Figure 1 illustrates the X-ray diffraction patterns of pristine In2S3 and MoS2 modified In2S3. XRD for sample IP indicating ten reflections (220), (311), (222), (400), (422), (511), (440), (531), (553), and (622), which assured the presence of β-phase of In2S3 (JCPDS-500814). XRD spectra for sample IPM1, IPM2, and IPM3 show two additional minor peaks compared to the XRD spectrum of sample IP. These two peaks (100) and (106) confirm the presence of the 2H phase of MoS2 in sample IPM1, IPM2, and IPM3 (JCPDS-371492). Raman spectra of sample IP, IPM1, IPM2, and IPM3 are presented in Fig. 2. Raman spectrum for sample IP indicating the four distinct peaks at 181 cm− 1, 249 cm− 1, 306 cm− 1, and 369 cm− 1 which attributed to the fingerprint vibrations of β-phase of In2S3 [37]. Raman result for sample IPM1 indicates the slight shift in the Raman peaks corresponds to the β-phase of In2S3. The observed peaks are 196 cm− 1, 223 cm− 1, 286 cm− 1 and 347 cm− 1 implies the formation of MoS2-In2S3 nanohybrids [18, 33]. Apart from this, two distinct peaks at 379 cm− 1 and 405 cm− 1 arise in the Raman curve of sample IPM1 further confirmed the existence of MoS2. Raman spectrum for sample IPM2, the peaks correspond to the β-In2S3 and 2H-MoS2 are found slightly shifted compared to Raman spectrum of sample IPM1. The observed peaks for sample IPM2 are 197 cm− 1, 230 cm− 1, 343 cm− 1, 378 cm− 1 and 403 cm− 1. Interestingly, It can be seen that the intensity of peaks at 378 cm− 1 and 403 cm− 1 is higher as than sample IPM1, which suggested the high-density MoS2 in sample IPM2 as compared to sample IPM1. For sample IPM3, the Raman studies reveals the six peak at 196 cm− 1, 232 cm− 1, 298 cm− 1, 347 cm− 1, 378 cm− 1 and 405 cm− 1. The intensity of two peaks for MoS2 at 378 cm− 1 and 405 cm− 1 in sample IPM3 is higher than that of sample IPM1 and IPM2. The significant shift in the Raman peaks of sample IPM1, IPM2, and IPM3 compared to sample IP affirms the attachment of MoS2 with In2S3 [33]. The shift in the Raman spectra of MoS2 modified In2S3 can be ascribed due to the lattice vibrations enable through the variation in the bond force constant of Mo-S and In-S bond. Modulation in bond force constant generated due to the modification of Mo4+ atom in In2S3 [18, 33, 38].
Figure 3(a-d) depicts the modulations in the morphologies of sample IP, IPM1, IPM2, and IPM3. For sample IP, 2D- nanosheets can appear with an average width of 162 nm (Fig. 3(a)).FESEM result for sample IPM1 illustrates the functionlization of 2D-layered structured aggregates on sheet-like nanostructures Fig. 3(b). With the increment in the concentration of MoS2, a more distinct flower-like morphology can be seen in the FESEM image of sample IPM2. Apart from this, a 2D-layered sheet can also be seen attached to the surface of flowers-like morphology (Fig. 3(c)). To further increase the Mo+ 4 ions concentration in In2S3, the 2D sheet mediated flowers-like structures attached with the sheet-like nanostructures can be observed for sample IPM3 (Fig. 3(d)). To confirm the formation of MoS2-In2S3 nanohybrids, elemental mapping for sample IPM3 was carried out and illustrated in Fig. 4. The broader view of surface morphologies of sample IPM3 has been presented in Fig. 4(a), while elemental mapping of the specific atom and corresponding nanohybrid is depicted in Fig. 4(b-e). Elemental mapping studies affirm the uniform spreading of Mo, In, and S atoms in sample IPM3. Surface morphology results confirm the formation of MoS2-In2S3 nanohybrids. To explore the crystal structures and the attachment of In2S3 and MoS2, TEM studies were employed. Figure 4(f) shows the addition of MoS2 nanoparticles in In2S3 nanosheets in sample IPM1. The computed average size of MoS2 nanoparticles in sample IPM1 is 24 nm. TEM images for sample IPM3 (Fig. 4(g-h)) reveal the growth of MoS2 flakes on the surface of the In2S3 sheets. The assembly of MoS2 nanosheets formed the flowers-like nanostructures on the surface of In2S3 sheets (Fig. 4(h)).
The optical profile of pristine In2S3 nanosheets and MoS2 modified In2S3 is illustrated in Fig. 5(a). The optical absorption curve of sample IP reveals a broad peak form 250–600 nm. With the modification of MoS2 in In2S3, the optical absorption enhanced significantly (sample IPM1). For sample IPM2, the optical absorption is further increased and found higher than IP and IPM1. The optical absorption spectrum for sample IPM3 indicates the highest absorption compared to sample IP, IPM1, and IPM2. Optical absorption studies manifest a remarkable improvement in MoS2-In2S3 nanohybrids as compared to pristine In2S3. To explore the bandgap engineering in MoS2-In2S3 nanohybrids Tauc plots, studies were carried out and presented in Fig. 5 (b). Tauc plots suggest the narrowing in the bandgap in MoS2-In2S3 nanohybrids as compared to pristine In2S3. The computed bandgap for sample IP, IPM1, IPM2 and IPM3 are 2.25 eV, 1.94 eV, 1.89 eV and 1.82 eV, respectively.
The valance state and surface chemical composition of MoS2-In2S3 nanohybrids were determined through the XPS and illustrated in Fig. 6. Gaussian fitted XPS spectra for In3d, S2p and Mo3d were presented in Fig. 6(a-c), respectively. Figure 6(a) reveals that the XPS spectrum for In3d consists of two distinct peaks at 444.6 eV and 452.2 eV, which can be assigned to the In3d5/2 and In3d5/2, respectively [39]. Gaussian fitted S2s spectrum reveals the three peaks at 161.8 eV, 164.3 eV and 168.2 eV (Fig. 6(b)). The peaks at 161.8 eV and 164.3 eV can be attributed to the S2P3/2 and S2P1/2 sequentially and affirms the presence of the S2p state of the sulfur atom [33, 34]. Apart from this, a broad peak at 168.2 eV assured the existence of the S-O bond [40]. The fitted spectrum of Mo3d shows the distinct four peaks at 228.3 eV, 231.0 eV, 232.1 eV, and 234.5 eV. The major peaks at 228.4 eV and 231.3 eV corresponds to the Mo3d5/2 and Mo3d3/2, respectively [41]. The peaks at 232.1 eV and 234.5 eV can be attributed to the existence of Mo+ 6 states in the Mo3d spectrum [41, 42]. XPS studies indicate the presence of Mo, In, and S in the state of Mo+ 4, In + 3, and S− 2 state in the MoS2-In2S3 nanohybrids, respectively. XPS results explicitly suggest that no shift takes place in the recombination process of MoS2-In2S3 nanohybrids.
We investigated the sunlight-induced photodegradation ability of the In2S3 nanosheets and MoS2 modified In2S3 samples by the degradation of MB molecule solution. Figure 7(a-d) depicts the optical absorption spectrum of MB molecule solution employing sample IP, IPM1, IPM2, and IPM3. A UV-visible absorption study implies that sample IPM3 exhibits the highest photocatalytic efficiency compared to sample IP, IPM1, and IPM2. Sample IPM3 is found 96.8% efficient for the breakdown of 10µM MB molecule solution in 8 minutes, while sample IP, IPM1, and IPM2 are found capable of decomposing 35.3%, 77.7%, and 95.3% of 10µM MB molecule solution in 8 minutes (Fig. 8(a)). Figure 7(e) shows the modulations among the photodegradation kinetics of sample IP, IPM1, IPM2, and IPM3. Photodegradation rate kinetics reveals the higher photodegradation performance of sample IPM3 as compared to other photocatalyst samples. In order to found out the rate constant value of each photocatalyst sample, linear fitting of logarithm value of C/Co as a function ofexposure time were plotted. The rate constant value for sample IP, IPM1, IPM2, and IPM3 is depicted in Fig. 7(f). The calculated k values for sample IP, IPM1, IPM2, and IPM3 are 0.0475/ min, 0.1724/min, 0.357/min, and 0.3421/min, sequentially. Photocatalytic studies affirm that sample IPM3 attain 2.7 times better photodecomposition pefrormance as compared to pristine sample IP. The rate constant value for sample IPM3 is 7.2 times the K value of sample IP. Photodecomposition results assure the improved degradation nature of the IPM3 sample as compared to other prepared photocatalyst samples. The stability of the most efficient synthesized sample (IPM3) was explored through the usage of sample IPM3 for three cycles of the photocatalytic reaction process. After three runs of photodegradation reaction, the efficiency of sample IPM3 is constant, which indicates their stable nature (Fig. 8(b)). To reveal the high photodegradation capability of the most efficient sample IPM3, the photodecomposition of OTC-HCl molecules was explored. Figure 9(a-b) indicates the optical absorption results of photodegradation results of OTC-HCl molecule solution using sample IP and IPM3, respectively. Photodegradation rate kinetics assures the high photodecomposition performance of sample IPM3 as compare to sample IP. It has been computed that sample IP can decompose the 27.1% efficient while sample IPM3 shows the decomposition of 76.3% of 0.3mg/mL of OTC-HCl solution in sunlight (Fig. 9(c)). The k-values for sample IP and IPM3 are 0.00621/min and 0.0308/min sequentially (Fig. 9(d)).
To determine the charge transfer profile in In2S3-MoS2 nanohybrids during the photocatalytic reaction, a schematic diagram has been presented (Fig. 10). Under sunlight irradiation, In2S3 sheets and MoS2 flakes are get activated and generate the electron-hole pair in their respective conduction and valence bands. In the next step, the photoinduced electron exists in the conduction band of In2S3 move towards the conduction band of MoS2 due to the band alignment positions, which in turn maintain the synergistic effect and control the recombination rate in In2S3 nanosheets. The VB and CB positions for In2S3 nanosheets and MoS2 nanoflakes are obtained through the following relation (1).
Where Eg indicates the bandgap value of the 2D layered materials (In2S3 and MoS2) while Ee stands for the energy on the hydrogen scale (4.5 eV), and χ shows the electronegativity of the 2D layered nanostrucutres (In2S3 ~ 4.7 eV and MoS2 ~ 5.32 eV). The computed VB and CB values for In2S3 nanosheets are 1.32 eV and − 0.92 eV, sequentially, while CB and VB values for MoS2 nanoflakes are − 0.13 eV and 1.77 eV, sequentially.
Apart from the efficient charge separation process, the density of electrons in CB of MoS2 enhanced remarkably; consequently, the formation rate of superoxide radicals concentration increased. The high concentration of superoxide radicals primarily affects the photocatalytic reaction and accelerates it significantly. Similarly, due to the synergistic effect among In2S3 and MoS2, the enhanced production of holes interacted with the water molecule and created the hydroxyl radicals with high concentration. The high density of hydroxyl radicals interacts with the MB molecule and degrades it. Thus the synergistic effect in In2S3 sheets - MoS2 flakes control the charge separation and improve the photodegradation ability.
In the present report, an In2S3-MoS2 nanohybrids with an effective charge separation effect is successfully fabricated. The density of MoS2 nanostructures over In2S3 nanosheets was precisely varied and used for the decomposition of MB molecules under solar light illumination. Sample IP consists of the sheet-like structures of In2S3. Pristine In2S3 sheets interact with the pollutant molecules and degrade only 35.3% of 10 µM of MB molecule solution in solar light illumination. For sample IPM1 MoS2 nanoparticles functionalized In2S3 sheets with effective charge separation effect decompose 77.7% of MB molecule solution. In sample IPM2, the density of MoS2 nanostructures enhanced, which further increases the density of interface among In2S3 and MoS2; consequently, charge separation increase significantly. Therefore sample IPM2 can decompose 95.3% of 10 µM methylene blue (MB) molecules solution in 8 minutes of sunlight irradiation. At the highest density of MoS2 in sample IPM3, the MoS2 flakes were assembled on the surface of In2S3 nanosheets and significantly improve the charge separation and reduce the recombination rate. UV-DRS results also indicate that sample IPM3 indicates the highest optical absorption compared to sample IP, IPM1, and IPM2.
Moreover, the highest bandgap narrowing occurs in sample IPM3, which also indicates the efficient charge separation. Raman and XRD studies also reveal that the density of MoS2 over In2S3 nanosheets was increased from sample IPM1 to IPM3. For sample IPM3, high numbers of heterojunction of MoS2 and In2S3 are responsible for the extremely high sunlight-induced photodegradation capability towards MB molecules solution. Thus sample IPM3 decomposes 96.8% 10 µM of MB and 76.3% of 0.3 gm/mL OTC molecules solution in 8 minutes and 40 minutes sequentially under solar light illumination. Liu et al. [33] reported the formation of MoS2 nanodots functionalized In2S3 nanoplates and used them for the photoelectrochemical application. They have found that MoS2/In2S3 heterojunction with effective charge exhibited better performance as compared to pristine MoS2 and In2S3. Wang et al. [34] synthesized MoS2 functionalized In2S3 nanostructures for the Cr+ 6 removal using a photocatalysis process. They have demonstrated that MoS2 functionalized In2S3 nanostructures exhibit 3.2 times better photocatalytic activity than pristine In2S3.