Inorganic–organic nanohybrid of MoS2-PANI for advanced photocatalytic application
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In this work, the fabrication of MoS2-Polyaniline (PANI) nanocomposites, by in situ polymerization, has been described and its application as a photocatalyst for the degradation of organic pollutants such as methylene blue (MB) and 4-chlorophenol (4-CP), has been reported. The disadvantages of MoS2 such as poor electronic conductivity and agglomeration between the nanosheets, was overcome by intercalating 2D MoS2 layers with 1D conducting polymer, Polyaniline (PANI). The composite material was characterised by SEM, TEM, BET, XRD, FTIR, UV–Vis spectroscopy and XPS, indicating proper intercalation between MoS2 nanosheets and PANI. Consequently, the synthesized composites were used for the degradation of MB and 4-CP, followed by kinetic investigations to determine the rate kinetics. The photoactivity of the nanocomposite is explained by the transfer of photogenerated electrons from PANI to the CB of MoS2, thus preventing the direct recombination of electrons and holes. Hence, a positive synergistic effect between MoS2 and PANI resulted in efficient photocatalytic degradation of organic dyes like MB.
KeywordsHydrothermal MoS2-PANI hybrid Photocatalysis Pseudo first order rate kinetics
In search of highly efficient photocatalytic materials, 2D nanostructures have attracted tremendous interest as a possible replacement of existing photocatalytic materials due to their visible light driven photocatalytic activity. Two dimensional (2D) metal dichalcogenide (MoS2), is known for its remarkable photocatalytic properties arising from its unique physicochemical characteristics such as porous structure, large surface areas and active sites, good crystallinity and better separation of charge carriers, etc [1, 2, 3]. Despite all its advantages, poor electronic conductivity acts as a major drawback in employing MoS2 as a photocatalytic material . Also, the weak interlayer bonding and large interlayer spacing causes stacking of MoS2 nanosheets [5, 6]. Several attempts have been made in the direction to improve conductivity of MoS2 and prevent its aggregation with carbon nanotubes and stacking with graphene [7, 8]. However, there has been limited improvement due to relative decline in number of interconnected conductive pathways. The use of numerous existing metal oxides, e.g., WO3, TiO2, ZnO, and SnO2 as photocatalyst, is also restricted due to: (1) wide band gap which hinders the complete use of sunlight for charge carrier generation, (2) low mobility which prevents charge transport, (3) diminishing photocatalytic activity due to rapid recombination of photogenerated electron–hole pairs, and (4) agglomeration of nanomaterials leading to poor dispersion [9, 10]. In view of the above, the work presented here reports a simple synthesis of 3D architectures composed of two-dimensional MoS2 nanosheets and one-dimensional conducting polymer polyaniline (PANI) as an effective approach to solve the aforementioned issues.
Polyaniline offers an extended p-conjugated electron system and remarkable properties viz. visible region absorption and generation of highly mobile charge carriers [11, 12]. It has found extensive use in solar energy and photocatalytic activity owing to its reasonable cost, excellent environmental stability and facile synthesis methods [13, 14]. PANI is treated equivalent to p-type inorganic semiconductors due to its high hole concentration. Integration of MoS2 nanosheets with PANI improves the electrical conductivity of the nanosheets, reduces the electron hole recombination probability, prevents MoS2 from agglomeration, leading to enhancement in surface area . It is, therefore, believed that the proposed hybrid, MoS2-PANI nanostructure, will have the potential to overcome the challenges prevalent in existing photocatalytic materials and also provide an optimum solution for the elimination of dyes and pollutants from waste water for water purification.
Herein, MoS2-PANI hybrid is used as a catalyst for the photodegradation of 4-chlorophenol (4-CP) and methylene blue (MB). MB is an organic photoactive dye which is used for biological applications. However, MB is hazardous to living beings as it can cause ailments such as tissue necrosis, diarrhoea, cyanosis and jaundice while 4-CP is a major toxic chemical pollutant which causes severe health issues including histopathological alterations, genotoxicity, mutagenicity and carcinogenicity [13, 14]. The removal of these pollutants is therefore crucial in restoring water quality. A study of the intermediates that were formed during degradation of 4-CP were also processed using high performance liquid chromatography (HPLC).
Materials and reagents
Synthesis of MoS2: sodium molybdate (Na2MoO4·2H2O, Thermo Fisher Scientific India Pvt. Ltd), thioacetamide (C2H5NS, Central Drug House Pvt. Ltd.) and silicotungstic acid AR(H[Si(W3O10)4]. XH2O with max impurity limit of 0.009%, Central Drug House Pvt. Ltd.).
Synthesis of PANI: aniline (C6H5NH2 with max impurity limit of 0.02%, Central Drug House Pvt. Ltd.), hydrochloric acid (HCl.H2O with max impurity limit of 0.0316%, Thermo Fisher Scientific India Pvt. Ltd.) and ammonium persulphate [(NH4)2S2O8 with max impurity limit of 0.005%, Sisco Research Laboratories Pvt. Ltd.].
Synthesis of MoS2-PANI
The morphology and structural properties were examined using a Quanta 3D FEG, (FEI’s) Scanning Electron Microscope (SEM) and TECNAI F30 S-Twin High Transmission electron microscopy (HRTEM) working at 300 kV. For TEM analysis, the sample was ultrasonically dispersed in ethanol. Cu-coated TEM grid was then dipped into the sample solution, followed by drying for 24 h. Brunauer–Emmett–Teller (BET) (NOVA-1000 version 370) using N2 as analysis gas, measured at 77.4 K, was used to determine the pore size distribution of the sample. The elemental constituents of the nanocomposite were determined by energy dispersive spectroscopy (EDS) that was integrated into the SEM system. The crystal structure of the nanocomposite was analysed using Rigaku Smart Lab X–ray diffractometer with Cu Kα radiation at 1.540 Å with 2θ varying between 5° and 90°. A Bruker Optik, Vertex 70V spectrometer scanning in the range of 400–4000 cm−1 with a step size of 4 cm−1 was used for High Resolution Fourier Transform Infrared Spectroscopy (HR-FTIR). UV–Vis spectrophotometer (Agilent technologies, model no: Cary 100 series) was used to determine the band gap of the synthesized material. Elemental identification and the oxidation states of the different elements in the sample were determined by X-Ray Photoelectron Spectroscopy (Multiprobe Surface analysis system, Omicron). The nanocomposite was also characterised by SEM, XRD and FTIR after the photocatalytic reactions.
The testing of the photocatalytic efficiency of MoS2-PANI composite was performed using Newport Research Arch Lamp Housing, model no. 66902 (50–500 W) which was connected to an external power supply. A catalyst concentration of 10 mg/100 mL was maintained for all the photocatalytic degradation experiments. A quantity of 0.33 mg MB was dissolved in the solution which was irradiated with UV light (100 W/cm2) for over 2 h. At time intervals of 15 min, 1 ml aliquots were sampled out and the concentration change of MB was determined by observing the absorption spectra at 664 nm using UV–Vis spectroscopy. Similarly, for the degradation of 4-CP, 50 mg of the catalyst was dissolved in 100 ml solution containing 0.005 g of 4-CP. Characteristic absorption at 225 nm indicated the change in the concentration of 4-CP.
Analysis of intermediates
The intermediates that were formed during the degradation of 4-CP in the presence of MoS2-PANI, were analysed through HPLC using N2000 Chromatography Data System (Spectra Lab Scientific Inc.) DI water was used as the mobile phase with a flow rate of 0.6 mL/min. The samples were injected at a temperature of 65 °C.
Results and discussion
Photocatalytic experiments and kinetic analysis
Pseudo first order represents a process which depends on Van der Waals interaction between the adsorbate (MB) and adsorbent (MoS2-PANI). This is a weak adsorption process involving no chemical interaction between the adsorbate and adsorbent . The fitting results of pseudo first order model are shown in Fig. 10b. First order rate constant (k1) is found to be 0.0163 g mg−1 min−1 from the slope of the above graph. Moreover, R2 (correlation coefficient) value of the plot is found to be 0.9623. High value of R2 reveals that adsorption kinetics is following the first order adsorption model.
Figure 10c shows the fitting results of pseudo second order model. Low values of R2, i.e., 0.1104 indicate that pseudo second order adsorption model is not applicable. Additionally, the theoretical and experimental values of Qe were not found to be in good match which implied that adsorption kinetics cannot be well explained by pseudo second order adsorption model . Pseudo second order model presents the chemisorption process which involves the formation of relatively strong bonds with the surfaces .
The fitting results of all the models
First order model
K1 (g mg−1 min−1)
Qe (mg g−1)
Second order model
K2 (g mg−1 min−1)
Qe (mg g−1)
Intra-particle diffusion method
K1 (g mg−1 min−1)
K2 (g mg−1 min−1)
K3 (g mg−1 min−1)
Photocatalysis and analysis of intermediates of 4-CP degradation
Analysis of MoS2-PANI after photocatalytic activity
In this paper, we report the synthesis of a novel MoS2-PANI nanocomposite through an in situ polymerization process. The sample was successfully characterised by SEM, TEM, XRD, FTIR, BET pore size analysis, XPS and UV–Vis spectroscopy techniques. The structural and morphological properties in addition to pore size and volume were also determined. The oxidation states of different elements present in the sample, were confirmed by XPS. The photocatalytic efficiency of the synthesized nanocomposite, indicates that the composite favours dye adsorption and is found to be photocatalytically active, stable and capable of degradation of organic dyes. Kinetic investigations indicate that the photocatalytic reaction is driven by pseudo first order (k1 = 0.0163 g mg−1 min−1 and R2 = 0.9623 g mg−1 min−1) adsorption model. The synergy between MoS2 nanosheets and PANI may have an important role in improving photocatalytic efficiency and rate kinetics as a result of which, the synthesized MoS2-PANI sample can act as a promising photocatalyst.
This work by supported by Science and Engineering Research Board, India (SERB, ECR/2017/001222).
Compliance with ethical standards
Conflict of interest
The authors declare no conflicts of interest.
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