Low Temperature, Concentration Dependent Synthesis of PEDOT:PSS/MoS2 Nanocomposites: Effect on Morphological and Optical properties

doi:10.21203/rs.3.rs-5007781/v1

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Low Temperature, Concentration Dependent Synthesis of PEDOT:PSS/MoS2 Nanocomposites: Effect on Morphological and Optical properties

https://doi.org/10.21203/rs.3.rs-5007781/v1

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In this work, MoS₂ nanosheets with diverse morphologies and their nanocomposites with PEDOT:PSS are synthesized at relatively low temperature via a hydrothermal process using ammonium heptamolybdate tetrahydrate (AHT), thiourea (Th), and citric acid as precursors. The study investigates the effects of varying concentrations of AHT and Th in acidic condition on the growth, morphology, and optical properties of MoS₂ nanosheets and their nanocomposites with PEDOT:PSS. Morphology, chemical composition, and optical properties of the MoS₂/PEDOT:PSS nanocomposites are studied using Field Emission Scanning Electron Microscopy(FESEM), FTIR, UV-Vis, and photoluminescence spectroscopy. The FESEM images indicate that the MoS₂ nanosheets exhibited large, uniform rectangular and leaf-like structures formed through the stacking of multiple nanosheets. Moreover, the concentration of the PEDOT:PSS polymer is found to have a significant impact on the morphological and optical properties of the MoS₂/PEDOT:PSS nanocomposites.

hydrothermal synthesis

MoS2

PEDOT:PSS

nanocomposite

Transition metal dichalcogenides (TMDs) have emerged as a fascinating class of materials, particularly in the context of two-dimensional (2D) nanostructures, due to their exceptional electrical, optical, and mechanical properties[1, 2]. Among the various TMDs, molybdenum disulfide (MoS₂) stands out due to its unique structural characteristics and superior electronic as well as optoelectronic properties in various applications[3, 4]. MoS₂, with its layered structure and strong in-plane covalent bonding coupled with weak van der Waals interactions between layers allows for the creation of stable, atomically thin layers that exhibit remarkable properties distinct from their bulk counterparts. The 2D structure of MoS₂ confers several advantageous properties such as energy storage devices, field effect transistor (FET) devices, chemical sensors, photodetectors, and light emitting devices etc[4, 5].

Since the advent of organic molecular and polymeric semiconductors, researchers have found many types of semiconducting systems known as conjugated materials. These materials are exciting for use in photonic devices. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is a conducting polymer that stands out as one of the most widely studied and utilized conducting polymer. Its exceptional electrical conductivity, high transparency, flexibility, and stability make it an ideal candidate for a variety of energy applications, including solar cells, supercapacitors, batteries etc[6, 7]. The integration of 2D-MoS₂ into PEDOT:PSS nanocomposites has shown great promise in wide band gap optoelectronic devices because of bandgap tunability, efficient charge separation and transport facilitated by the MoS₂ nanosheets[8]. Their wide bandgap energy allows devices to operate at higher temperatures, voltages, and frequencies, showing significant potential[9].

We report here a facile hydrothermal method for the synthesis of a wide bandgap MoS₂ and its nanocomposites with PEDOT:PSS. The primary objective of this study is to investigate the concentration dependent morphological and optical properties of MoS₂/PEDOT:PSS nanocomposite. This research aims to improve understanding of how wide bandgap MoS₂ and PEDOT:PSS conductive polymer synergically be utilized for application in optoelectronic devices.

2.1 Materials used

The precursors used for the synthesis of MoS₂ nanosheets are Ammonium heptamolybdate tetrahydrate[AHT] (NH₄)₆Mo₇O₂₄.4H₂O, Thiourea (CH₄N₂S), citric acid (C₆H₈O₇), Ammonia water solution(NH₃OH). For synthesis of nanocomposite, PEDOT:PSS polymer is used as phase. PEDOT:PSS(3–4% in water) and Ammonium heptamolybdate tetrahydrate with 99.9% purity is purchased from Sigma-Aldrich Company. Thiourea (Th), Citric acid and Ammonia water solution are obtained from E-Mark company. The materials used all have 99.9% purity and are used without any further treatment.

2.2 Preparation of MoS₂ nanosheets

The MoS₂ nanosheets are synthesized using a facile hydrothermal method. Three different samples of MoS₂ nanosheets (namely M1, M2 and M3) are synthesised by varying the precursor amount but keeping the AHT:Th ratio same. Three concentrations of AHT: 28.33 mM, 40.46 mM and 80.92 mM are separately dissolved in 20 ml of de-ionized water and stirred for 30 mins at temperature 80˚C in acidic conditions provided by adding citric acid. 0.299M, 0.426M and 0.854M of thiourea in 10 ml DI are then added to above three solutions, respectively. The pH of the final solutions is maintained at 5 by adding ammonium water dropwise with constant stirring for 1.5 hours. It is then transferred to a 40 ml hydrothermal bomb to autoclave for 2 hours at temperature ~ 130˚C. After allowing it to cool down at room temperature blue coloured MoS₂ nanosheets dispersion is obtained.

2.3 Preparation of composite films

0.5 ml of each of as-prepared MoS₂ nanosheets M1, M2 and M3 are mixed with 0.01ml and 0.1 ml of PEDOT:PSS respectively in 3 ml solution to obtain M1P1, M1P2, M2P1, M2P2, M3P1 and M3P2 nanocomposite systems. The mixing of MoS₂ with PEDOT:PSS is done by magnetic stirring followed by sonication for 30 minutes. The nanocomposite films are made by simple drop casting method in glass substrates for further studies.

The morphology of the three hydrothermally prepared samples of MoS₂ nanosheets (M1, M2, M3) and their composites with different concentrations of PEDOT:PSS are studied using Field Emission Scanning Electron microscopy (FESEM)(ZEISS at accelerating voltage of 5kV). For optical properties, UV-Vis optical absorption spectra are recorded by Shimadzu UV-1900i spectrophotometer and Photoluminescence(PL) spectra are recorded with Horiba(QM08075-21-C)Spectrofluorometer. For FTIR spectroscopy, PerkinElmer spectrophotometer is used. In this work, we focus on the synthesis of MoS₂ incorporated PEDOT:PSS nanocomposites without any additives and to observe the morphological and optical properties of the prepared nanocomposites.

4.1 UV-Visible spectroscopy

Figure 1(i-iv) shows the UV-Visible absorption spectra of pristine MoS₂, PEDOT:PSS and various MoS₂/PEDOT:PSS nanocomposites. It is observed that absorption is significantly more in UV region than in visible region of electromagnetic spectrum for MoS₂, PEDOT:PSS, and their different nanocomposites. UV-Vis absorption peaks are observed at 211 nm and 273 nm for PEDOT:PSS. These two peaks are attributed to the aromatic ring in PSS structure assigned to n → π* and π → π* transition [10–12]. For all three MoS₂ samples, the straight transition from the deep valence band to the conduction band causes the UV-Vis absorption spectra to exhibit absorption at about 310 nm[13, 14]. The indirect bandgap of samples is calculated using Tauc relation[9].

The optical bandgap PEDOT:PSS is calculated to be 5.17 eV whereas for MoS₂ and MoS₂/PEDOT:PSS nanocomposites it ranges from 3.2–3.8 eV. The calculated band gap indicates that the samples can be classified as wide band gap semiconductors[15]. The bandgap of nanocomposites is observed to be increasing with increase in concentration of PEDOT:PSS. This observed variation in bandgap is attributed to the variation in MoS₂-nanosheet sizes in nanocomposites[9] and possible modifications to the electrical structure of the nanocomposite brought about by incorporation of PEDOT:PSS[16].

4.2 FE-SEM

The surface morphology of pure PEDOT:PSS, MoS₂ nanosheets and MoS₂/PEDOT:PSS nanocomposites are examined using field emission scanning electron microscopy (FESEM). The FESEM images are depicted in Fig. 3.(a)-(j). FESEM images confirms that the morphology of as-synthesized MoS₂ nanosheets changes remarkably with different concentrations of precursors (Molybdenum source and Sulphur source) prepared at same conditions. Hence, adjusting the precursor concentration allows for easy control of the morphology. The synthesised MoS₂ samples are not monolayered but multilayered; stacked together. The MoS₂ nanosheets(M1, M2 and M3) are mostly rectangular with length in the range of 3.5–27µm and width in the range 1.8–23µm. The dimensions of the MoS₂ nanosheet are getting smaller after incorporation of PEDOT:PSS in M1P1, M2P2(in comparison to M1) and M2P1, M2P2(in comparison to M2) nanocomposites. Whereas MoS₂ nanosheets with erratic sizes are observed in M3P1 and M3P2 nanocomposites.

The electron dispersion X-ray (EDX) is used to characterize the elemental composition of hydrothermally synthesized MoS₂ nanosheets and its MoS₂/PEDOT:PSS nanocomposites. EDX spectra of all the MoS₂/PEDOT:PSS nanocomposites reveal similar elemental composition. Figure 4.(a)-(c) shows EDX spectra of PEDOT:PSS, M3 and one of its nanocomposites M3P2. EDX results confirm the presence of Mo and S elements in pure MoS₂ sample, whereas PEDOT:PSS mainly consisted of C and O due to its organic nature. Additionally, EDX spectrum analysis reveal presence of C, N and O in MoS₂. Presence of carbon is due to the carbon-tape used while recording the spectra[17]. Whereas presence Nitrogen may be attributed to residual nitrogen components of Thiourea, and presence of Oxygen is due to high surface energy of MoS₂ nanosheets that quickly interact with water molecules and form bonds with oxygen[18].

Figure 5. shows FTIR spectra of pristine MoS₂, PEDOT:PSS and various MoS₂ /PEDOT:PSS nanocomposites. FTIR results show crosslinking between PEDOT:PSS and in MoS₂. PEDOT:PSS peaks are observed at 699 cm^− 1, 1082 cm^− 1, 1635 cm^− 1, 2925 cm^− 1, 3413 cm^− 1 corresponding to C-S, C-O-C, O-H, C-H and O-H functional groups [19, 20]. The peaks of MoS₂/PEDOT:PSS nanocomposites at 908 cm^− 1, 1072 cm^− 1, 1396 cm^− 1, 1622 cm^− 1 & 3408 cm^− 1 correspond to S-C, C-O-C, C-C stretching of Thiophene ring and O-H functional groups present in PEDOT:PSS respectively[20]. And peaks at 921 cm^− 1, 1071cm^− 1, 1627 cm^− 1, 3194 cm^− 1 belongs to S-S, C-O, C-O-C and O-H groups present in MoS₂ nanosheets which indicate successful formation of MoS₂/PEDOT:PSS nanocomposites[21, 22].

Figure 6. PL spectra of different samples (i) PEDOT:PSS (ii)M1, M1P1, M1P2 (iii) M2, M2P1, M2P2 (iv) M3, M3P1, M3P2

4.4 Photoluminescence (PL):

Photoluminescence spectral data can be utilized to calculate chromaticity coordinates for the prepared samples based on the International Commission on Illumination (CIE) 1931 chromaticity co-ordinates[24]. These coordinates are determined based on the eye response of standard observers to three specific wavelengths of light in the red, green, and blue (RGB) regions to illustrate the color of the light emitted from the sample[25]. Here, we calculated the CIE coordinates at an excitation wavelength of 300nm. The color coordinates for various samples are depicted in the CIE chromaticity diagram using solid symbols[Fig. 7] which indicate the color of the synthesized samples upon excitation at 300nm. From Fig. 7, it is evident that the colors of the prepared MoS₂ and its composites with PEDOT:PSS fall within the blue region, displaying various intensities of blue color. The variation in the chromaticity coordinates indicates a shift in the emission wavelength for the same excitation wavelength of 300nm. Hence, these nanocomposites show potential for applications in optoelectronic devices such as OLED panels for their tuneable shades of blue light.

In this work, we have successfully synthesized MoS₂/PEDOT:SS nanocomposites with varying morphologies and study their optical properties depending on the precursor concentration at a low temperature of 130˚C. The UV-Visible absorption spectra of pristine MoS₂, PEDOT:PSS, and various MoS₂/PEDOT:PSS nanocomposites have exhibited higher UV absorption compared to visible light. PEDOT:PSS showed absorption peaks at 211 nm and 273 nm due to aromatic ring transitions. MoS₂ displayed UV-Vis absorption at 310 nm due to deep valance to conduction band transitions. The nanocomposites exhibited a slight blue shift and varied bandgap, influenced by MoS₂ nanosheet sizes and electrical modifications. The optical bandgap ranged from 3.2 to 3.8 eV, indicating wide band gap semiconductors. Studies of surface morphology using FESEM have revealed that MoS₂ nanosheets changed significantly with different precursor concentrations, allowing control over their morphological properties. Incorporating PEDOT:PSS resulted in smaller nanosheet dimensions in some nanocomposites, while others showed erratic sizes. Elemental composition analysis using EDX confirmed the presence of Mo, S ,and C, N, and O elements in the samples attributed to PEDOT:PSS. FTIR spectra demonstrated successful crosslinking between PEDOT:PSS and MoS₂ with specific peaks indicating nanocomposite formation. PL spectra at room temperature showed distinct peaks for pure PEDOT:PSS and slightly shifted peaks for nanocomposites. The highest PL intensity is observed in pure MoS₂ samples. UV-Vis absorption spectra yielded a higher bandgap than PL spectra, suggesting multilayered nanosheets. The synthesized samples emitted blue light, making them suitable for optoelectronic devices. The work demonstrates how controlled morphological changes in nanocomposites can be used to tailor electronic and optical properties, providing valuable insights for the development of innovative optoelectronic devices, sensors, and energy solutions.

Acknowledgement

The authors express their gratitude to CIF, Gauhati University for FESEM measurements, SAIF-IASST for PL measurements, and Department of Chemical Sciences, Gauhati University for FTIR measurement. The authors also like to acknowledge the financial support from DST, Govt. of India under INSPIRE scheme.

Statements and Declarations:

Ethical approval Not applicable

Competing interests

The authors declare that they have no competing interests.

Author’s contribution

All authors contributed to the study, conceptualization and design of the work. I.P. Borgohain is involved in synthesis, characterization, investigation & writing—original draft preparation. P. Gogoi has contributed to synthesis and formatting manuscript. Data analysis, supervision, writing—review & editing is done by S. Deb.

Funding

One of the authors I.P. Borgohain acknowledge financial support from DST(Govt. of India) through DST-INSPIRE fellowship grant.

Availability of data and materials

Data presented in the study are available with the corresponding author upon reasonable request.

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No competing interests reported.

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Low Temperature, Concentration Dependent Synthesis of PEDOT:PSS/MoS2 Nanocomposites: Effect on Morphological and Optical properties

Status:

Version 1

Abstract

Figures

1 Introduction

2 Experimental

2.1 Materials used

2.2 Preparation of MoS₂ nanosheets

2.3 Preparation of composite films

3 Characterization

4 Results and discussion

4.1 UV-Visible spectroscopy

4.2 FE-SEM

4.4 Photoluminescence (PL):

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1

Low Temperature, Concentration Dependent Synthesis of PEDOT:PSS/MoS2 Nanocomposites: Effect on Morphological and Optical properties

Status:

Version 1

Abstract

Figures

1 Introduction

2 Experimental

2.1 Materials used

2.2 Preparation of MoS2 nanosheets

2.3 Preparation of composite films

3 Characterization

4 Results and discussion

4.1 UV-Visible spectroscopy

4.2 FE-SEM

4.4 Photoluminescence (PL):

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1

2.2 Preparation of MoS₂ nanosheets