation of a heterostructure and the effective separation of photogenerated electrons (e) and holes (h+). In addition, it was found that the photocatalytic activity of CC@SnS2/SnO2-2 is slightly higher than that of SnS2/SnO2-2 under the same experimental condition (Fig. S6). Therefore, another potential contributing factor is that the CC provides a conducting support to transport electrons and to some extent prevents the combination of electrons and holes. The separation efficiency of the electron-hole pairs (e-h+) was evaluated by measuring the photocurrent density vs time curve. Fig. 8a shows the density – time curves for SnS2, SnS2/SnO2-2 and SnO2 under visible light irradiation. As shown in Fig. 8a, the photocurrent densities of SnO2 were approximately zero due to the lack of response to visible light. At the same time, the order of photocurrent intensity is SnS2/SnO2-2 > SnS2/SnO2-1 > SnS2 > SnS2/SnO2-3 > SnS2/SnO2-4, which agrees well with the observed experiment results of their photocatalytic activity mentioned
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Fig. 4. XPS spectra of CC@SnS2, CC@SnS2/SnO2-2, and CC@SnO2 (a) and (b) and high-resolution XPS spectra of C 1s (c), Sn 3d (d), S 2p (e), O 1s (f).
Fig. 5. (a) UV–vis diffuse reflectance spectra of SnS2, SnS2/SnO2-(1–4), and SnO2 nanosheets. (b) Plots of (F(R1)hm)2 versus (hm) for estimation of the optical band gaps of SnS2, SnS2/SnO2-(1–4), and SnO2.
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Fig. 6. (a) Photocatalytic reduction of 50 mL of 10 mg/L Cr(VI) solution at pH 2, 5, and 8 under visible light irradiation with 120 mg of CC@SnS2/SnO2-2. The UV–vis absorption spectral changes of Cr(VI) solution over 120 min of irradiation at (b) pH 2, (c) pH 5, and (d) pH 8.
Fig. 7. Photocatalytic activities of the samples for Cr(VI) solution (a), (b). UV–vis absorption spectra of Cr(VI) in presence of (c) CC@SnS2 and (d) CC@SnS2/SnO2-2.
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Fig. 8. (a) The transient photocurrent responses of the SnS2, SnS2/SnO2-(1–4) and SnO2. (b) Cyclic performance of CC@SnS2/SnO2-2 for the reduction of Cr(VI) solution under visible light irradiation.
above. In addition, SnS2/SnO2-2 showed an approximately twofold improvement in photocurrent response when compared with SnS2 under the same conditions. This supports the band gap data which suggested that the SnS2/SnO2-2 composite would show more efficient charge separation and migration. The stability of the catalyst is also an important parameter in practical applications, therefore recycle experiments for the reduction of Cr(VI) were conducted with CC@SnS2/SnO2-2. As presented in Fig. 8b, the reduction rate of Cr(VI) remained high, 91.16%, after three cycling runs. The XRD pattern (Fig. S7) of the recycled CC@SnS2/SnO2-2-AP was consistent with that of the initial sample with no obvious deviations in the locations of the peaks, indicating that CC@SnS2/SnO2-2 composite is a stable photocatalyst. Moreover, the CC@SnS2/SnO2-2 after three cycle photocatalytic use was further characterized by XPS. As can be seen in Fig. S8, the CC@SnS2/SnO2-2-AP contained
C, Sn, S and O components, as well as Cr contaminants. Meanwhile, the binding energy of Cr 2p3/2 over CC@SnS2/SnO2-2-AP was observed at 577.4 eV (Fig. S8b), which was consistent with Cr(III) in Cr(OH)3 [16]. Additionally, the solution after the cycle photocatalytic reduction of Cr (VI) over CC@SnS2/SnO2-2 was detected three times using ICP-MS with Cr(III) standard curve, in order to confirm the content of Cr ions in solution. The results show that the average contents of Cr ions are 0.150 mg/L, which indicate the Cr(III) was mainly located on the surface of photocatalyst rather than in solution. In conclusion, the above results demonstrate that CC@SnS2/ SnO2-2 has a good stability, and the appearance of Cr(III) further confirms the reduction of Cr(VI) under visible light irradiation. Scheme 2 shows a proposed mechanism for the photocatalytic reduction of Cr(VI) based on the experimental data reported, In general, upon irradiation the semiconductor photocatalyst absorbs
Scheme 2. Schematic illustration of the reaction mechanism for reduction of Cr(VI) over CC@SnS2/SnO2 composite under visible-light irradiation.
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light energy and the electrons (e) in the valence band (VB) are excited to the conduction band (CB), simultaneously producing the same number of holes (h+) in the VB. The e and h+ can either move to the surface of the semiconductor photocatalyst and take part in redox reactions with adsorbed species; or photogenerated e and h+ can recombine, decreasing photocatalytic activity, which is a significant limitation of semiconductor photocatalysts [1,2,16,19,20,33,34]. For CC@SnS2/SnO2 composites under visiblelight irradiation (Eq. (4)), the e in the VB of SnS2 can be excited to the CB and an equal amount of h+ are generated in the VB, but SnO2 shows no capacity to absorb visible light. However, e from SnS2 can transfer to the CB of SnO2 and h+ remain in the VB of SnS2 due to the VB and CB potentials of SnS2 both being more negative than those of SnO2. To some extent, the carbon cloth (CC) in this system transports e and prevents the combination of e and h+. Therefore, e and h+ can be effectively separated and their recombination is retarded, leading to greater photocatalytic activ3+ ity. Finally, Cr2O2 by the photogenerated elec7 is reduced to Cr trons and O2 is produced by oxidation of H2O, as shown in Eqs. (5) and (6).
CC@SnS2 =SnO2 þ hm ! e þ h
ð4Þ
3þ þ Cr2 O2 þ 7H2 O 7 þ 14H þ 6e ! 2Cr
ð5Þ
þ
þ
2H2 O þ 4h ! O2 þ 4Hþ
ð6Þ
4. Conclusions In summary, a series of CC@SnS2/SnO2 composites were successfully prepared using an efficient method. Carbon cloth (CC) not only favored the uniform and vertical growth of SnS2 nanosheets, but also effectively supported catalysts leading to unchanged morphology and mechanical properties during the calcination process. Of the composites prepared (CC@SnS2/SnO2-(1– 4)), CC@SnS2/SnO2-2 exhibited the highest photocatalytic activity compared with pure CC@SnS2. It is believed that the matched band potentials and good interfacial contact between the SnS2 and SnO2 on carbon cloth (CC) can facilitate charge transfer and suppress recombination of electrons and holes, leading to the enhanced photocatalytic reduction of Cr(VI) solution. Importantly, catalyst stability was maintained after three cycles. This study demonstrates that CC@SnS2/SnO2 composites are a promising alternative for the reduction of Cr(VI) in wastewater in the future. Furthermore, the current work could provide a general guiding line for designing catalysts on suitable conductive supports (for example, carbon cloth (CC), carbon fibers (CNF), nickel foam or stainless steel mesh), which would provide opportunities for both fundamental research and promising applications in environmental purification or energy conversion.
Acknowledgements We gratefully acknowledge the financial support provided by the National Key R&D Program of China (2017YFC0210901, 2017YFC0210906), National Natural Science Foundation of China (51573122, 21722607, 21776190), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (17KJA430014, 17KJA150009), the Science and Technology Program for Social Development of Jiangsu (BE2015637) and the project supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2017.12.045.
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