Enhanced photocatalytic activity of BiOI under visible light irradiation by the modification of MoS₂

Abstract

3D hierarchical BiOI nanostructures modified with MoS₂ were successfully fabricated through a simple solvent thermal process. The prepared heterostructured BiOI–MoS₂ nanocomposite exhibited excellent photocatalytic performance in the degradation of methyl orange under visible light irradiation. The photocatalytic activity of BiOI–MoS₂ increases with the increase of MoS₂ content first, then it decreases when MoS₂ content is over 0.5 wt%. The degradation efficiency of methyl orange could achieve 95% within 90 min in the presence of BiOI–0.5 wt% MoS₂, which is about two times higher than that of pristine BiOI nanostructures. The obviously improved photocatalytic performance of BiOI–MoS₂ could be mainly attributed to the significantly enhanced separation efficiency of photogenerated charge carriers. However, too much MoS₂ loading on the surface of BiOI will prevent light from reaching surface of BiOI and limit the efficiency of charge separation. Based on active species trapping experiments, holes (h⁺) were proved to be the main reactive species in the prepared BiOI–MoS₂ system.

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1 Introduction

Bismuth oxyhalides (BiOX), in which X is a halogen element, is a layered compound having an open, layered crystal structure consisting of positive [Bi₂O₂] layers sandwiched between two slabs of negative halogen ions.^1,2 These unique layered structures make BiOX exhibit good adsorption properties and excellent photocatalytic activities in waste water and indoor air purification.^3–5 Among the BiOX family, BiOI is found to be a promising photocatalyst for the removal of organic pollutants under visible light irradiation since BiOI possesses the smallest band gap (E_g = 1.73–1.92 eV) and strong absorption.^6–8 However, pure BiOI is always poor in photocatalytic activity because of the rapid recombination of photoinduced electron–hole pairs.^9,10 It is important to facilitate the separation of electron–hole pairs and thus improve the photocatalytic activities of BiOI. Development of a highly efficient visible light response of BiOI based catalysts to meet the requirement of practical application still remains a challenge.

It is well known that semiconductor combination is an efficient way for the separation of photoinduced carries, and thus raises the photocatalytic efficiency since a heterojunction structure may be formed between different semiconductors.^11,12 This heterostructure composite photocatalyst results in an efficient charge separation at the interface. Several heterostructure based on BiOI have been reported which showed higher photocatalytic activity than pure BiOI, such as BiOI/WO₃,^13,14 BiOI/AgI,^15–17 BiOI/Ag,^18–20 BiOI/Ag/AgI,^21–23 AgI/BiOI–Bi₂O₃,²⁴ BiOI/Bi₂WO₆,²⁵ BiOI/Ag₃PO₄,²⁶ BiOI/Bi₂MoO₆,²⁷ BiOI/ZnSn(OH)₆,²⁸ BiOI/BiOBr.²⁹

MoS₂ has a sandwich interlayer structure composed of three stacked atom layers (S–Mo–S) which are bonded together by weak van der Waals forces.³⁰ The monolayer MoS₂ has been successfully synthesized, and get particular attention in photovoltaic application due to its distinctive electronic, optical, large surface area and catalytically active sites.^31–33

In view of the fact that MoS₂ and BiOI are similar in structure, it is envisaged that a BiOI/MoS₂ composite can have the merits of MoS₂ and BiOI, consequently showing high photocatalytic activity under visible light. However, seldom research has been reported on BiOI/MoS₂ composite photocatalytic performance until now. Recently, 2D structure BiOI/MoS₂ (ref. 34) and 3D hierarchical BiOI/MoS₂/AgI³⁵ were reported which exhibited good photocatalytic activity towards rhodamine B degradation. The degradation efficiencies of RhB after 50 min were 20%, 40% and 96% by 3D hierarchical BiOI, BiOI/AgI and MoS₂/BiOI/AgI under simulated solar irradiation, respectively. However, the photocatalytic activity of 3D hierarchical BiOI/MoS₂ under visible light irradiation has not been reported yet. Therefore, 3D hierarchical BiOI/MoS₂ composite was fabricated by a simple solvent thermal process in this work, and the results show that the as-prepared BiOI/MoS₂ composites exhibited an excellent photocatalytic activity for the degradation of methyl orange (MO) in visible light irradiation. The possible photocatalytic mechanism of BiOI/MoS₂ heterostructure to MO degradation is proposed by adding radical scavengers in the photocatalytic system.

2 Experimental methods

2.1 Synthesis of MoS₂, BiOI and BiOI–MoS₂ composites

All chemical reagents were of analytical grade and used without further purification in this study. For the preparation of MoS₂, 5 mmol Na₂MoO₄ and 20 mmol CS(NH₂)₂ were dissolved in deionized water with vigorous stirring for 1 h at room temperature. Then, the mixture was transferred into a 100 ml Teflon-lined stainless-steel autoclave. The autoclave was kept at 483 K for 24 h. The sample was obtained by filtration, washed with deionized water and absolute ethanol many times, and then dispersed into absolute ethanol and dried for 12 h at 323 K.

BiOI/MoS₂ composite were synthesized by an ethylene glycol (EG)-assisted solvothermal method. In a typical synthesis procedure, 2 mmol Bi(NO₃)₃·5H₂O and 2 mmol KI was dissolved in 15 ml ethylene glycol under ultrasonication for 20 min respectively first. Then, the two solutions were mixed and some MoS₂ was added to the mixture. The mixture was under vigorous stirring for 15 min, then it was transferred into a100 ml Teflon-lined stainless-steel autoclave. The autoclave was kept at 433 K for 12 h. The resultant precipitates were collected, repeatedly washed with deionized water and ethanol, and dried for 12 h at 333 K. BiOI/MoS₂ composites were named as BiOI- (wt% of MoS₂) MoS₂. For comparison, BiOI was also fabricated as the same method without the presence of MoS₂.

2.2 Characterization

The morphologies of the prepared BiOI, MoS₂ and BiOI–(0.5%) MoS₂ samples were characterized by a PANalytical B.V. MPDDY2094 X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5406 Å). Scanning electron microscopy (SEM) pictures and quantitative standard microanalyses were obtained using an energy dispersive X-ray analysis (EDS) with a Zeiss ultra plus FESEM apparatus. UV-vis diffuse reflectance spectrum (DRS) was recorded on a Perkin Elmer Lambda 35 UV-vis spectrophotometer with BaSO₄ as a reference, and the scanned range being 200–800 nm against barium sulfate standard. N₂ adsorption–desorption isotherm was conducted on a Micromeritics ASAP-2020-HD88 volumetric gas sorption apparatus using 99.999% pure N₂.

The photoluminescence (PL) measurements were performed on a Hitachi F7000 Fluorescence Spectrophotometer with an excitation wavelength of 325 nm at room temperature. The photocurrents were performed in 0.5 mol l⁻¹ Na₂SO₄ solution using an electrochemistry workstation (CHI 660, China) with a three-electrode system. The obtained sample was served as the working electrode (1 mg of the as-prepared sample was dispersed in 0.2 ml of ethanol and 0.2 ml of ethanol mixture to produce a suspension, which was then dip-coated onto an ITO glass electrode). A platinum plate and a saturated Ag/AgCl were used as the counter electrode and reference electrode, respectively. A 300 W Xe lamp with main emission wavelength 420 nm in the range of 420–780 nm along with a power of 80 mW cm⁻² was utilized as the photosource.

2.3 Photocatalytic activity tests

The photocatalytic activity tests of the obtained samples were investigated by evaluating in terms of the degradation of methyl orange (MO) solution under the simulated sunlight irradiation. The photochemical reactor contains 40 mg catalysts and 5.0 × 10⁻⁵ mol l⁻¹ of 100 ml MO. A 300 W Xe lamp with main emission wavelength 420 nm in the range of 420–780 nm along with a power of 20 mW cm⁻² (Beijing Science and Technology Co., Ltd. Park Philae), was set inside a cylindrical reactor, and surrounded by a circulating water jacket to cool the lamp and minimize infrared radiation. The methyl orange aqueous solutions with the added catalysts were kept in the dark for some minutes to establish the adsorption equilibrium of the methyl orange for the catalyst before exposure to the simulated sunlight. After exposure to the radiation for different intervals, the UV-vis spectrophotometer (TU-1900, Beijing Purkinje General Instrument Co., Ltd.) was used to determine the solution concentration of methyl orange. The absorbance accuracy is ±0.002 Abs (0–0.5 Abs), and ±0.004 Abs (0.5–1.0 Abs).

3 Results and discussion

3.1 Structure and morphology

Fig. 1 shows the XRD patterns of prepared MoS₂, BiOI and BiOI–MoS₂ samples. The diffraction peaks of 14.43° (002), 33.61° (100), 39.88° (103), 58.87° (110) can be indexed to MoS₂ (JCPDS card no. 01-087-2416), while 24.57° (101), 29.55° (102), 32.13° (110), 36.56° (103), 45.59° (104), 50.39° (114), 55.19° (212), 66.44° (214), 75.48° (310) diffraction peaks are belong to tetragonal phase BiOI (JCPDS card no. 00-010-0445). No peaks of other impurities are observed in the diffraction peaks of the as prepared BiOI and MoS₂, indicating that the high purity and single phase of the prepared BiOI and MoS₂ products. Moreover, diffraction peaks of MoS₂ can be found hardly because of its relatively low content in the prepared BiOI–MoS₂ samples.

Fig. 1 X-ray diffraction patterns of prepared MoS₂, BiOI and BiOI–MoS₂ samples.

FESEM images of the MoS₂, BiOI and BiOI–0.5% MoS₂ samples are shown in Fig. 2.

Fig. 2 FESEM images of MoS₂ (a), BiOI (b) and BiOI–0.5% MoS₂ (c and d).

As shown in Fig. 2(a), the synthesized MoS₂ sample structure is composed of curled and interlaced nanosheets, while BiOI and BiOI–0.5% MoS₂ samples exhibit a similar 3D flower-like hierarchical spherical structure. The flower-like structures have wide size distribution and diameters ranging from approximately 600 nm to 1.5 μm. Moreover, these microspheres are not completely isolated from each other, but often coupled together. Compare with pristine BiOI, BiOI–0.5% MoS₂ has smaller spherical size. Close examination of the morphology of reveals that the hierarchical microsphere consisting of loosely stacked nanoplates with an average slide length of about 200 nm. This specific hierarchical nanostructures make the samples have an obvious high surface-to-volume ratio, which is favourable for pollutant molecules transportation and light injection.^36,37

3.2 Optical properties

The optical properties of the samples were investigated by ultraviolet-visible diffuse reflectance spectra and depicted in Fig. 3(a), and the curves of (αhν)^1/2 versus hν derived from the UV-visible spectra are presented in Fig. 3(b). As shown in Fig. 3(b), the loading of MoS₂ can significantly affect the band gap energy compare to pristine BiOI, and the extent of band gap energy greatly depends upon the loading level. The pristine BiOI and MoS₂ exhibit a band gap energy of 1.87 eV and 1.71 eV respectively, which is close to the values reported in other literatures.^1,38,39 Moreover, BiOI–MoS₂ composites exhibit larger band gap energy than that of BiOI, as BiOI–MoS₂ composites exhibit a band gap energy of about 2.25 eV (BiOI–0.1% MoS₂), 2.30 eV (BiOI–0.5% MoS₂) and 2.25 eV (BiOI–1% MoS₂) respectively. Although the addition of MoS₂ in BiOI enlarged the band gap of BiOI, the absorbance of BiOI–MoS₂ samples are much stronger than that of pristine BiOI in the visible region, which allows the BiOI–MoS₂ catalysts to utilize more visible light during photocatalytic degradation and is expected to enhance the photocatalytic behavior.⁴⁰

Fig. 3 (a) UV-Vis diffused reflectance spectra of pristine MoS₂, BiOI and BiOI–MoS₂ samples; (b) the plots of transformed Kubelka–Munk function versus the energy of light.

3.3 Adsorption–desorption analysis

The N₂ adsorption–desorption isotherms of the pristine BiOI and BiOI–0.5% MoS₂ samples are shown in Fig. 4. Both of the isotherms exhibit a typical IV isotherm with a hysteresis loop, suggesting their mesoporous features, which is also confirmed by the corresponding pore size distribution curve (inset in Fig. 4). As shown in Table 1, the BET surface areas were calculated to be 52.87 and 48.81 m² g⁻¹ for pristine BiOI and BiOI–0.5% MoS₂ respectively. In addition, the average mesoporous diameter of BiOI–0.5% MoS₂ is larger than that of pristine BiOI. The BET surface area values did not have obvious difference, which indicates that the BET surface area has little effect on the photocatalytic performance of obtained BiOI and BiOI–0.5% MoS₂ samples.

Fig. 4 N₂ adsorption and desorption isotherms and the corresponding pore-size distribution (inset) for the BiOI and BiOi–0.5% MoS₂ samples.

Table 1 Textural features of BiOI and BiOI–0.5% MoS₂ samples

Semiconductor	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore diameter (nm)
BiOI	52.87	0.23	13.60
BiOI–0.5% MoS2	48.81	0.24	16.57

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3.4 FTIR analysis

The FTIR spectra of pure BiOI and BiOI–MoS₂ hybrids are shown in Fig. 5. The peak at around 520 cm⁻¹ is corresponded to the stretching vibration of the Bi–O.⁴¹ Compared with pure BiOI, the stretching vibration of Bi–O in BiOI–MoS₂ samples shift to higher frequency region slightly, indicating that there are strong interaction between BiOI and MoS₂.^34,42 The strong interaction between BiOI and MoS₂ may be beneficial to the separation of photo-induced charge carriers. In addition, the peak at around 1620 cm⁻¹ is ascribed to the bending vibration absorption of free water molecules, and the broad peak at around 3420 cm⁻¹ is related to the stretching vibrations of O–H,^43,44 which could serve as anchor to the contaminant molecules.

Fig. 5 FTIR spectra of BiOI, MoS₂ and BiOI–MoS₂ samples.

3.5 Photocatalytic performance

The degradations of methyl orange dye by BiOI–MoS₂ samples under visible light irradiation are shown in Fig. 6(a). Error bars in Fig. 6 represent SDs calculated from three parallel experiments. About 50% MO was photodegraded by pristine BiOI, and the introduction of MoS₂ into BiOI can enhance the degradation efficiency. BiOI–0.5% MoS₂ sample exhibits the best excellent photocatalytic activity towards MO, and the photodegradation rate could reach 95% under 90 min of visible light irradiation. Compared to other reported BiOI composites, the prepared BiOI–MoS₂ composites shows higher photodegradation properties to MO as shown in Table 2.

Fig. 6 Degradation properties of MO by BiOI–MoS₂ samples under visible light irradiation. (a) The relationship between degradation efficiency of MO and irradiation time. (b) Recycling property of BiOI–0.5% MoS₂ in the degradation of MO.

Table 2 Photodegradation properties of some reported BiOI based composites to MO solution under visible light irradiation

Photocatalyst	Photodegradation conditions (photocatalyst mass, MO solution concentration, MO solution volume, photodegradation time)	Photodegradation rate	Reference number
BiOI–1 wt% WO3	100 mg, 10 mg l^−1, 150 ml, 100 min	70%	14
BiOI–20 wt% AgI	100 mg, 20 mg l^−1, 100 ml, 180 min	90%	15
BiOI–Ag	100 mg, 10 mg l^−1, 50 ml, 120 min	90%	18
BiOI–0.6 wt% Ag	50 mg, 10 mg l^−1, 50 ml, 240 min	80%	20
BiOI–Ag–AgI (6 wt% Ag–AgI)	100 mg, 10 mg l^−1, 50 ml, 180 min	93%	22
BiOI–Ag3PO4 (1 : 2 mol ratio)	50 mg, 3.2 mg l^−1, 50 ml, 100 min	70%	26
BiOI–0.5 wt% MoS2	40 mg, 16 mg l^−1, 100 ml, 90 min	95%	This report

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The stability is one of the important considerations for the application of a photocatalyst. Recycling experiments on the photocatalytic degradation of methyl orange dye were carried out as shown in Fig. 6(b). The degradation rate is about 80% for the second and third cycles, while it could maintain at 60% after four successive cycles. Fig. 7 shows the microstructure of BiOI–0.5% MoS₂ which had gone through four cycles photocatalysis tests. Compared with the microstructure of BiOI–0.5% MoS₂ before cycles (Fig. 2(c) and (d)), there are more small particles on the surface of spherical BiOI, which indicates the disaggregation of the sponge structure of BiOI. The decrease of the photocatalytic activity may be due to the disaggregation of the sponge structure of BiOI.

Fig. 7 FESEM images of BiOI–0.5% MoS₂ after four photocatalysis cycles.

Photoluminescence (PL) spectrum and photocurrent generation are effective techniques to evaluate the efficiency of charge carrier trapping, transfer and separation in semiconductor materials and surface defects of samples. Fig. 8(a) shows the room temperature PL spectra of prepared BiOI and BiOI–MoS₂ composites with an excitation wavelength of 325 nm. There are three intense emission peaks centered around 370 nm, 590 nm and 620 nm. The BiOI–0.5% MoS₂ exhibited the lowest peak intensity, while BiOI exhibited the highest peak intensity. This result implies BiOI–0.5% MoS₂ has the lowest rate of recombination of photogenerated charge carriers, while BiOI has the highest rate of recombination of photogenerated charge carriers.

Fig. 8 (a) Photoluminescence spectra of BiOI and BiOI–MoS₂ composites; (b) transient photocurrent response of BiOI, MoS₂ and BiOI–0.5% MoS₂ under visible light irradiation.

Fig. 8(b) showed the photocurrent response of BiOI, MoS₂ and BiOI–0.5% MoS₂ with illumination of visible light. The rapid increase photocurrent response from a light-off to light-on state was mainly due to the fast separation and transfer of the photogenerated electrons on the surface of the working electrode. As shown in Fig. 8(b), the photocurrent density generated by BiOI–0.5% MoS₂ was much higher than that by BiOI, which indicate that BiOI–0.5% MoS₂ has more efficiency separation rate of photogenerated carriers than BiOI. This result is consistent with the results of PL, and it could be speculated that the separation of photogenerated carriers may be one of the reasons for the highest photocatalytic performance of BiOI–0.5% MoS₂ on MO degradation.

Moreover, the energy of conduction band edge (E_CB) and the energy of valence band edge (E_VB) can be calculated by using the following equations:⁴⁵

E_CB = −χ + 0.5E_g (1)

and

E_VB = −χ − 0.5E_g (2)

where, χ is the absolute electronegativity of the semiconductor, and E_g is the band gap energy of the semiconductor. The absolute electronegativity of BiOI and MoS₂ is 5.99 eV (ref. 8) and 5.32 eV (ref. 45) respectively. As the pristine BiOI and MoS₂ exhibit a band gap energy of 1.87 eV and 1.71 eV respectively as shown in Fig. 3(b), the calculated E_CB and E_VB of BiOI is about −5.05 eV and −6.93 eV, while it is −4.46 eV and −6.18 eV for MoS₂ respectively. Therefore, it is thermodynamically favorable for the direct electron transfer from the conduction band of MoS₂ to the conduction band of BiOI, while the holes transfer from the valance band of BiOI to the valance band of MoS₂ as shown in Fig. 9.

Fig. 9 Schematic diagram illustrating the principle of photo-induced charge transfer in BiOI–MoS₂ semiconductor.

3.6 Photocatalytic mechanism

To survey the photocatalytic disinfection mechanism, active species trapping experiments were conducted and the results are shown in Fig. 10. Error bars in Fig. 10 represent SDs calculated from three parallel experiments. Ammonium oxalate and isopropanol were chose as h⁺ scavengers and OH scavengers respectively. As shown in Fig. 10, comparing with the photodegradation efficiency without scavenger, the photodegradation efficiency is decreased obviously when ammonium oxalate was added into the MO solution. However, it is only a little decreased when isopropanol was added into the MO solution. This result indicates that h⁺ was the main reactive species in the photodegradation of MO for the prepared BiOI–MoS₂ samples under visible light irradiation.

Fig. 10 Scavenger effects on the degradation of MO solution under visible light irradiation.

3.7 Discussion on the effects of MoS₂ on the photocatalytic activity

From Fig. 6, it is deduced that the photocatalytic activity of BiOI increased with the loading of MoS₂ on BiOI. The photocatalytic activity of BiOI–MoS₂ increases with the increase of MoS₂ contents first, then it decreases when MoS₂ content is over 0.5 wt%. This can be attributed to two main reasons. First, it is thermodynamically favorable for the direct electron transfer from the conduction band of MoS₂ to the conduction band of BiOI, while the holes transfer from the valance band of BiOI to the valance band of MoS₂ as shown in Fig. 9. Thus, the modification of MoS₂ on BiOI can accelerate the division and restrain the recombination of photogenerated electron–hole pairs, resulting in the improved photocatalytic activity. Second, too much MoS₂ loading on the surface of BiOI will prevent light from reaching surface of BiOI, and thus reduce the generation of electrons and holes. In addition, too much MoS₂ loading will also limit the efficiency of charge separation, which is confirmed by the photoluminescence spectra of pure BiOI and BiOI–MoS₂ composites as shown in Fig. 8(a).

4 Conclusions

3D hierarchical BiOI modified by MoS₂ with excellent photocatalytic performance in the degradation of methyl orange under visible light irradiation were successfully fabricated through a simple solvent thermal process. The photocatalytic activity of BiOI–MoS₂ for the degradation of methyl orange increases with the increase of MoS₂ contents first, then it decreases when MoS₂ content is over 0.5 wt%. The degradation efficiency of methyl orange for BiOI–0.5% MoS₂ is about two times higher than that of bare BiOI nanostructures. The enhanced photocatalytic activities could be attributed to higher separation efficiency of photogenerated charge carriers. However, radicals and holes trapping experiments showed that h⁺ dominates the photodegradation process of MO.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 51574071).

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