Hydrothermally Synthesized MoS 2 as Electrochemical Catalyst for the Fabrication of Thiabendazole Electrochemical Sensor and Dye Sensitized Solar Cells

: In this work we reported the hydrothermal preparation of molybdenum disulfide (MoS 2 ). The phase purity and crystalline nature of the synthesized MoS 2 were examined via the powder X-ray diffraction method. The surface morphological structure of the MoS 2 was examined using scanning electron microscopy and transmission electron microscopy. The specific surface area of the MoS 2 was calculated using the Brunauer-Emmett-Teller method. The elemental composition and distribution of the Mo and S elements were determined using energy-dispersive X-ray spectroscopy. The oxidation states of the Mo and S elements were studied through employing X-ray photoelectron spectroscopy. In further studies, we modified the active surface area (3 mm) of the glassy carbon (GC) electrode using MoS 2 as an electrocatalyst. The MoS 2 modified GC electrode (MSGC) was used as an electrochemical sensor for the detection of thiabendazole (TBZ). Linear sweep voltammetry (LSV) was used as the electrochemical sensing technique. The MSGC exhibited good performance in the detection of TBZ. A limit of detection of 0.1 µ M with a sensitivity of 7.47 µ A/ µ M.cm 2 was obtained for the detection of TBZ using the LSV method. The MSGC also showed good selectivity for the detection of TBZ in the presence of various interfering compounds. The obtained results showed that MoS 2 has good electrocatalytic properties. This motivated us to explore the catalytic properties of MoS 2 in dye sensitized solar cells (DSSCs). Thus, we have fabricated DSSCs using MoS 2 as a platinum-free counter electrode material. The MoS 2 counter electrode-based DSSCs showed good power conversion efficiency of more than 5%. We believe that the present work is beneficial for the scientific community, and especially for research surrounding the design and fabrication of catalysts for electrochemical sensing and DSSC applications.


Introduction
At present, the energy crisis, environmental pollution, and food safety related issues are major concerns which demand extensive attention [1][2][3].Ensuring the safety of food is a critical priority for public health, especially when it comes to agricultural products that play a vital role in providing essential nutrition to the population [4].Although there has been a historical emphasis on the safety of veterinary drugs, recent research has highlighted a new potential hazard: the existence of various pesticides in crops cultivated in soil [5].The application of pesticides on crops is a vital element in cultivating agricultural goods, providing advantages such as enhanced product quality [6][7][8].However, if pesticides are applied incorrectly or excessively, it can lead to the accumulation of pesticide residues, which pose a substantial threat to global environmental safety and public health [9].Prolonged exposure to these pesticides has been associated with various adverse health effects, encompassing neurodegenerative, respiratory, reproductive, and developmental toxicity [10].Of particular concern are the effects observed during crucial developmental phases, which can have enduring repercussions on individuals, especially concerning brain development and the endocrine system [11].A notable harmful pesticide is the fungicide and insecticide thiabendazole (TBZ), extensively used in agriculture to combat mildew and blight on crops [12].TBZ residues have been identified in both crops and farm runoff, and its biological activity and persistence enhance its potential to interfere with thyroid hormone balance, induce liver damage, and potentially lead to severe cases of cancer [10].Consequently, it is essential to establish accurate and dependable methods for identifying and measuring TBZ in food matrices [9].Ensuring the safety of agricultural products mandates thorough pesticide residue testing before the products enter the market [13].Several effective assessment techniques have been documented for identifying pesticide residues in fruits, encompassing chromatographic methods, spectrometric approaches, and immunoassays [13][14][15][16].However, these methods often necessitate advanced instruments, costly reagents, and extensive sample preparation, constraining their suitability for onthe-spot detection and quick screening [17][18][19].In contrast, electrochemical sensors offer a straightforward, cost-efficient, and rapid alternative method for the detection of pesticide residues in food samples [20][21][22].In recent times, numerous electrochemical sensors have emerged as candidates for determining the presence of TBZ and other pesticide residues in fruits, utilizing various nanomaterials and sensing strategies [18].These encompass sensors based on graphene, metal oxide nanocomposites, conducting polymers, and more.Nevertheless, challenges such as limitations surrounding selectivity, reproducibility, and long-term stability still need to be resolved in order to guarantee dependable and precise detection of pesticide residues in practical, real-world scenarios [19].
To deal with the energy crisis, various technologies such as hydrogen production devices [23,24], energy storage devices (super-capacitors) [25,26], batteries [27], light emitting diodes [28], electrochromic devices [29,30], and solar cells [31] have been developed.In particular, solar cells possess unique advantages over energy-related optoelectronic devices due to their simple methods of fabrication, cost-effectiveness, and eco-friendliness [32].Dye sensitized solar cells (DSSCs) have received extensive attention because of their stability, performance, low cost, and environmentally friendly nature [33].DSSCs consist of various components, which can be categorized as a photoanode, light absorber material, electrolyte, and counter electrode [34].Counter electrodes typically consist of platinum (Pt), which catalyzes the redox process during the operation of DSSCs.Specifically, counter electrode materials accept electrons from the photoanode and transfer them to the to the redox electrolyte by completing the electric circuit [35].Pt has been proven to be the most promising and efficient counter electrode material for DSSCs and their applications [36].Unfortunately, Pt is a precious metal and is of high cost and low abundance [37].Thus, it is difficult to commercialize Pt-based DSSCs, and it is necessary to discover alternative Pt-free counter electrode materials for DSSCs.In this regard, various transition metal oxides such as nickel oxide [38,39], manganese dioxide [40], tin oxide [41], copper oxide [42], cobalt oxide [43], zinc oxide [44], tungsten oxide [44], and polymers such as polyaniline [45], polypyrrole [46], and polythiophene [47] have been explored as potential Pt-free counter electrode materials.Some of these Pt-free counter electrode materials have shown good photovoltaic performance, but overall, the performance of Pt-free DSSCs needs to be improved.
Recently, transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS 2 ), tungsten disulfide (WS 2 ), and molybdenum diselenide (MoSe 2 ) have attracted research interest due to their excellent optical and electrical properties [34,48,49].MoS 2 has been widely used as an electrocatalyst for the fabrication of electrochemical sensors due to its excellent catalytic properties [50].The presence of covalent bonding between Mo and S and van der Waals bonds between the layers in this compound make it a suitable candidate for sensing applications [37].Previously, MoS 2 and its hybrid composite materials have been used in the fabrication of dopamine [50], uric acid [51], ascorbic acid [52], nitrophenol [53], hydrazine [54], and glucose [55].The above discussed results indicated the presence of excellent electrocatalytic properties in MoS 2 .Moreover, MoS 2 has a work function of 5.3 eV, which is comparable to Pt [56,57].Thus, it can be assumed that MoS 2 can be used as a catalyst for sensing and DSSC applications.
This paper reports the bi-functional properties of hydrothermally grown MoS 2 for use in the construction of a TBZ sensor and Pt-free DSSCs.The fabricated sensor exhibited a reasonably good limit of detection, with good sensitivity for the presence of TBZ.The MoS 2 based DSSCs also demonstrated good power conversion efficiency and open circuit voltage., which can be assigned to the presence of (002), (100), ( 103), (105), and (110) diffraction planes, respectively.The above peaks exist due to the interference taking place within the layers.shown good photovoltaic performance, but overall, the performance of Pt-free DSSCs needs to be improved.Recently, transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2), tungsten disulfide (WS2), and molybdenum diselenide (MoSe2) have attracted research interest due to their excellent optical and electrical properties [34,48,49].MoS2 has been widely used as an electrocatalyst for the fabrication of electrochemical sensors due to its excellent catalytic properties [50].The presence of covalent bonding between Mo and S and van der Waals bonds between the layers in this compound make it a suitable candidate for sensing applications [37].Previously, MoS2 and its hybrid composite materials have been used in the fabrication of dopamine [50], uric acid [51], ascorbic acid [52], nitrophenol [53], hydrazine [54], and glucose [55].The above discussed results indicated the presence of excellent electrocatalytic properties in MoS2.Moreover, MoS2 has a work function of 5.3 eV, which is comparable to Pt [56,57].Thus, it can be assumed that MoS2 can be used as a catalyst for sensing and DSSC applications.

Materials Characterization
This paper reports the bi-functional properties of hydrothermally grown MoS2 for use in the construction of a TBZ sensor and Pt-free DSSCs.The fabricated sensor exhibited a reasonably good limit of detection, with good sensitivity for the presence of TBZ.The MoS2 based DSSCs also demonstrated good power conversion efficiency and open circuit voltage.

Materials Characterization
Figure 1 exhibits the P-XRD pattern of the as-obtained MoS2 sample.The P-XRD pattern of the synthesized MoS2 shows diffraction peaks at 13.89°, 32.99°, 39.46°, 49.52°, and 58.88°, which can be assigned to the presence of (002), (100), ( 103), (105), and (110) diffraction planes, respectively.The above peaks exist due to the interference taking place within the layers.The surface morphology of the synthesized MoS2 was examined utilizing scanning electron microscopy (SEM).The SEM images of the as-synthesized MoS2 are displayed in The surface morphology of the synthesized MoS 2 was examined utilizing scanning electron microscopy (SEM).The SEM images of the as-synthesized MoS 2 are displayed in Figure 2a,b.It was observed that MoS 2 has sheet-like surface morphological characteristics.Thus, it can be said that MoS 2 has a uniform sheet-like surface structure, which consists of flower-like self-assembled MoS 2 particles.TEM images of the MoS 2 were also recorded to further confirm its morphological features.Figure S1a,b shows that MoS 2 has flower-like sheets, which is in agreement with the SEM results.It was observed that MoS2 has sheet-like surface morphological characteristics.Thus, it can be said that MoS2 has a uniform sheet-like surface structure, which consists of flower-like self-assembled MoS2 particles.TEM images of the MoS2 were also recorded to further confirm its morphological features.Figure S1a,b shows that MoS2 has flower-like sheets, which is in agreement with the SEM results.The phase purity and presence of the Mo and S elements in the compound were also examined using energy X-ray dispersive spectroscopy (EDX).The EDX spectrum and EDX mapping images of the synthesized MoS2 are summarized in Figure 3a-d.Figure 3b demonstrates the presence of Mo and S elements and shows that no other element has been detected.This suggests that the MoS2 sample has good phase purity, and no residual element or impurities were present in the synthesized MoS2 sample.The uniform distribution of the Mo and S elements can be seen in Figures 3c and 3d, respectively.The EDX mapping results suggest uniform particle distribution.Furthermore, a high resolution XPS spectrum for Mo3d and S2p was also recorded.Figure S2a shows the obtained Mo3d XPS spectrum of the MoS2.The Mo3d spectrum shows XPS peaks at the binding energies of 231.37 eV and 228.21 eV, which can be assigned to Mo(IV)3/2 and Mo(IV)/5/2, respectively.However, the peak at 235.41 eV was ascribed to the presence of Mo(VI)3/2.Figure S2b shows the S2p XPS spectrum of MoS2, and two XPS peaks were observed at binding energies of 161.8 eV and 160.6 eV.These peaks were ascribed to the presence of S2p1/2 and S2p3/2, respectively.These results confirmed the formation of MoS2.The specific surface area may play a vital role in the catalysis applications.Thus, it is of great importance to know the specific surface area of the as-prepared MoS2 sample.The BET method was adopted to calculate the specific surface area of the as-prepared MoS2 sample.The nitrogen (N2) adsorption-desorption isotherm of MoS2 was also recorded to obtain the specific surface area of the synthesized MoS2 sample.The phase purity and presence of the Mo and S elements in the compound were also examined using energy X-ray dispersive spectroscopy (EDX).The EDX spectrum and EDX mapping images of the synthesized MoS 2 are summarized in Figure 3a-d.Figure 3b demonstrates the presence of Mo and S elements and shows that no other element has been detected.This suggests that the MoS 2 sample has good phase purity, and no residual element or impurities were present in the synthesized MoS 2 sample.The uniform distribution of the Mo and S elements can be seen in Figures 3c and 3d, respectively.The EDX mapping results suggest uniform particle distribution.Furthermore, a high resolution XPS spectrum for Mo3d and S2p was also recorded.Figure S2a shows the obtained Mo3d XPS spectrum of the MoS 2 .The Mo3d spectrum shows XPS peaks at the binding energies of 231.37 eV and 228.21 eV, which can be assigned to Mo(IV) 3/2 and Mo(IV)/ 5/2 , respectively.However, the peak at 235.41 eV was ascribed to the presence of Mo(VI) 3/2 .Figure S2b shows the S2p XPS spectrum of MoS 2 , and two XPS peaks were observed at binding energies of 161.8 eV and 160.6 eV.These peaks were ascribed to the presence of S2p 1/2 and S2p 3/2 , respectively.These results confirmed the formation of MoS 2 .
Figure 2a,b.It was observed that MoS2 has sheet-like surface morphological characteristics.Thus, it can be said that MoS2 has a uniform sheet-like surface structure, which consists of flower-like self-assembled MoS2 particles.TEM images of the MoS2 were also recorded to further confirm its morphological features.Figure S1a,b shows that MoS2 has flower-like sheets, which is in agreement with the SEM results.The phase purity and presence of the Mo and S elements in the compound were also examined using energy X-ray dispersive spectroscopy (EDX).The EDX spectrum and EDX mapping images of the synthesized MoS2 are summarized in Figure 3a-d.Figure 3b demonstrates the presence of Mo and S elements and shows that no other element has been detected.This suggests that the MoS2 sample has good phase purity, and no residual element or impurities were present in the synthesized MoS2 sample.The uniform distribution of the Mo and S elements can be seen in Figures 3c and 3d, respectively.The EDX mapping results suggest uniform particle distribution.Furthermore, a high resolution XPS spectrum for Mo3d and S2p was also recorded.Figure S2a shows the obtained Mo3d XPS spectrum of the MoS2.The Mo3d spectrum shows XPS peaks at the binding energies of 231.37 eV and 228.21 eV, which can be assigned to Mo(IV)3/2 and Mo(IV)/5/2, respectively.However, the peak at 235.41 eV was ascribed to the presence of Mo(VI)3/2.Figure S2b shows the S2p XPS spectrum of MoS2, and two XPS peaks were observed at binding energies of 161.8 eV and 160.6 eV.These peaks were ascribed to the presence of S2p1/2 and S2p3/2, respectively.These results confirmed the formation of MoS2.The specific surface area may play a vital role in the catalysis applications.Thus, it is of great importance to know the specific surface area of the as-prepared MoS2 sample.The BET method was adopted to calculate the specific surface area of the as-prepared MoS2 sample.The nitrogen (N2) adsorption-desorption isotherm of MoS2 was also recorded to obtain the specific surface area of the synthesized MoS2 sample.The specific surface area may play a vital role in the catalysis applications.Thus, it is of great importance to know the specific surface area of the as-prepared MoS 2 sample.The BET method was adopted to calculate the specific surface area of the as-prepared MoS 2 sample.The nitrogen (N 2 ) adsorption-desorption isotherm of MoS 2 was also recorded to obtain the specific surface area of the synthesized MoS 2 sample.
The N 2 adsorption-desorption isotherm of MoS 2 is depicted in Figure 4.The obtained results demonstrate that MoS 2 has a specific surface area of 57.5 m 2 /g.The aforementioned characterization studies showed that MoS 2 has a layered structure with good specific surface area and sheet-like surface morphology.These properties may be beneficial for improving electron transportation.Thus, the as-synthesized MoS 2 has been explored as a catalyst for the development of a TBZ sensor and Pt-free DSSCs.
results demonstrate that MoS2 has a specific surface area of 57.5 m 2 /g.The aforemention characterization studies showed that MoS2 has a layered structure with good specific su face area and sheet-like surface morphology.These properties may be beneficial for im proving electron transportation.Thus, the as-synthesized MoS2 has been explored as catalyst for the development of a TBZ sensor and Pt-free DSSCs.

Electrochemical Performance of the MSGC
The MoS2-modified GC (MSGC) electrode was used as a working electrode, with s ver/silver chloride (Ag/AgCl) as a reference electrode and Pt as a counter electrode.T bare GC electrode was also used as a working electrode for the control experiment.T sensing performance of the bare GC electrode was evaluated for 20 µM TBZ in the pre ence of 0.1 M PBS of pH 2.0 at an applied scan rate of 50 mVs −1 .Linear sweep voltammet (LSV) was used as the sensing technology.The bare GC exhibited a current response 8.7 µA for the oxidation of TBZ.The obtained LSV graph of the bare GC electrode h been illustrated in Figure 5a, and its oxidation peak value is presented in Figure 5b.further studies, the LSV of the MSGC was also recorded under similar conditions an concentration as was used for the bare GC electrode.It can be noted that the MSGC exhi its an improved current response of 20.4 µA for the oxidation of 20 µM TBZ, as shown Figure 5a,b.This interesting and significant improvement in the current response MSGC may be attributed to the presence of electrochemically active MoS2 catalyst wi high surface area on the surface of GC electrode.This suggests the presence of good cat lytic properties in the hydrothermally prepared MoS2.

Electrochemical Performance of the MSGC
The MoS 2 -modified GC (MSGC) electrode was used as a working electrode, with silver/silver chloride (Ag/AgCl) as a reference electrode and Pt as a counter electrode.The bare GC electrode was also used as a working electrode for the control experiment.The sensing performance of the bare GC electrode was evaluated for 20 µM TBZ in the presence of 0.1 M PBS of pH 2.0 at an applied scan rate of 50 mVs −1 .Linear sweep voltammetry (LSV) was used as the sensing technology.The bare GC exhibited a current response of 8.7 µA for the oxidation of TBZ.The obtained LSV graph of the bare GC electrode has been illustrated in Figure 5a, and its oxidation peak value is presented in Figure 5b.In further studies, the LSV of the MSGC was also recorded under similar conditions and concentration as was used for the bare GC electrode.It can be noted that the MSGC exhibits an improved current response of 20.4 µA for the oxidation of 20 µM TBZ, as shown in Figure 5a,b.This interesting and significant improvement in the current response of MSGC may be attributed to the presence of electrochemically active MoS 2 catalyst with high surface area on the surface of GC electrode.This suggests the presence of good catalytic properties in the hydrothermally prepared MoS 2 .
The concentration of TBZ present may play a significant role in the catalytic activity of the MSGC.It is of great importance to study the effect of various concentrations of TBZ on the catalytic behavior of the MSGC.Thus, we have recorded LSV graphs of the MSGC in the presence of different concentrations (0.03, 0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 2, 3, 5, 7, 9, 11, 13, 15, and 20 µM) of TBZ.The obtained LSV graphs have been summarized in Figure 6a.The obtained results show that current response increases with increasing concentration of TBZ.The MSGC exhibits decent linear range from 0.03 to 2 µM, and 2 to 20 µM.The linear calibration graph was plotted between the oxidation current responses of the MSGC with respect to the concentration of TBZ, as shown in Figure 6b.The charge transfer kinetic process of the MSGC was also studied for 20 µM TBZ at different applied scan rates (50-500 mV/s).Figure 7a shows the obtained LSV curves of the MSGC at different applied scan rates and reveals that current response was enhanced as the applied scan rate rose from 50 to 500 mV/s.The calibration plot between the square root of the applied scan rate and oxidation current response of the MSGC is presented in Figure 7b.It was noted that current response linearly increases with a regression coefficient (R 2 ) value of 0.99.This suggests that the sensing of TBZ at the MSGC surface may occur via a diffusion controlled-process.The charge transfer kinetic process of the MSGC was also studied for 20 µM TBZ at different applied scan rates (50-500 mV/s).Figure 7a shows the obtained LSV curves of the MSGC at different applied scan rates and reveals that current response was enhanced as the applied scan rate rose from 50 to 500 mV/s.The calibration plot between the square root of the applied scan rate and oxidation current response of the MSGC is presented in Figure 7b.It was noted that current response linearly increases with a regression coefficient (R 2 ) value of 0.99.This suggests that the sensing of TBZ at the MSGC surface may occur via a diffusion controlled-process.The charge transfer kinetic process of the MSGC was also studied for 20 µM TBZ at different applied scan rates (50-500 mV/s).Figure 7a shows the obtained LSV curves of the MSGC at different applied scan rates and reveals that current response was enhanced as the applied scan rate rose from 50 to 500 mV/s.The calibration plot between the square root of the applied scan rate and oxidation current response of the MSGC is presented in Figure 7b.It was noted that current response linearly increases with a regression coefficient (R 2 ) value of 0.99.This suggests that the sensing of TBZ at the MSGC surface may occur via a diffusion controlled-process.The reproducibility of the MSGC was examined for 20 µM TBZ (scan rate = 50 mV/s).The results suggested a good response, as described in Figure 8a.For the reproducibility study, five MSGC electrodes were fabricated under the same conditions and their performance was examined for the same analyte solution (20 µM TBZ, 0.1 M PBS, and pH = 2.0).The repeatability of the MSGC was also determined via running 40 consecutive LSV responses.The current values for TBZ oxidation for different cycle numbers (1, 10, 20, 30, and 40) are summarized in Figure 8b.The MSGC exhibited good repeatability for 40 cycles.The storage stability is also an important feature of electrochemical sensors, and thus the storage stability of the MSGC was studied for 28 days.The MSGC was stored for 28 days, and then its current response for the oxidation of 20 µM TBZ was examined.Observations revealed good stability at 28 days (Figure 8c).The reproducibility of the MSGC was examined for 20 µM TBZ (scan rate = 50 mV/s).The results suggested a good response, as described in Figure 8a.For the reproducibility study, five MSGC electrodes were fabricated under the same conditions and their performance was examined for the same analyte solution (20 µM TBZ, 0.1 M PBS, and pH = 2.0).The repeatability of the MSGC was also determined via running 40 consecutive LSV responses.The current values for TBZ oxidation for different cycle numbers (1, 10, 20, 30, and 40) are summarized in Figure 8b.The MSGC exhibited good repeatability for 40 cycles.The storage stability is also an important feature of electrochemical sensors, and thus the storage stability of the MSGC was studied for 28 days.The MSGC was stored for 28 days, and then its current response for the oxidation of 20 µM TBZ was examined.Observations revealed good stability at 28 days (Figure 8c).Selectivity is one of the most important properties for any sensor in practical applications.The LSV response of the MSGC for 10 µM TBZ was recorded.Further, 50 µM of interfering compounds (AA + UA + DA + Glu + 4-CP) were introduced to the TBZ solution (Figure 9a).The obtained results showed no change in the LSV response.Similarly, a second group comprising 50 µM of alternative interfering compounds (CA + Cl -+ Na + + AtCl + HZ + H2O2) were spiked to the TBZ solution and no significant change in the LSV re- Selectivity is one of the most important properties for any sensor in practical applications.The LSV response of the MSGC for 10 µM TBZ was recorded.Further, 50 µM of interfering compounds (AA + UA + DA + Glu + 4-CP) were introduced to the TBZ solution (Figure 9a).The obtained results showed no change in the LSV response.Similarly, a second group comprising 50 µM of alternative interfering compounds (CA + Cl − + Na + + AtCl + HZ + H 2 O 2 ) were spiked to the TBZ solution and no significant change in the LSV response was observed.Thus, it was revealed that MSGC has reasonable selectivity for the detection of TBZ.The selectivity responses are shown in Figure 9a,b.

DSSC Applications
In the first step, we examined the catalytic behavior of FTO@MoS2 in the presence of liquid electrolyte solution (0.05 M LiI, 0.01 M I2, and 0.5 M LiClO4 in acetonitrile; scan rate = 50 mV/s).The obtained CV curve of FTO@MoS2 is displayed in Figure 10a.The obtained results exhibit good electrocatalytic activity towards the redox reaction at the surface of FTO@MoS2.The CV of FTO@Pt was also recorded under similar conditions as were used for FTO@MoS2.FTO@Pt exhibited higher electrocatalytic redox properties compared to FTO@MoS2.However, the electrocatalytic activity of FTO@MoS2 is overall reasonable compared to the FTO@Pt.
Thus, it can be concluded that FTO@MoS2 can be applied as a counter electrode for the fabrication of DSSCs.A schematic diagram of the DSSC is shown in Figure 10b.The redox reactions at the surface of FTO@MoS2 can be seen in the inset of Figure 10b.The photovoltaic performance of DSSCs can be evaluated in terms of power conversion effi- The probable mechanism for the sensing of TBZ is described in Scheme 2. In the first step, TBZ is oxidized and releases one electron during the electro-oxidation process.The positively charged TBZ further accepts the released electron to form TBZ. It can be seen that electro-oxidation of TBZ at the MSGC surface is a reversible process.
The performance of the MSGC was calculated using the equations listed below: where σ e is the standard error for the calibration plot between the peak current response and concentration of the TBZ, and S is the slope of the calibration plot between the peak current response and concentration of the TBZ.

Sensitivity = S/area of the GC electrode (2)
The MSGC demonstrated good LOD of 0.1 µM and sensitivity of 7.47 µA/µM.cm 2 .The obtained results have been summarized in Table 1, and are reasonable compared to those in the literature [19,[58][59][60].

DSSC Applications
In the first step, we examined the catalytic behavior of FTO@MoS 2 in the presence of liquid electrolyte solution (0.05 M LiI, 0.01 M I 2 , and 0.5 M LiClO 4 in acetonitrile; scan rate = 50 mV/s).The obtained CV curve of FTO@MoS 2 is displayed in Figure 10a.The obtained results exhibit good electrocatalytic activity towards the redox reaction at the surface of FTO@MoS 2 .The CV of FTO@Pt was also recorded under similar conditions as were used for FTO@MoS 2 .FTO@Pt exhibited higher electrocatalytic redox properties compared to FTO@MoS 2 .However, the electrocatalytic activity of FTO@MoS 2 is overall reasonable compared to the FTO@Pt.Thus, it can be concluded that FTO@MoS 2 can be applied as a counter electrode for the fabrication of DSSCs.A schematic diagram of the DSSC is shown in Figure 10b.The redox reactions at the surface of FTO@MoS 2 can be seen in the inset of Figure 10b.The photovoltaic performance of DSSCs can be evaluated in terms of power conversion efficiency (PCE).The photocurrent density versus voltage (JV) results of the FTO@Pt and FTO@MoS 2 counter electrode-based DSSCs were recorded under 1 sun conditions.The obtained JV results are shown in Figure 11.The photovoltaic parameters of the FTO@Pt and FTO@MoS 2 counter electrode-based DSSCs are summarized in Figure 12a-d.It can be seen that FTO@MoS2 counter electrode-based DSSCs exhibited open circuit voltage (Voc) of 0.73 V (Figure 12a) with photocurrent density (Jsc) of 13.48 mA/cm 2 (Figure 12c).The fill factor (FF) of the photovoltaic cells measures the quality of the cell.The FF has significant impact on the efficiency of DSSCs.The FF can be defined as given below: where Pmax is the maximum power point and Isc is the short circuit current.

Hydrothermal Synthesis of MoS 2
The present work adopted the hydrothermal method for the fabrication of MoS 2 , which has been reported elsewhere [28].In brief, 1.6 g thiourea was dissolved in 25 mL deionized water (DI water).In another beaker, 1.4 g of sodium molybdate was dissolved in 25 mL DI water.The aqueous solution of thiourea was added to the aqueous solution of sodium molybdate.This reaction mixture was stirred at room temperature for 30 min, transferred to a Teflon cup, and covered tightly with a stainless steel autoclave.The autoclave was put into the muffle furnace and heated at 180 • C for 24 h.The muffle furnace was cooled down to room temperature following this.After cooling down the muffle furnace, the autoclave was opened.MoS 2 was collected using centrifugation and washed with DI water and ethanol.After washing, the MoS 2 was dried in a vacuum oven at 60 was cooled down to room temperature following this.After cooling down the muffle furnace, the autoclave was opened.MoS2 was collected using centrifugation and washed with DI water and ethanol.After washing, the MoS2 was dried in a vacuum oven at 60 °C for 12 h.The synthesis of MoS2 is schematically described in Scheme 1.
Scheme 1. Schematic representation of the synthesis of MoS2.

Fabrication of MSGC
The synthesized MoS2 (1.5 mgmL −1 ) was dispersed in DI water via ultrasonication for 2 h (0.1 wt% of the Nafion was added as a binder).7 µL of the prepared dispersion was drop-casted on the active surface area (3 mm) of the glassy carbon (GC) electrode and dried at RT for 3 h.The modified GC electrode was used as the working electrode, and Ag/AgCl was adopted as the reference electrode.A Pt wire-based electrode was used as a counter electrode.The sensing activity of the MoS2 modified GC electrode (MSGC) was determined in a three electrode assembly.A CH instruments-based potentiostat system was used as the sensing platform.The fabrication of MSGC is described in Scheme 2.

Fabrication of MSGC
The synthesized MoS 2 (1.5 mgmL −1 ) was dispersed in DI water via ultrasonication for 2 h (0.1 wt% of the Nafion was added as a binder).7 µL of the prepared dispersion was drop-casted on the active surface area (3 mm) of the glassy carbon (GC) electrode and dried at RT for 3 h.The modified GC electrode was used as the working electrode, and Ag/AgCl was adopted as the reference electrode.A Pt wire-based electrode was used as a counter electrode.The sensing activity of the MoS modified GC electrode (MSGC) was determined in a three electrode assembly.A CH instruments-based potentiostat system was used as the sensing platform.The fabrication of MSGC is described in Scheme 2.

Fabrication of MSGC
The synthesized MoS2 (1.5 mgmL −1 ) was dispersed in DI water via ultrasonication for 2 h (0.1 wt% of the Nafion was added as a binder).7 µL of the prepared dispersion was drop-casted on the active surface area (3 mm) of the glassy carbon (GC) electrode and dried at RT for 3 h.The modified GC electrode was used as the working electrode, and Ag/AgCl was adopted as the reference electrode.A Pt wire-based electrode was used as a counter electrode.The sensing activity of the MoS2 modified GC electrode (MSGC) was determined in a three electrode assembly.A CH instruments-based potentiostat system was used as the sensing platform.The fabrication of MSGC is described in Scheme 2. Scheme 2. Schematic illustration of the fabrication of MSGC for application in a TBZ sensor.

Fabrication of DSSCs
In the first step, a photoanode was fabricated.FTO coated glass substrates (Sigma, 2 × 2 cm pieces) were cleaned in an ultrasonic bath using a mild alkaline solution.The cleaning of the FTO substrates was completed via the use of DI water, acetone, and isopropyl alcohol for 10 min each.The blocking layer of TiO 2 was coated on to the cleaned FTO to stop recombination reactions.The FTO glass substrate was immersed (facing conductive side upwards) in a 45 mM aqueous solution of TiCl 4 for 25 min at 60 • C. The TiCl 4 -immersed FTO glass substrate was heated at 100 • C for 10 min.The doctor-blade method was used for the fabrication of the TiO 2 layer, using an invisible tape as the spacer to maintain the uniform thickness of the TiO 2 film.The TiO 2 paste was prepared by mixing 0.15 mg of polyethylene glycol (PEG; MW 10,000) in 1 molar HNO 3 , followed by a spike of 0.21 mL acetylacetone, 0.04 mL Triton-X, and TiO 2 (P25 Deguusa) powder.This solution was sonicated for 60 min and stirred for 1 day to obtain the colloidal TiO 2 solution.The TiO 2 coated FTO substrates were sintered at 450 • C for 40 min.In another step, the counter electrode was fabricated using MoS 2 as the base material and Pt as a control counter electrode.The MoS 2 (15 mg) was dispersed in 2-propanol for 30 min using sonication.50 µL of the prepared dispersion was drop-casted on to the pre-cleaned FTO glass substrate, which was used as a Pt-free counter electrode.The Pt counter electrode was also fabricated as a control.A 5 mM solution of H 2 [Pt(Cl) 6 ].6H 2 O was prepared in 2-propanol, drop-casted on to the pre-cleaned FTO glass substrates, and sintered at 450 • C for 30 min.
The fabricated photoanode (FTO/TiO 2 ) was dipped in the N3 dye solution [5 mM N3 dye in acetonitrile/tert-butanol (ratio = 1:1)] for 1 day under dark conditions.After 24 h of dipping in N3 dye solution, the photoanode was rinsed with ethanol to remove the extra dye molecules from the photoanode surface.In the final step, the photoanode and counter electrode were clipped together to assemble DSSCs.Parafilm was used as a separator to avoid contact/short circuit between the two conductive surfaces.The liquid electrolyte was prepared using 0.05 M I 2, 0.5 M LiI, and 0.5 M 4-tert-butylpyridine in 3-methoxypropionitrile.The liquid electrolyte was injected in between the photoanode and counter electrode.

Instruments
The XRD pattern of the prepared MoS 2 was recorded on a Rigaku instrument (Cu/Kα radiation (wavelength = 1.54 Å)) (Japan).Scanning electron microscope (SEM) images were captured on a Zeiss Supra 55 microscope.Energy dispersive X-ray (EDX) studies were carried out using an Oxford EDX instrument.The Brunauer-Emmett-Teller (BET) analysis was carried out using nitrogen isotherm (adsorption/desorption) on Quantachrome Instruments: Autosorb iQ (version 1.11).The cyclic voltammograms (CV) of the counter electrodes were obtained in the presence of liquid electrolyte solution (0.05 M LiI, 0.01 M I 2 , and 0.5 M LiClO 4 in acetonitrile; scan rate = 50 mV/s).The efficiency of the fabricated DSSCs was examined using a solar simulator (Xe arc lamp, 100 mW/cm 2 , AM 1.5 G).The XPS scan was obtained using a Thermo Fischer Scientific Xray photoelectron spectrometer.The transmission electron microscope images were obtained using a TEM CM 200 instrument.

Conclusions
Finally, it can be concluded that MoS 2 flowers have been synthesized using the hydrothermal method and characterized using various sophisticated techniques.The synthesized MoS 2 has a high surface area and sheet-like surface morphology.Further, an electrochemical sensor was fabricated using MoS 2 as an electrocatalyst for the determination of the presence of TBZ.The MoS 2 -modified GC electrode exhibited a limit of detection (LOD) of 0.1 µM.The MoS 2 -modified GC electrode also showed good selectivity and sensitivity for the determination of the presence of TBZ.MoS 2 counter electrode-based dye sensitized solar cells were also fabricated.The fabricated dye sensitized solar cells showed good efficiency of 5.01%.This work reported the bi-functional applications of hydrothermally synthesized MoS 2 for the fabrication of TBZ sensors and Pt-free dye sensitized solar cells.

Figure
Figure 2a,b.It was observed that MoS2 has sheet-like surface morphological characteristics.Thus, it can be said that MoS2 has a uniform sheet-like surface structure, which consists of flower-like self-assembled MoS2 particles.TEM images of the MoS2 were also recorded to further confirm its morphological features.FigureS1a,bshows that MoS2 has flower-like sheets, which is in agreement with the SEM results.

Figure 3 .
Figure 3. EDX electron image (a), EDX spectrum (b) and EDX mapping image of Mo (c) and S (d) elements.

Figure 3 .
Figure 3. EDX electron image (a), EDX spectrum (b) and EDX mapping image of Mo (c) and S (d) elements.

Figure 3 .
Figure 3. EDX electron image (a), EDX spectrum (b) and EDX mapping image of Mo (c) and S (d) elements.

Figure 5 .
Figure 5. LSV graph (a) and current value (b) of the GC and MSGC for 20 µM TBZ (conditions; 50 mVs −1 ; 0.1 M PBS; pH = 2.0).The concentration of TBZ present may play a significant role in the catalytic activity of the MSGC.It is of great importance to study the effect of various concentrations of TBZ on the catalytic behavior of the MSGC.Thus, we have recorded LSV graphs of the MSGC in the presence of different concentrations (0.03, 0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 2, 3, 5, 7, 9, 11, 13, 15, and 20 µM) of TBZ.The obtained LSV graphs have been summarized in Figure 6a.The obtained results show that current response increases with increasing concentration of TBZ.The MSGC exhibits decent linear range from 0.03 to 2 µM, and 2 to 20 µM.The linear calibration graph was plotted between the oxidation current responses of the MSGC with respect to the concentration of TBZ, as shown in Figure 6b.The charge transfer kinetic process of the MSGC was also studied for 20 µM TBZ at different applied scan rates (50-500 mV/s).Figure7ashows the obtained LSV curves of the MSGC at different applied scan rates and reveals that current response was enhanced as the applied scan rate rose from 50 to 500 mV/s.The calibration plot between the square root of the applied scan rate and oxidation current response of the MSGC is presented in Figure7b.It was noted that current response linearly increases with a regression coefficient (R 2 ) value of 0.99.This suggests that the sensing of TBZ at the MSGC surface may occur via a diffusion controlled-process.

Figure
CV graphs (a) of the FTO@Pt and FTO@MoS2 in presence of liquid electrolyte solution (0.05 M LiI, 0.01 M I2, and 0.5 M LiClO4 in acetonitrile; scan rate = 50 mV/s).Schematic picture (b) of the DSSCs.

Figure 11 .
Figure 11.J-V graphs of the FTO@Pt and FTO@MoS2 counter electrode based fabricated DSSCs under 1 sun conditions (AM 1.5 G).

Figure 11 .
Figure 11.J-V graphs of the FTO@Pt and FTO@MoS 2 counter electrode based fabricated DSSCs under 1 sun conditions (AM 1.5 G).

Scheme 1 .
Scheme 1. Schematic representation of the synthesis of MoS 2 .

Scheme 2 .
Scheme 2. Schematic illustration of the fabrication of MSGC for application in a TBZ sensor.

Table 1 .
Comparison of the LOD and linear range of the MSGC with previously reported TBZ sensors.

Table 1 .
Comparison of the LOD linear range of the MSGC with previously reported TBZ sensors.
• C for 12 h.The synthesis of MoS 2 is schematically described in Scheme 1.