Facile synthesis and defect optimization of 2D-layered MoS2 on TiO2 heterostructure for industrial effluent, wastewater treatments

Current research is paying much attention to heterojunction nanostructures. Owing to its versatile characteristics such as stimulating morphology, affluent surface-oxygen-vacancies and chemical compositions for enhanced generation of reactive oxygen species. Herein, we report the hydrothermally synthesized TiO2@MoS2 heterojunction nanostructure for the effective production of photoinduced charge carriers to enhance the photocatalytic capability. XRD analysis illustrated the crystalline size of CTAB capped TiO2, MoS2@TiO2 and L-Cysteine capped MoS2@TiO2 as 12.6, 11.7 and 10.2 nm, respectively. The bandgap of the samples analyzed by UV–Visible spectroscopy are 3.57, 3.66 and 3.94 eV. PL spectra of anatase phase titania shows the peaks present at and above 400 nm are ascribed to the defects in the crystalline structure in the form of oxygen vacancies. HRTEM reveals the existence of hexagonal layered MoS2 formation on the spherical shaped TiO2 nanoparticles at the interface. X-ray photoelectron spectroscopy recommends the chemical interactions between MoS2 and TiO2, specifically, oxygen vacancies. In addition, the electrochemical impedance spectroscopy studies observed that L-MT sample performed low charge transfer resistance (336.7 Ω cm2) that promotes the migration of electrons and interfacial charge separation. The photocatalytic performance is evaluated by quantifying the rate of Congo red dye degradation under visible light irradiation, and the decomposition efficiency was found to be 97%. The electron trapping recombination and plausible photocatalytic mechanism are also explored, and the reported work could be an excellent complement for industrial wastewater treatment.


Results and discussion
XRD analysis. The structural and physical analysis of the samples were studied by X-ray diffraction (XRD) using Bruker advance diffractometer with a scanning rate of 5° per min with Cuk α radiation source (λ = 1.54060 Å) operating at 40 kV. Figure 1a shows the XRD pattern of prepared CTAB capped TiO 2 within the 2θ range between 10 and 80°. The diffraction peaks located at 25.28°, 38.57°, 48.05°, 55.06° and 62.68° corresponds to the planes (101), (103), (200), (105), (211) and (204), respectively which follows the standard JCPDS pattern . The XRD patterns of MoS 2 @TiO 2 and L-Cysteine capped MoS 2 @TiO 2 heterostructure has indexed as a tetragonal lattice and body centered phase of CTAB capped TiO 2 with lattice constants as a = b = 3.785 nm, c = 9.513 nm, lattice angles as α = β = γ = 90° and space group as I4 1 /amd. The presence of diffraction of anatase TiO 2 suggests that 2D-MoS 2 nanosheets loading does not change the crystal phase of TiO 2 . On the other hand, no apparent peaks for MoS 2 could be detected, due to its relatively lower amount along with its high dispersity and week intensity that is in well agreed with previous reports 37,38 . Further, no other impurity peaks are observed in the XRD patterns evidencing the single-phase formation of the samples. The main diffraction peak of TiO 2 at 25.32 and 37.84° are ascribed to (101) and (103) planes, respectively 39,40. The neglectable influence of thermal reduction parades on the crystal phase and crystallinity of TiO 2 . The crystallite size was calculated using Debye Scherrer equation D = kλ/βcosθ. The crystalline size of CTAB capped TiO 2 (PT), MoS 2 @ TiO 2 (MT) and L-Cysteine capped MoS 2 @TiO 2 (L-MT) are 12.6, 11.7 and 10.2 nm respectively. Interestingly, the grain surface relaxation contributes to the line broadening resulting in the reduction of the measured value of dislocation density 38 Fig. 1b.The intense absorption peak is found between 200 and 275 nm in the ultraviolet region. It is evident from this spectrum that the prepared CTAB capped TiO 2 , MoS 2 @TiO 2, and L-cysteine capped MoS 2 @ TiO 2 nanoparticles are found to have higher absorbance in UV-region. The absorption edges of PT, MT and L-MT are estimated, and their corresponding energy band gaps are 3.57, 3.66 and 3.94 eV, respectively. The energy bandgap of the material is related to its absorption coefficient, and energy of the photon, as explained by the Tauc's relation is shown in Fig. 1c. The increasing absorption/bandgap values show the increment of MoS 2 on TiO 2 surface 9,41 . The L-MT heterostructure could be well controlled by the existence of oxygen vacancies that were introduced during the synthesis. This was established by the extent of the absorption of light from UV to visible region 42 .
The changes in the optical properties of L-MT heterostructure can be controlled by defects such as oxygen vacancies is furtherly investigated through photoluminescence spectra 43 . The spectra of the prepared PT, MT and L-MT 44 with an excitation wavelength of 320 nm in the range between 350 and 600 nm are given in Fig. 1d. The peaks observed in the evident region are connected to the subsistence of oxygen defects in the MT, L-MT. The peaks presented at 361, 377, 410, 437, and 490 nm corresponds to the ultra-violet, violet and blue region, respectively 45,46 . The high photoluminescence intensity for PT, MT sample explored the hasty electron-hole pair recombination. Whereas, the intensity of L-MT emission peaks are found to be tuned down, due to the efficient photo-carrier separation at heterojunction interfaces 47 . Additionally, the drop in photoluminescence intensity occurs due to chemisorption of oxygen molecules leading to an increase in conductivity and it helps to avoid the recombination process 48 . The luminescence is related to the recombination of electrons in single occupied oxygen vacancies with photoexcited holes in the valance band. Photoluminescence spectra of anatase phase titania shows that the peaks present at above 400 nm is ascribed to defects in the crystalline structure such as oxygen vacancies, which also reported by He et al., Fang et al. and B. Choudhury et al 49,50 . These defects accept electrons in the photoinduced reaction with a reduction in the recombination rate of the exciton. The blue emissions peak is observed at 490 nm, and it might indicate a profound level of visible emission to localize levels in the bandgap power 42 . The sample MT and L-MT show lower intensity because of defects. These defects may leads to rarer electron-hole pair recombination possibility [51][52][53][54][55] . The lower intensity indicates the more efficient separation of photoinduced electrons (e−) holes (h+), thereby expecting higher photocatalytic activity [56][57][58][59] . These results demonstrate that the developed MoS 2 hexagonal sheets have efficient light-harvesting in the visible region 60 .
X-ray photoelectron spectroscopy (XPS) analysis. X-ray photoelectron spectroscopy shows the surface composed elements in L-cysteine capped MoS 2 @TiO 2 (L-MT) nanoparticles. The characteristic peaks clearly evidence the presence of Mo, S, Ti and O elements. The high resolution XPS spectra of Mo 3d, S 2p and Ti 3p, O 1s binding energy confirms the formation of MoS 2 @TiO 2 heterostructure. The deconvolution peaks of Mo provide information about the Mo 4+ oxidation state and the corresponding peaks presented in ~ 231.4 eV and ~ 234.53 eV respective to Mo 3d 5/2 and Mo 3d 3/2 are shown in Fig. 2a with the slightly shifted peaks resulting from the composition of TiO 2 . In general, the standard energy separation difference between Mo 3d 5/2 and  Figure 4, represents the hexagonal layered MoS 2 decorated on spherical shaped TiO 2 nanoparticles. It is interesting to emphasize that most of the nanosheets and nanoparticles are overlapped towards the edge site. The HRTEM images of the heterostructure display two kind of lattice fringes as shown in the heterostructure. The attachment between MoS 2 and TiO 2 nanoparticles well aggregate the interparticle adhesive nature 61 with few coarsening as seen from the observed irregular profile that might be attributed to the thermal flux effect of heterostructures synthesis. The selected area electron diffraction (SAED) pattern suggests the existence of numerous ring patterns to explore the crystalline nature of the synthesized MoS 2 nanosheets on the TiO 2 nanostructure in detail.
HRTEM observation revealed a greater number of MoS 2 nanosheets grown on the surface of TiO 2 nanoparticles together with the minimal observation of an elevated aggregation level of the heterostructure. The HRTEM images presented in Fig. 3d of MT disclose two kinds of lattice fringes confirming the presence of 5-6 individual layers. Furthermore, HRTEM images are given in Fig. 3e,h shows a discontinuous area at the interface between MoS 2 @TiO 2 which clearly indicates the presence of oxygen defects in the structure. The large separation seen between TiO 2 and MoS 2 nanosheets also poses defects resulting from their lattice mismatch. The crystal interface between MoS 2 @TiO 2 shows a distorted atomic pattern and significant lattice distortions, www.nature.com/scientificreports/ which consequences in a change of periodicity and the formation of structural defects. This interface improves the photoinduced charge carrier transfer and significantly increases the number of active catalytic sites [62][63][64][65] .
Densely packed disordered areas have been observed around the interface because of lattice stress and robust interfaces between MoS 2 and TiO 2 . This is consistent with photoluminescence spectra arguments as stated above. The nanosheet grown on the surface of TiO 2 with lattice spacing of 6.14 Å correlates to the (001) plane of MoS 2 . The set of major fringes  www.nature.com/scientificreports/ spacing measured as 3.52 Å could be related to the (101) lattice spacing of anatase TiO 2. The lattice fringes are well matching with previous research work 61 . Similarly, XPS spectrum ( Fig. 2) displays the shifting of Ti 2P peaks to the lower binding energies after the deposition of TiO 2 onto MoS 2 nanosheets. The above-mentioned result evidence that titanium (Ti) atoms accept electrons from MoS 2 , resulting in increased Ti 3+ in the heterostructures. The coupling of MoS 2 with (101) faced TiO 2 results in a totally different change of spectra. The shift of Ti2p peaks to higher binding energies indicates the functioning of Ti atom as an electron donor. The surface dependent interfacial electronic structure implies the exchange charge transfer behaviour between MoS 2 and TiO 2 heterostructures 37 .
Moreover, the charge carriers in semiconductor photocatalyst strongly depends on the exposed facets and structural defects. Consider the example of (001) and (101) facets/phase of TiO 2 , two most common facets of anatase phase, usually exhibit different adsorption characteristics and redox abilities during the photocatalytic reaction 37,66,67 .
In the present work, its directly observed that the HRTEM images of 3b,e,h have similar characteristic facets of (001) and (101). It indicates the exposure of (101) facets as more favourable for the formation of surface oxygen vacancies. The marked red coloured circles of HRTEM images represent the actual defects sites. This defect modulation is an effective strategy that helps to improve photocatalytic activity of (101) faced TiO 2 molecules. In order to study the synergetic effect between oxygen defects and MoS 2 , the charge carrier's behaviour was studied by PL measurements. Obviously, the deposition of MoS 2 and oxygen vacancy formation results in the PL emission of (001) and (101) faced TiO 2 37,68,69 . Eventually, the mid gap between MoS 2 and TiO 2 oxygen defects states can act as an electron mediator to facilitate this charge transfer 37 .
The peak position of 410 nm established in the current work corroborates well with the emission band at ~ 412 nm referred in the previous literature 3 . Hence, it is clearly evident that 412 nm band was assigned to self-trapped excitons localized on TiO 6 octahedra 3 . The PL bands at the long wavelength side of anatase TiO 2 nanoparticles have been attributed to the oxygen vacancies(OVs) 76 . The OVs sites were occupied by O 2 ions in the -Ti-O network and are ascribed to F + centres 76 . Furthermore, the emission peak centred at 437 nm is also very close to 433 nm as given in the reported literature which could be assigned to self-trapped exciton 52 .
In a similar manner, the peaks position at 490 nm is also much related to the reported value of 492 nm 52 . This peak position occurs charge transfer transition from Ti 3+ to TiO 6 2complex and are well associated with oxygen defects 77,78 . The efficiency of the PL emission is accounted to both radiative and non-radiative recombination process.
Visible light induced degradation of Congo red using L-MT heterostructure. The degradation of highly carcinogenic effluents such as Congo red dye was explored using the prepared PT, MT and L-MT as given in Fig. 5a-c. These azodic groups of dyes are highly carcinogenic and genotoxic to human health causing various diseases. Hence, the photocatalytic decomposition of Congo red was examined with the irradiation of visible light (λ = 400 nm) via L-MT heterostructure 31,33 , which shows the higher degradation nature among all the three samples. The well-arranged crystalline structure with more active surface area and smaller size effect of the hexagonal layered MoS 2 attached on the spherical shaped TiO 2 nanoparticles in L-MT samples leads to excellent decomposition nature than that of the other two samples. The HRTEM images (Fig. 3b,e,h) corroborate the layers on spherical or vice versa. Eventually, the surface of TiO 2 nanoparticles was decorated with thin MoS 2 nanosheets is a major source of more reactive sites. The enduring destruction and increased removal capacities of harmful chemicals using L-MT were found to be more due to its shapes, porous and crystalline nature.
The absorbance spectra of CR dye (15 ppm) during the decomposition process with 60 mg of catalyst on the illumination of visible light at pH-3 is given in Fig. 5c. The absorption spectra show the two peaks at 350 nm and 495 nm corresponding to the aromatic ring and π-π* transitions emerging from the azodic group. The decomposition of Congo red dye was proved by the reduction in peaks intensity and color change from red to a colorless solution with increasing the time interval under visible light irradiation 79,80 . Similarly, the above-mentioned experiment was carried out in the absence of the photocatalyst and in the dark condition as shown in Fig. 5d. In addition, the same experiment is also carried out in neutral and basic medium by the addition of HCl/NaOH which differ in the time of decomposition. The plot between C/Co and time in a minute is portrayed in Fig. 5d. Furthermore, the decomposition efficiency of Congo red dye was shown in Fig. 6a and found to be 97% in the acidic medium with 120 min of irradiation of light source, whereas the degradation was taking a longer time duration in the basic medium [81][82][83][84] . The complete mineralization of the CR was attained within 120 min in acidic conditions. To conclude, the above results of the synthesized photocatalyst holds extensive photocatalytic activity not only for Congo red and have great potentiality for decomposing other colored dyes too 85-91 . Effect of pH on the photocatalytic activity of L-MT. The degradation efficiency of L-MT heterostructure on photodecomposition of CR is detailed for a range of pH from 3 to 9 to examine the effect of pH. Figure 6b is evidencing that the pH drastically pretentious the rate of degradation of CR. The rate of reaction decreased from 97 to 46% with the change in pH value from 3 to 11 upon illumination of visible light with a time interval of 120 min. The highest rate of reaction is obtained for the degradation of CR nearly 97% at pH3. In acidic medium, the reaction mixture consists of more protons than hydroxide groups; implies that the positive charge is increased on the surface of L-MT heterostructure which in turn results in an attraction of the anionic dye with the positive surface of the catalyst. As a result, complete decomposition occurs with a minimum duration of nearly 120 min. When the pH is above 7, the decomposition desires longer time for the degradation of CR. The rate of decomposition of anionic CR dye is greater in acidic medium as compared with basic and neutral medium. Consequently, the adsorption and decomposition of CR dye on the catalyst on illumination of visible light becomes higher in acidic medium. Thus, the decomposition process revealed that the catalyst has a great ability to be used as an efficient catalyst for the various industrial effluents. The electrostatic interaction occurs between the negatively charged dye and the positively charged surface of the catalyst in the acidic environment. In the case of basic condition, the heterogeneous catalyst carries a negative charge on the surface with the intention of repulsions by the anionic dye solution, thereby decreasing the decomposition of dye. Furthermore, the decomposition is possible in different pH due to hydrogen bonding, hydrophobic-hydrophobic interactions and Van der Walls forces etc. 81 , The mechanism of degradation in acidic and basic medium can be well-understood from the following equation: In acidic medium, In basic medium,

Effect of catalytic dose in the photo-decomposition.
To evaluate the effect of the amount of catalyst in the decomposition of CR, the experiment was conceded with the various amount of catalyst from 40 to 80 mg. The intensity of visible light source, the concentration of CR dye solution (15 ppm) and pH of the solution remains constant in the degradation process. The catalytic dose is increased from 40 to 60 mg in dye solution with increasing decomposition rate of the reaction as evidenced from Fig. 6c However, the degradation is reduced by raising the concentration of catalyst above 60 mg due to the scattering of light and poor penetra- www.nature.com/scientificreports/ tion of light in the reaction mixture. When the catalytic amount increased gradually in the reaction mixture, the energetic molecules of the catalyst reduced owing to the aggregation of catalyst that further leads to turbidity causing subsequent minimization of the dispersion of light in the reaction medium. As a result, the complete catalytic decomposition of CR dye can be achieved with the use of 60 mg as a suitable catalytic dose.

Effect of initial dye concentration. The strength of the industrial effluent like CR dye plays a vital role
in the photodecomposition process. Hence, the effect of initial strength of dye solution in the reaction process was examined with different strength of CR dye solution (7, 10, 15, 20 mg/L) as presented in Fig. 6d. During this decomposition process, the dose of catalyst and pH of the reaction mixture remains constant throughout the overall process. The strength of the dye solution is found to increase from 7 to 15 mg/L with an increase in the efficiency of decomposition of CR dye. However, the rate of decomposition of CR dye is observed to decrease corresponding to the strength of dye solution greater than 15 mg/L, which confirms the fact that the removal of CR dye depends on the initial strength. Further, with the increase in the concentration of CR dye, the degradation underway to decrease 82 .

Reusability of L-MT heterostructure photocatalyst.
In order to identify the reusability nature of the used catalyst after the photo degradation reaction, the used catalyst was removed from the reaction mixture, washed with distilled water and dried in the oven at 120 °C to further estimate its reusability. The reusability and consistent nature of the used L-MT catalyst after the photodegradation reaction was tested with four cycles by removing the catalyst from the reaction mixture. At the end of the last cycle, the decomposition efficiency was found to be 95% under illumination of visible light as seen in Fig. 6e Furthermore, Fig. 6f proves that the occurrence of typical peaks in the XRD patterns for the L-MT sample after fourth cycle. In addition, the good quantity of used catalyst was separated from the reaction mixture at the end of fourth cycle. The physical property of the fresh and used photocatalyst remains almost the same, which is also evidence for the decomposition of CR and the absence of adsorption of dye on the photocatalyst. The well-established stability of the catalyst increases its practical usage as a catalyst in photo decomposition of CR 49 . Thus, the as-synthesized L-MT sample was stable and reusable. Consequently, the results corroborate the stability of the catalyst and its reusability for long duration.  Table. ST1 (supplementary table)  Photodegradation of CR mechanism. The degradation of major industrial effluents such as CR dye was enhanced by the surplus generation of excitons on the surface of the catalyst under irradiation of visible light source. The electrons in the ground state of the surface of the L-MT catalyst promotes to excited state under visible light 9,10,50 , the possible schematic mechanism shown in Fig. 8. The charge separation occurs in the valence and conduction band of the catalyst. Hence, the excited electrons by absorbing photons occupy the conduction band (CB) by leaving the holes in the valance band (VB) of the catalytic surface. In the PL spectra of PT (pristine TiO 2 ) and L-MT sample, the intensity of L-MT sample is lower than that of the PT 58,92 . These observed results confirm that the L-MT heterostructure possessing more competent charge carrier separation, which leads to suppression of the exciton recombination in hexagonal 2D-layered MoS 2 decorated on spherical shaped TiO 2 heterostructures 84 .
In a similar manner, Congo red dye absorbs high energy photons from visible light through photosensitization progression and undergoes autooxidative revolution giving rise to circumlocutory creation of oxidizing hydroxyl (OH · ) radicals. The electron in the highest occupied molecular orbital (HOMO) shifted to lowest unoccupied molecular orbital (LUMO) of the CR dye on visible light irradiation. The photo induced electron in the excited CR* was migrated to catalyst surface by leaving CR*+ dye to strengthen the generation of exciton. Furthermore, CR*+ reacts with active hydroxide radicals to form smaller fragments product via breaking of ring structure as shown in Fig. 9. The mechanism implicated in the progression of photosensitization is given in below Eqs. (4) and (5).
X. Zhang et al. reported the delocalization of electrons with reduction in the recombination electron-hole pairs leads to higher catalytic activity due to the presence of surface defects on the nanorods 93 . The EIS spectra confirm the more efficient separation of photoinduced electron-hole pairs and rapid interfacial charge transfer for the L-cystine MoS 2 doped TiO 2 heterojunction surface than the other two samples. These generated excitons position in the VB and CB of L-MT catalyst plays a vital role to increase the photocatalytic efficiency of the L-MT catalyst 51 . The reduction and oxidation of congo red in aqueous reaction mixture solution were carried out through the excitons, ensuing in significant enhancement in the catalytic performance. The band gap of TiO 2  www.nature.com/scientificreports/ and MoS 2 are 3.28 eV and 1.89 eV which is well consistent with many reported work 94,95 . The VB and CB edge position is concurrence with their electronegativity 96 . The CB and VB potentials of semiconductors are calculated using the following empirical equations: E e is the energy of free electrons versus hydrogen (4.5 eV). Finally, χ is the electronegativity of semiconductor and it was calculated by the following equation: In which a, b, and c are the number of atoms in the compounds. The generated excitons position in the VB and CB of L-MT catalyst plays a vital role to increase the photocatalytic efficiency of the L-MT catalyst 51 . These excitons assist the reduction and oxidation of Congo red in aqueous reaction mixture solution, ensuring in significant enhancement of the photocatalytic performance. In L-MT sample, VB is to be found at 2.96 eV and CB is at − 0.32 eV for titania and MoS 2 VB (1.77 eV) / CB (− 0.12 eV) against normal hydrogen electrode(NHE) 26,38 . The aforementioned result confirms the CB edge of titania to be less negative than that of the redox potential of O 2 / O 2 ·− (− 0.33 V). This process slows down the electron in the conduction band reacts with oxygen molecule to form superoxide anion radicals (O 2 ·− ) 24,32 . The electron in the conduction band must be transferred to CB of MoS 2 that has to be used by H 2 O 2 to generate more OH · radical which is involved in the decomposition of CR dye 29,97 . The hole (h + ) with higher oxidation potential can contribute to the oxidation of the CR dye 27 . The VB edge of catalyst is greater than the redox potential of OH · /OH − (1.99 V). These positive holes are required to oxidize water to form a OH · radical leading to oxidation of CR dye solution into non-toxic products like H 2 O, NO 3 − , NH 4 + and CO 2 etc 31,32,36 . The resultant generated oxidizing hydroxyl (OH · ) radicals and reducing superoxide anions facilitates the complete decomposition of organic contaminants, as reported in previous litreatures 98,99 . In the case of MoS 2 , the mechanism procedure is followed by the generation of ROS for the further degradation of CR dye. The oxygen vacancies accept electrons in the photoinduced reaction with the reduction in recombination rate of exciton as evidenced in photoluminescence spectra. The presence of oxygen vacancies in the L-MT sample plays an imperative task for the efficiency of degradation of dye. These oxygen vacancies on the surface level of the catalyst are accountable for trapping the electron from the conduction band and play down the excitonic recombination resulting in a superior photoinduced catalytic effect. The reactive oxygen species (ROS) generated in the reaction decomposes the CR dye into smaller units. Therefore, in the L-MT heterostructure, the recombination of excitons is reduced, further generating the strong oxidative radicals for the degradation 80 . The various parameters like pH, catalytic dose and initial concentration of dye on the photocatalytic decomposition process via L-MT heterostructure reveal the adsorption capability and high destruction performance in the degradation of Congo red. When oxygen vacancies are debuted into the L-MT, the defects can act as an electron facilitator to assist the charge transfer and separation of photoinduced electron-hole pairs 37,100 . The synergetic effect between oxygen vacancies, crystal surfaces and narrow bandgap leads to significant photocatalytic activity 60,101 . The obtained results from optical and EIS analysis are in good agreement with the results of the photocatalytic efficacy. Consequently, the generation of photoinduced excitons will be influenced by the internal electric field in the heterostructures. Hence, evidently the present work revealed that this stable catalyst in future potentiality can act as an efficient photocatalyst for environmental wastewater treatment and its purification. Moreover, the present nanocomposite with subsequent functionalization has futuristic scope for antireflective coatings too. The synergetic effect of MoS 2 nanosheets and TiO 2 nanoparticles results in a large number of reactive sites and poor exciton recombination for adsorption followed by decomposition 102 , enhance the photocatalytic nature. Table.ST2 (Supporting information) shows the evaluation of synthesised photocatalyst with other photocatalysts that recently used for degradation of dyes. The photo-decomposition of CR dye mechanism of L-MT heterostructures has been projected as follows:

Conclusion
In summary, the present study demonstrates the novel MoS 2 nanosheets decorated on spherical shaped TiO 2 heterojunction photocatalysts prepared through hydrothermal approach for photocatalytic degradation of Congo red dye in visible light. It is evident from HRTEM analysis that the attachment between MoS 2 and TiO 2 nanoparticles well aggregated the interparticle adhesive nature. The influence of capping ligand binder on TiO 2 @MoS 2 heterostructure was investigated. Moreover, based on the EIS analyses, the diameter of a semicircle of L-MT is very smaller that indicates the increase in separation of the photogenerated electrons and holes on the surface of L-MT heterostructure. The L-MT heterostructure exhibits strong adsorption ability and high photocatalytic performance in the degradation of Congo red that obviously revealed its future potentiality as an efficient photocatalyst for environmental applications. We have clearly outlined the effects of pH, catalytic dose and initial concentration of dye on the photocatalytic degradation process. The enhancement in photocatalytic activity of the proposed heterostructured photocatalyst is ascribed to the complementing synergetic effects of MoS 2 nanosheets on TiO 2 nanoparticles resulting in a large number of active sites for adsorption. From our study, it is well proved that L-cysteine capped MoS 2 @TiO 2 heterostructure have better removal of Congo red with minimum 120 min with maximum efficiency of 97%.

Methods
All the chemicals were used as-received and without further purification. The absolute ethanol (99.99%) was obtained from Merck chemicals Ltd. Titanium isopropoxide, cetyltrimethylammonium bromide (CTAB), potassium iodide (99.9%), citric acid, thiourea, L-cysteine and ammonium heptamolybdate were purchased from Aldrich. The entire synthesis process was performed using deionized water.

Synthesis of CTAB capped TiO 2 nanoparticles. The preparation of CTAB capped TiO 2 nanoparticles
was performed by sol-gel method where 2.87 mL of titaniumisopropoxide was mixed in the CTAB solution.
3.64 g of CTAB was dissolved in the mixture of 25 ml of absolute ethanol and 100 ml of deionized water (1:4 volume ratio) and the solution was stirred for 1 h to form a clear solution. Subsequently, titanium isopropoxide was added drop wise in the CTAB solution with vigorous stirring for 24 h. The resulting gel was centrifuged and washed several times with ethanol.The final product was calcined at 400 °C for 3 h in static air and the collected sample was designated as pure TiO 2 (PT).

Synthesis of MoS 2 doped TiO 2 nanoparticles.
The simple co-precipitation method adopted for the synthesis of MoS 2 doped TiO 2 nanoparticlesis explained: 1.3 g of ammonium heptamolybdate and 0.49 g of citric acid were dissolved in 50 ml of water understirring at 90 °C for 30 min. Moreover, the pH was adjusted to 4 using ammonia. The as-prepared TiO 2 nanoparticles (PT) and 1.27 g of thiourea was mixed in 20 ml of deionized water. The two solutions were mixed together and stirred at 90 °C for 1 h. Finally, the precipitated powder was centrifuged and washed with water/ethanol which was further annealed at 160 °C for 3 h. The sample prepared using citric acid was labelled as MoS 2 doped TiO 2 (MT). For comparison, 0.49 g of L-Cysteine was used instead of citric acid in the same synthesis procedure as mentioned above was labelled as L-Cysteine MoS 2 doped TiO 2 (L-MT). The details of the characterization tools are provided in the supplementary details.
Photocatalytic experiment. The photocatalytic decomposition of highly carcinogenic pollutant like Congo red was examined with the irradiation of visible light (λ = 400 nm) via L-MT. In the experimental procedure, 60 mg of the synthesized photocatalyst was dispersed in 100 mL of Congo red dye solution (CR) (15 mg/L) on irradiation of visible light at various pH medium. Initially, the reaction mixture was stirred in dark without the visible light using magnetic stirrer in order to attain adsorption-desorption between the Congo red dye solution and the catalyst. From the basic mixture solution, 3 mL of reacted solution was taken for each 25 min, centrifuged and filtered for further analysis. The aliquot from the reaction was used to analyze the strength of Congo Red dye solution with the help of UV spectrophotometer (Shimadzu UV mini-1240, 200-800 nm). The effectiveness of degradation rate was derived from the following equation: Photodegradation efficiency = 1-[C/ Co], where C and Co are the initial and final absorption intensity of dye solution, respectively. At the end of the photodegradation process, the used catalyst was removed, washed with distilled water and dried in oven at 120 °C to understand the reusability of the sample.