S-scheme heterojunction of MoO₃ nanobelts and MoS₂ nanoflowers for photocatalytic degradation

Abstract

This project presents the fabrication of an efficient heterojunction photocatalyst through combining 3D MoS₂ nanoflowers with 2D MoO₃ nanobelts, both having highly prominent photocatalytic features. The prepared MoS₂@MoO₃ heterojunction exhibited superior photocatalytic activity towards the degradation of Azo dye under visible light irradiation and attained about 96% degradation within four hours. Such a high photocatalytic activity might be associated with the high BET surface area, and especially with the S-scheme mechanism that occurs between p-type MoS₂ and p-type MoO₃, probably due to the fact that this offers effectively separated and transitioned photogenerated electron-hole pairs, while the recombination rate is reduced. The addition of MoO₃ increased the bandgap of MoS₂ and consequently enhanced the photoinduced electron transfer rate and prolonged the lifetime of the charge carriers. In a word, the generation of hole and ^•O₂^– radicals in the whole process of degradation, which have been proved by scavenger tests and Mott-Schottky analysis, proved the MoS₂@MoO₃ p-p heterojunction to be photocatalytically active. This work underlines the successful application of bandgap and morphological engineering in the design of photocatalysts and points out the 3D/2D MoS₂@MoO₃ heterojunction structure as the basis for further development of transition metal chalcogenide (TMC)/transition metal oxide (TMO) photocatalysts with a view to tackling important environmental problems by means of sustainable technologies.

Introduction

Recently, some cost-effective transition metal chalcogenides such as selenides, phosphates, and sulfides are considered to exhibit excellent catalytic activities¹. Among such materials, molybdenum disulfide (MoS₂) drew the attention of many researchers due to its distinctive layered structure². MoS₂, representing the transition metal disulfides in a 3D form, is a semiconductor with a narrow band gap of 1.2–1.9 eV and an anisotropic layered structure, with weak van der Waals interactions between the S-Mo-S layers in each sandwich structure^3–5. These provide it with an extremely large specific surface area and thus offer a permeable pathway during the adsorption and transport processes of molecules^6–10. Consequently, the narrow absorption range and fast recombination of photoinduced electron-hole pairs reduce the photocatalytic removal efficiency of 3D-MoS₂ for organic pollutants. In addition, MoS₂ can shorten electron transport distances and increase charge carrier separation to lessen photo-corrosion in photocatalytic reactions¹¹. Unfortunately, the practical photocatalytic efficiency of MoS₂ is rather unsatisfactory because its photogenerated carriers may recombine very rapidly. Recent studies have demonstrated that the optimization of morphology, creation of vacancies, and doping may improve the actual catalytic performance^12–14. In this regard, Zheng et al.¹⁵ modified S, N, P-co-doped carbon (SNP–C) nanofibers with MoS2 using electrospinning and nonthermal plasma-sulfurization methods to enhance their adsorption and photocatalytic efficiency. In the case of MoS₂/SNP-C composites, the RhB removal under visible light is heavily dependent on the mass ratio of Mo/P and the time for sulfurization. Xie et al.¹⁶ prepared a 3D flower-shaped g-C₃N₄/MoS₂ composite through in-situ hydrothermal synthesis, incorporating MoS₂ onto carbon nitride with structural defects. The catalyst was able to degrade 98% of MB within 60 min and showed a removal efficiency of 96.4% after five cycles, revealing its excellent stability and regeneration.

The constructing heterostructurs with different morphologies to adjust the surface electronic structure has proven to be the most effective and straightforward method of improving catalytic performance. Electron flow between semiconductors in the process of forming heterojunctions is quite crucial to determining catalytic efficiency. S-scheme heterojunctions, having the shape of staircase-like electron transfer, are formed by an oxidation photocatalyst (OP) and a reduction photocatalyst (RP)¹². When they get in contact, electrons from the higher Fermi level of RP migrate to the lower Fermi level of OP, forming a built-in electric field that forces photogenerated electrons from the conduction band (CB) of the OP into the valence band (VB) of the RP, which is facilitated by Coulombic attraction and band bending. Such charge transfer impedes reverse electron transfer, such that highly energetic photogenerated electrons and holes are confined in the RP CB and OP VB, respectively, for boosting redox capacity and photocatalytic performance. Despite initial use for n-type semiconductors¹⁷, the concept has since been adapted^19,20 for application to p-type semiconductors with only the need being that RP CB and the Fermi level need to be greater than for OP. These yield four potential S-scheme heterojunctions: n-n, p-p, p-n, and n-p¹⁸, with the first being OP and the second RP¹⁹. Moreover, the semiconductor redox potential and the construction of a built-in electric field at the interface may further enhance the photocatalytic reaction and optimize the catalytic activities effectively^20–23.

Two-dimensional (2D) materials have emerged recently as a new class of materials that have been considered the most attractive categories because of their unique properties and wide range of applications in electronics, photonics, and energy storage^24–26. Atomic-scale thickness, combined with exceptional mechanical, electrical, and thermal properties, enables new functionalities not accessible using conventional materials²⁷. MoO₃ as a 2D material is a transition metal oxide and is reported to be a photocatalytic material with immense capacity for contaminant adsorption. In the case of a MoO₃ lattice, significant tetrahedral and octahedral gaps exist, acting as a perfect ion location and useful channel in particular carrier transitions during the catalytic process. Compared with other photocatalytic semiconductor materials, MoO₃ possesses the advantages of high chemical stability, low preparation cost, and rich reserve. The wide band gap of about 3.1 eV makes it hard for MoO₃ to respond to visible light photocatalytically. However, that fact does not impede combining it with some semiconductors in order to improve the photocatalytic performance^28–32. For example, Chuang et al.³³ prepared Fe₂O₃-decorated MoO₃ rods by using a two-step process and evaluated their photocatalytic activity in the photodegradation of methylene blue (MB). The results demonstrated that the photodegradation efficiency of Fe₂O₃/MoO₃ was improved owing to the decrease in band gap energy and the stability of the 1D hexagonal prism structure. Zhang et al.³⁴ fabricated a novel 0D-2D Z-scheme heterojunction photocatalyst (MoO₃/g-C₃N₄) by a simple hydrothermal calcination method. They tested the catalytic activity of MoO₃/g-C₃N₄ by investigating its efficiency in the degradation of tetracycline. The results indicated that the 0D-2D MoO₃/g-C₃N₄ Z-scheme heterojunction showed much higher activity than the original g-C₃N₄.

This work will try to design an efficient photocatalyst by combining bandgap engineering and morphology engineering for methylene blue (MB) photodegradation. The 3D MoS₂ nanoflowers were, therefore, synthesized by the hydrothermal oxidation method. Later, MoO₃ nanobelt was grown by using self-assembly electrostatic method onto the surface of the 3D MoS₂ nanoflowers for making the S-scheme heterojunction photocatalyst, which was confirmed by the experimental analysis. The visible light photocatalytic performance of the 3D/2D MoS₂@MoO₃ heterojunction was thereby enhanced. The enhancement of redox capability may be under the synergistic effect of two kinds of semiconductors with different redox energy levels and the designed heterojunction structure. At the end, this work could extend the heterojunction structure to the design of other MoS₂-based photocatalysts used to combat environmental issues.

Experimental details

Synthesis of MoS₂ nanoflowers

A facile hydrothermal process was used to make the flower-like MoS₂ nanostructure.1.2 g of thiourea, and 0.8 g of (NH₄)₆ Mo₇O₂₄.4H₂O powders were added to a beaker containing 50 ml of deionized (DI) water and stirred for 10 min. Then, 0.02 g of Cetyltrimethylammonium bromide (CTAB) as a surfactant was added to the solution and stirred at low speed for 5 min, producing a relatively clear suspension. The pH was reduced to around two by adding HCl (12 M), resulting in a clear solution. The prepared solution was placed in a Teflon-lined stainless-steel autoclave and heated to 200 °C for 24 h. Finally, after reaching room temperature, the sediments were rinsed five times with DI water and ethanol. The sediments were dried in an oven for 24 h at 80 ° C.

Synthesis of MoO₃ nanobelts

The MoO₃ nanobelts were created using a hydrothermal technique. In a beaker containing 25 ml distilled water, 2.04 g sodium molybdate was added and stirred for 5 min until completely dissolved. The pH of the solution was then brought close to 1 by using 4 M HNO₃. The obtained solution was then transferred to an autoclave with a 35 ml Teflon container and placed in a hot box for 24 h at 180 °C. It was naturally cooled to room temperature after this operation. The resultant white precipitates were centrifuged six times at 4000 rpm for approximately 6 min with distilled water and ethanol. The washing was then continued with 500 ml of distilled water using a vacuum pump and a Buchner funnel. The sediments were put in an oven and dried for 12 h at 80 °C.

Synthesis of MoS₂@xMoO₃ (x = 1, 5, and 10%) heterojunction

MoS₂@1%MoO₃ heterojunctions were synthesized using self-assembly electrostatic method. To generate MoS₂@1%MoO₃ heterojunctions, 0.5 g of MoS₂ and 0.005 MoO₃ were added to a solution comprising 28 ml of ethanol and 8 ml of DI water 80 µl of 1 M HCl and stirred for 5 min. The samples were then dispersed in an ultrasonic bath for 30 min before being placed in a Teflon-lined stainless-steel autoclave and heated for 12 h at 160 °C. It was then allowed to cool to ambient temperature naturally after this operation. The precipitates were collected and centrifuged three times with distilled water at 4000 rpm for about 10 min each, then dried for 8 h in a 60 °C oven. Also, the same process was utilized to make MoS₂@5%MoO₃ and MoS₂/10%MoO₃ nanocomposites with the amounts of 0.025 and 0.05 g of MoO₃ replaced, respectively.

Material characterization

Using a field-emission scanning electron microscope (FE-SEM, ZEISS Sigma 300) and an X-ray diffractometer (XRD, Bruker D8 Advance) with Cu Kα radiation (λ = 1.5418 A˚), the samples’ morphology and structure were characterized. Attenuated total reflectance (ATR) spectroscopy, specifically using the Bruker Invenio S, was employed to identify the functional groups present in each of the generated samples. The microscopic structure, chemical composition, and distribution of elements in the synthesised photocatalysts were studied with a field emission scanning electron microscope (FE-SEM, ZEISS Sigma 300) equipped with an EDX elemental mapping system. The size of the nanoparticles and their zeta potential (ζ) were evaluated using a Zetasizer Nano ZSU3100 from Malvern Instruments Ltd. A UV-Vis spectrophotometer (Shimadzu UV- DR 2800) was used to measure the absorbance. A UV-Vis spectrometer (Specord 210 plus, Analytikjena, Germany) performed UV-Vis diffuse reflectance spectra (UV-Vis DRS). To investigate the photoluminescence (PL) spectrum of the produced photocatalysts, which excited wavelength at 420 nm, the fluorescence spectrometer (G9800A, Agilent, USA) was used. A Belsorp mini II analyzer (Microtrac Bel Corp, Japan) was used to determine the synthesized samples’ Brunauer-Emmett-Teller (BET) surface area and the N₂ adsorption-desorption isotherm.

Evaluation of the photocatalytic activity

The MB solution’s degradation was provided to measure the samples’ photocatalytic activity. To reach adsorption-desorption equilibrium, 0.01 mg of synthesized photocatalysts was added to 50 ml of MB solution (20 ppm) and stirred in the dark for 30 min. An LED lamp (30 W) was utilized as a visible light source to illuminate the solution. Centrifugation was carried out at 5000 rpm to separate suspension particles from the samples, and the MB concentration was read at 664 nm. The removal percentage of MB (%R) can be determined using Eq. (1)³⁵:

1

Also, the photocatalytic activity of the optimized nanocomposite was investigated in acidic, neutral, and alkaline environmental conditions. Solutions of HCl and NaOH were used to study the photocatalytic activity of the optimal combination at various pH. The solutions’ pH was adjusted to 5 and 9 using the mentioned acid and base, and the amount of degradation was measured and compared to this amount at neutral pH conditions. The stability of the optimal photocatalyst was confirmed by performing three cycles of MB photodegradation. For this purpose, the used photocatalyst was recovered by centrifugation, washed with deionized water, and then dried at 60 °C before the next run.

Role of active species and mechanism determining

To evaluate the active species participating in the process, scavenging studies with the addition of different scavengers to the photocatalytic system were conducted under the same experimental circumstances as those indicated above. The hydroxyl radicals (^•OH), superoxide anion radicals (O₂^•−), and holes (h⁺) were each trapped by different scavengers in this study, including tert-butyl alcohol (t-BuOH), 1, 4-benzoquinone (BQ), and ammonium oxalate (AO), respectively³⁶.

Results and discussion

Structure analysis

Figure 1a shows the XRD patterns of MoS₂, MoO₃, and their nanocomposites MoS₂@xMoO₃ (x = 1, 5, and 10%). Pure MoS₂ nanoflowers exhibited distinctive peaks appeared at 2θ = 14.38^◦, 32.68^◦, 39.54^◦, 47.76^◦ and 58.33^◦ corresponded to (002), (100), (103), (105), and (110) crystallographic planes of MoS₂, respectively. The XRD diffraction peaks of MoS₂ nanoflowers are well congruent with the crystallographic planes of the MoS₂ JCPDS card (No. 37-1492). Furthermore, the broadening of the peaks suggests that the particles have been synthesized in nano dimensions, consistent with the Debye-Scherrer equation³⁷. The XRD pattern of hydrothermally synthesized MoO₃ nanobelts is displayed in Fig. 1a. As shown in the Fig. 1a, the peaks of 2θ: 12.6˚, 23.2˚, 25.6˚, 27.2˚, 33.6˚, 38.9˚, 45.7˚, 49.1˚, 58.8˚, and 64.4˚, respectively, correspond to plates (020), (110), (040), (021), (111), (060), (200), (002), (081), and (062). This pattern also complies with the JCPDS standard card No. 89-5108. The XRD pattern of MoS₂ nanoflowers and pure MoO₃ are compared to the XRD patterns of their nanocomposites in Fig. 1a. It is clear from these comparisons that the 2θ diffraction angles of the MoO₃ nanobelts are well placed on the XRD pattern of the MoS₂ nanoflowers. This indicates that the two materials are well-composited together. XRD pattern also shows that the intensity of the peaks related to this material has increased as the percentage of MoO₃ nanobelts in manufacturing nanocomposites has increased.

Fig. 1.

(a) XRD patterns of MoS₂, MoO₃, and MoS₂@xMoO₃ (x = 1, 5, and 10%), (b) ATR- FTIR spectrum of MoS₂, MoO₃, and MoS₂@5%MoO₃, (c) N₂ adsorption-desorption isotherms and pore size distribution plots of MoS₂, MoO₃, and MoS₂@5%MoO₃, Size distribution plot (from DLS) of (d) MoS₂, (e) MoO₃, and (f) MoS₂@5%MoO₃.

Figure 1b shows the ATR-FTIR spectrum of MoS₂, MoO₃, and their nanocomposites MoS₂@5%MoO₃. In the case of MoO₃, specific spectral peaks in distinct ranges point toward characteristic structural features. Strong peaks between 900 and 1000 cm⁻¹ are associated with Mo = O stretching vibrations, confirming the presence of molybdenum-oxygen double bonds. Peaks between 600 and 800 cm⁻¹ correspond to Mo-O-Mo stretching vibrations, indicating oxygen bridges between molybdenum atoms. Also, peaks below 500 cm⁻¹ are assigned to Mo-O bending vibrations. In the case of MoS₂, the main peaks are between 600 and 800 cm⁻¹, which correspond to Mo-S stretching vibrations. Peaks below 500 cm⁻¹ are assigned to other Mo-S bond vibrations. In the composite of MoS₂ and MoO₃, peaks from both materials are observed. The peaks between 900 and 1000 cm⁻¹ confirm the presence of MoO₃, while peaks between 600 and 800 cm⁻¹ and below 500 cm⁻¹ confirm the presence of MoS₂.

The nitrogen gas adsorption and desorption isotherm diagram for pure MoS₂ and optimized photocatalysts is shown in Fig. 1c. Also, Table 1 gives details on the specific surface area, volume, and radius of the pores. According to the surface area analysis results, the synthesized photocatalysts have an appropriate surface area, and surface adsorption does not have much effect on removing pollutants in the presence of photocatalyst. It is possible to see the improvement of the optimized photocatalyst compared to pure MoS₂ by comparing the volume of pores and the specific surface area of the optimal photocatalysts and pure MoS₂. In this way, with the pore radius remaining constant, the average parameters of the specific surface area and the volume of the pores of the optimized photocatalyst increased by 35% and 17%, respectively. This improvement can also be inferred from comparing the nitrogen gas adsorption-desorption isotherm graphs of different photocatalysts, as shown in Fig. 1c. This is because the optimal photocatalyst has a lower average pressure of nitrogen gas entering its pores than the inferior one. The Barrett-Joyner-Halenda (BJH) test, which determines the distribution and average size of the pores in a structure, was carried out for a more thorough analysis of the morphology of the compounds, as mentioned earlier. The curve obtained by BJH test revealed that the maximum size distribution of the pores is less than 10 nm and that the photocatalysts’ structure is mesoporous. Additionally, by comparing the BJH graphs of different photocatalysts, the most suitable photocatalyst has the most significant distribution of pores with a size of less than 10 nm.

Table 1.

Surface properties attained from the BET study for MoS₂ and MoS₂@5%MoO₃ photocatalysts.

Sample	BET surface area (m2g− 1)	Pore volume (cm3g− 1)	Mean pore diameter (nm)
MoS2	59.33	0.37	25.25
MoS2@5%MoO3	69.11	0.50	29.29

Figure 1d-f show the size and zeta potential characteristics of the samples MoS₂, MoO₃, and MoS₂@5%MoO₃, respectively. In the case of MoS₂, the Z-average was 568.2 nm, which suggests that this is a polydisperse system with aggregates, which agreed with the high polydispersity index (PDI) of 0.8749. The mean zeta potential of -51.25 mV indicates high stability of suspension because of enough electrostatic repulsion. Sample MoO₃ had a Z-average of 438.7 nm, with a PDI of 0.23, suggesting a size distribution narrower than that for MoS₂. Mean zeta potential of -57.07 mV confirmed its stability. The Sample MoS₂@MoO₃ had a Z-average of 547.8 nm, indicating that the particles are polydispersed, with a PDI of 0.4986. The mean value of zeta potential determined as -55.59 mV showed stable particle dispersion. These results demonstrate the size distribution and electrostatic stability of the samples.

Morphological analysis

The morphology and particle size of MoS₂, MoO₃, and MoS₂@5%MoO₃ heterojunction were explored by SEM images, as shown in Fig. 2a–c. MoS₂ nanoparticles are flower-like, as shown in Fig. 2a. On the other hand, the uneven nanoplates of this shape give MoS₂ nanoparticles a relatively large surface area, which is critical for photocatalytic activity. Additionally, Fig. 2a shows that the particles are nanostructured because of their 12–80 nm size range. These micrographs also show the distribution and uniform size of the particles. MoO₃ nanoparticles with nano-belt morphology were successfully synthesized, as shown in Fig. 2b. The size of the nanoparticles, which ranges from 40 to 90 nm, is shown in Fig. 2b. Figure 2c displays micrographs of nanocomposite of MoS₂@5%MoO₃. The presence of MoO₃ nanoparticles with approximate dimensions of 50 nm between MoS₂ flower-liked branches can be identified by comparing the FE-SEM images obtained from the surface of MoO₃ and MoS₂ nanoparticles with the nanocomposite micrographs of these two materials. Nanoparticles of MoO₃ are broken by the ultrasonic and hydrothermal processes during the composite process and are scattered on the surface of MoS₂. Most of the MoS₂ surface’s photocatalytic active sites are near its side edges, where many van der Waals bonds with crystal defects cause accumulation. This phenomenon reduces the number of edge active sites, and as a result, the photocatalytic activity decreases. However, they are sparsely protected by MoO₃-loaded nanoparticles, which prevents the decrease of the photocatalytic active sites and enhances photocatalytic activity. EDX analysis was performed for elemental composition of MoS₂, MoO₃, and MoS₂@5%MoO₃ nanocomposites, as shown in Fig. 3a-c. The EDX spectrum for the MoS₂ nanoflower confirms the existence of Mo and S elements, showing a Mo/S atomic ratio of about 0.47 in the MoS₂ structure. In addition, the absence of peaks for other elements in EDX spectra indicates that the particles of MoS₂ have been synthesized in a pure form. Elemental mappings confirm the even distribution of Mo and S in the MoS₂ structure (Fig. 3a). From Fig. 3b, there are Mo and O components in the MoO₃ sample, and the Mo/O atomic ratio is 0.17, which agrees with the elemental mappings and confirms that Mo and O elements are dispersed evenly in the MoO₃ structure. The elemental mappings for the MoS₂@5%MoO₃ sample, as shown in Fig. 3c, indicate that the Mo, S, and O elements are homogeneously distributed in the nanocomposite structure. Besides, elemental mappings also confirm the homogeneous distribution of Mo, O, and S in the MoS₂@5%MoO₃ structure.

Fig. 2.

FE-SEM images of (a) MoS₂, (b) MoO₃, and (c) MoS₂@5%MoO₃.

Fig. 3.

EDS spectrum and SEM-elemental maps of (a) MoS₂, (b) MoO₃, and (c) MoS₂@5%MoO₃.

Photoelectrochemistry and optical absorption analysis

The optical absorption properties and band gap energy (E_g) of the prepared photocatalysts were characterized by the UV–vis DRS technique in the range from 200 to 1200 nm, as shown in Fig. 4a-d. The MoS₂ sample showed significant absorption in the visible light region, and its absorption edge is approximately at 420 nm. In contrast, MoO₃ exhibited significant absorption spanning the UV-vis-NIR range without a well-defined absorption edge. Moreover, MoS₂@5%MoO₃ showed significant absorption in the UV-vis-NIR region, with an absorption peak at about 320 nm. This suggests that the absorption intensity of MoS₂@5%MoO₃ was higher than that of MoS₂, which further resulted in a higher yield of photoinduced electron-hole pairs. This may be ascribed to the broader band gap of MoS₂@5%MoO₃ due to the attachment of MoO₃ onto MoS₂. The amount of the band gap energy of photocatalysts can be calculated using the following Eqs^38,39:

Fig. 4.

(a) UV–vis adsorption spectra, (b-d) Tauc plots, (e-g) Mott-Schottky plots, (h) PL spectra, (i) EIS plots related to synthesized photocatalysts.

1

The absorption coefficient (α), Planck’s constant (h), frequency of light (υ), band gap energy (E_g), and type of electron transfer (n = 1/2, 2/3, 2, and 3 for allowed, forbidden, and indirect transfers, respectively) are all represented in this equation. The energy gap is obtained by drawing the curve (αhυ)^1/2 against hυ and extrapolating the linear part (αhυ)^1/2 to zero (x-axis). This curve is known as the Tauc diagram. In this investigation, the band gap energy was calculated from the following equation:

2

where λ_g is the peak wavelength of the band gap energy. Figure 4b-d show Tauc diagram used to determine the band gap energy of the synthesized photocatalysts. In this study, the band gap energy value for MoS₂, which corresponds to a wavelength of 750 nm, is equivalent to 1.7 eV; for MoO₃, which corresponds to a wavelength of 393.65 nm, equal to 3.3 eV; for MoS₂@5%MoO₃, which corresponds to a wavelength of 692 nm, is equal to 1.8 eV. According to the values, the increase in absorption in the visible region is observed due to the composite process. A shift towards shorter wavelengths and an increase in the band gap energy are observed in the case of photocatalysts MoS₂ and MoS₂@5%MoO₃, respectively. As a result of these modifications, there is less recombination and the creation of holes with higher oxidizing potential, which enhances the photocatalytic activities and more efficiently eliminates the pollutant.

Understanding the energy level of VB and CB is necessary to investigate the photocatalytic mechanism. In this work, Mott-Schottky plots were used to estimate the flat band potential of the synthesized samples (Fig. 4e-g). From the x-axis intercept of the Mott-Schottky plot, the calculated E_fb for MoS₂, MoO₃, and MoS₂@5%MoO₃ are − 0.54, -0.12, and − 0.55 V (vs. NHE), respectively. The negative slopes of the plots in Fig. 4e-g reveal that all samples are p-type semiconductors. The calculated E_fb, E_CB, and E_VB values are listed in Table 2 for each sample. The CB values of MoS₂, MoO₃, and MoS₂@5%MoO₃ were calculated to be -0.54, -12, and − 0.55 V versus NHE, respectively. Similarly, using the calculated Eg values, the corresponding VB values were determined as VB = CB + E_g. Calculated VB values for MoS₂, MoO₃, and MoS₂@5%MoO₃ are + 1.16, + 3.18, and + 1.12 V versus NHE, respectively. A favorable band alignment is achieved between MoS₂ and MoO₃ allowing efficient charge transfer between the two.

Table 2.

The band structure properties of MoS₂, MoO₃, and MoS₂@5%MoO₃.

Sample	Band gap (eV)	Semiconductor type	Flat band (V vs. NHE)	Conduction band (V vs. NHE)	Valance band (V vs. NHE)
MoS2	1.7	P	-0.54	-0.54	+ 1.16
MoO3	3.3	P	-0.12	-0.12	+ 3.18
MoS2@5%MoO3	1.8	P	-0.55	-0.55	+ 1.12

The spectrum of photoluminescence emission for MoS₂ and the MoS₂@5%MoO₃ within the wavelength range of 380 to 630 nm can be seen in Fig. 4h. These photocatalysts were first excited by 360 nm photon radiation, and their photoluminescence emission was subsequently evaluated in the visible spectrum. The Fig. 4h shows that the MoS₂ emission intensity is higher than the MoS₂@5%MoO₃ nanocomposite. This indicates that the MoS₂ photocatalyst has a higher recombination rate than the modified sample. In other words, the MoS₂ photocatalyst’s photoluminescence emission intensity decreases when the nanocomposite fabrication method is applied to modify it. This tells us how well the modification technique worked to separate the electron holes produced by visible light radiation, reducing the recombination rate. This can increase the dye degradation efficiency by improving the photocatalytic properties.

The efficiency of the charge transfer and separation after photogenerated processes was further investigated by EIS and photocurrent measurements. Figure 4i illustrates the Nyquist plots of the synthesized photocatalysts, which show resistance to photocharge transfer at the liquid-solid interface. Smaller arc radii in the Nyquist plot usually correspond to more effective photocharge separation and a faster photocharge transfer across the electrode/electrolyte interface. It is observed that the addition of MoO₃ to MoS₂ decreased the arc radius in the Nyquist plot, which is consistent with the enhanced conductivity of MoS₂@5%MoO₃. This suggests an improved charge separation during the photocatalytic process for MoS₂@5%MoO₃ and reduced resistance to charge transfer at the electrode/electrolyte interface.

Photocatalytic activity

MB degradation

The study examined the synthesized photocatalysts’ photocatalytic activity under visible light and dark conditions for degrading MB dye. According to the test results, the adsorption of synthesized photocatalysts in the dark is calculated at about 20% in 30 min. To achieve a higher degree of pollutant removal, including a photocatalyst with light irradiation is imperative. According to Fig. 5a, the dye removal was enhanced in the presence of the MoS₂ photocatalyst to 63.2%. The amount of removal was calculated as 91.1%, 98.2%, and 51.6%, respectively, for nanocomposites containing 1, 5, and 10% of MoO₃, as shown in Fig. 5a; it can be said that the modification of MoS₂ nanoflowers was effective based on the general rise in the degradation rate. Of course, the degradation percentage is lower in the case of a 10% by-weight nanocomposite; this can be explained by the phenomenon of agglomeration brought on by an excessive amount. Figure 5b shows the degradation kinetics of MB by the synthesized photocatalysts. The concentration changes of MB over time follow a quasi-first-order kinetic model. The reaction rate constant (k) increases with the addition of MoO₃ nanobelts to the MoS₂ structure. Notably, the impact of MoO₃ nanobelts on increasing k is more pronounced than that of MoS₂ nanoflowers, potentially due to structural defects introduced by MoO₃ in the MoS₂ structure. The highest k value is observed for the binary nanocomposite MoS₂@5%MoO₃ (0.6486), which is approximately 5 times greater than that of pristine MoS₂ (0.1135).The pH of the solution can significantly influence the photocatalytic process. This effect can be attributed to the electrostatic interactions between the catalyst surface and the target substrate. The findings (Fig. 5c) showed that the degradation percentage rose under alkaline conditions, and in about 6 h, the dye was removed entirely. In an acidic environment, this value dropped, and in 6 h, it reached 74%. This might be because MoS₂ has an isoelectric point (IEP) of approximately 3. Consequently, more removal happens at pH values higher than seven because the nanoparticle performs better at eliminating cationic dyes. It is clear from these data that the photocatalyst under consideration has the highest removal rate and functions best in alkaline environments. For practical photocatalytic applications, not only high photocatalytic activity but also stability and recyclability are necessary. Figure 5d depicts the photostability of the MoS₂@5%MoO₃ heterostructure in UV-Vis light during five successive cycles of MB degradation. Under five successive cycles of solar irradiation, the photocatalyst could retain up to 75% MB degradation efficiency, with only a 21% decrease from the fresh performance. The slight reduction in performance may be attributed to the recovery process-washing and drying of the recovered catalyst. The synthetic composite is reusable without a significant loss in activity, reflecting its stability for long-term applications. The scavenger test was employed to look into the mechanism of the photocatalytic process by the optimal photocatalyst under visible light illumination. Some experiments were carried out to trap reactive species to determine how MB is destroyed. To achieve this, ammonium oxalate (AO), tert-butyl alcohol (t-BA), and benzoquinone (BQ) have been employed as the scavengers of the active species, reactive hole species (h⁺), hydroxide radical (^•OH), and superoxide ion (O₂^•−). So, when initiating the photocatalytic process, each scavenger was added independently to each solution containing the dye pollutant at a concentration of one millimolar. It is possible to identify the effective active species on the degradation process by analyzing the impact of each of these scavengers on the efficiency of photocatalytic removal. In this way, reducing removal efficiency by adding a scavenger associated with an active species indicates its effectiveness and participation in photocatalytic degradation. The impact of these scavengers on the MoS₂@5%MoO₃ photocatalytic process is shown in Fig. 5e. The removal efficiency of MB is reduced from 98.2 to 55% by the ammonium oxalate scavenger with the highest inhibition rate, as can be observed. Of this removal amount, approximately 20% is attributed to dye photolysis and the remaining portion to other removal mechanisms. This scavenger acts as a hole scavenger. As a result, the blocking of the hole is the main reason for the reduction of the removal efficiency, and as a result, the main factor of destruction in this study is the hole. This created hole and electron are directly the result of light irradiation to the photocatalyst, and the direct reduction of dissolved oxygen forms the superoxide radical⁴⁰.

Fig. 5.

(a) MB removal (%) plot, and (b) the quasi-first-order reaction kinetics of MB with prepared photocatalysts under visible light, (c) MB removal (%) in the different pH values, (d) MB removal (%) in recycling tests of MoS₂@5%MoO₃ heterojunction, (e) the active species trapping experiments for MB degradation over MoS₂@5%MoO₃ heterojunction.

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Hydroxyl radicals are obtained by direct oxidation of the hole or by multi-step reduction of superoxide anion radicals caused by electrons created by light. These steps include the following:

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Adding other scavengers reduced the removal efficiency to 73 and 68%, suggesting that superoxide anion radical and hydroxide radical are other factors that contribute to dye destruction, however, to a lesser extent. This may be due to the combination of MoS₂ and MoO₃ as electron acceptors, which facilitates the transfer of electrons excited by visible light into the solution during the photocatalytic process and the reduction of the hole and electron recombination rate. These findings suggest that molybdenum disulfide’s photocatalytic properties can be effectively enhanced by modifying it by combining it with MoO₃ as a composite.

Photocatalytic mechanism

For the photocatalytic pollutant degradation, two photo-charge transfer mechanisms were proposed using the MoS₂@5%MoO₃ heterojunction (Fig. 6). In these two mechanisms, MoS₂ and MoO₃ can be simultaneously activated by visible light. In the I-type system, the photo-excited electrons move from CB of MoS₂ (-0.54 V vs. NHE) to CB of MoO₃ (-0.12 V vs. NHE). This hinders the formation of ^•O₂⁻ radicals, as the CB potential of MoO₃ is more positive than that of O₂/^•O₂⁻ (-0.33 V vs. NHE)⁴¹. Holes shift from VB of MoO₃ (+ 3.18 V vs. NHE) to VB of MoS₂ (+ 1.16 V vs. NHE), but the ^•OH formation is not possible since the VB potential of MoS₂ is lower than that of H₂O/^•OH (+ 2.72 V vs. NHE) and ^•OH/OH⁻ (+ 2.38 V vs. NHE)⁴¹. Consequently, electron-hole pairs are generated by direct light irradiation, and dissolved oxygen is reduced directly by hole on VB of MoS₂ to form ^•O₂⁻ and ^•OH radicals. This system is highly inefficient in nature because of the recombination of electron-hole pairs on their respective semiconductors.

Fig. 6.

Possible photocatalytic mechanism of MB degradation by MoS₂@5%MoO₃ heterojunction (a) Z-scheme and (b) I-type.

In the S-scheme system, electrons excited from CB of MoS₂ recombine with holes on VB of MoO₃, leading to an effective separation of charge carriers and a reduction in recombination. This enhances the redox capabilities of electrons on CB of MoO₃ and holes on VB of MoS2. Moreover, the ^•O₂⁻ formation is prevented by the lower CB potential of MoO₃ (-0.12 V vs. NHE), and the higher potential of ^•OH/⁻OH and H₂O/^•OH compared to the VB potential of MoS₂ hinders ^•OH formation. As a result, electron-hole pairs are generated by direct light irradiation, and dissolved oxygen is directly reduced by holes on VB of MoS₂ to form ^•O₂⁻ and ^•OH radicals. In other words, most active species photodegradation arise essentially from VB of MoS₂. The proposed mechanism agrees with the obtained result in scavenger tests indicating holes are major active species responsible and ^•O₂⁻and ^•OH as other factors participate in the degradation processes.

Conclusions

In this work, MoS₂ nanoflowers and MoO₃ nanobelts with their various weight% nanocomposites were synthesized via hydrothermal and ultrasonic techniques, respectively, and photocatalytic properties were evaluated. Indeed, the quasi-first-order kinetic model has shown a 96% removal of 20 mg/L MB in 6 h of visible light irradiation by the 3D/2D MoS₂@MoO₃ heterojunction, which is higher when compared with pure MoS₂ and MoO₃. This is due to the synergistic effect of p-type MoS₂ and p-type MoO₃ and the increased band gap of pure MoS₂ caused by the incorporation of MoO₃, which facilitates the separation and transfer of photo-induced electron-hole pairs. Furthermore, the photocatalytic mechanism of the S-scheme MoS₂@5%MoO₃ heterojunction is linked to the generation of ^•OH and ^•O₂⁻ radicals throughout the degradation process. Scavenger tests on the best photocatalyst showed that the holes, generated as a result of enhanced oxidizing potential and reduced hole-electron recombination because of the formation of the nanocomposite, were the most effective species in dye degradation. Finally, the design concepts investigated in this work can be applied to improve the performance of other low-efficiency photocatalysts intended for organic pollutant removal in water.

Author contributions

Mohammad Mahdi Rezaei & Seyyedeh Fatemeh Hosseini: Investigation, writing – original draft preparationMir Saeed Seyed Dorraji & Mohammad Hossein Rasoulifard: Supervision, Writing – review and editing. All authors reviewed the manuscript.

Data availability

All data associated with this study are present in the paper.

Declarations

Competing interests

The authors declare no competing interests.