Defect mediated visible light induced photocatalytic activity of Co₃O₄ nanoparticle decorated MoS₂ nanoflower: A combined experimental and theoretical study

Abstract

In this work, Co₃O₄ nanoparticle-decorated MoS₂ (MoS₂@Co₃O₄) hetero-nanoflowers were synthesized by a facile hydrothermal method, and the effect of Co₃O₄ on the morphological, structural, optical, electronic, and photocatalytic properties of MoS₂ was analyzed. The surface morphology of MoS₂ and MoS₂@Co₃O₄ was studied via field emission electron microscopy (FE-SEM) and transmission electron microscopy (TEM), which revealed a strong interaction between the MoS₂ nanoflower and the nanoparticles. The X-ray diffraction pattern showed a decrease in the crystallite sizes from 7.35 nm to 6.26 nm due to the incorporation of Co₃O₄. The UV–Vis spectroscopy of the analysis revealed that the indirect band gap of MoS₂ was reduced from 1.89 eV to 1.65 eV with the incorporation of Co₃O₄ nanoparticles. Density functional theory (DFT) calculations were used to investigate the electronic properties of MoS₂ and MoS₂@Co₃O₄ hetero-nanoflowers, which also showed a reduction in the electronic band gap for the Co₃O₄ nanoparticles that were injected. The presence of defect states was also observed in the electronic property of MoS₂@Co₃O₄. The photocatalytic activity of the prepared composite and nanoflower is studied using an aqueous solution of methylene blue (MB), and the efficiencies are found to be 27.96% for MoS₂ and 78.89% for MoS₂@Co₃O₄. The improved photocatalytic efficiency of MoS₂@Co₃O₄ hetero-nanoflower can be attributed to narrowing the band gap together with the creation of defect states by the injection of nanoparticles that slows down electron-hole recombination rate by trapping charge carrier. The degradation analysis of the composite provides a new route for the purification of polluted water.

1. Introduction

A large number of industries are being built all over the world to meet the basic needs of the growing population [1]. These industries release harmful substances, namely, dyes and chemicals into the environment without any sort of treatment. A large amount of these dyes and chemicals goes into the wastewater that seriously contaminated the water supplies. These compounds have an extended negative effect on the biological components and human beings [[2], [3], [4]]. Pure water is an essential requirement for the survival of living beings. It is therefore indispensable to remove the dye from the water [5]. A number of methods including coagulation, adsorption, foam flotation, osmosis, photocatalysis, piezocatalysis, etc. have been used for the removal of toxic pollutants from water and wastewater [6]. Among them, photocatalysis is considered as the most durable, environmentally friendly, and cost-effective method to remove contaminant from water [7].

During photocatalysis, the photon-generated electrons and holes from the photocatalytic materials take part into the oxidation process and clean up pollutants present in the water [8]. Three crucial steps are typically involved in the photocatalysis process: (i) production of charge carriers due to the absorption of the incident light, (ii) separation of electron-hole pairs and transfer towards the catalytic surface, and (iii) utilization of the photo-generated carriers during phtocatalytic redox reaction [8,9]. The performance of photocatalytic degradation is greatly influenced by the choice of photocatalyst materials. Traditionally, transition materials such as ZnO, TiO₂, CeO₂, etc. are widely used for the light-induced catalytic removal of chemical compounds from water [10,11,12,13]. These materials exhibit high band gap causing a high rate of electron-hole recombination under the visible UV light response that reduces the photocatalytic performance.

Recently, MoS₂, a low band gap (∼1.80 eV) two-dimensional (2D) layered-structure materials, has gained considerable research interest as a photocatalyst due to their inter-layer distance, optimal band gap and excellent co-catalytic supports for effective electron transfer [[14], [15], [16]]. In addition, MoS₂ significantly absorb the electromagnetic spectrum in the visible region, which makes them an ideal candidate for photocatalytic applications [17]. The previous study showed that MoS₂ exhibits 49.28% degradation efficiency [18,19]. The photocatalytic efficiency of MoS₂ can further be improved by incorporating transition metal oxide nanoparticles into it. Recently the photocatalytic performance of several MoS₂-based heterostructures, such as MoS₂@NiS, MoS₂@ZnO, MoS₂@BiVO₄, MoS₂@C₃N₄, etc., have been studied, which shows an increase in the degradation efficiency due to the incorporation of nanomaterials [[19], [20], [21], [22], [23]]. When incorporated with the semiconductors or transition metal oxides nanomaterials, MoS₂ creates heterojunction with them that (i) reduces band gap, (ii) creates defect states, (iii) increases surface area, and tunes the band gap [24,25]. Due to the low band gap, the nanocomposite is active in the visible light region and improves the photocatalytic performance. Moreover, the incorporated Co₃O₄ (4 wt%) nanoparticles create defects in the crystal structure that can trap electrons least charge resistance thus photocatalytic properties are enhanced by electron-hole separation and the development of the Schottky barrier at the interface of the composite [26,27].

Various types of nanoparticles, more specifically, transition metal and its oxide, can serve as traps for the excited electrons and promote charge separation efficiency [28]. Among transition metal oxide, Co₃O₄, a p-type semiconductor, is widely used for the photodegradation of pollutants because of its environmental friendliness, unique electronic properties, extraordinary redox ability, excellent chemical, and physical stability, etc. [29]. It was observed that the particle-shaped Co₃O₄ nanoparticles exhibited better photocatalytic performance compared to other types of Co₃O₄ nanostructures. Recently it was observed that when incorporated with MoS₂ nanosheets, the Co₃O₄ nanostructure increases the surface area of heterojunction, which may expedite the charge transfer and thereby improve the photocatalytic efficiency [30]. However, the synthesis of MoS₂ nanosheet requires painstaking processes and provides low yields, which might limit the widespread applications. In this regard, MoS₂ nanoflowers are fabricated using low-cost one-step hydrothermal synthesis because other methods, such as green synthesis, do not control morphology and give a low amount of yield, but hydrothermal processing may offer a cost-effective route for the production of high-yield of MoS₂ [27,31].

In this paper, Co₃O₄ nanoparticles decorated on MoS₂ nanoflowers are prepared via a one-step hydrothermal method. The hydrothermal method is an easy, low cost and low-temperature process used for the synthesis of nanomaterials. The surface morphological, structural, and optical properties of the as-synthesized nanostructure material were studied. The optical band gap of the nanocomposite was found to be reduced from 1.89 eV to 1.65 eV due to the incorporation of Co₃O₄ nanoparticles. The photocatalytic performance of MoS₂ nanoflowers and MoS₂@Co₃O₄ nanocomposite were studied by the degradation of Methylene blue (MB) solution under visible light irradiation. The results demonstrated that the synthesized MoS₂@Co₃O₄ hetero-nanoflowers had a better photocatalytic activity in comparison with MoS₂ nanoflowers. The degradation of MB is 27.96% of MoS₂ and 78.89% of the composite. Moreover, an investigation of the electronic properties of MoS₂ and MoS₂@Co₃O₄ was performed using the density functional theory (DFT), which is enormously helpful for predicting the properties of the materials. The possible photocatalytic reaction mechanism is also investigated deeply in this paper. The defect-rich MoS₂@Co₃O₄ composite nanostructure will offer an easy, simple economic route for the removal of pollutants from the surface water.

2. Materials & method

2.1. Materials

Sodium molybdate dehydrate (Na₂MoO₄⋅2H₂O), Methylene Blue (MB) dye were obtained from Merck, Mumbi, India. Thiourea (CH₄N₂S), Urea (CH₄N₂O), and Cobalt nitrate hexahydrate (Co(NO₃)₂⋅6H₂O) were purchased from Research Lab, India and SRL, India. Without extra purification, all materials were used as received.

2.2. Synthesis of MoS₂, Co₃O₄, and MoS₂@Co₃O₄

A hydrothermal method was used to synthesize MoS₂ nanoflower. Initially, 120 ml of deionized (DI) water was employed to dissolve sodium molybdate dihydrate (Na₂MoO₄⋅2H₂O) and thiourea (CH₄N₂S). The mixture was vigorously agitated to create a homogeneous mixture, and the resulting suspension was then put into a 200 ml Teflon-lined stainless steel autoclave, which was then heated to 200 °C for 24 h and cooled to ambient temperature. To obtain the MoS₂ nanoflower, the dark yield was repeatedly rinsed with DI water and ethanol before being dried for 16 h at 60 °C.

Secondly, to prepare Co₃O₄ nanoparticles, we used 120 ml of DI water, a suitable amount of cobalt nitrate hexahydrate (Co (NO₃)₂·6H₂O), and urea (CH₄N₂O) were dissolved into 120 ml DI water, and then continuously stirred until a homogeneous and transparent solution was formed. After that, the mixture was loaded into a Teflon-lined stainless steel autoclave and heated via oven at 200 °C for 72 h. The resultant precipitation was centrifuged and cleaned with DI water and ethanol. Co₃O₄ nanoparticles are finally obtained after 14 h of drying precipitation at 60 °C.

To prepare MoS₂@Co₃O₄ nanocomposite, a certain amount of Co₃O₄ (4 wt%) nanoparticles was poured into 50 ml DI water and sonicated for 1 h by ultra-sonication. 70 ml Sodium molybdate dihydrate (Na₂MoO₄⋅2H₂O) solution and thiourea (CH₄N₂S) were vigorously stirred. Then 50 ml solution of Co₃O₄ was mixed with 70 ml of MoS₂ precursor and was stirred for 30 min. The resulting suspension was put into a Teflon tube autoclave made of stainless steel and heated to 200 °C for 24 h. The produced MoS₂@Co₃O₄ nanocomposite was washed with DI water and ethanol and dried for 16 h at 80 °C.

2.3. Characterization

The surface characteristics of MoS₂ and MoS₂@Co₃O₄ were investigated by field emission electron microscopy (FE-SEM) (JSM 7600, Jeol). To analyze micromorphology, a JEOL, JEM 2100 F transmission electron microscope was used. The crystal structure, crystallite size, microstrain, and dislocation density of MoS₂ and MoS₂@Co₃O₄ were studied using the X-ray diffraction (XRD) technique. XRD data was taken for 2θ values between 10 and 80° using an X-ray diffractometer (3040 XPert PRO, Philips) using CuKα radiation (λ = 1.5406 Å). The diffuse reflectance spectra (DRS) (UV-2450; Shimadzu, Japan) was taken for estimation of the optical band gap of the as-prepared MoS₂ nanoflower and MoS₂@Co₃O₄ nanocomposite.

2.3.1. Photocatalytic activity measurement

The photocatalytic performance of MoS₂ nanoflower and MoS₂@Co₃O₄ nanocomposite was evaluated by examining the photo-degradation of the aqueous phase methylene blue (MB) dye under visible light. The methylene blue solution (MB) was magnetically stirred and kept in the dark for 60 min to achieve adsorption-desorption equilibrium and photolysis. To get the data on photocatalytic performance, MoS₂ and MoS₂@Co₃O₄ nanocomposite were immersed in a beaker filled with a 100 mL aqueous solution of MB. The beaker was then irradiated by a 20-W visible source of light that was positioned 20 cm above it. The UV–visible spectrophotometer was employed to determine the absorbance spectra of the illuminated MB solution, which was collected at different points in time.

2.4. Computational method

The density functional theory has been used in all calculations implemented by the Doml3 package of material studio. To take into account van der Waals interactions, Grimme's (DFT-D) was imposed due to a long-range electron effect [[32], [33], [34], [35]]. The exchange and correlation energy was calculated by generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) for better results because localized density approximation (LDA) did not enumerate the exact energy and equilibrium state of the nanosheet [36]. Under atomic orbital consideration, we employed double numerical plus polarization in Material studio software (MS). At the same time, the DFT semi-core pseudopotential (DSSP) method was under consideration to deal with the influence of core electron relativity. The convergence criteria of geometry optimizations were set to 5 × 10⁻³ A for displacement and 4 × 10⁻³ Ha/Å for force, respectively [37]. A global orbital cut-off radius of 5.0 and a smearing value of 0.005 Ha for orbital occupancy were applied where the self-consistent field (SCF) convergence accuracy was 1 × 10⁻⁵ Ha. We set P1 symmetry and 3 × 3 × 1 supercell for a single layer with a vacuum space above 25 Å to restrict interactions between MoS₂ periodic nanosheet and Co₃O₄ nanoparticle. The spin polarization effect was not considered in all the calculations. The Brillouin zone is sampled with a grid parameter 5 × 5 × 1 k points by the Monkhorst-Pack scheme method [37]. Our measured energy gap of the MoS₂ single layer was 1.83eV with a definite tolerance range compared with other theoretical results [38]. In the case of the Co₃O₄ nanoparticle-like molecular structure, we used a procedure for geometrical optimization. The absorption energy of Co₃O₄ nanoparticles on the MoS₂ monolayer is determined by the following equation:

$${\mathrm{E}}_{\mathrm{a}\mathrm{d}\mathrm{s}}={\mathrm{E}}_{{\mathrm{C}\mathrm{o}}_3{\mathrm{O}}_4/{\mathrm{M}\mathrm{o}\mathrm{S}}_2}-{\mathrm{E}}_{{\mathrm{C}\mathrm{o}}_3{\mathrm{O}}_4}-{\mathrm{E}}_{{\mathrm{M}\mathrm{o}\mathrm{S}}_2}$$ (1)

where in equation (1) $E_{{Co}_3{O}_4/{MoS}_2}$ is the total energy of Co₃O₄ nanoparticle absorbed on MoS₂. $E_{{MoS}_2}\mathrm{a}\mathrm{n}\mathrm{d}{E}_{{Co}_3{O}_4}$ represent individual energy of MoS₂ nanosheet and Co₃O₄ nanoparticle. According to the above equation, small negative adsorption energy demonstrates robust interaction between the nanoparticle and nanosheet [39]. The schematic structure of the MoS₂ nanosheet and Co₃O₄ and MoS₂/Co₃O₄ are shown in Fig. 1(a), 1(b) and 1(c), respectively.

Fig. 1.

Geometric structure (a) MoS₂ monolayer, (b) Co₃O₄ and (c) MoS₂@Co₃O₄ nanocomposite.

3. Results and discussion

3.1. Scanning electron microscopy

The surface morphology of hydrothermally synthesized MoS₂ and MoS₂@Co₃O₄ hetero-nanostructure was investigated through field emission electron microscopy. Fig. 2(a) represents the FE-SEM images of MoS₂, which demonstrates 3D flower-like structures with numerous nanoscale petals. Fig. 2(b) shows the prism-like structure of Co₃O₄ nanoparticles. The Co₃O₄ nanoparticles are smoothly attached to each other and possess different size distributions among them, with an average diameter of 25–250 nm. From Fig. 2(c), the higher magnification images of the MoS₂ nanoflower show that the petals are made of 2D nanosheets. The petals are disorderly intersected and are tightly assembled into a common center, and the average diameter of the nanoflowers is about 2–5 μm [40].

Fig. 2.

FE-SEM images of MoS₂ nanoflower (a) low magnification, (b) FE-SEM image of Co₃O₄ nanoparticles, (c) high magnification image of MoS₂ nanoflower (d) FE-SEM image of MoS₂@Co₃O₄ nanocomposite.

Fig. 2(d) represents the FE-SEM image of MoS₂@Co₃O₄ nanocomposite where the Co₃O₄ nanoparticles are found to be attached to the upper, lower, and inner surfaces of the petals of MoS₂ nanoflower. After incorporation of the Co₃O₄ nanoparticle, the nano petals of MoS₂ become distorted, which elucidates the defect-rich nature of the composite nanomaterials that can induce the formation of more active edges which can help to expedite the photocatalytic performance. Moreover, the average thickness of the petals increases from 3-6 nm to 5–8 nm which is estimated by using ImageJ software. This indicates the presence of strong interaction between the nanoparticles and nanoflower [41]. As a result, the electrons can rapidly transfer from the MoS₂ nanoflower to the Co₃O₄ nanoparticle, which can shorten the charge transfer distance, strengthen the separation of electrons and holes, and effectively improve the photocatalytic efficiency [42].

3.1.1. Transmission electron microscopy

The surface micrograph of the material was also examined using a transmission electron microscope. Fig. 3(a) shows a TEM image of a MoS₂ nanoflower composed of a few layers of petals. Additionally, the TEM image of a MoS₂ nanoflower reveals that the petals are twisted together due to their nanoscale thickening [43,44]. The inset shows high-resolution TEM images (HRTEM) of a selected portion of the images. The lattice spacing of the MoS₂ nanoflower was estimated from the HRTEM images and was found to be 0.628 nm which is a little larger than that of the typical MoS₂ nanostructure (0.615 nm) [45]. This observed expansion in the estimated interplanar distance can be attributed to the disordered structure of the MoS₂ nanoflower [46]. Fig. 3(b) shows a TEM image of MoS₂@Co₃O₄ nanocomposite. The surface of the MoS₂ nanoflower has some dark patches on it that are generated by Co₃O₄ nanoparticles. Traces of Co₃O₄ nanoparticles are observed on the petals of the MoS₂ nanoflowers. Moreover, Co₃O₄ nanoparticles strongly interact with MoS₂ nanoflower and are enveloped by it, which makes it possible that a heterostructure between Co₃O₄ and MoS₂ nanoflower has formed. The interlayer distance of the nanocomposite estimated from the corresponding HRTEM image (inset of Fig. 3(b)) is found to be 0.633 nm. The creation of defects in the nanoflower due to the incorporation of nanoparticles is responsible for strain relief and thereby enhances heterostructure lattice spacing [47].

Fig. 3.

TEM image of (a) MoS₂ nanoflower, and (b) MoS₂@Co₃O_4. Inset shows the HRTEM images of the corresponding sample.

3.2. Structural analysis

The crystal structure of MoS₂ nanoflower and MoS₂@Co₃O₄ nanocomposite was studied by X-ray diffraction, and the corresponding XRD patterns are shown in Fig. 4. For the MoS₂ nanoflower diffraction peaks are obtained at the 2θ values of 13.99 (002), 33.14 (100), 39.46 (103), 49.09 (105), 58.8 (110) that correspond to the JCPDS card number 37–1492. The XRD pattern indicates good crystallinity, well–stacked layer, and hexagonal crystal structure. The most intense peak (002) position provide lattice constants a = b = 3.16, and c = 12.29 Å [[48], [49], [50]]. No additional diffraction peaks were detected from the XRD pattern of MoS₂@Co₃O₄ nanocomposite, which may be due to the tiny concentration of Co₃O₄ nanoparticle used [51]. The diffraction peaks height for the MoS₂@Co₃O₄ is smaller compared to that of MoS₂, indicating the reduction of crystallinity due to the incorporation of Co₃O₄. When added to the MoS₂ structure, Co₃O₄ nanoparticles create lattice distortion, and provide faulty structures that cause the diffraction line to be expanded and shrank, resulting in a decrease in crystallinity [[47], [48], [49], [50]]. The Scherrer equation was utilized to calculate the crystallite size (L) of the sample (using the 002 planes) [52].

$$\frac{0.94\lambda }{\beta \mathit{cos}Ɵ}$$

where, λ, θ and β represent the wavelength of the X-ray, diffraction angle, and full width at half maximum, respectively. The estimated crystallite size of MoS₂ and MoS₂@Co₃O₄ was 7.35 nm and 6.26 nm, respectively. The crystallite size of MoS₂@Co₃O₄ nanocomposite got reduced because of the decoration of nanoparticles. The displacements of atoms in the nanocomposite created lattice defects and produced microstrain [53]. The microstrain (ε) of MoS₂ and MoS₂@Co₃O₄ nanocomposite were measured by the formulation [54].

$$\epsilon =\frac{\beta }{4\mathit{tan}Ɵ}$$

Fig. 4.

XRD patterns of MoS₂ and Co₃O₄ decorated MoS₂ (MoS₂@Co₃O₄).

The estimated microstrain for pristine MoS₂ nanoflower and MoS₂@Co₃O₄ are 40.41 and 47.31, respectively. Due to the structural defects in the nanocomposite, the interplanar distance becomes increased. The interplanar distance of MoS₂ nanoflower and MoS₂@Co₃O₄ are found to be 0.628 nm and 0.633 nm. The lattice constant has been calculated by the equation:

$$\frac{1}{d^2}=\frac{4}{3}\frac{h^2+k^2+hk}{a^2}+\frac{l^2}{c^2}$$

where d is the interplanar distance, (hkl) denotes the miller indices, and (a, b, c) are lattice parameters.

3.3. Optical properties

The optical band gap of MoS₂ nanoflowers and MoS₂@Co₃O₄ nanocomposite were calculated using the Modified Kubelleka- Munk (K-M) function. The Kubelleka-Munk function is described by the equation [55].

$$F\left(R\right)=\frac{K}{S}=\frac{{\left(1-R\right)}^2}{2R}=\frac{\alpha }{S}$$

where F(R) is the (K-M) function, R is the diffused reflectance, α is the absorption coefficient, and S is the scattering coefficient. The K-M function is roughly proportional to the absorption coefficient or absorbance of the sample. The Modified Kubelleka- Munk function is obtained by multiplying F(R) with hѵ and employing the relevant coefficient (n) linked to an electronic transition. The direct and indirect band gap density depends on the value of n, for a direct transition n = 2, and for an indirect transition n = 1/2. The bandgap of the nanomaterials can be obtained by plotting the modified K-M function and projecting the linear portion at (F(R)hv)^1/2 = 0. From Fig. 5, we found that the band gap of MoS₂ nanoflower reduces from 1.89 eV to 1.65 eV due to the incorporation of Co₃O₄ nanoparticles. Such a reduction in band gap can be attributed to the presence of defect states of the composite [56]. When nanoparticles are incorporated with the MoS₂ nanoflower new electronic states are created that stay in between the conduction and valence band and thereby reduce the width of the forbidden energy gap.

Fig. 5.

Kubelka- Munk plots for determination of optical band gap of MoS₂ nanoflower and MoS₂@Co₃O₄ nanocomposite.

3.4. Electronic properties

The calculated electronic band structure of pristine MoS₂ and MoS₂@Co₃O₄ is illustrated in Fig. 6. (a) and (b) within the first Brillouin zone along highly symmetry directions G-F-Q-Z-G with zero pressure. The Fermi level (E_f) is set to 0 eV with the energy range from -6eV to 3eV for MoS₂ and -2eV to 0.5 eV for MoS₂@Co₃O₄ nanocomposite. The calculated energy gap of the MoS₂ monolayer is 1.83eV, which is close to the other experimental and theoretical values of the MoS₂ monolayer [57]. After gas adsorption or MoS₂-based heterostructure, the band structure of the MoS₂ monolayer significantly decreased [58]. In the case of a Co₃O₄ molecular structure like nanoparticles adsorbed on a MoS₂ monolayer, the energy gap of pristine MoS₂ is remarkably reduced to 0.019eV. Due to the decrease in energy gap, the conductivity of the MoS₂@ Co₃O₄ nanocomposite significantly increased, and resistance decreased. Generally, conductivity is reciprocal of the resistivity and expressed as σ = ne where n is the carrier concentration, and e is the electron charge, respectively. The smaller band gap corresponds to a higher carrier concentration and, consequently, high conductivity. MoS₂@Co₃O₄ nanocomposite's band structure shifts in the direction of least energy, which also suggests an exothermic process [37]. Additionally, defect states are created near the fermi level, which significantly changes the transport properties.

Fig. 6.

The electronic band structure (a) MoS₂, and (b) MoS₂@Co₃O₄ nanocomposite.

The DOS for MoS₂ and MoS₂@Co₃O₄ at ambient pressure is shown in Fig. 7(a) and 7(b). The calculated energy gap (E_g) from the density of states (DOS) of the MoS₂ monolayer is 1.83 eV and MoS₂@Co₃O₄ nanocomposite is 0.019 eV. Similar results were obtained from the band structure calculation. The density of states reveals p-type semiconducting behavior for MoS₂ and n-type semiconducting behavior for MoS₂@Co₃O₄. The DOS of MoS₂ had eight peaks and after the adsorption of Co₃O₄ nanoparticles, the heterostructure showed three dominating peaks which affects the conductivity of the system. Furthermore, as a result of adsorption, the density of states of the nanocomposite shifted towards the Fermi level compared to the MoS₂ suggesting electrons gathered near the Fermi level due to the adsorption of nanoparticles. The DOS of the composite shows a slight zigzag that evident that the defect states are created in the heterostructure due to the effect of Co₃O₄ nanoparticles. These defect states help to create trapping which hindrance the electron-hole pairs that increase the photocatalytic performance.

Fig. 7.

Density of states of (a) MoS₂, and (b) MoS₂@Co₃O₄ nanocomposite.

3.5. Photocatalytic properties

The Photocatalytic activity of MoS₂ nanoflower and MoS₂@Co₃O₄ composite were evaluated by studying the disintegration characteristics of MB dye under visible light illumination. The degradation of the MB solution was determined by the intensity of the absorption spectrum at 664 nm with a definite interval of time. The absorption intensity of MB solution changes with MoS₂ and MoS₂@Co₃O₄ under different illumination times is presented in Fig. 8(a) and 8(b). From the figure, it is evident that the peak intensity decreases faster for MoS₂@Co₃O₄ composite than MoS₂. After 500 min of visible light illumination into the MB solution, the degradation efficiency was found to be 27.96% for MoS₂, which became 78.89% due to the nanocomposite. The rate of degradation MoS₂@Co₃O₄ nanocomposite is superior to the previous work shown in Table. S1 [59,60] This indicates that the incorporation of Co₃O₄ nanoparticles significantly improves the photocatalytic performance of MoS₂ nanoflower.

Fig. 8.

The effect of (a) MoS₂ nanoflower and (b) MoS₂@Co₃O₄ nanocomposite on the absorption spectra of MB solution for different reaction time under visible light illumination and, (c)) ln(C/C₀) as function of visible light irradiation time for the degradation of MB dye with the presence MoS₂ and MoS₂@Co₃O₄ as catalyst.

3.6. Kinetics of photocatalytic activity

The kinematics of the removal of different organic dyes are explained by the Langmuir-Hinshelwood (L-H) model. The L-H model predicts that the reaction rate, R, during degradation of organic pollutants is proportional to the portion of the surface of the reactant, θ [61],

$$R=-\frac{dC}{dt}=k_r\theta \text{;}$$ (2)

Here, the reactant concentration C, and $k_r$ is the reaction rate constant. The absorption coefficient, K of the reactant, helps determine the value of θ and the above equation (2) can be written as -

$$R=-\frac{dC}{dt}=k_r\theta =k_r\frac{KC}{1+KC}\text{;}$$ (3)

In case of very low starting concentration $C_0$ integrating of equation (3) yields:

$$\ln \left(\frac{C}{C_0}\right)=kt\text{;}$$

Where, $k$ is the first-order reaction rate constant.

Fig. 8(c) shows the plot of $\ln \left(\frac{C}{C_0}\right)$vs irradiation time, the graph is linear, and the slope provides the reaction rate of the corresponding sample. The curve is linear, which indicates that the kinetics of the photocatalytic degradation of MB follows a pseudo-first-order reaction rate [62]. The first-order reaction rate indicates the degradation efficiency [63]. The higher reaction constant was found for MoS₂@Co₃O₄, than MoS₂; their values are 0.00059 min⁻¹ and 0.0026 min⁻¹.

3.7. Mechanism of photocatalytic activity

Fig. 9 represents a schematic diagram that depicts a model that can be used to describe the mechanism for the photocatalytic performance of MoS₂, MoS₂@Co₃O₄. When light incident upon the MoS₂ it produces an electron (e⁻) from the valence band that leaves a hole (h⁺) in the valence band. The as-produced electrons and holes then move to the surface. The holes oxidize water molecules ($O{H}^{-}\mathrm{i}\mathrm{o}\mathrm{n})$ and produces Hydroxyl radical $\left(\bullet OH\right)$ whereas the electrons interact with atmospheric oxygen and form super-oxide (${\bullet O}_2^{-}$). These radicals react with the dye, and the dye starts to degrade. The reaction that occurs during the degradation is given below [64]:

$$Mo{S}_2+h\nu \to e^{-}+h^{+}$$

$$H_{2}O+h^{+}\to O{H}^{-}+H^{+}$$

$$O_2+e_{\to }^{-}{\bullet O}_2^{-}$$

$${\bullet O}_2^{-}+H^{+}\to H{O}_2$$

$$H{O}_2+\bullet O_2^{-}+H^{+}\to H_2{O}_2+O_2$$

$$H_2{O}_2+e^{-}\to O{H}^{-}+\bullet OH$$

$$\bullet O_2^{-}+O{H}^{-}+Dye\to Degradedye$$

Fig. 9.

Schematic diagrams showing the photo-catalytic mechanism of (a) MoS₂ and (b) MoS₂@Co₃O₄ composite.

The photo-generated charge carriers are unstable, and they jump back from the conduction band and recombine with the hole and thereby decreasing the photocatalytic efficiency. To enhance the degradation efficiency the recombination process must need to be decelerated. From the DFT analysis, it was observed that when Co₃O₄ was incorporated with the MoS₂ then defect states were created. These defects can act as charge traps for the photo-generated charge carrier and hence reduce the electron-hole recombination and thereby increase the photocatalytic efficiency [65]. Furthermore, from the XRD analysis, it is observed that the crystallite size of the nanomaterials reduces due to the decoration of Co₃O₄ nanoparticles. The reduction of the particle size provides a large surface area, and as a result, more active sites become available for the absorption of dye molecules which helps to improve the photocatalytic performance [66]. Additionally, the UV–vis analysis reveals that the band gap and a narrowing of the optical band gap of MoS₂ occur due to the incorporation of Co₃O₄ nanoparticles. The reduced band gap permits more charge carriers to be able to participate in the photocatalytic reaction and, as a result, increase the photocatalytic efficiency [45].

4. Conclusion

To encapsulate, MoS₂ nanoflower and Co₃O₄ decorated MoS₂ nanoflower were prepared via a simple hydrothermal method. The effect of decoration Co₃O₄ nanoparticles on the surface morphology, crystallite size, optical band gap, electronic properties, and photocatalytic efficiency of MoS₂ was studied. The composite shows better photocatalytic activity than MoS₂. The improvement of the photocatalytic activity can be attributed to the lessening of the charge-carrier recombination rate due to the creation of defect states, lowering of the band gap, and increased surface area as a consequence of Co₃O₄ decoration. The Co₃O₄ decorated MoS₂ nanoflower synthesized by a simple and economic route may offer a sustainable route to improve the photocatalytic performance of MoS₂ and can find their applications in water purification and environment cleaning.

Author contribution statement

Mizanur Rahaman: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper. Md. Hasive Ahmed, Sarker Md. Sadman: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data. Muhammad Rakibul Islam: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

Data will be made available on request.

Declaration of interest's statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

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