English

• 0.   Introduction

  Cr(Ⅵ) is a priority contaminant in the wastewaters arising from mining, paint making, electroplating and chromate manufacturing^[1]. Nowadays the pollution of Cr(Ⅵ) which seriously exerts passive effects on human life has been a severe social issue in many countries due to its high mobility and strong toxicity in water^[2-3]. A variety of methods such as chemical reduction, adsorption, membrane separation, bacterial degradation and photocatalytic reduction have been researched in order to dispose Cr(Ⅵ)^[4-7]. Among the above technologies, the chemical method converting Cr(Ⅵ) into Cr(Ⅲ) in wastewater has been widely used because in neutral or alkaline solutions Cr(Ⅲ) is precipitated as Cr(OH)₃^[8]. However, it have proved that there are two inevitable drawbacks. Firstly, it requires lots of reductants that are not cost-effective^[2]. What′s more, the latest research has proved that soluble Cr(Ⅲ) still has high toxicity to some microor-ganisms and aquatic organisms^[9-10] and may be oxidized to Cr(Ⅵ) again by some kind of bacteria^[11]. Above all, in order to maintain the ecological balance and consider cost savings, it is indispensable and urgent to develop new materials that can both remove Cr(Ⅵ) and Cr(Ⅲ) efficiently.

  Recently, photocatalytic reduction of Cr(Ⅵ) has attracted more attention in green chemistry technology on account of its low cost, reusability, use of clean solar energy and no need of any hazardous chemicals^[12-13]. Several photocatalysts have been put into use in dealing with environmental pollution, such as TiO₂^[14], SnO₂^[15], CdS^[16], CuO^[17], WO₃^[18], ZnO^[19], CdO^[20], Fe₂O₃^[21], C₃N₄^[22], BiVO₄^[23], BiOI^[24], Bi₂S₃^[25] and SnS₂^[26]. However, the photocatalytic efficiency, limited by light absorption ability and separation efficiency of photogenerated electron-hole pairs, is a primary drawback of semi-conductor photocatalysts^[27]. Among those photocata-lysts, it has been proved that SnS₂ can reduce Cr(Ⅵ) with high efficiency due to its wider light absorption and adsorb Cr(Ⅲ) ability^[28]. In this case, separation of photogenerated electrons and holes is vital factor to improve the photocatalytic activity of SnS₂. In addition, modifying the band structure of photocatalyst by doping elements is an effective approach to enhance separation of photoinduced electrons and holes. At present a few researches have been reported about doping SnS₂ with In, Ce, Zn, Cu and C as photo-catalysts^[29-33]. Among them, only Kiruthigaa et al. investigated the Ce doped SnS₂^[29]. It reported the synthesis, structural and optical properties of Ce-doped SnS₂ nanoflakes and measured photocatalytic ability to degrade reactive red 120 dye^[29]. However, in this previous paper Ce-doped SnS₂ was synthesized by solid state reaction requiring higher temperature and longer heating/cooling times. The photocatalytic process was driven by UV light and the work did not evaluate photocatalytic reduction ability of Ce-doped SnS₂. Hence, it is significant to develop a new method to synthesize Ce-doped SnS₂ nanomaterials with low cost and study its photocatalytic reduction ability under the visible-light irradiation.

  In this paper, Ce-doped SnS₂ was synthesized successfully via a facile hydrothermal method and investigated in terms of microstructure, morphology, light absorption and the electrochemical properties. Photocatalytic activities were researched by the reduction of Cr(Ⅵ) under visible-light irradiation.

  1.   Experimental

  1.1   Preparation

  All reagents used in the experiments were of analytical grade and used as received without further purification. The Ce-doped SnS₂ was prepared simply by a direct hydrothermal method. In a typical synthetic process, 5 mmol of citric acid (C₆H₈O₇) and 1 mmol of tin(Ⅳ) chloride pentahydrate (SnCl₄·5H₂O) were dissolved into 30 mL of deionized water. Afterward, 8 mmol of thiourea (CH₄N₂S) was added into the above solution. Then, different molar ratio of cerium(Ⅲ) nitrate hexahydrate (Ce(NO₃)₃·6H₂O) was dissolved, and the solution was exposed to magnetic stirring treatment for 30 min. Finally, the mixture was transferred to 50 mL Teflon lined stainless steel autoclaves for the reaction at 180 ℃ for 20 h. The sample was then cooled to room temperature naturally and washed with deionized water and ethanol for several times by centrifugation at 7 000 r·min^-1 for 5 min. The collected yellow product was dried at 80 ℃ for 12 h in a dryer at last. The similar procedures were carried out to prepare the SnS₂ with different molar ratio of Ce as 0%, 1%, 3%, 5% and 7%. The as-synthesized samples were denoted as x% Ce/SnS₂, where x% refers to the molar ratio of Ce to SnS₂.

  1.2   Characterization

  The phase and composition of as-prepared samples were examined by an Rigaku Ultima Ⅳ XRD equipped with Cu Kα radiation (λ=0.154 18 nm) at a scan rate of 10°·min^-1 with 2θ=10°~80°. The operating voltage was 40 kV and the current was 40 mA. The morphology of the samples was performed on JSM-7100F field emission scanning electron microscopy (FESEM), and the operating voltage was 5 kV. The OXFORD INCA X-act EDS equipped on FESEM was used for chemical analysis and elemental mapping. The N₂ adsorption-desorption isotherms were performed to measure pore size distribution and the Brunauer-Emmett-Teller (BET) surface area of as-prepared samples by a Quantachrome Autosorb. UV-Vis DRS of the sample were recorded with a UV-Vis spectro-photometer (UV-1950, Beijing Purkinje General Instrument Co. Ltd., China). The chemical state of Cr after photocatalytic reaction was determined by AXIS SUPRA X-ray photoelectron spectroscopy (XPS) with Al Kα X-ray source (λ=0.833 89 nm).

  Electrochemical measurements were carried out on a CHI660D electrochemical workstation by a standard three-electrode system with a saturated calomel electrode (SCE) as reference electrode, a platinum wire as counter electrode, and a working electrode. The working electrodes were processed by coating method. Briefly, 10 mg of photocatalyst was suspended in the slurry of the combination of 5% (w/w) nafion solution and anhydrous ethanol with volume radio of 0.25 which was then dropped onto a 2.5 cm×1.5 cm fluorine-tin oxide (FTO) glass electrode with a sheet resistance of 15 Ω. Afterwards, the films were dried under ambient conditions. The electrochemical impedance spectroscopies (EIS) were measured at the open circuit potential. The Mott-Schottky measure-ments were executed at fixed frequencies of 500 and 1 000 Hz with 5 mV amplitude. And the flat band potentials (V_fb) were obtained from Mott-Schottky plots. Before and during all measurements, the electrolyte (0.1 mol·L^-1 Na₂SO₄) was purged with N₂. Each experiment was repeated three times to guarantee the accuracy of date acquisition. The current-time curve tests were conducted by applying a potential (+0.5 V vs SCE) and extra light source to the working electrode.

  1.3   Photocatalytic reduction of Cr(Ⅵ)

  The photocatalytic activities of as-prepared Ce-doped SnS₂ and pure SnS₂ were carried out by the photocatalytic reduction of Cr(Ⅵ). Cr(Ⅵ) mainly exists in the form of Cr₂O₇^2- in wastewater^[10], so potassium dichromate solution is selected to imitate aqueous Cr(Ⅵ). Typically, 300 mg photocatalyst was added into 100 mL of 40 mg·L^-1 K₂Cr₂O₇ aqueous solution adjusted pH=3 with HCOOH in a customized photocatalytic reactor. Then, the mixed suspension was magnetically stirred in the dark for 1 h to reach adsorption and desorption equilibrium. The suspension was irradiated by a 300 W Xe lamp equipped with a cut-off filter (λ > 420 nm) under magnetic stirring, and the distance from the light source to the reactor was about 10 cm. During illumination, at certain time intervals, 5 mL of suspension was taken from the reactor and centrifuged to remove the photocatalyst. The Cr(Ⅵ) content variation was evaluated using a UV-Vis spectroscopy (UV-1950, Beijing Purkinje General Instrument Co. Ltd., China) via a 1, 5-diphenylcarbazide method by testing the absorbance at 540 nm^[34]. The reduction efficiency of Cr(Ⅵ) was calculated by C/C₀, where C₀ is the initial Cr(Ⅵ) concentration and C is the actual Cr(Ⅵ) concen-tration.

  2.   Results and discussion

  The XRD patterns of undoped SnS₂ and Ce-doped SnS₂ are displayed in Fig. 1. The patterns of SnS₂ and Ce/SnS₂ match well with that of berndtite-2T (PDF No.23-0677) without any other phases formed. It indicates that the Ce doping had not damaged the crystal structure of SnS₂. Furthermore, the chara-cteristic diffraction peaks of undoped SnS₂ were sharper and stronger than that of Ce-doped SnS₂, which confirmed that Ce doping was effective and decreased crystallite size of SnS₂. To further research the surface structure of the composites, the EDS spectrum of 5% Ce/SnS₂ is shown in Fig. 2. It confirms the presence of Ce elements in the Ce/SnS₂ products. The calculated atomic ratio of Sn to S is 1:1.63 for SnS₂ and 1:1.71 for 5% Ce/SnS₂, respectively, which implies the possibility that some Sn ions may be replaced by Ce ions in Ce doped SnS₂ samples. The elemental mappings of 5% Ce/SnS₂ was shown in Fig. 3. The results clearly revealed the presence and uniform distribution of Ce in 5% Ce/SnS₂.

  Figure 1

  

  Figure 1.  XRD patterns of undoped SnS₂ and Ce-doped SnS₂

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  Figure 2

  

  Figure 2.  EDS spectra of 5% Ce/SnS₂

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  Figure 3

  

  Figure 3.  Elemental mappings of 5% Ce/SnS₂

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  The morphology of the as-prepared products was investigated by SEM as shown in Fig. 4(a~f). The images (Fig. 4(a~b)) exhibit the undoped SnS₂ poss-essing rose-like structures with an average size of 900 nm. Each uniform rose-like structure was composed of hexagonal platelets with smooth surfaces. Scheme 1 displays the formation process of the rose-like structure of SnS₂. From the Scheme 1, it can be observed that the SnS₂ grow up based on screw growth theory^[35]. However, with the increasing of Ce-doped contents rose-like structures of SnS₂ were gradually destroyed, which was probably attributed to that Ce doping causes lattice distortion and restrains the spiral growth. Moreover, the effects of Ce doping on surface structure of SnS₂ were investigated and the specific surface areas of undoped SnS₂ and Ce-doped SnS₂ were listed in Table 1. It reveals that the specific surface area of SnS₂ increased with varied Ce contents from 0% to 5%, while decreased from 5% to 7%. The enlargement of specific surface area can generate more active site, and further enhance the photocatalytic efficiency of SnS₂.

  Figure 4

  

  Figure 4.  SEM images of undoped SnS₂ (a, b), 1% Ce/SnS₂ (c), 3% Ce/SnS₂ (d), 5% Ce/SnS₂ (e) and 7% Ce/SnS₂ (f)

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  Scheme 1

  

  Scheme 1.  Formation process of the rose-like structure

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  Table 1

  

  Table 1.  Specific surface area of undoped SnS₂ and Ce-doped SnS₂

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  Samples	Specific surface area / (m2·g-1)
  SnS2	18
  1% Ce/SnS2	24
  3% Ce/SnS2	39
  5% Ce/SnS2	44
  7% Ce/SnS2	36

  As well-known, the optical absorption property which is related to the electronic structure feature plays a significant role in photocatalytic performance. The optical absorption properties of undoped SnS₂ and Ce-doped SnS₂ powders were examined by UV-Vis DRS as shown in Fig. 5. It could be observed that undoped SnS₂ and Ce-doped SnS₂ all exhibited broad response spectrum in the visible light region, which implied the possibility of reduction of Cr(Ⅵ) under visible-light irradiation. Furthermore, it reveals that Ce-doped SnS₂ exhibited enhanced absorption in visible-light region due to the fact that Ce doping affects the electronic structure of SnS₂ leading to the stronger absorption. The band gap energy (E_g) of the direct band gap semiconductors material could be calculated based on the following equation^[36]:

  $ \alpha h\nu = B{\left( {h\nu - {E_{\rm{g}}}} \right)^{1/2}} $

  Figure 5

  

  Figure 5.  UV-Vis DRS spectra of undoped SnS₂ and Ce-doped SnS₂

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  where α, h, ν and B are absorption coefficient, planck constant, light frequency and a constant related to the material, respectively. The curves of (αhν)² versus hν for undoped and Ce-doped SnS₂ are plotted in Fig. 6. The values of E_g are determined by extrapolating the linear portion of the curves to (αhν)²=0. The E_g values of SnS₂, 1% Ce/SnS₂, 3% Ce/SnS₂, 5% Ce/SnS₂ and 7% Ce/SnS₂ are estimated to be 2.05, 2.03, 1.98, 1.98 and 2.02 eV, respectively. The E_g of undoped SnS₂ is smaller than that of previous reports^[37], which is more beneficial to produce photogenerated carriers. Moreover, it is observed that the E_g values of Ce-doped SnS₂ decreased slightly with Ce-doped contents from 0% to 3%, while increased with Ce-doped contents from 5% to 7%, indicating that the change of E_g values is not homogeneous with increasing Ce doping concentration, most probably due to a combination of several contributions, such as substitution of Ce³⁺ ions, Sn⁴⁺ vacancies, distortion of the crystal lattice and microstructural defects^[29].

  Figure 6

  

  Figure 6.  Band gap energies determined by (αhν)² versus hν of undoped SnS₂ and Ce-doped SnS₂

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  In order to calculate the potential of the photo-induced electrons in the sample, the impedance spectrum of undoped SnS₂ and 5% Ce/SnS₂ was measured and plotted based on Mott-Schottky principle as shown in Fig. 7. The results reveal that the flat band potential of undoped SnS₂ and 5% Ce/SnS₂ was -0.32 V (-0.08 V vs NHE) (Fig. 7a) and -0.25 V (-0.01 V vs NHE) (Fig. 7b), respectively. That is to say, the conduction band potentials of undoped SnS₂ and 5% Ce/SnS₂ samples are approximately equal to -0.08 V (vs NHE) and -0.01 V (vs NHE), respectively, given that it has been well accepted that the flat band potential is regarded as the conduction band potential. Because the potentials of the photo-induced carriers are approximately equal to that of the energy bands which they reside in^[38], the potentials of the photo-induced electrons of undoped SnS₂ and 5% Ce/SnS₂ samples were about -0.08 V (vs NHE) and -0.01 V (vs NHE), respectively, which was far lower than the reduction potential of K₂Cr₂O₇ (1.36 V vs NHE, in acid solution), indicating that it likely played a part in photocatalytic reduction of aqueous Cr(Ⅵ) under visible-light irradiation. Meanwhile, according to band gap energies of SnS₂ and 5% Ce/SnS₂ calculated by DRS, the valence band potentials of undoped SnS₂ and 5% Ce/SnS₂ were both about 1.97 V (vs NHE), that means Ce doping did not change the potential of photo-generated holes. From the above, it can be testified that Ce doping resulted in the appearance of doping energy level at the bottom of the conduction band, and further causes the conduction band potentials of 5% Ce/SnS₂ is lower than that of SnS₂. On the basis of conduction band potentials and band gap energy, the energy band structures of undoped SnS₂ and 5% Ce/SnS₂ are exhibited in Fig. 8.

  Figure 7

  

  Figure 7.  Mott-Schottky plots of undoped SnS₂ (a) and 5% Ce/SnS₂ (b)

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  Figure 8

  

  Figure 8.  Energy band structures of undoped SnS₂ and 5% Ce/SnS₂

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  In order to prove that the induced electrons participate in photoreduction of aqueous Cr(Ⅵ), the current-time curve of the undoped SnS₂ and Ce-doped SnS₂ was recorded under lamp irradiation with 20 s on/off cycles as shown in Fig. 9. When the lamp was turned on, the current density of undoped SnS₂ and Ce-doped SnS₂ enlarged to a maximum point which was much higher than the current density at the situation that the lamp was turned off. This showed the excellent ability of as-made samples responding to visible-light and transferring the photo-induced electrons^[39]. And it is observed that the visible-light response of 5% Ce/SnS₂ was far stronger than that of other samples, which implies that 5% Ce/SnS₂ maybe possess higher photocatalytic efficiency than others. In addition, it should be noted that the photo current density kept constant as before after 40 cycles, which demonstrated the commendable photostability of the as-prepared samples. In conclusion, undoped SnS₂ and Ce-doped SnS₂ all could realize photocatalytic reduction of Cr(Ⅵ) in theory, and 5% Ce-doped SnS₂ likely had the best photocatalytic property due to its good light harvesting ability and separation efficiency of photo induced carriers.

  Figure 9

  

  Figure 9.  Photocurrent responses of undoped SnS₂ and Ce-doped SnS₂ at the external bias of 0.5 V versus SCE electrode

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  In order to prove the above reasoning, the photocatalytic activities of undoped SnS₂ and Ce-doped SnS₂ were conducted by reduction of Cr(Ⅵ) with HCOOH as a hole scavenger and pH value regulator. In previous reports, it was proved that pH of the solution is a key parameter for photocatalytic efficiency, and the photocatalytic efficiency of reduction of Cr(Ⅳ) in low pH level was far higher than that in high pH level^[40]. Hence, the following photocatalytic reduction tests of soluble Cr(Ⅵ) were executed under pH=3. Fig. 10 shows time-dependence of photocatalytic reduction of aqueous Cr(Ⅵ) under visible-light irradiation in the presence of 5% Ce/SnS₂ sample. It is observed that the characteristic peak of soluble Cr(Ⅵ) was 540 nm using the diphenylcarbazide method, which is consistent with previous reports^[41]. Typically, the characteristic peak of 40 mg·L^-1 K₂Cr₂O₇ solution almost disappear under visible-light irradiation for mere 20 min in the presence of 5% Ce/SnS₂. Fig. 11 shows that the photocatalytic activities of undoped SnS₂ and Ce-doped SnS₂ in the reduction of Cr(Ⅵ) under visible light were affected by the amount of Ce doping. It can be seen that in the absence of any photocatalyst the reduction of aqueous Cr(Ⅵ) hardly occurred under visible-light irradiation for 25 min and in the presence of undoped 5% Ce/SnS₂ the amount of aqueous Cr(Ⅵ) changed very little under dark cond-itions, while it proceeded quite rapidly in the presence of undoped SnS₂ and Ce-doped SnS₂. Nevertheless, the photocatalytic activities of as-prepared samples were greatly different, indicating that the amount of Ce doping played a vital role in photocatalytic process. Accompanied by the increase of Ce ions concentration the photocatalytic capability of SnS₂ increased and the 5% Ce/SnS₂ sample showed the best photocatalytic reduction ability. And further the increased concen-tration of Ce ions resulted in the decrease of photo-catalytic activity. Based on these results, it can be seen that undoped SnS₂ and Ce-doped SnS₂ could catalyze reduction of Cr(Ⅵ) under visible-light irradia-tion, and 5% Ce-doped SnS₂ had the best photo-catalytic property which agreed well with the above inference. Then, the chemical state of Cr after reaction was characterized by XPS. The binding energy of Cr2p_3/2 and Cr2p_1/2 at 577.4 and 586.4 eV respectively were observed from Fig. 12, which corre-sponded to Cr(Ⅲ), indicating that the Cr(Ⅵ) was reduced to Cr(Ⅲ) successfully^[26].

  Figure 10

  

  Figure 10.  Time dependent UV-Vis absorption spectrum in the presence of 5% Ce/SnS₂

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  Figure 11

  

  Figure 11.  Variation of concentration of Cr(Ⅵ) under visible-light irradiation time for different samples

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  Figure 12

  

  Figure 12.  XPS spectra of Cr on 5% Ce/SnS₂ photocatalyst after reaction

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  3.   Conclusions

  Based on the screw dislocation growth mechanism, the rose-like Ce-doped SnS₂ were obtained through a one-step hydrothermal method and the rose-like morphology was ruined gradually with the increment of Ce ions concentration. The doping element of Ce was checked by EDS and it could enhance photo-absorption ability of SnS₂ effectively because Ce doping narrows the energy band gap probably due to the appearance of doping energy level. Moreover, the Ce doping of different proportions could improve the separation efficiency of photo-generated carriers to varying degrees. And the 5% Ce/SnS₂ sample possessed the highest photocurrent indicating the best activities which was verified by photocatalytic reduction of Cr(Ⅵ) under visible-light irradiation (λ > 420 nm).

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• 

  Figure 1  XRD patterns of undoped SnS₂ and Ce-doped SnS₂

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  Figure 2  EDS spectra of 5% Ce/SnS₂

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  Figure 3  Elemental mappings of 5% Ce/SnS₂

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  Figure 4  SEM images of undoped SnS₂ (a, b), 1% Ce/SnS₂ (c), 3% Ce/SnS₂ (d), 5% Ce/SnS₂ (e) and 7% Ce/SnS₂ (f)

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  Scheme 1  Formation process of the rose-like structure

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  Figure 5  UV-Vis DRS spectra of undoped SnS₂ and Ce-doped SnS₂

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  Figure 6  Band gap energies determined by (αhν)² versus hν of undoped SnS₂ and Ce-doped SnS₂

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  Figure 7  Mott-Schottky plots of undoped SnS₂ (a) and 5% Ce/SnS₂ (b)

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  Figure 8  Energy band structures of undoped SnS₂ and 5% Ce/SnS₂

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  Figure 9  Photocurrent responses of undoped SnS₂ and Ce-doped SnS₂ at the external bias of 0.5 V versus SCE electrode

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  Figure 10  Time dependent UV-Vis absorption spectrum in the presence of 5% Ce/SnS₂

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  Figure 11  Variation of concentration of Cr(Ⅵ) under visible-light irradiation time for different samples

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  Figure 12  XPS spectra of Cr on 5% Ce/SnS₂ photocatalyst after reaction

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  Table 1.  Specific surface area of undoped SnS₂ and Ce-doped SnS₂

  Samples	Specific surface area / (m2·g-1)
  SnS2	18
  1% Ce/SnS2	24
  3% Ce/SnS2	39
  5% Ce/SnS2	44
  7% Ce/SnS2	36

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