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Lin, C., Liu, Y., Su, Q., Liu, Y., Lv, Y., and Liu, M. (2019). "One-pot synthesis of cellulose/MoS2 composite for efficient visible-light photocatalytic reduction of Cr(VI)," BioRes. 14(3), 6114-6133.


An effective cellulose/MoS2 (Ce/MoS2) composite was synthesized via a one-pot microwave-assisted ionic liquid method for the photocatalytic reduction of toxic Cr(VI). Effects of ionic liquids (ILs) on the MoS2 nanostructure were considered, and the obtained composite was characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectrometry (XPS), and electrochemical impedance spectroscopy (EIS). The results indicated that the MoS2 nanoplates were anchored and dispersed on the surface of the cellulose. Compared with the pristine MoS2, the support of the cellulose greatly enhanced the photocatalytic reduction efficiency of Cr(VI) ions in solution, from 65.9% to nearly 100%. The reduction mechanism was considered, and the results implied that the simultaneous reduction of Cr(VI) during the initial dark adsorption process was observed due to the effect of citric acid as a hole scavenger. Finally, regeneration tests revealed that the Ce/MoS2 composite could be recycled and reused.

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One-pot Synthesis of Cellulose/MoS2 Composite for Efficient Visible-light Photocatalytic Reduction of Cr(VI)

Chunxiang Lin,a,b Yushi Liu,Qiaoquan Su,a Yifan Liu,a Yuancai Lv,a and Minghua Liu a,*

An effective cellulose/MoS2 (Ce/MoS2) composite was synthesized via a one-pot microwave-assisted ionic liquid method for the photocatalytic reduction of toxic Cr(VI). Effects of ionic liquids (ILs) on the MoSnanostructure were considered, and the obtained composite was characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectrometry (XPS), and electrochemical impedance spectroscopy (EIS). The results indicated that the MoS2 nanoplates were anchored and dispersed on the surface of the cellulose. Compared with the pristine MoS2, the support of the cellulose greatly enhanced the photocatalytic reduction efficiency of Cr(VI) ions in solution, from 65.9% to nearly 100%. The reduction mechanism was considered, and the results implied that the simultaneous reduction of Cr(VI) during the initial dark adsorption process was observed due to the effect of citric acid as a hole scavenger. Finally, regeneration tests revealed that the Ce/MoS2 composite could be recycled and reused.

Keywords: Cellulose; MoS2; One-pot; Photocatalytic reduction; Cr(VI)

Contact information: a: College of Environment & Resources, Fuzhou University, 350108, Fuzhou, China; b: Shishi Xinggang Plastic Packaging Co., Ltd., 362700, Fuzhou, China;

* Corresponding author:


The improper discharge of metal ions into aquatic bodies causes pollution. Hexavalent chromium (Cr(VI)), known as a carcinogen and mutagen, is an issue of concern due to its highly toxic, potentially mutagenic, and carcinogenic nature (Zhang et al. 2012; Ge and Ma 2015; Periyasamy et al. 2017). It is mobile in nature and difficult to remove from water by some common physical or chemical methods. However, another oxidation state of chromium, Cr(III), is much less toxic and carcinogenic than Cr(VI), and also much less soluble in water, so it can be easily removed in an alkaline medium (Chebeir and Liu 2016). Therefore, redox treatment is an effective and easy way to remove Cr(VI) from solution, including chemical reduction, biosorption coupled reduction, microreduction, photocatalysis, etc. (López-García et al. 2010; Bertoni et al. 2014; Kafilzadeh et al. 2016; Wang et al. 2016; Jiang et al. 2018). Chemical reduction is a traditional procedure that produces large quantities of solid sludge containing toxic chromium compounds with high cost of disposal (Remoundaki et al. 2010). Biomaterials and microorganisms are promising techniques for the adsorption and reduction of Cr(VI) pollution due to their low cost, environmentally-friendly character, and absence of secondary pollution. However, the efficiency to remove high concentration levels of Cr(VI) is often limited by the slow reduction kinetics (Hasin et al. 2010); notably, the reduction of Cr(VI) by biomaterials only has been found to occur only under strongly acidic conditions (Park et al. 2008 ). Compared with the above technologies, photocatalytic reduction of Cr(VI) to Cr(III) using photocatalysts is more effective and faster due to the remarkable chemical and photochemical stability of a semiconductor (Liu et al. 2011). It is a popular and promising technology for Cr(VI) removal (Bora and Mewada 2017; Christoforidis and Fornasiero 2017; Nahar et al. 2017; Szczepanik 2017) on account of its economical, simple, and efficient nature. The photoreduction process is often accompanied by simultaneous oxidation of organic matter in solution, which plays the role of ligand and/or sacrificial electron donor (CIESLAet al. 2004). Therefore, this method is especially suitable for the treatment of organic wastewater containing chromium (Mitra et al. 2013; Qu et al. 2017; Zhang et al. 2018). Various photocatalysts such as TiO2, Bi-based catalyst, CdS, ZnS, and MoS2 have been synthesized and used for photocatalytic Cr(VI) reduction and other organic pollutants degradation (Hu et al.2017; Li et al. 2018; Soto et al. 2018; Wang et al. 2018; Yin et al. 2018; Shindume et al. 2019; Sun et al. 2019).

Recently, much attention has been paid to MoS2 photocatalyst, owing to its excellent properties such as a narrow band gap (1.75 eV), large surface area, and unique morphology (Zhang et al. 2016). It has a special sandwich structure of three stacked atomic layers (S-Mo-S) held together by covalent bonding and an interlayer link through weak van der Waals forces (Ai et al.2016). The special properties and strong absorption in the visible region of the solar spectrum make it a potential candidate as a visible-light-driven photocatalyst for water treatment.

However, some problems limit the application of MoS2, including easy aggregation of nanoparticles and difficulty in the recovery process. To solve these problems, MoS2 nanoparticles are often immobilized onto a support (Zhao et al. 2015; Li et al. 2016). Biomass including cellulose has been used as a support for the photocatalyst because of its excellent properties, such as biodegradability, accessibility, bio-compatibility, and high mechanical properties (Jonoobi et al. 2015). There are many reports about immobilization of semiconductor nanoparticles on a cellulose support with enhanced photocatalytic properties. For example, BiOBr/regenerated cellulose composites have been successfully synthesized in situ, and the composites show excellent photocatalytic activity for degradation of Rhodamine B (Du et al. 2018). Nanocellulose was used as a host for the synthesis of zinc oxide (ZnO) nanostructures through in situ solution casting method (Lefatshe et al. 2017). Mohamed et al. (2016) developed a regenerated cellulose/N-doped TiO2 membrane via phase inversion method. However, the synthesis of cellulose-based photocatalyst materials often requires complicated, time-consuming, harsh conditions or multiple-step processes (Yang et al. 2016; Zhou et al. 2016; Wang and Mi 2017). In this regard, mild reaction conditions, time-effectiveness, and methods with low energy consumption are highly desirable.

The microwave-assisted ionic-liquid method (MAIL) has received significant attention worldwide owing to its combination of the benefits of microwave heating and ionic liquids. It is a powerful tool for the synthesis of the nanostructured materials due to the advantageous properties of rapidity and reduced energy consumption. Much progress on the preparation of nanomaterials by using this rapid method has been published, such as metal oxides (Schütz et al. 2017), metal sulfides (Ma et al. 2014), metal selenides (Tyrrell et al. 2015), metal tellurides (Schaumann et al. 2017), etc. A previous study revealed that ionic liquids have obvious effects on both the crystal structure and morphology of the nanomaterials (Lin et al.2017).

Herein, the MAIL method was used to prepare the cellulose and immobilize MoS2 nanoparticles with lower temperature and shorter time under ambient pressure, after which the effects of ionic liquid types on the shapes of MoS2 were investigated. The obtained cellulose/MoScomposite was characterized and then applied to the reduction of Cr(VI) to Cr(III) in aqueous solution. Moreover, the reduction mechanism of Cr(VI) via Ce/MoScomposite was considered, and the stability of the composite was also evaluated.


Materials and Reagents

Cotton linter was used as the cellulose material (Ce). A series of 1-butyl-3-methylimidazolium salts [BMIM]X ionic liquids (X = Cl, PF4, BF4, HSO4; purity = 99%; named IL(1)–IL(4), representative [BMIM]Cl, [BMIM]PF6, [BMIM]BF4, [BMIM]HSO4, respectively) were used in the experiments. Among these ILs, IL(2) through IL(4) were purchased from Shanghai Chengjie Chemical Co., Ltd (Shanghai, China), and IL(1) was obtained from Henan Lihua Pharmaceutical Co., Ltd (Henan, China). Ammonium paramolybdate ((NH4)6Mo7O2, ≥ 99.5%) and diphenylcarbazide were supplied by Aladdin Reagent Co., Ltd. (Shanghai, China). All other reagents were of analytical grade and utilized as received without further purification.

One-pot Synthesis of Ce/MoS2 Composite

First, 1 g of MoS2 precursor (m((NH4)6Mo7O24):m(CH4N2S) = 0.6:1) was added to 50 mL of deionized water to form transparent and uniform solution and then added to the Ce/IL mixture (in a 0.5:20 Ce–IL ratio, m/m). The mixture was stirred and sealed in a microwave reactor at 180 °C for 60 min. The resulting products were collected by centrifuging, washing repeatedly with distilled water and alcohol, and drying in a vacuum oven under 60 °C for 24 h.

To study the effects of ILs on MoS2 structure performance, the pristine MoSmaterial was prepared by a similar procedure without the addition of cellulose. The corresponding products were labeled from MoS2-IL1 to MoS2-IL4, respectively. Also, pure cellulose products (Ce) were prepared by microwave-assisted ionic liquid method without adding MoSprecursor solution.

Materials Characterization

The crystal structures of MoS2 and Ce/MoS2 composites were determined with a Min Flex 600 (Rigaku, Tokyo, Japan) X-ray diffractometer (XRD). The morphologies and microstructures of the samples were investigated by field-emission scanning electron microscopy (SEM, Nova NanoSEM 230) equipped with an energy-dispersive X-ray spectroscopy (EDX) system. Transmission electron microscopy (TEM) measurements were obtained using a Tecnai G2-F20 microscope (Ames Laboratory, Ames, IA, USA). Electrochemical impedance spectroscopy (EIS) was carried out using a CHI650E electrochemical analyzer (CH Instruments, Austin, TX, USA). X-ray photoelectron spectrometry (XPS, ESCALAB 250Xi, ThermoFisher, Waltham, MA, USA) was applied to the surface compositions analysis of the pristine MoS2 and Ce/MoS2 composites. UV-visible diffuse reflectance spectroscopy (DRS) was recorded on a Carry 500-Scan spectrophotometer.

Photocatalytic Activity Measurement

The photocatalytic activity of the composite was evaluated by the reduction of Cr(VI) (mg/L) under a Xe lamp (300 W) equipped with a UV-cut off filter (λ ≥ 420 nm). The impact of thermal catalysis was prevented by using a ventilating fan. All experiments were conducted at room temperature in air. In a typical photocatalytic experiment, 0.03 g of composites were totally dispersed in 100 mL of 50 mg/L Cr(VI) aqueous solution with 20 mmol/L of citric acid. After stirring for 60 min in the dark to ensure absorption-desorption equilibrium, the suspensions were exposed to visible-light irradiation under magnetic stirring. During the illumination reaction process, about 4.0 mL reaction solution was collected at 30 min intervals and then centrifuged to remove the catalyst completely. The remaining amount of Cr(VI) was analyzed using an ultraviolet-visible-light (UV-vis) spectrophotometer (T6 new century, Beijing Puyi General Instrument Co. LTD., Beijing, China), determined at 540 nm. The concentration of total (Cr(III) + Cr(VI)) in aqueous phase was measured using an inductively coupled plasma atomic emission spectroscopy (ICP-OES Optima 3000, Perkin Elmer Inc., Waltham, MA, USA).


Effect of ILs on the Morphology and Properties of MoS2

To investigate the influence of ILs on the properties of MoS2 under microwave radiation, the pristine MoS2 samples were prepared via MAIL method using IL(1) through IL(4) as the medium without adding cellulose.

The crystalline phases of the MoS2 samples prepared in different ILs were studied by XRD characterization. Figure 1 shows the XRD spectra of MoS2-IL1 through MoS2-IL4, in which diffraction peaks appearing at 2θ = 8.76 °, 17.46 °, 33.50 °, and 58.33 ° are observed in each sample, indexing to (002), (004), (100), and (110) reflections from the layered MoS2 structure (Liu et al. 2015). The intercalation of NH4+ ions in layered MoS2 widens the spacing of the layers, resulting in the shift of (002) diffraction towards lower angles (Bissessur et al. 2002). Moreover, the XRD peak intensity of MoS2-IL1 is obviously higher than those of the others. No impurity peak was found in MoS2-IL1, which suggests higher crystallinity and higher purity of products prepared in IL(1) by MAIL method.

Fig. 1. XRD patterns of MoSprepared in different ionic liquids

Fig. 2. SEM images of MoS2-IL1 (a), MoS2-IL2 (b), MoS2-IL3 (c) and MoS2-IL4 (d)

The effects of the ILs on the size and morphology of the MoSsamples (MoS2-IL1 through MoS2-IL4) were examined by SEM. As shown in Fig. 2a, the SEM image of MoS2-IL1 displays three-dimensional (3D) metal sulfide flower-like microspheres assembled tightly from many curved two-dimensional (2D) MoSnanoparticles. The MoS2-IL2 was constructed of rough clusters having irregular surface structure, as illustrated in Fig. 2b. Figure 2c shows the SEM image of MoS2-IL3, which displays microspheric particles with a mean diameter of 1.5 μm. The MoS2-IL4 had carnation flower-like structure, and the surface of the sample was constructed of sheet-like structures, as shown in Fig. 2d. The results indicates that the ionic liquid had an important influence on the morphology of MoS2. Different anions bring about different interactions between IL and ammonium parmolybdate and then cause different morphologies of MoS(Ma et al. 2008; Lin et al. 2017). Then, TEM characterization was used to further observe the morphology of the MoS(see supporting information Fig. S1 in the Appendix), and the results are corresponding to that of the SEM micrographs.

The charge transfer and recombined processes of MoS2 samples were evaluated by electrochemical impedance spectroscopy (EIS), and the obtained results were fitted using an equivalent circuit, as shown in Fig. S2. The radius of the semicircle reflects the charge-transfer resistance, Rct, which is an important parameter for characterizing the semiconductor-electrolyte charge-transfer process. Higher values of Rct mean lower rates of charge transfer and recombined process (Cheng et al. 2016; Finn and Macdonald 2016). In Fig. S2, MoS2-IL1 displays a narrower semicircle radius than that of others, which indicates lower Rct of MoSand thus lower recombination of photogenerated carriers when IL(1) is used. The results mean that MoS2-IL1 will show stronger photocurrent and higher photocatalytic performance (Okafor et al. 2011; Farag and Hegazy 2013).

Photocatalytic performance of as-prepared MoS2 samples was evaluated by the degradation of 20 mg/L RhB solutions under visible light irradiation. The results are presented in Fig. S3. Before visible light irradiation, the mixture of samples and RhB solution were shaken in the dark for 60 min to achieve adsorption-desorption balance. The MoS2 samples show different adsorption and catalytic behaviors, which could be roughly determined by the slope of the curves. This result demonstrates that MoS2-IL1 presented higher photocatalytic activity than others. The probable reason might be due to the lower recombination of photogenerated carriers of MoS2-IL1, and also to the open porous structure of the IL1 form by the 2D nanosheets assembled together (Fig. 2(a)).

Based on the above results, the MoS2 prepared in IL(1) via MAIL exhibited better crystallinity and higher photocatalytic activity. Hence, IL1, which was also excellent for cellulose dissolution, would be used further to prepare Ce/MoS2 composite materials by MAIL for subsequent experiments. Effects of cellulose support on the structures, morphologies, and photocatalytic properties of the composites will be discussed.

Effect of Temperature on the Morphology of MoS2 Prepared in IL(1)

Effect of the temperature on the morphology of MoS2 nanoparticles was also investigated using IL(1) as solvent. As shown in Fig. S4, the MoS2 samples were present as bulk when the microwave irradiation temperature was 100 °C. The reason might be that low temperature (100 °C) is not sufficient for effective MoS2 nucleation and crystal growth. With the irradiation temperature was increased (100 to 180 °C), the 2D interweaved nanosheets gradually appeared, finally forming a flower-like architecture assembled from building blocks of 2D nanopetals. However, higher temperature ( ≥ 180 °C) would lead to easily decomposition of cellulose support.

Preparation and Characterization of Ce/MoS2 Composite

The successfully synthesized Ce/MoS2 composite through MAIL method was first demonstrated by XRD characterization, and the spectrum is illustrated in Fig. 3. Compared to the XRD pattern of cellulose, the spectrum of the composite shows the main diffraction peak of MoSphase along with the Ce characteristic peaks, which appeared at 2θ = 16.3 °, 22.5 °, and 35.7 °.

Fig. 3. XRD patterns of cellulose, MoS2-IL1, and Ce/MoS2 composite

The general morphology of the Ce after microwave-assisted ionic liquid treatment is shown in Fig. 4a, in which Ce is present as bulk with nothing observed on it. Figures 4b and 4c present the SEM images of Ce/MoScomposite, in which many 2D nanoplate-like MoS2 are tightly anchored on the surface of cellulose, which provide a place for MoS2 to growWith the presence of cellulose in the synthetic process, the MoS2 in the composite presents differently from the pristine MoS2, which indicates that the growth and assembly process of MoS2 on the surface of cellulose is hindered. Furthermore, the energy dispersive X-ray spectroscopy (EDS) and corresponding elemental mapping images of the composite (Fig. 4d) exhibit matched spatial distribution of C, O, Mo, and S, which implies the formation of MoS2 on the surface of the cellulose.

Fig. 4. SEM images of Ce (a), Ce/MoS2 composite (b, c) and the corresponding elemental mapping images (d)

Additionally, high resolution TEM (HRTEM) observation was performed to further reveal the crystal structure of the MoS2 with the Ce support (Fig. 5). The lattice spacing of 0.95 nm and 0.56 nm of composite corresponded to the values of (002) and (004) for MoScrystal structure (Zhang et al. 2016, 2017).

Similarly, the charge transfer process of Ce/MoS2 composite materials was measured by EIS, as shown in Fig. S5. Compared with MoS2-IL1, the narrower radius of Ce/MoS2 means that the composite is more conductive to the migration of composite carriers, and thus might possess better photocatalytic performance.