SERS-based detection of efficient removal of organic dyes using molybdenum dichalcogenide nanostructures

Two-dimensional materials have been popular in recent times owing to their special properties that can lead to several applications. In particular, transition metal dichalcogenides have been reported to be potential candidates for photocatalytic degradation and adsorptive removal of organic pollutants. Molybdenum-based chalcogenides have shown to be very efficient in removing pollutant dyes from aqueous solutions. Here, we report a facile method for the removal of organic dyes from aqueous solution using molybdenum dichalcogenide (MoX2; X = S, Se, Te) based nanostructures. The molybdenum dichalcogenide nanostructures were synthesized chemically using the simple hydrothermal method. The samples were characterized by X-ray diffraction, Raman Spectroscopy, UV–visible spectroscopy, and scanning electron microscopy. The as-prepared samples have been utilized as an adsorbent for the removal of common organic dyes such as methylene blue (MB), methyl orange (MO), malachite green (MG), rhodamine B (RhB), rhodamine 6 G (R6G) and mixtures of these organic dyes from aqueous solution. It was observed that among the synthesized samples, molybdenum disulfide (MoS2) presented excellent adsorption affinity towards these dyes. In addition, selective adsorption of MB in the presence of MO and RhB was demonstrated. Furthermore, the application of surface-enhanced Raman scattering (SERS) to monitor the degradation of the dyes in the experiments was also investigated.


Introduction
The unique structural, optical, and electronic properties of 2D materials offer new opportunities in a large number of applications ranging from optoelectronics, nanoelectronics to sensing and catalysis [1][2][3][4][5][6]. The interesting properties of these materials arise from interlayer Van der Waals (VdW) forces between atomic sheets of covalently bonded atoms [7][8][9]. Among various emerging materials, 2D materials ranging from bulk to nanoscale, transition metal dichalcogenides (TMDs) have gained popularity due to their excellent structural and physical features such as stability, compositional flexibility, large surface area, electronic conductivity, porosity, and layered assemblies [10][11][12][13][14]. They represent an affordable and abundant alternative to replace the expensive as well as relatively rare Pt group of metals for catalysis applications. When compared to their bulk counterparts, TMD nanostructures possess excellent catalytic properties with greater activity, selectivity as well as stability along with extremely high specific surface area with the capability of tuning their catalytically active sites [15]. 2D-TMDs have been explored worldwide for their potential in applications such as catalysis, energy storage, electronics, optoelectronics, photonics, and gas sensors. TMDs have an X-M-X layered structure so that its single layer includes a layer of the transition metal (M), which is between two layers of chalcogenide (X) atoms [16,17]. Although several different techniques such as sputtering, chemical vapor deposition, electrodeposition, and spray pyrolysis have been utilized for the synthesis of 2D materials, the hydrothermal method is relatively Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. simple for large-scale, cost-effective preparation [18,19]. Out of the different TMDs, molybdenum disulfide (MoS 2 ) is one of the most stable systems that can be easily synthesized by hydrothermal process. It is composed of layers having a prismatic arrangement involving an atomic plane of Mo sandwiched between two atomic planes of S. Here the MoS 2 layers are bound to each other by weak VdW forces, while the S and Mo atoms have strong covalent bonds between them. In the 2H phase of MoS 2 , the Mo atom is surrounded in trigonal prismatic coordination with two S-Mo-S units per elemental cell [20]. Considerable effort has been made to study the H-phase MoS 2 in the fields of electronics, photonics, and photo-catalytic chemistry [21]. Molybdenum selenide (MoSe 2 ) has also been extensively researched due to its strong absorption of sunlight and unique band structure while molybdenum telluride (MoTe 2 ) is gaining interest in electronic and optoelectronic devices [22]. These 2D materials show interesting properties that can be harnessed to remove environmental pollutants by catalysis and adsorption.
Environmental pollution has been a great concern across the globe and organic dyes are serious threats as water pollutant which is jeopardizing natural resources. In particular, persistent organic pollutants (POPs) such as organic dyes can have long-lasting impacts on aquatic as well as human beings by way of bioaccumulation. Therefore, there has been considerable effort in the efficient treatment of water as well as the removal of such pollutants [23]. In consequence, various techniques have been developed for the removal of dye, including precipitation, ultrafiltration, photocatalysis, and adsorption [23][24][25][26]. The adsorption technique deserves extensive attention due to its low consumption of energy, simple operation, high efficiency, low cost, and also its wide suitability for diverse organic dyes [27]. Massey et al have shown that hierarchical microspheres of MoS 2 nanosheets are efficient and regenerative adsorbents for the removal of methylene blue (MB), malachite green (MG), rhodamine 6 G (R6G), fuchsin acid, and congo red dyes [28]. They illustrated a kinetic study of MO dye on MoS 2 nanosheets. Also, Han and co-authors have shown that MoS 2 possessed superior dye adsorption behavior for rhodamine B (RhB), MB, and methyl orange (MO) [29]. Tang et al have investigated the adsorption process of MO on 3D hierarchical flower-like MoSe 2 microspheres [30]. In all these reports, the adsorption process was studied by monitoring the UV-visible absorption spectrum of the dye in regular intervals after the addition of the TMD. While absorption measurements can provide information on the removal of the dye from the solution, it lacks sensitivity to detection. Spectroscopic tools such as FTIR, Raman, and mass spectrometry can detect the presence of trace amounts of the dye in the solution. In this regard, surface-enhanced Raman scattering (SERS) has emerged as an efficient method to detect ultralow concentrations of analyte molecules [31][32][33][34]. By harnessing the surface plasmon excitations in noble metals it is possible to enhance Raman scattering from analytes that are in close vicinity of these metal nanostructures. Although there have been previous works on using SERS to detect dye adsorption and removal, real-time monitoring of the degradation process using Raman spectra was not reported [35,36]. Here we explore the possibility of using SERS as a tool to detect the adsorption and removal of organic dyes using TMDs by the addition of gold nanoparticles as SERS substrates.
We study the use of different TMD nanostructures for efficient adsorption and removal of different organic dyes. In this work, MoS 2 , MoSe 2, and MoTe 2 were synthesized using the hydrothermal method. The samples were characterized using X-ray diffraction, Raman Spectroscopy, UV-visible spectroscopy, and scanning electron microscopy. The as-prepared samples have been utilized as an adsorbent for the removal of common organic dyes such as MB, MO, MG, RhB, and R6G from aqueous solution without the use of any photocatalytic activity. Among the various samples, MoS 2 showed remarkable adsorption activity towards all these dyes. We have tested the adsorption properties of MoS 2 toward each of them in single and combined dye systems such as

Synthesis of MoS 2
The schematic of the synthesis process is shown in figure 1. Few-layered MoS 2 nanosheets were synthesized by a one-step hydrothermal reaction method reported previously [37]. In a typical experiment, 1.288 g Na 2 MoO 4 (Sigma-Aldrich, 99%) and 1.906 g thioacetamide (C 2 H 5 NS, Sigma-Aldrich, 99%) were dissolved in 80 ml deionized (DI) water and stirred for 1 h at 27°C by using a magnetic stirrer. The solution was transferred to a 100 ml stainless steel autoclave, which was then heated up to 200°C and kept for 24 h. After cooling naturally, the product was filtered, washed with DI water followed by ethanol, and dried at 70°C for 4 h. MoS 2 was also synthesized at a higher temperature of 210°C with a precursor ratio of 1:5.

Synthesis of MoSe 2
In this method, Se nanoparticles were prepared by making an aqueous stock solution of 100 mM sodium selenite and 400 mM ascorbic acid. Ascorbic acid was added dropwise to the sodium selenite solution under magnetic stirring (300 rpm) at room temperature for 20 min. The mixtures were allowed to react till the color change was observed from colorless to orange. Soon after the change was observed the mixtures were diluted to 25 ml DI water and washed several times with DI water. These Se nanoparticles were used in the synthesis of MoSe 2 . The process involves the formation of a black solution by adding 0.25 g of Se nanoparticles, 0.1 g of sodium borohydride, and 0.65 g of sodium molybdate in 40 ml of DI water at room temperature. The as-formed mixture was stirred for 10 min, ensuring the complete dispersion of selenium, and then transferred into the Teflon-lined stainless-steel autoclave of 100 ml capacity, sealed, and placed in the oven at 200°C for 24 h. After the autoclave cooled down naturally, the precipitate was cooled by washing with excess DI water and dried at 50°C for 1 h.

Synthesis of MoTe 2
For the synthesis of MoTe 2 1.45 g of sodium molybdate, 1.53 g of tellurium metal powder, and 0.34 g of sodium borohydride were added in 80 ml of DI water at room temperature. The mixture was stirred for 10 min followed by transfer of the mixture to a Teflon-lined stainless-steel autoclave and hydrothermal process at 200°C for 48 h. The sample was collected, washed with DI water followed by ethanol, and finally dried at 70°C for 4 h.

Materials characterization
UV-Vis absorbance spectra of the synthesized samples were recorded using a Shimadzu UV-2401 PC UV-Vis spectrophotometer. Powder XRD pattern was obtained in the angle range of 10°-70°, using Cu-K α radiation (1.5406 Å) with an operation voltage and current maintained at 40 kV and 50 mA. SEM images of the samples were taken using Zeiss EVO 18 system. The Raman spectra were obtained using the LabRAM HR Evolution Raman system with a 532 nm laser source.

Results and discussion
Characterization of MoS 2 samples The plots in figure 2(a) show the UV-Vis diffuse reflection spectrum of the MoS 2 sample. Although MoS 2 has almost a flat absorption in the visible, it becomes stronger towards UV. An excitonic absorption peak around 680 nm can be observed in the plot [38]. The energy band gap from the Kubelka-Munk model ( figure 2(b)) was found to be 2.11 eV for MoS 2 which is in accordance with the values reported in the literature.62, 63 The XRD pattern of MoS 2 (shown in figure 2(c)) exhibited the main peaks at 14.2°, 33.20°, 39.5°, and 57.23°which is attributed to (002), (100), (103), and (110) planes of 2H-MoS 2 [39]. The Raman spectra of MoS 2 depict two prominent phonon modes E 2g 1 which appears 380 cm −1 due to in-plane vibration and A g 1 which appears 405 cm −1 due to out-of-plane vibration mode [40][41][42][43][44][45]. The appearance of two first-order Raman active modes   3(a)) of MoS 2 synthesized at 200°C shows a typical 3D-flower-like structure that is obtained by hydrothermal process at high pressure and temperatures [30]. The zoomed-in SEM image is given in figure 3(b). The corresponding images for the MoS 2 sample synthesized at 210°C is shown in figures 3(c) and (d). In both cases, the flower-like assembly can be observed which provides a large surface area needed for efficient adsorption.
Similar measurements were also carried out on MoSe 2 and MoTe 2 samples (See Supplementary  Information). The energy band gap of MoSe 2 from UV-Vis spectroscopy is found to be 1.55 eV which is in agreement with previous reports [46]. The Raman spectrum of MoSe 2 was recorded at room temperature with an excitation wavelength of 532 nm. Peaks position at 287 cm −1 and 239 cm −1 correspond to that of 2H MoSe 2 commensurate with those found in the literature [47]. The XRD pattern of MoSe 2 is indexed to its hexagonal phase while that of MoTe 2 indicates the peaks corresponding to MoTe 2 and weaker Te-rings with the pattern indexed to their hexagonal phase similar to that reported previously [10].

Adsorption experiments
The results of adsorption measurements for RhB, R6G, MG, and MO are shown in figure 4. All adsorption experiments were carried out in the dark and at room temperature. At first, a 40 mg MoS 2 sample was added to 100 ml of 10 ppm solution of RhB, R6G, MO, MB, and MG. Due to the lower adsorption capacity observed in RhB the concentration of the solution was decreased to 5 ppm. Also, due to rapid adsorption of MG and MB, the concentration of solution was increased to 20 ppm for MG to slow down the adsorption process while the amount of sample was reduced to 15 mg for MB. For the dye mixture, 40 mg of MoS 2 was added to a solution containing 5 ml of MO (100 ppm), and 5 ml of MB (100 ppm) diluted to get 100 ml of total solution. A similar experiment was also carried out with a solution containing 5 ml of RhB (100 ppm), and 5 ml of MB (100 ppm) diluted to get 100 ml of total solution. The concentration of the solution was determined using the UV-Visible spectrophotometer. Selective adsorption of MB in the presence of MO and RhB was observed.
The physisorption process is regarded to be the key driving force for adsorption of the organic dyes [25]. It is a weak adsorption process in which the adsorbate is adsorbed onto the surface of the adsorbent either by van der Waals or dipole-driven interaction. In this process, the chemical nature of the adsorbing pair remains intact [28]. The adsorption kinetics and the adsorption mechanism of the dyes on the MoS 2 nanosheets were investigated by linear pseudo-first-order (equation 1) and linear pseudo second-order (equation 2) adsorption models using UV-Vis absorption results.
The pseudo first order adsorption model is expressed by the equation [48]: where Q e and Q t are the amount of dye adsorbed (mg.g −1 ) at equilibrium and at time t (min.), respectively. k 1 is the pseudo first order rate constant (min −1 ). The pseudo second order adsorption model gives where k 2 is the pseudo second order adsorption process rate contact (g.mg −1 .min −1 ). The pseudo first order kinetic model for RhB gave a regression coefficient (R 2 value) of 0.9862 which indicated that the adsorption process fitted well to the pseudo first-order adsorption kinetic model ( figure 4(c)). It was also observed that the recovered MoS 2 can be further reused for the dye adsorption of subsequent batches without compromising the adsorption capacity.
A similar experiment was conducted with R6G and it was found that MoS 2 could remove the dye successfully from the solution ( figure 4(d)). The corresponding adsorption kinetics (figure 4(e)) and the pseudo-first-order fit (figure 4(f)) were also obtained. Figures 4(g) and (j) show the results of absorption spectra of two other common dyes: MG and MO respectively after adding MoS 2 . The adsorption kinetics is plotted in figures 4(h) and (k). The adsorption process in these cases fitted well with a second-order kinetic model (see figures 4(i) and (l)). As in the case of rhodamine dyes, it was possible to remove the organic dye from the solution and reuse the recovered MoS 2 particles for further adsorption process. The evolution of absorption spectra and adsorption kinetics for MB is shown in figures 5(a)−(c).
We also explored the possibility of selective adsorption of a mixture of dyes by MoS 2 structures. Figure 5(d) shows the selective adsorption of MB in the presence of MB and MO. Also, the selective adsorption of MB in the presence of RhB is shown in figure 5(e). Furthermore, it was observed that recovered MoS 2 can be reused for the dye adsorption of subsequent batches without compromising the adsorption capacity. Such high adsorption capacity of these MoS 2 nanostructures can be attributed to their large surface area and layered structure [28]. From these experiments, we inferred that MoS 2 can be used for the selective removal of a mixture of dyes.
SERS is a very powerful spectral analysis technique and has exhibited remarkable application prospects in various fields such as surface and interface chemistry, catalysis, nanotechnology, biology, biomedicine, food science, environmental analysis and other areas [33,[49][50][51][52]. It is a highly sensitive technique that enhances the Raman signal supported by some nanostructured materials and has many advantages over its parent technique of Raman spectroscopy [32,53]. It is an effective vibrational spectroscopic tool that provides several orders of magnitude higher sensitivity than inherently weak Raman scattering by exciting localized surface plasmon resonance (LSPR) on the metal substrates [54][55][56][57][58]. High sensitivity and ease of use can be exploited in the detection of analyte molecules of ultra-low concentrations in combination with other techniques such as electrokinetics, dielectrophoresis, and microfluidics [59][60][61]. SERS can find applications in the detection of organic dyes as the dye molecules efficiently get adsorbed onto the plasmonic metal nanoparticles. In this work, we investigated the application of SERS in the detection of the removal of dyes from aqueous solutions. The schematic of the experiment is shown in figure 6. In a typical experiment, 100 μl of goldnanoparticle solution ( -10 10 M concentration) was mixed with 100 μl of the dye solutions collected at different time intervals after the addition of MoS 2 . The mixture was then sonicated for 20 min and drop-casted on a clean silicon wafer. SERS was also performed to compare the activity of various molybdenum dichalcogenides towards the removal of MB from aqueous solution and it was observed that MoS 2 was most efficient. The time evolution of the SERS spectra for the adsorption of RhB, R6G, MB, and MO was taken before and after the adsorption experiment (i.e., the addition of MoS 2 ) clearly indicates the removal of the dye from the solution on the addition of MoS 2 ( Supplementary Information figure S8). It can be observed that with time the intensity of the Raman peaks gets reduced indicating a decrease in the dye concentration. As the dye gets adsorbed onto the MoS 2 surface, the number of molecules in solution rapidly reduces leading to a low SERS signal.  In figure 7, the evolution of SERS spectra of MG after the addition of MoS 2 with time is shown. The intensity of the characteristic peaks is found to decrease with time after the addition of MoS 2 indicating the removal of dye from the aqueous solution. The SERS spectra of a combination of dyes MO+MB, RhB+MB, and MG+MB reveal the rate of removal of the dyes from the solution after the addition of the TMD (Supplementary data figure S9).
One added advantage of using TMD-based materials for dye removal is the stability of the compound which allows for recovery and reuse. In order to test this, we recovered the MoS 2 after an adsorption process and then used the recovered MoS 2 to remove the dye from a fresh solution. For the recovery of molybdenum disulfide (MoS 2 ), the desorption of a dye adsorbed on molybdenum disulfide (MoS 2 ), an aqueous solution of sodium hydroxide (NaOH) was first used for desorption of the dye. The dye was removed from the solution, and the residual MoS 2 was recovered and washed. The samples were then used for further cycles of dye adsorption. The SERS spectra of MoS 2 recovered after the adsorption of MB dye demonstrate the characteristic peaks of MB dye adsorbed onto the MoS 2 nanosheets ( figure S9 (d)).
In addition, SERS was also performed to compare the activity of other molybdenum dichalcogenides (MoTe 2 and MoSe 2 ) towards the removal of MB from aqueous solution and it was observed that MoS 2 was most efficient in the removal of the organic dyes. In figure 8, SERS spectra showing degradation of MB using MoS 2 , MoSe 2, and MoTe 2 nanostructures are shown. It can be observed that MoS 2 shows better adsorption of the dye compared to the other metal dichalcogenides.

Conclusion
In summary, molybdenum dichalcogenide (MoX 2 ; X = S, Se, Te) nanostructures were synthesized using the hydrothermal method and characterized by x-ray diffraction (XRD), Raman Spectroscopy, UV-Visible spectroscopy and scanning electron microscopy (SEM). Among the various samples, MoS 2 showed remarkable adsorption activity towards MO, MB, MG, RhB, and R6G. We have tested the adsorption properties of MoS 2 toward each of them in single and combined dye systems. Furthermore, it was observed that recovered MoS 2 can be reused for the dye adsorption of subsequent batches without compromising the adsorption capacity. The application of SERS in the detection of the degradation of the dyes in the experiments was also investigated. The intensities of the characteristic peaks were found to decrease with time after the addition of MoS 2 indicating the removal of dye from the aqueous solution. In addition, SERS was also performed to compare the activity of various molybdenum dichalcogenides towards the removal of MB from aqueous solution and it was observed that MoS 2 was most efficient. The SERS spectra of MoS 2 recovered after adsorption of MB dye demonstrate the characteristic peaks of MB dye adsorbed onto the MoS 2 nanosheets.
Supporting data, is available online. Characterization plots of MoSe 2 , MoTe 2, and Au nanoparticles, the evolution of SERS spectra for various dyes, and combination of dyes.